First Detection of Interstellar S₂H

Abstract

We present the first detection of gas phase S₂H in the Horsehead, a moderately UV-irradiated nebula. This confirms the presence of doubly sulfuretted species in the interstellar medium and opens a new challenge for sulfur chemistry. The observed S₂H abundance is ~5×10⁻¹¹, only a factor 4-6 lower than that of the widespread H₂S molecule. H₂S and S₂H are efficiently formed on the UV-irradiated icy grain mantles. We performed ice irradiation experiments to determine the H₂S and S₂H photodesorption yields. The obtained values are ~1.2×10⁻³ and <1×10⁻⁵ molecules per incident photon for H₂S and S₂H, respectively. Our upper limit to the S₂H photodesorption yield suggests that photo-desorption is not a competitive mechanism to release the S₂H molecules to the gas phase. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption and grain shattering can increase the gaseous S₂H abundance to some extent. Alternatively, S₂H can be formed via gas phase reactions involving gaseous H₂S and the abundant ions S⁺ and SH⁺. The detection of S₂H in this nebula could be therefore the result of the coexistence of an active grain surface chemistry and gaseous photo-chemistry.

1. Introduction

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 sulphur 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 sulphur 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 it is 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 interestellar 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 laboratoty 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 that 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) unsuccesfully 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 photo-dissociation region, the Horsehead. In Sect. 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 Sect. 5.

2. Observations and Data Reduction

The data used in this work are from the Horsehead WHISPER (Wide-band High-resolution Iram-30m Surveys at two Positions with Emir Receivers, PI: J. Pety) project and the Director’s Discrectionary 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 30m telescope. Two positions are observed: i) the HCO peak (RA=5^h40^m53^s.936, Dec=2°28′00″, J2000), which is characteristic of the photo-dissociation region at the UV-illuminated surface of the Horsehead nebula (Gerin et al. 2009) (also referred to as PDR position), and ii) the DCO⁺ peak (RA=5^h40^m55^s.61, Dec=2°27′38″, J2000), which corresponds to a cold and UV-shielded condensation located less than 40″ 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 RA: 05^h40^m54^s.27 Dec: −02°28′00″.0. Several lines of S₂H were tentatively detected towards the two positions observed in the WHISPER survey. In order to confirm the S₂H detection, we requested Director’s Discrectionay Time (D11-16) to observe a single setup covering the frequencies listed in Table 1. The merged S₂H spectra are shown in Fig. 1. Line intensities are given in main brightness temperature (T_MB) and the lines were observed with a frequency resolution of 49 kHz.

Table 1.

Gaussian fits

Freq(MHz)	Area(K kms−1)	vlsr(km s−1)	Δv(km s−1)	TMB(K)	rms (K)
DCO+ peak (core)
94526.32	0.019 (0.002)	10.90 (0.05)	0.9 (0.1)	0.018	0.004
94731.21	0.020 (0.003)	11.39 (0.13)	1.7 (0.2)	0.011	0.004
110294.15	0.024 (0.004)	10.86 (0.05)	0.7 (0.1)	0.033	0.008
110498.11	0.029 (0.004)	11.09 (0.06)	0.9 (0.1)	0.029	0.007
HCO peak (PDR)
94526.32	0.016 (0.002)	10.97 (0.06)	0.9 (0.1)	0.018	0.004
94731.21	0.023 (0.003)	11.03 (0.05)	0.9 (0.1)	0.023	0.004
110294.15	0.026 (0.003)	10.87 (0.04)	0.7 (0.1)	0.037	0.008
110498.11	0.019 (0.004)	11.07 (0.09)	0.9 (0.2)	0.020	0.007

Figure 1.

In the central panel, we show the integrated intensity map of the H₂S 1_1,0→1_0,1 line (168.763 GHz). UV-illumination from σOri comes from the west (right). The beam is drawn in the bottom-left corner. Black circles around the surveyed positions indicate the beam of the S₂H detections. Spectra of the four S₂H lines detected towards the two positions targetted in the Whisper spectral are plotted in the left (DCO⁺ peak) and right (HCO peak) panels. The frequency in GHz is indicated in the top-left corner. In red, the Gaussian fits shown in Table 1.

In order to have a deeper insight into the S₂H chemistry, we compare the new S₂H observations with the H₂S 1_1,0→1_0,1 map observed during April 2006 with the IRAM 30m telescope. These observations were done using the frequency switching mode and a spectral resolution of 40 kHz. 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^−1.5 K km s⁻¹ across the molecular cloud with an abrupt border in the west (see Fig. 1). The H₂S emission presents a local minimum towards the DCO⁺ peak, similar to the morphology observed in other species such as CH₃OH (Guzmán et al. 2011, 2013), suggesting gaseous H₂S depletion towards this cold dense core.

3. Column Densities and Abundances

The rotational spectrum of S₂H was calculated by Tanimoto et al. (2000). The spectroscopic data can be found in the CDMS catalogue (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 Fig. 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 central frequency the one of 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 catalogues. There is no other good candidate to be a carrier of these lines. The large linewidth of the 94.731 GHz line towards 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 1. 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 linewidths 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 towards the DCO⁺ peak than towards the PDR (Goicoechea et al. 2009). However, others PDR-like species such as HCO present similar linewidths towards both positions (Gerin et al. 2009). This suggests that even towards the core position, the S₂H emission is mainly coming from the UV-illuminated layers of the cloud along the line of sight.

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 (wrt hydrogen nuclei) is ~5×10⁻¹¹ towards the DCO⁺ peak and about a factor of 2 larger towards the HCO peak.

From the chemical point of view, it is interesting to compare the S₂H abundance with those of the related species H₂S. Unfortunately there is only one transition of H₂S that is easily observable with the 30m telescope given the physical conditions in the Horsehead, the o-H₂S 1_1,0→1_0,1 line. Thus, we need to assume a rotation temperature to derive the H₂S column density. Since the H₂S dipole moment (µ_b=0.978 D; Viswanathan et al. 1984) is similar to those of S₂H (µ_a=1.161 D, µ_b=0.827 D; Peterson et al. 2008), we assume the same rotation temperature for both molecules. With these assumptions and adopting an ortho-to-para ratio of 3, we derive a H₂S abundance of ~3×10⁻¹⁰ towards the two positions. This would imply that [S₂H]/[H₂S]=0.15±0.09 in the DCO⁺ peak and [S₂H]/[H₂S]=0.27±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.

4. S₂H Formation

The formation of S₂H is a intricate problem due to the low H-SS energy bonding. Evidences for the formation of S₂H during irradiation of pure H₂S and H₂S:H₂O ice mixtures were 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 barrier, so likely also on surface. An alternative surface induced S₂H production may be s-H₂S₂ photo-dissociation-desorption: s-H₂S₂ + hν→S₂H+H. s-H₂S₂ may be efficiently produced by the s-HS + s-HS reaction but it needs mobile HS on ice and so a high grain temperature.

In the gas-phase, S₂H may be produced by the electronic dissociative recombination of ${\text{H}}_2{\text{S}}_2^{+}.$ 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 ${\text{H}}_2{\text{S}}_2^{+}$ production pathways: the ${\text{S}}^{+}+{\text{H}}_2\text{S}\to {\text{H}}_2{\text{S}}_2^{+}+\text{h}\nu$ reaction (Anicich, V.G. 2003), despite the fact that the reference is an unpublished work and previous experimental studies did not identify this channel (Smith et al. 2004), and the $\text{S}{\text{H}}^{+}+{\text{H}}_2\text{S}\to {\text{H}}_2{\text{S}}_2^{+}+\text{H}$ reaction which is well characterized (Anicich, V.G. 2003). We note that 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.

5. Experimental Study of the Photodesorption of S₂H and H₂S

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 vs 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 vaccum-ultraviolet flux of 2×10¹⁴ photons cm² 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 monitore S₂H directly. However, if H₂S₂ or S₂H were desorbed, we would expect to detect all the fragments derived from these species, in particular ${\text{S}}_2^{+}.$ While photodesorption of H₂S was detected, no gaseous ${\text{S}}_2^{+}$ was observed (see Fig. 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 photo-product, 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 photo-dissociation-desorption are therefore not expected to be the origin of the gaseous S₂H.

Figure 2.

Photodesorption of H₂S (black) detected by the QMS during irradiation of a pure H₂S ice sample. No increase of the measured ion current for the mass fragment m/z = 64 (blue, corresponding to any sulfur-sulfur photo-product) was detected. Irradiation intervals are indicated with vertical dashed lines. Signals are shifted for clarity.

6. Discussion and Conclusions

At a distance of 400 pc, the Horsehead is a PDR viewed nearly edge-on and illuminated by the O9.5V star σOri 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 HII 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⁺ towards the HCO peak are well understood in terms of gas-phase photochemistry (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 photo-destroyed 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) towards the warm (T_kin ~ 60 K) PDR and its associated cold (T_kin ~ 20 K) core was unexpected. In fact, the chemical complexity reached in the Horsehead is extraordinarily high with COMs of up to 7 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 Horse-head 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 Horse-head. The observed S₂H abundance is ~5×10⁻¹¹, only a factor 4-6 lower than that of H₂S. Our laboratory experiments show that the H₂S and S₂H photodesortion 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 suggest that this mechamism 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 gas phase. S₂H can also be formed in 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 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 gas phase, S₂H is mainly formed through ${\text{S}}^{+}+{\text{H}}_2\text{S}\to {\text{H}}_2{\text{S}}_2^{+}+\text{h}\nu$ and $\text{S}{\text{H}}^{+}+{\text{H}}_2\text{S}\to {\text{H}}_2{\text{S}}_2^{+}+\text{H}$ followed by dissociative recombination of ${\text{H}}_2{\text{S}}_2^{+}.$ Oxygen and sulfur have indeed similar reactivity but, due to their different ionization potentials, O⁺ is expected less abundant than S⁺ and then the O⁺ and OH⁺ reactions play a smaller role. 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 towards HSO⁺ + H₂ reaction is endothermic in opposite to the channels towards S₂H⁺ + H₂ and ${\text{S}}_2{\text{H}}_2^{+}+\text{H}.$ Therefore, in 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. Photo-desorption 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 a H₂S and H₂S-H₂O ices are irradiated, providing a path to form species with two sulfur atoms. Summarizing, sulfur and oxygen are not analogues in the gas-phase and surface chemistry, and the comparison of their related species requires the full chemical modelling of the region.

Table 3.

Column density upper limits

Molecule	Freq (GHz)	rms1 (mK)	NX2 (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

⁽¹⁾The rms has been calculated for a channel width of ≈0.3 km s⁻¹. The obtained rms is similar in the two surveyed positions.

⁽²⁾3σ upper limits assuming LTE, T_rot=10 K and a linewidth of 0.6 km s⁻¹

Table 2.

Summay of column densities and fractional abundances

Molecule			DCO+ peak			HCO peak
	HPBW (″)	Trot (K)	N(X) (cm–2)	N(X)/NH	Trot (K)	N(X) (cm–2)	N(X)/NH
H2	12		2.9×1022	0.5		1.9×1022	0.5
S2H	22 - 26	${8.73}_{-1.10}^{+1.36}$	${3.0}_{-0.6}^{+0.9}\times {10}^{12}$	${5.2}_{-1.0}^{+1.5}\times {10}^{-11}$	${12.69}_{-1.54}^{+1.78}$	${3.3}_{-0.7}^{+1.2}\times {10}^{12}$	${8.7}_{-1.9}^{+3.1}\times {10}^{-11}$
H2S1	14	$9_{-1}^{+1}$	${1.9}_{-0.3}^{+0.4}\times {10}^{13}$	${3.3}_{-0.6}^{+0.7}\times {10}^{-10}$	${12}_{-2}^{+2}$	${1.2}_{-0.2}^{+0.1}\times {10}^{13}$	${3.1}_{-0.5}^{+0.3}\times {10}^{-10}$

⁽¹⁾We assume the rotation temperatures derived from S₂H.