Disks, toroids, and the formation of high-mass stars

                         Riccardo Cesaroni
              INAF - Osservatorio Astrofisico di Arcetri

Notwithstanding the great progress made in the last decade, the
formation of high-mass stars remains to be properly understood. The
term "high-mass" is commonly used to indicate OB-type stars with
luminosities above several 1000 Lsun. For these, radiation pressure
should halt the infall of circumstellar material and hence prevent
further growth of the stellar mass.  Consequently, it is not obvious
that stars in excess of 6-8 Msun can form through accretion as well as
solar-type stars. Recent observations and theoretical studies have
begun to shed light on this intriguing topic.  After a short
introduction to the problem of star formation, I will discuss the most
relevant findings obtained by us and the role these may play in
discriminating between different models of OB star formation.  In
particular, I will illustrate how (sub)millimeter interferometric
imaging and VLBI maser observations have allowed the detection and
investigation of accretion disks and disk-like structures (toroids)
rotating about high-mass (proto)stars.  Finally, I will show how these
results support the (non obvious) idea that B-type stars can form
through a mechanism similar to that of low-mass stars.  The formation
of early O-type stars remains instead an open issue, which might
involve more ``exotic'' scenarios.