Obliquity Tides May Drive WASP-12b’s Rapid Orbital Decay. (arXiv:1812.01624v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Millholland_S/0/1/0/all/0/1">Sarah Millholland</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Laughlin_G/0/1/0/all/0/1">Gregory Laughlin</a>
Recent analyses have revealed a mystery. The orbital period of the highly
inflated hot Jupiter, WASP-12b, is decreasing rapidly. The rate of inspiral,
however, is too fast to be explained by either eccentricity tides or
equilibrium stellar tides. While dynamical stellar tides are possible, they
require a subgiant structure for the star, whereas stellar models point toward
a main sequence host. Here, we show that these hitherto irreconcilable
observations might be explained by planetary obliquity tides if planet b’s spin
vector is trapped in a high-obliquity state maintained by a secular spin-orbit
resonance with an unseen exterior perturbing planet. We derive constraints on
the obliquity ($epsilongtrsim50^{circ}$), reduced tidal quality factor
($Q^{prime}sim10^{6}-10^{7}$), and perturbing planet parameters
($M_{2}sim10-20M_{oplus}$, $a_2lesssim0.04,{rm AU}$) required to generate
the observed orbital decay. Direct N-body simulations that include tidal and
spin dynamics reinforce the plausibility of the scenario. Furthermore, we show
that the resonance could have been captured when planet b’s obliquity was
small, making the proposed sequence of events easy to explain. The hypothetical
perturbing planet is within the limits of current radial velocity constraints
on the system yet is also detectable. If it exists, it could provide evidence
in favor of the in situ formation hypothesis for hot Jupiters.
Recent analyses have revealed a mystery. The orbital period of the highly
inflated hot Jupiter, WASP-12b, is decreasing rapidly. The rate of inspiral,
however, is too fast to be explained by either eccentricity tides or
equilibrium stellar tides. While dynamical stellar tides are possible, they
require a subgiant structure for the star, whereas stellar models point toward
a main sequence host. Here, we show that these hitherto irreconcilable
observations might be explained by planetary obliquity tides if planet b’s spin
vector is trapped in a high-obliquity state maintained by a secular spin-orbit
resonance with an unseen exterior perturbing planet. We derive constraints on
the obliquity ($epsilongtrsim50^{circ}$), reduced tidal quality factor
($Q^{prime}sim10^{6}-10^{7}$), and perturbing planet parameters
($M_{2}sim10-20M_{oplus}$, $a_2lesssim0.04,{rm AU}$) required to generate
the observed orbital decay. Direct N-body simulations that include tidal and
spin dynamics reinforce the plausibility of the scenario. Furthermore, we show
that the resonance could have been captured when planet b’s obliquity was
small, making the proposed sequence of events easy to explain. The hypothetical
perturbing planet is within the limits of current radial velocity constraints
on the system yet is also detectable. If it exists, it could provide evidence
in favor of the in situ formation hypothesis for hot Jupiters.
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