Magnetic White Dwarfs in the SDSS and Estimating the Mean Mass of Normal DA and DB WDs

Abstract

Abstract. When classifying by eye more than 22 000 spectra selected as possible white dwarf stars from the Sloan Digital Sky Survey Data Release 7, we detected Zeeman splittings in more than 800 stars, increasing by a factor of five the number of known magnetic white dwarfs. Our field estimations range from 90 MG to less than 1 MG, complementing the detections by Külebi et al. [1]. These magnetic white dwarf stars cover the whole range of temperature and spectral classes observed.As the Zeeman splittings broadens the lines, we cannot use the line profiles to estimate surface gravity directly. We therefore excluded the magnetic white dwarfs from our average mass estimate of normal DAs and DBs. Analysis of the remaining 1505 bright and hot DA white dwarfs, i.e., those with S/N ≥ 20 and T eff = 12 000 K, results in a mean mass M DA = 0.604 ± 0.003M ⊙ , while that of our 82 bright DBs with T eff = 16 000 K is M DB = 0.646 ± 0.006M ⊙ . Kleinman et al. [2] is forming a catalog of white dwarf stars with spectra from the Sloan Digital Sky Survey Data Release 7. Using careful human inspections of our more than 23,000 white dwarf spectra, much more than were looked at in past SDSS white dwarf catalogs, we found evidence of magnetic field -Zeeman splittings -in somewhere between 5-10% of our white dwarf sample. The vast majority of these stars are low field. Keywords MAGNETIC WHITE DWARF STARSIndependent of Kleinman et al.[2], Külebi et al.[1] found 44 new magnetic white dwarfs in the SDSS DR7 sample, and used log g=8.0 models to estimate the fields of the 141 known magnetic white dwarfs (1 to 900 MG).Donati & Landstreet [3] discuss extensively that magnetic fields are found in all types of stars and play a role in their formation and evolution, influencing accretion, diffusion, mass loss, turbulence, pulsations, their rotation rates and even their masses and surface chemical composition. The fraction of white dwarf stars that are magnetic is quoted in the literature as anywhere between 2% to 20%. An accurate estimate of this percentage is crucial to understanding the origin of the magnetic field in these stars. We find 5% of the SDSS white dwarf stars show clear evidence of magnetic fields with another 4%

Gravitational waves from fast-spinning white dwarfs

“…For the three binary systems investigated here, we conclude that from the magnetic deformation mechanism, the WDs require a magnetic field above ∼ 10 9 G to produce GW amplitudes that can be detectable by BBO, for example. These fields are quite intense, but not unrealistic, since these WDs with surface magnetic fields from 10 6 G up to 10 9 G have been confirmed by the recent results of SDSS (Külebi et al 2009;Kuelebi et al 2010;Kepler et al 2010Kepler et al , 2013Kepler et al , 2015.…”

“…There is an increasing interest of the astrophysics community on highly magnetized white dwarfs (HMWDs) both from the theoretical and observational points of view. These WDs with surface magnetic fields from 10 6 G up to 10 9 G have been confirmed by the recent results of the Sloan Digital Sky Survey (SDSS) (see e.g., Külebi et al 2009;Kuelebi et al 2010;Kepler et al 2010Kepler et al , 2013Kepler et al , 2015. Besides their high magnetic fields, most of them have been shown to be massive, and responsible for the high-mass peak at 1 M of the WD mass distribution; for instance: REJ 0317-853 has M ≈ 1.35 M and B ≈ (1.7-6.6) × 10 8 G ( Barstow et al 1995;Külebi et al 2010); PG 1658+441 has M ≈ 1.31 M and B ≈ 2.3 × 10 6 G (Liebert et al 1983;Schmidt et al 1992); and PG 1031+234 has the highest magnetic field B ≈ 10 9 G (Schmidt et al 1986;Külebi et al 2009).…”

SGR 0418+5729,SwiftJ1822.3-1606, and 1E 2259+586 as massive, fast-rotating, highly magnetized white dwarfs

“…The range we obtained for such frequencies falls between the infrared and UV bands. It is important to remark that magnetic fields in white dwarfs ranging from 10 7 G up to 10 9 G are routinely observed; see, e.g., Külebi et al (2009), Külebi et al (2010a), Kepler et al (2010), and very recently Kepler et al (2013), where from the Data Release 7 of the Sloan Digital Sky Survey, white dwarfs with magnetic fields in the range from around 10 6 G to 7.3×10 8 G have been found from the analysis of the Zeeman splitting of the Balmer absorption lines. Deep photometric and spectrometric observations in the range of cyclotron frequencies predicted in this work are therefore highly recommended to detect possible absorptions and line-splitting features in the spectra of SGRs and AXPs.…”

“…It is remarkable that white dwarfs with large magnetic fields from 10 7 G all the way up to 10 9 G have been indeed observed (see e.g., Külebi et al 2009Külebi et al , 2010aKepler et al 2010Kepler et al , 2013. Also, most of the observed magnetized white dwarfs are massive; e.g., REJ 0317-853 with M ∼ 1.35 M and B ∼ (1.7−6.6) × 10 8 G (Barstow et al 1995;Külebi et al 2010b); PG 1658+441 with M ∼ 1.31 M and B ∼ 2.3 × 10 6 G (Liebert et al 1983;Schmidt et al 1992); and PG 1031+234 with the highest magnetic field ∼10 9 G (Schmidt et al 1986;Külebi et al 2009).…”

Theoretical study of the line profiles of the hydrogen perturbed by collisions with protons

“…Kleinman et al (2004) and Liebert, Bergeron & Holberg (2005) derive masses from spectroscopic fits, and Bergeron, Wesemael & Fontaine (1991) and Koester (1991) discuss the apparent increase in the average mass of DAs with T eff < 12 000 K. Kepler et al (2006) compare SDSS spectra with high signal‐to‐noise ratio (S/N) spectra obtained with the 8‐m Gemini telescope for four WDs with T eff ∼ 12 000 K and determine that the masses derived from fitting the relatively low S/N SDSS spectra are systematically overestimated by Δ M ≃ 0.13 M ⊙ . Due to a correlation between T eff and log g (a small increase in T eff can be offset by a small decrease in log g ), such discrepancies are concentrated only in the region around the maximum of the Balmer lines, 14 000 ≥ T eff ≥ 12 000 K. To study the trend of the apparent increased average mass of DAs, Kepler et al (2010) analyse 1505 such WDs with S/N ≥ 20 and T eff > 12 000 K and determined an average DA mass of M /M ⊙ = 0.604 ± 0.003. They observe that the distribution of the sample is similar to that of the Palomar Green survey published by Liebert et al (2005).…”