The nature of molecular cloud boundary layers from SOFIA [O I] observations

Abstract

Context. Dense highly ionized boundary layers (IBLs) outside of the neutral Photon Dominated Regions (PDRs) have recently been detected via the 122 and 205 μm transitions of ionized nitrogen. These layers have higher densities than in the Warm Ionized Medium (WIM) but less than typically found in H II regions. Observations of [C II] emission, which is produced in both the PDR and IBL, do not fully define the characteristics of these sources. Observations of additional probes which just trace the PDRs, such as the fine structure lines of atomic oxygen, are needed derive their properties and distinguish among different models for [C II] and [N II] emissison. Aims. We derive the properties of the PDRs adjacent to dense highly ionized boundary layers of molecular clouds. Methods. We combine high-spectral resolution observations of the 63 μm [O I] fine structure line taken with the upGREAT HFA-band instrument on SOFIA with [C II] observations to constrain the physical conditions in the PDRs. The observations consist of samples along four lines of sight (LOS) towards the inner Galaxy containing several dense molecular clouds. We interpret the conditions in the PDRs using radiative transfer models for [C II] and [O I]. Results. We have a 3.5-σ detection of [O I] toward one source but only upper limits towards the others. We use the [O I] to [C II] ratio, or their upper limits, and the column density of C+ to estimate the thermal pressure, Pth, in these PDRs. In two LOS the thermal pressure is likely in the range 2–5 × 105 in units of K cm−3, with kinetic temperatures of order 75–100 K and H2 densities, n(H2) ~ 2–4 × 103 cm−3. For the other two sources, where the upper limits on [O I] to [C II] are larger, Pth ≲105 (K cm−3). We have also used PDR models that predict the [O I] to [C II] ratio, along with our observations of this ratio, to limit the intensity of the Far UV radiation field. Conclusions. The [C II] and [N II] emission with either weak, or without any, evidence of [O I] indicates that the source of dense highly ionized gas traced by [N II] most likely arises from the ionized boundary layers of clouds rather than from H II regions.

The ALPINE-ALMA [C II] survey

“…On the other hand, as C + is emitted only in regions where star formation is taking place, the molecular gas not illuminated by stars would be undetected. Finally, [C ii] emission is also found in the outer parts of nearby galaxies, where low-density H ii regions were reported to contribute up to ∼50% of the extended [C ii] emission (Madden et al 1993; see also Langer et al 2016Langer et al , 2018 for their studies of the Milky Way). More recently, [C ii] emission has also been observed in the outer parts of high-redshift SFGs (Fujimoto et al 2019;Ginolfi et al 2020b).…”

Opening the Treasure Chest in Carina

“…The pillar so formed is supported against radial collapse by magnetic field but undergoes longitudinal erosion by stellar winds and radiation. There exist multiple analytical (Bertoldi 1989;Bertoldi & McKee 1990) and numerical models (Lefloch & Lazareff 1994;Bisbas et al 2011), which explain and reproduce many of the observed features of these structures sculpted by the radiation and wind from massive O/B stars starting from either an isolated or an embedded pre-existing dense clump or core. Additionally, there are observations which find the presence of sequential star formation in such structures consistent with triggered formation of stars due to the photoionization-induced shocks (Sugitani et al 2002;Ikeda et al 2008).…”