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.


Physics, Astronomy, and Materials Science

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Publications of the Astronomical Society of the Pacific