1998A&A...337..539B

Result. are presented of an extensive, multi-wavelength study of the area around the evolved HII region Sh 2-151. We discuss the excitation of the HII region, and analyze its interaction with molecular clouds in the vicinity, the clouds themselves, and their embedded objects. We present KOSMA 3-m ¹²CO(2-1) and (3-2) observations of a 26'x46' region around Sh 2-151. These observations reveal five clouds, with a total mass of 2.9x10⁴M_☉. A small (2'x4') region to the SW of Sh 2-151 was mapped in several CO and CS transitions with the IRAM 30-m telescope. The latter region contains 1.8x10³M_☉ of molecular mass, and hosts an IRAS- and H₂O maser source (WB89-234). Tracers of higher (column-) densities, such as C¹⁸O and CS, show the existence of two clumps (N(H₂)≃1-2x10²²cm^–2) connected by a ridge of warm (T_kin∼36K) narrow-line (Δv(¹²CO)<1km/s) gas. The southern clump contains the IRAS/maser source, exhibits a molecular outflow (M≃5M_☉, τ_dyn≃2x10⁵yr), and is therefore probably a region of active star formation. In the northern clump the line profiles have an intermediate width (1-3km/s; partly due to the presence of two emission components) and are non-gaussian (shoulders, wings) at several positions. IRAS (HIRES) observations show there is also FIR emission associated with the northern clump. NIR (J, H, K, H₂) observations show the presence of several red star-like objects and diffuse emission in both clumps; some of the diffuse emission is stronger and more extended in K and H₂, and may be due to dust emission around embedded objects. HIRES data reveal that IRAS 23004+5642 (WB 89-234) nearly coincides with the H₂O maser and the outflow, and not with the strong, narrow-line CO-emitting gas detected towards the point source catalog position. The linewidth of this relatively low-density warm gas, the heating source of which is as yet unknown, indicates the absence of dynamical interaction with both Sh 2-151 and the embedded IR sources. From the HIRES 60µm and 100µm data we derive dust temperatures of 30-40K, the highest values occurring near the maser, and a dust mass of ∼30M_☉; assuming a gas-to-dust ratio of 100, this implies we detect ∼10% of the dust. Evidence from the available (literature) data leads to the conclusion that the exciting star(s) of Sh 2-151 has (have) not yet been identified. We suspect that one or more early type (<B0) stars may be hidden behind the molecular cloud associated with WB 89-234. The ionized gas mass of Sh 2-151 is ∼700M_☉. Star formation in the Sh 2-151 area may occur in the whole region of strong FIR emission (as seen in the HIRES maps: ∼0.5 degrees, or 50pc, in length); north of Sh 2-151, activity is seen near the edge of a large (20pc diameter) molecular cloud.