Observational Constraints on the Dust Albedo Spectrum in a Protoplanetary Disk

speaker: Tomohiro Yoshida

Planets and planetary cores are formed by the coagulation of dust grains in a protoplanetary disk. Therefore, it is crucial to constrain the dust composition. Such dust grains in the disk midplane are observed at mm wavelengths. However, it is challenging to observationally constrain the composition directly because the mm-wavelength continuum emission of the dust does not show features as seen in infrared wavelengths. Meanwhile, recent observations and theory show that scattering significantly affects the emission even in the mm-wavelengths when the grain size is large. Since the scattering albedo as a function of wavelength (“albedo spectrum”) depends on the dust composition and grain size distributions, the constraints on the albedo can provide a hint on the nature of the dust. However, observational constraints on the albedo have been also difficult due to its degeneracy with temperature. In this talk, we propose to use the pressure broadened CO line wings, which are optically thin and trace the deepest region of the disk, as a thermometer of the disk midplane. We apply the method to ALMA archival observations of CO J=2-1 and 3-2 lines in the inner region of the TW Hya disk, and derive the mm-wavelength albedo spectrum for the first time by combining observations of other bands. We also discuss the constraints on the dust properties.