FAUST

X. Formaldehyde in the protobinary system [BHB2007] 11: Small-scale deuteration

¹ IRAP, Université de Toulouse, CNRS, CNES, UPS, 9 av. du Colonel Roche, BP 44346, 31028 Toulouse, France
² INAF-Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Florence, Italy
³ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
e-mail: l.e.evans@leeds.ac.uk
⁴ Max-Planck-Institut für extraterrestrische Physik (MPE), Gießenbachstr. 1, 85741 Garching, Germany
⁵ Centro de Astrobiologia (CSIC-INTA), Ctra. de Torrejon a Ajalvir, km 4, 28850 Torrejon de Ardoz, Spain
⁶ Graduate School of Informatics and Engineering, The University of Electro-Communications, Chofu, Tokyo 182-8585, Japan
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ Université Grenoble Alpes, CNRS, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG), 38058 Grenoble Cedex 9, France
⁹ National Radio Astronomy Observatory, PO Box O, Socorro, NM 87801, USA
¹⁰ European Southern Observatory, Karl-Schwarzschild Str. 2, 85748 Garching bei München, Germany
¹¹ RIKEN Cluster for Pioneering Research, 2-1, Hirosawa, Wako-shi, Saitama 351-0198, Japan
¹² Institut de Radioastronomie Millimétrique (IRAM), 300 rue de la Piscine, 38406 Saint-Martin-d’Hères, France
¹³ Department of Physics, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
¹⁴ Research Center for the Early Universe, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan

Received: 15 March 2023
Accepted: 1 August 2023

Abstract

Context. Deuterium in H-bearing species is enhanced during the early stages of star formation. However, only a small number of high-spatial-resolution deuteration studies exist towards protostellar objects, leaving the small-scale structures of these objects unrevealed and understudied.

Aims. We aim to constrain the deuterium fractionation ratios in a Class 0/I protostellar object in formaldehyde (H₂CO), which has abundant deuterated isotopologues in this environment.

Methods. We used the Atacama Large Millimeter Array (ALMA) within the context of the Large Program Fifty AU STudy of the chemistry in the disk/envelope system of Solar-like protostars (FAUST) to observe the Class 0/I protobinary system [BHB2007] 11, whose emission components are embedded in circumstellar disks that have radii of 2 to 3 au. The system is surrounded by a complex filamentary structure (the so-called streamers) connected to the larger circumbinary disk. In this work, we present the first study of formaldehyde D-fractionation towards this source with detections of H₂CO 3(0,3)–2(0,2), combined with HDCO 4(2,2)–3(2,1), HDCO 4(1,4)–3(1,3) and D₂CO 4(0,4)–3(0,3). These observations probe the structures of the protobinary system, enabling us to resolve multiple velocity components associated with the methanol hot spots also uncovered by FAUST data, as well as the colder external envelope. In addition, based on the kinematics seen in our observations of the H₂CO emission, we propose the presence of a second large-scale outflow.

Results. The results derived from our ALMA observations agree with the current literature in that we only find the deuterated species HDCO and D₂CO in the central regions of the core, while undeuterated H₂CO is found more ubiquitously. From our radiative transfer modelling, we the column density of H₂CO to be in the range of (3-8) × 10¹⁴ cm⁻² and that of HDCO to be within (0.8−2.9) × 10¹³ cm⁻². The column density for the single detected velocity component of D₂CO is within (2.6–1.3) × 10¹² cm⁻². This yields an average D/H ratio for formaldehyde in [BHB2007] 11 of ${0.02}_{-0.01}^{+0.02}$ from HDCO. The results of our kinematic model suggest that the dynamic feature is inconsistent with a streamer-like nature given the flat and outflowing velocity relation; we therefore tentatively conclude that the feature is an asymmetric molecular outflow launched by a wide-angle disk wind.