Stellar Population Astrophysics (SPA) with the TNG. Measurement of the He I 10830 angstrom 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 angstrom 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 (loggf ) by employing corresponding abundances obtained from simultaneously observed optical spectra. Our observations reveal that the 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 strength and the Ca ii logRpHK 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 as a probe for the stellar helium abundance. The procedure of measuring the 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.