Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Multiple shock waves in the atmosphere of the Cepheid X Sagittarii?
Context: .Shock waves in Cepheids have often been invoked, both fromobservational and theoretical points of view. However, classical shockwave signatures, such as emission or line doubling, have hardly beendetected. Aims: .In this paper, we suggest that our spectra of theclassical Cepheid X Sgr can be interpreted by means of the passage of 2shock waves per pulsation period. Methods: .We study new,high-resolution (120 000) spectra of X Sgr that show very complicatedpatterns within metallic lines. Results: .Spectra show up to 3components in most of the lines of the spectra during most of thepulsation cycle. These components seem to follow a pulsation motion. Inthe blue wing, the appearance of a new component is observed twice perpulsation period, which can be interpreted by 2 consecutive shock waves,one being apparently related to the classical κ-mechanism at workin these stars. The origin of the second shock is still unclear. Conclusions: .X Sgr is an exceptional Cepheid according to its observedmulti-components behaviour. Additional observations are requested inorder to establish the eventual effect of the binary orbit on thepulsation motion.
| Galactic Cepheids. II. Lithium
We report on the discovery of two lithium Cepheids in the Galaxy, basedon observations made with the echelle spectrograph of the Apache PointObservatory. We have used high-resolution, high signal-to-noise ratiospectra to determine abundances of chemical elements in 16 classicalCepheids. Only two of our program stars show a lithium line, RX Aur andYZ Aur (RX Aur has been also classified by us as a new nonradialpulsator). For the others, including the stars with [N/C]<0.2, Li isdepleted up to logN(Li)<1.0. Hence, it appears that mixing depletesLi before stars enter the instability strip. According to stellarmodels, the main mixing event takes place when Teff dropsbelow 4000 K, which is outside the red edge of the instability strip;i.e., after stars have crossed the instability strip for the first time.
| High-Mass Triple Systems: The Classical Cepheid Y Carinae
We have obtained a Hubble Space Telescope STIS ultraviolethigh-dispersion echelle-mode spectrum of the binary companion of thedouble-mode classical Cepheid Y Car. The velocity measured for the hotcompanion from this spectrum is very different from reasonablepredictions for binary motion, implying that the companion is itself ashort-period binary. The measured velocity changed by 7 kms-1 during the 4 days between two segments of theobservation, confirming this interpretation. We summarize ``binary''Cepheids that are in fact members of a triple system and find that atleast 44% are triples. The summary of information on Cepheids withorbits makes it likely that the fraction is underestimated.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Mean Angular Diameters and Angular Diameter Amplitudes of Bright Cepheids
We predict mean angular diameters and amplitudes of angular diametervariations for all monoperiodic PopulationI Cepheids brighter than=8.0 mag. The catalog is intended to aid selecting mostpromising Cepheid targets for future interferometric observations.
| The Nature of the Companion to the Eclipsing Overtone Cepheid MACHO 81.8997.87
The light curve of the Large Magellanic Cloud variable star MACHO81.8997.87 shows evidence for photometric variations due to both stellarpulsation, with a 2.035 day period, and eclipsing behavior, with an800.4 day period. The primary star of the system has been identified asa first-overtone Cepheid, but the nature of the secondary star has notbeen determined. Here we present multicolor BVI photometry of a primaryeclipse of the system and fit a model to the complete light curve toproduce an updated set of elements. These results are combined with TwoMicron All Sky Survey (2MASS) JHK photometry to give further insightinto the identity of the companion star. We find that the companion ismost consistent with a late K or an early M giant but also that thereare a number of problems with this interpretation. The prospects forfuture observations of this system are also discussed.
| New and Confirmed Triple Systems with Luminous Cool Primaries and Hot Companions
In the course of comparing parameters of evolved cool star plus hotmain-sequence star binaries with theoretical isochrones, somediscrepancies are found between implied stellar masses and thespectroscopic binary mass function or the measured angular separation.These are naturally explained if there is a third star in the system.Multiplicity is also required to explain some comparisons of ``cool plushot binary'' IUE and optical spectral energy distribution analysis withmeasured flux ratios, especially Tycho's two-color photometry ofseparate components. Out of a sample of 136 cool-plus-hot binary starsystems under study, measurements are now indicating several systemsconsidered double (HD 5373, 23089, 26673, 29094, 49126, 71129, 149379,179002, 187299), and probably a few others (including HD 136415), tohave at least three stellar components. Several other cases of suspectedtriple systems are confirmed. For comparison, there are eight knowntriples included in the project. In all, about 25% of the systemscontain three or more components within a few arcseconds. Estimatedseparations are provided, which may be of use when not known frominterferometry. In general, the triple systems have onepost-main-sequence component and two upper main-sequence components,usually revolving around each other. One new triple system, HD 149379,has as its middle component an F giant in the brief first crossing ofthe Hertzsprung gap.
| Sodium enrichment of the stellar atmospheres. II. Galactic Cepheids
The present paper is a continuation of our study of the sodium abundancein supergiant atmospheres (Andrievsky et al. 2002a). We present theresults on the NLTE abundance determination in Cepheids, and the derivedrelation between the sodium overabundance and their masses.
| The Spectra of Type II Cepheids. II. The Hα Line in Intermediate-Period Stars
We present 98 Hα profiles for 21 pulsating variable stars withperiods from 3 to 8 days. The strength, depth, and shape of Hαvary throughout the cycles of the stars in a way consistent with thetemperature changes. Otherwise, they are quite uniform among all thestars, with a single exception. In FM Del, Hα is weaker and has asmaller central depth than in the other stars. This and the wavelengthshifts of the core are attributed to incipient emission. Thedifferential velocity of Hα relative to the metal lines is lessthan 25 km s-1 for all the stars except QY Cyg, FM Del, andEF Tau. We suggest that this indicates that only these stars are type IICepheids despite the large distances of some of the others from theGalactic plane.Based in part on observations obtained with the Apache Point Observatory3.5 m telescope, which is owned and operated by the AstrophysicalResearch Consortium.
| Line profile variations in classical Cepheids. Evidence for non-radial pulsations?
We have investigated line profiles in a large sample of Cepheid spectra,and found four stars that show unusual (for Cepheids) line profilestructure (bumps or/and asymmetries). The profiles can be phasedependent but the behavior persists over many cycles. The asymmetriesare unlikely to be due to the spectroscopic binarity of these stars orthe specific velocity field in their atmospheres caused by shock waves.As a preliminary hypothesis, we suggest that the observed features onthe line profiles in the spectra of X Sgr, V1334 Cyg, EV Sct and BG Crucan be caused by the non-radial oscillations. It is possible that thesenon-radial oscillations are connected to resonances between the radialmodes (3fd2 , 7fd5 or 10fd0 ).Based on spectra collected at McDonald 2.1 m and Kitt Peak 4 m (USA),CTIO 4 m (Chile) and MSO 1.8 m (Australia).
| Using Cepheids to determine the galactic abundance gradient. I. The solar neighbourhood
A number of studies of abundance gradients in the galactic disk havebeen performed in recent years. The results obtained are ratherdisparate: from no detectable gradient to a rather significant slope ofabout -0.1 dex kpc-1. The present study concerns theabundance gradient based on the spectroscopic analysis of a sample ofclassical Cepheids. These stars enable one to obtain reliable abundancesof a variety of chemical elements. Additionally, they have welldetermined distances which allow an accurate determination of abundancedistributions in the galactic disc. Using 236 high resolution spectra of77 galactic Cepheids, the radial elemental distribution in the galacticdisc between galactocentric distances in the range 6-11 kpc has beeninvestigated. Gradients for 25 chemical elements (from carbon togadolinium) are derived. The following results were obtained in thisstudy. Almost all investigated elements show rather flat abundancedistributions in the middle part of galactic disc. Typical values foriron-group elements lie within an interval from ~-0.02 to ~-0.04 dexkpc-1 (in particular, for iron we obtainedd[Fe/H]/dRG =-0.029 dex kpc-1). Similar gradientswere also obtained for O, Mg, Al, Si, and Ca. For sulphur we have founda steeper gradient (-0.05 dex kpc-1). For elements from Zr toGd we obtained (within the error bars) a near to zero gradient value.This result is reported for the first time. Those elements whoseabundance is not expected to be altered during the early stellarevolution (e.g. the iron-group elements) show at the solargalactocentric distance [El/H] values which are essentially solar.Therefore, there is no apparent reason to consider our Sun as ametal-rich star. The gradient values obtained in the present studyindicate that the radial abundance distribution within 6-11 kpc is quitehomogeneous, and this result favors a galactic model including a barstructure which may induce radial flows in the disc, and thus may beresponsible for abundance homogenization. Based on spectra collected atMcDonald - USA, SAORAS - Russia, KPNO - USA, CTIO - Chile, MSO -Australia, OHP - France. Full Table 1 is only available in electronicform at http://www.edpsciences.org Table A1 (Appendix) is only, andTable 2 also, available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/32
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| A photometric and spectroscopic study of the brightest northern Cepheids - III. A high-resolution view of Cepheid atmospheres
We present new high-resolution spectroscopic observations(λ/Δλ~40000) of 18 bright northern Cepheids carriedout at the David Dunlap Observatory in 1997. The measurements mainlyextend those presented in Paper I of this series, adding three morestars (AW Per, SV Vul, T Mon). The spectra were obtained in theyellow-red spectral region in the interval of 5900 and 6660Å,including strong lines of sodium D and Hα. New radial velocitiesdetermined with the cross-correlation technique and the bisectortechnique are presented. The new data are compared with those recentlypublished by several groups. We found systematic differences between thespectroscopic and CORAVEL-type measurements as large as1-3kms-1 in certain phases. We performed Baade-Wesselinkanalysis for CK Cam discovered by the Hipparcos satellite. The resultingradius is 31+/-1Rsolar, which is in very good agreement withrecent period-radius relation by Gieren, Moffett & Barnes III. It isshown that the systematic velocity differences do not affect theBaade-Wesselink radius more than 1per cent for CK Cam. Observationalpieces of evidence of possible velocity gradient affecting theindividual line profiles are studied. The full-width at half minimum(FWHM) of the metallic lines, similarly to the velocity differences,shows a very characteristic phase dependence, illustrating the effect ofglobal compression in the atmosphere. The smallest line widths alwaysoccur around the maximal radius, while the largest FWHM is associatedwith velocity reversal before the minimal radius. Three first overtonepulsators do not follow the general trend: the largest FWHM in SU Casand SZ Tau occurs after the smallest radius, during the expansion, whilein V1334 Cyg there are only barely visible FWHM variations. Thepossibility of a bright yellow companion of V1334 Cyg is brieflydiscussed. The observed line-profile asymmetries exceed the valuespredicted with a simple projection effect by a factor of 2-3. This couldbe associated with the velocity gradient, which is also supported by thedifferences between individual line velocities of different excitationpotentials.
| V1334 Cygni: A Triple System Containing a Classical Cepheid
We have observed the small-amplitude Cepheid V1334 Cyg A (pulsationperiod 3.3 days) for nearly 30 years. From these radial velocity data wehave derived an orbit with a period of 5 years. From this orbit we havederived limits on the mass of the companion (V1334 Cyg C) of 3.1 to 4.4Msolar. The system is also a marginally resolved visualbinary with a much longer period that has been observed for nearly acentury. We have used an IUE high-resolution spectrum to conclude thatthe hottest star in the system (V1334 Cyg B), which dominates thespectrum in the ultraviolet, is the wide companion, since the velocityis very near the systemic velocity.
| Galactic Cepheids. Catalogue of light-curve parameters and distances
We report a new version of the catalogue of distances and light-curveparameters for Galactic classical Cepheids. The catalogue listsamplitudes, magnitudes at maximum light, and intensity means for 455stars in BVRI filters of the Johnson system and (RI)_C filters of theCron-Cousins system. The distances are based on our new multicolour setof PL relations and on our Cepheid-based solution for interstellarextinction law parameters and are referred to an LMC distance modulus of18.25. The catalogue is only available in electronic form at the CDS viaanonymous ftp (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Direct calibration of the Cepheid period-luminosity relation
After the first release of Hipparcos data, Feast & Catchpole gave anew value for the zero-point of the visual Cepheid period-luminosityrelation, based on trigonometric parallaxes. Because of the largeuncertainties on these parallaxes, the way in which individualmeasurements are weighted is of crucial importance. We thereforeconclude that the choice of the best weighting system can be aided by aMonte Carlo simulation. On the basis of such a simulation, it is shownthat (i) a cut-off in π or in σ_ππ introduces a strongbias; (ii) the zero-point is more stable when only the brightestCepheids are used; and (iii) the Feast & Catchpole weighting givesthe best zero-point and the lowest dispersion. After correction, theadopted visual period-luminosity relation is=-2.77logP-1.44+/-0.05. Moreover, we extend this study to thephotometric I band (Cousins) and obtain=-3.05logP-1.81+/-0.09.
| Toward an Orbit for the High-Luminosity Cepheid T Monocerotis
We have obtained new velocities of the long-period Cepheid T Mon fromthe ground and velocities of its hot companion with the Hubble SpaceTelescope (HST) and the International Ultraviolet Explorer (IUE).Although observations do not cover a full orbit, both the maximum andminimum orbital velocities have now been obtained. We present apreliminary orbit and discuss the uncertainties in the orbitalparameters. The velocities for the companion appear to be inconsistentwith binary orbital motion, and it is likely that the companion isitself a binary in a short-period orbit. The HST spectrum of thecompanion shows that it is a chemically peculiar star, probablymagnetic. Because it is coupled with the more massive Cepheid, it mustbe very close to the zero-age main sequence. The well-determined massfunction from the preliminary orbit implies that the inclination of thelong-period system is close to 90 deg.
| I- and JHK-band photometry of classical Cepheids in the HIPPARCOS catalog
By correlating the \cite[Fernie et al. (1995)]{F95} electronic databaseon Cepheids with the ``resolved variable catalog'' of the hipparcosmission and the simbad catalog one finds that there are 280 Cepheids inthe hipparcos catalog. By removing W Vir stars (Type ii Cepheids),double-mode Cepheids, Cepheids with an unreliable solution in thehipparcos catalog, and stars without photometry, it turns out that thereare 248 classical Cepheids left, of which 32 are classified asfirst-overtone pulsators. For these stars the literature was searchedfor I-band and near-infrared data. Intensity-mean I-band photometry onthe Cousins system is derived for 189 stars, and intensity-mean JHK dataon the Carter system is presented for 69 stars.
| A catalog of Cepheid radial velocities measured in 1995-1998 with the correlation spectrometer.
Not Available
| A photometric and spectroscopic study of the brightest northern Cepheids - II. Fundamental physical parameters
We present a new Cepheid reddening and effective temperature scale basedon the uvby photometry published in the first paper of this series.Using all available information about the companion stars in Cepheidswith bright blue secondaries, we remove their light from the observedlight and colour curves. The resulting corrections are as large as0.05-0.15 mag in several cases for different colour indices. A newphotometric approach based on the (b -y) versus (B-V) two-colour diagramis tested with three other previous calibrations taken from theliterature. Two uvby relations in earlier studies turn out to be themost reliable and consistent, and so they are used in deriving colourexcesses. We determine systematically higher reddenings for Cepheidswith a significant secondary light correction. The dereddened Stromgrencolours are calibrated in terms of T_eff and logg using the most recentsynthetic colour grids. Our temperature scale is very close to that ofKraft, which is supported by other recent temperature determinationsusing the infrared flux method or Geneva photometry. The photometricgravities fit some of the earlier theoretical and observational (mainlyspectroscopic) results very well.
| A photometric and spectroscopic study of the brightest northern Cepheids - I. Observations
We present simultaneous UBVuvby photometry for the 18 brightest northernCepheids carried out between 1995 and 1997. Additionally, two fainterstars have been observed in the Johnson system only. The wholephotometric data base contains about 3500 individual data points for 20stars. The accuracy has been carefully tested with different methods. Aserious systematic difference has been found between the present dataset and the Stromgren photometry available in the literature, which hasprobably been caused by the peculiar filter set used in the earlierstudy. As an extension to the photometry, we took high-resolutionoptical spectra at David Dunlap Observatory in the red spectral region(lambda/Deltalambda~ 40000, in the interval of 6200and6600Angstromsincluding Hα). The spectroscopic programme contained 12stars from the photometric programme, the newly discovered brightclassical Cepheid CK Cam and two double-mode Cepheids (TU Cas and COAur). New radial velocities obtained with the cross-correlationtechnique are presented. We found significant velocity differencesbetween two cross-correlated spectral regions (6188-6220and 6405-6435Angstroms) as large as 0.8-1.2 km s^-1, which show very characteristicphase dependence in certain Cepheids. Finally, recent period variationsare briefly discussed in terms of phase jumps and duplicity.
| Spectroscopic survey of field Type II Cepheids
A sample of relatively bright, short- and intermediate-period (P=1-10d)Type II Cepheids in the Galactic field have been observedspectroscopically with an intermediate-resolution(lambda/Deltalambda=11000) spectrograph. The wavelength region was6500-6700A, including the Hα line and some photospheric ironlines. The signal-to-noise ratio (S/N) was usually between 50 and 100,depending on weather conditions and the brightness of target stars.Radial velocities were determined by cross-correlating the Cepheidspectra with those of selected IAU velocity standard stars having F-Gspectral types. The internal error of the velocity determination processwas calculated to be about 1 km s^-1. Hα emission and strong linesplitting were observed in BL Her during the expansion phase, but nosimilar phenomenon was detected in any other stars in this programme,except for AU Peg which has an unusual Hα line showing a PCygni-like profile. The velocity curve agrees well with recent CORAVELmeasurements. The velocity gradients in Cepheid atmospheres are studiedusing the Hα minus metallic velocities. Similar data are collectedfrom the literature. It seems that having large velocity differences(v_Hα-v_metal>40 km s^-1) is a characteristic feature of thevery short-period (P<1.5d) and longer period (P>10d) Cepheids.Between these period regions the Cepheid atmospheres exhibit smallervelocity differences. Most of the Type II Cepheids observed in thepresent study fall into this latter category. There might be a tendencyfor classical Cepheids of intermediate period to have larger maximumvelocity differences.
| Ultraviolet and Optical Studies of Binaries with Luminous Cool Primaries and Hot Companions. V. The Entire IUE Sample
We have obtained or retrieved IUE spectra for over 100 middle- andlate-type giant and supergiant stars whose spectra indicate the presenceof a hot component earlier than type F2. The hot companions areclassified accurately by temperature class from their far-UV spectra.The interstellar extinction of each system and the relative luminositiesof the components are derived from analysis of the UV and opticalfluxes, using a grid of UV intrinsic colors for hot dwarfs. We find thatthere is fair agreement in general between current UV spectralclassification and ground-based hot component types, in spite of thedifficulties of assigning the latter. There are a few cases in which thecool component optical classifications disagree considerably with thetemperature classes inferred from our analysis of UV and opticalphotometry. The extinction parameter agrees moderately well with otherdeterminations of B-V color excess. Many systems are worthy of furtherstudy especially to establish their spectroscopic orbits. Further workis planned to estimate luminosities of the cool components from the dataherein; in many cases, these luminosities' accuracies should becomparable to or exceed those of the Hipparcos parallaxes.
| The shape and scale of Galactic rotation from Cepheid kinematics
A catalog of Cepheid variables is used to probe the kinematics of theGalactic disk. Radial velocities are measured for eight distant Cepheidstoward l = 300 deg; these new Cepheids provide a particularly goodconstraint on the distance to the Galactic center, R0. We model the diskwith both an axisymmetric rotation curve and one with a weak ellipticalcomponent, and find evidence for an ellipticity of 0.043 +/- 0.016 nearthe sun. Using these models, we derive R0 = 7.66 +/- 0.32 kpc andv(circ) = 237 +/- 12 km/s. The distance to the Galactic center agreeswell with recent determinations from the distribution of RR Lyraevariables and disfavors most models with large ellipticities at thesolar orbit.
| A search for evolutionary changes in the periods of low-amplitude Cepheids.
Not Available
| The Angular Momentum of Main Sequence Stars and Its Relation to Stellar Activity
Rotational velocities are reported for intermediate-mass main sequencestars it the field. The measurements are based on new, high S/N CCDspectra from the Coudé Feed Telescope of the Kitt Peak NationalObservatory. We analyze these rotation rates for a dependence on bothmass and age. We compare the average rotation speeds of the field starswith mean velocities for young stars in Orion, the Alpha Persei cluster,the Pleiades, and the Hyades. The average rotation speeds of stars moremassive than $\sim1.6$ \msun\experience little or no change during theevolutionary lifetimes of these stars on the zero age main sequence orwithin the main sequence band. Less massive stars in the range betwee n1.6\msun\ and 1.3\msun\ also show little decline in mean rotation ratewhile they are on the main sequence, and at most a factor of 2 decreasein velocity as they evolve off the main sequence. The {\it e}-foldingtime for the loss of angular momentum b y the latter group of stars isat least 1--2 billion years. This inferred characteristic time scale forspindown is far longer than the established rotational braking time forsolar-type stars with masses below $\sim1.3$ \msun. We conclude from acomparison of the trends in rotation with trends in chromospheric andcoronal activity that the overall decline in mean rotation speed alongthe main sequence, from $\sim2$ \msun\ down to $\sim1.3$ \msun, isimposed during the pre-main sequence phase of evolution, and that thispattern changes little thereafter while the star resides on the mainsequence. The magnetic activity implicated in the rotational spindown ofthe Sun and of similar stars during their main sequence lifetimes mus ttherefore play only a minor role in determining the rotation rates ofthe intermediate mass stars, either because a solar-like dynamo is weakor absent, or else the geometry of the magnetic field is appreciablyless effective in removing angular momentu m from these stars. (SECTION:Stars)
| Classification of Population II Stars in the Vilnius Photometric System. II. Results
The results of photometric classification of 848 true and suspectedPopulation II stars, some of which were found to belong to Population I,are presented. The stars were classified using a new calibrationdescribed in Paper I (Bartkevicius & Lazauskaite 1996). We combinethese results with our results from Paper I and discuss in greaterdetail the following groups of stars: UU Herculis-type stars and otherhigh-galactic-latitude supergiants, field red horizontal-branch stars,metal-deficient visual binaries, metal-deficient subgiants, stars fromthe Catalogue of Metal-deficient F--M Stars Classified Photometrically(MDPH; Bartkevicius 1993) and stars from one of the HIPPARCOS programs(Bartkevicius 1994a). It is confirmed that high galactic latitudesupergiants from the Bartaya (1979) catalog are giants or even dwarfs.Some stars, identified by Rose (1985) and Tautvaisiene (1996a) as fieldRHB stars, appear to be ordinary giants according to our classification.Some of the visual binaries studied can be considered as physical pairs.Quite a large fraction of stars from the MDPH catalog are found to havesolar metallicity. A number of new possible UU Herculis-type stars, RHBstars and metal-deficient subgiants are identified.
| The chemical composition of the s-Cepheids. III.
The hypothesis about s-Cepheids' first time crossing of the instabilitystrip is checked by spectroscopic testing. On the basis of CCD spectraanalysis, we have derived the abundances for seven s-Cepheids. Thefollowing results have been obtained: 1) V473 Lyr, IR Cep, UY Mon, BYCas, V636 Cas have solar iron abundance, while V526 Mon and V924 Cygshow moderate iron deficiency. 2) The absolute carbon deficiency(relatively to the solar (C/H) value) found for all program stars(excepting V636 Cas) and a nitrogen overabundance show that theses-Cepheids are not crossing the instability strip for the first time. 3)Similarly to previously investigated Cepheid V1162 Aql, V636 Cas alsodemonstrates rather high carbon content. The plausible explanation ofthis phenomenon is that this star is crossing the instability strip forthe first time. 4) Na and Al are overabundant for all program starswhere these elements were measured. Sodium overabundance takes placealso for two first- time crossing Cepheids. Possibly such anoverabundance arises during the main-sequence phase. 5) α-elementsshow practically solar ratios with respect to Fe. 6) For iron-groupelements the (M/Fe) values are close to the solar ones. 7) s-processelements are perhaps slightly enhanced in the program stars. The resultsof the present paper are discussed together with the results of therecent paper by Andrievsky et al. (1996A&A...305..551A) devoted tos-Cepheids.
| Derivation of the Galactic rotation curve using space velocities
We present rotation curves of the Galaxy based on the space-velocitiesof 197 OB stars and 144 classical cepheids, respectively, which rangeover a galactocentric distance interval of about 6 to 12kpc. Nosignificant differences between these rotation curves and rotationcurves based solely on radial velocities assuming circular rotation arefound. We derive an angular velocity of the LSR of{OMEGA}_0_=5.5+/-0.4mas/a (OB stars) and {OMEGA}_0_=5.4+/-0.5mas/a(cepheids), which is in agreement with the IAU 1985 value of{OMEGA}_0_=5.5mas/a. If we correct for probable rotations of the FK5system, the corresponding angular velocities are {OMEGA}_0_=6.0mas/a (OBstars) and {OMEGA}_0_=6.2mas/a (cepheids). These values agree betterwith the value of {OMEGA}_0_=6.4mas/a derived from the VLA measurementof the proper motion of SgrA^*^.
| The chemical composition of the s-Cepheids. II.
On the base of photographic, Reticon and CCD spectra analysis we havederived the abundances for 8 s-Cepheids and V1162 Aql (before classifiedas s-Cepheid). The following results have been obtained: 1) all Cepheids(excluding only EU Tau) have solar-like abundances of α- andiron-group elements, 2) Na is overabundant for all program stars, 3) thecarbon deficiency found for EU Tau, DT Cyg, V440 Per and nitrogenoverabundance (DT Cyg), show that these s-Cepheids are not crossing theinstability strip for first time, 4) s-process elements are slightlyenhanced in the program stars, 5) V1162 Aql does not show any changes inC and N abundances. The most plausible explanation is that this star isa normal Cepheid (Cδ), but it is firstly crossing the instabilitystrip toward the giant branch. This conclusion is also confirmed by itsposition on the evolutionary diagram.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Cigno |
| Ascensione retta: | 21h19m22.20s |
| Declinazione: | +38°14'15.0" |
| Magnitudine apparente: | 5.83 |
| Distanza: | 1075.269 parsec |
| Moto proprio RA: | 1.4 |
| Moto proprio Dec: | 1.1 |
| B-T magnitude: | 6.46 |
| V-T magnitude: | 5.938 |
Cataloghi e designazioni:
|
