A search for main-sequence companions to subdwarf B stars using the Two Micron All Sky Survey

We explore the binary fraction of subdwarf B (sdB) stars by using the Two Micron All Sky Survey (2MASS) to search for main-sequence companions. Subdwarf B stars are extreme horizontal branch (EHB) stars with effective temperatures between 25,000 and 40,000 K. As such, they are extremely blue, making infrared color excess a useful method for detecting main-sequence (MS) companions between A0 and M2. We have convolved Kurucz models with appropriate 2MASS and B-filter bandpasses to examine how various combinations separate single sdB stars from sdB+MS binaries. We find that most infrared color combinations produce similar separations, with (BJ)-(JH) having the greatest separation. For our determination of the sdB+MS binary fraction, we use three different color index combinations as well as several tests to look for biases and selection effects. We notice that as sdB+MS sources are brighter than single sdB sources, they should preferentially be observed, increasing the fraction of inferred sdB+MS binaries. As such, we examine a magnitude-limited sample appropriate for single sdB; stars. We also examine the Palomar-Green (PG) survey selection effect against sdB+MS binaries by comparing it with newer surveys that can better detect sdB+MS binaries. From our complete sample, we infer an sdB+MS (A0 to M2) binary fraction of 53%+/-6%, whereas from our magnitude-limited sample, we infer an sdB+MS binary fraction of 20%+/-2% (including the PG selection effect). We discuss the implications for producing sdB stars via binary interactions.