What are the spectroscopic binaries with high mass functions near the Gaia DR3 main sequence?

Abstract

The 3rd data release of the Gaia mission includes orbital solutions for $> 10^5$ single-lined spectroscopic binaries, representing more than an order of magnitude increase in sample size over all previous studies. This dataset is a treasure trove for searches for quiescent black hole + normal star binaries. We investigate one population of black hole candidate binaries highlighted in the data release: sources near the main sequence in the color-magnitude diagram (CMD) with dynamically-inferred companion masses $M_2$ larger than the CMD-inferred mass of the luminous star. We model light curves, spectral energy distributions, and archival spectra of the 14 such objects in DR3 with high-significance orbital solutions and inferred $M_2 > 3\,M_{\odot}$. We find that 100\% of these sources are mass-transfer binaries containing a highly stripped lower giant donor ($0.2 \lesssim M/M_{\odot} \lesssim 0.4$) and much more massive ($2 \lesssim M/M_{\odot} \lesssim 2.5$) main-sequence accretor. The Gaia orbital solutions are for the donors, which contribute about half the light in the Gaia RVS bandpass but only $\lesssim 20\%$ in the $g-$band. The accretors' broad spectral features likely prevented the sources from being classified as double-lined. The donors are all close to Roche lobe-filling ($R/R_{\rm Roche\,lobe}>0.8$), but modeling suggests that a majority are detached ($R/R_{\rm Roche\,lobe}<1$). Binary evolution models predict that these systems will soon become detached helium white dwarf + main sequence "EL CVn" binaries. Our investigation highlights both the power of Gaia data for selecting interesting sub-populations of binaries and the ways in which binary evolution can bamboozle standard CMD-based stellar mass estimates.