The physical and chemical status of pre-protostellar core B68

We have investigated the physical and chemical status of the pre-protostellar core B68. A previous extinction study suggested that the density pro. le 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) 70 m 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 pre-protostellar cores (L1498 and 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 34 m telescope. We find that the kinetic temperature of B68 is only 11K, which is significantly lower than that previously assumed. We also derive the nonthermal line width in B68 and show that B68 is thermally dominated with little contribution from turbulence support ( less than 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 similar to95 pc with a corresponding mass of similar to1.0 M-.. If some magnetic field is present, it can be further away ( beyond similar to100 pc) and still satisfy the density structure of a Bonnor-Ebert 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).