The search for substellar common proper motion companions to white dwarfs

A large sample of white dwarfs within the solar neighbourhood has been examined to search for resolved common proper motion companions. The luminosity of these white dwarfs make them ideal candidates for detecting low mass objects such as brown dwarfs and gas giant planets. Theoretical predictions generally agree that a star will consume and destroy close-in, low mass planets as it ascends the red giant and asymptotic giant branch evolutionary tracks, but larger mass objects and those further out will survive. A substellar companion detected around a white dwarf would prove that it could survive the final stages of stellar evolution and place constraints on the frequency of planetary systems around their progenitors. Ultra cool brown dwarfs are particularly interesting due to their scarcity and offer a unique opportunity to test properties predicted for them by atmospheric models. They provide a crucial link between the colder gas giant planets and hotter T spectral type brown dwarfs that can be imaged orbiting nearby stars. Possible companions were identified from the reduction and analysis of archived Spitzer data. Their masses and temperatures were determined using their apparent magnitudes and the COND evolutionary models. The results present evidence of the detection of potential companions to five white dwarfs; four of these companions have masses within the range associated with giant planets. It is inferred that < 15% of white dwarfs have unresolved companions with masses between 5 - 13 MJup while < 8% of white dwarfs have companions with masses above the deuterium burning limit (~ 13 MJup). Also, the search could detect companions with effective temperatures in the range of 300 - 450 K (later Y spectral type) for < 45% of the white dwarf targets. Future work should confirm these objects and extend the search to other white dwarfs taken from the Spitzer Data Archive.