Hot Super-Earths with Hydrogen Atmospheres: A Model Explaining Their Paradoxical Existence. (arXiv:1912.05884v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Modirrousta_Galian_D/0/1/0/all/0/1">Darius Modirrousta-Galian</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Locci_D/0/1/0/all/0/1">Daniele Locci</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Tinetti_G/0/1/0/all/0/1">Giovanna Tinetti</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Micela_G/0/1/0/all/0/1">Giuseppina Micela</a>
In this paper we propose a new mechanism that could explain the survival of
hydrogen atmospheres on some hot super-Earths. We argue that on close-orbiting
tidally-locked super-Earths the tidal forces with the orbital and rotational
centrifugal forces can partially confine the atmosphere on the nightside.
Assuming a super terran body with an atmosphere dominated by volcanic species
and a large hydrogen component, the heavier molecules can be shown to be
confined within latitudes of $lesssim 80^{circ}$ whilst the volatile hydrogen
is not. Because of this disparity the hydrogen has to slowly diffuse out into
the dayside where XUV irradiation destroys it. For this mechanism to take
effect it is necessary for the exoplanet to become tidally locked before losing
the totality of its hydrogen envelop. Consequently, for super-Earths with this
proposed configuration it is possible to solve the tidal-locking and mass-loss
timescales in order to constrain their formation `birth’ masses. Our model
predicts that 55 Cancri e formed with a day-length between approximately
$17-18.5$ hours and an initial mass less than $rm sim12 M_{oplus}$ hence
allowing it to become tidally locked before the complete destruction of its
atmosphere. For comparison, CoRoT-7b, an exoplanet with very similar properties
to 55 Cancri e but lacking an atmosphere, formed with a day-length
significantly different from $sim 20.5$ hours whilst also having an initial
mass smaller than $rm sim9 M_{oplus}$
In this paper we propose a new mechanism that could explain the survival of
hydrogen atmospheres on some hot super-Earths. We argue that on close-orbiting
tidally-locked super-Earths the tidal forces with the orbital and rotational
centrifugal forces can partially confine the atmosphere on the nightside.
Assuming a super terran body with an atmosphere dominated by volcanic species
and a large hydrogen component, the heavier molecules can be shown to be
confined within latitudes of $lesssim 80^{circ}$ whilst the volatile hydrogen
is not. Because of this disparity the hydrogen has to slowly diffuse out into
the dayside where XUV irradiation destroys it. For this mechanism to take
effect it is necessary for the exoplanet to become tidally locked before losing
the totality of its hydrogen envelop. Consequently, for super-Earths with this
proposed configuration it is possible to solve the tidal-locking and mass-loss
timescales in order to constrain their formation `birth’ masses. Our model
predicts that 55 Cancri e formed with a day-length between approximately
$17-18.5$ hours and an initial mass less than $rm sim12 M_{oplus}$ hence
allowing it to become tidally locked before the complete destruction of its
atmosphere. For comparison, CoRoT-7b, an exoplanet with very similar properties
to 55 Cancri e but lacking an atmosphere, formed with a day-length
significantly different from $sim 20.5$ hours whilst also having an initial
mass smaller than $rm sim9 M_{oplus}$
http://arxiv.org/icons/sfx.gif
