2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/318The results are still valid, however this letter is superceeded by a significantly extended version which is available at astro-ph/0003358 and is scheduled for publication in the ApJ Dec. 10, 2000. We have withdrawn this letter to avoid confusion.O pages, superceeded by arXiv:astro-ph/0003358Astrophysicstext