Rotation of Low-mass Stars in Upper Scorpius and ρ Ophiuchus with K2

Abstract

We present an analysis of K2 light curves (LCs) for candidate members of the young Upper Sco (USco) association (∼8 Myr) and the neighboring ρ Oph embedded cluster (∼1 Myr). We establish ∼1300 stars as probable members, ∼80% of which are periodic. The phased LCs have a variety of shapes which can be attributed to physical causes ranging from stellar pulsation and stellar rotation to disk-related phenomena. We identify and discuss a number of observed behaviors. The periods are ∼0.2-30 days with a peak near 2 days and the rapid period end nearing breakup velocity. M stars in the young USco region rotate systematically faster than GK stars, a pattern also present in K2 data for the older Pleiades and Praesepe systems. At higher masses (types FGK), the well-defined period-color relationship for slowly rotating stars seen in the Pleiades and Praesepe systems is not yet present in USco. Circumstellar disks are present predominantly among the more slowly rotating M stars in USco, with few disks in the subday rotators. However, M dwarfs with disks rotate faster on average than FGK systems with disks. For four of these disked M dwarfs, we provide direct evidence for disk locking based on the K2 LC morphologies. Our preliminary analysis shows a relatively mass-independent spin-up by a factor of ∼3.5 between USco and the Pleiades, then mass-dependent spin-down between Pleiades and Praesepe.

Rotation of Late-type Stars in Praesepe with K2

“…Much of our current analysis is very similar to that we conducted in the Pleiades (Rebull et al 2016a(Rebull et al , 2016bStauffer et al 2016), and we often make comparisons to the Pleiades analysis and results. In Section 2, we present the observations and data reduction, as well as the range of data from the literature that we assembled, and define the final sample of members that we analyze in the rest of the paper.…”

“…In the Pleiades, we determined 92% of the sample to be periodic (Rebull et al 2016b); here, we obtain 809/941=86% of the member sample to be periodic.…”

“…In the subsequent analysis here, we often compare Praesepe results to those from the Pleiades (Rebull et al 2016a(Rebull et al , 2016b. We take the age of the Pleiades to be 125 Myr , the distance to be 136 pc (Melis et al 2014), and the metallicity to be [Fe/H]=+0.04 (Soderblom et al 2009 BVRI photometry on the Kron system has been published by Upgren et al (1979), Weis (1981), Stauffer (1982), and Mermilliod et al (1990).…”

“…Our approach for finding periods was identical to what we used in the Pleiades in Rebull et al (2016b). In summary, we used the Lomb-Scargle (LS; Scargle 1982) approach as implemented by the NASA Exoplanet Archive Periodogram Service 12 (Akeson et al 2013).…”

“…Since these data were reduced at the same time as we reduced our Pleiades data (Rebull et al 2016b), we have the same sets of LCs available to us: (1) the pre-search data conditioning (PDC) version generated by the Kepler project and obtained from MAST, the Mikulski Archive for Space Telescopes, (2) a version with moving apertures obtained following A. M. Cody et al (2017, in preparation), (3) the version using a semiparametric Gaussian process model used by Aigrain et al (2015Aigrain et al ( , 2016, and (4) the "self-flat-fielding" approach used by Vanderburg & Johnson (2014) as obtained from MAST. To this, late in the process, we added (5) the LCs from the EVEREST pipeline (Luger et al 2016), which uses pixel level decorrelation.…”