Stellar population astrophysics (SPA) with the TNG. The Arcturus Lab

Detailed, high resolution optical and NIR spectroscopy of stellar calibrators is fundamental to define optimal diagnostics for atmospheric parameters and chemical abundances of stars and stellar populations with different ages, metallicities and evolutionary properties.

While diagnostic tools from high resolution optical spectroscopy are well established and calibrated since a longtime, those from NIR spectroscopy started to be explored only recently, thanks to the new generation of NIR echelle spectrographs with adequate performances for such quantitative studies.

Within the Stellar Population Astrophysics Large Programme at the TNG, a high-resolution (R=50000) NIR spectrum of Arcturus has been acquired with the GIANO-B echelle spectrograph. This spectrum has been used as a laboratory to define and calibrate an optimal linelist and new diagnostic tools to derive accurate stellar parameters and chemical abundances.

We inspected several hundreds of NIR atomic and molecular lines to derive abundances of 26 different chemical species, including CNO, iron-group, alpha, Z-odd and neutron-capture elements. We then performed a similar analysis in the optical using VLT-UVES spectra of Arcturus.

A new thermometer and a new gravitometer for giant stars has been defined by comparing the Carbon and Oxygen abundances as derived from atomic and molecular lines in the combined optical and NIR spectrum. A number of problematic lines, which can be affected by deviations from thermal equilibrium and/or chromospheric activity, as traced by the observed variability of the He I line at 1083 nm, have been also discussed.

The value of such a combined optical and NIR study has been multifold and can be summarised as follows.

1. it maximises the set of diagnostic lines to sample almost all the chemical elements of interest and with a statistically significant number of lines for most of them;
2. it enables to sample lines of a given species at different wavelengths, extending the range of excitation potentials and transition probabilities for a better understanding of the physics of line formation and the modelling of the observed spectrum;
3. it drives the analysis towards a physical, self-consistent solution over the entire spectrum of the degeneracy problem among stellar parameters and chemical abundances.

YJHK spectrum of Arcturus acquired with the GIANO-B high resolution spectrograph at the TNG.
Black line is the telluric-corrected Arcturus spectrum, red line the telluric one.
Several hundreds of NIR atomic and molecular lines have been inspected (bottom-left panel) and detailed abundances for 26 chemical elements have been derived from the NIR GIANO-B spectrum (red symbols in the bottom-right panel) and compared with those derived from optical UVES spectra (bottom-right panel, blue symbols).
Adapted from C. Fanelli, L. Origlia, E. Oliva, A. Mucciarelli, N. Sanna, E. Dalessandro and D. Romano, 2020, A&A, in press, arXiv:2011.12321.