The initial gas-phase sulfur abundance in the Orion Molecular Cloud from sulfur radio recombination lines

Abstract

The abundances of chemical elements and their depletion factors are essential parameters for understanding the composition of the gas and dust that are ultimately incorporated into stars and planets. Sulfur is an abundant but peculiar element in the sense that, despite being less volatile than other elements (e.g., carbon), it is not a major constituent of dust grains in diffuse interstellar clouds. Here, we determine the gas-phase carbon-to-sulfur abundance ratio, [C]/[S], and the [S] in a dense star-forming cloud from new radio recombination lines (RRLs) detected with the Yebes 40m telescope – at relatively high frequencies (∼40 GHz ≃ 7 mm) and angular resolutions (down to 36″) – in the Orion Bar, a rim of the Orion Molecular Cloud (OMC). We detect nine Cnα RRLs (with n = 51–59) as well as nine narrow line features separated from the Cnα lines by δv = −8.4 ± 0.3 km s−1. Based on this velocity separation, we assign these features to sulfur RRLs, with little contribution of RRLs from the more condensable elements Mg, Si, or Fe. Sulfur RRLs lines trace the photodissociation region of the OMC. In these neutral gas layers, up to AV ≃ 4, the ions C+ and S+ lock in most of the C and S gas-phase reservoir. We determine a relative abundance of [C]Ori/[S]Ori = 10.4 ± 0.6 and, adopting the same [C]Ori measured in the translucent gas toward star θ1 Ori B, an absolute abundance of [S]Ori = (1.4 ± 0.4)⋅10−5. This value is consistent with emission models of the observed sulfur RRLs if N(S+) ≃ 7⋅1017 cm−2 (beam-averaged). The [S]Ori is the “initial” sulfur abundance in the OMC, before an undetermined fraction of the [S]Ori goes into molecules and ice mantles in the cloud interior. The inferred abundance [S]Ori matches the solar abundance, thus implying that there is little depletion of sulfur onto rocky dust grains, with D(S) = 0.0 ± 0.2 dex.