Spectroscopic temperature determination of M-giant stars in the K-band

Sammanfattning

Determining the effective temperature (Teff) of stars is fundamental to further study stellar populations. For cool M-giants this is a challenge due to their crowded optical spectra, caused by closely spaced absorption lines from molecular bands. These complicate existing spectroscopic methods, limiting observations to the near-infrared (NIR). This is further motivated by the fact that NIR wavelengths can penetrate dust-obscured regions such as the Galactic center, where M-giants can serve as important probes of stellar populations. The project investigates whether the temperature-sensitive neutral scandium (Sc I) lines in the K-band can be used to determine Teff empirically. Due to its unique atomic structure, resulting in hyperfine splitting of energy levels and a metastable lower state, its spectral lines are especially strong. This combined with the lines' temperature sensitivity in the 3000–4000 K range makes them promising candidates for a spectroscopic thermometer. Nine Sc I lines in the 2.17–2.27 µm range were selected and their strength measured by the area under a Gaussian profile fitted to the lines. This was done for 44 M-giant stars provided by Nandakumar et al. (2023). Empirical calibration equations of the form Teff = a1 · area + a2 · [M/H] + a0 were derived for each line using two calibration sets, from Nandakumar et al. (2023) and APOGEE. The resulting equations were tested against 19 stars from Thorsbro et al. (2020), and the calculated Teff agreed with the literature values within the uncertainty of ±150 K for most stars. Metallicity was found to have a small but noticeable effect on the line areas, corresponding to a Teff difference of 50–100 K per 0.5 dex in metallicity. The method provides a simple and practical tool for estimating Teff of M-giant stars from K-band spectra, with applications for large stellar surveys and studies of dust-obscured regions.