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Amplitude variability or close frequencies in pulsating stars - the δ Scuti star FG Vir
The nature of the observed amplitude variability of several modes in theδ Scuti star FG Vir is examined. This is made possible by theextensive photometry obtained during 2002-04, as well as the longphotometric time-base starting in 1992. In this star, three frequenciesshow strong amplitude and phase variations. In the power spectrum, thesefrequencies also show up as frequency doublets. However, since trueamplitude variability of a single frequency can also lead to (false)frequency doublets in the power spectrum, a specific test examining indetail the observed amplitude and the phase variations of an assumedsingle frequency is applied. For the frequencies at 12.15 and 23.40cycle d-1, it is shown that the amplitude variability of asingle mode can be ruled out. In particular, an important property ofbeating between two modes is fulfilled: the amplitude and the phase varysynchronously with a phase shift close to 90°. The origin of theamplitude variability of a third mode, namely, near 19.86 cycled-1, is not clear due to the long beat period of 20+ yr, forwhich the amplitude-phase test suffers from gaps in the coverage.However, even for this frequency, the amplitude variations can beexpressed well by a mathematical two-mode model.If we examine these three close frequency pairs together with other(usually more widely separated) close frequencies in FG Vir, 18 pairs offrequencies with separations closer than 0.10 cycle d-1 havebeen detected. It is shown that the majority of the pairs occur near thetheoretically expected frequencies of radial modes. Mode identificationsare available for only a few modes: the only detected radial mode at12.15 cycle d-1 is part of a close pair.It is shown that accidental agreements between the frequencies ofexcited modes can be ruled out because of the large number of detectedclose frequency doublets.
| 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.
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| Cepheidenbeobachtung in der BAV: Ruckblick und Ausblick.
Not Available
| WWW Database of Variable Star Fourier Coefficients
Fourier coefficients are a valuable tool in the study of a wide varietyof pulsating stars. In the case of RR Lyrae stars they, can be used toestimate various physical parameters, such as mass, luminosity,metallicity, and effective temperature. They are frequently used todiscriminate between different pulsation modes for both RR Lyrae starsand Cepheids. Fourier coefficients have been published for stars basedon many different photometric systems as well as for radial velocitymeasurements. With the release of coefficients from large-scale surveysand the availability of these data on the Internet, the number ofFourier coefficients available for study has significantly increased,and it is difficult to obtain all available data for individual stars ora given subset of stars. To assist researchers in obtaining and makinguse of Fourier coefficients, an on-line archive of published values ofFourier coefficients has been established. Users can search the databaseusing a variety of tools, and the data sets extend beyond the Milky Wayto include extragalactic variables. The archive is located at the Website http://nitro9.earth.uni.edu/fourier/.
| 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.
| A Photometric and Spectroscopic Study of 3 Vulpeculae: An Observer's Nightmare
We describe photometry of 3 Vulpeculae obtained with the Four CollegeAutomated Photoelectric Telescope and spectroscopy obtained with the1.22 m telescope of the Dominion Astrophysical Observatory. We haveanalyzed differential uvby photometric observations obtained over 7 yr.Three main frequencies (f1=0.9719, f2=0.7923, andf3=0.8553 cycles day-1) were found, as well as asum frequency (f1+f2=1.76420 cyclesday-1). A study of the photographic region usinghigh-dispersion spectrograms obtained with a Reticon detector at thecoudé spectrograph confirms the variable nature of 3 Vul as a 53Persei star and indicates that the star's abundances are normal formain-sequence band B stars. The new spectra were combined with thosepreviously published to confirm an orbital period very close to 1 yr.Finally, comparison with models yielded both a mass (4.16Msolar) and an age (25 Myr), and a mass range for thecompanion (0.6-1.1 Msolar). With an orbital period of almost1 yr and a pulsation period of almost 1 day, this star is indeed anobserver's nightmare.
| Period Changes in Galactic Classical Cepheids. Slow Evolution of Long-Period Cepheids
We compared period changes derived from O-C diagrams for 63 classicalCepheids from our Galaxy with model calculations. We found that forCepheids with log P > 1.0 the observed changes are smaller thanpredicted values, except variable SZ Cas. However some of the firstovertone Cepheids, particularly EU Tau and Polaris, change its periodmuch faster than it follows from theory. Summary of the known data onperiod changes in Cepheids from the Galaxy and from the MagellanicClouds (previous papers) leads to conclusion that none of the 999Cepheids is undergoing the first crossing of the instability strip. Alsothe observed period changes for long-period Cepheids are a few timesslower than predicted by the models. These results imply that muchlarger fraction of helium is burned in the Cepheid stage than it ispredicted by models.
| Measuring stellar magnetic fields from high resolution spectroscopy of near-infrared lines
Zeeman splitting of otherwise degenerate levels provides astraight-forward method of measuring stellar magnetic fields. In theoptical, the relative displacements of the Zeeman components are quitesmall compared to the rotational line broadening, and thereforeobservations of Zeeman splitting are usually possible only for ratherstrong magnetic fields in very slowly rotating stars. However, themagnitude of the Zeeman splitting is proportional to the square of thewavelength, whereas rotational line broadening mechanisms are linear inwavelength; therefore, there is a clear advantage in using near-infraredspectral lines to measure surface stellar magnetic fields. We haveobtained high resolution (R >= 25 000) spectra in the 15 625-15 665Å region for two magnetic chemically peculiar stars, viz. HD176232 and HD 201601, and for the suspected magnetic chemically peculiarstar HD 180583, as part of a pilot study aimed at determining theaccuracy with which we can measure stellar magnetic fields using theZeeman splitting of near-infrared lines. We confirm that in principlethe magnetic field strength can be estimated from the magneticintensification of spectral lines, i.e. the increase in equivalent widthof a line over the zero-field value. However, due to line blending aswell as the dependence of this intensification on abundance and fieldgeometry, accurate estimates of the magnetic field strengths can beobtained only by modelling the line profiles by means of spectralsynthesis techniques. Using this approach, we find a 1.4 kG magneticfield modulus in HD 176132 and an upper limit of 0.2 kG in HD 180583.The very weak infrared lines in the spectrum of HD 201601 are consistentwith a 3.9 kG field modulus estimated from the splitting of the Fe II6149.258 Å line seen in an optical spectrum. Finally, we wouldlike to draw attention to the fact that there are no sufficientlydetailed and reliable atomic line lists available for the near-infraredregion that can be used in high resolution work; a large fraction of thefeatures observed in our spectra remains to be identified.Based on observations made with the NASA Infrared Telescope Facility.
| 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).
| System Description and First Light Curves of the Hungarian Automated Telescope, an Autonomous Observatory for Variability Search
Having been operational at Kitt Peak for more than a year, the prototype(HAT-1) of the Hungarian Automated Telescope (HAT) has been used for anall-sky variability search of the northern hemisphere. This smallautonomous observatory is recording the brightness of stars in the rangeof IC~6-13 mag with a telephoto lens and its9deg×9deg field of view, yielding a datarate of ~106 photometric measurements per night. We givebrief hardware and software descriptions of the system, controlled by asingle PC running RealTime Linux OS. We provide an overview ofsite-specific details and quantify the astrometric and photometriccapabilities of HAT. As a demonstration of system performance, we give asample of 60 short-period variables in a single selected field, allbright, with I<13 mag, where only 14 were known before. Depending onthe observing strategy, a search for extrasolar planet transits is alsoa feasible observing program. We conclude with a short discussion onfuture directions. Further information can be found at the HAT Webpage.3
| Cepheiden: was wird beobachtet - was nicht ?
Not Available
| Fundamental Parameters of Cepheids. V. Additional Photometry and Radial Velocity Data for Southern Cepheids
I present photometric and radial velocity data for Galactic Cepheids,most of them being in the southern hemisphere. There are 1250 Genevaseven-color photometric measurements for 62 Cepheids, the averageuncertainty per measurement is better than 0.01 mag. A total of 832velocity measurements have been obtained with the CORAVEL radialvelocity spectrograph for 46 Cepheids. The average accuracy of theradial velocity data is 0.38 km s-1. There are 33 stars withboth photometry and radial velocity data. I discuss the possiblebinarity or period change that these new data reveal. I also presentreddenings for all Cepheids with photometry. The data are availableelectronically. Based on observations obtained at the European SouthernObservatory, La Silla.
| Polaris: Amplitude, Period Change, and Companions
Polaris has presented us with the rare phenomenon of a Cepheid with apulsation amplitude that has decreased over the last 50 yr. In thisstudy we have used this property to see whether the amplitude decreaseduring the last 15 yr has had any effect on upper atmosphere heating. Weobtained IUE high- and low-resolution spectra but found no change ineither the Mg II chromospheric emission or the flux at 1800 Åbetween 1978 and 1993 when the pulsation amplitude dropped by 50% (from2.8 to 1.6 km s-1). The energy distribution from 1700 Åthrough V, B, R(KC), and I(KC) is like that of a nonvariable supergiantof the same color rather than a full amplitude Cepheid in that it hasmore flux at 1800 Å than the full amplitude Cepheid δ Cep.Polaris also has a rapidly changing period (3.2 s yr-1), incommon with other overtone pulsators. We argue that this is a naturalconsequence of the different envelope locations that dominate pulsationgrowth rates in fundamental and overtone pulsation. In fundamental modepulsators, the deeper envelope is more important in determining growthrates than for overtone pulsators. For fundamental mode pulsators,evolutionary changes in the radius produce approximately linear changesin period. In overtone pulsators, pulsation reacts to small evolutionarychanges in a more unstable way because the modes are more sensitive tohigh envelope features such as opacity bumps, and the growth rates forthe many closely spaced overtone modes change easily. Finally, the upperlimit to the X-ray flux from an Einstein observation implies that thecompanion in the astrometric orbit is earlier than F4 V. The combinationof upper and lower limits on the companion from IUE and Einsteinrespectively catch the companion mass between 1.7 and 1.4Msolar. The X-ray limit is consistent with the more distantcompanion α UMi B being a physical companion in a hierarchaltriple system. However the X-ray limits require that the even moredistant companions α UMi C and D are too old to be physicallyassociated with Polaris.
| Sodium enrichment of stellar atmospheres. I. Non-variable supergiants and bright giants
48 supergiants and bright giants have been observed in order toinvestigate the sodium enrichment of their atmospheres and itsconnection with stellar gravity. We present the equivalent widths of the6154 Å and 6160 Å Na I lines measured from the programspectra, the results of effective temperature determinations, the NLTEsodium abundances, and the derived relation between the sodiumoverabundance and surface gravity.
| CHARM: A Catalog of High Angular Resolution Measurements
The Catalog of High Angular Resolution Measurements (CHARM) includesmost of the measurements obtained by the techniques of lunaroccultations and long-baseline interferometry at visual and infraredwavelengths, which have appeared in the literature or have otherwisebeen made public until mid-2001. A total of 2432 measurements of 1625sources are included, along with extensive auxiliary information. Inparticular, visual and infrared photometry is included for almost allthe sources. This has been partly extracted from currently availablecatalogs, and partly obtained specifically for CHARM. The main aim is toprovide a compilation of sources which could be used as calibrators orfor science verification purposes by the new generation of largeground-based facilities such as the ESO Very Large Interferometer andthe Keck Interferometer. The Catalog is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/386/492, and from theauthors on CD-Rom.
| Close frequency pairs in Delta Scuti stars
The majority of the well-studied delta Scuti stars shows frequency pairsin the power spectra with frequency separations less than 0.06cd-1 (0.7 mu Hz) as well as amplitude variability. We examinethe interpretation in terms of separate excited stellar pulsation modes,single modes with variable amplitudes, and observational problems. Thevariable-phase technique, which examines the phase jumps near the timesof minimum amplitude of an assumed single frequency, is applied to theextensive data of the star BI CMi, which shows some of the most extremebehavior. The following results are found for the 5 features in thepower spectrum which could be explained as single modes with variableamplitudes or as double modes: for three features it can be shown thatthese are indeed pairs of separate pulsation modes beating with eachother: at times of minimum amplitude the phase jumps are observed andboth the observed amplitude and phase variations can be predictedcorrectly by assuming two separate modes of nearly equal frequencies.Artifacts caused by observational error, insufficient frequencyresolution or variable amplitudes can be ruled out. A fourth pair has aprobable origin in two excited modes, while a 5th case is inconclusivedue to long time scales of variability and small amplitudes. Theexistence of close frequency needs to be taken into account in planningthe lengths of earth-based as well as space campaigns so that sufficientfrequency resolution is obtained. Possible reasons for the existence ofclose frequencies in delta Scuti stars are considered. They include thedense frequency spacing caused by the presence of mixed modes,rotational splitting as well as near-coincidence of the frequencies ofmodes with different l values (the so-called Small Spacing).
| Time series analysis of V511 Lyrae photometry
Our time series analysis of sixteen BV light curves of thechromospherically active binary V511 Lyr confirmed the 2fd7 rotationperiod uniquely. The seasonal periodicity changes of 3.8% indicated thepresence of detectable surface differential rotation. The significant2fd67455 periodicity in the light curve minimum epochs suggested thatthe strongest starspot formation in V511 Lyr was concentrated on onestable active longitude. Table 2 is only available in electronic form atthe CDS via anonymous ftp to cdarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A&A/383/197. Thephotometric data are also available athttp://schwab.tsuniv.edu/t3/v511lyr/v511lyr.html
| 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
| V473 Lyrae: a Blazhko-effect Cepheid?
The amplitude spectra of the light curves of RR Lyrae Blazhko stars canoften be typified by symmetrical triplets. It is shown that the sameapplies to V473 Lyr. Furthermore, the star is probably a radial pulsator- as is thought to be the case for the RR Lyrae stars - thus ruling outrotational splitting of a purely non-radial mode.
| Multiperiodicities from the Hipparcos epoch photometry and possible pulsation in early A-type stars
A selection criterion based on the relative strength of the largestpeaks in the amplitude spectra, and an information criterion are used incombination to search for multiperiodicities in Hipparcos epochphotometry. The method is applied to all stars which have beenclassified as variable in the Hipparcos catalogue: periodic, unsolvedand microvariables. Results are assessed critically: although there aremany problems arising from aliasing, there are also a number ofinteresting frequency combinations which deserve further investigation.One such result is the possible occurrence of multiple periods of theorder of a day in a few early A-type stars. The Hipparcos catalogue alsocontains a number of these stars with single periodicities: such starswith no obvious variability classifications are listed, and informationabout their properties (e.g., radial velocity variations) discussed.These stars may constitute a new class of pulsators.
| The Amplitude Decrease of V Bootis
We Explain the amplitude decrease in V Bootis by interference betweentwo close peiods. We also discuss V Bootis in comparison withtheoretical predictions and with other stars that have undergoneamplitude decrease.
| A Cepheid is No More: Hubble's Variable 19 in M33
We report on the remarkable evolution in the light curve of a variablestar discovered by Hubble in M33 and classified by him as a Cepheid.Early in the 20th century, the variable, designated as V19, exhibited a54.7 day period, an intensity-weighted mean B magnitude of 19.59+/-0.23mag, and a B amplitude of 1.1 mag. Its position in the period-luminosityplane was consistent with the relation derived by Hubble from a total of35 variables. Modern observations by the DIRECT project show a dramaticchange in the properties of V19: its mean B magnitude has risen to19.08+/-0.05 mag, and its B amplitude has decreased to less than 0.1mag. V19 does not appear to be a classical (Population I) Cepheidvariable at present, and its nature remains a mystery. It is not clearhow frequent such objects are or how often they could be mistaken forclassical Cepheids.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. II. Basic Parameters of Program Stars and the Role of Microturbulence
Paper I of this series presented precise MK spectral types for 372 lateA-, F-, and early G-type stars with the aim of understanding the natureof luminosity classification on the MK spectral classification systemfor this range of spectral types. In this paper, a multidimensionaldownhill simplex technique is introduced to determine the basicparameters of the program stars from fits of synthetic spectra andfluxes with observed spectra and fluxes from Strömgren uvbyphotometry. This exercise yields useful calibrations of the MK spectralclassification system but, most importantly, gives insight into thephysical nature of luminosity classification on the MK spectralclassification system. In particular, we find that in this range ofspectral types, microturbulence appears to be at least as important asgravity in determining the MK luminosity type.
| 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.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Fourier Analysis of Hipparcos Photometry of Cepheid Variables
Fourier parameters have been computed for 240 field Cepheids observed bythe Hipparcos satellite. We have identified three new PopulationIovertone Cepheids: V411Lac, V898 Cen and V572 Aql. We have comparedFourier progressions of Population I and Population II Cepheids. Thepossibility of using Fourier parameters to distinguish between the twotypes is discussed.
| Multi-colour PL-relations of Cepheids in the bt HIPPARCOS catalogue and the distance to the LMC
We analyse a sample of 236 Cepheids from the hipparcos catalog, usingthe method of ``reduced parallaxes'' in V, I, K and the reddening-free``Wesenheit-index''. We compare our sample to those considered by Feast& Catchpole (1997) and Lanoix et al. (1999), and argue that oursample is the most carefully selected one with respect to completeness,the flagging of overtone pulsators, and the removal of Cepheids that mayinfluence the analyses for various reasons (double-mode Cepheids,unreliable hipparcos solutions, possible contaminated photometry due tobinary companions). From numerical simulations, and confirmed by theobserved parallax distribution, we derive a (vertical) scale height ofCepheids of 70 pc, as expected for a population of 3-10 Msunstars. This has consequences for Malmquist- and Lutz-Kelker (Lutz &Kelker 1973, Oudmaijer et al. 1998) type corrections which are smallerfor a disk population than for a spherical population. The V and I datasuggest that the slope of the Galactic PL-relations may be shallowerthan that observed for LMC Cepheids, either for the whole period range,or that there is a break at short periods (near log P_0 ~ 0.7-0.8). Westress the importance of two systematic effects which influence thedistance to the LMC: the slopes of the Galactic PL-relations andmetallicity corrections. In order to assess the influence of thesevarious effects, we present 27 distance moduli (DM) to the LMC. Theseare based on three different colours (V,I,K), three different slopes(the slope observed for Cepheids in the LMC, a shallower slope predictedfrom one set of theoretical models, and a steeper slope as derived forGalactic Cepheids from the surface-brightness technique), and threedifferent metallicity corrections (no correction as predicted by one setof theoretical models, one implying larger DM as predicted by anotherset of theoretical models, and one implying shorter DM based onempirical evidence). We derive DM between 18.45 +/- 0.18 and 18.86 +/-0.12. The DM based on K are shorter than those based on V and I andrange from 18.45 +/- 0.18 to 18.62 +/- 0.19, but the DM in K could besystematically too low by about 0.1 magnitude because of a bias due tothe fact that NIR photometry is available only for a limited number ofstars. From the Wesenheit-index we derive a DM of 18.60 +/- 0.11,assuming the observed slope of LMC Cepheids and no metallicitycorrection, for want of more information. The DM to the LMC based on theparallax data can be summarised as follows. Based on the PL-relation inV and I, and the Wesenheit-index, the DM is 18.60 ± 0.11(± 0.08 slope)(^{+0.08}_{-0.15} ;metallicity), which is ourcurrent best estimate. Based on the PL-relation in K the DM is ;;;;18.52 +/- 0.18 (± 0.03 ;slope) (± 0.06 ;metallicity)(^{+0.10}_{-0} ;sampling ;bias). The random error is mostly due to thegiven accuracy of the hipparcos parallaxes and the number of Cepheids inthe respective samples. The terms between parentheses indicate thepossible systematic uncertainties due to the slope of the GalacticPL-relations, the metallicity corrections, and in the K-band, due to thelimited number of stars. Recent work by Sandage et al. (1999) indicatesthat the effect of metallicity towards shorter distances may be smallerin V and I than indicated here. From this, we point out the importanceof obtaining NIR photometry for more (closeby) Cepheids, as for themoment NIR photometry is only available for 27% of the total sample.This would eliminate the possible bias due to the limited number ofstars, and would reduce the random error estimate from 0.18 to about0.10 mag. Furthermore, the sensitivity of the DM to reddening,metallicity correction and slope are smallest in the K-band. Based ondata from the ESA HP astrometry satellite.
| 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.
| The MACHO Project LMC Variable Star Inventory. VI. The Second Overtone Mode of Cepheid Pulsation from First/Second Overtone Beat Cepheids
MACHO Project photometry of 45 LMC first and second overtone (FO and SO,respectively) beat Cepheids that pulsate in the FO and SO has beenanalyzed to determine the light-curve characteristics for the SO mode ofCepheid pulsation. We predict that singly periodic SO Cepheids will havenearly sinusoidal light curves; that we will only be able to discern SOCepheids from fundamental (F) and FO Cepheids for P<~1.4 days; andthat the SO distribution will overlap the short-period edge of the LMCFO Cepheid period-luminosity relation (when both are plotted as afunction of photometric period). We also report the discovery of one SOCepheid candidate, MACHO*05:03:39.6-70:04:32, with a photometric periodof 0.775961+/-0.000019 days and an instrumental amplitude of0.047+/-0.009 mag in V.
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Osservazione e dati astrometrici
| Costellazione: | Lira |
| Ascensione retta: | 19h15m59.40s |
| Declinazione: | +27°55'37.0" |
| Magnitudine apparente: | 6.16 |
| Distanza: | 515.464 parsec |
| Moto proprio RA: | 6.6 |
| Moto proprio Dec: | 0.8 |
| B-T magnitude: | 6.906 |
| V-T magnitude: | 6.224 |
Cataloghi e designazioni:
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