Discovery of interstellar mercapto radicals (SH) with the GREAT instrument on SOFIA

We report the discovery of interstellar mercapto radicals (SH) along the sight-line to the submillimeter continuum source W49N. We have used the GREAT instrument on SOFIA to observe the 1383 GHz 2 Π3/2 J = 5/2 ← 3/2 lambda doublet in the upper sideband of the L1 receiver. The resulting spectrum reveals SH absorption in material local to W49N, as well as in foreground gas, unassociated with W49N, that is located along the sight-line. For the foreground material at velocities in the range 37–44 km s −1 with respect to the local standard of rest, we infer a total SH column density ∼4.6 × 10 12 cm −2 , corresponding to an abundance of ∼7 × 10 −9 relative to H2, and yielding an SH/H2S abundance ratio ∼0.13. The observed SH/H2S abundance ratio is much smaller than that predicted by standard models for the production of SH and H2S in turbulent dissipation regions and shocks, and suggests that the endothermic neutral-neutral reaction SH + H2 → H2S + H must be enhanced along with the ion-neutral reactions believed to produce CH + and SH + in diffuse molecular clouds.

[1]  Discovery of Interstellar Hydrogen Fluoride , 1997, astro-ph/9708013.

[2]  P. Hennebelle,et al.  Detection of hydrogen fluoride absorption in diffuse molecular clouds with Herschel/HIFI: an ubiquitous tracer of molecular gas , 2010, 1007.2148.

[3]  W. Meerts,et al.  A Molecular Beam Electric Resonance Study of the Hyperfine Λ Doubling Spectrum of OH, OD, SH, and SD , 1975 .

[4]  È. Roueff,et al.  Theoretical studies of interstellar molecular shocks – IV. The sulphur chemistry in diffuse clouds , 1986 .

[5]  A. H. Barrett,et al.  Radio Observations of OH in the Interstellar Medium , 1963, Nature.

[6]  K. Roth,et al.  Discovery of interstellar NH , 1991 .

[7]  E. Falgarone,et al.  Models of turbulent dissipation regions in the diffuse interstellar medium , 2009, 0901.3712.

[8]  N. Abel,et al.  Ultraviolet Survey of CO and H2 in Diffuse Molecular Clouds: The Reflection of Two Photochemistry Regimes in Abundance Relationships , 2008, 0807.0940.

[9]  J. Cernicharo,et al.  The molecular universe : proceedings of the 280th Symposium of the International Astronomical Union held in Toledo, Spain, May 30th - June 3 2011 , 2011 .

[10]  Comparative chemistry of diffuse clouds. III. Sulfur-bearing molecules. , 2002, astro-ph/0201163.

[11]  G. Herzberg,et al.  Note on CH^{+} in Interstellar Space and in the Laboratory. , 1941 .

[12]  P. Hennebelle,et al.  Interstellar OH+, H2O+ and H3O+ along the sight-line to G10.6–0.4 , 2010, 1005.5653.

[13]  L. Ziurys,et al.  Abundances of hydrogen sulfide in star-forming regions. , 1991, The Astrophysical journal.

[14]  A. Belloche,et al.  First interstellar detection of OH , 2010, 1004.2627.

[15]  J. L. Bourlot,et al.  A Model for Atomic and Molecular Interstellar Gas: The Meudon PDR Code , 2006, astro-ph/0602150.

[16]  G. Blake,et al.  Chlorine in dense interstellar clouds : the abundance of HCl in OMC-1. , 1985 .

[17]  K. Menten,et al.  Submillimeter absorption from SH+, a new widespread interstellar radical, 13CH+ and HCl , 2010, 1009.2825.

[18]  G. P. Forêts,et al.  C‐type shocks in the interstellar medium: profiles of CH+ and CH absorption lines , 1998 .

[19]  M. Asplund,et al.  The chemical composition of the Sun , 2009, 0909.0948.

[20]  G. P. Forêts,et al.  Theoretical studies of interstellar molecular shocks – I. General formulation and effects of the ion–molecule chemistry , 1985 .

[21]  E. Dishoeck,et al.  Rotationally inelastic and hyperfine resolved cross sections for OH–H2 collisions. Calculations using a new ab initio potential surface , 1994 .

[22]  K. Menten,et al.  Comparative study of CH+ and SH+ absorption lines observed towards distant star-forming regions , 2012, 1201.5457.

[23]  D. Lis,et al.  A Line Survey of Orion-KL from 607 to 725 GHz , 2001 .

[24]  G. Winnewisser,et al.  The Rotational Spectrum of SH and SD , 1996 .

[25]  FOURIER TRANSFORM FAR-INFRARED SPECTROSCOPY OF THE SH RADICAL , 1995 .

[26]  P. Hennebelle,et al.  Interstellar CH absorption in the diffuse interstellar medium along the sight-lines to G10.6-0.4 (W31C), W49N, and W51 , 2010 .

[27]  U. U. Graf,et al.  GREAT: the SOFIA high-frequency heterodyne instrument , 2012, 1203.2845.

[28]  M. Gerin,et al.  Molecular absorption lines toward star-forming regions: a comparative study of HCO+, HNC, HCN, and CN , 2010, 1006.0582.

[29]  P. Swings,et al.  Considerations Regarding Interstellar Molecules , 1937 .

[30]  George R. Carruthers,et al.  Rocket observation of interstellar molecular hydrogen , 1970 .