Astrochemistry of Solar System precursors: the missing evidence of large carbon chains

Chemical processes taking place during the early protostellar phases strongly affect the final chemical composition of a forming planetary system. Indeed, while close to the protostar a chemical reset is expected, in the outer protostellar disk regions a consistent part of the molecular complexity is expected to be inherited from the early stages. The complex molecules formed in the disk are thus incorporated in planetesimals and planets, possibly triggering a more complex chemistry of prebiotic compounds.

The vast majority of the observations that led the recognition the chemical variety of solar-type protostars has been obtained via millimeter wavelengths telescopes, where several relatively light molecules, like the iCOMs or the small carbon chains have their peak of emission. In contrast, the lines of heavy molecules (e.g. chains with more than seven carbon atoms) at mm wavelengths are substantially weaker. Their observation could add an important piece of the overall puzzle as they might have a crucial role in the heritage of organic material from the pre- and proto- stellar phase to the objects of the newly formed planetary system, like asteroids and comets.

In order to explore the complex carbon chains content of Solar System precursor, I performed new observations with the 100m Green Bank Telescope. More specifically, I observed several crucial carbon bearing chains in the 8.0 – 11.5 GHz and 14.0 — 15.4 GHz intervals, in L1544 and IRAS16293-2422, which are the two archetypes of prestellar cores and protostars, respectively. This pilot study revealed an impressive molecular richness of C-chains (e.g. C$_4$H, C$_6$H, HC$_7$N, HC$_9$N, C$_3$S) and a chemical differentiation between the two sources at large angular scales.

In my recent work, published in The Astrophysical Journal [1], I focus on the cyanopolyyne chemistry in the prestellar core L1544. I detected several emission lines from HC$_3$N, HC$_5$N, HC$_7$N, and HC$_9$N, detected for the first time towards the source. The resolved spectral profiles show a double-peak profile, suggesting that the bulk of the cyanopolyyne emission is associated to the southern region of the core, where other smaller carbon chains peak. This supports the idea that cyanopolyynes are mainly formed in the external part of the core, where the interstellar field radiation increases the free carbon atoms available to form long chains. Even if the measurements in other star forming regions are scarce, the results obtained in L1544 suggest that a complex C-chain chemistry is active in other sources and it is related to the presence of free gaseous carbon. The latter can be abundant either because the core is very young and the conversion to CO is not completed, or because the CO is destroyed by UV illumination or cosmic-ray irradiation. We suggest that the column density of heavy cyanopolyynes (larger that HC$_5$N) could be a proxy to discriminate between these two regimes.

These observations are even more important because they cover part of the frequency range of the future Square Kilometer Array (SKA), a new generation radio telescope currently in construction. Thanks to this new radio telescope, in the coming years we will image the spatial distribution of the observed C-chains and understand if large carbon chains and iCOMs coexist in the planets formation region. This work inspired a scientific use case for SKA, in the framework of the "Cradle of Life" working group, in which I am actively involved.

Thanks to my collaborators for this work: Anthony Remijan (NRAO), Claudio Codella (INAF-OA, Univ. Grenoble Alpes), Cecilia Ceccarelli (Univ. Grenoble Alpes), François Lique (Univ. Rennes), Silvia Spezzano (MPE), Nadia Balucani (Univ. Perugia, INAF-OA), Paola Caselli (MPE), Eric Herbst (Univ. of Virginia), Linda Podio (INAF-OA), Charlotte Vastel (IRAP), Brett McGuire (MIT, NRAO).



[1] Bianchi, E., Remijan, A., Codella, C., et al. 2023, ApJ, in press (arXiv:2301.10106)