Astronomy and Astrophysics – Astronomy
Scientific paper
Sep 2011
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011ess.....2.1004a&link_type=abstract
American Astronomical Society, ESS meeting #2, #10.04
Astronomy and Astrophysics
Astronomy
Scientific paper
Recent observations that indicate that some extrasolar planets observed in transit can experience mass loss from their surfaces. Motivated by these findings, we consider outflows from Hot Jupiters in the regime where the flow is controlled by magnetic fields. Given the mass loss rates estimated from current observations (and from theory) magnetic fields will dominate the flow for planets with surface fields greater than 1 gauss, comparable to the fields of the Sun and Jupiter. More specifically, the magnetic pressure is larger than the ram pressure of the wind by a factor of 1,000,000 at the planet surface and a factor of 10,000 at the sonic surface. The problem can be separated into an inner regime, near the planet, where the outflow is launched, and an outer regime where the flow follows (primarily) stellar field lines and interacts with the stellar wind. We consider a variety of field configurations, but start with a dipole planetary field with a spatially constant background contribution from the star. For each field configuration, we construct a set of orthogonal coordinates that follow the field lines and determine the corresponding differential operators. Under the assumption of polytropic flow, we can analytically find the conditions required for escaping material to pass smoothly through the sonic transition, and can then estimate the mass outflow rates. These magnetically controlled outflows differ significantly from previous spherical models: The outflow rates are somewhat smaller, and the flow is launched primarily from the polar regions of the planet. In addition, if the stellar wind is strong enough, the flow could be reversed and the planet could gain mass from the star; this latter scenario may operate in the earliest (and hence most extreme) stages of evolution.
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