Orbital Evolution of Planets around Intermediate-Mass Giants

Around low- and intermediate-mass (1.5-3 M_sun) red giants, no planets have been found inside 0.6 AU. Such a paucity is not seen in the case of 1 M_sun main sequence stars. In this study, we examine the possibility that short-period planets were engulfed by their host star evolving off the main sequence. To do so, we have simulated the orbital evolution of planets, including the effects of stellar tide and mass loss, to determine the critical semimajor axis, a_crit, beyond which planets survive the RGB expansion of their host star. We have found that a_crit changes drastically around 2 M_sun: In the lower-mass range, a_crit is more than 1 AU, while a_crit is as small as about 0.2 AU in the higher-mass range. Comparison with measured semimajor axes of known planets suggests that there is a lack of planets that only planet engulfment never accounts for in the higher-mass range. Whether the lack is real affects our understanding of planet formation. Therefore, increasing the number of planet samples around evolved intermediate-mass stars is quite meaningful to confirm robustness of the lack of planets.