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To understand how adiabatic compression influences mantle convection in super-Earths, we carried out linear stability analysis and non-linear numerical simulation of thermal convection for constant viscosity infinite Prandtl number fluid with both constant and pressure-dependent thermal expansivity. The mantle is basally heated and internal heating is not considered. In the case of constant thermal expansivity, thermal convection is totally inhibited in super-Earths of more than about 5 times the Earth’s mass owing to the strong effect of adiabatic compression, when the surface temperature is sufficiently high. Pressure-dependence of thermal expansivity is crucial for the onset of convection in massive super-Earths. Even when the thermal expansivity depends on pressure, our numerical simulation shows that the effect of adiabatic compression reduces the efficiency of convective heat transport by up to about 60%, depending on the planetary mass and the surface temperature. The reduction in the efficiency of convective heat transport makes cooling of the mantle more difficult in massive super-Earths, especially when the surface temperature is high. The surface temperature of a planet may affect its thermal history not only through its effects on the mechanical properties of convecting mantle materials, but also through its influence on adiabatic compression of convecting materials.

We evaluate a numerical model on the thermal evolution of terrestrial planets to estimate life-time of planetary intrinsic magnetic field for various mass planets. In this model, we take into account the pressure-dependency of density profile of the planet by using Birch-Murnaghun equation of state, and simulate thermal evolution of the planet by means of mixing length theory. According to our numerical results, the planetary mass must be between 0.1 and 1.4 Earth mass to sustain the intrinsic magnetic field for 4.5Gyr. If existence of intrinsic magnetic field were a key factor to make the planet habitable, the mass range above indicates that super-Earths would not be habitable.

We report the current status of the Infrared Doppler (IRD) instrument for the Subaru telescope, which aims at detecting Earth-like planets around nearby M darwfs via the radial velocity (RV) measurements. IRD is a fiber-fed, near infrared spectrometer which enables us to obtain high-resolution spectrum (R~70000) from 0.97 to 1.75 μm. We have been developing new technologies to achieve 1m/s RV measurement precision, including an original laser frequency comb as an extremely stable wavelength standard in the near infrared. To achieve ultimate thermal stability, very low thermal expansion ceramic is used for most of the optical components including the optical bench.

IRD is the near-infrared high-precision radial velocity instrument for the Subaru 8.2-m telescope. It is a relatively compact (~1m size) spectrometer with a new echelle-grating and Volume-Phase Holographic gratings covering 1-2 micron wavelengths combined with an original frequency comb using optical pulse synthesizer. The spectrometer will employ a 4096x4096-pixel HgCdTe array under testing at IfA, University of Hawaii. Both the telescope/Adaptive Optics and comb beams are fed to the spectrometer via optical fibers, while the instrument is placed at the Nasmyth platform of the Subaru telescope. Expected accuracy of the Doppler-shifted velocity measurements is about 1 m s-1. Helped with the large collecting area and high image quality of the Subaru telescope, IRD can conduct systematic radial velocity surveys of nearby middle-to-late M stars aiming for down to one Earth-mass planet. Systematic observational and theoretical studies of M stars and their planets for the IRD science are also ongoing. We will report the design and preliminary development progresses of the whole and each component of IRD.

We have numerically studied the thermal evolution of various-mass terrestrial planets in habitable zones, focusing on duration of dynamo activity to generate their intrinsic magnetic fields, which may be one of key factors in habitability on the planets. In particular, we are concerned with super-Earths, observations of which are rapidly developing. We calculated evolution of temperature distributions in planetary interior, using Vinet equations of state, Arrhenius-type formula for mantle viscosity, and the astrophysical mixing length theory for convective heat transfer modified for mantle convection. After calibrating the model with terrestrial planets in the Solar system, we apply it for 0.1--$10M_{\oplus}$ rocky planets with surface temperature of $300~\mbox{K}$ (in habitable zones) and the Earth-like compositions. With the criterion for heat flux at the CMB (core-mantle boundary), the lifetime of the magnetic fields is evaluated from the calculated thermal evolution. We found that the lifetime slowly increases with the planetary mass ($M_p$) independent of initial temperature gap at the core-mantle boundary ($\Delta T_{\rm CMB}$) but beyond a critical value $M_{c,p}$ ($\sim O(1)M_{\oplus}$) it abruptly declines by the mantle viscosity enhancement due to the pressure effect. We derived $M_{c,p}$ as a function of $\Delta T_{\rm CMB}$ and a rheological parameter (activation volume, $V^*$). Thus, the magnetic field lifetime of super-Earths with $M_p > M_{p,c}$ sensitively depends on $\Delta T_{\rm CMB}$, which reflects planetary accretion, and $V^*$, which has uncertainty at very high pressure. More advanced high-pressure experiments and first-principle simulation as well as planetary accretion simulation are needed to discuss habitability of super-Earths., Comment: 19pages, 15 figures, accepted for publication in ApJ

It is expected that many Super‐Earths will be detected by upcoming space transit surveys such as the Kepler mission. Since the probability of planetary transit is highly proportional to the orbital radius of a planet, the planets detected via the transit survey may include a lot of hot Super‐Earths, which are terrestrial planets orbiting very close to their central stars and with high surface temperatures around 1000 K.In this study, we model the internal thermal evolution of such hot Super‐Earths. In our model, the internal density structure is computed taking into account the compressibility of rocks and metal (assuming an Earth‐like composition), and the internal thermal transfer through the rocky mantle is simulated by using a mixing length theory modified for the mantle convection that takes into account the temperature‐ and pressure‐dependency of mantle viscosity. As the result of these calculations, we have evaluated the lifetime of magnetic fields of various hot Super‐Earths as a function of plane...

Numerical models are presented to clarify how adiabatic compression affects thermal convection in the mantle of super-Earths ten times the Earth's mass. The viscosity strongly depends on temperature, and the Rayleigh number is much higher than that of the Earth's mantle. The strong effect of adiabatic compression reduces the activity of mantle convection; hot plumes ascending from the bottom of the mantle lose their thermal buoyancy in the middle of the mantle owing to adiabatic decompression, and do not reach the surface. A thick lithosphere, as thick as 0.1 times the depth of the mantle, develops along the surface boundary, and the efficiency of convective heat transport measured by the Nusselt number is reduced by a factor of about four compared with the Nusselt number for thermal convection of incompressible fluid. The strong effect of adiabatic decompression is likely to inhibit hot spot volcanism on the surface and is also likely to affect the thermal history of the mantle, and hence, the generation of magnetic field in super-Earths.