EXOA3
Either the hypothesis that the supply of organic molecules necessary for the emergence of life is of extraterrestrial origin or the hypothesis that such organic molecules were synthesized locally on Earth as it is done in other planets or moons of our Solar System (or beyond) require convincing evidence that only space exploration in search of molecular complexity can provide. In this symposium, we will analyze the state of the art of our knowledge on the degree of complexity that organic molecules can reach in the interstellar medium (with particular attention to star-forming regions and protoplanetary disks), in comets, asteroids, meteorites, and interplanetary dust particles as well as in the planets and moons of the Solar System. Contributions on the detection of organic molecules in space, on laboratory experiments to determine their formation pathways, on astrochemical or photochemical models as well as on implications in astrobiology are welcome.
Session assets
Polycyclic aromatic hydrocarbons (PAHs) are widely observed in space both in the gas phase and as components of interstellar dust. PAHs account for ~20% of the carbon (C) in space. Understanding the mechanism to unlock C from PAHs is important to shed light on the formation of complex organic molecules (COMs), which play a crucial role in regulating the physics and chemistry of our universe and aiding in the origin of life [1]. While COM formation often begins with CO activation via hydrogenation on the surface of interstellar grains, the breakdown of gas-phase PAHs could provide an alternative pathway. PAHs often host radicals such as oxygen (O) and hydrogen (H), which weaken their aromatic bonds. While superhydrogenation of PAHs is well-studied [2], their interaction with atomic O as a possible route for their fragmentation remains underexplored [3].
Using density functional theory (DFT) and instanton theory [4], we predicted intermediate formations resulting from the reaction of O with naphthalene in the gas phase (Figure 1) and computed accurate tunneling rates. Similar to PAH hydrogenation, DFT suggests a sequence of O attachment. Once atomic oxygen reacts with the first C atom of naphthalene, intersystem crossing occurs from the triplet to the singlet spin state. O initially bridges Cs in naphthalene, catalyzing the barrier-less breakdown of a C-C bond, thus forming a heterocyclic ring. Subsequent oxygenation breaks down the PAH structure. Computed instanton rates reveal that tunneling is dominant at temperatures below 50K, with both O and C atoms tunneling through the potential barrier, aiding in the opening of the aromatic ring. Once one of the PAH rings is opened, the attack of two H leads to the fragmentation of the PAH, forming formaldehyde. These findings may clarify the role of PAHs in the formation of COMs, potentially linking them to the organic inventory formation in star-forming regions [1].
Figure 1: Oxygenation (in red) and hydrogenation (in black) tunneling of naphthalene aiding in fragmentation in the gas phase.
References
[1] Tielens, A. G. G. M. Rev. Mod. Phys. 2013, 85, 1021.
[2] Campisi, D.; et al. Phys. Chem. Chem. Phys. 2020, 22, 1557-1565.
[3] Cavallotti, C.; et. al. J. Phys. Chem. Lett. 2020, 11, 22, 9621–9628.
[4] Rommel, J. B.; et. al. J. Chem. Theory Comput. 2011, 7, 3, 690–698.
How to cite: Campisi, D.: Tunneling Toward Interstellar PAHs: O and H’s Quantum Leap , Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-887, https://doi.org/10.5194/epsc2024-887, 2024.
The process that led to the formation of our Solar System and the origin of the observed prebiotic compounds are among the most exciting open questions in modern astrochemistry. The first results of the Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like protostars (FAUST; PI: S. Yamamoto; Codella et al. 2021) ALMA Large Program suggest that the chemical composition and fate of future planetary systems strongly depend on the history of the parental protostellar envelope.
I will present new FAUST high-angular resolution (50 au) observations of interstellar Complex Organic Molecules (iCOMs; i.e. organic molecules with at least 6 atoms; Ceccarelli et al. 2017) and dust continuum emission towards the Corona Australis (CrA) star cluster (see Figure 1). The CH₃OH emission reveals an arc-structure at ~1800 au from the protostellar system IRS7B along the direction perpendicular to the disk major axis (see Figure 2a). The arc is located at the edge of two elongated continuum structures that define a cone emerging from IRS7B (see Figure 1). The region inside the cone is probed by H ₂ CO (see Figure 2d), while the eastern wall of the arc shows bright emission in SiO, a typical shock tracer (see Figure 2e). Taking into account the association with a previously discovered radio jet imaged with JVLA at 6 cm, the molecular arc reveals for the first time a bow shock driven by IRS7B and a two-sided dust cavity opened by the mass-loss process.
We derive for each cavity wall an average H2 column density of ∼ 7×1021 cm-2 , a mass of ∼ 9×10-3 M⊙ , and a lower limit on the dust spectral index of 1.4.
These observations provide the first evidence of the shock and the conical dust cavity opened by the jet driven by IRS7B, with important implications for the chemical enrichment and grain growth in the envelope of Solar-System analogues.
Figure 1: Left: SCUBA map at 450µm; Central: ALMA map of 1.3mm continuum emission from Sabatini et al (2024). Right: Zoom around IRS7B. Our ALMA continuum map reveals for the first time the dust grains in the walls of the cavity (cyan dashed lines) opened by the jet driven by IRS7B (cyan solid line).
Figure 2: Moment 0 of (a) CH3OH-E (42,3 -31,2), (b) CH3OH-A (51,4-41,3), (d) p-H2CO (30,3-20,2), (e) SiO (5-4) lines, around the molecular arc (integrated from 0 to +12 km s−1). Cyan lines and arrows follow Figure 1. The white contours mark the 5σ emission. Small circles indicate the positions of the brightest spots in CH3OH (42,3-31,2), SiO (5-4), and p-H2CO (30,3-20,2), i.e. labelled “A”, “B” and “C”, respectively. Red cross indicates the position of SMM 1A (Figure 1). The green semicircle shows the ALMA Band 6 FoV, while the grey background delimits the region inside each ALMA pointing.
How to cite: Sabatini, G., Podio, L., Codella, C., Ceccarelli, C., Chandler, C. J., Sakai, N., and Yamamoto, S.: Dusty cavity and molecular shock driven by IRS7B in the CoronaAustralis Cluster, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-1108, https://doi.org/10.5194/epsc2024-1108, 2024.
Over the last decade, there has been an important advance in the comprehension of the formation and destruction routes of the most common interstellar chemical species. However, a significant lack of knowledge concerning the chemistry of other minor species is still present, often due to the limited amount of computational and experimental data. In this contribution, the possible role of the electronically-excited sulphur S(1D) and oxygen O(1D) in the chemistry of interstellar or cometary ice has been explored through a theoretical characterization of the S(1D) reaction with H2O and CH3OH, and the O(1D) reaction with CH3SH, in the gas-phase and in the presence of a cluster of four water molecules. All the stationary points of the investigated potential energy surface (PES) were optimized at the density functional (DFT) level of theory, using the B3LYP functional and the correlation-consistent valence-polarized basis set aug-cc-pVTZ basis set, augmented with a tight d function on sulphur atoms. The presence of the 4-water-molecules cluster drastically changes the reaction mechanisms since the SO + H2 channel, which is the only open channel in the gas-phase S(1D) + H2O reaction, cannot occur due to the hindrance caused by the H-bonds between the involved reaction intermediates and the water molecules of the cluster. As regards the S(1D) + CH3OH and O(1D) + CH3SH, the only H-displacement channels together with SH- and OH-group ejection were found to be occurring, while all the other dissociation routes, which are thermally and kinetically active in the gas phase, were found to be hindered due to the presence of the water cluster. In addition, a global reduction of the energy content with respect to the reactants makes most of the H-displacement channels allowed from a thermodynamic standpoint, whereas in some cases they are endothermic, thus unfavored, for the isolated system. Overall, the ice matrix has been predicted to stabilize the reactive intermediates HSOH, H2SO, and CH2OHSH. Therefore, we started to investigate the effect of adding more water molecules by including an 18-water-molecule cluster in the case of the S(1D) + H2O system finding an enzyme-like effect of the cluster. In fact, the reaction pathways are found to be consistently favored because of the substantial reduction of the energy barriers connecting the minima of the potential energy surface. Additional work is necessary to simulate the ice environment better and confirm these preliminary results.
Figure 1 illustrates PES arising from the S(1D) + H2O reaction in the gas phase. The bold lines of Figure 1 highlight the exothermic pathway leading to the single set of reaction products SO + H2.
Figure 2 illustrates the PES calculated at the same level of theory arising from the S(1D) + H2O reaction considering a 4-membered cluster of H2O interacting
with the reaction intermediates, reactants, and products.
Figure 2 illustrates the PES calculated at the wB97XD functional level arising from the S(1D) + H2O reaction considering a 18-membered cluster of H2O interacting with the reaction intermediates, evidencing the decrease of the minimum energy path between each intermediate of the reaction.
How to cite: Giustini, A., Balucani, N., Rosi, M., and Di Genova, G.: Theoretical investigation on reactions involving electronically-excited atoms occurring in the gas phase of extraterrestrial environments and at the surface of interstellar icy grains, Europlanet Science Congress 2024, Berlin, Germany, 8–13 Sep 2024, EPSC2024-765, https://doi.org/10.5194/epsc2024-765, 2024.