A Detailed Model Atmosphere Analysis of Cool White Dwarfs in the Sloan Digital Sky Survey

Abstract

We present optical spectroscopy and near-infrared photometry of 126 cool white dwarfs (WDs) in the Sloan Digital Sky Survey (SDSS). Our sample includes high proper motion targets selected using the SDSS and USNO-B astrometry and a dozen previously known ultracool WD candidates. Our optical spectroscopic observations demonstrate that a clean selection of large samples of cool WDs in the SDSS (and the SkyMapper, Pan-STARRS, and the Large Synoptic Survey Telescope data sets) is possible using a reduced proper motion diagram and a tangential velocity cut-off (depending on the proper motion accuracy) of 30 km s −1 . Our near-infrared observations reveal eight new stars with significant absorption. We use the optical and near-infrared photometry to perform a detailed model atmosphere analysis. More than 80% of the stars in our sample are consistent with either pure hydrogen or pure helium atmospheres. However, the eight stars with significant infrared absorption and the majority of the previously known ultracool WD candidates are best explained with mixed hydrogen and helium atmosphere models. The age distribution of our sample is consistent with a Galactic disk age of 8 Gyr. A few ultracool WDs may be as old as 12-13 Gyr, but our models have problems matching the spectral energy distributions of these objects. There are only two halo WD candidates in our sample. However, trigonometric parallax observations are required for accurate mass and age determinations and to confirm their membership in the halo.

Broad-band spectral energy distribution of 3000 Å break quasars from the Sloan Digital Sky Survey

“…J095051.70+122758.5 looks quite similar, with pronounced band heads at roughly 5600 Å, 5200 Å, and 4700 Å, and is obviously a newly discovered peculiar DQ 3 . J124739.04+064604.5 and J180302.57+232043.3, also classified as peculiar DQs (Kilic et al 2010;Limoges et al 2013;Kleinman et al 2013), look similar to the former two spectra, but display a declining flux density towards the shortest wavelengths. All four stars have significant proper motions between 200 and 400 mas yr .1 .…”

The WFCAM transit survey and cool white dwarfs

“…The pure-H models used are those of [14], but now including the red wing opacity from Ly . The pure-He model atmospheres considered are the same models as those used in [5]. The surface gravity is fixed to log g = 8 (the most typical value for white dwarfs).…”

The brightest pure-H ultracool white dwarf

“…Blue squares correspond to high proper motion stars in this sample (μ > 100 mas/yr). For the sake of comparison we also show the objects studied in Kilic et al (2010a), confirmed cool white dwarfs as magenta asterisks and subdwarfs that contaminated their sample as green asterisks (IR photometry for the subdwarfs is taken from the 2MASS catalogue, Cutri et al 2003). The solid lines correspond to the theoretical tracks for pure-H composition with tangential velocities of V tan = 40 km s −1 (top) and 160 km s −1 (bottom).…”

“…3). Asterisks correspond to objects from Kilic et al (2010a): cool white dwarfs are depicted in magenta and subdwarfs in green. Fig.…”

“…More recently, Kilic et al (2010a) carried out a study of cool white dwarf candidates found in SDSS ). They performed a detailed model atmosphere analysis providing temperatures and compositions, as well as obtaining spectroscopic observations to confirm the white dwarf nature of the candidates.…”