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://ucrisportal.univie.ac.at/en/publications/be839990-e9e4-4361-8d5c-47537e042723