Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
The colours of the Sun
We compile a sample of Sun-like stars with accurate effectivetemperatures, metallicities and colours (from the ultraviolet to thenear-infrared). A crucial improvement is that the effective temperaturescale of the stars has recently been established as both accurate andprecise through direct measurement of angular diameters obtained withstellar interferometers. We fit the colours as a function of effectivetemperature and metallicity, and derive colour estimates for the Sun inthe Johnson-Cousins, Tycho, Strömgren, 2MASS and SDSS photometricsystems. For (B-V)solar, we favour the `red' colour 0.64versus the `blue' colour 0.62 of other recent papers, but both valuesare consistent within the errors; we ascribe the difference to theselection of Sun-like stars versus interpolation of widercolour-Teff-metallicity relations.
| Anharmonic and standing dynamo waves: theory and observation of stellar magnetic activity
The familiar decadal cycle of solar activity is one expression ofinterannual variability of surface magnetism observed in stars on ornear the lower main sequence. From studies of time-series of CaII H andK emission fluxes that go back more than 35 yr and have been accumulatedfor such stars at the Mount Wilson Observatory by the HK Project, wedefine a quantitative measure, called anharmonicity, of the cycliccomponent of interannual magnetic variability. Anharmonicity provides aconnection between observed variations in magnetic activity and thetwo-dimensional description of a Parker dynamo model. We explore theparameter space of the Parker dynamo model and find an excellentcounterpart in the records of several of the lowest-mass (late K-type toearly M-type) active stars in the HK Project sample to the solutionscontaining highly anharmonic, standing dynamo waves. We interpretanharmonicity apparent in the records as resulting from non-propagatingor standing dynamo waves, which operate in a regime that issubstantially supercriticial. There, for the majority of a cycle, orpulse of decadal-to-interdecadal variability, the large-scale magneticfields are generated and maintained by winding of field by differentialrotation rather than by the joint action of differential rotation andhelical convection. Among the less active stars (the Sun is consideredsuch a star in the HK Project sample) we find a correspondence betweenanharmonicity and Parker dynamo model solutions that include simpleharmonic, migratory and/or intermediate-type dynamo wave patterns over abroad range of dynamo parameters.
| HD 98618: A Star Closely Resembling Our Sun
Despite the observational effort carried out in the last few decades, noperfect solar twin has been found to date. An important milestone wasachieved a decade ago by Porto de Mello & da Silva, who showed that18 Sco is almost a solar twin. In the present work, we use extremelyhigh resolution (R=105), high signal-to-noise ratio KeckHIRES spectra to carry out a differential analysis of 16 solar-twincandidates. We show that HD 98618 is the second-closest solar twin andthat the fundamental parameters of both HD 98618 and 18 Sco are verysimilar (within a few percent) to the host star of our solar system,including the likelihood of hosting a terrestrial planet within theirhabitable zones. We suggest that these stars should be given toppriority in exoplanet and SETI surveys.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration.
| IRS Spectra of Solar-Type Stars: A Search for Asteroid Belt Analogs
We report the results of a spectroscopic search for debris diskssurrounding 41 nearby solar-type stars, including eight planet-bearingstars, using the Infrared Spectrometer (IRS) on the Spitzer SpaceTelescope. With the accurate relative photometry of the IRS between 7and 34 μm we are able to look for excesses as small as ~2% ofphotospheric levels, with particular sensitivity to weak spectralfeatures. For stars with no excess, the 3 σ upper limit in a bandat 30-34 μm corresponds to ~75 times the brightness of our zodiacaldust cloud. Comparable limits at 8.5-13 μm correspond to ~1400 timesthe brightness of our zodiacal dust cloud. These limits correspond tomaterial located within the <1 to ~5 AU region that, in our solarsystem, originates predominantly from debris associated with theasteroid belt. We find excess emission longward of ~25 μm from fivestars, of which four also show excess emission at 70 μm. Thisemitting dust must be located in a region starting around 5-10 AU. Onestar has 70 μm emission but no IRS excess. In this case, the emittingregion must begin outside 10 AU; this star has a known radial velocityplanet. Only two stars of the five show emission shortward of 25 μm,where spectral features reveal the presence of a population of small,hot dust grains emitting in the 7-20 μm band. One of these stars, HD72905, is quite young (300 Myr), while the other, HD 69830, is olderthan 2 Gyr. The data presented here strengthen the results of previousstudies to show that excesses at 25 μm and shorter are rare: only 1out of 40 stars older than 1 Gyr or ~2.5% shows an excess. Asteroidbelts 10-30 times more massive than our own appear are rare amongmature, solar-type stars.
| Frequency of Debris Disks around Solar-Type Stars: First Results from a Spitzer MIPS Survey
We have searched for infrared excesses around a well-defined sample of69 FGK main-sequence field stars. These stars were selected withoutregard to their age, metallicity, or any previous detection of IRexcess; they have a median age of ~4 Gyr. We have detected 70 μmexcesses around seven stars at the 3 σ confidence level. Thisextra emission is produced by cool material (<100 K) located beyond10 AU, well outside the ``habitable zones'' of these systems andconsistent with the presence of Kuiper Belt analogs with ~100 times moreemitting surface area than in our own planetary system. Only one star,HD 69830, shows excess emission at 24 μm, corresponding to dust withtemperatures >~300 K located inside of 1 AU. While debris disks withLdust/L*>=10-3 are rare around oldFGK stars, we find that the disk frequency increases from 2%+/-2% forLdust/L*>=10-4 to 12%+/-5% forLdust/L*>=10-5. This trend in thedisk luminosity distribution is consistent with the estimated dust inour solar system being within an order of magnitude greater or less thanthe typical level around similar nearby stars. Although there is nocorrelation of IR excess with metallicity or spectral type, there is aweak correlation with stellar age, with stars younger than a gigayearmore likely to have excess emission.
| Dwarfs in the Local Region
We present lithium, carbon, and oxygen abundance data for a sample ofnearby dwarfs-a total of 216 stars-including samples within 15 pc of theSun, as well as a sample of local close giant planet (CGP) hosts (55stars) and comparison stars. The spectroscopic data for this work have aresolution of R~60,000, a signal-to-noise ratio >150, and spectralcoverage from 475 to 685 nm. We have redetermined parameters and derivedadditional abundances (Z>10) for the CGP host and comparison samples.From our abundances for elements with Z>6 we determine the meanabundance of all elements in the CGP hosts to range from 0.1 to 0.2 dexhigher than nonhosts. However, when relative abundances ([x/Fe]) areconsidered we detect no differences in the samples. We find nodifference in the lithium contents of the hosts versus the nonhosts. Theplanet hosts appear to be the metal-rich extension of local regionabundances, and overall trends in the abundances are dominated byGalactic chemical evolution. A consideration of the kinematics of thesample shows that the planet hosts are spread through velocity space;they are not exclusively stars of the thin disk.
| 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.
| 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.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| The dependence of the Rossby number and XUV-Lyα emission flux with age for solar-like G-type stars
Stellar parameters of 11 G-type stars with ages ranging from 0.1 to 8.5Gyr, from the Sun in Time programme, were used to compute the Rossbynumber, ℜ, for each star. The Rossby number for each star wascalculated from the rotation period and the convective overturn timederived from spectral type (B-V). It was found to vary essentially ast0.5, where t is the stellar age in Gyr. The Rossby number isused as an index of X-ray-ultraviolet (XUV) (1-1200 Å) andLyα activity, defined as the ratio of the total emission flux inthese spectral regions to the total bolometric emission. Expressions forthe ratio of the stellar surface XUV and Lyα emission fluxrelative to present mean solar surface flux values are given in terms ofℜ. It is shown that the observed activity in these stars varies asℜ-β, where β takes values of 2.5 and 1.5 forXUV and Lyα, respectively. Expressions for deriving the Rossbynumber from B-V and age are also given. Thus, one can use the stellarB-V and effective temperature variation with age to calculate the XUVand Lyα emission flux relative to present solar values. As anexample, the evolution of the solar XUV and Lyα with age from 0.1to 8.5 Gyr is given. The variation of the stellar ultraviolet flux withage can be used in photochemical models to study the evolution ofplanetary atmospheres orbiting such stars.
| 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 Effective Temperature Scale of FGK Stars. II. Teff:Color:[Fe/H] Calibrations
We present up-to-date metallicity-dependent temperature versus colorcalibrations for main-sequence and giant stars based on temperaturesderived with the infrared flux method (IRFM). Seventeen colors in thephotometric systems UBV, uvby, Vilnius, Geneva, RI(Cousins), DDO,Hipparcos-Tycho, and Two Micron All Sky Survey (2MASS) have beencalibrated. The spectral types covered by the calibrations range from F0to K5 (7000K>~Teff>~4000K) with some relationsextending below 4000 K or up to 8000 K. Most of the calibrations arevalid in the metallicity range -3.5>~[Fe/H]>~0.4, although some ofthem extend to as low as [Fe/H]~-4.0. All fits to the data have beenperformed with more than 100 stars; standard deviations range from 30 to120 K. Fits were carefully performed and corrected to eliminate thesmall systematic errors introduced by the calibration formulae. Tablesof colors as a function of Teff and [Fe/H] are provided. Thiswork is largely based on the study by A. Alonso and collaborators; thus,our relations do not significantly differ from theirs except for thevery metal-poor hot stars. From the calibrations, the temperatures of 44dwarf and giant stars with direct temperatures available are obtained.The comparison with direct temperatures confirms our finding in Paper Ithat the zero point of the IRFM temperature scale is in agreement, tothe 10 K level, with the absolute temperature scale (that based onstellar angular diameters) within the ranges of atmospheric parameterscovered by those 44 stars. The colors of the Sun are derived from thepresent IRFM Teff scale and they compare well with those offive solar analogs. It is shown that if the IRFM Teff scaleaccurately reproduces the temperatures of very metal-poor stars,systematic errors of the order of 200 K, introduced by the assumption of(V-K) being completely metallicity independent when studying verymetal-poor dwarf stars, are no longer acceptable. Comparisons with otherTeff scales, both empirical and theoretical, are also shownto be in reasonable agreement with our results, although it seems thatboth Kurucz and MARCS synthetic colors fail to predict the detailedmetallicity dependence, given that for [Fe/H]=-2.0, differences as highas approximately +/-200 K are found.
| Single-Visit Photometric and Obscurational Completeness
We report a method that uses ``completeness'' to estimate the number ofextrasolar planets discovered by an observing program with adirect-imaging instrument. We develop a completeness function forEarth-like planets on ``habitable'' orbits for an instrument with acentral field obscuration, uniform sensitivity in an annular detectionzone, and limiting sensitivity that is expressed as a ``deltamagnitude'' with respect to the star, determined by systematic effects(given adequate exposure time). We demonstrate our method of estimationby applying it to our understanding of the coronagraphic version of theTerrestrial Planet Finder (TPF-C) mission as of 2004 October. Weestablish an initial relationship between the size, quality, andstability of the instrument's optics and its ability to meet missionscience requirements. We provide options for increasing the fidelity andversatility of the models on which our method is based, and we discusshow the method could be extended to model the TPF-C mission as a wholeto verify that its design can meet the science requirements.
| 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.
| Evolution of the Solar Activity over Time and Effects on Planetary Atmospheres. I. High-Energy Irradiances (1-1700 Å)
We report on the results of the Sun in Time multiwavelength program(X-rays to UV) of solar analogs with ages covering ~0.1-7 Gyr. The chiefscience goals are to study the solar magnetic dynamo and to determinethe radiative and magnetic properties of the Sun during its evolutionacross the main sequence. The present paper focuses on the latter goal,which has the ultimate purpose of providing the spectral irradianceevolution of solar-type stars to be used in the study and modeling ofplanetary atmospheres. The results from the Sun in Time program suggestthat the coronal X-ray-EUV emissions of the young main-sequence Sun were~100-1000 times stronger than those of the present Sun. Similarly, thetransition region and chromospheric FUV-UV emissions of the young Sunare expected to be 20-60 and 10-20 times stronger, respectively, than atpresent. When we consider the integrated high-energy emission from 1 to1200 Å, the resulting relationship indicates that about 2.5 Gyrago the solar high-energy flux was about 2.5 times the present value andabout 3.5 Gyr ago was about 6 times the present value (when lifesupposedly arose on Earth). The strong radiation emissions inferredshould have had major influences on the thermal structure,photochemistry, and photoionization of planetary atmospheres and haveplayed an important role in the development of primitive life in thesolar system. Some examples of the application of the Sun in Timeresults on exoplanets and on early solar system planets are discussed.
| Stellar activity cycles: observing the dynamo?
The enormous complexity of the atmospheric structure observed on the Sunmakes it very difficult to compare the Sun with ``solar-type stars''.Clearly, we need to identify parameters that can be observed on the Sunas well as on other stars which can be interpreted unambiguously. Themost widely accepted dynamo signature is the presence of an activitycycle, well documented for the Sun and for main-sequence stars due tothe Mount Wilson Ca II H&K project. Only recently have we detectedspatial information, differential rotation and possibly meridional flowson other stars and thereby adding another constraint for itsinterpretation within a dynamo theory. Again, the picture is notcomplete yet, despite that there is just a single main ingredient thatacts as the driving mechanism for activity in all atmospheric layers andthe convective envelope of a solar-type star: the dynamo-relatedmagnetic field. I stress the importance of mapping stellar surfaces asfingerprints of the underlying dynamo action over long periods of time.
| Keck HIRES Spectroscopy of Four Candidate Solar Twins
We use high signal-to-noise ratio, high-resolution Keck HIRESspectroscopy of four solar twin candidates (HIP 71813, 76114, 77718, and78399) pulled from our Hipparcos-based Ca II H and K survey to carry outparameter and abundance analyses of these objects. Our spectroscopicTeff estimates are ~100 K hotter than the photometric scale of therecent Geneva-Copenhagen survey; several lines of evidence suggest thephotometric temperatures are too cool at solar Teff. At the same time,our abundances for the three solar twin candidates included in theGeneva-Copenhagen survey are in outstanding agreement with thephotometric metallicities; there is no sign of the anomalously lowphotometric metallicities derived for some late-G UMa group and Hyadesdwarfs. A first radial velocity determination is made for HIP 78399 andUVW kinematics derived for all stars. HIP 71813 appears to be akinematic member of the Wolf 630 moving group (a structure apparentlyreidentified in a recent analysis of late-type Hipparcos stars), but itsmetallicity is 0.1 dex higher than the most recent estimate of thisgroup's metallicity. While certainly solar-type stars, HIP 76114 and77718 are a few percent less massive, significantly older, andmetal-poor compared to the Sun; they are neither good solar twincandidates nor solar analogs providing a look at the Sun at some otherpoint in its evolution. HIP 71813 appears to be an excellent solaranalog of ~8 Gyr age. Our results for HIP 78399 suggest the promise ofthis star as a solar twin may be equivalent to the ``closest ever solartwin,'' HR 6060; follow-up study of this star is encouraged.
| Stars within 15 Parsecs: Abundances for a Northern Sample
We present an abundance analysis for stars within 15 pc of the Sunlocated north of -30° declination. We have limited our abundancesample to absolute magnitudes brighter than +7.5 and have eliminatedseveral A stars in the local vicinity. Our final analysis list numbers114 stars. Unlike Allende Prieto et al. in their consideration of a verysimilar sample, we have enforced strict spectroscopic criteria in thedetermination of atmospheric parameters. Nevertheless, our results arevery similar to theirs. We determine the mean metallicity of the localregion to be <[Fe/H]>=-0.07 using all stars and -0.04 when interlopersfrom the thick disk are eliminated.
| Chromospheric models of solar analogues with different activity levels
We computed chromospheric models of the Sun as a star and of nine solaranalogues. The atmospheric models were constructed to obtain the bestpossible match with the Ca II K and Hβ lines, including theasymmetry of the lines due to macroscopic velocity fields. The starswere chosen with 0.62 < B-V< 0.68 (the solar B-V=0.65) and have awide variety of magnetic activity levels, which allows us to study thedifferences in atmospheric structures induced by activity. For the lessactive stars we found that the changes with activity are in the regionof the temperature minimum, while the most active stars show changes allalong their atmospheric structures, mainly in the upper chromosphere.Regarding the macroscopic velocity fields, we can distinguish betweenthe two groups. The most active group has a velocity field in thetemperature-minimum region, and the other group in the chromosphericplateau. We also computed the net radiative losses for each model, andfound that they depend linearly on the usual index of chromosphericactivity, SCa II.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| 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.
| Fine structure of the chromospheric activity in Solar-type stars - The Hα line
A calibration of Hα as both a chromospheric diagnostic and an ageindicator is presented, complementing the works previously done on thissubject (\cite{herbig}; \cite{luca1}). The chromospheric diagnostic wasbuilt with a statistically significant sample, covering nine years ofobservations, and including 175 solar neighborhood stars. Regarding theage indicator, the presence of stars for which very accurate ages aredetermined, such as those belonging to clusters and kinematic groups,lends confidence to our analysis. We also investigate the possibilitythat stars of the same age might have gone through different tracks ofchromospheric decay, identifying - within the same age range - effectsof metallicity and mass. These parameters, however, as well as age, seemto be significant only for dwarf stars, losing their meaning when weanalyze stars in the subgiant branch. This result suggests that, inthese evolved stars, the emission mechanism cannot bemagnetohydrodynamical in nature, in agreement with recent models (Fawzyet al. 2002c, and references therein). The Sun is found to be a typicalstar in its Hα chromospheric flux, for its age, mass andmetallicity. As a byproduct of this work, we developed an automaticmethod to determine temperatures from the wings of Hα, which meansthe suppression of the error inherent to the visual procedure used inthe literature.Based on observations collected at Observatório do Pico dos Dias,operated by the Laboratório Nacional de Astrofísica, CNPq,Brazil.Table 5 is only available in electronic form at thehttp://www.edpsciences.org
| Time-spectra of chromospheric activity of old solar-type stars: detection of rotational signals from double wavelet analysis
We introduce a novel technique, called the double wavelet analysis(DWA), for the determination of stellar rotation periods from timeserial data. This first paper aims narrowly at the discussion,introduction and application of the DWA technique to records of surfacemagnetism in solar-type (relatively old) lower main sequence stars thatare obtained by the Mount Wilson Observatory (MWO) HK Project. Thetechnique takes a series of careful steps that seek to optimize waveletparameters and normalization schemes, ultimately allowing fine-tuned,arguably more accurate, estimates of rotation-modulated signals (with,e.g., periods of days to months) in records that contain longerperiodicities such as stellar magnetic activity cycles (with, e.g.,period of years). The apparent rotation periods estimated from the DWAtechnique are generally consistent with results from both ``first-pass''(i.e., ordinary) global wavelet spectrum and earlier classicalperiodogram analyses. But there are surprises as well. For example, therotation period of the ancient subdwarf Goombridge 1830 (HD 103095),previously identified as ~31 days, suggests under the DWAtechnique a significantly slower period of 60 days. DWA spectra alsogenerally reveal a shift in the cycle period toward high frequencies(hence shorter periods) compared to the first-pass wavelet spectrum. Forsolar-type stars analyzed here, the character of the DWA spectrum andslope of the first-pass global wavelet spectrum produce a classificationscheme that allows a star's record to be placed into one of threecategories.
| Near-infrared spectral monitoring of Triton with IRTF/SpeX I: establishing a baseline for rotational variability
We present eight new 0.8 to 2.4 μm spectral observations ofNeptune's satellite Triton, obtained at IRTF/SpeX during 2002 July15-22 UT. Our objective was to determine how Triton's near-infraredspectrum varies as Triton rotates, and to establish an accurate baselinefor comparison with past and future observations. The most strikingspectral change detected was in Triton's nitrogen ice absorptionband at 2.15 μm; its strength varies by about a factor of two asTriton rotates. Maximum N2 absorption approximately coincideswith Triton's Neptune-facing hemisphere, which is also thelongitude where the polar cap extends nearest Triton's equator.More subtle rotational variations are reported for Triton'sCH4 and H2O ice absorption bands. Unlike the otherices, Triton's CO2 ice absorption bands remain nearlyconstant as Triton rotates. Triton's H2O ice is shown tobe crystalline, rather than amorphous. Triton's N2 iceis confirmed to be the warmer, hexagonal, β N2 phase,and its CH4 is confirmed to be highly diluted inN2 ice.
| The Atmospheric Lithium Abundances of Solar Analogues
We have analyzed the lithium abundance in the atmospheres of 20 starsthat are solar analogues based on high-resolution echelle spectra usingmodel atmospheres in a non-LTE approach. In terms of their lithiumabundances, the stars (which are located in a narrow range oftemperatures of 5650 5900 K) can be divided into two groups: one withlow lithium abundances, as in the solar atmosphere, and one with lithiumabundances that are higher than the solar value by about 1 dex (with theaccuracy of the lithium abundances being 0.15 dex).
| Chemical Composition of 15 Photometric Analogues of the Sun
The results of a spectroscopic analysis of 15 stars that are photometricanalogues of the Sun are reported. The effective temperatures andsurface gravities in the stellar atmospheres are derived from publishedphotometric indices and the HIPPARCOS parallaxes. The abundances of 33elements ranging from lithium to europium are analyzed based onhigh-dispersion spectra taken with the new Coudé echellespectrometer of the Terskol Observatory in the northern Caucasus. Themain parameters of most of the stars agree with the data of an [Fe/H]catalog published in 2001. Our study of the chemical compositions of thesample stars indicates that photometric analogues of the Sun can bedivided into three groups according to their elemental abundances: sixstars have solar chemical composition, four have abundance excesses, andfive have some abundance deficiencies. The sample contains twometal-deficient subgiants (HD 133002 and HD 225239). Our resultsdemonstrate that photometric similarity is not a sufficient criterion toconsider a star as solar analogue. When several criteria, includingchemical composition, are simultaneously taken into account, only fourstars from the sample can be considered true solar analogues: HD 10307,HD 34411, HD 146233 (18 Sco), and HD 186427 (16 Cyg B). These resultsconfirm the previously published suggestion that 18 Sco is the mostprobable twin of the Sun: essentially all the parameters of the twostars coincide within the errors.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| The Chromospheric Activity and Variability of Cycling and Flat Activity Solar-Analog Stars
We present an analysis of more than 3700 observations of the Ca II H andK lines in 57 Sun-like stars and over 3000 analogous observations of theSun. Ten of the 57 stars under consideration are observed in flatstates, but these stars do not always exhibit overall Ca II H and K corebrightness below that of solar minimum. Solar activity minimum lies nearthe lowest level observed for stars with cyclic or irregularvariability, but many flat stars have HK activity levels comparable toor exceeding that of solar minimum. While flat activity stars may be inperiods of extended activity minima analogous to the solar Maunderminimum, a significant reduction in magnetic activity during suchperiods is not implied (although it is also not rejected) by the data.
| Synthetic Lick Indices and Detection of α-enhanced Stars. II. F, G, and K Stars in the -1.0 < [Fe/H] < +0.50 Range
We present an analysis of 402 F, G, and K solar neighborhood stars, withaccurate estimates of [Fe/H] in the range -1.0 to +0.5 dex, aimed at thedetection of α-enhanced stars and at the investigation of theirkinematical properties. The analysis is based on the comparison of 571sets of spectral indices in the Lick/IDS system, coming from fourdifferent observational data sets, with synthetic indices computed withsolar-scaled abundances and with α-element enhancement. We useselected combinations of indices to single out α-enhanced starswithout requiring previous knowledge of their main atmosphericparameters. By applying this approach to the total data set, we obtain alist of 60 bona fide α-enhanced stars and of 146 stars withsolar-scaled abundances. The properties of the detected α-enhancedand solar-scaled abundance stars with respect to their [Fe/H] values andkinematics are presented. A clear kinematic distinction betweensolar-scaled and α-enhanced stars was found, although a one-to-onecorrespondence to ``thin disk'' and ``thick disk'' components cannot besupported with the present data.
| Boron Benchmarks for the Galactic Disk
Sixteen Population I solar-type dwarfs have been selected to ascertainthe baseline B abundance in the Galactic disk for a range of a factor of4 in metallicity: from [Fe/H] of -0.5 to +0.1. All the stars selectedare undepleted in Be, which ensures that they have also retained theirfull initial abundance of B. Evaluation of the trend of B with Feprovides a means to study the evolution of B in the Galactic disk. Weobserved 16 bright stars around the B I 2497 Å line, using theSTIS echelle spectrograph on HST. New observations of Li and Be in somestars were made, and previous abundance studies of Li and Be in thesestars were reevaluated using revised parameters and a modified spectralsynthesis code for consistency with the B measurements. Abundances of Bwere calculated by spectrum synthesis with the revised MOOG code, whichaccounts for the increased opacity in the UV due to metals; the LTE Babundances were then corrected for non-LTE effects. Four additionalstars with undepleted Be have HST B observations, which increase oursample to 20. For these disk stars there is a shallow slope for B versusFe and Be versus Fe, such that as Fe increases by a factor of 4, B andBe increase by 1.7 times. The slope for BLTE versus Fe is0.31+/-0.09, for BNLTE versus Fe 0.40+/-0.12, and for Beversus Fe 0.38+/-0.14. We have estimated the effect of additional UVopacity from Mg and find that an increase of 0.3 dex in Mg results in ahigher B abundances by 0.1 dex for all the disk stars. Individual starsare not consistently above (or below) the mean in both B and Be,implying that the star-to-star differences are not due to variations inthe elemental content of the ``natal'' clouds. We find that the trend ofB abundance with [Fe/H] is consistent with the general trend observed inhalo stars. If we connect the halo and disk stars, then an increase inthe Fe abundance by 103 is accompanied by increases of 100times in B and 550 times in Be. However, fitting two separate relationsfor the disk and the halo stars results in a somewhat steeper slope forBe for the halo stars (1.08+/-0.07) relative to the disk stars(0.38+/-0.14). This is the case for B also in LTE, with Bhalo(0.90+/-0.07) versus Bdisk (0.32+/-0.12). However, the NLTE Babundance increases more slowly for halo stars than the Be abundancedoes; since this is not predicted by light-element synthesis ordepletion, we suggest that a full NLTE analysis would be preferable tomaking the (small) corrections to the LTE abundances. Some of the lowestmetallicity stars are thought to have only upper limits on the Babundance; if that is the case, the NLTE B slope is steeper, nearing1.0. The abundance of B in the disk stars is observed to be a factor of~15+7-5 more than the abundance of Be in thesestars, a result consistent with the predictions of Galactic cosmic-ray(GCR) spallation, B/Be=15+/-5. The upper envelope for Li versus Feyields Li/B and Li/Be ratios that, when coupled with models andpredictions, indicate that 20%-45% of Li might be produced by GCRs.While there is no evidence to support the production of B by neutrinospallation, we cannot rule it out.Based on observations obtained with the NASA/ESA Hubble Space Telescope(HST) through the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS5-26555.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Scorpius |
| Ascensió Recta: | 16h15m37.30s |
| Declinació: | -08°22'10.0" |
| Magnitud Aparent: | 5.5 |
| Distancia: | 14.025 parsecs |
| Moviment propi RA: | 231.1 |
| Moviment propi Dec: | -494.6 |
| B-T magnitude: | 6.292 |
| V-T magnitude: | 5.57 |
Catàlegs i designacions:
|
