Surveying the Giant HII Regions of the Milky Way with SOFIA: II. M17

TL;DR

This study uses mid-infrared imaging from SOFIA to analyze the structure, stellar content, and evolutionary stages of the M17 giant HII region, identifying new massive young stellar objects and clarifying emission characteristics.

Contribution

It provides new mid-infrared observations of M17, identifies four new MYSO candidates, and offers insights into the region's star formation and emission properties.

Findings

Four new MYSO candidates identified

The most massive object, UC1, has a stellar mass of 64 solar masses

Southern bar of M17 is likely younger than the northern bar

Abstract

We present our second set of results from our mid-infrared imaging survey of Milky Way Giant HII regions. We used the FORCAST instrument on the Stratospheric Observatory For Infrared Astronomy to obtain 20 and 37$\mu$m images of the central ~10'X10' area of M17. We investigate the small- and large-scale properties of M17 using our data in conjunction with previous multi-wavelength observations. The spectral energy distributions of individual compact sources were constructed with Spitzer-IRAC, SOFIA-FORCAST, and Herschel-PACS photometry data and fitted with massive young stellar object (MYSO) models. Seven sources were found to match the criteria for being MYSO candidates, four of which are identified here for the first time, and the stellar mass of the most massive object, UC1, is determined to be 64 solar mass. We resolve the extended mid-infrared emission from the KW Object, and suggest that the angle of this extended emission is influenced by outflow. IRS5 is shown to decrease in brightness as a function of wavelength from the mid- to far-infrared, and has several other indicators that point to it being an intermediate mass Class II object and not a MYSO. We find that the large-scale appearance of emission in M17 at 20$\mu$m is significantly affected by contamination from the [SIII] emission line from the ionized gas of the Giant HII region. Finally, a number of potential evolutionary tracers yield a consistent picture suggesting that the southern bar of M17 is likely younger than the northern bar.