The evidence for a shortage of exosolar planets with semimajor axes -1.1 \leq \log ( a / AU ) \leq - 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 _ { * } \geq 1.2 M _ { \odot } ( 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 ) \geq 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 \times 10 ^ { 6 } -3 \times 10 ^ { 7 } yrs .
This might explain the observed trend with stellar mass if disks around stars with masses M _ { * } \geq 1.2 M _ { \odot } 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 .