Using the Atacama Large Millimeter/submillimeter Array (ALMA), Ruobing Dong and collaborators (Astrophys. J., in the press; preprint at https://arxiv.org/abs/1701.05189) have imaged three isotopologues of carbon monoxide and the 1.3 mm dust continuum emission in the transitional disk J160421.7-213028 (pictured). The cavity or gap that defines this disk as transitional is clearly visible with ALMA’s 0.25’’ spatial resolution, which at the distance of the disk is ∼35 au. Combined with an H-band scattered light image (of wavelength 1.5–1.8 μm; not shown), the authors are able to identify the distributions of three of the main components of this transitional disk: sub-micrometre-sized (small) dust, millimetre-sized (large) dust and the molecular gas.

Both dust populations indicate a sharp transition in dust density at a radius of ∼70 au, with the cavity at least 1,000 times less concentrated in dust than the surrounding disk. In the molecular gas observations, the cavity is significantly smaller, extending only ∼15 au from the central star. Its edge is also more gradual, with the surface density increasing steadily between 15 and 70 au.

Credit: RUOBING DONG

The cause of the central cavities in transitional disks is still an open question. A leading theory is that orbiting planets might sculpt a cavity opening through tidal interactions with the disk material. A single planet might open up a gap (such as those seen in the gas-rich disk of TW Hydrae) and multiple planets might open up a cavity like the one seen here. Alternatively, strong radiation from the star might indirectly ‘blow’ inner material away in a process called photoevaporation. This can cause a cavity to form if the disk accretion rate is sufficiently small. In this scenario, however, the inner edges of both the dust disk and the gas disk are sharply defined.