Rotation of Late-type Stars in Praesepe with K2

Abstract

We have Fourier-analyzed 941 K2 light curves (LCs) of likely members of Praesepe, measuring periods for 86% and increasing the number of rotation periods (P) by nearly a factor of four. The distribution of P versus -( ) V K s , a mass proxy, has three different regimes:, where the rotation rate rapidly slows as mass decreases; 1.3< -( ) V K s <4.5, where the rotation rate slows more gradually as mass decreases; andwhere the rotation rate rapidly increases as mass decreases. In this last regime, there is a bimodal distribution of periods, with few between ∼2 and ∼10 days. We interpret this to mean that once M stars start to slow down, they do so rapidly. The K2 period-color distribution in Praesepe (∼790 Myr) is much different than that in the Pleiades (∼125 Myr) for late F, G, K, and early-M stars; the overall distribution moves to longer periods and is better described by two line segments. For mid-M stars, the relationship has a similarly broad scatter and is steeper in Praesepe. The diversity of LCs and of periodogram types is similar in the two clusters; about a quarter of the periodic stars in both clusters have multiple significant periods. Multi-periodic stars dominate among the higher masses, starting at a bluer color in Praesepe ( -( ) V K s ∼1.5) than in the Pleiades ( -( ) V K s ∼2.6). In Praesepe, there are relatively more LCs that have two widely separated periods, D > P 6 days. Some of these could be examples of M star binaries where one star has spun down but the other has not.

M Dwarf Rotation from the K2 Young Clusters to the Field. I. A Mass–Rotation Correlation at 10 Myr

“…To demonstrate this, we show in Figure 9 empirical estimates of the amount of AM lost from Pleiades stars since the age of Upper Sco (blue), assuming solid-body rotation. We also show a similar calculation for stars in the ∼700Myr Praesepe cluster (purple), with rotation rates from Rebull et al (2017), and masses estimated in the same fashion as the Pleiades. On the left, we compare these data to predictions from Kawaler (1988) solid-body models (green) for fast, median, and slow initial rotation rates.…”

“…However, the fidelity of these models at early ages is largely due to the simplistic physics of PMS contraction, and does not suggest that the models make accurate and precise predictions thereafter, when the details of the wind laws and internal AM transport become all important. In upcoming work (G. Somers et al 2017, in preparation), we will examine the longer term spin down of low-mass stars using Praesepe rotation rates from K2 (Rebull et al 2017) and field rotation rates from Mdwarfs in the solar neighborhood (e.g., Newton et al 2016;Stelzer et al 2016).…”

“…K2 has also observed at least three associations with ages 10 Myr  , namely Taurus-Auriga, ρ Ophiuchus (Oph), and Upper Scorpius. Already, rotation periods for thousands of stars in these systems have been deduced, including ∼750 members of the Pleiades (Rebull et al 2016a(Rebull et al , 2016bStauffer et al 2016), ∼800 members of Praesepe (Rebull et al 2017;Douglas et al 2017), 65 members of Hyades (Douglas et al 2017), and 16 brown dwarfs in the Upper Sco association (Scholz et al 2015). Moreover, our team has derived as-yetunpublished rotation periods for hundreds of additional Upper Sco members from ∼0.05 to 2 M  (L. Rebull et al 2017, in preparation).…”

“…This method involves tuning crit, w  such that stars with rapid initial rotation rates reproduce the fastest rotating 1 M  stars in a young (t 100 Myr) cluster, and tuning F K so that stars with average initial rotation rates produce the rotation rate of the slowest stars in an older cluster. For this purpose, we use the stars of mass 0.95-1.05 M  in the K2 rotation distributions of the Pleiades and Praesepe (Rebull et al 2017), and determine fast and median initial conditions from the rotation rates given by Moraux et al (2013) for the 13Myr open cluster hPersei (see Somers & Pinsonneault 2015b, for more details). The final calibrated parameters are listed in Table 1.…”

New Low-mass Eclipsing Binary Systems in Praesepe Discovered by K2

“…Praesepe is a relatively nearby, metal-rich, several hundred Myr cluster hosting >1000 high-probability members (>80%) and more than 100 candidate members (>50% probability; Kraus & Hillenbrand 2007;Rebull et al 2017). Given its richness and proximity, Praesepe is a well-studied benchmark cluster.…”