Universität zu Köln

Langer, W. D., Goldsmith, P. F., Pineda, J. L., Chambers, E. T., Jacobs, K. and Richter, H. (2018). The nature of molecular cloud boundary layers from SOFIA [O I] observations. Astron. Astrophys., 617. LES ULIS CEDEX A: EDP SCIENCES S A. ISSN 1432-0746

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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 mu m 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 mu m [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-sigma 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 x 10(5) in units of Kcm(-3), with kinetic temperatures of order 75-100K and H-2 densities, n(H-2) similar to 2-4 x 10(-3) cm(-3). For the other two sources, where the upper limits on [O I] to [C II] are larger, Pth less than or similar to 10(5) (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.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Langer, W. D. UNSPECIFIED UNSPECIFIED UNSPECIFIED Goldsmith, P. F. UNSPECIFIED UNSPECIFIED UNSPECIFIED Pineda, J. L. UNSPECIFIED UNSPECIFIED UNSPECIFIED Chambers, E. T. UNSPECIFIED UNSPECIFIED UNSPECIFIED Jacobs, K. UNSPECIFIED UNSPECIFIED UNSPECIFIED Richter, H. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-172603
DOI:	10.1051/0004-6361/201832691
Journal or Publication Title:	Astron. Astrophys.
Volume:	617
Date:	2018
Publisher:	EDP SCIENCES S A
Place of Publication:	LES ULIS CEDEX A
ISSN:	1432-0746
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language PHOTODISSOCIATION REGIONS; GAS; MODEL; LINE Multiple languages Astronomy & Astrophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/17260