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Register a new userAnalysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS
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We have calculated optical spectra of hydrogen-rich (DA) white dwarfs with magnetic field strengths between 1 MG and 1000 MG for temperatures between 7000 K and 50000 K. Through a least-squares minimization scheme with an evolutionary algorithm, we have analyzed the spectra of 114 magnetic DAs from the SDSS (95 previously published plus 14 newly discovered within SDSS, and five discovered by SEGUE). Since we were limited to a single spectrum for each object we used only centered magnetic dipoles or dipoles which were shifted along the magnetic dipole axis. We also statistically investigated the distribution of magnetic-field strengths and geometries of our sample.
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We model the structure of the surface magnetic fields of the hydrogen-rich white dwarfs in the SDSS. We have calculated a grid of state-of-the-art theoretical optical spectra of hydrogen-rich magnetic white dwarfs with magnetic field strengths between 1 MG and 1200 MG for different angles, and for effective temperatures between 7000 K and 50000 K. We used a least-squares minimization scheme with an evolutionary algorithm in order to find the magnetic field geometry best fitting the observed data. We used simple centered dipoles or dipoles which were shifted along the dipole axis to model the coadded SDSS fiber spectrum of each object. We have analysed the spectra of all known magnetic DAs from the SDSS (97 previously published plus 44 newly discovered) and also investigated the statistical properties of magnetic field geometries of this sample. The total number of known magnetic white dwarfs already more than tripled by the SDSS and more objects are expected from a more systematic search. The magnetic fields span a range between ~1 and 900 MG. Our results further support the claim that Ap/Bp population is insufficient in generating the numbers and field strength distributions of the observed MWDs, and either another source of progenitor types or binary evolution is needed. Moreover clear indications for non-centered dipoles exist in about ~50% of the objects which is consistent with the magnetic field distribution observed in Ap/Bp stars.
We present the results of the asteroseismological analysis of two rich DAVs, G38-29 and R808, recent targets of the Whole Earth Telescope. 20 periods between 413 s and 1089 s were found in G38-29s pulsation spectrum, while R808 is an even richer pulsator, with 24 periods between 404 s and 1144 s. Traditionally, DAVs that have been analyzed asteroseismologically have had fewer than half a dozen modes. Such a large number of modes presents a special challenge to white dwarf asteroseismology, but at the same time has the potential to yield a detailed picture of the interior chemical make-up of DAVs.We explore this possibility by varying the core profiles as well as the layer masses.We use an iterative grid search approach to find best fit models for G38-29 and R808 and comment on some of the intricacies of fine grid searches in white dwarf asteroseismology.
A large fraction of white dwarf stars shows photospheric chemical composition polluted by heavy elements accreted from a debris disk. Such debris disks result from the tidal disruption of rocky planetesimals which had survived to whole stellar evolution from the main sequence to the final white dwarf stage. Determining the accretion rate of this material is an important step towards estimating the mass of the planetesimals and towards understanding the ultimate fate of the planetary systems. The accretion of heavy material with a mean molecular weight, $mu$, higher than the mean molecular weight of the white dwarf outer layers, induces a double-diffusive instability producing fingering convection and extra-mixing. As a result, the accreted material is diluted deep into the star. We explore the effect of this extra-mixing on the abundance evolution of Mg, O, Ca, Fe and Si in the cases of the two well studied polluted DAZ white dwarfs: GD~133 and G~29-38. We performed numerical simulations of the accretion of material with a chemical composition similar to the bulk Earth one. We considered accretion rates from $10^{4}$~g/s to $10^{10}$~g/s. The double-diffusive instability develops on a very short time scale. The accretion rate needed to reproduce the observed abundances exceeds by more than 2 orders of magnitude the rate estimated by neglecting the fingering convection in the case of GD~133, and by approximately 1.7 dex in the case of G~29-38. Our numerical simulations show that fingering convection is an efficient mechanism to mix the accreted material and that it must be taken into account in the determination of accretion rates.
We present a homogeneous analysis of 1023 DBZ/DZ(A) and 319 DQ white dwarf stars taken from the Montreal White Dwarf Database. This represents a significant increase over the previous comprehensive studies on these types of objects. We use new trigonometric parallax measurements from the Gaia second data release, together with photometry from the Sloan Digital Sky Survey, Pan-STARRS, Gaia, or BVRI from the literature, which allow the determination of the mass for the majority of the objects in our sample. We use the photometric and spectroscopic techniques with the most recent model atmospheres available, which include high-density effects, to accurately determine the effective temperature, surface gravity, and heavy element abundances for each object. We study the abundance of hydrogen in DBZ/DZ white dwarfs and the properties of the accreted planetesimals. We explore the nature of the second sequence of DQ stars using proper motions from Gaia, and highlight evidence of crystallization in massive DQ stars. We also present mass distributions for both spectral types. Finally, we discuss the implications of our findings in the context of the spectral evolution of white dwarfs, and provide the atmospheric parameters for each star.
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T. Rauch (Dr.-Remeis-Sternwarte
, Bamberg
, Germany; Institut fuern Astronomie und Astrophysik
2002
Previous investigations on hydrogen-rich white dwarfs generally yield only very small rotational velocities (v_rot sin i). We have analyzed line profiles in high-resolution optical spectra of eight hydrogen-deficient (pre-) white dwarfs and find deviations from the dominant Stark line broadening in five cases which, interpreted as an effect of stellar rotation, indicate projected rotational velocities of 40 - 70 km/sec. For the three least luminous stars upper limits of v_rot sin i = 15 - 25 km/sec could be derived only. The resulting velocities correlate with luminosity and mass. However, since the mass-loss rate is correlated to the luminosity of a star, the observed line profiles may be affected by a stellar wind as well. In the case of RX J2117.1+3412, this would solve discrepancies to results of pulsational modeling (v_rot sin i ~ 0).
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