Stellar Population Astrophysics with TNG: The old open clusters Collinder 350, Gulliver 51, NGC 7044, Ruprecht 171

Star clusters are among the most versatile astronomical objects. Within our Galaxy, they play a key-role in the study of both stellar and Galactic chemical evolution, star formation history and dynamical evolution of the Milky Way.

Recent results obtained within the Stellar Population Astrophysics (SPA) project, are presented. SPA is an on-going Large Programme carried out with the 3.6 m Telescopio Nazionale Galileo (TNG) at the Roque de los Muchachos Observatory (La Palma, Spain). The project is led by L. Origlia (INAF-OA Bologna), with A. Bragaglia (INAF-OA Bologna) as leader of the star cluster group.

The project aims to contribute to our understanding of the star formation and chemical enrichment history of our Galaxy, providing high-resolution spectra of a sample of stars in the Solar neighborhood. Its intent is to obtain a large variety of elemental abundances to seek possible global trends in the Galactic disc and, combined with the kinematic counterpart from Gaia and other surveys, to provide a framework for a comprehensive chemo-dynamical modeling of the disc formation and evolution in the Solar vicinity.

It is obtaining high-resolution spectra of approximately 500 stars near to the Sun, covering a wide range of ages and properties, such as Cepheids and stars in both young and old open clusters, of spectral type from A to K. These stars are observed in the optical and near-infrared bands at high-spectral resolution by using GIARPS, a combination of the HARPS-N and GIANO-B spectrographs mounted at the Telescopio Nazionale Galileo.

In a recent paper, a team led by G. Casali (PhD student at the University of Florence) and her supervisor Dr. L. Magrini (INAF-OA Arcetri) analysed the high-resolution HARPS-N spectra of some member stars of four open clusters: Collinder 350, Gulliver 51, NGC7044, Ruprecht 171. These open clusters allow us to characterize the nearby regions of our Galaxy due their location (300 - 3300 pc from the Sun) and age (0.3-3 Gyr), so far little-studied from a spectroscopic point of view.

The team performed a spectral analysis based on the EW measurements and spectral fitting to characterize the chemical composition of several stars in each cluster. They found, for stars belonging to the same cluster, a correlation between stellar parameters and metallicity: the coolest stars (Teff < 4300 K and log g < 1.8 dex) appear more metal-poor than the hottest ones. They investigated several possible aspects that can influence the analysis of the coolest giant stars, concluding that a combination of continuum placement and inaccurate model atmospheres might affect the final metallicity.

Discarding those stars, the team derived abundances for several elements. In particular, they investigated the Galactic radial metallicity gradient by comparing the SPA clusters with those of Gaia-ESO iDR5 and APOGEE DR16: Collinder 350, Gulliver 51, NGC7044 and Ruprecht 17, together with other star clusters analyzed in previous SPA papers (ASCC123 and Praesepe; Frasca et al. 2019, D’Orazi et al. 2020) follow well the global radial metallicity gradient (see Figure). Also the abundances of the other elements, belonging to different nucleosynthesis channels, are in agreement with the Galactic trends.

Finally, thanks to the wide spectral coverage of the SPA spectra, they derived abundances of some light elements, as C and Li, which are modified in giant stars by stellar evolution. The comparison between the observations and theoretical models confirms the necessity of some extra-mixing process, as the rotation-induced mixing.

Related paper:
Casali, G., et al., A&A in press, https://arxiv.org/pdf/2009.06695.pdf

Radial metallicity gradient

Figure. Radial metallicity gradient: in red the results of Gaia-ESO iDR5 from Magrini et al. (2018), in green the results of APOGEE DR16, and in blue the four SPA clusters analysed in this work. The blue square is the mean [Fe/H] of Rup 171 obtained with only the hottest stars analysed by spectral fitting. The clusters in common between APOGEE and Gaia-ESO are linked by a black line. The magenta and black circles are the SPA clusters ASCC 123 and Praesepe studied by Frasca et al. (2019) and D’Orazi et al. (2020), respectively. The light green diamonds are the young Gaia-ESO OCs by Baratella et al. (2020).