VLA Observations of 9 Extended Green Objects in the Milky Way: Ubiquitous Weak, Compact Continuum Emission, and Multi-Epoch Emission from CH$_3$OH, H$_2$O, and NH$_3$ Masers

TL;DR

This study used the VLA to observe 9 Extended Green Objects in the Milky Way, revealing ubiquitous faint, compact continuum emission and multi-epoch maser activity, providing insights into the physical processes in massive protoclusters.

Contribution

First detailed VLA observations of EGOs showing diverse continuum processes and maser activity, advancing understanding of massive star formation environments.

Findings

Continuum emission detected in all EGOs, typically faint and compact.

Multiple physical processes, including thermal and non-thermal emissions, are present.

All EGOs exhibit methanol and water masers, indicating active star formation.

Abstract

We have observed a sample of 9 Extended Green Objects (EGOs) at 1.3 and 5 cm with the VLA with sub-arcsecond resolution and ~7-14 uJy/beam sensitivities in order to characterize centimeter continuum emission as it first appears in these massive protoclusters. We find EGO-associated continuum emission - within 1 arcsec of the extended 4.5 um emission - in every field, which is typically faint (order 10^1-10^2 uJy) and compact (unresolved at 0.3-0.5 arcsec). The derived spectral indices of our 36 total detections are consistent with a wide array of physical processes, including both non-thermal (19% of detections) and thermal free-free processes (e.g. ionized jets and compact HII regions, 78% of sample), and warm dust (1 source). We also find EGO-associated 6.7 GHz CH$_3$OH and 22 GHz H$_2$O maser emission in 100% of the sample, and NH$_3$ (3,3) masers in ~45%; we do not detect any NH$_3$ (6,6) masers at ~5.6 mJy/beam sensitivity. We find statistically-significant correlations between radio-distance and bolometric luminosities at two physical scales and three frequencies, consistent with thermal emission from ionized jets, but no correlation between water-maser and radio-distance luminosities for our sample. From these data, we conclude that EGOs likely host multiple different centimeter continuum-producing processes simultaneously. Additionally, at our ~1000 au resolution, we find that all EGOs except G18.89$-$0.47 contain 1 to 2 massive sources based on the presence of methanol maser groups, which is consistent with our previous work suggesting that these are typical massive protoclusters in which only one to a few of the YSOs are massive.