The initial gas-phase sulfur abundance in the Orion Molecular Cloud from
  sulfur radio recombination lines
  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 sulfur abundance [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 Cn\alpha RRLs (with n=51 to 59) as well as nine
narrow line features separated from the Cn\alpha lines by delta v=-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 (PDR) of the
OMC. In these predominantly neutral gas layers, up to A_V~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 theta^1 Ori B, an absolute abundance of
[S]_Ori=(1.4+/-0.4)x10^-5. This value is consistent with emission models of the
observed sulfur RRLs if N(S+)~7x10^17 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.
