Exploring the formation by core accretion and the luminosity evolution of directly-imaged planets: The case of HIP 65426 b. (arXiv:1902.01869v1 [astro-ph.EP])

Exploring the formation by core accretion and the luminosity evolution of directly-imaged planets: The case of HIP 65426 b. (arXiv:1902.01869v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Marleau_G/0/1/0/all/0/1">Gabriel-Dominique Marleau</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Coleman_G/0/1/0/all/0/1">Gavin A. L. Coleman</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Leleu_A/0/1/0/all/0/1">Adrien Leleu</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Mordasini_C/0/1/0/all/0/1">Christoph Mordasini</a>

A low-mass companion to the two-solar mass star HIP65426 was recently
detected by SPHERE at around 100 au from its host. Explaining the presence of
super-jovian planets at large separations, as revealed by direct imaging, is
currently an open question.

We want to derive statistical constraints on the mass and initial entropy of
HIP65426b and to explore possible formation pathways of directly-imaged objects
within the core-accretion paradigm, focusing on HIP65426b.

Constraints on the planet’s mass and post-formation entropy are derived from
its age and luminosity combined with cooling models. For the first time, the
results of population synthesis are also used to inform the results. Secondly,
a formation model which includes N-body dynamics with several embryos per disc
is used to study possible formation histories and the properties of possible
additional companions. Finally, the outcomes of two- and three-planet
scattering in the post-disc phase are analysed, taking tides into account.

The mass of HIP65426b is found to be Mp = 9.9 +1.1 -1.8 MJ using the hot
population and Mp = 10.9 +1.4 -2.0 MJ with the cold-nominal population. Core
formation at small separations from the star followed by outward scattering and
runaway accretion at a few 100s of AU succeeds in reproducing the mass and
separation of HIP65426b. Alternatively, systems having two or more giant
planets close enough to be on an unstable orbit at disc dispersal are likely to
end up with one planet on a wide HIP65426b-like orbit with a relatively large
eccentricity (>~ 0.5).

If this scattering scenario explains its formation, HIP65426b is predicted to
have a high eccentricity and to be accompanied by one or several roughly
jovian-mass planets at smaller semimajor axes, which also could have a high
eccentricity. This could be tested by further observations via direct imaging
as well as radial velocity.

A low-mass companion to the two-solar mass star HIP65426 was recently
detected by SPHERE at around 100 au from its host. Explaining the presence of
super-jovian planets at large separations, as revealed by direct imaging, is
currently an open question.

We want to derive statistical constraints on the mass and initial entropy of
HIP65426b and to explore possible formation pathways of directly-imaged objects
within the core-accretion paradigm, focusing on HIP65426b.

Constraints on the planet’s mass and post-formation entropy are derived from
its age and luminosity combined with cooling models. For the first time, the
results of population synthesis are also used to inform the results. Secondly,
a formation model which includes N-body dynamics with several embryos per disc
is used to study possible formation histories and the properties of possible
additional companions. Finally, the outcomes of two- and three-planet
scattering in the post-disc phase are analysed, taking tides into account.

The mass of HIP65426b is found to be Mp = 9.9 +1.1 -1.8 MJ using the hot
population and Mp = 10.9 +1.4 -2.0 MJ with the cold-nominal population. Core
formation at small separations from the star followed by outward scattering and
runaway accretion at a few 100s of AU succeeds in reproducing the mass and
separation of HIP65426b. Alternatively, systems having two or more giant
planets close enough to be on an unstable orbit at disc dispersal are likely to
end up with one planet on a wide HIP65426b-like orbit with a relatively large
eccentricity (>~ 0.5).

If this scattering scenario explains its formation, HIP65426b is predicted to
have a high eccentricity and to be accompanied by one or several roughly
jovian-mass planets at smaller semimajor axes, which also could have a high
eccentricity. This could be tested by further observations via direct imaging
as well as radial velocity.

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