Chemical Differentiation in Regions of Massive Star Formation

Abstract

We have reexamined the origin of the apparent differentiation between nitrogen-bearing molecules and complex oxygen-bearing molecules that is observed in hot molecular cores associated with massive protostars. Observations show that methanol is an ubiquitous and abundant component of protostellar ices. Recent observations suggest that ammonia may constitute an appreciable fraction of the ices toward some sources. In contrast to previous theories that suggested that N/O differentiation was caused by an anticorrelation between methanol and ammonia in the precursor grain mantles, we show that the presence of ammonia in mantles and the core temperature are key quantities in determining N/O differentiation. Calculations are presented which show that when large amounts of ammonia are evaporated alkyl cation transfer reactions are suppressed and the abundances of complex O-bearing organic molecules greatly reduced. Cooler cores (100 K) eventually evolve to an oxygen-rich chemical state similar to that attained when no ammonia was injected, but on a timescale that is an order of magnitude longer (~10(5) yr). Hotter cores (300 K) never evolve an O-rich chemistry unless ammonia is almost absent from the mantles. In this latter case, a complex O-rich chemistry develops on a timescale of ~10(4) yr, as in previous models, but disappears in about 2 x 10(5) yr, after which time the core is rich in NH3, HCN, and other N-bearing molecules. There are thus two ways in which N-rich cores can occur. We briefly discuss the implications for the determination of hot-core ages and for explaining N/O differentiation in several well-studied sources.

Gas-phase CO$_\mathsf{2}$, C$_\mathsf{2}$H$_\mathsf{2}$, and HCN toward Orion-KL

“…This could be a possible explanation for the high HCN abundance found from the absorption lines, which is an order of magnitude higher than found in the more extended Orion ridge (Bergin et al 1997). The HCN and C 2 H 2 abundances also agree well with predictions from hot core models by Rodgers & Charnley (2001). Abundances taken at a single temperature/density point from envelope models following Doty et al (2002) are in agreement with the low end of the observed range (see Table 2 and Sect.…”

“…These are higher than predicted by gas-phase models at T < ∼ 200 K, which predict abundances of < ∼ 5 × 10 −8 at late times. For T > ∼ 200 K HCN and C 2 H 2 abundances of ∼10 −7 are easily produced by gas-phase models (Rodgers & Charnley 2001;Doty et al 2002). The HCN abundance is also higher than the values of ∼10 −8 derived from submillimeter observations by Blake et al (1987) and Schilke et al (1992) for the extended ridge.…”

“…b Average column density from Blake et al (1987) and Schilke et al (2001) for the plateau gas. c HCN and C 2 H 2 from Rodgers & Charnley (2001) for t ∼ 10 5 yr at T = 300 K for HCN and T = 100-300 K for C 2 H 2 . CO 2 from Charnley (1997) at T = 200 K and t ∼ 10 5 yr.…”

“…It is assumed that these molecules are originally not present in ices. d HCN and C 2 H 2 from Rodgers & Charnley (2001) for t ∼ 10 5 yr at T = 300 K for HCN and T = 100-300 K for C 2 H 2 . CO 2 from Charnley (1997) at T = 200 K and t ∼ 10 5 yr.…”

Observations of chemical differentiation in clumpy molecular clouds

“…17]. The chemical model is based on the model of Rodgers & Charnley [60], and has been updated to incorporate recent laboratory measurements of the dissociative recombination channels for several ions, including CH 3 OH + 2 [44,61,62,63,64]. The initial molecular abundances in the newly-formed clump are determined by those in the interclump medium, which are themselves controlled by both (i) the chemistry in these regions, and (ii) whether the material previously passed through a dense phase.…”