Paper: astro-ph/0512091
Date: Mon, 5 Dec 2005 09:41:44 GMT (49kb)

Title: Birth and fate of hot-Neptune planets

Authors: I. Baraffe, Y. Alibert, G. Chabrier, W. Benz

Comments: 4 figures, accepted for publication in Astronomy and Astrophysics
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This paper presents a consistent description of the formation and the
subsequent evolution of gaseous planets, with special attention to
short-period, low-mass hot-Neptune planets characteristic of $\mu$ Ara-like
systems. We show that core accretion including migration and disk evolution and
subsequent evolution taking into account irradiation and evaporation provide a
viable formation mechanism for this type of strongly irradiated light planets.
At an orbital distance $a \simeq$ 0.1 AU, this revised core accretion model
leads to the formation of planets with total masses ranging from $\sim$ 14
$\mearth$ (0.044 $\mjup$) to $\sim$ 400 $\mearth$ (1.25 $\mjup$). The newly
born planets have a dense core of $\sim$ 6 $\mearth$, independent of the total
mass, and heavy element enrichments in the envelope, $M_{\rm Z,env}/M_{\rm env}
$, varying from 10% to 80% from the largest to the smallest planets. We examine
the dependence of the evolution of the born planet on the evaporation rate due
to the incident XUV stellar flux. In order to reach a $\mu$ Ara-like mass
($\sim$ 14 $\mearth$) after $\sim $ 1 Gyr, the initial planet mass must range
from 166 $\mearth$ ($\sim$ 0.52 $\mjup$) to about 20 $\mearth$, for evaporation
rates varying by 2 orders of magnitude, corresponding to 90% to 20% mass loss
during evolution. The presence of a core and heavy elements in the envelope
affects appreciably the structure and the evolution of the planet and yields
$\sim 8%-9%$ difference in radius compared to coreless objects of solar
composition for Saturn-mass planets. These combinations of evaporation rates
and internal compositions translate into different detection probabilities, and
thus different statistical distributions for hot-Neptunes and hot-Jupiters.

\\ ( http://arXiv.org/abs/astro-ph/0512091 , 49kb)