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Sample records for accretion rate drops

  1. MASS ACCRETION RATE OF ROTATING VISCOUS ACCRETION FLOW

    SciTech Connect

    Park, Myeong-Gu

    2009-11-20

    The mass accretion rate of transonic spherical accretion flow onto compact objects such as black holes is known as the Bondi accretion rate, which is determined only by the density and the temperature of gas at the outer boundary. A rotating accretion flow has angular momentum, which modifies the flow profile from the spherical Bondi flow, and hence its mass accretion rate, but most work on disc accretion has taken the mass flux to be given with the relation between that parameter and external conditions left uncertain. Within the framework of a slim alpha disk, we have constructed global solutions of the rotating, viscous, hot accretion flow in the Paczynski-Wiita potential and determined its mass accretion rate as a function of density, temperature, and angular momentum of gas at the outer boundary. We find that the low angular momentum flow resembles the spherical Bondi flow and its mass accretion rate approaches the Bondi accretion rate for the same density and temperature at the outer boundary. The high angular momentum flow on the other hand is the conventional hot accretion disk with advection, but its mass accretion rate can be significantly smaller than the Bondi accretion rate with the same boundary conditions. We also find that solutions exist only within a limited range of dimensionless mass accretion rate m-dotident toM-dot/M-dot{sub B}, where M-dot is the mass accretion rate and M-dot{sub B} is the Bondi accretion rate: when the temperature at the outer boundary is equal to the virial temperature, solutions exist only for 0.05approxaccretion rate is roughly independent of the radius of the outer boundary but inversely proportional to the angular momentum at the outer boundary and proportional to the viscosity parameter, m-dotapprox =9.0 alphalambda{sup -1} when 0.1 approx

  2. Teen Birth Rates Drop, But Disparities Persist

    MedlinePlus

    ... Features Teen Birth Rates Drop, But Disparities Persist Language: English Español (Spanish) Recommend on Facebook Tweet Share Compartir The feature you selected is no longer available. In 10 seconds you will be automatically redirected to the CDC. ...

  3. Lifetimes and Accretion Rates of Protoplanetary Disks

    NASA Astrophysics Data System (ADS)

    Li, Min; Xiao, Lin

    2016-03-01

    Protoplanetary disks originate in the collapse of molecular cloud cores. The formation and evolution of disks are influenced by the properties of molecular cloud cores. In this paper we investigate the dependence of disk lifetimes and accretion rates on cloud core properties. We find that the lifetime increases as the angular velocities and the mass of cloud cores increase and that the lifetime decreases as the core temperature increases. We have calculated the distribution of disk lifetimes and disk fractions with stellar age. Our calculations show that the lifetime is in the range of 1-15 Myr and that the typical lifetime is 1-3 Myr. There are a few disks with lifetimes greater than 10 Myr and ˜ 30% of the disks have lifetimes less than 1 Myr. We also fit the disk fraction by an exponential decay curve with characteristic time ˜3.7 Myr. Our results explain the observations of disk lifetimes. We also find that the accretion rate does not change significantly with ω and generally decreases with {T}{{cd}}. At the early evolution of the disks, the \\dot{M}{--}{M}* relation is about \\dot{M}\\propto {M}*1.2-2. Since the effects of the photoevaporation are weak at this stage, this relation is the consequence of the cloud core properties. At the late evolution of the disks, the \\dot{M}{--}{M}* relation is about \\dot{M}\\propto {M}*1.2-1.7. For low accretion rates at this stage, the \\dot{M}{--}{M}* relation results from the effects of X-ray photoevaporation. The calculated \\dot{M}{--}{M}* relations are consistent with the observations.

  4. LAUNCHING AND QUENCHING OF BLACK HOLE RELATIVISTIC JETS AT LOW ACCRETION RATE

    SciTech Connect

    Pu, Hung-Yi; Chang, Hsiang-Kuang; Hirotani, Kouichi

    2012-10-20

    Relativistic jets are launched from black hole (BH) X-ray binaries and active galactic nuclei when the disk accretion rate is below a certain limit (i.e., when the ratio of the accretion rate to the Eddingtion accretion rate, m-dot , is below about 0.01) but quenched when above. We propose a new paradigm to explain this observed coupling between the jet and the accretion disk by investigating the extraction of the rotational energy of a BH when it is surrounded by different types of accretion disk. At low accretion rates (e.g., when m-dot {approx}<0.1), the accretion near the event horizon is quasi-spherical. The accreting plasmas fall onto the event horizon in a wide range of latitudes, breaking down the force-free approximation near the horizon. To incorporate the plasma inertia effect, we consider the magnetohydrodynamical (MHD) extraction of the rotational energy from BHs by the accreting MHD fluid, as described by the MHD Penrose process. It is found that the energy extraction operates, and hence a relativistic jet is launched, preferentially when the accretion disk consists of an outer Shakura-Sunyaev disk (SSD) and an inner advection-dominated accretion flow. When the entire accretion disk type changes into an SSD, the jet is quenched because the plasmas bring more rest-mass energy than what is extracted from the hole electromagnetically to stop the extraction. Several other issues related to observed BH disk-jet couplings, such as why the radio luminosity increases with increasing X-ray luminosity until the radio emission drops, are also explained.

  5. Low Accretion Rate Expected From G2 Gas Cloud

    NASA Astrophysics Data System (ADS)

    Gracey, Brandon; Morsony, Brian; Workman, Jared

    2015-08-01

    We present high-resolution simulations of the encounter of the G2 gas cloud with Sag A*, focusing on the mass that can be accreted onto the supermassive black hole. Even assuming G2 is a gas cloud of a few time the mass of Earth, we find that very little material should be expected to be accreted. From 5 years before to 5 years after pericenter passage, at most 0.1% of the cloud mass is accreted. The total amount of material accreted by Sag A* increases by at most 20% over this period, and in many cases actually decreases due to the passage of G2. Even over very long timescales, out to 30 years after pericenter passage, only a few 10th's of a percent of the cloud will be accreted, with no significant increase in the overall mass accretion rate of Sag A*.We find that the size of the accretion radius in our simulations has a large effect on the accretion rate, with a smaller accretion radius having a smaller accretion rate. Changing the size of the accretion radius has a larger effect than changing the density profile of the cloud or changing the structure of the background material around Sag A*.

  6. Coastal erosion and accretion rates in Greece

    NASA Astrophysics Data System (ADS)

    Foteinis, Spyros; Papadopoulos, Costas; Koutsogiannaki, Irini; Synolakis, Costas

    2010-05-01

    Erosion threatens many coastal regions of Greece. Anthropogenic changes of landforms such as coastal roads built on even narrow beaches, sand mining for construction, poor design of coastal structures that interfere with sediment, and dams without sediment bypasses have significantly reduced beach widths. We present erosion rates for different beaches, some of which are in sensitive ecosystems, otherwise "protected" by local and EU ordinances. By comparing inferences of beach widths in varying intervals from 1933 to 2006, we infer that the construction of dams in Acheloos river in western Greece, built in a faraonic attempt to partially divert its flows to eastern Greece, this is responsible for up to 20m/year erosion rates observed in certain locales in the Acheloos delta. More characteristic erosion rates in the region are ~ 2m/year. By contrast, there appears rapid accretion of up to 4m/year in the beaches around the Nestos delta in northern Greece (Papadopoulos, 2009). In beaches that are not near large river deltas, erosion rates range from 0.5m/year to 1m/year. While we have not done comprehensive comparisons among coastlines with different levels of coastal development, it does appear that rapid coastal development correlates well with erosion rates. The underlying problem is the complete lack of any semblance of coastal zone management in Greece and substandard design of coastal structures, which are often sited without any measurements of waves and currents offshore (Synolakis et al, 2008). Beach maintenance remains an exotic concept for most local authorities, who invariably prefer to build hard coastal structures to "protect" versus nourish, siting lack of experience with nourishment and "environmental" concerns. In certain cases, choices are dictated by costs, the larger the cost the easier the project gets approved by regulatory authorities, hence the preference for concrete or rubble structures. We conclude that, unless urgent salvage measures are

  7. Jets at lowest mass accretion rates

    NASA Astrophysics Data System (ADS)

    Maitra, Dipankar; Cantrell, Andrew; Markoff, Sera; Falcke, Heino; Miller, Jon; Bailyn, Charles

    2011-02-01

    We present results of recent observations and theoretical modeling of data from black holes accreting at very low luminosities (L/LEdd <~ 10-8). We discuss our newly developed time-dependent model for episodic ejection of relativistic plasma within a jet framework, and a successful application of this model to describe the origin of radio flares seen in Sgr A*, the Galactic center black hole. Both the observed time lags and size-frequency relationships are reproduced well by the model. We also discuss results from new Spitzer data of the stellar black hole X-ray binary system A0620-00. Complemented by long term SMARTS monitoring, these observations indicate that once the contribution from the accretion disk and the donor star are properly included, the residual mid-IR spectral energy distribution of A0620-00 is quite flat and consistent with a non-thermal origin. The results above suggest that a significant fraction of the observed spectral energy distribution originating near black holes accreting at low luminosities could result from a mildly relativistic outflow. The fact that these outflows are seen in both stellar-mass black holes as well as in supermassive black holes at the heart of AGNs strengthens our expectation that accretion and jet physics scales with mass.

  8. Binary accretion rates: dependence on temperature and mass ratio

    NASA Astrophysics Data System (ADS)

    Young, M. D.; Clarke, C. J.

    2015-09-01

    We perform a series of 2D smoothed particle hydrodynamics simulations of gas accretion on to binaries via a circumbinary disc, for a range of gas temperatures and binary mass ratios (q). We show that increasing the gas temperature increases the accretion rate on to the primary for all values of the binary mass ratio: for example, for q = 0.1 and a fixed binary separation, an increase of normalized sound speed by a factor of 5 (from our `cold' to `hot' simulations) changes the fraction of the accreted gas that flows on to the primary from 10 to ˜40 per cent. We present a simple parametrization for the average accretion rate of each binary component accurate to within a few per cent and argue that this parametrization (rather than those in the literature based on warmer simulations) is relevant to supermassive black hole accretion and all but the widest stellar binaries. We present trajectories for the growth of q during circumbinary disc accretion and argue that the period distribution of stellar `twin' binaries is strong evidence for the importance of circumbinary accretion. We also show that our parametrization of binary accretion increases the minimum mass ratio needed for spin alignment of supermassive black holes to q ˜ 0.4, with potentially important implications for the magnitude of velocity kicks acquired during black hole mergers.

  9. MEASURING TINY MASS ACCRETION RATES ONTO YOUNG BROWN DWARFS

    SciTech Connect

    Herczeg, Gregory J.; Cruz, Kelle L.; Hillenbrand, Lynne A.

    2009-05-10

    We present low-resolution Keck I/LRIS spectra spanning from 3200 to 9000 A of nine young brown dwarfs and three low-mass stars in the TW Hya Association and in Upper Sco. The optical spectral types of the brown dwarfs range from M5.5 to M8.75, though two have near-IR spectral types of early L dwarfs. We report new accretion rates derived from excess Balmer continuum emission for the low-mass stars TW Hya and Hen 3-600A and the brown dwarfs 2MASS J12073347-3932540, UScoCTIO 128, SSSPM J1102-3431, USco J160606.29-233513.3, DENIS-P J160603.9-205644, and Oph J162225-240515B, and upper limits on accretion for the low-mass star Hen 3-600B and the brown dwarfs UScoCTIO 112, Oph J162225-240515A, and USco J160723.82-221102.0. For the six brown dwarfs in our sample that are faintest at short wavelengths, the accretion luminosity or upper limit is measurable only when the image is binned over large wavelength intervals. This method extends our sensitivity to accretion rate down to {approx}10{sup -13} M{sub sun}yr{sup -1} for brown dwarfs. Since the ability to measure an accretion rate from excess Balmer continuum emission depends on the contrast between excess continuum emission and the underlying photosphere, for objects with earlier spectral types the upper limit on accretion rate is much higher. Absolute uncertainties in our accretion rate measurements of {approx}3-5 include uncertainty in accretion models, brown dwarf masses, and distance. The accretion rate of 2 x 10{sup -12} M {sub sun} yr{sup -1} onto 2MASS J12073347-3932540 is within 15% of two previous measurements, despite large changes in the H{alpha} flux.

  10. ACCRETION RATE AND THE PHYSICAL NATURE OF UNOBSCURED ACTIVE GALAXIES

    SciTech Connect

    Trump, Jonathan R.; Impey, Christopher D.; Gabor, Jared M.; Diamond-Stanic, Aleksandar M.; Kelly, Brandon C.; Civano, Francesca; Hao, Heng; Lanzuisi, Giorgio; Merloni, Andrea; Salvato, Mara; Urry, C. Megan; Jahnke, Knud; Nagao, Tohru; Taniguchi, Yoshi; Koekemoer, Anton M.; Liu, Charles; Mainieri, Vincenzo; Scoville, Nick Z.

    2011-05-20

    We show how accretion rate governs the physical properties of a sample of unobscured broad-line, narrow-line, and lineless active galactic nuclei (AGNs). We avoid the systematic errors plaguing previous studies of AGN accretion rates by using accurate intrinsic accretion luminosities (L{sub int}) from well-sampled multiwavelength spectral energy distributions from the Cosmic Evolution Survey, and accurate black hole masses derived from virial scaling relations (for broad-line AGNs) or host-AGN relations (for narrow-line and lineless AGNs). In general, broad emission lines are present only at the highest accretion rates (L{sub int}/L{sub Edd} > 10{sup -2}), and these rapidly accreting AGNs are observed as broad-line AGNs or possibly as obscured narrow-line AGNs. Narrow-line and lineless AGNs at lower specific accretion rates (L{sub int}/L{sub Edd} < 10{sup -2}) are unobscured and yet lack a broad-line region. The disappearance of the broad emission lines is caused by an expanding radiatively inefficient accretion flow (RIAF) at the inner radius of the accretion disk. The presence of the RIAF also drives L{sub int}/L{sub Edd} < 10{sup -2} narrow-line and lineless AGNs to have ratios of radio-to-optical/UV emission that are 10 times higher than L{sub int}/L{sub Edd} > 10{sup -2} broad-line AGNs, since the unbound nature of the RIAF means it is easier to form a radio outflow. The IR torus signature also tends to become weaker or disappear from L{sub int}/L{sub Edd} < 10{sup -2} AGNs, although there may be additional mid-IR synchrotron emission associated with the RIAF. Together, these results suggest that specific accretion rate is an important physical 'axis' of AGN unification, as described by a simple model.

  11. Binary interactions with high accretion rates onto main sequence stars

    NASA Astrophysics Data System (ADS)

    Shiber, Sagiv; Schreier, Ron; Soker, Noam

    2016-07-01

    Energetic outflows from main sequence stars accreting mass at very high rates might account for the powering of some eruptive objects, such as merging main sequence stars, major eruptions of luminous blue variables, e.g., the Great Eruption of Eta Carinae, and other intermediate luminosity optical transients (ILOTs; red novae; red transients). These powerful outflows could potentially also supply the extra energy required in the common envelope process and in the grazing envelope evolution of binary systems. We propose that a massive outflow/jets mediated by magnetic fields might remove energy and angular momentum from the accretion disk to allow such high accretion rate flows. By examining the possible activity of the magnetic fields of accretion disks, we conclude that indeed main sequence stars might accrete mass at very high rates, up to ≈ 10‑2 M ⊙ yr‑1 for solar type stars, and up to ≈ 1 M ⊙ yr‑1 for very massive stars. We speculate that magnetic fields amplified in such extreme conditions might lead to the formation of massive bipolar outflows that can remove most of the disk's energy and angular momentum. It is this energy and angular momentum removal that allows the very high mass accretion rate onto main sequence stars.

  12. The Mass Accretion Rate of Galaxy Clusters: A Measurable Quantity

    NASA Astrophysics Data System (ADS)

    De Boni, C.; Serra, A. L.; Diaferio, A.; Giocoli, C.; Baldi, M.

    2016-02-01

    We explore the possibility of measuring the mass accretion rate (MAR) of galaxy clusters from their mass profiles beyond the virial radius R200. We derive the accretion rate from the mass of a spherical shell whose inner radius is 2R200, whose thickness changes with redshift, and whose infall velocity is assumed to be equal to the mean infall velocity of the spherical shells of dark matter halos extracted from N-body simulations. This approximation is rather crude in hierarchical clustering scenarios where both smooth accretion and aggregation of smaller dark matter halos contribute to the mass accretion of clusters. Nevertheless, in the redshift range z = [0, 2], our prescription returns an average MAR within 20%-40% of the average rate derived from the merger trees of dark matter halos extracted from N-body simulations. The MAR of galaxy clusters has been the topic of numerous detailed numerical and theoretical investigations, but so far it has remained inaccessible to measurements in the real universe. Since the measurement of the mass profile of clusters beyond their virial radius can be performed with the caustic technique applied to dense redshift surveys of the cluster outer regions, our result suggests that measuring the mean MAR of a sample of galaxy clusters is actually feasible. We thus provide a new potential observational test of the cosmological and structure formation models.

  13. ACCRETION RATES OF MOONLETS EMBEDDED IN CIRCUMPLANETARY PARTICLE DISKS

    SciTech Connect

    Ohtsuki, Keiji; Yasui, Yuki; Daisaka, Hiroshi

    2013-08-01

    We examine the gravitational capture probability of colliding particles in circumplanetary particle disks and accretion rates of small particles onto an embedded moonlet, using analytic calculation, three-body orbital integrations, and N-body simulations. Expanding our previous work, we take into account the Rayleigh distribution of particles' orbital eccentricities and inclinations in our analytic calculation and orbital integration and confirm agreement between them when the particle velocity dispersion is comparable to or larger than their mutual escape velocity and the ratio of the sum of the physical radii of colliding particles to their mutual Hill radius (r-tilde{sub p}) is much smaller than unity. As shown by our previous work, the capture probability decreases significantly when the velocity dispersion is larger than the escape velocity and/or r-tilde{sub p}{approx}>0.7. Rough surfaces of particles can enhance the capture probability. We compare the results of three-body calculations with N-body simulations for accretion of small particles by an embedded moonlet and find agreement at the initial stage of accretion. However, when particles forming an aggregate on the moonlet surface nearly fill the Hill sphere, the aggregate reaches a quasi-steady state with a nearly constant number of particles covering the moonlet, and the accretion rate is significantly reduced compared to the three-body results.

  14. Accretion rate of extraterrestrial 41Ca in Antarctic snow samples

    NASA Astrophysics Data System (ADS)

    Gómez-Guzmán, J. M.; Bishop, S.; Faestermann, T.; Famulok, N.; Fimiani, L.; Hain, K.; Jahn, S.; Korschinek, G.; Ludwig, P.; Rodrigues, D.

    2015-10-01

    Interplanetary Dust Particles (IDPs) are small grains, generally less than a few hundred micrometers in size. Their main source is the Asteroid Belt, located at 3 AU from the Sun, between Mars and Jupiter. During their flight from the Asteroid Belt to the Earth they are irradiated by galactic and solar cosmic rays (GCR and SCR), thus radionuclides are formed, like 41Ca and 53Mn. Therefore, 41Ca (T1/2 = 1.03 × 105 yr) can be used as a key tracer to determine the accretion rate of IDPs onto the Earth because there are no significant terrestrial sources for this radionuclide. The first step of this study consisted to calculate the production rate of 41Ca in IDPs accreted by the Earth during their travel from the Asteroid Belt. This production rate, used in accordance with the 41Ca/40Ca ratios that will be measured in snow samples from the Antarctica will be used to calculate the amount of extraterrestrial material accreted by the Earth per year. There challenges for this project are, at first, the much longer time for the flight needed by the IDPs to travel from the Asteroid Belt to the Earth in comparison with the 41Ca half-life yields an early saturation for the 41Ca/40Ca ratio, and second, the importance of selecting the correct sampling site to avoid a high influx of natural 40Ca, preventing dilution of the 41Ca/40Ca ratio, the quantity measured by AMS.

  15. Rate of organic film formation and oxidation on aqueous drops

    NASA Astrophysics Data System (ADS)

    Aumann, E.; Tabazadeh, A.

    2008-12-01

    Previous studies suggest that saturated fatty acids or other lipids, which are known to be strong film-forming agents, form condensed films on aqueous drops. Specifically, stearic acid (SA) has been used in laboratory and modeling studies to mimic the surface composition of some particles in the atmosphere. In this study, laboratory measurements were used to determine the rate of SA spreading from a solid on aqueous surfaces,ranging in composition from ammonium sulfate to highly acidic sulfuric acid. Maximum spreading rates were measured on neutral electrolyte solutions, while spreading was not observed on aqueous sulfuric and hydrochloric acids (pH < 0). Also, the spreading rates on water and electrolyte surfaces declined sharply as the solution pH was lowered from 7 to 3. Spreading rates are reported with a dependence on the length of solid-aqueous-air boundary (triple interface perimeter). Spreading rates measured on bulk solutions were modeled on atmospheric particles to determine the time constant of organic film formation on aqueous drops. The time required for a saturated fatty acid to spread and coat a submicron salt particle or a cloud drop is on the order of seconds to minutes or minutes to hours, respectively. In conclusion, lipid coatings can form quickly on neutral or slightly acidic salt drops if a sufficient amount of lipid is present in the drop and the lipid is in direct contact with the aqueous solution surface. Rapid film formation and fast heterogeneous oxidation can provide an efficient way of converting water-insoluble organic films into more water-soluble components in aerosols or cloud drops.

  16. Drop-interface coalescence rate in tertiary amine solvent extraction

    SciTech Connect

    Moyer, B.; McDowell, W.J.

    1983-01-01

    An important requirement for economical application of solvent extraction technology is rapid and efficient phase disengagement. However, much progress toward a clear fundamental understanding of the factors that affect phase disengagement rate will be needed before a logical approach to the problem will be possible. In this paper, the conceptual framework for the study of drop-interface coalescence in collapsing liquid/liquid dispersions is developed and the details of the experimental setup employed in the initial work are presented. The method for determining the drop-interface coalescence rate requires measurement of the average volume of drops (v/sub f/) adjacent to the interface, their number (n) per unit area of interface, and dispersed-phase throughput (Q) per unit area. Recording videomicrography for measurement of v/sub f/ and n is employed while Q is found from the changing position of the major interface as the dispersion band collapses (batch mode). Experimental results are presented for the highly purified system 0.1 M trioctylamine in o-xylene vs 0.1 M HCl, 0.9 M LiCl. Successive batch phase-mixing runs (1:1 phase ratio) using this system produced highly reproducible results, attributed to the use of high-purity chemicals and noncontaminating cell construction materials. Wall effects were found to be negligible. The results indicated that the changing throughput observed in batch experiments is brought about not only by drop size growth rates, but also by drop packing behavior and the fact that drop-interface coalescence rate changes with time.

  17. Observe Z sources at High Mass Accretion Rates

    NASA Astrophysics Data System (ADS)

    Canizares, Claude

    2008-09-01

    We propose to test a new interpretation that links mass accretion rate to observed spectral changes in Z-sources in a diffwrent way than previously though. Integral part of the test is to catch Z-source on the horizontal branch (HB). There are a few sources where RXTE and previous observatories established a fairly accurate record of how often they appear on a specific spectral branch. 4 observations for 8 ks each has a 50% chance to observe GX 5-1 on the HB.

  18. Emotional Competence and Drop-Out Rates in Higher Education

    ERIC Educational Resources Information Center

    Kingston, Emma

    2008-01-01

    Purpose: The purpose of this paper is to compare the emotional competence of first year undergraduates enrolled on a high or low drop-out rate (HDR and LDR, respectively) course, at a newly established university within the UK. Design/methodology/approach: A mixed methods approach using both quantitative and qualitative data collection methods was…

  19. OBSERVATIONAL LIMITS ON TYPE 1 ACTIVE GALACTIC NUCLEUS ACCRETION RATE IN COSMOS

    SciTech Connect

    Trump, Jonathan R.; Impey, Chris D.; Gabor, Jared; Kelly, Brandon C.; Elvis, Martin; Hao Heng; Huchra, John P.; Merloni, Andrea; Bongiorno, Angela; Brusa, Marcella; Cappelluti, Nico; McCarthy, Patrick J.; Koekemoer, Anton; Nagao, Tohru; Salvato, Mara; Scoville, Nick Z.

    2009-07-20

    We present black hole masses and accretion rates for 182 Type 1 active galactic nuclei (AGNs) in COSMOS. We estimate masses using the scaling relations for the broad H {beta}, Mg II, and C IV emission lines in the redshift ranges 0.16 < z < 0.88, 1 < z < 2.4, and 2.7 < z < 4.9. We estimate the accretion rate using an Eddington ratio L{sub I}/L{sub Edd} estimated from optical and X-ray data. We find that very few Type 1 AGNs accrete below L{sub I} /L{sub Edd} {approx} 0.01, despite simulations of synthetic spectra which show that the survey is sensitive to such Type 1 AGNs. At lower accretion rates the broad-line region may become obscured, diluted, or nonexistent. We find evidence that Type 1 AGNs at higher accretion rates have higher optical luminosities, as more of their emission comes from the cool (optical) accretion disk with respect to shorter wavelengths. We measure a larger range in accretion rate than previous works, suggesting that COSMOS is more efficient at finding low accretion rate Type 1 AGNs. However, the measured range in accretion rate is still comparable to the intrinsic scatter from the scaling relations, suggesting that Type 1 AGNs accrete at a narrow range of Eddington ratio, with L{sub I} /L{sub Edd} {approx} 0.1.

  20. Smearing of mass accretion rate variation by viscous processes in accretion disks in compact binary systems

    NASA Astrophysics Data System (ADS)

    Ghosh, A.; Chakrabarti, Sandip K.

    2016-09-01

    Variation of mass supply rate from the companion can be smeared out by viscous processes inside an accretion disk. Hence, by the time the flow reaches the inner edge, the variation in X-rays need not reflect the true variation of the mass supply rate at the outer edge. However, if the viscosity fluctuates around a mean value, one would expect the viscous time scale t_{{visc}} also to spread around a mean value. In high mass X-ray binaries, which are thought to be primarily wind-fed, the size of the viscous Keplerian disk is smaller and thus such a spread could be lower as compared to the low mass X-ray binaries which are primarily fed by Roche lobe overflow. If there is an increasing or decreasing trend in viscosity, the interval between enhanced emission would be modified systematically. In the absence of a detailed knowledge about the variation of mass supply rates at the outer edge, we study ideal circumstances where modulation must take place exactly in orbital time scales, such as when there is an ellipticity in the orbit. We study a few compact binaries using long term All Sky monitor (ASM) data (1.5-12 keV) of Rossi X-ray Timing Explorer (RXTE) and all sky survey data (15-50 keV) of Swift satellites by different methods to look for such smearing effects and to infer what these results can tell us about the viscous processes inside the respective disks. We employ three different methods to seek imprints of periodicity on the X-ray variation and found that in all the cases, the location of the peak in the power density spectra is consistent with the orbital frequencies. Interestingly, in high mass X-ray binaries the peaks are sharp with high rms values, consistent with a small Keplerian disk in a wind fed system. However, in low mass X-ray binaries with larger Keplerian disk component, the peaks are spreaded out with much lower rms values. X-ray reflections, or superhump phenomena which may also cause such X-ray modulations would not be affected by the size of

  1. Reaction rate and composition dependence of the stability of thermonuclear burning on accreting neutron stars

    SciTech Connect

    Keek, L.; Cyburt, R. H.; Heger, A.

    2014-06-01

    The stability of thermonuclear burning of hydrogen and helium accreted onto neutron stars is strongly dependent on the mass accretion rate. The burning behavior is observed to change from Type I X-ray bursts to stable burning, with oscillatory burning occurring at the transition. Simulations predict the transition at a 10 times higher mass accretion rate than observed. Using numerical models we investigate how the transition depends on the hydrogen, helium, and CNO mass fractions of the accreted material, as well as on the nuclear reaction rates of 3α and the hot-CNO breakout reactions {sup 15}O(α, γ){sup 19}Ne and {sup 18}Ne(α, p){sup 21}Na. For a lower hydrogen content the transition is at higher accretion rates. Furthermore, most experimentally allowed reaction rate variations change the transition accretion rate by at most 10%. A factor 10 decrease of the {sup 15}O(α, γ){sup 19}Ne rate, however, produces an increase of the transition accretion rate of 35%. None of our models reproduce the transition at the observed rate, and depending on the true {sup 15}O(α, γ){sup 19}Ne reaction rate, the actual discrepancy may be substantially larger. We find that the width of the interval of accretion rates with marginally stable burning depends strongly on both composition and reaction rates. Furthermore, close to the stability transition, our models predict that X-ray bursts have extended tails where freshly accreted fuel prolongs nuclear burning.

  2. Does livestock grazing affect sediment deposition and accretion rates in salt marshes?

    NASA Astrophysics Data System (ADS)

    Nolte, Stefanie; Müller, Frauke; Schuerch, Mark; Wanner, Antonia; Esselink, Peter; Bakker, Jan P.; Jensen, Kai

    2013-12-01

    Accretion rates, defined as the vertical growth of salt marshes measured in mm per year, may be influenced by grazing livestock in two ways: directly, by increasing soil compaction through trampling, and indirectly, by reducing aboveground biomass and thus decreasing sediment deposition rates measured in g/m² per year. Although accretion rates and the resulting surface elevation change largely determine the resilience of salt marshes to sea-level rise (SLR), the effect of livestock grazing on accretion rates has been little studied. Therefore, this study aimed to investigate the effect of livestock grazing on salt-marsh accretion rates. We hypothesise that accretion will be lower in grazed compared to ungrazed salt marshes. In four study sites along the mainland coast of the Wadden Sea (in the south-eastern North Sea), accretion rates, sediment deposition rates, and soil compaction of grazed and ungrazed marshes were analysed using the 137Cs radionuclide dating method. Accretion rates were on average 11.6 mm yr-1 during recent decades and thus higher than current and projected rates of SLR. Neither accretion nor sediment deposition rates were significantly different between grazing treatments. Meanwhile, soil compaction was clearly affected by grazing with significantly higher dry bulk density on grazed compared to ungrazed parts. Based on these results, we conclude that other factors influence whether grazing has an effect on accretion and sediment deposition rates and that the effect of grazing on marsh growth does not follow a direct causal chain. It may have a great importance when interacting with other biotic and abiotic processes on the marsh.

  3. THE RATE OF GAS ACCRETION ONTO BLACK HOLES DRIVES JET VELOCITY

    SciTech Connect

    King, Ashley L.; Miller, Jon M.; Gültekin, Kayhan; Reynolds, Mark; Bietenholz, Michael; Bartel, Norbert; Mioduszewski, Amy; Rupen, Michael

    2015-01-20

    Accreting black holes are observed to launch relativistic, collimated jets of matter and radiation. In some sources, discrete ejections have been detected with highly relativistic velocities. These particular sources typically have very high mass accretion rates, while sources lower knot velocities are predominantly associated with black holes with relatively low mass accretion rates. We quantify this behavior by examining knot velocity with respect to X-ray luminosity, a proxy for mass accretion rate onto the black hole. We find a positive correlation between the mass-scaled X-ray luminosity and jet knot velocity. In addition, we find evidence that the jet velocity is also a function of polar angle, supporting the ''spine-sheath'' model of jet production. Our results reveal a fundamental aspect of how accretion shapes mechanical feedback from black holes into their host environments.

  4. Star-disc interaction in galactic nuclei: orbits and rates of accreted stars

    NASA Astrophysics Data System (ADS)

    Kennedy, Gareth F.; Meiron, Yohai; Shukirgaliyev, Bekdaulet; Panamarev, Taras; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-07-01

    We examine the effect of an accretion disc on the orbits of stars in the central star cluster surrounding a central massive black hole by performing a suite of 39 high-accuracy direct N-body simulations using state-of-the art software and accelerator hardware, with particle numbers up to 128k. The primary focus is on the accretion rate of stars by the black hole (equivalent to their tidal disruption rate for black holes in the small to medium mass range) and the eccentricity distribution of these stars. Our simulations vary not only the particle number, but disc model (two models examined), spatial resolution at the centre (characterized by the numerical accretion radius) and softening length. The large parameter range and physically realistic modelling allow us for the first time to confidently extrapolate these results to real galactic centres. While in a real galactic centre both particle number and accretion radius differ by a few orders of magnitude from our models, which are constrained by numerical capability, we find that the stellar accretion rate converges for models with N ≥ 32k. The eccentricity distribution of accreted stars, however, does not converge. We find that there are two competing effects at work when improving the resolution: larger particle number leads to a smaller fraction of stars accreted on nearly circular orbits, while higher spatial resolution increases this fraction. We scale our simulations to some nearby galaxies and find that the expected boost in stellar accretion (or tidal disruption, which could be observed as X-ray flares) in the presence of a gas disc is about a factor of 10. Even with this boost, the accretion of mass from stars is still a factor of ˜100 slower than the accretion of gas from the disc. Thus, it seems accretion of stars is not a major contributor to black hole mass growth.

  5. Star-disc interaction in galactic nuclei: orbits and rates of accreted stars

    NASA Astrophysics Data System (ADS)

    Kennedy, Gareth F.; Meiron, Yohai; Shukirgaliyev, Bekdaulet; Panamarev, Taras; Berczik, Peter; Just, Andreas; Spurzem, Rainer

    2016-07-01

    We examine the effect of an accretion disc on the orbits of stars in the central star cluster surrounding a central massive black hole by performing a suite of 39 high-accuracy direct N-body simulations using state-of-the art software and accelerator hardware, with particle numbers up to 128k. The primary focus is on the accretion rate of stars by the black hole (equivalent to their tidal disruption rate for black holes in the small to medium mass range) and the eccentricity distribution of these stars. Our simulations vary not only the particle number, but disc model (two models examined), spatial resolution at the centre (characterized by the numerical accretion radius) and softening length. The large parameter range and physically realistic modelling allow us for the first time to confidently extrapolate these results to real galactic centres. While in a real galactic centre both particle number and accretion radius differ by a few orders of magnitude from our models, which are constrained by numerical capability, we find that the stellar accretion rate converges for models with N ≥ 32k. The eccentricity distribution of accreted stars, however, does not converge. We find that there are two competing effects at work when improving the resolution: larger particle number leads to a smaller fraction of stars accreted on nearly circular orbits, while higher spatial resolution increases this fraction. We scale our simulations to some nearby galaxies and find that the expected boost in stellar accretion (or tidal disruption, which could be observed as X-ray flares) in the presence of a gas disc is about a factor of 10. Even with this boost, the accretion of mass from stars is still a factor of ∼100 slower than the accretion of gas from the disc. Thus, it seems accretion of stars is not a major contributor to black hole mass growth.

  6. A Stellar-mass-dependent Drop in Planet Occurrence Rates

    NASA Astrophysics Data System (ADS)

    Mulders, Gijs D.; Pascucci, Ilaria; Apai, Dániel

    2015-01-01

    The Kepler spacecraft has discovered a large number of planets with up to one-year periods and down to terrestrial sizes. While the majority of the target stars are main-sequence dwarfs of spectral type F, G, and K, Kepler covers stars with effective temperatures as low as 2500 K, which corresponds to M stars. These cooler stars allow characterization of small planets near the habitable zone, yet it is not clear if this population is representative of that around FGK stars. In this paper, we calculate the occurrence of planets around stars of different spectral types as a function of planet radius and distance from the star and show that they are significantly different from each other. We further identify two trends. First, the occurrence of Earth- to Neptune-sized planets (1-4 R ⊕) is successively higher toward later spectral types at all orbital periods probed by Kepler; planets around M stars occur twice as frequently as around G stars, and thrice as frequently as around F stars. Second, a drop in planet occurrence is evident at all spectral types inward of a ~10 day orbital period, with a plateau further out. By assigning to each spectral type a median stellar mass, we show that the distance from the star where this drop occurs is stellar mass dependent, and scales with semi-major axis as the cube root of stellar mass. By comparing different mechanisms of planet formation, trapping, and destruction, we find that this scaling best matches the location of the pre-main-sequence co-rotation radius, indicating efficient trapping of migrating planets or planetary building blocks close to the star. These results demonstrate the stellar-mass dependence of the planet population, both in terms of occurrence rate and of orbital distribution. The prominent stellar-mass dependence of the inner boundary of the planet population shows that the formation or migration of planets is sensitive to the stellar parameters.

  7. A STELLAR-MASS-DEPENDENT DROP IN PLANET OCCURRENCE RATES

    SciTech Connect

    Mulders, Gijs D.; Pascucci, Ilaria; Apai, Dániel

    2015-01-10

    The Kepler spacecraft has discovered a large number of planets with up to one-year periods and down to terrestrial sizes. While the majority of the target stars are main-sequence dwarfs of spectral type F, G, and K, Kepler covers stars with effective temperatures as low as 2500 K, which corresponds to M stars. These cooler stars allow characterization of small planets near the habitable zone, yet it is not clear if this population is representative of that around FGK stars. In this paper, we calculate the occurrence of planets around stars of different spectral types as a function of planet radius and distance from the star and show that they are significantly different from each other. We further identify two trends. First, the occurrence of Earth- to Neptune-sized planets (1-4 R {sub ⊕}) is successively higher toward later spectral types at all orbital periods probed by Kepler; planets around M stars occur twice as frequently as around G stars, and thrice as frequently as around F stars. Second, a drop in planet occurrence is evident at all spectral types inward of a ∼10 day orbital period, with a plateau further out. By assigning to each spectral type a median stellar mass, we show that the distance from the star where this drop occurs is stellar mass dependent, and scales with semi-major axis as the cube root of stellar mass. By comparing different mechanisms of planet formation, trapping, and destruction, we find that this scaling best matches the location of the pre-main-sequence co-rotation radius, indicating efficient trapping of migrating planets or planetary building blocks close to the star. These results demonstrate the stellar-mass dependence of the planet population, both in terms of occurrence rate and of orbital distribution. The prominent stellar-mass dependence of the inner boundary of the planet population shows that the formation or migration of planets is sensitive to the stellar parameters.

  8. ACCRETION RATES FOR T TAURI STARS USING NEARLY SIMULTANEOUS ULTRAVIOLET AND OPTICAL SPECTRA

    SciTech Connect

    Ingleby, Laura; Calvet, Nuria; Blaty, Alex; Herczeg, Gregory; Walter, Frederick; Ardila, David; Alexander, Richard; Edwards, Suzan; Espaillat, Catherine; Gregory, Scott G.; Hillenbrand, Lynne; Brown, Alexander E-mail: ncalvet@umich.edu

    2013-04-20

    We analyze the accretion properties of 21 low-mass T Tauri stars using a data set of contemporaneous near-UV (NUV) through optical observations obtained with the Hubble Space Telescope Imaging Spectrograph and the ground-based Small and Medium Aperture Research Telescope System, a unique data set because of the nearly simultaneous broad wavelength coverage. Our data set includes accreting T Tauri stars in Taurus, Chamaeleon I, {eta} Chamaeleon, and the TW Hydra Association. For each source we calculate the accretion rate (M-dot ) by fitting the NUV and optical excesses above the photosphere, produced in the accretion shock, introducing multiple accretion components characterized by a range in energy flux (or density) for the first time. This treatment is motivated by models of the magnetospheric geometry and accretion footprints, which predict that high-density, low filling factor accretion spots coexist with low-density, high filling factor spots. By fitting the UV and optical spectra with multiple accretion components, we can explain excesses which have been observed in the near-IR. Comparing our estimates of M-dot to previous estimates, we find some discrepancies; however, they may be accounted for when considering assumptions for the amount of extinction and variability in optical spectra. Therefore, we confirm many previous estimates of the accretion rate. Finally, we measure emission line luminosities from the same spectra used for the M-dot estimates, to produce correlations between accretion indicators (H{beta}, Ca II K, C II], and Mg II) and accretion properties obtained simultaneously.

  9. MEASURING THE STELLAR ACCRETION RATES OF HERBIG Ae/Be STARS

    SciTech Connect

    Donehew, Brian; Brittain, Sean E-mail: sbritt@clemson.edu

    2011-02-15

    The accretion rate of young stars is a fundamental characteristic of these systems. While accretion onto T Tauri stars has been studied extensively, little work has been done on measuring the accretion rate of their intermediate-mass analogs, the Herbig Ae/Be stars. Measuring the stellar accretion rate of Herbig Ae/Bes is not straightforward both because of the dearth of metal absorption lines available for veiling measurements and the intrinsic brightness of Herbig Ae/Be stars at ultraviolet wavelengths where the brightness of the accretion shock peaks. Alternative approaches to measuring the accretion rate of young stars by measuring the luminosity of proxies such as the Br {gamma} emission line have not been calibrated. A promising approach is the measurement of the veiling of the Balmer discontinuity. We present measurements of this veiling as well as the luminosity of Br {gamma}. We show that the relationship between the luminosity of Br {gamma} and the stellar accretion rate for classical T Tauri stars is consistent with Herbig Ae stars but not Herbig Be stars. We discuss the implications of this finding for understanding the interaction of the star and disk for Herbig Ae/Be stars.

  10. Baseline Assessment of Net Calcium Carbonate Accretion Rates on U.S. Pacific Reefs.

    PubMed

    Vargas-Ángel, Bernardo; Richards, Cristi L; Vroom, Peter S; Price, Nichole N; Schils, Tom; Young, Charles W; Smith, Jennifer; Johnson, Maggie D; Brainard, Russell E

    2015-01-01

    This paper presents a comprehensive quantitative baseline assessment of in situ net calcium carbonate accretion rates (g CaCO3 cm(-2) yr(-1)) of early successional recruitment communities on Calcification Accretion Unit (CAU) plates deployed on coral reefs at 78 discrete sites, across 11 islands in the central and south Pacific Oceans. Accretion rates varied substantially within and between islands, reef zones, levels of wave exposure, and island geomorphology. For forereef sites, mean accretion rates were the highest at Rose Atoll, Jarvis, and Swains Islands, and the lowest at Johnston Atoll and Tutuila. A comparison between reef zones showed higher accretion rates on forereefs compared to lagoon sites; mean accretion rates were also higher on windward than leeward sites but only for a subset of islands. High levels of spatial variability in net carbonate accretion rates reported herein draw attention to the heterogeneity of the community assemblages. Percent cover of key early successional taxa on CAU plates did not reflect that of the mature communities present on surrounding benthos, possibly due to the short deployment period (2 years) of the experimental units. Yet, net CaCO3 accretion rates were positively correlated with crustose coralline algae (CCA) percent cover on the surrounding benthos and on the CAU plates, which on average represented >70% of the accreted material. For foreeefs and lagoon sites combined CaCO3 accretion rates were statistically correlated with total alkalinity and Chlorophyll-a; a GAM analysis indicated that SiOH and Halimeda were the best predictor variables of accretion rates on lagoon sites, and total alkalinity and Chlorophyll-a for forereef sites, demonstrating the utility of CAUs as a tool to monitor changes in reef accretion rates as they relate to ocean acidification. This study underscores the pivotal role CCA play as a key benthic component and supporting actively calcifying reefs; high Mg-calcite exoskeletons makes CCA

  11. Baseline Assessment of Net Calcium Carbonate Accretion Rates on U.S. Pacific Reefs

    PubMed Central

    Vargas-Ángel, Bernardo; Richards, Cristi L.; Vroom, Peter S.; Price, Nichole N.; Schils, Tom; Young, Charles W.; Smith, Jennifer; Johnson, Maggie D.; Brainard, Russell E.

    2015-01-01

    This paper presents a comprehensive quantitative baseline assessment of in situ net calcium carbonate accretion rates (g CaCO3 cm-2 yr-1) of early successional recruitment communities on Calcification Accretion Unit (CAU) plates deployed on coral reefs at 78 discrete sites, across 11 islands in the central and south Pacific Oceans. Accretion rates varied substantially within and between islands, reef zones, levels of wave exposure, and island geomorphology. For forereef sites, mean accretion rates were the highest at Rose Atoll, Jarvis, and Swains Islands, and the lowest at Johnston Atoll and Tutuila. A comparison between reef zones showed higher accretion rates on forereefs compared to lagoon sites; mean accretion rates were also higher on windward than leeward sites but only for a subset of islands. High levels of spatial variability in net carbonate accretion rates reported herein draw attention to the heterogeneity of the community assemblages. Percent cover of key early successional taxa on CAU plates did not reflect that of the mature communities present on surrounding benthos, possibly due to the short deployment period (2 years) of the experimental units. Yet, net CaCO3 accretion rates were positively correlated with crustose coralline algae (CCA) percent cover on the surrounding benthos and on the CAU plates, which on average represented >70% of the accreted material. For foreeefs and lagoon sites combined CaCO3 accretion rates were statistically correlated with total alkalinity and Chlorophyll-a; a GAM analysis indicated that SiOH and Halimeda were the best predictor variables of accretion rates on lagoon sites, and total alkalinity and Chlorophyll-a for forereef sites, demonstrating the utility of CAUs as a tool to monitor changes in reef accretion rates as they relate to ocean acidification. This study underscores the pivotal role CCA play as a key benthic component and supporting actively calcifying reefs; high Mg-calcite exoskeletons makes CCA

  12. The Influence of Accretion Rate and Metallicity on Thermonuclear Bursts: Predictions from KEPLER Models

    NASA Astrophysics Data System (ADS)

    Lampe, Nathanael; Heger, Alexander; Galloway, Duncan K.

    2016-03-01

    Using the KEPLER hydrodynamics code, 464 models of thermonuclear X-ray bursters were performed across a range of accretion rates and compositions. We present the library of simulated burst profiles from this sample, and examine variations in the simulated light curve for different model conditions. We find that the recurrence time varies as a power law against accretion rate, and measure its slope while mixed H/He burning is occurring for a range of metallicities, finding the power law gradient to vary from η =1.1 to 1.24. We identify the accretion rates at which mixed H/He burning stops and a transition occurs to different burning regimes. We explore how varying the accretion rate and metallicity affects burst morphology in both the rise and tail.

  13. Effect of the Heat Flux Density on the Evaporation Rate of a Distilled Water Drop

    NASA Astrophysics Data System (ADS)

    Ponomarev, Konstantin; Orlova, Evgeniya; Feoktistov, Dmitry

    2016-02-01

    This paper presents the experimental dependence of the evaporation rate of a nondeaerated distilled water drop from the heat flux density on the surfaces of non-ferrous metals (copper and brass). A drop was placed on a heated substrate by electronic dosing device. To obtain drop profile we use a shadow optical system; drop symmetry was controlled by a high-speed video camera. It was found that the evaporation rate of a drop on a copper substrate is greater than on a brass. The evaporation rate increases intensively with raising volume of a drop. Calculated values of the heat flux density and the corresponding evaporation rates are presented in this work. The evaporation rate is found to increase intensively on the brass substrate with raising the heat flux density.

  14. The Accretion Rates and White Dwarf Components of Nova-Like Cataclysmic Variables

    NASA Astrophysics Data System (ADS)

    Mizusawa, Trisha; Merritt, J.; Bonaro, M.; Foran, S.; Plumberg, C.; Stewart, H.; Wiley, T.; Ballouz, R.; Sion, E.

    2009-01-01

    We present the results of a multi-component synthetic spectral analysis of the archival far ultraviolet spectra of several key nova-like variables including members of the SW Sex, RW Tri, UX UMa and VY Scl subclasses: KR Aur, V795 Her, BP Lyn, V825 Her, HL Aqr, RW Tri and V425 Cas. Accretion rates as well as the flux contribution of the accreting white dwarf are included in our analysis. Except for RW Tri which has a reliable trigonometric parallax, we computed the distances to the nova-like systems using the method of Knigge(2006, MNRAS, 373, 484). For KR Aur, we find that the white dwarf has T_eff = 29,000 +/- 2000K, log g = 8.4 and contributes 18% of the FUV flux while an accretion disk with accretion rate Mdot = 3 x 10-10 Msun/yr at an inclination of 41 degrees, contributes the remainder. Our analysis of seven archival IUE spectra of RW Tri at its parallax distance consistently yields a low mass (0.4 Msun) white dwarf and an average accretion rate, Mdot = 6.3 x 10-9 Msun/yr. We find that an accreting white dwarf rather than accretion disk dominates the far UV spectrum of V425 Cas while HL Aqr's and V825 Her's FUV spectra are dominated by an accretion disk with Mdot = 1 x 10-9 Msun/yr and 3 x 10-9 Msun/yr, respectively. For BP Lyn we find Mdot = 1 x 10-8 Msun/yr and we explore the possiblity that V795 Her is an intermediate polar. We discuss the implications of our results for the evolutionary status of nova-like variables. This work is supported by NSF grant AST-0807892 to Villanova University and by the Delaware Space Grant Consortium

  15. On the relation between Seyfert 2 accretion rate and environment at z < 0.1

    NASA Astrophysics Data System (ADS)

    Coldwell, Georgina V.; Gurovich, Sebastián; Díaz Tello, Jorge; Söchting, Ilona K.; Lambas, Diego G.

    2014-01-01

    We analyse different properties of the small-scale environment of Seyfert 2 for two samples selected according to the accretion rate parameter , R, from the Sloan Digital Sky Survey, Data Release 7 survey. We compare the results with two control samples of non-active galaxies that cover the same redshift range, luminosity, colours, morphology, age and stellar mass content. Our study shows that both high and low accretion rate subsamples reside in bluer and lower density environments than the control samples. However, we find that this difference is at least two times stronger for the low accretion rate Seyferts. In the vicinity of Seyfert 2, red galaxies have systematically lower values of stellar mass as compared with corresponding control samples. The lower values of stellar mass for red neighbours is more significant at higher density environments and it is more evident for low accretion rate Seyfert. We also find that this effect is independent of the host's stellar mass. Our results are consistent with a scenario where active galactic nucleus occurrence is higher in lower/medium density environments with a higher merger rate and a lack of a dense intergalactic medium (that can strip gas from these systems) that provide suitable conditions for the central black hole feeding. We find this particularly evident for the low accretion rate Seyferts that could compensate through the intergalactic medium the lack of gas of their hosts.

  16. The rate of collisions due to Brownian or gravitational motion of small drops

    NASA Technical Reports Server (NTRS)

    Zhang, Xiaoguang; Davis, Robert H.

    1991-01-01

    Quantitative predictions of the collision rate of two spherical drops undergoing Brownian diffusion or gravitational sedimentation are presented. The diffusion equation for relative Brownian motion of two drops is derived, and the relative motion of pairs of drops in gravitational sedimentation is traced via a trajectory analysis in order to develop theoretical models to determine the collision efficiencies, both with and without interparticle forces applied between the drops. It is concluded that finite collision rates between nondeforming fluid drops are possible for Brownian diffusion or gravitational sedimentation in the absence of attractive forces, in stark contrast to the prediction that lubrication forces prevent rigid spheres from contacting each other unless an attractive force that becomes infinite as the separation approaches zero is applied. Collision rates are shown to increase as the viscosity of the drop-phase decreases. In general, hydrodynamic interactions reduce the collision rates more for gravitational collisions than for Brownian collisions.

  17. Effects of livestock species and stocking density on accretion rates in grazed salt marshes

    NASA Astrophysics Data System (ADS)

    Nolte, Stefanie; Esselink, Peter; Bakker, Jan P.; Smit, Christian

    2015-01-01

    Coastal ecosystems, such as salt marshes, are threatened by accelerated sea-level rise (SLR). Salt marshes deliver valuable ecosystem services such as coastal protection and the provision of habitat for a unique flora and fauna. Whether salt marshes in the Wadden Sea area are able to survive accelerated SLR depends on sufficient deposition of sediments which add to vertical marsh accretion. Accretion rate is influenced by a number of factors, and livestock grazing was recently included. Livestock grazing is assumed to reduce accretion rates in two ways: (a) directly by increasing soil compaction through trampling, and (b) indirectly by affecting the vegetation structure, which may lower the sediment deposition. For four years, we studied the impact of two livestock species (horse and cattle) at two stocking densities (0.5 and 1.0 animal ha-1) on accretion in a large-scale grazing experiment using sedimentation plates. We found lower cumulative accretion rates in high stocking densities, probably because more animals cause more compaction and create a lower canopy. Furthermore, a trend towards lower accretion rates in horse-compared to cattle-grazed treatments was found, most likely because (1) horses are more active and thus cause more compaction, and (2) herbage intake by horses is higher than by cattle, which causes a higher biomass removal and shorter canopy. During summer periods, negative accretion rates were found. When the grazing and non-grazing seasons were separated, the impact of grazing differed among years. In summer, we only found an effect of different treatments if soil moisture (precipitation) was relatively low. In winter, a sufficiently high inundation frequency was necessary to create differences between grazing treatments. We conclude that stocking densities, and to a certain extent also livestock species, affect accretion rates in salt marshes. Both stocking densities and livestock species should thus be taken into account in management

  18. Chernobyl {sup 137}Cs used to determine sediment accretion rates at selected northern European coastal wetlands

    SciTech Connect

    Callaway, J.C.; DeLaune, R.D.; Patrick, W.H. Jr.

    1996-05-01

    Sediment cores were collected form five coastal wetlands along the North Sea (England and Netherlands) and Baltic Sea (Poland). {sup 137}Cs dating was used to assess sediment accretion rates, including rates based on the {sup 137}Cs peak from the 1986 accident at Chernobyl. Peaks form the Chernobyl fallout were found in cores from the Oder and Vistula Rivers in Poland, from the Eastern Scheldt in the Netherlands, and in one of the two cores from Stiffkey Marsh, UK. No evidence of Chernobyl fallout was found in cores from Dengie Marsh, UK. The Chernobyl {sup 137}Cs peak serves as an excellent marker for short-term accretion rates because of its high activity. Vertical accretion rates (cm yr{sup {minus}1}) based on 1963 and 1986 peaks were similar at most sites; differences may be due to large inputs of sediment from storms or recent accumulation of organic matter. Large differences in sediment characteristics and accretion rates were found between samples from Poland and western Europe. Vertical accretion rates over the period 1963-1986 ranged from 0.26 to 0.85 cm{sup {minus}1} and from 0.30 to 1.90 cm yr{sup {minus}1} over the 1986-1991 period. Vertical accretion rates for the period these sites are in imminent danger of excessive flooding. The Chernobyl {sup 137}Cs peak will be especially useful for studies of short-term (i.e. very recent) sedimentation in the near future and for comparisons of sediment processes over different time scales. 33 refs., 4 figs., 4 tabs.

  19. Effects of long-term grazing on sediment deposition and salt-marsh accretion rates

    NASA Astrophysics Data System (ADS)

    Elschot, Kelly; Bouma, Tjeerd J.; Temmerman, Stijn; Bakker, Jan P.

    2013-11-01

    Many studies have attempted to predict whether coastal marshes will be able to keep up with future acceleration of sea-level rise by estimating marsh accretion rates. However, there are few studies focussing on the long-term effects of herbivores on vegetation structure and subsequent effects on marsh accretion. Deposition of fine-grained, mineral sediment during tidal inundations, together with organic matter accumulation from the local vegetation, positively affects accretion rates of marsh surfaces. Tall vegetation can enhance sediment deposition by reducing current flow and wave action. Herbivores shorten vegetation height and this could potentially reduce sediment deposition. This study estimated the effects of herbivores on 1) vegetation height, 2) sediment deposition and 3) resulting marsh accretion after long-term (at least 16 years) herbivore exclusion of both small (i.e. hare and goose) and large grazers (i.e. cattle) for marshes of different ages. Our results firstly showed that both small and large herbivores can have a major impact on vegetation height. Secondly, grazing processes did not affect sediment deposition. Finally, trampling by large grazers affected marsh accretion rates by compacting the soil. In many European marshes, grazing is used as a tool in nature management as well as for agricultural purposes. Thus, we propose that soil compaction by large grazers should be taken in account when estimating the ability of coastal systems to cope with an accelerating sea-level rise.

  20. ACCRETION RATES ON PRE-MAIN-SEQUENCE STARS IN THE YOUNG OPEN CLUSTER NGC 6530

    SciTech Connect

    Gallardo, Jose; Del Valle, Luciano; Ruiz, Maria Teresa E-mail: ldelvall@das.uchile.cl

    2012-01-15

    It is well accepted that during the star formation process, material from a protoplanetary disk is accreted onto the central object during the first {approx}1-5 Myr. Different authors have published measurements of accretion rates for young low- and intermediate-mass stars in several nearby star-forming regions (SFRs). Due to its somewhat larger distance, the SFR M8 (the Lagoon Nebula) has not been studied to the same extent, despite its abundant population of young stellar objects. We have obtained optical band low-resolution spectra of a sample of pre-main-sequence stars in the open cluster NGC 6530 located in the aforementioned nebulae using the Gemini Multi Object Spectrograph at Gemini-South in multi-object mode. Spectra cover the H{sub {alpha}} emission line used to measure the accretion rate, following the method presented by Natta et al. The observed spectral characteristics are fully consistent with pre-main-sequence stars, showing lithium absorption lines, which are very common in young stellar objects, as well as prominent and broad H{sub {alpha}} emission lines, indicating a T Tauri evolutionary stage. This work presents the first determinations of mass accretion rates of young stellar objects in the open cluster NGC 6530, confirming that they are classical T Tauri stars going through the accretion phase. These observations contribute to a better understanding of the stellar content and evolutionary phase of the very active Lagoon Nebula SFR.

  1. Weak Line Quasars at High Redshift: Extremely High Accretion Rate Sources?

    NASA Astrophysics Data System (ADS)

    Shemmer, Ohad

    2011-10-01

    The Sloan Digital Sky Survey has recently discovered a remarkable group of ~80 quasars at z=2.2-5.9 with extremely weak emission lines in their rest-frame UV spectra. We propose to extend our XMM-Newton observations of such sources and obtain imaging spectroscopy of four quasars of this class with a total exposure time of 165 ks, providing ~1000 photons per source. This will enable an accurate measurement of the hard-X-ray photon index required for a robust determination of the accretion rate in each source. Steep spectral slopes will indicate that high accretion rates may be responsible for the intrinsic weakness of the UV emission lines. The proposed observations will lead to new insights about the accretion process and broad emission line formation in all active galactic nuclei.

  2. Weak Line Quasars at High Redshift: Extremely High Accretion Rate Sources?

    NASA Astrophysics Data System (ADS)

    Shemmer, Ohad

    2010-10-01

    The Sloan Digital Sky Survey has recently discovered a remarkable group of ~80 quasars at z=2.2-5.9 with extremely weak emission lines in their rest-frame UV spectra. We propose to extend our XMM-Newton observations of such sources and obtain imaging spectroscopy of four quasars of this class with a total exposure time of 165 ks, providing ~1000 photons per source. This will enable an accurate measurement of the hard-X-ray photon index required for a robust determination of the accretion rate in each source. Steep spectral slopes will indicate that high accretion rates may be responsible for the intrinsic weakness of the UV emission lines. The proposed observations will lead to new insights about the accretion process and broad emission line formation in all active galactic nuclei.

  3. Weak Line Quasars at High Redshift: Extremely High Accretion Rate Sources?

    NASA Astrophysics Data System (ADS)

    Shemmer, Ohad

    2013-10-01

    The Sloan Digital Sky Survey has discovered a remarkable group of ~100 quasars with extremely weak emission lines in their rest-frame optical-UV spectra. We propose to extend our XMM-Newton observations of such sources and obtain imaging spectroscopy of six quasars of this class with a total exposure time of 128 ks, providing ~1000 photons per source. This will enable an accurate measurement of the hard-X-ray photon index required for a robust determination of the accretion rate in each source. Steep spectral slopes will indicate that high accretion rates may be responsible for the intrinsic weakness of the optical-UV emission lines. The proposed observations will lead to new insights about the accretion process and broad emission line formation in all active galactic nuclei.

  4. Suppression of the accretion rate in thin discs around binary black holes.

    NASA Astrophysics Data System (ADS)

    Ragusa, Enrico; Lodato, Giuseppe; Price, Daniel J.

    2016-05-01

    We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with H/R ≳ 0.1, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs, in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on H/R) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of systems accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed after binary coalescence, thus influencing also the evolutionary path both of the binary and of the host galaxy. Our results, being scale-free, are also applicable to equal mass, circular binaries of stellar mass black holes, such as the progenitor of the recently discovered gravitational wave source GW150914.

  5. Suppression of the accretion rate in thin discs around binary black holes

    NASA Astrophysics Data System (ADS)

    Ragusa, Enrico; Lodato, Giuseppe; Price, Daniel J.

    2016-08-01

    We present three-dimensional Smoothed Particle Hydrodynamics (SPH) simulations investigating the dependence of the accretion rate on the disc thickness around an equal-mass, circular black hole binary system. We find that for thick/hot discs, with H/R ≳ 0.1, the binary torque does not prevent the gas from penetrating the cavity formed in the disc by the binary (in line with previous investigations). The situation drastically changes for thinner discs; in this case the mass accretion rate is suppressed, such that only a fraction (linearly dependent on H/R) of the available gas is able to flow within the cavity and accrete on to the binary. Extrapolating this result to the cold and thin accretion discs expected around supermassive black hole binary systems implies that this kind of system accretes less material than predicted so far, with consequences not only for the electromagnetic and gravitational waves emissions during the late inspiral phase but also for the recoil speed of the black hole formed after binary coalescence, thus influencing also the evolutionary path both of the binary and of the host galaxy. Our results, being scale-free, are also applicable to equal-mass, circular binaries of stellar mass black holes, such as the progenitor of the recently discovered gravitational wave source GW150914.

  6. An Analysis of Hispanic Students' Drop Out Rates.

    ERIC Educational Resources Information Center

    Egemba, Maria Olivia; Crawford, James R.

    This study examined factors associated with Hispanic students' high dropout rates, considering not only demographic factors but also the role of family background, early school experiences, and social influences in the high Hispanic student dropout rate. Data came from students in the High School and Beyond (HS&B) longitudinal study of the 1980…

  7. Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

    NASA Astrophysics Data System (ADS)

    Chattopadhyay, Indranil; Kumar, Rajiv

    2016-07-01

    We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion-ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von Zeipel surfaces and projected the jet equations of motion on to the streamline and solved them simultaneously with the accretion disc equations of motion. We found that steady shock cannot exist beyond α ≳ 0.06 in the general relativistic prescription, but is lower if mass-loss is considered too. We showed that for fixed outer boundary, the shock moves closer to the horizon with increasing viscosity parameter. The mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock location. The jet terminal speed increases with stronger shocks; quantitatively speaking, the terminal speed of jets vj∞ > 0.1 if rsh < 20rg. The maximum of the outflow rate obtained in the general relativistic regime is less than 6 per cent of the mass accretion rate.

  8. Monitoring the Mass Accretion Rate in Scorpius X-1 Using the Optical Johnson B Filter

    NASA Astrophysics Data System (ADS)

    McNamara, B. J.; Norwood, J.; Harrison, T. E.; Holtzman, J.; Dukes, R.; Barker, T.

    2005-04-01

    The emission from low-mass X-ray binaries (LMXBs) arises from the accretion of mass onto a neutron star or black hole. A knowledge of the amount of mass being accreted as well as changes in this value are therefore essential inputs into models of these systems. Despite the need for this information, we currently lack an easily applied method that allows the accretion rate to be measured. X-ray color-color plots and UV observations can be used for this purpose, but these methods require access to oversubscribed satellites. Even if time is granted on these facilities, there is no guarantee that the source will be in a desired state when the observations take place. In this paper we show that an estimate of the ratio of the mass accretion rate to the Eddington rate can be obtained for Sco X-1 by using the Johnson B magnitude. Based on correlated X-ray and ground-based observations, we find that for Sco X-1, M˙/M˙E=-(0.123+/-0.007)B+2.543+/-0.085. This relation is valid when the system is on its normal and lower flaring branches. Based on theoretical models, we suggest that similar relations should also exist for other LMXBs.

  9. Estimation of mass outflow rates from viscous relativistic accretion discs around black holes

    NASA Astrophysics Data System (ADS)

    Chattopadhyay, Indranil; Kumar, Rajiv

    2016-04-01

    We investigated flow in Schwarzschild metric, around a non-rotating black hole and obtained self-consistent accretion-ejection solution in full general relativity. We covered the whole of parameter space in the advective regime to obtain shocked, as well as, shock-free accretion solution. We computed the jet streamline using von-Zeipel surfaces and projected the jet equations of motion onto the streamline and solved them simultaneously with the accretion disc equations of motion. We found that steady shock cannot exist beyond α ≲ 0.06 in the general relativistic prescription, but is lower if massloss is considered too. We showed that for fixed outer boundary, the shock moves closer to the horizon with increasing viscosity parameter. The mass outflow rate increases as the shock moves closer to the black hole, but eventually decreases, maximizing at some intermediate value of shock. The jet terminal speed increases with stronger shocks, quantitatively speaking, the terminal speed of jets vj∞ > 0.1 if rsh < 20rg. The maximum of the outflow rate obtained in the general relativistic regime is less than 6% of the mass accretion rate.

  10. The Relation Between Accretion Rate And Jet Power in X-Ray Luminous Elliptical Galaxies

    SciTech Connect

    Allen, Steven W.; Dunn, R.J.H.; Fabian, A.C.; Taylor, G.B.; Reynolds, C.S.; /Maryland U.

    2006-03-10

    Using Chandra X-ray observations of nine nearby, X-ray luminous elliptical galaxies with good optical velocity dispersion measurements, we show that a tight correlation exists between the Bondi accretion rates calculated from the observed gas temperature and density profiles and estimated black hole masses, and the power emerging from these systems in relativistic jets. The jet powers, which are inferred from the energies and timescales required to inflate cavities observed in the surrounding X-ray emitting gas, can be related to the accretion rates using a power law model of the form log (P{sub Bondi}/10{sup 43} erg s{sup -1}) = A + B log (P{sub jet}/10{sup 43} erg s{sup -1}), with A = 0.62 {+-} 0.15 and B = 0.77 {+-} 0.18. Our results show that a significant fraction of the energy associated with the rest mass of material entering the Bondi accretion radius (2.4{sub -0.7}{sup +1.0} per cent, for P{sub jet} = 10{sup 43} erg s{sup -1}) eventually emerges in the relativistic jets. Our results have significant implications for studies of accretion, jet formation and galaxy formation. The observed tight correlation suggests that the Bondi formulae provide a reasonable description of the accretion process in these systems, despite the likely presence of magnetic pressure and angular momentum in the accreting gas. The similarity of the P{sub Bondi} and P{sub jet} values argues that a significant fraction of the matter entering the accretion radius flows down to regions close to the black holes, where the jets are presumably formed. The tight correlation between P{sub Bondi} and P{sub jet} also suggests that the accretion flows are approximately stable over timescales of a few million years. Our results show that the black hole ''engines'' at the hearts of large elliptical galaxies and groups feed back sufficient energy to stem cooling and star formation, leading naturally to the observed exponential cut off at the bright end of the galaxy luminosity function.

  11. ER Death Rate in U.S. Drops by Nearly Half

    MedlinePlus

    ... https://medlineplus.gov/news/fullstory_159738.html ER Death Rate in U.S. Drops by Nearly Half Study ... July 6, 2016 (HealthDay News) -- Hospital emergency room deaths in the United States plummeted by nearly half ...

  12. ER Death Rate in U.S. Drops by Nearly Half

    MedlinePlus

    ... nih.gov/medlineplus/news/fullstory_159738.html ER Death Rate in U.S. Drops by Nearly Half Study ... July 6, 2016 (HealthDay News) -- Hospital emergency room deaths in the United States plummeted by nearly half ...

  13. Report to the Nation shows cancer death rates dropping

    Cancer.gov

    The Annual Report to the Nation on the Status of Cancer, 1975–2009, shows that overall cancer death rates continued to decline in the United States among both men and women, among all major racial and ethnic groups, and for all of the most common cancer s

  14. Accretion Rates on Pre-main-sequence Stars in the Young Open Cluster NGC 6530

    NASA Astrophysics Data System (ADS)

    Gallardo, José; del Valle, Luciano; Ruiz, María Teresa

    2012-01-01

    It is well accepted that during the star formation process, material from a protoplanetary disk is accreted onto the central object during the first ~1-5 Myr. Different authors have published measurements of accretion rates for young low- and intermediate-mass stars in several nearby star-forming regions (SFRs). Due to its somewhat larger distance, the SFR M8 (the Lagoon Nebula) has not been studied to the same extent, despite its abundant population of young stellar objects. We have obtained optical band low-resolution spectra of a sample of pre-main-sequence stars in the open cluster NGC 6530 located in the aforementioned nebulae using the Gemini Multi Object Spectrograph at Gemini-South in multi-object mode. Spectra cover the Hα emission line used to measure the accretion rate, following the method presented by Natta et al. The observed spectral characteristics are fully consistent with pre-main-sequence stars, showing lithium absorption lines, which are very common in young stellar objects, as well as prominent and broad Hα emission lines, indicating a T Tauri evolutionary stage. This work presents the first determinations of mass accretion rates of young stellar objects in the open cluster NGC 6530, confirming that they are classical T Tauri stars going through the accretion phase. These observations contribute to a better understanding of the stellar content and evolutionary phase of the very active Lagoon Nebula SFR. Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciencia e Tecnologia (Brazil), and Ministerio de Ciencia, Tecnología e Innovación Productiva

  15. RADIATIVELY EFFICIENT MAGNETIZED BONDI ACCRETION

    SciTech Connect

    Cunningham, Andrew J.; Klein, Richard I.; McKee, Christopher F.; Krumholz, Mark R.; Teyssier, Romain

    2012-01-10

    We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion from a uniform, isothermal gas onto a resistive, stationary point mass. Only mass, not magnetic flux, accretes onto the point mass. The simulations for this study avoid complications arising from boundary conditions by keeping the boundaries far from the accreting object. Our simulations leverage adaptive refinement methodology to attain high spatial fidelity close to the accreting object. Our results are particularly relevant to the problem of star formation from a magnetized molecular cloud in which thermal energy is radiated away on timescales much shorter than the dynamical timescale. Contrary to the adiabatic case, our simulations show convergence toward a finite accretion rate in the limit in which the radius of the accreting object vanishes, regardless of magnetic field strength. For very weak magnetic fields, the accretion rate first approaches the Bondi value and then drops by a factor of {approx}2 as magnetic flux builds up near the point mass. For strong magnetic fields, the steady-state accretion rate is reduced by a factor of {approx}0.2 {beta}{sup 1/2} compared to the Bondi value, where {beta} is the ratio of the gas pressure to the magnetic pressure. We give a simple expression for the accretion rate as a function of the magnetic field strength. Approximate analytic results are given in the Appendices for both time-dependent accretion in the limit of weak magnetic fields and steady-state accretion for the case of strong magnetic fields.

  16. MEASURING MASS ACCRETION RATE ONTO THE SUPERMASSIVE BLACK HOLE IN M87 USING FARADAY ROTATION MEASURE WITH THE SUBMILLIMETER ARRAY

    SciTech Connect

    Kuo, C. Y.; Asada, K.; Rao, R.; Nakamura, M.; Algaba, J. C.; Liu, H. B.; Inoue, M.; Koch, P. M.; Ho, P. T. P.; Matsushita, S.; Pu, H.-Y.; Nishioka, H.; Pradel, N.; Akiyama, K.

    2014-03-10

    We present the first constraint on the Faraday rotation measure (RM) at submillimeter wavelengths for the nucleus of M87. By fitting the polarization position angles (χ) observed with the Submillimeter Array at four independent frequencies around ∼230 GHz and interpreting the change in χ as a result of external Faraday rotation associated with accretion flow, we determine the RM of the M87 core to be between –7.5 × 10{sup 5} and 3.4 × 10{sup 5} rad m{sup –2}. Assuming a density profile of the accretion flow that follows a power-law distribution and a magnetic field that is ordered, radial, and has equipartition strength, the limit on the RM constrains the mass accretion rate M-dot to be below 9.2 × 10{sup –4} M {sub ☉} yr{sup –1} at a distance of 21 Schwarzschild radii from the central black hole. This value is at least two orders of magnitude smaller than the Bondi accretion rate, suggesting significant suppression of the accretion rate in the inner region of the accretion flow. Consequently, our result disfavors the classical advection-dominated accretion flow and prefers the adiabatic inflow-outflow solution or convection-dominated accretion flow for the hot accretion flow in M87.

  17. Mass accretion rates from multiband photometry in the Carina Nebula: the case of Trumpler 14

    NASA Astrophysics Data System (ADS)

    Beccari, G.; De Marchi, G.; Panagia, N.; Valenti, E.; Carraro, G.; Romaniello, M.; Zoccali, M.; Weidner, C.

    2015-01-01

    Context. We present a study of the mass accretion rates of pre-main sequence (PMS) stars in the cluster Trumpler 14 (Tr 14) in the Carina Nebula. Using optical multiband photometry we were able to identify 356 PMS stars showing Hα excess emission with equivalent width EW(Hα) > 20 Å. We interpret this observational feature as an indication that these objects are still actively accreting gas from their circumstellar medium. From a comparison of the HR diagram with PMS evolutionary models we derive ages and masses of the PMS stars. We find that most of the PMS objects are younger than 10 Myr with a median age of ~3 Myr. Surprisingly, we also find that ~20% of the mass accreting objects are older than 10 Myr. For each PMS star in Trumpler 14 we determine the mass accretion rate (Ṁacc) and discuss its dependence on mass and age. We finally combine the optical photometry with near-IR observations to build the spectral energy distribution (SED) for each PMS star in Tr 14. The analysis of the SEDs suggests the presence of transitional discs in which a large amount of gas is still present and sustains accretion onto the PMS object at ages older than 10 Myr. Our results, discussed in light of recent recent discoveries with Herschel of transitional discs containing a massive gas component around the relatively old PMS stars TW Hydrae, 49 Ceti, and HD 95086, support a new scenario n which old and evolved debris discs still host a significant amount of gas. Aims: Methods: Results:

  18. On the stream-accretion disk interaction - Response to increased mass transfer rate

    NASA Technical Reports Server (NTRS)

    Dgani, Ruth; Livio, Mario; Soker, Noam

    1989-01-01

    The time-dependent interaction between the stream of mass from the inner Lagrangian point and the accretion disk, resulting from an increasing mass transfer rate is calculated. The calculation is fully three-dimensional, using a pseudoparticle description of the hydrodynamics. It is demonstrated that the results of such calculations, when combined with specific observations, have the potential of both determining essential parameters, such as the viscosity parameter alpha, and can distinguish between different models of dwarf nova eruptions.

  19. NOVAE WITH LONG-LASTING SUPERSOFT EMISSION THAT DRIVE A HIGH ACCRETION RATE

    SciTech Connect

    Schaefer, Bradley E.; Collazzi, Andrew C.

    2010-05-15

    We identify a new class of novae characterized by the post-eruption quiescent light curve being more than roughly a factor of 10 brighter than the pre-eruption light curve. Eight novae (V723 Cas, V1500 Cyg, V1974 Cyg, GQ Mus, CP Pup, T Pyx, V4633 Sgr, and RW UMi) are separated out as being significantly distinct from other novae. This group shares a suite of uncommon properties, characterized by the post-eruption magnitude being much brighter than before eruption, short orbital periods, long-lasting supersoft emission following the eruption, a highly magnetized white dwarf (WD), and secular declines during the post-eruption quiescence. We present a basic physical picture which shows why all five uncommon properties are causally connected. In general, novae show supersoft emission due to hydrogen burning on the WD in the final portion of the eruption, and this hydrogen burning will be long-lasting if new hydrogen is poured onto the surface at a sufficient rate. Most novae do not have adequate accretion for continuous hydrogen burning, but some can achieve this if the companion star is nearby (with short orbital period) and a magnetic field channels the matter onto a small area on the WD so as to produce a locally high accretion rate. The resultant supersoft flux irradiates the companion star and drives a higher accretion rate (with a brighter post-eruption phase), which serves to keep the hydrogen burning and the supersoft flux going. The feedback loop cannot be perfectly self-sustaining, so the supersoft flux will decline over time, forcing a decline in the accretion rate and the system brightness. We name this new group after the prototype, V1500 Cyg. V1500 Cyg stars are definitely not progenitors of Type Ia supernovae. The V1500 Cyg stars have similar physical mechanisms and appearances as predicted for nova by the hibernation model, but with this group accounting for only 14% of novae.

  20. Conditions for Circumstellar Disc Formation II: Effects of Initial Cloud Stability and Mass Accretion Rate

    NASA Astrophysics Data System (ADS)

    Machida, Masahiro N.; Matsumoto, Tomoaki; Inutsuka, Shu-ichiro

    2016-09-01

    Disc formation in strongly magnetized cloud cores is investigated using a three-dimensional magnetohydrodynamic simulation with a focus on the effects of the initial cloud stability and the mass accretion rate. The initial cloud stability greatly alters the disc formation process even for prestellar clouds with the same mass-to-flux ratio. A high mass accretion rate onto the disc-forming region is realized in initially unstable clouds, and a large angular momentum is introduced into the circumstellar region in a short time. The region around the protostar has both a thin infalling envelope and a weak magnetic field, which both weaken the effect of magnetic braking. The growth of the rotation-supported disc is promoted in such unstable clouds. Conversely, clouds in an initially near-equilibrium state show lower accretion rates of mass and angular momentum. The angular momentum is transported to the outer envelope before protostar formation. After protostar formation, the circumstellar region has a thick infalling envelope and a strong magnetic field that effectively brake the disc. As a result, disc formation is suppressed when the initial cloud is in a nearly stable state. The density distribution of the initial cloud also affects the disc formation process. Disc growth strongly depends on the initial conditions when the prestellar cloud has a uniform density, whereas there is no significant difference in the disc formation process in prestellar clouds with nonuniform densities.

  1. The Properties of Low-Luminosity AGN: Variability, Accretion Rate, Black Hole Mass and Color

    NASA Astrophysics Data System (ADS)

    Oleas, Juan; Podjed, Stephanie; Sarajedini, Vicki

    2016-01-01

    We present the results from a study of ~5000 Broad-Line selected AGN from the Sloan Digital Sky Survey DR7. Galaxy and AGN templates have been fit to the SDSS spectra to isolate the AGN component. The sources have absolute magnitudes in the range -23 < Mi < -18 and lie at redshifts less than z ~ 0.8. A variability analysis reveals that the anti-correlation between luminosity and variability amplitude continues to the faintest AGN in our sample (Gallastegui-Aizpun & Sarajedini 2014), though the underlying cause of the relation is still poorly understood. To address this, we further explore the connection between AGN luminosity and variability through measurement of the Hβ line width to determine black hole mass and accretion rate. We find that AGN with the highest variability amplitudes at a given luminosity appear to have lower accretion rates compared to low amplitude variables. We also investigate correlations with AGN color and accretion rate among these low-luminosity AGN.

  2. The radial dependence of pebble accretion rates: A source of diversity in planetary systems. I. Analytical formulation

    NASA Astrophysics Data System (ADS)

    Ida, S.; Guillot, T.; Morbidelli, A.

    2016-06-01

    Context. The classical planetesimal accretion scenario for the formation of planets has recently evolved with the idea that pebbles, centimeter- to meter-sized icy grains migrating in protoplanetary disks, can control planetesimal and/or planetary growth. Aims: We investigate how pebble accretion depends on disk properties and affects the formation of planetary systems. Methods: We construct analytical models of pebble accretion onto planetary embryos that consistently account for the mass and orbital evolution of the pebble flow and reflect disk structure. Results: We derive simple formulas for pebble accretion rates in the so-called settling regime for planetary embryos that are more than 100 km in size. For relatively smaller embryos or in outer disk regions, the accretion mode is three-dimensional (3D), meaning that the thickness of the pebble flow must be taken into account, and resulting in an accretion rate that is independent of the embryo mass. For larger embryos or in inner regions, the accretion is in a two-dimensional (2D) mode, i.e., the pebble disk may be considered infinitely thin. We show that the radial dependence of the pebble accretion rate is different (even the sign of the power-law exponent changes) for different disk conditions such as the disk heating source (viscous heating or stellar irradiation), drag law (Stokes or Epstein, and weak or strong coupling), and in the 2D or 3D accretion modes. We also discuss the effect of the sublimation and destruction of icy pebbles inside the snow line. Conclusions: Pebble accretion easily produces a large diversity of planetary systems. In other words, to infer the results of planet formation through pebble accretion correctly, detailed prescriptions of disk evolution and pebble growth, sublimation, destruction and migration are required.

  3. Dependence of the outer density profiles of halos on their mass accretion rate

    SciTech Connect

    Diemer, Benedikt; Kravtsov, Andrey V.

    2014-07-01

    We present a systematic study of the density profiles of ΛCDM halos, focusing on the outer regions, 0.1 < r/R {sub vir} < 9. We show that the median and mean profiles of halo samples of a given peak height exhibit significant deviations from the universal analytic profiles discussed previously in the literature, such as the Navarro-Frenk-White and Einasto profiles, at radii r ≳ 0.5R {sub 200m}. In particular, at these radii the logarithmic slope of the median density profiles of massive or rapidly accreting halos steepens more sharply than predicted. The steepest slope of the profiles occurs at r ≈ R {sub 200m}, and its absolute value increases with increasing peak height or mass accretion rate, reaching slopes of –4 and steeper. Importantly, we find that the outermost density profiles at r ≳ R {sub 200m} are remarkably self-similar when radii are rescaled by R {sub 200m}. This self-similarity indicates that radii defined with respect to the mean density are preferred for describing the structure and evolution of the outer profiles. However, the inner density profiles are most self-similar when radii are rescaled by R {sub 200c}. We propose a new fitting formula that describes the median and mean profiles of halo samples selected by their peak height or mass accretion rate with accuracy ≲ 10% at all radii, redshifts, and masses we studied, r ≲ 9R {sub vir}, 0 < z < 6, and M {sub vir} > 1.7 × 10{sup 10} h {sup –1} M {sub ☉}. We discuss observational signatures of the profile features described above and show that the steepening of the outer profile should be detectable in future weak-lensing analyses of massive clusters. Such observations could be used to estimate the mass accretion rate of cluster halos.

  4. Evidence for a correlation between mass accretion rates onto young stars and the mass of their protoplanetary disks

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Rosotti, G.; Testi, L.; Natta, A.; Alcalá, J. M.; Williams, J. P.; Ansdell, M.; Miotello, A.; van der Marel, N.; Tazzari, M.; Carpenter, J.; Guidi, G.; Mathews, G. S.; Oliveira, I.; Prusti, T.; van Dishoeck, E. F.

    2016-06-01

    A relation between the mass accretion rate onto the central young star and the mass of the surrounding protoplanetary disk has long been theoretically predicted and observationally sought. For the first time, we have accurately and homogeneously determined the photospheric parameters, mass accretion rate, and disk mass for an essentially complete sample of young stars with disks in the Lupus clouds. Our work combines the results of surveys conducted with VLT/X-Shooter and ALMA. With this dataset we are able to test a basic prediction of viscous accretion theory, the existence of a linear relation between the mass accretion rate onto the central star and the total disk mass. We find a correlation between the mass accretion rate and the disk dust mass, with a ratio that is roughly consistent with the expected viscous timescale when assuming an interstellar medium gas-to-dust ratio. This confirms that mass accretion rates are related to the properties of the outer disk. We find no correlation between mass accretion rates and the disk mass measured by CO isotopologues emission lines, possibly owing to the small number of measured disk gas masses. This suggests that the mm-sized dust mass better traces the total disk mass and that masses derived from CO may be underestimated, at least in some cases.

  5. Soil and phosphorus accretion rates in sub-tropical wetlands: Everglades Stormwater Treatment Areas as a case example.

    PubMed

    Bhomia, R K; Inglett, P W; Reddy, K R

    2015-11-15

    Wetlands are known to serve as sinks for particulate matter and associated nutrients and contaminants. Consequently rate of soil accretion is critical for continued performance of wetlands to provide ecosystem services including water quality improvement and reduce excess contaminant loads into downstream waters. Here we demonstrate a new technique to determine rate of soil accretion in selected subtropical treatment wetlands located in southern USA. We also report changes in soil accretion rates and subsequent phosphorus (P) removal efficiency with increasing operational history of these treatment wetlands. Utilizing discernible signatures preserved within the soil depth profiles, 'change points' (CP) that corresponded to specific events in the life history of a wetland were determined. The CP was observed as an abrupt transition in the physico-chemical properties of soil as a manifestation of prevailing historical conditions (e.g. startup of treatment wetlands in this case). Vertical depth of CP from the soil surface was equivalent to the depth of recently accreted soil (RAS) and used for soil accretion rate calculations. Annual soil and P accretion rates determined using CP technique (CPT) in studied wetlands ranged from 1.0±0.3 to 1.7±0.8 cm yr(-1) and 1.3±0.6 to 3.3±2 g m(-2) yr(-1), respectively. There was no difference in RAS depth between emergent and submerged aquatic vegetation communities found at the study location. Our results showed that soil and P accretion rates leveled off after 10 yr of treatment wetlands' operation. On comparison, soil accretion rates and RAS depth determined by CPT were commensurate with that measured by other techniques. CPT can be easily used where a reliable record of wetland establishment date or some significant alteration/perturbation is available. This technique offers a relatively simple alternative to determine vertical accretion rates in free-water surface wetlands. PMID:26172597

  6. Modelling aperiodic X-ray variability in black hole binaries as propagating mass accretion rate fluctuations: A short review

    NASA Astrophysics Data System (ADS)

    Ingram, A. R.

    2016-05-01

    Black hole binary systems can emit very bright and rapidly varying X-ray signals when material from the companion accretes onto the black hole, liberating huge amounts of gravitational potential energy. Central to this process of accretion is turbulence. In the propagating mass accretion rate fluctuations model, turbulence is generated throughout the inner accretion flow, causing fluctuations in the accretion rate. Fluctuations from the outer regions propagate towards the black hole, modulating the fluctuations generated in the inner regions. Here, I present the theoretical motivation behind this picture before reviewing the array of statistical variability properties observed in the light curves of black hole binaries that are naturally explained by the model. I also discuss the remaining challenges for the model, both in terms of comparison to data and in terms of including more sophisticated theoretical considerations.

  7. An upper limit on the contribution of accreting white dwarfs to the type Ia supernova rate.

    PubMed

    Gilfanov, Marat; Bogdán, Akos

    2010-02-18

    There is wide agreement that type Ia supernovae (used as standard candles for cosmology) are associated with the thermonuclear explosions of white dwarf stars. The nuclear runaway that leads to the explosion could start in a white dwarf gradually accumulating matter from a companion star until it reaches the Chandrasekhar limit, or could be triggered by the merger of two white dwarfs in a compact binary system. The X-ray signatures of these two possible paths are very different. Whereas no strong electromagnetic emission is expected in the merger scenario until shortly before the supernova, the white dwarf accreting material from the normal star becomes a source of copious X-rays for about 10(7) years before the explosion. This offers a means of determining which path dominates. Here we report that the observed X-ray flux from six nearby elliptical galaxies and galaxy bulges is a factor of approximately 30-50 less than predicted in the accretion scenario, based upon an estimate of the supernova rate from their K-band luminosities. We conclude that no more than about five per cent of type Ia supernovae in early-type galaxies can be produced by white dwarfs in accreting binary systems, unless their progenitors are much younger than the bulk of the stellar population in these galaxies, or explosions of sub-Chandrasekhar white dwarfs make a significant contribution to the supernova rate. PMID:20164924

  8. Pebble Accretion Rates for Planetesimals: Hydrodynamics Calculations with Direct Particle Integration

    NASA Astrophysics Data System (ADS)

    Hughes, Anna; Boley, Aaron

    2015-12-01

    The formation and growth of planetesimals are fundamental to planet building. However, in our understanding of planet formation, there are a number of processes that limit the formation of planetesimals such as particle bouncing, fragmentation, and inward radial drift due to gas drag. Such processes seemingly make growth beyond mm to cm sizes difficult. In this case, the protoplanetary disk may become rich in pebble-sized solids as opposed to km-sized planetesimals. If a small number of large planetesimals do manage to form, then gas-drag effects can allow those seeds to efficiently accrete the abundant pebbles from the nebula and grow to planet sizes. We present self-consistent hydrodynamic simulations with direct particle integration and gas-drag coupling to evaluate the rate of planetesimal growth due to pebble accretion. We explore a range of particle sizes and nebular conditions using wind tunnel numerical experiments.

  9. Determining Seed Cotton Mass Flow Rate by Pressure Drop Across a Blowbox: Gin Testing

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Accurate measurement of the mass flow rate of seed cotton is needed for control and monitoring purposes in gins. A system was developed that accurately predicted mass flow rate based on the static pressure drop measured across the blowbox and the air velocity and temperature entering the blowbox. Ho...

  10. Stellar parameters and accretion rate of the transition disk star HD 142527 from X-shooter

    SciTech Connect

    Mendigutía, I.; Fairlamb, J.; Oudmaijer, R. D.; Montesinos, B.; Najita, J. R.; Brittain, S. D.; Van den Ancker, M. E.

    2014-07-20

    HD 142527 is a young pre-main-sequence star with properties indicative of the presence of a giant planet and/or a low-mass stellar companion. We have analyzed an X-Shooter/Very Large Telescope spectrum to provide accurate stellar parameters and accretion rate. The analysis of the spectrum, together with constraints provided by the spectral energy distribution fitting, the distance to the star (140 ± 20 pc), and the use of evolutionary tracks and isochrones, led to the following set of parameters: T{sub eff} = 6550 ± 100 K, log g = 3.75 ± 0.10, L{sub *}/L{sub ☉} = 16.3 ± 4.5, M{sub *}/M{sub ☉} = 2.0 ± 0.3, and an age of 5.0 ± 1.5 Myr. This stellar age provides further constraints to the mass of the possible companion estimated by Biller et al., being between 0.20 and 0.35 M{sub ☉}. Stellar accretion rates obtained from UV Balmer excess modeling and optical photospheric line veiling, and from the correlations with several emission lines spanning from the UV to the near-IR, are consistent with each other. The mean value from all previous tracers is 2 (±1) × 10{sup –7} M{sub ☉} yr{sup –1}, which is within the upper limit gas flow rate from the outer to the inner disk recently provided by Cassasus et al.. This suggests that almost all gas transferred between both components of the disk is not trapped by the possible planet(s) in between but fall onto the central star, although it is discussed how the gap flow rate could be larger than previously suggested. In addition, we provide evidence showing that the stellar accretion rate of HD 142527 has increased by a factor ∼7 on a timescale of 2 to 5 yr.

  11. Stellar Parameters and Accretion Rate of the Transition Disk Star HD 142527 from X-Shooter

    NASA Astrophysics Data System (ADS)

    Mendigutía, I.; Fairlamb, J.; Montesinos, B.; Oudmaijer, R. D.; Najita, J. R.; Brittain, S. D.; van den Ancker, M. E.

    2014-07-01

    HD 142527 is a young pre-main-sequence star with properties indicative of the presence of a giant planet and/or a low-mass stellar companion. We have analyzed an X-Shooter/Very Large Telescope spectrum to provide accurate stellar parameters and accretion rate. The analysis of the spectrum, together with constraints provided by the spectral energy distribution fitting, the distance to the star (140 ± 20 pc), and the use of evolutionary tracks and isochrones, led to the following set of parameters: T eff = 6550 ± 100 K, log g = 3.75 ± 0.10, L */L ⊙ = 16.3 ± 4.5, M */M ⊙ = 2.0 ± 0.3, and an age of 5.0 ± 1.5 Myr. This stellar age provides further constraints to the mass of the possible companion estimated by Biller et al., being between 0.20 and 0.35 M ⊙. Stellar accretion rates obtained from UV Balmer excess modeling and optical photospheric line veiling, and from the correlations with several emission lines spanning from the UV to the near-IR, are consistent with each other. The mean value from all previous tracers is 2 (±1) × 10-7 M ⊙ yr-1, which is within the upper limit gas flow rate from the outer to the inner disk recently provided by Cassasus et al.. This suggests that almost all gas transferred between both components of the disk is not trapped by the possible planet(s) in between but fall onto the central star, although it is discussed how the gap flow rate could be larger than previously suggested. In addition, we provide evidence showing that the stellar accretion rate of HD 142527 has increased by a factor ~7 on a timescale of 2 to 5 yr.

  12. Rates of floodplain accretion in a tropical island river system impacted by cyclones and large floods

    NASA Astrophysics Data System (ADS)

    Terry, James P.; Garimella, Sitaram; Kostaschuk, Ray A.

    2002-01-01

    Fluvial processes, especially rates of floodplain accretion, are less well understood in the wet tropics than in other environments. In this study, the caesium-137 ( 137Cs) method was used to examine the recent historical sedimentation rate on the floodplain of the Wainimala River, in the basin of the Rewa River, the largest fluvial system in Fiji and the tropical South Pacific Islands. 137Cs activity in the floodplain stratigraphy showed a well-defined profile, with a clear peak at 115 cm depth. Our measured accretion rate of 3.2 cm year -1 over the last ca. 45 years exceeds rates recorded in humid regions elsewhere. This is explained by the high frequency of tropical cyclones near Fiji (40 since 1970) which can produce extreme rainfalls and large magnitude floods. Since the beginning of hydrological records, large overbank floods have occurred every 2 years on average at the study site. The biggest floods attained peak flows over 7000 m 3 s -1, or six times the bankfull discharge. Concentrations of suspended sediments are very high (max. 200-500 g l -1), delivered mainly by channel bank erosion. In the future, climatic change in the tropical South Pacific region may be associated with greater tropical cyclone intensities, which will probably increase the size of floods in the Rewa Basin and rates of floodplain sedimentation.

  13. The distribution of Extremely High Accretion Rates and Metallicities of QSO's as a Function of Redshift over Cosmic Evolution

    NASA Astrophysics Data System (ADS)

    Abu Seif, Nasser; Kazanas, Demosthenes

    2016-07-01

    The investigation of how QSOs' extremity of accretion rates vary with redshift has remained a major focus of our study in the last five years. How does the evolution of QSOs trace the accretion history of early SMBH? What does accretion at super-Eddington rates look like? Does the correlation between SMBHs and metallicity of QSOs emission line evolve differently at high redshift? Is it a surprise that metallicity is high at high redshift, or is this expected? Here, we establish a new database for the width of an emission line (e.g., Hβ, Mg II and C IV) to obtain a large statistical sample of QSOs at different redshifts. We calculated L/LEdd that determined mass from previous studies (Sloan Digital Sky Survey (SDSS)). We investigated the significant evolution of L/ LEdd for any value of MBH as a function of redshift. Also, we investigated the evolution and distribution of the accretion rate (L/LEdd) over cosmic time with a concentration on the extremely high accretion rate sources at high redshift. The current study investigated the accretion rate (L/LEdd) correlation to other QSO properties and investigated how the accretion of Black Holes L/LEdd and MBH occurs within heavily obscured environments. Our research found that some QSOs are radiating near the Eddington limit with L/ Ledd ~ 1 and those QSOs have extreme accretion. We also found that the lowest M BH has the highest accretion rate, a result that was already noted by McClure & Dunlop (2004). The distribution of Eddington ratio displayed by QSOs clearly shows that all luminous QSOs accreted at their Eddington limit have a poor approximation. This result is important because it is often assumed that optically luminous QSOs are accreting at their Eddington limit within the models of QSOs evolution. We determined the peak of the L/LEdd versus redshift and we found the largest of those peaks to be at the interval of redshift (1< Z < 2). We noted that the highest peak of the distribution of L/LEdd at all

  14. The Accretion Rate Independence of Horizontal Branch Oscillation in XTE J1701-462

    NASA Astrophysics Data System (ADS)

    Li, Zhaosheng; Chen, Li; Qu, Jinlu; Bu, Qingcui; Wang, Dehua; Xu, Renxin

    2014-05-01

    We study the temporal and energy spectral properties of the unique neutron star low-mass X-ray binary XTE J1701-462. Assuming the horizontal branch/normal branch (HB/NB) vertex as a reference position of the accretion rate, the horizontal branch oscillation (HBO) of the HB/NB vertex is roughly 50 Hz. It indicates that the HBO is independent of the accretion rate or the source intensity. The spectral analysis shows R_{in}\\propto \\dot{M}_{Disk}^{2.9+/- 0.09} in the HB/NB vertex and R_{in}\\propto \\dot{M}_{Disk}^{1.7+/- 0.06} in the NB/flaring branch (FB) vertex, which implies that different accretion rates may be produced in the HB/NB and NB/FB vertex. The Comptonization component could be fitted by a constrained broken power law or nthComp. Unlike GX 17+2, the frequencies of HBO positively correlate with the inner disk radius, which contradict with the prediction of the Lense-Thirring precession model. XTE J1701-462, both in the Cyg-like phase and in the Sco-like phase, follows a positive correlation between the break frequency of broadband noise and the characteristic frequency of HBO, which is called the W-K relation. An anticorrelation between the frequency of HBO and photon energy is observed. Moreover, the rms of HBO increases with photon energy until ~10 keV. We discuss the possible origin of HBO from the corona in XTE J1701-462.

  15. Is the Oort A-value a universal growth rate limit for accretion disk shear instabilities?

    NASA Technical Reports Server (NTRS)

    Balbus, Steven A.; Hawley, John F.

    1992-01-01

    A weak-field local MHD instability that is of importance to accretion disks is examined. The maximum growth rate of the instability is found to be not only independent of the magnetic field strength but independent of field geometry as well. In particular, all Keplerian disks are unstable in the presence of any weak poloidal field, with the ratio of the maximum growth rate to disk angular velocity given by 3/4. The maximum growth rate of any weak field configuration that is not purely toroidal is given by the local Oort A-value of the disk. The behavior is studied by using a form of the dynamical Hill equations. It is conjectured that the Oort A-value is an upper bound to the growth rate of any instability feeding upon the free energy of differential rotation.

  16. The Effects of Retention on Drop-Out and Graduation Rates. Research Brief

    ERIC Educational Resources Information Center

    Bleyaert, Barbara

    2009-01-01

    What is the relationship between retention, drop-out and graduation rates? Retention of low-achieving students is one of the most controversial and complex issues educational leaders face, despite a half century of research that has shown consistently that any gains in achievement are short-lived, and the long-term effects for retained students…

  17. Locations of accretion shocks around galaxy clusters and the ICM properties: insights from self-similar spherical collapse with arbitrary mass accretion rates

    NASA Astrophysics Data System (ADS)

    Shi, Xun

    2016-09-01

    Accretion shocks around galaxy clusters mark the position where the infalling diffuse gas is significantly slowed down, heated up, and becomes a part of the intracluster medium (ICM). They play an important role in setting the ICM properties. Hydrodynamical simulations have found an intriguing result that the radial position of this accretion shock tracks closely the position of the `splashback radius' of the dark matter, despite the very different physical processes that gas and dark matter experience. Using the self-similar spherical collapse model for dark matter and gas, we find that an alignment between the two radii happens only for a gas with an adiabatic index of γ ≈ 5/3 and for clusters with moderate mass accretion rates. In addition, we find that some observed ICM properties, such as the entropy slope and the effective polytropic index lying around ˜1.1-1.2, are captured by the self-similar spherical collapse model, and are insensitive to the mass accretion history.

  18. ACCRETION ONTO BLACK HOLES FROM LARGE SCALES REGULATED BY RADIATIVE FEEDBACK. II. GROWTH RATE AND DUTY CYCLE

    SciTech Connect

    Park, Kwang Ho; Ricotti, Massimo E-mail: ricotti@astro.umd.edu

    2012-03-01

    This paper, the second in a series on radiation-regulated accretion onto black holes (BHs) from galactic scales, focuses on the effects of radiation pressure and angular momentum of the accreting gas. We simulate accretion onto intermediate-mass black holes, but we derive general scaling relationships that are solutions of the Bondi problem with radiation feedback valid for any mass of the BH M{sub bh}. Thermal pressure of the ionized sphere around the BH regulates the accretion rate, producing periodic and short-lived luminosity bursts. We find that for ambient gas densities exceeding n{sup cr}{sub H,{infinity}}{proportional_to}M{sup -1}{sub bh}, the period of the oscillations decreases rapidly and the duty cycle increases from 6%, in agreement with observations of the fraction of active galactic nuclei at z {approx} 3, to 50%. The mean accretion rate becomes Eddington limited for n{sub H,{infinity}} > n{sup Edd}{sub H,{infinity}} {approx_equal} n{sup cr}{sub H,{infinity}} T{sub {infinity},4}{sup -1} where T{sub {infinity},4} is the gas temperature in units of 10{sup 4} K. In the sub-Eddington regime, the mean accretion rate onto BHs is about 1%T{sup 2.5}{sub {infinity},4} of the Bondi rate, and thus is proportional to the thermal pressure of the ambient medium. The period of the oscillations coincides with the depletion timescale of the gas inside the ionized bubble surrounding the BH. Gas depletion is dominated by a pressure gradient pushing the gas outward if n{sub H,{infinity}} < n{sup cr}{sub H,{infinity}} and by accretion onto the BH otherwise. Generally, for n{sub H,{infinity}} < n{sup cr}{sub H,{infinity}} angular momentum does not significantly affect the accretion rate and period of the oscillations.

  19. GX 3+1: THE STABILITY OF SPECTRAL INDEX AS A FUNCTION OF MASS ACCRETION RATE

    SciTech Connect

    Seifina, Elena; Titarchuk, Lev E-mail: titarchuk@fe.infn.it

    2012-03-10

    We present an analysis of the spectral and timing properties observed in X-rays from neutron star (NS) binary GX 3+1 (4U 1744-26) during long-term transitions between the faint and bright phases superimposed on short-term transitions between lower banana (LB) and upper banana (UB) branches in terms of its color-color diagram. We analyze all observations of this source obtained with the Rossi X-ray Timing Explorer and Beppo SAX satellites. We find that the X-ray broadband energy spectra during these spectral transitions can be adequately reproduced by a composition of a low-temperature blackbody component, a Comptonized component (COMPTB), and a Gaussian component. We argue that the electron temperature kT{sub e} of the Compton cloud monotonically increases from 2.3 keV to 4.5 keV, when GX 3+1 makes a transition from UB to LB. We also detect an evolution of noise components (a very low frequency noise and a high-frequency noise) during these LB-UB transitions. Using a disk seed photon normalization of COMPTB, which is proportional to the mass accretion rate, we find that the photon power-law index {Gamma} is almost constant ({Gamma} = 2.00 {+-} 0.02) when mass accretion rate changes by a factor of four. In addition, we find that the emergent spectrum is dominated by the strong Comptonized component. We interpret this quasi-stability of the index {Gamma} and a particular form of the spectrum in the framework of a model in which the energy release in the transition layer located between the accretion disk and NS surface dominates that in the disk. Moreover, this index stability effect now established for GX 3+1 was previously found in the atoll source 4U 1728-34 and suggested for a number of other low-mass X-ray NS binaries (see Farinelli and Titarchuk). This intrinsic behavior of NSs, in particular for atoll sources, is fundamentally different from that seen in black hole binary sources where the index monotonically increases during spectral transition from the low

  20. GX 3+1: The Stability of Spectral Index as a Function of Mass Accretion Rate

    NASA Technical Reports Server (NTRS)

    Seifana, Elena; Titarchuk, Lev

    2012-01-01

    We present an analysis of the spectral and timing properties observed in X-rays from neutron star (NS) binary GX 3+1 (4U 1744-26) during long-term transitions between the faint and bright phases superimposed on short-term transitions between lower banana (LB) and upper banana (UB) branches in terms of its color-color diagram, We analyze all observations of this source obtained with the Rossi X-ray Timing Explorer and BeppoSAX satellites, We find that the X-ray broadband energy spectra during these spectral transitions can be adequately reproduced by a composition of a low-temperature blackbody component, a Comptonized component (COMPTB), and Gaussian component We argue that the electron temperature kTe of the Compton cloud monotonically increases from 2.3 keY to 4.5 keY, when GX 3+1 makes a transition from UB to LB. We also detect an evolution of noise components (a very low frequency noise and a high-frequency noise) during these LB-UB transitions. Using a disk seed photon normalization of COMPTB, which is proportional to the mass accretion rate, we find that the photon power-law index Gamma is almost constant (Gamma = 2.00 +/- 0.02) when mass accretion rate changes by factor four. In addition, we find that the emergent spectrum is dominated by the strong Comptonized component We interpret this quasi-stability of the index Gamma and a particular form of the spectrum in the framework of a model in which the energy release in the transition layer located between the accretion disk and NS surface dominates that in the disk. Moreover, this index stability effect now established for GX 3+ I was previously found in the atoll source 4U 1728-34 and suggested for a number of other low-mass X-ray NS binaries. This intrinsic behavior of NSs, in particular for atoll sources, is fundamentally different from that seen in black hole binary sources where the index monotonically increases during spectral transition from the low state to the high state and then finally saturates at

  1. POISSON project. III. Investigating the evolution of the mass accretion rate

    NASA Astrophysics Data System (ADS)

    Antoniucci, S.; García López, R.; Nisini, B.; Caratti o Garatti, A.; Giannini, T.; Lorenzetti, D.

    2014-12-01

    Context. As part of the Protostellar Optical-Infrared Spectral Survey On NTT (POISSON) project, we present the results of the analysis of low-resolution near-IR spectroscopic data (0.9-2.4 μm) of two samples of young stellar objects in the Lupus (52 objects) and Serpens (17 objects) star-forming clouds, with masses in the range of 0.1 to 2.0 M⊙ and ages spanning from 105 to a few 107 yr. Aims: After determining the accretion parameters of the targets by analysing their H i near-IR emission features, we added the results from the Lupus and Serpens clouds to those from previous regions (investigated in POISSON with the same methodology) to obtain a final catalogue (143 objects) of mass accretion rate values (Ṁacc) derived in a homogeneous and consistent fashion. Our final goal is to analyse how Ṁacc correlates with the stellar mass (M∗) and how it evolves in time in the whole POISSON sample. Methods: We derived the accretion luminosity (Lacc) and Ṁacc for Lupus and Serpens objects from the Brγ (Paβ in a few cases) line by using relevant empirical relationships available in the literature that connect the H i line luminosity and Lacc. To minimise the biases that arise from adopting literature data that are based on different evolutionary models and also for self-consistency, we re-derived mass and age for each source of the POISSON samples using the same set of evolutionary tracks. Results: We observe a correlation Ṁacc~M*2.2 between mass accretion rate and stellar mass, similarly to what has previously been observed in several star-forming regions. We find that the time variation of Ṁacc is roughly consistent with the expected evolution of the accretion rate in viscous disks, with an asymptotic decay that behaves as t-1.6. However, Ṁacc values are characterised by a large scatter at similar ages and are on average higher than the predictions of viscous models. Conclusions: Although part of the scattering may be related to systematics due to the

  2. A New Paradigm for Gamma Ray Bursts: Long Term Accretion Rate Modulation by an External Accretion Disk

    NASA Technical Reports Server (NTRS)

    Cannizzo, John; Gehrels, Neil

    2009-01-01

    We present a new way of looking at the very long term evolution of GRBs in which the disk of material surrounding the putative black hole powering the GRB jet modulates the mass flow, and hence the efficacy of the process that extracts rotational energy from the black hole and inner accretion disk. The pre-Swift paradigm of achromatic, shallow-to-steep "breaks" in the long term GRB light curves has not been borne out by detailed Swift data amassed in the past several years. We argue that, given the initial existence of a fall-back disk near the progenitor, an unavoidable consequence will be the formation of an "external disk" whose outer edge continually moves to larger radii due to angular momentum transport and lack of a confining torque. The mass reservoir at large radii moves outward with time and gives a natural power law decay to the GRB light curves. In this model, the different canonical power law decay segments in the GRB identified by Zhang et al. and Nousek et al. represent different physical states of the accretion disk. We identify a physical disk state with each power law segment.

  3. A NEW PARADIGM FOR GAMMA-RAY BURSTS: LONG-TERM ACCRETION RATE MODULATION BY AN EXTERNAL ACCRETION DISK

    SciTech Connect

    Cannizzo, J. K. E-mail: gehrels@milkyway.gsfc.nasa.gov

    2009-08-01

    We present a new way of looking at the very long-term evolution of gamma-ray bursts (GRBs) in which the disk of material surrounding the putative black hole powering the GRB jet modulates the mass flow, and hence the efficacy of the process that extracts rotational energy from the black hole and inner accretion disk. The pre-Swift paradigm of achromatic, shallow-to-steep 'breaks' in the long-term GRB light curves has not been borne out by detailed Swift data amassed in the past several years. We argue that, given the initial existence of a fall-back disk near the progenitor, an unavoidable consequence will be the formation of an 'external disk' whose outer edge continually moves to larger radii due to angular momentum transport and lack of a confining torque. The mass reservoir at large radii moves outward with time and gives a natural power-law decay to the GRB light curves. In this model, the different canonical power-law decay segments in the GRB identified by Zhang et al. and Nousek et al. represent different physical states of the accretion disk. We identify a physical disk state with each power-law segment.

  4. Differential rates of vertical accretion and elevation change among aerial root types in Micronesian mangrove forests

    USGS Publications Warehouse

    Krauss, K.W.; Allen, J.A.; Cahoon, D.R.

    2003-01-01

    Root systems in mangrove swamps have captured the attention of scientists for decades. Among the postulated roles of root structures include a contribution to the geomorphological stability of mangrove soils through sediment trapping and binding. In this study, we used feldspar marker horizons and sediment pins to investigate the influence of three different functional root types - prop roots in Rhizophora spp., root knees in Bruguiera gymnorrhiza, and pneumatophores in Sonneratia alba - on vertical accretion and elevation change in three mangrove forests in the Federated States of Micronesia. Prop roots facilitated vertical accretion (11.0 mm year-1) more than pneumatophores or bare soil controls (mean, 8.3 mm year-1). Sediment elevation, on the other hand, increased at an average rate of only 1.3 mm year-1 across all root types, with rate differences by root type, ranging from -0.2 to 3.4 mm year-1, being detected within river basins. This investigation demonstrates that prop roots can assist in the settling of suspended sediments from estuarine waters, yet prop root structures are not as successful as pneumatophores in maintaining sediment elevation over 2.5 years. As root densities increase over time, an increase in turbulence-induced erosion and in shallow subsidence as organic peat layers form is expected in Micronesian mangrove forests. ?? 2003 Elsevier Science B.V. All rights reserved.

  5. New Low Accretion Rate Magnetic Binary Systems and their Significance for the Evolution of Cataclysmic Variables

    NASA Astrophysics Data System (ADS)

    Schmidt, Gary D.; Szkody, Paula; Vanlandingham, Karen M.; Anderson, Scott F.; Barentine, J. C.; Brewington, Howard J.; Hall, Patrick B.; Harvanek, Michael; Kleinman, S. J.; Krzesinski, Jurek; Long, Dan; Margon, Bruce; Neilsen, Eric H., Jr.; Newman, Peter R.; Nitta, Atsuko; Schneider, Donald P.; Snedden, Stephanie A.

    2005-09-01

    Discoveries of two new white dwarf plus M star binaries with striking optical cyclotron emission features from the Sloan Digital Sky Survey (SDSS) brings to six the total number of X-ray-faint, magnetic accretion binaries that accrete at rates M˙<~10-13 Msolar yr-1, or <1% of the values normally encountered in cataclysmic variables. This fact, coupled with donor stars that underfill their Roche lobes and very cool white dwarfs, brand the binaries as post-common-envelope systems whose orbits have not yet decayed to the point of Roche lobe contact. They are premagnetic cataclysmic variables, or pre-Polars. The systems exhibit spin-orbit synchronism and apparently accrete by efficient capture of the stellar wind from the secondary star, a process that has been dubbed a ``magnetic siphon.'' Because of this, period evolution of the binaries will occur solely by gravitational radiation, which is very slow for periods >3 hr. Optical surveys for the cyclotron harmonics appear to be the only means of discovery, so the space density of pre-Polars could rival that of Polars, and the binaries provide an important channel of progenitors (in addition to the asynchronous intermediate Polars). Both physical and SDSS observational selection effects are identified that may help to explain the clumping of all six systems in a narrow range of magnetic field strength around 60 MG. A portion of the results presented here was obtained with the MMT Observatory, a facility operated jointly by the University of Arizona and the Smithsonian Institution. Based in part on observations with the Apache Point Observatory 3.5 m telescope and the Sloan Digital Sky Survey, which are owned and operated by the Astrophysical Research Consortium (ARC).

  6. EVIDENCE FOR ACCRETION RATE CHANGE DURING TYPE I X-RAY BURSTS

    SciTech Connect

    Worpel, Hauke; Galloway, Duncan K.; Price, Daniel J.

    2013-08-01

    The standard approach for time-resolved X-ray spectral analysis of thermonuclear bursts involves subtraction of the pre-burst emission as background. This approach implicitly assumes that the persistent flux remains constant throughout the burst. We reanalyzed 332 photospheric radius expansion bursts observed from 40 sources by the Rossi X-Ray Timing Explorer, introducing a multiplicative factor f{sub a} to the persistent emission contribution in our spectral fits. We found that for the majority of spectra the best-fit value of f{sub a} is significantly greater than 1, suggesting that the persistent emission typically increases during a burst. Elevated f{sub a} values were not found solely during the radius expansion interval of the burst, but were also measured in the cooling tail. The modified model results in a lower average value of the {chi}{sup 2} fit statistic, indicating superior spectral fits, but not yet to the level of formal statistical consistency for all the spectra. We interpret the elevated f{sub a} values as an increase of the mass accretion rate onto the neutron star during the burst, likely arising from the effects of Poynting-Robertson drag on the disk material. We measured an inverse correlation of f{sub a} with the persistent flux, consistent with theoretical models of the disk response. We suggest that this modified approach may provide more accurate burst spectral parameters, as well as offering a probe of the accretion disk structure.

  7. Sensitivity analysis of the Wetland Accretion Rate Model for Ecosystem Resilience (WARMER)

    NASA Astrophysics Data System (ADS)

    Swanson, K.; Drexler, J. Z.; Schoellhamer, D. H.; Thorne, K.; Spragens, K.; Takekawa, J.

    2010-12-01

    The San Francisco Estuary contains the largest extent of tidal marsh in the western United States. It is home to several state and federally listed species that are threatened or endangered. Climate change is a potential threat to these tidal marsh habitats through accelerated sea-level rise. The Wetland Accretion Rate Model for Ecosystem Resilience, or WARMER, is a 1-D vertical model of elevation at a point representative of target wetland habitat. WARMER incorporates both biological and physical components of vertical marsh accretion processes based on previous wetland models and is modified to incorporate mechanistic organic matter and inorganic deposition and the predicted SLR curve for San Francisco Estuary. Processes that are currently being modified include relative sea-level rise, inorganic sediment deposition, organic matter production, decomposition, and compaction. The model will be applied to marshes across the San Francisco Estuary and results will be used to evaluate the extent sea-level rise will reduce the functional habitat of the threatened black rail (Laterallus jamaicensis coturniculus), the endangered California clapper rail (Rallus longirostris obsoletus), and the endangered salt marsh harvest mouse (Reithrodontomys raviventris). Here we present a sensitivity analysis of key model parameters. Previous studies have noted that inorganic sediment deposition, initial elevation and pore space are the most sensitive parameters. Consistent with these studies, sensitivity analysis shows that pore space is the most sensitive parameter in the current model and the modified inorganic sediment deposition subroutine is particularly sensitive to the parameterization of settling velocity. Perturbations to initial elevation, the rate of sea level rise, organic matter input rates and percent refractory organic matter had small impacts on the modeled final elevation. Proper characterization of marsh sediment pore space and temporally variable sediment

  8. Testing propagating mass accretion rate fluctuations model PROPFLUC on black hole X-ray binaries

    NASA Astrophysics Data System (ADS)

    Rapisarda, S.; Ingram, A.; van der Klis, M.

    2016-05-01

    Over the past 20 years, a consistent phenomenology has been established to describe the variability properties of black hole X-ray binaries. However, the physics behind the observational data is still poorly understood. The recently proposed model PROPFLUC assumes a truncated disc/hot inner flow geometry, with mass accretion rate fluctuations propagating through a precessing inner flow. These two processes give rise respectively to broad band variability and a quasi-periodic oscillation (QPO) on the precession frequency. We recently applied systematically for the first time PROPFLUC on a black hole candidate (MAXI J1543-564) in order to compare the results of phenomenological and physical modeling of the source power spectrum and to give a physical interpretation of the rising phase of the source outburst. Here we resume the results of our study on MAXI J1543-564 and we discuss future PROPFLUC implementations.

  9. Drop size distributions and kinetic energy rates in variable intensity rainfall

    NASA Astrophysics Data System (ADS)

    Assouline, Shmuel

    2016-04-01

    Temporal variability in rainfall intensity reflects on the drop size distribution (DSD), and affects the rainfall kinetic energy during the event. Smith et al. (2009) reported on 1-min interval rainfall intensity and corresponding DSD variability during a storm on the 22/7/06 at Princeton, NJ. They reported also on DSDs characteristics of heavy convective rainfall events during the whole summer. Applying the DSD model of Assouline and Mualem (1997), it is shown that: (a) a similar relationship between the mean drop size and the rainfall intensity characterized the local rainfall at both the seasonal and the single storm scale; (b) using the mean drop size as a scaling factor of the DSD removes the rainfall intensity dependence at the intrastorm scale, providing a powerful tool to deal with temporal variability of rainfall rates during rainfall events. For a storm characterized by a given temporal variability of intensities, three different ways of evaluating kinetic energy per unit mass or time were applied. By comparison to estimates accounting for rainfall temporal variability and related full DSDs, representing the storm by mean intensity and drop diameter tends to overestimate kinetic energy for low intensities and underestimate it for the higher ones. The relative error for the kinetic energy per unit of mass is ±45% and shifts from negative to positive sign for I>25 mm/h. For the kinetic energy per unit of time, the relative error ranges from -100% to +210% and changes sign for I>45 mm/h. When temporal variation of intensity is accounted for but drops are characterized by their mean values instead of the full DSD, kinetic energy is underestimated by 20% on average. Consequently, accounting for temporal variability in rainfall intensity during a storm has a notable impact on the erosive power of the rainfall.

  10. Hospital addresses ED overcrowding, sees treatment times and walkout rates drop.

    PubMed

    2005-11-01

    Mary Washington Hospital in Fredericksburg, VA, addressed the new accreditation standard on overcrowding and saw its treatment times drop by about two hours and its walkout rate drop to 2%, despite increases in volume. A Patient Flow Excellence and Accountability Team focused on getting patients upstairs in under one hour, which was achieved through nurse and supervisor buy-in. Monthly staff meetings were added for all shifts. Computers and equipment for trauma and codes were placed closer to where they were needed. The staffing ratio on the acute side was reduced to 1:4 for nurses and 1:12 for physicians. The ED converted to a five-level triage system, and the hospital is experimenting with a triage nurse focused on lower acuity areas and a "virtual bed system" in which patients are seen by a physician/nurse team at triage and start having labs and X-rays done immediately. PMID:16752445

  11. Whole body synthesis rates of DHA from α-linolenic acid are greater than brain DHA accretion and uptake rates in adult rats[S

    PubMed Central

    Domenichiello, Anthony F.; Chen, Chuck T.; Trepanier, Marc-Olivier; Stavro, P. Mark; Bazinet, Richard P.

    2014-01-01

    Docosahexaenoic acid (DHA) is important for brain function, however, the exact amount required for the brain is not agreed upon. While it is believed that the synthesis rate of DHA from α-linolenic acid (ALA) is low, how this synthesis rate compares with the amount of DHA required to maintain brain DHA levels is unknown. The objective of this work was to assess whether DHA synthesis from ALA is sufficient for the brain. To test this, rats consumed a diet low in n-3 PUFAs, or a diet containing ALA or DHA for 15 weeks. Over the 15 weeks, whole body and brain DHA accretion was measured, while at the end of the study, whole body DHA synthesis rates, brain gene expression, and DHA uptake rates were measured. Despite large differences in body DHA accretion, there was no difference in brain DHA accretion between rats fed ALA and DHA. In rats fed ALA, DHA synthesis and accretion was 100-fold higher than brain DHA accretion of rats fed DHA. Also, ALA-fed rats synthesized approximately 3-fold more DHA than the DHA uptake rate into the brain. This work indicates that DHA synthesis from ALA may be sufficient to supply the brain. PMID:24212299

  12. A CORRELATION BETWEEN STAR FORMATION RATE AND AVERAGE BLACK HOLE ACCRETION IN STAR-FORMING GALAXIES

    SciTech Connect

    Chen, Chien-Ting J.; Hickox, Ryan C.; Alberts, Stacey; Pope, Alexandra; Brodwin, Mark; Jones, Christine; Forman, William R.; Goulding, Andrew D.; Murray, Stephen S.; Alexander, David M.; Mullaney, James R.; Assef, Roberto J.; Gorjian, Varoujan; Brown, Michael J. I.; Dey, Arjun; Jannuzi, Buell T.; Le Floc'h, Emeric

    2013-08-10

    We present a measurement of the average supermassive black hole accretion rate (BHAR) as a function of the star formation rate (SFR) for galaxies in the redshift range 0.25 < z < 0.8. We study a sample of 1767 far-IR-selected star-forming galaxies in the 9 deg{sup 2} Booetes multi-wavelength survey field. The SFR is estimated using 250 {mu}m observations from the Herschel Space Observatory, for which the contribution from the active galactic nucleus (AGN) is minimal. In this sample, 121 AGNs are directly identified using X-ray or mid-IR selection criteria. We combined these detected AGNs and an X-ray stacking analysis for undetected sources to study the average BHAR for all of the star-forming galaxies in our sample. We find an almost linear relation between the average BHAR (in M{sub Sun} yr{sup -1}) and the SFR (in M{sub Sun} yr{sup -1}) for galaxies across a wide SFR range 0.85 < log SFR < 2.56: log BHAR = (- 3.72 {+-} 0.52) + (1.05 {+-} 0.33)log SFR. This global correlation between SFR and average BHAR is consistent with a simple picture in which SFR and AGN activity are tightly linked over galaxy evolution timescales.

  13. Growing black holes and galaxies: black hole accretion versus star formation rate

    NASA Astrophysics Data System (ADS)

    Volonteri, Marta; Capelo, Pedro R.; Netzer, Hagai; Bellovary, Jillian; Dotti, Massimo; Governato, Fabio

    2015-05-01

    We present a new suite of hydrodynamical simulations and use it to study, in detail, black hole and galaxy properties. The high time, spatial and mass resolution, and realistic orbits and mass ratios, down to 1:6 and 1:10, enable us to meaningfully compare star formation rate (SFR) and BH accretion rate (BHAR) time-scales, temporal behaviour, and relative magnitude. We find that (i) BHAR and galaxy-wide SFR are typically temporally uncorrelated, and have different variability time-scales, except during the merger proper, lasting ˜0.2-0.3 Gyr. BHAR and nuclear (<100 pc) SFR are better correlated, and their variability are similar. Averaging over time, the merger phase leads typically to an increase by a factor of a few in the BHAR/SFR ratio. (ii) BHAR and nuclear SFR are intrinsically proportional, but the correlation lessens if the long-term SFR is measured. (iii) Galaxies in the remnant phase are the ones most likely to be selected as systems dominated by an active galactic nucleus, because of the long time spent in this phase. (iv) The time-scale over which a given diagnostic probes the SFR has a profound impact on the recovered correlations with BHAR, and on the interpretation of observational data.

  14. Accretion rate of extraterrestrial matter: Iridium deposited over the last 70 million years

    NASA Technical Reports Server (NTRS)

    Kyte, Frank T.

    1988-01-01

    In order to quantify the accretion rate of extraterrestrial matter during the Cenozoic, Ir concentrations were measured in a continuous series of 450 samples across most of the length of piston core LL44-GPC3. LL44-GPC3 is a 25-meter-long, large-diameter piston core of abyssal clay from the central North Pacific. This core contains a nearly continuous record of sedimentation over the last 70 Ma, as this site migrated from a region near the Equator in the late Cretaceous to its present position north of Hawaii. The first-cut survey across the core is nearing completion, and all of the conclusions of the earlier study, in which was reported the concentrations of Ir, Co, and Sb across 9 meters of this core, remain unchanged. The only strongly enhanced Ir concentrations occur at the Cretaceous-Tertiary (K-T) boundary and outside the K-T boundary Ir correlates well with Co, a terrestrial element which is largely present in hydrogenous ferromanganese oxide precipitates from seawater. Concentrations of both elements appear to be inversely correlated with the sedimentation rate. Although the K-T Ir anomaly is unique in magnitude in this core, there are several small bumps in the Ir profile which may reflect smaller accretionary events. The most promising Ir enhancement was observed in a 30 cm section approximately 1 m below the K-T boundary. Preliminary data suggest deposition of an excess across this interval at a time estimate to be approximate 1 Ma before the K-T impact event, but there is insufficient evidence at present to prove that this reflects enhanced accretion of extraterrestrial matter. A detailed model is being prepared of the chemical record of sedimentation in this core using a combined database of 39 elements in approximately 450 samples across the Cenozoic. Preliminary working model indicates that the only sedimentary sources which contribute significantly to the Ir budget in this core are the hydrogenous precipitates and extraterrestrial particulates.

  15. Sediment accretion rates and sediment composition in Prairie Pothole wetlands under varying land use practices, Montana, United States

    USGS Publications Warehouse

    Preston, T.M.; Sojda, R.S.; Gleason, R.A.

    2013-01-01

    Increased sedimentation and nutrient cycle changes in Prairie Pothole Region wetlands associated with agriculture threaten the permanence and ecological functionality of these important resources. To determine the effects of land use on sedimentation and nutrient cycling, soil cores were analyzed for cesium-137 (137Cs), lead-210 (210Pb), and potassium-40 (40K) activities; textural composition; organic and inorganic carbon (C); and total nitrogen (N) from twelve wetlands surrounded by cropland, Conservation Reserve Program (CRP) lands, or native prairie uplands. Separate soil cores from nine of these wetlands were also analyzed for phosphorus (P), nitrate (NO3), and ammonium (NH4) concentrations. Wetlands surrounded by cropland had significantly greater linear sediment accretion rates than wetlands surrounded by CRP or native prairie. Linear sediment accretion rates from wetlands surrounded by cropland were 2.7 and 6 times greater than wetlands surrounded by native prairie when calculated from the initial and peak occurrence of 137Cs, respectively, and 0.15 cm y−1 (0.06 in yr−1) greater when calculated from 210Pb. Relative to wetlands surrounded by CRP, linear sediment accretion rates for wetlands surrounded by cropland were 4.4 times greater when calculated from the peak occurrence of 137Cs. No significant differences existed between the linear sediment accretion rates between wetlands surrounded by native prairie or CRP uplands. Wetlands surrounded by cropland had increased clay, P, NO3, and NH4, and decreased total C and N concentrations compared to wetlands surrounded by native prairie. Wetlands surrounded by CRP had the lowest P and NO3 concentrations and had clay, NH4, C, and N concentrations between those of cropland and native prairie wetlands. We documented increased linear sediment accretion rates and changes in the textural and chemical properties of sediments in wetlands with cultivated uplands relative to wetlands with native prairie uplands. These

  16. Estimating the long-term phosphorus accretion rate in the Everglades: A Bayesian approach with risk assessment

    NASA Astrophysics Data System (ADS)

    Qian, Song S.; Richardson, Curtis J.

    Using wetlands as a sink of nutrients, phosphorus in particular, is becoming an increasingly attractive alternative to conventional wastewater treatment technology. In this paper, we briefly review the mechanism of phosphorus retention in wetlands, as well as previous modeling efforts. A Bayesian method is then proposed for estimating the long-term phosphorus accretion rate in wetlands through a piecewise linear model of outflow phosphorus concentration and phosphorus mass loading rate. The Bayesian approach was used for its simplicity in computation and its ability to accurately represent uncertainty. Applied to an Everglades wetland, the Bayesian method not only produced the probability distribution of the long-term phosphorus accretion rate but also generated a relationship of acceptable level of ``risk'' and optimal phosphorus mass loading rate for the proposed constructed wetlands in south Florida. The latter is a useful representation of uncertainty which is of interest to decision makers.

  17. Weak Line Quasars at High Redshift: Extremely High Accretion Rates or Anemic Broad-line Regions?

    NASA Astrophysics Data System (ADS)

    Shemmer, Ohad; Trakhtenbrot, Benny; Anderson, Scott F.; Brandt, W. N.; Diamond-Stanic, Aleksandar M.; Fan, Xiaohui; Lira, Paulina; Netzer, Hagai; Plotkin, Richard M.; Richards, Gordon T.; Schneider, Donald P.; Strauss, Michael A.

    2010-10-01

    We present Gemini-North K-band spectra of two representative members of the class of high-redshift quasars with exceptionally weak rest-frame ultraviolet emission lines (WLQs), SDSS J114153.34+021924.3 at z = 3.55 and SDSS J123743.08+630144.9 at z = 3.49. In both sources, we detect an unusually weak broad Hβ line and place tight upper limits on the strengths of their [O III] lines. Virial, Hβ-based black hole mass determinations indicate normalized accretion rates of L/L Edd=0.4 for these sources, which is well within the range observed for typical quasars with similar luminosities and redshifts. We also present high-quality XMM-Newton imaging spectroscopy of SDSS J114153.34+021924.3 and find a hard-X-ray photon index of Γ = 1.91+0.24 -0.22, which supports the virial L/L Edd determination in this source. Our results suggest that the weakness of the broad emission lines in WLQs is not a consequence of an extreme continuum-emission source but instead due to abnormal broad emission line region properties.

  18. WEAK LINE QUASARS AT HIGH REDSHIFT: EXTREMELY HIGH ACCRETION RATES OR ANEMIC BROAD-LINE REGIONS?

    SciTech Connect

    Shemmer, Ohad; Trakhtenbrot, Benny; Netzer, Hagai; Anderson, Scott F.; Brandt, W. N.; Schneider, Donald P.; Diamond-Stanic, Aleksandar M.; Fan Xiaohui; Lira, Paulina; Plotkin, Richard M.; Richards, Gordon T.; Strauss, Michael A.

    2010-10-20

    We present Gemini-North K-band spectra of two representative members of the class of high-redshift quasars with exceptionally weak rest-frame ultraviolet emission lines (WLQs), SDSS J114153.34+021924.3 at z = 3.55 and SDSS J123743.08+630144.9 at z = 3.49. In both sources, we detect an unusually weak broad H{beta} line and place tight upper limits on the strengths of their [O III] lines. Virial, H{beta}-based black hole mass determinations indicate normalized accretion rates of L/L {sub Edd}=0.4 for these sources, which is well within the range observed for typical quasars with similar luminosities and redshifts. We also present high-quality XMM-Newton imaging spectroscopy of SDSS J114153.34+021924.3 and find a hard-X-ray photon index of {Gamma} = 1.91{sup +0.24} {sub -0.22}, which supports the virial L/L {sub Edd} determination in this source. Our results suggest that the weakness of the broad emission lines in WLQs is not a consequence of an extreme continuum-emission source but instead due to abnormal broad emission line region properties.

  19. SUSTAINING STAR FORMATION RATES IN SPIRAL GALAXIES: SUPERNOVA-DRIVEN TURBULENT ACCRETION DISK MODELS APPLIED TO THINGS GALAXIES

    SciTech Connect

    Vollmer, Bernd; Leroy, Adam K.

    2011-01-15

    Gas disks of spiral galaxies can be described as clumpy accretion disks without a coupling of viscosity to the actual thermal state of the gas. The model description of a turbulent disk consisting of emerging and spreading clumps contains free parameters, which can be constrained by observations of molecular gas, atomic gas, and the star formation rate for individual galaxies. Radial profiles of 18 nearby spiral galaxies from THINGS, HERACLES, SINGS, and GALEX data are used to compare the observed star formation efficiency, molecular fraction, and velocity dispersion to the model. The observed radially decreasing velocity dispersion can be reproduced by the model. In the framework of this model, the decrease in the inner disk is due to the stellar mass distribution which dominates the gravitational potential. Introducing a radial break in the star formation efficiency into the model improves the fits significantly. This change in the star formation regime is realized by replacing the free-fall time in the prescription of the star formation rate with the molecule formation timescale. Depending on the star formation prescription, the break radius is located near the transition region between the molecular-gas-dominated and atomic-gas-dominated parts of the galactic disk or closer to the optical radius. It is found that only less massive galaxies (log M(M{sub sun}) {approx}< 10) can balance gas loss via star formation by radial gas accretion within the disk. These galaxies can thus access their gas reservoirs with large angular momentum. On the other hand, the star formation of massive galaxies is determined by the external gas mass accretion rate from a putative spherical halo of ionized gas or from satellite accretion. In the absence of this external accretion, star formation slowly exhausts the gas within the optical disk within the star formation timescale.

  20. Enhanced Accretion Rates of Stars on Supermassive Black Holes by Star-Disk Interactions in Galactic Nuclei

    NASA Astrophysics Data System (ADS)

    Just, Andreas; Yurin, Denis; Makukov, Maxim; Berczik, Peter; Omarov, Chingis; Spurzem, Rainer; Vilkoviskij, Emmanuil Y.

    2012-10-01

    We investigate the dynamical interaction of a central star cluster surrounding a supermassive black hole (SMBH) and a central accretion disk (AD). The dissipative force acting on stars in the disk leads to an enhanced mass flow toward the SMBH and to an asymmetry in the phase space distribution due to the rotating AD. The AD is considered as a stationary Keplerian rotating disk, which is vertically extended in order to employ a fully self-consistent treatment of stellar dynamics including the dissipative force originating from star-gas ram pressure effects. The stellar system is treated with a direct high-accuracy N-body integration code. A star-by-star representation, desirable in N-body simulations, cannot be extended to real particle numbers yet. Hence, we carefully discuss the scaling behavior of our model with regard to particle number and tidal accretion radius. The main idea is to find a family of models for which the ratio of two-body relaxation time and dissipation time (for kinetic energy of stellar orbits) is constant, which then allows us to extrapolate our results to real parameters of galactic nuclei. Our model is derived from basic physical principles and as such it provides insight into the role of physical processes in galactic nuclei, but it should be regarded as a first step toward more realistic and more comprehensive simulations. Nevertheless, the following conclusions appear to be robust: the star accretion rate onto the AD and subsequently onto the SMBH is enhanced by a significant factor compared to purely stellar dynamical systems neglecting the disk. This process leads to enhanced fueling of central disks in active galactic nuclei (AGNs) and to an enhanced rate of tidal stellar disruptions. Such disruptions may produce electromagnetic counterparts in the form of observable X-ray flares. Our models improve predictions for their rates in quiescent galactic nuclei. We do not yet model direct stellar collisions in the gravitational potential

  1. ENHANCED ACCRETION RATES OF STARS ON SUPERMASSIVE BLACK HOLES BY STAR-DISK INTERACTIONS IN GALACTIC NUCLEI

    SciTech Connect

    Just, Andreas; Yurin, Denis; Makukov, Maxim; Berczik, Peter; Omarov, Chingis; Spurzem, Rainer; Vilkoviskij, Emmanuil Y.

    2012-10-10

    We investigate the dynamical interaction of a central star cluster surrounding a supermassive black hole (SMBH) and a central accretion disk (AD). The dissipative force acting on stars in the disk leads to an enhanced mass flow toward the SMBH and to an asymmetry in the phase space distribution due to the rotating AD. The AD is considered as a stationary Keplerian rotating disk, which is vertically extended in order to employ a fully self-consistent treatment of stellar dynamics including the dissipative force originating from star-gas ram pressure effects. The stellar system is treated with a direct high-accuracy N-body integration code. A star-by-star representation, desirable in N-body simulations, cannot be extended to real particle numbers yet. Hence, we carefully discuss the scaling behavior of our model with regard to particle number and tidal accretion radius. The main idea is to find a family of models for which the ratio of two-body relaxation time and dissipation time (for kinetic energy of stellar orbits) is constant, which then allows us to extrapolate our results to real parameters of galactic nuclei. Our model is derived from basic physical principles and as such it provides insight into the role of physical processes in galactic nuclei, but it should be regarded as a first step toward more realistic and more comprehensive simulations. Nevertheless, the following conclusions appear to be robust: the star accretion rate onto the AD and subsequently onto the SMBH is enhanced by a significant factor compared to purely stellar dynamical systems neglecting the disk. This process leads to enhanced fueling of central disks in active galactic nuclei (AGNs) and to an enhanced rate of tidal stellar disruptions. Such disruptions may produce electromagnetic counterparts in the form of observable X-ray flares. Our models improve predictions for their rates in quiescent galactic nuclei. We do not yet model direct stellar collisions in the gravitational potential

  2. Epidural insertion simulator of higher insertion resistance & drop rate after puncture.

    PubMed

    Naemura, K; Sakai, A; Hayashi, T; Saito, H

    2008-01-01

    Accidents such as dural puncture remain one of the problems of epidural anesthesia, and unskilled doctors can repeat such accidents. The purpose of the current research was to provide a new simulator for epidural insertion training. No reference data regarding the resistance force used when inserting a needle into patients have been reported. A comparative study was conducted to aid in the development of a new simulator. Pork loin (n=5) were employed as a substitute for patients. Thickness was set at 2 cm so as to improve the reproducibility. The authors took the conventional simulator apart, and picked a block as an analogue of muscle and ligamentum flavum. A new simulator was made of a melamine foam resin block and a latex rubber sheet. An epidural needle fixed on a motorized stage was inserted at the speed of 2 mm per second. The reaction force was measured while the needle was inserted into each specimen. Waveform of the pork loin exhibited two slopes of different inclines up to peaks and then falls after puncture. The conventional simulator showed a simple increase up to peak and a slow fall after puncture. In contrast, the new simulator showed two slopes up to peak and then a sudden fall after puncture. The insertion resistances were 2.5 N/s for the porcine, 0.8 N/s for the conventional and 2.1 N/s for the new simulator. The drop rates were 5 N/s for the porcine, 0.6 N/s for the conventional and 24 N/s for the new simulator. The higher insertion resistance and drop rate for the new simulator than the conventional simulator will be suitable for epidural insertion training. PMID:19163400

  3. Ground reaction forces and loading rates associated with parkour and traditional drop landing techniques.

    PubMed

    Puddle, Damien L; Maulder, Peter S

    2013-01-01

    Due to the relative infancy of Parkour there is currently a lack of empirical evidence on which to base specific technique instruction upon. The purpose of this study was to compare the ground reaction forces and loading rates involved in two Parkour landing techniques encouraged by local Parkour instructors and a traditional landing technique recommended in the literature. Ten male participants performed three different drop landing techniques (Parkour precision, Parkour roll, and traditional) onto a force plate. Compared to the traditional technique the Parkour precision technique demonstrated significantly less maximal vertical landing force (38%, p < 0.01, ES = 1.76) and landing loading rate (54%, p < 0.01, ES = 1.22). Similarly, less maximal vertical landing force (43%, p < 0.01, ES = 2.04) and landing loading rate (63%, p < 0.01, ES = 1.54) were observed in the Parkour roll technique compared to the traditional technique. It is unclear whether or not the Parkour precision technique produced lower landing forces and loading rates than the Parkour roll technique as no significant differences were found. The landing techniques encouraged by local Parkour instructors such as the precision and roll appear to be more appropriate for Parkour practitioners to perform than a traditional landing technique due to the lower landing forces and loading rates experienced. Key pointsParkour precision and Parkour roll landings were found to be safer than a traditional landing technique, resulting in lower maximal vertical forces, slower times to maximal vertical force and ultimately lesser loading rates.Parkour roll may be more appropriate (safer) to utilize than the Parkour precision during Parkour landing scenarios.The Parkour landing techniques investigated n this study may be beneficial for landing by non-Parkour practitioners in everyday life. PMID:24149735

  4. Ground Reaction Forces and Loading Rates Associated with Parkour and Traditional Drop Landing Techniques

    PubMed Central

    Puddle, Damien L.; Maulder, Peter S.

    2013-01-01

    Due to the relative infancy of Parkour there is currently a lack of empirical evidence on which to base specific technique instruction upon. The purpose of this study was to compare the ground reaction forces and loading rates involved in two Parkour landing techniques encouraged by local Parkour instructors and a traditional landing technique recommended in the literature. Ten male participants performed three different drop landing techniques (Parkour precision, Parkour roll, and traditional) onto a force plate. Compared to the traditional technique the Parkour precision technique demonstrated significantly less maximal vertical landing force (38%, p < 0.01, ES = 1.76) and landing loading rate (54%, p < 0.01, ES = 1.22). Similarly, less maximal vertical landing force (43%, p < 0.01, ES = 2.04) and landing loading rate (63%, p < 0.01, ES = 1.54) were observed in the Parkour roll technique compared to the traditional technique. It is unclear whether or not the Parkour precision technique produced lower landing forces and loading rates than the Parkour roll technique as no significant differences were found. The landing techniques encouraged by local Parkour instructors such as the precision and roll appear to be more appropriate for Parkour practitioners to perform than a traditional landing technique due to the lower landing forces and loading rates experienced. Key points Parkour precision and Parkour roll landings were found to be safer than a traditional landing technique, resulting in lower maximal vertical forces, slower times to maximal vertical force and ultimately lesser loading rates. Parkour roll may be more appropriate (safer) to utilize than the Parkour precision during Parkour landing scenarios. The Parkour landing techniques investigated n this study may be beneficial for landing by non-Parkour practitioners in everyday life. PMID:24149735

  5. Flow Visualization in Evaporating Liquid Drops and Measurement of Dynamic Contact Angles and Spreading Rate

    NASA Technical Reports Server (NTRS)

    Zhang, Neng-Li; Chao, David F.

    2001-01-01

    A new hybrid optical system, consisting of reflection-refracted shadowgraphy and top-view photography, is used to visualize flow phenomena and simultaneously measure the spreading and instant dynamic contact angle in a volatile-liquid drop on a nontransparent substrate. Thermocapillary convection in the drop, induced by evaporation, and the drop real-time profile data are synchronously recorded by video recording systems. Experimental results obtained from this unique technique clearly reveal that thermocapillary convection strongly affects the spreading process and the characteristics of dynamic contact angle of the drop. Comprehensive information of a sessile drop, including the local contact angle along the periphery, the instability of the three-phase contact line, and the deformation of the drop shape is obtained and analyzed.

  6. Formation Rates of Black Hole Accretion Disk Gamma-Ray Bursts

    NASA Astrophysics Data System (ADS)

    Fryer, Chris L.; Woosley, S. E.; Hartmann, Dieter H.

    1999-11-01

    The cosmological origin of at least an appreciable fraction of classical gamma-ray bursts (GRBs) is now supported by redshift measurements for a half-dozen faint host galaxies. Still, the nature of the central engine (or engines) that provide the burst energy remains unclear. While many models have been proposed, those currently favored are all based upon the formation of and/or rapid accretion into stellar-mass black holes. Here we discuss a variety of such scenarios and estimate the probability of each. Population synthesis calculations are carried out using a Monte Carlo approach in which the many uncertain parameters intrinsic to such calculations are varied. We estimate the event rate for each class of model as well as the propagation distances for those having significant delay between formation and burst production, i.e., double neutron star (DNS) mergers and black hole-neutron star (BH/NS) mergers. One conclusion is a 1-2 order of magnitude decrease in the rate of DNS and BH/NS mergers compared to that previously calculated using invalid assumptions about common envelope evolution. Other major uncertainties in the event rates and propagation distances include the history of star formation in the universe, the masses of the galaxies in which merging compact objects are born, and the radii of the hydrogen-stripped cores of massive stars. For reasonable assumptions regarding each, we calculate a daily event rate in the universe for (1) merging neutron stars: ~100 day-1 (2) neutron star-black hole mergers: ~450 day-1 (3) collapsars: ~104 day-1 (4) helium star black hole mergers: ~1000 day-1 and (5) white dwarf-black hole mergers: ~20 day-1. The range of uncertainty in these numbers, however, is very large, typically 2-3 orders of magnitude. These rates must additionally be multiplied by any relevant beaming factor (fΩ<1) and sampling fraction (if the entire universal set of models is not being observed). Depending upon the mass of the host galaxy, one-half of

  7. Stemming Down High Rates of Truancy, Drop-Out and Failure in Secondary Schools in Osun State

    ERIC Educational Resources Information Center

    Alaba, Sofowora O.

    2008-01-01

    The purpose of this study was to find out whether: (1) truancy, drop-out and failure rates was as a result of the teacher teaching strategy in Secondary Schools in Osun State, (2) If skill in instructional design has any relationship with failure rate; and (3) whether teachers that studied Educational Technology would perform significantly better…

  8. Exposing the Myth: Advanced Math Does Not Increase Drop out Rates. Math Works

    ERIC Educational Resources Information Center

    Achieve, Inc., 2013

    2013-01-01

    A common argument against raising math course-taking requirements for all students is that it will cause more students to drop out of high school. But most students who drop out for academic reasons do so not because they are being "too challenged," but rather because they are not being challenged enough. It is important to raise the rigor and…

  9. Classical T Tauri stars with VPHAS+ - I. H α and u-band accretion rates in the Lagoon Nebula M8

    NASA Astrophysics Data System (ADS)

    Kalari, V. M.; Vink, J. S.; Drew, J. E.; Barentsen, G.; Drake, J. J.; Eislöffel, J.; Martín, E. L.; Parker, Q. A.; Unruh, Y. C.; Walton, N. A.; Wright, N. J.

    2015-10-01

    We estimate the accretion rates of 235 Classical T Tauri star (CTTS) candidates in the Lagoon Nebula using ugri H α photometry from the VST Photometric H α survey+. Our sample consists of stars displaying H α excess, the intensity of which is used to derive accretion rates. For a subset of 87 stars, the intensity of the u-band excess is also used to estimate accretion rates. We find the mean variation in accretion rates measured using H α and u-band intensities to be ˜0.17 dex, agreeing with previous estimates (0.04-0.4 dex) but for a much larger sample. The spatial distribution of CTTS align with the location of protostars and molecular gas suggesting that they retain an imprint of the natal gas fragmentation process. Strong accretors are concentrated spatially, while weak accretors are more distributed. Our results do not support the sequential star-forming processes suggested in the literature.

  10. A New Approach to Measure Contact Angle and Evaporation Rate with Flow Visualization in a Sessile Drop

    NASA Technical Reports Server (NTRS)

    Zhang, Nengli; Chao, David F.

    1999-01-01

    The contact angle and the spreading process of sessile droplet are very crucial in many technological processes, such as painting and coating, material processing, film-cooling applications, lubrication, and boiling. Additionally, as it is well known that the surface free energy of polymers cannot be directly, measured for their elastic and viscous restraints. The measurements of liquid contact angle on the polymer surfaces become extremely important to evaluate the surface free energy of polymers through indirect methods linked with the contact angle data. Due to the occurrence of liquid evaporation is inevitable, the effects of evaporation on the contact angle and the spreading become very important for more complete understanding of these processes. It is of interest to note that evaporation can induce Marangoni-Benard convection in sessile drops. However, the impacts of the inside convection on the wetting and spreading processes are not clear. The experimental methods used by previous investigators cannot simultaneously measure the spreading process and visualize the convection inside. Based on the laser shadowgraphic system used by the present author, a very simple optical procedure has been developed to measure the contact angle, the spreading speed, the evaporation rate, and to visualize inside convection of a sessile drop simultaneously. Two CCD cameras were used to synchronously record the real-time diameter of the sessile drop, which is essential for determination of both spreading speed and evaporation rate, and the shadowgraphic image magnified by the sessile drop acting as a thin plano-convex lens. From the shadowgraph, the inside convection of the drop can be observed if any and the image outer diameter, which linked to the drop profile, can be measured. Simple equations have been derived to calculate the drop profile, including the instantaneous contact angle, height, and volume of the sessile drop, as well as the evaporation rate. The influence of

  11. Models of the hard X-ray spectrum of AM Herculis and implications for the accretion rate

    NASA Technical Reports Server (NTRS)

    Swank, J. H.; Fabian, A. C.; Ross, R. R.

    1983-01-01

    Phenomenological fits to the hard X-ray spectrum of AM Herculis left unexplained the high equivalent width (0.8 + or - 0.1 keV) of Fe K alpha emission. A purely thermal origin implies a much steeper spectrum than was observed. With Monte Carlo calculations, scattering and fluorescent line production in a cold or partially ionized accretion column of hard X-rays emitted at the base were investigated. The strength of the iron emission and the flat spectral continuum can be explained by the effects of fluorescence and absorption within the accretion column and the surface of the white dwarf on a thermal X-ray spectrum. Thomson optical depths across the column in the range 0.2 to 0.7 are acceptable. The accretion rate and gravitational power can be deduced from the optical depth across the column, if the column size is known, and, together with the observed hard X-ray and polarized light luminosities, imply a lower limit for the luminosity in the UV to soft X-ray range, for which the observations give model-dependent values. Estimates of the column size differ by a factor of 40. Small spot sizes and low luminosities would be consistent with the soft component being the expected reprocessed bremsstrahlung and cyclotron radiation, although the constraint of matching the spectrum confines one to solutions with fluxes exceeding 20% the Eddington limits.

  12. UNLEASHING POSITIVE FEEDBACK: LINKING THE RATES OF STAR FORMATION, SUPERMASSIVE BLACK HOLE ACCRETION, AND OUTFLOWS IN DISTANT GALAXIES

    SciTech Connect

    Silk, Joseph

    2013-08-01

    Pressure-regulated star formation is a simple variant on the usual supernova-regulated star formation efficiency that controls the global star formation rate as a function of cold gas content in star-forming galaxies, and accounts for the Schmidt-Kennicutt law in both nearby and distant galaxies. Inclusion of active galactic nucleus (AGN) induced pressure, by jets and/or winds that flow back onto a gas-rich disk, can lead, under some circumstances, to significantly enhanced star formation rates, especially at high redshift and most likely followed by the more widely accepted phase of star formation quenching. Simple expressions are derived that relate supermassive black hole growth, star formation, and outflow rates. The ratios of black hole to spheroid mass and of both black hole accretion and outflow rates to star formation rate are predicted as a function of time. I suggest various tests of the AGN-triggered star formation hypothesis.

  13. Acidity variations across the cloud drop size spectrum and their influence on rates of atmospheric sulfate production

    SciTech Connect

    Collett, J.L. Jr.; Bator, A.; Rao, Xin; Demoz, B.

    1994-11-01

    Measurements of pH variations within natural cloud drop populations reveal that small drops are often more acidic than large drops. Cloud samples collected from coastal stratus clouds, frontal clouds, and radiation fogs, from heavily polluted and pristine locations, had pH values ranging from below three to more than seven. Differences between small and large cloud drop acidities as large as two pH units were observed, although differences were generally below one pH unit. This chemical heterogenity can significantly enhance oxidation of sulfur dioxide to sulfate within clouds, relative to oxidation rates predicted from the average cloudwater composition. One-third of the sampled clouds were estimated to experience an increase of at least 20% in the rate of sulfur oxidation by ozone (8% of the clouds had increases exceeding 100%) as a result of acidity differences between large and small cloud drops. These findings suggest that sulfate production within clouds, a critical component of the global sulfur cycle, may be more rapid than previously though. 20 refs., 3 figs.

  14. Asymmetric Accretion Flows within a Common Envelope

    NASA Astrophysics Data System (ADS)

    MacLeod, Morgan; Ramirez-Ruiz, Enrico

    2015-04-01

    This paper examines flows in the immediate vicinity of stars and compact objects dynamically inspiralling within a common envelope (CE). Flow in the vicinity of the embedded object is gravitationally focused, leading to drag and potentially to gas accretion. This process has been studied numerically and analytically in the context of Hoyle-Lyttleton accretion (HLA). Yet, within a CE, accretion structures may span a large fraction of the envelope radius, and in so doing sweep across a substantial radial gradient of density. We quantify these gradients using detailed stellar evolution models for a range of CE encounters. We provide estimates of typical scales in CE encounters that involve main sequence stars, white dwarfs, neutron stars, and black holes with giant-branch companions of a wide range of masses. We apply these typical scales to hydrodynamic simulations of three-dimensional HLA with an upstream density gradient. This density gradient breaks the symmetry that defines HLA flow, and imposes an angular momentum barrier to accretion. Material that is focused into the vicinity of the embedded object thus may not be able to accrete. As a result, accretion rates drop dramatically, by one to two orders of magnitude, while drag rates are only mildly affected. We provide fitting formulae to the numerically derived rates of drag and accretion as a function of the density gradient. The reduced ratio of accretion to drag suggests that objects that can efficiently gain mass during CE evolution, such as black holes and neutron stars, may grow less than implied by the HLA formalism.

  15. The White Dwarf Mass and the Accretion Rate of Recurrent Novae: An X-ray Perspective

    NASA Technical Reports Server (NTRS)

    Mukai, Koji; Sokoloski, Jennifer L.; Nelson, Thomas; Luna, Gerardo J. M.

    2011-01-01

    We present recent results of quiescent X-ray observations of recurrent novae (RNe) and related objects. Several RNe are luminous hard X-ray sources in quiescence, consistent with accretion onto a near Chandrasekhar mass white dwarf. Detection of similar hard X-ray emissions in old novae and other cataclysmic variables may lead to identification of additional RN candidates. On the other hand, other RNe are found to be comparatively hard X-ray faint. We present several scenarios that may explain this dichotomy, which should be explored further.

  16. 2500 years of changing shoreline accretion rates at the mouths of the Mekong River delta

    NASA Astrophysics Data System (ADS)

    Besset, Manon; Tamura, Toru; Anthony, Edward; Brunier, Guillaume; Saito, Yoshiki; Dussouillez, Philippe; Lap Nguyen, Van; Ta, Oahn

    2016-04-01

    The Mekong River delta prograded rapidly in a relatively sheltered bight in the South China Sea under the influence of high fluvial sediment supply 5300 to 3500 years ago, developing from an estuary into a delta. This >200 km seaward growth resulted in increasing exposure of the delta to ocean waves that led to a more wave-influenced mode of progradation characterized by the construction of numerous sets of beach ridges in the eastern sector of the delta, which shows a system of multiple distributary mouths. The growth pattern of this river-mouth sector over the last 2500 years has been determined from OSL dating of these beach-ridge deposits, while the most up-to-date trends (1950-2014) have been highlighted from the analysis of maps and satellite images. The OSL ages show that the area of the delta in the mouths sector remained nearly constant till about 500 yr BP, following which significant accretion occurred, possibly in response to changes in catchment land-use and monsoon rainfall and attendant river water and sediment discharge. A fine-tuned analysis of changes since 1950 shows dominant but fluctuating accretion, with two periods of erosion. The first (1965-1973) occurred in the course of the second Indochina war, and the second more recently from 2003 to 2011, followed by mild recovery between 2011 and 2014. These fluctuations most likely reflect changes in sediment supply caused by the vicissitudes of war and its effect on vegetation cover, as well as variations in monsoon rainfall and discharge, and, for the most recent period, massive sand mining in the river and deltaic channels. Accretion of the mouths sector has gone apace, over the same recent multi-decadal period, with large-scale erosion of the muddy shores of the delta in the western South China Sea and the Gulf of Thailand, thus suggesting that the mouths sector may be increasingly sequestering sediment to the detriment of the rest of the delta shoreline. The accretion in the mouths sector is

  17. VizieR Online Data Catalog: RM AGNs accretion rates and BH masses (Du+, 2016)

    NASA Astrophysics Data System (ADS)

    Du, P.; Wang, J.-M.; Hu, C.; Ho, L. C.; Li, Y.-R.; Bai, J.-M.

    2016-05-01

    We select all AGNs with reverberation mapping (RM) data (here only broad Hβ line), which yield robust BH mass estimates needed for our analysis. All RM AGNs before 2013 are summarized by Bentz et al. (2013ApJ...767..149B). Our project to search for super-Eddington accreting massive black holes (SEAMBHs) has monitored about 25 candidates and successfully measured Hβ lags ({tau}Hβ) in 14 AGNs to date (Du et al. 2015, J/ApJ/806/22) and other five objects monitored between 2014 and 2015 (to be submitted). See section 2 for further explanations. (2 data files).

  18. Supermassive black holes with high accretion rates in active galactic nuclei. I. First results from a new reverberation mapping campaign

    SciTech Connect

    Du, Pu; Hu, Chen; Qiu, Jie; Li, Yan-Rong; Wang, Jian-Min; Lu, Kai-Xing; Wang, Fang; Bai, Jin-Ming; Kaspi, Shai; Netzer, Hagai; Collaboration: SEAMBH collaboration

    2014-02-10

    We report first results from a large project to measure black hole (BH) mass in high accretion rate active galactic nuclei (AGNs). Such objects may be different from other AGNs in being powered by slim accretion disks and showing saturated accretion luminosities, but both are not yet fully understood. The results are part of a large reverberation mapping (RM) campaign using the 2.4 m Shangri-La telescope at the Yunnan Observatory in China. The goals are to investigate the gas distribution near the BH and the properties of the central accretion disks, to measure BH mass and Eddington ratios, and to test the feasibility of using such objects as a new type of cosmological candles. The paper presents results for three objects, Mrk 335, Mrk 142, and IRAS F12397+3333, with Hβ time lags relative to the 5100 Å continuum of 10.6{sub −2.9}{sup +1.7}, 6.4{sub −2.2}{sup +0.8} and 11.4{sub −1.9}{sup +2.9} days, respectively. The corresponding BH masses are (8.3{sub −3.2}{sup +2.6})×10{sup 6} M{sub ⊙}, (3.4{sub −1.2}{sup +0.5})×10{sup 6} M{sub ⊙}, and (7.5{sub −4.1}{sup +4.3})×10{sup 6} M{sub ⊙}, and the lower limits on the Eddington ratios are 0.6, 2.3, and 4.6 for the minimal radiative efficiency of 0.038. Mrk 142 and IRAS F12397+333 (extinction corrected) clearly deviate from the currently known relation between Hβ lag and continuum luminosity. The three Eddington ratios are beyond the values expected in thin accretion disks and two of them are the largest measured so far among objects with RM-based BH masses. We briefly discuss implications for slim disks, BH growth, and cosmology.

  19. Coronal geometry at low mass-accretion rates from XMM and NuSTAR spectra

    NASA Astrophysics Data System (ADS)

    Fuerst, F.; NuSTAR Binaries Team; NuSTAR AGN Team

    2016-06-01

    At very low Eddington luminosities the structure and physics of the accretion flow around a black hole are still debated, in particular in the inner most regions. By making sensitive measurements of the relativistic blurring of the X-ray reflection spectrum we investigate these physics, a task for which XMM-Newton, in combination with hard X-ray coverage provided by NuSTAR or Hitomi, is ideally suited and will continue to be unique for years to come. I will present results from XMM and NuSTAR observations of the radio-galaxy Cen A and of the X-ray binary GRS 1739-278 during the decline of its outburst. While Cen A shows a prominent iron line, the broad-band spectrum shows no evidence of reflection. This lack of reflection can best be explained by a jet origin of the hard X-rays or a significantly truncated accretion disk. The iron line can be self-consistently explained when assuming an optically thick torus surrounding the super-massive black-hole. The broad-band X-ray spectrum of GRS 1739-278 can be well described by a simple power-law or Comptonization continuum. A weak relativistic reflection model results in a small but significant improvement of the statistical quality of the fit. This relativistic model indicates a strongly truncated disk.

  20. PHOTOMETRIC DETERMINATION OF THE MASS ACCRETION RATES OF PRE-MAIN-SEQUENCE STARS. IV. RECENT STAR FORMATION IN NGC 602

    SciTech Connect

    De Marchi, Guido; Beccari, Giacomo; Panagia, Nino E-mail: gbeccari@eso.org

    2013-09-20

    We have studied the young stellar populations in NGC 602, in the Small Magellanic Cloud, using a novel method that we have developed to combine Hubble Space Telescope photometry in the V, I, and Hα bands. We have identified about 300 pre-main-sequence (PMS) stars, all of which are still undergoing active mass accretion, and have determined their physical parameters (effective temperature, luminosity, age, mass, and mass accretion rate). Our analysis shows that star formation has been present in this field over the last 60 Myr. In addition, we can recognize at least two clear, distinct, and prominent episodes in the recent past: one about 2 Myr ago, but still ongoing in regions of higher nebulosity, and one (or more) older than 30 Myr, encompassing both stars dispersed in the field and two smaller clusters located about 100'' north of the center of NGC 602. The relative locations of younger and older PMS stars do not imply a causal effect or triggering of one generation on the other. The strength of the two episodes appears to be comparable, but the episodes occurring more than 30 Myr ago might have been even stronger than the current one. We have investigated the evolution of the mass accretion rate, M-dot{sub acc}, as a function of the stellar parameters finding that log M-dot{sub acc}≅-0.6 log t + log m + c, where t is the age of the star, m is its mass, and c is a decreasing function of the metallicity.

  1. An empirically derived basis for calculating the area, rate, and distribution of water-drop impingement on airfoils

    NASA Technical Reports Server (NTRS)

    Bergrun, Norman R

    1952-01-01

    An empirically derived basis for predicting the area, rate, and distribution of water-drop impingement on airfoils of arbitrary section is presented. The concepts involved represent an initial step toward the development of a calculation technique which is generally applicable to the design of thermal ice-prevention equipment for airplane wing and tail surfaces. It is shown that sufficiently accurate estimates, for the purpose of heated-wing design, can be obtained by a few numerical computations once the velocity distribution over the airfoil has been determined. The calculation technique presented is based on results of extensive water-drop trajectory computations for five airfoil cases which consisted of 15-percent-thick airfoils encompassing a moderate lift-coefficient range. The differential equations pertaining to the paths of the drops were solved by a differential analyzer.

  2. Influence of Sea-Level Rise and Storms on Soil Accretion Rates in the Mangrove Forests of Everglades National Park, USA

    NASA Astrophysics Data System (ADS)

    Smoak, J. M.; Breithaupt, J.; Smith, T., III; Sanders, C. J.; Peterson, L. C.

    2014-12-01

    Mangrove forests provide a range of valuable ecosystem services including sequestering large quantities of organic carbon (OC) in their soils at rates higher than other forests. Whether or not mangrove soils continue to be a sink for OC will be determined by the mangrove ecosystems' response to climate change-induced stressors. The threats of rising sea level outpacing mangrove forest soil accretion and increased wave energy associated with this rise may become the primary climate change-induced stressors on mangrove ecosystems. The threat from wave energy is amplified during storm events, which could increasingly damage mangrove forests along the coastline. However, storms may enhance accretion rates at some sites due to delivery of storm surge material, which could increase the system's ability to keep pace with sea-level rise (SLR). To investigate these processes we measure soil accretion rates over the last 100 years (via 210Pb dating) within the mangrove forests of Everglades National Park, which are situated within the largest contiguous mangrove forest in North America. Accretion rates range from 2 to 2.8 mm per year for sites within 10 km of the Gulf of Mexico. These rates match (within error) or exceed SLR over the last 100 years. Sites farther inland than 10 km have slightly lower accretion rates. Throughout the system organic matter accumulation is the most important source material contributing to accretion. The more seaward sites also show an important contribution from carbonate material. Soil cores from the most seaward sites exhibited visual laminations and Ca peaks (determined via x-ray fluorescence). These are indicators of storm surge deposits. While higher sea level might produce more damage and loss of mangrove forest along open water (e.g., Gulf of Mexico), our findings suggest some sites will have enhanced accretion rates due to supplementation with storm surge material.

  3. Drop-box Weir for Measuring Flow Rates Under Extreme Flow Conditions

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Sediment and large rocks often are transported in runoff during extreme events. The sediment can deposit in a runoff-measuring structure and give erroneous readings. The drop-box weir (DBW) is one of only a few flow-measuring devices capable of measuring sediment-laden flows. Recent studies have ...

  4. Supermassive black holes with high accretion rates in active galactic nuclei. II. The most luminous standard candles in the universe

    SciTech Connect

    Wang, Jian-Min; Du, Pu; Hu, Chen; Qiu, Jie; Li, Yan-Rong; Netzer, Hagai; Kaspi, Shai; Bai, Jin-Ming; Wang, Fang; Lu, Kai-Xing; Collaboration: SEAMBH collaboration

    2014-10-01

    This is the second in a series of papers reporting on a large reverberation mapping (RM) campaign to measure black hole (BH) mass in high accretion rate active galactic nuclei (AGNs). The goal is to identify super-Eddington accreting massive black holes (SEAMBHs) and to use their unique properties to construct a new method for measuring cosmological distances. Based on theoretical models, the saturated bolometric luminosity of such sources is proportional to the BH mass, which can be used to obtain their distance. Here we report on five new RM measurements and show that in four of the cases, we can measure the BH mass and three of these sources are SEAMBHs. Together with the three sources from our earlier work, we now have six new sources of this type. We use a novel method based on a minimal radiation efficiency to identify nine additional SEAMBHs from earlier RM-based mass measurements. We use a Bayesian analysis to determine the parameters of the new distance expression and the method uncertainties from the observed properties of the objects in the sample. The ratio of the newly measured distances to the standard cosmological ones has a mean scatter of 0.14 dex, indicating that SEAMBHs can be use as cosmological distance probes. With their high luminosity, long period of activity, and large numbers at high redshifts, SEAMBHs have a potential to extend the cosmic distance ladder beyond the range now explored by Type Ia supernovae.

  5. Experimental and theoretical analysis of the rate of solvent equilibration in the hanging drop method of protein crystal growth

    NASA Astrophysics Data System (ADS)

    Fowlis, William W.; DeLucas, Lawrence J.; Twigg, Pamela J.; Howard, Sandra B.; Meehan, Edward J.; Baird, James K.

    1988-07-01

    In the hanging drop method of protein crystal growth, a water droplet containing protein, buffer and a precipitating agent, such as ammonium sulfate, is suspended from a glass coverslip above a well containing an aqueous solution of the precipitating agent at a concentration double that in the drop. We present a comprehensive theoretical study of the rate of water evaporation in the hanging drop method. We find that in earth's gravity the rate controlling step in the evaporation is the rate of diffusion of water vapor across the air space separating the drop from the well. Using ammonium sulfate as the precipitating agent, we have made careful measurements at both 4°C and 25°C of the evaporation times for some 25 μL droplets at various concentrations. These results are in good agreement with our theory. As determined by the theory, the parameters affecting the rate of evaporation include the temperature, the vapor pressure of water, the ionization constant of the salt, the volume of the droplet, the contact angle between the droplet and the coverslip, the number of moles of salt in the droplet, the number of moles of water and salt in the well, the molar volumes of water and salt, the distance from the droplet to the well, and the coefficient of diffusion of water vapor through air. These parameters do not act independently; rather, they combine to form three dimensionless groups upon which the rate of evaporation depends. We evaluate numerically 18 different drop and well arrangements commonly encountered in the laboratory. In all cases considered at 25°C, the number of moles of water in the droplet achieves 95% of its final value in 3—30 h, after which further evaporation is quite slow. Our experiments confirm this. We consider qualitatively the effect of weightlessness (spaceflight) on the rate of evaporation and find it to be most likely controlled by the rate of the interdiffusion of salt and water in the droplet.

  6. Experimental and theoretical analysis of the rate of solvent equilibration in the hanging drop method of protein crystal growth

    NASA Technical Reports Server (NTRS)

    Fowlis, William W.; Delucas, Lawrence J.; Twigg, Pamela J.; Howard, Sandra B.; Meehan, Edward J.

    1988-01-01

    The principles of the hanging-drop method of crystal growth are discussed, and the rate of water evaporation in a water droplet (containing protein, buffer, and a precipitating agent) suspended above a well containing a double concentration of precipitating agent is investigated theoretically. It is shown that, on earth, the rate of evaporation may be determined from diffusion theory and the colligative properties of solutions. The parameters affecting the rate of evaporation include the temperature, the vapor pressure of water, the ionization constant of the salt, the volume of the drop, the contact angle between the droplet and the coverslip, the number of moles of salt in the droplet, the number of moles of water and salt in the well, the molar volumes of water and salt, the distance from the droplet to the well, and the coefficient of diffusion of water vapor through air. To test the theoretical equations, hanging-drop experiments were conducted using various reagent concentrations in 25-microliter droplets and measuring the evaporation times at 4 C and 25 C. The results showed good agreement with the theory.

  7. Evolution of Nova TrA 2008 into a High Mass-Accretion Rate Post-Nova

    NASA Astrophysics Data System (ADS)

    Walter, Frederick M.

    2015-06-01

    NR TrA (Nova TrA 2008) was a normal slow Fe II novae for its first year of evolution. During its third year eclipses appeared, and optical spectra revealed the presence of hot permitted lines of C IV, N V, and O VI in addition to the usual nebular lines. The light curve and spectra resemble those of the V Sge stars. The orbital period is 5.25 hours. The time-resolved spectra show a prominent S-wave in the hot lines with an amplitude of about 100 km/s. We conclude that the system is a CV with a high mass accretion rate that has persisted for some 6 years after the explosion.

  8. Mapping the average AGN accretion rate in the SFR-M* plane for Herschel-selected galaxies at 0 < z ≤ 2.5

    NASA Astrophysics Data System (ADS)

    Delvecchio, I.; Lutz, D.; Berta, S.; Rosario, D. J.; Zamorani, G.; Pozzi, F.; Gruppioni, C.; Vignali, C.; Brusa, M.; Cimatti, A.; Clements, D. L.; Cooray, A.; Farrah, D.; Lanzuisi, G.; Oliver, S.; Rodighiero, G.; Santini, P.; Symeonidis, M.

    2015-05-01

    We study the relation of AGN accretion, star formation rate (SFR) and stellar mass (M*) using a sample of ≈8600 star-forming galaxies up to z = 2.5 selected with Herschel imaging in the GOODS and COSMOS fields. For each of them we derive SFR and M*, both corrected, when necessary, for emission from an active galactic nucleus (AGN), through the decomposition of their spectral energy distributions (SEDs). About 10 per cent of the sample are detected individually in Chandra observations of the fields. For the rest of the sample, we stack the X-ray maps to get average X-ray properties. After subtracting the X-ray luminosity expected from star formation and correcting for nuclear obscuration, we derive the average AGN accretion rate for both detected sources and stacks, as a function of M*, SFR and redshift. The average accretion rate correlates with SFR and with M*. The dependence on SFR becomes progressively more significant at z > 0.8. This may suggest that SFR is the original driver of these correlations. We find that average AGN accretion and star formation increase in a similar fashion with offset from the star-forming `main-sequence'. Our interpretation is that accretion on to the central black hole and star formation broadly trace each other, irrespective of whether the galaxy is evolving steadily on the main-sequence or bursting.

  9. Gas accretion from halos to disks: observations, curiosities, and problems

    NASA Astrophysics Data System (ADS)

    Elmegreen, Bruce G.

    2016-08-01

    Accretion of gas from the cosmic web to galaxy halos and ultimately their disks is a prediction of modern cosmological models but is rarely observed directly or at the full rate expected from star formation. Here we illustrate possible large-scale cosmic HI accretion onto the nearby dwarf starburst galaxy IC10, observed with the VLA and GBT. We also suggest that cosmic accretion is the origin of sharp metallicity drops in the starburst regions of other dwarf galaxies, as observed with the 10-m GTC. Finally, we question the importance of cosmic accretion in normal dwarf irregulars, for which a recent study of their far-outer regions sees no need for, or evidence of, continuing gas buildup.

  10. Charge Accretion Rate and Injection Radius of Ionized-Induced Injections in Laser Wakefield Accelerators

    NASA Astrophysics Data System (ADS)

    Zeng, Ming; Chen, Min; Sheng, Zheng-Ming

    2016-03-01

    Ionization-induced injection has recently been proved to be a stable injection method with several advantages in laser wakefield accelerators. However, the controlling of this injection process aiming at producing high quality electron beams is still challenging. In this paper, we examine the ionization injection processes and estimate the injection rate with two-dimensional particle-in-cell simulations. The injection rate is shown to increase linearly with the high-Z gas density as long as its ratio is smaller than some threshold in the mix gases. It is also shown that by changing the transverse mode of the driving lasers one can control the injection rate.

  11. On the Disappearance of Kilohertz Quasi-periodic Oscillations at a High Mass Accretion Rate in Low-Mass X-Ray Binaries

    NASA Astrophysics Data System (ADS)

    Cui, Wei

    2000-05-01

    For all sources in which the phenomenon of kilohertz quasi-periodic oscillation (kHz QPO) is observed, the QPOs disappear abruptly when the inferred mass accretion rate exceeds a certain threshold. Although the threshold cannot at present be accurately determined (or even quantified) observationally, it is clearly higher for bright Z sources than for faint atoll sources. Here we propose that the observational manifestation of kHz QPOs requires direct interaction between the neutron star magnetosphere and the Keplerian accretion disk and that the cessation of kHz QPOs at a high accretion rate is due to the lack of such an interaction when the Keplerian disk terminates at the last stable orbit and yet the magnetosphere is pushed farther inward. The threshold is therefore dependent on the magnetic field strength-the stronger the magnetic field, the higher the threshold. This is certainly in agreement with the atoll/Z paradigm, but we argue that it is also generally true, even for individual sources within each (atoll or Z) category. For atoll sources, the kHz QPOs also seem to vanish at a low accretion rate. Perhaps the ``disengagement'' between the magnetosphere and the Keplerian disk also takes place under such circumstances because of, for instance, the presence of quasi-spherical advection-dominated accretion flow (ADAF) close to the neutron star. Unfortunately, in this case, the estimation of the accretion rate threshold would require a knowledge of the physical mechanisms that cause the disengagement. If the ADAF is responsible, the threshold is likely dependent on the magnetic field of the neutron star.

  12. Determining seed cotton mass flow rate by pressure drop across a blowbox

    Technology Transfer Automated Retrieval System (TEKTRAN)

    A seed cotton mass flow rate sensor would offer useful feedback for gin managers and provide a critical input for advanced process control systems. Several designs of seed cotton mass flow rate sensors have been evaluated in the laboratory, but none have found acceptance in commercial gins. The obje...

  13. Drop-Out Rates among American Indian and Alaska Native Students: Beyond Cultural Discontinuity. ERIC Digest.

    ERIC Educational Resources Information Center

    St. Germaine, Richard

    American Indian and Alaska Native (AI/AN) students regularly face obstacles during the transition to high school and have the highest dropout rate of all U.S. racial and ethnic groups. Educational theorists and researchers have various explanations for this high failure rate, each with its own prescriptions. These explanations include: (1) deficit…

  14. A Mixed Methods Explanatory Study of the Failure/Drop Rate for Freshman STEM Calculus Students

    ERIC Educational Resources Information Center

    Worthley, Mary

    2013-01-01

    In a national context of high failure rates in freshman calculus courses, the purpose of this study was to understand who is struggling, and why. High failure rates are especially alarming given a local environment where students have access to a variety of academic, and personal, assistance. The sample consists of students at Colorado State…

  15. Default Rate on Student Loans Drops for 6th Consecutive Year.

    ERIC Educational Resources Information Center

    Burd, Stephen

    1998-01-01

    The student loan default rate was 9.6% in 1996, a decline of 12.8% since the 1990 peak, saving the federal government $3 billion over six years. Since 1993 the Department of Education has barred 1065 institutions, mostly for-profit trade schools, from participating in federal student loan programs; institutions with a 40% default rate can lose…

  16. Determining seed cotton mass flow rate by pressure drop across a blowbox

    Technology Transfer Automated Retrieval System (TEKTRAN)

    A seed cotton mass flow rate sensor would offer useful feedback for gin managers and provide a critical input for advanced process control systems. Several designs of seed cotton mass flow rate sensors have been evaluated in the laboratory, but none have found acceptance in commercial gins. The ob...

  17. Short term Heart Rate Variability to predict blood pressure drops due to standing: a pilot study

    PubMed Central

    2015-01-01

    Background Standing from a bed or chair may cause a significant lowering of blood pressure (ΔBP), which may have severe consequences such as, for example, falls in older subjects. The goal of this study was to develop a mathematical model to predict the ΔBP due to standing in healthy subjects, based on their Heart Rate Variability, recorded in the 5 minutes before standing. Methods Heart Rate Variability was extracted from an electrocardiogram, recorded from 10 healthy subjects during the 5 minutes before standing. The blood pressure value was measured before and after rising. A mathematical model aiming to predict ΔBP based on Heart Rate Variability measurements was developed using a robust multi-linear regression and was validated with the leave-one-subject-out cross-validation technique. Results The model predicted correctly the ΔBP in 80% of experiments, with an error below the measurement error of sphygmomanometer digital devices (±4.5 mmHg), a false negative rate of 7.5% and a false positive rate of 10%. The magnitude of the ΔBP was associated with a depressed and less chaotic Heart Rate Variability pattern. Conclusions The present study showes that blood pressure lowering due to standing can be predicted by monitoring the Heart Rate Variability in the 5 minutes before standing. PMID:26391336

  18. PHOTOMETRIC DETERMINATION OF THE MASS ACCRETION RATES OF PRE-MAIN-SEQUENCE STARS. I. METHOD AND APPLICATION TO THE SN 1987A FIELD

    SciTech Connect

    De Marchi, Guido; Panagia, Nino; Romaniello, Martino E-mail: panagia@stsci.ed

    2010-05-20

    We have developed and successfully tested a new self-consistent method to reliably identify pre-main-sequence (PMS) objects actively undergoing mass accretion in a resolved stellar population, regardless of their age. The method does not require spectroscopy and combines broadband V and I photometry with narrowband H{alpha} imaging to (1) identify all stars with excess H{alpha} emission, (2) convert the excess H{alpha} magnitude into H{alpha} luminosity L(H{alpha}), (3) estimate the H{alpha} emission equivalent width, (4) derive the accretion luminosity L{sub acc} from L(H{alpha}), and finally (5) obtain the mass accretion rate M-dot{sub acc} from L{sub acc} and the stellar parameters (mass and radius). By selecting stars with an accuracy of 15% or better in the H{alpha} photometry, the statistical uncertainty on the derived M-dot{sub acc} is typically {approx_lt}17% and is dictated by the precision of the H{alpha} photometry. Systematic uncertainties, of up to a factor of 3 on the value of M-dot{sub acc}, are caused by our incomplete understanding of the physics of the accretion process and affect all determinations of the mass accretion rate, including those based on a spectroscopic H{alpha} line analysis. As an application of our method, we study the accretion process in a field of 9.16 arcmin{sup 2} around SN 1987A, using existing Hubble Space Telescope photometry. We identify as bona fide PMS stars a total of 133 objects with a H{alpha} excess above the 4{sigma} level and a median age of 13.5 Myr. Their median mass accretion rate of 2.6 x 10{sup -8} M{sub sun} yr{sup -1} is in excellent agreement with previous determinations based on the U-band excess of the stars in the same field, as well as with the value measured for G-type PMS stars in the Milky Way. The accretion luminosity of these PMS objects shows a strong dependence on their distance from a group of hot massive stars in the field and suggests that the ultraviolet radiation of the latter is rapidly

  19. Emergency Department Death Rates Dropped By Nearly 50 Percent, 1997-2011.

    PubMed

    Kanzaria, Hemal K; Probst, Marc A; Hsia, Renee Y

    2016-07-01

    Between 1997 and 2011, there was a nearly 50 percent reduction in US emergency department mortality rates for adults. This trend likely has many causes, related to advances in palliative, prehospital, and emergency care. PMID:27385248

  20. Weak-Line Quasars at High Redshift: Extremely High Accretion Rates or Anemic Broad-Line Regions?

    NASA Astrophysics Data System (ADS)

    Shemmer, Ohad; Trakhtenbrot, B.; Anderson, S. F.; Brandt, W. N.; Diamond-Stanic, A. M.; Fan, X.; Lira, P.; Netzer, H.; Plotkin, R. M.; Richards, G. T.; Schneider, D. P.; Strauss, M. A.

    2011-01-01

    We present Gemini-North K-band spectra of two representative members of the class of high-redshift quasars with exceptionally weak rest-frame ultraviolet emission lines (WLQs), SDSS J114153.34+021924.3 at z=3.55 and SDSS J123743.08+630144.9 at z=3.49. In both sources we detect an unusually weak broad Hβ line and we place tight upper limits on the strengths of their [O III] lines. Virial, Hβ-based black-hole mass determinations indicate normalized accretion rates of L/LEdd=0.4 for these sources, which is well within the range observed for typical quasars with similar luminosities and redshifts. We also present high-quality XMM-Newton imaging spectroscopy of SDSS J114153.34+021924.3 and find a hard-X-ray photon index of Γ=1.91+0.24-0.22which supports the virial L/LEdd determination in this source. Our results suggest that the weakness of the broad-emission lines in WLQs is not a consequence of an extreme continuum-emission source but instead due to abnormal broad-emission line region properties.

  1. Combining Hf-W Ages, Cooling Rates, and Thermal Models to Estimate the Accretion Time of Iron Meteorite Parent Bodies

    NASA Astrophysics Data System (ADS)

    Qin, L.; Dauphas, N.; Wadhwa, M.; Masarik, J.; Janney, P. E.

    2007-12-01

    The 182Hf-182W short-lived chronometer has been widely used to date metal-silicate differentiation processes in the early Solar System. However the presence of cosmogenic effects from exposure to GCR can potentially hamper the use of this system for chronology purposes (e.g. [1,2]). These effects must be corrected for in order to calculate metal-silicate differentiation ages. In this study, high-precision W isotope measurements are presented for 32 iron meteorites from 8 magmatic and 2 non-magmatic groups. Exposure ages and pre- atmospheric size estimates are available for most of these samples [3]. Our precision is better than or comparable to the currently most precise literature data and our results agree with previous work [4]. All magmatic irons have ɛ182W equal within error to or more negative than the Solar System initial derived from a CAI isochron [5]. Iron meteorites from the same magmatic groups show variations in ɛ182W. These are most easily explained by exposure to cosmic rays in space. A correction method was developed to estimate pre-exposure ɛ182W for individual iron meteorite groups. Metal-silicate differentiation in most iron meteorite parent bodies must have occurred within 2 Myr of formation of refractory inclusions. For the first time, we combine 182Hf-182W ages with parent body sizes inferred from metallographic cooling rates in a thermal model to constrain the accretion time of iron meteorite parent bodies. The estimated accretion ages are within 1.5 Myr for most magmatic groups, and could be as early as 0.2 Myr after CAI formation. This is consistent with the study of Bottke et al. [6] who argued that iron meteorite parent bodies could represent an early generation of planetesimals formed in the inner region of the Solar System. [1] Masarik J. (1997) EPSL 152, 181-185. [2] Markowski A. et al. (2006) EPSL 250,104-115. [3] Voshage H. (1984) EPSL 71, 181-194. [4] Markowski A. et al. (2006) EPSL 242, 1-15. [5] Kleine T. et al. (2005) GCA 69

  2. PHOTOMETRIC DETERMINATION OF THE MASS ACCRETION RATES OF PRE-MAIN-SEQUENCE STARS. II. NGC 346 IN THE SMALL MAGELLANIC CLOUD

    SciTech Connect

    De Marchi, Guido; Sirianni, Marco; Panagia, Nino; Sabbi, Elena; Romaniello, Martino; Prada Moroni, Pier Giorgio; Degl'Innocenti, Scilla E-mail: panagia@stsci.edu

    2011-10-10

    We have studied the properties of the stellar populations in the field of the NGC 346 cluster in the Small Magellanic Cloud, using a novel self-consistent method that allows us to reliably identify pre-main-sequence (PMS) objects actively undergoing mass accretion, regardless of their age. The method does not require spectroscopy and combines broadband V and I photometry with narrowband H{alpha} imaging to identify all stars with excess H{alpha} emission and derive the accretion luminosity L{sub acc} and mass accretion rate M-dot{sub acc} for all of them. The application of this method to existing Hubble Space Telescope (HST)/Advanced Camera for Surveys photometry of the NGC 346 field has allowed us to identify and study 680 bona fide PMS stars with masses from {approx}0.4 M{sub sun} to {approx}4 M{sub sun} and ages in the range from {approx}1 Myr to {approx}30 Myr. Previous investigations of this region, based on the same data, had identified young ({approx}3 Myr old) candidate PMS stars on the basis of their broadband colors. In this study, we show that there are at least two, almost equally numerous, young populations with distinct ages of, respectively, {approx}1 and {approx}20 Myr. We provide accurate physical parameters for all of them. We take advantage of the unprecedented size of our PMS sample and of its spread in mass and age to study the evolution of the mass accretion rate as a function of stellar parameters. We find that, regardless of stellar mass, the mass accretion rate decreases with roughly the square root of the age, or about three times slower than predicted by current models of viscous disk evolution, and that more massive stars systematically have a higher mass accretion rate in proportion to their mass. A multivariate linear regression fit reveals that log M-dot{sub acc}{approx_equal}-0.6 log t + log m + c, where t is the age of the star, m is its mass, and c is a quantity that is higher at lower metallicity. This result is consistent with

  3. Hispanic High School Graduates Pass Whites in Rate of College Enrollment: High School Drop-out Rate at Record Low

    ERIC Educational Resources Information Center

    Fry, Richard; Taylor, Paul

    2013-01-01

    A record seven-in-ten (69%) Hispanic high school graduates in the class of 2012 enrolled in college that fall, two percentage points higher than the rate (67%) among their white counterparts, according to a Pew Research Center analysis of new data from the U.S. Census Bureau. This milestone is the result of a long-term increase in Hispanic…

  4. Theory of wind accretion

    NASA Astrophysics Data System (ADS)

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.

    2014-01-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus attention to different regimes of quasi-spherical accretion onto the neutron star: the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically toward NS magnetospghere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. Two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg/s. In the subsonic case, which sets in at low luminosities, a hot quasi-spherical shell must be formed around the magnetosphere, and the actual accretion rate onto NS is determined by ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. We calculate the rate of plasma entry the magnetopshere and the angular momentum transfer in the shell due to turbulent viscosity appearing in the convective differentially rotating shell. We also discuss and calculate the structure of the magnetospheric boundary layer where the angular momentum between the rotating magnetosphere and the base of the differentially rotating quasi-spherical shell takes place. We show how observations of equilibrium X-ray pulsars Vela X-1 and GX 301-2 can be used to estimate dimensionless parameters of the subsonic settling accretion theory, and obtain the width of the magnetospheric boundary layer for these pulsars.

  5. The crime drop in comparative perspective: the impact of the economy and imprisonment on American and European burglary rates.

    PubMed

    Rosenfeld, Richard; Messner, Steven F

    2009-09-01

    Influential statements on recent American crime reductions maintain that the crime drop was confined to the USA. Yet other research has revealed comparable crime decreases in Europe. We suggest that the USA and European crime declines occurred in tandem because they were both brought about by upturns in the economy. In light of US research showing crime reductions resulting from growth in imprisonment, we also examine the possibility that rising imprisonment rates reduced European crime rates. We test these hypotheses in a pooled cross-sectional time-series analysis of burglary rates in the USA and nine European nations between 1993 and 2006. The results indicate that burglary declines in the US and Europe were associated with rising consumer confidence. By contrast, imprisonment appears to be significantly related to burglary rates only after unusual policy interventions, such as Italy's 2006 clemency measure that dramatically reduced the size of its prison population. We interpret these findings as reflecting the structural similarity and economic integration of the world's developed nations and the uneven convergence in US and European punishment policies. PMID:19703170

  6. Magnetospheric accretion in EX Lupi

    NASA Astrophysics Data System (ADS)

    Abraham, Peter; Kospal, Agnes; Bouvier, Jerome

    2016-08-01

    We propose to observe EX Lup, the prototype of the EXor class of young eruptive stars, in order to understand how the accretion process works in the quiescent system. Here, we request 2.6 hours of telescope time on Spitzer, to carry out a mid-infrared photometric monitoring, which we will supplement with simultaneous ground-based optical and near-infrared data. The multi-wavelength light curves will allow us to reliably separate the effects of fluctuating accretion rate from the rotation of the star. By analyzing the variations of the accretion rate we will determine whether EX Lup accretes through a few stable accretion columns or several short-lived random accretion streams. With this campaign, EX Lup will become one of the T Tauri systems where the accretion process is best understood.

  7. Turbulent Distortion of Condensate Accretion

    NASA Technical Reports Server (NTRS)

    Hazoume, R.; Orou Chabi, J.; Johnson, J. A., III

    1997-01-01

    When a simple model for the relationship between the density-temperature fluctuation correlation and mean values is used, we determine that the rate of change of turbulent intensity can influence directly the accretion rate of droplets. Considerable interest exists in the accretion rate for condensates in nonequilibrium flow with icing and the potential role which reactant accretion can play in nonequilibrium exothermic reactant processes. Turbulence is thought to play an important role in such flows. It has already been experimentally determined that turbulence influences the sizes of droplets in the heterogeneous nucleation of supersaturated vapors. This paper addresses the issue of the possible influence of turbulence on the accretion rate of droplets.

  8. Foot Drop

    MedlinePlus

    ... Awards Enhancing Diversity Find People About NINDS NINDS Foot Drop Information Page Table of Contents (click to ... research is being done? Clinical Trials What is Foot Drop? Foot drop describes the inability to raise ...

  9. Wetland Accretion Rate Model of Ecosystem Resilience (WARMER) and its application to habitat sustainability for endangered species in the San Francisco Estuary

    USGS Publications Warehouse

    Swanson, Kathleen M.; Drexler, Judith Z.; Schoellhamer, David H.; Thorne, Karen M.; Casazza, Michael L.; Overton, Cory T.; Callaway, John C.; Takekawa, John Y.

    2014-01-01

    Salt marsh faunas are constrained by specific habitat requirements for marsh elevation relative to sea level and tidal range. As sea level rises, changes in relative elevation of the marsh plain will have differing impacts on the availability of habitat for marsh obligate species. The Wetland Accretion Rate Model for Ecosystem Resilience (WARMER) is a 1-D model of elevation that incorporates both biological and physical processes of vertical marsh accretion. Here, we use WARMER to evaluate changes in marsh surface elevation and the impact of these elevation changes on marsh habitat for specific species of concern. Model results were compared to elevation-based habitat criteria developed for marsh vegetation, the endangered California clapper rail (Rallus longirostris obsoletus), and the endangered salt marsh harvest mouse (Reithrodontomys raviventris) to determine the response of marsh habitat for each species to predicted >1-m sea-level rise by 2100. Feedback between vertical accretion mechanisms and elevation reduced the effect of initial elevation in the modeled scenarios. Elevation decreased nonlinearly with larger changes in elevation during the latter half of the century when the rate of sea-level rise increased. Model scenarios indicated that changes in elevation will degrade habitat quality within salt marshes in the San Francisco Estuary, and degradation will accelerate in the latter half of the century as the rate of sea-level rise accelerates. A sensitivity analysis of the model results showed that inorganic sediment accumulation and the rate of sea-level rise had the greatest influence over salt marsh sustainability.

  10. The deepest X-ray view of high-redshift galaxies: constraints on low-rate black-hole accretion

    NASA Astrophysics Data System (ADS)

    Vito, F.; Gilli, R.; Vignali, C.; Brandt, W. N.; Comastri, A.; Yang, G.; Lehmer, B. D.; Luo, B.; Basu-Zych, A.; Bauer, F. E.; Cappelluti, N.; Koekemoer, A.; Mainieri, V.; Paolillo, M.; Ranalli, P.; Shemmer, O.; Trump, J.; Wang, J. X.; Xue, Y. Q.

    2016-08-01

    We exploit the 7 Ms Chandra observations in the Chandra Deep Field-South (CDF-S), the deepest X-ray survey to date, coupled with CANDELS/GOODS-S data, to measure the total X-ray emission arising from 2076 galaxies at 3.5 ≤ z < 6.5. This aim is achieved by stacking the Chandra data at the positions of optically selected galaxies, reaching effective exposure times of ≥109s. We detect significant (>3.7σ) X-ray emission from massive galaxies at z ≈ 4. We also report the detection of massive galaxies at z ≈ 5 at a 99.7% confidence level (2.7σ), the highest significance ever obtained for X-ray emission from galaxies at such high redshifts. No significant signal is detected from galaxies at even higher redshifts. The stacking results place constraints on the BHAD associated with the known high-redshift galaxy samples, as well as on the SFRD at high redshift, assuming a range of prescriptions for X-ray emission due to X- ray binaries. We find that the X-ray emission from our sample is likely dominated by processes related to star formation. Our results show that low-rate mass accretion onto SMBHs in individually X-ray-undetected galaxies is negligible, compared with the BHAD measured for samples of X-ray detected AGN, for cosmic SMBH mass assembly at high redshift. We also place, for the first time, constraints on the faint-end of the AGN X-ray luminosity function (logLX ˜ 42) at z > 4, with evidence for fairly flat slopes. The implications of all of these findings are discussed in the context of the evolution of the AGN population at high redshift.

  11. Chandra and MMT observations of low-mass black hole active galactic nuclei accreting at low rates in dwarf galaxies

    SciTech Connect

    Yuan, W.; Zhou, H.; Dou, L.; Dong, X.-B.; Wang, T.-G.; Fan, X.

    2014-02-10

    We report on Chandra X-ray observations of four candidate low-mass black hole (M {sub bh} ≲ 10{sup 6} M {sub ☉}) active galactic nuclei (AGNs) that have the estimated Eddington ratios among the lowest (∼10{sup –2}) found for this class. The aims are to validate the nature of their AGNs and to confirm the low Eddington ratios that are derived from the broad Hα line, and to explore this poorly studied regime in the AGN parameter space. Among them, two objects with the lowest significance of the broad lines are also observed with the Multi-Mirror Telescope, and the high-quality optical spectra taken confirm them as Seyfert 1 AGNs and as having small black hole masses. X-ray emission is detected from the nuclei of two of the galaxies, which is variable on timescales of ∼10{sup 3} s, whereas no significant (or only marginal at best) detection is found for the remaining two. The X-ray luminosities are on the order of 10{sup 41} erg s{sup –1} or even lower, on the order of 10{sup 40} erg s{sup –1} for non-detections, which are among the lowest regimes ever probed for Seyfert galaxies. The low X-ray luminosities, compared to their black hole masses derived from Hα, confirm their low accretion rates assuming typical bolometric corrections. Our results hint at the existence of a possibly large population of under-luminous low-mass black holes in the local universe. An off-nucleus ultra-luminous X-ray source in one of the dwarf galaxies is detected serendipitously, with a luminosity (6-9)× 10{sup 39} erg s{sup –1} in 2-10 keV.

  12. The Phase Space of z=1.2 Clusters: Probing Dust Temperature and Star Formation Rate as a Function of Environment and Accretion History

    NASA Astrophysics Data System (ADS)

    Noble, Allison; SpARCS Collaboration

    2016-01-01

    Understanding the influence of environment is a fundamental goal in studies of galaxy formation and evolution, and galaxy clusters offer ideal laboratories with which to examine environmental effects on their constituent members. Clusters continually evolve and build up mass through the accumulation of galaxies and groups, resulting in distinct galaxy populations based on their accretion history. In Noble et al. 2013, we presented a novel definition for environment using the phase space of line-of-sight velocity and clustercentric radius, which probes the time-averaged density to which a galaxy has been exposed and traces out accretion histories. Using this dynamical definition of environment reveals a decline in specific star formation towards the cluster core in the earliest accreted galaxies, and was further shown to isolate post-starburst galaxies within clusters (Muzzin et al. 2014). We have now extended this work to higher-redshift clusters at z=1.2 using deep Herschel-PACS and -SPIRE data. With a sample of 120 spectroscopically-confirmed cluster members, we investigate various galaxy properties as a function of phase-space environment. Specifically, we use 5-band Herschel photometry to estimate the dust temperature and star formation rate for dynamically distinct galaxy populations, namely recent infalls and those that were accreted into the cluster at an earlier epoch (Noble et al. submitted). These properties are then compared to a field sample of star-forming galaxies at 1.1 < z < 1.2 to shed light on cluster-specific processes in galaxy evolution. In this talk I will discuss the various implications of a phase-space definition for environment, and present our most recent results, focusing on how this accretion-based definition aids our understanding of quenching mechanisms within z=1.2 galaxies.

  13. Building bones in babies: can and should we exceed the human milk-fed infant's rate of bone calcium accretion?

    Technology Transfer Automated Retrieval System (TEKTRAN)

    Increasing calcium absorption and bone calcium accretion to levels above those achieved by human milk-fed, full-term infants is possible with infant formulas. However, no data support such a goal or suggest that it is beneficial to short- or long-term bone health. Small differences in the bioavailab...

  14. Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. V. A New Size–Luminosity Scaling Relation for the Broad-line Region

    NASA Astrophysics Data System (ADS)

    Du, Pu; Lu, Kai-Xing; Zhang, Zhi-Xiang; Huang, Ying-Ke; Wang, Kai; Hu, Chen; Qiu, Jie; Li, Yan-Rong; Fan, Xu-Liang; Fang, Xiang-Er; Bai, Jin-Ming; Bian, Wei-Hao; Yuan, Ye-Fei; Ho, Luis C.; Wang, Jian-Min; SEAMBH collaboration

    2016-07-01

    This paper reports results of the third-year campaign of monitoring super-Eddington accreting massive black holes (SEAMBHs) in active galactic nuclei (AGNs) between 2014 and 2015. Ten new targets were selected from the quasar sample of the Sloan Digital Sky Survey (SDSS), which have generally been more luminous than the SEAMBH candidates in the last two years. Hβ lags ({τ }{{H}β }) in five of the 10 quasars have been successfully measured in this monitoring season. We find that the lags are generally shorter, by large factors, than those of objects with same optical luminosity, in light of the well-known R H β–L 5100 relation. The five quasars have dimensionless accretion rates of \\dot{{M}\\quad }=10–103. Combining these with measurements of the previous SEAMBHs, we find that the reduction of Hβ lags depends tightly on accretion rates, {τ }{{H}β }/{τ }R-L\\propto {\\dot{{M}}}-0.42, where {τ }R-L is the Hβ lag from the normal R H β–L 5100 relation. Fitting 63 mapped AGNs, we present a new scaling relation for the broad-line region: {R}{{H}β }={α }1{{\\ell }}44{β 1} {min} [1,{(\\dot{{M}}/{\\dot{{M}}}c)}-{γ 1}], where {{\\ell }}44={L}5100/{10}44 {erg} {{{s}}}-1 is the 5100 Å continuum luminosity, and the coefficients are {α }1={29.6}-2.8+2.7 lt-day, {β }1={0.56}-0.03+0.03, {γ }1={0.52}-0.16+0.33, and {\\dot{{M}}}c={11.19}-6.22+2.29. This relation is applicable to AGNs over a wide range of accretion rates, from 10‑3 to 103. Implications of this new relation are briefly discussed.

  15. Massive star formation by accretion. I. Disc accretion

    NASA Astrophysics Data System (ADS)

    Haemmerlé, L.; Eggenberger, P.; Meynet, G.; Maeder, A.; Charbonnel, C.

    2016-01-01

    Context. Massive stars likely form by accretion and the evolutionary track of an accreting forming star corresponds to what is called the birthline in the Hertzsprung-Russell (HR) diagram. The shape of this birthline is quite sensitive to the evolution of the entropy in the accreting star. Aims: We first study the reasons why some birthlines published in past years present different behaviours for a given accretion rate. We then revisit the question of the accretion rate, which allows us to understand the distribution of the observed pre-main-sequence (pre-MS) stars in the HR diagram. Finally, we identify the conditions needed to obtain a large inflation of the star along its pre-MS evolution that may push the birthline towards the Hayashi line in the upper part of the HR diagram. Methods: We present new pre-MS models including accretion at various rates and for different initial structures of the accreting core. We compare them with previously published equivalent models. From the observed upper envelope of pre-MS stars in the HR diagram, we deduce the accretion law that best matches the accretion history of most of the intermediate-mass stars. Results: In the numerical computation of the time derivative of the entropy, some treatment leads to an artificial loss of entropy and thus reduces the inflation that the accreting star undergoes along the birthline. In the case of cold disc accretion, the existence of a significant swelling during the accretion phase, which leads to radii ≳ 100 R⊙ and brings the star back to the red part of the HR diagram, depends sensitively on the initial conditions. For an accretion rate of 10-3M⊙ yr-1, only models starting from a core with a significant radiative region evolve back to the red part of the HR diagram. We also obtain that, in order to reproduce the observed upper envelope of pre-MS stars in the HR diagram with an accretion law deduced from the observed mass outflows in ultra-compact HII regions, the fraction of the

  16. Supermassive Black Holes with High Accretion Rates in Active Galactic Nuclei. VI. Velocity-resolved Reverberation Mapping of the Hβ Line

    NASA Astrophysics Data System (ADS)

    Du, Pu; Lu, Kai-Xing; Hu, Chen; Qiu, Jie; Li, Yan-Rong; Huang, Ying-Ke; Wang, Fang; Bai, Jin-Ming; Bian, Wei-Hao; Yuan, Ye-Fei; Ho, Luis C.; Wang, Jian-Min; SEAMBH Collaboration

    2016-03-01

    In the sixth of a series of papers reporting on a large reverberation mapping (RM) campaign of active galactic nuclei (AGNs) with high accretion rates, we present velocity-resolved time lags of Hβ emission lines for nine objects observed in the campaign during 2012-2013. In order to correct the line broadening caused by seeing and instruments before analyzing the velocity-resolved RM, we adopt the Richardson-Lucy deconvolution to reconstruct their Hβ profiles. The validity and effectiveness of the deconvolution are checked using Monte Carlo simulation. Five among the nine objects show clear dependence of the time delay on velocity. Mrk 335 and Mrk 486 show signatures of gas inflow whereas the clouds in the broad-line regions (BLRs) of Mrk 142 and MCG +06-26-012 tend to be radial outflowing. Mrk 1044 is consistent with having virialized motions. The lags of the remaining four are not velocity-resolvable. The velocity-resolved RM of super-Eddington accreting massive black holes (SEAMBHs) shows that they have diverse kinematics in their BLRs. Comparing with the AGNs with sub-Eddington accretion rates, we do not find significant differences in the BLR kinematics of SEAMBHs.

  17. Rotating Bondi Accretion Flow

    NASA Astrophysics Data System (ADS)

    Park, Myeong-Gu; Han, Du-Hwan

    2016-06-01

    The characteristics of accretion flow onto a black hole are determined by the physical condition of gas at large radius. When the gas has no angular momentum and is polytropic, the accretion flow becomes the classic Bondi flow. The mass accretion rate in such case is an eigenvalue and uniquely determined by the density and the temperature of the surrounding gas for a given black hole mass. When the gas has angular momentum above some critical value, the angular momentum of the gas should be removed by viscosity to reach the black hole horizon. We study, within the slim disk approximation, rotating polytropic accretion flow with alpha viscosity as an an extension of the Bondi flow. The characteristics of the accretion flow are now determined by the temperature, density, and angular momentum of the gas at the outer boundary. We explore the effects of the viscosity parameter and the outer boundary radius on the physical characteristic of the flow, especially on the mass accretion rate, and compare the result with previous works of Park (2009) and Narayan & Fabian (2011).

  18. The Effect of Diagnostic Delays on the Drop-Out Rate and the Total Delay to Diagnosis of Tuberculosis

    PubMed Central

    Millen, Stephen J.; Uys, Pieter W.; Hargrove, John; van Helden, Paul D.; Williams, Brian G.

    2008-01-01

    Background Numerous patient and healthcare system-related delays contribute to the overall delay experienced by patients from onset of TB symptoms to diagnosis and treatment. Such delays are critical as infected individuals remain untreated in the community, providing more opportunities for transmission of the disease and adversely affecting the epidemic. Methodology/Principal Findings We present an analysis of the factors that contribute to the overall delay in TB diagnosis and treatment, in a resource-poor setting. Impact on the distribution of diagnostic delay times was assessed for various factors, the sensitivity of the diagnostic method being found to be the most significant. A linear relationship was found between the sensitivity of the test and the predicted mean delay time, with an increase in test sensitivity resulting in a reduced mean delay time and a reduction in the drop-out rate. Conclusions/Significance The results show that in a developing country a number of delay factors, particularly the low sensitivity of the initial sputum smear microscopy test, potentially increase total diagnostic delay times experienced by TB patients significantly. The results reinforce the urgent need for novel diagnostic methods, both for smear positive and negative TB, that are highly sensitive, accessible and point of care, in order to reduce mean delay times. PMID:18398459

  19. Accreting X-ray Pulsars

    NASA Technical Reports Server (NTRS)

    Wilson-Hodge, Colleen A.

    2009-01-01

    This presentation describes the behavior of matter in environments with extreme magnetic and gravitational fields, explains the instability/stability of accretion disks in certain systems, and discusses how emergent radiation affects accretion flow. Magnetic field measurements are obtained by measuring the lowest cyclotron absorption line energy, observing the cutoff of accretion due to centrifugal inhibition and measuring the spin-up rate at high luminosity.

  20. Evolution of Massive Protostars Via Disk Accretion

    NASA Astrophysics Data System (ADS)

    Hosokawa, Takashi; Yorke, Harold W.; Omukai, Kazuyuki

    2010-09-01

    Mass accretion onto (proto-)stars at high accretion rates \\dot{M}_* > 10^{-4} M_{⊙} yr^{-1} is expected in massive star formation. We study the evolution of massive protostars at such high rates by numerically solving the stellar structure equations. In this paper, we examine the evolution via disk accretion. We consider a limiting case of "cold" disk accretion, whereby most of the stellar photosphere can radiate freely with negligible backwarming from the accretion flow, and the accreting material settles onto the star with the same specific entropy as the photosphere. We compare our results to the calculated evolution via spherically symmetric accretion, the opposite limit, whereby the material accreting onto the star contains the entropy produced in the accretion shock front. We examine how different accretion geometries affect the evolution of massive protostars. For cold disk accretion at 10-3 M sun yr-1, the radius of a protostar is initially small, R *sime a few R sun. After several solar masses have accreted, the protostar begins to bloat up and for M * ~= 10 M sun the stellar radius attains its maximum of 30-400 R sun. The large radius ~100 R sun is also a feature of spherically symmetric accretion at the same accreted mass and accretion rate. Hence, expansion to a large radius is a robust feature of accreting massive protostars. At later times, the protostar eventually begins to contract and reaches the zero-age main sequence (ZAMS) for M * ~= 30 M sun, independent of the accretion geometry. For accretion rates exceeding several 10-3 M sun yr-1, the protostar never contracts to the ZAMS. The very large radius of several hundreds R sun results in the low effective temperature and low UV luminosity of the protostar. Such bloated protostars could well explain the existence of bright high-mass protostellar objects, which lack detectable H II regions.

  1. Structure of the Upper Crust Exposed at Endeavor Deep: Implications for Crustal Accretion at Ultra-Fast Spreading Rates

    NASA Astrophysics Data System (ADS)

    Popham, C. T.; Pockalny, R. A.; Larson, R. L.

    2004-12-01

    Endeavor Deep lies at the tip of the propagating spreading center defining the Juan Fernandez/Nazca plate boundary. This 3 km-deep, amagmatic basin,which rifted into ultra-fast spread 3Myr old Nazca Plate crust, was recently surveyed and sampled with Simrad EM300, DSL-120 and ROV Jason II. Over 140 structural orientations measured from Jason II video of the south rift wall show that flows in extrusive layer 2a strike north-south and dip shallowly to the west, while dikes in intrusive layer 2b strike east-west and dip steeply to the south. Using a general model for crustal accretion in which dikes are emplaced vertically and extrusives horizontally, a rotational history is determined for the 3 Myr old crust exposed in the walls of the deep. Multiple rotations are necessary with two-fold intent, first to return structure to the original off-axis orientation prior to tectonic reorganization; second to account for rotations involved in the process of accretion. Tectonic events are first addressed with a 10-25 degree rotation about a horizontal rift parallel axis to account for flexural uplift. Next a 65-degree rotation applied about a vertical axis to return magnetic lineation 2a to its proposed paleo-off axis orientation. After rotation, intrusive and extrusive populations are strike parallel (N5E). In this orientation, dikes average 65-degree dip away from and extrusives 25-degree dip towards the ridge axis. This generally conforms to observations at Hess deep, Blanco FZ, and ODP hole 801C. The second goal of rotation is to account for off axis adjustments during crustal accretion; a 25-degree rotation about a ridge parallel, horizontal axis returns the average dike inclination to vertical and the extrusive dip to horizontal.

  2. Launching jets from accretion belts

    NASA Astrophysics Data System (ADS)

    Schreier, Ron; Soker, Noam

    2016-05-01

    We propose that sub-Keplerian accretion belts around stars might launch jets. The sub-Keplerian inflow does not form a rotationally supported accretion disk, but it rather reaches the accreting object from a wide solid angle. The basic ingredients of the flow are a turbulent region where the accretion belt interacts with the accreting object via a shear layer, and two avoidance regions on the poles where the accretion rate is very low. A dynamo that is developed in the shear layer amplifies magnetic fields to high values. It is likely that the amplified magnetic fields form polar outflows from the avoidance regions. Our speculative belt-launched jets model has implications on a rich variety of astrophysical objects, from the removal of common envelopes to the explosion of core collapse supernovae by jittering jets.

  3. Intrinsic Absorption in Quasars (AAL & BAL) and its Relation to Outflows, BH Mass, Accretion Rate, Spin, Orientation, and Radio Properties

    NASA Astrophysics Data System (ADS)

    Stone, Robert Bernard; Richards, Gordon T.

    2016-01-01

    Despite the fact that quasars are fueled by matter falling into supermassive black holes, this process spews out considerable mass and energy. We investigate the nature of these outflows in the form of both broad and narrow absorption lines using data taken as part of the Sloan Digital Sky Survey (SDSS). Although these outflows are seen to have ejection speeds of up to 60,000 km/s, it is still unclear how they affect the quasar's host-galaxy and its evolution. We look for correlations of these outflows with the radio properties of the quasars, which can potentially reveal a physical connection between the quasar's accretion physics and its outflows. We also investigate how relaxing the traditional criteria for defining both radio loud and broad absorption line quasars impacts our understanding of these classes and quasars in general. Our ultimate goal is to understand how outflows from quasars change as a function of line-of-sight orientation, mass, accretion, and spin of the black holes that fuel them.

  4. Igneous Cooling Rate constraints on the Accretion of the lower Oceanic Crust in Mid-ocean Ridges: Insights from a new Thermo-mechanical Model

    NASA Astrophysics Data System (ADS)

    Garrido, C. J.; Machetel, P.

    2005-12-01

    We report the results of a new thermo-mechanical model of crustal flow beneath fast spreading mid-ocean ridges to investigate both the effect of deep, near off-axis hydrothermal convection on the thermal structure of the magma chamber and the role of variable number of melt intrusions on the accretion of the oceanic crust. In our model the melt is injected at the center of the axial magma chamber with a 'needle' with adjustable porosity at different depths allowing the simulation of different arrangements of melt injection and supply within the magma chamber. Conversely to previous models, the shape of the magma chamber -defined as the isotherm where 95% solidification of the melt occurs- is not imposed but computed from the steady state reached by the thermal field considering the heat diffusion and advection and the latent heat of crystallization. The motion equation is solved for a temperature and phase dependent viscosity. The thermal diffusivity is also dependent on temperature and depth, with a higher diffusivity in the upper plutonic crust to account for more efficient hydrothermal cooling at these crustal levels. In agreement with previous non-dynamic thermal models, our results show that near, deep off-axis hydrothermal circulation strongly affects the shape of the axial magma by tightening isotherms in the upper half of the plutonic oceanic crust where hydrothermal cooling is more efficient. Different accretion modes have however little effect on the shape of the magma chamber, but result in variable arrangements of flow lines ranging from tent-shape in a single-lens accretion scenario to sub-horizontal in "sheeted-sill" intrusion models. For different intrusion models, we computed the average Igneous Cooling Rates (ICR) of gabbros by dividing the crystallization temperature interval of gabbros by the integrated time, from the initial intrusion to the point where it crossed the 950 °C isotherm where total solidification of gabbro occurs, along individual

  5. Did Social Safety Net Scholarships Reduce Drop-Out Rates during the Indonesian Economic Crisis? Policy Research Working Paper.

    ERIC Educational Resources Information Center

    Cameron, Lisa A.

    This paper uses regression and matching techniques to evaluate Indonesia's Social Safety Net Scholarships Program. The program aimed to prevent large numbers of children from dropping out of school as a result of the Asian economic crisis of 1998. It was expected that families would find it difficult to keep their children in school and that…

  6. Experimental study on latent heat storage characteristics of W/O emulsion -Supercooling rate of dispersed water drops by direct contact heat exchange-

    NASA Astrophysics Data System (ADS)

    Morita, Shin-ichi; Hayamizu, Yasutaka; Horibe, Akihiko; Haruki, Naoto; Inaba, Hideo

    2013-04-01

    Recently, much attention has been paid to investigate the latent heat storage system. Using of ice heat storage system brings an equalization of electric power demand, because it will solved the electric -power-demand-concentration on day-time of summer by the air conditioning. The flowable latent heat storage material, Oil/Water type emulsion, microencapsulated latent heat material-water mixture or ice slurry, etc., is enable to transport the latent heat in a pipe. The flowable latent heat storage material can realize the pipe size reduction and system efficiency improvement. Supercooling phenomenon of the dispersed latent heat storage material in continuous phase brings the obstruction of latent heat storage. The latent heat storage rates of dispersed water drops in W/O (Water/Oil) emulsion are investigated experimentally in this study. The water drops in emulsion has the diameter within 3 ˜ 25μm, the averaged water drop diameter is 7.3μm and the standard deviation is 2.9μm. The direct contact heat exchange method is chosen as the phase change rate evaluation of water drops in W/O emulsion. The supercooled temperature and the cooling rate are set as parameters of this study. The evaluation is performed by comparison between the results of this study and the past research. The obtained experimental result is shown that the 35K or more degree from melting point brings 100% latent heat storage rate of W/O emulsion. It was clarified that the supercooling rate of dispersed water particles in emulsion shows the larger value than that of the bulk water.

  7. Drop dynamics

    NASA Technical Reports Server (NTRS)

    Elleman, D. D.

    1981-01-01

    The drop dynamics module is a Spacelab-compatible acoustic positioning and control system for conducting drop dynamics experiments in space. It consists basically of a chamber, a drop injector system, an acoustic positioning system, and a data collection system. The principal means of collecting data is by a cinegraphic camera. The drop is positioned in the center of the chamber by forces created by standing acoustic waves generated in the nearly cubical chamber (about 12 cm on a side). The drop can be spun or oscillated up to fission by varying the phse and amplitude of the acoustic waves. The system is designed to perform its experiments unattended, except for start-up and shutdown events and other unique events that require the attention of the Spacelab payload specialist.

  8. The ultraluminous X-ray source NGC 5643 ULX1: a large stellar mass black hole accreting at super-Eddington rates?

    NASA Astrophysics Data System (ADS)

    Pintore, Fabio; Zampieri, Luca; Sutton, Andrew D.; Roberts, Timothy P.; Middleton, Matthew J.; Gladstone, Jeanette C.

    2016-06-01

    A sub-set of the brightest ultraluminous X-ray sources (ULXs), with X-ray luminosities well above 1040 erg s-1, typically have energy spectra which can be well described as hard power laws, and short-term variability in excess of ˜10 per cent. This combination of properties suggests that these ULXs may be some of the best candidates to host intermediate-mass black holes (IMBHs), which would be accreting at sub-Eddington rates in the hard state seen in Galactic X-ray binaries. In this work, we present a temporal and spectral analysis of all of the available XMM-Newton data from one such ULX, the previously poorly studied 2XMM J143242.1-440939, located in NGC 5643. We report that its high-quality EPIC spectra can be better described by a broad, thermal component, such as an advection-dominated disc or an optically thick Comptonizing corona. In addition, we find a hint of a marginal change in the short-term variability which does not appear to be clearly related to the source unabsorbed luminosity. We discuss the implications of these results, excluding the possibility that the source may be host an IMBH in a low state, and favouring an interpretation in terms of super-Eddington accretion on to a black hole of stellar origin. The properties of NGC 5643 ULX1 allow us to associate this source to the population of the hard/ultraluminous ULX class.

  9. ACCRETION OUTBURSTS IN CIRCUMPLANETARY DISKS

    SciTech Connect

    Lubow, S. H.; Martin, R. G.

    2012-04-20

    We describe a model for the long-term evolution of a circumplanetary disk that is fed mass from a circumstellar disk and contains regions of low turbulence (dead zones). We show that such disks can be subject to accretion-driven outbursts, analogous to outbursts previously modeled in the context of circumstellar disks to explain FU Ori phenomena. Circumplanetary disks around a proto-Jupiter can undergo outbursts for infall accretion rates onto the disks in the range M-dot{sub infall} approx. 10{sup -9} to 10{sup -7} M{sub Sun} yr{sup -1}, typical of accretion rates in the T Tauri phase. During outbursts, the accretion rate and disk luminosity increases by several orders of magnitude. Most of the planet mass growth during planetary gas accretion may occur via disk outbursts involving gas that is considerably hotter than predicted by steady state models. For low infall accretion rates M-dot{sub infall} {approx}< 10{sup -10} M{sub sun} yr{sup -1} that occur in late stages of disk accretion, disk outbursts are unlikely to occur, even if dead zones are present. Such conditions are favorable for the formation of icy satellites.

  10. Controls on the Geometry of Accretion Reflectors

    NASA Astrophysics Data System (ADS)

    Wolovick, M.; Bell, R. E.; Buck, W. R.; Creyts, T. T.

    2012-12-01

    Basal accretion occurs when meltwater refreezes onto the base of an ice sheet. Thick packages (900-1100m) of accretion ice are identified in radio-echo sounding data as plume-shaped reflectors above the basal reflector and below isochronous layers of meteoric ice. Accretion reflectors have been imaged in both Antarctica and Greenland rising to a height of 1/3-1/2 of the ice sheet thickness and extending in the flow direction as far as 100 km. Here we use a two-dimensional thermomechanical higher order flowline model coupled to a basal hydrology model to investigate the freezing rates and energy budgets of basal accretion processes. Simple order-of-magnitude estimates for the freezing rate based on the observed height of the reflectors and the assumption that all ice under the observed reflector consists of accretion ice indicate very large freezing rates, on the order of 10-100 cm/yr. We test two end-member possibilities for the formation of the basal accretion bodies: high accretion rates and complex basal deformation. The first possibility is that the freezing rates are very large. The second possibility is that the ice under the observed reflector is a mixture of accreted and meteoric ice. If the ice below the accretion reflector is a mixture, the freezing rates can be much smaller than the simple estimates. If the freezing rates are small, then complex basal deformation must be invoked to cause accretion ice to override meteoric ice to a height of 1/3-1/2 the ice thickness. In the basal deformation case, low freezing rates predict a maximum thickness of 100-200m of accretion ice. The remaining ice beneath the reflector will be deformed meteoric ice. Both cases make testable predictions. If the accretion rates are very high and supercooling is the dominant process, accretion cannot use up all of the subglacial water. In this high rate scenario there will be water at the melting point exiting the accretion site. Alternatively if the accretion is part of a complex

  11. Heat transfer, pressure drop, and mass flow rate in pin fin channels with long and short trailing edge ejection holes

    NASA Astrophysics Data System (ADS)

    Lau, S. C.; Han, J. C.; Batten, T.

    1988-06-01

    The turbulent heat transfer and friction characteristics in the pin fin channels with small trailing edge ejection holes found in internally-cooled turbine airfoils have been experimentally investigated. It is found that the overall heat transfer increases when the length of the trailing edge ejection holes is increased and when the trailing edge ejection holes are configured such that much of the cooling air is forced to flow further downstream in the radial flow direction prior to exiting. The increase in the overall heat transfer is shown to be accompanied by an increase in the overall pressure drop.

  12. PS1-10jh Continues to Follow the Fallback Accretion Rate of a Tidally Disrupted Star

    NASA Astrophysics Data System (ADS)

    Gezari, S.; Chornock, R.; Lawrence, A.; Rest, A.; Jones, D. O.; Berger, E.; Challis, P. M.; Narayan, G.

    2015-12-01

    We present late-time observations of the tidal disruption event candidate PS1-10jh. UV and optical imaging with Hubble Space Telescope/WFC3 localize the transient to be coincident with the host galaxy nucleus to an accuracy of 0.023 arcsec, corresponding to 66 pc. The UV flux in the F225W filter, measured 3.35 rest-frame years after the peak of the nuclear flare, is consistent with a decline that continues to follow a t‑5/3 power-law with no spectral evolution. Late epochs of optical spectroscopy obtained with MMT ∼ 2 and 4 years after the peak, enable a clean subtraction of the host galaxy from the early spectra, revealing broad helium emission lines on top of a hot continuum, and placing stringent upper limits on the presence of hydrogen line emission. We do not measure Balmer Hδ absorption in the host galaxy that is strong enough to be indicative of a rare, post-starburst “E+A” galaxy as reported by Arcavi et al. The light curve of PS1-10jh over a baseline of 3.5 years is best modeled by fallback accretion of a tidally disrupted star. Its strong broad helium emission relative to hydrogen (He iiλ4686/Hα > 5) could be indicative of either the hydrogen-poor chemical composition of the disrupted star, or certain conditions in the tidal debris of a solar-composition star in the presence of an optically thick, extended reprocessing envelope.

  13. An Empirical Method Permitting Rapid Determination of the Area, Rate and Distribution of Water-Drop Impingement on an Airfoil of Arbitrary Section at Subsonic Speeds

    NASA Technical Reports Server (NTRS)

    Bergrun, N. R.

    1951-01-01

    An empirical method for the determination of the area, rate, and distribution of water-drop impingement on airfoils of arbitrary section is presented. The procedure represents an initial step toward the development of a method which is generally applicable in the design of thermal ice-prevention equipment for airplane wing and tail surfaces. Results given by the proposed empirical method are expected to be sufficiently accurate for the purpose of heated-wing design, and can be obtained from a few numerical computations once the velocity distribution over the airfoil has been determined. The empirical method presented for incompressible flow is based on results of extensive water-drop. trajectory computations for five airfoil cases which consisted of 15-percent-thick airfoils encompassing a moderate lift-coefficient range. The differential equations pertaining to the paths of the drops were solved by a differential analyzer. The method developed for incompressible flow is extended to the calculation of area and rate of impingement on straight wings in subsonic compressible flow to indicate the probable effects of compressibility for airfoils at low subsonic Mach numbers.

  14. Pressure Drop

    NASA Technical Reports Server (NTRS)

    Lawson, Mike

    2010-01-01

    Mike Lawson briefly discussed pressure drop for aerospace applications and presented short stories about adventures experienced while working at NASA and General Dynamics, including exposure to technologies like the Crew and Equipment Translation Aid (CETA) cart and the SWME.

  15. Relationship between star formation rate and black hole accretion at z=3: the different contributions in quiescent, normal, and starburst galaxies

    SciTech Connect

    Rodighiero, G.; Franceschini, A.; Baronchelli, I.; Brusa, M.; Delvecchio, I.; Pozzi, F.; Cimatti, A.; Mullaney, J. R.; Lutz, D.; Gruppioni, C.; Silverman, J.

    2015-02-10

    We investigate the co-evolution of the black hole accretion rate (BHAR) and the star formation rate (SFR) in 1.5accretion density of the universe at z∼2 is associated with normal star-forming systems, with only ∼6(±1)% and ∼11(±1)% associated with starburst and quiescent galaxies, respectively.

  16. Relationship between Star Formation Rate and Black Hole Accretion At Z = 2: the Different Contributions in Quiescent, Normal, and Starburst Galaxies

    NASA Astrophysics Data System (ADS)

    Rodighiero, G.; Brusa, M.; Daddi, E.; Negrello, M.; Mullaney, J. R.; Delvecchio, I.; Lutz, D.; Renzini, A.; Franceschini, A.; Baronchelli, I.; Pozzi, F.; Gruppioni, C.; Strazzullo, V.; Cimatti, A.; Silverman, J.

    2015-02-01

    We investigate the co-evolution of the black hole accretion rate (BHAR) and the star formation rate (SFR) in 1.5\\lt z\\lt 2.5 galaxies displaying a greater diversity of star-forming properties compared to previous studies. We combine X-ray stacking and far-IR photometry of stellar mass-limited samples of normal star-forming, starburst, and quiescent/quenched galaxies in the COSMOS field. We corroborate the existence of a strong correlation between BHAR (i.e., the X-ray luminosity, LX) and stellar mass (M*) for normal star-forming galaxies, though we find a steeper relation than previously reported. We find that starbursts show a factor of three enhancement in BHAR compared to normal SF galaxies (against a factor of six excess in SFR), while quiescents show a deficit of a factor times 5.5 at a given mass. One possible interpretation of this is that the starburst phase does not coincide with cosmologically relevant BH growth, or that starburst-inducing mergers are more efficient at boosting SFR than BHAR. Contrary to studies based on smaller samples, we find that the BHAR/SFR ratio of main-sequence (MS) galaxies is not mass invariant, but scales weakly as M*0.43+/- 0.09, implying faster BH growth in more massive galaxies at z∼ 2. Furthermore, BHAR/SFR during the starburst is a factor of two lower than in MS galaxies, at odds with the predictions of hydrodynamical simulations of merger galaxies that foresee a sudden enhancement of LX/SFR during the merger. Finally, we estimate that the bulk of the accretion density of the universe at z∼ 2 is associated with normal star-forming systems, with only ∼ 6(+/- 1)% and ∼ 11(+/- 1)% associated with starburst and quiescent galaxies, respectively.

  17. MAGNETICALLY REGULATED GAS ACCRETION IN HIGH-REDSHIFT GALACTIC DISKS

    SciTech Connect

    Birnboim, Yuval

    2009-09-10

    Disk galaxies are in hydrostatic equilibrium along their vertical axis. The pressure allowing for this configuration consists of thermal, turbulent, magnetic, and cosmic-ray components. For the Milky Way the thermal pressure contributes {approx}10% of the total pressure near the plane, with this fraction dropping toward higher altitudes. Out of the rest, magnetic fields contribute {approx}1/3 of the pressure to distances of {approx}3 kpc above the disk plane. In this Letter, we attempt to extrapolate these local values to high-redshift, rapidly accreting, rapidly star-forming disk galaxies and study the effect of the extra pressure sources on the accretion of gas onto the galaxies. In particular, magnetic field tension may convert a smooth cold-flow accretion to clumpy, irregular star formation regions and rates. The infalling gas accumulates on the edge of the magnetic fields, supported by magnetic tension. When the mass of the infalling gas exceeds some threshold mass, its gravitational force cannot be balanced by magnetic tension anymore, and it falls toward the disk's plane, rapidly making stars. Simplified estimations of this threshold mass are consistent with clumpy star formation observed in SINS, UDF, GOODS, and GEMS surveys. We discuss the shortcomings of pure hydrodynamic codes in simulating the accretion of cold flows into galaxies, and emphasize the need for magnetohydrodynamic simulations.

  18. Impact of initial models and variable accretion rates on the pre-main-sequence evolution of massive and intermediate-mass stars and the early evolution of H II regions

    NASA Astrophysics Data System (ADS)

    Haemmerlé, Lionel; Peters, Thomas

    2016-05-01

    Massive star formation requires the accretion of gas at high rate while the star is already bright. Its actual luminosity depends sensitively on the stellar structure. We compute pre-main-sequence tracks for massive and intermediate-mass stars with variable accretion rates and study the evolution of stellar radius, effective temperature and ionizing luminosity, starting at 2 M⊙ with convective or radiative structures. The radiative case shows a much stronger swelling of the protostar for high accretion rates than the convective case. For radiative structures, the star is very sensitive to the accretion rate and reacts quickly to accretion bursts, leading to considerable changes in photospheric properties on time-scales as short as 100-1000 yr. The evolution for convective structures is much less influenced by the instantaneous accretion rate, and produces a monotonically increasing ionizing flux that can be many orders of magnitude smaller than in the radiative case. For massive stars, it results in a delay of the H II region expansion by up to 10 000 yr. In the radiative case, the H II region can potentially be engulfed by the star during the swelling, which never happens in the convective case. We conclude that the early stellar structure has a large impact on the radiative feedback during the pre-main-sequence evolution of massive protostars and introduces an important uncertainty that should be taken into account. Because of their lower effective temperatures, our convective models may hint at a solution to an observed discrepancy between the luminosity distribution functions of massive young stellar objects and compact H II regions.

  19. Analysis of the Hydrogen Reduction Rate of Magnetite Concentrate Particles in a Drop Tube Reactor Through CFD Modeling

    NASA Astrophysics Data System (ADS)

    Fan, Deqiu; Mohassab, Yousef; Elzohiery, Mohamed; Sohn, H. Y.

    2016-06-01

    A computational fluid dynamics (CFD) approach, coupled with experimental results, was developed to accurately evaluate the kinetic parameters of iron oxide particle reduction. Hydrogen reduction of magnetite concentrate particles was used as a sample case. A detailed evaluation of the particle residence time and temperature profile inside the reactor is presented. This approach eliminates the errors associated with assumptions like constant particle temperature and velocity while the particles travel down a drop tube reactor. The gas phase was treated as a continuum in the Eulerian frame of reference, and the particles are tracked using a Lagrangian approach in which the trajectory and velocity are determined by integrating the equation of particle motion. In addition, a heat balance on the particle that relates the particle temperature to convection and radiation was also applied. An iterative algorithm that numerically solves the governing coupled ordinary differential equations was developed to determine the pre-exponential factor and activation energy that best fit the experimental data.

  20. Analysis of the Hydrogen Reduction Rate of Magnetite Concentrate Particles in a Drop Tube Reactor Through CFD Modeling

    NASA Astrophysics Data System (ADS)

    Fan, Deqiu; Mohassab, Yousef; Elzohiery, Mohamed; Sohn, H. Y.

    2016-02-01

    A computational fluid dynamics (CFD) approach, coupled with experimental results, was developed to accurately evaluate the kinetic parameters of iron oxide particle reduction. Hydrogen reduction of magnetite concentrate particles was used as a sample case. A detailed evaluation of the particle residence time and temperature profile inside the reactor is presented. This approach eliminates the errors associated with assumptions like constant particle temperature and velocity while the particles travel down a drop tube reactor. The gas phase was treated as a continuum in the Eulerian frame of reference, and the particles are tracked using a Lagrangian approach in which the trajectory and velocity are determined by integrating the equation of particle motion. In addition, a heat balance on the particle that relates the particle temperature to convection and radiation was also applied. An iterative algorithm that numerically solves the governing coupled ordinary differential equations was developed to determine the pre-exponential factor and activation energy that best fit the experimental data.

  1. Wind accretion: Theory and observations

    NASA Astrophysics Data System (ADS)

    Shakura, N. I.; Postnov, K. A.; Kochetkova, A. Yu.; Hjalmarsdotter, L.; Sidoli, L.; Paizis, A.

    2015-07-01

    A review of wind accretion in high-mass X-ray binaries is presented. We focus on different regimes of quasi-spherical accretion onto the neutron star (NS): the supersonic (Bondi) accretion, which takes place when the captured matter cools down rapidly and falls supersonically towards the NS magnetosphere, and subsonic (settling) accretion which occurs when plasma remains hot until it meets the magnetospheric boundary. These two regimes of accretion are separated by an X-ray luminosity of about 4 × 1036 erg s-1. In the subsonic case, which sets in at lower luminosities, a hot quasi-spherical shell must form around the magnetosphere, and the actual accretion rate onto NS is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instability. In turn, two regimes of subsonic accretion are possible, depending on plasma cooling mechanism (Compton or radiative) near the magnetopshere. The transition from the high-luminosity with Compton cooling to the lowluminosity (Lx ≲ 3 × 1035 erg s-1) with radiative cooling can be responsible for the onset of the off states repeatedly observed in several low-luminosity slowly accreting pulsars, such as Vela X-1, GX 301-2, and 4U 1907+09. The triggering of the transitionmay be due to a switch in the X-ray beam pattern in response to a change in the optical depth in the accretion column with changing luminosity. We also show that in the settling accretion theory, bright X-ray flares (~1038-1040 erg) observed in supergiant fast X-ray transients (SFXT) can be produced by sporadic capture of magnetized stellar wind plasma. At sufficiently low accretion rates, magnetic reconnection can enhance the magnetospheric plasma entry rate, resulting in copious production of X-ray photons, strong Compton cooling and ultimately in unstable accretion of the entire shell. A bright flare develops on the free-fall time scale in the shell, and the typical energy released in an SFXT bright flare corresponds to the mass

  2. Influence of solidification on the impact of supercooled water drops onto cold surfaces

    NASA Astrophysics Data System (ADS)

    Li, Hai; Roisman, Ilia V.; Tropea, Cameron

    2015-06-01

    This study presents an experimental investigation of the impact of a supercooled drop onto hydrophilic and superhydrophobic substrates. The aim is to better understand the process of airframe icing caused by supercooled large droplets, which has been recently identified as a severe hazard in aviation. The Weber number and Reynolds number of the impinging drop ranged from 200 to 300 and from 2600 to 5800, respectively. Drop impact, spreading, and rebound were observed using a high-speed video system. The maximum spreading diameter of an impacting drop on hydrophilic surfaces was measured. The temperature effect on this parameter was only minor for a wide range of the drop and substrate temperatures. However, ice/water mixtures emerged when both the drop and substrate temperatures were below 0 °C. Similarly, drop rebound on superhydrophobic substrates was significantly hindered by solidification when supercooled drop impacted onto substrates below the freezing point. The minimum receding diameter and the speed of ice accretion on the substrate were measured for various wall temperatures. Both parameters increased almost linearly with decreasing wall temperature, but eventually leveled off beyond a certain substrate temperature. The rate of ice formation on the substrate was significantly higher than the growth rate of free ice dendrites, implying that multiple nucleation sites were present.

  3. He-accreting white dwarfs: accretion regimes and final outcomes

    NASA Astrophysics Data System (ADS)

    Piersanti, L.; Tornambé, A.; Yungelson, L. R.

    2014-12-01

    The behaviour of carbon-oxygen (CO) white dwarfs (WDs) subject to direct helium accretion is extensively studied. We aim to analyse the thermal response of an accreting WD to mass deposition at different timescales. The analysis has been performed for initial WD masses and accretion rates in the range 0.60-1.02 M⊙ and 10-9-10-5 M⊙ yr-1, respectively. Thermal regimes in the parameter space MWD-dot{M}_He leading to formation of red-giant-like structures, steady burning of He, and mild, strong and dynamical flashes have been identified and the transition between these regimes has been studied in detail. In particular, the physical properties of WDs experiencing the He-flash accretion regime have been investigated to determine the mass retention efficiency as a function of the accretor total mass and accretion rate. We also discuss to what extent the building up of a He-rich layer via H burning could be described according to the behaviour of models accreting He-rich matter directly. Polynomial fits to the obtained results are provided for use in binary population synthesis computations. Several applications for close binary systems with He-rich donors and CO WD accretors are considered and the relevance of the results for interpreting He novae is discussed.

  4. Accretion disk boundary layers in cataclysmic variables. 1: Optically thick boundary layers

    NASA Technical Reports Server (NTRS)

    Popham, Robert; Narayan, Ramesh

    1995-01-01

    We develop numerical models of accretions disks in cataclysmic variables (CVs), including and emphasizing the boundary layer region where the accretion disk meets the accreting white dwarf. We confine ourselves to solutions where the boundary layer region is vertically optically thick, and find that these solutions share several common features. The angular and radial velocities of the accreting material drop rapidly in a dynamical boundary layer, which has a radial width approximately 1%-3% of the white dwarf radius. The energy dissipated in this region diffuses through the inner part of the disk and is radiated from the disk surface in a thermal boundary layer, which has a radial width comparable to the disk thickness, approximately 5%-15% of the white dwarf radius. We examine the dependence of the boundary layer structure on the mass accretion rate, the white dwarf mass and rotation rate, and the viscosity parameter alpha. We delineate the boundary between optically thick and optically thin boundary layer solutions as a function of these parameters and suggest that by means of a careful comparison with observations it may be possible to estimate alpha in CVs. We derive an expression for the total boundary layer luminosities as a function of the parameters and show that it agrees well with the luminosites of our numerical solutions. Finally, we calcuate simple blackbody continuum spectra of the boundary layer and disk emission for our solutions and compare these to soft X-ray, EUV, and He II emission-line observations of CVs. We show that, through such comparisons, it may be possible to determine the rotation rates of the accreting stars in CVs, and perhaps also the white dwarf masses and the accretion rates. The spectra are quite insensitive to alpha, so the uncertainty in this parameter does not affect such comparisons.

  5. Galactic Fountains and Gas Accretion

    NASA Astrophysics Data System (ADS)

    Marinacci, F.; Binney, J.; Fraternali, F.; Nipoti, C.; Ciotti, L.; Londrillo, P.

    2010-06-01

    Star-forming disc galaxies such as the Milky Way need to accrete >~1 Msolar of gas each year to sustain their star formation. This gas accretion is likely to come from the cooling of the hot corona, however it is still not clear how this process can take place. We present simulations supporting the idea that this cooling and the subsequent accretion are caused by the passage of cold galactic-fountain clouds through the hot corona. The Kelvin-Helmholtz instability strips gas from these clouds and the stripped gas causes coronal gas to condense in the cloud's wake. For likely parameters of the Galactic corona and of typical fountain clouds we obtain a global accretion rate of the order of that required to feed the star formation.

  6. Radar-based rainfall estimation: Improving Z/R relations through comparison of drop size distributions, rainfall rates and radar reflectivity patterns

    NASA Astrophysics Data System (ADS)

    Neuper, Malte; Ehret, Uwe

    2014-05-01

    Gini index, Rosenbluth index) were calculated and compared to the synoptic situation in general and the atmospheric stability in special. The indices were then related to the drop size distributions and the rain rate. Special emphasis was laid in an objective distinction between stratiform and convective precipitation and hereby altered droplet size distribution, respectively Z/R relationship. In our presentation we will show how convective and stratiform precipitation becomes manifest in the different distribution indices, which in turn are thought to represent different patterns in the radar image. We also present and discuss the correlation between these distribution indices and the evolution of the drop size distribution and the rain rate and compare a dynamically adopted Z/R relation to the standard Marshall-Palmer Z/R relation.

  7. Modeling the shear rate and pressure drop in a hydrodynamic cavitation reactor with experimental validation based on KI decomposition studies.

    PubMed

    Badve, Mandar P; Alpar, Tibor; Pandit, Aniruddha B; Gogate, Parag R; Csoka, Levente

    2015-01-01

    A mathematical model describing the shear rate and pressure variation in a complex flow field created in a hydrodynamic cavitation reactor (stator and rotor assembly) has been depicted in the present study. The design of the reactor is such that the rotor is provided with surface indentations and cavitational events are expected to occur on the surface of the rotor as well as within the indentations. The flow characteristics of the fluid have been investigated on the basis of high accuracy compact difference schemes and Navier-Stokes method. The evolution of streamlining structures during rotation, pressure field and shear rate of a Newtonian fluid flow have been numerically established. The simulation results suggest that the characteristics of shear rate and pressure area are quite different based on the magnitude of the rotation velocity of the rotor. It was observed that area of the high shear zone at the indentation leading edge shrinks with an increase in the rotational speed of the rotor, although the magnitude of the shear rate increases linearly. It is therefore concluded that higher rotational speeds of the rotor, tends to stabilize the flow, which in turn results into less cavitational activity compared to that observed around 2200-2500RPM. Experiments were carried out with initial concentration of KI as 2000ppm. Maximum of 50ppm of iodine liberation was observed at 2200RPM. Experimental as well as simulation results indicate that the maximum cavitational activity can be seen when rotation speed is around 2200-2500RPM. PMID:24924259

  8. To accrete or not accrete, that is the question

    USGS Publications Warehouse

    von, Huene R.

    1986-01-01

    Along modern convergent margins tectonic processes span a spectrum from accretion to erosion. The process of accretion is generally recognized because it leaves a geologic record, whereas the process of erosion is generally hypothetical because it produces a geologic hiatus. Major conditions that determine the dominance of accretion or erosion at modern convergent margins are: 1) rate and direction of plate convergence, 2) sediment supply and type in the trench, and 3) topography of the subducting ocean floor. Most change in structure has been ascribed to plate motion, but both erosion and accretion are observed along the same convergence margin. Thus sediment supply and topography are probably of equivalent importance to plate motion because both erosion and accretion are observed under constant conditions of plate convergence. The dominance of accretion or erosion at a margin varies with the thickness of trench sediment. In a sediment flooded trench, the proportions of subducted and accreted sediment are commonly established by the position of a decollement along a weak horizon in the sediment section. Thus, the vertical variation of sediment strength and the distribution of horizontal stress are important factors. Once deformation begins, the original sediment strength is decreased by sediment remolding and where sediment thickens rapidly, increases in pore fluid pressure can be pronounced. In sediment-starved trenches, where the relief of the subducting ocean floor is not smoothed over, the front of the margin must respond to the topography subducted as well as that accreted. The hypothesized erosion by the drag of positive features against the underside of the upper plate (a high stress environment) may alternate with erosion due to the collapse of a margin front into voids such as graben (a low stress environment). ?? 1986 Ferdinand Enke Verlag Stuttgart.

  9. At-Risk Students and the Dropout Rate: What Influences Student Decisions to Remain in School or Drop-Out in a Suburban High School?

    ERIC Educational Resources Information Center

    McIntyre, Kevin

    2013-01-01

    The drop out phenomenon is a persistent problem in public schools across the United States. The consequence of a student dropping out of school negatively impacts both the individual who chooses to drop out and the society that ultimately is responsible for supporting that individual. The purpose of this study is to analyze the decision making…

  10. Pulsed Accretion onto Eccentric and Circular Binaries

    NASA Astrophysics Data System (ADS)

    Muñoz, Diego J.; Lai, Dong

    2016-08-01

    We present numerical simulations of circumbinary accretion onto eccentric and circular binaries using the moving-mesh code AREPO. This is the first set of simulations to tackle the problem of binary accretion using a finite-volume scheme on a freely moving mesh, which allows for accurate measurements of accretion onto individual stars for arbitrary binary eccentricity. While accretion onto a circular binary shows bursts with period of ∼ 5 times the binary period P b, accretion onto an eccentric binary is predominantly modulated at the period ∼ 1{P}{{b}}. For an equal-mass circular binary, the accretion rates onto individual stars are quite similar to each other, following the same variable pattern in time. By contrast, for eccentric binaries, one of the stars can accrete at a rate 10–20 times larger than its companion. This “symmetry breaking” between the stars, however, alternates over timescales of order 200P b and can be attributed to a slowly precessing, eccentric circumbinary disk. Over longer timescales, the net accretion rates onto individual stars are the same, reaching a quasi-steady state with the circumbinary disk. These results have important implications for the accretion behavior of binary T Tauri stars and supermassive binary black holes.

  11. Pulsed Accretion onto Eccentric and Circular Binaries

    NASA Astrophysics Data System (ADS)

    Muñoz, Diego J.; Lai, Dong

    2016-08-01

    We present numerical simulations of circumbinary accretion onto eccentric and circular binaries using the moving-mesh code AREPO. This is the first set of simulations to tackle the problem of binary accretion using a finite-volume scheme on a freely moving mesh, which allows for accurate measurements of accretion onto individual stars for arbitrary binary eccentricity. While accretion onto a circular binary shows bursts with period of ˜ 5 times the binary period P b, accretion onto an eccentric binary is predominantly modulated at the period ˜ 1{P}{{b}}. For an equal-mass circular binary, the accretion rates onto individual stars are quite similar to each other, following the same variable pattern in time. By contrast, for eccentric binaries, one of the stars can accrete at a rate 10–20 times larger than its companion. This “symmetry breaking” between the stars, however, alternates over timescales of order 200P b and can be attributed to a slowly precessing, eccentric circumbinary disk. Over longer timescales, the net accretion rates onto individual stars are the same, reaching a quasi-steady state with the circumbinary disk. These results have important implications for the accretion behavior of binary T Tauri stars and supermassive binary black holes.

  12. Ordovician ocean plate stratigraphy and thrust duplexes of the Ballantrae Complex, SW Scotland: Implications for the pelagic deposition rate and forearc accretion in the closing Iapetus Ocean

    NASA Astrophysics Data System (ADS)

    Fujisaki, Wataru; Asanuma, Hisashi; Suzuki, Kazue; Sawaki, Yusuke; Sakata, Shuhei; Hirata, Takafumi; Maruyama, Shigenori; Windley, Brian F.

    2015-11-01

    The Ballantrae Complex (at Bennane Lea in SW Scotland) contains important ocean plate stratigraphy (basalt, chert, mudstone, sandstone) in an accretionary prism that is associated with a classic Ordovician ophiolite. We used the ocean plate stratigraphy to sub-divide the prism into 11 tectonic units. To determine the depositional age of bedded cherts, zircons were separated from 9 tuff beds from 6 different units. All the tuffs have early to middle Ordovician ages, even though their present positions are mutually distant. These ages are consistent with microfossil records of radiolaria and graptolites. The stratigraphic-structural relationships demonstrate that the ocean plate stratigraphy has been repeated by bedding-parallel thrusts; this is typical of a modern accretionary duplex. We calculated the sedimentation rate of Early to Middle Ordovician bedded cherts at Bennane Lea on the basis of U-Pb zircon ages obtained from several tuff beds; the data indicate that the depositional rate (0.6-3 m/myr) was as slow as that of Mesozoic-Cenozoic equivalents defined by radiolaria. The age spectra of detrital zircons from Ballantrae sandstones show prominent single peaks at ca. 467 and 478 Ma, and a lack of Precambrian zircons. Integration of our new zircon ages with published isotopic data and palaeo-geographic maps indicates that the sandstones were deposited near an intra-oceanic arc and far from any continent containing Precambrian rocks. The pelagic-to-clastic sediments at Bennane Lea were deposited in the closing Iapetus Ocean from ca. 477 Ma to ca. 464 Ma, when they were accreted with the intra-oceanic arc of Ballantrae.

  13. Bondi accretion onto cosmological black holes

    NASA Astrophysics Data System (ADS)

    Karkowski, Janusz; Malec, Edward

    2013-02-01

    In this paper we investigate a steady accretion within the Einstein-Straus vacuole, in the presence of the cosmological constant. The dark energy damps the mass accretion rate and—above a certain limit—completely stops the steady accretion onto black holes, which, in particular, is prohibited in the inflation era and after (roughly) 1012 years from the big bang (assuming the presently known value of the cosmological constant). Steady accretion would not exist in the late phases of the Penrose’s scenario—known as the Weyl curvature hypothesis—of the evolution of the Universe.

  14. Star formation sustained by gas accretion

    NASA Astrophysics Data System (ADS)

    Sánchez Almeida, Jorge; Elmegreen, Bruce G.; Muñoz-Tuñón, Casiana; Elmegreen, Debra Meloy

    2014-07-01

    Numerical simulations predict that metal-poor gas accretion from the cosmic web fuels the formation of disk galaxies. This paper discusses how cosmic gas accretion controls star formation, and summarizes the physical properties expected for the cosmic gas accreted by galaxies. The paper also collects observational evidence for gas accretion sustaining star formation. It reviews evidence inferred from neutral and ionized hydrogen, as well as from stars. A number of properties characterizing large samples of star-forming galaxies can be explained by metal-poor gas accretion, in particular, the relationship among stellar mass, metallicity, and star-formation rate (the so-called fundamental metallicity relationship). They are put forward and analyzed. Theory predicts gas accretion to be particularly important at high redshift, so indications based on distant objects are reviewed, including the global star-formation history of the universe, and the gas around galaxies as inferred from absorption features in the spectra of background sources.

  15. A remarkable sea-level drop and relevant biotic responses across the Guadalupian-Lopingian (Permian) boundary in low-latitude mid-Panthalassa: Irreversible changes recorded in accreted paleo-atoll limestones in Akasaka and Ishiyama, Japan

    NASA Astrophysics Data System (ADS)

    Kofukuda, Daisuke; Isozaki, Yukio; Igo, Hisayoshi

    2014-03-01

    The Capitanian (Upper Guadalupian) to Wuchiapingian (Lower Lopingian) shallow-marine limestones at Akasaka and Ishiyama in central Japan record unique aspects of the extinction-related Guadalupian-Lopingian boundary (G-LB) interval. The ca. 140 m-thick Akasaka Limestone consists of the Capitanian black limestone (Unit B; 112 m) and the Wuchiapingian light gray dolomitic limestone (Unit W; 21 m), with a black/white striped limestone (Unit S; 9 m) between them. The G-LB horizon is assigned at the base of Unit W, on the basis of the first occurrence of the Wuchiapingian fusulines. The Capitanian Unit B and the Wuchiapingian Unit W were deposited mostly in the subtidal zone of a lagoon, whereas the intervened Unit S and the lowermost Unit W were in the intertidal zone. A hiatus with a remarkable erosional feature was newly identified at the top of Unit S. These records indicate that the sea-level has dropped significantly around the G-LB to have exposed the top of the atoll complex above the sea-level. The Ishiyama Limestone, located ca. 10 km to the north of the Akasaka limestone, retains almost the same depositional records. The extinction of large-tested fusuline (Yabeina) and large bivalves (Alatoconchidae) occurred in the upper part of Unit B, and the overlying 20 m-thick limestone (the uppermost Unit B and Unit S) below the hiatus represents a unique barren interval. The upper half of the barren interval is more depleted in fossils than the lower half, and this likely represents a duration of the severest environmental stress(es) for the shallow-marine protists/animals on the mid-oceanic paleo-atoll complex. Small-tested fusulines re-appeared at the base of Unit W above the hiatus. These facts prove that the elimination of shallow-marine biota occurred during the Capitanian shallowing of Akasaka paleo-atoll before the subaerial exposure/erosion across the G-LB. The overall shallowing and the development of such a clear hiatus at the top of a mid-oceanic seamount

  16. Scaling of the photon index vs. mass accretion rate correlation and estimate of black hole mass in M101 ULX-1

    NASA Astrophysics Data System (ADS)

    Titarchuk, Lev; Seifina, Elena

    2016-01-01

    We report the results of Swift and Chandra observations of an ultraluminous X-ray source, ULX-1 in M101. We show strong observational evidence that M101 ULX-1 undergoes spectral transitions from the low/hard state to the high/soft state during these observations. The spectra of M101 ULX-1 are well fitted by the so-called bulk motion Comptonization (BMC) model for all spectral states. We have established the photon index (Γ) saturation level, Γsat = 2.8 ± 0.1, in the Γ versus mass accretion rate (Ṁ) correlation. This Γ-Ṁ correlation allows us to evaluate black hole (BH) mass in M101 ULX-1 to be MBH ~ (3.2-4.3) × 104 M⊙, assuming the spread in distance to M101 (from 6.4 ± 0.5 Mpc to 7.4 ± 0.6 Mpc). For this BH mass estimate we apply the scaling method, using Galactic BHs XTE J1550-564, H 1743-322 and 4U 1630-472 as reference sources. The Γ vs. Ṁ correlation revealed in M101 ULX-1 is similar to that in a number of Galactic BHs and clearly exhibits the correlation along with the strong Γ saturation at ≈ 2.8. This is robust observational evidence for the presence of a BH in M101 ULX-1. We also find that the seed (disk) photon temperatures are low, on the order of 40-100 eV, which is consistent with high BH mass in M101 ULX-1. Thus, we suggest that the central object in M101 ULX-1 has intermediate BH mass on the order of 104 solar masses.

  17. Perturbation growth in accreting filaments

    NASA Astrophysics Data System (ADS)

    Clarke, S. D.; Whitworth, A. P.; Hubber, D. A.

    2016-05-01

    We use smoothed particle hydrodynamic simulations to investigate the growth of perturbations in infinitely long filaments as they form and grow by accretion. The growth of these perturbations leads to filament fragmentation and the formation of cores. Most previous work on this subject has been confined to the growth and fragmentation of equilibrium filaments and has found that there exists a preferential fragmentation length-scale which is roughly four times the filament's diameter. Our results show a more complicated dispersion relation with a series of peaks linking perturbation wavelength and growth rate. These are due to gravo-acoustic oscillations along the longitudinal axis during the sub-critical phase of growth. The positions of the peaks in growth rate have a strong dependence on both the mass accretion rate onto the filament and the temperature of the gas. When seeded with a multiwavelength density power spectrum, there exists a clear preferred core separation equal to the largest peak in the dispersion relation. Our results allow one to estimate a minimum age for a filament which is breaking up into regularly spaced fragments, as well as an average accretion rate. We apply the model to observations of filaments in Taurus by Tafalla & Hacar and find accretion rates consistent with those estimated by Palmeirim et al.

  18. Episodic Accretion in Young Stars

    NASA Astrophysics Data System (ADS)

    Audard, M.; Ábrahám, P.; Dunham, M. M.; Green, J. D.; Grosso, N.; Hamaguchi, K.; Kastner, J. H.; Kóspál, Á.; Lodato, G.; Romanova, M. M.; Skinner, S. L.; Vorobyov, E. I.; Zhu, Z.

    In the last 20 years, the topic of episodic accretion has gained significant interest in the star-formation community. It is now viewed as a common, although still poorly understood, phenomenon in low-mass star formation. The FU Orionis objects (FUors) are long-studied examples of this phenomenon. FU Orionis objects are believed to undergo accretion outbursts during which the accretion rate rapidly increases from typically 10-7 to a few 10-4 M⊙ yr-1, and remains elevated over several decades or more. EXors, a loosely defined class of pre-main-sequence stars, exhibit shorter and repetitive outbursts, associated with lower accretion rates. The relationship between the two classes, and their connection to the standard pre-main-sequence evolutionary sequence, is an open question: Do they represent two distinct classes, are they triggered by the same physical mechanism, and do they occur in the same evolutionary phases? Over the past couple of decades, many theoretical and numerical models have been developed to explain the origin of FUor and EXor outbursts. In parallel, such accretion bursts have been detected at an increasing rate, and as observing techniques improve, each individual outburst is studied in increasing detail. We summarize key observations of pre-main-sequence star outbursts, and review the latest thinking on outburst triggering mechanisms, the propagation of outbursts from star/disk to disk/jet systems, the relation between classical EXors and FUors, and newly discovered outbursting sources — all of which shed new light on episodic accretion. We finally highlight some of the most promising directions for this field in the near- and long-term.

  19. Accretion flows govern black hole jet properties

    NASA Astrophysics Data System (ADS)

    Koljonen, K.; Russell, D.; Fernández Ontiveros, J.; Miller-Jones, J.; Russell, T.; Curran, P.; Soria, R.; Markoff, S.; van der Horst, A.; Casella, P.

    2015-07-01

    The process of jet formation in accreting black holes, and the conditions under which it occurs is currently hotly debated, with competing models predicting the jet power to be governed by black hole spin, the magnetic field strength, the location of the jet base, the mass accretion rate and/or the properties of the inner accretion flow. We present new results that show empirical correlations between the accretion flow properties and the spectral energy distribution of the jets launched from accreting black holes. The X-ray power law is directly related to the particle energy distribution in the hot accretion flow. We find that the photon index of this power law correlates with the characteristic break frequency in the jet spectrum emitted near the jet base, and the jet luminosity up to the break frequency. The observed correlations can be explained by the energy distribution of electrons in the hot accretion flow being subsequently channeled into the jet. These correlations represent a new inflow--outflow connection in accreting black holes, and demonstrate that the spectral properties of the jet rely most critically on the conditions in the inner accretion flow, rather than other parameters such as the black hole mass or spin.

  20. Jets from magnetized accretion disks

    NASA Astrophysics Data System (ADS)

    Matsumoto, Ryoji

    When an accretion disk is threaded by large scale poloidal magnetic fields, the injection of magnetic helicity from the accretion disk drives bipolar outflows. We present the results of global magnetohydrodynamic (MHD) simulations of jet formation from a torus initially threaded by vertical magnetic fields. After the torsional Alfvén waves generated by the injected magnetic twists propagate along the large-scale magnetic field lines, magnetically driven jets emanate from the surface of the torus. Due to the magnetic pinch effect, the jets are collimated along the rotation axis. Since the jet formation process extracts angular momentum from the disk, it enhances the accretion rate of the disk material. Through three-dimensional (3D) global MHD simulations, we confirmed previous 2D results that the magnetically braked surface of the disk accretes like an avalanche. Owing to the growth of non-axisymmetric perturbations, the avalanche flow breaks up into spiral channels. Helical structure also appears inside the jet. When magnetic helicity is injected into closed magnetic loops connecting the central object and the accretion disk, it drives recurrent magnetic reconnection and outflows.

  1. Get a Vocation: Keeping on Top of Studies--Reducing the Drop-Out Rate in Vocational Upper Secondary Education and Training

    ERIC Educational Resources Information Center

    Jappinen, Aini-Kristiina

    2009-01-01

    The Finnish Ministry of education commissioned a study in 2006-07 on reducing the number of drop-outs, "Get a vocation--keeping on top of studies". The aim of the study was to establish the good practices and operating models underpinning reduction in drop-outs. The study indicates that an educational institution that fosters commitment of…

  2. Counter-rotating accretion discs

    NASA Astrophysics Data System (ADS)

    Dyda, S.; Lovelace, R. V. E.; Ustyugova, G. V.; Romanova, M. M.; Koldoba, A. V.

    2015-01-01

    Counter-rotating discs can arise from the accretion of a counter-rotating gas cloud on to the surface of an existing corotating disc or from the counter-rotating gas moving radially inwards to the outer edge of an existing disc. At the interface, the two components mix to produce gas or plasma with zero net angular momentum which tends to free-fall towards the disc centre. We discuss high-resolution axisymmetric hydrodynamic simulations of viscous counter-rotating discs for the cases where the two components are vertically separated and radially separated. The viscosity is described by an isotropic α-viscosity including all terms in the viscous stress tensor. For the vertically separated components, a shear layer forms between them and the middle part of this layer free-falls to the disc centre. The accretion rates are increased by factors of ˜102-104 over that for a conventional disc rotating in one direction with the same viscosity. The vertical width of the shear layer and the accretion rate are strongly dependent on the viscosity and the mass fraction of the counter-rotating gas. In the case of radially separated components where the inner disc corotates and the outer disc rotates in the opposite direction, a gap between the two components opens and closes quasi-periodically. The accretion rates are ≳25 times larger than those for a disc rotating in one direction with the same viscosity.

  3. The accretion regimes of a highly magnetized NS: the unique case of NuSTAR J095551+6940.8

    NASA Astrophysics Data System (ADS)

    Dall'Osso, Simone; Perna, Rosalba; Papitto, Alessandro; Bozzo, Enrico; Stella, Luigi

    2016-04-01

    The ultraluminous accreting pulsar M82-X2 (NuSTAR J095551+6940.8) offers an unprecedented opportunity to study the disc-magnetosphere interaction in a new regime of supercritical accretion. The source X-ray emission has been highly variable during the last 15 yrs. It ranged from a maximum of ˜2 × 1040 erg s-1 through intermediate values ˜ a few × 1039 erg s-1, and down to a minimum below 2 × 1038 erg s-1 that we have determined here, by analysing archival Chandra HRC observations of the source at an epoch at which it was undetected. We interpret the source variability via a magnetically threaded disc model: when at peak luminosity, the neutron star (NS) is close to spin equilibrium, its inner disc edge rm ˜ 108 cm is approximately half the corotation radius rco, and radiation pressure dominates the disc out to rtr ≲ 109 cm. In the radiation-pressure-dominated regime, rm grows very slowly as the mass inflow rate drops: as a result, rm < rco remains valid until dot{M} ≳dot{M}_E, the Eddington accretion rate, allowing a wide range of accretion luminosities to the NS. Once dot{M} < dot{M}_E accretion on to the NS is inhibited because rm > rco, and the source luminosity is expected to drop by a large factor. We conclude that a magnetically threaded accretion disc surrounding a highly magnetized NS (B ≲ 1013 G), and transitioning between the radiation-pressure and gas-pressure dominated regimes, offers the best interpretation for all the currently observed properties of NuSTAR J095551+6940.8.

  4. Pressure gradient torque in highly supersonic nonaxisymmetric accretion

    NASA Technical Reports Server (NTRS)

    Ho, Cheng; Taam, Ronald E.; Fryxell, Bruce A.; Matsuda, Takuya; Koide, Hiroshi

    1989-01-01

    The contribution of a pressure gradient torque to the angular momentum transfer rate in highly supersonic nonaxisymmetric accretion flows is considered. This study takes into account the contribution due to the pressure variation in the postaccretion-shock region which is significant for high Mach number accretion. For the case of accretion flow with Mach (infinity) of not less than 5, the overall accretion torque is shown to approach a constant value.

  5. A wind accretion model for HLX-1

    SciTech Connect

    Miller, M. Coleman; Farrell, Sean A.; Maccarone, Thomas J.

    2014-06-20

    The brightest ultraluminous X-ray source currently known, HLX-1, has been observed to undergo five outburst cycles. The periodicity of these outbursts, and their high inferred maximum accretion rates of ∼few × 10{sup –4} M {sub ☉} yr{sup –1}, naturally suggest Roche lobe overflow at the pericenter of an eccentric orbit. It is, however, difficult for the Roche lobe overflow model to explain the apparent trend of decreasing decay times over the different outbursts while the integrated luminosity also drops. Thus, if the trend is real rather than simply being a reflection of the complex physics of accretion disks, a different scenario may be necessary. We present a speculative model in which, within the last decade, a high-mass giant star had most of its envelope tidally stripped by the ∼10{sup 4–5} M {sub ☉} black hole in HLX-1, and the remaining core plus low-mass hydrogen envelope now feeds the hole with a strong wind. This model can explain the short decay time of the disk, and could explain the fast decrease in decay time if the wind speed changes with time. A key prediction of this model is that there will be excess line absorption due to the wind; our analysis does in fact find a flux deficit in the ∼0.9-1.1 keV range that is consistent with predictions, albeit at low significance. If this idea is correct, we also expect that within years to dacades the bound material from the original disruption will return and will make HLX-1 a persistently bright source.

  6. Pulsed accretion in a variable protostar.

    PubMed

    Muzerolle, James; Furlan, Elise; Flaherty, Kevin; Balog, Zoltan; Gutermuth, Robert

    2013-01-17

    Periodic increases in luminosity arising from variable accretion rates have been predicted for some pre-main-sequence close binary stars as they grow from circumbinary disks. The phenomenon is known as pulsed accretion and can affect the orbital evolution and mass distribution of young binaries, as well as the potential for planet formation. Accretion variability is a common feature of young stars, with a large range of amplitudes and timescales as measured from multi-epoch observations at optical and infrared wavelengths. Periodic variations consistent with pulsed accretion have been seen in only a few young binaries via optical accretion tracers, albeit intermittently with accretion luminosity variations ranging from zero to 50 per cent from orbit to orbit. Here we report that the infrared luminosity of a young protostar (of age about 10(5) years) increases by a factor of ten in roughly one week every 25.34 days. We attribute this to pulsed accretion associated with an unseen binary companion. The strength and regularity of this accretion signal is surprising; it may be related to the very young age of the system, which is a factor of ten younger than the other pulsed accretors previously studied. PMID:23283175

  7. Microwave ice accretion meter

    NASA Technical Reports Server (NTRS)

    Magenheim, Bertram (Inventor); Rocks, James K. (Inventor)

    1984-01-01

    A system for indicating ice thickness and rate of ice thickness growth on surfaces is disclosed. The region to be monitored for ice accretion is provided with a resonant surface waveguide which is mounted flush, below the surface being monitored. A controlled oscillator provides microwave energy via a feed point at a controllable frequency. A detector is coupled to the surface waveguide and is responsive to electrical energy. A measuring device indicates the frequency deviation of the controlled oscillator from a quiescent frequency. A control means is provided to control the frequency of oscillation of the controlled oscillator. In a first, open-loop embodiment, the control means is a shaft operated by an operator. In a second, closed-loop embodiment, the control means is a processor which effects automatic control.

  8. Observations of accreting pulsars

    NASA Technical Reports Server (NTRS)

    Prince, Thomas A.; Bildsten, Lars; Chakrabarty, Deepto; Wilson, Robert B.; Finger, Mark H.

    1994-01-01

    We discuss recent observations of accreting binary pulsars with the all-sky BATSE instrument on the Compton Gamma Ray Observatory. BATSE has detected and studied nearly half of the known accreting pulsar systems. Continuous timing studies over a two-year period have yielded accurate orbital parameters for 9 of these systems, as well as new insights into long-term accretion torque histories.

  9. Dilating Eye Drops

    MedlinePlus

    ... Frequently Asked Questions Español Condiciones Chinese Conditions Dilating Eye Drops En Español Read in Chinese What are dilating eye drops? Dilating eye drops contain medication to enlarge ( ...

  10. Morphodynamics of Accreting Beaches

    NASA Astrophysics Data System (ADS)

    Ruggiero, P.; Gelfenbaum, G.; Sherwood, C. R.; Kaminsky, G. M.

    2002-12-01

    Beaches along the Pacific Northwest coast of the US have been shown to have large seasonal variability in shoreline position with several 10's of meters of recession occurring during the winter (high-energy waves) and typically similar scales of beach recovery during the summer (low-energy waves). However, many beaches along the Columbia River littoral cell (northwest Oregon and southwest Washington) have exhibited net residual progradation of several meters per year over decades, resulting in significant shoreline realignment. This historical shoreline advance has been primarily due to the dispersal of sand from the flanks of the ebb-tidal deltas following jetty construction at the entrances to the Columbia River and Grays Harbor. The installation of jetties removed the shallow shoals from the influence of tidal currents, resulting in a shoreface profile that was too shallow for the inherent wave energy. Onshore transport of large quantities of sand occurred over the next several decades, decreasing through time. While much of the original source material is now exhausted, many beaches today are still rapidly accreting on inter-annual time scales. Gradients in alongshore sediment transport, net onshore directed cross-shore sediment transport within the surf zone, and cross-shore feeding from a shoreface out of equilibrium with forcing conditions may each be partially responsible for this continued accretion. The primary morphodynamic mechanism for sub-aerial beach growth, and shoreline progradation on a seasonal scale, is hypothesized to be the development, onshore migration, and welding of inter-tidal (swash) bars to the upper beach face. To investigate the processes and morphodynamics associated with accreting beaches we have completed two field experiments and are applying computational models that link measured sediment transport to wave and current forcing. Experiments completed in Spring 2001 and Summer 2002 combined process measurements with observations of

  11. Transient jet formation and state transitions from large-scale magnetic reconnection in black hole accretion discs

    NASA Astrophysics Data System (ADS)

    Dexter, Jason; McKinney, Jonathan C.; Markoff, Sera; Tchekhovskoy, Alexander

    2014-05-01

    Magnetically arrested accretion discs (MADs), where the magnetic pressure in the inner disc is dynamically important, provide an alternative mechanism for regulating accretion to what is commonly assumed in black hole systems. We show that a global magnetic field inversion in the MAD state can destroy the jet, significantly increase the accretion rate, and move the effective inner disc edge in to the marginally stable orbit. Reconnection of the MAD field in the inner radii launches a new type of transient outflow containing hot plasma generated by magnetic dissipation. This transient outflow can be as powerful as the steady magnetically dominated Blandford-Znajek jet in the MAD state. The field inversion qualitatively describes many of the observational features associated with the high-luminosity hard-to-soft state transition in black hole X-ray binaries: the jet line, the transient ballistic jet, and the drop in rms variability. These results demonstrate that the magnetic field configuration can influence the accretion state directly, and hence the magnetic field structure is an important second parameter in explaining observations of accreting black holes across the mass and luminosity scales.

  12. Accretion of Ghost Condensate by Black Holes

    SciTech Connect

    Frolov, A

    2004-06-02

    The intent of this letter is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as tenth of a solar mass per second for 10MeV-scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model.

  13. Exploding Water Drops

    NASA Astrophysics Data System (ADS)

    Reich, Gary

    2016-01-01

    Water has the unusual property that it expands on freezing, so that ice has a specific gravity of 0.92 compared to 1.0 for liquid water. The most familiar demonstration of this property is ice cubes floating in a glass of water. A more dramatic demonstration is the ice bomb shown in Fig. 1. Here a cast iron flask is filled with water and tightly stoppered. The flask is then cooled, either by leaving it outdoors in winter or by immersing it in a cryogenic fluid, until the water freezes. As the water freezes and expands, the pressure inside the flask increases dramatically, eventually becoming sufficient to fracture the metal walls of the enclosure. A related, but much less familiar, phenomenon is the explosive fracturing of small water drops upon freezing. That water drops can fracture in this way has been known for many years, and the phenomenon has been described in detail in the atmospheric sciences literature, where it is seen as relevant to the freezing of raindrops as they fall through cold air. Carefully controlled experiments have been done documenting how the character and frequency of fracture is affected by such variables as drop size, rate of cooling, chemistry of dissolved gases, etc. Here I describe instead a simple demonstration of fracture suitable for video analysis and appropriate for study at the introductory physics level. Readers may also be interested in other characteristics of freezing and fragmenting water drops, for example, charge separation upon fracture and the appearance of spikes and bulges on the surface.

  14. The co-evolution of the obscured quasar PKS 1549-79 and its host galaxy: evidence for a high accretion rate and warm outflow

    NASA Astrophysics Data System (ADS)

    Holt, J.; Tadhunter, C.; Morganti, R.; Bellamy, M.; González Delgado, R. M.; Tzioumis, A.; Inskip, K. J.

    2006-08-01

    We use deep optical, infrared and radio observations to explore the symbiosis between nuclear activity and galaxy evolution in the southern compact radio source PKS 1549-79 (z = 0.1523). The optical imaging observations reveal the presence of tidal tail features which provide strong evidence that the host galaxy has undergone a major merger in the recent past. The merger hypothesis is further supported by the detection of a young stellar population (YSP), which, on the basis of spectral synthesis modelling of our deep Very Large Telescope (VLT) optical spectra, was formed 50-250 Myr ago and makes up a significant fraction of the total stellar mass (1-30 per cent). Despite the core-jet structure of the radio source, which is consistent with the idea that the jet is pointing close to our line of sight, our HI 21-cm observations reveal significant HI absorption associated with both the core and the jet. Moreover, the luminous, quasar-like active galactic nucleus (AGN) (MV < -23.5) is highly extinguished (Av > 6.4) at optical wavelengths and show many properties in common with narrow-line Seyfert 1 galaxies (NLS1), including relatively narrow permitted lines [full width at half-maximum (FWHM) ~ 1940 km s-1], highly blueshifted [OIII] λλ5007,4959 lines (ΔV ~ 680 km s-1) and evidence that the putative supermassive black hole is accreting at a high Eddington ratio (0.3 < Lbol/Ledd < 11). The results suggest that accretion at high Eddington ratio does not prevent the formation of powerful relativistic jets. Together, the observations lend strong support to the predictions of some recent numerical simulations of galaxy mergers in which the black hole grows rapidly through merger-induced accretion following the coalescence of the nuclei of two merging galaxies, and the major growth phase is largely hidden at optical wavelengths by the natal gas and dust. Although the models also predict that AGN-driven outflows will eventually remove the gas from the bulge of the host

  15. Formation of Jupiter's Core and Early Stages of Envelope Accretion

    NASA Astrophysics Data System (ADS)

    D'Angelo, G.; Weidenschilling, S.; Lissauer, J. J.; Bodenheimer, P.; Hubickyj, O.

    2012-12-01

    , envelope-enhanced cross-section leads to the growth of a core of 3 Earth masses in less than 1e5 years and of a core of 5 Earth masses in less than 2e5 years. By the time the total planet mass reaches about 6 Earth masses, the accretion rate of solids has dropped below ~1e-6 Earth masses per year. Support for this research from NASA Outer Planets Research Program is gratefully acknowledged.

  16. Disk accretion by magnetic neutron stars

    NASA Technical Reports Server (NTRS)

    Ghosh, P.; Lamb, F. K.

    1978-01-01

    A model for disk accretion by a rotating magnetic neutron star is proposed which includes a detailed description of matter flow in the transition region between the disk and the magnetosphere. It is shown that the disk plasma cannot be completely screened from the stellar magnetic field and that the resulting magnetic coupling between the star and the disk exerts a significant torque on the star. On the assumption that the distortion of the residual stellar field lines threading the disk is limited by reconnection, the total accretion torque on the star is calculated. The calculated torque gives period changes in agreement with those observed in the pulsating X-ray sources and provides a natural explanation of why a fast rotator like Her X-1 has a spin-up rate much below the conventional estimate for slow rotators. It is shown that for such fast rotators, fluctuations in the mass-accretion rate can produce fluctuations in the accretion torque about 100 times larger. For sufficiently fast rotators or, equivalently, for sufficiently low accretion rates, the star experiences a braking torque even while accretion continues and without any mass ejection from its vicinity.

  17. Quasi-stars: accreting black holes inside massive envelopes

    NASA Astrophysics Data System (ADS)

    Begelman, Mitchell C.; Rossi, Elena M.; Armitage, Philip J.

    2008-07-01

    We study the structure and evolution of `quasi-stars', accreting black holes embedded within massive hydrostatic gaseous envelopes. These configurations may model the early growth of supermassive black hole seeds. The accretion rate on to the black hole adjusts so that the luminosity carried by the convective envelope equals the Eddington limit for the total mass, M* + MBH ~ M*. This greatly exceeds the Eddington limit for the black hole mass alone, leading to rapid growth of the black hole. We use analytic models and numerical stellar structure calculations to study the structure and evolution of quasi-stars. We show that the photospheric temperature of the envelope scales as Tph ~ M-2/5BHM7/20*, and decreases with time while the black hole mass increases. Once Tph < 104 K, the photospheric opacity drops precipitously and Tph hits a limiting value, analogous to the Hayashi track for red giants and protostars, below which no hydrostatic solution for the convective envelope exists. For metal-free (Population III) opacities, this limiting temperature is approximately 4000 K. After a quasi-star reaches this limiting temperature, it is rapidly dispersed by radiation pressure. We find that black hole seeds with masses between 103 and 104Msolar could form via this mechanism in less than a few Myr.

  18. Alternate drop pulse polarography

    USGS Publications Warehouse

    Christie, J.H.; Jackson, L.L.; Osteryoung, R.A.

    1976-01-01

    The new technique of alternate drop pulse polarography is presented. An experimental evaluation of alternate drop pulse polarography shows complete compensation of the capacitative background due to drop expansion. The capillary response phenomenon was studied in the absence of faradaic reaction and the capillary response current was found to depend on the pulse width to the -0.72 power. Increased signal-to-noise ratios were obtained using alternate drop pulse polarography at shorter drop times.

  19. Accretion, winds and outflows in young stars

    NASA Astrophysics Data System (ADS)

    Günther, H. M.

    2013-02-01

    Young stars and planetary systems form in molecular clouds. After the initial radial infall an accretion disk develops. For classical T Tauri stars (CTTS, F-K type precursors) the accretion disk does not reach down to the central star, but it is truncated near the co-rotation radius by the stellar magnetic field. The inner edge of the disk is ionized by the stellar radiation, so that the accretion stream is funneled along the magnetic field lines. On the stellar surface an accretion shock develops, which is observed over a wide wavelength range as X-ray emission, UV excess, optical veiling and optical and IR emission lines. Some of the accretion tracers, e.g. Hα, can be calibrated to measure the accretion rate. This accretion process is variable on time scales of hours to years due to changing accretion rates, stellar rotation and reconfiguration of the magnetic field. Furthermore, many (if not all) accreting systems also drive strong outflows which are ultimately powered by accretion. However, the exact driving mechanism is still unclear. Several components could contribute to the outflows: slow, wide-angle disk winds, X-winds launched close to the inner disk rim, and thermally driven stellar winds. In any case, the outflows contain material of very different temperatures and speeds. The disk wind is cool and can have a molecular component with just a few tens of km s-1, while the central component of the outflow can reach a few 100 km s-1. In some cases the inner part of the outflow is collimated to a small-angle jet. These jets have an onion-like structure, where the inner components are consecutively hotter and faster. The jets can contain working surfaces, which show up as Herbig-Haro knots. Accretion and outflows in the CTTS phase do not only determine stellar parameters like the rotation rate on the main-sequence, they also can have a profound impact on the environment of young stars. This review concentrates on CTTS in near-by star forming regions where

  20. Bondi accretion in early-type galaxies

    NASA Astrophysics Data System (ADS)

    Korol, Valeriya; Ciotti, Luca; Pellegrini, Silvia

    2016-05-01

    Accretion on to central massive black holes in galaxies is often modelled with the Bondi solution. In this paper we study a generalization of the classical Bondi accretion theory, considering the additional effects of the gravitational potential of the host galaxy, and of electron scattering in the optically thin limit. We provide a general analysis of the bias in the estimates of the Bondi radius and mass accretion rate, when adopting as fiducial values for the density and temperature at infinity the values of these quantities measured at finite distance from the central black hole. We also give general formulae to compute the correction terms of the critical accretion parameter in relevant asymptotic regimes. A full analytical discussion is presented in the case of an Hernquist galaxy, when the problem reduces to the discussion of a cubic equation, therefore allowing for more than one critical point in the accretion structure. The results are useful for observational works (especially in the case of systems with a low Eddington ratio), as well as for numerical simulations, where accretion rates are usually defined in terms of the gas properties near the black hole.

  1. Bondi accretion in early-type galaxies

    NASA Astrophysics Data System (ADS)

    Korol, Valeriya; Ciotti, Luca; Pellegrini, Silvia

    2016-08-01

    Accretion on to central massive black holes in galaxies is often modelled with the Bondi solution. In this paper, we study a generalization of the classical Bondi accretion theory, considering the additional effects of the gravitational potential of the host galaxy, and of electron scattering in the optically thin limit. We provide a general analysis of the bias in the estimates of the Bondi radius and mass accretion rate, when adopting as fiducial values for the density and temperature at infinity the values of these quantities measured at finite distance from the central black hole. We also give general formulae to compute the correction terms of the critical accretion parameter in relevant asymptotic regimes. A full analytical discussion is presented in the case of a Hernquist galaxy, when the problem reduces to the discussion of a cubic equation, therefore, allowing for more than one critical point in the accretion structure. The results are useful for observational works (especially in the case of systems with a low Eddington ratio), as well as for numerical simulations, where accretion rates are usually defined in terms of the gas properties near the black hole.

  2. Accretion onto Planetary Mass Companions of Low-mass Young Stars

    NASA Astrophysics Data System (ADS)

    Zhou, Yifan; Herczeg, Gregory J.; Kraus, Adam L.; Metchev, Stanimir; Cruz, Kelle L.

    2014-03-01

    Measurements of accretion rates onto planetary mass objects may distinguish between different planet formation mechanisms, which predict different accretion histories. In this Letter, we use Hubble Space Telescope (HST)/WFC3 UVIS optical photometry to measure accretion rates onto three accreting objects, GSC 06214-00210 b, GQ Lup b, and DH Tau b, that are at the planet/brown dwarf boundary and are companions to solar mass stars. The excess optical emission in the excess accretion continuum yields mass accretion rates of 10-9-10-11 M ⊙ yr-1 for these three objects. Their accretion rates are an order of magnitude higher than expected from the correlation between mass and accretion rates measured from the UV excess, which is applicable if these wide planetary mass companions formed by protostellar core fragmentation. The high accretion rates and large separation from the central star demonstrate the presence of massive disks around these objects. Models for the formation and evolution of wide planetary mass companions should account for their large accretion rates. High ratios of Hα luminosity over accretion luminosity for objects with low accretion rates suggest that searches for Hα emission may be an efficient way to find accreting planets.

  3. Self-Diffusion of Drops in a Dilute Sheared Emulsion

    NASA Technical Reports Server (NTRS)

    Loewenberg, Michael; Hinch, E. J.

    1996-01-01

    Self-diffusion coefficients that describe cross-flow migration of non-Brownian drops in a dilute sheared emulsion were obtained by trajectory calculations. A boundary integral formulation was used to describe pairwise interactions between deformable drops; interactions between undeformed drops were described with mobility functions for spherical drops. The results indicate that drops have large anisotropic self-diffusivities which depend strongly on the drop viscosity and modestly on the shear-rate. Pairwise interactions between drops in shear-flow do not appreciably promote drop breakup.

  4. Rapidly Accreting Supergiant Protostars: Embryos of Supermassive Black Holes?

    NASA Astrophysics Data System (ADS)

    Hosokawa, Takashi; Omukai, Kazuyuki; Yorke, Harold W.

    2012-09-01

    Direct collapse of supermassive stars (SMSs) is a possible pathway for generating supermassive black holes in the early universe. It is expected that an SMS could form via very rapid mass accretion with \\dot{M}_*\\sim 0.1{--}1 \\,M_\\odot \\,yr^{-1} during the gravitational collapse of an atomic-cooling primordial gas cloud. In this paper, we study how stars would evolve under such extreme rapid mass accretion, focusing on the early evolution until the stellar mass reaches 103 M ⊙. To this end, we numerically calculate the detailed interior structure of accreting stars with primordial element abundances. Our results show that for accretion rates higher than 10-2 M ⊙ yr-1, stellar evolution is qualitatively different from that expected at lower rates. While accreting at these high rates, the star always has a radius exceeding 100 R ⊙, which increases monotonically with the stellar mass. The mass-radius relation for stellar masses exceeding ~100 M ⊙ follows the same track with R *vpropM 1/2 * in all cases with accretion rates >~ 10-2 M ⊙ yr-1 at a stellar mass of 103 M ⊙, the radius is ~= 7000 R ⊙ (sime 30 AU). With higher accretion rates, the onset of hydrogen burning is shifted toward higher stellar masses. In particular, for accretion rates exceeding \\dot{M}_*\\gtrsim 0.1 \\,M_\\odot \\,yr^{-1}, there is no significant hydrogen burning even after 103 M ⊙ have accreted onto the protostar. Such "supergiant" protostars have effective temperatures as low as T eff ~= 5000 K throughout their evolution and because they hardly emit ionizing photons, they do not create an H II region or significantly heat their immediate surroundings. Thus, radiative feedback is unable to hinder the growth of rapidly accreting stars to masses in excess of 103 M ⊙ as long as material is accreted at rates \\dot{M}_*\\gtrsim 10^{-2} \\,M_\\odot \\,yr^{-1}.

  5. Attracting Water Drops

    NASA Video Gallery

    Astronauts Cady Coleman and Ron Garan perform the Attracting Water Drops experiment from Chabad Hebrew Academy in San Diego, Calif. This research determines if a free-floating water drop can be att...

  6. Dilating Eye Drops

    MedlinePlus

    ... Conditions Most Common Searches Adult Strabismus Amblyopia Cataract Conjunctivitis Corneal Abrasions Dilating Eye Drops Lazy eye (defined) ... Loading... Most Common Searches Adult Strabismus Amblyopia Cataract Conjunctivitis Corneal Abrasions Dilating Eye Drops Lazy eye (defined) ...

  7. Ternary drop collisions

    NASA Astrophysics Data System (ADS)

    Hinterbichler, Hannes; Planchette, Carole; Brenn, Günter

    2015-10-01

    It has been recently proposed to use drop collisions for producing advanced particles or well-defined capsules, or to perform chemical reactions where the merged drops constitute a micro-reactor. For all these promising applications, it is essential to determine whether the merged drops remain stable after the collision, forming a single entity, or if they break up. This topic, widely investigated for binary drop collisions of miscible and immiscible liquid, is quite unexplored for ternary drop collisions. The current study aims to close this gap by experimentally investigating collisions between three equal-sized drops of the same liquid arranged centri-symmetrically. Three drop generators are simultaneously operated to obtain controlled ternary drop collisions. The collision outcomes are observed via photographs and compared to those of binary collisions. Similar to binary collisions, a regime map is built, showing coalescence and bouncing as well as reflexive and stretching separation. Significant differences are observed in the transitions between these regimes.

  8. Torque Reversals in Disk Accreting Pulsars

    NASA Astrophysics Data System (ADS)

    Li, Jianke; Wickramasinghe, Dayal T.

    1998-07-01

    X-ray binaries in which the accreting component is a neutron star commonly exhibit significant changes in their spin. In the system Cen X-3, a disk accreting binary system, the pulsar was observed to spin up at a rate ḟ = 8 × 10-13 Hz s-1 when averaged over the past twenty years, but significant fluctuations were observed above this mean. Recent BASTE observations have disclosed that these fluctuations are much larger than previously noted, and appeared to be a system characteristic. The change in the spin state from spin-up to spin-down or vice-versa occurs on a time scale that is much shorter than the instrument can resolve (≤1 d), but appears always to be a similar amplitude, and to occur stochastically. These observations have posed a problem for the conventional torque-mass accretion relation for accreting pulsars, because in this model the spin rate is closely related to the accretion rate, and the latter needs to be finely tuned and to change abruptly to explain the observations. Here we review recent work in this direction and present a coherent picture that explains these observations. We also draw attention to some outstanding problems for future studies.

  9. The Infrared Signature of Accretion Luminosity in Protostars

    NASA Astrophysics Data System (ADS)

    Terebey, Susan; Villarama, Ethan G.; Flores-Rivera, Lizxandra

    2016-06-01

    Mass accretion from the disk onto the star is an important mechanism by which a star increases in mass during the formation phase. If the mass accretion rate is time variable then the brightness of the star should also change with time. We use the HOCHUNK3D radiative transfer code to investigate how disk accretion rate (Mdot) affects the protostar spectral energy distribution (SED). The biggest changes in brightness occur at infrared wavelengths ranging from approximately 5 to 100 microns. The results show that the protostar luminosity doubles from 1 to 2 L⊙ when the disk accretion rate is increased to Mdot=3.0e-7 M⊙/year. We conclude that the models are a useful tool to study mass accretion rates and time variability in protostars.

  10. BONDI-HOYLE-LYTTLETON ACCRETION ONTO A PROTOPLANETARY DISK

    SciTech Connect

    Moeckel, Nickolas; Throop, Henry B.

    2009-12-10

    Young stellar systems orbiting in the potential of their birth cluster can accrete from the dense molecular interstellar medium during the period between the star's birth and the dispersal of the cluster's gas. Over this time, which may span several Myr, the amount of material accreted can rival the amount in the initial protoplanetary disk; the potential importance of this 'tail-end' accretion for planet formation was recently highlighted by Throop and Bally. While accretion onto a point mass is successfully modeled by the classical Bondi-Hoyle-Lyttleton solutions, the more complicated case of accretion onto a star-disk system defies analytic solution. In this paper, we investigate via direct hydrodynamic simulations the accretion of dense interstellar material onto a star with an associated gaseous protoplanetary disk. We discuss the changes to the structure of the accretion flow caused by the disk, and vice versa. We find that immersion in a dense accretion flow can redistribute disk material such that outer disk migrates inward, increasing the inner disk surface density and reducing the outer radius. The accretion flow also triggers the development of spiral density features, and changes to the disk inclination. The mean accretion rate onto the star remains roughly the same with and without the presence of a disk. We discuss the potential impact of this process on planet formation, including the possibility of triggered gravitational instability, inclination differences between the disk and the star, and the appearance of spiral structure in a gravitationally stable system.

  11. An unexpected drop in the magnetic field of the X-ray pulsar V0332+53 after the bright outburst occurred in 2015

    NASA Astrophysics Data System (ADS)

    Cusumano, G.; La Parola, V.; D'Aì, A.; Segreto, A.; Tagliaferri, G.; Barthelmy, S. D.; Gehrels, N.

    2016-07-01

    How the accreted mass settling on the surface of a neutron star affects the topology of the magnetic field and how the secular evolution of the binary system depends on the magnetic field change is still an open issue. We report evidence for a clear drop in the observed magnetic field in the accreting pulsar V0332+53 after undergoing a bright 3-month long X-ray outburst. We determine the field from the position of the fundamental cyclotron line in its X-ray spectrum and relate it to the luminosity. For equal levels of luminosity, in the declining phase we measure a systematically lower value of the cyclotron line energy with respect to the rising phase. This results in a drop of ˜1.7 × 1011 G of the observed field between the onset and the end of the outburst. The settling of the accreted plasma on to the polar cap seems to induce a distortion of the magnetic field lines weakening their intensity along the accretion columns. Therefore, the dissipation rate of the magnetic field could be much faster than previously estimated, unless the field is able to restore its original configuration on a time-scale comparable with the outbursts recurrence time.

  12. ACCRETION ONTO INTERMEDIATE-MASS BLACK HOLES REGULATED BY RADIATIVE FEEDBACK. I. PARAMETRIC STUDY FOR SPHERICALLY SYMMETRIC ACCRETION

    SciTech Connect

    Park, Kwang Ho; Ricotti, Massimo E-mail: ricotti@astro.umd.edu

    2011-09-20

    We study the effect of radiative feedback on accretion onto intermediate-mass black holes (IMBHs) using the hydrodynamical code ZEUS-MP with a radiative transfer algorithm. In this paper, the first of a series, we assume accretion from a uniformly dense gas with zero angular momentum and extremely low metallicity. Our one-dimensional (1D) and 2D simulations explore how X-ray and UV radiation emitted near the black hole regulates the gas supply from large scales. Both 1D and 2D simulations show similar accretion rates and periods between peaks in accretion, meaning that the hydro-instabilities that develop in 2D simulations do not affect the mean flow properties. We present a suite of simulations exploring accretion across a large parameter space, including different radiative efficiencies and radiation spectra, black hole masses, density, and temperature, T{sub {infinity}}, of the neighboring gas. In agreement with previous studies, we find regular oscillatory behavior of the accretion rate, with duty cycle {approx}6%, mean accretion rate 3% (T{sub {infinity}}/10{sup 4} K){sup 2.5} of the Bondi rate and peak accretion {approx}10 times the mean for T{sub {infinity}} ranging between 3000 K and 15, 000 K. We derive parametric formulae for the period between bursts, the mean accretion rate, and the peak luminosity of the bursts and thus provide a formulation of how feedback-regulated accretion operates. The temperature profile of the hot ionized gas is crucial in determining the accretion rate, while the period of the bursts is proportional to the mean size of the Stroemgren sphere, and we find qualitatively different modes of accretion in the high versus low density regimes. We also find that a softer radiation spectrum produces a higher mean accretion rate. However, it is still unclear what the effect of a significant time delay is between the accretion rate at our inner boundary and the output luminosity. Such a delay is expected in realistic cases with non

  13. Supernova Light Curves Powered by Fallback Accretion

    NASA Astrophysics Data System (ADS)

    Dexter, Jason; Kasen, Daniel

    2013-07-01

    Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time (gsimdays) power potentially associated with the accretion of this "fallback" material could significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as \\dot{M} \\propto t^{-5/3} at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous (gsim 1044 erg s-1) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

  14. SUPERNOVA LIGHT CURVES POWERED BY FALLBACK ACCRETION

    SciTech Connect

    Dexter, Jason; Kasen, Daniel

    2013-07-20

    Some fraction of the material ejected in a core collapse supernova explosion may remain bound to the compact remnant, and eventually turn around and fall back. We show that the late time ({approx}>days) power potentially associated with the accretion of this 'fallback' material could significantly affect the optical light curve, in some cases producing super-luminous or otherwise peculiar supernovae. We use spherically symmetric hydrodynamical models to estimate the accretion rate at late times for a range of progenitor masses and radii and explosion energies. The accretion rate onto the proto-neutron star or black hole decreases as M-dot {proportional_to}t{sup -5/3} at late times, but its normalization can be significantly enhanced at low explosion energies, in very massive stars, or if a strong reverse shock wave forms at the helium/hydrogen interface in the progenitor. If the resulting super-Eddington accretion drives an outflow which thermalizes in the outgoing ejecta, the supernova debris will be re-energized at a time when photons can diffuse out efficiently. The resulting light curves are different and more diverse than previous fallback supernova models which ignored the input of accretion power and produced short-lived, dim transients. The possible outcomes when fallback accretion power is significant include super-luminous ({approx}> 10{sup 44} erg s{sup -1}) Type II events of both short and long durations, as well as luminous Type I events from compact stars that may have experienced significant mass loss. Accretion power may unbind the remaining infalling material, causing a sudden decrease in the brightness of some long duration Type II events. This scenario may be relevant for explaining some of the recently discovered classes of peculiar and rare supernovae.

  15. Thermal history of chondrites - Hot accretion vs. metamorphic reheating

    NASA Technical Reports Server (NTRS)

    Haack, Henning; Taylor, G. J.; Scott, E. R. D.; Keil, Klaus

    1992-01-01

    The thermal evolution of chondrules is investigated for the stages including primary heating through accretion to parent-body processing to determine whether the chondrules could be hot during accretion. Theoretical attention is given to whether chondrites of different petrologic types could have originated by means of hot accretion or metamorphic reheating. Data are presented from cooling-rate experiments and from calculations of heat retention required for the hot-accretion scenario. The accretion of chondrules hotter than 800 C is shown to be inconsistent with constraints on chondrule thermal evolution, in particular the slow cooling environment of chondrules vs the apparent cooling of chondrites in cold environments. It is argued that petrologic chondrites are formed by cold accretion and subsequently by metamorphic heating.

  16. Multi-dimensional structure of accreting young stars

    NASA Astrophysics Data System (ADS)

    Geroux, C.; Baraffe, I.; Viallet, M.; Goffrey, T.; Pratt, J.; Constantino, T.; Folini, D.; Popov, M. V.; Walder, R.

    2016-04-01

    This work is the first attempt to describe the multi-dimensional structure of accreting young stars based on fully compressible time implicit multi-dimensional hydrodynamics simulations. One major motivation is to analyse the validity of accretion treatment used in previous 1D stellar evolution studies. We analyse the effect of accretion on the structure of a realistic stellar model of the young Sun. Our work is inspired by the numerical work of Kley & Lin (1996, ApJ, 461, 933) devoted to the structure of the boundary layer in accretion disks, which provides the outer boundary conditions for our simulations. We analyse the redistribution of accreted material with a range of values of specific entropy relative to the bulk specific entropy of the material in the accreting object's convective envelope. Low specific entropy accreted material characterises the so-called cold accretion process, whereas high specific entropy is relevant to hot accretion. A primary goal is to understand whether and how accreted energy deposited onto a stellar surface is redistributed in the interior. This study focusses on the high accretion rates characteristic of FU Ori systems. We find that the highest entropy cases produce a distinctive behaviour in the mass redistribution, rms velocities, and enthalpy flux in the convective envelope. This change in behaviour is characterised by the formation of a hot layer on the surface of the accreting object, which tends to suppress convection in the envelope. We analyse the long-term effect of such a hot buffer zone on the structure and evolution of the accreting object with 1D stellar evolution calculations. We study the relevance of the assumption of redistribution of accreted energy into the stellar interior used in the literature. We compare results obtained with the latter treatment and those obtained with a more physical accretion boundary condition based on the formation of a hot surface layer suggested by present multi

  17. Accretion disks in luminous young stellar objects

    NASA Astrophysics Data System (ADS)

    Beltrán, M. T.; de Wit, W. J.

    2016-01-01

    An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and optically revealed phases. The properties were derived from spatially resolved observations and, therefore, predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates.

  18. Planetary migration, accretion, and atmospheres

    NASA Astrophysics Data System (ADS)

    Dobbs-Dixon, Ian M.

    This dissertation explores three distinct projects in the field of planetary formation and evolution: type I migration, cessation of mass accretion, and the atmospheric dynamics of hot Jupiters. All three of these projects touch on outstanding or unresolved issues in the field. Each attempts to unify analytic and numerical approaches in order to physically motivate solutions while simultaneously probing areas currently inaccessible to purely analytic approaches. The first section, type I migration, explores the outstanding problem of the rapid inward migration of low mass planets embedded in protoplanetary disks. Analytic estimates of migration predict characteristic timescales that are much shorter then either observed disk lifetimes or theoretical core-accretion formation timescales. If migration is actually as efficient as these analytic estimates predict, planet formation across the observed range of masses and semimajor axis' is difficult. Here I introduce several new formalisms to both allow the disk to adiabatically adjust to the presence of a planet and include the effect of axisymmetric disk self-gravity. I find that these modifications increase migration timescales by approximately 4 times. In addition to these numerical improvements, I present simulations of migration in lower sound-speed regions of the disk on the grounds that self shadowing within the disk could yield substantially cooler gas temperatures then those derived by most irradiated disk models. In such regions the planetary perturbation excites a secondary instability, leading to the formation of vortices. These vortices cause a substantial reduction in the net torque, increasing migration timescales by up to approximately 200 times the analytically predicted rate. The second section addresses the mechanism for shutting off accretion onto giant planets. According to the conventional sequential accretion scenario, giant planets acquire a majority of their gas in a runaway phase. Conventional

  19. Evolution and precession of accretion disk in tidal disruption events

    NASA Astrophysics Data System (ADS)

    Shen, R.-F.; Matzner, C. D.

    2012-12-01

    In a supermassive black hole (BH) tidal disruption event (TDE), the tidally disrupted star feeds the BH via an accretion disk. Most often it is assumed that the accretion rate history, hence the emission light curve, tracks the rate at which new debris mass falls back onto the disk, notably the t-5/3 power law. But this is not the case when the disk evolution due to viscous spreading - the driving force for accretion - is carefully considered. We construct a simple analytical model that comprehensively describes the accretion rate history across 4 different phases of the disk evolution, in the presence of mass fallback and disk wind loss. Accretion rate evolves differently in those phases which are governed by how the disk heat energy is carried away, early on by advection and later by radiation. The accretion rate can decline as steeply as t-5/3 only if copious disk wind loss is present during the early advection-cooled phase. Later, the accretion rate history is t-8/7 or shallower. These have great implications on the TDE flare light curve. A TDE accretion disk is most likely misaligned with the equatorial plane of the spinning BH. Moreover, in the TDE the accretion rate is super- or near-Eddington thus the disk is geometrically thick, for which case the BH's frame dragging effect may cause the disk precess as a solid body, which may manifest itself as quasi-periodic signal in the TDE light curve. Our disk evolution model predicts the disk precession period increases with time, typically as ∝ t. The results are applied to the recently jetted TDE flare Swift transient J1644 + 57 which shows numerous, quasi-periodic dips in its long-term X-ray light curve. As the current TDE sample increases, the identification of the disk precession signature provides a unique way of measuring BH spin and studying BH accretion physics.

  20. SPIN EVOLUTION OF ACCRETING YOUNG STARS. II. EFFECT OF ACCRETION-POWERED STELLAR WINDS

    SciTech Connect

    Matt, Sean P.; Greene, Thomas P.; Pudritz, Ralph E. E-mail: thomas.p.greene@nasa.gov E-mail: pudritz@physics.mcmaster.ca

    2012-01-20

    We present a model for the rotational evolution of a young, solar-mass star interacting magnetically with an accretion disk. As in a previous paper (Paper I), the model includes changes in the star's mass and radius as it descends the Hayashi track, a decreasing accretion rate, and a prescription for the angular momentum transfer between the star and disk. Paper I concluded that, for the relatively strong magnetic coupling expected in real systems, additional processes are necessary to explain the existence of slowly rotating pre-main-sequence stars. In the present paper, we extend the stellar spin model to include the effect of a spin-down torque that arises from an accretion-powered stellar wind (APSW). For a range of magnetic field strengths, accretion rates, initial spin rates, and mass outflow rates, the modeled stars exhibit rotation periods within the range of 1-10 days in the age range of 1-3 Myr. This range coincides with the bulk of the observed rotation periods, with the slow rotators corresponding to stars with the lowest accretion rates, strongest magnetic fields, and/or highest stellar wind mass outflow rates. We also make a direct, quantitative comparison between the APSW scenario and the two types of disk-locking models (namely, the X-wind and Ghosh and Lamb type models) and identify some remaining theoretical issues for understanding young star spins.

  1. Chaotic cold accretion on to black holes

    NASA Astrophysics Data System (ADS)

    Gaspari, M.; Ruszkowski, M.; Oh, S. Peng

    2013-07-01

    Bondi theory is often assumed to adequately describe the mode of accretion in astrophysical environments. However, the Bondi flow must be adiabatic, spherically symmetric, steady, unperturbed, with constant boundary conditions. Using 3D adaptive mesh refinement simulations, linking the 50 kpc to the sub-parsec (sub-pc) scales over the course of 40 Myr, we systematically relax the classic assumptions in a typical galaxy hosting a supermassive black hole. In the more realistic scenario, where the hot gas is cooling, while heated and stirred on large scales, the accretion rate is boosted up to two orders of magnitude compared with the Bondi prediction. The cause is the non-linear growth of thermal instabilities, leading to the condensation of cold clouds and filaments when tcool/tff ≲ 10. The clouds decouple from the hot gas, `raining' on to the centre. Subsonic turbulence of just over 100 km s-1 (M > 0.2) induces the formation of thermal instabilities, even in the absence of heating, while in the transonic regime turbulent dissipation inhibits their growth (tturb/tcool ≲ 1). When heating restores global thermodynamic balance, the formation of the multiphase medium is violent, and the mode of accretion is fully cold and chaotic. The recurrent collisions and tidal forces between clouds, filaments and the central clumpy torus promote angular momentum cancellation, hence boosting accretion. On sub-pc scales the clouds are channelled to the very centre via a funnel. In this study, we do not inject a fixed initial angular momentum, though vorticity is later seeded by turbulence. A good approximation to the accretion rate is the cooling rate, which can be used as subgrid model, physically reproducing the boost factor of 100 required by cosmological simulations, while accounting for the frequent fluctuations. Since our modelling is fairly general (turbulence/heating due to AGN feedback, galaxy motions, mergers, stellar evolution), chaotic cold accretion may be common in

  2. Ringed Accretion Disks: Instabilities

    NASA Astrophysics Data System (ADS)

    Pugliese, D.; Stuchlík, Z.

    2016-04-01

    We analyze the possibility that several instability points may be formed, due to the Paczyński mechanism of violation of mechanical equilibrium, in the orbiting matter around a supermassive Kerr black hole. We consider a recently proposed model of a ringed accretion disk, made up by several tori (rings) that can be corotating or counter-rotating relative to the Kerr attractor due to the history of the accretion process. Each torus is governed by the general relativistic hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. We prove that the number of the instability points is generally limited and depends on the dimensionless spin of the rotating attractor.

  3. Accretion dynamics of EX Lupi in quiescence. The star, the spot, and the accretion column

    NASA Astrophysics Data System (ADS)

    Sicilia-Aguilar, Aurora; Fang, Min; Roccatagliata, Veronica; Collier Cameron, Andrew; Kóspál, Ágnes; Henning, Thomas; Ábrahám, Peter; Sipos, Nikoletta

    2015-08-01

    Context. EX Lupi is a young, accreting M0 star and the prototype of EXor variable stars. Its spectrum is very rich in emission lines, including many metallic lines with narrow and broad components. The presence of a close companion has also been proposed, based on radial velocity signatures. Aims: We use the metallic emission lines to study the accretion structures and to test the companion hypothesis. Methods: We analyse 54 spectra obtained during five years of quiescence time. We study the line profile variability and the radial velocity of the narrow and broad metallic emission lines. We use the velocity signatures of different species with various excitation conditions and their time dependency to track the dynamics associated with accretion. Results: We observe periodic velocity variations in the broad and the narrow line components, consistent with rotational modulation. The modulation is stronger for lines with higher excitation potentials (e.g. He II), which are likely produced in a confined area very close to the accretion shock. Conclusions: We propose that the narrow line components are produced in the post-shock region, while the broad components originate in the more extended, pre-shock material in the accretion column. All the emission lines suffer velocity modulation due to the rotation of the star. The broad components are responsible for the line-dependent veiling observed in EX Lupi. We demonstrate that a rotationally modulated line-dependent veiling can explain the radial velocity signature of the photospheric absorption lines, making the close-in companion hypothesis unnecessary. The accretion structure is locked to the star and very stable during the five years of observations. Not all stars with similar spectral types and accretion rates show the same metallic emission lines, which could be related to differences in temperature and density in their accretion structure(s). The contamination of photospheric signatures by accretion

  4. University Drop-Out: An Italian Experience

    ERIC Educational Resources Information Center

    Belloc, Filippo; Maruotti, Antonello; Petrella, Lea

    2010-01-01

    University students' drop-out is a crucial issue for the universities' efficiency evaluation and funding. In this paper, we analyze the drop-out rate of the Economics and Business faculty of Sapienza University of Rome. We use administrative data on 9,725 undergraduates students enrolled in three-years bachelor programs from 2001 to 2007 and…

  5. Micro-explosion of compound drops

    NASA Astrophysics Data System (ADS)

    Chen, Chun-Kuei; Lin, Ta-Hui

    2014-08-01

    and inserted manners. The drop ignition delay time increased with increasing water content. The average burning rate of alkane-water drops decreased with increasing water content. In the burning process, hexadecane-water drops exhibited flash vaporization or flame extinction. Heterogeneous explosion was occasionally observed in drops with trapped air bubbles. The air bubbles were assumed to be the nucleation points of the heterogeneous explosions. Chen and Lin[11 studied the characteristics of water-in-dodecane compound drop with different water content, diameter of drop and environmental oxygen concentration. The vaporization rate increased with increasing environmental oxygen concentration. The compound drops micro-exploded during the burning process in a random way. The number of micro-explosions was majorly influenced by drop diameter, followed by environmental oxygen concentration. Water content had a weaker effect on micro-explosion. As available literature and research results of compound drop burning are scarce, their combustion and micro-explosion behaviors are still poorly understood. In this regard, we changed the drop nature as compound drops to study their combustion characteristics and micro-explosion phenomena.

  6. Disk Accretion and the Stellar Birthline

    NASA Astrophysics Data System (ADS)

    Hartmann, Lee; Cassen, Patrick; Kenyon, Scott J.

    1997-02-01

    We present a simplified analysis of some effects of disk accretion on the early evolution of fully convective, low-mass pre-main-sequence stars. Our analysis builds on the previous seminal work of Stahler, but it differs in that the accretion of material occurs over a small area of the stellar surface, such as through a disk or magnetospheric accretion column, so that most of the stellar photosphere is free to radiate to space. This boundary condition is similar to the limiting case considered by Palla & Stahler for intermediate-mass stars. We argue that for a wide variety of disk mass accretion rates, material will be added to the star with relatively small amounts of thermal energy. Protostellar evolution calculated assuming this ``low-temperature'' limit of accretion generally follows the results of Stahler because of the thermostatic nature of deuterium fusion, which prevents protostars from contracting below a ``birthline'' in the H-R diagram. Our calculated protostellar radii tend to fall below Stahler's at higher masses; the additional energy loss from the stellar photosphere in the case of disk accretion tends to make the protostar contract. The low-temperature disk accretion evolutionary tracks never fall below the deuterium-fusion birthline until the internal deuterium is depleted, but protostellar tracks can lie above the birthline in the H-R diagram if the initial radius of the protostellar core is large enough or if rapid disk accretion (such as might occur during FU Ori outbursts) adds significant amounts of thermal energy to the star. These possibilities cannot be ruled out by either theoretical arguments or observational constraints at present, so that individual protostars might evolve along a multiplicity of birthlines with a modest range of luminosity at a given mass. Our results indicate that there are large uncertainties in assigning ages for the youngest stars from H-R diagram positions, given the uncertainty in birthline positions. Our

  7. Drag on Sessile Drops

    NASA Astrophysics Data System (ADS)

    Milne, Andrew J. B.; Fleck, Brian; Nobes, David; Sen, Debjyoti; Amirfazli, Alidad; University of Alberta Mechanical Engineering Collaboration

    2013-11-01

    We present the first ever direct measurements of the coefficient of drag on sessile drops at Reynolds numbers from the creeping flow regime up to the point of incipient motion, made using a newly developed floating element differential drag sensor. Surfaces of different wettabilities (PMMA, Teflon, and a superhydrophobic surface (SHS)), wet by water, hexadecane, and various silicone oils, are used to study the effects of drop shape, and fluid properties on drag. The relation between drag coefficient and Reynolds number (scaled by drop height) varies slightly with liquid-solid system and drop volume with results suggesting the drop experiences increased drag compared to similar shaped solid bodies due to drop oscillation influencing the otherwise laminar flow. Drops adopting more spherical shapes are seen to experience the greatest force at any given airspeed. This indicates that the relative exposed areas of drops is an important consideration in terms of force, with implications for the shedding of drops in applications such as airfoil icing and fuel cell flooding. The measurement technique used in this work can be adapted to measure drag force on other deformable, lightly adhered objects such as dust, sand, snow, vesicles, foams, and biofilms. The authours acknowledge NSERC, Alberta Innovates Technology Futures, and the Killam Trusts.

  8. Sedimentation, accretion, and subsidence in marshes of Barataria Basin, Louisiana

    SciTech Connect

    Hatton, R.S.; DeLaune, R.D.; Patrick, W.H. Jr.

    1983-05-01

    Vertical accretion and sediment accumulation rates were determined from the distribution of /sup 137/Cs in cores collected from fresh water, intermediate, brackish, and salt marshes in the Barataria Basin, Louisiana. Vertical accretion rates vary from about 1.3 cm.yr/sup -1/ in levee areas to 0.7 in backmarshes. Mineral sediment content of the marsh soil profile decreased with distance from the coast. Except in natural levee areas, marsh accretion rates are less than subsidence measured by water level data, however this alone cannot account for observed land-loss patterns in the basin area.

  9. Episodic Accretion among the Orion Protostars

    NASA Astrophysics Data System (ADS)

    Fischer, William J.; Safron, Emily; Megeath, S. Thomas

    2016-06-01

    Episodic accretion, where a young stellar object undergoes stochastic spikes in its disk-to-star accretion rate one or more times over its formation period, may be a crucial process in the formation of low-mass stars. These spikes result in a factor of 10 to 100 increase in the source luminosity over the course of several months that may persist for years. Six years after the Spitzer survey of the Orion molecular clouds, the WISE telescope mapped Orion with similar wavelength coverage. Thus, the two surveys can be used to explore the mid-infrared variability of young stars on this timescale, which is suitable for discovering episodic accretion events. Out of 319 Orion protostars that were targets of the Herschel Orion Protostar Survey, we identified two examples of episodic accretion with this method. One of them, HOPS 223, was previously known. The other, HOPS 383, is the first known example of episodic accretion in a Class 0 protostar (age < 0.2 Myr). With these and one other outburst that began early in the Spitzer mission, we estimate that the most likely interval between protostellar outbursts is 740 years, with a 90% confidence interval of 470 to 6200 years. These outbursts are weaker than the optically revealed FU Ori events. We will update the mid-infrared light curves of HOPS 223 and HOPS 383 with recent data from FORCAST aboard SOFIA; HOPS 223 shows signs of fading.

  10. The accretion column of AE Aqr

    NASA Astrophysics Data System (ADS)

    Rodrigues, Claudia; Costa, D. Joaquim; Luna, Gerardo; Lima, Isabel J.; Silva, Karleyne M. G.; De Araujo, Jose Carlos N.; Coelho, Jaziel

    2016-07-01

    AE Aqr is a magnetic cataclysmic variable, whose white dwarf rotates at the very fast rate of 33 s modulating the flux from high-energies to optical wavelengths. There are many studies of the origin of its emission, which consider emission from a rotating magnetic field or from an accretion column. Recently, MAGIC observations have discarded AE Aqr emission in very high energy gamma-rays discarding non-thermal emission. Furthermore, soft and hard X-ray data from Swift and NuSTAR were fitted using thermal models. Here we present the modelling of AE Aqr X-ray spectra and light curve considering the emission of a magnetic accretion column using the Cyclops code. The model takes into consideration the 3D geometry of the system, allowing to properly represent the white-dwarf auto eclipse, the pre-shock column absorption, and the varying density and temperature of a tall accretion column.

  11. Exploring the disk accretion in DI Cep

    NASA Astrophysics Data System (ADS)

    Parihar, Padmakar Singh; Shantikumar, N. S.

    The low mass young stellar objects of class-II, popularly known as classical T Tauri stars (CTTS) supposed to be surrounded by thick flared disk and accretes disk material through strong stellar dipolar magnetic field. The disk accretion rate and its variation with time is poorly know. DI Cep is an interesting object, found to have unexpected hump around 5300 Å in the continuum excess emission spectrum, which cannot be explained by current models of YSOs. Over the last six years this object is being spectroscopically as well as photometrically monitored using HCT. The data have been analyzed and modeled using a simple modeling technique developed by us. In this paper, we report for the first time our results related to the disk accretion phenomena in DI Cep.

  12. Radiative and gas cooling of falling molten drops

    NASA Technical Reports Server (NTRS)

    Robinson, M. B.

    1978-01-01

    The supercooling rate and solidification time for molten drops of niobium, copper, and lead are calculated. Calculations for both radiation and helium gas cooling are presented in order to estimate the influence that the presence of helium gas would have upon the cooling rate of falling drops in the Marshall Space Flight Center space processing drop tube.

  13. Youth Crime Drop. Report.

    ERIC Educational Resources Information Center

    Butts, Jeffrey A.

    This report examines the recent drop in violent crime in the United States, discussing how much of the decrease seen between 1995-99 is attributable to juveniles (under age 18 years) and older youth (18-24 years). Analysis of current FBI arrest data indicates that not only did America's violent crime drop continue through 1999, but falling youth…

  14. Axisymmetric Liquid Hanging Drops

    ERIC Educational Resources Information Center

    Meister, Erich C.; Latychevskaia, Tatiana Yu

    2006-01-01

    The geometry of drops hanging on a circular capillary can be determined by numerically solving a dimensionless differential equation that is independent on any material properties, which enables one to follow the change of the height, surface area, and contact angle of drops hanging on a particular capillary. The results show that the application…

  15. Drop Tower Physics

    ERIC Educational Resources Information Center

    Dittrich, William A.

    2014-01-01

    The drop towers of yesteryear were used to make lead shot for muskets, as described in "The Physics Teacher" in April 2012. However, modern drop towers are essentially elevators designed so that the cable can "break" on demand, creating an environment with microgravity for a short period of time, currently up to nine seconds at…

  16. Bondi-Hoyle accretion in an isothermal magnetized plasma

    SciTech Connect

    Lee, Aaron T.; McKee, Christopher F.; Klein, Richard I.; Cunningham, Andrew J.

    2014-03-01

    In regions of star formation, protostars and newborn stars will accrete mass from their natal clouds. These clouds are threaded by magnetic fields with a strength characterized by the plasma β—the ratio of thermal and magnetic pressures. Observations show that molecular clouds have β ≲ 1, so magnetic fields have the potential to play a significant role in the accretion process. We have carried out a numerical study of the effect of large-scale magnetic fields on the rate of accretion onto a uniformly moving point particle from a uniform, non-self-gravitating, isothermal gas. We consider gas moving with sonic Mach numbers of up to M≈45; magnetic fields that are either parallel, perpendicular, or oriented 45° to the flow; and β as low as 0.01. Our simulations utilize adaptive mesh refinement in order to obtain high spatial resolution where it is needed; this also allows the boundaries to be far from the accreting object to avoid unphysical effects arising from boundary conditions. Additionally, we show that our results are independent of our exact prescription for accreting mass in the sink particle. We give simple expressions for the steady-state accretion rate as a function of β and M for the parallel and perpendicular orientations. Using typical molecular cloud values of M∼5 and β ∼ 0.04 from the literature, our fits suggest that a 0.4 M {sub ☉} star accretes ∼4 × 10{sup –9} M {sub ☉} yr{sup –1}, almost a factor of two less than accretion rates predicted by hydrodynamic models. This disparity can grow to orders of magnitude for stronger fields and lower Mach numbers. We also discuss the applicability of these accretion rates versus accretion rates expected from gravitational collapse, and under what conditions a steady state is possible. The reduction in the accretion rate in a magnetized medium leads to an increase in the time required to form stars in competitive accretion models, making such models less efficient than predicted by

  17. Nanofluid Drop Evaporation: Experiment, Theory, and Modeling

    NASA Astrophysics Data System (ADS)

    Gerken, William James

    Nanofluids, stable colloidal suspensions of nanoparticles in a base fluid, have potential applications in the heat transfer, combustion and propulsion, manufacturing, and medical fields. Experiments were conducted to determine the evaporation rate of room temperature, millimeter-sized pendant drops of ethanol laden with varying amounts (0-3% by weight) of 40-60 nm aluminum nanoparticles (nAl). Time-resolved high-resolution drop images were collected for the determination of early-time evaporation rate (D2/D 02 > 0.75), shown to exhibit D-square law behavior, and surface tension. Results show an asymptotic decrease in pendant drop evaporation rate with increasing nAl loading. The evaporation rate decreases by approximately 15% at around 1% to 3% nAl loading relative to the evaporation rate of pure ethanol. Surface tension was observed to be unaffected by nAl loading up to 3% by weight. A model was developed to describe the evaporation of the nanofluid pendant drops based on D-square law analysis for the gas domain and a description of the reduction in liquid fraction available for evaporation due to nanoparticle agglomerate packing near the evaporating drop surface. Model predictions are in relatively good agreement with experiment, within a few percent of measured nanofluid pendant drop evaporation rate. The evaporation of pinned nanofluid sessile drops was also considered via modeling. It was found that the same mechanism for nanofluid evaporation rate reduction used to explain pendant drops could be used for sessile drops. That mechanism is a reduction in evaporation rate due to a reduction in available ethanol for evaporation at the drop surface caused by the packing of nanoparticle agglomerates near the drop surface. Comparisons of the present modeling predictions with sessile drop evaporation rate measurements reported for nAl/ethanol nanofluids by Sefiane and Bennacer [11] are in fairly good agreement. Portions of this abstract previously appeared as: W. J

  18. Accretion Processes in the Nucleus of M31

    NASA Astrophysics Data System (ADS)

    Liu, Siming; Melia, Fulvio

    2001-04-01

    The hypothesized supermassive black hole in the nucleus of M31 (M31*) has many features in common with Sgr A* at the Galactic center, yet they differ in several significant and important ways. Although M31* is probably 10 times heavier, its radio luminosity at 3.6 cm is only one-third that of Sgr A*. At the same time, M31* is apparently thousands of times more luminous in X-rays than its Galactic center counterpart. Thus, a comparative study of these objects can be valuable in helping us to understand the underlying physical basis for their activity. We show here that the accretion model being developed for Sgr A* comprises two branches of solutions, distinguished by the relative importance of cooling compared to compressional heating at the radius rC where the ambient gas is captured by the black hole. For typical conditions in the interstellar medium, the initial temperature [T(rC)~106-107 K] sits on the unstable branch of the cooling function. Depending on the actual value of T(rC) and the accretion rate, the plasma settles onto either a hot branch (attaining a temperature as high as 1010 K or so at small radii) or a cold branch, in which T drops to ~104 K. Sgr A* is presumably a ``hot'' black hole. We show here that Very Large Array, Hubble Space Telescope, and Chandra observations of M31* reveal it to be a member of the ``cold'' black hole family. We discuss several predicted features in the spectrum of M31* that may be testable by future multiwavelength observations, including the presence of a prominent UV spike (from hydrogen line emission) that would be absent on the hot branch.

  19. Accretion of southern Alaska

    USGS Publications Warehouse

    Hillhouse, J.W.

    1987-01-01

    Paleomagnetic data from southern Alaska indicate that the Wrangellia and Peninsular terranes collided with central Alaska probably by 65 Ma ago and certainly no later than 55 Ma ago. The accretion of these terranes to the mainland was followed by the arrival of the Ghost Rocks volcanic assemblage at the southern margin of Kodiak Island. Poleward movement of these terranes can be explained by rapid motion of the Kula oceanic plate, mainly from 85 to 43 Ma ago, according to recent reconstructions derived from the hot-spot reference frame. After accretion, much of southwestern Alaska underwent a counterclockwise rotation of about 50 ?? as indicated by paleomagnetic poles from volcanic rocks of Late Cretaceous and Early Tertiary age. Compression between North America and Asia during opening of the North Atlantic (68-44 Ma ago) may account for the rotation. ?? 1987.

  20. Hoyle-Lyttleton Accretion from a Planar Wind

    NASA Astrophysics Data System (ADS)

    Raymer, Eric

    2014-01-01

    Two-dimensional hydrodynamic simulations of Hoyle-Lyttleton accretion have informed predictions about the evolution of wind-driven accretion systems for over two decades. These simulations frequently exhibit dramatic nonlinear behavior such as the flip-flop instability and the formation of transient accretion disks. During disk accretion, the mass accretion rate is suppressed and angular momentum accretion occurs at quasi-Keplerian rates. These results have been used to interpret neutron star accretion from the equatorially enhanced wind of a Be star in Be/X-ray Binaries. We employ large-scale hydrodynamic simulations to investigate whether the flip-flop instability is possible in three dimensions or is simply a consequence of the restrictions on a 2D flow. We do not observe the flip-flop instability in 3D for any values of the wind scale height or density. Moreover, the angular momentum vector of the accreting gas is typically found to be in the plane of the disk wind rather than perpendicular to it as one might expect based on the results of 2D planar simulations. We measure large-scale asymmetries about the plane of the disk wind that arise due to rotational flow near the accretor. Gas is driven above and below the plane, where it interacts with the bow shock and results in a time-varying shock structure. Winds with scale heights of 0.25 Ra enter locked rotation modes that remain stable for the duration of our computational runs. During this phase, the mass accretion rate is suppressed by up to two orders of magnitude below the analytical prediction and angular momentum accretion occurs at sub-Keplerian values.

  1. Accretion disk electrodynamics

    NASA Technical Reports Server (NTRS)

    Coroniti, F. V.

    1985-01-01

    Accretion disk electrodynamic phenomena are separable into two classes: (1) disks and coronas with turbulent magnetic fields; (2) disks and black holes which are connected to a large-scale external magnetic field. Turbulent fields may originate in an alpha-omega dynamo, provide anomalous viscous transport, and sustain an active corona by magnetic buoyancy. The large-scale field can extract energy and angular momentum from the disk and black hole, and be dynamically configured into a collimated relativistic jet.

  2. Matter accreting neutron stars

    NASA Technical Reports Server (NTRS)

    Meszaros, P.

    1981-01-01

    Some of the fundamental neutron star parameters, such as the mass and the magnetic field strength, were experimentally determined in accreting neutron star systems. Some of the relevant data and the models used to derive useful information from them, are reviewed concentrating mainly on X-ray pulsars. The latest advances in our understanding of the radiation mechanisms and the transfer in the strongly magnetized polar cap regions are discussed.

  3. Accretion disk coronae

    NASA Technical Reports Server (NTRS)

    White, N. E.; Holt, S. S.

    1981-01-01

    Recent observations of partial X-ray eclipses from 4U1822-37 have shown that the central X-ray source in this system is diffused by a large Compton-thick accretion disk corona (ADC). Another binary, 4U2129-47, also displays a partial eclipse and contains an ADC. The possible origin of an ADC is discussed and a simple hydrostatic evaporated ADC model is developed which, when applied to 4U1822-37, 4U2129+47 and Cyg X-3, can explain their temporal and spectral properties. The quasi-sinusoidal modulation of all three sources can be reconciled with the partial occultation of the ADC by a bulge at the edge of the accretion disk which is caused by the inflowing material. The height of this bulge is an order of magnitude larger than the hydrostatic disk height and is the result of turbulence in the outer region of the disk. The spectral properties of all three sources can be understood in terms of Compton scattering of the original source spectrum by the ADC. Spectral variations with epoch in Cyg X-3 are probably caused by changes in the optical depth of the corona. A consequence of our model is that any accreting neutron star X-ray source in a semi-detached binary system which is close to its Eddington limit most likely contains an optically thick ADC.

  4. Drop Tower Physics

    NASA Astrophysics Data System (ADS)

    Dittrich, William A. Toby

    2014-10-01

    The drop towers of yesteryear were used to make lead shot for muskets, as described in The Physics Teacher1 in April 2012. However, modern drop towers are essentially elevators designed so that the cable can "break" on demand, creating an environment with microgravity for a short period of time, currently up to nine seconds at the drop tower in Bremen, Germany. Using these drop towers, one can briefly investigate various physical systems operating in this near zero-g environment. The resulting "Drop Tower Physics" is a new and exciting way to challenge students with a physical example that requires solid knowledge of many basic physics principles, and it forces them to practice the scientific method. The question is, "How would a simple toy, like a pendulum, behave when it is suddenly exposed to a zero-g environment?" The student must then postulate a particular behavior, test the hypothesis against physical principles, and if the hypothesis conforms to these chosen physical laws, the student can formulate a final conclusion. At that point having access to a drop tower is very convenient, in that the student can then experimentally test his or her conclusion. The purpose of this discussion is to explain the response of these physical systems ("toys") when the transition is made to a zero-g environment and to provide video demonstrations of this behavior to support in-class discussions of Drop Tower Physics.

  5. Early Pan-African evolution of the basement around Elat, Israel, and the Sinai Peninsula revealed by single-zircon evaporation dating, and implications for crustal accretion rates

    SciTech Connect

    Kroener, A. ); Eyal, M.; Eyal, Y. )

    1990-06-01

    The authors report {sup 207}Pb/{sup 206}Pb single-zircon evaporation ages for early Pan-African rocks from southern Israel and the northeastern Sinai Peninsula, the northernmost extension of the Arabian-Nubian shield. The oldest rocks are metamorphic schists of presumed island-arc derivation; detrital zircons date the source terrain at ca. 800-820 Ma. A major phase of tonalite-trondhjemite plutonism occurred at ca. 760-780 Ma; more evolved granitic rocks were emplaced at about 745 Ma. A metagabbro-metadiorite complex reflects the youngest igneous phase at ca. 640 Ma. We find no evidence for pre-Pan-African crust, and our data document important crust-forming events that correlate with similar episodes elsewhere in the shield. The widespread presence of early Pan-African juvenile rocks (i.e., ca. 760-850 Ma) in many parts of the Arabian-Nubian shield makes this period the most important in the magmatic history of the shield and supports earlier suggestions for unusually high crust-production rates.

  6. Drop impact of suspensions

    NASA Astrophysics Data System (ADS)

    Thoraval, M.-J.; Boyer, F.; Sandoval Nava, E.; Dijksman, J. F.; Lohse, D.; Snoeijer, J. H.

    2014-11-01

    Drop impact studies have a wide range of applications, many of which involve complex fluids. We study here the liquid drop impact of a silver nano-particles dispersion on a solid glass surface. This dispersion is used for inkjet printing of functional electronic materials. When the impact velocity increases, the drop classically splashes into smaller droplets. However, it surprisingly stops splashing above a critical impact velocity. We combine high-speed imaging experiments with different characterizations of the dispersion to understand this transition to non-splashing.

  7. Rayleigh-Taylor-Unstable Accretion and Variability of Magnetized Stars: Global Three-Dimensional Simulations

    SciTech Connect

    Kulkarni, A. K.; Romanova, M. M.

    2008-10-29

    We present results of 3D simulations of MHD instabilities at the accretion disk-magnetosphere boundary. The instability is Rayleigh-Taylor, and develops for a fairly broad range of accretion rates and stellar rotation rates and magnetic fields. It produces tall, thin tongues of plasma that penetrate the magnetosphere in the equatorial plane. The shape and number of the tongues changes with time on the inner-disk dynamical timescale. In contrast with funnel flows, which deposit matter mainly in the polar region, the tongues deposit matter much closer to the stellar equator. The instability appears for relatively small misalignment angles, {theta} < or approx. 30 deg., between the star's rotation and magnetic axes, and is associated with higher accretion rates. The hot spots and light curves during accretion through instability are generally much more chaotic than during stable accretion. The unstable state of accretion has possible implications for quasi-periodic oscillations and intermittent pulsations from accreting systems.

  8. Hubble Space Telescope Observations of Accretion-Induced Star Formation in the Tadpole Galaxy Kiso 5639

    NASA Astrophysics Data System (ADS)

    Elmegreen, Debra Meloy; Elmegreen, Bruce G.; Sánchez Almeida, Jorge; Muñoz-Tuñón, Casiana; Mendez-Abreu, Jairo; Gallagher, John S.; Rafelski, Marc; Filho, Mercedes; Ceverino, Daniel

    2016-07-01

    The tadpole galaxy Kiso 5639 has a slowly rotating disk with a drop in metallicity at its star-forming head, suggesting that star formation was triggered by the accretion of metal-poor gas. We present multi-wavelength Hubble Space Telescope Wide Field Camera 3 images of UV through I band plus Hα to search for peripheral emission and determine the properties of various regions. The head has a mass in young stars of ∼ {10}6 {M}ȯ and an ionization rate of 6.4× {10}51 s‑1, equivalent to ∼2100 O9-type stars. There are four older star-forming regions in the tail, and an underlying disk with a photometric age of ∼1 Gyr. The mass distribution function of 61 star clusters is a power law with a slope of ‑1.73 ± 0.51. Fourteen young clusters in the head are more massive than {10}4 {M}ȯ , suggesting a clustering fraction of 30%–45%. Wispy filaments of Hα emission and young stars extend away from the galaxy. Shells and holes in the head H ii region could be from winds and supernovae. Gravity from the disk should limit the expansion of the H ii region, although hot gas might escape through the holes. The star formation surface density determined from Hα in the head is compared to that expected from likely pre-existing and accreted gas. Unless the surface density of the accreted gas is a factor of ∼3 or more larger than what was in the galaxy before, the star formation rate has to exceed the usual Kennicutt–Schmidt rate by a factor of ≥slant 5.

  9. Hubble Space Telescope Observations of Accretion-Induced Star Formation in the Tadpole Galaxy Kiso 5639

    NASA Astrophysics Data System (ADS)

    Elmegreen, Debra Meloy; Elmegreen, Bruce G.; Sánchez Almeida, Jorge; Muñoz-Tuñón, Casiana; Mendez-Abreu, Jairo; Gallagher, John S.; Rafelski, Marc; Filho, Mercedes; Ceverino, Daniel

    2016-07-01

    The tadpole galaxy Kiso 5639 has a slowly rotating disk with a drop in metallicity at its star-forming head, suggesting that star formation was triggered by the accretion of metal-poor gas. We present multi-wavelength Hubble Space Telescope Wide Field Camera 3 images of UV through I band plus Hα to search for peripheral emission and determine the properties of various regions. The head has a mass in young stars of ˜ {10}6 {M}ȯ and an ionization rate of 6.4× {10}51 s‑1, equivalent to ˜2100 O9-type stars. There are four older star-forming regions in the tail, and an underlying disk with a photometric age of ˜1 Gyr. The mass distribution function of 61 star clusters is a power law with a slope of ‑1.73 ± 0.51. Fourteen young clusters in the head are more massive than {10}4 {M}ȯ , suggesting a clustering fraction of 30%–45%. Wispy filaments of Hα emission and young stars extend away from the galaxy. Shells and holes in the head H ii region could be from winds and supernovae. Gravity from the disk should limit the expansion of the H ii region, although hot gas might escape through the holes. The star formation surface density determined from Hα in the head is compared to that expected from likely pre-existing and accreted gas. Unless the surface density of the accreted gas is a factor of ˜3 or more larger than what was in the galaxy before, the star formation rate has to exceed the usual Kennicutt–Schmidt rate by a factor of ≥slant 5.

  10. Drop Tower Workshop

    NASA Technical Reports Server (NTRS)

    Urban, David

    2013-01-01

    Ground based microgravity facilities are an important proving ground for space experiments, ground-based research and space hardware risk mitigation. An overview of existing platforms will be discussed with an emphasis on drop tower capabilities. The potential for extension to partial gravity conditions will be discussed. Input will be solicited from attendees for their potential to use drop towers in the future and the need for enhanced capabilities (e.g. partial gravity)

  11. Accretion of radiation and rotating primordial black holes

    NASA Astrophysics Data System (ADS)

    Mahapatra, S.; Nayak, B.

    2016-02-01

    We consider rotating primordial black holes (PBHs) and study the effect of accretion of radiation in the radiation-dominated era. The central part of our analysis deals with the role of the angular momentum parameter on the evolution of PBHs. We find that both the accretion and evaporation rates decrease with an increase in the angular momentum parameter, but the rate of evaporation decreases more rapidly than the rate of accretion. This shows that the evaporation time of PBHs is prolonged with an increase in the angular momentum parameter. We also note that the lifetime of rotating PBHs increases with an increase in the accretion efficiency of radiation as in the case of nonrotating PBHs.

  12. Flow rate/pressure drop data gathered from testing a sample of the Space Shuttle Strain Isolation Pad (SIP): Effects of ambient pressure combined with tension and compression conditions

    NASA Technical Reports Server (NTRS)

    Springfield, R. D.; Lawing, P. L.

    1983-01-01

    Tests were conducted on a sample of strain isolation pad (SIP) typical of that used in the shuttle orbiter thermal protection system to determine the characteristics of SIP internal flow. Data obtained were pressure drop as a function of flow rate for a range of ambient pressures representing various points along the Shuttle trajectory and for stretched and compressed conditions of the SIP. Flow was in the direction of the weave parallel to most of the fibers. The data are plotted in several standard engineering formats in order to be of maximum utility to the user. In addition to providing support to the Space Shuttle Program, these data are a source of experimental information on flow through fiberous (rather than the more usual sand bed type) porous media.

  13. Rain Drop Charge Sensor

    NASA Astrophysics Data System (ADS)

    S, Sreekanth T.

    begin{center} Large Large Rain Drop Charge Sensor Sreekanth T S*, Suby Symon*, G. Mohan Kumar (1) , S. Murali Das (2) *Atmospheric Sciences Division, Centre for Earth Science Studies, Thiruvananthapuram 695011 (1) D-330, Swathi Nagar, West Fort, Thiruvananthapuram 695023 (2) Kavyam, Manacaud, Thiruvananthapuram 695009 begin{center} ABSTRACT To study the inter-relations with precipitation electricity and precipitation microphysical parameters a rain drop charge sensor was designed and developed at CESS Electronics & Instrumentation Laboratory. Simultaneous measurement of electric charge and fall speed of rain drops could be done using this charge sensor. A cylindrical metal tube (sensor tube) of 30 cm length is placed inside another thick metal cover opened at top and bottom for electromagnetic shielding. Mouth of the sensor tube is exposed and bottom part is covered with metal net in the shielding cover. The instrument is designed in such a way that rain drops can pass only through unhindered inside the sensor tube. When electrically charged rain drops pass through the sensor tube, it is charged to the same magnitude of drop charge but with opposite polarity. The sensor tube is electrically connected the inverted input of a current to voltage converter operational amplifier using op-amp AD549. Since the sensor is electrically connected to the virtual ground of the op-amp, the charge flows to the ground and the generated current is converted to amplified voltage. This output voltage is recorded using a high frequency (1kHz) voltage recorder. From the recorded pulse, charge magnitude, polarity and fall speed of rain drop are calculated. From the fall speed drop diameter also can be calculated. The prototype is now under test running at CESS campus. As the magnitude of charge in rain drops is an indication of accumulated charge in clouds in lightning, this instrument has potential application in the field of risk and disaster management. By knowing the charge

  14. Dropping out from School. Policy Brief Number 8

    ERIC Educational Resources Information Center

    Hunt, Frances

    2009-01-01

    While initial access to education is increasing in many countries, drop out rates continue to be high. This seriously affects MDG and EFA goals around educational access. This briefing paper looks at the issue of dropping out from school. It is based on the CREATE Pathways to Access Research Monograph, "Dropping out from school: a cross country…

  15. Liquid metal drop ejection

    NASA Technical Reports Server (NTRS)

    Khuri-Yakub, B. T.

    1993-01-01

    The aim of this project was to demonstrate the possibility of ejecting liquid metals using drop on demand printing technology. The plan was to make transducers for operation in the 100 MHz frequency range and to use these transducers to demonstrate the ability to eject drops of liquid metals such as gallium. Two transducers were made by indium bonding piezoelectric lithium niobate to quartz buffer rods. The lithium niobate plates were thinned by mechanical polishing to a thickness of 37 microns for operation at 100 MHz. Hemispherical lenses were polished in the opposite ends of the buffer rods. The lenses, which focus the sound waves in the liquid metal, had an F-number equals 1. A mechanical housing was made to hold the transducers and to allow precise control over the liquid level above the lens. We started by demonstrating the ability to eject drops of water on demand. The drops of water had a diameter of 15 microns which corresponds to the wavelength of the sound wave in the water. A videotape of this ejection was made. We then used a mixture of Gallium and Indium (used to lower the melting temperature of the Gallium) to demonstrate the ejection of liquid metal drops. This proved to be difficult because of the oxide skin which forms on the surface of the liquid. In some instances, we were able to eject metal drops, however, this was not consistent and reproducible. An experiment was set up at NASA-Lewis to stabilize the process of drop on demand liquid metal ejection. The object was to place the transducer and liquid metal in a vacuum station so that no oxide would form on the surface. We were successful in demonstrating that liquid metals could be ejected on demand and that this technology could be used for making sheet metal in space.

  16. Coronal Neutrino Emission in Hypercritical Accretion Flows

    NASA Astrophysics Data System (ADS)

    Kawabata, R.; Mineshige, S.; Kawanaka, N.

    2008-03-01

    Hypercritical accretion flows onto stellar mass black holes (BHs) are commonly believed to be as a promising model of central engines of gamma-ray bursts (GRBs). In this model a certain fraction of the gravitational binding energy of accreting matter is deposited to the energy of relativistic jets via neutrino annihilation and/or magnetic fields. However, some recent studies have indicated that the energy deposition rate by neutrino annihilation is somewhat smaller than that needed to power a GRB. To overcome this difficulty, Ramirez-Ruiz and Socrates proposed that high-energy neutrinos from the hot corona above the accretion disk might enhance the efficiency of the energy deposition. We elucidate the disk corona model in the context of hypercritical accretion flows. From the energy balance in the disk and the corona, we can calculate the disk and coronal temperature, Td and Tc, and neutrino spectra, taking into account the neutrino cooling processes by neutrino-electron scatterings and neutrino pair productions. The calculated neutrino spectra consist of two peaks: one by the neutrino emission from the disk and the other by that from the corona. We find that the disk corona can enhance the efficiency of energy release but only by a factor of 1.5 or so, unless the height of the corona is very small, Hll r. This is because the neutrino emission is very sensitive to the temperature of the emitting region, and then the ratio Tc/Td cannot be very large.

  17. Spectroscopic Detection of a Stellar-like Photosphere in an Accreting Protostar

    NASA Technical Reports Server (NTRS)

    Greene, Thomas P.; Lada, Charles J.; DeVincenzi, Donald L. (Technical Monitor)

    2002-01-01

    We present high-resolution (R is approximately equal to 18,000), high signal-to-noise 2 micron spectra of two luminous, X-ray flaring Class I protostars in the rho Ophiuchi cloud acquired with the NIRSPEC (near infrared spectrograph) of the Keck II telescope. We present the first spectrum of a highly veiled, strongly accreting protostar which shows photospheric absorption features and demonstrates the stellar nature of its central core. We find the spectrum of the luminous (L (sub bol) = 10 solar luminosity) protostellar source, YLW 15, to be stellar-like with numerous atomic and molecular absorption features, indicative of a K5 IV/V spectral type and a continuum veiling r(sub k) = 3.0. Its derived stellar luminosity (3 stellar luminosity) and stellar radius (3.1 solar radius) are consistent with those of a 0.5 solar mass pre-main-sequence star. However, 70% of its bolometric luminosity is due to mass accretion, whose rate we estimate to be 1.7 x 10(exp -6) solar masses yr(exp -1). We determine that excess infrared emission produced by the circumstellar accretion disk, the inner infalling envelope, and accretion shocks at the surface of the stellar core of YLW 15 all contribute significantly to its near-IR (infrared) continuum veiling. Its rotational velocity v sin i = 50 km s(exp -1) is comparable to those of flat-spectrum protostars but considerably higher than those of classical T Tauri stars in the rho Oph cloud. The protostar may be magnetically coupled to its circumstellar disk at a radius of 2 - 3 R(sub *). It is also plausible that this protostar can shed over half its angular momentum and evolve into a more slowly rotating classical T Tauri star by remaining coupled to its circumstellar disk (at increasing radius) as its accretion rate drops by an order of magnitude during the rapid transition between the Class I and Class II phases of evolution. The spectrum of WL 6 does not show any photospheric absorption features, and we estimate that its continuum

  18. Student Drop-Out Trends at Sultan Qaboos University and Kuwait University: 2000-2011

    ERIC Educational Resources Information Center

    Al Ghanboosi, Salim Saleem; Alqahtani, Abdulmuhsen Ayedh

    2013-01-01

    The current study aims to explore the drop-out trends at Sultan Qaboos. University and Kuwait University. Archival data of the period 2000-2011 were used to achieve this goal. Main findings showed that (a) male drop-out rates are higher than female drop-out rates; (b) drop- out rates at scientific colleges are higher; (c) drop-out rates of…

  19. Spherical Accretion in a Uniformly Expanding Universe

    NASA Astrophysics Data System (ADS)

    Colpi, Monica; Shapiro, Stuart L.; Wasserman, Ira

    1996-10-01

    We consider spherically symmetric accretion of material from an initially homogeneous, uniformly expanding medium onto a Newtonian point mass M. The gas is assumed to evolve adiabatically with a constant adiabatic index F, which we vary over the range Γ ɛ [1, 5/3]. We use a one-dimensional Lagrangian code to follow the spherical infall of material as a function of time. Outflowing shells gravitationally bound to the point mass fall back, giving rise to a inflow rate that, after a rapid rise, declines as a power law in time. If there were no outflow initially, Bondi accretion would result, with a characteristic accretion time-scale ta,0. For gas initially expanding at a uniform rate, with a radial velocity U = R/t0 at radius R, the behavior of the flow at all subsequent times is determined by ta,0/t0. If ta,0/t0 ≫ 1, the gas has no time to respond to pressure forces, so the fluid motion is nearly collisionless. In this case, only loosely bound shells are influenced by pressure gradients and are pushed outward. The late-time evolution of the mass accretion rate Mdot is close to the result for pure dust, and we develop a semianalytic model that accurately accounts for the small effect of pressure gradients in this limit. In the opposite regime, ta,0/t0 ≪ 1, pressure forces significantly affect the motion of the gas. At sufficiently early times, t ≤ ttr, the flow evolved along a sequence of quasi-stationary, Bondi-like states, with a time-dependent Mdot determined by the slowly varying gas density at large distances. However, at later times, t ≥ ttr, the fluid flow enters a dustllke regime; ttr is the time when the instantaneous Bondi accretion radius reaches the marginally bound radius. The transition time ttr depends sensitively on ta,0/t0 for a given Γ and can greatly exceed t0. We show that there exists a critical value Γ = 11/9, below which the transition from fluid to ballistic motion disappears. As one application of our calculations, we consider the

  20. Drying drops of blood

    NASA Astrophysics Data System (ADS)

    Brutin, David; Sobac, Benjamin; Loquet, Boris; Sampol, José.

    2010-11-01

    The drying of a drop of human blood is fascinating by the complexity of the physical mechanisms that occur as well as the beauty of the phenomenon which has never been previously evidenced in the literature. The final stage of full blood evaporation reveals for a healthy person the same regular pattern with a good reproducibility. Other tests on anemia and hyperlipidemic persons were performed and presented different patterns. By means of digital camera, the influence of the motion of red blood cells (RBCs) which represent about 50% of the blood volume, is revealed as well as its consequences on the final stages of drying. The mechanisms which lead to the final pattern of dried blood drops are presented and explained on the basis of fluid and solid mechanics in conjunction with the principles of hematology. Our group is the first to evidence that the specific regular patterns characteristic of a healthy individual do not appear in a dried drop of blood from a person with blood disease. Blood is a complex colloidal suspension for which the flow motion is clearly non-Newtonian. When drops of blood evaporate, all the colloids are carried by the flow motion inside the drop and interact.

  1. Recent Observational Progress on Accretion Disks Around Compact Objects

    NASA Astrophysics Data System (ADS)

    Miller, Jon M.

    2016-04-01

    Studies of accretion disks around black holes and neutron stars over the last ten years have made remarkable progress. Our understanding of disk evolution as a function of mass accretion rate is pushing toward a consensus on thin/thick disk transitions; an apparent switching between disk-driven outflow modes has emerged; and monitoring observations have revealed complex spectral energy distributions wherein disk reprocessing must be important. Detailed studies of disk winds, in particular, have the potential to reveal the basic physical processes that mediate disk accretion, and to connect with numerical simulations. This talk will review these developments and look ahead to the potential of Astro-H.

  2. Accretion tori and cones of ionizing radiation in Seyfert galaxies

    NASA Technical Reports Server (NTRS)

    Acosta-Pulido, Jose A.; Perez-Fournon, Ismael; Calvani, Massimo; Wilson, Andrew S.

    1990-01-01

    The photoionization of extended narrow-line regions in Seyfert galaxies by the radiation produced in a thick accretion disk is studied. The emission-line spectrum is calculated for a range of black hole masses, varying the values of the ionization parameter and the disk size. It is found that models with a million solar masses fit observations of very large accretion disk sizes, while models with 10 million solar masses fit them better with smaller disks. The latter models are preferable since they have lower super-Eddington accretion rates.

  3. Lambda Boo Abundance Patterns: Accretion from Orbiting Sources

    NASA Astrophysics Data System (ADS)

    Jura, M.

    2015-12-01

    The abundance anomalies in λ Boo stars are popularly explained by element-specific mass inflows at rates that are much greater than empirically inferred bounds for interstellar accretion. Therefore, a λ Boo star’s thin outer envelope must derive from a companion star, planet, analogs to Kuiper Belt objects or a circumstellar disk. Because radiation pressure on gas-phase ions might selectively allow the accretion of carbon, nitrogen, and oxygen and inhibit the inflow of elements such as iron, the source of the acquired matter need not contain dust. We propose that at least some λ Boo stars accrete from the winds of hot Jupiters.

  4. ACCRETION DISK TEMPERATURES OF QSOs: CONSTRAINTS FROM THE EMISSION LINES

    SciTech Connect

    Bonning, E. W.; Shields, G. A.; Stevens, A. C.; Salviander, S. E-mail: shields@astro.as.utexas.edu E-mail: triples@astro.as.utexas.edu

    2013-06-10

    We compare QSO emission-line spectra to predictions based on theoretical ionizing continua of accretion disks. The observed line intensities do not show the expected trend of higher ionization with theoretical accretion disk temperature as predicted from the black hole mass and accretion rate. Consistent with earlier studies, this suggests that the inner disk does not reach temperatures as high as expected from standard disk theory. Modified radial temperature profiles, taking account of winds or advection in the inner disk, achieve better agreement with observation. The emission lines of radio-detected and radio-undetected sources show different trends as a function of the theoretically predicted disk temperature.

  5. Bondi-like Accretion in Magnetized Supersonic Isothermal Turbulence

    NASA Astrophysics Data System (ADS)

    Burleigh, Kaylan J.; McKee, Christopher F.; Klein, Richard I.

    2016-01-01

    The Bondi and Bondi-Hoyle-Lytlleton formulas give the order of magnitude steady-accretion rate onto a point mass at rest or moving, respectively, in a uniform density gas in the limit of negligible gas self-gravity. This applies in star-forming clouds where self-gravity is negligible near protostars and new-born stars, but instead of being uniform the gas is supersonically turbulent and threaded by dynamically important (Alven Mach number ˜ 1) large-scale magnetic fields. To determine the Bondi-like accretion rate in these environments, we used the ORION2 code to carry out grid-based 3D adaptive mesh refinement (AMR) magnetohydrodynamic (MHD) simulations of accretion onto sink particles embedded in an environment of fully developed, magnetized supersonic isothermal turbulence. We evolved the models until the median and mean accretion rates, over particles, became steady. We present a simple semi-analytic model that predicts the median and mean accretion rate from the turbulent properties of the background medium, such as the 3D Mach number and RMS plasma-β, and show that it is highly consistent with our simulations. Numerical codes can use our semi-analytic model as an accurate sub-grid model for accretion in magnetized supersonic isothermal turbulence.

  6. Accretion of a ghost condensate by black holes

    SciTech Connect

    Frolov, Andrei V.

    2004-09-15

    The intent of this paper is to point out that the accretion of a ghost condensate by black holes could be extremely efficient. We analyze steady-state spherically symmetric flows of the ghost fluid in the gravitational field of a Schwarzschild black hole and calculate the accretion rate. Unlike minimally coupled scalar field or quintessence, the accretion rate is set not by the cosmological energy density of the field, but by the energy scale of the ghost condensate theory. If hydrodynamical flow is established, it could be as high as a tenth of a solar mass per second for 10 MeV scale ghost condensate accreting onto a stellar-sized black hole, which puts serious constraints on the parameters of the ghost condensate model.

  7. Compton heated winds and coronae above accretion disks. I Dynamics

    NASA Technical Reports Server (NTRS)

    Begelman, M. C.; Mckee, C. F.; Shields, G. A.

    1983-01-01

    X rays emitted in the inner part of an accretion disk system can heat the surface of the disk farther out, producing a corona and possibly driving off a strong wind. The dynamics of Compton-heated coronae and winds are analyzed using an approximate two-dimensional technique to estimate the mass loss rate as a function of distance from the source of X rays. The findings have important dynamical implications for accretion disks in quasars, active galactic nuclei, X ray binaries, and cataclysmic variables. These include: mass loss from the disk possibly comparable with or exceeding the net accretion rate onto the central compact object, which may lead to unstable accretion; sufficient angular momentum loss in some cases to truncate the disk in a semidetached binary at a smaller radius than that predicted by tidal truncation theories; and combined static plus ram pressure in the wind adequate to confine line-emitting clouds in quasars and Seyfert galaxies.

  8. Magnetically driven accretion in protoplanetary discs

    NASA Astrophysics Data System (ADS)

    Simon, Jacob B.; Lesur, Geoffroy; Kunz, Matthew W.; Armitage, Philip J.

    2015-11-01

    We characterize magnetically driven accretion at radii between 1 and 100 au in protoplanetary discs, using a series of local non-ideal magnetohydrodynamic (MHD) simulations. The simulations assume a minimum mass solar nebula (MMSN) disc that is threaded by a net vertical magnetic field of specified strength. Confirming previous results, we find that the Hall effect has only a modest impact on accretion at 30 au, and essentially none at 100 au. At 1-10 au the Hall effect introduces a pronounced bimodality in the accretion process, with vertical magnetic fields aligned to the disc rotation supporting a strong laminar Maxwell stress that is absent if the field is anti-aligned. In the anti-aligned case, we instead find evidence for bursts of turbulent stress at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear instability. The presence or absence of these bursts depends upon the details of the adopted chemical model, which suggests that appreciable regions of actual protoplanetary discs might lie close to the borderline between laminar and turbulent behaviour. Given the number of important control parameters that have already been identified in MHD models, quantitative predictions for disc structure in terms of only radius and accretion rate appear to be difficult. Instead, we identify robust qualitative tests of magnetically driven accretion. These include the presence of turbulence in the outer disc, independent of the orientation of the vertical magnetic fields, and a Hall-mediated bimodality in turbulent properties extending from the region of thermal ionization to 10 au.

  9. EARTH, MOON, SUN, AND CV ACCRETION DISKS

    SciTech Connect

    Montgomery, M. M.

    2009-11-01

    accretion disks are present or not. Our results suggest that the accretion disk's geometric shape directly affects the disk's precession rate.

  10. Earth, Moon, Sun, and CV Accretion Disks

    NASA Astrophysics Data System (ADS)

    Montgomery, M. M.

    2009-11-01

    accretion disks are present or not. Our results suggest that the accretion disk's geometric shape directly affects the disk's precession rate.

  11. AGN flickering and chaotic accretion

    NASA Astrophysics Data System (ADS)

    King, Andrew; Nixon, Chris

    2015-10-01

    Observational arguments suggest that the growth phases of the supermassive black holes in active galactic nuclei have a characteristic time-scale ˜105 yr. We show that this is the time-scale expected in the chaotic accretion picture of black hole feeding, because of the effect of self-gravity in limiting the mass of any accretion-disc feeding event.

  12. Accretion in brown dwarfs: An infrared view

    NASA Astrophysics Data System (ADS)

    Natta, A.; Testi, L.; Muzerolle, J.; Randich, S.; Comerón, F.; Persi, P.

    2004-09-01

    This paper presents a study of the accretion properties of 19 very low mass objects (M*˜ 0.01-0.1 M⊙) in the regions Chamaeleon I and ρ Oph. For 8 objects we obtained high resolution Hα profiles and determined mass accretion rate \\dot Mac and accretion luminosity Lac. Pa\\beta is detected in emission in 7 of the 10 ρ Oph objects, but only in one in Cha I. Using objects for which we have both a determination of Lac from Hα and a Paβ detection,} we show that the correlation between the Paβ luminosity and luminosity Lac, found by Muzerolle et al. (\\cite{Mea98}) for T Tauri stars in Taurus, extends to objects with mass ˜0.03 M⊙; L(Paβ) can be used to measure Lac also in the substellar regime. The results were less conclusive for Brγ, which was detected only in 2 objects, neither of which had an Hα estimate of \\dot Mac. Using the relation between L(Pa\\beta) and Lac we determined the accretion rate for all the objects in our sample (including those with no Hα spectrum), } more than doubling the number of substellar objects with known \\dot Mac. When plotted as a function of the mass of the central object together with data from the literature, our results confirm the trend of lower \\dot Mac for lower M*, although with a large spread. Some of the spread is probably due to an age effect; our very young objects in ρ Oph have on average an accretion rate at least one order of magnitude higher than objects of similar mass in older regions. As a side product, we found that the width of Hα measured at 10% peak intensity is not only a qualitative indicator of the accreting nature of very low mass objects, but can be used to obtain a quantitative, although not very accurate, estimate of \\dot Mac over a large mass range, from T Tauri stars to brown dwarfs. Finally, we found that some of our objects show evidence of mass-loss in their optical spectra. Based on observations collected at the European Southern Observatory, Chile. Table 2 is only available in

  13. Accreting planets as dust dams in 'transition' disks

    SciTech Connect

    Owen, James E.

    2014-07-01

    We investigate under what circumstances an embedded planet in a protoplanetary disk may sculpt the dust distribution such that it observationally presents as a 'transition' disk. We concern ourselves with 'transition' disks that have large holes (≳ 10 AU) and high accretion rates (∼10{sup –9}-10{sup –8} M {sub ☉} yr{sup –1}), particularly, those disks which photoevaporative models struggle to explain. Adopting the observed accretion rates in 'transition' disks, we find that the accretion luminosity from the forming planet is significant, and can dominate over the stellar luminosity at the gap edge. This planetary accretion luminosity can apply a significant radiation pressure to small (s ≲ 1 μm) dust particles provided they are suitably decoupled from the gas. Secular evolution calculations that account for the evolution of the gas and dust components in a disk with an embedded, accreting planet, show that only with the addition of the radiation pressure can we explain the full observed characteristics of a 'transition' disk (NIR dip in the spectral energy distribution (SED), millimeter cavity, and high accretion rate). At suitably high planet masses (≳ 3-4 M{sub J} ), radiation pressure from the accreting planet is able to hold back the small dust particles, producing a heavily dust-depleted inner disk that is optically thin to infrared radiation. The planet-disk system will present as a 'transition' disk with a dip in the SED only when the planet mass and planetary accretion rate are high enough. At other times, it will present as a disk with a primordial SED, but with a cavity in the millimeter, as observed in a handful of protoplanetary disks.

  14. Using NPOL (the NASA S-band polarimetric radar), and a network of 2D video disdrometers for external radar calibration and rain rate estimation, and to determine spatial correlation of rain drop size distribution parameters

    NASA Astrophysics Data System (ADS)

    Thurai, M.; Bringi, V. N.; Tolstoy, L.; Petersen, W. A.

    2012-12-01

    On two days during the MC3E campaign in northern Oklahoma, NASA's S-band polarimetric radar (NPOL) performed repeated PPI scans over a network of six 2D video disdrometer (2DVD) sites, located 20 to 30 km from the radar. The scans were repeated approximately every 40 seconds. We consider here the two cases, one a rapidly evolving multi-cell rain event (with large drops) on 24 April 2011 and the second a somewhat more uniform rain event on 11 May 2011. For both events, the external calibration offsets for radar reflectivity and differential reflectivity were determined by comparing the radar data extracted over the disdrometer sites with those determined from scattering simulations using the 2DVD data. Time series comparisons show excellent agreement for all six sites, and a technique was developed to determine the offsets for the NPOL data quantitatively from the comparisons. The radar data were then used to determine the rain rates over the six sites and compared with those derived from the 2DVD measurements. Once again, excellent agreement was obtained for all six sites, both in terms of rain fall rates and rain accumulations (see Fig. 1). Comparisons have also been made over many rain gauges located within ground validation network area. The repeated PPI scans were also used to determine the spatial correlations of two of the main rain drop-size distribution (DSD) parameters (Do and log Nw) as well as rainfall rate (R). The correlations were determined along the radial over the whole azimuthal range of the PPI scans. The spatial correlation of R shows azimuthal dependence particularly for the first event. However, the 50 percentile levels are similar between the two events, at least up to 4 km. For the DSD parameters, reasonable agreement with 2DVD-based spatial correlations were obtained As part of the abovementioned scan sequence, the NPOL had also made repeated RHI scans along one azimuth. These scans were used to determine the vertical correlations of the

  15. Factors Influencing University Drop Out Rates

    ERIC Educational Resources Information Center

    Araque, Francisco; Roldan, Concepcion; Salguero, Alberto

    2009-01-01

    This paper develops personalized models for different university degrees to obtain the risk of each student abandoning his degree and analyzes the profile for undergraduates that abandon the degree. In this study three faculties located in Granada, South of Spain, were involved. In Software Engineering three university degrees with 10,844…

  16. Observational diagnostics of accretion on young stars and brown dwarfs

    NASA Astrophysics Data System (ADS)

    Stelzer, Beate; Argiroffi, Costanza

    I present a summary of recent observational constraints on the accretion properties of young stars and brown dwarfs with focus on the high-energy emission. In their T Tauri phase young stars assemble a few percent of their mass by accretion from a disk. Various observational signatures of disks around pre-main sequence stars and the ensuing accretion process are found in the IR and optical regime: e.g. excess emission above the stellar photosphere, strong and broad emission lines, optical veiling. At high energies evidence for accretion is less obvious, and the X-ray emission from stars has historically been ascribed to magnetically confined coronal plasmas. While being true for the bulk of the emission, new insight obtained from XMM-Newton and Chandra observations has unveiled contributions from accretion and outflow processes to the X-ray emission from young stars. Their smaller siblings, the brown dwarfs, have been shown to undergo a T Tauri phase on the basis of optical/IR observations of disks and measurements of accretion rates. Most re-cently, first evidence was found for X-rays produced by accretion in a young brown dwarf, complementing the suspected analogy between stars and substellar objects.

  17. The Evolution of Accretion and Activity Signatures in Young A Stars

    NASA Astrophysics Data System (ADS)

    Williger, G. M.; Grady, C. A.; Hamaguchi, K.; Hubrig, S.; Bouret, J.-C.; Roberge, A.; Sahu, M.; Woodgate, B.; Kimble, R.

    2005-12-01

    FUV spectroscopy obtained with FUSE reveals excess continuum light in 12 lightly reddened Herbig Ae stars, as well as the routine presence of emission in a range of ionization stages sampling material from neutral atomic gas to transition region temperature plasma. The FUV excess light is correlated with the near IR colors of the stars which has previously been noted as a tracer of mass accretion rate. In several cases, sufficient data exist to demonstrate that FUV continuum variability is present and is correlated with changes in the FUV emission lines, particularly red-shifted material. Combining the FUV spectra with disk inclination data, we find that the red-shifted C III 1176 emission is seen for inclinations between 0 and 60 degrees with no dependence upon inclination in that range, as expected for funneled accretion scenarios. The FUV excess light and X-ray luminosity show the same evolutionary trend, dropping gradually over the 1st 10 Myr as long as the star is accreting material from the disk. Centrally-cleared A debris disk systems have X-ray luminosities which are at least 3 orders of magnitude fainter than the Herbig Ae stars, demonstrating that the X-ray emission is related to accretion and not to more conventional stellar activity. Plasma at transition region and chromospheric temperatures persists longer, at least in some systems. Recent magnetic field detections for 5 of the FUSE Herbig Ae stars and Beta Pictoris indicate that magnetic fields with typical field strengths of 50 to several hundred Gauss are present over the entire age range where the accretion signatures are seen. This study is based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by the Johns Hopkins University under NASA contract NAS5-32985. Data included in this study were obtained under FUSE GO Programs C126, D065, and the FUSE Legacy program E510. HST observations of HD 163296 and HD 104237 were obtained under HST

  18. X-Shooter study of accretion in Chamaeleon I

    NASA Astrophysics Data System (ADS)

    Manara, C. F.; Fedele, D.; Herczeg, G. J.; Teixeira, P. S.

    2016-01-01

    We present the analysis of 34 new VLT/X-Shooter spectra of young stellar objects in the Chamaeleon I star-forming region, together with four more spectra of stars in Taurus and two in Chamaeleon II. The broad wavelength coverage and accurate flux calibration of our spectra allow us to estimate stellar and accretion parameters for our targets by fitting the photospheric and accretion continuum emission from the Balmer continuum down to ~700 nm. The dependence of accretion on stellar properties for this sample is consistent with previous results from the literature. The accretion rates for transitional disks are consistent with those of full disks in the same region. The spread of mass accretion rates at any given stellar mass is found to be smaller than in many studies, but is larger than that derived in the Lupus clouds using similar data and techniques. Differences in the stellar mass range and in the environmental conditions between our sample and that of Lupus may account for the discrepancy in scatter between Chamaeleon I and Lupus. Complete samples in Chamaeleon I and Lupus are needed to determine whether the difference in scatter of accretion rates and the lack of evolutionary trends are not influenced by sample selection. This work is based on observations made with ESO Telescopes at the Paranal Observatory under programme ID 084.C-1095 and 094.C-0913.

  19. Azimuthal Stress and Heat Flux In Radiatively Inefficient Accretion Flows

    NASA Astrophysics Data System (ADS)

    Devlen, Ebru

    2016-07-01

    Radiatively Inefficient Accretion Flows (RIAFs) have low radiative efficiencies and/or low accretion rates. The accreting gas may retain most of its binding energy in the form of heat. This lost energy for hot RIAFs is one of the problems heavily worked on in the literature. RIAF observations on the accretion to super massive black holes (e.g., Sagittarius A* in the center of our Galaxy) have shown that the observational data are not consistent with either advection-dominated accretion flow (ADAF) or Bondi models. For this reason, it is very important to theoretically comprehend the physical properties of RIAFs derived from observations with a new disk/flow model. One of the most probable candidates for definition of mass accretion and the source of excess heat energy in RIAFs is the gyroviscous modified magnetorotational instability (GvMRI). Dispersion relation is derived by using MHD equations containing heat flux term based on viscosity in the energy equation. Numerical solutions of the disk equations are done and the growth rates of the instability are calculated. This additional heat flux plays an important role in dissipation of energy. The rates of the angular momentum and heat flux which are obtained from numerical calculations of the turbulence brought about by the GVMRI are also discussed.

  20. Effects of surfactants on the deformation of microfluidic drops

    NASA Astrophysics Data System (ADS)

    Cordero, Maria Luisa; Ulloa, Camilo

    2013-11-01

    A microfluidic analog of the four-roll-mill experiment is used to study the deformation and breakup of microfluidic drops. The behavior of water drops flowing in mineral oil is quantified as a function of the capillary number, Ca , which is based on the oil viscosity, drop radius, flow shear rate and equilibrium interfacial tension, both in the presence and absence of surfactants. In the absence of surfactants the deformation of the drops increases linearly with Ca . If surfactants are added to the carrier oil then, for the same value of Ca , drops deform less if the flow velocity is larger. Moreover, for a given drop size in the presence of surfactants, drops begin to split at a threshold shear rate but stop breaking if the shear rate is increased beyond a second threshold. These observations are explained by a decrease in the surfactant concentration at the surface of the drop due to advection of surfactant molecules by the oil flow. This increases the interfacial tension, thus making the drop less deformable for higher flow velocities. We use the deformation of the drops to infer the mean interfacial tension and from this we quantify the surface concentration of surfactants at the drop interface. Work supported by FONDECYT 11100204.

  1. On the Stability of Rotating Drops

    PubMed Central

    Nurse, A. K.; Coriell, S. R.; McFadden, G. B.

    2015-01-01

    We consider the equilibrium and stability of rotating axisymmetric fluid drops by appealing to a variational principle that characterizes the equilibria as stationary states of a functional containing surface energy and rotational energy contributions, augmented by a volume constraint. The linear stability of a drop is determined by solving the eigenvalue problem associated with the second variation of the energy functional. We compute equilibria corresponding to both oblate and prolate shapes, as well as toroidal shapes, and track their evolution with rotation rate. The stability results are obtained for two cases: (i) a prescribed rotational rate of the system (“driven drops”), or (ii) a prescribed angular momentum (“isolated drops”). For families of axisymmetric drops instabilities may occur for either axisymmetric or non-axisymmetric perturbations; the latter correspond to bifurcation points where non-axisymmetric shapes are possible. We employ an angle-arc length formulation of the problem which allows the computation of equilibrium shapes that are not single-valued in spherical coordinates. We are able to illustrate the transition from spheroidal drops with a strong indentation on the rotation axis to toroidal drops that do not extend to the rotation axis. Toroidal drops with a large aspect ratio (major radius to minor radius) are subject to azimuthal instabilities with higher mode numbers that are analogous to the Rayleigh instability of a cylindrical interface. Prolate spheroidal shapes occur if a drop of lower density rotates within a denser medium; these drops appear to be linearly stable. This work is motivated by recent investigations of toroidal tissue clusters that are observed to climb conical obstacles after self-assembly [Nurse et al., Journal of Applied Mechanics 79 (2012) 051013]. PMID:26958440

  2. Quantitative assessment of ribosome drop-off in E. coli.

    PubMed

    Sin, Celine; Chiarugi, Davide; Valleriani, Angelo

    2016-04-01

    Premature ribosome drop-off is one of the major errors in translation of mRNA by ribosomes. However, repeated analyses of Ribo-seq data failed to quantify its strength inE. coli Relying on a novel highly sensitive data analysis method we show that a significant rate of ribosome drop-off is measurable and can be quantified also when cells are cultured under non-stressing conditions. Moreover, we find that the drop-off rate is highly variable, depending on multiple factors. In particular, under environmental stress such as amino acid starvation or ethanol intoxication, the drop-off rate markedly increases. PMID:26935582

  3. Quantitative assessment of ribosome drop-off in E. coli

    PubMed Central

    Sin, Celine; Chiarugi, Davide; Valleriani, Angelo

    2016-01-01

    Premature ribosome drop-off is one of the major errors in translation of mRNA by ribosomes. However, repeated analyses of Ribo-seq data failed to quantify its strength in E. coli. Relying on a novel highly sensitive data analysis method we show that a significant rate of ribosome drop-off is measurable and can be quantified also when cells are cultured under non-stressing conditions. Moreover, we find that the drop-off rate is highly variable, depending on multiple factors. In particular, under environmental stress such as amino acid starvation or ethanol intoxication, the drop-off rate markedly increases. PMID:26935582

  4. Accretion Acceleration of Neutron Stars and Effects of Gravitational Radiation

    NASA Astrophysics Data System (ADS)

    Fu, Yan-yan; Zhang, Yue-zhu; Wei, Yi-huan; Zhang, Cheng-min; Yu, Shao-hua; Pan, Yuan-yue; Guo, Yuan-qi; Wang, De-hua

    2016-01-01

    In this paper we studied the neutron star's spin acceleration in the accretion process of the neutron star binary system, and the relation how the spin period changes with the accreted mass. We analyzed further the evolutions of both magnetic field and spin period of a neutron star, and compared the modeled results with the observational data of pulsars, to show that they are consistent with each other. Based on above studies, we investigated the effect of gravitational radiation on the spin-up process of a neutron star, and derived the change rate of the neutron star's spin period in the accretion process. We also estimated the critical angular velocity Ωcr, at which the accretion torque is balanced by that of gravitational radiation, and discussed the influence of gravitational radiation on the neutron star's spin evolution.

  5. Properties of accretion disk coronae

    NASA Technical Reports Server (NTRS)

    Wilms, J.; Dove, J.; Staubert, R.; Begelman, M. C.

    1997-01-01

    The properties of accretion disk corona in a parameter regime suitable for Galactic black hole candidates are considered and the results of an analysis of these properties using a self-consistent Monte Carlo code are presented. Examples of the coronal temperature structure, the shape and angular dependency of the spectrum and the maximum temperature allowed for each optical depth of the corona are presented. It is shown that the observed spectrum of the Galactic black hole candidate Cygnus X-1 cannot be explained by accreting disk corona models with a slab geometry, where the accretion disk is sandwiched by the comptonizing medium.

  6. Digitizing of drop table output

    SciTech Connect

    Muncy, K.

    1984-01-01

    The method for monitoring and analyzing the drop pulses from the MTS1212 drop table system has been upgraded from a labor intensive manual system to an automatic digital system. The pulse from each drop is recorded, analyzed and printed out. The data printed out includes all product information, the drop parameters calculated and a plot of the drop pulse. Some of the advantages of this system, besides the replacement of old and obsolete equipment, include the dropping of the repeatability check requirement, ease of operation, complete automatic documentation of each drop, no need to take Polaroid pictures of a drop nor is it necessary to have a drop film read by the film analysis group. Data comparisons between the old method and the new digital method have been very favorable.

  7. The Drop Tower Bremen -An Overview

    NASA Astrophysics Data System (ADS)

    von Kampen, Peter; Könemann, Thorben; Rath, Hans J.

    The Center of Applied Space Technology and Microgravity (ZARM) was founded in 1985 as an institute of the University of Bremen, which focuses on research on gravitational and space-related phenomena. In 1988, the construction of ZARM`s drop tower began. Since its inau-guration in September 1990, the eye-catching Drop Tower Bremen with a height of 146m and its characteristic glass roof has become twice a landmark on the campus of the University of Bremen and the emblem of the technology park Bremen. As such an outstanding symbol of space science in Bremen the drop tower provides an european unique facility for experiments under conditions of high-quality weightlessness with residual gravitational accelerations in the microgravity regime. The period of maximum 4.74s of each freely falling experiment at the Drop Tower Bremen is only limited by the height of the drop tower vacuum tube, which was fully manufactured of steal and enclosed by an outer concrete shell. Thus, the pure free fall height of each microgravity drop experiment is approximately 110m. By using the later in-stalled catapult system established in 2004 ZARM`s short-term microgravity laboratory is able to nearly double the time of free fall. This world-wide inimitable capsule catapult system meets scientists` demand of extending the period of weightlessness. During the catapult operation the experiment capsule performs a vertical parabolic flight within the drop tower vacuum tube. In this way the time of microgravity can be extended to slightly over 9s. Either in the drop or in the catapult operation routine the repetition rates of microgravity experiments at ZARM`s drop tower facility are the same, generally up to 3 times per day. In comparison to orbital platforms the ground-based laboratory Drop Tower Bremen represents an economic alternative with a permanent access to weightlessness on earth. Moreover, the exceptional high quality of weightlessness in order of 1e-6 g (in the frequency range below 100

  8. The Dripping Handrail Model: Transient Chaos in Accretion Systems

    NASA Technical Reports Server (NTRS)

    Young, Karl; Scargle, Jeffrey D.; Cuzzi, Jeffrey (Technical Monitor)

    1995-01-01

    We define and study a simple dynamical model for accretion systems, the "dripping handrail" (DHR). The time evolution of this spatially extended system is a mixture of periodic and apparently random (but actually deterministic) behavior. The nature of this mixture depends on the values of its physical parameters - the accretion rate, diffusion coefficient, and density threshold. The aperiodic component is a special kind of deterministic chaos called transient chaos. The model can simultaneously exhibit both the quasiperiodic oscillations (QPO) and very low frequency noise (VLFN) that characterize the power spectra of fluctuations of several classes of accretion systems in astronomy. For this reason, our model may be relevant to many such astrophysical systems, including binary stars with accretion onto a compact object - white dwarf, neutron star, or black hole - as well as active galactic nuclei. We describe the systematics of the DHR's temporal behavior, by exploring its physical parameter space using several diagnostics: power spectra, wavelet "scalegrams," and Lyapunov exponents. In addition, we note that for large accretion rates the DHR has periodic modes; the effective pulse shapes for these modes - evaluated by folding the time series at the known period - bear a resemblance to the similarly- determined shapes for some x-ray pulsars. The pulsing observed in some of these systems may be such periodic-mode accretion, and not due to pure rotation as in the standard pulsar model.

  9. CONSTRAINT ON THE GIANT PLANET PRODUCTION BY CORE ACCRETION

    SciTech Connect

    Rafikov, Roman R.

    2011-02-01

    The issue of giant planet formation by core accretion (CA) far from the central star is rather controversial because the growth of a massive solid core necessary for triggering the gas runaway can take longer than the lifetime of the protoplanetary disk. In this work, we assess the range of separations at which CA may operate by (1) allowing for an arbitrary (physically meaningful) rate of planetesimal accretion by the core and (2) properly taking into account the dependence of the critical mass for the gas runaway on the planetesimal accretion luminosity. This self-consistent approach distinguishes our work from similar studies in which only a specific planetesimal accretion regime was explored and/or the critical core mass was fixed at some arbitrary level. We demonstrate that the largest separation at which the gas runaway can occur within 3 Myr corresponds to the surface density of solids in the disk {approx}>0.1 g cm{sup -2} and is 40-50 AU in the minimum mass solar nebula. This limiting separation is achieved when the planetesimal accretion proceeds at the fastest possible rate, even though the high associated accretion luminosity increases the critical core mass, delaying the onset of gas runaway. Our constraints are independent of the mass of the central star and vary only weakly with the core density and its atmospheric opacity. We also discuss various factors that can strengthen or weaken our limits on the operation of CA.

  10. Turbulent Mixing on Helium-accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    Piro, Anthony L.

    2015-03-01

    An attractive scenario for producing Type Ia supernovae (SNe Ia) is a double detonation, where detonation of an accreted helium layer triggers ignition of a C/O core. Whether or not such a mechanism can explain some or most SNe Ia depends on the properties of the helium burning, which in turn is set by the composition of the surface material. Using a combination of semi-analytic and simple numerical models, I explore when turbulent mixing due to hydrodynamic instabilities during the accretion process can mix C/O core material up into the accreted helium. Mixing is strongest at high accretion rates, large white dwarf (WD) masses, and slow spin rates. The mixing would result in subsequent helium burning that better matches the observed properties of SNe Ia. In some cases, there is considerable mixing that can lead to more than 50% C/O in the accreted layer at the time of ignition. These results will hopefully motivate future theoretical studies of such strongly mixed conditions. Mixing also has implications for other types of WD surface explosions, including the so-called .Ia supernovae, the calcium-rich transients (if they arise from accreting WDs), and metal-enriched classical novae.

  11. Driving of Accretion Disk Variability by the Disk Dynamo

    NASA Astrophysics Data System (ADS)

    Hogg, J. Drew; Reynolds, Christopher S.

    2016-01-01

    Variability is a univeral feature of emission from accreting objects, but many questions remain as to how the variability is driven and how it relates to the underlying accretion physics. We use a long, semi-global MHD simulation of a thin accretion disk around a black hole to perform a detailed study of the fluctuations in the internal disk stress and the affect these fluctuations have on the accretion flow. In this poster, we show that low frequency fluctuations in the effective α-parameter in the disk are due to oscillations of the disk dynamo. Additionally, we show that fluctuations in the effective α-parameter drive "propagating fluctuations" in mass accretion rate through the disk that qualitatively resemble the variability from astrophysical black hole systems. In particular, we show that several of the ubiquitous phenomenological properties of black hole variability, including log-normal flux distributions, RMS-flux relationships, and radial coherence are present in the mass accretion rate fluctuations of our simulation.

  12. Drop tube technical tasks

    NASA Technical Reports Server (NTRS)

    Workman, G. L.

    1986-01-01

    Criteria, using fundamental thermochemical dynamics, were developed to assist a scientist using the Drop Tube Facility in designing a good experiment. The types of parameters involved in designing the experiments include the type of furnace, the type of atmosphere, and in general which materials are better behaved than others as determined by past experience in the facility. One of the major advantages of the facility lies in its ability to provide large undercoolings in the cooling curve during the drops. A beginning was to consider the effect of oxygen and other gases upon the amount of undercooling observed. The starting point of the thermochemistry was given by Ellingham and later transformed into what is known as the Richardson Chart. The effect of surface oxidations upon the nucleation phenomena can be observed in each specimen.

  13. Thermonuclear bursts from slowly and rapidly accreting neutron stars

    NASA Astrophysics Data System (ADS)

    Linares, Manuel

    2012-07-01

    Models of thermonuclear burning on accreting neutron stars predict different ignition regimes, depending mainly on the mass accretion rate per unit area. For more than three decades, testing these regimes observationally has met with only partial success. I will present recent results from the Fermi-GBM all-sky X-ray burst monitor, which is yielding robust measurements of recurrence time of rare and highly energetic thermonuclear bursts at the lowest mass accretion rates. I will also present RXTE observations of thermonuclear bursts at high mass accretion rates, including the discovery of millihertz quasi-periodic oscillations and several bursting regimes in a neutron star transient and 11 Hz X-ray pulsar. This unusual neutron star, with higher magnetic field and slower rotation than any other known burster, showed copious bursting activity when the mass accretion rate varied between 10% and 50% of the Eddington rate. I will discuss the role of fuel composition and neutron star spin in setting the burst properties of this system, and the possible implications for the rest of thermonuclear bursters.

  14. FILAMENTARY ACCRETION FLOWS IN THE EMBEDDED SERPENS SOUTH PROTOCLUSTER

    SciTech Connect

    Kirk, Helen; Myers, Philip C.; Bourke, Tyler L.; Gutermuth, Robert A.; Wilson, Grant W.; Hedden, Abigail

    2013-04-01

    One puzzle in understanding how stars form in clusters is the source of mass-is all of the mass in place before the first stars are born, or is there an extended period when the cluster accretes material which can continuously fuel the star formation process? We use a multi-line spectral survey of the southern filament associated with the Serpens South embedded cluster-forming region in order to determine if mass is accreting from the filament onto the cluster, and whether the accretion rate is significant. Our analysis suggests that material is flowing along the filament's long axis at a rate of {approx}30 M{sub Sun} Myr{sup -1} (inferred from the N{sub 2}H{sup +} velocity gradient along the filament), and radially contracting onto the filament at {approx}130 M{sub Sun} Myr{sup -1} (inferred from HNC self-absorption). These accretion rates are sufficient to supply mass to the central cluster at a similar rate to the current star formation rate in the cluster. Filamentary accretion flows may therefore be very important in the ongoing evolution of this cluster.

  15. Two-dimensional vortices and accretion disks

    NASA Astrophysics Data System (ADS)

    Nauta, Michiel Doede

    2000-01-01

    Observations show that there are disks around certain stars that slowly rain down on the central (compact) object: accretion disks. The rate of depletion of the disk might be slow but is still larger than was expected on theoretical grounds. That is why it has been suggested that the disks are turbulent. Because the disk is thin and rotating this turbulence might be related to two-dimensional (2D) turbulence which is characterized by energy transfers towards small wave numbers and the formation of 2D-vortices. This hypothesis is investigated in this thesis by numerical simulations. After an introduction, the numerical algorithm that was inplemented is discussed together with its relation to an accretion disk. It performs well under the absence of discontinuities. The code is used to study 2D-turbulence under the influence of background rotation with compressibility and a shearing background flow. The first is found to be of little consequence but the shear flow alters 2D-turbulence siginificantly. Only prograde vortices of enough strength are able to withstand the shear flow. The size of the vortices in the cross stream direction is also found to be smaller than the equivalent of the thickness of an accretion disk. These circulstances imply that the assumption of two-dimensionality is questionable so that 2D-vortices might not abound in accretion disks. However, the existence of such vortices is not ruled out and one such a cortex is studied in detail in chapter 4. The internal structure of the vortex is well described by a balance between Coriolis, centrifugal and pressure forces. The vortex is also accompanied by two spiral compressible waves. These are not responsible for the azimuthal drift of the vortex, which results from secondary vortices, but they might be related to the small radial drift that is observed. Radial drift leads to accretion but it is not very efficient. Multiple vortex interactions are the topic of tha last chapter and though interesting the

  16. Dynamics of continental accretion.

    PubMed

    Moresi, L; Betts, P G; Miller, M S; Cayley, R A

    2014-04-10

    Subduction zones become congested when they try to consume buoyant, exotic crust. The accretionary mountain belts (orogens) that form at these convergent plate margins have been the principal sites of lateral continental growth through Earth's history. Modern examples of accretionary margins are the North American Cordilleras and southwest Pacific subduction zones. The geologic record contains abundant accretionary orogens, such as the Tasmanides, along the eastern margin of the supercontinent Gondwana, and the Altaïdes, which formed on the southern margin of Laurasia. In modern and ancient examples of long-lived accretionary orogens, the overriding plate is subjected to episodes of crustal extension and back-arc basin development, often related to subduction rollback and transient episodes of orogenesis and crustal shortening, coincident with accretion of exotic crust. Here we present three-dimensional dynamic models that show how accretionary margins evolve from the initial collision, through a period of plate margin instability, to re-establishment of a stable convergent margin. The models illustrate how significant curvature of the orogenic system develops, as well as the mechanism for tectonic escape of the back-arc region. The complexity of the morphology and the evolution of the system are caused by lateral rollback of a tightly arcuate trench migrating parallel to the plate boundary and orthogonally to the convergence direction. We find geological and geophysical evidence for this process in the Tasmanides of eastern Australia, and infer that this is a recurrent and global phenomenon. PMID:24670638

  17. ACCRETING CIRCUMPLANETARY DISKS: OBSERVATIONAL SIGNATURES

    SciTech Connect

    Zhu, Zhaohuan

    2015-01-20

    I calculate the spectral energy distributions of accreting circumplanetary disks using atmospheric radiative transfer models. Circumplanetary disks only accreting at 10{sup –10} M {sub ☉} yr{sup –1} around a 1 M{sub J} planet can be brighter than the planet itself. A moderately accreting circumplanetary disk ( M-dot ∼10{sup −8} M{sub ⊙} yr{sup −1}; enough to form a 10 M{sub J} planet within 1 Myr) around a 1 M{sub J} planet has a maximum temperature of ∼2000 K, and at near-infrared wavelengths (J, H, K bands), this disk is as bright as a late-M-type brown dwarf or a 10 M{sub J} planet with a ''hot start''. To use direct imaging to find the accretion disks around low-mass planets (e.g., 1 M{sub J} ) and distinguish them from brown dwarfs or hot high-mass planets, it is crucial to obtain photometry at mid-infrared bands (L', M, N bands) because the emission from circumplanetary disks falls off more slowly toward longer wavelengths than those of brown dwarfs or planets. If young planets have strong magnetic fields (≳100 G), fields may truncate slowly accreting circumplanetary disks ( M-dot ≲10{sup −9} M{sub ⊙} yr{sup −1}) and lead to magnetospheric accretion, which can provide additional accretion signatures, such as UV/optical excess from the accretion shock and line emission.

  18. Hanging drop crystal growth apparatus and method

    NASA Technical Reports Server (NTRS)

    Carter, Daniel C. (Inventor); Smith, Robbie E. (Inventor)

    1989-01-01

    An apparatus (10) is constructed having a cylindrical enclosure (16) within which a disc-shaped wicking element (18) is positioned. A well or recess (22) is cut into an upper side (24) of this wicking element, and a glass cover plate or slip (28) having a protein drop disposed thereon is sealably positioned on the wicking element (18), with drop (12) being positioned over well or recess (22). A flow of control fluid is generated by a programmable gradient former (16), with this control fluid having a vapor pressure that is selectively variable. This flow of control fluid is coupled to the wicking element (18) where control fluid vapor diffusing from walls (26) of the recess (22) is exposed to the drop (12), forming a vapor pressure gradient between the drop (12) and the control fluid vapor. Initially, this gradient is adjusted to draw solvent from the drop (12) at a relatively high rate, and as the critical supersaturation point is approached (the point at which crystal nucleation occurs), the gradient is reduced to more slowly draw solvent from the drop (12). This allows discrete protein molecules more time to orient themselves into an ordered crystalline lattice, producing protein crystals which, when processed by X-ray crystallography, possess a high degree of resolution.

  19. NMR velocity imaging of single liquid drops

    NASA Astrophysics Data System (ADS)

    Amar, A.; Stapf, S.; Bluemich, B.

    2007-03-01

    Liquid-liquid extraction processes are often found in industrial applications when a bulk phase needs to be purified from dissolved components. The extraction strategy consists of dissolving the impurities into a second, carrier phase, with optimal performance being guaranteed by maximizing both contact interface area and mass transfer rate, in the shape of a swarm of dispersed droplets. Their buoyancy-driven flow within the continuous medium induces internal fluid motion driven by momentum transfer at the drop surface. This convective transport enhances mass transfer and the efficiency of an extraction column. However, understanding mass transfer depends on a proper description of the flow field inside and outside the drops. For that purpose, a cell was built that enables the levitation of a single drop within a counterstream of water. NMR velocity imaging was then applied to drops of different fluids to monitor the internal dynamics as a function of drop size, age, and interface tension. Vortex-type patterns in at least part of the drop were observed where their size and velocity magnitude depended on the system impurity concentration.

  20. Energy flows in thick accretion discs and their consequences for black hole feedback

    NASA Astrophysics Data System (ADS)

    Sądowski, Aleksander; Lasota, Jean-Pierre; Abramowicz, Marek A.; Narayan, Ramesh

    2016-03-01

    We study energy flows in geometrically thick accretion discs, both optically thick and thin, using general relativistic, three-dimensional simulations of black hole accretion flows. We find that for non-rotating black holes the efficiency of the total feedback from thick accretion discs is 3 per cent - roughly half of the thin disc efficiency. This amount of energy is ultimately distributed between outflow and radiation, the latter scaling weakly with the accretion rate for super-critical accretion rates, and returned to the interstellar medium. Accretion on to rotating black holes is more efficient because of the additional extraction of rotational energy. However, the jet component is collimated and likely to interact only weakly with the environment, whereas the outflow and radiation components cover a wide solid angle.

  1. How do accretion discs break?

    NASA Astrophysics Data System (ADS)

    Dogan, Suzan

    2016-07-01

    Accretion discs are common in binary systems, and they are often found to be misaligned with respect to the binary orbit. The gravitational torque from a companion induces nodal precession in misaligned disc orbits. In this study, we first calculate whether this precession is strong enough to overcome the internal disc torques communicating angular momentum. We compare the disc precession torque with the disc viscous torque to determine whether the disc should warp or break. For typical parameters precession wins: the disc breaks into distinct planes that precess effectively independently. To check our analytical findings, we perform 3D hydrodynamical numerical simulations using the PHANTOM smoothed particle hydrodynamics code, and confirm that disc breaking is widespread and enhances accretion on to the central object. For some inclinations, the disc goes through strong Kozai cycles. Disc breaking promotes markedly enhanced and variable accretion and potentially produces high-energy particles or radiation through shocks. This would have significant implications for all binary systems: e.g. accretion outbursts in X-ray binaries and fuelling supermassive black hole (SMBH) binaries. The behaviour we have discussed in this work is relevant to a variety of astrophysical systems, for example X-ray binaries, where the disc plane may be tilted by radiation warping, SMBH binaries, where accretion of misaligned gas can create effectively random inclinations and protostellar binaries, where a disc may be misaligned by a variety of effects such as binary capture/exchange, accretion after binary formation.

  2. Electromagnetic Spindown of a Transient Accreting Millisecond Pulsar During Quiescence

    NASA Astrophysics Data System (ADS)

    Melatos, A.; Mastrano, A.

    2016-02-01

    The measured spindown rates in quiescence of the transient accreting millisecond pulsars IGR J00291+5934, XTE J1751-305, SAX J1808.4-3658, and Swift J1756.9-2508 have been used to estimate the magnetic moments of these objects assuming standard magnetic dipole braking. It is shown that this approach leads to an overestimate if the amount of residual accretion is enough to distort the magnetosphere away from a force-free configuration through magnetospheric mass loading or crushing, so that the lever arm of the braking torque migrates inside the light cylinder. We derive an alternative spindown formula and calculate the residual accretion rates where the formula is applicable. As a demonstration we apply the alternative spindown formula to produce updated magnetic moment estimates for the four objects above. We note that based on current uncertain observations of quiescent accretion rates, magnetospheric mass loading and crushing are neither firmly indicated nor ruled out in these four objects. Because quiescent accretion rates are not measured directly (only upper limits are placed), without more data it is impossible to be confident about whether the thresholds for magnetospheric mass loading or crushing are reached or not.

  3. Leidenfrost Drop on a Step

    NASA Astrophysics Data System (ADS)

    Lagubeau, Guillaume; Le Merrer, Marie; Clanet, Christophe; Quere, David

    2008-11-01

    When deposited on a hot plate, a water droplet evaporates quickly. However, a vapor film appears under the drop above a critical temperature, called Leidenfrost temperature, which insulates the drop from its substrate. Linke & al (2006) reported a spontaneous movement of such a drop, when deposited on a ratchet. We study here the case of a flat substrate decorated with a single micrometric step. The drop is deposited on the lower part of the plate and pushed towards the step at small constant velocity. If the kinetic energy of the drop is sufficient, it can climb up the step. In that case, depending on the substrate temperature, the drop can either be decelerated or accelerated by the step. We try to understand the dynamics of these drops, especially the regime where they accelerate. Taking advantage of this phenomenon, we could then build a multiple-step setup, making it possible for a Leidenfrost drop to climb stairs.

  4. Evolution of Accretion Disks in Tidal Disruption Events

    NASA Astrophysics Data System (ADS)

    Shen, Rong-Feng; Matzner, Christopher D.

    2014-04-01

    During a stellar tidal disruption event (TDE), an accretion disk forms as stellar debris returns to the disruption site and circularizes. Rather than being confined within the circularizing radius, the disk can spread to larger radii to conserve angular momentum. A spreading disk is a source of matter for re-accretion at rates that may exceed the later stellar fallback rate, although a disk wind can suppress its contribution to the central black hole accretion rate. A spreading disk is detectible through a break in the central accretion rate history or, at longer wavelengths, by its own emission. We model the evolution of TDE disk size and accretion rate by accounting for the time-dependent fallback rate, for the influence of wind losses in the early advective stage, and for the possibility of thermal instability for accretion rates intermediate between the advection-dominated and gas-pressure-dominated states. The model provides a dynamic basis for modeling TDE light curves. All or part of a young TDE disk will precess as a solid body because of the Lense-Thirring effect, and precession may manifest itself as a quasi-periodic modulation of the light curve. The precession period increases with time. Applying our results to the jetted TDE candidate Swift J1644+57, whose X-ray light curve shows numerous quasi-periodic dips, we argue that the data best fit a scenario in which a main-sequence star was fully disrupted by an intermediate mass black hole on an orbit significantly inclined from the black hole equator, with the apparent jet shutoff at t = 500 days corresponding to a disk transition from the advective state to the gas-pressure-dominated state.

  5. Evolution of accretion disks in tidal disruption events

    SciTech Connect

    Shen, Rong-Feng; Matzner, Christopher D. E-mail: matzner@astro.utoronto.ca

    2014-04-01

    During a stellar tidal disruption event (TDE), an accretion disk forms as stellar debris returns to the disruption site and circularizes. Rather than being confined within the circularizing radius, the disk can spread to larger radii to conserve angular momentum. A spreading disk is a source of matter for re-accretion at rates that may exceed the later stellar fallback rate, although a disk wind can suppress its contribution to the central black hole accretion rate. A spreading disk is detectible through a break in the central accretion rate history or, at longer wavelengths, by its own emission. We model the evolution of TDE disk size and accretion rate by accounting for the time-dependent fallback rate, for the influence of wind losses in the early advective stage, and for the possibility of thermal instability for accretion rates intermediate between the advection-dominated and gas-pressure-dominated states. The model provides a dynamic basis for modeling TDE light curves. All or part of a young TDE disk will precess as a solid body because of the Lense-Thirring effect, and precession may manifest itself as a quasi-periodic modulation of the light curve. The precession period increases with time. Applying our results to the jetted TDE candidate Swift J1644+57, whose X-ray light curve shows numerous quasi-periodic dips, we argue that the data best fit a scenario in which a main-sequence star was fully disrupted by an intermediate mass black hole on an orbit significantly inclined from the black hole equator, with the apparent jet shutoff at t = 500 days corresponding to a disk transition from the advective state to the gas-pressure-dominated state.

  6. Quasispherical subsonic accretion in X-ray pulsars

    NASA Astrophysics Data System (ADS)

    Shakura, Nikolai I.; Postnov, Konstantin A.; Kochetkova, A. Yu; Hjalmarsdotter, L.

    2013-04-01

    A theoretical model is considered for quasispherical subsonic accretion onto slowly rotating magnetized neutron stars. In this regime, the accreting matter settles down subsonically onto the rotating magnetosphere, forming an extended quasistatic shell. Angular momentum transfer in the shell occurs via large-scale convective motions resulting, for observed pulsars, in an almost iso-angular-momentum \\omega \\sim 1/R^2 rotation law inside the shell. The accretion rate through the shell is determined by the ability of the plasma to enter the magnetosphere due to Rayleigh-Taylor instabilities, with allowance for cooling. A settling accretion regime is possible for moderate accretion rates \\dot M \\lesssim \\dot M_* \\simeq 4\\times 10^{16} g s ^{-1}. At higher accretion rates, a free-fall gap above the neutron star magnetosphere appears due to rapid Compton cooling, and the accretion becomes highly nonstationary. Observations of spin-up/spin-down rates of quasispherically wind accreting equilibrium X-ray pulsars with known orbital periods (e.g., GX 301-2 and Vela X-1) enable us to determine the main dimensionless parameters of the model, as well as to estimate surface magnetic field of the neutron star. For equilibrium pulsars, the independent measurements of the neutron star magnetic field allow for an estimate of the stellar wind velocity of the optical companion without using complicated spectroscopic measurements. For nonequilibrium pulsars, a maximum value is shown to exist for the spin-down rate of the accreting neutron star. From observations of the spin-down rate and the X-ray luminosity in such pulsars (e.g., GX 1+4, SXP 1062, and 4U 2206+54), a lower limit can be put on the neutron star magnetic field, which in all cases turns out to be close to the standard value and which agrees with cyclotron line measurements. Furthermore, both explains the spin-up/spin-down of the pulsar frequency on large time-scales and also accounts for the irregular short

  7. Drop foot corrective device

    NASA Technical Reports Server (NTRS)

    Deis, B. C. (Inventor)

    1986-01-01

    A light weight, economical device to alleviate a plurality of difficulties encountered in walking by a victim suffering from a drop foot condition is discussed. A legband girdles the leg below the knee and above the calf providing an anchor point for the upper end of a ligament having its lower end attached to a toe of a shoe or a toe on the foot. The ligament is of such length that the foot is supported thereby and retained in a normal position during walking.

  8. How to Use Ear Drops

    MedlinePlus

    How to Use Ear Drops(Having someone else give you the ear drops may make this procedure easier.) Wash your hands thoroughly with soap and ... facecloth and then dry your ear. Warm the drops to near body temperature by holding the container ...

  9. Liquid drops impacting superamphiphobic coatings.

    PubMed

    Deng, Xu; Schellenberger, Frank; Papadopoulos, Periklis; Vollmer, Doris; Butt, Hans-Jürgen

    2013-06-25

    The dynamics of liquid drops impacting superamphiphobic coatings is studied by high-speed video microscopy. Superamphiphobic coatings repel water and oils. The coating consists of a fractal-like hydrophobized silica network. Mixtures of ethanol-water and glycerin-water are chosen to investigate the influence of interfacial tension and viscosity on spreading and retraction dynamics. Drop spreading is dominated by inertia. At low impact velocity, the drops completely rebound. However, the contact time increases with impact velocity, whereas the restitution coefficient decreases. We suggest that the drop temporarily impales the superamphiphobic coating, although the drop completely rebounds. From an estimate of the pressure, it can be concluded that impalement is dominated by depinning rather than sagging. With increasing velocity, the drops partially pin, and an increasing amount of liquid remains on the coating. A time-resolved study of the retraction dynamics reveals two well-separated phases: a fast inertia-dominated phase followed by a slow decrease of the contact diameter of the drop. The crossover occurs when the diameter of the retracting drop matches the diameter of the drop before impact. We suggest that the depth of impalement increases with impact velocity, where impalement is confined to the initial impact zone of the drop. If the drop partially pins on the coating, the depth of impalement exceeds a depth, preventing the whole drop from being removed during the retraction phase. PMID:23697383

  10. A Systems-Level Perspective on Engine Ice Accretion

    NASA Technical Reports Server (NTRS)

    May, Ryan D.; Guo, Ten-Huei; Simon, Donald L.

    2013-01-01

    The accretion of ice in the compression system of commercial gas turbine engines operating in high ice water content conditions is a safety issue being studied by the aviation sector. While most of the research focuses on the underlying physics of ice accretion and the meteorological conditions in which accretion can occur, a systems-level perspective on the topic lends itself to potential near-term operational improvements. This work focuses on developing an accurate and reliable algorithm for detecting the accretion of ice in the low pressure compressor of a generic 40,000 lbf thrust class engine. The algorithm uses only the two shaft speed sensors and works regardless of engine age, operating condition, and power level. In a 10,000-case Monte Carlo simulation, the detection approach was found to have excellent capability at determining ice accretion from sensor noise with detection occurring when ice blocks an average of 6.8% of the low pressure compressor area. Finally, an initial study highlights a potential mitigation strategy that uses the existing engine actuators to raise the temperature in the low pressure compressor in an effort to reduce the rate at which ice accretes.

  11. TEMPERATURE STRUCTURE OF PROTOPLANETARY DISKS UNDERGOING LAYERED ACCRETION

    SciTech Connect

    Lesniak, M. V.; Desch, S. J.

    2011-10-20

    We calculate the temperature structures of protoplanetary disks (PPDs) around T Tauri stars heated by both incident starlight and viscous dissipation. We present a new algorithm for calculating the temperatures in disks in hydrostatic and radiative equilibrium, based on Rybicki's method for iteratively calculating the vertical temperature structure within an annulus. At each iteration, the method solves for the temperature at all locations simultaneously, and converges rapidly even at high (>>10{sup 4}) optical depth. The method retains the full frequency dependence of the radiation field. We use this algorithm to study for the first time disks evolving via the magnetorotational instability. Because PPD midplanes are weakly ionized, this instability operates preferentially in their surface layers, and disks will undergo layered accretion. We find that the midplane temperatures T{sub mid} are strongly affected by the column density {Sigma}{sub a} of the active layers, even for fixed mass accretion rate M-dot . Models assuming uniform accretion predict midplane temperatures in the terrestrial planet forming region several x 10{sup 2} K higher than our layered accretion models do. For M-dot < 10{sup -7} M{sub sun} yr{sup -1} and the column densities {Sigma}{sub a} < 10 g cm{sup -2} associated with layered accretion, disk temperatures are indistinguishable from those of a passively heated disk. We find emergent spectra are insensitive to {Sigma}{sub a}, making it difficult to observationally identify disks undergoing layered versus uniform accretion.

  12. TW Hya: SPECTRAL VARIABILITY, X-RAYS, AND ACCRETION DIAGNOSTICS

    SciTech Connect

    Dupree, A. K.; Brickhouse, N. S.; Cranmer, S. R.; Luna, G. J. M.; Schneider, E. E.; Bessell, M. S.; Bonanos, A.; Crause, L. A.; Lawson, W. A.; Mallik, S. V.; Schuler, S. C.

    2012-05-01

    The nearest accreting T Tauri star, TW Hya was intensively and continuously observed over {approx}17 days with spectroscopic and photometric measurements from four continents simultaneous with a long segmented exposure using the Chandra satellite. Contemporaneous optical photometry from WASP-S indicates a 4.74 day period was present during this time. The absence of a similar periodicity in the H{alpha} flux and the total X-ray flux which are dominated by accretion processes and the stellar corona, respectively, points to a different source of photometric variations. The H{alpha} emission line appears intrinsically broad and symmetric, and both the profile and its variability suggest an origin in the post-shock cooling region. An accretion event, signaled by soft X-rays, is traced spectroscopically for the first time through the optical emission line profiles. After the accretion event, downflowing turbulent material observed in the H{alpha} and H{beta} lines is followed by He I ({lambda}5876) broadening near the photosphere. Optical veiling resulting from the heated photosphere increases with a delay of {approx}2 hr after the X-ray accretion event. The response of the stellar coronal emission to an increase in the veiling follows {approx}2.4 hr later, giving direct evidence that the stellar corona is heated in part by accretion. Subsequently, the stellar wind becomes re-established. We suggest a model that incorporates the dynamics of this sequential series of events: an accretion shock, a cooling downflow in a supersonically turbulent region, followed by photospheric and later, coronal heating. This model naturally explains the presence of broad optical and ultraviolet lines, and affects the mass accretion rates determined from emission line profiles.

  13. TW Hya: Spectral Variability, X-Rays, and Accretion Diagnostics

    NASA Astrophysics Data System (ADS)

    Dupree, A. K.; Brickhouse, N. S.; Cranmer, S. R.; Luna, G. J. M.; Schneider, E. E.; Bessell, M. S.; Bonanos, A.; Crause, L. A.; Lawson, W. A.; Mallik, S. V.; Schuler, S. C.

    2012-05-01

    The nearest accreting T Tauri star, TW Hya was intensively and continuously observed over ~17 days with spectroscopic and photometric measurements from four continents simultaneous with a long segmented exposure using the Chandra satellite. Contemporaneous optical photometry from WASP-S indicates a 4.74 day period was present during this time. The absence of a similar periodicity in the Hα flux and the total X-ray flux which are dominated by accretion processes and the stellar corona, respectively, points to a different source of photometric variations. The Hα emission line appears intrinsically broad and symmetric, and both the profile and its variability suggest an origin in the post-shock cooling region. An accretion event, signaled by soft X-rays, is traced spectroscopically for the first time through the optical emission line profiles. After the accretion event, downflowing turbulent material observed in the Hα and Hβ lines is followed by He I (λ5876) broadening near the photosphere. Optical veiling resulting from the heated photosphere increases with a delay of ~2 hr after the X-ray accretion event. The response of the stellar coronal emission to an increase in the veiling follows ~2.4 hr later, giving direct evidence that the stellar corona is heated in part by accretion. Subsequently, the stellar wind becomes re-established. We suggest a model that incorporates the dynamics of this sequential series of events: an accretion shock, a cooling downflow in a supersonically turbulent region, followed by photospheric and later, coronal heating. This model naturally explains the presence of broad optical and ultraviolet lines, and affects the mass accretion rates determined from emission line profiles.

  14. Drop Testing Representative Multi-Canister Overpacks

    SciTech Connect

    Snow, Spencer D.; Morton, Dana K.

    2015-06-01

    The objective of the work reported herein was to determine the ability of the Multi- Canister Overpack (MCO) canister design to maintain its containment boundary after an accidental drop event. Two test MCO canisters were assembled at Hanford, prepared for testing at the Idaho National Engineering and Environmental Laboratory (INEEL), drop tested at Sandia National Laboratories, and evaluated back at the INEEL. In addition to the actual testing efforts, finite element plastic analysis techniques were used to make both pre-test and post-test predictions of the test MCOs structural deformations. The completed effort has demonstrated that the canister design is capable of maintaining a 50 psig pressure boundary after drop testing. Based on helium leak testing methods, one test MCO was determined to have a leakage rate not greater than 1x10-5 std cc/sec (prior internal helium presence prevented a more rigorous test) and the remaining test MCO had a measured leakage rate less than 1x10-7 std cc/sec (i.e., a leaktight containment) after the drop test. The effort has also demonstrated the capability of finite element methods using plastic analysis techniques to accurately predict the structural deformations of canisters subjected to an accidental drop event.

  15. Accretion and canal impacts in a rapidly subsiding wetland II: Feldspar marker horizon technique

    USGS Publications Warehouse

    Cahoon, D.R.; Turner, R.E.

    1989-01-01

    Recent (6-12 months) marsh sediment accretion and accumulation rates were measured with feldspar marker horizons in the vicinity of natural waterways and man-made canals with spoil banks in the rapidly subsiding environment of coastal Louisiana. Annual accretion rates in a Spartina alterniflora salt marsh in the Mississippi deltaic plain averaged 6 mm in marsh adjacent to canals compared to 10 mm in marsh adjacent to natural waterways. The rates, however, were not statistically significantly different. The average rate of sediment accretion in the same salt marsh region for a transect perpendicular to a canal (13 mm yr-1) was significantly greater than the rate measured for a transect perpendicular to a natural waterway (7 mm yr-1). Measurements of soil bulk density and organic matter content from the two transects were also different. This spatial variability in accretion rates is probably related to (1) spoil bank influences on local hydrology; and (2) a locally high rate of sediment input from lateral erosion associated with pond enlargement. In a brackish Spatina patens marsh on Louisiana's chenier plain, vertical accretion rates were the same along natural and canal waterways (3-4 mm yr-1) in a hydrologically restricted marsh region. However, the accretion rates for both waterways were significantly lower than the rates along a nonhydrologically restricted natural waterway nearby (11 mm yr-1). The vertical accretion of matter displayed semi-annual differences in the brackish marsh environment.

  16. Hyper-Eddington accretion flows on to massive black holes

    NASA Astrophysics Data System (ADS)

    Inayoshi, Kohei; Haiman, Zoltán; Ostriker, Jeremiah P.

    2016-07-01

    We study very high rate, spherically symmetric accretion flows on to massive black holes (BHs; 102 ≲ MBH ≲ 106 M⊙) embedded in dense metal-poor clouds, performing one-dimensional radiation hydrodynamical simulations. We find solutions from outside the Bondi radius at hyper-Eddington rates, unimpeded by radiation feedback when (n∞/105 cm-3) > (MBH/104 M⊙)-1(T∞/104 K)3/2, where n∞ and T∞ are the density and temperature of ambient gas. Accretion rates in this regime are steady, and larger than 5000LEdd/c2, where LEdd is the Eddington luminosity. At lower Bondi rates, the accretion is episodic due to radiative feedback and the average rate is below the Eddington rate. In the hyper-Eddington case, the solution consists of a radiation-dominated central core, where photon trapping due to electron scattering is important, and an accreting envelope which follows a Bondi profile with T ≃ 8000 K. When the emergent luminosity is limited to ≲ LEdd because of photon trapping, radiation from the central region does not affect the gas dynamics at larger scales. We apply our result to the rapid formation of massive BHs in protogalaxies with a virial temperature of Tvir ≳ 104K. Once a seed BH forms at the centre of the galaxy, it can grow to a maximum ˜105(Tvir/104 K) M⊙ via gas accretion independent of the initial BH mass. Finally, we discuss possible observational signatures of rapidly accreting BHs with/without allowance for dust. We suggest that these systems could explain Lyα emitters without X-rays and nearby luminous infrared sources with hot dust emission, respectively.

  17. Hyper-Eddington accretion flows onto massive black holes

    NASA Astrophysics Data System (ADS)

    Inayoshi, Kohei; Haiman, Zoltán; Ostriker, Jeremiah P.

    2016-04-01

    We study very-high rate, spherically symmetric accretion flows onto massive black holes (BH; 10^2 ⪉ M_BH ⪉ 10^6~M_⊙) embedded in dense metal-poor clouds, performing one-dimensional radiation hydrodynamical simulations. We find solutions from outside the Bondi radius at hyper-Eddington rates, unimpeded by radiation feedback when (n∞/105~cm-3) > (MBH/104~M⊙)-1(T∞/104~K)3/2, where n∞ and T∞ are the density and temperature of ambient gas. Accretion rates in this regime are steady, and larger than 5000~LEdd/c2, where LEdd is the Eddington luminosity. At lower Bondi rates, the accretion is episodic due to radiative feedback and the average rate is below the Eddington rate. In the hyper-Eddington case, the solution consists of a radiation-dominated central core, where photon trapping due to electron scattering is important, and an accreting envelope which follows a Bondi profile with T ≃ 8000~K. When the emergent luminosity is limited to ⪉ L_Edd because of photon trapping, radiation from the central region does not affect the gas dynamics at larger scales. We apply our result to the rapid formation of massive BHs in protogalaxies with a virial temperature of T_vir⪆ 10^4~K. Once a seed BH forms at the center of the galaxy, it can grow to a maximum ˜105~(Tvir/104~K)~M⊙ via gas accretion independent of the initial BH mass. Finally, we discuss possible observational signatures of rapidly accreting BHs with/without allowance for dust. We suggest that these systems could explain Lyα emitters without X-rays and nearby luminous infrared sources with hot dust emission, respectively.

  18. Influence of hydrodynamic energy on Holocene reef flat accretion, Great Barrier Reef

    NASA Astrophysics Data System (ADS)

    Dechnik, Belinda; Webster, Jody M.; Nothdurft, Luke; Webb, Gregory E.; Zhao, Jian-xin; Duce, Stephanie; Braga, Juan C.; Harris, Daniel L.; Vila-Concejo, Ana; Puotinen, Marji

    2016-01-01

    The response of platform reefs to sea-level stabilization over the past 6 ka is well established for the Great Barrier Reef (GBR), with reefs typically accreting laterally from windward to leeward. However, these observations are based on few cores spread across reef zones and may not accurately reflect a reef's true accretional response to the Holocene stillstand. We present a new record of reef accretion based on 49 U/Th ages from Heron and One Tree reefs in conjunction with re-analyzed data from 14 reefs across the GBR. We demonstrate that hydrodynamic energy is the main driver of accretional direction; exposed reefs accreted primarily lagoon-ward while protected reefs accreted seawards, contrary to the traditional growth model in the GBR. Lateral accretion rates varied from 86.3 m/ka-42.4 m/ka on the exposed One Tree windward reef and 68.35 m/ka-15.7 m/ka on the protected leeward Heron reef, suggesting that wind/wave energy is not a dominant control on lateral accretion rates. This represents the most comprehensive statement of lateral accretion direction and rates from the mid-outer platform reefs of the GBR, confirming great variability in reef flat growth both within and between reef margins over the last 6 ka, and highlighting the need for closely-spaced transects.

  19. Oscillating and star-shaped drops levitated by an airflow.

    PubMed

    Bouwhuis, Wilco; Winkels, Koen G; Peters, Ivo R; Brunet, Philippe; van der Meer, Devaraj; Snoeijer, Jacco H

    2013-08-01

    We investigate the spontaneous oscillations of drops levitated above an air cushion, eventually inducing a breaking of axisymmetry and the appearance of "star drops". This is strongly reminiscent of the Leidenfrost stars that are observed for drops floating above a hot substrate. The key advantage of this work is that we inject the airflow at a constant rate below the drop, thus eliminating thermal effects and allowing for a better control of the flow rate. We perform experiments with drops of different viscosities and observe stable states, oscillations, and chimney instabilities. We find that for a given drop size the instability appears above a critical flow rate, where the latter is largest for small drops. All these observations are reproduced by numerical simulations, where we treat the drop using potential flow and the gas as a viscous lubrication layer. Qualitatively, the onset of instability agrees with the experimental results, although the typical flow rates are too large by a factor 10. Our results demonstrate that thermal effects are not important for the formation of star drops and strongly suggest a purely hydrodynamic mechanism for the formation of Leidenfrost stars. PMID:24032934

  20. Magnetic fields in primordial accretion disks

    NASA Astrophysics Data System (ADS)

    Latif, M. A.; Schleicher, D. R. G.

    2016-01-01

    Magnetic fields are considered a vital ingredient of contemporary star formation and may have been important during the formation of the first stars in the presence of an efficient amplification mechanism. Initial seed fields are provided via plasma fluctuations and are subsequently amplified by the small-scale dynamo, leading to a strong, tangled magnetic field. We explore how the magnetic field provided by the small-scale dynamo is further amplified via the α-Ω dynamo in a protostellar disk and assess its implications. For this purpose, we consider two characteristic cases, a typical Pop. III star with 10M⊙ and an accretion rate of 10-3M⊙ yr-1, and a supermassive star with 105M⊙ and an accretion rate of 10-1M⊙ yr-1. For the 10M⊙ Pop. III star, we find that coherent magnetic fields can be produced on scales of at least 100 AU, which are sufficient to drive a jet with a luminosity of 100L⊙ and a mass outflow rate of 10-3.7M⊙ yr-1. For the supermassive star, the dynamical timescales in its environment are even shorter, implying smaller orbital timescales and an efficient magnetization out to at least 1000 AU. The jet luminosity corresponds to ~106.0L⊙ and a mass outflow rate of 10-2.1M⊙ yr-1. We expect that the feedback from the supermassive star can have a relevant impact on its host galaxy.

  1. Use and abuse of crustal accretion calculations

    NASA Astrophysics Data System (ADS)

    Pallister, John S.; Cole, James C.; Stoeser, Douglas B.; Quick, James E.

    1990-01-01

    Recent attempts to calculate the average growth rate of continental crust for the Late Proterozoic shield of Arabia and Nubia are subject to large geological uncertainties, and widely contrasting conclusions result from dissimilar boundary conditions. The four greatest sources of divergence are (1) the extent of 620-920 Ma arc-terrane crust beneath Phanerozoic cover; (2) the extent of pre-920 Ma continental crust within the arc terranes; (3) the amount of postaccretion magmatic addition and erosion; and (4) the aggregate length and average life span of Late Proterozoic magmatic-arc systems that formed the Arabian-Nubian Shield. Calculations restricted to the relatively well known Arabian segment of the Arabian-Nubian Shield result in average crustal growth rates and arc accretion rates comparable to rates for modern arc systems, but we recognize substantial uncertainty in such results. Critical review of available geochemical, isotopic, and geochronological evidence contradicts the often stated notion that intact, pre-920 Ma crust is widespread in the eastern Arabian Shield. Instead, the arc terranes of the region apparently were "contaminated" with sediments derived, in part, from pre-920 Ma crust. Available geologic and radiometric data indicate that the Arabian-Nubian Shield and its "Pan-African" extensions constitute the greatest known volume of arc-accreted crust on Earth that formed in the period 920-620 Ma. Thus, the region may truly represent a disproportionate share of Earth's crustal growth budget for this time period.

  2. An experimental study of dynamics of drop formation

    SciTech Connect

    Zhang, X.; Basaran, O.A.

    1995-06-01

    A liquid being ejected from a nozzle emanates from it as discrete, uniformly sized drops when the flow rate is sufficiently low. In this paper, an experimental study is presented of the dynamics of a viscous liquid drop that is being formed directly at the tip of a vertical tube into ambient air. The evolution in time of the drop shape and volume is monitored with a time resolution of 1/12 to 1 ms. Following the detachment of the previous drop, the profile of the new growing drop at first changes from spherical to pear-shaped. As time advances, the throat of the pear-shaped drop takes on the appearance of a liquid thread that connects the bottom portion of the drop that is about to detach to the rest of the liquid that is pendant from the tube. The focus here is on probing the effects of physical and geometric parameters on the universal features of drop formation, paying special attention to the development, extension, and breakup of the liquid thread and the satellite drops that are formed subsequent to its breakup. The role of surfactants in modifying the dynamics of drop formation is also studied. The effects of finite inertial, capillary, viscous, and gravitational forces are all accounted for to classify drastically different formation dynamics and to elucidate the fate of satellite drops following thread rupture.

  3. Academic Achievement as a Drop Out Predictor.

    ERIC Educational Resources Information Center

    Tanner, David E.

    Improving graduation rates among U.S. elementary and secondary school students requires that one be able to detect which students are at greatest risk for dropping out. There may be a variety of social and psychological differences between those who leave and those who graduate, but there is also evidence that those who abandon school are…

  4. Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion

    PubMed Central

    Johansen, Anders; Low, Mordecai-Mark Mac; Lacerda, Pedro; Bizzarro, Martin

    2015-01-01

    Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas drag–assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo runaway accretion of chondrules within ~3 My, forming planetary embryos up to Mars’s size along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size sorting of chondrules consistent with chondrites. Accretion of millimeter-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disc lifetime outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles in the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts. PMID:26601169

  5. Growth of asteroids, planetary embryos, and Kuiper belt objects by chondrule accretion.

    PubMed

    Johansen, Anders; Low, Mordecai-Mark Mac; Lacerda, Pedro; Bizzarro, Martin

    2015-04-01

    Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas drag-assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo runaway accretion of chondrules within ~3 My, forming planetary embryos up to Mars's size along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size sorting of chondrules consistent with chondrites. Accretion of millimeter-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disc lifetime outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles in the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts. PMID:26601169

  6. Controlling charge on levitating drops.

    PubMed

    Hilger, Ryan T; Westphall, Michael S; Smith, Lloyd M

    2007-08-01

    Levitation technologies are used in containerless processing of materials, as microscale manipulators and reactors, and in the study of single drops and particles. Presented here is a method for controlling the amount and polarity of charge on a levitating drop. The method uses single-axis acoustic levitation to trap and levitate a single, initially neutral drop with a diameter between 400 microm and 2 mm. This drop is then charged in a controllable manner using discrete packets of charge in the form of charged drops produced by a piezoelectric drop-on-demand dispenser equipped with a charging electrode. The magnitude of the charge on the dispensed drops can be adjusted by varying the voltage applied to the charging electrode. The polarity of the charge on the added drops can be changed allowing removal of charge from the trapped drop (by neutralization) and polarity reversal. The maximum amount of added charge is limited by repulsion of like charges between the drops in the trap. This charging scheme can aid in micromanipulation and the study of charged drops and particles using levitation. PMID:17580951

  7. Disk Accretion of Tidally Disrupted Rocky Bodies onto White Dwarfs

    NASA Astrophysics Data System (ADS)

    Feng, Wanda; Desch, Steven; Turner, Neal; Kalyaan, Anusha

    2016-06-01

    About 1/3 of white dwarfs (WDs) are polluted with heavy elements (e.g., Koester et al., 2014; Zuckerman et al., 2010) that should sediment out of their atmospheres on astronomically short timescales unless replenished by accretion from a reservoir, at rates that for many WDs must exceed ~1010 g/s (Farihi et al., 2010). Direct accretion of planetesimals is too improbable and Poynting-Robertson drag of dust is too slow (due to the low luminosity of WDs) (Jura, 2003), so it is often assumed that WDs accrete from a disk of gas and solid particles, fed by tidal disruption of planeteismals inside the WD Roche limit (e.g. Debes et al., 2012; Rafikov, 2011a, 2011b). A few such gaseous disks have been directly observed, through emission from Ca II atoms in the disk (e.g. Manser et al., 2016; Wilson et al. 2014). Models successfully explain the accretion rates of metals onto the WD, provided the gaseous disk viscously spreads at rates consistent with a partially suppressed magnetorotational instability (Rafikov, 2011a, 2011b). However, these models currently do not explore the likely extent of the magnetorotational instability in disks by calculating the degree of ionization, or suppression by strong magnetic field.We present a 1-D model of a gaseous WD disk accretion, to assess the extent of the magnetorotational instability in WD disks. The composition of the disk, the ionization and recombination mechanisms, and the degree of ionization of the disk are explored. Magnetic field strengths consistent with WD dipolar magnetic fields are assumed. Elsasser numbers are calculated as a function of radius in the WD disk. The rate of viscous spreading is calculated, and the model of Rafikov (2011a, 2011b) updated to compute likely accretion rates of metals onto WDs.

  8. Classical Accreting Pulsars with NICER

    NASA Technical Reports Server (NTRS)

    Wilson-Hodge, Colleen A.

    2014-01-01

    Soft excesses are very common center dot Lx > 1038 erg/s - reprocessing by optically thick material at the inner edge of the accretion disk center dot Lx < 1036 erg/s - photoionized or collisionally heated diffuse gas or thermal emission from the NS surface center dot Lx 1037 erg/s - either or both types of emission center dot NICER observations of soft excesses in bright X-ray pulsars combined with reflection modeling will constrain the ionization state, metalicity and dynamics of the inner edge of the magnetically truncated accretion disk Reflection models of an accretion disk for a hard power law - Strong soft excess below 3 keV from hot X-ray heated disk - For weakly ionized case: strong recombination lines - Are we seeing changes in the disk ionization in 4U1626-26? 13 years of weekly monitoring with RXTE PCA center dot Revealed an unexpectedly large population of Be/X-ray binaries compared to the Milky Way center dot Plotted luminosities are typical of "normal" outbursts (once per orbit) center dot The SMC provides an excellent opportunity to study a homogenous population of HMXBs with low interstellar absorption for accretion disk studies. Monitoring with NICER will enable studies of accretion disk physics in X-ray pulsars center dot The SMC provides a potential homogeneous low-absorption population for this study center dot NICER monitoring and TOO observations will also provide measurements of spinfrequencies, QPOs, pulsed fluxes, and energy spectra.

  9. Characterizing Accreting White Dwarf Pulsators

    NASA Astrophysics Data System (ADS)

    Szkody, Paula; Mukadam, Anjum

    2014-02-01

    Understanding the population, mass distribution, and evolution of accreting white dwarfs impacts the entire realm of binary interaction, including the creation of Type Ia supernovae. We are concentrating on accreting white dwarf pulsators, as the pulsation properties allow us a view of how the accretion affects the interior of the star. Our ground- based photometry on 11 accreting pulsators with corresponding temperatures from HST UV spectra suggest a broad instability strip in the range of 10500 to 16000K. Additionally, tracking a post-outburst heated white dwarf as it cools and crosses the blue edge and resumes pulsation provides an independent method to locate the empirical instability strip. Determining a post-outburst cooling curve yields an estimate of the amount of heating and the accreted mass during the outburst. We request additional photometry of 2 objects that present unique properties: GW Lib which has not yet returned to its pre-outburst pulsation spectrum after 6 yrs, and EQ Lyn which returned to its pre- outburst pulsation after 3 yrs but is now turning on and off without ongoing outbursts. Following the pulsation spectrum changes over stretches of several nights in a row will provide specific knowledge of the stability of the observed modes.

  10. Accretion flow diagnostics with X-ray spectral timing: the hard state of SWIFT J1753.5-0127

    NASA Astrophysics Data System (ADS)

    Cassatella, P.; Uttley, P.; Maccarone, T. J.

    2012-12-01

    Recent XMM-Newton studies of X-ray variability in the hard states of black hole X-ray binaries (BHXRBs) indicate that the variability is generated in the 'standard' optically thick accretion disc that is responsible for the multi-colour blackbody emission. The variability originates in the disc as mass-accretion fluctuations and propagates through the disc to 'light up' inner disc regions, eventually modulating the power-law emission that is produced relatively centrally. Both the covariance spectra and time-lags that cover the soft bands strongly support this scenario. Here, we present a comparative spectral-timing study of XMM-Newton data from the BHXRB SWIFT J1753.5-0127 in a bright 2009 hard state with that from the significantly fainter 2006 hard state to show for the first time the change in disc spectral-timing properties associated with a global increase in both the accretion rate and the relative contribution of the disc emission to the bolometric luminosity. We show that, although there is strong evidence for intrinsic disc variability in the more luminous hard state, the disc variability amplitude is suppressed relative to that of the power-law emission, which contrasts with the behaviour at lower luminosities where the disc variability is slightly enhanced when compared with the power-law variations. Furthermore, in the higher luminosity data the disc variability below 0.6 keV becomes incoherent with the power-law and higher energy disc emission at frequencies below 0.5 Hz, in contrast with the coherent variations seen in the 2006 data. We explain these differences and the associated complex lags in the 2009 data in terms of the fluctuating disc model, where the increase in accretion rate seen in 2009 leads to more pronounced and extended disc emission. If the variable signals are generated at small radii in the disc, the variability of disc emission can be naturally suppressed by the fraction of unmodulated disc emission arising from larger radii

  11. Testing theories for longterm accretion variability in black hole X-ray binaries

    NASA Astrophysics Data System (ADS)

    Cambier, Hal J.

    Many X-ray sources are now understood to be "black hole X-ray binaries'' in which a stellar remnant black hole either tidally "squeezes'' gas off a companion star, or pulls in some fraction the companion's wind. This gas can drain inward through a dense, thin disk characterized by thermalized radiation, or a sparse and radiatively-inefficient flow, or some combination of the two. Observations at other energies often provide crucial information, but our primary tools to study accretion, especially closest to the black hole, are X-ray spectra and their time evolution. This evolution includes numerous behaviors spanning orders of magnitude in timescale and luminosity, and also hints at spatial structure since draining is generally faster at smaller radii. This includes variability at time-scales of weeks to months which remains difficult to explain despite an abundance of possible variability mechanisms since direct simulations covering the full spatial and temporal range remain impractical. After reviewing general aspects of accretion, I present both more and less familiar forms of longterm variability. Based on these, I argue the problem involves finding a physical process (or combination) that can generate repeatable yet adjustable cycles in luminosity and evolution of low and high energy spectral components, while letting the ionization instability dominate conventional outbursts. Specific models examined include: disks embedded in, and interacting with, hot, sparse flows, and another instability that quenches viscous-draining of the disk at more fundamental level. Testing these theories, alone and in combination, motivates building a very general and simplified numerical model presented here. I find that two-phase flow models still predict excessive recondensation in LMC X-3 among other problems, while the viscosity-quenching instability may account for rapid drops and slow recoveries in disk accretion rate but also likely requires diffusivity orders of magnitude

  12. Chondrule formation in the radiative accretional shock

    NASA Technical Reports Server (NTRS)

    Ruzmaikina, T. V.; Ip, W.

    1994-01-01

    The physical, mineralogical, and isotopic properties of chondrules strongly indicate that they were formed by the rapid melting and resolidification of preexisting solids composed of primitive material. The chondrule precursors were heated to temperatures of about 1800 K in short high-temperature events, followed by cooling with a rate of 10(exp 2)-10(exp 3) K/hr. A heat input of about 1500 J/g is required to heat chondrule precursors to such a temperature and melt them. Lightning discharges and flares in the solar nebula, and heating of the chondrule precursors by friction with gas decelerated in the accretional shock or in a shock (of unspecified origin) within the solar nebula, have been discussed as possible mechanisms for chondrule formation. One advantage of chondrule formation in large-scale shocks is that a lot of dust material can be processed. An accretional shock, which is produced by infalling gas of the presolar cloud when it collides with the solar nebula, belongs to this type of shock. In 1984 Wood considered the possibility of chondrule formation in the accretional shock by heating of chondrule precursors by gas drag. He concluded that the density in the accreting material is much lower than needed to melt silicates at the distance of the asteroid belt if the accreting matter had the cosmic ratio of dust to gas, and the mass of the solar nebula did not exceed 2 solar mass units. Melting of chondrule precursors is difficult because of their effective cooling by thermal radiation. Suppression of the radiative cooling of individual grains in dust swarms, which are opaque to thermal emission, was considered to be the only possible means of chondrule formation in solar nebula shocks. Previous models of solid grain melting in solar nebula shocks have neglected gas cooling behind the shock front, i.e., they considered adiabatic shocks. In this paper we show that large dust grains could be heated much stronger than was supposed by these authors, because of

  13. Winds and accretion in delta Sagittae

    NASA Astrophysics Data System (ADS)

    Eaton, Joel A.; Hartkopf, William I.; McAlister, Harold A.; Mason, Brian D.

    1995-04-01

    The ten-year binary delta Sge (M2 Ib-II+B9.5 V) is a zeta Aur binary containing an abnormally cool component. Combining our analysis of the system as a visual binary with Batten's radial-velocity solution leads to the following properties: i = 40 deg, a = 51 mas = 8.83 A.U. = 1893 solar radius, hence d = 173 pc; MB = 2.9 solar mass and MM = 3.8 solar mass; and RB = 2.6 solar radius and RM = 152 solar radius. This interpretation of the orbit places the M supergiant on the asymptotic giant branch. We have collected ultraviolet spectra throughout the star's 1980-90 orbit, concentrated around the conjuction of 1990. The wind of the M giant appears in these as narrow shell lines of singly ionized metals, chiefly Fe II, with P-Cyg profiles at many phases, which show the slow variation in strength expected for the orbit but no pronounced atmospheric eclipse. The terminal velocity of the wind is 16-18 km/s, and its excitation temperature is approximately 10,000 K. Most of the broadening of the wind lines is caused by differential expansion of the atmosphere, with (unmeasurably) low turbulent velocities. Nontheless, the mass loss rate (1.1 +/- 0.4 X 10 -8 solar mas/yr) is almost the same as found previously by Reimers and Schroder for very different assumptions about the velocity structure. Also seen in the spectrum throughout the orbit are the effects of a variable, high-speed wind as well as evidence for accretion onto the B9.5 star. This high-speed wind absorbs in species of all ionization stages observed, e. g., C II, Mg II, Al III, SI IV, C IV, and has a terminaal velocity in the range 200-450 km/s. We presume this wind originates at the B dwarf, not the M supergiant, and speculate that it comes from an accretion disk, as suggested by recent models of magnetically moderated accretion. Evidence for accretion is redshifted absorption in the same transitions formed in the high-speed wind, as well as broad emission lines of singly ionized metals. This emission seems to be

  14. Winds and accretion in delta Sagittae

    NASA Technical Reports Server (NTRS)

    Eaton, Joel A.; Hartkopf, William I.; Mcalister, Harold A.; Mason, Brian D.

    1995-01-01

    The ten-year binary delta Sge (M2 Ib-II+B9.5 V) is a zeta Aur binary containing an abnormally cool component. Combining our analysis of the system as a visual binary with Batten's radial-velocity solution leads to the following properties: i = 40 deg, a = 51 mas = 8.83 A.U. = 1893 solar radius, hence d = 173 pc; M(sub B) = 2.9 solar mass and M(sub M) = 3.8 solar mass; and R(sub B) = 2.6 solar radius and R(sub M) = 152 solar radius. This interpretation of the orbit places the M supergiant on the asymptotic giant branch. We have collected ultraviolet spectra throughout the star's 1980-90 orbit, concentrated around the conjuction of 1990. The wind of the M giant appears in these as narrow shell lines of singly ionized metals, chiefly Fe II, with P-Cyg profiles at many phases, which show the slow variation in strength expected for the orbit but no pronounced atmospheric eclipse. The terminal velocity of the wind is 16-18 km/s, and its excitation temperature is approximately 10,000 K. Most of the broadening of the wind lines is caused by differential expansion of the atmosphere, with (unmeasurably) low turbulent velocities. Nontheless, the mass loss rate (1.1 +/- 0.4 X 10 (exp -8) solar mas/yr) is almost the same as found previously by Reimers and Schroder for very different assumptions about the velocity structure. Also seen in the spectrum throughout the orbit are the effects of a variable, high-speed wind as well as evidence for accretion onto the B9.5 star. This high-speed wind absorbs in species of all ionization stages observed, e. g., C II, Mg II, Al III, SI IV, C IV, and has a terminaal velocity in the range 200-450 km/s. We presume this wind originates at the B dwarf, not the M supergiant, and speculate that it comes from an accretion disk, as suggested by recent models of magnetically moderated accretion. Evidence for accretion is redshifted absorption in the same transitions formed in the high-speed wind, as well as broad emission lines of singly ionized

  15. Lessons from accretion disks in cataclysmic variables

    NASA Astrophysics Data System (ADS)

    Horne, Keith

    1998-04-01

    We survey recent progress in the interpretation of observations of cataclysmic variables, whose accretion disks are heated by viscous dissipation rather than irradiation. Many features of standard viscous accretion disk models are confirmed by tomographic imaging studies of dwarf novae. Eclipse maps indicate that steady disk temperature structures are established during outbursts. Doppler maps of double-peaked emission lines suggest disk chromospheres heated by magnetic activity. Gas streams impacting on the disk rim leave expected signatures both in the eclipses and emission lines. Doppler maps of dwarf nova IP Peg at the beginning of an outburst show evidence for tidally-induced spiral shocks. While enjoying these successes, we must still face up to the dreaded ``SW Sex syndrome'' which afflicts most if not all cataclysmic variables in high accretion states. The anomalies include single-peaked emission lines with skewed kinematics, flat temperature-radius profiles, shallow offset line eclipses, and narrow low-ionization absorption lines at phase 0.5. The enigmatic behavior of AE Aqr is now largely understood in terms of a magnetic propeller model in which the rapidly spinning white dwarf magnetosphere expels the gas stream out of the system before an accretion disk can form. A final piece in this puzzle is the realization that an internal shock zone occurs in the exit stream at just the right place to explain the anomalous kinematics and violent flaring of the single-peaked emission lines. Encouraged by this success, we propose that disk-anchored magnetic propellers operate in the high accretion rate systems afflicted by the SW Sex syndrome. Magnetic fields anchored in the Keplerian disk sweep forward and apply a boost that expels gas stream material flowing above the disk plane. This working hypothesis offers a framework on which we can hang all the SW Sex anomalies. The lesson for theorists is that magnetic links appear to be transporting energy and angular

  16. Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni

    NASA Astrophysics Data System (ADS)

    Muñoz-Darias, T.; Casares, J.; Mata Sánchez, D.; Fender, R. P.; Armas Padilla, M.; Linares, M.; Ponti, G.; Charles, P. A.; Mooley, K. P.; Rodriguez, J.

    2016-06-01

    Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10‑8 solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes.

  17. Regulation of black-hole accretion by a disk wind during a violent outburst of V404 Cygni.

    PubMed

    Muñoz-Darias, T; Casares, J; Mata Sánchez, D; Fender, R P; Armas Padilla, M; Linares, M; Ponti, G; Charles, P A; Mooley, K P; Rodriguez, J

    2016-06-01

    Accretion of matter onto black holes is universally associated with strong radiative feedback and powerful outflows. In particular, black-hole transients have outflows whose properties are strongly coupled to those of the accretion flow. This includes X-ray winds of ionized material, expelled from the accretion disk encircling the black hole, and collimated radio jets. Very recently, a distinct optical variability pattern has been reported in the transient stellar-mass black hole V404 Cygni, and interpreted as disrupted mass flow into the inner regions of its large accretion disk. Here we report observations of a sustained outer accretion disk wind in V404 Cyg, which is unlike any seen hitherto. We find that the outflowing wind is neutral, has a large covering factor, expands at one per cent of the speed of light and triggers a nebular phase once accretion drops sharply and the ejecta become optically thin. The large expelled mass (>10(-8) solar masses) indicates that the outburst was prematurely ended when a sizeable fraction of the outer disk was depleted by the wind, detaching the inner regions from the rest of the disk. The luminous, but brief, accretion phases shown by transients with large accretion disks imply that this outflow is probably a fundamental ingredient in regulating mass accretion onto black holes. PMID:27251277

  18. Drop tube research

    NASA Technical Reports Server (NTRS)

    Workman, Gary L.

    1992-01-01

    This report covers the activities performed in the Drop Tube Study which The University of Alabama in Huntsville designed, fabricated and performed various low gravity experiments in materials processing from November 1, 1991 through October 30, 1992. During the performance of this contract the utilization of these ground-based containerless processing facilities has been instrumental in providing the opportunity to determine the feasibility of performing a number of solidification experiments in a simulated space environment, without the expense of a space-based experiment. A number of periodic reports have been given to the TCOR during the course of this contract hence this final report is meant only to summarize the many activities performed and not redundantly cover materials already submitted.

  19. The Influence of Outflow in Supercritical Accretion Flows

    NASA Astrophysics Data System (ADS)

    Zahra Zeraatgari, Fatemeh; Abbassi, Shahram; Mosallanezhad, Amin

    2016-06-01

    We solve the radiation-hydrodynamic equations of supercritical accretion flows in the presence of radiation force and outflow by using self-similar solutions. Similar to the pioneering works, in this paper we consider a power-law function for mass inflow rate as \\dot{M}\\propto {r}s. We found that s = 1 when the radiative cooling term is included in the energy equation. Correspondingly, the effective temperature profile with respect to the radius was obtained as {T}{eff}\\propto {r}-1/2. In addition, we investigated the influence of the outflow on the dynamics of the accretion flow. We also calculated the continuum spectrum emitted from the disk surface as well as the bolometric luminosity of the accretion flow. Furthermore, our results show that the advection parameter, f, depends strongly on mass inflow rate.

  20. Supercooled large drop detection with NASA's Icing Remote Sensing System

    NASA Astrophysics Data System (ADS)

    Serke, David J.; Reehorst, Andrew L.; Politovich, Marcia K.

    2010-10-01

    In-flight icing occurs when aircraft impact supercooled liquid drops. The supercooled liquid freezes on contact and the accreted ice changes a plane's aerodynamic characteristics, which can lead to dangerous loss of control. NASA's Icing Remote Sensing System consists of a multi-channel radiometer, a laser ceilometer and a vertically-pointing Kaband radar, whos fields are merged with internal software logic to arrive at a hazard classification for in-flight icing. The radiometer is used to derive atmospheric temperature soundings and integrated liquid water and the ceilometer and radar are used to define cloud boundaries. The integrated liquid is then distributed within the determined cloud boundaries and layers to arrive at liquid water content profiles, which if present below freezing are categorized as icing hazards. This work outlines how the derived liquid water content and measured Ka-band reflectivity factor profiles can be used to derive a vertical profile of radar-estimated particle size. This is only possible because NASA's system arrives at independent and non-correlated measures of liquid water and reflectivity factor for a given range volume. The size of the drops significantly effect the drop collection efficiency and the location that icing accretion occurs on the craft's superstructure and thus how a vehicle's performance is altered. Large drops, generally defined as over 50 μm in diameter, tend to accrete behind the normal ice protected areas of the leading edge of the wing and other control surfaces. The NASA Icing Remote Sensing System was operated near Montreal, Canada for the Alliance Icing Research Study II in 2003 and near Cleveland, Ohio from 2006 onward. In this study, we present case studies to show how NASA's Icing Remote Sensing System can detect and differentiate between no icing, small drop and large drop in-flight icing hazards to aircraft. This new product provides crucial realtime hazard detection capabilities which improve

  1. Black hole accretion disc impacts

    NASA Astrophysics Data System (ADS)

    Pihajoki, P.

    2016-04-01

    We present an analytic model for computing the luminosity and spectral evolution of flares caused by a supermassive black hole impacting the accretion disc of another supermassive black hole. Our model includes photon diffusion, emission from optically thin regions and relativistic corrections to the observed spectrum and time-scales. We test the observability of the impact scenario with a simulated population of quasars hosting supermassive black hole binaries. The results indicate that for a moderate binary mass ratio of 0.3, and impact distances of 100 primary Schwarzschild radii, the accretion disc impacts can be expected to equal or exceed the host quasar in brightness at observed wavelength λ = 510 nm up to z = 0.6. We conclude that accretion disc impacts may function as an independent probe for supermassive black hole binaries. We release the code used for computing the model light curves to the community.

  2. An evaporation model of multicomponent solution drops

    NASA Astrophysics Data System (ADS)

    Sartori, Silvana; Liñán, Amable; Lasheras, Juan C.

    2010-11-01

    Solutions of polymers are widely used in the pharmaceutical industry as tablets coatings. These allow controlling the rate at which the drug is delivered, taste or appearance. The coating is performed by spraying and drying the tablets at moderate temperatures. The wetting of the coating solution on the pill's surface depends on the droplet Webber and Re numbers, angle of impact and on the rheological properties of the droplet. We present a model for the evaporation of multicomponent solutions droplets in a hot air environment with temperatures substantially lower than the boiling temperature of the solvent. As the liquid vaporizes from the surface the fluid in the drop increases in concentration, until reaching its saturation point. After saturation, precipitation occurs uniformly within the drop. As the surface regresses, a compacting front formed by the precipitate at its maximum packing density advances into the drop, while the solute continues precipitating uniformly. This porous shell grows fast due to the double effect of surface regression and precipitation. The evaporation rate is determined by the rates at which heat is transported to the droplet surface and at which liquid vapor diffuses away from it. When the drop is fully compacted, the evaporation is drastically reduced.

  3. Ubiquitous equatorial accretion disc winds in black hole soft states

    NASA Astrophysics Data System (ADS)

    Ponti, G.; Fender, R. P.; Begelman, M. C.; Dunn, R. J. H.; Neilsen, J.; Coriat, M.

    2012-05-01

    High-resolution spectra of Galactic black holes (GBHs) reveal the presence of highly ionized absorbers. In one GBH, accreting close to the Eddington limit for more than a decade, a powerful accretion disc wind is observed to be present in softer X-ray states and it has been suggested that it can carry away enough mass and energy to quench the radio jet. Here we report that these winds, which may have mass outflow rates of the order of the inner accretion rate or higher, are a ubiquitous component of the jet-free soft states of all GBHs. We furthermore demonstrate that these winds have an equatorial geometry with opening angles of few tens of degrees, and so are only observed in sources in which the disc is inclined at a large angle to the line of sight. The decrease in Fe XXV/Fe XXVI line ratio with Compton temperature, observed in the soft state, suggests a link between higher wind ionization and harder spectral shapes. Although the physical interaction between the wind, accretion flow and jet is still not fully understood, the mass flux and power of these winds and their presence ubiquitously during the soft X-ray states suggest they are fundamental components of the accretion phenomenon.

  4. Accretion disk emission from a BL Lacertae object

    NASA Technical Reports Server (NTRS)

    Wandel, Amri; Urry, C. Megan

    1991-01-01

    It is suggested here that the UV and X-ray emission of BL Lac objects may originate in an accretion disk. Using detailed calculations of accretion disk spectra, the best-measured ultraviolet and soft X-ray spectra of the BL Lac object PKS 2155-304 are fitted, and the mass and accretion rate required is determined. The ultraviolet through soft X-ray continuum is well fitted by the spectrum of an accretion disk, but near-Eddington accretion rates are required to produce the soft X-ray excess. A hot disk or corona could Comptonize soft photons from the cool disk and produce the observed power-law spectrum in the 1-10 keV range. The dynamic time scale in the disk regions that contribute most of the observed ultraviolet and soft X-ray photons are consistent with the respective time scales for intensity variations observed in these two wave bands; the mass derived from fitting the continuum spectrum is consistent with the limit derived from the fastest hard X-ray variability.

  5. Evolution of Accreting White Dwarfs: Some of Them Continue to Grow

    NASA Astrophysics Data System (ADS)

    Newsham, G.; Starrfield, S.; Timmes, F. X.

    2014-12-01

    Novae are cataclysmic variable binary systems in which a white dwarf (WD) primary is accreting material from a low mass companion. The importance of this accretion takes on added significance if the WD can increase its mass to reach the Chandrasekhar limit thus exploding as a Type Ia supernova. In this study we accrete material of Solar composition onto carbon/oxygen (CO) WDs of 0.70, 1.00 and 1.35 M⊙ with accretion rates from 1.6×10-10 to 1.6×10-6 M⊙ yr-1. We have utilized the MESA stellar evolution code for our modeling and evolve them for many nova cycles or, in some cases, evolution to a red giant stage. Differing behaviors occur as a function of both the WD mass and the accretion rate. For the lower WD masses, the models undergo recurrent hydrogen flashes at low accretion rates; for higher accretion rates, steady-burning of hydrogen occurs and eventually gives way to recurrent hydrogen flashes. At the highest accretion rates, these models go through a steady-burning phase but eventually transition into red giants. For the highest WD mass recurrent hydrogen flashes occur at lower accretion rates but for higher rates the models exhibit steady-burning interspersed with helium flashes. We find that for all our models that undergo recurrent hydrogen flashes, as well as the steady-burning models that exhibit helium flashes, the mass of the WD continues to grow toward the Chandrasekhar limit. These results suggest that the accretion of Solar abundance material onto CO WDs in cataclysmic variable systems, the single degenerate scenario, is a viable channel for progenitors of Type Ia supernova explosions.

  6. The interplay of sedimentation and carbon accretion in riparian forests

    NASA Astrophysics Data System (ADS)

    Rieger, Isaak; Lang, Friederike; Kowarik, Ingo; Cierjacks, Arne

    2014-06-01

    Sediment trapping and organic carbon (OC) accretion in soil are crucial ecosystem services of floodplain forests. However, interactions between the two processes have scarcely been analyzed at the ecosystem level. This study aimed at quantifying OC accretion parameters (CAP, including sedimentation rate, OC concentration, OC accretion) over roughly the last 50 years on both sides of a dike in a Danubian floodplain forest in Austria. Additionally, we determined soil OC stocks (0-100 cm in depth) and modeled both CAP and OC stocks in relation to environmental parameters. Overall, mean sedimentation rate and OC accretion of the riparian forest were 0.8 cm y- 1 and 3.3 t OC ha- 1 y- 1 and significantly higher in flooded riparian forest (FRF; 1.0 cm y- 1 and 4.1 t OC ha- 1 y- 1) than in diked riparian forest (DRF; 0.3 cm y- 1 and 1.5 t OC ha- 1 y- 1). In contrast, mean OC concentration (0.05 t OC m- 3) and OC stocks (238 t OC ha- 1) were significantly higher in the DRF than in FRF (0.05 vs. 0.04 t OC m- 3 and 286 vs. 201 t OC ha- 1). Modeling revealed tree species, fluctuation of groundwater table, and the distance to the river as valuable indicators for OC accretion rate. The OC concentration and distance to the river were positively and sedimentation negatively correlated with OC stock. The dike was consistently ruled out as a significant predictor variable. Consequently, differences among FRF and DRF seem to be related rather to longer term processes during the last centuries than directly to the dike. Our findings highlight the relevance of sediment quality (i.e., OC concentration) for building up long-term soil OC stocks, whereas sediment quantity is the main driver of recent OC accretion rates.

  7. First drop dissimilarity in drop-on-demand inkjet devices

    SciTech Connect

    Famili, Amin; Palkar, Saurabh A.; Baldy, William J. Jr.

    2011-01-15

    As inkjet printing technology is increasingly applied in a broader array of applications, careful characterization of its method of use is critical due to its inherent sensitivity. A common operational mode in inkjet technology known as drop-on-demand ejection is used as a way to deliver a controlled quantity of material to a precise location on a target. This method of operation allows ejection of individual or a sequence (burst) of drops based on a timed trigger event. This work presents an examination of sequences of drops as they are ejected, indicating a number of phenomena that must be considered when designing a drop-on-demand inkjet system. These phenomena appear to be driven by differences between the first ejected drop in a burst and those that follow it and result in a break-down of the linear relationship expected between driving amplitude and drop mass. This first drop, as quantified by high-speed videography and subsequent image analysis, can be different in morphology, trajectory, velocity, and volume from subsequent drops within a burst. These findings were confirmed orthogonally by both volume and mass measurement techniques which allowed quantitation down to single drops.

  8. Optofluidic ultrahigh-throughput detection of fluorescent drops.

    PubMed

    Kim, Minkyu; Pan, Ming; Gai, Ya; Pang, Shuo; Han, Chao; Yang, Changhuei; Tang, Sindy K Y

    2015-03-21

    This paper describes an optofluidic droplet interrogation device capable of counting fluorescent drops at a throughput of 254,000 drops per second. To our knowledge, this rate is the highest interrogation rate published thus far. Our device consists of 16 parallel microfluidic channels bonded directly to a filter-coated two-dimensional Complementary Metal-Oxide-Semiconductor (CMOS) sensor array. Fluorescence signals emitted from the drops are collected by the sensor that forms the bottom of the channel. The proximity of the drops to the sensor facilitates efficient collection of fluorescence emission from the drops, and overcomes the trade-off between light collection efficiency and field of view in conventional microscopy. The interrogation rate of our device is currently limited by the acquisition speed of CMOS sensor, and is expected to increase further as high-speed sensors become increasingly available. PMID:25588522

  9. On the growth of pebble-accreting planetesimals

    NASA Astrophysics Data System (ADS)

    Visser, Rico G.; Ormel, Chris W.

    2016-02-01

    Context. Pebble accretion is a newly discovered mechanism to quickly grow the cores of planets. In pebble accretion, gravity and gas drag conspire to yield large collisional cross sections for small particles in protoplanetary disks. However, before pebble accretion commences, aerodynamic deflection may act to prevent planetesimals from becoming large, because particles tend to follow gas streamlines. Aims: We derive the planetesimal radius where pebble accretion is initiated and determine the growth timescales of planetesimals by sweep-up of small particles. Methods: The equation of motion for a pebble, including gas drag and gravitational interactions, was integrated in three dimensions at distances of 1, 3, and 10 AU from the star. We obtained the collision efficiency factor as the ratio of the numerically obtained collisional cross section to the planetesimal surface area, from which we obtained the growth timescales. Integrations were conducted in the potential flow limit (steady, inviscid) and in the Stokes flow regime (steady, viscid). Results: Only particles of stopping time ts ≪ tX where tX ≈ 103 s experience aerodynamic deflection. Even in this case, the planetesimal's gravity always ensures positive collision factors. The planetesimal radius where growth proceeds slowest is ≈ 100 km (less for colder disks) corresponding to interactions shifting from the geometric to the Safronov focusing regime. For particles ts ≫ tX pebble accretion only commences after this phase and is characterized by a steep drop in growth timescales. At 1 AU, growth timescales are shorter than the disk lifetime for pebbles larger than 0.03 cm. The planetesimal radius RPA where pebble accretion commences increases with disk orbital radius. At distances beyond ~ 10 AU, sweep-up growth times are always longer than 10 Myr, while in the inner disk (≲3 AU) the viability of the sweep-up scenario is determined by the outcome of pebble-planetesimal collisions in the geometric

  10. Student Drop-Out from German Higher Education Institutions

    ERIC Educational Resources Information Center

    Heublein, Ulrich

    2014-01-01

    28% of students of any one year currently give up their studies in bachelor degree programmes at German higher education institutions. Drop-out is to be understood as the definite termination in the higher education system without obtaining an academic degree. The drop-out rate is thereby calculated with the help of statistical estimation…

  11. Mass Outflows from Dissipative Shocks in Hot Accretion Flows

    NASA Astrophysics Data System (ADS)

    Fukumura, Keigo; Kazanas, Demosthenes

    2007-11-01

    We consider stationary, axisymmetric hydrodynamic accretion flows in Kerr geometry. As a plausible means of efficiently separating a small population of nonthermal particles from the bulk accretion flows, we investigate the formation of standing dissipative shocks, i.e., shocks at which fraction of the energy, angular momentum, and mass fluxes do not participate in the shock transition of the flow that accretes onto the compact object but are lost into collimated (jets) or uncollimated (winds) outflows. The mass-loss fraction (at a shock front) is found to vary over a wide range (0%-95%), depending on flow's angular momentum and energy. On the other hand, the associated energy-loss fraction appears to be relatively low (<~1%) for a flow onto a nonrotating black hole case, whereas the fraction could be an order of magnitude higher (<~10%) for a flow onto a rapidly rotating black hole. By estimating the escape velocity of the outflowing particles with a mass-accretion rate relevant for typical active galactic nuclei, we find that nearly 10% of the accreting mass could escape to form an outflow in a disk around a nonrotating black hole, while as much as 50% of the matter may contribute to outflows in a disk around a rapidly rotating black hole. In the context of disk-jet paradigm, our model suggests that shock-driven outflows from accretion can occur in regions not too far from a central engine. Our results imply that a shock front under some conditions could serve as a plausible site where (nonthermal) seed particles of the outflows (jets/winds) are efficiently decoupled from bulk accretion.

  12. Star Formation in Massive Clusters via Bondi Accretion

    NASA Astrophysics Data System (ADS)

    Murray, Norman; Chang, Philip

    2012-02-01

    Essentially all stars form in giant molecular clouds (GMCs). However, inside GMCs, most of the gas does not participate in star formation; rather, denser gas accumulates in clumps in the GMC, with the bulk of the stars in a given GMC forming in a few of the most massive clumps. In the Milky Way, these clumps have masses M cl <~ 5 × 10-2 of the GMC, radii r cl ~ 1 pc, and free-fall times τcl ~ 2 × 105 yr. We show that clumps inside GMCs should accrete at a modified Bondi accretion rate, which depends on clump mass as \\dot{M}_{cl}\\sim M_{cl}^{5/4}. This rate is initially rather slow, usually slower than the initial star formation rate inside the clump (we adopt the common assumption that inside the clump, \\dot{M}_*=\\epsilon _ffM_{cl}/\\tau _{cl}, with epsilonff ≈ 0.017). However, after ~2 GMC free-fall times τGMC, the clump accretion rate accelerates rapidly; formally, the clump can accrete the entire GMC in ~3τGMC. At the same time, the star formation rate accelerates, tracking the Bondi accretion rate. If the GMC is disrupted by feedback from the largest clump, half the stars in that clump form in the final τGMC before the GMC is disrupted. The theory predicts that the distribution of effective star formation rates, measured per GMC free-fall time, is broad, ranging from ~0.001 up to 0.1 or larger and that the mass spectrum of star clusters is flatter than that of clumps, consistent with observations.

  13. Elemental Fractionation During Rapid Accretion of the Moon Triggered by a Giant Impact

    NASA Technical Reports Server (NTRS)

    Abe, Y.; Zahnle, K. J.; Hashimoto, A.

    1998-01-01

    . Viewed globally, the accretional energy is about half the energy required to vaporize the entire Moon. Thus to first approximation, half of the Moon-forming material can be vaporized and lost during accretion. During this process, we would expect preferential loss of relatively volatile elements. Escape will retard the rate of accretion. To test these ideas, we computed detailed models of the thermal state of the Moon during accretion. We pay special attention to the structure of the silicate atmosphere and its loss rate by calculating the chemical species at equilibrium. We used the PHEQ program which includes 12 elements (H,O,C,Mg,Si,Fe,Ca, Al, Na,Ti, and N.) and 272 compounds (including ionic compounds). Because of the large heats of vaporization and ionization, the adiabatic atmosphere is nearly isothermal and massive escape is expected. The pressure of the atmosphere is determined by the balance between vaporization of a accreting material and escape. If the accretion time is one month, a 0.3 bar atmosphere is expected. Elemental fractionation depends strongly on the temperature of the accreting material. The initial temperature of the material can be estimated from the condition of gravitational instability in the protolunar disk. As shown by Ida et al, accretion starts when gravitational instability occurs when more than 99% of the material condenses. At this point, all of Ca, Al, Si, Mg, and Fe, and 95% of Na (probably K also), are in condensed phases. If the moon is formed from the accretion of such material, volatile elements such as Na, and K are retained by the moon only early in accretion. At later times, K and Na are lost and a fraction of the MG, Si and Fe is lost. However, refractory elements such as Ca and Al are retained and so achieve a mild degree (factor 2) of superabundance.

  14. Large amplitude drop shape oscillations

    NASA Technical Reports Server (NTRS)

    Trinh, E. H.; Wang, T. G.

    1982-01-01

    An experimental study of large amplitude drop shape oscillation was conducted in immiscible liquids systems and with levitated free liquid drops in air. In liquid-liquid systems the results indicate the existence of familiar characteristics of nonlinear phenomena. The resonance frequency of the fundamental quadrupole mode of stationary, low viscosity Silicone oil drops acoustically levitated in water falls to noticeably low values as the amplitude of oscillation is increased. A typical, experimentally determined relative frequency decrease of a 0.5 cubic centimeters drop would be about 10% when the maximum deformed shape is characterized by a major to minor axial ratio of 1.9. On the other hand, no change in the fundamental mode frequency could be detected for 1 mm drops levitated in air. The experimental data for the decay constant of the quadrupole mode of drops immersed in a liquid host indicate a slight increase for larger oscillation amplitudes. A qualitative investigation of the internal fluid flows for such drops revealed the existence of steady internal circulation within drops oscillating in the fundamental and higher modes. The flow field configuration in the outer host liquid is also significantly altered when the drop oscillation amplitude becomes large.

  15. Stellar X-ray accretion signatures

    NASA Astrophysics Data System (ADS)

    Schneider, C.; Guenther, M.

    2016-06-01

    Accretion is observed in a wide range objects with partially overlapping properties. In this contribution, we study accretion in young stars, where we can directly observe the accretion shock on the stellar surface in the X-ray regime. High-resolution grating spectroscopy allows us to infer the properties of the accretion streams. I will present results from our recent 250 ks XMM-Newton/Chandra program targeting the prototypical T Tau system such as strong X-ray variability despite constant mass accretion, abundances typical for accreting stars, but line ratios typically not found in accreting stars. Finally, I will compare these results with other systems focusing on potentially different accretion modes.

  16. ACCRETION OF ROCKY PLANETS BY HOT JUPITERS

    SciTech Connect

    Ketchum, Jacob A.; Adams, Fred C.; Bloch, Anthony M.

    2011-11-01

    The observed population of Hot Jupiters displays a stunning variety of physical properties, including a wide range of densities and core sizes for a given planetary mass. Motivated by the observational sample, this Letter studies the accretion of rocky planets by Hot Jupiters, after the Jovian planets have finished their principal migration epoch and become parked in {approx}4 day orbits. In this scenario, rocky planets form later and then migrate inward due to torques from the remaining circumstellar disk, which also damps the orbital eccentricity. This mechanism thus represents one possible channel for increasing the core masses and metallicities of Hot Jupiters. This Letter determines probabilities for the possible end states for the rocky planet: collisions with the Jovian planets, accretion onto the star, ejection from the system, and long-term survival of both planets. These probabilities depend on the mass of the Jovian planet and its starting orbital eccentricity, as well as the eccentricity damping rate for the rocky planet. Since these systems are highly chaotic, a large ensemble (N {approx} 10{sup 3}) of simulations with effectively equivalent starting conditions is required. Planetary collisions are common when the eccentricity damping rate is sufficiently low, but are rare otherwise. For systems that experience planetary collisions, this work determines the distributions of impact velocities-both speeds and impact parameters-for the collisions. These velocity distributions help determine the consequences of the impacts, e.g., where energy and heavy elements are deposited within the giant planets.

  17. Sliding viscoelastic drops on slippery surfaces

    NASA Astrophysics Data System (ADS)

    Xu, H.; Clarke, A.; Rothstein, J. P.; Poole, R. J.

    2016-06-01

    We study the sliding of drops of constant-viscosity dilute elastic liquids (Boger fluids) on various surfaces caused by sudden surface inclination. For smooth or roughened hydrophilic surfaces, such as glass or acrylic, there is essentially no difference between these elastic liquids and a Newtonian comparator fluid (with identical shear viscosity, surface tension, and static contact angle). In contrast for embossed polytetrafluoroethylene superhydrophobic surfaces, profound differences are observed: the elastic drops slide at a significantly reduced rate and complex branch-like patterns are left on the surface by the drop's wake including, on various scales, beads-on-a-string like phenomena. Microscopy images indicate that the strong viscoelastic effect is caused by stretching filaments of fluid from isolated islands, residing at pinning sites on the surface pillars, of the order ˜30 μm in size. On this scale, the local strain rates are sufficient to extend the polymer chains, locally increasing the extensional viscosity of the solution, retarding the drop and leaving behind striking branch-like structures on much larger scales.

  18. The dropped big toe.

    PubMed

    Satku, K; Wee, J T; Kumar, V P; Ong, B; Pho, R W

    1992-03-01

    Surgical procedures for exposure of the upper third of the fibula have been known to cause weakness of the long extensor of the big toe post-operatively. The authors present three representative cases of surgically induced dropped big toe. From cadaveric dissection, an anatomic basis was found for this phenomenon. The tibialis anterior and extensor digitorum longus muscles have their origin at the proximal end of the leg and receive their first motor innervation from a branch that arises from the common peroneal or deep peroneal nerve at about the level of the neck of the fibula. However, the extensor hallucis longus muscle originates in the middle one-third of the leg and the nerves innervating this muscle run a long course in close proximity to the fibula for up to ten centimeters from a level below the neck of the fibula before entering the muscle. Surgical intervention in the proximal one-third of the fibula just distal to the origin of the first motor branch to the tibialis anterior and extensor digitorum longus muscles carries a risk of injury to the nerves innervating the extensor hallucis longus. PMID:1519891

  19. Viscosity in spherically symmetric accretion

    NASA Astrophysics Data System (ADS)

    Ray, Arnab K.

    2003-10-01

    The influence of viscosity on the flow behaviour in spherically symmetric accretion has been studied here. The governing equation chosen has been the Navier-Stokes equation. It has been found that at least for the transonic solution, viscosity acts as a mechanism that detracts from the effectiveness of gravity. This has been conjectured to set up a limiting scale of length for gravity to bring about accretion, and the physical interpretation of such a length scale has been compared with the conventional understanding of the so-called `accretion radius' for spherically symmetric accretion. For a perturbative presence of viscosity, it has also been pointed out that the critical points for inflows and outflows are not identical, which is a consequence of the fact that under the Navier-Stokes prescription, there is a breakdown of the invariance of the stationary inflow and outflow solutions - an invariance that holds good under inviscid conditions. For inflows, the critical point gets shifted deeper within the gravitational potential well. Finally, a linear stability analysis of the stationary inflow solutions, under the influence of a perturbation that is in the nature of a standing wave, has indicated that the presence of viscosity induces greater stability in the system than has been seen for the case of inviscid spherically symmetric inflows.

  20. Theoretical Researches on Hot Accretion Flows around Black Holes

    NASA Astrophysics Data System (ADS)

    Xie, F. G.

    2010-10-01

    Black hole accretion systems, which are widely believed to be harbored in the central regions of active galactic nuclei (AGNs), low-luminosity AGNs (LLAGNs) as well as some X-ray binaries (XRBs), are the key physical processes to understand their observational phenomena, like spectral energy distribution, radiative variability, etc. In this thesis, we focus on the hot accretion flow models, including advection-dominated accretion flow (ADAF) and luminous hot accretion flow (LHAF). These models are the foundations to explain the observations of LLAGNs and XRBs in hard state. In Chapter 1, a detailed description of the background is presented. First the astrophysical black holes and the systems in which they reside are discussed. Then, an extensive discussion on the accretion process is presented. The basic concepts, 4 well-known accretion models and the mechanism of the transition between ADAF and standard thin disk are focused on. After this, we further describe the properties of ADAF - the basic model of this thesis, e.g., the dynamics, the radiative processes and several recent progresses: outflow, direct turbulent heating to the electrons, as well as LHAF at relatively high accretion rate. In Chapter 2, the influences of outflow on the dynamics of inflow are explored. As indicated through observations (e.g., towards the Galactic center), theoretical researches and (magneto-) hydrodynamical simulations, outflow is a common phenomenon in accretion systems. However, most researches in this field, especially when aiming at explaining/fitting observational data, incline to only include the mass loss due to the existence of outflow, while all the other effects like the angular momentum transport are totally neglected. This obviously conflicts with the results from simulations. Since outflow is not fully understood currently, we here parameterize its properties. Our results are shown as follows: (1) under current status of observations and theories, it is acceptable to

  1. Accretion onto some well-known regular black holes

    NASA Astrophysics Data System (ADS)

    Jawad, Abdul; Shahzad, M. Umair

    2016-03-01

    In this work, we discuss the accretion onto static spherically symmetric regular black holes for specific choices of the equation of state parameter. The underlying regular black holes are charged regular black holes using the Fermi-Dirac distribution, logistic distribution, nonlinear electrodynamics, respectively, and Kehagias-Sftesos asymptotically flat regular black holes. We obtain the critical radius, critical speed, and squared sound speed during the accretion process near the regular black holes. We also study the behavior of radial velocity, energy density, and the rate of change of the mass for each of the regular black holes.

  2. Evolution of Pre-Main Sequence Accretion Disks

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee W.

    2003-01-01

    The aim of this project is to develop a comprehensive global picture of the physical conditions in, and evolutionary timescales of, pre-main sequence accretion disks. The results of this work will help constrain the initial conditions for planet formation. To this end we are developing much larger samples of 3-10 Myr-old stars to provide better empirical constraints on protoplanetary disk evolution; measuring disk accretion rates in these systems; and constructing detailed model disk structures consistent with observations to infer physical conditions such as grain growth in protoplanetary disks.

  3. Evolution of Pre-Main Sequence Accretion Disks

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee W.

    2005-01-01

    The aim of this project was to develop a comprehensive global picture of the physical conditions in, and evolutionary timescales of, premain sequence accretion disks. The results of this work will help constrain the initial conditions for planet formation. To this end we developed much larger samples of 3-10 Myr-old stars to provide better empirical constraints on protoplanetary disk evolution; measured disk accretion rates in these systems; and constructed detailed model disk structures consistent with observations to infer physical conditions such as grain growth in protoplanetary disks.

  4. Evolution of Pre-Main Sequence Accretion Disks

    NASA Technical Reports Server (NTRS)

    Hartmann, Lee W.

    2004-01-01

    The aim of this project is to develop a comprehensive global picture of the physical conditions in, and evolutionary timescales of, pre-main sequence accretion disks. The results of this work will help constrain the initial conditions for planet formation. To this end we are developing much larger samples of 3-10 Myr-old stars to provide better empirical constraints on protoplanetary disk evolution; measuring disk accretion rates in these systems; and constructing detailed model disk structures consistent with observations to infer physical conditions such as grain growth in protoplanetary disks.

  5. Importance of the accretion process in asteroid thermal evolution: 6 Hebe as an example

    NASA Astrophysics Data System (ADS)

    Ghosh, A.; Weidenschilling, S. J.; McSween, H. Y., Jr.

    2003-05-01

    Widespread evidence exists for heating that caused melting, thermal metamorphism, and aqueous alteration in meteorite parent bodies. Previous simulations of asteroid heat transfer have assumed that accretion was instantaneous. For the first time, we present a thermal model that assumes a realistic (incremental) accretion scenario and takes into account the heat budget produced by decay of 26Al during the accretion process. By modeling 6 Hebe (assumed to be the H chondrite parent body), we show that, in contrast to results from instantaneous accretion models, an asteroid may reach its peak temperature during accretion, the time at which different depth zones within the asteroid attain peak metamorphic temperatures may increase from the center to the surface, and the volume of high-grade material in the interior may be significantly less than that of unmetamorphosed material surrounding the metamorphic core. We show that different times of initiation and duration of accretion produce a spectrum of evolutionary possibilities, and thereby, highlight the importance of the accretion process in shaping an asteroid's thermal history. Incremental accretion models provide a means of linking theoretical models of accretion to measurable quantities (peak temperatures, cooling rates, radioisotope closure times) in meteorites that were determined by their thermal histories.

  6. Dropping Out of School: Issues Affecting Culturally, Ethnically, and Linguistically Distinct Student Groups.

    ERIC Educational Resources Information Center

    Chavez, Rudolfo Chavez; And Others

    1991-01-01

    A discussion of issues surrounding and reasons for school drop-outs explores various definitions of dropping out, rate calculations, and drop-out rates among Native Americans, Hispanics, African Americans, and Asian Americans. It is concluded that society and schools have not been culturally and contextually sensitive to the diverse needs of the…

  7. The Stability of Hoyle-Lyttleton Accretion in Three Dimensions

    NASA Astrophysics Data System (ADS)

    Blondin, John M.; Raymer, E.

    2011-05-01

    The gravitational accretion of gas onto a compact star moving supersonically through a uniform ambient medium is dynamically unstable in the restricted case of two-dimensional planar geometry (a cylindrical star). Numerical simulations in 3D (e.g., the series of papers by Ruffert) show some hint of instability, but not the dramatic flip-flop seen in 2D planar simulations. We extend the recent 2D numerical simulations of Blondin and Pope (2009) to 3D using the overset spherical grid approach developed by Kageyama and Sato (2004). Using this grid geometry on current supercomputers allows us to simulate the smallest accretors studied in previous 3D work, but with an order of magnitude higher spatial resolution. For an ideal gas with a ratio of specific heats of 5/3, we find relatively minor time variability in the subsonic flow between the head of the accretion bow shock and the accreting star. Overall the bow shock and mass accretion rate remain nearly constant in time, with negligible angular momentum accreted onto the compact star.

  8. Identification of 23 accreting binaries in the Galactic Bulge Survey

    NASA Astrophysics Data System (ADS)

    Torres, M. A. P.; Jonker, P. G.; Britt, C. T.; Johnson, C. B.; Hynes, R. I.; Greiss, S.; Steeghs, D.; Maccarone, T. J.; Özel, F.; Bassa, C.; Nelemans, G.

    2014-05-01

    We present the identification of optical counterparts to 23 GBS X-ray sources. All sources are classified as accreting binaries according to the emission-line characteristics inferred from medium-resolution spectroscopy. To distinguish accreting binaries from chromospherically active objects, we develop criteria based on Hα and He I λλ5786, 6678 emission-line properties available in the literature. The spectroscopic properties and photometric variability of each object is discussed and a classification is given where possible. At least 12 of the 23 systems show an accretion-dominated optical spectrum and another 6 show stellar absorption features in addition to emission lines indicating that they are probably accreting binaries in quiescence or in a low accretion rate state. Two sources are confirmed to be eclipsing: CX207 and CX794. CX207 is likely a magnetic cataclysmic variable (CV), while CX794 is a nova-like CV in the period gap. Finally, the large broadening (2100 km s-1 FWHM) of the Hα emission lines in CX446 and CX1004 suggests that they are also high-inclination or even eclipsing systems. Whether the compact object is a white dwarf in an eclipsing CV, a neutron star or a black hole in a high-inclination low-mass X-ray binary remains to be established.

  9. Gas Accretion and Mergers in Massive Galaxies at z ~ 2

    NASA Astrophysics Data System (ADS)

    Conselice, C. J.; Ownsworth, Jamie; Mortlock, Alice; Bluck, Asa F. L.

    2013-07-01

    Galaxy assembly is an unsolved problem, with ΛCDM theoretical models unable to easily account for among other things, the abundances of massive galaxies, and the observed merger history. We show here how the problem of galaxy formation can be addressed in an empirical way without recourse to models. We discuss how galaxy assembly occurs at 1.5 < z < 3 examining the role of major and minor mergers, and gas accretion from the intergalactic medium in forming massive galaxies with log M* > 11 found within the GOODS NICMOS Survey (GNS). We find that major mergers, minor mergers and gas accretion are roughly equally important in the galaxy formation process during this epoch, with 64% of the mass assembled through merging and 36% through accreted gas which is later converted to stars, while 58% of all new star formation during this epoch arises from gas accretion. We also discuss how the total gas accretion rate is measured as Ṁ = 90+/-40 M⊙ yr-1 at this epoch, a value close to those found in some hydrodynamical simulations.

  10. Radial accretion flows on static spherically symmetric black holes

    NASA Astrophysics Data System (ADS)

    Chaverra, Eliana; Sarbach, Olivier

    2015-08-01

    We analyze the steady radial accretion of matter into a nonrotating black hole. Neglecting the self-gravity of the accreting matter, we consider a rather general class of static, spherically symmetric and asymptotically flat background spacetimes with a regular horizon. In addition to the Schwarzschild metric, this class contains certain deformation of it, which could arise in alternative gravity theories or from solutions of the classical Einstein equations in the presence of external matter fields. Modeling the ambient matter surrounding the black hole by a relativistic perfect fluid, we reformulate the accretion problem as a dynamical system, and under rather general assumptions on the fluid equation of state, we determine the local and global qualitative behavior of its phase flow. Based on our analysis and generalizing previous work by Michel, we prove that for any given positive particle density number at infinity, there exists a unique radial, steady-state accretion flow which is regular at the horizon. We determine the physical parameters of the flow, including its accretion and compression rates, and discuss their dependency on the background metric.

  11. Dynamical structure of magnetized dissipative accretion flow around black holes

    NASA Astrophysics Data System (ADS)

    Sarkar, Biplob; Das, Santabrata

    2016-06-01

    We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accretion solutions including shocks. We estimate the energy dissipation at the PSC from where a part of the accreting matter can deflect as outflows and jets. We compare the maximum energy that could be extracted from the PSC and the observed radio luminosity values for several super-massive black hole sources and the observational implications of our present analysis are discussed.

  12. Dynamical structure of magnetized dissipative accretion flow around black holes

    NASA Astrophysics Data System (ADS)

    Sarkar, Biplob; Das, Santabrata

    2016-09-01

    We study the global structure of optically thin, advection dominated, magnetized accretion flow around black holes. We consider the magnetic field to be turbulent in nature and dominated by the toroidal component. With this, we obtain the complete set of accretion solutions for dissipative flows where bremsstrahlung process is regarded as the dominant cooling mechanism. We show that rotating magnetized accretion flow experiences virtual barrier around black hole due to centrifugal repulsion that can trigger the discontinuous transition of the flow variables in the form of shock waves. We examine the properties of the shock waves and find that the dynamics of the post-shock corona (PSC) is controlled by the flow parameters, namely viscosity, cooling rate and strength of the magnetic field, respectively. We separate the effective region of the parameter space for standing shock and observe that shock can form for wide range of flow parameters. We obtain the critical viscosity parameter that allows global accretion solutions including shocks. We estimate the energy dissipation at the PSC from where a part of the accreting matter can deflect as outflows and jets. We compare the maximum energy that could be extracted from the PSC and the observed radio luminosity values for several supermassive black hole sources and the observational implications of our present analysis are discussed.

  13. Structural properties of impact ices accreted on aircraft structures

    NASA Technical Reports Server (NTRS)

    Scavuzzo, R. J.; Chu, M. L.

    1987-01-01

    The structural properties of ice accretions formed on aircraft surfaces are studied. The overall objectives are to measure basic structural properties of impact ices and to develop finite element analytical procedures for use in the design of all deicing systems. The Icing Research Tunnel (IRT) was used to produce simulated natural ice accretion over a wide range of icing conditions. Two different test apparatus were used to measure each of the three basic mechanical properties: tensile, shear, and peeling. Data was obtained on both adhesive shear strength of impact ices and peeling forces for various icing conditions. The influences of various icing parameters such as tunnel air temperature and velocity, icing cloud drop size, material substrate, surface temperature at ice/material interface, and ice thickness were studied. A finite element analysis of the shear test apparatus was developed in order to gain more insight in the evaluation of the test data. A comparison with other investigators was made. The result shows that the adhesive shear strength of impact ice typically varies between 40 and 50 psi, with peak strength reaching 120 psi and is not dependent on the kind of substrate used, the thickness of accreted ice, and tunnel temperature below 4 C.

  14. Mapping the QCD Phase Transition with Accreting Compact Stars

    SciTech Connect

    Blaschke, D.; Poghosyan, G.; Grigorian, H.

    2008-10-29

    We discuss an idea for how accreting millisecond pulsars could contribute to the understanding of the QCD phase transition in the high-density nuclear matter equation of state (EoS). It is based on two ingredients, the first one being a ''phase diagram'' of rapidly rotating compact star configurations in the plane of spin frequency and mass, determined with state-of-the-art hybrid equations of state, allowing for a transition to color superconducting quark matter. The second is the study of spin-up and accretion evolution in this phase diagram. We show that the quark matter phase transition leads to a characteristic line in the {omega}-M plane, the phase border between neutron stars and hybrid stars with a quark matter core. Along this line a drop in the pulsar's moment of inertia entails a waiting point phenomenon in the accreting millisecond pulsar (AMXP) evolution: most of these objects should therefore be found along the phase border in the {omega}-M plane, which may be viewed as the AMXP analog of the main sequence in the Hertzsprung-Russell diagram for normal stars. In order to prove the existence of a high-density phase transition in the cores of compact stars we need population statistics for AMXPs with sufficiently accurate determination of their masses, spin frequencies and magnetic fields.

  15. The minimum mantle viscosity of an accreting earth

    NASA Technical Reports Server (NTRS)

    Cooperman, S. A.

    1983-01-01

    The minimum mantle viscosity in an earth accreting from planetesimals is estimated. A plausible distribution of planetesimal sizes deposits enough energy to melt the outer nine-tenths of earth's mass; however, vigorous convection keeps temperatures near the solidus. Viscosity is significantly lower than prevails now. The temperature-dependent viscosity provides self-regulation so there is a continuing balance between accretional energy input and heat transfer out. This allows calculation of the minimum viscosity necessary to transfer out heat by a Nu/Ra-number relation. Typical viscosities are 0.1 to a million sq m/sec, lowest at mid-accretion when the mass growth rate is largest. Terrestrial planets are compared, and minimum iron descent times to central lithospheres are calculated.

  16. Accretion disk emission from a BL Lacertae object

    NASA Technical Reports Server (NTRS)

    Urry, C. Megan; Wandel, Amri

    1990-01-01

    The accretion disk is an attractive model for BL Lac objects because of its preferred axis and high efficiency. While the smooth continuum spectra of BL Lacs do not show large UV bumps, in marked contrast to quasars, high quality simultaneous data do reveal deviations from smoothness. Using detailed calculations of cool accretion disk spectra, the best measured ultraviolet and soft x ray spectra of the BL Lac object PKS 2155-304 are fitted. The mass and accretion rate required are determined. A hot disk or corona could comptonize soft photons from the cool disk and produce the observed power law spectrum in the 1 to 10 keV range. The dynamic time scales in the disk regions that contribute most of the observed ultraviolet and soft x ray photons are consistent with the respective time scales for intensity variations. The mass derived from fitting the continuum spectrum is consistent with the limit derived from the fastest hard x ray variability.

  17. Nonlinear dynamics of accretion disks with stochastic viscosity

    SciTech Connect

    Cowperthwaite, Philip S.; Reynolds, Christopher S.

    2014-08-20

    We present a nonlinear numerical model for a geometrically thin accretion disk with the addition of stochastic nonlinear fluctuations in the viscous parameter. These numerical realizations attempt to study the stochastic effects on the disk angular momentum transport. We show that this simple model is capable of reproducing several observed phenomenologies of accretion-driven systems. The most notable of these is the observed linear rms-flux relationship in the disk luminosity. This feature is not formally captured by the linearized disk equations used in previous work. A Fourier analysis of the dissipation and mass accretion rates across disk radii show coherence for frequencies below the local viscous frequency. This is consistent with the coherence behavior observed in astrophysical sources such as Cygnus X-1.

  18. Star Formation and Gas Accretion in Nearby Galaxies

    NASA Astrophysics Data System (ADS)

    Yim, Kijeong; van der Hulst, J. M.

    2016-08-01

    In order to quantify the relationship between gas accretion and star formation, we analyse a sample of 29 nearby galaxies from the WHISP survey which contains galaxies with and without evidence for recent gas accretion. We compare combined radial profiles of FUV (GALEX) and IR 24 μm (Spitzer) characterizing distributions of recent star formation with radial profiles of CO (IRAM, BIMA, or CARMA) and H I (WSRT) tracing molecular and atomic gas contents to examine star formation efficiencies in symmetric (quiescent), asymmetric (accreting), and interacting (tidally disturbed) galaxies. In addition, we investigate the relationship between star formation rate and H I in the outer discs for the three groups of galaxies. We confirm the general relationship between gas surface density and star formation surface density, but do not find a significant difference between the three groups of galaxies.

  19. Hanging drop crystal growth apparatus

    NASA Technical Reports Server (NTRS)

    Naumann, Robert J. (Inventor); Witherow, William K. (Inventor); Carter, Daniel C. (Inventor); Bugg, Charles E. (Inventor); Suddath, Fred L. (Inventor)

    1990-01-01

    This invention relates generally to control systems for controlling crystal growth, and more particularly to such a system which uses a beam of light refracted by the fluid in which crystals are growing to detect concentration of solutes in the liquid. In a hanging drop apparatus, a laser beam is directed onto drop which refracts the laser light into primary and secondary bows, respectively, which in turn fall upon linear diode detector arrays. As concentration of solutes in drop increases due to solvent removal, these bows move farther apart on the arrays, with the relative separation being detected by arrays and used by a computer to adjust solvent vapor transport from the drop. A forward scattering detector is used to detect crystal nucleation in drop, and a humidity detector is used, in one embodiment, to detect relative humidity in the enclosure wherein drop is suspended. The novelty of this invention lies in utilizing angular variance of light refracted from drop to infer, by a computer algorithm, concentration of solutes therein. Additional novelty is believed to lie in using a forward scattering detector to detect nucleating crystallites in drop.

  20. Gas Pressure-Drop Experiment

    ERIC Educational Resources Information Center

    Luyben, William L.; Tuzla, Kemal

    2010-01-01

    Most chemical engineering undergraduate laboratories have fluid mechanics experiments in which pressure drops through pipes are measured over a range of Reynolds numbers. The standard fluid is liquid water, which is essentially incompressible. Since density is constant, pressure drop does not depend on the pressure in the pipe. In addition, flow…

  1. Binary drop coalescence in liquids

    NASA Astrophysics Data System (ADS)

    Kim, Jungyong

    Experiments on binary drop collisions within an index-matched liquid were conducted for Weber numbers (We) of 1-50 and collision angles of 15-80° below the horizontal. Drop pairs of water/glycerin mixture were injected into silicone oil and, due to gravitational effects, traveled on downward trajectories before colliding. A dual-field high-speed PIV measurement system was employed to quantify drop trajectories and overall collision conditions while simultaneously examining detailed velocity fields near the collision interface. In the We range examined, for equal size drops, both rebounding and coalescing behavior occurred. The drops coalesced for We > 10 and rebounded for We < 10, and this boundary was found to be insensitive to collision angle. Coalescence was found to result from a combination of vortical flow within drops and strong drop deformation characteristic of higher We. Flow through the centers of opposing ring vortices, strengthened by drop deformation, enhanced drainage of the thin film in the impact region, leading to film rupture and coalescence. The collision angle affected the eventual location of film rupture, with the rupture location moving higher in the thin film region as the collision angle increased. The film rupture location correlated closely with the location of maximum downward velocity in the thin film. The time between collision and rupture increases with We until We = 30. For We > 30, the time decreases as We increases. Unequal size drop collisions with drop size ratios (Ds/D L) of 0.7 and 0.5 were also examined. Coalescence occurs above We* = 11 similar to equal size drops. As drop size ratio decreases, the intervening film deforms more. If the velocity ratio uL/u s < 1, the deformed interface becomes flat before coalescence. The rupture location varies due to the asymmetry of the drops. As collision offset increases (B > 0), the film rupture time is shortened and mixing of the fluid from both drops is enhanced after coalescence

  2. Magnetic control of Leidenfrost drops.

    PubMed

    Piroird, Keyvan; Clanet, Christophe; Quéré, David

    2012-05-01

    We show how a magnetic field can influence the motion of a paramagnetic drop made of liquid oxygen in a Leidenfrost state on solids at room temperature. It is demonstrated that the trajectory can be modified in both direction and velocity and that the results can be interpreted in terms of classical mechanics as long as the drop does not get too close to the magnet. We study the deviation and report that it can easily overcome 180∘ and even diverge under certain conditions, leading to situations where a drop gets captured. In the vicinity of the magnet, another type of trapping is observed, due to the deformation of the drop in this region, which leads to a strong energy dissipation. Conversely, drops can be accelerated by moving magnets (slingshot effect). PMID:23004866

  3. The LINER Nucleus of M87: A Shock-excited Dissipative Accretion Disk

    NASA Astrophysics Data System (ADS)

    Dopita, Michael A.; Koratkar, Anuradha P.; Allen, Mark G.; Tsvetanov, Zlatan I.; Ford, Holland C.; Bicknell, Geoffrey V.; Sutherland, Ralph S.

    1997-11-01

    We present long-baseline Faint Object Spectrograph (FOS) spectra of the nuclear accretion disk in M87 (NGC 4486), offset from the nucleus by 0.6" (42.7 pc) in order to avoid the nuclear continuum. Even so close to the nucleus, the optical spectrum has the appearance of a normal LINER galaxy. We show that the presence of strong UV emission lines provides a definitive test of the excitation mechanism; the disk is shock excited, not photoionized by a UV continuum from the central source. The shock velocity inferred (265 km s-1) is about one-half of the Keplerian rotation velocity found earlier by Ford et al. Since shock dissipation appears to be the principal means of increasing the binding energy of the accreting gas, we can use the FOS data and the luminosity profile of the accretion disk to estimate the rate of mass accretion as a function of radius. We find that this rate decreases with decreasing distance from the nucleus, as the material becomes organized into a cool and thin classical accretion disk in the inner regions. In the outer disk, the accretion rate (~4 M⊙ yr-1) is comparable to that determined for the X-ray-emitting cooling flow, showing that a large fraction of the cooling gas can find its way into the nuclear regions. The accretion rate near the nucleus (~3 × 10-2 M⊙ yr-1) is consistent with the properties of the relativistic jet and its associated radio emission. Over the lifetime of the jets, about 107 M⊙ of cool material may have accumulated in the nuclear regions, allowing the formation of a disk that is optically thick to Thomson scattering where it becomes ionized close to the nucleus. We speculate that LINER emission is a general property of the shocked dissipative regions of accretion disks in active galaxies with strongly sub-Eddington accretion and may therefore be used as a diagnostic of these dissipative accretion flows.

  4. Three-dimensional Hydrodynamic Simulations of Accretion in High-mass X-ray Binaries

    NASA Astrophysics Data System (ADS)

    Raymer, Eric John

    Wind accretion in high-mass X-ray binaries (HMXBs) often results in highly variable X-ray behavior, the nature of which is not well understood. Most models of wind accretion are based on the analytical predictions of Hoyle-Lyttleton accretion (HLA), which assumes a steady axisymmetric flow. Surprisingly little is known about the structure, stability, and time-evolution of HLA in three dimensions, particularly in the presence of non-uniform winds. This work describes hydrodynamic simulations of idealized HLA in three-dimensions, then applies these simulations to two HMXB subclasses that exhibit unexplained X-ray behavior. Our idealized HLA models show that the accretion flow remains steady and stable in two-dimensional axisymmetric and three dimensional grid geometries, assuming a uniform upstream flow. We test the stability of the model with linear upstream density gradients and find that they are able to induce rotational flow around the accretor that reduces the mass accretion rate by up to an order of magnitude. We apply our 3D model to accretion in the context of Be/X-ray binaries, in which the accreting neutron star is immersed in the dense decretion disk of the Be donor star. These systems have traditionally been described with 2D models that exhibit the flip-flop instability. This instability results in the formation and destruction of transient accretion disks with accompanying bursts of mass accretion. Our 3D models show no sign of the flip-flop instability, but instead display rotation about the neutron star directed primarily out of the plane of the decretion disk. This rotation generates large-scale asymmetries in the bow shock and suppresses mass accretion by up to two orders of magnitude. The accretion of a clumped stellar wind is one of the primary mechanisms proposed to explain the high-luminosity X-ray flares of supergiant fast X-ray transients. We model clump accretion in 3D to determine whether the impact of a clump can produce flares with a

  5. Revisiting the "Flip-Flop" Instability of Hoyle-Lyttleton Accretion

    NASA Astrophysics Data System (ADS)

    Blondin, John M.; Pope, T. Chris

    2009-07-01

    We revisit the flip-flop instability of two-dimensional planar accretion using high-fidelity numerical simulations. By starting from an initially steady-state axisymmetric solution, we are able to follow the growth of this overstability from small amplitudes. In the small-amplitude limit, before any transient accretion disk is formed, the oscillation period of the accretion shock is comparable to the Keplerian period at the Hoyle-Lyttleton accretion radius (Ra ), independent of the size of the accreting object. The growth rate of the overstability increases dramatically with decreasing size of the accretor, but is relatively insensitive to the upstream Mach number of the flow. We confirm that the flip-flop does not require any gradient in the upstream flow. Indeed, a small density gradient as used in the discovery simulations has virtually no influence on the growth rate of the overstability. The ratio of specific heats does influence the overstability, with smaller γ leading to faster growth of the instability. For a relatively large accretor (a radius of 0.037 Ra ) planar accretion is unstable for γ = 4/3, but stable for γ >= 1.6. Planar accretion is unstable even for γ = 5/3 provided the accretor has a radius of < 0.0025 Ra . We also confirm that when the accretor is sufficiently small, the secular evolution is described by sudden jumps between states with counter-rotating quasi-Keplerian accretion disks.

  6. The structure and appearance of winds from supercritical accretion disks. I - Numerical models

    NASA Technical Reports Server (NTRS)

    Meier, D. L.

    1979-01-01

    Equations for the structure and appearance of supercritical accretion disks and the radiation-driven winds which emanate from them are derived and solved by a steady-state hydrodynamic computer code with a relaxation technique used in stellar structure problems. The present model takes into account the mass of the accreting star, the total accretion rate, a generalization of the disk alpha parameter which accounts for heating by processes in addition to viscosity, and the ratio of the total luminosity to the Eddington luminosity. Solutions indicate that for accretion onto a hard-surfaced star, steady, optically thick winds result for even slightly supercritical accretion, and the object will appear as a supergiant star with a high mass loss rate and a nonblackbody spectrum. Winds from black hole accretion disks are expected to depend on the form of the accretion interior to the critical radius, possibly consisting of no ejection at all, a wind similar to that of a hard-surfaced star, or a column of material ejected from a hole in the accretion disk.

  7. Ground Motion Prediction Equations Empowered by Stress Drop Measurement

    NASA Astrophysics Data System (ADS)

    Miyake, H.; Oth, A.

    2015-12-01

    Significant variation of stress drop is a crucial issue for ground motion prediction equations and probabilistic seismic hazard assessment, since only a few ground motion prediction equations take into account stress drop. In addition to average and sigma studies of stress drop and ground motion prediction equations (e.g., Cotton et al., 2013; Baltay and Hanks, 2014), we explore 1-to-1 relationship for each earthquake between stress drop and between-event residual of a ground motion prediction equation. We used the stress drop dataset of Oth (2013) for Japanese crustal earthquakes ranging 0.1 to 100 MPa and K-NET/KiK-net ground motion dataset against for several ground motion prediction equations with volcanic front treatment. Between-event residuals for ground accelerations and velocities are generally coincident with stress drop, as investigated by seismic intensity measures of Oth et al. (2015). Moreover, we found faster attenuation of ground acceleration and velocities for large stress drop events for the similar fault distance range and focal depth. It may suggest an alternative parameterization of stress drop to control attenuation distance rate for ground motion prediction equations. We also investigate 1-to-1 relationship and sigma for regional/national-scale stress drop variation and current national-scale ground motion equations.

  8. The lamppost model of accreting black holes

    NASA Astrophysics Data System (ADS)

    Zdziarski, A.

    2016-06-01

    Niedzwiecki, Zdziarski & Szanecki (2016, ApJL, submitted) have studied the lamppost model, in which the X-ray source in accreting black-hole systems is located on the rotation axis close to the horizon. We point out a number of inconsistencies in the widely used lamppost model relxilllp. They appear to invalidate those model fitting results for which the source distances from the horizon are within several gravitational radii. Furthermore, we note that if those results were correct, most of the photons produced in the lamppost would be trapped by the black hole, and the source luminosity as measured at infinity would be much larger than that observed. This appears to be in conflict with the observed smooth state transitions between the hard and soft states of X-ray binaries. The required increase of the accretion rate and the associated efficiency reduction present also a problem for AGNs. Then, those models imply the luminosity measured in the local frame much higher than the dissipated power due to time dilation and redshift, and the electron temperature significantly higher than that observed. We show that these conditions imply that the fitted sources would be out of the pair equilibrium.

  9. Transient Marangoni convection in hanging evaporating drops

    NASA Astrophysics Data System (ADS)

    Savino, R.; Fico, S.

    2004-10-01

    A combined experimental and numerical analysis has been carried out to study Marangoni effects during the evaporation of droplets. The experiments are performed with pendant drops of silicone oils (with different viscosities) and hydrocarbons. The temperature of the disk sustaining the drop is rapidly increased or decreased in order to study transient heating or cooling processes. The velocity field in the droplet is evaluated monitoring the motion of tracers in the meridian plane, using a laser sheet illumination system and a video camera. Surface temperature distributions of the drops are detected by infrared thermocamera. The numerical model is based on axisymmetric Navier-Stokes equations, taking into account the presence of Marangoni shear stresses and evaporative cooling at the liquid-air interface. Marangoni flows cause a larger, more uniform surface temperature, increasing heat transfer from disk to droplet, as well as evaporation rate. When Marangoni effects are negligible, larger surface temperature differences occur along the drop surface and heat transfer is relatively small. The role of Marangoni and buoyancy flows in silicone oils with different viscosities and hydrocarbons is discussed and correlations are presented between experimental and numerical results.

  10. X-ray deficiency on strongly accreting T Tauri stars. Comparing Orion with Taurus

    NASA Astrophysics Data System (ADS)

    Bustamante, I.; Merín, B.; Bouy, H.; Manara, C. F.; Ribas, Á.; Riviere-Marichalar, P.

    2016-03-01

    Context. Depending on whether a T Tauri star accretes material from its circumstellar disk or not, different X-ray emission properties can be found. The accretion shocks produce cool heating of the plasma, contributing to the soft X-ray emission from the star. Aims: Using X-ray data from the Chandra Orion Ultra-deep Project and accretion rates that were obtained with the Hubble Space Telescope/WFPC2 photometric measurements in the Orion Nebula Cluster (ONC), we studied the relation between the accretion processes and the X-ray emissions of a coherent sample of T Tauri sources in the region. Methods: We performed regression and correlation analyses of our sample of T Tauri stars between the X-ray parameters, stellar properties, and the accretion measurements. Results: We find that a clear anti-correlation is present between the residual X-ray luminosity and the accretion rates in our samples in Orion that is consistent with that found on the XMM-Newton Extended Survey of the Taurus molecular cloud (XEST) study. A considerable number of classified non-accreting sources show accretion rates comparable to those of classical T Tauri Stars (CTTS). Our data do not allow us to confirm the classification between classical and weak-line T Tauri stars (WTTS), and the number of WTTS in this work is small compared to the complete samples. Thus, we have used the entire samples as accretors in our analysis. We provide a catalog with X-ray luminosities (corrected from distance) and accretion measurements of an ONC T Tauri stars sample. Conclusions: Although Orion and Taurus display strong differences in their properties (total gas and dust mass, star density, strong irradiation from massive stars), we find that a similar relation between the residual X-ray emission and accretion rate is present in the Taurus molecular cloud and in the accreting samples from the ONC. The spread in the data suggests dependencies of the accretion rates and the X-ray luminosities other than the

  11. Controlling Vapor Pressure In Hanging-Drop Crystallization

    NASA Technical Reports Server (NTRS)

    Carter, Daniel C.; Smith, Robbie

    1988-01-01

    Rate of evaporation adjusted to produce larger crystals. Device helps to control vapor pressure of water and other solvents in vicinity of hanging drop of solution containing dissolved enzyme protein. Well of porous frit (sintered glass) holds solution in proximity to drop of solution containing protein or enzyme. Vapor from solution in frit controls evaporation of solvent from drop to control precipitation of protein or enzyme. With device, rate of nucleation limited to decrease number and increase size (and perhaps quality) of crystals - large crystals of higher quality needed for x-ray diffraction studies of macromolecules.

  12. Instant freezing of impacting wax drops

    NASA Astrophysics Data System (ADS)

    Ponomarenko, Alexandre; Virot, Emmanuel; Rubinstein, Shmuel

    2015-11-01

    We present the impact of hot liquid drops of wax on surfaces whose temperature is below the solidifying temperature of the drops. During the fall the drops remain mostly liquid, but upon impact, their temperature quickly decreases resulting in the solidification of the drop. Depending on the impact energy, drops size and the temperature difference between the drop and the surface this results in plethora of solid shapes: simple lenses, triangular drops, spherical caps and popped popcorn shapes.

  13. Obscured accretion from AGN surveys

    NASA Astrophysics Data System (ADS)

    Vignali, Cristian

    2014-07-01

    Recent models of super-massive black hole (SMBH) and host galaxy joint evolution predict the presence of a key phase where accretion, traced by obscured Active Galactic Nuclei (AGN) emission, is coupled with powerful star formation. Then feedback processes likely self-regulate the SMBH growth and quench the star-formation activity. AGN in this important evolutionary phase have been revealed in the last decade via surveys at different wavelengths. On the one hand, moderate-to-deep X-ray surveys have allowed a systematic search for heavily obscured AGN, up to very high redshifts (z~5). On the other hand, infrared/optical surveys have been invaluable in offering complementary methods to select obscured AGN also in cases where the nuclear X-ray emission below 10 keV is largely hidden to our view. In this review I will present my personal perspective of the field of obscured accretion from AGN surveys.

  14. Cyclotron Resonance in Accreting Pulsars

    NASA Astrophysics Data System (ADS)

    Bhattacharya, Dipankar

    2016-07-01

    Cyclotron Resonance Absorption/Scattering features provide direct measurement of magnetic field strength in the line forming region. This has enabled the estimation of magnetic field strengths of nearly two dozen neutron stars in accreting high mass binary systems. With improved spectroscopic sensitivity, new X-ray observatories such as NuSTAR, Astrosat and Hitomi are opening the doors to studying detailed features such as the line shape and phase dependence with high significance. Such studies will help understand the nature of matter accumulation in, and outflow from, the magnetically confined accretion column on the neutron star. This talk will describe the results of MHD simulations of the matter flow in such systems, the diagnostics of such flows using cyclotron lines, and comparison with recent observations from NuSTAR and Astrosat.

  15. Accreting neutron stars by QFT

    NASA Astrophysics Data System (ADS)

    Chen, Shao-Guang

    I deduce the new gravitational formula from the variance in mass of QFT and GR (H05-0029-08, E15-0039 -08, E14-0032-08, D31-0054-10) in the partial differential: f (QFT) = f (GR) = delta∂ (m v)/delta∂ t = f _{P} + f _{C} , f _{P} = m delta∂ v / delta∂ t = - ( G m M /r (2) ) r / r, f _{C} = v delta∂ m / delta∂ t = - ( G mM / r (2) ) v / c (1). f (QFT) is the quasi-Casimir pressure of net virtual neutrinos nuν _{0} flux (after counteract contrary direction nuν _{0}). f (GR) is equivalent to Einstein’s equation as a new version of GR. GR can be inferred from Eq.(1) thereby from QFT, but QFT cannot be inferred from Eq.(1) or GR. f (QFT) is essential but f (GR) is phenomenological. Eq.(1) is obtained just by to absorb the essence of corpuscule collided gravitation origin ism proposed by Fatio in 1690 and 1920 Majorana’s experiment concept about gravitational shield effect again fuse with QFT. Its core content is that the gravity produced by particles collide cannot linear addition, i.e., Eq.(1) with the adding nonlinearity caused by the variable mass to replace the nonlinearity of Einstein’s equation. The nonlinear gravitation problems can be solved using the classical gradual approximation of alone f _{P} and alone f _{C}. Such as the calculation of advance of the perihelion of QFT, let the gravitational potential U = - G M /r which is just the distribution density of net nuν _{0} flux. From SR we again get Eq.(1): f (QFT) = f _{P} + f _{C}, f _{P} = - m ( delta∂ U / delta∂ r) r / r, f _{C} = - m ( delta∂U / delta∂ r) v / c , U = (1 - betaβ (2) )V, V is the Newtonian gravitational potential. f_{ P} correspond the change rate of three-dimensional momentum p, f_{C} correspond the change rate of fourth dimensional momentum i m c which show directly as a dissipative force of mass change. In my paper ‘To cross the great gap between the modern physics and classic physics, China Science &Technology Overview 129 85-91 (2011)’ with the

  16. Accretion physics in the Galaxy - Swift J045106.8-694803 a possible accreting magnetar in the Large Magellanic Cloud.

    NASA Astrophysics Data System (ADS)

    Klus, Helen

    2012-09-01

    We report the possibility of an accreting magnetar in the Large Magellanic Cloud (LMC). The High Mass X-ray Binary pulsar Swift J045106.8-694803 has been observed with Swift XRT in 2008 and RXTE in 2011. The change in spin period over these three years indicates a spin up rate of 5.68 seconds a year, amongst the highest observed for an accreting pulsar. This spin up rate can be accounted for using Ghosh and Lamb's (1979) accretion theory assuming it has a magnetic field of over 10^14 Gauss. This would make it an accreting magnetar, only the second of which to be discovered and the first in the LMC. The large spin up rate is consistent with Swift BAT observations that show that Swift J045106.8-694803 has had a continually high X-ray luminosity for at least five years. The orbital period is also reported. Further investigation of this source is required to confirm its change in spin period and hence its magnetic field.

  17. Electrokinetics of isolated electrified drops.

    PubMed

    Pillai, Rohit; Berry, Joseph D; Harvie, Dalton J E; Davidson, Malcolm R

    2016-04-14

    Using a recently developed multiphase electrokinetic model, we simulate the transient electrohydrodynamic response of a liquid drop containing ions, to both small and large values of electric field. The temporal evolution is found to be governed primarily by two dimensionless groups: (i) Ohnesorge number (Oh), a ratio of viscous to inertio-capillary effects, and (ii) inverse dimensionless Debye length (κ), a measure of the diffuse regions of charge that develop in the drop. The effects of dielectric polarization dominate at low Oh, while effects of separated charge gain importance with increase in Oh. For small values of electric field, the deformation behaviour of a drop is shown to be accurately described by a simple analytical expression. At large electric fields, the drops are unstable and eject progeny drops. Depending on Oh and κ this occurs via dripping or jetting; the regime transitions are shown by a Oh-κ phase map. In contrast to previous studies, we find universal scaling relations to predict size and charge of progeny drops. Our simulations suggest charge transport plays a significant role in drop dynamics for 0.1 ≤ Oh ≤ 10, a parameter range of interest in microscale flows. PMID:26954299

  18. Pool impacts of Leidenfrost drop

    NASA Astrophysics Data System (ADS)

    Darbois Texier, Baptiste; Maquet, Laurent; Dorbolo, Stephane; Dehandschoewercker, Eline; Pan, Zhao; Truscott, Tadd

    2015-11-01

    This work concerns the impact of a droplet made of a volatile liquid (typically HFE) on a pool of an other liquid (typically silicone oil) which temperature is above the boiling point of the drop. Depending on the properties of the two liquids and the impacting conditions, four different regimes are observed. For low impacting speeds, the droplet bounces on the surface of the bath and finally levitates above it in a Leidenfrost state. Such a regime occurs as soon as the pool temperature exceeds the boiling point of the drop. This observation means that there is no threshold in temperature for a Leidenfrost effect on a liquid surface contrary to the case of a solid substrate. For intermediate impacting velocities, the pinch-off of the surface of the pool entraps the drop in the liquid bulk. The entrapped drop is separated from the pool by a layer of its own vapour in a similar way of antibulles. For increasing impacting speeds, the vapour layer between the drop and the pool does not hold during the pinch-off event. The contact of the drop with the hot liquid provokes a sudden and intense evaporation. At very large impacting speeds, the drop rapidely contacts the pool, spreads and finally induces a hemi-spherical cavity. In the end, these four different regimes are summarized in a Froud-Weber diagram which boundaries are discussed.

  19. MN Lup: X-RAYS FROM A WEAKLY ACCRETING T TAURI STAR

    SciTech Connect

    Guenther, H. M.; Wolk, S. J.; Wolter, U.; Robrade, J.

    2013-07-01

    Young T Tauri stars (TTS) are surrounded by an accretion disk, which over time disperses due to photoevaporation, accretion, and possibly planet formation. The accretion shock on the central star produces an UV/optical veiling continuum, line emission, and X-ray signatures. As the accretion rate decreases, the impact on the central star must change. In this article we study MN Lup, a young star where no indications of a disk are seen in IR observations. We present XMM-Newton and VLT/UVES observations, some of them taken simultaneously. The X-ray data show that MN Lup is an active star with L{sub X} /L{sub bol} close to the saturation limit. However, we find high densities (n{sub e} > 3 Multiplication-Sign 10{sup 10} cm{sup -3}) in the X-ray grating spectrum. This can be well fitted using an accretion shock model with an accretion rate of 2 Multiplication-Sign 10{sup -11} M{sub Sun} yr{sup -1}. Despite the simple H{alpha} line profile which has a broad component, but no absorption signatures as typically seen on accreting TTS, we find rotational modulation in Ca II K and in photospheric absorption lines. These line profile modulations do not clearly indicate the presence of a localized hot accretion spot on the star. In the H{alpha} line we see a prominence in absorption about 2R{sub *} above the stellar surface-the first of its kind on a TTS. MN Lup is also the only TTS where accretion is seen, but no dust disk is detected that could fuel it. We suggest that MN Lup presents a unique and short-lived state in the disk evolution. It may have lost its dust disk only recently and is now accreting the remaining gas at a very low rate.

  20. Persistent Patterns in Accretion Disks

    SciTech Connect

    Amin, Mustafa A.; Frolov, Andrei V.; /KIPAC, Menlo Park

    2006-04-03

    We present a set of new characteristic frequencies associated with accretion disks around compact objects. These frequencies arise from persistent rotating patterns in the disk that are finite in radial extent and driven purely by the gravity of the central body. Their existence depends on general relativistic corrections to orbital motion and, if observed, could be used to probe the strong gravity region around a black hole. We also discuss a possible connection to the puzzle of quasi-periodic oscillations.

  1. Spiral Waves in Accretion Disks

    NASA Astrophysics Data System (ADS)

    Harlaftis, Emilios

    A review with the most characteristic spiral waves in accretion disks of cataclysmic variables will be presented. Recent work on experiments targeting the detection of spiral waves from time lapse movies of real disks and the study of permanent spiral waves will be discussed. The relevance of spiral waves with other systems such as star-planet X-ray binaries and Algols will be reviewed.

  2. Accretion disks around black holes

    NASA Technical Reports Server (NTRS)

    Abramowicz, M. A.

    1994-01-01

    The physics of accretion flow very close to a black hole is dominated by several general relativistic effects. It cannot be described by the standard Shakura Sunyaev model or by its relativistic version developed by Novikov and Thome. The most important of these effects is a dynamical mass loss from the inner edge of the disk (Roche lobe overflow). The relativistic Roche lobe overflow induces a strong advective cooling, which is sufficient to stabilize local, axially symmetric thermal and viscous modes. It also stabilizes the non-axially-symmetric global modes discovered by Papaloizou and Pringle. The Roche lobe overflow, however, destabilizes sufficiently self-gravitating accretion disks with respect to a catastrophic runaway of mass due to minute changes of the gravitational field induced by the changes in the mass and angular momentum of the central black hole. One of the two acoustic modes may become trapped near the inner edge of the disk. All these effects, absent in the standard model, have dramatic implications for time-dependent behavior of the accretion disks around black holes.

  3. Magnetically Torqued Thin Accretion Disks

    NASA Astrophysics Data System (ADS)

    Kluźniak, W.; Rappaport, S.

    2007-12-01

    We compute the properties of a geometrically thin, steady accretion disk surrounding a central rotating, magnetized star. The magnetosphere is assumed to entrain the disk over a wide range of radii. The model is simplified in that we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal component of the magnetic field as a function of radial distance. We find a solution for the angular velocity profile tending to corotation close to the central star and smoothly matching a Keplerian curve at a radius where the viscous stress vanishes. The value of this ``transition'' radius is nearly the same for both of our adopted B-field models. We then solve analytically for the torques on the central star and for the disk luminosity due to gravity and magnetic torques. When expressed in a dimensionless form, the resulting quantities depend on one parameter alone, the ratio of the transition radius to the corotation radius. For rapid rotators, the accretion disk may be powered mostly by spin-down of the central star. These results are independent of the viscosity prescription in the disk. We also solve for the disk structure for the special case of an optically thick alpha disk. Our results are applicable to a range of astrophysical systems including accreting neutron stars, intermediate polar cataclysmic variables, and T Tauri systems.

  4. Infall of Planetesimals onto Growing Giant Planets: Onset of Runaway Gas Accretion and Metallicity of Their Gas Envelopes

    NASA Astrophysics Data System (ADS)

    Shiraishi, Masakazu; Ida, Shigeru

    2008-09-01

    We have investigated the planetesimal accretion rate onto giant planets that are growing through gas accretion, using numerical simulations and analytical arguments. We derived the condition for the opening of a gap in the planetesimal disk, which is determined by a competition between the expansion of the planet's Hill radius due to the planet's growth and the damping of planetesimal eccentricity due to gas drag. We also derived the semianalytical formula for the planetesimal accretion rate as a function of the ratios of the rates of the Hill radius expansion, the damping, and planetesimal scattering by the planet. The predicted low planetesimal accretion rate due to the opening of the gap in early gas accretion stages quantitatively shows that "phase 2," which is a long (more than a Myr), slow gas accretion phase before the onset of runaway gas accretion, is not likely to occur. In late stages, rapid Hill radius expansion fills the gap, resulting in significant planetesimal accretion, which is as large as several M⊕ for Jupiter and Saturn. The efficient onset of runaway gas accretion and the late pollution may reconcile the ubiquity of extrasolar giant planets with the metal-rich envelopes of Jupiter and Saturn inferred from interior structure models. These formulae will give deep insights into the formation of extrasolar gas giants and the diversity in the metallicities of transiting gas giants.

  5. Parsec-scale Accretion and Winds Irradiated by a Quasar

    NASA Astrophysics Data System (ADS)

    Dorodnitsyn, A.; Kallman, T.; Proga, D.

    2016-03-01

    We present numerical simulations of properties of a parsec-scale torus exposed to illumination by the central black hole in an active galactic nucleus (AGN). Our physical model allows to investigate the balance between the formation of winds and accretion simultaneously. Radiation-driven winds are allowed by taking into account radiation pressure due to UV and IR radiation along with X-ray heating and dust sublimation. Accretion is allowed through angular momentum transport and the solution of the equations of radiative, viscous radiation hydrodynamics. Our methods adopt flux-limited diffusion radiation hydrodynamics for the dusty, infrared pressure driven part of the flow, along with X-ray heating and cooling. Angular momentum transport in the accreting part of the flow is modeled using effective viscosity. Our results demonstrate that radiation pressure on dust can play an important role in shaping AGN obscuration. For example, when the luminosity illuminating the torus exceeds L\\gt 0.01 {L}{{Edd}}, where LEdd is the Eddington luminosity, we find no episodes of sustained disk accretion because radiation pressure does not allow a disk to form. Despite the absence of the disk accretion, the flow of gas to smaller radii still proceeds at a rate 10-4-10-1{M}⊙ {{{yr}}}-1 through the capturing of the gas from the hot evaporative flow, thus providing a mechanism to deliver gas from a radiation-pressure dominated torus to the inner accretion disk. As L/{L}{{edd}} increases, larger radiation input leads to larger torus aspect ratios and increased obscuration of the central black hole. We also find the important role of the X-ray heated gas in shaping the obscuring torus.

  6. Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. II. Optical Study and Interpretation

    NASA Astrophysics Data System (ADS)

    Soria, R.; Graham, Alister W.; Fabbiano, G.; Baldi, A.; Elvis, M.; Jerjen, H.; Pellegrini, S.; Siemiginowska, A.

    2006-03-01

    Our X-ray study of the nuclear activity in a new sample of six quiescent early-type galaxies, as well as in a larger sample from the literature, confirmed (Paper I) that the Bondi accretion rate of diffuse hot gas is not a good indicator of the SMBH X-ray luminosity. Here we suggest that a more reliable estimate of the accretion rate must include the gas released by the stellar population inside the sphere of influence of the SMBH, in addition to the Bondi inflow of hot gas across that surface. We use optical surface brightness profiles to estimate the mass-loss rate from stars in the nuclear region: we show that for our sample of galaxies it is an order of magnitude higher (~10-4 to 10-3 Msolar yr-1) than the Bondi inflow rate of hot gas, as estimated from Chandra (Paper I). Only by taking into account both sources of fuel can we constrain the true accretion rate, the accretion efficiency, and the power budget. Radiatively efficient accretion is ruled out, for quiescent SMBHs. For typical radiatively inefficient flows, the observed X-ray luminosities of the SMBHs imply accretion fractions ~1%-10% (i.e., ~90%-99% of the available gas does not reach the SMBH) for at least five of our six target galaxies and most of the other galaxies with known SMBH masses. We discuss the conditions for mass conservation inside the sphere of influence, so that the total gas injection is balanced by accretion plus outflows. We show that a fraction of the total accretion power (mechanical plus radiative) would be sufficient to sustain a self-regulating, slow outflow that removes from the nuclear region all the gas that does not sink into the BH (``BH feedback''). The rest of the accretion power may be carried out in a jet or advected. We also discuss scenarios that would lead to an intermittent nuclear activity.

  7. CONSTRAINTS ON THE VISCOSITY AND MAGNETIC FIELD IN HOT ACCRETION FLOWS AROUND BLACK HOLES

    SciTech Connect

    Liu, B. F.; Taam, Ronald E. E-mail: r-taam@northwestern.edu

    2013-07-15

    The magnitude of the viscosity and magnetic field parameters in hot accretion flows is investigated in low luminosity active galactic nuclei (LLAGNs). Theoretical studies show that a geometrically thin, optically thick disk is truncated at mass accretion rates less than a critical value by mass evaporated vertically from the disk to the corona, with the truncated region replaced by an advection dominated accretion flow (ADAF). The critical accretion rate for such a truncation is a function of the viscosity and magnetic field. Observations of X-ray photon indices and spectral fits of a number of LLAGNs published in the literature provide an estimate of the critical rate of mass accretion and the truncation radius, respectively. By comparing the observational results with theoretical predictions, the viscosity and magnetic field parameters in the hot accretion flow region are estimated. Specifically, the mass accretion rates inferred in different sources constrain the viscosity parameter, whereas the truncation radii of the disk, as inferred from spectral fits, further constrain the magnetic field parameter. It is found that the value of the viscosity parameter in the corona/ADAF ranges from 0.17 to 0.5, with values clustered about 0.2-0.3. Magnetic pressure is required by the relatively small truncation radii for some LLAGNs and is found to be as high as its equipartition value with the gas pressure. The inferred values of the viscosity parameter are in agreement with those obtained from the observations of non-stationary accretion in stellar mass black hole X-ray transients. This consistency provides support for the paradigm that a geometrically thin disk is truncated by means of a mass evaporation process from the disk to the corona at low mass accretion rates.

  8. Black Hole Accretion and Feedback Driven by Thermal Instability

    NASA Astrophysics Data System (ADS)

    Gaspari, M.; Ruszkowski, M.; Oh, S. P.; Churazov, E.; Brighenti, F.; Ettori, S.; Sharma, P.; Temi, P.

    2013-03-01

    Multiwavelength data indicate that the cores of several galaxy clusters are moderately cooling, though not catastrophically, showing signs of filamentary extended multiphase gas. Through 3D AMR hydrodynamic simulations, we study the impact of thermal instability in the evolution of the intracluster medium. Common moderate turbulence of just over 100 km/s leads to the growth of nonlinear thermal instability within the central few tens kpc. In the presence of a global counterbalancing heating, the condensation of extended filamentary cold gas is violent, occurring when the cooling time falls below 10 times the free-fall time. The frequent stochastic collisions, fragmentations and shearing motions between the cold clouds, filaments and the central torus, efficiently reduce angular momentum. Tracking the accreting gas with a dynamical range of 10 million, we find that the accretion rate is boosted up to 100 times with respect to the Bondi rate. In a commonly turbulent and quasi-stable atmosphere, the mode of black accretion is cold and chaotic, substantially different from the classic idealized scenario. Only in the transonic regime, turbulent dissipation starts to inhibit thermal instability. On sub-parsec scales the cold phase is channeled via a funnel, triggering the black hole feedback likely linked to mechanical jets/outflows. As shown by long-term self-regulated simulations, the interplay of chaotic cold accretion and AGN feedback is crucial in order to avoid the cooling catastrophe and to reproduce the key thermodynamical features of observed clusters.

  9. Gravitational torques in spiral galaxies: Gas accretion as a driving mechanism of galactic evolution

    NASA Astrophysics Data System (ADS)

    Block, D. L.; Bournaud, F.; Combes, F.; Puerari, I.; Buta, R.

    2002-11-01

    The distribution of gravitational torques and bar strengths in the local Universe is derived from a detailed study of 163 galaxies observed in the near-infrared. The results are compared with numerical models for spiral galaxy evolution. It is found that the observed distribution of torques can be accounted for only with external accretion of gas onto spiral disks. Accretion is responsible for bar renewal - after the dissolution of primordial bars - as well as the maintenance of spiral structures. Models of isolated, non-accreting galaxies are ruled out. Moderate accretion rates do not explain the observational results: it is shown that galactic disks should double their mass in less than the Hubble time. The best fit is obtained if spiral galaxies are open systems, still forming today by continuous gas accretion, doubling their mass every 10 billion years.

  10. Spherical steady accretion flows: Dependence on the cosmological constant, exact isothermal solutions, and applications to cosmology

    NASA Astrophysics Data System (ADS)

    Mach, Patryk; Malec, Edward; Karkowski, Janusz

    2013-10-01

    We investigate spherical, isothermal and polytropic steady accretion models in the presence of the cosmological constant. Exact solutions are found for three classes of isothermal fluids, assuming the test gas approximation. The cosmological constant damps the mass accretion rate and—above a certain limit—completely stops the steady accretion onto black holes. A “homoclinic-type” accretion flow of polytropic gas has been discovered in anti-de Sitter spacetimes in the test-gas limit. These results can have cosmological connotation, through the Einstein-Straus vacuole model of embedding local structures into Friedman-Lemaitre-Robertson-Walker spacetimes. In particular, one infers that steady accretion would not exist in the late phases of Penrose’s scenario of the evolution of the Universe, known as the Weyl curvature hypothesis.

  11. Drop breakup and deformation in sudden onset strong flows

    NASA Astrophysics Data System (ADS)

    Marks, Charles Raphael

    This work characterizes the deformation and breakup of a single drop subjected to a sudden onset shear flow. The drop is immersed in a second fluid (the matrix) with which it is immiscible. A cylindrical couette device is used to create a flow field which, in the absence of the drop, would constitute a close approximation of simple shear flow. The magnitude of the imposed shear rate was greater than that which would be necessary to just break the drop. The experiments conducted were limited to matrix fluid viscosities above 7Pa˙ s and shear rates below 15/s, ensuring that the flows considered were inertialess. The matrix fluid was a corn syrup solution. The drop fluids were polybutadiene, paraffin oil and silicone oil, leading to a range of interfacial tensions. At the shear rates used in these experiments the fluids used Newtonian. Viscosity ratios (drop/matrix) ranging from 0.01 to 1 were considered. Two breakup mechanisms were observed to contribute to the dispersion of the original drop. In all cases elongative end pinching, defined by this study, caused the ends of a stretching drop to break off and form daughter drops. Breakup due to elongative end pinching was always the first breakup observed. The daughter drops formed by elongative end pinching were always the largest daughter drops formed. In cases when the experimental conditions were sufficiently stronger than the critical conditions (needed to just barely break up the drop), a second type of breakup, capillary wave breakup, was also observed. Measurement of the characteristic time scales and length scales were made of each type of breakup. The lengths (a) were found to scale as capillary numbers: Ca=a mg/s. The times (t) were found to scale as strains: s=t g. A qualitative explanation for the capillary number scaling is presented and quantitatively compared to predictions based on small deformation analysis. Additionally the daughter drop size distributions resulting from drop breakup is characterized

  12. Leidenfrost drops: Effect of gravity

    NASA Astrophysics Data System (ADS)

    Maquet, L.; Brandenbourger, M.; Sobac, B.; Biance, A.-L.; Colinet, P.; Dorbolo, S.

    2015-04-01

    A specific experimental set-up has been installed in a large centrifuge facility in order to study different aspects of Leidenfrost drops under high-gravity conditions (5, 10, 15 and 20 times the Earth gravity). In particular, the drop lifetime and more precisely the variations of drop diameter vs. time have shown to be in good agreement with previous experiments and scaling analysis (Biance A.-L. et al., Phys. Fluids, 15 (2003) 1632). Moreover, so-called chimneys are expectedly observed in the large puddles, the distance between two chimneys depending linearly on the capillary length. Finally, the Leidenfrost point, i.e. the temperature above which the Leidenfrost effect takes place, was unexpectedly found to increase slightly with gravity. A qualitative explanation based on a refined model (Sobac B. et al., Phys. Rev. E, 90 (2014) 053011) recognizing the non-trivial shape of the vapor film under the drop is proposed to explain this observation.

  13. Orion Capsule Mockup is Dropped

    NASA Video Gallery

    An Orion capsule mockup is dropped from a plane 25,000 feet above the Arizona desert to test its parachute design. Orion will return to Earth at speeds faster than previous human spacecraft, and wi...

  14. Drop oscillation and mass transfer in alternating electric fields

    SciTech Connect

    Carleson, T.E.

    1992-06-24

    In certain cases droplet direct contact heat transfer rates can be significantly enhanced by the application of an alternating electric field. This field can produce shape oscillations in a droplet which will enhance mixing. The theoretical evaluation of the effect of the interaction of the field with drop charge on the hydrodynamics has been completed for small amplitude oscillations. Previous work with a zero order perturbation method was followed up with a first order perturbation method to evaluate the effect of drop distortion on drop charge and field distribution. The first order perturbation results show secondary drop oscillations of four modes and two frequencies in each mode. The most significant secondary oscillation has the same mode and frequency as the second mode oscillation predicted from the first order perturbation work. The resonant frequency of all oscillations decrease with increasing electric field strength and drop charge. Work is currently underway to evaluate the heat transfer enhancement from an applied alternating electric field.

  15. Solidification Dynamics of Spherical Drops in a Free Fall Environment

    NASA Technical Reports Server (NTRS)

    Grugel, Richard N.; Brush, Lucien N.

    2006-01-01

    Silver drops (99.9%, 4, 5, 7, and 9 mm diameter) were levitated, melted, and released to fall through Marshall Space Flight Center's 105 meter drop tube in helium - 6% hydrogen and pure argon atmospheres. By varying a drop s initial superheat the extent of solidification prior to impact ranged from complete to none during the approx. 4.6s of free fall time. Comparison of the experimental observations is made with numerical solutions to a model of the heat transfer and solidification kinetics associated with cooling of the drop during free fall, particularly with regard to the fraction of liquid transformed. Analysis reveals the relative importance ,of the initial parameters affecting the cooling and solidification rates within the drop. A discussion of the conditions under which the actual observations deviate from the assumptions used in the model is presented.

  16. METAL ACCRETION ONTO WHITE DWARFS CAUSED BY POYNTING-ROBERTSON DRAG ON THEIR DEBRIS DISKS

    SciTech Connect

    Rafikov, Roman R.

    2011-05-01

    Recent discoveries of compact (sizes {approx}accretion of high-Z material from the disk. But the mechanism responsible for efficient transfer of mass from a particulate disk to the WD atmosphere has not yet been identified. Here we demonstrate that radiation of the WD can effectively drive accretion of matter through the disk toward the sublimation radius (located at several tens of WD radii), where particles evaporate, feeding a disk of metal gas accreting onto the WD. We show that, contrary to some previous claims, Poynting-Robertson (PR) drag on the debris disk is effective at providing metal accretion rate M-dot{sub PR}{approx}10{sup 8} g s{sup -1} and higher, scaling quadratically with WD effective temperature. We compare our results with observations and show that, as expected, no WD hosting a particulate debris disk shows evidence of metal accretion rate below that produced by the PR drag. Existence of WDs accreting metals at rates significantly higher than M-dot{sub PR} suggests that another mechanism in addition to the PR drag drives accretion of high-Z elements in these systems.

  17. Accretion of Jupiter-mass planets in the limit of vanishing viscosity

    SciTech Connect

    Szulágyi, J.; Morbidelli, A.; Crida, A.; Masset, F.

    2014-02-20

    In the core-accretion model, the nominal runaway gas-accretion phase brings most planets to multiple Jupiter masses. However, known giant planets are predominantly Jupiter mass bodies. Obtaining longer timescales for gas accretion may require using realistic equations of states, or accounting for the dynamics of the circumplanetary disk (CPD) in the low-viscosity regime, or both. Here we explore the second way by using global, three-dimensional isothermal hydrodynamical simulations with eight levels of nested grids around the planet. In our simulations, the vertical inflow from the circumstellar disk (CSD) to the CPD determines the shape of the CPD and its accretion rate. Even without a prescribed viscosity, Jupiter's mass-doubling time is ∼10{sup 4} yr, assuming the planet at 5.2 AU and a Minimum Mass Solar Nebula. However, we show that this high accretion rate is due to resolution-dependent numerical viscosity. Furthermore, we consider the scenario of a layered CSD, viscous only in its surface layer, and an inviscid CPD. We identify two planet-accretion mechanisms that are independent of the viscosity in the CPD: (1) the polar inflow—defined as a part of the vertical inflow with a centrifugal radius smaller than two Jupiter radii and (2) the torque exerted by the star on the CPD. In the limit of zero effective viscosity, these two mechanisms would produce an accretion rate 40 times smaller than in the simulation.

  18. A Simple test for the existence of two accretion modes in active galactic nuclei

    SciTech Connect

    Jester, Sebastian; /Fermilab

    2005-02-01

    By analogy to the different accretion states observed in black-hole X-ray binaries (BHXBs), it appears plausible that accretion disks in active galactic nuclei (AGN) undergo a state transition between a radiatively efficient and inefficient accretion flow. If the radiative efficiency changes at some critical accretion rate, there will be a change in the distribution of black hole masses and bolometric luminosities at the corresponding transition luminosity. To test this prediction, the author considers the joint distribution of AGN black hole masses and bolometric luminosities for a sample taken from the literature. The small number of objects with low Eddington-scaled accretion rates m < 0.01 and black hole masses M{sub BH} < 10{sup 9} M{sub {circle_dot}} constitutes tentative evidence for the existence of such a transition in AGN. Selection effects, in particular those associated with flux-limited samples, systematically exclude objects in particular regions of the (M{sub BH}, L{sub bol}) plane. Therefore, they require particular attention in the analysis of distributions of black hole mass, bolometric luminosity, and derived quantities like the accretion rate. The author suggests further observational tests of the BHXB-AGN unification scheme which are based on the jet domination of the energy output of BHXBs in the hard state, and on the possible equivalence of BHXB in the very high (or steep power-law) state showing ejections and efficiently accreting quasars and radio galaxies with powerful radio jets.

  19. Testing the Propagating Fluctuations Model with a Long, Global Accretion Disk Simulation

    NASA Astrophysics Data System (ADS)

    Hogg, J. Drew; Reynolds, Christopher S.

    2016-07-01

    The broadband variability of many accreting systems displays characteristic structures; log-normal flux distributions, root-mean square (rms)-flux relations, and long inter-band lags. These characteristics are usually interpreted as inward propagating fluctuations of the mass accretion rate in an accretion disk driven by stochasticity of the angular momentum transport mechanism. We present the first analysis of propagating fluctuations in a long-duration, high-resolution, global three-dimensional magnetohydrodynamic (MHD) simulation of a geometrically thin (h/r ≈ 0.1) accretion disk around a black hole. While the dynamical-timescale turbulent fluctuations in the Maxwell stresses are too rapid to drive radially coherent fluctuations in the accretion rate, we find that the low-frequency quasi-periodic dynamo action introduces low-frequency fluctuations in the Maxwell stresses, which then drive the propagating fluctuations. Examining both the mass accretion rate and emission proxies, we recover log-normality, linear rms-flux relations, and radial coherence that would produce inter-band lags. Hence, we successfully relate and connect the phenomenology of propagating fluctuations to modern MHD accretion disk theory.

  20. Evaporation of pure liquid sessile and spherical suspended drops: a review.

    PubMed

    Erbil, H Yildirim

    2012-01-15

    A sessile drop is an isolated drop which has been deposited on a solid substrate where the wetted area is limited by a contact line and characterized by contact angle, contact radius and drop height. Diffusion-controlled evaporation of a sessile drop in an ambient gas is an important topic of interest because it plays a crucial role in many scientific applications such as controlling the deposition of particles on solid surfaces, in ink-jet printing, spraying of pesticides, micro/nano material fabrication, thin film coatings, biochemical assays, drop wise cooling, deposition of DNA/RNA micro-arrays, and manufacture of novel optical and electronic materials in the last decades. This paper presents a review of the published articles for a period of approximately 120 years related to the evaporation of both sessile drops and nearly spherical droplets suspended from thin fibers. After presenting a brief history of the subject, we discuss the basic theory comprising evaporation of micrometer and millimeter sized spherical drops, self cooling on the drop surface and evaporation rate of sessile drops on solids. The effects of drop cooling, resultant lateral evaporative flux and Marangoni flows on evaporation rate are also discussed. This review also has some special topics such as drop evaporation on superhydrophobic surfaces, determination of the receding contact angle from drop evaporation, substrate thermal conductivity effect on drop evaporation and the rate evaporation of water in liquid marbles. PMID:22277832

  1. Small-Scale Variability of Large Cloud Drops

    NASA Technical Reports Server (NTRS)

    Marshak, Alexander; Knyazikhin, Y.; Wiscombe, Warren

    2004-01-01

    Cloud droplet size distribution is one of the most fundamental subjects in cloud physics. Understanding of spatial distribution and small-scale fluctuations of cloud droplets is essential for both cloud physics and atmospheric radiation. For cloud physics, it relates to the coalescence growth of raindrops while for radiation, it has a strong impact on a cloud's radiative properties. Most of the existing cloud radiation and precipitation formation models assume that the mean number of drops with a given radius varies proportionally to volume. The analysis of microphysical data on liquid water drop sizes shows that, for sufficiently small volumes, the number is proportional to the drop size dependent power of the volume. For abundant small drops present, the exponent is 1 as assumed in the conventional approach. However, for rarer large drops, the exponents fall below unity. At small scales, therefore, the mean number of large drops decreases with volume at a slower rate than the conventional approach assumes, suggesting more large drops at these scales than conventional models account for; their impact is consequently underestimated. Size dependent models of spatial distribution of cloud drops that simulate the observed power laws show strong drop clustering, the more so the larger the drops. The degree of clustering is determined by the observed exponents. The strong clustering of large drops arises naturally from the observed power-law statistics. Current theories of photon-cloud interaction and warm rain formation will need radical revision in order to produce these statistics; their underlying equations are unable to yield the observed power law.

  2. Organization of microbeads in Leidenfrost drops.

    PubMed

    Maquet, Laurent; Colinet, Pierre; Dorbolo, Stéphane

    2014-06-21

    We investigated the organization of micrometric hydrophilic beads (glass or basalt) immersed in Leidenfrost drops. Starting from a large volume of water compared to the volume of the beads, while the liquid evaporates, we observed that the grains are eventually trapped at the interface of the droplet and accumulate. At a moment, the grains entirely cover the droplet. We measured the surface area at this moment as a function of the total mass of particles inserted in the droplet. We concluded that the grains form a monolayer around the droplet assuming (i) that the packing of the beads at the surface is a random close packing and (ii) that the initial surface of the drop is larger than the maximum surface that the beads can cover. Regarding the evaporation dynamics, the beads are found to reduce the evaporation rate of the drop. The slowdown of the evaporation is interpreted as being the consequence of the dewetting of the particles located at the droplet interface which makes the effective surface of evaporation smaller. As a matter of fact, contact angles of the beads with the water deduced from the evaporation rates are consistent with contact angles of beads directly measured at a flat air-water interface of water in a container. PMID:24705688

  3. Accretion disk dynamics in X-ray binaries

    NASA Astrophysics Data System (ADS)

    Peris, Charith Srian

    Accreting X-ray binaries consist of a normal star which orbits a compact object with the former transferring matter onto the later via an accretion disk. These accretion disks emit radiation across the entire electromagnetic spectrum. This thesis exploits two regions of the spectrum, exploring the (1) inner disk regions of an accreting black hole binary, GRS1915+105, using X-ray spectral analysis and (2) the outer accretion disks of a set of neutron star and black hole binaries using Doppler Tomography applied on optical observations. X-ray spectral analysis of black hole binary GRS1915+105: GRS1915+105 stands out as an exceptional black hole primarily due to the wild variability exhibited by about half of its X-ray observations. This study focused on the steady X-ray observations of the source, which were found to exhibit significant curvature in the harder coronal component within the RXTE/PCA band-pass. The roughly constant inner-disk radius seen in a majority of the steady-soft observations is strongly reminiscent of canonical soft state black-hole binaries. Remarkably, the steady-hard observations show the presence of growing truncation in the inner-disk. A majority of the steady observations of GRS1915+105 map to the states observed in canonical black hole binaries which suggests that within the complexity of this source is a simpler underlying basis of states. Optical tomography of X-ray binary systems: Doppler tomography was applied to the strong line features present in the optical spectra of X-ray binaries in order to determine the geometric structure of the systems' emitting regions. The point where the accretion stream hits the disk, also referred to as the "hotspot'', is clearly identified in the neutron star system V691 CrA and the black hole system Nova Muscae 1991. Evidence for stream-disk overflows exist in both systems, consistent with relatively high accretion rates. In contrast, V926 Sco does not show evidence for the presence of a hotspot which

  4. Leidenfrost effect: accurate drop shape modeling and new scaling laws

    NASA Astrophysics Data System (ADS)

    Sobac, Benjamin; Rednikov, Alexey; Dorbolo, Stéphane; Colinet, Pierre

    2014-11-01

    In this study, we theoretically investigate the shape of a drop in a Leidenfrost state, focusing on the geometry of the vapor layer. The drop geometry is modeled by numerically matching the solution of the hydrostatic shape of a superhydrophobic drop (for the upper part) with the solution of the lubrication equation of the vapor flow underlying the drop (for the bottom part). The results highlight that the vapor layer, fed by evaporation, forms a concave depression in the drop interface that becomes increasingly marked with the drop size. The vapor layer then consists of a gas pocket in the center and a thin annular neck surrounding it. The film thickness increases with the size of the drop, and the thickness at the neck appears to be of the order of 10--100 μm in the case of water. The model is compared to recent experimental results [Burton et al., Phys. Rev. Lett., 074301 (2012)] and shows an excellent agreement, without any fitting parameter. New scaling laws also emerge from this model. The geometry of the vapor pocket is only weakly dependent on the superheat (and thus on the evaporation rate), this weak dependence being more pronounced in the neck region. In turn, the vapor layer characteristics strongly depend on the drop size.

  5. Emulsification in turbulent flow: 3. Daughter drop-size distribution.

    PubMed

    Tcholakova, Slavka; Vankova, Nina; Denkov, Nikolai D; Danner, Thomas

    2007-06-15

    Systematic set of experiments is performed to clarify the effects of several factors on the size distribution of the daughter drops, which are formed as a result of drop breakage during emulsification in turbulent flow. The effects of oil viscosity, etaD, interfacial tension, sigma, and rate of energy dissipation in the turbulent flow, epsilon, are studied. As starting oil-water premixes we use emulsions containing monodisperse oil drops, which have been generated by membrane emulsification. By passing these premixes through a narrow-gap homogenizer, working in turbulent regime of emulsification, we monitor the changes in the drop-size distribution with the emulsification time. The experimental data are analyzed by using a new numerical procedure, which is based on the assumption (supported by the experimental data) that the probability for formation of daughter drops with diameter smaller than the maximum diameter of the stable drops, ddrop number concentrations in the final emulsions, which are obtained after a long emulsification time. We found that the breakage of a single "mother" drop leads to the formation of multiple daughter drops, and that the number and size distribution of these daughter drops depend strongly on the viscosity of the dispersed phase. Different scaling laws are found to describe the experimental results for the oils of low and high viscosity. The obtained results for the daughter drop-size distribution are in a reasonably good agreement with the experimental results reported by other authors. In contrast, the comparison with several basic model functions, proposed in the literature, does not show good agreement and the possible reasons are discussed. The proposed numerical procedure allows us to describe accurately the evolution of all main characteristics of the drop-size distribution during emulsification, such as the number and volume averaged diameters, and the distributive and cumulative functions by

  6. Electrostatic Liquid-Drop-Levitation System

    NASA Technical Reports Server (NTRS)

    Rhim, Won Kyu; Chung, San Kun; Hyson, Michael T.; Elleman, Daniel D.

    1988-01-01

    Electrostatic levitator has levitated drops of liquid up to 4 mm in diameter while maintaining spherical drop shapes. Stable levitation of spherical drops valuable in experiments involving super-cooling, solidification, and crystal growth.

  7. THE VERTICAL COMPOSITION OF NEUTRINO-DOMINATED ACCRETION DISKS IN GAMMA-RAY BURSTS

    SciTech Connect

    Liu, Tong; Xue, Li; Gu, Wei-Min; Lu, Ju-Fu

    2013-01-10

    We investigate the vertical structure and element distribution of neutrino-dominated accretion flows around black holes in spherical coordinates using the reasonable nuclear statistical equilibrium. According to our calculations, heavy nuclei tend to be produced in a thin region near the disk surface, whose mass fractions are primarily determined by the accretion rate and vertical distribution of temperature and density. In this thin region, we find that {sup 56}Ni is dominant for the flow with a low accretion rate (e.g., 0.05 M {sub Sun} s{sup -1}), but {sup 56}Fe is dominant for the flow with a high accretion rate (e.g., 1 M {sub Sun} s{sup -1}). The dominant {sup 56}Ni in the aforementioned region may provide a clue to understanding the bumps in the optical light curve of core-collapse supernovae.

  8. Short time interval for condensation of high-temperature silicates in the solar accretion disk.

    PubMed

    Luu, Tu-Han; Young, Edward D; Gounelle, Matthieu; Chaussidon, Marc

    2015-02-01

    Chondritic meteorites are made of primitive components that record the first steps of formation of solids in our Solar System. Chondrules are the major component of chondrites, yet little is known about their formation mechanisms and history within the solar protoplanetary disk (SPD). We use the reconstructed concentrations of short-lived (26)Al in chondrules to constrain the timing of formation of their precursors in the SPD. High-precision bulk magnesium isotopic measurements of 14 chondrules from the Allende chondrite define a (26)Al isochron with (26)Al/(27)Al = 1.2(±0.2) × 10(-5) for this subset of Allende chondrules. This can be considered to be the minimum bulk chondrule (26)Al isochron because all chondrules analyzed so far with high precision (∼50 chondrules from CV and ordinary chondrites) have an inferred minimum bulk initial ((26)Al/(27)Al) ≥ 1.2 × 10(-5). In addition, mineral (26)Al isochrons determined on the same chondrules show that their formation (i.e., fusion of their precursors by energetic events) took place from 0 Myr to ∼2 Myr after the formation of their precursors, thus showing in some cases a clear decoupling in time between the two events. The finding of a minimum bulk chondrule (26)Al isochron is used to constrain the astrophysical settings for chondrule formation. Either the temperature of the condensation zone dropped below the condensation temperature of chondrule precursors at ∼1.5 My after the start of the Solar System or the transport of precursors from the condensation zone to potential storage sites stopped after 1.5 My, possibly due to a drop in the disk accretion rate. PMID:25605942

  9. Short time interval for condensation of high-temperature silicates in the solar accretion disk

    PubMed Central

    Luu, Tu-Han; Young, Edward D.; Gounelle, Matthieu; Chaussidon, Marc

    2015-01-01

    Chondritic meteorites are made of primitive components that record the first steps of formation of solids in our Solar System. Chondrules are the major component of chondrites, yet little is known about their formation mechanisms and history within the solar protoplanetary disk (SPD). We use the reconstructed concentrations of short-lived 26Al in chondrules to constrain the timing of formation of their precursors in the SPD. High-precision bulk magnesium isotopic measurements of 14 chondrules from the Allende chondrite define a 26Al isochron with 26Al/27Al = 1.2(±0.2) × 10−5 for this subset of Allende chondrules. This can be considered to be the minimum bulk chondrule 26Al isochron because all chondrules analyzed so far with high precision (∼50 chondrules from CV and ordinary chondrites) have an inferred minimum bulk initial (26Al/27Al) ≥ 1.2 × 10−5. In addition, mineral 26Al isochrons determined on the same chondrules show that their formation (i.e., fusion of their precursors by energetic events) took place from 0 Myr to ∼2 Myr after the formation of their precursors, thus showing in some cases a clear decoupling in time between the two events. The finding of a minimum bulk chondrule 26Al isochron is used to constrain the astrophysical settings for chondrule formation. Either the temperature of the condensation zone dropped below the condensation temperature of chondrule precursors at ∼1.5 My after the start of the Solar System or the transport of precursors from the condensation zone to potential storage sites stopped after 1.5 My, possibly due to a drop in the disk accretion rate. PMID:25605942

  10. Forced Oscillations of Supported Drops

    NASA Technical Reports Server (NTRS)

    Wilkes, Edward D.; Basaran, Osman A.

    1996-01-01

    Oscillations of supported liquid drops are the subject of wide scientific interest, with applications in areas as diverse as liquid-liquid extraction, synthesis of ceramic powders, growing of pure crystals in low gravity, and measurement of dynamic surface tension. In this research, axisymmetric forced oscillations of arbitrary amplitude of viscous liquid drops of fixed volume which are pendant from or sessile on a rod with a fixed or moving contact line and surrounded by an inviscid ambient gas are induced by moving the rod in the vertical direction sinusiodally in time. In this paper, a preliminary report is made on the computational analysis of the oscillations of supported drops that have 'clean' interfaces and whose contact lines remain fixed throughout their motions. The relative importance of forcing to damping can be increased by either increasing the amplitude of rod motion A or Reynolds number Re. It is shown that as the ratio of forcing to damping rises, for drops starting from an initial rest state a sharp increase in deformation can occur when they are forced to oscillate in the vicinity of their resonance frequencies, indicating the incipience of hysteresis. However, it is also shown that the existence of a second stable limit cycle and the occurrence of hysteresis can be observed if the drop is subjected to a so-called frequency sweep, where the forcing frequency is first increased and then decreased over a suitable range. Because the change in drop deformation response is abrupt in the vicinity of the forcing frequencies where hysteresis occurs, it should be possible to exploit the phenomenon to accurately measure the viscosity and surface tension of the drop liquid.

  11. ROTATING ACCRETION FLOWS: FROM INFINITY TO THE BLACK HOLE

    SciTech Connect

    Li, Jason; Ostriker, Jeremiah; Sunyaev, Rashid

    2013-04-20

    Accretion onto a supermassive black hole of a rotating inflow is a particularly difficult problem to study because of the wide range of length scales involved. There have been broadly utilized analytic and numerical treatments of the global properties of accretion flows, but detailed numerical simulations are required to address certain critical aspects. We use the ZEUS code to run hydrodynamical simulations of rotating, axisymmetric accretion flows with Bremsstrahlung cooling, considering solutions for which the centrifugal balance radius significantly exceeds the Schwarzschild radius, with and without viscous angular momentum transport. Infalling gas is followed from well beyond the Bondi radius down to the vicinity of the black hole. We produce a continuum of solutions with respect to the single parameter M-dot{sub B}/ M-dot{sub Edd}, and there is a sharp transition between two general classes of solutions at an Eddington ratio of M-dot{sub B}/M-dot{sub Edd}{approx}few Multiplication-Sign 10{sup -2}. Our high inflow solutions are very similar to the standard Shakura and Sunyaev results. But our low inflow results are to zeroth order the stationary Papaloizou and Pringle solution, which has no accretion. To next order in the small, assumed viscosity they show circulation, with disk and conical wind outflows almost balancing inflow. These solutions are characterized by hot, vertically extended disks, and net accretion proceeds at an extremely low rate, only of order {alpha} times the inflow rate. Our simulations have converged with respect to spatial resolution and temporal duration, and they do not depend strongly on our choice of boundary conditions.

  12. Production of ultra-small ink jet drops using drop-on-demand (DOD) drop formation

    NASA Astrophysics Data System (ADS)

    Gao, Haijing; Xu, Qi; Harris, Michael; Basaran, Osman

    2009-11-01

    The formation of drops having radii that are smaller than the radii of the nozzle from which they are ejected is an active area of research in drop-on-demand (DOD) ink jet printing. In the last decade, Chen and Basaran (Phys Fluids, 2002; US patent, 2003) showed experimentally and computationally that several fold reduction in drop radius R (an order of magnitude reduction in drop volume V) is possible by judicious use of waveform modulation in which one or more intrinsic time scales such as capillary time, time for vorticity diffusion, and time for piezo actuation are varied. In this paper, we report the results of a computational study through which we have uncovered a novel method for achieving a factor of 5-10 reduction in R (about two to three orders of magnitude reduction in V). Scaling arguments are also developed which yield a simple expression for the size of the ultra-small drops formed as a function of the governing dimensionless groups. Formation of such small drops using DOD technology may prove especially attractive in applications involving direct printing of flexible electronics and solar cells.

  13. Lithium synthesis in microquasar accretion.

    PubMed

    Iocco, Fabio; Pato, Miguel

    2012-07-13

    We study the synthesis of lithium isotopes in the hot tori formed around stellar mass black holes by accretion of the companion star. We find that sizable amounts of both stable isotopes 6Li and 7Li can be produced, the exact figures varying with the characteristics of the torus and reaching as much as 10(-2) M⊙ for each isotope. This mass output is enough to contaminate the entire Galaxy at a level comparable with the original, pregalactic amount of lithium and to overcome other sources such as cosmic-ray spallation or stellar nucleosynthesis. PMID:23030150

  14. Impact of a single drop on a flowing liquid film.

    PubMed

    Gao, Xuan; Li, Ri

    2015-11-01

    The impact of a single liquid drop on a flowing liquid film is experimentally and theoretically studied. The drop impact produces a crownlike rising liquid sheet, which radially expands. Small droplets can be formed from the crown sheet, resulting in splash. The present study results in three major contributions. (1) A theoretical model is developed to predict the expansion of the crown base. The model with an introduced energy loss factor is shown to be in satisfactory agreement with our experimental observations of drop impact on both stationary and flowing films. The energy loss factor is correlated to the properties of the film and drop. (2) Analysis is conducted to derive an equation for evaluating the stretching rate of the rising crown sheet, which is the local gradient of the rising velocity at the top edge of the crown sheet. It shows that the highest stretching rate appears where the drop spreading flow is right opposite to the film flow, which helps explain why the same location is most probable for splash to take place. (3) A parameter as a function of modified Weber and Reynolds numbers is defined to predict splash and nonsplash of drop impact on flowing films. The two nondimensional numbers evaluate the competition of the two flows (drop and film) against viscosity and surface tension effects. A threshold value of the parameter is found for the occurrence of splash impact on flowing films. PMID:26651777

  15. Coalescence of Drops of a Power-law Fluid

    NASA Astrophysics Data System (ADS)

    Kamat, Pritish; Thete, Sumeet; Basaran, Osman

    2014-11-01

    Drop coalescence is crucial in a host of industrial, household, and natural processes that involve dispersions. Coalescence is a rate-controlling process in breaking emulsions and strongly influences drop-size-distributions in sprays. In a continuum approach, coalescence begins by the formation of a microscopic, non-slender bridge connecting the two drops. Indefinitely large axial curvature at the neck results in local lowering of pressure that drives fluid from the bulk of the drops toward the neck, thereby causing the bridge radius r (t) and height z (t) to increase in time t. The coalescence of Newtonian drops in air has heretofore been thoroughly studied. Here, we extend these earlier studies by analyzing the coalescence of drops of power-law fluids because many fluids encountered in real applications, including cosmetic creams, shampoos, grease, and paint, exhibit power-law (deformation-rate thinning) rheology. On account of the non-slender geometry of the liquid bridge connecting the two drops (z << r) , we analyze the resulting free surface flow problem by numerical simulation. Among other results, we present and discuss the nature of flows and scaling behaviors for r and z as functions of the initial viscosity and power-law index (0 < n <= 1) .

  16. Measurement of an Evaporating Drop on a Reflective Substrate

    NASA Technical Reports Server (NTRS)

    Chao, David F.; Zhang, Nengli

    2004-01-01

    A figure depicts an apparatus that simultaneously records magnified ordinary top-view video images and laser shadowgraph video images of a sessile drop on a flat, horizontal substrate that can be opaque or translucent and is at least partially specularly reflective. The diameter, contact angle, and rate of evaporation of the drop as functions of time can be calculated from the apparent diameters of the drop in sequences of the images acquired at known time intervals, and the shadowgrams that contain flow patterns indicative of thermocapillary convection (if any) within the drop. These time-dependent parameters and flow patterns are important for understanding the physical processes involved in the spreading and evaporation of drops. The apparatus includes a source of white light and a laser (both omitted from the figure), which are used to form the ordinary image and the shadowgram, respectively. Charge-coupled-device (CCD) camera 1 (with zoom) acquires the ordinary video images, while CCD camera 2 acquires the shadowgrams. With respect to the portion of laser light specularly reflected from the substrate, the drop acts as a plano-convex lens, focusing the laser beam to a shadowgram on the projection screen in front of CCD camera 2. The equations for calculating the diameter, contact angle, and rate of evaporation of the drop are readily derived on the basis of Snell s law of refraction and the geometry of the optics.

  17. A growing-drop technique for measuring dynamic interfacial tension

    SciTech Connect

    MacLeod, C.A.; Radke, C.J.

    1993-10-01

    A novel, growing-drop technique is described for measuring dynamic interfacial tension due to sorption of surface-active solutes. The proposed method relates the instantaneous pressure and size of expanding liquid drops to interfacial tension and is useful for measuring both liquid/gas and liquid/liquid tensions over a wide range of time scales, currently from 10 ms to several hours. Growing-drop measurements on surfactant-free water/air and water/octanol interfaces yield constant tensions equal to their known literature values. For surfactant-laden, liquid drops, the growing-drop technique captures the actual transient tension evolution of a single interface, rather than interval times as with the classic maximum-drop-pressure and drop.-volume tension measurements. Dynamic tensions measured for 0.25 mM aqueous 1-decanol solution/air and 0.02 kg/m{sup 3} aqueous Triton X-100 solution/dodecane interfaces show nonmonotonic behavior, indicating slow surfactant transport relative to the imposed rates of interfacial dilatation. The dynamic tension of a purified and fresh 6 mM aqueous sodium dodecyl sulfate (SDS) solution/air interface shows only a monotonic decrease, indicating rapid surfactant transport relative to the imposed rates of dilatation. ConverselY, an aged SDS solution, naturally containing trace dodecanol impurities, exhibits dynamic tensions which reflect a superposition of the rapidly equilibrating SDS and the slowly adsorbing dodecanol.

  18. The effect of pressure drop on respirator faceseal leakage.

    PubMed

    Janssen, Larry; Weber, Robert

    2005-07-01

    Users of particulate air-purifying respirators are typically told to change their filters when breathing resistance becomes uncomfortable. It has been proposed that a noticeable increase in breathing resistance (pressure drop) may increase airflow through respirator faceseal leaks. This logic has been extended to suggest that respirator user exposure to contaminants may increase because of this theoretical increase in air leakage. Procedures similar to those of previous investigators were used to study this issue. Repeated faceseal leak rate measurements were made at -5.6 through -20.1 mm water pressure drops across the faceseal. Subjects were divided into two groups, representing acceptable fit or unacceptable fit, based on leak rate criteria prescribed by the Occupational Safety and Health Administration (OSHA). Subjects with acceptable fit did not experience an increase in faceseal leak rate with increased pressure drop. Leak rates for subjects with unacceptable fit were highly variable and did not show an association with pressure drop. Results of this study do not support the concept of increased faceseal leakage with increased pressure drop. The evidence does not suggest increased risk of contaminant exposure through the face seal as pressure drop increases. PMID:16020096

  19. Precession and accretion in circumbinary discs: the case of HD 104237

    NASA Astrophysics Data System (ADS)

    Dunhill, A. C.; Cuadra, J.; Dougados, C.

    2015-04-01

    We present the results of smoothed particle hydrodynamics (SPH) simulations of the disc around the young, eccentric stellar binary HD 104237. We find that the binary clears out a large cavity in the disc, driving a significant eccentricity at the cavity edge. This then precesses around the binary at a rate of dot{\\varpi } = 0.48°Tb^{-1}, which for HD 104237 corresponds to a precession period of 40 years. We find that the accretion pattern into the cavity and on to the binary changes with this precession, resulting in a periodic accretion variability driven purely by the physical parameters of the binary and its orbit. For each star we find that this results in order of magnitude changes in the accretion rate. We also find that the accretion variability allows the primary to accrete gas at a higher rate than the secondary for approximately half of each precession period. Using a large number of three-body integrations of test particles orbiting different binaries, we find good agreement between the precession rate of a test particle and our SPH disc precession. These rates also agree very well with the precession rates predicted by the analytic theory of Leung & Lee, showing that their prescription can be accurately used to predict long-term accretion variability time-scales for eccentric binaries accreting from a disc. We discuss the implications of our result, and suggest that this process provides a viable way of preserving unequal-mass ratios in accreting eccentric binaries in both the stellar and supermassive black hole regimes.

  20. ACCRETION VARIABILITY OF HERBIG Ae/Be STARS OBSERVED BY X-SHOOTER HD 31648 AND HD 163296

    SciTech Connect

    Mendigutía, I.; Brittain, S.; Eiroa, C.; Meeus, G.; Montesinos, B.; Mora, A.; Muzerolle, J.; Oudmaijer, R. D.; Rigliaco, E.

    2013-10-10

    This work presents X-Shooter/Very Large Telescope spectra of the prototypical, isolated Herbig Ae stars HD 31648 (MWC 480) and HD 163296 over five epochs separated by timescales ranging from days to months. Each spectrum spans over a wide wavelength range covering from 310 to 2475 nm. We have monitored the continuum excess in the Balmer region of the spectra and the luminosity of 12 ultraviolet, optical, and near-infrared spectral lines that are commonly used as accretion tracers for T Tauri stars. The observed strengths of the Balmer excesses have been reproduced from a magnetospheric accretion shock model, providing a mean mass accretion rate of 1.11 × 10{sup –7} and 4.50 × 10{sup –7} M{sub ☉} yr{sup –1} for HD 31648 and HD 163296, respectively. Accretion rate variations are observed, being more pronounced for HD 31648 (up to 0.5 dex). However, from the comparison with previous results it is found that the accretion rate of HD 163296 has increased by more than 1 dex, on a timescale of ∼15 yr. Averaged accretion luminosities derived from the Balmer excess are consistent with the ones inferred from the empirical calibrations with the emission line luminosities, indicating that those can be extrapolated to HAe stars. In spite of that, the accretion rate variations do not generally coincide with those estimated from the line luminosities, suggesting that the empirical calibrations are not useful to accurately quantify accretion rate variability.

  1. Thermonuclear processes on accreting neutron stars - A systematic study

    NASA Technical Reports Server (NTRS)

    Ayasli, S.; Joss, P. C.

    1982-01-01

    A series of model calculations for the evolution of the surface layers of an accreting neutron star is carried out. The neutron star mass, radius, core temperature, and surface magnetic field strength are systematically varied, as are the accretion rate onto the neutron star surface and the metallicity of the accreting matter, in order to determine the effects of these parameters on the properties of thermonuclear flashes in the surface layers and the emitted X-ray bursts that result from such flashes. The core temperatures required for thermal equilibrium are found to be approximately a factor of 2 lower than estimated in earlier work. Owing to the effects of the gravitational redshift, the emitted X-ray bursts have lower peak luminosities and longer durations than those calculated in the Newtonian approximation. The entrainment of hydrogen into helium flashes can cause the flashes to exhibit a rather wide range of observable effects and can decrease by a factor of more than 2 the ratio of persistent accretion-driven luminosity to time-averaged burst luminosity emitted by the neutron star.

  2. Peculiarities of the accretion flow in the system HL CMa

    NASA Astrophysics Data System (ADS)

    Semena, A. N.; Revnivtsev, M. G.; Buckley, D.; Lutovinov, A. A.; Breitenbach, H.

    2016-06-01

    The properties of the aperiodic luminosity variability for the dwarf novaHLCMa are considered. The variability of the system HL CMa is shown to be suppressed at frequencies above 0.7 × 10-2 Hz. Different variability suppression mechanisms related to the radiation reprocessing time, partial disk evaporation, and characteristic variability formation time are proposed. It has been found that the variability suppression frequency does not change when the system passes from the quiescent state to the outburst one, suggesting that the accretion flow geometry is invariable. It is concluded from the optical and Xray luminosities of the system that the boundary layer on the white dwarf surface is optically thick in both quiescent and outburst states. The latter implies that the optically thick part of the accretion flow (disk) reaches the white dwarf surface. The accretion rate in the system and the accretion flow geometry and temperature have been estimated from the variability power spectra and spectral characteristics in a wide energy range, from the optical to X-ray ones.

  3. Review on drop towers and long drop tubes

    NASA Technical Reports Server (NTRS)

    Bayuzick, R. J.; Hofmeister, W. H.; Robinson, M. B.

    1987-01-01

    A drop tube is an enclosure in which a molten sample can be solidified while falling; three such large tubes are currently in existence, all at NASA research facilities, and are engaged in combustion and fluid physics-related experiments rather than in materials research. JPL possesses smaller tubes, one of which can be cryogenically cooled to produce glass and metal microshells. A new small drop tube will soon begin operating at NASA Lewis that is equipped with four high-speed two-color pyrometers spaced equidistantly along the column.

  4. Sediment accretion in tidal freshwater forests and oligohaline marshes of the Waccamaw and Savannah Rivers, USA

    USGS Publications Warehouse

    Ensign, Scott H.; Hupp, Cliff R.; Noe, Gregory B.; Krauss, Ken W.; Stagg, Camille L.

    2014-01-01

    Sediment accretion was measured at four sites in varying stages of forest-to-marsh succession along a fresh-to-oligohaline gradient on the Waccamaw River and its tributary Turkey Creek (Coastal Plain watersheds, South Carolina) and the Savannah River (Piedmont watershed, South Carolina and Georgia). Sites included tidal freshwater forests, moderately salt-impacted forests at the freshwater–oligohaline transition, highly salt-impacted forests, and oligohaline marshes. Sediment accretion was measured by use of feldspar marker pads for 2.5 year; accessory information on wetland inundation, canopy litterfall, herbaceous production, and soil characteristics were also collected. Sediment accretion ranged from 4.5 mm year−1 at moderately salt-impacted forest on the Savannah River to 19.1 mm year−1 at its relict, highly salt-impacted forest downstream. Oligohaline marsh sediment accretion was 1.5–2.5 times greater than in tidal freshwater forests. Overall, there was no significant difference in accretion rate between rivers with contrasting sediment loads. Accretion was significantly higher in hollows than on hummocks in tidal freshwater forests. Organic sediment accretion was similar to autochthonous litter production at all sites, but inorganic sediment constituted the majority of accretion at both marshes and the Savannah River highly salt-impacted forest. A strong correlation between inorganic sediment accumulation and autochthonous litter production indicated a positive feedback between herbaceous plant production and allochthonous sediment deposition. The similarity in rates of sediment accretion and sea level rise in tidal freshwater forests indicates that these habitats may become permanently inundated if the rate of sea level rise increases.

  5. The pulse amplitude variation with QPO frequency in SAX J1808.4-3658: Resonances with the accretion disk

    NASA Astrophysics Data System (ADS)

    Caliskan, Sirin; Alpar, Mehmet Ali; Sasmaz Mus, Sinem

    2016-07-01

    SAX J1808.4-3658 is an accreting millisecond pulsar with a spin period of 401 Hz. The pulsed amplitudes of this source vary with its kHz QPO frequencies (Bult & van der Klis 2015). The pulsed amplitude peaks at certain upper kHz QPO frequencies which we associate with boundary layer modes of the viscous accretion disk (Erkut et al. 2008). We model this as peaks in the energy dissipation rate at the accretion caps due to resonances between the accretion column and the driving modes of the boundary layer.

  6. Static Magnetowetting of Ferrofluid Drops.

    PubMed

    Rigoni, Carlo; Pierno, Matteo; Mistura, Giampaolo; Talbot, Delphine; Massart, René; Bacri, Jean-Claude; Abou-Hassan, Ali

    2016-08-01

    We report results of a comprehensive study of the wetting properties of sessile drops of ferrofluid water solutions at various concentrations deposited on flat substrates and subjected to the action of permanent magnets of different sizes and strengths. The amplitude and the gradient of the magnetic field experienced by the ferrofluid are changed by varying the magnets and their distance to the surface. Magnetic forces up to 100 times the gravitational one and magnetic gradients up to 1 T/cm are achieved. A rich phenomenology is observed, ranging from flattened drops caused by the magnetic attraction to drops extended normally to the substrate because of the normal traction of the magnetic field. We find that the former effect can be conveniently described in terms of an effective Bond number that compares the effective drop attraction with the capillary force, whereas the drop's vertical elongation is effectively expressed by a dimensionless number S, which compares the pressure jump at the ferrofluid interface because of the magnetization with the capillary pressure. PMID:27385506

  7. Air induced breakup of drops.

    NASA Astrophysics Data System (ADS)

    Han, Jaehoon; Tryggvason, Gretar

    1997-11-01

    The deformation and breakup of drops subject to both sudden and gradual acceleration is examined by axisymmetric inviscid and full numerical simulations. In the full simulations, the Navier Stokes equations are solved for the fluid inside and outside of the drop by a Front Tracking/Finite Difference Method. In the limit of small density stratification, inviscid simulations show the formation of a toroidal drop for small surface tension and the formation of skirts as the surface tension is increased. The viscous computations show a similar transition plus a RbagS break up for a relatively high surface tension, but not high enough so that the drop reaches a steady state deformation. The RbagS break up mode appears when the drop slows down due to viscous dissipation after most of its fluid has accumulated in the rim, forming a torous connected by a thin film. A RbagS is formed when the rim starts to fall faster than the film. The various break up modes, as a function of the Ohnesorge and Weber (or Eotvos) numbers as well as property ratios is discussed. Supported by AFOSR.

  8. Universal evolution of a viscous-capillary spreading drop.

    PubMed

    Thampi, Sumesh P; Pagonabarraga, Ignacio; Adhikari, Ronojoy; Govindarajan, Rama

    2016-07-13

    The rate of spreading or retraction of a drop on a flat substrate is determined through a balance of surface tension and hydrodynamic flow. While asymptotic regimes are known, no general rate equation has hitherto been available. Here, we revisit this classic problem, in a regime governed by capillary and viscous forces, by performing an exhaustive numerical study of drop evolution as a function of the contact angle with the substrate. Our study reveals a universal evolution of the drop radius parameterised only by the substrate wettability. Two limits of this evolution recover the familiar exponential and algebraic regimes. Our results show quantitative comparison with the evolution derived from lubrication theory, indicating that dissipation at the contact line is the key determinant in drop evolution. Our work, both numerical and theoretical, provides a foundation for studying the full temporal dynamics of droplet evolution under the influence of external fields and thermal fluctuations, which are of importance in nanofluidics. PMID:27374245

  9. 1 Hz Flaring in the Accreting Millisecond Pulsar NGC 6440 X-2: Disk Trapping and Accretion Cycles

    NASA Astrophysics Data System (ADS)

    Patruno, Alessandro; D'Angelo, Caroline

    2013-07-01

    The dynamics of the plasma in the inner regions of an accretion disk around accreting millisecond X-ray pulsars (AMXPs) is controlled by the magnetic field of the neutron star. The interaction between an accretion disk and a strong magnetic field is not well understood, particularly at low accretion rates (the so-called propeller regime). This is due in part to the lack of clear observational diagnostics to constrain the physics of the disk-field interaction. Here, we associate the strong ~1 Hz modulation seen in the AMXP NGC 6440 X-2 with an instability that arises when the inner edge of the accretion disk is close to the corotation radius (where the stellar rotation rate matches the Keplerian speed in the disk). A similar modulation has previously been observed in another AMXP (SAX J1808.4-3658) and we suggest that the two phenomena are related and that this may be a common phenomenon among other magnetized systems. Detailed comparisons with theoretical models suggest that when the instability is observed, the interaction region between the disk and the field is very narrow—of the order of 1 km. Modeling further suggests that there is a transition region (~1-10 km) around the corotation radius where the disk-field torque changes sign from spin-up to spin-down. This is the first time that a direct observational constraint has been placed on the width of the disk-magnetosphere interaction region, in the frame of the trapped-disk instability model.

  10. Swept wing ice accretion modeling

    NASA Technical Reports Server (NTRS)

    Potapczuk, Mark G.; Bidwell, Colin S.

    1990-01-01

    An effort to develop a three-dimensional modeling method was initiated. This first step towards creation of a complete aircraft icing simulation code builds on previously developed methods for calculating three-dimensional flow fields and particle trajectories combined with a two-dimensional ice accretion calculation along coordinate locations corresponding to streamlines. This work is a demonstration of the types of calculations necessary to predict a three-dimensional ice accretion. Results of calculations using the 3-D method for a MS-317 swept wing geometry are projected onto a 2-D plane normal to the wing leading edge and compared to 2-D results for the same geometry. It is anticipated that many modifications will be made to this approach, however, this effort will lay the groundwork for future modeling efforts. Results indicate that the flow field over the surface and the particle trajectories differed for the two calculations. This led to lower collection efficiencies, convective heat transfer coefficients, freezing fractions, and ultimately ice accumulation for the 3-D calculation.

  11. Ringed Accretion Disks: Equilibrium Configurations

    NASA Astrophysics Data System (ADS)

    Pugliese, D.; Stuchlík, Z.

    2015-12-01

    We investigate a model of a ringed accretion disk, made up by several rings rotating around a supermassive Kerr black hole attractor. Each toroid of the ringed disk is governed by the general relativity hydrodynamic Boyer condition of equilibrium configurations of rotating perfect fluids. Properties of the tori can then be determined by an appropriately defined effective potential reflecting the background Kerr geometry and the centrifugal effects. The ringed disks could be created in various regimes during the evolution of matter configurations around supermassive black holes. Therefore, both corotating and counterrotating rings have to be considered as being a constituent of the ringed disk. We provide constraints on the model parameters for the existence and stability of various ringed configurations and discuss occurrence of accretion onto the Kerr black hole and possible launching of jets from the ringed disk. We demonstrate that various ringed disks can be characterized by a maximum number of rings. We present also a perturbation analysis based on evolution of the oscillating components of the ringed disk. The dynamics of the unstable phases of the ringed disk evolution seems to be promising in relation to high-energy phenomena demonstrated in active galactic nuclei.

  12. Pressure drop in tubing in aircraft instrument installations

    NASA Technical Reports Server (NTRS)

    Wildhack, W A

    1937-01-01

    The theoretical basis of calculation of pressure drop in tubing is reviewed briefly. The effect of pressure drop in connecting tubing upon the operation and indication of aircraft instruments is discussed. Approximate equations are developed, and charts and tables based upon them are presented for use in designing installations of altimeters, air-speed indicators, rate-of-climb indicators, and air-driven gyroscopic instruments.

  13. Beta blocker eye drops for treatment of acute migraine.

    PubMed

    Migliazzo, Carl V; Hagan, John C

    2014-01-01

    We report seven cases of successful treatment of acute migraine symptoms using beta blocker eye drops. The literature on beta blockers for acute migraine is reviewed. Oral beta blocker medication is not effective for acute migraine treatment. This is likely due to a relatively slow rate of achieving therapeutic plasma levels when taken orally. Topical beta blocker eye drops achieve therapeutic plasma levels within minutes of ocular administration which may explain their apparent effectiveness in relief of acute migraine symptoms. PMID:25211851

  14. Testing Convergence for Global Accretion Disks

    NASA Astrophysics Data System (ADS)

    Hawley, John F.; Richers, Sherwood A.; Guan, Xiaoyue; Krolik, Julian H.

    2013-08-01

    Global disk simulations provide a powerful tool for investigating accretion and the underlying magnetohydrodynamic turbulence driven by magneto-rotational instability (MRI). Using them to accurately predict quantities such as stress, accretion rate, and surface brightness profile requires that purely numerical effects, arising from both resolution and algorithm, be understood and controlled. We use the flux-conservative Athena code to conduct a series of experiments on disks having a variety of magnetic topologies to determine what constitutes adequate resolution. We develop and apply several resolution metrics: langQz rang and langQ phirang, the ratio of the grid zone size to the characteristic MRI wavelength, αmag, the ratio of the Maxwell stress to the magnetic pressure, and \\langle B_R^2\\rangle /\\langle B_\\phi ^2\\rangle, the ratio of radial to toroidal magnetic field energy. For the initial conditions considered here, adequate resolution is characterized by langQz rang >= 15, langQ phirang >= 20, αmag ≈ 0.45, and \\langle B_R^2\\rangle /\\langle B_\\phi ^2\\rangle \\approx 0.2. These values are associated with >=35 zones per scaleheight H, a result consistent with shearing box simulations. Numerical algorithm is also important. Use of the Harten-Lax-van Leer-Einfeldt flux solver or second-order interpolation can significantly degrade the effective resolution compared to the Harten-Lax-van Leer discontinuities flux solver and third-order interpolation. Resolution at this standard can be achieved only with large numbers of grid zones, arranged in a fashion that matches the symmetries of the problem and the scientific goals of the simulation. Without it, however, quantitative measures important to predictions of observables are subject to large systematic errors.

  15. TESTING CONVERGENCE FOR GLOBAL ACCRETION DISKS

    SciTech Connect

    Hawley, John F.; Richers, Sherwood A.; Guan Xiaoyue; Krolik, Julian H. E-mail: xg3z@virginia.edu

    2013-08-01

    Global disk simulations provide a powerful tool for investigating accretion and the underlying magnetohydrodynamic turbulence driven by magneto-rotational instability (MRI). Using them to accurately predict quantities such as stress, accretion rate, and surface brightness profile requires that purely numerical effects, arising from both resolution and algorithm, be understood and controlled. We use the flux-conservative Athena code to conduct a series of experiments on disks having a variety of magnetic topologies to determine what constitutes adequate resolution. We develop and apply several resolution metrics: (Q{sub z} ) and (Q{sub {phi}}), the ratio of the grid zone size to the characteristic MRI wavelength, {alpha}{sub mag}, the ratio of the Maxwell stress to the magnetic pressure, and /, the ratio of radial to toroidal magnetic field energy. For the initial conditions considered here, adequate resolution is characterized by (Q{sub z} ) {>=} 15, (Q{sub {phi}}) {>=} 20, {alpha}{sub mag} Almost-Equal-To 0.45, and /{approx}0.2. These values are associated with {>=}35 zones per scaleheight H, a result consistent with shearing box simulations. Numerical algorithm is also important. Use of the Harten-Lax-van Leer-Einfeldt flux solver or second-order interpolation can significantly degrade the effective resolution compared to the Harten-Lax-van Leer discontinuities flux solver and third-order interpolation. Resolution at this standard can be achieved only with large numbers of grid zones, arranged in a fashion that matches the symmetries of the problem and the scientific goals of the simulation. Without it, however, quantitative measures important to predictions of observables are subject to large systematic errors.

  16. Isoelectric Focusing in a Drop

    PubMed Central

    Weiss, Noah G.; Hayes, Mark A.; Garcia, Antonio A.; Ansari, Rafat R.

    2010-01-01

    A novel approach to molecular separations is investigated using a technique termed droplet-based isoelectric focusing. Drops are manipulated discretely on a superhydrophobic surface, subjected to low voltages for isoelectric focusing, and split—resulting in a preparative separation. A universal indicator dye demonstrates the generation of stable, reversible pH gradients (3–10) in ampholyte buffers and these gradients lead to protein focusing within the drop length. Focusing was visually characterized, spectroscopically verified, and assessed quantitatively by non-invasive light scattering measurements. It was found to correlate with a quantitative model based on 1D steady state theory. This work illustrates that molecular separations can be deployed within a single open drop and the differential fractions can be separated into new discrete liquid elements. PMID:21117663

  17. The Phase Space of z~1.2 SpARCS Clusters: Using Herschel to Probe Dust Temperature as a Function of Environment and Accretion History

    NASA Astrophysics Data System (ADS)

    Noble, A. G.; Webb, T. M. A.; Yee, H. K. C.; Muzzin, A.; Wilson, G.; van der Burg, R. F. J.; Balogh, M. L.; Shupe, D. L.

    2016-01-01

    We present a five-band Herschel study (100-500 μm) of three galaxy clusters at z˜ 1.2 from the Spitzer Adaptation of the Red-Sequence Cluster Survey. With a sample of 120 spectroscopically confirmed cluster members, we investigate the role of environment on galaxy properties utilizing the projected cluster phase space (line-of-sight velocity versus clustercentric radius), which probes the time-averaged galaxy density to which a galaxy has been exposed. We divide cluster galaxies into phase-space bins of (r/{r}200)× ({{Δ }}v/{σ }v), tracing a sequence of accretion histories in phase space. Stacking optically star-forming cluster members on the Herschel maps, we measure average infrared star formation rates, and, for the first time in high-redshift galaxy clusters, dust temperatures for dynamically distinct galaxy populations—namely, recent infalls and those that were accreted onto the cluster at an earlier epoch. Proceeding from the infalling to virialized (central) regions of phase space, we find a steady decrease in the specific star formation rate and increase in the stellar age of star-forming cluster galaxies. We perform a probability analysis to investigate all acceptable infrared spectral energy distributions within the full parameter space and measure a ˜ 4σ drop in the average dust temperature of cluster galaxies in an intermediate phase-space bin, compared to an otherwise flat trend with phase space. We suggest one plausible quenching mechanism which may be consistent with these trends, invoking ram-pressure stripping of the warmer dust for galaxies within this intermediate accretion phase. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

  18. Dynamics of polymeric drop breakup in microchannels

    NASA Astrophysics Data System (ADS)

    Arratia, Paulo; Gollub, Jerry; Durian, Douglas

    2006-11-01

    The dynamics of drop formation of sheared polymeric and Newtonian fluids are investigated in a 50 μm microchannel. Inverse emulsions are obtained in a cross-like geometry by impinging a continuous oil phase (with surfactant) onto either a polymeric or a Newtonian aqueous solution. The viscosity ratio between the continuous and dispersed phases is kept close to unity, and both flow rates are varied. Solutions containing small amounts (100 ppm) of flexible polymers strongly affect the filament and drop breakup processes when compared to a Newtonian solution of similar viscosity. We find that the thinning of the filament for the Newtonian case is characterized by linear decline followed by a rapid approach to breakup. The polymeric case shows an initial Newtonian-like thinning followed by a slower, elasticity- dominated thinning. Consequently, the filament breakup time and length are considerably increased for the polymeric solutions. Also, larger primary drops and beads-on-string phenomena are found for the polymer solutions.

  19. Star accretion onto supermassive black holes in axisymmetric galactic nuclei

    NASA Astrophysics Data System (ADS)

    Zhong, Shiyan; Berczik, Peter; Spurzem, Rainer

    2016-02-01

    Tidal Disruption (TD) of stars by supermassive central black holes from dense rotating star clusters is modeled by high-accuracy direct N-body simulation. We study the time evolution of the stellar tidal disruption rate and the origin of tidally disrupted stars. Compared with that in spherical systems, we found a higher TD rate in axisymmetric systems. The enhancement can be explained by an enlarged loss-cone in phase space which is raised from the fact that total angular momentum J is not conserved. As in the case of spherical systems, the distribution of the last apocenter distance of tidally accreted stars peaks at the classical critical radius. However, the angular distribution of the origin of the accreted stars reveals bimodal features. We show that the bimodal structure can be explained by the presence of two families of regular orbits, namely short axis tube and saucer orbits.

  20. Vegetation Influences on Tidal Freshwater Marsh Sedimentation and Accretion

    NASA Astrophysics Data System (ADS)

    Cadol, D. D.; Elmore, A. J.; Engelhardt, K.; Palinkas, C. M.

    2011-12-01

    Continued sea level rise, and the potential for acceleration over the next century, threatens low-lying natural and cultural resources throughout the world. In the national capital region of the United States, for example, the National Park Service manages over 50 km^2 of land along the shores of the tidal Potomac River and its tributaries that may be affected by sea level rise. Dyke Marsh Wildlife Preserve on the Potomac River south of Washington, DC, is one such resource with a rich history of scientific investigation. It is a candidate for restoration to replace marsh area lost to dredging in the 1960s, yet for restoration to succeed in the long term, accretion must maintain the marsh surface within the tidal range of rising relative sea level. Marsh surface accretion rates tend to increase with depth in the tidal frame until a threshold depth is reached below which marsh vegetation cannot be sustained. Suspended sediment concentration, salinity, tidal range, and vegetation community all influence the relationship between depth and accretion rate. The complex interactions among these factors make sedimentation rates difficult to generalize across sites. Surface elevation tables (SET) and feldspar marker horizons have been monitored at 9 locations in Dyke Marsh for 5 years, providing detailed data on sedimentation, subsidence, and net accretion rates at these locations. We combine these data with spatially rich vegetation surveys, a LiDAR derived 1-m digital elevation model of the marsh, and temperature-derived inundation durations to model accretion rates across the marsh. Temperature loggers suggest a delayed arrival of tidal water within the marsh relative to that predicted by elevation alone, likely due to hydraulic resistance caused by vegetation. Wave driven coastal erosion has contributed to bank retreat rates of ~2.5 m/yr along the Potomac River side of the marsh while depositing a small berm of material inland of the retreating shoreline. Excluding sites

  1. Occurrence of instability through the protostellar accretion disks by landing of low-mass condensations

    NASA Astrophysics Data System (ADS)

    Elyasi, Mahjubeh; Nejad-Asghar, Mohsen

    2016-06-01

    Low-mass condensations (LMCs) are observed inside the envelope of the collapsing molecular cloud cores. In this research, we investigate the effects of landing LMCs for occurrence of instability through the protostellar accretion disks. We consider some regions of the disk where duration of infalling and landing of the LMCs are shorter than the orbital period. In this way, we can consider the landing LMCs as density bumps and grooves in the azimuthal direction of an initial thin axisymmetric steady state self-gravitating protostellar accretion disk (nearly Keplerian). Using the linear effects of the bump quantities, we obtain a characteristic equation for growth/decay rate of bumps; we numerically solve it to find occurrence of instability. We also evaluate the minimum-growth-time-scale (MGTS) and the enhanced mass accretion rate. The results show that infalling and landing of the LMCs in the inner regions of the protostellar accretion disks can cause faster unstable modes and less enhanced accretion rates relative to the outer regions. Also, more fragmentation of landed LMCs in the azimuthal direction have less chance for instability, and then can produce more values of enhanced mass accretion rate.

  2. Ultraviolet observations of accretion disk in LMC X-3

    NASA Technical Reports Server (NTRS)

    Cowley, A. P.; Schmidthe, P. C.; Hutchings, J. B.; Crampton, D.

    1994-01-01

    We report information obtained from a series of International Ultraviolet Explorer (IUE) low-resolution spectra and two HST UV spectra of LMC X-3. The HST spectra are used to identify disk emission lines and interstellar absorptions as well as to fit continuum models, which indicate the presence of a hot disk component. The IUE observations, mainly taken over approximately 1.5 years, were intended to study how the observed characteristics of the accretion disk change through the precessional cycle. It is shown that although the emission line strengths and short-wavelength ultraviolet flux are well correlated, both optical and UV data show little long-term periodic modulation was present during our observations, indicating LMC X-3 had dropped into a 'low' state.

  3. Formation of the Giant Planets by Concurrent Accretion of Solids and Gas

    NASA Technical Reports Server (NTRS)

    Hubickyj, Olenka

    1997-01-01

    Models were developed to simulate planet formation. Three major phases are characterized in the simulations: (1) planetesimal accretion rate, which dominates that of gas, rapidly increases owing to runaway accretion, then decreases as the planet's feeding zone is depleted; (2) occurs when both solid and gas accretion rates are small and nearly independent of time; and (3) starts when the solid and gas masses are about equal and is marked by runaway gas accretion. The models applicability to planets in our Solar System are judged using two basic "yardsticks". The results suggest that the solar nebula dissipated while Uranus and Neptune were in the second phase, during which, for a relatively long time, the masses of their gaseous envelopes were small but not negligible compared to the total masses. Background information, results and a published article are included in the report.

  4. Hard apex transition in quasi-periodic oscillators - Closing of the accretion gap

    NASA Technical Reports Server (NTRS)

    Biehle, Garrett T.; Blandford, Roger D.

    1993-01-01

    We propose that the 'hard apex' transition in the X-ray two-color diagrams for low-mass X-ray binaries exhibiting quasi-periodic oscillation is associated with closure of a gap between the accretion disk and the star. At low accretion rates, gas crosses this gap intermittently. However, when the mass accretion rate increases, the disk thickens and its inner edge touches the star, thus forming a boundary layer through which the gas flows steadily. This explanation is viable provided that the equation of state of nuclear matter is not significantly harder than the Bethe-Johnson I prescription. Accretion gap scenarios are possibly distinguishable from models which invoke a small magnetosphere around the neutron star, in that they preclude large stellar magnetic fields and associate the high-frequency (horizontal-branch) oscillations with different sites.

  5. Effect of wettability on sessile drop freezing: when superhydrophobicity stimulates an extreme freezing delay.

    PubMed

    Boinovich, Ludmila; Emelyanenko, Alexandre M; Korolev, Vadim V; Pashinin, Andrei S

    2014-02-18

    An increasing number of studies directed at supercooling water droplets on surfaces with different wettabilities have appeared in recent years. This activity has been stimulated by the recognition that water supercooling phenomena can be effectively used to develop methods for protecting outdoor equipment and infrastructure elements against icing and snow accretion. In this article, we discuss the nucleation kinetics of supercooled sessile water droplets on hydrophilic, hydrophobic, and superhydrophobic surfaces under isothermal conditions at temperatures of -8, -10, and -15 °C and a saturated water vapor atmosphere. The statistics of nucleation events for the ensembles of freezing sessile droplets is completed by the detailed analysis of the contact angle temperature dependence and freezing of individual droplets in a saturated vapor atmosphere. We have demonstrated that the most essential freezing delay is characteristic of the superhydrophobic coating on aluminum, with the texture resistant to contact with ice and water. This delay can reach many hours at T = -8 °C and a few minutes at -23 °C. The observed behavior is analyzed on the basis of different nucleation mechanisms. The dissimilarity in the total nucleation rate, detected for two superhydrophobic substrates having the same apparent contact angle of the water drop but different resistivities of surface texture to the contact with water/ice, is associated with the contribution of heterogeneous nucleation on external centers located at the water droplet/air interface. PMID:24491217

  6. A Lagrangian drop model to study warm rain microphysical processes in shallow cumulus

    NASA Astrophysics Data System (ADS)

    Naumann, Ann Kristin; Seifert, Axel

    2015-09-01

    In this study, we introduce a Lagrangian drop (LD) model to study warm rain microphysical processes in shallow cumulus. The approach combines Large-Eddy Simulations (LES) including a bulk microphysics parameterization with an LD model for raindrop growth. The LD model is one-way coupled with the Eulerian LES and represents all relevant rain microphysical processes such as evaporation, accretion, and selfcollection among LDs as well as dynamical effects such as sedimentation and inertia. To test whether the LD model is fit for purpose, a sensitivity study for isolated shallow cumulus clouds is conducted. We show that the surface precipitation rate and the development of the raindrop size distribution are sensitive to the treatment of selfcollection in the LD model. Some uncertainty remains for the contribution of the subgrid-scale turbulence to the relative velocity difference of a pair of LDs, which appears as a factor in the collision kernel. Sensitivities to other model parameters such as the initial multiplicity or the initial mass distribution are small. Overall, sensitivities of the LD model are small compared to the uncertainties in the assumptions of the bulk rain microphysics scheme, and the LD model is well suited for particle-based studies of raindrop growth and dynamics. This opens up the opportunity to study effects like recirculation, deviations from terminal fall velocity and other microphysical phenomena that so far were not accessible for bin, bulk, or parcel models.

  7. Accretion onto the first stellar mass black holes

    SciTech Connect

    Alvarez, Marcelo A.; Wise, John H.; Abel, Tom

    2009-08-05

    The first stars, forming at redshifts z > 15 in minihalos with M {approx} 10{sup 5-6} M{sub {circle_dot}} may leave behind remnant black holes, which could conceivably have been the 'seeds' for the supermassive black holes observed at z {approx}< 7. We study remnant black hole growth through accretion, including for the first time the radiation emitted due to accretion, with adaptive mesh refinement cosmological radiation-hydrodynamical simulations. The effects of photo-ionization and heating dramatically affect the large-scale inflow, resulting in negligible mass growth. We compare cases with accretion luminosity included and neglected to show that accretion radiation drastically changes the environment within 100 pc of the black hole, increasing gas temperatures by an order of magnitude. Gas densities are reduced and further star formation in the same minihalo is prevented for the two hundred million years we followed. Without radiative feedback included most seed black holes do not gain mass as efficiently as has been hoped for in previous theories, implying that black hole remnants of Pop III stars in minihalos are not likely to be miniquasars. Most importantly, however, our calculations demonstrate that if these black holes are indeed accreting close to the Bondi-Hoyle rate with ten percent radiative efficiency they have a dramatic local effect in regulating star formation in the first galaxies. This suggests a novel mechanism for massive black hole formation - stellar-mass black holes may have suppressed fragmentation and star formation after falling into halos with virial temperatures {approx} 10{sup 4} K, facilitating intermediate mass black hole formation at their centers.

  8. Turbulence in Accretion Discs. The Global Baroclinic Instability

    NASA Astrophysics Data System (ADS)

    Klahr, Hubert; Bodenheimer, Peter

    The transport of angular momentum away from the central object is a sufficient condition for a protoplanetary disk to accrete matter onto the star and spin it down. Magnetic fields cannot be of importance for this process in a large part of the cold and dusty disk where the planets supposedly form. Our new hypothesis on the angular momentum transport based on radiation hydro simulations is as follows: We present the global baroclinic instability as a source for vigorous turbulence leading to angular momentum transport in Keplerian accretion disks. We show by analytical considerations and three-dimensional radiation hydro simulations that, in particular, protoplanetary disks have a negative radial entropy gradient, which makes them baroclinic. Two-dimensional numerical simulations show that this baroclinic flow is unstable and produces turbulence. These findings are currently tested for numerical effects by performing barotropic simulations which show that imposed turbulence rapidly decays. The turbulence in baroclinic disks draws energy from the background shear, transports angular momentum outward and creates a radially inward bound accretion of matter, thus forming a self consistent process. Gravitational energy is transformed into turbulent kinetic energy, which is then dissipated, as in the classical accretion paradigm. We measure accretion rates in 2D and 3D simulations of dot M= - 10-9 to -10-7 Msolar yr-1 and viscosity parameters of α = 10-4 - 10-2, which fit perfectly together and agree reasonably with observations. The turbulence creates pressure waves, Rossby waves, and vortices in the (r-φ) plane of the disk. We demonstrate in a global simulation that these vortices tend to form out of little background noise and to be long-lasting features, which have already been suggested to lead to the formation of planets.

  9. Fueling galaxy growth through gas accretion in cosmological simulations

    NASA Astrophysics Data System (ADS)

    Nelson, Dylan Rubaloff

    Despite significant advances in the numerical modeling of galaxy formation and evolution, it is clear that a satisfactory theoretical picture of how galaxies acquire their baryons across cosmic time remains elusive. In this thesis we present a computational study which seeks to address the question of how galaxies get their gas. We make use of new, more robust simulation techniques and describe the first investigations of cosmological gas accretion using a moving-mesh approach for solving the equations of continuum hydrodynamics. We focus first on a re-examination of past theoretical conclusions as to the relative importance of different accretion modes for galaxy growth. We study the rates and nature of gas accretion at z=2, comparing our new simulations run with the Arepo code to otherwise identical realizations run with the smoothed particle hydrodynamics code Gadget. We find significant physical differences in the thermodynamic history of accreted gas, explained in terms of numerical inaccuracies in SPH. In contrast to previous results, we conclude that hot mode accretion generally dominates galaxy growth, while cold gas filaments experience increased heating and disruption. Next, we consider the impact of feedback on our results, including models for galactic-scale outflows driven by stars as well as the energy released from supermassive black holes. We find that feedback strongly suppresses the inflow of "smooth" mode gas at all redshifts, regardless of its temperature history. Although the geometry of accretion at the virial radius is largely unmodified, strong galactic-fountain recycling motions dominate the inner halo. We measure a shift in the characteristic timescale of accretion, and discuss implications for semi-analytical models of hot halo gas cooling. To overcome the resolution limitations of cosmological volumes, we simulate a suite of eight individual 1012 solar mass halos down to z=2. We quantify the thermal and dynamical structure of the gas in

  10. Plasma physics of accreting neutron stars

    NASA Technical Reports Server (NTRS)

    Ghosh, Pranab; Lamb, Frederick K.

    1991-01-01

    Plasma concepts and phenomena that are needed to understand X- and gamma-ray sources are discussed. The capture of material from the wind or from the atmosphere or envelope of a binary companion star is described and the resulting types of accretion flows discussed. The reasons for the formation of a magnetosphere around the neutron star are explained. The qualitative features of the magnetospheres of accreting neutron stars are then described and compared with the qualitative features of the geomagnetosphere. The conditions for stable flow and for angular and linear momentum conservation are explained in the context of accretion by magnetic neutron stars and applied to obtain rough estimates of the scale of the magnetosphere. Accretion from Kepleri