  The evidence for a shortage of exosolar planets with semimajor axes -1.1 <=
log (a/AU) <= -0.2 is investigated. It is shown that this valley results from a
gap in the radial distribution of planets, orbiting stars with masses M >= 1.2
Msolar (the high-mass sample, HMS). No underabundance is found for planets
orbiting stars with smaller masses. The observational data also indicate that
within the HMS population it is preferentially the more massive planets with M
sin(i) >= 0.8 M_J that are missing. Monte-Carlo simulations of planet formation
and migration are presented that reproduce the observed shortage of planets in
the observed radius regime. A dependence on the disk depletion timescale
tau_dep is found. The gap is more pronounced for tau_dep = 10^6 - 10^7 yrs than
for tau_dep = 3*10^6 - 3*10^7 yrs. This might explain the observed trend with
stellar mass if disks around stars with masses M* >= 1.2 Msolar have shorter
depletion timescales than those around less massive stars. Possible reasons for
such a dependence are a decrease of disk size and an increase of stellar EUV
flux with stellar mass.
