The Physical and Chemical Status of Pre - protostellar Core B 68

We have investigated the physical and chemical status of the pre-protostellar core B68. A previous extinction study suggested that the density profile of B68 is remarkably consistent with a Bonnor-Ebert sphere with 2.1 M at 16 K. We mapped B68 in C3H2, CCS, and NH3 with the Deep Space Network (DSN) 70m telescope at Goldstone. Our results show that the NH3 peak coincides with the dust continuum peak, whereas CCS and C3H2 are offset from the NH3 and dust peaks. The B68 chemical structure is consistent with that seen in other such preprotostellar cores (L1498, L1544) and is explained by time dependent chemical models that include depletion. We measured the kinetic temperature of B68 with NH3 (1,1) and (2,2) spectra obtained with a DSN 34m telescope. We find that the kinetic temperature of B68 is only 11 K which is significantly lower than that previously assumed. We also derive the non-thermal linewidth in B68, and show that B68 is thermally dominated with little contribution from turbulence support (< 10%). We consider a modified Bonnor-Ebert sphere to include effects of turbulence and magnetic fields and use it to constrain the uncertainties in its distance determination. We conclude that the distance to B68 is ∼ 95pc with a corresponding mass of ∼ 1.0 M . If some magnetic field is present it can be further away (beyond ∼ 100pc) and still satisfy the density structure of a BonnorEbert sphere. The sulfur (CS and CCS) and carbon chain (C3H2) molecules are heavily depleted in B68 and do not trace the dense interior region. We see some evidence for depletion of NH3 at the core center roughly on a scale similar to that of N2H . Our observations do not preclude any instability such as the onset of collapse, or slow contraction, occurring in the center of the core, which cannot be resolved with our beam size (45′′). Subject headings: ISM: molecules – ISM: abundances – ISM: globules – ISM: individual (B68) – stars: formation