Stellar Population Astrophysics (SPA) with the TNG. Measurement of the He I 10 830 Å line in the open cluster Stock 2

The precise measurement of stellar abundances plays a pivotal role in providing constraints on the chemical evolution of the Galaxy. However, before spectral lines can be employed as reliable abundance indicators, particularly for challenging elements such as helium, they must undergo thorough scrutiny. Galactic open clusters, representing well-defined single stellar populations, offer an ideal setting for unfolding the information stored in the helium spectral line feature. In this study, we characterise the profile and strength of the helium transition at around 10 830 Å (He 10 830) in nine giant stars in the Galactic open cluster Stock 2. To remove the influence of weak blending lines near the helium feature, we calibrated their oscillator strengths (log 𝑔f) by employing corresponding abundances obtained from simultaneously observed optical spectra. Our observations reveal that the He 10 830 in all the targets is observed in absorption, with line strengths categorised into two groups. Three stars exhibit strong absorption, including a discernible secondary component, while the remaining stars exhibit weaker absorption. The lines are in symmetry and align with or near their rest wavelengths, suggesting a stable upper chromosphere without a significant systematic mass motion. We find a correlation between the He 10 830 strength and the Ca II log R′_HK index, with a slope similar to that reported in previous studies on dwarf stars. This correlation underscores the necessity of accounting for the stellar chromosphere structure when employing He 10 830 as a probe for the stellar helium abundance. The procedure of measuring the He 10 830 we developed in this study is applicable not only to other Galactic open clusters but also to field stars, and we plan to use it to map the helium abundance across various types of stars in the future.

Based on observations made with the Italian Telescopio Nazionale Galileo (TNG) operated on the island of La Palma by the Fundación Galileo Galilei of the INAF (Istituto Nazionale di Astrofisica) at the Spanish Observatorio del Roque de los Muchachos. This study is part of the Large Programme entitled Stellar Population Astrophysics (SPA): this study of the detailed, age-resolved chemistry of the Milky Way disc (A37TAC_31, PI: L. Origlia) started in 2018 and was granted observing time with HARPS-N and GIANO-B echelle spectrographs at the TNG.