Dust, ice and gas in time (DIGIT): Herschel and Spitzer spectro-imaging of SMM3 and SMM4 in Serpens

Author(s)

Odysseas Dionatos, J. K. Jørgensen, G. J. Herczeg, N. J. Evans, L. E. Kristensen, J. E. Lindberg, E. F. van Dishoeck

Abstract

Context. Mid- and far-infrared observations of the environment around

embedded protostars reveal a plethora of high excitation molecular and

atomic emission lines. Different mechanisms for the origin of these

lines have been proposed, including shocks induced by protostellar jets

and radiation or heating by the embedded protostar of its immediate

surroundings. Aims: By studying of the most important molecular

and atomic coolants, we aim at constraining the physical conditions

around the embedded protostars SMM3 and SMM4 in the Serpens molecular

cloud core and measuring the CO/H2 ratio in warm gas.

Methods: Spectro-imaging observations from the Spitzer Infrared

Spectrograph (IRS) and the Herschel Photodetector Array Camera and

Spectrometer (PACS) provide an almost complete wavelength coverage

between 5 and 200 μm. Within this range, emission from all major

molecular (H2, CO, H2O and OH) and many atomic

([OI], [CII], [FeII], [SiII] and [SI]) coolants of excited gas are

detected. Emission line maps reveal the morphology of the observed

emission and indicate associations between the different species. The

excitation conditions for molecular species are assessed through

rotational diagrams. Emission lines from major coolants are compared to

the results of steady-state C- and J-type shock models. Results:

Line emission tends to peak at distances of ~10-20″ from the

protostellar sources with all but [CII] peaking at the positions of

outflow shocks seen in near-IR and sub-millimeter interferometric

observations. The [CII] emission pattern suggests that it is most likely

excited from energetic UV radiation originating from the nearby

flat-spectrum source SMM6. Excitation analysis indicates that

H2 and CO originate in gas at two distinct rotational

temperatures of ~300 K and 1000 K, while the excitation temperature for

H2O and OH is ~100-200 K. The morphological and physical

association between CO and H2 suggests a common excitation

mechanism, which allows direct comparisons between the two molecules.

The CO/H2 abundance ratio varies from ~10-5 in the

warmer gas up to ~10-4 in the hotter regions. Shock models

indicate that C-shocks can account for the observed line intensities if

a beam filling factor and a temperature stratification in the shock

front are considered. C-type shocks can best explain the emission from

H2O. The existence of J-shocks is suggested by the strong

atomic/ionic (except for [CII]) emission and a number of line ratio

diagnostics. Dissociative shocks can account for the CO and

H2 emission in a single excitation temperature structure. Conclusions: The bulk of cooling from molecular and atomic lines is

associated with gas excited in outflow shocks. The strong association

between H2 and CO constrain their abundance ratio in warm

gas. Both C- and J-type shocks can account for the observed molecular

emission; however, J-shocks are strongly suggested by the atomic

emission and provide simpler and more homogeneous solutions for CO and

H2. The variations in the CO/H2 abundance ratio

for gas at different temperatures can be interpreted by their

reformation rates in dissociative J-type shocks, or the influence of

both C and J shocks.

 

Herschel is an ESA space observatory with science instruments provided

by European-led Principal Investigator consortia and with important

participation from NASA.Appendices A-C are available in electronic form

at www.aanda.org

Organisation(s)

Department of Astrophysics

External organisation(s)

University of Copenhagen, Natural History Museum of Denmark, Peking University, University of Texas, Austin, Leiden University, Max-Planck-Institut für extraterrestrische Physik

Journal

Astronomy & Astrophysics

Volume

558

No. of pages

22

ISSN

0004-6361

DOI

https://doi.org/10.1051/0004-6361/201220452

Publication date

10-2013

Peer reviewed

Yes

Austrian Fields of Science 2012

103004 Astrophysics, 103003 Astronomy

Keywords

Portal url

https://ucrisportal.univie.ac.at/en/publications/7688b840-9c3f-4564-95f6-1d8a5201dd68