Stellar Population Astrophysics (SPA) with the TNG

Stock 2, a little-studied open cluster with an eMSTO^⋆

¹ INAF–Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
e-mail: javier.alonso@inaf.it
² INAF–Osservatorio di Astrofisica e Scienza dello Spazio, Via P. Gobetti 93/3, 40129 Bologna, Italy
³ Fundación Galileo Galilei–INAF, Rambla José Ana Fernández Pérez 7, 38712 Breña Baja, Tenerife, Spain
⁴ INAF–Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
⁵ INAF–Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁶ Dipartimento di Fisica e Astronomia, Università degli Studi di Bologna, Via Gobetti 93/2, 40129 Bologna, Italy
⁷ The Kavli Institute for Astronomy and Astrophysics at Peking University, 100871 Beijing, PR China
⁸ INFN, Laboratori Nazionali del Sud, Via S. Sofia 62, 95123 Catania, Italy
⁹ Dipartamento di Fisica e Astronomia, Università degli Studi di Firenze, via G. Sansone 1, 50019 Sesto Fiorentino, Firenze, Italy
¹⁰ Dipartamento di Fisica e Astronomia, Università di Padova, vicolo Osservatorio 2, 35122 Padova, Italy

Received: 8 July 2021
Accepted: 28 September 2021

Abstract

Stock 2 is a little-studied open cluster that shows an extended main-sequence turnoff (eMSTO). In order to investigate this phenomenon and characterise the cluster itself, we performed high-resolution spectroscopy in the framework of the Stellar Population Astrophysics project. We employed the High Accuracy Radial velocity Planet Searcher in the Northern hemisphere spectrograph at the Telescopio Nazionale Galileo (TNG). We completed our observations with additional spectra taken with the Catania Astrophysical Observatory Spectropolarimeter. We observed 46 stars (dwarfs and giants). They represent by far the largest sample collected for this cluster to date. We provide the stellar parameters, the extinction, and the radial and projected rotational velocities for most of the stars. Chemical abundances for 21 species with atomic numbers up to 56 have also been derived. We note a differential reddening in the cluster field with an average value of 0.27 mag. This appears the main cause for the observed eMSTO because it cannot be explained as the result of different rotational velocities, as found in other clusters. We estimate an age for Stock 2 of 450 ± 150 Ma, which corresponds to a MSTO stellar mass of ≈2.8 M_⊙. The cluster mean radial velocity is about 8.0 km s⁻¹. We find a solar-like metallicity for the cluster, [Fe/H] = −0.07 ± 0.06, compatible with its galactocentric distance. The chemical abundances of main-sequence stars and giants are compatible within the errors. The exceptions are barium and strontium, which are clearly overabundant in giants, and cobalt, which is only marginally overabundant. Finally, the chemical composition of Stock 2 is fully compatible with the composition observed in other open clusters of the Galactic thin-disc population.