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Sample records for accreting pulsating white

  1. 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.

  2. An Update on the Quirks of Pulsating, Accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    Szkody, Paula; Mukadam, Anjum S.; Gänsicke, Boris T.; Hermes, J. J.; Toloza, Odette

    2015-06-01

    At the 18th European White Dwarf Workshop, we reported results for several dwarf novae containing pulsating white dwarfs that had undergone an outburst in 2006-2007. HST and optical data on the white dwarfs in GW Lib, EQ Lyn and V455 And all showed different behaviors in the years following their outbursts. We continued to follow these objects for the last 2 years, providing timescales of 6-7 years past outburst. All three reached their optical quiescent values within 4 years but pulsational stability has not returned. EQ Lyn showed its pre-outburst pulsation period after 3 years, but it continues to show photometric variability that alternates between pulsation and disk superhump periods while remaining at quiescence. V455 And has almost reached its pre-outburst pulsation period, while GW Lib still remains heated and with a different pulsation spectrum than at quiescence. These results indicate that asteroseismology provides a unique picture of the effects of outburst heating on the white dwarf.

  3. GW Librae: a unique laboratory for pulsations in an accreting white dwarf

    NASA Astrophysics Data System (ADS)

    Toloza, O.; Gänsicke, B. T.; Hermes, J. J.; Townsley, D. M.; Schreiber, M. R.; Szkody, P.; Pala, A.; Beuermann, K.; Bildsten, L.; Breedt, E.; Cook, M.; Godon, P.; Henden, A. A.; Hubeny, I.; Knigge, C.; Long, K. S.; Marsh, T. R.; de Martino, D.; Mukadam, A. S.; Myers, G.; Nelson, P.; Oksanen, A.; Patterson, J.; Sion, E. M.; Zorotovic, M.

    2016-07-01

    Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of Hubble Space Telescope (HST) ultraviolet spectroscopy taken in 2002, 2010, and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in 2013 May, we obtained new HST/Cosmic Origin Spectrograph ultraviolet observations that displayed unexpected behaviour: besides showing variability at ≃275 s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits high-amplitude variability on an ≃4.4 h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to ≃30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.

  4. GW Librae: A unique laboratory for pulsations in an accreting white dwarf

    NASA Astrophysics Data System (ADS)

    Toloza, O.; Gänsicke, B. T.; Hermes, J. J.; Townsley, D. M.; Schreiber, M. R.; Szkody, P.; Pala, A.; Beuermann, K.; Bildsten, L.; Breedt, E.; Cook, M.; Godon, P.; Henden, A. A.; Hubeny, I.; Knigge, C.; Long, K. S.; Marsh, T. R.; de Martino, D.; Mukadam, A. S.; Myers, G.; Nelson, P.; Oksanen, A.; Patterson, J.; Sion, E. M.; Zorotovic, M.

    2016-04-01

    Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in atmospheres with mixed compositions of hydrogen, helium, and metals, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Librae stands out by having a well-established observational record of three independent pulsation modes that disappeared when the white dwarf temperature rose dramatically following its 2007 accretion outburst. Our analysis of HST ultraviolet spectroscopy taken in 2002, 2010 and 2011, showed that pulsations produce variations in the white dwarf effective temperature as predicted by theory. Additionally in May 2013, we obtained new HST/COS ultraviolet observations that displayed unexpected behaviour: besides showing variability at ≃275 s, which is close to the post-outburst pulsations detected with HST in 2010 and 2011, the white dwarf exhibits high-amplitude variability on a ≃4.4 h time-scale. We demonstrate that this variability is produced by an increase of the temperature of a region on white dwarf covering up to ≃30 per cent of the visible white dwarf surface. We argue against a short-lived accretion episode as the explanation of such heating, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.

  5. The post-outburst pulsations of the accreting white dwarf in the cataclysmic variable GW Librae

    NASA Astrophysics Data System (ADS)

    Chote, P.; Sullivan, D. J.

    2016-05-01

    We present new time series photometry of the accreting pulsating white dwarf system GW Librae obtained in 2012 and 2013 at the University of Canterbury Mt John Observatory in New Zealand. Our 2012 data show the return of a ˜19 min periodicity that was previously detected in 2008. This pulsation mode was a dominant feature of our quality 2012 May data set, which consisted of six contiguous nights; a detailed analysis indicated a degree of frequency variability. We show by comparison with the previously identified pulsation modes that this periodicity is best explained as a new mode, and that the quasi-stability of the periods appears to be a general feature of the pulsations in these systems. We also find a previously unreported 3-h modulation period, which we believe to be related to the known two and four hour periods of so far unknown origin.

  6. Enigmatic Recurrent Pulsational Variability of the Accreting White Dwarf EQ Lyn (SDSS J074531.92+453829.6)

    NASA Astrophysics Data System (ADS)

    Mukadam, Anjum S.; Townsley, D. M.; Szkody, Paula; Gänsicke, B. T.; Southworth, J.; Brockett, T.; Parsons, S.; Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Harrold, S.; Tovmassian, G.; Zharikov, S.; Drake, A. J.; Henden, A.; Rodriguez-Gil, P.; Sion, E. M.; Zola, S.; Szymanski, T.; Pavlenko, E.; Aungwerojwit, A.; Qian, S.-B.

    2013-09-01

    Photometric observations of the cataclysmic variable EQ Lyn (SDSS J074531.92+453829.6), acquired from 2005 October to 2006 January, revealed high-amplitude variability in the range 1166-1290 s. This accreting white dwarf underwent an outburst in 2006 October, during which its brightness increased by at least five magnitudes, and it started exhibiting superhumps in its light curve. Upon cooling to quiescence, the superhumps disappeared and it displayed the same periods in 2010 February as prior to the outburst within the uncertainties of a couple of seconds. This behavior suggests that the observed variability is likely due to nonradial pulsations in the white dwarf star, whose core structure has not been significantly affected by the outburst. The enigmatic observations begin with an absence of pulsational variability during a multi-site campaign conducted in 2011 January-February without any evidence of a new outburst; the light curve is instead dominated by superhumps with periods in the range of 83-87 minutes. Ultraviolet Hubble Space Telescope time-series spectroscopy acquired in 2011 March reveals an effective temperature of 15,400 K, placing EQ Lyn within the broad instability strip of 10,500-16,000 K for accreting pulsators. The ultraviolet light curve with 90% flux from the white dwarf shows no evidence of any pulsations. Optical photometry acquired during 2011 and Spring 2012 continues to reflect the presence of superhumps and an absence of pulsations. Subsequent observations acquired in 2012 December and 2013 January finally indicate the disappearance of superhumps and the return of pulsational variability with similar periods as previous data. However, our most recent data from 2013 March to May reveal superhumps yet again with no sign of pulsations. We speculate that this enigmatic post-outburst behavior of the frequent disappearance of pulsational variability in EQ Lyn is caused either by heating the white dwarf beyond the instability strip due to an

  7. ENIGMATIC RECURRENT PULSATIONAL VARIABILITY OF THE ACCRETING WHITE DWARF EQ LYN (SDSS J074531.92+453829.6)

    SciTech Connect

    Mukadam, Anjum S.; Szkody, Paula; Townsley, D. M.; Brockett, T.; Gaensicke, B. T.; Parsons, S.; Southworth, J.; Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Harrold, S.; Tovmassian, G.; Zharikov, S.; Drake, A. J.; Henden, A.; Rodriguez-Gil, P.; Sion, E. M.; Zola, S.; Szymanski, T.; Pavlenko, E.; and others

    2013-09-15

    Photometric observations of the cataclysmic variable EQ Lyn (SDSS J074531.92+453829.6), acquired from 2005 October to 2006 January, revealed high-amplitude variability in the range 1166-1290 s. This accreting white dwarf underwent an outburst in 2006 October, during which its brightness increased by at least five magnitudes, and it started exhibiting superhumps in its light curve. Upon cooling to quiescence, the superhumps disappeared and it displayed the same periods in 2010 February as prior to the outburst within the uncertainties of a couple of seconds. This behavior suggests that the observed variability is likely due to nonradial pulsations in the white dwarf star, whose core structure has not been significantly affected by the outburst. The enigmatic observations begin with an absence of pulsational variability during a multi-site campaign conducted in 2011 January-February without any evidence of a new outburst; the light curve is instead dominated by superhumps with periods in the range of 83-87 minutes. Ultraviolet Hubble Space Telescope time-series spectroscopy acquired in 2011 March reveals an effective temperature of 15,400 K, placing EQ Lyn within the broad instability strip of 10,500-16,000 K for accreting pulsators. The ultraviolet light curve with 90% flux from the white dwarf shows no evidence of any pulsations. Optical photometry acquired during 2011 and Spring 2012 continues to reflect the presence of superhumps and an absence of pulsations. Subsequent observations acquired in 2012 December and 2013 January finally indicate the disappearance of superhumps and the return of pulsational variability with similar periods as previous data. However, our most recent data from 2013 March to May reveal superhumps yet again with no sign of pulsations. We speculate that this enigmatic post-outburst behavior of the frequent disappearance of pulsational variability in EQ Lyn is caused either by heating the white dwarf beyond the instability strip due to an

  8. A First Look at the Nonadiabatic Properties of Pulsating Accreting White Dwarfs of the GW Lib Type

    NASA Astrophysics Data System (ADS)

    Van Grootel, V.; Fontaine, G.; Brassard, P.; Dupret, M.-A.

    2015-06-01

    We present results of a detailed stability survey of the pulsation properties of accreting white dwarfs of the GW Lib type. This is based on several state-of-the-art white dwarf evolutionary sequences with varying envelope compositions, from pure hydrogen to pure helium. Using the same tools as in Van Grootel et al. (2013), where we have presented the first consistent view of the ZZ Ceti instability strip, we have mapped the GW Lib instability strip over the effective temperature-surface gravity plane, and as a function of envelope composition. We find that the location of the GW Lib instability domain is a strong and continuous function of the assumed envelope composition. We can accomodate all of the known GW Lib pulsators in various strips according to their atmospheric compositions.

  9. FIRST UNAMBIGUOUS DETECTION OF THE RETURN OF PULSATIONS IN THE ACCRETING WHITE DWARF SDSS J074531.92+453829.6 AFTER AN OUTBURST

    SciTech Connect

    Mukadam, Anjum S.; Szkody, P.; Townsley, D. M.; Gaensicke, B. T.; Winget, D. E.; Hermes, J. J.; Howell, Steve B.; Teske, J.; Patterson, Joseph; Armstrong, Eve; Kemp, Jonathan

    2011-02-20

    The primary white dwarf of the cataclysmic variable SDSS J074531.92+453829.6 was discovered to exhibit non-radial pulsations in 2006 January. This accreting white dwarf underwent its first recorded dwarf nova outburst in 2006 October, during which its brightness increased by more than 5 mag. A Hubble Space Telescope (HST) ultraviolet spectrum, obtained one year after the outburst, revealed a white dwarf temperature of 16,500 K, hotter than all other known accreting white dwarf pulsators. This implies that the accreting primary white dwarf of SDSS J074531.92+453829.6 was heated to temperatures beyond the instability strip during the outburst. Optical observations acquired a year after the outburst did not reveal any evidence of pulsations, suggesting that the white dwarf had not cooled to quiescence by then. We recently acquired optical high-speed time-series photometry on this cataclysmic variable SDSS J074531.92+453829.6 more than three years after its outburst to find that pulsations have now returned to the primary white dwarf. Moreover, the observed pulsation periods agree with pre-outburst periods within the uncertainties of a few seconds. This discovery is significant because it indicates that the outburst did not affect the interior stellar structure, which governs the observed pulsation frequencies. It also suggests that the surface of the white dwarf has now cooled to quiescence. Using this discovery in addition to the prior HST temperature measurement of 16,500 K, we have been able to constrain the matter accreted during the 2006 outburst. This is the first time an accreting white dwarf was unambiguously observed to resume pulsating after an outburst.

  10. The Pulsating White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Brassard, P.

    2008-10-01

    We present a summary of what is currently known about the three distinct families of isolated pulsating white dwarfs. These are the GW Vir stars (He/C/O-atmosphere stars with Teff sime 120,000 K), the V777 Her stars (He-atmosphere, Teff sime 25,000 K), and the ZZ Ceti stars (H-atmosphere, Teff sime 12,000 K), all showing multiperiodic luminosity variations caused by low-order and low-degree g-mode instabilities. We also provide, in an Appendix, a very brief overview of the newly found evidence in favor of the existence of a fourth category of oscillating white dwarfs bearing strong similarities with these families of pulsators. We begin our survey with a short historical introduction, followed by a general discussion of pulsating white dwarfs as compact pulsators. We then discuss the class properties of these objects, including an updated census. We next focus on the instability domains for each family of pulsators in the log g - Teff diagram, and present their time-averaged properties in more detail. This is followed by a section on excitation physics, i.e., the causes of the pulsational instabilities, with emphasis on the common properties of the different types of pulsator. We then discuss the time-dependent properties of the pulsating white dwarfs featuring, among other things, a brief "picture tour" across the ZZ Ceti instability strip. We next review the methods used to infer or constrain the angular geometry of a pulsation mode in a white dwarf. These include multicolor photometry and time-resolved spectroscopy, the exploitation of the nonlinear features in the observed light curves, and rotational splitting. We also consider basic adiabatic asteroseismology starting with a discussion of the reaction of the period spectrum to variations of model parameters. We next review the various asteroseismological inferences that have so far been claimed for white dwarfs. We also discuss the potential of exploiting the rates of period change. We finally provide some

  11. Pulsating Helium Atmosphere White Dwarfs

    NASA Astrophysics Data System (ADS)

    Provencal, Judith; Montgomery, Michael H.; Bischoff-Kim, Agnes; Shipman, Harry; Nitta, Atsuko; Whole Earth Telescope Collaboration

    2015-08-01

    The overwhelming majority of all stars currently on the main sequence as well as those from earlier generations will or have ended their stellar lives as white dwarf stars. White dwarfs are rich forensic laboratories linking the history and future evolution of our Galaxy. Their structure and atmospheric composition provide evidence of how the progenitors lived, how they evolved, and how they died. This information reveals details of processes governing the behavior of contemporary main sequence stars. Combined with their distribution in luminosity/temperature, white dwarfs strongly constrain models of galactic and cosmological evolution.GD358 is among the brightest (mv =13.7) and best studied of the pulsating white dwarfs. This helium atmoshere pulsator (DBV) has an extensive photometric database spanning 30 years, including nine multisite Whole Earth Telescope campaigns. GD358 exhibits a range of behaviors, from drastic changes in excited pulsation modes to variable multiplet splittings. We use GD358 as a template for an examination of the DBV class, combining photometric results with recent COS spectroscopy. The results present new questions concerning DB formation and evolution.

  12. Nonlinear Analysis of Pulsating White Dwarf Lightcurves

    NASA Astrophysics Data System (ADS)

    Provencal, J. L.; Montgomery, M. H.; Shipman, H.; WET TEam

    2015-06-01

    Convection remains one of the largest sources of theoretical uncertainty in our understanding of stellar physics. For example, Bergeron (1995) show that basic parameters such as flux, line profiles, energy distribution, color indices, and equivalent widths are extremely sensitive to the assumed convective parameterization. This is compelling, since we use our knowledge of these basic parameters to calibrate white dwarf cooling sequences, provide detailed estimates for the ages of individual white dwarfs, and determine the age of the Galactic disk. The Whole Earth Telescope (WET) is engaged in a long term project to empirically calibrate the physical properties of convection in pulsating white dwarfs by combining asteroseismology and analysis of nonlinear light curves. Nonsinusoidal distortions, in the form of narrow peaks and wider valleys, are observed in many pulsating white dwarf light curves. These are a reflection of the local depth of the convection zone, a value which varies during a pulsation cycle. Applying asteroseismology and convective light curve fitting to a wide sample of pulsating white dwarfs provides an empirical map of how the convective response time (the convection zone “depth”) varies as a function of effective temperature, and this can be compared with theoretical models, both MLT and hydrodynamic. This project has resulted in a large database of white dwarf lightcurves and pulsation frequencies. We present current results for DA and DB pulsators, and provide a few examples of interesting pulsation behavior seen along the way.

  13. Radial pulsations in DB white dwarfs?

    NASA Technical Reports Server (NTRS)

    Kawaler, Steven D.

    1993-01-01

    Theoretical models of DB white dwarfs are unstable against radial pulsation at effective temperatures near 20,000-30,000 K. Many high-overtone modes are unstable, with periods ranging from 12 s down to the acoustic cutoff period of approximately 0.1 s. The blue edge for radial instability lies at slightly higher effective temperatures than for nonradial pulsations, with the temperature of the blue edge dependent on the assumed efficiency of convection. Models with increased convective efficiency have radial blue edges that are increasingly closer to the nonradial blue edge; in all models the instability persists into the nonradial instability strip. Radial pulsations therefore may exist in the hottest DB stars that lie below the DB gap; the greatest chance for detection would be observations in the ultraviolet. These models also explain why searches for radial pulsations in DA white dwarfs have failed: the efficient convection needed to explain the blue edge for nonradial DA pulsation means that the radial instability strip is 1000 K cooler than found in previous investigations. The multiperiodic nature of the expected pulsations can be used to advantage to identify very low amplitude modes using the uniform spacing of the modes in frequency. This frequency spacing is a direct indicator of the mass of the star.

  14. Two new extremely hot pulsating white dwarfs

    NASA Technical Reports Server (NTRS)

    Bond, H. E.; Grauer, A. D.; Green, R. F.; Liebert, J. W.

    1984-01-01

    High speed photometry of the extremely hot, nearly degenerate stars PG 1707 + 427 and PG 2131 + 066 reveals that they are low-amplitude pulsating variables. Power spectral analysis shows both to be multiperiodic, with dominant periods of 7.5 and 6.4-6.9 minutes, respectively. Together with the known pulsators PG 1159 - 035 and the central star of the planetary nebula Kohoutek 1-16, these objects define a new pulsational instability strip at the hot edge of the H-R diagram. The variations of these objects closely resemble those of the much cooler pulsating ZZ Ceti DA white dwarfs; both groups are probably nonradial g-mode pulsators. Evolutionary contraction of the PG 1159 - 035 variables may lead to period changes that would be detectable in as little as 1 year. The optical and IUE spectra of the PG 1159 - 035 variables are characterized by absorption lines of C IV and other CNO ions, indicating radiative levitation of species heavier than helium. He II is also present in the spectra, but the hydrogen Balmer lines are absent. Effective temperatures near 100,000 K are required, and the He II 4686 A profiles indicate log g greater than 6. These helium-rich pulsators form the hottest known subgroup of the DO white dwarfs.

  15. New pulsating white dwarfs in cataclysmic variables

    NASA Astrophysics Data System (ADS)

    Nilsson, R.; Uthas, H.; Ytre-Eide, M.; Solheim, J.-E.; Warner, B.

    2006-07-01

    The number of discovered non-radially pulsating white dwarfs (WDs) in cataclysmic variables (CVs) is increasing rapidly by the aid of the Sloan Digital Sky Survey (SDSS). We performed photometric observations of two additional objects, SDSS J133941.11+484727.5 (SDSS 1339), independently discovered as a pulsator by Gänsicke et al., and SDSS J151413.72+454911.9, which we identified as a CV/ZZ Ceti hybrid. In this Letter we present the results of the remote observations of these targets performed with the Nordic Optical Telescope (NOT) during the Nordic-Baltic Research School at Molėtai Observatory, and follow-up observations executed by NOT in service mode. We also present three candidates we found to be non-pulsating. The results of our observations show that the main pulsation frequencies agree with those found in previous CV/ZZ Ceti hybrids, but specifically for SDSS 1339 the principal period differs slightly between individual observations and also from the recent independent observation by Gänsicke et al. Analysis of SDSS colour data for the small sample of pulsating and non-pulsating CV/ZZ Ceti hybrids found so far seems to indicate that the r - i colour could be a good marker for the instability strip of this class of pulsating WDs. Based on observations made with the Nordic Optical Telescope, operated on the island of La Palma jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. E-mail: ricky@astro.lu.se

  16. Pulsating White Dwarf Star GD99

    NASA Astrophysics Data System (ADS)

    Chynoweth, K. M.; Thompson, S.; Mullally, F.; Yeates, C.

    2004-12-01

    We present 15 hours of time-series photometry of the variable white dwarf star GD99. These data were obtained at the McDonald Observatory 2.1m Otto Struve Telescope in January 2003, using the Argos CCD photometer. We achieved a noise level as low as 0.07 %, as measured from the power spectrum of our first night. Our observations confirm that GD99 is a unique pulsating white dwarf whose modes show characteristics of both the hot and cold type of DA variable stars. Additionally, GD99 has a large number of modes, making it a good candidate for asteroseismological study. Our preliminary results indicate that this star merits further study to decipher its abundant set of unusual modes. With such a rich period structure, longer continuous data sets will be required to fully resolve the pulsation spectrum.

  17. DISCOVERY OF AN ULTRAMASSIVE PULSATING WHITE DWARF

    SciTech Connect

    Hermes, J. J.; Castanheira, Barbara G.; Winget, D. E.; Montgomery, M. H.; Harrold, Samuel T.; Kepler, S. O.; Gianninas, A.; Brown, Warren R.

    2013-07-01

    We announce the discovery of the most massive pulsating hydrogen-atmosphere white dwarf (WD) ever discovered, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a 12, 030 {+-} 210 K WD with a log g =9.08 {+-} 0.06, which corresponds to a mass of 1.20 {+-} 0.03 M{sub Sun }. Stellar evolution models indicate that the progenitor of such a high-mass WD endured a stable carbon-burning phase, producing an oxygen-neon-core WD. The discovery of pulsations in GD 518 thus offers the first opportunity to probe the interior of a WD with a possible oxygen-neon core. Such a massive WD should also be significantly crystallized at this temperature. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 to 595 s and amplitudes up to 0.7%, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, which exhibit non-radial g-mode pulsations driven by a hydrogen partial ionization zone. Successfully unraveling both the total mass and core composition of GD 518 provides a unique opportunity to investigate intermediate-mass stellar evolution, and can possibly place an upper limit to the mass of a carbon-oxygen-core WD, which in turn constrains Type Ia supernovae progenitor systems.

  18. White dwarf evolution - Cradle-to-grave constraints via pulsation

    NASA Technical Reports Server (NTRS)

    Kawaler, Steven D.

    1990-01-01

    White dwarf evolution, particularly in the early phases, is not very strongly constrained by observation. Fortunately, white dwarfs undergo nonradial pulsation in three distinct regions of the H-R diagram. These pulsations provide accurate masses, surface compositional structure and rotation velocities, and help constrain other important physical properties. We demonstrate the application of the tools of stellar seismology to white dwarf evolution using the hot white dwarf star PG 1159-035 and the cool DAV (or ZZ Ceti) stars as examples. From pulsation studies, significant challenges to the theory of white dwarf evolution emerge.

  19. Excitation and Saturation of White Dwarf Pulsations

    NASA Astrophysics Data System (ADS)

    Wu, Yanqin

    1998-06-01

    Variable hydrogen white dwarfs (DAV) pulsate in a number of low-order gravity-modes with periods from 100 s to 1200 s and amplitudes no larger than a few percent. We answer two questions in this thesis: the driving for these pulsations, and the saturation of their amplitudes. The surface convection zone in these stars, which adjusts its entropy level instantaneously during the pulsation, can drive the observed modes. This mechanism (called 'convective driving') was discovered by Brickhill but has been largely neglected so far. We find that modes with periods shorter than the thermal adjustment time of the convection zone can become overstable, but those with very short periods are hardly visible at the surface. As the star cools and the convection zone deepens, longer period modes can be excited. The driving rates increase sharply with period. We relate these to the time-scale of mode variability. We include complications arising from nonadiabaticity in the radiative interior and turbulent damping at the convective-radiative boundary. The former limits the driving and damping rates for strongly nonadiabatic modes, and relates the phase and amplitude of surface horizontal velocity in a gravity-mode to those of its flux variation. The turbulent damping results from the horizontal velocity shear below the convection zone, inside which there is little velocity shear and negligible damping. This suppresses the amplitudes of long period modes to below detection. The width of the theoretical DAV instability strip is about 1000 K. The growth of an overstable mode can be saturated by parametric instability, where energy transfers resonantly into two damped modes of roughly half its frequency. This occurs above a critical amplitude which depends on the 3-mode coupling coefficient and the nonadiabatic damping rates. The critical amplitudes all fall below a few percent, with longer period modes having larger surface amplitudes. Combined with the amplitude limits due to

  20. Constraining the neutrino magnetic dipole moment from white dwarf pulsations

    NASA Astrophysics Data System (ADS)

    Córsico, A. H.; Althaus, L. G.; Miller Bertolami, M. M.; Kepler, S. O.; García-Berro, E.

    2014-08-01

    Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment (μν) using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. We employ state-of-the-art evolutionary and pulsational codes which allow us to perform a detailed asteroseismological period fit based on fully DB white dwarf evolutionary sequences. Plasmon neutrino emission is the dominant cooling mechanism for this class of hot pulsating white dwarfs, and so it is the main contributor to the rate of change of period with time (Pi dot) for the DBV class. Thus, the inclusion of an anomalous neutrino emission through a non-vanishing magnetic dipole moment in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DBV stars. By comparing the theoretical Pi dot value with the rate of change of period with time of PG 1351+489, we assess the possible existence of additional cooling by neutrinos with magnetic dipole moment. Our models suggest the existence of some additional cooling in this pulsating DB white dwarf, consistent with a non-zero magnetic dipole moment with an upper limit of μν lesssim 10-11 μB. This bound is somewhat less restrictive than, but still compatible with, other limits inferred from the white dwarf luminosity function or from the color-magnitude diagram of the Globular cluster M5. Further improvements of the measurement of the rate of period change of the dominant pulsation mode of PG 1351+489 will be necessary to confirm our bound.

  1. Constraining the neutrino magnetic dipole moment from white dwarf pulsations

    SciTech Connect

    Córsico, A.H.; Althaus, L.G.; García-Berro, E. E-mail: althaus@fcaglp.unlp.edu.ar E-mail: kepler@if.ufrgs.br

    2014-08-01

    Pulsating white dwarf stars can be used as astrophysical laboratories to constrain the properties of weakly interacting particles. Comparing the cooling rates of these stars with the expected values from theoretical models allows us to search for additional sources of cooling due to the emission of axions, neutralinos, or neutrinos with magnetic dipole moment. In this work, we derive an upper bound to the neutrino magnetic dipole moment (μ{sub ν}) using an estimate of the rate of period change of the pulsating DB white dwarf star PG 1351+489. We employ state-of-the-art evolutionary and pulsational codes which allow us to perform a detailed asteroseismological period fit based on fully DB white dwarf evolutionary sequences. Plasmon neutrino emission is the dominant cooling mechanism for this class of hot pulsating white dwarfs, and so it is the main contributor to the rate of change of period with time (Pidot) for the DBV class. Thus, the inclusion of an anomalous neutrino emission through a non-vanishing magnetic dipole moment in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DBV stars. By comparing the theoretical Pidot value with the rate of change of period with time of PG 1351+489, we assess the possible existence of additional cooling by neutrinos with magnetic dipole moment. Our models suggest the existence of some additional cooling in this pulsating DB white dwarf, consistent with a non-zero magnetic dipole moment with an upper limit of μ{sub ν} ∼< 10{sup -11} μ{sub B}. This bound is somewhat less restrictive than, but still compatible with, other limits inferred from the white dwarf luminosity function or from the color-magnitude diagram of the Globular cluster M5. Further improvements of the measurement of the rate of period change of the dominant pulsation mode of PG 1351+489 will be necessary to confirm our bound.

  2. Accretion Flows in Magnetic White Dwarf Systems

    NASA Technical Reports Server (NTRS)

    Imamura, James N.

    2005-01-01

    We received Type A and B funding under the NASA Astrophysics Data Program for the analysis and interpretation of hard x-ray data obtained by the Rossi X-ray Timing Explorer and other NASA sponsored missions for Intermediate Polars (IPS) and Polars. For some targets, optical data was available. We reduced and analyzed the X-ray spectra and the X-ray and optical (obtained at the Cerro Tololo Inter-American Observatory) timing data using detailed shock models (which we constructed) to place constraints on the properties of the accreting white dwarfs, the high energy emission mechanisms of white dwarfs, and the large-scale accretion flows of Polars and IPS. IPS and Polars are white dwarf mass-transfer binaries, members of the larger class of cata,clysmic variables. They differ from the bulk of the cataclysmic variables in that they contain strongly magnetic white dwarfs; the white dwarfs in Polars have B, = 7 to 230 MG and those in IPS have B, less than 10 MG. The IPS and Polars are both examples of funneled accretion flows in strong magnetic field systems. The IPS are similar to x-ray pulsars in that accretion disks form in the systems which are disrupted by the strong stellar magnetic fields of the white dwarfs near the stellar surface from where the plasma is funneled to the surface of the white dwarf. The localized hot spots formed at the footpoints of the funnels coupled with the rotation of the white dwarf leads to coherent pulsed x-ray emission. The Polars offer an example of a different accretion topology; the magnetic field of the white dwarf controls the accretion flow from near the inner Lagrangian point of the system directly to the stellar surface. Accretion disks do not form. The strong magnetic coupling generally leads to synchronous orbital/rotational motion in the Polars. The physical system in this sense resembles the Io/Jupiter system. In both IPS and Polars, pulsed emission from the infrared to x-rays is produced as the funneled flows merge onto the

  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 by rotating magnetic neutron stars. III - Accretion torques and period changes in pulsating X-ray sources

    NASA Technical Reports Server (NTRS)

    Ghosh, P.; Lamb, F. K.

    1979-01-01

    The solutions of the two-dimensional hydromagnetic equations are used to calculate the torque on a magnetic neutron star accreting from a Keplerian disk. It is found that the magnetic coupling between the star and the plasma in the outer transition zone is appreciable; that as a result, the spin-up torque on fast rotators is substantially less than that on slow rotators, and that for sufficiently high stellar angular velocities or sufficiently low mass accretion rates, the rotation of the star can be braked while accretion continues. These results are applied to pulsating X-ray sources, revealing that at high luminosities a star of given spin period rotating in the same direction as the disk can experience either spin-up or spin-down, depending on its luminosity. Also considered are the general problem of interpreting period changes in pulsating X-ray sources, and the dipole magnetic moments of nine pulsating X-ray sources are estimated by fitting the theoretical spin-up equation to estimates of the average luminosity and spin-up rate of each source.

  5. SEISMOLOGY OF A MASSIVE PULSATING HYDROGEN ATMOSPHERE WHITE DWARF

    SciTech Connect

    Kepler, S. O.; Pelisoli, Ingrid; Pecanha, Viviane; Costa, J. E. S.; Fraga, Luciano; Hermes, J. J.; Winget, D. E.; Castanheira, Barbara; Corsico, A. H.; Romero, A. D.; Althaus, Leandro; Kleinman, S. J.; Nitta, A.; Koester, D.; Kuelebi, Baybars; Kanaan, Antonio

    2012-10-01

    We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1 m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1 m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M{sub *} = 0.88 {+-} 0.02 M{sub Sun} and T{sub eff} = 12, 100 {+-} 140 K. These values are consistent with those derived from the optical spectra and photometric colors.

  6. WD 0158-160, a new pulsating DB white dwarf

    NASA Astrophysics Data System (ADS)

    Kilkenny, D.

    2016-03-01

    The DB star, WD 0158-160 (=EC 01585-1600) is shown to be a variable white dwarf with a rich pulsation spectrum, dominated by a strong variation with a frequency near 1637 μHz (amplitude ˜0.024 mag, period 598 s), though at least 10 frequencies are detected between about 1285 and 5747 μHz (780-173 s) in a relatively small data set. At ˜14.5 mag, the star is one of the brightest known DBV stars.

  7. Peculiar variations of white dwarf pulsation frequencies and maestro

    NASA Astrophysics Data System (ADS)

    Dalessio, James Ruland

    In Part I we report on variations of the normal mode frequencies of the pulsating DB white dwarfs EC 20058-5234 and KIC 8626021 and the pulsating DA white dwarf GD 66. The observations of EC 20058-5234 and KIC 8626021 were motivated by the possibility of measuring the plasmon neutrino production rate of a white dwarf, while the observations of GD 66 were part of a white dwarf pulsation timing based planet search. We announce the discovery of periodic and quasi-periodic variations of multiple normal mode frequencies that cannot be due to the presence of planetary companions. We note the possible signature of a planetary companion to EC 20058-5234 and show that GD 66 cannot have a planet in a several AU orbit down to half a Jupiter mass. We also announce the discovery of secular variations of the normal mode frequencies of all three stars that are inconsistent with cooling alone. Importantly, the rates of period change of several modes of KIC 8626021 are consistent with evolutionary cooling, but are not yet statistically significant. These modes offer the best possibility of measuring the neutrino production rate in a white dwarf. We also observe periodic and secular variations in the frequency of a combination mode that exactly matches the variations predicted by the parent modes, strong observational evidence that combination modes are created by the convection zone and are not normal modes. Periodic variations in the amplitudes of many of these modes is also noted. We hypothesize that these frequency variations are caused by complex variations of the magnetic field strength and geometry, analogous to behavior observed in the Sun. In Part II we describe the MAESTRO software framework and the MAESTRO REDUCE algorithm. MAESTRO is a collection of astronomy specific MatLab software developed by the Whole Earth Telescope. REDUCE is an an algorithm that can extract the brightness of stars on a set of CCD images with minimal configuration and human interaction. The key to

  8. A Pulsational Study of Crystallized White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Montgomery, M. H.; Winget, D. E.

    1998-03-01

    The DAV BPM 37093 should have a substantially crystallized core based on its mass and temperature. Using this as a motivation, we examine the way in which a crystalline interior affects the nonradial g-mode frequencies of a white dwarf star. We confine ourselves to a relatively massive model within the DA instability strip (M_⋆ = 1.1Msun), since crystallization in this temperature range should be important only for high-mass white dwarfs. We find that crystallization has a significant effect on the mean period spacing of adjacent radial overtones, of order 10--30 %. Thus, a correct pulsational treatment of crystallization is vital if we are to make reliable asteroseismological measurements of a given stars' properties.

  9. Panel 1: A pulsating red giant star and a compact, hot white dwarf star orbit each other.

    NASA Technical Reports Server (NTRS)

    2002-01-01

    Panel 1: A pulsating red giant star and a compact, hot white dwarf star orbit each other. Panel 2: The red giant sheds much of its outer layers in a stellar wind. The white dwarf helps concentrate the wind along a thin equatorial plane. The white dwarf accretes some of this escaping gas forming a disk around the itself. Panel 3: When enough gas accumulates on the white dwarf's surface it explodes as a nova outburst. Most of the hot gas forms a pair of expanding bubbles above and below the equatorial disk. Panel 4: A few thousand years after the bubbles expand into space, the white dwarf goes through another nova outburst and makes another pair of bubbles, which form a distinctive hourglass shape.

  10. A Second Case of Outbursts in a Pulsating White Dwarf Observed by Kepler

    NASA Astrophysics Data System (ADS)

    Hermes, J. J.; Montgomery, M. H.; Bell, Keaton J.; Chote, P.; Gänsicke, B. T.; Kawaler, Steven D.; Clemens, J. C.; Dunlap, Bart H.; Winget, D. E.; Armstrong, D. J.

    2015-09-01

    We present observations of a new phenomenon in pulsating white dwarf stars: large-amplitude outbursts at timescales much longer than the pulsation periods. The cool ({T}{eff} = 11,060 K), hydrogen-atmosphere pulsating white dwarf PG 1149+057 was observed nearly continuously for more than 78.8 day by the extended Kepler mission in K2 Campaign 1. The target showed 10 outburst events, recurring roughly every 8 day and lasting roughly 15 hr, with maximum flux excursions up to 45% in the Kepler bandpass. We demonstrate that the outbursts affect the pulsations and therefore must come from the white dwarf. Additionally, we argue that these events are not magnetic reconnection flares, and are most likely connected to the stellar pulsations and the relatively deep surface convection zone. PG 1149+057 is now the second cool pulsating white dwarf to show this outburst phenomenon, after the first variable white dwarf observed in the Kepler mission, KIC 4552982. Both stars have the same effective temperature, within the uncertainties, and are among the coolest known pulsating white dwarfs of typical mass. These outbursts provide fresh observational insight into the red edge of the DAV instability strip and the eventual cessation of pulsations in cool white dwarfs.

  11. PULSATIONS IN HYDROGEN BURNING LOW-MASS HELIUM WHITE DWARFS

    SciTech Connect

    Steinfadt, Justin D. R.; Bildsten, Lars; Arras, Phil E-mail: bildsten@kitp.ucsb.ed

    2010-07-20

    Helium core white dwarfs (WDs) with mass M {approx}< 0.20 M {sub sun} undergo several Gyr of stable hydrogen burning as they evolve. We show that in a certain range of WD and hydrogen envelope masses, these WDs may exhibit g-mode pulsations similar to their passively cooling, more massive carbon/oxygen core counterparts, the ZZ Cetis. Our models with stably burning hydrogen envelopes on helium cores yield g-mode periods and period spacings longer than the canonical ZZ Cetis by nearly a factor of 2. We show that core composition and structure can be probed using seismology since the g-mode eigenfunctions predominantly reside in the helium core. Though we have not carried out a fully nonadiabatic stability analysis, the scaling of the thermal time in the convective zone with surface gravity highlights several low-mass helium WDs that should be observed in search of pulsations: NLTT 11748, SDSS J0822+2753, and the companion to PSR J1012+5307. Seismological studies of these He core WDs may prove especially fruitful, as their luminosity is related (via stable hydrogen burning) to the hydrogen envelope mass, which eliminates one model parameter.

  12. The discovery of non-radially pulsating white dwarfs in cataclysmic variables from the Sloan Digital Sky Survey

    NASA Astrophysics Data System (ADS)

    Woudt, P. A.; Warner, B.; Pretorius, M. L.; Dale, D.

    2005-08-01

    The group of known non-radially pulsating white dwarfs in cataclysmic variables (CVs) has quintupled in the last year. These are all systems of low mass-transfer rate in which the central white dwarf is located in the ZZ Ceti instability strip (Teff ˜ 11,000 - 12,500 K). Due to the low mass-transfer rate, these systems are intrinsically faint and it is only due to surveys such as the Sloan Digital Sky Survey (SDSS) that this population of CV/ZZ hybrids is being uncovered in a systematic way. It is estimated that among the ˜ 400 CVs expected in the SDSS, about ˜ 32 systems will be CV/ZZ stars. Within such a sample undoubtedly some (low amplitude ZZ Ceti pulsators) will exist that are suitable for detailed analysis and identification of pulsation modes in order to study the influence of accretion on the internal structure of white dwarfs. Here we present results from our ongoing high-speed photometric survey (using the UCT CCD) of the newly identified SDSS CV/ZZ stars, including two CV/ZZ candidates.

  13. Information about accretion flows from X-ray timing of pulsating sources

    NASA Technical Reports Server (NTRS)

    Lamb, F. K.; Pines, D.; Shaham, J.

    1976-01-01

    The response was studied of a rotating neutron star to fluctuating torques and it was found that the observed variations in the pulsation periods of the compact X-ray sources Cen X-3 and Her X-1 could be caused by short time scale fluctuations in the accretion torques acting on the neutron stars. The sizes and rates of the required fluctuations are consistent with current accretion models. Such fluctuations can cause period variations either (a) directly, by causing a random walk of the star's angular velocity or (b) indirectly, by exciting a long-period mode of the neutron star, such as the Tkachenko mode of the rotating neutron superfluid. Phenomena in compact X-ray sources and cataclysmic variables which may be caused by fluctuating mass flow rates are also discussed.

  14. An analytical model of accretion onto white dwarfs

    NASA Astrophysics Data System (ADS)

    Ospina, N.; Hernanz, M.

    2013-05-01

    The analytical model of Frank et al. (2002) has been used to investigate the structure of the accretion stream onto white dwarfs (WD). In particular, the post-shock region (temperature, density and gas velocity distributions) and X-ray spectrum emitted by this region. We have obtained the temperature, density and gas velocity distributions of the emission region for different masses of white dwarfs and at different positions in the shock coordinate. Also, we calculated the emitted spectrum for different WD masses and at different positions of the shock with the principal objective of study the accretion at different points of the emission region.

  15. Active states and structure transformations in accreting white dwarfs

    NASA Astrophysics Data System (ADS)

    Boneva, Daniela; Kaygorodov, Pavel

    2016-07-01

    Active states in white dwarfs are usually associated with light curve's effects that concern to the bursts, flickering or flare-up occurrences. It is common that a gas-dynamics source exists for each of these processes there. We consider the white dwarf binary stars with accretion disc around the primary. We suggest a flow transformation modeling of the mechanisms that are responsible for ability to cause some flow instability and bring the white dwarfs system to the outburst's development. The processes that cause the accretion rate to sufficiently increase are discussed. Then the transition from a quiescent to an active state is realized. We analyze a quasi-periodic variability in the luminosity of white dwarf binary stars systems. The results are supported with an observational data.

  16. First axion bounds from a pulsating helium-rich white dwarf star

    NASA Astrophysics Data System (ADS)

    Battich, T.; Córsico, A. H.; Althaus, L. G.; Miller Bertolami, M. M.

    2016-08-01

    The Peccei-Quinn mechanism proposed to solve the CP problem of Quantum Chromodynamics has as consequence the existence of axions, hypothetical weakly interacting particles whose mass is constrained to be on the sub-eV range. If these particles exist and interact with electrons, they would be emitted from the dense interior of white dwarfs, becoming an important energy sink for the star. Due to their well known physics, white dwarfs are good laboratories to study the properties of fundamental particles such as the axions. We study the general effect of axion emission on the evolution of helium-rich white dwarfs and on their pulsational properties. To this aim, we calculate evolutionary helium-rich white dwarf models with axion emission, and assess the pulsational properties of this models. Our results indicate that the rates of change of pulsation periods are significantly affected by the existence of axions. We are able for the first time to independently constrain the mass of the axion from the study of pulsating helium-rich white dwarfs. To do this, we use an estimation of the rate of change of period of the pulsating white dwarf PG 1351+489 corresponding to the dominant pulsation period. From an asteroseismological model of PG 1351+489 we obtain gae < 3.3 × 10‑13 for the axion-electron coupling constant, or macos2β lesssim 11.5 meV for the axion mass. This constraint is relaxed to gae < 5.5 × 10‑13 (macos2β lesssim 19.5 meV), when no detailed asteroseismological model is adopted for the comparison with observations.

  17. Spin-up and mixing in accreting white dwarfs

    SciTech Connect

    Livio, M.; Truran, J.W.

    1987-07-01

    It is demonstrated that existing theories of mixing in accreting white dwarfs encounter difficulties when confronted with observations of enrichments in nova ejecta. Arguments are presented, based on the Ekman spin-up process, which suggest that angular momentum transport from the accreted material to the white dwarf is more efficient than previously thought. This should lead to matter spreading over the entire white dwarf surface, as well as inward mixing. It is shown that when efficient transfer of angular momentum is taken into account, the gross features of nova outbursts can be reproduced, with the runaway occuring in a mixed layer. Some implications of the results for DQ Her, the hibernation model of novae, recurrent novae, and soft X-ray emission are discussed. 63 references.

  18. Searching for Pulsations from a Helium White Dwarf Companion to a Millisecond Pulsar

    NASA Astrophysics Data System (ADS)

    Bildsten, Lars

    2009-07-01

    The low mass white dwarf {WD} companion to the 3.26 ms pulsar PSR J1911-5958A offers an unprecedented opportunity for seismological study of the interior of a helium core WD. While much more massive carbon/oxygen core WDs are observed to pulsate in normal modes of oscillation called g-modes {known as ZZ Ceti stars}, no helium core pulsator is known. By extrapolating the boundaries of the ZZ Ceti instability strip downward in surface gravity by a factor of 20 below any known pulsator, we find that the effective temperature of this WD makes it an excellent candidate to search for pulsation. Detection of g-mode pulsations in the lightcurve would have a transformative effect on the field of WD pulsations, as this would allow the first seismological study of the interior of a helium core WD, and the low gravity strongly constrains theories for the driving and amplitudes of pulsations. We show that with 3 orbits of HST, we will detect photometric variations with amplitudes of 1%, lower than typically seen in other hydrogen-dominated ZZ Ceti stars. A set of measured mode periods would also constrain the thickness of the presumed stably hydrogen burning shell, and help us determine its age more securely.

  19. 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.

  20. New Insights on Pulsating White Dwarfs from 3D Radiation-Hydrodynamical Simulations

    NASA Astrophysics Data System (ADS)

    Tremblay, Pier-Emmanuel; Fontaine, Gilles; Ludwig, Hans-Günter

    2015-08-01

    We have recently computed a grid of 3D radiation-hydrodynamical simulations for the atmosphere of 70 pure-hydrogen DA white dwarfs in the range 7.0 < log g < 9.0. This includes the full ZZ Ceti instability strip where DA white dwarfs are pulsating, by far the most common type of degenerate pulsators. We have significantly improved the theoretical framework to study these objects by removing the free parameters of 1D convection, which were previously a major modeling hurdle. We will compare our new models with the observed sample of ZZ Ceti stars and highlight the improved derived properties of these objects. In particular, the new spectroscopically determined 3D atmospheric parameters allow for an improved definition of instability strip edges. We have also made new predictions for the size of convection zones, which significantly impact the position where the pulsations are driven, and the region of the HR diagram where white dwarfs are expected to pulsate. Finally, we will present new results from non-adiabatic pulsation calculations.

  1. A Survey of Helium Accreting White Dwarfs

    NASA Technical Reports Server (NTRS)

    Steeghs, Daniel

    2005-01-01

    We have initiated a survey of the X-ray spectral properties of double white binaries using XMM-Newton. Three of our sources were indeed observed with XMM during AO-3. We have analyzed these data using the latest data reduction software and have also extracted several archival data sets of similar systems from the XMM archive. The first paper presenting the X-ray spectral and variability properties of four of these binary systems has been submitted in January 2005. We have also secured some optical ground-based spectroscopy and are currently analyzing the spectral signatures in more detail. We are also continuing this survey with additional approved XMM observations during AO-4 and anticipate additional publications in the near future.

  2. On interstellar accretion and the rejuvenation of white dwarfs

    NASA Technical Reports Server (NTRS)

    Truran, J. W.; Wyatt, S. P.; Starrfield, S. G.; Strittmatter, P. A.; Sparks, W. M.

    1977-01-01

    The paper investigates physical conditions which can give rise to thermonuclear runaways in the hydrogen envelopes of low-luminosity white dwarfs. Specifically, calculations are performed for white dwarfs of 0.5, 0.75, 1, and 1.25 solar masses with envelope masses in the range from 0.00001 to 0.001 solar mass and initial luminosities of 0.00001 to 0.01 times the solar value. It is found that envelope masses as low as 0.0001 solar mass are sufficient to initiate thermonuclear runaways on more massive white dwarfs with luminosities as low as about 0.001 the solar value. The runaway time scales under these conditions, typically of the order of a billion years, are comparable to the time scales for cooling of white dwarfs to these low luminosities. Since time-averaged accretion rates as low as a few times 10 to the -14th power solar mass per year are sufficient to reconstitute such envelopes, also on a time scale of several billion years, it is suggested that the association of novalike events with binary systems may not be unique. Accretion of interstellar matter onto isolated white dwarfs may, under some circumstances, be sufficient to rekindle and perhaps rejuvenate the dwarf. Such evolutionary behavior might define a new and distinct class of objects.

  3. Amplitude Variability as Evidence of Crystallization in GD 518 and Other Massive Pulsating White Dwarfs

    NASA Astrophysics Data System (ADS)

    Hermes, J. J.; Kepler, S. O.; Montgomery, M. H.; Gianninas, A.; Castanheira, Barbara G.; Winget, D. E.

    2015-06-01

    In 2013 March we discovered pulsations in the most massive pulsating hydrogen-atmosphere white dwarf to date, GD 518. Model atmosphere fits to the optical spectrum of this star show it is a Teff = 12,030±210 K, log g = 9.08±0.06 white dwarf, which corresponds to a mass of 1.20±0.03 M⊙. Such a massive WD should also be significantly crystallized at this temperature, and may possibly contain an oxygen-neon core. The star exhibits multi-periodic luminosity variations at timescales ranging from roughly 425 to 595 s and amplitudes up to 0.7% in a given night, consistent in period and amplitude with the observed variability of typical ZZ Ceti stars, although the pulsation amplitudes change drastically over the 33 days of our discovery observations. We investigate the possibility that these amplitude variations are a consequence of the pulsation modes sampling only the non-crystallized outer mass fraction of the white dwarf (perhaps <0.05 M⊙ of material), and thus have very low mode inertia. Amplitude variability could be an observational consequence of a significantly crystallized stellar interior.

  4. 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.

  5. High-Speed Hubble Space Telescope Ultraviolet photometry of two DB white dwarfs: Nonradial and radial pulsations

    NASA Technical Reports Server (NTRS)

    Kawaler, Steven D.; Bond, Howard E.; Sherbert, Lisa E.; Watson, Todd K.

    1994-01-01

    We observed two DB white dwarf stars with the High Speed Photometer aboard the Hubble Space Telescope. The two targets, the nonradial pulsator GD 358, and PG 0112+104, a non-pulsating white dwarf with similar temperature, were each observed for 1800 s with a time resolution of 10 ms. We used the F140LP configuration, which gives a broadband response in the UV between 1400 and 3000 A. The data clearly show the long period (about 700 s) pulsations in GD 358. Comparison with optical observations obtained two weeks earlier shows that the amplitude of the pulsations in the UV is approximately 1.4 times higher, consistent with nonradial pulsations due solely to temperature changes at constant radius. The high time resolution of these observations allows us to search for high-frequency pulsations (such as p modes or high overtone radial modes). No firm evidencefor high-frequency pulsations was seen in either object between 1 and 12 Hz. Correlation analysis of GD 358 shows no clear signal of multifrequency high overtone radial pulsations at the 0.00075 mag level, with no individual modes above the 0.0016 mag level. Upper limits for PG 0112+104 are approximately 2 times higher than for GD 358. Implications of this study for the theory of white dwarf pulsations are discussed.

  6. A NEW TIMESCALE FOR PERIOD CHANGE IN THE PULSATING DA WHITE DWARF WD 0111+0018

    SciTech Connect

    Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Mullally, Fergal; Bischoff-Kim, A.

    2013-03-20

    We report the most rapid rate of period change measured to date for a pulsating DA (hydrogen atmosphere) white dwarf (WD), observed in the 292.9 s mode of WD 0111+0018. The observed period change, faster than 10{sup -12} s s{sup -1}, exceeds by more than two orders of magnitude the expected rate from cooling alone for this class of slow and simply evolving pulsating WDs. This result indicates the presence of an additional timescale for period evolution in these pulsating objects. We also measure the rates of period change of nonlinear combination frequencies and show that they share the evolutionary characteristics of their parent modes, confirming that these combination frequencies are not independent modes but rather artifacts of some nonlinear distortion in the outer layers of the star.

  7. Prevention of accretion onto white dwarfs by stellar winds

    NASA Technical Reports Server (NTRS)

    Macdonald, James

    1992-01-01

    There is indirect observational evidence that hot white dwarfs may have weak stellar winds. In this paper, the interaction between such a wind and the flow of ISM material in the gravitational field of the white dwarf is investigated with the aim of finding limits on the mass-loss rate and terminal velocity of winds capable of preventing accretion from the ISM. The limiting cases of no relative motion of the star and the ISM and supersonic relative motion of the star through ISM are separately investigated. Each case is treated by generalizing models for the interaction between the solar wind and the local ISM to include the effects of gravity. It is found that, for wind velocities expected for radiatively driven winds, mass-loss rates as low as 10 exp -21 solar mass/yr are sufficient to prevent accretion from the hot phase of the ISM. To prevent accretion during passages through cold clouds, wind mass-loss rates of order 10 exp -18 to 10 exp -17 are required.

  8. Discovery of Three Pulsating, Mixed-atmosphere, Extremely Low-mass White Dwarf Precursors

    NASA Astrophysics Data System (ADS)

    Gianninas, A.; Curd, Brandon; Fontaine, G.; Brown, Warren R.; Kilic, Mukremin

    2016-05-01

    We report the discovery of pulsations in three mixed-atmosphere, extremely low-mass white dwarf (ELM WD, M ≤slant 0.3 M ⊙) precursors. Following the recent discoveries of pulsations in both ELM and pre-ELM WDs, we targeted pre-ELM WDs with mixed H/He atmospheres with high-speed photometry. We find significant optical variability in all three observed targets with periods in the range 320-590 s, consistent in timescale with theoretical predictions of p-mode pulsations in mixed-atmosphere ≈0.18 M ⊙ He-core pre-ELM WDs. This represents the first empirical evidence that pulsations in pre-ELM WDs can only occur if a significant amount of He is present in the atmosphere. Future, more extensive, timeseries photometry of the brightest of the three new pulsators offers an excellent opportunity to constrain the thickness of the surface H layer, which regulates the cooling timescales for ELM WDs. 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 National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil).

  9. Discovery of Three Pulsating, Mixed-atmosphere, Extremely Low-mass White Dwarf Precursors

    NASA Astrophysics Data System (ADS)

    Gianninas, A.; Curd, Brandon; Fontaine, G.; Brown, Warren R.; Kilic, Mukremin

    2016-05-01

    We report the discovery of pulsations in three mixed-atmosphere, extremely low-mass white dwarf (ELM WD, M ≤slant 0.3 M ⊙) precursors. Following the recent discoveries of pulsations in both ELM and pre-ELM WDs, we targeted pre-ELM WDs with mixed H/He atmospheres with high-speed photometry. We find significant optical variability in all three observed targets with periods in the range 320–590 s, consistent in timescale with theoretical predictions of p-mode pulsations in mixed-atmosphere ≈0.18 M ⊙ He-core pre-ELM WDs. This represents the first empirical evidence that pulsations in pre-ELM WDs can only occur if a significant amount of He is present in the atmosphere. Future, more extensive, timeseries photometry of the brightest of the three new pulsators offers an excellent opportunity to constrain the thickness of the surface H layer, which regulates the cooling timescales for ELM WDs. 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 National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tecnologia e Inovação (Brazil).

  10. Two new pulsating low-mass pre-white dwarfs or SX Phoenicis stars?

    NASA Astrophysics Data System (ADS)

    Corti, M. A.; Kanaan, A.; Córsico, A. H.; Kepler, S. O.; Althaus, L. G.; Koester, D.; Sánchez Arias, J. P.

    2016-03-01

    Context. The discovery of pulsations in low-mass stars opens an opportunity to probe their interiors and determine their evolution by employing the tools of asteroseismology. Aims: We aim to analyse high-speed photometry of SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25 and discover brightness variabilities. In order to locate these stars in the Teff - log g diagram, we fit optical spectra (SDSS) with synthetic non-magnetic spectra derived from model atmospheres. Methods: To carry out this study, we used the photometric data we obtained for these stars with the 2.15 m telescope at CASLEO, Argentina. We analysed their light curves and applied the discrete Fourier transform (FT) to determine the pulsation frequencies. Finally, we compare both stars in the Teff - log g diagram, with two known pre-white dwarfs and seven pulsating pre-ELM white dwarf stars, δ Scuti, and SX Phe stars Results: We report the discovery of pulsations in SDSS J145847.02+070754.46 and SDSS J173001.94+070600.25. We determine their effective temperature and surface gravity to be Teff = 7972 ± 200 K, log g = 4.25 ± 0.5 and Teff = 7925 ± 200 K, log g = 4.25 ± 0.5, respectively. With these parameters, these new pulsating low-mass stars can be identified with either ELM white dwarfs (with ~0.17 M⊙) or more massive SX Phe stars. We identified pulsation periods of 3278.7 and 1633.9 s for SDSS J145847.02+070754.46 and a pulsation period of 3367.1 s for SDSS J173001.94+070600.25. These two new objects, together with those of Maxted et al. (2013, 2014), indicate the possible existence of a new instability domain towards the late stages of evolution of low-mass white dwarf stars, although their identification with SX Phe stars cannot be discarded. Visiting Astronomer, Complejo Astronómico El Leoncito operated under agreement between the Consejo Nacional de Investigaciones Científicas y Técnicas de la República Argentina and the National Universities of La Plata, Córdoba, and San Juan.

  11. Magnetohydrostatics in the polar caps of accreting magnetized white dwarfs

    NASA Astrophysics Data System (ADS)

    Hameury, J. M.; Lasota, J. P.

    1985-04-01

    The mechanism of quasi-periodic luminosity variations seen in cataclysmic variable stars is investigated theoretically. Numerical calculations are performed for the model proposed by Livio (1984), in which matter accreted onto the polar caps of a magnetized white dwarf causes repeated breaking of the magnetic-field lines; the method is based (by analogy) on the analysis of magnetic-field disruption in magnetized neutron stars of Hameury et al. (1983). It is found that any such magnetic-field breaking gives recurrence times orders of magnitude greater than those actually observed in magnetized cataclysmic variables suggesting that some other mechanism must be found for the quasi-periodic variations.

  12. Temperatures for hot and pulsating DB white dwarfs obtained with the IUE Observatory

    NASA Technical Reports Server (NTRS)

    Liebert, James; Wesemael, F.; Hansen, C. J.; Fontaine, G.; Shipman, Harry L.

    1986-01-01

    Ultraviolet energy distributions are presented for 12 hot, helium-atmosphere DB white dwarfs, including the four known pulsating stars which define an empirical DB instability strip. The UV spectrophotometry is reported, and the energy distributions are matched with two separate grids of DB model atmosphere calculations, so that effective temperature estimates may be derived. The sources of uncertainty in the UV temperature fits are assessed, and a comparison with estimates from optical data is made.

  13. Period variations in pulsating X-ray sources. I - Accretion flow parameters and neutron star structure from timing observations

    NASA Technical Reports Server (NTRS)

    Lamb, F. K.; Shaham, J.; Pines, D.

    1978-01-01

    Torque fluctuations which can lead to variations in the periods of pulsating X-ray sources are examined. A description of torque variations in terms of noise processes is developed, and the resulting noise models are applied to observations of several pulsating X-ray sources. It is shown that fluctuations in accretion torque could account for the observed period variations and spindown episodes in Her X-1 and Cen X-3. The values of the torque noise strengths inferred from either a nonresonant response or, in the case of Her X-1, a Tkachenko-mode interpretation of the data are found to be consistent with those expected from processes at the magnetospheric boundary of an accreting neutron star. Ways to distinguish among the various interpretations of the period variations are considered. It is noted that fluctuating mass-flow rates may be responsible for other phenomena observed in compact X-ray sources, such as wobble with zero initial amplitude and binary period variations in close binary systems experiencing mass transfer.

  14. Limits from the Ongoing Search for Planets Around White Dwarf Stars Using Pulsation Timings

    NASA Astrophysics Data System (ADS)

    Winget, D. E.; Hermes, J. J.; Mullally, Fergal; Bell, K. J.; Montgomery, M. H.; Williams, S. G.; Harrold, S. T.; Kepler, S. O.; Castanheira, B.; Chandler, D. W.; Winget, K. I.; Mukadam, A. S.; Nather, R. E.

    2015-06-01

    Evidence from searches of stars in our galaxy for exoplanet companions suggests that most lower main sequence stars likely have one or more planets; the vast majority of these planet-hosting stars will evolve into white dwarf stars. Some planets may survive this process and new ones may form in a sort of second generation from the cast-off material. If we combine this argument with evidence of a substantial population of metal polluted white dwarf stars, we may plausibly expect that planets may be common around white dwarf stars. Empirically, however, little is known about the presence of planets, new or old around white dwarf stars. Our search is small (˜15 white dwarf stars), but sensitive. Using pulsation arrival times we reach a large search volume around each star: we are sensitive to 1 MJupiter planets at distances ranging from 1- 100AU. In this context, our tightening constraints from pulsation timings become increasingly important to the broader study of planet formation, dynamical evolution, and ultimate survival.

  15. The Driving Mechanism of Pulsating Pre-White Dwarfs: Variability of the "Hybrid" PG 1159 Star HS 2324+3944

    NASA Astrophysics Data System (ADS)

    Handler, G.; Silvotti, R.

    We acquired 8 nights of time-series photometric observations of the variable RhybridS PG 1159 star HS 2324+3944. These data allow us to demonstrate the presence of four frequencies in the stellar light variations with evidence for more. The dominating time scale of the variability (around 35 minutes) is much longer than that of GW Vir pulsators. Binarity is not likely to cause the object's light variations. A pulsational origin of the variability seems more attractive. Recent theoretical investigations suggest that pre-white dwarf pulsations can be excited despite the presence of hydrogen in the model's driving region.

  16. SDSS J184037.78+642312.3: THE FIRST PULSATING EXTREMELY LOW MASS WHITE DWARF

    SciTech Connect

    Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Brown, Warren R.; Kenyon, Scott J.; Kilic, Mukremin

    2012-05-10

    We report the discovery of the first pulsating extremely low mass (ELM) white dwarf (WD), SDSS J184037.78+642312.3 (hereafter J1840). This DA (hydrogen-atmosphere) WD is by far the coolest and the lowest-mass pulsating WD, with T{sub eff} = 9100 {+-} 170 K and log g = 6.22 {+-} 0.06, which corresponds to a mass of {approx}0.17 M{sub Sun }. This low-mass pulsating WD greatly extends the DAV (or ZZ Ceti) instability strip, effectively bridging the log g gap between WDs and main-sequence stars. We detect high-amplitude variability in J1840 on timescales exceeding 4000 s, with a non-sinusoidal pulse shape. Our observations also suggest that the variability is multi-periodic. The star is in a 4.6 hr binary with another compact object, most likely another WD. Future, more extensive time-series photometry of this ELM WD offers the first opportunity to probe the interior of a low-mass, presumably He-core WD using the tools of asteroseismology.

  17. The pulsation index, effective temperature, and thickness of the hydrogen layer in the pulsating DA white dwarf G117-B15A

    NASA Technical Reports Server (NTRS)

    Robinson, E. L.; Mailloux, T. M.; Zhang, E.; Koester, D.; Stiening, R. F.; Bless, R. C.; Percival, J. W.; Taylor, M. J.; Van Citters, G. W.

    1995-01-01

    We have measured the amplitude of the 215 s pulsation of the pulsating DA white dwarf, or ZZ Ceti star, G117-B15A in six passbands with effective wavelengths from 1570 to 6730 A. We find that the index of the pulsation is l = 1 with a high degree of confidence, the first unambiguous determination of l for a pulsation of a ZZ Ceti star. We also find that log g and T(sub eff) are tightly correlated for model atmospheres that fit the data, such that at log g = 7.5 the temperature is 11,750 K and at log g = 8.0 the temperature is 12,375 K. Adopting log g = 7.97 +/- 0.06 from published observations of the optical spectrum of G117-B15A, the correlation yields T(sub eff) = 12,375 +/- 125 K. This temperature is free of flux calibration errors and should be substantially more reliable than temperatures derived for IUE spectra. Since G117-B15A is thought to lie close to the blue edge of the ZZ Ceti instability strip, this low temperature also implies a low temperature for the blue edge. Using pulsation models calculated by Fontaine et al. (1992) and Bradley (1994), we find that the mass of the hydrogen layer in G117-B15A lies between 1.0 x 10(exp -6) solar mass (for k = 1) and 8 x 10(exp -5) solar mass (for k = 2). This range of masses is (barely) consistent with the masses predicted by recent models for the ejection of planetary nebulae, (8-13) x 10(exp -5) solar mass. The mass is too large to be consistent with models invoking thin hydrogen layers to explain the spectral evolution of white dwarfs.

  18. PERIODIC VARIATIONS IN THE O - C DIAGRAMS OF FIVE PULSATION FREQUENCIES OF THE DB WHITE DWARF EC 20058-5234

    SciTech Connect

    Dalessio, J.; Provencal, J. L.; Shipman, H. L.; Sullivan, D. J.; Sullivan, T.; Kilkenny, D.; Fraga, L.; Sefako, R.

    2013-03-01

    Variations in the pulsation arrival time of five independent pulsation frequencies of the DB white dwarf EC 20058-5234 individually imitate the effects of reflex motion induced by a planet or companion but are inconsistent when considered in unison. The pulsation frequencies vary periodically in a 12.9 year cycle and undergo secular changes that are inconsistent with simple neutrino plus photon-cooling models. The magnitude of the periodic and secular variations increases with the period of the pulsations, possibly hinting that the corresponding physical mechanism is located near the surface of the star. The phase of the periodic variations appears coupled to the sign of the secular variations. The standards for pulsation-timing-based detection of planetary companions around pulsating white dwarfs, and possibly other variables such as subdwarf B stars, should be re-evaluated. The physical mechanism responsible for this surprising result may involve a redistribution of angular momentum or a magnetic cycle. Additionally, variations in a supposed combination frequency are shown to match the sum of the variations of the parent frequencies to remarkable precision, an expected but unprecedented confirmation of theoretical predictions.

  19. Precision asteroseismology of the pulsating white dwarf GD 1212 using a two-wheel-controlled Kepler spacecraft

    SciTech Connect

    Hermes, J. J.; Charpinet, S.; Barclay, Thomas; Mullally, Fergal; Huber, Daniel; Still, Martin; Howell, Steve B.; Caldwell, Douglas A.; Haas, Michael R.; Bryson, Stephen T.; Pakštienė, E.; Kawaler, Steven D.; Bloemen, S.; Castanheira, Barbara G.; Winget, D. E.; Montgomery, M. H.; Van Grootel, V.

    2014-07-01

    We present a preliminary analysis of the cool pulsating white dwarf (WD) GD 1212, enabled by more than 11.5 days of space-based photometry obtained during an engineering test of the two-reaction-wheel-controlled Kepler spacecraft. We detect at least 19 independent pulsation modes, ranging from 828.2-1220.8 s, and at least 17 nonlinear combination frequencies of those independent pulsations. Our longest uninterrupted light curve, 9.0 days in length, evidences coherent difference frequencies at periods inaccessible from the ground, up to 14.5 hr, the longest-period signals ever detected in a pulsating WD. These results mark some of the first science to come from a two-wheel-controlled Kepler spacecraft, proving the capability for unprecedented discoveries afforded by extending Kepler observations to the ecliptic.

  20. SHORT-PERIOD g-MODE PULSATIONS IN LOW-MASS WHITE DWARFS TRIGGERED BY H-SHELL BURNING

    SciTech Connect

    Córsico, A. H.; Althaus, L. G.

    2014-09-20

    The detection of pulsations in white dwarfs with low mass offers the possibility of probing their internal structures through asteroseismology and placing constraints on the binary evolutionary processes involved in their formation. In this Letter, we assess the impact of stable H burning on the pulsational stability properties of low-mass He-core white dwarf models resulting from binary star evolutionary calculations. We found that besides a dense spectrum of unstable radial modes and nonradial g and p modes driven by the κ mechanism due to the partial ionization of H in the stellar envelope, some unstable g modes with short pulsation periods are also powered by H burning via the ε mechanism of mode driving. This is the first time that ε destabilized modes are found in models representative of cool white dwarf stars. The short periods recently detected in the pulsating low-mass white dwarf SDSS J111215.82+111745.0 could constitute the first evidence of the existence of stable H burning in these stars, in particular in the so-called extremely low-mass white dwarfs.

  1. Precise atmospheric parameters for the shortest-period binary white dwarfs: gravitational waves, metals, and pulsations

    SciTech Connect

    Gianninas, A.; Kilic, Mukremin; Dufour, P.; Bergeron, P.; Brown, Warren R.; Hermes, J. J.

    2014-10-10

    We present a detailed spectroscopic analysis of 61 low-mass white dwarfs and provide precise atmospheric parameters, masses, and updated binary system parameters based on our new model atmosphere grids and the most recent evolutionary model calculations. For the first time, we measure systematic abundances of He, Ca, and Mg for metal-rich, extremely low mass white dwarfs and examine the distribution of these abundances as a function of effective temperature and mass. Based on our preliminary results, we discuss the possibility that shell flashes may be responsible for the presence of the observed He and metals. We compare stellar radii derived from our spectroscopic analysis to model-independent measurements and find good agreement except for white dwarfs with T {sub eff} ≲ 10,000 K. We also calculate the expected gravitational wave strain for each system and discuss their significance to the eLISA space-borne gravitational wave observatory. Finally, we provide an update on the instability strip of extremely low mass white dwarf pulsators.

  2. Precise Atmospheric Parameters for the Shortest-period Binary White Dwarfs: Gravitational Waves, Metals, and Pulsations

    NASA Astrophysics Data System (ADS)

    Gianninas, A.; Dufour, P.; Kilic, Mukremin; Brown, Warren R.; Bergeron, P.; Hermes, J. J.

    2014-10-01

    We present a detailed spectroscopic analysis of 61 low-mass white dwarfs and provide precise atmospheric parameters, masses, and updated binary system parameters based on our new model atmosphere grids and the most recent evolutionary model calculations. For the first time, we measure systematic abundances of He, Ca, and Mg for metal-rich, extremely low mass white dwarfs and examine the distribution of these abundances as a function of effective temperature and mass. Based on our preliminary results, we discuss the possibility that shell flashes may be responsible for the presence of the observed He and metals. We compare stellar radii derived from our spectroscopic analysis to model-independent measurements and find good agreement except for white dwarfs with T eff <~ 10,000 K. We also calculate the expected gravitational wave strain for each system and discuss their significance to the eLISA space-borne gravitational wave observatory. Finally, we provide an update on the instability strip of extremely low mass white dwarf pulsators. Based on observations obtained at the MMT Observatory, a joint facility of the Smithsonian Institution and the University of Arizona.

  3. Pulsating low-mass white dwarfs in the frame of new evolutionary sequences. III. The pre-ELM white dwarf instability strip

    NASA Astrophysics Data System (ADS)

    Córsico, A. H.; Althaus, L. G.; Serenelli, A. M.; Kepler, S. O.; Jeffery, C. S.; Corti, M. A.

    2016-04-01

    Context. Many low-mass (M⋆/M⊙ ≲ 0.45) and extremely low-mass (ELM, M⋆/M⊙ ≲ 0.18-0.20) white-dwarf stars are currently being found in the field of the Milky Way. Some of these stars exhibit long-period gravity-mode (g-mode) pulsations, and constitute the class of pulsating white dwarfs called ELMV stars. In addition, two low-mass pre-white dwarfs, which could be precursors of ELM white dwarfs, have been observed to show multiperiodic photometric variations. They could constitute a new class of pulsating low-mass pre-white dwarf stars. Aims: Motivated by this finding, we present a detailed nonadiabatic pulsation study of such stars, employing full evolutionary sequences of low-mass He-core pre-white dwarf models. Methods: Our pulsation stability analysis is based on a set of low-mass He-core pre-white dwarf models with masses ranging from 0.1554 to 0.2724 M⊙, which were derived by computing the nonconservative evolution of a binary system consisting of an initially 1 M⊙ ZAMS star and a 1.4 M⊙ neutron star companion. We have considered models in which element diffusion is accounted for and also models in which it is neglected. Results: We confirm and explore in detail a new instability strip in the domain of low gravities and low effective temperatures of the Teff - log g diagram, where low-mass pre-white dwarfs are currently found. The destabilized modes are radial and nonradial p and g modes excited by the κ - γ mechanism acting mainly at the zone of the second partial ionization of He, with non-negligible contributions from the region of the first partial ionization of He and the partial ionization of H. The computations with element diffusion are unable to explain the pulsations observed in the two known pulsating pre-white dwarfs, suggesting that element diffusion might be inhibited at these stages of the pre-white dwarf evolution. Our nonadiabatic models without diffusion, on the other hand, naturally explain the existence and range of

  4. An independent constraint on the secular rate of variation of the gravitational constant from pulsating white dwarfs

    SciTech Connect

    Córsico, Alejandro H.; Althaus, Leandro G.

    2013-06-01

    A secular variation of the gravitational constant modifies the structure and evolutionary time scales of white dwarfs. Using an state-of-the-art stellar evolutionary code and an up-to-date pulsational code we compute the effects of a secularly varying G on the pulsational properties of variable white dwarfs. Comparing the the theoretical results obtained taking into account the effects of a running G with the observed periods and measured rates of change of the periods of two well studied pulsating white dwarfs, G117-B15A and R548, we place constraints on the rate of variation of Newton's constant. We derive an upper bound Ġ/G ∼ −1.8 × 10{sup −10} yr{sup −1} using the variable white dwarf G117-B15A, and Ġ/G ∼ −1.3 × 10{sup −10} yr{sup −1} using R548. Although these upper limits are currently less restrictive than those obtained using other techniques, they can be improved in a future measuring the rate of change of the period of massive white dwarfs.

  5. The Effect of 22NE Diffusion in the Evolution and Pulsational Properties of White Dwarfs with Solar Metallicity Progenitors

    NASA Astrophysics Data System (ADS)

    Camisassa, María E.; Althaus, Leandro G.; Córsico, Alejandro H.; Vinyoles, Núria; Serenelli, Aldo M.; Isern, Jordi; Miller Bertolami, Marcelo M.; García–Berro, Enrique

    2016-06-01

    Because of the large neutron excess of 22Ne, sedimentation of this isotope occurs rapidly in the interior of white dwarfs. This process releases an additional amount of energy, thus delaying the cooling times of the white dwarf. This influences the ages of different stellar populations derived using white dwarf cosmochronology. Furthermore, the overabundance of 22Ne in the inner regions of the star modifies the Brunt–Väisälä frequency, thus altering the pulsational properties of these stars. In this work we discuss the impact of 22Ne sedimentation in white dwarfs resulting from solar metallicity progenitors (Z = 0.02). We performed evolutionary calculations of white dwarfs with masses of 0.528, 0.576, 0.657, and 0.833 {M}ȯ derived from full evolutionary computations of their progenitor stars, starting at the zero-age main sequence all the way through the central hydrogen and helium burning, the thermally pulsing asymptotic giant branch (AGB), and post-AGB phases. Our computations show that at low luminosities ({log}(L/{L}ȯ )≲ -4.25), 22Ne sedimentation delays the cooling of white dwarfs with solar metallicity progenitors by about 1 Gyr. Additionally, we studied the consequences of 22Ne sedimentation on the pulsational properties of ZZ Ceti white dwarfs. We find that 22Ne sedimentation induces differences in the periods of these stars larger than the present observational uncertainties, particularly in more massive white dwarfs.

  6. DISCOVERY OF PULSATIONS, INCLUDING POSSIBLE PRESSURE MODES, IN TWO NEW EXTREMELY LOW MASS, He-CORE WHITE DWARFS

    SciTech Connect

    Hermes, J. J.; Montgomery, M. H.; Winget, D. E.; Bell, Keaton J.; Harrold, Samuel T.; Brown, Warren R.; Kenyon, Scott J.; Gianninas, A.; Kilic, Mukremin

    2013-03-10

    We report the discovery of the second and third pulsating extremely low mass (ELM) white dwarfs (WDs), SDSS J111215.82+111745.0 (hereafter J1112) and SDSS J151826.68+065813.2 (hereafter J1518). Both have masses < 0.25 M{sub Sun} and effective temperatures below 10, 000 K, establishing these putatively He-core WDs as a cooler class of pulsating hydrogen-atmosphere WDs (DAVs, or ZZ Ceti stars). The short-period pulsations evidenced in the light curve of J1112 may also represent the first observation of acoustic (p-mode) pulsations in any WD, which provide an exciting opportunity to probe this WD in a complimentary way compared to the long-period g-modes that are also present. J1112 is a T{sub eff} =9590 {+-} 140 K and log g =6.36 {+-} 0.06 WD. The star displays sinusoidal variability at five distinct periodicities between 1792 and 2855 s. In this star, we also see short-period variability, strongest at 134.3 s, well short of the expected g-modes for such a low-mass WD. The other new pulsating WD, J1518, is a T{sub eff} =9900 {+-} 140 K and log g =6.80 {+-} 0.05 WD. The light curve of J1518 is highly non-sinusoidal, with at least seven significant periods between 1335 and 3848 s. Consistent with the expectation that ELM WDs must be formed in binaries, these two new pulsating He-core WDs, in addition to the prototype SDSS J184037.78+642312.3, have close companions. However, the observed variability is inconsistent with tidally induced pulsations and is so far best explained by the same hydrogen partial-ionization driving mechanism at work in classic C/O-core ZZ Ceti stars.

  7. 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

  8. Accreting white dwarf models for type 1 supernovae. 1: Presupernova evolution and triggering mechanisms

    NASA Technical Reports Server (NTRS)

    Nomoto, K.

    1981-01-01

    As a plausible explosion model for a Type I supernova, the evolution of carbon-oxygen white dwarfs accreting helium in binary systems was investigated from the onset of accretion up to the point at which a thermonuclear explosion occurs. The relationship between the conditions in the binary system and the triggering mechanism for the supernova explosion is discussed, especially for the cases with relatively slow accretion rate. It is found that the growth of a helium zone on the carbon-oxygen core leads to a supernova explosion which is triggered either by the off-center helium detonation for slow and intermediate accretion rates or by the carbon deflagration for slow and rapid accretion rates. Both helium detonation and carbon deflagration are possible for the case of slow accretion, since in this case the initial mass of the white dwarf is an important parameter for determining the mode of ignition. Finally, various modes of building up the helium zone on the white dwarf, namely, direct transfer of helium from the companion star and the various types and strength of the hydrogen shell flashes are discussed in some detail.

  9. Statistical properties of quasi-periodic pulsations in white-light flares observed with Kepler

    NASA Astrophysics Data System (ADS)

    Pugh, C. E.; Armstrong, D. J.; Nakariakov, V. M.; Broomhall, A.-M.

    2016-07-01

    We embark on a study of quasi-periodic pulsations (QPPs) in the decay phase of white-light stellar flares observed by Kepler. Out of the 1439 flares on 216 different stars detected in the short-cadence data using an automated search, 56 flares are found to have pronounced QPP-like signatures in the light curve, of which 11 have stable decaying oscillations. No correlation is found between the QPP period and the stellar temperature, radius, rotation period and surface gravity, suggesting that the QPPs are independent of global stellar parameters. Hence they are likely to be the result of processes occurring in the local environment. There is also no significant correlation between the QPP period and flare energy, however there is evidence that the period scales with the QPP decay time for the Gaussian damping scenario, but not to a significant degree for the exponentially damped case. This same scaling has been observed for MHD oscillations on the Sun, suggesting that they could be the cause of the QPPs in those flares. Scaling laws of the flare energy are also investigated, supporting previous reports of a strong correlation between the flare energy and stellar temperature/radius. A negative correlation between the flare energy and stellar surface gravity is also found.

  10. Statistical Properties of Quasi-Periodic Pulsations in White-Light Flares Observed With Kepler

    NASA Astrophysics Data System (ADS)

    Pugh, C. E.; Armstrong, D. J.; Nakariakov, V. M.; Broomhall, A.-M.

    2016-04-01

    We embark on a study of quasi-periodic pulsations (QPPs) in the decay phase of white-light stellar flares observed by Kepler. Out of the 1439 flares on 216 different stars detected in the short-cadence data using an automated search, 56 flares are found to have pronounced QPP-like signatures in the light curve, of which 11 have stable decaying oscillations. No correlation is found between the QPP period and the stellar temperature, radius, rotation period and surface gravity, suggesting that the QPPs are independent of global stellar parameters. Hence they are likely to be the result of processes occurring in the local environment. There is also no significant correlation between the QPP period and flare energy, however there is evidence that the period scales with the QPP decay time for the Gaussian damping scenario, but not to a significant degree for the exponentially damped case. This same scaling has been observed for MHD oscillations on the Sun, suggesting that they could be the cause of the QPPs in those flares. Scaling laws of the flare energy are also investigated, supporting previous reports of a strong correlation between the flare energy and stellar temperature/radius. A negative correlation between the flare energy and stellar surface gravity is also found.

  11. Evaporation and accretion of extrasolar comets following white dwarf kicks

    NASA Astrophysics Data System (ADS)

    Stone, Nicholas; Metzger, Brian D.; Loeb, Abraham

    2015-03-01

    Several lines of observational evidence suggest that white dwarfs receive small birth kicks due to anisotropic mass-loss. If other stars possess extrasolar analogues to the Solar Oort cloud, the orbits of comets in such clouds will be scrambled by white dwarf natal kicks. Although most comets will be unbound, some will be placed on low angular momentum orbits vulnerable to sublimation or tidal disruption. The dusty debris from these comets will manifest itself as an IR excess temporarily visible around newborn white dwarfs; examples of such discs may already have been seen in the Helix Nebula, and around several other young white dwarfs. Future observations with the James Webb Space Telescope may distinguish this hypothesis from alternatives such as a dynamically excited Kuiper Belt analogue. Although competing hypotheses exist, the observation that ≳15 per cent of young white dwarfs possess such discs, if interpreted as indeed being cometary in origin, provides indirect evidence that low-mass gas giants (thought necessary to produce an Oort cloud) are common in the outer regions of extrasolar planetary systems. Hydrogen abundances in the atmospheres of older white dwarfs can, if sufficiently low, also be used to place constraints on the joint parameter space of natal kicks and exo-Oort cloud models.

  12. Accretion of rotating fluids by barytropes - Numerical results for white-dwarf models

    NASA Technical Reports Server (NTRS)

    Durisen, R. H.

    1977-01-01

    Numerical sequences of rotating axisymmetric nonmagnetic equilibrium models are constructed which represent the evolution of a barytropic star as it accretes material from a rotating medium. Two accretion geometries are considered - one approximating accretion from a rotating cloud and the other, accretion from a Keplerian disk. It is assumed that some process, such as Ekman spin-up or nonequilibrium oscillations, maintains nearly constant angular velocity along cylinders about the rotation axis. Transport of angular momentum in the cylindrically radial direction by viscosity is included. Fluid instabilities and other physical processes leading to enhancement of this transport are discussed. Particular application is made to zero-temperature white-dwarf models, using the degenerate electron equation of state. An initially nonrotating 0.566-solar-mass white dwarf is followed during the accretion of more than one solar mass of material. Applications to degenerate stellar cores, to mass-transfer binary systems containing white dwarfs, such as novae and dwarf novae, to Type I supernovae, and to galactic X-ray sources are considered.

  13. 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.

  14. 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

  15. 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}white dwarfs (WDs) suggest that pollution of these stars with metals may be caused by 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.

  16. Compressional heating of accreting white dwarfs in CVs

    NASA Astrophysics Data System (ADS)

    Townsley, D. M.; Bildsten, L.

    2002-01-01

    In recent years several Dwarf Novae (DN) systems have been observed in quiescence, when the accretion rate is low and the WD photosphere can be directly detected. The WDs are observed to cool after the DN outburst from high effective temperatures to lower effective temperatures (Teff) thought to be indicative of the thermal state of the deep interior of the WD. Sion has argued that the most likely energy source for this quiescent luminosity is the gravitational compression of the WD interior, which rejuvenates an otherwise cold WD into a much hotter state. We are undertaking a theoretical study of the compressional heating of WD's, extending down to the very low time averaged accretion rates, < dot M > ~10-11 Modot yr-1, applicable to the post-turnaround CV's (the ``TOADS''). Nuclear burning is unstable at these < dot M >'s, so we have incorporated the recurrent heating and cooling of the WD throughout the classical novae limit cycle. In addition to self-consistently finding the range of Teff as a function of < dot M > during the cycle, we also self-consistently find the ignition masses. Comparing these theoretical masses to the observed ejected masses will tell us whether the WD mass in CV's is secularly increasing or decreasing. We close by comparing our results to the accumulated observations of quiescent DN and making predictions for the colors of low < dot M > CV's in quiescence that are applicable to searches for faint CVs in the field and galactic globular clusters.

  17. FOLLOW-UP STUDIES OF THE PULSATING MAGNETIC WHITE DWARF SDSS J142625.71+575218.3

    SciTech Connect

    Green, E. M.; Dufour, P.; Fontaine, G.; Brassard, P. E-mail: dufourpa@astro.umontreal.ca E-mail: brassard@astro.umontreal.ca

    2009-09-10

    We present a follow-up analysis of the unique magnetic luminosity-variable carbon-atmosphere white dwarf SDSS J142625.71+575218.3. This includes the results of some 106.4 hr of integrated light photometry which have revealed, among other things, the presence of a new periodicity at 319.720 s which is not harmonically related to the dominant oscillation (417.707 s) previously known in that star. Using our photometry and available spectroscopy, we consider the suggestion made by Montgomery et al. that the luminosity variations in SDSS J142625.71+575218.3 may not be caused by pulsational instabilities, but rather by photometric activity in a carbon-transferring analog of AM CVn. This includes a detailed search for possible radial velocity variations due to rapid orbital motion on the basis of Multiple Mirror Telescope spectroscopy. At the end of the exercise, we unequivocally rule out the interacting binary hypothesis and conclude instead that, indeed, the luminosity variations are caused by g-mode pulsations as in other pulsating white dwarfs. This is in line with the preferred possibility put forward by Montgomery et al.

  18. Accretion-induced variability links young stellar objects, white dwarfs, and black holes

    PubMed Central

    Scaringi, Simone; Maccarone, Thomas J.; Körding, Elmar; Knigge, Christian; Vaughan, Simon; Marsh, Thomas R.; Aranzana, Ester; Dhillon, Vikram S.; Barros, Susana C. C.

    2015-01-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies. PMID:26601307

  19. Accretion-induced variability links young stellar objects, white dwarfs, and black holes.

    PubMed

    Scaringi, Simone; Maccarone, Thomas J; Körding, Elmar; Knigge, Christian; Vaughan, Simon; Marsh, Thomas R; Aranzana, Ester; Dhillon, Vikram S; Barros, Susana C C

    2015-10-01

    The central engines of disc-accreting stellar-mass black holes appear to be scaled down versions of the supermassive black holes that power active galactic nuclei. However, if the physics of accretion is universal, it should also be possible to extend this scaling to other types of accreting systems, irrespective of accretor mass, size, or type. We examine new observations, obtained with Kepler/K2 and ULTRACAM, regarding accreting white dwarfs and young stellar objects. Every object in the sample displays the same linear correlation between the brightness of the source and its amplitude of variability (rms-flux relation) and obeys the same quantitative scaling relation as stellar-mass black holes and active galactic nuclei. We also show that the most important parameter in this scaling relation is the physical size of the accreting object. This establishes the universality of accretion physics from proto-stars still in the star-forming process to the supermassive black holes at the centers of galaxies. PMID:26601307

  20. Gravitational waves, pulsations, and more : high-speed photometry of low-mass, He-core white dwarfs

    NASA Astrophysics Data System (ADS)

    Hermes, J. J.

    2013-08-01

    This dissertation is an observational exploration of the exciting physics that can be enabled by high-speed photometric monitoring of extremely low-mass (< 0.25 Msun) white dwarf stars, which are found in some of the most compact binaries known. It includes the cleanest indirect detection of gravitational waves at visible wavelengths, the discovery of pulsations in He-core WDs, the strongest evidence for excited p-mode pulsations in a WD, the discovery of the first tidally distorted WDs and their use to constrain the low-end of the WD mass-radius relationship, and the strongest cases of Doppler beaming observed in a binary system. It is the result of the more than 220 nights spent at McDonald Observatory doing high-speed photometry with the Argos instrument on the 2.1 m Otto Struve telescope, which has led to a number of additional exciting results, including the discovery of an intermediate timescale in the evolution of cooling DA WDs and the discovery of the most massive pulsating WD, which should have an ONe-core and should be highly crystallized.

  1. A numerical study of the stability of radiative shocks. [in accretion flows onto white dwarf stars

    NASA Technical Reports Server (NTRS)

    Imamura, J. N.; Wolff, M. T.; Durisen, R. H.

    1984-01-01

    Attention is given to the oscillatory instability of optically thin radiative shocks in time-dependent numerical calculations of accretion flows onto degenerate dwarfs. The present nonlinear calculations yield good quantitative agreement with the linear results obtained for oscillation frequencies, damping rates, and critical alpha-values. The fundamental mode and the first overtone in the shock radius and luminosity variations can be clearly identified, and evidence is sometimes seen for the second overtone. Time-dependent calculations are also performed which include additional physics relevant to degenerate dwarf accretion, such as electron thermal conduction, unequal electron and ion temperatures, Compton cooling, and relativistic corrections to the bremsstrahlung cooling law. All oscillatory modes are found to be damped, and hence stable, in the case of a 1-solar mass white dwarf accreting in spherical symmetry.

  2. THE NEWLY DISCOVERED PULSATING LOW-MASS WHITE DWARFS: AN EXTENSION OF THE ZZ CETI INSTABILITY STRIP

    SciTech Connect

    Van Grootel, V.; Dupret, M.-A.; Fontaine, G.; Brassard, P.

    2013-01-01

    In light of the exciting discovery of g-mode pulsations in extremely low-mass, He-core DA white dwarfs, we report on the results of a detailed stability survey aimed at explaining the existence of these new pulsators as well as their location in the spectroscopic Hertzsprung-Russell diagram. To this aim, we calculated some 28 evolutionary sequences of DA models with various masses and chemical layering. These models are characterized by the so-called ML2/{alpha} = 1.0 convective efficiency and take into account the important feedback effect of convection on the atmospheric structure. We pulsated the models with the nonadiabatic code MAD, which incorporates a detailed treatment of time-dependent convection. On the other hand, given the failure of all nonadiabatic codes, including MAD, to account properly for the red edge of the strip, we resurrect the idea that the red edge is due to energy leakage through the atmosphere. We thus estimated the location of that edge by requiring that the thermal timescale in the driving region-located at the base of the H convection zone-be equal to the critical period beyond which l = 1 g-modes cease to exist. Using this approach, we find that our theoretical ZZ Ceti instability strip accounts remarkably well for the boundaries of the empirical strip, including the low-gravity, low-temperature regime where the three new pulsators are found. We also account for the relatively long periods observed in these stars, and thus conclude that they are true ZZ Ceti stars, but with low masses.

  3. Flickering of accreting white dwarfs: the remarkable amplitude-flux relation and disc viscosity

    NASA Astrophysics Data System (ADS)

    Zamanov, R. K.; Boeva, S.; Latev, G.; Sokoloski, J. L.; Stoyanov, K. A.; Genkov, V.; Tsvetkova, S. V.; Tomov, T.; Antov, A.; Bode, M. F.

    2016-03-01

    We analyse optical photometric data of short term variability (flickering) of accreting white dwarfs in cataclysmic variables (KR Aur, MV Lyr, V794 Aql, TT Ari, V425 Cas), recurrent novae (RS Oph and T CrB) and jet-ejecting symbiotic stars (CH Cyg and MWC 560). We find that the amplitude-flux relationship is visible over four orders of magnitude, in the range of fluxes from 1029 to 1033 erg s-1 Å-1, as a `statistically perfect correlation with correlation coefficient 0.96 and p-value ˜10-28. In the above range, the amplitude of variability for any of our 9 objects is proportional to the flux level with (almost) one and the same factor of proportionality for all nine accreting white dwarfs with ΔF = 0.36(±0.05)Fav, σrms = 0.086(±0.011)Fav, and σrms/ΔF = 0.24 ± 0.02. Overall, our results indicate that the viscosity in the accretion discs is practically the same for all nine objects in our sample, in the mass accretion rate range 2 × 10-11 - 2 × 10-7 M⊙ yr-1.

  4. HYDROGEN BURNING ON ACCRETING WHITE DWARFS: STABILITY, RECURRENT NOVAE, AND THE POST-NOVA SUPERSOFT PHASE

    SciTech Connect

    Wolf, William M.; Bildsten, Lars; Brooks, Jared; Paxton, Bill

    2013-11-10

    We examine the properties of white dwarfs (WDs) accreting hydrogen-rich matter in and near the stable burning regime of accretion rates as modeled by time-dependent calculations done with Modules for Experiments in Stellar Astrophysics (MESA). We report the stability boundary for WDs of masses between 0.51 M{sub ☉} and 1.34 M{sub ☉} as found via time-dependent calculations. We also examine recurrent novae that are accreting at rates close to, but below, the stable burning limit and report their recurrence times. Our dense grid in accretion rates finds the expected minimum possible recurrence times as a function of the WD mass. This enables inferences to be made about the minimum WD mass possible to reach a specific recurrence time. We compare our computational models of post-outburst novae to the stably burning WDs and explicitly calculate the duration and effective temperature (T{sub eff}) of the post-nova WD in the supersoft phase. We agree with the measured turnoff time-T{sub eff} relation in M31 by Henze and collaborators, infer WD masses in the 1.0-1.3 M{sub ☉} range, and predict ejection masses consistent with those observed. We close by commenting on the importance of the hot helium layer generated by stable or unstable hydrogen burning for the short- and long-term evolution of accreting WDs.

  5. Nickel-rich outflows from accretion discs formed by the accretion-induced collapse of white dwarfs

    NASA Astrophysics Data System (ADS)

    Metzger, B. D.; Piro, A. L.; Quataert, E.

    2009-07-01

    A white dwarf (WD) approaching the Chandrasekhar mass may in several circumstances undergo accretion-induced collapse (AIC) to a neutron star (NS) before a thermonuclear explosion ensues. It has generally been assumed that such an AIC does not produce a detectable supernova (SN). If, however, the progenitor WD is rapidly rotating (as may be expected due to its prior accretion), a centrifugally supported disc forms around the NS upon collapse. We calculate the subsequent evolution of this accretion disc and its nuclear composition using time-dependent height-integrated simulations with initial conditions taken from the AIC calculations of Dessart and collaborators. Soon after its formation, the disc is cooled by neutrinos and its composition is driven neutron rich (electron fraction Ye ~ 0.1) by electron captures. However, as the disc viscously spreads, it is irradiated by neutrinos from the central proto-NS, which dramatically alters its neutron-to-proton ratio. We find that electron neutrino captures increase Ye to ~0.5 by the time that weak interactions in the disc freeze out. Because the disc becomes radiatively inefficient and begins forming α-particles soon after freeze out, powerful winds blow away most of the disc's remaining mass. These Ye ~ 0.5 outflows synthesize up to a few times 10-2Msolar in 56Ni. As a result, AIC may be accompanied by a radioactively powered SN-like transient that peaks on a time-scale of ~1 d. Since few intermediate mass elements are likely synthesized, these nickel-rich explosions should be spectroscopically distinct from other SNe. The time-scale, velocity and composition of the AIC transient can be modified if the disc wind sweeps up a ~0.1Msolar remnant disc created by a WD-WD merger; such an `enshrouded' AIC may account for sub-luminous, sub-Chandrasekhar Type I SNe. Optical transient surveys such as the Panoramic Survey Telescope and Rapid Response System and the Palomar Transient Factory should detect a few AIC transients per

  6. Eclipse mapping of the accreting magnetic white dwarf in DP Leonis with HST

    NASA Technical Reports Server (NTRS)

    Stockman, H. S.; Schmidt, Gary D.; Liebert, James; Holberg, J. B.

    1994-01-01

    We present time-resolved ultraviolet spectrophotometry of the AM Her system DP Leo (E1114+182), obtained with the Faint Object Spectrograph on the Hubble Space Telescope (HST) in 1991 October. During this period, the binary was in a low-activity state. Complementary optical spectrophotometry and spectopolarimetry were obtained in 1991 October and December, as well as in 1992 April when the object had entered a high-accretion state. The HST spectrophotometry reveals two UV-emitting components-a hot 'spot' (T approximately 50,000 K, very approximately) near the magnetic pole into which most material is accreted, and the white dwarf photosphere (T approximately 16,000 K), which dominates when the spot is not in view. Both components appear as rapid and gradual components in the ingress and egress of secondary eclipse, the latter indicating a normal white dwarf radius of 0.8 x 10(exp 9) cm. The fractional area of the spot (f approximately 0.006) implied by the eclipse data is consistent with the observed UV flux and a minimum distance of 380 pc is similar to those found in other AM Her systems. The nearly identical eclipse light curves from the HST and earlier optical observations imply that the UV-emitting reprocessing area is not more extended than the cyclotron source of optical radiation in the accretion funnel. The timing of the onset of the bright phase indicated that the spot leads the secondary in orbital azimuth by approximately 3 deg.

  7. Accretion of planetary debris onto the unique white dwarf GD394

    NASA Astrophysics Data System (ADS)

    Wilson, David

    2014-10-01

    It is now undisputed that metal-pollution of the H or He atmospheres observed in a substantial fraction of white dwarfs is due to accretion of planetary debris. While significant progress has been made in establishing the frequency of such evolved planetary systems, and statistics on the abundances of the debris material are beginning to emerge, very little is known regarding the delivery mechanism of the debris. Given that the red giant progenitors of the metal-polluted white dwarfs evaporated all planetary material within >1 AU, the debris found at many white dwarfs must originate originally from a substantial distance of the star. The canonical model is that remaining planets perturb the orbits of astroids, sending them onto a white dwarf grazing course, where the extreme gravity will result in their tidal disruption. The debris then must undergo complex dynamical evolution, and eventually fall into the white dwarf atmosphere. EUV light curves of GD394, an extremely metal-polluted white dwarf, suggest a strong variation of the metal abundances over the stellar surface. We propose to obtain spin-phase resolved high-resolution spectroscopy of GD394 over its 1.15d rotation period to resolve the abundance pattern on its surface from variations in the photospheric metal lines, and thereby probe the geometry of the circumstellar debris. Line profile variations can be used to reconstruct the surface distribution of the metals using Roche tomography. In addition, comparison to a single archive HST/GHRS observation from 1992 will place tight constraints on the long-term variation of the accretion rate, and the life time of the circumstellar debris.

  8. The asteroseismological potential of the pulsating DB white dwarf stars CBS 114 and PG 1456+103

    NASA Astrophysics Data System (ADS)

    Handler, G.; Metcalfe, T. S.; Wood, M. A.

    2002-09-01

    We have acquired 65 h of single-site time-resolved CCD photometry of the pulsating DB white dwarf star (DBV) CBS 114 and 62 h of two-site high-speed CCD photometry of another DBV, PG 1456+103. The pulsation spectrum of PG 1456+103 is complicated and variable on time-scales of approximately 1 week and could only partly be deciphered with our measurements. The modes of CBS 114 are more stable in time and we were able to arrive at a frequency solution somewhat affected by aliasing, but still satisfactory, involving seven independent modes and two combination frequencies. These frequencies also explain the discovery data of the star, taken 13 yr earlier. We find a mean period spacing of 37.1 +/- 0.7 s significant at the 98 per cent level between the independent modes of CBS 114 and argue that they are caused by non-radial g-mode pulsations of spherical degree l= 1. We performed a global search for asteroseismological models of CBS 114 using a genetic algorithm, and we examined the susceptibility of the results to the uncertainties of the observational frequency determinations and mode identifications (we could not provide m values). The families of possible solutions are identified correctly even without knowledge of m. Our best-fitting model suggests Teff= 21 000 K, M*= 0.730 Msolar and log(MHe/M*) =-6.66, XO= 0.61. The latter value of the central oxygen mass fraction implies a rate for the 12C(α,γ)16O nuclear reaction near S300= 180 keV b, consistent with laboratory measurements.

  9. Self-Consistent Simulations of Accretion-Induced Collapse of White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kleiser, Io; Ott, Christian; Abdikamalov, Ernazar; O'Connor, Evan

    2013-04-01

    Accreting white dwarfs and white dwarf mergers are commonly thought to end in thermonuclear explosions that produce Type Ia supernovae (SNe Ia). However, there is an alternative outcome for these systems that has not been theoretically explored as thoroughly, nor has it been securely identified observationally. Some white dwarfs, rather than exploding, should undergo electron capture and collapse to neutron stars. This accretion-induced collapse (AIC) scenario is expected to be intrinsically rare compared to SNe Ia, and past studies indicate that the associated optical transient would be faint and short-lived, near the detection limits of current surveys. However, until now there have not been self-consistent numerical studies of AIC that examine the explosion dynamics, subsequent evolution, and all resulting observables. We use GR1D, a one-dimensional general-relativistic hydrodynamics code, to follow AIC through collapse, core bounce, explosion, and shock breakout and to present new results on its neutrino signature and nucleosynthetic yields. This study is preliminary to the goal of developing fully self-consistent three-dimensional models that will yield predictions for electromagnetic, neutrino, and gravitational-wave signals form AIC events.

  10. Self-Consistent Simulations of Accretion-Induced Collapse of White Dwarfs

    NASA Astrophysics Data System (ADS)

    Kleiser, Io; Ott, C. D.

    2013-01-01

    Accreting white dwarfs and white dwarf mergers are commonly thought to end in thermonuclear explosions that produce Type Ia supernovae (SNe Ia). However, there is an alternative outcome for these systems that has not been theoretically explored as thoroughly, nor has it been securely identified observationally. Some white dwarfs, rather than exploding, should undergo electron capture and collapse to neutron stars. This accretion-induced collapse (AIC) scenario is expected to be intrinsically rare compared to SNe Ia, and past studies indicate that the associated optical transient would be faint and short-lived, near the detection limits of current surveys. However, until now there have not been self-consistent numerical studies of AIC that examine the explosion dynamics, subsequent evolution, and all resulting observables. We use GR1D, a one-dimensional general-relativistic hydrodynamics code, to follow AIC through collapse, core bounce, explosion, and shock breakout and to present new results on its neutrino signature. This study is preliminary to the goal of developing fully self-consistent three-dimensional models that will yield predictions for electromagnetic, neutrino, and gravitational-wave signals form AIC events.

  11. Potential Impacts of ASTRO-H on the Studies of Accreting White Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Mukai, Koji; Yuasa, Tadayuki; Harayama, Atsushi; Hayashi, Takayuki; Ishida, Manabu; Long, Knox S.; Terada, Yukikatsu; Tsujimoto, Masahiro; ASTRO-H Team

    2015-01-01

    Interacting binaries in which a white dwarf accretes material from a companion - cataclysmic variables (CVs) in which the mass loss is via Roche-lobe overflow, and symbiotic stars in which the white dwarf captures the wind of a late type giant - are important populations of X-ray sources. Accretion onto the white dwarf surface often creates shocks with temperatures in the 10-50 keV range. If the post-shock region stays optically thin, it produces multi-temperature plasma emission over the medium to hard X-ray band (~0.5-50 keV). This makes them well-matched to the capabilities of the upcoming ASTRO-H mission, which will allow high-resolution spectroscopy in the 0.3-10 keV range with the microcalorimeter instrument, Soft X-ray Spectrometer (SXS), and simultaneous imaging spectroscopy in the 5-80 keV range with the Hard X-ray Imager (HXI). We will highlight several areas in which ASTRO-H can make unique contributions to the studies of these binaries. For example, X-ray emitting plasma in many of these systems are expected to have such high densities that only the SXS can provide density diagnostics. The prominent Fe K lines will allow dynamical studies of the X-ray emitting plasma for which velocities of order 1,000 km s-2 are expected. Finally, we discuss the potential of ASTRO-H to study the reflection off the white dwarf surface, both via the continuum bump observable with the HXI and the 6.4 keV fluorescent iron line with the SXS. For near Chandrasekhar-mass white dwarfs, the gravitational redshift of the latter is within reach of the instrumental capability and may provide the best direct measurement of their masses.

  12. A search for p-mode pulsations in white dwarf stars using the Berkeley Visible Imaging Tube detector

    NASA Astrophysics Data System (ADS)

    Kilkenny, D.; Welsh, B. Y.; Koen, C.; Gulbis, A. A. S.; Kotze, M. M.

    2014-01-01

    We present high-speed photometry (resolution 0.1 s) obtained during the commissioning of the Berkely Visible Imaging Tube system on the Southern African Large Telescope (SALT). The observations were an attempt to search for very rapid p-mode oscillations in white dwarf stars and included three DA stars known to be g-mode pulsators (ZZ Cet, HK Cet and AF Pic), one other DA star (WD 1056-384) not known to be variable and one AM Cvn star (HP Lib). No evidence was found for any variations greater than about 1 mmag in amplitude (˜0.1 per cent) at frequencies in excess of 60 mHz (periods <17 s) in any of the target stars, though several previously known g-mode frequencies were recovered.

  13. 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.

  14. Carbon Shell or Core Ignitions in White Dwarfs Accreting from Helium Stars

    NASA Astrophysics Data System (ADS)

    Brooks, Jared; Bildsten, Lars; Schwab, Josiah; Paxton, Bill

    2016-04-01

    White dwarfs accreting from helium stars can stably burn at the accreted rate and avoid the challenge of mass loss associated with unstable helium burning that is a concern for many SNe Ia scenarios. We study binaries with helium stars of mass 1.25{M}ȯ ≤slant {M}{{He}}≤slant 1.8{M}ȯ , which have lost their hydrogen rich envelopes in an earlier common envelope event and now orbit with periods ({P}{{orb}}) of several hours with non-rotating 0.84 and 1.0{M}ȯ C/O WDs. The helium stars fill their Roche lobes after exhaustion of central helium and donate helium on their thermal timescales (∼ {10}5 years). As shown by others, these mass transfer rates coincide with the steady helium burning range for WDs, and grow the WD core up to near the Chandrasekhar mass ({M}{{Ch}}) and a core carbon ignition. We show here, however, that many of these scenarios lead to an ignition of hot carbon ashes near the outer edge of the WD and an inward going carbon flame that does not cause an explosive outcome. For {P}{{orb}}=3 hr, 1.0{M}ȯ C/O WDs with donor masses {M}{{He}}≳ 1.8{M}ȯ experience a shell carbon ignition, while {M}{{He}}≲ 1.3{M}ȯ will fall below the steady helium burning range and undergo helium flashes before reaching core C ignition. Those with 1.3{M}ȯ ≲ {M}{{He}}≲ 1.7{M}ȯ will experience a core C ignition. We also calculate the retention fraction of accreted helium when the accretion rate leads to recurrent weak helium flashes.

  15. Axisymmetric general relativistic simulations of the accretion-induced collapse of white dwarfs

    SciTech Connect

    Abdikamalov, E. B.; Ott, C. D.; Rezzolla, L.; Dessart, L.; Dimmelmeier, H.; Marek, A.; Janka, H.-T.

    2010-02-15

    The accretion-induced collapse (AIC) of a white dwarf may lead to the formation of a protoneutron star and a collapse-driven supernova explosion. This process represents a path alternative to thermonuclear disruption of accreting white dwarfs in type Ia supernovae. In the AIC scenario, the supernova explosion energy is expected to be small and the resulting transient short-lived, making it hard to detect by electromagnetic means alone. Neutrino and gravitational-wave (GW) observations may provide crucial information necessary to reveal a potential AIC. Motivated by the need for systematic predictions of the GW signature of AIC, we present results from an extensive set of general-relativistic AIC simulations using a microphysical finite-temperature equation of state and an approximate treatment of deleptonization during collapse. Investigating a set of 114 progenitor models in axisymmetric rotational equilibrium, with a wide range of rotational configurations, temperatures and central densities, and resulting white dwarf masses, we extend previous Newtonian studies and find that the GW signal has a generic shape akin to what is known as a 'type III' signal in the literature. Despite this reduction to a single type of waveform, we show that the emitted GWs carry information that can be used to constrain the progenitor and the postbounce rotation. We discuss the detectability of the emitted GWs, showing that the signal-to-noise ratio for current or next-generation interferometer detectors could be high enough to detect such events in our Galaxy. Furthermore, we contrast the GW signals of AIC and rotating massive star iron core collapse and find that they can be distinguished, but only if the distance to the source is known and a detailed reconstruction of the GW time series from detector data is possible. Some of our AIC models form massive quasi-Keplerian accretion disks after bounce. The disk mass is very sensitive to progenitor mass and angular momentum

  16. A subsynchronously rotating pulsating subdwarf B star in a short-period binary with a white dwarf companion

    NASA Astrophysics Data System (ADS)

    Baran, A. S.; Telting, J. H.; Németh, P.; Østensen, R. H.; Reed, M. D.; Kiaeerad, F.

    2016-01-01

    We present our analysis of KIC 7664467, an sdB pulsator that we have found to be residing in a 1.56-day binary system with a white-dwarf companion. This system was observed photometrically with the Kepler spacecraft and spectroscopically with ground-based telescopes. We analyzed the amplitude spectra detecting 61 periods, rotationally split multiplets, and an equally spaced sequence in period. These two features helped with the mode identification. We derived both the binary and rotation periods showing that this is another binary system with a subsynchronous sdB star. From our spectroscopy of the sdB star, we determined Teff = 27440 ± 120 K, log g = 5.38 ± 0.02 dex. The abundance pattern follows the general trend observed in sdB stars, where light metals are subsolar, while the Fe abundance is very close to the solar value. We found the N enrichment and low abundances of C and O that resemble the equilibrium abundances of the CNO cycle. We could also measure the Mg and Si abundances. Using the radial velocity amplitude K1 = 57(3) km s-1 and the Doppler boosting-dominated photometric signal at the orbital period, we constrained the companion to be a compact object, almost certainly a white dwarf.

  17. Metal Accretion onto White Dwarfs. I. The Approximate Approach Based on Estimates of Diffusion Timescales

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Brassard, P.; Dufour, P.; Tremblay, P.-E.

    2015-06-01

    The accretion-diffusion picture is the model par excellence for describing the presence of planetary debris polluting the atmospheres of relatively cool white dwarfs. Some important insights into the process may be derived using an approximate approach which combines static stellar models with estimates of diffusion timescales at the base of the outer convection zone or, in its absence, at the photosphere. Until recently, and to our knowledge, values of diffusion timescales in white dwarfs have all been obtained on the basis of the same physics as that developed initially by Paquette et al., including their diffusion coefficients and thermal diffusion coefficients. In view of the recent exciting discoveries of a plethora of metals (including some never seen before) polluting the atmospheres of an increasing number of cool white dwarfs, we felt that a new look at the estimates of settling timescales would be worthwhile. We thus provide improved estimates of diffusion timescales for all 27 elements from Li to Cu in the periodic table in a wide range of the surface gravity-effective temperature domain and for both DA and non-DA stars.

  18. STRENGTHENING THE CASE FOR ASTEROIDAL ACCRETION: EVIDENCE FOR SUBTLE AND DIVERSE DISKS AT WHITE DWARFS

    SciTech Connect

    Farihi, J.; Jura, M.; Zuckerman, B.; Lee, J.-E. E-mail: jura@astro.ucla.ed E-mail: jelee@sejong.ac.k

    2010-05-10

    Spitzer Space Telescope IRAC 3-8 {mu}m and AKARI IRC 2-4 {mu}m photometry are reported for 10 white dwarfs with photospheric heavy elements; nine relatively cool stars with photospheric calcium and one hotter star with a peculiar high carbon abundance. A substantial infrared excess is detected at HE 2221-1630, while modest excess emissions are identified at HE 0106-3253 and HE 0307+0746, implying these latter two stars have relatively narrow ({Delta}r < 0.1 R{sub sun}) rings of circumstellar dust. A likely 7.9 {mu}m excess is found at PG 1225-079 and may represent, together with G166-58, a sub-class of dust ring with a large inner hole. The existence of attenuated disks at white dwarfs substantiates the connection between their photospheric heavy elements and the accretion of disrupted minor planets, indicating many polluted white dwarfs may harbor orbiting dust, even those lacking an obvious infrared excess.

  19. ON THE EFFECT OF EXPLOSIVE THERMONUCLEAR BURNING ON THE ACCRETED ENVELOPES OF WHITE DWARFS IN CATACLYSMIC VARIABLES

    SciTech Connect

    Sion, Edward M.; Sparks, Warren E-mail: warrensparks@comcast.net

    2014-11-20

    The detection of heavy elements at suprasolar abundances in the atmospheres of some accreting white dwarfs in cataclysmic variables (CVs), coupled with the high temperatures needed to produce these elements, requires explosive thermonuclear burning. The central temperatures of any formerly more massive secondary stars in CVs undergoing hydrostatic CNO burning are far too low to produce these elements. Evidence is presented that at least some CVs contain donor secondaries that have been contaminated by white dwarf remnant burning during the common envelope phase and are transferring this material back to the white dwarf. This scenario does not exclude the channel in which formerly more massive donor stars underwent CNO processing in systems with thermal timescale mass transfer. Implications for the progenitors of CVs are discussed and a new scenario for the white dwarf's accretion-nova-outburst is given.

  20. Population synthesis of accreting white dwarfs - II. X-ray and UV emission

    NASA Astrophysics Data System (ADS)

    Chen, Hai-Liang; Woods, T. E.; Yungelson, L. R.; Gilfanov, M.; Han, Zhanwen

    2015-11-01

    Accreting white dwarfs (WDs) with non-degenerate companions are expected to emit in soft X-rays and the UV, if accreted H-rich material burns stably. They are an important component of the unresolved emission of elliptical galaxies, and their combined ionizing luminosity may significantly influence the optical line emission from warm interstellar medium (ISM). In an earlier paper, we modelled populations of accreting WDs, first generating WD with main-sequence, Hertzsprung gap and red giant companions with the population synthesis code BSE, and then following their evolution with a grid of evolutionary tracks computed with MESA. Now we use these results to estimate the soft X-ray (0.3-0.7 keV), H- and He II-ionizing luminosities of nuclear burning WDs and the number of supersoft X-ray sources for galaxies with different star formation histories. For the starburst case, these quantities peak at ˜1 Gyr and decline by ˜1-3 orders of magnitude by the age of 10 Gyr. For stellar ages of ˜10 Gyr, predictions of our model are consistent with soft X-ray luminosities observed by Chandra in nearby elliptical galaxies and He II 4686 Å/H β line ratio measured in stacked Sloan Digital Sky Survey spectra of retired galaxies, the latter characterizing the strength and hardness of the UV radiation field. However, the soft X-ray luminosity and He II 4686 Å/H β ratio are significantly overpredicted for stellar ages of ≲4-8 Gyr. We discuss various possibilities to resolve this discrepancy and tentatively conclude that it may be resolved by a modification of the typically used criteria of dynamically unstable mass-loss for giant stars.

  1. Further Studies of Accretion onto Hot, Massive, White Dwarfs: The Growth to the Chandrasekhar Limit

    NASA Astrophysics Data System (ADS)

    Starrfield, S.; Timmes, F. X.; Hix, W. R.; Sion, E. M.; Sparks, W. M.; Dwyer, S.

    2004-05-01

    We report on further studies of accretion (Solar Composition material) at a variety of rates onto hot, massive white dwarfs (WDs: Starrfield et al. 2003, Atlanta Meeting, poster 45.10). Our initial models are 1.0 M⊙, 1.25 M⊙, and 1.35 M⊙ WDs with luminosities of 30 L⊙. We have now run evolutionary sequences with mass accretion rates varying from 1.6×10-10 M⊙ yr-1 to 3.5×10-7 M⊙ yr-1 and, in all cases, the WDs grow in mass to near the Chandrasekhar Limit. We use the 1D, hydrodynamic computer code as described in Starrfield et al. (APJS, 127,485, 2000) and have updated the nuclear reaction network, since the calculations reported at Atlanta, to that found at www.cococubed.com/code_pages/net_hhe.shtml. This network combines the pp + hot cno + rp breakout network used previously with an alpha-chain that runs to 56Ni. This will allow us to report on whether or not elements such as Si, S, and Ca are produced in the outer layers during the evolution to explosion so that they do not have to be produced by the explosion. We will also report on simulations done with a lower metallicity for the accreting material - characteristic of that of the LMC. S. Starrfield acknowledges support from NSF and NASA grants to ASU, FXT is supported by the National Security Fellow program at Los Alamos National Laboratory, WRH is partially supported by the National Science Foundation under contracts PHY-0244783 and AST-9877130 and by the Department of Energy, through the Scientific Discovery through Advanced Computing Program. Oak Ridge National Laboratory is managed by UT-Battelle, LLC, for the U.S. Department of Energy under contract DE-AC05-00OR22725. EMS is supported by NASA ADP grant NAG5-11182.

  2. Pulsating white dwarfs in cataclysmic variables: The marriage of ZZ Cet and dwarf nova

    NASA Astrophysics Data System (ADS)

    Warner, Brian; Woudt, Patrick A.

    2004-05-01

    There are now four dwarf novae known with white dwarf primaries that show large amplitude non-radial oscillations of the kind seen in ZZ Cet stars. We compare the properties of these stars and point out that by the end of Sloan Digital Sky Survey more than 30 should be known.

  3. Evidence for Neutron Star Formation from Accretion Induced Collapse of a White Dwarf

    NASA Technical Reports Server (NTRS)

    Paradijis, J. Van; VanDenHeuvel, E. P. J.; Kouveliotou, C.; Fishman, G. J.; Finger, M. H.; Lewin, W. H. G.

    1997-01-01

    The orbital parameters of the recently discovered transient burster/pulsar GRO J1744-28 indicate that this system is a low-mass X-ray binary in an advanced stage of its mass transfer, with several tenths of a solar mass already transferred from the donor to the compact star. All neutron stars known to have accreted such an amount have very weak magnetic fields, and this has led to the idea that the magnetic fields of neutron stars decay as a result of accretion. The observation of a strongly magnetized neutron star in GRO J1744-28 then suggests that this neutron star was formed recently as a result of the collapse of a white dwarf during an earlier stage of the current phase of mass transfer. It is shown that this model can consistently explain the observed characteristics of GRO J1744-28. Attractive progenitors for such an evolution are the luminous supersoft X-ray sources detected with ROSAT.

  4. The Stability of Double White Dwarf Binaries Undergoing Direct-Impact Accretion

    NASA Astrophysics Data System (ADS)

    Motl, Patrick M.; Frank, Juhan; Tohline, Joel E.; D'Souza, Mario C. R.

    2007-12-01

    We present numerical simulations of dynamically unstable mass transfer in a double white dwarf binary with initial mass ratio q=0.4. The binary components are approximated as polytropes of index n=3/2, and the initially synchronously rotating, semidetached equilibrium binary is evolved hydrodynamically, with the gravitational potential being computed through the solution of Poisson's equation. Upon initiating deep contact in our baseline simulation, the mass transfer rate grows by more than an order of magnitude over approximately 10 orbits, as would be expected for dynamically unstable mass transfer. However, the mass transfer rate then reaches a peak value, the binary expands, and the mass transfer event subsides. The binary must therefore have crossed the critical mass ratio for stability against dynamical mass transfer. Despite the initial loss of orbital angular momentum into the spin of the accreting star, we find that the accretor's spin saturates and that angular momentum is returned to the orbit more efficiently than has been previously suspected for binaries in the direct-impact accretion mode. To explore this surprising result, we directly measure the critical mass ratio for stability by imposing artificial angular momentum loss at various rates to drive the binary to an equilibrium mass transfer rate. For one of these driven evolutions, we attain equilibrium mass transfer and deduce that, effectively, qcrit has evolved to approximately 2/3. Despite the absence of a fully developed disk, tidal interactions appear to be effective in returning excess spin angular momentum to the orbit.

  5. A SEARCH FOR RAPIDLY ACCRETING WHITE DWARFS IN THE SMALL MAGELLANIC CLOUD

    SciTech Connect

    Lepo, Kelly; Van Kerkwijk, Marten E-mail: mhvk@astro.utoronto.ca

    2013-07-01

    The nature of the progenitors of Type Ia supernovae (SNe Ia) is still a mystery. While plausible candidates are known for both the single-degenerate and double-degenerate models, the observed numbers fall significantly short of what is required to reproduce the SNe Ia rate. Some of the most promising single-degenerate Type Ia progenitors are recurrent novae and super-soft sources (SSS). White dwarfs (WDs) with higher mass transfer rates can also be SN Ia progenitors. For these rapidly accreting white dwarfs (RAWDs), more material than is needed for steady burning accretes on the WD, and extends the WD's photosphere. Unlike SSS, such objects will likely not be detectable at soft X-ray energies, but will be bright at longer wavelengths, such as the far-ultraviolet (UV). Possible examples include LMC N66 and the V Sagittae stars. We present a survey using multi-object spectrographs looking for RAWDs in the central core of the Small Magellanic Cloud (SMC), from objects selected to be bright in the far-UV and with blue far UV - V colors. While we find some unusual objects, and recover known planetary nebula and Wolf-Rayet (WR) stars, we detect no candidate RAWD. The upper limits from this non-detection depend on our expectations of what an RAWD should look like, as well assumptions about the internal extinction of the SMC. Assuming they resemble LMC N66 or fainter versions of WR stars we set an upper limit of 10-14 RAWDs in the SMC. However, our survey is unlikely to detect objects like V Sge, and hence we cannot set meaningful upper limits if RAWDs generally resemble V Sge.

  6. Angular momentum exchange in white dwarf binaries accreting through direct impact

    SciTech Connect

    Sepinsky, J. F.; Kalogera, V. E-mail: vicky@northwestern.edu

    2014-04-20

    We examine the exchange of angular momentum between the component spins and the orbit in semi-detached double white dwarf binaries undergoing mass transfer through direct impact of the transfer stream. We approximate the stream as a series of discrete massive particles ejected in the ballistic limit at the inner Lagrangian point of the donor toward the accretor. This work improves upon similar earlier studies in a number of ways. First, we self-consistently calculate the total angular momentum of the orbit at all times. This includes changes in the orbital angular momentum during the ballistic trajectory of the ejected mass, as well as changes during the ejection/accretion due to the radial component of the particle's velocity. Second, we calculate the particle's ballistic trajectory for each system, which allows us to determine the precise position and velocity of the particle upon accretion. We can then include specific information about the radius of the accretor as well as the angle of impact. Finally, we ensure that the total angular momentum is conserved, which requires the donor star spin to vary self-consistently. With these improvements, we calculate the angular momentum change of the orbit and each binary component across the entire parameter space of direct impact double white dwarf binary systems. We find a significant decrease in the amount of angular momentum removed from the orbit during mass transfer, as well as cases where this process increases the angular momentum of the orbit at the expense of the spin angular momentum of the donor. We conclude that, unlike earlier claims in the literature, mass transfer through direct impact need not destabilize the binary and that the quantity and sign of the orbital angular momentum transfer depends on the binary properties, particularly the masses of the double white dwarf binary component stars. This stabilization may significantly impact the population synthesis calculations of the expected numbers of

  7. EVOLUTION OF THE SYMBIOTIC NOVA PU VUL-OUTBURSTING WHITE DWARF, NEBULAE, AND PULSATING RED GIANT COMPANION

    SciTech Connect

    Kato, Mariko; Mikolajewska, Joanna; Hachisu, Izumi

    2012-05-01

    We present a composite light-curve model of the symbiotic nova PU Vul (Nova Vulpeculae 1979) that shows a long-lasting flat optical peak followed by a slow decline. Our model light curve consists of three components of emission, i.e., an outbursting white dwarf (WD), its M-giant companion, and the nebulae. The WD component dominates in the flat peak while the nebulae dominate after the photospheric temperature of the WD rises to log T (K) {approx}> 4.5, suggesting its WD origin. We analyze the 1980 and 1994 eclipses to be total eclipses of the WD occulted by the pulsating M-giant companion with two sources of the nebular emission; one is an unocculted nebula of the M-giant's cool-wind origin and the other is a partially occulted nebula associated to the WD. We confirmed our theoretical outburst model of PU Vul by new observational estimates, which spanned 32 yr, of the temperature and radius. Also our eclipse analysis confirmed that the WD photosphere decreased by two orders of magnitude between the 1980 and 1994 eclipses. We obtain the reddening E(B - V) {approx} 0.3 and distance to PU Vul d {approx} 4.7 kpc. We interpret the recent recovery of brightness in terms of eclipse of the hot nebula surrounding the WD, suggesting that hydrogen burning is ongoing. To detect supersoft X-rays, we recommend X-ray observations around 2014 June when absorption by neutral hydrogen is minimum.

  8. An Accreting White Dwarf near the Chandrasekhar Limit in the Andromeda Galaxy

    NASA Technical Reports Server (NTRS)

    Tang, Sumin; Bildsten, Lars; Wolf, William M.; Li, K. L.; Kong, Albert K. H.; Cao, Yi; Cenko, S. Bradley; De Cia, Annalisa; Kasliwal, Mansi M.; Kulkarni, Shrinivas R.; Laher, Russ R.; Masci, Frank; Nugent, Peter E.; Perley, Daniel A.; Prince, Thomas A.; Surace, Jason

    2014-01-01

    The iPTF (Intermediate Palomar Transient Factory) detection of the most recent outburst of the recurrent nova system RX J0045.4+4154 in the Andromeda Galaxy has enabled the unprecedented study of a massive (mass is greater than 1.3 solar masses) accreting white dwarf (WD). We detected this nova as part of the near daily iPTF monitoring of M31 to a depth of R (red band-pass filter) approximately equal to magnitude 21 and triggered optical photometry, spectroscopy and soft X-ray monitoring of the outburst. Peaking at an absolute magnitude of MR (red, mid-infrared band-pass filter) equals magnitude -6.6, and with a decay time of 1 magnitude per day, it is a faint and very fast nova. It shows optical emission lines of He/N and expansion velocities of 1900 to 2600 kilometers per second 1-4 days after the optical peak. The Swift monitoring of the X-ray evolution revealed a supersoft source (SSS) with kT (energy: Boltzmann constant times temperature) (sub eff (effective)) approximately equal to 90-110 electronvolts that appeared within 5 days after the optical peak, and lasted only 12 days. Most remarkably, this is not the first event from this system, rather it is a recurrent nova with a time between outbursts of approximately 1 year, the shortest known. Recurrent X-ray emission from this binary was detected by ROSAT in 1992 and 1993, and the source was well characterized as a mass greater than 1.3 solar masses WD SSS. Based on the observed recurrence time between different outbursts, the duration and effective temperature of the SS phase, MESA models of accreting WDs allow us to constrain the accretion rate to mass greater than 1.7x10 (sup -7) solar masses per year and WD mass greater than 1.30 solar masses. If the WD keeps 30 percent of the accreted material, it will take less than a million years to reach core densities high enough for carbon ignition (if made of C/O) or electron capture (if made of O/Ne) to end the binary evolution.

  9. An accreting white dwarf near the Chandrasekhar limit in the Andromeda galaxy

    SciTech Connect

    Tang, Sumin; Bildsten, Lars; Wolf, William M.; Li, K. L.; Kong, Albert K. H.; Cao, Yi; Kulkarni, Shrinivas R.; Perley, Daniel A.; Prince, Thomas A.; Cenko, S. Bradley; De Cia, Annalisa; Kasliwal, Mansi M.; Laher, Russ R.; Surace, Jason; Nugent, Peter E.

    2014-05-01

    The intermediate Palomar Transient Factory (iPTF) detection of the most recent outburst of the recurrent nova (RN) system RX J0045.4+4154 in the Andromeda galaxy has enabled the unprecedented study of a massive (M > 1.3 M {sub ☉}) accreting white dwarf (WD). We detected this nova as part of the near-daily iPTF monitoring of M31 to a depth of R ≈ 21 mag and triggered optical photometry, spectroscopy and soft X-ray monitoring of the outburst. Peaking at an absolute magnitude of M{sub R} = –6.6 mag, and with a decay time of 1 mag per day, it is a faint and very fast nova. It shows optical emission lines of He/N and expansion velocities of 1900-2600 km s{sup –1} 1-4 days after the optical peak. The Swift monitoring of the X-ray evolution revealed a supersoft source (SSS) with kT {sub eff} ≈ 90-110 eV that appeared within 5 days after the optical peak, and lasted only 12 days. Most remarkably, this is not the first event from this system, rather it is an RN with a time between outbursts of approximately 1 yr, the shortest known. Recurrent X-ray emission from this binary was detected by ROSAT in 1992 and 1993, and the source was well characterized as a M > 1.3 M {sub ☉} WD SSS. Based on the observed recurrence time between different outbursts, the duration and effective temperature of the SS phase, MESA models of accreting WDs allow us to constrain the accretion rate to M-dot >1.7×10{sup −7} M{sub ⊙} yr{sup −1} and WD mass >1.30 M {sub ☉}. If the WD keeps 30% of the accreted material, it will take less than a Myr to reach core densities high enough for carbon ignition (if made of C/O) or electron capture (if made of O/Ne) to end the binary evolution.

  10. An Accreting White Dwarf near the Chandrasekhar Limit in the Andromeda Galaxy

    NASA Astrophysics Data System (ADS)

    Tang, Sumin; Bildsten, Lars; Wolf, William M.; Li, K. L.; Kong, Albert K. H.; Cao, Yi; Cenko, S. Bradley; De Cia, Annalisa; Kasliwal, Mansi M.; Kulkarni, Shrinivas R.; Laher, Russ R.; Masci, Frank; Nugent, Peter E.; Perley, Daniel A.; Prince, Thomas A.; Surace, Jason

    2014-05-01

    The intermediate Palomar Transient Factory (iPTF) detection of the most recent outburst of the recurrent nova (RN) system RX J0045.4+4154 in the Andromeda galaxy has enabled the unprecedented study of a massive (M > 1.3 M ⊙) accreting white dwarf (WD). We detected this nova as part of the near-daily iPTF monitoring of M31 to a depth of R ≈ 21 mag and triggered optical photometry, spectroscopy and soft X-ray monitoring of the outburst. Peaking at an absolute magnitude of MR = -6.6 mag, and with a decay time of 1 mag per day, it is a faint and very fast nova. It shows optical emission lines of He/N and expansion velocities of 1900-2600 km s-1 1-4 days after the optical peak. The Swift monitoring of the X-ray evolution revealed a supersoft source (SSS) with kT eff ≈ 90-110 eV that appeared within 5 days after the optical peak, and lasted only 12 days. Most remarkably, this is not the first event from this system, rather it is an RN with a time between outbursts of approximately 1 yr, the shortest known. Recurrent X-ray emission from this binary was detected by ROSAT in 1992 and 1993, and the source was well characterized as a M > 1.3 M ⊙ WD SSS. Based on the observed recurrence time between different outbursts, the duration and effective temperature of the SS phase, MESA models of accreting WDs allow us to constrain the accretion rate to \\dot{M} \\gt 1.7\\times 10^{-7}\\ {M_{\\odot }\\ yr}^{-1} and WD mass >1.30 M ⊙. If the WD keeps 30% of the accreted material, it will take less than a Myr to reach core densities high enough for carbon ignition (if made of C/O) or electron capture (if made of O/Ne) to end the binary evolution.

  11. Super-Eddington wind scenario for the progenitors of type Ia supernovae: Accreting He-rich matter onto white dwarfs

    NASA Astrophysics Data System (ADS)

    Wang, B.; Li, Y.; Ma, X.; Liu, D.-D.; Cui, X.; Han, Z.

    2015-12-01

    Context. Supernovae of type Ia (SNe Ia) are believed to be thermonuclear explosions of carbon-oxygen white dwarfs (CO WDs). However, the mass accretion process onto CO WDs is still not completely understood. Aims: In this paper, we study the accretion of He-rich matter onto CO WDs and explore a scenario in which a strong wind forms on the surface of the WD if the total luminosity exceeds the Eddington limit. Methods: Using a stellar evolution code called modules for experiments in stellar astrophysics (MESA), we simulated the He accretion process onto CO WDs for WDs with masses of 0.6-1.35 M⊙ and various accretion rates of 10-8-10-5 M⊙ yr-1. Results: If the contribution of the total luminosity is included when determining the Eddington accretion rate, then a super-Eddington wind could be triggered at relatively lower accretion rates than those of previous studies based on steady-state models. The super-Eddington wind can prevent the WDs with high accretion rates from evolving into red-giant-like He stars. We found that the contributions from thermal energy of the WD are non-negligible, judging by our simulations, even though the nuclear burning energy is the dominating source of luminosity. We also provide the limits of the steady He-burning regime in which the WDs do not lose any accreted matter and increase their mass steadily, and calculated the mass retention efficiency during He layer flashes for various WD masses and accretion rates. These obtained results can be used in future binary population synthesis computations.

  12. Radio Observations as a Tool to Investigate Shocks and Asymmetries in Accreting White Dwarf Binaries

    NASA Astrophysics Data System (ADS)

    Weston, Jennifer H. S.

    2016-07-01

    This dissertation uses radio observations with the Karl G. Jansky Very Large Array (VLA) to investigate the mechanisms that power and shape accreting white dwarfs (WD) and their ejecta. We test the predictions of both simple spherical and steady-state radio emission models by examining nova V1723 Aql, nova V5589 Sgr, symbiotic CH Cyg, and two small surveys of symbiotic binaries. First, we highlight classical nova V1723 Aql with three years of radio observations alongside optical and X-ray observations. We use these observations to show that multiple outflows from the system collided to create early non-thermal shocks with a brightness temperature of ≥106 K. While the late-time radio light curve is roughly consistent an expanding thermal shell of mass 2x10-4 M⊙ solar masses, resolved images of V1723 Aql show elongated material that apparently rotates its major axis over the course of 15 months, much like what is seen in gamma-ray producing nova V959 Mon, suggesting similar structures in the two systems. Next, we examine nova V5589 Sgr, where we find that the early radio emission is dominated by a shock-powered non-thermal flare that produces strong (kTx > 33 keV) X-rays. We additionally find roughly 10-5 M⊙ solar masses of thermal bremsstrahlung emitting material, all at a distance of ~4 kpc. The similarities in the evolution of both V1723 Aql and V5589 Sgr to that of nova V959 Mon suggest that these systems may all have dense equatorial tori shaping faster flows at their poles. Turning our focus to symbiotic binaries, we first use our radio observations of CH Cyg to link the ejection of a collimated jet to a change of state in the accretion disk. We additionally estimate the amount of mass ejected during this period (10-7 M⊙ masses), and improve measurements of the period of jet precession (P=12013 ± 74 days). We then use our survey of eleven accretion-driven symbiotic systems to determine that the radio brightness of a symbiotic system could potentially

  13. Pulsating Stars

    NASA Astrophysics Data System (ADS)

    Catelan, M.; Smith, H. A.

    2015-03-01

    This book surveys our understanding of stars which change in brightness because they pulsate. Pulsating variable stars are keys to distance scales inside and beyond the Milky Way galaxy. They test our understanding not only of stellar pulsation theory but also of stellar structure and evolution theory. Moreover, pulsating stars are important probes of the formation and evolution of our own and neighboring galaxies. Our understanding of pulsating stars has greatly increased in recent years as large-scale surveys of pulsating stars in the Milky Way and other Local Group galaxies have provided a wealth of new observations and as space-based instruments have studied particular pulsating stars in unprecedented detail.

  14. An asteroseismic constraint on the mass of the axion from the period drift of the pulsating DA white dwarf star L19-2

    NASA Astrophysics Data System (ADS)

    Córsico, Alejandro H.; Romero, Alejandra D.; Althaus, Leandro G.; García-Berro, Enrique; Isern, Jordi; Kepler, S. O.; Miller Bertolami, Marcelo M.; Sullivan, Denis J.; Chote, Paul

    2016-07-01

    We employ an asteroseismic model of L19-2, a relatively massive (Mstar ~ 0.75 Msolar) and hot (Teff ~ 12 100 K) pulsating DA (H-rich atmosphere) white dwarf star (DAV or ZZ Ceti variable), and use the observed values of the temporal rates of period change of its dominant pulsation modes (Π ~ 113 s and Π ~ 192 s), to derive a new constraint on the mass of the axion, the hypothetical non-barionic particle considered as a possible component of the dark matter of the Universe. If the asteroseismic model employed is an accurate representation of L19-2, then our results indicate hints of extra cooling in this star, compatible with emission of axions of mass ma cos2β lesssim 25 meV or an axion-electron coupling constant of gae lesssim 7 × 10‑13.

  15. Is the Oxygen-Rich White Dwarf SDSS J1242+5226 Accreting Water-Abundant Debris?

    NASA Astrophysics Data System (ADS)

    Raddi, R.; Gänsicke, B. T.; Koester, D.; Farihi, J.; Hermes, J. J.; Scaringi, S.; Breedt, E.; Girven, J.; EGAPS Consortium

    2015-06-01

    We identified a new strongly metal polluted white dwarf, and report the determination of hydrogen and traces of O, Na, Mg, Si, Ca, Ti, Cr, and Fe in a helium-dominated atmosphere. The four most common rock-forming elements, i.e. O, Mg, Si, and Fe embody almost entirely the 1024 g of metals that are mixed in the convection zone. Oxygen is the most abundant of these four elements and we estimate that about 50-60% of it is in excess with respect to the amount expected from the accretion of mineral oxides. We suggest that the parent body that of the planetary debris detected in this white dwarf was composed by 28-48% water. We also note that a handful of other known debris-polluted white dwarfs, like GD 61, GD 16, and GD 362 may be the actively accreting examples of a larger number of stars that previously accreted water-rich debris. We speculate that the hydrogen content of DBA and DZ white dwarfs could have a similar origin.

  16. Relation between the X-ray and optical luminosities in binary systems with accreting nonmagnetic white dwarfs

    NASA Astrophysics Data System (ADS)

    Revnivtsev, M. G.; Filippova, E. V.; Suleimanov, V. F.

    2014-04-01

    We investigate the relation between the optical ( g-band) and X-ray (0.5-10 keV) luminosities of accreting nonmagnetic white dwarfs. According to the present-day counts of the populations of star systems in our Galaxy, these systems have the highest space density among the close binary systems with white dwarfs. We show that the dependence of the optical luminosity of accreting white dwarfs on their X-ray luminosity forms a fairly narrow one-parameter curve. The typical half-width of this curve does not exceed 0.2-0.3 dex in optical and X-ray luminosities, which is essentially consistent with the amplitude of the aperiodic flux variability for these objects. At X-ray luminosities L x ˜ 1032 erg s-1 or lower, the optical g-band luminosity of the accretion flow is shown to be related to its X-ray luminosity by a factor ˜2-3. At even lower X-ray luminosities ( L x ≲ 1030 erg s-1), the contribution from the photosphere of the white dwarf begins to dominate in the optical spectrum of the binary system and its optical brightness does not drop below M g ˜ 13-14. Using the latter fact, we show that in current and planned X-ray sky surveys, the family of accreting nonmagnetic white dwarfs can be completely identified to the distance determined by the sensitivity of an optical sky survey in this region. For the Sloan Digital Sky Survey (SDSS) with a limiting sensitivity m g ˜ 22.5, this distance is ˜400-600 pc.

  17. The accretion of solar material onto white dwarfs: No mixing with core material implies that the mass of the white dwarf is increasing

    SciTech Connect

    Starrfield, Sumner

    2014-04-15

    Cataclysmic Variables (CVs) are close binary star systems with one component a white dwarf (WD) and the other a larger cooler star that fills its Roche Lobe. The cooler star is losing mass through the inner Lagrangian point of the binary and some unknown fraction of this material is accreted by the WD. One consequence of the WDs accreting material, is the possibility that they are growing in mass and will eventually reach the Chandrasekhar Limit. This evolution could result in a Supernova Ia (SN Ia) explosion and is designated the Single Degenerate Progenitor (SD) scenario. This paper is concerned with the SD scenario for SN Ia progenitors. One problem with the single degenerate scenario is that it is generally assumed that the accreting material mixes with WD core material at some time during the accretion phase of evolution and, since the typical WD has a carbon-oxygen CO core, the mixing results in large amounts of carbon and oxygen being brought up into the accreted layers. The presence of enriched carbon causes enhanced nuclear fusion and a Classical Nova explosion. Both observations and theoretical studies of these explosions imply that more mass is ejected than is accreted. Thus, the WD in a Classical Nova system is losing mass and cannot be a SN Ia progenitor. However, the composition in the nuclear burning region is important and, in new calculations reported here, the consequences to the WD of no mixing of accreted material with core material have been investigated so that the material involved in the explosion has only a Solar composition. WDs with a large range in initial masses and mass accretion rates have been evolved. I find that once sufficient material has been accreted, nuclear burning occurs in all evolutionary sequences and continues until a thermonuclear runaway (TNR) occurs and the WD either ejects a small amount of material or its radius grows to about 10{sup 12} cm and the evolution is ended. In all cases where mass ejection occurs, the

  18. Metal Accretion onto White Dwarfs. II. A Better Approach Based on Time-Dependent Calculations in Static Models

    NASA Astrophysics Data System (ADS)

    Fontaine, G.; Dufour, P.; Chayer, P.; Dupuis, J.; Brassard, P.

    2015-06-01

    The accretion-diffusion picture is the model par excellence for describing the presence of planetary debris polluting the atmospheres of relatively cool white dwarfs. Inferences on the process based on diffusion timescale arguments make the implicit assumption that the concentration gradient of a given metal at the base of the convection zone is negligible. This assumption is, in fact, not rigorously valid, but it allows the decoupling of the surface abundance from the evolving distribution of a given metal in deeper layers. A better approach is a full time-dependent calculation of the evolution of the abundance profile of an accreting-diffusing element. We used the same approach as that developed by Dupuis et al. to model accretion episodes involving many more elements than those considered by these authors. Our calculations incorporate the improvements to diffusion physics mentioned in Paper I. The basic assumption in the Dupuis et al. approach is that the accreted metals are trace elements, i.e, that they have no effects on the background (DA or non-DA) stellar structure. This allows us to consider an arbitrary number of accreting elements.

  19. Metal Accretion onto White Dwarfs. III. A Still Better Approach Based on the Coupling of Diffusion with Evolution

    NASA Astrophysics Data System (ADS)

    Brassard, Pierre; Fontaine, Gilles

    2015-06-01

    The accretion-diffusion picture is the model par excellence for describing the presence of planetary debris polluting the atmospheres of relatively cool white dwarfs. In the time-dependent approach used in Paper II of this series (Fontaine et al. 2014), the basic assumption is that the accreted metals are trace elements and do not influence the background structure, which may be considered static in time. Furthermore, the usual assumption of instantaneous mixing in the convection zone is made. As part of the continuing development of our local evolutionary code, diffusion in presence of stellar winds or accretion is now fully coupled to evolution. Convection is treated as a diffusion process, i.e., the assumption of instantaneous mixing is relaxed, and, furthermore, overshooting is included. This allows feedback on the evolving structure from the accreting metals. For instance, depending of its abundance, a given metal may contribute enough to the overall opacity (especially in a He background) to change the size of the convection zone as a function of time. Our better approach also allows to include in a natural way the mechanism of thermohaline convection, which we discuss at some length. Also, it is easy to consider sophisticated time-dependent models of accretion from circumstellar disks, such as those developed by Roman Rafikov at Princeton for instance. The current limitations of our approach are 1) the calculations are extremely computer-intensive, and 2) we have not yet developed detailed EOS megatables for metals beyond oxygen.

  20. An irradiated brown-dwarf companion to an accreting white dwarf

    NASA Astrophysics Data System (ADS)

    Hernández Santisteban, Juan V.; Knigge, Christian; Littlefair, Stuart P.; Breton, Rene P.; Dhillon, Vikram S.; Gänsicke, Boris T.; Marsh, Thomas R.; Pretorius, Magaretha L.; Southworth, John; Hauschildt, Peter H.

    2016-05-01

    Interacting compact binary systems provide a natural laboratory in which to study irradiated substellar objects. As the mass-losing secondary (donor) in these systems makes a transition from the stellar to the substellar regime, it is also irradiated by the primary (compact accretor). The internal and external energy fluxes are both expected to be comparable in these objects, providing access to an unexplored irradiation regime. The atmospheric properties of donors are largely unknown, but could be modified by the irradiation. To constrain models of donor atmospheres, it is necessary to obtain accurate observational estimates of their physical properties (masses, radii, temperatures and albedos). Here we report the spectroscopic detection and characterization of an irradiated substellar donor in an accreting white-dwarf binary system. Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of 0.055 ± 0.008 solar masses and an average spectral type of L1 ± 1, supporting both theoretical predictions and model-dependent observational constraints that suggest that the donor is a brown dwarf. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the dayside and nightside of the substellar donor (57 kelvin) and the maximum difference between the hottest and coolest parts of its surface (200 kelvin). The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of less than 0.54 at the 2σ confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the atmosphere of the brown-dwarf donor. These results add to our knowledge of binary evolution, in that the donor has survived the transition from the stellar to the substellar regime, and of substellar atmospheres, in that we have been able to test a regime in which the irradiation and the internal energy of a brown dwarf are

  1. An irradiated brown-dwarf companion to an accreting white dwarf.

    PubMed

    Santisteban, Juan V Hernández; Knigge, Christian; Littlefair, Stuart P; Breton, Rene P; Dhillon, Vikram S; Gänsicke, Boris T; Marsh, Thomas R; Pretorius, Magaretha L; Southworth, John; Hauschildt, Peter H

    2016-05-19

    Interacting compact binary systems provide a natural laboratory in which to study irradiated substellar objects. As the mass-losing secondary (donor) in these systems makes a transition from the stellar to the substellar regime, it is also irradiated by the primary (compact accretor). The internal and external energy fluxes are both expected to be comparable in these objects, providing access to an unexplored irradiation regime. The atmospheric properties of donors are largely unknown, but could be modified by the irradiation. To constrain models of donor atmospheres, it is necessary to obtain accurate observational estimates of their physical properties (masses, radii, temperatures and albedos). Here we report the spectroscopic detection and characterization of an irradiated substellar donor in an accreting white-dwarf binary system. Our near-infrared observations allow us to determine a model-independent mass estimate for the donor of 0.055 ± 0.008 solar masses and an average spectral type of L1 ± 1, supporting both theoretical predictions and model-dependent observational constraints that suggest that the donor is a brown dwarf. Our time-resolved data also allow us to estimate the average irradiation-induced temperature difference between the dayside and nightside of the substellar donor (57 kelvin) and the maximum difference between the hottest and coolest parts of its surface (200 kelvin). The observations are well described by a simple geometric reprocessing model with a bolometric (Bond) albedo of less than 0.54 at the 2σ confidence level, consistent with high reprocessing efficiency, but poor lateral heat redistribution in the atmosphere of the brown-dwarf donor. These results add to our knowledge of binary evolution, in that the donor has survived the transition from the stellar to the substellar regime, and of substellar atmospheres, in that we have been able to test a regime in which the irradiation and the internal energy of a brown dwarf are

  2. RADIO TRANSIENTS FROM THE ACCRETION-INDUCED COLLAPSE OF WHITE DWARFS

    SciTech Connect

    Piro, Anthony L.; Kulkarni, S. R.

    2013-01-10

    It has long been expected that in some scenarios when a white dwarf (WD) grows to the Chandrasekhar limit, it can undergo an accretion-induced collapse (AIC) to form a rapidly rotating neutron star. Nevertheless, the detection of such events has so far evaded discovery, likely because the optical, supernova-like emission is expected to be dim and short-lived. Here we propose a novel signature of AIC: a transient radio source lasting for a few months. Rapid rotation along with flux freezing and dynamo action can grow the WD's magnetic field to magnetar strengths during collapse. The spin-down of this newly born magnetar generates a pulsar wind nebula (PWN) within the {approx}10{sup -3}-10{sup -1} M{sub Sun} of ejecta surrounding it. Our calculations show that synchrotron emission from the PWN may be detectable in the radio, even if the magnetar has a rather modest magnetic field of {approx}2 Multiplication-Sign 10{sup 14} G and an initial spin period of {approx}10 ms. An all-sky survey with a detection limit of 1 mJy at 1.4 GHz would see {approx}4(f/10{sup -2}) above threshold at any given time, where f is the ratio of the AIC rate to Type Ia supernova rate. A similar scenario may result from binary neutron stars if some mergers produce massive neutron stars rather than black holes. We conclude with a discussion of the detectability of these types of transient radio sources in an era of facilities with high mapping speeds.

  3. Adiabatic properties of pulsating DA white dwarfs. I - The treatment of the Brunt-Vaisala frequency and the region of period formation

    NASA Technical Reports Server (NTRS)

    Brassard, P.; Fontaine, G.; Wesemael, F.; Kawaler, S. D.; Tassoul, M.

    1991-01-01

    The fundamental issue of the region of period formaton in a degenerate star is examined, with special attention given to the treatment of the Brunt-Vaisala frequency. It is shown that, in order to obtain reliable numerical results in degenerate stellar models, the Brunt-Vaisala frequency must be appropriately transformed, because it is defined in terms of a difference between two numbers which become nearly equal in highly degenerate matter, causing serious numerical problems and systematic errors. An alternative expression is derived, which is valid for multicomponent nonideal partially degenerate and partially ionized plasmas such as those encountered in white dwarf envelopes. This expression is used to compute the period structure of the same white dwarf considered by Pesnell (1987). It is shown that the implicit numerical differencing used in the Lagrangian pulsation code of Pesnell leads to very serious difficulties when used with models of degenerate stars.

  4. The rms-flux relation in accreting white dwarfs: another nova-like variable and the first dwarf nova

    NASA Astrophysics Data System (ADS)

    Van de Sande, M.; Scaringi, S.; Knigge, C.

    2015-04-01

    We report on the detection of the linear rms-flux relation in two accreting white dwarf binary systems: V1504 Cyg and KIC 8751494. The rms-flux relation relates the absolute root-mean-square (rms) variability of the light curve to its mean flux. The light curves analysed were obtained with the Kepler satellite at a 58.8 s cadence. The rms-flux relation was previously detected in only one other cataclysmic variable (CV), MV Lyr. This result reinforces the ubiquity of the linear rms-flux relation as a characteristic property of accretion-induced variability, since it has been observed in several black hole binaries, neutron star binaries and active galactic nuclei. Moreover, its detection in V1504 Cyg is the first time the rms-flux relation has been detected in a dwarf nova-type CV during quiescence. This result, together with previous studies, hence points towards a common physical origin of accretion-induced variability, independent of the size, mass or type of the central accreting compact object.

  5. X-ray and ultraviolet radiation from accreting white dwarfs. IV - Two-temperature treatment with electron thermal conduction

    NASA Technical Reports Server (NTRS)

    Imamura, J. N.; Durisen, R. H.; Lamb, D. Q.; Weast, G. J.

    1987-01-01

    Results are reported from two-temperature calculations of the structures and X-ray spectra of radiation shocks generated by accretion onto nonmagnetic white dwarfs. The approach was necessitated by the domination of bremsstrahlung in the emission region by Compton cooling. Features of the shock model, which includes steady, spherical infall of fully ionized plasma and dominance of the stand-off shock by collisional processes, are summarized. A maximum hard X-ray temperature of about 50 keV and a maximum hard X-ray luminosity of 2 x 10 to the 36th ergs/sec were obtained. The results prove that the bulk of accretion energy cannot be transported to the star by electron thermal conduction, provided that bremsstrahlung cooling is dominant over cyclotron cooling.

  6. Laterally Propagating Detonations in Thin Helium Layers on Accreting White Dwarfs

    NASA Astrophysics Data System (ADS)

    Townsley, Dean M.; Moore, Kevin; Bildsten, Lars

    2012-08-01

    Theoretical work has shown that intermediate mass (0.01 M ⊙ < M He < 0.1 M ⊙) helium shells will unstably ignite on the accreting white dwarf (WD) in an AM CVn binary. For more massive (M > 0.8 M ⊙) WDs, these helium shells can be dense enough (>5 × 105 g cm-3) that the convectively burning region runs away on a timescale comparable to the sound travel time across the shell, raising the possibility for an explosive outcome rather than an Eddington limited helium novae. The nature of the explosion (i.e., deflagration or detonation) remains ambiguous, is certainly density dependent, and likely breaks spherical symmetry. In the case of detonation, this causes a laterally propagating front whose properties in these geometrically thin and low-density shells we begin to study here. Our calculations show that the radial expansion time of <0.1 s leads to incomplete helium burning, in agreement with recent work by Sim and collaborators, but that the nuclear energy released is still adequate to realize a self-sustaining laterally propagating detonation. These detonations are slower than the Chapman-Jouguet speed of 1.5 × 109 cm s-1, but still fast enough at 0.9 × 109 cm s-1 to go around the star prior to the transit through the star of the inwardly propagating weak shock. Our simulations resolve the subsonic region behind the reaction front in the detonation wave. The two-dimensional nucleosynthesis is shown to be consistent with a truncated one-dimensional Zeldovich-von Neumann-Döring calculation at the slower detonation speed. The ashes from the lateral detonation are typically He rich, and consist of predominantly 44Ti, 48Cr, along with a small amount of 52Fe, with very little 56Ni and with significant 40Ca in carbon-enriched layers. If this helium detonation results in a Type Ia supernova, its spectral signatures would appear for the first few days after explosion.

  7. LATERALLY PROPAGATING DETONATIONS IN THIN HELIUM LAYERS ON ACCRETING WHITE DWARFS

    SciTech Connect

    Townsley, Dean M.; Moore, Kevin; Bildsten, Lars

    2012-08-10

    Theoretical work has shown that intermediate mass (0.01 M{sub Sun} < M{sub He} < 0.1 M{sub Sun }) helium shells will unstably ignite on the accreting white dwarf (WD) in an AM CVn binary. For more massive (M > 0.8 M{sub Sun }) WDs, these helium shells can be dense enough (>5 Multiplication-Sign 10{sup 5} g cm{sup -3}) that the convectively burning region runs away on a timescale comparable to the sound travel time across the shell, raising the possibility for an explosive outcome rather than an Eddington limited helium novae. The nature of the explosion (i.e., deflagration or detonation) remains ambiguous, is certainly density dependent, and likely breaks spherical symmetry. In the case of detonation, this causes a laterally propagating front whose properties in these geometrically thin and low-density shells we begin to study here. Our calculations show that the radial expansion time of <0.1 s leads to incomplete helium burning, in agreement with recent work by Sim and collaborators, but that the nuclear energy released is still adequate to realize a self-sustaining laterally propagating detonation. These detonations are slower than the Chapman-Jouguet speed of 1.5 Multiplication-Sign 10{sup 9} cm s{sup -1}, but still fast enough at 0.9 Multiplication-Sign 10{sup 9} cm s{sup -1} to go around the star prior to the transit through the star of the inwardly propagating weak shock. Our simulations resolve the subsonic region behind the reaction front in the detonation wave. The two-dimensional nucleosynthesis is shown to be consistent with a truncated one-dimensional Zeldovich-von Neumann-Doering calculation at the slower detonation speed. The ashes from the lateral detonation are typically He rich, and consist of predominantly {sup 44}Ti, {sup 48}Cr, along with a small amount of {sup 52}Fe, with very little {sup 56}Ni and with significant {sup 40}Ca in carbon-enriched layers. If this helium detonation results in a Type Ia supernova, its spectral signatures would

  8. Amplitude and frequency variations of oscillation modes in the pulsating DB white dwarf star KIC 08626021. The likely signature of nonlinear resonant mode coupling

    NASA Astrophysics Data System (ADS)

    Zong, W.; Charpinet, S.; Vauclair, G.; Giammichele, N.; Van Grootel, V.

    2016-01-01

    Context. The signatures of nonlinear effects affecting stellar oscillations are difficult to observe from ground observatories because of the lack of continuous high-precision photometric data spanning extended enough time baselines. The unprecedented photometric quality and coverage provided by the Kepler spacecraft offers new opportunities to search for these phenomena. Aims: We use the Kepler data accumulated on the pulsating DB white dwarf KIC 08626021 to explore in detail the stability of its oscillation modes, searching, in particular, for evidence of nonlinear behaviors. Methods: We analyze nearly two years of uninterrupted short-cadence data, concentrating on identified triplets that are caused by stellar rotation and that show intriguing behaviors during the course of the observations. Results: We find clear signatures of nonlinear effects that could be attributed to resonant mode coupling mechanisms. These couplings occur between the components of the triplets and can induce different types of behaviors. We first notice that a structure at 3681 μHz, identified as a triplet in previous published studies, is in fact forming a doublet, with the third component being an independent mode. We find that a triplet at 4310 μHz and this doublet at 3681 μHz (most likely the two visible components of an incomplete triplet) have clear periodic frequency and amplitude modulations, which are typical of the so-called intermediate regime of the resonance, with timescales consistent with theoretical expectations. Another triplet at 5073 μHz is likely in a narrow transitory regime in which the amplitudes are modulated while the frequencies are locked. Using nonadiabatic pulsation calculations, based on a model representative of KIC 08626021 to evaluate the linear growth rates of the modes in the triplets, we also provide quantitative information that could be useful for future comparisons with numerical solutions of the amplitude equations. Conclusions: The observed

  9. The Instability Strip of ZZ Ceti White Dwarfs and Its Extension to the Extremely Low Mass Pulsators

    NASA Astrophysics Data System (ADS)

    Van Grootel, V.; Fontaine, G.; Brassard, P.; Dupret, M.-A.

    2013-12-01

    The determination of the location of the theoretical ZZ Ceti instability strip in the log g - Teff diagram has remained a challenge over the years, due to the lack of a suitable treatment for convection in these stars. We report here a detailed stability survey over the whole ZZ Ceti regime, including the very low masses where three pulsators have recently been found. With this in mind, we computed twenty-nine evolutionary sequences of DA models with various masses and chemical layering. These models are characterized by the so-called ML2/α = 1.0 convective efficiency and take into account the important feedback effect of convection on the atmospheric structure. We computed power spectra for these models with the Liège nonadiabatic pulsation code MAD, which is the only one to conveniently incorporate a full time-dependent convection treatment and, thus, provides the best available description of the blue edge of the instability strip. On the other hand, given the failure of all nonadiabatic codes to properly account for the red edge of the strip, including MAD, we tested the idea that the red edge is due to energy leakage through the atmosphere. Using this approach, we found that our theoretical ZZ Ceti instability strip accounts remarkably well for the boundaries of the empirical strip.

  10. Aperiodic X-ray flux variability of EX Hya and the area of the base of the accretion column at the white dwarf surface

    NASA Astrophysics Data System (ADS)

    Semena, A. N.; Revnivtsev, M. G.

    2014-08-01

    The goal of this paper is to determine the characteristic cooling time of the accretion flowmatter near the surface of the magnetic white dwarf in the binary system EX Hya. Most of the X-ray photons in such binary systems are produced in an optically thin hot plasma with a temperature above 10 keV heated when the matter passes through the shock near the white dwarf surface. The total X-ray luminosity is determined by the matter accumulated below the shock in its cooling time. Thus, the X-ray luminosity variability related to the variations in the accretion rate onto the white dwarf surface must be suppressed at frequencies higher than the inverse cooling time. If the optically thin plasma radiation dominates in the rate of energy losses by the heated matter, which is true for white dwarfs with moderately strong magnetic fields, less than 1-10 MG, then the matter cooling time can give an estimate of the matter density in the accretion column. Given the accretion rate and the matter density in the accretion column at the white dwarf surface, the area of the accretion channel can be estimated. We have analyzed all of the currently available observational data for one of the brightest intermediate polars in the X-ray sky, EX Hya, from the RXTE and XMM-Newton observatories. The power spectra of its aperiodic variability have given an upper limit on the cooling time of the hot plasma: <1.5-2 s. For the observed accretion rate, ×1015 g s-1, this corresponds to a matter density below the shock surface ≳1016 cm-3 and an area of the base of the accretion channel no more than <4.6 × 1015 cm2. Using the information about the maximum geometrical size of the accretion channel obtained by analyzing X-ray eclipses in the binary system EX Hya, we have derived an upper limit on the thickness of the flow over the surface of the magnetosphere near the white dwarf surface, ≲3 × 106 cm, and the plasma penetration depth at the magnetospheric boundary, Δ r/r ≲ 6 × 10-3.

  11. Thermonuclear flashes on hydrogen/helium accreting carbon monoxide white dwarfs and structure of exotic nuclei

    NASA Astrophysics Data System (ADS)

    Mitchell, Joseph P.

    We studied H-shell flashes on CO WDs accreting Hydrogen rich matter in regimes where they are believed to be on the border of stable accretion and of having dynamical mass loss. These systems are believed to be progenitors of SNe Ia, however, there is still some question of what range of accretion rates and WD masses allow for growth to the Chandrasekhar mass, if any do at all. Flashes that result in mass loss are also of interest as they enrich the Inter Stellar Medium. Use of an explicit hydro code has allowed for the observation of a new physical effect from wave dissipation. With our high time resolution, energy transport via waves, and detailed EOS, we found that at the onset of the flash, a reduction in the degeneracy pressure due to electron captures, results in a reduction of the total pressure. With a gravitational acceleration on the order of 108 in the shell, a reduction of the total pressure by 1% results in an in fall acceleration of 10 kms2 . With such a strong in fall, compressional heating results in a hotter flash, with results showing temperatures over a billion degrees in all models. These high temperatures had consequences on the nucleosynthesis, as they allowed for rp-breakout during the flash. The effect of a "double" flash was found in one model. This resulted when the flash stalled in the H-shell, resulting in high temperature burning in only a portion of the shell. Once the H was exhausted in the flash region, cooling occurred and there was contraction of the H exhausted region. This contraction caused an in fall of the un-exhausted region which via compressional heating resulted in the flash to occur in the un-exhausted region. Such an effect may happen in any progenitor system in which the flash stalls and compression afterwards is suitable for a re-start of the flash. This effect may be observable with the current generation of instruments. With the high temperatures found in the flashes, rp-breakout nucleosynthesis was found to occur

  12. Modeling the Effect of Kick Velocity during the Accretion Induced Collapse of White Dwarfs on Binary Pulsars

    NASA Astrophysics Data System (ADS)

    Taani, Ali

    2016-07-01

    The kick velocity which arises during the binary interaction plays an important role in disruption or surviving the binary systems. This paper attempts to draw an evolutionary connection of the long-period (Porb ≥ 2 d) millisecond pulsars (MSPs) with orbits of low eccentricity (e ≤ 0.2). We propose that a kick velocity caused by dynamical effects of asymmetric collapse imparted to the companion star through an accretion induced collapse (AIC) of white dwarfs-that become unstable once they approach the Chandrasekhar limit-can account for the differences in their orbital period distributions. Furthermore, in some cases, an appropriate kick can disrupt the binary system and result in the birth of isolated MSPs. Otherwise, the binary survives and an eccentric binary MSP is formed. In this case only the binding energy equivalent (0.2M⊙) of mass is lost and the system remains intact in a symmetric collapse. Consequently, the AIC decreases the mass of the neutron star and increases the orbital period leading to orbit circularization. We present the results of our model and discuss the possible implications for the binary MSPs in galactic disk and globular clusters.

  13. The SW Sextantis-type star 2MASS J01074282+4845188: an unusual bright accretion disk with non-steady emission and a hot white dwarf

    NASA Astrophysics Data System (ADS)

    Khruzina, T.; Dimitrov, D.; Kjurkchieva, D.

    2013-03-01

    Context. Cataclysmic variables (CVs) present a short evolutional stage of binary systems. The nova-like stars are rare objects, especially those with eclipses (only several tens). But precisely these allow to determine the global parameters of their configurations and to learn more about the late stage of stellar evolution. Aims: The light curve solution allows one to determine the global parameters of the newly discovered nova-like eclipsing star 2MASS J01074282+4845188 and to estimate the contribution of the different light sources. Methods: We present new photometric and spectral observations of 2MASS J01074282+4845188. To obtain a light curve solution we used a model of a nova-like star whose emission sources are a white dwarf surrounded by an accretion disk, a secondary star filling its Roche lobe, a hot spot and a hot line. The obtained global parameters are compared with those of the eclipsing nova-like UX UMa. Results: 2MASS J01074282+4845188 shows the deepest permanent eclipse among the known nova-like stars. It is reproduced by covering the very bright accretion disk by the secondary component. The luminosity of the disk is much bigger than that of the rest light sources. The determined high temperature of the disk is typical for that observed during the outbursts of CVs. The primary of 2MASS J01074282+4845188 is one of the hottest white dwarfs in CVs. The temperature of 5090 K of its secondary is also quite high and more appropriate for a long-period SW Sex star. It might be explained by the intense heating from the hot white dwarf and the hot accretion disk of the target. Conclusions: The high mass accretion rate Ṁ = 8 × 10-9 M⊙ yr-1, the broad and single-peaked Hα emission profile, and the presence of an S-wave are sure signs for the SW Sex classification of 2MASS J01074282+4845188. The obtained flat temperature distribution along the disk radius as well as the deviation of the energy distribution from the black-body law are evidence of the non

  14. Asteroseismology of White Dwarf Stars

    NASA Technical Reports Server (NTRS)

    Hansen, Carl J.

    1997-01-01

    The primary purpose of this investigation has been to study various aspects of multimode pulsations in variable white dwarfs. In particular, nonlinear interactions among pulsation modes in white dwarfs (and, to some extent, in other variable stars), analysis of recent observations where such interactions are important, and preliminary work on the effects of crystallization in cool white dwarfs are reported.

  15. X-ray Pulsation Searches with NICER

    NASA Astrophysics Data System (ADS)

    Ray, Paul S.; Arzoumanian, Zaven

    2016-04-01

    The Neutron Star Interior Composition Explorer (NICER) is an X-ray telescope with capabilities optimized for the study of the structure, dynamics, and energetics of neutron stars through high-precision timing of rotation- and accretion-powered pulsars in the 0.2-12 keV band. It has large collecting area (twice that of the XMM-Newton EPIC-pn camera), CCD-quality spectral resolution, and high-precision photon time tagging referenced to UTC through an onboard GPS receiver. NICER will begin its 18-month prime mission as an attached payload on the International Space Station around the end of 2016. I will describe the science planning for the pulsation search science working group, which is charged with searching for pulsations and studying flux modulation properties of pulsars and other neutron stars. A primary goal of our observations is to detect pulsations from new millisecond pulsars that will contribute to NICER’s studies of the neutron star equation of state through pulse profile modeling. Beyond that, our working group will search for pulsations in a range of source categories, including LMXBs, new X-ray transients that might be accreting millisecond pulsars, X-ray counterparts to unassociated Fermi LAT sources, gamma-ray binaries, isolated neutron stars, and ultra-luminous X-ray sources. I will survey our science plans and give an overview of our planned observations during NICER’s prime mission.

  16. Radial pulsation stability as a function of hydrogen abundance

    NASA Astrophysics Data System (ADS)

    Jeffery, Simon; Saio, Hideyuki

    2015-08-01

    Following the discovery of pulsation in an extremely low-mass pre-white dwarf by Maxted et al. (2011, 2013), Jeffery & Saio (2013) showed that pulsations in such stars would be excited in high radial overtones provided that the driving zone was sufficiently depleted in hydrogen. Following previous work which shows that pulsations are more easily excited in stars where the damping effects of hydrogen are somehow reduced (Jeffery & Saio 2006), we have completed a survey of radial pulsation stability across a substantially larger parameter space. The object is to identify new regions of the HR diagram where stars should be unstable to radial pulsations, or where closely related p-modes might be excited. These would enable targeted surveys for new classes of pulsating variable. This poster reports the survey results and the identification of new instability regions.

  17. Substellar companions to white dwarves

    NASA Astrophysics Data System (ADS)

    Mullally, Fergal Robert

    2007-08-01

    We search for planets and brown dwarves around white dwarves (WDs). Finding extra-solar planets is the first step toward establishing the existence and abundance of life in the Universe. The low mass and luminosity of WDs make them ideal stars to search for low mass companion objects. Theoretical predictions generally agree that a star will consume and destroy close-in, low mass planets as it ascends the red giant and asymptotic giant branch evolutionary tracks, but larger mass objects and those further out will survive. The matter ejected from the star as it evolves into a white dwarf may also be accreted onto daughter planets, or may coalesce into a disk from which planets can then form. We employ two techniques to search for planets and brown dwarves (BDs) around WDs. A subset of pulsating white dwarf stars have a pulsational stability that rivals pulsars and atomic clocks. When a planet is in orbit around a such a star the orbital motion of the star around the centre of mass is detectable as a change in arrival times of the otherwise stable pulsations. We search for, and find, a sample of suitable pulsators, monitor them for between three and four years, and place limits on companions by constraining the variation in the pulse arrival times. For one star, we detect a variation consistent with a 2.4M J planet in a 4.6 year orbit. We also observe a large sample of WDs to search for a mid-infrared excess caused by the presence of sub-stellar companions. We present evidence for a potential binary system consisting of a WD and a BD on the basis of an observed excess flux at near and mid-infrared wavelengths. We also place limits on the presence of planetary mass companions around these stars and compare our results to predictions of planetary survival theories. Our findings do not support suggestions of planet formation or accretion of extra mass during stellar death.

  18. Quasi-periodic oscillations in accreting magnetic white dwarfs. I. Observational constraints in X-ray and optical

    NASA Astrophysics Data System (ADS)

    Bonnet-Bidaud, J. M.; Mouchet, M.; Busschaert, C.; Falize, E.; Michaut, C.

    2015-07-01

    Quasi-periodic oscillations (QPOs) are observed in the optical flux of some polars with typical periods of 1 to 3 s but none have been observed yet in X-rays where a significant part of the accreting energy is released. QPOs are expected and predicted from shock oscillations. Most of the polars have been observed by the XMM-Newton satellite. We made use of the homogeneous set of observations of the polars by XMM-Newton to search for the presence of QPOs in the (0.5-10 keV) energy range and to set significant upper limits for the brightest X-ray polars. We extracted high time-resolution X-ray light curves by taking advantage of the 0.07 s resolution of the EPIC-PN camera. Among the 65 polars observed with XMM-Newton from 1998 to 2012, a sample of 24 sources was selected on the basis of their counting rate in the PN instrument to secure significant limits. We searched for QPOs using Fast Fourier Transform (FFT) methods and defined limits of detection using statistical tools. Among the sample surveyed, none shows QPOs at a significant level. Upper limits to the fractional flux in QPOs range from 7% to 71%. These negative results are compared to the detailed theoretical predictions of numerical simulations based on a 2D hydrodynamical code presented in Paper II. Cooling instabilities in the accretion column are expected to produce shock quasi-oscillations with a maximum amplitude reaching ~40% in the bremsstrahlung (0.5-10 keV) X-ray emission and ~20% in the optical cyclotron emission. The absence of X-ray QPOs imposes an upper limit of ~(5-10) g cm-2 s-1 on the specific accretion rate but this condition is found inconsistent with the value required to account for the amplitudes and frequencies of the observed optical QPOs. This contradiction outlines probable shortcomings with the shock instability model. Figures 1-3 are available in electronic form at http://www.aanda.org

  19. Time-dependent models of accretion discs with nuclear burning following the tidal disruption of a white dwarf by a neutron star

    NASA Astrophysics Data System (ADS)

    Margalit, Ben; Metzger, Brian D.

    2016-09-01

    We construct time-dependent one-dimensional (vertically averaged) models of accretion discs produced by the tidal disruption of a white dwarf (WD) by a binary neutron star (NS) companion. Nuclear reactions in the disc mid-plane burn the WD matter to increasingly heavier elements at sequentially smaller radii, releasing substantial energy which can impact the disc dynamics. A model for disc outflows is employed, by which cooling from the outflow balances other sources of heating (viscous, nuclear) in regulating the Bernoulli parameter of the mid-plane to a fixed value ≲0. We perform a comprehensive parameter study of the compositional yields and velocity distributions of the disc outflows for WDs of different initial compositions. For C/O WDs, the radial composition profile of the disc evolves self-similarly in a quasi-steady-state manner, and is remarkably robust to model parameters. The nucleosynthesis in helium WD discs does not exhibit this behaviour, which instead depends sensitively on factors controlling the disc mid-plane density (e.g. the strength of the viscosity, α). By the end of the simulation, a substantial fraction of the WD mass is unbound in outflows at characteristic velocities of ˜109 cm s-1. The outflows from WD-NS merger discs contain 10-4-3 × 10-3 M⊙ of radioactive 56Ni, resulting in fast (˜ week long) dim (˜1040 erg s-1) optical transients; shock heating of the ejecta by late-time outflows may increase the peak luminosity to ˜1043 erg s-1. The accreted mass on to the NS is probably not sufficient to induce gravitational collapse, but may be capable of spinning up the NS to periods of ˜10 ms, illustrating the feasibility of this channel in forming isolated millisecond pulsars.

  20. Detection of the optical counterparts of the 555 second X-ray pulsations from V471 Tauri

    SciTech Connect

    Robinson, E.L.; Clemens, J.C.; Hine, B.P.

    1988-08-01

    The optical counterpart of the 555-s X-ray periodicity from V471 Tau has been detected based on high-speed photometry acquired using the 0.76 and 0.92 m McDonald Observatory telescopes. The fractional semiamplitude of the periodicity in the Johnson U band is found to vary from 9.7 x 10 to the -4th to less than 3.8 x 10 to the -4th. It is suggested that the observed periodicity could be caused either by rotation of the white dwarf, if it is magnetized and accretes material from the K2 V star, or by g-mode pulsations of the white dwarf. 25 references.

  1. DETECTION OF WEAK CIRCUMSTELLAR GAS AROUND THE DAZ WHITE DWARF WD 1124-293: EVIDENCE FOR THE ACCRETION OF MULTIPLE ASTEROIDS

    SciTech Connect

    Debes, J. H.; Kilic, M.; Faedi, F.; Shkolnik, E. L.; Lopez-Morales, M.; Weinberger, A. J.; Slesnick, C.; West, R. G.

    2012-07-20

    Single metal-polluted white dwarfs with no dusty disks are believed to be actively accreting metals from a circumstellar disk of gas caused by the destruction of asteroids perturbed by planetary systems. We report, for the first time, the detection of circumstellar Ca II gas in absorption around the DAZ WD 1124-293, which lacks an infrared excess. We constrain the gas to >7 R{sub WD} and <32000 AU, and estimate it to be at {approx}54 R{sub WD}, well within WD 1124-293's tidal disruption radius. This detection is based on several epochs of spectroscopy around the Ca II H and K lines ({lambda} = 3968 A, 3933 A) with the MIKE spectrograph on the Magellan/Clay Telescope at Las Campanas Observatory. We confirm the circumstellar nature of the gas by observing nearby sightlines and finding no evidence for gas from the local interstellar medium. Through archival data we have measured the equivalent width of the two photospheric Ca lines over a period of 11 years. We see <5%-7% epoch-to-epoch variation in equivalent widths over this time period, and no evidence for long term trends. The presence of a circumstellar gas implies a near edge-on inclination to the system, thus we place limits to short period transiting planetary companions with R > R{sub Circled-Plus} using the Wide Angle Search for Planets survey. The presence of gas in orbit around WD 1124-293 implies that most DAZs could harbor planetary systems. Since 25%-30% of white dwarfs show metal line absorption, the dynamical process for perturbing small bodies must be robust.

  2. Pulsations, Boundary Layers, and Period Bounce in the Cataclysmic Variable RE J1255+266

    NASA Astrophysics Data System (ADS)

    Patterson, Joseph; Thorstensen, John R.; Kemp, Jonathan

    2005-05-01

    We report time-series photometry of the quiescent optical counterpart of the EUV transient RE J1255+266. The star appears as a DA white dwarf with bright emission lines and a complex spectrum of periodic signals in its light curve. A signal at 0.0829 days is likely to be the orbital period of the underlying cataclysmic binary (probably a dwarf nova). Characteristic periods of 1344, 1236, and 668 s are seen, as well as a host of weaker signals. We interpret these noncommensurate signals as (nonradial) pulsation periods of the white dwarf. The donor star is unseen at all wavelengths, and the accretion rate is very low. We estimate a distance of 180+/-50 pc and MV=14.6+/-1.3 for the accretion light. The binary probably represents a very late stage of evolution, with the donor star whittled down to M2<0.05 Msolar. Such binaries stubbornly resist discovery due to their faintness and reluctance to erupt, but are probably a very common type of cataclysmic variable. If the signal at 0.0829 days is indeed the orbital period, then the binary is an excellent candidate as a ``period bouncer.'' Plausible colleagues in this club include four dwarf novae and the (so far) noneruptive stars GD 552 and 1RXS J105010.3-140431. The 1994 EUV eruption implies a soft X-ray/EUV luminosity of 1034-1035 ergs s-1, greater than that of any other dwarf nova. We attribute that to a favorable blend of properties: a high-mass white dwarf, a very transparent line of sight (the ``local chimney''), and a low binary inclination. The first maximizes the expected temperature and luminosity of boundary-layer emission; the other two increase the likelihood that soft X-rays can survive their perilous passage through an accretion-disk wind and the interstellar medium.

  3. A new driving mechanism for stellar pulsations

    NASA Astrophysics Data System (ADS)

    Pesnell, W. Dean

    1987-03-01

    A new driving mechanism termed "convective blocking", a variation of the normal κ- and γ-mechanisms in Cepheids, is demonstrated using two models of hydrogen white dwarf stars. This mechanism is shown to be physically reasonable in the limit of frozen convection (implying the time scale for convective readjustment is long compared to a pulsation period). Some qualitative effects are given for when the two time scales are not as disparate.

  4. Characteristics of Pulsating Aurora

    NASA Astrophysics Data System (ADS)

    Humberset, B. K.; Gjerloev, J. W.; Mann, I. R.; Samara, M.; Michell, R.

    2013-12-01

    We have investigated the spatiotemporal characteristics of pulsating auroral patches observed with an all-sky imager located at Poker Flat, Alaska. Pulsating aurora often covers the entire sky with intermixed large and small-scale patches that vary in intensity or disappear and reappear on different time scales and timings. The broad definition of pulsating aurora covers patches and bands from tens to several tens of km which have a quasi-periodic temporal variation from 1 s to tens of seconds. In this paper we examine >15 patches from different events. We analyze all-sky movies (557.7 nm, 3.31 Hz) with a simple, yet robust, technique that allows us to determine the scale size dependent variability of the >15 individual patches. A spatial 2D Fourier Transform is used to separate the aurora into different horizontal scale sizes, and by correlating each patch for all image separations and available scale sizes smaller than the patch itself, we reveal what scale sizes are pulsating and their variability. The patches are found to be persistent, meaning that we can follow them for typically 5 minutes. The period of the pulsations is often remarkably variable and it seems that only certain scale sizes pulsate (typically the size of the patch). The patches drift with the background ExB plasma drift indicating that the magnetospheric source mechanism drifts with the field lines.

  5. KIC 4552982: outbursts and pulsations in the longest-ever pseudo-continuous light curve of a ZZ Ceti

    NASA Astrophysics Data System (ADS)

    Bell, K. J.; Hermes, J. J.; Bischoff-Kim, A.; Moorhead, S.; Castanheira, B. G.; Montgomery, M. H.; Winget, D. E.

    2015-09-01

    KIC 4552982 was the first ZZ Ceti (hydrogen-atmosphere pulsating white dwarf) identified to lie in the Kepler field, resulting in the longest pseudo-continuous light curve ever obtained for this type of variable star. In addition to the pulsations, this light curve exhibits stochastic episodes of brightness enhancement unlike any previously studied white dwarf phenomenon. We briefly highlight the basic outburst and pulsation properties in these proceedings.

  6. Interaction Between Convection and Pulsation

    NASA Astrophysics Data System (ADS)

    Houdek, Günter; Dupret, Marc-Antoine

    2015-12-01

    This article reviews our current understanding of modelling convection dynamics in stars. Several semi-analytical time-dependent convection models have been proposed for pulsating one-dimensional stellar structures with different formulations for how the convective turbulent velocity field couples with the global stellar oscillations. In this review we put emphasis on two, widely used, time-dependent convection formulations for estimating pulsation properties in one-dimensional stellar models. Applications to pulsating stars are presented with results for oscillation properties, such as the effects of convection dynamics on the oscillation frequencies, or the stability of pulsation modes, in classical pulsators and in stars supporting solar-type oscillations.

  7. White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.

    2014-10-01

    White dwarfs are the evolutionary endpoint for nearly 95% of all stars born in our Galaxy, the final stages of evolution of all low- and intermediate mass stars, i.e., main sequence stars with masses below (8.5± 1.5) M_{odot}, depending on metallicity of the progenitor, mass loss and core overshoot. Massive white dwarfs are intrinsically rare objects, tand produce a gap in the determination of the initial vs. final mass relation at the high mass end (e.g. Weidemann 2000 A&A, 363, 647; Kalirai et al. 2008, ApJ, 676, 594; Williams, Bolte & Koester 2009, ApJ, 693, 355). Main sequences stars with higher masses will explode as SNII (Smartt S. 2009 ARA&A, 47, 63), but the limit does depend on the metallicity of the progenitor. Massive white dwarfs are probably SNIa progenitors through accretion or merger. They are rare, being the final product of massive stars (less common) and have smaller radius (less luminous). Kepler et al. 2007 (MNRAS, 375, 1315), Kleinman et al. 2013 (ApJS, 204, 5) estimate only 1-2% white dwarfs have masses above 1 M_{odot}. The final stages of evolution after helium burning are a race between core growth and loss of the H-rich envelope in a stellar wind. When the burning shell is exposed, the star rapidly cools and burning ceases, leaving a white dwarf. As they cool down, the magnetic field freezes in, ranging from a few kilogauss to a gigagauss. Peculiar type Ia SN 2006gz, SN 2007if, SN 2009dc, SN 2003fg suggest progenitors in the range 2.4-2.8 M_{odot}, and Das U. & Mukhopadhyay B. (2012, Phys. Rev. D, 86, 042001) estimate that the Chandrasekhar limit increases to 2.3-2.6 M_{odot} for extremely high magnetic field stars, but differential rotation induced by accretion could also increase it, according to Hachisu I. et al. 2012 (ApJ, 744, 69). García-Berro et al. 2012, ApJ, 749, 25, for example, proposes double degenerate mergers are the progenitors of high-field magnetic white dwarfs. We propose magnetic fields enhance the line broadening in

  8. Pulsating aurora: Source region & morphology

    NASA Astrophysics Data System (ADS)

    Jaynes, Allison

    Pulsating aurora, a common phenomenon in the polar night sky, offers a unique opportunity to study the precipitating particle populations responsible for this subtle yet fascinating display of lights. The conjecture that the source of these electrons originates near the equator, made decades ago, has now been confirmed using in-situ measurements. In this thesis, we present these results that compare the frequencies of equatorial electron flux pulsations and pulsating aurora luminosity fluctuations at the ionospheric footprint. We use simultaneous satellite-based data from GOES 13 and ground-based data from the THEMIS allsky imager array to show that there is a direct correlation between luminosity fluctuations near the ground and particle pulsations in equatorial space; the source region of the pulsating aurora. Pulsating aurora almost exclusively occurs embedded within a region of diffuse aurora. By studying the two particle populations, one can contribute to the theory behind auroral pulsations. The interplay between the two auroral types, and the systems that control them, are not yet well known. We analyze ground optical observations of pulsating aurora events to attempt to characterize the relationship between the two types of auroral precipitation. Pulsating aurora is a significant component of energy transfer within the framework of magnetosphere-ionosphere coupling. Further study of the morphology, total energy deposition, and the pulsation mechanism of pulsating aurora is key to a better understanding of our earth-sun system.

  9. Search for optical pulsations in PSR J0337+1715

    DOE PAGESBeta

    Strader, M. J.; Archibald, A. M.; Meeker, S. R.; Szypryt, P.; Walter, A. B.; van Eyken, J. C.; Ulbricht, G.; Stoughton, C.; Bumble, B.; Kaplan, D. L.; et al

    2016-03-20

    In this study, we report on a search for optical pulsations from PSR J0337+1715 at its observed radio pulse period. PSR J0337+1715 is a millisecond pulsar (2.7 ms spin period) in a triple hierarchical system with two white dwarfs, and has a known optical counterpart with g-band magnitude 18. The observations were done with the Array Camera for Optical to Near-IR Spectrophotometry (ARCONS) at the 200" Hale telescope at Palomar Observatory. No significant pulsations were found in the range 4000-11000 angstroms, and we can limit pulsed emission in g-band to be fainter than 25 mag.

  10. Search for optical pulsations in PSR J0337+1715

    NASA Astrophysics Data System (ADS)

    Strader, M. J.; Archibald, A. M.; Meeker, S. R.; Szypryt, P.; Walter, A. B.; van Eyken, J. C.; Ulbricht, G.; Stoughton, C.; Bumble, B.; Kaplan, D. L.; Mazin, B. A.

    2016-06-01

    We report on a search for optical pulsations from PSR J0337+1715 at its observed radio pulse period. PSR J0337+1715 is a millisecond pulsar (2.7 ms spin period) in a triple hierarchical system with two white dwarfs, and has a known optical counterpart with g-band magnitude 18. The observations were done with the ARray Camera for Optical to Near-IR Spectrophotometry at the 200 arcsec Hale telescope at Palomar Observatory. No significant pulsations were found in the range 4000-11 000 Å, and we can limit pulsed emission in g band to be fainter than 25 mag.

  11. DETECTION OF A WHITE DWARF COMPANION TO THE WHITE DWARF SDSSJ125733.63+542850.5

    SciTech Connect

    Marsh, T. R.; Gaensicke, B. T.; Steeghs, D.; Southworth, J.; Koester, D.; Harris, V.; Merry, L.

    2011-08-01

    SDSSJ125733.63+542850.5 (hereafter SDSSJ1257+5428) is a compact white dwarf binary from the Sloan Digital Sky Survey that exhibits high-amplitude radial velocity variations on a period of 4.56 hr. While an initial analysis suggested the presence of a neutron star or black hole binary companion, a follow-up study concluded that the spectrum was better understood as a combination of two white dwarfs. Here we present optical spectroscopy and ultraviolet fluxes which directly reveal the presence of the second white dwarf in the system. SDSSJ1257+5428's spectrum is a composite, dominated by the narrow-lined spectrum from a cool, low-gravity white dwarf (T{sub eff} {approx_equal} 6300 K, log g = 5-6.6) with broad wings from a hotter, high-mass white dwarf companion (11, 000-14, 000 K; {approx}1 M{sub sun}). The high-mass white dwarf has unusual line profiles which lack the narrow central core to H{alpha} that is usually seen in white dwarfs. This is consistent with rapid rotation with vsin i = 500-1750 km s{sup -1}, although other broadening mechanisms such as magnetic fields, pulsations, or a helium-rich atmosphere could also be contributory factors. The cool component is a puzzle since no evolutionary model matches its combination of low gravity and temperature. Within the constraints set by our data, SDSSJ1257+5428 could have a total mass greater than the Chandrasekhar limit and thus be a potential Type Ia supernova progenitor. However, SDSSJ1257+5428's unusually low-mass ratio q {approx} 0.2 suggests that it is more likely that it will evolve into an accreting double white dwarf (AM CVn star).

  12. OGLE and pulsating stars

    NASA Astrophysics Data System (ADS)

    Udalski, A.

    2016-05-01

    OGLE-IV is currently one of the largest sky variability surveys worldwide, focused on the densest stellar regions of the sky. The survey covers over 3000 square degrees and monitors regularly over a billion sources. The main targets include the inner Galactic bulge and the Magellanic System. Supplementary shallower Galaxy Variability Survey covers the extended Galactic bulge and 2/3 of the whole Galactic disk. The current status, prospects, and the latest results of the OGLE-IV survey focused on pulsating stars, in particular RR Lyrae variables, are presented.

  13. Formation of Primordial Supermassive Stars by Rapid Mass Accretion

    NASA Astrophysics Data System (ADS)

    Hosokawa, Takashi; Yorke, Harold W.; Inayoshi, Kohei; Omukai, Kazuyuki; Yoshida, Naoki

    2013-12-01

    Supermassive stars (SMSs) forming via very rapid mass accretion (\\dot{M}_*\\gtrsim 0.1 \\,M_\\odot \\,yr^{-1}) could be precursors of supermassive black holes observed beyond a redshift of about six. Extending our previous work, here we study the evolution of primordial stars growing under such rapid mass accretion until the stellar mass reaches 104 - 5 M ⊙. Our stellar evolution calculations show that a star becomes supermassive while passing through the "supergiant protostar" stage, whereby the star has a very bloated envelope and a contracting inner core. The stellar radius increases monotonically with the stellar mass until ~= 100 AU for M * >~ 104 M ⊙, after which the star begins to slowly contract. Because of the large radius, the effective temperature is always less than 104 K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M * >~ 105 M ⊙ can stellar UV feedback operate and disturb the mass accretion flow. We also examine the pulsation stability of accreting SMSs, showing that the pulsation-driven mass loss does not prevent stellar mass growth. Observational signatures of bloated SMSs should be detectable with future observational facilities such as the James Webb Space Telescope. Our results predict that an inner core of the accreting SMS should suffer from the general relativistic instability soon after the stellar mass exceeds 105 M ⊙. An extremely massive black hole should form after the collapse of the inner core.

  14. 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.

  15. A search for pulsations in planetary nebulae nuclei

    SciTech Connect

    Hine, B.P.A. III.

    1988-01-01

    The author presents the results of a survey of the central stars of planetary nebulae design to detect g-mode pulsations driven by hydrogen and/or helium shell burning. Using newly developed high-speed photometric instrumentation to overcome the inherent difficulties in observing these central stars in the presence of their nebulae, he has obtained time-series photometric data for 51 central stars in an effort to detect the g-mode pulsations predicted by Kawaler and his colleagues. He detects no periodic variations, for periods between 40 and 500 seconds, in the data down to a limit of approximately 0.5 (average) millimagnitudes. Since the theoretical calculations require these pulsations in the presence of shell burning, he must conclude that either the shell burning sources are extinguished prior to this evolutionary stage, or some mechanism is inhibiting the growth of these pulsations. If the shell burning source is indeed extinguished prior to the central star becoming a white dwarf, then this implies that white dwarfs are formed with hydrogen layer masses less than 10{sup {minus}6}M mass of sum.

  16. Pulsating incinerator hearth

    SciTech Connect

    Basic, J.N. Sr.

    1984-10-09

    A pulsating hearth for an incinerator wherein the hearth is suspended on a fixed frame for movement in a limited short arc to urge random size particles burning in a pile on the hearth in a predetermined path intermittently across the surface of the heart. Movement is imparted to the hearth in periodic pulses preferably by inflating sets of air bags mounted on the frame, which stroke the hearth to move it a short distance from an initial position and jar it against the frame, thus impelling the burning particles a short distance by inertia and concurrently stoking the burning pile upon each stroke, and then returning the hearth to its initial position. The hearth may also have a plurality of nozzles connected to a source of air for delivering gently flowing air to the burning pile on the hearth.

  17. Optical pulsations from the anomalous X-ray pulsar 4U0142+61.

    PubMed

    Kern, B; Martin, C

    2002-05-30

    Anomalous X-ray pulsars (AXPs) differ from ordinary radio pulsars in that their X-ray luminosity is orders of magnitude greater than their rate of rotational energy loss, and so they require an additional energy source. One possibility is that AXPs are highly magnetized neuron stars or 'magnetars' having surface magnetic fields greater than 10(14) G. This would make them similar to the soft gamma-ray repeaters (SGRs), but alternative models that do not require extreme magnetic fields also exist. An optical counterpart to the AXP 4U0142+61 was recently discovered, consistent with emission from a magnetar, but also from a magnetized hot white dwarf, or an accreting isolated neutron star. Here we report the detection of optical pulsations from 4U0142+61. The pulsed fraction of optical light (27 per cent) is five to ten times greater than that of soft X-rays, from which we conclude that 4U0142+61 is a magnetar. Although this establishes a direct relationship between AXPs and the soft gamma-ray repeaters, the evolutionary connection between AXPs, SGRs and radio pulsars remains controversial. PMID:12037561

  18. Whole Earth Telescope observations of V471 Tauri - The nature of the white dwarf variations

    NASA Technical Reports Server (NTRS)

    Clemens, J. C.; Nather, R. E.; Winget, D. E.; Robinson, E. L.; Wood, M. A.; Claver, C. F.; Provencal, J.; Kleinman, S. J.; Bradley, P. A.; Frueh, M. L.

    1992-01-01

    Time-series photometric observations of the binary star V471 Tauri were conducted using the Whole Earth Telescope observing network. The purpose was to determine the mechanism responsible for causing the 555 and 277 s periodic luminosity variations exhibited by the white dwarf in this binary. Previous observers have proposed that either g-mode pulsations or rotation of an accreting magnetic white dwarf could cause the variations, but were unable to decide which was the correct model. The present observations have answered this question. Learning the cause of the white dwarf variations has been possible because of the discovery of a periodic signal at 562 s in the Johnson U-band flux of the binary. By identifying this signal as reprocessed radiation and using its phase to infer the phase of the shorter wavelength radiation which produces it, made it possible to compare the phase of the 555 s U-band variations to the phase of the X-ray variations. It was found that U-band maximum coincides with X-ray minimum. From this result it was concluded that the magnetic rotator model accurately describes the variations observed, but that models involving g-mode pulsations do not.

  19. HST AND OPTICAL DATA REVEAL WHITE DWARF COOLING, SPIN, AND PERIODICITIES IN GW LIBRAE 3-4 YEARS AFTER OUTBURST

    SciTech Connect

    Szkody, Paula; Mukadam, Anjum S.; Gaensicke, Boris T. E-mail: mukadam@astro.washington.edu; and others

    2012-07-10

    Since the large amplitude 2007 outburst which heated its accreting, pulsating white dwarf, the dwarf nova system GW Librae has been cooling to its quiescent temperature. Our Hubble Space Telescope ultraviolet spectra combined with ground-based optical coverage during the third and fourth year after outburst show that the fluxes and temperatures are still higher than quiescence (T = 19,700 K and 17,300 K versus 16,000 K pre-outburst for a log g = 8.7 and d = 100 pc). The K{sub wd} of 7.6 {+-} 0.8 km s{sup -1} determined from the C I {lambda}1463 absorption line, as well as the gravitational redshift implies a white dwarf mass of 0.79 {+-} 0.08 M{sub Sun }. The widths of the UV lines imply a white dwarf rotation velocity v sin i of 40 km s{sup -1} and a spin period of 209 s (for an inclination of 11 deg and a white dwarf radius of 7 Multiplication-Sign 10{sup 8} cm). Light curves produced from the UV spectra in both years show a prominent multiplet near 290 s, with higher amplitude in the UV compared to the optical, and increased amplitude in 2011 versus 2010. As the presence of this set of periods is intermittent in the optical on weekly timescales, it is unclear how this relates to the non-radial pulsations evident during quiescence.

  20. Decoding the Rich Pulsation Spectrum of EC 14012-1446

    NASA Astrophysics Data System (ADS)

    Bischoff-Kim, A.

    2013-12-01

    EC 14012-1446 is a ZZ Ceti star (DAV) that was the object of a Whole Earth Telescope run in 2008. The extended coverage run provided a detailed and well resolved period spectrum for the star, confirming and revealing twenty independent modes of vibration, including one triplet and a few more incomplete triplets. With a large number of modes (for pulsating white dwarfs) and good clues for some of the mode identifications from independent methods, EC 14012-1446 is a good candidate for “fast” asteroseismology, where we try to infer interior structure based on a minimal set of assumptions about stellar evolution. The method also allows some numerical experiments that test the validity of asteroseismic techniques used on white dwarfs. Here we experiment with using modified Echelle diagrams on the pulsation spectrum of EC 14012-1446 to aid mode identification.

  1. Modules for Experiments in Stellar Astrophysics (MESA): Binaries, Pulsations, and Explosions

    NASA Astrophysics Data System (ADS)

    Paxton, Bill; Marchant, Pablo; Schwab, Josiah; Bauer, Evan B.; Bildsten, Lars; Cantiello, Matteo; Dessart, Luc; Farmer, R.; Hu, H.; Langer, N.; Townsend, R. H. D.; Townsley, Dean M.; Timmes, F. X.

    2015-09-01

    We substantially update the capabilities of the open-source software instrument Modules for Experiments in Stellar Astrophysics (MESA). MESA can now simultaneously evolve an interacting pair of differentially rotating stars undergoing transfer and loss of mass and angular momentum, greatly enhancing the prior ability to model binary evolution. New MESA capabilities in fully coupled calculation of nuclear networks with hundreds of isotopes now allow MESA to accurately simulate the advanced burning stages needed to construct supernova progenitor models. Implicit hydrodynamics with shocks can now be treated with MESA, enabling modeling of the entire massive star lifecycle, from pre-main-sequence evolution to the onset of core collapse and nucleosynthesis from the resulting explosion. Coupling of the GYRE non-adiabatic pulsation instrument with MESA allows for new explorations of the instability strips for massive stars while also accelerating the astrophysical use of asteroseismology data. We improve the treatment of mass accretion, giving more accurate and robust near-surface profiles. A new MESA capability to calculate weak reaction rates “on-the-fly” from input nuclear data allows better simulation of accretion induced collapse of massive white dwarfs and the fate of some massive stars. We discuss the ongoing challenge of chemical diffusion in the strongly coupled plasma regime, and exhibit improvements in MESA that now allow for the simulation of radiative levitation of heavy elements in hot stars. We close by noting that the MESA software infrastructure provides bit-for-bit consistency for all results across all the supported platforms, a profound enabling capability for accelerating MESA's development.

  2. Pulsating Star Mystery Solved

    NASA Astrophysics Data System (ADS)

    2010-11-01

    By discovering the first double star where a pulsating Cepheid variable and another star pass in front of one another, an international team of astronomers has solved a decades-old mystery. The rare alignment of the orbits of the two stars in the double star system has allowed a measurement of the Cepheid mass with unprecedented accuracy. Up to now astronomers had two incompatible theoretical predictions of Cepheid masses. The new result shows that the prediction from stellar pulsation theory is spot on, while the prediction from stellar evolution theory is at odds with the new observations. The new results, from a team led by Grzegorz Pietrzyński (Universidad de Concepción, Chile, Obserwatorium Astronomiczne Uniwersytetu Warszawskiego, Poland), appear in the 25 November 2010 edition of the journal Nature. Grzegorz Pietrzyński introduces this remarkable result: "By using the HARPS instrument on the 3.6-metre telescope at ESO's La Silla Observatory in Chile, along with other telescopes, we have measured the mass of a Cepheid with an accuracy far greater than any earlier estimates. This new result allows us to immediately see which of the two competing theories predicting the masses of Cepheids is correct." Classical Cepheid Variables, usually called just Cepheids, are unstable stars that are larger and much brighter than the Sun [1]. They expand and contract in a regular way, taking anything from a few days to months to complete the cycle. The time taken to brighten and grow fainter again is longer for stars that are more luminous and shorter for the dimmer ones. This remarkably precise relationship makes the study of Cepheids one of the most effective ways to measure the distances to nearby galaxies and from there to map out the scale of the whole Universe [2]. Unfortunately, despite their importance, Cepheids are not fully understood. Predictions of their masses derived from the theory of pulsating stars are 20-30% less than predictions from the theory of the

  3. On the Role of Resonances in Nonradial Pulsators

    NASA Technical Reports Server (NTRS)

    Buchler, J. R.; Goupil, M. J.; Hansen, C. J.

    1997-01-01

    Resonances or near resonances are ubiquitous among the excited nonradial pulsation modes of variable stars and they must play an important role in determining their pulsational behavior. Here in a first step at nonlinear asteroseismology, we explore some of the consequences of resonances by means of the amplitude equation formalism. We show how parity and angular momentum constraints can be used to eliminate many of the possible nonlinear resonant couplings between modes (and multiplets of modes), and how the amplitude equations can thus be simplified. Even when we may not be able, nor wish, to make an ab initio computation of the values of the coupling coefficients, it is still possible to obtain constraints on the nature of the excited modes if a resonance between observed frequencies can be identified. Resonances can cause nonlinear frequency locking of modes. This means that the observed frequencies appear in exact resonance even though the linear frequencies are only approximately in resonance. The nonlinear frequency lock, when it occurs, it does so over a range of departures from linear resonance, and it is accompanied by constant pulsation amplitudes. The locked, nonlinear frequencies can differ noticeably from their nonresonant counterparts which are usually used in seismology. This is particularly true for multiplets of modes split by rotation. Beyond the regime of the frequency lock, amplitude and frequency modulations can appear in the pulsations. Far from the resonance condition one recovers the regime of steady pulsations with nonresonant frequencies for which the seismological studies, as they are presently carried out, are justified (provided furthermore, of course, that nonlinear frequency shifts are negligible). Success in identifying a resonance in an observed power spectrum depends on the quality of the data. While keeping this limitation in mind, ew discuss the possible existence of peculiar resonances the pulsations specific variable white

  4. Theoretical Period Changes in Yellow Giant Pulsators

    SciTech Connect

    Cox, A.N.

    1998-03-01

    Period changes in RR Lyrae variables and Cepheids, known for more than 60 years, can possibly be explained by small changes in a helium composition gradient below the hydrogen and helium convection zones. The particular cases for the globular cluster M15 double-mode RR Lyrae variable V53 and the Cepheid Polaris are studied. For the last 80 years, the fundamental mode period of V53 has been decreasing while the overtone mode period in this same star has been increasing. The rather steady overtone mode period increase for Polaris stopped very recently, and the period now seems constant. Diffusive settling of helium in these kinds of stars has been known to be slight because of the two convection zones and the long diffusion timescale below them. But a small amount of helium settling, even before the star begins to pulsate, and then a dredge-up of just a little helium by an occasional overshooting can change surface layer structures and periods. This dredge-up can have a timescale as short as the convection turnover time, i.e., a few days. A slight helium dredge-up episode may now have temporarily stopped the decaying pulsations and period increase of Polaris. Such an episode cannot explain the double-mode V53 case, but possibly the helium composition gradient is deepened enough by matter accretion in only 80 years to explain its observed opposite period changes. Another mechanism that might be important for period changes is tidal mixing of the small composition gradients caused by occasional close encounters of stars in clusters. Significant stellar rotation would keep the surface layer composition homogeneous and not allow the anomalous period changes discussed here. {copyright} {ital {copyright} 1998.} {ital The American Astronomical Society}

  5. Evidence of Fast Magnetic Field Evolution in an Accreting Millisecond Pulsar

    NASA Astrophysics Data System (ADS)

    Patruno, A.

    2012-07-01

    The large majority of neutron stars (NSs) in low-mass X-ray binaries (LMXBs) have never shown detectable pulsations despite several decades of intense monitoring. The reason for this remains an unsolved problem that hampers our ability to measure the spin frequency of most accreting NSs. The accreting millisecond X-ray pulsar (AMXP) HETE J1900.1-2455 is an intermittent pulsar that exhibited pulsations at about 377 Hz for the first two months and then turned into a nonpulsating source. Understanding why this happened might help us to understand why most LMXBs do not pulsate. We present a seven-year coherent timing analysis of data taken with the Rossi X-ray Timing Explorer. We discover new sporadic pulsations that are detected on a baseline of about 2.5 years. We find that the pulse phases anti-correlate with the X-ray flux as previously discovered in other AMXPs. We place stringent upper limits of 0.05% rms on the pulsed fraction when pulsations are not detected and identify an enigmatic pulse phase drift of ~180° in coincidence with the first disappearance of pulsations. Thanks to the new pulsations we measure a long term spin frequency derivative whose strength decays exponentially with time. We interpret this phenomenon as evidence of magnetic field burial.

  6. Discovery of X-ray pulsations from a massive star.

    PubMed

    Oskinova, Lidia M; Nazé, Yael; Todt, Helge; Huenemoerder, David P; Ignace, Richard; Hubrig, Swetlana; Hamann, Wolf-Rainer

    2014-01-01

    X-ray emission from stars much more massive than the Sun was discovered only 35 years ago. Such stars drive fast stellar winds where shocks can develop, and it is commonly assumed that the X-rays emerge from the shock-heated plasma. Many massive stars additionally pulsate. However, hitherto it was neither theoretically predicted nor observed that these pulsations would affect their X-ray emission. All X-ray pulsars known so far are associated with degenerate objects, either neutron stars or white dwarfs. Here we report the discovery of pulsating X-rays from a non-degenerate object, the massive B-type star ξ(1) CMa. This star is a variable of β Cep-type and has a strong magnetic field. Our observations with the X-ray Multi-Mirror (XMM-Newton) telescope reveal X-ray pulsations with the same period as the fundamental stellar oscillations. This discovery challenges our understanding of stellar winds from massive stars, their X-ray emission and their magnetism. PMID:24892504

  7. [Bachelard and the mathematical pulsation].

    PubMed

    Guitart, René

    2015-01-01

    The working mathematician knows a specific gesture named « mathematical pulsation », a necessary creative moving in diagrams of thoughts and interpretations of mathematical writings. In this perspective the fact of being an object is definitely undecided, and related to the game of relations. The purpose of this paper today is to construct this pulsation, starting from the epistemology of Bachelard, concerning mathematics as well as mathematical physics. On the way, we recover links between ideas of Bachelard and more recent specific propositions by Gilles Ch-let, Charles Alunni, or René Guitart. Also are used authors like Jacques Lacan, Arthur Koestler, Alfred N. Whitehead, Charles S. Peirce. We conclude that the mathematical work consists with pulsative moving in the space of diagrams; we claim that this view is well compatible with the Bachelard's analysis of scientific knowledge: the intellectual or formal mathematical data preceeds the empirical objects, and in some sense these objects result from the pulsative gestures of the thinkers. So we finish with a categorical scheme of the pulsation. PMID:26223414

  8. High latitude pulsating aurorae revisited

    SciTech Connect

    Wu, Q.; Rosenberg, T.J. )

    1992-01-03

    Dayside auroral pulsations (10-40 s periods) have been studied for different levels of geomagnetic disturbance with N{sub 2}{sup +} 427.8 nm emission data obtained at South Pole station, Antarctica ({minus}74.2{degree} MLAT). The occurrence distribution exhibits a single peak at magnetic noon under geomagnetically quiet conditions (0 {le} Kp < 1). With increased Kp, the distribution shifts to earlier times, the peak occurring at 1000-1030 MLT for 1 {le} Kp < 4. At these higher Kp levels a secondary occurrence peak is evident in the afternoon sector between 1400 and 1600 MLT, occurring earlier as Kp increases. These results are compared with those obtained separately for pre-noon pulsations observed at Ny Alesund and post-noon pulsations observed at Ny Alesund and post-noon pulsations observed at Davis, northern and southern hemisphere sites at approximately the same magnetic latitude as South Pole. South Pole and Ny Alesund observe morning peaks at the same time and with a similar lack of Kp dependence; South Pole and Davis observe afternoon peaks with similar Kp dependence, though the peak occurs earlier at Davis. In contrast to the results from the earlier studies, the South Pole observations show larger pulsation amplitudes in the morning sector and significantly higher occurrence rates overall.

  9. Synchronization Model for Pulsating Variables

    NASA Astrophysics Data System (ADS)

    Takahashi, S.; Morikawa, M.

    2013-12-01

    A simple model is proposed, which describes the variety of stellar pulsations. In this model, a star is described as an integration of independent elements which interact with each other. This interaction, which may be gravitational or hydrodynamic, promotes the synchronization of elements to yield a coherent mean field pulsation provided some conditions are satisfied. In the case of opacity driven pulsations, the whole star is described as a coupling of many heat engines. In the case of stochastic oscillation, the whole star is described as a coupling of convection cells, interacting through their flow patterns. Convection cells are described by the Lorentz model. In both models, interactions of elements lead to various pulsations, from irregular to regular. The coupled Lorenz model also describes a light curve which shows a semi-regular variability and also shows a low-frequency enhancement proportional to 1/f in its power spectrum. This is in agreement with observations (Kiss et al. 2006). This new modeling method of ‘coupled elements’ may provide a powerful description for a variety of stellar pulsations.

  10. A COMPREHENSIVE SPECTROSCOPIC ANALYSIS OF DB WHITE DWARFS

    SciTech Connect

    Bergeron, P.; Wesemael, F.; Dufour, Pierre; Beauchamp, A.; Hunter, C.; Gianninas, A.; Limoges, M.-M.; Dufour, Patrick; Fontaine, G.; Saffer, Rex A.; Ruiz, M. T.; Liebert, James E-mail: wesemael@astro.umontreal.ca E-mail: limoges@astro.umontreal.ca E-mail: fontaine@astro.umontreal.ca E-mail: chris.hunter@yale.edu E-mail: mtruiz@das.uchile.cl

    2011-08-10

    We present a detailed analysis of 108 helium-line (DB) white dwarfs based on model atmosphere fits to high signal-to-noise optical spectroscopy. We derive a mean mass of 0.67 M{sub sun} for our sample, with a dispersion of only 0.09 M{sub sun}. White dwarfs also showing hydrogen lines, the DBA stars, comprise 44% of our sample, and their mass distribution appears similar to that of DB stars. As in our previous investigation, we find no evidence for the existence of low-mass (M < 0.5 M{sub sun}) DB white dwarfs. We derive a luminosity function based on a subset of DB white dwarfs identified in the Palomar-Green Survey. We show that 20% of all white dwarfs in the temperature range of interest are DB stars, although the fraction drops to half this value above T{sub eff} {approx} 20,000 K. We also show that the persistence of DB stars with no hydrogen features at low temperatures is difficult to reconcile with a scenario involving accretion from the interstellar medium, often invoked to account for the observed hydrogen abundances in DBA stars. We present evidence for the existence of two different evolutionary channels that produce DB white dwarfs: the standard model where DA stars are transformed into DB stars through the convective dilution of a thin hydrogen layer and a second channel where DB stars retain a helium atmosphere throughout their evolution. We finally demonstrate that the instability strip of pulsating V777 Her white dwarfs contains no non-variables, if the hydrogen content of these stars is properly accounted for.

  11. The Pulsating Pulsar Magnetosphere

    NASA Astrophysics Data System (ADS)

    Tsui, K. H.

    2015-06-01

    Following the basic principles of a charge-separated pulsar magnetosphere, we consider the magnetosphere to be stationary in space, instead of corotating, and the electric field to be uploaded from the potential distribution on the pulsar surface, set up by the unipolar induction. Consequently, the plasma of the magnetosphere undergoes guiding center drifts of the gyromotion due to the forces transverse to the magnetic field. These forces are the electric force, magnetic gradient force, and field line curvature force. Since these plasma velocities are of drift nature, there is no need to introduce an emf along the field lines, which would contradict the {{E}\\parallel }={\\boldsymbol{E}} \\cdot {\\boldsymbol{B}} =0 plasma condition. Furthermore, there is also no need to introduce the critical field line separating the electron and ion open field lines. We present a self-consistent description where the magnetosphere is described in terms of electric and magnetic fields and also in terms of plasma velocities. The fields and velocities are then connected through the space-charge densities self-consistently. We solve the pulsar equation analytically for the fields and construct the standard steady-state pulsar magnetosphere. By considering the unipolar induction inside the pulsar and the magnetosphere outside the pulsar as one coupled system, and under the condition that the unipolar pumping rate exceeds the Poynting flux in the open field lines, plasma pressure can build up in the magnetosphere, in particular, in the closed region. This could cause a periodic opening up of the closed region, leading to a pulsating magnetosphere, which could be an alternative to pulsar beacons. The closed region can also be opened periodically by the build up of toroidal magnetic field through a positive feedback cycle.

  12. 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.

  13. Pulsating aurora: The importance of the ionosphere

    SciTech Connect

    Stenbaek-Nielsen, H.C.

    1980-05-01

    A number of different, but mainly optical, observations made in pulsating auroras are presented. These observations indicate that active ionospheric processes are likely to play an important role in causing and/or modifying pulsating aurora.

  14. Type Ia supernovae: Pulsating delayed detonation models, IR light curves, and the formation of molecules

    NASA Technical Reports Server (NTRS)

    Hoflich, Peter; Khokhlov, A.; Wheeler, C.

    1995-01-01

    We computed optical and infrared light curves of the pulsating class of delayed detonation models for Type Ia supernovae (SNe Ia). It is demonstrated that observations of the IR light curves can be used to identify subluminous SNe Ia by testing whether secondary maxima occur in the IR. Our pulsating delayed detonation models are in agreement with current observations both for subluminous and normal bright SN Ia, namely SN1991bg, SN1992bo, and SN1992bc. Observations of molecular bands provide a test to distinguish whether strongly subluminous supernovae are a consequence of the pulsating mechanism occurring in a high-mass white dwarf (WD) or, alternatively, are formed by the helium detonation in a low-mass WD as was suggested by Woosley. In the latter case, no carbon is left after the explosion of low-mass WDs whereas a log of C/O-rich material is present in pulsating delayed detonation models.

  15. The temperatures, masses and pulsation modes of three ZZ Cetis in the Kepler field

    NASA Astrophysics Data System (ADS)

    Greiss, Sandra

    2013-10-01

    Most stars in our Galaxy, including all known planet hosts, will end or have already ended their lives as white dwarfs, dense stellar remnants sustained by electron degeneracy. Here, we propose to obtain COS far-ultraviolet spectroscopy of three pulsating hydrogen-atmosphere {DA} white dwarfs {ZZ Ceti stars} that for which we are obtaining Kepler short-cadence data. Far-ultraviolet spectroscopy of white dwarfs, covering the H2/H2+ quasi-molecular satellites around 1400A and 1600A, is essential to determine accurate atmospheric parameters, and precision asteroseismology of white dwarfs has the potential to probe in detail the structure of their cores and envelopes that is not possible in any other way. A succesful asteroseismologial analysis requires, however, the correct identification of the pulsation modes. Because ZZ Ceti stars have typically only few large-amplitude modes, the mode identification based on their optical light curves is often ambiguous. Because the ratio of ultraviolet-to-optical pulsation amplitudes depends strongly on the pulsatoin mode, our COS data will also enable us to identify the pulsation modes in the Kepler light curves of these three stars. The unique combination of HST and Kepler observations will enable to investigate the atmospheric and internal structure of three white dwarfs with an unprecedented accuracy. The proposed research is part of the PI's doctoral thesis.

  16. 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.

  17. The superslow pulsation X-ray pulsars in high mass X-ray binaries

    NASA Astrophysics Data System (ADS)

    Wang, Wei

    2013-03-01

    There exists a special class of X-ray pulsars that exhibit very slow pulsation of P spin > 1000 s in the high mass X-ray binaries (HMXBs). We have studied the temporal and spectral properties of these superslow pulsation neutron star binaries in hard X-ray bands with INTEGRAL observations. Long-term monitoring observations find spin period evolution of two sources: spin-down trend for 4U 2206+54 (P spin ~ 5560 s with Ṗ spin ~ 4.9 × 10-7 s s-1) and long-term spin-up trend for 2S 0114+65 (P spin ~ 9600 s with Ṗ spin ~ -1 × 10-6 s s-1) in the last 20 years. A Be X-ray transient, SXP 1062 (P spin ~ 1062 s), also showed a fast spin-down rate of Ṗ spin ~ 3 × 10-6 s s-1 during an outburst. These superslow pulsation neutron stars cannot be produced in the standard X-ray binary evolution model unless the neutron star has a much stronger surface magnetic field (B > 1014 G). The physical origin of the superslow spin period is still unclear. The possible origin and evolution channels of the superslow pulsation X-ray pulsars are discussed. Superslow pulsation X-ray pulsars could be younger X-ray binary systems, still in the fast evolution phase preceding the final equilibrium state. Alternatively, they could be a new class of neutron star system - accreting magnetars.

  18. Formation of primordial supermassive stars by rapid mass accretion

    SciTech Connect

    Hosokawa, Takashi; Yoshida, Naoki; Yorke, Harold W.; Inayoshi, Kohei; Omukai, Kazuyuki E-mail: hosokwtk@gmail.com

    2013-12-01

    Supermassive stars (SMSs) forming via very rapid mass accretion ( M-dot {sub ∗}≳0.1 M{sub ⊙} yr{sup −1}) could be precursors of supermassive black holes observed beyond a redshift of about six. Extending our previous work, here we study the evolution of primordial stars growing under such rapid mass accretion until the stellar mass reaches 10{sup 4–5} M {sub ☉}. Our stellar evolution calculations show that a star becomes supermassive while passing through the 'supergiant protostar' stage, whereby the star has a very bloated envelope and a contracting inner core. The stellar radius increases monotonically with the stellar mass until ≅ 100 AU for M {sub *} ≳ 10{sup 4} M {sub ☉}, after which the star begins to slowly contract. Because of the large radius, the effective temperature is always less than 10{sup 4} K during rapid accretion. The accreting material is thus almost completely transparent to the stellar radiation. Only for M {sub *} ≳ 10{sup 5} M {sub ☉} can stellar UV feedback operate and disturb the mass accretion flow. We also examine the pulsation stability of accreting SMSs, showing that the pulsation-driven mass loss does not prevent stellar mass growth. Observational signatures of bloated SMSs should be detectable with future observational facilities such as the James Webb Space Telescope. Our results predict that an inner core of the accreting SMS should suffer from the general relativistic instability soon after the stellar mass exceeds 10{sup 5} M {sub ☉}. An extremely massive black hole should form after the collapse of the inner core.

  19. Auroral pulsations from ionospheric winds

    SciTech Connect

    Nakada, M.P. )

    1989-11-01

    The possibility that auroral pulsations are due to oscillatory electrical circuits in the ionosphere that are driven by the negative resistance of jet stream winds is examined. For the condenser plates, the highly conducting surfaces above the edges of the jet stream are postulated. The dielectric constant of the plasma between the plates is quite large. The current that is driven perpendicular to and by the jet stream closes along the plates and through Pederson currents in the F region above the stream. This closed loop gives the inductance and resistance for the circuit. Periods of oscillation for this circuit appear to be in the range of Pc 1 to Pc 3. In accord with observations, this circuit appears to be able to limit the brightness of pulsations.

  20. Chaotic pulsations in stellar models

    SciTech Connect

    Buchler, J.R. )

    1990-12-01

    The irregular behavior of large-amplitude pulsating stars undergoing radial oscillations is examined theoretically, with a focus on hydrodynamic simulations of the W Virginis population II Cepheids (stars which show both regular and RV Tau characteristics). Sequences of models are constructed as one-parameter families (with luminosity, mass, and composition fixed and Teff as the control parameter) and analyzed to derive a systematic map of the bifurcation set; i.e., of the possible types of pulsations. The results are presented graphically, and it is shown that both cascades of period doubling (via destabilization of an overtone through a half-integer-type resonance) and tangent bifurcation are possible routes to chaos in these systems, depending on the stellar parameters. The general robustness of the chaotic behavior and the existence of a 'chaotic blue edge' in stellar-parameter space are demonstrated. 55 refs.

  1. Discovery of a new PG 1159 (GW Vir) pulsator

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Fraga, Luciano; Winget, Don Earl; Bell, Keaton; Córsico, Alejandro H.; Werner, Klaus

    2014-08-01

    We report the discovery of pulsations in the spectroscopic PG 1159 type pre-white dwarf SDSS J075415.12 + 085232.18. Analysis of the spectrum by Werner et al. indicated Teff = 120 000 ± 10 000 K, log g = 7.0 ± 0.3, mass {M}=0.52 ± 0.02 M_{⊙}, C/He = 0.33 by number. We obtained time series images with the SOAR 4.1 m telescope and 2.1 m Otto Struve telescope at McDonald Observatory and show the star is also a variable PG 1159 type star, with dominant period of 525 s.

  2. The morphology of displays of pulsating auroras.

    NASA Technical Reports Server (NTRS)

    Cresswell, G. R.

    1972-01-01

    An auroral substorm generates displays of pulsating auroras in ways which show a dependence upon both local time and latitude relative to the auroral oval. For several hours after midnight pulsating auroras can be observed in the wake of poleward expansions or within equatorward spreading diffuse envelopes of meridional extent of several hundred kilometers. As the dawn meridian is approached the displays of pulsating auroras tend increasingly to be comprised of distinct eastward drifting patches easily recorded by all-sky cameras.

  3. Dayside Pi 2 pulsations at low altitudes

    SciTech Connect

    Sutcliffe, P.R. ); Yumoto, Kiyohumi )

    1989-08-01

    In this paper the authors investigate the occurrence of dayside Pi 2 geomagnetic pulsations at low and mid latitudes. The technique of data adaptive filtering is used to identify Pi 2's concealed by the presence of typical daytime Pc type pulsations. Convincing new evidence is presented demonstrating that Pi 2 pulsations occur simultaneously in both the nightside and dayside hemispheres at low latitudes. Dayside Pi 2's are occasionally identified at mid latitudes. These results have implications with regard to the source mechanism for low latitude Pi 2 pulsations and allude to a global cavity mode.

  4. DARK STARS: IMPROVED MODELS AND FIRST PULSATION RESULTS

    SciTech Connect

    Rindler-Daller, T.; Freese, K.; Montgomery, M. H.; Winget, D. E.; Paxton, B.

    2015-02-01

    We use the stellar evolution code MESA to study dark stars (DSs). DSs, which are powered by dark matter (DM) self-annihilation rather than by nuclear fusion, may be the first stars to form in the universe. We compute stellar models for accreting DSs with masses up to 10{sup 6} M {sub ☉}. The heating due to DM annihilation is self-consistently included, assuming extended adiabatic contraction of DM within the minihalos in which DSs form. We find remarkably good overall agreement with previous models, which assumed polytropic interiors. There are some differences in the details, with positive implications for observability. We found that, in the mass range of 10{sup 4}-10{sup 5} M {sub ☉}, our DSs are hotter by a factor of 1.5 than those in Freese et al., are smaller in radius by a factor of 0.6, denser by a factor of three to four, and more luminous by a factor of two. Our models also confirm previous results, according to which supermassive DSs are very well approximated by (n = 3)-polytropes. We also perform a first study of DS pulsations. Our DS models have pulsation modes with timescales ranging from less than a day to more than two years in their rest frames, at z ∼ 15, depending on DM particle mass and overtone number. Such pulsations may someday be used to identify bright, cool objects uniquely as DSs; if properly calibrated, they might, in principle, also supply novel standard candles for cosmological studies.

  5. Disk-overflow accretion in GK Persei?

    NASA Technical Reports Server (NTRS)

    Hellier, Coel; Livio, Mario

    1994-01-01

    We reanalyze the 1983 European X-ray Observatory Satellite (EXOSAT) observations of GK Per during an outburst to investigate the approximately 5000 s quasiperiodic modulation. We find that the spectral behavior is reminiscent of dipping low-mass X-ray binaries and note that the time scale is characteristic of the radius where an accretion stream overflowing the disk would collide back onto the disk. We suggest that structure caused by such disk-overflow accretion was periodically obscuring the white dwarf, producing the modulation.

  6. 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.

  7. Transition to turbulence in pulsating pipe flow

    NASA Astrophysics Data System (ADS)

    Xu, Duo; Warnecke, Sascha; Hof, Bjoern; Avila, Marc

    2014-11-01

    We report an experimental investigation of the transition to turbulence in a pulsating pipe flow. This flow is a prototype of various pulsating flows in both nature and engineering, such as in the cardiovascular system where the onset of turbulence is often possibly related to various diseases (e.g., the formation of aneurysms). The experiments are carried out in a straight rigid pipe using water with a sinusoidal modulation of the flow rate. The governing parameters, Reynolds number, Womersley number α (dimensionless pulsating frequency) and the pulsating amplitude A, cover a wide range 3 < α < 23 and 0 < A < 1 . To characterize the transition to turbulence, we determine how the characteristic lifetime of turbulent spots (/puffs) are affected by the pulsation. While at high pulsation frequencies (α > 12) lifetimes of turbulent spots are entirely unaffected by the pulsation, at lower frequencies they are substantially affected. With decreasing frequency much larger Reynolds numbers are needed to obtain spots of the same characteristic lifetime. Hence at low frequency transition is delayed significantly. In addition the effect of the pulsation amplitude on the transition delay is quantified. Duo Xu would like to acknowledge the support from Humboldt Foundation.

  8. Constant auroral forms during regular pulsations

    NASA Astrophysics Data System (ADS)

    Roldugin, V. K.; Roldugin, A. V.

    2016-01-01

    A case is described in which complex auroral forms varied slightly at Lovozero Observatory over the course of more than an hour in the morning hours during the auroral recovery phase. Pc3 and Pc5 auroral and geomagnetic pulsations were observed during the event. The phenomenon is compared with recurrent pulsating auroras, which are described in the literature.

  9. 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.

  10. Pi2 pulsations in the magnetosphere

    NASA Technical Reports Server (NTRS)

    Lin, C. C.; Cahill, L. J., Jr.

    1975-01-01

    Several substorms were observed at Explorer 45 in November and December 1971, and January and February 1972, while the satellite was in the evening quadrant near L = 5. These same substorms were identified in ground level magnetograms from auroral zone and low latitude stations. The satellite vector magnetic field records and rapid run ground magnetograms were examined for evidence of simultaneous occurrence of Pi2 magnetic pulsations. Pulsations which began abruptly were observed at the satellite during 7 of the 13 substorms studied and the pulsations occurred near the estimated time of substorm onset. These 7 pulsation events were also observed on the ground and 6 were identified in station comments as Pi2. All of the events observed were principally compressional waves, that is, pulsations in field magnitude. There were also transverse components elliptically polarized counter-clockwise looking along the field line. Periods observed ranged from 40 to 200 sec with 80 sec often the dominant period.

  11. The accretion halo in AM Herculis systems

    NASA Technical Reports Server (NTRS)

    Achilleos, N.; Wickramasinghe, D. T.; Wu, Kinwah

    1992-01-01

    Previous phase-resolved spectropolarimetric observations of the AM Herculis systems V834 Centauri (E1405-451) and EF Eridani have shown broad, Zeeman-shifted absorption features during the bright phases. These features are thought to be nonphotospheric in origin, and to arise from a cool 'halo' of unshocked gas surrounding the accretion shock on the surface of the white dwarf primary. Preliminary models for the accretion halo region are presented and these models are used to perform a more detailed analysis of the relevant data for these two systems than has previously been done. To explain the observed halo Zeeman features, geometries which are consistent with the presence of linearly extended cyclotron emission regions are required. Such regions have previously been deduced from different considerations by other investigators. The estimated masses for the accretion halo are comparable to the mass of the cyclotron emission region.

  12. Models of cylindrical bubble pulsation

    PubMed Central

    Ilinskii, Yurii A.; Zabolotskaya, Evgenia A.; Hay, Todd A.; Hamilton, Mark F.

    2012-01-01

    Three models are considered for describing the dynamics of a pulsating cylindrical bubble. A linear solution is derived for a cylindrical bubble in an infinite compressible liquid. The solution accounts for losses due to viscosity, heat conduction, and acoustic radiation. It reveals that radiation is the dominant loss mechanism, and that it is 22 times greater than for a spherical bubble of the same radius. The predicted resonance frequency provides a basis of comparison for limiting forms of other models. The second model considered is a commonly used equation in Rayleigh-Plesset form that requires an incompressible liquid to be finite in extent in order for bubble pulsation to occur. The radial extent of the liquid becomes a fitting parameter, and it is found that considerably different values of the parameter are required for modeling inertial motion versus acoustical oscillations. The third model was developed by V. K. Kedrinskii [Hydrodynamics of Explosion (Springer, New York, 2005), pp. 23–26] in the form of the Gilmore equation for compressible liquids of infinite extent. While the correct resonance frequency and loss factor are not recovered from this model in the linear approximation, it provides reasonable agreement with observations of inertial motion. PMID:22978863

  13. Limestone calcination during pulsating combustion

    SciTech Connect

    James, R.E. III ); Richards, G.A. )

    1992-01-01

    METC is currently conducting research on enhanced calcination during pulsating combustion as part of the Heat Engines program. It has been shown elsewhere that rapid, high temperature calcination will result in a calcined product with relatively large surface area, as desired for sulfur capture. It is proposed that such a process may occur during pulsating combustion where the oscillating pressure/velocity field around a particle increases the heat/mass transfer to and from the particle. To test this hypothesis, calcination tests in progress at METC use a novel form of pulse combustion called thermal'' pulse combustion, operating at 60000 BTUH, 100 Hz, and 5--15 psig peak-to- peak amplitude. Two configurations are being studied during the testing: one configuration is injection of sorbent into a refractory lined drop tube being heated by the pulse combustor, and the other configuration is injection of the sorbent into the pulse combustor through its centerbody and along the tailpipe at various positions. To understand the observed behavior, a characterization study of the pulse combustor is being conducted. Different flow rates, equivalence ratios, and injection positions are being tested.

  14. A study of the pulsation driving mechanism in pulsating combustors

    NASA Astrophysics Data System (ADS)

    Goldman, Y.; Timnat, Y. M.

    Experiments performed in a facility consisting of a Schmidt-type pulsating combustor, in which high-speed photographs were taken and pressure, temperature and gas composition measured, showed that the air supply conditions at the inlet and the volume of the combustor strongly influence the oscillation frequency. From the measurements, the existence of two separate regions, one containing cold air and the other containing fuel-rich gas, was found, and a pressure-volume diagram was drawn, showing the effect of chemical energy release and heat supply during the compression stroke and differentiating it from the expansion. A model of the interaction between the cyclic combustion process and the acoustic oscillations of the gas volume within the chamber and the tail-pipe is presented. The conditions for chemical energy release that result in high-pressure amplitude are described.

  15. Swings between rotation and accretion power in a binary millisecond pulsar.

    PubMed

    Papitto, A; Ferrigno, C; Bozzo, E; Rea, N; Pavan, L; Burderi, L; Burgay, M; Campana, S; Di Salvo, T; Falanga, M; Filipović, M D; Freire, P C C; Hessels, J W T; Possenti, A; Ransom, S M; Riggio, A; Romano, P; Sarkissian, J M; Stairs, I H; Stella, L; Torres, D F; Wieringa, M H; Wong, G F

    2013-09-26

    It is thought that neutron stars in low-mass binary systems can accrete matter and angular momentum from the companion star and be spun-up to millisecond rotational periods. During the accretion stage, the system is called a low-mass X-ray binary, and bright X-ray emission is observed. When the rate of mass transfer decreases in the later evolutionary stages, these binaries host a radio millisecond pulsar whose emission is powered by the neutron star's rotating magnetic field. This evolutionary model is supported by the detection of millisecond X-ray pulsations from several accreting neutron stars and also by the evidence for a past accretion disc in a rotation-powered millisecond pulsar. It has been proposed that a rotation-powered pulsar may temporarily switch on during periods of low mass inflow in some such systems. Only indirect evidence for this transition has hitherto been observed. Here we report observations of accretion-powered, millisecond X-ray pulsations from a neutron star previously seen as a rotation-powered radio pulsar. Within a few days after a month-long X-ray outburst, radio pulses were again detected. This not only shows the evolutionary link between accretion and rotation-powered millisecond pulsars, but also that some systems can swing between the two states on very short timescales. PMID:24067710

  16. Pulsations and outbursts of luminous blue variables

    SciTech Connect

    Cox, A.N.; Guzik, J.A.; Soukup, M.S.; Despain, K.M.

    1997-06-01

    We propose an outburst mechanism for the most luminous stars in our and other galaxies. These million solar luminosity stars, with masses (after earlier mass loss) of between 20 and maybe 70 solar masses, are pulsationally unstable for both radial and low-degree nonradial modes. Some of these modes are ``strange,`` meaning mostly that the pulsations are concentrated near the stellar surface and have very rapid growth rates in linear theory. The pulsation driving is by both the high iron line opacity (near 150,000 K) and the helium opacity (near 30,000 K) kappa effects. Periods range from 5 to 40 days. Depending on the composition, pulsations periodically produce luminosities above the Eddington limit for deep layers. The radiative luminosity creates an outward push that readily eases the very low gamma envelope to very large outburst radii. A key point is that a super-Eddington luminosity cannot be taken up by the sluggish convection rapidly enough to prevent an outward acceleration of much of the envelope. As the helium abundance in the envelope stellar material increases by ordinary wind mass loss and the luminous blue variable outbursts, the opacity in the deep pulsation driving layers decreases. This makes the current Eddington luminosity even higher so that pulsations can then no longer give radiative luminosities exceeding the limit. For the lower mass and luminosity luminous blue variables there is considerably less iron line opacity driving, and pulsations are almost all caused by the helium ionization kappa effect.

  17. Persistent, widespread pulsating aurora: A case study

    NASA Astrophysics Data System (ADS)

    Jones, S. L.; Lessard, M. R.; Rychert, K.; Spanswick, E.; Donovan, E.; Jaynes, A. N.

    2013-06-01

    Observations of a pulsating aurora event occurring on 11 February 2008, using the Time History of Events and Macroscale Interactions during Substorms (THEMIS) All-Sky Imager (ASI) array, indicate a spatially and temporally continuous event with a duration of greater than 15 h and covering a region with a maximum size of greater than 10 h magnetic local time. The optical pulsations are at times locally interrupted or drowned out by auroral substorm activity but are observed in the same location once the discrete aurora recedes. The pulsations following the auroral breakup appear to be brighter and have a larger patch size than before breakup. This suggests that, while the onset of pulsating aurora is not necessarily dependent upon a substorm precursor, the pulsations are affected and possibly enhanced by the substorm process. The long duration of this pulsating aurora event, lasting approximately 8 h without interruption as imaged from Gillam station, is significantly longer than the typical 2-3 h substorm recovery phase, suggesting that pulsating aurora is not strictly a recovery phase phenomenon. This paper is accompanied by a movie of the THEMIS ASI array data, from 0000 to 1715 UT, plotted in mosaic and superimposed onto a map of North America.

  18. Multi-periodic pulsations of a stripped red-giant star in an eclipsing binary system.

    PubMed

    Maxted, Pierre F L; Serenelli, Aldo M; Miglio, Andrea; Marsh, Thomas R; Heber, Ulrich; Dhillon, Vikram S; Littlefair, Stuart; Copperwheat, Chris; Smalley, Barry; Breedt, Elmé; Schaffenroth, Veronika

    2013-06-27

    Low-mass white-dwarf stars are the remnants of disrupted red-giant stars in binary millisecond pulsars and other exotic binary star systems. Some low-mass white dwarfs cool rapidly, whereas others stay bright for millions of years because of stable fusion in thick surface hydrogen layers. This dichotomy is not well understood, so the potential use of low-mass white dwarfs as independent clocks with which to test the spin-down ages of pulsars or as probes of the extreme environments in which low-mass white dwarfs form cannot fully be exploited. Here we report precise mass and radius measurements for the precursor to a low-mass white dwarf. We find that only models in which this disrupted red-giant star has a thick hydrogen envelope can match the strong constraints provided by our data. Very cool low-mass white dwarfs must therefore have lost their thick hydrogen envelopes by irradiation from pulsar companions or by episodes of unstable hydrogen fusion (shell flashes). We also find that this low-mass white-dwarf precursor is a type of pulsating star not hitherto seen. The observed pulsation frequencies are sensitive to internal processes that determine whether this star will undergo shell flashes. PMID:23803845

  19. Pulsations in close binaries: challenges and opportunities

    NASA Astrophysics Data System (ADS)

    Maceroni, C.; Lehmann, H.; Da Silva, R.; Montalbán, J.

    2015-09-01

    CoRoT and Kepler provided a precious by-product: a number of eclipsing binaries containing variable stars and, among these, non-radial pulsators. This providential occurrence allows combining independent information from two different phenomena whose synergy yields scientific results well beyond those from the single sources. In particular, the analysis of pulsations in eclipsing binary components throws light on the internal structure of the pulsating star, on the system evolution, and on the role of tidal forces in exciting the oscillations. The case study of the Kepler target KIC 3858884 is illustrative of the difficulties of analysis and of the achievements in this rapidly developing field.

  20. Accretion stream mapping with `genetically modified fireflies'

    NASA Astrophysics Data System (ADS)

    Bridge, C. M.; Hakala, Pasi; Cropper, Mark; Ramsay, Gavin

    2004-07-01

    We apply an eclipse mapping technique using `genetically modified fireflies' to the eclipse light curves of HU Aqr and EP Dra. The technique makes as few assumptions as possible about the location of accretion stream material, allowing the emission to be located anywhere within the Roche lobe of the white dwarf. We model two consecutive eclipses in the UBVRC band for HU Aqr, and four consecutive `white'-light eclipses for EP Dra, to determine the changing brightness distribution of stream material. We find firefly distributions which are consistent with accretion through a curtain of material in both HU Aqr and EP Dra, and show that the previously assumed two-part ballistic and magnetic trajectory is a good approximation for polars. Model fits to the colour-band data of HU Aqr indicate that the material confined to the magnetic field lines is brightest, and most of the emission originates from close to the white dwarf. There is evidence for emission from close to a calculated ballistic stream in both HU Aqr and EP Dra. We propose that a change in the stream density causes a change in the location of the bright material in the accretion stream in EP Dra.

  1. CHARACTERIZING PULSATING MIXING OF SLURRIES

    SciTech Connect

    Bamberger, Judith A.; Meyer, Perry A.

    2007-12-01

    This paper describes the physical properties for defining the operation of a pulse jet mixing system. Pulse jet mixing operates with no moving parts located in the vessel to be mixed. Pulse tubes submerged in the vessel provide a pulsating flow due to a controlled combination of applied pressure to expel the fluid from the pulse tube nozzle followed by suction to refill the pulse tube through the same nozzle. For mixing slurries nondimensional parameters to define mixing operation include slurry properties, geometric properties and operational parameters. Primary parameters include jet Reynolds number and Froude number; alternate parameters may include particle Galileo number, particle Reynolds number, settling velocity ratio, and hindered settling velocity ratio. Rating metrics for system performance include just suspended velocity, concentration distribution as a function of elevation, and blend time.

  2. 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.

  3. A Semi-coherent Search for Weak Pulsations in AQUILA X--1

    NASA Astrophysics Data System (ADS)

    Messenger, C.; Patruno, A.

    2015-06-01

    Non-pulsating neutron stars in low mass X-ray binaries largely outnumber those that show pulsations. The lack of detectable pulses represents a big open problem for two important reasons. The first is that the structure of the accretion flow in the region closest to the neutron star is not well understood and it is therefore unclear what is the mechanism that prevents the pulse formation. The second is that the detection of pulsations would immediately reveal the spin of the neutron star. AQUILA X-1 is a special source among low mass X-ray binaries because it has showed the unique property of pulsating for only ˜150 s out of a total observing time of more than 1.5 million seconds. However, the existing upper limits on the pulsed fraction leave open two alternatives. Either AQUILA X-1 has very weak pulses which have been undetected, or it has genuinely pulsed only for a tiny amount of the observed time. Understanding which of the two scenarios is the correct one is fundamental to increase our knowledge about the pulse formation process and understand the chances we have to detect weak pulses in other low-mass X-ray binaries. In this paper we perform a semi-coherent search on the entire X-ray data available for AQUILA X-1. We find no evidence for (new) weak pulsations with the most stringent upper limits being of the order of 0.3% in the 7-25 keV energy band.

  4. Statistical study of dayside pulsating aurora

    NASA Astrophysics Data System (ADS)

    Kanmae, T.; Kadokura, A.; Ogawa, Y.; Ebihara, Y.; Motoba, T.; Gerrard, A. J.; Weatherwax, A. T.

    2015-12-01

    Pulsating aurora normally occurs after a substorm breakup in the midnight sector, often observed to persist through the morning sector and beyond. Indeed, it has also been observed on the dayside; however, the characteristics of the dayside pulsating aurora are poorly known. A handful of observational studies have been reported, but the results are somewhat disputable because most of the studies had non-uniform sampling of the dark dayside region. Furthermore, the previous studies used photometer data, with which the spatial characteristics of the pulsating aurora cannot be examined. To determine both temporal and spatial characteristics of the pulsating aurora, we have studied three years of all-sky image data obtained at the South Pole station. Because of its unique geographical location, the station has 24 hours of darkness during the austral winter from April to August, providing an ideal platform for studying dayside aurora. In a preliminary survey of the data, we have identified the pulsating auroras in 198 days out of 365 days of observations. The magnetic local time (MLT) distribution of the occurrence peaks between 9:00 and 11:00, but shows no or little dependence on the geomagnetic activity. In many events, pulsating patches initially appear as east-west aligned arc segments and later in the afternoon sector develop into large, diffuse patches, which occasionally fill a large part of the field of view. Using the long-term data, we will statistically examine both temporal (occurrence rate, duration and pulsation period) and spatial (sizes and shapes) characteristics of the dayside pulsating aurora.

  5. A motion picture presentation of magnetic pulsations

    NASA Technical Reports Server (NTRS)

    Suzuki, A.; Kim, J. S.; Sugura, M.; Nagano, H.

    1981-01-01

    Using the data obtained from the IMS North American magnetometer network stations at high latitudes, a motion picture was made by a computer technique, describing time changes of Pc5 and Pi3 magnetic pulsation vectors. Examples of pulsation characteristics derived from this presentation are regional polarization changes including shifts of polarization demarcation lines, changes in the extent of an active region and its movement with time.

  6. Soft X-ray production by photon scattering in pulsating binary neutron star sources

    NASA Technical Reports Server (NTRS)

    Bussard, R. W.; Meszaros, P.; Alexander, S.

    1985-01-01

    A new mechanism is proposed as a source of soft (less than 1 keV) radiation in binary pulsating X-ray sources, in the form of photon scattering which leaves the electron in an excited Landau level. In a plasma with parameters typical of such sources, the low-energy X-ray emissivity of this mechanism far exceeds that of bremsstrahlung. This copious source of soft photons is quite adequate to provide the seed photons needed to explain the power-law hard X-ray spectrum by inverse Comptonization on the hot electrons at the base of the accretion column.

  7. 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).

  8. Accretion mode of the Ultra-Luminous X-ray source M82 X-2

    NASA Astrophysics Data System (ADS)

    Karino, S.; Miller, J. C.

    2016-05-01

    Periodic pulsations have been found in emission from the ultra-luminous X-ray source (ULX) M82 X-2, strongly suggesting that the emitter is a rotating neutron star rather than a black hole. However, the radiation mechanisms and accretion mode involved have not yet been clearly established. In this paper, we examine the applicability to this object of standard accretion modes for high mass X-ray binaries (HMXBs). We find that spherical wind accretion, which drives OB-type HMXBs, cannot apply here but that there is a natural explanation in terms of an extension of the picture for standard Be-type HMXBs. We show that a neutron star with a moderately strong magnetic field, accreting from a disc-shaped wind emitted by a Be-companion, could be compatible with the observed relation between spin and orbital period. A Roche lobe overflow picture is also possible under certain conditions.

  9. The pulsations of ZZ Ceti stars. III - The driving mechanism

    NASA Astrophysics Data System (ADS)

    Brickhill, A. J.

    1991-08-01

    The outer layers of the variable white dwarfs are in a state of partial ionization. During the pulsation cycle the base of the ionization zone is strongly heated by the radiative layers below, in phase with the pressure perturbation. If this excess heat is not quickly lost at the surface, then the driving effect is strong. The surface flux perturbation tends to be small and delayed in phase because the surface flux is remarkably insensitive to temperature changes in the deeper layers of the ionization zone. This insensitivity is closely associated with the well known inward divergence of the solutions for the equilibrium thermal structure in the convective layers. The mechanism which excites the oscillations could be called 'convective driving'.

  10. AT LAST-A V777 HER PULSATOR IN THE KEPLER FIELD

    SciTech Connect

    Oestensen, R. H.; Bloemen, S.; Vuckovic, M.; Aerts, C.; Oreiro, R.; Kinemuchi, K.; Still, M.

    2011-08-01

    We present the discovery of the first-and so far the only-pulsating white dwarf star located in the field of view of the Kepler spacecraft. During our ongoing effort to search for compact pulsator candidates that can benefit from the near-continuous coverage of Kepler, we recently identified a faint DB star from spectroscopy obtained with the William Herschel Telescope. After establishing its physical parameters to be T{sub eff} = 24,950 K and log g = 7.91 dex, placing it right in the middle of the V777 Her instability strip, we immediately submitted the target for follow-up space observations. The Kepler light curve reveals a pulsation spectrum consisting of five modes that follow a sequence roughly equally spaced in period with a mean spacing of 37 s. The three strongest modes show a triplet structure with a mean splitting of 3.3 {mu}Hz. We conclude that this object is a V777 Her pulsator with a mass of {approx}0.56 M{sub sun}, and very similar to the class prototype.

  11. The pulsating central star of the planetary nebula Kohoutek 1-16

    NASA Astrophysics Data System (ADS)

    Grauer, A. D.; Bond, H. E.

    1984-02-01

    High-speed photometry of the central star of the planetary nebula Kohoutek 1-16 shows it to be a low-amplitude pulsating variable. The dominant period is 28.3 minutes, with a semiamplitude that is usually about 0.01 mag. However, several additional periods sometimes appear in power spectra computed from light curves, and on two occasions a rapid drop into, or emergence from, a state in which no detectable variations were present was observed. Such 'mode switching' is typical of some of the ZZ Ceti-type white dwarf nonradial pulsators, but, at effective temperatures higher than 80,000 K, K1-16 is much too hot to be a ZZ Ceti variable. Spectroscopically and photometrically, the central star of K1-16 closely resembles the previously known hot pulsator PG 1159-035; these two objects represent a new pulsational instability mechanism for extremely hot degenerate or predegenerate stars. It is predicted that the evolutionary contraction of K1-16 will lead to a period decrease so rapid that it should be detectable over an interval of about 2 yr.

  12. Thermal Management Using Pulsating Jet Cooling Technology

    NASA Astrophysics Data System (ADS)

    Alimohammadi, S.; Dinneen, P.; Persoons, T.; Murray, D. B.

    2014-07-01

    The existing methods of heat removal from compact electronic devises are known to be deficient as the evolving technology demands more power density and accordingly better cooling techniques. Impinging jets can be used as a satisfactory method for thermal management of electronic devices with limited space and volume. Pulsating flows can produce an additional enhancement in heat transfer rate compared to steady flows. This article is part of a comprehensive experimental and numerical study performed on pulsating jet cooling technology. The experimental approach explores heat transfer performance of a pulsating air jet impinging onto a flat surface for nozzle-to-surface distances 1 <= H/D <= 6, Reynolds numbers 1,300 <= Re <= 2,800 pulsation frequency 2Hz <= f <= 65Hz, and Strouhal number 0.0012 <= Sr = fD/Um <= 0.084. The time-resolved velocity at the nozzle exit is measured to quantify the turbulence intensity profile. The numerical methodology is firstly validated using the experimental local Nusselt number distribution for the steady jet with the same geometry and boundary conditions. For a time-averaged Reynolds number of 6,000, the heat transfer enhancement using the pulsating jet for 9Hz <= f <= 55Hz and 0.017 <= Sr <= 0.102 and 1 <= H/D <= 6 are calculated. For the same range of Sr number, the numerical and experimental methods show consistent results.

  13. Modeling of pulsating heat pipes.

    SciTech Connect

    Givler, Richard C.; Martinez, Mario J.

    2009-08-01

    This report summarizes the results of a computer model that describes the behavior of pulsating heat pipes (PHP). The purpose of the project was to develop a highly efficient (as compared to the heat transfer capability of solid copper) thermal groundplane (TGP) using silicon carbide (SiC) as the substrate material and water as the working fluid. The objective of this project is to develop a multi-physics model for this complex phenomenon to assist with an understanding of how PHPs operate and to be able to understand how various parameters (geometry, fill ratio, materials, working fluid, etc.) affect its performance. The physical processes describing a PHP are highly coupled. Understanding its operation is further complicated by the non-equilibrium nature of the interplay between evaporation/condensation, bubble growth and collapse or coalescence, and the coupled response of the multiphase fluid dynamics among the different channels. A comprehensive theory of operation and design tools for PHPs is still an unrealized task. In the following we first analyze, in some detail, a simple model that has been proposed to describe PHP behavior. Although it includes fundamental features of a PHP, it also makes some assumptions to keep the model tractable. In an effort to improve on current modeling practice, we constructed a model for a PHP using some unique features available in FLOW-3D, version 9.2-3 (Flow Science, 2007). We believe that this flow modeling software retains more of the salient features of a PHP and thus, provides a closer representation of its behavior.

  14. 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.

  15. Transition to turbulence in pulsating pipe flow

    NASA Astrophysics Data System (ADS)

    Hof, Bjorn; Warnecke, Sascha; Xu, Duo

    2013-11-01

    We report an experimental study of the transition to turbulence in a pulsating pipe flow the most important example of pulsating flows is the cardiovascular system where the onset of fluctuations and turbulence can be a possible cause for various diseases such as the formation of aneurysms. The present study is limited to a straight rigid pipe, sinusoidal modulation of the flow rate and a Newtonian fluid. The dimensionless parameters (Womersley and Reynolds numbers) were chosen to include the parameter range encountered in larger arteries. We observe that at large frequencies the critical point for the onset of turbulence remains completely unaffected by pulsation for all amplitudes investigated (up to 40%). However for smaller frequencies (Womersley numbers below 10) the critical point considerably increases. Furthermore we investigate how the transition scenario is affected for a fixed frequency and increasing amplitudes (approaching oscillatory flow).

  16. Unveiling Accretion Disks - Physical Parameter Eclipse Mapping of Accretion Disks in Dwarf Novae

    NASA Astrophysics Data System (ADS)

    Vrielmann, S.

    1997-06-01

    This work presents a new tomography algorithm, the Physical Parameter Eclipse Mapping method. It has been designed to reconstruct the structure of accretion disks in cataclysmic variables in terms of the basic physical parameters. Cataclysmic variables are close interacting binaries, in which mass transfer from one of the stars, typically a main sequence star, to the other star, a white dwarf, proceeds via an accretion disk around the white dwarf. Accretion disks are of general importance in astrophysics, since they occur in a number of objects with mass accretion, like active galactic nuclei and young stellar objects. The eclipsing cataclysmic variables are ideal systems to study such accretion process, since with the varying orbital phase different parts of the accretion disk can be viewed. The tomography method is based on the classical Eclipse Mapping algorithm which yields intensity distributions in the accretion disk by fitting the observed eclipse light curve. In order to avoid ambiguities this back-projection is using a maximum entropy algorithm, with selects the smoothest solution still compatible with the data. In my new method the intensity distributions are replaced by distributions of physical parameters, using a specific theoretical model spectrum to fit a set of eclipse light curves at various wavelengths. The spectrum is chosen according to the type of cataclysmic variable under investigation and its state at the time of observation. This work shows through application to synthetic data that with such an approach given distributions in physical parameters can be reproduced, as long as the parameters assume values in the parameter space outside of regions where ambiguities arise. Versions with two simple models are tested, but in principle this method can cope with any given model spectrum. The Physical Parameter Eclipse Mapping method is applied to two sets of real data of the dwarf nova IP Pegasi on decline from outburst and HT Cassiopeiae in

  17. Benefit of pulsation in soft corals

    PubMed Central

    Kremien, Maya; Shavit, Uri; Mass, Tali; Genin, Amatzia

    2013-01-01

    Soft corals of the family Xeniidae exhibit a unique, rhythmic pulsation of their tentacles (Movie S1), first noted by Lamarck nearly 200 y ago. However, the adaptive benefit of this perpetual, energetically costly motion is poorly understood. Using in situ underwater particle image velocimetry, we found that the pulsation motions thrust water upward and enhance mixing across the coral–water boundary layer. The induced upward motion effectively prevents refiltration of water by neighboring polyps, while the intensification of mixing, together with the upward flow, greatly enhances the coral’s photosynthesis. A series of controlled laboratory experiments with the common xeniid coral Heteroxenia fuscescens showed that the net photosynthesis rate during pulsation was up to an order of magnitude higher than during the coral’s resting, nonpulsating state. This enhancement diminished when the concentration of oxygen in the ambient water was artificially raised, indicating that the enhancement of photosynthesis was due to a greater efflux of oxygen from the coral tissues. By lowering the internal oxygen concentration, pulsation alleviates the problem of reduced affinity of ribulose-1,5-bisphosphate carboxylase oxygenase (RuBisCO) to CO2 under conditions of high oxygen concentrations. The photosynthesis–respiration ratio of the pulsating H. fuscescens was markedly higher than the ratios reported for nonpulsating soft and stony corals. Although pulsation is commonly used for locomotion and filtration in marine mobile animals, its occurrence in sessile (bottom-attached) species is limited to members of the ancient phylum Cnidaria, where it is used to accelerate water and enhance physiological processes. PMID:23610420

  18. Accretion Disks in Supersoft X-ray Sources

    NASA Technical Reports Server (NTRS)

    Popham, Robert; DiStefano, Rosanne

    1996-01-01

    We examine the role of the accretion disk in the steady-burning white dwarf model for supersoft sources. The accretion luminosity of the disk is quite small compared to the nuclear burning luminosity of the central source. Thus, in contrast to standard accretion disks, the main role of the disk is to reprocess the radiation from the white dwarf. We calculate models of accretion disks around luminous white dwarfs and compare the resulting disk fluxes to optical and UV observations of the LMC supersoft sources CAL 83, CAL 87, and RX J0513.9-6951. We find that if the white dwarf luminosity is near the upper end of the steady-burning region, and the flaring of the disk is included, then reprocessing by the disk can account for the UV fluxes and a substantial fraction of the optical fluxes of these systems. Reprocessing by the companion star can provide additional optical flux, and here too the disk plays an important role: since the disk is fairly thick, it shadows a significant fraction of the companion's surface.

  19. Gas compressor with side branch absorber for pulsation control

    SciTech Connect

    Harris, Ralph E.; Scrivner, Christine M.; Broerman, III, Eugene L.

    2011-05-24

    A method and system for reducing pulsation in lateral piping associated with a gas compressor system. A tunable side branch absorber (TSBA) is installed on the lateral piping. A pulsation sensor is placed in the lateral piping, to measure pulsation within the piping. The sensor output signals are delivered to a controller, which controls actuators that change the acoustic dimensions of the SBA.

  20. SuperDARN observations of pulsating aurora

    NASA Astrophysics Data System (ADS)

    Clausen, L. B. N.; Yeoman, T. K.; Hosokawa, K.; Yukimatu, A. S.; Sato, N.; Milan, S. E.; Lester, M.

    2009-04-01

    On 25 September 2006 the all-sky camera located in Tjornes, Iceland observed pulsating aurora. During the event, the SuperDARN radar at Pykkvibaer was running in a high time, high spatial resolution mode and observed oscillating Doppler velocities. The pulsating velocities were observed in two separate patches of backscatter at different range gates, with different velocities. Backscattered power and spectral width as well as elevation angle data suggest that the power associated with each patch travelled along different ray paths. We discuss possible ray paths as well as the mechanisms that could have led to the difference in Doppler velocity observed for each patch.

  1. Ionospheric variation during pulsating aurora

    NASA Astrophysics Data System (ADS)

    Hosokawa, K.; Ogawa, Y.

    2015-07-01

    We have statistically analyzed data from the European Incoherent Scatter (EISCAT) UHF/VHF radars in Tromsø (69.60°N, 19.20°E), Norway, to reveal how the occurrence of pulsating auroras (PsAs) modifies the electron density profile in the ionosphere. By checking five winter seasons' (2007-2012) observations of all-sky aurora cameras of the National Institute of Polar Research in Tromsø, we have extracted 21 cases of PsA. During these PsA events, either the UHF or VHF radar of EISCAT was operative and the electron density profiles were obtained along the field-aligned or vertical direction near the zenith. From these electron density measurements, we calculated hmE (E region peak height) and NmE (E region peak density), which are proxies for the energy and flux of the precipitating PsA electrons, respectively. Then, we examined how these two parameters changed during the evolution of 21 PsA events in a statistical fashion. The results can be summarized as follows: (1) hmE is lower (the energy of precipitation electrons is higher) during the periods of PsA than that in the surrounding interval; (2) when NmE is higher (flux of PsA electrons is larger), hmE tends to be lower (precipitation is harder); (3) hmE is lower and NmE is larger in the later magnetic local time; and (4) when the AE index during the preceding substorm is larger, hmE is lower and NmE is larger. These tendencies are discussed in terms of the characteristics of particles and plasma waves in the source of PsA in the magnetosphere. In addition to the statistics of the EISCAT data, we carried out several detailed case studies, in which the altitude profiles of the electron density were derived by separating the On and Off phases of PsA. This allows us to estimate the true altitude profiles of the PsA ionization, which can be used for estimating the characteristic energy of the PsA electrons and better understanding the wave-particle interaction process in the magnetosphere.

  2. Simulations of Double White Dwarf Mergers

    NASA Astrophysics Data System (ADS)

    Motl, Patick; Staff, Jan; Marcello, Dominic; Clayton, Geoffrey; Frank, Juhan

    2016-03-01

    We present numerical simulations of double white dwarf mergers initiated by mass transfer instability. In particular, we are interested in the possible connection between such double degenerate mergers and the peculiar irregular variable R Corona Borealis stars. For the merger of a Carbon-Oxygen white dwarf with a Helium white dwarf, the degree to which Carbon from the accreting star is dredged up plays a crucial role in the appearance of the rejuvenated, merged object. We explore the amount of dredge up in the accreting star and its influence in stellar evolution models initialized from the merged object resulting from dynamical evolutions.

  3. Large Variety of New Pulsating Stars in the OGLE-III Galactic Disk Fields

    NASA Astrophysics Data System (ADS)

    Pietrukowicz, P.; Dziembowski, W. A.; Mróz, P.; Soszyński, I.; Udalski, A.; Poleski, R.; Szymański, M. K.; Kubiak, M.; Pietrzyński, G.; Wyrzykowski, Ł.; Ulaczyk, K.; Kozłowski, S.; Skowron, J.

    2013-12-01

    We present the results of a search for pulsating stars in the 7.12 deg2 OGLE-III Galactic disk area in the direction tangent to the Centaurus Arm. We report the identification of 20 Classical Cepheids, 45 RR Lyr type stars, 31 Long-Period Variables, such as Miras and Semi-Regular Variables, one pulsating white dwarf, and 58 very likely δ Sct type stars. Based on asteroseismic models constructed for one quadruple-mode and six triple-mode δ Sct type pulsators, we estimated masses, metallicities, ages, and distance moduli to these objects. The modeled stars have masses in the range 0.9-2.5 MSun and are located at distances between 2.5 kpc and 6.2 kpc. Two triple-mode and one double-mode pulsators seem to be Population II stars of the SX Phe type, probably from the Galactic halo. Our sample also includes candidates for Type II Cepheids and unclassified short-period (P<0.23 d) multi-mode stars which could be either δ Sct or β Cep type stars. One of the detected variables is a very likely δ Sct star with an exceptionally high peak-to-peak I-band amplitude of 0.35 mag at the very short period of 0.0196 d. All reported pulsating variable stars but one object are new discoveries. They are included in the OGLE-III Catalog of Variable Stars. Finally, we introduce the on-going OGLE-IV Galactic Disk Survey, which covers more than half of the Galactic plane. For the purposes of future works on the spiral structure and star formation history of the Milky Way, we have already compiled a list of known Galactic Classical Cepheids.

  4. Evaluation of Pump Pulsation in Respirable Size-Selective Sampling: Part I. Pulsation Measurements

    PubMed Central

    Lee, Eun Gyung; Lee, Larry; Möhlmann, Carsten; Flemmer, Michael M.; Kashon, Michael; Harper, Martin

    2015-01-01

    Pulsations generated by personal sampling pumps modulate the airflow through the sampling trains, thereby varying sampling efficiencies, and possibly invalidating collection or monitoring. The purpose of this study was to characterize pulsations generated by personal sampling pumps relative to a nominal flow rate at the inlet of different respirable cyclones. Experiments were conducted using a factorial combination of 13 widely used sampling pumps (11 medium and 2 high volumetric flow rate pumps having a diaphragm mechanism) and 7 cyclones [10-mm nylon also known as Dorr-Oliver (DO), Higgins-Dewell (HD), GS-1, GS-3, Aluminum, GK2.69, and FSP-10]. A hot- wire anemometer probe cemented to the inlet of each cyclone type was used to obtain pulsation readings. The three medium flow rate pump models showing the highest, a midrange, and the lowest pulsations and two high flow rate pump models for each cyclone type were tested with dust-loaded filters (0.05, 0.21, and 1.25 mg) to determine the effects of filter loading on pulsations. The effects of different tubing materials and lengths on pulsations were also investigated. The fundamental frequency range was 22–110 Hz and the magnitude of pulsation as a proportion of the mean flow rate ranged from 4.4 to 73.1%. Most pump/cyclone combinations generated pulse magnitudes >10% (48 out of 59 combinations), while pulse shapes varied considerably. Pulsation magnitudes were not considerably different for the clean and dust-loaded filters for the DO, HD, and Aluminum cyclones, but no consistent pattern was observed for the other cyclone types. Tubing material had less effect on pulsations than tubing length; when the tubing length was 183 cm, pronounced damping was observed for a pump with high pulsation (>60%) for all tested tubing materials except for the Tygon Inert tubing. The findings in this study prompted a further study to determine the possibility of shifts in cyclone sampling efficiency due to sampling pump pulsations

  5. VOLUME COMPENSATING MEANS FOR PULSATING PUMPS

    DOEpatents

    Weaver, D.L.W.; MacCormack, R.S. Jr.

    1959-12-01

    A double diaphragm, two-liquid pulsating pump for remote control use, having as an improvement an apparatus for maintaining constant the volume of the liquid such as kerosene between the two diaphragms is described. Phase difficulties encountered in the operation of such pumps when the volume of the liquid is altered by changes in temperature are avoided.

  6. Pulsations in total columnar electron content

    NASA Technical Reports Server (NTRS)

    Okuzawa, T.; Davies, K.

    1981-01-01

    Radio signals from the ATS 6 beacon received at Boulder reveal small-amplitude, quasi-sinusoidal fluctuations with periods in the range of 10 to 50 s. Visual comparisons of these data (116 events for October 1974 to April 1975) shows a good correspondence with simultaneous geomagnetic pulsations at Boulder in two thirds of the cases for which Boulder magnetograms were available, but they do not necessarily correspond with magnetic pulsations on ATS 6. Spectral analyses, by the method of maximum entropy, were made on sample records. The principal results are the following: (1) The occurrence of the pulsations is higher on magnetically disturbed days. (2) The maximum likelihood of occurrence is around 2100 UT (1400 LT). (3) The dominant spectrum peaks of the radio fluctuations and geomagnetic field on the ground generally coincide. Cases are found also in which temporal characteristics of the spectra are similar. These results indicate a close association of the radio fluctuations with the Pc 3-4 type pulsations of the geomagnetic field on the ground. It is suggested that the radio fluctuations originate mainly in the F region of the ionosphere, while some of them could be due to plasmapause effects.

  7. Blackworms, Blood Vessel Pulsations and Drug Effects.

    ERIC Educational Resources Information Center

    Lesiuk, Nalena M.; Drewes, Charles D.

    1999-01-01

    Introduces the freshwater oligochaete worm, lumbriculus variegatus (common name: blackworms), an organism that is well suited for classroom study because of its closed circulatory system. Describes a set of simple, fast, noninvasive, and inexpensive methods for observing pulsations of the worm's dorsal blood vessels under baseline conditions, and…

  8. On the standing wave mode of giant pulsations

    NASA Technical Reports Server (NTRS)

    Takahashi, K.; Sato, N.; Warnecke, J.; Luehr, H.; Spence, H. E.; Tonegawa, Y.

    1992-01-01

    In order to determine the standing wave mode of giant pulsations, a systematic survey of magnetic field data from the AMPTE CCE spacecraft and from ground stations located near the geomagnetic foot point of CCE was made. One giant pulsation was associated with a compressional wave, while no giant pulsation was observed in association with transverse wave events. The CCE magnetic field record for the giant pulsation exhibited a remarkable similarity to a giant pulsation observed from the ATS 6 geostationary satellite near the magnetic equator. It is concluded that the compressional nature of the giant pulsation is due to an odd-mode standing wave structure, which places a strong constraint on the generation mechanism of giant pulsations.

  9. 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.

  10. 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

  11. 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.

  12. The Origin and Evolution of the White Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Clemens, J. C.

    1994-05-01

    The secret of how white dwarf stars form and evolve is hidden in their interiors. There, gravity separates the constituent elements into layers; the lighter elements float to the top and the heavier ones sink. Consequently, a white dwarf's structure depends on the quantity of the elements present. Measuring that structure can tell us about the processes which formed white dwarfs and allow us to calculate how fast they cool. The latter is indispensable for measuring the age of our galaxy using the oldest white dwarfs as chronometers. Because some white dwarfs pulsate, we can exploit the resulting luminosity variations to measure their internal structure using asteroseismology. Exploring white dwarf structure via asteroseismology poses a difficult observational task: acquiring essentially uninterrupted time series measurements of the brightness changes of pulsating white dwarf stars. We have accomplished this task using an instrument we call the Whole Earth Telescope (WET). By combining data from the WET with published measurements, we have detected a common pattern in the pulsation spectra of all the variable, hydrogen spectra white dwarfs (DAVs), implying that they have similar surface hydrogen layer masses. Because we have identified the degree (l) and the radial overtone (k) of the modes in the pattern detected, we have been able to compare their periods to published pulsation models to find the mass of the hydrogen layer; it is about 10(-4) times the total stellar mass. This result will require adjustments to published estimates of the age of the galaxy which use theoretical cooling times of the oldest white dwarfs as a time standard; the theoretical models typically assume much thinner hydrogen layers. We have also investigated the two classes of pulsating helium spectra white dwarfs (DOVs and DBVs). From their pulsation properties, and the mass of the hydrogen layer measured for the DAVs, we have concluded that the helium surface white dwarfs do not form via

  13. The Origin and Evolution of the White-Dwarf Stars

    NASA Astrophysics Data System (ADS)

    Clemens, J. C.

    1994-12-01

    The secret of how white dwarf stars form and evolve is hidden in their interiors. There, gravity separates the constituent elements into layers; the lighter elements float to the top and the heavier ones sink. Consequently, a white dwarf's structure depends on the quantity of the elements present. Measuring that structure can tell us about the processes which formed white dwarfs and allow us to calculate how fast they cool. The latter is indispensable for measuring the age of our galaxy using the oldest white dwarfs as chronometers. Because some white dwarfs pulsate, we can exploit the resulting luminosity variations to measure their internal structure using "asteroseismology," a procedure analogous to terrestrial seismology. Exploring white dwarf structure via asteroseismology poses a difficult observational task: acquiring essentially uninterrupted time series measurements of the brightness changes of pulsating white dwarf stars. We have accomplished this task using an instrument we developed for this purpose, the Whole Earth Telescope. By combining data from the Whole Earth Telescope with published measurements, we have detected a common pattern in the pulsation spectra of all the variable, hydrogen spectra white dwarfs (DAVs), implying that they have similar surface hydrogen layer masses. Because we have identified the degree (l) and the radial overtone (k) of the modes in the pattern detected, we have been able to compare their periods to published pulsation models to find the mass of the hydrogen layer; it is about 10^-4 times the total stellar mass. This result will require adjustments to published estimates of the age of the galaxy which use theoretical cooling times of the oldest white dwarfs as a time standard; the theoretical models typically assume much thinner hydrogen layers. We have also investigated the two classes of pulsating helium spectra white dwarfs (DOVs and DBVs). From their pulsation properties, and the mass of the hydrogen layer measured

  14. The origin and evolution of the white dwarf stars

    NASA Astrophysics Data System (ADS)

    Clemens, James Christopher

    1994-01-01

    The secret of how white dwarf stars form and evolve is hidden in their interiors. There, gravity separates the constituent elements into layers; the lighter elements float to the top and the heavier ones sink. Consequently, a white dwarf's structure depends on the quantity of the elements present. Measuring that structure can tell Us about the processes which formed white dwarfs and allow us to calculate how fast they cool. The latter is indispensable for measuring the age of our galaxy using the oldest white dwarfs as chronometers. Because some white dwarfs pulsate, we can exploit the resulting luminosity variations to measure their internal structure using 'asteroseismology', a procedure analogous to terrestrial seismology. Exploring white dwarf structure via asteroseismology poses a difficult observational task: acquiring essentially uninterrupted time series measurements of the brightness changes of pulsating white dwarf stars. We have accomplished this task using an instrument we developed for this purpose, the Whole Earth Telescope. By combining data from the Whole Earth Telescope with published measurements, we have detected a common pattern in the pulsation spectra of all the variable, hydrogen spectra white dwarfs (DAVs), implying that they have similar surface hydrogen layer masses. Because we have identified the degree (l) and the radial overtone (k) of the modes in the pattern detected, we have been able to compare their periods to published pulsation models to find the mass of the hydrogen layer, it is about 10-4 times the total stellar mass. This result will require adjustments to published estimates of the age of the galaxy which use theoretical cooling times of the oldest white dwarfs as a time standard; the theoretical models typically assume much thinner hydrogen layers. We have also investigated the two classes of pulsating helium spectra white dwarfs (DOVs and DBVs). From their pulsation properties and the mass of the hydrogen layer measured for

  15. 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.

  16. 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.

  17. 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.

  18. An accurate geometric distance to the compact binary SS Cygni vindicates accretion disc theory.

    PubMed

    Miller-Jones, J C A; Sivakoff, G R; Knigge, C; Körding, E G; Templeton, M; Waagen, E O

    2013-05-24

    Dwarf novae are white dwarfs accreting matter from a nearby red dwarf companion. Their regular outbursts are explained by a thermal-viscous instability in the accretion disc, described by the disc instability model that has since been successfully extended to other accreting systems. However, the prototypical dwarf nova, SS Cygni, presents a major challenge to our understanding of accretion disc theory. At the distance of 159 ± 12 parsecs measured by the Hubble Space Telescope, it is too luminous to be undergoing the observed regular outbursts. Using very long baseline interferometric radio observations, we report an accurate, model-independent distance to SS Cygni that places the source substantially closer at 114 ± 2 parsecs. This reconciles the source behavior with our understanding of accretion disc theory in accreting compact objects. PMID:23704566

  19. 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

  20. 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.

  1. Evaluation of hydro-mechanical pulsation for rocket injector research

    NASA Astrophysics Data System (ADS)

    Wilson, Matthew B.

    The Propulsion Research Center at the University of Alabama in Huntsville has designed and built a hydro-mechanical pulsator to simulate the pressure fluctuations created by high frequency combustion instability. The pressure response characteristics were evaluated in an atmospheric test rig using filtered de-ionized water as the working fluid. The outlet of the pulsator was connected to a swirl injector post to provide downstream flow resistance. Previous low pressure and mass flow experimental data revealed a complex relationship between the control parameters and the pulsation response. For each test, the average mass flow rates of the waste water, water lost through the seals, and injector mass flow rates are measured. A dynamic pressure transducer at the pulsator exit measures and records the pressure waveform. Pulsation magnitude, reliability, repeatability, pulsation effects, and detailed variable control are examined. The data shows the pulsator is capable of generating 30% pulsation at 1575 Hz input. The repeatability of the pulsator is questionable because the standard deviations exceeded 40% of the average. The detailed data obtained during this research provides is sufficient to develop a pulsator tuning procedure for future applications.

  2. Parametric study of flow patterns behind the standing accretion shock wave for core-collapse supernovae

    SciTech Connect

    Iwakami, Wakana; Nagakura, Hiroki; Yamada, Shoichi

    2014-05-10

    In this study, we conduct three-dimensional hydrodynamic simulations systematically to investigate the flow patterns behind the accretion shock waves that are commonly formed in the post-bounce phase of core-collapse supernovae. Adding small perturbations to spherically symmetric, steady, shocked accretion flows, we compute the subsequent evolutions to find what flow pattern emerges as a consequence of hydrodynamical instabilities such as convection and standing accretion shock instability for different neutrino luminosities and mass accretion rates. Depending on these two controlling parameters, various flow patterns are indeed realized. We classify them into three basic patterns and two intermediate ones; the former includes sloshing motion (SL), spiral motion (SP), and multiple buoyant bubble formation (BB); the latter consists of spiral motion with buoyant-bubble formation (SPB) and spiral motion with pulsationally changing rotational velocities (SPP). Although the post-shock flow is highly chaotic, there is a clear trend in the pattern realization. The sloshing and spiral motions tend to be dominant for high accretion rates and low neutrino luminosities, and multiple buoyant bubbles prevail for low accretion rates and high neutrino luminosities. It is interesting that the dominant pattern is not always identical between the semi-nonlinear and nonlinear phases near the critical luminosity; the intermediate cases are realized in the latter case. Running several simulations with different random perturbations, we confirm that the realization of flow pattern is robust in most cases.

  3. Pulsation and mass loss in Mira variables

    NASA Technical Reports Server (NTRS)

    Wood, P. R.

    1980-01-01

    The behavior of pulsation in the outer layers of a typical Mira variable was studied in the adiabatic and isothermal limits. A shock wave propagates outward once per period and the radial velocity obtained from observations of hydrogen emission lines is identified with the velocity of gas in the post shock region. In the adiabatic case, mass loss in the form of a steady stellar wind was produced. In the isothermal case, no continuous mass loss was produced but occasional ejection of shells occur. Pulsation introduced into a star undergoing steady mass loss as a result of radiation pressure acting on grains caused the mass loss rate to increase by a factor of approximately 40, while the terminal velocity of the flow was almost unaltered.

  4. The eight-schwabe-cycle pulsation

    NASA Astrophysics Data System (ADS)

    Richard, Jean-Guillaume

    2004-09-01

    The shape of the Sun’s secular activity cycle is found to be a saw-tooth curve. The additional Schwabe cycle 4‧ (1793 1799) suggested by Usoskin, Mursula, and Kovaltsov (2001a) is taken into account in the telescopic sunspot record (1610 2001). Instead of a symmetrical Gleissberg cycle, a saw-tooth of exactly eight Schwabe sunspot maxima (‘Pulsation’) is found. On average, the last sunspot maximum of an eight-Schwabe-cycle saw-tooth pulsation has been about three times as high as its first maximum. The Maunder Minimum remains an exception to this pattern. The Pulsation is defined as a secular-scale envelope of Schwabe-cycle maxima, whereas the Gleissberg cycle is a result of long-term smoothing of the sunspot series.

  5. 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.

  6. White dwarf models for type 1 supernovae and quiet supernovae, and presupernova evolution

    NASA Technical Reports Server (NTRS)

    Nomoto, K.

    1980-01-01

    Supernova mechanisms in accreting white dwarfs are considered with emphasis on deflagration as a plausible mechanism for producing Type I supernovae and electron captures to form quiet supernovae leaving neutron stars. These outcomes depend on accretion rate of helium, initial mass and composition of the white dwarf. The various types of hydrogen shell burning in the presupernova stage are also discussed.

  7. 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.

  8. Pulsating aurorae: Evidence for flux limiting

    SciTech Connect

    Davidson, G.T.; Sears, R.D.

    1980-03-01

    Theoretical models based upon the concept of self-modulated VLF wave-electron interactions have been proposed to explain pulsating aurorae. These models incorporate the idea of a trapping limit, above which strong diffusion into the loss cone rapidly removes any excess electrons. At flux values near the trapping limit, perturbations of the trapped electron distribution can result in cyclic wave growth and electron precipitation. The trapping limit is thus related to the energy deposited and the characteristic energy of electrons precipitated in pulsating aurorae. Photometric measurements of the total energy deposit and of the mean energy parameter made at Chatanika, Alaska (invariant geomagnetic latitude, 65 /sup 0/) indicate that the well-developed pulsations are caused mainly by a modulation of the mean energy parameter. Thus, a nearly constant ''limiting'' value for the precipitating flux is measured F=7 x 10/sup 8/ el/cm/sup 2/ sec from which a trapped flux limit of Japprox. =3 x 10/sup 9/ el/cm/sup 2/ sec can be inferred.

  9. Simultaneous observation of monochromatic and variable period geomagnetic pulsations

    SciTech Connect

    McDiarmid, D.R.; Nielsen, E. )

    1987-05-01

    On February 4, 1983, following a storm sudden commencement, a monochromatic and a variable period pulsation were simultaneously observed by the Scandinavian Twin Auroral Radar Experiment (STARE) and Sweden and Britain Radar Experiment (SABRE) radar systems. Both pulsations differed from previously analyzed examples of their class. The phase of the monochromatic pulsation increased linearly with latitude rather than decreased. Its amplitude remained relatively constant over the latitude interval of linear phase change. The variable period pulsation experienced a change of orientation of its essentially linear polarization diagram in association with a discontinuity of its period. The variable period pulsation was thus manifest in both the toroidal and poloidal components. The results are discussed in terms of recent developments in theoretical pulsation modeling.

  10. 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.

  11. White dwarfs in Be star binary systems

    NASA Technical Reports Server (NTRS)

    Apparao, K. M. V.

    1991-01-01

    An evaluation is made of possible reasons for the persistent inability to identify white dwarf stars in the Be binary systems. It is noted that many Be stars exhibiting large optical enhancements may be Be + WD and Be + He systems, and that observations of pulsations in the H-alpha emission, as well as observation of time delays between enhancements of optical line and continuum, can identify such systems.

  12. The δ Scuti Pulsation Periods in KIC 5197256

    NASA Astrophysics Data System (ADS)

    Turner, G.; Holaday, J.

    2015-06-01

    In this paper we present the pulsational spectrum for KIC 5197256. This object is an eclipsing binary system with a period of 6.96 days. We demonstrate that the light curve shows presence of δ Scuti pulsations with a dominant period of 0.1015 day. The object should therefore be included in the ever-growing class of eclipsing binary systems with at least one pulsating component.

  13. Detection and characterization of geomagnetic pulsations using HF ionospheric heating

    SciTech Connect

    Lee, H.S.; Ferraro, A.J.; Olson, J.V. Alaska Univ., Fairbanks )

    1990-12-01

    This paper describes the geomagnetic pulsations observed in the high-latitude ionosphere during an experiment dealing with the ionospheric generation of ELF/VLF EM waves in June and October 1987. There was clear evidence of geomagnetic pulsations intermixed with the ELF/VLF signals in both the magnitude and phase data. A simple simulation model is introduced to facilitate the interpretation of the data, and a procedure for characterizing the pulsation is described. 5 refs.

  14. Helium abundance effects on RR Lyrae pulsation properties

    NASA Astrophysics Data System (ADS)

    Marconi, M.; Coppola, G.; Bono, G.; Braga, V.; Pietrinferni, A.

    2016-05-01

    A new set of nonlinear convective pulsation models of RR Lyrae stars has been computed varying both the metallicity and the helium content. To constrain the helium dependence of pulsation observables we adopted, for each metal content, at least three different helium abundances. We provide for the first time a homogeneous evolutionary and pulsation framework covering the entire range of cluster and field variables. The implications for the use of RR Lyrae as stellar population tracers and distance indicators are briefly discussed.

  15. On the pulsation and evolutionary properties of helium burning radially pulsating variables

    NASA Astrophysics Data System (ADS)

    Bono, G.; Pietrinferni, A.; Marconi, M.; Braga, V. F.; Fiorentino, G.; Stetson, P. B.; Buonanno, R.; Castellani, M.; Dall'Ora, M.; Fabrizio, M.; Ferraro, I.; Giuffrida, G.; Iannicola, G.; Marengo, M.; Magurno, D.; Martinez-Vazquez, C. E.; Matsunaga, N.; Monelli, M.; Neeley, J.; Rastello, S.; Salaris, M.; Short, L.; Stellingwerf, R. F.

    2016-05-01

    We discuss pulsation and evolutionary properties of low- (RR Lyrae, Type II Cepheids) and intermediate-mass (Anomalous Cepheids) radial variables. We focus our attention on the topology of the instability strip and the distribution of the quoted variables in the Hertzsprung-Russell diagram. We discuss their evolutionary status and the dependence on the metallicity. Moreover, we address the diagnostics (period derivative, difference in luminosity, stellar mass) that can provide solid constraints on their progenitors and on the role that binarity and environment have in shaping their current pulsation characteristics. Finally, we briefly outline their use as standard candles.

  16. Large-Scale Aspects and Temporal Evolution of Pulsating Aurora

    NASA Technical Reports Server (NTRS)

    Jones, S. L.; Lessard, M. R.; Rychert, K.; Spanswick, E.; Donovan, E.

    2010-01-01

    Pulsating aurora is a common phenomenon generally believed to occur mainly in the aftermath of a, substorm, where dim long-period pulsating patches appear. The study determines the temporal and spatial evolution of pulsating events using two THEN IIIS ASI stations, at Gillam (66.18 mlat, 332.78 mlon, magnetic midnight at 0634 UT) and Fort Smith, (67.38 mlat, 306.64 mlon, magnetic midnight at, 0806 UT) along roughly the same invariant latitude. Parameters have been calculated from a database of 74 pulsating aurora events from 119 days of good optical data within the period from September 2007 through March 2008 as identified with the Gillam camera. It is shown that the source region of pulsating aurora drifts or expands eastward, away from magnetic midnight, for pre-midnight onsets and that the spatial evolution is more complicated for post midnight onsets, which has implications for the source mechanism. The most probable duration of a pulsating aurora event is roughly 1.5 hours while the distribution of possible event durations includes many long (several hours) events. This may suggest that pulsating aurora is not strictly a substorm recovery phase phenomenon but rather a persistent, long-lived phenomenon that may be temporarily disrupted by auroral substorms. Observations from the Gillam station show that in fact, pulsating aurora is quite common with the occurrence rate increasing to around 60% for morning hours, with 6910 of pulsating aurora onsets occurring after substorm breakup.

  17. Periodic stellar pulsations - Stability analysis and amplitude equations

    NASA Astrophysics Data System (ADS)

    Buchler, J. R.; Moskalik, Pawel; Kovacs, Geza

    1991-10-01

    The stability properties of nonlinear periodic stellar pulsations are studied within the amplitude equation formalism. Both nonresonant and resonant pulsations are considered. A comparison to a sequence of classical Cepheid models shows that the formalism provides a good qualitative and quantitative description of the behavior of the Floquet coefficients and that it also captures the most important features of the Floquet eigenvectors. It thus helps shed new light on the behavior (bifurcations) of pulsating stars. In addition, the predictive powers of the analytical approach allow a systematic search for models with specific pulsational properties.

  18. Magnetic pulsations at the quasi-parallel shock

    NASA Technical Reports Server (NTRS)

    Thomsen, M. F.; Gosling, J. T.; Bame, S. J.; Russell, C. T.

    1990-01-01

    The plasma and field properties of large-amplitude magnetic field pulsatins upstream from the quasi-parallel region of the earth's bow shock are examined in high time resolution using data from ISEE 1 and 2. The relative timing of the magnetic field profiles observed at the two spacecraft shows that some of the pulsations are convecting antisunward across the spacecraft while others are brief out/in motions of bow shock across the spacecraft. Pulsations with both timing signatures are the site of slowing and heating of the solar wind plasma. The ions tend to be only weakly heated in the convecting pulsations, while within the out/in pulsations the ion heating can be quite substantial but variable. This variation occurs not only from pulsation to pulsation but also from point to point within a given pulsation. In general, the hottest distributions within the out/in pulsations tend to occur in regions of lower density and field strength. Magnetic pulsations bear a number of similarities to previously identified hot diamagnetic cavity events as well as to more durable crossings of the quasi-parallel shock itself. These various phenomena may be different manifestations of the same basic physical processes, in particular the coupling of coherently reflected ions to the solar wind beam.

  19. DRIVING G-MODE PULSATIONS IN GAMMA DORADUS VARIABLES

    SciTech Connect

    J. GUZIK; A. KAYE; ET AL

    2000-10-10

    The {gamma} Doradus stars are a newly-discovered class of gravity-mode pulsators which lie just at or beyond the red edge of the {delta} Scuti instability strip. We present the results of calculations which predict pulsation instability of high-order g-modes with periods between 0.4 and 3 days, as observed in these stars. The pulsations are driven by the modulation of radiative flux by convection at the base of a deep envelope convection zone. Pulsation instability is predicted only for models with temperatures at the convection zone base between {approximately}200,000 and {approximately}480,000 K. The estimated shear dissipation due to turbulent viscosity within the convection zone, or in an overshoot region below the convection zone, can be comparable to or even exceed the predicted driving, and is likely to reduce the number of unstable modes, or possibly to quench the instability. Additional refinements in the pulsation modeling are required to determine the outcome. A few Doradus stars have been observed that also pulsate in {delta} Scuti-type p-modes, and at least two others have been identified as chemically peculiar. Since our calculated driving region is relatively deep, Doradus pulsations are not necessarily incompatible with surface abundance peculiarities or with {delta} Scuti p-mode pulsations driven by the H and He-ionization {kappa} effect. Such stars will provide useful observational constraints on the proposed Doradus pulsation mechanism.

  20. 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.

  1. A search for pulsations from the compact object of GRB 060218

    NASA Astrophysics Data System (ADS)

    Mirabal, N.; Gotthelf, E. V.

    2010-02-01

    Aims: A fraction of massive stars are expected to collapse into compact objects (accreting black holes or rapidly rotating neutron stars) that successfully produce gamma-ray bursts (GRBs). We examine the possibility of directly observing these gamma-ray burst compact objects (GCOs) using post-explosion observations of past and future GRB sites. Methods: We present a search for early pulsations from the nearby (z=0.0335) gamma-ray burst GRB 060218, which exhibited features possibly consistent with a rapidly spinning neutron star as its underlying GCO. We also consider alternative techniques that could potentially achieve a detection of GCOs either in the Local Volume or near the plane of our own Galaxy. Results: We report the non-detection of pulsations from the GCO of GRB 060218. In particular, fast fourier transform analysis applied to the light curve shows no significant power over the range of frequencies 0.78 mHz < f < 227 Hz with an upper limit on the pulsed fraction of ~2%. In addition, we present detection limits of current high-resolution archival X-ray images of galaxies within the Local Volume. The existing data could be harnessed to rule out the presence of any background contaminants at the GRB position of future nearby events. Conclusions: The null detection of pulsations from the GCO of GRB 060218 is most likely explained by the fact that the afterglow emission occurs near the head of the jet and should be far removed from the compact object. We also find that the comparison of pre- and post-explosion explosion images of future GRBs within the Local Volume, as well as the firm identification of a GCO within an ancient GRB remnant near the Galactic plane are extremely challenging with current GeV/TeV capabilities. Finally, we conclude that only under some very exceptional circumstances will it be possible to directly detect the compact object responsible for gamma-ray bursts.

  2. 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.

  3. Thermonuclear reactions in cool accreting neutron stars and burst phenomena

    NASA Technical Reports Server (NTRS)

    Miyaji, S.; Nomoto, K.

    1985-01-01

    The ignition of accreting materials on neutron stars is explored using strongly coupled plasma analytical techniques. The calculations cover the ignition temperature and density at the bottom of the accreted envelope of a neutron star. Emphasis is placed on low-temperature ignitions which take place at high densities. The investigation is extended to the accretion of material from a white dwarf in the form of pure He, C + O, or O + Ne + Mg. It is shown that electrons are strongly degenerate in low-temperature flashes, where the ignition is more dependent on density than on temperature. Precursor flashes 0.4-0.7 the intensity of the main burst will appear before the main bursts. The intensity relationship indicates that the appropriate model for an X-ray burst from a neutron star accreting from a white dwarf is a He shell flash in the presence of a hydrogen-rich atmosphere. The flash will have a maximum energy of 2 x 10 to the 43 ergs and could last as long as 40,000 sec.

  4. 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.

  5. 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

  6. The Boundary Layer-White Dwarf Connection: CV White Dwarf Rotation and Line Formation Regions in the Prototype Dwarf Nova U Geminorum

    NASA Astrophysics Data System (ADS)

    Sion, Edward

    1994-01-01

    The central accreting object in compact binaries and other accreting systems is usually hidden from direct observation, enshrouded by optically thick accretion disks which are opaque to the radiation emitted by the central object as it accretes matter. In the dwarf nova U Gem the underlying white dwarf accreter is clearly exposed in the far UV during dwarf nova quiescence. By probing the white dwarf with the sensitivity and velocity resolution of GHRS, just after the deposition of mass, energy and angular momentum into its envelope by the dwarf nova outburst event and also later in quiescence, we will obtain fundamental information such as: (1) the first rate of rotation of a CV white dwarf and hence the expected temperature and extent of the boundary layer during the high accretion (dwarf nova outburst) episode; (2) the delineation of the lower temperature line formation regions associated with the rotating, average (30-40,000K), photosphere of the white dwarf from the line formation region(s) of higher temperature ion species (e.g. NV 1238,1242) (boundary layer/accretion belt or putative hot corona?) surrounding the white dwarf; (3) the velocity amplitude and hence mass and gravitational redshift of the U Gem white dwarf; (4) the possible presence of persisting (but weaker in quiescence) radiation- driven selective ion wind (5) line profile variations to test physical mech- anisms that control the flow of accreted elements and hence abundance changes (diffusion, mixing, dilution, dredgeup and ongoing

  7. A pulsating star inside η Carinae. I. Light variations, 1992-1994.

    NASA Astrophysics Data System (ADS)

    van Genderen, A. M.; Sterken, C.; de Groot, M.; Stahl, O.; Andersen, J.; Andersen, M. I.; Caldwell, J. A. R.; Casey, B.; Clement, R.; Corradi, W. J. B.; Cuypers, J.; Debehogne, H.; Garcia de Maria, J. M.; Jonch-Sorensen, H.; Vaz, L. P. R.; Stefl, S.; Suso Lopez, J.; Beele, D.; Eggenkamp, I. M. M. G.; Goecking, K.-D.; Jorissen, A.; de Koff, S.; Kuss, C.; Schoenmakers, A. P.; Vink, J.; Waelde, E.

    1995-12-01

    We present and analyze two seasons of intense photometric monitoring in the Stroemgren uvbyHβ system of η Carinae (October 1992-August 1994). The luminous blue variable (LBV) in the core did not show much S Dor activity, i.e. it was in a relatively quiescent stage. This situation was very favourable for studying its optical micro variations. It appears that the central LBV pulsates (presumably in a non-radial mode) like other massive evolved stars, the α Cyg variables. The quasi-period is 58.56d. The linear ephemeris is: JD_max_ = 2448875.0 +58.56 E. Support was found for the existence of the presumed periodicity of 52.4d of the so-called "dimples", shallow dips in the light curve which only last for a few days. The cause may be the eclipse of a small companion or of a hot spot in an accretion disk. The Hβ index became bluer by ~0.07mag during the last 11/2 years and shows an oscillation in anti-phase with the 58.56d pulsation, suggesting that the HII region(s) responsible for the hydrogen line emission has a relatively high luminosity. This is another reason to suppose that a second luminous source, perhaps a luminous disk (with a hot spot), may be present in the η Car system.

  8. 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

  9. Convective heat transfer characteristics of laminar pulsating pipe air flow

    NASA Astrophysics Data System (ADS)

    Habib, M. A.; Attya, A. M.; Eid, A. I.; Aly, A. Z.

    Heat transfer characteristics to laminar pulsating pipe flow under different conditions of Reynolds number and pulsation frequency were experimentally investigated. The tube wall of uniform heat flux condition was considered. Reynolds number was varied from 780 to 1987 while the frequency of pulsation ranged from 1 to 29.5Hz. The results showed that the relative mean Nusselt number is strongly affected by pulsation frequency while it is slightly affected by Reynolds number. The results showed enhancements in the relative mean Nusselt number. In the frequency range of 1-4Hz, an enhancement up to 30% (at Reynolds number of 1366 and pulsation frequency of 1.4Hz) was obtained. In the frequency range of 17-25Hz, an enhancement up to 9% (at Reynolds number of 1366 and pulsation frequency of 17.5Hz) was indicated. The rate of enhancement of the relative mean Nusselt number decreased as pulsation frequency increased or as Reynolds number increased. A reduction in relative mean Nusselt number occurred outside these ranges of pulsation frequencies. A reduction in relative mean Nusselt number up to 40% for pulsation frequency range of 4.1-17Hz and a reduction up to 20% for pulsation frequency range of 25-29.5Hz for Reynolds numbers range of 780-1987 were considered. This reduction is directly proportional to the pulsation frequency. Empirical dimensionless equations have been developed for the relative mean Nusselt number that related to Reynolds number (750

  10. 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.

  11. Kronos: A Multiwavelength Observatory for Mapping Accretion-Driven Sources

    NASA Technical Reports Server (NTRS)

    Peterson, Bradley M.; Polidan, Ronald S.; Robinson, Edward L.

    2002-01-01

    Kronos is a multiwavelength observatory proposed as a NASA Medium Explorer. Kronos is designed to make use of the natural variability of accreting sources to create microarcsecond-resolution maps of the environments of supermassive black holes in active galaxies and stella-size black holes in binary systems and to characterize accretion processes in Galactic compact binaries. Kronos will obtain broad energy range spectroscopic data with co-aligned X-ray, ultraviolet, and optical spectrometers. The high-Earth orbit of Kronos enables well-sampled, high time-resolution observations, critical for the innovative and sophisticated methods that are used to understand the accretion flows, mass outflows, jets, and other phenomena found in accreting sources. By utilizing reverberation mapping analysis techniques, Kronos produces advanced high-resolution maps of unprecedented resolution of the extreme environment in the inner cores of active galaxies. Similarly, Doppler tomography and eclipse mapping techniques characterize and map Galactic binary systems, revealing the details of the physics of accretion processes in black hole, neutron star, and white dwarf binary systems. The Kronos instrument complement, sensitivity, and orbital environment make it suitable to aggressively address time variable phenomena in a wide range of astronomical objects from nearby flare stars to distant galaxies.

  12. 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.

  13. Spiral shocks in a solar-size accretion disc

    NASA Astrophysics Data System (ADS)

    Harlaftis, E. T.; Steeghs, D.

    Accretion discs are fundamental in understanding astrophysical phenomena such as AGNs, novae outbursts and star formation. In interacting binaries, a compact star accretes matter from a donor star through an accretion disc. The outburst origin (disc or secondary star) and the mechanism for the angular momentum transport of the disc material (`viscosity') are still controversial subjects. The rarely-observed rise to outburst may hold the key to a better understanding. Imaging of the Balmer and He{I} emission lines of the dwarf nova IP Peg, during such a rise to outburst, shows a two-arm spiral pattern on the accretion disc around the white dwarf and provides the first convincing observational evidence for spiral waves in a stellar accretion disc (Steeghs, Harlaftis, Horne, 1997, Nature, submitted). Recent observations during the recent November 1996 outburst (Harlaftis, Steeghs, Horne, Martin, ApJ, 1997, in preparation) also demonstrate spiral arms in high-ionization lines such as HeII and the Bowen fluorescence lines which suggests that the spiral pattern may provide an efficient mechanism for trasport of angular momentum out of the disc through spiral shocks. We discuss the origin and location of the spiral arms. The tidal interaction of the secondary star with the enlarged (0.6 Rodot) outburst disc can raise such spiral waves in the outer disc. Comparison and implications for theories of spiral galaxies and planet formation is briefly outlined.

  14. Identification of pulsational modes in rotating slowly pulsating B-type stars

    NASA Astrophysics Data System (ADS)

    Szewczuk, W.; Daszyńska-Daszkiewicz, J.

    2015-06-01

    Knowledge of the geometry of pulsational modes is a prerequisite for seismic modelling of stars. In the case of slowly pulsating B-type (SPB) pulsators, the simple zero-rotation approach so far used for mode identification is usually not valid because pulsational frequencies are often of the order of the rotational frequency. Moreover, this approach allows us to determine only the spherical harmonic degree, ℓ, while the azimuthal order, m, is beyond its reach. On the other hand, because of the density of oscillation spectra of SPB stars, knowledge of m is indispensable if one wants to assign the radial order, n, to the observed frequency peaks. Including the effects of rotation via the traditional approximation, we perform identification of the mode angular numbers (ℓ, m) for 31 SPB stars with available multicolour time series photometry. Simultaneously, constraints on the rotational velocity, Vrot, and the inclination angle, i, are determined assuming uniform rotation and a constant value of Vrot sin i. Dependence of the results on the adopted model is tested using HD 21071 as an example. Despite some model uncertainties and limitations of the method, our studies show the correct approach to identifying the low-frequency oscillation modes.

  15. Cold Accretion from the Cosmic Web

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2016-06-01

    The cosmic web is a vast, foam-like network of filaments and voids stretching throughout the universe. How did the first galaxies form within the cosmic web, at the intersections of filaments? New observations of a protodisk a galaxy in the early stages of formation may provide a clue.Models for Galaxy FormationNarrowband image of the candidate protodisk (marked with a white ellipse) and filaments (outlined in white). [Adapted from Martin et al. 2016]The standard model for galaxy formation, known as the hot accretion model, argues that galaxies form out of collapsing, virialized gas that forms a hot halo and then slowly cools, fueling star and galaxy formation at its center.But what if galaxies are actually formed from cool gas? In this contrasting picture, the cold accretion model, cool (temperature of ~104 K) unshocked gas from cosmic web filaments flows directly onto galactic disks forming at the filamentary intersections. The narrow streams of cold gas deliver fuel for star formation.A signature of the cold accretion model is that the streams of cold gas form a disk as the gas spirals inward, sinking toward the central protogalaxy. Detecting these cold-flow disks could be strong evidence in support of this model and last year, a team of authors reported just such a detection! This year theyre back again with a second object that may provide confirmation of cold accretion from the cosmic web.A Candidate ProtodiskThe team, led by Christopher Martin (California Institute of Technology), made the discovery using the Palomar Cosmic Web Imager, an instrument designed to observe faint emission from the intergalactic medium. Martin and collaborators found a large (R 100 kpc, more than six times the radius of the Milky Way), rotating structure of hydrogen gas, illuminated by the nearby quasi-stellar object QSO HS1549+1919. The system is located at a redshift of z~2.8.The authors testthree potential kinematic models of the candidate protodisk and filaments. In (a) two

  16. Construction of Database for Pulsating Variable Stars

    NASA Astrophysics Data System (ADS)

    Chen, B. Q.; Yang, M.; Jiang, B. W.

    2011-07-01

    A database for the pulsating variable stars is constructed for Chinese astronomers to study the variable stars conveniently. The database includes about 230000 variable stars in the Galactic bulge, LMC and SMC observed by the MACHO (MAssive Compact Halo Objects) and OGLE (Optical Gravitational Lensing Experiment) projects at present. The software used for the construction is LAMP, i.e., Linux+Apache+MySQL+PHP. A web page is provided to search the photometric data and the light curve in the database through the right ascension and declination of the object. More data will be incorporated into the database.

  17. Design of a Hydrogen Pulsating Heat Pipe

    NASA Astrophysics Data System (ADS)

    Liu, Yumeng; Deng, Haoren; Pfotenhauer, John; Gan, Zhihua

    In order to enhance the application of a cryocooler that provides cooling capacity at the cold head location, and effectively spread that cooling over an extended region, one requires an efficient heat transfer method. The pulsating heat pipe affords a highly effective heat transfer component that has been extensively researched at room temperature, but is recently being investigated for cryogenic applications. This paper describes the design. The experimental setup is designed to characterize the thermal performance of the PHP as a function of the applied heat, number of turns, filling ratio, inclination angle, and length of adiabatic section.

  18. Optical multichannel sensing of skin blood pulsations

    NASA Astrophysics Data System (ADS)

    Spigulis, Janis; Erts, Renars; Kukulis, Indulis; Ozols, Maris; Prieditis, Karlis

    2004-09-01

    Time resolved detection and analysis of the skin back-scattered optical signals (reflection photoplethysmography or PPG) provide information on skin blood volume pulsations and can serve for cardiovascular assessment. The multi-channel PPG concept has been developed and clinically verified in this study. Portable two- and four-channel PPG monitoring devices have been designed for real-time data acquisition and processing. The multi-channel devices were successfully applied for cardiovascular fitness tests and for early detection of arterial occlusions in extremities. The optically measured heartbeat pulse wave propagation made possible to estimate relative arterial resistances for numerous patients and healthy volunteers.

  19. Multidimensional modelling of classical pulsating stars

    NASA Astrophysics Data System (ADS)

    Muthsam, H. J.; Kupka, F.

    2016-05-01

    After an overview of general aspects of modelling the pulsation- convection interaction we present reasons why such simulations (in multidimensions) are needed but, at the same time, pose a considerable challenge. We then discuss, for several topics, what insights multidimensional simulations have either already provided or can be expected to yield in the future. We finally discuss properties of our ANTARES code. Many of these features can be expected to be characteristic of other codes which may possibly be applied to these physical questions in the foreseeable future.

  20. Pulsating laminar fully developed channel and pipe flows.

    PubMed

    Haddad, Kais; Ertunç, Ozgür; Mishra, Manoranjan; Delgado, Antonio

    2010-01-01

    Analytical investigations are carried out on pulsating laminar incompressible fully developed channel and pipe flows. An analytical solution of the velocity profile for arbitrary time-periodic pulsations is derived by approximating the pulsating flow variables by a Fourier series. The explicit interdependence between pulsations of velocity, mass-flow rate, pressure gradient, and wall shear stress are shown by using the proper dimensionless parameters that govern the flow. Utilizing the analytical results, the scaling laws for dimensionless pulsation amplitudes of the velocity, mass-flow rate, pressure gradient, and wall shear stress are analyzed as functions of the dimensionless pulsation frequency. Special attention has been given to the scaling laws describing the flow reversal phenomenon occurring in pulsating flows, such as the condition for flow reversal, the dependency of the reversal duration, and the amplitude. It is shown that two reversal locations away from the wall can occur in pulsating flows in pipes and channels and the reversed amount of mass per period reaches a maximum at a certain dimensionless frequency for a given amplitude of mass-flow rate fluctuations. These analyses are numerically conducted for pipe and channel flows over a large frequency range in a comparative manner. PMID:20365456

  1. Pulsating Instability of Turbulent Thermonuclear Flames in Type Ia Supernovae

    NASA Astrophysics Data System (ADS)

    Poludnenko, Alexei Y.

    2014-01-01

    Presently, one of the main explosion scenarios of type Ia supernovae (SNIa), aimed at explaining both "normal" and subluminous events, is the thermonuclear incineration of a white-dwarf in a single-degenerate system. The underlying engine of such explosions is the turbulent thermonuclear flame. Modern, large-scale, multidimensional simulations of SNIa cannot resolve the internal flame structure, and instead must include a subgrid-scale prescription for the turbulent-flame properties. As a result, development of robust, parameter-free, large-scale models of SNIa crucially relies on the detailed understanding of the turbulent flame properties during each stage of the flame evolution. Due to the complexity of the flame dynamics, such understanding must be validated by the first-principles direct numerical simulations (DNS). In our previous work, we showed that sufficiently fast turbulent flames are inherently susceptible to the development of detonations, which may provide the mechanism for the deflagration-to-detonation transition (DDT) in the delayed-detonation model of SNIa. Here we extend this study by performing detailed analysis of the turbulent flame properties at turbulent intensities below the critical threshold for DDT. We carried out a suite of 3D DNS of turbulent flames for a broad range of turbulent intensities and system sizes using a simplified, single-step, Arrhenius-type reaction kinetics. Our results show that at the later stages of the explosion, as the turbulence intensity increases prior to the possible onset of DDT, the flame front will become violently unstable. We find that the burning rate exhibits periodic pulsations with the energy release rate varying by almost an order of magnitude. Furthermore, such flame pulsations can produce pressure waves and shocks as the flame speed approaches the critical Chapman-Jouguet deflagration speed. Finally, in contrast with the current theoretical understanding, such fast turbulent flames can propagate at

  2. Chandra Discovers Eruption and Pulsation in Nova Outburst

    NASA Astrophysics Data System (ADS)

    2001-09-01

    NASA's Chandra X-ray Observatory has discovered a giant outburst of X-rays and unusual cyclical pulsing from a white dwarf star that is closely orbiting another star -- the first time either of these phenomena has been seen in X-rays. The observations are helping scientists better understand the thermonuclear explosions that occur in certain binary star systems. The observations of Nova Aquilae were reported today at the "Two Years of Science with Chandra" symposium by an international team led by Sumner Starrfield of Arizona State University. "We found two important results in our Chandra observations. The first was an underlying pulsation every 40 minutes in the X-ray brightness, which we believe comes from the cyclical expansion and contraction of the outer layers of the white dwarf," said Starrfield. "The other result was an enormous flare of X-rays that lasted for 15 minutes. Nothing like this has been seen before from a nova, and we don't know how to explain it." Novas occur on a white dwarf (a star which used up all its nuclear fuel and shrank to roughly the size of the Earth) that is orbiting a normal size star. Strong gravity tides drag hydrogen gas off the normal star and onto the white dwarf, where it can take more than 100,000 years for enough hydrogen to accumulate to ignite nuclear fusion reactions. Gradually, these reactions intensify until a cosmic-sized hydrogen bomb blast results. The outer layers of the white dwarf are then blown away, producing a nova outburst that can be observed for a period of months to years as the material expands into space. "Chandra has allowed us to see deep into the gases ejected by this giant explosion and extract unparalleled information on the evolution of the white dwarf whose surface is exploding," said Jeremy Drake of the Harvard-Smithsonian Center for Astrophysics. The brightening of Nova Aquilae was first detected by optical astronomers in December 1999. "Although this star is at a distance of more than 6

  3. 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.

  4. 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.

  5. Pulsation, mass loss, and evolution of upper asymptotic giant branch stars

    NASA Astrophysics Data System (ADS)

    Bedijn, P. J.

    1988-10-01

    model of the evolution on the upper AGB for stars with various main sequence masses. We incorporate mass loss according to the expression derived beginning at the moment a star starts to pulsate in the fundamental mode; this we assume is the onset of AGB evolution as a Mira variable. The moment when fundamental mode pulsation sets in is obtained from recent stellar pulsation models. We use this model of upper AGB evolution together with an essentially constant past star formation rate in the solar neighbourhood to obtain a model fit to the period distribution of classical Miras as well as to the 1612 MHz OH luminosity function of OH emitting Miras and OH/IR stars in the solar neighbourhood. The average past star formation rate as obtained from fitting the observed period distribution of classical Miras is in good agreement with that obtained by analysing the present day mass function of solar neighbourhood stars as outlined in Scalo (1986). We also calculate a model mass distribution for white dwarfs and a local white dwarf formation rate in good agreement with observations.

  6. Heat transfer characteristics of pulsated turbulent pipe flow

    NASA Astrophysics Data System (ADS)

    Habib, M. A.; Said, S. A. M.; Al-Farayedhi, A. A.; Al-Dini, S. A.; Asghar, A.; Gbadebo, S. A.

    Heat Transfer characteristics of pulsated turbulent pipe flow under different conditions of pulsation frequency, amplitude and Reynolds number were experimentally investigated. The pipe wall was kept at uniform heat flux. Reynolds number was varied from 5000 to 29 000 while frequency of pulsation ranged from 1 to 8 Hz. The results show an enhancement in the local Nusselt number at the entrance region. The rate of enhancement decreased as Re increased. Reduction of heat transfer coefficient was observed at higher frequencies and the effect of pulsation is found to be significant at high Reynolds number. It can be concluded that the effect of pulsation on the mean Nusselt numbers is insignificant at low values of Reynolds number.

  7. 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.

  8. 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

  9. 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.

  10. Recurrent pulsations in Saturn's high latitude magnetosphere

    NASA Astrophysics Data System (ADS)

    Mitchell, D. G.; Carbary, J. F.; Bunce, E. J.; Radioti, A.; Badman, S. V.; Pryor, W. R.; Hospodarsky, G. B.; Kurth, W. S.

    2016-01-01

    Over the course of about 6 h on Day 129, 2008, the UV imaging spectrograph (UVIS) on the Cassini spacecraft observed a repeated intensification and broadening of the high latitude auroral oval into the polar cap. This feature repeated at least 5 times with about a 1 h period, as it rotated in the direction of corotation, somewhat below the planetary rotation rate, such that it moved from noon to post-dusk, and from roughly 77° to 82° northern latitudes during the observing interval. The recurring UV observation was accompanied by pronounced ∼1 h pulsations in auroral hiss power, magnetic perturbations consistent with small-scale field aligned currents, and energetic ion conics and electrons beaming upward parallel to the local magnetic field at the spacecraft location. The magnetic field and particle events are in phase with the auroral hiss pulsation. This event, taken in the context of the more thoroughly documented auroral hiss and particle signatures (seen on many high latitude Cassini orbits), sheds light on the possible driving mechanisms, the most likely of which are magnetopause reconnection and/or Kelvin Helmholtz waves.

  11. Computational model of miniature pulsating heat pipes.

    SciTech Connect

    Martinez, Mario J.; Givler, Richard C.

    2013-01-01

    The modeling work described herein represents Sandia National Laboratories (SNL) portion of a collaborative three-year project with Northrop Grumman Electronic Systems (NGES) and the University of Missouri to develop an advanced, thermal ground-plane (TGP), which is a device, of planar configuration, that delivers heat from a source to an ambient environment with high efficiency. Work at all three institutions was funded by DARPA/MTO; Sandia was funded under DARPA/MTO project number 015070924. This is the final report on this project for SNL. This report presents a numerical model of a pulsating heat pipe, a device employing a two phase (liquid and its vapor) working fluid confined in a closed loop channel etched/milled into a serpentine configuration in a solid metal plate. The device delivers heat from an evaporator (hot zone) to a condenser (cold zone). This new model includes key physical processes important to the operation of flat plate pulsating heat pipes (e.g. dynamic bubble nucleation, evaporation and condensation), together with conjugate heat transfer with the solid portion of the device. The model qualitatively and quantitatively predicts performance characteristics and metrics, which was demonstrated by favorable comparisons with experimental results on similar configurations. Application of the model also corroborated many previous performance observations with respect to key parameters such as heat load, fill ratio and orientation.

  12. ACCRETION TORQUES AND MOTION OF THE HOT SPOT ON THE ACCRETING MILLISECOND PULSAR XTE J1807-294

    SciTech Connect

    Patruno, Alessandro; Wijnands, R.; Van der Klis, Michiel; Hartman, Jacob M.; Chakrabarty, Deepto

    2010-07-10

    We present a coherent timing analysis of the 2003 outburst of the accreting millisecond pulsar (AMXP) XTE J1807-294. We find a 95% confidence interval for the pulse frequency derivative of (+0.7, + 4.7) x 10{sup -14} Hz s{sup -1} and (-0.6, + 3.8) x 10{sup -14} Hz s{sup -1} for the fundamental and second harmonics, respectively. The sinusoidal fractional amplitudes of the pulsations are the highest observed among AMXPs and can reach values of up to 27% (2.5-30 keV). The pulse arrival time residuals of the fundamental frequency follow a linear anti-correlation with the fractional amplitudes that suggests hot spot motion both in longitude and latitude over the surface of the neutron star. An anti-correlation between residuals and X-ray flux suggests an influence of the accretion rate on pulse phase and casts doubts on the interpretation of pulse frequency derivatives in terms of changes of spin rates and torques on the neutron star.

  13. 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

  14. 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.

  15. Propeller effect in action in the ultraluminous accreting magnetar M82 X-2

    NASA Astrophysics Data System (ADS)

    Tsygankov, Sergey S.; Mushtukov, Alexander A.; Suleimanov, Valery F.; Poutanen, Juri

    2016-03-01

    We present here the first convincing observational manifestation of a magnetar-like magnetic field in an accreting neutron star in binary system - the first pulsating ultraluminous X-ray source X-2 in the galaxy M82. Using the Chandra X-ray observatory data, we show that the source exhibit the bimodal distribution of the luminosity with two well-defined peaks separated by a factor of 40. This behaviour can be interpreted as the action of the `propeller regime' of accretion. The onset of the propeller in a 1.37 s pulsar at luminosity of ˜1040 erg s-1 implies the dipole component of the neutron star magnetic field of ˜1014 G.

  16. 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.

  17. 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.

  18. KIC 7668647: a 14 day beaming sdB+WD binary with a pulsating subdwarf

    NASA Astrophysics Data System (ADS)

    Telting, J. H.; Baran, A. S.; Nemeth, P.; Østensen, R. H.; Kupfer, T.; Macfarlane, S.; Heber, U.; Aerts, C.; Geier, S.

    2014-10-01

    The recently discovered subdwarf B (sdB) pulsator KIC 7668647 is one of the 18 pulsating sdB stars detected in the Kepler field. It features a rich g-mode frequency spectrum, with a few low-amplitude p-modes at short periods. This makes it a promising target for a seismic study aiming to constrain the internal structure of this star, and of sdB stars in general. We use new ground-based low-resolution spectroscopy, and the near-continuous 2.88 year Kepler light curve, to reveal that KIC 7668647 consists of a subdwarf B star with an unseen white-dwarf companion with an orbital period of 14.2 d. An orbit with a radial-velocity amplitude of 39 km s-1 is consistently determined from the spectra, from the orbital Doppler beaming seen by Kepler at 163 ppm, and from measuring the orbital light-travel delay of 27 s by timing of the many pulsations seen in the Kepler light curve. The white dwarf has a minimum mass of 0.40 M⊙. We use our high signal-to-noise average spectra to study the atmospheric parameters of the sdB star, and find that nitrogen and iron have abundances close to solar values, while helium, carbon, oxygen and silicon are underabundant relative to the solar mixture. We use the full Kepler Q06-Q17 light curve to extract 132 significant pulsation frequencies. Period-spacing relations and multiplet splittings allow us to identify the modal degree ℓ for the majority of the modes. Using theg-mode multiplet splittings we constrain the internal rotation period at the base of the envelope to 46-48 d as a first seismic result for this star. The few p-mode splittings may point at a slightly longer rotation period further out in the envelope of the star. From mode-visibility considerations we derive that the inclination of the rotation axis of the sdB in KIC 7668647 must be around ~60°. Furthermore, we find strong evidence for a few multiplets indicative of degree 3 ≤ ℓ ≤ 8, which is another novelty in sdB-star observations made possible by Kepler. Based on

  19. The ionospheric signature of Pi 2 pulsations observed by STARE

    SciTech Connect

    Sutcliffe, P.R. ); Nielsen, E. )

    1992-07-01

    This study extends the work of Sutcliffe and Nielsen (1990) in which a classical Pi 2 pulsation was first isolated in Scandinavian Twin Auroral Radar Experiment (STARE) data. A high-pass-filtering technique is used to remove the background electric field in the STARE data and so reveal the spatial and temporal ionospheric signatures of the Pi 2 pulsation electric fields. A number of events are identified and examples presented in which pulsation electric fields up to 50 mV/m are observed. Magnetic field oscillations computed from the filtered STARE data using the Biot-Savart law correlate well with pulsation magnetometer data. A 180 {degree} phase difference is observed between high- and low-altitude X component pulsations. The ionospheric signature of a Pi 2 is located slightly poleward of the core of the auroral breakup region where the southward, westward, and northward directed background electric fields coverage; the strongest pulsation fields occur in the region of equatorward directed electric fields. The ionospheric electric field patterns of the Pi 2 pulsations determined from the STARE data correlate well with those modeled for a transverse Alfven wave incident on an east-west aligned high-conductivity strip in the ionosphere.

  20. On the standing wave mode of giant pulsations

    SciTech Connect

    Takahashi, K. ); Sato, N. ); Warnecke, J.; Luehr, H. ); Spence, H.E. ); Tonegawa, Y. )

    1992-07-01

    Both odd-mode and even-mode standing were structures have been proposed for giant pulsations. Unless a conclusion is drawn on the field-aligned mode structure, little progress can be made in understanding the excitation mechanism of giant pulsations. In order to determine the standing wave mode, the authors have made a systematic survey of magnetic field data from the AMPTE CCE spacecraft and from ground stations located near the geomagnetic foot point of CCE. They selected time intervals when CCE was close to the magnetic equator and also magnetically close to Syowa and stations in Iceland, and when either transverse or compressional Pc 4 waves were observed at CCE. Magnetograms from the ground stations were then examined to determine if there was a giant pulsation was observed in association with transverse wave events. The CCE magnetic field record for the giant pulsation exhibited a remarkable similarity to a giant pulsation observed from the ATS 6 geostationary satellite near the magnetic equator (Hillebrand et at., 1982). In agreement with Hillebrand et al., they conclude that the compressional nature of the giant pulsation is due to an odd-mode standing wave structure. This conclusion places a strong constraint on the generation mechanism of giant pulsations.

  1. Pulsations in the free oscillations of the Earth

    NASA Astrophysics Data System (ADS)

    Sobolev, G. A.

    2015-05-01

    The records from wideband IRIS stations after a strong earthquake are analyzed. A few days after the earthquake, pulsations with a period of 128 min arise and last for about a week. They appear as a periodical variation in the amplitude of the free radial oscillation of the Earth 0S0 having a period of 20.46 min. The period of the pulsations is more than double the period of the lowest-frequency free spheroidal oscillations of the Earth (53.9 min). The pulsations are most pronounced at the mid-latitudinal and equatorial stations and less distinct near the poles. The pulsations are phase synchronous at the nearby stations and antiphase at the stations located in the western and eastern hemispheres. The pulsation amplitude does not depend on the phase of the Earth's tide. The shape and period of the pulsations are fitted by the model of beatings appearing in the Van der Pol oscillator with periodic forcing. The pulsations are hypothesized to result from asynchronous interaction between the free oscillations of the Earth.

  2. DETECTION OF ACCRETION X-RAYS FROM QS Vir: CATACLYSMIC OR A LOT OF HOT AIR?

    SciTech Connect

    Matranga, Marco; Drake, Jeremy J.; Kashyap, Vinay; Steeghs, Danny

    2012-03-10

    An XMM-Newton observation of the nearby 'pre-cataclysmic' short-period (P{sub orb} = 3.62 hr) binary QS Vir (EC 13471-1258) revealed regular narrow X-ray eclipses when the white dwarf passed behind its M2-4 dwarf companion. The X-ray emission provides a clear signature of mass transfer and accretion onto the white dwarf. The low-resolution XMM-Newton EPIC spectra are consistent with a cooling flow model and indicate an accretion rate of M-dot = 1.7 Multiplication-Sign 10{sup -13} M{sub sun} yr{sup -1}. At 48 pc distant, QS Vir is then the second nearest accreting cataclysmic variable known, with one of the lowest accretion rates found to date for a non-magnetic system. To feed this accretion through a wind would require a wind mass-loss rate of M-dot {approx}2 Multiplication-Sign 10{sup -12} M{sub sun} yr{sup -1} if the accretion efficiency is of the order of 10%. Consideration of likely mass-loss rates for M dwarfs suggests this is improbably high and pure wind accretion unlikely. A lack of accretion disk signatures also presents some difficulties for direct Roche lobe overflow. We speculate that QS Vir is on the verge of Roche lobe overflow, and that the observed mass transfer could be supplemented by upward chromospheric flows on the M dwarf, analogous to spicules and mottles on the Sun, that escape the Roche surface to be subsequently swept up into the white dwarf Roche lobe. If so, QS Vir would be in a rare evolutionary phase lasting only a million years. The X-ray luminosity of the M dwarf estimated during primary eclipse is L{sub X} = 3 Multiplication-Sign 10{sup 28} erg s{sup -1}, which is consistent with that of rapidly rotating 'saturated' K and M dwarfs.

  3. The stellar seismology of hot white dwarfs and planetary nebula nuclei

    NASA Technical Reports Server (NTRS)

    Kawaler, Steven D.

    1987-01-01

    The pulsation properties of hot white dwarfs make it possible to determine their mass, surface composition, rotation, and rate of evolution, and provide constraints on their internal structure. Period spacings are sensitive measures of stellar mass and indicate surface layer structure. Measurement of the rate of period change for these stars provide a way to determine their cooling rates. Attention is also given to how well (or poorly) models of excitation of the pulsations fit within current models of planetary nebula nuclei and hot white dwarfs.

  4. Accretion Disk Outflows from Compact Object Mergers

    NASA Astrophysics Data System (ADS)

    Metzger, Brian

    Nuclear reactions play a key role in the accretion disks and outflows associated with the merger of binary compact objects and the central engines of gamma-ray bursts and supernovae. The proposed research program will investigate the impact of nucleosynthesis on these events and their observable signatures by means of analytic calculations and numerical simulations. One focus of this research is rapid accretion following the tidal disruption of a white dwarf (WD) by a neutron star (NS) or black hole (BH) binary companion. Tidal disruption shreds the WD into a massive torus composed of C, O, and/or He, which undergoes nuclear reactions and burns to increasingly heavier elements as it flows to smaller radii towards the central compact object. The nuclear energy so released is comparable to that released gravitationally, suggesting that burning could drastically alter the structure and stability of the accretion flow. Axisymmetric hydrodynamic simulations of the evolution of the torus including nuclear burning will be performed to explore issues such as the mass budget of the flow (accretion vs. outflows) and its thermal stability (steady burning and accretion vs. runaway explosion). The mass, velocity, and composition of outflows from the disk will be used in separate radiative transfer calculations to predict the lightcurves and spectra of the 56Ni-decay powered optical transients from WD-NS/WD-BH mergers. The possible connection of such events to recently discovered classes of sub-luminous Type I supernovae will be assessed. The coalescence of NS-NS/NS-BH binaries also results in the formation of a massive torus surrounding a central compact object. Three-dimensional magnetohydrodynamic simulations of the long-term evolution of such accretion disks will be performed, which for the first time follow the effects of weak interactions and the nuclear energy released by Helium recombination. The nucleosynthetic yield of disk outflows will be calculated using a detailed

  5. Heat transfer coefficients for drying in pulsating flows

    SciTech Connect

    Fraenkel, S.L.

    1998-05-01

    Pulsating flows generated by a Rijke type combustor are studied for drying of grains and food particles. It is assumed that the velocity fluctuations are the main factor in the enhancement of the drying process. The heat transfer coefficients for drying in vibrating beds are utilized to estimate the heat transfer coefficients of fixed beds in pulsating and permeating flows and are compared to the steady flow heat transfer coefficients obtained for solid porous bodies, after perturbing the main flow. The cases considered are compared to the convective heat transfer coefficients employed in non-pulsating drying.

  6. The attractor dimension of solar decimetric radio pulsations

    NASA Technical Reports Server (NTRS)

    Kurths, J.; Benz, A. O.; Aschwanden, M. J.

    1991-01-01

    The temporal characteristics of decimetric pulsations and related radio emissions during solar flares are analyzed using statistical methods recently developed for nonlinear dynamic systems. The results of the analysis is consistent with earlier reports on low-dimensional attractors of such events and yield a quantitative description of their temporal characteristics and hidden order. The estimated dimensions of typical decimetric pulsations are generally in the range of 3.0 + or - 0.5. Quasi-periodic oscillations and sudden reductions may have dimensions as low as 2. Pulsations of decimetric type IV continua have typically a dimension of about 4.

  7. Ambiguity of mapping the relative phase of blood pulsations

    PubMed Central

    Teplov, Victor; Nippolainen, Ervin; Makarenko, Alexander A.; Giniatullin, Rashid; Kamshilin, Alexei A.

    2014-01-01

    Blood pulsation imaging (BPI) is a non-invasive optical method based on photoplethysmography (PPG). It is used for the visualization of changes in the spatial distribution of blood in the microvascular bed. BPI specifically allows measurements of the relative phase of blood pulsations and using it we detected a novel type of PPG fast waveforms, which were observable in limited areas with asynchronous regional blood supply. In all subjects studied, these fast waveforms coexisted with traditional slow waveforms of PPG. We are therefore presenting a novel lock-in image processing technique of blood pulsation imaging, which can be used for detailed temporal characterization of peripheral microcirculation. PMID:25401026

  8. Total-pressure averaging in pulsating flows.

    NASA Technical Reports Server (NTRS)

    Krause, L. N.; Dudzinski, T. J.; Johnson, R. C.

    1972-01-01

    A number of total-pressure tubes were tested in a nonsteady flow generator in which the fraction of period that pressure is a maximum is approximately 0.8, thereby simulating turbomachine-type flow conditions. Most of the tubes indicated a pressure which was higher than the true average. Organ-pipe resonance which further increased the indicated pressure was encountered with the tubes at discrete frequencies. There was no obvious combination of tube diameter, length, and/or geometry variation used in the tests which resulted in negligible averaging error. A pneumatic-type probe was found to measure true average pressure and is suggested as a comparison instrument to determine whether nonlinear averaging effects are serious in unknown pulsation profiles.

  9. THE PULSATION MODE OF THE CEPHEID POLARIS

    SciTech Connect

    Turner, D. G.; Kovtyukh, V. V.; Usenko, I. A.; Gorlova, N. I.

    2013-01-01

    A previously derived photometric parallax of 10.10 {+-} 0.20 mas, d = 99 {+-} 2 pc, is confirmed for Polaris by a spectroscopic parallax derived using line ratios in high dispersion spectra for the Cepheid. The resulting estimates for the mean luminosity of (M{sub V} ) = -3.07 {+-} 0.01 s.e., average effective temperature of (T{sub eff}) = 6025 {+-} 1 K s.e., and intrinsic color of ((B) - (V)){sub 0} = +0.56 {+-} 0.01 s.e., which match values obtained previously from the photometric parallax for a space reddening of E{sub B-V} = 0.02 {+-} 0.01, are consistent with fundamental mode pulsation for Polaris and a first crossing of the instability strip, as also argued by its rapid rate of period increase. The systematically smaller Hipparcos parallax for Polaris appears discrepant by comparison.

  10. Optical noninvasive monitoring of skin blood pulsations

    NASA Astrophysics Data System (ADS)

    Spigulis, Janis

    2005-04-01

    Time-resolved detection and analysis of skin backscattered optical signals (remission photoplethysmography or PPG) provide rich information on skin blood volume pulsations and can serve for reliable cardiovascular assessment. Single- and multiple-channel PPG concepts are discussed. Simultaneous data flow from several locations on the human body allows us to study heartbeat pulse-wave propagation in real time and to evaluate vascular resistance. Portable single-, dual-, and four-channel PPG monitoring devices with special software have been designed for real-time data acquisition and processing. The prototype devices have been clinically studied, and their potential for monitoring heart arrhythmias, drug-efficiency tests, steady-state cardiovascular assessment, body fitness control, and express diagnostics of the arterial occlusions has been confirmed.

  11. Interplanetary navigation using pulsating radio sources

    NASA Technical Reports Server (NTRS)

    Downs, G. S.

    1974-01-01

    Radio beacons with distinguishing signatures exist in nature as pulsating radio sources (pulsars). These objects radiate well determined pulse trains over hundreds of megahertz of bandwidth at radio frequencies. Since they are at known positions, they can also be used as navigation beacons in interplanetary space. Pulsar signals are weak and dispersive when viewed from earth. If an omnidirectional antenna is connected to a wideband receiver (200 MHz bandwidth centered at 200 MHz) in which dispersion effects are removed, nominal spacecraft position errors of 1500 km can be obtained after 24 h of signal integration. An antenna gain of 10 db would produce errors as low as 150 km. Since the spacecraft position is determined from the measurement of the phase of a periodic signal, ambiguities occur in the position measurement. Simultaneous use of current spacecraft navigation schemes eliminates these ambiguities.

  12. Total pressure averaging in pulsating flows

    NASA Technical Reports Server (NTRS)

    Krause, L. N.; Dudzinski, T. J.; Johnson, R. C.

    1972-01-01

    A number of total-pressure tubes were tested in a non-steady flow generator in which the fraction of period that pressure is a maximum is approximately 0.8, thereby simulating turbomachine-type flow conditions. Most of the tubes indicated a pressure which was higher than the true average. Organ-pipe resonance which further increased the indicated pressure was encountered within the tubes at discrete frequencies. There was no obvious combination of tube diameter, length, and/or geometry variation used in the tests which resulted in negligible averaging error. A pneumatic-type probe was found to measure true average pressure, and is suggested as a comparison instrument to determine whether nonlinear averaging effects are serious in unknown pulsation profiles. The experiments were performed at a pressure level of 1 bar, for Mach number up to near 1, and frequencies up to 3 kHz.

  13. White Dwarfs in Intermediate Polars

    NASA Astrophysics Data System (ADS)

    Belle, Kunegunda E.; Sion, E. M.

    2009-01-01

    Intermediate polars (IPs), magnetic cataclysmic variables (CVs) in which the white dwarf (WD) has an intermediate strength magnetic field (B< 5 MG), present an interesting laboratory for the study of the evolution of CVs as they contain elements of both non-magnetic and magnetic systems. Do magnetic CVs and IPs evolve in the same manner as non-magnetic systems? One answer in this puzzle may come from understanding the nature of the white dwarf in a magnetic CV. Standard CV evolution theory predicts a white dwarf temperature for a given CV orbital period and accretion rate. By investigating the temperature of white dwarfs in IPs and comparing the temperatures to those predicted from theory, we can learn where IPs fit into the model of CV evolution. Here we present the results of our continued study of the nature of WDs in IPs. We compare temperatures derived from model fits to UV spectra with temperatures calculated based on the accretion rate and binary orbital period. Our preliminary results indicate that IPs follow the general trend of magnetic CVs containing cooler WDs than non-magnetic CVs.

  14. Gaia --- A White Dwarf Discovery Machine

    NASA Astrophysics Data System (ADS)

    Jordan, S.

    2007-09-01

    Gaia is a satellite mission of the ESA, aiming at absolute astrometric measurements of about one billion stars (V<20) with unprecedented accuracy. Additionally, magnitudes and colors will be obtained for all these stars, while radial-velocities will be determined only for bright objects (V<17.5). However, the wavelength range for the radial-velocity instrument is rather unsuitable for most white dwarfs. Gaia will probably discover about 400,000 white dwarfs; up to 100 pc the detection probability for white dwarfs is almost 100 %. This survey of white dwarfs will have very clear, easy to understand selection criteria, and will therefore be very suitable for statistical investigations. The Gaia data will help to improve the construction of a luminosity function for the disk and the halo and will provide a more accurate determination of the age of our solar neighborhood. Moreover, reliable stellar dynamical investigations of the disk and halo components will be possible. For the first time it will be possible to test the mass-radius relation of white dwarfs in great detail. Moreover, more accurate masses of magnetic and cool white dwarfs can be expected. Gaia is also expected to discover many new pulsating white dwarfs. The Gaia measurements can also complement the measurements of gravitational waves from close white dwarf binaries with Lisa.

  15. Revealing the pulsational properties of the V777 Herculis star KUV 05134+2605 by its long-term monitoring

    NASA Astrophysics Data System (ADS)

    Bognár, Zs.; Paparó, M.; Córsico, A. H.; Kepler, S. O.; Győrffy, Á.

    2014-10-01

    Context. KUV 05134+2605 is one of the 21 pulsating DB white dwarfs (V777 Her or DBV variables) known so far. The detailed investigation of the short-period and low-amplitude pulsations of these relatively faint targets requires considerable observational efforts from the ground, long-term single-site or multi-site observations. The observed amplitudes of excited modes undergo short-term variations in many cases, which makes determining pulsation modes difficult. Aims: We aim to determine the pulsation frequencies of KUV 05134+2605, find regularities between the frequency and period components, and perform an asteroseismic investigation for the first time. Methods: We re-analysed the published data and collected new measurements. We compared the frequency content of the different datasets from the different epochs and performed various tests to check the reliability of the frequency determinations. The mean period spacings were investigated with linear fits to the observed periods, Kolmogorov-Smirnov and inverse variance significance tests, and with a Fourier analysis of different period sets, including a Monte Carlo test that simulated the effect of alias ambiguities. We employed fully evolutionary DB white dwarf models for the asteroseismic investigations. Results: We identified 22 frequencies between 1280 and 2530 μHz. These form 12 groups, which suggests at least 12 possible frequencies for the asteroseismic investigations. Thanks to the extended observations, KUV 05134+2605 joined the group of rich white dwarf pulsators. We identified one triplet and at least one doublet with a ≈ 9 μHz frequency separation, from which we derived a stellar rotation period of 0.6 d. We determined the mean period spacings of ≈ 31 s and 18 s for the modes we propose as dipole and quadrupole. We found an excellent agreement between the stellar mass derived from the ℓ = 1 period spacing and the period-to-period fits, all providing M∗ = 0.84 - 0.85 M⊙ solutions. Our study

  16. A NEW ACCRETION DISK AROUND THE MISSING LINK BINARY SYSTEM PSR J1023+0038

    SciTech Connect

    Patruno, A.; Archibald, A. M.; Hessels, J. W. T.; Bassa, C. G.; Janssen, G. H.; Bogdanov, S.; Stappers, B. W.; Lyne, A. G.; Kaspi, V. M.; Tendulkar, S.

    2014-01-20

    PSR J1023+0038 is an exceptional system for understanding how slowly rotating neutron stars are spun up to millisecond rotational periods through accretion from a companion star. Observed as a radio pulsar from 2007-2013, optical data showed that the system had an accretion disk in 2000/2001. Starting at the end of 2013 June, the radio pulsar has become undetectable, suggesting a return to the previous accretion-disk state, where the system more closely resembles an X-ray binary. In this Letter we report the first targeted X-ray observations ever performed of the active phase and complement them with UV/optical and radio observations collected in 2013 October. We find strong evidence that indeed an accretion disk has recently formed in the system and we report the detection of fast X-ray changes spanning about two orders of magnitude in luminosity. No radio pulsations are seen during low flux states in the X-ray light curve or at any other times.

  17. 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.

  18. 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.

  19. Photometric study of the pulsating, eclipsing binary OO DRA

    SciTech Connect

    Zhang, X. B.; Deng, L. C.; Tian, J. F.; Wang, K.; Yan, Z. Z.; Luo, C. Q.; Sun, J. J.; Liu, Q. L.; Xin, H. Q.; Zhou, Q.; Luo, Z. Q.

    2014-12-01

    We present a comprehensive photometric study of the pulsating, eclipsing binary OO Dra. Simultaneous B- and V-band photometry of the star was carried out on 14 nights. A revised orbital period and a new ephemeris were derived from the data. The first photometric solution of the binary system and the physical parameters of the component stars are determined. They reveal that OO Dra could be a detached system with a less-massive secondary component nearly filling its Roche lobe. By subtracting the eclipsing light changes from the data, we obtained the intrinsic pulsating light curves of the hotter, massive primary component. A frequency analysis of the residual light yields two confident pulsation modes in both B- and V-band data with the dominant frequency detected at 41.865 c/d. A brief discussion concerning the evolutionary status and the pulsation nature of the binary system is finally given.

  20. Pulsations of B stars: A review of observations and theories

    SciTech Connect

    Cox, A.N.

    1986-01-01

    The observational and theoretical status are discussed for several classes of variable B stars. The older classes now seem to be better understood in terms of those stars that probably have at least one radial mode and those that have only nonradial modes. The former are the ..beta.. Cephei variables, and the latter are the slowly rotating 53 Persei and the rapidly rotating zeta Ophiuchi variables. It seems that in this last class there are also some Be stars that show nonradial pulsations from the variations of the line shapes and their light. Among the nonradial pulsators, we must also include the supergiants which show pulsations with very short lifetimes. A review of the present observational and theoretical problems is given. The most persistent problem of the cause for the pulsations is briefly discussed, and many proposed mechanisms plus some new thoughts are presented. 57 refs., 4 figs.

  1. Report of geomagnetic pulsation indices for space weather applications

    USGS Publications Warehouse

    Xu, Z.; Gannon, Jennifer L.; Rigler, Erin J.

    2013-01-01

    The phenomenon of ultra-low frequency geomagnetic pulsations was first observed in the ground-based measurements of the 1859 Carrington Event and has been studied for over 100 years. Pulsation frequency is considered to be “ultra” low when it is lower than the natural frequencies of the plasma, such as the ion gyrofrequency. Ultra-low frequency pulsations are considered a source of noise in some geophysical analysis techniques, such as aeromagnetic surveys and transient electromagnetics, so it is critical to develop near real-time space weather products to monitor these geomagnetic pulsations. The proper spectral analysis of magnetometer data, such as using wavelet analysis techniques, can also be important to Geomagnetically Induced Current risk assessment.

  2. Studying geomagnetic pulsation characteristics with the local approximation method

    NASA Astrophysics Data System (ADS)

    Getmanov, V. G.; Dabagyan, R. A.; Sidorov, R. V.

    2016-03-01

    A local approximation method based on piecewise sinusoidal models has been proposed in order to study the frequency and amplitude characteristics of geomagnetic pulsations registered at a network of magnetic observatories. It has been established that synchronous variations in the geomagnetic pulsation frequency in the specified frequency band can be studied with the use of calculations performed according to this method. The method was used to analyze the spectral-time structure of Pc3 geomagnetic pulsations registered at the network of equatorial observatories. Local approximation variants have been formed for single-channel and multichannel cases of estimating the geomagnetic pulsation frequency and amplitude, which made it possible to decrease estimation errors via filtering with moving weighted averaging.

  3. Unilateral Loss of Spontaneous Venous Pulsations in an Astronaut

    NASA Technical Reports Server (NTRS)

    Mader, Thomas H.; Gibson, C. Robert; Lee, Andrew G.; Patel, Nimesh; Hart, Steven; Pettit, Donald R.

    2014-01-01

    Spontaneous venous pulsations seen on the optic nerve head (optic disc) are presumed to be caused by fluctuations in the pressure gradient between the intraocular and retrolaminar venous systems. The disappearance of previously documented spontaneous venous pulsations is a well-recognized clinical sign usually associated with a rise in intracranial pressure and a concomitant bilateral elevation of pressure in the subarachnoid space surrounding the optic nerves. In this correspondence we report the unilateral loss of spontaneous venous pulsations in an astronaut 5 months into a long duration space flight. We documented a normal lumbar puncture opening pressure 8 days post mission. The spontaneous venous pulsations were also documented to be absent 21 months following return to Earth.. We hypothesize that these changes may have resulted from a chronic unilateral rise in optic nerve sheath pressure caused by a microgravity-induced optic nerve sheath compartment syndrome.

  4. Canards in a rheodynamic model of cardiac pressure pulsations

    NASA Astrophysics Data System (ADS)

    Xie, Feng; Chen, Xian-Feng

    2007-09-01

    This paper reports on the canard phenomenon occurring in a rheodynamic model of cardiac pressure pulsations. By singular perturbation techniques the corresponding parameter value at which canards exist is obtained. The physiological significance of canards in this model is given.

  5. 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.

  6. Observation and modeling of compressional Pi 3 magnetic pulsations

    NASA Technical Reports Server (NTRS)

    Matsuoka, Hitoshi; Takahashi, K.; Yumoto, K.; Anderson, B. J.; Sibeck, D. G.

    1995-01-01

    Compressional magnetic pulsations with irregular waveforms and periods longer than 150 s (here termed Pi 3) have been studied by using data from Active Magnetospheric Particle Tracer Explorers Charge Composition Explorer (AMPTE/CCE) and GOES 5 and 6 in the dayside magnetosphere and compared with signatures on the ground at low latitudes by using data from Kakioka station (L = 1.25). On the ground, the pulsations appear in the horizontal component. A study of 17 such concurrent events during a 2-month period in 1986 reveals the following pulsation characteristics. (1) The peak-to-peak amplitudes in space (delta B(sub T)) and on the ground (delta H) are comparable and are in the range of 0.5-7 nT. (2) On the ground the pulsations can be seen at all local times, even at midnight, while at geostationary orbit they are observed only on the dayside with a clear amplitude maximum at noon. (3) The pulsations on the ground lag those observed by CCE near local noon, and the lag increases as the local time separation between CCE and the ground station increases. The time lag is 1-2 min longer when the ground station is on the nightside than when it is on the dayside. (4) The time lag between pulsations observed at geostationary orbit and near noon by CCE varies systematically with local time and is about 2 min per 6 hours of local time separation. These observations indicate that some nightside pulsations in the Pi 3 band have dayside origins. The position dependence of the pulsation amplitude can be explained well by changes in the magnetopause current, which are in turn presumably caused by changes in the solar wind dynamic pressure. The time lags observed in space are consistent with signal propagation in the MHD fast mode, but the variation in space-ground time lags with ground station local time must be attributed to another mechanism.

  7. Ionospheric ion heating by ULF Pc 5 magnetic pulsations

    SciTech Connect

    Lathuillere, C.; Glangeaud, F.; Zhao, Z.Y.

    1986-02-01

    Frictional heating of the ions resulting from dc ionospheric electric fields is experimentally and theoretically well known. We extend these results to ion heating due to ULF magnetic pulsations of periods as low as 3 min. Ion temperature fluctuations as measured by the European incoherent scatter facility are very well correlated to magnetic Pc 5 pulsations. We present a method which estimates these ion temperature enhancements from ion velocity measurements.

  8. 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.

  9. Non-Invasive Measurement of Intracranial Pressure Pulsation using Ultrasound

    NASA Technical Reports Server (NTRS)

    Ueno, Toshiaki; Ballard, R. E.; Yost, W. T.; Hargens, A. R.

    1997-01-01

    Exposure to microgravity causes a cephalad fluid shift which may elevate intracranial pressure (ICP). Elevation in ICP may affect cerebral hemodynamics in astronauts during space flight. ICP is, however, a difficult parameter to measure due to the invasiveness of currently available techniques. We already reported our development of a non-invasive ultrasound device for measurement of ICP. We recently modified the device so that we might reproducibly estimate ICP changes in association with cardiac cycles. In the first experiment, we measured changes in cranial distance with the ultrasound device in cadavera while changing ICP by infusing saline into the lateral ventricle. In the second experiment, we measured changes in cranial distance in five healthy volunteers while placing them in 60 deg, 30 deg head-up tilt, supine, and 10 deg head-down tilt position. In the cadaver study, fast Fourier transformation revealed that cranial pulsation is clearly associated with ICP pulsation. The ratio of cranial distance and ICP pulsation is 1.3microns/mmHg. In the tilting study, the magnitudes of cranial pulsation are linearly correlated to tilt angles (r=0.87). The ultrasound device has sufficient sensitivity to detect cranial pulsation in association with cardiac cycles. By analyzing the magnitude of cranial pulsation, estimates of ICP during space flight are possible.

  10. Structure of black aurora associated with pulsating aurora

    NASA Astrophysics Data System (ADS)

    Fritz, Bruce A.; Lessard, Marc L.; Blandin, Matthew J.; Fernandes, Philip A.

    2015-11-01

    Morphological behavior of black aurora as it relates to pulsating aurora is investigated by examining a collection of ground-based observations from January 2007 in support of the Rocket Observations of Pulsating Aurora rocket campaign. Images were sampled from video recorded by a Xybion intensified camera (30 fps) at Poker Flat Research Range, AK. The primary observations of black aurora recorded during the substorm recovery phase were between separate patches of pulsating aurora as well as pulsating aurora separated from diffuse aurora. In these observations the black aurora forms an apparent firm boundary between the auroral forms in a new behavior that is in contrast with previously reported observations. Also presented for the first time are black curls in conjunction with pulsating aurora. Curl structures that indicate shear plasma flows in the ionosphere may be used as a proxy for converging/diverging electric fields in and above the ionosphere. This new subset of black auroral behavior may provide visual evidence of black aurora as an ionospheric feedback mechanism as related to pulsating aurora.

  11. Optical emissions and ionization profiles during an intense pulsating aurora

    SciTech Connect

    Sears, R.D.; Vondrak, R.R.

    1981-08-01

    Coordinated spectrophotometric and incoherent scatter radar measurements were made of an intense early-morning pulsating aurora at Chatanika, Alaska. Both instruments were operated simultaneously with temporal resolution of 1 s and were boresighted at geomagnetic zenith. The goals of the collaborative experimental were to determine the total energy flux E/sub t/ and the characteristic energy parameter ..cap alpha.. of the precipitating electrons on a time scale smaller than the pulsation duration and to investigate the response of the ionospheric electron density on a comparable time scale. The relationship between total energy flux and the characteristic energy parameter suggests that during these observations the pulsations were caused by a modulation of ..cap alpha.. rather than by a modulation of the total precipitating particle flux. The values of E/sub t/ and ..cap alpha.. derived from photometric data were used to model the electron density versus altitude in the ionosphere for comparison with the incoherent scatter radar measurements. Measured altitude profiles of ionization did not change significantly on the time scale of the pulsations. The E region maximum electron density typically was 7 x 10/sup 5/ at an altitude of 99 km. Variations in ionospheric conductivity were less than 15% on the pulsation time scale. These results suggest that a magnetospheric modulation mechanism is required to explain diffuse pulsating auroras.

  12. On the temporal fluctuations of pulsating auroral luminosity

    SciTech Connect

    Yamamoto, Tatsundo )

    1988-02-01

    From a study of all-sky TV records, it is shown that the luminosity fluctuations of pulsating auroras can be understood in terms of a series of pulses with rapid on-off switchings in burstlike fashion and that the widths of successive pulses (pulsation on times) are fairly constant. This is common even when luminosity fluctuations consist of complex-irregular variations, in contrast to the pulsation off time that is significantly variable. Complex-irregular variations are ground to be due to simultaneous appearance of more pulsating patches that exhibit movements eastward and westward over the site, and each of the patches shows primarily isolated luminosity pulses. Several examples are presented and described in detail. A natural consequence of these observations is that the classical concept of period does not mean much and the luminosity fluctuations should be treated as a series of individual isolated pulses where the pulsation on time is the most essential quantity. These characteristics are briefly discussed in relation to VLF/ELF wave-particle interactions in the magnetosphere. Then a new interpretation of the nonlinear relaxation oscillation model is proposed, where the propagation effect of VLF/ELF waves in low energy plasm irregularities near the magnetospheric equatorial plane plays an essential role to produce rapid on-off switchings of precipitating energetic electron fluxes. Both electromagnetic and electrostatic waves are possibly related to the precipitation pulsations.

  13. A brown dwarf mass donor in an accreting binary.

    PubMed

    Littlefair, S P; Dhillon, V S; Marsh, T R; Gänsicke, Boris T; Southworth, John; Watson, C A

    2006-12-01

    A long-standing and unverified prediction of binary star evolution theory is the existence of a population of white dwarfs accreting from substellar donor stars. Such systems ought to be common, but the difficulty of finding them, combined with the challenge of detecting the donor against the light from accretion, means that no donor star to date has a measured mass below the hydrogen burning limit. We applied a technique that allowed us to reliably measure the mass of the unseen donor star in eclipsing systems. We were able to identify a brown dwarf donor star, with a mass of 0.052 +/- 0.002 solar mass. The relatively high mass of the donor star for its orbital period suggests that current evolutionary models may underestimate the radii of brown dwarfs. PMID:17158322

  14. 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.

  15. Accretion Column Structure of Magnetic Cataclysmic Variables from X-ray Spectroscopy

    SciTech Connect

    Hoogerwerf, R; Brickhouse, N S; Mauche, C W

    2006-02-27

    Using Chandra HETG data we present light curves for individual spectral lines of Mg XI and Mg XII for EX Hydrae, an intermediate-polar type cataclysmic variable. The Mg XI light curve, folded on the white dwarf spin period, shows two spikes that are not seen in the Mg XII or broad-band light curves. Occultation of the accretion column by the body of the white dwarf would produce such spikes for an angle between the rotation axis and the accretion columns of {alpha} = 18{sup o} and a height of the Mg XI emission above the white dwarf surface of {approx}< 0.0004 white dwarf radii or {approx}< 4 km. The absence of spikes in the Mg XII and broad-band light curves could then be explained if the bulk of its emission forms at much larger height, > 0.004 white dwarf radii or > 40 km, above the white dwarf surface. The technique described in this letter demonstrates that high signal-to-noise ratio and high spectral resolution X-ray spectra can be used to map the temperature and density structure of accretion flows in magnetic cataclysmic variables. The Mg XI and Mg XII light curves are not consistent with the temperature and density structure predicted by the standard Aizu model.

  16. The pulsating/ellipsoidal variable hot B subdwarf KPD 1930+2752

    NASA Astrophysics Data System (ADS)

    Fontaine, G.

    We propose to measure the projected rotational velocity of KPD 1930+2752, a unique binary system consisting of a sdB primary undergoing acoustic mode pulsations and ellipsoidally distorted by the presence of an unseen white dwarf companion. The orbital period of the system is 2.383 hrs. The measurement of v sin i is the one datum currently missing that prevents us from modeling the ellipsoidal light curve in terms of the basic parameters of the two stellar components. Once this measurement is secured, we should be able to derive the values of the masses and radii of these two stars. We should be able to verify if indeed KPD 1930+2752 is a genuine progenitor of a Type Ia supernova as was recently suggested by a British group. More importantly, by combining the values of the mass and the radius of the sdB pulsator with the period data, we should be able to model, for the first time, the interior structure of a sdB star through asteroseismological techniques. FUSE time tag observations are unique for this type of measurement. They allow us to remove the star's orbital velocity (348 km/s) that inevitably smears out the photospheric lines. We have already successfully implemented such a procedure in the CalFUSE pipeline. The projected equatorial velocity, v sin i, is then determined by comparing the star's line profiles to computed rotation profiles.

  17. PULSE AMPLITUDE DEPENDS ON kHz QPO FREQUENCY IN THE ACCRETING MILLISECOND PULSAR SAX J1808.4-3658

    SciTech Connect

    Bult, Peter; Van der Klis, Michiel

    2015-01-10

    We study the relation between the 300-700 Hz upper kHz quasi-periodic oscillation (QPO) and the 401 Hz coherent pulsations across all outbursts of the accreting millisecond X-ray pulsar SAX J1808.4-3658 observed with the Rossi X-ray Timing Explorer. We find that the pulse amplitude systematically changes by a factor of ∼2 when the upper kHz QPO frequency passes through 401 Hz: it halves when the QPO moves to above the spin frequency and doubles again on the way back. This establishes for the first time the existence of a direct effect of kHz QPOs on the millisecond pulsations and provides a new clue to the origin of the upper kHz QPO. We discuss several scenarios and conclude that while more complex explanations can not formally be excluded, our result strongly suggests that the QPO is produced by azimuthal motion at the inner edge of the accretion disk, most likely orbital motion. Depending on whether this azimuthal motion is faster or slower than the spin, the plasma then interacts differently with the neutron-star magnetic field. The most straightforward interpretation involves magnetospheric centrifugal inhibition of the accretion flow that sets in when the upper kHz QPO becomes slower than the spin.

  18. Ultra high energy gamma rays, cosmic rays and neutrinos from accreting degenerate stars

    NASA Technical Reports Server (NTRS)

    Brecher, K.; Chanmugam, G.

    1985-01-01

    Super-Eddington accretion for a recently proposed unipolar induction model of cosmic ray acceleration in accreting binary star systems containing magnetic white dwarfs or neutron stars is considered. For sufficiently high accretion rates and low magnetic fields, the model can account for: (1) acceleration of cosmic ray nuclei up to energies of 10 to the 19th power eV; (2) production of more or less normal solar cosmic ray composition; (3) the bulk of cosmic rays observed with energies above 1 TeV, and probably even down to somewhat lower energies as well; and (4) possibly the observed antiproton cosmic ray flux. It can also account for the high ultra high energy (UHE) gamma ray flux observed from several accreting binary systems (including Cygnus X-3), while allowing the possibility of an even higher neutrino flux from these sources, with L sub nu/L sub gamma is approximately 100.

  19. ORIGIN OF INTERMITTENT ACCRETION-POWERED X-RAY OSCILLATIONS IN NEUTRON STARS WITH MILLISECOND SPIN PERIODS

    SciTech Connect

    Lamb, Frederick K.; Boutloukos, Stratos; Van Wassenhove, Sandor; Chamberlain, Robert T.; Lo, Ka Ho; Coleman Miller, M.

    2009-11-01

    We have shown previously that many of the properties of persistent accretion-powered millisecond pulsars can be understood if their X-ray emitting areas are near their spin axes and move as the accretion rate and structure of the inner disk vary. Here, we show that this 'nearly aligned moving spot model' may also explain the intermittent accretion-powered pulsations that have been detected in three weakly magnetic accreting neutron stars. We show that movement of the emitting area from very close to the spin axis to approx10 deg. away can increase the fractional rms amplitude from approx<0.5%, which is usually undetectable with current instruments, to a few percent, which is easily detectable. The second harmonic of the spin frequency usually would not be detected, in agreement with observations. The model produces intermittently detectable oscillations for a range of emitting area sizes and beaming patterns, stellar masses and radii, and viewing directions. Intermittent oscillations are more likely in stars that are more compact. In addition to explaining the sudden appearance of accretion-powered millisecond oscillations in some neutron stars with millisecond spin periods, the model explains why accretion-powered millisecond oscillations are relatively rare and predicts that the persistent accretion-powered millisecond oscillations of other stars may become undetectable for brief intervals. It suggests why millisecond oscillations are frequently detected during the X-ray bursts of some neutron stars but not others and suggests mechanisms that could explain the occasional temporal association of intermittent accretion-powered oscillations with thermonuclear X-ray bursts.

  20. PULSATION FREQUENCIES AND MODES OF GIANT EXOPLANETS

    SciTech Connect

    Le Bihan, Bastien; Burrows, Adam E-mail: burrows@astro.princeton.edu

    2013-02-10

    We calculate the eigenfrequencies and eigenfunctions of the acoustic oscillations of giant exoplanets and explore the dependence of the characteristic frequency {nu}{sub 0} and the eigenfrequencies on several parameters: the planet mass, the planet radius, the core mass, and the heavy element mass fraction in the envelope. We provide the eigenvalues for degree l up to 8 and radial order n up to 12. For the selected values of l and n, we find that the pulsation eigenfrequencies depend strongly on the planet mass and radius, especially at high frequency. We quantify this dependence through the calculation of the characteristic frequency {nu}{sub 0} which gives us an estimate of the scale of the eigenvalue spectrum at high frequency. For the mass range 0.5 M{sub J} {<=} M{sub P} {<=} 15 M{sub J} , and fixing the planet radius to the Jovian value, we find that {nu}{sub 0} {approx} 164.0 Multiplication-Sign (M{sub P} /M{sub J} ){sup 0.48}{mu}Hz, where M{sub P} is the planet mass and M{sub J} is Jupiter's mass. For the radius range from 0.9 to 2.0 R{sub J} , and fixing the planet's mass to the Jovian value, we find that {nu}{sub 0} {approx} 164.0 Multiplication-Sign (R{sub P} /R{sub J} ){sup -2.09}{mu}Hz, where R{sub P} is the planet radius and R{sub J} is Jupiter's radius. We explore the influence of the presence of a dense core on the pulsation frequencies and on the characteristic frequency of giant exoplanets. We find that the presence of heavy elements in the envelope affects the eigenvalue distribution in ways similar to the presence of a dense core. Additionally, we apply our formalism to Jupiter and Saturn and find results consistent with both the observational data of Gaulme et al. and previous theoretical work.

  1. White Dwarf Pulsars

    NASA Technical Reports Server (NTRS)

    Patterson, Joseph

    1999-01-01

    This proposal was designed to study pulse and orbital modulations in candidate DQ Herculis stars. Data on 5 stars were obtained. The best results were obtained on YY Draconis, which exhibited a strongly pulsed hard X-ray flux, and even suggested a transition between one-pole and two-pole emission during the course of the observation. This result is being readied for inclusion in a comprehensive study of YY Draconis. A strong pulsation appeared to be present also in H0857-242, but with a period of about 50 minutes, confusion with the first harmonic of the satellite's orbital frequency is possible. So that result is uncertain. A negative result was obtained on 4UO608-49 (V347 Pup), suggesting either that the X-ray identification is incorrect, or that the source is very transient. Finally, data was obtained on V1432 Aql and WZ Sge, respectively the slowest and fastest of these stars. Combined with the Advanced Satellite for Cosmology and Astrophysics (ASCA) data, the high-energy data demonstrates the latter to contain a white dwarf rotating with P = 27.87 s. Optical photometry contemporaneous with the X-ray data was obtained of V1432 Aql, in order to study the variations in the eclipse waveform. As anticipated, the width and centroid of the eclipse appeared to vary with the 50-day "supercycle". A paper reporting this study is now in preparation.

  2. 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.

  3. The Evolved Pulsating CEMP Star HD 112869

    NASA Astrophysics Data System (ADS)

    Začs, Laimons; Sperauskas, Julius; Grankina, Aija; Deveikis, Viktoras; Kaminskyi, Bogdan; Pavlenko, Yakiv; Musaev, Faig A.

    2015-04-01

    Radial velocity measurements, BVRC photometry, and high-resolution spectroscopy in the wavelength region from blue to near-infrared are employed in order to clarify the evolutionary status of the carbon-enhanced metal-poor star HD 112869 with a unique ratio of carbon isotopes in the atmosphere. An LTE abundance analysis was carried out using the method of spectral synthesis and new self-consistent 1D atmospheric models. The radial velocity monitoring confirmed semiregular variations with a peak-to-peak amplitude of about 10 km {{s}-1} and a dominating period of about 115 days. The light, color, and radial velocity variations are typical of the evolved pulsating stars. The atmosphere of HD 112869 appears to be less metal-poor than reported before, [Fe/H] = -2.3 ± 0.2 dex. Carbon-to-oxygen and carbon isotope ratios are found to be extremely high, C/O ≃ 12.6 and12C/13C ≳ 1500, respectively. The s-process elements yttrium and barium are not enhanced, but neodymium appears to be overabundant. The magnesium abundance seems to be lower than the average found for CEMP stars, [Mg/Fe] < +0.4 dex. HD 112869 could be a single low-mass halo star in the stage of asymptotic giant branch evolution.

  4. Fine droplet generation using tunable electrohydrodynamic pulsation

    NASA Astrophysics Data System (ADS)

    Yuan, Xin; Ba, Zhengyu; Xiong, Zhenhua

    2015-07-01

    High-efficiency generation of fine droplets is significant for many microfluidic chips and sensor applications. To produce fine droplets, nozzles with small diameters are needed, which results in a high cost for nozzles and low efficiency of droplet generation. In this paper, a tunable electrohydrodynamic pulsation method which can generate fine droplets with high frequency and controllable size is presented using low conductivity liquids. The effects of flow rates and voltage parameters with respect to deposition frequency and droplet size are investigated. The influence of these parameters on Taylor cone formation time are also discussed and simple scaling laws are proposed to reveal and guide the droplet generation process. Experimental results show that single cycle deposition frequency decreases with increasing voltage frequency, but is only slightly influenced by the flow rates. The droplet size also decreases with voltage frequency, while large flow rates can make this decline gradual allowing better control. Moreover, the Taylor cone formation time may greatly affect the stability of the deposition frequency when the voltage frequency is larger than 30 Hz. Due to the short cycle time of high voltage frequencies, the hydrodynamic behavior in the emission process may be considerably affected by the increase of volume, which is also related to the flow rates. Tunable micropatterns consisting of fine droplets can be achieved by using this method in combination with motion stages.

  5. 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.

  6. 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.

  7. 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.

  8. Theories of white dwarf oscillations

    NASA Technical Reports Server (NTRS)

    Vanhorn, H. M.

    1980-01-01

    The current status of theoretical understanding of the oscillations observed in the ZZ Ceti stars and cataclysmic variables is briefly reviewed. Nonradial g-mode oscillations appear to provide a satisfactory explanation for the low amplitude variables such as R548, with periods in the range of approximately 200 to 300 seconds, but for the longer period (800 to 1000 seconds) oscillators, the situation is still unclear. Rotation may play an important role in this problem, and the effects of both slow and fast rotation upon the mode structure are discussed. In the cataclysmic variables, both accretion and thermonuclear burning may act to excite oscillations of the white dwarf.

  9. Theoretical rates of pulsation period change in the Galactic Cepheids

    NASA Astrophysics Data System (ADS)

    Fadeyev, Yu. A.

    2014-05-01

    Theoretical estimates of the rates of radial pulsation period change in Galactic Cepheids with initial masses 5.5 M ⊙ ≤ M ZAMS ≤ 13 M ⊙, chemical composition X = 0.7, Z = 0.02 and periods 1.5 day ≤ Π ≤ 100 day are obtained from consistent stellar evolution and nonlinear stellar pulsation computations. Pulsational instability was investigated for three crossings of the instability strip by the evolutionary track in the HR diagram. The first crossing occurs at the post-main sequence helium core gravitational contraction stage which proceeds in the Kelvin-Helmholtz timescale whereas the second and the third crossings take place at the evolutionary stage of thermonuclear core helium burning. During each crossing of the instability strip the period of radial pulsations is a quadratic function of the stellar evolution time. Theoretical rates of the pulsation period change agree with observations but the scatter of observational estimates of noticeably exceeds the width of the band confining evolutionary tracks in the period-period change rate diagram. One of the causes of the large scatter with very high values of in Cepheids with increasing periods might be the stars that cross the instability strip for the first time. Their fraction ranges from 2% for M ZAMS = 5.5 M ⊙ to 9% for M ZAMS = 13 M ⊙ and variables α UMi and IX Cas seem to belong to such objects.

  10. Lattice melting and rotation in perpetually pulsating equilibria

    SciTech Connect

    Pichon, C.; Lynden-Bell, D.; Pichon, J.; Lynden-Bell, R.

    2007-01-15

    Systems whose potential energies consists of pieces that scale as r{sup -2} together with pieces that scale as r{sup 2}, show no violent relaxation to Virial equilibrium but may pulsate at considerable amplitude forever. Despite this pulsation these systems form lattices when the nonpulsational ''energy'' is low, and these disintegrate as that energy is increased. The ''specific heats'' show the expected halving as the ''solid'' is gradually replaced by the ''fluid'' of independent particles. The forms of the lattices are described here for N{<=}18 and they become hexagonal close packed for large N. In the larger N limit, a shell structure is formed. Their large N behavior is analogous to a {gamma}=5/3 polytropic fluid with a quasigravity such that every element of fluid attracts every other in proportion to their separation. For such a fluid, we study the ''rotating pulsating equilibria'' and their relaxation back to uniform but pulsating rotation. We also compare the rotating pulsating fluid to its discrete counterpart, and study the rate at which the rotating crystal redistributes angular momentum and mixes as a function of extra heat content.

  11. The nature of the rapidly oscillating Ap stars' pulsations

    NASA Astrophysics Data System (ADS)

    Cunha, M. S.; Perraut, K.

    2013-12-01

    Chemically peculiar stars are stage to a wide variety of physical phenomena, including diffusion, convection, magnetism and pulsation. Progress in the understanding of these objects, through the study of their oscillations, can help us to characterize these physical phenomena and better understand the way they are coupled in stars. A number of chemically peculiar A-type stars, known as rapidly oscillating Ap (roAp) stars, have been known to exhibit high frequency oscillations since the early 80s. Despite this, the mechanism responsible for driving these oscillations is not fully understood. Currently, the most widely accepted theory states that oscillations in this class of pulsators are excited by the opacity mechanism acting on the hydrogen ionization region, in an envelope where convection has been suppressed by a strong magnetic field. Nevertheless, this theory fails to correctly predict some of the observations for this class of pulsators. In this paper we briefly review the current status of understanding of the driving of pulsations in roAp stars. In particular, we shall emphasize the comparison between predictions of nonadiabatic models of roAp stars with observations of a subset of pulsators of this class for which stringent data on global parameters are available.

  12. The Discovery of Pulsating Hot Subdwarfs in NGC 2808

    NASA Technical Reports Server (NTRS)

    Brown, Thomas M.; Landsman, Wayne B.; Randall, Suzanna K.; Sweigert, Allen V.; Lanz, Thierry

    2013-01-01

    We present the results of a Hubble Space Telescope program to search for pulsating hot subdwarfs in the core of NGC 2808. These observations were motivated by the recent discovery of such stars in the outskirts of Omega Cen. Both NGC 2808 and ? Cen are massive globular clusters exhibiting complex stellar populations and large numbers of extreme horizontal branch stars. Our far-UV photometric monitoring of over 100 hot evolved stars has revealed six pulsating subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of 2.0%-6.8%. In the UV color-magnitude diagram of NGC 2808, all six of these stars lie immediately below the canonical horizontal branch, a region populated by the subluminous "blue-hook" stars. For three of these six pulsators, we also have low-resolution far-UV spectroscopy that is sufficient to broadly constrain their atmospheric abundances and effective temperatures. Curiously, and in contrast to the ? Cen pulsators, the NGC 2808 pulsators do not exhibit the spectroscopic or photometric uniformity one might expect from a well-defined instability strip, although they all fall within a narrow band (0.2 mag) of far-UV luminosity.

  13. Heartbeat Stars and the Ringing of Tidal Pulsations

    NASA Astrophysics Data System (ADS)

    Hambleton, Kelly; Kurtz, Don; Prša, Andrej; Fuller, Jim; Thompson, Susan

    2015-09-01

    With the advent of high precision photometry from satellites such as Kepler and CoRoT, a whole new layer of interesting and astounding astronomical objects has been revealed: heartbeat stars are an example of such objects. Heartbeat stars are eccentric ellipsoidal variables that undergo strong tidal interactions when the stars are almost in contact at the time of closest approach. These interactions deform of the stars and cause a notable light curve variation in the form of a tidal pulse. A subset of these objects (~20%) show prominent tidally induced pulsations: pulsations forced by the binary orbit. We now have a fully functional code that models binary star features (using PHOEBE) and stellar pulsations simultaneously, enabling a complete and accurate heartbeat star model to be determined. In this paper we show the results of our new code, which uses emcee, a variant of mcmc, to generate a full set of stellar parameters. We further highlight the interesting features of KIC 8164262, including its tidally induced pulsations and resonantly locked pulsations.

  14. Pc 3 magnetic pulsations and precipitation of energetic electrons

    NASA Technical Reports Server (NTRS)

    Arthur, C. W.; Bjordal, J.; Rosenberg, T. J.

    1979-01-01

    The synchronous altitude satellite ATS 1 data and near-conjugate measurements of bremsstrahlung X-rays and ground magnetic variations were used to analyze an event of modulated auroral zone electron precipitation and magnetic pulsations in the Pc range. Transverse, azimuthal, nearly linearly polarized waves observed at ATS 1, ground magnetic pulsations at College, Alaska, and intervals of modulated electron precipitation centered on local magnetic moon, and noted in the X-ray data from Fort Yukon, Alaska, are discussed, noting that the origin of the Pc 3 waves is attributed to local field line resonances induced by Kelvin-Helmholtz instability at the magnetopause. The wave resonance model can explain observed differences in the pulsation activity at the ground, balloon, and satellite if account is taken of the spatial sensitivities of the techniques and the location of observing sites with respect to the probable location of resonant field lines. The data suggest that electron precipitation pulsations will correlate with Pc 3 magnetic pulsations when substorm injections coupled with azimuthal drift provide enhanced energetic particle fluxes with dayside resonance regions.

  15. Low Latitude Pulsations Associated with Different Phases of Geomagnetic Storms

    NASA Astrophysics Data System (ADS)

    Bulusu, J.; Vankayala, R. C.; Sinha, A. K.; Vichare, G.; Thomas, N.

    2014-12-01

    During geomagnetic storm lot of free energy is available in the magnetosphere and this energy can act as feeder to electromagnetic waves in different frequency bands. A classical geomagnetic storm consists mainly of four phases i.e. SSC (Sudden Storm commencement), initial Phase, main phase and recovery phase. In this paper, we investigate the characteristics of electromagnetic waves in ULF (ultra low frequency) band associated with different phases of geomagnetic storms. Electromagnetic waves in ULF band (Period~ 10-100s) in the Earth's magnetosphere are generally termed as geomagnetic pulsations. A detailed statistical analysis has been performed over ten years of geomagnetic data from low latitude ground stations in Indian and Japanese sectors. The study reveals that storms in general, are accompanied with continuous pulsations of different frequency bands during different phases. In particular, the main phase of 91 % of intense storms was accompanied with pulsations in Pc5 band (frequency~ 2-7 mHz). However, the occurrence of these pulsations was less frequent during main phase of weak to moderate storms. Further, the amplitude of these pulsations increased with the intensity of storm.

  16. THE DISCOVERY OF PULSATING HOT SUBDWARFS IN NGC 2808

    SciTech Connect

    Brown, Thomas M.; Landsman, Wayne B.; Randall, Suzanna K.; Sweigart, Allen V.; Lanz, Thierry E-mail: Wayne.Landsman@nasa.gov E-mail: allen.sweigart@gmail.com

    2013-11-10

    We present the results of a Hubble Space Telescope program to search for pulsating hot subdwarfs in the core of NGC 2808. These observations were motivated by the recent discovery of such stars in the outskirts of ω Cen. Both NGC 2808 and ω Cen are massive globular clusters exhibiting complex stellar populations and large numbers of extreme horizontal branch stars. Our far-UV photometric monitoring of over 100 hot evolved stars has revealed six pulsating subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of 2.0%-6.8%. In the UV color-magnitude diagram of NGC 2808, all six of these stars lie immediately below the canonical horizontal branch, a region populated by the subluminous 'blue-hook' stars. For three of these six pulsators, we also have low-resolution far-UV spectroscopy that is sufficient to broadly constrain their atmospheric abundances and effective temperatures. Curiously, and in contrast to the ω Cen pulsators, the NGC 2808 pulsators do not exhibit the spectroscopic or photometric uniformity one might expect from a well-defined instability strip, although they all fall within a narrow band (0.2 mag) of far-UV luminosity.

  17. Ionospheric signatures of cusp latitude Pc 3 pulsations

    NASA Technical Reports Server (NTRS)

    Engebretson, M. J.; Anderson, B. J.; Cahill, L. J., Jr.; Arnoldy, R. L.; Rosenberg, T. J.

    1990-01-01

    Search coil magnetometer, riometer, photometer, and ELF-VLF receiver data obtained at South Pole Station and McMurdo, Antarctica during selected days in March and April 1986 are compared. Narrow-band magnetic pulsations in the Pc 3 period range are observed simultaneously at both stations in the dayside sector during times of low IMF cone angle, but are considerably stronger at South Pole, which is located at a latitude near the nominal foot point of the dayside cusp/cleft region. Pulsations in auroral light at 427.8 nm wavelength are often observed with magnetic pulsations at South Pole, but such optical pulsations are not observed at McMurdo. The observations suggest that precipitating magnetosheathlike electrons at nominal dayside cleft latitudes are at times modulated with frequencies similar to those of upstream waves. These particles may play an important role, via modification of ionospheric currents and conductivities, in the transmission of upstream wave signals into the magnetosphere and in the generation of dayside high-latitude Pc 3 pulsations.

  18. Epsilon Aur monitoring during predicted pulsation phase

    NASA Astrophysics Data System (ADS)

    Waagen, Elizabeth O.; Templeton, Matthew R.

    2014-09-01

    Dr. Robert Stencel (University of Denver Astronomy Program) has requested that AAVSO observers monitor epsilon Aurigae from now through the end of the observing season. "Studies of the long-term, out-of-eclipse photometry of this enigmatic binary suggest that intervals of coherent pulsation occur at roughly 1/3 of the 27.1-year orbital period. Kloppenborg, et al. noted that stable variation patterns develop at 3,200-day intervals' implying that 'the next span of dates when such events might happen are circa JD ~2457000 (2014 December)'. "These out-of-eclipse light variations often have amplitudes of ~0.1 magnitude in U, and ~0.05 in V, with characteristic timescales of 60-100 days. The AAVSO light curve data to the present may indicate that this coherent phenomenon has begun, but we encourage renewed efforts by observers...to help deduce whether these events are internal to the F star, or externally-driven by tidal interaction with the companion star." Nightly observations or one observation every few days (CCD/PEP/DSLR, VUBR (amplitude too small for visual)) are requested. Finder charts with sequence may be created using the AAVSO Variable Star Plotter (http://www.aavso.org/vsp). Observations should be submitted to the AAVSO International Database. Epsilon Aur was the subject of major international campaigns and the AAVSO's Citizen Sky project as it went through its 27.1-year eclipse in 2009-2011. Over 700 observers worldwide submitted over 20,000 multicolor observations to the AAVSO International Database for this project. Much information on eps Aur is available from the AAVSO, including material on the Citizen Sky website (http://www.aavso.org/epsilon-aurigae and http://www.citizensky.org/content/star-our-project). The Journal of the AAVSO, Volume 40, No. 2 (2012) was devoted to discussion of and research results from this event. See full Alert Notice for more details and observations.

  19. High-speed Photometric Observations of ZZ Ceti White Dwarf Candidates

    NASA Astrophysics Data System (ADS)

    Green, E. M.; Limoges, M.-M.; Gianninas, A.; Bergeron, P.; Fontaine, G.; Dufour, P.; O'Malley, C. J.; Guvenen, B.; Biddle, L. I.; Pearson, K.; Deyoe, T. W.; Bullivant, C. W.; Hermes, J. J.; Van Grootel, V.; Grosjean, M.

    2015-06-01

    We present high-speed photometric observations of ZZ Ceti white dwarf candidates drawn from the spectroscopic survey of bright DA stars from the Villanova White Dwarf Catalog by Gianninas et al., and from the recent spectroscopic survey of white dwarfs within 40 parsecs of the Sun by Limoges et al. We report the discovery of six new ZZ Ceti pulsators from these surveys, and several photometrically constant DA white dwarfs, which we then use to refine the location of the ZZ Ceti instability strip.

  20. 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.

  1. 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.

  2. Substorm onset identification using neural networks and Pi2 pulsations

    NASA Astrophysics Data System (ADS)

    Sutcliffe, P. R.

    1997-10-01

    The pattern recognition capabilities of artificial neural networks (ANNs) have for the first time been used to identify Pi2 pulsations in magnetometer data, which in turn serve as indicators of substorm onsets and intensifications. The pulsation spectrum was used as input to the ANN and the network was trained to give an output of +1 for Pi2 signatures and -1 for non-Pi2 signatures. In order to evaluate the degree of success of the neural-network procedure for identifying Pi2 pulsations, the ANN was used to scan a number of data sets and the results compared with visual identification of Pi2 signatures. The ANN performed extremely well with a success rate of approximately 90% for Pi2 identification and a timing accuracy generally within 1 min compared to visual identification. A number of potential applications of the neural-network Pi2 scanning procedure are discussed.

  3. Ground magnetic field fluctuations associated with pulsating aurora

    NASA Astrophysics Data System (ADS)

    Michell, R. G.; Samara, M.

    2015-10-01

    A case study of an intense pulsating auroral event is presented where the large-scale (100-200 km) optical intensity variations are anticorrelated with fluctuations in the ground magnetometer data at a frequency of 0.1 Hz. The auroral event occurred over Poker Flat, Alaska, on 1 March 2012 and was imaged optically with several different fields of view and filters. The fluctuations in the magnetometer data were most prominent in the D component and had magnitudes of 1 to 5 nT. The auroral intensity variations had amplitudes of 200 to 400 R, comprising 25% to 50% of the total auroral luminosity at 427.8 nm. The direction of the magnetometer deflections is consistent with a south-to-north ionospheric current present when each pulsation is on, thus providing closure for the field-aligned currents associated with each of the pulsating patches.

  4. An Adaptive Code for Radial Stellar Model Pulsations

    NASA Astrophysics Data System (ADS)

    Buchler, J. Robert; Kolláth, Zoltán; Marom, Ariel

    1997-09-01

    We describe an implicit 1-D adaptive mesh hydrodynamics code that is specially tailored for radial stellar pulsations. In the Lagrangian limit the code reduces to the well tested Fraley scheme. The code has the useful feature that unwanted, long lasting transients can be avoided by smoothly switching on the adaptive mesh features starting from the Lagrangean code. Thus, a limit cycle pulsation that can readily be computed with the relaxation method of Stellingwerf will converge in a few tens of pulsation cycles when put into the adaptive mesh code. The code has been checked with two shock problems, viz. Noh and Sedov, for which analytical solutions are known, and it has been found to be both accurate and stable. Superior results were obtained through the solution of the total energy (gravitational + kinetic + internal) equation rather than that of the internal energy only.

  5. Geomagnetic pulsations observed simultaneously on three geostationary satellites

    NASA Technical Reports Server (NTRS)

    Hughes, W. J.; Mcpherron, R. L.; Barfield, J. N.

    1978-01-01

    Simultaneous observations of magnetic pulsations have been made by three geostationary satellites carrying similar magnetometers and acting as an azimuthal array. Autospectral and cross-spectral analysis yields coherence and phase differences between the pulsations at the satellite positions. The majority of the data fit the Kelvin-Helmholtz generation mechanism. The azimuthal wave number changes sign near noon and corresponds to propagation away from noon. Usually, the wave number is less than 10 deg per degree of longitude. Later in the afternoon, however, strong pulsations are observed with low coherence, implying large wave numbers. This suggests an instability driven by a gradient in the intensity of energetic protons, which may be expected at this local time. The data also suggest that some of the observed phase difference between the azimuthal components is due to small separations in magnetic shell, whereas this is not the case for the radial components. This implies a localized field-line resonance.

  6. Ultra-low-frequency magnetic pulsations in the earth's magnetosphere

    NASA Technical Reports Server (NTRS)

    Anderson, Brian J.

    1990-01-01

    Spacecraft observations have shown that geomagnetic pulsations originating in magnetospheric processes, in spite of their small amplitude on the ground, have amplitudes in space relative to the local magnetic field of 5-10 percent and occasionally up to about 50 percent. It is noted that by studying geomagnetic pulsations, a detailed comparison can be made between plasma physics theory and observations that are not possible in laboratory experiments. Also geomagnetic pulsations play a role in magnetospheric dynamics and energy transport, and their study forms an integral part of enhancing the knowledge of the magnetosphere. The importance of spacecraft observations are discussed and attention is given to such topics as waves in the magnetosphere, field-line resonances, the quantitative analysis of a dipole field, plasma instabilities, and energy flow.

  7. On permanent and sporadic pulsations of the magnetosphere

    NASA Astrophysics Data System (ADS)

    Guglielmi, A. V.

    2015-05-01

    A question concerning the influence of permanent Pc3-band pulsations (periods from 10 to 45 s) on the excitation of sporadic Pi2-band pulsations (40 to 150 s) is raised. It is hypothesized that, being generated ahead of the front of the Earth's magnetosphere, the Pc3 penetrate into the geomagnetic tail where they cause local depression in the electric current in the neutral sheet and, under favorable conditions, initiate tearing instability. This leads to the reconnection of the magnetic field lines and explosion-like release of the magnetic energy stored in the tail. As a result, a substorm arises with the sporadic Pi2 pulsations being its important element. Ways are suggested to theoretically substantiate and experimentally validate this hypothesis.

  8. Ultra-low-frequency magnetic pulsations in the earth's magnetosphere

    SciTech Connect

    Anderson, B.J. )

    1990-12-01

    Spacecraft observations have shown that geomagnetic pulsations originating in magnetospheric processes, in spite of their small amplitude on the ground, have amplitudes in space relative to the local magnetic field of 5-10 percent and occasionally up to about 50 percent. It is noted that by studying geomagnetic pulsations, a detailed comparison can be made between plasma physics theory and observations that are not possible in laboratory experiments. Also geomagnetic pulsations play a role in magnetospheric dynamics and energy transport, and their study forms an integral part of enhancing the knowledge of the magnetosphere. The importance of spacecraft observations are discussed and attention is given to such topics as waves in the magnetosphere, field-line resonances, the quantitative analysis of a dipole field, plasma instabilities, and energy flow. 28 refs.

  9. On the Stenbaek-Nielsen and Hallinan pulsating auroras

    SciTech Connect

    D'Angelo, N. )

    1991-02-01

    Stenbaek-Nielsen and Hallinan (1979) argued that if the pulsating auroras are caused by precipitating electrons, the data suggest that noncollisional interactions, localized in the lower E region (90-107 km), where most of the pulsations are observed, may play a dominant role in thermalizing the particles. They also stated that one type of process that comes to mind is a wave-particle interaction in the ionosphere. If, through one or more instabilities, waves grow at the expense of beam energy and, in turn, energize ambient electrons, the energized ambient electrons could ionize the background gas and, in process, produce luminosity. The exceptionally thin pulsating auroral patches observed by Stenbaek-Nielsen and Hallinan (1979) may be caused by an ionization instability occurring at the end of the range of precipitating electrons. A comparison is made to laboratory results reported by Johnson et al. (1990).

  10. 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.

  11. 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.

  12. White House

    MedlinePlus

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  13. Accretion of gas and comets onto a nearby degenerate star

    NASA Astrophysics Data System (ADS)

    Pineault, S.

    1987-04-01

    Conditions under which accretion onto a nearby degenerate star, i.e., a white dwarf (WD) or neutron star (NS), could produce a sufficient flux of high-energy radiation to threaten the Earth's protective ozone layer are investigated. Both the case of a field star making a brief encounter with the Solar System and that of a degenerate solar companion ("Nemesis") are considered. For steady accretion from the interstellar medium (ISM), no significant flux is expected from a WD or a low-mass NS. A 1 M_sun; NS could deplete the ozone layer but only if either its closest approach is on the order of 1000 AU or the local ISM density is somewhat higher than average. A field star has a probability of about 2% of making such a close encounter over the lifetime of the Solar System. In the Nemesis case, an ellipticity of 0.99 is implied for a canonical period of 26 myr. In both cases, accretion of comets from the Oort cloud could result in γ-ray bursts, whose fluence could reach a significant level if the star came near the inner edge of the comet cloud. A degenerate Nemesis, if now at the aphelion of its proposed orbit, could be potentially observable as an X-ray or γ-ray source.

  14. UV line diagnostics of accretion disk winds in cataclysmic variables

    NASA Technical Reports Server (NTRS)

    Vitello, Peter; Shlosman, Isaac

    1992-01-01

    The IUE data base is used to analyze the UV line shapes of cataclysmic variables RW Sex, RW Tri, and V Sge. Observed lines are compared to synthetic line profiles computed using a model of rotating bi-conical winds from accretion disks. The wind model calculates the wind ionization structure self-consistently including photoionization from the disk and boundary layer and treats 3-D line radiation transfer in the Sobolev approximation. It is found that winds from accretion disks provide a good fit for reasonable parameters to the observed UV lines which include the P Cygni profiles for low inclination systems and pure emission at large inclination. Disk winds are preferable to spherical winds which originate on the white dwarf because they (1) require a much lower ratio of mass loss rate to accretion rate and are therefore more plausible energetically, (2) provide a natural source for a bi-conical distribution of mass outflow which produces strong scattering far above the disk leading to P Cygni profiles for low inclination systems, and pure line emission profiles at high inclination with the absence of eclipses in UV lines, and (3) produce rotation broadened pure emission lines at high inclination.

  15. Ultraviolet line diagnostics of accretion disk winds in cataclysmic variables

    NASA Technical Reports Server (NTRS)

    Vitello, Peter; Shlosman, Isaac

    1993-01-01

    The IUE data base is used to analyze the UV line shapes of the cataclysmic variables RW Sex, RW Tri, and V Sge. Observed lines are compared to synthetic line profiles computed using a model of rotating biconical winds from accretion disks. The wind model calculates the wind ionization structure self-consistently including photoionization from the disk and boundary layer and treats 3D line radiation transfer in the Sobolev approximation. It is found that winds from accretion disks provide a good fit for reasonable parameters to the observed UV lines which include the P Cygni profiles for low-inclination systems and pure emission at large inclination. Disk winds are preferable to spherical winds which originate on the white dwarf because they: (1) require a much lower ratio of mass-loss rate to accretion rate and are therefore more plausible energetically; (2) provide a natural source for a biconical distribution of mass outflow which produces strong scattering far above the disk leading to P Cygni profiles for low-inclination systems and pure line emission profiles at high inclination with the absence of eclipses in UV lines; and (3) produce rotation-broadened pure emission lines at high inclination.

  16. Detection of Anisotropic Pulsating Flow and Its Velocity-Fluctuation Rate in Fertilized Bird Eggs by NMR Microimaging

    NASA Astrophysics Data System (ADS)

    Görke, U.; Kimmich, R.; Weis, J.

    1996-06-01

    Coherent and incoherent flows in fertilized quail and bantam eggs have been studied with the aid of NMR microimaging techniques in the course of incubation until the end of the sixth day. The methods employed were multiplane tagging NMR imaging and a NMR gradient-echo imaging pulse sequence supplemented by bipolar gradient pulses in the coherence-evolution interval. The latter technique is suited for recording of velocity maps as well as for localizing of regions with enhanced echo attenuation by incoherent motions. Slight coherent displacements in the middle of the upper part of the egg white were found after the fourth day of incubation with the aid of both pulse schemes. The maximum velocity was estimated to be 1 mm/s. More pronounced effects revealed themselves in the examination of incoherent motions. After the same time of incubation and in a somewhat more restricted area of the upper part of the egg white, distinct motions could be localized consistently with either technique. It is shown that these motions are directed to and from the yolk. Furthermore, the analysis of the time fluctuations of the local signals with the aid of a Fourier transformation showed that the flow is largely of a pulsating nature. The pulsation frequency was found to be 0.4 Hz.

  17. 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.

  18. 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.

  19. Extreme ultraviolet spectroscopy and photometry of VV Puppis during a high accretion state

    NASA Technical Reports Server (NTRS)

    Vennes, Stephane; Szkody, Paula; Sion, Edward M.; Long, Knox S.

    1995-01-01

    We determine the physical properties of the accretion region of the AM Her-type binary VV Puppis using extreme ultraviolet (EUV) medium-resolution spectroscopy and photometry obtained with the Extreme Ultraviolet Explorer (EUVE) observatory. The EUV continuum from VV Pup was detected in the wavelength range from 75 to 135 A and was simultaneously recorded with the Deep Survey/Spectrometer (DS/S) imaging telescope, allowing for the extraction of an accurate light curve. VV Pup appeared to have entered a high-accretion state just prior to the pointed EUVE observations. We use the EUV light curve to infer the diameter of the accretion region (d = 220 km) assuming a hemispherical geometry and a radius of 9000 km for the white dwarf. We perform a model atmosphere analysis and, based on the light curve properties and assuming a distance of 145 pc, we derive an effective temperature of the accretion region in the range 270,000 is less than T(sub eff) is less than 360,000 K and a neutral hydrogen column density in the local interstellar medium of n(sub H) = 1.9 - 3.7 x 10(exp 19)/sq cm. The total EUV/soft X-ray energy radiated by the accretion region is approximately 3.5 x 10(exp 32) ergs/s. Our results provide a first verification of past suggestions that deep heating of the white dwarf surface produces the soft X-ray flux from the polars. We present a possible detection of O VI absortion features, and we suggest that extensive EUVE observations targeting high-accretion events may result in oxygen and heavier element abundance determination in the accretion region.

  20. The 2002 Outburst of the Millisecond Accreting Pulsar XTE J1751-305

    NASA Astrophysics Data System (ADS)

    Markwardt, C. B.; Swank, J. H.

    2002-12-01

    The millisecond accreting pulsar XTE J1751--305 was discovered in the galactic bulge region by the RXTE PCA in early 2002. It is one of only a handful of now-known millisecond pulsars that are presumably spinning up by mass accretion (along with SAX J1808.4--3658 and XTE J0929--314). We will present an analysis of the complete outburst of XTE J1751--305, including spectroscopy and timing. The outburst followed a similar track to the first known millisecond accreting pulsar, SAX J1808.4--3658, with a fast rise, exponential decay (time constant ~ 7 day), and a sudden cut-off. Over the outburst, the energy spectral shape remained essentially constant, and showed no strong line features. Aside from the pulsations, XTE J1751--305 also exhibited lower frequency fluctuations in the power spectrum, which are typical of low mass X-ray binaries. While there appears to be no strong kiloHertz quasiperiodic oscillations, there is some evidence for a weak and broad power spectral excess feature centered on a few hundred Hertz.

  1. Unusual pulsating states in hydrocarbon-oxygen premixed flames.

    PubMed

    Gorman, M; Perrollier, S

    2006-12-01

    An unusual type of pulsating state has been observed using a new ignition protocol for heavy hydrocarbon-oxygen premixed flames on a circular porous plug burner. The shape and motion of these states are quasicircular, luminous, pulsating regions of M (M=1, 2, 3, or 4) lobes that increase in size as the flame propagates outward. As the lobes expand, they break apart near their midpoints and form counterpropagating spiral-like arms. These spiral arms rotate, "collide" with arms generated by adjacent lobes, and are extinguished. We will describe the unusual characteristics of the dynamics of these states. PMID:17199402

  2. Photometric Survey to Search for Field sdO Pulsators

    NASA Astrophysics Data System (ADS)

    Johnson, C.; Green, E.; Wallace, S.; O'Malley, C.; Amaya, H.; Biddle, L.; Fontaine, G.

    2014-04-01

    We present the results of a campaign to search for subdwarf O (sdO) star pulsators among bright field stars. The motivation for this project is the recent discovery by Randall et al. (2011) of four rapidly pulsating sdO stars in the globular cluster ω Cen, with Teff near 50,000 K, 5.4 < log g < 6.0, and hydrogen-rich atmospheres. The only previously known sdO pulsator is significantly hotter at 68,500 K and log g = 6.1. All of the sdO pulsators identified so far are fainter than V≍17.4 and, thus, are poor candidates for an in-depth follow-up with asteroseismology. We therefore obtained high S/N light curves and spectroscopy for a number of field sdO stars to attempt to discover bright counterparts to these stars, particularly the ω Cen pulsators. Our primary sample consisted of 19 sdO stars with hydrogen-rich atmospheres, log N(He)/N(H) < -1.0, effective temperatures in the range 40,000 K < Teff < 67,000 K, and surface gravities 5.3 < log g < 6.1. We also observed 17 additional helium-rich sdO stars with log N(He)/N(H) > -0.1 and similar temperatures and gravities. To date, we have found no detectable pulsations at amplitudes above 0.08% (4 times the mean noise level) in any of the 36 field sdO stars that we observed. The presence of pulsations in ω Cen sdO stars and their apparent absence in seemingly comparable field sdO stars is perplexing. While very suggestive, the significance of this result is difficult to assess more completely right now due to remaining uncertainties about the temperature width and purity of the ω Cen instability strip and the existence of any sdO pulsators with weaker amplitudes than the current detection limit in globular clusters.

  3. Resonant self-pulsations in coupled nonlinear microcavities

    SciTech Connect

    Grigoriev, Victor; Biancalana, Fabio

    2011-04-15

    A different point of view on the phenomenon of self-pulsations is presented, which shows that they are a balanced state formed by two counteracting processes: beating of modes and bistable switching. A structure based on two coupled nonlinear microcavities provides a generic example of a system with enhanced ability to support this phenomenon. The specific design of such a structure in the form of multilayered media is proposed, and the coupled-mode theory is applied to describe its dynamical properties. It is emphasized that the frequency of self-pulsations is related to the frequency splitting between resonant modes and can be adjusted over a broad range.

  4. Pulsation research during the IMS. [International Magnetospheric Study

    NASA Technical Reports Server (NTRS)

    Hughes, W. J.

    1982-01-01

    After describing the development status of the field of magnetic pulsations in 1975, before the initiation of the International Magnetospheric Study (IMS), attention is given to the IMS's novel observational results and an attempt is made to identify the most effective research methods employed. It is found that the most fruitful work involved small-scale collaboration between a few individuals or a few groups possessing complementary data sets. Consideration is restricted to research on the long period pulsations which can be broadly classified as field line resonances. Recommendations are made for future research efforts.

  5. Effects of pulsating flow on current meter performance

    USGS Publications Warehouse

    Fulford, Janice M.

    1995-01-01

    Summarized are laboratory tests for current meter response to pulsating flows. Included are results for mechanical and electromagnetic water-current meters that are commonly used for stream gaging. Most of the vertical-axis and horizontal-axis types of mechanical meters that were tested significantly underregistered the mean flow velocity when the magnitude of the pulsating portion of the flow velocity was greater than half the mean velocity but less than the mean velocity. Errors for all meters tested were largest at the lowest mean flow velocity, 0.076 m/s.

  6. Search for pulsating stars in multiple stellar systems

    NASA Astrophysics Data System (ADS)

    Antonello, E.; Pastori, L.; Fracassini, M.; Pasinetti, L. E.

    Two lists of possible Delta Scuti stars are compiled, one for the wide visual binaries, the other for the spectroscopic binaries in the catalogue of Batten et al. (1978). For companions with normal spectral type A, F belonging to the instability strip, the expected periods and maximum amplitudes of pulsation are calculated and shown. A list of 21 spectroscopic binaries is presented containing the star identifications, apparent visual magnitudes, spectral type, possible period and maximum visual amplitude of pulsation, sepration of components, and remarks.

  7. Mode identification from spectroscopy of gravity-mode pulsators

    NASA Astrophysics Data System (ADS)

    Pollard, K. R.; Brunsden, E.; Cottrell, P. L.; Davie, M.; Greenwood, A.; Wright, D. J.; De Cat, P.

    2014-02-01

    The gravity modes present in γ Doradus stars probe the deep stellar interiors and are thus of particular interest in asteroseismology. For the MUSICIAN programme at the University of Canterbury, we obtain extensive high-resolution echelle spectra of γ Dor stars from the Mt John University Observatory in New Zealand. We analyze these to obtain the pulsational frequencies and identify these with the multiple pulsational modes excited in the star. A summary of recent results from our spectroscopic mode-identification programme is given.

  8. An ultraluminous X-ray source powered by an accreting neutron star.

    PubMed

    Bachetti, M; Harrison, F A; Walton, D J; Grefenstette, B W; Chakrabarty, D; Fürst, F; Barret, D; Beloborodov, A; Boggs, S E; Christensen, F E; Craig, W W; Fabian, A C; Hailey, C J; Hornschemeier, A; Kaspi, V; Kulkarni, S R; Maccarone, T; Miller, J M; Rana, V; Stern, D; Tendulkar, S P; Tomsick, J; Webb, N A; Zhang, W W

    2014-10-01

    The majority of ultraluminous X-ray sources are point sources that are spatially offset from the nuclei of nearby galaxies and whose X-ray luminosities exceed the theoretical maximum for spherical infall (the Eddington limit) onto stellar-mass black holes. Their X-ray luminosities in the 0.5-10 kiloelectronvolt energy band range from 10(39) to 10(41) ergs per second. Because higher masses imply less extreme ratios of the luminosity to the isotropic Eddington limit, theoretical models have focused on black hole rather than neutron star systems. The most challenging sources to explain are those at the luminous end of the range (more than 10(40) ergs per second), which require black hole masses of 50-100 times the solar value or significant departures from the standard thin disk accretion that powers bright Galactic X-ray binaries, or both. Here we report broadband X-ray observations of the nuclear region of the galaxy M82 that reveal pulsations with an average period of 1.37 seconds and a 2.5-day sinusoidal modulation. The pulsations result from the rotation of a magnetized neutron star, and the modulation arises from its binary orbit. The pulsed flux alone corresponds to an X-ray luminosity in the 3-30 kiloelectronvolt range of 4.9 × 10(39) ergs per second. The pulsating source is spatially coincident with a variable source that can reach an X-ray luminosity in the 0.3-10 kiloelectronvolt range of 1.8 × 10(40) ergs per second. This association implies a luminosity of about 100 times the Eddington limit for a 1.4-solar-mass object, or more than ten times brighter than any known accreting pulsar. This implies that neutron stars may not be rare in the ultraluminous X-ray population, and it challenges physical models for the accretion of matter onto magnetized compact objects. PMID:25297433

  9. Three-dimensional radiative transfer calculations on an SIMD machine applied to accretion disks

    NASA Astrophysics Data System (ADS)

    Vath, H.

    We have developed a tool to solve the radiative transfer equation for a three-dimensional astrophysical object on the SIMD computer MasPar MP-1. With this tool we can rapidly calculate the image of such an object as seen from an arbitrary direction and at an arbitrary wavelength. Such images and spectra can then be used to directly compare observations with the model. This tool can be applied to many different areas in astrophysics, e.g., HI disks of galaxies and polarized radiative transfer of accretion columns onto white dwarfs. Here we use this tool to calculate the image and spectrum of a simple model of an accretion disk.

  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. Probing accretion on the high-magnetized polar RX J1007.5-2017

    NASA Astrophysics Data System (ADS)

    Rodrigues, C. V.; Cieslinski, D.; Ribeiro, T.; Silva, K. M. G.; Baptista, R.; Oliveira, A. S.; Costa, J. E. R.; Campbell, R.

    2014-10-01

    RX J1007.5-2017 is a polar: a compact binary system in which matter flows from a low-mass main-sequence star to a magnetized white dwarf without the formation of an accretion disk. RX J1007.5-2017 has some observational peculiarities (conspicuous optical cyclotron harmonics, a very soft X-ray spectrum, and no polarization in R and I bands), which may be related to extreme conditions at the accretion flow: a very strong white-dwarf magnetic field (around 100 MG on surface) and a low accretion rate. To study the accretion, from the mass-donor star to the white dwarf, we obtained time-resolved spectroscopy using the Goodman spectrograph at the SOAR telescope in observing runs distributed around the first semester of 2012. We found the object in different brightness states. In the low state, we gathered data with two spectral resolutions (219 km/s and 170 km/s). In a brighter state, the spectral resolution was ≍ 170 km/s. The low (high) spectral resolution data cover the spectral region from 360 to 760 nm (435 to 700 nm). The continuum varies in both states and the cyclotron humps are visible at some orbital phases. The low-state spectra show Balmer emission lines superimposed on absorption features from the mass-donor star. The bright-state spectra show strong Balmer, HeI, and HeII emission lines. The Balmer and HeII lines are not single Gaussians: in bright state the lines are broader and have three components; in low state, the lines are narrower and two components are distinguished in some phases. Doppler tomography of the low state reveals that line emission arises mainly from a region near the white dwarf. The orbital dependence of the cyclotron emission was modeled using the Cyclops code, which adopts a 3D representation of the accretion column.

  12. 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

  13. The signature of single-degenerate accretion-induced collapse

    SciTech Connect

    Piro, Anthony L.

    2014-10-10

    The accretion-induced collapse (AIC) of a white dwarf to a neutron star has long been suggested as a natural theoretical outcome in stellar evolution, but there has never been a direct detection of such an event. This is not surprising since the small amount of radioactive nickel synthesized (∼10{sup –3} M {sub ☉}) implies a relatively dim optical transient. Here we argue that a particularly strong signature of an AIC would occur for an oxygen-neon-magnesium (ONeMg) white dwarf accreting from a star that is experiencing Roche-lobe overflow as it becomes a red giant. In such cases, the ∼10{sup 50} erg explosion from the AIC collides with and shock-heats the surface of the extended companion, creating an X-ray flash lasting ∼1 hr followed by an optical signature that peaks at an absolute magnitude of ∼ – 16 to –18 and lasts for a few days to a week. These events would be especially striking in old stellar environments where hydrogen-rich supernova-like transients would not normally be expected. Although the rate of such events is not currently known, we describe observing strategies that could be utilized with high cadence surveys that should either detect these events or place strong constraints on their rates.

  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. White Rock

    NASA Technical Reports Server (NTRS)

    2002-01-01

    (Released 19 April 2002) The Science 'White Rock' is the unofficial name for this unusual landform which was first observed during the Mariner 9 mission in the early 1970's. As later analysis of additional data sets would show, White Rock is neither white nor dense rock. Its apparent brightness arises from the fact that the material surrounding it is so dark. Images from the Mars Global Surveyor MOC camera revealed dark sand dunes surrounding White Rock and on the floor of the troughs within it. Some of these dunes are just apparent in the THEMIS image. Although there was speculation that the material composing White Rock could be salts from an ancient dry lakebed, spectral data from the MGS TES instrument did not support this claim. Instead, the White Rock deposit may be the erosional remnant of a previously more continuous occurrence of air fall sediments, either volcanic ash or windblown dust. The THEMIS image offers new evidence for the idea that the original deposit covered a larger area. Approximately 10 kilometers to the southeast of the main deposit are some tiny knobs of similarly bright material preserved on the floor of a small crater. Given that the eolian erosion of the main White Rock deposit has produced isolated knobs at its edges, it is reasonable to suspect that the more distant outliers are the remnants of a once continuous deposit that stretched at least to this location. The fact that so little remains of the larger deposit suggests that the material is very easily eroded and simply blows away. The Story Fingers of hard, white rock seem to jut out like icy daggers across a moody Martian surface, but appearances can be deceiving. These bright, jagged features are neither white, nor icy, nor even hard and rocky! So what are they, and why are they so different from the surrounding terrain? Scientists know that you can't always trust what your eyes see alone. You have to use other kinds of science instruments to measure things that our eyes can

  16. Constraints on pre-main-sequence evolution from stellar pulsations

    NASA Astrophysics Data System (ADS)

    Casey, M. P.; Zwintz, K.; Guenther, D. B.

    2014-02-01

    Pulsating pre-main-sequence (PMS) stars afford the earliest opportunity in the lifetime of a star to which the concepts of asteroseismology can be applied. PMS stars should be structurally simpler than their evolved counterparts, thus (hopefully!) making any asteroseismic analysis relatively easier. Unfortunately, this isn't necessarily the case. The majority of these stars (around 80) are δ Scuti pulsators, with a couple of γ Doradus, γ Doradus - δ Scuti hybrids, and slowly pulsating B stars thrown into the mix. The majority of these stars have only been discovered within the last ten years, with the community still uncovering the richness of phenomena associated with these stars, many of which defy traditional asteroseismic analysis. A systematic asteroseismic analysis of all of the δ Scuti PMS stars was performed in order to get a better handle on the properties of these stars as a group. Some strange results have been found, including one star pulsating up to the theoretical acoustic cut-off frequency of the star, and a number of stars in which the most basic asteroseismic analysis suggests problems with the stars' positions in the Hertzsprung-Russell diagram. From this we get an idea of the\\break constraints - or lack thereof - that these results can put on PMS stellar evolution.

  17. Experimental and numerical study of pulsating transversal jets

    NASA Astrophysics Data System (ADS)

    Goldfeld, M. A.; Fedorova, N. N.; Fedorchenko, I. A.; Pozdnyakov, G. A.; Timofeev, K. Yu.; Zhakharova, Yu. V.

    2015-06-01

    Paper presents results of joint experimental and numerical investigation of pulsating jet penetration into still air and supersonic flow. Goal of the study is to investigate two-dimensional (2D) Hartmann generator (HG) properties and clear up its possibilities in providing better mixing between air and secondary (injected) gases.

  18. Radar auroral observations during a burst of irregular magnetic pulsations

    SciTech Connect

    Haldoupis, C.I.; Nielsen, E.; Holtet, J.A.; Egeland, A.; Chivers, H.A.

    1982-03-01

    Micropulsation data from an auroral state are compared with concurrent STARE radar observations from the E region above the station during a substorm event. The substorm onset is marked by a strong burst of irregular pulsations, (Pi B) accompanied by abrupt intensifications in the equivalent current, the backscatter intensity, and the riometer absorption. The magnetic Px and Py pulsation components have a reasonable degree of correlation and the polarization properties of the horizontal disturbance vetor exhibit well-defined changes during the first few minutes after onset. The radio signal undergoes deep quasiperiodic fading that is closely related to simultaneous PiB amplitude variations. The radar Dopplar data, which show normal fluctuations in the electron drift velocity, exclude modulation of radio backscatter by directional variations of the electric field. It is argued that the PiB pulsations and the variations seen in the backscatter are reflecting changes of the ionospheric currents due to conductivity modifications dictated by variations in the field-aligned currents flowing in the region. The possibility exists that the Pi B is closely related to precipitation pulsations as reported by Heacock and Hunsucker (1977).

  19. Electron precipitation response to geomagnetic pulsations: Riometer revelation

    NASA Astrophysics Data System (ADS)

    Honary, Farideh; Kavanagh, Andrew

    Electron precipitation modulations by geomagnetic pulsation have been observed in cosmic noise absorption (CNA) as early as 1965 by widebeam riometers (Barcus and Rosenberg, 1965). The first observation of pulsation with high m-number was reported by Kikuchi et al.(1988) em-ploying a scanning narrow-beam riometer to investigate the spatial structure in one dimension with a high resolution. However, the advances in high spatial resolution imaging riometers has provided the ability to observe pulsating cosmic noise absorption with azimuthal wave numbers as high as 380 as well as providing the capability of mapping their structures. These waves are commonly observed during the morning and early afternoon and exhibit eastward propagation. In this presentation a complete generating mechanism for these high m-number waves is dis-cussed as a five step process, beginning with the solar wind as a source for the excitation of dayside magnetospheric cavity modes, mode conversion, energisation of drift-bounce protons by Landau damping, followed by inverse Landau damping as a driving mechanism for the high m number secondary waves that ultimately modulate the electron precipitation. This modulation is observed as pulsations in cosmic noise absorption.

  20. A statistical method for draft tube pressure pulsation analysis

    NASA Astrophysics Data System (ADS)

    Doerfler, P. K.; Ruchonnet, N.

    2012-11-01

    Draft tube pressure pulsation (DTPP) in Francis turbines is composed of various components originating from different physical phenomena. These components may be separated because they differ by their spatial relationships and by their propagation mechanism. The first step for such an analysis was to distinguish between so-called synchronous and asynchronous pulsations; only approximately periodic phenomena could be described in this manner. However, less regular pulsations are always present, and these become important when turbines have to operate in the far off-design range, in particular at very low load. The statistical method described here permits to separate the stochastic (random) component from the two traditional 'regular' components. It works in connection with the standard technique of model testing with several pressure signals measured in draft tube cone. The difference between the individual signals and the averaged pressure signal, together with the coherence between the individual pressure signals is used for analysis. An example reveals that a generalized, non-periodic version of the asynchronous pulsation is important at low load.

  1. 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

  2. Observations of Cepheids with the MOST satellite: contrast between pulsation modes

    NASA Astrophysics Data System (ADS)

    Evans, N. R.; Szabó, R.; Derekas, A.; Szabados, L.; Cameron, C.; Matthews, J. M.; Sasselov, D.; Kuschnig, R.; Rowe, J. F.; Guenther, D. B.; Moffat, A. F. J.; Rucinski, S. M.; Weiss, W. W.

    2015-02-01

    The quantity and quality of satellite photometric data strings is revealing details in Cepheid variation at very low levels. Specifically, we observed a Cepheid pulsating in the fundamental mode and one pulsating in the first overtone with the Canadian MOST (Microvariability and Oscillations of Stars) satellite. The 3.7-d period fundamental mode pulsator (RT Aur) has a light curve that repeats precisely, and can be modelled by a Fourier series very accurately. The overtone pulsator (SZ Tau, 3.1 d period) on the other hand shows light-curve variation from cycle to cycle which we characterize by the variations in the Fourier parameters. We present arguments that we are seeing instability in the pulsation cycle of the overtone pulsator, and that this is also a characteristic of the O - C curves of overtone pulsators. On the other hand, deviations from cycle to cycle as a function of pulsation phase follow a similar pattern in both stars, increasing after minimum radius. In summary, pulsation in the overtone pulsator is less stable than that of the fundamental mode pulsator at both long and short time-scales.

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. Conjugate Event Study of Geomagnetic ULF Pulsations with Wavelet-based Indices

    NASA Astrophysics Data System (ADS)

    Xu, Z.; Clauer, C. R.; Kim, H.; Weimer, D. R.; Cai, X.

    2013-12-01

    The interactions between the solar wind and geomagnetic field produce a variety of space weather phenomena, which can impact the advanced technology systems of modern society including, for example, power systems, communication systems, and navigation systems. One type of phenomena is the geomagnetic ULF pulsation observed by ground-based or in-situ satellite measurements. Here, we describe a wavelet-based index and apply it to study the geomagnetic ULF pulsations observed in Antarctica and Greenland magnetometer arrays. The wavelet indices computed from these data show spectrum, correlation, and magnitudes information regarding the geomagnetic pulsations. The results show that the geomagnetic field at conjugate locations responds differently according to the frequency of pulsations. The index is effective for identification of the pulsation events and measures important characteristics of the pulsations. It could be a useful tool for the purpose of monitoring geomagnetic pulsations.

  8. INTRODUCING CAFein, A NEW COMPUTATIONAL TOOL FOR STELLAR PULSATIONS AND DYNAMIC TIDES

    SciTech Connect

    Valsecchi, F.; Farr, W. M.; Willems, B.; Rasio, F. A.; Kalogera, V.

    2013-08-10

    Here we present CAFein, a new computational tool for investigating radiative dissipation of dynamic tides in close binaries and of non-adiabatic, non-radial stellar oscillations in isolated stars in the linear regime. For the latter, CAFein computes the non-adiabatic eigenfrequencies and eigenfunctions of detailed stellar models. The code is based on the so-called Riccati method, a numerical algorithm that has been successfully applied to a variety of stellar pulsators, and which does not suffer from the major drawbacks of commonly used shooting and relaxation schemes. Here we present an extension of the Riccati method to investigate dynamic tides in close binaries. We demonstrate CAFein's capabilities as a stellar pulsation code both in the adiabatic and non-adiabatic regimes, by reproducing previously published eigenfrequencies of a polytrope, and by successfully identifying the unstable modes of a stellar model in the {beta} Cephei/SPB region of the Hertzsprung-Russell diagram. Finally, we verify CAFein's behavior in the dynamic tides regime by investigating the effects of dynamic tides on the eigenfunctions and orbital and spin evolution of massive main sequence stars in eccentric binaries, and of hot Jupiter host stars. The plethora of asteroseismic data provided by NASA's Kepler satellite, some of which include the direct detection of tidally excited stellar oscillations, make CAFein quite timely. Furthermore, the increasing number of observed short-period detached double white dwarfs (WDs) and the observed orbital decay in the tightest of such binaries open up a new possibility of investigating WD interiors through the effects of tides on their orbital evolution.

  9. On the Polarization Properties of Magnetar Giant Flare Pulsating Tails

    NASA Astrophysics Data System (ADS)

    Yang, Yuan-Pei; Zhang, Bing

    2015-12-01

    Three giant flares have been detected so far from soft gamma-ray repeaters, each characterized by an initial short hard spike and a pulsating tail. The observed pulsating tails are characterized by a duration of ˜100 s, an isotropic energy of ˜1044 erg, and a pulse period of a few seconds. The pulsating tail emission likely originates from the residual energy after the intense energy release during the initial spike, which forms a trapped fireball composed of a photon-pair plasma in a closed-field-line region of the magnetars. Observationally the spectra of pulsating tails can be fitted by the superposition of a thermal component and a power-law component, with the thermal component dominating the emission in the early and late stages of the pulsating-tail observations. In this paper, assuming that the trapped fireball is from a closed-field-line region in the magnetosphere, we calculate the atmospheric structure of the optically thick trapped fireball and the polarization properties of the trapped fireball. By properly treating the photon propagation in a hot, highly magnetized, electron-positron pair plasma, we tally photons in two modes (O mode and E mode) at a certain observational angle through Monte Carlo simulations. Our results suggest that the polarization degree depends on the viewing angle with respect to the magnetic axis of the magnetar, and can be as high as Π ≃ 30% in the 1-30 keV band, and Π ≃ 10% in the 30-100 keV band, if the line of sight is perpendicular to the magnetic axis.

  10. Kappa effect pulsational instability for hot extreme helium stars

    SciTech Connect

    Cox, A.N.

    1990-01-01

    A long standing problem for the hydrogen deficient stars has been the mechanism for the pulsation instability for the hottest members of this class. The usual {kappa} mechanism works well for stars that are in the hydrogen and helium ionization instability strip, and this strip extends to perhaps 20,000K at high luminosity. However, several stars are definitely hotter. Investigations for another ionization instability strip, such as for carbon, have always shown that there is not enough carbon to produce a rapid enough increase of opacity with temperature to give the well-known {kappa} effect. This is so even though these hydrogen deficient stars do show enhanced carbon in their spectra. A strong stellar wind can produce the observed hydrogen deficiency. Another popular mechanism is mass loss in a binary system through the Roche lobe. It now is possible that the missing pulsational instability mechanism is the rapid increase of iron lines absorption as the temperature increases above about 150,000K in the low density envelopes of these luminous stars. Recent calculations shows that the n = 3 to n = 3 transitions in iron that were assumed unimportant in the earlier Los Alamos calculations can double or triple the opacity suddenly as the iron lines appear in a very sensitive part of the spectrum of the diffusing photons. It has been proposed that these iron lines also cause the many varieties of normal B star pulsations, and the hydrogen deficient stars are merely another example of this new {kappa} effect for pulsating stars. The extreme helium star V2076 Oph at 31,900K, and 38,900 L{sub {circle dot}} for a mass of 1.4 M{sub {circle dot}} pulsates in the radial fundamental model at about 1 day period with a very large linear growth rate when the iron lines more than double the opacity, but is stable otherwise.

  11. Formation of accretion disks in close-binary systems with magnetic fields

    NASA Astrophysics Data System (ADS)

    Zhilkin, A. G.; Bisikalo, D. V.

    2010-12-01

    We have developed a three-dimensional numerical model and applied it to simulate plasma flows in semi-detached binary systems whose accretor possesses a strong intrinsic magnetic field. The model is based on the assumption that the plasma dynamics are determined by the slow mean flow, which forms a backdrop for the rapid propagation of MHD waves. The equations describing the slow motion of matter were obtained by averaging over rapidly propagating pulsations. The numerical model includes the diffusion of magnetic field by current dissipation in turbulent vortices, magnetic buoyancy, and wave MHD turbulence. A modified three-dimensional, parallel, numerical code was used to simulate the flow structure in close binary systems with various accretor magnetic fields, from 105 to 108 G. The conditions for the formation of the accretion disk and the criteria distinguishing the two types of flow corresponding to intermediate polars and polars are discussed.

  12. A Study of Short-term White Dwarf Variability Using gPhoton

    NASA Astrophysics Data System (ADS)

    Tucker, Michael; Fleming, Scott W.; Caton, Daniel B.; Million, Chase; Shiao, Bernie

    2016-01-01

    The Galaxy Evolution Explorer (GALEX) was a UV space telescope that operated from 2003 until 2013. A new project at MAST, gPhoton takes advantage of the microchannel-plate photon detector aboard GALEX, which catalogued and time-stamped every photon event by putting the one trillion photon events into a database. Utilizing associated open-source software, gPhoton can create coadd images, movies and light curves at user-defined spatial and temporal scales. As part of early science investigations with gPhoton, 364 white dwarf stars from the McCook-Sion catalog with ample GALEX coverage were photometrically inspected for inter-visit variations during an REU program at STScI. Out of the 364 white dwarfs that were studied, three previously documented pulsating white dwarf stars were confirmed in the UV and (at least) three new pulsating white dwarf stars were discovered. Follow-up observations are conducted at Appalachian State University using optical telescopes at the Dark Sky Observatory. We compare optical and UV light curves of these new white dwarf pulsators and show a selection of other variables found with gPhoton.

  13. Fast modulations of pulsating proton aurora related to subpacket structures of Pc1 geomagnetic pulsations at subauroral latitudes

    DOE PAGESBeta

    Ozaki, M.; Shiokawa, K.; Miyoshi, Y.; Kataoka, R.; Yagitani, S.; Inoue, T.; Ebihara, Y.; Jun, C. -W; Nomura, R.; Sakaguchi, K.; et al

    2016-08-14

    To understand the role of electromagnetic ion cyclotron (EMIC) waves in determining the temporal features of pulsating proton aurora (PPA) via wave-particle interactions at subauroral latitudes, high-time-resolution (1/8 s) images of proton-induced N2>+ emissions were recorded using a new electron multiplying charge-coupled device camera, along with related Pc1 pulsations on the ground. The observed Pc1 pulsations consisted of successive rising-tone elements with a spacing for each element of 100 s and subpacket structures, which manifest as amplitude modulations with a period of a few tens of seconds. In accordance with the temporal features of the Pc1 pulsations, the auroral intensitymore » showed a similar repetition period of 100 s and an unpredicted fast modulation of a few tens of seconds. Furthermore, these results indicate that PPA is generated by pitch angle scattering, nonlinearly interacting with Pc1/EMIC waves at the magnetic equator.« less

  14. RADIAL STELLAR PULSATION AND THREE-DIMENSIONAL CONVECTION. II. TWO-DIMENSIONAL CONVECTION IN FULL AMPLITUDE RADIAL PULSATION

    SciTech Connect

    Geroux, Chris M.; Deupree, Robert G.

    2013-07-10

    We have developed a three-dimensional radiation hydrodynamics code to simulate the interaction of convection and radial pulsation in classical variable stars. One key goal is the ability to carry these simulations to full amplitude in order to compare them with observed light curves. Previous multi-dimensional calculations were prevented from reaching full amplitude because of drift in the radial coordinate system, due to the algorithm defining radial movement of the coordinate system during the pulsation cycle. We have removed this difficulty by defining our radial coordinate flow algorithm to require that the mass in a spherical shell remain constant for every time step throughout the pulsation cycle. We have used our new code to perform two-dimensional (2D) simulations of the interaction of radial pulsation and convection. We have made comparisons between light curves from our 2D convective simulations with observed light curves and find that our 2D simulated light curves are better able to match the observed light curve shape near the red edge of the RR Lyrae instability strip than light curves from previous one-dimensional time-dependent convective models.

  15. White phosphorus

    Integrated Risk Information System (IRIS)

    White phosphorus ; CASRN 7723 - 14 - 0 Human health assessment information on a chemical substance is included in the IRIS database only after a comprehensive review of toxicity data , as outlined in the IRIS assessment development process . Sections I ( Health Hazard Assessments for Noncarcinogenic

  16. 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.

  17. 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.

  18. A DOUBLE OUTBURST FROM IGR J00291+5934: IMPLICATIONS FOR ACCRETION DISK INSTABILITY THEORY

    SciTech Connect

    Hartman, Jacob M.; Galloway, Duncan K.; Chakrabarty, Deepto

    2011-01-01

    The accretion-powered millisecond pulsar IGR J00291+5934 underwent two {approx}10 day long outbursts during 2008, separated by 30 days in quiescence. Such a short quiescent period between outbursts has never been seen before from a neutron star X-ray transient. X-ray pulsations at the 599 Hz spin frequency are detected throughout both outbursts. For the first time, we derive a pulse phase model that connects two outbursts, providing a long baseline for spin frequency measurement. Comparison with the frequency measured during the 2004 outburst of this source gives a spin-down during quiescence of -(4 {+-} 1) x 10{sup -15} Hz s{sup -1}, approximately an order of magnitude larger than the long-term spin-down observed in the 401 Hz accretion-powered pulsar SAX J1808.4-3658. If this spin-down is due to magnetic dipole radiation, it requires a 2 x 10{sup 8} G field strength, and its high spin-down luminosity may be detectable with the Fermi Large Area Telescope. Alternatively, this large spin-down could be produced by gravitational wave emission from a fractional mass quadrupole moment of Q/I = 1 x 10{sup -9}. The rapid succession of the outbursts also provides a unique test of models for accretion in low-mass X-ray binaries. Disk instability models generally predict that an outburst will leave the accretion disk too depleted to fuel a second outburst after such a brief quiescence. We suggest a modification in which the outburst is shut off by the onset of a propeller effect before the disk is depleted. This model can explain the short quiescence and the unusually slow rise of the light curve of the second 2008 outburst.

  19. Metal Lines in DA White Dwarfs

    NASA Astrophysics Data System (ADS)

    Zuckerman, B.; Koester, D.; Reid, I. N.; Hünsch, M.

    2003-10-01

    We report Keck telescope HIRES echelle observations of DA white dwarfs in a continuation of an extensive search for metals. These spectra are supplemented with new JHK magnitudes that are used to determine improved atmospheric parameters. Of the DA white dwarfs not in binary or common proper motion systems, about 25% show Ca II lines. For these, Ca abundances are determined from comparison with theoretical equivalent widths from model atmosphere calculations; in a few cases we also obtain Mg, Fe, Si, and Al abundances. If Ca is not observed, we generally determine very stringent upper limits. We compare the data to predictions of previously published models involving the accretion/diffusion of interstellar matter and of comets. The derived abundances are not obviously compatible with the predictions of either model, which up to now could only be tested with traces of metals in helium-rich white dwarfs. By modifying certain assumptions in the published interstellar accretion model we are able to match the distribution of the elements in the white dwarf atmospheres, but, even so, tests of other expectations from this scenario are less successful. Because comet accretion appears unlikely to be the primary cause of the DAZ phenomenon, the data suggest that no more than about 20% of F-type main-sequence stars are accompanied by Oort-like comet clouds. This represents the first observational estimate of this fraction. A plausible alternative to the accretion of cometary or interstellar matter is disruption and accretion of asteroidal material, a model first suggested in 1990 to explain excess near-infrared emission from the DAZ G29-38. An asteroidal debris model to account for the general DAZ phenomenon does not presently disagree with the HIRES data, but neither is there any compelling evidence in support of such a model. The HIRES data indicate that in close red dwarf/white dwarf binaries not known to be cataclysmic variables there is, nonetheless, significant mass

  20. 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 Keplerian disks is then considered in some detail. The radial structure of geometrically thin disk flows, the interaction of disk flows with the neutron star magnetosphere, and models of steady accretion from Keplerian disks are described. Accretion torques and the resulting changes in the spin frequencies of rotating neutron stars are considered. The predicted behavior is then compared with observations of accretion-powered pulsars. Magnetospheric processes that may accelerate particles to very high energies, producing GeV and, perhaps, TeV gamma-rays are discussed. Finally, the mechanisms that decelerate and eventually stop accreting plasma at the surfaces of strongly magnetic neutron stars are described.

  1. 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

  2. On the area of accretion curtains from fast aperiodic time variability of the intermediate polar EX Hya

    NASA Astrophysics Data System (ADS)

    Semena, Andrey N.; Revnivtsev, Mikhail G.; Buckley, David A. H.; Kotze, Marissa M.; Khabibullin, Ildar I.; Breytenbach, Hannes; Gulbis, Amanda A. S.; Coppejans, Rocco; Potter, Stephen B.

    2014-08-01

    We present results of a study of the fast timing variability of the magnetic cataclysmic variable (mCV) EX Hya. It was previously shown that one may expect the rapid flux variability of mCVs to be smeared out at time-scales shorter than the cooling time of hot plasma in the post-shock region of the accretion curtain near the white dwarf (WD) surface. Estimates of the cooling time and the mass accretion rate, thus provide us with a tool to measure the density of the post-shock plasma and the cross-sectional area of the accretion funnel at the WD surface. We have probed the high frequencies in the aperiodic noise of one of the brightest mCV EX Hya with the help of optical telescopes, namely Southern African Large Telescope and the South African Astronomical Observatory 1.9 m telescope. We place upper limits on the plasma cooling time-scale τ < 0.3 s, on the fractional area of the accretion curtain footprint f < 1.6 × 10-4, and a lower limit on the specific mass accretion rate Ṁ/A>3 g s-1 cm-2. We show that measurements of accretion column footprints via eclipse mapping highly overestimate their areas. We deduce a value of Δr/r ≲ 10- 3 as an upper limit to the penetration depth of the accretion disc plasma at the boundary of the magnetosphere.

  3. Disk Variability and Pulsation in the Be Star π Aquarii

    NASA Astrophysics Data System (ADS)

    Peters, Geraldine J.; Gies, Douglas R.; Wang, Luqian

    2015-01-01

    π Aqr is a bright Be star that lost its circumstellar disk in the late-1990s after showing strong disk emission lines for about five decades. We have analyzed spectra in the Hα/He I 6678 region that were obtained during the hiatus in its mass loss and the epoch of early disk buildup afterwards to investigate the star's pulsation and its possible connection with mass loss activity. The spectra were obtained with the Coudé Feed Telescope at KPNO during three observing runs on 1999 November 20-29, 2000 October 29 - November 3, and 2001 January 4-8. A total of 55 images with a S/N~350 and spectral resolution of 0.103 Å/pixel were obtained. The time resolution was 15 m and the observation sets spanned 1.5-3.0 hr. Rapid nonradial pulsations (NRP) with l=|m| =5 were observed with a period of 1.88 ± 0.02 hours. The motion was prograde for a rotation period of 1.8 days. Pulsation amplitudes were largest during the middle observing run. The power in the high frequency signal declined in the final run accompanied by an increase in the low frequency power (as in HD 49330, Huat et al. 2009) suggesting that p waves may have been replaced with g waves. The photospheric lines are broader during a mass loss episode (increased Hα emission). The NRP variations in Hα during 2000 Nov. 1 suggest a formation in a low pressure gas perhaps at the equator. The NRP bumps are slightly broader in Hα than in He I and C II, which implies a photospheric origin. Since the structure is quite visible in Hα, the apparent NRP is probably occurring in the upper atmosphere, as Stark line broadening would render the features more diffuse if they prevailed at deep layers. Narrow stationary violet and red-shifted features that varied in strength on the time scale of the pulsations were observed in Hα, and suggest that disk changes may be driven by pulsation. Additional spectra from KPNO and the BeSS archive reveal that Hα disk emission peaked in 2011 July (comparable to that observed in 1993

  4. Accretion in the galactic halo

    NASA Astrophysics Data System (ADS)

    Stephens, Alex Courtney

    2000-10-01

    The Milky Way disk is enveloped in a diffuse, dynamically-hot collection of stars and star clusters collectively known as the ``stellar halo''. Photometric and chemical analyses suggest that these stars are ancient fossils of the galaxy formation epoch. Yet, little is known about the origin of this trace population. Is this system merely a vestige of the initial burst of star formation within the decoupled proto-Galaxy, or is it the detritus of cannibalized satellite galaxies? In an attempt to unravel the history of the Milky Way's stellar halo, I performed a detailed spectroscopic analysis of 55 metal-poor stars possessing ``extreme'' kinematic properties. It is thought that stars on orbits that either penetrate the remote halo or exhibit large retrograde velocities could have been associated with assimilated (or ``accreted'') dwarf galaxies. The hallmark of an accreted halo star is presumed to be a deficiency (compared with normal stars) of the α-elements (O, Mg, Si, Ca, Ti) with respect to iron, a consequence of sporadic bursts of star formation within the diminutive galaxies. Abundances for a select group of light metals (Li, Na, Mg, Si, Ca, Ti), iron-peak nuclides (Cr, Fe, Ni), and neutron-capture elements (Y, Ba) were calculated using line-strengths measured from high-resolution, high signal-to-noise spectral observations collected with the Keck I 10-m and KPNO 4-m telescopes. The abundances extracted from the spectra reveal: (1)The vast majority of outer halo stars possess supersolar [α/Fe] > 0.0) ratios. (2)The [α/Fe] ratio appears to decrease with increasing metallicity. (3)The outer halo stars have lower ratios of [α/Fe] than inner halo stars at a given metallicity. (4)At the largest metallicities, there is a large spread in the observed [α/Fe] ratios. (5)[α/Fe] anti-correlates with RAPO. (6)Only one star (BD+80° 245) exhibits the peculiar abundances expected of an assimilated star. The general conclusion extracted from these data is that the

  5. Features of Pc5 pulsations in the geomagnetic field, auroral luminosity, and Riometer absorption

    NASA Astrophysics Data System (ADS)

    Belakhovsky, V. B.; Pilipenko, V. A.; Samsonov, S. N.; Lorentsen, D.

    2016-01-01

    Simultaneous morning Pc5 pulsations ( f ~ 3-5 mHz) in the geomagnetic field, aurora intensities (in the 557.7 and 630.0 nm oxygen emissions and the 471.0 nm nitrogen emission), and riometer absorption, were studied based on the CARISMA, CANMOS, and NORSTAR network data for the event of January 1, 2000. According to the GOES-8 satellite observations, these Pc5 geomagnetic pulsations are observed as incompressible Alfvén waves with toroidal polarization in the magnetosphere. Although the Pc5 pulsation frequencies in auroras, the geomagnetic field, and riometer absorption are close to one another, stable phase relationships are not observed between them. Far from all trains of geomagnetic Pc5 pulsations are accompanied by corresponding auroral pulsations; consequently, geomagnetic pulsations are primary with respect to auroral pulsations. Both geomagnetic and auroral pulsations propagate poleward, and the frequency decreases with increasing geomagnetic latitude. When auroral Pc5 pulsations appear, the ratio of the 557.7/630.0 nm emission intensity sharply increases, which indicates that auroral pulsations result from not simply modulated particle precipitation but also an additional periodic acceleration of auroral electrons by the wave field. A high correlation is not observed between Pc5 pulsations in auroras and the riometer absorption, which indicates that these pulsations have a common source but different generation mechanisms. Auroral luminosity modulation is supposedly related to the interaction between Alfvén waves and the region with the field-aligned potential drop above the auroral ionosphere, and riometer absorption modulation is caused by the scattering of energetic electrons by VLF noise pulsations.

  6. Red-Line (630nm) Pulsating Auroras And Their Possible Magnetospheric Driver

    NASA Astrophysics Data System (ADS)

    Liang, J.; Donovan, E.; Zhang, X.; Spanswick, E.; Gillies, M.; Jackel, B. J.

    2015-12-01

    Pulsating auroras are usually known to be led by the precipitation of energetic electrons from the central plasma sheet, and are thus often displayed in blue-line and/or green-line auroral emission lines. In this study we report the observations of 630nm oxygen red-line auroras by REGO imager, and explore their possible underlying mechanisms. Upon inspecting the temporal/spatial pattern of the red-line pulsating auroras and comparing with concurrent/collocated observations from THEMIS and RAINBOW imagers, we distinguish two types of red-line pulsating auroras. In one type of red-line pulsating aurora, the emission is weak in intensity, but its pulsation is synchronous with that of the collocated green-line pulsating aurora. The other type of red-line pulsating aurora is fairly strong in intensity, but its temporal pattern contains much longer-period components as compared to the green-line pulsating aurora. In both types of pulsating auroras, the spatial dimension and motion speed of the red- and green-line auroral patches are rather comparable. We suggest that the first type of red-line pulsating aurora is caused by secondary electrons and/or a cascading excitation of O(1D) as the byproduct of the primary electron precipitation (and the resulting green-line emission), while the second type of red-line pulsating aurora is directly led by the low-energy part (hundreds of eV) of the electron precipitation in the lower F-region ionosphere. In the latter regard, we present in-situ RBSP observations conjugate to the pulsating auroral patches in a few events, and explore a potential relationship between the red-line pulsating aurora and the electron cyclotron harmonic (ECH) wave, which is capable of scattering the low-energy electrons into the loss-cone in the inner magnetosphere.

  7. White Rock

    NASA Technical Reports Server (NTRS)

    2005-01-01

    14 November 2005 This Mars Global Surveyor (MGS) Mars Orbiter Camera (MOC) image shows a portion of the famous 'White Rock' feature in Pollack Crater in the Sinus Sabaeus region of Mars. The light-toned rock is not really white, but its light tone caught the eye of Mars geologists as far back as 1972, when it was first spotted in images acquired by Mariner 9. The light-toned materials are probably the remains of a suite of layered sediments that once spread completely across the interior of Pollack Crater. Dark materials in this image include sand dunes and large ripples.

    Location near: 8.1oS, 335.1oW Image width: width: 3 km (1.9 mi) Illumination from: lower left Season: Southern Summer

  8. White Blood Cell Count

    MedlinePlus

    ... Home Visit Global Sites Search Help? White Blood Cell Count Share this page: Was this page helpful? Also ... Leukocyte Count; White Count Formal name: White Blood Cell Count Related tests: Complete Blood Count , Blood Smear , White ...

  9. Search for Pulsations from a Nearby Millisecond Pulsar and Wasilewski 49: Mirror for a Hidden Seyfert 1 Nucleus

    NASA Technical Reports Server (NTRS)

    Halpern, Jules P.

    1999-01-01

    , such modulation will be further evidence that surface reheating by the impact of particles accelerated along open fiel;d lines operates in these approximately 10(exp -9) year old pulsars. In a second study, a new AM Her star serendipitously in a 25 day observation was detected with the EUVE satellite. A coherent period of 85.82 min is present in the EUVE Deep Survey imager light curve of this source. A spectroscopic optical identification is made with a 19th magnitude blue star that has H and He emission lines, and broad cyclotron humps typical of a magnetic cataclysmic variable. A lower limit to the polar magnetic field of 50 MG is estimated from the spacing of the cyclotron harmonics. EUVE J0425.6-5714 is also detected in archival ROSAT HRI observations spanning two months, and its stable and highly structured light curve permits us to fit a coherent ephemeris linking the ROSAT and EUVE data over a 1.3 yr gap. The derived period is 85.82107 +/- 0.00020 min, and the ephemeris should be accurate to 0.1 cycles until the year 2005. A narrow but partial X-ray eclipse suggests that this object belongs to the group of AM Her stars whose viewing geometry is such that the accretion stream periodically occults the soft X-ray emitting accretion spot on the surface of the white dwarf. A non-detection of hard X-rays from ASCA observations that are contemporaneous with the ROSAT HRI shows that the soft X-rays must dominate by at least an order of magnitude, which is consistent with a known trend among AM Her stars with large magnetic field. This object should not be confused with the Seyfert galaxy IH 0419-577 (= LB 1727), another X-ray/EUV source which lies only 3'95 away, and was the principal target of these monitoring observations. In a third report; the identity of the persistent high-energy (> 100 MeV) gamma-ray sources in the Galaxy, still largely a mystery is investigated. The second installment of the EGRET (2EG) lists a total of 128 sources, of which 51 are likely or

  10. Workshop on Physics of Accretion Disks Around Compact and Young Stars

    NASA Technical Reports Server (NTRS)

    Liang, E (Editor); Stepinski, T. F. (Editor)

    1995-01-01

    The purpose of the two-day Workshop on Physics of Accretion Disks Around Compact and Young Stars was to bring together workers on accretion disks in the western Gulf region (Texas and Louisiana). Part 2 presents the workshop program, a list of poster presentations, and a list of workshop participants. Accretion disks are believed to surround many stars. Some of these disks form around compact stars, such as white dwarfs, neutron stars, or black holes that are members of binary systems and reveal themselves as a power source, especially in the x-ray and gamma regions of the spectrum. On the other hand, protostellar disks are believed to be accretion disks associated with young, pre-main-sequence stars and manifest themselves mostly in infrared and radio observations. These disks are considered to be a natural outcome of the star formation process. The focus of this workshop included theory and observations relevant to accretion disks around compact objects and newly forming stars, with the primary purpose of bringing the two communities together for intellectual cross-fertilization. The nature of the workshop was exploratory, to see how much interaction is possible between distinct communities and to better realize the local potential in this subject. A critical workshop activity was identification and documentation of key issues that are of mutual interest to both communities.

  11. The occurrence of non-pulsating stars in the γ Dor and δ Sct pulsation instability regions: Results from Kepler quarter 14–17 data

    SciTech Connect

    Guzik, J. A.; Bradley, P. A.; Jackiewicz, J.; Molenda-Zakowicz, J.; Uytterhoeven, K.; Kinemuchi, K.

    2015-04-21

    In this study, the high precision long time-series photometry of the NASA Kepler spacecraft provides an excellent means to discover and characterize variability in main-sequence stars, and to make progress in interpreting the pulsations to derive stellar interior structure and test stellar models. For stars of spectral types A–F, the Kepler data revealed a number of surprises, such as more hybrid pulsating Sct and Dor pulsators than expected, pulsators lying outside of the instability regions predicted by theory, and stars that were expected to pulsate, but showed no variability. In our 2013 Astronomical Review article, we discussed the statistics of variability for 633 faint (Kepler magnitude 14–16) spectral type A–F stars observed by Kepler during Quarters 6–13 (June 2010–June 2012).

  12. The occurrence of non-pulsating stars in the γ Dor and δ Sct pulsation instability regions: Results from Kepler quarter 14–17 data

    DOE PAGESBeta

    Guzik, J. A.; Bradley, P. A.; Jackiewicz, J.; Molenda-Zakowicz, J.; Uytterhoeven, K.; Kinemuchi, K.

    2015-04-21

    In this study, the high precision long time-series photometry of the NASA Kepler spacecraft provides an excellent means to discover and characterize variability in main-sequence stars, and to make progress in interpreting the pulsations to derive stellar interior structure and test stellar models. For stars of spectral types A–F, the Kepler data revealed a number of surprises, such as more hybrid pulsating Sct and Dor pulsators than expected, pulsators lying outside of the instability regions predicted by theory, and stars that were expected to pulsate, but showed no variability. In our 2013 Astronomical Review article, we discussed the statistics ofmore » variability for 633 faint (Kepler magnitude 14–16) spectral type A–F stars observed by Kepler during Quarters 6–13 (June 2010–June 2012).« less

  13. WHITE DWARF/M DWARF BINARIES AS SINGLE DEGENERATE PROGENITORS OF TYPE Ia SUPERNOVAE

    SciTech Connect

    Wheeler, J. Craig

    2012-10-20

    Limits on the companions of white dwarfs in the single-degenerate scenario for the origin of Type Ia supernovae (SNe Ia) have gotten increasingly tight, yet igniting a nearly Chandrasekhar mass C/O white dwarf from a condition of near hydrostatic equilibrium provides compelling agreement with observed spectral evolution. The only type of non-degenerate stars that survive the tight limits, M{sub V} {approx}> 8.4 on the SN Ia in SNR 0509-67.5 and M{sub V} {approx}> 9.5 in the remnant of SN 1572, are M dwarfs. While M dwarfs are observed in cataclysmic variables, they have special properties that have not been considered in most work on the progenitors of SNe Ia: they have small but finite magnetic fields and they flare frequently. These properties are explored in the context of SN Ia progenitors. White dwarf/M dwarf pairs may be sufficiently plentiful to provide, in principle, an adequate rate of explosions even with slow orbital evolution due to magnetic braking or gravitational radiation. Even modest magnetic fields on the white dwarf and M dwarf will yield adequate torques to lock the two stars together, resulting in a slowly rotating white dwarf, with the magnetic poles pointing at one another in the orbital plane. The mass loss will be channeled by a 'magnetic bottle' connecting the two stars, landing on a concentrated polar area on the white dwarf. This enhances the effective rate of accretion compared to spherical accretion. Luminosity from accretion and hydrogen burning on the surface of the white dwarf may induce self-excited mass transfer. The combined effects of self-excited mass loss, polar accretion, and magnetic inhibition of mixing of accretion layers give possible means to beat the 'nova limit' and grow the white dwarf to the Chandrasekhar mass even at rather moderate mass accretion rates.

  14. White Dwarf/M Dwarf Binaries as Single Degenerate Progenitors of Type Ia Supernovae

    NASA Astrophysics Data System (ADS)

    Wheeler, J. Craig

    2012-10-01

    Limits on the companions of white dwarfs in the single-degenerate scenario for the origin of Type Ia supernovae (SNe Ia) have gotten increasingly tight, yet igniting a nearly Chandrasekhar mass C/O white dwarf from a condition of near hydrostatic equilibrium provides compelling agreement with observed spectral evolution. The only type of non-degenerate stars that survive the tight limits, MV >~ 8.4 on the SN Ia in SNR 0509-67.5 and MV >~ 9.5 in the remnant of SN 1572, are M dwarfs. While M dwarfs are observed in cataclysmic variables, they have special properties that have not been considered in most work on the progenitors of SNe Ia: they have small but finite magnetic fields and they flare frequently. These properties are explored in the context of SN Ia progenitors. White dwarf/M dwarf pairs may be sufficiently plentiful to provide, in principle, an adequate rate of explosions even with slow orbital evolution due to magnetic braking or gravitational radiation. Even modest magnetic fields on the white dwarf and M dwarf will yield adequate torques to lock the two stars together, resulting in a slowly rotating white dwarf, with the magnetic poles pointing at one another in the orbital plane. The mass loss will be channeled by a "magnetic bottle" connecting the two stars, landing on a concentrated polar area on the white dwarf. This enhances the effective rate of accretion compared to spherical accretion. Luminosity from accretion and hydrogen burning on the surface of the white dwarf may induce self-excited mass transfer. The combined effects of self-excited mass loss, polar accretion, and magnetic inhibition of mixing of accretion layers give possible means to beat the "nova limit" and grow the white dwarf to the Chandrasekhar mass even at rather moderate mass accretion rates.

  15. Quantitative results of stellar evolution and pulsation theories.

    NASA Technical Reports Server (NTRS)

    Fricke, K.; Stobie, R. S.; Strittmatter, P. A.

    1971-01-01

    The discrepancy between the masses of Cepheid variables deduced from evolution theory and pulsation theory is examined. The effect of input physics on evolutionary tracks is first discussed; in particular, changes in the opacity are considered. The sensitivity of pulsation masses to opacity changes and to the ascribed values of luminosity and effective temperature are then analyzed. The Cepheid mass discrepancy is discussed in the light of the results already obtained. Other astronomical evidence, including the mass-luminosity relation for main sequence stars, the solar neutrino flux, and cluster ages are also considered in an attempt to determine the most likely source of error in the event that substantial mass loss has not occurred.

  16. Comparative pulsation calculations with OP and OPAL opacities

    NASA Technical Reports Server (NTRS)

    Kanbur, Shashi M.; Simon, Norman R.

    1994-01-01

    Comparative linear nonadiabatic pulsation calculations are presented using the OPAL and Opacity Project opacities. The two sets of opacities include effects due to intermediate coupling and fine structure as well as new abundances. We used two mass luminosity (M-L) relations, one standard (BIT), and one employing substantial convective core overshoot (COV). The two sets of opacities cannot be differentiated on the basis of the stellar pulsation calculations presented here. The BIT relation can model the beat and bump Cepheids with masses between 4 and 7 solar mass, while if the overshoot relation is used, masses between 2 and 6 solar mass are required. In the RR Lyrae regime, we find the inferred masses of globular cluster RRd stars to be little influenced by the choice of OPAL or OP. Finally, the limited modeling we have done is not able to constrain the Cepheid M-L relation based upon period ratios observed in the beat and bump stars.

  17. Experimental investigation on a pulsating heat pipe with hydrogen

    NASA Astrophysics Data System (ADS)

    Deng, H. R.; Liu, Y. M.; Ma, R. F.; Han, D. Y.; Gan, Z. H.; Pfotenhauer, J. M.

    2015-12-01

    The pulsating heat pipe (PHP) has been increasingly studied in cryogenic application, for its high transfer coefficient and quick response. Compared with Nb3Sn and NbTi, MgB2 whose critical transformation temperature is 39 K, is expected to replace some high-temperature superconducting materials at 25 K. In order to cool MgB2, this paper designs a Hydrogen Pulsating Heat Pipe, which allows a study of applied heat, filling ratio, turn number, inclination angle and length of adiabatic section on the thermal performance of the PHP. The thermal performance of the hydrogen PHP is investigated for filling ratios of 35%, 51%, 70% at different heat inputs, and provides information regarding the starting process is received at three filling ratios.

  18. Pulsation effects on the air fuel ratio of carburetor engines

    SciTech Connect

    Tanaka, M.; Sato, T.; Watanabe, K.

    1986-01-01

    A significant wavewise change of air fuel ratio in line with the engine speed having a long intake pipe was studied experimentally and theoretically. The results show the pulsewise change in fuel flow plays the dominant role in the wavewise change in the air fuel ratio. It is found that this pulsewise fluctuation of the fuel flow forms an oscillation wave with both the amplitude and frequency becoming larger according to the engine speed resulted by the phase change of the pulsation wave in the intake pipe according to the engine speed. A modified frequency ratio of gas vibration in the intake pipe to that of engine intake stroke is proposed to explain this pulsation effect on the fuel flow and an effective simulator for this phenomena is established.

  19. Motion analysis of artery pulsation in neonatal cranial ultrasonogram

    NASA Astrophysics Data System (ADS)

    Fukuzawa, Masayuki; Kubo, Hiroki; Kitsunezuka, Yoshiki; Yamada, Masayoshi

    1999-05-01

    Using an optical-flow technique, we have quantitatively analyzed tissue motion due to artery pulsation accompanied with blood flow in a neonatal cranial ultrasonogram. The tissue motion vector was successfully calculated at each pixel in a series of echo images (32 frames, 640 X 480 pixels/frame, 8 bits/pixel, 33 ms/frame) taken in the brightness mode by using an ultrasound probe of 5.0 MHz. The optical-flow technique used was a gradient method combined with local optimization for 3 X 3 neighbors. From 2D mappings of tissue motion vectors and their time-sequence variations, it was found that the tissue motion due to artery pulsation revealed periodic to-and-fro motion synchronized with heartbeat (300 - 500 ms), clearly distinguishing from unwanted non-periodic motion due to the sway of neonatal head during diagnosis.

  20. Multiple satellite observations of pulsation resonance structure in the magnetosphere

    NASA Technical Reports Server (NTRS)

    Hughes, W. J.; Mcpherron, R. L.; Russell, C. T.

    1977-01-01

    Data from two intervals when pulsation activity was simultaneously observed on both ATS 1 and Ogo 5 satellites are presented. The first example, a Pc 4, indicates that this pulsation is caused by a field line near L = 7 resonating in its second-harmonic mode. This is inferred from both plasma density measurements and polarization characteristics. The wave was not observed at three ground stations in the vicinity of the satellite conjugate points. This indicates that Pc 4 waves are very localized in latitude and that a close array (less than 100 km) is needed to perform effective correlation with satellites. The second event, which is also in the Pc 4 band, can again be inferred to be a field line resonance from the polarization characteristics

  1. Pc 3 pulsation eigenperiod determination at low latitudes

    SciTech Connect

    Hattingh, S.K.F.; Sutcliffe, P.R. )

    1987-11-01

    A realistic method of calculating the eigenperiod of Pc 3 pulsations at low latitudes is discussed. Solution of the problem requires a magnetic field model and a model for the plasma distribution along the resonating field line. The calculated eigenperiods obtained using the dipole field model and the IGRF model are found to be similar. The inclusion of the F region O{sup +} in the plasma distribution noticeably affects the calculated eigenperiod at low latitudes. This effect decreases with increasing L value. Pulsation periods obtained from recordings made at four stations lying on a geomagnetic meridian demonstrate the importance of including O{sup +} in the plasma model if realistic periods are to be calculated at low latitudes.

  2. Impulse-excited pulsations during the July 29, 1977 event

    SciTech Connect

    Nopper, R.W. Jr.; Hughes, W.J.; Maclennan, C.G.; McPherron, R.L.

    1982-08-01

    The propagation of a geomagnetic sudden impulse (si) and the magnetic field pulsations excited by it in the magnetosphere is traced from the bow shock in the solar wind, through the magnetosphere, to the ground. Within the magnetosphere the impulse appears as a compressive magnetohydrodynamic (MHD) impulse that travels rapidly (approx.1500 km/s) tailward. A resonant oscillation observed both in space and on the ground is excited near geostationary orbit. The effect of the si is enhanced by a factor of at least 5 on the ground near the geomagnetic equator. We suggest that discontinuities in the solar wind may be a more important source of exciting dayside pulsations than has been commonly assumed.

  3. Long-period geomagnetic pulsations as solar flare precursors

    NASA Astrophysics Data System (ADS)

    Barkhatov, N. A.; Obridko, V. N.; Revunov, S. E.; Snegirev, S. D.; Shadrukov, D. V.; Sheiner, O. A.

    2016-03-01

    We compare long-period pulsations of the horizontal component of the geomagnetic field at intervals that precede extreme solar flares. To this end, we use the wavelet-skeleton technique to process the geomagnetic field disturbances recorded at magnetic stations over a wide geographical range. The synchronization times of wavelet-skeleton spectral distributions of long-period pulsations of geomagnetic oscillations over all magnetic stations are shown as normalized histograms. A few days before an intense solar flare, the histograms show extremes. This means that these extremes can be regarded as flare precursors. The same technique is used to analyze the parameters of near-Earth space. The histograms obtained in this case are free of the aforementioned extrema and, therefore, cannot point to an upcoming flare. The goal of this study is to construct a correlation-spectral method for the short-term prediction of solar flare activity.

  4. The unique dynamical system underlying RR Lyrae pulsations

    NASA Astrophysics Data System (ADS)

    Kollath, Z.

    2016-05-01

    Hydrodynamic models of RR Lyrae pulsation display a very rich behaviour. Contrary to earlier expectations, high order resonances play a crucial role in the nonlinear dynamics representing the interacting modes. Chaotic attractors can be found at different time scales: both in the pulsation itself and in the amplitude equations shaping the possible modulation of the oscillations. Although there is no one-to-one connection between the nonlinear features found in the numerical models and the observed behaviour, the richness of the found phenomena suggests that the interaction of modes should be taken seriously in the study of the still unsolved puzzle of Blazhko effect. One of the main lessons of this complex system is that we should rethink the simple interpretation of the observed effect of resonances.

  5. Pulsating aurora induced by upper atmospheric barium releases

    NASA Technical Reports Server (NTRS)

    Deehr, C.; Romick, G.

    1977-01-01

    The paper reports the apparent generation of pulsating aurora by explosive releases of barium vapor near 250 km altitude. This effect occurred only when the explosions were in the path of precipitating electrons associated with the visible aurora. Each explosive charge was a standard 1.5 kg thermite mixture of Ba and CuO with an excess of Ba metal which was vaporized and dispersed by the thermite explosion. Traces of Sr, Na, and Li were added to some of the charges, and monitoring was achieved by ground-based spectrophotometric observations. On March 28, 1976, an increase in emission at 5577 A and at 4278 A was observed in association with the first two bursts, these emissions pulsating with roughly a 10 sec period for approximately 60 to 100 sec after the burst.

  6. Characteristics of velocity pulsations in a turbulent recirculated melt flow

    NASA Astrophysics Data System (ADS)

    Kirpo, M.; Jakovics, A.; Baake, E.

    2005-06-01

    For the modern industrial applications it is necessary to develop and investigate metallic and oxide materials of high purity or predicted composition. Such materials can be produced by the induction melting method, especially in inductor and cold crucible furnaces. Measurements taken in experimental furnaces show that the velocity pulsations dominate for the heat and mass exchange in the melt, especially in a zone between typical upper and lower eddies. Understanding of the pulsation mechanism and development of models to estimate the exchange characteristics are very important for the development and optimisation of industrial furnaces. The authors present experimental results and propose a simple 3D large eddy simulation (LES) model of the induction furnace that can be adapted to qualitative analysis of experimental data. Tables 2, Figs 13, Refs 7.

  7. Experimental research on heat transfer of pulsating heat pipe

    NASA Astrophysics Data System (ADS)

    Li, Jia; Yan, Li

    2008-06-01

    Experimental research was conducted to understand heat transfer characteristic of pulsating heat pipe in this paper, and the PHP is made of high quality glass capillary tube. Under different fill ratio, heat transfer rate and many other influence factors, the flow patterns were observed in the start-up, transition and stable stage. The effects of heating position on heat transfer were discussed. The experimental results indicate that no annular flow appears in top heating condition. Under different fill ratios and heat transfer rate, the flow pattern in PHP is transferred from bulk flow to semi-annular flow and annular flow, and the performance of heat transfer is improved for down heating case. The experimental results indicate that the total heat resistant of PHP is increased with fill ratio, and heat transfer rate achieves optimum at filling rate 50%. But for pulsating heat pipe with changing diameters the thermal resistance is higher than that with uniform diameters.

  8. Hydrodynamic Modeling of Pulsation-Initiated Luminous Blue Variable Outbursts

    NASA Astrophysics Data System (ADS)

    Onifer, Andrew J.; Guzik, J. A.

    2007-12-01

    Luminous Blue Variables (LBVs) are characterized by semi-periodic episodes of outburst. The cause of these outbursts has thus far been a mystery. One possible explanation is that they are initiated by pulsations in the atmosphere caused by a buildup of luminosity at temperatures near the so-called "iron bump" ( 200,000 K). Due to a lag in the onset of convection in the presence of these pulsations, this luminosity can build until it exceeds the Eddington limit locally, driving some mass from the star. We present results from a parameter study focusing on the conditions necessary to trigger normal AG Car-like (as opposed to extreme η Carinae-like) outbursts. The implications for mass loss will also be discussed. This work was performed under the auspices of the U. S. Department of Energy by the Los Alamos National Security (LANS), LLC under contract No. DE-AC52-06NA25396.

  9. Analysis of variability in the burst oscillations of the accreting millisecond pulsar XTE J1814-338

    NASA Technical Reports Server (NTRS)

    Watts, Anna L.; Strohmayer, Tod E.; Markwardt, Craig B.

    2005-01-01

    The accreting millisecond pulsar XTE J1814-338 exhibits oscillations at the known spin frequency during Type I X-ray bursts. The properties of the burst oscillations reflect the nature of the thermal asymmetry on the stellar surface. We present an analysis of the variability of the burst oscillations of this source, focusing on three characteristics: fractional amplitude, harmonic content and frequency. Fractional amplitude and harmonic content constrain the size, shape and position of the emitting region, whilst variations in frequency indicate motion of the emitting region on the neutron star surface. We examine both long-term variability over the course of the outburst, and short-term variability during the bursts. For most of the bursts, fractional amplitude is consistent with that of the accretion pulsations, implying a low degree of fuel spread. There is however a population of bursts whose fractional amplitudes are substantially lower, implying a higher degree of fuel spread, possibly forced by the explosive burning front of a precursor burst. For the first harmonic, substantial differences between the burst and accretion pulsations suggest that hotspot geometry is not the only mechanism giving rise to harmonic content in the latter. Fractional amplitude variability during the bursts is low; we can only rule out the hypothesis that the fractional amplitude remains constant at the l(sigma) level for bursts that do not exhibit photospheric radius expansion (PRE). There are no significant variations in frequency in any of the bursts except for the one burst that exhibits PRE. This burst exhibits a highly significant but small (= 0.1Hz) drop in frequency in the burst rise. The timescale of the frequency shift is slower than simple burning layer expansion models predict, suggesting that other mechanisms may be at work.

  10. Self-Sustained Ultrafast Pulsation in Coupled VCSELs

    NASA Technical Reports Server (NTRS)

    Ning, Cun-Zheng

    2001-01-01

    High frequency, narrow-band self-pulsating operation is demonstrated in two coupled vertical-cavity surface-emitting lasers (VCSELs). The coupled VCSELs provide an ideal source for high-repetition rate (over 40 GHz), sinusoidal-like modulated laser source with Gaussian-like near- and far-field profiles. We also show that the frequency of the modulation can be tuned by the inter-VCSEL separation or by DC-bias level.

  11. A Disintegrating Minor Planet Transiting a White Dwarf

    NASA Astrophysics Data System (ADS)

    Vanderburg, Andrew; Johnson, John Asher; Rappaport, Saul; Bieryla, Allyson; Irwin, Jonathan; Lewis, John; Charbonneau, David; Latham, David W.; Ciardi, David; Schaefer, Laura; Kipping, David; Angus, Ruth; Eastman, Jason; Wright, Jason; McCrady, Nate; Wittenmyer, Robert; Dufour, Patrick

    2015-12-01

    Over the past decade, evidence has accumulated suggesting that the photospheres of many white dwarfs are polluted by the remnants of small rocky bodies leftover from the progenitors' planetary systems. The evidence for this scenario is typically indirect and circumstantial. We report observations of a disintegrating minor planet transiting a polluted white dwarf. The transits are 5 minutes long, up to 40% deep, have an asymmetric profile and highly variable transit depths. This system provides strong corroborating evidence for the planet accretion model for white dwarf pollution and lets us watch the destruction of a solar system in real time.

  12. Are dayside long-period pulsations related to the cusp?

    NASA Astrophysics Data System (ADS)

    Pilipenko, V.; Belakhovsky, V.; Engebretson, M. J.; Kozlovsky, A.; Yeoman, T.

    2015-03-01

    We compare simultaneous observations of long-period ultra-low-frequency (ULF) wave activity from a Svalbard/IMAGE fluxgate magnetometer latitudinal profile covering the expected cusp geomagnetic latitudes. Irregular Pulsations at Cusp Latitudes (IPCL) and narrowband Pc5 waves are found to be a ubiquitous element of ULF activity in the dayside high-latitude region. To identify the ionospheric projections of the cusp, we use the width of return signal of the Super Dual Auroral Radar Network (SuperDARN) radar covering the Svalbard archipelago, predictions of empirical cusp models, augmented whenever possible by Defense Meteorological Satellite Program (DMSP) identification of magnetospheric boundary domains. The meridional spatial structure of broadband dayside Pc5-6 pulsation spectral power has been found to have a localized latitudinal peak, not under the cusp proper as was previously thought, but several degrees southward from the equatorward cusp boundary. The earlier claims of the dayside monochromatic Pc5 wave association with the open-closed boundary also seems doubtful. Transient currents producing broadband Pc5-6 probably originate at the low-latitude boundary layer/central plasma sheet (LLBL/CPS) interface, though such identification with available DMSP data is not very precise. The occurrence of broadband Pc5-6 pulsations in the dayside boundary layers is a challenge to modelers because so far their mechanism has not been firmly identified.

  13. The anticorrelation of auroral arc and Pc5 pulsation occurrence

    NASA Astrophysics Data System (ADS)

    Donovan, E.; Knudsen, D.; Rankin, R.; Baker, G.; Jackel, B.; Cogger, L.; Wallis, D.

    2003-04-01

    Using extensive data sets from the CANOPUS All-Sky Imager (ASI) and magnetometer at Gillam, Canada (manetic latitude 67 degrees), we have compiled occurrence statistics of Pc5 pulsations, and auroral arcs. For our purpose, Pc5 pulsations were defined as monochromatic, quasisunsoidal magnetic perturbations, with a frequency between 1.7 and 6.7 mHz, and that underwent at least four complete cycles. Auroral arcs were defined to be elongated auroral features. We find, consistent with the results of previous studies, that Pc5 pulsation occurrence peaks near both the dawn and dusk meridians, and auroral arc occurrence in the late evening sector, near 2300 hours MLT. We discuss the implications of these results for candidate auroral mechanism, in particular those which demand time variation ( ie., the field line resonance) versus those that rely on static processes, showing examples of auroral arcs which display characteristics which could be attributed to mechanisms from one or the other category. We conclude that while it is clear that field-line resonances with frequencies in the Pc5 band cause or at least modulate electron precipitation in some arcs, there are equally clearly arcs for which this is not true.

  14. Pulsating proton aurora caused by rising tone Pc1 waves

    NASA Astrophysics Data System (ADS)

    Nomura, R.; Shiokawa, K.; Omura, Y.; Ebihara, Y.; Miyoshi, Y.; Sakaguchi, K.; Otsuka, Y.; Connors, M.

    2016-02-01

    We found rising tone emissions with a dispersion of ˜1 Hz per several tens of seconds in the dynamic spectrum of a Pc1 geomagnetic pulsation (Pc1) observed on the ground. These Pc1 rising tones were successively observed over ˜30 min from 0250 UT on 14 October 2006 by an induction magnetometer at Athabasca, Canada (54.7°N, 246.7°E, magnetic latitude 61.7°N). Simultaneously, a Time History of Events and Macroscale Interactions during Substorms panchromatic (THEMIS) all-sky camera detected pulsations of an isolated proton aurora with a period of several tens of seconds, ˜10% variations in intensity, and fine structures of 3° in magnetic longitudes. The pulsations of the proton aurora close to the zenith of ATH have one-to-one correspondences with the Pc1 rising tones. This suggests that these rising tones scatter magnetospheric protons intermittently at the equatorial region. The radial motion of the magnetospheric source, of which the isolated proton aurora is a projection, can explain the central frequency increase of Pc1, but not the shorter period (tens of seconds) frequency increase of ˜1 Hz in Pc1 rising tones. We suggest that EMIC-triggered emissions generate the frequency increase of Pc1 rising tones on the ground and that they also cause the Pc1 pearl structure, which has a similar characteristic time.

  15. An Observational Study of Pulsations in Proto-Planetary Nebulae

    NASA Astrophysics Data System (ADS)

    Hrivnak, Bruce J.; Lu, Wenxian; Henson, Gary D.; Hillwig, Todd C.

    2016-01-01

    We have been carrying out a long-term monitoring program to study the light variability in proto-planetary nebulae (PPNe). PPNe are post-Asymptotic Giant Branch objects in transition between the AGB and PN phases in the evolution of low and intermediate-mass stars. As such, it is not surprising that they display pulsational variability. We have been carrying out photometric monitoring of 30 of these at the Valparaiso University campus observatory over the last 20 years, with the assistance of undergraduate students. The sample size has been enlarged over the past six years by observations made using telescopes in the SARA consortium at KPNO and CTIO. Periods have been determined for those of F-G spectral types. We have also enlarged the sample with PPNe from outside the Milky Way by determining periods of eight PPNe in the lower metalicity environment of the Magellanic Clouds. Periods for the entire sample range from 35 to 160 days. Some clear patterns have emerged, with those of higher temperature possessing shorter periods and smaller amplitudes, indicating a reduction in period and pulsation amplitude as the objects evolve. Radial velocity monitoring of several of the brightest of these has allowed us to document their changes in brightness, color, and size during a pulsation cycle. The results of this study will be presented. This research is supported by grants from the National Science Foundation (most recently AST 1413660), with additional student support from the Indiana Space Grant Consortium.

  16. The pulsating laminar flow in a rectangular channel

    NASA Astrophysics Data System (ADS)

    Valueva, E. P.; Purdin, M. S.

    2015-11-01

    The finite difference method is used to solve the task of the developed pulsating laminar flow in a rectangular channel. The optimum of the difference scheme parameters was determined. Data on the amplitude and phase of the longitudinal velocity oscillations, the hydraulic and friction drag coefficients, the shear stress on the wall have been obtained. Using the dimensionless value of the frequency pulsations two characteristic regimes — the quasisteady-state regime and the high-frequency regime have been identified. In the quasi-steady-state regime, the values of all hydrodynamic quantities at each instant of time correspond to the velocity value averaged over the cross section at a given moment of time. It is shown that in the high-frequency regime, the dependences on the dimensionless oscillation frequency of oscillating components of hydrodynamic quantities are identical for rectilinear channels with a different cross-sectional form (round pipe, flat and a rectangular channels). The effect of the aspect ratio of the rectangular channel sides channel on the pulsating flow dynamics has been analyzed.

  17. Russian Pulsating Mixer Pump. Innovative Technology Summary Report

    SciTech Connect

    2002-03-01

    This sludge mixing/mobilization system was developed in Russia. A prototype system was evaluated by the Tanks Focus Area (TFA) and Industry and University Programs (INDP). The Russian Pulsating Mixer Pump showed promise for mixing highly viscous sludges. This project is to refine the system design (especially the control subsystem) and manufacture the system in Russia in accordance with quality standards required for deployment in radioactive waste storage tanks. Specifications and requirements are being developed by the TFA and INDP. The requirements may call for two or three of the sludge mixing systems to be delivered to Oak Ridge. DOE-Oak Ridge and the Oak Ridge National Laboratory will deploy the pulsating mixing pump system in their Gunite Tanks. These tanks are being emptied and cleaned prior to closure. Oak Ridge has deployed a number of innovative technologies in these efforts. If successful at Oak Ridge, the pulsating mixing pump system has potential application at several other DOE sites, including Savannah River, Hanford, and Idaho.

  18. THE PULSATION MODE AND DISTANCE OF THE CEPHEID FF AQUILAE

    SciTech Connect

    Turner, D. G.; Kovtyukh, V. V.; Luck, R. E.; Berdnikov, L. N. E-mail: val@deneb1.odessa.ua E-mail: leonid.berdnikov@gmail.com

    2013-07-20

    The determination of pulsation mode and distance for field Cepheids is a complicated problem best resolved by a luminosity estimate. For illustration a technique based on spectroscopic luminosity discrimination is applied to the 4.47 day s-Cepheid FF Aql. Line ratios in high dispersion spectra of the variable yield values of (M{sub V} ) = -3.40 {+-} 0.02 s.e. ({+-}0.04 s.d.), average effective temperature T{sub eff} = 6195 {+-} 24 K, and intrinsic color ((B) - (V)){sub 0} = +0.506 {+-} 0.007, corresponding to a reddening of E{sub B-V} = 0.25 {+-} 0.01, or E{sub B-V}(B0) = 0.26 {+-} 0.01. The skewed light curve, intrinsic color, and luminosity of FF Aql are consistent with fundamental mode pulsation for a small-amplitude classical Cepheid on the blue side of the instability strip, not a sinusoidal pulsator. A distance of 413 {+-} 14 pc is estimated from the Cepheid's angular diameter in conjunction with a mean radius of (R) = 39.0 {+-} 0.7 R{sub Sun} inferred from its luminosity and effective temperature. The dust extinction toward FF Aql is described by a ratio of total-to-selective extinction of R{sub V} = A{sub V} /E(B - V) = 3.16 {+-} 0.34 according to the star's apparent distance modulus.

  19. Ionospheric signatures of cusp-latitude Pc 3 pulsations

    SciTech Connect

    Engebretson, M.J. ); Cahill, L.J. Jr. ); Arnoldy, R.L. )

    1988-01-01

    It has been well established that many of the disturbances in the Earth's magnetosphere, such as auroral substorms, are a response to variations in the solar wind that continually sweeps from the Sun past the Earth and other planets. Studies over the past several years, most recently reviewed by Odera (1986) and Arnoldy at el. (1988), have shown that Pc 3 pulsations, a class of ultra-low-frequency waves in the Earth's magnetic field with periods between 15 and 40 seconds, are also directly related to activity in the solar wind just upstream of the Earth. The authors present in this report new observations from South Pole Station, Antarctica, which during certain hours every day is located under the nominal position of the magnetospheric cleft/cusp region. There has been ample evidence that plasmas from interplanetary space can penetrate to ionospheric altitudes in the cusp region. Two earlier papers based on South Pole data noted that large-amplitude, narrowband Pc 3 magnetic pulsations occurred at South Pole Station near local magnetic noon when the interplanetary magnetic field was aligned near the Earth-Sun direction (low interplanetary magnetic field cone angle). They have now found evidence of these pulsations in data from other South Pole instruments as well.

  20. Harmonic structure of Pc 3--4 pulsations

    SciTech Connect

    Takahashi, K.; McPherron, R.L.

    1982-03-01

    Power spectra of magnetic pulsations observed at synchronous orbit by the ATS 6 satellite often show several spectral peaks simultaneously. Such pulsations, which we call harmonic events because of the nearly constant separation between successive peaks, are continuously observed in the dayside in the Pc 3--4 frequency range (6.6--100 mHz). The harmonic events are seen clearly only in the east-west magnetic field component. The spectral peaks are regularly spaced with a typical minimum separation of 14 mHz in the morining gradually decreasing to 10 mHz in the afternoon. In the dynamic spectra of harmonic events, the fundamental mode is usually absent. In addition, the relative amplitudes of the higher harmonics depend on the magnetic latitude. These observed features can be explained by a standing Alfven wave consisting of many discrete harmonic frequencies. A statistical analysis of power spectra demonstrates that at least 10--30% of Pc 3 pulsations can be classified as harmonic events. Using the harmonic events, we are able to estimate the plasma mass density. For a selected event on August 7, 1975, the plasma mass density at the synchronous orbit is estimated to be 3--8 hydrogen mass/cm/sup 3/.

  1. [Relation between microcirculation parameters and Pc3 geomagnetic pulsations].

    PubMed

    Zenchehko, T A; Poskotinova, L V; Rekhtina, A G; Zaslavskaia, R M

    2010-01-01

    An individual analysis of long-term monitoring of microcirculation parameters of nine healthy volunteers showed that an increase in the geomagnetic activity led to an increase in tissue perfusion, variability of blood flow and growth of the amplitude of neurogenic and myogenic oscillations in four volunteers. It was found that the degree of microcirculation sensitivity to the level of geomagnetic activity values with time and is proportional to its average level in the period of measurement. A comparison of frequency ranges of oscillations of blood flow and variations of the geomagnetic activity shows that neurogenic and myogenic oscillations showing the highest sensitivity to the geomagnetic activity have the same frequency as geomagnetic Pc3 pulsations. The pulsations of this frequency range are excited mainly during geomagnetic disturbances, which may explain the correlation between the microcirculation parameters and the Kp index. The relation of the amplitude-frequency characteristics of Pc3-pulsations can explain the results obtained using the alternating magnetic fields. PMID:20968090

  2. Synchronous manifestations of 160-min pulsations of the ground pressure

    NASA Astrophysics Data System (ADS)

    Timofeev, V.; Miroshnichenko, L.; Samsonov, S.; Skryabin, N.

    The oscillations of ground pressure with a period of sim 160 min in December 2003 and March 2004 relatively to the zero meridian are studied using 5-min data of 4 stations Moskow Yakutsk Apatity and Tixie separated in longitude The choice of time is caused by the fact that in December the territory of Russia is the nearest to the direction to the Galaxy center under such a choice of the reper point through the Earth The most removal of the zero meridian from this direction is realized in March If we suppose that 160-min pulsations arrive from the Galaxy center then they synchronously manifest themselves most of all on the territory of Russia only in December As the analysis has shown really in December the mentioned oscillations are synchronously manifested in Moscow Yakutsk Apatity and Tixie mainly in the form of packets quanta in 2-5 impulses The mean amplitude of synchronous ground pressure variations on the territory of Russia is approx 0 0115 mb During other seasons the synchronism is observed considerably worse The manifestation of oscillations in the form of packets is also observed worse The authors suppose that 160-min pulsations of the ground pressure are not related to pulsations of the Sun s brightness The the most favorable time for their observations coincides with the moments of appearance of the stations near the direction to the Galaxy center

  3. Mass-spring model of a self-pulsating drop.

    PubMed

    Antoine, Charles; Pimienta, Véronique

    2013-12-01

    Self-pulsating sessile drops are a striking example of the richness of far-from-equilibrium liquid/liquid systems. The complex dynamics of such systems is still not fully understood, and simple models are required to grasp the mechanisms at stake. In this article, we present a simple mass-spring mechanical model of the highly regular drop pulsations observed in Pimienta, V.; Brost, M.; Kovalchuk, N.; Bresch, S.; Steinbock, O. Complex shapes and dynamics of dissolving drops of dichloromethane. Angew. Chem., Int. Ed. 2011, 50, 10728-10731. We introduce an effective time-dependent spreading coefficient that sums up all of the forces (due to evaporation, solubilization, surfactant transfer, coffee ring effect, solutal and thermal Marangoni flows, drop elasticity, etc.) that pull or push the edge of a dichloromethane liquid lens, and we show how to account for the periodic rim breakup. The model is examined and compared against experimental observations. The spreading parts of the pulsations are very rapid and cannot be explained by a constant positive spreading coefficient or superspreading. PMID:24200165

  4. Stellar Pulsations Excited by Planetary Tides in WASP-33

    NASA Astrophysics Data System (ADS)

    Cameron, Andrew; Guenther, E.; Matthews, J. M.; Amado, P. J.; McDonald, I.; Shkolnik, E.; Smith, A. M. S.; Telting, J.; Walker, G. A. H.; MOST Science Team

    2011-09-01

    The bright, rapidly-rotating A5 star HD 15082 (= WASP-33) has a transiting gas-giant planet in a 1.22-day retrograde orbit, only 5.5 stellar radii from the stellar photosphere (Collier Cameron et al 2010, MNRAS 407, 507). Time-resolved spectra of the system during several transits revealed a complex pattern of non-radial pulsations of the gamma Dor and/or delta Scuti type. The extreme proximity of the planet to the host star raises the possibility that some of these pulsation modes could be excited by planetary tides (Herrero et al 2011 A&A 526, L10). The system was observed continuously by the MOST satellite(*) from 2010 October 07.0 to October 31.0. The MOST data establish the frequency spectrum of the stellar pulsations, providing a direct test of theories of planetary tidal evolution via excitation of inertial waves in the host star. The ellipsoidal variation of the host star places limits on the mass of the planet. During the MOST run, a ground-based support campaign of time-resolved echelle spectroscopy yielded tomographic data sets suitable for mode identification and precise determination of the orientation of the planet's orbit. (*) MOST is a Canadian Space Agency mission, operated by Microsat Systems Canada Inc. (formerly the space division of Dynacon Inc.), the University of Toronto Institute for Aerospace Studies and the University of British Columbia, with support from the University of Vienna.

  5. Searching for pulsations in Kepler eclipsing binary stars

    NASA Astrophysics Data System (ADS)

    Gaulme, Patrick; Guzik, Joyce A.

    2014-02-01

    Eclipsing binaries can in principle provide additional constraints to facilitate asteroseismology of one or more pulsating components. We have identified 94 possible eclipsing binary systems in a sample of over 1800 stars observed in long cadence as part of the Kepler Guest Observer Program to search for γ Doradus and δ Scuti star candidates. We show the results of a procedure to fold the light curve to identify the potential binary period, subtract a fit to the binary light curve, and perform a Fourier analysis on the residuals to search for pulsation frequencies that may arise in one or both of the stellar components. From this sample, we have found a large variety of light curve types; about a dozen stars show frequencies consistent with δ Sct or γ Dor pulsations, or light curve features possibly produced by stellar activity (rotating spots). For several stars, the folded candidate `binary' light curve resembles more closely that of an RR Lyr, Cepheid, or high-amplitude δ Sct star. We show highlights of our results and discuss the potential for asteroseismology of the most interesting objects.

  6. Investigations on the Aerodynamic Characteristics and Blade Excitations of the Radial Turbine with Pulsating Inlet Flow

    NASA Astrophysics Data System (ADS)

    Liu, Yixiong; Yang, Ce; Yang, Dengfeng; Zhang, Rui

    2016-04-01

    The aerodynamic performance, detailed unsteady flow and time-based excitations acting on blade surfaces of a radial flow turbine have been investigated with pulsation flow condition. The results show that the turbine instantaneous performance under pulsation flow condition deviates from the quasi-steady value significantly and forms obvious hysteretic loops around the quasi-steady conditions. The detailed analysis of unsteady flow shows that the characteristic of pulsation flow field in radial turbine is highly influenced by the pulsation inlet condition. The blade torque, power and loading fluctuate with the inlet pulsation wave in a pulse period. For the blade excitations, the maximum and the minimum blade excitations conform to the wave crest and wave trough of the inlet pulsation, respectively, in time-based scale. And toward blade chord direction, the maximum loading distributes along the blade leading edge until 20% chord position and decreases from the leading to trailing edge.

  7. PREFACE: 16th European White Dwarfs Workshop

    NASA Astrophysics Data System (ADS)

    Garcia-Berro, Enrique; Hernanz, Margarita; Isern, Jordi; Torres, Santiago

    2009-07-01

    The 16th European Workshop on White Dwarfs was held in Barcelona, Spain, from 30 June to 4 July 2008 at the premises of the UPC. Almost 120 participants from Europe (France, Germany, United Kingdom, Italy, and several others), America (USA, Canada, Argentina, Brazil, and Chile), and other continents (Australia, South Africa, . . . ) attended the workshop. Among these participants were the most relevant specialists in the field. The topics covered by the conference were: White dwarf structure and evolution Progenitors and Planetary Nebulae White dwarfs in binaries: cataclysmic variables, double degenerates and other binaries White dwarfs, dust disks and planetary systems Atmospheres, chemical composition, magnetic fields Variable white dwarfs White dwarfs in stellar clusters and the halo White Dwarfs as SNIa progenitors The programme included 54 talks, and 45 posters. The oral presentations were distributed into the following sessions: Luminosity function, mass function and populations White dwarf structure and evolution White dwarf ages White dwarf catalogs and surveys Central stars of planetary nebulae Supernovae progenitors White dwarfs in novae and CVs Physical processes in white dwarfs and magnetic white dwarfs Disks, dust and planets around white dwarfs Pulsating white dwarfs Additionally we had a special open session about Spitzer and white dwarfs. The Proceedings of the 16th European Workshop on White Dwarfs are representative of the current state-of-the-art of the research field and include new and exciting results. We acknowledge the very positive attitude of the attendants to the workshop, which stimulated very fruitful discussions that took place in all the sessions and after the official schedule. Also, the meeting allowed new collaborations tp start that will undoubtedly result in significant advances in the research field. We also acknowledge the willingness of the participants to deliver their contributions before the final deadline. We sincerely

  8. An independent limit on the axion mass from the variable white dwarf star R548

    NASA Astrophysics Data System (ADS)

    Córsico, A. H.; Althaus, L. G.; Romero, A. D.; Mukadam, A. S.; García-Berro, E.; Isern, J.; Kepler, S. O.; Corti, M. A.

    2012-12-01

    Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as astrophysical laboratories to constrain the properties of fundamental particles like axions. Comparing the measured cooling rates of these stars with the expected values from theoretical models allows us to search for sources of additional cooling due to the emission of weakly interacting particles. In this paper, we present an independent inference of the mass of the axion using the recent determination of the evolutionary cooling rate of R548, the DAV class prototype. We employ a state-of-the-art code which allows us to perform a detailed asteroseismological fit based on fully evolutionary sequences. Stellar cooling is the solely responsible of the rates of change of period with time (\\dot\\Pi}) for the DAV class. Thus, the inclusion of axion emission in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DAV stars. This allows us to compare the theoretical \\dot\\Pi} values to the corresponding empirical rate of change of period with time of R548 to discern the presence of axion cooling. We found that if the dominant period at 213.13 s in R548 is associated with a pulsation mode trapped in the hydrogen envelope, our models indicate the existence of additional cooling in this pulsating white dwarf, consistent with axions of mass macos 2β ~ 17.1 meV at a 2σ confidence level. This determination is in agreement with the value inferred from another well-studied DAV, G117-B15A. We now have two independent and consistent estimates of the mass of the axion obtained from DAVs, although additional studies of other pulsating white dwarfs are needed to confirm this value of the axion mass.

  9. An independent limit on the axion mass from the variable white dwarf star R548

    SciTech Connect

    Córsico, A.H.; Althaus, L.G.; Mukadam, A.S.; García-Berro, E.; Corti, M.A. E-mail: althaus@fcaglp.unlp.edu.ar E-mail: anjum@astro.washington.edu E-mail: isern@ice.cat E-mail: mariela@fcaglp.fcaglp.unlp.edu.ar

    2012-12-01

    Pulsating white dwarfs with hydrogen-rich atmospheres, also known as DAV stars, can be used as astrophysical laboratories to constrain the properties of fundamental particles like axions. Comparing the measured cooling rates of these stars with the expected values from theoretical models allows us to search for sources of additional cooling due to the emission of weakly interacting particles. In this paper, we present an independent inference of the mass of the axion using the recent determination of the evolutionary cooling rate of R548, the DAV class prototype. We employ a state-of-the-art code which allows us to perform a detailed asteroseismological fit based on fully evolutionary sequences. Stellar cooling is the solely responsible of the rates of change of period with time (.Π)) for the DAV class. Thus, the inclusion of axion emission in these sequences notably influences the evolutionary timescales, and also the expected pulsational properties of the DAV stars. This allows us to compare the theoretical .Π) values to the corresponding empirical rate of change of period with time of R548 to discern the presence of axion cooling. We found that if the dominant period at 213.13 s in R548 is associated with a pulsation mode trapped in the hydrogen envelope, our models indicate the existence of additional cooling in this pulsating white dwarf, consistent with axions of mass m{sub a}cos {sup 2}β ∼ 17.1 meV at a 2σ confidence level. This determination is in agreement with the value inferred from another well-studied DAV, G117-B15A. We now have two independent and consistent estimates of the mass of the axion obtained from DAVs, although additional studies of other pulsating white dwarfs are needed to confirm this value of the axion mass.

  10. Effects of ice accretions on aircraft aerodynamics

    NASA Astrophysics Data System (ADS)

    Lynch, Frank T.; Khodadoust, Abdollah

    2001-11-01

    This article is a systematic and comprehensive review, correlation, and assessment of test results available in the public domain which address the aerodynamic performance and control degradations caused by various types of ice accretions on the lifting surfaces of fixed wing aircraft. To help put the various test results in perspective, overviews are provided first of the important factors and limitations involved in computational and experimental icing simulation techniques, as well as key aerodynamic testing simulation variables and governing flow physics issues. Following these are the actual reviews, assessments, and correlations of a large number of experimental measurements of various forms of mostly simulated in-flight and ground ice accretions, augmented where appropriate by similar measurements for other analogous forms of surface contamination and/or disruptions. In-flight icing categories reviewed include the initial and inter-cycle ice accretions inherent in the use of de-icing systems which are of particular concern because of widespread misconceptions about the thickness of such accretions which can be allowed before any serious consequences occur, and the runback/ridge ice accretions typically associated with larger-than-normal water droplet encounters which are of major concern because of the possible potential for catastrophic reductions in aerodynamic effectiveness. The other in-flight ice accretion category considered includes the more familiar large rime and glaze ice accretions, including ice shapes with rather grotesque features, where the concern is that, in spite of all the research conducted to date, the upper limit of penalties possible has probably not been defined. Lastly, the effects of various possible ground frost/ice accretions are considered. The concern with some of these is that for some types of configurations, all of the normally available operating margins to stall at takeoff may be erased if these accretions are not

  11. The frequency of planetary debris around young white dwarfs

    NASA Astrophysics Data System (ADS)

    Koester, D.; Gänsicke, B. T.; Farihi, J.

    2014-06-01

    Context. Heavy metals in the atmospheres of white dwarfs are thought in many cases to be accreted from a circumstellar debris disk, which was formed by the tidal disruption of a rocky planetary body within the Roche radius of the star. The abundance analysis of photospheric elements and conclusions about the chemical composition of the accreted matter are a new and promising method of studying the composition of extrasolar planetary systems. However, ground-based searches for metal-polluted white dwarfs that rely primarily on the detection of the Ca ii K line become insensitive at Teff > 15 000 K because this ionization state depopulates. Aims: We present the results of the first unbiased survey for metal pollution among hydrogen-atmosphere (DA type) white dwarfs with cooling ages in the range 20-200 Myr and 17 000 K white dwarfs studied, or 56% show traces of heavy elements. In 25 stars (showing only Si and occasionally C), the elements can be explained by radiative levitation alone, although we argue that accretion has very likely occurred recently. The remaining 23 white dwarfs (27%), however, must be currently accreting. Together with previous studies from the ground and adopting bulk Earth abundances for the debris, accretion rates range from a few 105 g s-1 to a few 108 g s-1, with no evident trend in cooling age from ≈40 Myr to ≈2 Gyr. Only a single, modest case of metal pollution (Ṁ < 106 g s-1) is found among ten white dwarfs with Teff > 23 000 K, in excellent agreement

  12. Some spectral and pulsation characteristics of a horizontal gas-liquid stream

    NASA Astrophysics Data System (ADS)

    Krokovnyi, P. M.

    1980-07-01

    In the experiments described, the turbulence characteristics of a two-phase gas-liquid pipe flow were studied, using a 6 m long, 19-mm-diam tube. The inlet temperature of the suspension was maintained at 25 C. The friction energy spectra and the relative intensity of the friction pulsations were measured. The spectral and pulsation characteristics were obtained by an electrodiffusion technique which provided reliable data on the pulsations of the wall shear stress.

  13. An experimental investigation of heat transfer to pulsating pipe air flow with different amplitudes

    NASA Astrophysics Data System (ADS)

    Zohir, A. E.; Habib, M. A.; Attya, A. M.; Eid, A. I.

    2006-05-01

    Heat transfer characteristics to both laminar and turbulent pulsating pipe flows under different conditions of Reynolds number, pulsation frequency, pulsator location and tube diameter were experimentally investigated. The tube wall of uniform heat flux condition was considered for both cases. Reynolds number varied from 750 to 12,320 while the frequency of pulsation ranged from 1 to 10 Hz. With locating the pulsator upstream of the inlet of the test section tube, results showed an increase in heat transfer rate due to pulsation by as much as 30% with flow Reynolds number of 1,643 and pulsation frequency of 1 Hz, depending on the upstream location of the pulsator valve. Closer the valve to the tested section inlet, the better improvement in the heat transfer coefficient is achieved. Upon comparing the heat transfer results of the upstream and the downstream pulsation, at Reynolds number of 1,366 and 1,643, low values of the relative mean Nusselt number were obtained with the upstream pulsation. Comparing the heat transfer results of the two studied test sections tubes for Reynolds number range from 8,000 to 12,000 and pulsation frequency range from 1.0 to 10 Hz showed that more improvement in heat transfer rate was observed with a larger tube diameter. For Reynolds number ranging from 8,000 to 12,000 and pulsation frequency of 10 Hz, an improvement in the relative mean Nusselt number of about 50% was obtained at Reynolds number of 8,000 for the large test section diameter of 50 mm. While, for the small test section diameter of 15 mm, at same conditions of Reynolds number and frequency, a reduction in the relative mean Nusselt number of up to 10% was obtained.

  14. Some spectral and pulsation characteristics of the horizontal flow of a gas-liquid suspension

    NASA Astrophysics Data System (ADS)

    Krokovnyi, P. M.

    1980-02-01

    In the experiments described, the turbulence characteristics of a two-phase gas-liquid pipe flow were studied, using a 6 m long, 19-mm-diam tube. The inlet temperature of the suspension was maintained at 25 C. The friction energy spectra and the relative intensity of the friction pulsations were measured. The spectral and pulsation characteristics were obtained by an electrodiffusion technique which provided reliable data on the pulsations of the wall shear stress.

  15. Accreting Neutron Stars as Astrophysical Laboratories

    NASA Technical Reports Server (NTRS)

    Chakrabarty, Deepto

    2004-01-01

    In the last year, we have made an extremely important breakthrough in establishing the relationship between thermonuclear burst oscillations in accreting neutron stars and the stellar spin. More broadly, we have continued t o make significant scientific progress in all four of the key focus areas identified in our original proposal: (1) the disk-magnetosphere interaction in neutron stars, (2) rapid variability in accreting neutron stars, (3) physics of accretion flows, and (4) fundamental properties of neutron stars. A list of all publications that have arising from this work since the start of our program is given.

  16. Lyman edges - Signatures of accretion disks

    NASA Astrophysics Data System (ADS)

    Kinney, A. L.

    1992-05-01

    Accretion disks are thought to provide the ultraviolet emission seen in the big blue bump of quasars. However, observations of the UV spectra of quasars do not show the additional signatures predicted by the accretion disk models. This paper will concentrate on just one of those signatures - the Lyman edge. Two studies are briefly discussed which explore the Lyman edge region of both high and low redshift quasars (Antonucci, Kinney, and Ford 1989 and Koratkar, Kinney, and Bohlin 1992). Both studies find that Lyman edges are not present in quasar spectra as frequently as predicted by the models or at the strength predicted by accretion disk models.

  17. A new pulsation spectrum and asteroseismology of {delta} Scuti

    SciTech Connect

    Templeton, M.R.; McNamara, B.J.; Guzik, J.A.; Bradley, P.A.; Cox, A.N.; Middleditch, J.

    1997-10-01

    We present the results of a five-year Str{umlt o}mgren y photometric campaign on {delta} Scuti. Our data set consists of 6515 discrete differential magnitudes, and spans the period of 1983 June to 1988 September. We found the primary pulsation mode at 59.731129{plus_minus}0.000002 {mu}Hz, in close agreement with the frequency determination of Fitch (1976, IAU Colloquium, 29, 167), but we find our best-fit observed frequencies for other pulsation modes differ by 0.5{endash}2 cycles per year from Fitch{close_quote}s results. In the case of the second strongest pulsation mode, we found a frequency of 61.936104{plus_minus}0.000009 {mu}Hz{emdash}one cycle per year off of the commonly quoted frequency. All of the other modes not classified as harmonics or beating modes were identified in our data, as well as a new pulsation frequency at 96.21443{plus_minus}0.00005 {mu}Hz discovered in both Str{umlt o}mgren y and b observations. We measured the phase differences between our Str{umlt o}mgren y data and a short string of Str{umlt o}mgren b data taken during the 1987 multisite campaign, and find phase differences ranging from 0 to 0.33 radians, suggesting that there are modes of different spherical harmonic order present in {delta} Scuti. Finally, we evolved a set of M=1.8{endash}2.4 M{sub {circle_dot}} models with solar abundances (X=0.7,Z=0.02) and two (M=2.2 and M=2.4 M{sub {circle_dot}}) models with solar abundances scaled to (X=0.66,Z=0.06), using recent opacity and reaction rate data, and applied linear, nonadiabatic pulsation analysis to models in the shell hydrogen burning phase. The Z=0.02 model which best fit the observed spectral type of F2III, the {ital Hipparcos} absolute magnitude of M{sub V}=1.0, and the radius estimate of Cugier and Monier of R=4.1 R{sub {circle_dot}}, and which has a pure radial mode at 59.731 {mu}Hz has a mass of 2.1 M{sub {circle_dot}}, with T{sub eff}=6894 K, R=4.14 R{sub {circle_dot}}, and M{sub V}=1.0. (Abstract Truncated)

  18. Investigation on the Possible Relationship between Magnetic Pulsations and Earthquakes

    NASA Astrophysics Data System (ADS)

    Jusoh, M.; Liu, H.; Yumoto, K.; Uozumi, T.; Takla, E. M.; Yousif Suliman, M. E.; Kawano, H.; Yoshikawa, A.; Asillam, M.; Hashim, M.

    2012-12-01

    The sun is the main source of energy to the solar system, and it plays a major role in affecting the ionosphere, atmosphere and the earth surface. The connection between solar wind and the ground magnetic pulsations has been proven empirically by several researchers previously (H. J. Singer et al., 1977, E. W. Greenstadt, 1979, I. A. Ansari 2006 to name a few). In our preliminary statistical analysis on relationship between solar and seismic activities (Jusoh and Yumoto, 2011, Jusoh et al., 2012), we observed a high possibility of solar-terrestrial coupling. We observed high tendency of earthquakes to occur during lower phase solar cycles which significantly related with solar wind parameters (i.e solar wind dynamic pressure, speed and input energy). However a clear coupling mechanism was not established yet. To connect the solar impact on seismicity, we investigate the possibility of ground magnetic pulsations as one of the connecting agent. In our analysis, the recorded ground magnetic pulsations are analyzed at different ranges of ultra low frequency; Pc3 (22-100 mHz), Pc4 (6.7-22 mHz) and Pc5 (1.7-6.7 mHz) with the occurrence of local earthquake events at certain time periods. This analysis focuses at 2 different major seismic regions; north Japan (mid latitude) and north Sumatera, Indonesia (low latitude). Solar wind parameters were obtained from the Goddard Space Flight Center, NASA via the OMNIWeb Data Explorer and the Space Physics Data Facility. Earthquake events were extracted from the Advanced National Seismic System (ANSS) database. The localized Pc3-Pc5 magnetic pulsations data were extracted from Magnetic Data Acquisition System (MAGDAS)/Circum Pan Magnetic Network (CPMN) located at Ashibetsu (Japan); for earthquakes monitored at north Japan and Langkawi (Malaysia); for earthquakes observed at north Sumatera. This magnetometer arrays has established by International Center for Space Weather Science and Education, Kyushu University, Japan. From the

  19. 1H 1752 + 081: An eclipsing cataclysmic variable with a small accretion disk

    NASA Technical Reports Server (NTRS)

    Silber, Andrew D.; Remillard, Ronald A.; Horne, Keith; Bradt, Hale V.

    1994-01-01

    We announce the discovery of an eclipsing nova-like cataclysmic variable (CV) as the optical counterpart to the HEAO 1 X-ray source 1H1752 + 081. This CV has an orbital period of 1.882801 hr, a high equivalent width of H-beta, and an average m(sub v) of 16.4 out of the eclipse. A geometric model is constructed from observations of the eclipse ingress and egress in many optical bandpasses. The broad-band emission originates primarily in two regions; the disk/accretion stream 'hot spot' and a compact central component, which may be a spot on the white dwarf surface, the entire white dwarf surface or the boundary layer between the accretion disk and the white dwarf surface. Based on the durations and offsets of the two eclipses we determined the mass ratio q = 2.5 +/- 0.6 and the angle of inclination i = 77 deg +/- 2 deg. If the central component is the entire white dwarf surface the masses of the stars are M(sub 1) = 0.80 +/- 0.06 solar masses and M(sub 2) = 0.32 +/- 0.06 solar masses. The disk is faint and small (R(sub D) = 0.25 +/- 0.05 r(sub L1), where r(sub L1) is the distance from the primary to the L(sub 1) point), compared to other eclipsing CVs. The small disk may result from the removal of angular momentum from the accretion disk by the magnetic field of the white dwarf; this CV may be a DQ Her type with a slowly rotating white dwarf. The emission-line velocities do not show the 'Z-wave' expected from the eclipse of a Keplerian accretion disk, nor do they have the correct phasing to originate near the white dwarf. The most likely origin of the line emission is the hot spot. The secondary star is visible at wavelengths greater than or equal to 6000 A during eclipse. We estimate a spectral type approximately M6 which, together with the observed m(sub 1) = 16.94 during eclipse, results in a distance estimate of 150 +/- 27 pc.

  20. 1H 1752 + 081: an eclipsing cataclysmic variable with a small accretion disk

    NASA Astrophysics Data System (ADS)

    Silber, Andrew D.; Remillard, Ronald A.; Horne, Keith; Bradt, Hale V.

    1994-04-01

    We announce the discovery of an eclipsing nova-like cataclysmic variable (CV) as the optical counterpart to the HEAO 1 X-ray source 1H1752 + 081. This CV has an orbital period of 1.882801 hr, a high equivalent width of H-beta, and an average mv of 16.4 out of the eclipse. A geometric model is constructed from observations of the eclipse ingress and egress in many optical bandpasses. The broad-band emission originates primarily in two regions; the disk/accretion stream 'hot spot' and a compact central component, which may be a spot on the white dwarf surface, the entire white dwarf surface or the boundary layer between the accretion disk and the white dwarf surface. Based on the durations and offsets of the two eclipses we determined the mass ratio q = 2.5 +/- 0.6 and the angle of inclination i = 77 deg +/- 2 deg. If the central component is the entire white dwarf surface the masses of the stars are M1 = 0.80 +/- 0.06 solar masses and M2 = 0.32 +/- 0.06 solar masses. The disk is faint and small (RD = 0.25 +/- 0.05 rL1, where rL1 is the distance from the primary to the L1 point), compared to other eclipsing CVs. The small disk may result from the removal of angular momentum from the accretion disk by the magnetic field of the white dwarf; this CV may be a DQ Her type with a slowly rotating white dwarf. The emission-line velocities do not show the 'Z-wave' expected from the eclipse of a Keplerian accretion disk, nor do they have the correct phasing to originate near the white dwarf. The most likely origin of the line emission is the hot spot. The secondary star is visible at wavelengths greater than or equal to 6000 A during eclipse. We estimate a spectral type approximately M6 which, together with the observed m1 = 16.94 during eclipse, results in a distance estimate of 150 +/- 27 pc.

  1. Cepheids in Magellanic Cloud star clusters - Fundamental and overtone pulsators in NGC 2157

    NASA Technical Reports Server (NTRS)

    Mateo, Mario; Olszewski, Edward W.; Madore, Barry F.

    1990-01-01

    CCD survey data are employed to examine Cepheids in young Magellanic Cloud star clusters. The properties of three Cepheids observed in NGC 2157 are described. It is detected that the two short-period (3 days) Cepheids have photometric properties that correspond to overtone pulsators and the long-period (7.7 days) Cepheid pulses in the fundamental mode. The pulsational masses for the three Cepheids are calculated to be about 5 solar masses. This mass value does not correlate with the average pulsational mass for Cepheids of 3.0 + or - 0.4 solar masses. The potential cause of this deviation in evolutionary/pulsational mass is investigated.

  2. Analysis of a subdwarf B pulsator observed during Campaign 2 of K2

    NASA Astrophysics Data System (ADS)

    Ketzer, Laura; Baran, Andrzej; Reed, Mike; Telting, John H.; Nemeth, Peter

    2016-06-01

    We present an analysis of the pulsating subdwarf B (sdB) star EPIC 203948264, observed during Campaign 2 of the extended Kepler mission. A time series analysis of the short cadence data set has revealed a rich g-mode pulsation spectrum with 17 independent pulsation periods between 0.5 and 2.8 hours. All of the pulsations fit the asymptotic period sequences for ell=1 or 2, with average period spacings of 259+/-1.4 and 149+/-0.3 s, respectively. The pulsation amplitudes range from 0.77 to the detection limit at 0.26 ppt, with amplitudes that vary over time. Radial velocity measurements give no indication for binarity in this star. We did not find rotationally induced pulsation multiplets, which indicates that the rotation period of the star is longer than about 45 days, which would make the data too short to resolve multiplets. By characterizing the various pulsation modes present in pulsating sdB stars, and by examining the time-dependence of pulsation amplitudes, we can constrain structural models of the interiors of sdB stars. This is a promising approach to enhancing our understanding of these stars.

  3. Observations of candidate oscillating eclipsing binaries and two newly discovered pulsating variables

    NASA Astrophysics Data System (ADS)

    Liakos, A.; Niarchos, P.

    2009-03-01

    CCD observations of 24 eclipsing binary systems with spectral types ranging between A0-F0, candidate for containing pulsating components, were obtained. Appropriate exposure times in one or more photometric filters were used so that short-periodic pulsations could be detected. Their light curves were analyzed using the Period04 software in order to search for pulsational behaviour. Two new variable stars, namely GSC 2673-1583 and GSC 3641-0359, were discov- ered as by-product during the observations of eclipsing variables. The Fourier analysis of the observations of each star, the dominant pulsation frequencies and the derived frequency spectra are also presented.

  4. First Satellite Imaging of Auroral Pulsations by the Fast Auroral Imager on e-POP

    NASA Astrophysics Data System (ADS)

    Lui, A.; Cogger, L.; Howarth, A. D.; Yau, A. W.

    2015-12-01

    We report the first satellite imaging of auroral pulsations by the Fast Auroral Imager (FAI) onboard the Enhanced Polar Outflow Probe (e-POP) satellite. The near-infrared camera of FAI is capable of providing up to two auroral images per second, ideal for investigation of pulsating auroras. The auroral pulsations were observed within the auroral bulge formed during a substorm interval on 2014 February 19. This first satellite view of these pulsations from FAI reveals that (1) several pulsating auroral channels (PACs) occur within the auroral bulge, (2) periods of the intensity pulsations span over one decade within the auroral bulge, and (3) there is no apparent trend of longer pulsation periods associated with higher latitudes for these PACs. Although PACs resemble in some respect stable pulsating auroras reported previously but they have several important differences in characteristics.PACs are not embedded in or emerging from omega bands or torches and are located at significant distances from the equatorward boundary of the auroral oval, unlike the characteristics of stable pulsating auroras.

  5. On resonances in the pulsations of stars - II. Canonical perturbation theories

    NASA Astrophysics Data System (ADS)

    Vandervoort, Peter O.

    2015-10-01

    This is a study of stellar pulsations that are dominated by the non-linear interaction of a pair of nearly degenerate modes of infinitesimal pulsation. We describe two examples in which the equations that govern the adiabatic, non-linear pulsations of a star admit of Hamiltonian formulations, and we construct canonical perturbation theories for the solution of the canonical equations of motion in those examples. The primary example is a model of non-linear pulsations described in an earlier paper, in which we have represented the pulsations as quasi-homologous oscillations of a compressible, heterogeneous spheroid. The tensor virial equations of the second order and an equation representing an integral form of the first law of thermodynamics govern the pulsations of that model. The second example is a Hamiltonian representation of stellar pulsations of the kind originally formulated by J. Woltjer. In these examples the pulsations are quasi-periodic in two or more degrees of freedom. Two degrees of freedom characterize the non-linear interaction of the nearly degenerate modes of infinitesimal pulsation. The period of the motion in one of those degrees of freedom is a non-linear counterpart of the beat period of a superposition of the two nearly degenerate modes. It appears that episodes of this non-linear beat phenomenon must occur during the evolution of β Cephei stars.

  6. Interaction between ELF-VLF emission and magnetic pulsations: quasi-periodic ELF-VLF emissions associated with Pc 3--4 magnetic pulsations and their geomagnetic conjugacy

    SciTech Connect

    Sato, N.; Kokubun, S.

    1980-01-01

    The characteristics of quasi-periodic (QP) ELF-VLF emissions with periods of 10--150 s and their relationships to magnetic pulsations are studied by using data obtained from Syowa and Mizuho Stations in Antarctica and at Husafell in Iceland, which is located near the geomagnetic conjugate point of Syowa. From the coherency analysis between QP emissions and Pc 3--4 magnetic pulsations it is found that the coherency between the D component of magnetic pulsations and the intensity fluctuations of QP's is much higher than that between the H component of magnetic pulsations and QP's. It is also found that the propagation time of magnetic pulsations (HM waves) from the interaction region between magnetic pulsations and QP's in the magnetosphere to the ground is 20--30 s. These properties are observed at conjugate-pair stations with good conjugacy. The results strongly suggest that QP emissions are modulated by compressional mode Pc 3--4 magnetic pulsations near the equatorial plane in the outer magnetosphere.

  7. Mystery of a Dimming White Dwarf

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2015-12-01

    In the wake of the recent media attention over an enigmatic, dimming star, another intriguing object has been discovered: J1529+2928, a white dwarf that periodically dims. This mystery, however, may have a simple solution with interesting consequences for future surveys of white dwarfs.Unexpected VariabilityJ1529+2928 is an isolated white dwarf that appears to have a mass of slightly more than the Sun. But rather than radiating steadily, J1529+2928 dims once every 38 minutes almost as though it were being eclipsed.The team that discovered these variations, led by Mukremin Kilic (University of Oklahoma), used telescopes at the Apache Point Observatory and the McDonald Observatory to obtain follow-up photometric data of J1529+2928 spread across 66 days. The team also took spectra of the white dwarf with the Gemini North telescope.Kilic and collaborators then began, one by one, to rule out possible causes of this objects variability.Eliminating OptionsThe period of the variability is too long for J1529+2928 to be a pulsating white dwarf with luminosity variation caused by gravity-wave pulsations.The variability cant be due to an eclipse by a stellar or brown-dwarf companion, because there isnt any variation in J1529+2928s radial velocity.Its not due to the orbit of a solid-body planetary object; such a transit would be too short to explain observations.It cant be due to the orbit of a disintegrated planet; this wouldnt explain the light curves observed in different filters plus the light curve doesnt change over the 66-day span.Spotty SurfaceTop and middle two panels: light curves from three different nights observing J1529+2928s periodic dimming. Bottom panel: The Fourier transform shows a peak at 37.7 cycles/day (and another, smaller peak at its first harmonic). [Kilic et al. 2015]So what explanation is left? The authors suggest that J1529+2928s variability is likely caused by a starspot on the white dwarfs surface that rotates into and out of our view. Estimates

  8. Accretion Timescales from Kepler AGN

    NASA Astrophysics Data System (ADS)

    Kasliwal, Vishal P.; Vogeley, Michael S.; Richards, Gordon T.

    2015-01-01

    We constrain AGN accretion disk variability mechanisms using the optical light curves of AGN observed by Kepler. AGN optical fluxes are known to exhibit stochastic variations on timescales of hours, days, months and years. The excellent sampling properties of the original Kepler mission - high S/N ratio (105), short sampling interval (30 minutes), and long sampling duration (~ 3.5 years) - allow for a detailed examination of the differences between the variability processes present in various sub-types of AGN such as Type I and II Seyferts, QSOs, and Blazars. We model the flux data using the Auto-Regressive Moving Average (ARMA) representation from the field of time series analysis. We use the Kalman filter to determine optimal mode parameters and use the Akaike Information Criteria (AIC) to select the optimal model. We find that optical light curves from Kepler AGN cannot be fit by low order statistical models such as the popular AR(1) process or damped random walk. Kepler light curves exhibit complicated power spectra and are better modeled by higher order ARMA processes. We find that Kepler AGN typically exhibit power spectra that change from a bending power law (PSD ~ 1/fa) to a flat power spectrum on timescales in the range of ~ 5 - 100 days consistent with the orbital and thermal timescales of a typical 107 solar mass black hole.

  9. 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.

  10. Heat transfer on accreting ice surfaces

    NASA Technical Reports Server (NTRS)

    Yamaguchi, Keiko; Hansman, R. John, Jr.

    1993-01-01

    Based on previous observations of glaze ice accretion on aircraft surfaces, a multizone model with distinct zones of different surface roughness is demonstrated. The use of surface roughness in the LEWICE ice accretion prediction code is examined. It was found that roughness is used in two ways: (1) to determine the laminar to turbulent boundary-layer transition location; and (2) to calculate the convective turbulent heat-transfer coefficient. A two-zone version of the multizone model is implemented in the LEWICE code, and compared with experimental convective heat-transfer coefficient and ice accretion results. The analysis of the boundary-layer transition, surface roughness, and viscous flowfield effects significantly increased the accuracy in predicting heat-transfer coefficients. The multizone model was found to significantly improve the ice accretion prediction for the cases compared.

  11. Gravitomagnetic acceleration from black hole accretion disks

    NASA Astrophysics Data System (ADS)

    Poirier, J.; Mathews, G. J.

    2016-05-01

    We demonstrate how the motion of the neutral masses in an accretion disk orbiting a black hole creates a general-relativistic magnetic-like (gravitomagnetic) field that vertically accelerates neutral particles near an accretion disk upward and then inward toward the axis of the accretion disk. Even though this gravitomagnetic field is not the only mechanism contributing to the production of jets, it presents a novel means to identify one general relativistic effect from a much more complicated problem. In addition, as the accelerated material above or below the accretion disk nears the axis with a nearly vertical direction, a frame-dragging effect twists the trajectories around the axis thus contributing to the collimation of the jet.

  12. Planetary science: Iron fog of accretion

    DOE PAGESBeta

    Anderson, William W.

    2015-03-02

    Here, pinpointing when Earth's core formed depends on the extent of metal–silicate equilibration in the mantle. Vaporization and recondensation of impacting planetesimal cores during accretion may reconcile disparate lines of evidence.

  13. 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

  14. Thermonuclear flashes on accreting neutron stars

    NASA Technical Reports Server (NTRS)

    Joss, P. C.

    1979-01-01

    Observations of X-ray bursts from binary pulsars and globular clusters are reviewed. The previously proposed hypothesis is considered that such X-ray bursts result from thermonuclear flashes on accreting neutron stars. A general scenario for this mechanism is outlined, and numerical computations of the evolution of the surface layers of an accreting neutron star are discussed. The relation of these calculations to X-ray bursts and other phenomena is examined. Possible improvements in the numerical calculations are suggested.

  15. He-accreting WDs: AM CVn stars with WD donors

    NASA Astrophysics Data System (ADS)

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

    2015-09-01

    We study the physical and evolutionary properties of the `white dwarf (WD) family' of AM CVn stars by computing realistic models of interacting double-degenerate systems. We evaluate self-consistently both the mass-transfer rate from the donor, as determined by gravitational wave emission and interaction with the binary companion, and the thermal response of the accretor to mass deposition. We find that, after the onset of mass transfer, all the considered systems undergo a strong non-dynamical He-flash. However, due to the compactness of these systems, the expanding accretors fill their Roche lobe very soon, thus preventing the efficient heating of the external layers of the accreted CO WDs. Moreover, due to the loss of matter from the systems, the orbital separations enlarge and mass transfer comes to a halt. The further evolution depends on the value of dot{M} after the donors fill again their lobe. On one hand, if the accretion rate, as determined by the actual value of (Mdon, Macc), is high enough, the accretors experience several He-flashes of decreasing strength and then quiescent He-burning sets in. Later on, since the mass-transfer rate in IDD is a permanently decreasing function of time, accretors experience several recurrent strong flashes. On the other hand, for intermediate and low values of dot{M} the accretors enter directly the strong flashes accretion regime. As expected, in all the considered systems the last He-flash is the strongest one, even if the physical conditions suitable for a dynamical event are never attained. When the mass accretion rate decreases below (2-3) × 10-8 M⊙ yr-1, the compressional heating of the He-shell becomes less efficient than the neutrino cooling, so that all the accretors in the considered systems evolve into massive degenerate objects. Our results suggest that SNe .Ia or Type Ia Supernovae due to Edge-Lit Detonation in the WD family of AM CVn stars should be much more rare than previously expected.

  16. Post-flare Formation of the Accretion Stream and a dip in Pulse Profiles of LMC X-4

    NASA Astrophysics Data System (ADS)

    Beri, Aru; Paul, Biswajit

    2016-07-01

    We will present results from a pulse profile evolution study of an accreting X-ray pulsar LMC X-4 during and after the large X-ray flares using data from two observatories XMM-Newton and RXTE. During the flares, the pulse profiles was found to have a significant phase offset and also some intensity dependence of the pulse amplitude. Moreover, a phase shift of nearly 180 between the pulse profiles from the persistent emission (just before and after the flares) was found. Investigating the pulse profiles for a long duration after the flares we estimated the time required for the modified accretion column to return to its normal structure and formation of accretion stream that causes dip in the pulse profile of LMC X-4. We will also discuss the results from a pulse phase resolved spectroscopy in narrow phase bins using data from EPIC-PN which confirmed a pulsating nature of the soft spectral component having a pulse shape and phase different form the rest.

  17. NuSTAR Discovery of a Cyclotron Line in the Accreting X-Ray Pulsar IGR J16393-4643

    NASA Astrophysics Data System (ADS)

    Bodaghee, Arash; Tomsick, John A.; Fornasini, Francesca M.; Krivonos, Roman; Stern, Daniel; Mori, Kaya; Rahoui, Farid; Boggs, Steven E.; Christensen, Finn E.; Craig, William W.; Hailey, Charles J.; Harrison, Fiona A.; Zhang, William W.

    2016-06-01

    The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was observed by the Nuclear Spectroscope Telescope Array in the 3–79 keV energy band for a net exposure time of 50 ks. We present the results of this observation which enabled the discovery of a cyclotron resonant scattering feature with a centroid energy of {29.3}-1.3+1.1 keV. This allowed us to measure the magnetic field strength of the neutron star for the first time: B = (2.5 ± 0.1) × 1012 G. The known pulsation period is now observed at 904.0 ± 0.1 s. Since 2006, the neutron star has undergone a long-term spin-up trend at a rate of \\dot{P}=-2× {10}-8 s s‑1 (‑0.6 s per year, or a frequency derivative of \\dot{ν }=3× {10}-14 Hz s‑1). In the power density spectrum, a break appears at the pulse frequency which separates the zero slope at low frequency from the steeper slope at high frequency. This addition of angular momentum to the neutron star could be due to the accretion of a quasi-spherical wind, or it could be caused by the transient appearance of a prograde accretion disk that is nearly in corotation with the neutron star whose magnetospheric radius is around 2 × 108 cm.

  18. 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.

  19. 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.

  20. Heating and Cooling in Accreting Neutron Stars

    NASA Astrophysics Data System (ADS)

    Cumming, Andrew

    2015-10-01

    Neutron stars in low mass X-ray binaries accrete enough mass over their lifetimes to replace their entire crust. The accreted matter undergoes a series of nuclear reactions in the crust as it is compressed by continued accretion to higher density. These reactions, which include electron captures, neutron emissions, and pycnonuclear reactions, heat the crust and core of the neutron star. In this talk I will discuss what we can learn from observations of transiently accreting neutron stars in quiescence, when accretion has turned off and we can see emission from the neutron star directly. The quiescent luminosity of these neutron stars constrains the neutrino emissivity in the neutron star core. In systems with long accretion outbursts, observations of thermal relaxation of the crust in quiescence enable, for the first time, constraints on the thermal conductivity and heat capacity of the crust. In this way, low mass X-ray binary neutron stars offer a remarkable chance to constrain the properties of dense neutron-rich matter, such as neutron superfluidity and pasta phases in the inner crust of neutron stars.