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If star formation proceeds by thermal fragmentation and the subsequentgravitational collapse of the individual fragments, how is it possible to formfragments massive enough for O and B stars in a typical star-forming molecularcloud 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 oflow-mass stars may heat the gas enough that subsequent thermal fragmentationresults in fragments >=10Msun, sufficient to form B stars. We combine ATCA andSMA observations of the massive star-forming region G8.68-0.37 with radiativetransfer modeling to derive the present-day conditions in the region and usethis to infer the conditions in the past, at the time of core formation.Assuming the current mass/separation of the observed cores equals thefragmentation Jeans mass/length and the region's average density has notchanged, requires the gas temperature to have been 100K at the time offragmentation. The postulated first-generation of low-mass stars would still bearound today, but the number required to heat the cloud exceeds the limitsimposed by the observations. Several lines of evidence suggest the observedcores in the region should eventually form O stars yet none have sufficient rawmaterial. Even if feedback may have suppressed fragmentation, it was notsufficient to halt it to this extent. To develop into O stars, the cores mustobtain additional mass from outside their observationally defined boundaries.The observations suggest they are currently fed via infall from the verymassive reservoir (~1500Msun) of gas in the larger pc scale cloud around thestar-forming cores. This suggests that massive stars do not form in thecollapse of individual massive fragments, but rather in smaller fragments thatthemselves continue to gain mass by accretion from larger scales.

Is protostellar heating sufficient to halt fragmentation? A case study of the massive protocluster G8.68-0.37