PDRs4All. IX. Sulfur elemental abundance in the Orion Bar

Abstract

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 H$_2$S 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<!PCT!> 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 mu m line. 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. We used the S III 18.7 mu m S IV 10.5 mu m, and S l 25.249 mu m 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. The S III 18.7 mu m and S IV 10.5 mu m 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 sim 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 mu m 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 mu m line in a PDR. The detailed modeling of DF3 using the Meudon PDR code shows that the emission of the S l 25.249 mu m line is coming from warm ($>$ 40 K) molecular gas located at A$_ V $ sim 1$-$5 mag from the ionization front. Moreover, the intensity of the S l 25.249 mu m line is only accounted for if we assume the presence of undepleted sulfur. 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.