A Universal Spin–Mass Relation for Brown Dwarfs and Planets

Abstract

While brown dwarfs show similarities to stars early in their lives, their spin evolutions are much more akin to those of planets. We have used light curves from the K2 mission to measure new rotation periods for 18 young brown dwarfs in the Taurus star-forming region. Our sample spans masses from 0.02 to 0.08 M e and has been characterized extensively in the past. To search for periods, we utilize three different methods (autocorrelation, periodogram, Gaussian processes). The median period for brown dwarfs with disks is twice as long as for those without (3.1 versus 1.6 days), a signature of rotational braking by the disk, albeit with small numbers. With an overall median period of 1.9 days, brown dwarfs in Taurus rotate slower than their counterparts in somewhat older (3-10 Myr) star-forming regions, consistent with spin-up of the latter due to contraction and angular momentum conservation, a clear sign that disk braking overall is inefficient and/or temporary in this mass domain. We confirm the presence of a linear increase of the typical rotation period as a function of mass in the substellar regime. The rotational velocities, when calculated forward to the age of the solar system, assuming angular momentum conservation, fit the known spin-mass relation for solar system planets and extra-solar planetary-mass objects. This spin-mass trend holds over six orders of magnitude in mass, including objects from several different formation paths. Our result implies that brown dwarfs by and large retain their primordial angular momentum through the first few Myr of their evolution.

Cloud Atlas: High-contrast Time-resolved Observations of Planetary-mass Companions

“…E.g., Snellen et al (2014); Biller et al (2015); Zhou et al (2016) pointed out a linear trend between mass and rotational rate -the more massive the planet, the faster it spins. Scholz et al (2018) found a ∝ 1∕2 relationship between spin velocity and mass that fits simultaneously for solar system planets, planetary- Figure 18. Spin velocity and mass relationship for solar system planets, exoplanets, planetary-mass companions, and brown dwarfs.…”

“…The lower panel zooms in for gas giants or more massive substellar objects. For the brown dwarfs that are in sample of Scholz et al (2018), we plot both the observed spin velocity (pink open triangles) and the derived final spin velocity (solid open triangles) assuming angular momentum conserved contraction. The orange line and shade are the best-fit linear relation between rotational period and log( ) for five solar system planets (Mars, Jupiter, Saturn, Uranus, Neptune) and the 1 uncertainty.…”

“…To investigate the spin-mass relation for planetary-mass companions and brown dwarfs, we collected the entire sample of planetary-mass objects that have rotation measurements available as well as two representative samples for brown dwarfs (Metchev et al 2015;Scholz et al 2018) and plot the spin velocity mass relation in a log − log plot (Figure18). We reaffirm the caveat and limitation of our Lomb-Scargle periodogram based measurements and the possibility that the detected quasi-periodic signals are lower limit of the real rotational periods.…”

“…For the field brown dwarf sample from Metchev et al (2015), we use a radius of 1 Jup for the conversion. For the young brown dwarf sample from Scholz et al (2018), we use 4 Jup as their current radii and 1 Jup as the final radii and plot spin velocities for both calculations. We note that the 4 Jup is an estimate from substellar evolution models assuming "hot start" formation scenario.…”

Evidence for disc regulation in the lowest mass stars of the young stellar cluster NGC 2264

“…Rodríguez-Ledesma et al (2010) find a disc-rotation connection in the 0.075 to 0.4 M mass regime from a study of the K-band excesses of the periodic stars in the ONC, while Cody & Hillenbrand (2010) find that stars in this mass regime with and without a disc have very similar period distributions in the σ-Ori cluster. Scholz et al (2018) also find hints for disc regulation in the brown dwarf regime, although with a very small sample of only 25 objects.…”