archive

2019/12/18 (水)

UTAPwiki/セミナー/planet lunch
担当者：Yuting, Lu
時間　：12:15-13:00
場所　：理学部1号館908室
内容　： The detection of a dust disk around the white dwarf star G29-38 and transits from debris orbiting the white dwarf WD 1145+017 confirmed that the photospheric trace metals found in many white dwarfs arise from the accretion of tidally disrupted planetesimals. The composition of these planetesimals is similar to that of rocky bodies in the inner Solar System. Gravitational scattering of planetesimals towards the white dwarf requires the presence of more massive bodies, yet no planet has so far been detected at a white dwarf. Here we report optical spectroscopy of a hot (about 27,750 kelvin) white dwarf, WD J091405.30+191412.25, that is accreting from a circumstellar gaseous disk composed of hydrogen, oxygen and sulfur at a rate of about 3.3 × 109 grams per second. The composition of this disk is unlike all other known planetary debris around white dwarfs, but resembles predictions for the makeup of deeper atmospheric layers of icy giant planets, with H2O and H2S being major constituents. A giant planet orbiting a hot white dwarf with a semi-major axis of around 15 solar radii will undergo substantial evaporation with expected mass loss rates comparable to the accretion rate that we observe onto the white dwarf. The orbit of the planet is most probably the result of gravitational interactions, indicating the presence of additional planets in the system. We infer an occurrence rate of approximately 1 in 10,000 for spectroscopically detectable giant planets in close orbits around white dwarfs.

2019/12/11 (水)

UTAPwiki/セミナー/planet lunch
担当者：Yasushi, Suto
時間　：12:15-13:00
場所　：理学部1号館908室
内容　： The presentation on the first two papers is based on JC20191209 by Kazumi Kashiyama.

2019/12/4 (水)

UTAPwiki/セミナー/planet lunch
担当者：Toshinori, Hayashi
時間　：12:15-13:00
場所　：理学部1号館908室
内容　： We present a numerical study of the application of the Shannon entropy technique to the planar restricted three-body problem in the vicinity of first-order interior mean-motion resonances with the perturber. We estimate the diffusion coefficient for a series of initial conditions and compare the results with calculations obtained from the time evolution of the variance in the semimajor-axis and eccentricity plane. Adopting adequate normalization factors, both methods yield comparable results, although much shorter integration times are required for entropy calculations. A second advantage of the use of entropy is that it is possible to obtain reliable results even without the use of ensembles or analysis restricted to surfaces of section or representative planes. This allows for a much more numerically efficient tool that may be incorporated into a working N-body code and applied to numerous dynamical problems in planetary dynamics. Finally, we estimate instability times for a series of initial conditions in the 2/1 and 3/2 mean-motion resonances and compare them with times of escape obtained from directed N-body simulations. We find very good agreement in all cases, not only with respect to average values but also in their dispersion for near-by trajectories.

2019/11/27 (水)

UTAPwiki/セミナー/planet lunch
担当者：Yuta, Nakagawa
時間　：12:15-13:00
場所　：理学部1号館908室
内容　： WASP-12b is a transiting hot Jupiter on a 1.09-day orbit around a late-F star. Since the planet's discovery in 2008, the time interval between transits has been decreasing by 29±2 msec year−1. This is a possible sign of orbital decay, although the previously available data left open the possibility that the planet's orbit is slightly eccentric and is undergoing apsidal precession. Here, we present new transit and occultation observations that provide more decisive evidence for orbital decay, which is favored over apsidal precession by a ΔBIC of 22.3 or Bayes factor of 70,000. We also present new radial-velocity data that rule out the Rømer effect as the cause of the period change. This makes WASP-12 the first planetary system for which we can be confident that the orbit is decaying. The decay timescale for the orbit is P/P˙=3.25±0.23 Myr. Interpreting the decay as the result of tidal dissipation, the modified stellar tidal quality factor is Q′⋆=1.8×105.

2019/11/20 (水)

UTAPwiki/セミナー/planet lunch
担当者：Shijie, Wang
時間　：12:15-13:00
場所　：理学部1号館908室
内容　： As a natural consequence of the elementary processes of dust growth, we discovered that a new class of planets can be formed around supermassive black holes (SMBHs). We investigated a growth path from sub-micron sized icy dust monomers to Earth-sized bodies outside the "snow line", located several parsecs from SMBHs in low luminosity active galactic nuclei (AGNs). In contrast to protoplanetary disks, the "radial drift barrier" does not prevent the formation of planetesimals. In the early phase of the evolution, low collision velocity between dust particles promotes sticking; therefore, the internal density of the dust aggregates decreases with growth. When the porous aggregate's size reaches 0.1--1 cm, the collisional compression becomes effective, and the decrease in internal density stops. Once 10--100 m sized aggregates are formed, they are decoupled from gas turbulence, and the aggregate layer becomes gravitationally unstable, leading to the formation of planets by the fragmentation of the layer, with ten times the mass of the earth. The growth time scale depends on the turbulent strength of the circumnuclear disk and the black hole mass MBH, and it is comparable to the AGN's lifetime (∼108 yr) for low mass (MBH∼106M⊙) SMBHs.

2019/11/6 (水)

UTAPwiki/セミナー/planet lunch
担当者：Masataka, Aizawa
時間　：12:15-13:00
場所　：理学部1号館908室
内容　： Oxygen fugacity is a measure of rock oxidation that influences planetary structure and evolution. Most rocky bodies in the Solar System formed at oxygen fugacities approximately five orders of magnitude higher than a hydrogen-rich gas of solar composition. It is unclear whether this oxidation of rocks in the Solar System is typical among other planetary systems. We exploit the elemental abundances observed in six white dwarfs polluted by the accretion of rocky bodies to determine the fraction of oxidized iron in those extrasolar rocky bodies and therefore their oxygen fugacities. The results are consistent with the oxygen fugacities of Earth, Mars, and typical asteroids in the Solar System, suggesting that at least some rocky exoplanets are geophysically and geochemically similar to Earth.

2019/10/30 (水)

UTAPwiki/セミナー/planet lunch
担当者：Kazuhiro, Kanagawa
時間　：12:15-13:00
場所　：理学部1号館908室
内容　： Atmospheric chemical abundances of giant planets lead to important constraints on planetary formation and migration. Studies have shown that giant planets that migrate through the protoplanetary disc can accrete substantial amounts of oxygen-rich planetesimals, leading to supersolar metallicities in the envelope and solar or subsolar C/O ratios. Pebble accretion has been demonstrated to play an important role in core accretion and to have growth rates that are consistent with planetary migration. The high pebble accretion rates allow planetary cores to start their growth beyond 10 au and subsequently migrate to cold (≳1 au), warm (̃0.1-1 au) or hot (≲0.1 au) orbits. In this work we investigate how the formation of giant planets via pebble accretion influences their atmospheric chemical compositions. We find that under the standard pebble accretion scenario, where the core is isolated from the envelope, the resulting metallicities (O/H and C/H ratios) are subsolar, while the C/O ratios are supersolar. Planets that migrate through the disc to become hot Jupiters accrete substantial amounts of water vapour, but still acquire slightly subsolar O/H and supersolar C/O of 0.7-0.8. The metallicity can be substantially subsolar (̃0.2-0.5 × solar) and the C/O can even approach 1.0 if the planet accretes its envelope mostly beyond the CO2 ice line, I.e. cold Jupiters or hot Jupiters that form far out and migrate in by scattering. Allowing for core erosion yields significantly supersolar metallicities and solar or subsolar C/O, which can also be achieved by other means, e.g. photoevaporation and late-stage planetesimal accretion.

2019/10/9 (水)

UTAPwiki/セミナー/planet lunch
担当者：Shoji, Mori
時間　：12:15-13:00
場所　：理学部1号館908室
内容　： We report the detection of spiral substructure in both the gas velocity and temperature structure of the disk around TW~Hya, suggestive of planet-disk interactions with an unseen planet. Perturbations from Keplerian rotation tracing out a spiral pattern are observed in the SE of the disk, while significant azimuthal perturbations in the gas temperature are seen in the outer disk, outside 90~au, extending the full azimuth of the disk. The deviation in velocity is either Δvϕ/vkep∼0.1 or Δvz/vkep∼0.01 depending on whether the perturbation is in the rotational or vertical direction, while radial perturbations can be ruled out. Deviations in the gas temperature are ±4 K about the azimuthally averaged profile, equivalent to deviations of ΔTgas/Tgas∼0.05. Assuming all three structures can be described by an Archimedean spiral, measurements of the pitch angles of both velocity and temperature spirals show a radially decreasing trend for all three, ranging from 9∘ at 70 au, dropping to 3∘ at 200 au. Such low pitch-angled spirals are not readily explained through the wake of an embedded planet in the location of previously reported at 94 au, but rather require a launching mechanism which results in much more tightly wound spirals. Molecular emission tracing distinct heights in the disk is required to accurately distinguish between spiral launching mechanisms.

2019/10/2 (水)

UTAPwiki/セミナー/planet lunch
担当者：Yasushi, Suto
時間　：12:15-13:00
場所　：理学部1号館908室
内容　：
Asteroseismology has revealed that cores of red giants rotate about one order of magnitude faster than their convective envelopes. This paper attempts an explanation for this rotational state in terms of the theory of angular momentum transport in stellar convection zones. A differential rotation model based on the theory is applied to a sequence of evolutionary states of a red giant of one solar mass. The model computations show a rotation of about ten times faster in the cores compared to the stellar surface. This rotational state is caused by the non-diffusive downward convective transport of angular momentum. The contrast in rotational rates between core and envelope increases with the radius (age) of the star. Seismologically detected scaling for the spindown of the giants' cores is also reproduced.

2019/7/17 (水)

UTAPwiki/セミナー/planet lunch
担当者：Hayashi, Toshinori
時間　：12:15-13:00
場所　：理学部1号館908室
内容： We here revisit the essential problem of dynamical stability of planetary orbits around stellar binaries. We build on the coplanar three-body system of the Dvorak(1986), extending his stability diagram to both corotation and counter rotation of P-type orbits. His stability diagram express the change of stability across the gap between upper(UCO) and lower(LCO) critical orbits. By radius, this gap has a width of about 8% in the corotation case and 24% in the counter rotation case. As the gap of the second lies below the first, counter rotation is more stable, yet by width it is more chaotic. The gap between UCO and LCO follows a transition radius r+g=2.39+2.53e−1.40e2 and r−g=0.92−2.47e for corotation and respectively, counter rotation of the third body (the planet). Our r+g agrees with the same of Dvorak to within 0.35%. As a result, we discover r+g/r−g≲2.57 for all e. Around dim binaries, therefore, a relatively close in habitable zone may still be populated with planets on counter rotating orbits. The accurate numerical results presented here based on adaptive integration using MATLAB ODE45 may also serve as a novel benchmark of accurate \textit{N}-body integrators of exosolar systems more generally.

2019/7/10 (水)

UTAPwiki/セミナー/planet lunch
担当者：Hayashi, Toshinori
時間　：12:15-13:00
場所　：理学部1号館908室
内容：Exomoons orbiting terrestrial or super-terrestrial exoplanets have not yet been discovered; their possible existence and properties are therefore still an unresolved question. Here we explore the collisional formation of exomoons through giant planetary impacts. We make use of smooth particle hydrodynamical (SPH) collision simulations and survey a large phase-space of terrestrial/super-terrestrial planetary collisions. We characterize the properties of such collisions, including the debris mass, the long-term tidal-stability and the expected properties of the formed exomoons. We find our giant-impact models to generate relatively iron-rich massive disks, however even under the most optimistic assumptions these disks would not form exomoons exceeding ~Mars mass. Most of the modelled exomoons are likely to form beyond the synchronous radius of the planet, and would tidally evolve outward rather than be destroyed. We also find one rare case in which an exomoon forms through a graze \& capture scenario between a super-Earth and an Earth-sized planet. The result is an intact, planet-sized exomoon, containing about half the mass of the Earth. Our results suggest that it is extremely difficult to collisionally form currently-detectable exomoons orbiting super-terrestrial planets, through single giant impacts. It might be possible to form more massive, detectable exomoons through several mergers of smaller exomoons, formed by multiple impacts, however more studies are required in order to reach a conclusion. Given the current observational initiatives, the search should focus on more massive planet categories. However, typical exomoons predicted by our models are very likely to be detectable, given an order of magnitude improvement in the detection capability of future instruments.

2019/7/3 (水)

UTAPwiki/セミナー/planet lunch
担当者：Wang,Shijie
時間　：12:15-13:00
場所　：理学部1号館908室
内容：
The Galactic center is dominated by the gravity of a super-massive black hole (SMBH), Sagittarius A*, and is suspected to contain a sizable population of binary stars. Such binaries form hierarchical triples with the SMBH, undergoing Eccentric Kozai–Lidov (EKL) evolution, which can lead to high-eccentricity excitations for the binary companions' mutual orbit. This effect can lead to stellar collisions or Roche-lobe crossings, as well as orbital shrinking due to tidal dissipation. In this work we investigate the dynamical and stellar evolution of such binary systems, especially with regards to the binaries' post-main-sequence evolution. We find that the majority of binaries (~75%) is eventually separated into single stars, while the remaining binaries (~25%) undergo phases of common-envelope evolution and/or stellar mergers. These objects can produce a number of different exotic outcomes, including rejuvenated stars, G2-like infrared-excess objects, stripped giant stars, Type Ia supernovae (SNe), cataclysmic variables, symbiotic binaries, or compact object binaries. We estimate that, within a sphere of 250 Mpc radius, about 7.5–15 SNe Ia per year should occur in galactic nuclei due to this mechanism, potentially detectable by the Zwicky Transient Facility and ASAS-SN. Likewise, we estimate that, within a sphere of 1 Gpc3 volume, about 10–20 compact object binaries form per year that could become gravitational wave sources. Based on results of EKL-driven compact object binary mergers in galactic nuclei by Hoang et al., this compact object binary formation rate translates to about 15–30 events per year that are detectable by Advanced LIGO.

2019/6/26 (水)

UTAPwiki/セミナー/planet lunch
担当者：Aizawa, Masataka
時間　：12:15-13:00
場所　：理学部1号館908室

2019/6/19 (水)

UTAPwiki/セミナー/planet lunch
担当者：Nakagawa, Yuta
時間　：12:15-13:00
場所　：理学部1号館908室
内容：Thermal light-curve analysis is a powerful approach to probe the thermal structures of exoplanetary atmospheres, which are greatly influenced by the planetary obliquity and eccentricity. Here we investigate the thermal light curves of eccentric-tilted exoplanets across various radiative timescales, eccentricities, obliquities, and viewing geometries using results of shallow-water simulations presented in Ohno & Zhang. We also achieve an analytical theory of the thermal light curve that can explain general trends in the light curves of tilted exoplanets. For tilted planets in circular orbits, the orbital phase of the flux peak is largely controlled by either the flux from the hot spot projected onto the orbital plane or the pole heated at the summer solstice, depending on the radiative timescale τ rad, planetary day P orb, and obliquity θ. We find that tilted planets potentially produce the flux peak after the secondary eclipse when obliquity is θ gsim 90° for the hot regime τ rad Lt P rot or θ gsim 18° for the cool regime τ rad Gt P rot. For tilted planets in eccentric orbits, the shape of the light curve is considerably influenced by the heating at the periapse. The flux peak occurring after the secondary eclipse can be used to distinguish tilted planets from nontilted planets when the periapse takes place before the secondary eclipse. Our results could help to constrain exoplanet obliquities in future observations.

2019/6/12 (水)

UTAPwiki/セミナー/planet lunch
担当者：Mori, Shoji
時間　：12:15-13:00
場所　：理学部1号館908室
内容　：
The poor stickiness of silicate dust grains is a major obstacle to the formation of rocky planetesimals. In this study, we examine the possibility that silicate grains with an organic mantle, which we call Organic-mantled Grains (OMGs), form planetesimals through direct coagulation. Organic mantles are commonly found in interplanetary dust particles, and laboratory experiments show that they are softer than silicates, in particular in warm environments. This, combined with the theory of particle adhesion, implies that OMGs are stickier than bare silicate grains. Because organic mantles can survive up to 400 K, silicate grains inside the water snow line in protoplanetary disks can in principle hold such mantles. We construct a simple grain adhesion model to estimate the threshold collision velocity below which aggregates of OMGs can grow. The model shows that aggregates of 0.1 μm-sized OMGs can overcome the fragmentation barrier in protoplanetary disks if the mantles are as thick as those in interplanetary dust particles and if the temperature is above ∼200 K. We use this adhesion model to simulate the global evolution of OMG aggregates in the inner part of a protoplanetary disk, demonstrating that OMG aggregates can indeed grow into planetesimals under favorable conditions. Because organic matter is unstable at excessively high temperatures, rocky planetesimal formation by the direct sticking of OMGs is expected to occur in a disk annulus corresponding to the temperature range ∼200–400 K. The organic-rich planetesimals may grow into carbon-poor rocky planetesimals by accreting a large amount of carbon-poor chondrules.

2019/6/5 (水)

UTAPwiki/セミナー/planet lunch
担当者：Kanagawa, Kazuhiro
時間　：12:15-13:00
場所　：理学部1号館908室
内容　：
The composition of gas and solids in protoplanetary discs sets the composition of planets that form out of them. Recent chemical models have shown that the composition of gas and dust in discs evolves on Myr time-scales, with volatile species disappearing from the gas phase. However, discs evolve due to gas accretion and radial drift of dust on time-scales similar to these chemical time-scales. Here we present the first model coupling the chemical evolution in the disc mid-planes with the evolution of discs due to accretion and radial drift of dust. Our models show that transport will always overcome the depletion of CO2 from the gas phase, and can also overcome the depletion of CO and CH4 unless both transport is slow (viscous α ≲ 10−3) and the ionization rate is high (ζ ≈ 10−17). Including radial drift further enhances the abundances of volatile species because they are carried in on the surface of grains before evaporating left at their ice lines. Due to large differences in the abundances within 10 au for models with and without efficient radial drift, we argue that composition can be used to constrain models of planet formation via pebble accretion.

2019/5/29 (水)

UTAPwiki/セミナー/planet lunch
担当者：Suto, Yasushi
時間　：12:15-13:00
場所　：理学部1号館908室

2019/5/15 (水)

UTAPwiki/セミナー/planet lunch
担当者：Hayashi, Toshinori
時間　：12:15-13:00
場所　：理学部1号館908室
内容：To date more than 3500 exoplanets have been discovered orbiting a large variety of stars. Due to the sensitivity limits of the currently used detection techniques, these planets populate zones restricted either to the solar neighbourhood or towards the Galactic bulge. This selection problem prevents us from unveiling the true Galactic planetary population and is not set to change for the next two decades. Here we present a new detection method that overcomes this issue and that will allow us to detect gas giant exoplanets using gravitational wave astronomy. We show that the Laser Interferometer Space Antenna (LISA) mission can characterise hundreds of new circumbinary exoplanets orbiting white dwarf binaries everywhere in our Galaxy - a population of exoplanets so far completely unprobed - as well as detecting extragalactic bound exoplanets in the Magellanic Clouds. Such a method is not limited by stellar activity and, in extremely favourable cases, will allow LISA to detect super-Earths down to 10 Earth masses.

2019/5/8 (水)

UTAPwiki/セミナー/planet lunch
担当者：Nakagawa, Yuta
時間　：12:15-13:00
場所　：理学部1号館908室
内容 ：Relatively long-period, nonsynchronized planets—such as warm Jupiters—potentially retain the primordial rotation, eccentricity, and obliquity that might encapsulate information on planetary climate and formation processes. To date, there has not been a systematic study on climate patterns on these planets that will significantly influence their observations. Here we investigate the atmospheric dynamics of nonsynchronized, fast-rotating exoplanets across various radiative timescales, eccentricities, and obliquities using a shallow water model. The dynamical pattern can be demarcated into five regimes in terms of radiative timescale τ rad and obliquity θ. An atmosphere with τ rad shorter than a planetary day usually exhibits a strong day–night temperature contrast and a day-to-night flow pattern. In the intermediate τ rad regime between a planetary day and a year, the atmosphere is dominated by steady temperature and eastward jet patterns for θ ≤ 18° but shows a strong seasonal variation for θ ≥ 18° because the polar region undergoes an intense heating at around the summer solstice. If τ rad is larger than a year, seasonal variation is very weak. In this regime, eastward jets are developed for θ ≤ 54° and westward jets are developed for θ ≥ 54°. These dynamical regimes are also applicable to planets in eccentric orbits. The large effects of exoplanetary obliquities on circulation patterns might offer observational signatures, which will be investigated in Paper II of this study.

2019/4/24 (水)

UTAPwiki/セミナー/planet lunch
担当者：Aizawa, Masataka
時間　：12:15-13:00
場所　：理学部1号館908室
内容 ：Numerous exoplanets have been detected around Sun-like stars. These stars end their lives as white dwarfs, which should inherit any surviving planetary systems. Manser et al. found periodic shifts in emission lines from a disc of gas orbiting around a white dwarf (see the Perspective by Fossati). They used numerical simulations to show that the most likely explanation for the spectral shifts is a low-mass planet orbiting within the disc. The planet must be unusually small and dense to avoid being ripped apart by tidal forces. The authors speculate that it may be the leftover core of a planet whose outer layers have been removed.