PDRs4All IX. Sulfur elemental abundance in the Orion Bar
Journal article, 2024
Context. One of the main problems in astrochemistry is determining the amount of sulfur in volatiles and refractories in the interstellar medium. The detection of the main sulfur reservoirs (icy H2S and atomic gas) has been challenging, and estimates are based on the reliability of models to account for the abundances of species containing less than 1% of the total sulfur. The high sensitivity of the James Webb Space Telescope provides an unprecedented opportunity to estimate the sulfur abundance through the observation of the [S I] 25.249 µm line. Aims. Our aim is to determine the amount of sulfur in the ionized and warm molecular phases toward the Orion Bar as a template to investigate sulfur depletion in the transition between the ionized gas and the molecular cloud in HII regions.
Methods. We used the [S III] 18.7 µm, [S IV] 10.5 µm, and [S l] 25.249 µm lines to estimate the amount of sulfur in the ionized and molecular gas along the Orion Bar. For the theoretical part, we used an upgraded version of the Meudon photodissociation region (PDR) code to model the observations. New inelastic collision rates of neutral atomic sulfur with ortho- and para- molecular hydrogen were calculated to predict the line intensities.
Results. The [S III] 18.7 µm and [S IV] 10.5 µm lines are detected over the imaged region with a shallow increase (by a factor of 4) toward the HII region. This suggests that their emissions are partially coming from the Orion Veil. We estimate a moderate sulfur depletion, by a factor of ∼2, in the ionized gas. The corrugated interface between the molecular and atomic phases gives rise to several edge-on dissociation fronts we refer to as DF1, DF2, and DF3. The [S l] 25.249 µm line is only detected toward DF2 and DF3, the dissociation fronts located farthest from the HII region. This is the first ever detection of the [S l] 25.249 µm line in a PDR. The detailed modeling of DF3 using the Meudon PDR code shows that the emission of the [S l] 25.249 µm line is coming from warm (>40 K) molecular gas located at AV ∼1–5 mag from the ionization front. Moreover, the intensity of the [S l] 25.249 µm line is only accounted for if we assume the presence of undepleted sulfur.
Conclusions. Our data show that sulfur remains undepleted along the ionic, atomic, and molecular gas in the Orion Bar. This is consistent with recent findings that suggest that sulfur depletion is low in massive star-forming regions because of the interaction of the UV photons coming from the newly formed stars with the interstellar matter.
Methods. We used the [S III] 18.7 µm, [S IV] 10.5 µm, and [S l] 25.249 µm lines to estimate the amount of sulfur in the ionized and molecular gas along the Orion Bar. For the theoretical part, we used an upgraded version of the Meudon photodissociation region (PDR) code to model the observations. New inelastic collision rates of neutral atomic sulfur with ortho- and para- molecular hydrogen were calculated to predict the line intensities.
Results. The [S III] 18.7 µm and [S IV] 10.5 µm lines are detected over the imaged region with a shallow increase (by a factor of 4) toward the HII region. This suggests that their emissions are partially coming from the Orion Veil. We estimate a moderate sulfur depletion, by a factor of ∼2, in the ionized gas. The corrugated interface between the molecular and atomic phases gives rise to several edge-on dissociation fronts we refer to as DF1, DF2, and DF3. The [S l] 25.249 µm line is only detected toward DF2 and DF3, the dissociation fronts located farthest from the HII region. This is the first ever detection of the [S l] 25.249 µm line in a PDR. The detailed modeling of DF3 using the Meudon PDR code shows that the emission of the [S l] 25.249 µm line is coming from warm (>40 K) molecular gas located at AV ∼1–5 mag from the ionization front. Moreover, the intensity of the [S l] 25.249 µm line is only accounted for if we assume the presence of undepleted sulfur.
Conclusions. Our data show that sulfur remains undepleted along the ionic, atomic, and molecular gas in the Orion Bar. This is consistent with recent findings that suggest that sulfur depletion is low in massive star-forming regions because of the interaction of the UV photons coming from the newly formed stars with the interstellar matter.
HII regions
ISM: individual objects: Orion Bar
astrochemistry
ISM: abundances
photon-dominated region (PDR)
Author
A. Fuente
Centro de Astrobiologia (CAB)
Evelyne Roueff
Paris Observatory
Franck Le Petit
Paris Observatory
Jacques Le Bourlot
Paris Observatory
Paris Descartes University
E. Bron
Paris Observatory
M.G. Wolfire
College of Computer, Mathematical, & Natural Sciences
James F. Babb
Harvard-Smithsonian Center for Astrophysics
Pei Gen Yan
Harvard-Smithsonian Center for Astrophysics
Takashi Onaka
University of Tokyo
John H Black
Chalmers, Space, Earth and Environment, Astronomy and Plasmaphysics
Ilane Schroetter
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Dries Van De Putte
Space Telescope Science Institute (STScI)
Ameek Sidhu
Western University
Amelie Canin
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Boris Trahin
Institut d'Astrophysique Spatiale
Felipe Alarcon
University of Michigan
Ryan Chown
Western University
Olga Kannavou
Institut d'Astrophysique Spatiale
O. Berné
Institut de Recherche en Astrophysique et Planétologie (IRAP)
Emilie Habart
Institut d'Astrophysique Spatiale
Els Peeters
SETI Institute
Western University
J.R. Goicoechea
CSIC - Instituto de Fisica Fundamental (IFF)
Marion Zannese
Institut d'Astrophysique Spatiale
Raphael Meshaka
Paris Observatory
Institut d'Astrophysique Spatiale
Yoko Okada
University of Cologne
M. Rollig
Physikalischer Verein
Goethe University Frankfurt
R. Le Gal
Institut de Radioastronomie Millimétrique (IRAM)
Institut de Planétologie et d’Astrophysique de Grenoble (IPAG)
Dinalva A. Sales
Estatística e Física
Maria Elisabetta Palumbo
Istituto nazionale di astrofisica (INAF)
Giuseppe Antonio Baratta
Istituto nazionale di astrofisica (INAF)
S. C. Madden
University Paris-Saclay
Naslim Neelamkodan
United Arab Emirates University
Ziwei E. Zhang
RIKEN
P. C. Stancil
University of Georgia
Astronomy and Astrophysics
0004-6361 (ISSN) 1432-0746 (eISSN)
Vol. 687 A87Subject Categories
Meteorology and Atmospheric Sciences
Astronomy, Astrophysics and Cosmology
Atom and Molecular Physics and Optics
DOI
10.1051/0004-6361/202449229