The APOGEE Data Release 16 Spectral Line List

The updated H -band spectral-line list (from λ 15000–17000) adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) for the SDSS-IV Data Release 16 (DR16) is presented in this work. The APOGEE line list is a combination of atomic and molecular lines, with data drawn from laboratory, theoretical, and astrophysical sources. Oscillator strengths and damping constants are adjusted using high signal-to-noise, high-resolution spectra of the Sun, and α Boo (Arcturus), as “standard stars.” Updates to the DR16 line list, as compared to the previous DR14 version, include the addition of molecular H 2 O and FeH lines, as well as a much larger (by a factor of ∼4) atomic line list, including a significantly greater number of transitions with hyperfine splitting. More recent references and line lists for the crucial molecules, CO and OH, as well as for C 2 and SiH, are also included. In contrast to DR14, DR16 contains measurable lines from the heavy neutron-capture elements cerium (as Ce ii ), neodymium (as Nd ii ), and ytterbium (as Yb ii ), as well as one line from rubidium (as Rb i ), which may be detectable in a small fraction of APOGEE red giants.