Chemical environments of 6.7 GHz methanol maser sources

TL;DR

This study investigates the chemical environments of 6.7 GHz methanol maser sources, revealing molecular abundance ratios that trace their evolutionary stages in high-mass star formation.

Contribution

It provides the first detailed molecular line analysis of a large sample of methanol maser hosts, linking chemical ratios to evolutionary stages.

Findings

Abundance ratios increase or decrease with source evolution.

Methanol maser sources are at an intermediate evolutionary stage.

Chemical ratios suggest progression from quiescent to HII regions.

Abstract

6.7 GHz methanol masers are the brightest of class II methanol masers that are regarded as excellent signposts in the formation of young massive stars. We present here a molecular line study of 68 6.7 GHz methanol maser hosts chosen from the MMB catalogue, that have MALT90 data available. We performed (1) pixel-by-pixel study of 9 methanol maser sources that have high signal-to-noise ratio and (2) statistical study taking into account the entire 68 sources. We estimated the molecular column densities and abundances of N$_2$H$^+$(1-0), HCO$^+$(1-0), HCN(1-0) and HNC(1-0) lines. The derived abundances are found to be in congruence with the typical values found towards high mass star forming regions. We derived the column density and abundance ratios between these molecular species as an attempt to unveil the evolutionary stage of methanol maser sources. We found the column density and abundance ratio of HCN to HNC to increase and that of N$_2$H$^+$ to HCO$^+$ to decline with source evolution, as suggested by the chemical models. The HCN/HNC, N$_2$H$^+$/HCO$^+$, HNC/HCO$^+$ and N$_2$H$^+$/HNC ratios of the methanol maser sources are consistent with them being at a later evolutionary state than quiescent phase and possibly protostellar phase, but at an earlier stage than HII regions and PDRs.