N[TINF]2[/TINF]H[TSUP]+[/TSUP] and C[TSUP]18[/TSUP]O Depletion in a Cold Dark Cloud

We present sensitive, high angular resolution molecular-line observations ofC18O and N2H+ toward the dark globule B68. We directly compare these data withthe near-infrared extinction measurements of Alves, Lada, & Lada (2001) toderive the first evidence for the depletion of N2H+, and by inference N2, in apre-stellar dark cloud. We also find widespread C18O depletion throughout thecentrally condensed core of the B68 cloud. Specifically, we find the N2H+emission to peak in a shell partially surrounding the peak of dust extinction.Moreover, the N2H+ peaks inside the much larger C18O depletion hole and has asmaller depletion zone, confirming theoretical predictions. These data areanalyzed through a direct coupling of time dependent chemical models to aradiation transfer code. This analysis highlights the importance ofphotodissociation at cloud edges and suggests that the CO abundance declines bytwo orders of magnitude from edge to center. In contrast N2H+ declines inabundance, at minimum, by at least a factor of two. Indeed it is entirelypossible that both N2H+ and N2 are completely absent from the central regionsof the B68 core. The depletion of N2H+, and its parent molecule N2, opens thepossibility that the centers of dense cores, prior to the formation of a star,may evade detection by conventional methods of probing cores using molecularemission. Under these conditions H2D+ may be the sole viable molecular probe ofthe innermost regions of star forming cores.Comment: 11 pages, 3 figures (2 color). Astrophysical Journal Letters, accepte