Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Evolution of Low Mass Contact Binaries
VFTS 682, a very massive and very hot Wolf-Rayet (WR) star recentlydiscovered in the Large Magellanic Cloud near the famous star clusterR136, might be providing us with a glimpse of a missing link in ourunderstanding of Long Gamma-Ray Bursts (LGRBs), including dark GRBs. Itis likely its properties result from chemically homogeneous evolution(CHE), believed to be a key process for a massive star to become a GRB.It is also heavily obscured by dust extinction, which could make it adark GRB upon explosion. Using Spitzer data we investigate theproperties of interstellar dust in the vicinity of R136, and argue thatits high obscuration is not unusual for its environment and that itcould indeed be a slow runaway (``walkaway'') from R136. Unfortunately,based on its current mass loss rate, VFTS 682 is unlikely to become aGRB, because it will lose too much angular momentum at its death. If itwere to become a GRB, it probably would also not be dark, eitherescaping or destroying its surrounding dusty region. Nevertheless, it isa very interesting star, deserving further studies, and being one ofonly three presently identified WR stars (two others in the SmallMagellanic Cloud) that seems to be undergoing CHE.
| Physical parameters of 62 eclipsing binary stars using the All Sky Automated Survey-3 data - I
We present a detailed light-curve analysis of publicly available V-bandobservations of 62 binary stars, mostly contact binaries, obtained bythe All Sky Automated Survey (ASAS)-3 project between 2000 and 2009.Eclipsing binaries are important astronomical targets for determiningthe physical parameters of component stars from the geometry of theirorbits. They provide an independent direct method of measuring the radiiof stars. We improved the ASAS determined periods and ephemerides, andobtained the Fourier parameters from the phased light curves of these 62stars. These Fourier parameters were used for preliminary classificationof the stars in our sample. The phased light curves were then analysedwith the aid of the Wilson-Devinney light-curve modelling technique inorder to obtain various geometrical and physical parameters of thesebinaries. The spectroscopic mass ratios as determined from the radialvelocity measurements available in the literature were used as one ofthe inputs to the light-curve modelling. Thus reliable estimations ofparameters of these binaries were obtained with combined photometric andspectroscopic data, and error estimates were made using the heuristicscan method. For several systems in the sample, the solutions wereobtained for the first time and would serve as a good source in thefuture for light-curve analysis based on more precise follow-up CCDphotometric observations. Out of 62 stars in the sample, photometricanalysis of 39 stars is presented here for the first time using the ASASphotometry and precise spectroscopic mass ratios. From the analysis, wefound 54 contact binaries, six semidetached binaries and two detachedbinaries. The Fourier parameters in the a2-a4plane were used for preliminary classification, and the finalclassification was done based on the Roche lobe geometry obtained fromthe light-curve modelling.
| Photometric Study of the Very Short Period Shallow Contact Binary DD Comae Berenices
The first photometric solutions of the very short period (VSP) closebinary DD Comae Berenices (P = 0fd26920811) based on our new complete(IR)C light curves are derived by the 2003 version Wilson-VanHamme code. They show that the system belongs to shallow contact W-typeW UMa systems with a degree of overcontact of 8.7%. The observed lightcurve distortions are explained by employing the spots model due to thelate-type nature of both components. We have collected all availablephotometric data about the system with emphasis on the individualobservational data, which we treated simultaneously using our own methodbased on the usage of computed model light curves as templates. Werecalculated published times of light minimum and added new ones of ourown to construct an O - C diagram that spans over 70 years. Using aleast squares method orthogonal quadratic model function, we found thatthe orbital period of DD Com is continuously increasing with\dot{P}=0.00401(22) s yr-1. The period increase may be causedby the mass transfer from the less-massive component to the more-massiveone. With the period increase, the binary is evolving from the presentshallow contact phase to the broken stage predicted by the thermalrelaxation oscillation (TRO) theory. Compared with other VSP systems, DDCom is a rare system that lies on the expanding phase of the TRO cycle.Until now, only four such systems including DD Com are found in thisstage. Thus, this target is another good observational proof of the TROtheory in a very short period region.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Coronal Activity from the ASAS Eclipsing Binaries
We combine the catalog of eclipsing binaries from the All Sky AutomatedSurvey (ASAS) with the ROSAT All Sky Survey (RASS). The combinationresults in 836 eclipsing binaries that display coronal activity and isthe largest sample of active binary stars assembled to date. By usingthe (V-I) colors of the ASAS eclipsing binary catalog, we are able todetermine the distances and thus bolometric luminosities for themajority of eclipsing binaries that display significant stellaractivity. A typical value for the ratio of soft X-ray to bolometricluminosity is LX/Lbol? a few×10-4, similar to the ratio of soft X-ray to bolometricflux FX/Fbol in the most active regions of theSun. Unlike rapidly rotating isolated late-type dwarfs - stars withsignificant outer convection zones - a tight correlation between Rossbynumber and activity of eclipsing binaries is absent. We find evidencefor the saturation effect and marginal evidence for the so-called``super-saturation'' phenomena. Our work shows that wide-field stellarvariability searches can produce a high yield of binary stars withstrong coronal activity.The combined ASAS and RASS catalog, as well as the results of this workare available for download.
| Angular momentum and mass evolution of contact binaries
Various scenarios of contact binary evolution have been proposed in thepast, giving hints of (sometimes contradictory) evolutionary sequencesconnecting A- and W-type systems. As the components of close detachedbinaries approach each other and contact binaries are formed, followingevolutionary paths transforms them into systems of two categories:A-type and W-type. The systems evolve in a similar way but underslightly different circumstances. The mass/energy transfer rate isdifferent, leading to quite different evolutionary results. Analternative scenario of evolution in contact is presented and discussed,based on the observational data of over one hundred low-temperaturecontact binaries. It results from the observed correlations amongcontact binary physical and orbital parameters. Theoretical tracks arecomputed assuming angular momentum loss from a system via stellar wind,accompanied by mass transfer from an advanced evolutionary secondary tothe main-sequence primary. A good agreement is seen between the tracksand the observed graphs. Independently of details of the evolution incontact and a relation between A- and W-type systems, the ultimate fateof contact binaries involves the coalescence of both components into asingle fast rotating star.
| The Properties of Long-Period Variables in the Large Magellanic Cloud from MACHO
We present a new analysis of the long-period variables in the LargeMagellanic Cloud (LMC) from the MACHO Variable Star Catalog.Three-quarters of our sample of evolved, variable stars have periodiclight curves. We characterize the stars in our sample using the multipleperiods found in their frequency spectra. Additionally, we usesingle-epoch Two Micron All Sky Survey measurements to construct theaverage infrared light curves for different groups of these stars.Comparison with evolutionary models shows that stars on the red giantbranch (RGB) or the early asymptotic giant branch (AGB) often shownon-periodic variability, but begin to pulsate with periods on the twoshortest period-luminosity (P-L) sequences (3 & 4) when theybrighten to K s ? 13. The stars on the thermally pulsingAGB are more likely to pulsate with longer periods that lie on the nexttwo P-L sequences (1 & 2), including the sequence associated withthe Miras in the LMC. The Petersen diagram and its variants show thatmulti-periodic stars on each pair of these sequences (3 & 4, and 1& 2) typically pulsate with periods associated only with that pair.The periods in these multi-periodic stars become longer and stronger asthe star evolves. We further constrain the mechanism behind the longsecondary periods (LSPs) seen in half of our sample, and find that thereis a close match between the luminosity functions of the LSP stars andall of the stars in our sample, and that these star's pulsationamplitudes are relatively wavelength independent. Although this ischaracteristic of stellar multiplicity, the large number of thesevariables is problematic for that explanation.
| Observation and Analysis of RW Doradus
Photometry of the short-period (P ~0.285 d) low-mass (M1~0.67, M2 ~0.48 Msolar) W UMa-type eclipsingbinary RW Dor, mainly involving a good series of observations in1987-88, using standard B and V filters at the Auckland Observatory, isexamined. Analysis via light curve fittings alone permits both transit(near main sequence) and occultation (W-type contact binary) solutions.Consideration of proximity or other effects on the relatively smallnumber of measured radial velocities gives rise to a more detailedreview of the alternatives, though the (W-type) configuration determinedby Hilditch, Hill & Bell (1992) is still confirmed. Furtherimplications of this configuration are examined, given an apparentgeneral trend to period decrease (? P/P ~6 ×10-11), and some light curve asymmetry. This is consistentwith the initially low-mass companion, Case-B scenario of Budding(1984). The evidence does suggest irregularity of the period variationhowever, and further detailed surveillance, including more spectroscopy,is desirable.
| Radial Velocity Studies of Southern Close Binary Stars. II. Spring/Summer Systems
Radial velocity measurements and sine-curve fits to the orbital velocityvariations are presented for 14 close binary stars, S Ant, TT Cet, TWCet, AA Cet, RW Dor, UX Eri, YY Eri, BV Eri, CT Eri, SZ Hor, AD Phe, TYPup, HI Pup, and TZ Pyx. All are double-lined binaries, and all exceptthe last one are contact binaries. The orbital data must be consideredpreliminary because of the relatively small number of observations(6-12), a circumstance that is partly compensated by the good definitionof the broadening functions used for the radial velocity determinations.Based on data obtained at the European Southern Observatory.
| Dynamical evolution of active detached binaries on the logJo-logM diagram and contact binary formation
Orbital angular momentum (OAM, Jo), systemic mass (M) andorbital period (P) distributions of chromospherically active binaries(CAB) and W Ursae Majoris (W UMa) systems were investigated. Thediagrams of and logJo-logM were formed from 119 CAB and 102 WUMa stars. The logJo-logM diagram is found to be mostmeaningful in demonstrating dynamical evolution of binary star orbits. Aslightly curved borderline (contact border) separating the detached andthe contact systems was discovered on the logJo-logM diagram.Since the orbital size (a) and period (P) of binaries are determined bytheir current Jo, M and mass ratio, q, the rates of OAM loss(dlogJo/dt) and mass loss (dlogM/dt) are primary parametersto determine the direction and the speed of the dynamical evolution. Adetached system becomes a contact system if its own dynamical evolutionenables it to pass the contact border on the logJo-logMdiagram. The evolution of q for a mass-losing detached system is unknownunless the mass-loss rate for each component is known. Assuming q isconstant in the first approximation and using the mean decreasing ratesof Jo and M from the kinematical ages of CAB stars, it hasbeen predicted that 11, 23 and 39 per cent of current CAB stars wouldtransform to W UMa systems if their nuclear evolution permits them tolive 2, 4 and 6 Gyr, respectively.
| Search for associations containing young stars (SACY). I. Sample and searching method
We report results from a high-resolution optical spectroscopic surveyaimed to search for nearby young associations and young stars amongoptical counterparts of ROSAT All-Sky Survey X-ray sources in theSouthern Hemisphere. We selected 1953 late-type (B-V~≥~0.6),potentially young, optical counterparts out of a total of 9574 1RXSsources for follow-up observations. At least one high-resolutionspectrum was obtained for each of 1511 targets. This paper is the firstin a series presenting the results of the SACY survey. Here we describeour sample and our observations. We describe a convergence method in the(UVW) velocity space to find associations. As an example, we discuss thevalidity of this method in the framework of the β Pic Association.
| W Ursae Majoris Contact Binary Variables as X-Ray Sources
We present cross-identification of archived ROSAT X-ray point sourceswith W UMa variable stars found in the All-Sky Automated Survey. A totalof 34 W UMa stars have been found associated with X-ray emission. Wecompute the distances of these W UMa systems and hence their X-rayluminosities. Our data support the ``supersaturation'' phenomenon seenin these fast rotators, namely that the faster a W UMa star rotates, theweaker its X-ray luminosity.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Evolution of Close Binary Systems
We collect data on the masses, radii, etc., of three classes of closebinary stars: low-temperature contact binaries (LTCBs), near-contactbinaries (NCBs), and detached close binaries (DCBs). We restrictourselves to systems in which (1) both components are, at leastarguably, near the main sequence, (2) the periods are less than a day,and (3) there is both spectroscopic and photometric analysis leading toreasonably reliable data. We discuss the possible evolutionaryconnections between these three classes, emphasizing the roles played bymass loss and angular momentum loss in rapidly rotating cool stars. Wedescribe a new mechanism, differential rotation as observed in the Sun,which can explain the remarkable efficiency of heat transport in theouter envelopes of contact binaries.
| Kinematics of W Ursae Majoris type binaries and evidence of the two types of formation
We study the kinematics of 129 W UMa binaries and we discuss itsimplications on the contact binary evolution. The sample is found to beheterogeneous in the velocity space. That is, kinematically younger andolder contact binaries exist in the sample. A kinematically young (0.5Gyr) subsample (moving group) is formed by selecting the systems thatsatisfy the kinematical criteria of moving groups. After removing thepossible moving group members and the systems that are known to bemembers of open clusters, the rest of the sample is called the fieldcontact binary (FCB) group. The FCB group is further divided into fourgroups according to the orbital period ranges. Then, a correlation isfound in the sense that shorter-period less-massive systems have largervelocity dispersions than the longer-period more-massive systems.Dispersions in the velocity space indicate a 5.47-Gyr kinematical agefor the FCB group. Compared with the field chromospherically activebinaries (CABs), presumably detached binary progenitors of the contactsystems, the FCB group appears to be 1.61 Gyr older. Assuming anequilibrium in the formation and destruction of CAB and W UMa systems inthe Galaxy, this age difference is treated as an empirically deducedlifetime of the contact stage. Because the kinematical ages (3.21, 3.51,7.14 and 8.89 Gyr) of the four subgroups of the FCB group are muchlonger than the 1.61-Gyr lifetime of the contact stage, the pre-contactstages of the FCB group must dominantly be producing the largedispersions. The kinematically young (0.5 Gyr) moving group covers thesame total mass, period and spectral ranges as the FCB group. However,the very young age of this group does not leave enough room forpre-contact stages, and thus it is most likely that these systems wereformed in the beginning of the main sequence or during thepre-main-sequence contraction phase, either by a fission process or mostprobably by fast spiralling in of two components in a common envelope.
| On the properties of contact binary stars
We have compiled a catalogue of light curve solutions of contact binarystars. It contains the results of 159 light curve solutions. Theproperties of contact binary stars were studied using the cataloguedata. As is well known since Lucy's (\cite{Lucy68a},b) and Mochnacki's(\cite{Mochnacki81}) studies, primary components transfer their ownenergy to the secondary star via the common envelope around the twostars. This transfer was parameterized by a transfer parameter (ratio ofthe observed and intrinsic luminosities of the primary star). We provethat this transfer parameter is a simple function of the mass andluminosity ratios. We introduced a new type of contact binary stars: Hsubtype systems which have a large mass ratio (q>0.72). These systemsshow behaviour in the luminosity ratio- transfer parameter diagram thatis very different from that of other systems and according to ourresults the energy transfer rate is less efficient in them than in othertypes of contact binary stars. We also show that different types ofcontact binaries have well defined locations on the mass ratio -luminosity ratio diagram. Several contact binary systems do not followLucy's relation (L2/L1 =(M2/M1)0.92). No strict mass ratio -luminosity ratio relation of contact binary stars exists.Tables 2 and 3 are available in electronic form athttp://www.edpsciences.org
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| Period and light variations for the cool, overcontact binary BX Pegasi
New charge-coupled device photometric observations of the W UMa-typebinary BX Pegasi (BX Peg) were collected on four nights from 1999October to 2000 September. The light curve was covered completely ineach season. Seven new times of minimum light were determined. It wasfound that the orbital period of the system has varied recently in asinusoidal way, superimposed on a downward parabolic variation. Thelong-term period decrease rate is deduced as dP/dt=-8.62 or 9.59 ×10-8 d yr-1, which can be interpreted as eithermass transfer from the more massive cool star to the less massive hotcomponent, or as the combination of mass transfer and angular momentumloss due to a magnetic stellar wind. The period and amplitude of thesinusoidal period variation were calculated to be about 35.3 yr and0.015 d, respectively. The light curves of BX Peg are asymmetric andshow year-to-year light variability. A spot model has been applied toanalyse these light curves. After using the light curves of 1999 asreference ones, we solve those of 2000 by adjusting only the spotparameters. One cool-spot model on the cool secondary satisfies theobserved light curves of both 1999 and 2000 quite well and shows a goodrepresentation of the BX Peg system for both the photospheric and spotdescriptions. The brightness variations of BX Peg are not coincidentwith the period variations and so do not conform to a prediction of theApplegate mechanism. We think the most likely cause of the cyclicalvariation is the light-time effect due to a third body, although nothird light was detected in the light-curve analysis. If it exists, thehypothetical object could be a very red main-sequence star or a whitedwarf. We have solved anew the historical published light curve for onlythe spot parameters and these closely resemble our spot parameters. Wespeculate that this result is associated with the small coronalsaturation of the cool star of the system.
| Up-to-Date Linear Elements of Eclipsing Binaries
About 1800 O-C diagrams of eclipsing binaries were analyzed and up-todate linear elements were computed. The regularly updated ephemerides(as a continuation of SAC) are available only in electronic form at theInternet address: http://www.as.ap.krakow.pl/ephem/.
| Minima of Eclipsing Binaries in the ASAS-2 Database
Not Available
| Catalogue of the field contact binary stars
A catalogue of 361 galactic contact binaries is presented. Listedcontact binaries are divided into five groups according to the type andquality of the available observations and parameters. For all systemsthe ephemeris for the primary minimum, minimum and maximum visualbrightness and equatorial coordinates are given. If available,photometric elements, (m1+m2)sin3i,spectral type, parallax and magnitude of the O'Connell effect are alsogiven. Photometric data for several systems are augmented by newobservations. The quality of the available data is assessed and systemsrequiring modern light-curve solutions are selected. Selectedstatistical properties of the collected data are discussed.
| CN Andromedae: A Broken-Contact Binary?
We solve new UBV light curves of the eclipsing binary CN Andromedaesimultaneously with radial velocities by the method of differentialcorrections. We find it semidetached with the more massive star fillingits limiting lobe and the less massive star very close to lobe-filling.Our solutions of earlier light curves have the same configuration.Within the uncertainties, the system may be in a broken or a marginalcontact stage as it undergoes thermal relaxation oscillations. The lightcurves are strongly affected by a large dark spot at high latitude onthe more massive star and by a bright substellar spot on the companion.Spot locations and temperatures do not differ significantly from epochto epoch. Based on single star evolutionary models, the primary's mass(1.299+/-0.045 Msolar) and radius (1.425+/-0.016Rsolar) are consistent with those of a star of age2.9×109 yr that will leave the main sequence in another2.0×109 yr. The orbital period has decreased during thepast 50 yr according to traditional eclipse timings and also accordingto our generalized light and velocity solutions that include a referenceepoch T0 and a rate of period change dP/dt. We finddP/dt=-0.01951+/-0.00054 s yr-1, which is consistent withmass transfer from the more to the less massive star of1.4×10-7 Msolar yr-1, assuming nomass is lost from the system. Reliable absolute dimensions aredetermined.
| A CCD Photometric Study of the Contact Binary V396 Monocerotis
Complete BV light curves of the W Ursae Majoris binary V396 Mon arepresented. The present CCD photometric observations reveal that thelight curves of the system are obviously asymmetric, with the primarymaximum brighter than the secondary maximum (the ``O'Connell effect'').The light curves are analyzed by means of the latest version of theWilson-Devinney code. The results show that V396 Mon is a W-subtype WUMa contact binary with a mass ratio of 0.402. The asymmetry of thelight curves is explained by a cool spot on the secondary component. Thenature of the overluminosity of the secondary of a W UMa-type system isanalyzed. It is shown that the overluminosity of the secondary isclearly related to the mass of the primary and that, for a W UMa system,the higher the mass of the primary, the greater the overluminosity ofthe secondary. In addition, the overluminosity of the secondary is alsorelated to its own density: the lower the density of the secondary, thegreater its overluminosity.
| ROSAT all-sky survey of W Ursae Majoris stars and the problem of supersaturation
From ROSAT all-sky survey (RASS) data we obtained X-ray fluxes for 57 WUMa type contact systems. In our sample we detected three stars whichare the shortest period main sequence binaries ever found as X-raysources. For stars with (B-V)_0 < 0.6 the normalized X-ray fluxdecreases with a decreasing color index but for (B-V)_0 > 0.6 aplateau is reached, similar to the saturation level observed for single,rapidly rotating stars. The X-ray flux of W UMa stars is about 4-5 timesweaker than that of the fastest rotating single stars. Because earlytype, low activity variables have longer periods, an apparentperiod-activity relation is seen among our stars, while cool stars with(B-V)_0 > 0.6 and rotation periods between 0.23 and 0.45 days do notshow any such relation. The lower X-ray emission of the single, ultrafast rotators (UFRs) and W UMa stars is interpreted as the result of adecreased coronal filling factor. The physical mechanisms responsiblefor the decreased surface coverage differs for UFRs and W UMa systems.For UFRs we propose strong polar updrafts within a convection zone,driven by nonuniform heating from below. The updrafts should beaccompanied by large scale poleward flows near the bottom of theconvective layer and equatorward flows in the surface layers. The flowsdrag dynamo generated fields toward the poles and create a field-freeequatorial region with a width depending on the stellar rotation rate.For W UMa stars we propose that a large scale horizontal flow embracingboth stars will prevent the magnetic field from producing long-livedstructures filled with hot X-ray emitting plasma. The decreased activityof the fastest rotating UFRs increases the angular momentum loss timescale of stars in a supersaturated state. Thus the existence of a periodcutoff and a limiting mass of W UMa stars can be naturally explained.
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| The All Sky Automated Survey. Catalog of about 3800 Variable Stars
Results of the first two years of observations using the All SkyAutomated Survey prototype camera are presented. More than 140000 starsin 50 Selected Fields covering 300 square degrees were monitored eachclear night in the I-band resulting in the ASAS Photometric I-bandCatalog containing over 5*10^7 individual measurements. Nightlymonitoring of over 100 standard stars confirms that most of our dataremains within sigma_I=0.03 mag of the standard I system. Search for thestars varying on the time scales longer than a day revealed about 3800variable stars (mostly irregular, pulsating and binaries) brighter than13 mag. Only 630 of them are known or suspected variables included inthe GCVS (Kholopov 1985). Among the stars brighter than I approx 7.5(which are saturated on our frames) we have found about 50 variables (12are in the GCVS, 6 others in the Hipparcos catalog). Because of thelarge volume of data we present here only selected tables and lightcurves, but the complete ASAS Catalog of Variable Stars (currentlydivided into Periodic and Miscellaneous sections) and all photometricdata are available on the Internethttp://www.astrouw.edu.pl/~gp/asas/asas.html orhttp://archive.princeton.edu/~asas/
| ROSAT HRI catalogue of X-ray sources in the LMC region
All 543 pointed observations of the ROSAT High Resolution Imager (HRI)with exposure times higher than 50 s and performed between 1990 and 1998in a field of 10°\ x 10°\ covering the Large Magellanic Cloud(LMC) were analyzed. A catalogue was produced containing 397 X-raysources with their properties measured by the HRI. The list wascross-correlated with the ROSAT Position Sensitive Propotional Counter(PSPC) source catalogue presented by Haberl & Pietsch (1999) inorder to obtain the hardness ratios for the X-ray sources detected byboth instruments. 138 HRI sources are contained in the PSPC catalogue,259 sources are new detections. The spatial resolution of the HRI washigher than that of the PSPC and the source position could be determinedwith errors mostly smaller than 15'' which are dominated by systematicattitude errors. After cross-correlating the source catalogue with theSIMBAD data base and the TYCHO catalogue 94 HRI sources were identifiedwith known objects based on their positional coincidence and X-rayproperties. Whenever more accurate coordinates were given in cataloguesor literature for identified sources, the X-ray coordinates werecorrected and the systematic error of the X-ray position was reduced.For other sources observed simultaneously with an identified source thecoordinates were improved as well. In total the X-ray position of 254sources could be newly determined. The catalogue contains 39 foregroundstars, 24 supernova remnants (SNRs), five supersoft sources (SSSs), nineX-ray binaries (XBs), and nine AGN well known from literature. Anothereight sources were identified with known candidates for these sourceclasses. Additional 21 HRI sources are suggested in the present work ascandidates for SNR, X-ray binary in the LMC, or background AGN becauseof their extent, hardness ratios, X-ray to optical flux ratio, or fluxvariability. Table 4 is only and Tables 1--3 are also available 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
| A ROSAT PSPC catalogue of X-ray sources in the LMC region
We analyzed more than 200 ROSAT PSPC observations in a 10 by 10 degreefield centered on the Large Magellanic Cloud (LMC) and performed between1990 and 1994 to derive a catalogue of X-ray sources. The list contains758 sources with their X-ray properties. From cross-correlations of thePSPC catalogue with the SIMBAD data base and literature searches we givelikely identifications for 144 X-ray sources based on positionalcoincidence, but taking into account X-ray properties like hardnessratios and source extent. 46 known sources are associated with supernovaremnants (SNRs) and candidates in the LMC, most of them already detectedby previous X-ray missions. Including the new candidates from\cite[Haberl & Pietsch (1999)]{HP99} based on variability studies ofthe sources in our PSPC catalogue, the number of X-ray binaries in theLMC increased to 17 and that of the supersoft sources (SSSs) to 9. Theremaining ~ 50% of the identified sources comprise mainly foregroundstars (up to 57) and background extragalactic objects (up to 15). Theoften distinguished X-ray properties of the different source types wereused for a first classification of new, unknown X-ray sources. Eight newPSPC sources are classified as SNRs from their hardness ratios and onepromising new SNR candidate with extended X-ray emission is foundfurther north than all known SNRs. Three soft X-ray sources havehardness ratios compatible to those of the known SSSs. A selection onhardness ratios and X-ray to optical flux ratio further suggests 27foreground stars and 3 AGN.
| The All Sky Automated Survey. The Catalog of the Periodic Variable Stars in the Selected Fields
Results of the first two months of observations using the All SkyAutomated Survey prototype camera are presented. More than 45000 starsin 24 Selected Fields covering 140 square degrees were monitored a fewtimes per night resulting in the I-band catalog containing 10^7individual measurements. Period search revealed 126 variable starsbrighter than 13 mag with periods less than 20^d. Only 30 of them areknown variable stars included in the General Catalogue of VariableStars. The other 90 objects are newly detected variable stars -- mainlyeclipsing binaries (75%) and pulsating stars (17%). We estimate thatcompleteness of the current catalogs of variable stars is smaller than50% already for the stars brighter than 9mag. The Catalog is accessibleover the WWW: http://www.astrouw.edu.pl/~gp/asas/asas.html
| Absolute Magnitude Calibration for the W UMa-Type Systems Based on HIPPARCOS Data
Hipparcos parallax data for 40 contact binary stars of the W UMa-type(with epsilon M_V < 0.5) are used to derive a new, (B-V)-basedabsolute-magnitude calibration of the form M_V = M_V(log P,B-V). Thecalibration covers the ranges 0.26 < (B-V)_0 < 1.14, 0.24 < P< 1.15 day, and 1.4 < M_V < 6.1; it is based on a solutionweighted by relative errors in the parallaxes (2.7% to 24%). Previouscalibrations have not been based on such a wide period and color space,and while they have been able to predict M_V with sufficient accuracyfor systems closely following the well-known period-color relation, thenew calibration should be able to give also good predictions for moreexotic ``outlying'' contact binary systems. The main limitations of thiscalibration are the inadequate quality of the ground-based photometricdata, and the restriction to the (B-V) index, which is more sensitive tometallicity effects than the (V-I) index; metallicities are, however,basically unknown for the local W UMa-type systems. (SECTION: Stars)
| The properties of W Ursae Majoris contact binaries: new results and old problems.
The physical properties of W UMa binary systems are revisited on thebasis of the observational data published in the last decade and of therecent theoretical studies on angular-momentum-loss-driven secularevolution. The absolute elements (masses, radii, luminosities) arederived by an inference method and a calibration based on the availablehigh quality spectroscopic orbits. The derived age (8Gy) agrees with theestimate of Guinan and Bradstreet from space motions. The analysis ofthe resulting physical parameters shows little correlation between thestandard classification in A and W subtype (first proposed by Binnendijk(1970) and only related to the light curve morphology) and theevolutionary status and origin of the systems. Most A-subtype systemsseem to have no evolutionary link with W-subtype ones. The relationbetween total mass and mass ratio for the "bona fide" sample alsosuggests that mass loss from the system may play an important role.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
| Constel·lació: | Dorado |
| Ascensió Recta: | 05h18m32.53s |
| Declinació: | -68°13'32.7" |
| Magnitud Aparent: | 11.164 |
| Moviment propi RA: | 100.5 |
| Moviment propi Dec: | 52.8 |
| B-T magnitude: | 12.005 |
| V-T magnitude: | 11.234 |
Catàlegs i designacions:
|
