Testing models for fully and partially stripped low-mass stars with Gaia: Implications for hot subdwarfs, binary RR Lyrae, and black hole impostors

Abstract

When low-mass ($\lesssim 2$ $M_{\odot}$) red giants lose their envelopes to a companion just before the helium flash, the resulting mass transfer can produce binaries hosting hot subdwarfs, horizontal branch stars, and undermassive red clump stars. Recent work predicts a continuum of such products, from fully stripped hot subdwarfs to partially stripped horizontal branch and red clump stars, and suggests that young, metal-rich RR Lyrae can form when partial stripping leaves a helium-burning star in the instability strip. To enable direct comparison with observations, we model these binaries in a simulated Milky Way-like galaxy with a realistic metallicity-dependent star formation history and 3D dust map, generate epoch astrometry using Gaia's scanning law, and fit it with the cascade of astrometric models applied in Gaia DR3. We compare the simulated population to DR3 observations of hot subdwarfs, RR Lyrae, and red giants with high astrometric mass functions. The model significantly overpredicts the number of hot subdwarfs with astrometric binary solutions, partly because the predicted flux ratios are more unequal than observed. It also predicts $\gtrsim 100$ RR Lyrae with DR3 astrometric orbital solutions, while none are observed. We conclude that RR Lyrae in au-scale binaries may be substantially rarer than predicted. In contrast, the model plausibly explains the population of red clump stars with high astrometric mass functions, which we interpret as potential black hole impostors. We predict that $\sim 10 \times$ more stripped-star binaries will be detectable in DR4, whose sensitivity to longer periods will more strongly test wide-orbit systems.

Figures (11)

• Figure 1: Extinction-corrected color-magnitude diagrams (CMDs) of both the simulated stripped star binaries and their individual components. We show a random sample of sources from Gaia DR3 within 100 pc for comparison. The stripped stars lie in the red clump, horizontal branch, and hot subdwarf regions of the CMD. The light contributions of the luminous companions, which are MS stars and giants, add scatter to the CMD positions of the unresolved sources.
• Figure 2: Orbital period vs. distance for the predicted population of stripped star binaries. For clarity, we show a random sample of 1% of the binaries. We plot curves corresponding to $a_0 = 0.1$ mas (which is an approximate lower limit on the angular semi-major axis of detectable photocenter orbits in DR3) for different flux ratios $S \equiv F_2 / F_1$, assuming a typical stripped star mass of $0.5\,M_{\odot}$ and a typical main sequence star mass of $1\,M_{\odot}$. Binaries with $a_0 < 0.1$ mas are plotted in gray with lower opacity. We show the approximate upper detectable limit on the orbital period in DR3 ($\sim 1000$ d) with a dash-dotted line and the corresponding limit in DR4 ($\sim 2000$ d) with a dashed line. The majority of stripped star binaries are amenable to detection via Gaia astrometry.
• Figure 3: Mock DR3 observations of two unresolved $\approx 0.47\,M_{\odot}$ sdB binaries. In the left panel, the binary hosts an $\approx 0.6\,M_{\odot}$ companion, leading to a small secondary-to-primary flux ratio of $\approx 0.136$. In the right panel, the companion mass is doubled to $\approx 1.2\,M_{\odot}$ with all other parameters held constant, leading to a larger flux ratio of $\approx 2.403$. Due to the larger photocenter orbit, the gaiamock pipeline predicts that only the binary on the left receives an astrometric binary solution in DR3, underscoring the fact that detectability depends significantly on flux ratio.
• Figure 4: Simulated sdB binaries within 500 pc in the Gaia color-magnitude diagram (CMD). Many of these binaries are duplicates located at different Galactic coordinates. For comparison, we also show the known wide composite sdB binaries with published astrometric geier_dr2_2019culpan_subdwarf_2022 or spectroscopic barlow_two_2013molina_wide_2026 orbital solutions. The majority of the observed systems pass the cuts used by vos_bobrick_2020 to identify composite sdB binaries. On the other hand, many simulated sdB binaries fall outside the red, dotted line, which is used by vos_bobrick_2020 (following geier_dr2_2019) to exclude main sequence stars. It appears that this color cut is rather conservative, since several known composite sdBs with published orbits fall redward of it. We use open circles to denote simulated sdB binaries that are not predicted to show infrared excess, and hence would not be observationally identified as composite nemeth_hot_2012. These systems lie toward the blue end of the CMD.
• Figure 5: Median number of sdBs in composite binaries predicted to receive astrometric binary solutions in DR3 across 10 realizations. The error bars signify the middle 68% of counts. The hatched bars show the number of sdB binaries in each category before the cuts used to identify composite systems are applied. We compare predictions against observations, finding that the adopted model significantly overestimates the number of composite sdB systems that receive astrometric binary solutions.