Protostars and Planets VI, Heidelberg, July 15-20, 2013
Poster 2B068
The bright end of the exo-Zodi luminosity function: Disk evolution and implications for exo-Earth detectability
Kennedy, Grant (Institute of Astronomy, University of Cambridge)
Wyatt, Mark (Institute of Astronomy, University of Cambridge)
Abstract:
We present the first characterisation of the 12$um warm dust (``exo-Zodi\'\') luminosity
function around Sun-like stars, focussing on the dustiest systems that can be identified
by the WISE mission. We use the sample of main-sequence stars observed by Hipparcos
within 150pc as an unbiased sample, and report the detection of six new warm dust
candidates. The ages of five of these new sources are unknown, meaning that they may be
sites of terrestrial planet formation or rare analogues of other old warm dust
systems. We show that the dustiest old (>Gyr) systems such as BD+20 307 are 1 in 10,000
occurrences. Bright warm dust is much more common around young (<120Myr) systems, with a
~1% occurrence rate. We show that a two component in situ model where all stars have
initially massive warm disks and in which warm debris is also generated at some random
time along the stars\' main-sequence lifetime, perhaps due to a collision, can explain the
observations. However, if all stars only have initially massive warm disks these would
not be visible at Gyr ages, and random collisions on the main-sequence are too infrequent
to explain the high disk occurrence rate for young stars. That is, neither component can
explain the observations on their own. Despite these conclusions, we cannot rule out an
alternative dynamical model in which comets are scattered in from outer regions because
the distribution of systems with the appropriate dynamics is unknown. Our in situ model
predicts that the fraction of stars with exo-Zodi bright enough to cause problems for
future exo-Earth imaging attempts is at least roughly 10%, and is higher for populations
of stars younger than a few Gyr. This prediction of roughly 10% also applies to old stars
because bright systems like BD+20 307 imply a population of fainter systems that were
once bright, but are now decaying through fainter levels. Our prediction should be
strongly tested by the Large Binocular Telescope Interferometer, which will provide
valuable constraints and input for more detailed evolution models. A detection fraction
lower than our prediction could indicate that the hot dust in systems like BD+20 307 has
a cometary origin due to the quirks of the planetary dynamics. Population models of comet
delivery need to be developed to help distinguish between different possible origins of
warm dust.
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