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Colour-differential interferometry for the observation of extrasolar planets
We present the high angular resolution technique of colour-differentialinterferometry for direct detection of extrasolar giant planets (EGPs).The measurement of differential phase with long-baseline ground-basedinterferometers in the near-infrared could allow the observation ofseveral hot giant extrasolar planets in tight orbit around the nearbystars, and thus yield their low- or mid-resolution spectroscopy,complete orbital data set and mass. Estimates of potentially achievablesignal-to-noise ratios are presented for a number of planets alreadydiscovered by indirect methods. The limits from the instrumental andatmospheric instability are discussed, and a subsequent observationalstrategy is proposed.
| An Upper Limit on the Albedo of HD 209458b: Direct Imaging Photometry with the MOST Satellite
We present space-based photometry of the transiting exoplanetary systemHD 209458 obtained with the Microvariablity and Oscillations of Stars(MOST) satellite, spanning 14 days and covering 4 transits and 4secondary eclipses. The HD 209458 photometry was obtained in MOST'slower precision direct imaging mode, which is used for targets in thebrightness range 6.5>=V>=13. We describe the photometric reductiontechniques for this mode of observing, in particular the corrections forstray earthshine. We do not detect the secondary eclipse in the MOSTdata, to a limit in depth of 0.053 mmag (1 σ). We set a 1 σupper limit on the planet-star flux ratio of 4.88×10-5corresponding to a geometric albedo upper limit in the MOST bandpass(400-700 nm) of 0.25. The corresponding numbers at the 3 σ levelare 1.34×10-4 and 0.68, respectively. HD 209458b ishalf as bright as Jupiter in the MOST bandpass. This low geometricalbedo value is an important constraint for theoretical models of the HD209458b atmosphere, in particular ruling out the presence of reflectiveclouds. A second MOST campaign on HD 209458 is expected to be sensitiveto an exoplanet albedo as low as 0.13 (1 σ), if the star does notbecome more intrinsically variable in the meantime.MOST is a Canadian Space Agency mission, operated jointly by Dynacon,Inc., and the Universities of Toronto and British Columbia, withassistance from the University of Vienna.
| Two Suns in The Sky: Stellar Multiplicity in Exoplanet Systems
We present results of a reconnaissance for stellar companions to all 131radial velocity-detected candidate extrasolar planetary systems known asof 2005 July 1. Common proper-motion companions were investigated usingthe multiepoch STScI Digitized Sky Surveys and confirmed by matching thetrigonometric parallax distances of the primaries to companion distancesestimated photometrically. We also attempt to confirm or refutecompanions listed in the Washington Double Star Catalog, in the Catalogsof Nearby Stars Series by Gliese and Jahreiß, in Hipparcosresults, and in Duquennoy & Mayor's radial velocity survey. Ourfindings indicate that a lower limit of 30 (23%) of the 131 exoplanetsystems have stellar companions. We report new stellar companions to HD38529 and HD 188015 and a new candidate companion to HD 169830. Weconfirm many previously reported stellar companions, including six starsin five systems, that are recognized for the first time as companions toexoplanet hosts. We have found evidence that 20 entries in theWashington Double Star Catalog are not gravitationally bound companions.At least three (HD 178911, 16 Cyg B, and HD 219449), and possibly five(including HD 41004 and HD 38529), of the exoplanet systems reside intriple-star systems. Three exoplanet systems (GJ 86, HD 41004, andγ Cep) have potentially close-in stellar companions, with planetsat roughly Mercury-Mars distances from the host star and stellarcompanions at projected separations of ~20 AU, similar to the Sun-Uranusdistance. Finally, two of the exoplanet systems contain white dwarfcompanions. This comprehensive assessment of exoplanet systems indicatesthat solar systems are found in a variety of stellar multiplicityenvironments-singles, binaries, and triples-and that planets survive thepost-main-sequence evolution of companion stars.
| Catalog of Nearby Exoplanets
We present a catalog of nearby exoplanets. It contains the 172 knownlow-mass companions with orbits established through radial velocity andtransit measurements around stars within 200 pc. We include fivepreviously unpublished exoplanets orbiting the stars HD 11964, HD 66428,HD 99109, HD 107148, and HD 164922. We update orbits for 83 additionalexoplanets, including many whose orbits have not been revised sincetheir announcement, and include radial velocity time series from theLick, Keck, and Anglo-Australian Observatory planet searches. Both thesenew and previously published velocities are more precise here due toimprovements in our data reduction pipeline, which we applied toarchival spectra. We present a brief summary of the global properties ofthe known exoplanets, including their distributions of orbital semimajoraxis, minimum mass, and orbital eccentricity.Based on observations obtained at the W. M. Keck Observatory, which isoperated jointly by the University of California and the CaliforniaInstitute of Technology. The Keck Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation.
| A Stellar Companion in the HD 189733 System with a Known Transiting Extrasolar Planet
We show that the very close-by (19 pc) K0 star HD 189733, already foundto be orbited by a transiting giant planet, is the primary of a doublestar system, with the secondary being a mid-M dwarf with projectedseparation of about 216 AU from the primary. This conclusion is based onastrometry, proper-motion and radial velocity measurements, spectraltype determination, and photometry. We also detect differential propermotion of the secondary. The data appear consistent with the secondary'sorbiting the primary in a clockwise orbit, lying nearly in the plane ofthe sky (i.e., nearly perpendicular to the orbital plane of thetransiting planet), and with period of about 3200 years.
| Chemical Composition of the Planet-harboring Star TrES-1
We present a detailed chemical abundance analysis of the parent star ofthe transiting extrasolar planet TrES-1. Based on high-resolution KeckHIRES and Hobby-Eberly Telescope HRS spectra, we have determinedabundances relative to the Sun for 16 elements (Na, Mg, Al, Si, Ca, Sc,Ti, V, Cr, Mn, Co, Ni, Cu, Zn, Y, and Ba). The resulting averageabundance of <[X/H]>=-0.02+/-0.06 is in good agreement withinitial estimates of solar metallicity based on iron. We compare theelemental abundances of TrES-1 with those of the sample of stars withplanets, searching for possible chemical abundance anomalies. TrES-1appears not to be chemically peculiar in any measurable way. Weinvestigate possible signs of selective accretion of refractory elementsin TrES-1 and other stars with planets and find no statisticallysignificant trends of metallicity [X/H] with condensation temperatureTc. We use published abundances and kinematic information forthe sample of planet-hosting stars (including TrES-1) and severalstatistical indicators to provide an updated classification in terms oftheir likelihood to belong to either the thin disk or the thick disk ofthe Milky Way. TrES-1 is found to be very likely a member of thethin-disk population. By comparing α-element abundances of planethosts and a large control sample of field stars, we also find thatmetal-rich ([Fe/H]>~0.0) stars with planets appear to besystematically underabundant in [α/Fe] by ~0.1 dex with respect tocomparison field stars. The reason for this signature is unclear, butsystematic differences in the analysis procedures adopted by differentgroups cannot be ruled out.
| Frequency of Hot Jupiters and Very Hot Jupiters from the OGLE-III Transit Surveys toward the Galactic Bulge and Carina
We derive the frequencies of hot Jupiters (HJs) with 3-5 day periods andvery hot Jupiters (VHJs) with 1-3 day periods by comparing the planetsactually detected in the OGLE-III survey with those predicted by ourmodels. The models are constructed following Gould and Morgan (2003) bypopulating the line of sight with stars drawn from the HipparcosCatalogue. Using these, we demonstrate that the number of stars withsensitivity to HJs and VHJs is only 5-16% of those in the OGLE-IIIfields satisfying the spectroscopic-follow-up limit of V_max < 17.5mag. Hence, the frequencies we derive are much higher than a naiveestimate would indicate. We find that at 90% confidence the fraction ofstars with planets in the two period ranges is (1/320)(1^+1.37_-0.59)for HJs and (1/710)(1^+1.10_-0.54) for VHJs. The HJ rate isstatistically indistinguishable from that found in radial velocity (RV)studies. However, we note that magnitude-limited RV samples are heavilybiased toward metal-rich (hence, planet-bearing) stars, while transitsurveys are not, and therefore we expect that more sensitive transitsurveys should find a deficit of HJs as compared to RV surveys. Thedetection of three transiting VHJs, all with periods less than 2 days,is marginally consistent with the complete absence of such detections inRV surveys. The planets detected are consistent with being uniformlydistributed between 1.00 and 1.25 Jovian radii, but there are too few inthe sample to map this distribution in detail.
| A search for water masers toward extrasolar planets
Context: .Water is the most common triatomic molecule in the universeand the basis of life on Earth. Astrophysical masers have been widelystudied in recent years and have been shown to be invaluable probes ofthe details of the environment in which they are found. Water masers,for instance, are often detected toward low-mass star-forming regions.Doppler radial-velocity surveys have detected about 160exoplanets.Aims.Observations of water masers from exoplanetary systemswould give us a new detailed window through which to explorethem.Methods.We present a search for water masers toward eighteenextrasolar planets using the newly upgraded Australia Telescope CompactArray at 12 mm. A sensitivity of 25 mJy beam-1 and anangular resolution of ~10'' were achieved at 22.235 GHz. Results.Nomaser lines are clearly observed.
| Abundances of refractory elements in the atmospheres of stars with extrasolar planets
Aims.This work presents a uniform and homogeneous study of chemicalabundances of refractory elements in 101 stars with and 93 without knownplanetary companions. We carry out an in-depth investigation of theabundances of Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Na, Mg and Al. The newcomparison sample, spanning the metallicity range -0.70< [Fe/H]<0.50, fills the gap that previously existed, mainly at highmetallicities, in the number of stars without known planets.Methods.Weused an enlarged set of data including new observations, especially forthe field "single" comparison stars . The line list previously studiedby other authors was improved: on average we analysed 90 spectral linesin every spectrum and carefully measured more than 16 600 equivalentwidths (EW) to calculate the abundances.Results.We investigate possibledifferences between the chemical abundances of the two groups of stars,both with and without planets. The results are globally comparable tothose obtained by other authors, and in most cases the abundance trendsof planet-host stars are very similar to those of the comparison sample.Conclusions.This work represents a step towards the comprehension ofrecently discovered planetary systems. These results could also beuseful for verifying galactic models at high metallicities andconsequently improve our knowledge of stellar nucleosynthesis andgalactic chemical evolution.
| Ground-based direct detection of close-in extra-solar planets with nulling and high order adaptive optics
Ground-based direct detection of extra-solar planets is very challengingdue to high planet to star brightness contrasts. For giant close-inplanets, such as have been discovered by the radial velocity method,closer than 0.1 AU, the reflected light is predicted to be fairly highyielding a contrast ratio ranging from 10-4 to10-5 at near infra-red wavelengths. In this paper, weinvestigate direct detection of reflected light from such planets usingnulling interferometry, and high-order adaptive optics in conjunctionwith large double aperture ground-based telescopes. In thisconfiguration, at least 10-3 suppression of the entirestellar Airy pattern with small loss of planet flux as close as 0.03arcsec is achievable. Distinguishing residual starlight from the planetsignal is achieved by using the center of gravity shift method ormulticolor differential imaging. Using these assumptions, we deriveexposure times from a few minutes to several hours for direct detectionof many of the known extra-solar planets with several short-baselinedouble aperture telescopes such as the Large Binocular Telescope (LBT),the Very Large Telescope (VLT) and the Keck Telescope.
| Oxygen abundances in planet-harbouring stars. Comparison of different abundance indicators
We present a detailed and uniform study of oxygen abundances in 155solar type stars, 96 of which are planet hosts and 59 of which form partof a volume-limited comparison sample with no known planets. EWmeasurements were carried out for the [O I] 6300 Å line and the OI triplet, and spectral synthesis was performed for several OH lines.NLTE corrections were calculated and applied to the LTE abundanceresults derived from the O I 7771-5 Å triplet. Abundances from [OI], the O I triplet and near-UV OH were obtained in 103, 87 and 77dwarfs, respectively. We present the first detailed and uniformcomparison of these three oxygen indicators in a large sample ofsolar-type stars. There is good agreement between the [O/H] ratios fromforbidden and OH lines, while the NLTE triplet shows a systematicallylower abundance. We found that discrepancies between OH, [O I] and the OI triplet do not exceed 0.2 dex in most cases. We have studied abundancetrends in planet host and comparison sample stars, and no obviousanomalies related to the presence of planets have been detected. Allthree indicators show that, on average, [O/Fe] decreases with [Fe/H] inthe metallicity range -0.8< [Fe/H] < 0.5. The planet host starspresent an average oxygen overabundance of 0.1-0.2 dex with respect tothe comparison sample.
| A link between the semimajor axis of extrasolar gas giant planets and stellar metallicity
The fact that most extrasolar planets found to date are orbitingmetal-rich stars lends credence to the core accretion mechanism of gasgiant planet formation over its competitor, the disc instabilitymechanism. However, the core accretion mechanism is not refined to thepoint of explaining orbital parameters such as the unexpected semimajoraxes and eccentricities. We propose a model that correlates themetallicity of the host star with the original semimajor axis of itsmost massive planet, prior to migration, assuming that the coreaccretion scenario governs giant gas planet formation. The modelpredicts that the optimum regions for planetary formation shift inwardsas stellar metallicity decreases, providing an explanation for theobserved absence of long-period planets in metal-poor stars. We compareour predictions with the available data on extrasolar planets for starswith masses similar to the mass of the Sun. A fitting procedure producesan estimate of what we define as the zero-age planetary orbit (ZAPO)curve as a function of the metallicity of the star. The model hints thatthe lack of planets circling metal-poor stars may be partly caused by anenhanced destruction probability during the migration process, becausethe planets lie initially closer to their central star.
| Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs
We present a uniform catalog of stellar properties for 1040 nearby F, G,and K stars that have been observed by the Keck, Lick, and AAT planetsearch programs. Fitting observed echelle spectra with synthetic spectrayielded effective temperature, surface gravity, metallicity, projectedrotational velocity, and abundances of the elements Na, Si, Ti, Fe, andNi, for every star in the catalog. Combining V-band photometry andHipparcos parallaxes with a bolometric correction based on thespectroscopic results yielded stellar luminosity, radius, and mass.Interpolating Yonsei-Yale isochrones to the luminosity, effectivetemperature, metallicity, and α-element enhancement of each staryielded a theoretical mass, radius, gravity, and age range for moststars in the catalog. Automated tools provide uniform results and makeanalysis of such a large sample practical. Our analysis method differsfrom traditional abundance analyses in that we fit the observed spectrumdirectly, rather than trying to match equivalent widths, and wedetermine effective temperature and surface gravity from the spectrumitself, rather than adopting values based on measured photometry orparallax. As part of our analysis, we determined a new relationshipbetween macroturbulence and effective temperature on the main sequence.Detailed error analysis revealed small systematic offsets with respectto the Sun and spurious abundance trends as a function of effectivetemperature that would be inobvious in smaller samples. We attempted toremove these errors by applying empirical corrections, achieving aprecision per spectrum of 44 K in effective temperature, 0.03 dex inmetallicity, 0.06 dex in the logarithm of gravity, and 0.5 kms-1 in projected rotational velocity. Comparisons withprevious studies show only small discrepancies. Our spectroscopicallydetermined masses have a median fractional precision of 15%, but theyare systematically 10% higher than masses obtained by interpolatingisochrones. Our spectroscopic radii have a median fractional precisionof 3%. Our ages from isochrones have a precision that variesdramatically with location in the Hertzsprung-Russell diagram. We planto extend the catalog by applying our automated analysis technique toother large stellar samples.
| The Planet-Metallicity Correlation
We have recently carried out spectral synthesis modeling to determineTeff, logg, vsini, and [Fe/H] for 1040 FGK-type stars on theKeck, Lick, and Anglo-Australian Telescope planet search programs. Thisis the first time that a single, uniform spectroscopic analysis has beenmade for every star on a large Doppler planet search survey. We identifya subset of 850 stars that have Doppler observations sufficient todetect uniformly all planets with radial velocity semiamplitudes K>30m s-1 and orbital periods shorter than 4 yr. From this subsetof stars, we determine that fewer than 3% of stars with-0.5<[Fe/H]<0.0 have Doppler-detected planets. Above solarmetallicity, there is a smooth and rapid rise in the fraction of starswith planets. At [Fe/H]>+0.3 dex, 25% of observed stars have detectedgas giant planets. A power-law fit to these data relates the formationprobability for gas giant planets to the square of the number of metalatoms. High stellar metallicity also appears to be correlated with thepresence of multiple-planet systems and with the total detected planetmass. This data set was examined to better understand the origin of highmetallicity in stars with planets. None of the expected fossilsignatures of accretion are observed in stars with planets relative tothe general sample: (1) metallicity does not appear to increase as themass of the convective envelopes decreases, (2) subgiants with planetsdo 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 oreccentricity could be identified. We conclude that stars with extrasolarplanets do not have an accretion signature that distinguishes them fromother stars; more likely, they are simply born in higher metallicitymolecular clouds.Based on observations obtained at Lick and Keck Observatories, operatedby the University of California, and the Anglo-Australian Observatories.
| Prospects for Habitable ``Earths'' in Known Exoplanetary Systems
We have examined whether putative Earth-mass planets could remainconfined to the habitable zones (HZs) of the 111 exoplanetary systemsconfirmed by 2004 August. We find that in about half of these systemsthere could be confinement for at least the past 1000 Myr, though insome cases only in variously restricted regions of the HZ. The HZmigrates outward during the main-sequence lifetime, and we find that inabout two-thirds of the systems an Earth-mass planet could be confinedto the HZ for at least 1000 Myr sometime during the main-sequencelifetime. Clearly, these systems should be high on the target list forexploration for terrestrial planets. We have reached our conclusions bydetailed investigations of seven systems, which has resulted in anestimate of the distance from the giant planet within which orbitalstability is unlikely for an Earth-mass planet. This distance is givenby nRH, where RH is the Hill radius of the giantplanet and n is a multiplier that depends on the giant's orbitaleccentricity and on whether the Earth-mass planet is interior orexterior to the giant planet. We have estimated n for each of the sevensystems by launching Earth-mass planets in various orbits and followingtheir fate with a hybrid orbital integrator. We have then evaluated thehabitability of the other exoplanetary systems using nRHderived from the giant's orbital eccentricity without carrying outtime-consuming orbital integrations. A stellar evolution model has beenused to obtain the HZs throughout the main-sequence lifetime.
| Hot Jupiters and Hot Spots: The Short- and Long-Term Chromospheric Activity on Stars with Giant Planets
We monitored the chromospheric activity in the Ca II H and K lines of 13solar-type stars (including the Sun): 8 of them over 3 years at theCanada-France-Hawaii Telescope (CFHT) and 5 in a single run at the VeryLarge Telescope (VLT). A total of 10 of the 13 targets have closeplanetary companions. All of the stars observed at the CFHT showlong-term (months to years) changes in H and K intensity levels. Fourstars display short-term (days) cyclical activity. For two, HD 73256 andκ1 Cet, the activity is likely associated with anactive region rotating with the star; however, the flaring in excess ofthe rotational modulation may be associated with a hot Jupiter. Aplanetary companion remains a possibility for κ1 Cet.For the other two, HD 179949 and υ And, the cyclic variation issynchronized to the hot Jupiter's orbit. For both stars thissynchronicity with the orbit is clearly seen in two out of three epochs.The effect is only marginal in the third epoch at which the seasonallevel of chromospheric activity had changed for both stars. Short-termchromospheric activity appears weakly dependent on the mean K linereversal intensities for the sample of 13 stars. In addition, asuggestive correlation exists between this activity and theMpsini of the star's hot Jupiter. Because of their smallseparation (<=0.1 AU), many of the hot Jupiters lie within theAlfvén radius of their host stars, which allows a direct magneticinteraction with the stellar surface. We discuss the conditions underwhich a planet's magnetic field might induce activity on the stellarsurface and why no such effect was seen for the prime candidate, τBoo. This work opens up the possibility of characterizing planet-starinteractions, with implications for extrasolar planet magnetic fieldsand the energy contribution to stellar atmospheres.Based on observations collected at the Canada-France-Hawaii Telescopeoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii, aswell as data from the European Southern Observatory's Very LargeTelescope, Chile (programme ESO 73.C-0694).
| Radial Velocity Detectability of Low-Mass Extrasolar Planets in Close Orbits
Detection of Jupiter-mass companions to nearby solar-type stars withprecise radial velocity measurements is now routine, and Doppler surveysare moving toward lower velocity amplitudes. The detection of severalNeptune-mass planets with orbital periods of less than a week has beenreported. The drive toward the search for close-in, Earth-mass planetsis on the agenda. Successful detection or meaningful upper limits willplace important constraints on the process of planet formation. In thispaper, we quantify the statistics of detection of low-mass planets inclose orbits, showing how the detection threshold depends on the numberand timing of the observations. In particular, we consider the case of alow-mass planet close to but not on the 2:1 mean motion resonance with ahot Jupiter. This scenario is a likely product of the core-accretionhypothesis for planet formation coupled with migration of Jupiters inthe protoplanetary disk. It is also advantageous for detection becausethe orbital period is well constrained. We show that the minimumdetectable mass is ~4 M⊕(N/20)-1/2(σ/ms-1)(P/days)1/3(M*/Msolar)2/3for N>=20, where N is the number of observations, P is the orbitalperiod, σ is the quadrature sum of Doppler velocity measurementerrors and stellar jitter, and M* is the stellar mass.Detection of few Earth-mass rocky cores will require ~1 m s-1velocity precision and, most important, a better understanding ofstellar radial velocity ``jitter.''
| On the ages of exoplanet host stars
We obtained spectra, covering the CaII H and K region, for 49 exoplanethost (EH) stars, observable from the southern hemisphere. We measuredthe chromospheric activity index, R'{_HK}. We compiled previouslypublished values of this index for the observed objects as well as theremaining EH stars in an effort to better smooth temporal variations andderive a more representative value of the average chromospheric activityfor each object. We used the average index to obtain ages for the groupof EH stars. In addition we applied other methods, such as: Isochrone,lithium abundance, metallicity and transverse velocity dispersions, tocompare with the chromospheric results. The kinematic method is a lessreliable age estimator because EH stars lie red-ward of Parenago'sdiscontinuity in the transverse velocity dispersion vs dereddened B-Vdiagram. The chromospheric and isochrone techniques give median ages of5.2 and 7.4 Gyr, respectively, with a dispersion of 4 Gyr. The medianage of F and G EH stars derived by the isochrone technique is 1-2 Gyrolder than that of identical spectral type nearby stars not known to beassociated with planets. However, the dispersion in both cases is large,about 2-4 Gyr. We searched for correlations between the chromosphericand isochrone ages and L_IR/L* (the excess over the stellarluminosity) and the metallicity of the EH stars. No clear tendency isfound in the first case, whereas the metallicy dispersion seems toslightly increase with age.
| Sulphur abundance in Galactic stars
We investigate sulphur abundance in 74 Galactic stars by using highresolution spectra obtained at ESO VLT and NTT telescopes. For the firsttime the abundances are derived, where possible, from three opticalmultiplets: Mult. 1, 6, and 8. By combining our own measurements withdata in the literature we assemble a sample of 253 stars in themetallicity range -3.2 [Fe/H] +0.5. Two important features,which could hardly be detected in smaller samples, are obvious from thislarge sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H]-1; 2) at low metallicities we observe stars with [S/Fe] 0.4, aswell as stars with higher [S/Fe] ratios. The latter do not seem to bekinematically different from the former ones. Whether the latter findingstems from a distinct population of metal-poor stars or simply from anincreased scatter in sulphur abundances remains an open question.
| Four new wide binaries among exoplanet host stars
In our ongoing survey for wide (sub)stellar companions of exoplanet hoststars we have found 4 new co-moving stellar companions of the stars HD114729, HD 16141, HD 196050 and HD 213240 with projected separationsfrom 223 up to 3898 AU. The companionship of HD 114729 B, HD 196050 Band HD 213240 C is confirmed by photometry and spectroscopy, all beingearly M dwarfs. The masses of the detected companions are derived fromtheir infrared JHK magnitudes and range between 0.146 and 0.363Mȯ. Our first and second epoch observations can rule outadditional stellar companions around the primaries from 200 up to 2400AU ({S/N}=10). In our survey we have found so far 6 new binaries amongthe exoplanet host stars. According to these new detections, thereported differences between single-star and binary-star planets withorbital periods short than 40 days remain significant in both themass-period and eccentricity-period distribution. In contrast, allexoplanets with orbital periods longer than 100 days tend to displaysimilar distributions.
| A photometric survey of stars with circumstellar material
We present the result of a follow-up Strömgren photometric surveyof sixteen southern bright stars with circumstellar material, in orderto detect possible weak photometric variations. We found new variationsof the β~ Pictoris brightness from 1999 to 2002 with a weaklong-term variation of ~-0.8× 10-3 mag per year, overabout 3 years. These variations look similar to those seen from 1975 to1981 and from 1995 to 1998 (Nitschelm et al. 2000, A&AS, 145, 275).They can be due to differential occultation by dust inhomogeneitiestransiting the star through the years. We detected new periodicvariations for HD 256 (HR 10) with periods ranging from 0.35 day to 6.69days during several months. These variations may also be interpreted interms of variable obscuration due to structures in the circumstellardisk suspected to be surrounding this star.
| Abundances of Na, Mg and Al in stars with giant planets
We present Na, Mg and Al abundances in a set of 98 stars with knowngiant planets, and in a comparison sample of 41 “single”stars. The results show that the [X/H] abundances (with X = Na, Mg andAl) are, on average, higher in stars with giant planets, a resultsimilar to the one found for iron. However, we did not find any strongdifference in the [X/Fe] ratios, for a fixed [Fe/H], between the twosamples of stars in the region where the samples overlap. The data wasused to study the Galactic chemical evolution trends for Na, Mg and Aland to discuss the possible influence of planets on this evolution. Theresults, similar to those obtained by other authors, show that the[X/Fe] ratios all decrease as a function of metallicity up to solarvalues. While for Mg and Al this trend then becomes relatively constant,for Na we find indications of an upturn up to [Fe/H] values close to0.25 dex. For metallicities above this value the [Na/Fe] becomesconstant.
| Spectroscopic metallicities for planet-host stars: Extending the samples
We present stellar parameters and metallicities for 29 planet-hoststars, as well as for a large volume-limited sample of 53 stars notknown to be orbited by any planetary-mass companion. These stars add tothe results presented in our previous series of papers, providing twolarge 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 withplanets 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 singleplanet-hosts and planet-hosts in multiple stellar systems. The resultsshow that a small difference cannot be excluded, in the sense that thelatter sample is slighly overmetallic. However, more data are needed toconfirm this correlation.
| Stellar wind regimes of close-in extrasolar planets
Close-in extrasolar planets of Sun-like stars are exposed to stellarwind conditions that differ considerably from those for planets in thesolar system. Unfortunately, these stellar winds belong to the stillunknown parameters of these planetary systems. On the other hand, theyplay a crucial role in a number of star-planet interaction processesthat may lead to observable radiation events. In order to lay afoundation for the investigation of such interaction processes, weestimate stellar wind parameters on the basis of the solar wind model byWeber & Davis and study the implications of the stellar magneticfields. Our results suggest that in contrast to the solar systemplanets, some close-in extrasolar planets may be obstacles in asub-Alfvénic stellar wind flow. In this case, the stellar windmagnetic pressure is comparable to or even larger than the dynamic flowpressure. We discuss possible consequences of these findings for thewind-exoplanet interactions. Further, we derive upper limit estimatesfor the energies such stellar winds can deposit in the exoplanetarymagnetospheres. We finally discuss the implications thesub-Alfvénic environment may have on the star-planet interaction.
| Search for (Sub)stellar Companions of Exoplanet Host Stars
Not Available
| On the possible correlation between the orbital periods of extrasolar planets and the metallicity of the host stars
We investigate a possible correlation between the orbital periods P ofthe extrasolar planet sample and the metallicity [Fe/H] of their parentstars. Close-in planets, on orbits of a few days, are more likely to befound around metal-rich stars. Simulations show that a weak correlationis present. This correlation becomes stronger when only single starswith one detected planet are considered. We discuss several potentialsources of bias that might mimic the correlation, and find that they canbe ruled out, but not with high significance. If real, the absence ofvery short-period planets around the stellar sample with [Fe/H] < 0.0can be interpreted as evidence of a metallicity dependence of themigration rates of giant planets during formation in the protoplanetarydisc. The observed P-[Fe/H] correlation can be falsified or confirmed byconducting spectroscopic or astrometric surveys of metal-poor stars([Fe/H] < -0.5) in the field.
| Capture and escape in the elliptic restricted three-body problem
Several families of irregular moons orbit the giant planets. These moonsare thought to have been captured into planetocentric orbits afterstraying into a region in which the gravitation of the planet dominatessolar perturbations (the Hill sphere). This mechanism requires a sourceof dissipation, such as gas drag, in order to make capture permanent.However, capture by gas drag requires that particles remain inside theHill sphere long enough for dissipation to be effective. Recently wehave proposed that in the circular restricted three-body problem (CRTBP)particles may become caught up in sticky chaotic layers, which tends toprolong their sojourn within the Hill sphere of the planet therebyassisting capture. Here, we show that this mechanism survivesperturbations due to the ellipticity of the orbit of the planet.However, Monte Carlo simulations indicate that the ability of the planetto capture moons decreases with increasing orbital eccentricity. At theactual orbital eccentricity of Jupiter, this results in approximately anorder of magnitude lower capture probability than estimated in thecircular model. Eccentricities of planetary orbits in the Solar systemare moderate but this is not necessarily the case for extrasolarplanets, which typically have rather eccentric orbits. Therefore, ourfindings suggest that these extrasolar planets are unlikely to havesubstantial populations of irregular moons.
| Obliquity variations of terrestrial planets in habitable zones
We have investigated obliquity variations of possible terrestrialplanets in habitable zones (HZs) perturbed by a giant planet(s) inextrasolar planetary systems. All the extrasolar planets so fardiscovered are inferred to be jovian-type gas giants. However,terrestrial planets could also exist in extrasolar planetary systems. Inorder for life, in particular for land-based life, to evolve and surviveon a possible terrestrial planet in an HZ, small obliquity variations ofthe planet may be required in addition to its orbital stability, becauselarge obliquity variations would cause significant climate change. It isknown that large obliquity variations are caused by spin-orbitresonances where the precession frequency of the planet's spin nearlycoincides with one of the precession frequencies of the ascending nodeof the planet's orbit. Using analytical expressions, we evaluated theobliquity variations of terrestrial planets with prograde spins in HZs.We found that the obliquity of terrestrial planets suffers largevariations when the giant planet's orbit is separated by several Hillradii from an edge of the HZ, in which the orbits of the terrestrialplanets in the HZ are marginally stable. Applying these results to theknown extrasolar planetary systems, we found that about half of thesesystems can have terrestrial planets with small obliquity variations(smaller than 10°) over their entire HZs. However, the systems withboth small obliquity variations and stable orbits in their HZs are only1/5 of known systems. Most such systems are comprised of short-periodgiant planets. If additional planets are found in the known planetarysystems, they generally tend to enhance the obliquity variations. On theother hand, if a large/close satellite exists, it significantly enhancesthe precession rate of the spin axis of a terrestrial planet and islikely to reduce the obliquity variations of the planet. Moreover, if aterrestrial planet is in a retrograde spin state, the spin-orbitresonance does not occur. Retrograde spin, or a large/close satellitemight be essential for land-based life to survive on a terrestrialplanet in an HZ.
| The Radiometric Bode's Law and Extrasolar Planets
We predict the radio flux densities of the extrasolar planets in thecurrent census, making use of an empirical relation-the radiometricBode's law-determined from the five ``magnetic'' planets in the solarsystem (the Earth and the four gas giants). Radio emission from theseplanets results from solar wind-powered electron currents depositingenergy in the magnetic polar regions. We find that most of the knownextrasolar planets should emit in the frequency range 10-1000 MHz and,under favorable circumstances, have typical flux densities as large as 1mJy. We also describe an initial, systematic effort to search for radioemission in low radio frequency images acquired with the Very LargeArray (VLA). The limits set by the VLA images (~300 mJy) are consistentwith, but do not provide strong constraints on, the predictions of themodel. Future radio telescopes, such as the Low Frequency Array and theSquare Kilometer Array, should be able to detect the known extrasolarplanets or place austere limits on their radio emission. Planets withmasses much lower than those in the current census will probably radiatebelow 10 MHz and will require a space-based array.
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| Сазвежђа: | Једра |
| Ректацензија: | 08h47m40.50s |
| Deклинација: | -41°44'14.0" |
| Apparent магнитуда: | 6.36 |
| Даљина: | 28.944 parsecs |
| Proper motion RA: | -19.5 |
| Proper motion Dec: | -227.7 |
| B-T magnitude: | 7.042 |
| V-T magnitude: | 6.408 |
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