Spin-orbit alignment in the very low mass binary regime

The L dwarf tight binary 2MASSW J0746425+200032AB

¹ Cahill Center for Astrophysics, California Institute of Technology, 1200 E. California Blvd., MC 249-17, Pasadena, CA 91125, USA
e-mail: lkh@astro.caltech.edu; gh@astro.caltech.edu
² Centre for Astronomy, National University of Ireland, University Road, Galway, Ireland
e-mail: ray.butler@nuigalway.ie
³ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto M5S 3H4, Ontario, Canada
e-mail: konopacky@di.utoronto.ca
⁴ JILA, University of Colorado, Boulder, CO 80309-0440, USA
e-mail: kaitlin.kratter@jila.colorado.edu
⁵ Vatican Observatory Research Group, Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
e-mail: boyle@ricci.as.arizona.edu
⁶ Department of Genetics (Computational Genetics), Albert Einstein College of Medicine, Bronx NY 10461, USA
e-mail: aaron.golden@einstein.yu.edu

Received: 7 December 2012
Accepted: 17 April 2013

Abstract

Studies of solar-type binaries have found coplanarity between the equatorial and orbital planes of systems with <40 AU separation. By comparison, the alignment of the equatorial and orbital axes in the substellar regime, and the associated implications for formation theory, are relatively poorly constrained. Here we present the discovery of the rotation period of 3.32 ±  0.15 h from 2MASS J0746+20A – the primary component of a tight (2.7 AU) ultracool dwarf binary system (L0+L1.5). The newly discovered period, together with the established period via radio observations of the other component, and the well constrained orbital parameters and rotational velocity measurements, allow us to infer alignment of the equatorial planes of both components with the orbital plane of the system to within 10 degrees. This result suggests that solar-type binary formation mechanisms may extend down into the brown dwarf mass range, and we consider a number of formation theories that may be applicable in this case. This is the first such observational result in the very low mass binary regime. In addition, the detected period of 3.32 ± 0.15 h implies that the reported radio period of 2.07 ± 0.002 h is associated with the secondary star, not the primary, as was previously claimed. This in turn refutes the claimed radius of 0.78 ± 0.1 R_J for 2MASS J0746+20A, which we demonstrate to be 0.99 ± 0.03 R_J.