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.

ABSTRACT: Our understanding of the formation and evolution of magnetic cataclysmic variables from initially detached, post-common envelope binary stars containing a magnetic white dwarf and a main sequence star is currently poorly constrained due to the lack of observational identification of the progenitor systems. Very few potential pre-cataclysmic variables containing a magnetic white dwarf are known, compared with approximately 25% of the cataclysmic variable population that contain magnetic white dwarfs. We present the results of a search for candidate binary systems containing a magnetic white dwarf, which utilized photometric data from the Two Micron All Sky Survey (as well as Hubble Space Telescope and WIYN Observatory follow-up data for several objects). Our target sample was drawn primarily from the comprehensive list of magnetic white dwarfs by Wickramasinghe & Ferrario, plus several recently identified objects selected from the literature. Out of 51 2MASS detections, no convincing binary candidates were found. However, six objects merit additional observation to determine the origin of a small near-infrared excess that could be attributed to a very low mass stellar or substellar companion. An additional four white dwarfs are possibly at the centers of previously unknown, and likely unresolved, planetary nebulae. (Support for this research was provided by NASA under an ADP grant issued through the Office of Space Science.)

ABSTRACT: We present near-infrared magnitudes for all white dwarfs (selected from the catalog of McCook & Sion) contained in the Two Micron All Sky Survey (2MASS) Second Incremental Data Release (2IDR). We show that the near-IR color-color diagram is an effective means of identifying candidate binary stars containing a WD and a low mass main sequence star. The loci of single WDs and WD + red dwarf binaries occupy distinct regions of the near-IR color-color diagram. We recovered all known unresolved WD + red dwarf binaries located in the 2IDR sky coverage, and also identified as many new candidate binaries (47 new candidates out of 95 total). Using observational near-IR data for WDs and M–L dwarfs, we have compared a sample of simulated WD + red dwarf binaries with our 2MASS data. The colors of the simulated binaries are dominated by the low mass companion through the late-M to early-L spectral types. As the spectral type of the companion becomes progressively later, however, the colors of unresolved binaries become progressively bluer. Binaries containing the lowest mass companions will be difficult to distinguish from single WDs solely on the basis of their near-IR colors.