Searching for long period subdwarf B systems using TESS data

Subdwarf B (sdB) stars are extreme horizontal branch stars with high temperature and gravity. The most promising formation scenarios involve close binary star evolution with three different channels: (1) a Common Envelope (CE) channel, which can produce short period (P=0.1–10 d) sdB + white dwarf (WD) or main sequence (MS) binaries, (2) a Roche lobe overflow (RLOF) channel, which results in a long period (450 < P < 1400 d) sdB + MS binaries, and (3) a WD merger channel, which can produce single sdB stars. To test these scenarios, population studies to estimate the member percentages in each channel are essential. Prior observational studies focused mostly on the CE channel because the short period binaries produced by this channel are easier to detect and characterize using eclipse or radial velocity (RV) methods, and as a result approximately 150 of these systems are currently known. Observational studies of the RLOF and merger channels are difficult because confirming long-period binary systems or single stars requires long-term observing campaigns, and the eclipse and RV methods are inefficient for such cases. Approximately 30% of sdB stars show stable pulsations and for these, the pulsation timing method remains the only effective tool to search for long-period binary systems or single stars. Positive detection of a binary system can be obtained from a periodic change of a pulsation mode phase, which is a consequence of a star’s reflex motion. Consequently, we can confirm single stars by non-detection of such a phase variation. TESS photometry is a powerful tool for this study because TESS data are continuous for 27 to as many as 324 days. We employ TESS and follow-up ground-based observations to search for companions to sdB stars via this pulsating timing variation method. We further supplement these constraints by comparing our targets in color-color diagrams to different types of sdB binaries. Our study increases the sample of long-period binaries and single sdBs, allowing for important new constraints on the formation channels of sdB stars.