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Register a new userRetired A Stars and Their Companions. III. Comparing the Mass-Period Distributions of Planets Around A-Type Stars and Sun-Like Stars
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We present an analysis of ~5 years of Lick Observatory radial velocity measurements targeting a uniform sample of 31 intermediate-mass subgiants (1.5 < M*/Msun < 2.0) with the goal of measuring the occurrence rate of Jovian planets around (evolved) A-type stars and comparing the distributions of their orbital and physical characteristics to those of planets around Sun-like stars. We provide updated orbital solutions incorporating new radial velocity measurements for five known planet-hosting stars in our sample; uncertainties in the fitted parameters are assessed using a Markov Chain Monte Carlo method. The frequency of Jovian planets interior to 3 AU is 26 (+9,-8)%, which is significantly higher than the ~5-10% frequency observed around solar-mass stars. The median detection threshold for our sample includes minimum masses down to {0.2, 0.3, 0.5, 0.6, 1.3} MJup within {0.1, 0.3, 0.6, 1.0, 3.0} AU. To compare the properties of planets around intermediate-mass stars to those around solar-mass stars we synthesize a population of planets based on the parametric relationship dN ~ M^{alpha}P^{beta} dlnM dlnP, the observed planet frequency, and the detection limits we derived. We find that the values of alpha and beta for planets around solar-type stars from Cumming et al. fail to reproduce the observed properties of planets in our sample at the 4 sigma level, even when accounting for the different planet occurrence rates. Thus, the properties of planets around A stars are markedly different than those around Sun-like stars, suggesting that only a small (~ 50%) increase in stellar mass has a large influence on the formation and orbital evolution of planets.
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We report the detection of eighteen Jovian planets discovered as part of our Doppler survey of subgiant stars at Keck Observatory, with follow-up Doppler and photometric observations made at McDonald and Fairborn Observatories, respectively. The host stars have masses 0.927 < Mstar /Msun < 1.95, radii 2.5 < Rstar/Rsun < 8.7, and metallicities -0.46 < [Fe/H] < +0.30. The planets have minimum masses 0.9 MJup < MP sin i <3 MJup and semima jor axes a > 0.76 AU. These detections represent a 50% increase in the number of planets known to orbit stars more massive than 1.5 Msun and provide valuable additional information about the properties of planets around stars more massive thantheSun.
We report radial velocity measurements of the G-type subgiants 24 Sextanis (=HD90043) and HD200964. Both are massive, evolved stars that exhibit periodic variations due to the presence of a pair of Jovian planets. Photometric monitoring with the T12 0.80m APT at Fairborn Observatory demonstrates both stars to be constant in brightness to <= 0.002 mag, thus strengthening the planetary interpretation of the radial velocity variations. 24 Sex b,c have orbital periods of 453.8 days and 883~days, corresponding to semimajor axes 1.333 AU and 2.08 AU, and minimum masses (Msini) 1.99 Mjup and 0.86 Mjup, assuming a stellar mass 1.54 Msun. HD200964 b,c have orbital periods of 613.8 days and 825 days, corresponding to semimajor axes 1.601 AU and 1.95 AU, and minimum masses 1.85 Mjup and 0.90 Mjup, assuming M* = 1.44 Msun. We also carry out dynamical simulations to properly account for gravitational interactions between the planets. Most, if not all, of the dynamically stable solutions include crossing orbits, suggesting that each system is locked in a mean motion resonance that prevents close encounters and provides long-term stability. The planets in the 24 Sex system likely have a period ratio near 2:1, while the HD200964 system is even more tightly packed with a period ratio close to 4:3. However, we caution that further radial velocity observations and more detailed dynamical modelling will be required to provide definitive and unique orbital solutions for both cases, and to determine whether the two systems are truly resonant.
We expand on the results of Nielsen et al. (2008), using the null result for giant extrasolar planets around the 118 target stars from the VLT NACO H and Ks band planet search (Masciadri et al. 2005), the VLT and MMT Simultaneous Differential Imaging (SDI) survey (Biller et al. 2007), and the Gemini Deep Planet Survey (Lafreniere et al. 2007) to set constraints on the population of giant extrasolar planets. Our analysis is extended to include the planet luminosity models of Fortney et al. (2008), as well as the correlation between stellar mass and frequency of giant planets found by Johnson et al. (2007). Doubling the sample size of FGKM stars strengthens our conclusions: a model for extrasolar giant planets with power-laws for mass and semi-major axis as giving by Cumming et al. (2008) cannot, with 95% confidence, have planets beyond 65 AU, compared to the value of 94 AU reported in Nielsen et al. (2008), using the models of Baraffe et al. (2003). When the Johnson et al. (2007) correction for stellar mass (which gives fewer Jupiter-mass companions to M stars with respect to solar-type stars) is applied, however, this limit moves out to 82 AU. For the relatively new Fortney et al. (2008) models, which predict fainter planets across most of parameter space, these upper limits, with and without a correction for stellar mass, are 182 and 234 AU, respectively.
The search for life in the universe is currently focused on Earth-analog planets. However, we should be prepared to find a diversity of terrestrial exoplanets not only in terms of host star but also in terms of surface environment. Simulated high-resolution spectra of habitable planets covering a wide parameter space are essential in training retrieval tools, optimizing observing strategies, and interpreting upcoming observations. Ground-based extremely large telescopes like ELT, GMT, and TMT; and future space-based mission concepts like Origins, HabEx, and LUVOIR are designed to have the capability of characterizing a variety of potentially habitable worlds. Some of these telescopes will use high precision radial velocity techniques to obtain the required high-resolution spectra ($Rapprox100,000$) needed to characterize potentially habitable exoplanets. Here we present a database of high-resolution (0.01 cm$^{-1}$) reflection and emission spectra for simulated exoplanets with a wide range of surfaces, receiving similar irradiation as Earth around 12 different host stars from F0 to K7. Depending on surface type and host star, we show differences in spectral feature strength as well as overall reflectance, emission, and star to planet contrast ratio of terrestrial planets in the Habitable zone of their host stars. Accounting for the wavelength-dependent interaction of the stellar flux and the surface will help identify the best targets for upcoming spectral observations in the visible and infrared. All of our spectra and model profiles are available online.
Small planets, 1-4x the size of Earth, are extremely common around Sun-like stars, and surprisingly so, as they are missing in our solar system. Recent detections have yielded enough information about this class of exoplanets to begin characterizing their occurrence rates, orbits, masses, densities, and internal structures. The Kepler mission finds the smallest planets to be most common, as 26% of Sun-like stars have small, 1-2 R_e planets with orbital periods under 100 days, and 11% have 1-2 R_e planets that receive 1-4x the incident stellar flux that warms our Earth. These Earth-size planets are sprinkled uniformly with orbital distance (logarithmically) out to 0.4 AU, and probably beyond. Mass measurements for 33 transiting planets of 1-4 R_e show that the smallest of them, R < 1.5 R_e, have the density expected for rocky planets. Their densities increase with increasing radius, likely caused by gravitational compression. Including solar system planets yields a relation: rho = 2.32 + 3.19 R/R_e [g/cc]. Larger planets, in the radius range 1.5-4.0 R_e, have densities that decline with increasing radius, revealing increasing amounts of low-density material in an envelope surrounding a rocky core, befitting the appellation mini-Neptunes. Planets of ~1.5 R_e have the highest densities, averaging near 10 g/cc. The gas giant planets occur preferentially around stars that are rich in heavy elements, while rocky planets occur around stars having a range of heavy element abundances. One explanation is that the fast formation of rocky cores in protoplanetary disks enriched in heavy elements permits the gravitational accumulation of gas before it vanishes, forming giant planets. But models of the formation of 1-4 R_e planets remain uncertain. Defining habitable zones remains difficult, without benefit of either detections of life elsewhere or an understanding of lifes biochemical origins.
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