Deuterated methanol in Orion BN/KL^⋆,⋆⋆

¹ Univ. Bordeaux, LAB, UMR 5804, 33270 Floirac, France
e-mail: Tzu-Cheng.Peng@obs.u-bordeaux1.fr
² CNRS, LAB, UMR 5804, 33270 Floirac, France
³ Max-Planck-Institut für Radioastronomie (MPIfR), Auf dem Hügel 69, 53121 Bonn, Germany

Received: 21 October 2011
Accepted: 30 May 2012

Abstract

Aims. Deuterated molecules have been detected and studied toward Orion BN/KL in the past decades, mostly with single-dish telescopes. However, high angular resolution data are critical not only for interpreting the spatial distribution of the deuteration ratio but also for understanding this complex region in terms of cloud evolution involving star-forming activities and stellar feedbacks. Therefore, it is important to investigate the deuterated ratio of methanol, one of the most abundant grain-surface species, on a scale of a few arcseconds to better understand the physical conditions related to deuteration in Orion BN/KL.

Methods. Orion BN/KL was extensively observed with the IRAM Plateau de Bure Interferometer from 1999 to 2007 in the 1 to 3 mm range. The angular resolution varies from 1.″8×0.″8 to 3.″6×2.″3 and the spectral resolution varies from 0.4 to 1.9 km s^-1. Deuterated methanol CH₂DOH and CH₃OD and CH₃OH lines were searched for within our 3 mm and 1.3 mm data sets.

Results. We present here the first high angular resolution (1.″8×0.″8) images of deuterated methanol CH₂DOH in Orion BN/KL. Six CH₂DOH lines were detected around 105.8, 223.5, and 225.9 GHz. In addition, three E-type methanol lines around 101–102 GHz were detected and were used to derive the corresponding CH₃OH rotational temperatures and column densities toward different regions across Orion BN/KL. The strongest CH₂DOH and CH₃OH emissions come from the Hot Core southwest region with a velocity that is typical of the Compact Ridge (V_LSR ≈ 8 km s^-1). We derive [CH₂DOH]/[CH₃OH] abundance ratios of 0.8–1.3 × 10^-3 toward three CH₂DOH emission peaks. A new transition of CH₃OD was detected at 226.2 GHz for the first time in the interstellar medium. Its distribution is similar to that of CH₂DOH. Besides, we find that the [CH₂DOH]/[CH₃OD] abundance ratios are lower than unity in the central part of BN/KL. Furthermore, the HDO 3_1,2−2_2,1 line at 225.9 GHz was detected and its emission distribution shows a shift of a few arcseconds with respect to the deuterated methanol emission that likely results from different excitation effects. The deuteration ratios derived along Orion BN/KL are not markedly different from one clump to another. However, various processes such as slow heating due to ongoing star formation, heating by luminous infrared sources, or heating by shocks could be competing to explain some local differences observed for these ratios.