SETI bioastro: On the Origin of HD149026b

From: LARRY KLAES (ljk4_at_msn.com)
Date: Thu Jul 13 2006 - 12:51:22 PDT

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    Astrophysics, abstract
    astro-ph/0607212

    From: Masahiro Ikoma [view email]

    Date: Tue, 11 Jul 2006 02:59:14 GMT (743kb)

    On the Origin of HD149026b

    Authors: M. Ikoma, T. Guillot, H. Genda, T. Takayuki, S. Ida

    Comments: 29 pages, 8 figures, to appear in the 10 October 2006 issue of ApJ

    The high density of the close-in extrasolar planet HD149026b suggests the
    presence of a huge core in the planet, which challenges planet formation
    theory. We first derive constraints on the amount of heavy elements and
    hydrogen/helium present in the planet: We find that preferred values of the
    core mass are between 50 and 80 M_E. We then investigate the possibility of
    subcritical core accretion as envisioned for Uranus and Neptune and find
    that the subcritical accretion scenario is unlikely in the case of HD149026b
    for at least two reasons: (i) Subcritical planets are such that the ratio of
    their core mass to their total mass is above ~0.7, in contradiction with
    constraints for all but the most extreme interior models of HD149026b; (ii)
    High accretion rates and large isolation mass required for the formation of
    a subcritical core of 30 M_E are possible only at specific orbital distances
    in a disk with a surface density of dust equal to at least 10 times that of
    the minimum mass solar nebula. This value climbs to 30 when considering a 50
    M_E core. These facts point toward two main routes for the formation of this
    planet: (i) Gas accretion that is limited by a slow viscous inflow of gas in
    an evaporating disk; (ii) A significant modification of the composition of
    the planet after as accretion has stopped. These two routes are not mutually
    exclusive. Illustrating the second route, we show that for a wide range of
    impact parameters, giant impacts lead to a loss of the gas component of the
    planet and thus may lead to planets that are highly enriched in heavy
    elements. In the giant impact scenario, we expect an outer giant planet to
    be present. Observational studies by imaging, astrometry and long term
    interferometry of this system are needed to better narrow down the ensemble
    of possibilities.

    http://arxiv.org/abs/astro-ph/0607212


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