Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1
Context: .About 5% of upper main sequence stars are permeated by astrong magnetic field, the origin of which is still matter of debate. Aims: . With this work we provide observational material to studyhow magnetic fields change with the evolution of stars on the mainsequence, and to constrain theory explaining the presence of magneticfields in A and B-type stars. Methods: . Using FORS1 inspectropolarimetric mode at the ESO VLT, we have carried out a survey ofmagnetic fields in early-type stars belonging to open clusters andassociations of various ages. Results: . We have measured themagnetic field of 235 early-type stars with a typical uncertainty of 100 G. In our sample, 97 stars are Ap or Bp stars. For thesetargets, the median error bar of our field measurements was 80 G.A field has been detected in about 41 of these stars, 37 of which werenot previously known as magnetic stars. For the 138 normal A and B-typestars, the median error bar was 136 G, and no field was detected in anyof them.
| Evolution of interacting binaries with a B type primary at birth
We revisited the analytical expression for the mass ratio distributionfor non-evolved binaries with a B type primary. Selection effectsgoverning the observations were taken into account in order to comparetheory with observations. Theory was optimized so as to fit best withthe observed q-distribution of SB1s and SB2s. The accuracy of thistheoretical mass ratio distribution function is severely hindered by theuncertainties on the observations. We present a library of evolutionarycomputations for binaries with a B type primary at birth. Some liberalcomputations including loss of mass and angular momentum during binaryevolution are added to an extensive grid of conservative calculations.Our computations are compared statistically to the observeddistributions of orbital periods and mass ratios of Algols. ConservativeRoche Lobe Over Flow (RLOF) reproduces the observed distribution oforbital periods but fails to explain the observed mass ratios in therange q in [0.4-1]. In order to obtain a better fit the binaries have tolose a significant amount of matter, without losing much angularmomentum.
| Eclipsing binaries in open clusters - I. V615 Per and V618 Per in h Persei
We derive absolute dimensions for two early-type main-sequence detachedeclipsing binaries in the young open cluster h Persei (NGC 869). V615Persei has a spectral type of B7 V and a period of 13.7 d. V618 Perseiis A2 V and has a period of 6.4 d. New ephemerides are calculated forboth systems. The masses of the component stars have been derived usinghigh-resolution spectroscopy and are 4.08 +/- 0.06 and 3.18 +/- 0.05Msolar for V615 Per and 2.33 +/- 0.03 and 1.56 +/- 0.02Msolar for V618 Per. The radii have been measured by fittingthe available light curves using EBOP and are 2.29 +/- 0.14 and 1.90 +/-0.09 Rsolar for V615 Per and 1.64 +/- 0.07 and 1.32 +/- 0.07Rsolar for V618 Per. By comparing the observed spectra ofV615 Per with synthetic spectra from model atmospheres we find that theeffective temperatures of the stars are 15000 +/- 500 K for the primaryand 11000 +/- 500 K for the secondary. The equatorial rotationalvelocities of the primary and secondary components of V615 Per are 28+/- 5 and 8 +/- 5 km s-1, respectively. Both components ofV618 Per rotate at 10 +/- 5 km s-1. The equatorial rotationalvelocities for synchronous rotation are about 10 km s-1 forall four stars. The time-scales for orbital circularization for bothsystems, and the time-scale for rotational synchronization of V615 Per,are much greater than the age of h Per. Their negligible eccentricitiesand equatorial rotational velocities therefore support the hypothesisthat they were formed by delayed break-up. We have compared the radii ofthese stars with models by the Granada and the Padova groups for starsof the same masses but different compositions. We conclude that themetallicity of the stars is Z~ 0.01. This appears to be the firstestimate of the bulk metallicity of h Per. Recent photometric studieshave assumed a solar metallicity so their results should be reviewed.
| 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
| The mass dependence of the overshooting parameter determined from eclipsing binary data
High-precision data on absolute dimensions of evolved eclipsing binarieshave been used for a quantitative evaluation of the significance ofconvective overshoot in the stellar core. Eight detached double-linedeclipsing binaries with components close to or beyond the terminal agemain sequence (TAMS) and masses between 2 and 12Msolar havebeen compared with evolutionary models using different overshootingparameters. The results are robust and indicate a systematic increase ofthe amount of convective overshoot with the stellar mass. Suchdetermination constitutes a fundamental point for stellar structure andevolution theory.
| Chemical composition of eclipsing binaries: a new approach to the helium-to-metal enrichment ratio
The chemical enrichment law Y(Z) is studied by using detacheddouble-lined eclipsing binaries with accurate absolute dimensions andeffective temperatures. A sample of 50 suitable systems was collectedfrom the literature, and their effective temperatures were carefullyre-determined. The chemical composition of each of the systems wasobtained by comparison with stellar evolutionary models, under theassumption that they should fit an isochrone to the observed propertiesof the components. Evolutionary models covering a wide grid in Z and Ywere adopted for our study. An algorithm was developed for searching thebest-fitting chemical composition (and the age) for the systems, basedon the minimization of a χ2 function. The errors (andbiases) of these parameters were estimated by means of Monte Carlosimulations, with special care put on the correlations existing betweenthe errors of both components. In order to check the physicalconsistency of the results, we compared our metallicity values withempirical determinations, obtaining excellent coherence. Theindependently derived Z and Y values yielded a determination of thechemical enrichment law via weighted linear least-squares fit. Our valueof the slope, ΔY/ΔZ=2.2+/-0.8, is in good agreement withrecent results, but it has a smaller formal error and it is free ofsystematic effects. Linear extrapolation of the enrichment law to zerometals leads to an estimation of the primordial helium abundance ofYp=0.225+/-0.013, possibly affected by systematics in theeffective temperature determination.
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Do the physical properties of Ap binaries depend on their orbital elements?
We reveal sufficient evidence that the physical characteristics of Apstars are related to binarity. The Ap star peculiarity [represented bythe Δ(V1-G) value and magnetic field strength] diminishes witheccentricity, and it may also increase with orbital period(Porb). This pattern, however, does not hold for largeorbital periods. A striking gap that occurs in the orbital perioddistribution of Ap binaries at 160-600d might well mark a discontinuityin the above-mentioned behaviour. There is also an interestingindication that the Ap star eccentricities are relatively lower thanthose of corresponding B9-A2 normal binaries for Porb>10d.All this gives serious support to the pioneering idea of Abt &Snowden concerning a possible interplay between the magnetism of Apstars and their binarity. Nevertheless, we argue instead in favour ofanother mechanism, namely that it is binarity that affects magnetism andnot the opposite, and suggest the presence of a newmagnetohydrodynamical mechanism induced by the stellar companion andstretching to surprisingly large Porb.
| V907 Scorpii: A Remarkable Binary Star Whose Eclipses Turn On and Off and On and Off
V907 Scorpii, near the open cluster M7, and possibly a member, is uniqueamong all known eclipsing binary stars because its eclipses have turnedon and off twice within modern history. By using all availablephotometric and spectroscopic data, we have discovered that it is atleast a triple star and possibly a quadruple star system consisting of avisual binary with a very long orbital period, the brighter member ofwhich is itself the triple star. The triple star contains an eclipsingbinary star (B9.5 V) with an orbital period of 3.78 days and a faint,distant companion (late K, or perhaps a white dwarf) with an orbitalperiod of 99.3 days around the center of mass of the triple star system.Radial velocity measurements allow the masses to be estimated. Becausethe orbital planes of the eclipsing binary and its triple companion arenot coplanar, the orbital plane of the eclipsing binary shows nodalregression with a period of 68 yr. For about one-third of this time, theclose binary is eclipsing; the rest of the time the inclination is toosmall for eclipses to occur. The earliest observations of the system inthe year 1899 show eclipses; the eclipses stopped about 1918, startedagain about 1963, and stopped again in about 1986. We predict that theeclipses should start occurring once again in the year 2030+/-5.
| Five-colour photometry of OB-stars in the Southern Hemisphere
Observations of OB-stars, made in 1959 and 1960 at the Leiden SouthernStation near Hartebeespoortdam, South Africa, with the VBLUW photometerattached to the 90 cm light-collector, are given in this paper. They arecompared with photometry obtained by \cite[Graham (1968),]{gra68}\cite[Walraven & Walraven (1977),]{wal77} \cite[Lub & Pel(1977)]{lub77} and \cite[Van Genderen et al. (1984).]{gen84} Formulaefor the transformation of the present observations to those of\cite[Walraven & Walraven (1977)]{wal77} and \cite[Lub & Pel(1977)]{lub77} are given. Table 4 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/Abstract.html
| Open clusters with Hipparcos. I. Mean astrometric parameters
New memberships, mean parallaxes and proper motions of all 9 openclusters closer than 300 pc (except the Hyades) and 9rich clusters between 300 and 500 pc have been computed using Hipparcosdata. Precisions, ranging from 0.2 to 0.5 mas for parallaxes and 0.1 to0.5 mas/yr for proper motions, are of great interest for calibratingphotometric parallaxes as well as for kinematical studies. Carefulinvestigations of possible biases have been performed and no evidence ofsignificant systematic errors on the mean cluster parallaxes has beenfound. The distances and proper motions of 32 more distant clusters,which may be used statistically, are also indicated. Based onobservations made with the ESA Hipparcos astrometry satellite
| Four-colour photometry of eclipsing binaries. XXXVIII. Light curves of the triple system V906 Scorpii
Complete $uvby$ light curves of the detached triple-lined late B-typeeclipsing binary V906 Scorpii, secured from 1987 to 1991, are presented.A detailed photometric analysis based on these observations and on newspectroscopic material yields accurate masses and radii (errors <~2%) for the components, confirms that the system is a member of NGC 6475(Messier 7), and is published separately \cite[(Alencar et al.1997)]{ale97}. Based on observations done with the Danish 50 cmTelescope (SAT) at the European Southern Observatory (ESO), La Silla,Chile. Table 2 presented in this paper will only be accessible inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| MSC - a catalogue of physical multiple stars
The MSC catalogue contains data on 612 physical multiple stars ofmultiplicity 3 to 7 which are hierarchical with few exceptions. Orbitalperiods, angular separations and mass ratios are estimated for eachsub-system. Orbital elements are given when available. The catalogue canbe accessed through CDS (Strasbourg). Half of the systems are within 100pc from the Sun. The comparison of the periods of close and widesub-systems reveals that there is no preferred period ratio and allpossible combinations of periods are found. The distribution of thelogarithms of short periods is bimodal, probably due to observationalselection. In 82\% of triple stars the close sub-system is related tothe primary of a wide pair. However, the analysis of mass ratiodistribution gives some support to the idea that component masses areindependently selected from the Salpeter mass function. Orbits of wideand close sub-systems are not always coplanar, although thecorresponding orbital angular momentum vectors do show a weak tendencyof alignment. Some observational programs based on the MSC aresuggested. Tables 2 and 3 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| V 3903 Sagittarii: a massive main-sequence (O7V+O9V) detached eclipsing binary
We present for the first time an analysis based on uvby light curves,Hβ indices and on new spectroscopic data of the massive detacheddouble-lined O-type eclipsing binary V 3903Sgr. The uvby light curvesare analysed with the WINK (initial solutions) and the Wilson-Devinney(WD, final solution) programs. Both codes were used in their extendedversions, with stellar atmospheres and taking into account the geometricdistortions and photometric effects caused by proximity of thecomponents. The spectroscopic CCD observations were analysed with theharmonic ``Wilsing-Russell'' and the ``Lehman-Filhes'' methods. Weconclude that V 3903Sgr is one of the rare O-type detached systems whereboth components are still on the initial phases of the main sequence,with an age of either 1.6x10(6) yrs or 2.5x10(6) yrs (depending on theevolutionary model adopted) at a distance of ~1500pc, the same as forthe Lagoon Nebula (Messier8) complex, of which the system is probably amember. We determine the absolute dimensions: M_A=27.27+/-0.55,R_A=8.088+/-% 0.086, M_B=19.01+/-0.44 and R_B=6.125+/-0.060 (solarunits). There is no evidence of mass transfer and the system isdetached. The orbit is circular, and both components show synchronousrotation, despite their early evolutionary stage. The absolutedimensions determined should be representative for normal single stars.Amongst the massive systems (M>17Msun) with preciseabsolute dimensions (errors <2%), V 3903Sgr is that with the mostmassive primary, with the largest mass difference between thecomponents, and it is the youngest one. Based on data collected with the60$\,$cm and 1.6$\,$m telescopes at the Pico dos Dias Observatory,Na\-tional Laboratory of Astrophysics, LNA-CNPq, Bra\-só\-polis,MG, Brazil and with the Danish 50$\,$cm telescope (SAT) at the EuropeanSouthern Observatory (ESO), La Silla, Chile
| Absolute dimensions of eclipsing binaries. XXI. V906 Scorpii: a triple system member of M 7.
We present an analysis based on new uvby light curves and spectroscopicdata of the detached triple-lined B-type eclipsing binary V906 Sco. Theuvby light curves are analysed with an extended version of theWilson-Devinney program. The spectroscopic CCD observations are analysedwith both the Sterne and the Lehmann-Filhes methods. We conclude fromthe combined analysis that the triple system V906 Sco is a member of theopen cluster M 7 and that its B-type eclipsing components are still onthe main sequence, at an age of (2.4+/-0.3)x10^8^yrs, already close tothe TAMS (especially the more massive one). The system is older than thetime for circularization of the orbit, and the small eccentricity isprobably caused by the third component, which, however, is unlikely tobe the main responsible for the apsidal motion, probably more influencedby tidal and rotational deformation. We determine absolute dimensions ofhigh precision (errors <2%): M_A_=3.25+/-0.07, R_A_=3.52+/-0.04,M_B_=3.38+/-0.07 and R_B_=4.52+/-0.04, in solar units. The system isdetached, with both components in synchronous rotation, and thereforerepresentative for normal stars.
| A Catalogue of Correlations Between Eclipsing Binaries and Other Categories of Double Stars
Among the 9110 stars in The Bright Star Catalogue, there are 225eclipsing or ellipsoidal variables. A search has been made for these incatalogues of spectroscopic binaries, visual double or multiple stars,speckle interferometry, occulation binaries, and galatic clusters. Themajority of the photometric binaries are also members of groups ofhigher multiplicity. The variables are in systems ranging from one to 91stars, five on the average. 199 are either spectroscopic binaries (SB)or stars with variable radial velocity, with orbital periods known for160. Photometric periods are lacking for 48 while SB periods areavailable for 23 of these. Observers with photoelectric equipment areencouraged to plan observations to test if the SB periods are consistentwith photometric data. Observers are likewise encouraged to examinethose stars for which the photometric and SB periods appear to beinconsistent. Parallaxes are available for 86 of the stars, 41 of themindicating distances nearer than 50 parsecs.
| The ROSAT all-sky survey catalogue of optically bright OB-type stars.
For the detailed statistical analysis of the X-ray emission of hot starswe selected all stars of spectral type O and B listed in the Yale BrightStar Catalogue and searched for them in the ROSAT All-Sky Survey. Inthis paper we describe the selection and preparation of the data andpresent a compilation of the derived X-ray data for a complete sample ofbright OB stars.
| An X-Ray Survey of the Open Cluster NGC 6475 (M7) With ROSAT
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995AJ....110.1229P&db_key=AST
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| SANTIAGO 91, a right ascension catalogue of 3387 stars (equinox J2000).
The positions in right ascension of 3387 stars belonging to the Santiago67 Catalogue, observed with the Repsold Meridian Circle at Cerro Calan,National Astronomical Observatory, during the period 1989 to 1994, aregiven. The average mean square error of a position, for the wholeCatalogue, is +/-0.009 s. The mean epoch of the catalogue is 1991.84.
| Relative Orientation of Angular Momenta in Multiple Stellar Systems
Not Available
| Predicting the Secondary Minima Depths and Spectra of the Main Components for the Dm-Type Eclipsing Variable Stars
Not Available
| ICCD speckle observations of binary stars. VIII - Measurements during 1989-1991 from the Cerro Tololo 4 M telescope
One-thousand eighty-eight observations of 947 binary star systems,observed by means of speckle interferometry with the 4 m telescope onCerro Tololo, are presented. These measurements, made during the period1989-1991, comprise the second installment of results stemming from theexpansion of our speckle program to the southern hemisphere.
| Speckle observations of visual and spectroscopic binaries. IV
This is the fourth paper of this series giving results of speckleobservations for 22 visual and 161 spectroscopic binaries. Theobservation was carried out by using the 212 cm telescope of San PedroMartir Observatory in Mexico on 7 nights from July 20 to July 26, 1991.We obtained fringes in power spectra of 19 visual and 11 spectroscopicbinaries (6 newly resolved ones) with angular separation larger than0.06 arcsec. We introduced a new ICCD TV camera in this observation, andwere able to achieve the diffraction-limit resolution of the 212 cmtelescope.
| Speckle observations of visual and spectroscopic binaries. III
This is the third paper of this series giving results of speckleobservations carried out for seven visual and 119 spectroscopic binariesat seven nights from May 20 to May 27, 1989, and for 30 visual and 272spectroscopic binaries at 12 nights from June 11 to June 15, and fromAugust 28 to September 3, 1990, using the 212-cm telescope at San PedroMartir Observatory in Mexico. Fringes in the lower spectrum of 31 visualand spectroscopic binaries with angular separation larger than 21 arcsecare obtained. Additionally to two spectroscopic binaries, HD41116 andHD206901, named in the second paper of this series, six spectroscopicbinaries are found each of which has the third component starsurrounding two stars of spectroscopic binary having periodic variationof radial velocity.
| ICCD speckle observations of binary stars. V - Measurements during 1988-1989 from the Kitt Peak and the Cerro Tololo 4 M telescopes
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1990AJ.....99..965M&db_key=AST
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| A Study of the DM and SD Type Eclipsing Variables by Means of the Cluster Analysis Technique
Not Available
| Experiments with CP stars
A method for fine-classification of stars is applied to a sample ofabout 100 stars within the approximate spectral type range B6-A5.Experiments have been performed to find a method for fine-classificationof A stars, both normal and chemically peculiar, within the MK system.It is found that, independent of the definition of the degree ofpeculiarity, no sharp border exists separating the CP stars from thenormal ones. It is also shown that spectral classification can, onaverage, be performed with almost the same accuracy for CP stars as forthe normal ones if cases of extreme peculiarity are avoided.
| On the (B-V) colors of the bright stars
The possible causes of the dispersion of (B-V) colors of nearby stars inthe Bright Star Catalog are investigated. The distribution of (B-V)colors is presented for the entire range of spectral classes.Explanations for the dispersion in terms of a nonuniform distribution ofinterstellar absorbing material and a variability of metallicity areaddressed. A new statistical model for reddening by interstellar dustclouds is developed. It is concluded that extinction by nonuniforminterstellar matter is an important contribution to the reddening ofnearby stars, and that a part of the dispersion of (B-V) colors of Kand, possibly, M giants may be due to some unidentified variableproperty of those stars.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
INHABITED SKY
