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Register a new userTwo new young, wide magnetic + non-magnetic double-degenerate binary systems
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Abridged: We report the discovery of two, new, rare, wide, double-degenerate binaries that each contain a magnetic and a non-magnetic star. The components of SDSSJ092646.88+132134.5 + J092647.00+132138.4 and SDSSJ150746.48+521002.1 + J150746.80+520958.0 have angular separations of only 4.6 arcsec (a~650AU) and 5.1 arcsec (a~750AU), respectively. They also appear to share common proper motions. Follow-up optical spectroscopy reveals each system to consist of a DA and a H-rich high-field magnetic white dwarf (HFMWD). Our measurements of the effective temperatures and the surface gravities of the DA components reveal both to have larger masses than are typical of field white dwarfs. By assuming that these degenerates have evolved essentially as single stars, due to their wide orbital separations, we use them to place limits on the total ages of our stellar systems. These argue that in each case the HFMWD is probably associated with an early type progenitor (M_init > 2M_solar). We find that the cooling time of SDSSJ150746.80+520958.0 (DAH) is somewhat lower than might be expected had it followed the evolutionary path of a typical single star. This mild discord is in the same sense as that observed for two of the small number of other HFMWDs for which progenitor mass estimates have been made, REJ0317-853 and EG59. The mass of the other DAH, SDSSJ092646.88+132134.5, appears to be smaller than expected on the basis of single star evolution. If this object was/is a member of a hierarchical triple system it may have experienced greater mass loss during an earlier phase of its life as a result of it having a close companion. The large uncertainties on our estimates of the parameters of the HFMWDs suggest a larger sample of these objects is required to firmly identify any trends in their inferred cooling times and progenitor masses.
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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 microG (e.g., in molecular clouds) to TeraG and more (e.g., in magnetic neutron stars); in non-degenerate stars in particular, they feature large-scale topologies varying from simple nearly-axisymmetric dipoles to complex non-axsymmetric structures, and from mainly poloidal to mainly toroidal topology. After recalling the main techniques of detecting and modelling stellar magnetic fields, we review the existing properties of magnetic fields reported in cool, hot and young non-degenerate stars and protostars, and discuss our understanding of the origin of these fields and their impact on the birth and life of stars.
Characterizing the local space density of double degenerate binary systems is a complementary approach to broad sky surveys of double degenerates to determine the expected rates of white dwarf binary mergers, in particular those that may evolve into other observable phenomena such as extreme helium stars, Am CVn systems, and supernovae Ia. However, there have been few such systems detected in local space. We report here the discovery that WD 1242$-$105, a nearby bright WD, is a double-line spectroscopic binary consisting of two degenerate DA white dwarfs of similar mass and temperature, despite it previously having been spectroscopically characterized as a single degenerate. Follow-up photometry, spectroscopy, and trigonometric parallax have been obtained in an effort to determine the fundamental parameters of each component of this system. The binary has a mass ratio of 0.7 and a trigonometric parallax of 25.5 mas, placing it at a distance of 39 pc. The systems total mass is 0.95 M$_odot$ and has an orbital period of 2.85 hours, making it the strongest known gravitational wave source ($log h = -20.78$) in the mHz regime. Because of its orbital period and total mass, WD 1242$-$105 is predicted to merge via gravitational radiation on a timescale of 740 Myr, which will most likely not result in a catastrophic explosion.
We present a spectroscopic component analysis of 18 candidate young, wide, non-magnetic, double-degenerate binaries identified from a search of the Sloan Digital Sky Survey Data Release 7 (DR7). All but two pairings are likely to be physical systems. We show SDSS J084952.47+471247.7 + SDSS J084952.87+471249.4 to be a wide DA+DB binary, only the second identified to date. Combining our measurements for the components of 16 new binaries with results for three similar, previously known systems within the DR7, we have constructed a mass distribution for the largest sample to date (38) of white dwarfs in young, wide, non-magnetic, double-degenerate pairings. This is broadly similar in form to that of the isolated field population with a substantial peak around M~0.6 Msun. We identify an excess of ultra-massive white dwarfs and attribute this to the primordial separation distribution of their progenitor systems peaking at relatively larger values and the greater expansion of their binary orbits during the final stages of stellar evolution. We exploit this mass distribution to probe the origins of unusual types of degenerates, confirming a mild preference for the progenitor systems of high-field-magnetic white dwarfs, at least within these binaries, to be associated with early-type stars. Additionally, we consider the 19 systems in the context of the stellar initial mass-final mass relation. None appear to be strongly discordant with current understanding of this relationship.
HD 144941 is an extreme He (EHe) star, a rare class of subdwarf OB star formed from the merger of two white dwarf (WD) stars. Uniquely amongst EHe stars, its light curve has been reported to be modulated entirely by rotation, suggesting the presence of a magnetic field. Here we report the first high-resolution spectropolarimetric observations of HD 144941, in which we detect an extremely strong magnetic field both in circular polarization (with a line-of-sight magnetic field averaged over the stellar disk $langle B_z rangle sim -8$ kG) and in Zeeman splitting of spectral lines (yielding a magnetic modulus of $langle B rangle sim 17$ kG). We also report for the first time weak H$alpha$ emission consistent with an origin an a Centrifugal Magnetosphere (CM). HD 144941s atmospheric parameters could be consistent with either a subdwarf or a main sequence (MS) star, and its surface abundances are neither similar to other EHe stars nor to He-strong magnetic stars. However, its H$alpha$ emission properties can only be reproduced if its mass is around 1 M$_odot$, indicating that it must be a post-MS object. Since there is no indication of binarity, it is unlikely to be a stripped star, and was therefore most likely produced in a WD merger. HD 144941 is therefore further evidence that mergers are a viable pathway for the generation of fossil magnetic fields.
We present a numerical study of the mass transfer dynamics prior to the gravitational wave-driven merger of a double white dwarf system. Recently, there has been some discussion about the dynamics of these last stages, different methods seemed to provide qualitatively different results. While earlier SPH simulations indicated a very quick disruption of the binary on roughly the orbital time scale, more recent grid-based calculations find long-lived mass transfer for many orbital periods. Here we demonstrate how sensitive the dynamics of this last stage is to the exact initial conditions. We show that, after a careful preparation of the initial conditions, the reportedly short-lived systems undergo mass transfer for many dozens of orbits. The reported numbers of orbits are resolution-biased and therefore represent only lower limits to what is realized in nature. Nevertheless, the study shows convincingly the convergence of different methods to very similar results.
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