Line Expansion Opacity in Relativistically Expanding Media
Abstract
Spectral lines of heavy atomic elements in the ejecta of supernovae and neutron star mergers can have important contribution to the opacity of the ejecta matter even when the abundance of the elements is very small. Under favorable conditions, the line expansion opacity arising from spectral lines and the expansion of the medium can be orders of magnitude larger than the opacity of electron scattering. In this paper, we derive the formulae for evaluating the line expansion opacity and its Rosseland mean in an expanding medium in the framework of special relativity, which can be considered as a generalization of previous work in the Newtonian approximation. Then we compare the derived relativistic formulae to the Newtonian ones to explore the relativistic effect on the opacity and test the new formulae with the spectral lines of some heavy atomic elements. We also derive some approximation formulae for the Rosseland mean of the line expansion opacity that are easy to use in numerical works while still maintaining a highenough accuracy relative to exact solutions. The formulae derived in this paper are expected to have important applications in radiative problems related to relativistic astrophysical phenomena such as neutron star mergers, supernovae, and gammaray bursts where relativistic or subrelativistic expansions are involved.
 Publication:

The Astrophysical Journal
 Pub Date:
 December 2019
 DOI:
 10.3847/15384357/ab5306
 arXiv:
 arXiv:1910.13619
 Bibcode:
 2019ApJ...887...60L
 Keywords:

 Atomic physics;
 Stellar atmospheric opacity;
 Radiative transfer;
 Radiative transfer equation;
 Neutron stars;
 Supernovae;
 Relativistic mechanics;
 2063;
 1585;
 1335;
 1336;
 1108;
 1668;
 1391;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 37 pages, including 15 figures. Accepted for publication in the Astrophysical Journal