Institute of Astronomy

Planet Traps and Stellar Metallicity: Application to the Planet-Metallicity Relation and the Formation of First Gas Giants

SpeakerTalk DateTalk Series
Yasuhiro Hasegawa28 July 2014Across HR 2014 Posters


Stellar metallicity is one of the fundamental parameters for planet formation in protoplanetary disks. We present a statistical analysis for planetary populations generated by the combination of the core accretion scenario with planet traps at which rapid type I migration is halted. We compute the planet formation frequency (PFF) and the planetary core mass, as a function of metallicity ([Fe/H]). The analysis is applied for three major populations inferred from the exoplanet observations: hot Jupiters, exo-Jupiters that are densely populated around 1 AU, and low-mass planets such as super-Earths. We show that the PFFs for both types of Jovian planets correlate positively with metallicity whereas the low-mass planets form efficiently for a wide range of metallicities. We find that these trends originate from the planetary core mass: the core mass of both kinds of Jovian planets increases steadily with metallicity while that of the low-mass planets is almost constant. These behaviors on the core mass define transition metallicity (TM) above which the core mass of Jovian planets exceeds that of the low-mass ones, and hence gas giant formation becomes efficient. We demonstrate that the properties of TM depend on the critical core mass that initiates gas accretion onto the cores. Comparing the radial velocity observations, our results show that the typical critical core mass for observed exoplanets is likely to be about 5 Earth-masse. Finally, we apply the calculations to metal-poor stars, and find that the critical metallicity for forming first gas giants is likely to be [Fe/H] = -1.2.