Molecular abundances in OMC-1 - The chemical composition of interstellar molecular clouds and the influence of massive star formation

Abstract

We present here an investigation of the chemical composition of the various regions in the core of the Orion molecular cloud (OMC-1) based on results from the Caltech Owens Valley Radio Observatory (OVRO) millimeter-wave spectral line survey (Sutton et al.; Blake et al.). This survey covered a 55 GHz interval in the 1.3 rom (230 GHz) atmospheric window and contained emission from over 800 resolved spectral features. Of the 29 identified species 14 have a sufficient number of detected transitions to be investigated with an LTE "rotation diagram" technique, in which large numbers of lines are used to estimate both the rotational excitation and the overall abundance. The rotational temperatures and column densities resulting from these fits have then been used to model the emission from those remaining species which either have too few lines or which are too weak to be so analyzed. When different kinematic sources of emission are blended to produce a single feature, Gaussian fits have been used to derive the individual contributions to the total line profile. The uniformly calibrated data in the unique and extensive Caltech spectral line survey lead to accurate estimates of the chemical and physical parameters of the Orion molecular cloud, and place significant constraints on models of interstellar chemistry. A global analysis of the observed abundances shows that the markedly different chemical compositions of the kinematically and spatially distinct Orion subsources may be interpreted in the framework of an evolving, initially quiescent, gas-phase chemistry influenced by the process of massive star formation. The chemical composition of the extended Orion cloud complex is similar to that found in a number of other objects, but the central regions of OMC-1 have had their chemistry selectively altered by the radiation and high-velocity outflow from the young stars embedded deep within the interior of the molecular cloud. Specifically, the extended ridge clouds are inferred to have a low (subsolar) gas-phase oxygen content from the prevalence of reactive carbon-rich species like CN, CCH, and C 3 H 2 also found in more truly quiescent objects such as TMC-1. The similar abundances of these and other simple species in clouds like OMC-1, Sgr B2, and TMC-1 lend support to gas-phase ion-molecule models of interstellar chemistry, but grain processes may also play a significant role in maintaining the overall chemical balance in such regions through selective depletion mechanisms and grain mantle processing. In contrast, the chemical compositions of the more turbulent plateau and hot core components of OMC-1 are dominated by high-temperature, shock-induced gas and grain surface neutral-neutral reaction processes. The high silicon/sulfur oxide and water content of the plateau gas is best modeled by fast shock disruption of smaller grain cores to release the more refractory elements followed by a predominantly neutral chemistry in the cooling postshock regions, while a more passive release of grain mantle products driven towar...

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

“…Millimeter and submillimeter single-dish surveys show thousands of lines of nearly a hundred different molecules (e.g., Blake et al 1987;Sutton et al 1995;Schilke et al 1997Schilke et al , 2001), whereas interferometer studies reveal intriguing chemical differentiation over scales of less than 2000 AU (e.g., Wright et al 1996;Blake et al 1996). In spite of this wealth of data, molecules such as CO 2 and C 2 H 2 , which are symmetric and thus lack a dipole moment, cannot be observed through rotational transitions at millimeter Send offprint requests to: A. M. S. Boonman, e-mail: boonman@strw.leidenuniv.nl Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, The Netherlands and the UK) and with the participation of ISAS and NASA. wavelengths.…”

“…In contrast, the gas-phase CO 2 abundance is surprisingly low, ∼10 −7 , toward massive YSOs (van Dishoeck et al 1996;van Dishoeck 1998;Dartois et al 1998;Boonman et al 2000). Since the abundances of many gas-phase species are enhanced toward IRc2, in particular those of species involved in the gas-grain chemistry (e.g., Blake et al 1987;Charnley et al 1992), it is important to investigate whether the CO 2 chemistry follows this trend. Observations of C 2 H 2 and HCN are interesting because they are both significant in the carbon-and nitrogen chemistry, and because their excitation provides information on the physical conditions .…”

The Effect of Violent Star Formation on the State of the Molecular Gas in M 82

“…The best agreement with the observed line ratios and absolute intensities is found for a beam filling of 0.4. Positions 6 and 7 at the eastern MIR peak (Telesco & Gezari 1992) (Blake et al 1987) and were suggested by chemical models (Farquhar et al 1994). …”

Models and Observations of the Chemistry Near Young Stellar Objects

“…Observationally, hot cores are characterized by high abundances of fully hydrogenated molecules such as NH 3 , H 2 S, and H 2 O (e.g., Gensheimer et al 1996), as well as saturated complex organic molecules like CH 3 OH, CH 3 CN, CH 3 OCH 3 and HCOOCH 3 (e.g., Hatchell et al 1998). Examples of hot cores include the Orion hot core and compact ridge (e.g., Blake et al 1987, Sutton et al 1995, SgrB2(N) (e.g., Kuan & Snyder 1996), W 3(H 2 O) (e.g., and objects near ultracompact H II regions such as G34.3+0.15 (Macdonald et al 1996; see chapter by Churchwell). The observed abundances of the species can vary significantly from region to region.…”