White dwarfs whose atmospheres are polluted by terrestrial-like planetary debris have become a powerful and unique tool to study evolved planetary systems .
This paper presents results for an unbiased Spitzer IRAC search for circumstellar dust orbiting a homogeneous and well-defined sample of 134 single white dwarfs .
The stars were selected without regard to atmospheric metal content but were chosen to have 1 ) hydrogen rich atmospheres , 2 ) 17 000 K < T _ { \mathrm { eff } } < 25 000 K and correspondingly young post main-sequence ages of 15–270 Myr , and 3 ) sufficient far-ultraviolet brightness for a corresponding Hubble Space Telescope COS Snapshot .
Five white dwarfs were found to host an infrared bright dust disc , three previously known , and two reported here for the first time , yielding a nominal 3.7 ^ { +2.4 } _ { -1.0 } % of white dwarfs in this post-main sequence age range with detectable circumstellar dust .
Remarkably , the complementary Hubble observations indicate that a fraction of 27 % show metals in their photosphere that can only be explained with ongoing accretion from circumstellar material , indicating that nearly 90 % of discs escape detection in the infrared , likely due to small emitting surface area .
This paper also presents the distribution of disc fractional luminosity as a function of cooling age for all known dusty white dwarfs , suggesting possible disc evolution scenarios and indicating an undetected population of circumstellar discs .