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

TL;DR

This study uses Gaia DR3 data to identify and analyze a population of mass-transfer binaries near the main sequence, revealing their true nature as stripped giants with massive companions and predicting their future evolution.

Contribution

The paper demonstrates that Gaia DR3 data can uncover hidden binary populations and clarifies the nature of high mass function systems as mass-transfer binaries with stripped donors.

Findings

All 14 high-mass-function systems studied are mass-transfer binaries with stripped giant donors.

Donors are close to Roche lobe-filling but mostly detached, affecting mass estimates.

Future evolution predicts these systems will become helium white dwarf + main sequence binaries.

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.