Singly and doubly deuterated formaldehyde in massive star-forming regions

¹ Department of Physical Science, Graduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
e-mail: s.zahorecz@p.s.osakafu-u.ac.jp
² Chile Observatory, National Astronomical Observatory of Japan, National Institutes of Natural Science, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
³ Centro de Astrobiología (CSIC/INTA), Ctra de Torrejón a Ajalvir, km 4, E-28850 Torrejón de Ardoz, Spain
⁴ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748, Garching bei München, Germany
⁵ Joint Institute for VLBI ERIC (JIVE), Postbus 2, 7990 AA Dwingeloo, The Netherlands
⁶ INAF-Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, 50125 Firenze, Italy
⁷ Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching bei München, Germany
⁸ Kavli Institute for Astronomy and Astrophysics, Peking University, 5 Yiheyuan Road, Haidian District, Beijing 100871, PR China

Received: 29 October 2019
Accepted: 8 June 2021

Abstract

Context. Deuterated molecules are good tracers of the evolutionary stage of star-forming cores. During the star formation process, deuterated molecules are expected to be enhanced in cold, dense pre-stellar cores and to deplete after protostellar birth.

Aims. In this paper, we study the deuteration fraction of formaldehyde in high-mass star-forming cores at different evolutionary stages to investigate whether the deuteration fraction of formaldehyde can be used as an evolutionary tracer.

Methods. Using the APEX SEPIA Band 5 receiver, we extended our pilot study of the J = 3 →2 rotational lines of HDCO and D₂CO to eleven high-mass star-forming regions that host objects at different evolutionary stages. High-resolution follow-up observations of eight objects in ALMA Band 6 were performed to reveal the size of the H₂CO emission and to give an estimate of the deuteration fractions HDCO/H₂CO and D₂CO/HDCO at scales of ~6″ (0.04–0.15 pc at the distance of our targets).

Results. Our observations show that singly and doubly deuterated H₂CO are detected towards high-mass protostellar objects (HMPOs) and ultracompact H II regions (UC H II regions), and the deuteration fraction of H₂CO is also found to decrease by an order of magnitude from the earlier HMPO phases to the latest evolutionary stage (UC H II), from ~0.13 to ~0.01. We have not detected HDCO and D₂CO emission from the youngest sources (i.e. high-mass starless cores or HMSCs).

Conclusions. Our extended study supports the results of the previous pilot study: the deuteration fraction of formaldehyde decreases with the evolutionary stage, but higher sensitivity observations are needed to provide more stringent constraints on the D/H ratio during the HMSC phase. The calculated upper limits for the HMSC sources are high, so the trend between HMSC and HMPO phases cannot be constrained.