Spin-orbit alignment in very low mass tight binaries: results for the 2MASS J0746425+200032AB and 2MASS J1314203+132001AB systems using spectroscopic and optically derived rotational estimates

¹ Department of Physics, College of Science, Al-Nahrain University, Baghdad, Iraq
² Vatican Observatory Research Group, Steward Observatory, University of Arizona, Tucson, AZ 85721, USA
³ Department of Computer and Software Engineering, Technological University of the Shannon, Athlone, Co. Westmeath, Ireland
⁴ Physics, School of Natural Sciences, College of Science & Engineering, University of Galway, Galway, Ireland
⁵ Centre for Astronomy, College of Science & Engineering, University of Galway, Galway, Ireland

Received: 9 March 2024
Accepted: 9 August 2024

Abstract

Context. Constraining the coplanarity status of very low mass (VLM) tight binary systems provides valuable information towards understanding the dominant mechanisms in star and planetary formation at the lower end of the initial mass function.

Aims. We sought to constrain the v sin i degeneracies of two nearby tight VLM binary systems, 2MASS J0746+20AB and 2MASS J1314+13AB, by independently determining each companion’s rotational period and so characterize each system’s spin-orbit alignment.

Methods. Long observational baseline high cadence I band photometry data were obtained for both systems using the Galway Ultra Fast Imager (GUFI) on the Mount Graham International Observatory’s Vatican Advanced Technology Telescope. Previously known rotational periods determined in other passbands were used as a basis for recovering additional periodic modulations in the time-series data.

Results. Using the known rotational period of 3.32 hours for 2MASS J0746425+200032A, we recovered an underlying periodic modulation of 2.14 ± 0.11 hours, which we associate with 2MASS J0746425+200032B. Breaking each components’ v sin i corresponds to equatorial inclination angles of 32 ± 4 degrees and 37 ± 4 degrees for components A & B respectively. We recover a weaker 2.06 ± 0.05 hours modulation separate from the known 3.79 hour signature for J1314203+132001B, which we associate with J1314203+132001A. We place a lower limit on J1314203+132001A’s equatorial inclination angle to be in the range of 24.5₋₃^+3.5 degrees, which deviates from the system’s orbital plane previously determined to be 49.34_−0.23^+0.28 degrees.

Conclusions. We confirm long term, consistent periodic modulations from both binary systems and report the first definitive rotational period for J1314203+132001A of 2.06 ± 0.05 hours, in addition to coplanarity to within 10 degrees in the spin-orbit alignment of the 2MASS J0746425+200032AB system. The lack of separate v sin i values for the 2MASS J1314203+132001AB system limits a definitive assessment but best estimates suggest coplanarity to be unlikely.