N2H+ and C18O Depletion in a Cold Dark Cloud

Author(s)
Edwin A. Bergin, João Alves, Tracy Huard, Charles J. Lada
Abstract

We present sensitive, high angular resolution molecular line

observations of C18O and N2H+ toward

the dark globule B68. We directly compare these data with the

near-infrared extinction measurements of Alves, Lada, & Lada to

derive the first evidence for the depletion of

N2H+, and by inference N2, in a

prestellar dark cloud. We also find widespread C18O depletion

throughout the centrally 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,

N2H+ peaks inside the much larger C18O

depletion hole and has a smaller depletion zone, confirming theoretical

predictions. These data are analyzed through a direct coupling of

time-dependent chemical models to a radiation transfer code. This

analysis highlights the importance of photodissociation at cloud edges

and suggests that the CO abundance declines by 2 orders of magnitude

from edge to center. In contrast, N2H+ declines in

abundance, at minimum, by at least a factor of 2. Indeed, it is entirely

possible that both N2H+ and N2 are

completely absent from the central regions of the B68 core. The

depletion of N2H+, and its parent molecule

N2, opens up the possibility that the centers of dense cores,

prior to the formation of a star, may evade detection by conventional

methods of probing cores using molecular emission. Under these

conditions, H2D+ may be the sole viable molecular

probe of the innermost regions of star-forming cores.

Organisation(s)
Department of Astrophysics
External organisation(s)
Harvard-Smithsonian Center for Astrophysics
Journal
The Astrophysical journal Letters
Volume
570
Pages
L101-L104
ISSN
2041-8205
DOI
https://doi.org/10.1086/340950
Publication date
05-2002
Peer reviewed
Yes
Austrian Fields of Science 2012
103004 Astrophysics
Keywords
Portal url
https://ucris.univie.ac.at/portal/en/publications/n2h-and-c18o-depletion-in-a-cold-dark-cloud(be839990-e9e4-4361-8d5c-47537e042723).html