The APOGEE Data Release 16 Spectral Line List

TL;DR

The paper presents the updated APOGEE DR16 spectral line list for the H-band, incorporating new molecular and atomic lines, and improved data sources, enhancing stellar spectral analysis capabilities.

Contribution

It introduces a significantly expanded and refined spectral line list for APOGEE DR16, including molecular lines, hyperfine splitting, and heavy element lines, based on laboratory, theoretical, and astrophysical data.

Findings

Inclusion of molecular H2O and FeH lines.

Larger atomic line list with hyperfine splitting.

Detection of heavy neutron-capture elements in spectra.

Abstract

The updated H-band spectral line list (from \lambda 15,000 - 17,000\AA) adopted by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) for the SDSS IV Data Release 16 (DR16) is presented here. The APOGEE line list is a combination of atomic and molecular lines with data 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 alpha Boo (Arcturus) as "standard stars". Updates to the DR16 line list, when compared to the previous DR14 version, are the inclusion of molecular H_2O and FeH lines, as well as a much larger (by a factor of ~4) atomic line list, which includes significantly more transitions with hyperfine splitting. More recent references and line lists for the crucial molecules CO and OH were used, as well as for C_2 and SiH. 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), that may be detectable in a small fraction of APOGEE red giants.