내용
사진
사진 업로드
DSS Images Other Images
관련 글
Observational results for northern and southern (candidate) gamma Doradus stars .
We report on observational results obtained for 78 objects originallyclassified as bona-fide or candidate gamma Doradus stars. For thesouthern objects, we gathered echelle spectra with the CORALIEspectrograph attached to the Euler telescope in 1998-2003 and/orJohnson-Cousins B,V,I_c observations with the MODULAR photometerattached to the 0.5-m SAAO telescope in 1999-2000. For the northernobjects, we obtained Geneva U,B,B_1,B_2,V,V_1,G observations with the P7photometer attached to the 1.2-m Mercator telescope in 2001-2004. Atleast 15 of our objects are binaries, of which 7 are new. For 6binaries, we determined the orbit for the first time. At least 17objects show profile variations and at least 12 objects aremultiperiodic photometric variables. Our results allow us to upgrade 11objects to bona-fide gamma Doradus stars and to downgrade 8 objects toconstant up to the current detection limits. Mode identification isstill ongoing, but so far, only ℓ = 1 and 2 modes have beenidentified.
| 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.
| A spectroscopic study of southern (candidate) γ Doradus stars. I. Time series analysis
We present the results of a spectroscopic study of 37 southern(candidate) γ Doradus stars based on échelle spectra. Theobserved spectra were cross-correlated with the standard templatespectrum of an F0-type star for easier detection of binary and intrinsicvariations. We identified 15 objects as spectroscopic binaries,including 7 new ones, while another 3 objects are binary suspects. Atleast 12 objects show composite spectra. We could determine the orbitalparameters for 9 binaries, of which 4 turned out to be ellipsoidalvariables. For 6 binaries, we estimated the expected time-base of theorbital variations. Clear profile variations were observed for 17objects, pointing towards stellar pulsation. For 8 of them, we haveevidence that the main spectroscopic and photometric periods coincide.Our results, in combination with prior knowledge from the literature,lead to the classification of 10 objects as new bona fide γDoradus stars, 1 object as a new bona fide δ Scuti star, and 8objects as constant stars. Finally, we determined the projectedrotational velocity by two independent methods. The resulting v sin ivalues range from 3 to 135 {km s-1}. For the bona fideγ Doradus stars, the majority has v sin i below 60 {kms-1}.
| Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| 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.
| 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.
| 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.
| The Structure of the Local Interstellar Medium. III. Temperature and Turbulence
We present 50 individual measurements of the gas temperature andturbulent velocity in the local interstellar medium (LISM) within 100pc. By comparing the absorption line widths of many ions with differentatomic masses, we can satisfactorily discriminate between the twodominant broadening mechanisms, thermal broadening and macroscopicnonthermal, or turbulent, broadening. We find that the successful use ofthis technique requires a measurement of a light ion, such as D I, andan ion at least as heavy as Mg II. However, observations of more linesprovide an important consistency check and can also improve theprecision and accuracy of the measurement. Temperature and turbulentvelocity measurements are vital to understanding the physical propertiesof the gas in our local environment and can provide insight into thethree-dimensional morphological structure of the LISM. The weighted meangas temperature in the LISM warm clouds is 6680 K and the dispersionabout the mean is 1490 K. The weighted mean turbulent velocity is 2.24km s-1 and the dispersion about the mean is 1.03 kms-1. The ratio of the mean thermal pressure to the meanturbulent pressure is PT/Pξ~26. Turbulentpressure in LISM clouds cannot explain the difference in the apparentpressure imbalance between warm LISM clouds and the surrounding hot gasof the Local Bubble. Pressure equilibrium among the warm clouds may bethe source of a moderately negative correlation between temperature andturbulent velocity in these clouds. However, significant variations intemperature and turbulent velocity are observed. The turbulent motionsin the warm partially ionized clouds of the LISM are definitelysubsonic, and the weighted mean turbulent Mach number for clouds in theLISM is 0.19 with a dispersion of 0.11. These measurements provideimportant constraints on models of the evolution and origin of warmpartially ionized clouds in our local environment.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS AR-09525.01A. Theseobservations are associated with program 9525.
| The Structure of the Local Interstellar Medium. II. Observations of D I, C II, N I, O I, Al II, and Si II toward Stars within 100 Parsecs
Moderate- and high-resolution measurements(λ/Δλ>~40,000) of interstellar resonance lines ofD I, C II, N I, O I, Al II, and Si II (hereafter called light ions) arepresented for all available observed targets located within 100 pc thatalso have high-resolution observations of interstellar Fe II or Mg II(heavy ions) lines. All spectra were obtained with the Goddard HighResolution Spectrograph or the Space Telescope Imaging Spectrographinstrument aboard the Hubble Space Telescope. Currently, there are 41sight lines to targets within 100 pc with observations that include aheavy ion at high resolution and at least one light ion at moderate orhigh resolution. We present new measurements of light ions along 33 ofthese sight lines and collect from the literature results for theremaining sight lines that have already been analyzed. For all of thenew observations we provide measurements of the central velocity,Doppler width parameter, and column density for each absorptioncomponent. We greatly increase the number of sight lines with usefullocal interstellar medium (LISM) absorption-line measurements of lightions by using knowledge of the kinematic structure along a line of sightobtained from high-resolution observations of intrinsically narrowabsorption lines, such as Fe II and Mg II. We successfully fit theabsorption lines with this technique, even with moderate-resolutionspectra. Because high-resolution observations of heavy ions are criticalfor understanding the kinematic structure of local absorbers along theline of sight, we include 18 new measurements of Fe II and Mg II in anAppendix. We present a statistical analysis of the LISM absorptionmeasurements, which provides an overview of some physicalcharacteristics of warm clouds in the LISM, including temperature andturbulent velocity. This complete collection and reduction of allavailable LISM absorption measurements provides an important databasefor studying the structure of nearby warm clouds, including ionization,abundances, and depletions. Subsequent papers will present models forthe morphology and physical properties of individual structures (clouds)in the LISM.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS AR-09525.01A. Theseobservations are associated with program 9525.
| S4N: A spectroscopic survey of stars in the solar neighborhood. The Nearest 15 pc
We report the results of a high-resolution spectroscopic survey of allthe stars more luminous than M_V = 6.5 mag within 14.5 pc from the Sun.The Hipparcos catalog's completeness limits guarantee that our survey iscomprehensive and free from some of the selection effects in othersamples of nearby stars. The resulting spectroscopic database, which wehave made publicly available, includes spectra for 118 stars obtainedwith a resolving power of R ≃ 50 000, continuous spectral coveragebetween 362-921 nm, and typical signal-to-noise ratios in therange 150-600. We derive stellar parameters and perform a preliminaryabundance and kinematic analysis of the F-G-K stars in the sample. Theinferred metallicity ([Fe/H]) distribution is centered at about -0.1dex, and shows a standard deviation of 0.2 dex. A comparison with largersamples of Hipparcos stars, some of which have been part of previousabundance studies, suggests that our limited sample is representative ofa larger volume of the local thin disk. We identify a number ofmetal-rich K-type stars which appear to be very old, confirming theclaims for the existence of such stars in the solar neighborhood. Withatmospheric effective temperatures and gravities derived independentlyof the spectra, we find that our classical LTE model-atmosphere analysisof metal-rich (and mainly K-type) stars provides discrepant abundancesfrom neutral and ionized lines of several metals. This ionizationimbalance could be a sign of departures from LTE or inhomogeneousstructure, which are ignored in the interpretation of the spectra.Alternatively, but seemingly unlikely, the mismatch could be explainedby systematic errors in the scale of effective temperatures. Based ontransitions of majority species, we discuss abundances of 16 chemicalelements. In agreement with earlier studies we find that the abundanceratios to iron of Si, Sc, Ti, Co, and Zn become smaller as the ironabundance increases until approaching the solar values, but the trendsreverse for higher iron abundances. At any given metallicity, stars witha low galactic rotational velocity tend to have high abundances of Mg,Si, Ca, Sc, Ti, Co, Zn, and Eu, but low abundances of Ba, Ce, and Nd.The Sun appears deficient by roughly 0.1 dex in O, Si, Ca, Sc, Ti, Y,Ce, Nd, and Eu, compared to its immediate neighbors with similar ironabundances.Based on observations made with the 2.7 m telescope at the McDonaldObservatory of the University of Texas at Austin (Texas), and the 1.52 mtelescope at the European Southern Observatory (La Silla, Chile) underthe agreement with the CNPq/Observatorio Nacional (Brazil).Tables 3-5 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/420/183
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| NEXXUS: A comprehensive ROSAT survey of coronal X-ray emission among nearby solar-like stars
We present a final summary of all ROSAT X-ray observations of nearbystars. All available ROSAT observations with the ROSAT PSPC, HRI and WFChave been matched with the CNS4 catalog of nearby stars and the resultsgathered in the Nearby X-ray and XUV-emitting Stars data base, availablevia www from the Home Page of the Hamburger Sternwarte at the URLhttp://www.hs.uni-hamburg.de/DE/For/Gal/Xgroup/nexxus. Newvolume-limited samples of F/G-stars (dlim = 14 pc), K-stars(dlim = 12 pc), and M-stars (dlim = 6 pc) areconstructed within which detection rates of more than 90% are obtained;only one star (GJ 1002) remains undetected in a pointed follow-upobservation. F/G-stars, K-stars and M-stars have indistinguishablesurface X-ray flux distributions, and the lower envelope of the observeddistribution at FX ≈ 104 erg/cm2/sis the X-ray flux level observed in solar coronal holes. Large amplitudevariations in X-ray flux are uncommon for solar-like stars, but maybemore common for stars near the bottom of the main sequence; a largeamplitude flare is reported for the M star LHS 288. Long term X-raylight curves are presented for α Cen A/B and Gl 86, showingvariations on time scales of weeks and demonstrating that α Cen Bis a flare star.Tables 1-3 are also 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/417/651
| Another single hydrogen-rich Wolf-Rayet star in the SMC?
A 12th Wolf-Rayet star in the SMC has recently been discovered by\cite{Massey-etal-2003}. To determine its spectral type and preliminarybinary status, we obtained 3 high signal-to-noise spectra separated intime at the ESO-NTT. Compared to other WR stars in the SMC, SMC-WR12appears to belong to the subgroup of faint, single and hydrogen-rich WNstars. We discuss the evolutionary status of WR12 and show thatrelatively low mass rotating progenitors can better account for theproperties of single hydrogen-rich WN stars in the SMC.Based on observations made at the European Southern Observatory, LaSilla, Chile.
| Wolf-Rayet binaries in the Magellanic Clouds and implications for massive-star evolution - I. Small Magellanic Cloud
We have carried out an intensive spectroscopic campaign to search forbinaries via periodic radial velocity (RV) variations among all thenitrogen-rich WN Wolf-Rayet (WR) stars in the Small Magellanic Cloud(SMC), and all WNE stars in the Large Magellanic Cloud (LMC). We presentin this first paper the results for the SMC. Along with the results ofBartzakos et al. on the only carbon/oxygen-rich WR star (AB8, WO4+O4),the whole WR population of the SMC (11 stars) has now been investigatedintensively for periodic RV variability. We have also retrievedtime-dependent photometric data in the public domain from the OGLE andMACHO projects, and X-ray data from the ROSAT and Chandra archives, toprovide additional constraints on the binary character. Contrary totheoretical expectations that predict a virtually 100 per cent binaryfrequency in the SMC, we find a normal (~40 per cent) WR binaryfrequency in this galaxy. We also find the clear presence of hydrogen inthe winds of the single WR stars in the SMC, even for the stars with anearly spectral subtype. We discuss the possible reasons and implicationsof this for stellar evolution of massive stars in such a low-metallicityenvironment, e.g. the influence of rotation versus the necessity of avery high initial mass of the progenitors for single stars, and thepossible past occurrence of Roche lobe overflow for binaries.
| Space Telescope Imaging Spectrograph Survey of Far-Ultraviolet Coronal Forbidden Lines in Late-Type Stars
We describe an extensive search with the Space Telescope ImagingSpectrograph (STIS) for ultraviolet coronal (T>106 K)forbidden lines in a sample of 29 F-M dwarfs, giants, and supergiants.Measuring coronal lines in the 1150-1700 Å band with STIS hasimportant advantages of superior velocity resolution and an absolutewavelength calibration compared with using the Chandra or XMM-Newtongrating spectrometers to observe permitted transitions of the same ionstages in the kilovolt X-ray region. Fe XII λλ1242, 1349(T~2×106 K) and Fe XXI λ1354 (107 K)are well known from solar studies and have been reported in previousstellar work. A search for other coronal forbidden lines in the1200-1600 Å region was largely negative. The few candidateidentifications (e.g., Ar XIII λ1330 and Ca XV λ1375) aretoo faint to be diagnostically useful. We add new dwarfs to the list ofFe XII detections, including the nearby solar twin α Cen A (G2 V).Clear detections of Fe XXI were obtained in dMe stars, active giants, ashort-period RS CVn binary, and possibly in active solar-type dwarfs. Wedeveloped a semiempirical method for removing the C I blend thatpartially affects the Fe XXI λ1354 profile. As discussed recentlyby Johnson et al., Capella (α Aur; G8 III+G1 III) displays clearFe XXI variability between Goddard High-Resolution Spectograph (GHRS)and STIS observations 4 yr apart, which is apparently due to asubstantial decline in the contribution from the G8 primary. We presentan alternative model of the GHRS and STIS era profiles using informationin the two sets of line shapes jointly, as well as knowledge of thebehavior of Fe XXI profiles of other late-G ``clump'' giants similar toCapella G8. The full survey sample also provides a context for theapparent variability: the Fe XXI flux of the G8 star in the GHRSspectrum is nearly identical (in LFeXXI/Lbol) toother clump giants of similar LX/Lbol, but it haddropped at least a factor of 6 in the STIS measurement. The He IIλ1640 Bα feature-which is thought to be responsive tocoronal irradiation-also showed significant changes between the GHRS andSTIS epochs, but the decrease in the G8 star was much smaller than FeXXI. The Fe XII flux displays a correlation with the ROSAT 0.2-2 keVX-ray flux that can be described by an α=0.5 power law. Fe XXIexhibits a steeper, perhaps linear (α=1), correlation with theROSAT flux down to an activity level ofLX/Lbol~10-5, below which detections ofthe coronal forbidden line are rare. There is no evidence of large,systematic Doppler shifts in either Fe XII λ1242 or Fe XXIλ1354. This suggests that the emissions arise dominantly inconfined structures, analogous to magnetic loops on the Sun, ratherthan, say, in a hot wind. The Fe XII and Fe XXI line widths generallyare close to thermal (FWHM~40-90 km s-1 atT~106.2-107.0 K), except for the Hertzsprung-gapgiants 31 Comae (G0 III) and Capella G1 and the K1 subgiant primary ofHR 1099, all of which show evidence for excess broadening in Fe XXI (FeXII is obscured in these objects by broad N V λ1242 features). Ifthe excess broadening is rotational, it implies that the hot coronae of``X-ray-deficient'' 31 Com and Capella G1 are highly extended, contraryto the compact structures suggested by recent density estimates in anumber of active coronal sources.
| Differential rotation in rapidly rotating F-stars
We obtained high quality spectra of 135 stars of spectral types F andlater and derived ``overall'' broadening functions in selectedwavelength regions utilizing a Least Squares Deconvolution (LSD)procedure. Precision values of the projected rotational velocity v \siniwere derived from the first zero of the Fourier transformed profiles andthe shapes of the profiles were analyzed for effects of differentialrotation. The broadening profiles of 70 stars rotating faster than v\sini = 45 km s-1 show no indications of multiplicity nor ofspottedness. In those profiles we used the ratio of the first two zerosof the Fourier transform q_2/q_1 to search for deviations from rigidrotation. In the vast majority the profiles were found to be consistentwith rigid rotation. Five stars were found to have flat profilesprobably due to cool polar caps, in three stars cuspy profiles werefound. Two out of those three cases may be due to extremely rapidrotation seen pole on, only in one case (v \sini = 52 km s-1)is solar-like differential rotation the most plausible explanation forthe observed profile. These results indicate that the strength ofdifferential rotation diminishes in stars rotating as rapidly as v \sini>~ 50 km s-1.Table A.1 is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/412/813Based on observations collected at the European Southern Observatory, LaSilla, 69.D-0015(B).
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is 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/403/247
| Statistical cataloging of archival data for luminosity class IV-V stars. II. The epoch 2001 [Fe/H] catalog
This paper describes the derivation of an updated statistical catalog ofmetallicities. The stars for which those metallicities apply are ofspectral types F, G, and K, and are on or near the main sequence. Theinput data for the catalog are values of [Fe/H] published before 2002February and derived from lines of weak and moderate strength. Theanalyses used to derive the data have been based on one-dimensional LTEmodel atmospheres. Initial adjustments which are applied to the datainclude corrections to a uniform temperature scale which is given in acompanion paper (see Taylor \cite{t02}). After correction, the data aresubjected to a statistical analysis. For each of 941 stars considered,the results of that analysis include a mean value of [Fe/H], an rmserror, an associated number of degrees of freedom, and one or moreidentification numbers for source papers. The catalog of these resultssupersedes an earlier version given by Taylor (\cite{t94b}).Catalog is only available in electronic form at the CDS via anonymousftp cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/731
| Statistical cataloging of archival data for luminosity class IV-V stars. I. The epoch 2001 temperature catalog
This paper is one of a pair in which temperatures and metallicitycatalogs for class IV-V stars are considered. The temperature catalogdescribed here is derived from a calibration based on stellar angulardiameters. If published calibrations of this kind are compared by usingcolor-index transformations, temperature-dependent differences among thecalibrations are commonly found. However, such differences are minimizedif attention is restricted to calibrations based on Johnson V-K. Acalibration of this sort from Di Benedetto (\cite{dib98}) is thereforetested and adopted. That calibration is then applied to spectroscopicand photometric data, with the latter predominating. Cousins R-Iphotometry receives special attention because of its high precision andlow metallicity sensitivity. Testing of temperatures derived from thecalibration suggests that their accuracy and precision are satisfactory,though further testing will be warranted as new results appear. Thesetemperatures appear in the catalog as values of theta equiv5040/T(effective). Most of these entries are accompanied by measured orderived values of Cousins R-I. Entries are given for 951 stars.Catalog is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/398/721
| New periodic variables from the Hipparcos epoch photometry
Two selection statistics are used to extract new candidate periodicvariables from the epoch photometry of the Hipparcos catalogue. Theprimary selection criterion is a signal-to-noise ratio. The dependenceof this statistic on the number of observations is calibrated usingabout 30000 randomly permuted Hipparcos data sets. A significance levelof 0.1 per cent is used to extract a first batch of candidate variables.The second criterion requires that the optimal frequency be unaffectedif the data are de-trended by low-order polynomials. We find 2675 newcandidate periodic variables, of which the majority (2082) are from theHipparcos`unsolved' variables. Potential problems with theinterpretation of the data (e.g. aliasing) are discussed.
| The Structure of the Local Interstellar Medium. I. High-Resolution Observations of Fe II, Mg II, and Ca II toward Stars within 100 Parsecs
High-resolution absorption measurements(λ/Δλ>~100,000) of the resonance lines of Fe II,Mg II, and Ca II are presented for all available observed targets within100 pc. The Fe II and Mg II spectra were obtained with the Goddard HighResolution Spectrograph (GHRS) and the Space Telescope ImagingSpectrograph (STIS) instruments aboard the Hubble Space Telescope (HST).Of the 63 observations of targets within 100 pc, we present newmeasurements for 24 lines of sight. We also compiled all publishedabsorption measurements based on Ca II spectra obtained by variousground-based instruments. For each observation we provide measurementsof the central velocity, Doppler parameter, and column density for eachabsorption component. These three ions provide the best opportunity tomeasure the component velocity structure. Because these are the heaviestions observed in absorption through the warm local interstellar medium(LISM), the narrow line widths minimize significant blending ofcomponents and allow for accurate measurements of the central velocity.We present a statistical analysis of the LISM absorption measurements,which provides an overview of some physical characteristics of warmclouds in the LISM, such as, temperature, turbulent velocity, ionizationdegree, and depletion. The complete collection and reduction of all LISMabsorption measurements provides an important database for studying thestructure of nearby warm clouds. Subsequent papers will present modelsfor the morphology and physical properties of individual structures(clouds) in the LISM.
| HIPPARCOS age-metallicity relation of the solar neighbourhood disc stars
We derive age-metallicity relations (AMRs) and orbital parameters forthe 1658 solar neighbourhood stars to which accurate distances aremeasured by the HIPPARCOS satellite. The sample stars comprise 1382 thindisc stars, 229 thick disc stars, and 47 halo stars according to theirorbital parameters. We find a considerable scatter for thin disc AMRalong the one-zone Galactic chemical evolution (GCE) model. Orbits andmetallicities of thin disc stars show now clear relation each other. Thescatter along the AMR exists even if the stars with the same orbits areselected. We examine simple extension of one-zone GCE models whichaccount for inhomogeneity in the effective yield and inhomogeneous starformation rate in the Galaxy. Both extensions of the one-zone GCE modelcannot account for the scatter in age - [Fe/H] - [Ca/Fe] relationsimultaneously. We conclude, therefore, that the scatter along the thindisc AMR is an essential feature in the formation and evolution of theGalaxy. The AMR for thick disc stars shows that the star formationterminated 8 Gyr ago in the thick disc. As already reported by Grattonet al. (\cite{Gratton_et.al.2000}) and Prochaska et al.(\cite{Prochaska_et.al.2000}), thick disc stars are more Ca-rich thanthin disc stars with the same [Fe/H]. We find that thick disc stars showa vertical abundance gradient. These three facts, the AMR, verticalgradient, and [Ca/Fe]-[Fe/H] relation, support monolithic collapseand/or accretion of satellite dwarf galaxies as likely thick discformation scenarios. Tables 2 and 3 are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http:/ /cdsweb.u-strasbg.fr/ cgi-bin/qcat?J/ A+A/394/927
| A 25 micron search for Vega-like disks around main-sequence stars with ISO
We present an ISO 25 mu m photometric survey of a sample of 81 nearbymain-sequence stars in order to determine the incidence of ``warm'' dustdisks. All stars were detected by ISO. We used an empirical relation toestimate the photospheric flux of the stars at 25 mu m. We find 5 stars(6%) with excess above the photospheric flux which we attribute to aVega-like disk. These stars show disk temperatures not warmer than 120K. Our study indicates that warm disks are relatively rare. Not a singlestar in our sample older than 400 Myr has a warm disk. We find an upperlimit of Mdisk = 2x 10-5 Moplus forthe mass of the disks which we did not detect. ISO is an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the United Kingdom) andwith the participation of ISAS and NASA.
| On X-ray variability in ROSAT-PSPC observations of F7-K2 stars
We have analyzed the X-ray variability of dF7-dK2 stars in the solarneighborhood detected with the pointed ROSAT-PSPC observations. Our database is the sample of all stars listed in the CNS3 catalog (Gliese &Jahreibeta 1991) having a B-V color between 0.5 and 0.9; it includes 70pointed observations of 40 distinct stars or multiple systems. We haveapplied the unbinned Kolmogorov-Smirnov test on all X-ray photon timeseries of our sample: only 10 observations relative to 8 distinct starsare variable at a confidence level greater than 99% and 4 of them belongto multiple systems. For the subsample of 9 stars observed both at thebeginning and at the end of the mission, we can study the variability ontime scale of years and compare amplitude variations at short and longtime scales. Our analysis suggests that, for these stars, the X-rayvariability is more likely on longer time scale. All the stars variableon long time scale, and not on short time scale, are relatively quietand similar to the Sun, suggesting that the variations may be due tocycles. The comparison of our results with those previously obtained fordM stars shows that the amplitude of variability of X-ray emission fromdF7-dK2 stars is smaller than that observed in dM stars.
| A revision of the solar neighbourhood metallicity distribution
We present a revised metallicity distribution of dwarfs in the solarneighbourhood. This distribution is centred on solar metallicity. Weshow that previous metallicity distributions, selected on the basis ofspectral type, are biased against stars with solar metallicity orhigher. A selection of G-dwarf stars is inherently biased againstmetal-rich stars and is not representative of the solar neighbourhoodmetallicity distribution. Using a sample selected on colour, we obtain adistribution where approximately half the stars in the solarneighbourhood have metallicities higher than [Fe/H]=0. The percentage ofmid-metal-poor stars ([Fe/H]<-0.5) is approximately 4 per cent, inagreement with present estimates of the thick disc. In order to have ametallicity distribution comparable to chemical evolution modelpredictions, we convert the star fraction to mass fraction, and showthat another bias against metal-rich stars affects dwarf metallicitydistributions, due to the colour (or spectral type) limits of thesamples. Reconsidering the corrections resulting from the increasingthickness of the stellar disc with age, we show that the simpleclosed-box model with no instantaneous recycling approximation gives areasonable fit to the observed distribution. Comparisons with theage-metallicity relation and abundance ratios suggest that the simpleclosed-box model may be a viable model of the chemical evolution of theGalaxy at solar radius.
| Stellar Encounters with the β Pictoris Planetesimal System
We use data from the Hipparcos catalog and the Barbier-Brossat &Figon catalog of stellar radial velocities to test the hypothesis thatthe β Pic planetesimal disk was disrupted by a close stellarencounter. We trace the space motions of 21,497 stars and discover 18that have passed within 5 pc of β Pic in the past 1 Myr. βPic's closest encounter is with the K2 III star HIP 27628 (~0.6 pc), butdynamically the most important encounter is with the F7 V star HIP 23693(~0.9 pc). We calculate the velocity and eccentricity changes induced bythe 18 perturbations and conclude that they are dynamically significantif planetesimals exist in a β Pic Oort cloud. We provide afirst-order estimate for the evolutionary state of a β Pic Oortcloud and conclude that the primary role of these stellar perturbationswould be to help build a comet cloud rather than destroy a preexistingstructure. The stellar sample is ~20% complete and motivates future workto identify less common close interactions that would significantlymodify the observed circumstellar disk. For future radial velocitystudy, we identify six stars in the Hipparcos catalog that may haveapproached β Pic to within 0.1 pc and therefore remain as candidatedisk perturbers.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Incidence and survival of remnant disks around main-sequence stars
We present photometric ISO 60 and 170 μm measurements, complementedby some IRAS data at 60 μm, of a sample of 84 nearby main-sequencestars of spectral class A, F, G and K in order to determine theincidence of dust disks around such main-sequence stars. Fifty starswere detected at 60 μm. 36 of these emit a flux expected from theirphotosphere while 14 emit significantly more. The excess emission weattribute to a circumstellar disk like the ones around Vega and betaPictoris. Thirty four stars were not detected at all; the expectedphotospheric flux, however, is so close to the detection limit that thestars cannot have an excess stronger than the photospheric flux densityat 60 μm. Of the stars younger than 400 Myr one in two has a disk;for the older stars this is true for only one in ten. We conclude thatmost stars arrive on the main sequence surrounded by a disk; this diskthen decays in about 400 Myr. Because (i) the dust particles disappearand must be replenished on a much shorter time scale and (ii) thecollision of planetesimals is a good source of new dust, we suggest thatthe rapid decay of the disks is caused by the destruction and escape ofplanetesimals. We suggest that the dissipation of the disk is related tothe heavy bombardment phase in our Solar System. Whether all starsarrive on the main sequence surrounded by a disk cannot be established:some very young stars do not have a disk. And not all stars destroytheir disk in a similar way: some stars as old as the Sun still havesignificant disks. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, The Netherlands and the UK) and with the participationof ISAS and NASA. Tables 2, 3 and 4 are also available in electronicform at the CDS via anonymous ftp cdsarc.u-strasbg.fr (130.79.128.5)} orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/365/545
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| The Disk of β Pictoris in the Light of Polarimetric Data
We model the linear polarization of the radiation of β Picscattered by dust particles in the circumstellar disk. The observedspatial distribution and the wavelength dependence of the polarizationtogether with the colors of the β Pic disk require that particlesin a wide size range be present in the disk, with the grains smallerthan a few microns in size being somewhat depleted but still ofimportance for the polarization and colors. The inferred sizedistribution is consistent with the production and loss mechanisms: thesources-presumably collisions and evaporation of largebodies-continuously produce dust with a power-law size distribution withthe exponent ~3.5 over a broad range of sizes, but the particles smallerthan a few microns are blown away by the radiation pressure, whichshortens the time they spend in the disk and decreases their numberdensities. Compact (or slightly porous) silicates are found to givebetter agreement with the observations, although other materials arestill not ruled out and a high fluffiness of the large particles ispossible. The observed asymmetry in the polarization of two wings can beexplained if more small grains (by 20%-30%) are present on the northeastside of the disk. We show that such an asymmetry in the sizedistributions in two wings might be caused by an influence of theinterstellar medium; a required amount of small grains could be producedby destructive collisions of interstellar grains with the circumstellardust particles.
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
INHABITED SKY
