First science results from SOFIA/FORCAST: The mid-infrared view of the compact HII region W3A

TL;DR

This paper presents initial mid-infrared observations of the W3A star-forming region using SOFIA/FORCAST, revealing dust structures, PAH emission, and the effects of stellar feedback on the surrounding gas and dust.

Contribution

First mid-infrared imaging results from SOFIA/FORCAST of W3A, providing detailed dust and PAH emission maps and insights into dust properties and star feedback effects.

Findings

IRS2 has created a dust-free bubble of ~0.05 pc radius.

Dust in the ionized shell has a dust-to-gas ratio similar to the diffuse ISM.

IR-to-UV opacity of dust in the shell is about three times higher than in the diffuse ISM.

Abstract

The massive star forming region W3 was observed with the faint object infrared camera for the SOFIA telescope (FORCAST) as part of the Short Science program. The 6.4, 6.6, 7.7, 19.7, 24.2, 31.5 and 37.1 \um bandpasses were used to observe the emission of Polycyclic Aromatic Hydrocarbon (PAH) molecules, Very Small Grains and Big Grains. Optical depth and color temperature maps of W3A show that IRS2 has blown a bubble devoid of gas and dust of $\sim$0.05 pc radius. It is embedded in a dusty shell of ionized gas that contributes 40% of the total 24 \um emission of W3A. This dust component is mostly heated by far ultraviolet, rather than trapped Ly$\alpha$ photons. This shell is itself surrounded by a thin ($\sim$0.01 pc) photodissociation region where PAHs show intense emission. The infrared spectral energy distribution (SED) of three different zones located at 8, 20 and 25\arcsec from IRS2, show that the peak of the SED shifts towards longer wavelengths, when moving away from the star. Adopting the stellar radiation field for these three positions, DUSTEM model fits to these SEDs yield a dust-to-gas mass ratio in the ionized gas similar to that in the diffuse ISM. However, the ratio of the IR-to-UV opacity of the dust in the ionized shell is increased by a factor $\simeq$3 compared to the diffuse ISM.