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S. Ida

Publications and source records attributed to S. Ida.

36 records · Page 2Linked to original sources
arXiv2010

A Population of Very-Hot Super-Earths in Multiple-Planet Systems Should be Uncovered by Kepler

We simulate a Kepler-like observation of a theoretical exoplanet population and we show that the observed orbital period distribution of the Kepler giant planet candidates is best matched by an average stellar specific dissipation function Q_* in the interval 10^6 ~< Q_* ~< 10^7. In that situation, the few super-Earths that are driven to orbital periods P < 1 day by dynamical interactions in multiple-planet systems will survive tidal disruption for a significant fraction of the main-sequence lifetimes of their stellar hosts. Consequently, though these very-hot super-Earths are not characteristic of the overall super-Earth population, their substantial transit probability implies that they should be significant contributors to the full super-Earth population uncovered by Kepler. As a result, the CoRoT-7 system may be the first representative of a population of very-hot super-Earths that we suggest should be found in multiple-planet systems preferentially orbiting the least-dissipative stellar hosts in the Kepler sample.

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arXiv2010

Toward a Deterministic Model of Planetary Formation VI: Dynamical Interaction and Coagulation of Multiple Rocky Embryos and Super-Earth Systems around Solar Type Stars

Radial velocity and transit surveys indicate that solar-type stars bear super-Earths, with mass and period up to ~ 20 M_E and a few months, are more common than those with Jupiter-mass gas giants. In many cases, these super-Earths are members of multiple-planet systems in which their mutual dynamical interaction has influenced their formation and evolution. In this paper, we modify an existing numerical population synthesis scheme to take into account protoplanetary embryos' interaction with their evolving natal gaseous disk, as well as their close scatterings and resonant interaction with each other. We show that it is possible for a group of compact embryos to emerge interior to the ice line, grow, migrate, and congregate into closely-packed convoys which stall in the proximity of their host stars. After the disk-gas depletion, they undergo orbit crossing, close scattering, and giant impacts to form multiple rocky Earths or super-Earths in non-resonant orbits around ~ 0.1AU with moderate eccentricities of ~0.01-0.1. We suggest that most refractory super-Earths with period in the range of a few days to weeks may have formed through this process. These super-Earths differ from Neptune-like ice giants by their compact sizes and lack of a substantial gaseous envelope.

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arXiv2009

On the various origins of close-in extrasolar planets

The extrasolar planets (EPs) so far detected are very different to the planets in our own Solar System. Many of them have Jupiter-like masses and close-in orbits (the so-called hot planets, HPs), with orbital periods of only a few days. In this paper, we present a new statistical analysis of the observed EPs, focusing on the origin of the HPs. Among the several HP formation mechanisms proposed so far, the two main formation mechanisms are type II migration and scattering. In both cases, planets form beyond the so-called snow-line of the protoplanetary disk and then migrate inward due to angular momentum and energy exchange with either the protoplanetary disk or with companion planets. Although theoretical studies produce a range of observed features, no firm correspondence between the observed EPs and models has yet been established. In our analysis, by means of principal component analysis and hierarchical cluster analysis, we find convincing indications for the existence of two types of HPs, whose parameters reflect physical mechanisms of type II migration and scattering.

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arXiv2008

Modification of Angular Velocity by Inhomogeneous MRI Growth in Protoplanetary Disks

We have investigated evolution of magneto-rotational instability (MRI) in protoplanetary disks that have radially non-uniform magnetic field such that stable and unstable regions coexist initially, and found that a zone in which the disk gas rotates with a super-Keplerian velocity emerges as a result of the non-uniformly growing MRI turbulence. We have carried out two-dimensional resistive MHD simulations with a shearing box model. We found that if the spatially averaged magnetic Reynolds number, which is determined by widths of the stable and unstable regions in the initial conditions and values of the resistivity, is smaller than unity, the original Keplerian shear flow is transformed to the quasi-steady flow such that more flattened (rigid-rotation in extreme cases) velocity profile emerges locally and the outer part of the profile tends to be super-Keplerian. Angular momentum and mass transfer due to temporally generated MRI turbulence in the initially unstable region is responsible for the transformation. In the local super-Keplerian region, migrations due to aerodynamic gas drag and tidal interaction with disk gas are reversed. The simulation setting corresponds to the regions near the outer and inner edges of a global MRI dead zone in a disk. Therefore, the outer edge of dead zone, as well as the inner edge, would be a favorable site to accumulate dust particles to form planetesimals and retain planetary embryos against type I migration.

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arXiv2008

The Signature of the Ice Line and Modest Type I Migration in the Observed Exoplanet Mass-Semimajor Axis Distribution

Existing exoplanet radial velocity surveys are complete in the planetary mass-semimajor axis (Mp-a) plane over the range 0.1 AU < a < 2.0 AU where Mp >~ 100 M_Earth. We marginalize over mass in this complete domain of parameter space and demonstrate that the observed semimajor axis distribution is inconsistent with models of planet formation that use the full Type I migration rate derived from a linear theory and that do not include the effect of the ice line on the disk surface density profile. However, the efficiency of Type I migration can be suppressed by both nonlinear feedback and the barriers introduced by local maxima in the disk pressure distribution, and we confirm that the synthesized Mp-a distribution is compatible with the observed data if we account for both retention of protoplanetary embryos near the ice line and an order-of-magnitude reduction in the efficiency of Type I migration. The validity of these assumption can be checked because they also predict a population of short-period rocky planets with a range of masses comparable to that of the Earth as well as a "desert" in the Mp-a distribution centered around Mp ~ 30-50 M_Earth and a < 1 AU. We show that the expected "desert" in the Mp-a plane will be discernible by a radial velocity survey with 1 m/s precision and n ~ 700 radial velocity observations of program stars.

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arXiv2008

Formation of Hot Planets by a combination of planet scattering, tidal circularization, and Kozai mechanism

We have investigated the formation of close-in extrasolar giant planets through a coupling effect of mutual scattering, Kozai mechanism, and tidal circularization, by orbital integrations. We have carried out orbital integrations of three planets with Jupiter-mass, directly including the effect of tidal circularization. We have found that in about 30% runs close-in planets are formed, which is much higher than suggested by previous studies. We have found that Kozai mechanism by outer planets is responsible for the formation of close-in planets. During the three-planet orbital crossing, the Kozai excitation is repeated and the eccentricity is often increased secularly to values close enough to unity for tidal circularization to transform the inner planet to a close-in planet. Since a moderate eccentricity can remain for the close-in planet, this mechanism may account for the observed close-in planets with moderate eccentricities and without nearby secondary planets. Since these planets also remain a broad range of orbital inclinations (even retrograde ones), the contribution of this process would be clarified by more observations of Rossiter-McLaughlin effects for transiting planets.

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arXiv2007

On gas drag in a circular binary system

We investigate both analytically and numerically the motion of massless particles orbiting primary star in a close circular binary system with particular focus on the gas drag effects. These are the first calculations with particles ranging in size from 1m to 10km, which account for the presence of a tidally perturbed gaseous disk. We have found numerically that the radial mass transport by the tidal waves plays a crucial role in the orbital evolution of particles. Numerical results are confirmed analytical calculations that do not assume anything about origin of the radial flow in the disk. We demonstrate that the migration rate of a particle in a disk out of radial equilibrium is enhanced due to the enhanced mass flux of gas colliding with the particle and the migration is always directed inward regardless of the sign of the radial gas velocity. Within the framework of the perturbation theory we derive general, formulae for short-term variations of the particle semi-major axis, eccentricity and inclination in such disk. The formulae account for departures from axial symmetry by introducing effective components of the gas velocity. They agree with numerical results within several percent. We have also found in numerical simulations that the tidal waves introduce coherence in periastron longitude and eccentricity for particles on neighbouring orbits. The degree of the coherence depends on the particle size and on the distance from the primary star, being most prominent for particles with 10m radius. The results are important mainly in the context of planetesimal formation and, to a lesser degree, during the early planetesimal accretion stage.

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arXiv2006

On the Origin of HD149026b

The high density of the close-in extrasolar planet HD149026b suggests the presence of a huge core in the planet, which challenges planet formation theory. We first derive constraints on the amount of heavy elements and hydrogen/helium present in the planet: We find that preferred values of the core mass are between 50 and 80 M_E. We then investigate the possibility of subcritical core accretion as envisioned for Uranus and Neptune and find that the subcritical accretion scenario is unlikely in the case of HD149026b for at least two reasons: (i) Subcritical planets are such that the ratio of their core mass to their total mass is above ~0.7, in contradiction with constraints for all but the most extreme interior models of HD149026b; (ii) High accretion rates and large isolation mass required for the formation of a subcritical core of 30 M_E are possible only at specific orbital distances in a disk with a surface density of dust equal to at least 10 times that of the minimum mass solar nebula. This value climbs to 30 when considering a 50 M_E core. These facts point toward two main routes for the formation of this planet: (i) Gas accretion that is limited by a slow viscous inflow of gas in an evaporating disk; (ii) A significant modification of the composition of the planet after as accretion has stopped. These two routes are not mutually exclusive. Illustrating the second route, we show that for a wide range of impact parameters, giant impacts lead to a loss of the gas component of the planet and thus may lead to planets that are highly enriched in heavy elements. In the giant impact scenario, we expect an outer giant planet to be present. Observational studies by imaging, astrometry and long term interferometry of this system are needed to better narrow down the ensemble of possibilities.

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arXiv2005

The N2K Consortium. II. A Transiting Hot Saturn Around HD 149026 With a Large Dense Core

Doppler measurements from Subaru and Keck have revealed radial velocity variations in the V=8.15, G0IV star HD 149026 consistent with a Saturn-Mass planet in a 2.8766 day orbit. Photometric observations at Fairborn Observatory have detected three complete transit events with depths of 0.003 mag at the predicted times of conjunction. HD 149026 is now the second brightest star with a transiting extrasolar planet. The mass of the star, based on interpolation of stellar evolutionary models, is 1.3 +/- 0.1 solar masses; together with the Doppler amplitude, K=43.3 m s^-1, we derive a planet mass Msin(i)=0.36 Mjup, and orbital radius of 0.042 AU. HD 149026 is chromospherically inactive and metal-rich with spectroscopically derived [Fe/H]=+0.36, Teff=6147 K, log g=4.26 and vsin(i)=6.0 km s^-1. Based on Teff and the stellar luminosity of 2.72 Lsun, we derive a stellar radius of 1.45 Rsun. Modeling of the three photometric transits provides an orbital inclination of 85.3 +/- 1.0 degrees and (including the uncertainty in the stellar radius) a planet radius of 0.725 +/- 0.05 Rjup. Models for this planet mass and radius suggest the presence of a ~67 Mearth core composed of elements heavier than hydrogen and helium. This substantial planet core would be difficult to construct by gravitational instability.

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arXiv2005

Towards a deterministic model of planetary formation. III. Mass distribution of short-period planets around stars of various masses

The origin of a recently discovered close-in Neptune-mass planet around GJ436 poses a challenge to the current theories of planet formation. Based on the sequential accretion hypothesis and the standard theory of gap formation and orbital migration, we show that around M dwarf stars, close-in Neptune-mass ice-giant planets may be relatively common, while close-in Jupiter-mass gas-giant planets are relatively rare. The mass distribution of close-in planets generally has two peaks at about Neptune mass and Jupiter mass. The lower-mass peak takes the maximum frequency for M dwarfs. Around more massive solar-type stars (G dwarfs), the higher-mass peak is much more pronounced. These are because planets tend to undergo type II migration after fully accreting gas around G dwarfs while they tend to migrate faster than gas accretion around M stars. Close-in Neptune-mass planets may also exist around G dwarfs, though they tend to be mostly composed of silicates and iron cores and their frequency is expected to be much smaller than that of Neptune-mass planets around M dwarfs and that of gas giants around G dwarfs. We also show that the conditions for planets' migration due to their tidal interaction with the disk and the stellar-mass dependence in the disk-mass distribution can be calibrated by the mass distribution of short-period planets around host stars with various masses.

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arXiv2004

A Hot Saturn Planet Orbiting HD 88133, from the N2K Consortium

The N2K consortium is carrying out a distributed observing campaign with the Keck, Magellan and Subaru telescopes, as well as the automatic photometric telescopes of Fairborn Observatory, in order to search for short-period gas giant planets around metal-rich stars. We have established a reservoir of more than 14,000 main sequence and subgiant stars, closer than 110 pc, brighter than V=10.5 and with $0.4 < B-V < 1.2$. Because the fraction of stars with planets is a sensitive function of stellar metallicity, a broadband photometric calibration has been developed to identify a subset of 2000 stars with [Fe/H] $> 0.1$ dex for this survey. We outline the strategy and report the detection of a planet orbiting the metal-rich G5IV star HD 88133 with a period of 3.41 days, semi-velocity amplitude, K=35.7 \mse and \msini = 0.29 \mjupe. Photometric observations reveal that HD 88133 is constant on the 3.415-day radial velocity period to a limit of 0.0005 mag. Despite a transit probability of 15.6%, our photometry rules out the shallow transits predicted by the large stellar radius.

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arXiv2004

The formation and retention of gas giant planets around stars with a range of metallicities

The apparent dependence of detection frequency of extrasolar planets on the metallicity of their host stars is investigated with Monte Carlo simulations using a deterministic core-accretion planet formation model. According to this model, gas giants formed and acquired their mass $M_{\rm p}$ through planetesimal coagulation followed by the emergence of cores onto which gas is accreted. These protoplanets migrate and attain their asymptotic semi-major axis $a$ through their tidal interaction with their nascent disk. Based on the observed properties of protostellar disks, we generate $M_{\rm p}$-$a$ distribution. Our results reproduce the observed lack of planets with intermediate mass $M_{\rm p} = 10$--100$M_{\oplus}$ and $a \la 3$AU and with large mass $M_{\rm p} \ga 10^3 M_{\oplus}$ and $a \la 0.2$AU. Based on the simulated $M_{\rm p}$-$a$ distributions, we also evaluate the metallicity dependence of fraction of stars harboring planets that are detectable with current radial velocity survey. If protostellar disks attain the same fraction of heavy elements which are contained in their host stars, the detection probability around metal-rich stars would be greatly enhanced because protoplanetary cores formed in them can grow to several Earth masses prior to their depletion. These large masses are required for the cores to initiate rapid gas accretion and to transform into giant planets. The theoretically extrapolated metallicity dependence is consistent with the observation. This correlation does not arise naturally in the gravitational-instability scenario. We also suggest other metallicity dependence of the planet distributions that can be tested by on-going observations.

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arXiv2003

Towards a Deterministic Model of Planetary Formation I: a Desert in the Mass and Semi Major Axis Distributions of Extra Solar Planets

We examine the accretion of cores of giant planets from planetesimals, gas accretion onto the cores, and their orbital migration. We adopt a working model for nascent protostellar disks with a wide variety of surface density distributions in order to explore the range of diversity among extra solar planetary systems. If some cores can acquire more mass than a critical value of several Earth masses during the persistence of the disk gas, they would be able to rapidly accrete gas and evolve into gas giant planets. The gas accretion process is initially regulated by the Kelvin-Helmholtz contraction of the planets' gas envelope. Based on the assumption that the exponential decay of the disk-gas mass occurs on the time scales $\sim 10^{6}-10^{7}$ years and that the disk mass distribution is comparable to those inferred from the observations of circumstellar disks of T Tauri stars, we carry out simulations to predict the distributions of masses and semi major axes of extra solar planets. Since planets' masses grow rapidly from $10 M_{\oplus}$ to $100 M_{\oplus}$, the gas giant planets rarely form with asymptotic masses in this intermediate range. Our model predicts a paucity of extra solar planets with mass in the range 10-$100 M_{\oplus}$ and semi major axis less than 3AU. We refer to this deficit as a ``planet desert''. The effect of migration is to sharpen the boundaries and to enhance the contrast of the planet desert. The mass and semi major axis distributions generated in our simulations for the gas giants are consistent with those of the known extra solar planets.

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arXiv2002

Eccentricity Evolution of Extrasolar Multiple Planetary Systems due to the Depletion of Nascent Protostellar Disks

Most extrasolar planets are observed to have eccentricities much larger than those in the solar system. Some of these planets have sibling planets, with comparable masses, orbiting around the same host stars. In these multiple planetary systems, eccentricity is modulated by the planets' mutual secular interaction as a consequence of angular momentum exchange between them. For mature planets, the eigenfrequencies of this modulation are determined by their mass and semi-major axis ratios. But, prior to the disk depletion, self gravity of the planets' nascent disks dominates the precession eigenfrequencies. We examine here the initial evolution of young planets' eccentricity due to the apsidal libration or circulation induced by both the secular interaction between them and the self gravity of their nascent disks. We show that as the latter effect declines adiabatically with disk depletion, the modulation amplitude of the planets' relative phase of periapse is approximately invariant despite the time-asymmetrical exchange of angular momentum between planets. However, as the young planets' orbits pass through a state of secular resonance, their mean eccentricities undergo systematic quantitative changes. For applications, we analyze the eccentricity evolution of planets around Upsilon Andromedae and HD168443 during the epoch of protostellar disk depletion. We find that the disk depletion can change the planets' eccentricity ratio. However, the relatively large amplitude of the planets' eccentricity cannot be excited if all the planets had small initial eccentricities.

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arXiv1999

Spatial Structure and Coherent Motion in Dense Planetary Rings Induced by Self-Gravitational Instability

We investigate the formation of spatial structure in dense, self-gravitating particle systems such as Saturn's B-ring through local $N$-body simulations to clarify the intrinsic physics based on individual particle motion. In such a system, Salo (1995) showed that the formation of spatial structure such as wake-like structure and particle grouping (clump) arises spontaneously due to gravitational instability and the radial velocity dispersion increases as the formation of the wake structure. However, intrinsic physics of the phenomena has not been clarified. We performed local $N$-body simulations including mutual gravitational forces between ring particles as well as direct (inelastic) collisions with identical (up to $N\sim40000$) particles. In the wake structure particles no longer move randomly but coherently. We found that particle motion was similar to Keplerian motion even in the wake structure and that the coherent motion was produced since the particles in a clump had similar eccentricity and longitude of perihelion. This coherent motion causes the increase and oscillation in the radial velocity dispersion. The mean velocity dispersion is rather larger in a more dissipative case with a smaller restitution coefficient and/or a larger surface density since the coherence is stronger in the more dissipative case. Our simulations showed that the wavelength of the wake structure was approximately given by the longest wavelength $\hsλ{cr} = 4π^2 GΣ/κ^2$ in the linear theory of axisymmetric gravitational instability in a thin disk, where $G$, $Σ$, and $κ$ are the gravitational constant, surface density, and a epicyclic frequency.

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arXiv1999

Evolution of the Velocity Dispersion of Self-Gravitating Particles in Disc Potentials

The ratio of the vertical velocity dispersion to radial one (sigma_z / sigma_R) of self-gravitating bodies in various disc potentials is investigated through two different numerical methods (statistical compilation of two-body encounters and N-body simulations). The velocity dispersion generated by two-body relaxation is considered. The ratio is given as a function of a disc potential parameter, kappa/Omega, where kappa and Omega are the epicycle and circular frequencies (the parameters kappa/Omega=1 and 2 correspond to Kepler rotation and solid-body rotation). For 1 <= kappa/Omega <~ 1.5, the velocity dispersion increases keeping some anisotropy (sigma_z / sigma_R ~ 0.5-0.7) if the amplitude of radial excursion is larger than tidal radius, while sigma_z / sigma_R << 1 for smaller amplitude. On the other hand, for 1.5 <~ kappa/Omega <= 2.0, we found isotropic state (sigma_z / sigma_R ~ 1) in the intermediate velocity regime, while anisotropic state (sigma_z / sigma_R < 1) still exists for higher and lower velocity regimes. The range of the intermediate velocity regime expands with kappa/Omega. In the limit of solid-body rotation, the regime covers all over the velocity space. Thus, the velocity dispersion generally has two different anisotropic states for each disc potential (1 <= kappa/Omega < 2) and one isotropic state for 1.5 <~ kappa/Omega < 2 where the individual states correspond to different amplitude of velocity dispersion, while in the limit of solid-body rotation (kappa/Omega = 2.0), entire velocity space is covered by the isotropic state.

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arXiv1999

Evidence for early stellar encounters in the orbital distribution of Edgeworth-Kuiper Belt objects

We have investigated effects of early stellar encounters on a protoplanetary disk (planetesimal disk) and found that they can explain the high eccentricities and inclinations observed in the outer part ($>42$AU) of the Edgeworth-Kuiper Belt (EKB). The proto-sun is considered as a member of a stellar aggregation that undergoes dissolution on a timescale $\sim 10^8$yrs, such that a planetesimal disk experiences a flyby encounter at pericenter distance ($q$) on the order of 100AU. We simulated dynamical evolution of a planetesimal (test particle) disk perturbed by a passing star. We show that the stellar encounter pumps the velocity dispersion in the disk in the outer parts ($> 0.25q$). Planet formation is forestalled in that region. We also find that a stellar encounter with pericenter distance $q \sim 100-200$AU could have pumped up the velocity dispersion of EKB objects outside 42AU to the observed magnitude while preserving that inside Neptune's 3:2 mean-motion resonance (located at 39.5AU), which allows for the efficient capture of objects by the sweeping of the 3:2 resonance during orbital migration by proto-Neptune.

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