A bright inner disk and structures in the transition disk around the very low-mass star CIDA 1

The frequency of Earth-sized planets in habitable zones appears to be higher around M-dwarfs, making these systems exciting laboratories to investigate planet formation. Observations of protoplanetary disks around very low-mass stars and brown dwarfs remain challenging and little is known about their properties. The disk around CIDA 1 (similar to 0.1-0.2 M-circle dot) is one of the very few known disks that host a large cavity (20 au radius in size) around a very low-mass star. We present new ALMA observations at Band 7 (0.9 mm) and Band 4 (2.1 mm) of CIDA 1 with a resolution of similar to 0.05 '' x 0.034 ''. These new ALMA observations reveal a very bright and unresolved inner disk, a shallow spectral index of the dust emission (similar to 2), and a complex morphology of a ring located at 20 au. We also present X-shooter (VLT) observations that confirm the high accretion rate of CIDA 1 of (M)over dot(acc) = 1.4 x 10(-8) M-circle dot. yr(-1). This high value of (M)over dot(acc), the observed inner disk, and the large cavity of 20 au exclude models of photo-evaporation to explain the observed cavity. When comparing te these observations with models that combine planet-disk interaction, dust evolution, and radiative transfer, we exclude planets more massive than 0.5 M-Jup as the potential origin of the large cavity because with these it is difficult to maintain a long-lived and bright inner disk. Even in this planet mass regime, an additional physical process may be needed to stop the particles from migrating inwards and to maintain a bright inner disk on timescales of millions of years. Such mechanisms include a trap formed by a very close-in extra planet or the inner edge of a dead zone. The low spectral index of the disk around CIDA1 is difficult to explain and challenges our current dust evolution models, in particular processes like fragmentation, growth, and diffusion of particles inside pressure bumps.