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Astron. Astrophys. 362, 1109-1121 (2000)

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1. Introduction

The molecular cloud Cepheus B, at a distance of 730 pc (Blaauw 1964), can be regarded as a typical example of sequential star formation. For a comprehensive review of earlier observations see Testi et al. (1995). Located at the edge of the HII region S155 and the Cepheus OB3 association, it comprises a compact HII region/hot core near the S155 interface, where young stars have already started to form (Moreno-Corral et al. 1993), and one might suspect that the process of star formation continues further inside the cloud. Cepheus B is an ideal target to investigate the spatial variations of excitation conditions, starting at a site of recent star formation and extending far into quiet parts of the cloud (see Fig. 1).

[FIGURE] Fig. 1. Velocity integrated 12CO 3-2 intensities in contours overlayed on an optical picture (red POSS plate) of Cepheus B and the HII region S155. The range of integration is -18 to -8 km s-1. Contour levels are 4 by 6 to 94 Kkm s-1. The white crosses indicate the O7 star HD217086 (north-west) and the B1 star HD217061; the white [FORMULA] denotes the hot core region. In addition, we marked, as reference for the further analysis, the positions of strongest 12CO, 13CO, and C18O peak temperatures by a [FORMULA], a [FORMULA] and a filled [FORMULA]; a fourth position in the south western part is marked by a filled [FORMULA].

First large scale observations of the molecular material in the whole Cepheus region were conducted by Sargent et al. (1977) with the strongest 12CO 1-0 detection in Cepheus B. Later Minchin et al. (1992) observed the region around the previously detected compact HII region (Felli et al. 1978; in the following we refer to this region as the hot core) in 12CO 3-2 with a spatial resolution of [FORMULA] and found kinetic temperatures as high as 60 K.

Testi et al. (1995) resolved the hot core at 8.4 GHz and 14.9 GHz with VLA continuum observations into 4 substructures: one compact blister type HII region (heated probably by a B1 ZAMS star), confirming former predictions by Minchin et al. (1992), two pre-main-sequence stars and one smaller ionization front. Furthermore they detected a small cluster of young stars visible in the NIR and associated with the hot core. However, no other signposts of star formation like water masers or outflows were found in Cepheus B (Hughes 1988).

Olmi & Felli (1998) observed CS to study the dense gas in the immediate vicinity of the hot core and detected weaker CS emission than observed in other massive star forming regions (Cesaroni et al. 1999). They find that the hot core is in an advanced state of star forming activity where new stars have destroyed the molecular material and thus reduced the column densities.

Most of the recent investigations concentrated on the Cepheus B hot core and its immediate surroundings. In contrast to this, the observations presented here deal with structures of Cepheus B on larger scales. Our main interest is the variation of excitation conditions of the molecular material from the hot core to inner regions of the cold molecular cloud.

The outline of this paper is as follows: Sect. 2 gives a description of the observations, Sect. 3 presents the observational results, in Sect. 4 a multi-line-analysis of the data set is conducted, Sect. 5 discusses our results and Sect. 6 summarizes the output of our analysis.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000