Ring-Gap Structures in the Class I Circumstellar Disk of CrA IRS 2 Associated with Magnetic Flux-Driven Bubble

TL;DR

This study applies PRIISM super-resolution imaging to ALMA Band 6 continuum data of CrA IRS 2, revealing a young, nearly face-on disk with an inner hole and an outer ring-gap. The authors quantify the outer gap's depth and width and show these properties are consistent with planet-disk interaction models, inferring a nascent planet of roughly $0.22^{+0.33}_{-0.14}\,M_{ m Jup}$ (PRIISM) to $0.71^{+1.05}_{-0.44}\,M_{ m Jup}$ (deconvolved) under reasonable disk-parameter assumptions. They further propose that interchange-instability–driven magnetic flux dissipation can suppress MRI turbulence, extend the dead zone, and enable rapid dust growth and early planet formation, linking magnetic processes to substructure emergence. Overall, CrA IRS 2 provides a crucial case where substructures and planet formation may arise during the early, accreting phase of disk evolution, with implications for the role of magnetic fields in planet formation timescales.

Abstract

Recent ALMA observations with 0''.1 resolution reveal characteristic substructures in circumstellar disks around young Class I sources, providing clues to the early stages of morphological disk evolution. In this paper, we applied PRIISM imaging to ALMA archival Band 6 continuum data of the circumstellar disk around the Class I protostar CrA IRS 2, located in the Corona Australis molecular cloud, which is associated with an extended gas ring attributed to magnetic flux advection driven by interchange instability. The dust continuum image with 1.5 times higher spatial resolution than conventional imaging revealed, for the first time, the early-phase circumstellar disk with both inner central hole and outer ring-gap structures, making CrA IRS 2 the youngest system exhibiting such features based on the bolometric temperature of $T_{\rm bol}$=235 K. To examine planet-disk interaction as one possible origin of the outer ring-gap structure, we found the measured depth and width to be consistent with planet-disk interaction models, suggesting the existence of a giant planet with a mass of 0.1-1.8 $M_{\rm Jup}$. The additional mechanism required for rapid planet formation could be the magnetic flux dissipation driven by the interchange instability, which suppresses MRI-driven turbulence and extends the dead zone, allowing efficient dust growth and the early formation of planets. This system thus provides new insight into how substructures and planet formation can emerge during the early, accreting phase of disk evolution.

Figures (8)

• Figure 1: ALMA 1.3 mm (Band 6) dust continuum around the Class I protostar CrA IRS 2. The centr position corresponds to the J2000 coordinate ($19^{\rm h}01^{\rm m}41.48^{\rm s}$$-36^\circ58^\prime31\farcs8$). (a) CLEAN image reconstructed with Briggs weighting (robust=0.5). The white ellipse denotes the synthesized beam. (b) PRIISM image reconstructed with hyper-parameters of $(\Lambda_l, \Lambda_{tsv})$=(10$^4$, 10$^{11}$). The white ellipse represents the effective spatial resolution $\theta_{\rm eff}$, determined by the point source injection method. (c) Disk model image generated using the best-fit geometric parameters and Gaussian kernel hyper-parameters with protomidpy.
• Figure 2: Radial intensity profile averaged over the full azimuthal angle. The violet solid curves in all panels are the profile of the PRIISM image. In the middle panel, the blue and green solid curves represent the two components determined by a multiple-Gaussian fitting approach (see §\ref{['subsec:morphology']}). The corresponding deconvolved profiles, with the deconvolved standard deviations $\sigma_{\rm dec}$ and the original amplitudes $I_0$ described in Table \ref{['table:ring']}, are shown as dotted lines. The deconvolved profile corresponds to the orange curve in the bottom panel (see §\ref{['subsec:gap_formation']}). All the radial profiles are linearly interpolated onto radial grid points spaced by 0.1 au using interpolate.interpld from the SciPy module.
• Figure 3: Relationship between gap width and depth based on the model from Zhang_2018 (shaded region), overlaid with the gap properties of the circumstellar disk around CrA IRS 2 (this study) and of Taurus protoplanetary disks reported by Yamaguchi_2024. Using fixed values of a maximum dust grain size of $s_{\rm max}$=0.1 mm and a viscosity parameter of $\alpha_{\rm vis}=10^{-3}$, we present models for gas surface densities $\Sigma_{\rm gas}$ of 100 g cm$^{-2}$ (green) and 10 g cm$^{-2}$ (blue), with the disk scale height ranging from 0.05 to 0.10, which is used in Zhang_2018.
• Figure 4: (Top) Radial profiles of the brightness temperature in the original (PRIISM; purple) and the deconvolved (orange) cases. The gray dotted and dashed lines represent disk temperature models $T_{\rm dust}(r)$ of $\varphi$=0.07 and 0.14, respectively. (Middle) Optical depths of two cases $\tau_\nu$. The purple line was derived from the original brightness temperature profile and the dust disk temperature model of $\varphi$=0.07. In contrast, the orange line was estimated from the deconvolved profile and the model of $\varphi$=0.14. (Bottom) Dust surface densities of the two models $\Sigma_{\rm dust}(r)$ estimated with the dust opacity $\kappa_\nu$=0.43 cm$^2$ g$^{-1}$Birnstiel_2018.
• Figure 5: ALMA Band 6 (1.3 mm) dust continuum images obtained using PRIISM. (a) Image produced with the hyperparameters $(\Lambda_l, \Lambda_{tsv}) = (10^4, 10^{10})$. (b) Same as panel (a), but with $(\Lambda_l, \Lambda_{tsv}) = (10^4, 10^{11})$. Both images share the same color scale. The white ellipse in each panel indicates the effective spatial resolution $\theta_{\rm eff}$ for each image.