By Wanggi Lim and Joan Schmelz
Paper: Surveying the Giant H ii Regions of the Milky Way with SOFIA. II. M17
Lim, et al., 2020, ApJ, 888, 98.

SOFIA scientists have been able to detect deeply embedded, massive young stellar objects and derive their physical properties in the extreme environment of the Omega Nebula (M17). With SOFIA providing the sharpest mid-infrared image ever obtained of this region, the study was able to reveal a number of objects for the first time and deliver important clues of the global evolutionary history.

M17 is an iconic celestial structure in the northern sky that has been studied for more than 250 years. Even with this long history of investigation, researchers have been able to understand only limited physical properties of the nebula due to its complex structure with high extinction.

As massive young stars start to ionize hydrogen in their immediate environment, the resulting ionized hydrogen (HII) regions become bigger as the stars evolve. These individual regions sometimes intersect and interact with their neighbors forming structures that span large areas. These giant HII regions are believed to be the most active massive star forming regions in the Milky Way. They are extremely bright at all wavelengths and, thus, easily detected in other galaxies.

M17 is the closest of the ~50 giant HII regions in the Galaxy, about 6,500 light years from the Sun. The central stellar cluster is home to more than a hundred massive stars, which are responsible for the formation of the giant HII region and possibly for triggering new generations of stars.

The global evolutionary history of the entire M17 system is under active investigation. By utilizing the Faint Object infraRed Camera for the SOFIA Telescope, or FORCAST, instrument’s 20 micron (20μm) and 37μm band images, researchers were able to peer deep into the nebula with high angular resolution. Images from The Spitzer Space Telescope and the Wide-field Infrared Survey Explorer (WISE) telescope were highly saturated at comparable wavelengths. The data reveal nine new mid-infrared point sources in addition to the seven already known.

Using these data in conjunction with non-saturated images from Spitzer at wavelengths ≤ 8μm and Herschel at wavelengths ≥70μm, the team was able to construct spectral energy distributions for these 16 sources. The results were then compared with theoretical models of massive star formation to determine physical properties, including the mass of central stars.

The star formation history of M17 was then investigated by comparing two different evolutionary tracers of the proto-clusters: (1) the luminosity-to-mass ratio and (2) virial states of corresponding proto-clusters, which are defined from the SOFIA-FORCAST extended sources. In this analysis, in-depth studies toward the northern bar of M17 were carried out for the first time, while previous studies focused mainly on southern bar. Results indicate that the northern bar is more evolved than the southern bar but younger than the central cluster. The virial states of these regions indicate that the northern bar underwent extreme kinematic episodes, possibly at the colliding interface of two different large-scale wind bubbles.

Since star formation is less active in the northern bar, the virial states revealed by the global kinematic analyses may indicate a negative feedback effect on star and star-cluster formation in such environments. SOFIA’s ability to search inside dense molecular clouds with its mid-infrared vision allows researchers to move one step closer to understanding the past and present of M17.