Far-Infrared Emission from a Late Supernova Remnant in an Inhomogeneous Medium

Abstract

The interstellar dust grains are swept up during the expansion of the supernova (SN) remnant, they penetrate behind the shock front, where they are heated and destroyed in the hot gas. This leads to a change in emissivity of such grains. In this work, we consider the evolution of the infrared (IR) luminosity of the SN remnant expanding into an inhomogeneous interstellar medium with lognormal distribution of the density fluctuations. The IR luminosity of the swept-up interstellar dust rapidly increases during the first several thousand years after the SN explosion, and reaches the maximum value. Afterwards, it decreases due to the destruction of the dust grains in hot gas and their declining emissivity in the cooling down gas of the shell. We show how the IR luminosity of dust in the SN remnant depends on the dispersion of the gas density in front of the SN shock front. We find that for the significant period of time (40 - 50 kyr) the maximum of the dust IR luminosity peaks at the range centered at 70$\mu$m. Therefore, this band can be considered as the most optimal range for studying the late SN remnants. We illustrate that during evolution, the dust temperature changes from 70 to 20 K, and only slightly depends on the inhomogeneity of the medium. In the radiative phase, the strong emission lines of metal ions emerge above the dust continuum. Their luminosity rapidly increases and exceeds the dust continuum luminosity by $\sim 10-10^3$ times. The point in time when the high luminosity in the lines is reached strongly depends on the inhomogeneity of the medium. We discuss possibilities for detection of the IR emission both in dust continuum and in lines. We expect that their ratios will allow to estimate the inhomogeneity of the medium, where the remnant is expanding.