A New Look at an Old Cluster: The Membership, Rotation, and Magnetic Activity of Low-mass Stars in the 1.3 Gyr Old Open Cluster NGC 752

Abstract

The nearby open cluster NGC 752 presents a rare opportunity to study stellar properties at ages >1 Gyr. However, constructing a membership catalog for it is challenging; most surveys have been limited to identifying its giants and dwarf members earlier than mid-K. We supplement past membership catalogs with candidates selected with updated photometric and proper-motion criteria, generating a list of 258 members, a >50% increase over previous catalogs. Using a Bayesian framework to fit MESA Isochrones & Stellar Tracks evolutionary models to literature photometry and the Tycho-Gaia Astrometric Solution data available for 59 cluster members, we infer the age of, and distance to, NGC 752: 1.34 ± 0.06 Gyr and 438 +8 −6 pc. We also report the results of our optical monitoring of the cluster using the Palomar Transient Factory. We obtain rotation periods for 12 K and M cluster members, the first periods measured for such low-mass stars with a well-constrained age >1 Gyr. We compare these new periods to data from the younger clusters Praesepe and NGC 6811, and to a theoretical model for angular-momentum loss, to examine stellar spin down for low-mass stars over their first 1.3 Gyr. While on average NGC 752 stars are rotating more slowly than their younger counterparts, the difference is not significant. Finally, we use our spectroscopic observations to measure Hα for cluster stars, finding that members earlier than ≈M2 are magnetically inactive, as expected at this age. Forthcoming Gaia data should solidify and extend the membership of NGC 752 to lower masses, thereby increasing its importance for studies of low-mass stars.

Spatial distribution of exoplanet candidates based on Kepler and Gaia data

“…Barnes et al (2016) found that rotation periods of M67 members delineate a sequence in the CPD reminiscent of that discovered first in the Hyades cluster. Similar sequence we can see in the mass-period diagram (which is equivalent to CPD) of NGC 752 (Agüeros et al 2018). In general, the rotational evolution theory is described in Barnes (2010), van Saders et al (2019.…”

First grids of low-mass stellar models and isochrones with self-consistent treatment of rotation

“…At the age of Praesepe (580 Myr), the fit to the observed dispersion is very good down to 0.4 M , but the models fail to reproduce the short rotation period of the less massive stars. This discrepancy between models predictions and observations has been discussed in Agüeros et al (2018) and appears at the mass transition where the star remains fully convective. For these very low-mass stars, our braking prescription is too efficient and/or happens too early.…”

“…The cluster ages reported in Fig. 7 are taken from the literature 7 , and the masses for the sample stars are from Gallet & Bouvier (2013); Bouvier et al (2014); Douglas et al (2016Douglas et al ( , 2017; Agüeros et al (2018) and references therein. The ages of the youngest clusters (up to hPer) are relatively uncertain with sometimes a factor of two uncertainty depending on the sets of isochrones used to fit their color-magnitude diagram (CMD).…”

TESS Reveals that the Nearby Pisces–Eridanus Stellar Stream is only 120 Myr Old

“…There are also theoretical models that pair stellar evolution with a magnetic torque law to predict angular momentum evolution (e.g., van Saders & Pinsonneault 2013;Matt et al 2015;Gallet & Bouvier 2015). However, no model has been published that can explain all of the cluster rotation data (see Curtis et al 2019;Douglas et al 2019;Agüeros et al 2018). Instead, we suggest that the best way to constrain the age of the Psc-Eri stream with gyrochronology is by comparing its P rot distribution directly to the distributions measured for benchmark clusters.…”