Sequential star formation at the periphery of the HII regions Sh 217 and Sh 219

The HII regions Sh 217 and Sh 219 are textbook examples of a Stromgren sphere surrounded by an annular photodissociation region (PDR). The annular PDR is observed in both the 21 cm atomic hydrogen emission and the dust (PAH) emission near 8 micron (MSX Survey). An ultracompact radio continuum source is observed in the direction of the annular PDR, in both Sh 217 and Sh 219. JHKobservations show the presence of highly reddened stellar clusters (AV ~ 20 mag) in the directions of these radio sources. These clusters are also IRAS sources, of luminosities 22700 Lo for Sh 217 and 5900 Lo for Sh 219. Each cluster contains at least one luminous star with an IR colour excess; the one in the Sh 219 cluster shows H-alpha emission. The cluster associated with Sh 217 is almost spherical and contains luminous objects at its centre. The cluster associated with Sh 219 is elongated along the ionization front of this HII region. We argue that these are `second-generation clusters', which means that the physical conditions present in the PDRs, close to the ionization fronts, have favoured the formation of clusters containing massive objects. We discuss the physical mechanisms which may be at the origin of the observed triggered star formation.Comment: 12 pages, Late

Damage identification in gravity dams using dynamic coupled hydro-mechanical XFEM

Damage identification in gravity dams using dynamic coupled hydro-mechanical XFEM

The Identification of Diffuse Bands

Our present knowledge of the diffuse interstellar bands (two of which were first noted by Wilson in 1958) is briefly summarized. Other broad and very broad interstellar features (the 220nm extinction bump, the very broad structure in the extinction curve, the extended red emission in reflection and other nebulae, and the unidentified - sometimes called the "overidentified" - infrared bands) are also briefly described. The origins of all these features remain unknown, in spite of intensive study. Possible relations between these various features and families is briefly discussed. Recent observations are shown to support the hypothesis that the carriers of the diffuse bands are free-flying molecules, whereas the alternative hypothesis of dust grains now seems to be untenable. Candidate types of molecular carriers are mentioned, and the possible source of such molecules in the diffuse interstellar medium is briefly discussed