Evidence from stellar rotation of enhanced disc dispersal

Abstract

Context. The early stage of stellar evolution is characterized by a magnetic coupling between a star and its accretion disc, known as a star-disc locking mechanism. The disc-locking prevents the star to spin its rotation up, and its timescale depends on the disc lifetime, which should not be longer than about 10 Myr. Some mechanisms can significantly shorten this lifetime, allowing a few stars to start spinning up much earlier than other stars and increasing the observed rotation period dispersion among coeval stars. Aims. In the present study, we aim to investigate how the properties of the circumstellar environment can shorten the disc lifetime, more specifically the presence of a close stellar companion. Methods. We have identified a few multiple stellar systems, composed of stars with similar masses, which belong to associations with a known age. Since all parameters that are responsible for the rotational evolution, with the exception of environment properties and initial stellar rotation, are similar for all components, we expect that significant differences among the rotation periods can only arise from differences in the disc lifetimes. A photometric timeseries allowed us to measure the rotation periods of each component, while high-resolution spectra provided us with the fundamental parameters, v sin i and chromospheric line fluxes. Results. In the present study, we have collected timeseries photometry of BD−21 1074, a member of the 21 Myr old β Pictoris association, and measured the rotation periods of its brightest components A and B. They differ significantly, and the component B, which has a closer companion C, rotates faster than the more distant and isolated component A. It also displays a slightly higher chromospheric activity level. Conclusions. Since components A and B have similar mass, age, and initial chemical composition, we can ascribe the rotation period difference to either different initial rotation periods or different disc-locking phases arising from the presence of the close companion C. In the specific case of BD−21 1074, the second scenario seems to be more favored. However, a statistically meaningful sample is yet needed to be able to infer which scenario is more likely. In our hypothesis of different disc-locking phase, any planet orbiting this star, if found by future investigations, is likely formed very rapidly owing to a gravitational instability mechanism, rather than core accretion. Only a large difference of initial rotation periods alone could account for the observed period difference, leaving comparable disc lifetimes.

Is the β Pictoris member GJ 3039AB a physical binary? What the rotation periods tell us?

“…Moreover, when a star has a companion in a close orbit, the rotation period allows us to shed light on the early stage of its stellar evolution, specifically on the primordial disc lifetime. In fact, the faster rotation that we observe in a close binary with respect to an equal-mass single star reveals that gravitational effects have enhanced the disc dispersal, and shortened the disc-locking duration (see, e.g., Messina et al 2014Messina et al , 2015aMessina et al , 2016a. The rotation period combined with the v sin i can put constraints on the value of the stellar radius since the inclination of the stellar rotation axis must be i < 90…”

Reaching the boundary between stellar kinematic groups and very wide binaries

“…When available, we gathered masses of single early type stars and close binaries from the literature (e.g., Strassmeier & Rice 2000;Neuhäuser et al 2002;Caballero 2009;Donati et al 2011;Janson et al 2012;Elliott et al 2015;Montet et al 2015) or suitable information that allowed us to make a precise derivation (e.g., magnitude differences from adaptive optics or lucky imaging, mass ratios from spectroscopic monitoring - Chauvin et al 2010;Neuhäuser et al 2011;Janson et al 2012;Messina et al 2014;Bowler et al 2015;Elliott et al 2015). Masses range approximately from 2.4 M for η Tel A to well below the substellar boundary for L 186-67 B with a broad maximum of the distribution at 0.5-1.0 M .…”

“…Derived masses reasonably match those expected from spectral types, when available. For the sake of completeness, we also list spectral types compiled from a number of sources in Table A.5 (Riaz et al 2006;Reid et al 2007;Pickles & Depagne 2010;Janson et al 2012;Kraus et al 2014;Messina et al 2014;Rodríguez et al 2014;Mason et al 2015;I. Gallardo & M. Gómez Garrido, priv.…”

HADES RV Programme with HARPS-N at TNG

“…Despite these difficulties, some studies suggest that the connection between age, rotation, and activity may also hold in early-M dwarfs (e.g. Delfosse et al 1998;Messina et al 2003;Mohanty & Basri 2003;Pizzolato et al 2003;West et al 2004;Kiraga & Stepien 2007;Browning et al 2010;Reiners et al 2012;Stelzer et al 2013b;West et al 2015), although deviations in the case of close M binaries have been reported (Messina et al 2014).…”