Astron. Astrophys. 339, 872-879 (1998)

1. Introduction

The Cepheus molecular cloud, at a distance of 725 pc, is a well known star forming region (Hartigan et al. 1986, Sargent 1977, 1979). The O and B stars of the Cepheus OB3 association are responsible for ionizing a large diffuse HII region (S 155), which lies at the interface between the large scale molecular cloud and the OB association (Sargent 1979).

The relationship between the Cep B molecular cloud and S 155 has been studied using radio continuum (Felli et al. 1978, and Testi et al. 1995, hereafter TOHTFG), recombination lines (Panagia & Thum 1981), combined far-infrared (FIR) and CO (Minchin et al. 1992, hereafter MWW), and optical and near-infrared (NIR) observations (Moreno-Corral et al. 1993, and TOHTFG). A summary of all these observations can be found in TOHTFG.

Here, we want to concentrate on a small region () in S 155 close to Cep B, which is the hottest ¹²CO component and is located to the NW edge of the molecular complex. MWW found that the spatial distribution of the ¹²CO emission in Cep B has an elephant trunk appearance with a peak brightness temperature of 48 K. The hotspot is close to a small bright HII region, which is believed to be ionized by an internal B1-B0.5 star rather than from the stars of the OB association (Felli et al. 1978, MWW). MWW proposed that the hotspot and the bright HII region are the product of sequential star formation due to shock compression of the molecular cloud induced by the radiation from HD217086, the brightest star of the Ceph OB3 association.

More recently, TOHTFG using NIR and high-resolution radio continuum observations were able to resolve this bright HII region into four distinct components (see Fig. 1): source A, which contains most of the radio flux and has a blister-type morphology, an extended ionization front (called the RIDGE), and two unresolved sources (B and C). NIR observations revealed a miniature stellar cluster, with three bright objects near the positions of the radio sources A, B and C. Source B is non-thermal (with ) and its connection with the star forming complex is unclear. All available indications suggest a rather evolved nature of the blister HII region: no H₂O maser or molecular outflow are present and the near IR source located at its centre (called A-NIR) does not show any IR excess.

Fig. 1. K-band (top panel ) and optical (bottom panel ) images (grey scale, logarithmic in both images) overlayed with the FCRAO map (resolution ) of the CS(2-1) integrated intensity (dashed contours), in the range -15 to -11 km s-1, and with the VLA-D map (resolution ) from TOHTFG (solid contours). The CS(2-1) levels vary from 0.2 to 2.2 by 0.2 K km s-1, and the continuum emission is in the range from 0.3 mJy/beam to 7.5 mJy/beam. One of the brightest stars of the OB3 association, HD217061, is barely visible at the W edge of the map. The other bright member, HD217086, far more north, is not visible in this map.

In this paper we focus on the hotspot (see Fig. 1) with sligltly higher resolution molecular observations using a molecular species, CS, which allows to better define the morphology, physical parameters and kinematics of the high density molecular gas in a region of strong interaction between ionized and molecular gas.

The outline of the paper is the following: in Sect. 2 we give a brief description of the observations, and in Sect. 3 we describe the results. Finally, in Sect. 4, we draw our conclusions.

© European Southern Observatory (ESO) 1998

Online publication: October 22, 1998
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