Increased complexity in interstellar chemistry: detection and chemical modeling of ethyl formate andn-propyl cyanide in Sagittarius B2(N)

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 (C 2 H 5 OCHO) and n-propyl cyanide (C 3 H 7 CN) 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 × 10 16 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 × 10 16 cm −2 and 6.6 × 10 15 cm −2 , respectively, for a source size of 3 , a temperature of 150 K, and a linewidth of 7 km s −1 . The abundances of ethyl formate and n-propyl cyanide relative to H 2 are estimated to be 3.6 × 10 −9 and 1.0 × 10 −9 , respectively. We derived column density ratios of 0.8/15/1 for the related species t-HCOOH/CH 3 OCHO/C 2 H 5 OCHO and 108/80/1 for CH 3 CN/C 2 H 5 CN/C 3 H 7 CN. 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 CH 3 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.

Rotational spectra of isotopic species of methyl cyanide, CH3CN, in their ground vibrational states up to terahertz frequencies

“…With rotational temperatures of CH 3 CN in hot cores reaching at least 150−200 K (Sutton et al 1985;Belloche et al 2009), these data will be of great relevance for the Atacama Large Millimeter Array (ALMA). The main isotopolog and possibly even the 13 C isotopomers or the 8 = 1 excited vibrational state will be seen with the HIFI instrument (Heterodyne Instrument for the Far-Infrared) on board of the recently launched Herschel satellite or with SOFIA (Stratospheric Observatory For Infrared Astronomy).…”

Rotational spectrum of a chiral amino acid precursor, 2-aminopropionitrile, and searches for it in Sagittarius B2(N)

“…This number takes into account the contribution of the lowest energy vibrational-torsional modes 3 . Belloche et al (2009) reported an abundance ratio of 1.4 between methyl cyanide (CH 3 CN) and ethyl cyanide (C 2 H 5 CN) in Sgr B2(N). Because aminoacetonitrile and 2-aminopropionitrile are based on the same carbon chains as these two alkyl cyanides, but with an additional amine functional group, one could naively expect the same abundance ratio.…”

“…Therefore we used our complete model which includes all species detected in our survey so far (Belloche et al, in prep. ; see Belloche et al 2007Belloche et al , 2008aBelloche et al , 2009 for more details) to point out possible blends with transitions of other species. Within the frequency ranges of our survey, 182 transitions of 2-aminopropionitrile are predicted to be stronger than 20 mK on the main-beam temperature scale, which is a conservative limit given the typical noise level of this survey (see Sect.…”

The snow border

“…Several observations of these hot parts have shown that water plays a major role in the formation of complex molecules (e.g. Belloche et al 2009). This is due to the high dipole moment of this molecule that makes it active in both gas phase and surface chemistry.…”