Is protostellar heating sufficient to halt fragmentation? A case study
  of the massive protocluster G8.68-0.37
  If star formation proceeds by thermal fragmentation and the subsequent
gravitational collapse of the individual fragments, how is it possible to form
fragments massive enough for O and B stars in a typical star-forming molecular
cloud where the Jeans mass is about 1Msun at the typical densities (10^4 cm^-3)
and temperatures (10K)? We test the hypothesis that a first generation of
low-mass stars may heat the gas enough that subsequent thermal fragmentation
results in fragments >=10Msun, sufficient to form B stars. We combine ATCA and
SMA observations of the massive star-forming region G8.68-0.37 with radiative
transfer modeling to derive the present-day conditions in the region and use
this to infer the conditions in the past, at the time of core formation.
Assuming the current mass/separation of the observed cores equals the
fragmentation Jeans mass/length and the region's average density has not
changed, requires the gas temperature to have been 100K at the time of
fragmentation. The postulated first-generation of low-mass stars would still be
around today, but the number required to heat the cloud exceeds the limits
imposed by the observations. Several lines of evidence suggest the observed
cores in the region should eventually form O stars yet none have sufficient raw
material. Even if feedback may have suppressed fragmentation, it was not
sufficient to halt it to this extent. To develop into O stars, the cores must
obtain additional mass from outside their observationally defined boundaries.
The observations suggest they are currently fed via infall from the very
massive reservoir (~1500Msun) of gas in the larger pc scale cloud around the
star-forming cores. This suggests that massive stars do not form in the
collapse of individual massive fragments, but rather in smaller fragments that
themselves continue to gain mass by accretion from larger scales.
