How Chemical Complexity Flourishes on Dust Grains: CH3SH Formation in Cold Molecular Clouds
Abstract
The molecular makeup of the Interstellar Medium (ISM) holds the key to tracing the pathways that lead to star- and planet-forming regions, including our own. In dense molecular clouds, chemical complexity flourishes on the surfaces of cold dust grains, which act as catalytic sites that enable the formation of icy mantles containing molecules like water (H2O), methane (CH4), and ammonia (NH3), as well as interstellar Complex Organic Molecules (iCOMs). However, the precise formation mechanisms for iCOMs, particularly sulfur-bearing carriers in the solid state, remain elusive. In this study, we experimentally investigate the role of atomic carbon in driving solid-state reactions forming S-bearing iCOMs, uncovering new potential pathways for molecular complexity in the ISM. Using the SURFRESIDE3 setup, interstellar ice analogues are grown on a cold (10 K) gold-plated substrate within an ultra-high vacuum (UHV) chamber (P_base = 10^-10 mbar) by the co-deposition atomic (H/D, C) and molecular species (H2S, H2). The ices are monitored via reflection absorption infrared spectroscopy (RAIRS), and analyzed with a quadrupole mass spectrometer (QMS) during temperature-programmed desorption (TPD) measurements. I will discuss the formation pathways of CH3SH and H2CS in prestellar cores and the implications their formation routes have on deuteration ratios and gas-phase abundances found at later stages of star formation.
Full text
Dr. Franciele Kruczkiewicz How chemical complexity flourishes on dust grains: CH3SH formation on cold molecular clouds Towards New Frontiers 10 – 14 March, ESO Garching Marie Curie Skłodowska Actions (MSCA) Individual Fellow Laboratory for Astrophysics (LfA), Leiden University The Netherlands
How do molecules form in astrophysical environments, and what can they tell us about the physical and chemical conditions of the regions where they are detected?
Sulfur-bearing species: Simple: H2S, OCS, H2CS, CS₂, SO₂, S₈, NH₄SH Complex: CH3SH,CH3CH2SH, CH2CHSH, HSCH2CH2SH How do molecules form in astrophysical environments, and what can they tell us about the physical and chemical conditions of the regions where they are detected?
Sulfur chemistry … and how different works this week address it S-depletion problem Angèle Taillard Julia Santos Katerina Slavicinska diffuse clouds [S] [H] ≈ 10-5 Std ratio ~ Std ratio 1% std. 10% std. Solar System protoplanetary disk dense clouds
Sulfur chemistry … and how different works this week address it S-depletion problem S solid-state chemistry Angèle Taillard Julia Santos Katerina Slavicinska Rafael Martin Domenech Julia Santos Katerina Slavicinska diffuse clouds [S] [H] ≈ 10-5 Std ratio ~ Std ratio 1% std. S+gas ⟶ S-precursorice energetic processing (UV/CRs) non-energetic processing (atoms/radicals) ⟶⟶ S-bearing-product 10% std. Solar System protoplanetary disk dense clouds
Sulfur chemistry … and how different works this week address it S-depletion problem S solid-state chemistry S-bearing species as a tracer Angèle Taillard Julia Santos Katerina Slavicinska Maria Teresa Valdivia Mena Laura Colzi Rafael Martin Domenech Julia Santos Katerina Slavicinska diffuse clouds [S] [H] ≈ 10-5 Std ratio ~ Std ratio 1% std. Tychoenic+21 S+gas ⟶ S-precursorice energetic processing (UV/CRs) non-energetic processing (atoms/radicals) ⟶⟶ S-bearing-product +Posters: Miguel Sanz-Novo, Laura Schöller 10% std. Solar System protoplanetary disk dense clouds
S-depletion problem S-bearing species as a tracer diffuse clouds [S] [H] ≈ 10-5 Std ratio ~ Std ratio 1% std. Tychoenic+21 10% std. Solar System protoplanetary disk dense clouds Sulfur chemistry … and how different works this week address it S solid-state chemistry S+gas ⟶ S-precursorice energetic processing (UV/CRs) non-energetic processing (atoms/radicals) ⟶⟶ S-bearing-product a.k.a Meth yl Mer ca pta n H2S + H2/H/C ⟶ CH3SH this work
CH3SH: the target molecule •First CH3SH detection: •Sgr B2 Turner 1977; Linke+1979 •First CH2DSH detection: •IRAS 16293-2422 Bunn+2025 CH3OH Meth anol Watanabe+02; Fuchs+09; Chuang+16, 17; Fedoseev+17; Santos+22 CO ⟶ HCO ⟶ H2CO ⟶ CH3O ⟶ CH3OH +H +H +H +H CH3SH Meth yl Mer captan CS ⟶ HCS ⟶ H2CS ⟶ CH3S ⟶ CH3SH +H +H +H +H Lamberts+18 CH3SH is the S-analogue of CH3OH
CH3SH: the target molecule •First CH3SH detection: •Sgr B2 Turner 1977; Linke+1979 •First CH2DSH detection: •IRAS 16293-2422 Bunn+2025 CH3OH Meth anol Watanabe+02; Fuchs+09; Chuang+16, 17; Fedoseev+17; Santos+22 CO ⟶ HCO ⟶ H2CO ⟶ CH3O ⟶ CH3OH +H +H +H +H CH3SH Meth yl Mer captan CS ⟶ HCS ⟶ H2CS ⟶ CH3S ⟶ CH3SH +H +H +H +H Lamberts+18 CH3SH is the S-analogue of CH3OH Beyond CS hydrogenation: What alternative pathways lead to CH₃SH formation, and what do they reveal about sulfur chemistry in starforming environments?
C + H + H2S: TPD H2CS and CH3SH desorb at different temperatures with distinct fragmentation patterns -CH3SH - IR spectra and fragments match with NIST database - H2CS: - C + H2S - Tdes higher than expected, might be forming a trimer or bigger oligomers
1400 1200 1000 0.000 0.002 0.004 H2S Absorbance Wavenumber (cm-1) C + H2S + H(H2) C + H2S + H2 C + H2S+(trace H2) C + H2S @35 K CH4 CH3SH CH3SH CH3SH CH3SH H2CS H2CS H2CS Less H2 More H2 C + H/H2 + H2S: IR SPECTRA H2CS and CH3SH formation in various H/H2 conditions
1400 1200 1000 0.000 0.002 0.004 H2S Absorbance Wavenumber (cm-1) C + H2S + H(H2) C + H2S + H2 C + H2S+(trace H2) C + H2S @35 K CH4 CH3SH CH3SH CH3SH CH3SH H2CS H2CS H2CS Less H2 More H2 C + H/H2 + H2S: IR SPECTRA H2CS and CH3SH formation in various H/H2 conditions
Less H/H2More H/H2 0.0 0.2 0.4 0.6 0.8 1.0 C + H2S + H/H2 C + H2S + H2 C + H2S + (trance H2) Normalized abundance* Experiment CH3SH H2CS *Normalized to the N(H2CS) for C+H2S@35 K C + H2S (@35K) C + H/H2 + H2S H2CS and CH3SH formation with different efficiencies the absence or with very low H/H₂, H₂CS dominates formation of CH₃SH predominates C + H2S ⟶ H2CS C ⟶ CH2 CH2 ⟶ CH3SH C ⟶ CH2 ⟶ CH4 +H2 Contributes to CH4 formation Lamberts+2022 +H2 +H2 +H2S
Less H/H2More H/H2 0.0 0.2 0.4 0.6 0.8 1.0 C + H2S + H/H2 C + H2S + H2 C + H2S + (trance H2) Normalized abundance* Experiment CH3SH H2CS *Normalized to the N(H2CS) for C+H2S@35 K C + H2S (@35K) C + H/H2 + H2S H2CS and CH3SH formation with different efficiencies the absence or with very low H/H₂, H₂CS dominates formation of CH₃SH predominates C + H2S ⟶ H2CS C ⟶ CH2 CH2 ⟶ CH3SH C ⟶ CH2 ⟶ CH4 +H2 Contributes to CH4 formation Lamberts+2022 +H2 +H2 +H2S Beyond CS hydrogenation
Prestellar core chemistry …and where C + H2S + H2 is relevant Snow & McCall, 2006 van Dishoeck & Black (1989) H2S(s) + H2 (g) + C(g) Where is available simultaneously? H₂ is 2 - 4 x more abundant than H!
Prestellar core chemistry …and where C + H2S + H2 is relevant Ruffle+99 At low abundances CO can be destroyed by He+ and C atoms are available at denser regions chemical model of a collapsing cloud
Prestellar core chemistry Increasing density, decreasing temperature H2 ⟶ H2+ ⟶ H3+ cr H2 Central in gas-phase reactions H3+ + HD ⟶ H2D+ + H2 + 230 K Watson+1974 Ceccarelli+PPVI 2014 H2S(s) + H2/H/C(g)
Silvia Spezzano Paola Caselli H2/HD remains constant! Lab Astro meeting Ringberg Castle, Oct24
Silvia Spezzano Paola Caselli H2/HD remains constant! cv cv So solid-state reactions with H2/HD wouldn’t affect D-fractionation! Franciele Kruczkiewicz Lab Astro meeting Ringberg Castle, Oct24
Which mechanisms and barriers are into play? C + H2 + H2S 2) “H-assisted” mechanism e.g. Molpecere+21, Ferrero+23 C + H2O → H2CO proceeds barrierless due to a catalytic effect provided by the H-bonded networks from water Molpeceres+21 Molpeceres+21