Cold gas as an ice diagnostic toward low mass protostars

📝 Abstract

Up to 90% of the chemical reactions during star formation occurs on ice surfaces, probably including the formation of complex organics. Only the most abundant ice species are however observed directly by infrared spectroscopy. This study aims to develop an indirect observational method of ices based on non-thermal ice desorption in the colder part of protostellar envelopes. For that purpose the IRAM 30m telescope was employed to observe two molecules that can be detected both in the gas and the ice, CH3 OH and HNCO, toward 4 low mass embedded protostars. Their respective gas-phase column densities are determined using rotational diagrams. The relationship between ice and gas phase abundances is subsequently determined. The observed gas and ice abundances span several orders of magnitude. Most of the CH3OH and HNCO gas along the lines of sight is inferred to be quiescent from the measured line widths and the derived excitation temperatures, and hence not affected by thermal desorption close to the protostar or in outflow shocks. The measured gas to ice ratio of ~10-4 agrees well with model predictions for non-thermal desorption under cold envelope conditions and there is a tentative correlation between ice and gas phase abundances. This indicates that non-thermal desorption products can serve as a signature of the ice composition. A larger sample is however necessary to provide a conclusive proof of concept.

💡 Analysis

Up to 90% of the chemical reactions during star formation occurs on ice surfaces, probably including the formation of complex organics. Only the most abundant ice species are however observed directly by infrared spectroscopy. This study aims to develop an indirect observational method of ices based on non-thermal ice desorption in the colder part of protostellar envelopes. For that purpose the IRAM 30m telescope was employed to observe two molecules that can be detected both in the gas and the ice, CH3 OH and HNCO, toward 4 low mass embedded protostars. Their respective gas-phase column densities are determined using rotational diagrams. The relationship between ice and gas phase abundances is subsequently determined. The observed gas and ice abundances span several orders of magnitude. Most of the CH3OH and HNCO gas along the lines of sight is inferred to be quiescent from the measured line widths and the derived excitation temperatures, and hence not affected by thermal desorption close to the protostar or in outflow shocks. The measured gas to ice ratio of ~10-4 agrees well with model predictions for non-thermal desorption under cold envelope conditions and there is a tentative correlation between ice and gas phase abundances. This indicates that non-thermal desorption products can serve as a signature of the ice composition. A larger sample is however necessary to provide a conclusive proof of concept.

📄 Content

arXiv:0901.1019v1 [astro-ph.SR] 8 Jan 2009 Astronomy & Astrophysics manuscript no. 11228 c⃝ESO 2018 November 1, 2018 Cold gas as an ice diagnostic toward low mass protostars Karin I. ¨Oberg1, Sandrine Bottinelli1, and Ewine F. van Dishoeck1,2 1 Leiden Observatory, Leiden University, P.O. Box 9513, NL 2300 RA Leiden, The Netherlands 2 Max-Planck-Institut f¨ur extraterrestrische Physik (MPE), Giessenbachstraat 1, 85748 Garching, Germany ABSTRACT Context. Up to 90% of the chemical reactions during star formation occurs on ice surfaces, proba- bly including the formation of complex organics. Only the most abundant ice species are however observed directly by infrared spectroscopy. Aims. This study aims to develop an indirect observational method of ices based on non-thermal ice desorption in the colder part of protostellar envelopes. Methods. The IRAM 30m telescope was employed to observe two molecules that can be detected both in the gas and the ice, CH3OH and HNCO, toward 4 low mass embedded protostars. Their respective gas-phase column densities are determined using rotational diagrams. The relationship between ice and gas phase abundances is subsequently determined. Results. The observed gas and ice abundances span several orders of magnitude. Most of the CH3OH and HNCO gas along the lines of sight is inferred to be quiescent from the measured line widths and the derived excitation temperatures, and hence not affected by thermal desorption close to the protostar or in outflow shocks. The measured gas to ice ratio of ∼10−4 agrees well with model predictions for non-thermal desorption under cold envelope conditions and there is a tentative correlation between ice and gas phase abundances. This indicates that non-thermal desorption products can serve as a signature of the ice composition. A larger sample is however necessary to provide a conclusive proof of concept. Key words. Astrochemistry, Molecular processes, Molecular data, ISM: molecules, Circumstellar matter, Radio lines: ISM

1. Introduction In cold pre-stellar cores, more than 90% of all molecules, except for H2, are found in ices (Caselli et al. 1999; Bergin et al. 2002). These ices build up through accretion of atoms and molecules onto cold (sub)micron-sized silicate particles and subsequent hydrogenation to form e.g. H2O from O (L´eger et al. 1985; Boogert & Ehrenfreund 2004). Observations show that H2O is the main ice constituent in most lines of sight, with a typical abundance of 1 × 10−4 with respect to H2, followed by CO, CO2 and CH3OH (Gibb et al. 2004; Pontoppidan et al. 2004). During star formation, these ices may be modified by interactions with cosmic rays, UV ir- radiation, and heating to form complex organic species (Garrod et al. 2008). Gas phase complex 2 Karin I. ¨Oberg et al.: Cold gas as an ice diagnostic toward low mass protostars species have been observed toward several high and low mass protostars, so-called hot cores and corinos (Bisschop et al. 2007; Bottinelli et al. 2007). Whether these molecules are formed in the ice and subsequently evaporated, or formed in the hot gas phase from desorbed simpler ices such as CH3OH is still debated. This is not easily resolved because the abundances of the solid complex molecules are too low to be detected with infrared observations of ices even if they are present in the ice. Therefore, observing gas-phase abundances in the cold envelope may be the most robust constraint on complex ice processes available. Experimental investigations have concluded that non-thermal desorption is efficient for several common ice molecules, such as CO, CO2, and H2O, with photodesorption yields of ∼10−3 per inci- dent photon (Westley et al. 1995; ¨Oberg et al. 2008, 2009). Photodesorption is possible inside cold dark cloud cores and protostellar envelopes because of constant UV fields generated from cosmic ray interactions with H2 (Shen et al. 2004). Thus a small, but significant, part of the molecules formed in the ice should always be present in the gas phase. This explains observed abundances of gas phase CH3OH in translucent clouds, dark cloud cores and protostellar envelopes (Turner 1998; Maret et al. 2005; Requena-Torres et al. 2007). The amount of CH3OH gas observed in these en- vironments suggests that complex molecules (e.g. methyl formate) that form in the ice should be observable in the gas phase due to ice photodesorption, if their abundance ratios with respect to CH3OH in the ice are the same as observed in hot cores and corinos. For the first time, we combine infrared ice observations and millimeter gas observations for the same lines of sight to investigate the connection between ice and quiescent gas abundances. We focus on the only commonly observed ice components that have rotational transitions in the mil- limeter spectral range – CH3OH and HNCO. The CH3OH ice abundances in low mass protostellar envelopes vary between 1–30% with respect to H2O ice (Boogert et al. 2008). It is also one of the most

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