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

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: pinilla@mpia.de
² Mullard Space Science Laboratory, University College London, Holmbury St Mary, Dorking, Surrey RH5 6NT, UK
³ Unidad Mixta Internacional Franco-Chilena de Astronomía, CNRS/INSU UMI 3386, Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁶ Dublin Institute for Advanced Studies, School of Cosmic Physics, 31 Fitzwilliam Place, Dublin 2, Ireland
⁷ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
⁸ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK
⁹ Department of Physics and Astronomy, California State University Northridge, 18111 Nordhoff Street, Northridge, CA 91130, USA
¹⁰ INAF – Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy

Received: 18 January 2021
Accepted: 17 March 2021

Abstract

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 (~0.1–0.2 M_⊙) 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 ~0.05″ × 0.034″. These new ALMA observations reveal a very bright and unresolved inner disk, a shallow spectral index of the dust emission (~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 Ṁ_acc = 1.4 × 10⁻⁸ M_⊙ yr⁻¹. This high value of Ṁ_acc, the observed inner disk, and the large cavity of 20 au exclude models of photo-evaporation to explain the observed cavity. When comparing 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 CIDA 1 is difficult to explain and challenges our current dust evolution models, in particular processes like fragmentation, growth, and diffusion of particles inside pressure bumps.