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

Abstract

The third data release of the Gaia mission includes orbital solutions for >10⁵ single-lined spectroscopic binaries, representing more than an order of magnitude increase in sample size over all previous studies. This data set 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 colour–magnitude diagram (CMD) with dynamically inferred companion masses M₂ 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 M₂ > 3 M_⊙⁠. We find that 100 per cent of these sources are mass-transfer binaries containing a highly stripped lower giant donor (0.2 ≲ M/M_⊙ ≲ 0.4) and much more massive (2 ≲ M/M_⊙ ≲ 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 ≲20 per cent 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_(Roche lobe) > 0.8⁠), but modelling suggests that a majority are detached (⁠R/R_(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 subpopulations of binaries and the ways in which binary evolution can bamboozle standard CMD-based stellar-mass estimates.

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