Testing Models for Stripped Low-mass Stars with Gaia

When low-mass red giants lose their envelopes to a companion just before the helium flash, the resulting mass transfer can result in a continuum of products ranging from fully stripped hot subdwarfs to partially stripped horizontal branch stars and undermassive red clump stars. Population synthesis models also suggest that young, metal-rich RR Lyrae can form when partial stripping leaves a helium-burning star in the instability strip. To test these predictions, I generated epoch astrometry for a simulated Galactic population of low-mass stripped star binaries using Gaia’s scanning law and fitted it with the cascade of astrometric models applied in Gaia DR3. I compared the simulated population to DR3 observations of hot subdwarfs, RR Lyrae, and red giants with high astrometric mass functions. I found that the adopted population synthesis model significantly overpredicted the number of hot subdwarfs with astrometric binary solutions, partly because the predicted flux ratios were more unequal than observed. It also predicted more than 100 RR Lyrae with DR3 astrometric orbital solutions, while none are observed. I concluded that RR Lyrae in au-scale binaries may be substantially rarer than predicted. In contrast, the model plausibly explained the population of red clump stars with high astrometric mass functions, which I interpreted to be potential black hole impostors. I predicted that about 10 times as many stripped-star binaries will be detectable in DR4, whose sensitivity to longer periods will more strongly test wide-orbit systems.

Read more about this research project in the associated publication here.