Interstellar medium in the M 43 nebula

TL;DR

This paper catalogs interstellar absorption lines toward HD 37061 in the M 43 nebula, revealing excited atomic and molecular levels, including fluorescence-populated Fe II and vibrationally excited H2 with high rotational temperature.

Contribution

It provides a detailed list of interstellar absorption lines and insights into the excitation mechanisms in the M 43 nebula's interstellar medium.

Findings

Detection of uncommon excited atomic levels in interstellar clouds

Identification of vibrationally excited H2 with high rotational temperature

Measurement of ortho/para H2 ratio as 2.7

Abstract

We present a list of interstellar absorption lines in the direction of HD 37061 in the M 43 nebula. Some of the absorption lines arise from atomic excited levels that are uncommon in interstellar clouds. The excited levels of Fe II are populated by fluorescence. We found a large number of H2 molecular absorption lines arising from vibrationally excited levels. The ortho/para H2 ratio is equal to 2.7. The H2 rotational temperature of vibrational levels 1 - 5 exceeds 2000 K.