Increased complexity in interstellar chemistry: Detection and chemical modeling of ethyl formate and n-propyl cyanide in Sgr B2(N)

Abstract: In recent years, organic molecules of increasing complexity have been found toward the prolific Galactic center source Sagittarius B2. We wish to explore the degree of complexity that the interstellar chemistry can reach in star-forming regions. 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. We analyzed this spectral survey in the LTE 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. We report the first detection in space of ethyl formate (C2H5OCHO) and n-propyl cyanide (C3H7CN) toward Sgr B2(N). The abundances of ethyl formate and n-propyl cyanide relative to H2 are estimated to be 3.6e-9 and 1.0e-9, respectively. Our chemical modeling 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. 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. {Abridged}

• The paper presents the first detection of ethyl formate in the interstellar medium and models its formation in Sgr B2(N).
• It employs LTE analysis of IRAM 30 m spectral data to determine column densities and temperatures of complex organic molecules.
• The findings support grain-surface chemistry pathways as efficient routes to forming complex organics in star-forming regions.

Overview of Detection and Chemical Modeling of Complex Molecules in Sgr B2(N)

The study addresses the complexity of interstellar chemistry by reporting the detection and subsequent chemical modeling of two complex organic molecules, ethyl formate (C$_2$H$_5$OCHO) and n-propyl cyanide (C$_3$H$_7$CN), in the Sagittarius B2(N) region, a prolific source for molecular discoveries in the interstellar medium (ISM). This investigation was conducted using spectral data obtained from the IRAM 30 m telescope.

Methodological Approach

The researchers executed a comprehensive spectral line survey of the Sgr B2(N) and Sgr B2(M) star-forming regions, examining the 3 mm wavelength range. They utilized the local thermodynamical equilibrium (LTE) approximation to analyze the spectral data, which included simultaneous modeling of emissions from known molecules. This strategic approach was essential in identifying the signatures of less abundant and complex molecules such as ethyl formate and n-propyl cyanide.

Key Findings

• Ethyl Formate (C$_2$H$_5$OCHO): The investigation led to the first detection of ethyl formate in the ISM. For an assumed source size of 3", the ethyl formate was found with a column density of $5.4 \times 10^{16}$ cm$^{-2}$, at a temperature of 100 K.
• n-Propyl Cyanide (C$_3$H$_7$CN): Similarly detected in the ISM, n-propyl cyanide was observed in two velocity components with column densities of $1.5 \times 10^{16}$ cm$^{-2}$ and $6.6 \times 10^{15}$ cm$^{-2}$ for a source size of 3", at a temperature of 150 K.

Implications for Interstellar Chemistry

The research supports the concept that complex molecules can form in interstellar environments through grain-surface chemistry. The observed formation route includes a piecewise assembly of functional molecular groups on dust grains, facilitated by thermal diffusions at adequate dust temperatures. For instance, ethyl formate might form from methyl formate-derived products or involve ethanol as a precursor.

The observed column densities suggest complex organic species can arise efficiently under interstellar conditions, implying a robust chemical network.

Significance and Future Directions

The detection of ethyl formate and n-propyl cyanide not only extends the chemical complexity observed in Sgr B2(N), but it also hints at the possible presence of even more complex organic compounds in ISM. The results align well with chemical models incorporating both gas-phase and grain-surface processes, providing a pivotal test for theoretical predictions of interstellar chemistry.

Future investigations may leverage high-resolution observations to further unravel the spatial distribution of such complex organics, enhancing our understanding of molecular synthesis pathways in star-forming regions. Additionally, extending this work to other potential complex molecules in different astronomical environments could yield more insights into the universal nature of chemical processes in the cosmos.