Empirical determination of the lithium 6707.856 Å wavelength in young stars^⋆

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
e-mail: jcampbel@eso.org
² SUPA, School of Science and Engineering, University of Dundee, Nethergate, Dundee DD1 4HN, UK
³ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia, 78, 95123 Catania, Italy
⁴ Hamburg Observatory, Gojenbergsweg 11, 21029 Hamburg, Germany
⁵ INAF – Osservatorio Astronomico di Roma: Monte Porzio Catone, Lazio, Italy
⁶ Instituto de Física y Astronomía, Universidad de Valparaíso, Gran Bretaña, 1111 Valparaíso, Chile
⁷ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
⁸ Excellence Cluster ‘Origins’, Boltzmannstr. 2, 85748 Garching, Germany
⁹ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Eötvös Loránd Research Network (ELKH), Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
¹⁰ CSFK, MTA Centre of Excellence, Konkoly-Thege Miklós út 15-17, 1121 Budapest, Hungary
¹¹ ELTE Eötvös Loránd University, Institute of Physics, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
¹² Purple Mountain Observatory, Chinese Academy of Sciences, 10 Yuanhua Road, Nanjing 210023, PR China
¹³ INAF-Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, 50125 Firenze, Italy
¹⁴ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
¹⁵ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, rua do Campo Alegre 687, 4169-007 Porto, Portugal
¹⁶ ASI, Italian Space Agency, Via del Politecnico snc, 00133 Rome, Italy
¹⁷ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹⁸ Instituto de Astronom ia, Universidad Nacional Autónoma de México, Carr. Tijuana-Ensenada km107, C.I.C.E.S.E., 22860 Ensenada, B.C., Mexico
¹⁹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’osservatorio 5, 35122 Padova, Italy
²⁰ Department of Physics & Astronomy, Amherst College, C025 Science Center 25 East Drive, Amherst, MA 01002, USA
²¹ SETI Institute, 339 Bernardo Ave, Suite 200, Mountain View, CA 94043, USA

Received: 15 December 2022
Accepted: 7 March 2023

Abstract

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.