Chemical differentiation in regions of massive star formation

被引:119
|
作者
Rodgers, SD [1 ]
Charnley, SB [1 ]
机构
[1] NASA, Ames Res Ctr, Div Space Sci, Moffett Field, CA 94035 USA
来源
ASTROPHYSICAL JOURNAL | 2001年 / 546卷 / 01期
关键词
ISM : abundances; ISM : clouds; ISM : molecules; molecular processes; stars : formation;
D O I
10.1086/318263
中图分类号
P1 [天文学];
学科分类号
0704 ;
摘要
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 (similar to 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 similar to 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.
引用
收藏
页码:324 / 329
页数:6
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