On the Properties of the HI shells in the Small Magellanic CLoud

Abstract

There are 509 expanding neutral hydrogen shells catalogued in the Small Magellanic Cloud (SMC), all apparently very young, with dynamical ages of a few Myr. To examine their relationship with young stellar objects we cross-correlated the shell catalogue with various catalogues of OB associations, super giants, Cepheids, WR stars, supernova remnants, and star clusters. The incidence of chance line-ups was estimated via Monte-Carlo simulations, and found to be high. However, it is important that there are 1.5 times more shells that are {\em not} spatially correlated to an OB association, than shells that are. Moreover, 59 of the 509 shells lie mainly in low stellar density fields and have no young stellar objects associated with them, and therefore no obvious energy source. It is shown that, on the whole, the properties of these "empty" shells are very similar to the properties of the rest of the shells, once selection biases are taken into account. Theoretical implications of the results are discussed.