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Home All issues Volume 542 (June 2012) A&A, 542 (2012) L6 Full HTML
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A&A
Volume 542, June 2012
GREAT: early science results
Article Number L6
Number of page(s) 4
Section Letters
DOI https://doi.org/10.1051/0004-6361/201218870
Published online 10 May 2012

© ESO, 2012

1. Introduction

In the seven decades following the discovery of interstellar CH (Swings & Rosenfeld 1937) – the first identified interstellar molecule – five or six additional neutral diatomic hydrides have been discovered in the interstellar gas: OH (Weinreb 1963), H2 (Carruthers 1970), HCl (Blake et al. 1985), NH (Meyer & Roth 1991), HF (Neufeld et al. 1995), and SiH (Schilke et al. 2001; a tentative detection). These discoveries were obtained from observations over a remarkably wide range of wavelengths, from the far-ultraviolet (H2, discovered at 1013–1110 Å), through the near-UV (NH at 3358 Å), the optical (CH at 4300 Å), the far-infrared (HF at 122 μm), the submillimeter (SiH at 478 μm, and HCl at 479 μm), and the radio (OH at 18 cm) spectral regions. In addition to these neutral molecules, four hydride molecular ions have now been discovered: CH+ (Douglas & Herzberg 1941), OH+ (Gerin et al. 2010a; Wyrowski et al. 2010), SH+ (Benz et al. 2010), and HCl+ (DeLuca et al. 2012; Gupta et al., in prep.), the latter three in just the past two years.

The diatomic hydrides represent the simplest of interstellar molecules, and – carefully interpreted in the context of astrochemical models – may provide key information about the interstellar environment. For example, the OH+ abundance yields constraints on the rate of cosmic ray ionization, which initiates the chemical network leading to OH+ (e.g. Neufeld et al. 2010). The CH+, SH+ and OH abundances, by contrast, probe the influence of shocks and turbulent dissipation, which heat the interstellar gas and/or lead to ambipolar diffusion; these processes may thereby drive endothermic chemical reactions that lead to enhanced abundances of CH+, SH+ and OH (e.g. Flower et al. 1985; Godard et al. 2009).

The mercapto radical, SH – which, like CH+ and SH+, is expected to show a strong abundance enhancement in warm regions that are heated by shocks or turbulent dissipation – has been conspicuously absent from the list of previously detected molecules. In its ground rotational state, SH can absorb radiation in the lambda doublet near 1383 GHz, a frequency that is completely inaccessible from the ground, due to atmospheric absorption, and which – by bad fortune – falls right in the gap between bands 5 and 6 of the Herschel Space Observatory’s Heterodyne Instrument for the Far-Infrared (HIFI). Fortunately, this spectral region is now accessible to the GREAT (German REceiver for Astronomy at Terahertz frequencies) instrument1 on SOFIA (Stratospheric Observatory For Infrared Astronomy), and in this Letter we report the first detection of interstellar SH with the use of that instrument. The observations and data reduction are described in Sect. 2, and the results presented in Sect. 3. In Sect. 4 we discuss the implications of the measured SH column density in the context of astrochemical models.

2. Observations and data reduction

We observed the 2Π3/2 J = 5/2 ← 3/2 transition of SH, a lambda doublet for which the strongest hyperfine components (F = 3 ← 2) lie at frequencies of 1382.910 and 1383.241 GHz, in the upper sideband of the L1 receiver. A broad atmospheric ozone feature is present in the lower sideband, at a frequency of 1379.50 GHz. The LO settings were selected to separate this feature as far as possible (in IF frequency) from the target SH transition. These observations, with a combined on-source integration time of 4.0 min, were carried out on 2011 September 28 as part of the SOFIA basic science program. The telescope beam, of diameter ~21′′ HPBW, was centered on W49N at α = 19h10m13.2s,δ = + 09006′12.0′′ (J2000). The observations were performed in dual beam switch mode, with a chopper frequency of 1 Hz and the reference positions located 75′′ on either side of the source along an east-west axis. The AFFTS backend provided 8192 spectral channels at a spacing of 183.1 kHz. Thanks to laboratory spectroscopy performed by Morino & Kawaguchi (1995) and by Klisch et al. (1996), the SH rest frequencies are known to an estimated accuracy of < 2 MHz, The separation of the lambda doublet corresponds to a velocity shift of 71.8 km s-1, and each doublet member is additionally split into three hyperfine components; the rest frequencies and spontaneous radiative rates are listed in Table 1 for an assumed SH dipole moment of 0.758 D (Meerts & Dynamus 1975).

Table 1

SH lines observed with GREAT.

The raw data were calibrated to the (“forward beam brightness temperature”) scale, fitting independently the dry and the wet content of the atmospheric emission. Here, the assumed forward efficiency was 0.95 and the assumed beam efficiency for the L1 band was 0.54. The uncertainty in the flux calibration is estimated to be ~20% (Heyminck et al. 2012). Additional data reduction – performed using CLASS2 – entailed second-order baseline removal, averaging the data (with a weighting inversely proportional to the square of the rms noise), and spectral smoothing to a 3.7 MHz channel spacing.

In addition to the SOFIA observations of SH that are the primary subject of this Letter, we have carried out ancillary observations of the 110 − 101 168.763 GHz transition of H2S, using the IRAM 30 m telescope located in Pico Veleta near Granada (Spain) in December 2006 under good weather conditions. We used the C150 receiver, tuned in single sideband, coupled to two spectrometers: the high-resolution correlator VESPA with 40 kHz spectral resolution, and a broad-band filter bank with 1 MHz spectral resolution. The data were acquired using the wobbler with a frequency of 0.5 Hz, for a total time of 32 min, and were analyzed with the CLASS software.

thumbnail Fig. 1

Spectrum of SH obtained by GREAT toward W49N. Note that because GREAT employs double sideband receivers, the complete absorption of radiation at a single frequency will reduce the measured antenna temperature to one-half the apparent continuum level. The lambda doubling and hyperfine splittings are indicated by the red bars for a component at an LSR velocity of 40 km s-1.

3. Results

Figure 1 shows the observed spectrum of SH , with the frequency scale corrected to the Local Standard of Rest (LSR). The double sideband continuum antenna temperature is  K, and the rms noise is 0.12 K in a 3.7 MHz channel. Because GREAT employs double sideband receivers, the complete absorption of radiation at a single frequency will reduce the measured antenna temperature to one-half the apparent continuum level.

In Fig. 2, the fractional transmission is shown separately for each of the lambda doublets (top two panels), with the frequency scale expressed as Doppler velocities relative to the LSR for the strongest hyperfine component of each doublet member. The transmission is given by , given the assumption that the sideband gain ratio is unity. Analogous spectra are shown for the 110 − 101 (ground state) transition of ortho-H2S, and for the transition of CH (Gerin et al. 2010b), believed to be a good tracer of H2 (e.g. Sheffer et al. 2008).

thumbnail Fig. 2

Ratio of flux to continuum flux, for SH (), ortho-H2S (110 ← 101), and CH (). The CH spectrum has been hyperfine-deconvolved.

Absorption by SH is clearly detected in the range vLSR ~ 5−20 km s-1, near the systemic velocity of the source. In addition, a narrow absorption feature is detected unequivocally near vLSR ~ 39 km s-1, a component that is clearly present in the spectra of CH, H2S, and many other molecules (e.g. Godard et al. 2010; Sonnentrucker et al. 2010). This component arises in a foreground cloud unassociated with W49N, which has an (kinematically-) estimated Galactocentric distance of ~6.7 kpc (Godard et al. 2012).

In contrast to the case of CH, there is an absence of strong SH or H2S absorption in the 60−65 km s-1 range. This behavior is similar to that observed for CS (Miyawaki et al. 1988) and for the nitrogen hydrides NH, NH2, and NH3 (Persson et al. 2012); it may suggest that those molecules – such as SH – for which the vLSR ~ 39 km s-1 absorption is much stronger than that in the 60−65 km s-1 range all originate in material with a larger molecular fraction.

Because of its large spontaneous radiative decay rate (4.7 × 10-3 s-1), the SH transition we have observed possesses a high critical density3 (≳107 cm-3, for an assumed collisional deexcitation rate ~10-10 cm3 s-1, similar to that computed by Offer et al. 1994, for the analogous transition of OH). Therefore, in the foreground material unassociated with W49N, we expect that SH will be almost entirely in its ground rotational state, . In that case, the absorption optical depth, integrated over velocity and summed over the the six components listed in Table 1, is given by (1)(For the 5–20 km s-1 velocity interval, Eq. (1) would likely yield an underestimate of the SH column density, because – for gas associated with the dense W49N cloud itself – there is likely to be a significant SH population in excited rotational states.)

In the present study4, we confine our attention to the narrow absorption feature appearing near vLSR = 39 km s-1. In Table 2, we present estimates of the SH column density in the 39 km s-1 absorbing cloud, along with analogous results obtained for ortho-H2S from IRAM 30 m observations and for SH+, para-H2O and CH from Herschel/HIFI observations. Given an estimated H2 column density of 6.6 × 1020 cm-2 for this cloud (Godard et al. 2012), the observed SH column density implies an SH/H2 abundance ratio ~7 × 10-9, corresponding to ~0.003% of the solar abundance of elemental sulfur. The SH/SH+ ratio is ~1.8. For an assumed H2S ortho-to-para of 3 (the value expected in equilibrium), the SH/H2S ratio is 0.13, a value that is signficantly smaller than the OH/H2O ratio ~1.0 observed by SOFIA for this absorbing cloud (Wiesemeyer et al. 2012).

Table 2

Column densities for vLSR in the 37–44 km s-1 range.

4. Discussion

Sulfur is unusual among the abundant elements in having a set of hydrides and hydride cations with relatively small bond energies. Accordingly, none of the species S, SH, S+, SH+, or H2S+ can undergo an exothermic H atom abstraction reaction with H2. (In other words, X + H2 → XH + H is endothermic for X = S, SH, S+, SH+, or H2S+.) Figure 3 illustrates the thermochemistry of the sulphur-bearing hydrides by displaying the heats of formation of various sulphur-bearing species (with associated hydrogen atoms and H2 molecules involved in their formation). The endothermicities of the reactions X + H2 → XH + H, expressed in temperature units, ΔE/k, are 9641, 6984, 10117, 7634, and 4096 K respectively for X = S, SH, S+, SH+, and H2S+.

thumbnail Fig. 3

Heats of formation of various S-bearing species (with associated H atoms produced during their formation), based upon thermochemical data from the NIST Chemistry Web book.

Clearly, these H atom abstraction reactions leading to the formation of the sulfur-containing hydrides are negligibly slow at the temperatures (<100 K) typical of diffuse or dense molecular clouds. Nevertheless, surprisingly high column densities of H2S have previously been observed in both diffuse molecular clouds and dense regions of active star-formation. For example, based on observations of diffuse foreground material along the sight-lines to Sgr B2 (M) and W49N, Tieftrunk et al. (1994) measured H2S abundances (of several × 10-9) relative to H2 that were considerably larger than those predicted in standard chemical models, they suggested that H2S might have been enhanced by high temperature reactions in shocks (e.g. Pineau des Forêts et al. 1986) or by hydrogenation reactions on grain surfaces. Some smaller H2S abundances (of several times 10-10) were observed in diffuse clouds at high Galactic latitude by Lucas & Liszt (2002), while even larger H2S abundances (up to ~10-6) have been inferred for dense regions of active star-formation (Minh et al. 1991).

More recently, enhanced SH+ abundances ~few × 10-9 have been measured in the diffuse ISM, using APEX (Menten et al. 2011) and Herschel/HIFI (Godard et al. 2012). These measurements have been interpreted as resulting from an increased reaction rate for S +  + H2 → SH +  + H in turbulent dissipation regions where the gas temperature is elevated and significant ion-neutral drift is present. The small SH/H2S abundance ratio ~0.13 inferred in Sect. 3 above for the 39 km s-1 absorbing cloud suggests that the reaction SH + H2 → H2S + H must be similarly increased, and argues that endothermic neutral-neutral reactions are enhanced along with ion-neutral reactions. As described below, this conclusion is supported by detailed modeling of (1) standard photodissociation regions (PDRs); (2) turbulent dissipation regions (TDRs); and (3) “C”- and “J”-type shock waves.

With the use of the Meudon PDR model (Le Petit et al. 2006), we have found that standard PDR models – i.e. models that do not include ion-neutral drift or additional heating mechanisms beyond those associated with ultraviolet irradiation and cosmic rays – greatly overpredict the SH/H2S ratio in low-density diffuse gas, yielding typical values of 104, and greatly underpredict the SH/H2 and H2S/H2 ratios. We found that SH/H2S ratios as small as unity are predicted only in gas clouds with densities, nH, greater than 104 cm-3, and visual extinctions, AV greater than 4, values that are both unreasonably high for the 39 km s-1 absorbing cloud toward W49N. We also investigated PDR models that include the effects of grain surface hydrogenation of S and SH and the subsequent photodesorption of H2S (Vasyunin & Herbst 2011); these too underpredict the SH/H2 and H2S/H2 ratios, unless an unreasonably high gas density is assumed. Using models for the chemistry of sulfur-bearing molecules in TDRs (Godard et al. 2009) and “C”- and “J”-type shocks (Pineau des Forêts et al. 1986), we determined that the predicted SH/H2S ratios in diffuse molecular clouds are ~10, significantly smaller than those for standard PDRs models without grain surface reactions, but nevertheless a factor ~100 larger than what is observed. Here, the production of SH is enhanced by the sequence S + (H2,H)SH + (H2,H)H2S + (e,H)SH, in which the first two endothermic reactions are significantly enhanced by ion-neutral drift. While the predicted SH and SH+ abundances are broadly consistent with the observations, H2S is underpredicted by a factor of 100. As in the PDR models, smaller SH/H2S abundances are only predicted in clouds with implausibly large nH and AV. Apparently, standard models for PDRs, TDRs, or “C”- or “J”-type shocks cannot account for the observed SH/H2S ratio, because they fail to predict a sufficient enhancement in the rates of neutral-neutral reactions.

We note, however, that existing models for sulfur chemistry in TDRs and “C”-type shocks assume that all neutral species share a common velocity. As originally pointed out by Flower & Pineau des Forêts (1998) in the context of CH and CH+ chemistry, neutral molecules that are produced by dissociative recombination of molecular ions in environments where there is significant ion-neutral drift – such as S and SH in the case of present interest – initially carry an imprint of the velocity of the ionized parents from which they formed. This effect may enhance endothermic reactions with H2 that occur before the newly-formed neutral species has undergone sufficient elastic collisions to acquire the velocity of the neutral fluid. Detailed modeling, which we defer to a future publication, will be needed to determine whether the SH/H2S ratio serves as a diagnostic of this effect.

1

GREAT is a development by the MPI für Radioastronomie and the KOSMA/Universität zu Köln, in cooperation with the MPI für Sonnensystemforschung and the DLR Institut für Planetenforschung.

2

Continuum and Line Analysis Single-dish Software – http://www.iram.fr/IRAMFR/GILDAS

3

Here, “critical density” is defined as the gas density at which the collisional deexcitation rate equals the spontaneous radiative decay rate.

4

A future study, requiring detailed modeling of the excitation of SH in W49N, will be needed to obtain an estimate of the SH+ column density in the source itself.

Acknowledgments

Based on observations made with the NASA/DLR Stratospheric Observatory for Infrared Astronomy. SOFIA Science Mission Operations are conducted jointly by the Universities Space Research Association, Inc., under NASA contract NAS2-97001, and the Deutsches SOFIA Institut under DLR contract 50 OK 0901. This research was supported by USRA through a grant for Basic Science Program 81-0014. E.F., M.G. and B.G. acknowledge support from the French CNRS/INSU Programme PCMI (Physique et Chimie du Milieu Interstellaire).

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All Tables

Table 1

SH lines observed with GREAT.

In the text
Table 2

Column densities for vLSR in the 37–44 km s-1 range.

In the text

All Figures

thumbnail Fig. 1

Spectrum of SH obtained by GREAT toward W49N. Note that because GREAT employs double sideband receivers, the complete absorption of radiation at a single frequency will reduce the measured antenna temperature to one-half the apparent continuum level. The lambda doubling and hyperfine splittings are indicated by the red bars for a component at an LSR velocity of 40 km s-1.
In the text
thumbnail Fig. 2

Ratio of flux to continuum flux, for SH (), ortho-H2S (110 ← 101), and CH (). The CH spectrum has been hyperfine-deconvolved.
In the text
thumbnail Fig. 3

Heats of formation of various S-bearing species (with associated H atoms produced during their formation), based upon thermochemical data from the NIST Chemistry Web book.
In the text

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