Showing papers on "Planetary system published in 2009"

Protoplanetary disk structures in ophiuchus

Abstract: We present the results of a high angular resolution (0.''3 {approx} 40 AU) Submillimeter Array survey of the 345 GHz (870 {mu}m) thermal continuum emission from nine of the brightest, and therefore most massive, circumstellar disks in the {approx}1 Myr-old Ophiuchus star-forming region. Using two-dimensional radiative transfer calculations, we simultaneously fit the observed continuum visibilities and broadband spectral energy distribution for each disk with a parametric structure model. Compared to previous millimeter studies, this survey includes significant upgrades in modeling, data quality, and angular resolution that provide improved constraints on key structure parameters, particularly those that characterize the spatial distribution of mass in the disks. In the context of a surface density profile motivated by similarity solutions for viscous accretion disks, {sigma} {proportional_to} (R/R{sub c} ){sup -{gamma}}exp [ - (R/R{sub c} ){sup 2-{gamma}}], the best-fit models for the sample disks have characteristic radii R{sub c} {approx} 20-200 AU, high disk masses M{sub d} {approx} 0.005-0.14 M {sub sun} (a sample selection bias), and a narrow range of radial {sigma} gradients ({gamma} {approx} 0.4-1.0) around a median {gamma} = 0.9. These density structures are used in conjunction with accretion rate estimates from the literature to help characterize the viscous evolution ofmore » the disk material. Using the standard prescription for disk viscosities, those combined constraints indicate that {alpha} {approx} 0.0005-0.08. Three of the sample disks show large (R {approx} 20-40 AU) central cavities in their continuum emission morphologies, marking extensive zones where dust has been physically removed and/or has significantly diminished opacities. Based on the current requirements of planet formation models, these emission cavities and the structure constraints for the sample as a whole suggest that these young disks may eventually produce planetary systems, and have perhaps already started.« less

Transiting exoplanets from the CoRoT space mission VIII. CoRoT-7b: the first Super-Earth with measured radius

Abstract: Aims. We report the discovery of very shallow (ΔF/F ≈ 3.4× 10 −4 ), periodic dips in the light curve of an active V = 11.7 G9V star observed by the CoRoT satellite, which we interpret as caused by a transiting companion. We describe the 3-colour CoRoT data and complementary ground-based observations that support the planetary nature of the companion. Methods. We used CoRoT colours information, good angular resolution ground-based photometric observations in- and out- of transit, adaptive optics imaging, near-infrared spectroscopy, and preliminary results from radial velocity measurements, to test the diluted eclipsing binary scenarios. The parameters of the host star were derived from optical spectra, which were then combined with the CoRoT light curve to derive parameters of the companion. Results. We examined all conceivable cases of false positives carefully, and all the tests support the planetary hypothesis. Blends with separation >0.40 �� or triple systems are almost excluded with a 8 × 10 −4 risk left. We conclude that, inasmuch we have been exhaustive, we have discovered a planetary companion, named CoRoT-7b, for which we derive a period of 0.853 59 ± 3 × 10 −5 day and a radius of Rp = 1.68 ± 0.09 REarth .A nalysis of preliminary radial velocity data yields an upper limit of 21 MEarth for the companion mass, supporting the finding. Conclusions. CoRoT-7b is very likely the first Super-Earth with a measured radius. This object illustrates what will probably become a common situation with missions such as Kepler, namely the need to establish the planetary origin of transits in the absence of a firm radial velocity detection and mass measurement. The composition of CoRoT-7b remains loosely constrained without a precise mass. A very high surface temperature on its irradiated face, ≈1800–2600 K at the substellar point, and a very low one, ≈50 K, on its dark face assuming no atmosphere, have been derived.

A probable giant planet imaged in the β Pictoris disk. VLT/NaCo deep L'-band imaging

Abstract: Context. Since the discovery of its dusty disk in 1984, β Pictoris has become the prototype of young early-type planetary systems, and there are now various indications that a massive Jovian planet is orbiting the star at ∼10 AU. However, no planets have been detected around this star so far. Aims. Our goal was to investigate the close environment of β Pic, searching for planetary companion(s). Methods. Deep adaptive-optics L � -band images of β Pic were recorded using the NaCo instrument at the Very Large Telescope. Results. A faint point-like signal is detected at a projected distance of � 8 AU from the star, within the northeastern extension of the dust disk. Various tests were made to rule out possible instrumental or atmospheric artefacts at a good confidence level. The probability of a foreground or background contaminant is extremely low, based in addition on the analysis of previous deep HST images. Its L � = 11.2 apparent magnitude would indicate a typical temperature of ∼1500 K and a mass of ∼8 MJup. If confirmed, it could explain the main morphological and dynamical peculiarities of the β Pic system. The present detection is unique among A-stars by the proximity of the resolved planet to its parent star. Its closeness and location inside the β Pic disk suggest a formation process by core accretion or disk instabilities rather than binary-like formation processes.

Extrasolar planet population synthesis - I. Method, formation tracks, and mass-distance distribution

Abstract: Context. With the high number of extrasolar planets discovered by now, it has become possible to use the properties of this planetary population to constrain theoretical formation models in a statistical sense. This paper is the first in a series in which we carry out a large number of planet population synthesis calculations within the framework of the core accretion scenario. We begin the series with a paper mainly dedicated to the presentation of our approach, but also the discussion of a representative synthetic planetary population of solar like stars. In the second paper we statistically compare the subset of detectable planets to the actual extrasolar planets. In subsequent papers, we shall extend the range of stellar masses and the properties of protoplanetary disks.Aims. The last decade has seen a large observational progress in characterizing both protoplanetary disks, and extrasolar planets. Concurrently, progress was made in developing complex theoretical formation models. The combination of these three developments allows a new kind of study: the synthesis of a population of planets from a model, which is compared with the actual population. Our aim is to obtain a general overview of the population, to check if we quantitatively reproduce the most important observed properties and correlations, and to make predictions about the planets that are not yet observable.Methods. Based as tightly as possible on observational data, we have derived probability distributions for the most important initial conditions for the planetary formation process. We then draw sets of initial conditions from these distributions and obtain the corresponding synthetic planets with our formation model. By repeating this step many times, we synthesize the populations.Results. Although the main purpose of this paper is the description of our methods, we present some key results: we find that the variation of the initial conditions in the limits occurring in nature leads to the formation of planets of wide diversity. This formation process is best visualized in planetary formation tracks in the mass-semimajor axis diagram, where different phases of concurrent growth and migration can be identified. These phases lead to the emergence of sub-populations of planets distinguishable in a mass-semimajor axis diagram. The most important ones are the “failed cores”, a vast group of core-dominated low mass planets, the “horizontal branch”, a sub-population of Neptune mass planets extending out to 6 AU, and the “main clump”, a concentration of giant gaseous planets at around 0.3-2 AU.

The HARPS search for southern extra-solar planets - XVII. Super-Earth and Neptune-mass planets in multiple planet systems HD 47 186 and HD 181 433

Abstract: We report on the detection of two new multiple planet systems orbiting solar-like stars HD 47 186 and HD 181 433. The first system contains a hot Neptune of 22.78 M ⊕ with a 4.08-day period and a Saturn of 0.35 M JUP with a 3.7-year period. The second system contains a Super-Earth of 7.5 M ⊕ with a 9.4-day period, a 0.64 M JUP with a 2.6-year period, and a third companion of 0.54 M JUP with a period of about 6 years. These detections increase to 20 the number of close-in low-mass exoplanets (below 0.1 M JUP) and strengthen the fact that 80% of these planets are in multiple planetary systems.

Protoplanetary Disk Structures in Ophiuchus

Abstract: We present a high angular resolution (0.3" = 40 AU) SMA survey of the 870 micron thermal continuum emission from 9 of the brightest, and therefore most massive, circumstellar disks in the ~1 Myr-old Ophiuchus star-forming region. Using 2-D radiative transfer calculations, we simultaneously fit the observed continuum visibilities and broadband spectral energy distribution for each disk with a parametric structure model. Compared to previous millimeter studies, this survey includes significant upgrades in modeling, data quality, and angular resolution that provide improved constraints on key structure parameters, particularly those that characterize the spatial distribution of mass in the disks. In the context of a surface density profile motivated by similarity solutions for viscous accretion disks, the best-fit models for the sample disks have characteristic radii R_c = 20-200 AU, high disk masses M_d = 0.005-0.14 M_sun, and a narrow range of radial surface density gradients around a median $\gamma$ = 0.9. These density structures are used in conjunction with accretion rate estimates from the literature to help characterize the viscous evolution of the disk material. Using the standard prescription for disk viscosities, those combined constraints indicate that $\alpha$ = 0.0005-0.08. Three of the sample disks show large (R = 20-40 AU) central cavities in their continuum emission morphologies, marking extensive zones where dust has been physically removed and/or has significantly diminished opacities. Based on the current requirements of planet formation models, these emission cavities and the structure constraints for the sample as a whole suggest that these young disks may eventually produce planetary systems, and have perhaps already started. (abridged)

The CoRoT-7 planetary system: two orbiting super-Earths

Abstract: We report on an intensive observational campaign carried out with HARPS at the 3.6 m telescope at La Silla on the star CoRoT-7. Additional simultaneous photometric measurements carried out with the Euler Swiss telescope have demonstrated that the observed radial velocity variations are dominated by rotational modulation from cool spots on the stellar surface. Several approaches were used to extract the radial velocity signal of the planet(s) from the stellar activity signal. First, a simple pre-whitening procedure was employed to find and subsequently remove periodic signals from the complex frequency structure of the radial velocity data. The dominant frequency in the power spectrum was found at 23 days, which corresponds to the rotation period of CoRoT-7. The 0.8535 day period of CoRoT-7b planetary candidate was detected with an amplitude of 3.3 m s −1 . Most other frequencies, some with amplitudes larger than the CoRoT-7b signal, are most likely associated with activity. A second approach used harmonic decomposition of the rotational period and up to the first three harmonics to filter out the activity signal from radial velocity variations caused by orbiting planets. After correcting the radial velocity data for activity, two periodic signals are detected: the CoRoT-7b transit period and a second one with a period of 3.69 days and an amplitude of 4 m s −1 . This second signal was also found in the pre-whitening analysis. We attribute the second signal to a second, more remote planet CoRoT-7c . The orbital solution of both planets is compatible with circular orbits. The mass of CoRoT-7b is 4.8 ± 0. 8( M⊕) and that of CoRoT-7c is 8.4 ± 0. 9( M⊕), assuming both planets are on coplanar orbits. We also investigated the false positive scenario of a blend by a faint stellar binary, and this may be rejected by the stability of the bisector on a nightly scale. According to their masses both planets belong to the super-Earth planet category. The average density of CoRoT-7b is ρ = 5.6 ± 1. 3gc m −3 , similar to the Earth. The CoRoT-7 planetary system provides us with the first insight into the physical nature of short period super-Earth planets recently detected by radial velocity surveys. These planets may be denser than Neptune and therefore likely made of rocks like the Earth, or a mix of water ice and rocks.

The Peculiar Solar Composition and Its Possible Relation to Planet Formation

Abstract: We have conducted a differential elemental abundance analysis of unprecedented accuracy (~0.01 dex) of the Sun relative to 11 solar twins from the Hipparcos catalog and 10 solar analogs from planet searches. We find that the Sun shows a characteristic signature with a 20% depletion of refractory elements relative to the volatile elements in comparison with the solar twins. The abundance differences correlate strongly with the condensation temperatures of the elements. This peculiarity also holds in comparisons with solar analogs known to have close-in giant planets while the majority of solar analogs found not to have such giant planets in radial velocity monitoring show the solar abundance pattern. We discuss various explanations for this peculiarity, including the possibility that the differences in abundance patterns are related to the formation of planetary systems like our own, in particular to the existence of terrestrial planets.

The HARPS search for southern extra-solar planets: XVIII. An Earth-mass planet in the GJ 581 planetary system

Abstract: The GJ 581 planetary system was already known to harbour three planets, including two presumably rocky planets which straddle its habitable zone. We report here the detection of an additional planet ‐ GJ 581e ‐ with a minimum mass of 1.9 M" . With a period of 3.15 days, it is the innermost planet of the system and has a # 5% transit probability. We also correct our previous confusion of the orbital period of GJ 581d (the outermost planet) with a one-year alias, thanks to an extended time span and many more measurements. The revised period is 66.8 days, and locates the semi-major axis inside the habitable zone of the low mass star. The dynamical stability of the 4-planet system imposes an upper bound on the orbital plane inclination. The planets cannot be more massive than approximately 1.6 times their minimum mass.

WASP-12b: the hottest transiting extrasolar planet yet discovered

Abstract: We report on the discovery of WASP-12b, a new transiting extrasolar planet with R pl = 1.79+0.09 –0.09 RJ and M pl = 1.41+0.10 –0.10 M J. The planet and host star properties were derived from a Monte Carlo Markov Chain analysis of the transit photometry and radial velocity data. Furthermore, by comparing the stellar spectrum with theoretical spectra and stellar evolution models, we determined that the host star is a supersolar metallicity ([M/H] = 0.3+0.05 –0.15), late-F (T eff = 6300+200 –100 K) star which is evolving off the zero-age main sequence. The planet has an equilibrium temperature of T eq = 2516 K caused by its very short period orbit (P = 1.09 days) around the hot, twelfth magnitude host star. WASP-12b has the largest radius of any transiting planet yet detected. It is also the most heavily irradiated and the shortest period planet in the literature.

Extrasolar planet population synthesis I: Method, formation tracks and mass-distance distribution

Abstract: With the high number of extrasolar planets discovered by now, it becomes possible to constrain theoretical formation models in a statistical sense. This paper is the first in a series in which we carry out a large number of planet population synthesis calculations. We begin the series with a paper mainly dedicated to the presentation of our approach, but also the discussion of a representative synthetic planetary population of solar like stars. Based as tightly as possible on observational data, we have derived probability distributions for the most important initial conditions for the planetary formation process. We then draw sets of initial conditions from these distributions and obtain the corresponding synthetic planets with our formation model. Although the main purpose of this paper is the description of our methods, we present some key results: We find that the variation of the initial conditions in the limits occurring in nature leads to the formation of planets of large diversity. This formation process is best visualized in planetary formation tracks, where different phases of concurrent growth and migration can be identified. These phases lead to the emergence of sub-populations of planets distinguishable in a mass-semimajor axis diagram. The most important ones are the "failed cores", a vast group of core-dominated low mass planets, the "horizontal branch", a sub-population of Neptune mass planets extending out to 6 AU, and the "main clump", a concentration of giant gaseous giants planets at around 0.3-2 AU.

Ten New and Updated Multiplanet Systems and a Survey of Exoplanetary Systems

Abstract: We present the latest velocities for ten multiplanet systems, including a re-analysis of archival Keck and Lick data, resulting in improved velocities that supersede our previously published measurements. We derive updated orbital fits for 10 Lick and Keck systems, including two systems (HD 11964, HD 183263) for which we provide confirmation of second planets only tentatively identified elsewhere, and two others (HD 187123 and HD 217107) for which we provide a major revision of the outer planet's orbit. We compile orbital elements from the literature to generate a catalog of the 28 published multiple-planet systems around stars within 200 pc. From this catalog we find several intriguing patterns emerging: (1) including those systems with long-term radial velocity trends, at least 28% of known planetary systems appear to contain multiple planets; (2) planets in multiple-planet systems have somewhat smaller eccentricities than single planets; and (3) the distribution of orbital distances of planets in multiplanet systems and single planets are inconsistent: single-planet systems show a pileup at P ~ 3 days and a jump near 1 AU, while multiplanet systems show a more uniform distribution in log-period. In addition, among all planetary systems we find the following. (1) There may be an emerging, positive correlation between stellar mass and giant-planet semimajor axis. (2) Exoplanets with M sin i > 1 M_(Jup) more massive than Jupiter have eccentricities broadly distributed across 0 < e < 0.5, while lower mass exoplanets exhibit a distribution peaked near e = 0.

Extrasolar planet population synthesis. II. Statistical comparison with observations

Abstract: Context. This is the second paper in a series of papers showing the results of extrasolar planet population synthesis calculations using our extended core accretion model. In the companion paper (Paper I), we have presented in detail the methods we use. In subsequent papers, we shall discuss the e ect of the host star’s mass on the planetary population and the influence of various properties of protoplanetary disks. Aims. In this second paper, we focus on planets orbiting solar-like stars. The goal is to use the main characteristics of the actually observed extrasolar planet population to derive in a statistical manner constraints on the planet formation models. Methods. Drawing initial conditions for our models at random from probability distributions derived as closely as possible from observations, we synthesize a number of planetary populations. By applying an observational detection bias appropriate for radial velocity surveys, we identify the potentially detectable synthetic planets. The properties of these planets are compared in quantitative statistical tests with the properties of a carefully selected sub-population of actually observed extrasolar planets. Results. We use a two dimensional Kolmogorov-Smirnov test to compare the mass-distance distributions of synthetic and observed planets, as well as the one dimensional version of the test to compare the M sini, the semimajor axis and the [Fe/H] distribution. We find that while many combinations of parameters lead to unacceptable distributions, a number of models can account to a reasonable degree of statistical significance for most of the properties of the observed sample. We concurrently account for many other observed features, e.g. the “metallicity e ect”. This gives us confidence that our model captures several essential features of giant planet formation. In addition, the fact that many parameter combination could be rejected, indicates that planet population synthesis is indeed a promising approach to constrain formation models. Our simulations allow us also to extract a number of properties of the underlying exoplanet population that are not yet directly detectable. For example, we have derived the planetary initial mass function (PIMF) and have been led to conclude that the planets detected so far represent only the tip of the iceberg (9%) of all the existing planets. The PIMF can also be used to predict how the detectable extrasolar planet population will change as the instrumental precision of radial velocity surveys improves from 10 m/s to 1 m/s, or even to an extreme precision of 0.1 m/s.

Structure and evolution of pre-main-sequence circumstellar disks

Abstract: We present new subarcsecond (~0".7) Combined Array for Research in Millimeter-wave Astronomy (CARMA) observations of the 1.3 mm continuum emission from circumstellar disks around 11 low- and intermediate-mass pre-main-sequence stars. High-resolution observations for three additional sources were obtained from the literature. In all cases the disk emission is spatially resolved. We adopt a self-consistent accretion disk model based on the similarity solution for the disk surface density and constrain the dust radial density distribution on spatial scales of about 40 AU. Disk surface densities appear to be correlated with the stellar ages where the characteristic disk radius increases from ~20 AU to ~100 AU over about 5 Myr. This disk expansion is accompanied by a decrease in the mass accretion rate, suggesting that our sample disks form an evolutionary sequence. Interpreting our results in terms of the temporal evolution of a viscous α-disk, we estimate (1) that at the beginning of the disk evolution about 60% of the circumstellar material was located inside radii of 25-40 AU, (2) that disks formed with masses from 0.05 to 0.4 M⊙ , and (3) that the viscous timescale at the disk initial radius is about 0.1-0.3 Myr. Viscous disk models tightly link the surface density Σ(R) with the radial profile of the disk viscosity ν(R) ∝ R^γ. We find values of γ ranging from –0.8 to 0.8, suggesting that the viscosity dependence on the orbital radius can be very different in the observed disks. Adopting the α parameterization for the viscosity, we argue that α must decrease with the orbital radius and that it may vary between 0.5 and 10^(–4). From the inferred disk initial radii we derive specific angular momenta, j, for parent cores of (0.8 – 4) × 10^(–4) km s^(–1) pc. Comparison with the values of j in dense cores suggests that about 10% of core angular momentum and 30% of the core mass are conserved in the formation of the star/disk system. We demonstrate that the similarity solution for the surface density for γ < 0 can explain the properties of some "transitional disks" without requiring discontinuities in the disk surface density. In the case of LkCa 15, a smooth distribution of material from few stellar radii to about 240 AU can produce both the observed spectral energy distribution and the spatially resolved continuum emission at millimeter wavelengths. Finally we show that among the observed sample, TW Hya is the only object that has a disk radius comparable with the early solar nebula.

The HARPS search for southern extra-solar planets - XIII. A planetary system with 3 super-Earths (4.2, 6.9, and 9.2 M)

Abstract: Received ; accepted To be inserted later Abstract. This paper reports on the detection of a planetary system with three Super-Earths orbiting HD 40307. HD 40307 is a K2 V metal-deficient star at a distance of only 13 parsec, part of the HARPS GTO high-precision planet-search programme. The three planets on circular orbits have very low minimum masses of respectively 4.2, 6.9 and 9.2 Earth masses and periods of 4.3, 9.6 and 20.5 days. The planet with the shortest period is the lightest planet detected to-date orbiting a main sequence star. The detection of the correspondingly low amplitudes of the induced radial-velocity variations is completely secured by the 135 very high-quality HARPS observations illustrated by the radial-velocity residuals around the 3-Keplerian solution of only 0.85 ms 1 . Activity and bisector indicators exclude any significant perturbations of stellar intrinsic origin, which supports the planetary interpretation. Contrary to most planet-host stars, HD 40307 has a marked sub-solar metallicity ((Fe/H) = 0.31), further supporting the already raised possibility that the occurrence of very light planets might show a di erent dependence on host star's metallicity compared to the population of gas giant planets. In addition to the 3 planets close to the central star, a small drift of the radial-velocity residuals reveals the presence of another companion in the system the nature of which is still unknown.

Infrared signatures of disrupted minor planets at white dwarfs

Abstract: Spitzer Space Observatory IRAC and MIPS photometric observations are presented for 20 white dwarfs with T eff 20, 000 K and metal-contaminated photospheres. A warm circumstellar disk is detected at GD 16 and likely at PG 1457?086, while the remaining targets fail to reveal mid-infrared excess typical of dust disks, including a number of heavily polluted stars. Extending previous studies, over 50% of all single white dwarfs with implied metal-accretion rates dM/dt 3 ? 108 g s?1 display a warm infrared excess from orbiting dust; the likely result of a tidally destroyed minor planet. This benchmark accretion rate lies between the dust production rates of 106 g s?1 in the solar system zodiacal cloud and 1010 g s?1 often inferred for debris disks at main-sequence A-type stars. It is estimated that between 1% and 3% of all single white dwarfs with cooling ages less than around 0.5 Gyr possess circumstellar dust, signifying an underlying population of minor planets.

PROSAC: a submillimeter array survey of low-mass protostars - II. The mass evolution of envelopes, disks, and stars from the class 0 through I stages

Abstract: Context. The key question about early protostellar evolution is how matter is accreted from the large-scale molecular cloud, through the circumstellar disk onto the central star. Aims. We constrain the masses of the envelopes, disks, and central stars of a sample of low-mass protostars and compare the results to theoretical models for the evolution of young stellar objects through the early protostellar stages. Methods. A sample of 20 Class 0 and I protostars has been observed in continuum at (sub)millimeter wavelengths at high angular resolution (typically 2 �� ) with the submillimeter array. Using detailed dust radiative transfer models of the interferometric data, as well as single-dish continuum observations, we have developed a framework for disentangling the continuum emission from the envelopes and disks, and from that estimated their masses. For the Class I sources in the sample HCO + 3–2 line emission was furthermore observed with the submillimeter array. Four of these sources show signs of Keplerian rotation, making it possible to determine the masses of the central stars. In the other sources the disks are masked by optically thick envelope and outflow emission. Results. Both Class 0 and I protostars are surrounded by disks with typical masses of about 0.05 M� , although significant scatter is seen in the derived disk masses for objects within both evolutionary stages. No evidence is found for a correlation between the disk . .. . ..

The Two Modes of Gas Giant Planet Formation

Abstract: I argue for two modes of gas giant planet formation and discuss the conditions under which each mode operates. Gas giant planets at disk radii r>100 AU are likely to form in situ by disk instability, while core accretion plus gas capture remains the dominant formation mechanism for r < 100 AU. During the mass accretion phase, mass loading can push disks toward fragmentation conditions at large r. Massive, extended disks can fragment into clumps of a few to tens of Jupiter masses. This is confirmed by radiation hydrodynamics simulations. The two modes of gas giant formation should lead to a bimodal distribution of gas giant semimajor axes. Because core accretion is expected to be less efficient in low-metallicity systems, the ratio of gas giants at large r to planets at small r should increase with decreasing metallicity.

The orbital evolution of gas giant planets around giant stars

Abstract: Recent surveys have revealed a lack of close-in planets around evolved stars more massive than 1.2 M ☉. Such planets are common around solar-mass stars. We have calculated the orbital evolution of planets around stars with a range of initial masses, and have shown how planetary orbits are affected by the evolution of the stars all the way to the tip of the red giant branch. We find that tidal interaction can lead to the engulfment of close-in planets by evolved stars. The engulfment is more efficient for more-massive planets and less-massive stars. These results may explain the observed semimajor axis distribution of planets around evolved stars with masses larger than 1.5 M ☉. Our results also suggest that massive planets may form more efficiently around intermediate-mass stars.

Time Evolution of Viscous Circumstellar Disks due to Photoevaporation by Far-Ultraviolet, Extreme-Ultraviolet, and X-ray Radiation from the Central Star

Abstract: We present the time evolution of viscously accreting circumstellar disks as they are irradiated by ultraviolet and X-ray photons from a low-mass central star Our model is a hybrid of a one-dimensional (1D) time-dependent viscous disk model coupled to a 1+1D disk vertical structure model used for calculating the disk structure and photoevaporation rates We find that disks of initial mass 01 M ☉ around ~1 M ☉ stars survive for ~4 × 106 yr, assuming a viscosity parameter α = 001, a time-dependent FUV luminosity L FUV ~ 10–2-10–3 L ☉ and with X-ray and EUV luminosities LX ~ L EUV ~ 10–3 L ☉ We find that FUV/X-ray-induced photoevaporation and viscous accretion are both important in depleting disk mass Photoevaporation rates are most significant at ~1-10 AU and at 30 AU Viscosity spreads the disk which causes mass loss by accretion onto the central star and feeds mass loss by photoevaporation in the outer disk We find that FUV photons can create gaps in the inner, planet-forming regions of the disk (~1-10 AU) at relatively early epochs in disk evolution while disk masses are still substantial EUV and X-ray photons are also capable of driving gaps, but EUV can only do so at late, low accretion-rate epochs after the disk mass has already declined substantially Disks around stars with predominantly soft X-ray fields experience enhanced photoevaporative mass loss We follow disk evolution around stars of different masses, and find that disk survival time is relatively independent of mass for stars with M * 3 M ☉; for M * 3 M ☉ the disks are short-lived (~105 yr)

The atmospheric signatures of super-earths: how to distinguish between hydrogen-rich and hydrogen-poor atmospheres

Abstract: Extrasolar super-Earths (1–10 M⊕) are likely to exist with a wide range of atmospheres. Some super-Earths may be able to retain massive hydrogen-rich atmospheres. Others might never accumulate hydrogen or experience significant escape of lightweight elements, resulting in atmospheres more like those of the terrestrial planets in our solar system. We examine how an observer could differentiate between hydrogen-rich and hydrogenpoor atmospheres by modeling super-Earth emission and transmission spectra, and we find that discrimination is possible by observing the transmission spectrum alone. An Earth-like atmosphere, composed of mostly heavy elements and molecules, will have a very weak transmission signal due to its small atmospheric scale height (since the scale height is inversely proportional to molecular weight). However, a large hydrogen-rich atmosphere reveals a relatively large transmission signal. The super-Earth emission spectrum can additionally constrain the atmospheric composition and temperature structure. Super-Earths with massive hydrogen atmospheres will reveal strong spectral features due to water, whereas those that have lost most of their hydrogen (and have no liquid ocean) will be marked by CO2 features and a lack of H2O. We apply our study specifically to the low-mass planet orbiting an M star, Gl 581c (M sin i = 5 M⊕), although our conclusions are relevant for super-Earths in general. The ability to distinguish hydrogen-rich atmospheres might be essential for interpreting mass and radius observations of planets in the transition between rocky super-Earths and Neptune-like planets.

Fomalhaut's debris disk and planet: constraining the mass of fomalhaut b from disk morphology

Abstract: Following the optical imaging of exoplanet candidate Fomalhaut b (Fom b), we present a numerical model of how Fomalhaut's debris disk is gravitationally shaped by a single interior planet. The model is simple, adaptable to other debris disks, and can be extended to accommodate multiple planets. If Fom b is the dominant perturber of the belt, then to produce the observed disk morphology it must have a mass M(sub pl) 101.5AU, and an orbital eccentricity e(sub pl) = 0.11 - 0.13. These conclusions are independent of Fom b's photometry. To not disrupt the disk, a greater mass for Fom b demands a smaller orbit farther removed from the disk; thus, future astrometric measurement of Fom b's orbit, combined with our model of planet-disk interaction, can be used to determine the mass more precisely. The inner edge of the debris disk at a approximately equals 133AU lies at the periphery of Fom b's chaotic zone, and the mean disk eccentricity of e approximately equals 0.11 is secularly forced by the planet, supporting predictions made prior to the discovery of Fom b. However, previous mass constraints based on disk morphology rely on several oversimplifications. We explain why our constraint is more reliable. It is based on a global model of the disk that is not restricted to the planet's chaotic zone boundary. Moreover, we screen disk parent bodies for dynamical stability over the system age of approximately 100 Myr, and model them separately from their dust grain progeny; the latter's orbits are strongly affected by radiation pressure and their lifetimes are limited to approximately 0.1 Myr by destructive grain-grain collisions. The single planet model predicts that planet and disk orbits be apsidally aligned. Fomalhaut b's nominal space velocity does not bear this out, but the astrometric uncertainties are difficult to quantify. Even if the apsidal misalignment proves real, our calculated upper mass limit of 3 M(sub J) still holds. Parent bodies are evacuated from mean-motion resonances with Fom b; these empty resonances are akin to the Kirkwood gaps opened by Jupiter. The belt contains at least 3M(sub Earth) of solids that are grinding down to dust, their velocity dispersions stirred so strongly by Fom b that collisions are destructive. Such a large mass in solids is consistent with Fom b having formed in situ.

Exoplanetary spin-orbit alignment: results from the ensemble of rossiter-mclaughlin observations

Abstract: One possible diagnostic of planet formation, orbital migration, and tidal evolution is the angle ψ between a planet's orbital axis and the spin axis of its parent star. In general, ψ cannot be measured, but for transiting planets one can measure the angle λ between the sky projections of the two axes via the Rossiter-McLaughlin effect. Here, we show how to combine measurements of λ in different systems to derive statistical constraints on ψ. We apply the method to 11 published measurements of λ, using two different single-parameter distributions to describe the ensemble. First, assuming a Rayleigh distribution (or more precisely, a Fisher distribution on a sphere), we find that the peak value is less than 22° with 95% confidence. Second, assuming that a fraction f of the orbits have random orientations relative to the stars, and the remaining fraction (1 – f) are perfectly aligned, we find f < 0.36 with 95% confidence. This latter model fits the data better than the Rayleigh distribution, mainly because the XO-3 system was found to be strongly misaligned while the other 10 systems are consistent with perfect alignment. If the XO-3 result proves robust, then our results may be interpreted as evidence for two distinct modes of planet migration.

Hat-p-7: a retrograde or polar orbit, and a third body

Abstract: We show that the exoplanet HAT-P-7b has an extremely tilted orbit, with a true angle of at least 86° with respect to its parent star's equatorial plane, and a strong possibility of retrograde motion. We also report evidence for an additional planet or companion star. The evidence for the unparalleled orbit and the third body is based on precise observations of the star's apparent radial velocity (RV). The anomalous RV due to rotation (the Rossiter-McLaughlin effect) was found to be a blueshift during the first half of the transit and a redshift during the second half, an inversion of the usual pattern, implying that the angle between the sky-projected orbital and stellar angular momentum vectors is 182°.5 ± 9°.4. The third body is implicated by excess RV variation of the host star over 2 yr. Some possible explanations for the tilted orbit of HAT-P-7b are a close encounter with another planet, the Kozai effect, and resonant capture by an inward-migrating outer planet.

The chemical history of molecules in circumstellar disks - I. Ices

Abstract: Context. Many chemical changes occur during the collapse of a molecular cloud to form a low-mass star and the surrounding disk. One-dimensional models have been used so far to analyse these chemical processes, but they cannot properly describe the incorporation of material into disks. Aims. The goal of this work is to understand how material changes chemically as it is transported from the cloud to the star and the disk. Of special interest is the chemical history of the material in the disk at the end of the collapse. Methods. A two-dimensional, semi-analytical model is presented that, for the first time, follows the chemical evolution from the pre-stellar core to the protostar and circumstellar disk. The model computes infall trajectories from any point in the cloud and tracks the radial and vertical motion of material in the viscously evolving disk. It includes a full time-dependent radiative transfer treatment of the dust temperature, which controls much of the chemistry. A small parameter grid is explored to understand the effects of the sound speed and the mass and rotation of the cloud. The freeze-out and evaporation of carbon monoxide (CO) and water (H 2 O), as well as the potential for forming complex organic molecules in ices, are considered as important first steps towards illustrating the full chemistry. Results. Both species freeze out towards the centre before the collapse begins. Pure CO ice evaporates during the infall phase and re-adsorbs in those parts of the disk that cool below the CO desorption temperature of ∼ 18 K. Water remains solid almost everywhere during the infall and disk formation phases and evaporates within ∼10 AU of the star. Mixed CO-H 2 O ices are important in keeping some solid CO above 18 K and in explaining the presence of CO in comets. Material that ends up in the planet- and comet-forming zones of the disk (∼5-30 AU from the star) is predicted to spend enough time in a warm zone (several 104 yr at a dust temperature of 20-40 K) during the collapse to form first-generation complex organic species on the grains. The dynamical timescales in the hot inner envelope (hot core or hot corino) are too short for abundant formation of second-generation molecules by high-temperature gas-phase chemistry.

An Infrared/X-ray Survey for New Members of the Taurus Star-Forming Region

Abstract: We present the results of a search for new members of the Taurus star-forming region using data from the Spitzer Space Telescope and the XMM-Newton Observatory. We have obtained optical and near-infrared spectra of 44 sources that exhibit red Spitzer colors that are indicative of stars with circumstellar disks and 51 candidate young stars that were identified by Scelsi and coworkers using XMM-Newton. We also performed spectroscopy on four possible companions to members of Taurus that were reported by Kraus and Hillenbrand. Through these spectra, we have demonstrated the youth and membership of 41 sources, 10 of which were independently confirmed as young stars by Scelsi and coworkers. Five of the new Taurus members are likely to be brown dwarfs based on their late spectral types (>M6). One of the brown dwarfs has a spectral type of L0, making it the first known L-type member of Taurus and the least massive known member of the region (M ~ 4-7 M_(Jup)). Another brown dwarf exhibits a flat infrared spectral energy distribution, which indicates that it could be in the protostellar class I stage (star+disk+envelope). Upon inspection of archival images from various observatories, we find that one of the new young stars has a large edge-on disk (r = 25 = 350 AU). The scattered light from this disk has undergone significant variability on a timescale of days in optical images from the Canada-France-Hawaii Telescope. Using the updated census of Taurus, we have measured the initial mass function for the fields observed by XMM-Newton. The resulting mass function is similar to previous ones that we have reported for Taurus, showing a surplus of stars at spectral types of K7-M1 (0.6-0.8 M ) relative to other nearby star-forming regions, such as IC 348, Chamaeleon I, and the Orion Nebula Cluster.

THE sdB+M ECLIPSING SYSTEM HW VIRGINIS AND ITS CIRCUMBINARY PLANETS

Abstract: For the very short period subdwarf B eclipsing binary HW Vir, we present new CCD photometry made from 2000 through 2008. In order to obtain consistency of the binary parameters, our new light curves, showing sharp eclipses and a striking reflection effect, were analyzed simultaneously with previously published radial velocity data. The secondary star parameters of M 2 = 0.14 M ?, R 2 = 0.18 R ?, and T 2 = 3084 K are consistent with those of an M6-7 main-sequence star. A credibility issue regarding bolometric corrections is emphasized. More than 250 times of minimum light, including our 41 timings and spanning more than 24 yr, were used for a period study. From a detailed analysis of the O ? C diagram, it emerged that the orbital period of HW Vir has varied as a combination of a downward-opening parabola and two sinusoidal variations, with cycle lengths of P 3 = 15.8 yr and P 4 = 9.1 yr and semiamplitudes of K 3 = 77 s and K 4 = 23 s, respectively. The continuous period decrease with a rate of ?8.28 ? 10?9 days yr?1 may be produced by angular momentum loss due to magnetic stellar wind braking but not by gravitational radiation. Of the possible causes of the cyclical components of the period change, apsidal motion and magnetic period modulation can be ruled out. The most reasonable explanation of both cyclical variations is a pair of light-travel-time effects driven by the presence of two substellar companions with projected masses of M 3sin i 3 = 19.2 M Jup and M 4sin i 4 = 8.5 M Jup. The two objects are the first circumbinary planets known to have been formed in a protoplanetary disk as well the first ones discovered by using the eclipse-timing method. The detection implies that planets could be common around binary stars just as are planets around single stars and demonstrates that planetary systems formed in a circumbinary disk can survive over long timescales. Depending on the thermal inertia of their massive atmospheres, the hemispheres of the planets turned toward the stars can experience substantial reciprocating temperature changes during the minutes-long primary eclipse intervals.

The debris disk around hr 8799

Abstract: We have obtained a full suite of Spitzer observations to characterize the debris disk around HR 8799 and to explore how its properties are related to the recently discovered set of three massive planets orbiting the star. We distinguish three components to the debris system: (1) warm dust (T ~ 150 K) orbiting within the innermost planet; (2) a broad zone of cold dust (T ~ 45 K) with a sharp inner edge orbiting just outside the outermost planet and presumably sculpted by it; and (3) a dramatic halo of small grains originating in the cold dust component. The high level of dynamical activity implied by this halo may arise due to enhanced gravitational stirring by the massive planets. The relatively young age of HR 8799 places it in an important early stage of development and may provide some help in understanding the interaction of planets and planetary debris, an important process in the evolution of our own solar system.

Formation of the terrestrial planets from a narrow annulus

Abstract: We show that the assembly of the Solar System terrestrial planets can be successfully modelled with all of the mass initially confined to a narrow annulus between 0.7 and 1.0 AU. With this configuration, analogues of Mercury and Mars often form from the collisional evolution of material diffusing out of the annulus under the scattering of the forming Earth and Venus analogues. The final systems also possess eccentricities and inclinations that match the observations, without recourse to dynamical friction from remnant small body populations. Finally, the characteristic assembly timescale for Earth analogues is rapid in this model, and consistent with cosmochemical models based on the 182 Hf– 182 W isotopes. The agreement between this model and the observations suggests that terrestrial planet systems may also be formed in ‘planet traps’, as has been proposed recently for the cores of giant planets in our solar system and others.

Alien Maps of an Ocean-Bearing World

Abstract: When Earth-mass extrasolar planets first become detectable, one challenge will be to determine which of these worlds harbor liquid water, a widely used criterion for habitability. Some of the first observations of these planets will consist of disc-averaged, time-resolved broadband photometry. To simulate such data, the Deep Impact spacecraft obtained light curves of Earth at seven wavebands spanning 300-1000 nm as part of the EPOXI mission of opportunity. In this paper, we analyze disc-integrated light curves, treating Earth as if it were an exoplanet, to determine if we can detect the presence of oceans and continents. We present two observations each spanning 1 day, taken at gibbous phases of 57° and 77°, respectively. As expected, the time-averaged spectrum of Earth is blue at short wavelengths due to Rayleigh scattering, and gray redward of 600 nm due to reflective clouds. The rotation of the planet leads to diurnal albedo variations of 15%-30%, with the largest relative changes occurring at the reddest wavelengths. To characterize these variations in an unbiased manner, we carry out a principal component analysis of the multi-band light curves; this analysis reveals that 98% of the diurnal color changes of Earth are due to only two dominant eigencolors. We use the time variations of these two eigencolors to construct longitudinal maps of the Earth, treating it as a non-uniform Lambert sphere. We find that the spectral and spatial distributions of the eigencolors correspond to cloud-free continents and oceans despite the fact that our observations were taken on days with typical cloud cover. We also find that the near-infrared wavebands are particularly useful in distinguishing between land and water. Based on this experiment, we conclude that it should be possible to infer the existence of water oceans on exoplanets with time-resolved broadband observations taken by a large space-based coronagraphic telescope.