Why Canonical Disks Cannot Produce Advection-dominated Flows

Using simple arguments we show that the canonical thin keplerian accretiondisks cannot smoothly match any plain advection dominated flow (ADAF) model. By'plain' ADAF model we mean the ones with zero cooling. The existence of sonicpoints in exact solutions is critical and imposes constraints that cannot besurpassed adopting 'reasonable' physical conditions at the hypothetical matchpoint. Only the occurrence of new critical physical phenomena may produce atransition. We propose that exact advection models are a class of solutionswhich don't necessarily involve the standard thin cool disks and suggest adifferent scenario in which good ADAF solutions could eventually occur.Comment: 12 pages, 2 figures, Accepted for publication in Ap.J. Letter