Investigating the Early Stages of Massive Star Formation in Protocluster Environments: Multiwavelength Studies of Extended Green Objects

Massive stars have a predilection for forming in clustered environments with other protostars. Therefore, the study of Massive Young Stellar Objects (MYSOs) necessarily requires the study of clusters of forming stars (protoclusters). Distinguishing between competing theories of massive star formation requires observing massive protoclusters at various stages in their evolutionary process. Extended Green Objects (EGOs) are signposts of massive young protoclusters believed to be in an evolutionary state just prior to the emergence of Ultracompact (UC) HII regions. This phase of protocluster evolution - in which the presence of massive stars can be confirmed but the natal clump has not yet been destroyed by the radiative feedback from MYSOs - is a critical one for distinguishing between competing star-formation theories. We have been conducting a multiwavelength study of a sample of 20 nearby EGOs with the goal of determining source bolometric luminosity, source multiplicity, and the evolutionary state of the most massive protocluster members. We have examined this sample using proprietary and archival infrared data (3.6 to 870 um) and modeled their Spectral Energy Distributions (SEDs). We have further examined this sample at 1.3 and 5 cm with the VLA, and determined continuum-source flux densities, spectral indices, and spatial distributions within these regions, as well as associations with water, methanol, and ammonia masers.
The average massive-source multiplicities (number of massive sources) are 1~2 per EGO, regardless of whether infrared or radio selection criteria are used. This is consistent with the model of EGOs as massive protoclusters which will form one to a few massive stars only, rather than extreme star-forming regions (e.g. 30 Doradus). Their luminosity-to-mass ratios (L/M) are broadly consistent with other massive protoclusters, but specifically seem to straddle the "IR-quiet" and "IR-bright" categories of MYSO classification. Generally, the difference between "IR-quiet" and "IR-bright" sources is the presence of a protostar which has started to produce significant radiative feedback, i.e. M > 8Msun in the central star. This suggests that EGOs may be in a particularly consequential stage of evolution compared to other samples with different selection criteria. We also find ubiquitous weak, compact 1.3 and 5 cm continuum emission, as well as a plethora of water masers, a mild association with ammonia (3,3) masers, and a 100% association with 6.7 GHz methanol masers (which are exclusively associated with massive protostars). Even at our high angular resolutions (~1000 au), most of our continuum sources remain unresolved. Their spectral indices span a broad range (-0.5 < alpha < 2) and are consistent with multiple possible emission mechanisms, from synchrotron to thermal free-free to thermal dust emission. In order to distinguish between these possibilities, high-resolution millimeter data are needed in order to perform detailed modeling of the radio SEDs.