A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

Gal, Romane Le; Herbst, Eric; Dufour, Gwenaëlle; Gratier, P.; Ruaud, M.; Vidal, Thomas; Wakelam, Valentine

doi:10.1051/0004-6361/201730980

Cited by 27 publications

(7 citation statements)

References 94 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Current pure gas-phase models cannot reproduce the inferred H 2 CO, CH 3 OH 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. CH 3 CN and CH 3 NC, key species for the formation of prebiotic molecules, seem to have a very specific formation pathway in the PDR (Gratier et al 2013).…”

Section: Discussionmentioning

confidence: 99%

First Detection of Interstellar S₂H

Fuente

Goicoechea

Pety

et al. 2017

ApJL

View full text Add to dashboard Cite

We present the first detection of gas phase S 2 H 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 S 2 H abundance is ∼5×10 −11 , only a factor 4-6 lower than that of the widespread H 2 S molecule. H 2 S and S 2 H are efficiently formed on the UV-irradiated icy grain mantles. We performed ice irradiation experiments to determine the H 2 S and S 2 H photodesorption yields. The obtained values are ∼1.2×10 −3 and <1×10 −5 molecules per incident photon for H 2 S and S 2 H, respectively. Our upper limit to the S 2 H photodesorption yield suggests that photo-desorption is not a competitive mechanism to release the S 2 H molecules to the gas phase. Other desorption mechanisms such as chemical desorption, cosmic-ray desorption and grain shattering can increase the gaseous S 2 H abundance to some extent. Alternatively, S 2 H can be formed via gas phase reactions involving gaseous H 2 S and the abundant ions S + and SH + . The detection of S 2 H in this nebula could be therefore the result of the coexistence of an active grain surface chemistry and gaseous photo-chemistry.

show abstract

Section: Discussionmentioning

confidence: 99%

First Detection of Interstellar S₂H

Fuente

Goicoechea

Pety

et al. 2017

ApJL

View full text Add to dashboard Cite

show abstract

“…Rubin et al (1971);Cazaux et al Kahane et al (2013); Mendoza et al (2014); López-Sepulcre et al (2015); Belloche et al (2017); Ligterink et al (2017); McGuire et al (2018)), in the circumstellar envelopes of AGB stars (Cernicharo et al (2000)), shocked regions (Arce et al (2008); Codella et al (2017); Lefloch et al (2017)) and even in external galaxies (Muller et al (2013)). Despite their presence has been known for decades, how iCOMs are synthesized is still an open question and under debate (Herbst & van Dishoeck (2009) ;Caselli & Ceccarelli (2012); Woods et al (2013); Balucani et al (2015); Butscher et al (2017); Butscher et al (2019); Fedoseev et al (2015); Enrique-Romero et al (2016); Enrique-Romero et al (2019); Gal et al (2017); Rivilla et al (2017); Vasyunin et al (2017); Rimola et al (2018); Lamberts et al (2019)). Two different paradigms have been proposed: i) on the surfaces of grains (either during the cold prestellar or warmer collapse phase (e.g.…”

Section: Introductionmentioning

confidence: 99%

Revisiting the reactivity between HCO and CH3 on interstellar grain surfaces

Enrique-Romero

Álvarez‐Barcia

Kolb

et al. 2020

Monthly Notices of the Royal Astronomical Society

View full text Add to dashboard Cite

Formation of interstellar complex organic molecules is currently thought to be dominated by the barrierless coupling between radicals on the interstellar icy grain surfaces. Previous standard DFT results on the reactivity between CH 3 and HCO on amorphous water surfaces, showed that formation of CH 4 + CO by H transfer from HCO to CH 3 assisted by water molecules of the ice was the dominant channel. However, the adopted description of the electronic structure of the biradical (i.e., CH 3 /HCO) system was inadequate (without the broken-symmetry (BS) approach). In this work, we revisit the original results by means of BS-DFT both in gas phase and with one water molecule simulating the role of the ice. Results indicate that adoption of BS-DFT is mandatory to describe properly biradical systems. In the presence of the single water molecule, the water-assisted H transfer exhibits a high energy barrier. In contrast, CH 3 CHO formation is found to be barrierless. However, direct H transfer from HCO to CH 3 to give CO and CH 4 presents a very low energy barrier, hence being a potential competitive channel to the radical coupling and indicating, moreover, that the physical insights of the original work remain valid.

show abstract

“…Because of its low ionization potential, sulfur is expected to be ionized up to A v ~ 7 mag (see e.g. Le Gal et al 2017) and is a plausible precursor of NS + . The abundance of the ion H 2 S + , formed by the reaction S+H3+, might be important in deeper layers of the molecular cloud depending on the cosmic ray ionization rate (Sternberg & Dalgarno 1995).…”

Section: Discussionmentioning

confidence: 99%

“…Dust grain temperatures in this low-UV PDR are ~20-30 K, lower than the evaporation temperature of H 2 S, the main sulphur reservoir in solid state. In the gas phase, S and S + are expected to be the main sulphur reservoirs (Le Gal et al 2017; Goicoechea et al 2006). Unfortunately the observation of these species is not easy in the (sub-)millimeter wavelength range.…”

Section: Discussionmentioning

confidence: 99%

“…Current pure gas-phase models cannot reproduce the inferred H 2 CO, CH 3 OH, and COMs abundances in the Horsehead PDR (Guzmán et al 2011, 2013), which supports the theory of a grain surface origin for these molecules. Le Gal et al (2017) were able to reproduce the observed COMs abundances using a chemical model with grain surface chemistry and proposed that chemical desorption, instead of photodesorption, was most likely the dominant process to release COMs to the gas phase. However, the efficiency of chemical desorption seems to be extremely dependent on the species involved and the chemical composition of the icy mantle (Minissale et al.…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Abundances of sulphur molecules in the Horsehead nebula

Rivière-Marichalar

Fuente

Goicoechea

et al. 2019

A&A

Self Cite

View full text Add to dashboard Cite

Context Sulphur 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. The understanding of its chemistry is therefore of major importance. Aims Our goal is to complete the inventory of S-bearing molecules and their abundances in the prototypical photodissociation region (PDR) the Horsehead nebula to gain insight into sulphur chemistry in UV irradiated regions. Based on the WHISPER (Wide-band High-resolution Iram-30m Surveys at two positions with Emir Receivers) millimeter (mm) line survey, our goal is to provide an improved and more accurate description of sulphur species and their abundances towards the core and PDR positions in the Horsehead Methods The Monte Carlo Markov Chain (MCMC) methodology and the molecular excitation and radiative transfer code RADEX were used to explore the parameter space and determine physical conditions and beam-averaged molecular abundances. Results A total of 13 S-bearing species (CS, SO, SO2, OCS, H2CS – both ortho and para – HDCS, C2S, HCS+, SO+, H2S, S2H, NS and NS+) have been detected in the two targeted positions. This is the first detection of SO+ in the Horsehead and the first detection of NS+ in any PDR. We find a differentiated chemical behaviour between C-S and O-S bearing species within the nebula. The C-S bearing species C2S and o-H2CS present fractional abundances a factor of > two higher in the core than in the PDR. In contrast, the O-S bearing molecules SO, SO2, and OCS present similar abundances towards both positions. A few molecules, SO+, NS, and NS+, are more abundant towards the PDR than towards the core, and could be considered as PDR tracers. Conclusions This is the first complete study of S-bearing species towards a PDR. Our study shows that CS, SO, and H2S are the most abundant S-bearing molecules in the PDR with abundances of ∼ a few 10−9. We recall that SH, SH+, S, and S+ are not observable at the wavelengths covered by the WHISPER survey. At the spatial scale of our observations, the total abundance of S atoms locked in the detected species is < 10−8, only ∼0.1% of the cosmic sulphur abundance.

show abstract

A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

Cited by 27 publications

References 94 publications

First Detection of Interstellar S₂H

First Detection of Interstellar S₂H

Revisiting the reactivity between HCO and CH3 on interstellar grain surfaces

Abundances of sulphur molecules in the Horsehead nebula

Contact Info

Product

Resources

About

A new study of the chemical structure of the Horsehead nebula: the influence of grain-surface chemistry

Cited by 27 publications

References 94 publications

First Detection of Interstellar S2H

First Detection of Interstellar S2H

Revisiting the reactivity between HCO and CH3 on interstellar grain surfaces

Abundances of sulphur molecules in the Horsehead nebula

Contact Info

Product

Resources

About

First Detection of Interstellar S₂H

First Detection of Interstellar S₂H