The Separation/Period Gap in the Distribution of Extrasolar Planets around Stars with Masses M >= 1.2 Msolar

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.