Do two-temperature debris discs have multiple belts?

Abstract

We present a study of debris discs whose spectra are well modelled by dust emission at two different temperatures. These discs are typically assumed to be a sign of multiple belts, which in only a few cases have been confirmed via high-resolution observations. We first compile a sample of two-temperature discs to derive their properties, summarized by the ratios of the warm and cool component temperatures and fractional luminosities. The ratio of warm to cool temperatures is constant in the range 2-4, and the temperature of both warm and cool components increases with stellar mass. We then explore whether this emission can arise from dust in a single narrow belt, with the range of temperatures arising from the size variation of grain temperatures. This model can produce two-temperature spectra for Sun-like stars, but is not supported where it can be tested by observed disc sizes and far-infrared/mm spectral slopes. Therefore, while some two-temperature discs arise from single belts, it is probable that most have multiple spatial components. These discs are plausibly similar to the outer Solar system's configuration of Asteroid and Edgeworth-Kuiper belts separated by giant planets. Alternatively, the inner component could arise from inward scattering of material from the outer belt, again due to intervening planets. In either case, we suggest that the ratio of warm/cool component temperatures is indicative of the scale of outer planetary systems, which typically span a factor of about 10 in radius.