Turbulent infall onto class 0 disks as cause of CAI brief condensation episode in the solar system

Technical Summary: Turbulent Infall onto Class 0 Disks as Cause of CAI Brief Condensation Episode in the Solar System

Problem Statement
Calcium-aluminum-rich inclusions (CAIs) found in carbonaceous chondritic meteorites represent the oldest solids in the solar system, characterized by a remarkably brief condensation episode lasting approximately $10^5$ years. A central challenge in solar system formation theory is reconciling this short formation timescale with the long-duration (Myr) pollution of star-forming regions by short-lived radionuclides (SLRs) from external sources like supernovae or asymptotic giant branch stars. While previous models, such as the X-wind model, attempted to explain CAI formation via magnetocentrifugal winds, precise isotopic dating has constrained CAI formation to a narrow window that challenges models relying on prolonged solar irradiation. Furthermore, while various heating mechanisms (e.g., FU Orionis outbursts, nearby supernovae) have been proposed, they often struggle to simultaneously explain the spatial extent (several AU), the transient nature, and the intensity of the heating required to sublimate and re-condense refractory grains. The authors investigate whether the dynamic infall of gas onto Class 0 protostellar disks can generate the necessary thermal conditions for this brief, high-temperature episode.

Methodology
The study employs high-resolution hydrodynamic simulations using the GIZMO code in Meshless Finite Mass (MFM) mode. This Lagrangian scheme is selected for its superior shock capturing, reduced numerical dissipation, and excellent angular momentum conservation, which are critical for modeling warped disks and turbulent infall.

• Initial Conditions: The simulations utilize initial conditions derived from a population synthesis study of star cluster formation (Bate 2012, 2019). The authors trace the evolution of solar-mass stars back to the Class 0 stage, capturing realistic, time-dependent, and anisotropic infall histories from filamentary accretion streams.
• Simulation Setup: Two primary simulations are conducted:
  • Run 1: Features a rapid, chaotic infall phase with significant misaligned angular momentum streams impacting the disk.
  • Run 2: A control case with a similar initial setup but lacking significant vertical gas infall from streamers, serving as a baseline for isolated disk evolution.
• Physics Models:
  • Radiative Transfer: Instead of a simple barotropic equation of state, the authors employ an approximate radiative cooling method (Young et al. 2024) that hybridizes column density estimators (Stamatellos et al. 2007; Lombardi Jr et al. 2015) to handle both spherical and disk-like geometries. This allows for realistic thermal evolution where viscous heating dominates over irradiation in the optically thick midplane.
  • Resolution: A resolution study (Appendix A) confirms that the key physical features, specifically spiral density waves and associated shock heating, converge at particle counts of $\sim 10$ million. The final simulations utilize 10 million particles.

Key Results
The simulations reveal a distinct three-phase evolution in Run 1 (the infall scenario), contrasting sharply with the stable evolution of Run 2.

1. Global Evolution and Disk Reorientation:

   • Run 1: The system undergoes a "main heating phase" ($t \approx 6000-9000$ yr) driven by vigorous, misaligned infall. The interaction between infalling streams and the rotating disk causes a global reorientation of the disk by nearly $30^\circ$. This leads to the formation of warps and global spiral density waves.
   • Run 2: Without significant external infall, the disk remains geometrically thin and stable, with only minor tilts ($10^\circ-20^\circ$) and no large-scale reorientation.

2. Thermal Evolution:

   • Run 1: The main heating phase is characterized by a rapid temperature rise. The maximum disk temperature reaches $\sim 2000$ K, sufficient to sublimate CAI grains. This heating is sustained for several thousand years. The heating mechanism is identified as shock dissipation at the disk-envelope interface and within the disk midplane due to spiral shocks. The dense, vertically thickened disk during the infall peak reduces radiative cooling efficiency, trapping heat.
   • Run 2: The disk temperature remains significantly lower, peaking only around $400-600$ K. The absence of external infall prevents the generation of the intense shock heating observed in Run 1.

3. Heating Mechanisms:

   • Surface Shocks: Vertical slices show converging infall flows colliding with the disk surface, creating strong shocks and high-entropy zones.
   • Spiral Shocks: Analysis of the midplane reveals that spiral density waves (driven by gravitational instability with Toomre $Q \approx 1$) generate localized entropy jumps and heating. In the inner disk ($r \lesssim 3$ AU), temperatures reach $\sim 2000$ K, while spiral shocks in the 2–3 AU range create hotspots exceeding 1500 K.
   • Angular Momentum: The misalignment of infalling angular momentum vectors drives the reorientation and turbulence, which in turn enhances viscous dissipation and shock formation.

Significance and Claims
The paper claims that turbulent infall onto Class 0 disks provides a viable mechanism for the brief, high-temperature condensation episode required to form CAIs. Specifically:

• Transient Heating: The infall-driven heating is transient, lasting $\sim 10^3-10^4$ years, which aligns with the radiogenic age constraints of CAIs.
• Spatial Extent: The heating extends to $\sim 2-3$ AU, covering the region where CAIs are thought to form and be preserved.
• Re-condensation: The model suggests a cycle where "pre-solar" CAI-loaded grains in infalling clouds are sublimated by the shock heating and subsequently re-condense as the system cools during the infall subsidence phase.
• Dichotomy of Systems: The comparison between Run 1 and Run 2 implies that the formation of a single-population CAI reservoir is not an inevitable outcome of all disk formation. It depends on specific boundary conditions, such as the presence of chaotic, misaligned infall streams. This may explain why CAIs are enriched in carbonaceous chondrites (CC) but scarce in non-carbonaceous chondrites (NC), suggesting that only a fraction of planetary systems experience the specific infall dynamics necessary to reset the CAI clock globally.

The authors conclude that while their simulations cover the very early stage of disk formation (Class 0, central mass $\sim 0.2 M_\odot$), the infall-driven heating mechanism offers a robust explanation for the thermal conditions necessary for CAI formation, linking the dynamical history of the protostellar envelope directly to the meteoritic record.