PDRs4All IX. Sulfur elemental abundance in the Orion Bar

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_2S 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 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. 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. 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. 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. 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.