תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
HD 98618: A Star Closely Resembling Our Sun
Despite the observational effort carried out in the last few decades, noperfect solar twin has been found to date. An important milestone wasachieved a decade ago by Porto de Mello & da Silva, who showed that18 Sco is almost a solar twin. In the present work, we use extremelyhigh resolution (R=105), high signal-to-noise ratio KeckHIRES spectra to carry out a differential analysis of 16 solar-twincandidates. We show that HD 98618 is the second-closest solar twin andthat the fundamental parameters of both HD 98618 and 18 Sco are verysimilar (within a few percent) to the host star of our solar system,including the likelihood of hosting a terrestrial planet within theirhabitable zones. We suggest that these stars should be given toppriority in exoplanet and SETI surveys.The data presented herein were obtained at the W. M. Keck Observatory,which is operated as a scientific partnership among the CaliforniaInstitute of Technology, the University of California, and the NationalAeronautics and Space Administration.
| Kinematics, ages and metallicities for F- and G-type stars in the solar neighbourhood
A new metallicity distribution and an age-metallicity relation arepresented for 437 nearby F and G turn-off and sub-giant stars selectedfrom radial velocity data of Nidever et al. Photometric metallicitiesare derived from uvby- Hβ photometry, and the stellar ages from theisochrones of Bergbusch & VandenBerg as transformed to uvbyphotometry using the methods of Clem et al.The X (stellar population) criterion of Schuster et al., which combinesboth kinematic and metallicity information, provides 22 thick-discstars. σW= 32 +/- 5 km s-1,= 154 +/- 6 km s-1 and<[M/H]>=-0.55 +/- 0.03 dex for these thick-disc stars, which is inagreement with values from previous studies of the thick disc.α-element abundances which are available for some of thesethick-disc stars show the typical α-element signatures of thethick disc, supporting the classification procedure based on the Xcriterion.Both the scatter in metallicity at a given age and the presence of old,metal-rich stars in the age-metallicity relation make it difficult todecide whether or not an age-metallicity relation exists for the olderthin-disc stars. For ages greater than 3 Gyr, our results agree with theother recent studies that there is almost no correlation between age andmetallicity, Δ([M/Fe])/Δ(age) =-0.01 +/- 0.005 dexGyr-1. For the 22 thick-disc stars there is a range in agesof 7-8 Gyr, but again almost no correlation between age and metallicity.For the subset of main-sequence stars with extra-solar planets, theage-metallicity relation is very similar to that of the total sample,very flat, the main difference being that these stars are mostlymetal-rich, [M/H]>~-0.2 dex. However, two of these stars have[M/H]~-0.6 dex and have been classified as thick-disc stars. As for thetotal sample, the range in ages for these stars with extra-solarplanetary systems is considerable with a nearly uniform distributionover 3 <~ age <~ 13 Gyr.
| Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs
We present a uniform catalog of stellar properties for 1040 nearby F, G,and K stars that have been observed by the Keck, Lick, and AAT planetsearch programs. Fitting observed echelle spectra with synthetic spectrayielded effective temperature, surface gravity, metallicity, projectedrotational velocity, and abundances of the elements Na, Si, Ti, Fe, andNi, for every star in the catalog. Combining V-band photometry andHipparcos parallaxes with a bolometric correction based on thespectroscopic results yielded stellar luminosity, radius, and mass.Interpolating Yonsei-Yale isochrones to the luminosity, effectivetemperature, metallicity, and α-element enhancement of each staryielded a theoretical mass, radius, gravity, and age range for moststars in the catalog. Automated tools provide uniform results and makeanalysis of such a large sample practical. Our analysis method differsfrom traditional abundance analyses in that we fit the observed spectrumdirectly, rather than trying to match equivalent widths, and wedetermine effective temperature and surface gravity from the spectrumitself, rather than adopting values based on measured photometry orparallax. As part of our analysis, we determined a new relationshipbetween macroturbulence and effective temperature on the main sequence.Detailed error analysis revealed small systematic offsets with respectto the Sun and spurious abundance trends as a function of effectivetemperature that would be inobvious in smaller samples. We attempted toremove these errors by applying empirical corrections, achieving aprecision per spectrum of 44 K in effective temperature, 0.03 dex inmetallicity, 0.06 dex in the logarithm of gravity, and 0.5 kms-1 in projected rotational velocity. Comparisons withprevious studies show only small discrepancies. Our spectroscopicallydetermined masses have a median fractional precision of 15%, but theyare systematically 10% higher than masses obtained by interpolatingisochrones. Our spectroscopic radii have a median fractional precisionof 3%. Our ages from isochrones have a precision that variesdramatically with location in the Hertzsprung-Russell diagram. We planto extend the catalog by applying our automated analysis technique toother large stellar samples.
| The Planet-Metallicity Correlation
We have recently carried out spectral synthesis modeling to determineTeff, logg, vsini, and [Fe/H] for 1040 FGK-type stars on theKeck, Lick, and Anglo-Australian Telescope planet search programs. Thisis the first time that a single, uniform spectroscopic analysis has beenmade for every star on a large Doppler planet search survey. We identifya subset of 850 stars that have Doppler observations sufficient todetect uniformly all planets with radial velocity semiamplitudes K>30m s-1 and orbital periods shorter than 4 yr. From this subsetof stars, we determine that fewer than 3% of stars with-0.5<[Fe/H]<0.0 have Doppler-detected planets. Above solarmetallicity, there is a smooth and rapid rise in the fraction of starswith planets. At [Fe/H]>+0.3 dex, 25% of observed stars have detectedgas giant planets. A power-law fit to these data relates the formationprobability for gas giant planets to the square of the number of metalatoms. High stellar metallicity also appears to be correlated with thepresence of multiple-planet systems and with the total detected planetmass. This data set was examined to better understand the origin of highmetallicity in stars with planets. None of the expected fossilsignatures of accretion are observed in stars with planets relative tothe general sample: (1) metallicity does not appear to increase as themass of the convective envelopes decreases, (2) subgiants with planetsdo not show dilution of metallicity, (3) no abundance variations for Na,Si, Ti, or Ni are found as a function of condensation temperature, and(4) no correlations between metallicity and orbital period oreccentricity could be identified. We conclude that stars with extrasolarplanets do not have an accretion signature that distinguishes them fromother stars; more likely, they are simply born in higher metallicitymolecular clouds.Based on observations obtained at Lick and Keck Observatories, operatedby the University of California, and the Anglo-Australian Observatories.
| Chromospheric Ca II Emission in Nearby F, G, K, and M Stars
We present chromospheric Ca II H and K activity measurements, rotationperiods, and ages for ~1200 F, G, K, and M type main-sequence stars from~18,000 archival spectra taken at Keck and Lick Observatories as a partof the California and Carnegie Planet Search Project. We have calibratedour chromospheric S-values against the Mount Wilson chromosphericactivity data. From these measurements we have calculated medianactivity levels and derived R'HK, stellar ages,and rotation periods from general parameterizations for 1228 stars,~1000 of which have no previously published S-values. We also presentprecise time series of activity measurements for these stars.Based on observations obtained at Lick Observatory, which is operated bythe University of California, and on observations obtained at the W. M.Keck Observatory, which is operated jointly by the University ofCalifornia and the California Institute of Technology. The KeckObservatory was made possible by the generous financial support of theW. M. Keck Foundation.
| The 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
| Radial Velocities for 889 Late-Type Stars
We report radial velocities for 844 FGKM-type main-sequence and subgiantstars and 45 K giants, most of which had either low-precision velocitymeasurements or none at all. These velocities differ from the standardstars of Udry et al. by 0.035 km s-1 (rms) for the 26 FGKstandard stars in common. The zero point of our velocities differs fromthat of Udry et al.: =+0.053km s-1. Thus, these new velocities agree with the best knownstandard stars both in precision and zero point, to well within 0.1 kms-1. Nonetheless, both these velocities and the standardssuffer from three sources of systematic error, namely, convectiveblueshift, gravitational redshift, and spectral type mismatch of thereference spectrum. These systematic errors are here forced to be zerofor G2 V stars by using the Sun as reference, with Vesta and day sky asproxies. But for spectral types departing from solar, the systematicerrors reach 0.3 km s-1 in the F and K stars and 0.4 kms-1 in M dwarfs. Multiple spectra were obtained for all 889stars during 4 years, and 782 of them exhibit velocity scatter less than0.1 km s-1. These stars may serve as radial velocitystandards if they remain constant in velocity. We found 11 newspectroscopic binaries and report orbital parameters for them. Based onobservations obtained at the W. M. Keck Observatory, which is operatedjointly by the University of California and the California Institute ofTechnology, and on observations obtained at the Lick Observatory, whichis operated by the University of California.
| Further images of α Persei G dwarfs
We present two images of intermediate and low axial inclination G dwarfs(AP 149 and AP 193) in the young open cluster α Persei, andcompare these with previous images of intermediate and high axialinclination objects in this cluster. All stars show starspots at highlatitudes, with one star exhibiting a strong polar spot. Althoughlow-latitude features are found on all stars to some degree, thedetection of spots on AP 193 is marginal. The apparent difference instarspot morphology from one object to the next is probably the resultof a stellar magnetic cycle, although the exact effect on the starspotdistribution throughout a cycle is unknown. Polar spots are found inmany Doppler images of rapidly rotating cool stars. In the past, theirexistence has been called into question, and it has been suggested thatthey could be the manifestations of NLTE (e.g. chromospheric filling inof line profiles) effects rather than real photospheric features. Weassume the polar spots to be real photospheric features, and concludethat the flat-bottomed nature of the profile shape can be attributed tophotospheric polar spots. The degree of truncation of the profiledepends not only on spot size and strength, but also on the effectiveforeshortening of the polar region, a function of axial inclination.Hα is in emission on AP 149 which shows a double peak at mostphases. The time-series of the profile shows an s-wave pattern as theposition of these peaks changes throughout the rotation cycle. Weattribute this to coronal clouds located above the stellar surface insynchronous orbit. A maximum-entropy tomogram is derived revealing fourdistinct emission regions located near and above the corotation radius.
| Lithium in the Coma Berenices open cluster
Lithium abundances, radial velocities, and rotational velocities arereported for 20 candidate F, G and K stars of the sparse Coma Berenicesopen cluster. All the stars are proper-motion selected, and our radialvelocities support the membership credentials of at least 12 of thecandidates. Combining our data with that in the literature, we have aclose-to-complete census of Li abundances for late-type stars withMV<5.8. These data show that the Li-depletion pattern inComa Ber is similar, but not identical, to that in the Hyades cluster,which has a similar age but higher metallicity. Several Coma Ber F starshave suffered significantly more Li depletion than their counterparts inthe Hyades. The G and early-K stars of Coma Ber have undergone less Lidepletion than those of the Hyades, but much more than that predicted bystandard evolutionary models featuring only convective mixing. Thisprovides strong evidence for additional mixing and Li depletionoperating in these stars during their first 400 Myr on the mainsequence, amounting to 0.3 dex at 6000 K and rising to 0.8-1.2 dex at5400 K. We find that 4 of the radial-velocity non-members are among asmall number of low-mass stars which were previously reported as part ofan extra-tidal moving group associated with the Coma Ber cluster. As aresult, we now find that the luminosity function of this moving group isindistinguishable from that of the central cluster. It is uncertainwhether there may be a significant number of stars with even lowermasses among the moving group.
| Radial Velocity Studies of Close Binary Stars. I.
Radial velocity data are presented for 10 W UMa-type systems-GZ And,V417 Aql, LS Del, EF Dra, V829 Her, FG Hya, AP Leo, UV Lyn, BB Peg, andAQ Psc-together with preliminary circular-orbit determinations ofspectroscopic elements, with the main goal of obtaining mean radialvelocities and mass ratios for these systems. This is the first part ofa series that will contain radial velocity data for northern hemisphere,short-period eclipsing binaries, accessible to medium-resolutionspectroscopic studies with 1.8 m-class telescopes.
| Radial velocities. Measurements of 2800 B2-F5 stars for HIPPARCOS
Radial velocities have been determined for a sample of 2930 B2-F5 stars,95% observed by the Hipparcos satellite in the north hemisphere and 80%without reliable radial velocity up to now. Observations were obtainedat the Observatoire de Haute Provence with a dispersion of 80Ä,mm(-1) with the aim of studying stellar and galactic dynamics.Radial velocities have been measured by correlation with templates ofthe same spectral class. The mean obtained precision is 3.0 km s(-1)with three observations. A new MK spectral classification is estimatedfor all stars. Based on observations made at the Haute ProvenceObservatory, France and on data from The Hipparcos Catalogue, ESA.Tables 4, 5 and 6 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr or viahttp://cdsweb.u-strasbg.fr/Abstract.htm
| The Stellar Kinematic Fields of NGC 3379
We have measured the stellar kinematic profiles of NGC 3379 along fourposition angles, using absorption lines in spectra obtained with theMultiple Mirror Telescope. We derive a far more detailed description ofthe kinematic fields through the main body of the galaxy than could beobtained from previous work. Our data extend 90" from the center, atessentially seeing-limited resolution out to 17". The derived meanvelocities and dispersions have total errors (internal and systematic)better than +/-10 km s^-1, and frequently better than 5 km s^-1, out to55". We find very weak (3 km s^-1) rotation on the minor axis interiorto 12" and no detectable rotation above 6 km s^-1 from 12" to 50" orabove 16 km s^-1 out to 90" (95% confidence limits). However, a Fourierreconstruction of the mean velocity field from all four sampled PAs doesindicate a ~5 deg twist of the kinematic major axis, in the directionopposite to the known isophotal twist. The h_3 and h_4 parameters arefound to be generally small over the entire observed region. Theazimuthally averaged dispersion profile joins smoothly at large radiiwith the velocity dispersions of planetary nebulae. Unexpectedly, wefind sharp bends in the major axis rotation curve, also visible (thoughless pronounced) on the diagonal position angles. The outermost bendclosely coincides in position with other sharp kinematic features: anabrupt flattening of the dispersion profile, and local peaks in h_3 andh_4. All of these features are in a photometrically interesting regionin which the surface brightness profile departs significantly from anr^1/4 law. Features such as these are not generally known in ellipticalgalaxies owing to a lack of data at comparable resolution. Very similarbehavior, however, is seen the kinematics of the edge-on S0 galaxy NGC3115. We discuss the suggestion that NGC 3379 could be a misclassifiedS0 galaxy; preliminary results from dynamical modeling indicate that itmay be a flattened, weakly triaxial system seen in an orientation thatmakes it appear round.
| High-speed optical spectroscopy of a cataclysmic variable wind - BZ Camelopardalis
BZ Camelopardalis is the first cataclysmic variable star with anaccretion disk wind evident in its optical spectrum. The wind was foundby Thorstensen, who discovered intermittent P Cygni profiles occurringsimultaneously in He I 5876 and H. We have since obtained spectra with0.4 A/pixel dispersion and 60 s time resolution. We find a much fasterand more rapidly variable wind than the radiatively accelerated ones ofOB stars, Wolf-Rayet stars, or luminous blue variables. Instead ofshowing blob ejection, the whole wind of BZ Cam appears to turn on andoff. We use this to measure the acceleration law of a CV wind for thefirst time. The velocity increases linearly with time, attaining blueedge velocities near about 3000 km/s, and absorption velocities nearabout 1700 km/s, in 6 to 8 minutes after starting near rest. We alsofind a subsequent linear deceleration to nearly rest in 30-40 minutes,perhaps an effect of dilution as the wind expands. No periodicity fromrotational outflow is obvious. This wind is erratic and incessantlyvariable, and perhaps bipolar and face-on, but not highly collimated.The P Cygni absorption events trace out sawtooth waves, occurring within30 to 40 white dwarf radii from the disk. This is the approximate sizeof the disk, as well as the disk/wind transition region recentlypostulated by Knigge and Drew (1997). We estimate a distance of 830 160pc and an orbital inclination i such that i is between about 12 deg and40 deg.
| Variable Radial Velocities Among Composite-Spectrum Binaries in the PG Catalog
Ferguson, Green, & Liebert (1984\markcite{FGL}; FGL) identified aset of stars from the PG catalog that are composite-spectrum binariesconsisting of a hot subdwarf star and a late-type (G-K) star. During twoseasons at Kitt Peak National Observatory (KPNO), we examined theFGL\markcite{FGL} sample for close binaries by searching for largeradial velocity changes among typically three or four differentobservations. Only one of the twelve stars sampled so far has shownsignificant radial velocity variations (greater than 3sigma ): DeltaV_max = 83+/- 11 km s(-1) for PG 1224+309. Two other stars showedmarginal velocity changes: PG 0825+428, with Delta V_max = 24+/- 16 kms(-1) (1.6sigma ), and PG 1210+429, with Delta V_max = 39+/- 29 km s(-1)(1.3sigma ). The other nine stars did not display variations larger than0.9sigma (Delta V_max ≲ 15 km s(-1) ). We have reclassified thecool components of these twelve FGL\markcite{FGL} stars and made newestimates of the fractional contributions of the cool components to thetotal flux in the wavelength interval 5320-6532 Angstroms, generallyfinding spectral types slightly earlier, and flux fractions less, thanfound by FGL\markcite{FGL}. Both of these effects lead to reviseddistances to the binaries, mostly larger than those estimated byFGL\markcite{FGL}. An interesting and unexpected result from this workis the discovery of several rapidly rotating secondary stars. Thesestars may be related to the wind-accretion induced rapidly rotatingstars (WIRRing stars) discussed by Jeffries & Stevens(1996)\markcite{js96}. At least two systems, PG 1309-078 and PG1502-103, have large positive mean radial velocities, possiblyindicating halo orbits. We also tabulate mean apparent velocities forthe hot subdwarfs, measured from the He I D_3 line (based on an adoptedwavelength of 5875.697 Angstroms) which may be useful in assessing thegravitational redshifts of these stars. Four of the hot subdwarfs didnot exhibit a measurable D_3 line.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| The Mass-To-Light Ratios of the Draco and Ursa Minor Dwarf Spheroidal Galaxies.I. Radial Velocities from Multifiber Spectroscopy
We have measured 206 radial velocities for 94 probable members in theUrsa Minor dwarf spheroidal galaxy and 167 velocities for 91 probablemembers in the Draco dwarf spheroidal galaxy. These velocities wereobtained using the KPNO 4 m telescope with the Hydra multifiberpositioner and bench spectrograph in 1992,1993, and 1994. Bothsimulations and repeated measurements show that the median uncertaintyin these velocities is 4.0 km/s. Our velocities are compared with theother major datasets for Draco and Ursa Minor. Aside from a smallzero-point difference, there is excellent agreement between the Hydravelocities and the Olszewski et al. MMT echelle velocities. Comparisonwith the Hargreaves et al. Ursa Minor velocities reveals inconsistencieswhich we ascribe to underestimated errors in the Hargreaves et al. data,After correcting for zero-point differences, we combine the three majorsets of velocities. Our spectra recovered all of the known Carbon (C)stars in Draco and Ursa Minor and revealed one new C star in Draco andtwo in Ursa Minor. In addition, one star whose velocity is consistentwith membership in Ursa Minor has a deep and wide Hβ absorptionline in its spectrum. An initial discussion of the kinematics of the twogalaxies is presented. The systemic velocities of Draco and Ursa Minorare -293.3 +/- 1.0 and -247.4 +/- 1.0 km s^-1^, respectively. Ursa Minorshows apparent rotation about a position angle of 75^deg^ with anamplitude of about 3 km s^-1^; the morphological major axis is 53^deg^+/-5^deg^. No such rotation is found in Draco. We find a velocitydispersion of 10.4 +/- 0.9 km s^-1^ for Ursa Minor including all starsand 8.8 +/- 0.8 km s^-1^ excluding the star with the most extremevelocity. In Draco we find 10.7 +/- 0.9 km s^-1^ including all stars and8.5 +/- 0.7 km s^-1^ excluding the three stars with the most extremevelocities. These dispersions are consistent with the dispersionsmeasured in the other two studies and confirm that Draco and Ursa Minorhave the largest mass-to-light ratios of any of the dwarf spheroidals.
| Nine Seasons of Velocity Measurements in the Draco and Ursa Minor Dwarf Spheroidal Galaxies with the MMT Echelle
We have used the Multiple Mirror Telescope echelle spectrograph tomeasure 112 velocities of 42 stars in the Draco and Ursa Minor dwarfspheroidal galaxies and three velocities of three foreground starsbetween 1982 April and 1990 September. We used 11 A resolution spectraobtained with the MX multifiber spectrograph at the Steward 90" to findadditional giant candidates; 5 UMi and 13 Draco stars were then observedat the MMT and added to the original sample of velocity members. Inaddition, the MX spectra were used to eliminate 74 stars in thedirection of UMI and 59 stars in Draco as likely foreground dwarfs. Wedetected 7 velocity variables, defined as those stars whose probabilityof exceeding the measured X^2^ by chance is less than 1.5%. Three ofthese stars are Carbon (C) stars (UMi K and VA 335 and Draco C1); twohave emission lines (Draco CI and UMi M). We show that the C star DracoC4, with a proper motion membership probability of 7%, has a velocityconsistent with membership. It is not surprising that these C (mostlikely CH) stars are binaries because McClure has shown that mostGalactic CH stars are in binary systems. Of the remaining 35 stars, only4 are velocity variables, with measured velocity extrema of 29.1 kms^-1^ (UMi M), 7.2 km s^-1^ (Draco XI-2), 9.0 km s^-1^ (Draco 24), and8.3 km s^-1^ (Draco 473). The velocity dispersions are 10.1 +/- 1.7 kms^-1^ for UMi, and 9.9 +/- 1.4 km s^-1^ for Draco. These dispersionschange to 10.5 +/- 2.0 for UMi, and 8.2 +?- 1.3 for Draco if weeliminate the velocity variables. Our dispersion for UMi differs fromthat of Hargreaves et al. [MNRAS, 271,693 (1994b)] by 1.3σ of thecombined errors. These velocities are combined with the one-componentKing models of Pryor & Kormendy [AJ, 100,127 (1990)] to give M/L =73 for UMi, and 77 for Draco.
| Internal kinematics of the Leo II dwarf spherodial galaxy
We have obtained radial velocities of 31 red giants in the Leo II dwarfspheroidal (dSph) galaxy using the HIRES echelle spectrograph of theKeck 10 m telescope. These stars were selected using CCD photometryobtained with the Palomar 1.5 m telescope. The radial velocity of errorswere determined from repeat measurements of Leo II giants and faintradial velocity standards; the average error for the Leo II giants is2.4 km/s. The systemic velocity of Leo II is 76.0 +/- 1.3 km/s, and thevelocity dispersion is 6.7 +/- 1.1 km/s. Adopting the standardassumptions (mass follows light, isotropy, and equilibrium) we derive aglobal V-band mass-to-light ratio (Msub tot)/Ltot = 11.1 +/-3.8, and a central value rho0/I0 = 11.6 +/- 3.2(both in solar units). These values are larger than the M/L ratiosmeasured for local stellar populations (star clusters or the solarneighborhood). Monte Carlo simulations show that a large population ofbinaries probably has not inflated a small intrinsic velocity dispersionto the observed value. A model-independent lower limit of the centralmass density of Leo II yeilds rho0,min = solar masspc-3, larger than the central luminous mass density in Leo IIfor (M/L)lum = 2.2. This demands an extensive dark halo inthe outer regions of the galaxy to account for the observed dispersion.We conclude that Leo II contains a significant dark matter component.The total mass of the dark halo is 0.9X177 solar mass,consistent with the masses inferred in all well-studied dSph galaxies.Because it is remote from the Galaxy, tide cannot plausibly affect thevelocity dispersion of Leo II. However, we cannot use these results torule out the possibility that tides may influence the kinematics ofother dSph galaxies found closer to the Milky Way.
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| The spotted contact binary SS ARIETIS - Spectroscopy and infrared photometry
The first infrared photometry for the W-UMa system SS Ari is presented.An analysis based on medium-resolution spectroscopy presented here showsthat SS Ari is a W-type system with a mass ratio of 0.33. It seemscertain that the asymmetry in the published light curves and thoseobtained for this study can be explained by the effect of spots on oneor possibly both components of the system. The precise location, sizeand temperature of these spots require the use of Doppler Imagingtechniques in conjunction with high-quality multiband photometry.
| A truly triple system DI Pegasi
Radial velocity measurements of DI Peg are derived based onspectroscopic observations and existing photometric observations whichhelp confirm the existence of a third body. A total of 30 spectra areobtained at 150 A, 240 A, and 100 A centered at 5050 A, and the phasesare computed from the time of deeper eclipse with the residuals invelocity computed from the adopted sine curves. Data reduction is basedon overclock-mean subtraction, field flattening, cosmic-ray deletion,and the extraction of 1D spectra from 2D components. The spectroscopicelements and mass ratio are given, and the radial velocity of the thirdbody is found to be +40.2 +/- 0.3 km/s suggesting that it is a physicalcompanion to DI Peg. A linear ephemeris is given for the eclipsemoments, and the photometric solutions for the close pair are solved.
| Radial-velocity observations and absolute dimensions of eclipsing binaries - SS Ari
Radial-velocity observations of the binary SS Ari were carried out inthe last quarter of 1990 and on the first clear night of 1991 at theDominion Astrophysical Observatory. A spectroscopic orbit was determinedon the assumption that both components of the binary are undistorted andrevolve in circular orbits. It is shown that SS Ari belongs to the Wsubclass of the W UMa systems and has properties that are typical ofthat subclass.
| Spectroscopic orbits of eclipsing binaries - WX CANCRI
A spectropic orbit is determined for WX Cnc. The following orbitalelements are obtained: V(0) = 9.8 km/s, K1 = 110.2 km/s, K2 = 149.0km/s, and T(0) = HJD 2446 480.0309. Absolute parameters are derived andthe spectroscopic mass ratio is determined to be 0.74.
| Velocities of stars in remote Galactic satellites and the mass of the Galaxy
Observations of the two most distant known dwarf spheroidal Galacticsatellites, Leo I and II, and of two remote globular clusters, Eridanusand Pal 14, are presented. It is demonstrated that etalon spectra can beused for accurate wavelength calibration across the entire observedspectral range. Measured velocities of three stars in Eridanus, twostars in Pal 14, six stars in Leo I, and five stars in Leo II are usedto derive that the heliocentric systemic velocities of these systems are- 21 + or - 4, 72 + or - 4, 285 + or - 3, and 70 + or - 4 km/s,respectively. The value of the velocity for Leo I is in significantdisagreement with previously published values. Timing arguments are usedto estimate that the mass of the Galaxy is at least 13 x 10 to the 11thsolar. This value is valid only if Leo I is gravitationally bound to theGalaxy, and arguments supporting this assumption are presented.
| Fermi paradox and alternative strategies for SETI programs - The anthropic principle and the search for close solar analogs
The anthropic principle, a new trend of modern cosmology, claims thatthe origin of life and the development of intelligent beings on theearth is the result of highly selective biological processes, strictlytuned in the fundamental physical characteristics of the universe. Thisprinciple could account for the failure of some programs of search forextraterrestrial intelligences (SETI) and suggests that the search forstrict solar analogs should be a primary target for SETI strategies. Inthis connection, 22 solar analogs are selected and their choice isdiscussed.
| Spectroscopy of V471 Tau. I - Review of basic properties
Spectroscopic observations of the eclipsing binary V471 Tau are reportedand analyzed. Data obtained mainly in the red band at Mt. WilsonObservatory and KPNO during the period 1975-1983 are compiled inextensive tables and graphs, and the radial velocity of the K dwarfcomponent is determined using a Griffin-mask technique. Resultsdiscussed include: (1) distance 44 + or - 6 pc, (consistent withmembership in the Hyades), (2) apparent period variation consistent witha third component, (3) emissionlike features affecting theradial-velocity determination, (4) transient features consistent withthe presence of flares, and (5) phase-coherent variation in H-alpha(attributed to the action of the white-dwarf Lyman continuum emission onthe K dwarf).
| Spectroscopic orbits of eclipsing binaries - AA UMa
High dispersion and time resolution spectra for the eclipsing binary AAUMa, obtained using an intensified Reticon, were measured bycross-correlation techniques. The spectra of the two components arefound to be very similar, and AA UMa is shown to belong to Subtype W ofW UMa binaries. The orbital elements of the system are determined andcompared with other W UMa binaries. It is found that the orbital periodis 0.48171 days and that the estimated mean mass center velocity for thesystem is -34.8 km/s. The mass ratio between the components isdetermined to be 0.54 and the magnitude difference in the wavelengthobserved is estimated to be 0.36.
| Photometry of the comet Halley : solar analogs selected along the path (November 1985 - Mat 1986)
Not Available
| MKJ and MSS classification of solar-type stars within 100 parsecs of the sun - Preliminary results
Not Available
| A calibration of the gnkmfu system for dwarf stars
Calibrations to R-I and Fe/H abundance have been established for Gdwarfs in the gnkmfu narrow band system. The accuracy obtained for R-Iand Fe/H is + or - 0.02 and + or - 0.15, respectively. Also, a measureof internal consistency between the narrow band indices, Res (k), hasbeen derived. A catalogue of Res (k), R-I and Fe/H, derived from thephotoelectric indices, is given for 253 G dwarfs.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | שור |
| התרוממות ימנית: | 05h07m55.77s |
| סירוב: | +26°19'40.7" |
| גודל גלוי: | 7.587 |
| מרחק: | 35.236 פארסק |
| תנועה נכונה: | -67.8 |
| תנועה נכונה: | -66.5 |
| B-T magnitude: | 8.427 |
| V-T magnitude: | 7.657 |
קטלוגים וכינוים:
|
