astronomy.system - Astronomy Astrophysics A&A 457...

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A&A 457, 927–936 (2006) DOI: 10.1051 / 0004-6361:20065560 c ESO 2006 Astronomy & Astrophysics Formation of methyl formate and other organic species in the warm-up phase of hot molecular cores R. T. Garrod 1 and E. Herbst 1 , 2 1 Department of Physics, The Ohio State University, Columbus, OH 43210, USA e-mail: [email protected] 2 Departments of Astronomy and Chemistry, The Ohio State University, Columbus, OH 43210, USA Received 5 May 2006 / Accepted 10 July 2006 ABSTRACT Aims. The production of saturated organic molecules in hot cores and corinos is not well understood. The standard approach is to assume that, as temperatures heat up during star formation, methanol and other species evaporate from grain surfaces and undergo a warm gas-phase chemistry at 100 K or greater to produce species such as methyl formate, dimethyl ether, and others. But a series of laboratory results shows that protonated ions, typical precursors to final products in ion-molecule schemes, tend to fragment upon dissociative recombination with electrons rather than just ejecting a hydrogen atom. Moreover, the specific proposed reaction to produce protonated methyl formate is now known not to occur at all. Methods. We utilize a gas-grain chemical network to probe the chemistry of the relatively ignored stage of hot core evolution during which the protostar switches on and the temperature of the surrounding gas and dust rises from 10 K to over 100 K. During this stage, surface chemistry involving heavy radicals becomes more important as surface hydrogen atoms tend to evaporate rather than react. Results. Our results show that complex species such as methyl formate, formic acid, and dimethyl ether can be produced in large abundance during the protostellar switch-on phase, but that both grain-surface and gas-phase processes help to produce most species. The longer the timescale for protostellar switch-on, the more important the surface processes. Key words. astrochemistry – stars: formation – ISM: abundances – ISM: clouds – ISM: molecules 1. Introduction Various large molecules including methyl formate (HCOOCH 3 ) have been detected in a number of hot molecular cores and corinos (Blake et al. 1987; Hatchell et al. 1998; Nummelin et al. 2000; Cazaux et al. 2003; Bottinelli et al. 2004). These are chemically rich regions in dense interstellar clouds which are warmed by an associated protostar and have typical densi- ties of 10 6 –10 8 cm 3 and temperatures on the order of 100 K. Observations suggest methyl formate abundances can be as high as 10 8 × n (H 2 ) in these regions. The standard view of the complex chemistry that pertains in hot cores requires that dust grains build up icy mantles at early times, when the temperature is low ( 10 K) and the core is in a state of collapse (Brown et al. 1988). Later, the forma- tion of a nearby protostar quickly warms the gas and dust, re- injecting the grain mantle material into the gas phase, and stim- ulating a rich chemistry. Various hot core models (Millar et al.
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