Empirical Determination of the Lithium 6707.856 Å Wavelength in Young Stars

• Published in: arXiv.org 2023-03
• Main Authors: Campbell-White, Justyn, Manara, Carlo F, Sicilia-Aguilar, Aurora, Frasca, Antonio, Nielsen, Louise D, Schneider, P Christian, Nisini, Brunella, Bayo, Amelia, Ercolano, Barbara, Ábrahám, Péter, Claes, Rik, Fang, Min, Fedele, Davide, Jorge Filipe Gameiro, Gangi, Manuele, Kóspál, Ágnes, Maucó, Karina, Petr-Gotzens, Monika G, Rigliaco, Elisabetta, Robinson, Connor, Siwak, Michal, Tychoniec, Lukasz, Venuti, Laura
• Format: Article
• Language: English
• Subjects: Absorption; Empirical analysis; Goodness of fit; Kinematics; Line spectra; Lithium; Space telescopes; Standard error; Stars; Stellar atmospheres; Velocity errors
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2303.03843

Absorption features in stellar atmospheres are often used to calibrate photocentric velocities for kinematic analysis of further spectral lines. The Li feature at \(\sim\) 6708 Å is commonly used, especially in the case of young stellar objects for which it is one of the strongest absorption lines. However, this is a complex line comprising two isotope fine-structure doublets. We empirically measure the wavelength of this Li feature in a sample of young stars from the PENELLOPE/VLT programme (using X-Shooter, UVES and ESPRESSO data) as well as HARPS data. For 51 targets, we fit 314 individual spectra using the STAR-MELT package, resulting in 241 accurately fitted Li features, given the automated goodness-of-fit threshold. We find the mean air wavelength to be 6707.856 Å, with a standard error of 0.002 Å (0.09 km/s) and a weighted standard deviation of 0.026 Å (1.16 km/s). The observed spread in measured positions spans 0.145 Å, or 6.5 km/s, which is up to a factor of six higher than typically reported velocity errors for high-resolution studies. We also find a correlation between the effective temperature of the star and the wavelength of the central absorption. We discuss how exclusively using this Li feature as a reference for photocentric velocity in young stars could potentially be introducing a systematic positive offset in wavelength to measurements of further spectral lines. If outflow tracing forbidden lines, such as [O i] 6300 Å, are actually more blueshifted than previously thought, this then favours a disk wind as the origin for such emission in young stars.