Deuterated methanol in the pre-stellar core L1544^⋆

¹ Centro de Astronomia e Astrofísica, Observatório Astronómico de Lisboa, Tapada da Ajuda, 1349-018 Lisboa, Portugal
e-mail: bizzocchi@oal.ul.pt; elle@oal.ul.pt
² Centre for Astrochemical Studies, Max-Planck-Institut für Extraterrestrische Physik, Gießenbachstraße 1, 85748 Garching, Germany
e-mail: caselli@mpe.mpg.de; bizzocchi@mpe.mpg.de
³ I.Physikalisches Institut, Universität zu Köln Zülpicherstraße 77, 50937 Köln, Germany
e-mail: spezzano@ph1.uni-koeln.de

Received: 21 March 2014
Accepted: 21 July 2014

Abstract

Context. High methanol (CH₃OH) deuteration has been revealed in Class 0 protostars with the detection of singly, doubly, and even triply D-substituted forms. Methanol is believed to form during the pre-collapse phase via gas-grain chemistry and then eventually injected into the gas when the heating produced by the newly formed protostar sublimates the grain mantles. The molecular deuterium fraction of the warm gas is thus a relic of the cold pre-stellar era and provides hints of the past history of the protostars.

Aims. Pre-stellar cores represent the preceding stages in the process of star formation. We aim at measuring methanol deuteration in L1544, a prototypical dense and cold core on the verge of gravitational collapse. The aim is to probe the deuterium fractionation process while the “frozen” molecular reservoir is accumulated onto dust grains.

Methods. Using the IRAM 30 m telescope, we mapped the methanol emission in the pre-stellar core L1544 and observed singly deuterated methanol (CH₂DOH and CH₃OD) towards the dust peak of L1544. Non-LTE radiative transfer modelling was performed on three CH₃OH emissions lines at 96.7 GHz, using a Bonnor–Ebert sphere as a model for the source. We have also assumed a centrally decreasing abundance profile to take the molecule freeze-out in the inner core into account. The column density of CH₂DOH was derived assuming LTE excitation and optically thin emission.

Results. The CH₃OH emission has a highly asymmetric morphology, resembling a non-uniform ring surrounding the dust peak, where CO is mainly frozen onto dust grains. The observations provide an accurate measure of methanol deuteration in the cold pre-stellar gas. The derived abundance ratio is [CH₂DOH]/[CH₃OH] = 0.10 ± 0.03, which is significantly smaller than the ones found in low-mass Class 0 protostars and smaller than the deuterium fraction measured in other molecules towards L1544. The singly-deuterated form CH₃OD was not detected at 3σ sensitivity of 7 mK km s^-1, yielding a lower limit of [CH₂DOH]/[CH₃OD] ≥ 10, consistent with previous measurements towards Class 0 protostars.

Conclusions. The low deuterium fractionation observed in L1544 and the morphology of the CH₃OH emission suggest that we are mainly tracing the outer parts of the core, where CO just started to freeze-out onto dust grains.