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Planets and Debris Disks: Results from a Spitzer/MIPS Search for Infrared Excess
Using the MIPS camera on the Spitzer Space Telescope, we have searchedfor debris disks around 104 stars known from radial velocity studies tohave one or more planets. Combining this new data with 42 alreadypublished observations of planet-bearing stars, we find that 14 of the146 systems have IR excess at 24 and/or 70 μm. Only one star, HD69830, has IR excess exclusively at 24 μm, indicative of warm dust inthe inner system analogous to that produced by collisions in the solarsystem's asteroid belt. For the other 13 stars with IR excess theemission is stronger at 70 μm, consistent with cool dust (<100 K)located beyond 10 AU, well outside of the orbital location of the knownplanets. Selection effects inhibit detection of faint disks around theplanet-bearing stars (e.g., the stars tend to be more distant),resulting in a lower detection rate for IR excess than in acorresponding control sample of nearby stars not known to have planets(9% ± 3% versus 14% ± 3%). Even taking into account theselection bias, we find that the difference between the dust emissionaround stars with planets and stars without known planets is notstatistically significant.
| Survey of Nearby FGK Stars at 160 μm with Spitzer
The Spitzer Space Telescope has advanced debris disk sciencetremendously with a wealth of information on debris disks around nearbyA, F, G, K, and M stars at 24 and 70 μm with the MIPS photometerand at 8-34 μm with IRS. Here we present 160 μmobservations of a small subset of these stars. At this wavelength, thestellar photospheric emission is negligible and any detected emissioncorresponds to cold dust in extended Kuiper Belt analogs. However, theSpitzer 160 μm observations are limited in sensitivity by thelarge beam size which results in significant "noise" due to cirrus andextragalactic confusion. In addition, the 160 μm measurementssuffer from the added complication of a light leak next to the star'sposition whose flux is proportional to the near-infrared flux of thestar. We are able to remove the contamination from the leak and report160 μm measurements or upper limits for 24 stars. Three stars(HD 10647, HD 207129, and HD 115617) have excesses at 160 μmthat we use to constrain the properties of the debris disks around them.A more detailed model of the spectral energy distribution of HD 10647reveals that the 70 and 160 μm emission could be due to smallwater ice particles at a distance of 100 AU, consistent with HubbleSpace Telescope optical imaging of circumstellar material in the system.
| Speckle Interferometry at Mount Wilson Observatory: Observations Obtained in 2006-2007 and 35 New Orbits
Results are presented for 607 speckle interferometric observations ofdouble stars, as well as 222 measures of single stars or unresolvedpairs. All data were obtained in 2006 and 2007 at the Mount WilsonObservatory, using the 2.5 m Hooker telescope. Separations range from0farcs06 to 6farcs31, with a median of 0farcs34. These three observingruns concentrated on binaries in need of confirmation (mainly Hipparcosand Tycho pairs), as well as systems in need of improved orbitalelements. New orbital solutions have been determined for 35 systems as aresult.
| On the Relationship Between Debris Disks and Planets
Dust in debris disks is generated by collisions among planetesimals. Theexistence of these planetesimals is a consequence of the planetformation process, but the relationship between debris disks and planetshas not been clearly established. Here we analyze Spitzer/MIPS 24 and 70μm data for 150 planet-bearing stars, and compare the incidence ofdebris disks around these stars with a sample of 118 stars around whichplanets have been searched for, but not found. Together they comprisethe largest sample ever assembled to deal with this question. The use ofsurvival analysis techniques allows us to account for the large numberof nondetections at 70 μm. We discovered 10 new debris disks aroundstars with planets and one around a star without known planets. We foundthat the incidence of debris disks is marginally higher among stars withplanets, than among those without, and that the brightness of theaverage debris disk is not significantly different in the two samples.We conclude that the presence of a planet that has been detected viacurrent radial velocity techniques is not a good predictor of thepresence of a debris disk detected at infrared wavelengths.
| A k-NN Method to Classify Rare Astronomical Sources: Photometric Search of Brown Dwarfs with Spitzer/IRAC
We present a statistical method for the photometric search of rareastronomical sources based on the weighted k-Nearest Neighbors method. Ametric is defined in a multidimensional color-magnitude space based onlyon the photometric properties of template sources and the photometricuncertainties of both templates and data, without the need to define adhoc color and magnitude cuts which could bias the search. The metric isdefined as a function of two parameters, the number of neighbors k and athreshold distance D th that can be optimized for maximumselection efficiency and completeness. We apply the method to the searchof L and T dwarfs in the Spitzer Extragalactic First Look Survey and theBoötes field of the Spitzer Shallow Survey, as well as to thesearch of substellar mass companions around nearby stars. With highlevel of completeness, we confirm the absence of late-T dwarfs detectedin at least two bands in the First Look Survey, and only one in theShallow Survey (previously discovered by Stern et al.). This result isin agreement with the expected statistics for late-T dwarfs. OneL/early-T candidate is found in the First Look Survey, and three in theShallow Surveys, currently undergoing follow-up spectroscopicverification. Finally, we discuss the potential for brown dwarf searcheswith this method in the Spitzer warm mission Exploration Scienceprograms.
| A Search for Multi-Planet Systems Using the Hobby-Eberly Telescope
Extrasolar multiple-planet systems provide valuable opportunities fortesting theories of planet formation and evolution. The architectures ofthe known multiple-planet systems demonstrate a fascinating level ofdiversity, which motivates the search for additional examples of suchsystems in order to better constrain their formation and dynamicalhistories. Here we describe a comprehensive investigation of 22planetary systems in an effort to answer three questions: (1) are thereadditional planets? (2) where could additional planets reside in stableorbits? and (3) what limits can these observations place on suchobjects? We find no evidence for additional bodies in any of thesesystems; indeed, these new data do not support three previouslyannounced planets (HD 20367 b: Udry et al. HD 74156 d: Beanet al. and 47 UMa c: Fischer et al.). The dynamicalsimulations show that nearly all of the 22 systems have large regions inwhich additional planets could exist in stable orbits. Thedetection-limit computations indicate that this study is sensitive toclose-in Neptune-mass planets for most of the systems targeted. Weconclude with a discussion on the implications of these nondetections.Based on observations obtained with the Hobby-Eberly Telescope, which isa joint project of the University of Texas at Austin, the PennsylvaniaState University, Stanford University,Ludwig-Maximilians-Universität München, andGeorg-August-Universität Göttingen.
| The young active star SAO 51891 (V383 Lacertae)
Aims: The aim of this work is to investigate the surface inhomogeneitiesof a young, late-type star, SAO 51891, at different atmospheric levels,from the photosphere to the upper chromosphere, analyzingcontemporaneous optical high-resolution spectra and broad-bandphotometry. Methods: The full spectral range of FOCES@CAHA (R≃ 40 000) is used to perform the spectral classification and todetermine the rotational and radial velocities. The lithium abundance ismeasured to obtain an age estimate. The {BVRIJHK}sphotometric bands are used to construct the spectral energy distribution(SED). The variations in the observed BV fluxes and effectivetemperature are used to infer the presence of photospheric spots andobserve their behavior over time. The chromospheric activity is studiedapplying the spectral subtraction technique to Hα, Ca ii H &K, Hɛ, and Ca ii IRT lines. Results: We find SAO 51891 tobe a young K0-1V star with a lithium abundance close to the Pleiadesupper envelope, confirming its youth ( 100 Myr), which is also inferredfrom its kinematical membership of the Local Association. No infraredexcess is detected from analysis of its SED, limiting the amount ofremaining circumstellar dust. We detect a rotational modulation of theluminosity, effective temperature, Ca ii H & K, Hɛ, and Caii IRT total fluxes. A simple spot model with two main active regions,about 240 K cooler than the surrounding photosphere, fits the observedlight and temperature curves very well. The small-amplitude radialvelocity variations are also well reproduced by our spot model. Theanti-correlation of light curves and chromospheric diagnostics indicateschromospheric plages spatially associated with the spots. The largestmodulation amplitude is observed for the Hɛ flux suggesting thatthis line is very sensitive to the presence of chromospheric plages. Conclusions: SAO 51891 is a young active star, lacking significantamounts of circumstellar dust or any evidence of low mass companions,and displays the typical phenomena produced by magnetic activity. Thespots are larger and warmer than those in less active main-sequencestars. If some debris is still present around the star, it will only bedetectable by future far-infrared and sub-mm observations (e.g.,Herschel or ALMA). The RV variation produced by the starspots has anamplitude comparable with those induced by Jupiter-mass planets orbitingclose to the host star. SAO 51891 is another good example of an activestar in which the detection of planets may be hampered by the highactivity level.Based on observations collected at Calar Alto Astronomical Observatory(Spain) and Catania Astrophysical Observatory (Italy).
| Directly Determined Linear Radii and Effective Temperatures of Exoplanet Host Stars
We present interferometric angular sizes for 12 stars with knownplanetary companions, for comparison with 28 additional main-sequencestars not known to host planets. For all objects we estimate bolometricfluxes and reddenings through spectral-energy distribution (SED) fits,and in conjunction with the angular sizes, measurements of effectivetemperature. The angular sizes of these stars are sufficiently smallthat the fundamental resolution limits of our primary instrument, thePalomar Testbed Interferometer, are investigated at thesub-milliarcsecond level and empirically established based upon knownperformance limits. We demonstrate that the effective temperature scaleas a function of dereddened (V – K)0 color isstatistically identical for stars with and without planets. A usefulbyproduct of this investigation is a direct calibration of the TEFF scale for solarlike stars, as a function of both spectraltype and (V – K)0 color, with an precision of\overline{\Delta T}_{\it {(V-K)}_0} = 138\,K over the range (V –K)0 = 0.0-4.0 and \overline{\Delta T}_{SpType} = 105\,K forthe range F6V-G5V. Additionally, in an Appendix we provide SED fits forthe 166 stars with known planets which have sufficient photometryavailable in the literature for such fits; this derived "XO-Rad"database includes homogeneous estimates of bolometric flux, reddening,and angular size.
| Multiwavelength Optical Observations of Two Chromospherically Active Binary Systems: V789 Mon and GZ Leo
This paper describes a multiwavelength optical study of chromospheres intwo X-ray/EUV-selected active binary stars with strong Hαemission, V789 Mon (2RE J0725 - 002) and GZ Leo (2RE J1101+223).The goal of the study is to determine radial velocities and fundamentalstellar parameters in chromospherically active binary systems in orderto include them in the activity-rotation and activity-age relations. Wecarried out high-resolution echelle spectroscopic observations andapplied spectral-subtraction technique in order to measure emissionexcesses due to chromosphere. The detailed study of activity indicatorsallowed us to characterize the presence of different chromosphericfeatures in these systems and enabled to include them in a largeractivity-rotation survey. We computed radial velocities of the systemsusing cross-correlation with the radial velocity standards. Thedouble-line spectral binarity was confirmed and the orbital solutionsimproved for both systems. In addition, other stellar parameters such asspectral types, projected rotational velocities (vsin i) and theequivalent width of the lithium Li I λ6707.8 Åabsorption line were determined.Based on observations collected with the 2.2 m telescope at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto(Almería, Spain), operated jointly by the Max-Planck Institutfür Astronomie and the Instituto de Astrofísica deAndalucía (CSIC) and with the 2.1 m Otto Struve Telescope atMcDonald Observatory of the University of Texas at Austin (USA).
| Stellar and Circumstellar Properties of Class I Protostars
We present a study of the stellar and circumstellar properties of ClassI sources using low-resolution (R ~ 1000) near-infrared (near-IR) K- andL-band spectroscopy. We measure prominent spectral lines and features ineight objects and use fits to standard star spectra to determinespectral types, visual extinctions, K-band excesses, and water iceoptical depths. Four of the seven systems studied are close binarypairs; only one of these systems, Haro 6-10, was angularly resolvable.For certain stars, some properties found in our analysis differsubstantially from published values; we analyze the origin of thesedifferences. We determine extinction to each source using threedifferent methods and compare and discuss the resulting values. Onehypothesis that we were testing, that extinction dominates over theK-band excess in obscuration of the stellar photospheric absorptionlines, appears not to be true. Accretion luminosities and mass accretionrates calculated for our targets are highly uncertain, in part theresult of our inexact knowledge of extinction. For the six targets wewere able to place on a Hertzsprung-Russell diagram, our age estimates,<2 Myr, are somewhat younger than those from comparable studies. Ourresults underscore the value of low-resolution spectroscopy in the studyof protostars and their environments; however, the optimal approach tothe study of Class I sources likely involves a combination of high- andlow-resolution near-IR, mid-IR, and millimeter wavelength observations.Accurate and precise measurements of extinction in Class I protostarswill be key to improving our understanding of these objects.
| The multiplicity of exoplanet host stars. New low-mass stellar companions of the exoplanet host stars HD 125612 and HD 212301
Aims: We present new results from our ongoing multiplicity study ofexoplanet host stars, carried out with SofI/NTT. We provide the mostrecent list of confirmed binary and triple star systems that harborexoplanets. Methods: We use direct imaging to identify widestellar and substellar companions as co-moving objects to the observedexoplanet host stars, whose masses and spectral types are determinedwith follow-up photometry and spectroscopy. Results: We found twonew co-moving companions of the exoplanet host stars HD 125612 and HD212301. HD 125612 B is a wide M 4 dwarf (0.18 M_ȯ) companion of theexoplanet host star HD 125612, located about 1.5 arcmin (~4750 AU ofprojected separation) south-east of its primary. In contrast, HD 212301B is a close M 3 dwarf (0.35 M_ȯ), which is found about 4.4 arcsec(~230 AU of projected separation) north-west of its primary. Conclusions: The binaries HD 125612 AB and HD 212301 AB are newmembers in the continuously growing list of exoplanet host star systemsof which 43 are presently known. Hence, the multiplicity rate ofexoplanet host stars is about 17%.Based on observations obtained on La Silla in ESO programs079.C-0099(A), 080.C-0312(A).
| The minimum Jeans mass, brown dwarf companion IMF, and predictions for detection of Y-type dwarfs
Cool L- and T-type objects were discovered first as companions to starsin 1988 and 1995, respectively. A certain example of the even coolerY-type spectral class (T_eff ⪉ 500 K) has not been seen. Recentinfrared-imaging observations of stars and brown dwarfs indicate thatsubstellar companions with large semi-major axes and with masses lessthan the brown dwarf/giant planet dividing line ( 13.5{M}_J) are rare.Theoretical considerations of the Jeans mass fragmentation of molecularclouds are consistent with this minimum mass cutoff and also with thesemi-major axis (hundreds of AU) characteristic of the lowest massimaged companions. As a consequence, Y-class companions with largesemi-major axes should be scarce around stars <2 Gyr old, and alsoaround substellar primaries of all ages. By focusing on brown dwarfcompanions to young stellar primaries, it is possible to derive a firstestimate of the brown dwarf IMF over the entire range of brown dwarfmasses (13{M}J to 79{M}_J) - the number of companion browndwarfs is proportional to the mass to the -1.2±0.2 power.
| The angular sizes of dwarf stars and subgiants. Non-linear surface brightness relations in BVR_cIc from interferometry
Context: The prediction of stellar angular diameters from broadbandphotometry plays an important role in different applications. Inparticular, long-baseline interferometry, gravitational microlensing,extrasolar planet transits, and many other observing techniques requireaccurate predictions of the angular size of stars. These predictions arebased on the surface brightness-color (SBC) relations. Aims: Ourgoal is to calibrate general-purpose SBC relations using visible colors,the most commonly available data for most stars. Methods: Wecompiled the existing long-baseline interferometric observations ofnearby dwarf and subgiant stars and the corresponding broadbandphotometry in the Johnson BV and Cousins R_cIc bands. We thenadjusted polynomial SBC models to these data. Results: Due to thepresence of spectral features that depend on the effective temperature,the SBC relations are usually not linear for visible colors. We presentpolynomial fits that can be employed with BVR_cIc basedcolors to predict the limb-darkened angular diameters (i.e.photospheric) of dwarf and subgiant stars with a typical accuracy of 5%. Conclusions: The derived polynomial relations provide asatisfactory approximation to the observed surface brightness of dwarfsand subgiants. For distant stars, the interstellar reddening should betaken into account.Tables [see full textsee full text] and [see full textsee full text] areonly available in electronic form at http://www.aanda.org
| Extrasolar Giant Planets and X-Ray Activity
We have carried out a survey of X-ray emission from stars with giantplanets, combining both archival and targeted surveys. Over 230 starshave been currently identified as possessing planets, and roughlyone-third of these have been detected in X-rays. We carry out detailedstatistical analysis on a volume-limited sample of main-sequence starsystems with detected planets, comparing subsamples of stars that haveclose-in planets with stars that have more distant planets. Thisanalysis reveals strong evidence that stars with close-in giant planetsare on average more X-ray active by a factor of ~4 than those withplanets that are more distant. This result persists for various sampleselections. We find that even after accounting for observational samplebias, a significant residual difference still remains. Thisobservational result is consistent with the hypothesis that giantplanets in close proximity to the primary stars influence the stellarmagnetic activity.
| Two Jupiter-Mass Planets Orbiting HD 154672 and HD 205739
We report the detection of the first two planets from the N2K Dopplerplanet search program at the Magellan telescopes. The first planet has amass of Msin i = 4.96 M Jup and orbits the G3 IV star HD154672 with an orbital period of 163.9 days. The second planet orbitsthe F7 V star HD 205739 with an orbital period of 279.8 days and has amass of Msin i = 1.37 M Jup. Both planets are in eccentricorbits, with eccentricities e = 0.61 and e = 0.27, respectively. Bothstars are metal rich and appear to be chromospherically inactive, basedon inspection of their Ca II H and K lines. Finally, the best Keplerianmodel fit to HD 205739b shows a trend of 0.0649 m s-1day-1, suggesting the presence of an additional outerbody in that system.This paper is based on data gathered with the 6.5 m Magellan Telescopeslocated at Las Campanas Observatory, Chile.
| Spectroscopic investigation of stars on the lower main sequence
Aims. The aim of this paper is to provide fundamental parameters andabundances with a high accuracy for a large sample of cool main sequencestars. This study is part of wider project, in which the metallicitydistribution of the local thin disc is investigated from a completesample of G and K dwarfs within 25 pc. Methods: The stars were observedat high resolution and a high signal-to-noise ratio with the ELODIEechelle spectrograph. The V sin i were obtained with a calibration ofthe cross-correlation function. Effective temperatures were estimated bythe line depth ratio method. Surface gravities (log g) were determinedby two methods: parallaxes and ionization balance of iron. The Mg and Naabundances were derived using a non-LTE approximation. Abundances ofother elements were obtained by measuring equivalent widths. Results:Rotational velocities, atmospheric parameters (T_eff, log g, [Fe/H],V_t), and Li, O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Co, Ni, and Znabundances are provided for 131 stars. Among them, more than 30 starsare active stars with a fraction of BY Dra and RS CVn type stars forwhich spectral peculiarities were investigated. We find the meanabundances of the majority of elements in active and nonactive stars tobe similar, except for Li, and possibly for Zn and Co. The lithium isreliably detected in 54% of active stars but only in 20% of nonactivestars. No correlation is found between Li abundances and rotationalvelocities. A possible anticorrelation of log A(Li) with the index ofchromospheric activity GrandS is observed. Conclusions: Active andnonactive cool dwarfs show similar dependencies of most elemental ratiosvs. [Fe/H]. This allows us to use such abundance ratios to study thechemical and dynamical evolution of the Galaxy. Among active stars, noclear correlation has been found between different indicators ofactivity for our sample stars.Based on spectra collected with the ELODIE spectrograph at the 1.93-mtelescope of the Observatoire de Haute Provence (France). Tables A.1-A3are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/489/923
| HN Peg B: A Test of Models of the L to T Dwarf Transition
Luhman and collaborators recently discovered an early-T dwarf companionto the G0 dwarf star HN Peg, using Spitzer Infrared Array Camera (IRAC)images. Companionship was established on the basis of the common propermotion inferred from 1998 Two Micron All Sky Survey images and the 2004IRAC images. In this paper we present new near-infrared imaging datawhich confirm the common proper motion of the system. We also presentnew 3-4 μm spectroscopy of HN Peg B, which provides tighterconstraints on both the bolometric luminosity determination and thecomparison to synthetic spectra. New adaptive optics imaging data arealso presented, which show the T dwarf to be unresolved, providinglimits on the multiplicity of the object. We use the age, distance, andluminosity of the solar-metallicity T dwarf to determine its effectivetemperature and gravity, and compare synthetic spectra with thesevalues, and a range of grain properties and vertical mixing, to theobserved 0.8-4.0 μm spectra and mid-infrared photometry. We find thatmodels with temperature and gravity appropriate for the older end of theage range of the system (0.5 Gyr) can do a reasonable job of fitting thedata, but only if the photospheric condensate cloud deck is thin, and ifthere is significant vertical mixing in the atmosphere. Dwarfs such asHN Peg B, with well-determined metallicity, radius, gravity, andtemperature, will allow development of dynamical atmosphere models,leading to the solution of the puzzle of the L to T dwarf transition.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation. Some data were also obtained at the Canada-France-HawaiiTelescope (CFHT), which is operated by the National Research Council ofCanada, the Institut National des Sciences de l'Univers of the CentreNational de la Recherche Scientifique of France, and the University ofHawaii.
| The Search for Stellar Companions to Exoplanet Host Stars Using the CHARA Array
Most exoplanets have been discovered via radial velocity studies, whichare inherently insensitive to orbital inclination. Interferometricobservations will show evidence of a stellar companion if it issufficiently bright, regardless of the inclination. Using the CHARAArray, we observed 22 exoplanet host stars to search for stellarcompanions in low-inclination orbits that may be masquerading asplanetary systems. While no definitive stellar companions werediscovered, it was possible to rule out certain secondary spectral typesfor each exoplanet system observed by studying the errors in thediameter fit to calibrated visibilities and by searching for separatedfringe packets.For preprints, please email baines@chara.gsu.edu.
| CHARA Array Measurements of the Angular Diameters of Exoplanet Host Stars
We have measured the angular diameters for a sample of 24 exoplanet hoststars using Georgia State University's CHARA Array interferometer. Weuse these improved angular diameters together with Hipparcos parallaxmeasurements to derive linear radii and to estimate the stars'evolutionary states.
| Mg II h+k emission lines as stellar activity indicators of main sequence F-K stars
Context: The largest dataset of stellar activity measurements availableat present is the one obtained at the Mount Wilson Observatory, wherehigh-precision Ca II H+K fluxes have been measured from 1966 for about2200 stars. Since the Mg II h and k lines at λ2800 Å areformed in a similar way to the Ca II H+K emission lines, they are alsogood indicators of chromospheric structure. The InternationalUltraviolet Explorer (IUE) provides a large database of UV spectra inthe band 1150-3350 Å from 1978 to 1995, which can also be used tostudy stellar activity. Aims: The main purpose of this study is to usethe IUE spectra in the analysis of magnetic activity of main sequenceF-K stars. Combining IUE observations of Mg II and optical spectroscopyof Ca II, the registry of activity of stars can be extended in time. Methods: We retrieved all the high-resolution spectra of F, G, and Kmain sequence stars observed by IUE (i.e. 1623 spectra of 259 F to Kdwarf stars). We obtained the continuum surface flux near the Mg II h+klines near λ2800 Å and the Mg II line-core surface fluxfrom the IUE spectra. Results: We obtained a relation between the meancontinuum flux near the Mg II lines with the colour B-V of the star. Fora set of 117 nearly simultaneous observations of Mg II and Ca II fluxesof 21 F5 to K3 main sequence stars, we obtained a colour dependentrelation between the Mount Wilson Ca II S-index and the Mg II emissionline-core flux. As an application of this calibration, we computed theMount Wilson index for all the dF to dK stars which have high resolutionIUE spectra. For some of the most frequently observed main sequencestars, we analysed the Mount Wilson index S from the IUE spectra,together with the ones derived from visible spectra. We confirm thecyclic chromospheric activity of ɛ Eri (HD 22049) and βHydri (HD 2151), and we find a magnetic cycle in α Cen B (HD128621)Table 3 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/483/903
| Vertical distribution of Galactic disk stars. IV. AMR and AVR from clump giants
We present the parameters of 891 stars, mostly clump giants, includingatmospheric parameters, distances, absolute magnitudes, spatialvelocities, galactic orbits and ages. One part of this sample consistsof local giants, within 100 pc, with atmospheric parameters eitherestimated from our spectroscopic observations at high resolution andhigh signal-to-noise ratio, or retrieved from the literature. The otherpart of the sample includes 523 distant stars, spanning distances up to1 kpc in the direction of the North Galactic Pole, for which we haveestimated atmospheric parameters from high resolution but lowsignal-to-noise Echelle spectra. This new sample is kinematicallyunbiased, with well-defined boundaries in magnitude and colours. Werevisit the basic properties of the Galactic thin disk as traced byclump giants. We find the metallicity distribution to be different fromthat of dwarfs, with fewer metal-rich stars. We find evidence for avertical metallicity gradient of -0.31 dex kpc-1 and for atransition at ~4-5 Gyr in both the metallicity and velocities. Theage-metallicity relation (AMR), which exhibits a very low dispersion,increases smoothly from 10 to 4 Gyr, with a steeper increase for youngerstars. The age-velocity relation (AVR) is characterized by thesaturation of the V and W dispersions at 5 Gyr, and continuous heatingin U.
| Nearby stars of the Galactic disc and halo - IV
The Milky Way Galaxy has an age of about 13 billion years. Solar-typestars evolve all the long way to the realm of degenerate objects onessentially this time-scale. This, as well as the particular advantagethat the Sun offers through reliable differential spectroscopicanalyses, render these stars the ideal tracers for the fossil record ofour parent spiral. Astrophysics is a science that is known to benotoriously plagued by selection effects. The present work - with amajor focus in this fourth contribution on model atmosphere analyses ofspectroscopic binaries and multiple star systems - aims at avolume-complete sample of about 300 nearby F-, G-, and K-type stars thatparticularly avoids any kinematical or chemical pre-selection from theoutset. It thereby provides an unbiased record of the local stellarpopulations - the ancient thick disc and the much younger thin disc. Onthis base, the detailed individual scrutiny of the long-lived stars ofboth populations unveils the thick disc as a single-burst component witha local normalization of no less than 20 per cent. This enormousfraction, combined with its much larger scaleheight, implies a mass forthe thick disc that is comparable to that of the thin disc. On accountof its completely different mass-to-light ratio the thick disc therebybecomes the dark side of the Milky Way, an ideal major source forbaryonic dark matter. This massive, ancient population consequentlychallenges any gradual build-up scenario for our parent spiral. Evenmore, on the supposition that the Galaxy is not unusual, the thick disc- as it emerges from this unbiased spectroscopic work - particularlychallenges the hierarchical cold-dark-matter-dominated formation picturefor spiral galaxies in general.
| Debris Disks around Sun-like Stars
We have observed nearly 200 FGK stars at 24 and 70 μm with theSpitzer Space Telescope. We identify excess infrared emission, includinga number of cases where the observed flux is more than 10 times brighterthan the predicted photospheric flux, and interpret these signatures asevidence of debris disks in those systems. We combine this sample of FGKstars with similar published results to produce a sample of more than350 main sequence AFGKM stars. The incidence of debris disks is4.2+2.0-1.1% at 24 μm for a sample of 213Sun-like (FG) stars and 16.4+2.8-2.9% at 70 μmfor 225 Sun-like (FG) stars. We find that the excess rates for A, F, G,and K stars are statistically indistinguishable, but with a suggestionof decreasing excess rate toward the later spectral types; this may bean age effect. The lack of strong trend among FGK stars of comparableages is surprising, given the factor of 50 change in stellar luminosityacross this spectral range. We also find that the incidence of debrisdisks declines very slowly beyond ages of 1 billion years.
| The helium abundance and ΔY/ΔZ in lower main-sequence stars
We use nearby K dwarf stars to measure the helium-to-metal enrichmentratio ΔY/ΔZ, a diagnostic of the chemical history of thesolar neighbourhood. Our sample of K dwarfs has homogeneously determinedeffective temperatures, bolometric luminosities and metallicities,allowing us to fit each star to the appropriate stellar isochrone anddetermine its helium content indirectly. We use a newly computed set ofPadova isochrones which cover a wide range of helium and metal content.Our theoretical isochrones have been checked against a congruous set ofmain-sequence binaries with accurately measured masses, to discuss andvalidate their range of applicability. We find that the stellar massesdeduced from the isochrones are usually in excellent agreement withempirical measurements. Good agreement is also found with empiricalmass-luminosity relations.Despite fitting the masses of the stars very well, we find thatanomalously low helium content (lower than primordial helium) isrequired to fit the luminosities and temperatures of the metal-poor Kdwarfs, while more conventional values of the helium content are derivedfor the stars around solar metallicity.We have investigated the effect of diffusion in stellar models and theassumption of local thermodynamic equilibrium (LTE) in derivingmetallicities. Neither of these is able to resolve the low-heliumproblem alone and only marginally if the cumulated effects are included,unless we assume a mixing-length which is strongly decreasing withmetallicity. Further work in stellar models is urgently needed.The helium-to-metal enrichment ratio is found to be ΔY/ΔZ =2.1 +/- 0.9 around and above solar metallicity, consistent with previousstudies, whereas open problems still remain at the lowest metallicities.Finally, we determine the helium content for a set of planetary hoststars.
| UV habitable zones around M stars
During the last decade there was a change in paradigm, which led toconsider that terrestrial-type planets within liquid-water habitablezones (LW-HZ) around M stars can also be suitable places for theemergence and evolution of life. Since many dMe stars emit large amountof UV radiation during flares, in this work we analyze the UV constrainsfor living systems on Earth-like planets around dM stars. We apply ourmodel of UV habitable zone (UV-HZ; Buccino, A.P., Lemarchand, G.A.,Mauas, P.J.D., 2006. Icarus 183, 491 503) to the three planetary systemsaround dM stars (HIP 74995, HIP 109388 and HIP 113020) observed by IUEand to two M-flare stars (AD Leo and EV Lac). In particular, HIP 74995hosts a terrestrial planet in the LW-HZ, which is the exoplanet thatmost resembles our own Earth. We show, in general, that during thequiescent state there would not be enough UV radiation within the LW-HZto trigger the biogenic processes and that this energy could be providedby flares of moderate intensity, while strong flares do not necessarilyrule-out the possibility of life-bearing planets.
| Hot Jupiters in Binary Star Systems
Radial velocity surveys find Jupiter-mass planets with semimajor axes aless than 0.1 AU around ~1% of solar-type stars; counting planets with aas large as 5 AU, the fraction of stars having planets reaches ~10% (asfound by Marcy et al. and Butler et al.). An examination of thedistribution of semimajor axes shows that there is a clear excess ofplanets with orbital periods around 3 or 4 days, corresponding to a~0.03AU, with a sharp cutoff at shorter periods (see Fig. 1). It is believedthat Jupiter-mass planets form at large distances from their parentstars; some fraction then migrates in to produce the short-periodobjects. We argue that a significant fraction of the hot Jupiters(a<0.1 AU) may arise in binary star systems in which the orbit of thebinary is highly inclined to the orbit of the planet. Mutual torquesbetween the two orbits drive down the minimum separation or periapsisrp between the planet and its host star (the Kozaimechanism). This periapsis collapse is halted when tidal friction on theplanet circularizes the orbit faster than Kozai torque can excite it.The same friction then circularizes the planet orbit, producing hotJupiters with the peak of the semimajor axis distribution lying around 3days. For the observed distributions of binary separation, eccentricity,and mass ratio, roughly 2.5% of planets with initial semimajor axisap~5 AU will migrate to within 0.1 AU of their parent star.Kozai migration could account for 10% or more of the observed hotJupiters.
| Ages for Illustrative Field Stars Using Gyrochronology: Viability, Limitations, and Errors
We here develop an improved way of using a rotating star as a clock, setit using the Sun, and demonstrate that it keeps time well. Thistechnique, called gyrochronology, derives ages for low-massmain-sequence stars using only their rotation periods and colors. Thetechnique is developed here and used to derive ages for illustrativegroups of nearby field stars with measured rotation periods. We firstdemonstrate the reality of the interface sequence, the unifying featureof the rotational observations of cluster and field stars that makes thetechnique possible, and extend it beyond the proposal of Skumanich byspecifying the mass dependence of rotation for these stars. We delineatewhich stars it cannot currently be used on. We then calibrate the agedependence using the Sun. The errors are propagated to understand theirdependence on color and period. Representative age errors associatedwith the technique are estimated at ~15% (plus possible systematicerrors) for late F, G, K, and early M stars. Gyro ages for the MountWilson stars are shown to be in good agreement with chromospheric agesfor all but the bluest stars, and probably superior. Gyro ages are thencalculated for each of the active main-sequence field stars studied byStrassmeier and collaborators. These are shown to have a median age of365 Myr. The sample of single field stars assembled by Pizzolato andcollaborators is then assessed and shown to have gyro ages ranging fromunder 100 Myr to several Gyr, with a median age of 1.2 Gyr. Finally, wedemonstrate that the individual components of the three wide binariesξ Boo AB, 61 Cyg AB, and α Cen AB yield substantially the samegyro ages.
| Characterization of the long-period companions of the exoplanet host stars: HD 196885, HD 1237 and HD 27442. VLT/NACO and SINFONI near-infrared, follow-up imaging and spectroscopy
Aims:We present the results of near-infrared, follow-up imaging andspectroscopic observations at VLT, aimed at characterizing thelong-period companions of the exoplanet host stars HD 196885, HD 1237and HD 27442. The three companions were previously discovered in thecourse of our CFHT and VLT coronographic imaging survey dedicated to thesearch for faint companions of exoplanet host stars. Methods: Weused the NACO near-infrared adaptive optics instrument to obtainastrometric follow-up observations of HD 196885 A and B. The long-slitspectroscopic mode of NACO and the integral field spectrograph SINFONIwere used to carry out a low-resolution spectral characterization of thethree companions HD 196885 B, HD 1237 B and HD 27442 B between 1.4 and2.5 μm. Results: We can now confirm that the companion HD196885 B is comoving with its primary exoplanet host star, as previouslyshown for HD 1237 B and HD 27442 B. We find that both companions HD196885 B and HD 1237 B are low-mass stars of spectral type M1±1Vand M4±1V respectively. HD 196885 AB is one of the closer (~23AU) resolved binaries known to host an exoplanet. This system is thenideal for carrying out a combined radial velocity and astrometricinvestigation of the possible impact of the binary companion on theplanetary system formation and evolution. Finally, we confirm viaspectroscopy that HD 27442 B is a white dwarf companion, the third oneto be discovered orbiting an exoplanet host star, following HD 147513and Gliese 86. The detection of the broad Brγ line of hydrogenindicates a white dwarf atmosphere dominated by hydrogen.Based on ESO observing programs 075.C-0825(A), 275.B-5057A and077.C-0444(A).
| The impact of stellar duplicity on planet occurrence and properties. I. Observational results of a VLT/NACO search for stellar companions to 130 nearby stars with and without planets
Context: Although it is commonly agreed that the presence of a closestellar companion is likely to affect planet formation and evolution,the precise effects and their actual impact on planet occurrence arestill debated. Different conclusions have been reached on thetheoretical side, while observational constraints are sparse, aconsequence of the discrimination against close binaries in Dopplerplanet searches. Accordingly, basic questions such as how hospitablebinaries are to planets and how binary separation and mass ratio impacton planet formation, remain poorly known. Aims: In an effort to bringobservational constraints on the occurrence and properties of planets inbinaries and multiple stars, we have been conducting a dedicatedinvestigation, the results of which will be presented in this series. Methods: Our investigation follows two different approaches, one basedon radial-velocity monitoring, the other based on direct imaging. Inthis first paper, we present the observational results from oursystematic adaptive optics search with VLT/NACO for close stellarcompanions to 130 nearby stars, 57 with planets and 73 without, forcomparison. The inclusion of a control sub-sample is a unique feature ofour program that will enable a meaningful and rigorous comparisonbetween the properties of planet-host stars and the properties of fieldstars subject to the same selection effects against close binaries, butshowing no evidence for planetary companions. Results: Our data reveal95 companion candidates found in the vicinity of 33 of our targets.Nineteen of these candidates are true companions and 2 are likely boundobjects. Among planet-host stars, we discovered a tight pair of very lowmass companions to HD 65216 (projected separation of 255 AU), an early Mcompanion to HD 177830 (projected separation of 97 AU), and we resolvedthe previously known companion to HD 196050 into a close pair of Mdwarfs. Our data additionally confirm the bound nature of the companionsto HD 142, HD 16141, and HD 46375. Among control stars, we detected truecompanions to HD 7895, HD 24331, HD 31412, HD 40397, HD 43834, HD 70923,HD 78351, HD 104263, HD 129642, HD 154682, and HD 223913, and likelybound companions to HD 82241 and HD 134180. Most of these objects are Mdwarfs and have projected separations between 7 and 505 AU.Based on observations collected at the ESO VLT Yepun telescope,proposals 70.C-0557, 71.C-0125, 73.C-0124, 74.C-0048, 75.C-0069, and76.C-0057. Tables 1, 2, and Figs. 3, 4 are only available in electronicform at http://www.aanda.org
| Physical and Spectral Characteristics of the T8 and Later Type Dwarfs
We use newly observed and published near-infrared spectra, together withsynthetic spectra obtained from model atmospheres, to derive physicalproperties of three of the latest type T dwarfs. A new R~1700 spectrumof the T7.5 dwarf HD 3651B, together with existing data, allows adetailed comparison to the well-studied and very similar dwarf Gl 570D.We find that HD 3651B has both higher gravity and higher metallicitythan Gl 570D, with best-fit atmospheric parameters ofTeff=820-830 K, logg=5.4-5.5, [m/H]=+0.2, andKzz=104 cm2 s-1. Its age is8-12 Gyr, and its implied mass is 60-70 MJ. We perform asimilar analysis of the T8 and T7.5 dwarfs 2MASS J09393548-2448279 and2MASS J11145133-2618235 using published data, comparing them to thewell-studied T8, 2MASS J04151954-0935066. We find that these two dwarfshave effectively the same Teff as the reference dwarf, andsimilar or slightly higher gravities, but lower metallicities. Thederived parameters are Teff=725-775 K and [m/H]=-0.3logg=5.3-5.45 for 2MASS J09393548-2448279 and logg=5.0-5.3 for 2MASSJ11145133-261823. The age and mass are ~10 Gyr and 60 MJ for2MASS J09393548-2448279, and ~5 Gyr and 40 MJ for 2MASSJ11145133-261823. A serious limitation to such analyses is theincompleteness of the line lists for transitions of CH4 andNH3 at λ<=1.7 μm, which are also needed forsynthesizing the spectrum of the later, cooler, Y type. Spectra ofSaturn and Jupiter, and of laboratory CH4 and NH3gas, suggest that NH3 features in the Y and J bands may beuseful as indicators of the next spectral type, and not features in theH and K bands, as previously thought. However, until cooler objects arefound, or the line lists improve, large uncertainties remain, as theabundance of NH3 is likely to be significantly below thechemical equilibrium value. Moreover, inclusion of laboratoryNH3 opacities in our models predicts band shapes that arediscrepant with existing data. It is possible that the T spectral classwill have to be extended to temperatures around 400 K, when water cloudscondense in the atmosphere and dramatically change the spectral energydistribution of the brown dwarf.
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