Empirical determination of the lithium 6707.856? wavelength in young stars

Absorption features in stellar atmospheres are often used to calibrate photocentric velocities for the kinematic analysis of further spectral lines. The Li feature at similar to 6708 angstrom 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 degrees, with a standard error of 0.002 angstrom(0.09 km s(-1)), and a weighted standard deviation of 0.026 angstrom(1.16 km s(-1)). The observed spread in measured positions spans 0.145 angstrom, 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 angstrom, is more blueshifted than previously thought, this then favours a disc wind as the origin for this emission in young stars.