Оглавление
Изображения
Загрузить ваше изображение
DSS Images Other Images
Публикации по объекту
First unambiguous asteroseismologic modelling of a gamma Doradus star .
An asteroseismologic model of the gamma Dor star 9 Aurigae is discussedin this work. This is the first time a complete asteroseismologic schemefor analyzing gamma Dor stars is applied. From standard photometricobservations (at least three oscillation frequencies, a photometricerror box in the HR diagram and multicolor photometric observations, inthis case in the Strömgren system) we can reduce the possibletheoretical models for this star, providing constraints to the stellarparameters as mass, overshooting, metallicity, MLT parameter alpha ,Brunt-Väiälä integral (Ith), etc.Simultaneously, an estimate of the modal identification of the observedfrequencies is also obtained.This can be possible by the application, for the first time, of acomplete procedure where different theoretical and computationaltechniques, recently developed, are linked and compared with photometricobservations. The Frequency Ratio Method (FRM) and the Time DependentConvection (TDC) theory are the basis of this complete procedure.
| An X-Ray Search for Compact Central Sources in Supernova Remnants. II. Six Large-Diameter SNRs
We present the second in a series of studies in which we have searchedfor undiscovered neutron stars in supernova remnants (SNRs). This paperdeals with the six largest SNRs in our sample, too large for Chandra orXMM-Newton to cover in a single pointing. These SNRs are nearby, withtypical distances of <1 kpc. We therefore used the ROSAT BrightSource Catalog and past observations in the literature to identify X-raypoint sources in and near the SNRs. Out of 54 sources, we wereimmediately able to identify optical/IR counterparts to 41 from existingdata. We obtained Chandra snapshot images of the remaining 13 sources.Of these, 10 were point sources with readily identified counterparts,two were extended, and one was not detected in the Chandra observationbut is likely a flare star. One of the extended sources may be a pulsarwind nebula, but if so it is probably not associated with the nearbySNR. We are then left with no identified neutron stars in these six SNRsdown to luminosity limits of ~1032 ergs s-1. Theselimits are generally less than the luminosities of typical neutron starsof the same ages, but are compatible with some lower luminosity sourcessuch as the neutron stars in the SNRs CTA 1 and IC 443.
| The frequency ratio method and the new multiperiodic γ Doradus star HD 218427
Oscillations of γ Dor-type were discovered in the star HD 218427through simultaneous uvby photometric observations carried out in theyear 2003. A few Hβ-Crawford measurements were also collected forcalibration purposes and they locate this star well inside the γDor instability region. We find HD 218427 to be deficient in metals,similar to other well-defined γ Dor stars, and discuss thepossibility that it has a λ Boo nature. We carried out frequencyanalysis for different filters, including the combined "vby" filter, andfive frequencies were found as significant with periods ranging between0.3 and 0.8 days. The recently-developed frequency ratio method is usedin order to identify the excited modes. The results are consistent withan l=2 identification for all the modes and with high radial quantumnumbers (n40) for the three main observed periodicities. Thepossibility of multiplet structures is also discussed. However, noconsistency is found when using the time-dependent convection treatmentto discriminate modes. This disagreement can be due to the largerotation velocity taking place in HD 218427 and, consequently, thesignificant coupling between the modes.
| Time-dependent convection seismic study of five γ Doradus stars
We apply for the first time the time-dependent convection (TDC)treatment of Gabriel and Grigahcène et al. to the photometricmode identification in γ Doradus (γ Dor) stars. We considerthe influence of this treatment on the theoretical amplitude ratios andphase differences. Comparison with the observed amplitudes and phases ofthe stars γ Dor, 9 Aurigae, HD 207223 = HR 8330, HD 12901 and48501 is presented and enables us to identify the degree l of thepulsation modes for four of them. We also determine the mode stabilityfor different models of these stars. We show that our TDC models agreebetter with observations than with frozen convection models. Finally, wecompare the results obtained with different values of the mixing-lengthparameter α.
| A high-resolution stellar library for evolutionary population synthesis
We present a library of 1654 high-resolution stellar spectra, with asampling of 0.3 Åand covering the wavelength range from 3000 to7000 Å. The library was computed with the latest improvements instellar atmospheres, incorporating non-local thermodynamic equilibrium(non-LTE) line-blanketed models for hot, massive (Teff>=27500K) and line-blanketed models for cool (3000<=Teff<= 4500K) stars. The total coverage of the gridis 3000 K <=Teff<= 55000 K and -0.5 <= log g<=5.5, for four chemical abundance values: twice solar, solar, half solarand 1/10 solar. Evolutionary synthesis models using this library arepresented in a companion paper. We tested the general behaviour of thelibrary by calculating and comparing equivalent widths of numerous H andHeI lines, and some of the commonly used metallic indices. We alsocompared the library with the empirical libraries STELIB and Indo-US.The full set of the synthetic stellar spectra is available from ourwebsites (http://www.iaa.csic.es/~rosa andhttp://www.astro.iag.usp.br/~lucimara/library.htm).
| HD 153720 - A SB2 system with twin metallic-line components
We report the results of abundance determinations for the components ofthe SB2 star HD 153720 from high resolution (R=60 000) echelle highsignal-to-noise spectra of the wavelength region 3595-10 260 Åtaken with the 2.7 m telescope of the McDonald Observatory We found thevalues of the atmospheric parameters of the primary to be effectivetemperature Teff = 7425 K and surface gravity log g,= 4.0cgs, and of the secondary to be Teff = 7125 K and log g,= 3.9cgs. The microturbulent velocity is vmicro= 2.7 kms-1 for both components, and the projected rotationalvelocity is v sin i ,= 15 km s-1 also for both components.The abundances of about 20 elements were determined with the method ofspectrum synthesis. The components of HD 153720 are metallic-linestars. Possible inconsistencies between old and new measurements ofradial velocities may be explained by the existence of third body inthis system. A review of recent high resolution spectral observations ofeight A4-F1 binaries shows that only one of these systems can beclassified as normal.Based on observations obtained at the 2.7-m telescope of the McDonaldObservatory.The data are only available in electronic form athttp://www.edpsciences.org
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| Rotational Velocities of B, A, and Early-F Narrow-lined Stars
Projected rotational velocities for 58 B, A, and early-F stars have beendetermined from high-resolution spectroscopic observations made at KittPeak National Observatory with the coudé feed telescope. All thestars are slowly rotating with vsini<60 km s-1. Because oftheir low rotational velocities, 15 of the stars have been observed asprospective, early-type, radial velocity standards.
| A Theoretical γ Doradus Instability Strip
In this paper, we present the first theoretical γ Doradusinstability strip. We find that our model instability strip agrees verywell with the previously established, observationally based, instabilitystrip of Handler & Shobbrook. We stress, as do Guzik et al., thatthe convection zone depth plays the major role in the determination ofour instability strip. Once this depth becomes too deep or too shallow,the convection zone no longer allows for pulsational instability. Ourtheoretical γ Dor instability strip is bounded by ~6850 and 7360 Kat the red and blue edge, respectively, on the zero-age main sequenceand by ~6560 and 7000 K at the red and blue edge, respectively,approximately 2 mag more luminous. This theoretical strip, transformedto the observer's color-magnitude diagram, overlays the region wheremost of the 30 bona fide γ Dor stars are found.
| A Dozen New γ Doradus Stars
We use new high-dispersion spectroscopic and precise photometricobservations to identify 12 new γ Doradus stars. Two of the 12systems are double-lined binaries that show obvious velocityvariability. Five other stars have metallic lines with compositeprofiles characterized by a narrow feature near the center of each broadcomponent. Spectrograms of the Hα line indicate that all fivestars are binaries rather than shell stars. The remaining five stars inour sample are probably single. All 12 stars are photometricallyvariable with amplitudes between 6 and 87 mmag in Johnson B and periodsbetween 0.3 and 1.2 days. Four stars are monoperiodic; the rest havebetween two and five independent periods. The variability at all periodsapproximates a sinusoid. Although many of the stars lie within theδ Scuti instability strip, none exhibit the higher frequencyvariability seen in δ Scuti stars. We have increased the sample ofknown γ Doradus stars by 40% and revised the positions of a numberof variables in the H-R diagram by accounting for duplicity. Our list of42 confirmed γ Doradus variables gives some of their properties.All are dwarfs or subgiants and lie within a well-defined region of theH-R diagram that overlaps the cool edge of the δ Scuti instabilitystrip. We compare the observed location of the γ Doradus variableswith a recently published theoretical γ Doradus instability stripand find good agreement.
| Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.
We have embarked on a project, under the aegis of the Nearby Stars(NStars)/Space Interferometry Mission Preparatory Science Program, toobtain spectra, spectral types, and, where feasible, basic physicalparameters for the 3600 dwarf and giant stars earlier than M0 within 40pc of the Sun. In this paper, we report on the results of this projectfor the first 664 stars in the northern hemisphere. These resultsinclude precise, homogeneous spectral types, basic physical parameters(including the effective temperature, surface gravity, and overallmetallicity [M/H]), and measures of the chromospheric activity of ourprogram stars. Observed and derived data presented in this paper arealso available on the project's Web site.
| The Orbit and Pulsation Periods of the γ Doradus Variable HR 6844 (V2502 Ophiuchi)
We obtained spectroscopic and photometric observations of the γDoradus variable HR 6844 (=V2502 Ophiuchi). Radial velocities show thatthis star is a single-lined binary with a period of 4.4852 days. Theprimary is an F1 V star, while the secondary is likely an M dwarf.Velocity residuals to a circular orbit have a period of 1.3071 days andan amplitude of ~3 km s-1. Three periods of light variationwere detected, 1.30702, 1.4350, and 0.62286 days. The first period isessentially identical to that found in the radial velocities and has thelargest amplitude, a peak-to-peak value of 0.067 mag in B. Thephotometric check star, 73 Oph (=HR 6795), has light variations with aperiod of 0.61439 in B. Although the star is a close visual binary, thelight variations are ascribed to the primary, making it most likely anewly discovered γ Doradus variable.
| STELIB: A library of stellar spectra at R ~ 2000
We present STELIB, a new spectroscopic stellar library, available athttp://webast.ast.obs-mip.fr/stelib. STELIB consists of an homogeneouslibrary of 249 stellar spectra in the visible range (3200 to 9500Å), with an intermediate spectral resolution (la 3 Å) andsampling (1 Å). This library includes stars of various spectraltypes and luminosity classes, spanning a relatively wide range inmetallicity. The spectral resolution, wavelength and spectral typecoverage of this library represents a substantial improvement overprevious libraries used in population synthesis models. The overallabsolute photometric uncertainty is 3%.Based on observations collected with the Jacobus Kaptein Telescope,(owned and operated jointly by the Particle Physics and AstronomyResearch Council of the UK, The Nederlandse Organisatie voorWetenschappelijk Onderzoek of The Netherlands and the Instituto deAstrofísica de Canarias of Spain and located in the SpanishObservatorio del Roque de Los Muchachos on La Palma which is operated bythe Instituto de AstrofÃsica de Canarias), the 2.3 mtelescope of the Australian National University at Siding Spring,Australia, and the VLT-UT1 Antu Telescope (ESO).Tables \ref{cat1} to \ref{cat6} and \ref{antab1} to A.7 are onlyavailable in electronic form at http://www.edpsciences.org. The StellarLibrary STELIB library is also available 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/402/433
| Six New γ Doradus Stars
We present high-resolution spectroscopy and precision photometry of sixnew γ Doradus stars, one of which was independently discovered byanother group. This brings the total number of confirmed γ Doradusvariables to 30. All six of these variables fall in the spectral classrange F0-F2 all but one are subgiants. The six stars have between oneand five photometric periods in the range 0.3-1.2 days. We find noevidence for higher frequency δ Scuti pulsations in any of thesesix stars. Our spectroscopic observations reveal HD 108100 to be thefirst confirmed γ Doradus variable with composite broad and narrowline profiles suggesting the presence of a circumstellar shell or disk.HD 221866 has the most asymmetric absorption lines of the six stars inthis paper and also the largest photometric amplitude. Most of the 30confirmed γ Doradus variables lie in a fairly tight region of theH-R diagram on or just above the main sequence that partially overlapsthe cool edge of the δ Scuti instability strip. However, threestars, including two of the new variables in this paper, are subgiantsthat lie well within the δ Scuti strip. Among the 30 confirmedγ Doradus variables, we find no correlation between thephotometric periods and intrinsic color, absolute magnitude, orluminosity.
| Discovery and analysis of p-mode and g-mode oscillations in the A-type primary of the eccentric binary HD 209295*
We have discovered both intermediate-order gravity mode and low-orderpressure mode pulsation in the same star, HD 209295. It is thereforeboth a γ Doradus and a δ Scuti star, which makes it thefirst pulsating star to be a member of two classes. The analysis of our128h of multisite spectroscopic observations carried out over twoseasons reveals that the star is a single-lined spectroscopic binarywith an orbital period of 3.10575+/-0.00010d and an eccentricity of0.352+/-0.011. Only weak pulsational signals are found in both theradial velocity and line-profile variations, but we have succeeded inshowing that the two highest-amplitude γ Doradus pulsation modesare consistent with l=1 and |m|=1. These two modes dominated our 280h ofBVIC multisite photometry, also obtained over two seasons. Wedetected altogether ten frequencies in the light variations, one in theδ Scuti regime and nine in the γ Doradus domain. Five of theγ Doradus frequencies are exact integer multiples of the orbitalfrequency. This observation leads us to suspect they are tidallyexcited. Attempts to identify modes from the multicolour photometryfailed. We performed model calculations and a stability analysis of thepulsations. The frequency range in which δ Scuti modes are excitedagrees well with observations. However, our models do not showexcitation of γ Doradus pulsations, although the damping issmaller in the observed range. We also investigated tidal excitation ofγ Doradus modes. Some of the observed harmonics of the orbitalperiod were found to be unstable. The observed orbital harmonics whichare stable in the models can be understood as linear combinations of theunstable modes. We could not detect the secondary component of thesystem in infrared photometry, suggesting that it may not be amain-sequence star. Archival data of this star show that it has a strongultraviolet (UV) excess, the origin of which is not known. The orbit ofthe primary is consistent with a secondary mass ofM>1.04Msolar, which is indicative of a neutron star,although a white dwarf companion is not ruled out.
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| The multiperiodicity of the gamma Doradus stars HD 224945 and HD 224638 as detected from a multisite campaign
We discuss new photometric data collected on the gamma Dor variables HD224945 and HD 224638. Multiperiodicity was detected in both stars,thanks to the clear spectral window of a multisite campaign thatinvolved five observatories. HD 224945 shows the shortest period amongthe gamma Dor stars, i.e., 0.3330 d. The pulsation behaviour is verydifferent: HD 224945 displays a set of frequencies spread over aninterval much larger than that of HD 224638. We clearly found evidencefor amplitude variations in the excited modes by comparing data fromdifferent years. HD 224945 and HD 224638 are among the best examples ofgamma Dor stars that show multimode pulsations, which make them veryinteresting from an asteroseismological point of view. Based onobservations partially collected at ESO-La Silla (Proposals 54.E-018 and56.E-0308).
| Astrophysics in 2000
It was a year in which some topics selected themselves as importantthrough the sheer numbers of papers published. These include theconnection(s) between galaxies with active central engines and galaxieswith starbursts, the transition from asymptotic giant branch stars towhite dwarfs, gamma-ray bursters, solar data from three major satellitemissions, and the cosmological parameters, including dark matter andvery large scale structure. Several sections are oriented aroundprocesses-accretion, collimation, mergers, and disruptions-shared by anumber of kinds of stars and galaxies. And, of course, there are theusual frivolities of errors, omissions, exceptions, and inventories.
| A Spectroscopic Analysis of the γ Doradus Star HD 207223 = HR 8330
We present a detailed analysis of 76 high-resolution, highsignal-to-noise spectra of the γ Doradus star HD 207223=HR 8330.Periodogram analysis of the moments of the line profiles reveals asingle period of 2.67+/-0.01 days, a result consistent with that foundin photometric studies. Mode identification was accomplished with boththe moment method and from direct line-profile fitting. Both methodsclearly point towards a sectoral mode of degree l>=2. The inclusionof rotational effects, by taking the Coriolis force into account,reveals a single consistent solution: an l=m=+2 retrograde sectoral modethat has a period of 1.4 days in the corotating frame. The star is seenat an inclination of 30°. From this, we derive a rotational periodof 4.9 days.
| 10 New γ Doradus and δ Scuti Stars
We present high-resolution spectroscopy and precision photometry of fivenew γ Doradus and five new δ Scuti variables. The five newγ Doradus variables substantially increase the number of confirmedstars of this class. All 10 stars fall in the spectral class rangeF0-F2, but they are cleanly separated into two groups by theirluminosity and photometric periods. However, the period gap between theγ Doradus and δ Scuti stars is becoming very narrow since weconfirm that HD 155154 is a γ Doradus star with the shortestperiods reported to date (the shortest of its four periods is ~0.312days). We do not find any evidence in our sample for stars exhibitingboth δ Scuti- and γ Doradus-type pulsations.
| 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
| delta Scuti and related stars: Analysis of the R00 Catalogue
We present a comprehensive analysis of the properties of the pulsatingdelta Scuti and related variables based mainly on the content of therecently published catalogue by Rodríguez et al.(\cite{retal00a}, hereafter R00). In particular, the primaryobservational properties such as visual amplitude, period and visualmagnitude and the contributions from the Hipparcos, OGLE and MACHOlong-term monitoring projects are examined. The membership of thesevariables in open clusters and multiple systems is also analyzed, withspecial attention given to the delta Scuti pulsators situated ineclipsing binary systems. The location of the delta Scuti variables inthe H-R diagram is discussed on the basis of HIPPARCOS parallaxes anduvbybeta photometry. New borders of the classical instability arepresented. In particular, the properties of the delta Scuti pulsatorswith nonsolar surface abundances (SX Phe, lambda Boo, rho Pup, delta Deland classical Am stars subgroups) are examined. The Hipparcos parallaxesshow that the available photometric uvbybeta absolute magnitudecalibrations by Crawford can be applied correctly to delta Scutivariables rotating faster than v sin i ~ 100 km s{-1} withnormal spectra. It is shown that systematic deviations exist for thephotometrically determined absolute magnitudes, which correlate with vsin i and delta m1. The photometric calibrations are found tofit the lambda Boo stars, but should not be used for the group ofevolved metallic-line A stars. The related gamma Dor variables and thepre-main-sequence delta Scuti variables are also discussed. Finally, thevariables catalogued with periods longer than 0fd 25 are examined on astar-by-star basis in order to assign them to the proper delta Scuti, RRLyrae or gamma Dor class. A search for massive, long-period delta Scutistars similar to the triple-mode variable AC And is also carried out.
| On the Variability of A3-F0 Luminosity Class III-V Stars
I investigate the Hipparcos Satellite photometry of A3-F0 stars ofluminosity classes III-V to learn about their variability and identify afew stars for which further study is desirable.
| Metal abundances of field A and Am stars
Detailed abundances of 9 field A and Am stars are derived for Mg, Ca,Sc, Cr, Fe, and Ni using high resolution spectroscopy. Most of oursample Am stars show the characteristic deficiencies of Ca and Sc on onehand, and the overabundances of the iron peak elements on the other,which is consistent with the predictions of the diffusion model for MainSequence or slightly evolved stars. There seems to be a correlationbetween abundances of Ca and Sc and the effective temperature for starsof similar age: the abundance of Ca increases with increasing masswhereas that of Sc decreases. Two of our Am stars have a differentabundance pattern: Ca is overabundant in the hot magnetic Am star o Pegand its abundance is nearly normal in the metallic giant star rho Pup.The other objects of our sample have globally a solar composition except28 And, which shows significant deficiencies for most the studiedelements. Based on observations collected at the Observatoire deHaute-Provence (France), and on data from the ESA\protect\linebreakHIPPARCOS astrometry satellite.
| Search for gamma Doradus variable stars in the Pleiades cluster
Photometric observations in the uvbybeta system of A-F type stars in thePleiades cluster have been performed in order to detect pulsatingvariable stars of gamma Doradus type in the lower part of the Cepheidinstability strip. In order to obtain more information about thebehaviour of the studied objects and to be able to distinguish betweenlong period variable and non-variable stars, two statistical methodshave been developed. Several of these stars show some type ofvariability but only two of the observed objects, H1284 and S29, can besafely classified as gamma Dor stars. Furthermore, these observationshave provided us with Strömgren and Hβ photometry,non-existing up to now for some of them, which permitted us to perform aphotometric study of the Pleiades cluster.
| Gamma Doradus Stars: Defining a New Class of Pulsating Variables
In this paper we describe a new class of pulsating stars, the prototypeof which is the bright, early, F-type dwarf gamma Doradus. These starstypically have between 1 and 5 periods ranging from 0.4 to 3 days withphotometric amplitudes up to 0.1 mag in Johnson V. The mechanism forthese observed variations is high-order, low-degree, nonradial,gravity-mode pulsation.
| The gamma DOR variable HR 8799: results from a multisite campaign
In this paper, we present the results of a multisite photometriccampaign devoted to the gamma Doradus type variable HR 8799. FromJohnson and Stromgren data, we were able to identify three independentfrequencies (f_1= 1.9791 cycle d^-1, f_2=1.7268 cycle d^-1 andf_3=1.6498 cycle d^-1) as well as another signal, which we are able toidentify as the coupling term between two of the frequencies(f_4=f_1-f_2=0.2479 cycle d^-1). These four frequencies are able toaccount for all of the observed variations down to the 1sigmasignificance level. We discuss another possible interpretation of thesefrequencies using a model of quasi-stochastic amplitude modulation. Inthis scenario, we are able to show that HR 8799 might be pulsating withtwo independent frequencies, one of which undergoes amplitude modulationsimilar to other gamma Dor objects. In addition, we discuss apreliminary mode identification based on the observed colour curves.Finally, 18 simultaneous, high-resolution echelle spectra were collectedon two nights and we qualitatively compare the radial velocities fromthese data with our photometry.
| Photometric Measurements of the Fields of More than 700 Nearby Stars
In preparation for optical/IR interferometric searches for substellarcompanions of nearby stars, we undertook to characterize the fields ofall nearby stars visible from the Northern Hemisphere to determinesuitable companions for interferometric phase referencing. Because theKeck Interferometer in particular will be able to phase-reference oncompanions within the isoplanatic patch (30") to about 17th magnitude atK, we took images at V, r, and i that were deep enough to determine iffield stars were present to this magnitude around nearby stars using aspot-coated CCD. We report on 733 fields containing 10,629 measurementsin up to three filters (Gunn i, r and Johnson V) of nearby stars down toabout 13th magnitude at V.
| The ROSAT all-sky survey catalogue of the nearby stars
We present X-ray data for all entries of the Third Catalogue of NearbyStars \cite[(Gliese & Jahreiss 1991)]{gli91} that have been detectedas X-ray sources in the ROSAT all-sky survey. The catalogue contains1252 entries yielding an average detection rate of 32.9 percent. Inaddition to count rates, source detection parameters, hardness ratios,and X-ray fluxes we also list X-ray luminosities derived from Hipparcosparallaxes. Catalogue also available at CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Oxygen and Nitrogen Abundances in Hyades F Type Dwarfs
The abundances of oxygen and nitrogen were investigated for elevenHyades F type main-sequence stars, based on the line-profile of O I (+FeI) 8446 and the equivalent-widths of N I 8629/8683, in order to examinewhether any conspicuous abundance deficiency exists for O and N at T_eff~ 6700 K similar to that observed for Li, since its possibility wassuggested by the diffusion interpretation of this ``Li-chasm''. Itturned out, however, that the abundances appear to be nearly independentof T_eff (and v_e sin i ) for both O and N, without showing any apparentdip larger than ~ 0.2 dex (for O) or ~ 0.3 dex (for N), being consistentwith the results of García-Lopez et al.'s (1993, AAA 58.114.068)study for oxygen. This may indicate that the diffusion process alone isprobably not responsible for the origin of the Li-gap. Regarding theaveraged O and N abundances in these Hyades F stars, a moderate N-excessby ~ 0.3 dex relative to the Sun was suggested (though not definite),with oxygen being nearly normal.
|
Добавить новую статью
Внешние ссылки
- - Внешних ссылок не найдено -
Добавить внешнюю ссылку
Группы:
|
Наблюдательные данные и астрометрия
| Созвездие: | Возничий |
| Прямое восхождение: | 05h06m40.60s |
| Склонение: | +51°35'52.0" |
| Видимая звёздная величина: | 5 |
| Расстояние: | 26.219 парсек |
| Собственное движение RA: | -29.9 |
| Собственное движение Dec: | -174.7 |
| B-T magnitude: | 5.356 |
| V-T magnitude: | 5.014 |
Каталоги и обозначения:
|
