Abstract
Abstract In the core-accretion formation scenario of gas giants, most of the gas accreting onto a planet is processed through an accretion shock. In this series of papers we study this shock because it is key in setting the structure of the forming planet and thus its postformation luminosity, with dramatic observational consequences. We perform one-dimensional gray radiation-hydrodynamical simulations with nonequilibrium (two-temperature) radiation transport and up-to-date opacities. We survey the parameter space of accretion rate, planet mass, and planet radius and obtain postshock temperatures, pressures, and entropies, as well as global radiation efficiencies. We find that the shock temperature is usually given by the “free-streaming” limit. At low temperatures the dust opacity can make the shock hotter but not significantly so. We corroborate this with an original semianalytical derivation of . We also estimate the change in luminosity between the shock and the nebula. Neither nor the luminosity profile depend directly on the optical depth between the shock and the nebula. Rather, depends on the immediate preshock opacity, and the luminosity change on the equation of state. We find quite high immediate postshock entropies ( –20 ), which makes it seem unlikely that the shock can cool the planet. The global radiation efficiencies are high ( ), but the remainder of the total incoming energy, which is brought into the planet, exceeds the internal luminosity of classical cold starts by orders of magnitude. Overall, these findings suggest that warm or hot starts are more plausible.
Highlights
With its first direct detections already some 10 to 15 years ago (Chauvin et al 2004; Marois et al 2008), the technique of direct imaging has started to reveal a scarce but interesting population of planets or substellar objects of very low mass at large separations from their host stars (Bowler 2016; Bowler & Nielsen 2018; Wagner et al 2019)
When we focus on M 10-3 MÅ yr-1 and masses above a few MJ, simulations with Rp » 1.5–3 RJ should be those in which the accretion shock is the most relevant
Note that the grid is irregular in shock position because we considered the same rmin values for all masses and accretion rates but the shock moves at different rates drshock dt; over the course of 2 × 107 s, which we use as the maximal simulation time because it is more than enough for the profiles to reach a quasi-steady state, the ranges of radial positions that the shock covers often do not overlap between different (M, Mp, rmin)
Summary
With its first direct detections already some 10 to 15 years ago (Chauvin et al 2004; Marois et al 2008), the technique of direct imaging has started to reveal a scarce but interesting population of planets or substellar objects of very low mass at large separations from their host stars (Bowler 2016; Bowler & Nielsen 2018; Wagner et al 2019). To interpret the brightness measurements requires knowing the postformation luminosity of planets of different masses Formation models, principally those of the California (Pollack et al 1996; Bodenheimer et al 2000, 2013; Marley et al 2007; Lissauer et al 2009) and the Bern group (Alibert et al 2005; Mordasini et al 2012a, 2012b, 2017), seek to predict this luminosity within the approximation of spherical accretion. Principally those of the California (Pollack et al 1996; Bodenheimer et al 2000, 2013; Marley et al 2007; Lissauer et al 2009) and the Bern group (Alibert et al 2005; Mordasini et al 2012a, 2012b, 2017), seek to predict this luminosity within the approximation of spherical accretion They need to assume something about the efficiency of the gasaccretion shock at the surface of the planet during runaway gas accretion.
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