2006   2008   2010   2012   2014   2016   2018   2020  
2006   2008   2010   2012   2014   2016   2018   2020
The MPIA conference series
at Ringberg Castle
The MPIA conference series
at Ringberg Castle
 
EPoS 2020 Suggested Contribution
The sulphur depletion problem in molecular clouds: the H2S case

David Navarro
OAN, Madrid, ES
Sulphur is one of the most abundant elements in the Universe and plays a crucial role in biological systems. It is therefore of great interest to track its chemical history in space. However, sulphuretted molecules are not as abundant as expected in the ISM - the Sulphur depletion problem - and there is no clear answer of where the missing Sulphur is yet. To shed light onto this open question, we focus our attention on the chemistry of H2S in dark clouds. This molecule is thought to be an important reservoir of Sulphur, mainly in solid state, locked in grain ices. Using a subset of the GEMS IRAM Large Program data, which comprises IRAM 30m telescope millimeter observations of CS, SO and H2S, in this work we have determined the physical conditions and modeled the H2S chemistry in the TMC 1-C, TMC 1-CP and Barnard 1b cores. The NAUTILUS chemical code is used to model the sulfur chemistry and explore the impact of photo-desorption and chemical desorption on the H2S abundance. Our results show that chemical desorption is the main formation mechanism of H2S in dark cores. Our results, at densities n(H) < 2×104 cm-3, are well fitted assuming the chemical desorption efficiency as proposed by for bare grains. For higher densities, our model overestimates the H2S abundance, suggesting that chemical desorption becomes less active. According to our model, the decrease of the H2S chemical desorption occurs when the abundance of H2O and CO ices achieves their maximum value in both molecular clouds. We propose that this change in the chemical desorption efficiency is related to a change in the chemical composition of grains, produced by the formation of a thick H2O and CO ice mantle on their surfaces when n(H) > 2×104 cm-3. Therefore, H2S might be tracing the snowline of dark clouds. Additionally, our model predicts that H2S is the main reservoir of S in icy mantles, with a similar abundance in both targets of around one fifth of the Sulphur cosmic abundance. Finally, our model yields an elemental abundance of S/H of around the cosmic value within a factor of ten.
Caption: Top panels: total visual extinction maps of TMC1 (left) and Barnard 1b (right), and the observed positions. Second row shows our observations (purple dots) compared to our simulations with Nautilus and different desorption schemes (solid and dashed lines). In the third and fourth rows, we simulate of the composition of ices in the surface of grains and their mantles in TMC 1 (left) and Barnard 1b (right), respectively. We find that solid H2S is the most abundant S-bearing molecule. Furthermore, the decreasing abundance of gas-phase H2S, and therefore, the desorption efficiency, together with the maximum H2O and CO ice abundance that occurs at n~2×104, suggest a change in the composition on grain surfaces. These simulations where carried out as a one-dimensional static cloud model. We have followed the abundance of chemical species along the static density, temperature, and extinction structures derived for TMC 1-C and Barnard 1b for 10e6 yr.
Collaborators:
D.G. Navarro, OAN, ES
A. Fuente, OAN, ES
R. Le Gal, CfA, USA
GEMS Team
Key publication

Suggested Session: Chemistry