摘要：Context. Barnard B1b has been revealed as one of the most interesting globules from the chemical and dynamical point of view. It presents a rich molecular chemistry characterized by large abundances of deuterated and complex molecules. Furthermore, this globule hosts an extremely young Class 0 object and one candidate for the first hydrostatic core (FHSC) proving the youth of this star-forming region.
Aims. Our aim is to determine the cosmic ray ionization rate, ζH2, and the depletion factors in this extremely young star-forming region. These parameters determine the dynamical evolution of the core.
Methods. We carried out a spectral survey toward Barnard 1b as part of the IRAM large program “IRAM Chemical survey of sun-like star-forming regions” (ASAI) using the IRAM 30-m telescope at Pico Veleta (Spain). This provided a very complete inventory of neutral and ionic C-, N-, and S- bearing species with, from our knowledge, the first secure detections of the deuterated ions DCS+ and DOCO+. We use a state-of-the-art pseudo-time-dependent gas-phase chemical model that includes the ortho and para forms of H2, H, D, H, H2D+, D2H+, D2, and D to determine the local value of the cosmic ray ionization rate and the depletion factors.
Results. Our model assumes n(H2) = 105 cm-3 and Tk = 12 K, as derived from our previous works. The observational data are well fitted with ζH2 between 3 × 10-17 s-1 and 10-16 s-1 and the elemental abundances O/H = 3 × 10-5, N/H = 6.4−8 × 10-5, C/H = 1.7 × 10-5, and S/H between 6.0 × 10-7 and 1.0 × 10-6. The large number of neutral/protonated species detected allows us to derive the elemental abundances and cosmic ray ionization rate simultaneously. Elemental depletions are estimated to be ~10 for C and O, ~1 for N, and ~25 for S.
Conclusions. Barnard B1b presents similar depletions of C and O as those measured in prestellar cores. The depletion of sulfur is higher than that of C and O, but not as extreme as in cold cores. In fact, it is similar to the values found in some bipolar outflows, hot cores, and photon-dominated regions. Several scenarios are discussed to account for these peculiar abundances. We propose that it is the consequence of the initial conditions (important outflows and enhanced UV fields in the surroundings) and a rapid collapse (~0.1 Myr) that allows most S- and N-bearing species to remain in the gas phase to great optical depths. The interaction of the compact outflow associated with B1b-S with the surrounding material could enhance the abundances of S-bearing molecules, as well.
