Spitzer Space Telescope - General Observer Proposal #30434


The brown dwarf that was swallowed by a red giant and survived

Principal Investigator: Matt Burleigh
           Institution: University of Leicester

Co-Investigators: 
Paul Dobbie, University of Leicester (UK)                             
Ralf Napiwotzki, University of Hertfordshire (UK)                     
Pierre Maxted, University of Keele (UK)                               
Carolyn Brinkworth, Spitzer Science Center / JPL                      
Donald Hoard, Spitzer Science Center                                  
Stefanie Wachter, Spitzer Science Center                              

Science Category: circumstellar/debris disks 
 Observing Modes: IracMap 
  Hours Approved: 0.7 

Abstract:
We have discovered, through optical radial velocity measurements, the
first confirmed close white dwarf + brown dwarf binary.  Since the
system is double-lined, we are able to determine the companion's mass
as 0.055+/-0.005Msun, placing it firmly in the substellar regime.  The
orbital period is 115 minutes, and no mass transfer is taking place.
Therefore, the brown dwarf must have survived a previous common
envelope phase of evolution, during which it was engulfed by the red
giant precursor's envelope.  The discovery of this system raises the
possibility that lower mass substellar objects, including giant
planets, may also survive the red giant phase to become close
companions to white dwarfs.  Indeed, they may accrete a significant
amount of mass during this phase.  A small excess K band flux above
that expected from the white dwarf is found in 2MASS photometry,
enabling us to place a tentative upper limit on the brown dwarf's
spectral type as mid-L.  We request Spitzer IRAC photometry of the
binary to more accurately determine the brown dwarf's spectral type
(to one subclass), and to measure precise fluxes and an effective
temperature.  These data will be used to understand how this unique
binary has been created:  has the brown dwarf accreted a significant
amount of matter during the common envelope phase?  Was it originally
a lower mass brown dwarf or even a giant planet?  If yes, this has
implications for the fate of all planets and brown dwarfs in the post
main-sequence phase of evolution.