Image copyright: Rafael Defavari, via apod.com
SOFIA FORCAST and Archival Infrared Luminosity Measurements of 12 Extended Green Objects (EGOs)
In order to fully explore the possibility and properties of high-mass star formation in the EGO sample for the 1.3 cm project (below), I conducted a multi-wavelength analysis of 12 of the previous 20 EGOs from the near-infrared to sub-millimeter regimes using archival data from GLIMPSE, MIPSGAL, HiGAL, and ATLASGAL images, as well as new proprietary data at 19.7 and 37.1 um from the SOFIA FORCAST instrument. We used these data to construct SEDs for the dominant 37.1 um source in each EGO and performed radiative-transfer modeling (using three different publicly-available radiative-transfer modeling packages in addition to single-component greybody fitting) to extract well-constrained source luminosities for all 12 sources. We also calculated source masses based on the 870 um ATLASGAL emission using both the greybody-returned temperature results and the fitted ammonia temperatures of Cyganowski et al. (2013), to control for the possibility of different gas and dust temperatures.
We found minimal differences between dust and gas temperature for our sample. We did, however, find that all 12 EGOs occupy a specific range of luminosity-to-mass ratio (L/M) values. L/M is a possible indicator of evolutionary state for massive star-forming regions, based on the relatively quick increase in protostellar luminosity compared to mass at particular evolutionary stages. We found that, compared to other samples which have been divided into "infrared-quiet" and "infrared-bright" categories, the L/M values of our sources lie in an intermediate range between the characteristic L/M values of either infrared-based category. This is consistent with the fact that, according to the IR-quiet/IR-bright selection criteria, half of our sources would be classified as IR-quiet and half as IR-bright. We found similar, though less conclusive, trends in the dust and gas temperatures of the sample.
These trends, taken together, led us to suggest that it is possible that EGOs represent a particular transitional state in massive star formation in which the massive protostar transitions from an IR-quiet to an IR-bright state; most likely, such a shift would be due to a concrete physical change such as stellar bloating or reaching a critical protostellar mass (e.g. the "swelling" phase in the evolutionary tracks of Hosokawa & Omukai 2009). We note that this is a small sample size, and that this possibility should be more thoroughly explored with a larger sample and, ideally, higher angular resolution in the mid-infrared to submillimeter regimes.
These results were published earlier this year in Towner et al. (2019).
Class I Methanol Maser and 1.3 cm Continuum Observations of 20 EGOs in the Milky Way
My first project in graduate school was an examination of K-band (1.3 cm) maser and continuum data from the VLA. These observations were taken in C- and D-configurations toward 20 Extended Green Objects (EGOs) in the Milky Galaxy, which were selected from the original ~300-source EGO catalog (Cyganowski et al. 2008) for detailed multiwavelength follow-up. EGOs are defined by their extended emission in the 4.5 um Spitzer IRAC band, are strongly associated with both Class II (radiatively pumped) 6.7 GHz and Class I (collisionally pumped) 44 GHz methanol and 22 GHz water masers, and are thought to be sites of ongoing high-mass star formation in protocluster environments. In particular, EGOs are thought to occupy a specific evolutionary state in which active protostellar accretion is driving outflow activity.
Our data revealed unexpectedly ubiquitous emission from the 25 GHz ladder of Class I methanol masers, which is excited by a similar but narrower range of temperatures and densities as the more well-known 44 GHz Class I methanol line and is typically weaker. Consequently, 25 GHz maser emission has been rarely observed, and these results significantly increased the number of 25~GHz methanol masers observed at interferometric angular resolutions. Our data also revealed weak, compact 1.3 cm continuum emission associated with our target sources in 80% of the EGOs. In most cases, this emission exceeded a 4-sigma detection threshold but was unresolved, so fitted sizes of the emitting regions could not be determined. In twelve cases, the 1.3 cm continuum emission was coincident with previously observed 6.7 GHz maser spots, indicating that the centimeter-wavelength continuum is tracing deeply embedded massive protostars.
These results satisfied the research requirements for my Master's degree, and were published in Towner et al. (2017).