Cool DZ white dwarfs in the SDSS

Aims. We report the identification of 26 cool DZ white dwarfs that lie across and below the main sequence in the Sloan Digital Sky Survey (SDSS) (u − g) vs. (g − r) two-color diagram; 21 of these stars are new discoveries. Methods. The sample was identified by visual inspection of all spectra of objects that fall below the main sequence in the two-color diagram, as well as by an automated search for characteristic spectral features over a large area in color space that included the main sequence. The spectra and photometry provided by the SDSS project are interpreted with model atmospheres, including all relevant metals. Effective temperatures and element abundances are determined, while the surface gravity has to be assumed and was fixed at the canonical value of logg = 8. Results. These stars represent the extension of the well-known DZ sequence towards cooler temperatures and fill the gap around Teff = 6500 K present in a previous study. The metal abundances are similar to those in the hotter DZ, but the lowest abundances are missing, probably because of our selection procedures. The interpretation is complicated in terms of the accretion/diffusion scenario, because we do not know if accretion is still occurring or has ended long ago. Independent of that uncertainty, the masses of the metals currently present in the convection zones – and thus an absolute lower limit of the total accreted masses – of these stars are similar to the largest asteroids in our solar system.

[1]  R. Napiwotzki,et al.  The magnetic and metallic degenerate G77-50 , 2011, 1101.2203.

[2]  D. Koester,et al.  Accretion and diffusion in white dwarfs - New diffusion timescales and applications to GD 362 and G 29-38 , 2009, 0903.1499.

[3]  T. Marsh,et al.  A Gaseous Metal Disk Around a White Dwarf , 2006, Science.

[4]  F. Allard,et al.  A new model for brown dwarf spectra including accurate unified line shape theory for the Na I and K I resonance line profiles , 2003 .

[5]  Marc J. Kuchner,et al.  The New Class of Dusty DAZ White Dwarfs , 2007, astro-ph/0703473.

[6]  T. S. Monteiro,et al.  Broadening of the Ca+ and Mg+ resonance lines by collisions with helium , 1986 .

[7]  M. Barstow,et al.  The Discovery of Mg II λ4481 in the White Dwarf EG 102: Evidence for Ongoing Accretion , 1997 .

[8]  James Liebert,et al.  Model Atmosphere Analysis of the Weakly Magnetic DZ White Dwarf G165-7 , 2006 .

[9]  List of Known White Dwarfs , 1941 .

[10]  A. Nitta,et al.  NEAR-INFRARED CONSTRAINTS ON THE PRESENCE OF WARM DUST AT METAL-RICH, HELIUM ATMOSPHERE WHITE DWARFS , 2008, 0804.3398.

[11]  James Liebert,et al.  On the Spectral Evolution of Cool, Helium-Atmosphere White Dwarfs: Detailed Spectroscopic and Photometric Analysis of DZ Stars , 2007, astro-ph/0703758.

[12]  B. Zuckerman,et al.  INFRARED SIGNATURES OF DISRUPTED MINOR PLANETS AT WHITE DWARFS , 2009, 0901.0973.

[13]  D. Koester,et al.  The accretion-diffusion scenario for metals in cool white dwarfs , 2006 .

[14]  Gilles Fontaine,et al.  A study of metal abundance patterns in cool white dwarfs. II - Simulations of accretion episodes , 1993 .

[15]  John H. Debes,et al.  Are There Unstable Planetary Systems around White Dwarfs , 2002 .

[16]  M. Jura,et al.  POLLUTION OF SINGLE WHITE DWARFS BY ACCRETION OF MANY SMALL ASTEROIDS , 2008, 0802.4075.

[17]  F. Kupka,et al.  The Vienna Atomic Line Database: Present State and Future Development , 1997 .

[18]  M. SubbaRao,et al.  Spectroscopic Target Selection in the Sloan Digital Sky Survey: The Quasar Sample , 2002, astro-ph/0202251.

[19]  John F. Kielkopf,et al.  The effect of neutral nonresonant collisions on atomic spectral lines , 1982 .

[20]  J. Southworth,et al.  SDSS J104341.53+085558.2: a second white dwarf with a gaseous debris disc , 2007, 0705.0447.

[21]  A. Unsöld,et al.  Physik der Sternatmosphären , 1938 .

[22]  G. Fontaine,et al.  THE DISCOVERY OF THE MOST METAL-RICH WHITE DWARF: COMPOSITION OF A TIDALLY DISRUPTED EXTRASOLAR DWARF PLANET , 2010, 1006.3710.

[23]  H. C. Stempels,et al.  VALD{2: Progress of the Vienna Atomic Line Data Base ? , 1999 .

[24]  D. Pritchard,et al.  Collisional line broadening due to van der Waals potentials , 1984 .

[25]  Albrecht W. Hofmann,et al.  The chemical composition of the Earth , 1995 .

[26]  B. Zuckerman,et al.  CHEMICAL ABUNDANCES IN THE EXTERNALLY POLLUTED WHITE DWARF GD 40: EVIDENCE OF A ROCKY EXTRASOLAR MINOR PLANET , 2009, 0912.1422.

[27]  M. Asplund,et al.  The chemical composition of the Sun , 2009, 0909.0948.

[28]  Gilles Fontaine,et al.  A study of metal abundance patterns in cool white dwarfs. I: Time-dependent calculations of gravitational settling , 1992 .

[29]  S. K. Leggett,et al.  Photometric and Spectroscopic Analysis of Cool White Dwarfs with Trigonometric Parallax Measurements , 2000, astro-ph/0011286.

[30]  G. Fontaine,et al.  Carbon pollution in helium-rich white dwarf atmospheres: time-dependent calculations of the dredge-up process , 1986 .

[31]  An Initial Survey of White Dwarfs in the Sloan Digital Sky Survey , 2003, astro-ph/0305347.

[32]  J. Thorstensen,et al.  Observations of White Dwarfs in the Solar Neighborhood , 2004 .

[33]  Gilles Fontaine,et al.  A study of metal abundance patterns in cool white dwarfs. III: Comparison of the predictions of the two-phase accretion model with the observations , 1993 .

[34]  J. Farihi,et al.  STRENGTHENING THE CASE FOR ASTEROIDAL ACCRETION: EVIDENCE FOR SUBTLE AND DIVERSE DISKS AT WHITE DWARFS , 2010, 1003.2627.

[35]  E. Sion,et al.  THE WHITE DWARFS WITHIN 20 PARSECS OF THE SUN: KINEMATICS AND STATISTICS , 2009, 0910.1288.

[36]  D. Steeghs,et al.  Two White Dwarfs with Oxygen-Rich Atmospheres , 2009, Science.

[37]  M. A. Barstow,et al.  Rocky planetesimals as the origin of metals in DZ stars , 2010, 1001.5025.

[38]  M. Jura.  A Tidally Disrupted Asteroid around the White Dwarf G29-38 , 2003 .

[39]  M. Jura,et al.  Carbon Deficiency in Externally Polluted White Dwarfs: Evidence for Accretion of Asteroids , 2006, astro-ph/0609045.