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Ýlgili Makaleler
A Survey of Stellar Families: Multiplicity of Solar-type Stars
We present the results of a comprehensive assessment of companions tosolar-type stars. A sample of 454 stars, including the Sun, was selectedfrom the Hipparcos catalog with ?>40 mas,??/? < 0.05, 0.5 <= B - V <= 1.0(~F6-K3), and constrained by absolute magnitude and color to excludeevolved stars. These criteria are equivalent to selecting all dwarf andsubdwarf stars within 25 pc with V-band flux between 0.1 and 10 timesthat of the Sun, giving us a physical basis for the term "solar-type."New observational aspects of this work include surveys for (1) veryclose companions with long-baseline interferometry at the Center forHigh Angular Resolution Astronomy Array, (2) close companions withspeckle interferometry, and (3) wide proper-motion companions identifiedby blinking multi-epoch archival images. In addition, we include theresults from extensive radial-velocity monitoring programs and evaluatecompanion information from various catalogs covering many differenttechniques. The results presented here include four new commonproper-motion companions discovered by blinking archival images.Additionally, the spectroscopic data searched reveal five new stellarcompanions. Our synthesis of results from many methods and sourcesresults in a thorough evaluation of stellar and brown dwarf companionsto nearby Sun-like stars. The overall observed fractions of single,double, triple, and higher-order systems are 56% ± 2%, 33%± 2%, 8% ± 1%, and 3% ± 1%, respectively, countingall confirmed stellar and brown dwarf companions. If all candidate,i.e., unconfirmed, companions identified are found to be real, thepercentages would change to 54% ± 2%, 34% ± 2%, 9%± 2%, and 3% ± 1%, respectively. Our completeness analysisindicates that only a few undiscovered companions remain in thiswell-studied sample, implying that the majority (54% ± 2%) ofsolar-type stars are single, in contrast to the results of priormultiplicity studies. Our sample is large enough to enable a check ofthe multiplicity dependence on various physical parameters by analyzingappropriate subsamples. Bluer, more massive stars are seen as morelikely to have companions than redder, less massive ones, consistentwith the trend seen over the entire spectral range. Systems with largerinteraction cross sections, i.e., those with more than two components orlong orbital periods, are preferentially younger, suggesting thatcompanions may be stripped over time by dynamical interactions. Weconfirm the planet-metallicity correlation (i.e., higher metallicitystars are more likely to host planets), but are unable to check it forbrown dwarfs due to the paucity of such companions, implying that thebrown dwarf desert extends over all separation regimes. We find nocorrelation between stellar companions and metallicity for B - V< 0.625, but among the redder subset, metal-poor stars ([Fe/H]<-0.3) are more likely to have companions with a 2.4?significance. The orbital-period distribution of companions is unimodaland roughly log normal with a peak and median of about 300 years. Theperiod-eccentricity relation shows the expected circularization forperiods below 12 days, caused by tidal forces over the age of theGalaxy, followed by a roughly flat distribution. The mass-ratiodistribution shows a preference for like-mass pairs, which occur morefrequently in relatively close pairs. The fraction of planet hosts amongsingle, binary, and multiple systems are statisticallyindistinguishable, suggesting that planets are as likely to form aroundsingle stars as they are around components of binary or multiple systemswith sufficiently wide separations. This, along with the preference oflong orbital periods among stellar systems, increases the space aroundstars conducive for planet formation, and perhaps life.
| Chromospheric activity and rotation of FGK stars in the solar vicinity. An estimation of the radial velocity jitter
Context. Chromospheric activity produces both photometric andspectroscopic variations that can be mistaken as planets. Large spotscrossing the stellar disc can produce planet-like periodic variations inthe light curve of a star. These spots clearly affect the spectral lineprofiles, and their perturbations alter the line centroids creating aradial velocity jitter that might “contaminate” thevariations induced by a planet. Precise chromospheric activitymeasurements are needed to estimate the activity-induced noise thatshould be expected for a given star. Aims: We obtain precisechromospheric activity measurements and projected rotational velocitiesfor nearby (d ? 25 pc) cool (spectral types F to K) stars, toestimate their expected activity-related jitter. As a complementaryobjective, we attempt to obtain relationships between fluxes indifferent activity indicator lines, that permit a transformation oftraditional activity indicators, i.e., Ca ii H & K lines, to othersthat hold noteworthy advantages. Methods: We used high resolution(~50 000) echelle optical spectra. Standard data reduction was performedusing the IRAF echelle package. To determine the chromospheric emissionof the stars in the sample, we used the spectral subtraction technique.We measured the equivalent widths of the chromospheric emission lines inthe subtracted spectrum and transformed them into fluxes by applyingempirical equivalent width and flux relationships. Rotational velocitieswere determined using the cross-correlation technique. To inferactivity-related radial velocity (RV) jitter, we used empiricalrelationships between this jitter and the R'_HK index. Results:We measured chromospheric activity, as given by different indicatorsthroughout the optical spectra, and projected rotational velocities for371 nearby cool stars. We have built empirical relationships among themost important chromospheric emission lines. Finally, we used themeasured chromospheric activity to estimate the expected RV jitter forthe active stars in the sample.Based on observations made with the 2.2 m telescope at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto (Spain)and the Telescopio Nazionale Galileo (TNG) operated on the island of LaPalma by the Istituto Nazionale de Astrofisica Italiano (INAF), in theSpanish Observatorio del Roque de los Muchachos. This research has beensupported by the Programa de Acceso a InfraestructurasCientíficas y Tecnológicas Singulares (ICTS).Tables A1 toA4 are only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/520/A79
| Orbital Elements of the Symbiotic Star Z Andromedae from Optical Linear Polarization During the Quiescent Phase
We present low-resolution spectropolarimetry for the symbiotic star ZAndromedae at four different epochs during the quiescent phase. Thelinear polarization of the continuum showed a temporal variation; thedifference between the maximum and the minimum is 0.3%-0.6% in Stokes qand is larger with shorter wavelengths. Applying scattering models tothis variation, we found the variation in the continuum may becorrelated with the orbital motion of the binary and estimated theorbital inclination angle i c = 73° ± 14° andthe orientation angle ?c = 80° ± 5°. Wealso confirmed that the intrinsic linear polarization of the Raman line?683 varies with the orbital phase; from this modulation, theorbital elements were derived as i r = 41° ±8° and ?r = 82° ± 2°. Theinclination derived from the continuum has a large error, and the valueis larger by twice the error than the inclination angle value derivedfrom the Raman line. The derived orientation, in contrast, is comparablewith that derived from the Raman line. The possible inconsistency in theinclination may be due to the simplicity of our adopted model, or it maybe caused by a bias effect due to the low quality of the observedcontinuum polarization data. An accurate estimation of the inclinationfrom the continuum polarization could settle the question, but thatestimation requires more frequent observations that cover at least morethan a few orbital cycles during the quiescent phase when theobservations are not interrupted by the activity of the hot component.
| The Infrared Telescope Facility (IRTF) Spectral Library: Cool Stars
We present a 0.8-5 μm spectral library of 210 cool stars observed ata resolving power of R ≡ λ/Δλ ~ 2000 with themedium-resolution infrared spectrograph, SpeX, at the 3.0 m NASAInfrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. The stars havewell-established MK spectral classifications and are mostly restrictedto near-solar metallicities. The sample not only contains the F, G, K,and M spectral types with luminosity classes between I and V, but alsoincludes some AGB, carbon, and S stars. In contrast to some otherspectral libraries, the continuum shape of the spectra is measured andpreserved in the data reduction process. The spectra are absolutely fluxcalibrated using the Two Micron All Sky Survey photometry. Potentialuses of the library include studying the physics of cool stars,classifying and studying embedded young clusters and optically obscuredregions of the Galaxy, evolutionary population synthesis to studyunresolved stellar populations in optically obscured regions of galaxiesand synthetic photometry. The library is available in digital form fromthe IRTF Web site.
| REM near-IR and optical photometric monitoring of pre-main sequence stars in Orion. Rotation periods and starspot parameters
Aims. We aim at determining the rotational periods and the starspotproperties in very young low-mass stars belonging to the Ori OB1c starforming region, contributing to the study of the angular momentum andmagnetic activity evolution in these objects. Methods: Weperformed an intensive photometric monitoring of the PMS stars fallingin a field of about 10 arcmin× 10 arcmin in the vicinity of theOrion nebula cluster (ONC), also containing the BD eclipsing system2MASS J05352184-0546085. Photometric data were collected betweenNovember 2006 and January 2007 with the REM telescope in the {VRIJHK}'bands. The largest number of observations is in the I band (about 2700images) and in J and H bands (about 500 images in each filter). From theobserved rotational modulation, induced by the presence of surfaceinhomogeneities, we derived the rotation periods. The long time-baseline(nearly three months) allowed us to detect rotation periods, also forthe slowest rotators, with sufficient accuracy (? P/P<2%). Theanalysis of the spectral energy distributions and, for some stars, ofhigh-resolution spectra provided us with the main stellar parameters(luminosity, effective temperature, mass, age, and v sin i) which areessential for the discussion of our results. Moreover, the simultaneousobservations in six bands, spanning from optical to near-infraredwavelengths, enabled us to derive the starspot properties for these veryyoung low-mass stars. Results: In total, we were able to determinethe rotation periods for 29 stars, spanning from about 0.6 to 20 days.Thanks to the relatively long time-baseline of our photometry, wederived periods for 16 stars and improved previous determinations forthe other 13. We also report the serendipitous detection of two strongflares in two of these objects. In most cases, the light-curveamplitudes decrease progressively from the R to H band as expected forcool starspots, while in a few cases, they can only be modelled by thepresence of hot spots, presumably ascribable to magnetosphericaccretion. The application of our own spot model to the simultaneouslight curves in different bands allowed us to deduce the spot parametersand particularly to disentangle the spot temperature and size effects onthe observed light curves. Based on observations collected at the ESO REM telescope (La Silla,Chile) and at the MMT Observatory, a joint facility of the SmithsonianInstitution and the University of Arizona. Tables 6, 7 and the lightcurves of the variable stars are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/508/1313 Figures 21-24 areonly available in electronic form at http://www.aanda.org
| 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.
| 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.
| The Geneva-Copenhagen survey of the solar neighbourhood. III. Improved distances, ages, and kinematics
Context: Ages, chemical compositions, velocity vectors, and Galacticorbits for stars in the solar neighbourhood are fundamental test datafor models of Galactic evolution. The Geneva-Copenhagen Survey of theSolar Neighbourhood (Nordström et al. 2004; GCS), amagnitude-complete, kinematically unbiased sample of 16 682 nearby F andG dwarfs, is the largest available sample with complete data for starswith ages spanning that of the disk. Aims: We aim to improve theaccuracy of the GCS data by implementing the recent revision of theHipparcos parallaxes. Methods: The new parallaxes yield improvedastrometric distances for 12 506 stars in the GCS. We also use theparallaxes to verify the distance calibration for uvby? photometryby Holmberg et al. (2007, A&A, 475, 519; GCS II). We add newselection criteria to exclude evolved cool stars giving unreliableresults and derive distances for 3580 stars with large parallax errorsor not observed by Hipparcos. We also check the GCS II scales of T_effand [Fe/H] and find no need for change. Results: Introducing thenew distances, we recompute MV for 16 086 stars, and U, V, W,and Galactic orbital parameters for the 13 520 stars that also haveradial-velocity measurements. We also recompute stellar ages from thePadova stellar evolution models used in GCS I-II, using the new valuesof M_V, and compare them with ages from the Yale-Yonsei andVictoria-Regina models. Finally, we compare the observed age-velocityrelation in W with three simulated disk heating scenarios to show thepotential of the data. Conclusions: With these revisions, thebasic data for the GCS stars should now be as reliable as is possiblewith existing techniques. Further improvement must await consolidationof the T_eff scale from angular diameters and fluxes, and the Gaiatrigonometric parallaxes. We discuss the conditions for improvingcomputed stellar ages from new input data, and for distinguishingdifferent disk heating scenarios from data sets of the size andprecision of the GCS.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/501/941
| An evolved donor star in the long-period cataclysmic variable HS 0218+3229
Context: We present time-resolved spectroscopy and photometry of HS0218+3229, a new long-period cataclysmic variable discovered within theHamburg Quasar Survey. It is one of the few systems that allow adynamical measurement of the masses of the stellar components. Aims: We combine the analysis of time-resolved optical spectroscopyand R-band photometry with the aim of measuring the mass of the whitedwarf and the donor star and the orbital inclination. Methods:Cross-correlation of the spectra with K-type dwarf templates is used toderive the radial velocity curve of the donor star. An optimalsubtraction of the broadened templates is performed to measure therotational broadening and constrain the spectral type of the donor.Finally, an ellipsoidal model is fitted to the R-band light curve toobtain constraints upon the orbital inclination of the binary system. Results: The orbital period of HS 0218+3229 is found to be0.297229661 ± 0.000000001 d (7.13351186 ± 0.00000002 h),and the amplitude of the donor's radial velocity curve is K2= 162.4 ± 1.4 km~s-1. Modelling the ellipsoidal lightcurves gives an orbital inclination in the range i = 59° ±3°. A rotational broadening between 82.4 ± 1.2km~s-1 and 89.4 ± 1.3 km~s-1 is found whenassuming zero and continuum limb darkening, respectively. The secondarystar has most likely a spectral type K5 and contributes ~80-85% to theR-band light. Our analysis yields a mass ratio of 0.52 < q < 0.65,a white dwarf mass of 0.44 < M1 (M_ȯ) < 0.65, anda donor star mass of 0.23 < M2 (M_ȯ) < 0.44. Conclusions: We find that the donor star in HS 0218+3229 issignificantly undermassive for its spectral type. It is therefore verylikely that it has undergone nuclear evolution prior to the onset ofmass transfer.
| H? spectropolarimetry of RY Tauri and PX Vulpeculae
Aims: To detect line effects using spectropolarimetry in order to findevidence of rotating disks and their respective symmetry axes in T Tauristars. Methods: We used the IAGPOL imaging polarimeter along withthe Eucalyptus-IFU to obtain spectropolarimetric measurements of the TTauri stars RY Tau (two epochs) and PXVul (one epoch). Evidence of line effects showing a loop inthe Q-U diagram favors a compact rather than an extended source for theline photons in a rotating disk. In addition, the polarization positionangle (PA) obtained using the line effect can constrain the symmetryaxis of the disk. Results: RY Tau shows avariable H? double peak in 2004-2005 data. A polarization lineeffect is evident in the Q-U diagram for both epochs confirming aclockwise rotating disk. A single loop is evident in 2004 changing to alinear excursion plus a loop in 2005. Interestingly, the intrinsic PAcalculated using the line effect is consistent between our two epochs(~167°). An alternative intrinsic PA computed from the interstellarpolarization-corrected continuum and averaged between 2001-2005 yieldeda PA ~ 137°. This last value is closer to perpendicular to theobserved disk direction (~25°), as expected from single scatteringin an optically thin disk. For PX Vul, we detectedspectral variability in H? along with non-variable continuumpolarization when compared with previous data. The Q-U diagram shows awell-defined loop in H? associated with a counter-clockwiserotating disk. The symmetry axis inferred from the line effect has a PA~ 91°(with an ambiguity of 90°). Our results confirm previousevidence that the emission line in T Tauri stars has its origin in acompact source scattered off a rotating accretion disk.Based on observations obtained at the Observatório do Pico dosDias, LNA/MCT, Itajubá, Brazil.
| A catalogue of multiplicity among bright stellar systems
We consider the multiplicity of stellar systems with (combined)magnitude brighter than 6.00 in Hipparcos magnitudes. We identify 4559such bright systems (including the Sun), and the frequencies ofmultiplicities 1, 2,..., 7 are found to be 2718, 1437, 285, 86, 20, 11and 2. We discuss the uncertainties, which are substantial. We alsoconsider the distributions of periods of orbits and suborbits. We notethat for even more restricted set of 478 systems with VH<= 4.00, the proportions of higher multiples up to sextuple areprogressively larger (213, 179, 54, 19, 8, 5), suggesting substantialincompleteness in even the reasonably well studied larger sample.This sample can be seen as relatively thoroughly studied formultiplicity, and reasonably representative of stars more massive thanthe Sun. But the restriction to VH <= 6 means that oursample contains hardly any systems where all components are low-massmain-sequence stars (K or M).Data on multiplicity are important as a constraint on (i) the starformation problem, (ii) the problem of the evolution of the Galacticstellar population and (iii) the interaction of dynamics and evolutionthrough the effect of Kozai cycles. We discuss these topics briefly.
| The Jupiter Twin HD 154345b
We announce the discovery of a twin of Jupiter orbiting the slightlymetal-poor ([Fe/H]=-0.1) nearby (d=18 pc) G8 dwarf HD 154345. Thisplanet has a minimum mass of 0.95 MJup and a 9.2 year,circular orbit with radius 4.2 AU. There is currently little or noevidence for other planets in the system, but smaller or exteriorplanets cannot yet be ruled out. We also detect a ~ 9 year activitycycle in this star photometrically and in chromospheric emission. Werule out activity cycles as the source of the radial velocity variationsby comparison with other cycling late G dwarfs.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.
| The Last Measurements Made with the Wampler Scanner. II. Tests of Temperature Determinations from Flux Data for FGK IV-V Stars
New flux data are presented for 60 FGK stars on and near the mainsequence. All the measurements are made at wavelengths suitable fortemperature determinations. Measurements of all but seven program starsinclude four passbands in the Brackett continuum as well as 12 passbandsin the Paschen continuum. After the accuracy of the new data is shown tobe satisfactory, measurements of three "solar proxies" are compared totheoretical flux data from a solar model atmosphere calculated byKurucz. In this case, agreement with rms scatter at the 5-7 mmag levelis obtained. In addition, a possible zero-point problem with syntheticphotometry of two Kurucz solar atmospheres is detected. Usingmeasurements for 48 of the program stars and the Kurucz model grid, twosets of temperature data are then derived. The first set is based solelyon data for the Paschen continuum, and the second set is derived fromdata for both the Paschen and Brackett continua. When the resultingtemperatures are compared to temperature data on an angular-diameterscale, some data in the first data set yield offsets of about 40 K.However, no such offsets are found for the second data set. It isconcluded that the Kurucz models yield correct continua between 6056 and10404 Å for the program stars. In contrast, it appears that someof those models do not quite reproduce the line blocking in thatwavelength range. The latter deduction is supported by analyzing fluxdata for a star (61 Cyg A) with relatively large amounts of lineblocking. It is recommended that the measurements reported here be usedagain when further tests of theoretical flux curves for FGK stars areperformed.
| BF Eridani: a cataclysmic variable with a massive white dwarf and an evolved secondary
We present high- and medium-resolution spectroscopic observations of thecataclysmic variable BF Eridani (BF Eri) during its low and brightstates. The orbital period of this system was found to be 0.270881(3) d.The secondary star is clearly visible in the spectra through theabsorption lines of the neutral metals MgI, FeI and CaI. Its spectraltype was found to be K3+/-0.5. A radial velocity study of the secondaryyielded a semi-amplitude of K2 = 182.5 +/- 0.9 kms-1. The radial velocity semi-amplitude of the white dwarfwas found to be K1 = 74 +/- 3 km s-1 from themotion of the wings of the Hα and Hβ emission lines. Fromthese parameters, we have obtained that the secondary in BF Eri is anevolved star with a mass of 0.50-0.59 Msolar, whose size isabout 30 per cent larger than a zero-age main-sequence single star ofthe same mass. We also show that BF Eri contains a massive white dwarf(M1 >= 1.2 Msolar), which allows us to considerthe system as a Type Ia supernova progenitor. BF Eri also shows a highγ-velocity (γ = -94 km s-1) and substantialproper motion. With our estimation of the distance to the system (d ~700 +/- 200 pc), this corresponds to a space velocity of ~350 kms-1 with respect to the dynamical local standard of rest. Thecumulative effect of repeated nova eruptions with asymmetric envelopeejection might explain the high space velocity of the system. We analysethe outburst behaviour of BF Eri and question the current classificationof the system as a dwarf nova. We propose that BF Eri might be an oldnova exhibiting `stunted' outbursts.
| Starspot-Induced Radial Velocity Variability in LkCa 19
We describe a new radial velocity survey of T Tauri stars and presentthe first results. Our search is motivated by an interest in detectingmassive young planets, as well as investigating the origin of the browndwarf desert. As part of this survey, we discovered large-amplitude,periodic, radial velocity variations in the spectrum of the weak-line TTauri star LkCa 19. Using line bisector analysis and a new simulation ofthe effect of starspots on the photometric and radial velocityvariability of T Tauri stars, we show that our measured radialvelocities for LkCa 19 are fully consistent with variations caused bythe presence of large starspots on this rapidly rotating young star.These results illustrate the level of activity-induced radial velocitynoise associated with at least some very young stars. Thisactivity-induced noise will set lower limits on the mass of a companiondetectable around LkCa 19 and similarly active young stars.
| Automated classification of ELODIE stellar spectral library using probabilistic artificial neural networks
A Probabilistic Neural Network model has been used for automatedclassification of ELODIE stellar spectral library consisting of about2000 spectra into 158 known spectro-luminosity classes. The full spectrawith 561 flux bins and a PCA reduced set of 57, 26 and 16 componentshave been used for the training and test sessions. The results show aspectral type classification accuracy of 3.2 sub-spectral type andluminosity class accuracy of 2.7 for the full spectra and an accuracy of3.1 and 2.6 respectively with the PCA set. This technique will be usefulfor future upcoming large databases and their rapid classification.
| 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.
| 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 main sequence from F to K stars of the solar neighbourhood in SDSS colours
For an understanding of Galactic stellar populations in the SDSS filtersystem well defined stellar samples are needed. The nearby stars providea complete stellar sample representative for the thin disc population.We compare the filter transformations of different authors applied tothe main sequence stars from F to K dwarfs to SDSS filter system anddiscuss the properties of the main sequence. The location of the meanmain sequence in colour-magnitude diagrams is very sensitive tosystematic differences in the filter transformation. A comparison withfiducial sequences of star clusters observed in g', r', and i' show goodagreement. Theoretical isochrones from Padua and from Dartmouth havestill some problems, especially in the (r-i) colours.
| Speckle interferometry of nearby multiple stars. IV. Measurements in 2004 and new orbits
The results of speckle interferometric observations of 104 binary and 6triple stars performed at the BTA 6 m telescope in 2004 October arepresented. Nearby low-mass stars are mostly observed for the program,among which 59 there are new binaries recently discovered by theHipparcos astrometric satellite. Concurrently with thediffraction-limited position measurements we obtained 154 brightnessratio measurements of binary and multiple star components in differentbands of the visible spectrum. New, first-resolved binaries are thesymbiotic star CH Cyg with a weak companion at 0.043″ separationand the pair of red dwarfs, GJ 913 = HIP 118212. In addition, we derivedthe orbital parameters for two interferometric systems: the CN-giantpair HD 210211 = HIP 109281 (P = 10.7 yr) and the G2V-K2V G2V-K2V binaryGJ 9830 = HIP 116259 (P = 15.7 yr).
| 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.
| Patterns of activity in stars with cycles becoming established
Late-type stars with chromospheric and coronal activities exceedingthose of the Sun and other stars with well-defined cycles areconsidered. These rotate more rapidly than stars with well establishedcycles; for single stars, this appears to be due to their younger ages.The spots on such stars cover several per cent of the total area, whichis an order of magnitude higher than for the Sun at its activitymaximum. Our wavelet analysis of the chromospheric-emission variability,which has been observed since 1965 in the framework of the HK project,indicates that the period of the axial rotation of some of these startsvaries from year to year. This is most pronounced in two“Good” stars according to the classification of Baliunas etal., HD 149661 and HD 115404, and also in a star with a more complexvariability, HD 101501. No similar effect is exhibited by the“Excellent” cyclic-activity stars. Such variations in theperiod can be observed during epochs of appreciable rotationalmodulations of the chromospheric-emission fluxes, most likely,immediately after the maximum of a long-period wave (cycle?). This seemsto provide evidence for the existence of huge activity complexes in thechromospheres of these stars, whose longitudes remain virtually constantover several years; they drift from fairly high latitudes to the equatorat speeds close to the value typical of sunspots. The observed periodvariations are most likely due to differential rotation of the same signthat is known for the Sun. Our results provide independent confirmationof similar conclusions obtained by us previously using zonal models forhighly spotted stars. Other activity features of a selected star groupand the implications of the results for the theory of stellar and solardynamos are discussed.
| Patterns of Photometric and Chromospheric Variation among Sun-like Stars: A 20 Year Perspective
We examine patterns of variation of 32 primarily main-sequence Sun-likestars [selected at project onset as stars on or near the main sequenceand color index 0.42<=(B-V)<=1.4], extending our previous 7-12 yrtime series to 13-20 yr by combining Strömgren b, y photometry fromLowell Observatory with similar data from Fairborn Observatory. Parallelchromospheric Ca II H and K emission data from the Mount WilsonObservatory span the entire interval. The extended data strengthen therelationship between chromospheric and brightness variability at visiblewavelengths derived previously. We show that the full range ofphotometric variation has probably now been observed for a majority ofthe program stars. Twenty-seven stars are deemed variable according toan objective statistical criterion. On a year-to-year timescale, youngactive stars become fainter when their Ca II emission increases, whileolder less active stars such as the Sun become brighter when their Ca II emission increases. The Sun's total irradiance variation, scaled tothe b and y stellar filter photometry, still appears to be somewhatsmaller than stars in our limited sample with similar mean chromosphericactivity, but we now regard this discrepancy as probably due mainly toour limited stellar sample.
| Population Synthesis in the Blue. IV. Accurate Model Predictions for Lick Indices and UBV Colors in Single Stellar Populations
We present a new set of model predictions for 16 Lick absorption lineindices from Hδ through Fe5335 and UBV colors for single stellarpopulations with ages ranging between 1 and 15 Gyr, [Fe/H] ranging from-1.3 to +0.3, and variable abundance ratios. The models are based onaccurate stellar parameters for the Jones library stars and a new set offitting functions describing the behavior of line indices as a functionof effective temperature, surface gravity, and iron abundance. Theabundances of several key elements in the library stars have beenobtained from the literature in order to characterize the abundancepattern of the stellar library, thus allowing us to produce modelpredictions for any set of abundance ratios desired. We develop a methodto estimate mean ages and abundances of iron, carbon, nitrogen,magnesium, and calcium that explores the sensitivity of the variousindices modeled to those parameters. The models are compared to high-S/Ndata for Galactic clusters spanning the range of ages, metallicities,and abundance patterns of interest. Essentially all line indices arematched when the known cluster parameters are adopted as input.Comparing the models to high-quality data for galaxies in the nearbyuniverse, we reproduce previous results regarding the enhancement oflight elements and the spread in the mean luminosity-weighted ages ofearly-type galaxies. When the results from the analysis of blue and redindices are contrasted, we find good consistency in the [Fe/H] that isinferred from different Fe indices. Applying our method to estimate meanages and abundances from stacked SDSS spectra of early-type galaxiesbrighter than L*, we find mean luminosity-weighed ages of theorder of ~8 Gyr and iron abundances slightly below solar. Abundanceratios, [X/Fe], tend to be higher than solar and are positivelycorrelated with galaxy luminosity. Of all elements, nitrogen is the morestrongly correlated with galaxy luminosity, which seems to indicatesecondary nitrogen enrichment. If that interpretation is correct, thisresult may impose a lower limit of 50-200 Myr to the timescale of starformation in early-type galaxies. Unlike clusters, galaxies show asystematic effect whereby higher order, bluer, Balmer lines yieldyounger ages than Hβ. This age discrepancy is stronger for lowerluminosity galaxies. We examine four possible scenarios to explain thistrend. Contamination of the bluer indices by a metal-poor stellarpopulation with a blue horizontal branch cannot account for the data.Blue stragglers and abundance-ratio effects cannot be ruled out, as theycan potentially satisfy the data, even though this can only be achievedby resorting to extreme conditions, such as extremely high [O/Fe] orspecific blue-straggler frequencies. The most likely explanation is thepresence of small amounts of a young/intermediate-age stellar populationcomponent. We simulate this effect by producing two-component models andshow that they provide a reasonably good match to the data when the massfraction of the young component is typically a few percent. Ifconfirmed, this result implies star formation has been extended inearly-type galaxies, and more so in less massive galaxies, which seemsto lend support to the ``downsizing'' scenario. Moreover, it impliesthat stellar population synthesis models are capable of constraining notonly the mean ages of stellar populations in galaxies, but also theirage spread.
| On prospects for sounding activity cycles of Sun-like stars with acoustic modes
Data are now available on the Sun-like, p-mode oscillations of a growingnumber of late-type stars. With extension of these observations todedicated, long-term campaigns, it will soon become possible to probeacoustically magnetic activity, and stellar cycles, by observation ofsystematic shifts in the mode frequencies giving additional informationto the stellar dynamo theorists. Here, we use model computations of thedamping rates of stochastically excited radial p modes to makepredictions of the precision with which it will be possible to measurestellar-cycle frequency shifts of Sun-like stars along the lower mainsequence. We assume the first analyses will average shifts across themost prominent modes to reduce uncertainties. We also make somepredictions of the expected frequency shifts, based on existing stellarCaII H& K data. Our main conclusion is that the basic properties ofthe acoustic signatures of the cycles should be measurable to reasonablyhigh precision given only a few multimonth segments of data. It shouldalso be possible to make inference on the surface distribution of theactivity, through measurement of shifts of modes of different degree.Our computations also reveal an interesting feature in the predictedappearance of the acoustic mode spectra of stars cooler than about5400K: the modelled power spectral density of the modes shows twomaxima, at different frequencies. By computing average shifts of modesacross the two maxima, where the signal-to-noise ratio is highest, itshould be possible to get the first measures of the frequency dependenceof the p-mode shifts. This dependence provides information on themechanism responsible for driving these shifts.
| Fundamental Parameters and Elemental Abundances of 160 F-G-K Stars Based on OAO Spectrum Database
The properties of 160 F, G, and K disk dwarfs/subgiants (including 27planet-host stars) mostly within ?0.6 ? [Fe/H] ? +0.4,the Okayama Astrophysical Observatory spectrum collection of which hadbeen made open to the public recently, were extensively investigatedwith particular attention to determining (1) the mass and the age withthe help of theoretical stellar evolution calculations, (2) thekinematic parameters of orbital motions in the Galaxy, and (3) theabundances of 15 elements (Na, Mg, Al, Si, S, Ca, Sc, Ti, V, Cr, Mn, Co,Ni, Cu, and Zn) by analyzing the spectra in this database. The resultingcharacteristics are discussed in terms of several relevant topics ofinterest, such as a validity check for assuming LTE, the [X/Fe] vs.[Fe/H] diagram containing information on the chemical evolution of theGalactic disk, the age-metallicity-kinematics relation, and thedifference/similarity between stars with and without planets.
| Oxygen abundances in nearby stars. Clues to the formation and evolution of the Galactic disk
The abundances of iron and oxygen are homogeneously determined in asample of 523 nearby (d<150 pc) FGK disk and halo stars withmetallicities in the range -1.5<[Fe/H]<0.5. Iron abundances wereobtained from an LTE analysis of a large set of Fe I and Fe II lineswith reliable atomic data. Oxygen abundances were inferred from arestricted non-LTE analysis of the 777 nm O I triplet. We adopted theinfrared flux method temperature scale and surface gravities based onHipparcos trigonometric parallaxes. Within this framework, theionization balance of iron lines is not satisfied: the mean abundancesfrom the Fe I lines are systematically lower by 0.06 dex than those fromthe Fe II lines for dwarf stars of Teff>5500 K and[Fe/H]<0.0, and giant stars of all temperatures and metallicitiescovered by our sample. The discrepancy worsens for cooler and metal-richmain-sequence stars. We use the stellar kinematics to compute theprobabilities of our sample stars to be members of the thin disk, thickdisk, or halo of the Galaxy. We find that the majority of thekinematically-selected thick-disk stars show larger [O/Fe] ratioscompared to thin-disk stars while the rest show thin-disk abundances,which suggests that the latter are thin-disk members with unusual(hotter) kinematics. A close examination of this pattern for disk starswith ambiguous probabilities shows that an intermediate population withproperties between those of the thin and thick disks does not exist, atleast in the solar neighborhood. Excluding the stars with unusualkinematics, we find that thick-disk stars show slowly decreasing [O/Fe]ratios from about 0.5 to 0.4 in the -0.8<[Fe/H]<-0.3 range. Usinga simple model for the chemical evolution of the thick disk we show thatthis trend results directly from the metallicity dependence of the TypeII supernova yields. At [Fe/H]>-0.3, we find no obvious indication ofa sudden decrease (i.e., a "knee") in the [O/Fe] vs. [Fe/H] pattern ofthick-disk stars that would connect the thick and thin disk trends at ahigh metallicity. We conclude that Type Ia supernovae (SN Ia) did notcontribute significantly to the chemical enrichment of the thick disk.In the -0.8<[Fe/H]<+0.3 range, thin-disk stars show decreasing[O/Fe] ratios from about 0.4 to 0.0 that require a SN Ia contribution.The implications of these results for studies of the formation andevolution of the Galactic disk are discussed.Tables 4-6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/465/271 Partially based onobservations obtained with the Hobby-Eberly Telescope, which is a jointproject of the University of Texas at Austin, the Pennsylvania StateUniversity, Stanford University, Ludwig-Maximilians-UniversitätMünchen, and Georg-August-Universität Göttingen; and datafrom the UVES Paranal Observatory Project (ESO DDT Program ID266.D-5655).
| Chromospheric Activity in G and K Main-Sequence Stars, and What It Tells Us about Stellar Dynamos
For main-sequence G and K stars we study again the empirical relationsbetween the periods of the activity cycles, Pcyc, and therotational periods, prot. We use the high-quality dataselected by Brandenburg, Saar, and Turpin. As found by those authors``the Pcyc increase proportional to the prot,along two distinctly different sequences,'' the active ``A'' sequence,and the inactive ``'I'' sequence with cooler and more slowly rotatingstars. It is found here that along each sequence the number of rotationperiods per activity cycle is nearly the same, but the numbers aredifferent for the different sequences, indicating that probablydifferent kinds of dynamos are working for the stars on the differentsequences. The transition from one sequence to the other occurs at arotation period of 21 days. The rotation periods then increase abruptlyby about a factor of 2 for the cooler stars. We suggest that thisindicates abruptly increased deep mixing. Along the I sequence theoverall dependence of the Ca II emission line fluxes, F(Ca II), onrotation and Teff is consistent with F(CaII)~T4effp-4/3rot. For theA-sequence stars the dependence of F(Ca II) on rotation seems to bestronger than for the I-sequence stars.
| The Activity and Variability of the Sun and Sun-like Stars. I. Synoptic Ca II H and K Observations
Synoptic measurements of activity in Sun-like stars have been performedcontinuously since 1966, and the largest set comes from the Mount WilsonHK project, in the form of the well-known S index. We have beenmonitoring the activity and variability of the Sun and a large sample ofSun-like stars, in terms of S and absolute flux, since 1994 with theSolar-Stellar Spectrograph (SSS) at Lowell Observatory. Directlyinspired by the similar long-term program at Mount Wilson Observatory,the SSS incorporates both an HK spectrograph and an echelle for visibleand far-red observations. This is the first of three papers presentingthe results of some 20,000 observations of the Sun and Sun-like starswith the SSS. In this paper we describe our program, review thecalibration of solar and stellar fluxes to S and the chromosphericemission fraction R'HK, compare our derived stellar activitymeasures to those from other programs, and discuss the broadcharacteristics of the activity and variability in our target set, withparticular attention to good solar analogs and noncycling stars. Insubsequent papers we will discuss the echelle data and present detailedexaminations of stars of particular interest.
| Structure and Evolution of Nearby Stars with Planets. II. Physical Properties of ~1000 Cool Stars from the SPOCS Catalog
We derive detailed theoretical models for 1074 nearby stars from theSPOCS (Spectroscopic Properties of Cool Stars) Catalog. The Californiaand Carnegie Planet Search has obtained high-quality (R~=70,000-90,000,S/N~=300-500) echelle spectra of over 1000 nearby stars taken with theHamilton spectrograph at Lick Observatory, the HIRES spectrograph atKeck, and UCLES at the Anglo Australian Observatory. A uniform analysisof the high-resolution spectra has yielded precise stellar parameters(Teff, logg, vsini, [M/H], and individual elementalabundances for Fe, Ni, Si, Na, and Ti), enabling systematic erroranalyses and accurate theoretical stellar modeling. We have created alarge database of theoretical stellar evolution tracks using the YaleStellar Evolution Code (YREC) to match the observed parameters of theSPOCS stars. Our very dense grids of evolutionary tracks eliminate theneed for interpolation between stellar evolutionary tracks and allowprecise determinations of physical stellar parameters (mass, age,radius, size and mass of the convective zone, surface gravity, etc.).Combining our stellar models with the observed stellar atmosphericparameters and uncertainties, we compute the likelihood for each set ofstellar model parameters separated by uniform time steps along thestellar evolutionary tracks. The computed likelihoods are used for aBayesian analysis to derive posterior probability distribution functionsfor the physical stellar parameters of interest. We provide a catalog ofphysical parameters for 1074 stars that are based on a uniform set ofhigh-quality spectral observations, a uniform spectral reductionprocedure, and a uniform set of stellar evolutionary models. We explorethis catalog for various possible correlations between stellar andplanetary properties, which may help constrain the formation anddynamical histories of other planetary systems.
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| Takýmyýldýz: | Baliklar |
| Sað Açýklýk: | 01h42m29.76s |
| Yükselim: | +20°16'06.7" |
| Görünürdeki Parlaklýk: | 5.25 |
| özdevim Sað Açýklýk: | -300.6 |
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