Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Photoelectric Minima of Some Eclipsing Binary Stars
We present 119 minima times of 47 eclipsing binaries.
| A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| On the Temperature-Emission Measure Distribution in Stellar Coronae
Strong peaks in the emission measure-temperature (EM-T ) distributionsin the coronae of some binary stars are associated with the presence ofhot (107 K), dense (up to 1013 cm -3)plasma. These peaks are very reminiscent of those predicted to arise inan impulsively heated solar corona. A coronal model comprised of manyimpulsively heated strands is adapted to stellar parameters. It is shownthat the properties of the EM-T distribution can be accounted for ingeneral terms provided the emission comes from many very small loops(length under 103 km) with intense magnetic fields (1 kG)distributed across part of the surface of the star. The heating requiresevents that generally dissipate between 1026 and 1028 ergs, which is in the range of solar microflares. This impliesthat such stars must be capable of generating regions of localizedintense magnetic fields.
| Mass loss and orbital period decrease in detached chromospherically active binaries
The secular evolution of the orbital angular momentum (OAM), thesystemic mass (M=M1+M2) and the orbital period of114 chromospherically active binaries (CABs) were investigated afterdetermining the kinematical ages of the subsamples which were setaccording to OAM bins. OAMs, systemic masses and orbital periods wereshown to be decreasing by the kinematical ages. The first-orderdecreasing rates of OAM, systemic mass and orbital period have beendetermined as per systemic OAM, per systemic mass and per orbitalperiod, respectively, from the kinematical ages. The ratio of d logJ/dlogM= 2.68, which were derived from the kinematics of the presentsample, implies that there must be a mechanism which amplifies theangular momentum loss (AML) times in comparison to isotropic AML ofhypothetical isotropic wind from the components. It has been shown thatsimple isotropic mass loss from the surface of a component or bothcomponents would increase the orbital period.
| 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.
| New Minima of Selected Eclipsing Close Binaries
We present 180 CCD and photoelectric times of minima of selected closeeclipsing binaries.
| Optical and EUV observations of solar flare kernels
We present high-resolution spectral observations, covering the entireoptical region (3800-9000 Å), of a solar flare observed during amulti-wavelength campaign. The flare, recorded on 2002 January 11, was amedium solar flare event (GOES class C7.5). The spectral observationswere carried out using the Hamilton echelle spectrograph on thecoudé auxiliary telescope at Lick Observatory and with theCoronal Diagnostic Spectrometer (CDS) on board SoHO. The highsignal-to-noise optical spectra are analysed using the same techniquesas we applied to stellar flare data. Hα images obtained at BigBear Solar Observatory (BBSO), plus magnetograms obtained with theMichelson Doppler Imager (MDI) on board SoHO and Transition Region AndCoronal Explorer (TRACE) 1600 Å were used in the flare analysis.We observe stellar-like behaviour in the main solar chromosphericactivity indicators, which show either filling-in or emission during theflare. We find that the Balmer and Ca II lines show asymmetric profiles,with red-shifted wings and blue-shifted cores. This behaviour could beexplained by material expanding. During the flare, the Mg i and Fe ilines show a filling-in of the line profile indicating that the flareaffected the lower atmosphere. There is some evidence for pre-flareheating as seen in Fe xix 592 Å. Furthermore, O v 629 Åshows an increase in flux some 10 min. before the coronal lines, perhapsindicating particle beam heating in the initial stages of the flare. Wehave also determined the main physical parameters at flare maximum. Theelectron densities and electron temperatures found for the flare implythat the Balmer emitting plasma originates in the chromosphere. Thephysical parameters obtained for the modelled flare are consistent withpreviously derived values for solar flares.
| Photoelectric Minima of Some Eclipsing Binary Stars
We present 70 minima times of 35 eclipsing binaries.
| Investigating Ca II Emission in the RS Canum Venaticorum Binary ER Vulpeculae Using the Broadening Function Formalism
The synchronously rotating G stars in the detached, short-period (0.7days), partially eclipsing binary ER Vul are the most chromosphericallyactive solar-type stars known. We have monitored activity in the Ca II Hand K reversals for almost an entire orbit. Rucinski's broadeningfunction formalism allows the photospheric contribution to beobjectively subtracted from the highly blended spectra. The power of thebroadening function technique is also demonstrated by the good agreementof radial velocities with those measured by others from less crowdedspectral regions. In addition to strong Ca II emission from the primaryand secondary, there appears to be a high-velocity stream flowing ontothe secondary, where it stimulates a large active region on the surface30°-40° in advance of the subbinary longitude. A model lightcurve with a spot centered on the same longitude also gives the best fitto the observed light curve. A flare with ~13% more power than at otherphases was detected in one spectrum. We suggest that ER Vul may offer amagnified view of the more subtle chromospheric effects synchronized toplanetary revolution seen in certain 51 Peg-type systems.
| The `solar model problem' solved by the abundance of neon in nearby stars
The interior structure of the Sun can be studied with great accuracyusing observations of its oscillations, similar to seismology of theEarth. Precise agreement between helioseismological measurements andpredictions of theoretical solar models has been a triumph of modernastrophysics. A recent downward revision by 25-35 per cent of the solarabundances of light elements such as C, N, O and Ne (ref. 2) has,however, broken this accordance: models adopting the new abundancesincorrectly predict the depth of the convection zone, the depth profilesof sound speed and density, and the helium abundance. The discrepanciesare far beyond the uncertainties in either the data or the modelpredictions. Here we report neon-to-oxygen ratios measured in a sampleof nearby solar-like stars, using their X-ray spectra. The abundanceratios are all very similar and substantially larger than the recentlyrevised solar value. The neon abundance in the Sun is quite poorlydetermined. If the Ne/O abundance in these stars is adopted for the Sun,the models are brought back into agreement with helioseismologymeasurements.
| Catalogue of the orbital elements, masses, and luminosities for short-periodic RS CVn-type eclipsing systems
New data on the orbital elements, masses, and luminosities werecollected for 31 pre-contact binary systems of short-periodic {RS}{CVn}-type. We treat the catalogued data statistically in order toaccurately define the properties and evolutionary status of each classsystem. The ages of pre-contact systems were estimated by the isochronemethod. Numerous comments and bibliographic references to the catalogueare also included.
| Hot Jupiters and Hot Spots: The Short- and Long-Term Chromospheric Activity on Stars with Giant Planets
We monitored the chromospheric activity in the Ca II H and K lines of 13solar-type stars (including the Sun): 8 of them over 3 years at theCanada-France-Hawaii Telescope (CFHT) and 5 in a single run at the VeryLarge Telescope (VLT). A total of 10 of the 13 targets have closeplanetary companions. All of the stars observed at the CFHT showlong-term (months to years) changes in H and K intensity levels. Fourstars display short-term (days) cyclical activity. For two, HD 73256 andκ1 Cet, the activity is likely associated with anactive region rotating with the star; however, the flaring in excess ofthe rotational modulation may be associated with a hot Jupiter. Aplanetary companion remains a possibility for κ1 Cet.For the other two, HD 179949 and υ And, the cyclic variation issynchronized to the hot Jupiter's orbit. For both stars thissynchronicity with the orbit is clearly seen in two out of three epochs.The effect is only marginal in the third epoch at which the seasonallevel of chromospheric activity had changed for both stars. Short-termchromospheric activity appears weakly dependent on the mean K linereversal intensities for the sample of 13 stars. In addition, asuggestive correlation exists between this activity and theMpsini of the star's hot Jupiter. Because of their smallseparation (<=0.1 AU), many of the hot Jupiters lie within theAlfvén radius of their host stars, which allows a direct magneticinteraction with the stellar surface. We discuss the conditions underwhich a planet's magnetic field might induce activity on the stellarsurface and why no such effect was seen for the prime candidate, τBoo. This work opens up the possibility of characterizing planet-starinteractions, with implications for extrasolar planet magnetic fieldsand the energy contribution to stellar atmospheres.Based on observations collected at the Canada-France-Hawaii Telescopeoperated by the National Research Council of Canada, the Centre Nationalde la Recherche Scientifique of France, and the University of Hawaii, aswell as data from the European Southern Observatory's Very LargeTelescope, Chile (programme ESO 73.C-0694).
| Photoelectric Minima of Eclipsing Binaries
Not Available
| Spectroscopic and Photometric Observations of the Short-Period RS CVn-Type Star UV Piscium
High-resolution spectroscopic observations around the Hα line andlong-term BVRI photometry of the eclipsing short-period RS CVn star UVPsc are presented. The simultaneous solution of our radial velocitycurves and light curves yields the following values for the globalparameters of the components: M1=1.1 Msolar,M2=0.81 Msolar, R1=1.14Rsolar, and R2=0.85 Rsolar. Themeasured rotational broadening of the spectral lines corresponds toequatorial velocities V1=68.3 and V2=53.3 kms-1. Our spectral data reveal high activity of the twostellar components and very fast variability of the Hα line in thecenter of the primary eclipse. Modeling our photometric data showstrends in the starspot behavior. The trend toward active longitude beltscan have occasional exceptions. The observed secular luminosity decreasein 1999 may signal the onset of a new magnetic activity cycle.
| The Density of Coronal Plasma in Active Stellar Coronae
We have analyzed high-resolution X-ray spectra of a sample of 22 activestars observed with the High Energy Transmission Grating Spectrometer onChandra in order to investigate their coronal plasma density. Densitieswere investigated using the lines of the He-like ions O VII, Mg XI, andSi XIII. Si XIII lines in all stars of the sample are compatible withthe low-density limit (i.e., ne<~1013cm-3), casting some doubt on results based on lowerresolution Extreme Ultraviolet Explorer (EUVE) spectra finding densitiesne>1013 cm-3. Mg XI lines betray thepresence of high plasma densities up to a few times 1012cm-3 for most of the sources with higher X-ray luminosity(>~1030 ergs s-1) stars with higherLX and LX/Lbol tend to have higherdensities at high temperatures. Ratios of O VII lines yield much lowerdensities of a few times 1010 cm-3, indicatingthat the ``hot'' and ``cool'' plasma resides in physically differentstructures. In the cases of EV Lac, HD 223460, Canopus, μ Vel, TYPyx, and IM Peg, our results represent the first spectroscopic estimatesof coronal density. No trends in density-sensitive line ratios withstellar parameters effective temperature and surface gravity were found,indicating that plasma densities are remarkably similar for stars withpressure scale heights differing by up to 3 orders of magnitude. Ourfindings imply remarkably compact coronal structures, especially for thehotter (~7 MK) plasma emitting the Mg XI lines characterized by thecoronal surface filling factor, fMgXI, ranging from10-4 to 10-1, while we find fOVIIvalues from a few times 10-3 up to ~1 for the cooler (~2 MK)plasma emitting the O VII lines. We find that fOVIIapproaches unity at the same stellar surface X-ray flux level ascharacterizes solar active regions, suggesting that these stars becomecompletely covered by active regions. At the same surface flux level,fMgXI is seen to increase more sharply with increasingsurface flux. These results appear to support earlier suggestions thathot 107 K plasma in active coronae arises from flaringactivity and that this flaring activity increases markedly once thestellar surface becomes covered with active regions. Comparison of ourmeasured line fluxes with theoretical models suggests that significantresidual model inaccuracies might be present and, in particular, thatcascade contributions to forbidden and intercombination lines resultingfrom dielectronic recombination might be to blame.
| On the sizes of stellar X-ray coronae
Spatial information from stellar X-ray coronae cannot be assesseddirectly, but scaling laws from the solar corona make it possible toestimate sizes of stellar coronae from the physical parameterstemperature and density. While coronal plasma temperatures have longbeen available, we concentrate on the newly available densitymeasurements from line fluxes of X-ray lines measured for a large sampleof stellar coronae with the Chandra and XMM-Newton gratings. We compileda set of 64 grating spectra of 42 stellar coronae. Line counts of strongH-like and He-like ions and Fe XXI lines were measured with the CORAsingle-purpose line fitting tool by \cite{newi02}. Densities areestimated from He-like f/i flux ratios of O VII and Ne IX representingthe cooler (1-6 MK) plasma components. The densities scatter between logne ≈ 9.5-11 from the O VII triplet and between logne ≈ 10.5-12 from the Ne IX triplet, but we caution thatthe latter triplet may be biased by contamination from Fe XIX and Fe XXIlines. We find that low-activity stars (as parameterized by thecharacteristic temperature derived from H- and He-like line flux ratios)tend to show densities derived from O VII of no more than a few times1010 cm-3, whereas no definitive trend is foundfor the more active stars. Investigating the densities of the hotterplasma with various Fe XXI line ratios, we found that none of thespectra consistently indicates the presence of very high densities. Weargue that our measurements are compatible with the low-density limitfor the respective ratios (≈ 5× 1012cm-3). These upper limits are in line with constant pressurein the emitting active regions. We focus on the commonly used \cite{rtv}scaling law to derive loop lengths from temperatures and densitiesassuming loop-like structures as identical building blocks. We derivethe emitting volumes from direct measurements of ion-specific emissionmeasures and densities. Available volumes are calculated from theloop-lengths and stellar radii, and are compared with the emittingvolumes to infer filling factors. For all stages of activity we findsimilar filling factors up to 0.1.Appendix A is only available in electronic form athttp://www.edpsciences.org
| Spectroscopic and photometric observations of the short-period RS CVn-type star BH Virginis
High-resolution spectroscopic observations around theHα line and BVRI photometry from 1993 to 2003 of theeclipsing short-period RS CVn star BH Vir are presented. Thesimultaneous solution of our radial velocity curves and light curvesyielded the following values for global parameters of the components:M1= 1.173 ± 0.006 Mȯ; M2=1.046 ± 0.005 Mȯ; R1= 1.22 ±0.05 Rȯ; R2= 1.11 ± 0.04Rȯ; i= 87.5° ±0.8°. The measured rotational broadening of the spectrallines corresponds to equatorial velocities V1 = 79.8 kms-1 and V2= 68.4 km s-1. Our datareveal considerable Hα emission excess of the twostellar components. We modelled the photometric data to find the sizeand location of the starspots for each year. The established decreasingtrend of the spot latitudes may indicate a latitudinal cycle of at leasta decade.Based on spectral observations collected at the National AstronomicalObservatory at Rozhen and photometric observations collected at Mt.Laguna Observatory operated by San Diego State University, research wassupported in part by the NATO Linkage grant No. PST.CLG.978810, grantNo. 8/2003 of the Shoumen University, a Cottrell College Science Awardof the Research Corporation, and grants from NASA and the GaposchkinFund administered by the AAS Small Grants Program.Photometric and spectroscopic data are 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/cgi-bin/qcat?J/A+A/424/993Figures \ref{Fig2}-\ref{Fig7} and Figs. \ref{Fig9}-\ref{Fig16} are onlyavailable in electronic form at http://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
| 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.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| An improved ephemeris and physical elements of ER Vul
New photoelectric U B{V} and BV RI observations, secured duringAugust-November of 2002 at two observatories distant in local time areanalyzed together with two earlier photometric data sets and allavailable radial velocities to derive the most accurate ephemeris of thebinary and to set limits on its basic physical elements. Theseobservations were obtained as a support for a high-dispersionspectroscopic study, results of which will be published separately. Itwas found that radial velocities and photometric observations spanning53 years can be reconciled with the linear ephemeris T_prim.min.=HJD 2440 182.25628(46) + 0698095113(29)d × E, where the rms errors ofthe last digits are given in brackets. Masses of the stars are 1.02 \fmsand 0.97 \fms and the binary separation is 4.170 Rȯ.There is still a large uncertainty in the stellar radii. We also reportthe discovery of rapid light variations on a time scale of hours seen inthe U band light curve and offer a few thoughts on the futureinvestigation of this binary.This research is based on photoelectric observations from Hvar andUniversity of Victoria Observatories.Table 4 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/415/289
| Spectroscopic and photometric observations of the short-period RS CVn-type star WY Cnc
High-resolution spectroscopic observations around the H\alphaline and BVRI photometry of the eclipsing short-period RS CVn star WYCnc are presented. The solutions of our radial velocity curves and lightcurves yielded the following values for masses and radii of thecomponents: M1=0.84 Mȯ, M2=0.46Mȯ, R1=1.06 Rȯ,R2= 0.65 Rȯ. The measured rotationalbroadenings of the spectral lines correspond to equatorial velocitiesV1=78 km s-1 and V2=49 kms-1. The distortions of our multicolor light curve of WY Cncwere reproduced by two cool spots on the primary star. An increase ofthe out-of-eclipse brightness by 0.1 mag in February 2001 was detected.Our spectra show Hα line in emission from the secondarystar. The chromospheric activity appeared also in the observed emissioncores of the CaII H and K lines as well as the CaI 6494 line.Based on spectral observations collected at the National AstronomicalObservatory at Rozhen, research was supported in part by the NATOLinkage grant No. PST.CLG.978810 and grant No. 1/2002 of the ShoumenUniversity
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| Minimum Times of Several Eclipsing Binaries
We present 26 minima times of 11 eclipsing binaries, observed between1996 and 1999.
| The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun
Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSATsurveys, a sample of 100 stars most luminous in X-rays within or arounda distance of 50 pc is culled. The smallest X-ray luminosity in thesample, in units of 1029 ergs s-1, isLX=9.8 the strongest source in the solar neighborhood is IIPeg, a RS CVn star, at LX=175.8. With respect to the originof X-ray emission, the sample is divided into partly overlapping classesof pre-main-sequence, post-T Tauri, and very young ZAMS objects (typeXY), RS CVn-type binary stars (type RS), other active short-periodbinaries, including binary BY Dra-type objects (type XO), apparentlysingle or long-period binary active evolved stars (type XG), contactbinaries of WU UMa kind (type WU), apparently single or long-periodbinary variable stars of BY Dra kind (type BY), and objects of unknownnature (type X?). Chromospherically active, short-period binaries (RSand XO) make up 40% of the brightest X-ray emitters, followed by youngstars (XY) at 30% and unknown sources (X?) at 15%. The fraction ofspectroscopically single evolved X-ray emitters of spectral classes IVand III is quite large (10%). The sources identified as RS CVn-typestars (RS, 23 objects) are considerably stronger in X-ray than theXY-objects and the other active binaries (XO and WU, 20 objects). Sevenobjects have LX>100, all RS except one XY, viz., BO Mic. Onlyfive (22%) RS objects have LX<25, while only three (10%)XY stars have LX>25. Formally, the limit of LX=25could serve as a statistical criterion to differentiate RS and XY stars.However, the other short-period binaries (including eclipsing stars ofAlgol and β Lyr type) have a distribution of LX verysimilar to the XY objects. The contact binaries (WU) appear to be muchweaker in X-rays than their detached counterparts of RS type, but thesample of the former is too small (three objects) to reach a firmconclusion. Sources matched with giants (either single or in binaries)are found to be significantly harder, with only 7% of hardness ratiosbelow 0, than subgiants (66% of HR1<0) and dwarfs (59% of HR1<0).Almost all objects in the sample are binary or multiple stars; thefraction of components (FC), defined as the total number of componentsin all binary and multiple systems divided by the sum of the totalnumber of components and single stars, is at least 0.90. The FC for theXY objects reaches 0.81, and for the unknown type 0.89. About 70% of RSobjects have also visual or astrometric companions, which makes themhierarchical multiple systems. The RS objects (mostly old, evolvedstars) and the XY stars have quite different kinematics. While the RSobjects move at considerable velocities in apparently random directionswith respect to the local standard of rest, the young stars have smallerand orderly velocities and tend to comprise expanding mini-associationssuch as the β Pic and the Tucana groups. The majority of the youngX-ray active stars belong to the Pleiades stream with the meanheliocentric velocity (U,V,W)=(-9.6,-21.8,-7.7) km s-1.
| A period investigation of two chromospherically active binary stars: RT Coronae Borealis and PW Herculis
Orbital period variations of two chromospherically active binarysystems, RT CrBand PW Her, arepresented. It is shown that the orbital period of RT CrB undergoes acyclic oscillation with a period of 53.9years. For PW Her, an alternatechange, with a period of 42.7years, is found to superimpose on a rapidsecular increase (dP/dt=+3.53×10-6 days/year). If theperiod oscillations of those two systems are caused by the light-timeeffect of a third body, the analysis for RT CrB indicatesthat the third body would be a low-mass main-sequence star, while, forPW Her, the massof the third body should be no less than 7.8 Msolar. Since nospectral lines of the third body were seen in PW Her from thespectroscopic study by Popper [AJ 100 (1990) 247], if there is a thirdbody in the system, it can only be a black hole. However, as bothcomponents in the two binary stars were showing strong chromosphericactivity, the alternate period variations are more plausibly explainedas the result of magnetic activity cycles. No secular period changes ofRT CrB are found,which is in agreement with the detached evolved configuration of thesystem. The long-term period increase of PW Her may indicatethat it is on an active phase of mass transfer(dm/dt=2.17×10-6 Msolar/year).
| 162-nd List of Minima Timings of Eclipsing Binaries by BBSAG Observers
Not Available
| 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.
| Spectroscopic and photometric observations of the short-period RS CVn-type star ER Vulpeculae
Spectroscopic observations around the Halpha line and BVRphotometry of the eclipsing, short-period, RS CVn-type star ER Vul arepresented. The solutions of new radial velocity and light curves yieldedthe following masses and radii of the components: M1=1.16Msun, M2=1.05 Msun, R1=1.25Rsun, R2=1.12 Rsun. The equatorialvelocities corresponding to the measured rotational broadenings of thespectral lines are: Veq1=90 km s-1 andVeq2=80 km s-1. Their ratio is justequal to the ratio of the stellar radii. We detected excess emission inthe Halpha and CaI 6494 lines that changes during theorbital cycle. Our spectral data show the presence of absorbing matteraround the mass center of the system.Based on spectral observations collected at the National AstronomicalObservatory, Bulgaria.
| VLA multifrequency observations of RS CVn binaries
We present multiepoch Very Large Array (VLA) observations at 1.4 GHz,4.9 GHz, 8.5 GHz and 14.9 GHz for a sample of eight RS CVn binarysystems. Circular polarization measurements of these systems are alsoreported. Most of the fluxes observed are consistent with incoherentemission from mildly relativistic electrons. Several systems show anincrease of the degree of circular polarization with increasingfrequency in the optically thin regime, in conflict with predictions bygyrosynchrotron models. We observed a reversal in the sense of circularpolarization with increasing frequency in three non-eclipsing systems:EI Eri, DM UMa and HD8358. We find clear evidence for coherent plasma emission at1.4 GHz in the quiescent spectrum of HD 8358 duringthe helicity reversal. The degrees of polarization of the other twosystems could also be accounted for by a coherent emission process. Theobservations of ER Vul revealed two U-shaped fluxspectra at the highest frequencies. The U-shape of the spectra may beaccounted for by an optically thin gyrosynchrotron source for the lowfrequency part whereas the high frequency part is dominated by a thermalemission component.
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
| Costellazione: | Volpetta |
| Ascensione retta: | 21h02m25.91s |
| Declinazione: | +27°48'26.4" |
| Magnitudine apparente: | 7.356 |
| Distanza: | 49.85 parsec |
| Moto proprio RA: | 87.3 |
| Moto proprio Dec: | 6.2 |
| B-T magnitude: | 8.082 |
| V-T magnitude: | 7.416 |
Cataloghi e designazioni:
|
INHABITED SKY
