1998A&A...339..575F

We have observed with high angular resolution (10''-24'') an area of ≃18'x15' around HD 200775 in the J=2->1 line of ¹²CO and the J=1->0 lines of ¹³CO and C¹⁸O. An interferometric HI 21cm image with similar angular resolution (14'') is also presented. Molecular observations show that the star is located in a biconical cavity that has been very likely excavated by an energetic and bipolar outflow in an earlier evolutionary stage. The star is not located at the apex of this cavity but ∼50'' towards the eastern lobe. At the present stage, there is no evidence for high velocity gas within the lobes of the cavity. However, the morphology of the HI emission, two filaments arranged in a ``>'' shape feature, and the bow shock located at the tip of one of these filaments, show that high velocity atomic gas is outflowing in a shell adjacent to the walls of the cavity in the eastern lobe. Since this high velocity atomic gas has not been detected in the western lobe, the outflow has a cometary shape. We propose that this outflow is formed when the atomic gas in the inner walls of the cavity is accelerated by the stellar wind. The cometary shape is due to the off-center position of the star. This cometary HI region constitutes the latest evolutionary stage of the bipolar outflow associated with HD 200775. Based on these results we propose a simple model to explain the formation of cometary nebulae in massive star forming regions.