White Dwarfs and Hot Subdwarf Stars


Photo of Daniel Eisenstein
Daniel Eisenstein
Harvard-Smithsonian Center for Astrophysics


Spectra of white dwarf candidates in the SDSS Data Release 7 imaging area (7,430 deg2)

Finding Targets

An object whose ANCILLARY_TARGET1 value includes one or more of the bitmasks in the following table was targeted for spectroscopy as part of this ancillary target program. See SDSS bitmasks to learn how to use these values to identify objects in this ancillary target program.

Note the caveat regarding observations of these targets on some early BOSS plates

Program (bit name) Bit number Target Description Number of Fibers Number of Unique Primary Objects
WHITEDWARF_NEW 42 White dwarf candidate whose spectrum had not been observed previously by the SDSS 4,997 4,550
WHITEDWARF_SDSS 43 White dwarf candidate with a pre-existing SDSS spectrum 3,941 3,376


The original SDSS data massively increased the number of known white dwarfs, which had an enormous impact on the science of these stars. However, target selection considerations of the original SDSS meant that white dwarf selection was incomplete. This ancillary target program measured spectra of thousands of white dwarfs that were missed by prior SDSS spectroscopic surveys.

SDSS multi-color imaging efficiently distinguishes hot white dwarf and subdwarf stars from the bulk of the stellar and quasar loci in color-color space (Harris et al. 2003). Special target classes in SDSS produced the world’s largest spectroscopic samples of white dwarfs (Kleinman et al. 2004; Eisenstein et al. 2006). However, much of SDSS white dwarf targeting required that the objects be unblended, which caused many brighter white dwarfs to be skipped (for a detailed discussion, see Section 5.6 of Eisenstein et al. 2006). This BOSS ancillary targeting program relaxes this requirement and imposes color cuts to focus on warm and hot white dwarfs. Importantly, the BOSS spectral range extends further into the UV, allowing full coverage of the Balmer lines.

Target Selection

We require targets to be point sources with clean u, g, and r photometry (following the clean point source selection from the DR7 documentation), and USNO counterparts. We restrict to regions inside the DR7 imaging footprint and require that (all magnitudes quoted are extinction-corrected model magnitudes):

  • Identified as a star in imaging (type=6)
  • Clean photometry data (see the Clean Photometry tutorial)
  • Galactic extinction Ar < 0.5 mag (extinction_r < 0.5)
  • g < 19.2 (dered_g < 19.2)
  • (u-r) < 0.4 (dered_u - dered_r < 0.4)
  • -1 < (u-g) < 0.3 (dered_u - dered_g > -1 AND dered_u - dered_g < 0.3)
  • -1 < (g-r) < 0.5 (dered_g - dered_r > -1 AND dered_g - dered_r < 0.5)

Additionally, targets that do not have (u-r) < -0.1 and (g-r) < -0.1 must have USNO proper motions of more than 2 arcsec per century:

[propermotion > 2 || (dered_g-dered_r < -0.15 && dered_u-dered_r < -0.2)]

Objects satisfying the selection criteria that had not observed in previously by the SDSS are denoted by the WHITEDWARF_NEW target flag, while those with prior SDSS spectra are assigned the WHITEDWARF_SDSS flag. Some of the latter were re-observed with BOSS in order to obtain the extended wavelength coverage that the BOSS spectrograph offers.

The color selection used here includes DA stars with temperatures above ~14,000 K, helium atmosphere white dwarfs above ~8000 K, as well as many rarer classes of white dwarfs. Hot subdwarfs (sdB and sdO) are included as well. Many of these stars are excellent spectrophotometric standards, and can be tested in comparison to the BOSS F-star calibration.

SQL Query Used in Target Selection

   SELECT ................ 
   JOIN Star s
        ON u.objID = s.objID
   JOIN Field f
        ON f.fieldID = s.fieldID
   LEFT OUTER JOIN SpecObjAll sp 
       ON s.specObjID=sp.specObjID
       dered_g < 19.2 
       AND extinction_r < 0.5 
       AND dered_u-dered_r < 0.4 
       AND dered_g-dered_r < 0.5 
       AND dered_g-dered_r > -1 
       AND dered_u-dered_g < 0.3 
       AND dered_u-dered_g > -1 
       AND ( u.propermotion > 2 OR ( dered_g-dered_r < -0.1 AND dered_u-dered_r < -0.1)) AND ((flags_u & 0x10000000) != 0) AND ((flags_u & 0x8100000c00a4) = 0) AND (((flags_u & 0x400000000000) = 0) or (psfmagerr_u <= 0.2)) AND (((flags_u & 0x100000000000) = 0) or (flags_u & 0x1000) = 0) AND ((flags_g & 0x10000000) != 0) AND ((flags_g & 0x8100000c00a4) = 0) AND (((flags_g & 0x400000000000) = 0) or (psfmagerr_g <= 0.2)) AND (((flags_g & 0x100000000000) = 0) or (flags_g & 0x1000) = 0) AND ((flags_r & 0x10000000) != 0) AND ((flags_r & 0x8100000c00a4) = 0) AND (((flags_r & 0x400000000000) = 0) or (psfmagerr_r <= 0.2)) AND (((flags_r & 0x100000000000) = 0) or (flags_r & 0x1000) = 0) 


Eisenstein, D. J., et al., 2006, ApJS, 167, 40, doi:10.1086/507110
Harris, H. C., et al., 2003, AJ, 126, 1023, doi:10.1086/376842
Kleinman, S. J., et al., 2004, ApJ, 607, 426, doi:10.1086/383464