Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
Eleven Years of Radio Monitoring of the type IIn Supernova SN 1995N
We present radio observations of the optically bright Type IIn supernovaSN 1995N. We observed the SN at radio wavelengths with the Very LargeArray for 11 years. We also observed it at low radio frequencies withthe Giant Metrewave Radio Telescope at various epochs within 6.5 –10 years since explosion. Although there are indications of an earlyoptically thick phase, most of the data are in the optically thin regimeso it is difficult to distinguish between synchrotron self absorptionand free-free absorption (FFA) mechanisms. However, the information fromother wavelengths indicates that FFA is the dominant absorption process.Model fits of radio emission with FFA give reasonable physicalparameters. Making use of X-ray and optical observations, we derive thephysical conditions of the shocked ejecta and the shocked circumstellarmatter.
| On the Enhancement of Mass Loss in Cepheids due to Radial Pulsation. II. The Effect of Metallicity
It has been observed that Cepheids in the Magellanic Clouds have lowermasses for the same luminosity than those in the Milky Way. The model,from Neilson & Lester, of pulsation-driven mass loss for Cepheids isapplied to theoretical models of Cepheids with a metallicity consistentwith the Milky Way and Large and Small Magellanic Clouds. The mass-lossmodel is analyzed using the metallicity correction of theperiod-luminosity relation to compare the ratio of mass loss of Cepheidswith lower metallicity with that of Cepheids with solar metallicity. Itis determined that mass loss may be larger for the lower metallicityCepheids, counterintuitive to radiative driving estimates. Also themass-loss rates of theoretical Cepheid models are found to be up to 5× 10–9 M sun yr–1 forGalactic Cepheids, 5 × 10–8 M sunyr–1 for Large Magellanic Cloud Cepheids, and 2 ×10–7 M sun yr–1 for SmallMagellanic Cloud Cepheids. It is argued that mass loss increases asmetallicity decreases for Cepheids with periods less than 20 days andthat mass loss decreases for longer periods. Assuming dust forms in thewind of a Cepheid at some distance, the infrared excess of the models iscomputed, finding the infrared brightness is approximately a magnitudelarger due to mass loss. The infrared magnitudes are compared torecently published period-luminosity relations as a test of ourpredictions.
| High-resolution spectroscopy for Cepheids distance determination. IV. Time series of Hα line profiles
Context: In recent years, infrared interferometry has revealed thepresence of faint dusty circumstellar envelopes (CSE) around Cepheids.However the size, shape, chemical nature, and the interaction of the CSEwith the star itself are still under investigation. The presence of aCSE might have an effect on the angular diameter estimates used in theinterferometric Baade-Wesselink and surface-brightness methods ofdetermining the distance of Cepheids. Aims: By studying Hαprofiles as a function of the period, we investigate the permanent massloss and the CSE around Cepheids. Our high spectral- and time-resolutiondata, combined with a very good S/N, will be useful in constrainingfuture hydrodynamical models of Cepheids atmosphere and their closeenvironment. Methods: We present HARPS (High Accuracy Radial velocityPlanetary Search project developed by the European SouthernObservatory.) high-resolution spectroscopy (R = 120 000) of eightgalactic Cepheids: R Tra, S Cru, Y Sgr, β Dor, zeta Gem, RZ Vel,ell Car, and RS Pup, providing a good period sampling (P = 3.39 d to P =41.52 d). The Hα line profiles are described for all stars using a2D (wavelength versus pulsation phase) representation. For each star, anaverage spectral line profile is derived, together with its first moment(γ-velocity) and its asymmetry (γ-asymmetry). Results:Short-period Cepheids show Hα line profiles following thepulsating envelope of the star, while long-period Cepheids show verycomplex line profiles and, in particular, large asymmetries. We find anew relationship between the period of Cepheids and theirγ-velocities and -asymmetries. These results may be related to thedynamical structure of the atmosphere and to a permanent mass loss ofCepheids. In particular, we confirm for ell Car a dominant absorptioncomponent whose velocity is constant and nearly of zero kms-1 in the stellar rest frame. This component is attributedto the presence of circumstellar envelope. Conclusions: To understandthese very subtle γ effects, fully consistent hydrodynamicalmodels are required, including pulsating and evolutionary theories,convective energy transport, adaptive numerical meshes, and a refinedcalculation of the radiative transfer.Based on observations made with ESO telescopes at the Silla ParanalObservatory under program IDs 072.D-0419 and 073.D-0136.
| High-resolution spectroscopy for Cepheids distance determination. III. A relation between γ-velocities and γ-asymmetries
Context: Galactic Cepheids in the vicinity of the Sun have a residualline-of-sight velocity, or γ-velocity, which shows a systematicblueshift of about 2 km s-1 compared to an axisymmetricrotation model of the Milky Way. This term is either related to thespace motion of the star and, consequently, to the kinematic structureof our Galaxy, or it is the result of the dynamical structure of theCepheids' atmosphere. Aims: We aim to show that these residualγ-velocities are an intrinsic property of Cepheids. Methods: Weobserved eight galactic Cepheids with the HARPS (High Accuracy Radialvelocity Planetary Search project developed by the European SouthernObservatory.) spectroscope, focusing specifically on 17 spectral lines.For each spectral line of each star, we computed the γ-velocity(resp. γ-asymmetry) as an average value of the interpolated radialvelocity (resp. line asymmetry) curve. Results: For each Cepheid in oursample, a linear relation is found between the γ-velocities of thevarious spectral lines and their corresponding γ-asymmetries,showing that residual γ-velocities stem from the intrinsicproperties of Cepheids. We also provide a physical reference to thestellar γ-velocity: it should be zero when the γ-asymmetryis zero. Following this definition, we provide very precise andphysically calibrated estimates of the γ-velocities for all starsof our sample [ in km s-1] : -11.3 ± 0.3 [R TrA], -3.5± 0.4 [S Cru], -1.5 ± 0.2 [Y Sgr], 9.8 ± 0.1 [β Dor] , 7.1 ± 0.1 [ zeta Gem] , 24.6 ± 0.4 [RZ Vel],4.4 ± 0.1 [ ell Car] , 25.7 ± 0.2 [RS Pup]. Finally, weinvestigated several physical explanations for these γ-asymmetrieslike velocity gradients or the relative motion of the line-formingregion compared to the corresponding mass elements. However, none ofthese hypotheses seems to be entirely satisfactory to explain theobservations. Conclusions: To understand this very subtleγ-asymmetry effect, further numerical studies are needed.Cepheids' atmosphere are strongly affected by pulsational dynamics,convective flows, nonlinear physics, and complex radiative transport.Hence, all of these effects have to be incorporated simultaneously andconsistently into the numerical models to reproduce the observed lineprofiles in detail.Based on observations made with ESO telescopes at the Silla ParanalObservatory under programme IDs 072.D-0419 and 073.D-0136.Table 2 is only available in electronic form at http://www.aanda.org
| On the Enhancement of Mass Loss in Cepheids Due to Radial Pulsation
An analytical derivation is presented for computing mass-loss rates ofCepheids by using the method of Castor, Abbott, and Klein modified toinclude a term for momentum input from pulsation and shocks generated inthe atmosphere. Using this derivation, mass-loss rates of Cepheids aredetermined as a function of stellar parameters. When applied to a set ofknown Cepheids, the calculated mass-loss rates range from10-10 to 10-7 Msolar yr-1,larger than if the winds were driven by radiation alone. Infraredexcesses based on the predicted mass-loss rates are compared toobservations from optical interferometry and IRAS, and predictions aremade for Spitzer observations. The mass-loss rates are consistent withthe observations, within the uncertainties of each. The rate of periodchange of Cepheids is discussed and shown to relate to mass loss, albeitthe dependence is very weak. There is also a correlation between thelarge mass-loss rates and the Cepheids with slowest absolute rate ofperiod change due to evolution through the instability strip. Theenhanced mass loss helps illuminate the issue of infrared excess and themass discrepancy found in Cepheids.
| Direct Detection of the Close Companion of Polaris with the Hubble Space Telescope
Polaris, the nearest and brightest classical Cepheid, is a single-linedspectroscopic binary with an orbital period of 30 yr. Using the HighResolution Channel of the Advanced Camera for Surveys on board theHubble Space Telescope (HST) at a wavelength of ~2255 Å, we havedirectly detected the faint companion at a separation of 0farcs17. Asecond HST observation 1.04 yr later confirms orbital motion in aretrograde direction. By combining our two measures with thespectroscopic orbit of Kamper and an analysis of the Hipparcos and FK5proper motions by Wielen et al., we find a mass for Polaris Aa of4.5+2.2 1.4 M sun—the firstpurely dynamical mass determined for any Cepheid. For the faintcompanion Polaris Ab we find a dynamical mass of 1.26+0.14 0.07 M sun, consistent with an inferred spectraltype of F6 V and with a flux difference of 5.4 mag observed at 2255Å. The magnitude difference at the V band is estimated to be 7.2mag. Continued HST observations will significantly reduce the masserrors, which are currently still too large to provide criticalconstraints on the roles of convective overshoot, mass loss, rotation,and opacities in the evolution of intermediate-mass stars. Ourastrometry, combined with two centuries of archival measurements, alsoconfirms that the well-known, more distant (18'') visual companion,Polaris B, has a nearly common proper motion with that of the Aa, Abpair. This is consistent with orbital motion in a long-period boundsystem. The ultraviolet brightness of Polaris B is in accordance withits known F3 V spectral type if it has the same distance as Polaris Aa,Ab.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.
| The influence of chemical composition on the properties of Cepheid stars. II. The iron content
Context: The Cepheid period-luminosity (PL) relation is unquestionablyone of the most powerful tools at our disposal for determining theextragalactic distance scale. While significant progress has been madein the past few years towards its understanding and characterizationboth on the observational and theoretical sides, the debate on theinfluence that chemical composition may have on the PL relation is stillunsettled. Aims: With the aim to assess the influence of the stellariron content on the PL relation in the V and K bands, we have relatedthe V-band and the K-band residuals from the standard PL relations ofFreedman et al. (2001, ApJ, 553, 47) and Persson et al. (2004, AJ, 128,2239), respectively, to [Fe/H]. Methods: We used direct measurements ofthe iron abundances of 68 Galactic and Magellanic Cepheids from FEROSand UVES high-resolution and high signal-to-noise spectra. Results: Wefind a mean iron abundance ([Fe/H]) about solar (σ = 0.10) for ourGalactic sample (32 stars), ~-0.33 dex (σ = 0.13) for the LargeMagellanic Cloud (LMC) sample (22 stars) and ~-0.75 dex (σ = 0.08)for the Small Magellanic Cloud (SMC) sample (14 stars). Our abundancemeasurements of the Magellanic Cepheids double the number of starsstudied up to now at high resolution. The metallicity affects the V-bandCepheid PL relation and metal-rich Cepheids appear to be systematicallyfainter than metal-poor ones. These findings depend neither on theadopted distance scale for Galactic Cepheids nor on the adopted LMCdistance modulus. Current data do not allow us to reach a firmconclusion concerning the metallicity dependence of the K-band PLrelation. The new Galactic distances indicate a small effect, whereasthe old ones support a marginal effect. Conclusions: Recent robustestimates of the LMC distance and current results indicate that theCepheid PL relation is not Universal.Based on observations madewith ESO Telescopes at Paranal and La Silla Observatories underproposal ID 66.D-0571.Full Table [see full textsee full textsee full textsee full text] isonly available in electronic form at http://www.aanda.org
| Baade-Wesselink distances and the effect of metallicity in classical cepheids
Context: The metallicity dependence of the Cepheid PL-relation is ofimportance in establishing the extra-galactic distance scale. Aims: Theaim of this paper is to investigate the metallicity dependence of thePL-relation in V and K based on a sample of 68 Galactic Cepheids withindividual Baade-Wesselink distances (some of the stars also have anHST-based parallax) and individually determined metallicities fromhigh-resolution spectroscopy. Methods: Literature values of the V-band,K-band and radial velocity data have been collected for a sample of 68classical cepheids that have their metallicity determined in theliterature from high-resolution spectroscopy. Based on a (V-K)surface-brightness relation and a projection factor derived in aprevious paper, distances have been derived from a Baade-Wesselinkanalysis. PL- and PLZ-relations in V and K are derived. Results: Theeffect of the adopted dependence of the projection factor on period isinvestigated. The change from a constant p-factor to one recentlysuggested in the literature with a mild dependence on log P results in aless steep slope by 0.1 unit, which is about the 1-sigma error bar inthe slope itself. The observed slope in the PL-relation in V in the LMCagrees with both hypotheses. In K the difference between the Galacticand LMC slope is larger and would favour a mild period dependence of thep-factor. The dependence on metallicity in V and K is found to bemarginal, and independent of the choice of p-factor on period. Thisresult is severely limited by the small range in metallicity covered bythe Galactic Cepheids.
| The nebulosity and distance of the Cepheid RS Puppis
Adopting a distance for RS Pup derived from a period-luminosity relationbased on Cepheid parallaxes, it is shown that the phase-lag observationsof the surrounding nebulosity by Kervella et al. are well fitted by amodel of an equatorial disc at an angle of to the plane of the sky. Theastrophysical implications of this are briefly mentioned.
| The Peculiar Cepheid FN Aql
We draw attention to the singularly peculiar photometric properties ofthe 9.5-day Cepheid FN Aql. An initial study of its color-color plotsshows an unusually wide trajectory in the (U B)-(B V) diagnosticplane. A closer investigation reveals that the photometric residualsaround fits to the individual mean light curves are highly correlated,decreasing in amplitude as a function of increasing wavelength. Thatcorrelation and trend suggest time-variable dust extinction; however,the grain properties must be different from the general interstellarmedium. FN Aql is an IRAS source and also appears in the proto-planetarynebulae candidate in the 1989 listing of Volk & Kwok. The source ofthe changing extinction is probably localized and intrinsic to theCepheid itself, possibly a wind. Further photometric studies of thispeculiar Cepheid are recommended.
| The long-period Galactic Cepheid RS Puppis. I. A geometric distance from its light echoes
Context: The bright southern Cepheid RS Pup is surrounded by acircumstellar nebula reflecting the light from the central star. Thepropagation of the light variations from the Cepheid inside the dustynebula creates spectacular light echoes that can be observed up to largedistances from the star itself. This phenomenon is currently unique inthis class of stars. Aims: For this relatively distant star, thetrigonometric parallax available from Hipparcos has a low accuracy. Acareful observation of the light echoes has the potential to provide avery accurate, geometric distance to RS Pup. Methods: We obtained aseries of CCD images of RS Pup with the NTT/EMMI instrument covering thevariation period of the star (P = 41.4 d), allowing us to track theprogression of the light wavefronts over the nebular featuressurrounding the star. We measured precisely the phase lag of thephotometric variation in several regions of the circumstellar nebula. Results: From our phase lag measurements, we derive a geometric distanceof 1992 ± 28 pc to RS Pup. This distance is affected by a totaluncertainty of 1.4%, and corresponds to a parallax of π = 0.502± 0.007 mas and a distance modulus of μ = 11.50 ± 0.03. Conclusions: The geometric distance we derive is by far the mostaccurate to a Cepheid, and among the most accurate to any star. RS Pupappears both as somewhat neglected and particularly promising toinvestigate the mass-loss history of Cepheids. Thanks to its highlyaccurate distance, it is also bound to become an important luminosityfiducial for the long period part of the period-luminosity diagram.
| Color Excesses of Classical Cepheids in uvby Photometry
In order to determine color excess in the uvby color system forfundamental-mode classical Cepheids, 29 Cepheids whose reliable distancevalues were compiled by Ngeow and Kanbur were selected as calibrationstars. Then intrinsic photometric indices were calculated using givendistances to derive a calibrated empirical relation between(b-y)0 and period, [c1], and [m1]through a linear fit. This relation was used to determine color excessesof E(b-y) for 116 Cepheids, and the period-color relation was derived.
| Metallicity estimates of Galactic cepheids based on Walraven photometry .
We present new empirical and theoretical calibrations of two photometricmetallicity indices based on Walraven photometry. The empiricalcalibration relies on a sample of 48 Cepheids for which iron abundancesbased on high resolution spectra are available in the literature. Theycover a broad range in metal abundance (-0.5 le [Fe/H]le +0.5) and theintrinsic accuracy of the Metallicity Index Color (MIC) relations isbetter than 0.2 dex. The theoretical calibration relies on a homogeneousset of scaled-solar evolutionary tracks for intermediate-mass stars andon pulsation predictions concerning the topology of the instabilitystrip. The metal content of the adopted evolutionary tracks ranges fromZ=0.001 to Z=0.03 and the intrinsic accuracy of the MIC relations isbetter than 0.1 dex.
| Classical Cepheid pulsation models. XI. Effects of convection and chemical composition on the period-luminosity and period-Wesenheit relations
In spite of the relevance of classical Cepheids as primary distanceindicators, a general consensus on the dependence of theperiod-luminosity (PL) relation on the Cepheid chemical composition hasnot yet been achieved. From the theoretical point of view, our previousinvestigations were able to reproduce some empirical tests for suitableassumptions on the helium-to-metal relative enrichment, but theseresults relied on specific assumptions concerning the mass-luminosityrelation and the efficiency of the convective transfer in the pulsatingenvelopes. In this paper, we investigate the effects of the assumedvalue of the mixing-length parameter l/Hp on the pulsationproperties and we release the assumption of a fixed mass-luminosityrelation. To this purpose, new nonlinear convective fundamentalpulsation models have been computed for various chemical compositions(Z=0.004, 0.008, 0.01 and 0.02) and adopting l/H_p=1.7-1.8, which islarger than that (1.5) used in our previous papers. From the extendedmodel set, synthetic PL relations in the various photometric bands arederived using the predicted instability strip together with recentevolutionary tracks. We show that as the l/Hp value increasesthe pulsation region gets narrower, mostly due to the blueward shift ofthe red edge for fundamental pulsation, with the effect becomingstronger at the higher metal contents (Z≥ 0.01). However, thecomparison of the new models with previously computed models shows thatthe l/Hp variation has no consequence on the predictedperiod-Wesenheit (PW) relations, which instead are influenced by thepulsator metal content. On this basis, we present a straightforward wayto infer the distance and metal content of variables with observed BVIor BVK magnitudes. As for the PL relations, we show that either thezero-point and the slope are very slightly modified by thel/Hp variation, at constant chemical composition. We alsoconfirm that: (1) moving from visual to longer wavelengths, thepredicted period-magnitude distribution for a given metal contentbecomes narrower and its slope becomes steeper; (2) decreasing the metalcontent, the PL relations become steeper and brighter, with the amountof this metallicity effect decreasing from optical to near-infraredbands. Overall, we show that our pulsation relations appear fullyconsistent with the observed properties of Galactic and Magellanic CloudCepheids, supporting the predicted steepening and brightening of the PLrelations when moving from metal-rich to metal-poor variables. Moreover,we show that the distances inferred by the predicted PW relations agreewith recently measured trigonometric parallaxes, whereas they suggest acorrection to the values based on the Infrared Surface Brightnesstechnique, as already found from an independent method. Finally, alsothe pulsation metal contents suggested by the predicted PW relationsappear in statistical agreement with spectroscopic [Fe/H] measurements.
| A new calibration of Galactic Cepheid period-luminosity relations from B to K bands, and a comparison to LMC relations
Context: The universality of the Cepheid period-luminosity (PL)relations has been under discussion since metallicity effects wereassumed to play a role in the value of the intercept and, more recently,of the slope of these relations. Aims: The goal of the present study isto calibrate the Galactic PL relations in various photometric bands(from B to K) and to compare the results to the well-established PLrelations in the LMC. Methods: We use a set of 59 calibrating stars,the distances of which are measured using five different distanceindicators: Hubble Space Telescope and revised Hipparcos parallaxes,infrared surface brightness and interferometric Baade-Wesselinkparallaxes, and classical Zero-Age-Main-Sequence-fitting parallaxes forCepheids belonging to open clusters or OB stars associations. A detaileddiscussion of absorption corrections and projection factor to be used isgiven. Results: We find no significant difference in the slopes of thePL relations between LMC and our Galaxy. Conclusions: We conclude thatthe Cepheid PL relations have universal slopes in all photometric bands,not depending on the galaxy under study (at least for LMC and MilkyWay). The possible zero-point variation with metal content is notdiscussed in the present work, but an upper limit of 18.50 for the LMCdistance modulus can be deduced from our data.Tables 2, 6 and 7 are only available in electronic form athttp://www.aanda.org
| Cepheid parallaxes and the Hubble constant
Revised Hipparcos parallaxes for classical Cepheids are analysedtogether with 10 Hubble Space Telescope (HST)-based parallaxes. In areddening-free V, I relation we find that the coefficient of logP is thesame within the uncertainties in our Galaxy as in the Large MagellanicCloud (LMC), contrary to some previous suggestions. Cepheids in theinner region of NGC4258 with near solar metallicities confirm thisresult. We obtain a zero-point for the reddening-free relation and applyit to the Cepheids in galaxies used by Sandage et al. to calibrate theabsolute magnitudes of Type Ia supernova (SNIa) and to derive the Hubbleconstant. We revise their result for H0 from 62 to 70 +/-5kms-1Mpc-1. The Freedman et al. value is revisedfrom 72 to 76 +/- 8kms-1Mpc-1. These results areinsensitive to Cepheid metallicity corrections. The Cepheids in theinner region of NGC4258 yield a modulus of 29.22 +/- 0.03 (int.)compared with a maser-based modulus of 29.29 +/- 0.15. Distance modulifor the LMC, uncorrected for any metallicity effects, are 18.52 +/- 0.03from a reddening-free relation in V, I; 18.47 +/- 0.03 from aperiod-luminosity relation at K; 18.45 +/- 0.04 from aperiod-luminosity-colour relation in J, K. Adopting a metallicitycorrection in V, I from Macri et al. leads to a true LMC modulus of18.39 +/- 0.05.
| High-resolution spectroscopy for Cepheids distance determination. II. A period-projection factor relation
Context: The projection factor is a key quantity for the interferometricBaade-Wesselink (hereafter IBW) and surface-brightness (hereafter SB)methods of determining the distance of Cepheids. Indeed, it allows aconsistent combination of angular and linear diameters of the star. Aims: We aim to determine consistent projection factors that include thedynamical structure of the Cepheids' atmosphere. Methods:Hydrodynamical models of δ Cep and ℓ Car have been used tovalidate a spectroscopic method of determining the projection factor.This method, based on the amplitude of the radial velocity curve, isapplied to eight stars observed with the HARPS spectrometer. Theprojection factor is divided into three sub-concepts : (1) a geometricaleffect, (2) the velocity gradient within the atmosphere, and (3) therelative motion of the "optical" pulsating photosphere compared to thecorresponding mass elements (hereafter fo-g). Both, (1) and(3) are deduced from geometrical and hydrodynamical models,respectively, while (2) is derived directly from observations. Results:The Fe I 4896.439 Å line is found to be the best one to use in thecontext of IBW and SB methods. A coherent and consistentperiod-projection factor relation (hereafter Pp relation) is derived forthis specific spectral line: p = [-0.064 ± 0.020] log P + [1.376± 0.023]. This procedure is then extended to derive dynamicprojection factors for any spectral line of any Cepheid. Conclusions:This Pp relation is an important tool for removing bias in thecalibration of the period-luminosity relation of Cepheids. Moreover, itreveals a new physical quantity fo-g to investigate in thenear future.Table 1 is only available in electronic form at http://www.aanda.org
| The reliability of Cepheid reddenings based on BVIC photometry
Externally determined values of E(B - V) (Espacered) for 40Galactic Cepheids are compared to reddenings determined using B - V andV - IC colour indices and the method of Dean, Warren &Cousins (EBVIC), updated to allow for metallicitycorrections. With three stars omitted on the grounds of uncertainty intheir space reddenings, we find thatThe two scales agree well in scale and zero-point, and there is nosignificant trend with period. Given the non-zero errors in the Cepheidspace reddenings, the estimated error in BVIC Cepheidreddenings is no more than 0.02.The above results are not significantly changed whether one corrects thereddenings for metallicity using older Bell models, or using more recentmodels by Sandage, Bell & Tripicco. Using the SBT models to correctthe reddenings of Cloud Cepheids for metallicity gives slightly smallerreddenings at a given metal deficiency, yielding `new' median reddeningsof 0.056 (Small Magellanic Cloud) and 0.076 (Large Magellanic Cloud) ifwe assume the same metal deficiencies as Caldwell and Coulson. Withmetal deficiencies of [M/H] = -0.7 and -0.25, the median reddenings are0.040 and 0.058.
| Detailed chemical composition of Galactic Cepheids. A determination of the Galactic abundance gradient in the 8-12 kpc region
Aims.The recent introduction of high-resolution/large spectral-rangespectrographs has provided the opportunity to investigate the chemicalcomposition of classical Cepheids in detail. This paper focusses on newabundance determinations for iron and 6 light metals (O, Na, Mg, Al, Si,Ca) in 30 Galactic Cepheids. We also give a new estimate of the Galacticradial abundance gradient. Methods: The stellar effective temperatureswere determined using the method of line depth ratios, and the surfacegravity and the microturbulent velocity vt by imposing theionization balance between Fe I and Fe II with the help of curves ofgrowth. Abundances were calculated with classical LTE atmosphere models. Results: Abundances were obtained with rms accuracies of about0.05-0.10 dex for Fe, and 0.05-0.20 dex for the other elements. Cepheidsin our sample have solar-like abundances, and current measurements agreequite well with previous determinations. We computed "single zone"Galactic radial abundance gradients for the 8-12 kpc region and found aslope for iron of -0.061 dex kpc-1.Based on observations made with the 1.52 m ESO Telescope at La Silla,Chile.
| High resolution spectroscopy for Cepheids distance determination. I. Line asymmetry
Context: .The ratio of pulsation to radial velocity (the projectionfactor) is currently limiting the accuracy of the Baade-Wesselinkmethod, and in particular of its interferometric version recentlyapplied to several nearby Cepheids. Aims: .This work aims atestablishing a link between the line asymmetry evolution over theCepheids' pulsation cycles and their projection factor, with the finalobjective to improve the accuracy of the Baade-Wesselink method fordistance determinations. Methods: .We present HARPS high spectralresolution observations (R=120 000) of nine galactic Cepheids:R Tra, S Cru, YSgr, β Dor, ζGem, Y Oph, RZ Vel,ℓ Car and RS Pup, having agood period sampling (P=3.39d to P=41.52d). We fit spectral lineprofiles by an asymmetric bi-Gaussian to derive radial velocity,Full-Width at Half-Maximum in the line (FWHM) and line asymmetry for allstars. We then extract correlations curves between radial velocity andasymmetry. A geometric model providing synthetic spectral lines,including limb-darkening, a constant FWHM (hereafter σ_C) and therotation velocity is used to interpret these correlations curves.Results: .For all stars, comparison between observations and modellingis satisfactory, and we were able to determine the projected rotationvelocities and σC for all stars. We also find acorrelation between the rotation velocity (V_rot sin i) and the periodof the star: V_rot sin i= (-11.5 ± 0.9) log (P) + (19.8 ±1.0) [ km s-1] . Moreover, we observe a systematic shift inobservational asymmetry curves (noted γ_O), related to the periodof the star, which is not explained by our static model:γ_O=(-10.7 ± 0.1) log (P) + (9.7 ± 0.2) [in %]. Forlong-period Cepheids, in which velocity gradients, compression or shockwaves seem to be large compared to short- or medium-period Cepheids weobserve indeed a greater systematic shift in asymmetry curves.Conclusions: .This new way of studying line asymmetry seems to be verypromising for a better understanding of Cepheids atmosphere and todetermine, for each star, a dynamic projection factor.
| Period-colour and amplitude-colour relations in classical Cepheid variables - IV. The multiphase relations
The superb phase resolution and quality of the Optical GravitationalLensing Experiment (OGLE) data on the Large Magellanic Cloud (LMC) andSmall Magellanic Cloud (SMC) Cepheids, together with existing data onGalactic Cepheids, are combined to study the period-colour (PC) andamplitude-colour (AC) relations as a function of pulsation phase. Ourresults confirm earlier work that the LMC PC relation (at mean light) ismore consistent with two lines of differing slopes, separated at aperiod of 10 d. However, our multiphase PC relations reveal much newstructure which can potentially increase our understanding of Cepheidvariables. These multiphase PC relations provide insight into why theGalactic PC relation is linear but the LMC PC relation is non-linear.This is because the LMC PC relation is shallower for short (logP < 1)and steeper for long (logP > 1) period Cepheids than thecorresponding Galactic PC relation. Both of the short- and long-periodCepheids in all three galaxies exhibit the steepest and shallowestslopes at phases around 0.75-0.85, respectively. A consequence is thatthe PC relation at phase ~ 0.8 is highly non-linear. Further, theGalactic and LMC Cepheids with logP > 1 display a flat slope in thePC plane at phases close to the maximum light. When the LMCperiod-luminosity (PL) relation is studied as a function of phase, weconfirm that it changes with the PC relation. The LMC PL relation in Vand I band near the phase of 0.8 provides compelling evidence that thisrelation is also consistent with two lines of differing slopes joined ata period close to 10 d.
| Bump Cepheids in the Magellanic Clouds: Metallicities, the Distances to the LMC and SMC, and the Pulsation-Evolution Mass Discrepancy
We use nonlinear pulsation models to reproduce the observed light andcolor curves for two samples of bump Cepheid variables, 19 from theLarge Magellanic Cloud and 9 from the Small Magellanic Cloud. Thisanalysis determines the fundamental parameters mass, luminosity,effective temperature, metallicity, distance, and reddening for thesample of stars. The use of the light-curve shape alone to determinemetallicity is a new modeling technique introduced here. Themetallicity, distance, and reddening distributions for the two samplesare in agreement with those of similar stellar populations in theliterature. The distance modulus of the Large Magellanic Cloud isdetermined to be 18.54+/-0.018, and the distance modulus of the SmallMagellanic Cloud is determined to be 18.93+/-0.024. The mean Cepheidmetallicities are Z=0.0091+/-0.0007 and 0.0050+/-0.0005 for the LMC andSMC, respectively. The masses derived from pulsation analysis aresignificantly less than those predicted by stellar evolutionary modelswith no or mild convective core overshoot. We show that this discrepancycannot be accounted for by uncertainties in our input opacities or inmass-loss physics. We interpret the observed mass discrepancy in termsof enhanced internal mixing in the vicinity of the convective coreduring the main-sequence lifetime and find that the overshoot parameterΛc rises from 0.688+/-0.009Hp at the meanLMC metallicity to 0.746+/-0.009Hp in the SMC.
| Extended envelopes around Galactic Cepheids. I. ℓ Carinae from near and mid-infrared interferometry with the VLTI
We present the results of long-baseline interferometric observations ofthe bright southern Cepheid ℓ Carinae in the infrared N (8-13 μm)and K (2.0-2.4 μm) bands, using the MIDI and VINCI instruments of theVLT Interferometer. We resolve in the N band a large circumstellarenvelope (CSE) that we model with a Gaussian of 3 Rstar(≈500 Rȯ ≈ 2-3 AU) half width at half maximum. Thesignature of this envelope is also detected in our K band data as adeviation from a single limb darkened disk visibility function. Thesuperimposition of a Gaussian CSE on the limb darkened disk model of theCepheid star results in a significantly better fit of our VINCI data.The extracted CSE parameters in the K band are a half width at halfmaximum of 2 Rstar, comparable to the N band model, and atotal brightness of 4% of the stellar photosphere. A possibility is thatthis CSE is linked to the relatively large mass loss rate of ℓ Car.Though its physical nature cannot be determined from our data, wediscuss an analogy with the molecular envelopes of RV Tauri, redsupergiants and Miras.
| Angular diameter amplitudes of bright Cepheids.
Expected mean angular diameters and amplitudes of angular diametervariations are estimated for all monoperiodic Classical Cepheidsbrighter than < V > = 8.0 mag. The catalog is intended to helpselecting best Cepheid targets for interferometric observations.
| Stellar pulsation and evolution: a stepping-stone to match reality.
We discuss current status of evolutionary and pulsation predictions forintermediate-mass stars. In particular, we focus our attention on thedifferent physical mechanisms that might affect the current discrepancybetween evolutionary and pulsation estimates of Galactic and MagellanicCepheid masses. Theoretical findings and recent empirical evidenceindicate that the mass-loss may play a significan role in thislong-standing problem.
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veranderliche Sterne e.V.
Not Available
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| Infrared Surface Brightness Distances to Cepheids: A Comparison of Bayesian and Linear-Bisector Calculations
We have compared the results of Bayesian statistical calculations andlinear-bisector calculations for obtaining Cepheid distances and radiiby the infrared surface brightness method. We analyzed a set of 38Cepheids using a Bayesian Markov Chain Monte Carlo method that had beenrecently studied with a linear-bisector method. The distances obtainedby the two techniques agree to 1.5%+/-0.6%, with the Bayesian distancesbeing larger. The radii agree to 1.1%+/-0.7%, with the Bayesiandeterminations again being larger. We interpret this result asdemonstrating that the two methods yield the same distances and radii.This implies that the short distance to the Large Magellanic Cloud foundin recent linear-bisector studies of Cepheids is not caused bydeficiencies in the mathematical treatment. However, the computeduncertainties in distance and radius for our data set are larger in theBayesian calculation by factors of 1.4-6.7. We give reasons to favor theBayesian computations of the uncertainties. The larger uncertainties canhave a significant impact on interpretation of Cepheid distances andradii obtained from the infrared surface brightness method.
| Mean JHK Magnitudes of Fundamental-Mode Cepheids from Single-Epoch Observations
We present an empirical method for converting single-point near-infraredJ, H, and K measurements of fundamental-mode Cepheids to meanmagnitudes, using complete light curves in V or I bands. The algorithmis based on the template light curves in the near-infrared bandpasses.The mean uncertainty of the method is estimated to about 0.03 mag, whichis smaller than the uncertainties obtained in other approaches to theproblem in the literature.
| Pulsation and Evolutionary Masses of Classical Cepheids. I. Milky Way Variables
We investigate a selected sample of Galactic classical Cepheids withavailable distance and reddening estimates in the framework of thetheoretical scenario provided by pulsation models, computed with metalabundance Z=0.02, helium content in the range of Y=0.25-0.31, andvarious choices of the stellar mass and luminosity. After transformingthe bolometric light curve of the fundamental models into BVRIJKmagnitudes, we derived analytical relations connecting the pulsationperiod with the stellar mass, the mean (intensity averaged) absolutemagnitude, and the color of the pulsators. These relations are usedtogether with the Cepheid observed absolute magnitudes in order todetermine the ``pulsation'' mass, Mp, of each individualvariable. The comparison with the ``evolutionary'' masses,Me,can, given by canonical (no convective core overshooting,no mass loss) models of central He-burning stellar structures revealsthat the Mp/Me,can ratio is correlated with theCepheid period, ranging from ~0.8 at logP=0.5 to ~1 at logP=1.5. Wediscuss the effects of different input physics and/or assumptions on theevolutionary computations, as well as of uncertainties in the adoptedCepheid metal content, distance, and reddening. Eventually, we find thatthe pulsational results can be interpreted in terms of mass loss duringor before the Cepheid phase, whose amount increases as the Cepheidoriginal mass decreases. It vanishes around 13 Msolar andincreases up to ~20% at 4 Msolar.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Корма |
| Прямое восхождение: | 08h13m04.22s |
| Склонение: | -34°34'42.7" |
| Видимая звёздная величина: | 7.055 |
| Расстояние: | 2040.816 парсек |
| Собственное движение RA: | -3 |
| Собственное движение Dec: | 3.5 |
| B-T magnitude: | 8.973 |
| V-T magnitude: | 7.214 |
Каталоги и обозначения:
|
INHABITED SKY
