Absorption of scalars by extended objects

We derive the low energy absorption cross section of minimal scalars for a very generic class of geometries, that includes, among others, black holes, strings, p-branes, as well as intersecting brane configurations. The scalar field can be absorbed across a regular surface (a horizon) of finite area, or across a zero area throat, which may be singular or regular. Then we focus on some particular cases and compare with the results obtained using microscopic models at weak coupling. Exact agreement is found for the absorption by the non-degenerate ground state of effective strings. For geometries where the throat is singular, we show that absorption through the stretched horizon yields correct results for degenerate states. For non-degenerate (ground) states we introduce a stretched throat, and show that it accounts for the correct dependence on frequency, charges, and moduli for the absorption by ground states of BPS fundamental strings. It is also shown to lead to the correct dependence on frequency for extremal D-branes. (C) 1998 Published by Elsevier Science B.V.