Speaker
Description
The joint journey of rotational spectroscopy and radioastronomy is firmly pushing the frontiers of interstellar chemistry based on the detection of new molecules with ever-higher degrees of molecular complexity. However, for several families of compounds (e.g., carboxylic acids) the census of identified interstellar species has remained untouched for almost a quarter century. Herein, we present the discovery of cis-trans carbonic acid (HOCOOH) toward the molecular cloud G+0.693-0.027, which ranks as the first interstellar molecule containing three oxygen atoms and the third carboxylic acid detected in the interstellar medium to date. Although the available laboratory measurements ended at 65 GHz, we have used our spectral line survey as a “conventional” laboratory spectrum and managed to detect several clear and unblended spectroscopic features directly in the 3mm radio astronomical data. Hence, we were able to improve its set of rotational spectroscopic constants. We derive a column density for this conformer of N = (6.4 ± 0.4) x 1012 cm-2, which translates into an abundance with respect to H2 of 4.7 x 10-11. We also report the nondetection of the more stable cis-cis HOCOOH, showing an upper limit to the molecular abundance with respect to H2 of ≤ 1.2 x 10-9, because of the huge impact that its low dipole moment (about fifteen times lower than that of the cis-trans form) has on the overall detectability of the conformer. Nevertheless, we suggest that it may be efficiently generated under interstellar conditions, most likely via -OH radical addition of HOCO on the surface of dust grains, although it is nearly imperceptible to radio astronomical observations. We derive a cis-cis / cis-trans ratio of ≤ 25, consistent with the smaller energy difference between both conformers compared with the relative stability of trans- and cis-formic acid (HCOOH). Finally, we analyze the abundance of carboxylic acids in different astronomical environments, including star-forming regions, asteroids and comets, which enabled us to prove an overall good correlation between their relative molecular abundance; these acids seem to survive the star-formation process.
In summary, the interstellar detection of carbonic acid, HOCOOH, deepens our understanding of the levels of interstellar chemical complexity, and will be of great relevance to unveil the role of HOCOOH within both carbon and oxygen interstellar chemistry. This research work will also open a window to perform further coordinated observational, theoretical and laboratory efforts aiming to discover molecules with an extremely low dipole moment, based on the detection of other moderately higher-in-energy conformers with sizable dipole moments. This fact will be of great relevance for high-Tkin sources such as Galactic Center molecular clouds (i.e. G+0.693), hot cores and corinos, which are able to populate these high-energy species efficiently.
