2019ApJ...875..135T

Massive young stellar objects (MYSOs) are known to undergo an evolutionary phase in which high mass accretion rates drive strong outflows. A class of objects believed to trace this phase accurately is the Galactic Legacy Infrared Midplane Survey Extraordinaire (GLIMPSE) Extended Green Object (EGO) sample, so named for the presence of extended 4.5 µm emission on size scales of ∼0.1 pc in Spitzer images. We have been conducting a multiwavelength examination of a sample of 12 EGOs with distances of 1-5 kpc. In this paper, we present mid-infrared images and photometry of these EGOs obtained with the Stratospheric Observatory for Infrared Astronomy and subsequently construct spectral energy distributions (SEDs) for these sources from the near-infrared to submillimeter regimes using additional archival data. We compare the results from graybody models and several publicly available software packages that produce model SEDs in the context of a single massive protostar. The models yield typical R_* ∼ 10 R_☉, T_* ∼ 10³-10⁴ K, and L_* ∼ 1-40 x 10³ L_☉; the median L/M for our sample is 24.7 L_☉/M_☉. Model results rarely converge for R_* and T_*, but they do for L_*, which we take to be an indication of the multiplicity and inherently clustered nature of these sources even though, typically, only a single source dominates in the mid-infrared. The median L/M value for the sample suggests that these objects may be in a transitional stage between the commonly described "IR-quiet" and "IR-bright" stages of MYSO evolution. The median T_dust for the sample is less conclusive but suggests that these objects are either in this transitional stage or occupy the cooler (and presumably younger) part of the IR-bright stage.