Internal heating of molecular clouds by tidal fields

Abstract

We present a study of the effect of the galactic tidal field on the internal energy of molecular clouds, moving in the disk or central bar potential of a galaxy. During epicyclic motion or orbital motion in a bar, the tidal field across a cloud varies with time. This time variation of the tidal field across the cloud couples the rotational motion in the galaxy to the internal clump motion within the cloud. There will be a net exchange of internal energy between the cloud and the external gravitational field of the galaxy. We have examined this effect to see whether tidal fields are an important heating mechanism for molecular clouds and hence important for cloud support. An N-body simulation method has been used to treat the motion of clumps in molecular clouds. A cloud is initially virialized in a circular orbit to obtain a relaxed, clumpy system having a centrally peaked mass profile. The clouds were then evolved in epicyclic orbits and also closed orbits in a nonaxisymmetric bar potential. We find that the heating effect of the tidal field is not important for epicyclic motion, but it is significant for a cloud moving in a bar potential. For a cloud of mass 5 × 10⁴ M_sun, the change in internal energy is 10%-15% of the initial internal energy. Thus, although tidal fields alone cannot provide the energy required for cloud support, their contribution is significant in a bar potential. Also, some clumps become unbound from the cloud during the bar orbits. This can explain the origin of diffuse molecular gas which has been observed in the central regions of galaxies like our Galaxy, IC 342, and NGC 1808. We also find that an initially virialized cloud is not disrupted by the tidal field after a few rotations in the bar potential.