Thursday, February 7, 2013

1302.1517 (G. -D. Marleau et al.)

Constraining the initial entropy of directly-detected exoplanets    [PDF]

G. -D. Marleau, A. Cumming
The initial entropy S_i of a gas giant planet is a key witness to its formation history and a crucial quantity for its early evolution. However, formation models are not yet able to reliably predict S_i, making an observational determination of initial entropies essential in order to guide formation scenarios. Using a grid of models in mass and entropy, we show how to place joint constraints on the mass and initial entropy of an object from its observed luminosity and age. Moreover, we demonstrate that with mass information, e.g.\ from dynamical stability analyses or radial velocity, tighter bounds can be set on the initial entropy. We apply this procedure to 2M1207 b and find that its initial entropy is at least 9.2 $\Sunits$, assuming that it does not burn deuterium. For the planets of the HR 8799 system, we infer that they must have formed with $\Si > 9.2 \Sunits$, independent of the age uncertainties for the star. Finally, a similar analysis for $\beta$ Pic b reveals that it must have formed with $\Si >10.5 \Sunits$, using the radial-velocity mass upper limit. These initial entropy values are respectively ca.\ 0.7, 0.5, and 1.5 $\Sunits$ higher than the ones obtained from core accretion models by Marley et al.,thereby {\em quantitatively} ruling out the coldest starts for these objects and constraining warm starts, especially for $\beta$ Pic b.
View original: http://arxiv.org/abs/1302.1517

No comments:

Post a Comment