PyEMILI: A New Generation Computer-aided Spectral Line Identifier

Deep high-dispersion spectroscopy of Galactic photoionized gaseous nebulae, mainly planetary nebulae and HII regions, has revealed numerous emission lines. As a key step of spectral analysis, identification of emission lines hitherto has mostly been done manually, which is a tedious task, given that each line needs to be carefully checked against huge volumes of atomic transition/spectroscopic database to reach a reliable assignment of identity. Using Python, we have developed a line-identification code PyEMILI, which is a significant improvement over the Fortran-based package EMILI introduced ~20 years ago. In our new code PyEMILI, the major shortcomings in EMILI's line-identification technique have been amended. Moreover, the atomic transition database utilized by PyEMILI was adopted from Atomic Line List v3.00b4 but greatly supplemented with theoretical transition data from the literature. The effective recombination coefficients of the CII, OII, NII and NeII nebular lines are collected from the literature to form a subset of the atomic transition database to aid identification of faint optical recombination lines in the spectra of PNe and HII regions. PyEMILI is tested using the deep, high-dispersion spectra of two Galactic PNe, Hf2-2 and IC418, and gives better results of line identification than EMILI does. We also ran PyEMILI on the optical spectrum of a late-type [WC11] star UVQS J060819.93-715737.4 recently discovered in the Large Magellanic Cloud, and our results agree well with the previous manual identifications. The new identifier PyEMILI is applicable to not only emission-line nebulae but also emission stars, such as Wolf-Rayet stars.
Comment: Accepted for publication in ApJS; 34 pages including 2 figures and 10 tables. The PyEMILI code as well as the atomic transition database has been publicly available on GitHub, Zenodo and PyPI. This paper describes the PyEMILI code in thorough details (the criteria, methodology and technology developed for line identification), and provides an overview of emission line formation