Temperature and thermal emission of cosmic dust around the Sun, Vega and Fomalhaut

Abstract

Many stars are known to have dust disks, which are created through collisions between planetesimals, comparable to comets and asteroids in our solar system. Cosmic dust around a star absorbs electromagnetic radiation and re-radiates at a longer wavelength, determined by its temperature. The thermal emission of the dust can be observed. In certain systems, including the solar system, some of the dust is also observed in the close vicinity of the star, referred to as "hot dust".

In this work, model calculations of temperature and thermal emission of cosmic dust around Sun, Vega and Fomalhaut are presented. Absorption efficiencies were derived based on Mie theory. Calculations were done for dust with assumed composition of amorphous carbon, astronomical silicate, ice or a mixed iron/magnesium oxide (MgO/FeO). The size of the dust was 5-20 nm in a size distribution, 100 nm or 1 µm. It was assumed that the dust resides in a narrow ring between 0.18-0.2 AU. A ring at ~1 AU was tested for comparison. Spectral energy distributions (SEDs) of the thermal emission were computed with different parameters. These SEDs were compared to observations around Vega and Fomalhaut. The total dust mass determined the absolute brightness of the SEDs.

The results showed that computed dust temperatures were different from black body. Modelling of SEDs showed that dust located at 0.18-0.2 AU, which is composed of MgO/FeO or amorphous carbon, could explain the observations around Vega and Fomalhaut. The observations are best explained by thermal emission from dust with a size of 100 nm or smaller. The total dust mass required to explain the observations was found to be equivalent to the mass of ~60 Halley comets. Derived sublimations lifetimes indicate that dust with a size of 100 nm or smaller is influenced by sublimation inward of ~0.9 AU. Radiations pressure values indicate that dust with a size of 1 µm or smaller is likely to be ejected from Vega and Fomalhaut. Thus, the production rate of dust close to Vega and Fomalhaut has to be large. It is also possible that the hot dust component is a transient phenomenon or that the observed excess brightness is caused by another process. In this work, dust near the Sun was considered for a comparison.