# Revisiting the Temperature of the Diffuse ISM with CHESS Sounding Rocket Observations

## TL;DR

This study uses CHESS sounding rocket observations to measure the temperature and molecular hydrogen properties of diffuse interstellar clouds, revising previous temperature estimates and analyzing rotational level contributions.

## Contribution

It provides new measurements of H₂ excitation and temperature in the diffuse ISM using UV rocket data, correcting previous estimates by accounting for higher rotational level opacity effects.

## Key findings

- Revised average gas kinetic temperature T₀₁ = 68 ± 13 K, 12% lower than previous values.
- Discovered that higher rotational levels influence the absorption features, affecting column density estimates.
- Compared new measurements with past data, highlighting the importance of opacity effects in temperature determination.

## Abstract

Measuring the temperature and abundance patterns of clouds in the interstellar medium (ISM) provides an observational basis for models of the physical conditions within the clouds, which play an important role in studies of star and planet formation. The Colorado High-resolution Echelle Stellar Spectrograph (CHESS) is a far ultraviolet rocket-borne instrument designed to study the atomic-to-molecular transitions within diffuse molecular and translucent cloud regions. The final two flights of the instrument observed $\beta^{1}$ Scorpii ($\beta$ Sco) and $\gamma$ Arae. We present flight results of interstellar molecular hydrogen (H$_{\rm 2}$) excitation on the sightlines, including measurements of the column densities and temperatures. These results are compared to previous values that were measured using the damping wings of low J$^{\prime \prime}$ H$_{\rm 2}$ absorption features (Savage et al. 1977). For $\beta$ Sco, we find that the derived column density of the J$^{\prime \prime}$ = 1 rotational level differs by a factor of 2-3 when compared to the previous observations. We discuss the discrepancies between the two measurements and show that the source of the difference is due to the opacity of higher rotational levels contributing to the J$^{\prime \prime}$ = 1 absorption wing, increasing the inferred column density in the previous work. We extend this analysis to 9 $Copernicus$ and 13 $FUSE$ spectra to explore the interdependence of the column densities of different rotational levels and how the H$_{\rm 2}$ kinetic temperature is influenced by these relationships. We find a revised average gas kinetic temperature of the diffuse molecular ISM of T$_{01}$ = 68 $\pm$ 13 K, 12% lower than the value found previously.