On partition function in Astronomy \& Astrophysics

TL;DR

This paper compares semi-empirical and accurate calculations of the partition function for molecules in the interstellar medium, revealing significant deviations at higher temperatures, which impacts spectral analysis accuracy.

Contribution

It provides a detailed comparison between semi-empirical and accurate partition function calculations for interstellar molecules, highlighting temperature-dependent discrepancies.

Findings

Semi-empirical expression overestimates partition function above 120 K.

Deviation reaches about 25% at 400 K.

Implications for spectral analysis accuracy in astrophysics.

Abstract

In order to analyze spectrum from the interstellar medium (ISM), spectrum of the molecule of interest is recorded in a laboratory, and accurate rotational and centrifugal distortion constants are derived. By using these constants, one can calculate accurate partition function. However, in the same paper, where these constants are derived, the partition function is calculated by using a semi-empirical expression. We have looked into the details of this semi-empirical expression and compared the values, obtained from it, with the accurate ones. As an example, we have considered the case of Methanimine (CH$_2$NH) which is detected in a number of cosmic objects. It is found that for the kinetic temperature $T > 120$ K, the semi-empirical expression gives large value as compared to the accurate one. The deviation becomes about 25\% larger than the accurate one at the kinetic temperature of 400 K.