Streamers feeding the SVS13-A protobinary system: astrochemistry reveals accretion shocks?

The origins of our Solar System and extrasolar planetary systems is one of the most important open question in modern astrophysics. Our understanding of how a star and a planetary system form has evolved substantially in the latest years. Recent observations suggest that the first planetesimals start to form very early in a disk of gas and dust around the newborn star.

Since young disks are typically deeply embedded in the parent envelope, the characterisation of their physical and chemical properties is not trivial from an observational point of view. If simple parameters, such as the disk mass, are already difficult to measure, their chemical composition is even less explored. Nevertheless, the molecular complexity present in the disk will be inherited from the forming planets, at least in the outer regions. Observing the chemical composition of young disks allows us to investigate the initial chemical budget available for planets when they start to form.


In my recent work, published in the Faraday Discussions by the Royal Society of Chemistry [1], I report observations of the binary system SVS13-A performed with the Atacama Large Millimeter/submillimeter Array (ALMA) telescope. More specifically, I analyse deuterated water (HDO) and sulfur dioxide (SO$_2$) emission.

SVS13-A is a young binary system located in the Perseus star forming region, where two protostars (VLA4A and VLA4B) are forming inside a dense envelope. This system shows an extreme chemical complexity [2]. High-angular resolution observations have revealed streamers in the dust which suggest accretion from the envelope into the two protostars (see Fig. 1, [3]). In this new study, deuterated water reveals an additional emitting component spatially coincident with the dust accretion streamer, at a distance $\geq$ 120 au from the protostars, and at blue-shifted velocities ($>$ 3 km s$^{-1}$ from the systemic velocities).

The kinematic analysis and the distance from the protostar at which HDO, NH$_2$CHO and SO$_2$ are observed suggest that molecules cannot be produced simply by thermal sublimation, i.g. the increase of gas temperature due to the accretion.
In order to determine the origin of the emission, I used thermal sublimation temperatures calculated using updated binding energies distributions, calculated by experts in quantum chemistry computations.

The main conclusion is that the observed molecular emission is very likely produced by an accretion shock at the interface between the accretion streamer and the disk of VLA4A. This work suggest that accretion processes at the early stage of protostellar disks may alter their chemical composition and affect the planet formation process.


Thanks to my collaborators for this work: Ana López-Sepulcre ( Univ. Grenoble Alpes, IRAM), Cecilia Ceccarelli (Univ. Grenoble Alpes), Claudio Codella (INAF-OA, Univ. Grenoble Alpes), Linda Podio (INAF-OA), Mathilde Bouvier (Leiden Obs.), Joan Enrique-Romero (Leiden Inst. of Chemistry), Rafael Bachiller (OAN-IGN), Bertrand Lefloch (Univ. Grenoble Alpes, Laboratoire d'astrophysique de Bordeaux)


[1] Bianchi, E., López-Sepulcre, A., Ceccarelli, C., et al. 2023, Faraday Discussions (arXiv:2306.08539) DOI: 10.1039/D3FD00018D
[2] Bianchi, E., Codella, C., Ceccarelli, C., et al. 2019, MNRAS, 483, 1850 (arXiv:1810.11411)
[3] Bianchi, E., López-Sepulcre, A., Ceccarelli, C., et al. 2022, ApJL, 928, L3 (arXiv:2203.03412)