ABSTRACT: We present the culmination of our near-infrared survey of the optically spectroscopically identified white dwarf stars from the McCook & Sion catalog, conducted using photometric data from the Two Micron All Sky Survey final All Sky Data Release. The color-selection technique, which identifies candidate binaries containing a white dwarf and a low mass stellar (or sub-stellar) companion via their distinctive locus in the near-infrared color-color diagram, is demonstrated to be simple to apply and to yield candidates with a high rate of subsequent confirmation. We recover 105 confirmed binaries, and identify 27 firm candidates (19 of which are new to this work) and 21 tentative candidates (17 of which are new to this work) from the 2MASS data. Only a small number of candidates from our survey have likely companion spectral types later than M5, none of which is an obvious L type (i.e., potential brown dwarf) companion. Only one previously known WD + brown dwarf binary is detected. This result is discussed in the context of the 2MASS detection limits, as well as other recent observational surveys that suggest a very low rate of formation (or survival) for binary stars with extreme mass ratios.

Figure 1 — Near-IR color-color diagram for the white dwarfs (WDs) from the McCook & Sion (1999) catalog that are detected in the 2MASS All Sky Data Release. Filled black points with 1σ error bars are the Good detections (i.e., photometric uncertainties in all three bands ≤ 0.1 mag). Unfilled grey points are the Moderate detections (i.e., photometric uncertainty > 0.1 mag in at least one band). Poor detections (i.e., no formal photometric uncertainty in one or more bands, signifying a low signal-to-noise value more properly treated as an upper limit) are not shown, except as noted below.

The squares are the two known WDs with circumstellar dust disks: WD2326+049 (G29-38; filled square, Good detection) and WD1729+371 (GD362; unfilled square, Poor detection). The cross-hatched regions show the loci of empirical mean 2MASS colors of the main sequence (green ///), giant branch (blue \\\), and L dwarfs (red horizontal cross-hatches). Spectral types are labeled at the correct (J–H) value, but offset in (H–K_s); spectral types of the main sequence and L dwarfs are labeled with roman font, while those of giants are labeled with italic font. The red dotted lines mark the boundary between “normal” WDs and red-excess WDs. The red, vertical dashed line marks the (H–K_s) color of a dM5 star for comparison.

Figure 2 — As in Figure 1, but, for clarity, the error bars have not been plotted. The six objects plotted with large red symbols are confirmed WD + low mass companion binary stars in which the companion is of spectral type later than M5.