Masers as Probes of Massive Star Formation in the Nuclear Disk

TL;DR

This study uses masers and infrared data to investigate star formation in the Galaxy's nuclear disk, revealing low efficiency likely due to cosmic ray heating of molecular clouds.

Contribution

It combines maser observations with infrared data to analyze star formation activity and the impact of cosmic rays in the Galactic nuclear disk.

Findings

Methanol masers associate with mid-infrared green sources.

Star formation efficiency is low except in Sgr B2.

Cosmic rays significantly heat molecular clouds in the region.

Abstract

OH(1720 MHz) and methanol masers are now recognized to be excellent probes of the interactions of supernova remnants with molecular clouds and tracers of massive star formation, respectively. To better understand the nature of star formation activity in the central region of the Galaxy, we have used these two classes of masers combined with the IRAC and MIPS data to study prominent sites of ongoing star formation in the nuclear disk. The nuclear disk is characterized by massive GMCs with elevated gas temperatures, compared to their dust temperatures. We note an association between methanol masers and a class of mid-infrared ``green sources''. These highly embedded YSOs show enhanced 4.5micron emission due to excited molecular lines. The distribution of methanol masers and supernova remnants suggest a low efficiency of star formation (with the exception of Sgr B2), which we believe is due to an enhanced flux of cosmic ray electrons impacting molecular clouds in the nuclear disk. We also highlight the importance of cosmic rays in their ability to heat molecular clouds, and thus increase the gas temperature.