10:45 AM - 11:00 AM
[PCG20-07] Observational Constraint on the Dust Albedo in a Protoplanetary Disk
Keywords:Protoplanetary disk, Planetary system formation, Dust particles, ALMA
Terrestrial planets, cores of gas giant planets, and small bodies in planetary systems are formed by the accumulation of dust particles in protoplanetary disks. The accumulation processes depend on the dust material property, and the material property would be reflected in the optical property. Therefore, it is very important to observationally constrain the dust optical properties in disks. Indeed, it is known that the material-specific features in the infrared wavelength can tightly constrain the material property. However, the disk midplane, which is the most important region for planet formation, cannot be observed since it is optically thick in the infrared. Furthermore, there is almost no feature in wavelengths longer than sub-millimeters although it can trace the midplane, therefore, it is difficult to constrain the material property.
On the other hand, recent high-spatial-resolution observations with ALMA and theoretical studies have pointed out that the dust scattering albedo has non-negligible values even in submillimeter and millimeter wavelengths. Albedo depends on the composition, porosity, and size distribution; therefore, it can be a key to probing the dust material properties. However, albedo is poorly constrained by observations so far. This is because the temperature and the dust emitting surface degenerate in optically thick dust emission from disks. Meanwhile, the intensity ratios between different transitions of the same molecule depend on only the temperature if the line emission is optically thin. Therefore, by using multiple transitions of the same molecule, it would be possible to determine the temperature independently of the dust albedo. Such molecules must trace near the midplane and exhibit a sufficient signal-to-noise ratio. We found that the pressure-broadened carbon monoxide line wings can satisfy the conditions.
We analyzed ALMA archival data of the CO J=2-1 and 3-2 lines in the TW Hya disk and confirmed the pressure-broadened wings in both transitions near the disk center. Then, by constructing an emission model and fitting it to the two spectra simultaneously, we determined the temperature independently of the albedo. In addition, using multi-wavelength continuum observations from 0.4 mm to 3 mm, we derived the albedo spectrum for the first time. As a result, we found that the albedo is as high as ~0.8-0.9, which is inconsistent with low-albedo dust models such as carbon dust proposed by Zubko et al. (1996).
On the other hand, recent high-spatial-resolution observations with ALMA and theoretical studies have pointed out that the dust scattering albedo has non-negligible values even in submillimeter and millimeter wavelengths. Albedo depends on the composition, porosity, and size distribution; therefore, it can be a key to probing the dust material properties. However, albedo is poorly constrained by observations so far. This is because the temperature and the dust emitting surface degenerate in optically thick dust emission from disks. Meanwhile, the intensity ratios between different transitions of the same molecule depend on only the temperature if the line emission is optically thin. Therefore, by using multiple transitions of the same molecule, it would be possible to determine the temperature independently of the dust albedo. Such molecules must trace near the midplane and exhibit a sufficient signal-to-noise ratio. We found that the pressure-broadened carbon monoxide line wings can satisfy the conditions.
We analyzed ALMA archival data of the CO J=2-1 and 3-2 lines in the TW Hya disk and confirmed the pressure-broadened wings in both transitions near the disk center. Then, by constructing an emission model and fitting it to the two spectra simultaneously, we determined the temperature independently of the albedo. In addition, using multi-wavelength continuum observations from 0.4 mm to 3 mm, we derived the albedo spectrum for the first time. As a result, we found that the albedo is as high as ~0.8-0.9, which is inconsistent with low-albedo dust models such as carbon dust proposed by Zubko et al. (1996).