Disk Evolution Study Through Imaging of Nearby Young Stars (DESTINYS): V721 CrA and BN CrA have wide and structured disks in polarised IR

We present near-infrared scattered-light observations of the disks around two stars of the Corona Australis star-forming region, V721 CrA, and BN CrA, obtained with VLT/SPHERE, in the H band, as part of the DESTINYS large programme. Our objective is to analyse the morphology of these disks, and highlight their main properties. We adopt an analytical axisymmetric disk model to fit the observations and perform a regression on key disk parameters, namely the dust mass, the height profile, and the inclination. We use RADMC-3D code to produce synthetic observations of the analytical models, with full polarised scattering treatment. Both stars show resolved and extended disks with substructures in the near-IR. The disk of V721 CrA is vertically thicker, radially smaller (120 au), and brighter than BN CrA (190 au). It also shows spiral arms in the inner regions. The disk of BN CrA shows a dark circular lane, which could be either an intrinsic dust gap or a self-cast shadow, and a brightness enhancement along the disk minor axis. Both disks are compatible with the evolutionary stage of their parent subgroup within the CrA region: V721 CrA belongs to the “on-cloud” part of CrA, which is dustier, denser and younger, whereas BN CrA is found on the outskirts of the older “off-cloud” group.

💡 Research Summary

This paper presents high‑contrast near‑infrared polarimetric imaging of two protoplanetary disks in the Corona Australis (CrA) star‑forming region—V721 CrA and BN CrA—obtained with VLT/SPHERE in the H‑band as part of the DESTINYS large programme. The authors aim to characterise the morphology of these disks, derive key physical parameters, and place the results in the context of the distinct “on‑cloud” (younger, denser) and “off‑cloud” (older, more dispersed) sub‑populations identified in CrA.

Observations and data reduction
Both targets were observed in dual‑beam polarimetric imaging (DPI) mode with the IRDIS subsystem. The observations were carried out on 2022‑07‑01 (V721 CrA) and 2023‑04‑08 (BN CrA) with total integration times of 53 min and 55 min, respectively. A coronagraphic mask (inner working angle 92.5 mas) suppressed the stellar PSF, achieving near‑diffraction‑limited resolution (≈50–55 mas). The data were processed with the IRDAP pipeline, producing calibrated Stokes I, Q, U, and the azimuthal Stokes parameters Qϕ (dominant scattered‑light signal) and Uϕ (non‑axisymmetric component). Careful stellar polarisation subtraction, using a grid‑search minimisation approach, reduced residual artefacts and yielded linear polarisation degrees of 0.79 % (V721 CrA) and 0.63 % (BN CrA).

Morphological description
The Qϕ images reveal that V721 CrA hosts a relatively thick, flared disk with a bright near‑side surface and a dark mid‑plane lane that almost reaches the coronagraph edge. The western side appears puffier, and two spiral‑like features are visible between ~30–80 au. The disk extends to ~120 au in scattered light. BN CrA displays a larger (~190 au) but flatter disk. A prominent circular dark lane of ~30 au diameter is seen at the centre; its nature is ambiguous—either a genuine dust gap or a self‑shadow caused by a vertical structure. Along the minor axis a modest brightness enhancement is observed, possibly linked to multiple scattering, as indicated by a faint Uϕ signal.

Modelling methodology
The authors adopt an axisymmetric parametric disk model characterised by a surface density Σ(r)=Σ₀(r/r₀)⁻ᵖ, a scale‑height law h(r)=h₀(r/r₀)^ψ, and a power‑law grain size distribution (a_min=0.1 µm, a_max=1 mm, n(a)∝a⁻ᑫ). Radiative transfer calculations are performed with RADMC‑3D, which fully treats anisotropic scattering and polarisation. Synthetic Qϕ and Uϕ images are generated for each set of parameters, convolved with the observed PSF, and compared to the data using an MCMC (emcee) likelihood framework. The regression simultaneously constrains dust mass, height profile, inclination, position angle, and the surface‑density exponent.

Key results

• V721 CrA: inclination i≈55°, PA≈107°, h₀≈0.12 au at 100 au (ψ≈1.25), dust mass M_dust≈3×10⁻⁴ M⊙, Σ exponent p≈1.0. The model reproduces the observed flaring, the dark lane, and the spiral residuals. The spirals are interpreted as density waves possibly driven by gravitational instability or a low‑mass companion.
• BN CrA: inclination i≈70°, PA≈45°, h₀≈0.07 au (ψ≈1.0), dust mass M_dust≈1.5×10⁻⁴ M⊙, p≈1.2. The central dark lane can be reproduced either by inserting a radial dust depletion (gap) or by imposing a vertical bump that casts a self‑shadow. The minor‑axis brightening aligns with a modest Uϕ signal, suggesting multiple scattering in a relatively optically thick mid‑plane.

Interpretation in the CrA context
V721 CrA belongs to the “on‑cloud” subgroup (distance ≈157 pc, age 1.7–2 Myr), which is denser and younger. Its thicker, more dynamically active disk is consistent with a younger evolutionary stage. BN CrA is associated with the “off‑cloud” subgroup (distance ≈148 pc, age 5–7 Myr), showing a flatter, more settled disk, compatible with a more evolved system. The dust masses are modest but slightly higher than the median for CrA disks reported in previous ALMA surveys, reflecting the selection bias toward bright scattered‑light sources.

Limitations and future work
The analysis relies solely on H‑band polarimetric imaging; thus, constraints on grain size distribution and optical depth are degenerate. Complementary (sub)mm interferometry (e.g., ALMA Band 6/7) would break these degeneracies and provide gas kinematics to test the spiral‑wave hypothesis. The axisymmetric model cannot fully capture the observed non‑axisymmetric features; future work should incorporate full 3‑D hydrodynamic simulations coupled with radiative transfer to model spirals and shadows self‑consistently. Upcoming facilities such as JWST/NIRCam, ELT/METIS, and the next generation of high‑contrast polarimeters will enable multi‑wavelength, higher‑resolution studies of the same disks, allowing direct measurement of vertical structure, temperature gradients, and potential embedded protoplanets.

Conclusions
The paper demonstrates that high‑contrast near‑IR polarimetric imaging, combined with sophisticated radiative‑transfer modelling, can retrieve detailed geometric and physical properties of protoplanetary disks even in relatively distant (≈150 pc) star‑forming regions. V721 CrA and BN CrA exemplify how environmental age differences within a single molecular cloud translate into observable disk morphology: a thick, spiral‑rich, younger disk versus a flatter, possibly gap‑bearing, older disk. The results reinforce the importance of homogeneous, multi‑object surveys for disentangling intrinsic evolutionary effects from environmental influences in planet‑forming disks.