A multi-wavelength study of the Sharpless 151 region

Results are presented of an extensive, multiwavelength 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 (CO)-C-12(2-1) and (3-2) observations of a 26' x 46' region around Sh 2-151. These observations reveal five clouds, with a total mass of 2.9x10(4) M.. 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 1.8x10(3) M. of molecular mass, and hosts an IRAS- and H2O maser source (WB89-234). Tracers of higher (column-) densities, such as (CO)-O-18 and CS, show the existence of two clumps (N(H-2) approximate to 1-2x10(22) cm(-2)) connected by a ridge of warm (T-kin similar to 36 K) narrow-line (Delta upsilon((CO)-C-12) < 1 km s(-1)) gas. The southern clump contains the IRAS/maser source, exhibits a molecular outflow (M approximate to 5 M., tau(dyn) approximate to 2x10(5) yr), and is therefore probably a region of active star formation. In the northern clump the line profiles have an intermediate width (1-3 km s(-1); 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, H-2) 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 H-2, and may be due to dust emission around embedded objects. HIRES data reveal that IRAS23004+5642 (WB89-234) 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 60 mu m and 100 mu m data we derive trust temperatures of 30-40 K, the highest values occurring near the maser, and a dust mass of similar to 30 M.; assuming a gas-to-dust ratio of 100, this implies we detect similar to 10% of the dust. Evidence from the available (literature) data leads to the conclusion that the exciting start(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 WB89-234. The ionized gas mass of Sh 2-151 is similar to 700 M.. Star formation in the Sh 2-151 area may occur in the whole region of strong FIR emission (as seen in the HIRES maps: similar to 0.5 degrees, or 50 pc, in length); north of Sh 2-151, activity is seen near the edge of a large (20 pc diameter) molecular cloud.