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

Langer, W. D. and Goldsmith, P. F. and Pineda, J. L. and Chambers, E. T. and Jacobs, Karl and Richter, Heiko (2018) The nature of molecular cloud boundary layers from SOFIA [OI] observations. Astronomy & Astrophysics, 617 (A94). EDP Sciences. DOI: 10.1051/0004-6361/201832691 ISSN 0004-6361

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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. To 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 to 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.

Item URL in elib:https://elib.dlr.de/121949/
Document Type:Article
Title:The nature of molecular cloud boundary layers from SOFIA [OI] observations
AuthorsInstitution or Email of AuthorsAuthors ORCID iD
Langer, W. D.JPL, PasadenaUNSPECIFIED
Goldsmith, P. F.JPL, PasadenaUNSPECIFIED
Pineda, J. L.JPL, PasadenaUNSPECIFIED
Chambers, E. T.NASA Ames Research CenterUNSPECIFIED
Jacobs, KarlKOSMA, I. Physikalisches Institut Universität zu KölnUNSPECIFIED
Richter, HeikoHeiko.Richter (at) dlr.deUNSPECIFIED
Date:16 July 2018
Journal or Publication Title:Astronomy & Astrophysics
Refereed publication:Yes
Open Access:Yes
Gold Open Access:No
In ISI Web of Science:Yes
DOI :10.1051/0004-6361/201832691
Publisher:EDP Sciences
Keywords:THz, OI, molecular cloud
HGF - Research field:Aeronautics, Space and Transport
HGF - Program:Space
HGF - Program Themes:other
DLR - Research area:Raumfahrt
DLR - Program:R - no assignment
DLR - Research theme (Project):R - no assignment
Location: Berlin-Adlershof
Institutes and Institutions:Institute of Optical Sensor Systems > Terahertz and Laser Spectroscopy
Deposited By: Richter, Dr.rer.nat. Heiko
Deposited On:02 Oct 2018 08:32
Last Modified:01 Aug 2019 03:00

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