High-mass Star Formation: Coalescence vs. Accretion

Qizhou (Joe) Zhang
Harvard-Smithsonian Center for Astrophysics

High-mass stars are the key ingredients of galaxies.  They are responsible 
for the production of heavy elements and dominate the evolution of galaxies. 
To date, the process through which they form is not clear. There are two 
opposing views regarding high-mass star formation: One view proposes that  
high-mass stars form by coalescence of low- to intermediate-mass stars. 
The main argument for this is that a protostellar core, once reaching 
about 10 Msun, can exert enough radiation pressure to halt spherical 
infall and inhibit further growth of the core mass. The other view counters
that since the dynamical process near the (proto)star is not isotropic, 
high-mass stars could form via accretion.

In this talk, I describe a series of observations aiming at
resolving the two hypotheses. I will present spectroscopic evidence 
for gravitational collapse, rotating disks and molecular outflows 
toward the Galactic high-mass star-forming regions, and discuss 
the implication of these findings for the two prevailing views of 
high-mass star formation. Finally, I will discuss how high angular 
resolution studies in the submillimeter window will advance our knowledge 
of high-mass star formation.