Abstract
The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures much lower (< 500 K) than the coolest brown dwarfs currently observed. These ultra-low mass substellar objects would have spectral types > T8.5 and so could belong to the proposed Y dwarf spectral sequence. The detection of a planet around a white dwarf would prove that such objects can survive the final stages of stellar evolution and place constraints on the frequency of planetary systems around their progenitors (with masses between 1.5 - 8 solar masses, i.e., early B to mid F). This paper presents the results of a multi-epoch J band common proper motion survey of 23 nearby equatorial and northern hemisphere white dwarfs. We rule out the presence of any common proper motion companions, with limiting masses determined from the completeness limit of each observation, to 18 white dwarfs. For the remaining five targets, the motion of the white dwarf is not sufficiently separated from the non-moving background objects in each field. These targets require additional observations to conclusively rule out the presence of any common proper motion companions. From our completeness limits, we tentatively suggest that < 5% of white dwarfs have substellar companions with effective temperatures > 500 K between projected physical separations of 60 - 200 AU.
Highlights
Imaging the extrasolar planets found in orbit around solar type stars is difficult as these faint companions are too close to their bright parent stars
11 white dwarfs were subject to previous searches for substellar companions (Table 4)
Tentative conclusions regarding the frequency of substellar and planetary mass companions to white dwarfs and their main-sequence progenitors at wide separations can be made
Summary
Imaging the extrasolar planets found in orbit around solar type stars is difficult as these faint companions are too close to their bright parent stars. As the 1.5–8 M progenitor stars of white dwarfs have spectral types of early B, A and mid-F, searching for planetary mass companions in orbit around white dwarfs allows the examination of a currently inadequately explored region of parameter space, supplying new information on the frequency and mass distribution of extrasolar planets around intermediate-mass main-sequence stars. The age of an extrasolar planet found in orbit around a white dwarf equals the sum of the main-sequence progenitor lifetime and the white dwarf cooling age Using this age and the distance to the white dwarf, Burleigh et al (2002) used evolutionary models for cool brown dwarfs and extrasolar planets (Burrows et al 1997) to make initial predictions of the IR magnitudes of planetary mass companions around white dwarfs. Where tMS is measured in Gyr (Wood 1992)
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