Showing papers on "Planetary system published in 2005"

The Planet-Metallicity Correlation

Abstract: We have recently carried out spectral synthesis modeling to determine Teff, log g, v sin i, and [Fe/H] for 1040 FGK-type stars on the Keck, Lick, and Anglo-Australian Telescope planet search programs. This is the first time that a single, uniform spectroscopic analysis has been made for every star on a large Doppler planet search survey. We identify a subset of 850 stars that have Doppler observations sufficient to detect uniformly all planets with radial velocity semiamplitudes K > 30 m s-1 and orbital periods shorter than 4 yr. From this subset of stars, we determine that fewer than 3% of stars with -0.5 +0.3 dex, 25% of observed stars have detected gas giant planets. A power-law fit to these data relates the formation probability for gas giant planets to the square of the number of metal atoms. High stellar metallicity also appears to be correlated with the presence of multiple-planet systems and with the total detected planet mass. This data set was examined to better understand the origin of high metallicity in stars with planets. None of the expected fossil signatures of accretion are observed in stars with planets relative to the general sample: (1) metallicity does not appear to increase as the mass of the convective envelopes decreases, (2) subgiants with planets do not show dilution of metallicity, (3) no abundance variations for Na, Si, Ti, or Ni are found as a function of condensation temperature, and (4) no correlations between metallicity and orbital period or eccentricity could be identified. We conclude that stars with extrasolar planets do not have an accretion signature that distinguishes them from other stars; more likely, they are simply born in higher metallicity molecular clouds.

Circumstellar Dust Disks in Taurus-Auriga: The Submillimeter Perspective

Abstract: We present a sensitive, multiwavelength submillimeter continuum survey of 153 young stellar objects in the Taurus-Auriga star formation region. The submillimeter detection rate is 61% to a completeness limit of � 10 mJy (3 � )at850� m.Theinferredcircumstellardiskmassesarelognormallydistributedwithameanmassof � 5 ; 10 � 3 M� and a large dispersion (0.5 dex). Roughly one-third of the submillimeter sources have disk masses larger than the minimalnebulafromwhichthesolarsystemformed.Themediandisk-to-starmassratiois0.5%.Theempiricalbehavior of the submillimeter continuum is best described as F� / � 2:0� 0:5 between 350 � m and 1.3 mm, which we argue is duetothecombinedeffectsofthefractionofopticallythickemissionandaflatterfrequencybehavioroftheopacity compared to the interstellar medium. The latter effect could be due to a substantial population of large dust grains, which presumably would have grown through collisional agglomeration. In this sample, the only stellar property that is correlated with the outer disk is the presence of a companion. We find evidence for significant decreases in submillimeterfluxdensities,diskmasses,andsubmillimetercontinuumslopesalongthecanonicalinfraredspectral energy distribution evolution sequence for young stellar objects. The fraction of objects detected in the submillimeter isessentially identicalto the fractionwith excessnear-infrared emission,suggestingthatdustin the inner and outer disks is removed nearly simultaneously.

The use of transit timing to detect terrestrial-mass extrasolar planets.

Abstract: Future surveys for transiting extrasolar planets are expected to detect hundreds of jovian-mass planets and tens of terrestrial-mass planets. For many of these newly discovered planets, the intervals between successive transits will be measured with an accuracy of 0.1 to 100 minutes. We show that these timing measurements will allow for the detection of additional planets in the system (not necessarily transiting) by their gravitational interaction with the transiting planet. The transit-time variations depend on the mass of the additional planet, and in some cases terrestrial-mass planets will produce a measurable effect. In systems where two planets are seen to transit, the density of both planets can be determined without radial-velocity observations.

Detection of Thermal Emission from an Extrasolar Planet

Abstract: We present Spitzer Space Telescope infrared photometric time series of the transiting extrasolar planet system TrES-1. The data span a predicted time of secondary eclipse, corresponding to the passage of the planet behind the star. In both bands of our observations, we detect a flux decrement with a timing, amplitude, and duration as predicted by published parameters of the system. This signal represents the first direct detection of (i.e. the observation of photons emitted by) a planet orbiting another star. The observed eclipse depths (in units of relative flux) are 0.00066 ± 0.00013 at 4.5 µm and 0.00225±0.00036 at 8.0 µm. These estimates provide the first observational constraints on models of the thermal emission of hot Jupiters. Assuming that the planet emits as a blackbody, we estimate an effective temperature of Tp = 1060 ±50 K. Under the additional assumptions that the planet is in thermal equilibrium with the radiation from the star and emits isotropically, we find a Bond albedo of A = 0.31 ± 0.14. This would imply that the planet absorbs the majority of stellar radiation incident upon it, a conclusion of significant impact to atmospheric models of these objects. We also compare our data to a previously-published model of the planetary thermal emission, which predicts prominent spectral features in our observational bands due to water and carbon monoxide. This model adequately reproduces the observed planet-to-star flux ratio at 8.0 µm, however it significantly over-predicts the ratio at 4.5 µm. We also present an estimate of the timing of the secondary eclipse, which we use to place a

Models of giant planet formation with migration and disc evolution

Abstract: We present a new model of giant planet formation that extends the core-accretion model of Pollack et al. (1996, Icarus, 124, 62) to include migration, disc evolution and gap formation. We show that taking these effects into account can lead to much more rapid formation of giant planets, making it compatible with the typical disc lifetimes inferred from observations of young circumstellar discs. This speed up is due to the fact that migration prevents the severe depletion of the feeding zone as observed in in situ calculations. Hence, the growing planet is never isolated and it can reach cross-over mass on a much shorter timescale. To illustrate the range of planets that can form in our model, we describe a set of simulations in which we have varied some of the initial parameters and compare the final masses and semi-major axes with those inferred from observed extra-solar planets.

Biosignatures from Earth-like planets around M dwarfs

Abstract: Coupled one-dimensional photochemical-climate calculations have been performed for hypothetical Earth-like planets around M dwarfs. Visible/near-infrared and thermal-infrared synthetic spectra of these planets were generated to determine which biosignature gases might be observed by a future, space-based telescope. Our star sample included two observed active M dwarfs—AD Leo and GJ 643—and three quiescent model stars. The spectral distribution of these stars in the ultraviolet generates a different photochemistry on these planets. As a result, the biogenic gases CH4, N2O, and CH3Cl have substantially longer lifetimes and higher mixing ratios than on Earth, making them potentially observable by space-based telescopes. On the active M-star planets, an ozone layer similar to Earth's was developed that resulted in a spectroscopic signature comparable to the terrestrial one. The simultaneous detection of O2 (or O3) and a reduced gas in a planet's atmosphere has been suggested as strong evidence for life. Plane...

Toward a Deterministic Model of Planetary Formation. III. Mass Distribution of Short-Period Planets around Stars of Various Masses

Abstract: The origin of a recently discovered close-in Neptune-mass planet around GJ 436 poses a challenge to the current theories of planet formation. On the basis of 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. Planets around G dwarfs undergo orbital migration after fully accreting gas, while those around M dwarfs tend to migrate before starting rapid gas accretion. Close-in Neptune-mass planets may also exist around G dwarfs, although 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.

ELODIE metallicity-biased search for transiting Hot Jupiters. II. A very hot Jupiter transiting the bright K star HD 189733

Abstract: Among the 160 known exoplanets, mainly detected in large radial-velocity surveys, only 8 have a characterization of their actual mass and radius thanks to the two complementary methods of detection: radial velocities and photometric transit. We started in March 2004 an exoplanet-search programme biased toward high-metallicity stars which are more frequently host extra-solar planets. This survey aims to detect close-in giant planets, which are most likely to transit their host star. For this programme, high-precision radial velocities are measured with the ELODIE fiber-fed spectrograph on the 1.93-m telescope, and high-precision photometry is obtained with the CCD Camera on the 1.20-m telescope, both at the Haute-Provence Observatory. We report here the discovery of a new transiting hot Jupiter orbiting the star HD189733. The planetary nature of this object is confirmed by the observation of both the spectroscopic and photometric transits. The exoplanet HD189733b, with an orbital period of 2.219 days, has one of the shortest orbital periods detected by radial velocities, and presents the largest photometric depth in the light curve (~ 3%) observed to date. We estimate for the planet a mass of 1.15 +- 0.04 Mjup and a radius of 1.26 +- 0.03 RJup. Considering that HD189733 has the same visual magnitude as the well known exoplanet host star HD209458, further ground-based and space-based follow-up observations are very promising and will permit a characterization of the atmosphere and exosphere of this giant exoplanet.

A ~7.5 M⊕ Planet Orbiting the Nearby Star, GJ 876*

Abstract: High-precision, high-cadence radial velocity monitoring over the past 8 yr at the W. M. Keck Observatory reveals evidence for a third planet orbiting the nearby (4.69 pc) dM4 star GJ 876. The residuals of three-body Newtonian fits, which include GJ 876 and Jupiter-mass companions b and c, show significant power at a periodicity of 1.9379 days. Self-consistently fitting the radial velocity data with a model that includes an additional body with this period significantly improves the quality of the fit. These four-body (three-planet) Newtonian fits find that the minimum mass of companion "d" is m sin i = 5.89 ± 0.54 M⊕ and that its orbital period is 1.93776 (±7 × 10-5) days. Assuming coplanar orbits, an inclination of the GJ 876 planetary system to the plane of the sky of ~50° gives the best fit. This inclination yields a mass for companion d of m = 7.53 ± 0.70 M⊕, making it by far the lowest mass companion yet found around a main-sequence star other than our Sun. Precise photometric observations at Fairborn Observatory confirm low-level brightness variability in GJ 876 and provide the first explicit determination of the star's 96.7 day rotation period. Even higher precision short-term photometric measurements obtained at Las Campanas imply that planet d does not transit GJ 876.

Quantifying the uncertainty in the orbits of extrasolar planets

Abstract: Precise radial velocity measurements have led to the discovery of ~100 extrasolar planetary systems. We investigate the uncertainty in the orbital solutions that have been fitted to these observations. Understanding these uncertainties will become more and more important as the discovery space for extrasolar planets shifts to longer and longer periods. While detections of short-period planets can be rapidly refined, planets with long orbital periods will require observations spanning decades to constrain the orbital parameters precisely. Already in some cases, multiple distinct orbital solutions provide similarly good fits, particularly in multiple-planet systems. We present a method for quantifying the uncertainties in orbital fits and addressing specific questions directly from the observational data rather than relying on best-fit orbital solutions. This Markov chain Monte Carlo (MCMC) technique has the advantage that it is well suited to the high-dimensional parameter spaces necessary for the multiple-planet systems. We apply the MCMC technique to several extrasolar planetary systems, assessing the uncertainties in orbital elements for several systems. Our MCMC simulations demonstrate that for some systems there are strong correlations between orbital parameters and/or significant non-Gaussianities in parameter distributions, even though the measurement errors are nearly Gaussian. Once these effects are considered, the actual uncertainties in orbital elements can be significantly larger or smaller than the published uncertainties. We also present simple applications of our methods, such as predicting the times of possible transits for GJ 876.

A planetary system as the origin of structure in Fomalhaut's dust belt

Abstract: In 1983 the IRAS orbiting satellite detected excess infrared radiation from the direction of Fomalhaut, a first magnitude star in the otherwise dim constellation Piscis Austrinus. It was radiation from a huge dusty disk around the star, about four times the size of our Solar System. The Advanced Camera for Surveys onboard the Hubble Space Telescope has now detected Fomalhaut's dust complex at high resolution at optical wavelengths. The disk is offset from the star in a way that suggests the presence of several planets. The debris disks around Beta Pictoris and AU Microscopii are both edge-on, and the disk around HR 4796A has a small radius. So the Fomalhaut disk, seen on a slope rather like the ring around Saturn, older than the others and closer to us, may become the disk of choice for the study of planet formation. The Sun and >15 per cent of nearby stars are surrounded by dusty disks that must be collisionally replenished by asteroids and comets, as the dust would otherwise be depleted on timescales <107 years (ref. 1). Theoretical studies show that the structure of a dusty disk can be modified by the gravitational influence of planets2,3,4, but the observational evidence is incomplete, at least in part because maps of the thermal infrared emission from the disks have low linear resolution (35 au in the best case5). Optical images provide higher resolution, but the closest examples (AU Mic and β Pic) are edge-on6,7, preventing the direct measurement of the azimuthal and radial disk structure that is required for fitting theoretical models of planetary perturbations. Here we report the detection of optical light reflected from the dust grains orbiting Fomalhaut (HD 216956). The system is inclined 24° away from edge-on, enabling the measurement of disk structure around its entire circumference, at a linear resolution of 0.5 au. The dust is distributed in a belt 25 au wide, with a very sharp inner edge at a radial distance of 133 au, and we measure an offset of 15 au between the belt's geometric centre and Fomalhaut. Taken together, the sharp inner edge and offset demonstrate the presence of planetary-mass objects orbiting Fomalhaut.

On detecting terrestrial planets with timing of giant planet transits

Abstract: The transits of a distant star by a planet on a Keplerian orbit occur at time intervals exactly equal to the orbital period. If a second planet orbits the same star, the orbits are not Keplerian and the transits are no longer exactly periodic. We compute the magnitude of the variation in the timing of the transits, St. We investigate analytically several limiting cases: (i) interior perturbing planets with much smaller periods; (ii) exterior perturbing planets on eccentric orbits with much larger periods; (iii) both planets on circular orbits with arbitrary period ratio but not in resonance; (iv) planets on initially circular orbits locked in resonance. Using subscripts out' and 'in' for the exterior and interior planets, μ for planet-to-star mass ratio and the standard notation for orbital elements, our findings in these cases are as follows. (i) Planet-planet perturbations are negligible. The main effect is the wobble of the star due to the inner planet, and therefore St ∼ μ in (a in /a out ) P out . (ii) The exterior planet changes the period of the interior planet by μ out (a in /r out ) 3 P in . As the distance of the exterior planet changes due to its eccentricity, the inner planet's period changes. Deviations in its transit timing accumulate over the period of the outer planet, and therefore δt ∼ μ out e out (α in /α out ) 3 /P out . (iii) Halfway between resonances the perturbations are small, of the order of μ out a 2 in /(a in - a out ) 2 P in for the inner planet (switch 'out' and 'in' for the outer planet). This increases as one gets closer to a resonance. (iv) This is perhaps the most interesting case because some systems are known to be in resonances and the perturbations are the largest. As long as the perturber is more massive than the transiting planet, the timing variations would be of the order of the period regardless of the perturber mass. For lighter perturbers, we show that the timing variations are smaller than the period by the perturber-to-transiting-planet mass ratio. An earth-mass planet in 2: 1 resonance with a three-dimensional period transiting planet (e.g. HD 209458b) would cause timing variations of the order of 3 min, which would be accumulated over a year. This signal of a terrestrial planet is easily detectable with current ground-based measurements. For the case in which both planets are on eccentric orbits, we compute numerically the transit timing variations for several known multiplanet systems, assuming they are edge-on. Transit timing measurements may be used to constrain the masses, radii and orbital elements of planetary systems, and, when combined with radial velocity measurements, provide a new means of measuring the mass and radius of the host star.

Can Giant Planets Form by Direct Gravitational Instability

Abstract: Gravitational instability has been invoked as a possible mechanism of the giant planet production in protoplanetary disks. Here we critically revise its viability by noting that to form planets directly, it is not enough for protoplanetary disks to be gravitationally unstable. They must also be able to cool efficiently (on a timescale comparable to the local disk orbital period) to allow the formation of the bound clumps by fragmentation. A combination of the dynamical and thermal constraints puts very stringent lower limits on the properties of disks capable of fragmenting into the self-gravitating objects: for the gravitational instability to form giant planets at 10 AU in the disk cooled by the radiation transfer, the gas temperature must exceed 103 K with a minimum disk mass of 0.7 M☉ and a luminosity of 40 L☉. Although these requirements are relaxed in the more distant parts of the disk, masses of the bound objects formed as a result of instability are too large even at 100 AU (~10MJ) to explain the characteristics of known extrasolar giant planets. Such protoplanetary disks (and planets formed in them) have very unusual observational properties, and this severely constrains the possibility of giant planet formation by direct gravitational instability.

A ~ 7.5 Earth-Mass Planet Orbiting the Nearby Star, GJ 876

Abstract: High precision, high cadence radial velocity monitoring over the past 8 years at the W. M. Keck Observatory reveals evidence for a third planet orbiting the nearby (4.69 pc) dM4 star GJ 876. The residuals of three-body Newtonian fits, which include GJ 876 and Jupiter mass companions b and c, show significant power at a periodicity of 1.9379 days. Self-consistently fitting the radial velocity data with a model that includes an additional body with this period significantly improves the quality of the fit. These four-body (three-planet) Newtonian fits find that the minimum mass of companion ``d'' is m sin i = 5.89 +- 0.54 Earth masses and that its orbital period is 1.93776 (+- 7x10^-5) days. Assuming coplanar orbits, an inclination of the GJ 876 planetary system to the plane of the sky of ~ 50 degrees gives the best fit. This inclination yields a mass for companion d of m = 7.53 +- 0.70 Earth masses, making it by far the lowest mass companion yet found around a main sequence star other than our Sun. Precise photometric observations at Fairborn Observatory confirm low-level brightness variability in GJ 876 and provide the first explicit determination of the star's 96.7-day rotation period. Even higher precision short-term photometric measurements obtained at Las Campanas imply that planet d does not transit GJ 876.

Decay of Planetary Debris Disks

Abstract: We report new Spitzer 24 � m photometry of 76 main-sequence A-type stars. We combine these results with previously reportedSpitzer24 � m data and 24 and 25 � m photometry from theInfrared Space Observatoryand the InfraredAstronomySatellite.Theresultisasampleof266starswithmasscloseto2.5M� ,alldetectedtoatleastthe � 7 � level relative to their photospheric emission. We culled ages for the entire sample from the literature and/or estimated them using the H-R diagram and isochrones; they range from 5 to 850 Myr. We identified excess thermal emission using an internally derived K � 24 (or 25) � m photospheric color and then compared all stars in the sample tothatcolor.Becausewehaveexcludedstarswithstrongemissionlinesorextendedemission(associatedwithnearby interstellar gas), these excesses are likely to be generated by debris disks. Younger stars in the sample exhibit excess thermal emissionmore frequently andwithhigher fractional excess thandothe olderstars. However,asmanyas 50% oftheyoungerstarsdonotshowexcessemission.Thedeclineinthemagnitudeofexcessemission,forthosestarsthat show it, has a roughly t0/time dependence, with t0 � 150 Myr. If anything, stars in binary systems (including Algoltype stars) and k Boo stars show less excess emission than the other members of the sample. Our results indicate that (1) there is substantial variety among debris disks, including that a significant number of stars emerge from the protoplanetary stage of evolution with little remaining disk in the 10‐60 AU region and (2) in addition, it is likely that much of the dust we detect is generated episodically by collisions of large planetesimals during the planet accretion endgame,andthatindividualeventsoftendominatetheradiometricpropertiesofadebrissystem.Thislatterbehavior agrees generally withwhat weknowabouttheevolution of thesolar system, andalsowiththeoretical models ofplanetary system formation. Subject headingg circumstellar matter — infrared: stars — planetary systems: formation Online material: machine-readable table

Measurement of Spin‐Orbit Alignment in an Extrasolar Planetary System

Abstract: We determine the stellar, planetary, and orbital properties of the transiting planetary system HD 209458 through a joint analysis of high-precision radial velocities, photometry, and timing of the secondary eclipse. Of primary interest is the strong detection of the Rossiter-McLaughlin effect, the alteration of photospheric line profiles that occurs because the planet occults part of the rotating surface of the star. We develop a new technique for modeling this effect and use it to determine the inclination of the planetary orbit relative to the apparent stellar equator (λ = -4o.4 ± 1o.4), and the line-of-sight rotation speed of the star (v sin /_★ = 4.70 ± 0.16 km s^(-1)). The uncertainty in these quantities has been reduced by an order of magnitude relative to the pioneering measurements by Queloz and collaborators. The small but nonzero misalignment is probably a relic of the planet formation epoch, because the expected timescale for tidal coplanarization is larger than the age of the star. Our determination of v sin /★ is a rare case in which rotational line broadening has been isolated from other broadening mechanisms.

The t tauri phase down to nearly planetary masses : Echelle spectra of 82 very low mass stars and brown dwarfs

Abstract: Using the largest high-resolution spectroscopic sample to date of young, very low mass stars and brown dwarfs, we investigate disk accretion in objects ranging from just above the hydrogen-burning limit all the way to nearly planetary masses. Our 82 targets span spectral types from M5 to M9.5, or masses from 0.15 M? down to about 15 jupiters. They are confirmed members of the ? Ophiuchus, Taurus, Chamaeleon I, IC 348, R Coronae Australis, Upper Scorpius, and TW Hydrae star-forming regions and young clusters, with ages from <1 to ~10 Myr. The sample contains 41 brown dwarfs (spectral types ?M6.5). We have previously presented high-resolution optical spectra for roughly half the sample; the rest are new. This is a close to complete survey of all confirmed brown dwarfs known so far in the regions examined, except in ? Oph and IC 348 (where we are limited by a combination of extinction and distance). We find that (1) classical T Tauri-like disk accretion persists in the substellar domain down to nearly the deuterium-burning limit; (2) while an H? 10% width 200 km s-1 is our prime accretion diagnostic (following our previous work), permitted emission lines of Ca II, O I, and He I are also good accretion indicators, just as in classical T Tauri stars (we caution against a blind use of H? width alone, since inclination and rotation effects on the line are especially important at the low accretion rates in very low mass objects); (3) the Ca II ?8662 line flux is an excellent quantitative measure of the accretion rate in very low mass stars and brown dwarfs (as in higher mass classical T Tauri Stars), correlating remarkably well with the obtained from veiling and H? modeling; (4) the accretion rate diminishes rapidly with mass?our measurements support previous suggestions that (albeit with considerable scatter) and extend this correlation to the entire range of substellar masses; (5) the fraction of very low mass stellar and substellar accretors decreases substantially with age, as in higher mass stars; (6) at any given age, the fraction of very low mass stellar and substellar accretors is comparable to the accretor fraction in higher mass stars; and (7) a number of our sources with infrared excesses arising from dusty disks do not evince measurable accretion signatures, with the incidence of such a mismatch increasing with age: this implies that disks in the low-mass regime can persist beyond the main accretion phase and parallels the transition from the classical to post-T Tauri stage in more massive stars. These strong similarities at young ages, between higher mass stars on the one hand and low-mass bodies close to and below the hydrogen-burning limit on the other, are consistent with a common formation mechanism in the two mass regimes.

The Rossiter-Mclaughlin effect and analytic radial velocity curves for transiting extrasolar planetary systems

Abstract: A transiting extrasolar planet sequentially blocks off the light coming from the different parts of the disk of the host star in a time-dependent manner. Because of the spin of the star, this produces an asymmetric distortion in the line profiles of the stellar spectrum, leading to an apparent anomaly in the the radial velocity curves, known as the Rossiter-McLaughlin effect. Here, we derive approximate but accurate analytic formulae for the anomaly in the radial velocity curves, taking into account the stellar limb darkening. The formulae are particularly useful in extracting information on the projected angle between the planetary orbit axis and the stellar spin axis, λ, and the projected stellar spin velocity, V sin Is. We create mock samples for the radial curves for the transiting extrasolar system HD 209458 and demonstrate that constraints on the spin parameters (V sin Is, λ) can be significantly improved by combining our analytic template formulae and the precision velocity curves from high-resolution spectroscopic observations with 8-10 m class telescopes. Thus, future observational exploration of transiting systems using the Rossiter-McLaughlin effect will be one of the most important probes for a better understanding of the origin of extrasolar planetary systems, especially the origin of their angular momentum.

The effect of condensates on the characterization of transiting planet atmospheres with transmission spectroscopy

Abstract: Through a simple physical argument we show that the slant optical depth through the atmosphere of a ‘hot Jupiter’ planet is ∼35‐90 times greater than the normal optical depth. This not unexpected result has direct consequences for the method of transmission spectroscopy for characterizing the atmospheres of transiting giant planets. The atmospheres of these planets likely contain minor condensates and hazes, which at normal viewing geometry have negligible optical depth, but at slant viewing geometry have appreciable optical depth that can obscure absorption features of gaseous atmospheric species. We identify several possible condensates. We predict that this is a general masking mechanism for all planets, not just for HD 209458b, and will lead to weaker than expected or undetected absorption features. Constraints on an atmosphere from transmission spectroscopy are not the same as constraints on an atmosphere at normal viewing geometry. Ke yw ords: radiative transfer ‐ planetary systems.

Transonic hydrodynamic escape of hydrogen from extrasolar planetary atmospheres

Abstract: Hydrodynamic escape is an important process in the formation and evolution of planetary atmospheres. Transonic steady state solutions of the time-independent hydrodynamic equations are difficult to find because of the existence of a singularity point. A numerical model is developed to study the hydrodynamic escape of neutral gas from planetary atmospheres by solving the time-dependent hydrodynamic equations. The model is validated against an analytical solution of the escape from an isothermal atmosphere. The model uses a two-dimensional energy deposition calculation instead of the single-layer heating assumption, which is not sufficiently accurate for hydrodynamic escapefrom ahydrogen-richplanetaryatmosphere.Whenapplied totheatmospheresofextrasolar planets, themodel results are in good agreement with observations of the transiting extrasolar planet HD 209458b. The model predicts that hydrogen is escaping from HD 209458b at a maximum rate of 6 ; 10 10 gs � 1 . The extrasolar planet is stable under the hydrodynamic escape of hydrogen. The rate of hydrogen hydrodynamic escape from other possible extrasolar planets is investigated using the model. The importance of hydrogen hydrodynamic escape for the long-term evolution of extrasolar planets is discussed. Simulation shows that through hydrodynamic escape of hydrogen, a planet at the orbit of Mercury (0.4 AU) and with 0.5 Uranus mass can lose about 10% of its mass within 850 million yr if the solar EUV radiation is 10 times the present level. This calculation provides an indication of how Mercury may have evolved during the early days of the solar system. Subject heading gs: planetary systems — planets and satellites: general

Evolution of protoplanetary disks: Constraints from DM Tauri and GM Aurigae

Abstract: We present a one-dimensional model of the formation and viscous evolution of protoplanetary disks. The formation of the early disk is modeled as the result of the gravitational collapse of an isothermal molecular cloud. The disk’s viscous evolution is integrated according to two parameterizations of turbulence: the classical α representation and a β parameterization, representative of non-linear turbulence driven by the keplerian shear. We apply the model to DM Tau and GM Aur, two classical T-Tauri stars with relatively well-characterized disks, retrieving the evolution of their surface density with time. We perform a systematic Monte-Carlo exploration of the parameter space (i.e. values of the α-β parameters, and of the temperature and rotation rate in the molecular cloud) to find the values that are compatible with the observed disk surface density distribution, star and disk mass, age and present accretion rate. We find that the observations for DM Tau require 0.001 <α< 0. 1o r 2 × 10 −5 <β< 5 × 10 −4 . For GM Aur, we find that the turbulent viscosity is such that 4 × 10 −4 <α< 0.01 or 2 × 10 −6 <β< 8 × 10 −5 .T hese relatively large values show that an efficient turbulent diffusion mechanism is present at distances larger than ∼10 AU. This is to be compared to studies of the variations of accretion rates of T-Tauri stars versus age that mostly probe the inner disks, but also yield values of α ∼ 0.01. We show that the mechanism responsible for turbulent diffusion at large orbital distances most probably cannot be convection because of its suppression at low optical depths.

Improving the Efficiency of Markov Chain Monte Carlo for Analyzing the Orbits of Extrasolar Planets

Abstract: Precise radial velocity measurements have led to the discovery of ~170 extrasolar planetary systems. Understanding the uncertainties in the orbital solutions will become increasingly important as the discovery space for extrasolar planets shifts to planets with smaller masses and longer orbital periods. The method of Markov chain Monte Carlo (MCMC) provides a rigorous method for quantifying the uncertainties in orbital parameters in a Bayesian framework (Ford 2005a). The main practical challenge for the general application of MCMC is the need to construct Markov chains which quickly converge. The rate of convergence is very sensitive to the choice of the candidate transition probability distribution function (CTPDF). Here we explain one simple method for generating alternative CTPDFs which can significantly speed convergence by one to three orders of magnitude. We have numerically tested dozens of CTPDFs with simulated radial velocity data sets to identify those which perform well for different types of orbits and suggest a set of CTPDFs for general application. Additionally, we introduce other refinements to the MCMC algorithm for radial velocity planets, including an improved treatment of the uncertainties in the radial velocity observations, an algorithm for automatically choosing step sizes, an algorithm for automatically determining reasonable stopping times, and the use of importance sampling for including the dynamical evolution of multiple planet systems. Together, these improvements make it practical to apply MCMC to multiple planet systems. We demonstrate the improvements in efficiency by analyzing a variety of extrasolar planetary systems.

A Jovian-Mass Planet in Microlensing Event OGLE-2005-BLG-071

Abstract: We report the discovery of a several-Jupiter mass planetary companion to the primary lens star in microlensing event OGLE-2005-BLG-071. Precise (<1%) photometry at the peak of the event yields an extremely high signal-to-noise ratio detection of a deviation from the light curve expected from an isolated lens. The planetary character of this deviation is easily and unambiguously discernible from the gross features of the light curve. Detailed modeling yields a tightly-constrained planet-star mass ratio of q=m_p/M=0.0071+/-0.0003. This is the second robust detection of a planet with microlensing, demonstrating that the technique itself is viable and that planets are not rare in the systems probed by microlensing, which typically lie several kpc toward the Galactic center.

Comparative Planetary Atmospheres: Models of TrES-1 and HD 209458b

Abstract: We present new self-consistent atmosphere models for the transiting planets TrES-1 and HD 209458b. The planets were recently observed with the Spitzer Space Telescope in bands centered on 4.5 and 8.0 μm, for TrES-1, and 24 μm, for HD 209458b. We find that standard solar-metallicity models fit the observations for HD 209458b. For TrES-1, which has a Teff ~300 K cooler, we find that models with a metallicity 3-5 times enhanced over solar abundances can match the 1 σ error bar at 4.5 μm and 2 σ at 8.0 μm. Models with solar abundances that include energy deposition into the stratosphere give fluxes that fall within the 2 σ error bars in both bands. The best-fit models for both planets assume that reradiation of absorbed stellar flux occurs over the entire planet. For all models of both planets, we predict planet-to-star flux ratios in other Spitzer bandpasses.

The Truncated Disk of CoKu Tau/4

Abstract: We present a model of a dusty disk with an inner hole that accounts for the Spitzer Space Telescope Infrared Spectrograph observations of the low-mass pre-main-sequence star CoKu Tau/4. We have modeled the mid-infrared spectrum (between 8 and 25 μm) as arising from the inner wall of a disk. Our model disk has an evacuated inner zone of radius ~10 AU, with a dusty inner "wall" of half-height ~2 AU that is illuminated at normal incidence by the central star. The radiative equilibrium temperature decreases from the inner disk edge outward through the optically thick disk; this temperature gradient is responsible for the emission of the silicate bands at 10 and 20 μm. The observed spectrum is consistent with being produced by Fe-Mg amorphous glassy olivine and/or pyroxene, with no evidence of a crystalline component. The mid-infrared spectrum of CoKu Tau/4 is reminiscent of that of the much older star TW Hya, where it has been suggested that the significant clearing of its inner disk is due to planet formation. However, no inner disk remains in CoKu Tau/4, consistent with the star being a weak-emission (nonaccreting) T Tauri star. The relative youth of CoKu Tau/4 (~1 Myr) may indicate much more rapid planet formation than typically assumed.

Metallicity of M dwarfs - I. A photometric calibration and the impact on the mass-luminosity relation at the bottom of the main sequence

Abstract: We obtained high resolution ELODIE and CORALIE spectra for both components of 20 wide visual binaries composed of an F-, G- or K-dwarf primary and an M-dwarf secondary. We analyse the well-understood spectra of the primaries to determine metallicities ([Fe/H]) for these 20 systems, and hence for their M dwarf components. We pool these metallicities with determinations from the literature to obtain a precise (± 0.2 dex) photometric calibration of M dwarf metallicities. This calibration represents a breakthrough in a field where discussions have had to remain largely qualitative, and it helps us demonstrate that metallicity explains most of the large dispersion in the empirical V -band mass-luminosity relation. We examine the metallicity of the two known M-dwarf planet-host stars, Gl 876 (+0.02 dex) and Gl 436 (-0.03 dex), in the context of preferential planet formation around metal-rich stars. We finally determine the metallicity of the 47 brightest single M dwarfs in a volume-limited sample, and compare the metallicity distributions of solar-type and M-dwarf stars in the solar neighbourhood.

The HARPS search for southern extra-solar planets VI. A Neptune-mass planet around the nearby M dwarf Gl 581

Abstract: We report the discovery of a Neptune-mass planet around Gl 581 (M3V, M = 0.31 ), based on precise Doppler measurements with the HARPS spectrograph at La Silla Observatory. The radial velocities reveal a circular orbit of period P = 5.366 days and semi-amplitude K1 = 13.2 m s-1. The resulting minimum mass of the planet ( ) is only 0.052 = 0.97 = 16.6 making Gl 581b one of the lightest extra-solar planet known to date. The Gl 581 planetary system is only the third centered on an M dwarf, joining the Gl 876 three-planet system and the lone planet around Gl 436. Its discovery reinforces the emerging tendency of such planets to be of low mass, and found at short orbital periods. The statistical properties of the planets orbiting M dwarfs do not seem to match a simple mass scaling of their counterparts around solar-type stars.

CO emission from discs around isolated HAeBe and Vega-excess stars

Abstract: We describe results from a survey for J = 3‐2 12 CO emission from visible stars classified as having an infrared excess. The line is clearly detected in 21 objects, and significant molecular gas (10 −3 Jupiter masses) is found to be common in targets with infrared excesses 0.01 (56 per cent of objects), but rare for those with smaller excesses (∼10 per cent of objects). A simple geometrical argument based on the infrared excess implies that disc opening angles are typically 12 ◦ for objects with detected CO; within this angle, the disc is optically thick to stellar radiation and shields the CO from photodissociation. Two or three CO discs have an unusually low infrared excess (0.01), implying the shielding disc is physically very thin (1 ◦ ). Around 50 per cent of the detected line profiles are double-peaked, while many of the rest have significantly broadened lines, attributed to discs in Keplerian rotation. Simple model fits to the line profiles indicate outer radii in the range 30‐300 au, larger than found through fitting continuum SEDs, but similar to the sizes of debris discs around main-sequence stars. As many as five have outer radii smaller than the Solar System (50 au), with a further four showing evidence of gas in the disc at radii smaller than 20 au. The outer disc radius is independent of the stellar spectral type (from K through to B9), but there is evidence of a correlation between radius and total dust mass. Also the mean disc size appears to decrease with time: discs around stars of age 3-7 Myr have a mean radius ∼210 au, whereas discs of age 7-20 Myr are a factor of three smaller. This shows that a significant mass of gas (at least 2 M⊕ )e xists beyond the region of planet formation for up to ∼7 Myr, and may remain for a further ∼10 Myr within this region. The only bona fide debris disc with detected CO is HD9672; this shows a double-peaked CO profile and is the most compact gas disc observed, with a modelled outer radius of 17 au. In the case of HD141569, detailed modelling of the line profile indicates gas may lie in two rings, with radii of 90 and 250 au, similar to the dust structure seen in scattered light and the mid-infrared. In both AB Aur and HD163296 we also find that the sizes of the molecular disc and the dust scattering disc are similar; this suggests that the molecular gas and small dust grains are closely co-located. Ke yw ords: planetary systems: proto planetary discs ‐ submillimetre.

Spectroscopic metallicities for planet-host stars: Extending the samples

Abstract: We present stellar parameters and metallicities for 29 planet-host stars, as well as for a large volume-limited sample of 53 stars not known to be orbited by any planetary-mass companion. These stars add to the results presented in our previous series of papers, providing two large and uniform samples of 119 planet-hosts and 94 “single” stars with accurate stellar parameters and [Fe/H] estimates. The analysis of the results further confirms that stars with planets are metal-rich when compared with average field dwarfs. Important biases that may compromise future studies are also discussed. Finally, we compare the metallicity distributions for single planet-hosts and planet-hosts in multiple stellar systems. The results show that a small difference cannot be excluded, in the sense that the latter sample is slighly overmetallic. However, more data are needed to confirm this correlation.

Five New Multicomponent Planetary Systems

Abstract: We report Doppler measurements for six nearby G- and K-type main-sequence stars that show multiple low-mass companions, at least one of which has planetary mass. One system has three planets, the fourth triple-planet system known around a normal star, and another has an extremely low minimum mass of 18 M_⊕. HD 128311 (K0 V) has two planets (one previously known) with minimum masses (M sin i) of 2.18M_J and 3.21M_J and orbital periods of 1.26 and 2.54 yr, suggesting a possible 2 : 1 resonance. For HD 108874 (G5 V), the velocities reveal two planets (one previously known) having minimum masses and periods of (M sin i_b = 1.36M_J, P_b = 1.08 yr) and (M sin i_c = 1.02M_J, P_c = 4.4 yr). HD 50499 (G1 V) has a planet with P = 6.8 yr and M sin i = 1.7M_J, and the velocity residuals exhibit a trend of -4.8 m s^(-1) yr^(-1), indicating a more distant companion with P > 10 yr and minimum mass of 2M_J. HD 37124 (G4 IV-V) has three planets, one having M sin i = 0.61M_J and P = 154.5 days, as previously known. We find two plausible triple-planet models that fit the data, both having a second planet near P = 840 days, with the more likely model having its third planet in a 6 yr orbit and the other one in a 29 day orbit. For HD 190360, we confirm the planet having P = 7.9 yr and M sin i = 1.5M_J as found by the Geneva team, but we find a distinctly noncircular orbit with e = 0.36 ± 0.03, rendering this not an analog of Jupiter as had been reported. Our velocities also reveal a second planet with P = 17.1 days and M sin i = 18.1 M_⊕. HD 217107 (G8 IV) has a previously known "hot Jupiter" with M sin i = 1.4M_J and P = 7.13 days, and we confirm its high eccentricity, e = 0.13. The velocity residuals reveal an outer companion in an eccentric orbit, having minimum mass of M sin i > 2M_J, eccentricity e ~ 0.5, and a period P > 8 yr, implying a semimajor axis α > 4 AU and providing an opportunity for direct detection. We have obtained high-precision photometry of five of the six planetary host stars with three of the automated telescopes at Fairborn Observatory. We can rule out significant brightness variations in phase with the radial velocities in most cases, thus supporting planetary reflex motion as the cause of the velocity variations. Transits are ruled out to very shallow limits for HD 217107 and are also shown to be unlikely for the prospective inner planets of the HD 37124 and HD 108874 systems. HD 128311 is photometrically variable with an amplitude of 0.03 mag and a period of 11.53 days, which is much shorter than the orbital periods of its two planetary companions. This rotation period explains the origin of periodic velocity residuals to the two-planet model of this star. All of the planetary systems here would be further constrained with astrometry by the Space Interferometry Mission.