An unbiased study of debris discs around A-type stars with Herschel

The Herschel DEBRIS (Disc Emission via a Bias-free Reconnaissance in the Infrared/Submillimetre) survey brings us a unique perspective on the study of debris discs around main-sequence A-type stars. Bias-free by design, the survey offers a remarkable data set with which to investigate the cold disc properties. The statistical analysis of the 100 and 160 mu m data for 86 main-sequence A stars yields a lower than previously found debris disc rate. Considering better than 3 sigma excess sources, we find a detection rate >= 24 +/- 5 per cent at 100 mu m which is similar to the debris disc rate around main-sequence F/G/K-spectral type stars. While the 100 and 160 mu m excesses slowly decline with time, debris discs with large excesses are found around some of the oldest A stars in our sample, evidence that the debris phenomenon can survive throughout the length of the main sequence (similar to 1 Gyr). Debris discs are predominantly detected around the youngest and hottest stars in our sample. Stellar properties such as metallicity are found to have no effect on the debris disc incidence. Debris discs are found around A stars in single systems and multiple systems at similar rates. While tight and wide binaries (<1 and >100 au, respectively) host debris discs with a similar frequency and global properties, no intermediate separation debris systems were detected in our sample.