Poking the Beehive from Space: K2 Rotation Periods for Praesepe

Abstract

We analyze K2 light curves for 794 low-mass (1 > ∼ M * > ∼ 0.1 M ⊙ ) members of the ≈650-Myr-old open cluster Praesepe, and measure rotation periods (P rot ) for 677 of these stars. We find that half of the rapidly rotating > ∼ 0.3 M ⊙ stars are confirmed or candidate binary systems. The remaining > ∼ 0.3 M ⊙ fast rotators have not been searched for companions, and are therefore not confirmed single stars. We found previously that nearly all rapidly rotating > ∼ 0.3 M ⊙ stars in the Hyades are binaries, but we require deeper binary searches in Praesepe to confirm whether binaries in these two co-eval clusters have different P rot distributions. We also compare the observed P rot distribution in Praesepe to that predicted by models of angular-momentum evolution. We do not observe the clear bimodal P rot distribution predicted by Brown (2014) for >0.5 M ⊙ stars at the age of Praesepe, but 0.25−0.5 M ⊙ stars do show stronger bimodality. In addition, we find that >60% of early M dwarfs in Praesepe rotate more slowly than predicted at 650 Myr by Matt et al. (2015), which suggests an increase in braking efficiency for these stars relative to solar-type stars and fully convective stars. The incompleteness of surveys for binaries in open clusters likely impacts our comparison with these models, since the models only attempt to describe the evolution of isolated single stars.

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

“…These models predict much better the evolving AM distribution for both clusters, likely due to their more complex dependencies on the stellar properties (Section 2.3). The one exception is the middle-mass bin in Praesepe, which is significantly more drained of AM than the models predict, suggesting that stars around 0.4 M  spin down faster than expected after the age of the Pleiades-this has been noted by Douglas et al (2017), and will be explored in detail in an upcoming paper (G. Somers et al 2017, in preparation). Although our forward-modeling exercises above found little difference in the predictions of the models, a direct look at the evolving AM budget provides strong justification for using the newer models of AM loss.…”

“…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).…”

The Factory and the Beehive. III. PTFEB132.707+19.810, A Low-mass Eclipsing Binary in Praesepe Observed by PTF and K2

“…primary makes it a normal object on the P rot -M relation for Praesepe (Douglas et al 2017;Rebull et al 2017), but it is noticeably faster than the ensemble of field 0.4 M e stars (e.g., Harrison et al 2012;McQuillan et al 2013;Newton et al 2016), suggesting that it is a suitable representative of a young ZAMS star. There is only a lower limit on the rotational period of the 0.2 M e secondary; at that limit, it would sit on the slow edge of the Praesepe sequence but would not be unusually slow.…”

“…The light curve is dominated by flux from the primary star, contributing ∼75% of the flux in the red optical (Figure 6). If the observed sinusoidal variations (with full amplitude 6%) were caused by the secondary star, then its individual total amplitude of variation would be 26%; studies of rotational variability across the full sample by Douglas et al (2017) and Rebull et al (2017) found that the maximum amplitude seen for 0.2 M e stars was only 10%. A similar upper envelope was seen for periodic field stars observed by Kepler by Harrison et al (2012).…”

“…As we describe below, PTFEB132.707+19.810 also was identified to be an eclipsing binary with an orbital period of P 6.0 orb = days that has not locked its stars into synchronous rotation. The star has otherwise remained anonymous in the literature until this year, when its eclipsing nature was recognized by others studying K2 data in Praesepe (Douglas et al 2017;Gillen et al 2017;Rebull et al 2017). In this paper, we analyze our discovery light curve from PTF, follow-up spectroscopic observations, and the newly released K2 light curve of PTFEB132.707+19.810 in order to measure the stellar properties of this mid-M-type eclipsing binary system and test the predictions of stellar evolutionary models.…”