Discovery of an equal-mass ‘twin’ binary population reaching 1000 + au separations

Abstract

We use a homogeneous catalog of 42,000 main-sequence wide binaries identified by Gaia to measure the mass ratio distribution, p(q), of binaries with primary masses 0.1 < M 1 /M < 2.5, mass ratios 0.1 q < 1, and separations 50 < s/AU < 50, 000. A well-understood selection function allows us to constrain p(q) in 35 independent bins of primary mass and separation, with hundreds to thousands of binaries in each bin. Our investigation reveals a sharp excess of equal-mass "twin" binaries that is statistically significant out to separations of 1,000 to 10,000 AU, depending on primary mass. The excess is narrow: a steep increase in p(q) at 0.95 q < 1, with no significant excess at q 0.95. A range of tests confirm the signal is real, not a data artifact or selection effect. Combining the Gaia constraints with those from close binaries, we show that the twin excess decreases with increasing separation, but its width (q 0.95) is constant over 0.01 < a/AU < 10, 000. The wide twin population would be difficult to explain if the components of all wide binaries formed via core fragmentation, which is not expected to produce strongly correlated component masses. We conjecture that wide twins formed at closer separations (a 100 AU), likely via accretion from circumbinary disks, and were subsequently widened by dynamical interactions in their birth environments. The separation-dependence of the twin excess then constrains the efficiency of dynamical widening and disruption of binaries in young clusters. We also constrain p(q) across 0.1 q < 1. Besides changes in the twin fraction, p(q) is independent of separation at fixed primary mass over 100 s/AU < 50, 000. It is flatter than expected for random pairings from the IMF but more bottom-heavy for wide binaries than for binaries with a 100 AU.

Bayesian Characterization of Main-sequence Binaries in the Old Open Cluster NGC 188

“…in Geller & Mathieu 2012), as well as in the field(Raghavan et al 2010;Moe & Di Stefano 2017;El-Badry …”

Binary Star Population with Common Proper Motion in Gaia DR2

“…For such pairs, the masses of components are estimated. Independently from [21]], a confirmation of existence of an excess of binary systems"twins"-with close masses of components is obtained. It is shown that the detection of the peak in the catalog sample is not a consequence of its selectivity function.…”

“…Therefore, the smaller it is, the more pronounced the "twin peak" should be. A detailed review and discussion of possible channels for the formation of an excess of wide binary stars with close masses of components are presented in [21]and references therein. It is important to note that the most probable mechanism is the competitive accretion onto the forming pair components in a common protostellar disk, in which the component with an initially smaller mass, moving in the orbit with a large radius, can come up in mass with the initially more massive star.…”

Pairing function of visual binary stars

“…Important characteristic of the ( ) distribution is the probable increased frequency of equal-mass binaries ( ∼ 1). Originally found for close spectroscopic systems (Lucy & Ricco 1979), it is believed to be a common feature for a broad separation range (Söderhjelm 2007;El-Badry et al 2019). The origin of twin stars is being discussed (Tokovinin 2000;Tokovinin & Moe 2020;Adams et al 2020).…”

“…Fortunately, there are independent datasets containing wide systems which are used to verify the completeness of the WDS. I perform cross-identification of the WDS with Andrews et al (2017) and El-Badry et al (2019) lists of binaries based on the Tycho-Gaia Astrometric Solution (TGAS) (Gaia Collaboration et al 2016) and Gaia DR2 (Gaia Collaboration et al 2018) respectively. Particularly, I search for the systems which are present in the referred Gaia-based datasets but absent from the WDS.…”