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Detection and Chemical Modeling of Ethyl Formate and N-Propyl Cyanide in Sagittarius B2(N),

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Detection and Chemical Modeling of Ethyl Formate and N-Propyl Cyanide in Sagittarius B2(N)�, A&A 499, 215–232 (2009) Astronomy DOI: 10.1051/0004-6361/200811550 & c ESO 2009 Astrophysics Increased complexity in interstellar chemistry: detection and chemical modeling of ethyl formate and n-propyl cyanide in Sagittarius B2(N), A. Belloche1,R.T.Garrod2,1,H.S.P.Müller3,1,K.M.Menten1, C. Comito1, and P. Schilke1 1 Max-Planck Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany e-mail: [belloche;kmenten;ccomito;schilke]@mpifr-bonn.mpg.de 2 Department of Astronomy, Cornell University, 106 Space Sciences Building, Ithaca, NY 14853, USA e-mail: [email protected] 3 I. Physikalisches Institut, Universität zu Köln, Zülpicher Str. 77, 50937 Köln, Germany e-mail: [email protected] Received 19 December 2008 / Accepted 17 February 2009 ABSTRACT Context. In recent years, organic molecules of increasing complexity have been found toward the prolific Galactic center source Sagittarius B2. Aims. We wish to explore the degree of complexity that the interstellar chemistry can reach in star-forming regions. Methods. We carried out a complete line survey of the hot cores Sgr B2(N) and (M) with the IRAM 30 m telescope in the 3 mm range, plus partial surveys at 2 and 1.3 mm. We analyzed this spectral survey in the local thermodynamical equilibrium approximation. We modeled the emission of all known molecules simultaneously, which allows us to search for less abundant, more complex molecules. We compared the derived column densities with the predictions of a coupled gas-phase and grain-surface chemical code. Results. We report the first detection in space of ethyl formate (C2H5OCHO) and n-propyl cyanide (C3H7CN) toward Sgr B2(N). The detection of n-propyl cyanide is based on refined spectroscopic parameters derived from combined analyses of available laboratory spectroscopic data. For each molecule, we identified spectral features at the predicted frequencies having intensities compatible with a unique rotation temperature. For an assumed source size of 3, our modeling yields a column density of 5.4 × 1016 cm−2, a temperature of 100 K, and a linewidth of 7 km s−1 for ethyl formate. n-Propyl cyanide is detected with two velocity components having column densities of 1.5 × 1016 cm−2 and 6.6 × 1015 cm−2, respectively, for a source size of 3, a temperature of 150 K, and −1 −9 a linewidth of 7 km s . The abundances of ethyl formate and n-propyl cyanide relative to H2 are estimated to be 3.6 × 10 and −9 1.0 × 10 , respectively. We derived column density ratios of 0.8/15/1 for the related species t-HCOOH/CH3OCHO/C2H5OCHO and 108/80/1forCH3CN/C2H5CN/C3H7CN. Our chemical modeling reproduces these ratios reasonably well. It suggests that the sequential, piecewise construction of ethyl and n-propyl cyanide from their constituent functional groups on the grain surfaces is their most likely formation route. Ethyl formate is primarily formed on the grains by adding CH3 to functional-group radicals derived from methyl formate, although ethanol may also be a precursor. Conclusions. The detection in Sgr B2(N) of the next stage of complexity in two classes of complex molecule, esters and alkyl cyanides, suggests that greater complexity in other classes of molecule may be present in the interstellar medium. Key words. astrobiology – astrochemistry – line: identification – stars: formation – ISM: individual objects: Sagittarius B2 – ISM: molecules 1. Introduction complexity2. In addition, much larger molecules have been found in meteorites discovered on Earth, including more than More than 150 molecules have been discovered in the inter- 80 distinct amino acids. The non-terrestrial isotopic ratios of stellar medium or in circumstellar envelopes over the past four 3 1 these amino acids, as well as their racemic distributions , sug- decades (see, e.g., Müller et al. 2005 ). Among them, “com- gest that they, or at least their direct precursors, have an in- plex” organic molecules with up to 13 atoms have been found, terstellar origin (see, e.g., Ehrenfreund et al. 2001; Bernstein showing that the interstellar chemistry in some regions is ef- et al. 2002; Elsila et al. 2007, and references therein). Interstellar ficient enough to achieve a relatively high degree of chemical chemistry is therefore very likely capable of producing more complex organic molecules than those discovered in the inter- Based on observations carried out with the IRAM 30 m telescope. stellar medium so far. However, the degree of complexity that IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). Tables 1, 2, 6, 7, 9, 10,Figs.1, 3 and Appendix are only available in 2 These molecules are “complex” for astronomers, not for biologists! electronic form at http://www.aanda.org 3 A racemic distribution means equal amounts of left- and right- 1 Visit the Cologne Database for Molecular Spectroscopy (CDMS) at handed enantiomers. Enantiomers are stereoisomers that are mirror http://www.cdms.de for an updated list. images of each other and non-superposable. Article published by EDP Sciences 216 A. Belloche et al.: Detection and chemical modeling of ethyl formate and n-propyl cyanide in Sgr B2(N) may be reached is still an open question; the partition functions 2. Observations and data analysis of larger molecules are large, making it much more difficult to detect such species, even if they are present in reasonably large 2.1. Observations quantities. We observed the two hot core regions Sgr B2(N) and Sgr B2(M) Grain-surface chemistry is frequently invoked as the forma- in January 2004, September 2004, and January 2005 with the tion mechanism of many complex species, particularly following IRAM 30 m telescope on Pico Veleta, Spain. We carried out recent determinations of some key gas-phase reaction rates. Gas- a complete spectral survey toward both sources in the 3 mm phase production of methyl formate, a molecule ubiquitous in atmospheric window between 80 and 116 GHz. A complete hot-core spectra, appears prohibitively slow (Horn et al. 2004), survey was performed in parallel in the 1.3 mm window be- pointing to an efficient alternative. Additionally, the dissociative tween 201.8 and 204.6 GHz and between 205.0 and 217.7 GHz. recombination of large organic molecular ions with electrons, Additional selected spectra were also obtained in the 2 mm win- which is typically the final step in the gas-phase synthesis of dow and between 219 and 268 GHz. The coordinates of the ob- complex molecules, appears strongly to favor the fragmentation h m s ◦ served positions are αJ2000 = 17 47 20.0, δJ2000 = −28 22 19.0 of complex structure (Geppert et al. 2006). −1 for Sgr B2(N) with a systemic velocity Vlsr = 64 km s and In the case of hot cores, the granular ice mantles built up dur- h m s ◦ αJ2000 = 17 47 20.4, δJ2000 = −28 23 07.0 for Sgr B2(M) with ing prior phases of evolution present a rich source of simple sat- −1 Vlsr = 62 km s . More details about the observational setup and urated molecules from which more complex species may form, the data reduction can be found in Belloche et al. (2008a). An as has long been realized (Millar et al. 1991). However, while − ffi rms noise level of 15 20 mK on the Ta scale was achieved be- the e ciency of complex molecule formation in the gas phase is low 100 GHz, 20−30 mK between 100 and 114.5 GHz, about limited (not exclusively) by the need to stabilize the energized 50 mK between 114.5 and 116 GHz, and 25−60 mK in the 2 mm complex, often resulting in fragmentation, adhesion to a grain window. At 1.3 mm, the confusion limit was reached for most of surface allows an adduct to quickly thermalize. Thus, molecular the spectra obtained toward Sgr B2(N). radicals derived from the ice mantles may combine in situ on the grain surfaces to build up complex structures efficiently, if dust temperatures are sufficient for the reactants to meet by thermal 2.2. Modeling of the spectral survey diffusion. The hot-core models of Garrod & Herbst (2006)and Garrod et al. (2008) have demonstrated the plausibility of such The overall goal of our survey was to characterize the molecular mechanisms in reproducing observed abundances of many com- content of Sgr B2(N) and (M). It also allows searches for new plex organic species. species once lines emitted by known molecules have been iden- The detection of new complex molecules places valuable tified, including vibrationally and torsionally excited states, as constraints on the chemical models. In the context of the model well as less abundant isotopologues containing, e.g., 13C, 18O, employed, e.g., by Garrod et al. (2008), obtaining abundances 17O, 34S, 33S, or 15N. We detected about 3700 and 950 lines of structurally-related molecules allows one to isolate the chem- above 3σ over the whole 3 mm band toward Sgr B2(N) and (M), ical behavior of the functional groups from which they are con- respectively. These numbers correspond to an average line den- structed, and to relate these back to more fundamental model pa- sity of about 100 and 25 features per GHz. Given this high rameters such as photodissociation rates, binding energies, and line density, the assignment of a line to a given molecule can initial ice composition. Such an approach then allows further ob- be trusted only if all lines emitted by this molecule in our fre- servational predictions to be made. quency coverage are detected with the right intensity predicted One of the current best sources to search for new by a model (see below) and no predicted line is missing in the molecules in the interstellar medium is the hot dense core observed spectrum.
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