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

Using VLT/SPHERE near-infrared polarized observations as part of the DESTINYS program, this study reveals that the young stars V721 CrA and BN CrA in the Corona Australis region host wide, structured disks with distinct morphologies—such as spiral arms and potential gaps—that align with their respective evolutionary stages within the star-forming region.

Original authors: Gabriele Columba, Elisabetta Rigliaco, Raffaele Gratton, Christian Ginski, Antonio Garufi, Myriam Benisty, Stefano Facchini, Rob G. van Holstein, Alvaro Ribas, Jonathan Williams, Alice Zurlo

Published 2026-04-09

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CC BY 4.0

Original authors: Gabriele Columba, Elisabetta Rigliaco, Raffaele Gratton, Christian Ginski, Antonio Garufi, Myriam Benisty, Stefano Facchini, Rob G. van Holstein, Alvaro Ribas, Jonathan Williams, Alice Zurlo

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, cosmic nursery. In this nursery, stars are born from clouds of gas and dust, much like a baby is born from a womb. But before a baby is fully independent, it often has a "blanket" wrapped around it—a swirling disk of leftover dust and gas. This is called a protoplanetary disk, and it's the raw material from which planets (like Earth) will eventually form.

This paper is a detailed look at two specific "babies" in the Corona Australis (Southern Crown) star-forming region. The astronomers used a powerful telescope (the VLT in Chile) equipped with special "polarized sunglasses" to take high-definition pictures of the dust swirling around two young stars: V721 CrA and BN CrA.

Here is the story of what they found, explained simply:

1. The Two Neighbors: A Tale of Two Ages

Even though these two stars are neighbors in the same star-forming cloud, they are at very different stages of life. Think of them as two siblings growing up in the same house but with different personalities.

V721 CrA (The Toddler): This star is very young (about 2 million years old). It lives deep inside the dusty, dark part of the cloud (the "on-cloud" group). Because it's so young, it's still surrounded by a thick, fluffy blanket of fresh dust. It's like a toddler who just got out of the bath and is still covered in bubbles.
BN CrA (The Teenager): This star is older (about 5–7 million years old). It has moved to the edge of the cloud (the "off-cloud" group). It has grown up, lost some of its baby fat, and its dust disk is thinner and more settled. It's like a teenager who has cleaned up their room and is ready to move out.

2. The "Polarized Sunglasses" Trick

Why use special sunglasses? The central star shines so brightly that it usually blinds our telescopes, hiding the faint dust around it. However, the dust scatters light in a specific way that creates polarization (a specific orientation of light waves), while the star's direct light does not.

By using these "sunglasses" (polarimetry), the astronomers could filter out the blinding glare of the star and see the faint, beautiful patterns of the dust disk, revealing structures that were previously invisible.

3. What They Saw: The "Toddler's" Messy Room (V721 CrA)

The disk around the young star V721 CrA is a chaotic, exciting place:

Thick and Fluffy: It's a tall, puffy disk, like a fluffy pillow.
Spiral Arms: The most exciting discovery is the presence of spiral arms. Imagine a spinning record player where the grooves are spiraling outward. These spirals suggest that something massive (perhaps a forming planet or a passing star) is tugging on the dust, creating waves.
Feeding Time: There's a faint, extended feature on one side that looks like a stream of dust falling onto the disk. It's like a cosmic faucet still dripping water into the bathtub, feeding the growing system.

4. What They Saw: The "Teenager's" Organized Room (BN CrA)

The disk around the older star BN CrA is different:

Thin and Wide: It's flatter and stretches much further out (about 190 times the distance from Earth to the Sun).
The Dark Lane: The astronomers saw a dark circular ring or lane in the middle of the disk. This could be a gap where a planet has already formed and swept the dust away, or it could be a shadow cast by a bump in the inner disk.
Cleaned Up: There are no spiral arms or messy streams. The dust has settled down, and the disk looks much more orderly and symmetrical.

5. The "Dust Grain" Detective Work

The astronomers didn't just look at the shapes; they analyzed the quality of the dust itself by studying how the light bounced off the grains (the "scattering phase function").

V721 CrA's Dust: The light bounced off in a way that suggests the dust grains are small, fluffy, and fragile. This makes sense for a young system where the dust hasn't had time to clump together yet.
BN CrA's Dust: The light bounced differently, suggesting the grains are larger, clumpier, and more compact. This is like the difference between fresh, powdery flour (V721) and dough that has been kneaded and rolled into balls (BN CrA). The older system has had time for the dust to grow and stick together.

The Big Picture

This study is part of a larger project called DESTINYS, which aims to take a "class photo" of all the young stars in a region to understand how their environment shapes them.

The main takeaway is that environment and age matter.

The young star in the dense, dusty cloud is still messy, fluffy, and actively being fed.
The older star on the edge of the cloud has settled down, its dust has grown larger, and it has started to clear out gaps where planets might be hiding.

It's a beautiful reminder that even in the vastness of space, the story of how planets form is a story of growing up, cleaning up, and organizing your room.

1. Problem and Context

The study addresses the need for homogeneous analyses of protoplanetary disks within the same star-forming regions to understand how local and global environmental factors influence disk evolution and architecture. Specifically, the authors focus on the Corona Australis (CrA) star-forming region, which is known to host two distinct subgroups:

On-cloud: Younger, denser, and dustier stars located near the main molecular cloud.
Off-cloud: Older, more dispersed stars located on the outskirts.

While previous studies (e.g., ALMA surveys) have characterized the CrA population statistically, individual disks in this region remain poorly characterized, particularly in the near-infrared (NIR). The paper aims to fill this gap by presenting high-contrast, polarized NIR observations of two specific targets, V721 CrA and BN CrA, to analyze their morphology, physical properties, and evolutionary stages.

2. Methodology

The authors employed a multi-step approach combining observational data reduction, parametric modeling, and numerical simulations:

Observations:
- Instrument: VLT/SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) using the IRDIS subsystem.
- Mode: Dual-beam polarimetric imaging (DPI) in the H-band ( $\lambda_0 \approx 1.625 \, \mu\text{m}$ ).
- Targets: V721 CrA (observed 2022) and BN CrA (observed 2023).
- Data Reduction: Utilized the IRDAP pipeline to produce Stokes $Q_\phi$ and $U_\phi$ frames, applying polarimetric differential imaging (PDI) to suppress stellar light and enhance scattered light from dust. Special care was taken to subtract residual stellar polarization (spurious signals) to reveal true disk structures.
Stellar Characterization:
- Recalculated stellar parameters (mass, age, luminosity, extinction) using Gaia DR3 parallaxes, spectral energy distributions (SEDs), and pre-main-sequence evolutionary tracks (Siess, Bressan, Baraffe, Choi models).
- V721 CrA: $\sim1.0 \, M_\odot$ , age $\sim1.7\text{--}2.0$ Myr, on-cloud.
- BN CrA: $\sim1.1 \, M_\odot$ , age $\sim5.1\text{--}7.1$ Myr, off-cloud.
Modeling and Simulation:
- Parametric Model: Adopted an analytical, axisymmetric disk model assuming hydrostatic balance. Key free parameters included dust mass ( $M_d$ ), reference scale height ( $h_0$ ), flaring exponent ( $\beta$ ), and inclination ( $i$ ).
- Radiative Transfer: Used RADMC-3D (v2.0) to generate synthetic scattered-light observations with full anisotropic polarized scattering treatment.
- Dust Opacity: Tested DIANA standard dust vs. RADMC-3D default silicates; selected silicate grains (amorphous olivine, 0.1 $\mu$ m) as they provided better fits to the observed inclination and morphology.
- Statistical Analysis: Performed Markov Chain Monte Carlo (MCMC) regressions using the emcee Python module to find the best-fit parameters by comparing synthetic models pixel-by-pixel with observed $Q_\phi$ frames.

3. Key Contributions

First Resolved Imaging: This work presents the first spatially resolved images of the disks around V721 CrA and BN CrA in polarized NIR scattered light.
Homogeneous Comparison: It provides a direct, homogeneous comparison of two disks within the same star-forming region but belonging to different evolutionary subgroups (on-cloud vs. off-cloud), linking disk properties to the broader cluster environment.
Methodological Refinement: The study demonstrates the efficacy of combining parametric axisymmetric modeling with MCMC regression and RADMC-3D to constrain disk geometry and dust properties even in the presence of complex substructures.

4. Key Results

V721 CrA (On-cloud, Young)

Morphology: A thick, flared, and extended disk with a radius of $\sim120$ au.
Structure: Shows clear spiral arms in the inner regions and a "puffier" western side. The disk is less axisymmetric than BN CrA.
Physical Parameters:
- Dust Mass: High, estimated at $\sim2.8 \times 10^{-4} \, M_\odot$ (micron-sized dust).
- Geometry: Vertically thicker ( $h/r \approx 0.1$ at 115 au) with an inclination of $\sim57.6^\circ$ .
- Scattering: The scattering phase function (SPF) peaks near $90^\circ$ (bell-shaped), suggesting smaller, fluffier, low-porosity grains consistent with a young, less evolved population.
Environment: The disk shows signs of interaction with its surroundings, potentially including infalling material (streamers) or outer spiral structures, consistent with its location in the dense, gas-rich on-cloud region.

BN CrA (Off-cloud, Older)

Morphology: A wider ( $\sim190$ au), thinner, and more axisymmetric disk.
Structure: Features a prominent dark circular lane (gap or self-shadow) between $\sim70$ and $100$ au. No spiral structures were detected.
Physical Parameters:
- Dust Mass: Significantly lower, estimated at $\sim1.0 \times 10^{-6} \, M_\odot$ (two orders of magnitude less than V721 CrA).
- Geometry: Flatter ( $h/r \approx 0.07$ ) with a higher inclination of $\sim70.6^\circ$ .
- Scattering: The SPF shows a strong forward-scattering peak around $50^\circ$ , indicative of larger, more compact dust aggregates with lower porosity, consistent with an older, more processed population.
Environment: The disk appears isolated with no evidence of interaction with the surrounding medium, fitting the profile of the older, dispersed off-cloud subgroup.

5. Significance

Evolutionary Link: The results strongly support the hypothesis that the environment (on-cloud vs. off-cloud) dictates disk evolution. The younger, on-cloud V721 CrA retains more mass and smaller grains, while the older, off-cloud BN CrA has undergone significant dust growth and mass depletion.
Dust Evolution: The contrasting scattering phase functions provide direct evidence of dust grain evolution over time (from fluffy/small to compact/larger) within the same star-forming region.
Future Prospects: The detection of gaps and spirals highlights the potential for ongoing planet formation or dynamical interactions. The authors suggest that future ALMA observations (to probe larger grains/midplane) and radial velocity monitoring (to detect companions) are crucial for further characterizing these systems.

In conclusion, this study successfully characterizes the distinct evolutionary paths of two protoplanetary disks in Corona Australis, demonstrating that their morphological and physical differences are consistent with their respective ages and locations within the star-forming complex.