RX J0942.7−7726AB: an isolated pre-main-sequence wide binary

Authors

  • Simon J. Murphy,

    Corresponding author
    1. Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia
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  • Warrick A. Lawson,

    Corresponding author
    1. School of PEMS, University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia
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  • Michael S. Bessell

    Corresponding author
    1. Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia
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E-mail: murphysj@mso.anu.edu.au (SJM); w.lawson@adfa.edu.au (WAL); bessell@mso.anu.edu.au (MSB)

ABSTRACT

We report the discovery of two young M-dwarfs, RX J0942.7−7726 (M1) and 2MASS J09424157−7727130 (M4.5), that were found only 42 arcsec apart in a survey for pre-main-sequence stars surrounding the open cluster η Chamaeleontis. Both stars have congruent proper motions and near-infrared photometry. Medium-resolution spectroscopy reveals that they are coeval (age 8–12 Myr), codistant (100–150 pc) and thus almost certainly form a true wide binary with a projected separation of 4000–6000 au. The system appears too old and dynamically fragile to have originated in η Cha and a trace-back analysis argues for its birth in or near the Scorpius–Centaurus OB association (Sco–Cen). RX J0942.7−7726AB joins a growing group of wide binaries kinematically linked to Sco–Cen, suggesting that such fragile systems can survive the turbulent environment of their natal molecular clouds while still being dispersed with large velocities. Conversely, the small radial velocity difference between the stars (2.7 ± 1.0 km s−1) could mean the system is unbound, a result of the coincidental ejection of two single stars with similar velocity vectors from the OB association early in its evolution.

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