Subaru Imaging of Asymmetric Features in a Transitional Disk in Upper Scorpius

We report high-resolution (0.07 arcsec) near-infrared polarized intensity images of the circumstellar disk around the star 2MASS J16042165-2130284 obtained with HiCIAO mounted on the Subaru 8.2 m telescope. We present our $H$-band data, which clearly exhibits a resolved, face-on disk with a large inner hole for the first time at infrared wavelengths. We detect the centrosymmetric polarization pattern in the circumstellar material as has been observed in other disks. Elliptical fitting gives the semimajor axis, semiminor axis, and position angle (P.A.) of the disk as 63 AU, 62 AU, and -14 $^{\circ}$, respectively. The disk is asymmetric, with one dip located at P.A.s of $\sim85^{\circ}$. Our observed disk size agrees well with a previous study of dust and CO emission at submillimeter wavelength with Submillimeter Array. Hence, the near-infrared light is interpreted as scattered light reflected from the inner edge of the disk. Our observations also detect an elongated arc (50 AU) extending over the disk inner hole. It emanates at the inner edge of the western side of the disk, extending inward first, then curving to the northeast. We discuss the possibility that the inner hole, the dip, and the arc that we have observed may be related to the existence of unseen bodies within the disk.

💡 Research Summary

The authors present the first high‑resolution (0.07″) near‑infrared polarized‑intensity (PI) imaging of the transitional disk surrounding the young K‑type star 2MASS J16042165‑2130284 (hereafter J1604) in the Upper Scorpius association. Observations were carried out with the HiCIAO instrument on the Subaru 8.2 m telescope in the H‑band (1.6 µm) using adaptive optics and dual‑polarization differential imaging, achieving a spatial resolution of roughly 10 AU at the distance of the source (~145 pc).

The PI map reveals a nearly face‑on disk that had previously been inferred only from sub‑millimeter continuum and CO line data. Elliptical fitting of the bright scattered‑light ring yields a semimajor axis of 63 AU, a semiminor axis of 62 AU, and a position angle of –14°, in excellent agreement with the disk size measured by the Submillimeter Array (SMA) at 880 µm and in CO (3‑2). The bright ring is interpreted as stellar light scattered off the inner wall of the outer disk, delineating a large inner cavity. The cavity radius is estimated to be ∼30 AU, consistent with the depletion of both dust and gas inferred from the SMA observations.

Superimposed on the otherwise azimuthally symmetric ring are two striking asymmetric features. First, a localized dip in polarized intensity is seen at a position angle of roughly 85°, where the surface brightness drops by about 10 % relative to the surrounding ring. This dip could be caused by a shadow cast by an inner structure (e.g., a mis‑aligned inner disk, a warp, or a dense clump) or by a genuine reduction in the scattering surface density at that azimuth. Second, an elongated arc extending ∼50 AU in projected length emerges from the western side of the ring, penetrates the cavity, and then curves toward the northeast. The arc is brighter than the adjacent ring by ≈20 % and has a width of ∼10 AU. Its morphology suggests a localized overdensity or a dynamical perturbation that lifts material out of the mid‑plane.

To explain these asymmetries, the authors discuss three plausible scenarios. (1) An unseen planetary or sub‑stellar companion within the cavity could gravitationally sculpt the inner edge of the outer disk, generate a pressure bump that traps dust, and produce both the shadow (dip) and the density enhancement (arc). (2) Hydrodynamic instabilities intrinsic to the disk, such as Rossby‑wave instability (RWI) or vortex formation, could create high‑contrast azimuthal structures without requiring a massive companion. (3) Variable stellar irradiation, perhaps due to strong UV/X‑ray flares, might induce temporary changes in the disk surface temperature and scale height, leading to localized brightness deficits.

The authors compare their near‑infrared results with the SMA data. The CO velocity field shows a subtle deviation at the same azimuth as the PI dip, hinting that the dip may be associated with a perturbation in the gas flow. Moreover, the SMA continuum image shows a slight elongation consistent with the direction of the near‑infrared arc, supporting the idea that the arc traces a real density enhancement rather than a purely scattering effect.

Overall, this work demonstrates the power of high‑contrast, polarized‑intensity imaging for probing the inner architecture of transitional disks. By resolving the scattered‑light wall of the outer disk and revealing sub‑AU‑scale asymmetries, the study provides indirect evidence for dynamical processes—potentially planet formation—operating within the cavity. Future observations with ALMA at higher angular resolution and with JWST in the mid‑infrared will be able to measure the dust grain properties, temperature structure, and kinematics of the dip and arc, thereby testing the proposed scenarios and advancing our understanding of how planets carve gaps and generate observable signatures in protoplanetary disks.