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Astron. Astrophys. 339, 575-586 (1998)

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1. Introduction

Mass-loss phenomena are associated with the early stages of star formation both in low-mass and high-mass objects. In the last decade, a big observational and theoretical effort has been made to understand the structure and evolution of the outflows associated with low-mass YSOs. But very few works have been done to understand the intermediate and high-mass outflows. This is due mainly to observational problems: i) Massive star forming regions are located further away than low-mass star forming regions; ii) High-mass stars are usually formed in clusters where several stars could be driving bipolar outflows at the same time; iii) The creation and expansion of ionized regions increase the confusion when studying the kinematics of these massive regions; and finally iv) because of the fast evolution of high-mass stars, the mass-loss phase is expected to be shorter than in low-mass stars.

Herbig Ae-Be (HAEBE) stars are intermediate mass (1 - 10 [FORMULA]) pre-main sequence stars. They are associated with reflection nebulosities and obscuration regions, and share characteristics with T Tau and OB stars. Like low-mass stars, these objects present large IR, mm and submm continuum excess, rapid photometric and spectroscopy variability, stellar winds, and high velocity molecular outflows (see Catala, 1989 for a review). Like the more massive OB stars, some of these stars are formed in clusters. These stars are unable to create extended HII regions, but large HI regions can be found associated with some of them (Fuente et al. 1993, 1996, 1998; Blouin et al. 1997). Therefore, only a combined study of the atomic and molecular gas can provide a picture of the region. Although the star forming regions are located closer to the sun and are less complex than those of OB stars, very few intermediate-mass outflows have been studied in detail. Furthermore, none of these outflows has been studied combining the atomic and molecular data. The study of these outflows can give important clues for the understanding of the more massive outflows.

HD 200775 is a B3 star located at the northern edge of an elongated molecular cloud. Located at a distance of 430 pc (van den Ancker et al. 1997), it is the illuminating star of the well-known reflection nebula NGC 7023. The existence of a bipolar outflow associated with this star was first proposed by Watt et al. (1986) based on their low-angular resolution 12CO maps. Large scale maps of the region in 13CO show that the star is located in a biconical cavity whose walls delineate perfectly the borders of the optical nebula (Fuente et al. 1992). The shape of this cavity strongly suggests that a bipolar outflow has excavated it. Rogers et al. (1995) carried out an interferometric image of this nebula in the HI 21cm line with an angular resolution of [FORMULA]1'. They found that the HI emission is filling the molecular cavity and that the density inside the cavity is at least one order of magnitude lower than the density in the surrounding molecular cloud. Since the gas around the star is mainly atomic, the atomic data are crucial to understanding the kinematics of the region. We have mapped a region of [FORMULA] 18'[FORMULA]15' around the star with high angular resolution (beam [FORMULA] 12") in the J=2[FORMULA]1 rotational line of 12CO and the HI 21cm line. These observations have allowed a study of the structure of the high velocity atomic and molecular gas of this nebula. Furthermore, in order to study the densest regions, we have mapped the same region in the J=1[FORMULA]0 rotational lines of 13CO and C18O.

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© European Southern Observatory (ESO) 1998

Online publication: October 21, 1998