Observations and Analysis of Absorption Lines including J = K Rotational Levels of CH3CN: The Envelope of Sagittarius B2(M)

TL;DR

This study detects and analyzes absorption lines of acetonitrile (CH3CN) with J=K levels in the envelope of Sagittarius B2(M), revealing highly concentrated rotational states due to specific excitation conditions in the molecular cloud.

Contribution

It provides the first observational evidence of J=K level concentration of CH3CN in Sgr B2(M) and characterizes its physical conditions using radio telescope data.

Findings

Detected absorption lines of CH3CN with J=K levels in Sgr B2(M)

Derived physical parameters including temperature and column density

Confirmed the presence of concentrated J=K levels in the molecular envelope

Abstract

Molecules in diffuse and translucent clouds experience cooling as a result of radiation and less excitation from collisions. However, a rotation around a molecular axis of acetonitrile, CH3CN, cannot be cooled by radiation, causing rotational populations to concentrate at the J = K levels. We aim to search for absorption lines of CH3CN having J = K level concentrations in diffuse and translucent clouds. The JK = 43-33 transition at 73.6 GHz was investigated toward Sgr B2(M) in the Galactic Center region and other sources, using the Nobeyama 45 m telescope. Based on the detected absorption lines toward Sgr B2(M), a radiation temperature of 2.8 +/- 0.5 K, kinetic temperature of 88 +/- 29 K, and column density of (1.35 +/- 0.14) x 10^14 cm-2 were derived for this molecule, revealing extremely concentrated J = K levels due to the lower excitation temperature and the higher kinetic temperature. The absorption lines occurred at a velocity of 64 km s-1. The results confirm that CH3CN with J = K level concentrations exists in the envelope of Sgr B2(M).