PDRs4All

IX. Sulfur elemental abundance in the Orion Bar

¹ Centro de Astrobiología (CAB), CSIC-INTA, Ctra. de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Spain
e-mail: afuente@cab.inta-csic.es
² LERMA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, 92190 Meudon, France
³ Astronomy Department, University of Maryland, College Park, MD 20742, USA
⁴ Center for Astrophysics | Harvard & Smithsonian, MS 14, 60 Garden St., Cambridge, MA 02138, USA
⁵ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁶ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden
⁷ Institut de Recherche en Astrophysique et Planétologie, Université Toulouse III – Paul Sabatier, CNRS, CNES, 9 Av. du colonel Roche, 31028 Toulouse Cedex 04, France
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Department of Physics & Astronomy, The University of Western Ontario, London ON N6A 3K7, Canada
¹⁰ Institute for Earth and Space Exploration, The University of Western Ontario, London ON N6A 3K7, Canada
¹¹ Institut d'Astrophysique Spatiale, Université Paris-Saclay, CNRS, Bâtiment 121, 91405 Orsay Cedex, France
¹² Department of Astronomy, University of Michigan, 1085 South University Avenue, Ann Arbor, MI 48109, USA
¹³ Carl Sagan Center, SETI Institute, 339 Bernardo Avenue, Suite 200, Mountain View, CA 94043, USA
¹⁴ Instituto de Física Fundamental (CSIC), Calle Serrano 121–123, 28006 Madrid, Spain
¹⁵ I. Physikalisches Institut der Universität zu Köln, Zülpicher Straße 77, 50937 Köln, Germany
¹⁶ Physikalischer Verein – Gesellschaft für Bildung und Wissenschaft, Robert-Mayer-Str. 2, 60325 Frankfurt, Germany
¹⁷ Goethe-Universität, Physikalisches Institut, Frankfurt am Main, Germany
¹⁸ Institut de Planétologie et d'Astrophysique de Grenoble (IPAG), Université Grenoble Alpes, CNRS, 38000 Grenoble, France
¹⁹ Institut de Radioastronomie Millimétrique (IRAM), 300 Rue de la Piscine, 38406 Saint-Martin-d'Hères, France
²⁰ Instituto de Matemática, Estatística e Física, Universidade Federal do Rio Grande, 96201-900 Rio Grande, RS, Brazil
²¹ INAF – Osservatorio Astrofisico di Catania, Via Santa Sofia 78, 95123 Catania, Italy
²² AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
²³ Department of Physics, College of Science, United Arab Emirates University (UAEU), Al-Ain 15551, UAE
²⁴ Star and Planet Formation Laboratory, RIKEN Cluster for Pioneering Research, Wako, Saitama 351-0198, Japan
²⁵ Department of Physics and Astronomy, University of Georgia, Athens, GA 30602-2451, USA
²⁶ Université Paris Cité, 85 Bd. Saint-Germain, 75006 Paris, France

Received: 13 January 2024
Accepted: 25 March 2024

Abstract

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 A_V ~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.