Galactic fountains and their connection with high and intermediate velocity clouds^*

¹ Dipartimento di Astronomia, Università di Trieste, via G.B. Tiepolo 11, 34131, Italy e-mail: spitoni@oats.inaf.it
² INAF Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131, Italy

Received: 16 January 2008
Accepted: 17 March 2008

Abstract

Context. Sequential supernova explosions create supershells which can break out a stratified medium, producing bipolar outflows. The gas of the supershells can fragment into clouds which eventually fall toward the disk producing so-called galactic fountains.

Aims. The aim of this paper is to calculate the expansion law and chemical enrichment of a supershell powered by the energetic feedback of a typical Galactic OB association at various galactocentric radii. We study the orbits of the fragments created when the supershell breaks out and we compare their kinetic and chemical properties with the available observations of high – and intermediate – velocity clouds.

Methods. We use the Kompaneets (1960, Soviet Phys. Dokl., 5, 46) approximation for the evolution of the superbubble driven by sequential supernova explosions and we compute the abundances of oxygen and iron residing in the thin cold supershell. We assume that supershells are fragmented by Rayleigh-Taylor instabilities and we follow the orbit of the clouds either ballistically or by means of a hybrid model considering viscous interaction between the clouds and the extra-planar gas.

Results. Given the self-similarity of the solutions, clouds are always formed ~448 pc above the plane. If the initial metallicity is solar, the pollution from dying stars of the OB association has a negligible effect on the chemical composition of the clouds. The maximum height reached by the clouds above the plane seldom exceeds 2 kpc and when averaging over different throwing angles, the landing coordinate differs from the throwing coordinate by ~1 kpc at most.

Conclusions. The range of heights and [O/Fe] ratios spanned by our clouds suggest that the high velocity clouds cannot have a Galactic origin, whereas intermediate velocity clouds have kinematic properties similar to our modeled clouds but have overabundances observed for the [O/Fe] ratios that can be reproduced only with initial metallicities that are too low compared to those of the Galaxy disk.