Resolved gas temperatures and ¹²C/¹³C ratios in SVS13A from ALMA Observations of CH₃CN and CH₃¹³CN

¹ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
² Taiwan Astronomical Research Alliance (TARA), Taiwan
³ Institute of Astronomy and Astrophysics, Academia Sinica, PO Box 23-141, Taipei 106, Taiwan
⁴ Department of Physics and Astronomy, University of Rochester, Rochester, NY 14627, USA
⁵ European Southern Observatory, Karl-Schwarzschild-Strasse 2 85748 Garching bei Munchen, Munchen, Germany
⁶ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁷ Institut de Radioastronomie Millimétrique, 300 rue de la Piscine, Domaine Universitaire, 38406 Saint-Martin d’Hères, France

^★ Corresponding author; thhsieh@asiaa.sinica.edu.tw

Received: 3 December 2024
Accepted: 4 February 2025

Abstract

Context. Multiple systems are common in field stars and the frequency is found to be higher in early evolutionary stages. Thus, the study of young multiple systems during the embedded stages is key to a comprehensive understanding of star formation. In particular, the way material accretes from the large-scale envelope into the inner region and how this flow interacts with the system physically and chemically has not been well characterized observationally to date.

Aims. We aim to provide a snapshot of the forming protobinary system SVS13A, consisting of VLA4A and VLA4B. This includes a clear picture of its kinematic structures, physical conditions, and chemical properties.

Methods. We conducted ALMA observations toward SVS13A targeting CH₃CN and CH₃¹³CN J=12-11 K-ladder line emission with a high spatial resolution of ∼30 astronomical units (au) at a spectral resolution of ∼0.08 km s⁻¹ .

Results. We used local thermal equilibrium (LTE) radiative transfer models to fit the spectral features of the line emission. We found the two-layer LTE radiative model that includes dust absorption is essential to interpreting the CH₃CN and CH₃¹³CN line emission. We identified two major and four small kinematic components and derived their physical and chemical properties.

Conclusions. We identified a possible infalling signature toward the bursting secondary source VLA4A, which may be fed by an infalling streamer from the large-scale envelope. The mechanical heating in the binary system, as well as the infalling shocked gas, are likely to play a role in the thermal structure of the protobinary system. By accumulating mass from the streamer, it is plausible that the system experienced a gravitationally unstable phase before the accretion outburst. Finally, the derived CH₃CN/CH₃¹³CN ratio is lower than the canonical ratio in the ISM and varies between VLA4A and VLA4B.