First Detection of Interstellar S₂H

Sulfur is one of the most abundant elements in the universe (S/H ∼ 1.3 × 10⁻⁵) and plays a crucial role in biological systems on Earth, so it is important to follow its chemical history in space. Surprisingly, sulfuretted molecules are not as abundant as expected in the interstellar medium. A few sulfur compounds have been detected in diffuse clouds demonstrating that the sulfur abundance in these low-density regions is close to the cosmic value (Neufeld et al. 2015). A moderate sulfur depletion (a factor of 4) is observed in the external layers of the photodissociation region (PDR) in the Horsehead nebula as well (Goicoechea et al. 2006). In cold molecular clouds, a large depletion of sulfur is usually considered to reproduce the observations (see, for instance, Tieftrunk et al. 1994). However, recent models by Vidal et al. (2017) explained the observational data without or with little sulfur depletion after updating the gas and grain chemistry. In that case, HS and H₂S on the grains or atomic sulfur in the gas would contain most of the sulfur. In hot cores and corinos, Wakelam et al. (2004) found that observations of S-bearing molecules would be better reproduced if sulfur was sublimated from grains in the atomic form or if it was quickly converted into it. Thus far, the main solid or gas sulfur carrier is still debated.

With an adsorption energy of ∼1100 K (Hasegawa & Herbst 1993), sulfur atoms are expected to stick on the surfaces of grains with temperatures below ∼22 K. Here, because of the high hydrogen abundances and the mobility of hydrogen in the ice matrix, sulfur atoms are expected to form H₂S. Indeed, gaseous H₂S is the most abundant S-bearing molecule in comets, with an abundance of up to 1.5% relative to water (Bockelée-Morvan et al. 2000). A firm detection of H₂S in interstellar ices has not been reported yet. A realistic upper limit of the H₂S abundance in interstellar ices is 1% relative to water that is 10 times lower than the cosmic abundance (Jiménez-Escobar & Muñoz Caro 2011). One possibility to explain the low fraction of H₂S in interstellar ices is that H₂S is processed by UV photons or cosmic rays in the ice leading to the formation of other S-bearing species. OCS and, tentatively, SO₂ have been detected in icy mantles but their abundances are far too low to explain the missing S budget (Geballe et al. 1985; Palumbo et al. 1995; Boogert et al. 1997).

Trying to provide new insights into the ice composition, experimental simulations of the irradiation of interstellar ices containing H₂S under astrophysically relevant conditions have been performed in laboratory using UV photons (Jiménez-Escobar & Muñoz Caro 2011; Jiménez-Escobar et al. 2014), X-rays (Jiménez-Escobar et al. 2012), or ions (Moore et al. 2007; Ferrante et al. 2008; Garozzo et al. 2010). Energetic processing of H₂S-bearing ices readily generates sulfur–sulfur bonds, and the main S-bearing products in these experiments are H₂S₂ and S₂H, which were detected by Jiménez-Escobar & Muñoz Caro (2011) through their infrared absorption bands. The molecule H₂S₂ could subsequently photodissociate, forming S₂ and S₃ depending on the irradiation time. These molecules with two S atoms and even more could thus contain a significant fraction of the missing sulfur in dense clouds. In line with this work, Druard & Wakelam (2012) suggested that polysulphanes could be a sulfur reservoir in the ice and are rapidly converted into atomic sulfur once in the gas phase. Martín-Doménech et al. (2016a) unsuccessfully searched for S₂H and H₂S₂ in the gas phase toward the well-known hot corino, IRAS 16293−2422. The lack of gaseous S₂H and H₂S₂ was interpreted as the consequence of the rapid destruction of these species once sublimated in such a warm and dense environment (Martín-Doménech et al. 2016a; Fortenberry & Francisco 2017).

In this Letter, we report the first interstellar detection of S₂H in the prototypical PDR, the Horsehead. In Section 4, we discuss the possible grain-surface and gas-phase S₂H formation routes. New measurements of the photodesorption yields of S₂H and H₂S are presented in Section 5.

The data used in this work are from the Horsehead Wide-band High-resolution Iram-30 m Surveys at two Positions with Emir Receivers (WHISPER; PI: J. Pety) project and the Director's Discretionary Time project D11-16. The Horsehead WHISPER project is a complete, unbiased line survey of the 3, 2, and 1 mm bands using the IRAM 30 m telescope. Two positions are observed: (i) the HCO peak (R.A. = 5^h40^m53936, decl. = −2°28'00'', J2000), which is characteristic of the PDR at the UV-illuminated surface of the Horsehead nebula (Gerin et al. 2009; also referred to as the PDR position), and (ii) the DCO⁺ peak (R.A. = 5^h40^m5561, decl. = −2°27'38'', J2000), which corresponds to a cold and UV-shielded condensation located less than $40^{\prime\prime}$ away from the PDR edge (Pety et al. 2007). During the observations we used the Position-Switching procedure with the reference position located at an offset (−100'', 0) relative to R.A. = 05^h40^m5427, decl. = −02°28'000. Several lines of S₂H were tentatively detected toward the two positions observed in the WHISPER survey. In order to confirm the S₂H detection, we requested Director's Discretionary Time (D11-16) to observe a single setup covering the frequencies listed in Table 1. The merged S₂H spectra are shown in Figure 1. Line intensities are given in main brightness temperature (T_MB), and the lines were observed with a frequency resolution of 49 kHz.

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In order to have a deeper insight into the S₂H chemistry, we compare the new S₂H observations with the H₂S ${1}_{\mathrm{1,0}}\to {1}_{\mathrm{0,1}}$ map observed during 2006 April with the IRAM 30 m telescope. These observations were done using the frequency-switching mode and a spectral resolution of 40 kHz. The averaged noise level per resolution element at 168 GHz is rms(T_MB) = 170 mK. The integrated intensity emission of the H₂S line varies between 1.0 and 1.5 K km s⁻¹ across the molecular cloud with an abrupt border in the west (see Figure 1). The H₂S emission presents a local minimum toward the DCO⁺ peak, similar to the mor-phology observed in other species such as CH₃OH (Guzmán et al. 2011, 2013), suggesting gaseous H₂S depletion toward this cold dense core.

The rotational spectrum of S₂H was calculated by Tanimoto et al. (2000). The spectroscopic data can be found in the CDMS catalog (Müller et al. 2005). We have detected eight S₂H lines located at 94526.1508, 94526.3208, 94731.0115, 94731.2080, 110294.0282, 110294.1530, 110497.9666, and 110498.1104 MHz. The S₂H hyperfine transitions are forming doublets very close in frequency (∼0.12 MHz) that remain unresolved in our data (see Figure 1). In Table 1, we show the Gaussian fits to the observed line features, each one clearly detected with S/N > 5. We have adopted as the central frequency the most intense component of the doublet. For this reason the central velocity shown in Table 1 is systematically shifted by ∼0.2−0.5 km s⁻¹ from the Horsehead systemic velocity, 10.5 km s⁻¹. We have checked possible contamination by other compounds using the CDMS and JPL catalogs. There is no other good candidate to be a carrier of these lines. The large line width of the 94.731 GHz line toward the DCO⁺ peak position, ∼1.7 km s⁻¹, is more likely due to the poor baseline around this feature.

The WEEDS software has been used to simulate the S₂H spectrum in the whole frequency coverage of the WHISPER survey assuming LTE conditions. We have fitted the detections and upper limits of 97 lines with upper level energies lower than 75 K found in the frequency range of the full survey using the Bayesian method described by Majumdar et al. (2017). The whole spectrum can be fitted assuming that the emission uniformly fills the beam and the rotation temperatures and S₂H column densities listed in Table 2. The fitted line widths are 0.68 ± 0.12 km s⁻¹ for the core and 0.63 ± 0.1 km s⁻¹ for the PDR. The observed S₂H line widths are consistent with the emission coming from the UV-irradiated gas. Species that are more abundant in the cold and UV-shielded gas of the core as DCO⁺ and H¹³CO⁺ present narrower line widths toward the DCO⁺ peak than toward the PDR (Goicoechea et al. 2009). However, others PDR-like species such as HCO present similar line widths toward both positions (Gerin et al. 2009). This suggests that even toward the core position, the S₂H emission is mainly coming from the UV-illuminated layers of the cloud along the line of sight.

Table 2.  Summary of Column Densities and Fractional Abundances

Molecule	HPBW	DCO+ Peak			HCO Peak
		Trot	N(X)	N(X)/NH	Trot	N(X)	N(X)/NH
	('')	(K)	(cm−2)		(K)	(cm−2)
H2	12	⋯	2.9 × 1022	0.5	⋯	1.9 × 1022	0.5
S2H	22–26	${8.73}_{-1.10}^{+1.36}$	${3.0}_{-0.6}^{+0.9}$ × 1012	${5.2}_{-1.0}^{+1.5}$ × 10−11	${12.69}_{-1.54}^{+1.78}$	${3.3}_{-0.7}^{+1.2}$ × 1012	8.7${}_{-1.9}^{+3.1}$ × 10−11
H2Sa	14	${9}_{-1}^{+1}$	${1.9}_{-0.3}^{+0.4}$ × 1013	${3.3}_{-0.6}^{+0.7}$ × 10−10	12${}_{-2}^{+2}$	${1.2}_{-0.2}^{+0.1}$ × 1013	${3.1}_{-0.5}^{+0.3}$ × 10−10

Note.

^aWe assume the rotation temperatures derived from S₂H.

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The S₂H rotation temperatures reveal subthermal excitation and are similar to those derived for other high dipole moment compounds like o-H₂CO (Guzmán et al. 2011). The estimated abundance (w.r.t. hydrogen nuclei) is ∼5 × 10⁻¹¹ toward the DCO⁺ peak and about a factor of 2 larger toward the HCO peak.

From a chemical point of view, it is interesting to compare the S₂H abundance with those of the related species H₂S. Unfortunately, the only observable transition with the 30 m telescope, given the physical conditions in the Horsehead, is o-H₂S ${1}_{\mathrm{1,0}}\to {1}_{\mathrm{0,1}}$. Thus, we need to assume a rotation temperature to derive the H₂S column density. Since the H₂S dipole moment (${\mu }_{b}=0.978$ D; Viswanathan & Dyke 1984) is similar to those of S₂H (${\mu }_{a}=1.161$ D, ${\mu }_{b}=0.827$ D; Peterson et al. 2008), we assume the same rotation tem-perature for both molecules. With these assumptions and adopting an ortho-to-para ratio of 3, we derive a H₂S abundance of ∼3 × 10⁻¹⁰ toward the two positions. This would imply that $[{{\rm{S}}}_{2}{\rm{H}}]/[{{\rm{H}}}_{2}{\rm{S}}]=0.15\pm 0.09$ in the DCO⁺ peak and $[{{\rm{S}}}_{2}{\rm{H}}]/[{{\rm{H}}}_{2}{\rm{S}}]+0.27\pm 0.14$ toward the HCO peak. These numbers are consistent with the [S₂H]/[H₂S] ice ratio obtained by Jiménez-Escobar et al. (2012) in their simulations of UV irradiation of H₂S ices. In the following, we qualitatively explore the possible surface and gas-phase formation routes of S₂H.

The formation of S₂H is an intricate problem due to the low H-SS energy bonding. Evidence for the formation of S₂H during irradiation of pure H₂S and H₂S:H₂O ice mixtures was provided by Jiménez-Escobar & Muñoz Caro (2011) using the same experimental setup as the one described here. One way of forming S₂H could be the grain-surface reactions: s-H atom (hereafter, "s-" is used to refer to the solid phase) addition on s-S₂,and s-S + s-HS reaction, followed by chemical desorption. However, the low exothermicity of the first reaction should prevent efficient chemical desorption (Minissale et al. 2016; Wakelam et al. 2017). The second reaction should not be efficient in cold cores because, below 15 K, S atom and HS radical are not mobile on ice considering the adsorption energies given by Wakelam et al. (2017). Moreover, S₂H is a very reactive species in the gas phase, reacting with H, N, C, and O atoms without a barrier, so likely also on surface. An alternative surface-induced S₂H production may be ${\rm{s}} \mbox{-} {{\rm{H}}}_{2}{{\rm{S}}}_{2}$ photodissociation desorption: ${\rm{s}} \mbox{-} {{\rm{H}}}_{2}{{\rm{S}}}_{2}+h\nu \to {{\rm{S}}}_{2}{\rm{H}}+{\rm{H}}$. ${\rm{s}} \mbox{-} {{\rm{H}}}_{2}{{\rm{S}}}_{2}$ may be efficiently produced by the s-HS + s-HS reaction, but it needs mobile HS on ice and therefore a high grain temperature.

In the gas phase, S₂H may be produced by the electronic dissociative recombination of ${{\rm{H}}}_{2}{{\rm{S}}}_{2}^{+}$. Even if there is no data on this reaction, the loss of one H atom is always an important exit channel on dissociative recombination (Plessis et al. 2010). There are two known ${{\rm{H}}}_{2}{{\rm{S}}}_{2}^{+}$ production pathways: the ${{\rm{S}}}^{+}+{{\rm{H}}}_{2}{\rm{S}}\to {{\rm{H}}}_{2}{{\rm{S}}}_{2}^{+}+h\nu$ reaction (Anicich 2003), despite the fact that the reference is an unpublished work and previous experimental studies did not identify this channel (Smith et al. 1981), and the ${\mathrm{SH}}^{+}+{{\rm{H}}}_{2}{\rm{S}}\to {{\rm{H}}}_{2}{{\rm{S}}}_{2}^{+}+{\rm{H}}$ reaction, which is well characterized (Anicich 2003). We note that ${{\rm{S}}}^{+}$ and SH⁺ are only abundant in the UV-irradiated gas (Gerin et al. 2016). Therefore, in spite of the large uncertainties in the reaction rates, we can conclude that these formation routes are only efficient in the UV-illuminated cloud surfaces.

Jiménez-Escobar & Muñoz Caro (2011) showed that sulfur–sulfur bonds, in particular, H₂S₂ and S₂H, are formed in irradiated ices. Here we focus on the determination of the S₂H and H₂S photodesorption yields, which are key to determine the origin (surface versus gas-phase chemistry) of the observed S₂H. For this aim, we performed experimental simulations under astrophysically relevant conditions using the ISAC setup (Muñoz Caro et al. 2010), an ultra-high vacuum chamber with a work pressure on the order of 4 × 10⁻¹¹ mbar, corresponding to the pressure found in the interior of the pre-stellar cores. Sulfur is expected to be locked on the icy mantles in these regions, H₂S being the most abundant S-bearing molecule in cometary ices. Pure amorphous H₂S ice samples with thicknesses of about 40 × 10¹⁵ molecules cm⁻² were deposited from the gas phase (H₂S gas, Praxair, 99.8%) onto a KBr substrate at 8 K, and subsequently irradiated using an F-type microwave-discharged hydrogen flow lamp with a vacuum-ultraviolet flux of 2 × 10¹⁴ photons cm${}^{2}$ s⁻¹ at the sample position (Muñoz Caro et al. 2010). The emission spectrum of the lamp (reported in Chen et al. 2014, and Cruz-Diaz et al. 2014) resembles that of the secondary UV field in dense cloud interiors, calculated by Gredel et al. (1989). A Pfeiffer Prisma quadrupole mass spectrometer (QMS) was used during irradiation of the ice samples to monitor the mass fragments m/z = 34 (corresponding to photodesorbing H₂S molecules) and m/z = 64 (corresponding to any desorbing photoproduct with a sulfur–sulfur bond, observed to form in Jiménez-Escobar et al. 2012). In our experiment, we did not monitor S₂H directly. However, if H₂S₂ or S₂H were desorbed, we would expect to detect all the fragments derived from these species, in particular S₂⁺. While photodesorption of H₂S was detected, no gaseous ${{\rm{S}}}_{2}^{+}$ was observed (see Figure 2). The measured ion current was converted into a photodesorption yield following calibration of the QMS (see Martín-Doménech et al. 2015). Photodesorption of H₂S took place with a decreasing yield, reaching a steady-state value of 1.2 × 10⁻³ molecules per incident photon after ∼30 minutes of irradiation, which corresponds to the fluence experienced by ice mantles during the typical cloud lifetime (Shen et al. 2004). A factor of 2 is assumed as the error in the photodesorption yield values due to the uncertainties in the calibration process; see Martín-Doménech et al. (2016b). Following the non-detection of any sulfur–sulfur photoproduct, an upper limit of 1 × 10⁻⁵ molecules per incident photon (the sensitivity limit of our QMS) was assumed for the photodesorption of S₂H. Direct S₂H photodesorption or H₂S₂ photodissociation desorption are therefore not expected to be the origin of the gaseous S₂H.

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At a distance of 400 pc, the Horsehead is a PDR viewed nearly edge-on and illuminated by the O9.5V star σ Ori at a projected distance of ∼3.5 pc. The intensity of the incident FUV radiation field is χ = 60 relative to the interstellar radiation field in Draine units. This PDR presents a differentiated chemistry from others associated with nearby H ii regions such as the Orion Bar. One main difference is that the dust temperature is around ∼20–30 K in the PDR (Goicoechea et al. 2009), i.e., below or close to the sublimation temperature of many species, allowing a rich surface chemistry on the irradiated surfaces. Our unbiased line survey has provided valuable hints on the chemistry of this region. The detection of the molecular ions CF⁺ and HOC⁺ toward the HCO peak are well understood in terms of gas-phase pho-tochemistry (Guzmán et al. 2012). We learned that there is an efficient top-down chemistry in the PDR, in which large polyatomic molecules or small grains are photodestroyed into smaller hydrocarbon molecules/precursors, such as C₂H, C₃H₂, C₃H, and C₃H⁺ (Pety et al. 2012; Guzmán et al. 2015). The detection of several complex organic molecules (COMs) toward the warm (${T}_{\mathrm{kin}}\sim 60$ K) PDR and its associated cold (${T}_{\mathrm{kin}}\sim 20$ K) core was unexpected. In fact, the chemical complexity reached in the Horsehead is extraordinarily high with COMs of up to seven atoms: HCOOH, H₂CCO, CH₃CHO, and CH₃CCH (Guzmán et al. 2014). Current pure gas-phase models cannot reproduce the inferred H₂CO, CH₃OH, and COMs abundances in the Horsehead PDR (Guzmán et al. 2011, 2013), which supports the grain-surface origin of these molecules. Le Gal et al. (2017) was able to reproduce the observed COMs abundances using a chemical model with grain-surface chemistry and found that chemical desorption, instead of photodesorption, is the main process to release COMs to the gas phase. CH₃CN and CH₃NC, key species for the formation of prebiotic molecules, seem to have a very specific formation pathway in the PDR (Gratier et al. 2013). The Horsehead is therefore an excellent site to study the influence of UV radiation on the grain-surface chemistry and its subsequent impact on the gas phase.

We present the first detection of S₂H in the Horsehead. The observed S₂H abundance is ∼5 × 10⁻¹¹, only a factor of 4–6 lower than that of H₂S. Our laboratory experiments show that the H₂S and S₂H photodesorption yields are 1.2 × 10⁻³ and <1 × 10⁻⁵ molecules per incident photon, respectively. Although S₂H can be formed on warm (T_d > 15 K) grains, our upper limit to the S₂H photodesorption yield suggests that this mechanism is not efficient to release the S₂H molecules from the grain mantles. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption, and grain shattering could increase the S₂H abundance in the gas phase. S₂H can also be formed in the gas phase by reactions involving H₂S and the ions S⁺ and SH⁺. These ions are expected to be abundant in the external layers of the PDR (Goicoechea et al. 2006). The photodesorption of H₂S could hence boost the S₂H production in the gas phase. We conclude that the abundance of S₂H in the Horsehead is more likely the consequence of the favorable physical conditions prevailing in this nebula where grain mantles irradiated by UV photons coexist with the ions S⁺ and SH⁺ that are only abundant in PDRs.

One interesting issue is to compare the sulfur and oxygen chemistry. We have not detected H₂S₂, HSO, H₂O₂, and HO₂ in the Horsehead with the upper limits shown in Table 3. We find interesting that the column densities of HSO and HO₂ are lower than that of S₂H, although the oxygen elemental abundance is 30 times greater than that of sulfur. In the gas phase, S₂H is mainly formed through ${{\rm{S}}}^{+}+{{\rm{H}}}_{2}{\rm{S}}\to {{\rm{H}}}_{2}{{\rm{S}}}_{2}^{+}+h\nu$ and ${\mathrm{SH}}^{+}+{{\rm{H}}}_{2}{\rm{S}}\,\to {{\rm{H}}}_{2}{{\rm{S}}}_{2}^{+}+{\rm{H}}$, followed by dissociative recombination of H₂S₂⁺. Oxygen and sulfur have indeed similar reactivity, but due to their different ionization potentials, O⁺ is less abundant than S⁺ in the PDR. Hence, reactions involving O⁺ play a smaller role. In addition, reactions of OH⁺ and H₂O⁺ with H₂O do not lead to the formation of H₂O₂⁺ (Anicich 2003), thus to the formation of HO₂. We have also compared the SH⁺ + H₂O and SH⁺ + H₂S gas-phase reactions, which may be intermediate paths at work for producing SOH and S₂H, respectively. The channel toward the HSO⁺ + H₂ reaction is endothermic in opposition to the channels toward S₂H⁺ + H₂ and S₂H₂⁺ + H. Therefore, in the gas phase, the formation of S₂H is favored relative to HSO. HSO and related species have not been observed in space thus far (Cazzoli et al. 2016; Fortenberry & Francisco 2017). Laboratory experiments demonstrate that grain-surface chemistry involving H₂O and H₂S also present different pathways. Photodesorption experiments reported by Cruz-Diaz et al. (2017) show that H₂O₂ is not formed in UV-irradiated water ice. In contrast, Jiménez-Escobar & Muñoz Caro (2011) showed that H₂S₂ is formed when H₂S and H₂S–H₂O ices are irradiated, providing a path to form species with two sulfur atoms. In summary, sulfur and oxygen are not analogs in the gas-phase and surface chemistry, and the comparison of their related species requires the full chemical modeling of the region.

Table 3.  Column Density Upper Limits

Molecule	Frequency	rmsa	NXb
	(GHz)	(mK)	(cm−2)
S2H2	139.885	9	<8.5 × 1011
HSO	158.391	30	<1.5 × 1012
HO2	130.260	11	<4.7 × 1011
H2O2	90.365	4	<1.0 × 1012

Notes.

^aThe rms has been calculated for a channel width of $\approx 0.3$ km s⁻¹. The obtained rms is the same in the two surveyed positions. ^b3σ upper limits assuming LTE, T_rot = 10 K, and a line width of 0.6 km s⁻¹.

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V.W.'s research is funded by an ERC Starting Grant (3DICE, grant agreement 336474). We thank the Spanish MINECO for funding support from AYA2016-75066-C2-1/2-P, AYA2012-32032 and ERC under ERC-2013-SyG, G. A. 610256 NANOCOSMOS. This work was supported by the Programme National "Physique et Chimie du Milieu Interstellaire" (PCMI) of CNRS/INSU with INC/INP co-funded by CEA and CNES.