Effects of Stellar X-ray Photoevaporation on Planetesimal Formation via the Streaming Instability

TL;DR

The paper shows that stellar X-ray photoevaporation can induce a pressure bump at the edge of an expanding cavity, concentrating dust and enabling SI to form planetesimals during late disk dispersal. By coupling viscous gas evolution, multi-size dust coagulation/fragments, and wind losses in DustPy and applying a modern SI criterion, the authors quantify how final planetesimal masses depend on metallicity, X-ray luminosity, disk viscosity, and disk size. The fiducial model yields $\sim31\,M_{\oplus}$ of planetesimals (conversion efficiency $\sim20\%$), with higher dust content or favorable disk conditions increasing the yield, while low $L_X$ or high $\alpha$ suppress formation. This mechanism provides a plausible pathway for late-stage planetesimal formation in protoplanetary disks and informs how observable disk substructures and stellar properties relate to planetesimal inventories.

Abstract

The formation of planetesimals via the streaming instability (SI) is a crucial step in planet formation, yet its triggering conditions and efficiency are highly sensitive to both disk properties and specific evolutionary processes. We aim to study the planetesimal formation via the SI, driven by the stellar X-ray photoevaporation during the late stages of disk dispersal, and quantify its dependence on key disk and stellar parameters. We use the DustPy code to simulate the dust dynamics including coagulation, fragmentation, and radial drift in a viscously accreting disk undergoing stellar X-ray photoevaporation. Stellar X-rays drive the disk dispersal, opening a cavity at a few au orbital distance and inducing the formation of an associated local pressure maximum. This pressure maximum acts as a trap for radially drifting dust, therefore enhancing the dust density to the critical level required to initiate the streaming instability and the subsequent collapse into planetesimals. The fiducial model produces 31.4 M_\oplus of planetesimals with an initial dust to final planetesimal conversion efficiency of 20.4%. This pathway is most efficient in larger disks with higher metallicities, lower viscosities, higher dust fragmentation threshold velocities, and/or around stars with higher X-ray luminosities. This work demonstrates that stellar X-ray photoevaporation is a robust and feasible mechanism for triggering planetesimal formation via the SI during the final clearing phase of protoplanetary disk evolution.

Figures (9)

• Figure 1: The time and radial distance evolution of different quantities in the fiducial model. Panel a): Gas (dashed lines) and dust (solid lines) surface density. Panel b): Stokes number of the maximum-size dust particles (solid lines) and the mean Stokes number of the dust particles (dashed lines) at each radial bin. Maximum-size refers to the size of the particle whose surface density is at the peak of the mass bins hereafter. Panel c): Midplane density ratio $\epsilon$ (solid lines) and metallicity $Z$ (dashed lines).
• Figure 2: Synthetic $\rm 880\ \mu m$ continuum images from the fiducial model. Panel a): $\rm t{=}0.25\ Myr$, pre-cavity stage; Panel b): $\rm t{=}0.35\ Myr$, onset of photoevaporative cavity opening; Panel c): $\rm t{=}0.45\ Myr$, post-cavity stage.
• Figure 3: The evolution of dust mass (black lines), planetesimal mass (red lines), and dust mass fraction within the pressure bump in three Stokes number bins (light‑blue: $\rm{St}{<}10^{-2}$; blue: $10^{-2}{\leq}\rm{St}{\leq}10^{-1}$; deep‑blue: $\rm{St}{>}10^{-1}$), across four evolutionary stages: (i) pre‑cavity, (ii) cavity at 10 au, (iii) cavity at 20 au, and (iv) cavity at 100 au. These panels show how initial conditions affect dust retention and planetesimal formation: Panel a) Fiducial model ($Z=0.01$, $L_{\rm X}=2.6\,L_{\rm X,\odot}$, $M_{\rm disk}=0.05\,M_\odot$, $R_{\rm c}=60\,\rm au$, $\alpha=10^{-4}$) serves as the baseline for comparison, showing balanced dust evolution and planetesimal production across the disk. Panel b) Higher metallicity ($Z=0.02$) facilitates the retention of dust mass, thereby promoting planetesimal formation. Panel c) Decreased X‑ray luminosity (from $2.6\ L_{\rm X,\odot}$ to $1.0\ L_{\rm X,\odot}$) diminishes dust mass, and consequently impedes planetesimal formation. Panel d) Higher viscosity parameters ($\alpha{=}10^{-3}$, $\delta_{\rm t}{=}10^{-4}$) induce gas replenishment from the outer disk, delay cavity opening, reduce overall dust reservoir, and thus hinder planetesimal formation. Panel e) A more massive and extended disk ($M_{\rm disk}{=}0.1\ M_\odot$, $R_{\rm c}{=}\rm 120\ au$) prolongs dust retention, thereby extending the duration of planetesimal formation. Panel f) A less massive but extended disk ($M_{\rm disk}=0.05\,M_\odot$, $R_{\rm c}=120\,\rm au$) lowers the inner-disk dust density, causing photoevaporation to open a cavity earlier. This makes more dust retained at the onset of photoevaportative cavity, which also provides more building blocks for planetesimal formation.
• Figure 4: The distribution of dust as a function of radial distance and particle size at $t{=}0.30\ \rm Myr$ for run_fid (left) and run_frag (right). The color bar represents the surface density of dust per logarithmic mass interval. The green line is the drift limit (Eq. \ref{['St_drift']}), whereas blue line is the fragmentation limit (Eq. \ref{['St_frag']}). The four white lines correspond to $\rm St{=}[10^{-3}, 10^{-2}, 10^{-1}, 10^{0}]$
• Figure 5: Comparison of the radial mass distributions of planetesimals at different disk regions. The curves show the results for models with varying parameters: run_fid (black), run_metal (blue), run_lumi (orange), run_alpha (purple), run_dsize1 (red), and run_dsize2 (cyan).