The anomalously low (sub)millimeter spectral indices of some protoplanetary disks may be explained by dust self-scattering

Abstract

Previous (sub)millimeter observations have found that the spectral indices of dust emission from some young stellar objects are lower than that of the black body emission in the Rayleigh-Jeans limit (i.e., 2.0). In particular, the recent Atacama Large Millimeter Array observations have spatially resolved that the innermost regions of the protoplanetary disks TW\,Hya and HD\,163296 present anomalously low (i.e., $<$2.0) millimeter spectral indices. In some previous works, such anomalously low millimeter spectral indices were considered unphysical and were attributed to measurement errors. The present work clarifies that if the albedo is high and is increasing with frequency, it is possible to reproduce such anomalously low spectral indices when the emission source is optically thick. In addition, to yield lower than 2.0 spectral index at (sub)millimeter bands, the required dust maximum grain size \amax is on the order of 10-100 \micron, which is very well consistent with the previously derived \amax values based on multi-wavelength dust polarimetric observations. In light of this, measuring Stokes I spectral index may also serve as an auxiliary approach for assessing whether the observed dust polarization is mainly due to dust scattering or is due to the aligned dust grains.

Accepted to ApJL

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Cited by in corpus (12)

• The Radial Distribution of Dust Particles in the HL Tau Disk from ALMA and VLA Observations
• Annular substructures in the transition disks around LkCa 15 and J1610
• Nonsticky Ice at the Origin of the Uniformly Polarized Submillimeter Emission from the HL Tau Disk
• Orbital and mass constraints of the young binary system IRAS 16293-2422 A
• Predicting the observational signature of migrating Neptune-sized planets in low-viscosity disks
• Radial variations in grain sizes and dust scale heights in the protoplanetary disk around HD 163296 revealed by ALMA polarization observation
• Demographics of disks around young very low-mass stars and brown dwarfs in Lupus
• Dual-Wavelength ALMA Observations of Dust Rings in Protoplanetary Disks
• Solving grain size inconsistency between ALMA polarization and VLA continuum in the Ophiuchus IRS 48 protoplanetary disk
• Effects of Ringed Structures and Dust Size Growth on Millimeter Observations of Protoplanetary Disks
• Linear dust polarization during the embedded phase of protostar formation
• A Study of Millimeter Variability in FUor Objects