Methanol is believed to be formed on dust grains (Watanabe & Kouchi 2002) by subsequent hydrogenation of carbon monoxide, and its detection towards prestellar cores is already a challenge for current models given the absence of efficient desorption processes in these sources. Thermal desorption is out of question because of the low dust temperature. Recent laboratory studies showed that the photo-desorption of methanol from ices is likewise negligible (Bertin et al. 2016; Cruz-Diaz et al. 2016). The main desorption products when irradiating pure and mixed methanol ices are photo-fragments of methanol. An alternative route to explain the presence of methanol in the gas phase is the reactive/chemical desorption that has been theoretically proposed by Garrod et al. (2007) and Vasyunin & Herbst (2013) and experimentally studied by Dulieu et al. (2013) and Minissale et al. (2016). On the other hand, c-C3H2 is mainly formed in the gas phase (e.g. Spezzano et al. 2013) and is expected to preferentially trace dense and chemically young gas, that is, gas where C atoms have not yet been mainly locked into CO. This gas rich in C atoms is expected in the outer envelope of an externally illuminated dense core (e.g. Aikawa et al. 2001). However, towards L1544, c-C3H2 only appears to trace one side of the core, the one closer to the sharp N(H2) edge and away from the CH3OH peak. This indicates that photo-chemistry is not uniformly active around L1544, most likely because the distribution of the envelope material (belonging to the filament within which L1544 is embedded) is not uniform, as is clearly shown by the Herschel map in Fig. 1. This figure shows that methanol traces a region farther away from the southern sharp edge of the N(H2) map, which is possibly more shielded from the ISRF and where most of the carbon is locked in CO. CH3OH is preferentially found at the northern edge of L1544 because here photochemistry does not play a major role (so C is locked in CO) and the density is low enough to maintain a higher fraction of CH3OH in the gas phase, but above the threshold value for CO freeze-out, a few ×104 cm-3 (Caselli et al. 1999). Based on the Keto & Caselli (2010) model (that was updated by Keto et al. 2014), the volume density at the distance of the methanol peak is predicted to be 8 × 104 cm-3, which is just above the threshold value. In contrast, cyclopropenylidene has the most prominent peak towards the southern sharp edge of the H2 column density and extends along the semi-major axis of the core, almost parallel to the south-west edge of the N(H2) map. This behaviour is also clearly shown in Fig. 2, where the integrated intensities of both methanol and cyclopropenylidene are plotted against N(H2). c-C3H2 is also present at values of N(H2) that are lower than those of methanol, and it maintains a flat intensity profile, suggestive of a layer-like structure, with no significant increase towards the core centre. The CH3OH intensity instead shows a sharp rise up to column densities of about 1.6 × 1022 cm-2, and it declines at higher values.
Citation: Vasyunin & Herbst (2013)
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