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

TL;DR

This study uses VLT/SPHERE H-band polarimetric imaging to resolve two Corona Australis disks, V721 CrA and BN CrA, and characterizes their morphology with an axisymmetric parametric disk model coupled to RADMC-3D radiative transfer. An MCMC approach constrains the small-dust mass, scale height, and inclination, revealing V721 CrA as a thick, spiral-rich disk (Md ≈ 2.8×10^{-4} M⊙, i ≈ 57.6°) and BN CrA as a thinner disk with a potential gap (Md ≈ 1.0×10^{-6} M⊙, i ≈ 70.6°). The two disks show distinct scattering phase functions, suggesting different dust grain populations, with BN CrA toward larger, more compact grains and V721 CrA toward smaller, fluffy grains. The results align with the CrA subgroups’ ages and environments (on-cloud vs off-cloud) and highlight environmental influence on early disk evolution, motivating follow-up ALMA, optical/NIR, and radial velocity observations to probe larger solids and potential companions.

Abstract

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.

Figures (12)

• Figure 1: IRDIS $H$-band $Q_\phi$ frames for V721 CrA. The colour scale is logarithmic and cuts out the read-out noise. The coronagraph area is covered by a gray-filled circle; north is up and east to the left. The bottom panel is the annotated version of the frame above. The spiral features highlighted in the latter are more evident from the modelling residuals shown later in \ref{['fig:V721_mcmc-res']} and \ref{['fig:V721_r2']}.
• Figure 2: Same as \ref{['fig:V721_Qphi']} but for BN CrA.
• Figure 3: Brightness ratio around the minor disk axis in the $Q_\phi$ frames for V721 CrA and BN CrA (the latter after a gaussian smoothing with a 0.5-pixel sigma). The mirror axis inclination is orthogonal to the best PA values reported in Sec. \ref{['subsec:CrA_contourfitting']} (namely, 104.4 for V721 CrA and 117.5 for BN CrA, east-of-north to the disk major axis). The coronagraph area is covered by a gray-filled circle. North is up and east to the left.
• Figure 4: Best model for V721 CrA resulting from the MCMC procedure. Panel (a) illustrates the best model at $\lambda = 1.625\um$, convolved to the data resolution. The gray circle represents the coronagraph. Panel (b) shows the residuals between the data and the best model. North is up and east is left.
• Figure 5: Best model for BN CrA resulting from the MCMC procedure. Panel (a) illustrates the best model at $\lambda = 1.625\um$, convolved to the data resolution. The gray circle represents the coronagraph. Panel (b) shows the residuals between the data and the best model. North is up and east is left.