Incidence of debris discs around FGK stars in the solar neighbourhood

Abstract

Context. Debris discs are a consequence of the planet formation process and constitute the fingerprints of planetesimal systems. Their counterparts in the solar system are the asteroid and Edgeworth-Kuiper belts. Aims. The aim of this paper is to provide robust numbers for the incidence of debris discs around FGK stars in the solar neighbourhood. Methods. The full sample of 177 FGK stars with d <= 20 pc proposed for the DUst around Nearby Stars (DUNES) survey is presented. Herschel/PACS observations at 100 and 160 mu m were obtained, and were complemented in some cases with data at 70 mu m and at 250, 350, and 500 mu m SPIRE photometry. The 123 objects observed by the DUNES collaboration were presented in a previous paper. The remaining 54 stars, shared with the Disc Emission via a Bias-free Reconnaissance in IR and Sub-mm (DEBRIS) consortium and observed by them, and the combined full sample are studied in this paper. The incidence of debris discs per spectral type is analysed and put into context together with other parameters of the sample, like metallicity, rotation and activity, and age. Results. The subsample of 105 stars with d <= 15 pc containing 23 F, 33 G, and 49 K stars is complete for F stars, almost complete for G stars, and contains a substantial number of K stars from which we draw solid conclusions on objects of this spectral type. The incidence rates of debris discs per spectral type are 0.26(-0.14)(+0.21) (6 objects with excesses out of 23 F stars), 0.21(-0.11)(+0.17) (7 out of 33 G stars), and 0.20(-0.09)(+0.14) (10 out of 49 K stars); the fraction for all three spectral types together is 0.22(-0.07)(+0.08) (23 out of 105 stars). The uncertainties correspond to a 95% confidence level. The medians of the upper limits of L-dust/L-* for each spectral type are 7.8 x 10(-7) (F), 1.4 x 10(-6) (G), and 2.2 x 10(-6) (K); the lowest values are around 4.0 x 10(-7). The incidence of debris discs is similar for active (young) and inactive (old) stars. The fractional luminosity tends to drop with increasing age, as expected from collisional erosion of the debris belts.