Stellar rotation, binarity, and lithium in the open cluster IC 4756

Abstract

Context. An important aspect in the evolutionary scenario of cool stars is their rotation and the rotationally induced magnetic activity and interior mixing. Stars in open clusters are particularly useful tracers for these aspects because of their known ages. Aims. We aim to characterize the open cluster IC 4756 and measure stellar rotation periods and surface differential rotation for a sample of its member stars. Methods. Thirty-seven cluster stars were observed continuously with the CoRoT satellite for 78 days in 2010. Follow-up highresolution spectroscopy of the CoRoT targets and deep Strömgren uvbyβ and Hα photometry of the entire cluster were obtained with our robotic STELLA facility and its echelle spectrograph and wide-field imager, respectively. Results. We determined high-precision photometric periods for 27 of the 37 CoRoT targets and found values between 0.155 and 11.4 days. Twenty of these are rotation periods. Twelve targets are spectroscopic binaries of which 11 were previously unknown; orbits are given for six of them. Six targets were found that show evidence of differential rotation with ΔΩ/Ω in the range 0.04−0.15. Five stars are non-radially pulsating stars with fundamental periods of below 1 d, two stars are semi-contact binaries, and one target is a micro-flaring star that also shows rotational modulation. Nine stars in total were not considered members because of much redder color(s) and deviant radial velocities with respect to the cluster mean. Hα photometry indicates that the cluster ensemble does not contain magnetically over-active stars. The cluster average metallicity is -0.08 ± 0.06 (rms) and its logarithmic lithium abundance for 12 G-dwarf stars is 2.39 ± 0.17 (rms). Conclusions. The cluster is 890 ± 70 Myrs old with an average turn-off mass of 1.8 M and a solar or slightly subsolar metallicity. The distance modulus is 8. m 02 and the average reddening E(b − y) = 0. m 16. The cluster is masked by a very inhomogeneous foreground and background dust distribution and our survey covered 38% of the cluster. The average lithium abundance and the rotation periods in the range 1.7 to 11.4 d are consistent with the cluster age.

CoRoT photometry and STELLA spectroscopy of an eccentric, eclipsing, and spotted HgMn binary with sub-synchronized rotation

“…This constrains its X-ray luminosity to lower than ≈2 × 10 29 erg s −1 , even lower than what has been detected in the Hyades. With an age of 890 Myr (Strassmeier et al 2015a) IC 4756 is nearly coeval with the Hyades where magnetic activity is already weak. In general, we therefore do not expect to see coronal activity on HSS 348.…”

“…7. Strassmeier et al (2015a) redetermined the average cluster velocity of IC 4756 from 21 members to −24.0 km s −1 with a rms of around the mean of just ±1.3 km s −1 . Our systemic velocity of HSS 348 is in gross disagreement with this (γ = +21.8 km s −1 ) and we must conclude that HSS 348 is not a member of IC 4756 and most likely a background target.…”

Chemical composition of a sample of bright solar‐metallicity stars

“…This suggests that this star is younger than the Sun (and M 67). The logarithmic lithium abundance for 12 G-dwarf stars in the 800-Myr open cluster IC 4756 is 2.39 ± 0.17 (Strassmeier et al 2015). This suggests that HIP 80124 (HD 147443) may even be younger than that.…”

Project BMK10k: Zeiss Ballistische Messkammer with a 10k × 10k CCD for ground‐based PLATO support

“…Finally, the telescope had been mated with a cryogenic dewar holding the largest CCD available to date, a 10k × 10k STA1600LN CCD developed for others; for example, LBT's PEPSI (Strassmeier et al , ) and USNO's URAT (Zacharias et al ); therefore, it is called BMK10k.…”

“…The most important change to the original BMK was the replacement of its photographic plates with the large CCD detector. The CCD system now in the BMK is the PEPSI red spare 10,560 × 10,560 pixel STA1600LN device (Strassmeier et al , ). This device sets the effective FoV of the telescope to 7.25 × 7.25°, that is, a full 52.5 square degrees with a spatial sampling on the sky of 2.47″/pix.…”