Water and interstellar complex organics associated with the HH 212 protostellar disc

On disc atmospheres, disc winds, and accretion shocks

¹ INAF, Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: codella@arcetri.astro.it
³ Université Grenoble Alpes, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38401 Grenoble, France
² Università degli Studi di Firenze, Dipartimento di Fisica e Astronomia, Via G. Sansone 1, 50019 Sesto Fiorentino, Italy
⁴ LERMA, UMR 8112 du CNRS, Observatoire de Paris, École Normale Supérieure, 61 Av. de l’Observatoire, 75014 Paris, France
⁵ Academia Sinica Institute of Astronomy and Astrophysics, PO Box 23-141, Taipei 106, Taiwan
⁶ Graduate Institute of Astronomy and Astrophysics, National Taiwan University, No. 1, Sec. 4, Roosevelt Road, Taipei 10617, Taiwan
⁷ IGN, Observatorio Astronómico Nacional, Alfonso XII 3, 28014 Madrid, Spain
⁸ IRAM, 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
⁹ Laboratoire d’astrophysique de Bordeaux, Univ. de Bordeaux, CNRS, B18N, allée Geoffroy Saint–Hilaire, 33615 Pessac, France
¹⁰ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
¹¹ INAF – Istituto di Radioastronomia & Italian ALMA Regional Centre, Via P. Gobetti 101, 40129 Bologna, Italy

Received: 3 February 2018
Accepted: 12 June 2018

Abstract

Context. The unprecedented combination of high-sensitivity and high angular resolution provided by the ALMA interferometer allows us to shed light on the processes leading to the formation of the jet-disc system associated with a Sun-like mass protostar.

Aims. We investigate the physical and chemical properties of the gas associated with water and interstellar complex organic molecules around a protostar on solar system scales.

Methods. The HH 212 protostellar system, in Orion B, has been mapped, thanks to ALMA-Band 7 Cycle 1 and Cycle 4 observations of dueterated water (HDO) and acetaldehyde (CH₃CHO) emission with an angular resolution down to ~0.′′15 (60 au).

Results. Many emission lines due to 14 CH₃CHO and 1 HDO transitions at high excitation (E_u between 163 and 335 K) have been imaged in the inner ~70 au region. The local thermal equilibrium analysis of the CH₃CHO emission leads to a temperature of 78 ± 14 K and a column density of 7.6 ± 3.2 × 10¹⁵ cm⁻², which, when N_H2 of 10²⁴ cm⁻² is assumed, leads to an abundance of X_CH3CHO ≃ 8 × 10⁻⁹. The large velocity gradient analysis of the HDO emission also places severe constraints on the volume density, n_H2 ≥ 10⁸ cm⁻³. The line profiles are 5–7 km s⁻¹ wide, and CH₃CHO and HDO both show a ±2 km s⁻¹ velocity gradient over a size of ~70 au (blue-shifted emission towards the north-west and red-shifted emission towards the south-east) along the disc equatorial plane, in agreement with what was found so far using other molecular tracers.

Conclusions. The kinematics of CH₃CHO and HDO are consistent with the occurrence of a centrifugal barrier, that is, the infalling envelope-rotating disc ring, which is chemically enriched through low-velocity accretion shocks. The emission radius is ~60 au, in good agreement with what was found before for another interstellar complex organic molecule such as NH₂CHO. We support a vertical structure for the centrifugal barrier, suggesting the occurrence of two outflowing, expanding, and rotating rings above and below (of about 40–45 au) the optically thick equatorial disc plane. It is tempting to speculate that these rings could probe the basis of a wind launched from this region.