Steady-state evolution of debris discs around solar-type stars

We present an analysis of debris disc data around Solar-type stars (spectral types F0-K5) using the steady-state analytical model of Wyatt et al. Models are fitted to published data from the FEPS project and various GTO programmes obtained with MIPS on the Spitzer Space Telescope at 24 and 70 mu m, and compared to a previously published analysis of debris discs around A stars using the same evolutionary model. We find that the model reproduces most features found in the data sets, noting that the model disc parameters for solar-type stars are different to those of A stars. Although this could mean that discs around Solar-type stars have properties different from their counterparts around earlier-type stars, it is also possible that the properties of discs around stars of different spectral types appear more different than they are because the blackbody disc radius underestimates the true disc radius by a factor X-r which varies with spectral type. We use results from realistic grain modelling to quantify this effect for solar-type stars and for A stars. Our results imply that planetesimals around solar-type stars are on average larger than around A stars by a factor of a few but that the mass of the discs are lower for discs around FGK stars, as expected. We also suggest that discrepancies between the evolutionary time-scales of 24-mu m statistics predicted by our model and that observed in previous surveys could be explained by the presence of two-component discs in the samples of those surveys, or by transient events being responsible for the 24-mu m emission of cold discs beyond a few Myr. Further study of the prevalence of two-component discs, and of constraints on X-r, and increasing the size of the sample of detected discs, are important for making progress on interpreting the evolution of discs around solar-type stars.