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Empirical determination of the lithium 6707.856 Å wavelength in young stars

J. Campbell-White, C. F. Manara, A. Sicilia-Aguilar, A. Frasca, L. D. Nielsen, C. Schneider, B. Nisini, A. Bayo, B. Ercolano, P. Ábrahám, R. A. B. Claes, M. Fang, D. Fedele, J. F. Gameiro, M. Gangi, Á. Kóspál, K. Maucó, M. G. Petr-Gotzens, E. Rigliaco, C. Robinson, M. Siwak, L. Tychoniec, L. Venuti

Absorption features in stellar atmospheres are often used to calibrate photocentric velocities for the kinematic analysis of further spectral lines. The Li feature at ∼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 complex line comprises two isotope fine-structure doublets. We empirically measured 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−1), and a weighted standard deviation of 0.026 Å (1.16 km s−1). The observed spread in measured positions spans 0.145 Å, or 6.5 km s−1, which is higher by up to a factor of six than the 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 that exclusively using this Li feature as a reference for photocentric velocity in young stars might introduce a systematic positive offset in wavelength to measurements of further spectral lines. If outflow tracing forbidden lines, such as [O I] 6300 Å, is more blueshifted than previously thought, this then favours a disc wind as the origin for this emission in young stars.

stars: atmospheres / stars: pre-main sequence / stars: variables: T Tauri, Herbig Ae/Be / ISM: abundances

Based on observations collected at the European Southern Observatory under ESO programmes 105.207T and 106.20Z8.

Astronomy & Astrophysics
Volume 673, Article Number A80, Number of pages 13
2023 May

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Faculdade de Ciências da Universidade de Lisboa Universidade do Porto Faculdade de Ciências e Tecnologia da Universidade de Coimbra
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia