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Published November 2019 | public
Journal Article

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

Abstract

We use a homogeneous catalogue 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 < M₁/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 1000–10 000 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 artefact 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 discs, 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 initial mass function but more bottom-heavy for wide binaries than for binaries with a ≲100 au.

Additional Information

2019 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. This article is published and distributed under the terms of the Oxford University Press, Standard Journals Publication Model (https://academic.oup.com/journals/pages/open_access/funder_policies/chorus/standard_publication_model). We are grateful to the referee, Andrei Tokovinin, for thoughtful comments. We thank Matthew Bate, Anthony Brown, Eugene Chiang, Ian Czekala, Paul Duffel, Morgan Fouesneau, Harshil Kamdar, Tomoaki Matsumoto, Chris Mckee, Eliot Quataert, and Dan Weisz for helpful discussions. KE-B was supported in part by an NSF graduate research fellowship and by SFB 881. HJT acknowledges the National Natural Science Foundation of China (NSFC) under grant 11873034. MM acknowledges financial support from NASA under Grant No. ATP-170070. This project was developed in part at the 2019 Santa Barbara Gaia Sprint, hosted by the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara. This research was supported in part at KITP by the Heising-Simons Foundation and the National Science Foundation under Grant No. NSF PHY-1748958. This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement.

Additional details

Created:
August 22, 2023
Modified:
October 24, 2023