A (sub)millimetre study of dense cores in Orion B9⋆

Context. Studies of dense molecular-cloud cores at (sub)millimetre wavelengths are needed to understand the early stages of star formation.
Aims: We aim to further constrain the properties and evolutionary stages of dense cores in Orion B9. The prime objective of this study is to examine the dust emission of the cores near the peak of their spectral energy distributions, and to determine the degrees of CO depletion, deuterium fractionation, and ionisation.
Methods: The central part of Orion B9 was mapped at 350 μm with APEX/SABOCA. A sample of nine cores in the region were observed in C¹⁷O(2-1), H¹³CO⁺(4-3) (towards 3 sources), DCO⁺(4-3), N₂H⁺(3-2), and N₂D⁺(3-2) with APEX/SHFI. These data are used in conjunction with our previous APEX/LABOCA 870-μm dust continuum data.
Results: All the LABOCA cores in the region covered by our SABOCA map were detected at 350 μm. The strongest 350 μm emission is seen towards the Class 0 candidate SMM 3. Many of the LABOCA cores show evidence of substructure in the higher-resolution SABOCA image. In particular, we report on the discovery of multiple very low-mass condensations in the prestellar core SMM 6. Based on the 350-to-870 μm flux density ratios, we determine dust temperatures of T_dust ≃ 7.9-10.8 K, and dust emissivity indices of β ~ 0.5-1.8. The CO depletion factors are in the range f_D ~ 1.6-10.8. The degree of deuteration in N₂H⁺ is ≃0.04-0.99, where the highest value (seen towards the prestellar core SMM 1) is, to our knowledge, the most extreme level of N₂H⁺ deuteration reported so far. The level of HCO⁺ deuteration is about 1-2%. The fractional ionisation and cosmic-ray ionisation rate of H₂ could be determined only towards two sources with the lower limits of ~2-6 × 10^-8 and ~2.6 × 10^-17-4.8 × 10^-16 s^-1, respectively. We also detected D₂CO towards two sources.
Conclusions: The detected protostellar cores are classified as Class 0 objects, in agreement with our previous SED results. The detection of subcondensations within SMM 6 shows that core fragmentation can already take place during the prestellar phase. The origin of this substructure is likely caused by thermal Jeans fragmentation of the elongated parent core. Varying levels of f_D and deuteration among the cores suggest that they are evolving chemically at different rates. A low f_D value and the presence of gas-phase D₂CO in SMM 1 suggest that the core chemistry is affected by the nearby outflow. The very high N₂H⁺ deuteration in SMM 1 is likely to be remnant of the earlier CO-depleted phase.

This publication is based on data acquired with the Atacama Pathfinder EXperiment (APEX) under programmes 079.F-9313A, 084.F-9304A, and 084.F-9312A. APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.Our SABOCA map shown in Fig. 2 is available electronically in FITS format at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/538/A137