Jaime E Pineda

 

Direct Observation of the Transition to Coherence and Isothermal Filaments in a Dense Core

A. Goodman, H. Arce, P. Caselli, J. Foster, P.C Myers, E. Rosolowsky, S. Longmore, S. Corder

 

We present NH3 observations of the B5 region in Perseus obtained with the GBT and EVLA. The GBT map covers a region large enough (11'×14') 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 km s−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.

The EVLA observations (27 pointing mosaic) are combined with the GBT map to achieve a 6" beam. This map (~6.8'x8') covers the region of subsonic non-thermal velocity dispersion observed with the GBT. These observations reveal, for the first time, the presence of striking filamentary structure (20" wide or 5,000 AU at the distance of Perseus) in this low-mass star forming region. The integrated intensity profile of this structure is consistent with models of an isothermal filament in hydrostatic equilibrium. Also, the observed separation between the B5–IRS1 young stellar object (YSO), in the central region of the core, and the northern starless condensation matches the Jeans length of the dense gas. This suggests that the dense gas in the coherent region is fragmenting. The region observed displays a narrow velocity dispersion, where most of the gas shows evidence for subsonic turbulence, and where little spatial variations are present. It is only close to the YSO where an increase in the velocity dispersion is found, but still displaying subsonic non-thermal motions.

Finally, we'll discuss the implications of these results on the "core" identification/definition and the importance of the  region of subsonic turbulence in the formation process of low-mass stars.