A&A 624, A108 (2019) Astronomy https://doi.org/10.1051/0004-6361/201834446 & c J. C. Laas and P. Caselli 2019 Astrophysics Modeling sulfur depletion in interstellar clouds? Jacob C. Laas and Paola Caselli Max-Planck-Institut für extraterrestrische Physik, Garching 85748, Germany e-mail: [email protected],[email protected] Received 17 October 2018 / Accepted 25 February 2019 ABSTRACT Context. The elemental depletion of interstellar sulfur from the gas phase has been a recurring challenge for astrochemical models. Observations show that sulfur remains relatively non-depleted with respect to its cosmic value throughout the diffuse and translucent stages of an interstellar molecular cloud, but its atomic and molecular gas-phase constituents cannot account for this cosmic value toward lines of sight containing higher-density environments. Aims. We have attempted to address this issue by modeling the evolution of an interstellar cloud from its pristine state as a diffuse atomic cloud to a molecular environment of much higher density, using a gas-grain astrochemical code and an enhanced sulfur reac- tion network. Methods. A common gas-grain astrochemical reaction network has been systematically updated and greatly extended based on pre- vious literature and previous sulfur models, with a focus on the grain chemistry and processes. A simple astrochemical model was used to benchmark the resulting network updates, and the results of the model were compared to typical astronomical observations sourced from the literature. Results. Our new gas-grain astrochemical model is able to reproduce the elemental depletion of sulfur, whereby sulfur can be de- pleted from the gas-phase by two orders of magnitude, and that this process may occur under dark cloud conditions if the cloud has a chemical age of at least 106 years. The resulting mix of sulfur-bearing species on the grain ranges across all the most common chem- ical elements (H/C/N/O), not dissimilar to the molecules observed in cometary environments. Notably, this mixture is not dominated simply by H2S, unlike all other current astrochemical models. Conclusions. Despite our relatively simple physical model, most of the known gas-phase S-bearing molecular abundances are ac- curately reproduced under dense conditions, however they are not expected to be the primary molecular sinks of sulfur. Our model predicts that most of the “missing” sulfur is in the form of organo-sulfur species that are trapped on grains. Key words. astrochemistry – molecular processes – ISM: molecules 1. Introduction studies of photo-processed sulfur; the reaction network pertain- ing to certain categories of molecules has also been expanded Sulfur poses an interesting challenge to models of interstel- by the study reported by Vidal et al.(2017); and, a better under- lar chemistry. Within primitive interstellar environments, it is standing of dynamical models is provided by Vidal & Wakelam known that sulfur remains in ionized atomic form and close to (2018) to help put into perspective the apparent chemical vari- the cosmic abundance (Jenkins 2009). However, in molecular ability of sulfur chemistry across cloud evolution. clouds and star-forming regions, this cosmic abundance is dras- Laboratory studies are also continuously helping to shed tically reduced from the gas-phase molecular inventory. Many light on sulfur chemistry, which can be notoriously complex. simple organo-sulfur species can be detected, but not seemingly Sulfur bonds are generally not as strong as those of the first- enough to fully account for its cosmic abundances. and second-row elements (i.e., H, C, N, O), and processing of Modern astrochemical models still today predict that the interstellar analogs of ice mixtures containing simple S-bearing bulk of sulfur resides as condensed H2S, despite upper lim- molecules can yield a highly heterogeneous mixture of products its from observations (Smith 1991; van der Tak et al. 2003; (e.g., Jiménez-Escobar et al. 2014, and references therein). Qual- Jiménez-Escobar & Muñoz Caro 2011). The effective work- itatively, these processed ices sometimes even resemble the type around has been to use a severely depleted value of the elemental of chemistry that has been detected in both cometary ices (Cal- abundance (.1% with respect to the cosmic standard abundance) monte et al. 2016) and meteoritic material (see, e.g., Ehrenfreund for the initial sulfur content when modeling dense interstellar et al. 2002). However, many important laboratory studies have environments. This has resulted in a rather poor understanding not yet been incorporated into a modern gas-grain astrochemical of sulfur with respect to interstellar matter, and the inability to model. correctly model sulfur remains a severe shortcoming of astro- We have set out to update the sulfur chemistry within a mod- chemical models. That’s not to say that astrochemical models of ern astrochemical reaction network in a systematic and more sulfur are not improving. For example, a recent model (Woods complete way than has been done in the past, and then use this et al. 2015) helps to provide constraints on how much sulfur may gas-grain reaction network to model the evolution of an inter- be locked in the refractory residue that is known from laboratory stellar cloud from its pristine diffuse stage to a dense and dark ? Full Tables A.1 and B.4 are only available at the CDS via anony- quiescent state to check for clues pertaining to the gas-phase mous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http: depletion of sulfur. A collection of observational studies has //cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/624/A108 also been used to provide general constraints for benchmarking A108, page 1 of 17 Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Open Access funding provided by Max Planck Society. A&A 624, A108 (2019) this updated astrochemical model to ensure reasonable results Table 1. Initial elemental abundances. for all the sulfur-bearing interstellar molecules that are typi- cally observed in interstellar environments. We have found that Species Fractional Reference this updated sulfur network does in fact reproduce observa- abundance (a) tions of interstellar sulfur chemistry in dense environments, and this relatively simple astrochemical model also reproduces the H 0.9999 severe gas-phase depletion of sulfur thanks to the production H2 5e−5 a variety of (mostly-stable) organo-sulfur molecules on grains He 9.55e−2 1 (see Sect.3). O 5.7544e−4 1 + In Sect.2, we first present the astrochemical model in terms C 2.0893e−4 1 of its chemical and physical characteristics. In Sect.3, we N 5.7544e−5 1 Mg+ 3.6308e−5 1 present our results, beginning with a look at the question of + elemental depletion of sulfur during the dense stage (m ≈ Si 3.1623e−5 1 gas + 104−106 cm−3) of an interstellar cloud, and then followed up Fe 2.7542e−5 1 S+ 1.66e−5 2 with a brief analysis of commonly observed molecules across + a range of evolutionary stages and densities of interstellar clouds Na 1.74e−6 3 Cl+ 2.88e−7 2 with our model predictions. In Sect.4 we provide a summary + comparing our model with previous studies and to discuss key P 2.57e−7 3 results. Finally, Sect.5 contains some general conclusions. F 3.63e−8 3 e− 3.233e−4 (b) (a) 2. Astrochemical model Notes. We define the fractional abundance X(i) as the ratio of the abundance of species i wrt the total gas-phase hydrogen number density, (b) 2.1. Computer code which is initially equal to X(H) + 2X(H2). The total electron fraction is simply initialized as the sum of the gas-phase cations. Thevariousmodeltrialspresentedherehavebeenperformedusing References. (1) Przybilla et al.(2008); (2) Esteban et al.(2004); a new python-based code, which has not yet been described else- (3) Asplund et al.(2008). where1. The code is based on the OSU gas-grain astrochemical model presented by Garrod et al.(2008), which is a time- dependent, single-point, rate-based model for general application we felt detracts too much focus from development on the chem- to interstellar cloud environments. Benchmarks were performed ical network and the study of sulfur depletion. during its development to ensure that its operation is consistent We have used, when possible, known cosmic standard ele- with the code it is based on, and it has been further enhanced with mental abundances for the initial abundances of our model (i.e., a number of internal consistency checks and software tools for the for Stage 1), and these values are contained in Table1. Whereas development of a chemical network and for exploring its results. the details of the stages are presented below, in Sect. 2.3, we note here that the initial fractional abundances of subsequent stages are simply the final abundances of the preceding stages. 2.2. Chemical network The core reaction network was based on the OSU gas-grain 2.2.1. Sulfur chemistry model, which consists of ca. 8300 reactions and 860 chemical species, and remains in high circulation even a decade after being In order to allow for a number of chemical routes that a high released. Aside from a major revamping of the sulfur chemistry – (cosmic) sulfur abundance may take, we have sorted through a which we address in a separate section below – a number of wealth of literature data to systematically build a more complete modifications were made to the chemical inputs to reflect recent gas-grain sulfur network.