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M.A. Stark, R.A. Wade (Penn State)
Utilizing the Two Micron All Sky Survey (2MASS) Second Incremental Data Release Catalog, we have retrieved near-IR magnitudes for several hundred hot subdwarfs (sdO and sdB stars). Combining 2MASS photometry with visual photometry (Johnson BV or Strömgren by) available in the literature, we show that it is possible to identify hot subdwarf stars that exhibit IR excesses. This excess can be attributed to the presence of unresolved cool companions. The observed colors can be understood by considering contributions from two stars, for example: the J--KS and V--KS colors can be modeled as a combination of the hot subdwarf and a cool companion. Additionally, spectroscopic observations which verify the composite nature of the system exist for a number of subdwarfs we find have IR excess.
We find that a large fraction of the hot subdwarfs examined exhibit IR excess. Should all of the observed IR excesses be due to unresolved companions (~F and later spectral types), then the fraction of hot subdwarfs in composite-spectrum binary systems is significantly larger than that reported in other, smaller, spectroscopic surveys. If these unresolved pairs are in fact close binaries, this result may have interesting consequences for the hypothesis that hot subdwarfs originate via a binary process.
This work was supported in part by NASA, and made use of the 2MASS Second Incremental Data Release Point Source Catalog.