VLA Observations of Nine Extended Green Objects in the Milky Way: Ubiquitous Weak, Compact Continuum Emission, and Multi-epoch Emission from Methanol, Water, and Ammonia Masers

We have observed a sample of nine Extended Green Objects (EGOs) at 1.3 and 5 cm with the Very Large Array (VLA) with subarcsecond resolution and ~7-14 μJy beam^-1-sensitivities in order to characterize centimeter continuum emission as it first appears in these massive protoclusters. We find an EGO-associated continu um emission-within 1″ of the extended 4.5 μm emission-in every field, which is typically faint (order 10¹-10² μJy) and compact (unresolved at 0″.3-0″.5). 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 H II regions, 78% of sample) and warm dust (1 source). We also find an EGO-associated 6.7 GHz CH₃OH and 22 GHz H₂O maser emission in 100% of the sample and a NH₃ (3,3) masers in ~45%; we do not detect any NH₃ (6,6) masers at ~5.6 mJy beam^-1 sensitivity. We find statistically-significant correlations between L _radio and L _bol at two physical scales and three frequencies, consistent with thermal emission from ionized jets, but no correlation between ${L}_{{{\rm{H}}}_{2}{\rm{O}}}$ and L _radio 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 ~ 2 massive sources based on the presence of CH₃OH 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 young stellar objects are massive.