  We present NH3 observations of the B5 region in Perseus obtained with the
Green Bank Telescope (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 km/s/pc. 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.
