The Rare 23.1-GHz Methanol Masers in NGC 7538 IRS 1

TL;DR

This study reports high-resolution observations of 23.1-GHz methanol masers in NGC 7538 IRS 1, confirming their maser nature, spatially resolving components, and exploring their association with other masers and theoretical models.

Contribution

First high-resolution detection and spatial resolution of 23.1-GHz methanol masers in NGC 7538 IRS 1, revealing their distinct features and associations with other masers.

Findings

Confirmed maser nature with brightness temperatures >10^4 K.

Spatially resolved two velocity components as distinct features.

Observed positional offsets from other methanol masers, suggesting complex conditions.

Abstract

We present high angular resolution (FWHM_beam < 0.2") observations of the 23.1-GHz methanol (CH_3OH) transition toward the massive-star forming region NGC 7538 IRS 1. The two velocity components previously reported by Wilson et al. are resolved into distinct spatial features with brightness temperatures (T_B) greater than 10^4 K, proving their maser nature. Thus, NGC 7538 IRS 1 is the third region confirmed to show methanol maser emission at this frequency. The brighter 23.1-GHz spot coincides in position with a rare formaldehyde (H_2CO) maser, and marginally with a 22.2-GHz water (H_2O) maser, for which we report archival observations. The weaker CH_3OH spot coincides with an H_2O maser. The ratio of T_B for the 23.1-GHz masers to that of the well-known 12.2-GHz CH_3OH masers in this region roughly agrees with model predictions. However, the 23.1-GHz spots are offset in position from the CH_3OH masers at other frequencies. This is difficult to interpret in terms of models that assume that all the masers arise from the same clumps, but it may result from turbulent conditions within the gas or rapid variations in the background radiation field.