High resolution observations of the outer disk around T Cha : the view from ALMA

Abstract

CONTEXT. Transitional disks are circumstellar disks with dust gaps thought to be related in some cases to planet formation. They can shed light on the planet formation process by the analysis of their gas and dust properties. T Cha is a young star surrounded by a transitional disk with signatures of planet formation. AIMS. The aim of this work is to spatially resolve the outer disk around T Cha and to derive its main properties. METHODS. We have obtained high-resolution and high-sensitivity ALMA observations in the CO(3–2), ¹³CO(3–2), and CS(7–6) emission lines to reveal the spatial distribution of the gaseous disk around the star. In order to study the dust within the disk we have also obtained continuum images at 850 μm from the line-free channels. RESULTS. We have spatially resolved the outer disk around T Cha. Using the CO(3−2) emission we derive a radius of ~230 AU. We also report the detection of the ¹³CO(3−2) and the CS(7−8) molecular emissions, which show smaller radii than the CO(3−2) detection. The continuum observations at 850 μm allow the spatial resolution of the dusty disk, which shows two emission bumps separated by ~40 AU, consistent with the presence of a dust gap in the inner regions of the disk, and an outer radius of ~80 AU. Therefore, T Cha is surrounded by a compact dusty disk and a larger and more diffuse gaseous disk, as previously observed in other young stars. The continuum intensity profiles are different at both sides of the disk suggesting possible dust asymmetries. We derive an inclination of i(°) = 67 ± 5, and a position angle of PA(°) = 113 ± 6, for both the gas and dust disks. The comparison of the ALMA data with radiative transfer models shows that the gas and dust components can only be simultaneously reproduced when we include a tapered edge prescription for the surface density profile. The best model suggests that most of the disk mass is placed within a radius of R< 50 AU. Finally, we derive a dynamical mass for the central object of M∗ = 1.5 ± 0.2 M⊙, comparable to the one estimated with evolutionary models for an age of ~10 Myr.