20 research outputs found
Constraints on the Origins of Magnetic White Dwarfs
The central theme of this work is the most frequent final stage of the evolution of magnetic stars, the Magnetic White Dwarfs (MWDs). Improved statistical investigations coming from new surveys and very precise observations of unique MWDs offer the possibility to test various hypotheses on the evolution of these objects. In the first part of our work we identify hydrogen-rich MWDs (DAHs) in the Sloan Digital Sky Survey (SDSS) and investigate the population statistics of all known DAHs in the SDSS. Additionally, we investigate the evolutionary histories of a few of these objects using constraints from the observations of their binary counterparts or through their membership in open clusters. In the second part of our work, we investigate the unique MWD REJ 0317-853, by a parallax measurement with the Hubble Space Telescope and by time resolved spectro-polarimetric modeling. We show that the assumption of centered magnetic dipoles for the field geometry is not correct for more than half of the objects in our sample; this is in particular true for REJ 0317-853 which shows a very uniform field during one rotation phase. This is validated by the first observation of cyclotron absorption in the polarization spectrum of a white dwarf, which is explained with a new model for the self-consistent physical treatment of this absorption process. Furthermore, we study the possible influence of magnetism on the mass loss during the stellar evolution and on the structure of the star which is of importance to understand the nature of the massive white dwarf REJ 0317-853
Analysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS
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.Comment: to appear in the proceedings of the 16th European Workshop on White
Dwarfs, Barcelona, 200
Analysis of the hydrogen-rich magnetic White Dwarfs in the SDSS
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, we have analyzed the spectra of 114 magnetic DAs from the Sloan Digital Survey (SDSS; 95 previously published plus 14 newly discovered within SDSS
Catalog of magnetic white dwarfs with hydrogen dominated atmospheres
White dwarfs are excellent research laboratories as they reach temperatures,
pressures, and magnetic fields that are unattainable on Earth. To better
understand how these three physical parameters interact with each other and
with other stellar features, we determined the magnetic field strength for a
total of 804 hydrogen-rich white dwarfs of which 287 are not in the literature.
We fitted the spectra observed with the Sloan Digital Sky Survey using
atmospheric models that consider the Zeeman effect due to the magnetic field at
each point in the stellar disk. Comparing magnetic and non-magnetic WDs, the
literature already shows that the magnetic ones have on average higher mass
than the non-magnetic. In addition to that, magnetic fields are more common in
cooler WDs than in hotter WDs. In consonance, we found that those with higher
magnetic field strengths tend to have higher masses, and lower temperatures,
for which models indicate the crystallization process has already started. This
reinforces the hypothesis that the field is being generated and/or amplified in
the cooling process of the white dwarf. Our sample constitutes the largest
number of white dwarfs with determined magnetic fields to date.Comment: Accepted in ApJ on 27th December 202
Double degenerate mergers as progenitors of high-field magnetic white dwarfs
High-field magnetic white dwarfs have been long suspected to be the result of stellar mergers. However, the nature of the coalescing stars and the precise mechanism that produces the magnetic field are still unknown. Here, we show that the hot, convective, differentially rotating corona present in the outer layers of the remnant of the merger of two degenerate cores can produce magnetic fields of the required strength that do not decay for long timescales. Using a state-of-the-art Monte Carlo simulator, we also show that the expected number of high-field magnetic white dwarfs produced in this way is consistent with that found in the solar neighborhood.Instituto de Astrofísica de La PlataFacultad de Ciencias Astronómicas y Geofísica
Analysis of the Hydrogen-rich Magnetic White Dwarfs in the SDSS
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.Comment: to appear in the proceedings of the 16th European Workshop on White
Dwarfs, Barcelona, 200
Seismology of a massive pulsating hydrogen atmosphere white dwarf
We report our observations of the new pulsating hydrogen atmosphere white dwarf SDSS J132350.28+010304.22. We discovered periodic photometric variations in frequency and amplitude that are commensurate with nonradial g-mode pulsations in ZZ Ceti stars. This, along with estimates for the star's temperature and gravity, establishes it as a massive ZZ Ceti star. We used time-series photometric observations with the 4.1m SOAR Telescope, complemented by contemporary McDonald Observatory 2.1m data, to discover the photometric variability. The light curve of SDSS J132350.28+010304.22 shows at least nine detectable frequencies. We used these frequencies to make an asteroseismic determination of the total mass and effective temperature of the star: M= 0.88 ± 0.02 M and T eff = 12, 100 ± 140 K. These values are consistent with those derived from the optical spectra and photometric colors.Facultad de Ciencias Astronómicas y Geofísica