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Register a new userRotation periods for cool stars in the open cluster Ruprecht 147 (NGC 6774): Implications for gyrochronology
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Gyrochronology allows the derivation of ages for cool main sequence stars based on their observed rotation periods and masses, or a suitable proxy thereof. It is increasingly well-explored for FGK stars, but requires further measurements for older ages and K-M-type stars. We study the nearby, 3 Gyr-old open cluster Ruprecht 147 to compare it with the previously-studied, but far more distant, NGC 6819 cluster, and especially to measure cooler stars than was previously possible there. We constructed an inclusive list of 102 cluster members from prior work, including Gaia DR2, and for which light curves were also obtained during Campaign 7 of the Kepler/K2 space mission. [...] Periodic signals are found for 32 stars, 21 of which are considered to be both highly reliable and to represent single, or effectively single, Ru147 stars. These stars cover the spectral types from late-F to mid-M stars, and they have periods ranging from 6d-32d, allowing for a comparison of Ruprecht 147 to both of the other open clusters and to models of rotational spindown. The derived rotation periods connect reasonably to, overlap with, and extend to lower masses the known rotation period distribution of the 2.5 Gyr-old cluster NGC 6819. The data confirm that cool stars lie on a single surface in rotation period-mass-age space, and they simultaneously challenge its commonly assumed shape. The shape at the low mass region of the color-period diagram at the age of Ru147 favors a recently-proposed model, which requires a third mass-dependent timescale in addition to the two timescales required by a former model, suggesting that a third physical process is required to model rotating stars effectively.
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Employing photometric rotation periods for solar-type stars in NGC 1039 [M 34], a young, nearby open cluster, we use its mass-dependent rotation period distribution to derive the clusters age in a distance independent way, i.e., the so-called gyrochronology method. We present an analysis of 55 new rotation periods,using light curves derived from differential photometry, for solar type stars in M 34. We also exploit the results of a recently-completed, standardized, homogeneous BVIc CCD survey of the cluster in order to establish photometric cluster membership and assign B-V colours to each photometric variable. We describe a methodology for establishing the gyrochronology age for an ensemble of solar-type stars. Empirical relations between rotation period, photometric colour and stellar age (gyrochronology) are used to determine the age of M 34. Based on its position in a colour-period diagram, each M 34 member is designated as being either a solid-body rotator (interface or I-star), a differentially rotating star (convective or C-star) or an object which is in some transitory state in between the two (gap or g-star). Fitting the period and photometric colour of each I-sequence star in the cluster, we derive the clusters mean gyrochronology age. 47/55 of the photometric variables lie along the loci of the cluster main sequence in V/B-V and V/V-I space. We are further able to confirm kinematic membership of the cluster for half of the periodic variables [21/55], employing results from an on-going radial velocity survey of the cluster. For each cluster member identified as an I-sequence object in the colour-period diagram, we derive its individual gyrochronology age, where the mean gyro age of M 34 is found to be 193 +/- 9 Myr, formally consistent (within the errors) with that derived using several distance-dependent, photometric isochrone methods (250 +/- 67 Myr).
We report our spectroscopic monitoring of the detached, grazing, and slightly eccentric 12-day double-lined eclipsing binary EPIC 219568666 in the old nearby open cluster Ruprecht 147. This is the second eclipsing system to be analyzed in this cluster, following our earlier study of EPIC 219394517. Our analysis of the radial velocities combined with the light curve from the K2 mission yield absolute masses and radii for EPIC 219568666 of M1 = 1.121 +/- 0.013 M(Sun) and R1 = 1.1779 +/- 0.0070 R(Sun) for the F8 primary, and M2 = 0.7334 +/- 0.0050 M(Sun) and R2 = 0.640 +/- 0.017 R(Sun) for the faint secondary. Comparison with current stellar evolution models calculated for the known metallicity of the cluster points to a primary star that is oversized, as is often seen in active M dwarfs, but this seems rather unlikely for a star of its mass and with a low level of activity. Instead, we suspect a subtle bias in the radius ratio inferred from the photometry, despite our best efforts to avoid it, which may be related to the presence of spots on one or both stars. The radius sum for the binary, which bypasses this possible problem, indicates an age of 2.76 +/- 0.61 Gyr that is in good agreement with a similar estimate from the binary in our earlier study.
Eclipsing binaries in star clusters offer more stringent tests of stellar evolution theory than field binaries because models must not only match the binary properties, but also the radiative properties of all other cluster members at a single chemical composition and a single age. Here we report new spectroscopic observations of the G type, detached eclipsing binary EPIC 219394517 in the open cluster Ruprecht 147 ([Fe/H] = +0.10), which was observed in late 2015 by the K2 mission. A joint analysis of our radial-velocity measurements and the K2 light curve shows the 6.5 day orbit to be very nearly circular. We derive highly precise masses of 1.0782 +/- 0.0019 Msun and 1.0661 (+0.0027/-0.0021) Msun, radii of 1.055 +/- 0.011 Rsun and 1.042 +/- 0.012 Rsun, and effective temperatures of 5930 +/- 100 K and 5880 +/- 100 K for the primary and secondary, respectively. The distance we infer, 283 (+18/-16) pc, corresponds to a parallax in good agreement with the Gaia/DR2 value for the star. Current stellar evolution models from the MIST and PARSEC series match the above physical properties very well at ages of 2.48 and 2.65 Gyr. Isochrones for these same ages and the measured composition, along with our reddening estimate for EPIC 219394517, also show generally good agreement with the optical and near-infrared color-magnitude diagrams of the cluster, which can be constructed with no free parameters as the distances of all member stars are known from Gaia.
We report follow-up spectroscopic observations of the 1.62 day, K-type, detached, active, near-circular, double-lined eclipsing binary EPIC 219511354 in the open cluster Ruprecht 147, identified previously on the basis of photometric observations from the Kepler/K2 mission. This is the fourth eclipsing system analyzed in this cluster. A combined analysis of the light curve and radial velocities yields accurate masses of M(Aa) = 0.912 +/- 0.013 MSun and M(Ab) = 0.822 +/- 0.010 MSun for the primary (star Aa) and secondary (Ab), along with radii of R(Aa) = 0.920 +/- 0.016 RSun and R(Ab) = 0.851 +/- 0.016 RSun, and effective temperatures of 5035 +/- 150 and 4690 +/- 130 K, respectively. Comparison with current models of stellar evolution for the known age and metallicity of the cluster reveals that both radii are larger (by 10--14%) and both temperatures cooler (by $sim$6%) than theoretically predicted, as is often seen in M dwarfs. This is likely caused by the significant stellar activity in the system, manifested here by 6% peak-to-peak out-of-eclipse variability, a filled-in H$alpha$ line, and its detection as an X-ray source. We also find EPIC 219511354 to be a hierarchical triple system, with a low-mass tertiary in an eccentric 220 day orbit.
Spectroscopic observations are reported for the 2.75 day, double-lined, detached eclipsing binary EPIC 219552514 located at the turnoff of the old nearby open cluster Ruprecht 147. A joint analysis of our radial velocity measurements and the K2 light curve leads to masses of M1 = 1.509 (+0.063 / -0.056) MSun and M2 = 0.649 (+0.015 / -0.014) MSun for the primary and secondary, along with radii of R1 = 2.505 (+0.026 / -0.031) RSun and R2 = 0.652 (+0.013 / -0.012) RSun, respectively. The effective temperatures are 6180 +/- 100 K for the F7 primary and 4010 +/- 170 K for the late K secondary. The orbit is circular, and the stars rotation appears to be synchronized with the orbital motion. This is the third eclipsing system analyzed in the same cluster, following our earlier studies of EPIC 219394517 and EPIC 219568666. By comparison with stellar evolution models from the PARSEC series, we infer an age of 2.67 (+0.39 / -0.55) Gyr that is consistent with the estimates for the other two systems. EPIC 219552514 is a hierarchical triple system, with the period of the slightly eccentric outer orbit being 463 days. The unseen tertiary is either a low-mass M dwarf or a white dwarf.
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