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Sample records for a-type stars magnetic

  1. Magnetic fields in X-ray emitting A-type stars

    NASA Astrophysics Data System (ADS)

    Schröder, C.; Hubrig, S.; Schmitt, J. H. M. M.

    2008-04-01

    A common explanation for the observed X-ray emission of A-type stars is the presence of a hidden late-type companion. While this hypothesis can be shown to be correct in some cases, there is also evidence suggesting that low-mass companions cannot be the proper cause for the observed X-ray activity in all cases. Babel and Montmerle (1997) presented a theoretical framework to explain the X-ray emission from magnetic Ap/Bp stars, focusing on the A0p star IQ Aur. We test whether this theoretical model is capable of explaining the observed X-ray emissions. We present observations of 13 A-type stars that have been associated with X-ray emission detected by ROSAT. To determine the mean longitudinal magnetic field strength we measured the circular polarization in the wings of the Balmer lines using FORS 1. Although the emission of those objects with magnetic fields does fit the prediction of the Babel & Montmerle model, not all X-ray detections are related to the presence of a magnetic field. Additionally, the strengths of magnetic fields do not correlate with the X-ray luminosity and thus the magnetically-confined wind shock model cannot explain the X-ray emission from all investigated stars.

  2. Magnetic fields in A-type stars associated with X-ray emission

    NASA Astrophysics Data System (ADS)

    Schröder, C.; Hubrig, S.; Schmitt, J. H. M. M.

    2008-06-01

    A common explanation for the observed X-ray emission of A-type stars is the presence of a hidden late-type companion. While this assumption can be shown to be correct in some cases, a number of lines of evidence suggests that low-mass companions cannot be the correct cause for the observed activity in all cases. A model explains the X-ray emission for magnetic Ap/Bp stars, focusing on the A0p star IQ Aur. In this paper we test whether this theoretical model is able to explain the observed X-ray emission. We present the observations of 13 A-type stars that have been associated with X-ray emission detected by ROSAT. To determine the mean longitudinal magnetic field strength we measured the circular polarization in the wings of the Balmer lines using FORS1. Although the emission of those objects that possess magnetic fields fits the prediction of the Babel and Montmerle model, not all X-ray detections are connected to the presence of a magnetic field. Additionally, the measured magnetic fields do not correlate with the X-ray luminosity. Accordingly, the magnetically confined wind shock model cannot explain the X-ray emission from all the presented stars.

  3. Do A-type stars flare?

    NASA Astrophysics Data System (ADS)

    Pedersen, M. G.; Antoci, V.; Korhonen, H.; White, T. R.; Jessen-Hansen, J.; Lehtinen, J.; Nikbakhsh, S.; Viuho, J.

    2017-04-01

    For flares to be generated, stars have to have a sufficiently deep outer convection zone (F5 and later), strong large-scale magnetic fields (Ap/Bp-type stars) or strong, radiatively driven winds (B5 and earlier). Normal A-type stars possess none of these and therefore should not flare. Nevertheless, flares have previously been detected in the Kepler light curves of 33 A-type stars and interpreted to be intrinsic to the stars. Here, we present new and detailed analyses of these 33 stars, imposing very strict criteria for the flare detection. We confirm the presence of flare-like features in 27 of the 33 A-type stars. A study of the pixel data and the surrounding field of view reveals that 14 of these 27 flaring objects have overlapping neighbouring stars and five stars show clear contamination in the pixel data. We have obtained high-resolution spectra for 2/3 of the entire sample and confirm that our targets are indeed A-type stars. Detailed analyses revealed that 11 out of 19 stars with multiple epochs of observations are spectroscopic binaries. Furthermore, and contrary to previous studies, we find that the flares can originate from a cooler, unresolved companion. We note the presence of Hα emission in eight stars. Whether this emission is circumstellar or magnetic in origin is unknown. In summary, we find possible alternative explanations for the observed flares for at least 19 of the 33 A-type stars, but find no truly convincing target to support the hypothesis of flaring A-type stars.

  4. Magnetic Fields in Stars

    NASA Astrophysics Data System (ADS)

    Landstreet, J.; Murdin, P.

    2000-11-01

    Magnetism—the force that deflects the needle of a compass—and magnetic fields have been found in some hundreds of stars during the past 50 yr. Magnetic fields have been detected in T Tauri stars and other pre-main-sequence stars, several types of main sequence stars, white dwarfs and neutron stars. We now know a number of methods by which such magnetic fields may be detected, we are in the proces...

  5. Magnetized Compact Stars

    NASA Astrophysics Data System (ADS)

    Pérez Martínez, Aurora; González Felipe, Ricardo; Manreza Paret, Daryel

    2015-01-01

    The magnetized color flavor locked matter phase can be more stable than the unpaired phase, thus becoming the ground state inside neutron stars. In the presence of a strong magnetic field, there exist an anisotropy in the pressures. We estimate the mass-radius relation of magnetized compact stars taking into account the parallel and perpendicular (to the magnetic field) pressure components.

  6. AE and A type shell stars

    NASA Astrophysics Data System (ADS)

    Jaschek, C.; Andrillat, Y.

    1998-06-01

    We present and discuss the observations of 14 Ae and A type shell stars in the visual, the Hα region and the near infrared. At least 57% of these stars are spectrum variables. The Paschen lines are formed in a region which has the characterics of giant stars. We find that the systems lie one magnitude above the main sequence and that a large percentage belong to double and triple systems.

  7. The most magnetic stars

    NASA Astrophysics Data System (ADS)

    Wickramasinghe, Dayal T.; Tout, Christopher A.; Ferrario, Lilia

    2014-01-01

    Observations of magnetic A, B and O stars show that the poloidal magnetic flux per unit mass Φp/M appears to have an upper bound of approximately 10-6.5 G cm2 g-1. A similar upper bound to the total flux per unit mass is found for the magnetic white dwarfs even though the highest magnetic field strengths at their surfaces are much larger. For magnetic A and B stars, there also appears to be a well-defined lower bound below which the incidence of magnetism declines rapidly. According to recent hypotheses, both groups of stars may result from merging stars and owe their strong magnetism to fields generated by a dynamo mechanism as they merge. We postulate a simple dynamo that generates magnetic field from differential rotation. We limit the growth of magnetic fields by the requirement that the poloidal field stabilizes the toroidal and vice versa. While magnetic torques dissipate the differential rotation, toroidal field is generated from poloidal by an Ω dynamo. We further suppose that mechanisms that lead to the decay of toroidal field lead to the generation of poloidal. Both poloidal and toroidal fields reach a stable configuration which is independent of the size of small initial seed fields but proportional to the initial differential rotation. We pose the hypothesis that strongly magnetic stars form from the merging of two stellar objects. The highest fields are generated when the merge introduces differential rotation that amounts to critical break-up velocity within the condensed object. Calibration of a simplistic dynamo model with the observed maximum flux per unit mass for main-sequence stars and white dwarfs indicates that about 1.5 × 10-4 of the decaying toroidal flux must appear as poloidal. The highest fields in single white dwarfs are generated when two degenerate cores merge inside a common envelope or when two white dwarfs merge by gravitational-radiation angular momentum loss. Magnetars are the most magnetic neutron stars. Though these are

  8. On magnetized neutron stars

    SciTech Connect

    Lopes, Luiz; Menezes, Debora E-mail: debora.p.m@ufsc.br

    2015-08-01

    In this work we review the formalism normally used in the literature about the effects of density-dependent magnetic fields on the properties of neutron and quark stars, expose some ambiguities that arise and propose a way to solve the related problem. Our approach explores more deeply the concept of pressure, yielding the so called chaotic magnetic field formalism for the stress tensor. We also use a different way of introducing a variable magnetic field, which depends on the energy density rather than on the baryonic density, which allows us to build a parameter free model.

  9. Magnetic Fields of Nondegenerate Stars

    NASA Astrophysics Data System (ADS)

    Donati, J.-F.; Landstreet, J. D.

    2009-09-01

    Magnetic fields are present in a wide variety of stars throughout the HR diagram and play a role at basically all evolutionary stages, from very-low-mass dwarfs to very massive stars, and from young star-forming molecular clouds and protostellar accretion discs to evolved giants/supergiants and magnetic white dwarfs/neutron stars. These fields range from a few μG (e.g., in molecular clouds) to TG and more (e.g., in magnetic neutron stars); in nondegenerate stars in particular, they feature large-scale topologies varying from simple nearly axisymmetric dipoles to complex nonaxsymmetric structures, and from mainly poloidal to mainly toroidal topologies. After recalling the main techniques of detecting and modeling stellar magnetic fields, we review the existing properties of magnetic fields reported in cool, hot, and young nondegenerate stars and protostars, and discuss our understanding of the origin of these fields and their impact on the birth and life of stars.

  10. Magnetic Dynamos and Stars

    SciTech Connect

    Eggleton, P P

    2007-02-15

    Djehuty is a code that has been developed over the last five years by the Lawrence Livermore National Laboratory (LLNL), from earlier code designed for programmatic efforts. Operating in a massively parallel environment, Djehuty is able to model entire stars in 3D. The object of this proposal was to continue the effort to introduce magneto-hydrodynamics (MHD) into Djehuty, and investigate new classes of inherently 3D problems involving the structure, evolution and interaction of stars and planets. However, towards the end of the second year we discovered an unexpected physical process of great importance in the evolution of stars. Consequently for the third year we changed direction and concentrated on this process rather than on magnetic fields. Our new process was discovered while testing the code on red-giant stars, at the 'helium flash'. We found that a thin layer was regularly formed which contained a molecular-weight inversion, and which led therefore to Rayleigh-Taylor instability. This in turn led to some deeper-than-expected mixing, which has the property that (a) much {sup 3}He is consumed, and (b) some {sup 13}C is produced. These two properties are closely in accord with what has been observed over the last thirty years in red giants, whereas what was observed was largely in contradiction to what earlier theoretical models predicted. Thus our new 3D models with Djehuty explain a previously-unexplained problem of some thirty years standing.

  11. Accretion Onto Magnetic Degenerate Stars

    NASA Technical Reports Server (NTRS)

    Frank, Juhan

    2000-01-01

    While the original objectives of this research program included the study of radiative processes in cataclysmic variables and the evolution of neutron star magnetic fields, the scope of the reported research expanded to other related topics as this project developed. This final report therefore describes the results of our research in the following areas: 1) Irradiation-driven mass transfer cycles in cataclysmic variables and low-mass X-ray binaries; 2) Propeller effect and magnetic field decay in isolated old neutron stars; 3) Decay of surface magnetic fields in accreting neutron stars and pulsars; 4) Finite-Difference Hydrodynamic simulations of mass transfer in binary stars.

  12. Magnetic Fields of Neutron Stars

    NASA Astrophysics Data System (ADS)

    Konar, Sushan

    2017-09-01

    This article briefly reviews our current understanding of the evolution of magnetic fields in neutron stars, which basically defines the evolutionary pathways between different observational classes of neutron stars. The emphasis here is on the evolution in binary systems and the newly emergent classes of millisecond pulsars.

  13. Synchronization of magnetic stars in binary systems

    NASA Technical Reports Server (NTRS)

    Lamb, F. K.; Aly, J.-J.; Cook, M. C.; Lamb, D. Q.

    1983-01-01

    Asynchronous rotation of magnetic stars in close binary systems drives substantial field-aligned electrical currents between the magnetic star and its companion. The resulting magnetohydrodynamic torque is able to account for the heretofore unexplained synchronous rotation of the strongly magnetic degenerate dwarf component in systems like AM Her, VV Pup, AN UMa, and EF Eri as well as the magnetic A type component in systems like HD 98088 and 41 Tauri. The electric fields produced by even a small asynchronism are large and may accelerate some electrons to high energies, producing radio emission. The total energy dissipation rate in systems with degenerate dwarf spin periods as short as 1 minute may reach 10 to the 33rd ergs/s. Total luminosities of this order may be a characteristic feature of such systems.

  14. Destruction of a Magnetized Star

    NASA Astrophysics Data System (ADS)

    Kohler, Susanna

    2017-01-01

    What happens when a magnetized star is torn apart by the tidal forces of a supermassive black hole, in a violent process known as a tidal disruption event? Two scientists have broken new ground by simulating the disruption of stars with magnetic fields for the first time.The magnetic field configuration during a simulation of the partial disruption of a star. Top left: pre-disruption star. Bottom left: matter begins to re-accrete onto the surviving core after the partial disruption. Right: vortices form in the core as high-angular-momentum debris continues to accrete, winding up and amplifying the field. [Adapted from Guillochon McCourt 2017]What About Magnetic Fields?Magnetic fields are expected to exist in the majority of stars. Though these fields dont dominate the energy budget of a star the magnetic pressure is a million times weaker than the gas pressure in the Suns interior, for example they are the drivers of interesting activity, like the prominences and flares of our Sun.Given this, we can wonder what role stars magnetic fields might play when the stars are torn apart in tidal disruption events. Do the fields change what we observe? Are they dispersed during the disruption, or can they be amplified? Might they even be responsible for launching jets of matter from the black hole after the disruption?Star vs. Black HoleIn a recent study, James Guillochon (Harvard-Smithsonian Center for Astrophysics) and Michael McCourt (Hubble Fellow at UC Santa Barbara) have tackled these questions by performing the first simulations of tidal disruptions of stars that include magnetic fields.In their simulations, Guillochon and McCourt evolve a solar-mass star that passes close to a million-solar-mass black hole. Their simulations explore different magnetic field configurations for the star, and they consider both what happens when the star barely grazes the black hole and is only partially disrupted, as well as what happens when the black hole tears the star apart

  15. Magnetic fields around evolved stars

    NASA Astrophysics Data System (ADS)

    Leal-Ferreira, M.; Vlemmings, W.; Kemball, A.; Amiri, N.; Maercker, M.; Ramstedt, S.; Olofsson, G.

    2014-04-01

    A number of mechanisms, such as magnetic fields, (binary) companions and circumstellar disks have been suggested to be the cause of non-spherical PNe and in particular collimated outflows. This work investigates one of these mechanisms: the magnetic fields. While MHD simulations show that the fields can indeed be important, few observations of magnetic fields have been done so far. We used the VLBA to observe five evolved stars, with the goal of detecting the magnetic field by means of water maser polarization. The sample consists in four AGB stars (IK Tau, RT Vir, IRC+60370 and AP Lyn) and one pPN (OH231.8+4.2). In four of the five sources, several strong maser features were detected allowing us to measure the linear and/or circular polarization. Based on the circular polarization detections, we infer the strength of the component of the field along the line of sight to be between ~30 mG and ~330 mG in the water maser regions of these four sources. When extrapolated to the surface of the stars, the magnetic field strength would be between a few hundred mG and a few Gauss when assuming a toroidal field geometry and higher when assuming more complex magnetic fields. We conclude that the magnetic energy we derived in the water maser regions is higher than the thermal and kinetic energy, leading to the conclusion that, indeed, magnetic fields probably play an important role in shaping Planetary Nebulae.

  16. Magnetic Stars After the Hayashi Phase. II.

    NASA Astrophysics Data System (ADS)

    Glagolevskij, Yu. V.

    2016-09-01

    The properties of magnetic stars derived from observational data are analyzed. The degree of "magnetic" braking of parent protostars, which depends the magnetic field and mass, is studied. The conditions under which magnetic and "normal" nonmagnetic stars are separated, which appear to depend only on the rotational velocity of the protostars, are examined. The reasons for differences in the average magnitudes of the magnetic field in massive and low-mass magnetic stars are analyzed. The magnetic field structures of magnetic stars and their stability over time (rigidity of rotation) are examined.

  17. Abundances in A-type Horizontal Branch Stars

    NASA Astrophysics Data System (ADS)

    Wilhelm, R.; Smith, V. V.

    1998-12-01

    As part of a program to explore correlations between abundance anomilies and physical parameters (e.g. Teff, vsini) in horizontal branch stars, we present preliminary results from high-resolution (R ~ 18,000) spectral observations of a small sample of A-type, horizontal branch stars. The sample was obtained using the 2.1m telescope and Sandiford Echelle at McDonald Observatory. A total of six standard FHB stars were observed including two, HD 130095 and HD 167105, which have been previously shown by Adleman and Philip to posses anomalously low [Ca/Fe] values. We have also obtained observations of eight of the brighter (B = 11.5-12.5) FHB stars from the HK objective-prism survey and two BHB stars from the globular cluster, M4. We will present abundance results that include [Ca/Fe], [Mg/Fe], and vsini values for the sample along with O and Na results for the two M4 stars. Our findings will be compared to previously published results for cluster BHB and field HB stars.

  18. Quark Stars and Magnetic Collapse

    SciTech Connect

    Perez Martinez, A.; Perez Rojas, H.; Mosquera Cuesta, H. J.; Boligan, M.

    2005-09-28

    Quark matter is expected to exist in the interior of compact stellar objects as neutron stars or even the more exotic strange stars, based on the Bodmer-Witten conjecture. Bare strange quark stars and (normal) strange quark-matter stars, those possessing a baryon (electron) crust, are hypothesized as good candidates to explain the properties of a set of peculiar stellar sources as the enigmatic X-ray source RX J1856.5-3754, some pulsars as PSR B1828-11 and PSR B1642-03 (Xu 2003), and the anomalous X-ray pulsars and soft gamma-ray repeaters (Zhang et al. 2000). In the MIT bag model, quarks are treated as a degenerate Fermi gas confined to a region of space having a vacuum energy density Bbag (the Bag constant). In this note, we modified the MIT Bag Model by including the electromagnetic interaction. We also show that this version of the MIT model implies the anisotropy of the Bag pressure due to the presence of the magnetic field. The equations of state of degenerate quarks gases are studied in the presence of ultra strong magnetic fields. The behavior of a system made-up of quarks having (or not) an anomalous magnetic moment is reviewed. A structural instability is found, which is related to the anisotropic nature of the pressures in this highly magnetized matter. The conditions for the collapse of this system are obtained and compared to a previous model of neutron stars build-up on a neutron gas having anomalous magnetic moment.

  19. The magnetic activity sunlike stars.

    PubMed

    Vaughan, A H

    1984-08-24

    Sunspots, flares, and the myriad time-varying "events" observable in the Sun-the only star whose surface we can examine in detail-are testimony that the Sun is a magnetically variable or active star. Its magnetic field, carried into interplanetary space by the solar wind, produces observable changes in Earth's magnetosphere and variations in the flux of galactic cosmic-ray particles incident upon Earth's upper atmosphere. Centuries of observation have enabled solar scientists to recognize that the Sun's magnetism exists and varies in a globally organized pattern that is somehow coupled to the Sun's rotation. Within the past decade O. C. Wilson demonstrated that analogs of solar activity exist and can be studied in many other dwarf stars. From the continuing study, knowledge of the precise rates of rotation of the stars under investigation is being gained for the first time. The results are expected to increase our understanding of the origin of solar activity and stellar activity in general.

  20. Accretion, Outflows, and Winds of Magnetized Stars

    NASA Astrophysics Data System (ADS)

    Romanova, Marina M.; Owocki, Stanley P.

    2015-10-01

    Many types of stars have strong magnetic fields that can dynamically influence the flow of circumstellar matter. In stars with accretion disks, the stellar magnetic field can truncate the inner disk and determine the paths that matter can take to flow onto the star. These paths are different in stars with different magnetospheres and periods of rotation. External field lines of the magnetosphere may inflate and produce favorable conditions for outflows from the disk-magnetosphere boundary. Outflows can be particularly strong in the propeller regime, wherein a star rotates more rapidly than the inner disk. Outflows may also form at the disk-magnetosphere boundary of slowly rotating stars, if the magnetosphere is compressed by the accreting matter. In isolated, strongly magnetized stars, the magnetic field can influence formation and/or propagation of stellar wind outflows. Winds from low-mass, solar-type stars may be either thermally or magnetically driven, while winds from massive, luminous O and B type stars are radiatively driven. In all of these cases, the magnetic field influences matter flow from the stars and determines many observational properties. In this chapter we review recent studies of accretion, outflows, and winds of magnetized stars with a focus on three main topics: (1) accretion onto magnetized stars; (2) outflows from the disk-magnetosphere boundary; and (3) winds from isolated massive magnetized stars. We show results obtained from global magnetohydrodynamic simulations and, in a number of cases compare global simulations with observations.

  1. EVIDENCE FOR GRANULATION IN EARLY A-TYPE STARS

    SciTech Connect

    Kallinger, Thomas; Matthews, Jaymie M.

    2010-03-01

    Stars with spectral types earlier than about F0 on (or close) to the main sequence have long been believed to lack observable surface convection, although evolutionary models of A-type stars do predict very thin surface convective zones. We present evidence for granulation in two {delta} Scuti stars of spectral type A2: HD 174936 and HD 50844. Recent analyses of space-based CoRoT data revealed up to some 1000 frequencies in the photometry of these stars. The frequencies were interpreted as individual pulsation modes. If true, there must be large numbers of nonradial modes of very high degree l which should suffer cancellation effects in disk-integrated photometry (even of high space-based precision). The p-mode interpretation of all the frequencies in HD 174936 and HD 50844 depends on the assumption of white (frequency-independent) noise. Our independent analyses of the data provide an alternative explanation: most of the peaks in the Fourier spectra are the signature of non-white granulation background noise, and less than about 100 of the frequencies are actual stellar p-modes in each star. We find granulation timescales which are consistent with scaling relations that describe cooler stars with known surface convection. If the granulation interpretation is correct, the hundreds of low-amplitude Fourier peaks reported in recent studies are falsely interpreted as independent pulsation modes and a significantly lower number of frequencies are associated with pulsation, consistent with only modes of low degree.

  2. The STAR detector magnet subsystem

    SciTech Connect

    Brown, R.L.; Etkin, A.; Foley, K.J.

    1997-07-01

    The RHIC (Relativistic Heavy Ion Collider) Accelerator currently under construction at Brookhaven National Laboratory will have large detectors at two of its six intersection regions. One of these detectors, known as STAR (Solenoidal Tracker At RHIC), weighs 1100 tons and is being built around a large solenoid magnet. The magnet is 7.32 in in diameter, 7.25 m long and utilizes three different sizes of room temperature aluminum coils. The magnet will operate with a field set from 0.25 T to 0.5 T and have a field uniformity of better than 1000 ppm over a portion of its interior region. This paper describes the magnet design, fabrication and assembly requirements and presents the current construction status.

  3. EVOLUTION OF ROTATIONAL VELOCITIES OF A-TYPE STARS

    SciTech Connect

    Yang Wuming; Bi Shaolan; Tian Zhijia; Meng Xiangcun E-mail: yangwuming@bnu.edu.cn

    2013-03-10

    The equatorial velocity of A-type stars undergoes an acceleration in the first third of the main sequence (MS) stage, but the velocity decreases as if the stars were not undergoing any redistribution of angular momentum in the external layers in the last stage of the MS phase. Our calculations show that the acceleration and the decrease of the equatorial velocity can be reproduced by the evolution of the differential rotation zero-age MS model with the angular momentum transport caused by hydrodynamic instabilities during the MS stage. The acceleration results from the fact that the angular momentum stored in the interiors of the stars is transported outward. In the last stage, the core and the radiative envelope are uncoupling, and the rotation of the envelope is a quasi-solid rotation; the uncoupling and the expansion of the envelope indicate that the decrease of the equatorial velocity approximately follows the slope for the change in the equatorial velocity of the model without any redistribution of angular momentum. When the fractional age 0.3 {approx}< t/t{sub MS} {approx}< 0.5, the equatorial velocity remains almost constant for stars whose central density increases with age in the early stage of the MS phase, while the velocity decreases with age for stars whose central density decreases with age in the early stage of the MS phase.

  4. B- and A-Type Stars in the Taurus-Auriga Star-Forming Region

    NASA Technical Reports Server (NTRS)

    Mooley, Kunal; Hillenbrand, Lynne; Rebull, Luisa; Padgett, Deborah; Knapp, Gillian

    2013-01-01

    We describe the results of a search for early-type stars associated with the Taurus-Auriga molecular cloud complex, a diffuse nearby star-forming region noted as lacking young stars of intermediate and high mass. We investigate several sets of possible O, B, and early A spectral class members. The first is a group of stars for which mid-infrared images show bright nebulae, all of which can be associated with stars of spectral-type B. The second group consists of early-type stars compiled from (1) literature listings in SIMBAD, (2) B stars with infrared excesses selected from the Spitzer Space Telescope survey of the Taurus cloud, (3) magnitude- and color-selected point sources from the Two Micron All Sky Survey, and (4) spectroscopically identified early-type stars from the Sloan Digital Sky Survey coverage of the Taurus region. We evaluated stars for membership in the Taurus-Auriga star formation region based on criteria involving: spectroscopic and parallactic distances, proper motions and radial velocities, and infrared excesses or line emission indicative of stellar youth. For selected objects, we also model the scattered and emitted radiation from reflection nebulosity and compare the results with the observed spectral energy distributions to further test the plausibility of physical association of the B stars with the Taurus cloud. This investigation newly identifies as probable Taurus members three B-type stars: HR 1445 (HD 28929), t Tau (HD 29763), 72 Tau (HD 28149), and two A-type stars: HD 31305 and HD 26212, thus doubling the number of stars A5 or earlier associated with the Taurus clouds. Several additional early-type sources including HD 29659 and HD 283815 meet some, but not all, of the membership criteria and therefore are plausible, though not secure, members.

  5. B- AND A-TYPE STARS IN THE TAURUS-AURIGA STAR-FORMING REGION

    SciTech Connect

    Mooley, Kunal; Hillenbrand, Lynne; Rebull, Luisa; Padgett, Deborah; Knapp, Gillian

    2013-07-10

    We describe the results of a search for early-type stars associated with the Taurus-Auriga molecular cloud complex, a diffuse nearby star-forming region noted as lacking young stars of intermediate and high mass. We investigate several sets of possible O, B, and early A spectral class members. The first is a group of stars for which mid-infrared images show bright nebulae, all of which can be associated with stars of spectral-type B. The second group consists of early-type stars compiled from (1) literature listings in SIMBAD, (2) B stars with infrared excesses selected from the Spitzer Space Telescope survey of the Taurus cloud, (3) magnitude- and color-selected point sources from the Two Micron All Sky Survey, and (4) spectroscopically identified early-type stars from the Sloan Digital Sky Survey coverage of the Taurus region. We evaluated stars for membership in the Taurus-Auriga star formation region based on criteria involving: spectroscopic and parallactic distances, proper motions and radial velocities, and infrared excesses or line emission indicative of stellar youth. For selected objects, we also model the scattered and emitted radiation from reflection nebulosity and compare the results with the observed spectral energy distributions to further test the plausibility of physical association of the B stars with the Taurus cloud. This investigation newly identifies as probable Taurus members three B-type stars: HR 1445 (HD 28929), {tau} Tau (HD 29763), 72 Tau (HD 28149), and two A-type stars: HD 31305 and HD 26212, thus doubling the number of stars A5 or earlier associated with the Taurus clouds. Several additional early-type sources including HD 29659 and HD 283815 meet some, but not all, of the membership criteria and therefore are plausible, though not secure, members.

  6. Magnetic Field Distribution for Massive Stars

    NASA Astrophysics Data System (ADS)

    Medvedev, A.; Kholtygin, A.

    2017-06-01

    A model of the evolution of an ensemble of magnetic massive stars on the main sequence is developed. We use our own population synthesis code, which allows us to obtain distributions of stars by radii, ages, masses, temperatures, effective magnetic fields, and magnetic fluxes from the pre-main sequence up to the TAMS stages. We assume that magnetic fields in massive stars decrease with time. The rate of magnetic field dissipation may depend on the mass of a star on ZAMS. The distribution of magnetic fluxes of the ZAMS stars is assumed to be log-normal. We show that such kind of distribution may be a result of the dynamo action occurring at the pre-MS evolutionary stage of magnetic stars. Our model also includes capabilities for statistical simulations and parameter estimation necessary for the analysis of real data. Comparison of model magnetic field distributions with those obtained from recent measurements of stellar magnetic fields allows us to conclude that the evolution of magnetic fields of massive stars is very slow if not absent. The shape of the real magnetic field distribution has no indications of the “magnetic desert,” previously suggested by Lignieres et al. (2014). Based on those findings we argue that the observed fraction of magnetic stars is determined only by physical conditions at early stages of stellar evolution.

  7. Magnetic fields in Neutron Stars

    NASA Astrophysics Data System (ADS)

    Viganò, D.; Pons, J. A.; Miralles, J. A.; Rea, N.

    2015-05-01

    Isolated neutron stars show a diversity in timing and spectral properties, which has historically led to a classification in different sub-classes. The magnetic field plays a key role in many aspects of the neutron star phenomenology: it regulates the braking torque responsible for their timing properties and, for magnetars, it provides the energy budget for the outburst activity and high quiescent luminosities (usually well above the rotational energy budget). We aim at unifying this observational variety by linking the results of the state-of-the-art 2D magneto-thermal simulations with observational data. The comparison between theory and observations allows to place two strong constraints on the physical properties of the inner crust. First, strong electrical currents must circulate in the crust, rather than in the star core. Second, the innermost part of the crust must be highly resistive, which is in principle in agreement with the presence of a novel phase of matter so-called nuclear pasta phase.

  8. A new family of magnetic stars: the Am stars

    NASA Astrophysics Data System (ADS)

    Blazère, A.; Neiner, C.; Petit, P.; Lignières, F.

    2016-12-01

    We presented the discovery of an ultra-weak field in three Am stars, β UMa, θ Leo, and Alhena, thanks to ultra-deep spectropolarimetric observations. Two of the three stars of this study shown peculiar magnetic signatures with prominent positive lobes like the one of Sirius A that are not expected in the standard theory of the Zeeman effect. Alhena, contrary to Sirius A, β UMa and θ Leo, show normal signatures. These detections of ultra-weak fields in Am stars suggest the existence of a new family of magnetic intermediate-mass stars: the Am stars. However the various shapes of the signatures required further observation to identify the physical processes at work in these stars. A preliminary explanation is based on microturbulence.

  9. Evolutions of magnetized and rotating neutron stars

    SciTech Connect

    Liebling, Steven L.; Lehner, Luis; Neilsen, David; Palenzuela, Carlos

    2010-06-15

    We study the evolution of magnetized and rigidly rotating neutron stars within a fully general relativistic implementation of ideal magnetohydrodynamics with no assumed symmetries in three spatial dimensions. The stars are modeled as rotating, magnetized polytropic stars, and we examine diverse scenarios to study their dynamics and stability properties. In particular, we concentrate on the stability of the stars and possible critical behavior. In addition to their intrinsic physical significance, we use these evolutions as further tests of our implementation, which incorporates new developments to handle magnetized systems.

  10. The population of highly magnetized neutron stars

    NASA Astrophysics Data System (ADS)

    Gomes, R. O.; Dexheimer, V.; Franzon, B.; Schramm, S.

    2017-06-01

    In this work, we study the effects of strong magnetic field configurations on the population of neutron stars. The stellar matter is described within a relativistic mean field formalism which considers many-body force contributions in the scalar couplings. We choose the parametrization of the model that reproduces nuclear matter properties at saturation and also describes massive hyperon stars. Hadronic matter is modeled at zero temperature, in beta-equilibrium, charge neutral and populated by the baryonic octet, electrons and muons. Magnetic effects are taken into account in the structure of stars by the solution of the Einstein-Maxwell equations with the assumption of a poloidal magnetic field distribution. Our results show that magnetic neutron stars are populated essencialy by nucleons and leptons, due to the fact that strong magnetic fields decrease the central density of stars and, hence, supress the appearance of exotic particles.

  11. VizieR Online Data Catalog: VAST Survey. A-type stars multiplicity (De Rosa+, 2014)

    NASA Astrophysics Data System (ADS)

    De Rosa, R. J.; Patience, J.; Wilson, P. A.; Schneider, A.; Wiktorowicz, S. J.; Vigan, A.; Marois, C.; Song, I.; Macintosh, B.; Graham, J. R.; Doyon, R.; Bessell, M. S.; Thomas, S.; Lai, O.

    2014-11-01

    To measure the frequency of stellar binary companions, and the distribution of their separations and mass ratios, we have obtained observations of a sample of 435 nearby A-type stars. The sample is composed of two overlapping sets of A-type stars within 75pc: a 363 star sample observed with AO instrumentation and a 228 star sample investigated with astrometry obtained from all-sky photographic surveys, with an overlap of 156 stars. (5 data files).

  12. Non-rigid precession of magnetic stars

    NASA Astrophysics Data System (ADS)

    Lander, S. K.; Jones, D. I.

    2017-06-01

    Stars are, generically, rotating and magnetized objects with a misalignment between their magnetic and rotation axes. Since a magnetic field induces a permanent distortion to its host, it provides effective rigidity even to a fluid star, leading to bulk stellar motion that resembles free precession. This bulk motion is, however, accompanied by induced interior velocity and magnetic field perturbations, which are oscillatory on the precession time-scale. Extending previous work, we show that these quantities are described by a set of second-order perturbation equations featuring cross-terms scaling with the product of the magnetic and centrifugal distortions to the star. For the case of a background toroidal field, we reduce these to a set of differential equations in radial functions, and find a method for their solution. The resulting magnetic field and velocity perturbations show complex multipolar structure and are strongest towards the centre of the star.

  13. VLA observations of dwarf M flare stars and magnetic stars

    NASA Technical Reports Server (NTRS)

    Willson, R. F.; Lang, K. R.; Foster, P.

    1988-01-01

    The VLA has been used to search for 6 cm emission from 16 nearby dwarf M stars, leading to the detection of only one of them - Gliese 735. The dwarf M flare stars AD Leonis and YZ Canis Minoris were also monitored at 6 cm and 20 cm wavelength in order to study variability. Successive oppositely circularly polarized bursts were detected from AD Leo at 6 cm, suggesting the presence of magnetic fields of both magnetic polarities. An impulsive 20-cm burst from YZ CMi preceded slowly varying 6-cm emission. The VLA was also used, unsuccessfully, to search for 6-cm emission from 13 magnetic Ap stars, all of which exhibit kG magnetic fields. Although the Ap magnetic stars have strong dipolar magnetic fields, the failure to detect gyroresonant radiation suggests that these stars do not have hot, dense coronae. The quiescent microwave emission from GL 735 is probably due to nonthermal radiation, since unusually high (H = 50 kG or greater) surface magnetic fields are inferred under the assumption that the 6-cm radiation is the gyroresonant radiation of thermal electrons.

  14. Magnetic Color-flavor-locked Stars

    NASA Astrophysics Data System (ADS)

    Paulucci, Laura; Ferrer, Efrain J.; de la Incera, Vivian; Horvath, Jorge E.

    2013-03-01

    Using the solutions of the gap equations of the magnetic-color-flavor-locked (MCFL) phase of paired quark matter in a magnetic field, and taking into consideration the separation between the longitudinal and transverse pressures due to the field-induced breaking of the spatial rotational symmetry, the equation of state (EoS) of the MCFL phase is self-consistently determined. Implications for stellar models of magnetized (self-bound) strange stars and hybrid (MCFL core) stars are discussed.

  15. Magnetic fields in superconducting neutron stars.

    PubMed

    Lander, S K

    2013-02-15

    The interior of a neutron star is likely to be predominantly a mixture of superfluid neutrons and superconducting protons. This results in the quantization of the star's magnetic field into an array of thin flux tubes, producing a macroscopic force very different from the Lorentz force of normal matter. We show that in an axisymmetric superconducting equilibrium the behavior of a magnetic field is governed by a single differential equation. Solving this, we present the first self-consistent superconducting neutron star equilibria with poloidal and mixed poloidal-toroidal fields and also give the first quantitative results for the corresponding magnetically induced distortions to the star. The poloidal component is dominant in all our configurations. We suggest that the transition from normal to superconducting matter in a young neutron star may cause a large-scale field rearrangement.

  16. The Magnetic Field Geometry of Cool Stars

    NASA Astrophysics Data System (ADS)

    See, Victor; Jardine, Moira; Vidotto, Aline; Donati, Jean-Francois; Folsom, Colin; Boro Saikia, Sudeshna; Bouvier, Jerome; Fares, Rim; Gregory, Scott; Hussain, Gaitee; Jeffers, Sandra; Marsden, Stephen; Morin, Julien; Moutou, Claire; do Nascimento, Jose-Dias, Jr.; Petit, Pascal; Rosen, Lisa; Waite, Ian

    2016-06-01

    Zeeman-Doppler imaging has been used to map the large-scale surface magnetic fields of cool stars across a wide range of stellar masses and rotation periods. The derived field geometries are surprising, with many stars showing strong azimuthal fields that are not observed on the Sun. In this poster, using 100 magnetic maps of over 50 stars, we present results showing how the magnetic field geometry of cool stars varies as a function of fundamental parameters. The stellar mass, and hence internal structure, critically influences the field geometry, although this is modified by the stellar rotation rate. We discuss the implications of these results for dynamo theory and the nature of stellar magnetic activity.

  17. Are All Magnetic White Dwarf Stars Massive?

    NASA Astrophysics Data System (ADS)

    Nitta, A.; Kepler, S. O.; Kulebi, B.; Koester, D.; Kleinman, S. J.; Winget, D. E.; Castanheira, B. G.; Corsico, A. H.

    2017-03-01

    We obtained follow-up spectra on 25 white dwarf stars identified in our white dwarf catalog of Sloan Digital Sky Survey (SDSS) as massive or magnetic. We identified over 300 magnetic white dwarf stars from SDSS with some uncertainties due to the low S/N of the spectra. With much higher S/N Gemini data, our sample should be able to help us confirm accuracy of our determinations. We present here our results so far from the follow up observations.

  18. Compact Stars and Magnetized Cfl Matter

    NASA Astrophysics Data System (ADS)

    Pérez Martínez, Aurora; González Felipe, Ricardo; Manreza Paret, Daryel

    The stability of the color flavor locked phase in the presence of a strong magnetic field is investigated within the phenomenological MIT bag model. It is found that the minimum value of the energy per baryon in a color flavor locked state at vanishing pressure is lower than the corresponding one for unpaired magnetized strange quark matter and, as the magnetic field increases, the energy per baryon decreases. This implies that magnetized color flavor locked matter is more stable and could become the ground state inside neutron stars. The anisotropy of the pressures is discussed. The mass-radius relation for such stars is also studied.

  19. Magnetic Bubbles and Extramixing in Stars

    NASA Astrophysics Data System (ADS)

    Wasserburg, Gerald J.; Busso, Maurizio

    2008-04-01

    The possible role of magnetic flux tubes in transporting matter from near the H shell through the radiative zone and into the convective envelope is explored. It is shown that the required rates of mass transport necessary to provide nuclear processed material to the envelope can be achieved if large magnetic fields are present just above the H shell in AGB and RGB stars. The required fields in this zone reach 5 MG for the AGB case and 0.5-0.05 MG for the RGB case. It may thus be possible that magnetic bouyancy play a major role in providing the extra mixing needed for these stars.

  20. Anisotropic structure equations in magnetized quark stars

    NASA Astrophysics Data System (ADS)

    Manreza Paret, D.; Horvath, J. E.; Perez Martinez, A.

    2015-11-01

    It is well-known that a magnetic field in a fermion system breaks the SO(3) symmetry, giving rise to an anisotropy in the energy-momentum tensor and consequently in the equation of state (EoS). Thus, the stellar structure equations of magnetized quark stars (QS) demand to be derived from a more appropriate cylindrical metric. In this work we obtain these equations considering the magnetized quark matter in two phases: Magnetized Strange Quark Matter (MSQM) and Magnetized Color Flavor Locked Phase (MCFL). The main outcome of our work is the value of the maximum magnetic field that an star can hold, i.e. 1018 G. This upper bound is closely related to the violation of the virial relations and also with the instability associated with parity-violation weak processes during the collapse of supernovae.

  1. Magnetic field evolution of accreting neutron stars

    NASA Astrophysics Data System (ADS)

    Istomin, Y. N.; Semerikov, I. A.

    2016-01-01

    The flow of a matter, accreting on to a magnetized neutron star, is accompanied by an electric current. The closing of the electric current occurs in the crust of a neutron stars in the polar region across the magnetic field. But the conductivity of the crust along the magnetic field greatly exceeds the conductivity across the field, so the current penetrates deep into the crust down up to the superconducting core. The magnetic field, generated by the accretion current, increases greatly with the depth of penetration due to the Hall conductivity of the crust is also much larger than the transverse conductivity. As a result, the current begins to flow mainly in the toroidal direction, creating a strong longitudinal magnetic field, far exceeding an initial dipole field. This field exists only in the narrow polar tube of r width, narrowing with the depth, i.e. with increasing of the crust density ρ, r ∝ ρ-1/4. Accordingly, the magnetic field B in the tube increases with the depth, B∝ρ1/2, and reaches the value of about 1017 Gauss in the core. It destroys superconducting vortices in the core of a star in the narrow region of the size of the order of 10 cm. Because of generated density gradient of vortices, they constantly flow into this dead zone and the number of vortices decreases, the magnetic field of a star decreases as well. The attenuation of the magnetic field is exponential, B = B0(1 + t/τ)-1. The characteristic time of decreasing of the magnetic field τ is equal to τ ≃ 103 yr. Thus, the magnetic field of accreted neutron stars decreases to values of 108-109 Gauss during 107-106 yr.

  2. Magnetism and activity of planet hosting stars

    NASA Astrophysics Data System (ADS)

    Wright, Jason T.; Miller, Brendan P.

    The magnetic activity levels of planet host stars may differ from that of stars not known to host planets in several ways. Hot Jupiters may induce activity in their hosts through magnetic interactions, or through tidal interactions by affecting their host's rotation or convection. Measurements of photospheric, chromospheric, or coronal activity might then be abnormally high or low compared to control stars that do not host hot Jupiters, or might be modulated at the planet's orbital period. Such detections are complicated by the small amplitude of the expected signal, by the fact that the signals may be transient, and by the difficulty of constructing control samples due to exoplanet detection biases and the uncertainty of field star ages. We review these issues, and discuss avenues for future progress in the field.

  3. Magnetized strange quark matter and magnetized strange quark stars

    SciTech Connect

    Felipe, R. Gonzalez; Martinez, A. Perez; Rojas, H. Perez; Orsaria, M.

    2008-01-15

    Strange quark matter could be found in the core of neutron stars or forming strange quark stars. As is well known, these astrophysical objects are endowed with strong magnetic fields that affect the microscopic properties of matter and modify the macroscopic properties of the system. In this article we study the role of a strong magnetic field in the thermodynamical properties of a magnetized degenerate strange quark gas, taking into account {beta}-equilibrium and charge neutrality. Quarks and electrons interact with the magnetic field via their electric charges and anomalous magnetic moments. In contrast to the magnetic field value of 10{sup 19} G, obtained when anomalous magnetic moments are not taken into account, we find the upper bound B < or approx. 8.6x10{sup 17} G, for the stability of the system. A phase transition could be hidden for fields greater than this value.

  4. General properties of magnetic CP stars

    NASA Astrophysics Data System (ADS)

    Glagolevskij, Yu. V.

    2017-07-01

    We present the review of our previous studies related to observational evidence of the fossil field hypothesis of formation and evolution of magnetic and non-magnetic chemically peculiar stars. Analysis of the observed data shows that these stars acquire their main properties in the process of gravitational collapse. In the non-stationary Hayashi phase, a magnetic field becomes weakened and its configuration complicated, but the fossil field global orientation remains. After a non-stationary phase, relaxation of young star's tangled field takes place and by the time of joining ZAMS (Zero Age Main Sequence) it is generally restored to a dipole structure. Stability of dipole structures allows them to remain unchanged up to the end of their life on the Main Sequence which is 109 years at most.

  5. Magnetic fields and galactic star formation rates

    SciTech Connect

    Loo, Sven Van; Tan, Jonathan C.; Falle, Sam A. E. G.

    2015-02-10

    The regulation of galactic-scale star formation rates (SFRs) is a basic problem for theories of galaxy formation and evolution: which processes are responsible for making observed star formation rates so inefficient compared to maximal rates of gas content divided by dynamical timescale? Here we study the effect of magnetic fields of different strengths on the evolution of giant molecular clouds (GMCs) within a kiloparsec patch of a disk galaxy and resolving scales down to ≃0.5 pc. Including an empirically motivated prescription for star formation from dense gas (n{sub H}>10{sup 5} cm{sup −3}) at an efficiency of 2% per local free-fall time, we derive the amount of suppression of star formation by magnetic fields compared to the nonmagnetized case. We find GMC fragmentation, dense clump formation, and SFR can be significantly affected by the inclusion of magnetic fields, especially in our strongest investigated B-field case of 80 μG. However, our chosen kiloparsec-scale region, extracted from a global galaxy simulation, happens to contain a starbursting cloud complex that is only modestly affected by these magnetic fields and likely requires internal star formation feedback to regulate its SFR.

  6. Magnetic Fields and Galactic Star Formation Rates

    NASA Astrophysics Data System (ADS)

    Van Loo, Sven; Tan, Jonathan C.; Falle, Sam A. E. G.

    2015-02-01

    The regulation of galactic-scale star formation rates (SFRs) is a basic problem for theories of galaxy formation and evolution: which processes are responsible for making observed star formation rates so inefficient compared to maximal rates of gas content divided by dynamical timescale? Here we study the effect of magnetic fields of different strengths on the evolution of giant molecular clouds (GMCs) within a kiloparsec patch of a disk galaxy and resolving scales down to ≃ 0.5 pc. Including an empirically motivated prescription for star formation from dense gas ({{n}H}\\gt {{10}5} c{{m}-3}) at an efficiency of 2% per local free-fall time, we derive the amount of suppression of star formation by magnetic fields compared to the nonmagnetized case. We find GMC fragmentation, dense clump formation, and SFR can be significantly affected by the inclusion of magnetic fields, especially in our strongest investigated B-field case of 80 μG. However, our chosen kiloparsec-scale region, extracted from a global galaxy simulation, happens to contain a starbursting cloud complex that is only modestly affected by these magnetic fields and likely requires internal star formation feedback to regulate its SFR.

  7. Nuclear processes associated with peculiar A-type stars.

    NASA Technical Reports Server (NTRS)

    Cameron, A. G. W.

    1971-01-01

    A discussion is given of the various roles that nuclear reactions may play in production of anomalous abundances of elements in peculiar A stars. The effects which may be expected to occur both in the surface nuclear reactions and in some possible internal reactions that can occur in advanced stages of stellar evolution are considered. It is suggested that various features of peculiar A stars may require simultaneous operation of two or more of the processes of surface diffusion, surface nuclear reactions, and internal nuclear reactions.

  8. Central Magnetic Field of a Magnetic White Dwarf Star

    NASA Astrophysics Data System (ADS)

    Shah, Hridaya; Sebastian, Kunnat

    2017-07-01

    Observations of over-luminous Type 1a supernovae have prompted researchers to come up with various hypotheses in order to explain them. One hypothesis is based on the explosion of a progenitor super-massive magnetic white dwarf star. These stars are assumed to have very strong magnetic fields inside of them. However, there is a lack of analytic proof of the existence of such magnetic fields in the magnetic white dwarf stars. In this work, we plan to address an analytic proof of the existence of very strong magnetic fields in the center of these magnetic white dwarfs. We will see that for a one Landau-level white dwarf star, with central density {10}9{--}{10}11 {{g}} {{cm}}-3, it is possible to have central magnetic fields of the order of {10}13{--}{10}15G at least. In the presence of strong magnetic fields, the threshold densities chosen for this work that correspond to instabilities due to general relativity and pycnonuclear reactions have been found to increase so that the matter does not acquire instability at such central densities.

  9. Flares on A-type Stars: Evidence for Heating of Solar Corona by Nanoflares?

    NASA Astrophysics Data System (ADS)

    Švanda, Michal; Karlický, Marian

    2016-11-01

    We analyzed the occurrence rates of flares on stars of spectral types K, G, F, and A, observed by Kepler. We found that the histogram of occurrence frequencies of stellar flares is systematically shifted toward a high-energy tail for A-type stars compared to stars of cooler spectral types. We extrapolated the fitted power laws toward flares with smaller energies (nanoflares) and made estimates for total energy flux to stellar atmospheres by flares. We found that, for A-type stars, the total energy flux density was at least four-times smaller than for G stars. We speculate that this deficit in energy supply may explain the lack of hot coronae on A-type stars. Our results indicate the importance of nanoflares for heating and formation of the solar corona.

  10. Magnetic cycles at different ages of stars

    NASA Astrophysics Data System (ADS)

    Oláh, K.; Kővári, Zs.; Petrovay, K.; Soon, W.; Baliunas, S.; Kolláth, Z.; Vida, K.

    2016-06-01

    Aims: We study the different patterns of interannual magnetic variability in stars on or near the lower main sequence, approximately solar-type (G-K dwarf) stars in time series of 36 yr from the Mount Wilson Observatory Ca ii H&K survey. Our main aim is to search for correlations between cycles, activity measures, and ages. Methods: Time-frequency analysis has been used to discern and reveal patterns and morphology of stellar activity cycles, including multiple and changing cycles, in the datasets. Both the results from short-term Fourier transform and its refinement using the Choi-Williams distribution, with better frequency resolution, are presented in this study. Rotational periods of the stars were derived using multifrequency Fourier analysis. Results: We found at least one activity cycle on 28 of the 29 stars we studied. Twelve stars, with longer rotational periods (39.7 ± 6.0 days), have simple smooth cycles, and the remaining stars, with much faster rotation (18.1 ± 12.2 days) on average, show complex and sometimes vigorously changing multiple cycles. The cycles are longer and quite uniform in the first group (9.7 ± 1.9 yr), while they are generally shorter and vary more strongly in the second group (7.6 ± 4.9). The clear age division between stars with smooth and complex cycles follows the known separation between the older and younger stars at around 2 to 3 Gyr of age.

  11. A survey for pulsations in A-type stars using SuperWASP

    NASA Astrophysics Data System (ADS)

    Holdsworth, Daniel L.

    2015-12-01

    "It is sound judgement to hope that in the not too distant future we shall be competent to understand so simple a thing as a star." - Sir Arthur Stanley Eddington, The Internal Constitution of Stars, 1926 A survey of A-type stars is conducted with the SuperWASP archive in the search for pulsationally variable stars. Over 1.5 million stars are selected based on their (J-H) colour. Periodograms are calculated for light curves which have been extracted from the archive and cleaned of spurious points. Peaks which have amplitudes greater than 0.5 millimagnitude are identified in the periodograms. In total, 202 656 stars are identified to show variability in the range 5-300 c/d. Spectroscopic follow-up was obtained for 38 stars which showed high-frequency pulsations between 60 and 235 c/d, and a further object with variability at 636 c/d. In this sample, 13 were identified to be normal A-type δ Sct stars, 14 to be pulsating metallic-lined Am stars, 11 to be rapidly oscillating Ap (roAp) stars, and one to be a subdwarf B variable star. The spectra were used not only to classify the stars, but to determine an effective temperature through Balmer line fitting. Hybrid stars have been identified in this study, which show pulsations in both the high- and low-overtone domains; an observation not predicted by theory. These stars are prime targets to perform follow-up observations, as a confirmed detection of this phenomenon will have significant impact on the theory of pulsations in A-type stars. The detected number of roAp stars has expanded the known number of this pulsator class by 22 per cent. Within these results both the hottest and coolest roAp star have been identified. Further to this, one object, KIC 7582608, was observed by the Kepler telescope for 4 yr, enabling a detailed frequency analysis. This analysis has identified significant frequency variations in this star, leading to the hypothesis that this is the first close binary star of its type. The observational

  12. Magnetic field evolution in neutron stars

    NASA Astrophysics Data System (ADS)

    Castillo, F.; Reisenegger, A.; Valdivia, J. A.

    2017-07-01

    Neutron stars contain the strongest magnetic fields known in the Universe. Using numerical simulations restricted to axially symmetric geometry, we study the long-term evolution of the magnetic field in the interior of an isolated neutron star under the effect of ambipolar diffusion, i.e. the drift of the magnetic field and the charged particles relative to the neutrons. We model the stellar interior as an electrically neutral fluid composed of neutrons, protons and electrons; these species can be converted into each other by weak interactions (beta decays), suffer binary collisions, and be affected by each other's macroscopic electromagnetic fields. We show that, in the restricted case of pure ambipolar diffusion, neglecting weak interactions, the magnetic fields evolves towards a stable MHD equilibria configuration, in the timescales analytically expected.

  13. Physics in Strong Magnetic Fields Near Neutron Stars.

    ERIC Educational Resources Information Center

    Harding, Alice K.

    1991-01-01

    Discussed are the behaviors of particles and energies in the magnetic fields of neutron stars. Different types of possible research using neutron stars as a laboratory for the study of strong magnetic fields are proposed. (CW)

  14. Physics in Strong Magnetic Fields Near Neutron Stars.

    ERIC Educational Resources Information Center

    Harding, Alice K.

    1991-01-01

    Discussed are the behaviors of particles and energies in the magnetic fields of neutron stars. Different types of possible research using neutron stars as a laboratory for the study of strong magnetic fields are proposed. (CW)

  15. Gravitomagnetic effect in magnetized neutron stars

    NASA Astrophysics Data System (ADS)

    Chatterjee, Debarati; Chakraborty, Chandrachur; Bandyopadhyay, Debades

    2017-01-01

    Rotating bodies in General Relativity produce frame dragging, also known as the gravitomagnetic effect in analogy with classical electromagnetism. In this work, we study the effect of magnetic field on the gravitomagnetic effect in neutron stars with poloidal geometry, which is produced as a result of its rotation. We show that the magnetic field has a non-negligible impact on frame dragging. The maximum effect of the magnetic field appears along the polar direction, where the frame-dragging frequency decreases with increase in magnetic field, and along the equatorial direction, where its magnitude increases. For intermediate angles, the effect of the magnetic field decreases, and goes through a minimum for a particular angular value at which magnetic field has no effect on gravitomagnetism. Beyond that particular angle gravitomagnetic effect increases with increasing magnetic field. We try to identify this `null region' for the case of magnetized neutron stars, both inside and outside, as a function of the magnetic field, and suggest a thought experiment to find the null region of a particular pulsar using the frame dragging effect.

  16. Spin equilibrium in strongly magnetized accreting stars

    NASA Astrophysics Data System (ADS)

    D'Angelo, C. R.

    2017-09-01

    Strongly magnetized accreting stars are often hypothesized to be in 'spin equilibrium' with their surrounding accretion flows, which requires that the accretion rate changes more slowly than it takes the star to reach spin equilibrium. This is not true for most magnetically accreting stars, which have strongly variable accretion outbursts on time-scales much shorter than the time it would take to reach spin equilibrium. This paper examines how accretion outbursts affect the time a star takes to reach spin equilibrium and its final equilibrium spin period. I consider several different models for angular momentum loss - either carried away in an outflow, lost to a stellar wind, or transferred back to the accretion disc (the 'trapped disc'). For transient sources, the outflow scenario leads to significantly longer times to reach spin equilibrium (∼10 ×), and shorter equilibrium spin periods than would be expected from spin equilibrium arguments, while the 'trapped disc' does not. The results suggest that disc trapping plays a significant role in the spin evolution of strongly magnetic stars, with some caveats for young stellar objects.

  17. Disk accretion onto magnetic T Tauri stars

    SciTech Connect

    Koenigl, A. )

    1991-03-01

    The dynamical and radiative consequences of disk accretion onto magnetic T Tauri stars (TTS) are examined using the Ghosh and Lamb model. It is shown that a prolonged disk accretion phase is compatible with the low rotation rates measured in these stars if they possess a kilogauss strength field that disrupts the disk at a distance of a few stellar radii from the center. It is estimated that a steady state in which the net torque exerted on the star is zero can be attained on a time scale that is shorter than the age of the youngest visible TTS. Although the disk does not develop an ordinary shear boundary layer in this case, one can account for the observed UV excess and Balmer emission in terms of the shocks that form at the bottom of the high-latitude magnetic accretion columns on the stellar surface. This picture also provides a natural explanation of some of the puzzling variability properties of stars like DF Tau and RY Lup. YY Ori stars are interpreted as magnetic TTS in which the observer's line of sight is roughly parallel to an accretion column. 37 refs.

  18. A Catalog of Candidate Field Horizontal-Branch and A-Type Stars. II.

    NASA Astrophysics Data System (ADS)

    Beers, Timothy C.; Wilhelm, Ronald; Doinidis, Stephen P.; Mattson, Caroline J.

    1996-04-01

    We present coordinates and brightness estimates for 4175 candidate field horizontal-branch and A-type stars, in the magnitude range 10 ≤ B ≤ 15.5, selected using an objective-prism/interference-filter survey technique. The candidates lie primarily in the northern Galactic hemisphere and complement a previously published sample of southern Galactic hemisphere candidates. Available spectroscopy and photometry indicates that the great majority of the candidates are likely to be bona fide members of either the field blue horizontal-branch population or the blue, metal-deficient, high surface gravity stars referred to by Preston, Beers, & Shectman as BMP stars. The remaining stars in the catalog are likely to be a mix of metal-deficient turnoff stars, metallic-line (Am) stars, field red horizontal-branch stars, optical doubles with overlapping objective-prism spectra, and (particularly among the fainter candidates) inadvertently included late-type stars.

  19. Instability of magnetic equilibria in barotropic stars

    NASA Astrophysics Data System (ADS)

    Mitchell, J. P.; Braithwaite, J.; Reisenegger, A.; Spruit, H.; Valdivia, J. A.; Langer, N.

    2015-02-01

    In stably stratified stars, numerical magnetohydrodynamics simulations have shown that arbitrary initial magnetic fields evolve into stable equilibrium configurations, usually containing nearly axisymmetric, linked poloidal and toroidal fields that stabilize each other. In this work, we test the hypothesis that stable stratification is a requirement for the existence of such stable equilibria. For this purpose, we follow numerically the evolution of magnetic fields in barotropic (and thus neutrally stable) stars, starting from two different types of initial conditions, namely random disordered magnetic fields, as well as linked poloidal-toroidal configurations resembling the previously found equilibria. With many trials, we always find a decay of the magnetic field over a few Alfvén times, never a stable equilibrium. This strongly suggests that there are no stable equilibria in barotropic stars, thus clearly invalidating the assumption of barotropic equations of state often imposed on the search of magnetic equilibria. It also supports the hypothesis that, as dissipative processes erode the stable stratification, they might destabilize previously stable magnetic field configurations, leading to their decay.

  20. Magnetic buoyancy and the escape of magnetic fields from stars

    NASA Technical Reports Server (NTRS)

    Parker, E. N.

    1984-01-01

    A loss of magnetic flux through the free surface of a star into the surrounding space has important implications for the generation of the field within the star. The present investigation is concerned with the physics of the escape of net azimuthal flux from a star. The obtained results are used as a basis for the interpretation of some recent observations of the detailed behavior of magnetic fields emerging through the surface of the sun. The buoyancy of an isolated horizontal magnetic flux tube beneath the surface of a star causes the tube to rise at a rate comparable to the Alfven speed. The necessary conditions for escape of the flux are considered along with aspects of magnetic buoyancy, and the conditions on the sun. It appears that the observed retraction of bipolar magnetic fields at the end of their life at the surface is the one phenomenon which requires dynamical intervention. Attention is given to known dynamical effects which suppress the buoyant rise of an azimuthal magnetic field.

  1. ON MAGNETIC EQUILIBRIA IN BAROTROPIC STARS

    SciTech Connect

    Armaza, Cristóbal; Reisenegger, Andreas; Valdivia, Juan Alejandro

    2015-04-01

    Upper main-sequence stars, white dwarfs, and neutron stars are known to possess stable, large-scale magnetic fields. Numerical works have confirmed that stable magnetohydrodynamic equilibria can exist in non-barotropic, stably stratified stars. On the other hand, it is unclear whether stable equilibria are possible in barotropic stars, although the existing evidence suggests that they are all unstable. This work aims to construct barotropic equilibria in order to study their properties, as a first step to test their stability. We have assumed that the star is a perfectly conducting, axially symmetric fluid, allowing for both poloidal and toroidal components of the magnetic field. In addition, we made the astrophysically justified assumption that the magnetic force has a negligible influence on the fluid structure, in which case the equilibrium is governed by the Grad–Shafranov equation, involving two arbitrary functions of the poloidal flux. We built a numerical code to solve this equation, allowing for an arbitrary prescription for these functions. Taking particularly simple, but physically reasonable choices for these functions with a couple of adjustable parameters, all of the equilibria found present only a small (≲10%) fraction of the magnetic energy stored in the toroidal component, confirming previous results. We developed an analytical model in order to study in more detail the behavior of the magnetic energy over the full range of parameters. The model confirms that the toroidal fraction of the energy and the ratio of toroidal to poloidal flux are bounded from above for the whole range of parameters.

  2. Identification and Analysis of Horizontal-Branch and Other A-Type Stars in the Galaxy

    NASA Astrophysics Data System (ADS)

    Rossi, Silvia C. F.; Beers, Timothy C.; Wilhelm, Ronald J.

    New techniques are being developed for the identification of Field Horizontal-Branch (FHB) and other A-type stars based on moderate-resolution spectroscopic observations and broadband UBV photometry. Physical parameters (T_eff, log g, and feh\\ ) for these stars can be estimated with accuracy on the order of sigma (T_eff) = 250 K, sigma (log g) = 0.35 dex, and sigma (feh ) = 0.30 dex, respectively. Detailed analysis such as this is required in order to form a ``clean'' sample of FHB stars, as such samples are easily confounded by the presence of halo blue stragglers (or BMP stars) and other high-gravity A-type stars. We summarize these analysis techniques, and discuss their application to a new large sample of FHB/A stars identified as part of the ongoing HK interference-filter/objective-prism survey of Beers and collaborators.

  3. Magnetically driven crustquakes in neutron stars

    NASA Astrophysics Data System (ADS)

    Lander, S. K.; Andersson, N.; Antonopoulou, D.; Watts, A. L.

    2015-05-01

    Crustquake events may be connected with both rapid spin-up `glitches' within the regular slowdown of neutron stars, and high-energy magnetar flares. We argue that magnetic-field decay builds up stresses in a neutron star's crust, as the elastic shear force resists the Lorentz force's desire to rearrange the global magnetic-field equilibrium. We derive a criterion for crust-breaking induced by a changing magnetic-field configuration, and use this to investigate strain patterns in a neutron star's crust for a variety of different magnetic-field models. Universally, we find that the crust is most liable to break if the magnetic field has a strong toroidal component, in which case the epicentre of the crustquake is around the equator. We calculate the energy released in a crustquake as a function of the fracture depth, finding that it is independent of field strength. Crust-breaking is, however, associated with a characteristic local field strength of 2.4 × 1014 G for a breaking strain of 0.001, or 2.4 × 1015 G at a breaking strain of 0.1. We find that even the most luminous magnetar giant flare could have been powered by crustal energy release alone.

  4. Magnetic field decay in isolated neutron stars

    NASA Technical Reports Server (NTRS)

    Goldreich, Peter; Reisenegger, Andreas

    1992-01-01

    Three mechanisms that promote the loss of magnetic flux from an isolated neutron star - Ohmic decay, ambipolar diffusion, and Hall drift - are investigated. Equations of motions are solved for charged particles in the presence of a magnetic field and a fixed background of neutrons, while allowing for the creation and destruction of particles by weak interactions. Although these equations apply to normal neutrons and protons, the present interpretations of their solutions are extended to cover cases of neutron superfluidity and proton superconductivity. The equations are manipulated to prove that, in the presence of a magnetic force, the charged particles cannot be simultaneously in magnetostatic equilibrium and chemical equilibrium with the neutrons. The application of the results to real neutron stars is discussed.

  5. Magnetic fields and massive star formation

    SciTech Connect

    Zhang, Qizhou; Keto, Eric; Ho, Paul T. P.; Ching, Tao-Chung; Chen, How-Huan; Qiu, Keping; Girart, Josep M.; Juárez, Carmen; Liu, Hauyu; Tang, Ya-Wen; Koch, Patrick M.; Rao, Ramprasad; Lai, Shih-Ping; Li, Zhi-Yun; Frau, Pau; Li, Hua-Bai; Padovani, Marco; Bontemps, Sylvain

    2014-09-10

    Massive stars (M > 8 M {sub ☉}) typically form in parsec-scale molecular clumps that collapse and fragment, leading to the birth of a cluster of stellar objects. We investigate the role of magnetic fields in this process through dust polarization at 870 μm obtained with the Submillimeter Array (SMA). The SMA observations reveal polarization at scales of ≲0.1 pc. The polarization pattern in these objects ranges from ordered hour-glass configurations to more chaotic distributions. By comparing the SMA data with the single dish data at parsec scales, we found that magnetic fields at dense core scales are either aligned within 40° of or perpendicular to the parsec-scale magnetic fields. This finding indicates that magnetic fields play an important role during the collapse and fragmentation of massive molecular clumps and the formation of dense cores. We further compare magnetic fields in dense cores with the major axis of molecular outflows. Despite a limited number of outflows, we found that the outflow axis appears to be randomly oriented with respect to the magnetic field in the core. This result suggests that at the scale of accretion disks (≲ 10{sup 3} AU), angular momentum and dynamic interactions possibly due to close binary or multiple systems dominate over magnetic fields. With this unprecedentedly large sample of massive clumps, we argue on a statistical basis that magnetic fields play an important role during the formation of dense cores at spatial scales of 0.01-0.1 pc in the context of massive star and cluster star formation.

  6. Hidden Binaries Among Magnetic CP Stars in Two Examples

    NASA Astrophysics Data System (ADS)

    Semenko, E.

    2017-06-01

    Spectropolarimetric surveys of chemically peculiar or CP-stars that aim to the searches of stars hosting the strong and globally-organized magnetic field have increased the number of known magnetic CP's. At the same time, regular observations were carried out only for the limited sample of the stars. Hence, the problem of 'hidden' binary and multiple stellar systems among newly-discovered magnetic CP-stars is existing.

  7. The spectra of weakly magnetized neutron stars

    NASA Technical Reports Server (NTRS)

    Fu, Albert; Taam, Ronald E.

    1990-01-01

    The continuum spectrum of a rotating equatorial emitting region on the surface of an accreting neutron star is investigated within the Schwarzschild spacetime. It is found that the dominant general relativistic effect is the increase in apparent emission area from the neutron star surface due to gravitational light bending. For typical neutron star parameters, the apparent area of the accretion belt is remarkably independent of the viewing angle and, hence, the contribution for a boundary layer to the total X-ray luminosity is found to depend only weakly on the inclination angle of the binary system. For a blackbody spectrum in the local rest frame of the emitting surface, the distortion of the spectral shape by longitudinal and transverse Doppler shifts is minimal for neutron stars rotating at periods greater than about 2 ms. It is shown that the failure to detect a blackbody component from the weakly magnetized neutron star surface in some X-ray burst sources during their quiescent state may be related to the presence of temperature variations along the vertical extent of the boundary layer.

  8. Discussion - Winds and magnetic fields of active OB stars

    NASA Astrophysics Data System (ADS)

    Bouret, Jean-Claude; Cidale, Lydia

    2011-07-01

    The discussion on winds and magnetic fields of active OB stars was carried out by S. Owoki, G. Wade, M. Cantiello, O. Kochukhov, M. Smith, C. Neiner, T. Rivinius, H. Henrichs and R. Townsend. The topics were the ability to detect small and large scale magnetic fields in massive stars and the need to consider limits on photometric variability of the star surface brightness.

  9. On close-in magnetized star-planet interactions

    NASA Astrophysics Data System (ADS)

    Strugarek, A.; Brun, A. S.; Matt, S.

    2012-12-01

    We present 2D magnetohydrodynamic simulations performed with the PLUTO code to model magnetized star-planet interactions. We study two simple scenarios of magnetized star-planet interactions: the unipolar and dipolar} interactions.Despite the simplified hypotheses we consider in the model, the qualitative behavior of the interactions is very well recovered. These encouraging results promote further developments of the model to obtain predictions on the effect and the physical manifestation of magnetized star--close-in planet interactions.

  10. The discovery of nonthermal radio emission from magnetic Bp-Ap stars

    NASA Technical Reports Server (NTRS)

    Drake, Stephen A.; Abbott, David C.; Bastian, T. S.; Bieging, J. H.; Churchwell, E.

    1987-01-01

    In a VLA survey of chemically peculiar B- and A-type stars with strong magnetic fields, five of the 34 stars observed have been identified as 6 cm continuum sources. Three of the detections are helium-strong early Bp stars (Sigma Ori E, HR 1890, and Delta Ori C), and two are helium weak, silicon-strong stars with spectral types near A0p (IQ Aur = HD 34452, Babcock's star = HD 215441). The 6 cm luminosities L6 (ergs/s Hz) range from log L6 = 16.2 to 17.9, somewhat less than the OB supergiants and W-R stars. Three-frequency observations indicate that the helium-strong Bp stars are variable nonthermal sources.

  11. About the new B- and A-type periodic variable stars

    NASA Astrophysics Data System (ADS)

    Mowlavi, Nami; Saesen, Sophie; Barblan, Fabio; Semaan, Thierry; Eyer, Laurent

    2015-08-01

    New periodic variable stars of type B and A were discovered in 2013 in the open cluster NGC 3766 (Mowlavi et al. 2013), in a region of the Hertzsprung-Rusell diagram on the main sequence, between delta-Scuti and Slowly Pulsating B stars, where no periodic star was expected to be found. The observational properties of the potentially new class of variability challenged model predictions of pulsating stars that were available at the time of their discovery, and triggered new theoretical developments in the field.Meanwhile, we obtained additional observational elements that shed new light on these new periodic variable stars. First, we analyzed photometric data of three other young clusters with ages between about 15 to about 70 million years, for which we had up to seven years of observations. The analysis reveals the presence of the new periodic B- and A-type stars in those clusters as well, with properties similar to those found in NGC 3766, but with also new features. Second, we took spectra of all new periodic variables found in NGC 3766, using Giraffe multi-fiber spectrometer mounted on the VLT. Those data enable a better characterization of the nature of those stars.We present in this contribution the results of those two new studies, and their impacts on our understanding of the new periodic stars.

  12. Free fall onto magnetized neutron stars

    NASA Astrophysics Data System (ADS)

    Salpeter, E. E.

    Some compact X-ray sources show evidence of cyclotron line radiation from excited electron Landau orbits, powered by hydrogen and helium falling onto a neutron star atmosphere along the magnetic field. The slowing of the incident matter is discussed, including the spread in energy loss due to Coulomb scattering and direct nuclear reactions for disintegrating the α particles. The α disintegrations, followed by neutron capture, lead to nuclear γ rays; the γ-ray intensity is (indirectly) coupled to the Coulomb energy loss and the cyclotron line emission.

  13. MiMes and Magnetic Fields in Massive Stars

    NASA Astrophysics Data System (ADS)

    Petit, Veronique

    2013-06-01

    Massive star magnetism is often considered an astronomical ``wildcard'', as it is hard to predict in which stars it may occur. This reflects our fundamental ignorance of the origin of massive star magnetism, and compels us to better understand the scope of its influence on massive stars individually, and also as a population. In the last decade, our understanding of this phenomenon has made a giant leap forward thanks to a new generation of powerful spectropolarimeters capable of measuring the Zeeman effect in the spectra of these stars. Over the past 5 years, ambitious projects such as the Magnetism in Massive Stars (MiMeS) Collaboration have been seeking out magnetic massive stars in the Galaxy, to better understand their origins, physical properties, and how they influence observable stellar characteristics. In this talk, we review the general properties of OB star magnetism in the Galaxy, using recent MiMeS discoveries as examples. It is now clear that the magnetic properties of massive stars are established early in their evolution. This raises interesting and fundamental questions about the physics of stellar formation and connections with stellar magnetism, as MiMeS observations have established that about 1 in 15 Galactic OB stars hosts a magnetic field that is sufficiently strong to significantly influence its atmospheric and wind structure. Could your own mysterious and perplexing OB target be a magnetic massive star? To aid in answering this question, we review many of the outstanding or exotic properties exhibited by known magnetic OB stars that relate directly to their magnetic nature.

  14. Magnetic fields of spherical compact stars in a braneworld

    SciTech Connect

    Ahmedov, B. J.; Fattoyev, F. J.

    2008-08-15

    We study the stellar magnetic field configuration in dependence on brane tension and present solutions of Maxwell equations in the external background space-time of a magnetized spherical star in a Randall-Sundrum II type braneworld. The star is modeled as a sphere consisting of perfect highly magnetized fluid with infinite conductivity and a frozen-in magnetic field. With respect to solutions for magnetic fields found in the Schwarzschild space-time, brane tension introduces enhancing corrections to the exterior magnetic field which could be relevant for the magnetic fields of magnetized compact objects as pulsars and magnetars and may provide observational evidence for the brane tension.

  15. Magnetic buoyancy and the escape of magnetic fields from stars

    NASA Astrophysics Data System (ADS)

    Parker, E. N.

    1984-06-01

    Magnetic buoyancy causes the azimuthal magnetic fields of stars to rise rapidly to the surface, from where they are generally assumed to escape freely into space. However, a closer look at the problem reveals the simple fact that disengagement of the field from the gas, and escape into space, require a convoluted field configuration, producing neutral point reconnection of the flux in the tenuous gas above the surface of the star. Only that flux which reconnects can escape. Recent observations of the magnetic fields emerging through the surface of the Sun show that even at sunspot maximum the gaps in longitude between bipolar magnetic regions are so wide as to limit severely the reconnection between regions. We suggest from the observations that no more than perhaps 3% of the flux that is observed to emerge through the surface is able to reconnect and escape. Hence the surface of the Sun approximates to an impenetrable barrier rather than an open surface, with quantitative consequences for theoretical dynamo models. Recent observations of the retraction of bipolar fields at the end of their appearance at the surface suggest active dynamical control by the convection beneath the surface.

  16. Surface composition of magnetic neutron stars.

    NASA Technical Reports Server (NTRS)

    Rosen, L. C.; Cameron, A. G. W.

    1972-01-01

    The relative abundances of seven constituent nuclei, He4, C12, O16, Ne20, Mg24, Si28, and Fe56, are calculated as a function of time for neutron star atmospheres within which exist magnetic fields of the order of 10 to the 13th G. The opacity, equation of state of the electrons, and cooling rate of the magnetic star are discussed, and it is shown to be a reasonable approximation to assume an atmosphere to be isothermal. The effects of particle diffusion are included in the nuclear reaction network. Computations are performed both for a constant mass atmosphere and for an atmosphere in which mass is being ejected. It is found that the final abundances are model-independent, as long as the initial model contains predominantly He4. The relative abundances are compared to the cosmic ray spectrum. For both the constant-mass and mass-loss atmospheres, nucleosynthesis proceeds virtually completely to Fe56. However the outermost layers of the envelope, in which no mass is being ejected, are composed almost entirely of He4 with trace amounts of Fe56. After the loss of about 10 to the 21st g, only Fe56 is ejected from atmospheres expelling mass.

  17. Vorticity and magnetic shielding in a type-II superconductor.

    PubMed

    Cardoso, Marco; Bicudo, Pedro; Sacramento, Pedro D

    2006-09-20

    We study in detail, solving the Bogoliubov-de Gennes equations, the magnetic field, supercurrent and order parameter profiles originated by a solenoid or magnetic whisker inserted in a type-II superconductor. We consider solutions of different vorticities, n, in the various cases. The results confirm the connection between the vorticity, the internal currents and the boundstates in a self-consistent way. The number of boundstates is given by the vorticity of the phase of the gap function as in the case with no external solenoid. In the limiting case of an infinitely thin solenoid, like a Dirac string, the solution is qualitatively different. The quasiparticle spectrum and wavefunctions are a function of n-n(ext), where n(ext) is the vorticity of the solenoid. The flux is in all cases determined by the vorticity of the gap function.

  18. Neutron stars. [quantum mechanical processes associated with magnetic fields

    NASA Technical Reports Server (NTRS)

    Canuto, V.

    1978-01-01

    Quantum-mechanical processes associated with the presence of high magnetic fields and the effect of such fields on the evolution of neutron stars are reviewed. A technical description of the interior of a neutron star is presented. The neutron star-pulsar relation is reviewed and consideration is given to supernovae explosions, flux conservation in neutron stars, gauge-invariant derivation of the equation of state for a strongly magnetized gas, neutron beta-decay, and the stability condition for a neutron star.

  19. Transient phenomena from accreting magnetized neutron stars

    NASA Astrophysics Data System (ADS)

    Klochkov, Dmitry

    In this contribution, I will review the recent progress in the research of accreting magnetized neutron stars (observed as X-ray pulsars) based on the study of their variability on different time scales. Specifically, I will focus on the properties of the X-ray emitting region. In recent years, the high-quality observational data accumulated with the new generation of X-ray observatories have triggered a renewed interest in these systems. The new studies are primarily focused on the detailed structure of the two physical regions of the objects: (i) the emitting area above the polar caps of the neutron star and (ii) the magnetospheric boundary, where the infalling matter couples to the accretoŕs magnetic field. The modulation of the matter supply from the binary companion as well as the instabilities in the accretion flow lead to the transient character of the majority of X-ray pulsars. The observations show that the "persistent" pulsars also exhibit numerous types of variabilities over a broad range of time scales (off-states, pulse-to-pulse variability, switches of spectral states, alternation of the pulsar's spin-up/spin-down episodes etc.). Of particular importance are the observed variations of the cyclotron absorption features (cyclotron lines), whose centroid energies are directly proportional to the magnetic field strength at the site of the line formation. The detailed studies of these variabilities have lead to the development of new theoretical models describing the physics in the emitting region and at the magnetospheric boundary. It has been proposed that the configuration and geometry of the two areas may change abruptly when the mass accretion rate reaches certain critical values. Such changes cause transitions between different accretion modes. A particular mode is expected to be characterized by certain variability patterns and can thus be inferred from the observations. I will describe these recent observations and the models which are aimed at

  20. Observational Effects of Magnetism in O Stars: Surface Nitrogen Abundances

    NASA Technical Reports Server (NTRS)

    Martins, F.; Escolano, C.; Wade, G. A.; Donati, J. F.; Bouret, J. C.

    2011-01-01

    Aims. We investigate the surface nitrogen content of the six magnetic O stars known to date as well as of the early B-type star Tau Sco.. We compare these abundances to predictions of evolutionary models to isolate the effects of magnetic field on the transport of elements in stellar interiors. Methods. We conduct a quantitative spectroscopic analysis of the ample stars with state-of-the-art atmosphere models. We rely on high signal-to-noise ratio, high resolution optical spectra obtained with ESPADONS at CFHT and NARVAL at TBL. Atmosphere models and synthetic spectra are computed with the code CMFGEN. Values of N/H together with their uncertainties are determined and compared to predictions of evolutionary models. Results. We find that the magnetic stars can be divided into two groups: one with stars displaying no N enrichment (one object); and one with stars most likely showing extra N enrichment (5 objects). For one star (Ori C) no robust conclusion can be drawn due to its young age. The star with no N enrichment is the one with the weakest magnetic field, possibly of dynamo origin. It might be a star having experienced strong magnetic braking under the condition of solid body rotation, but its rotational velocity is still relatively large. The five stars with high N content were probably slow rotators on the zero age main sequence, but they have surface N/H typical of normal O stars, indicating that the presence of a (probably fossil) magnetic field leads to extra enrichment. These stars may have a strong differential rotation inducing shear mixing. Our results shOuld be viewed as a basis on which new theoretical simulations can rely to better understand the effect of magnetism on the evolution of massive stars.

  1. Gas and dust around A-type stars at tens of Myr: signatures of cometary breakup

    NASA Astrophysics Data System (ADS)

    Greaves, J. S.; Holland, W. S.; Matthews, B. C.; Marshall, J. P.; Dent, W. R. F.; Woitke, P.; Wyatt, M. C.; Matrà, L.; Jackson, A.

    2016-10-01

    Discs of dusty debris around main-sequence stars indicate fragmentation of orbiting planetesimals, and for a few A-type stars, a gas component is also seen that may come from collisionally released volatiles. Here we find the sixth example of a CO-hosting disc, around the ˜30 Myr-old A0-star HD 32997. Two more of these CO-hosting stars, HD 21997 and 49 Cet, have also been imaged in dust with SCUBA-2 within the SCUBA-2 Survey of Nearby Stars project. A census of 27 A-type debris hosts within 125 pc now shows 7/16 detections of carbon-bearing gas within the 5-50 Myr epoch, with no detections in 11 older systems. Such a prolonged period of high fragmentation rates corresponds quite well to the epoch when most of the Earth was assembled from planetesimal collisions. Recent models propose that collisional products can be spatially asymmetric if they originate at one location in the disc, with CO particularly exhibiting this behaviour as it can photodissociate in less than an orbital period. Of the six CO-hosting systems, only β Pic is in clear support of this hypothesis. However, radiative transfer modelling with the ProDiMo code shows that the CO is also hard to explain in a proto-planetary disc context.

  2. Spectropolarimetry of Giant stars: Probing the influence of magnetic field on evolved stars Spectropolarimetry of Giant stars: Probing the influence of magnetic field on evolved stars

    NASA Astrophysics Data System (ADS)

    da Costa, Jefferson; Castro, Matthieu; Petit, Pascal; do Nascimento, José-Dias, Jr.

    2015-08-01

    It is know that lithium is element easily destroyed in stellar interior, the existence of lithium rich stars means a great challenge in stellar evolution. In this context our observations ravels the serendipitous discovery of an unusually high lithium abundance star. This is a K0III HD 150050, which has strong deepening on lithium line (6707.8 Å) this means lithium abundance of 2.81 0.2 dex, therefore this star belong a rare group called super Li-Rich stars. A possible source of the non-standard episodes required to produce Li-rich stars were identified in magneto-thermohaline mixing accounted by models of extra-mixing induced by magnetic buoyancy. However to better understand this is necessary more observational data. In last three decades several studies has showed that late type red giant stars presents a remarkable modifications in these outer atmosphere layers when they become late type star in HR diagram. These changes are founded through X-ray, Ultraviolet, and Chromospheric activity analyses, and then we can establish the called “Dividing lines”. We made spectropalarimetric observations with ESPaDOnS@CFHT to achieve two main objectives: analyze the influence of magnetic field in the Li-rich giant stars, and understand how works the magnetic field in late type giants and supergiants across the “dividing line”.

  3. Magnetic fields and dense chromospheres in dMe stars

    NASA Technical Reports Server (NTRS)

    Mullan, D. J.

    1975-01-01

    The hypothesis is investigated that dense chromospheres of dMe stars are heated by dissipation of hydromagnetic waves which may be generated in active regions where the nonspot magnetic field strength can be as large as 5 to 10 kG. It is proposed that dMe stars are a set of magnetic stars on the lower main sequence which have strong fields generated by dynamo action in deep convective envelopes, while dM stars are nonmagnetic or weakly magnetic stars having no starspots on their surfaces. The combination of magnetic fields and dense chromospheres in dMe stars is shown to provide consistent evidence for several conclusions, including: (1) the dMe stars which are most likely to be flare stars are those with hydrogen emission lines and (2) propagation of flare-initiated coronal waves can trigger sympathetic stellar flares. It is suggested that grain formation occurs in starspots of dMe stars and that such grains in a circumstellar shell are responsible for the systematic IR excesses of dMe stars relative to dM stars.

  4. Magnetic Properties of Hydrothermalized A-type Red Granites

    NASA Astrophysics Data System (ADS)

    Trindade, R. I. F.; Nédélec, A.; Peschler, A.; Archanjo, C. J.; Poitrasson, F.; Bouchez, J. L.

    Hydrothermalized A-type granites are commonly identified by their pink to red-brick colour attributed to tiny flakes of hematite in the alkali feldspars. These inclusions can be of interest in magnetic studies, but their timing and process of formation are still unclear. Formation of chlorite after biotite is the commonest effect of hydrother- malization and may occur quite early after crystallization due to late-magmatic or externally-derived fluids. The reddish colour appears at a later stage. Five cases of A-type granites were investigated for their magnetic mineralogy and properties. The selected cases range from nearly unmodified granites (Panafrican stratoid granites of Madagascar) to strongly hydrothermalized ones (Meruoca, Brazil; Tana, Corsica); in- termediate cases are : Mount Scott (Oklahoma), Bushveld (granitic core kindly pro- vided by R.G. Cawthorn) and. Hydrothermal alteration is often associated to a de- crease of the magnetic susceptibility magnitude (K) and of the anisotropy degree (P). It also strongly affects the rockt's bulk coercivity parameters, since alteration changes the relative amounts of coarse-grained primary magnetite, fine-grained PSD to SD sec- ondary magnetite, and hematite. Correspondingly, most samples plot away from the magnetite trend in the Dayt's diagram, but the different groups identified after coer- civity parameters do not directly correlate with whole-rock colour. In addition, IRM- acquisition curves and thermal demagnetization of tri-axial IRM show that hematite occurs in almost all analysed samples despite their colour. Various hematite coercivity ranges are also evidenced. In fact, hematite can be formed either in feldspar crys- tals or after magnetite. Tiny hematite within feldspars can appear either by exsolu- tion process or, more likely, by precipitation from a fluid phase. For these reasons, hematite inclusions may carry a remanence acquired shortly after granite crystalliza- tion or, conversely, a recent

  5. Magnetic Fields in Population III Star Formation

    SciTech Connect

    Turk, Matthew J.; Oishi, Jeffrey S.; Abel, Tom; Bryan, Greg

    2012-02-22

    We study the buildup of magnetic fields during the formation of Population III star-forming regions, by conducting cosmological simulations from realistic initial conditions and varying the Jeans resolution. To investigate this in detail, we start simulations from identical initial conditions, mandating 16, 32 and 64 zones per Jeans length, and studied the variation in their magnetic field amplification. We find that, while compression results in some amplification, turbulent velocity fluctuations driven by the collapse can further amplify an initially weak seed field via dynamo action, provided there is sufficient numerical resolution to capture vortical motions (we find this requirement to be 64 zones per Jeans length, slightly larger than, but consistent with previous work run with more idealized collapse scenarios). We explore saturation of amplification of the magnetic field, which could potentially become dynamically important in subsequent, fully-resolved calculations. We have also identified a relatively surprising phenomena that is purely hydrodynamic: the higher-resolved simulations possess substantially different characteristics, including higher infall-velocity, increased temperatures inside 1000 AU, and decreased molecular hydrogen content in the innermost region. Furthermore, we find that disk formation is suppressed in higher-resolution calculations, at least at the times that we can follow the calculation. We discuss the effect this may have on the buildup of disks over the accretion history of the first clump to form as well as the potential for gravitational instabilities to develop and induce fragmentation.

  6. MAGNETIC FIELDS IN POPULATION III STAR FORMATION

    SciTech Connect

    Turk, Matthew J.; Bryan, Greg L.; Oishi, Jeffrey S.; Abel, Tom

    2012-02-01

    We study the buildup of magnetic fields during the formation of Population III star-forming regions by conducting cosmological simulations from realistic initial conditions and varying the Jeans resolution. To investigate this in detail, we start simulations from identical initial conditions, mandating 16, 32, and 64 zones per Jeans length, and study the variation in their magnetic field amplification. We find that, while compression results in some amplification, turbulent velocity fluctuations driven by the collapse can further amplify an initially weak seed field via dynamo action, provided there is sufficient numerical resolution to capture vortical motions (we find this requirement to be 64 zones per Jeans length, slightly larger than but consistent with previous work run with more idealized collapse scenarios). We explore saturation of amplification of the magnetic field, which could potentially become dynamically important in subsequent, fully resolved calculations. We have also identified a relatively surprising phenomenon that is purely hydrodynamic: the higher-resolved simulations possess substantially different characteristics, including higher infall velocity, increased temperatures inside 1000 AU, and decreased molecular hydrogen content in the innermost region. Furthermore, we find that disk formation is suppressed in higher-resolution calculations, at least at the times that we can follow the calculation. We discuss the effect this may have on the buildup of disks over the accretion history of the first clump to form as well as the potential for gravitational instabilities to develop and induce fragmentation.

  7. Circumstellar Gas-Disk Variability Around A-Type Stars: The Detection of Exocomets?

    NASA Astrophysics Data System (ADS)

    Welsh, Barry Y.; Montgomery, Sharon

    2013-07-01

    We present medium spectral resolution (R ∼ 60,000) observations of the CaII K-line (3,933 Å) absorption profiles observed toward 21 nearby A-type stars thought to possess circumstellar gas debris disks. The stars were repeatedly observed over two observing runs on the 2.1 m Otto Struve telescope at the McDonald Observatory, Texas in 2011 May and 2012 November. Nightly changes in the absorption strength of the CaII K-line near the stellar radial velocity were observed in four of the stars (HD 21620, HD 110411, HD 145964 and HD 183324). This type of absorption variability indicates the presence of a circumstellar gas disk around these stars. We also have detected weak absorption features that sporadically appear with velocities in the range ± 100 km s-1 of the main circumstellar K-line in the spectra of HD 21620, HD 42111, HD 110411 and HD 145964. Due to the known presence of both gas and dust disks surrounding these four stars, these transient absorption features are most probably associated with the presence of Falling Evaporated Bodies (FEBs, or exocomets) that are thought to liberate gas on their grazing trajectory toward and around the central star. This now brings the total number of A-type stars in which the evaporation of CaII gas from protoplanetary bodies (i.e., exocomets) has been observed to vary on a nightly basis to 10 systems. A statistical analysis of the 10 A-stars showing FEB-activity near the CaII K-line compared to 21 A-type stars that exhibit no measurable variability reveals that FEB-activity occurs in significantly younger stellar systems that also exhibit chemical peculiarities. The presence of FEB-activity does not appear to be associated with a strong mid-IR excess. This is probably linked to the disk inclination angle, since unless the viewing angle is favorable the detection of time-variable absorption may be unlikely. Additionally, if the systems are more evolved then the evaporation of gas due to FEB activity could have ceased

  8. Detection of Variable Gaseous Absorption Features in the Debris Disks Around Young A-type Stars

    NASA Astrophysics Data System (ADS)

    Montgomery, Sharon L.; Welsh, Barry Y.

    2012-10-01

    We present medium resolution (R = 60,000) absorption measurements of the interstellar Ca II K line observed towards five nearby A-type stars (49 Ceti, 5 Vul, ι Cyg, 2 And, and HD 223884) suspected of possessing circumstellar gas debris disks. The stars were observed on a nightly basis during a six night observing run on the 2.1-meter Otto Struve telescope at the McDonald Observatory, Texas. We have detected nightly changes in the absorption strength of the Ca II K line observed near the stellar radial velocity in three of the stars (49 Ceti, i Cyg and HD 223884). Such changes in absorption suggest the presence of a circumstellar (atomic) gas disk around these stars. In addition to the absorption changes in the main Ca II K line profile, we have also observed weak transient absorption features that randomly appear at redshifted velocities in the spectra of 49 Ceti, 5 Vul, and 2 And. These absorption features are most probably associated with the presence of falling evaporated bodies (exo-comets) that liberate evaporating gas on their approach to the central star. This now brings the total number of systems in which exocomet activity has been observed at Ca II or Na I wavelengths on a nightly basis to seven (β Pic, HR 10, HD 85905, β Car, 49 Ceti, 5 Vul, and 2 And), with 2 And exhibiting weaker and less frequent changes. All of the disk systems presently known to exhibit either type of short-term variability in Ca II K line absorption are rapidly rotating A-type stars (V sin i > 120 km s-1). Most exhibit mid-IR excesses, and many of them are very young (< 20 Myr), thus supporting the argument that many of them are transitional objects between Herbig Ae and “Vega-like” A-type stars with more tenuous circumstellar disks. No mid-IR excess (due to the presence of a dust disk) has yet been detected around either 2 And or HD 223884, both of which have been classified as λ Boötis-type stars. This may indicate that the observed changes in gas absorption for these

  9. IPHAS A-TYPE STARS WITH MID-INFRARED EXCESSES IN SPITZER SURVEYS

    SciTech Connect

    Hales, Antonio S.; Barlow, Michael J.; Drew, Janet E.; Unruh, Yvonne C.; Greimel, Robert; Irwin, Michael J.; Gonzalez-Solares, Eduardo E-mail: mjb@star.ucl.ac.uk E-mail: y.unruh@imperial.ac.uk

    2009-04-10

    We have identified 17 A-type stars in the Galactic Plane that have mid-infrared (mid-IR) excesses at 8 {mu}m. From observed colors in the (r' - H{alpha}) - (r' - i') plane, we first identified 23,050 early A-type main-sequence (MS) star candidates in the Isaac Newton Photometric H-Alpha Survey (IPHAS) point source database that are located in Spitzer Galactic Legacy Mid-Plane Survey Extraordinaire Galactic plane fields. Imposing the requirement that they be detected in all seven Two Micron All Sky Survey and Infrared Astronomical Satellite bands led to a sample of 2692 candidate A-type stars with fully sampled 0.6 to 8 {mu}m spectral energy distributions (SEDs). Optical classification spectra of 18 of the IPHAS candidate A-type MS stars showed that all but one could be well fitted using MS A-type templates, with the other being an A-type supergiant. Out of the 2692 A-type candidates 17 (0.6%) were found to have 8 {mu}m excesses above the expected photospheric values. Taking into account non-A-Type contamination estimates, the 8 {mu}m excess fraction is adjusted to {approx}0.7%. The distances to these sources range from 0.7 to 2.5 kpc. Only 10 out of the 17 excess stars had been covered by Spitzer MIPSGAL survey fields, of which five had detectable excesses at 24 {mu}m. For sources with excesses detected in at least two mid-IR wavelength bands, blackbody fits to the excess SEDs yielded temperatures ranging from 270 to 650 K, and bolometric luminosity ratios L {sub IR}/L {sub *} from 2.2 x 10{sup -3} - 1.9 x 10{sup -2}, with a mean value of 7.9 x 10{sup -3} (these bolometric luminosities are lower limits as cold dust is not detectable by this survey). Both the presence of mid-IR excesses and the derived bolometric luminosity ratios are consistent with many of these systems being in the planet-building transition phase between the early protoplanetary disk phase and the later debris disk phase.

  10. Observations of Strong Magnetic Fields in Nondegenerate Stars

    NASA Astrophysics Data System (ADS)

    Linsky, Jeffrey L.; Schöller, Markus

    2015-10-01

    We review magnetic-field measurements of nondegenerate stars across the Hertzprung-Russell diagram for main sequence, premain sequence, and postmain sequence stars. For stars with complex magnetic-field morphologies, which includes all G-M main sequence stars, the analysis of spectra obtained in polarized vs unpolarized light provides very different magnetic measurements because of the presence or absence of cancellation by oppositely directed magnetic fields within the instrument's spatial resolution. This cancellation can be severe, as indicated by the spatially averaged magnetic field of the Sun viewed as a star. These averaged fields are smaller by a factor of 1000 or more compared to spatially resolved magnetic-field strengths. We explain magnetic-field terms that characterize the fields obtained with different measurement techniques. Magnetic fields typically control the structure of stellar atmospheres in and above the photosphere, the heating rates of stellar chromospheres and coronae, mass and angular momentum loss through stellar winds, chemical peculiarity, and the emission of high energy photons, which is critically important for the evolution of protoplanetary disks and the habitability of exoplanets. Since these effects are governed by the star's magnetic energy, which is proportional to the magnetic-field strength squared and its fractional surface coverage, it is important to measure or reliably infer the true magnetic-field strength and filling factor across a stellar disk. We summarize magnetic-field measurements obtained with the different observing techniques for different types of stars and estimate the highest magnetic-field strengths. We also comment on the different field morphologies observed for stars across the H-R diagram, typically inferred from Zeeman-Doppler imaging and rotational modulation observations,

  11. Investigations of the Magnetic a and B Type Stars

    NASA Astrophysics Data System (ADS)

    Wade, Gregg Allan

    In an effort to explore further the mechanisms responsible for magnetism and chemical peculiarity in the early-type stars, I have constructed models of the magnetic fields of individual A and B type stars, studied the evolutionary dependence of magnetism in the Ap/Bp stars, undertaken the first systematic study of linear polarisation in the spectral lines of magnetic A and B type stars, and searched for magnetic fields in the photospheres of canonically non-magnetic upper-main sequence stars. I present detailed studies of 8 magnetic Ap, Bp and He-strong stars. For 7 of these objects (84 UMa, iota Cas, HD 115708, HD 184927, HD 200311, HD 81009, and HD 192678) new models of the surface magnetic field structure have been developed using longitudinal field, field modulus, and broadband linear polarisation measurements. These models represent a substantial contribution to the accumulated data regarding the surface magnetic fields of upper-main sequence stars. One object (HD 59435) is in fact a spectroscopic binary (SB2), the secondary component of which is a magnetic Ap star. A detailed study of the evolutionary state of the components and the mean magnetic field modulus variation of the secondary is presented. The results of this analysis are consistent with the suggestion by Hubrig & Mathys (1994) that magnetism (and perhaps chemical peculiarity) arise late in the evolution of A and B type stars. In order to test further this possibility, a more extensive study of the evolutionary states and magnetic fields of 10 magnetic Ap stars was undertaken. This study indicates that the magnetic stars are distributed across the entire width of the main sequence, a result inconsistent with the hypothesis of Hubrig & Mathys. I furthermore present investigations of the Zeeman circular and linear polarisation measured within stellar spectral lines. The MuSiCoS spectropolarimeter was used to conduct the first systematic, high signal-to-noise ratio observations of Zeeman linear

  12. Magnetic activity of F stars observed by Kepler

    NASA Astrophysics Data System (ADS)

    Mathur, S.; García, R. A.; Ballot, J.; Ceillier, T.; Salabert, D.; Metcalfe, T. S.; Régulo, C.; Jiménez, A.; Bloemen, S.

    2014-02-01

    Context. The study of stellar activity is important because it can provide new constraints for dynamo models when combined with surface rotation rates and the depth of the convection zone. We know that the dynamo mechanism, which is believed to be the main process that rules the magnetic cycle of solar-like stars, results from the interaction between (differential) rotation, convection, and magnetic field. The Kepler mission has already been collecting data for a large number of stars during four years allowing us to investigate magnetic stellar cycles. Aims: We investigated the Kepler light curves to look for magnetic activity or even hints of magnetic activity cycles. Based on the photometric data we also looked for new magnetic indexes to characterise the magnetic activity of the stars. Methods: We selected a sample of 22 solar-like F stars that have a rotation period shorter than 12 days. We performed a time-frequency analysis using the Morlet wavelet yielding a magnetic proxy for our sample of stars. We computed the magnetic index Sph as the standard deviation of the whole time series and the index ⟨ Sph ⟩, which is the mean of standard deviations measured in subseries of length five times the rotation period of the star. We defined new indicators, such as the contrast between high and low activity, to take into account the fact that complete magnetic cycles are not observed for all the stars. We also inferred the Rossby number of the stars and studied their stellar background. Results: This analysis shows different types of behaviour in the 22 F stars. Two stars show behaviour very similar to magnetic activity cycles. Five stars show long-lived spots or active regions suggesting the existence of active longitudes. Two stars in our sample seem to have a decreasing or increasing trend in the temporal variation of the magnetic proxies. Finally, the last group of stars shows magnetic activity (with the presence of spots) but no sign of cycle. Appendix A is

  13. On the anomaly of Balmer line profiles of A-type stars. Fundamental binary systems

    NASA Astrophysics Data System (ADS)

    Smalley, B.; Gardiner, R. B.; Kupka, F.; Bessell, M. S.

    2002-11-01

    In previous work, Gardiner et al. (\\cite{GKS99}) found evidence for a discrepancy between the Teff obtained from Balmer lines with that from photometry and fundamental values for A-type stars. An investigation into this anomaly is presented using Balmer line profiles of stars in binary system with fundamental values of both Teff and log g. A revision of the fundamental parameters for binary systems given by Smalley & Dworetsky (\\cite{SD95}) is also presented. The Teff obtained by fitting Hα and Hβ line profiles is compared to the fundamental values and those obtained from uvby photometry. We find that the discrepancy found by Gardiner et al. (\\cite{GKS99}) for stars in the range 7000 K <~ Teff <~ 9000 K is no longer evident. Partly based on DENIS data obtained at the European Southern Observatory.

  14. The origin of magnetic fields in hot stars

    NASA Astrophysics Data System (ADS)

    Neiner, Coralie; Mathis, Stéphane; Alecian, Evelyne; Emeriau, Constance; Grunhut, Jason; BinaMIcS; MiMeS Collaborations

    2015-10-01

    Observations of stable mainly dipolar magnetic fields at the surface of ~7% of single hot stars indicate that these fields are of fossil origin, i.e. they descend from the seed field in the molecular clouds from which the stars were formed. The recent results confirm this theory. First, theoretical work and numerical simulations confirm that the properties of the observed fields correspond to those expected from fossil fields. They also showed that rapid rotation does not modify the surface dipolar magnetic configurations, but hinders the stability of fossil fields. This explains the lack of correlation between the magnetic field properties and stellar properties in massive stars. It may also explain the lack of detections of magnetic fields in Be stars, which rotate close to their break-up velocity. In addition, observations by the BinaMIcS collaboration of hot stars in binary systems show that the fraction of those hosting detectable magnetic fields is much smaller than for single hot stars. This could be related to results obtained in simulations of massive star formation, which show that the stronger the magnetic field in the original molecular cloud, the more difficult it is to fragment massive cores to form several stars. Therefore, more and more arguments support the fossil field theory.

  15. Abundance analysis of two late A-type stars HD 32115 and HD 37594

    NASA Astrophysics Data System (ADS)

    Bikmaev, I. F.; Ryabchikova, T. A.; Bruntt, H.; Musaev, F. A.; Mashonkina, L. I.; Belyakova, E. V.; Shimansky, V. V.; Barklem, P. S.; Galazutdinov, G.

    2002-07-01

    We have performed abundance analysis of two slowly rotating, late A-type stars, HD 32115 (HR 1613) and HD 37594 (HR 1940), based on obtained echelle spectra covering the spectral range 4000-9850 Å. These spectra allowed us to identify an extensive line list for 31 chemical elements, the most complete to date for A-type stars. Two approaches to abundance analysis were used, namely a ``manual'' (interactive) and a semi-automatic procedure for comparison of synthetic and observed spectra and equivalent widths. For some elements non-LTE (NLTE) calculations were carried out and the corresponding corrections have been applied. The abundance pattern of HD 32115 was found to be very close to the solar abundance pattern, and thus may be used as an abundance standard for chemical composition studies in middle and late A stars. Further, its Hα line profile shows no core-to-wing anomaly like that found for cool Ap stars and therefore also may be used as a standard in comparative studies of the atmospheric structures of cool, slowly rotating Ap stars. HD 37594 shows a metal deficiency at the level of -0.3 dex for most elements and triangle-like cores of spectral lines. This star most probably belongs to the delta Sct group. Based on observations obtained at the 2-m telescope of Peak Terskol Observatory near Elbrus mountain, International Center of Astronomical and Medical-Ecological Researches, Russia. Table 4 is only available in electronic form at http://www.edpsciences.org

  16. GMC Collisions as Triggers of Star Formation. III. Density and Magnetically Regulated Star Formation

    NASA Astrophysics Data System (ADS)

    Wu, Benjamin; Tan, Jonathan C.; Christie, Duncan; Nakamura, Fumitaka; Van Loo, Sven; Collins, David

    2017-06-01

    We study giant molecular cloud (GMC) collisions and their ability to trigger star cluster formation. We further develop our three-dimensional magnetized, turbulent, colliding GMC simulations by implementing star formation subgrid models. Two such models are explored: (1) “Density-Regulated,” i.e., fixed efficiency per free-fall time above a set density threshold and (2) “Magnetically Regulated,” i.e., fixed efficiency per free-fall time in regions that are magnetically supercritical. Variations of parameters associated with these models are also explored. In the non-colliding simulations, the overall level of star formation is sensitive to model parameter choices that relate to effective density thresholds. In the GMC collision simulations, the final star formation rates and efficiencies are relatively independent of these parameters. Between the non-colliding and colliding cases, we compare the morphologies of the resulting star clusters, properties of star-forming gas, time evolution of the star formation rate (SFR), spatial clustering of the stars, and resulting kinematics of the stars in comparison to the natal gas. We find that typical collisions, by creating larger amounts of dense gas, trigger earlier and enhanced star formation, resulting in 10 times higher SFRs and efficiencies. The star clusters formed from GMC collisions show greater spatial substructure and more disturbed kinematics.

  17. The Onset of Chromospheres in A-Type Stars - the ALTAIR Affair

    NASA Astrophysics Data System (ADS)

    Catalano, S.; Gouttebroze, P.; Marilli, E.; Freire Ferrero, R.

    Here we present preliminary results on the study of the chromosphere of Altair from L high dispersion profiles. We report also the detection of L chromospheric emission from Cep (A7 V) and TrA (F0 V) with the ME. We show that chromosphere may exist up to B - V = 0.22. The possible heating mechanism, magnetic or nonmagnetic, for the chromosphere of these stars is also discussed.

  18. A high speed photometric survey of normal and peculiar A-type stars

    NASA Astrophysics Data System (ADS)

    Kurtanidze, Omar; Nikolashvili, Maria G.

    The rapidly-oscillating Ap stars represent the only main-sequence stars, despite the Sun, which pulsate in high-overtone low degree p-moges with their axis aligned with oblique ones. We have undertook a long-term programme of high-speed photometric observations about two hundred Normal and Peculiar A-type stars with 125cm RC telescope equipped by Two-Star Photometer. It enables to chop as frequently as need between objects, sky and dark due to effects of sky transparency and background variations are remouved and scintillation noise spectra are obtained. An attempt was made to include representative number of each spectral subtypes. The selected objects lie in the range of 8-10 magnitudes in the Jonhson wide-band B filter. At first stage it is planned to carry out a pilot survey with duration of 6-8 hours divided between two observing sessions. The objects with noticable oscillations will be studied photometrically as well as spectroscopically in detail. During the pleliminary observations one rapidly-oscillating Ap star HD231427 was revealed which should be considered as tentative.

  19. A High Speed Photometric Survey of Normal and Peculiar A-Type Stars

    NASA Astrophysics Data System (ADS)

    Kurtanidze, O. M.; Nikolashvili, M. G.

    The rapidly-oscillating Ap stars represent the only main-sequence stars, despite the Sun, which pulsate in high-overtone low degree p-moges with their axis aligned with oblique ones. We have undertook a long-term programme of high-speed photometric observations about two hundred Normal and Peculiar A-type stars with 125cm RC telescope equipped by Two-Star Photometer. It enables to chop as frequently as need between objects, sky and dark due to effects of sky transparency and background variations are remouved and scintillation noise spectra are obtained. An attempt was made to include representative number of each spectral subtypes. The selected objects lie in the range of 8-10 magnitudes in the Jonhson wide-band B filter. At first stage it is planned to carry out a pilot survey with duration of 6-8 hours divided between two observing sessions. The objects with noticable oscillations will be studied photometrically as well as spectroscopically in detail. During the preliminary observations one rapidly-oscillating Ap star HD231427 was revealed which should be considered as tentative.

  20. A High Speed Photometry Survey of Normal and Peculiar A-Type Stars

    NASA Astrophysics Data System (ADS)

    Kurtanidze, O. M.; Nikolashvili, M. G.

    The rapidly-oscillating Ap stars represent the only main-sequence stars, other than the Sun, which pulsate in high-overtone, low-degree, oblique-axis p-modes. We have undertaken a long-term programme of high-speed photometric observations of about two hundred normal and peculiar A-type stars with the 125 cm RC telescope equipped with a Two-Star Photometer. It enables chopping as frequently as needed between object, sky, and dark channels, so that effects of sky transparency and background variations are removed and scintillation noise spectra are obtained. An attempt was made to include a representative number of each spectral subtype. The selected objects lie in the magnitude range of 8-10 in Jonhson's B wide-band filter. For the first stage it is planned to carry out a pilot survey with a duration of 6-8 hours distributed over two observing runs. The objects with noticeable oscillations will be studied in detai photometrically as well as spectroscopically. During the preliminary observations one rapidly-oscillating Ap star, HDE 231427, was revealed which should be considered as tentative.

  1. An asymptotic-giant-branch star in the progenitor system of a type Ia supernova.

    PubMed

    Hamuy, Mario; Phillips, M M; Suntzeff, Nicholas B; Maza, José; González, L E; Roth, Miguel; Krisciunas, Kevin; Morrell, Nidia; Green, E M; Persson, S E; McCarthy, P J

    2003-08-07

    Stars that explode as supernovae come in two main classes. A type Ia supernova is recognized by the absence of hydrogen and the presence of elements such as silicon and sulphur in its spectrum; this class of supernova is thought to produce the majority of iron-peak elements in the Universe. They are also used as precise 'standard candles' to measure the distances to galaxies. While there is general agreement that a type Ia supernova is produced by an exploding white dwarf star, no progenitor system has ever been directly observed. Significant effort has gone into searching for circumstellar material to help discriminate between the possible kinds of progenitor systems, but no such material has hitherto been found associated with a type Ia supernova. Here we report the presence of strong hydrogen emission associated with the type Ia supernova SN2002ic, indicating the presence of large amounts of circumstellar material. We infer from this that the progenitor system contained a massive asymptotic-giant-branch star that lost several solar masses of hydrogen-rich gas before the supernova explosion.

  2. Analytical solutions of stellar winds in B-A type supergiants stars

    NASA Astrophysics Data System (ADS)

    Araya, Ignacio; Cure, Michel

    2013-06-01

    An analytical solution for the δ-slow hydrodynamic solution (Cure et al. 2011) in B-A type supergiants stars is developed. The methodology is based on the analytical solutions of a) Villata (1992), which is described in terms of the stellar and wind parameters and b) Muller & Vink (2008), which is described in terms of fitting parameters from a numerical solution (hydrodynamic). These methodologies only apply for fast solutions, for that reason the line acceleration term (gL) of Muller & Vink method is modified in order to obtain an analytical solution for the δ-slow solution. To find a relationship between the parameters from the fit and the stellar and wind parameters, a computational grid, based on the grid of stellar models from Ekstrom et al. (2012), is created for B-A type supergiants stars with δ-slow hydrodynamic solution. Finally, an analytical solution for B-A type supergiants stars is obtained based on the Lambert W function (Corless et al. 1996). Comparing with the numerical solutions, the terminal velocity has a median relative error below 4% and the mass loss rate has a median relative error below 5%. In addition, we calculated the wind-momentum luminosity relationship (WLR) with the models from the computational grid and compared with the observations, showing a very good agreement.

  3. Superfluid heat conduction and the cooling of magnetized neutron stars.

    PubMed

    Aguilera, Deborah N; Cirigliano, Vincenzo; Pons, José A; Reddy, Sanjay; Sharma, Rishi

    2009-03-06

    We report on a new mechanism for heat conduction in the neutron star crust. We find that collective modes of superfluid neutron matter, called superfluid phonons, can influence heat conduction in magnetized neutron stars. They can dominate the heat conduction transverse to the magnetic field when the magnetic field B> approximately 10(13) G. At a density of rho approximately 10(12)-10(14) g/cm3, the conductivity due to superfluid phonons is significantly larger than that due to lattice phonons and is comparable to electron conductivity when the temperature approximately 10(8) K. This new mode of heat conduction can limit the surface anisotropy in highly magnetized neutron stars. Cooling curves of magnetized neutron stars with and without superfluid heat conduction could show observationally discernible differences.

  4. Superfluid Heat Conduction and the Cooling of Magnetized Neutron Stars

    SciTech Connect

    Aguilera, Deborah N.; Cirigliano, Vincenzo; Reddy, Sanjay; Sharma, Rishi; Pons, Jose A.

    2009-03-06

    We report on a new mechanism for heat conduction in the neutron star crust. We find that collective modes of superfluid neutron matter, called superfluid phonons, can influence heat conduction in magnetized neutron stars. They can dominate the heat conduction transverse to the magnetic field when the magnetic field B > or approx. 10{sup 13} G. At a density of {rho}{approx_equal}10{sup 12}-10{sup 14} g/cm{sup 3}, the conductivity due to superfluid phonons is significantly larger than that due to lattice phonons and is comparable to electron conductivity when the temperature {approx_equal}10{sup 8} K. This new mode of heat conduction can limit the surface anisotropy in highly magnetized neutron stars. Cooling curves of magnetized neutron stars with and without superfluid heat conduction could show observationally discernible differences.

  5. Superfluid heat conduction and the cooling of magnetized neutron stars

    SciTech Connect

    Cirigliano, Vincenzo; Reddy, Sanjay; Sharma, Rishi; Aguilera, Deborah N

    2008-01-01

    We report on a new mechanism for heat conduction in the neutron star crust. We find that collective modes of superftuid neutron matter, called superfiuid phonons (sPhs), can influence heat conduction in magnetized neutron stars. They can dominate the heat conduction transverse to magnetic field when the magnetic field B {approx}> 10{sup 13} C. At density p {approx_equal} 10{sup 12}--10{sup 14} g/cm{sup 3} the conductivity due to sPhs is significantly larger than that due to lattice phonons and is comparable to electron conductivity at when temperature {approx_equal} 10{sup 8} K. This new mode of heat conduction can limit the surface anisotropy in highly magnetized neutron stars. Cooling curves of magnetized neutron stars with and without superfluid heat conduction show observationally discernible differences.

  6. Simulations of Magnetic Fields in Tidally Disrupted Stars

    NASA Astrophysics Data System (ADS)

    Guillochon, James; McCourt, Michael

    2017-01-01

    We perform the first magnetohydrodynamical simulations of tidal disruptions of stars by supermassive black holes. We consider stars with both tangled and ordered magnetic fields, for both grazing and deeply disruptive encounters. When the star survives disruption, we find its magnetic field amplifies by a factor of up to 20, but see no evidence for a self-sustaining dynamo that would yield arbitrary field growth. For stars that do not survive, and within the tidal debris streams produced in partial disruptions, we find that the component of the magnetic field parallel to the direction of stretching along the debris stream only decreases slightly with time, eventually resulting in a stream where the magnetic pressure is in equipartition with the gas. Our results suggest that the returning gas in most (if not all) stellar tidal disruptions is already highly magnetized by the time it returns to the black hole.

  7. Magnetic fields in non-convective regions of stars.

    PubMed

    Braithwaite, Jonathan; Spruit, Henk C

    2017-02-01

    We review the current state of knowledge of magnetic fields inside stars, concentrating on recent developments concerning magnetic fields in stably stratified (zones of) stars, leaving out convective dynamo theories and observations of convective envelopes. We include the observational properties of A, B and O-type main-sequence stars, which have radiative envelopes, and the fossil field model which is normally invoked to explain the strong fields sometimes seen in these stars. Observations seem to show that Ap-type stable fields are excluded in stars with convective envelopes. Most stars contain both radiative and convective zones, and there are potentially important effects arising from the interaction of magnetic fields at the boundaries between them; the solar cycle being one of the better known examples. Related to this, we discuss whether the Sun could harbour a magnetic field in its core. Recent developments regarding the various convective and radiative layers near the surfaces of early-type stars and their observational effects are examined. We look at possible dynamo mechanisms that run on differential rotation rather than convection. Finally, we turn to neutron stars with a discussion of the possible origins for their magnetic fields.

  8. Magnetic fields in non-convective regions of stars

    PubMed Central

    Braithwaite, Jonathan

    2017-01-01

    We review the current state of knowledge of magnetic fields inside stars, concentrating on recent developments concerning magnetic fields in stably stratified (zones of) stars, leaving out convective dynamo theories and observations of convective envelopes. We include the observational properties of A, B and O-type main-sequence stars, which have radiative envelopes, and the fossil field model which is normally invoked to explain the strong fields sometimes seen in these stars. Observations seem to show that Ap-type stable fields are excluded in stars with convective envelopes. Most stars contain both radiative and convective zones, and there are potentially important effects arising from the interaction of magnetic fields at the boundaries between them; the solar cycle being one of the better known examples. Related to this, we discuss whether the Sun could harbour a magnetic field in its core. Recent developments regarding the various convective and radiative layers near the surfaces of early-type stars and their observational effects are examined. We look at possible dynamo mechanisms that run on differential rotation rather than convection. Finally, we turn to neutron stars with a discussion of the possible origins for their magnetic fields. PMID:28386410

  9. Magnetic fields in non-convective regions of stars

    NASA Astrophysics Data System (ADS)

    Braithwaite, Jonathan; Spruit, Henk C.

    2017-02-01

    We review the current state of knowledge of magnetic fields inside stars, concentrating on recent developments concerning magnetic fields in stably stratified (zones of) stars, leaving out convective dynamo theories and observations of convective envelopes. We include the observational properties of A, B and O-type main-sequence stars, which have radiative envelopes, and the fossil field model which is normally invoked to explain the strong fields sometimes seen in these stars. Observations seem to show that Ap-type stable fields are excluded in stars with convective envelopes. Most stars contain both radiative and convective zones, and there are potentially important effects arising from the interaction of magnetic fields at the boundaries between them; the solar cycle being one of the better known examples. Related to this, we discuss whether the Sun could harbour a magnetic field in its core. Recent developments regarding the various convective and radiative layers near the surfaces of early-type stars and their observational effects are examined. We look at possible dynamo mechanisms that run on differential rotation rather than convection. Finally, we turn to neutron stars with a discussion of the possible origins for their magnetic fields.

  10. Sun-like Stars: magnetic fields, cycles and exoplanets

    NASA Astrophysics Data System (ADS)

    Fares, Rim

    2016-10-01

    In Sun-like stars, magnetic fields are generated in the outer convective layers. They shape the stellar environment, from the photosphere to planetary orbits. Studying the large-scale magnetic field of those stars enlightens our understanding of the field properties and gives us observational constraints for field generation dynamo models. It also sheds light on how ``normal'' the Sun is among Sun-like stars. In this contribution, I will review the field properties of Sun-like stars, focusing on solar twins and planet hosting stars. I will discuss the observed large-scale magnetic cycles, compare them to stellar activity cycles, and link that to what we know about the Sun. I will also discuss the effect of large-scale stellar fields on exoplanets, exoplanetary emissions (e.g. radio), and habitability.

  11. Neutron star dynamos and the origins of pulsar magnetism

    NASA Technical Reports Server (NTRS)

    Thompson, Christopher; Duncan, Robert C.

    1993-01-01

    Neutron star convection is a transient phenomenon and has an extremely high magnetic Reynolds number. In this sense, a neutron star dynamo is the quintessential fast dynamo. The convective motions are only mildly turbulent on scales larger than the approximately 100 cm neutrino mean free path, but the turbulence is well developed on smaller scales. Several fundamental issues in the theory of fast dynamos are raised in the study of a neutron star dynamo, in particular the possibility of dynamo action in mirror-symmetric turbulence. It is argued that in any high magnetic Reynolds number dynamo, most of the magnetic energy becomes concentrated in thin flux ropes when the field pressure exceeds the turbulent pressure at the smallest scale of turbulence. In addition, the possibilities for dynamo action during the various (pre-collapse) stages of convective motion that occur in the evolution of a massive star are examined, and the properties of white dwarf and neutron star progenitors are contrasted.

  12. Neutron star dynamos and the origins of pulsar magnetism

    NASA Technical Reports Server (NTRS)

    Thompson, Christopher; Duncan, Robert C.

    1993-01-01

    Neutron star convection is a transient phenomenon and has an extremely high magnetic Reynolds number. In this sense, a neutron star dynamo is the quintessential fast dynamo. The convective motions are only mildly turbulent on scales larger than the approximately 100 cm neutrino mean free path, but the turbulence is well developed on smaller scales. Several fundamental issues in the theory of fast dynamos are raised in the study of a neutron star dynamo, in particular the possibility of dynamo action in mirror-symmetric turbulence. It is argued that in any high magnetic Reynolds number dynamo, most of the magnetic energy becomes concentrated in thin flux ropes when the field pressure exceeds the turbulent pressure at the smallest scale of turbulence. In addition, the possibilities for dynamo action during the various (pre-collapse) stages of convective motion that occur in the evolution of a massive star are examined, and the properties of white dwarf and neutron star progenitors are contrasted.

  13. New bright optical spectrophotometric standards: A-type stars from the STIS Next Generation Spectral Library

    NASA Astrophysics Data System (ADS)

    Allende Prieto, C.; del Burgo, C.

    2016-02-01

    Exoplanets have sparked interest in extremely high signal-to-noise ratio spectroscopic observations of very bright stars, in a regime where flux calibrators, in particular DA white dwarfs, are not available. We argue that A-type stars offer a useful alternative and reliable space-based spectrophotometry is now available for a number of bright ones in the range 3 < V < 8 mag. By means of comparing observed spectrophotometry and model fluxes, we identify 18 new very bright trustworthy A-type flux standards for the optical range (400-800 nm), and provide scaled model fluxes for them. Our tests suggest that the absolute fluxes for these stars in the optical are reliable to within 3 per cent. We limit the spectral range to 400-800 nm, since our models have difficulties to reproduce the observed fluxes in the near-infrared and, especially, in the near-UV, where the discrepancies rise up to ˜10 per cent. Based on our model fits, we derive angular diameters with an estimated accuracy of about 1 per cent.

  14. Can neutron stars have auroras ? : electromagnetic coupling process between neutron star and magnetized accretion disk

    NASA Astrophysics Data System (ADS)

    Kimura, T.; Iwakiri, W. B.; Enoto, T.; Wada, T.; Tao, C.

    2015-12-01

    In the binary neutron star system, angular momentum transfer from accretion disk to a star is essential process for spin-up/down of stars. The angular momentum transfer has been well formulated for the accretion disk strongly magnetized by the neutron star [e.g., Ghosh and Lamb, 1978, 1979a, b]. However, the electromagnetic (EM) coupling between the neutron star and accretion disk has not been self-consistently solved in the previous studies although the magnetic field lines from the star are strongly tied with the accretion disk. In this study, we applied the planet-magnetosphere coupling process established for Jupiter [Hill, 1979] to the binary neutron star system. Angular momentum distribution is solved based on the torque balance between the neutron star's surface and accretion disk coupled by the magnetic field tensions. We found the EM coupling can transfer significantly larger fraction of the angular momentum from the magnetized accretion disk to the star than the unmagnetized case. The resultant spin-up rate is estimated to ~10^-14 [sec/sec] for the nominal binary system parameters, which is comparable with or larger than the other common spin-down/up processes: e.g., the magnetic dipole radiation spin-down. The Joule heating energy dissipated in the EM coupling is estimated to be up to ~10^36 [erg/sec] for the nominal binary system parameters. The release is comparable to that of gravitation energy directly caused by the matters accreting onto the neutron star. This suggests the EM coupling at the neutron star can accompany the observable radiation as auroras with a similar manner to those at the rotating planetary magnetospheres like Jupiter, Saturn, and other gas giants.

  15. VizieR Online Data Catalog: Horizontal-branch and A-type star catalog. II (Beers+ 1996)

    NASA Astrophysics Data System (ADS)

    Beers, T. C.; Wilhelm, R.; Doinidis, S. P.; Mattson, C. J.

    1996-09-01

    We present coordinates and brightness estimates for 4175 candidate field horizontal-branch and A-type stars, in the magnitude range 10<=B<=15.5, selected using an objective-prism/interference-filter survey technique. The candidates lie primarily in the northern Galactic hemisphere and complement a previously published sample of southern Galactic hemisphere candidates. Available spectroscopy and photometry indicates that the great majority of the candidates are likely to be bona fide members of either the field blue horizontal- branch population or the blue, metal-deficient, high surface gravity stars referred to by Preston, Beers, & Schectman (1994AJ....108..538P) as BMP stars. The remaining stars in the catalog are likely to be a mix of metal-deficient turnoff stars, metallic-line (Am) stars, field red horizontal-branch stars, optical doubles with overlapping objective-prism spectra, and (particularly among the fainter candidates) inadvertently included late-type stars. (1 data file).

  16. Strong dipole magnetic fields in fast rotating fully convective stars

    NASA Astrophysics Data System (ADS)

    Shulyak, D.; Reiners, A.; Engeln, A.; Malo, L.; Yadav, R.; Morin, J.; Kochukhov, O.

    2017-08-01

    M dwarfs are the most numerous stars in our Galaxy, with masses between approximately 0.5 and 0.1 solar masses. Many of them show surface activity qualitatively similar to our Sun and generate flares, high X-ray fluxes and large-scale magnetic fields1,2,3,4. Such activity is driven by a dynamo powered by the convective motions in their interiors2,5,6,7,8. Understanding properties of stellar magnetic fields in these stars finds a broad application in astrophysics, including theory of stellar dynamos and environment conditions around planets that may be orbiting these stars. Most stars with convective envelopes follow a rotation-activity relationship where various activity indicators saturate in stars with rotation periods shorter than a few days2,6,8. The activity gradually declines with rotation rate in stars rotating more slowly. It is thought that, due to a tight empirical correlation between X-ray radiance and magnetic flux9, the stellar magnetic fields will also saturate, to values around 4 kG (ref. 10). Here we report the detection of magnetic fields above the presumed saturation limit in four fully convective M dwarfs. By combining results from spectroscopic and polarimetric studies, we explain our findings in terms of bistable dynamo models11,12: stars with the strongest magnetic fields are those in a dipole dynamo state, whereas stars in a multipole state cannot generate fields stronger than about 4 kG. Our study provides observational evidence that the dynamo in fully convective M dwarfs generates magnetic fields that can differ not only in the geometry of their large-scale component, but also in the total magnetic energy.

  17. Star Catalogs on Punched Cards and Magnetic Tape

    NASA Technical Reports Server (NTRS)

    Berbert, J. H.

    1961-01-01

    In connection with the calibration of the Minitrack satellite tracking stations, the Goddard Space Flight Center has had the contents of a number of star catalogs put on punched cards and magnetic tape. This report discusses the plate data reduction procedures, briefly describes the information on the punched cards and magnetic tape, and calls attention to other applications of the card and tape star catalogs. The Goddard Space Flight Center has offered to prepare duplicate catalogs for qualified organizations.

  18. The MiMeS survey of Magnetism in Massive Stars: magnetic analysis of the O-type stars

    NASA Astrophysics Data System (ADS)

    Grunhut, J. H.; Wade, G. A.; Neiner, C.; Oksala, M. E.; Petit, V.; Alecian, E.; Bohlender, D. A.; Bouret, J.-C.; Henrichs, H. F.; Hussain, G. A. J.; Kochukhov, O.; MiMeS Collaboration

    2017-02-01

    We present the analysis performed on spectropolarimetric data of 97 O-type targets included in the framework of the Magnetism in Massive Stars (MiMeS) Survey. Mean least-squares deconvolved Stokes I and V line profiles were extracted for each observation, from which we measured the radial velocity, rotational and non-rotational broadening velocities, and longitudinal magnetic field Bℓ. The investigation of the Stokes I profiles led to the discovery of two new multiline spectroscopic systems (HD 46106, HD 204827) and confirmed the presence of a suspected companion in HD 37041. We present a modified strategy of the least-squares deconvolution technique aimed at optimizing the detection of magnetic signatures while minimizing the detection of spurious signatures in Stokes V. Using this analysis, we confirm the detection of a magnetic field in six targets previously reported as magnetic by the MiMeS collaboration (HD 108, HD 47129A2, HD 57682, HD 148937, CPD-28 2561, and NGC 1624-2), as well as report the presence of signal in Stokes V in three new magnetic candidates (HD 36486, HD 162978, and HD 199579). Overall, we find a magnetic incidence rate of 7 ± 3 per cent, for 108 individual O stars (including all O-type components part of multiline systems), with a median uncertainty of the Bℓ measurements of about 50 G. An inspection of the data reveals no obvious biases affecting the incidence rate or the preference for detecting magnetic signatures in the magnetic stars. Similar to A- and B-type stars, we find no link between the stars' physical properties (e.g. Teff, mass, and age) and the presence of a magnetic field. However, the Of?p stars represent a distinct class of magnetic O-type stars.

  19. Spectroscopy of Hot Stars in the Galactic Halo. III. Analysis of a Large Sample of Field Horizontal-Branch and Other A-Type Stars

    NASA Astrophysics Data System (ADS)

    Wilhelm, Ronald; Beers, Timothy C.; Sommer-Larsen, Jesper; Pier, Jeffrey R.; Layden, Andrew C.; Flynn, Chris; Rossi, Silvia; Christensen, Per Rex

    1999-05-01

    We present results from an analysis of medium-resolution spectroscopy and UBV photometry for a sample of 1121 A-type stars in the halo (and disk) of the Galaxy. A previously developed calibration technique is used to assign estimates of effective temperature, surface gravity, and stellar metal abundance, as parameterized by [Fe/H]. Radial velocities are reported with an accuracy of ~10 km s^-1. Distance estimates are obtained for the stars with well-determined luminosity classes. Note that although we refer to ``A-type'' stars, which dominate the present sample, the present data set includes roughly 100 stars of later spectral types, as a result of the temperature range we have chosen to explore in this paper (6000 K<=T_eff<=10,000 K). Included in the hot star sample are 444 stars we classify as field horizontal-branch stars, 416 we classify as main-sequence-gravity A-type (or slightly later) stars (including stars that are likely members of the blue metal-poor population, the so-called BMPs), 140 stars we classify as likely metallic-line (Am) or peculiar (Ap) stars, and 121 stars that cannot be unambiguously classified based on the present data. Examination of the distributions in metallicity and velocity indicates that the field horizontal-branch and main-sequence A-type samples are quite distinct; hence we expect only a modest amount of cross-contamination between the subsamples. We identify 58 RR Lyrae candidates among the hot star sample, based on incompatibilities in their photometric and spectroscopic data. There are 19 stars in the sample that have been previously classified as RR Lyrae variables, and one additional star that had been previously suggested as a variable, though not necessarily of the RR Lyrae class. There are 115 stars in the sample that were previously classified as BMPs by Preston, Beers, & Shectman, most of which fall into the main-sequence A-type category, but 10 of which are found among the Am/Ap classifications. Furthermore, 53 of

  20. Discovery of starspots on Vega. First spectroscopic detection of surface structures on a normal A-type star

    NASA Astrophysics Data System (ADS)

    Böhm, T.; Holschneider, M.; Lignières, F.; Petit, P.; Rainer, M.; Paletou, F.; Wade, G.; Alecian, E.; Carfantan, H.; Blazère, A.; Mirouh, G. M.

    2015-05-01

    Context. The theoretically studied impact of rapid rotation on stellar evolution needs to be compared with these results of high-resolution spectroscopy-velocimetry observations. Early-type stars present a perfect laboratory for these studies. The prototype A0 star Vega has been extensively monitored in recent years in spectropolarimetry. A weak surface magnetic field was detected, implying that there might be a (still undetected) structured surface. First indications of the presence of small amplitude stellar radial velocity variations have been reported recently, but the confirmation and in-depth study with the highly stabilized spectrograph SOPHIE/OHP was required. Aims: The goal of this article is to present a thorough analysis of the line profile variations and associated estimators in the early-type standard star Vega (A0) in order to reveal potential activity tracers, exoplanet companions, and stellar oscillations. Methods: Vega was monitored in quasi-continuous high-resolution echelle spectroscopy with the highly stabilized velocimeter SOPHIE/OHP. A total of 2588 high signal-to-noise spectra was obtained during 34.7 h on five nights (2 to 6 of August 2012) in high-resolution mode at R = 75 000 and covering the visible domain from 3895-6270 Å. For each reduced spectrum, least square deconvolved equivalent photospheric profiles were calculated with a Teff = 9500 and log g = 4.0 spectral line mask. Several methods were applied to study the dynamic behaviour of the profile variations (evolution of radial velocity, bisectors, vspan, 2D profiles, amongst others). Results: We present the discovery of a spotted stellar surface on an A-type standard star (Vega) with very faint spot amplitudes ΔF/Fc ~ 5 × 10-4. A rotational modulation of spectral lines with a period of rotation P = 0.68 d has clearly been exhibited, unambiguously confirming the results of previous spectropolarimetric studies. Most of these brightness inhomogeneities seem to be located in lower

  1. Spontaneous magnetization of solid quark-cluster stars

    NASA Astrophysics Data System (ADS)

    Lai, Xiao-Yu; Xu, Ren-Xin

    2016-09-01

    Pulsar-like compact stars usually have strong magnetic fields, with strengths from ˜ 108 to ˜ 1012 G on the surface. How such strong magnetic fields can be generated and maintained is still an unsolved problem, which is, in principle, related to the interior structure of compact stars, i.e., the equation of state of cold matter at supra-nuclear density. In this paper we are trying to solve the problem in the regime of solid quark-cluster stars. Inside quark-cluster stars, the extremely low ratio of number density of electrons to that of baryons ne/nb and the screening effect from quark-clusters could reduce the long-range Coulomb interaction between electrons to short-range interaction. In this case, Stoner’s model could apply, and we find that the condition for ferromagnetism is consistent with that for the validity of Stoner’s model. Under the screened Coulomb repulsion, the electrons inside the stars could be spontaneously magnetized and become ferromagnetic, and hence would contribute non-zero net magnetic momentum to the whole star. We conclude that, for most cases in solid quark-cluster stars, the amount of net magnetic momentum, which is proportional to the amount of unbalanced spins ξ = (n+ - n-)/ne and depends on the number density of electrons ne = n+ + n-, could be significant with non-zero ξ. The net magnetic moments of electron system in solid quark-cluster stars could be large enough to induce the observed magnetic fields for pulsars with B ˜ 1011 to ˜ 1013 G. Supported by 973 Program (2012CB821801), West Light Foundation (XBBS-2014-23), National Natural Science Foundation of China (11203018, 11225314, 11365022), Science Project of Universities in Xinjiang (XJEDU2012S02) and Doctoral Science Foundation of Xinjiang University (BS120107)

  2. The magnetic fields of forming solar-like stars

    NASA Astrophysics Data System (ADS)

    Gregory, S. G.; Jardine, M.; Gray, C. G.; Donati, J.-F.

    2010-12-01

    Magnetic fields play a crucial role at all stages of the formation of low-mass stars and planetary systems. In the final stages, in particular, they control the kinematics of in-falling gas from circumstellar discs, and the launching and collimation of spectacular outflows. The magnetic coupling with the disc is thought to influence the rotational evolution of the star, while magnetized stellar winds control the braking of more evolved stars and may influence the migration of planets. Magnetic reconnection events trigger energetic flares which irradiate circumstellar discs with high energy particles that influence the disc chemistry and set the initial conditions for planet formation. However, it is only in the past few years that the current generation of optical spectropolarimeters has allowed the magnetic fields of forming solar-like stars to be probed in unprecedented detail. In order to do justice to the recent extensive observational programs new theoretical models are being developed that incorporate magnetic fields with an observed degree of complexity. In this review we draw together disparate results from the classical electromagnetism, molecular physics/chemistry and the geophysics literature, and demonstrate how they can be adapted to construct models of the large scale magnetospheres of stars and planets. We conclude by examining how the incorporation of multipolar magnetic fields into new theoretical models will drive future progress in the field through the elucidation of several observational conundrums.

  3. X-ray emission from massive stars with magnetic fields

    NASA Astrophysics Data System (ADS)

    Oskinova, L. M.; Hamann, W.-R.; Cassinelli, J. P.; Brown, J. C.; Todt, H.

    2011-12-01

    We investigate the connections between the magnetic fields and the X-ray emission from massive stars. Our study shows that the X-ray properties of known strongly magnetic stars are diverse: while some comply to the predictions of the magnetically confined wind model, others do not. We conclude that strong, hard, and variable X-ray emission may be a sufficient attribute of magnetic massive stars, but it is not a necessary one. We address the general properties of X-ray emission from ``normal'' massive stars, especially the long standing mystery about the correlations between the parameters of X-ray emission and fundamental stellar properties. The recent development in stellar structure modeling shows that small-scale surface magnetic fields may be common. We suggest a ``hybrid'' scenario which could explain the X-ray emission from massive stars by a combination of magnetic mechanisms on the surface and shocks in the stellar wind. The magnetic mechanisms and the wind shocks are triggered by convective motions in sub-photospheric layers. This scenario opens the door for a natural explanation of the well established correlation between bolometric and X-ray luminosities. Based on observations obtained with \\xmm and \\cxo.

  4. Exploring the origin of magnetic fields in massive stars. II. New magnetic field measurements in cluster and field stars

    NASA Astrophysics Data System (ADS)

    Hubrig, S.; Schöller, M.; Ilyin, I.; Kharchenko, N. V.; Oskinova, L. M.; Langer, N.; González, J. F.; Kholtygin, A. F.; Briquet, M.; Magori Collaboration

    2013-03-01

    Context. Theories on the origin of magnetic fields in massive stars remain poorly developed, because the properties of their magnetic field as function of stellar parameters could not yet be investigated. Additional observations are of utmost importance to constrain the conditions that are conducive to magnetic fields and to determine first trends about their occurrence rate and field strength distribution. Aims: To investigate whether magnetic fields in massive stars are ubiquitous or appear only in stars with a specific spectral classification, certain ages, or in a special environment, we acquired 67 new spectropolarimetric observations for 30 massive stars. Among the observed sample, roughly one third of the stars are probable members of clusters at different ages, whereas the remaining stars are field stars not known to belong to any cluster or association. Methods: Spectropolarimetric observations were obtained during four different nights using the low-resolution spectropolarimetric mode of FOcal Reducer low dispersion Spectrograph (FORS 2) mounted on the 8-m Antu telescope of the VLT. Furthermore, we present a number of follow-up observations carried out with the high-resolution spectropolarimeters SOFIN mounted at the Nordic Optical Telescope (NOT) and HARPS mounted at the ESO 3.6 m between 2008 and 2011. To assess the membership in open clusters and associations, we used astrometric catalogues with the highest quality kinematic and photometric data currently available. Results: The presence of a magnetic field is confirmed in nine stars previously observed with FORS 1/2: HD 36879, HD 47839, CPD-28 2561, CPD-47 2963, HD 93843, HD 148937, HD 149757, HD 328856, and HD 164794. New magnetic field detections at a significance level of at least 3σ were achieved in five stars: HD 92206c, HD 93521, HD 93632, CPD-46 8221, and HD 157857. Among the stars with a detected magnetic field, five stars belong to open clusters with high membership probability. According to

  5. Magnetic moments and angular momenta of stars and planets

    NASA Technical Reports Server (NTRS)

    Arge, C. N.; Mullan, D. J.; Dolginov, A. Z.

    1995-01-01

    Using published data on magnetic fields, radii, masses, and rotation, we have compiled a data set of magnetic moments mu and angular momenta L for stars and planets. In our subsample of hotter stars (classes A, B, and O), there are 171 objects. In the subsample of cooler stars (classes F, G, K, and M), there are 54 objects. We include 33 white dwarfs, of which 19 are in cataclysmic variables. The pulsar subsample contains 32 pulsars in binaries and 429 isolated pulsars. Som subsamples exhibit significant empirical correlations between log mu and log L. For the hot and cool stars, the correlations are positive. However, the hot-star correlation is significantly shallower than for the cool stars. In the solar system subsample, the correlation has essentially the same slope as for the cool stars, although the magnetic moments are two to three orders of magnitude smaller for the solar system objects at a given L value. For isolated white dwarfs, the correlations are weak or absent. Pulsars and white dwarfs in close binaries show strong negative correlations: the results are quantitatively consistent with magnetically enforced synchronism with the orbital period. When we consider the centers of gravity of the different subsamples of objects, a significant positive correlation appears between log mu and log L.

  6. Electrodynamics of disk-accreting magnetic neutron stars

    NASA Technical Reports Server (NTRS)

    Miller, M. Coleman; Lamb, Frederick K.; Hamilton, Russell J.

    1994-01-01

    We have investigated the electrodynamics of magnetic neutron stars accreting from Keplerian disks and the implications for particle acceleration and gamma-ray emission by such systems. We argue that the particle density in the magnetospheres of such stars is larger by orders of magnitude than the Goldreich-Julian density, so that the formation of vacuum gaps is unlikely. We show that even if the star rotates slowly, electromotive forces (EMFs) of order 10(exp 15) V are produced by the interaction of plasma in the accretion disk with the magnetic field of the neutron star. The resistance of the disk-magnetosphere-star circuit is small, and hence these EMFs drive very large conduction currents. Such large currents are likely to produce magnetospheric instabilities, such as relativistic double layers and reconnection events, that can accelerate electrons or ions to very high energies.

  7. Energy flux determines magnetic field strength of planets and stars

    NASA Astrophysics Data System (ADS)

    Christensen, Ulrich R.; Holzwarth, Volkmar; Reiners, Ansgar

    2009-01-01

    The magnetic fields of Earth and Jupiter, along with those of rapidly rotating, low-mass stars, are generated by convection-driven dynamos that may operate similarly (the slowly rotating Sun generates its field through a different dynamo mechanism). The field strengths of planets and stars vary over three orders of magnitude, but the critical factor causing that variation has hitherto been unclear. Here we report an extension of a scaling law derived from geodynamo models to rapidly rotating stars that have strong density stratification. The unifying principle in the scaling law is that the energy flux available for generating the magnetic field sets the field strength. Our scaling law fits the observed field strengths of Earth, Jupiter, young contracting stars and rapidly rotating low-mass stars, despite vast differences in the physical conditions of the objects. We predict that the field strengths of rapidly rotating brown dwarfs and massive extrasolar planets are high enough to make them observable.

  8. Energy flux determines magnetic field strength of planets and stars.

    PubMed

    Christensen, Ulrich R; Holzwarth, Volkmar; Reiners, Ansgar

    2009-01-08

    The magnetic fields of Earth and Jupiter, along with those of rapidly rotating, low-mass stars, are generated by convection-driven dynamos that may operate similarly (the slowly rotating Sun generates its field through a different dynamo mechanism). The field strengths of planets and stars vary over three orders of magnitude, but the critical factor causing that variation has hitherto been unclear. Here we report an extension of a scaling law derived from geodynamo models to rapidly rotating stars that have strong density stratification. The unifying principle in the scaling law is that the energy flux available for generating the magnetic field sets the field strength. Our scaling law fits the observed field strengths of Earth, Jupiter, young contracting stars and rapidly rotating low-mass stars, despite vast differences in the physical conditions of the objects. We predict that the field strengths of rapidly rotating brown dwarfs and massive extrasolar planets are high enough to make them observable.

  9. Electrodynamics of disk-accreting magnetic neutron stars

    NASA Technical Reports Server (NTRS)

    Miller, M. Coleman; Lamb, Frederick K.; Hamilton, Russell J.

    1994-01-01

    We have investigated the electrodynamics of magnetic neutron stars accreting from Keplerian disks and the implications for particle acceleration and gamma-ray emission by such systems. We argue that the particle density in the magnetospheres of such stars is larger by orders of magnitude than the Goldreich-Julian density, so that the formation of vacuum gaps is unlikely. We show that even if the star rotates slowly, electromotive forces (EMFs) of order 10(exp 15) V are produced by the interaction of plasma in the accretion disk with the magnetic field of the neutron star. The resistance of the disk-magnetosphere-star circuit is small, and hence these EMFs drive very large conduction currents. Such large currents are likely to produce magnetospheric instabilities, such as relativistic double layers and reconnection events, that can accelerate electrons or ions to very high energies.

  10. RETIRED A STARS AND THEIR COMPANIONS. III. COMPARING THE MASS-PERIOD DISTRIBUTIONS OF PLANETS AROUND A-TYPE STARS AND SUN-LIKE STARS

    SciTech Connect

    Bowler, Brendan P.; Johnson, John Asher; Liu, Michael C.; Marcy, Geoffrey W.; Peek, Kathryn M. G.; Henry, Gregory W.; Fischer, Debra A.; Clubb, Kelsey I.; Reffert, Sabine; Schwab, Christian; Lowe, Thomas B.

    2010-01-20

    We present an analysis of approx5 years of Lick Observatory radial velocity measurements targeting a uniform sample of 31 intermediate-mass (IM) subgiants (1.5 approx< M{sub *}/M{sub sun}approx< 2.0) with the goal of measuring the occurrence rate of Jovian planets around (evolved) A-type stars and comparing the distributions of their orbital and physical characteristics to those of planets around Sun-like stars. We provide updated orbital solutions incorporating new radial velocity measurements for five known planet-hosting stars in our sample; uncertainties in the fitted parameters are assessed using a Markov-Chain Monte Carlo method. The frequency of Jovian planets interior to 3 AU is 26{sup +9}{sub -8}%, which is significantly higher than the 5%-10% frequency observed around solar-mass stars. The median detection threshold for our sample includes minimum masses down to left brace0.2, 0.3, 0.5, 0.6, 1.3right brace M{sub Jup} within left brace0.1, 0.3, 0.6, 1.0, 3.0right brace AU. To compare the properties of planets around IM stars to those around solar-mass stars we synthesize a population of planets based on the parametric relationship dN propor to M {sup a}lpha P {sup b}eta dlnMdlnP, the observed planet frequency, and the detection limits we derived. We find that the values of alpha and beta for planets around solar-type stars from Cumming et al. fail to reproduce the observed properties of planets in our sample at the 4sigma level, even when accounting for the different planet occurrence rates. Thus, the properties of planets around A stars are markedly different than those around Sun-like stars, suggesting that only a small (approx50%) increase in stellar mass has a large influence on the formation and orbital evolution of planets.

  11. Measuring surface magnetic fields of red supergiant stars

    NASA Astrophysics Data System (ADS)

    Tessore, B.; Lèbre, A.; Morin, J.; Mathias, P.; Josselin, E.; Aurière, M.

    2017-07-01

    Context. Red supergiant (RSG) stars are very massive cool evolved stars. Recently, a weak magnetic field was measured at the surface of α Ori and this is so far the only M-type supergiant for which a direct detection of a surface magnetic field has been reported. Aims: By extending the search for surface magnetic field in a sample of late-type supergiants, we want to determine whether the surface magnetic field detected on α Ori is a common feature among the M-type supergiants. Methods: With the spectropolarimeter Narval at Télescope Bernard-Lyot we undertook a search for surface magnetic fields in a sample of cool supergiant stars, and we analysed circular polarisation spectra using the least-squares deconvolution technique. Results: We detect weak Zeeman signatures of stellar origin in the targets CE Tau, α1 Her and μ Cep. For the latter star, we also show that cross-talk from the strong linear polarisation signals detected on this star must be taken into account. For CE Tau and μ Cep, the longitudinal component of the detected surface fields is at the Gauss-level, such as in α Ori. We measured a longitudinal field almost an order of magnitude stronger for α1 Her. We also report variability of the longitudinal magnetic field of CE Tau and α1 Her, with changes in good agreement with the typical atmospheric dynamics time-scales. We also report a non-detection of magnetic field at the surface of the yellow supergiant star ρ Cas. Conclusions: The two RSG stars of our sample, CE Tau and μ Cep, display magnetic fields very similar to that of α Ori. The non-detection of a magnetic field on the post-RSG star ρ Cas suggests that the magnetic field disappears, or at least becomes undetectable with present methods, at later evolutionary stages. Our analysis of α1 Her supports the proposed reclassification of the star as an M-type asymptotic giant branch star. Based on observations obtained at the Télescope Bernard Lyot (TBL) at the Observatoire du Pic du Midi

  12. Magnetic fields in the formation of sun-like stars.

    PubMed

    Girart, Josep M; Rao, Ramprasad; Marrone, Daniel P

    2006-08-11

    We report high-angular-resolution measurements of polarized dust emission toward the low-mass protostellar system NGC 1333 IRAS 4A. We show that in this system the observed magnetic field morphology is in agreement with the standard theoretical models of the formation of Sun-like stars in magnetized molecular clouds at scales of a few hundred astronomical units; gravity has overcome magnetic support, and the magnetic field traces a clear hourglass shape. The magnetic field is substantially more important than turbulence in the evolution of the system, and the initial misalignment of the magnetic and spin axes may have been important in the formation of the binary system.

  13. Magnetic propeller effect in the spectra of young stars

    NASA Astrophysics Data System (ADS)

    Grinin, V. P.; Potravnov, I. S.; Ilyin, I. V.; Shulman, S. G.

    2015-08-01

    The origin of the blueshifted narrow absorption components in the resonance sodium doublet lines observed in the spectra of some young stars is discussed. Such components are assumed to be formed by the interaction of the circumstellar gas with the stellar magnetosphere in the magnetic propeller regime. The results of observations for the post UX Ori star RZ Psc are discussed in detail. This star shows distinctive signatures of mass outflow in the absence of any clear accretion signatures. Such a picture is quite possible in the magnetic propeller regime. Estimates show that for this regime to be realized, the star must have a surface magnetic field of ~1 kG at an accretion rate that does not exceed 10-10 M . yr-1.

  14. Rotational and magnetic field instabilities in neutron stars

    SciTech Connect

    Kokkotas, Kostas D.

    2014-01-14

    In this short review we present recent results on the dynamics of neutron stars and their magnetic fields. We discuss the progress that has been made, during the last 5 years, in understanding the rotational instabilities with emphasis to the one due to the f-mode, the possibility of using gravitational wave detection in constraining the parameters of neutron stars and revealing the equation of state as well as the detectability of gravitational waves produced during the unstable phase of a neutron star’s life. In addition we discuss the dynamics of extremely strong magnetic fields observed in a class of neutron stars (magnetars). Magnetic fields of that strength are responsible for highly energetic phenomena (giant flares) and we demonstrate that the analysis of the emitted electromagnetic radiation can lead in constraining the parameters of neutron stars. Furthermore, we present our results from the study of such violent phenomena in association with the emission of gravitational radiation.

  15. Magnetic fields of weak line T-Tauri stars

    NASA Astrophysics Data System (ADS)

    Hill, Colin A.; MaTYSSE Collaboration

    2017-10-01

    T-Tauri stars (TTS) are late-type pre-main-sequence (PMS) stars that are gravitationally contracting towards the MS. Those that possess a massive accretion disc are known as classical T-Tauri stars (cTTSs), and those that have exhausted the gas in their inner discs are known as weak-line T-Tauri stars (wTTSs). Magnetic fields largely dictate the angular momentum evolution of TTS and can affect the formation and migration of planets. Thus, characterizing their magnetic fields is critical for testing and developing stellar dynamo models, and trialling scenarios currently invoked to explain low-mass star and planet formation. The MaTYSSE programme (Magnetic Topologies of Young Stars and the Survival of close-in Exoplanets) aims to determine the magnetic topologies of ~30 wTTSs and monitor the long-term topology variability of ~5 cTTSs. We present several wTTSs that have been magnetically mapped thus far (using Zeeman Doppler Imaging), where we find a much wider range of field topologies compared to cTTSs and MS dwarfs with similar internal structures.

  16. Magnetic fields in central stars of planetary nebulae?

    NASA Astrophysics Data System (ADS)

    Jordan, S.; Bagnulo, S.; Werner, K.; O'Toole, S. J.

    2012-06-01

    Context. Most planetary nebulae have bipolar or other non-spherically symmetric shapes. Magnetic fields in the central star may be responsible for this lack of symmetry, but observational studies published to date have reported contradictory results. Aims: We search for correlations between a magnetic field and departures from the spherical geometry of the envelopes of planetary nebulae. Methods: We determine the magnetic fields from spectropolarimetric observations of ten central stars of planetary nebulae. The results of the analysis of the observations of four stars were previously presented and discussed in the literature, while the observations of six stars, plus additional measurements of a star previously observed, are presented here for the first time. Results: All our determinations of magnetic field in the central planetary nebulae are consistent with null results. Our field measurements have a typical error bar of 150-300 G. Previous spurious field detections using data acquired with FORS1 (FOcal Reducer and low dispersion Spectrograph) of the Unit Telescope 1 (UT1) of the Very Large Telescope (VLT) were probably due to the use of different wavelength calibration solutions for frames obtained at different position angles of the retarder waveplate. Conclusions: There is currently no observational evidence of magnetic fields with a strength of the order of hundreds Gauss or higher in the central stars of planetary nebulae. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile, under programme ID 072.D-0089 (PI = Jordan) and 075.D-0289 (PI = Jordan).

  17. Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars

    NASA Astrophysics Data System (ADS)

    Bychkov, V. D.; Bychkova, L. V.; Madej, J.

    2003-08-01

    This paper presents the catalogue and the method of determination of averaged quadratic effective magnetic fields < B_e > for 596 main sequence and giant stars. The catalogue is based on measurements of the stellar effective (or mean longitudinal) magnetic field strengths B_e, which were compiled from the existing literature. We analysed the properties of 352 chemically peculiar A and B stars in the catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, and all ApSr type stars. We have found that the number distribution of all chemically peculiar (CP) stars vs. averaged magnetic field strength is described by a decreasing exponential function. Relations of this type hold also for stars of all the analysed subclasses of chemical peculiarity. The exponential form of the above distribution function can break down below about 100 G, the latter value representing approximately the resolution of our analysis for A type stars. Table A.1 and its references are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9 are only available in electronic form at http://www.edpsciences.org

  18. Monitoring of rotational period variations in magnetic chemically peculiar stars

    NASA Astrophysics Data System (ADS)

    Mikulášek, Z.

    2016-12-01

    A majority part of magnetic chemically peculiar (mCP) stars of the upper main sequence exhibits strictly periodic light, magnetic, radio, and spectral variations that can be fully explained by the model of a rigidly rotating main-sequence star with persistent surface structures and stable global magnetic field frozen into the body of the star. Nevertheless, there is an inhomogeneous group consisting of a few mCP stars whose rotation periods vary on timescales of decades, while the shapes of their phase curves remain nearly unchanged. Alternations in the rotational period variations, proven in the case of some of them, offer new insight on this theoretically unpredicted phenomenon. We present a novel and generally applicable method of period analysis based on the simultaneous exploitation of all available observational data containing phase information. This phenomenological method can monitor gradual changes in the observed instantaneous period very efficiently and reliably. We present up to date results of the period monitoring of V901 Ori, CU Vir, σ Ori E, and BS Cir, known to be mCP stars changing their observed periods and discuss the physics of this unusual behaviour. To compare the period behavior of those stars, we treated their data with an orthogonal polynomial model, which was specifically developed for this purpose. We confirmed period variations in all stars and showed that they reflect real changes in the angular velocity of outer layers of the stars, fastened by their global magnetic fields. However, the nature of the observed rotational instabilities has remained elusive up to now. The discussed group of mCP stars is inhomogeneous to such extent that each of the stars may experience a different cause for its period variations.

  19. Magnetic Doppler imaging of the chemically peculiar star HD 125248

    NASA Astrophysics Data System (ADS)

    Rusomarov, N.; Kochukhov, O.; Ryabchikova, T.; Ilyin, I.

    2016-04-01

    Context. Intermediate-mass, chemically peculiar stars with strong magnetic fields provide an excellent opportunity to study the topology of their surface magnetic fields and the interplay between magnetic geometries and abundance inhomogeneities in the atmospheres of these stars. Aims: We reconstruct detailed maps of the surface magnetic field and abundance distributions for the magnetic Ap star HD 125248. Methods: We performed the analysis based on phase-resolved, four Stokes parameter spectropolarimetric observations obtained with the HARPSpol instrument. These data were interpreted with the help of magnetic Doppler imaging techniques and model atmospheres taking the effects of strong magnetic fields and nonsolar chemical composition into account. Results: We improved the atmospheric parameters of the star, Teff = 9850 ± 250 K and log g = 4.05 ± 0.10. We performed detailed abundance analysis, which confirmed that HD 125248 has abundances typical of other Ap stars, and discovered significant vertical stratification effects for the Fe ii and Cr ii ions. We computed LSD Stokes profiles using several line masks corresponding to Fe-peak and rare earth elements, and studied their behavior with rotational phase. Combining previous longitudinal field measurements with our own observations, we improved the rotational period of the star Prot = 9.29558 ± 0.00006 d. Magnetic Doppler imaging of HD 125248 showed that its magnetic field is mostly poloidal and quasi-dipolar with two large spots of different polarity and field strength. The chemical maps of Fe, Cr, Ce, Nd, Gd, and Ti show abundance contrasts of 0.9-3.5 dex. Among these elements, the Fe abundance map does not show high-contrast features. Cr is overabundant around the negative magnetic pole and has 3.5 dex abundance range. The rare earth elements and Ti are overabundant near the positive magnetic pole. Conclusions: The magnetic field of HD 125248 has strong deviations from the classical oblique dipole field

  20. Observational evidence for enhanced magnetic activity of superflare stars

    PubMed Central

    Karoff, Christoffer; Knudsen, Mads Faurschou; De Cat, Peter; Bonanno, Alfio; Fogtmann-Schulz, Alexandra; Fu, Jianning; Frasca, Antonio; Inceoglu, Fadil; Olsen, Jesper; Zhang, Yong; Hou, Yonghui; Wang, Yuefei; Shi, Jianrong; Zhang, Wei

    2016-01-01

    Superflares are large explosive events on stellar surfaces one to six orders-of-magnitude larger than the largest flares observed on the Sun throughout the space age. Due to the huge amount of energy released in these superflares, it has been speculated if the underlying mechanism is the same as for solar flares, which are caused by magnetic reconnection in the solar corona. Here, we analyse observations made with the LAMOST telescope of 5,648 solar-like stars, including 48 superflare stars. These observations show that superflare stars are generally characterized by larger chromospheric emissions than other stars, including the Sun. However, superflare stars with activity levels lower than, or comparable to, the Sun do exist, suggesting that solar flares and superflares most likely share the same origin. The very large ensemble of solar-like stars included in this study enables detailed and robust estimates of the relation between chromospheric activity and the occurrence of superflares. PMID:27009381

  1. Observational evidence for enhanced magnetic activity of superflare stars.

    PubMed

    Karoff, Christoffer; Knudsen, Mads Faurschou; De Cat, Peter; Bonanno, Alfio; Fogtmann-Schulz, Alexandra; Fu, Jianning; Frasca, Antonio; Inceoglu, Fadil; Olsen, Jesper; Zhang, Yong; Hou, Yonghui; Wang, Yuefei; Shi, Jianrong; Zhang, Wei

    2016-03-24

    Superflares are large explosive events on stellar surfaces one to six orders-of-magnitude larger than the largest flares observed on the Sun throughout the space age. Due to the huge amount of energy released in these superflares, it has been speculated if the underlying mechanism is the same as for solar flares, which are caused by magnetic reconnection in the solar corona. Here, we analyse observations made with the LAMOST telescope of 5,648 solar-like stars, including 48 superflare stars. These observations show that superflare stars are generally characterized by larger chromospheric emissions than other stars, including the Sun. However, superflare stars with activity levels lower than, or comparable to, the Sun do exist, suggesting that solar flares and superflares most likely share the same origin. The very large ensemble of solar-like stars included in this study enables detailed and robust estimates of the relation between chromospheric activity and the occurrence of superflares.

  2. Are blue supergiants descendants of magnetic main sequence stars?

    NASA Astrophysics Data System (ADS)

    Petermann, Ilka; Langer, Norbert

    2013-06-01

    Red and blue supergiants are, together with luminous blue variables and Wolf-Rayet stars, evolved phases of massive (OB) stars. The position of blue supergiants (BSG) near the main sequence band cannot be reproduced by standard stellar evolution calculations. However, the assumption of a reduced convective core mass during the main sequence (MS) due to strong internal magnetic fields, established in roughly 10% of all stars on the upper MS, can recover this BSG population. For our calculations of the (non-rotating) massive stars at solar metallicity we used the 1D stellar evolution code MESA and compare their evolutionary tracks with positions from stars obtained from the VLT Flames survey of massive stars.

  3. Gluon Vortices and Induced Magnetic Field in Compact Stars

    SciTech Connect

    Ferrer, Efrain J.

    2007-10-26

    The natural candidates for the realization of color superconductivity are the extremely dense cores of compact stars, many of which have very large magnetic fields, especially the so called magnetars. In this paper we discuss how a color superconducting core can serve to generate and enhance the stellar magnetic field without appealing to a magnetohydrodynamic dynamo mechanism.

  4. Quark stars with strong magnetic fields: considering different magnetic field geometries

    NASA Astrophysics Data System (ADS)

    Wei, Wei; Liu, Xi-Wei; Zheng, Xiao-Ping

    2017-09-01

    We calculate the mass-radius relationship of quark stars with the magnetized density-dependent quark mass model in this work, considering two magnetic field geometries: a statistically isotropic, tangled field and a force-free configuration. In both cases, magnetic field production decreases in the case of maximum quark star mass. Furthermore, a tangled, isotropic magnetic field has a relatively smaller impact on the mass and radius, compared to the force-free configuration, which implies that the geometry of the interior magnetic field is at least as important as the field strength itself when the influence of the strong magnetic field on the mass and radius is assessed.

  5. Discovery of magnetic A supergiants: the descendants of magnetic main-sequence B stars

    NASA Astrophysics Data System (ADS)

    Neiner, Coralie; Oksala, Mary E.; Georgy, Cyril; Przybilla, Norbert; Mathis, Stéphane; Wade, Gregg; Kondrak, Matthias; Fossati, Luca; Blazère, Aurore; Buysschaert, Bram; Grunhut, Jason

    2017-10-01

    In the context of the high resolution, high signal-to-noise ratio, high sensitivity, spectropolarimetric survey BritePol, which complements observations by the BRITE constellation of nanosatellites for asteroseismology, we are looking for and measuring the magnetic field of all stars brighter than V = 4. In this paper, we present circularly polarized spectra obtained with HarpsPol at ESO in La Silla (Chile) and ESPaDOnS at CFHT (Hawaii) for three hot evolved stars: ι Car, HR 3890 and ε CMa. We detected a magnetic field in all three stars. Each star has been observed several times to confirm the magnetic detections and check for variability. The stellar parameters of the three objects were determined and their evolutionary status was ascertained employing evolution models computed with the Geneva code. ε CMa was already known and is confirmed to be magnetic, but our modelling indicates that it is located near the end of the main sequence, i.e. it is still in a core hydrogen burning phase. ι Car and HR 3890 are the first discoveries of magnetic hot supergiants located well after the end of the main sequence on the Hertzsprung-Russell diagram. These stars are probably the descendants of main-sequence magnetic massive stars. Their current field strength (a few G) is compatible with magnetic flux conservation during stellar evolution. These results provide observational constraints for the development of future evolutionary models of hot stars including a fossil magnetic field.

  6. Magnetic Fields and Atmospheric Motions in Pulsating Stars

    NASA Astrophysics Data System (ADS)

    Chadid, Merieme

    New series of high-precision longitudinal magnetic field measurements of RR Lyrae has been reported here, obtained with the MuSiCoS spectropolarimeter over a period of 4 years. These data provide no evidence whatsoever for a strong magnetic field in the photosphere of RR Lyrae, a result consistent with Preston’s (1967) results, but inconsistent with apparent magnetic field detections by Babcock (1958) and Romanov et al. (1987, 1994). Following discussion of these disparate results, we conclude that RR Lyrae is a bona fide non-magnetic star, a conclusion which leads to the general falsification of models of the Blazkho effect requiring strong photospheric magnetic fields.

  7. Vertical Structure of Magnetized Accretion Disks Around Young Stars

    NASA Astrophysics Data System (ADS)

    Tapia, Carlos; Lizano, Susana

    2016-01-01

    We model the vertical structure of magnetized accretion disks subject to viscous and resistive heating, and irradiation by the central star. We apply our formalism to the radial structure of magnetized accretion disks threaded by a poloidal magnetic field dragged during the process of star formation developed by Shu and coworkers. We consider disks around low mass protostars, T Tauri, and FU Orionis stars. We consider two levels of disk magnetization, λsys = 4 (strongly magnetized disks), and λsys = 12 (weakly magnetized disks). The rotation rates of strongly magnetized disks have large deviations from Keplerian rotation. In these models, resistive heating dominates the thermal structure for the FU Ori disk. The T Tauri disk is very thin and cold because it is strongly compressed by magnetic pressure; it may be too thin compared with observations. Instead, in the weakly magnetized disks, rotation velocities are close to Keplerian, and resistive heating is always less than 7% of the viscous heating. In these models, the T Tauri disk has a larger aspect ratio, consistent with that inferred from observations. All the disks have spatially extended hot atmospheres where the irradiation flux is absorbed, although most of the mass (~ 90 - 95 %) is in the disk midplane.

  8. SPINDOWN OF ISOLATED NEUTRON STARS: GRAVITATIONAL WAVES OR MAGNETIC BRAKING?

    SciTech Connect

    Staff, Jan E.; Jaikumar, Prashanth; Chan, Vincent; Ouyed, Rachid

    2012-05-20

    We study the spindown of isolated neutron stars from initially rapid rotation rates, driven by two factors: (1) gravitational wave emission due to r-modes and (2) magnetic braking. In the context of isolated neutron stars, we present the first study including self-consistently the magnetic damping of r-modes in the spin evolution. We track the spin evolution employing the RNS code, which accounts for the rotating structure of neutron stars for various equations of state. We find that, despite the strong damping due to the magnetic field, r-modes alter the braking rate from pure magnetic braking for B {<=} 10{sup 13} G. For realistic values of the saturation amplitude {alpha}{sub sat}, the r-mode can also decrease the time to reach the threshold central density for quark deconfinement. Within a phenomenological model, we assess the gravitational waveform that would result from r-mode-driven spindown of a magnetized neutron star. To contrast with the persistent signal during the spindown phase, we also present a preliminary estimate of the transient gravitational wave signal from an explosive quark-hadron phase transition, which can be a signal for the deconfinement of quarks inside neutron stars.

  9. Some magnetic null lines of astrophysical interest. II. [in magnetic Ap stars

    NASA Technical Reports Server (NTRS)

    Adelman, S. J.

    1974-01-01

    A list of magnetic null lines from selected spectroscopic analyses is presented for possible use in detailed studies of magnetic Ap stars. The presented data represent an extension of the magnetic null lines whose usefulness Shore and Adelman (1974) have recently demonstrated for the study of the overall physical conditions prevailing in the atmospheres of peculiar A stars and for testing the consequences of the mechanism of selective elemental diffusion.

  10. An infrared diagnostic for magnetism in hot stars

    NASA Astrophysics Data System (ADS)

    Oksala, M. E.; Grunhut, J. H.; Kraus, M.; Borges Fernandes, M.; Neiner, C.; Condori, C. A. H.; Campagnolo, J. C. N.; Souza, T. B.

    2015-06-01

    Magnetospheric observational proxies are used for indirect detection of magnetic fields in hot stars in the X-ray, UV, optical, and radio wavelength ranges. To determine the viability of infrared (IR) hydrogen recombination lines as a magnetic diagnostic for these stars, we have obtained low-resolution (R~ 1200), near-IR spectra of the known magnetic B2V stars HR 5907 and HR 7355, taken with the Ohio State Infrared Imager/Spectrometer (OSIRIS) attached to the 4.1 m Southern Astrophysical Research (SOAR) Telescope. Both stars show definite variable emission features in IR hydrogen lines of the Brackett series, with similar properties as those found in optical spectra, including the derived location of the detected magnetospheric plasma. These features also have the added advantage of a lowered contribution of stellar flux at these wavelengths, making circumstellar material more easily detectable. IR diagnostics will be useful for the future study of magnetic hot stars, to detect and analyze lower-density environments, and to detect magnetic candidates in areas obscured from UV and optical observations, increasing the number of known magnetic stars to determine basic formation properties and investigate the origin of their magnetic fields. Based on observations obtained at the Southern Astrophysical Research (SOAR) telescope, which is a joint project of the Ministério da Ciência, Tecnologia, e Inovação (MCTI) da República Federativa do Brasil, the US National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC), and Michigan State University (MSU).

  11. Vertical Structure of Magnetized Accretion Disks around Young Stars

    NASA Astrophysics Data System (ADS)

    Lizano, S.; Tapia, C.; Boehler, Y.; D'Alessio, P.

    2016-01-01

    We model the vertical structure of the magnetized accretion disks that are subject to viscous and resistive heating and irradiation by the central star. We apply our formalism to the radial structure of the magnetized accretion disks that are threaded by the poloidal magnetic field dragged during the process of star formation, which was developed by Shu and coworkers. We consider disks around low-mass protostars, T Tauri, and FU Orionis stars, as well as two levels of disk magnetization: {λ }{sys}=4 (strongly magnetized disks) and {λ }{sys}=12 (weakly magnetized disks). The rotation rates of strongly magnetized disks have large deviations from Keplerian rotation. In these models, resistive heating dominates the thermal structure for the FU Ori disk, and the T Tauri disk is very thin and cold because it is strongly compressed by magnetic pressure; it may be too thin compared with observations. Instead, in the weakly magnetized disks, rotation velocities are close to Keplerian, and resistive heating is always less than 7% of the viscous heating. In these models, the T Tauri disk has a larger aspect ratio, which is consistent with that inferred from observations. All the disks have spatially extended hot atmospheres where the irradiation flux is absorbed, although most of the mass (˜90%-95%) is in the disk midplane. With the advent of ALMA one expects direct measurements of magnetic fields and their morphology at disk scales. It will then be possible to determine the mass-to-flux ratio of magnetized accretion disks around young stars, an essential parameter for their structure and evolution. Our models contribute to the understanding of the vertical structure and emission of these disks.

  12. Neutron stars including the effects of chaotic magnetic fields and anomalous magnetic moments

    NASA Astrophysics Data System (ADS)

    Wu, Fei; Wu, Chen; Ren, Zhong-Zhou

    2017-04-01

    The relativistic mean field (RMF) FSUGold model extended to include hyperons is employed to study the properties of neutron stars with strong magnetic fields. The chaotic magnetic field approximation is utilized. The effect of anomalous magnetic moments (AMMs) is also investigated. It is shown that the equation of state (EOS) of neutron star matter is stiffened by the presence of the magnetic field, which increases the maximum mass of a neutron star by around 6%. The AMMs only have a small influence on the EOS of neutron star matter, and increase the maximum mass of a neutron star by 0.02M sun. Neutral particles are spin polarized due to the presence of the AMMs. Supported by National Natural Science Foundation of China (11535004, 11375086, 11120101005, 11175085, 11235001), 973 National Major State Basic Research and Development of China (2013CB834400), and Science and Technology Development Fund of Macau (068/2011/A)

  13. Circumstellar magnetic activity, flares and mass ejections in young stars

    NASA Astrophysics Data System (ADS)

    Foing, B. H.; Collier-Cameron, A.; Ehrenfreund, P.; Houdebine, E.

    1990-11-01

    Stellar activity studies are used for information on the early Sun and its environment, and for estimating activity effects on the young solar nebula. Large scale active structures are diagnosed from their rotational modulation signature, and reconstructed in three dimensions, using topographic or Doppler imaging techniques. Young stars undergo violent flares associated with coronal mass ejections leading to mass loss rates that can affect the stellar environment and stellar evolution. Young stars show large coronal condensations, magnetically linked with the stellar surface, that can be destabilized by energetic flares. From a joint ESA/AAT/IUE campaign, masses of such circumstellar clouds around the young star AB Doradus are estimated. These studies allow the problems of the link between circumstellar structures and magnetic activity and the role of mass ejections and flares in the context of mass loss and angular momentum braking in young stars, and early stellar evolution to be addressed.

  14. Magnetism and Activity of Planet-Hosting Stars

    NASA Astrophysics Data System (ADS)

    Wright, Jason Thomas; Miller, Brendan

    2015-08-01

    The magnetic activity levels of planet host stars may differ from that of stars not known to host planets in several ways. Hot jupiters may induce activity in their hosts through magnetic interactions, or through tidal interactions by affecting their host's rotation or convection. Measurements of photospheric, chromospheric, or coronal activity might then be abnormally abnormally high or low compared to control stars that do not host hot Jupiters, or might be modulated at the planet's orbital period. Such detections are complicated by the small amplitude of the expected signal, by the fact that the signals may be transient, and by the difficulty of constructing control samples due to exoplanet deteciton biases and the uncertainty of field star ages. I will review these issues, and discuss avenues for future progress in the field.

  15. The mean magnetic field modulus of AP stars

    NASA Astrophysics Data System (ADS)

    Mathys, G.; Hubrig, S.; Landstreet, J. D.; Lanz, T.; Manfroid, J.

    1997-06-01

    We present new measurements of the mean magnetic field modulus of a sample of Ap stars with spectral lines resolved into magnetically split components. We report the discovery of 16 new stars having this property. This brings the total number of such stars known to 42. We have performed more than 750 measurements of the mean field modulus of 40 of these 42 stars, between May 1988 and August 1995. The best of them have an estimated accuracy of 25 - 30 G. The availability of such a large number of measurements allows us to discuss for the first time the distribution of the field modulus intensities. A most intriguing result is the apparent existence of a sharp cutoff at the low end of this distribution, since no star with a field modulus (averaged over the rotation period) smaller than 2.8 kG has been found in this study. For more than one third of the studied stars, enough field determinations well distributed throughout the stellar rotation cycle have been achieved to allow us to characterize at least to some extent the variations of the field modulus. These variations are often significantly anharmonic, and it is not unusual for their extrema not to coincide in phase with the extrema of the longitudinal field (for the few stars for which enough data exist about the latter). This, together with considerations on the distribution of the relative amplitude of variation of the studied stars, supports the recently emerging evidence for markedly non-dipolar geometry and fine structure of the magnetic fields of most Ap stars. New or improved determinations of the rotation periods of 9 Ap stars have been achieved from the analysis of the variations of their mean magnetic field modulus. Tentative values of the period have been derived for 5 additional stars, and lower limits have been established for 10 stars. The shortest definite rotation period of an Ap star with magnetically resolved lines is 3.4 deg, while those stars that rotate slowest appear to have periods in

  16. Magnetic AP Stars in the Hertzsprung-Russell Diagram

    NASA Astrophysics Data System (ADS)

    Hubrig, S.; North, P.; Mathys, G.

    2000-08-01

    The evolutionary state of magnetic Ap stars is rediscussed using the recently released Hipparcos data. The distribution of the magnetic Ap stars of mass below 3 Msolar in the H-R diagram differs from that of the normal stars in the same temperature range at a high level of significance. Magnetic stars are concentrated toward the center of the main-sequence band. This is shown in two forms of the H-R diagram: one where logL is plotted against logTeff and a version more directly tied to the observed quantities, showing the astrometry-based luminosity (Arenou & Luri) against the (B2-G)0 index of Geneva photometry. In particular, it is found that magnetic fields appear only in stars that have already completed at least approximately 30% of their main-sequence lifetime. No clear picture emerges as to the possible evolution of the magnetic field across the main sequence. Hints of some (loose) relations between magnetic field strength and other stellar parameters are found: stars with shorter periods tend to have stronger fields, as do higher temperature and higher mass stars. A marginal trend of the magnetic flux to be lower in more slowly rotating stars may possibly be seen as suggesting a dynamo origin for the field. No correlation between the rotation period and the fraction of the main-sequence lifetime completed is observed, indicating that the slow rotation in these stars must already have been achieved before they became observably magnetic. Based on data from the ESA Hipparcos satellite and on observations collected at the European Southern Observatory (La Silla, Chile; ESO programs Nos. 43.7-004, 44.7-012, 49.7-030, 50.7-067, 51.7-041, 52.7-063, 53.7-028, 54.E-0416, and 55.E-0751), at the Observatoire de Haute-Provence (Saint-Michel l'Observatoire, France), at Kitt Peak National Observatory, and at the Canada-France-Hawaii Telescope.

  17. Magnetized color flavor locked state and compact stars

    NASA Astrophysics Data System (ADS)

    González Felipe, R.; Manreza Paret, D.; Pérez Martınez, A.

    2011-01-01

    The stability of the color flavor locked phase in the presence of a strong magnetic field is investigated within the phenomenological MIT bag model, taking into account the variation of the strange quark mass, the baryon density, the magnetic field, as well as the bag and gap parameters. It is found that the minimum value of the energy per baryon in a color flavor locked state at vanishing pressure is lower than the corresponding one for unpaired magnetized strange quark matter and, as the magnetic field increases, the energy per baryon decreases. This implies that magnetized color flavor locked matter is more stable and could become the ground state inside neutron stars. The mass-radius relation for such stars is also studied.

  18. Physics in strong magnetic fields near neutron stars

    NASA Technical Reports Server (NTRS)

    Harding, Alice K.

    1991-01-01

    Electromagnetic phenomena occurring in the strong magnetic fields of neutron stars are currently of great interest in high-energy astrophysics. Observations of rotation rate changes and cyclotron lines in pulsars and gamma-ray bursts indicate that surface magnetic fields of neutron stars often exceed a trillion gauss. In fields this strong, where electrons behave much as if they were in bound atomic states, familiar processes undergo profound changes, and exotic processes become important. Strong magnetic fields affect the physics in several fundamental ways: energies perpendicular to the field are quantized, transverse momentum is not conserved, and electron-positron spin is important. Neutron stars therefore provide a unique laboratory for the study of physics in extremely high fields that cannot be generated on earth.

  19. Detection of 610-MHz radio emission from hot magnetic stars

    NASA Astrophysics Data System (ADS)

    Chandra, P.; Wade, G. A.; Sundqvist, J. O.; Oberoi, D.; Grunhut, J. H.; ud-Doula, A.; Petit, V.; Cohen, D. H.; Oksala, M. E.; David-Uraz, A.

    2015-09-01

    We have carried out a study of radio emission from a small sample of magnetic O- and B-type stars using the Giant Metrewave Radio Telescope, with the goal of investigating their magnetospheres at low frequencies. These are the lowest frequency radio measurements ever obtained of hot magnetic stars. The observations were taken at random rotational phases in the 1390 and the 610 MHz bands. Out of the eight stars, we detect five B-type stars in both the 1390 and the 610 MHz bands. The three O-type stars were observed only in the 1390 MHz band, and no detections were obtained. We explain this result as a consequence of free-free absorption by the free-flowing stellar wind exterior to the confined magnetosphere. We also study the variability of individual stars. One star - HD 133880 - exhibits remarkably strong and rapid variability of its low-frequency flux density. We discuss the possibility of this emission being coherent emission as reported for CU Vir by Trigilio et al.

  20. A Review and Preview of Magnetic Star-Planet Interactions

    NASA Astrophysics Data System (ADS)

    Shkolnik, Evgenya

    2017-05-01

    Planets interact with their host stars through gravity, radiation and magnetic fields, and for those giant planets that orbit their stars within 20 stellar radii (=0.1 AU for a sun-like star), star-planet interactions (SPI) are observable with a wide variety of photometric, spectroscopic and spectropolarimetric studies. At such close distances, the planet orbits within the sub-alfvénic radius of the star in which the transfer of energy and angular momentum between the two bodies is particularly efficient. The nature of magnetic SPI is modeled to be strongly affected by both the stellar and planetary magnetic fields, possibly influencing the magnetic activity of both, as well as affecting the irradiation and even the migration of the planet. As we refine our observational techniques for hot Jupiter systems, we can begin to extend them to other tightly orbiting stellar systems, such as smaller planets close to M dwarfs where the region near tens of stellar radii begins to coincide with the classical habitable zone. Future studies of SPI with space-based telescopes and the next generation of ground-based telescopes will be informative pursuits for the study of the internal dynamics and atmospheric evolution of exoplanets.

  1. The sn stars - Magnetically controlled stellar winds among the helium-weak stars

    NASA Technical Reports Server (NTRS)

    Shore, Steven N.; Brown, Douglas N.; Sonneborn, George

    1987-01-01

    The paper reports observations of magnetically controlled stellar mass outflows in three helium-weak sn stars: HD 21699 = HR 1063; HD 5737 = Alpha Scl; and HD 79158 = 36 Lyn. IUE observations show that the C IV resonance doublet is variable on the rotational timescale but that there are no other strong-spectrum variations in the UV. Magnetic fields, which reverse sign on the rotational timescale, are present in all three stars. This phenomenology is interpreted in terms of jetlike mass loss above the magnetic poles, and these objects are discussed in the context of a general survey of the C IV and Si IV profiles of other more typical helium-weak stars.

  2. Chasing Low Frequency Radio Bursts from Magnetically Active Stars

    NASA Astrophysics Data System (ADS)

    Lynch, Christene; Murphy, Tara; Kaplan, David

    2017-05-01

    Flaring activity is a common characteristic of magnetically active stars. These events produce emission throughout the electromagnetic spectrum, implying a range of physical processes. A number of objects exhibit short-duration, narrow band, and highly circularly polarised (reaching 100%) radio bursts. The observed polarisation and frequency-time structure of these bursts points to a coherent emission mechanism such as the electron cyclotron maser. Due to the stochastic nature of these bursts and the sensitivity of current instruments, the number of stars where coherent emission has been detected is few, with numbers limited to a few tens of objects. Observations of a wider sample of active stars are necessary in order to establish the percentage that exhibit coherent radio bursts and to relate the observed emission characteristics to stellar magnetic properties. New wide-field, low frequency radio telescopes will probe a frequency regime that is mostly unexplored for many magnetically active stars and where coherent radio emissions are expected to be more numerous. M dwarf stars are of particular interest as they are currently favoured as most likely to host habitable planets. Yet the extreme magnetic activity observed for some M dwarf stars places some doubt on the ability of orbiting planets to host life. This presentation reports the first results from a targeted Murchison Widefield Array survey of M dwarf stars that were previously detected at 100 - 200 MHz using single dish telescopes. We will discuss robust flare-rate measurements over a high dynamic range of flare properties, as well as investigate the physical mechanism(s) behind the flares.

  3. ON THE PERIODIC VARIABILITY OF THE LONGITUDINAL MAGNETIC FIELDS OF STARS

    SciTech Connect

    Bychkov, V. D.; Bychkova, L. V.; Madej, J. E-mail: lbych@sao.ru

    2013-10-01

    There exist 218 stars with measured phase curves of their longitudinal (effective) magnetic field B{sub e} . In that group, 172 objects are classified as magnetic chemically peculiar stars. The remaining objects are stars of various spectral types, from the most massive hot Of?p supergiants to low-mass red dwarfs and stars with planets. In this paper, we discuss the behavior of the longitudinal magnetic field B{sub e} and present estimated parameters of the apparent magnetic variability for stars of each spectral type. This paper also aims to briefly review the properties of the observed magnetic behavior among various types of stars.

  4. Magnetic fields in the formation of massive stars.

    PubMed

    Girart, Josep M; Beltrán, Maria T; Zhang, Qizhou; Rao, Ramprasad; Estalella, Robert

    2009-06-12

    Massive stars play a crucial role in the production of heavy elements and in the evolution of the interstellar medium, yet how they form is still a matter of debate. We report high-angular-resolution submillimeter observations toward the massive hot molecular core (HMC) in the high-mass star-forming region G31.41+0.31. We find that the evolution of the gravitational collapse of the HMC is controlled by the magnetic field. The HMC is simultaneously contracting and rotating, and the magnetic field lines threading the HMC are deformed along its major axis, acquiring an hourglass shape. The magnetic energy dominates over the centrifugal and turbulence energies, and there is evidence of magnetic braking in the contracting core.

  5. Magnetic Models of Circumstellar Clouds around Massive Stars

    NASA Astrophysics Data System (ADS)

    Owocki, S.; Townsend, R.; Ud-Doula, A.

    2008-08-01

    This talk reviewed recent efforts to develop dynamical models for the effects of a surface dipole field on radiatively driven wind outflows. One particular project applies magnetohydrodynamic (MHD) simulations of a Magnetically Confined Wind Shock (MCWS) model (originally developed by Babel & Montmerle 1997) to explain X-ray emission observed by Rosat (Gagné et al. 1997) from the magnetic O7V star θ^{1 Ori C.

  6. Spin-down dynamics of magnetized solar-type stars

    SciTech Connect

    Oglethorpe, R. L. F.; Garaud, P.

    2013-12-01

    It has long been known that solar-type stars undergo significant spin-down, via magnetic braking, during their main-sequence lifetimes. However, magnetic braking only operates on the surface layers; it is not yet completely understood how angular momentum is transported within the star and how rapidly the spin-down information is communicated to the deep interior. In this work, we use insight from recent progress in understanding internal solar dynamics to model the interior of other solar-type stars. We assume, following Gough and McIntyre, that the bulk of the radiation zone of these stars is held in uniform rotation by the presence of an embedded large-scale primordial field, confined below a stably stratified, magnetic-free tachocline by large-scale meridional flows downwelling from the convection zone. We derive simple equations to describe the response of this model interior to spin-down of the surface layers, which are identical to the two-zone model of MacGregor and Brenner, with a coupling timescale proportional to the local Eddington-Sweet timescale across the tachocline. This timescale depends both on the rotation rate of the star and on the thickness of the tachocline, and it can vary from a few hundred thousand years to a few Gyr, depending on stellar properties. Qualitative predictions of the model appear to be consistent with observations, although they depend sensitively on the assumed functional dependence of the tachocline thickness on the stellar rotation rate.

  7. Spin-down Dynamics of Magnetized Solar-type Stars

    NASA Astrophysics Data System (ADS)

    Oglethorpe, R. L. F.; Garaud, P.

    2013-12-01

    It has long been known that solar-type stars undergo significant spin-down, via magnetic braking, during their main-sequence lifetimes. However, magnetic braking only operates on the surface layers; it is not yet completely understood how angular momentum is transported within the star and how rapidly the spin-down information is communicated to the deep interior. In this work, we use insight from recent progress in understanding internal solar dynamics to model the interior of other solar-type stars. We assume, following Gough & McIntyre, that the bulk of the radiation zone of these stars is held in uniform rotation by the presence of an embedded large-scale primordial field, confined below a stably stratified, magnetic-free tachocline by large-scale meridional flows downwelling from the convection zone. We derive simple equations to describe the response of this model interior to spin-down of the surface layers, which are identical to the two-zone model of MacGregor & Brenner, with a coupling timescale proportional to the local Eddington-Sweet timescale across the tachocline. This timescale depends both on the rotation rate of the star and on the thickness of the tachocline, and it can vary from a few hundred thousand years to a few Gyr, depending on stellar properties. Qualitative predictions of the model appear to be consistent with observations, although they depend sensitively on the assumed functional dependence of the tachocline thickness on the stellar rotation rate.

  8. Axisymmetric toroidal modes of general relativistic magnetized neutron star models

    SciTech Connect

    Asai, Hidetaka; Lee, Umin E-mail: lee@astr.tohoku.ac.jp

    2014-07-20

    We calculate axisymmetric toroidal modes of magnetized neutron stars with a solid crust in the general relativistic Cowling approximation. We assume that the interior of the star is threaded by a poloidal magnetic field, which is continuous at the surface with an outside dipole field. We examine the cases of the field strength B{sub S} ∼ 10{sup 16} G at the surface. Since separation of variables is not possible for the oscillations of magnetized stars, we employ finite series expansions for the perturbations using spherical harmonic functions. We find discrete normal toroidal modes of odd parity, but no toroidal modes of even parity are found. The frequencies of the toroidal modes form distinct mode sequences and the frequency in a given mode sequence gradually decreases as the number of radial nodes of the eigenfunction increases. From the frequency spectra computed for neutron stars of different masses, we find that the frequency is almost exactly proportional to B{sub S} and is well represented by a linear function of R/M for a given B{sub S}, where M and R are the mass and radius of the star. The toroidal mode frequencies for B{sub S} ∼ 10{sup 15} G are in the frequency range of the quasi-periodic oscillations (QPOs) detected in the soft-gamma-ray repeaters, but we find that the toroidal normal modes cannot explain all the detected QPO frequencies.

  9. Wind circulation in selected rotating magnetic early-B stars

    NASA Astrophysics Data System (ADS)

    Smith, M. A.; Groote, D.

    2001-06-01

    The rotating magnetic B stars are a class of variables consisting of He-strong and some beta Cep stars which have oblique dipolar magnetic fields. Such stars develop co-rotating, torus-shaped clouds by channeling wind particles from their magnetic poles to circumstellar regions centered around the plane of their magnetic equators. The rotation of the cloud-star complex permits the study of absorptions from the cloud as it occults the star. In this paper we describe a quantitative analysis of archival IUE data to map the properties of these clouds over four stars (HD 184927, sigma Ori E, beta Cep, and HR 6684). By computing spectral synthesis models for these stars, we find that only beta Cep has a solar-like metallicity. Our analysis also shows that the metal composition across the surfaces of all these stars is at least approximately homogeneous. Using the Hubeny code CIRCUS, we demonstrate that the periodic variations of broad-band ultraviolet continuum fluxes can be explained fully by the absorptions of the co-rotating cloud. We show next that among selected lines, those arising from low-excitation states are selectively affected by cloud absorption and turbulence. Our analysis also quantifies the cloud temperatures and column densities required to match the absorptions of a number of weak to moderate strength resonance lines. These temperatures increase with the ionization potential of the parent ions of these various lines, a result which is consistent with radiative equilibrium models in which temperature increases with proximity to the star's surface. Although these attributes appear stable from one epoch to another, dynamic processes are nonetheless at work. Both the strengths and widths of resonance lines at occultation phases indicate the presence of a turbulence in the cloud which increases inwards. The spectroscopic hallmark of this stellar class is the presence of strong C IV and N V resonance line absorptions at occultation phases and of redshifted

  10. Suppression of CMEs on active stars by overlying magnetic field

    NASA Astrophysics Data System (ADS)

    Drake, Jeremy J.; Garraffo, Cecilia; Cohen, Ofer; Alvarado-Gomez, Julian; Moschou, Sofia-Paraskevi

    2017-08-01

    On the Sun, the association rate of flares with coronal mass ejections (CMEs) increases with flare energy such that energetic X-class flares are nearly all associated with CMEs. Flares on active stars are commonly orders of magnitude more energetic than their solar counterparts, and extrapolating the solar trend suggests that all the flares we observe on active stars should be associated with CMEs. Such an association can imply uncomfortably high CME mass loss rates of more than 10^-11 Msun/yr for the most active stars. We suggest that, instead, only the more energetic CMEs escape and most are suppressed by strong overlying magnetic field. Here, we investigate the suppression threshold and its implications for CME rates and mass loss on active stars.

  11. X-RAY EMISSION FROM MAGNETIC MASSIVE STARS

    SciTech Connect

    Nazé, Yaël; Petit, Véronique; Rinbrand, Melanie; Owocki, Stan; Cohen, David; Ud-Doula, Asif; Wade, Gregg A.

    2014-11-01

    Magnetically confined winds of early-type stars are expected to be sources of bright and hard X-rays. To clarify the systematics of the observed X-ray properties, we have analyzed a large series of Chandra and XMM-Newton observations, corresponding to all available exposures of known massive magnetic stars (over 100 exposures covering ∼60% of stars compiled in the catalog of Petit et al.). We show that the X-ray luminosity is strongly correlated with the stellar wind mass-loss rate, with a power-law form that is slightly steeper than linear for the majority of the less luminous, lower- M-dot B stars and flattens for the more luminous, higher- M-dot O stars. As the winds are radiatively driven, these scalings can be equivalently written as relations with the bolometric luminosity. The observed X-ray luminosities, and their trend with mass-loss rates, are well reproduced by new MHD models, although a few overluminous stars (mostly rapidly rotating objects) exist. No relation is found between other X-ray properties (plasma temperature, absorption) and stellar or magnetic parameters, contrary to expectations (e.g., higher temperature for stronger mass-loss rate). This suggests that the main driver for the plasma properties is different from the main determinant of the X-ray luminosity. Finally, variations of the X-ray hardnesses and luminosities, in phase with the stellar rotation period, are detected for some objects and they suggest that some temperature stratification exists in massive stars' magnetospheres.

  12. ASCA Spectra of the Central Star of the Orion Nebula: a Magnetic O star

    NASA Astrophysics Data System (ADS)

    Gagne, M.; Caillault, J. P.; Song, I.; Tsuboi, Y.; Linsky, J. L.; Stauffer, J. R.

    1999-04-01

    The discovery of 15-day periodic H-alpha and He II emission on the central star of the Orion Nebula, theta 1 Orionis C (O7 V), suggests that this O star is an oblique magnetic rotator. The subsequent detection of large-amplitude, periodic X-ray variations led Babel and Montmerle to propose that the O star's magnetically confined wind is shocked near the magnetic equator, producing an X-ray cooling disk. In this poster, we present ASCA SIS spectra of the Trapezium obtained at rotational phases 0.01, 0.45, and 0.77. Because many unresolved T Tauri stars and OB stars produce about half the ASCA SIS flux, ROSAT HRI images obtained at similar phases and an ASCA raytrace code have been used to help reconstruct the SIS spectra. By all indications, theta 1 Ori C is a very hot source (kT > 3.0 keV). Suprisingly, we see little or no evidence of variable wind attenuation.

  13. DYNAMO ACTION AND MAGNETIC CYCLES IN F-TYPE STARS

    SciTech Connect

    Augustson, Kyle C.; Toomre, Juri; Brun, Allan Sacha

    2013-11-10

    Magnetic activity and differential rotation are commonly observed features on main-sequence F-type stars. We seek to make contact with such observations and to provide a self-consistent picture of how differential rotation and magnetic fields arise in the interiors of these stars. The three-dimensional magnetohydrodynamic anelastic spherical harmonic code is employed to simulate global-scale convection and dynamo processes in a 1.2 M{sub ☉} F-type star at two rotation rates. The simulations are carried out in spherical shells that encompass most of the convection zone and a portion of the stably stratified radiative zone below it, allowing us to explore the effects a stable zone has upon the morphology of the global-scale magnetic fields. We find that dynamo action with a high degree of time variation occurs in the star rotating more rapidly at 20 Ω{sub ☉}, with the polarity of the mean field reversing on a timescale of about 1600 days. Between reversals, the magnetic energy rises and falls with a fairly regular period, with three magnetic energy cycles required to complete a reversal. The magnetic energy cycles and polarity reversals arise due to a linking of the polar-slip instability in the stable region and dynamo action present in the convection zone. For the more slowly rotating case (10 Ω{sub ☉}), persistent wreaths of magnetism are established and maintained by dynamo action. Compared to their hydrodynamic progenitors, the dynamo states here involve a marked reduction in the exhibited latitudinal differential rotation, which also vary during the course of a cycle.

  14. Dynamo Action and Magnetic Cycles in F-type Stars

    NASA Astrophysics Data System (ADS)

    Augustson, Kyle C.; Brun, Allan Sacha; Toomre, Juri

    2013-11-01

    Magnetic activity and differential rotation are commonly observed features on main-sequence F-type stars. We seek to make contact with such observations and to provide a self-consistent picture of how differential rotation and magnetic fields arise in the interiors of these stars. The three-dimensional magnetohydrodynamic anelastic spherical harmonic code is employed to simulate global-scale convection and dynamo processes in a 1.2 M ⊙ F-type star at two rotation rates. The simulations are carried out in spherical shells that encompass most of the convection zone and a portion of the stably stratified radiative zone below it, allowing us to explore the effects a stable zone has upon the morphology of the global-scale magnetic fields. We find that dynamo action with a high degree of time variation occurs in the star rotating more rapidly at 20 Ω⊙, with the polarity of the mean field reversing on a timescale of about 1600 days. Between reversals, the magnetic energy rises and falls with a fairly regular period, with three magnetic energy cycles required to complete a reversal. The magnetic energy cycles and polarity reversals arise due to a linking of the polar-slip instability in the stable region and dynamo action present in the convection zone. For the more slowly rotating case (10 Ω⊙), persistent wreaths of magnetism are established and maintained by dynamo action. Compared to their hydrodynamic progenitors, the dynamo states here involve a marked reduction in the exhibited latitudinal differential rotation, which also vary during the course of a cycle.

  15. Magnetic white dwarf stars in the Sloan Digital Sky Survey

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Pelisoli, I.; Jordan, S.; Kleinman, S. J.; Koester, D.; Külebi, B.; Peçanha, V.; Castanheira, B. G.; Nitta, A.; Costa, J. E. S.; Winget, D. E.; Kanaan, A.; Fraga, L.

    2013-03-01

    To obtain better statistics on the occurrence of magnetism among white dwarfs, we searched the spectra of the hydrogen atmosphere white dwarf stars (DAs) in the Data Release 7 of the Sloan Digital Sky Survey (SDSS) for Zeeman splittings and estimated the magnetic fields. We found 521 DAs with detectable Zeeman splittings, with fields in the range from around 1 to 733 MG, which amounts to 4 per cent of all DAs observed. As the SDSS spectra have low signal-to-noise ratios, we carefully investigated by simulations with theoretical spectra how reliable our detection of magnetic field was.

  16. Thomson scattering in magnetic fields. [of white dwarf stars

    NASA Technical Reports Server (NTRS)

    Whitney, Barbara

    1989-01-01

    The equation of transfer in Thomson scattering atmospheres with magnetic fields is solved using Monte Carlo methods. Two cases, a plane parallel atmosphere with a magnetic field perpendicular to the atmosphere, and a dipole star, are investigated. The wavelength dependence of polarization from plane-parallel atmosphere is qualitatively similar to that observed in the magnetic white dwarf Grw+70 deg 8247, and the field strength determined by the calculation, 320 MG, is quantitatively similar to that determined from the line spectrum. The dipole model does not resemble the data as well as the single plane-parallel atmosphere.

  17. Magnetic fields in mixed neutron-star-plus-wormhole systems

    SciTech Connect

    Aringazin, Ascar; Dzhunushaliev, Vladimir; Folomeev, Vladimir; Kleihaus, Burkhard; Kunz, Jutta E-mail: v.dzhunushaliev@gmail.com E-mail: b.kleihaus@uni-oldenburg.de

    2015-04-01

    We consider mixed configurations consisting of a wormhole filled by a strongly magnetized isotropic or anisotropic neutron fluid. The nontrivial topology of the spacetime is allowed by the presence of exotic matter. By comparing these configurations with ordinary magnetized neutron stars, we clarify the question of how the presence of the nontrivial topology influences the magnetic field distribution inside the fluid. In the case of an anisotropic fluid, we find new solutions describing configurations, where the maximum of the fluid density is shifted from the center. A linear stability analysis shows that these mixed configurations are unstable.

  18. Magnetically Controlled Spasmodic Accretion during Star Formation. II. Results

    NASA Astrophysics Data System (ADS)

    Tassis, Konstantinos; Mouschovias, Telemachos Ch.

    2005-01-01

    The problem of the late accretion phase of the evolution of an axisymmetric, isothermal magnetic disk surrounding a forming star has been formulated in a companion paper. The ``central sink approximation'' is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 1011 cm-3 and radii smaller than a few AU. Only the electrons are assumed to be attached to the magnetic field lines, and the effects of both negatively and positively charged grains are accounted for. After a mass of 0.1 Msolar accumulates in the central cell (forming star), a series of magnetically driven outflows and associated outward-propagating shocks form in a quasi-periodic fashion. As a result, mass accretion onto the protostar occurs in magnetically controlled bursts. We refer to this process as spasmodic accretion. The shocks propagate outward with supermagnetosonic speeds. The period of dissipation and revival of the outflow decreases in time, as the mass accumulated in the central sink increases. We evaluate the contribution of ambipolar diffusion to the resolution of the magnetic flux problem of star formation during the accretion phase, and we find it to be very significant albeit not sufficient to resolve the entire problem yet. Ohmic dissipation is completely negligible in the disk during this phase of the evolution. The protostellar disk is found to be stable against interchange-like instabilities, despite the fact that the mass-to-flux ratio has temporary local maxima.

  19. Magnetic Doppler Imaging of He-strong star HD 184927

    NASA Astrophysics Data System (ADS)

    Yakunin, I.; Wade, G.; Bohlender, D.; Kochukhov, O.; Tsymbal, V.; Tsymbal

    2014-08-01

    We have employed an extensive new timeseries of Stokes I and V spectra obtained with the ESPaDOnS spectropolarimeter at the 3.6-m Canada-France-Hawaii Telescope to investigate the physical parameters, chemical abundance distributions and magnetic field topology of the slowly-rotating He-strong star HD 184927. We infer a rotation period of 9 d .53071 +/- 0.00120 from Hα, Hβ, LSD magnetic measurements and EWs of helium lines. We used an extensive NLTE TLUSTY grid along with the SYNSPEC code to model the observed spectra and find a new value of luminosity. In this poster we present the derived physical parameters of the star and the results of Magnetic Doppler Imaging analysis of the Stokes I and V profiles. Wide wings of helium lines can be described only under the assumption of the presence of a large, very helium-rich spot.

  20. Magnetized neutron stars with superconducting cores: effect of entrainment

    NASA Astrophysics Data System (ADS)

    Palapanidis, K.; Stergioulas, N.; Lander, S. K.

    2015-09-01

    We construct equilibrium configurations of magnetized, two-fluid neutron stars using an iterative numerical method. Working in Newtonian framework we assume that the neutron star has two regions: the core, which is modelled as a two-component fluid consisting of type-II superconducting protons and superfluid neutrons, and the crust, a region composed of normal matter. Taking a new step towards more complete equilibrium models, we include the effect of entrainment, which implies that a magnetic force acts on neutrons, too. We consider purely poloidal field cases and present improvements to an earlier numerical scheme for solving equilibrium equations, by introducing new convergence criteria. We find that entrainment results in qualitative differences in the structure of field lines along the magnetic axis.

  1. Collapse of magnetized hypermassive neutron stars in general relativity.

    PubMed

    Duez, Matthew D; Liu, Yuk Tung; Shapiro, Stuart L; Shibata, Masaru; Stephens, Branson C

    2006-01-27

    Hypermassive neutron stars (HMNSs)--equilibrium configurations supported against collapse by rapid differential rotation--are possible transient remnants of binary neutron-star mergers. Using newly developed codes for magnetohydrodynamic simulations in dynamical spacetimes, we are able to track the evolution of a magnetized HMNS in full general relativity for the first time. We find that secular angular momentum transport due to magnetic braking and the magnetorotational instability results in the collapse of an HMNS to a rotating black hole, accompanied by a gravitational wave burst. The nascent black hole is surrounded by a hot, massive torus undergoing quasistationary accretion and a collimated magnetic field. This scenario suggests that HMNS collapse is a possible candidate for the central engine of short gamma-ray bursts.

  2. Low-luminosity accretion onto magnetized neutron stars

    NASA Technical Reports Server (NTRS)

    Langer, S. H.; Rappaport, S.

    1982-01-01

    The behavior of matter accreting at low rates (M is less than 10 to the 16th g/s) onto the polar caps of a highly magnetized (B = 10 to the 12th G) neutron star is investigated. Flow solutions are found for the case in which the matter undergoes a stationary collisionless shock. It is found that the cyclotron emission is the dominant energy loss mechanism and can yield continuum spectra resembling those observed from X-ray pulsars. A number of relations among the accretion rate, the surface magnetic field, the shock height, and the characteristic electron and ion temperatures are obtained. For magnetic fields greater than 10 to the 12th G, typical values of KTe are several times the cyclotron energy at the surface of the neutron star. When the field drops below 10 to the 12th G, the electrons become very hot and emit gamma-rays.

  3. Torsional oscillations of neutron stars with highly tangled magnetic fields

    NASA Astrophysics Data System (ADS)

    Sotani, Hajime

    2015-11-01

    To determine the frequencies of magnetic oscillations in neutron stars with highly tangled magnetic fields, we derive the perturbation equations. We assume that the field strength of the global magnetic structure is so small that such fields are negligible compared with tangled fields, which may still be far from a realistic configuration. Then, we systematically examine the spectra of the magnetic oscillations, as varying the magnetic field strength and stellar mass. The frequencies without crust elasticity are completely proportional to the strength of the magnetic field, whose proportionality constant depends strongly on the stellar mass. On the other hand, the oscillation spectra with crust elasticity become more complicated, where the frequencies even for weak magnetic fields are different from the crustal torsional oscillations without magnetic fields. For discussing spectra, the critical field strength can play an important role, and it is determined in such a way that the shear velocity is equivalent to the Alfvén velocity at the crust basis. Additionally, we find that the effect of the crust elasticity can be seen strongly in the fundamental oscillations with a lower harmonic index, ℓ. Unlike the stellar models with a pure dipole magnetic field, we also find that the spectra with highly tangled magnetic fields become discrete, where one can expect many of the eigenfrequencies. Maybe these frequencies could be detected after the violent phenomena breaking the global magnetic field structure.

  4. Magnetic field models of nine CP stars from "accurate" measurements

    NASA Astrophysics Data System (ADS)

    Glagolevskij, Yu. V.

    2013-01-01

    The dipole models of magnetic fields in nine CP stars are constructed based on the measurements of metal lines taken from the literature, and performed by the LSD method with an accuracy of 10-80 G. The model parameters are compared with the parameters obtained for the same stars from the hydrogen line measurements. For six out of nine stars the same type of structure was obtained. Some parameters, such as the field strength at the poles B p and the average surface magnetic field B s differ considerably in some stars due to differences in the amplitudes of phase dependences B e (Φ) and B s (Φ), obtained by different authors. It is noted that a significant increase in the measurement accuracy has little effect on the modelling of the large-scale structures of the field. By contrast, it is more important to construct the shape of the phase dependence based on a fairly large number of field measurements, evenly distributed by the rotation period phases. It is concluded that the Zeeman component measurement methods have a strong effect on the shape of the phase dependence, and that the measurements of the magnetic field based on the lines of hydrogen are more preferable for modelling the large-scale structures of the field.

  5. Rotating and binary relativistic stars with magnetic field

    NASA Astrophysics Data System (ADS)

    Markakis, Charalampos

    We develop a geometrical treatment of general relativistic magnetohydrodynamics for perfectly conducting fluids in Einstein--Maxwell--Euler spacetimes. The theory is applied to describe a neutron star that is rotating or is orbiting a black hole or another neutron star. Under the hypotheses of stationarity and axisymmetry, we obtain the equations governing magnetohydrodynamic equilibria of rotating neutron stars with poloidal, toroidal or mixed magnetic fields. Under the hypothesis of an approximate helical symmetry, we obtain the first law of thermodynamics governing magnetized equilibria of double neutron star or black hole - neutron star systems in close circular orbits. The first law is written as a relation between the change in the asymptotic Noether charge deltaQ and the changes in the area and electric charge of black holes, and in the vorticity, baryon rest mass, entropy, charge and magnetic flux of the magnetofluid. In an attempt to provide a better theoretical understanding of the methods used to construct models of isolated rotating stars and corotating or irrotational binaries and their unexplained convergence properties, we analytically examine the behavior of different iterative schemes near a static solution. We find the spectrum of the linearized iteration operator and show for self-consistent field methods that iterative instability corresponds to unstable modes of this operator. On the other hand, we show that the success of iteratively stable methods is due to (quasi-)nilpotency of this operator. Finally, we examine the integrability of motion of test particles in a stationary axisymmetric gravitational field. We use a direct approach to seek nontrivial constants of motion polynomial in the momenta---in addition to energy and angular momentum about the symmetry axis. We establish the existence and uniqueness of quadratic constants and the nonexistence of quartic constants for stationary axisymmetric Newtonian potentials with equatorial symmetry

  6. WHY ARE THERE NORMAL SLOW ROTATORS AMONG A-TYPE STARS?

    SciTech Connect

    Abt, Helmut A.

    2009-07-15

    I wondered why there are still slowly rotating (V < 120 km s{sup -1}) normal A0-A9 field stars when such stars should have become Ap or Am stars by a diffusion mechanism. My first guess was that this was related to an unusually high binary frequency, but the frequency turned out to be normal, as shown in a paper in preparation. Next, I wondered whether all the slow rotators have had enough time to become Ap or Am stars. That process is rapid for Ap(Si), Ap(HgMn), and Am stars, but slow for Ap(SrCrEu) stars. For Ap(SrCrEu) stars, it takes about half of their main-sequence lifetime to show their abnormality. Under the assumption of a constant formation rate of field A stars, about half of the eventual Ap(SrCrEu) will appear as normal slow rotators. That is why there are still normal slow rotators in A0-A3. That is the main conclusion of this study. I discuss recent doubts about the length of time it takes to form an Ap(SrCr) star and found that the doubts are inappropriate. For the A4-F0 stars, I confirm that all the stars in binaries with orbital periods of 2-10 days became Am stars because their rotational velocities have been reduced by tidal interactions below V = 120 km s{sup -1}, a requirement for diffusion to act. It is also confirmed that all the normal stars in binaries have orbital periods above {approx}100 days because the tidal interactions in such binaries are inadequate to bring the rotational velocities below 120 km s{sup -1}. However, both Am and normal stars occur in binaries with orbital periods between 10 and 100 days, and at present we do not know why.

  7. Hall-drift induced magnetic field instability in neutron stars.

    PubMed

    Rheinhardt, M; Geppert, U

    2002-03-11

    In the presence of a strong magnetic field and under conditions as realized in the crust and the superfluid core of neutron stars, the Hall drift dominates the field evolution. We show by a linear analysis that, for a sufficiently strong large-scale background field depending at least quadratically on position in a plane conducting slab, an instability occurs which rapidly generates small-scale fields. Their growth rates depend on the choice of the boundary conditions, increase with the background field strength, and may reach 10(3) times the Ohmic decay rate. The effect of that instability on the rotational and thermal evolution of neutron stars is discussed.

  8. Surface structure of neutron stars with high magnetic fields

    NASA Technical Reports Server (NTRS)

    Fushiki, I.; Gudmundsson, E. H.; Pethick, C. J.

    1989-01-01

    The equation of state of cold dense matter in strong magnetic fields is calculated in the Thomas-Fermi and Thomas-Fermi-Dirac approximations. For use in the latter calculation, a new expression is derived for the exchange energy of the uniform electron gas in a strong magnetic field. Detailed calculations of the density profile in the surface region of a neutron star are described for a variety of equations of state, and these show that the surface density profile is strongly affected by the magnetic field, irrespective of whether or not matter in a magnetic field has a condensed state bound with respect to isolated atoms. It is also shown that, as a consequence of the field dependence of the screening potential, magnetic fields can significantly increase nuclear reaction rates.

  9. Ap stars with resolved magnetically split lines: Magnetic field determinations from Stokes I and V spectra⋆

    NASA Astrophysics Data System (ADS)

    Mathys, G.

    2017-05-01

    Context. Some Ap stars that have a strong enough magnetic field and a sufficiently low v sini show spectral lines resolved into their magnetically split components. Aims: We present the results of a systematic study of the magnetic fields and other properties of those stars. Methods: This study is based on 271 new measurements of the mean magnetic field modulus ⟨ B ⟩ of 43 stars, 231 determinations of the mean longitudinal magnetic field ⟨ Bz ⟩ and of the crossover ⟨ Xz ⟩ of 34 stars, and 229 determinations of the mean quadratic magnetic field ⟨ Bq ⟩ of 33 stars. Those data were used to derive new values or meaningful lower limits of the rotation periods Prot of 21 stars. Variation curves of the mean field modulus were characterised for 25 stars, the variations of the longitudinal field were characterised for 16 stars, and the variations of the crossover and of the quadratic field were characterised for 8 stars. Our data are complemented by magnetic measurements from the literature for 41 additional stars with magnetically resolved lines. Phase coverage is sufficient to define the curve of variation of ⟨ B ⟩ for 2 of these stars. Published data were also used to characterise the ⟨ Bz ⟩ curves of variation for 10 more stars. Furthermore, we present 1297 radial velocity measurements of the 43 Ap stars in our sample that have magnetically resolved lines. Nine of these stars are spectroscopic binaries for which new orbital elements were derived. Results: The existence of a cut-off at the low end of the distribution of the phase-averaged mean magnetic field moduli ⟨ B ⟩ av of the Ap stars with resolved magnetically split lines, at about 2.8 kG, is confirmed. This reflects the probable existence of a gap in the distribution of the magnetic field strengths in slowly rotating Ap stars, below which there is a separate population of stars with fields weaker than 2 kG. In more than half of the stars with magnetically resolved lines that have a

  10. Stellar parameters for Pop II A-type stars from IUE spectra and new-ODF ATLAS9 model atmospheres

    NASA Astrophysics Data System (ADS)

    Castelli, F.; Cacciari, C.

    2001-12-01

    Stellar parameters for twenty-seven field horizontal branch A-type stars, a post-AGB star (BD +32 2188), and a possible cool sdB star (BD +00 0145) were obtained by fitting the whole IUE energy distributions taken from the IUE-INES archive to the ultraviolet energy distributions predicted by new-ODF ATLAS9 model atmospheres, which include the Lyman-alpha H-H+ and H-H quasi-molecular absorptions near 1400 Å and 1600 Å. The sample of stars was extensively studied by Kinman et al. (2000), who derived stellar parameters for them by using visual observations and also an ultraviolet color index. The effective temperatures obtained by fitting the IUE spectra to the new-ODF models agree with T_eff derived by Kinman et al. (2000) for most of the stars in the sample. The gravities from UV agree with those from Kinman et al. (2000) for stars hotter than about 8700 K, while they are lower, on average, by 0.3 dex for the cooler stars. The same discrepancy is present when log g from the ultraviolet energy distribution is compared with log g from the visible energy distribution. The difference is insensitive to reddening, microturbulent velocity, metallicity, or mixing-length parameter for the treatment of the convection. Figures A.1 to A.15 are only available in electronic form at http://www.edpsciences.org

  11. Chiral magnetic effect in protoneutron stars and magnetic field spectral evolution

    SciTech Connect

    Sigl, Günter; Leite, Natacha E-mail: natacha.leite@desy.de

    2016-01-01

    We investigate the evolution of the chiral magnetic instability in a protoneutron star and compute the resulting magnetic power and helicity spectra. The instability may act during the early cooling phase of the hot protoneutron star after supernova core collapse, where it can contribute to the buildup of magnetic fields of strength up to the order of 10{sup 14} G. The maximal field strengths generated by this instability, however, depend considerably on the temperature of the protoneutron star, on density fluctuations and turbulence spectrum of the medium. At the end of the hot cooling phase the magnetic field tends to be concentrated around the submillimeter to cm scale, where it is subject to slow resistive damping.

  12. Gravitational waves from color-magnetic "mountains" in neutron stars.

    PubMed

    Glampedakis, K; Jones, D I; Samuelsson, L

    2012-08-24

    Neutron stars may harbor the true ground state of matter in the form of strange quark matter. If present, this type of matter is expected to be a color superconductor, a consequence of quark pairing with respect to the color and flavor degrees of freedom. The stellar magnetic field threading the quark core becomes a color-magnetic admixture and, in the event that superconductivity is of type II, leads to the formation of color-magnetic vortices. In this Letter, we show that the volume-averaged color-magnetic vortex tension force should naturally lead to a significant degree of nonaxisymmetry in systems such as radio pulsars. We show that gravitational radiation from such color-magnetic "mountains" in young pulsars, such as the Crab and Vela, could be observable by the future Einstein Telescope, thus, becoming a probe of paired quark matter in neutron stars. The detectability threshold can be pushed up toward the sensitivity level of Advanced LIGO if we invoke an interior magnetic field about a factor ten stronger than the surface polar field.

  13. Gravitational Waves from Color-Magnetic ``Mountains'' in Neutron Stars

    NASA Astrophysics Data System (ADS)

    Glampedakis, K.; Jones, D. I.; Samuelsson, L.

    2012-08-01

    Neutron stars may harbor the true ground state of matter in the form of strange quark matter. If present, this type of matter is expected to be a color superconductor, a consequence of quark pairing with respect to the color and flavor degrees of freedom. The stellar magnetic field threading the quark core becomes a color-magnetic admixture and, in the event that superconductivity is of type II, leads to the formation of color-magnetic vortices. In this Letter, we show that the volume-averaged color-magnetic vortex tension force should naturally lead to a significant degree of nonaxisymmetry in systems such as radio pulsars. We show that gravitational radiation from such color-magnetic “mountains” in young pulsars, such as the Crab and Vela, could be observable by the future Einstein Telescope, thus, becoming a probe of paired quark matter in neutron stars. The detectability threshold can be pushed up toward the sensitivity level of Advanced LIGO if we invoke an interior magnetic field about a factor ten stronger than the surface polar field.

  14. Dynamo action and magnetic activity of the giant star Pollux

    NASA Astrophysics Data System (ADS)

    Brun, Allan Sacha; Palacios, Ana

    2015-08-01

    Recent spectropolarimetric observations of the giant star Pollux have revealed that it possesses a weak global magnetic field of the order of a Gauss. Using 3-D nonlinear MHD simulations performed with the ASH code we study the source of this global magnetic field in this slowly rotating giant star (Omega*=Omega_sun/20). We find that the extended convective envelope is able to generate a multi-scales magnetic field reaching of the order of 10% of the kinetic energy contained in the envelope. This global field acts such as to suppress the strong differential rotation present in the purely hydrodynamical progenitor simulation. When filtering the large scale magnetic field components (dipole, quadrupole) we find magnetic field of the order of a few Gauss, hence in qualitative agreeement with observations. Our study confirms that such slowly rotating convective giants are likely to possess global magnetic field maintained through contemporaneous dynamo action and not as the vestige of their past main sequence activity.

  15. Rotation and magnetic field in the Be star omega Orionis

    NASA Astrophysics Data System (ADS)

    Neiner, C.; Hubert, A.-M.; Frémat, Y.; Floquet, M.; Jankov, S.; Preuss, O.; Henrichs, H. F.; Zorec, J.

    2003-10-01

    omega Ori is a B2IIIe star for which rotational modulation and non-radial pulsations (NRP) have been recently investigated from two independent observational campaigns in 1998 and 1999. Putting the data of these 2 campaigns together, and adding data obtained in 2001, we search for multiperiodicity in the line profile variations and evidence for outbursts. From new spectropolarimetric data obtained at the Télescope Bernard Lyot (TBL, Pic du Midi, France) in 2001 we also measure the Stokes V parameter in the polarised light. We find evidence for the presence of a weak magnetic field in omega Ori sinusoidally varying with a period of 1.29 d. The equivalent widths (EW) of the wind sensitive UV resonance lines also show a variation with the same period, which we identify as the rotational period of the star. We propose an oblique rotator model and derive Bpol =530 +/- 230 G to explain the observations. Moreover, we carry out an abundance analysis and find the star to be N-enriched, a property which is shared with other magnetic stars. We propose omega Ori as the first known classical Be star hosting a magnetic field. Based on observations obtained using the Musicos spectropolarimeter at the Observatoire du Pic du Midi (France), during the MuSiCoS 98 campaign \\citep{neiner}, and by \\cite{balona}. Based on INES data from the International Ultraviolet Explorer (IUE) satellite. Table 7 is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/409/275

  16. The link between magnetic field orientations and star formation rates

    NASA Astrophysics Data System (ADS)

    Li, Hua-Bai; Jiang, Hangjin; Fan, Xiaodan; Gu, Qilao; Zhang, Yapeng

    2017-08-01

    Understanding star formation rates (SFRs) is a central goal of modern star formation models, which mainly involve gravity, turbulence and, in some cases, magnetic fields (B-fields)1,2. However, a connection between B-fields and SFRs has never been observed. Here, a comparison between the surveys of SFRs3,4 and a study of cloud-field alignment5—which revealed a bimodal (parallel or perpendicular) alignment—shows consistently lower SFRs per solar mass for clouds almost perpendicular to the B-fields. This is evidence of B-fields being a primary regulator of SFRs. The perpendicular alignment possesses a significantly higher magnetic flux than the parallel alignment and thus a stronger support of the gas against self-gravity. This results in overall lower masses of the fragmented components, which are in agreement with lower SFRs.

  17. What is the unusual material orbiting the dustiest main sequence A-type stars HD 131488 and HD 121191?

    NASA Astrophysics Data System (ADS)

    Melis, Carl

    2015-10-01

    Only a small percentage of main sequence stars exhibit excess mid-infrared emission indicative of substantial quantities of warm (T >~ 300 K), inner planetary system material that likely originated in recent transient collisional processes. Detailed study of these events can provide us with insight into how rocky terrestrial-like planets form and evolve through collisional pathways. We have identified two young A-type stars with mid-infrared luminosity brighter than and spectrally distinct from that at any other known main-sequence A-type star. T-ReCS N-band and IRTF SpeX spectroscopy combined with IRAS, Herschel, WISE, and T-ReCS photometric measurements indicate that these stars host two distinct infrared emitting regions, one with characteristic temperatures of >300 K (equivalent to temperatures inside 1 AU in the solar system) and a second of ~100 K (equivalent to the temperature near Saturn). The T-ReCS N-band spectra present an enigma: a putative emission feature with peak wavelength near 6-7 microns is not reproducible with common silicate species. SOFIA-FORCAST narrow-band imaging is the only means available to settling the identity of these strange emission features and hence clarify the nature of the inner planetary system material around these two stars.

  18. Discovery of two new bright magnetic B stars: i Car and Atlas

    NASA Astrophysics Data System (ADS)

    Neiner, Coralie; Buysschaert, Bram; Oksala, Mary E.; Blazère, Aurore

    2015-11-01

    The BRITE (BRIght Target Explorer) constellation of nanosatellites performs seismology of bright stars via high-precision photometry. In this context, we initiated a high-resolution, high signal-to-noise, high-sensitivity, spectropolarimetric survey of all stars brighter than V = 4. The goal of this survey is to detect new bright magnetic stars and provide prime targets for both detailed magnetic studies and asteroseismology with BRITE. Circularly polarized spectra were acquired with Narval at TBL (Bernard Lyot Telescope, France) and HARPSpol at ESO (European Southern Observatory) in La Silla (Chile). We discovered two new magnetic B stars: the B3V star i Car and the B8V component of the binary star Atlas. Each star was observed twice to confirm the magnetic detections and check for variability. These bright magnetic B stars are prime targets for asteroseismology and for flux-demanding techniques, such as interferometry.

  19. Spin-up and magnetic fields in DQ Her stars

    NASA Technical Reports Server (NTRS)

    Lamb, D. Q.; Patterson, J.

    1983-01-01

    A detailed comparison of the spin-up behavior of the recently discovered faint long-period X-ray sources, using the accretion torque theory of Ghosh and Lamb has been carried out. It is confirmed that these sources are degenerate dwarfs. It is found that they have magnetic fields B about 1 billion G, an order of magnitude smaller than those of AM Her stars. On this basis, it is predicted that they will show significant polarization in the infrared.

  20. Electron transport through nuclear pasta in magnetized neutron stars

    NASA Astrophysics Data System (ADS)

    Yakovlev, D. G.

    2015-10-01

    We present a simple model for electron transport in a possible layer of exotic nuclear clusters (in the so-called nuclear pasta layer) between the crust and liquid core of a strongly magnetized neutron star. The electron transport there can be strongly anisotropic and gyrotropic. The anisotropy is produced by different electron effective collision frequencies along and across local symmetry axis in domains of exotic ordered nuclear clusters and by complicated effects of the magnetic field. We also calculate averaged kinetic coefficients in case local domains are freely oriented. Possible applications of the obtained results and open problems are outlined.

  1. Observations of magnetic fields on solar-type stars

    NASA Technical Reports Server (NTRS)

    Marcy, G. W.

    1982-01-01

    Magnetic-field observations were carried out for 29 G and K main-sequence stars. The area covering-factors of magnetic regions tends to be greater in the K dwarfs than in the G dwarfs. However, no spectral-type dependence is found for the field strengths, contrary to predictions that pressure equilibrium with the ambient photospheric gas pressure would determine the surface field strengths. Coronal soft X-ray fluxes from the G and K dwarfs correlate well with the fraction of the stellar surface covered by magnetic regions. The dependence of coronal soft X-ray fluxes on photospheric field strengths is consistent with Stein's predicted generation-rates for Alfven waves. These dependences are inconsistent with the one dynamo model for which a specific prediction is offered. Finally, time variability of magnetic fields is seen on the two active stars that have been extensively monitored. Significant changes in magnetic fields are seen to occur on timescales as short as one day.

  2. MAGNETIC ENERGY PRODUCTION BY TURBULENCE IN BINARY NEUTRON STAR MERGERS

    SciTech Connect

    Zrake, Jonathan; MacFadyen, Andrew I.

    2013-06-01

    The simultaneous detection of electromagnetic and gravitational wave emission from merging neutron star binaries would greatly aid in their discovery and interpretation. By studying turbulent amplification of magnetic fields in local high-resolution simulations of neutron star merger conditions, we demonstrate that magnetar-level ({approx}> 10{sup 16} G) fields are present throughout the merger duration. We find that the small-scale turbulent dynamo converts 60% of the randomized kinetic energy into magnetic fields on a merger timescale. Since turbulent magnetic energy dissipates through reconnection events that accelerate relativistic electrons, turbulence may facilitate the conversion of orbital kinetic energy into radiation. If 10{sup -4} of the {approx}10{sup 53} erg of orbital kinetic available gets processed through reconnection and creates radiation in the 15-150 keV band, then the fluence at 200 Mpc would be 10{sup -7} erg cm{sup -2}, potentially rendering most merging neutron stars in the advanced LIGO and Virgo detection volumes detectable by Swift BAT.

  3. GB 790305 as a very strongly magnetized neutron star

    NASA Technical Reports Server (NTRS)

    Paczynski, Bohdan

    1992-01-01

    The March 5 1979 event was the strongest gamma-ray burst ever observed. Its location in the sky is known with an accuracy of about 10 arcsec, and it coincides with the N49 supernova remnant in the Large Magellanic Cloud. The main burst was followed by a soft tail with the periodic 8 s modulation, and 16 soft gamma events over the following few years. If the source is a magnetic neutron star with the 8 s rotation period and the age of about 10 exp 4 years as indicated by N49 then the field strength of about 5 x 10 exp 14 gauss is implied. The corresponding critical luminosity is about 10 exp 4 L(Edd), as the electron scattering opacity is suppressed by the strong magnetic field. This luminosity is consistent with the observed peak flux of the soft tail and the soft repeaters. The soft spectrum may be approximated with the photospheric emission at kT(eff) = 17 keV. The corresponding photospheric radius is about 14 km, compatible with a neutron star hypothesis. The total magnetic energy of the star is about 4 x 10 exp 46 erg, more than enough to power the March 5 event and all its repeaters.

  4. Tracing magnetic fields and identifying star formation with velocity gradients

    NASA Astrophysics Data System (ADS)

    Lazarian, Alex; Gonzalez Casanova, Diego; Yuen, Ka Ho

    2017-01-01

    We are presenting a new technique of tracing magnetic fields utilizing Doppler broadened spectral lines. We demonstrate that for subAlfvenic turbulence, i.e. for regions with turbulence velocities less than Alfven speed, the velocity gradients (VGs) are well aligned with magnetic fields both in 3D data cubes and in synthetic observations. For the latter case the VGs are calculated using velocity centroids and their alignment with the projected magnetic field is studied. We demonstraed this by comparing the HI data with the PLANCK polarization maps. We conclude that velocity gradients present a new promissing way of studying magnetic fields in diffuse media. We also explored the properties of VGs in dense gas and found that starting with a particular density threshold value the VGs tend to be perpendicular to magnetic fields. We also compare the alignment of VGs and the density gradients (DGs) and find that these measures are well aligned in the absense of self-gravity, although in diffuse regions the VGs trace magnetic field better than the DGs. The advantage of VGs for tracing magnetic fields gets more obvious as the Mach number increases. Self-gravity acts differently on the VGs and the DGs. For self-gravitating regions the VGs and the DGs tend to get orthogonal to each other, revealing the the regions of ongoing star formation. This misalignment of the VGs and DGs is also evident in synthetic observations. We conclude that the VGs present(a) a new promising way of tracing magnetic field in diffuse media, (b) in combination with polarimety they reveal shocked gas,(c) in combination with the DGs they reveal star forming regions.

  5. Metal abundances of A-type stars in galactic clusters. III. alpha Persei: new results

    NASA Astrophysics Data System (ADS)

    Hui-Bon-Hoa, A.

    1999-03-01

    We complete our study (Hui-Bon-Hoa et al. 1997) of the abundances of Mg, Ca, Sc, Cr, Fe, and Ni in alpha Per cluster stars using new spectra of two stars of the previous sample and also adding two more objects. The new spectra of BD +48°894 and HD 21527 show that the former has an almost solar composition whereas the latter is an Am star with the iron peak elements being all enhanced and Ca marginally underabundant. As for the additional stars of this study, HD 19954 has a pattern very close to solar, and HD 20135 is an SB2 system where both components seem to be Am stars: Ca and Sc are strongly deficient; Cr and Ni are overabundant in one component. A more thorough study is needed for confirmation. Based on observations collected at the Observatoire de Haute-Provence (France)

  6. Tiny Stars, Strong Fields: Exploring the Origin of Intense Magnetism in M Stars

    NASA Astrophysics Data System (ADS)

    Toomre, Juri

    The M-type stars are becoming dominant targets in searches for Earth-like planets that could occupy their habitable zones. The low masses and luminosities of M-dwarf central stars make them very attractive for such exoplanetary hunts. The habitable zone of M dwarfs is close to the star due to their low luminosity. Thus possibly habitable planets will have short orbital periods, making their detection feasible both with the transit method (used by Kepler, K2 and soon with TESS) and with the radial velocity approaches. Yet habitability on a planet likely requires both solid surfaces and atmospheres, but also a favorable radiation environment. It is here that the M-dwarf central stars raise major theoretical puzzles, for many of them exhibit remarkably intense and frequent flaring, despite their modest intrinsic luminosities. The super-flares release their energy both in white light and in X-rays, and can be thousands of times brighter than the strongest solar flares. Such striking events must have magnetic origins, likely from fields built by convective dynamos operating in their interiors. Further, recent observations suggest that the surface of some M stars is carpeted with magnetic fields of 3 kG or more. Such field strengths are reminiscent of a sunspot, but here instead cover much of the stellar surface. With M stars now taking center stage in the search for Earthlike planets, it is crucial to begin to understand how convective dynamos may be able to build intense magnetic fields involved with super-flares and vast star spots, and how they depend upon the mass and rotation rate of these stars. We propose to use major 3-D MHD simulations with our Anelastic Spherical Harmonic (ASH) code to study the coupling of turbulent convection, rotation, and magnetism within full spherical domains such as the interior of an M dwarf. This permits the exploration of the magnetic dynamos that must be responsible for the evolving magnetism and intense activity of many M dwarfs

  7. The Magnetic Furnace: Intense Core Dynamos in B Stars

    NASA Astrophysics Data System (ADS)

    Augustson, Kyle C.; Brun, Allan Sacha; Toomre, Juri

    2016-10-01

    The dynamo action achieved in the convective cores of main-sequence massive stars is explored here through three-dimensional (3D) global simulations of convective core dynamos operating within a young 10 {M}⊙ B-type star, using the anelastic spherical harmonic code. These simulations capture the inner 65% of this star by radius, encompassing the convective nuclear-burning core (about 23% by radius) and a portion of the overlying radiative envelope. Eight rotation rates are considered, ranging from 0.05% to 16% of the surface breakup velocity, thereby capturing both convection that barely senses the effects of rotation and other situations in which the Coriolis forces are prominent. The vigorous dynamo action realized within all of these turbulent convective cores builds magnetic fields with peak strengths exceeding a megagauss, with the overall magnetic energy (ME) in the faster rotators reaching super-equipartition levels compared to the convective kinetic energy (KE). The core convection typically involves turbulent columnar velocity structures roughly aligned with the rotation axis, with magnetic fields threading through these rolls and possessing complex linkages throughout the core. The very strong fields are able to coexist with the flows without quenching them through Lorentz forces. The velocity and magnetic fields achieve such a state by being nearly co-aligned, and with peak magnetic islands being somewhat displaced from the fastest flows as the intricate evolution proceeds. As the rotation rate is increased, the primary force balance shifts from nonlinear advection balancing Lorentz forces to a magnetostrophic balance between Coriolis and Lorentz forces.

  8. Detection of a red supergiant progenitor star of a type II-plateau supernova.

    PubMed

    Smartt, Stephen J; Maund, Justyn R; Hendry, Margaret A; Tout, Christopher A; Gilmore, Gerard F; Mattila, Seppo; Benn, Chris R

    2004-01-23

    We present the discovery of a red supergiant star that exploded as supernova 2003gd in the nearby spiral galaxy M74. The Hubble Space Telescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9 months before the supernova explosion, and subsequent HST images confirm the positional coincidence of the supernova with a single resolved star that is a red supergiant of 8(+4)(-2) solar masses. This confirms both stellar evolution models and supernova theories predicting that cool red supergiants are the immediate progenitor stars of type II-plateau supernovae.

  9. Magnetic fields and star formation: New techniques and instrumentation

    NASA Astrophysics Data System (ADS)

    Hezareh, Talayeh

    Understanding the process of star formation is one of the fundamental challenges of modern astrophysics. Theoretical studies suggest that magnetic fields may play an important role in the formation and fragmentation of molecular clouds as well as for the support of clouds against gravitational collapse. Confirmation of these theoretical predictions can only be made through observations of magnetic field strengths and morphologies in star-forming regions. This thesis covers observational and instrumentation projects aimed at measuring the strength of the magnetic field in DR21(OH), an active star-forming region in the Galaxy. Chapter 1 provides an introduction on the physical conditions in molecular clouds and the problem of inefficient star formation observed in galaxies. Basics of astronomical polarimetry and its applications in magnetic field measurements are also discussed. Chapter 2 presents a new method for the simultaneous determination of the cosmic ray ionization rate of hydrogen molecules, zH2 , and the ionization fraction, chie, in DR21(OH) since the determination of the latter is essential in precisely estimating the magnetic strength. A simple network of chemical reactions dominant in the creation and destruction of two coexistent ions, HCNH+ and HCO +, is used in conjunction with observed pairs of rotational transitions of several molecular species in order to determine the electron and the H 3+ abundances. The cosmic ray ionization rate is then calculated knowing that in dark clouds it governs the rate of creation of H3+. The significance of our method lies in the ability to determine the H3+ abundance and chi e directly from observations, and estimate zH2 accordingly. Our results, zH2 = 3.1 x 10-18 s-1 and chi e = 3.2 x 10-8, are consistent with recent results in other objects. Chapter 3 presents a study based on a new method by Li & Houde (2008) towards measuring the strength of interstellar magnetic fields in turbulent clouds. We investigate the

  10. MAGNETIC ACTIVITY CYCLES IN THE EXOPLANET HOST STAR {epsilon} ERIDANI

    SciTech Connect

    Metcalfe, T. S.; Mathur, S.; Buccino, A. P.; Mauas, P. J. D.; Petrucci, R.; Brown, B. P.; Soderblom, D. R.; Henry, T. J.; Hall, J. C.; Basu, S.

    2013-02-01

    The active K2 dwarf {epsilon} Eri has been extensively characterized both as a young solar analog and more recently as an exoplanet host star. As one of the nearest and brightest stars in the sky, it provides an unparalleled opportunity to constrain stellar dynamo theory beyond the Sun. We confirm and document the 3-year magnetic activity cycle in {epsilon} Eri originally reported by Hatzes and coworkers, and we examine the archival data from previous observations spanning 45 years. The data show coexisting 3-year and 13-year periods leading into a broad activity minimum that resembles a Maunder minimum-like state, followed by the resurgence of a coherent 3-year cycle. The nearly continuous activity record suggests the simultaneous operation of two stellar dynamos with cycle periods of 2.95 {+-} 0.03 years and 12.7 {+-} 0.3 years, which, by analogy with the solar case, suggests a revised identification of the dynamo mechanisms that are responsible for the so-called 'active' and 'inactive' sequences as proposed by Boehm-Vitense. Finally, based on the observed properties of {epsilon} Eri, we argue that the rotational history of the Sun is what makes it an outlier in the context of magnetic cycles observed in other stars (as also suggested by its Li depletion), and that a Jovian-mass companion cannot be the universal explanation for the solar peculiarities.

  11. Absorption Features in Spectra of Magnetized Neutron Stars

    SciTech Connect

    Suleimanov, V.; Hambaryan, V.; Neuhaeuser, R.; Potekhin, A. Y.; Pavlov, G. G.; Adelsberg, M. van; Werner, K.

    2011-09-21

    The X-ray spectra of some magnetized isolated neutron stars (NSs) show absorption features with equivalent widths (EWs) of 50-200 eV, whose nature is not yet well known.To explain the prominent absorption features in the soft X-ray spectra of the highly magnetized (B{approx}10{sup 14} G) X-ray dim isolated NSs (XDINSs), we theoretically investigate different NS local surface models, including naked condensed iron surfaces and partially ionized hydrogen model atmospheres, with semi-infinite and thin atmospheres above the condensed surface. We also developed a code for computing light curves and integral emergent spectra of magnetized neutron stars with various temperature and magnetic field distributions over the NS surface. We compare the general properties of the computed and observed light curves and integral spectra for XDINS RBS 1223 and conclude that the observations can be explained by a thin hydrogen atmosphere above the condensed iron surface, while the presence of a strong toroidal magnetic field component on the XDINS surface is unlikely.We suggest that the harmonically spaced absorption features in the soft X-ray spectrum of the central compact object (CCO) 1E 1207.4-5209 (hereafter 1E 1207) correspond to peaks in the energy dependence of the free-free opacity in a quantizing magnetic field, known as quantum oscillations. To explore observable properties of these quantum oscillations, we calculate models of hydrogen NS atmospheres with B{approx}10{sup 10}-10{sup 11} G(i.e., electron cyclotron energy E{sub c,e}{approx}0.1-1 keV) and T{sub eff} = 1-3 MK. Such conditions are thought to be typical for 1E 1207. We show that observable features at the electron cyclotron harmonics with EWs {approx_equal}100-200 eV can arise due to these quantum oscillations.

  12. A magnetic study of spotted UV Ceti flare stars and related late-type dwarfs

    NASA Technical Reports Server (NTRS)

    Vogt, S. S.

    1980-01-01

    A multichannel photoelectric Zeeman analyzer has been used to investigate the magnetic nature of the spotted UV Ceti flare stars. Magnetic observations were obtained on a sample of 19 program objects, of which 5 were currently spotted dKe-dMe stars, 7 were normal dK-dM stars, 7 were UV Ceti flare stars, and 1 was a possible post-T Tauri star. Contrary to most previously published observations and theoretical expectations, no magnetic fields were detected on any of these objects from either the absorption lines or the H-alpha emission line down to an observational uncertainty level of 100-160 gauss (standard deviation).

  13. Magnetic Fields Sculpt Narrow Jets From Dying Star

    NASA Astrophysics Data System (ADS)

    2006-03-01

    Molecules spewed outward from a dying star are confined into narrow jets by a tightly-wound magnetic field, according to astronomers who used the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope to study an old star about 8,500 light-years from Earth. Magnetic Field Around Jet Artist's Conception Shows Tightly-Wound Magnetic Field Confining Jet CREDIT: NRAO/AUI/NSF (Click on image for larger version) The star, called W43A, in the constellation Aquila, is in the process of forming a planetary nebula, a shell of brightly-glowing gas lit by the hot ember into which the star will collapse. In 2002, astronomers discovered that the aging star was ejecting twin jets of water molecules. That discovery was a breakthrough in understanding how many planetary nebulae are formed into elongated shapes. "The next question was, what is keeping this outpouring of material confined into narrow jets? Theoreticians suspected magnetic fields, and we now have found the first direct evidence that a magnetic field is confining such a jet," said Wouter Vlemmings, a Marie Curie Fellow working at the Jodrell Bank Observatory of the University of Manchester in England. "Magnetic fields previously have been detected in jets emitted by quasars and protostars, but the evidence was not conclusive that the magnetic fields were actually confining the jets. These new VLBA observations now make that direct connection for the very first time," Vlemmings added. By using the VLBA to study the alignment, or polarization, of radio waves emitted by water molecules in the jets, the scientists were able to determine the strength and orientation of the magnetic field surrounding the jets. "Our observations support recent theoretical models in which magnetically-confined jets produce the sometimes-complex shapes we see in planetary nebulae," said Philip Diamond, also of Jodrell Bank Observatory. During their "normal" lives, stars similar to our Sun are powered by the nuclear fusion

  14. Super-Eddington accretion on to a magnetized neutron star

    NASA Astrophysics Data System (ADS)

    Chashkina, Anna; Abolmasov, Pavel; Poutanen, Juri

    2017-09-01

    Most of ultraluminous X-ray sources are thought to be objects accreting above their Eddington limits. In the recently identified class of ultraluminous X-ray pulsars, accretor is a neutron star and thus has a fairly small mass with a small Eddington limit. The accretion disc structure around such an object affects important observables such as equilibrium period, period derivative and the size of the magnetosphere. We propose a model of a nearly standard accretion disc interacting with the magnetosphere only in a thin layer near the inner disc rim. Our calculations show that the size of the magnetosphere may be represented as the classical Alfvén radius times a dimensionless factor ξ which depends only on the disc thickness. In the case of radiation-pressure-dominated disc, the size of the magnetosphere does not depend on the mass accretion rate. In general, increasing the disc thickness leads to a larger magnetosphere size in units of the Alfvén radius. For large enough mass accretion rates and magnetic moments, it is important to take into account not only the pressure of the magnetic field and the radiation pressure inside the disc, but also the pressure of the radiation produced close to the surface of the neutron star in accretion column. The magnetospheric size may increase by up to factor of 2 as a result of the effects related to the disc thickness and the irradiation from the central source. Accounting for these effects reduces the estimate of the neutron star magnetic moment by a factor of several orders.

  15. A Study of Magnetic CP Stars in Open Clusters and Associations with the 6-m Telescope

    NASA Astrophysics Data System (ADS)

    Romanyuk, I. I.; Semenko, E. A.; Yakunin, I. A.; Kudryavtsev, D. O.

    2017-06-01

    The study of magnetic CP stars in groups of different ages allows us to obtain data on the origin and evolution of large-scale magnetic fields. We selected 17 groups for observation with the 6-m telescope. Here we draw first conclusions from the study of the Orion OB1 association. Six new magnetic stars in it are added to those seventeen that had been known earlier, ten more CP stars were suspected to have fields. A complex structure of the magnetic field in the star HD 34736 has been found, which is indicative of its fossil origin.

  16. What is the magnetic field distribution for the equation of state of magnetized neutron stars?

    NASA Astrophysics Data System (ADS)

    Dexheimer, V.; Franzon, B.; Gomes, R. O.; Farias, R. L. S.; Avancini, S. S.; Schramm, S.

    2017-10-01

    In this Letter, we report a realistic calculation of the magnetic field profile for the equation of state inside strongly magnetized neutron stars. Unlike previous estimates, which are widely used in the literature, we find that magnetic fields increase relatively slowly with increasing baryon chemical potential (or baryon density) of magnetized matter. More precisely, the increase is polynomial instead of exponential, as previously assumed. Through the analysis of several different realistic models for the microscopic description of stellar matter (including hadronic, hybrid and quark models) combined with general relativistic solutions endowed with a poloidal magnetic field obtained by solving Einstein-Maxwell's field equations in a self-consistent way, we generate a phenomenological fit for the magnetic field distribution in the stellar polar direction to be used as input in microscopic calculations.

  17. Hydrogen in strong magnetic fields in neutron star surfaces

    NASA Astrophysics Data System (ADS)

    Salpeter, Edwin E.

    1998-12-01

    In magnetic fields of very much more than 0953-8984/10/49/017/img1 G, polyatomic hydrogen molecules, in the form of long chains, are stable. In neutron star surfaces, fields of 0953-8984/10/49/017/img2 G are commonplace and 0953-8984/10/49/017/img3 G has been reported. Liquid hydrogen can form at the higher field with a zero-pressure density of about 0953-8984/10/49/017/img4. At these densities, hydrogen can burn to helium by pycnonuclear reactions even at low temperatures - the `real cold fusion'.

  18. Radiatively driven winds from magnetic, fast-rotating stars

    NASA Technical Reports Server (NTRS)

    Nerney, S.

    1986-01-01

    An analytical procedure is developed to solve the magnetohydrodynamic equations for the stellar wind problem in the strong-magnetic field, optically thick limit for hot stars. The slow-mode, Alfven, and fast-mode critical points are modified by the radiation terms in the force equation but in a manner that can be treated relatively easily. Once the velocities at the critical points and the distances to the points are known, the streamline constants are determined in a straight-forward manner. This allows the structure of the wind to be elucidated without recourse to complicated computational schemes.

  19. Simulated magnetic field expulsion in neutron star cores

    NASA Astrophysics Data System (ADS)

    Elfritz, J. G.; Pons, J. A.; Rea, N.; Glampedakis, K.; Viganò, D.

    2016-03-01

    The study of long-term evolution of neutron star (NS) magnetic fields is key to understanding the rich diversity of NS observations, and to unifying their nature despite the different emission mechanisms and observed properties. Such studies in principle permit a deeper understanding of the most important parameters driving their apparent variety, e.g. radio pulsars, magnetars, X-ray dim isolated NSs, gamma-ray pulsars. We describe, for the first time, the results from self-consistent magnetothermal simulations considering not only the effects of the Hall-driven field dissipation in the crust, but also adding a complete set of proposed driving forces in a superconducting core. We emphasize how each of these core-field processes drive magnetic evolution and affect observables, and show that when all forces are considered together in vectorial form, the net expulsion of core magnetic flux is negligible, and will have no observable effect in the crust (consequently in the observed surface emission) on megayear time-scales. Our new simulations suggest that strong magnetic fields in NS cores (and the signatures on the NS surface) will persist long after the crustal magnetic field has evolved and decayed, due to the weak combined effects of dissipation and expulsion in the stellar core.

  20. Evidence of magnetic field decay in massive main-sequence stars

    NASA Astrophysics Data System (ADS)

    Fossati, L.; Schneider, F. R. N.; Castro, N.; Langer, N.; Simón-Díaz, S.; Müller, A.; de Koter, A.; Morel, T.; Petit, V.; Sana, H.; Wade, G. A.

    2016-08-01

    A significant fraction of massive main-sequence stars show strong, large-scale magnetic fields. The origin of these fields, their lifetimes, and their role in shaping the characteristics and evolution of massive stars are currently not well understood. We compile a catalogue of 389 massive main-sequence stars, 61 of which are magnetic, and derive their fundamental parameters and ages. The two samples contain stars brighter than magnitude 9 in the V-band and range in mass between 5 and 100 M⊙. We find that the fractional main-sequence age distribution of all considered stars follows what is expected for a magnitude limited sample, while that of magnetic stars shows a clear decrease towards the end of the main sequence. This dearth of old magnetic stars is independent of the choice of adopted stellar evolution tracks, and appears to become more prominent when considering only the most massive stars. We show that the decreasing trend in the distribution is significantly stronger than expected from magnetic flux conservation. We also find that binary rejuvenation and magnetic suppression of core convection are unlikely to be responsible for the observed lack of older magnetic massive stars, and conclude that its most probable cause is the decay of the magnetic field, over a time span longer than the stellar lifetime for the lowest considered masses, and shorter for the highest masses. We then investigate the spin-down ages of the slowly rotating magnetic massive stars and find them to exceed the stellar ages by far in many cases. The high fraction of very slowly rotating magnetic stars thus provides an independent argument for a decay of the magnetic fields.

  1. Stellar convection theory. III - Dynamical coupling of the two convection zones in A-type stars by penetrative motions

    NASA Technical Reports Server (NTRS)

    Latour, J.; Toomre, J.; Zahn, J.-P.

    1981-01-01

    The thermal convection occurring over many density scale heights in an A-type star outer envelope, encompassing both the hydrogen and helium convectively unstable zones, is examined by means of anelastic modal equations. The single-mode anelastic equations for such compressible convection display strong overshooting of the motions into adjacent radiative zones, which would preclude diffusive separation of elements in the supposedly quiescent region between the two unstable zones. In addition, the anelastic solutions reveal that the two zones of convective instability are dynamically coupled by the overshooting motions. The two solutions that the nonlinear single-mode equations admit for the same horizontal wavelength are distinguished by the sense of the vertical velocity at the center of the three-dimensional cell. It is suggested that strong horizontal shear flows should be present just below the surface of the star, and that the large-scale motions extending into the stable atmosphere would appear mainly as horizontal flows.

  2. A BCool survey of the magnetic fields of planet-hosting solar-type stars

    NASA Astrophysics Data System (ADS)

    Mengel, M. W.; Marsden, S. C.; Carter, B. D.; Horner, J.; King, R.; Fares, R.; Jeffers, S. V.; Petit, P.; Vidotto, A. A.; Morin, J.; BCool Collaboration

    2017-03-01

    We present a spectropolarimetric snapshot survey of solar-type planet-hosting stars. In addition to 14 planet-hosting stars observed as part of the BCool magnetic snapshot survey, we obtained magnetic observations of a further 19 planet-hosting solar-type stars in order to see if the presence of close-in planets had an effect on the measured surface magnetic field (|Bℓ|). Our results indicate that the magnetic activity of this sample is congruent with that of the overall BCool sample. The effects of the planetary systems on the magnetic activity of the parent star, if any, are too subtle to detect compared to the intrinsic dispersion and correlations with rotation, age and stellar activity proxies in our sample. Four of the 19 newly observed stars, two of which are subgiants, have unambiguously detected magnetic fields and are future targets for Zeeman-Doppler mapping.

  3. Deformed neutron stars due to strong magnetic field in terms of relativistic mean field theories

    NASA Astrophysics Data System (ADS)

    Yanase, Kota; Yoshinaga, Naotaka

    2014-09-01

    Some observations suggest that magnetic field intensity of neutron stars that have particularly strong magnetic field, magnetars, reaches values up to 1014-15G. It is expected that there exists more strong magnetic field of several orders of magnitude in the interior of such stars. Neutron star matter is so affected by magnetic fields caused by intrinsic magnetic moments and electric charges of baryons that masses of neutron stars calculated by using Tolman-Oppenheimer-Volkoff equation is therefore modified. We calculate equation of state (EOS) in density-dependent magnetic field by using sigma-omega-rho model that can reproduce properties of stable nuclear matter in laboratory Furthermore we calculate modified masses of deformed neutron stars.

  4. The prevalence of weak magnetic fields in Herbig Ae stars: the case of PDS 2

    NASA Astrophysics Data System (ADS)

    Hubrig, S.; Carroll, T. A.; Schöller, M.; Ilyin, I.

    2015-04-01

    Models of magnetically driven accretion and outflows reproduce many observational properties of T Tauri stars, but the picture is much less clear for the Herbig Ae/Be stars, due to the poor knowledge of their magnetic field strength and topology. The Herbig Ae star PDS 2 was previously included in two magnetic studies based on low-resolution spectropolarimetric observations. Only in one of these studies the presence of a weak mean longitudinal magnetic field was reported. In the present study, for the first time, high-resolution high accuracy radial velocity planet searcher (HARPS) spectropolarimetric observations of PDS 2 are used to investigate the presence of a magnetic field. A firm detection of a weak longitudinal magnetic field is achieved using the multiline singular value decomposition method for Stokes profile reconstruction ( = 33 ± 5 G). To gain better knowledge of typical magnetic field strengths in late Herbig Be and Herbig Ae stars, we compiled previous magnetic field measurements, revealing that only very few stars have fields stronger than 200 G, and half of the sample possesses fields of about 100 G and less. These results challenge our current understanding of the magnetospheric accretion in intermediate-mass pre-main-sequence stars as they indicate that the magnetic fields in Herbig Ae/Be stars are by far weaker than those measured in T Tauri stars.

  5. Anisotropic crystal structure of magnetized neutron star crust

    NASA Astrophysics Data System (ADS)

    Baiko, D. A.; Kozhberov, A. A.

    2017-09-01

    Although the crystallized neutron star crust is responsible for many fascinating observational phenomena, its actual microscopic structure in tremendous gravitational and magnetic fields is not understood. Here we show that in a non-uniform magnetic field, three-dimensional ionic Coulomb crystals comprising the crust may stretch or shrink while their electrostatic pressure becomes anisotropic. The pressure depends non-linearly on the magnitude of the stretch, so that a continuous magnetic field evolution may result in an abrupt crystal elongation or contraction. This may provide a trigger for magnetar activity. A phonon-mode instability is revealed, which sets the limits of magnetic field variation beyond which the crystal is destroyed. These limits sometimes correspond to surprisingly large deformations. It is not known what happens to crust matter subjected to a pressure anisotropy exceeding these limits. We hypothesize that the ion system then possesses a long-range order only in one or two dimensions, i.e. becomes a liquid crystal.

  6. Magnetic fields during the early phase of massive star formation

    NASA Astrophysics Data System (ADS)

    Seifried, Daniel Jürgen

    2013-01-01

    The goal of this work is to improve our current understanding of the formation process of massive stars in the presence of magnetic fields by means of numerical simulations. In particular, I focus on protostellar accretion rates, the evolution and the properties of protostellar discs and their associated outflows, and the interplay of turbulence and magnetic fields and its impact on protostellar disc formation. In a systematic parameter study I show that the accretion rates are remarkably constant over a wide range of initial conditions. Furthermore, I show that in the absence of turbulence for strong initial magnetic fields only sub-Keplerian discs can form which is attributed to the strong magnetic braking effect. This result seems to be in contrast to observational results. The morphology of the outflows, which shows a strong dependence on the initial conditions, can ultimately be linked to the structure of the underlying disc. Well-collimated outflows with high outflows velocities only develop if a Keplerian protostellar disc is present, otherwise slowly expanding, sphere-like outflows develop. Furthermore, I analyse the driving mechanism of outflows with an analytical criterion derived in the course of this work. When including supersonic, turbulent motions in the simulations, Keplerian protostellar discs form in contrast to the non-turbulent simulations. This result is in agreement with observations of early-type protostellar objects.

  7. Rotation and magnetic activity of oscillating solar-like stars with the Kepler mission

    NASA Astrophysics Data System (ADS)

    Mathur, Savita; Balliet, Lauren; García, Rafael A.; Salabert, David

    2017-09-01

    For the last few decades the investigation of stellar magnetic activity has been conducted through spectroscopic and spectropolarimetric surveys. This led not only to the detection of magnetic cycles in other stars but also to variable and magnetic activity. For the Sun, the magnetic activity is described as the interaction between convection, rotation, and magnetic field. To study magnetic activity of solar-like stars we need to have the knowledge of the surface rotation period, the properties of magnetic activity, and the structure of the stars. We present the results obtained from the studies of Kepler solarlike targets in terms of rotation periods, magnetic activity proxies and magnetic activity cycles detected. We can then combine this information with asteroseismic studies to have a broader picture of stellar magnetic activity.

  8. Modeling Formation of Clouds of Heavy Elements in the Magnetic Atmospheres of Hot Stars

    NASA Astrophysics Data System (ADS)

    Alecian, G.

    2017-07-01

    Magnetic A and B main-sequence stars present strange inhomogeneous distributions of metals on their surface. These stars constitute the well-known group of chemically peculiar magnetic stars (ApBp stars). The atmospheres of these stars are known to be strongly affected by atomic diffusion, and a new generation of modeling tools is appearing to take into account the complexity of physical processes in play. In this talk, I will present recent results of 3D numerical modeling of abundance distributions in these atmospheres.

  9. Generation and Distribution of a Magnetic Field in Superconducting Strange Stars

    NASA Astrophysics Data System (ADS)

    Sedrakian, D. M.; Hayrapetyan, M. V.; Baghdasaryan, D. S.

    2015-12-01

    Generation of a magnetic field and its distribution are considered within a rotating strange quark star with a crust. It is shown how, over time, a differential rotation is established between the superfluid and superconducting quark core and normal electron plasma, which leads to the generation of magnetic field. The magnetic field at the surface of a strange star may attain values of 1011-1015 G, depending on the star model. It is suggested that magnetars may be manifestations of strange stars, the cores of which rotate much faster than the observable part, i.e., the crust.

  10. The pulsating magnetosphere of the extremely slowly rotating magnetic β Cep star ξ1 CMa

    NASA Astrophysics Data System (ADS)

    Shultz, M.; Wade, G. A.; Rivinius, Th.; Neiner, C.; Henrichs, H.; Marcolino, W.; MiMeS Collaboration

    2017-10-01

    ξ1 CMa is a monoperiodically pulsating, magnetic β Cep star with magnetospheric X-ray emission that, uniquely amongst magnetic stars, is clearly modulated with the star's pulsation period. The rotational period Prot has yet to be identified, with multiple competing claims in the literature. We present an analysis of a large ESPaDOnS data set with a 9 yr baseline. The longitudinal magnetic field 〈Bz〉 shows a significant annual variation, suggesting that Prot is at least of the order of decades. The possibility that the star's H α emission originates around a classical Be companion star is explored and rejected based upon Very Large Telescope Interferometer AMBER and PIONIER interferometry, indicating that the emission must instead originate in the star's magnetosphere and should therefore also be modulated with Prot. Period analysis of H α equivalent widths measured from ESPaDOnS and CORALIE spectra indicates Prot > 30 yr. All evidence thus supports that ξ1 CMa is a very slowly rotating magnetic star hosting a dynamical magnetosphere. H α also shows evidence for modulation with the pulsation period, a phenomenon that we show cannot be explained by variability of the underlying photospheric line profile, i.e. it may reflect changes in the quantity and distribution of magnetically confined plasma in the circumstellar environment. In comparison to other magnetic stars with similar stellar properties, ξ1 CMa is by far the most slowly rotating magnetic B-type star, is the only slowly rotating B-type star with a magnetosphere detectable in H α (and thus, the coolest star with an optically detectable dynamical magnetosphere), and is the only known early-type magnetic star with H α emission modulated by both pulsation and rotation.

  11. A type of cylindrical Hall thruster with a magnetically insulated anode

    NASA Astrophysics Data System (ADS)

    Yongjie, Ding; Yu, Xu; Wuji, Peng; Liqiu, Wei; Hongbo, Su; Hezhi, Sun; Peng, Li; Hong, Li; Daren, Yu

    2017-04-01

    In this paper, a type of magnetically insulated anode structure is proposed for the design of a low-power cylindrical Hall thruster. The magnetic field distribution in the channel is guided by the magnetically insulated anode, altering the intersection status of the magnetic field line passing through the anode and wall. Experimental and simulation results show that a high potential is formed near the wall by the magnetically insulated anode. As the ionization moves towards the outlet, the energy and flux of the ions bombarding the channel wall can be reduced effectively. Due to the reduction in the bombardment of the wall from high-energy ions, the thrust and specific impulse greatly increase compared with those of the non-magnetically insulated anode. For anode mass flow rates of 0.3 and 0.35 mg s-1 and discharge voltages in the 100-200 V range, the thrust can be increased by more than 33% and the anode efficiency can be improved by more than 7%. Meanwhile, the length of the sputtering area is clearly reduced. The starting position of the sputtering area is in front of the magnetic pole, which can effectively prolong the service life of the thruster.

  12. Magnetic Complexity, Dynamos and Spin Down of Sun-like Stars

    NASA Astrophysics Data System (ADS)

    Garraffo, Cecilia; Drake, Jeremy J.; Cohen, Ofer

    2015-08-01

    We investigate the role of magnetic complexity on the behaviour of Sun-like stars. While the Sun exhibits small-scale magnetic spots, its large-scale field is dipolar. Observations of rotationally modulated brightness and polarization indicate that stars more active that the Sun have more complex large-scale surface fields. We investigate the implications of this field complexity on the magnetic evolution and rotation history of Sun-like stars. Using detailed MHD models, we argue that the theoretical wind-driven torque used in stellar spin evolution models is missing a term representing magnetic morphology that becomes dominant for young active stars. We discuss the implications for stellar dynamos, magnetic activity and the rotation evolution of stars.

  13. B fields in OB stars (BOB): on the detection of weak magnetic fields in the two early B-type stars β CMa and ɛ CMa. Possible lack of a "magnetic desert" in massive stars

    NASA Astrophysics Data System (ADS)

    Fossati, L.; Castro, N.; Morel, T.; Langer, N.; Briquet, M.; Carroll, T. A.; Hubrig, S.; Nieva, M. F.; Oskinova, L. M.; Przybilla, N.; Schneider, F. R. N.; Schöller, M.; Simón-Díaz, S.; Ilyin, I.; de Koter, A.; Reisenegger, A.; Sana, H.

    2015-02-01

    Only a small fraction of massive stars seem to host a measurable structured magnetic field, whose origin is still unknown and whose implications for stellar evolution still need to be assessed. Within the context of the "B fields in OB stars (BOB)" collaboration, we used the HARPSpol spectropolarimeter to observe the early B-type stars β CMa (HD 44743; B1 II/III) and ɛ CMa (HD 52089; B1.5II) in December 2013 and April 2014. For both stars, we consistently detected the signature of a weak (<30 G in absolute value) longitudinal magnetic field, approximately constant with time. We determined the physical parameters of both stars and characterise their X-ray spectrum. For the β Cep star β CMa, our mode identification analysis led to determining a rotation period of 13.6 ± 1.2 days and of an inclination angle of the rotation axis of 57.6 ± 1.7°, with respect to the line of sight. On the basis of these measurements and assuming a dipolar field geometry, we derived a best fitting obliquity of about 22° and a dipolar magnetic field strength (Bd) of about 100 G (60 magnetic massive stars. This conclusion is strengthened further by considerations of the star's X-ray spectrum. For ɛ CMa we could only determine a lower limit on the dipolar magnetic field strength of 13 G. For this star, we determine that the rotation period ranges between 1.3 and 24 days. Our results imply that both stars are expected to have a dynamical magnetosphere, so the magnetic field is not able to support a circumstellar disk. We also conclude that both stars are most likely core hydrogen burning and that they have spent more than 2/3 of their main sequence lifetime. Ahistogram of the distribution of the dipolar magnetic field strength for the magnetic massive stars known to date does not show the magnetic field "desert" observed instead for intermediate-mass stars. The biases involved in the detection of (weak) magnetic

  14. Magnetized Inertial Fusion (MIF) Research at the Shiva Star Facility

    NASA Astrophysics Data System (ADS)

    Degnan, James; Grabowski, C.; Domonkos, M.; Ruden, E. L.; Amdahl, D. J.; White, W. M.; Frese, M. H.; Frese, S. D.; Wurden, G. A.; Weber, T. E.

    2015-11-01

    The AFRL Shiva Star capacitor bank (1300 μF, up to 120 kV) used typically at 4 to 5 MJ stored energy, 10 to 15 MA current, 10 μs current rise time, has been used to drive metal shell (solid liner) implosions for compression of axial magnetic fields to multi-megagauss levels, suitable for compressing magnetized plasmas to MIF conditions. MIF approaches use magnetic field to reduce thermal conduction relative to inertial confinement fusion (ICF). MIF substantially reduces required implosion speed and convergence. Using profiled thickness liner enables large electrode apertures and field-reversed configuration (FRC) injection. Using a longer capture region, FRC trapped flux lifetime was made comparable to implosion time and an integrated compression test was conducted. The FRC was radially compressed a factor of ten, to 100x density >1018 cm-3 (a world FRC record), but temperatures were only 300-400 eV, compared to intended several keV. Compression to megabar pressures was inferred by the observed liner rebound, but the heating rate during the first half of the compression was less than the normal FRC decay rate. Principal diagnostics were soft x-ray imaging, soft x-ray diodes, and neutron measurements. This work has been supported by DOE-OFES.

  15. Magnetic fields in star-forming regions - Observations

    NASA Technical Reports Server (NTRS)

    Heiles, Carl; Goodman, Alyssa A.; Mckee, Christopher F.; Zweibel, Ellen G.

    1993-01-01

    We review the observational aspects of magnetic fields in dense, star-forming regions. First we discuss ways to observe the field. These include direct methods, which consist of the measurement of both linear and circular polarization of spectral line and continuum radiation; and indirect methods, consisting of the angular distribution of H2O masers on the sky and the measurement of ambipolar diffusion. Next we discuss selected observational results, focusing on detailed discussions of a small number of points rather than a generalized discussion that covers the waterfront. We discuss the Orion/BN-KL region in detail, both on the small and large scales. Next we discuss the derivation of the complete magnetic vector, including both the systematic and fluctuating component, from a large sample of Zeeman and linear polarization measurements for the L204 dark cloud. We examine the virial theorem as it applies to dark clouds in general and one dark cloud, Barnard 1, in particular. We critically discuss the numerous claims for alignment of cloud structural features with the plane-of-the-sky component of the magnetic field, and find that many of these have not been definitively established.

  16. Magnetic fields in star-forming regions - Observations

    NASA Technical Reports Server (NTRS)

    Heiles, Carl; Goodman, Alyssa A.; Mckee, Christopher F.; Zweibel, Ellen G.

    1993-01-01

    We review the observational aspects of magnetic fields in dense, star-forming regions. First we discuss ways to observe the field. These include direct methods, which consist of the measurement of both linear and circular polarization of spectral line and continuum radiation; and indirect methods, consisting of the angular distribution of H2O masers on the sky and the measurement of ambipolar diffusion. Next we discuss selected observational results, focusing on detailed discussions of a small number of points rather than a generalized discussion that covers the waterfront. We discuss the Orion/BN-KL region in detail, both on the small and large scales. Next we discuss the derivation of the complete magnetic vector, including both the systematic and fluctuating component, from a large sample of Zeeman and linear polarization measurements for the L204 dark cloud. We examine the virial theorem as it applies to dark clouds in general and one dark cloud, Barnard 1, in particular. We critically discuss the numerous claims for alignment of cloud structural features with the plane-of-the-sky component of the magnetic field, and find that many of these have not been definitively established.

  17. On the diversity of magnetic interactions in close-in star-planet systems

    SciTech Connect

    Strugarek, A.; Brun, A. S.; Réville, V.

    2014-11-01

    Magnetic interactions between close-in planets and their host star can play an important role in the secular orbital evolution of the planets, as well as the rotational evolution of their host. As long as the planet orbits inside the Alfvén surface of the stellar wind, the magnetic interaction between the star and the planet can modify the wind properties and also lead to direct angular momentum transfers between the two. We model these star-planet interactions using compressible magnetohydrodynamic (MHD) simulations, and quantify the angular momentum transfers between the star, the planet, and the stellar wind. We study the cases of magnetized and non-magnetized planets and vary the orbital radius inside the Alfvén surface of the stellar wind. Based on a grid of numerical simulations, we propose general scaling laws for the modification of the stellar wind torque, for the torque between the star and the planet, and for the planet migration associated with the star-planet magnetic interactions. We show that when the coronal magnetic field is large enough and the star is rotating sufficiently slowly, the effect of the magnetic star-planet interaction is comparable to tidal effects and can lead to a rapid orbital decay.

  18. [Nucleosynthesis, Rotation and Magnetism in Accreting Neutron Stars

    NASA Technical Reports Server (NTRS)

    Bildsten, Lars

    2004-01-01

    This is my final report on the NASA ATP grant on nucleosynthesis, rotation and magnetism in accreting neutron stars (NAG5-8658). In my last two reports, I summarized the science that I have accomplished, which covered a large range of topics. For this report, I want to point out the graduate students that were partially supported on this grant and where they are now. Andrew Cumming is an Assistant Professor of Physics at McGill University, Greg Ushomirsky is a researcher at MIT s Lincoln Laboratories, Dean Townsley is a postdoctoral researcher at Univ. of Chicago, Chris Deloye is a postdoctoral researcher at Northwestern University. The other two students, Phil Chang and Tony Piro, are still at UCSB and will be completing their PhD s in Summer 05 and Summer 06.

  19. IQ Aur: A new mode of X-ray generation in magnetic stars ?

    NASA Astrophysics Data System (ADS)

    Schmitt, Jurgen

    2007-10-01

    We propose to obtain the first high-resolution X-ray spectrum of the peculiar magnetic A-type star IQ Aur. From previous X-ray observations IQ Aur is known as a strong (LX ~ 4 10**29 erg/s), but very soft (TX ~ 0.29 keV) X-ray source. An attribution of IQ~Aur's X-ray emission to a low-mass companion would imply totally unusual properties of such an hypothesized object, thus IQ Aur is a good candidate for an A-type star with intrinsic X-ray emission.The XMM-Newton RGS spectrum will constrain the location of the X-ray emission site from a measurement or upper limit to the strength of the OVII f line, the overall RGS spectrum will determine the elemental abundances, which may be far away from solar,and the phase coverage will allow a search for rotational modulation of IQ Aur's X-ray flux.

  20. Internal composition of proto-neutron stars under strong magnetic fields

    NASA Astrophysics Data System (ADS)

    Franzon, B.; Dexheimer, V.; Schramm, S.

    2016-08-01

    In this work, we study the effects of magnetic fields and rotation on the structure and composition of proto-neutron stars. A hadronic chiral SU(3) model is applied to cold neutron stars and proto-neutron stars with trapped neutrinos and at fixed entropy per baryon. We obtain general relativistic solutions for neutron and proto-neutron stars endowed with a poloidal magnetic field by solving Einstein-Maxwell field equations in a self-consistent way. As the neutrino chemical potential decreases in value over time, this alters the chemical equilibrium and the composition inside the star, leading to a change in the structure and in the particle population of these objects. We find that the magnetic field deforms the star and significantly alters the number of trapped neutrinos in the stellar interior, together with strangeness content and temperature in each evolution stage.

  1. Magnetic properties of low mass stars: new discoveries and future prospects

    NASA Astrophysics Data System (ADS)

    Shulyak, Denis

    2017-05-01

    Measuring properties of surface magnetic fields is the path towards our understanding of stellar dynamos and environment of stars with convective envelopes. In this talk I will summarize what have we learned about magnetism in low-mass stars over last decades from numerous spectroscopic and polarimetric studies, and discuss on newest discoveries of strongest magnetic fields ever found in these objects since early eightees.

  2. A fossil origin for the magnetic field in A stars and white dwarfs.

    PubMed

    Braithwaite, Jonathan; Spruit, Hendrik C

    2004-10-14

    Some main-sequence stars of spectral type A are observed to have a strong (0.03-3 tesla), static, large-scale magnetic field, of a chiefly dipolar shape: they are known as 'Ap stars', such as Alioth, the fifth star in the Big Dipper. Following the discovery of these fields, it was proposed that they are remnants of the star's formation, a 'fossil' field. An alternative suggestion is that they could be generated by a dynamo process in the star's convective core. The dynamo hypothesis, however, has difficulty explaining high field strengths and the observed lack of a correlation with rotation. The weakness of the fossil-field theory has been the absence of field configurations stable enough to survive in a star over its lifetime. Here we report numerical simulations that show that stable magnetic field configurations, with properties agreeing with those observed, can develop through evolution from arbitrary, unstable initial fields. The results are applicable equally to Ap stars, magnetic white dwarfs and some highly magnetized neutron stars known as magnetars. This establishes fossil fields as the natural, unifying explanation for the magnetism of all these stars.

  3. Magnetically Fed Hot Star Keplerian Disks with Slow Outflow

    NASA Astrophysics Data System (ADS)

    Brown, J. C.; Cassinelli, J. P.; Maheswaran, M.

    2008-12-01

    The puzzle of the origin of Be star disks is discussed. Contrary to recently published claims, it is argued that the magnetically torqued disk (MTD) type models of Cassinelli et al. offer a viable scenario for a successful model with all the key ingredients. MTD models involve disk compression by equatorial collision of stellar wind streams that are steered and torqued by a dipole-like magnetic field. While the growing disk density tends to lead to the gas breaking out centrifugally from the field, it is proposed that the onset of viscous effects can lead to an eventual stable, slowly outflowing, Keplerian disk. It is then shown that the resulting very dense (wind compressed) disk need have only a very slow subsonic outflow to satisfy mass continuity. Consequently, line profile data do not preclude steadily expanding disks of high density. It is also shown that the time taken to reach the steady state would typically be of the order of 104 wind flow times R/v∞. This is far longer than the run times of recent numerical MHD simulations that displayed bursty breakout behavior, which may therefore only be transients induced by unrealistic initial conditions.

  4. Polarized x-ray emission from magnetized neutron stars: signature of strong-field vacuum polarization.

    PubMed

    Lai, Dong; Ho, Wynn C G

    2003-08-15

    In the atmospheric plasma of a strongly magnetized neutron star, vacuum polarization can induce a Mikheyev-Smirnov-Wolfenstein type resonance across which an x-ray photon may (depending on its energy) convert from one mode into the other, with significant changes in opacities and polarizations. We show that this vacuum resonance effect gives rise to a unique energy-dependent polarization signature in the surface emission from neutron stars. The detection of polarized x rays from neutron stars can provide a direct probe of strong-field quantum electrodynamics and constrain the neutron star magnetic field and geometry.

  5. The challenge of measuring magnetic fields in strongly pulsating stars: the case of HD 96446

    NASA Astrophysics Data System (ADS)

    Järvinen, S. P.; Hubrig, S.; Ilyin, I.; Schöller, M.; Briquet, M.

    2017-01-01

    Among the early B-type stars, He-rich Bp stars exhibit the strongest large-scale organized magnetic fields with a predominant dipole contribution. The presence of β Cep-like pulsations in the typical magnetic early Bp-type star HD 96446 was announced a few years ago, but the analysis of the magnetic field geometry was hampered by the absence of a reliable rotation period and a sophisticated procedure for accounting for the impact of pulsations on the magnetic field measurements. Using new spectropolarimetric observations and a recently determined rotation period based on an extensive spectroscopic time series, we investigate the magnetic field model parameters of this star under more detailed considerations of the pulsation behaviour of line profiles.

  6. The effects of intense magnetic fields on Landau levels in a neutron star

    NASA Astrophysics Data System (ADS)

    Gao, Z. F.; Wang, N.; Song, D. L.; Yuan, J. P.; Chou, C.-K.

    2011-08-01

    In this paper, an approximate method of calculating the Fermi energy of electrons ( E F ( e)) in a high-intensity magnetic field, based on the analysis of the distribution of a neutron star magnetic field, has been proposed. In the interior of a neutron star, different forms of intense magnetic field could exist simultaneously and a high electron Fermi energy could be generated by the release of magnetic field energy. The calculation results show that: E F ( e) is related to density ρ, the mean electron number per baryon Y e and magnetic field strength B.

  7. Radial modes of slowly rotating compact stars in the presence of magnetic field

    NASA Astrophysics Data System (ADS)

    Panda, N. R.; Mohanta, K. K.; Sahu, P. K.

    2016-09-01

    Compact stars are composed of very high-density hadron matter. When the matter is above nuclear matter density, then there is a chance of different phases of matter such as hadron matter to quark matter. There is a possible phase which, having the quark core surrounded by a mixed phase followed by hadronic matter, may be considered as a hybrid phase inside the stars called hybrid star (HS). The star which consists of only u, d and s quarks is called quark star (QS) and the star which has only hadronic matter is called neutron star (NS). For the equation of state (EOS) of hadronic matter, we have considered the Relativistic Mean Field (RMF) theory and we incorporated the effect of strong magnetic fields. For the EOS of the quark phase we use the simple MIT bag model. We have assumed Gaussian parametrization to make the density dependent for both bag pressure in quark matter and magnetic field. We have constructed the intermediate mixed phase by using the Glendenning conjecture. Eigenfrequencies of radial pulsations of slowly rotating magnetized compact stars (NS, QS, HS) are calculated in a general relativistic formalism given by Chandrasekhar and Friedman. We have studied the effect of central density on the square of the frequencies of the compact stars in the presence of zero and strong magnetic field.

  8. ON THE INDUCED GRAVITATIONAL COLLAPSE OF A NEUTRON STAR TO A BLACK HOLE BY A TYPE Ib/c SUPERNOVA

    SciTech Connect

    Rueda, Jorge A.; Ruffini, Remo E-mail: ruffini@icra.it

    2012-10-10

    It is understood that the supernovae (SNe) associated with gamma-ray bursts (GRBs) are of Type Ib/c. The temporal coincidence of the GRB and the SN continues to represent a major enigma of Relativistic Astrophysics. We elaborate here, from the earlier paradigm, that the concept of induced gravitational collapse is essential to explain the GRB-SN connection. The specific case of a close (orbital period <1 hr) binary system composed of an evolved star with a neutron star (NS) companion is considered. We evaluate the accretion rate onto the NS of the material expelled from the explosion of the core progenitor as a Type Ib/c SN and give the explicit expression of the accreted mass as a function of the nature of the components and binary parameters. We show that the NS can reach, in a few seconds, critical mass and consequently gravitationally collapse to a black hole. This gravitational collapse process leads to the emission of the GRB.

  9. Magnetic field measurements and wind-line variability of OB-type stars

    NASA Astrophysics Data System (ADS)

    Schnerr, R. S.; Henrichs, H. F.; Neiner, C.; Verdugo, E.; de Jong, J.; Geers, V. C.; Wiersema, K.; van Dalen, B.; Tijani, A.; Plaggenborg, B.; Rygl, K. L. J.

    2008-06-01

    Context: The first magnetic fields in O- and B-type stars that do not belong to the Bp-star class, have been discovered. The cyclic UV wind-line variability, which has been observed in a significant fraction of early-type stars, is likely to be related to such magnetic fields. Aims: We attempt to improve our understanding of massive-star magnetic fields, and observe twenty-five carefully-selected, OB-type stars. Methods: Of these stars we obtain 136 magnetic field strength measurements. We present the UV wind-line variability of all selected targets and summarise spectropolarimetric observations acquired using the MUSICOS spectropolarimeter, mounted at the TBL, Pic du Midi, between December 1998 and November 2004. From the average Stokes I and V line profiles, derived using the LSD method, we measure the magnetic field strengths, radial velocities, and first moment of the line profiles. Results: No significant magnetic field is detected in any OB-type star that we observed. Typical 1σ errors are between 15 and 200 G. A possible magnetic-field detection for the O9V star 10 Lac remains uncertain, because the field measurements depend critically on the fringe-effect correction in the Stokes V spectra. We find excess emission in UV-wind lines, centred about the rest wavelength, to be a new indirect indicator of the presence of a magnetic field in early B-type stars. The most promising candidates to host magnetic fields are the B-type stars δ Cet and 6 Cep, and a number of O stars. Conclusions: Although some O and B stars have strong dipolar field, which cause periodic variability in the UV wind-lines, such strong fields are not widespread. If the variability observed in the UV wind-lines of OB stars is generally caused by surface magnetic fields, these fields are either weak (⪉few hundred G) or localised. Figures [see full textsee full textsee full textsee full textsee full textsee full text]-[see full textsee full textsee full textsee full textsee full textsee

  10. MAGNETIC GRAIN TRAPPING AND THE HOT EXCESSES AROUND EARLY-TYPE STARS

    SciTech Connect

    Rieke, G. H.; Gáspár, András; Ballering, N. P. E-mail: agaspar@as.arizona.edu

    2016-01-10

    A significant fraction of main sequence stars observed interferometrically in the near-infrared have slightly extended components that have been attributed to very hot dust. To match the spectrum appears to require the presence of large numbers of very small (<200 nm in radius) dust grains. However, particularly for the hotter stars, it has been unclear how such grains can be retained close to the star against radiation pressure force. We find that the expected weak stellar magnetic fields are sufficient to trap nm-sized dust grains in epicyclic orbits for a few weeks or longer, sufficient to account for the hot excess emission. Our models provide a natural explanation for the requirement that the hot excess dust grains be smaller than 200 nm. They also suggest that magnetic trapping is more effective for rapidly rotating stars, consistent with the average vsini measurements of stars with hot excesses being larger (at ∼2σ) than those for stars without such excesses.

  11. Uncovering star formation feedback and magnetism in galaxies with radio continuum surveys

    NASA Astrophysics Data System (ADS)

    Tabatabaei, F. S.

    2017-03-01

    Recent studies show the importance of the star formation feedback in changing the energetic and structure of galaxies. Dissecting the physics of the feedback is hence crucial to understand the evolution of galaxies. Full polarization radio continuum surveys can be ideally performed to trace not only star formation but also the energetic components of the interstellar medium (ISM), the magnetic fields and cosmic ray electrons. Using the SKA precursors, we investigate the effect of the massive star formation on the ISM energy balance in nearby galaxies. Our multi-scale and multi-frequency surveys show that cosmic rays are injected in star forming regions and lose energy propagating away from their birth place. Due to the star formation feedback, cosmic ray electron population becomes younger and more energetic. Star formation also amplifies the turbulent magnetic field inserting a high pressure which is important in energy balance in the ISM and structure formation in the host galaxy.

  12. Assessing Magnetic Torques and Energy Fluxes in Close-in Star-Planet Systems

    NASA Astrophysics Data System (ADS)

    Strugarek, A.

    2016-12-01

    Planets in close-in orbit interact with the magnetized wind of their hosting star. This magnetic interaction was proposed to be a source for enhanced emissions in the chromosphere of the star, and to participate in setting the migration timescale of the close-in planet. The efficiency of the magnetic interaction is known to depend on the magnetic properties of the host star and of the planet, and on the magnetic topology of the interaction. We use a global, three-dimensional numerical model of close-in star-planet systems, based on the magnetohydrodynamics approximation, to compute a grid of simulations for varying properties of the orbiting planet. We propose a simple parametrization of the magnetic torque that applies to the planet, and of the energy flux generated by the interaction. The dependency upon the planet properties and the wind properties is clearly identified in the derived scaling laws, which can be used in secular evolution codes to take into account the effect of magnetic interactions in planet migration. They can also be used to estimate a potential magnetic source of enhanced emissions in observed close-in star-planet systems, in order to constrain observationally possible exoplanetary magnetic fields.

  13. Using red clump stars to decompose the galactic magnetic field with distance

    SciTech Connect

    Pavel, Michael D.

    2014-09-01

    A new method for measuring the large-scale structure of the Galactic magnetic field is presented. The Galactic magnetic field has been probed through the Galactic disk with near-infrared starlight polarimetry; however, the distance to each background star is unknown. Using red clump stars as near-infrared standard candles, this work presents the first attempt to decompose the line-of-sight structure of the sky-projected Galactic magnetic field. Two example lines of sight are decomposed: toward a field with many red clump stars and toward a field with few red clump stars. A continuous estimate of magnetic field orientation over several kiloparsecs of distance is possible in the field with many red clump stars, while only discrete estimates are possible in the sparse example. Toward the outer Galaxy, there is a continuous field orientation with distance that shows evidence of perturbation by the Galactic warp. Toward the inner Galaxy, evidence for a large-scale change in the magnetic field geometry is consistent with models of magnetic field reversals, independently derived from Faraday rotation studies. A photo-polarimetric method for identifying candidate intrinsically polarized stars is also presented. The future application of this method to large regions of the sky will begin the process of mapping the Galactic magnetic field in a way never before possible.

  14. Using Red Clump Stars to Decompose the Galactic Magnetic Field with Distance

    NASA Astrophysics Data System (ADS)

    Pavel, Michael D.

    2014-09-01

    A new method for measuring the large-scale structure of the Galactic magnetic field is presented. The Galactic magnetic field has been probed through the Galactic disk with near-infrared starlight polarimetry; however, the distance to each background star is unknown. Using red clump stars as near-infrared standard candles, this work presents the first attempt to decompose the line-of-sight structure of the sky-projected Galactic magnetic field. Two example lines of sight are decomposed: toward a field with many red clump stars and toward a field with few red clump stars. A continuous estimate of magnetic field orientation over several kiloparsecs of distance is possible in the field with many red clump stars, while only discrete estimates are possible in the sparse example. Toward the outer Galaxy, there is a continuous field orientation with distance that shows evidence of perturbation by the Galactic warp. Toward the inner Galaxy, evidence for a large-scale change in the magnetic field geometry is consistent with models of magnetic field reversals, independently derived from Faraday rotation studies. A photo-polarimetric method for identifying candidate intrinsically polarized stars is also presented. The future application of this method to large regions of the sky will begin the process of mapping the Galactic magnetic field in a way never before possible.

  15. Three-dimensional abundance distributions in ApBp star atmospheres: non-axisymmetric magnetic geometry

    NASA Astrophysics Data System (ADS)

    Alecian, G.; Stift, M. J.

    2017-06-01

    Numerical models for the atmospheres of magnetic ApBp stars have in the past dealt only with centred dipole magnetic field geometries. These models include atomic diffusion that stratifies the abundances of metals according to the local magnetic field strength and the direction with respect to the surface normal. The magnetic variations with rotational phase of most well observed stars, however, reveal that this assumption is far too simplistic. In this work, we establish for the first time a three-dimensional model with abundance stratifications arising from atomic diffusion of 16 metals, adopting a non-axisymmetric magnetic field geometry inspired by the configuration derived for a real ApBp star. We find that the chemical elements are distributed in complex patterns in all three dimensions, far from the simple rings that have been proposed as the dominant abundance structures from calculations that assume a perfectly centred dipolar magnetic geometry.

  16. MAGNETIC BRAKING FORMULATION FOR SUN-LIKE STARS: DEPENDENCE ON DIPOLE FIELD STRENGTH AND ROTATION RATE

    SciTech Connect

    Matt, Sean P.; Pinsonneault, Marc H.; Greene, Thomas P. E-mail: kmac@ucar.edu E-mail: thomas.p.greene@nasa.gov

    2012-08-01

    We use two-dimensional axisymmetric magnetohydrodynamic simulations to compute steady-state solutions for solar-like stellar winds from rotating stars with dipolar magnetic fields. Our parameter study includes 50 simulations covering a wide range of relative magnetic field strengths and rotation rates, extending from the slow- and approaching the fast-magnetic-rotator regimes. Using the simulations to compute the angular momentum loss, we derive a semi-analytic formulation for the external torque on the star that fits all of the simulations to a precision of a few percent. This formula provides a simple method for computing the magnetic braking of Sun-like stars due to magnetized stellar winds, which properly includes the dependence on the strength of the magnetic field, mass loss rate, stellar radius, surface gravity, and spin rate, and which is valid for both slow and fast rotators.

  17. The effect of the Coriolis force on the stability of rotating magnetic stars

    NASA Technical Reports Server (NTRS)

    Sakurai, K.

    1972-01-01

    The effect of the Coriolis force on the stability of rotating magnetic stars in hydrostatic equilibrium is investigated by using the method of the energy principle. It is shown that this effect is to inhibit the onset of instability.

  18. The effect of the Coriolis force on the stability of rotating magnetic stars.

    NASA Technical Reports Server (NTRS)

    Sakurai, K.

    1972-01-01

    The effect of the Coriolis force on the stability of rotating magnetic stars in hydrostatic equilibrium is investigated by using the method of the energy principle. It is shown that this effect is to inhibit the onset of instability.

  19. High resolution numerical relativity simulations for the merger of binary magnetized neutron stars

    NASA Astrophysics Data System (ADS)

    Kiuchi, Kenta; Kyutoku, Koutarou; Sekiguchi, Yuichiro; Shibata, Masaru; Wada, Tomohide

    2014-08-01

    We perform high-resolution magnetohydrodynamics simulations of binary neutron star mergers in numerical relativity on the Japanese supercomputer K. The neutron stars and merger remnants are covered by a grid spacing of 70 m, which yields the highest-resolution results among those derived so far. By an in-depth resolution study, we clarify several amplification mechanisms of magnetic fields during the binary neutron star merger for the first time. First, the Kelvin-Helmholtz instability developed in the shear layer at the onset of the merger significantly amplifies the magnetic fields. A hypermassive neutron star (HMNS) formed after the merger is then subject to the nonaxisymmetric magnetorotational instability, which amplifies the magnetic field in the HMNS. These two amplification mechanisms cannot be found with insufficient-resolution runs. We also show that the HMNS eventually collapses to a black hole surrounded by an accretion torus which is strongly magnetized at birth.

  20. VizieR Online Data Catalog: SDSS magnetic white dwarf stars (Kepler+, 2013)

    NASA Astrophysics Data System (ADS)

    Kepler, S. O.; Pelisoli, I.; Jordan, S.; Kleinman, S. J.; Koester, D.; Kulebi, B.; Pecanha, V.; Castanheira, B. G.; Nitta, A.; Costa, J. E. S.; Winget, D. E.; Kanaan, A.; Fraga, L.

    2017-07-01

    We classified more than 48000 spectra, selected as possible white dwarf stars from the SDSS DR7 by their colours, through visual inspection and detected Zeeman splittings in 521 DA stars. Table 1 shows the estimated values for the magnetic fields for the 521 spectra we measured. (1 data file).

  1. Ultraviolet variability of the magnetic chemically peculiar star 56 Arietis

    NASA Astrophysics Data System (ADS)

    Sokolov, N. A.

    2006-12-01

    The spectrophotometric variability of the magnetic chemically peculiar star 56 Arietis (56 Ari) in the far-ultraviolet spectral region from 1150 to 1980 Å is investigated. This study is based on the archival International Ultraviolet Explorer data obtained at different phases of the rotational cycle. The brightness of 56 Ari is not constant in the investigated wavelengths over the whole rotational period. The monochromatic light curves continuously change their shape with wavelength. The comparison of energy distributions at three phases shows that the first minimum of light curves at phase 0.25 is replaced by the maximum for λ > 1608 Å, but the second minimum of light curves at phase 0.65 is absent in the spectral region between λ1938 and λ1980 Å. Iron is responsible for the depression at λ1775 Å in the spectrum of 56 Ari. Based on IUE Newly Extracted Spectra (INES) data from the IUE satellite. E-mail: sokolov@gao.spb.ru

  2. Tunnel effect in molecules in strong magnetic fields of neutron stars

    NASA Astrophysics Data System (ADS)

    Khersonskii, V. K.

    1982-10-01

    The penetration coefficient and probability of tunnel transition in molecular ions H(2+) and HD(+), as functions of the magnetic field in the region B = 10 to the 11th-10 to the 13th G, is calculated. It is found that in this strong, neutron star magnetic field, the probability of tunnel effect increases significantly. It is probable that such an effect can catalyze nuclear reactions at the neutron star surface.

  3. External magnetic field of a static spherically symmetric star in Rosen's bimetric theory of gravitation

    SciTech Connect

    Prasanna, A.R.

    1982-05-15

    In this brief paper we present a complete exact solution for the external magnetic field (dipolar at infinity) of a static magnetic star on the spherically symmetric background metric solution of Rosen's bimetric theory of gravity. Unlike in general relativity the field is well behaved throughout the manifold except at r = 0, and thus allows one to consider the field for stars collapsed beyond 2m.

  4. A technique for measuring magnetic fields on solar-type stars

    NASA Technical Reports Server (NTRS)

    Marcy, G. W.

    1982-01-01

    A method is developed and tested by which magnetic-field strengths and area filling-factors of magnetic regions on late-type stars may be inferred from high-resolution, absorption-line profiles that have been broadened by the Zeeman effect. The technique involves fitting such profiles with a triplet of components, the shape of which is derived from profiles of lines of low Zeeman sensitivity. Tests of the systematic and random errors indicate that such magnetic flux measurements have an uncertainty of 20% for stars with field strengths of 2000 gauss if at least 10% of the stellar surface contains magnetic regions.

  5. A technique for measuring magnetic fields on solar-type stars

    NASA Technical Reports Server (NTRS)

    Marcy, G. W.

    1982-01-01

    A method is developed and tested by which magnetic-field strengths and area filling-factors of magnetic regions on late-type stars may be inferred from high-resolution, absorption-line profiles that have been broadened by the Zeeman effect. The technique involves fitting such profiles with a triplet of components, the shape of which is derived from profiles of lines of low Zeeman sensitivity. Tests of the systematic and random errors indicate that such magnetic flux measurements have an uncertainty of 20% for stars with field strengths of 2000 gauss if at least 10% of the stellar surface contains magnetic regions.

  6. Discovery of a Centrifugal Magnetosphere around the He-Strong Magnetic B1 Star ALS 3694

    NASA Astrophysics Data System (ADS)

    Shultz, M.; Wade, G.; Rivinius, T.; Sikora, J.; MiMeS Collaboration

    2016-11-01

    We report the results of 6 nights of Canada-France-Hawaii Telescope spectropolarimetric ESPaDOnS observations of the He-strong, magnetic B1 type star ALS 3694. The longitudinal magnetic field is approximately 2 kG in all 6 observations, showing essentially no variation between nights. The Hα line displays variable emission on all nights, peaking at high velocities (˜3 v sin i). Given the presence of a strong (Bd>6 kG) magnetic field, and the similarity of the emission profile to that of other magnetic B-type stars, we interpret the emission as a consequence of a centrifugal magnetosphere.

  7. Dipole magnetic field of neutron stars in f(R) gravity

    NASA Astrophysics Data System (ADS)

    Bakirova, Elizat; Folomeev, Vladimir

    2016-10-01

    The structure of an interior dipole magnetic field of neutron stars in f( R) gravity is considered. For this purpose, the perturbative approaches are used when both the deviations from general relativity and the deformations of spherically symmetric configurations associated with the presence of the magnetic field are assumed to be small. Solutions are constructed which describe relativistic, spherically symmetric configurations consisting of a gravitating magnetized perfect fluid modeled by a realistic equation of state. Comparing configurations from general relativity and modified gravity, we reveal possible differences in the structure of the magnetic field which occur in considering neutron stars in modified gravity.

  8. Magnetic Games between a Planet and Its Host Star: The Key Role of Topology

    NASA Astrophysics Data System (ADS)

    Strugarek, A.; Brun, A. S.; Matt, S. P.; Réville, V.

    2015-12-01

    Magnetic interactions between a star and a close-in planet are postulated to be a source of enhanced emissions and to play a role in the secular evolution of the orbital system. Close-in planets generally orbit in the sub-alfvénic region of the stellar wind, which leads to efficient transfers of energy and angular momentum between the star and the planet. We model the magnetic interactions occurring in close-in star-planet systems with three-dimensional, global, compressible magnetohydrodynamic numerical simulations of a planet orbiting in a self-consistent stellar wind. We focus on the cases of magnetized planets and explore three representative magnetic configurations. The Poynting flux originating from the magnetic interactions is an energy source for enhanced emissions in star-planet systems. Our results suggest a simple geometrical explanation for ubiquitous on/off enhanced emissions associated with close-in planets, and confirm that the Poynting fluxes can reach powers of the order of 1019 W. Close-in planets are also shown to migrate due to magnetic torques for sufficiently strong stellar wind magnetic fields. The topology of the interaction significantly modifies the shape of the magnetic obstacle that leads to magnetic torques. As a consequence, the torques can vary by at least an order of magnitude as the magnetic topology of the interaction varies.

  9. First discovery of a magnetic field in a main-sequence δ Scuti star: the Kepler star HD 188774

    NASA Astrophysics Data System (ADS)

    Neiner, C.; Lampens, P.

    2015-11-01

    The Kepler space mission provided a wealth of δ Sct-γ Dor hybrid candidates. While some may be genuine hybrids, others might be misclassified due to the presence of a binary companion or to rotational modulation caused by magnetism and related surface inhomogeneities. In particular, the Kepler δ Sct-γ Dor hybrid candidate HD 188774 shows a few low frequencies in its light and radial velocity curves, whose origin is unclear. In this work, we check for the presence of a magnetic field in HD 188774. We obtained two spectropolarimetric measurements with an Echelle SpectroPolarimetric Device for the Observation of Stars (ESPaDOnS) at Canada-France-Hawaii Telescope. The data were analysed with the least-squares deconvolution (LSD) method. We detected a clear magnetic signature in the Stokes V LSD profiles. The origin of the low frequencies detected in HD 188774 is therefore most probably the rotational modulation of surface spots possibly related to the presence of a magnetic field. Consequently, HD 188774 is not a genuine hybrid δ Sct-γ Dor star, but the first known magnetic main-sequence δ Sct star. This makes it a prime target for future asteroseismic and spot modelling. This result casts new light on the interpretation of the Kepler results for other δ Sct-γ Dor hybrid candidates.

  10. The complex magnetic field topology of the cool Ap star 49 Cam

    NASA Astrophysics Data System (ADS)

    Silvester, J.; Kochukhov, O.; Rusomarov, N.; Wade, G. A.

    2017-10-01

    49 Cam is a cool magnetic chemically peculiar star that has been noted for showing strong, complex Zeeman linear polarization signatures. This paper describes magnetic and chemical surface maps obtained for 49 Cam using the Invers10 magnetic Doppler imaging code and high-resolution spectropolarimetric data in all four Stokes parameters collected with the ESPaDOnS and Narval spectropolarimeters at the Canada-France-Hawaii Telescope and Pic du Midi Observatory. The reconstructed magnetic field maps of 49 Cam show a relatively complex structure. Describing the magnetic field topology in terms of spherical harmonics, we find significant contributions of modes up to ℓ = 3, including toroidal components. Observations cannot be reproduced using a simple low-order multipolar magnetic field structure. 49 Cam exhibits a level of field complexity that has not been seen in magnetic maps of other cool Ap stars. Hence, we concluded that relatively complex magnetic fields are observed in Ap stars at both low and high effective temperatures. In addition to mapping the magnetic field, we also derive surface abundance distributions of nine chemical elements, including Ca, Sc, Ti, Cr, Fe, Ce, Pr, Nd and Eu. Comparing these abundance maps with the reconstructed magnetic field geometry, we find no clear relationship of the abundance distributions with the magnetic field for some elements. However, for other elements some distinct patterns are found. We discuss these results in the context of other recent magnetic mapping studies and theoretical predictions of radiative diffusion.

  11. New measurements of photospheric magnetic fields in late-type stars and emerging trends

    NASA Technical Reports Server (NTRS)

    Saar, S. H.; Linsky, J. L.

    1986-01-01

    The magnetic fields of late-type stars are measured using the method of Saar et al. (1986). The method includes radiative transfer effects and compensation for line blending; the photospheric magnetic field parameters are derived by comparing observed and theoretical line profiles using an LTE code that includes line saturation and full Zeeman pattern. The preliminary mean active region magnetic field strengths (B) and surface area coverages for 20 stars are discussed. It is observed that there is a trend of increasing B towards the cooler dwarfs stars, and the linear correlation between B and the equipartition value of the magnetic field strength suggests that the photospheric gas pressure determines the photospheric magnetic field strengths. A tendency toward larger filling factors at larger stellar angular velocities is also detected.

  12. New measurements of photospheric magnetic fields in late-type stars and emerging trends

    NASA Technical Reports Server (NTRS)

    Saar, S. H.; Linsky, J. L.

    1986-01-01

    The magnetic fields of late-type stars are measured using the method of Saar et al. (1986). The method includes radiative transfer effects and compensation for line blending; the photospheric magnetic field parameters are derived by comparing observed and theoretical line profiles using an LTE code that includes line saturation and full Zeeman pattern. The preliminary mean active region magnetic field strengths (B) and surface area coverages for 20 stars are discussed. It is observed that there is a trend of increasing B towards the cooler dwarfs stars, and the linear correlation between B and the equipartition value of the magnetic field strength suggests that the photospheric gas pressure determines the photospheric magnetic field strengths. A tendency toward larger filling factors at larger stellar angular velocities is also detected.

  13. Magnetic field-related heating instabilities in the surface layers of the sun and stars

    NASA Technical Reports Server (NTRS)

    Ferrari, A.; Rosner, R.; Vaiana, G. S.

    1982-01-01

    The stability of a magnetized low-density plasma to current-driven filamentation instabilities is investigated and the results are applied to the surface layers of stars. Unlike previous studies, the initial (i.e., precoronal) state of the stellar surface atmosphere is taken to be a low-density, optically thin magnetized plasma in radiative equilibrium. The linear analysis shows that the surface layers of main-sequence stars (including the sun) which are threaded by magnetic fields are unstable; the instabilities considered lead to structuring perpendicular to the ambient magnetic fields. These results suggest that relatively modest surface motions, in conjunction with the presence of magnetic fields, suffice to account for the presence of inhomogeneous chromospheric and coronal plasma overlying a star's surface.

  14. Magnetic field-related heating instabilities in the surface layers of the sun and stars

    NASA Technical Reports Server (NTRS)

    Ferrari, A.; Rosner, R.; Vaiana, G. S.

    1982-01-01

    The stability of a magnetized low-density plasma to current-driven filamentation instabilities is investigated and the results are applied to the surface layers of stars. Unlike previous studies, the initial (i.e., precoronal) state of the stellar surface atmosphere is taken to be a low-density, optically thin magnetized plasma in radiative equilibrium. The linear analysis shows that the surface layers of main-sequence stars (including the sun) which are threaded by magnetic fields are unstable; the instabilities considered lead to structuring perpendicular to the ambient magnetic fields. These results suggest that relatively modest surface motions, in conjunction with the presence of magnetic fields, suffice to account for the presence of inhomogeneous chromospheric and coronal plasma overlying a star's surface.

  15. IUE observations of magnetically controlled stellar winds in the helium peculiar stars

    NASA Technical Reports Server (NTRS)

    Shore, Steven N.; Brown, Douglas N.

    1986-01-01

    Dramatic periodic variations in the C IV resonance lines of magnetic helium-weak sn stars HD 5737 = alpha Scl, HD 21699 = HR 1063, and HD 79158 = 36 Lyn are discussed. In all three cases, the 1548,50 doublet is the only non-negligibly variable UV spectral feature. The line profiles are consistent with outflow in a jet-like structure. In HD 21699 this outflow arises from one of the magnetic polar regions. Observations of two additional He-wk sn stars do not reveal strong C IV absorption, implying that the UV characteristics of these stars are less uniform than the optical phenomenology.

  16. The changing magnetic fields of intermediate-mass T Tauri stars

    NASA Astrophysics Data System (ADS)

    Villebrun, F.; Alecian, E.; Bouvier, J.; Hussain, G.; Folsom, C. P.

    2016-12-01

    Through their pre-main sequence phase (PMS phase), intermediate-mass stars evolve from a fully convective structure to a radiative structure in their interiors. During this transition, the occurence of strong magnetic fields drops from 100% to 7-8%. The reasons for this drop are yet unclear, and in order to understand it we need to characterise the magnetic fields of these intermediate-mass PMS stars, and compare them with the fundamental and evolutionary properties of the stars. As a first step, we determined their effective temperatures and luminosities. We show here the ESPaDOnS and HARPSpol spectra processing that brought up our results.

  17. Magnetic Field Measurements of T Tauri Stars in the Orion Nebula Cluster

    NASA Astrophysics Data System (ADS)

    Yang, Hao; Johns-Krull, Christopher M.

    2011-03-01

    We present an analysis of high-resolution (R ~ 50, 000) infrared K-band echelle spectra of 14 T Tauri stars (TTSs) in the Orion Nebula Cluster. We model Zeeman broadening in three magnetically sensitive Ti I lines near 2.2 μm and consistently detect kilogauss-level magnetic fields in the stellar photospheres. The data are consistent in each case with the entire stellar surface being covered with magnetic fields, suggesting that magnetic pressure likely dominates over gas pressure in the photospheres of these stars. These very strong magnetic fields might themselves be responsible for the underproduction of X-ray emission of TTSs relative to what is expected based on main-sequence star calibrations. We combine these results with previous measurements of 14 stars in Taurus and 5 stars in the TW Hydrae association to study the potential variation of magnetic field properties during the first 10 million years of stellar evolution, finding a steady decline in total magnetic flux with age. 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 (USA), the Science and Technology Facilities Council (UK), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência e Tecnologia (Brazil), and SECYT (Argentina).

  18. Generation of magnetic field on the accretion disk around a proto-first-star

    SciTech Connect

    Shiromoto, Yuki; Susa, Hajime; Hosokawa, Takashi

    2014-02-20

    The generation process of a magnetic field around a proto-first-star is studied. Utilizing the recent numerical results of proto-first-star formation based on radiation hydrodynamics simulations, we assess the magnetic field strength generated by the radiative force and the Biermann battery effect. We find that a magnetic field of ∼10{sup –9} G is generated on the surface of the accretion disk around the proto-first-star. The field strength on the accretion disk is smaller by two orders of magnitude than the critical value, above which the gravitational fragmentation of the disk is suppressed. Thus, the generated seed magnetic field hardly affect the dynamics of on-site first star formation directly, unless an efficient amplification process is taken into consideration. We also find that the generated magnetic field is continuously blown out from the disk on the outflows to the poles, that are driven by the thermal pressure of photoheated gas. The strength of the diffused magnetic field in low-density regions is ∼10{sup –14}-10{sup –13} G at n {sub H} = 10{sup 3} cm{sup –3}, which could play an important role in the next generation star formation, as well as the seeds of the magnetic field in the present-day universe.

  19. Magnetic field generation in PMS stars with and without radiative core

    NASA Astrophysics Data System (ADS)

    Zaire, B.; Guerrero, G.; Kosovichev, A. G.; Smolarkiewicz, P. K.; Landin, N. R.

    2017-10-01

    Recent observations of the magnetic field in pre-main sequence stars suggest that the magnetic field topology changes as a function of age. The presence of a tachocline could be an important factor in the development of magnetic field with higher multipolar modes. In this work we performed MHD simulations using the EULAG-MHD code to study the magnetic field generation and evolution in models that mimic stars at two evolutionary stages. The stratification for both stellar phases was computed by fitting stellar structure profiles obtained with the ATON stellar evolution code. The first stage is at 1.1Myr, when the star is completely convective. The second stage is at 14Myrs, when the star is partly convective, with a radiative core developed up to 30% of the stellar radius. In this proceedings we present a preliminary analysis of the resulting mean-flows and magnetic field. The mean-flow analysis shown that the star rotate almost rigidly on the fully convective phase, whereas at the partially convective phase there is differential rotation with conical contours of iso-rotation. As for the mean magnetic field both simulations show similarities with respect to the field evolution. However, the topology of the magnetic field is different.

  20. Magnetic, chemically peculiar (CP2) stars in the SuperWASP survey

    NASA Astrophysics Data System (ADS)

    Bernhard, K.; Hümmerich, S.; Paunzen, E.

    2015-12-01

    The magnetic chemically peculiar (CP2) stars of the upper main sequence are well-suited for investigating the impact of magnetic fields on the surface layers of stars, which leads to abundance inhomogeneities (spots) resulting in photometric variability. The light changes are explained in terms of the oblique rotator model; the derived photometric periods thus correlate with the rotational periods of the stars. CP2 stars exhibiting this kind of variability are classified as α2 Canum Venaticorum (ACV) variables. We have analysed around 3 850 000 individual photometric WASP measurements of magnetic chemically peculiar (CP2) stars and candidates selected from the catalogue of Ap, HgMn, and Am stars, with the ultimate goal of detecting new ACV variables. In total, we found 80 variables, from which 74 are reported here for the first time. The data allowed us to establish variability for 23 stars which had been reported as probably constant in the literature before. Light curve parameters were obtained for all stars by a least-squares fit with the fundamental sine wave and its first harmonic. Because of the scarcity of Strömgren uvbyβ measurements and the lack of parallax measurements with an accuracy better than 20%, we are not able to give reliable astrophysical parameters for the investigated objects.

  1. Using planetary transits to estimate magnetic cycles lengths in Kepler stars

    NASA Astrophysics Data System (ADS)

    Estrela, Raissa; Valio, Adriana

    2017-10-01

    Observations of various solar-type stars along decades showed that they could have magnetic cycles, just like our Sun. These observations yield a relation between the rotation period P rot and the cycle length P cycle of these stars. Two distinct branches for the cycling stars were identified: active and inactive, classified according to stellar activity level and rotation rate. In this work, we determined the magnetic activity cycle for 6 active stars observed by the Kepler telescope. The method adopted here estimates the activity from the excess in the residuals of the transit light curves. This excess is obtained by subtracting a spotless model transit from the light curve, and then integrating over all the residuals during the transit. The presence of long term periodicity is estimated from the analysis of a Lomb-Scargle periodogram of the complete time series. Finally, we investigate the rotation-cycle period relation for the stars analysed here.

  2. Mergers of magnetized neutron stars with spinning black holes: disruption, accretion, and fallback.

    PubMed

    Chawla, Sarvnipun; Anderson, Matthew; Besselman, Michael; Lehner, Luis; Liebling, Steven L; Motl, Patrick M; Neilsen, David

    2010-09-10

    We investigate the merger of a neutron star in orbit about a spinning black hole in full general relativity with a mass ratio of 5:1, allowing the star to have an initial magnetization of 10(12)  G. We present the resulting gravitational waveform and analyze the fallback accretion as the star is disrupted. We see no significant dynamical effects in the simulations or changes in the gravitational waveform resulting from the initial magnetization. We find that only a negligible amount of matter becomes unbound; 99% of the neutron star material has a fallback time of 10 seconds or shorter to reach the region of the central engine and that 99.99% of the star will interact with the central disk and black hole within 3 hours.

  3. The MiMeS survey of magnetism in massive stars: CNO surface abundances of Galactic O stars

    NASA Astrophysics Data System (ADS)

    Martins, F.; Hervé, A.; Bouret, J.-C.; Marcolino, W.; Wade, G. A.; Neiner, C.; Alecian, E.; Grunhut, J.; Petit, V.

    2015-03-01

    Context. The evolution of massive stars is still partly unconstrained. Mass, metallicity, mass loss, and rotation are the main drivers of stellar evolution. Binarity and the magnetic field may also significantly affect the fate of massive stars. Aims: Our goal is to investigate the evolution of single O stars in the Galaxy. Methods: For that, we used a sample of 74 objects comprising all luminosity classes and spectral types from O4 to O9.7. We relied on optical spectroscopy obtained in the context of the MiMeS survey of massive stars. We performed spectral modelling with the code CMFGEN. We determined the surface properties of the sample stars, with special emphasis on abundances of carbon, nitrogen, and oxygen. Results: Most of our sample stars have initial masses in the range of 20 to 50 M⊙. We show that nitrogen is more enriched and carbon and oxygen are more depleted in supergiants than in dwarfs, with giants showing intermediate degrees of mixing. CNO abundances are observed in the range of values predicted by nucleosynthesis through the CNO cycle. More massive stars, within a given luminosity class, appear to be more chemically enriched than lower mass stars. We compare our results with predictions of three types of evolutionary models and show that for two sets of models, 80% of our sample can be explained by stellar evolution including rotation. The effect of magnetism on surface abundances is unconstrained. Conclusions: Our study indicates that in the 20-50 M⊙ mass range, the surface chemical abundances of most single O stars in the Galaxy are fairly well accounted for by stellar evolution of rotating stars. Based on observations obtained at 1) the Telescope Bernard Lyot (USR5026) operated by the Observatoire Midi-Pyrénées, Université de Toulouse (Paul Sabatier), Centre National de la Recherche Scientifique of France; 2) at the Canada-France-Hawaii Telescope (CFHT) which is operated by the National Research Council (NRC) of Canada, the Institut

  4. Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars.

    PubMed

    van Saders, Jennifer L; Ceillier, Tugdual; Metcalfe, Travis S; Aguirre, Victor Silva; Pinsonneault, Marc H; García, Rafael A; Mathur, Savita; Davies, Guy R

    2016-01-14

    A knowledge of stellar ages is crucial for our understanding of many astrophysical phenomena, and yet ages can be difficult to determine. As they become older, stars lose mass and angular momentum, resulting in an observed slowdown in surface rotation. The technique of 'gyrochronology' uses the rotation period of a star to calculate its age. However, stars of known age must be used for calibration, and, until recently, the approach was untested for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for old field stars whose ages have been determined with asteroseismology. The data for the cluster agree with previous period-age relations, but these relations fail to describe the asteroseismic sample. Here we report stellar evolutionary modelling, and confirm the presence of unexpectedly rapid rotation in stars that are more evolved than the Sun. We demonstrate that models that incorporate dramatically weakened magnetic braking for old stars can--unlike existing models--reproduce both the asteroseismic and the cluster data. Our findings might suggest a fundamental change in the nature of ageing stellar dynamos, with the Sun being close to the critical transition to much weaker magnetized winds. This weakened braking limits the diagnostic power of gyrochronology for those stars that are more than halfway through their main-sequence lifetimes.

  5. Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars

    NASA Astrophysics Data System (ADS)

    van Saders, Jennifer L.; Ceillier, Tugdual; Metcalfe, Travis S.; Silva Aguirre, Victor; Pinsonneault, Marc H.; García, Rafael A.; Mathur, Savita; Davies, Guy R.

    2016-01-01

    A knowledge of stellar ages is crucial for our understanding of many astrophysical phenomena, and yet ages can be difficult to determine. As they become older, stars lose mass and angular momentum, resulting in an observed slowdown in surface rotation. The technique of ‘gyrochronology’ uses the rotation period of a star to calculate its age. However, stars of known age must be used for calibration, and, until recently, the approach was untested for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for stars in an open cluster of intermediate age (NGC 6819; 2.5 Gyr old), and for old field stars whose ages have been determined with asteroseismology. The data for the cluster agree with previous period-age relations, but these relations fail to describe the asteroseismic sample. Here we report stellar evolutionary modelling, and confirm the presence of unexpectedly rapid rotation in stars that are more evolved than the Sun. We demonstrate that models that incorporate dramatically weakened magnetic braking for old stars can—unlike existing models—reproduce both the asteroseismic and the cluster data. Our findings might suggest a fundamental change in the nature of ageing stellar dynamos, with the Sun being close to the critical transition to much weaker magnetized winds. This weakened braking limits the diagnostic power of gyrochronology for those stars that are more than halfway through their main-sequence lifetimes.

  6. The magnetic field of the BY Draconis flare star EQ Virginis

    NASA Technical Reports Server (NTRS)

    Saar, . H.; Linsky, J. L.; Beckers, J. M.

    1986-01-01

    High resolution, high S/N-ratio line profiles of the BY Draconis-type flare star EQ Vir obtained with the MMT are presently subjected to a novel Zeeman analysis procedure which includes radiative transfer effects and compensation for blends. A mean field of 2500 + or - 300 G covering 80 + or 15 percent of EQ Vir is determined. This constitutes the first positive detection of a magnetic field in a BY Draconis-type flare star, confirming that magnetic fields are present on these stars. The value of 2500 G obtained for the photospheric field strength is similar to the value derived by assuming equipartition of magnetic and thermal energy densities in the photosphere and scaling from the solar network fields, suggesting that equipartition may determine the mean field strength in the nonspotted portion of the flare star photosphere.

  7. The evolution of surface magnetic fields in young solar-type stars

    NASA Astrophysics Data System (ADS)

    Folsom, C. P.; Petit, P.; Bouvier, J.; Donati, J.-F.; Morin, J.

    2014-08-01

    The surface rotation rates of young solar-type stars decrease rapidly with age from the end of the pre-main sequence though the early main sequence. This suggests that there is also an important change in the dynamos operating in these stars, which should be observable in their surface magnetic fields. Here we present early results in a study aimed at observing the evolution of these magnetic fields through this critical time period. We are observing stars in open clusters and stellar associations to provide precise ages, and using Zeeman Doppler Imaging to characterize the complex magnetic fields. Presented here are results for six stars, three in the in the β Pic association (~10 Myr old) and three in the AB Dor association (~100 Myr old).

  8. Astronomers Find the First 'Wind Nebula' Around a Rare Ultra-Magnetic Neutron Star

    NASA Image and Video Library

    2016-06-21

    Astronomers have discovered a vast cloud of high-energy particles called a wind nebula around a rare ultra-magnetic neutron star, or magnetar, for the first time. The find offers a unique window into the properties, environment and outburst history of magnetars, which are the strongest magnets in the universe. A neutron star is the crushed core of a massive star that ran out of fuel, collapsed under its own weight, and exploded as a supernova. Each one compresses the equivalent mass of half a million Earths into a ball just 12 miles (20 kilometers) across, or about the length of New York's Manhattan Island. Neutron stars are most commonly found as pulsars, which produce radio, visible light, X-rays and gamma rays at various locations in their surrounding magnetic fields. When a pulsar spins these regions in our direction, astronomers detect pulses of emission, hence the name. Credit: ESA/XMM-Newton/Younes et al. 2016

  9. Magnetic STAR technology for real-time localization and classification of unexploded ordnance and buried mines

    NASA Astrophysics Data System (ADS)

    Wiegert, R. F.

    2009-05-01

    A man-portable Magnetic Scalar Triangulation and Ranging ("MagSTAR") technology for Detection, Localization and Classification (DLC) of unexploded ordnance (UXO) has been developed by Naval Surface Warfare Center Panama City Division (NSWC PCD) with support from the Strategic Environmental Research and Development Program (SERDP). Proof of principle of the MagSTAR concept and its unique advantages for real-time, high-mobility magnetic sensing applications have been demonstrated by field tests of a prototype man-portable MagSTAR sensor. The prototype comprises: a) An array of fluxgate magnetometers configured as a multi-tensor gradiometer, b) A GPS-synchronized signal processing system. c) Unique STAR algorithms for point-by-point, standoff DLC of magnetic targets. This paper outlines details of: i) MagSTAR theory, ii) Design and construction of the prototype sensor, iii) Signal processing algorithms recently developed to improve the technology's target-discrimination accuracy, iv) Results of field tests of the portable gradiometer system against magnetic dipole targets. The results demonstrate that the MagSTAR technology is capable of very accurate, high-speed localization of magnetic targets at standoff distances of several meters. These advantages could readily be transitioned to a wide range of defense, security and sensing applications to provide faster and more effective DLC of UXO and buried mines.

  10. MAGNETIZED NEUTRON STAR ATMOSPHERES: BEYOND THE COLD PLASMA APPROXIMATION

    SciTech Connect

    Suleimanov, V. F.; Werner, K.; Pavlov, G. G. E-mail: pavlov@astro.psu.edu

    2012-05-20

    All the neutron star (NS) atmosphere models published so far have been calculated in the 'cold plasma approximation', which neglects the relativistic effects in the radiative processes, such as cyclotron emission/absorption at harmonics of cyclotron frequency. Here, we present new NS atmosphere models which include such effects. We calculate a set of models for effective temperatures T{sub eff} = 1-3 MK and magnetic fields B {approx} 10{sup 10}-10{sup 11} G, typical for the so-called central compact objects (CCOs) in supernova remnants, for which the electron cyclotron energy E{sub c,e} and its first harmonics are in the observable soft X-ray range. Although the relativistic parameters, such as kT{sub eff}/m{sub e}c{sup 2} and E{sub c,e}/m{sub e}c{sup 2}, are very small for CCOs, the relativistic effects substantially change the emergent spectra at the cyclotron resonances, E Almost-Equal-To sE{sub c,e} (s = 1, 2, ...). Although the cyclotron absorption features can form in a cold plasma due to the quantum oscillations of the free-free opacity, the shape and depth of these features change substantially if the relativistic effects are included. In particular, the features acquire deep Doppler cores, in which the angular distribution of the emergent intensity is quite different from that in the cold plasma approximation. The relative contributions of the Doppler cores to the equivalent widths of the features grow with increasing quantization parameter b{sub eff} {identical_to} E{sub c,e}/kT{sub eff} and harmonic number s. The total equivalent widths of the features can reach {approx}150-250 eV; they increase with growing b{sub eff} and are smaller for higher harmonics.

  11. X-Ray Emission from Magnetically Torqued Disks of Oe/Be Stars

    NASA Astrophysics Data System (ADS)

    Li, Q.; Cassinelli, J. P.; Brown, J. C.; Waldron, W. L.; Miller, N. A.

    2008-01-01

    The near-main-sequence B stars show a sharp dropoff in their X-ray-to-bolometric luminosity ratio in going from B1 to later spectral types. Here we focus attention on the subset of these stars that are also Oe/Be stars, to test the concept that the disks of these stars form by magnetic channeling of wind material toward the equator. Calculations are made of the X-rays expected from the magnetically torqued disk (MTD) model for Be stars discussed by Cassinelli et al., Maheswaran, and Brown et al. In this model, the wind outflow from Be stars is channeled and torqued by a magnetic field such that the flows from the upper and lower hemispheres of the star collide as they approach the equatorial zone. X-rays are produced by the material that enters the shocks above and below the disk region and radiatively cools and compresses while moving toward the MTD central plane. The model predictions are compared with ROSAT observations obtained for an O9.5 star, ζ Oph, by Berghöfer et al. and for seven Be stars from Cohen et al. Two types of fitting models are used to compare predictions with observations of X-ray luminosity versus spectral type. Extra consideration is also given here to the well-studied Oe star ζ Oph, for which we have Chandra observations of the X-ray line profiles of the triad of He-like lines from the ion Mg XI. Thus, the X-ray properties add to the list of observables that can be explained within the context of the MTD concept. This list already includes the Hα equivalent widths and white-light polarization of Be stars.

  12. The Role of Magnetic Fields in Structuring Clouds and Forming Stars

    NASA Astrophysics Data System (ADS)

    Li, Zhi-Yun; Chen, C.-Y.; Fissel, L.; King, P.

    2017-06-01

    I will discuss the role of magnetic fields in structuring diffuse ISM and molecular clouds and in forming stars, with emphasis on the distribution of the relative orientation between the field direction as traced by dust polarization and cloud structures and the origin of the magnetically aligned striations.

  13. Hypercritical Accretion onto a Newborn Neutron Star and Magnetic Field Submergence

    NASA Astrophysics Data System (ADS)

    Bernal, Cristian G.; Page, Dany; Lee, William H.

    2013-06-01

    We present magnetohydrodynamic numerical simulations of the late post-supernova hypercritical accretion to understand its effect on the magnetic field of the newborn neutron star. We consider as an example the case of a magnetic field loop protruding from the star's surface. The accreting matter is assumed to be non-magnetized, and, due to the high accretion rate, matter pressure dominates over magnetic pressure. We find that an accretion envelope develops very rapidly, and once it becomes convectively stable, the magnetic field is easily buried and pushed into the newly forming neutron star crust. However, for low enough accretion rates the accretion envelope remains convective for an extended period of time and only partial submergence of the magnetic field occurs due to a residual field that is maintained at the interface between the forming crust and the convective envelope. In this latter case, the outcome should be a weakly magnetized neutron star with a likely complicated field geometry. In our simulations we find the transition from total to partial submergence to occur around \\dot{M} \\sim 10\\, M_\\odot yr-1. Back-diffusion of the submerged magnetic field toward the surface, and the resulting growth of the dipolar component, may result in a delayed switch-on of a pulsar on timescales of centuries to millennia.

  14. HYPERCRITICAL ACCRETION ONTO A NEWBORN NEUTRON STAR AND MAGNETIC FIELD SUBMERGENCE

    SciTech Connect

    Bernal, Cristian G.; Page, Dany; Lee, William H. E-mail: page@astro.unam.mx

    2013-06-20

    We present magnetohydrodynamic numerical simulations of the late post-supernova hypercritical accretion to understand its effect on the magnetic field of the newborn neutron star. We consider as an example the case of a magnetic field loop protruding from the star's surface. The accreting matter is assumed to be non-magnetized, and, due to the high accretion rate, matter pressure dominates over magnetic pressure. We find that an accretion envelope develops very rapidly, and once it becomes convectively stable, the magnetic field is easily buried and pushed into the newly forming neutron star crust. However, for low enough accretion rates the accretion envelope remains convective for an extended period of time and only partial submergence of the magnetic field occurs due to a residual field that is maintained at the interface between the forming crust and the convective envelope. In this latter case, the outcome should be a weakly magnetized neutron star with a likely complicated field geometry. In our simulations we find the transition from total to partial submergence to occur around M-dot {approx}10 M{sub sun} yr{sup -1}. Back-diffusion of the submerged magnetic field toward the surface, and the resulting growth of the dipolar component, may result in a delayed switch-on of a pulsar on timescales of centuries to millennia.

  15. Magnetic field estimates for accreting neutron stars in massive binary systems and models of magnetic field decay

    NASA Astrophysics Data System (ADS)

    Chashkina, A.; Popov, S. B.

    2012-08-01

    Some modern models of neutron star evolution predict that initially large magnetic fields rapidly decay down to some saturation value ˜ few ×1013 G and weaker magnetic fields do not decay significantly (Pons et al., 2009). It is difficult to check the predictions of this model for initially highly magnetized objects on the time scale of a few million years. We propose to use Be/X-ray binaries for this purpose. We apply several methods to estimate magnetic fields of neutron stars in these accreting systems using the data obtained by the RXTE satellite (Galache et al., 2008). Only using the most modern approach for estimating the magnetic field strengths of long period NSs as proposed by Shakura et al. (2012) we are able to obtain a field distribution compatible with predictions of the theoretical model of field decay of Pons et al. (2009).

  16. High-precision magnetic field measurements of Ap and Bp stars

    NASA Astrophysics Data System (ADS)

    Wade, G. A.; Donati, J.-F.; Landstreet, J. D.; Shorlin, S. L. S.

    2000-04-01

    In this paper we describe a new approach for measuring the mean longitudinal magnetic field and net linear polarization of Ap and Bp stars. As was demonstrated by Wade et al., least-squares deconvolution (LSD; Donati et al.) provides a powerful technique for detecting weak Stokes V, Q and U Zeeman signatures in stellar spectral lines. These signatures have the potential to apply strong new constraints to models of stellar magnetic field structure. Here we point out two important uses of LSD Stokes profiles. First, they can provide very precise determinations of the mean longitudinal magnetic field. In particular, this method allows one frequently to obtain 1σ error bars better than 50G, and smaller than 20G in some cases. This method is applicable to both broad- and sharp-lined stars, with both weak and strong magnetic fields, and effectively redefines the quality standard of longitudinal field determinations. Secondly, LSD profiles can in some cases provide a measure of the net linear polarization, a quantity analogous to the broad-band linear polarization recently used to derive detailed magnetic field models for a few stars (e.g. Leroy et al.). In this paper we report new high-precision measurements of the longitudinal fields of 14 magnetic Ap/Bp stars, as well as net linear polarization measurements for four of these stars, derived from LSD profiles.

  17. Magnetic Field and Rotation in Lower Main-Sequence Stars: an Empirical Time-dependent Magnetic Bode's Relation?

    NASA Astrophysics Data System (ADS)

    Baliunas, Sallie; Sokoloff, Dmitry; Soon, Willie

    1996-02-01

    We find a significant correlation between the magnetic and rotational moments for a sample of 112 lower main-sequence stars. The rotational moment is calculated from measurements of the rotation period in most of the stars (not from the projected rotational velocity inferred from Doppler broadening). The magnetic moment is computed from a database of homogeneous measurements of the mean level of Ca II H and K emission fluxes sampled for most of the stars over an interval of 25 yr. The slope connecting the logarithm of the magnetic moment and the logarithm of the rotational moment is about +0.5--0.6, with a Pearson correlation coefficient of about +0.9. The scatter of points from the mean relation has a component that is natural and caused by decade-long surface variability.

  18. Discovery of the magnetic field in the pulsating B star β Cephei

    NASA Astrophysics Data System (ADS)

    Henrichs, H. F.; de Jong, J. A.; Verdugo, E.; Schnerr, R. S.; Neiner, C.; Donati, J.-F.; Catala, C.; Shorlin, S. L. S.; Wade, G. A.; Veen, P. M.; Nichols, J. S.; Damen, E. M. F.; Talavera, A.; Hill, G. M.; Kaper, L.; Tijani, A. M.; Geers, V. C.; Wiersema, K.; Plaggenborg, B.; Rygl, K. L. J.

    2013-07-01

    Context. Although the star itself is not helium enriched, the periodicity and the variability in the UV wind lines of the pulsating B1 IV star β Cephei are similar to what is observed in magnetic helium-peculiar B stars, suggesting that β Cep is magnetic. Aims: We searched for a magnetic field using high-resolution spectropolarimetry. From UV spectroscopy, we analysed the wind variability and investigated the correlation with the magnetic data. Methods: We used 130 time-resolved circular polarisation spectra that were obtained from 1998 (when β Cep was discovered to be magnetic) to 2005, with the MuSiCoS échelle spectropolarimeter at the 2 m Télescope Bernard Lyot. We applied the least-square deconvolution method on the Stokes V spectra and derived the longitudinal component of the integrated magnetic field over the visible hemisphere of the star. We performed a period analysis on the magnetic data and on equivalent-width measurements of UV wind lines obtained over 17 years. We also analysed the short- and long-term radial velocity variations, which are due to the pulsations and the 90-year binary motion, respectively. Results: β Cep hosts a sinusoidally varying magnetic field with an amplitude 97 ± 4 G and an average value - 6 ± 3 G. From the UV wind line variability, we derive a period of 12.00075(11) days, which is the rotation period of the star, and is compatible with the observed magnetic modulation. Phases of maximum and minimum field match those of maximum emission in the UV wind lines, strongly supporting an oblique magnetic-rotator model. We discuss the magnetic behaviour as a function of pulsation behaviour and UV line variability. Conclusions: This paper presents the analysis of the first confirmed detection of a dipolar magnetic field in an upper main-sequence pulsating star. Maximum wind absorption originates in the magnetic equatorial plane. Maximum emission occurs when the magnetic north pole points to the Earth. Radial velocities agree with

  19. Discovery of a magnetic field in the δ Scuti F2m star ρ Pup

    NASA Astrophysics Data System (ADS)

    Neiner, C.; Wade, G. A.; Sikora, J.

    2017-06-01

    ρ Pup is a δ Scuti F2 pulsator, known to host a main radial mode as well as non-radial pulsations, with chemical peculiarities typical of evolved Am stars. We present a high-precision spectropolarimetric observations of this star, obtained with ESPaDOnS at the Canada-France-Hawaii Telescope in the frame of the BRITE spectropolarimetric survey. A magnetic field is clearly detected in ρ Pup, with a longitudinal field strength below 1 G. This makes ρ Pup the second known magnetic δ Scuti discovered, after HD 188774, and a possible cool evolved counterpart of the recently discovered ultraweakly magnetic Am family.

  20. Asteroseismology can reveal strong internal magnetic fields in red giant stars.

    PubMed

    Fuller, Jim; Cantiello, Matteo; Stello, Dennis; Garcia, Rafael A; Bildsten, Lars

    2015-10-23

    Internal stellar magnetic fields are inaccessible to direct observations, and little is known about their amplitude, geometry, and evolution. We demonstrate that strong magnetic fields in the cores of red giant stars can be identified with asteroseismology. The fields can manifest themselves via depressed dipole stellar oscillation modes, arising from a magnetic greenhouse effect that scatters and traps oscillation-mode energy within the core of the star. The Kepler satellite has observed a few dozen red giants with depressed dipole modes, which we interpret as stars with strongly magnetized cores. We find that field strengths larger than ~10(5) gauss may produce the observed depression, and in one case we infer a minimum core field strength of ≈10(7) gauss. Copyright © 2015, American Association for the Advancement of Science.

  1. Asteroseismology can reveal strong internal magnetic fields in red giant stars

    NASA Astrophysics Data System (ADS)

    Fuller, Jim; Cantiello, Matteo; Stello, Dennis; Garcia, Rafael A.; Bildsten, Lars

    2015-10-01

    Internal stellar magnetic fields are inaccessible to direct observations, and little is known about their amplitude, geometry, and evolution. We demonstrate that strong magnetic fields in the cores of red giant stars can be identified with asteroseismology. The fields can manifest themselves via depressed dipole stellar oscillation modes, arising from a magnetic greenhouse effect that scatters and traps oscillation-mode energy within the core of the star. The Kepler satellite has observed a few dozen red giants with depressed dipole modes, which we interpret as stars with strongly magnetized cores. We find that field strengths larger than ~105 gauss may produce the observed depression, and in one case we infer a minimum core field strength of ≈107 gauss.

  2. The Suppression and Promotion of Magnetic Flux Emergence in Fully Convective Stars

    NASA Astrophysics Data System (ADS)

    Weber, Maria A.; Browning, Matthew K.; Boardman, Suzannah; Clarke, Joshua; Pugsley, Samuel; Townsend, Edward

    2017-10-01

    Evidence of surface magnetism is now observed on an increasing number of cool stars. The detailed manner by which dynamo-generated magnetic fields giving rise to starspots traverse the convection zone still remains unclear. Some insight into this flux emergence mechanism has been gained by assuming bundles of magnetic field can be represented by idealized thin flux tubes (TFTs). Weber & Browning (2016) have recently investigated how individual flux tubes might evolve in a 0.3M⊙ M dwarf by effectively embedding TFTs in time-dependent flows representative of a fully convective star. We expand upon this work by initiating flux tubes at various depths in the upper ~50-75% of the star in order to sample the differing convective flow pattern and differential rotation across this region. Specifically, we comment on the role of differential rotation and time-varying flows in both the suppression and promotion of the magnetic flux emergence process.

  3. HERSCHEL-RESOLVED OUTER BELTS OF TWO-BELT DEBRIS DISKS AROUND A-TYPE STARS: HD 70313, HD 71722, HD 159492, AND F-TYPE: HD 104860

    SciTech Connect

    Morales, F. Y.; Bryden, G.; Werner, M. W.; Stapelfeldt, K. R.

    2013-10-20

    We present dual-band Herschel/Photodetector Array Camera and Spectrometer imaging for four stars whose spectral energy distributions (SEDs) suggest two-ring disk architectures that mirror that of the asteroid-Kuiper Belt geometry of our own solar system. The Herschel observations at 100 μm spatially resolve the cold/outer-dust component for each star-disk system for the first time, finding evidence of planetesimals at >100 AU, i.e., a larger size than assumed from a simple blackbody fit to the SED. By breaking the degeneracy between the grain properties and the dust's radial location, the resolved images help constrain the dust grain-size distribution for each system. Three of the observed stars are A-type and one solar-type. On the basis of the combined Spitzer/IRS+MIPS (5-70 μm), the Herschel/PACS (100 and 160 μm) dataset, and under the assumption of idealized spherical grains, we find that the cold/outer belts of the three A-type stars are well fit with a mixed ice/rock composition rather than pure rocky grains, while the debris around the solar-type star is consistent with either rock or ice/rock grains. For the solar-type star HD 104860, we find that the minimum grain size is larger than expected from the threshold set by radiative blowout. The A-type stars HD 71722 and HD 159492, on the other hand, require minimum grain sizes that are smaller than blowout for inner- and outer-ring populations. In the absence of spectral features for ice, we find that the behavior of the continuum can help constrain the composition of the grains (of icy nature and not pure rocky material) given the Herschel-resolved locations of the cold/outer-dust belts.

  4. Exploring the origin of magnetic fields in massive stars: a survey of O-type stars in clusters and in the field

    NASA Astrophysics Data System (ADS)

    Hubrig, S.; Schöller, M.; Kharchenko, N. V.; Langer, N.; de Wit, W. J.; Ilyin, I.; Kholtygin, A. F.; Piskunov, A. E.; Przybilla, N.; Magori Collaboration

    2011-04-01

    Context. Although the effects of magnetic fields in massive stars have been found to be substantial by recent models and observations, the magnetic fields of only a small number of massive O-type stars have so far been investigated. Additional observations are of the utmost importance to constraining the conditions that are conducive to magnetic fields and to determine the first trends about their occurrence rate and field strength distribution. Aims: To investigate statistically whether magnetic fields in massive stars are ubiquitous or appear only in stars with a specific spectral classification, certain ages, or in a special environment, we acquired 41 new spectropolarimetric observations for 36 stars. Among the observed sample, roughly half of the stars are probable members of clusters at different ages, whereas the remaining stars are field stars not known to belong to any cluster or association. Methods: Spectropolarimetric observations were obtained during three different nights using the low-resolution spectropolarimetric mode of FORS 2 (FOcal Reducer low dispersion Spectrograph) mounted on the 8-m Antu telescope of the VLT. To assess the membership in open clusters and associations, we used astrometric catalogues with the highest quality kinematic and photometric data currently available. Results: A field at a significance level of 3σ was detected in ten O-type stars. The strongest longitudinal magnetic fields were measured in two Of?p stars: ⟨ Bz ⟩ = -381 ± 122 G for CPD-28 2561 and ⟨ Bz ⟩ = -297 ± 62 G for HD 148937, the latter of which had previously been detected by ourselves as magnetic. The observations of HD 148937 obtained on three different nights indicate that the magnetic field is slightly variable. Our new measurements support our previous conclusion that large-scale organized magnetic fields with polar field strengths in excess of 1 kG are not widespread among O-type stars. Among the stars with a detected magnetic field, only one

  5. A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars

    NASA Astrophysics Data System (ADS)

    Stello, Dennis; Cantiello, Matteo; Fuller, Jim; Huber, Daniel; García, Rafael A.; Bedding, Timothy R.; Bildsten, Lars; Silva Aguirre, Victor

    2016-01-01

    Magnetic fields play a part in almost all stages of stellar evolution. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes. Intermediate-mass stars do not have deep convective envelopes, although 10 per cent exhibit strong surface fields that are presumed to be residuals from the star formation process. These stars do have convective cores that might produce internal magnetic fields, and these fields might survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface. Here we report the strength of dipolar oscillation modes for a sample of 3,600 red giant stars. About 20 per cent of our sample show mode suppression, by strong magnetic fields in the cores, but this fraction is a strong function of mass. Strong core fields occur only in red giants heavier than 1.1 solar masses, and the occurrence rate is at least 50 per cent for intermediate-mass stars (1.6-2.0 solar masses), indicating that powerful dynamos were very common in the previously convective cores of these stars.

  6. A prevalence of dynamo-generated magnetic fields in the cores of intermediate-mass stars.

    PubMed

    Stello, Dennis; Cantiello, Matteo; Fuller, Jim; Huber, Daniel; García, Rafael A; Bedding, Timothy R; Bildsten, Lars; Aguirre, Victor Silva

    2016-01-21

    Magnetic fields play a part in almost all stages of stellar evolution. Most low-mass stars, including the Sun, show surface fields that are generated by dynamo processes in their convective envelopes. Intermediate-mass stars do not have deep convective envelopes, although 10 per cent exhibit strong surface fields that are presumed to be residuals from the star formation process. These stars do have convective cores that might produce internal magnetic fields, and these fields might survive into later stages of stellar evolution, but information has been limited by our inability to measure the fields below the stellar surface. Here we report the strength of dipolar oscillation modes for a sample of 3,600 red giant stars. About 20 per cent of our sample show mode suppression, by strong magnetic fields in the cores, but this fraction is a strong function of mass. Strong core fields occur only in red giants heavier than 1.1 solar masses, and the occurrence rate is at least 50 per cent for intermediate-mass stars (1.6-2.0 solar masses), indicating that powerful dynamos were very common in the previously convective cores of these stars.

  7. Bright transients from strongly-magnetized neutron star-black hole mergers

    NASA Astrophysics Data System (ADS)

    D'Orazio, Daniel J.; Levin, Janna; Murray, Norman W.; Price, Larry

    2016-07-01

    Direct detection of black hole-neutron star pairs is anticipated with the advent of aLIGO. Electromagnetic counterparts may be crucial for a confident gravitational-wave detection as well as for extraction of astronomical information. Yet black hole-neutron star pairs are notoriously dark and so inaccessible to telescopes. Contrary to this expectation, a bright electromagnetic transient can occur in the final moments before merger as long as the neutron star is highly magnetized. The orbital motion of the neutron star magnet creates a Faraday flux and corresponding power available for luminosity. A spectrum of curvature radiation ramps up until the rapid injection of energy ignites a fireball, which would appear as an energetic blackbody peaking in the x ray to γ rays for neutron star field strengths ranging from 1012 to 1016 G respectively and a 10 M⊙ black hole. The fireball event may last from a few milliseconds to a few seconds depending on the neutron star magnetic-field strength, and may be observable with the Fermi Gamma-Ray Burst Monitor with a rate up to a few per year for neutron star field strengths ≳1014 G . We also discuss a possible decaying post-merger event which could accompany this signal. As an electromagnetic counterpart to these otherwise dark pairs, the black-hole battery should be of great value to the development of multi-messenger astronomy in the era of aLIGO.

  8. Searching for a magnetic field in Wolf-Rayet stars using FORS 2 spectropolarimetry

    NASA Astrophysics Data System (ADS)

    Hubrig, S.; Scholz, K.; Hamann, W.-R.; Schöller, M.; Ignace, R.; Ilyin, I.; Gayley, K. G.; Oskinova, L. M.

    2016-05-01

    To investigate if magnetic fields are present in Wolf-Rayet stars, we selected a few stars in the Galaxy and one in the Large Magellanic Cloud (LMC). We acquired low-resolution spectropolarimetric observations with the European Southern Observatory FORS 2 (FOcal Reducer low dispersion Spectrograph) instrument during two different observing runs. During the first run in visitor mode, we observed the LMC Wolf-Rayet star BAT99 7 and the stars WR 6, WR 7, WR 18, and WR 23 in our Galaxy. The second run in service mode was focused on monitoring the star WR 6. Linear polarization was recorded immediately after the observations of circular polarization. During our visitor observing run, the magnetic field for the cyclically variable star WR 6 was measured at a significance level of 3.3σ ( = 258 ± 78 G). Among the other targets, the highest value for the longitudinal magnetic field, = 327 ± 141 G, was measured in the LMC star BAT99 7. Spectropolarimetric monitoring of the star WR 6 revealed a sinusoidal nature of the variations with the known rotation period of 3.77 d, significantly adding to the confidence in the detection. The presence of the rotation-modulated magnetic variability is also indicated in our frequency periodogram. The reported field magnitude suffers from significant systematic uncertainties at the factor of 2 level, in addition to the quoted statistical uncertainties, owing to the theoretical approach used to characterize it. Linear polarization measurements showed no line effect in the stars, apart from WR 6. BAT99 7, WR 7, and WR 23 do not show variability of the linear polarization over two nights.

  9. Stability analysis of magnetized neutron stars - a semi-analytic approach

    NASA Astrophysics Data System (ADS)

    Herbrik, Marlene; Kokkotas, Kostas D.

    2017-04-01

    We implement a semi-analytic approach for stability analysis, addressing the ongoing uncertainty about stability and structure of neutron star magnetic fields. Applying the energy variational principle, a model system is displaced from its equilibrium state. The related energy density variation is set up analytically, whereas its volume integration is carried out numerically. This facilitates the consideration of more realistic neutron star characteristics within the model compared to analytical treatments. At the same time, our method retains the possibility to yield general information about neutron star magnetic field and composition structures that are likely to be stable. In contrast to numerical studies, classes of parametrized systems can be studied at once, finally constraining realistic configurations for interior neutron star magnetic fields. We apply the stability analysis scheme on polytropic and non-barotropic neutron stars with toroidal, poloidal and mixed fields testing their stability in a Newtonian framework. Furthermore, we provide the analytical scheme for dropping the Cowling approximation in an axisymmetric system and investigate its impact. Our results confirm the instability of simple magnetized neutron star models as well as a stabilization tendency in the case of mixed fields and stratification. These findings agree with analytical studies whose spectrum of model systems we extend by lifting former simplifications.

  10. H-beta line variability in magnetic Ap stars. I

    NASA Technical Reports Server (NTRS)

    Madej, J.; Jahn, K.; Stepien, K.

    1984-01-01

    Preliminary results of photometric measurements of H-beta in several Ap stars are presented. Periodic variations are found certainly in Theta Aur and Alpha (2) CVn, and possibly in Phi Dra. For the other stars upper limits for variations of H-beta are determined. Observed amplitudes are transformed into variations of equivalent width assuming specific profile variations. The results show that variations of equivalent width of H-beta in the stars investigated are of the order of 10 percent or less.

  11. First constraints on the magnetic field strength in extra-Galactic stars: FORS2 observations of Of?p stars in the Magellanic Clouds

    NASA Astrophysics Data System (ADS)

    Bagnulo, S.; Nazé, Y.; Howarth, I. D.; Morrell, N.; Vink, J. S.; Wade, G. A.; Walborn, N.; Romaniello, M.; Barbá, R.

    2017-05-01

    Massive O-type stars play a dominant role in our Universe, but many of their properties remain poorly constrained. In the last decade magnetic fields have been detected in all Galactic members of the distinctive Of?p class, opening the door to a better knowledge of all O-type stars. With the aim of extending the study of magnetic massive stars to nearby galaxies, to better understand the role of metallicity in the formation of their magnetic fields and magnetospheres, and to broaden our knowledge of the role of magnetic fields in massive star evolution, we carried out spectropolarimetry of five extra-Galactic Of?p stars, and of a couple of dozen neighbouring stars. We were able to measure magnetic fields with typical error bars from 0.2 to 1.0 kG, depending on the apparent magnitude and on weather conditions. No magnetic field was firmly detected in any of our measurements, but we were able to estimate upper limits on the field values of our target stars. One of our targets, 2dFS 936, exhibited an unexpected strengthening of emission lines. We confirm the unusual behaviour of BI 57, which exhibits a 787 d period with two photometric peaks and one spectroscopic maximum. The observed strengthening of the emission lines of 2dFS 936, and the lack of detection of a strong magnetic field in a star with such strong emission lines is at odd with expectations. Together with the unusual periodic behaviour of BI 57, it represents a challenge for the current models of Of?p stars. The limited precision that we obtained in our field measurements (in most cases as a consequence of poor weather) has led to field-strength upper limits that are substantially larger than those typically measured in Galactic magnetic O stars. Further higher precision observations and monitoring are clearly required.

  12. Magnetic models can not explain the Blazhko effect in RR Lyrae stars

    NASA Astrophysics Data System (ADS)

    Chadid, M.; Wade, G. A.; Shorlin, S. L. S.; Landstreet, J. D.

    2004-05-01

    We report a new series of high-precision Stokes V profiles and longitudinal magnetic field measurements of RR Lyrae, obtained with the MuSiCoS spectropolarimeter over a period of four years. These data provide no evidence whatsoever for a strong magnetic field in the photosphere of RR Lyrae, whish is consistent with Preston's (1967) results, but inconsistent with apparent magnetic field detections by Babcock (1958) and Romanov et a. (1987, 1994). Following discussion of these disparate results, we conclude that RR Lyrae is a bona fide non-magnetic star, a conclusion which leads to the general falsification of models of the Blazkho effect requiring strong photospheric magnetic fields.

  13. Radio-wavelength observations of magnetic fields on active dwarf-M, RS CVN and magnetic stars

    SciTech Connect

    Lang, K.R.

    1986-01-01

    The dwarf M stars YZ Canis Minoris and AD Leonis exhibit narrow band, slowly varying (hours) microwave emission that cannot be explained by conventional thermal radiation mechanisms. The dwarf M stars AD Leonis and Wolf 424 emit rapid spikes whose high brightness temperatures similarly require a nonthermal radiation process which could result from coherent mechanisms such as an electron-cyclotron maser or coherent-plasma radiation. If the electron-cyclotron maser emits at the second or third harmonic of the gyrofrequency, the coronal magnetic field strength H = 250 or 167 G and constraints on the plasma frequency imply an electron density of 6 x 10/sup 9//cm/sup 3/. Coherent-plasma radiation requires similar values of electron density but much weaker magnetic fields. Radio spikes from AD Leonis and Wolf 424 have rise times tau/sub R/ < 5 ms, indicating a linear size of L < 1.5 x 10/sup 8/ cm, or less than 0.005 of the stellar radius. Although Ap magnetic stars have strong dipole magnetic fields, they exhibit no detectable gyroresonant radiation, suggesting that these stars do not have hot, dense coronae. The binary RS CVn star UX Arietis exhibits variable emission at 6 cm wavelength on time scales ranging from 30 s to more than one hour. The shortest variation implies a linear size much less than that of the halo observed by VLBI techniques, and most probably sizes smaller than those of the component stars. The observed variations might be due to absorption by a thermal plasma located between the stars.

  14. ON THE MAGNETIC FIELD OF PULSARS WITH REALISTIC NEUTRON STAR CONFIGURATIONS

    SciTech Connect

    Belvedere, R.; Rueda, Jorge A.; Ruffini, R. E-mail: jorge.rueda@icra.it

    2015-01-20

    We have recently developed a neutron star model fulfilling global and not local charge neutrality, both in the static and in the uniformly rotating cases. The model is described by the coupled Einstein-Maxwell-Thomas-Fermi equations, in which all fundamental interactions are accounted for in the framework of general relativity and relativistic mean field theory. Uniform rotation is introduced following Hartle's formalism. We show that the use of realistic parameters of rotating neutron stars, obtained from numerical integration of the self-consistent axisymmetric general relativistic equations of equilibrium, leads to values of the magnetic field and radiation efficiency of pulsars that are very different from estimates based on fiducial parameters that assume a neutron star mass M = 1.4 M {sub ☉}, radius R = 10 km, and moment of inertia I = 10{sup 45} g cm{sup 2}. In addition, we compare and contrast the magnetic field inferred from the traditional Newtonian rotating magnetic dipole model with respect to the one obtained from its general relativistic analog, which takes into account the effect of the finite size of the source. We apply these considerations to the specific high-magnetic field pulsar class and show that, indeed, all of these sources can be described as canonical pulsars driven by the rotational energy of the neutron star, and have magnetic fields lower than the quantum critical field for any value of the neutron star mass.

  15. Magnetic Cycles in a Dynamo Simulation of Fully Convective M-star Proxima Centauri

    NASA Astrophysics Data System (ADS)

    Yadav, Rakesh K.; Christensen, Ulrich R.; Wolk, Scott J.; Poppenhaeger, Katja

    2016-12-01

    The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period ≲20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here, we present an anelastic dynamo simulation designed to mimic some of the physical characteristics of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone that drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the “activity” cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.

  16. B fields in OB stars (BOB): The discovery of a magnetic field in a multiple system in the Trifid nebula, one of the youngest star forming regions

    NASA Astrophysics Data System (ADS)

    Hubrig, S.; Fossati, L.; Carroll, T. A.; Castro, N.; González, J. F.; Ilyin, I.; Przybilla, N.; Schöller, M.; Oskinova, L. M.; Morel, T.; Langer, N.; Scholz, R. D.; Kharchenko, N. V.; Nieva, M.-F.

    2014-04-01

    Aims: Recent magnetic field surveys in O- and B-type stars revealed that about 10% of the core-hydrogen-burning massive stars host large-scale magnetic fields. The physical origin of these fields is highly debated. To identify and model the physical processes responsible for the generation of magnetic fields in massive stars, it is important to establish whether magnetic massive stars are found in very young star-forming regions or whether they are formed in close interacting binary systems. Methods: In the framework of our ESO Large Program, we carried out low-resolution spectropolarimetric observations with FORS 2 in 2013 April of the three most massive central stars in the Trifid nebula, HD 164492A, HD 164492C, and HD 164492D. These observations indicated a strong longitudinal magnetic field of about 500-600 G in the poorly studied component HD 164492C. To confirm this detection, we used HARPS in spectropolarimetric mode on two consecutive nights in 2013 June. Results: Our HARPS observations confirmed the longitudinal magnetic field in HD 164492C. Furthermore, the HARPS observations revealed that HD 164492C cannot be considered as a single star as it possesses one or two companions. The spectral appearance indicates that the primary is most likely of spectral type B1-B1.5 V. Since in both observing nights most spectral lines appear blended, it is currently unclear which components are magnetic. Long-term monitoring using high-resolution spectropolarimetry is necessary to separate the contribution of each component to the magnetic signal. Given the location of the system HD 164492C in one of the youngest star formation regions, this system can be considered as a Rosetta Stone for our understanding of the origin of magnetic fields in massive stars. Based on observations obtained in the framework of the ESO Prg. 191.D-0255(A,B).

  17. Discovery of a magnetic field in the early B-type star σ Lupi

    NASA Astrophysics Data System (ADS)

    Henrichs, H. F.; Kolenberg, K.; Plaggenborg, B.; Marsden, S. C.; Waite, I. A.; Landstreet, J. D.; Wade, G. A.; Grunhut, J. H.; Oksala, M. E.

    2012-09-01

    Context. Magnetic early B-type stars are rare. Indirect indicators are needed to identify them before investing in time-intensive spectropolarimetric observations. Aims: We use the strongest indirect indicator of a magnetic field in B stars, which is periodic variability of ultraviolet (UV) stellar wind lines occurring symmetric about the approximate rest wavelength. Our aim is to identify probable magnetic candidates which would become targets for follow-up spectropolarimetry to search for a magnetic field. Methods: From the UV wind line variability the B1/B2V star σ Lupi emerged as a new magnetic candidate star. AAT spectropolarimetric measurements with SEMPOL were obtained. The longitudinal component of the magnetic field integrated over the visible surface of the star was determined with the least-squares deconvolution method. Results: The UV line variations of σ Lupi are similar to what is known in magnetic B stars, but no periodicity could be determined. We detected a varying longitudinal magnetic field with amplitude of about 100 G with error bars of typically 20 G, which supports an oblique magnetic-rotator configuration. The equivalent width variations of the UV lines, the magnetic and the optical-line variations are consistent with the photometric period of 3.02 d, which we identify with the rotation period of the star. Additional observations with ESPaDOnS attached to the CFHT confirmed this discovery, and allowed the determination of a precise magnetic period. Analysis revealed that σ Lupi is a helium-strong star, with an enhanced nitrogen abundance and an underabundance of carbon, and has a chemically spotted surface. Conclusions.σ Lupi is a magnetic oblique rotator, and is a He-strong star. Like in other magnetic B stars the UV wind emission appears to originate close to the magnetic equatorial plane, with maximum emission occurring when a magnetic pole points towards the Earth. The 3.01972 ± 0.00043 d magnetic rotation period is consistent with

  18. Quark matter with strong magnetic field and possibility of the third family of compact stars

    NASA Astrophysics Data System (ADS)

    Sotani, Hajime; Tatsumi, Toshitaka

    2017-05-01

    We consider the possibility for the existence of the third family of compact objects, considering the effect of strong magnetic fields inside the hybrid stars. As a result, we demonstrate such new sequences of stable equilibrium configurations for some hadronic equations of state. Through the analysis of the adiabatic index inside stars, we find the conditions for appearing the third family of compact objects, i.e. for hadronic stars without quarks, that the maximum mass should be small, the central density for the maximum mass should be also small, and the radius for the maximum mass should be large. Even for soft hadronic equations of state, the two solar-mass stars might survive as the third family of compact objects, once quark matter with strong magnetic field, such as {˜ } O(10^{19} G), is taken into account. It might give a hint to solve the so-called hyperon puzzle in nuclear physics.

  19. Iron K photons from weakly magnetized neutron stars in X-ray binaries

    NASA Technical Reports Server (NTRS)

    Bai, T.

    1980-01-01

    The emission of iron K photons by the continuum X-ray source and the neutron star surface is considered. It is shown that the continuum sources of X-ray binaries produce negligible amounts of iron K photons because nearly all iron atoms in the continuum source are fully stripped due to the intense X-ray fluxes. In contrast, the atmosphere of the neutron star in an X-ray binary might be an important source of iron K photons (photon energy about 6.5 keV) because it is bombarded by a large number of hard X-rays capable of photo-ejecting K-shell electrons from iron atoms. Information is discussed concerning the magnetic field strength, the gravitational potential at the neutron star surface, and the direction of the magnetic dipole axis which are obtainable from the observations of K photons of the neutron star atmosphere.

  20. Magnetic Inhibition of Convection and the Fundamental Properties of Low-mass Stars. I. Stars with a Radiative Core

    NASA Astrophysics Data System (ADS)

    Feiden, Gregory A.; Chaboyer, Brian

    2013-12-01

    Magnetic fields are hypothesized to inflate the radii of low-mass stars—defined as less massive than 0.8 M ⊙—in detached eclipsing binaries (DEBs). We investigate this hypothesis using the recently introduced magnetic Dartmouth stellar evolution code. In particular, we focus on stars thought to have a radiative core and convective outer envelope by studying in detail three individual DEBs: UV Psc, YY Gem, and CU Cnc. Our results suggest that the stabilization of thermal convection by a magnetic field is a plausible explanation for the observed model-radius discrepancies. However, surface magnetic field strengths required by the models are significantly stronger than those estimated from observed coronal X-ray emission. Agreement between model predicted surface magnetic field strengths and those inferred from X-ray observations can be found by assuming that the magnetic field sources its energy from convection. This approach makes the transport of heat by convection less efficient and is akin to reduced convective mixing length methods used in other studies. Predictions for the metallicity and magnetic field strengths of the aforementioned systems are reported. We also develop an expression relating a reduction in the convective mixing length to a magnetic field strength in units of the equipartition value. Our results are compared with those from previous investigations to incorporate magnetic fields to explain the low-mass DEB radius inflation. Finally, we explore how the effects of magnetic fields might affect mass determinations using asteroseismic data and the implication of magnetic fields on exoplanet studies.

  1. Magnetic inhibition of convection and the fundamental properties of low-mass stars. I. Stars with a radiative core

    SciTech Connect

    Feiden, Gregory A.; Chaboyer, Brian E-mail: brian.chaboyer@dartmouth.edu

    2013-12-20

    Magnetic fields are hypothesized to inflate the radii of low-mass stars—defined as less massive than 0.8 M {sub ☉}—in detached eclipsing binaries (DEBs). We investigate this hypothesis using the recently introduced magnetic Dartmouth stellar evolution code. In particular, we focus on stars thought to have a radiative core and convective outer envelope by studying in detail three individual DEBs: UV Psc, YY Gem, and CU Cnc. Our results suggest that the stabilization of thermal convection by a magnetic field is a plausible explanation for the observed model-radius discrepancies. However, surface magnetic field strengths required by the models are significantly stronger than those estimated from observed coronal X-ray emission. Agreement between model predicted surface magnetic field strengths and those inferred from X-ray observations can be found by assuming that the magnetic field sources its energy from convection. This approach makes the transport of heat by convection less efficient and is akin to reduced convective mixing length methods used in other studies. Predictions for the metallicity and magnetic field strengths of the aforementioned systems are reported. We also develop an expression relating a reduction in the convective mixing length to a magnetic field strength in units of the equipartition value. Our results are compared with those from previous investigations to incorporate magnetic fields to explain the low-mass DEB radius inflation. Finally, we explore how the effects of magnetic fields might affect mass determinations using asteroseismic data and the implication of magnetic fields on exoplanet studies.

  2. A 40 Myr OLD GASEOUS CIRCUMSTELLAR DISK AT 49 CETI: MASSIVE CO-RICH COMET CLOUDS AT YOUNG A-TYPE STARS

    SciTech Connect

    Zuckerman, B.; Song, Inseok E-mail: song@physast.uga.edu

    2012-10-20

    The gaseous molecular disk that orbits the main-sequence A-type star 49 Ceti has been known since 1995, but the stellar age and the origin of the observed carbon monoxide molecules have been unknown. We now identify 49 Ceti as a member of the 40 Myr old Argus Association and present a colliding comet model to explain the high CO concentrations seen at 49 Ceti and the 30 Myr old A-type star HD 21997. The model suggests that massive-400 Earth mass-analogs of the Sun's Kuiper Belt are in orbit around some A-type stars, that these large masses are composed primarily of comet-like objects, and that these objects are rich in CO and perhaps also CO{sub 2}. We identify additional early-type members of the Argus Association and the Tucana/Horologium and Columba Associations; some of these stars display excess mid-infrared emission as measured with the Widefield Infrared Survey Explorer.

  3. Coupled Spin, Mass, Magnetic field, and Orbital Evolution of Accreting Neutron stars

    NASA Astrophysics Data System (ADS)

    Mirtorabi, M.; Javadi Khasraghi, A.; Abdolrahimi, S.

    2006-08-01

    The present study is mainly addressed to the coupled spin, mass, magnetic field, orbital separation, and orbital period evolution of a neutron star entering a close binary system with a low mass main sequence companion, which loses mass in form of homogenous stellar wind. We apply flux expulsion of the magnetic field from the superfluid superconductive core of a neutron star , based on different equation of states, and its subsequent decay in the crust , which also depends on conductivity of the crust, and hence on the temperature, T, and the neutron star age. The initial core and surface magnetic field are of the same order of magnitude. To derive the rate of expulsion of the magnetic flux out of the core we consider various forces which act on the fluxoids in the interior of a neutron star, including a force due to their pinning interaction with the moving neutron vortices, buoyancy force, curvature force, and viscous drag force due to magnetic scattering of electrons. Various effects accompanying mass exchange in binaries can influence the evolution of spin and magnetic field of the neutron star. The orbital separation of the binary clearly affects the estimated value of , and it itself evolves due to mass exchange between the components, mass loss from the system, and two other sinks of the orbital angular momentum namely magnetic braking and gravitational waves. The neutron star passes through four evolutionary phases ( isolated pulsar- propeller- accretion from the wind of a companion- accretion resulting from Roche-lobe overflow). Models for a range of parameters, and initial orbital period, magnetic field and spin period are constructed. The impurity parameter, Q, is assumed to be constant during the whole evolution of the star and range from 1 to 0.001. Final magnetic field, spin and orbital period are presented in this paper. The surface magnetic field weakens by a factor of .The suggested mechanism can explain the lower magnetic field and faster spins of

  4. Magnetic fields of chemically peculiar and related stars. III. Main results of 2016 and analysis of closest perspectives

    NASA Astrophysics Data System (ADS)

    Romanyuk, I. I.

    2017-07-01

    We have analyzed more than 90 papers in the area "Magnetic fields and physical parameters of chemically peculiar and related stars," published mainly in 2016. The main results of the period under survey are as follows. The search for new magnetic stars continued.Many measurements weremade at the 6-m BTA telescope of the SAO RAS, new data on stellar magnetism in the OrionOB1 association were obtained. A systematic study ofmagnetic fields of stars with large anomalies in the energy distribution in the continuum was started. New data on ultra-slowmagnetic rotators—chemically peculiar stars with rotation periods of years and decades are obtained. Successful observations on the search for new magnetic stars are performed among the objects of the southern sky in Chile at the FORS2 VLT spectropolarimeter. A new direction was developed, namely, the study of binarymagnetic stars. Depending on the mass-distance ratio between the components, interaction with the magnetosphere and, possibly, magnetic braking may occur. The study of the details of this process is important for the theory of formation of stellar magnetic fields. The search for large-scale, but weak magnetic fields (magnitude of unities and tens of G) in non-CP stars is ongoing. Such fields are found in Am stars. No fields were found in the classical Be stars. Cool stars of various types were studied in detail. They manifested magnetic fields of a complex structure. Their mapping was performed, changes in the topology of the field were found at timescales of several years. Spectral and photometric variability was studied. Dozens of new potentially magnetic stars are discovered as a result of the ASAS-3, SuperWASP, Stereo and Kepler surveys. High-accuracy observations of rapidly oscillating stars were performed with the BRITE nanosatellite.Work continued on the studies of magnetic and photometric variability of white dwarfs. Finally, an overview of several papers on exoplanets, related with the subject of our

  5. Coronal magnetic field and wind of an aging K-type star

    NASA Astrophysics Data System (ADS)

    Réville, Victor; Brun, Allan Sacha; Strugarek, Antoine; Jeffers, Sandra; Folsom, Colin; Marsden, Stephen C.; Petit, Pascal

    2015-08-01

    Created at the base of the convective envelope by a nonlinear dynamo process, the large scale magnetic field of a star evolves with its rotational history. Beyond the photosphere, magnetic processes heat the corona above one million Kelvin hence driving a magnetized wind responsible for the braking of main sequence stars. Hence a feedback loop tie those processes. Development of Zeeman-Doppler imaging through spectropolarimetry allows to precisely describe the surface magnetic field of a large sample of stars. Thus the study of the coronal structure and magnetic field with age, magnetochoronology, has developed to extend and complete gyrochronology. We propose a study of the corona and the wind of a sample of K-type stars of different age to follow the evolution of its properties from 20 Myr to 8 Gyr thanks to a set of 3D MHD simulations with the PLUTO code constrained by spectropolarimetric maps of the surface magnetic field obtained by the BCool consortium. To perform those simulations we developed a coherent framework to assess various stellar parameters such as the equilibrium coronal temperature driving the wind. Those assumptions have consequences on UV emissions, wind terminal speed and mass loss that impact planetary systems that could potentially host life.

  6. Triggered Collapse, Magnetic Fields, and Very Low Mass Star Formation (Invited Review)

    NASA Astrophysics Data System (ADS)

    Boss, A. P.

    Protostellar collapse calculations traditionally ignore the effects of nearby stars and of magnetic fields. Including these effects can help answer several questions, such as the origin of the Solar System and of free-floating planetary mass objects. Shock waves derived from massive stars can trigger the collapse of an otherwise stable dense cloud core, leading to the formation of single or binary low mass stars. Suitable sources of shock waves include distant supernovae, AGB winds, and nearby protostellar outflows. Short-lived radioisotopes produced by stellar nucleosynthesis or spallation can be injected into the collapsing cloud through Rayleigh-Taylor fingers, a process that may explain evidence for certain live radioisotopes in the early solar nebula. Magnetic fields are known to be dynamically important for dense cloud cores, yet have seldom been included in protostellar collapse and fragmentation calculations. The inclusion of magnetic field effects in a crude manner in these calculations suggests that, contrary to intuition, magnetic fields do not appear to inhibit fragmentation of a collapsing cloud into a binary or multiple protostar system. Rather, the ability of magnetic field tension effects to prevent the formation of a central density singularity seems to encourage fragmentation. Decompressional cooling during a magnetically-assisted rebound lowers the Jeans mass toward planetary values, suggesting that sub-brown dwarf stars could form from the collapse of solar mass dense clouds, provided that these very low mass protostars are dynamically ejected prior to accreting significantly more mass.

  7. Effect of radiation forces on disk accretion by weakly magnetic neutron stars

    NASA Technical Reports Server (NTRS)

    Miller, M. C.; Lamb, Frederick K.

    1993-01-01

    Radiation forces are shown to be more important than general relativistic corrections to Newtonian gravitational forces in determining the motion of particles accreting onto a slowly rotating neutron star if the luminosity of the star is greater than about 1 percent of the Eddington critical luminosity (ECL). This is so even if the radius of the star is less than the radius of the innermost stable orbit. In particular, radiation drag causes matter accreting from a disk to lose angular momentum and spiral inward. At luminosities greater than about 0.2 ECL, a substantial fraction of the accreting matter can transfer most of its angular momentum and gravitational binding energy to the radiation field before reaching the stellar surface. These results have important implications for the X-ray spectra, time variability, and spin evolution of neutron stars with very weak magnetic fields and the prospects for detecting general relativistic effects near such stars.

  8. Suppression of cooling by strong magnetic fields in white dwarf stars.

    PubMed

    Valyavin, G; Shulyak, D; Wade, G A; Antonyuk, K; Zharikov, S V; Galazutdinov, G A; Plachinda, S; Bagnulo, S; Machado, L Fox; Alvarez, M; Clark, D M; Lopez, J M; Hiriart, D; Han, Inwoo; Jeon, Young-Beom; Zurita, C; Mujica, R; Burlakova, T; Szeifert, T; Burenkov, A

    2014-11-06

    Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.

  9. Suppression of cooling by strong magnetic fields in white dwarf stars

    NASA Astrophysics Data System (ADS)

    Valyavin, G.; Shulyak, D.; Wade, G. A.; Antonyuk, K.; Zharikov, S. V.; Galazutdinov, G. A.; Plachinda, S.; Bagnulo, S.; Fox Machado, L.; Alvarez, M.; Clark, D. M.; Lopez, J. M.; Hiriart, D.; Han, Inwoo; Jeon, Young-Beom; Zurita, C.; Mujica, R.; Burlakova, T.; Szeifert, T.; Burenkov, A.

    2014-11-01

    Isolated cool white dwarf stars more often have strong magnetic fields than young, hotter white dwarfs, which has been a puzzle because magnetic fields are expected to decay with time but a cool surface suggests that the star is old. In addition, some white dwarfs with strong fields vary in brightness as they rotate, which has been variously attributed to surface brightness inhomogeneities similar to sunspots, chemical inhomogeneities and other magneto-optical effects. Here we describe optical observations of the brightness and magnetic field of the cool white dwarf WD 1953-011 taken over about eight years, and the results of an analysis of its surface temperature and magnetic field distribution. We find that the magnetic field suppresses atmospheric convection, leading to dark spots in the most magnetized areas. We also find that strong fields are sufficient to suppress convection over the entire surface in cool magnetic white dwarfs, which inhibits their cooling evolution relative to weakly magnetic and non-magnetic white dwarfs, making them appear younger than they truly are. This explains the long-standing mystery of why magnetic fields are more common amongst cool white dwarfs, and implies that the currently accepted ages of strongly magnetic white dwarfs are systematically too young.

  10. VLBA Provides Best Detail Yet of Star-Forming Cloud's Magnetic Field

    NASA Astrophysics Data System (ADS)

    2001-07-01

    Astronomers have used the National Science Foundation's Very Long Baseline Array (VLBA) radio telescope to do a very detailed map of the magnetic field within a star-forming cloud, an achievement that will help scientists unravel the mysterious first steps of the stellar birth process. "This study provides new and important data needed by theorists to understand how magnetic fields affect the early stages of star formation," said Anuj Sarma, an astronomer at the University of Illinois at Urbana-Champaign. Sarma worked with Thomas Troland of the University of Kentucky and Jonathan Romney of the National Radio Astronomy Observatory (NRAO) in Socorro, New Mexico. Their research results were published in the Astrophysical Journal Letters. Stars are formed when gas in giant interstellar clouds collapses gravitationally. Magnetic fields are believed to support such gas clouds, helping them resist gravitational collapse, so the beginning stages of star formation arise from a complex interplay of the magnetic fields and gravity that is not yet well understood. "In order to understand how star formation gets started, we need to know in detail the structure of the magnetic fields in a star-forming cloud," Sarma said. "Our observations with the VLBA have provided one more big step in this direction," he added. The astronomers studied a cloud of molecular gas more than 5,000 light- years from Earth in a spiral arm of our own Milky Way Galaxy. The cloud, known as W3 IRS5, contains seven newly-formed stars. In addition, it contains a number of regions, somewhat smaller than the diameter of Earth's orbit, in which water vapor molecules act to amplify, or strengthen, radio emission. Such regions, called masers, are a radio- wave parallel to lasers, which amplify light. The scientists used the VLBA to make a detailed study of the radio waves coming from these maser regions in the gas cloud. They detected a phenomenon called the Zeeman effect, in which a very precise frequency

  11. ACTIVITY ANALYSES FOR SOLAR-TYPE STARS OBSERVED WITH KEPLER. I. PROXIES OF MAGNETIC ACTIVITY

    SciTech Connect

    He, Han; Wang, Huaning; Yun, Duo

    2015-11-15

    Light curves of solar-type stars often show gradual fluctuations due to rotational modulation by magnetic features (starspots and faculae) on stellar surfaces. Two quantitative measures of modulated light curves are employed as the proxies of magnetic activity for solar-type stars observed with Kepler telescope. The first is named autocorrelation index i{sub AC}, which describes the degree of periodicity of the light curve; the second is the effective fluctuation range of the light curve R{sub eff}, which reflects the depth of rotational modulation. The two measures are complementary and depict different aspects of magnetic activities on solar-type stars. By using the two proxies i{sub AC} and R{sub eff}, we analyzed activity properties of two carefully selected solar-type stars observed with Kepler (Kepler ID: 9766237 and 10864581), which have distinct rotational periods (14.7 versus 6.0 days). We also applied the two measures to the Sun for a comparative study. The result shows that both the measures can reveal cyclic activity variations (referred to as i{sub AC}-cycle and R{sub eff}-cycle) on the two Kepler stars and the Sun. For the Kepler star with the faster rotation rate, i{sub AC}-cycle and R{sub eff}-cycle are in the same phase, while for the Sun (slower rotator), they are in the opposite phase. By comparing the solar light curve with simultaneous photospheric magnetograms, it is identified that the magnetic feature that causes the periodic light curve during solar minima is the faculae of the enhanced network region, which can also be a candidate of magnetic features that dominate the periodic light curves on the two Kepler stars.

  12. An investigation of the rotational properties of magnetic chemically peculiar stars

    NASA Astrophysics Data System (ADS)

    Netopil, Martin; Paunzen, Ernst; Hümmerich, Stefan; Bernhard, Klaus

    2017-07-01

    The magnetic chemically peculiar (mCP) stars of the upper main sequence exhibit strong, globally organized magnetic fields that are inclined to the rotational axis and facilitate the development of surface abundance inhomogeneities resulting in photometric and spectroscopic variability. Therefore, mCP stars are perfectly suited for a direct measurement of the rotational period without the need for any additional calibrations. We have investigated the rotational properties of mCP stars based on an unprecedentedly large sample consisting of more than 500 objects with known rotational periods. Using precise parallaxes from the Hipparcos and Gaia satellite missions, well-established photometric calibrations and state-of-the-art evolutionary models, we have determined the location of our sample stars in the Hertzsprung-Russell diagram and derived astrophysical parameters such as stellar masses, effective temperature, radii, inclinations and critical rotational velocities. We have confirmed the conservation of angular momentum during the main sequence evolution; no signs of additional magnetic braking were found. The inclination angles of the rotational axes are randomly distributed, although an apparent excess of fast rotators with comparable inclination angles has been observed. We have found a rotation rate of υ/υcrit ≥ 0.5 for several stars, whose characteristics cannot be explained by current models. For the first time, we have derived the relationship between mass and rotation rate of mCP stars, and provide an analysis that links mass and rotation with magnetic field strength. Our sample is unique and offers crucial input for forthcoming evolutionary models that include the effects of magnetic fields for upper main sequence stars.

  13. Low-metallicity star formation: relative impact of metals and magnetic fields

    NASA Astrophysics Data System (ADS)

    Peters, Thomas; Schleicher, Dominik R. G.; Smith, Rowan J.; Schmidt, Wolfram; Klessen, Ralf S.

    2014-08-01

    Low-metallicity star formation poses a central problem of cosmology, as it determines the characteristic mass scale and distribution for the first and second generations of stars forming in our Universe. Here, we present a comprehensive investigation assessing the relative impact of metals and magnetic fields, which may both be present during low-metallicity star formation. We show that the presence of magnetic fields generated via the small-scale dynamo stabilizes the protostellar disc and provides some degree of support against fragmentation. In the absence of magnetic fields, the fragmentation time-scale in our model decreases by a factor of ˜10 at the transition from Z = 0 to Z > 0, with subsequently only a weak dependence on metallicity. Similarly, the accretion time-scale of the cluster is set by the large-scale dynamics rather than the local thermodynamics. In the presence of magnetic fields, the primordial disc can become completely stable, therefore forming only one central fragment. At Z > 0, the number of fragments is somewhat reduced in the presence of magnetic fields, though the shape of the mass spectrum is not strongly affected in the limits of the statistical uncertainties. The fragmentation time-scale, however, increases by roughly a factor of 3 in the presence of magnetic fields. Indeed, our results indicate comparable fragmentation time-scales in primordial runs without magnetic fields and Z > 0 runs with magnetic fields.

  14. Influence of a magnetic field on the pulsational stability of stars

    NASA Technical Reports Server (NTRS)

    Strothers, R.

    1979-01-01

    Under certain simplifying assumptions the influence of a magnetic field on the pulsational stability of stars has been investigated, with a particular application to the problem of the stability of upper-main-sequence stars. It has been found that, if the magnetic field averaged over a spherical shell is either constant at all layers or distributed such that nu, the ratio of magnetic to thermodynamic pressure, is constant at all layers, the critical mass for stability against nuclear-energized pulsations is virtually unaffected by the presence of the field. On the other hand, if the field is strong in the envelope but weak in the core of the star, the critical mass is considerably increased; when nu exceeds about 0.1 in the envelope, stability is attained at all masses.

  15. DISCOVERY OF ROTATIONAL BRAKING IN THE MAGNETIC HELIUM-STRONG STAR SIGMA ORIONIS E

    SciTech Connect

    Townsend, R. H. D.; Oksala, M. E.; Owocki, S. P.; Cohen, D. H.; Ud-Doula, A.

    2010-05-10

    We present new U-band photometry of the magnetic helium-strong star {sigma} Ori E, obtained over 2004-2009 using the SMARTS 0.9 m telescope at Cerro Tololo Inter-American Observatory. When combined with historical measurements, these data constrain the evolution of the star's 1.19 day rotation period over the past three decades. We are able to rule out a constant period at the p {sub null} = 0.05% level, and instead find that the data are well described (p {sub null} = 99.3%) by a period increasing linearly at a rate of 77 ms per year. This corresponds to a characteristic spin-down time of 1.34 Myr, in good agreement with theoretical predictions based on magnetohydrodynamical simulations of angular momentum loss from magnetic massive stars. We therefore conclude that the observations are consistent with {sigma} Ori E undergoing rotational braking due to its magnetized line-driven wind.

  16. Three Dimensional Simulation of the Magnetic Stress in a Neutron Star Crust

    NASA Astrophysics Data System (ADS)

    Wood, T. S.; Hollerbach, R.

    2015-05-01

    We present the first fully self-consistent three dimensional model of a neutron star's magnetic field, generated by electric currents in the star's crust via the Hall effect. We find that the global-scale field converges to a dipolar Hall-attractor state, as seen in recent axisymmetric models, but that small-scale features in the magnetic field survive even on much longer time scales. These small-scale features propagate toward the dipole equator, where the crustal electric currents organize themselves into a strong equatorial jet. By calculating the distribution of magnetic stresses in the crust, we predict that neutron stars with fields stronger than 1 014 G can still be subject to starquakes more than 1 05 yr after their formation.

  17. The evolution of stable magnetic fields in stars: an analytical approach

    NASA Astrophysics Data System (ADS)

    Mestel, Leon; Moss, David

    2010-07-01

    The absence of a rigorous proof of the existence of dynamically stable, large-scale magnetic fields in radiative stars has been for many years a missing element in the fossil field theory for the magnetic Ap/Bp stars. Recent numerical simulations, by Braithwaite & Spruit and Braithwaite & Nordlund, have largely filled this gap, demonstrating convincingly that coherent global scale fields can survive for times of the order of the main-sequence lifetimes of A stars. These dynamically stable configurations take the form of magnetic tori, with linked poloidal and toroidal fields, that slowly rise towards the stellar surface. This paper studies a simple analytical model of such a torus, designed to elucidate the physical processes that govern its evolution. It is found that one-dimensional numerical calculations reproduce some key features of the numerical simulations, with radiative heat transfer, Archimedes' principle, Lorentz force and Ohmic decay all playing significant roles.

  18. Three dimensional simulation of the magnetic stress in a neutron star crust.

    PubMed

    Wood, T S; Hollerbach, R

    2015-05-15

    We present the first fully self-consistent three dimensional model of a neutron star's magnetic field, generated by electric currents in the star's crust via the Hall effect. We find that the global-scale field converges to a dipolar Hall-attractor state, as seen in recent axisymmetric models, but that small-scale features in the magnetic field survive even on much longer time scales. These small-scale features propagate toward the dipole equator, where the crustal electric currents organize themselves into a strong equatorial jet. By calculating the distribution of magnetic stresses in the crust, we predict that neutron stars with fields stronger than 10^{14} G can still be subject to starquakes more than 10^{5} yr after their formation.

  19. roAp stars: surface lithium abundance distribution and magnetic field configuration

    NASA Astrophysics Data System (ADS)

    Polosukhina, N.; Shulyak, D.; Shavrina, A.; Lyashko, D.; Drake, N. A.; Glagolevski, Yu.; Kudryavtsev, D.; Smirnova, M.

    2014-08-01

    High-resolution spectra obtained with the 6m BTA telescope, Russia, and with HARPS and VLT/UVES telescopes at ESO, Chile, were used for Doppler Imaging analysis of two roAp stars, HD 12098 and HD 60435, showing strong and variable Li resonance line in their spectra. We found that Li has highly inhomogeneous distribution on the surfaces of these stars. We compared our results with previously obtained Doppler Imaging mapping of two CP2 stars, HD 83368 and HD 3980, and discuss the correlation between the position of the high Li-abundance spots and magnetic field.

  20. Generation of strong magnetic fields in hybrid and quark stars driven by the electroweak interaction of quarks

    NASA Astrophysics Data System (ADS)

    Dvornikov, Maxim

    2017-01-01

    We study the generation of strong large scale magnetic fields in compact stars containing degenerate quark matter with unbroken chiral symmetry. The magnetic field growth is owing to the magnetic field instability driven by the electroweak interaction of quarks. In this system we predict the enhancement of the seed magnetic field 1012 G to the strengths (1014 - 1015) G. In our analysis we use the typical parameters of the quark matter in the core of a hybrid star or in a quark star. We also apply of the obtained results to model the generation of magnetic fields in magnetars.

  1. New Photometrically Variable Magnetic Chemically Peculiar Stars in the ASAS-3 Archive

    NASA Astrophysics Data System (ADS)

    Hümmerich, Stefan; Paunzen, Ernst; Bernhard, Klaus

    2016-10-01

    The magnetic Ap or CP2 stars are natural atomic and magnetic laboratories. Strictly periodic changes are observed in the spectra and brightness of these stars, which allow the derivation of rotational periods. Related to this group of objects are the He-weak (CP4) and He-rich stars, some of which also undergo brightness changes due to rotational modulation. Increasing the sample size of known rotational periods among CP2/4 stars is important and will contribute to our understanding of these objects and their evolution in time. We have compiled an extensive target list of CP2/4 stars from the General Catalog of Ap, HgMn, and Am stars, including several early-type (spectral types B/A) variables of undetermined type from the International Variable Star Index. We investigated our sample stars using publicly available observations from the ASAS-3 archive. Our previous efforts in this respect led to the discovery of 323 variable stars. Using a refined analysis approach, we were able to identify another 360 stars exhibiting photometric variability in ASAS-3 data. Summary data, folded light curves and, if available, information from the literature are presented for our final sample, which is composed of 334 bona-fide {α }2 Canum Venaticorum (ACV) variables, 23 ACV candidates, and 3 eclipsing binary systems. Interesting and unusual objects are discussed in detail. In particular, we call attention to HD 66051 (V414 Pup), which is an eclipsing binary system showing obvious rotational modulation of the light curve due to the presence of an ACV variable in the system.

  2. Compositional waves and variations in the atmospheric abundances of magnetic stars

    NASA Astrophysics Data System (ADS)

    Urpin, V.

    2016-02-01

    The stars of the middle main sequence often have relatively quiescent outer layers and spot-like chemical structures may develop in their atmospheres. Recent observations show that abundance peculiarities can change as stars evolve on the main sequence and the timescale of these changes lies in a wide range from million years to months. These observations imply that, perhaps, our understanding of diffusion processes at work in magnetic stars is incomplete and a more detailed analysis of these processes is required. In the present paper, we consider diffusion caused by a combined influence of the electric current and the Hall effect. Such diffusion has a number of very particular properties and, generally, can change the surface chemistry of stars in combination with other diffusion processes. For instant, current-driven diffusion is accompanied by a propagation of the special type of waves in which only the impurity number density oscillates. Propagation of such waves changes the shape and size of spots as well as chemical abundances within them. The period of compositional waves depends on the parameters of plasma (magnetic field, electric current, temperature, etc.) and can be different for the waves of different elements. Compositional waves exist in the regions where the magnetic pressure is greater than the gas pressure. These waves can be the reason of variations with different timescales in the abundance peculiarities of magnetic stars.

  3. Multiwavelength study of the magnetically active T Tauri star HD 283447

    NASA Technical Reports Server (NTRS)

    Feigelson, Eric D.; Welty, Alan D.; Imhoff, Catherine; Hall, Jeffrey C.; Etzel, Paul B.; Phillips, Robert B.; Lonsdale, Colin J.

    1994-01-01

    We observed the luminous T Tauri star HD 283447 = V773 Tauri simultaneously at X-ray, ultraviolet, optical photometric and spectroscopic, and radio wavelengths for several hours on UT 1992 September 11. ROSAT, IUE, Very Large Array (VLA) and an intercontinental Very Long Baseline Interferometry (VLBI) network, and three optical observatories participated in the campaign. The star is known for its unusually high and variable nonthermal radio continuum emission. High levels of soft X-ray and Mg II line emission are discovered, with luminosity L(sub x) = 5.5 x 10(exp 30) ergs/s (0.2 - 2 keV) and L(sub Mg II) = 1 x 10(exp 29) ergs/s, respectively. Optically, the spectrum exhibits rather weak characteristics of `classical' T Tauri stars. A faint, broad emission line component, probably due to a collimated wind or infall, is present. During the campaign, the radio luminosity decreased by a factor of 4, while optical/UV lines and X-ray emission remained strong but constant. The large gyrosynchrotron-emitting regions are therefore decoupled from the chromospheric and coronal emission. Five models for the magnetic geometry around the star are discussed; solar-type activity, dipole magnetosphere, star-disk magnetic coupling, disk magnetic fields, and close binary interaction. The data suggest that two magnetic geometries are simultaneously present: complex multipolar fields like those on the Sun, and a large-scale field possibly associated with the circumstellar disk.

  4. Multiwavelength study of the magnetically active T Tauri star HD 283447

    NASA Astrophysics Data System (ADS)

    Feigelson, Eric D.; Welty, Alan D.; Imhoff, Catherine; Hall, Jeffrey C.; Etzel, Paul B.; Phillips, Robert B.; Lonsdale, Colin J.

    1994-09-01

    We observed the luminous T Tauri star HD 283447 = V773 Tauri simultaneously at X-ray, ultraviolet, optical photometric and spectroscopic, and radio wavelengths for several hours on UT 1992 September 11. ROSAT, IUE, Very Large Array (VLA) and an intercontinental Very Long Baseline Interferometry (VLBI) network, and three optical observatories participated in the campaign. The star is known for its unusually high and variable nonthermal radio continuum emission. High levels of soft X-ray and Mg II line emission are discovered, with luminosity Lx = 5.5 x 1030 ergs/s (0.2 - 2 keV) and LMg II = 1 x 1029 ergs/s, respectively. Optically, the spectrum exhibits rather weak characteristics of `classical' T Tauri stars. A faint, broad emission line component, probably due to a collimated wind or infall, is present. During the campaign, the radio luminosity decreased by a factor of 4, while optical/UV lines and X-ray emission remained strong but constant. The large gyrosynchrotron-emitting regions are therefore decoupled from the chromospheric and coronal emission. Five models for the magnetic geometry around the star are discussed; solar-type activity, dipole magnetosphere, star-disk magnetic coupling, disk magnetic fields, and close binary interaction. The data suggest that two magnetic geometries are simultaneously present: complex multipolar fields like those on the Sun, and a large-scale field possibly associated with the circumstellar disk.

  5. Search for surface magnetic fields in Mira stars. First detection in χ Cygni

    NASA Astrophysics Data System (ADS)

    Lèbre, A.; Aurière, M.; Fabas, N.; Gillet, D.; Herpin, F.; Konstantinova-Antova, R.; Petit, P.

    2014-01-01

    Context. So far, surface magnetic fields have never been detected on Mira stars. Only recently have the spectropolarimetric capabilities of measuring it via the Zeeman effect become available to us. Then, to complete the knowledge of the magnetic field and of its influence during the transition from asymptotic giant branch to planetary nebulae stages, we have undertaken a search for magnetic fields on the surface of Mira stars. Aims: Our main goal is to constrain - at this stage of stellar evolution - the surface magnetic field (presence and strength) and to define the magnetic field strength dependence along the radial distance to the star, above the photosphere and across the circumstellar envelope of cool and evolved stars. Methods: We used spectropolarimetric observations (Stokes V spectra probing circular polarization), collected with the Narval instrument at TBL, in order to detect - with the least squares deconvolution (LSD) method - a Zeeman signature in the visible part of the spectrum. Results: We present the first spectropolarimetric observations of the S-type Mira star χ Cyg, performed around its maximum light. We detected a polarimetric signal in the Stokes V spectra and established its Zeeman origin. We claim that it is likely to be related to a weak magnetic field present at the photospheric level and in the lower part of the stellar atmosphere. We estimated the strength of its longitudinal component to about 2-3 gauss. This result favors a 1/r law for the variation in the magnetic field strength across the circumstellar envelope of χ Cyg. This is the first detection of a weak magnetic field on the stellar surface of a Mira star, and we discuss its origin in the framework of shock waves periodically propagating throughout the atmosphere of these radially pulsating stars. Conclusions: At the date of our observations of χ Cyg, the shock wave reaches its maximum intensity, and it is likely that the shock amplifies a weak stellar magnetic field during

  6. Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey

    NASA Astrophysics Data System (ADS)

    Alecian, E.; Kochukhov, O.; Petit, V.; Grunhut, J.; Landstreet, J.; Oksala, M. E.; Wade, G. A.; Hussain, G.; Neiner, C.; Bohlender, D.; MiMeS Collaboration

    2014-07-01

    Context. The Magnetism in Massive Stars (MiMeS) project aims at understanding the origin of the magnetic fields in massive stars as well as their impact on stellar internal structure, evolution, and circumstellar environment. Aims: One of the objectives of the MiMeS project is to provide stringent observational constraints on the magnetic fields of massive stars; however, identification of magnetic massive stars is challenging, as only a few percent of high-mass stars host strong fields detectable with the current instrumentation. Hence, one of the first objectives of the MiMeS project was to search for magnetic objects among a large sample of massive stars, and to build a sub-sample for in-depth follow-up studies required to test the models and theories of fossil field origins, magnetic wind confinement and magnetospheric properties, and magnetic star evolution. Methods: We obtained high-resolution spectropolarimetric observations of a large number of OB stars thanks to three large programs (LP) of observations that have been allocated on the high-resolution spectropolarimeters ESPaDOnS, Narval, and the polarimetric module HARPSpol of the HARPS spectrograph. We report here on the methods and first analysis of the HARPSpol magnetic detections. We identified the magnetic stars using a multi-line analysis technique. Then, when possible, we monitored the new discoveries to derive their rotation periods, which are critical for follow-up and magnetic mapping studies. We also performed a first-look analysis of their spectra and identified obvious spectral anomalies (e.g., surface abundance peculiarities, Hα emission), which are also of interest for future studies. Results: In this paper, we focus on eight of the 11 stars in which we discovered or confirmed a magnetic field from the HARPSpol LP sample (the remaining three were published in a previous paper). Seven of the fields were detected in early-type Bp stars, while the last field was detected in the Ap companion of

  7. Evidence for heating of neutron stars by magnetic-field decay.

    PubMed

    Pons, José A; Link, Bennett; Miralles, Juan A; Geppert, Ulrich

    2007-02-16

    We show the existence of a strong trend between neutron star (NS) surface temperature and the dipolar component of the magnetic field extending through three orders of field magnitude, a range that includes magnetars, radio-quiet isolated neutron stars, and many ordinary radio pulsars. We suggest that this trend can be explained by the decay of currents in the crust over a time scale of approximately 10(6) yr. We estimate the minimum temperature that a NS with a given magnetic field can reach in this interpretation.

  8. Modeling the magnetospheres of luminous stars: Interactions between supersonic radiation-driven winds and stellar magnetic fields

    NASA Astrophysics Data System (ADS)

    Owocki, Stan; Townsend, Rich; Ud-Doula, Asif

    2007-05-01

    Hot, luminous stars (spectral types O and B) lack the hydrogen recombination convection zones that drive magnetic dynamo generation in the sun and other cool stars. Nonetheless, observed rotational modulation of spectral lines formed in the strong, radiatively driven winds of hot stars suggests magnetic perturbations analogous to those that induce ``co-rotating interaction regions'' in the solar wind. Indeed, recent advances in spectropolarimetric techniques have now led to direct detection of moderate to strong (100-10 000 G), tilted dipole magnetic fields in several hot stars. Using a combination of analytic and numerical magnetohydrodynamic models, this paper focuses on the role of such magnetic fields in channeling, and sometimes confining, the radiatively driven mass outflows from such stars. The results show how ``magnetically confined wind shocks'' can explain the moderately hard x-ray emission seen from the O7V star Theta-1 Ori C, and how the trapping of material in a ``rigidly rotating magnetosphere'' can explain the periodically modulated Balmer line emission seen from the magnetic B2pV star Sigma Ori E. In addition, magnetic reconnection heating from episodic centrifugal breakout events might explain the occasional very hard x-ray flares seen from the latter star. The paper concludes with a brief discussion on the generation of hot-star fields and the broader relationship to other types of magnetospheres.

  9. MAGNETIC GAMES BETWEEN A PLANET AND ITS HOST STAR: THE KEY ROLE OF TOPOLOGY

    SciTech Connect

    Strugarek, A.; Brun, A. S.; Réville, V.

    2015-12-20

    Magnetic interactions between a star and a close-in planet are postulated to be a source of enhanced emissions and to play a role in the secular evolution of the orbital system. Close-in planets generally orbit in the sub-alfvénic region of the stellar wind, which leads to efficient transfers of energy and angular momentum between the star and the planet. We model the magnetic interactions occurring in close-in star–planet systems with three-dimensional, global, compressible magnetohydrodynamic numerical simulations of a planet orbiting in a self-consistent stellar wind. We focus on the cases of magnetized planets and explore three representative magnetic configurations. The Poynting flux originating from the magnetic interactions is an energy source for enhanced emissions in star–planet systems. Our results suggest a simple geometrical explanation for ubiquitous on/off enhanced emissions associated with close-in planets, and confirm that the Poynting fluxes can reach powers of the order of 10{sup 19} W. Close-in planets are also shown to migrate due to magnetic torques for sufficiently strong stellar wind magnetic fields. The topology of the interaction significantly modifies the shape of the magnetic obstacle that leads to magnetic torques. As a consequence, the torques can vary by at least an order of magnitude as the magnetic topology of the interaction varies.

  10. A Magnetic Ribbon Model for Star-forming Filaments

    NASA Astrophysics Data System (ADS)

    Auddy, Sayantan; Basu, Shantanu; Kudoh, Takahiro

    2016-11-01

    We develop a magnetic ribbon model for molecular cloud filaments. These result from turbulent compression in a molecular cloud in which the background magnetic field sets a preferred direction. We argue that this is a natural model for filaments and is based on the interplay between turbulence, strong magnetic fields, and gravitationally driven ambipolar diffusion, rather than pure gravity and thermal pressure. An analytic model for the formation of magnetic ribbons that is based on numerical simulations is used to derive a lateral width of a magnetic ribbon. This differs from the thickness along the magnetic field direction, which is essentially the Jeans scale. We use our model to calculate a synthetic observed relation between apparent width in projection versus observed column density. The relationship is relatively flat, similar to observations, and unlike the simple expectation based on a Jeans length argument.

  11. The role of magnetic damping in the r-mode evolution of accreting neutron stars

    NASA Astrophysics Data System (ADS)

    Cao, GuoJie; Zhou, Xia; Wang, Na

    2015-03-01

    The magnetic damping rate was introduced in the evolution equations of r-modes, which shows that r-modes can generate strong toroidal magnetic fields in the core of accreting millisecond pulsars inducing by differential rotation. With consideration of the coupling evolution of r-modes, spin and thermal evolution, we investigated the influence of the magnetic damping on the differential rotation of nonlinear r-modes of accreting neutron stars. We derived the coupling evolution equations of the star involving the magnetic damping rate in the framework of second-order r-mode theory. The numerical results show that the magnetic damping suppressed the nonlinear evolution of r-modes since the saturation amplitude is reduced to a great extent. In particular, because of the presence of the generated toroidal magnetic field, the spin-down of the stars is terminated and the viscous heating effects are also weakened. Moreover, we could obtain a stronger generated toroidal magnetic field in the second-order r-mode theory. The gravitational radiation may be detected by the advanced laser interferometer detector LIGO if the amount of differential rotation is small when the r-mode instability becomes active and the accretion rate is not very high.

  12. General relativistic models for rotating magnetized neutron stars in conformally flat space-time

    NASA Astrophysics Data System (ADS)

    Pili, A. G.; Bucciantini, N.; Del Zanna, L.

    2017-09-01

    The extraordinary energetic activity of magnetars is usually explained in terms of dissipation of a huge internal magnetic field of the order of 1015-16 G. How such a strong magnetic field can originate during the formation of a neutron star (NS) is still subject of active research. An important role can be played by fast rotation: if magnetars are born as millisecond rotators dynamo mechanisms may efficiently amplify the magnetic field inherited from the progenitor star during the collapse. In this case, the combination of rapid rotation and strong magnetic field determine the right physical condition not only for the development of a powerful jet-driven explosion, manifesting as a gamma-ray burst, but also for a copious gravitational waves emission. Strong magnetic fields are indeed able to induce substantial quadrupolar deformations in the star. In this paper, we analyse the joint effect of rotation and magnetization on the structure of a polytropic and axisymmetric NS, within the ideal magneto-hydrodynamic regime. We will consider either purely toroidal or purely poloidal magnetic field geometries. Through the sampling of a large parameter space, we generalize previous results in literature, inferring new quantitative relations that allow for a parametrization of the induced deformation, that takes into account also the effects due to the stellar compactness and the current distribution. Finally, in the case of purely poloidal field, we also discuss how different prescription on the surface charge distribution (a gauge freedom) modify the properties of the surrounding electrosphere and its physical implications.

  13. Plunging neutron stars as origin of organised magnetic field in galactic nuclei

    NASA Astrophysics Data System (ADS)

    Karas, V.; Kopáček, O.; Kunneriath, D.; Zajaček, M.; Araudo, A.; Eckart, A.; Kovář, J.

    2017-07-01

    Black holes cannot support their own internal magnetic field like, for example, compact stars can. Despite this fact observations indicate that event horizons of supermassive black holes (SMBH) are threaded by field lines along which plasma streams flow. Various magnetohydrodynamical mechanisms have been suggested to generate turbulent magnetic fields on small scales, however, the origin of the large-scale component is unclear. In this write-up we describe our progress in an on-going work and discuss the possibility of dipole-type magnetic fields being brought onto SMBH by magnetised neutron stars, which are expected to drift inward from a hidden population in the Nuclear Star Cluster. This can contribute to an organised component of the magnetic field on the characteristic length-scale of the stellar size, which thread the horizon during the final stages of the magnetized star plunge into or its close flyby around SMBH. Because of mass—size scaling relations for black holes, the effect is more important for lower-mass SMBH.

  14. Optically detected magnetic resonance study of a type-II GaAs/AlAs multiple quantum well

    NASA Astrophysics Data System (ADS)

    van Kesteren, H. W.; Cosman, E. C.; Greidanus, F. J. A. M.; Dawson, P.; Moore, K. J.; Foxon, C. T.

    1988-07-01

    In a type-II GaAs/AlAs multiple quantum well three optically detected magnetic resonance lines and two level anticrossings were observed. Two of the resonance lines and the two level anticrossings are in agreement with the electronic level scheme of the heavy-hole exciton. The third resonance line is in accordance with a magnetic spin resonance of an unbound electron. These optically detected magnetic resonance measurements open up the possibility to obtain detailed information about the excitons in and the band structure of type-II quantum wells.

  15. The nature of the light variability of magnetic Of?p star HD 191612

    NASA Astrophysics Data System (ADS)

    Krtička, J.

    2016-10-01

    Context. A small fraction of hot OBA stars host global magnetic fields with field strengths of the order of 0.1-10 kG. This leads to the creation of persistent surface structures (spots) in stars with sufficiently weak winds as a result of the radiative diffusion. These spots become evident in spectroscopic and photometric variability. This type of variability is not expected in stars with strong winds, where the wind inhibits the radiative diffusion. Therefore, a weak photometric variability of the magnetic Of?p star HD 191612 is attributed to the light absorption in the circumstellar clouds. Aims: We study the nature of the photometric variability of HD 191612. We assume that the variability results from variable wind blanketing induced by surface variations of the magnetic field tilt and modulated by stellar rotation. Methods: We used our global kinetic equilibrium (NLTE) wind models with radiative force determined from the radiative transfer equation in the comoving frame (CMF) to predict the stellar emergent flux. Our models describe the stellar atmosphere in a unified manner and account for the influence of the wind on the atmosphere. The models are calculated for different wind mass-loss rates to mimic the effect of magnetic field tilt on the emergent fluxes. We integrate the emergent fluxes over the visible stellar surface for individual rotational phases, and calculate the rotationally modulated light curve of HD 191612. Results: The wind blanketing that varies across surface of HD 191612 is able to explain a part of the observed light variability in this star. The mechanism is able to operate even at relatively low mass-loss rates. The remaining variability is most likely caused by the flux absorption in circumstellar clouds. Conclusions: The variable wind blanketing is an additional source of the light variability in massive stars. The presence of the rotational light variability may serve as a proxy for the magnetic field.

  16. Chandra Observations of a Young Embedded Magnetic B Star in the p Ophiuchus Cloud

    NASA Technical Reports Server (NTRS)

    Hamaguchi, Kenji; Imanishi, Kensuke

    2002-01-01

    This paper reports the analysis of two Chandra X-ray observations of the young magnetic B star rho Ophiuchus S1. X-ray emission from the star was detected in both observations. The average flux is almost the same in both, but during each observation the flux shows significant time variations by a factor of two on timescales of 20-40 ksec. Each spectrum can be fit by either an absorbed power law model with a photon index of approx. -3 or a thin-thermal plasma model with a temperature of approx. 2 keV and an extremely low metal abundance (approx. less than 0.1 solar). The spectrum of the first observation has an apparent line feature at about 6.8 keV, which likely corresponds to highly ionized iron K alpha. In contrast, the spectrum of the second observation shows an anomalous edge absorption component at E approx. 1 keV. The continuum emission and log (L(sub X)/L(sub bol)) approx. -6 are similar to those of young intermediate-mass stars (Herbig Ae/Be stars) although the presence of the magnetic field inferred from the detection of non-thermal radio emission has drawn an analogy between rho Ophiuchus S1 and magnetic chemically peculiar (MCP) stars. If the X-ray emission is thermal, the highest plasma temperature observed is too high to be explained by the conventional theories of magnetic stars, and favors some kind of magnetic dynamo activity, while if the emission is nonthermal, it might be related to mass infall. The 6.8 keV line and 4 keV edge features are marginal but they give important information near the stellar body if they are real. Their physical interpretation is discussed.

  17. The freedom to choose neutron star magnetic field equilibria: Table 1.

    NASA Astrophysics Data System (ADS)

    Glampedakis, Kostas; Lasky, Paul D.

    2016-12-01

    Our ability to interpret and glean useful information from the large body of observations of strongly magnetized neutron stars rests largely on our theoretical understanding of magnetic field equilibria. We answer the following question: is one free to arbitrarily prescribe magnetic equilibria such that fluid degrees of freedom can balance the equilibrium equations? We examine this question for various models for neutron star matter; from the simplest single-fluid barotrope to more realistic non-barotropic multifluid models with superfluid/superconducting components, muons and entropy. We do this for both axi- and non-axisymmetric equilibria, and in Newtonian gravity and general relativity. We show that, in axisymmetry, the most realistic model allows complete freedom in choosing a magnetic field equilibrium whereas non-axisymmetric equilibria are never completely arbitrary.

  18. Oscillations and Magnetic Fields in the G8 Star EK Eridani

    NASA Astrophysics Data System (ADS)

    Dall, T. H.; Cunha, M.; Strassmeier, K. G.; Stello, D.; Bruntt, H.

    2011-12-01

    Asteroseismology can provide information that is otherwise not easily accessible, like the stellar mass and the evolutionary stage. Strong magnetic fields are usually accompanied by rapid rotation, which makes asteroseismology difficult due to spectral line broadening. We have found what may turn out to be the Rosetta Stone of the stars: A slowly rotating solar-like star with a strong magnetic field. We have recently detected solar-like oscillations in this active sub-giant, but with amplitudes much lower than expected. We suggest that the large-scale magnetic field alters the pulsations, which become magnetoacoustic in nature. Here we present our results and discuss possible implications and how this may open up a new frontier in the studies of magnetic fields and stellar evolution.

  19. Massive outflows driven by magnetic effects in star-forming clouds with high mass accretion rates

    NASA Astrophysics Data System (ADS)

    Matsushita, Yuko; Machida, Masahiro N.; Sakurai, Yuya; Hosokawa, Takashi

    2017-09-01

    The relation between the mass accretion rate on to the circumstellar disc and the rate of mass ejection by magnetically driven winds is investigated using three-dimensional magnetohydrodynamics simulations. Using a spherical cloud core with a varying ratio of thermal to gravitational energy, which determines the mass accretion rate on to the disc, to define the initial conditions, the outflow propagation for approximately 104 yr after protostar formation is then calculated for several cloud cores. The mass ejection rate and accretion rate are comparable only when the magnetic energy of the initial cloud core is comparable to the gravitational energy. Consequently, in strongly magnetized clouds a higher mass accretion rate naturally produces both massive protostars and massive outflows. The simulated outflow mass, momentum, kinetic energy and momentum flux agree well with observations, indicating that massive stars form through the same mechanism as low-mass stars but require a significantly strong magnetic field to launch massive outflows.

  20. The effects of superhigh magnetic fields on the equations of state of neutron stars

    NASA Astrophysics Data System (ADS)

    Gao, Z. F.; Wang, N.; Xu, Y.; Shan, H.; Li, X.-D.

    2015-11-01

    By introducing Dirac's δ-function in superhigh magnetic fields, we deduce a general formula for the pressure of degenerate and relativistic electrons, Pe, which is suitable for superhigh magnetic fields, discuss the quantization of Landau levels of electrons, and consider the quantum electrodynamic(QED) effects on the equations of states (EOSs) for different matter systems. The main conclusions are as follows: the stronger the magnetic field strength, the higher the electron pressure becomes; compared with a common radio pulsar, a magnetar could be a more compact oblate spheroid-like deformed neutron star due to the anisotropic total pressure; and an increase in the maximum mass of a magnetar is expected because of the positive contribution of the magnetic field energy to the EoS of the star. Since this is an original work in which some uncertainties could exist, modifications and improvements of our theory should be considered in our future studies.

  1. The remarkably unremarkable global abundance variations of the magnetic Bp star HD 133652

    NASA Astrophysics Data System (ADS)

    Bailey, J. D.; Landstreet, J. D.

    2015-08-01

    Context. In recent years, significant effort has been made to understand how the magnetic field strengths and atmospheric chemical abundances of Ap/Bp stars evolve during their main sequence lifetime by identifying a large number of Ap/Bp stars with accurately known ages. As a next step, these stars should be studied individually and in detail to offer further insight into the physics of how such main sequence stars evolve. Aims: We have obtained high resolution spectra using the ESPaDOnS spectropolarimeter and FEROS spectrograph of the chemically peculiar, magnetic Bp star HD 133652. Using these data, we present a simple magnetic field model and abundance determinations of He, O, Mg, Si, Ti, Cr, Fe, Pr, and Nd. Methods: Abundance analysis was performed using zeeman.f, a spectral synthesis program that includes the effects of magnetic fields on line formation. The magnetic field structure is approximated as a simple, co-linear multipole expansion that reproduces the observed variations of the line-of-sight magnetic field with phase. The abundance distribution of each element was modelled using a uniform abundance in each of the two magnetic hemispheres. Results: Using the new magnetic field measurements, we were able to refine the rotation period of HD 133652 to P = 2.30405 ± 0.00002 d. The abundance analysis reveals that the elements modelled (except He, O and Mg) are overabundant compared to the Sun; however most elements studied do not show substantial differences in the large-scale mean abundances between the two magnetic hemispheres. The individual line profiles are very complex and clearly indicate the presence of significant small-scale abundance variations on the stellar surface. Conclusions: These data are adequate to perform a useful investigation of the magnetic field structure and abundance distribution over the stellar surface. HD 133652 is now one of a growing list of hotter Bp stars of known age for which this type of analysis has been performed

  2. MRI-driven accretion on to magnetized stars: global 3D MHD simulations of magnetospheric and boundary layer regimes

    NASA Astrophysics Data System (ADS)

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

    2012-03-01

    We discuss results of global three-dimensional magnetohydrodynamic simulations of accretion on to a rotating magnetized star with a tilted dipole magnetic field, where the accretion is driven by the magnetorotational instability (MRI). The simulations show that MRI-driven turbulence develops in the disc, and angular momentum is transported outwards primarily due to the magnetic stress. The turbulent flow is strongly inhomogeneous and the densest matter is in azimuthally stretched turbulent cells. We investigate two regimes of accretion: a magnetospheric regime and a boundary layer (BL) regime. In the magnetospheric regime, the magnetic field of the star is dynamically important: the accretion disc is truncated by the star's magnetic field within a few stellar radii from the star's surface, and matter flows to the star in funnel streams. The funnel streams flow towards the south and north magnetic poles but are not equal due to the inhomogeneity of the flow. The hotspots on the stellar surface are not symmetric as well. In the BL regime, the magnetic field of the star is dynamically unimportant, and matter accretes on to the surface of the star through the BL. The magnetic field in the inner disc is strongly amplified by the shear of the accretion flow, and the matter and magnetic stresses become comparable. Accreting matter forms a belt-shaped hot region on the surface of the star. The belt has inhomogeneous density distribution which varies in time due to variable accretion rate. The peaks in the variability curve are associated with accretion of individual turbulent cells. They show 20-50 per cent density amplifications at periods of ˜5-10 dynamical time-scales at the surface of the star. Spiral waves in the disc are excited in both magnetospheric and BL regimes of accretion. Results of simulations can be applied to classical T Tauri stars, accreting brown dwarfs, millisecond pulsars, dwarf novae cataclysmic variables and other stars with magnetospheres smaller

  3. Magnetic Field of the Solar-Like Star β Aql: a Study by Individual Spectral Lines

    NASA Astrophysics Data System (ADS)

    Butkovskaya, V. V.; Plachinda, S. I.; Baklanova, D. N.; Pankov, N. F.

    2017-06-01

    We present the results of the spectropolarimetric study of the solar-like star β Aql in individual spectral lines. The magnetic field on β Aql was detected with maximal value B =19.7 ±2.8 G, and minimal value B =-18.1 ±3.5 G. We suppose a difference in magnetic field value measured using spectral lines with normal and anomalous Zeeman spleeting.

  4. 3D magnetic interactions of stars with their close-in planets

    NASA Astrophysics Data System (ADS)

    Strugarek, Antoine; Brun, Allan Sacha; Matt, Sean; Réville, Victor

    2015-08-01

    Close-in planets generally orbit in a sub-alfv ´enic stellar wind, where the perturbations they excite in the corona are able to travel upwind to the stellar surface and potentially induce observable phenomena. The effective connection between the planet and its host takes the form of two Aflv ´en wings. Depending on the topology of the planetary and stellar magnetic fields, the rotation profile of the corona, and the orbital parameters, it is possible that none, one, or the two Aflv ´en wings connect together the star and the planet.We explore the formation and sustainment of Alfv ´en wings in global three dimensional simulations under the magneto-hydrodynamic formalism with the PLUTO code. We model the stellar wind of a typical cool star in which a close-in orbiting planet is introduced as a boundary condition. By varying the magnetic topologies of the planetary and stellar magnetic fields, we explore the variety of Alfv ´en wings that can develop and quantify the Poynting flux flowing through those wings. We thus provide estimates of the amount of magnetic energy these magnetic interactions can channel to the lower corona. We also quantify the phase and latitude offsets that can be expected between the planetary subpoint on the stellar surface and the actual location where energy is deposited. We summarize the typical situations (in terms of magnetic topology, stellar type, and orbital parameters) where the star-planet magnetic interaction could trigger observable flares. We conclude by extending our results to the cases of more complex, non-axisymmetric topologies of the observed magnetic fields for a few particular stars.

  5. Magnetic fields during the early stages of massive star formation - I. Accretion and disc evolution

    NASA Astrophysics Data System (ADS)

    Seifried, D.; Banerjee, R.; Klessen, R. S.; Duffin, D.; Pudritz, R. E.

    2011-10-01

    We present simulations of collapsing 100 M⊙ mass cores in the context of massive star formation. The effect of variable initial rotational and magnetic energies on the formation of massive stars is studied in detail. We focus on accretion rates and on the question under which conditions massive Keplerian discs can form in the very early evolutionary stage of massive protostars. For this purpose, we perform 12 simulations with different initial conditions extending over a wide range in parameter space. The equations of magnetohydrodynamics (MHD) are solved under the assumption of ideal MHD. We find that the formation of Keplerian discs in the very early stages is suppressed for a mass-to-flux ratio normalized to the critical value μ below 10, in agreement with a series of low-mass star formation simulations. This is caused by very efficient magnetic braking resulting in a nearly instantaneous removal of angular momentum from the disc. For weak magnetic fields, corresponding to μ≳ 10, large-scale, centrifugally supported discs build up with radii exceeding 100 au. A stability analysis reveals that the discs are supported against gravitationally induced perturbations by the magnetic field and tend to form single stars rather than multiple objects. We find protostellar accretion rates of the order of a few 10-4 M⊙ yr-1 which, considering the large range covered by the initial conditions, vary only by a factor of ˜ 3 between the different simulations. We attribute this fact to two competing effects of magnetic fields. On the one hand, magnetic braking enhances accretion by removing angular momentum from the disc thus lowering the centrifugal support against gravity. On the other hand, the combined effect of magnetic pressure and magnetic tension counteracts gravity by exerting an outward directed force on the gas in the disc thus reducing the accretion on to the protostars.

  6. Magnetic fields around evolved stars: further observations of H2O maser polarization

    NASA Astrophysics Data System (ADS)

    Leal-Ferreira, M. L.; Vlemmings, W. H. T.; Kemball, A.; Amiri, N.

    2013-06-01

    Context. A low- or intermediate-mass star is believed to maintain a spherical shape throughout the evolution from the main sequence to the asymptotic giant branch (AGB) phase. However, many post-AGB objects and planetary nebulae exhibit non-spherical symmetry. Several candidates have been suggested as factors that can play a role in this change of morphology, but the problem is still not well understood. Magnetic fields are one of these possible agents. Aims: We aim to detect the magnetic field and infer its properties around four AGB stars using H2O maser observations. The sample we observed consists of the following sources: the semi-regular variable RT Vir, and the Mira variables AP Lyn, IK Tau, and IRC+60370. Methods: We observed the 61,6 -52,3 H2O maser rotational transition in full-polarization mode to determine its linear and circular polarization. Based on the Zeeman effect, one can infer the properties of the magnetic field from the maser polarization analysis. Results: We detected a total of 238 maser features in three of the four observed sources. No masers were found toward AP Lyn. The observed masers are all located between 2.4 and 53.0 AU from the stars. Linear and circular polarization was found in 18 and 11 maser features, respectively. Conclusions: We more than doubled the number of AGB stars in which a magnetic field has been detected from H2O maser polarization. Our results confirm the presence of fields around IK Tau, RT Vir, and IRC+60370. The strength of the field along the line of sight is found to be between 47 and 331 mG in the H2O maser region. Extrapolating this result to the surface of the stars, assuming a toroidal field (∝ r-1), we find magnetic fields of 0.3-6.9 G on the stellar surfaces. If, instead of a toroidal field, we assume a poloidal field (∝ r-2), then the extrapolated magnetic field strength on the stellar surfaces are in the range between 2.2 and ~115 G. Finally, if a dipole field (∝ r-3) is assumed, the field

  7. X-rays from magnetic intermediate mass Ap/Bp stars

    NASA Astrophysics Data System (ADS)

    Robrade, Jan

    2016-09-01

    The X-ray emission of magnetic intermediate mass Ap/Bp stars is reviewed and put into context of intrinsic as well as extrinsic hypotheses for its origin. New X-ray observations of Ap/Bp stars are presented and combined with an updated analysis of the available datasets, providing the largest sample of its type that is currently available. In the studied stars the X-ray detections are found predominantly among the more massive, hotter and more luminous targets. Their X-ray properties are quite diverse and beside strong soft X-ray emission significant magnetic activity is frequently present. While a connection between more powerful winds and brighter X-ray emission is expected in intrinsic models, the scatter in X-ray luminosity at given bolometric luminosity is so far unexplained and several observational features like X-ray light curves and flaring, luminosity distributions and spectral properties are often similar to those of low-mass stars. It remains to be seen if these features can be fully reproduced by magnetospheres of intermediate mass stars. The article discusses implications for magnetically confined wind-shock models (MCWS) and stellar magnetospheres under the assumption that the intrinsic model is applicable, but also examines the role of possible companions. Further, related magnetospheric phenomena are presented and an outlook on future perspectives is given.

  8. The MiMeS survey of magnetism in massive stars: introduction and overview

    NASA Astrophysics Data System (ADS)

    Wade, G. A.; Neiner, C.; Alecian, E.; Grunhut, J. H.; Petit, V.; Batz, B. de; Bohlender, D. A.; Cohen, D. H.; Henrichs, H. F.; Kochukhov, O.; Landstreet, J. D.; Manset, N.; Martins, F.; Mathis, S.; Oksala, M. E.; Owocki, S. P.; Rivinius, Th.; Shultz, M. E.; Sundqvist, J. O.; Townsend, R. H. D.; ud-Doula, A.; Bouret, J.-C.; Braithwaite, J.; Briquet, M.; Carciofi, A. C.; David-Uraz, A.; Folsom, C. P.; Fullerton, A. W.; Leroy, B.; Marcolino, W. L. F.; Moffat, A. F. J.; Nazé, Y.; Louis, N. St; Aurière, M.; Bagnulo, S.; Bailey, J. D.; Barbá, R. H.; Blazère, A.; Böhm, T.; Catala, C.; Donati, J.-F.; Ferrario, L.; Harrington, D.; Howarth, I. D.; Ignace, R.; Kaper, L.; Lüftinger, T.; Prinja, R.; Vink, J. S.; Weiss, W. W.; Yakunin, I.

    2016-02-01

    The MiMeS (Magnetism in Massive Stars) project is a large-scale, high-resolution, sensitive spectropolarimetric investigation of the magnetic properties of O- and early B-type stars. Initiated in 2008 and completed in 2013, the project was supported by three Large Program allocations, as well as various programmes initiated by independent principal investigators, and archival resources. Ultimately, over 4800 circularly polarized spectra of 560 O and B stars were collected with the instruments ESPaDOnS (Echelle SpectroPolarimetric Device for the Observation of Stars) at the Canada-France-Hawaii Telescope, Narval at the Télescope Bernard Lyot and HARPSpol at the European Southern Observatory La Silla 3.6 m telescope, making MiMeS by far the largest systematic investigation of massive star magnetism ever undertaken. In this paper, the first in a series reporting the general results of the survey, we introduce the scientific motivation and goals, describe the sample of targets, review the instrumentation and observational techniques used, explain the exposure time calculation designed to provide sensitivity to surface dipole fields above approximately 100 G, discuss the polarimetric performance, stability and uncertainty of the instrumentation, and summarize the previous and forthcoming publications.

  9. Dynamos in asymptotic-giant-branch stars as the origin of magnetic fields shaping planetary nebulae.

    PubMed

    Blackman, E G; Frank, A; Markiel, J A; Thomas, J H; Van Horn, H M

    2001-01-25

    Planetary nebulae are thought to be formed when a slow wind from the progenitor giant star is overtaken by a subsequent fast wind generated as the star enters its white dwarf stage. A shock forms near the boundary between the winds, creating the relatively dense shell characteristic of a planetary nebula. A spherically symmetric wind will produce a spherically symmetric shell, yet over half of known planetary nebulae are not spherical; rather, they are elliptical or bipolar in shape. A magnetic field could launch and collimate a bipolar outflow, but the origin of such a field has hitherto been unclear, and some previous work has even suggested that a field could not be generated. Here we show that an asymptotic-giant-branch (AGB) star can indeed generate a strong magnetic field, having as its origin a dynamo at the interface between the rapidly rotating core and the more slowly rotating envelope of the star. The fields are strong enough to shape the bipolar outflows that produce the observed bipolar planetary nebulae. Magnetic braking of the stellar core during this process may also explain the puzzlingly slow rotation of most white dwarf stars.

  10. Rotation, pulsations and magnetic field in V 2052 Ophiuchi: A new He-strong star

    NASA Astrophysics Data System (ADS)

    Neiner, C.; Henrichs, H. F.; Floquet, M.; Frémat, Y.; Preuss, O.; Hubert, A.-M.; Geers, V. C.; Tijani, A. H.; Nichols, J. S.; Jankov, S.

    2003-12-01

    V 2052 Oph is a beta Cep star with v sin i ~ 60 km s-1. The behavior of its stellar wind was studied in the ultraviolet wavelength region with the IUE satellite. It revealed periodic variations in the equivalent widths (EW) of the resonance lines of wind-sensitive ions with a period of 3.638833 d, which is identified as the rotational period. These variations are typical of magnetic stars. Therefore time-resolved circular spectropolarimetric observations were obtained with the Musicos échelle spectropolarimeter at the 2-m Télescope Bernard Lyot (TBL) to search for a magnetic field in the star. Stokes V patterns were observed, the inclination and magnetic angles were derived and a value was determined for the polar magnetic field (250 +/- 190 G) using an oblique rotator dipole model. The spectroscopic information was used to search for periodicity in line-profile variations (lpv), radial velocity and minimum intensity curves. Multiperiodicity was found, corresponding to radial (f_1=7.15 c d-1) and non-radial (f_2=6.82 c d-1, l=3 or 4) pulsation modes. The rotation period is also detected in rotationally modulated observables because of the magnetic poles passing through the observer's line of sight. We determined the stellar parameters of the star, which was found to be chemically peculiar, in particular He-enriched. This makes V 2052 Oph the first magnetic He-strong early B star with known pulsational properties. Based on observations obtained with the MuSiCoS spectropolarimeter at the Observatoire du Pic du Midi, France, and by the International Ultraviolet Explorer, collected at NASA Goddard Space Flight Center and Villafranca Satellite Tracking Station of the European Space Agency, retrieved from the INES database.}\\fnmsep\\thanks{Table \\ref{tabmagV} is only available in electronic form at http://www.edpsciences.org

  11. Activity and magnetic field structure of the Sun-like planet-hosting star HD 1237

    NASA Astrophysics Data System (ADS)

    Alvarado-Gómez, J. D.; Hussain, G. A. J.; Grunhut, J.; Fares, R.; Donati, J.-F.; Alecian, E.; Kochukhov, O.; Oksala, M.; Morin, J.; Redfield, S.; Cohen, O.; Drake, J. J.; Jardine, M.; Matt, S.; Petit, P.; Walter, F. M.

    2015-10-01

    We analyse the magnetic activity characteristics of the planet-hosting Sun-like star, HD 1237, using HARPS spectro-polarimetric time-series data. We find evidence of rotational modulation of the magnetic longitudinal field measurements that is consistent with our ZDI analysis with a period of 7 days. We investigate the effect of customising the LSD mask to the line depths of the observed spectrum and find that it has a minimal effect on the shape of the extracted Stokes V profile but does result in a small increase in the S/N (~7%). We find that using a Milne-Eddington solution to describe the local line profile provides a better fit to the LSD profiles in this slowly rotating star, which also affects the recovered ZDI field distribution. We also introduce a fit-stopping criterion based on the information content (entropy) of the ZDI map solution set. The recovered magnetic field maps show a strong (+90 G) ring-like azimuthal field distribution and a complex radial field dominating at mid latitudes (~45 degrees). Similar magnetic field maps are recovered from data acquired five months apart. Future work will investigate how this surface magnetic field distribution affeccts the coronal magnetic field and extended environment around this planet-hosting star.

  12. Effects of magnetic field topology in black hole-neutron star mergers: Long-term simulations

    NASA Astrophysics Data System (ADS)

    Wan, Mew-Bing

    2017-05-01

    We report long-term simulations of black hole-neutron star binary mergers where the neutron star possesses an asymmetric magnetic field dipole. Focusing on the scenario where the neutron star is tidally disrupted by the black hole, we track the evolution of the binary up to ≈100 ms after the merger. We uncover more than one episode of thermally driven winds being launched along a funnel wall in all these cases beginning from ≈25 ms after the merger. On the other hand, we are unable to conclude presently whether the amount of ejected mass increases with the degree of asymmetry. A large-scale magnetic field configuration in the poloidal direction is formed along the funnel wall accompanied by the generation of a large Poynting flux. The magnetic field in the accretion disk around the black hole remnant is amplified by both magnetic winding and the nonaxisymmetric magnetorotational instability (MRI). The MRI growth is estimated to be in the ideal magnetohydrodynamics (MHD) regime and thus would be free from significant effects induced by potential neutrino radiation. However, the asymmetry in the magnetic field leads to increased turbulence, which causes the vertical magnetic field in the accretion disk to grow largely in a nonlinear manner.

  13. A search for photometric variability in magnetic chemically peculiar stars using ASAS-3 data

    NASA Astrophysics Data System (ADS)

    Bernhard, K.; Hümmerich, S.; Otero, S.; Paunzen, E.

    2015-09-01

    Context. The (magnetic) chemically peculiar (CP) stars of the upper main sequence are well-suited laboratories for investigating the influence of magnetic fields on the stellar surface because they produce abundance inhomogeneities (spots), which results in photometric variability that is explained in terms of the oblique rotator model. CP stars exhibiting this phenomenon are normally classified as α2 Canum Venaticorum (ACV) variables. It is important to increase the sample of known rotational periods among CP stars by discovering new ACV variables. An increased sample size will contribute to the understanding of the CP stars' evolution in time. Aims: We aim at discovering new ACV variables in the public data of the third phase of the All Sky Automated Survey (ASAS-3). Furthermore, by analysis of the available photometric data, we intend to derive rotational periods of the stars. Methods: The ASAS-3 data were cross-correlated with the Catalogue of Ap, HgMn, and Am stars in order to analyse the light curves of bona fide CP and related stars. The light curves were downloaded and cleaned of outliers and data points with a flag indicating bad quality. Promising candidates showing a larger scatter than observed for constant stars in the corresponding magnitude range were searched for periodic signals using a standard Fourier technique. Objects exhibiting periodic signals well above the noise level were considered and visually inspected, whereas borderline cases were rejected. Results: In total, we found 323 variables, from which 246 are reported here for the first time, and 77 were probably wrongly classified before. The observed variability pattern of most stars is in accordance with an ACV classification. For some cases, it is difficult to distinguish between the light curves of double-waved ACVs and the variability induced by orbital motion (ellipsoidal variables/eclipsing variables), especially for objects exhibiting very small amplitudes and/or significant scatter

  14. Magnetized neutron stars as gamma-ray bursters - Detection rates at high energies

    NASA Technical Reports Server (NTRS)

    Meszaros, P.; Bagoly, Z.; Riffert, H.

    1989-01-01

    Detailed calculations of the escape of high-energy gamma-rays from the dipolar magnetosphere of general relativistic neutron star models are used to model the detection rate of bursters at high photon energies between 0.3 and 10 MeV. This analysis shows the SMM detection rates to be compatible with a magnetized neutron star origin, with a distribution of magnetic field strengths extending at least up to about 4 x 10 to the 12th G, as expected if the (20-60) keV features reported from Konus and Ginga measurements are interpreted as cyclotron lines. Additional implications are discussed for the emission geometry and the neutron star radius.

  15. The Magnetic Activity of Solar-like Stars at Different Main-Sequence Ages

    NASA Astrophysics Data System (ADS)

    Lakatos, S. L.; Nandy, D.; Martens, P.

    2005-12-01

    We report on a study of modeling stellar magnetic activity inferred through CaII H+K and ROSAT X-ray emission. The purpose of this project is to create a subset of stars with similar properties to the Sun, but with a wide range of ages (0.6 - 10 Gyrs); to study the CaII H+K emission data and decipher how the stars' emission changes with age; and to compare the X-ray activity to the CaII H+K activity. The ultimate goal of this project is to determine and use the relationships between the stellar parameters to understand the evolution of the magnetic dynamo from an younger Sun to an older Sun. This research is supported by a NSF Research Experience for Undergraduates grant ATM-0243923 and a NASA Living With a Star grant NNG05GE47G to Montana State University.

  16. Formation of very strongly magnetized neutron stars - Implications for gamma-ray bursts

    NASA Technical Reports Server (NTRS)

    Duncan, Robert C.; Thompson, Christopher

    1992-01-01

    It is proposed that the main observational signature of magnetars, high-field neutron stars, is gamma-ray bursts powered by their vast reservoirs of magnetic energy. If they acquire large recoils, most magnetars are unbound from the Galaxy or reside in an extended, weakly bound Galactic corona. There is evidence that the soft gamma repeaters are young magnetars. It is argued that a convective dynamo can also generate a very strong dipole field after the merger of a neutron star binary, but only if the merged star survives for as long as about 10-100 ms. Several mechanisms which could impart a large recoil to these stars at birth, sufficient to escape from the Galactic disk, are discussed.

  17. Zeeman Effect in Sulfur Monoxide: a Probe to Observe Magnetic Fields in Star Forming Regions?

    NASA Astrophysics Data System (ADS)

    Cazzoli, Gabriele; Lattanzi, Valerio; Coriani, Sonia; Gauss, Jürgen; Codella, Claudio; Ramos, Andrés Asensio; Cernicharo, Jose; Puzzarini, Cristina

    2017-06-01

    Magnetic fields play a fundamental role in star formation processes and the best method to evaluate their intensity is is to measure the Zeeman effect of atomic and molecular lines. However, a direct measurement of the Zeeman spectral pattern from interstellar molecular species is challenging due to the high sensitivity and high spectral resolution required. So far, the Zeeman effect has been detected unambiguously in star forming regions for very few non-masing species, such as OH and CN. We decided to investigate the ability of sulfur monoxide (SO), which is one of the most abundant species in star forming regions, for probing the intensity of magnetic fields via Zeeman effect. The Zeeman effect for several rotational transitions of SO in the (sub-)mm spectral regions has been investigated by using a frequency-modulated, computer-controlled spectrometer, and by applying a magnetic field parallel to the radiation source. To support the experimental determination of the g factors of SO, a systematic quantum-chemical investigation of these parameters for both SO and O_2 has been carried out. An effective experimental-computational strategy for providing accurate g factors as well as for identifying the rotational transitions showing the strongest Zeeman effect has been presented. Our investigation supports SO as a good candidate for probing magnetic fields in high-density star forming regions.

  18. Force-Free Models of Magnetically Linked Star--Disk Systems

    NASA Astrophysics Data System (ADS)

    Uzdensky, D. A.; Königl, A.; Litwin, C.

    2000-12-01

    Disk accretion onto a magnetized star occurs in a variety of astrophysical contexts, from young stars to X-ray pulsars. The magnetohydrodynamic interaction between the stellar field and the accreting matter can have a strong effect on the disk structure, the transfer of mass and angular momentum between the disk and the star, and the production of bipolar outflows, e.g., plasma jets. We study a key element of this interaction --- the time evolution of the magnetic field configuration brought about by the relative rotation between the disk and the star --- using simplified, largely semianalytic, models. We first discuss the rapid inflation and opening up of the magnetic field lines in the corona above the accretion disk, which is caused by the differential rotation twisting. Then we consider additional physical effects that tend to limit this expansion, such as the effect of plasma inertia and the possibility of reconnection in the disk's corona, the latter possibly leading to repeated cycles in the evolution. We also derive the condition for the existence of a steady state for a resistive disk and conclude that a steady state configuration is not realistically possible. Finally, we generalize our analysis of the opening of magnetic field lines by using a non-self-similar numerical model that applies to an arbitrarily rotating (e.g. keplerian) disk. Work supported by NASA grants NAG 5-3687 and NAG 5-1485 and by ASCI/Alliances Center for Astrophysical Thermonuclear Flashes under DOE subcontract B341495.

  19. VizieR Online Data Catalog: New photometrically variable magnetic CP stars (Hummerich+, 2016)

    NASA Astrophysics Data System (ADS)

    Hummerich, S.; Paunzen, E.; Bernhard, K.

    2016-10-01

    The All Sky Automated Survey (ASAS) constantly monitored the entire southern sky and part of the northern sky to about δ<+28°. The third phase of the project, ASAS-3, lasted from 2000 until 2009 (Pojmanski 2002, Cat. II/264). The employed instrumentation, which was situated at the 10inch astrograph dome of the Las Campanas Observatory in Chile, consisted of two wide-field telescopes equipped with f/2.8 200mm Minolta lenses and 2048*2048 AP 10 Apogee detectors that covered a field of sky of 8.8°*8.8°. About 107 sources brighter than V~14mag were monitored in Johnson V. The achieved CCD resolution was about 14.8''/pixel, which led to an astrometric accuracy of around 3''-5'' for bright stars and up to 15.5'' for fainter stars. An initial list of target stars was created by selecting CP2 stars or CP2 star (the magnetic Bp/Ap stars) candidates and He-weak (CP4)/He-strong objects from the most recent version of the Catalog of Ap, HgMn, and Am stars (Renson & Manfroid 2009, Cat. III/260). (2 data files).

  20. A dominant magnetic dipole for the evolved Ap star candidate EK Eridani

    NASA Astrophysics Data System (ADS)

    Aurière, M.; Konstantinova-Antova, R.; Petit, P.; Roudier, T.; Donati, J.-F.; Charbonnel, C.; Dintrans, B.; Lignières, F.; Wade, G. A.; Morgenthaler, A.; Tsvetkova, S.

    2011-10-01

    Context. EK Eri is one of the most slowly rotating active giants known, and has been proposed to be the descendant of a strongly magnetic Ap star. Aims: We have performed a spectropolarimetric study of EK Eri over 4 photometric periods with the aim of inferring the topology of its magnetic field. Methods: We used the NARVAL spectropolarimeter at the Bernard Lyot telescope at the Pic du Midi Observatory, along with the least-squares deconvolution method, to extract high signal-to-noise ratio Stokes V profiles from a timeseries of 28 polarisation spectra. We have derived the surface-averaged longitudinal magnetic field Bℓ. We fit the Stokes V profiles with a model of the large-scale magnetic field and obtained Zeeman Doppler images of the surface magnetic strength and geometry. We studied the classical activity indicators, the Ca ii H and K lines, the Ca ii infrared triplet, and Hα line, as well as the stellar radial velocity. Results: Bℓ variations of up to about 80 G are observed without any reversal of its sign, and which are in phase with photometric ephemeris. The activity indicators are shown to vary smoothly on a timescale compatible with the rotational period inferred from photometry (308.8 d), however large deviations can occur from one rotation to another. The surface magnetic field variations of EK Eri appear to be dominated by a strong magnetic spot (of negative polarity) which is phased with the dark (cool) photometric spot. Our modeling shows that the large-scale magnetic field of EK Eri is strongly poloidal. For a rotational axis inclination of i = 60°, we obtain a model that is almost purely dipolar. Conclusions: In the dipolar model, the strong magnetic/photometric spot corresponds to the negative pole of the dipole, which could be the remnant of that of an Ap star progenitor of EK Eri. Our observations and modeling conceptually support this hypothesis, suggesting an explanation of the outstanding magnetic properties of EK Eri as the result of

  1. General relativistic simulations of black-hole-neutron-star mergers: Effects of magnetic fields

    NASA Astrophysics Data System (ADS)

    Etienne, Zachariah B.; Liu, Yuk Tung; Paschalidis, Vasileios; Shapiro, Stuart L.

    2012-03-01

    As a neutron star (NS) is tidally disrupted by a black hole (BH) companion at the end of a black-hole-neutron-star (BHNS) binary inspiral, its magnetic fields will be stretched and amplified. If sufficiently strong, these magnetic fields may impact the gravitational waveforms, merger evolution and mass of the remnant disk. Formation of highly-collimated magnetic field lines in the disk+spinning BH remnant may launch relativistic jets, providing the engine for a short-hard GRB. We analyze this scenario through fully general relativistic, magnetohydrodynamic BHNS simulations from inspiral through merger and disk formation. Different initial magnetic field configurations and strengths are chosen for the NS interior for both nonspinning and moderately spinning (aBH/MBH=0.75) BHs aligned with the orbital angular momentum. Only strong interior (Bmax⁡˜1017G) initial magnetic fields in the NS significantly influence merger dynamics, enhancing the remnant disk mass by 100% and 40% in the nonspinning and spinning BH cases, respectively. However, detecting the imprint of even a strong magnetic field may be challenging for Advanced LIGO. Though there is no evidence of mass outflows or magnetic field collimation during the preliminary simulations we have performed, higher resolution, coupled with longer disk evolutions and different initial magnetic field configurations, may be required to definitively assess the possibility of BHNS binaries as short-hard gamma-ray burst progenitors.

  2. The evolution of surface magnetic fields in young solar-type stars - I. The first 250 Myr

    NASA Astrophysics Data System (ADS)

    Folsom, C. P.; Petit, P.; Bouvier, J.; Lèbre, A.; Amard, L.; Palacios, A.; Morin, J.; Donati, J.-F.; Jeffers, S. V.; Marsden, S. C.; Vidotto, A. A.

    2016-03-01

    The surface rotation rates of young solar-type stars vary rapidly with age from the end of the pre-main sequence through the early main sequence. Important changes in the dynamos operating in these stars may result from this evolution, which should be observable in their surface magnetic fields. Here we present a study aimed at observing the evolution of these magnetic fields through this critical time period. We observed stars in open clusters and stellar associations of known ages, and used Zeeman Doppler imaging to characterize their complex magnetic large-scale fields. Presented here are results for 15 stars, from five associations, with ages from 20 to 250 Myr, masses from 0.7 to 1.2 M⊙, and rotation periods from 0.4 to 6 d. We find complex large-scale magnetic field geometries, with global average strengths from 14 to 140 G. There is a clear trend towards decreasing average large-scale magnetic field strength with age, and a tight correlation between magnetic field strength and Rossby number. Comparing the magnetic properties of our zero-age main-sequence sample to those of both younger and older stars, it appears that the magnetic evolution of solar-type stars during the pre-main sequence is primarily driven by structural changes, while it closely follows the stars' rotational evolution on the main sequence.

  3. Probing turbulent, magnetized star formation with ALMA observations and next-generation AREPO simulations

    NASA Astrophysics Data System (ADS)

    Hull, Charles L. H.; Mocz, Philip; Burkhart, Blakesley K.; Miquel Girart, Josep; Goodman, Alyssa A.; Cortes, Paulo; Li, Zhi-Yun; Lai, Shih-Ping; Hernquist, Lars; Springel, Volker

    2017-01-01

    The first polarization data from ALMA have been delivered, and are both expanding and confounding our understanding of the role of magnetic fields in low-mass star formation. Here I will show the highest resolution and highest sensitivity polarization images ever made of a Class 0 protostellar source. These new ALMA observations of the source, known as Ser-emb 8, achieve 140 AU resolution, allowing us to probe polarization -- and thus magnetic field orientation -- in the innermost regions surrounding the protostar. The collapse of strongly magnetized dense gas is predicted to pinch the magnetic field into an hourglass shape that persists down to scales <100 AU. However, in contrast with more than 50 years of theory, the ALMA data definitively rule out an hourglass morphology and instead reveal a chaotic magnetic field that has not been inherited from the field in the interstellar medium surrounding the source. We have simulated the star formation process with cutting-edge, moving-mesh AREPO simulations on scales from a million AU (5 pc) down to 60 AU. We find that only in the case of a very strong magnetic field (~100 microgauss on 5 pc scales) is the field direction preserved from cloud to disk scales. When the field is weak, turbulence in the interstellar gas shapes the field on large scales, and the forming star system re-shapes the field again on small scales, divorcing the field from its history on larger scales. We conclude that this is what has happened in Ser-emb 8. The main distinction from the strong-field star formation model is that in the weak-field case it is turbulence -- not the magnetic field -- that shapes the material that forms the protostar.

  4. Magnetic massive stars as progenitors of `heavy' stellar-mass black holes

    NASA Astrophysics Data System (ADS)

    Petit, V.; Keszthelyi, Z.; MacInnis, R.; Cohen, D. H.; Townsend, R. H. D.; Wade, G. A.; Thomas, S. L.; Owocki, S. P.; Puls, J.; ud-Doula, A.

    2017-04-01

    The groundbreaking detection of gravitational waves produced by the inspiralling and coalescence of the black hole (BH) binary GW150914 confirms the existence of 'heavy' stellar-mass BHs with masses >25 M⊙. Initial characterization of the system by Abbott et al. supposes that the formation of BHs with such large masses from the evolution of single massive stars is only feasible if the wind mass-loss rates of the progenitors were greatly reduced relative to the mass-loss rates of massive stars in the Galaxy, concluding that heavy BHs must form in low-metallicity (Z ≲ 0.25-0.5 Z⊙) environments. However, strong surface magnetic fields also provide a powerful mechanism for modifying mass-loss and rotation of massive stars, independent of environmental metallicity. In this paper, we explore the hypothesis that some heavy BHs, with masses >25 M⊙ such as those inferred to compose GW150914, could be the natural end-point of evolution of magnetic massive stars in a solar-metallicity environment. Using the MESA code, we developed a new grid of single, non-rotating, solar-metallicity evolutionary models for initial zero-age main sequence masses from 40 to 80 M⊙ that include, for the first time, the quenching of the mass-loss due to a realistic dipolar surface magnetic field. The new models predict terminal-age main-sequence (TAMS) masses that are significantly greater than those from equivalent non-magnetic models, reducing the total mass lost by a strongly magnetized 80 M⊙ star during its main-sequence evolution by 20 M⊙. This corresponds approximately to the mass-loss reduction expected from an environment with metallicity Z = 1/30 Z⊙.

  5. Modeling the Rise of Fibril Magnetic Fields in Fully Convective Stars

    NASA Astrophysics Data System (ADS)

    Weber, Maria A.; Browning, Matthew K.

    2016-08-01

    Many fully convective stars exhibit a wide variety of surface magnetism, including starspots and chromospheric activity. The manner by which bundles of magnetic field traverse portions of the convection zone to emerge at the stellar surface is not especially well understood. In the solar context, some insight into this process has been gleaned by regarding the magnetism as consisting partly of idealized thin flux tubes (TFTs). Here we present the results of a large set of TFT simulations in a rotating spherical domain of convective flows representative of a 0.3 M ⊙ main-sequence star. This is the first study to investigate how individual flux tubes in such a star might rise under the combined influence of buoyancy, convection, and differential rotation. A time-dependent hydrodynamic convective flow field, taken from separate 3D simulations calculated with the anelastic equations, impacts the flux tube as it rises. Convective motions modulate the shape of the initially buoyant flux ring, promoting localized rising loops. Flux tubes in fully convective stars have a tendency to rise nearly parallel to the rotation axis. However, the presence of strong differential rotation allows some initially low-latitude flux tubes of moderate strength to develop rising loops that emerge in the near-equatorial region. Magnetic pumping suppresses the global rise of the flux tube most efficiently in the deeper interior and at lower latitudes. The results of these simulations aim to provide a link between dynamo-generated magnetic fields, fluid motions, and observations of starspots for fully convective stars.

  6. A dynamo model of magnetic activity in solar-like stars with different rotational velocities

    SciTech Connect

    Karak, Bidya Binay; Choudhuri, Arnab Rai; Kitchatinov, Leonid L.

    2014-08-10

    We attempt to provide a quantitative theoretical explanation for the observations that Ca II H/K emission and X-ray emission from solar-like stars increase with decreasing Rossby number (i.e., with faster rotation). Assuming that these emissions are caused by magnetic cycles similar to the sunspot cycle, we construct flux transport dynamo models of 1 M{sub ☉} stars rotating with different rotation periods. We first compute the differential rotation and the meridional circulation inside these stars from a mean-field hydrodynamics model. Then these are substituted in our dynamo code to produce periodic solutions. We find that the dimensionless amplitude f{sub m} of the toroidal flux through the star increases with decreasing rotation period. The observational data can be matched if we assume the emissions to go as the power 3-4 of f{sub m}. Assuming that the Babcock-Leighton mechanism saturates with increasing rotation, we can provide an explanation for the observed saturation of emission at low Rossby numbers. The main failure of our model is that it predicts an increase of the magnetic cycle period with increasing rotation rate, which is the opposite of what is found observationally. Much of our calculations are based on the assumption that the magnetic buoyancy makes the magnetic flux tubes rise radially from the bottom of the convection zone. Taking into account the fact that the Coriolis force diverts the magnetic flux tubes to rise parallel to the rotation axis in rapidly rotating stars, the results do not change qualitatively.

  7. VizieR Online Data Catalog: Effective temperature of metal-poor A-type stars (Kinman+, 2002)

    NASA Astrophysics Data System (ADS)

    Kinman, T.; Castelli, F.

    2002-07-01

    Effective temperatures (Teff) can be determined from (V- (V-H)0 and (V-K)0 colours that are derived from 2MASS magnitudes. This gives another way to estimate the Teff of faint blue halo stars (V<~15) whose temperatures are now usually deduced from (BV)0_. Transformations (adapted from Carpenter, 2001AJ....121.2851C) are used to change colours derived from the 2MASS data to the Johnson system. Teff is then derived from these colours using an updated Kurucz model. Tables are given to derive Teff as a function of (V-J)0, (V-H)0 and (V-K)0 for a variety of metallicities and suitable for blue horizontal branch and main sequence stars. The temperatures obtained in this way are compared with those in the recent literature for various stars with 5<=V<=15 and Teff in the range 6500 to 9500K; systematic differences are ~100K. An exception is the sample of BHB stars observed by Wilhelm et al. (1999, Cat. ) whose Teff are significantly cooler than those we derive by an amount that increases with increasing temperature. Description: (2 data files).

  8. Magnetic stars with wide depressions in the continuum. 1. The Ap star with strong silicon lines HD5601

    NASA Astrophysics Data System (ADS)

    Romanyuk, I. I.; Kudryavtsev, D. O.; Semenko, E. A.; Moiseeva, A. V.

    2016-10-01

    Based on observations with the 6-m SAO RAS telescope, we have found that chemically peculiar star with a large depression of the continuum at λ5200 Å and strengthened silicon lines in the spectrum has a strong magnetic field. The longitudinal field component B e has a negative polarity and varies from -300 G to -2000 G with a period of 1.756 days. Photometric variations of brightness take place with the same period. We determined the variability of the radial velocity at times of about tens of years pointing to a possible binarity of the object. We have built a magnetic model of this star, determined the inclination angles of the rotation axis to the line of sight i = 20° and of the dipole axis to the rotation axis β = 116°, and the field strength at the pole is B p = 10 kG. We carried out a chemical composition analysis and found a lack of helium for almost an order of magnitude, some overabundance of silicon and metal elements for more than an order of magnitude, particularly, cobalt for three orders of magnitude.

  9. Magnetic effects on the low-T /|W | instability in differentially rotating neutron stars

    NASA Astrophysics Data System (ADS)

    Muhlberger, Curran D.; Nouri, Fatemeh Hossein; Duez, Matthew D.; Foucart, Francois; Kidder, Lawrence E.; Ott, Christian D.; Scheel, Mark A.; Szilágyi, Béla; Teukolsky, Saul A.

    2014-11-01

    Dynamical instabilities in protoneutron stars may produce gravitational waves whose observation could shed light on the physics of core-collapse supernovae. When born with sufficient differential rotation, these stars are susceptible to a shear instability (the "low-T /|W | instability"), but such rotation can also amplify magnetic fields to strengths where they have a considerable impact on the dynamics of the stellar matter. Using a new magnetohydrodynamics module for the Spectral Einstein Code, we have simulated a differentially-rotating neutron star in full 3D to study the effects of magnetic fields on this instability. Though strong toroidal fields were predicted to suppress the low-T /|W | instability, we find that they do so only in a small range of field strengths. Below 4 ×1 013 G , poloidal seed fields do not wind up fast enough to have an effect before the instability saturates, while above 5 ×1 014 G , magnetic instabilities can actually amplify a global quadrupole mode (this threshold may be even lower in reality, as small-scale magnetic instabilities remain difficult to resolve numerically). Thus, the prospects for observing gravitational waves from such systems are not in fact diminished over most of the magnetic parameter space. Additionally, we report that the detailed development of the low-T /|W | instability, including its growth rate, depends strongly on the particular numerical methods used. The high-order methods we employ suggest that growth might be considerably slower than found in some previous simulations.

  10. Bow shock nebulae of hot massive stars in a magnetized medium

    NASA Astrophysics Data System (ADS)

    Meyer, D. M.-A.; Mignone, A.; Kuiper, R.; Raga, A. C.; Kley, W.

    2017-01-01

    A significant fraction of OB-type, main-sequence massive stars are classified as runaway and move supersonically through the interstellar medium (ISM). Their strong stellar winds interact with their surroundings, where the typical strength of the local ISM magnetic field is about 3.5-7 μG, which can result in the formation of bow shock nebulae. We investigate the effects of such magnetic fields, aligned with the motion of the flow, on the formation and emission properties of these circumstellar structures. Our axisymmetric, magneto-hydrodynamical simulations with optically thin radiative cooling, heating and anisotropic thermal conduction show that the presence of the background ISM magnetic field affects the projected optical emission of our bow shocks at Hα and [O III] λ 5007 which become fainter by about 1-2 orders of magnitude, respectively. Radiative transfer calculations against dust opacity indicate that the magnetic field slightly diminishes their projected infrared emission and that our bow shocks emit brightly at 60 μm. This may explain why the bow shocks generated by ionizing runaway massive stars are often difficult to identify. Finally, we discuss our results in the context of the bow shock of ζ Ophiuchi and we support the interpretation of its imperfect morphology as an evidence of the presence of an ISM magnetic field not aligned with the motion of its driving star.

  11. The asteroseismic signature of strong magnetic fields in the cores of red giant stars

    NASA Astrophysics Data System (ADS)

    Fuller, Jim; Cantiello, Matteo; Stello, Dennis; Garcia, Rafael; Bildsten, Lars

    2016-01-01

    Internal stellar magnetic fields are inaccessible to direct observations and little is known about their amplitude, geometry and evolution. I will discuss how strong magnetic fields in the cores of red giant stars can be identified with asteroseismology. The fields manifest themselves via depressed dipole stellar oscillation modes, which arises from a magnetic greenhouse effect that scatters and traps oscillation mode energy within the core of the star. Physically, the effect stems from magnetic tension forces created by sufficiently strong fields, which break the spherical symmetry of the wave propagation cavity. The loss of wave energy within the core reduces the mode visibility at the stellar surface, and we find that our predicted visibilities are in excellent agreement with a class of red giants exhibiting depressed dipole oscillation modes. The Kepler satellite has already observed hundreds of these red giants, which we identify as stars with strongly magnetized cores. Field strengths larger than roughly 10^5 G can produce the observed depression, and in one case we measure a core field strength of 10^7 G.

  12. Modeling the structure of magnetic fields in Neutron Stars: from the interior to the magnetosphere

    NASA Astrophysics Data System (ADS)

    Bucciantini, Niccolò; Pili, Antonio G.; Del Zanna, Luca

    2016-05-01

    The phenomenology of the emission of pulsars and magnetars depends dramatically on the structure and properties of their magnetic field. In particular it is believed that the outbursting and flaring activity observed in AXPs and SRGs is strongly related to their internal magnetic field. Recent observations have moreover shown that charges are present in their magnetospheres supporting the idea that their magnetic field is tightly twisted in the vicinity of the star. In principle these objects offer a unique opportunity to investigate physics in a regime beyond what can be obtained in the laboratory. We will discuss the properties of equilibrium models of magnetized neutron stars, and we will show how internal and external currents can be related. These magnetic field configurations will be discussed considering also their stability, relevant for their origin and possibly connected to events like SNe and GRBs. We will also show what kind of deformations they induce in the star, that could lead to emission of gravitational waves. In the case of a twisted magnetosphere we will show how the amount of twist regulates their general topology. A general formalism based on the simultaneous numerical solution of the general relativistic Grad-Shafranov equation and Einstein equations will be presented.

  13. Magnetized hypermassive neutron-star collapse: a central engine for short gamma-ray bursts.

    PubMed

    Shibata, Masaru; Duez, Matthew D; Liu, Yuk Tung; Shapiro, Stuart L; Stephens, Branson C

    2006-01-27

    A hypermassive neutron star (HMNS) is a possible transient formed after the merger of a neutron-star binary. In the latest axisymmetric magnetohydrodynamic simulations in full general relativity, we find that a magnetized HMNS undergoes "delayed" collapse to a rotating black hole (BH) as a result of angular momentum transport via magnetic braking and the magnetorotational instability. The outcome is a BH surrounded by a massive, hot torus with a collimated magnetic field. The torus accretes onto the BH at a quasisteady accretion rate [FORMULA: SEE TEXT]; the lifetime of the torus is approximately 10 ms. The torus has a temperature [FORMULA: SEE TEXT], leading to copious ([FORMULA: SEE TEXT]) thermal radiation that could trigger a fireball. Therefore, the collapse of a HMNS is a promising scenario for generating short-duration gamma-ray bursts and an accompanying burst of gravitational waves and neutrinos.

  14. CARINA OB STARS: X-RAY SIGNATURES OF WIND SHOCKS AND MAGNETIC FIELDS

    SciTech Connect

    Gagne, Marc; Fehon, Garrett; Savoy, Michael R.; Cohen, David H.; Townsley, Leisa K.; Broos, Patrick S.; Povich, Matthew S.; Corcoran, Michael F.; Remage Evans, Nancy; Moffat, Anthony F. J.; Naze, Yael; Oskinova, Lida M.

    2011-05-01

    The Chandra Carina Complex contains 200 known O- and B-type stars. The Chandra survey detected 68 of the 70 O stars and 61 of 127 known B0-B3 stars. We have assembled a publicly available optical/X-ray database to identify OB stars that depart from the canonical L{sub X}/L{sub bol} relation or whose average X-ray temperatures exceed 1 keV. Among the single O stars with high kT we identify two candidate magnetically confined wind shock sources: Tr16-22, O8.5 V, and LS 1865, O8.5 V((f)). The O4 III(fc) star HD 93250 exhibits strong, hard, variable X-rays, suggesting that it may be a massive binary with a period of >30 days. The visual O2 If* binary HD 93129A shows soft 0.6 keV and hard 1.9 keV emission components, suggesting embedded wind shocks close to the O2 If* Aa primary and colliding wind shocks between Aa and Ab. Of the 11 known O-type spectroscopic binaries, the long orbital-period systems HD 93343, HD 93403, and QZ Car have higher shock temperatures than short-period systems such as HD 93205 and FO 15. Although the X-rays from most B stars may be produced in the coronae of unseen, low-mass pre-main-sequence companions, a dozen B stars with high L{sub X} cannot be explained by a distribution of unseen companions. One of these, SS73 24 in the Treasure Chest cluster, is a new candidate Herbig Be star.

  15. B field in OB stars (BOB): The outstandingly strong magnetic field in the evolved He-strong star CPD -62° 2124

    NASA Astrophysics Data System (ADS)

    Castro, N.; Fossati, L.; Hubrig, S.; Järvinen, S. P.; Przybilla, N.; Nieva, M.-F.; Ilyin, I.; Carroll, T. A.; Schöller, M.; Langer, N.; Schneider, F. R. N.; Simón-Díaz, S.; Morel, T.; Butler, K.; BOB Collaboration

    2017-01-01

    The origin and evolution of magnetism in OB stars is far from being well understood. With approximately 70 magnetic OB stars known, any new object with unusual characteristics may turn out to be a key piece of the puzzle. We report the detection of an exceptionally strong magnetic field in the He-strong B2IV star CPD -62° 2124. Spectropolarimetric FORS2 and HARPSpol observations were analysed by two independent teams and procedures, concluding on a strong longitudinal magnetic field of approximately 5.2 kG. The quantitative characterisation of the stellar atmosphere yields an effective temperature of 23 650 ± 250 K, a surface gravity of 3.95 ± 0.10 dex and a surface helium fraction of 0.35 ± 0.02 by number. The metal composition is in agreement with the cosmic abundance standard, except for Mg, Si and S, which are slightly non-solar. The strong and broad ( 300 km s-1) disc-like emission displayed by the Hα line suggests a centrifugal magnetosphere supported by the strong magnetic field. Our results imply that CPD -62° 2124 is an early B-type star hosting one of the strongest magnetic fields discovered to date, and one of the most evolved He-strong stars known, with a fractional main-sequence lifetime of approximately 0.6. Based on observations made with ESO telescopes at the La Silla and Paranal observatories under programme ID 191.D-0255(G,I).

  16. Spallation nucleosynthesis by accelerated charged-particles in stellar envelopes of magnetic stars

    SciTech Connect

    Goriely, S.

    2007-02-26

    Recent observations have suggested the presence of radioactive elements, such as Tc, Pm and 84 {<=} Z {<=} 99 elements at the surface of the chemically-peculiar magnetic star HD 101065, also known as Przybylski's star. The peculiar 35 < Z < 82 abundance pattern of HD 101065 has been so far explained by diffusion processes in the stellar envelope . However, such processes cannot be called for to explain the origin of short-lived radio-elements. The large magnetic field observed in Ap stars can be at the origin of a significant acceleration of charged-particles, mainly protons and {alpha}-particles, that in turn can by interaction with the stellar material modify the surface content.The present contribution explores to what extent the irradiation process corresponding to the interaction of the stellar material with energetic particles can by itself only explain the abundances determined by observation at the surface of the chemically peculiar star HD 101065, as well as other chemically peculiar star. Due to the unknown characteristics of the accelerated particles, a purely parametric approach is followed, taken as free parameters the proton and {alpha}-particle flux amplitude and energy distribution as well as the time of irradiation. The specific simulations considered here can explain many different observational aspects. In particular, it is shown that a significant production of Z > 30 heavy elements can be achieved. The most attractive feature of the irradiation process is the significant production of Tc and Pm, as well as actinides and sub-actinides.

  17. A FIVE-YEAR SPECTROSCOPIC AND PHOTOMETRIC CAMPAIGN ON THE PROTOTYPICAL {alpha} CYGNI VARIABLE AND A-TYPE SUPERGIANT STAR DENEB

    SciTech Connect

    Richardson, N. D.; Morrison, N. D.; Kryukova, E. E.; Adelman, S. J. E-mail: nmorris@utnet.utoledo.edu E-mail: adelmans@citadel.edu

    2011-01-15

    Deneb is often considered the prototypical A-type supergiant and is one of the visually most luminous stars in the Galaxy. A-type supergiants are potential extragalactic distance indicators, but the variability of these stars needs to be better characterized before this technique can be considered reliable. We analyzed 339 high-resolution echelle spectra of Deneb obtained over the five-year span of 1997 through 2001 as well as 370 Stroemgren photometric measurements obtained during the same time frame. Our spectroscopic analysis included dynamical spectra of the H{alpha} profile, H{alpha} equivalent widths, and radial velocities measured from Si II {lambda}{lambda} 6347, 6371. Time-series analysis reveals no obvious cyclic behavior that proceeds through multiple observing seasons, although we found a suspected 40 day period in two, non-consecutive observing seasons. Some correlations are found between photometric and radial velocity data sets and suggest radial pulsations at two epochs. No correlation is found between the variability of the H{alpha} profiles and that of the radial velocities or the photometry. Lucy found evidence that Deneb was a long-period single-lined spectroscopic binary star, but our data set shows no evidence for radial velocity variations caused by a binary companion.

  18. MOVES - I. The evolving magnetic field of the planet-hosting star HD189733

    NASA Astrophysics Data System (ADS)

    Fares, R.; Bourrier, V.; Vidotto, A. A.; Moutou, C.; Jardine, M. M.; Zarka, P.; Helling, Ch.; Lecavelier des Etangs, A.; Llama, J.; Louden, T.; Wheatley, P. J.; Ehrenreich, D.

    2017-10-01

    HD189733 is an active K dwarf that is, with its transiting hot Jupiter, among the most studied exoplanetary systems. In this first paper of the Multiwavelength Observations of an eVaporating Exoplanet and its Star (MOVES) programme, we present a 2-yr monitoring of the large-scale magnetic field of HD189733. The magnetic maps are reconstructed for five epochs of observations, namely 2013 June-July, 2013 August, 2013 September, 2014 September and 2015 July, using Zeeman-Doppler imaging. We show that the field evolves along the five epochs, with mean values of the total magnetic field of 36, 41, 42, 32 and 37 G, respectively. All epochs show a toroidally dominated field. Using previously published data of Moutou et al. and Fares et al., we are able to study the evolution of the magnetic field over 9 yr, one of the longest monitoring campaigns for a given star. While the field evolved during the observed epochs, no polarity switch of the poles was observed. We calculate the stellar magnetic field value at the position of the planet using the potential field source surface extrapolation technique. We show that the planetary magnetic environment is not homogeneous over the orbit, and that it varies between observing epochs, due to the evolution of the stellar magnetic field. This result underlines the importance of contemporaneous multiwavelength observations to characterize exoplanetary systems. Our reconstructed maps are a crucial input for the interpretation and modelling of our MOVES multiwavelength observations.

  19. Detecting planets around active stars: impact of magnetic fields on radial velocities and line bisectors

    NASA Astrophysics Data System (ADS)

    Hébrard, É. M.; Donati, J.-F.; Delfosse, X.; Morin, J.; Boisse, I.; Moutou, C.; Hébrard, G.

    2014-09-01

    Although technically challenging, detecting Earth-like planets around very low mass stars is in principle accessible to the existing velocimeters of highest radial-velocity (RV) precision. However, low-mass stars being active, they often feature dark spots and magnetic regions at their surfaces generating a noise level in RV curves (called activity jitter) that can severely limit our practical ability at detecting Earth-like planets. Whereas the impact of dark spots on RV data has been extensively studied in the literature, that of magnetic features only received little attention up to now. In this paper, we aim at quantifying the impact of magnetic fields (and the Zeeman broadening they induce) on line profiles, line bisectors and RV data. With a simple model, we quantitatively study the RV signals and bisector distortions that small magnetic regions or global magnetic dipoles can generate, especially at infrared wavelengths where the Zeeman broadening is much larger than that in the visible. We report in particular that the impact of magnetic features on line bisectors can be different from that of cool spots when the rotational broadening is comparable to or larger than the Zeeman broadening; more specifically, we find in this case that the top and bottom sections of the bisectors are anticorrelated, i.e. the opposite behaviour of what is observed for cool spots. We finally suggest new options to show and ultimately filter the impact of the magnetic activity on RV curves.

  20. High-field magnetic white dwarfs as the progeny of early-type stars?

    NASA Astrophysics Data System (ADS)

    Dobbie, P. D.; Külebi, B.; Casewell, S. L.; Burleigh, M. R.; Parker, Q. A.; Baxter, R.; Lawrie, K. A.; Jordan, S.; Koester, D.

    2013-01-01

    We present an analysis of the newly resolved components of two hot, double-degenerate systems, SDSS J074853.07+302543.5 + J074852.95+302543.4 and SDSS J150813.24+394504.9 + J150813.31+394505.6 (CBS 229). We confirm that each system has widely separated components (a > 100 au) consisting of a H-rich, non-magnetic white dwarf and a H-rich, high-field magnetic white dwarf (HFMWD). The masses of the non-magnetic degenerates are found to be larger than typical of field white dwarfs. We use these components to estimate the total ages of the binaries and demonstrate that both magnetic white dwarfs are the progeny of stars with Minit > 2 M⊙. We briefly discuss the traits of all known hot, wide, magnetic + non-magnetic double degenerates in the context of HFMWD formation theories. These are broadly consistent (chance probability, P ≈ 0.065) with HFMWDs forming primarily from early-type stars and, in the most succinct interpretation, link their magnetism to the fields of their progenitors. Our results do not, however, rule out that HFMWDs can form through close binary interactions and studies of more young, wide double degenerates are required to reach firm conclusions on these formation pathways.

  1. POSSIBLE CONSTRAINTS ON EXOPLANET MAGNETIC FIELD STRENGTHS FROM PLANET-STAR INTERACTION

    SciTech Connect

    Scharf, Caleb A.

    2010-10-20

    A small percentage of normal stars harbor giant planets that orbit within a few tenths of an astronomical unit. At such distances, the potential exists for significant tidal and magnetic field interaction resulting in energy dissipation that may manifest as changes within the stellar corona. We examine the X-ray emission of stars hosting planets and find a positive correlation between X-ray luminosity and the projected mass of the most closely orbiting exoplanets. We investigate possible systematics and observational biases that could mimic or confuse this correlation but find no strong evidence for any, especially for planets more massive than {approx}0.1 M{sub J} . Luminosities and upper limits are consistent with the interpretation that there is a lower floor to stellar X-ray emission dependent on close-in planetary mass. Under the hypothesis that this is a consequence of planet-star magnetic field interaction, and energy dissipation, we estimate a possible field strength increase of a factor of {approx}8 between planets of 1 and 10 M{sub J} . Intriguingly, this is consistent with recent geodynamo scaling law predictions. The high-energy photon emission of planet-star systems may therefore provide unique access to the detailed magnetic, and hence geodynamic, properties of exoplanets.

  2. The magnetic field of an isolated neutron star from X-ray cyclotron absorption lines.

    PubMed

    Bignami, G F; Caraveo, P A; De Luca, A; Mereghetti, S

    2003-06-12

    Isolated neutron stars are highly magnetized, fast-rotating objects that form as an end point of stellar evolution. They are directly observable in X-ray emission, because of their high surface temperatures. Features in their X-ray spectra could in principle reveal the presence of atmospheres, or be used to estimate the strength of their magnetic fields through the cyclotron process, as is done for X-ray binaries. Almost all isolated neutron star spectra observed so far appear as featureless thermal continua. The only exception is 1E1207.4-5209 (refs 7-9), where two deep absorption features have been detected, but with insufficient definition to permit unambiguous interpretation. Here we report a long X-ray observation of the same object in which the star's spectrum shows three distinct features, regularly spaced at 0.7, 1.4 and 2.1 keV, plus a fourth feature of lower significance, at 2.8 keV. These features vary in phase with the star's rotation. The logical interpretation is that they are features from resonant cyclotron absorption, which allows us to calculate a magnetic field strength of 8 x 10(10) G, assuming the absorption arises from electrons.

  3. Astronomers Find the First 'Wind Nebula' Around a Rare Ultra-Magnetic Neutron Star

    NASA Image and Video Library

    2017-09-28

    Astronomers have discovered a vast cloud of high-energy particles called a wind nebula around a rare ultra-magnetic neutron star, or magnetar, for the first time. The find offers a unique window into the properties, environment and outburst history of magnetars, which are the strongest magnets in the universe. A neutron star is the crushed core of a massive star that ran out of fuel, collapsed under its own weight, and exploded as a supernova. Each one compresses the equivalent mass of half a million Earths into a ball just 12 miles (20 kilometers) across, or about the length of New York's Manhattan Island. Neutron stars are most commonly found as pulsars, which produce radio, visible light, X-rays and gamma rays at various locations in their surrounding magnetic fields. When a pulsar spins these regions in our direction, astronomers detect pulses of emission, hence the name. Read more: go.nasa.gov/28PVUop Credit: ESA/XMM-Newton/Younes et al. 2016 NASA image use policy. NASA Goddard Space Flight Center enables NASA’s mission through four scientific endeavors: Earth Science, Heliophysics, Solar System Exploration, and Astrophysics. Goddard plays a leading role in NASA’s accomplishments by contributing compelling scientific knowledge to advance the Agency’s mission. Follow us on Twitter Like us on Facebook Find us on Instagram

  4. THE FRAGMENTATION OF MAGNETIZED, MASSIVE STAR-FORMING CORES WITH RADIATIVE FEEDBACK

    SciTech Connect

    Myers, Andrew T.; McKee, Christopher F.; Cunningham, Andrew J.; Klein, Richard I.; Krumholz, Mark R.

    2013-04-01

    We present a set of three-dimensional, radiation-magnetohydrodynamic calculations of the gravitational collapse of massive (300 M{sub Sun }), star-forming molecular cloud cores. We show that the combined effects of magnetic fields and radiative feedback strongly suppress core fragmentation, leading to the production of single-star systems rather than small clusters. We find that the two processes are efficient at suppressing fragmentation in different regimes, with the feedback most effective in the dense, central region and the magnetic field most effective in more diffuse, outer regions. Thus, the combination of the two is much more effective at suppressing fragmentation than either one considered in isolation. Our work suggests that typical massive cores, which have mass-to-flux ratios of about 2 relative to critical, likely form a single-star system, but that cores with weaker fields may form a small star cluster. This result helps us understand why the observed relationship between the core mass function and the stellar initial mass function holds even for {approx}100 M{sub Sun} cores with many thermal Jeans masses of material. We also demonstrate that a {approx}40 AU Keplerian disk is able to form in our simulations, despite the braking effect caused by the strong magnetic field.

  5. Can beating between different dynamo modes explain multiple magnetic cycles in solar - type stars

    NASA Astrophysics Data System (ADS)

    Simoniello, R.; Karoff, C.; Metcalfe, T. S.

    2015-09-01

    Stellar magnetic activity can be characterized by a chaotic, multiple or single cycle behavior. Sometimes cyclic activity can be interrupted by a flat behavior. The mechanism that produce such a diverse behavior in stellar atmosphere is a matter of debate. We decided to address this issue by investigating the properties of a sample of 40 stars with high quality cycles, selected from the original data provided by the Mount Wilson Observatory. This sample contains stars with single and secondary cycles, whose secondary periods are longer or shorter than the primary cycle.

  6. Magnetic field evolution and equilibrium configurations in neutron star cores: the effect of ambipolar diffusion

    NASA Astrophysics Data System (ADS)

    Castillo, F.; Reisenegger, A.; Valdivia, J. A.

    2017-10-01

    As another step towards understanding the long-term evolution of the magnetic field in neutron stars, we provide the first simulations of ambipolar diffusion in a spherical star. Restricting ourselves to axial symmetry, we consider a charged-particle fluid of protons and electrons carrying the magnetic flux through a motionless, uniform background of neutrons that exerts a collisional drag force on the former. We also ignore the possible impact of β decays, proton superconductivity and neutron superfluidity. All initial magnetic field configurations considered are found to evolve on the analytically expected time-scales towards 'barotropic equilibria' satisfying the 'Grad-Shafranov equation', in which the magnetic force is balanced by the degeneracy pressure gradient, so ambipolar diffusion is choked. These equilibria are so-called 'twisted torus' configurations, which include poloidal and toroidal components, the latter restricted to the toroidal volumes in which the poloidal field lines close inside the star. In axial symmetry, they appear to be stable, although they are likely to undergo non-axially symmetric instabilities.

  7. Model atmospheres and radiation of magnetic neutron stars: Anisotropic thermal emission

    NASA Technical Reports Server (NTRS)

    Pavlov, G. G.; Shibanov, Yu. A.; Ventura, J.; Zavlin, V. E.

    1994-01-01

    We investigate the anisotropy of the thermal radiation emitted by a surface element of a neutron star atmosphere (e.g., by a polar cap of a radio pulsar). Angular dependences of the partial fluxes at various photon energies, and spectra at various angles are obtained for different values of the effective temperature T(sub eff) and magnetic field strength B, and for different directions of the magnetic field. It is shown that the local radiation of the magnetized neutron star atmospheres is highly anisotropic, with the maximum flux emitted in the magnetic field direction. At high B the angular dependences in the soft X-ray range have two maxima, a high narrow peak along B and a lower and broader maximum at intermediate angles. The radiation is strongly polarized, the modulation of the degree of polarization due to the rotation of the neurtron star may be much higher than that for the radiative flux. The results obtained are compared with recent ROSAT observations of the thermal-like radiation from the radio pulsars PSR 1929+10 and PSR J0437-4715.

  8. Angular momentum loss and stellar spin-down in magnetic massive stars

    NASA Astrophysics Data System (ADS)

    ud-Doula, Asif; Owocki, Stanley P.; Townsend, Richard H. D.

    2009-04-01

    We examine the angular momentum loss and associated rotational spin-down for magnetic hot stars with a line-driven stellar wind and a rotation-aligned dipole magnetic field. Our analysis here is based on our previous 2-D numerical MHD simulation study that examines the interplay among wind, field, and rotation as a function of two dimensionless parameters, W(=Vrot/Vorb) and ‘wind magnetic confinement’, η∗ defined below. We compare and contrast the 2-D, time variable angular momentum loss of this dipole model of a hot-star wind with the classical 1-D steady-state analysis by Weber and Davis (WD), who used an idealized monopole field to model the angular momentum loss in the solar wind. Despite the differences, we find that the total angular momentum loss averaged over both solid angle and time follows closely the general WD scaling ~ ṀΩR2A. The key distinction is that for a dipole field Alfvèn radius RA is significantly smaller than for the monopole field WD used in their analyses. This leads to a slower stellar spin-down for the dipole field with typical spin-down times of order 1 Myr for several known magnetic massive stars.

  9. Model atmospheres and radiation of magnetic neutron stars: Anisotropic thermal emission

    NASA Technical Reports Server (NTRS)

    Pavlov, G. G.; Shibanov, Yu. A.; Ventura, J.; Zavlin, V. E.

    1994-01-01

    We investigate the anisotropy of the thermal radiation emitted by a surface element of a neutron star atmosphere (e.g., by a polar cap of a radio pulsar). Angular dependences of the partial fluxes at various photon energies, and spectra at various angles are obtained for different values of the effective temperature T(sub eff) and magnetic field strength B, and for different directions of the magnetic field. It is shown that the local radiation of the magnetized neutron star atmospheres is highly anisotropic, with the maximum flux emitted in the magnetic field direction. At high B the angular dependences in the soft X-ray range have two maxima, a high narrow peak along B and a lower and broader maximum at intermediate angles. The radiation is strongly polarized, the modulation of the degree of polarization due to the rotation of the neurtron star may be much higher than that for the radiative flux. The results obtained are compared with recent ROSAT observations of the thermal-like radiation from the radio pulsars PSR 1929+10 and PSR J0437-4715.

  10. Large and small-scale magnetic fields in star-forming regions

    NASA Astrophysics Data System (ADS)

    Reissl, Stefan; Bertrang, Gesa; Wolf, Sebastian; Banerjee, Robi; Das, Himadri Sekhar; Seifried, Daniel; Körtgen, Bastian

    2013-07-01

    We present numerical and observational studies aimed at analyzing the potential of multi-wavelength high-spatial resolution continuum polarization measurements for constraining the multi-scale structure of magnetic fields in the interior and environment of molecular clouds. Numerical simulations: We developed an extended, adaptive grid version of the 3D Monte-Carlo radiation transfer code MC3D (Wolf et al., 1999, 2003) for multi-wavelength polarization simulations. On the basis of theoretical dust grain models, polarization due to dichroic extinction and reemission as well as scattering is considered. Multi-scale magneto-hydrodynamical (MHD) simulations of the interstellar medium (ISM) provide the complex distributions of the density, temperature, and magnetic field in star-forming regions. This type of sophisticated synthetic polarization modeling will allow us to prepare and properly analyze existing and future observations of the three-dimensional magnetic field structure in the ISM. Various kinds of dust grain properties and advanced MHD scenarios are considered to cover the broad variety of observable ISM characteristics. Observations: Bok globules represent an ideal environment to study the influence of magnetic fields on the process of low-mass star formation. The magnetic field strength and structure in the dense inner regions of the globules can be determined by observing the polarized reemission radiation of aligned dust grains in the sub-mm wavelength range. The magnetic field in the outer, less dense parts of the globules can be traced by observing polarized radiation of background stars in the optical or near-IR. We present polarimetric observations of two Bok globules, CB68 and B335, carried out in the near-IR (ISAAC/VLT) and in the optical (IFOSC/IGO). Together with archival sub-mm data (SCUBA/JCMT), we trace the magnetic fields in these objects from 10^3 AU scales up to 10^5-10^6 scales for the first time.

  11. Chemical spots on the surface of the strongly magnetic Herbig Ae star HD 101412

    NASA Astrophysics Data System (ADS)

    Järvinen, S. P.; Hubrig, S.; Schöller, M.; Ilyin, I.; Carroll, T. A.; Korhonen, H.

    2016-03-01

    Due to the knowledge of the rotation period and the presence of a rather strong surface magnetic field, the sharp-lined young Herbig Ae star HD 101412 with a rotation period of 42 d has become one of the most well-studied targets among the Herbig Ae stars. High-resolution HARPS polarimetric spectra of HD 101412 were recently obtained on seven different epochs. Our study of the spectral variability over the part of the rotation cycle covered by HARPS observations reveals that the line profiles of the elements Mg, Si, Ca, Ti, Cr, Mn, Fe, and Sr are clearly variable while He exhibits variability that is opposite to the behaviour of the other elements studied. Since classical Ap stars usually show a relationship between the magnetic field geometry and the distribution of element spots, we used in our magnetic field measurements different line samples belonging to the three elements with the most numerous spectral lines, Ti, Cr, and Fe. Over the time interval covered by the available spectra, the longitudinal magnetic field changes sign from negative to positive polarity. The distribution of field values obtained using Ti, Cr, and Fe lines is, however, completely different compared to the magnetic field values determined in previous low-resolution FORS 2 measurements, where hydrogen Balmer lines are the main contributors to the magnetic field measurements, indicating the presence of concentration of the studied iron-peak elements in the region of the magnetic equator. Further, we discuss the potential role of contamination by the surrounding warm circumstellar matter in the appearance of Zeeman features obtained using Ti lines. Based on data obtained from the ESO Science Archive Facility under request MSCHOELLER 101895 (ESO programme Nos. 081.C-0410(A), 085.C-0137(A), and 187.D-0917(D)).

  12. X-ray emission from star-forming galaxies - signatures of cosmic rays and magnetic fields

    NASA Astrophysics Data System (ADS)

    Schober, J.; Schleicher, D. R. G.; Klessen, R. S.

    2015-01-01

    The evolution of magnetic fields in galaxies is still an open problem in astrophysics. In nearby galaxies the far-infrared-radio correlation indicates the coupling between magnetic fields and star formation. The correlation arises from the synchrotron emission of cosmic ray electrons travelling through the interstellar magnetic fields. However, with an increase of the interstellar radiation field (ISRF), inverse Compton scattering becomes the dominant energy loss mechanism of cosmic ray electrons with a typical emission frequency in the X-ray regime. The ISRF depends on the one hand on the star formation rate and becomes stronger in starburst galaxies, and on the other hand increases with redshift due to the higher temperature of the cosmic microwave background. With a model for the star formation rate of galaxies, the ISRF, and the cosmic ray spectrum, we can calculate the expected X-ray luminosity resulting from the inverse Compton emission. Except for galaxies with an active galactic nucleus the main additional contribution to the X-ray luminosity comes from X-ray binaries. We estimate this contribution with an analytical model as well as with an observational relation, and compare it to the pure inverse Compton luminosity. Using data from the Chandra Deep Field Survey and far-infrared observations from Atacama Large Millimeter/Submillimeter Array, we then determine upper limits for the cosmic ray energy. Assuming that the magnetic energy in a galaxy is in equipartition with the energy density of the cosmic rays, we obtain upper limits for the magnetic field strength. Our results suggest that the mean magnetic energy of young galaxies is similar to the one in local galaxies. This points towards an early generation of galactic magnetic fields, which is in agreement with current dynamo evolution models.

  13. SEARCH FOR A MAGNETIC FIELD VIA CIRCULAR POLARIZATION IN THE WOLF-RAYET STAR EZ CMa

    SciTech Connect

    De la Chevrotiere, A.; St-Louis, N.; Moffat, A. F. J.; Collaboration: MiMeS Collaboration

    2013-02-20

    We report on the first deep, direct search for a magnetic field via the circular polarization of Zeeman splitting in a Wolf-Rayet (W-R) star. Using the highly efficient ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope, we observed at three different epochs one of the best W-R candidates in the sky expected to harbor a magnetic field, the bright, highly variable WN4 star EZ CMa = WR6 = HD 50896. We looked for the characteristic circular polarization (Stokes V) pattern in strong emission lines that would arise as a consequence of a global, rotating magnetic field with a split monopole configuration. We also obtained nearly simultaneous linear polarization spectra (Stokes Q and U), which are dominated by electron scattering, most likely from a flattened wind with large-scale corotating structures. As the star rotates with a period of 3.766 days, our view of the wind changes, which in turn affects the value of the linear polarization in lines versus continuum at the {approx}0.2% level. Depending on the epoch of observation, our Stokes V data were affected by significant crosstalk from Stokes Q and U to V. We removed this spurious signal from the circular polarization data and experimented with various levels of spectral binning to increase the signal-to-noise ratio of our data. In the end, no magnetic field is unambiguously detected in EZ CMa. Assuming that the star is intrinsically magnetic and harbors a split monopole configuration, we find an upper limit of B {approx} 100 G for the intensity of its field in the line-forming regions of the stellar wind.

  14. Neutron stars in a perturbative f(R) gravity model with strong magnetic fields

    SciTech Connect

    Cheoun, Myung-Ki; Deliduman, Cemsinan; Güngör, Can; Keleş, Vildan; Ryu, C.Y.; Kajino, Toshitaka; Mathews, Grant J. E-mail: cemsinan@msgsu.edu.tr E-mail: kelesvi@itu.edu.tr E-mail: kajino@nao.ac.jp

    2013-10-01

    In Kaluza-Klein electromagnetism it is natural to associate modified gravity with strong electromagnetic fields. Hence, in this paper we investigate the combined effects of a strong magnetic field and perturbative f(R) gravity on the structure of neutron stars. The effect of an interior strong magnetic field of about 10{sup 17−18} G on the equation of state is derived in the context of a quantum hadrodynamics (QHD) equation of state (EoS) including effects of the magnetic pressure and energy along with occupied Landau levels. Adopting a random orientation of interior field domains, we solve the modified spherically symmetric hydrostatic equilibrium equations derived for a gravity model with f(R) = R+αR{sup 2}. Effects of both the finite magnetic field and the modified gravity are detailed for various values of the magnetic field and the perturbation parameter α along with a discussion of their physical implications. We show that there exists a parameter space of the modified gravity and the magnetic field strength, in which even a soft equation of state can accommodate a large ( > 2 M{sub s}un) maximum neutron star mass.

  15. Magnetic Cycles in a Wreath-Building Dynamo Simulation of a Young Solar-type Star

    NASA Astrophysics Data System (ADS)

    Brown, Benjamin; Miesch, M. S.; Browning, M. K.; Brun, A. S.; Nelson, N. J.; Toomre, J.

    2011-01-01

    Stars like the Sun build global-scale magnetic fields though dynamo processes in their convection zones. There, global-scale plasma motions couple with rotation and likely drive cycles of magnetic activity, though the exact processes at work in solar and stellar dynamos remain elusive. Observations of younger suns indicate that they rotate quite rapidly, have strong magnetic fields at their surfaces, and show signs of cyclic activity. Here we explore recent 3-D MHD simulations of younger, more rapidly rotating solar-type stars conducted with the anelastic spherical harmonic (ASH) code. These simulations of global-scale convection and dynamo action produce strikingly organized magnetic structures in the bulk of their convection zones. Wreaths of magnetic field fill the convection zone and can undergo regular cycles of polarity reversal. Indeed, we find that cyclic behavior is a common feature throughout the parameter space we have explored. Though these magnetic wreaths can coexist with tachoclines of penetration and shear, they do not rely on that internal boundary layer for their formation or persistence. Tachoclines may play a less critical role in the stellar dynamos of younger Suns than has been supposed in solar dynamo theory.

  16. No evidence of a strong magnetic field in the Blazhko star RR Lyrae

    NASA Astrophysics Data System (ADS)

    Chadid, M.; Wade, G. A.; Shorlin, S. L. S.; Landstreet, J. D.

    2004-01-01

    In this paper we report a new series of high-precision (median σB˜ 80 G) longitudinal magnetic field measurements of RR Lyrae, the brightest Blazhko star, obtained with the MuSiCoS spectropolarimeter over a period of almost 4 years from 1999-2002. These data provide no evidence whatsoever for a strong magnetic field in the photosphere of RR Lyrae, a result consistent with Preston's (\\cite{Preston67}) results, but inconsistent with apparent magnetic field detections by Babcock (\\cite{Babcock58}) and Romanov et al. (\\cite{Romanov87}, \\cite{Romanov94}). Following discussion of these disparate results, we conclude that RR Lyrae is a {bona fide} non-magnetic star, a conclusion which leads to the general falsification of models of the Blazkho effect requiring strong photospheric magnetic fields. Furthermore, due to the 4 year baseline of our observations, we can also dismiss the hypothesis that RR Lyrae undergoes a 4-year ``magnetic cycle''. Therefore the origin of the observed 4-year modulation of RR Lyrae remains unexplained, and we stress the need for additional theoretical efforts to come to a better understanding of this phenomenon. Based on observations obtained with the MuSiCoS spectropolarimeter attached to the 2 m Bernard-Lyot Telescope of the Pic du Midi Observatory, France.

  17. The influence of the magnetic topology on the wind braking of sun-like stars.

    NASA Astrophysics Data System (ADS)

    Réville, V.; Brun, A. S.; Matt, S. P.; Strugarek, A.; Pinto, R.

    2014-12-01

    Stellar winds are thought to be the main process responsible for the spin down of main-sequence stars. The extraction of angular momentum by a magnetized wind has been studied for decades, leading to several formulations for the resulting torque. However, previous studies generally consider simple dipole or split monopole stellar magnetic topologies. Here we consider in addition to a dipolar stellar magnetic field, both quadrupolar and octupolar configurations, while also varying the rotation rate and the magnetic field strength. 60 simulations made with a 2.5D, cylindrical and axisymmetric set-up and computed with the PLUTO code were used to find torque formulations for each topology. We further succeed to give a unique law that fits the data for every topology by formulating the torque in terms of the amount of open magnetic flux in the wind. We also show that our formulation can be applied to even more realistic magnetic topologies, with examples of the Sun in its minimum and maximum phase as observed at the Wilcox Solar Observatory, and of a young K-star (TYC-0486-4943-1) whose topology has been obtained by Zeeman-Doppler Imaging (ZDI).

  18. Magnetic Fields in the Universe III - From Laboratory and Stars to Primordial Structures

    NASA Astrophysics Data System (ADS)

    Soida, Marian; Otmianowska-Mazur, Katarzyna; de Gouveia Dal Pino, Elisabete M.; Lazarian, Alex

    2012-01-01

    The scientific aim of the conference is to provide a natural continuation to the former editions, putting forward the most recent advances of theoretical and numerical studies, as well as new information gathered from observations. Specific issues addressed include the role of magnetic fields in solar and stellar coronae, accretion disks, jets and outflows, stellar and galactic winds, massive stars and in black holes. We discussed its role in late stages of stellar evolution, supernova remnants, interstellar clouds and in star formation. Particular subjects involve the structure of spiral and irregular galaxies, galactic nuclei, clusters of galaxies, as well as high redshift and primordial magnetic fields. Basic plasma and MHD astrophysical processes were addressed. In particular, we mean phenomena such as: the MHD turbulence, dynamos, the magnetic reconnection, magnetic instabilities and a magnetized dusty gas,. We discussed these subjects using results of laboratory experiments and MHD of numerical modeling. The expected data on magnetic fields which will be gathered using new and forthcoming instruments (e.g. LOFAR, PLANCK, SOFIA, ALMA SKA, CTA, etc) were discussed as well.

  19. GLOBAL GALACTIC DYNAMO DRIVEN BY COSMIC RAYS AND EXPLODING MAGNETIZED STARS

    SciTech Connect

    Hanasz, Michal; Woltanski, Dominik; Kowalik, Kacper

    2009-11-20

    We report the first results of the first global galactic-scale cosmic ray (CR)-MHD simulations of CR-driven dynamo. We investigate the dynamics of magnetized interstellar medium (ISM), which is dynamically coupled with CR gas. We assume that exploding stars deposit small-scale, randomly oriented, dipolar magnetic fields into the differentially rotating ISM, together with a portion of CRs, accelerated in supernova shocks. We conduct numerical simulations with the aid of a new parallel MHD code PIERNIK. We find that the initial magnetization of galactic disks by exploding magnetized stars forms favorable conditions for the CR-driven dynamo. We demonstrate that dipolar magnetic fields supplied on small supernova remnant scales can be amplified exponentially by the CR-driven dynamo, to the present equipartition values, and transformed simultaneously to large galactic scales. The resulting magnetic field structure in an evolved galaxy appears spiral in the face-on view and reveals the so-called X-shaped structure in the edge-on view.

  20. Magnetic effects on the viscous boundary layer damping of the r-modes in neutron stars

    SciTech Connect

    Mendell, Gregory

    2001-08-15

    This paper explores the effects that magnetic fields have on the viscous boundary layers (VBLs) that can form in neutron stars at the crust-core interface, and it investigates the VBL damping of the gravitational-radiation driven r-mode instability. Approximate solutions to the magnetohydrodynamic equations valid in the VBL are found for ordinary-fluid neutron stars. It is shown that magnetic fields above 10{sup 9}(10{sup 10} K/T)G significantly change the structure of the VBL, and that magnetic fields decrease the VBL damping time. Furthermore, VBL damping completely suppresses the r-mode instability for B{approx}>10{sup 12} G (independent of the temperature). These bounds refer to the strength of the radial component of the equilibrium field at the location of the core radius. Magnetic effects on the VBL vanish wherever the radial component of the equilibrium field vanishes. Thus, magnetic fields will profoundly affect the VBL damping of the r-mode instability in hot young pulsars (that are cool enough to have formed a solid crust). One can speculate that magnetic fields can affect the VBL damping of this instability in low-mass x-ray binary systems and other cold old pulsars (if they have sufficiently large internal fields).

  1. Dynamical systems for modeling evolution of the magnetic field of the Sun, stars and planets

    NASA Astrophysics Data System (ADS)

    Popova, E.

    2016-12-01

    The magnetic activity of the Sun, stars and planets are connected with a dynamo process based on the combined action of the differential rotation and the alpha-effect. Application of this concept allows us to get different types of solutions which can describe the magnetic activity of celestial bodies. We investigated the dynamo model with the meridional circulation by the low-mode approach. This approach is based on an assumption that the magnetic field can be described by non-linear dynamical systems with a relatively small number of parameters. Such non-linear dynamical systems are based on the equations of dynamo models. With this method dynamical systems have been built for media which contains the meridional flow and thickness of the spherical shell where dynamo process operates. It was shown the possibility of coexistence of quiasi-biennial oscillations, 22-year cycle, and grand minima of magnetic activity which is consistent with the observational data for the solar activity. We obtained different regimes (oscillations, vacillations, dynamo-bursts) depending on a value of the dynamo-number, the meridional circulation, and thickness of the spherical shell. We discuss features of these regimes and compare them with the observed features of the magnetic fields of the Sun, stars and Earth. We built theoretical paleomagnetic time scale and butterfly-diagrams for the helicity and toroidal magnetic field for different regimes.

  2. General Relativistic Radiation MHD Simulations of Supercritical Accretion onto a Magnetized Neutron Star: Modeling of Ultraluminous X-Ray Pulsars

    NASA Astrophysics Data System (ADS)

    Takahashi, Hiroyuki R.; Ohsuga, Ken

    2017-08-01

    By performing 2.5-dimensional general relativistic radiation magnetohydrodynamic simulations, we demonstrate supercritical accretion onto a non-rotating, magnetized neutron star, where the magnetic field strength of dipole fields is 1010 G on the star surface. We found the supercritical accretion flow consists of two parts: the accretion columns and the truncated accretion disk. The supercritical accretion disk, which appears far from the neutron star, is truncated at around ≃3 R * (R * = 106 cm is the neutron star radius), where the magnetic pressure via the dipole magnetic fields balances with the radiation pressure of the disks. The angular momentum of the disk around the truncation radius is effectively transported inward through magnetic torque by dipole fields, inducing the spin up of a neutron star. The evaluated spin-up rate, ˜-10-11 s s-1, is consistent with the recent observations of the ultraluminous X-ray pulsars. Within the truncation radius, the gas falls onto a neutron star along the dipole fields, which results in a formation of accretion columns onto the northern and southern hemispheres. The net accretion rate and the luminosity of the column are ≃66 L Edd/c 2 and ≲10 L Edd, where L Edd is the Eddington luminosity and c is the light speed. Our simulations support a hypothesis whereby the ultraluminous X-ray pulsars are powered by the supercritical accretion onto the magnetized neutron stars.

  3. Magnetic field instability in a neutron star driven by the electroweak electron-nucleon interaction versus the chiral magnetic effect

    NASA Astrophysics Data System (ADS)

    Dvornikov, Maxim; Semikoz, Victor B.

    2015-03-01

    We show that the Standard Model electroweak interaction of ultrarelativistic electrons with nucleons (the e N interaction) in a neutron star (NS) permeated by a seed large-scale helical magnetic field provides its growth up to ≳1 015 G during a time comparable with the ages of young magnetars ˜1 04 yr . The magnetic field instability originates from the parity violation in the e N interaction entering the generalized Dirac equation for right and left massless electrons in an external uniform magnetic field. We calculate the average electric current given by the solution of the modified Dirac equation containing an extra current for right and left electrons (positrons), which turns out to be directed along the magnetic field. Such a current includes both a changing chiral imbalance of electrons and the e N potential given by a constant neutron density in a NS. Then we derive the system of the kinetic equations for the chiral imbalance and the magnetic helicity which accounts for the e N interaction. By solving this system, we show that a sizable chiral imbalance arising in a neutron protostar due to the Urca process eL-+p →N +νeL diminishes very rapidly because of a huge chirality-flip rate. Thus the e N term prevails over the chiral effect, providing a huge growth of the magnetic helicity and the helical magnetic field.

  4. Magnetic field in IRC+10216 and other C-rich evolved stars

    NASA Astrophysics Data System (ADS)

    Duthu, A.; Herpin, F.; Wiesemeyer, H.; Baudry, A.; Lèbre, A.; Paubert, G.

    2017-07-01

    Context. During the transition from the asymptotic giant branch (AGB) to planetary nebulae (PN), the circumstellar geometry and morphology change dramatically. Another characteristic of this transition is the high mass-loss rate, that can be partially explained by radiation pressure and a combination of various factors, such as the stellar pulsation, the dust grain condensation, and opacity in the upper atmosphere. The magnetic field can also be one of the main ingredients that shapes the stellar upper atmosphere and envelope. Aims: Our main goal is to investigate for the first time the spatial distribution of the magnetic field in the envelope of IRC+10216. More generally we intend to determine the magnetic field strength in the circumstellar envelope (CSE) of C-rich evolved stars, compare this field with previous studies for O-rich stars, and constrain the variation of the magnetic field with r the distance to the star's centre. Methods: We use spectropolarimetric observations of the Stokes V parameter, collected with Xpol on the IRAM-30 m radiotelescope, observing the Zeeman effect in seven hyperfine components of the CN J = 1-0 line. We use the Crutcher et al. (1996, ApJ, 456, 217) method to estimate the magnetic field. For the first time, the instrumental contamination is investigated, through dedicated studies of the power patterns in Stokes V and I in detail. Results: For C-rich evolved stars, we derive a magnetic field strength (B) between 1.6 and 14.2 mG while B is estimated to be 6 mG for the proto-PN (PPN) AFGL618, and an upper value of 8 mG is found for the PN NGC 7027. These results are consistent with a decrease of B as 1/r in the environment of AGB objects, that is, with the presence of a toroidal field. But this is not the case for PPN and PN stars. Our map of IRC+10216 suggests that the magnetic field is not homogeneously strong throughout or aligned with the envelope and that the morphology of the CN emission might have changed with time.

  5. Differential rotation as a model for starspots in magnetically active stars

    NASA Astrophysics Data System (ADS)

    Agostino, Christopher James; Basri, Gibor S.

    2017-01-01

    The Kepler mission has provided an opportunity to significantly expand our understanding of starspots. We have implemented a MCMC method to determine spot parameters of input light curves using a differential rotation spot model. We generated model light curves and explored parameter space in order to test the reliability of our method in retrieving input parameters and to investigate what constraints on spot parameters can be determined from photometric data. We also applied our method to light curves of magnetically active Kepler stars, using only a few spots. One interesting initial conclusion is that it is often possible to replicate complicated light curves over many rotation periods without the need for any spot evolution on stars with rotation periods less than 20 days. We have also begun investigating to what extent spot evolution is preferred as the alternative model for stellar variability. Of course, it is very likely that real stars exhibit both phenomena.

  6. Magnetic field topology of the unique chemically peculiar star CU Virginis

    NASA Astrophysics Data System (ADS)

    Kochukhov, O.; Lüftinger, T.; Neiner, C.; Alecian, E.; MiMeS Collaboration

    2014-05-01

    Context. The late-B, magnetic, chemically peculiar star CU Vir is one of the fastest rotators among the intermediate-mass stars with strong fossil magnetic fields. It shows a prominent rotational modulation of the spectral energy distribution and absorption line profiles due to chemical spots and exhibits a unique, strongly beamed variable radio emission. Aims: Little is known about the magnetic field topology of CU Vir. In this study, we aim to derive detailed maps of the magnetic field distribution over the surface of this star for the first time. Methods: We use high-resolution spectropolarimetric observations covering the entire rotational period. These data are interpreted using a multi-line technique of least-squares deconvolution (LSD) and a new Zeeman Doppler imaging code, which is based on detailed polarised radiative transfer modelling of the Stokes I and V LSD profiles. This new magnetic inversion approach relies on the spectrum synthesis calculations over the full wavelength range that is covered by observations and does not assume that the LSD profiles behave as a single spectral line with mean parameters. Results: We present magnetic and chemical abundance maps derived from the Si and Fe lines. Mean polarisation profiles of both elements reveal a significant departure of CU Vir's magnetic field topology from the commonly assumed axisymmetric dipolar configuration. The field of CU Vir is dipolar-like but clearly non-axisymmetric, showing a large difference in the field strength between the regions of opposite polarity. The main relative abundance depletion features in both Si and Fe maps coincide with the weak-field region in the magnetic map. Conclusions: The detailed information on the distorted dipolar magnetic field topology of CU Vir provided by our study is essential for understanding chemical spot formation, radio emission, and rotational period variation of this star. Based on observations obtained at the Bernard Lyot Telescope (USR5026

  7. γ Pegasi: testing Vega-like magnetic fields in B stars

    NASA Astrophysics Data System (ADS)

    Neiner, C.; Monin, D.; Leroy, B.; Mathis, S.; Bohlender, D.

    2014-02-01

    Context. The bright B pulsator γ Peg shows both p and g modes of β Cep and SPB types. It has also been claimed that it is a magnetic star, while others do not detect any magnetic field. Aims: We check for the presence of a magnetic field, with the aim to characterise it if it exists, or else provide a firm upper limit of its strength if it is not detected. If γ Peg is magnetic as claimed, it would make an ideal asteroseismic target for testing various theoretical scenarios. If it is very weakly magnetic, it would be the first observation of an extension of Vega-like fields to early B stars. Finally, if it is not magnetic and we can provide a very low upper limit on its non-detected field, it would make an important result for stellar evolution models. Methods: We acquired high resolution, high signal-to-noise spectropolarimetric Narval data at Telescope Bernard Lyot (TBL). We also gathered existing dimaPol spectropolarimetric data from the Dominion Astrophysical Observatory (DAO) and Musicos spectropolarimetric data from TBL. We analysed the Narval and Musicos observations using the least-squares deconvolution (LSD) technique to derive the longitudinal magnetic field and Zeeman signatures in lines. The longitudinal field strength was also extracted from the Hβ line observed with the DAO. With a Monte Carlo simulation we derived the maximum strength of the field possibly hosted by γ Peg. Results: We find that no magnetic signatures are visible in the very high quality spectropolarimetric data. The average longitudinal field measured in the Narval data is Bl = -0.1 ± 0.4 G. We derive a very strict upper limit of the dipolar field strength of Bpol ~ 40 G. Conclusions: We conclude that γ Peg is not magnetic: it hosts neither a strong stable fossil field as observed in a fraction of massive stars nor a very weak Vega-like field. There is therefore no evidence that Vega-like fields exist in B stars, contrary to the predictions by fossil field dichotomy scenarios

  8. VizieR Online Data Catalog: STEREO non-magnetic chemically peculiar stars (Paunzen+, 2013)

    NASA Astrophysics Data System (ADS)

    Paunzen, E.; Wraight, K. T.; Fossati, L.; Netopil, M.; White, G. J.; Bewsher, D.

    2014-03-01

    We have analysed the photometric data obtained with the STEREO spacecraft for 558 non-magnetic chemically peculiar (CP) stars to search for rotational and pulsational variability. Applying the Lomb-Scargle and the phase dispersion minimization methods, we have detected photometric variability for 44 objects from which 35 were previously unknown. The new objects are all bright stars on the ecliptic plane (magnitude range 4.7stars in our sample: the cool metallic line Am (CP1) and the hot HgMn (CP3) stars. These objects fall well inside the classical instability strip where δ Scuti, γ Doradus and slowly pulsating B-type stars are located. We also expect to find periods correlated to the orbital period for CP1 objects as they are mostly members of binary systems. For CP3 stars, rotationally induced variability is still a matter of debate. Although surface spots were detected, they are believed to produce only marginal photometric amplitudes. So, periods from several hours to a few days were expected for these two star groups. The STEREO/HI-1 data are well matched to studies of this frequency domain, owing to the cadence of approximately 40-min and multiple epochs over four and a half years. The remaining 514 stars are likely to be constant in the investigated range from 0.1 to 10d. In some cases, the presence of blending or systematic effects prevented us from detecting any reliable variability and in those cases we classified the star as constant. We discuss our results in comparison to already published ones and find a very good agreement. Finally, we have calibrated the variable stars in terms of the effective temperature and luminosity in

  9. Effects of neutrino emissivity on the cooling of neutron stars in the presence of a strong magnetic field

    SciTech Connect

    Coelho, Eduardo Lenho; Chiapparini, Marcelo; Negreiros, Rodrigo Picanço

    2015-12-17

    One of the most interesting kind of neutron stars are the pulsars, which are highly magnetized neutron stars with fields up to 10{sup 14} G at the surface. The strength of magnetic field in the center of a neutron star remains unknown. According to the scalar virial theorem, magnetic field in the core could be as large as 10{sup 18} G. In this work we study the influence of strong magnetic fields on the cooling of neutron stars coming from direct Urca process. Direct Urca process is an extremely efficient mechanism for cooling a neutron star after its formation. The matter is described using a relativistic mean-field model at zero temperature with eight baryons (baryon octet), electrons and muons. We obtain the relative population of each species of particles as function of baryon density for different magnetic fields. We calculate numerically the cooling of neutron stars for a parametrized magnetic field and compare the results for the case without a magnetic field.

  10. Tidal Heating of Earth-like Exoplanets around M Stars: Thermal, Magnetic, and Orbital Evolutions.

    PubMed

    Driscoll, P E; Barnes, R

    2015-09-01

    The internal thermal and magnetic evolution of rocky exoplanets is critical to their habitability. We focus on the thermal-orbital evolution of Earth-mass planets around low-mass M stars whose radiative habitable zone overlaps with the "tidal zone," where tidal dissipation is expected to be a significant heat source in the interior. We develop a thermal-orbital evolution model calibrated to Earth that couples tidal dissipation, with a temperature-dependent Maxwell rheology, to orbital circularization and migration. We illustrate thermal-orbital steady states where surface heat flow is balanced by tidal dissipation and cooling can be stalled for billions of years until circularization occurs. Orbital energy dissipated as tidal heat in the interior drives both inward migration and circularization, with a circularization time that is inversely proportional to the dissipation rate. We identify a peak in the internal dissipation rate as the mantle passes through a viscoelastic state at mantle temperatures near 1800 K. Planets orbiting a 0.1 solar-mass star within 0.07 AU circularize before 10 Gyr, independent of initial eccentricity. Once circular, these planets cool monotonically and maintain dynamos similar to that of Earth. Planets forced into eccentric orbits can experience a super-cooling of the core and rapid core solidification, inhibiting dynamo action for planets in the habitable zone. We find that tidal heating is insignificant in the habitable zone around 0.45 (or larger) solar-mass stars because tidal dissipation is a stronger function of orbital distance than stellar mass, and the habitable zone is farther from larger stars. Suppression of the planetary magnetic field exposes the atmosphere to stellar wind erosion and the surface to harmful radiation. In addition to weak magnetic fields, massive melt eruption rates and prolonged magma oceans may render eccentric planets in the habitable zone of low-mass stars inhospitable for life.

  11. Discovery of a magnetic field in the Slowly Pulsating B star zeta Cassiopeiae

    NASA Astrophysics Data System (ADS)

    Neiner, C.; Geers, V. C.; Henrichs, H. F.; Floquet, M.; Frémat, Y.; Hubert, A.-M.; Preuss, O.; Wiersema, K.

    2003-08-01

    zeta Cas is a B2 IV star with vsin i = 17 km s-1. Time-resolved circular spectropolarimetric observations of zeta Cas obtained in 2001 and 2002 with the Musicos échelle spectropolarimeter at the 2 m Télescope Bernard Lyot (TBL) show a sinusoidally varying longitudinal magnetic field with a strength between 10 G and -46 G for the averaged line-of-sight component, corresponding to B_pol=335+120-65 G. The period corresponds very accurately with the 5.37045 day period as derived from stellar wind variations observed in the ultraviolet. The epoch of the positive maximum field corresponds in phase with the maximum emission in the UV wind lines. This gives compelling evidence for a magnetic rotator model for this star, with an unambiguous rotation period of 5.37 days. We searched for periodicity in line-profile variations (lpv), radial velocity and minimum intensity curves in the ~ 400 optical spectra. We found a non-radial pulsation mode with l=2 +/- 1 at the frequency f=0.64 c d-1. From this periodicity and from stellar parameters derived from model fits, we propose to classify zeta Cas as a Slowly Pulsating B (SPB) star. This is the third detection of a magnetic field in an early B-type pulsating star and the first one in a SPB star. Based on observations obtained using the Musicos spectropolarimeter at the Observatoire du Pic du Midi, France, and on INES data from the International Ultraviolet Explorer (IUE) satellite.

  12. NON-DETECTION OF MAGNETIC FIELDS IN THE CENTRAL STARS OF THE PLANETARY NEBULAE NGC 1360 AND LSS 1362

    SciTech Connect

    Leone, Francesco; Privitera, Giovanni; Martinez Gonzalez, MarIa J.; Corradi, Romano L. M.; Sainz, Rafael Manso

    2011-04-20

    The presence of magnetic fields is an attractive hypothesis for shaping planetary nebulae (PNe). We report on observations of the central star of the two PNe NGC 1360 and LSS 1326. We performed spectroscopy on circularly polarized light with the Focal Reducer and Low Dispersion Spectrograph at the Very Large Telescope of the European Southern Observatory. Contrary to previous reports, we find that the effective magnetic field, which is the average over the visible stellar disk of longitudinal components of the magnetic fields, is null within errors for both stars. We conclude that direct evidence of magnetic fields on the central stars of PNe is still missing-either the magnetic field is much weaker (<600 G) than previously reported, or more complex (thus leading to cancellations), or both. Certainly, indirect evidence (e.g., MASER emission) fully justify further efforts to point out the strength and morphology of such magnetic fields.

  13. A type of novel fluorescent magnetic carbon quantum dots for cells imaging and detection.

    PubMed

    Su, Xi; Xu, Yi; Che, Yulan; Liao, Xin; Jiang, Yan

    2015-12-01

    A new type of multifunctional fluorescent magnetic carbon quantum dots SPIO@CQDs(n) ([superparamagnetic iron oxide nanoparticles (SPIO), carbon quantum dots, (CQDs)]) with magnetic and fluorescence properties was designed and prepared through layer-by-layer self-assembly method. The as-synthesized SPIO@CQDs(n) exhibited different emission colors including blue, green, and red when they were excited at different excitation wavelengths, and its fluorescent intensity increased as the increase of CQD layer (n). SPIO@CQDs(n) with quite low toxicity could mark cytoplasm with fluorescence by means of nonimmune markers. The mixture sample of liver cells L02 and hepatoma carcinoma cells HepG2 was taken as an example, and HepG2 cells were successfully separated and detected effectively by SPIO@CQDs(n), with a separation rate of 90.31%. Importantly, the designed and prepared SPIO@CQDs( n ) are certified to be wonderful biological imaging and magnetic separation regents.

  14. Surface temperature of a magnetized neutron star and interpretation of the ROSAT data. 1: Dipole fields

    NASA Technical Reports Server (NTRS)

    Page, Dany

    1995-01-01

    We model the temperature distribution at the surface of a magnetized neutron star and study the effects on the observed X-ray spectra and light curves. Generalrelativistic effects, i.e., redshift and lensing, are fully taken into account. Atmospheric effects on the emitted spectral flux are not included: we consider only blackbody emission at the local effective temperature. In this first paper we restrict ourselves to dipole fields. General features are studied and compared with the ROSAT data from the pulsars 0833 - 45 (Vela), 0656 + 14, 0630 + 178 (Geminga), and 1055 - 52, the four cases for which there is strong evidence that thermal radiation from the stellar surface is detected. The composite spectra we obtain are not very different from a blackbody spectrum at the star's effective temperature. We conclude that, as far as blackbody spectra are considered, temperature estimates using single-temperature models give results practically identical to our composite models. The change of the (composite blackbody) spectrum with the star's rotational phase is also not very large and may be unobservable inmost cases. Gravitational lensing strongly suppresses the light curve pulsations. If a dipole field is assumed, pulsed fractions comparable to the observed ones can be obtained only with stellar radii larger than those which are predicted by current models of neutron star struture, or with low stellar masses. Moreover, the shapes of the theoretical light curves with dipole fields do not correspond to the observations. The use of magnetic spectra may raise the pulsed fraction sufficiently but will certainly make the discrepancy with the light curve shapes worse: dipole fields are not sufficient to interpret the data. Many neutron star models with a meson condensate or hypersons predict very small radii, and hence very strong lensing, which will require highly nondipolar fields to be able to reproduce the observed pulsed fractions, if possible at all: this may be a new

  15. Surface temperature of a magnetized neutron star and interpretation of the ROSAT data. 1: Dipole fields

    NASA Technical Reports Server (NTRS)

    Page, Dany

    1995-01-01

    We model the temperature distribution at the surface of a magnetized neutron star and study the effects on the observed X-ray spectra and light curves. Generalrelativistic effects, i.e., redshift and lensing, are fully taken into account. Atmospheric effects on the emitted spectral flux are not included: we consider only blackbody emission at the local effective temperature. In this first paper we restrict ourselves to dipole fields. General features are studied and compared with the ROSAT data from the pulsars 0833 - 45 (Vela), 0656 + 14, 0630 + 178 (Geminga), and 1055 - 52, the four cases for which there is strong evidence that thermal radiation from the stellar surface is detected. The composite spectra we obtain are not very different from a blackbody spectrum at the star's effective temperature. We conclude that, as far as blackbody spectra are considered, temperature estimates using single-temperature models give results practically identical to our composite models. The change of the (composite blackbody) spectrum with the star's rotational phase is also not very large and may be unobservable inmost cases. Gravitational lensing strongly suppresses the light curve pulsations. If a dipole field is assumed, pulsed fractions comparable to the observed ones can be obtained only with stellar radii larger than those which are predicted by current models of neutron star struture, or with low stellar masses. Moreover, the shapes of the theoretical light curves with dipole fields do not correspond to the observations. The use of magnetic spectra may raise the pulsed fraction sufficiently but will certainly make the discrepancy with the light curve shapes worse: dipole fields are not sufficient to interpret the data. Many neutron star models with a meson condensate or hypersons predict very small radii, and hence very strong lensing, which will require highly nondipolar fields to be able to reproduce the observed pulsed fractions, if possible at all: this may be a new

  16. High-energy transients. [from weakly and strongly magnetic neutron stars

    NASA Technical Reports Server (NTRS)

    Lamb, D. Q.

    1991-01-01

    The observational characteristics of X-ray bursts are reviewed and the thermonuclear-flash model is examined to assess the suitability of the model and the sources of X- and gamma-ray bursts. The profiles of X-ray bursts from five different sources are delineated, and the global analysis of nuclear burning is discussed to review th range possible X-ray burst behavior. The analysis by Fushiki and Lamb (1987) is noted for the description of regions which lie away from the steady-state cut. The discussion of cyclotron lines in gamma-ray bursts emphasizes the source GB880205, and it is shown that Raman and cyclotron resonant scattering can explain the observed positions, strengths, and widths of the dips in the spectrum. X-ray bursts are theorized to emanate from weakly magnetic neutron stars, and gamma-ray bursts are shown to be the products of strongly magnetic neutron stars.

  17. On the magnetic field evolution time-scale in superconducting neutron star cores

    NASA Astrophysics Data System (ADS)

    Passamonti, Andrea; Akgün, Taner; Pons, José A.; Miralles, Juan A.

    2017-08-01

    We revisit the various approximations employed to study the long-term evolution of the magnetic field in neutron star cores and discuss their limitations and possible improvements. A recent controversy on the correct form of the induction equation and the relevant evolution time-scale in superconducting neutron star cores is addressed and clarified. We show that this ambiguity in the estimation of time-scales arises as a consequence of nominally large terms that appear in the induction equation, but which are, in fact, mostly irrotational. This subtlety leads to a discrepancy by many orders of magnitude when velocity fields are absent or ignored. Even when internal velocity fields are accounted for, only the solenoidal part of the electric field contributes to the induction equation, which can be substantially smaller than the irrotational part. We also argue that stationary velocity fields must be incorporated in the slow evolution of the magnetic field as the next level of approximation.

  18. Model atmospheres and radiation of magnetic neutron stars. I - The fully ionized case

    NASA Technical Reports Server (NTRS)

    Shibanov, Iu. A.; Zavlin, V. E.; Pavlov, G. G.; Ventura, J.

    1992-01-01

    Model neutron star atmospheres are calculated for typical cooling stars with a strong magnetic field and effective temperatures of 10 exp 5 to 10 exp 6 K. The effect of anisotropic photon diffusion in two normal modes are examined under the assumption that the opacity is due solely to the bremsstrahlung and Thomson scattering processes under conditions of LTE that are expected to prevail at the temperatures and densities obtained. The main aspects of anisotropic photon diffusion, and an original procedure for calculating model atmospheres and emitted spectra are discussed. Representative calculated spectra are given, and it is found that the hard spectral excess characterizing the nonmagnetic case, while still present, becomes less prominent in the presence of magnetic fields in the range of 10 exp 11 to 10 exp 13 G.

  19. The Role of Magnetic Fields in High-Mass Star-Forming Filaments

    NASA Astrophysics Data System (ADS)

    Stephens, Ian

    Filaments are ubiquitous in the star formation process. Planck has revealed that magnetic fields are perpendicular to the densest filaments, which are the birthplace of high-mass stars, suggesting that fields help funnel gas into the filaments. However, the resolved field morphologies and strengths in the dense filaments are unknown. We propose HAWC+ 53 and 214 um polarimetric observations toward two filaments, the Snake (G11.1) and G18.6, to unveil the field morphology. Such observations will probe the filament field morphology at the subarcminute scale over the largest spatial extent to date: 25 and 9 pc respectively. We expect to have over 400 independent beams worth of detections. From the field morphology, we will test the hub-filament theory and investigate how the magnetic field strength and morphology changes with evolution and size-scale.

  20. Model atmospheres and radiation of magnetic neutron stars. I - The fully ionized case

    NASA Technical Reports Server (NTRS)

    Shibanov, Iu. A.; Zavlin, V. E.; Pavlov, G. G.; Ventura, J.

    1992-01-01

    Model neutron star atmospheres are calculated for typical cooling stars with a strong magnetic field and effective temperatures of 10 exp 5 to 10 exp 6 K. The effect of anisotropic photon diffusion in two normal modes are examined under the assumption that the opacity is due solely to the bremsstrahlung and Thomson scattering processes under conditions of LTE that are expected to prevail at the temperatures and densities obtained. The main aspects of anisotropic photon diffusion, and an original procedure for calculating model atmospheres and emitted spectra are discussed. Representative calculated spectra are given, and it is found that the hard spectral excess characterizing the nonmagnetic case, while still present, becomes less prominent in the presence of magnetic fields in the range of 10 exp 11 to 10 exp 13 G.

  1. CONNECTING FLARES AND TRANSIENT MASS-LOSS EVENTS IN MAGNETICALLY ACTIVE STARS

    SciTech Connect

    Osten, Rachel A.; Wolk, Scott J.

    2015-08-10

    We explore the ramification of associating the energetics of extreme magnetic reconnection events with transient mass-loss in a stellar analogy with solar eruptive events. We establish energy partitions relative to the total bolometric radiated flare energy for different observed components of stellar flares and show that there is rough agreement for these values with solar flares. We apply an equipartition between the bolometric radiated flare energy and kinetic energy in an accompanying mass ejection, seen in solar eruptive events and expected from reconnection. This allows an integrated flare rate in a particular waveband to be used to estimate the amount of associated transient mass-loss. This approach is supported by a good correspondence between observational flare signatures on high flaring rate stars and the Sun, which suggests a common physical origin. If the frequent and extreme flares that young solar-like stars and low-mass stars experience are accompanied by transient mass-loss in the form of coronal mass ejections, then the cumulative effect of this mass-loss could be large. We find that for young solar-like stars and active M dwarfs, the total mass lost due to transient magnetic eruptions could have significant impacts on disk evolution, and thus planet formation, and also exoplanet habitability.

  2. The effect of rotational gravity darkening on magnetically torqued Be star discs

    NASA Astrophysics Data System (ADS)

    Brown, J. C.; Telfer, D.; Li, Q.; Hanuschik, R.; Cassinelli, J. P.; Kholtygin, A.

    2004-08-01

    In the magnetically torqued disc (MTD) model for hot star discs, as proposed and formulated by Cassinelli et al., stellar wind mass loss was taken to be uniform over the stellar surface. Here account is taken of the fact that as the stellar spin rate is increased, and the stellar equator is gravity darkened, the equatorial mass flux and terminal speed are reduced, compared with the poles, for a given total . As a result, the distribution of equatorial disc density, determined by the impact of northbound and southbound flows, is shifted further out from the star. This results, for high So(>~0.5), in a fall in the disc mass and emission measure, and hence in the observed emission line equivalent width, scattering polarization and infrared emission. Consequently, contrary to expectations, critical rotation So-> 1 is not the optimum for creation of hot star discs which, in terms of emission measure for example, is found to occur in a broad peak around So~ 0.5-0.6 depending slightly on the wind velocity law. The relationship of this analytic quasi-steady parametric MTD model to other work on magnetically guided winds is discussed. In particular, the failures of the MTD model for Be-star discs alleged by Owocki and ud-Doula are shown to revolve largely around open observational tests, rather than in the basic MTD physics, and around their use of insufficiently strong fields.

  3. Hunting For Strong Magnetic Fields In Rapidly Rotating Sun-Like Stars With Stokes-I Observations

    NASA Astrophysics Data System (ADS)

    Shulyak, Denis; Malo, Lison; Reiners, Ansgar; Kochukhov, Oleg; Jeffers, Sandra; Piskunov, Nikolai

    2016-06-01

    Stars with convective envelopes can generate strong magnetic fields through rotationally driven dynamos. Theory suggests that the maximum magnetic field strength depends on the energy budget stored in the stellar convective shell and can reach values of several kilogauss in fastest rotating stars. We test this predictions by measuring total magnetic flux and polarization in a sample of sun-like stars that rotate close to the activity saturation limit. We detect average magnetic flux densities of several hundred G in several of our targets,with the strongest field of about 1 kG in a K type star V383 Lac showing that young sun-like starscan produce average fields on the kG level.

  4. Self-similar fragmentation regulated by magnetic fields in a region forming massive stars.

    PubMed

    Li, Hua-bai; Yuen, Ka Ho; Otto, Frank; Leung, Po Kin; Sridharan, T K; Zhang, Qizhou; Liu, Hauyu; Tang, Ya-Wen; Qiu, Keping

    2015-04-23

    Most molecular clouds are filamentary or elongated. For those forming low-mass stars (<8 solar masses), the competition between self-gravity and turbulent pressure along the dynamically dominant intercloud magnetic field (10 to 100 parsecs) shapes the clouds to be elongated either perpendicularly or parallel to the fields. A recent study also suggested that on the scales of 0.1 to 0.01 parsecs, such fields are dynamically important within cloud cores forming massive stars (>8 solar masses). But whether the core field morphologies are inherited from the intercloud medium or governed by cloud turbulence is unknown, as is the effect of magnetic fields on cloud fragmentation at scales of 10 to 0.1 parsecs. Here we report magnetic-field maps inferred from polarimetric observations of NGC 6334, a region forming massive stars, on the 100 to 0.01 parsec scale. NGC 6334 hosts young star-forming sites where fields are not severely affected by stellar feedback, and their directions do not change much over the entire scale range. This means that the fields are dynamically important. The ordered fields lead to a self-similar gas fragmentation: at all scales, there exist elongated gas structures nearly perpendicular to the fields. Many gas elongations have density peaks near the ends, which symmetrically pinch the fields. The field strength is proportional to the 0.4th power of the density, which is an indication of anisotropic gas contractions along the field. We conclude that magnetic fields have a crucial role in the fragmentation of NGC 6334.

  5. Magnetic susceptibility oscillation in neutron stars with the hadron-quark transition

    SciTech Connect

    Jiang, W. Z.; Van Giai, N.

    2006-11-02

    We analyze the de Hass-van Alphen (HVA) oscillation of magnetic susceptibility in the nuetron star matter using an analytic relativistic expression obtained before, showing that the oscillation frequency is proportional to the squared chemical potential and the reciprocal of the field, and is independent of the temperature. The numerical results for the HVA oscillation are also shown. A superposition of the HVA oscillations changes the oscillation properties drastically if the color deconfinement occurs at high densities.

  6. Theoretical models of highly magnetic white dwarf stars that violate the Chandrasekhar Limit

    NASA Astrophysics Data System (ADS)

    Shah, Hridaya

    2017-08-01

    Until recently, white dwarf (WD) stars were believed to be no more massive than 1.44 solar masses (M ȯ ). This belief has been changed now with the observations of over-luminous or 'peculiar' Type la supernovae that have lead researchers to hypothesize the existence of WDs in the mass range 2.4 - 2.8 M ȯ . This discovery also raises some doubt over the reliability of the Type Ia supernova as a standard candle. It is thought that these super-massive WDs are their most likely progenitors and that they probably have a very strong magnetic field inside them. A degenerate electron gas in a magnetic field, such as that present inside this star, will be Landau quantized. Magnetic field changes the momentum space of electrons which in turn changes their density of states (DOS) and that in turn changes the equation of state (EoS) of matter inside the star, as opposed to that without a field. When this change in the DOS is taken into account and a link between the DOS and the EoS is established, as is done in this work, I find a physical reason behind the theoretical mass-radius (M-R) relations of a super-massive WD. I start with different equations of state with at most three Landau levels occupied and then construct stellar models of magnetic WDs (MWDs) using the same. I also show the M-R relations of these stars for a particular chosen value of maximum electron Fermi energy. Once a multiple Landau level system of electrons is considered, I find that it leads to such an EoS that gives multiple branches in the MR relations. Super-massive MWDs are obtained only when the Landau level occupancy is limited to just one level and some of the mass values fall within the mass range given above.

  7. CCO Pulsars as Anti-Magnetars: Evidence of Neutron Stars Weakly Magnetized at Birth

    NASA Astrophysics Data System (ADS)

    Gotthelf, E. V.; Halpern, J. P.

    2008-02-01

    Our new study of the two central compact object pulsars, PSR J1210-5226 (P = 424 ms) and PSR J1852+0040 (P = 105 ms), leads us to conclude that a weak natal magnetic field shaped their unique observational properties. In the dipole spin-down formalism, the 2-sigma upper limits on their period derivatives, <2×10-16 for both pulsars, implies surface magnetic field strengths of Bs<3×1011 G and spin periods at birth equal to their present periods to three significant digits. Their X-ray luminosities exceed their respective spin-down luminosities, implying that their thermal spectra are derived from residual cooling and perhaps partly from accretion of supernova debris. For sufficiently weak magnetic fields an accretion disk can penetrate the light cylinder and interact with the magnetosphere while resulting torques on the neutron star remain within the observed limits. We propose the following as the origin of radio-quiet CCOs: the magnetic field, derived from a turbulent dynamo, is weaker if the NS is formed spinning slowly, which enables it to accrete SN debris. Accretion excludes neutron stars born with both Bs<1011 G and P>0.1 s from radio pulsar surveys, where such weak fields are not encountered except among very old (>40 Myr) or recycled pulsars. We predict that these birth properties are common, and may be attributes of the youngest detected neutron star, the CCO in Cassiopeia A, as well as an undetected infant neutron star in the SN 1987A remnant. In view of the far-infrared light echo discovered around Cas A and attributed to an SGR-like outburst, it is especially important to determine via timing whether Cas A hosts a magnetar or not. If not a magnetar, the Cas A NS may instead have undergone a one-time phase transition (corequake) that powered the light echo.

  8. MAGNETIC COMPLEXITY AS AN EXPLANATION FOR BIMODAL ROTATION POPULATIONS AMONG YOUNG STARS

    SciTech Connect

    Garraffo, Cecilia; Drake, Jeremy J.; Cohen, Ofer

    2015-07-01

    Observations of young open clusters have revealed a bimodal distribution of fast and slower rotation rates that has proven difficult to explain with predictive models of spin down that depend on rotation rates alone. The Metastable Dynamo Model proposed recently by Brown, employing a stochastic transition probability from slow to more rapid spin down regimes, appears to be more successful but lacks a physical basis for such duality. Using detailed 3D MHD wind models computed for idealized multipole magnetic fields, we show that surface magnetic field complexity can provide this basis. Both mass and angular momentum losses decline sharply with increasing field complexity. Combined with observation evidence for complex field morphologies in magnetically active stars, our results support a picture in which young, rapid rotators lose angular momentum in an inefficient way because of field complexity. During this slow spin-down phase, magnetic complexity is eroded, precipitating a rapid transition from weak to strong wind coupling.

  9. Upper limits to the magnetic field in central stars of planetary nebulae

    SciTech Connect

    Asensio Ramos, A.; Martínez González, M. J.; Manso Sainz, R.; Corradi, R. L. M.; Leone, F.

    2014-06-01

    More than about 20 central stars of planetary nebulae (CSPNs) have been observed spectropolarimetrically, yet no clear, unambiguous signal of the presence of a magnetic field in these objects has been found. We perform a statistical (Bayesian) analysis of all the available spectropolarimetric observations of CSPN to constrain the magnetic fields in these objects. Assuming that the stellar field is dipolar and that the dipole axis of the objects is oriented randomly (isotropically), we find that the dipole magnetic field strength is smaller than 400 G with 95% probability using all available observations. The analysis introduced allows integration of future observations to further constrain the parameters of the distribution, and it is general, so that it can be easily applied to other classes of magnetic objects. We propose several ways to improve the upper limits found here.

  10. Evolution of dynamo-generated magnetic fields in accretion disks around compact and young stars

    NASA Technical Reports Server (NTRS)

    Stepinski, Tomasz F.

    1994-01-01

    Geometrically thin, optically thick, turbulent accretion disks are believed to surround many stars. Some of them are the compact components of close binaries, while the others are throught to be T Tauri stars. These accretion disks must be magnetized objects because the accreted matter, whether it comes from the companion star (binaries) or from a collapsing molecular cloud core (single young stars), carries an embedded magnetic field. In addition, most accretion disks are hot and turbulent, thus meeting the condition for the MHD turbulent dynamo to maintain and amplify any seed field magnetic field. In fact, for a disk's magnetic field to persist long enough in comparison with the disk viscous time it must be contemporaneously regenerated because the characteristic diffusion time of a magnetic field is typically much shorter than a disk's viscous time. This is true for most thin accretion disks. Consequently, studying magentic fields in thin disks is usually synonymous with studying magnetic dynamos, a fact that is not commonly recognized in the literature. Progress in studying the structure of many accretion disks was achieved mainly because most disks can be regarded as two-dimensional flows in which vertical and radial structures are largely decoupled. By analogy, in a thin disk, one may expect that vertical and radial structures of the magnetic field are decoupled because the magnetic field diffuses more rapidly to the vertical boundary of the disk than along the radius. Thus, an asymptotic method, called an adiabatic approximation, can be applied to accretion disk dynamo. We can represent the solution to the dynamo equation in the form B = Q(r)b(r,z), where Q(r) describes the field distribution along the radius, while the field distribution across the disk is included in the vector function b, which parametrically depends on r and is normalized by the condition max (b(z)) = 1. The field distribution across the disk is established rapidly, while the radial

  11. HD 18078: A very slowly rotating Ap star with an unusual magnetic field structure

    NASA Astrophysics Data System (ADS)

    Mathys, G.; Romanyuk, I. I.; Kudryavtsev, D. O.; Landstreet, J. D.; Pyper, D. M.; Adelman, S. J.

    2016-02-01

    Context. The existence of a significant population of Ap stars with very long rotation periods (up to several hundred years) has progressively emerged over the past two decades. However, only lower limits of the periods are known for most of them because their variations have not yet been observed over a sufficient timebase. Aims: We determine the rotation period of the slowly rotating Ap star HD 18078 and we derive constraints on the geometrical structure of its magnetic field. Methods: We combine measurements of the mean magnetic field modulus obtained from 1990 to 1997 with determinations of the mean longitudinal magnetic field spanning the 1999-2007 time interval to derive an unambiguous value of the rotation period. We show that this value is consistent with photometric variations recorded in the Strömgren uvby photometric system between 1995 and 2004. We fit the variations of the two above-mentioned field moments with a simple model to constrain the magnetic structure. Results: The rotation period of HD 18078 is (1358 ± 12) d. The geometrical structure of its magnetic field is consistent to first order with a colinear multipole model whose axis is offset from the centre of the star. Conclusions: HD 18078 is only the fifth Ap star with a rotation period longer than 1000 d for which the exact value of that period (as opposed to a lower limit) could be determined. The strong anharmonicity of the variations of its mean longitudinal magnetic field and the shift between their extrema and those of the mean magnetic field modulus are exceptional and indicative of a very unusual magnetic structure. Based in part on observations made at Observatoire de Haute Provence (CNRS), France; at Kitt Peak National Observatory, National Optical Astronomy Observatory (NOAO Prop. ID: KP2442; PI: T. Lanz), which is operated by the Association of Universities for Research in Astronomy (AURA) under cooperative agreement with the National Science Foundation; at the Canada

  12. A comprehensive analysis of the magnetic standard star HD 94660: Host of a massive compact companion?

    NASA Astrophysics Data System (ADS)

    Bailey, J. D.; Grunhut, J.; Landstreet, J. D.

    2015-03-01

    Aims: Detailed information about the magnetic geometry, atmospheric abundances and radial velocity variations has been obtained for the magnetic standard star HD 94660 based on high-dispersion spectroscopic and spectropolarimetric observations from the UVES, HARPSpol and ESPaDOnS instruments. Methods: We perform a detailed chemical abundance analysis using the spectrum synthesis code ZEEMAN for a total of 17 elements. Using both line-of-sight and surface magnetic field measurements, we derive a simple magnetic field model that consists of dipole, quadrupole and octupole components. Results: The observed magnetic field variations of HD 94660 are complex and suggest an inhomogeneous distribution of chemical elements over the stellar surface. This inhomogeneity is not reflected in the abundance analysis, from which all available spectra are modelled, but only a mean abundance is reported for each element. The derived abundances are mostly non-solar, with striking overabundances of Fe-peak and rare-earth elements. Of note are the clear signatures of vertical chemical stratification throughout the stellar atmosphere, most notably for the Fe-peak elements. We also report on the detection of radial velocity variations with a total range of 35 km s-1 in the spectra of HD 94660. A preliminary analysis shows the most likely period of these variations to be of order 840 d and, based on the derived orbital parameters of this star, suggests the first detection of a massive compact companion for a main sequence magnetic star. Conclusions: HD 94660 exhibits interestingly complex magnetic field variations and remarkable radial velocity variations. Long term monitoring is necessary to provide further constraints on the nature of these radial velocity variations. Detection of a companion will help establish the role of binarity in the origin of magnetism in stars with radiative envelopes. Based in part on our own observations made with the European Southern Observatory (ESO

  13. Can magnetic-field windup kill the r-mode instability of neutron stars?

    NASA Astrophysics Data System (ADS)

    Friedman, John; Lindblom, Lee; Rezzolla, Luciano

    2016-03-01

    At second order in perturbation theory, the unstable r-mode of a rotating star includes growing differential rotation whose form and growth rate are determined by gravitational radiation reaction. With no magnetic field, the angular velocity of a fluid element grows exponentially until the mode reaches its nonlinear saturation amplitude and remains nonzero after saturation. With a background magnetic field, the differential rotation winds up and amplifies the field, and previous work suggests that the amplification may damp out the instability. A background magnetic field, however, turns the time-independent perturbations corresponding to adding differential rotation into perturbations with characteristic frequencies of order the Alfven frequency. We argue that magnetic field growth stops soon after the mode reaches its saturation amplitude. We show that this is the case for a toy model, where magnetic amplification for small saturation amplitude is too small to damp the r-mode. For a more realistic model of a cold, rotating neutron star, an analogous upper limit depends on the assumption that there are no marginally unstable perturbations.

  14. Characterizing the magnetic field and spectral variability of the rigidly rotating magnetosphere star HD 345439

    NASA Astrophysics Data System (ADS)

    Hubrig, S.; Kholtygin, A. F.; Schöller, M.; Ilyin, I.

    2017-05-01

    A team involved in the Apache Point Observatory Galactic Evolution Experiment (APOGEE), one of the Sloan Digital Sky Survey III programmes, recently announced the discovery of two rare rigidly rotating magnetosphere stars, HD 345439 and HD 23478. Near-infrared spectra of these objects revealed emission-line behaviour identical to that previously discovered in the helium-strong star σ Ori E, which has a strong magnetic field and rotates fast. A single spectropolarimetric observation of HD 345439 with the FOcal Reducer low dispersion Spectrograph (FORS 2) at the Very Large Telescope (VLT) in 2014 over 88 min indicated that HD 345439 may host a strong, rapidly varying magnetic field. In this work, we present the results of our spectropolarimetric monitoring of this star with FORS 2, which revealed the presence of a strong longitudinal magnetic field dominated by a dipolar component. The analysis of spectral variability indicates an opposite behaviour of He and Si lines, which is usually attributed to differences in the distribution of surface He and Si abundance spots.

  15. Impact of Protostellar Outflows on Turbulence and Star Formation Efficiency in Magnetized Dense Cores

    NASA Astrophysics Data System (ADS)

    Offner, Stella S. R.; Chaban, Jonah

    2017-10-01

    The star-forming efficiency of dense gas is thought to be set within cores by outflow and radiative feedback. We use magnetohydrodynamic simulations to investigate the relation between protostellar outflow evolution, turbulence, and star formation efficiency. We model the collapse and evolution of isolated dense cores for ≳0.5 Myr including the effects of turbulence, radiation transfer, and both radiation and outflow feedback from forming protostars. We show that outflows drive and maintain turbulence in the core environment even with strong initial fields. The star formation efficiency decreases with increasing field strength, and the final efficiencies are 15%–40%. The Stage 0 lifetime, during which the protostellar mass is lower than that of the dense envelope, increases proportionally with the initial magnetic field strength and ranges from ∼ 0.1 {to} 0.4 {Myr}. The average accretion rate is well represented by a tapered turbulent core model, which is a function of the final protostellar mass and is independent of the magnetic field strength. By tagging material launched in the outflow, we demonstrate that the outflow entrains about three times the actual launched gas mass, a ratio that remains roughly constant in time regardless of the initial magnetic field strength. However, turbulent driving increases for stronger fields since momentum is more efficiently imparted to non-outflow material. The protostellar outflow momentum is highest during the first 0.1 Myr and declines thereafter by a factor of ≳ 10 as the accretion rate diminishes.

  16. Spin-Down Mechanisms in Neutron Stars with ``Anomalous'' Magnetic Fields

    NASA Astrophysics Data System (ADS)

    Rogers, Adam; Safi-Harb, Samar

    2015-08-01

    Energy losses from isolated neutron stars are attributed to a number of factors, the most common assumption being the emission of electromagnetic radiation from a rotating point-like magnetic dipole in vacuum. This energy loss mechanism predicts a braking index n = 3, which is not observed in highly magnetized neutron stars. Despite this fact, the assumptions of a dipole field and rapid early rotation are often assumed a priori. This typically causes a discrepancy in the characteristic age of these objects and the age of their associated Supernova Remnants (SNRs). In this work we consider neutron stars with ``anomalous'' magnetic fields - namely magnetars, high-B radio pulsars, and the Central Compact Objects (proposed to be `anti-magnetars’) that are securely associated with SNRs. Without making any assumptions about the initial spin periods of these objects and by constraining the SNR ages to match their associated pulsar ages, we compare the predictions of distinct energy loss mechanisms, such as field decay and the emission of relativistic winds using all observed data on the braking indices. This study has important implications on the proposed emission models for these exotic objects and helps in resolving the PSR-SNR age discrepancy.

  17. Results of magnetic field measurements in young stars DO Tau, DR Tau, and DS Tau

    NASA Astrophysics Data System (ADS)

    Dodin, A. V.; Lamzin, S. A.; Chuntonov, G. A.

    2013-04-01

    The results of longitudinal magnetic field measurements B z in the hot accretion spot in three classical T Tauri stars (CTTS) are reported. In all three stars the magnetic field is detected at a level above 2 σ in the formation region of the narrow component of the He I 5876 Å emission line. In the case of DS Tau the longitudinal field B z in the hot spot was also measured from the narrow emission components of the Na I D lines, implying +0.8 ± 0.3 kG, which is equal to the B z field component measured from the He I 5876 Å line. Our results suggest that the 6-m telescope of the Special Astrophysical Observatory can be used to study magnetic fields in the hot spots of CTTS with magnitudes down to 13m, making it possible to double the number of stars of this type with measured B z values in the accretion zone.

  18. THE HIDDEN MAGNETIC FIELD OF THE YOUNG NEUTRON STAR IN KESTEVEN 79

    SciTech Connect

    Shabaltas, Natalia; Lai Dong

    2012-04-01

    Recent observations of the central compact object in the Kesteven 79 supernova remnant show that this neutron star (NS) has a weak dipole magnetic field (a few Multiplication-Sign 10{sup 10} G) but an anomalously large ({approx}64%) pulse fraction in its surface X-ray emission. We explore the idea that a substantial sub-surface magnetic field exists in the NS crust, which produces diffuse hot spots on the stellar surface due to anisotropic heat conduction, and gives rise to the observed X-ray pulsation. We develop a general-purpose method, termed 'Temperature Template with Full Transport' (TTFT), that computes the synthetic pulse profile of surface X-ray emission from NSs with arbitrary magnetic field and surface temperature distributions, taking into account magnetic atmosphere opacities, beam pattern, vacuum polarization, and gravitational light bending. We show that a crustal toroidal magnetic field of order a few Multiplication-Sign 10{sup 14} G or higher, varying smoothly across the crust, can produce sufficiently distinct surface hot spots to generate the observed pulse fraction in the Kes 79 NS. This result suggests that substantial sub-surface magnetic fields, much stronger than the 'visible' dipole fields, may be buried in the crusts of some young NSs, and such hidden magnetic fields can play an important role in their observational manifestations. The general TTFT tool we have developed can also be used for studying radiation from other magnetic NSs.

  19. NuSTAR OBSERVATION OF A TYPE I X-RAY BURST FROM GRS 1741.9-2853

    SciTech Connect

    Barrière, Nicolas M.; Krivonos, Roman; Tomsick, John A.; Boggs, Steven E.; Craig, William W.; Bachetti, Matteo; Chakrabarty, Deepto; Christensen, Finn E.; Hailey, Charles J.; Mori, Kaya; Harrison, Fiona A.; Hong, Jaesub; Stern, Daniel; Zhang, William W.

    2015-02-01

    We report on two NuSTAR observations of GRS 1741.9-2853, a faint neutron star (NS) low-mass X-ray binary burster located 10' away from the Galactic center. NuSTAR detected the source serendipitously as it was emerging from quiescence: its luminosity was 6 × 10{sup 34} erg s{sup –1} on 2013 July 31 and 5 × 10{sup 35} erg s{sup –1} in a second observation on 2013 August 3. A bright, 800 s long, H-triggered mixed H/He thermonuclear Type I burst with mild photospheric radius expansion (PRE) was present during the second observation. Assuming that the luminosity during the PRE was at the Eddington level, an H mass fraction X = 0.7 in the atmosphere, and an NS mass M = 1.4 M {sub ☉}, we determine a new lower limit on the distance for this source of 6.3 ± 0.5 kpc. Combining with previous upper limits, this places GRS 1741.9-2853 at a distance of 7 kpc. Energy independent (achromatic) variability is observed during the cooling of the NS, which could result from the disturbance of the inner accretion disk by the burst. The large dynamic range of this burst reveals a long power-law decay tail. We also detect, at a 95.6% confidence level (1.7σ), a narrow absorption line at 5.46 ± 0.10 keV during the PRE phase of the burst, reminiscent of the detection by Waki et al. We propose that the line, if real, is formed in the wind above the photosphere of the NS by a resonant Kα transition from H-like Cr gravitationally redshifted by a factor 1 + z = 1.09, corresponding to a radius range of 29.0-41.4 km for a mass range of 1.4-2.0 M {sub ☉}.

  20. Physical properties of a complete volume-limited sample of Ap/Bp stars: Magnetic, rotational, multiplicity, and evolutionary properties of magnetic intermediate-mass stars within 100 pc of the sun

    NASA Astrophysics Data System (ADS)

    Power, Jennifer

    2007-08-01

    The population of all magnetic chemically peculiar stars (Ap stars) within 100 parsecs of the sun has been identified and investigated, determining fundamental parameters, rotational properties, and magnetic field characteristics. Using the HIPPARCOS Catalogue all intermediate mass stars within 100 pc of the sun have been identified. From published catalogues and other literature sources, we have identified 57 bona fide magnetic Ap/Bp stars in this distance- selected sample. Effective temperature, luminosity, radius, and mass were determined for each of the sample stars using published photometry and photometric calibrations, energy distributions, and HIPPARCOS parallaxes. Using the MuSiCoS spectropolarimeter at the Pic du Midi Observatory and the Least Squares Deconvolution procedure, Stokes I and V profiles were obtained for 26 of the 57 Ap sample stars. These observations were used in combination with previously published data to refine rotation periods, to determine projected rotational velocities, and to determine magnetic field strengths and geometries. Using the mass statistics of the sample Ap and non-Ap stars, the mass incidence distribution of magnetic intermediate mass stars in the solar neighbourhood has been derived. The Ap stars make up 1:7 to 2:8% (57 Ap stars out of 3904 intermediate mass stars) of all intermediate mass stars within 100 parsecs of the sun, and appear uniformly distributed across the main sequence. Statistical distributions of various properties, including distributions of the surface dipole field strength, rotational axis inclination, magnetic obliquity angle, and rotational period were analyzed. The rotation period distribution peaks at 2:1 ± 1:1 days, and displays an extended tail to very long periods. The majority of Ap stars in the sample have rotational velocities below 70 km/s, although some exhibit v sin i as large as 169 km/s. The histogram of obliquity angles shows a bimodal distribution, with a strong preference for b

  1. A JVLA survey of the high-frequency radio emission of the massive magnetic B- and O-type stars

    NASA Astrophysics Data System (ADS)

    Kurapati, Sushma; Chandra, Poonam; Wade, Gregg; Cohen, David H.; David-Uraz, Alexandre; Gagne, Marc; Grunhut, Jason; Oksala, Mary E.; Petit, Veronique; Shultz, Matt; Sundqvist, Jon; Townsend, Richard H. D.; ud-Doula, Asif

    2017-02-01

    We conducted a survey of seven magnetic O-type stars and eleven B-type stars with masses above 8 M⊙ using the Very Large Array in the 1, 3 and 13 cm bands. The survey resulted in a detection of two O- and two B-type stars. While the detected O-type stars - HD 37742 and HD 47129 - are in binary systems, the detected B-type stars, HD 156424 and ALS 9522, are not known to be in binaries. All four stars were detected at 3 cm, whereas three were detected at 1 cm and only one star was detected at 13 cm. The detected B-type stars are significantly more radio luminous than the non-detected ones, which is not the case for O-type stars. The non-detections at 13 cm are interpreted as due to thermal free-free absorption. Mass-loss rates were estimated using 3 cm flux densities and were compared with theoretical mass-loss rates, which assume free-free emission. For HD 37742, the two values of the mass-loss rates were in good agreement, possibly suggesting that the radio emission for this star is mainly thermal. For the other three stars, the estimated mass-loss rates from radio observations were much higher than those expected from theory, suggesting either a possible contribution from non-thermal emission from the magnetic star or thermal or non-thermal emission due to interacting winds of the binary system, especially for HD 47129. All the detected stars are predicted to host centrifugal magnetospheres except HD 37742, which is likely to host a dynamical magnetosphere. This suggests that non-thermal radio emission is favoured in stars with centrifugal magnetospheres.

  2. THE DISCOVERY OF A STRONG MAGNETIC FIELD AND COROTATING MAGNETOSPHERE IN THE HELIUM-WEAK STAR HD 176582

    SciTech Connect

    Bohlender, David A.; Monin, Dmitry

    2011-05-15

    We report the detection of a strong, reversing magnetic field and variable H{alpha} emission in the bright helium-weak star HD 176582 (HR 7185). Spectrum, magnetic, and photometric variability of the star are all consistent with a precisely determined period of 1.5819840 {+-} 0.0000030 days which we assume to be the rotation period of the star. From the magnetic field curve, and assuming a simple dipolar field geometry, we derive a polar field strength of approximately 7 kG and a lower limit of 52 deg. for the inclination of the rotation axis. However, based on the behavior of the H{alpha} emission, we adopt a large inclination angle of 85 deg. and this leads to a large magnetic obliquity of 77{sup 0}. The H{alpha} emission arises from two distinct regions located at the intersections of the magnetic and rotation equators and which corotate with the star at a distance of about 3.5 R{sub *} above its surface. We estimate that the emitting regions have radial and meridional sizes on the order of 2 R{sub *} and azimuthal extents (perpendicular to the magnetic equator) of less than approximately 0.6 R{sub *}. HD 176582 therefore appears to show many of the cool magnetospheric phenomena as that displayed by other magnetic helium-weak and helium-strong stars such as the prototypical helium-strong star {sigma} Ori E. The observations are consistent with current models of magnetically confined winds and rigidly rotating magnetospheres for magnetic Bp stars.

  3. Magnetically Controlled Spasmodic Accretion during Star Formation. I. Formulation of the Problem and Method of Solution

    NASA Astrophysics Data System (ADS)

    Tassis, Konstantinos; Mouschovias, Telemachos Ch.

    2005-01-01

    We formulate the problem of the late accretion phase of the evolution of an isothermal magnetic disk surrounding a forming star. The evolution is described by the six-fluid MHD equations, accounting for the presence of neutrals, atomic and molecular ions, electrons, and neutral, positively, and negatively charged grains. Only the electron fluid is assumed to be attached to the magnetic field, in order to investigate the effect of the detachment of the ions from the magnetic field lines that begins at densities as low as 108 cm-3. The ``central sink approximation'' is used to circumvent the problem of describing the evolution inside the opaque central region for densities greater than 1011 cm-3. In this way, the structure and evolution of the isothermal disk surrounding the forming star can be studied at late times without having to implement the numerically costly radiative transfer required by the physics of the opaque core. The mass and magnetic flux accumulating in the forming star are calculated, as are their effects on the structure & evolution of the surrounding disk. The numerical method of solution first uses an adaptive grid and later, after a central region a few AU in radius becomes opaque, switches to a stationary but nonuniform grid with a central sink cell. It also involves an implicit time integrator, an advective difference scheme that possesses the transportive property, a second-order difference approximation of forces inside a cell, an integral approximation of the gravitational and magnetic fields, and tensor artificial viscosity that permits an accurate investigation of the formation and evolution of shocks in the neutral fluid.

  4. East Meets West on "Double Star", a Joint Mission to Explore Earth's Magnetic Field

    NASA Astrophysics Data System (ADS)

    2001-07-01

    Cluster instruments has a number of advantages for both European and Chinese scientists. "By flying experiments identical to those on Cluster, we can reduce costs and development time," explained Alberto Gianolio, ESA Project Manager for Double Star. "This will minimise risk and help us to ensure that we are able to meet the spacecraft development schedule." ESA has agreed to contribute 8 million euros to the Double Star programme. This funding will be used for refurbishment and pre-integration of the European instruments, acquisition of data for 4 hours per day and coordination of scientific operations. Notes for Editors: Double Star will be the first mission launched by China to explore the Earth's magnetosphere - the magnetic bubble that surrounds our planet. As its name suggests, Double Star will involve two satellites - each designed, developed, launched and operated by the CNSA - flying in complementary orbits around the Earth. This orbital configuration will enable scientists to obtain simultaneous data on the changing magnetic field and population of electrified particles in different regions of the magnetosphere. The duo is expected to be launched by Chinese Long March 2C rockets in December 2002 and March 2003. This schedule may enable them to operate alongside ESA's Cluster mission - a mini-flotilla of four identical spacecraft launched into elliptical orbits around the Earth last summer. The "equatorial" spacecraft (DSP-1) will be launched into an elliptical orbit of 550 x 60,000 km, inclined at 28.5 degrees to the equator. This will enable it to investigate the Earth's huge magnetic tail, the region where particles are accelerated towards the planet's magnetic poles by a process known as reconnection. The "polar" satellite (DSP-2) will concentrate on physical processes taking place over the magnetic poles and the development of aurorae. It will have a 350 x 25,000 km orbit taking it round the Earth once every 7.3 hours.

  5. Long-term magnetic field monitoring of the Sun-like star ξ Bootis A

    NASA Astrophysics Data System (ADS)

    Morgenthaler, A.; Petit, P.; Saar, S.; Solanki, S. K.; Morin, J.; Marsden, S. C.; Aurière, M.; Dintrans, B.; Fares, R.; Gastine, T.; Lanoux, J.; Lignières, F.; Paletou, F.; Ramírez Vélez, J. C.; Théado, S.; Van Grootel, V.

    2012-04-01

    Aims: We aim to investigate the long-term temporal evolution of the magnetic field of the solar-type star ξ Bootis A, both from direct magnetic field measurements and from the simultaneous estimate of indirect activity indicators. Methods: We obtained seven epochs of high-resolution, circularly-polarized spectra from the NARVAL spectropolarimeter between 2007 and 2011, for a total of 76 spectra. Using approximately 6100 photospheric spectral lines covering the visible domain, we employed a cross-correlation procedure to compute a mean polarized line profile from each spectrum. The large-scale photospheric magnetic field of the star was then modelled by means of Zeeman-Doppler Imaging, allowing us to follow the year-to-year evolution of the reconstructed magnetic topology. Simultaneously, we monitored the width of several magnetically sensitive spectral lines, the radial velocity, the line asymmetry of intensity line profiles, and the chromospheric emission in the cores of the Ca II H and Hα lines. Results: During the highest observed activity states, in 2007 and 2011, the large-scale field of ξ Bootis A is almost completely axisymmetric and is dominated by its toroidal component. The toroidal component persists with a constant polarity, containing a significant fraction of the magnetic energy of the large-scale surface field through all observing epochs. The magnetic topologies reconstructed for these activity maxima are very similar, suggesting a form of short cyclicity in the large-scale field distribution. The mean unsigned large-scale magnetic flux derived from the magnetic maps varies by a factor of about 2 between the lowest and highest observed magnetic states. The chromospheric flux is less affected and varies by a factor of 1.2. Correlated temporal evolution, due to both rotational modulation and seasonal variability, is observed between the Ca II emission, the Hα emission and the width of magnetically sensitive lines. The rotational dependence of

  6. Hydrodynamic Stability and Magnetic Reconnection in Disks and Stars

    NASA Technical Reports Server (NTRS)

    Goodman, Jeremy; Kulsrud, Russell

    1999-01-01

    The purpose of this grant is to study parametric instability. The simplest example of parametric instability is a harmonic oscillator with a periodic modulation of the spring constant. If the modulation frequency is close to twice the natural frequency of the oscillator, the amplitude of oscillation tends to grow exponentially. The growth rate is proportional to the strength of the modulation, but it also depends upon the closeness to resonance of the two frequencies, and upon natural damping rate or "Q" of the oscillator. Parametric instabilities are very common in physics. A familiar example is a jogger's ponytail--normally a very strongly damped pendulum, it can be destabilized by the variation in effective gravity during the jogger's stride. Observation confirms that the period of the pendulum is half that of the jogger's vertical motion. In astrophysics, parametric instability may occur by external tidal forcing, or by interaction among eigenmodes. In the latter case, an energetic eigenmode may destabilize modes of half its frequency, provided some weak nonlinearity exists to couple them. Under a previous Astrophysical Theory grant (NAGW-2419), the PI discovered a parametric instability of tidally forced disks such as the accretion disks in cataclysmic variables and X ray binaries [2]. The destabilized modes are tightly-wound, incompressible, three-dimensional waves analogous to g-modes and r-modes in stars. Later work has confirmed our analysis [4]. It was hoped that these modes might provide a source of turbulence and angular momentum transport in accretion disks. However, a follow-up investigation of this instability by local numerical simulations, although confirming the analytically estimated growth rates, found negligible angular momentum flux [3]. Other work, partly supported by the ATP, now strongly indicates that the transport mechanism in such disks is magnetohydrodynamic turbulence [6]. Nevertheless, the parametric mechanism may truncate the outer

  7. Period04 FCAPT uvby Photometric Studies of Eight Magnetic CP Stars

    NASA Astrophysics Data System (ADS)

    Adelman, Saul J.; Dukes, Robert J.

    2014-06-01

    We present Four College Automated Photometric Telescope (FCAPT) differential Stromgren uvby photometry of 8 magnetic CP (mCP) stars: HD 5797 (V551 Cas), HD 26792 (DH Cam), HD 27309 (56 Tau, V724 Tau), HD 49713 (V740 Mon), HD 74521 (49 Cnc, BI Cnc), HD 120198 (84 UMa, CR UMa), HD 171263 (QU Ser), and HD 215441 (GL Lac, Babcock's star). Our data sets are larger than those of most mCP stars in the literature. These are the first FCAPT observations of HD 5797, HD 26792, HD 49713, and HD 171263. Those for the remaining four stars substantially extend published FCAPT data. The FCAPT observed some stars for a longer time range and with greater accuracy than other optical region automated photometric telescopes.Our goals were to determine very accurate periods, the u, v, b, and y amplitudes, and if there were any long period periods. In addition we wanted to compare our results with those of magnetic field measurements to help interpret the light curves.We used the Period04 computer program to analyze the light curves. This program provides errors for the derived quantities as it fits the light curve. Our derived periods of 68.046 +/- 0.008 days for HD 5797, 3.80205 +/- 0.00006 days for HD 26792, 1.56889 +/- 0.000002 days for HD 27309, 2.13536 +/- 0.00002 days for HD 49713, 7.0505 +/- 0.0001 days for HD 74521, 1.38577 +/- 0.000004 days for HD 120198, 3.9974 +/- 0.0001days for HD 171263, and 9.487792 +/- 0.00005 days for HD 215441 are refinements of the best determinations in the literature.

  8. The Effects of Magnetic Fields on the Dynamics of Radiation Pressure-dominated Massive Star Envelopes

    NASA Astrophysics Data System (ADS)

    Jiang, Yan-Fei; Cantiello, Matteo; Bildsten, Lars; Quataert, Eliot; Blaes, Omer

    2017-07-01

    We use three-dimensional radiation magnetohydrodynamic simulations to study the effects of magnetic fields on the energy transport and structure of radiation pressure-dominated main sequence massive star envelopes at the region of the iron opacity peak. We focus on the regime where the local thermal timescale is shorter than the dynamical timescale, corresponding to inefficient convective energy transport. We begin with initially weak magnetic fields relative to the thermal pressure, from 100 to 1000 G in differing geometries. The unstable density inversion amplifies the magnetic field, increasing the magnetic energy density to values close to equipartition with the turbulent kinetic energy density. By providing pressure support, the magnetic field’s presence significantly increases the density fluctuations in the turbulent envelope, thereby enhancing the radiative energy transport by allowing photons to diffuse out through low-density regions. Magnetic buoyancy brings small-scale magnetic fields to the photosphere and increases the vertical energy transport, with the energy advection velocity proportional to the Alfvén velocity, although in all cases we study, photon diffusion still dominates the energy transport. The increased radiative and advective energy transport causes the stellar envelope to shrink by several scale heights. We also find larger turbulent velocity fluctuations compared with the purely hydrodynamic case, reaching ≈ 100 {{{km}}{{s}}}-1 at the stellar photosphere. The photosphere also shows vertical oscillations with similar averaged velocities and periods of a few hours. The increased turbulent velocity and oscillations will have strong impacts on the line broadening and periodic signals in massive stars.

  9. Hall attractor in axially symmetric magnetic fields in neutron star crusts.

    PubMed

    Gourgouliatos, Konstantinos N; Cumming, Andrew

    2014-05-02

    We find an attractor for an axially symmetric magnetic field evolving under the Hall effect and subdominant Ohmic dissipation, resolving the question of the long-term fate of the magnetic field in neutron star crusts. The electron fluid is in isorotation, analogous to Ferraro's law, with its angular velocity being approximately proportional to the poloidal magnetic flux, Ω∝Ψ. This equilibrium is the long-term configuration of a magnetic field evolving because of the Hall effect and Ohmic dissipation. For an initial dipole-dominated field, the attractor consists mainly of a dipole and an octupole component accompanied by an energetically negligible quadrupole toroidal field. The field dissipates in a self-similar way: Although higher multipoles should decay faster, the toroidal field mediates transfer of energy into them from the lower ones, leading to an advection diffusion equilibrium and keeping the ratio of the poloidal multipoles almost constant. This has implications for the structure of the intermediate-age neutron stars, su