Resolved millimeter-dust continuum cavity around the very low mass young star CIDA 1

Context. Transition disks (TDs) are circumstellar disks with inner regions highly depleted in dust. TDs are observed in a small fraction of disk-bearing objects at ages of 1-10 Myr. They are important laboratories to study evolutionary effects in disks, from photoevaporation to planet-disk interactions. Aim. We report the discovery of a large inner dust-empty region in the disk around the very low mass star CIDA 1 (M-* similar to 0.1-0.2 M-circle dot). Methods. We used ALMA continuum observations at 887 mu m, which provide a spatial resolution of 0.'' 21 x 0.'' 12(similar to 15 x 8 au in radius at 140 pc). Results. The data show a dusty ring with a clear cavity of radius similar to 20 au, the typical characteristic of a TD. The emission in the ring is well described by a narrow Gaussian profile. The dust mass in the disk is similar to 17 M-circle plus. CIDA 1 is one of the lowest mass stars with a clearly detected millimeter cavity. When compared to objects of similar stellar mass, it has a relatively massive dusty disk (less than similar to 5% of Taurus Class II disks in Taurus have a ratio of M-disk/M-* larger than CIDA 1) and a very high mass accretion rate (CIDA 1 is a disk with one of the lowest values of M-disk/(M) over dot ever observed). In light of these unusual parameters, we discuss a number of possible mechanisms that can be responsible for the formation of the dust cavity (e.g. photoevaporation, dead zones, embedded planets, close binary). We find that an embedded planet of a Saturn mass or a close binary are the most likely possibilities.