Fast-rotating giant stars behind the Coma Berenices star cluster

In the frame of a study of the empirical isochrones of young stellar clusters, we have carried out BVIc Johnson-Cousins photometry of a sample of K and M stars of the Coma Berenices star cluster. All these stars have known rotational periods. Our main goal is to get a valuable reference on the colour-magnitude diagram, M-v versus B - V, for stars with ages within 400-800 Myr. For this purpose, we obtained BVIc photometry with an average upper limit for the precision of about 0.025 mag and used parallaxes from the Gaia Data Release 2. We found that one-third of our sample is located well above the cluster main sequence and these stars are confirmed as background giants by their radial velocities in the Gaia Data Release 2. This misclassification shows that giants with short-surface rotational periods can mimic main-sequence stars if they are located at the appropriate distance. We recommend caution when using rotational periods in order to determine cluster membership. Besides, the gyrochronology technique should be used only when the luminosity class of the stars is well known. Finally, our cleared sample supports an age of similar to 600 Myr for Coma Berenices, rather than an age of similar to 800 Myr. © 2020, Oxford University PressThis research was supported by the Spanish AYA2016-79425-C3-3-P grant of the Spanish Ministry of Economy and Competitiveness (MINECO), co-funded with EU FEDER funds. EC acknowledges financial support from the Spanish Ministry of Economy and Competitiveness under the FPI grant AYA2011-30147-C03-01. EJA acknowledges financial support from the State Agency for Research of the Spanish MCIU through grant PGC2018-095049-B-C21. AT acknowledges financial support from the Spanish National Research Council (CSIC) through the Intramural grant 201850E126. We also acknowledge support from the State Agency for Research of the Spanish MCIU through the `Center of Excellence Severo Ochoa' award for the Instituto de Astrof ' isica de Andalucia (SEV-2017-0709). This research was based on data taken at the Sierra Nevada Observatory, which is operated by the Instituto de Astrofisica de Andalucia (CSIC). This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/con sortium). Funding for the DPAC has been provided by national institutions, in particular, the institutions participating in the Gaia Multilateral Agreement. This research has made use of the SIMBAD database, the VizieR catalogue access tool, and 'Aladin sky atlas', CDS, Strasbourg, France; of TOPCAT (Taylor 2005); of PYTHON (http://www.python.org), NUMPY (van der Walt, Colbert & Varoquaux 2011), Pandas (Mckinney 2011), and MATPLOTLIB (Hunter 2007); and of IRAF, distributed by the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. We thank the anonymous referee for reading the paper carefully and providing thoughtful comments, many of which have resulted in changes in the paper.Peer reviewe