EPoS Contribution
EPoS Contribution
Direct Observation of a Sharp Transition to Coherence in Dense Cores

Jaime E. Pineda
Harvard-Smithsonian Center for Astrophysics,
We present NH3 observations of the B5 region in Perseus obtained with the GBT. The map covers a region large enough (11'x14') that it contains the entire dense core observed in previous dust continuum surveys. The dense gas traced by NH3(1,1) covers a much larger area than the dust continuum features found in bolometer observations. The velocity dispersion in the central region of the core is small, presenting subsonic non-thermal motions which are independent of scale. However, it is thanks to the coverage and high sensitivity of the observations that we present the detection, **for the first time**, of the transition between the coherent core and the dense but more turbulent gas surrounding it. This transition is sharp, increasing the velocity dispersion by a factor of 2 in less than 0.04 pc (the 31" beam size at the distance of Perseus, 250 pc).The change in velocity dispersion at the transition is ~3 kms-1 pc-1. The existence of the transition provides a natural definition of dense core: the region with nearly-constant subsonic non-thermal velocity dispersion. From the analysis presented here we can not confirm nor rule out a corresponding sharp density transition.
Caption: Velocity dispersion map derived from fitting all hyperfine components of NH3 simultaneously. The box on the map presents the region where we zoom in and present the corresponding spectra in the right panel, which shows only the main component of the NH3(1,1) line. The centroid velocity and velocity dispersion obtained from the fit is displayed for each position. Top spectra in the right panel display two main hyperfine components clearly separated thanks to their low velocity dispersion (the coherent core), while when moving to positions outside the core the lines get weaker and broader (evident by the disappearance of the gap between hyperfine components).
Collaborators:
A.A. Goodman, HSCfA, USA
H.G. Arce, Yale U, USA
P. Caselli, Leeds U, UK
J.B. Foster, Boston U, USA
P.C. Myers, HSCfA, USA
E.W. Rosolowsky, U of British Columbia Okanagan, Canada
Suggested Session: Chemistry, Cores and Collapse, Molecular Clouds, Turbulence