Genya Takeda

Structure and Evolution of Nearby Stars with Planets. II. Physical Properties of ~1000 Cool Stars from the SPOCS Catalog

We derive detailed theoretical models for 1074 nearby stars from the SPOCS (Spectroscopic Properties of Cool Stars) Catalog. The California and Carnegie Planet Search has obtained high-quality (R 70,000-90,000, S/N 300-500) echelle spectra of over 1000 nearby stars taken with the Hamilton spectrograph at Lick Observatory, the HIRES spectrograph at Keck, and UCLES at the Anglo Australian Observatory. A uniform analysis of the high-resolution spectra has yielded precise stellar parameters (Teff, log g, v sin i, [M/H], and individual elemental abundances for Fe, Ni, Si, Na, and Ti), enabling systematic error analyses and accurate theoretical stellar modeling. We have created a large database of theoretical stellar evolution tracks using the Yale Stellar Evolution Code (YREC) to match the observed parameters of the SPOCS stars. Our very dense grids of evolutionary tracks eliminate the need for interpolation between stellar evolutionary tracks and allow precise determinations of physical stellar parameters (mass, age, radius, size and mass of the convective zone, surface gravity, etc.). Combining our stellar models with the observed stellar atmospheric parameters and uncertainties, we compute the likelihood for each set of stellar model parameters separated by uniform time steps along the stellar evolutionary tracks. The computed likelihoods are used for a Bayesian analysis to derive posterior probability distribution functions for the physical stellar parameters of interest. We provide a catalog of physical parameters for 1074 stars that are based on a uniform set of high-quality spectral observations, a uniform spectral reduction procedure, and a uniform set of stellar evolutionary models. We explore this catalog for various possible correlations between stellar and planetary properties, which may help constrain the formation and dynamical histories of other planetary systems.

High Orbital Eccentricities of Extrasolar Planets Induced by the Kozai Mechanism

One of the most remarkable properties of extrasolar planets revealed by the ongoing radial velocity surveys is their high orbital eccentricities, which are difficult to explain with our current theoretical paradigm for planet formation. Observations have shown that at least ~20% of these planets, including some with particularly high eccentricities, are orbiting a component of a wide binary star system. The presence of a distant binary companion can cause significant secular perturbations to the orbit of a planet. In particular, at high relative inclinations, a planet can undergo a large-amplitude eccentricity oscillation. This so-called Kozai mechanism is effective at a very long range, and its amplitude is purely dependent on the relative orbital inclination. In this paper, we address the following simple question: assuming that every host star with a detected giant planet also has a (possibly unseen, e.g., substellar) distant companion, with reasonable distributions of orbital parameters and masses, how well could secular perturbations reproduce the observed eccentricity distribution of planets? Our calculations show that the Kozai mechanism consistently produces an excess of planets with very high (e 0.6) and very low (e 0.1) eccentricities. Assuming an isotropic distribution of relative orbital inclination, we would expect that 23% of planets do not have sufficiently high inclination angles to experience the eccentricity oscillation. By a remarkable coincidence, only 23% of currently known extrasolar planets have eccentricities e < 0.1. However, this paucity of near-circular orbits in the observed sample cannot be explained solely by secular perturbations. This is because, even with high enough inclinations, the Kozai mechanism often fails to produce significant eccentricity perturbations when there are other competing sources of orbital perturbations on secular timescales, such as general relativity. Our results show that, with any reasonable set of mass and initial orbital parameters, the Kozai mechanism always leaves more than 50% of planets on near-circular orbits. On the other hand, the Kozai mechanism can produce many highly eccentric orbits. Indeed, the overproduction of high eccentricities observed in our models could be combined with plausible circularizing mechanisms (e.g., friction from residual gas) to create more intermediate eccentricities (e 0.1-0.6).

Five intermediate-period planets from the N2K sample

We report the detection of five Jovian-mass planets orbiting high-metallicity stars. Four of these stars were first observed as part of the N2K program, and exhibited low rms velocity scatter after three consecutive observations. However, follow-up observations over the last 3 years now reveal the presence of longer period planets with orbital periods ranging from 21 days to a few years. HD 11506 is a G0 V star with a planet of M sin i = 4.74 M_(Jup) in a 3.85 yr orbit. HD 17156 is a G0 V star with a 3.12 M_(Jup) planet in a 21.2 day orbit. The eccentricity of this orbit is 0.67, one of the highest known for a planet with a relatively short period. The orbital period for this planet places it in a region of parameter space where relatively few planets have been detected. HD 125612 is a G3 V star with a planet of M sin i = 3.5 M_(Jup) in a 1.4 yr orbit. HD 170469 is a G5 IV star with a planet of M sin i = 0.67 M_(Jup) in a 3.13 year orbit. HD 231701 is an F8 V star with planet of 1.08 M_(Jup) in a 142 day orbit. All of these stars have supersolar metallicity. Three of the five stars were observed photometrically, but showed no evidence of brightness variability. A transit search conducted for HD 17156 was negative, but covered only 25% of the search space, and so is not conclusive.

Planetary Systems in Binaries. I. Dynamical Classification

Many recent observational studies have concluded that planetary systems commonly exist in multiple-star systems. At least ~20% of the known extrasolar planetary systems are associated with one or more stellar companions. The orbits of stellar binaries hosting planetary systems are typically wider than 100 AU and are often highly inclined with respect to the planetary orbits. The effect of secular perturbations from such an inclined binary orbit on a coupled system of planets, however, is little understood theoretically. In this paper we investigate various dynamical classes of double-planet systems in binaries through numerical integrations and provide an analytic framework based on secular perturbation theories. Differential nodal precession of the planets is the key property that separates two distinct dynamical classes of multiple planets in binaries: (1) dynamically rigid systems, in which the orbital planes of planets precess in concert as if they were embedded in a rigid disk, and (2) weakly coupled systems, in which the mutual inclination angle between initially coplanar planets grows to large values on secular timescales. In the latter case, the quadrupole perturbation from the outer planet induces additional Kozai cycles and causes the orbital eccentricity of the inner planet to oscillate with large amplitudes. The cyclic angular momentum transfer from a stellar companion propagating inward through planets can significantly alter the orbital properties of the inner planet on shorter timescales. This perturbation propagation mechanism may offer important constraints on the presence of additional planets in known single-planet systems in binaries.

On the Origins of Eccentric Close-In Planets

Strong tidal interaction with the central star can circularize the orbits of close-in planets. With the standard tidal quality factor Q of our solar system, estimated circularization times for close-in extrasolar planets are typically shorter than the ages of the host stars. While most extrasolar planets with orbital radii -->a 0.1 AU indeed have circular orbits, some close-in planets with substantial orbital eccentricities have recently been discovered. This new class of eccentric close-in planets implies that either their tidal Q factor is considerably higher, or circularization is prevented by an external perturbation. Here we constrain the tidal Q factor for transiting extrasolar planets by comparing their circularization times with accurately determined stellar ages. Using estimated secular perturbation timescales, we also provide constraints on the properties of hypothetical second planets exterior to the known ones.

Two Jovian-Mass Planets in Earthlike Orbits*

We report the discovery of two new planets: a 1.94 M_(Jup) planet in a 1.8 yr orbit of HD 5319, and a 2.51 M_(Jup) planet in a 1.1 yr orbit of HD 75898. The measured eccentricities are 0.12 for HD 5319b and 0.10 for HD 75898b, and Markov chain Monte Carlo simulations based on the derived orbital parameters indicate that the radial velocities of both stars are consistent with circular planet orbits. With low eccentricity and 1 AU < α < 2 AU, our new planets have orbits similar to terrestrial planets in the solar system. The radial velocity residuals of both stars have significant trends, likely arising from substellar or low-mass stellar companions.

Pervasive Orbital Eccentricities Dictate the Habitability of Extrasolar Earths

The long-term habitability of Earth-like planets requires low orbital eccentricities. A secular perturbation from a distant stellar companion is a very important mechanism in exciting planetary eccentricities, as many of the extrasolar planetary systems are associated with stellar companions. Although the orbital evolution of an Earth-like planet in a stellar binary system is well understood, the effect of a binary perturbation on a more realistic system containing additional gas-giant planets has been very little studied. Here, we provide analytic criteria confirmed by a large ensemble of numerical integrations that identify the initial orbital parameters leading to eccentric orbits. We show that an extrasolar earth is likely to experience a broad range of orbital evolution dictated by the location of a gas-giant planet, which necessitates more focused studies on the effect of eccentricity on the potential for life.

Two Jupiter-Mass Planets Orbiting HD 154672 and HD 205739

We report the detection of the first two planets from the N2K Doppler planet search program at the Magellan telescopes. The first planet has a mass of Msin i = 4.96 M Jup and orbits the G3 IV star HD 154672 with an orbital period of 163.9 days. The second planet orbits the F7 V star HD 205739 with an orbital period of 279.8 days and has a mass of Msin i = 1.37 M Jup. Both planets are in eccentric orbits, with eccentricities e = 0.61 and e = 0.27, respectively. Both stars are metal rich and appear to be chromospherically inactive, based on inspection of their Ca II H and K lines. Finally, the best Keplerian model fit to HD 205739b shows a trend of 0.0649 m s–1 day–1, suggesting the presence of an additional outer body in that system.

Eccentricities of Planets in Binary Systems

The most puzzling property of the extrasolar planets discovered by recent radial velocity surveys is their high orbital eccentricities, which are very difficult to explain within our current theoretical paradigm for planet formation. Current data reveal that at least 25% of these planets, including some with particularly high eccentricities, are orbiting a component of a binary star system. The presence of a distant companion can cause significant secular perturbations in the orbit of a planet. At high relative inclinations, large-amplitude, periodic eccentricity perturbations can occur. These are known as “Kozai cycles” and their amplitude is purely dependent on the relative orbital inclination. Assuming that every planet host star also has a (possibly unseen, e.g., substellar) distant companion, with reasonable distributions of orbital parameters and masses, we determine the resulting eccentricity distribution of planets and compare it to observations? We find that perturbations from a binary companion always appear to produce an excess of planets with both very high (≳0.6) and very low (e ≲ 0.1) eccentricities. The paucity of near-circular orbits in the observed sample implies that at least one additional mechanism must be increasing eccentricities. On the other hand, the overproduction of very high eccentricities observed in our models could be combined with plausible circularization mechanisms (e.g., friction from residual gas) to create more planets with intermediate eccentricities (e≃ 0.1–0.6).

Induced Kozai Migration and Formation of Close-in Planets in Binaries

Many recent observational studies have concluded that planetary systems commonly exist in multiple-star systems. At least �20%, and presumably a larger fraction of the known extrasolar planetary systems are associated with one or more stellar companions. These stel- lar companions normally exist at large distances from the planetary systems (typical projected binary separations are on the orders 10 2 -10 4 AU) and are often faint (ranging from F to T spectral types). Yet, secular cyclic angular momentum exchange with these distant stellar com- panions can significantly alter the orbital configuration of the planets around the primaries. One of the most interesting and fairly common outcomes seen in numerical simulations is the opening of a large mutual inclination angle between the planetary orbits, forced by differential nodal precessions caused by the binary companion. The growth of the mutual inclination angle between planetary orbits induces additional large-amplitude eccentricity oscillations of the inner planet due to the quadrupole gravitational perturbation by the outer planet. This eccentricity oscillation may eventually result in the orbital decay of the inner planet through tidal friction, as previously proposed as Kozai migration or Kozai cycles with tidal friction (KCTF). This orbital decay mechanism induced by the binary perturbation and subsequent tidal dissipation may stand as an alternative formation channel for close-in extrasolar planets.