[Paper Review] The effect of late giant collisions on the atmospheres of protoplanets and the formation of cold sub-Saturns

Mohamad Ali-Dib, A. Cumming+1 more|arXiv (Cornell University)|Oct 15, 2021

Astro and Planetary SciencePhysics and Astronomy85 references11 citations

TL;DR

This paper proposes that giant collisions between sub-critical protoplanets can strip away gaseous envelopes via thermally driven super-Eddington winds from the heated core, preventing runaway accretion and leading to the formation of cold sub-Saturns (CSS) with massive cores (~30 M⊕) and low-mass envelopes (~few M⊕). The process occurs during disc dispersal, and the resulting planets avoid becoming gas giants due to rapid disc depletion, explaining the observed excess of CSS over theoretical predictions.

ABSTRACT

We investigate the origins of cold sub-Saturns (CSS), an exoplanetary population inferred from microlensing surveys. If confirmed, these planets would rebut a theorised gap in planets' mass distribution between those of Neptune and Jupiter caused by the rapid runaway accretion of super-critical cores. In an attempt to resolve this theoretical-observational disparity, we examine the outcomes of giant collisions between sub-critical protoplanets. Due to the secular interaction among protoplanets, these events may occur in rapidly depleting discs. We show that impactors ~ 5% the mass of near-runaway envelopes around massive cores can efficiently remove these envelopes entirely via a thermally-driven super-Eddington wind emanating from the core itself, in contrast with the stellar Parker winds usually considered. After a brief cooling phase, the merged cores resume accretion. But, the evolution timescale of transitional discs is too brief for the cores to acquire sufficiently massive envelopes to undergo runaway accretion despite their large combined masses. Consequently, these events lead to the emergence of CSS without their transformation into gas giants. We show that these results are robust for a wide range of disc densities, grain opacities and silicate abundance in the envelope. Our fiducial case reproduces CSS with heavy (>= 30 M_Earth) cores and less massive (a few M_Earth) sub-critical envelopes. We also investigate the other limiting cases, where continuous mergers of comparable-mass cores yield CSS with wider ranges of core-to-envelope mass ratios and envelope opacities. Our results indicate that it is possible for CSS and Uranus and Neptune to emerge within the framework of well studied processes and they may be more common than previously postulated.

Motivation & Objective

• To resolve the theoretical-observational disparity between the predicted 'desert' of intermediate-mass planets and the observed abundance of cold sub-Saturns (CSS) from microlensing surveys.
• To investigate whether giant collisions during disc dispersal can prevent core runaway accretion and lead to CSS formation.
• To examine the role of core heating, envelope stripping, and disc depletion in shaping the final mass and structure of CSS.
• To determine under what conditions giant impacts can produce planets with core masses ≳30 M⊕ and sub-critical envelopes.
• To assess the robustness of this mechanism across varying disc densities, grain opacities, and silicate abundances.

Proposed method

• Modeling post-collision envelope evolution using a simplified thermal wind framework to simulate super-Eddington mass loss driven by core heating.
• Simulating energy dissipation from impacts to estimate core temperature rise and luminosity enhancement.
• Tracking envelope mass loss via shock wave propagation and subsequent thermal wind ejection, with focus on energy transfer efficiency.
• Varying initial conditions including core mass (15–30 M⊕), disc gas density (10−14 to 10−11 g/cm³), grain opacity (0.00–1.50 × 10−2), and metallicity (solar/sub-solar).
• Incorporating both 'dry' and 'wet' cases (with and without envelope recycling via disc flow) to assess impact on re-accretion.
• Comparing results with population synthesis models (e.g., Ida & Lin 2004) to validate outcomes under standard formation assumptions.

Experimental results

Research questions

• RQ1 Can giant collisions efficiently strip gaseous envelopes from protoplanets with near-critical cores, preventing runaway accretion?
• RQ2 What is the role of core-driven super-Eddington winds in envelope ejection following high-velocity impacts?
• RQ3 How does rapid disc depletion during the transitional phase affect the re-accretion of gaseous envelopes post-collision?
• RQ4 To what extent do variations in disc density, grain opacity, and silicate abundance affect the final mass and structure of the resulting planet?
• RQ5 Can this mechanism produce cold sub-Saturns with core masses ≳30 M⊕ and envelope masses ≲10 M⊕, consistent with microlensing observations?

Key findings

• Giant collisions with impactors of 0.5–1.5 M⊕ mass can completely strip 20–30 M⊕ protoplanetary envelopes via core-driven super-Eddington winds, preventing runaway accretion.
• The envelope stripping occurs in two phases: initial shock wave removal followed by thermal wind ejection driven by core heating, with luminosity exceeding the Eddington limit.
• Post-collision core cooling and re-accretion occur on timescales orders of magnitude shorter than the transitional disc lifetime (~10⁵ yr), limiting envelope growth.
• For a wide range of disc densities (10⁻¹⁴–10⁻¹¹ g/cm³), grain opacities (0.00–1.50 × 10⁻²), and silicate abundances, the final envelope mass remains sub-critical, yielding cold sub-Saturns with core masses ≳30 M⊕ and envelope masses of a few M⊕.
• Envelope recycling via disc flow slightly reduces re-accreted envelope mass but does not alter the fundamental outcome of sub-critical envelope growth.
• The mechanism produces planets consistent with microlensing statistics, explaining the observed overabundance of CSS relative to theoretical predictions of a planetary desert.