Stellar population astrophysics (SPA) with the TNG: Measurement of the He I 10830Å line in the open cluster Stock 2

Abstract

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 characterize the profile and strength of the helium transition at around 10830Å (He 10830) 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 He 10830 in all the targets is observed in absorption, with line strengths categorized 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 around their rest wavelengths, suggesting a stable upper chromosphere without a significant systematic mass motion. We found a correlation between He 10830 strength and Ca II $\log R_{\mathrm{HK}}^{\prime }$ index, with a slope similar to that reported in previous studies on dwarf stars. This correlation underscores the necessity of accounting for stellar chromosphere structure when employing He 10830 as a probe for stellar helium abundance. The procedure of measuring the He 10830 we developed in this study is applicable not only to other Galactic open clusters but also to field stars, with the aim of mapping helium abundance across various types of stars in the future.

15 pages, 10 figures, 4 tables, accepted for publication in A&A

References in corpus (11)

• What is a globular cluster? An observational perspective
• Correcting for Telluric Absorption: Methods, Case Studies, and Release of the TelFit Code
• YBC, a stellar bolometric corrections database with variable extinction coefficients: an application to PARSEC isochrones
• Structure and mass segregation in Galactic stellar clusters
• Active cool stars and He I 10830 Å: the coronal connection
• Stellar Population Astrophysics (SPA) with TNG Atmospheric parameters of members of 16 unstudied open clusters
• Stellar Population Astrophysics (SPA) with TNG. The old open clusters Collinder 350, Gulliver 51, NGC 7044, and Ruprecht 171
• FeI lines in 0.91-1.33 $\mu$m spectra of red giants for measuring the microturbulence and metallicities
• Comprehensive abundance analysis of red giants in the open clusters Stock 2, NGC 2168, 6475, 6991 and 7762
• Stellar Population Astrophysics (SPA) with the TNG: Stock 2, a little-studied open cluster with an eMSTO
• Rotation and activity in late-type members of the young cluster ASCC 123

Cited by in corpus (2)

• Nearby open clusters with tidal features: golden sample selection and 3D structure
• Stellar Population Astrophysics (SPA) with the TNG: NLTE atmospheric parameters and abundances of giant stars in 33 Open Clusters