The structure of the IC1396 region.

We have made an extensive study of the molecular clouds associated with the Hii region IC1396 in the rotational transitions J=1-0 and J=2-1 of ^12^CO and ^13^CO and J=3-2 of ^12^CO with an average spatial resolution of 2.5' and an average sampling of about 2.0', in order to get information on its structure and evolution. On the basis of our observations, which cover an area of more than 6deg^2^, we can classify the molecular clouds into those directly associated with the ionizing O6.5V star HD 206267, producing the bright-rimmed clouds, and the cold gas along the line of sight, which is mainly foreground material. The bright-rimmed clouds show the presence of warmer molecular gas through higher ^12^CO (2-1)/(1-0), ^13^CO (2-1)/(1-0) and ^12^CO (3-2)/(2-1) line ratios than the cold foreground gas. The warm clouds form roughly a shell-like arrangement with a diameter of 25 to 40pc around HD206267 (though most are slightly closer to the Sun than the O star), and they seem to be the remainder of the now dispersing molecular cloud which gave birth to the O6.5 star and the star cluster Tr37 associated with it. All bright-rimmed clouds show internal structure on all size scales, including bipolar outflows. Optical, FIR (IRAS 12 to 100μm) and CO maps are in close agreement over the whole region, especially for the bright-rimmed clouds: Exceptions are some optically bright (ionized) regions, which show FIR, but no CO emission, and the cold foreground clouds, which are very weak at FIR wavelengths. The entire mass of the mapped IC1396 region is estimated to be 12000M_sun_, which is composed of molecular (4000M_sun_), atomic (5000M_sun_), and ionized material (3000M_sun_) in nearly equal amounts. The masses of the bright-rimmed clouds range from a few to several 100M_sun_.