Abstract. We present results of a method for an automatic search for H  shells in 3D data cubes and apply it to the Leiden-Dwingeloo H  survey of the northern Milky Way. In the 2nd Galactic quadrant, where identifications of structures are not substantially influenced by overlapping, we find nearly 300 structures. The Galactic distribution of shells has an exponential profile in the radial direction with a scale length of σ gsh = 3 kpc. In the z direction, one half of the shells are found at distances smaller than 500 pc. We also calculate the energies necessary to create the shells: there are several structures with energies greater than 10E SN but only one with an energy exceeding 100E SN . Their size distribution, corrected for distance effects, is approximated by a power-law with an index α = 2.1. Our identifications provide a lower limit to the filling factor of shells in the outer Milky Way: f 2D = 0.4 and f 3D = 0.05.

Aims. We analyse the all-sky Leiden/Argentina/Bonn HI survey, where we identify shells belonging to the Milky Way. Methods. We used an identification method based on the search of continuous regions of a low brightness temperature that are compatible with given properties of HI shells. Results. We found 333 shells in the whole Galaxy. The size distribution of shells in the outer Galaxy is fitted by a power law with the coefficient of 2.6 corresponding to the index 1.8 in the distribution of energy sources. Their surface density decreases exponentially with a scale length of 2.8 kpc. The surface density of shells with radii ≥100 pc in the solar neighbourhood is ∼4 kpc −2 and the 2D porosity is ∼0.7.

We study a model of rapidly cooling shocked stellar winds in young massive clusters and estimate the circumstances under which secondary star formation, out of the reinserted winds from a first stellar generation (1G), is possible. We have used two implementations of the model: a highly idealized computationally inexpensive spherically symmetric semi-analytic model, and a complex three-dimensional radiation-hydrodynamic simulations, and they are in a good mutual agreement. The results confirm our previous findings that in a cluster with 1G mass 10 7 M and half-mass radius 2.38 pc, the shocked stellar winds become thermally unstable, collapse into dense gaseous structures that partially accumulate inside the cluster, self-shield against ionizing stellar radiation and form the second generation (2G) of stars. We have used the semi-analytic model to explore a subset of the parameter space covering a wide range of the observationally poorly constrained parameters: the heating efficiency, η he , and the mass loading, η ml . The results show that the fraction of the 1G stellar winds accumulating inside the cluster can be larger than 50% if η he 10% which is suggested by the observations. Furthermore, for low η he , the model provides a self-consistent mechanism predicting 2G stars forming only in the central zones of the cluster. Finally, we have calculated the accumulated warm gas emission in the H30α recombination line, analyzed its velocity profile and estimated its intensity for super star clusters in interacting galaxies NGC4038/9 (Antennae) showing that the warm gas should be detectable with ALMA.

Criteria for gravitational collapse of expanding shells in rotating, shearing galaxy discs were determined using three‐dimensional numerical simulations in the thin shell approximation. The simulations were run over a grid of seven independent variables, and the resultant probabilities for triggering and unstable masses were determined as functions of eight dimensionless parameters. When the ratio of the midplane gas density to the midplane total density is small, an expanding shell reaches the disc scaleheight and vents to the halo before it collapses. When the Toomre instability parameter Q, or a similar shear parameter, QA, is large, Coriolis forces and shear stall or reverse the collapse before the shell accumulates enough mass to be unstable. With large values of csh5/(GL), for rms velocity dispersion csh in the swept‐up matter and shell‐driving luminosity L, the pressure in the accumulated gas is too large to allow collapse during the expansion time. Considering ∼5000 models covering a wide range of parameter space, the common properties of shell collapse as a mechanism for triggered star formation are: (1) the time‐scale is ∼4(csh/2πGρ[GL]0.2)0.5 for ambient midplane density ρ, (2) the total fragment mass is ∼2 × 107 M⊙, of which only a small fraction is likely to be molecular, (3) the triggering radius is ∼2 times the scaleheight, and the triggering probability is ∼0.5 for large OB associations. Star formation triggered by shell collapse should be most common in gas‐rich galaxies, such as young galaxies or those with late Hubble types.

ABSTRACT13 CO(J = 2-1) and C 18 O(J = 2-1) observations of the molecular cloud G285.90+4.53 (Cloud 16) in the Carina Flare supershell (GSH287+04-17) with the APEX telescope are presented. With an algorithm DENDROFIND we identify 51 fragments and compute their sizes and masses. We discuss their mass spectrum and interpret it as being the result of the shell fragmentation process described by the pressure assisted gravitational instability -PAGI. We conclude that the explanation of the clump mass function needs a combination of gravity with pressure external to the shell.