On the chemical ladder of esters

Abstract

Context. In recent years, the detection of organic molecules with increasing complexity and potential biological relevance is opening the possibility to understand the formation of the building blocks of life in the interstellar medium. One of the families of molecules of substantial astrobiological interest are the esters. The simplest ester, methyl formate (CH 3 OCHO), is rather abundant in star-forming regions. The next step in the chemical complexity of esters is ethyl formate, C 2 H 5 OCHO. Despite the increase in sensitivity of current telescopes, the detection of complex molecules with more than ten atoms such as C 2 H 5 OCHO is still a challenge. Only two detections of this species have been reported so far, which strongly limits our understanding of how complex molecules are formed in the interstellar medium. New detections towards additional sources with a wide range of physical conditions are crucial to differentiate between competing chemical models based on dust grain surface and gas-phase chemistry. Aims. We have searched for ethyl formate towards the W51 e2 hot molecular core, one of the most chemically rich sources in the Galaxy and one of the most promising regions to study prebiotic chemistry, especially after the recent discovery of the P−O bond, key in the formation of DNA. Methods. We have analyzed a spectral line survey towards the W51 e2 hot molecular core, which covers 44 GHz in the 1, 2 and 3 mm bands, carried out with the IRAM 30 m telescope. Results. We report the detection of the trans and gauche conformers of ethyl formate. A local thermodynamic equilibrium analysis indicates that the excitation temperature is 78 ± 10 K and that the two conformers have similar source-averaged column densities of (2.0 ± 0.3) × 10 −16 cm −2 and an abundance of ∼10 −8. We compare for the first time the observed molecular abundances of ethyl formate with different competing chemical models based on grain surface and gas-phase chemistry. Conclusions. We propose that grain-surface chemistry may have a dominant role in the formation of ethyl formate (and other complex organic molecules) in hot molecular cores, rather than reactions in the gas phase.

The feedback of an HC HII region on its parental molecular core

“…From this comparison we deduce that the mass of molecular gas seen in projection over the HC HII region should be ≈2 M , and the total molecular mass inside core A1 ≈7 M . The estimated abundance of CH 3 OCHO is ≈2 × 10 −7 , in good agreement with the value recently derived by Rivilla et al (2017b) towards the massive star-forming region W51. The good correspondence between the CH 3 OCHO N tot map and the dust continuum map allows us to use the former to estimate the molecular mass of specific regions of core A1, taking advantage of the higher S/N of the former compared with the latter.…”

Abundant Z-cyanomethanimine in the interstellar medium: paving the way to the synthesis of adenine

“…We note that also the populations of other isomers in the ISM seem to be in thermodynamic equilibrium at T k , as e.g. the conformers of ethyl formate (C 2 H 5 OCHO) in the hot molecular cores located in the W51 and Orion KL regions (Rivilla et al 2017b;Tercero et al 2013). Since the isomerization barrier between the E− and Z−isomers of HNCHCN is very high (15.95 kK; Zaleski et al 2013) this process cannot occur in the ISM.…”

The First Detections of the Key Prebiotic Molecule Po in Star-Forming Regions

“…To firmly confirm the detection of PO, we have evaluated the based on the detection of multiple transitions, which has allowed us to derive its rotational temperature and column density. Using the physical parameters derived in Rivilla et al (2016) (N = 10 16.3 , T = 100 K), we have predicted the LTE line profile of the ethyl formate transition close to the 109.20620 GHz transition of PO. The result is shown in the left panel of Figure 3.…”