Contributed Talk - Splinter SOFIA

Thursday, 14 September 2023, 17:30   (H 3006)

The hidden neutral oxygen in M17SW

Cristian Guevara, Jürgen Stutzki, Volker Ossenkopf-Okada
I. Physikalisches Institut - Universität zu Köln

Our simultaneous observations of [12CII] and its isotope [13CII] towards M17SW and other Galactic star-forming regions (Guevara et al. 2020) have shown that C+ is optically thick, and even under self-absorption effects that mimic velocity components. Through a simple radiative transfer model composed of two layers, one background layer in emission and one foreground layer in absorption, we fit the [12CII] and [13CII] line emission simultaneously through multiple Gaussian components and to recover the "hidden" [12CII] emission at both layers in selected individual positions. The velocity profile is much simpler than expected, with column densities up to 10E19 cm −2. M17 is one of the brightest and most massive star-forming regions in the Galaxy, located at a distance of 1.9 kpcs (Wu et al. 2019). M17SW is where the HII region is localized and associated with a giant molecular cloud and PDR interface. M17SW is considered a prototype of an edge-on PDR. The HII region is ionized by a highly obscured (A V > 10 mag) cluster of many ( > 100) OB stars (Hoffmeister et al. 2008). New observations at high spectral resolution and signal-to-noise (S/N) towards M17SW in both [OI] transitions, 63 and 145 um, have shown strong correlations between the optically thin transitions [OI] 145 um and [13CII] and the optically thick transitions [OI] 63 um and [12CII]. Using both lines, we have recovered the hidden neutral oxygen through the abovementioned multi-component analysis. Background and foreground layers show high column density, above 10E19 cm-2 and 10E18 cm-2, respectively. The nature of the foreground layer is still under investigation.