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Register a new userClose encounters of a rotating star with planets in parabolic orbits of varying inclination and the formation of Hot Jupiters
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(abbreviated) We extend the theory of close encounters of a planet on a parabolic orbit with a star to include the effects of tides induced on the central rotating star. Orbits with arbitrary inclination to the stellar rotation axis are considered. We obtain results both from an analytic treatment and numerical one that are in satisfactory agreement. These results are applied to the initial phase of the tidal circularisation problem. We find that both tides induced in the star and planet can lead to a significant decrease of the orbital semi-major axis for orbits having periastron distances smaller than 5-6 stellar radii (corresponding to periods $sim 4-5$ days after the circularisation has been completed) with tides in the star being much stronger for retrograde orbits compared to prograde orbits. We use the simple Skumanich law for the stellar rotation with its rotational period equal to one month at the age of 5Gyr. The strength of tidal interactions is characterised by circularisation time scale, $t_{ev}$ defined as a time scale of evolution of the planets semi-major axis due to tides considered as a function of orbital period $P_{obs}$ after the process of tidal circularisation has been completed. We find that the ratio of the initial circularisation time scales corresponding to prograde and retrograde orbits is of order 1.5-2 for a planet of one Jupiter mass and $P_{obs}sim $ four days. It grows with the mass of the planet, being of order five for a five Jupiter mass planet with the same $P_{orb}$. Thus, the effect of stellar rotation may provide a bias in the formation of planetary systems having planets on close orbits around their host stars, as a consequence of planet-planet scattering, favouring systems with retrograde orbits. The results may also be applied to the problem of tidal capture of stars in young stellar clusters.
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Aims: We aim to investigate the consequences of a fast massive stellar remnant - a black hole (BH) or a neutron star (NS) - encountering a planetary system. Methods: We modelled a close encounter between the actual Solar System (SS) and a $2,M_odot$ NS and a $10,M_odot$ BH, using a few-body symplectic integrator. We used a range of impact parameters, orbital phases at the start of the simulation derived from the current SS orbital parameters, encounter velocities, and incidence angles relative to the plane of the SS. Results: We give the distribution of possible outcomes, such as when the SS remains bound, when it suffers a partial or complete disruption, and in which cases the intruder is able to capture one or more planets, yielding planetary systems around a BH or a NS. We also show examples of the long-term stability of the captured planetary systems.
Jovian planet formation has been shown to be strongly correlated with host star metallicity, which is thought to be a proxy for disk solids. Observationally, previous works have indicated that jovian planets preferentially form around stars with solar and super solar metallicities. Given these findings, it is challenging to form planets within metal-poor environments, particularly for hot Jupiters that are thought to form via metallicity-dependent core accretion. Although previous studies have conducted planet searches for hot Jupiters around metal-poor stars, they have been limited due to small sample sizes, which are a result of a lack of high-quality data making hot Jupiter occurrence within the metal-poor regime difficult to constrain until now. We use a large sample of halo stars observed by TESS to constrain the upper limit of hot Jupiter occurrence within the metal-poor regime (-2.0 $leq$ [Fe/H] $leq$ -0.6). Placing the most stringent upper limit on hot Jupiter occurrence, we find the mean 1-$sigma$ upper limit to be 0.18 $%$ for radii 0.8 -2 R$_{rm{Jupiter}}$ and periods $0.5- 10$ days. This result is consistent with previous predictions indicating that there exists a certain metallicity below which no planets can form.
The discovery of planetary systems outside of the solar system has challenged some of the tenets of planetary formation. Among the difficult-to-explain observations, are systems with a giant planet orbiting a very-low mass star, such as the recently discovered GJ~3512b planetary system, where a Jupiter-like planet orbits an $M$-star in a tight and eccentric orbit. Systems such as this one are not predicted by the core accretion theory of planet formation. Here we suggest a novel mechanism, in which the giant planet is born around a more typical Sun-like star ($M_{*,1}$), but is subsequently exchanged during a dynamical interaction with a flyby low-mass star ($M_{*,2}$). We perform state-of-the-art $N$-body simulations with $M_{*,1}=1M_odot$ and $M_{*,2}=0.1M_odot$ to study the statistical outcomes of this interaction, and show that exchanges result in high eccentricities for the new orbit around the low-mass star, while about half of the outcomes result in tighter orbits than the planet had around its birth star. We numerically compute the cross section for planet exchange, and show that an upper limit for the probability per planetary system to have undergone such an event is $Gammasim 4.4(M_{rm c}/100M_odot)^{-2}(a_{rm p}/{rm AU}) (sigma/1,{rm km},{rm s}^{-1})^{5}$Gyr$^{-1}$, where $a_{rm p}$ is the planet semi-major axis around the birth star, $sigma$ the velocity dispersion of the star cluster, and $M_{rm c}$ the total mass of the star cluster. Hence these planet exchanges could be relatively common for stars born in open clusters and groups, should already be observed in the exoplanet database, and provide new avenues to create unexpected planetary architectures.
The discovery of high incidence of hot Jupiters in dense clusters challenges the field-based hot Jupiter formation theory. In dense clusters, interactions between planetary systems and flyby stars are relatively common. This has a significant impact on planetary systems, dominating hot Jupiter formation. In this paper, we perform high precision, few-body simulations of stellar flybys and subsequent planet migration in clusters. A large parameter space exploration demonstrates that close flybys that change the architecture of the planetary system can activate high eccentricity migration mechanisms: Lidov-Kozai and planet-planet scattering, leading to high hot Jupiter formation rate in dense clusters. Our simulations predict that many of the hot Jupiters are accompanied by ultra-cold Saturns, expelled to apastra of thousands of AU. This increase is particularly remarkable for planetary systems originally hosting two giant planets with semi-major axis ratios $sim$ 4 and the flyby star approaching nearly perpendicular to the planetary orbital plane. The estimated lower limit to the hot Jupiter formation rate of a virialized cluster is $sim 1.6times10^{-4}({sigma}/{rm 1kms^{-1}})^5({a_{rm p}}/{rm 20 AU})({M_{rm c}}/{rm 1000M_odot})^{-2}$Gyr$^{-1}$ per star, where $sigma$ is the cluster velocity dispersion, $a_{rm p}$ is the size of the planetary system and $M_{rm c}$ is the mass of the cluster. Our simulations yield a hot Jupiter abundance which is $sim$ 50 times smaller than that observed in the old open cluster M67. We expect that interactions involving binary stars, as well as a third or more giant planets, will close the discrepancy.
Context: More than 40 planets have been found around giant stars, revealing a lack of systems orbiting interior to $sim$ 0.6 AU. This observational fact contrasts with the planetary population around solar-type stars and has been interpreted as the result of the orbital evolution of planets due to the interaction with the host star and/or because of a different formation/migration scenario of planets around more massive stars. Aims: We are conducting a radial velocity study of a sample of 166 giant stars aimed at studying the population of close-in planets orbiting post-main sequence stars. METHODS: We have computed precision radial velocities from multi-epoch spectroscopic data, in order to search for planets around giant stars. Results: In this paper we present the discovery of a massive planet around the intermediate-mass giant star HIP,63242. The best keplerian fit to the data lead to an orbital distance of 0.57 AU, an eccentricity of 0.23 and a projected mass of 9.2 mjup. HIP,63242,b is the innermost planet detected around any intermediate-mass giant star and also the first planet detected in our survey.
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