The NGC 281 West cluster .1. Star formation in photoevaporating clumps

The NGC 281 West molecular cloud is an excellent test case for studying star formation in the clumpy interface between a Hn region and a giant molecular cloud. We present here a study based on new high resolution radio and near-infrared data. Using the IRAM 30-meter telescope, we have mapped the interface in the (CO)-O-18 (2-->1), (CO)-O-18 (1-->0), and (CS)-S-34 (3-->2) transitions with FWHP beamwidths less than or equal to 22 ''. We have imaged the same region with the VLA in the 20, 6, and 2 cm continuum bands to obtain complementary maps of the ionized gas distribution with angular resolutions less than or equal to 13 ''. In addition, we have obtained near-infrared J and K'-band images to detect young stars in the interface. The 30-meter data shows the molecular gas is concentrated into three clumps with masses of 570, >210, and 300 M. and average volume densities of 1.4, > 1, and 2 x 10(4) cm(-3). We detect (CS)-S-34 (3-->2 emission in two of the clumps, indicating peak densities in excess of 5 x 10(5) cm(-3) an attained in the clumps. A comparison of the (CO)-O-18 line data with the 20 cm continuum image suggests that the molecular clumps are being photoevaporated through their direct exposure to the UV radiation from neighboring OB stars, The luminosity and extent of the observed 20 cm emission is in good agreement with models of photoevaporative flows. We use these models to estimate the pressure exerted on the clumps by the ionized gas and find that it exceeds the internal, turbulent pressure of the clumps by a factor of a 2.5. Although a pressure equilibrium is not excluded given the uncertainties inherent in determining the pressures of the ionized and molecular gases, our best estimates of the clump and flow parameters favor the existence of low velocity shocks (1.5 km s(-1)) in the clumps. The clumps exhibit broad, non-Gaussian line shapes and complex kinematical structures suggestive of shocks. Further evidence for shocks is found in a comparison of position-velocity diagrams with published numerical simulations of imploding spherical clumps. We discuss the possibility that the knots of (CS)-S-34 (3-->2) emission trace gas compressed by converging shock waves. The K'-band observations show a rich cluster of primarily low mass stars in the H II/molecular interface, which we argue is divided into two distinct sub-clusters. We associate one sub-cluster with the two clumps nearest the OB stars, and the second sub-cluster with the third clump. The two clumps nearest the OB stars contain an embedded stellar population, suggesting that star formation is ongoing. We discuss the impact photoevaporation is having on star formation in these two clumps. We find that photoevaporation is dispersing the molecular gas from which the cluster is forming and estimate that the molecular gas will be completely evaporated in 2.5 Myr. Deep K'-band imaging of the two clumps show that the stars are detected primarily on the sides of the clumps facing the OB stars and in the adjoining H II region. We examine three explanations for this asymmetry: the acceleration of the molecular clumps away from the stars by photoevaporation (i.e., the rocket effect), the unveiling of young embedded stars by ionization-shock fronts, and the triggered formation of stars by shocks advancing into the clumps. if shocks do indeed exist in the clumps, then we argue that shock triggered star formation is the best explanation of the asymmetry. (C) 1997 American Astronomical Society.