Structure formation in a colliding flow: The Herschel view of the Draco
  nebula
  The Draco nebula is a high Galactic latitude interstellar cloud likely to
have been formed by the collision of a Galactic halo cloud entering the disk of
the Milky Way. Such conditions are ideal to study the formation of cold and
dense gas in colliding flows of warm gas. We present Herschel-SPIRE
observations that reveal the fragmented structure of the interface between the
infalling cloud and the Galactic layer. This front is characterized by a
Rayleigh-Taylor instability structure. From the determination of the typical
length of the periodic structure (2.2 pc) we estimated the gas kinematic
viscosity and the turbulence dissipation scale (0.1 pc) that is compatible with
that expected if ambipolar diffusion is the main mechanism of energy
dissipation in the WNM. The small-scale structures of the nebula are typical of
that seen in some molecular clouds. The gas density has a log-normal
distribution with an average value of $10^3$ cm$^{-3}$. The size of the
structures is 0.1-0.2 pc but this estimate is limited by the resolution of the
observations. The mass ranges from 0.2 to 20 M$_{\odot}$ and the distribution
of the more massive clumps follows a power law $dN/d\log(M) \sim M^{-1.4}$. We
identify a mass-size relation with the same exponent as that found in GMCs
($M\sim L^{2.3}$) but only 15% of the mass of the cloud is in gravitationally
bound structures. We conclude that the increase of pressure in the collision is
strong enough to trigger the WNM-CNM transition caused by the interplay between
turbulence and thermal instability as self-gravity is not dominating the
dynamics.
