Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
B.R.N.O. Contributions #34
Not Available
| Photoelectric Minima of Selected Eclipsing Binaries and Maxima of Pulsating Stars
Not Available
| Multiwavelength optical observations of chromospherically active binary systems. V. FF UMa (2RE J0933+624): a system with orbital period variation
Context: This is the fifth paper in a series aimed at studying thechromospheres of active binary systems using several opticalspectroscopic indicators to obtain or improve orbital solution andfundamental stellar parameters. Aims: We present here the study ofFF UMa (2RE J0933+624), a recently discovered,X-ray/EUV selected, active binary with strong Hα emission. Theobjectives of this work are, to find orbital solutions and definestellar parameters from precise radial velocities and carry out anextensive study of the optical indicators of chromospheric activity. Methods: We obtained high resolution echelle spectroscopic observationsduring five observing runs from 1998 to 2004. We found radial velocitiesby cross correlation with radial velocity standard stars to achieve thebest orbital solution. We also measured rotational velocity bycross-correlation techniques and have studied the kinematic by galacticspace-velocity components (U, V, W) and Eggen criteria. Finally, we havedetermined the chromospheric contribution in optical spectroscopicindicators, from Ca ii H & K to Ca ii IRT lines, using the spectralsubtraction technique. Results: We have found that this system presentsan orbital period variation, higher than previously detected in other RSCVn systems. We determined an improved orbital solution, finding acircular orbit with a period of 3.274 days. We derived the stellarparameters, confirming the subgiant nature of the primary component(MP = 1.67 Mȯ and R sin{i}_P=2.17Rȯ) and obtained rotational velocities (v sin{i}), of33.57 ± 0.45 km s-1 and 32.38 ± 0.75 kms-1 for the primary and secondary components respectively.From our kinematic study, we can deduce its membership to the Castormoving group. Finally, the activity study has given us a betterunderstanding of the possible mechanisms that produce the orbital periodvariation.Based on observations collected with the 2.2 m telescope at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto(Almería, Spain), operated jointly by the Max-Planck Institutfür Astronomie and the Instituto de Astrofísica deAndalucía (CSIC); with the Nordic Optical Telescope (NOT),operated on the island of La Palma jointly by Denmark, Finland, Iceland,Norway and Sweden, in the Spanish Observatorio del Roque de LosMuchachos of the Instituto de Astrofísica de Canarias; with the2.1 m Otto Struve Telescope at McDonald Observatory of the University ofTexas at Austin (USA) and with Hobby-Eberly Telescope, which is a jointproject of the University of Texas at Austin, the Pennsylvania StateUniversity, Stanford University, Ludwig-Maximilians-UniversitätMünchen, and Georg-August-Universität Göttingen. Tables 7and 8 are only available in electronic form at http://www.aanda.org
| 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.
| Variations in the Solar Luminosity, Radius, and Quadrupole Moment as Effects of a Large-Scale Dynamo in the Solar Convection Zone
The effect of large-scale magnetic fields generated by the solar dynamoon the irradiance of the Sun and stratification of the solar convectionzone is studied using a numerical model of a spherical axisymmetricdynamo. This model provides a joint description of the generation oflarge-scale magnetic fields, differential rotation, and convective heattransfer taking into account energy transformations associated with thelarge-scale magnetic fields, as well as the stratification of theconvection zone. The model further develops a previously suggestedself-consistent approach to analyzing solar luminosity variations, basedon the conservation of the energy of the large-scale magnetic fields andturbulent flows. The results indicate that the increase in the solarluminosity near the maximum of the cycle is mainly due to thedissipation of the energy of magnetic fields escaping to above thephotosphere, with the partial conversion of this energy into radiation.In addition, near-photospheric magnetic fields strongly affect thelatitudinal nonuniformity of the cyclic variations in the radiativeflux. The large-scale magnetic field also influences the hydrostaticequilibrium of the convection zone and gives rise to 11-year variationsin the sound speed with a relative amplitude of 10-3. The cyclicmagnetic activity generates oscillations in the quadrupole moment withan amplitude of 4.5 × 10-9(GM ȯ/R ȯ). According to ourestimates, the variations in the solar radius are very small, 10-6R ȯ. Our numerical model is used to estimate the variations in theorbital periods of close binaries whose primaries have the same spectralclass as the Sun. Modulation of the centrifugal force by torsionaloscillations can provide a plausible explanation for variations in theorbital periods of the companion stars in these systems.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| A study of the Mg II 2796.34 Å emission line in late-type normal and RS CVn stars
We carry out an analysis of the Mg II 2796.34 Å emission line inRS CVn stars and make a comparison with the normal stars studied in aprevious paper (Paper I). The sample of RS CVn stars consists of 34objects with known HIPPARCOS parallaxes and observed at high resolutionwith IUE. We confirm that RS CVn stars tend to possess wider Mg II linesthan normal stars having the same absolute visual magnitude. However, wecould not find any correlation between the logarithmic line width logWdeg and the absolute visual magnitude MV (theWilson-Bappu relationship) for these active stars, contrary to the caseof normal stars addressed in Paper I. On the contrary, we find that astrong correlation exists in the (MV, log LMg II)plane (LMg II is the absolute flux in the line). In thisplane, normal and RS CVn stars are distributed along two nearly parallelstraight lines with RS CVn stars being systematically brighter by ~1dex. Such a diagram provides an interesting tool to discriminate activefrom normal stars. We finally analyse the distribution of RS CVn and ofnormal stars in the (log LMg II, log Wdeg) plane,and find a strong linear correlation for normal stars, which can be usedfor distance determinations.
| The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| VLA Radio Positions of Stars: 1978-1995
VLA astrometric positions of the radio emission from 52 stars arereported, from observations obtained between 1978 and 1995. Thepositions of these stars have been obtained and reduced in a uniformmanner. Based on our measurements, the offset of the optical (Hipparcos)frame from the radio reference frame is in agreement with the Hipparcosextragalactic link results, within their mean errors. Comparison of theVLA measurements with the Hipparcos optical positions confirms earlierestimates of the accuracy of these positions as 30 mas. Long-termmeasurements of UX Ari have improved its proper motion.
| Archive of Photometric Plates Obtained at Cluj Astronomical Observatory
The archive of photometric plates obtained at the AstronomicalObservatory of Cluj in the period 1952-1974 is presented. The plateinventory is made within the framework of the Wide-Field Plate Databaseproject. The plates were taken in the regions around 110 variable stars(eclipsing variables and RR Lyrae-type stars), as well as in somecomparison stars regions.
| WW Dra - ein heller BAV-Programmstern mit vielen Problemen.
Not Available
| Zur qualitat der visuellen Beobachtung kurzperiodisch Veranderlicher.
Not Available
| Beobachtungsegebnisse Bundesdeutsche Arbeitsgemainschaft fur Veranderliche Sterne e.V.
Not Available
| Orbital period modulation in close binaries due to cyclic alpha 2-dynamo activity
The question is answered whether dynamo-generated magnetic fields areable to produce such quadrupole terms in the gravity potential which canexplain the observed cyclic orbital variation of RS CVn stars. We startwith spherical dynamo models with outer convection zones but without anydifferential rotation, i.e. with alpha 2-dynamos which areknown as nonoscillating. With the known anisotropic alpha -tensor ofrapidly rotating stars the magnetic modes with the lowest dynamo numbersare nonaxisymmetric with a slow azimuthal drift. We also find, however,stable (i.e. with the lowest dynamo number) axisymmetric oscillatingmodes but only for a very special, highly inhomogeneous alpha -tensor.The dynamo model is a linear one with an arbitrary field amplitude whichcan be scaled in order to reproduce the observations. The star proves tobe as prolate during the maximum of the toroidal field energy and itproves to be as oblate during the maximum of the poloidal field energy.In the time average the influence of the toroidal field dominates andthe star is slightly prolate. From the computed temporal variations ofthe gravitation quadrupole moment a magnetic field of more than105 G is found in order to produce the period modulation oforder 10-5 which has been observed.
| Determination of the Ages of Close Binary Stars on the Main Sequence from Evolutionary Model Stars of Claret and Gimenez
A grid of isochrones, covering a wide range of stellar ages from thezero-age main sequence to 10 billion years, is calculated in the presentwork on the basis of the model stars of Claret and Gimenez withallowance for convective overshoot and mass loss by the components. Theages of 88 eclipsing variables on the main sequence from Andersen'scatalog and 100 chromospherically active stars from Strassmeier'scatalog are calculated with a description of the method of optimuminterpolation. Comparisons with age determinations by other authors aregiven and good agreement is established.
| Statistics of spectroscopic sub-systems in visual multiple stars
A large sample of visual multiples of spectral types F5-M has beensurveyed for the presence of spectroscopic sub-systems. Some 4200 radialvelocities of 574 components were measured in 1994-2000 with thecorrelation radial velocity meter. A total of 46 new spectroscopicorbits were computed for this sample. Physical relations are establishedfor most of the visual systems and several optical components areidentified as well. The period distribution of sub-systems has a maximumat periods from 2 to 7 days, likely explained by a combination of tidaldissipation with triple-star dynamics. The fraction of spectroscopicsub-systems among the dwarf components of close visual binaries withknown orbits is similar to that of field dwarfs, from 11% to 18% percomponent. Sub-systems are more frequent among the components of widevisual binaries and among wide tertiary components to the known visualor spectroscopic binaries - 20% and 30%, respectively. In triple systemswith both outer (visual) and inner (spectroscopic) orbits known, we findan anti-correlation between the periods of inner sub-systems and theeccentricities of outer orbits which must be related to dynamicalstability constraints. Tables 1, 2, and 6 are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/382/118
| 149 Bedeckungssterne der BAV-Programme. Eine Analyse der Beobachtungstatigkeit seit den Angangen.
Not Available
| Beobachtungsergebnisse Bundesdeutsche Arbeitsgemeinschaft fur Veraenderliche Sterne e.V.
Not Available
| The Catania Automatic Photoelectric Telescope on Mt. Etna: a systematic study of magnetically active stars
A photometric monitoring of about 50 magnetically active stars, that arespread almost all over the H-R diagram, was initiated at the mountainstation of Catania Observatory on Mt. Etna (1750-m a.s.l.) in 1992 withan 80-cm robotic telescope (APT-80) built by AutoScope Co. (USA). Thissystematic survey is now approaching its 10th year anniversary. For mostof the stars, quite well defined solar-like spot maps have been derivedfrom UBV data obtained in different epochs. These data have allowed usto investigate some relevant characteristics of spot activity andvariability on stars, and to obtain clear evidence of long-term activitycycles, in the range from a few to about 10 years, on some of theobserved targets. Starspot maps are constructed by using advanced tools,such as massive parallel computing and are based on Maximum Entropy andTikhonov regularization criteria. Selected results are here presented.Our systematic observation program is still underway and a secondAPT80/2, equipped with a CCD camera, will pair the APT80/1 on the samesite. Its operation is foreseen for mid 2002.
| The comparative accuracy of photographic observations of radio stars observed at the Engelhardt Astronomical Observatory
At the Engelhardt Astronomical Observatory (EAO), we observedphotographic positions of 113 Galactic Radio Sources (GRS) in the systemPPM catalogue (Rizvanov & Dautov 1998). Analysis of their accuracyis made by comparison with the Hipparcos catalogue (Perryman et al.1997) and astrometric catalogue of radio stars in the radio window fromthe article of Walter et al. (1991). Table 2 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/375/670
| 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
| 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 ROSAT Bright Survey: II. Catalogue of all high-galactic latitude RASS sources with PSPC countrate CR > 0.2 s-1
We present a summary of an identification program of the more than 2000X-ray sources detected during the ROSAT All-Sky Survey (Voges et al.1999) at high galactic latitude, |b| > 30degr , with countrate above0.2 s-1. This program, termed the ROSAT Bright Survey RBS, isto more than 99.5% complete. A sub-sample of 931 sources with countrateabove 0.2 s-1 in the hard spectral band between 0.5 and 2.0keV is to 100% identified. The total survey area comprises 20391deg2 at a flux limit of 2.4 x 10-12 ergcm-2 s-1 in the 0.5 - 2.0 keV band. About 1500sources of the complete sample could be identified by correlating theRBS with SIMBAD and the NED. The remaining ~ 500 sources were identifiedby low-resolution optical spectroscopy and CCD imaging utilizingtelescopes at La Silla, Calar Alto, Zelenchukskaya and Mauna Kea. Apartfrom completely untouched sources, catalogued clusters and galaxieswithout published redshift as well as catalogued galaxies with unusualhigh X-ray luminosity were included in the spectroscopic identificationprogram. Details of the observations with an on-line presentation of thefinding charts and the optical spectra will be published separately.Here we summarize our identifications in a table which contains opticaland X-ray information for each source. As a result we present the mostmassive complete sample of X-ray selected AGNs with a total of 669members and a well populated X-ray selected sample of 302 clusters ofgalaxies with redshifts up to 0.70. Three fields studied by us remainwithout optical counterpart (RBS0378, RBS1223, RBS1556). While the firstis a possible X-ray transient, the two latter are isolated neutron starcandidates (Motch et al. 1999, Schwope et al. 1999).
| On X-Ray Variability in Active Binary Stars
We have compared the X-ray emissions of active binary stars observed atvarious epochs by the Einstein and ROSAT satellites in order toinvestigate the nature of their X-ray variability. The primary aim ofthis work is to determine whether or not active binaries exhibitlong-term variations in X-ray emission, perhaps analogous to theobserved cyclic behavior of solar magnetic activity. We find that, whilethe mean level of emission of the sample remains steady, comparison ofdifferent ROSAT observations of the same stars shows significantvariation on timescales <~2 yr, with an ``effective variability''ΔI/I=0.32+/-0.04, where I and ΔI represent the mean emissionand variation from the mean emission, respectively. A comparison of theROSAT All-Sky Survey and later pointed observations with earlierobservations of the same stars carried out with Einstein yields onlymarginal evidence for a larger variation (ΔI/I=0.38+/-0.04 forEinstein vs. ROSAT All-Sky Survey and 0.46+/-0.05 for Einstein vs. ROSATpointed) at these longer timescales (~10 yr), thus indicating thepossible presence of a long-term component to the variability. Whetheror not this long-term component is due to the presence of cyclicvariability cannot be decided on the basis of existing data. However,assuming that this component is analogous to the observed cyclicvariability of the Sun, we find that the relative magnitude of thecyclic component in the ROSAT passband can, at most, be a factor of 4,i.e., I_cyc/I_min<4. This is to be compared with the correspondingbut significantly higher solar value of ~10-10^2 derived from GOES,Yohkoh, and Solrad data. These results are consistent with thesuggestions of earlier studies that a turbulent or distributive dynamomight be responsible for the observed magnetic activity on the mostactive, rapidly rotating stars.
| The Wilson-Bappu relation for RS CVn stars
We investigate the extent to which the Wilson-Bappu relationship holdsfor chromospherically active binaries using the Mg ii h&k lines of41 RS CVn stars observed with IUE. The resulting fits are different fromthe relationships obtained for single, less active stars. The parallaxused were those from the hipparcos catalogue, these give a much bettercorrelation than the magnitudes taken from CABS. Within a particularluminosity class the relationship is good, however it tends to breakdown when we incorporate objects ranging in luminosity from class i tov. From model calculations there is very little dependence of the Mg iiline width on effective temperature. The line width does however dependon the column mass at the transition region boundary showing increasedline width at lower column mass. There is also a dependence on thecolumn mass adopted for the temperature minimum, however, the major anddominant parameter is the surface gravity scaling as g(-1/4) . Within aluminosity class more active objects will show larger lines widthsreflecting a higher column mass deeper in the atmosphere, e.g. at thetemperature minimum level.
| Orbital period modulation and quadrupole moment changes in magnetically active close binaries
We discuss the main characteristics of the orbital period modulation inclose binaries with late-type components. We focus on the variousphysical scenarios proposed to explain this phenomenon and, inparticular, Hall's (1989) suggestion that it may be connected withmagnetic activity. Starting from the work of Applegate (1992) and Lanzaet al. (1998a), we develop an integral approach to evaluate thegravitational quadrupole moment of an active star and its variations,which we consider to be an important driver of the observed orbitalperiod changes. The method applies the tensor virial theorem afterChandrasekhar (1961) and directly relates the variation of thequadrupole moment with the changes of kinetic and magnetic energy of thestellar hydromagnetic dynamo. Particular effort has been applied inminimizing the number of free parameters entering the problem. A sampleof 46 close binaries with period changes of alternate signs has beenstudied by our method. The amplitude of the quadrupole moment changeappears to decrease with increasing angular velocity, implying that thetime-variable part of the kinetic energy of rotation varies as delta{cal T}/{cal T} ~ Omega ({-0.93+/-) 0.10}, with a correlationcoefficient of 0.83. The length of the cycle of the orbital periodmodulation seems to be correlated with the angular velocity asPmod ~ Omega ({-0.36+/-) 0.10}, but with a smallercorrelation coefficient of 0.62. These results support the suggestionthat a distributed non-linear dynamo is at work in the convectiveenvelopes of very active stars and that it strongly affects thedifferential rotation. We also discuss the energy budget of the processresponsible for the quadrupole moment variation and find that, onaverage, only ~ 10% of the energy required to maintain the differentialrotation may be lost by dissipation in the turbulent convective envelopeduring a cycle of the orbital period change. The problems of themagnetic field geometry and stability and the relationship between thelength of the activity cycle, as determined by the change of the area ofthe starspots and the orbital period modulation, respectively, are alsoaddressed.
| Period Variation of Six RS CVn-Type Binaries with Possible Light-Time Effect
Not Available
| Mutual irradiation and absorption and surface temperature variation in detached binary systems
In the paper, we calculate the absorbed flux of the mutual irradiationof the components in detached binary systems, and obtain the surfacetemperature distribution on the components. The maximum temperaturedifference over the surface is a measure of the deviation betweenisothermal and equipotential surfaces. For 33 late-type binary systemswith known elements, we evaluate this maximum difference. The resultshows that the isothermal-equipotential deviation is always larger inthe cooler than in the hotter components. This result supports Zhou andLeung's idea that the large difference between ``star-spots'' andsunspots is due to cyclones and anticyclones. The effect of limbdarkening (amounting to over 18% and always larger in the hottercomponent) is included in the consideration.
| Early Radio Positions of Stars
Early radio positions for a sample of 100 Hipparcos stars, threeTycho-only stars, and nine radio stars with optical positions referredto the Hipparcos/ICRS frame are analyzed. The optical proper motions areused to compare the optical and radio positions. From an original sampleof 247 radio positions for the above 112 stars, a set of 220 showscoincidence between the optical and radio centers of emission closerthan 500 mas. This set is analyzed for systematic departures between theoptical and radio positions. A smaller subset of 136 early radiopositions for 72 stars show radio-minus-optical offsets smaller than 100mas and are useful for monitoring of the spin of the Hipparcos frame.
| Mutual irradiation and absorption in detached binary systems and surface temperature variation.
Not Available
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Dragone |
| Ascensione retta: | 16h39m03.98s |
| Declinazione: | +60°41'58.8" |
| Magnitudine apparente: | 8.615 |
| Distanza: | 115.34 parsec |
| Moto proprio RA: | 22.2 |
| Moto proprio Dec: | -57.7 |
| B-T magnitude: | 9.515 |
| V-T magnitude: | 8.69 |
Cataloghi e designazioni:
|
