Envelope Structure of Starless Core L694-2 Derived from a Near-Infrared Extinction Map

Daniel W. A. Harvey, David J. Wilner, Charles J. Lada, Philip C. Myers, João F. Alves

We present a near-infrared extinction study of the dark globule L694-2,

a starless core that shows strong evidence for inward motions in the

profiles of molecular spectral lines. The J-, H-, and K-band data were

taken using the European Southern Observatory New Technology Telescope.

The best-fit simple spherical power-law model has index p=2.6+/-0.2,

over the ~0.036-0.1 pc range in radius sampled in extinction. This

power-law slope is steeper than the value of p=2 for a singular

isothermal sphere, the initial condition of the inside-out model for

protostellar collapse. Including an additional component of extinction

along the line of sight further steepens the inferred profile. A fit for

a Bonnor-Ebert sphere model results in a supercritical value of the

dimensionless radius ξmax=25+/-3. This unstable

configuration of material in the L694-2 core may be related to the

observed inward motions. The Bonnor-Ebert model matches the shape of the

observed density profile but significantly underestimates the amount of

extinction observed in the L694-2 core (by a factor of ~4). This

discrepancy in normalization has also been found for the nearby

protostellar core B335 (Harvey and coworkers). A cylindrical model with

scale height H=0.0164+/-0.002 pc (13.5"+/-5'') viewed at a

small inclination to the axis of the cylinder provides an equally good

radial profile as a power-law model, and it also reproduces the

asymmetry of the L694-2 core remarkably well. In addition, this model

provides a possible basis for understanding the discrepancy in the

normalization of the Bonnor-Ebert model, namely, that L694-2 has prolate

structure, with the full extent (mass) of the core being missed by

analysis that assumes symmetry between the profiles of the core in the

plane of the sky and along the line of sight. If the core is

sufficiently magnetized, then fragmentation may be avoided, and later

evolution might produce a protostar similar to B335.

Department of Astrophysics
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Harvard-Smithsonian Center for Astrophysics
The Astrophysical Journal
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