Physical and chemical fingerprint of protostellar disc formation

¹ Niels Bohr Institute & Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5–7, 1350 Copenhagen K., Denmark
e-mail: elizabeth.artur@nbi.ku.dk
² Department of Astronomy, University of Michigan, 311 West Hall, 1085 S. University Avenue, Ann Arbor, MI 48109, USA
³ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁴ The Institute of Physical and Chemical Research (RIKEN), 2–1 Hirosawa, Wako-shi, Saitama 351-0198, Japan
⁵ Max-Planck-Institut für extraterrestrische Physik, Giessenbachstraße 1, 85748 Garching bei München, Germany
⁶ Department of Physics, The University of Tokyo, Bunkyo-ku, Tokyo 113-0033, Japan

Received: 14 December 2018
Accepted: 12 April 2019

Abstract

Context. The structure and composition of emerging planetary systems are likely strongly influenced by their natal environment within the protoplanetary disc at the time when the star is still gaining mass. It is therefore essential to identify and study the physical processes at play in the gas and dust close to young protostars and investigate the chemical composition of the material that is inherited from the parental cloud.

Aims. The purpose of this paper is to explore and compare the physical and chemical structure of Class I low-mass protostellar sources on protoplanetary disc scales.

Methods. We present a study of the dust and gas emission towards a representative sample of 12 Class I protostars from the Ophiuchus molecular cloud with the Atacama Large Millimeter/submillimeter Array (ALMA). The continuum at 0.87 mm and molecular transitions from C¹⁷O, C³⁴S, H¹³CO⁺, CH₃OH, SO₂, and C₂H were observed at high angular resolution (0.′′4, ~60 au diameter) towards each source. The spectrally and spatially resolved maps reveal the kinematics and the spatial distribution of each species. Moreover, disc and stellar masses are estimated from the continuum flux and position-velocity diagrams, respectively.

Results. Six of the sources show disc-like structures in C¹⁷O, C³⁴S, or H¹³CO⁺ emission. Towards the more luminous sources, compact emission and large line widths are seen for transitions of SO₂ that probe warm gas (E_u ~ 200 K). In contrast, C¹⁷O emission is detected towards the least evolved and less luminous systems. No emission of CH₃OH is detected towards any of the continuum peaks, indicating an absence of warm CH₃OH gas towards these sources.

Conclusions. A trend of increasing stellar mass is observed as the envelope mass decreases. In addition, a power-law relation is seen between the stellar mass and the bolometric luminosity, corresponding to a mass accretion rate of (2.4 ± 0.6) × 10⁻⁷ M_⊙ yr⁻¹ for the Class I sources, with a minimum and maximum value of 7.5 × 10⁻⁸ and 7.6 × 10⁻⁷ M_⊙ yr⁻¹, respectively. This mass accretion rate is lower than the expected value if the accretion is constant in time and rather points to a scenario of accretion occurring in bursts. The differentiation between C¹⁷O and SO₂ suggests that they trace different physical components: C¹⁷O traces the densest and colder regions of the disc-envelope system, while SO₂ may be associated with regions of higher temperature, such as accretion shocks. The lack of warm CH₃OH emission suggests that there is no hot-core-like region around any of the sources and that the CH₃OH column density averaged over the disc is low. Finally, the combination of bolometric temperature and luminosity may indicate an evolutionarytrend of chemical composition during these early stages.