Planetesimal fragmentation and giant planet formation

Abstract

Context. Most planet formation models that incorporate planetesimal fragmentation consider a catastrophic impact energy threshold for basalts at a constant velocity of 3 km s−1 throughout the process of the formation of the planets. However, as planets grow, the relative velocities of the surrounding planetesimals increase from velocities of the order of meters per second to a few kilometers per second. In addition, beyond the ice line where giant planets are formed, planetesimals are expected to be composed roughly of 50% ices. Aims. We aim to study the role of planetesimal fragmentation on giant planet formation considering the planetesimal catastrophic impact energy threshold as a function of the planetesimal relative velocities and compositions. Methods. We improved our model of planetesimal fragmentation incorporating a functional form of the catastrophic impact energy threshold with the planetesimal relative velocities and compositions. We also improved in our model the accretion of small fragments produced by the fragmentation of planetesimals during the collisional cascade considering specific pebble accretion rates. Results. We find that a more accurate and realistic model for the calculation of the catastrophic impact energy threshold tends to slow down the formation of massive cores. Only for reduced grain opacity values at the envelope of the planet is the cross-over mass achieved before the disk timescale dissipation. Conclusions. While planetesimal fragmentation favors the quick formation of massive cores of 5–10 M⊕ the cross-over mass could be inhibited by planetesimal fragmentation. However, grain opacity reduction or pollution by the accreted planetesimals together with planetesimal fragmentation could explain the formation of giant planets with low-mass cores.

Giant planet formation at the pressure maxima of protoplanetary disks

“…Despite the fact that the analytical recipes for the gas accretion from Ikoma et al (2000) reproduce their numerical results, where also a Henyey method was applied to solve the constitutive envelope equations, some differences exist between the two models. While in our model we use the equation of state (EoS) from Saumon et al (1995), Ikoma et al (2000) used an ideal gas EoS considering hydrogen molecules and the ionization of hydrogen atoms. Another difference is related to the outer envelope boundary conditions.…”

Requirements for gravitational collapse in planetesimal formation --- the impact of scales set by Kelvin-Helmholtz and nonlinear streaming instability

“…Lastly, we acknowledge that the evolution of formed clumps has not been studied in our work. This includes planetesimal growth, mergers and accretion (e.g., Kokubo & Ida 2012;San Sebastián et al 2019;Liu et al 2019;Johansen & Bitsch 2019), possible fragmentation (Wakita et al 2017;Gerbig et al 2019)…”

The tensile strength of compressed dust samples and the catastrophic disruption threshold of pre-planetary matter

“…At present, whether primordial planetesimals are born small or big remains a matter of debate (Morbidelli et al 2009;Weidenschilling 2011). The difference between both cases resides mainly in the impact-velocity regime in which collisions among planetesimals take place (San Sebastián et al 2019). Assuming a Keplerian shear regime without significant gravitational stirring, collisions among planetesimals occur at low impact speeds and the outcome of an impact between two small planetesimals leads to growth (Weidenschilling 2011).…”

“…The tensile strength is the maximum tension that a material can resist before it breaks. This parameter is key for planet formation simulations that include models of planetesimal fragmentation (San Sebastián et al 2019). The tensile strength range of meteoroid-stream materials of likely cometary origin is between 0.4 kPa and 150 kPa (Blum et al 2014).…”