  There has been a long-standing discussion in the literature as to whether
core accretion or disk instability is the dominant mode of planet formation.
Over the last decade, several lines of evidence have been presented showing
that core accretion is most likely the dominant mechanism for the close-in
population of planets probed by radial velocity and transits. However, this
does not by itself prove that core accretion is the dominant mode for the total
planet population, since disk instability might conceivably produce and retain
large numbers of planets in the far-out regions of the disk. If this is a
relevant scenario, then the outer massive disks of B-stars should be among the
best places for massive planets and brown dwarfs to form and reside. In this
study, we present high-contrast imaging of 18 nearby massive stars, of which 15
are in the B2--A0 spectral type range and provide excellent sensitivity to wide
companions. By comparing our sensitivities to model predictions of disk
instability based on physical criteria for fragmentation and cooling, and using
Monte-Carlo simulations for orbital distributions, we find that ~85 % of such
companions should have been detected in our images on average. Given this high
degree of completeness, stringent statistical limits can be set from the
null-detection result, even with the limited sample size. We find that <32 % of
massive stars form and retain disk instability planets, brown dwarfs and very
low-mass stars of <100 Mjup within 300 AU, at 99 % confidence. These results,
combined with previous findings in the literature, lead to the conclusion that
core accretion is likely the dominant mode of planet formation.
