The importance of being stable: New results about the survival of PAHs in extreme astrophysical environments

Increasing observational evidence shows that a non-negligible fraction of the cosmic carbon is locked into macromolecules like Polycyclic Aromatic Hydrocarbons (PAHs). Interstellar PAHs live in extreme environments where there are processed by energetic photons (UV and X-rays) and by ions and electrons accelerated in hot shocked plasma and arising from cosmic rays. It is therefore important to quantify the capability of PAHs to survive under these extreme conditions and to determine the structural modifications induced by such energetic processing. I will present some novel results on this topic, focusing on the bombardment by ions and electrons in interstellar shocks. This work shows the importance of pairing an appropriate physical description of the interaction between target and projectiles with updated laboratory measurements of the relevant physical parameters. The results from physical modeling allowed to derive updated astronomical lifetimes for PAHs.

Models of irradiated molecular shocks

Context. The recent discovery of excited molecules in starburst galaxies observed with ALMA and the Herschel space telescope has highlighted the necessity to understand the relative contributions of radiative and mechanical energies in the formation of molecular lines and explore the conundrum of turbulent gas bred in the wake of galactic outflows. Aims. The goal of the paper is to present a detailed study of the propagation of low velocity (5–25 km s−1) stationary molecular shocks in environments illuminated by an external ultraviolet (UV) radiation field. In particular, we intend to show how the structure, dynamics, energetics, and chemical properties of shocks are modified by UV photons and to estimate how efficiently shocks can produce line emission. Methods. We implemented several key physico-chemical processes in the Paris-Durham shock code to improve the treatment of the radiative transfer and its impact on dust and gas particles. We propose a new integration algorithm to find the steady-state solutions of magnetohydrodynamics equations in a range of parameters in which the fluid evolves from a supersonic to a subsonic regime. We explored the resulting code over a wide range of physical conditions, which encompass diffuse interstellar clouds and hot and dense photon-dominated regions. Results. We find that C-type shock conditions cease to exist as soon as G0 > 0.2 (nH/cm−3)1/2. Such conditions trigger the emergence of another category of stationary solutions, called C*-type and CJ-type shocks, in which the shocked gas is momentarily subsonic along its trajectory. These solutions are shown to be unique for a given set of physical conditions and correspond to dissipative structures in which the gas is heated up to temperatures comprised between those found in C-type and adiabatic J-type shocks. High temperatures combined with the ambient UV field favour the production or excitation of a few molecular species to the detriment of others, hence leading to specific spectroscopic tracers such as rovibrational lines of H2 and rotational lines of CH+. Unexpectedly, the rotational lines of CH+ may carry as much as several percent of the shock kinetic energy. Conclusions. Ultraviolet photons are found to strongly modify the way the mechanical energy of interstellar shocks is processed and radiated away. In spite of what intuition dictates, a strong external UV radiation field boosts the efficiency of low velocity interstellar shocks in the production of several molecular lines which become evident tracers of turbulent dissipation.

Cloud-cloud collision in the Sgr B2 molecular cloud complex

We present results from a high-resolution wide-field imaging observation of the central molecular zone (CMZ) in H13CO+J = (1 − 0) and SiO v=0, J = (2−1) emission lines by using the Nobeyama 45-m telescope in order to depict the high-density molecular gas mass distribution and explore molecular gas affected by interstellar shocks. We found a candidate for ongoing cloud-cloud collision in the Sgr B2 complex. This is identified as a hollow paraboloid-like structure in the l−b−v data cube of both emission lines. The central part of the feature is denser and warmer than the outer envelope and contains a vast amount of shocked molecular gas. These properties are consistent with those expected from simulations of cloud-cloud collisions in the CMZ.

The Stability of Cosmic Fullerenes and Fullerenic Aggregates

Establishing the stability of cosmic fullerenes and fullerenic aggregates is extremely relevant for a variety of reasons. For instance, the emission features of C60 and C70 fall in the same spectral region as the Un-identified InfraRed (UIR) bands, which they could contribute to. To be able to contribute to the UIR emission, however, fullerenes must be able to survive long enough against the destruction mechanisms operating in the interstellar medium. In this study we focus on the effects of collisional processing, i.e., the bombardment by energetic ions and electrons. A recent experimental/theoretical study has shown that ion collisions with C60 clusters result in the dissociation of the cluster with the simultaneous formation of covalent fullerene dimers, which could play a role as DIBs carriers. We present here our first results about the collisional processing of C60 molecules and clusters by H, He and C ions in interstellar shocks. We have adapted the models that have previously been developed to successfully treat the collisional processing of PAHs in space. The nature of the interaction and the similarities between PAHs and fullerenes make this approach appropriate. In addition, our study shows that the formation of covalent dimers following ion collisions with C60 clusters is compatible with the astrophysical conditions under consideration.