Gas versus solid-phase deuterated chemistry: HDCO and D2CO in massive star-forming regions
1 Department of Physical ScienceGraduate School of Science, Osaka Prefecture University, 1-1 Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan
2 National Astronomical Observatory of Japan, National Institutes of Natural Science, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
3 Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, 1121 Budapest, Konkoly Thege Miklós út 15−17, Hungary
4 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
5 Eötvös Loránd University, Department of Astronomy, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
6 School of Physics and Astronomy, Queen Mary University of London, Mile End Road, E1 4 NS London, UK
7 INAF-Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, 50125 Firenze, Italy
8 Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching bei München, Germany
9 Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching bei München, Germany
Received: 27 September 2016
Accepted: 8 May 2017
Context. The formation of deuterated molecules is favoured at low temperatures and high densities. Therefore, the deuteration fraction (Dfrac) is expected to be enhanced in cold, dense prestellar cores and to decrease after protostellar birth. Previous studies have shown that the deuterated forms of species such as N2H+ (formed in the gas phase) and CH3OH (formed on grain surfaces) can be used as evolutionary indicators and to constrain their dominant formation processes and timescales.
Aims. Formaldehyde (H2CO) and its deuterated forms can be produced both in the gas phase and on grain surfaces. However, the relative importance of these two chemical pathways is unclear. Comparison of the deuteration fraction of H2CO with respect to that of N2H+, NH3, and CH3OH can help us to understand its formation processes and timescales.
Methods. With the new SEPIA Band 5 receiver on APEX, we have observed the J = 3 → 2 rotational lines of HDCO and D2CO at 193 GHz and 175 GHz toward three massive star-forming regions hosting objects at different evolutionary stages: two high-mass starless cores (HMSC), two high-mass protostellar objects (HMPOs), and one ultracompact HII region (UC HII). By using previously obtained H2CO J = 3 → 2 data, the deuteration fractions HDCO/H2CO and D2CO/HDCO are estimated.
Results. Our observations show that singly deuterated H2CO is detected toward all sources and that the deuteration fraction of H2CO increases from the HMSC to the HMPO phase and then sharply decreases in the latest evolutionary stage (UCHII). The doubly deuterated form of H2CO is detected only in the earlier evolutionary stages, with D2CO/H2CO showing a pattern that is qualitatively consistent with the pattern of HDCO/H2CO, within current uncertainties.
Conclusions. Our initial results show that H2CO may display a similar Dfrac pattern as that of CH3OH in massive young stellar objects. This finding suggests that solid-state reactions dominate its formation.
Key words: astrochemistry / ISM: molecules / stars: formation / radio lines: ISM
© ESO, 2017