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Are the W Ursae Majoris-type systems EK Comae Berenices and UX Eridani surrounded by circumstellar matter?
The variations of the orbital periods of two nearly neglected W UMa-typeeclipsing binaries, EK Comae Berenices and UX Eridani, are presentedthrough a detailed analysis of the O C diagrams. It is found that theorbital period of EK Com is decreasing and the period of UX Eridani isincreasing, and several sudden jumps have occurred in the orbitalperiods of both binaries. We analyze the mechanism(s), which mightunderlie the changes of the orbital periods of both systems, and obtainsome new results. The long-term decrease of the orbital period of EKComae Berenices might be caused by the decrease of the orbital angularmomentum due to a magnetic stellar wind (MSW) or by mass transfer fromthe more massive to the less massive component. The secular increase inthe orbital period of UX Eridani might be caused by mass transfer fromthe less massive to the more massive star. The possible mechanisms,which underlie the sudden changes in the orbital periods of the closebinary systems are as the followings: (1) the variations of thestructure due to the variation of the magnetic field; (2) the rapid massexchange between the close binaries and their circumstellar matter.Finally, the evolutionary status of the systems EK Comae Berenices andUX Eridani is discussed.
| 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.
| High-mass X-ray binaries in the Small Magellanic Cloud: the luminosity function
We study a population of compact X-ray sources in the Small MagellanicCloud (SMC) using archival data from the XMM-Newton observatory. Thetotal area of the survey is ~1.5 deg2 with the limitingsensitivity of ~10-14 erg s-1 cm-2,corresponding to a luminosity of ~4.3 × 1033 ergs-1 at the SMC distance. Out of ~150 point sources detectedin the 2-8 keV energy band, ~3/4 are cosmic X-ray background sources,observed through the SMC. Based on the properties of the optical andnear-infrared counterparts of the detected sources, we identified likelyhigh-mass X-ray binary (HMXB) candidates and sources whose nature isuncertain, thus providing lower and upper limits on the luminositydistribution of HMXBs in the observed part of the SMC.The observed number of HMXBs is consistent with the prediction based onstar formation rate (SFR) estimates derived from the supernovaefrequency and an analysis of colour-magnitude diagrams for the stellarpopulation. If, on the contrary, the true value of the SFR is betterrepresented by far-infrared-, Hα- and ultraviolet-basedestimators, then the abundance of HMXBs in the SMC may significantly (bya factor of as much as ~10) exceed the value derived for the Milky Wayand other nearby galaxies. The shape of the observed distribution at thebright end is consistent with the universal HMXB X-ray luminosityfunction. At the faint end, LX<~ 2 × 1035erg s-1, the upper limit on the luminosity function isconsistent with the L-0.6 power law, while the lower limit issignificantly flatter than this.
| Detection of red line asymmetries in LHS 2034
We report very pronounced line asymmetries during a long duration flareon the dM6 star LHS 2034 (AZ Cnc). While all lines of the Balmer seriesand all strong He i lines show these asymmetries, the metal lines donot. This can be explained with the help of PHOENIX model chromospheresconsidering the formation depth of the lines involved. Moreover, theasymmetries persist over about one hour changing shape and amplitude.Fitting the asymmetries with an additional broad Gaussian componentleads us to the scenario of a series of downward propagatingcondensations that decelerate due to the higher density of the lowerchromosphere. In addition, similar but weaker line asymmetries werefound in LHS 2397a.
| Improved astrophysical parameters for the overcontact binary FG Hydrae
Photometric data on FG Hya obtained in 2002 and 2004 are presented.Three data sets show the exchange between A-type, W-type and thevariable O'Connell effects. The photometric mass ratio (q= 0.1115 +/-0.0003) derived from B and V light curves is almost the same as thespectroscopic mass ratio (qsp= 0.112 +/- 0.004). The newphotometric solutions reveal that FG Hya is a deep overcontact binarysystem (f= 85.6 +/- 1.8 per cent) with a spotted massive component. Aperiod investigation, based on all available photoelectric or CCD timesof light minimum, shows that the O-C curve of FG Hya can be explained asa combination of a secular period decrease and a cyclic variation with aperiod of 36.4yr and an amplitude of 0.0289 d. By comparing thevariation of the depth of the primary minimum with the change of thecyclic period, it is discovered that both of them may vary with the samecycle length of 36.4yr and in the same phase. The variation of the lightcurve, the spotted primary component and the connection between thecyclic period change and the depth of the primary minimum, all maysuggest that the G0-type component displays solar-type magnetic activitywith a 36.4-yr cycle length. The long-time period decrease isinterpreted by mass transfer from the more massive component to the lessmassive one or/and angular momentum loss due to mass outflow from theouter Lagrangian point.
| Mg II chromospheric radiative loss rates in cool active and quiet stars
The Mg II k emission line is a good indicator of the level ofchromospheric activity in late-type stars. We investigate the dependenceof this activity indicator on fundamental stellar parameters. To thispurpose we use IUE observations of the Mg II k line in 225 late-typestars of luminosity classes I-V, with different levels of chromosphericactivity. We first re-analyse the relation between Mg II k lineluminosity and stellar absolute magnitude, performing linear fits to thepoints. The ratio of Mg II surface flux to total surface flux is foundto be independent of stellar luminosity for evolved stars and toincrease with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The Mg II k emissionlevel turns out to be not dependent on metallicity. Finally, the Mg II kline surface flux-temperature relation is investigated by treatingseparately, for the first time, a large sample of very active and normalstars. The stellar surface fluxes in the k line of normal stars arefound to be strongly dependent on the temperature and slightly dependenton the gravity, thus confirming the validity of recently proposedmodels. In contrast, data relative to RS CVn binaries and BY Dra stars,which show very strong chromospheric activity, are not justified in theframework of a description based only on acoustic waves and uniformlydistributed magnetic flux tubes so that they require more detailedmodels.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| The Chromospherically Active Contact Binary CE Leonis
We present complete VRI light curves of the contact binary CE Leo andprovide detailed photometric solutions, as well as an analysis of itsperiod variation. A total of 73 times of minima over 50 years, includingour observations, were used for the period study. The complex periodvariation can be sorted into a linear period improvement, a period ofP=0.30342771 days, a secular period increase at the rate ofP/P=+3.05×10-7 days yr-1, and a 22.6+/-0.5yr periodic component. The ~22.6 yr periodic variation in the O-Cresiduals most likely arises from the light-travel time effect from alow-mass (m3~0.3Msolarsini) dM tertiary companionmoving in an eccentric (e'=0.61+/-0.04) orbit. However, it isalso possible that this periodic variation arises from the effects of amagnetic activity cycle, known as the Applegate mechanism. The lightcurves show asymmetries in the two different maxima. The 1998 lightcurves show that primary maximunm was 0.042, 0.038, and 0.038 magbrighter than secondary maximum in V, R, and I, respectively. Thedurations of totality for three epochs were 18.3 minutes in 1989, 19.7minutes in 1998, and 18.8 minutes in 2001. The relative depth of primaryminimum was found to vary between 1.20 and 1.32 mag. We have analyzedthe light curves from the three epochs using the 1993 version of theWilson-Devinney differential corrections computer code to find a uniquesolution for CE Leo. The corresponding spectral type of the secondarystar from the colors and effective temperature is ~dK2. The asymmetriclight curves can be explained by the effects of starspots. We find thatthe light curves are best fitted by employing a cool spot on the cooler,larger component of the system. The spot-effect parameter SE=0.05 foundfor CE Leo is relatively large for W UMa binaries.
| Chemical enrichment and star formation in the Milky Way disk. III. Chemodynamical constraints
In this paper, we investigate some chemokinematical properties of theMilky Way disk, by using a sample composed by 424 late-type dwarfs. Weshow that the velocity dispersion of a stellar group correlates with theage of this group, according to a law proportional to t0.26,where t is the age of the stellar group. The temporal evolution of thevertex deviation is considered in detail. It is shown that the vertexdeviation does not seem to depend strongly on the age of the stellargroup. Previous studies in the literature seem to not have found it dueto the use of statistical ages for stellar groups, rather thanindividual ages. The possibility to use the orbital parameters of a starto derive information about its birthplace is investigated, and we showthat the mean galactocentric radius is likely to be the most reliablestellar birthplace indicator. However, this information cannot bepresently used to derive radial evolutionary constraints, due to anintrinsic bias present in all samples constructed from nearby stars. Anextensive discussion of the secular and stochastic heating mechanismscommonly invoked to explain the age-velocity dispersion relation ispresented. We suggest that the age-velocity dispersion relation couldreflect the gradual decrease in the turbulent velocity dispersion fromwhich disk stars form, a suggestion originally made by Tinsley &Larson (\cite{tinsley}, ApJ, 221, 554) and supported by several morerecent disk evolution calculations. A test to distinguish between thetwo types of models using high-redshift galaxies is proposed.Full Table 1 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/517
| 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.
| Coronal abundances from high-resolution X-ray data: The case of Algol
We discuss the determination of elemental abundances from highresolution X-ray data. We emphasize the need for an accuratedetermination of the underlying temperature structure and advocate theuse of a line ratio method which allows us to utilize, first, thestrongest lines observed in the X-ray spectra, and second, lines thatspan a rather wide temperature range. We point out the need to usecontinuous emission measure distributions and show via example thatmodeling in terms of individual temperature components yields errors ofmore than 50%. We stress the need to derive differential emissionmeasure distributions based on physical assumptions and considerations.We apply our methods to the Chandra LETGS spectrum of Algol and showthat nitrogen is considerably enhanced compared to cosmic abundances bya factor of 2 while carbon is depleted by at least a factor of 25. Iron,silicon, and magnesium, are all depleted compared to cosmic abundances,while the noble gas neon has the relatively highest abundance.
| From the Solar Corona to Clusters of Galaxies: The Radio Astronomy of Bruce Slee
Owen Bruce Slee is one of the pioneers of Australian radio astronomy.During World War II he independently discovered solar radio emission,and, after joining the CSIRO Division of Radiophysics, used a successionof increasingly more sophisticated radio telescopes to examine anamazing variety of celestial objects and phenomena. These ranged fromthe solar corona and other targets in our solar system, to differenttypes of stars and the ISM in our Galaxy, and beyond to distant galaxiesand clusters of galaxies. Although long retired, Slee continues to carryout research, with emphasis on active stars and clusters of galaxies. Aquiet and unassuming man, Slee has spent more than half a century makingan important, wide-ranging contribution to astronomy, and his workdeserves to be more widely known.
| VLA and MERLIN observations of RZ Cassiopeiae
We present radio interferometric observations of the Algol-type binarysystem RZ Cassiopeiae made with the VLA and MERLIN arrays at 6 cm overan incomplete orbital cycle of the system (1.195 d). We detected RZ Caswith both instruments. The images were unresolved in both cases, withangular extents comparable to the synthesized beams. The peak fluxdensity in the VLA image was 1.14 mJy beam-1 and in theMERLIN image it was 0.93 mJy beam-1. The derived brightnesstemperatures are 4.02 × 108 and 4.35 ×108 K and the effective electron energies are 0.347 and 0.346MeV for the MERLIN and VLA data respectively. The radio light curveshows an interesting modulation centred close to the primary eclipsewhich seems to correlate with ASCA SIS observations of the system. Theresults can be interpreted as an emitting region on the outer hemisphereof the cool component aligned along the centroid axis of the binarysystem.
| 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.
| Chromospheric activity and unique solution of SZ Psc
We have confirmed that the MgII h and k emission lines of SZ Psc aremainly enhanced by the K subgiant component of the system, using theradial velocities reduced from the emission lines. The intensityvariation of the emission line is correlated with the orbital phase ofSZ Psc. The shape of light curves of SZ Psc were changing every season.The three representative light curves of three different epochs, 1978,1979 and 1981, were selected to find a unique solution of SZ Psc whichcan explain the variation of light curves. The binary parameters,including inclination, mass ratio, temperatures and potentials of bothcomponents for the three epochs coincide, although the shapes of lightcurves for three epochs are different from each other. Changing onlyspot parameters, the model light curves fit to the observed light curvesfor three epochs. This means that the variation of spot location andsize is the main reason for changing the shape of light curves.
| 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.
| Period Change in an RS CVn BINARY - what drives CF Tuc?
The active binary CF Tuc has shown clear period changes over the lasttwo decades. The current period has been stable since about 1995.
| A systematic study of X-ray variability in the ROSAT all-sky survey
We present a systematic search for variability among the ROSAT All-SkySurvey (RASS) X-ray sources. We generated lightcurves for about 30 000X-ray point sources detected sufficiently high above background. For ourvariability study different search algorithms were developed in order torecognize flares, periods and trends, respectively. The variable X-raysources were optically identified with counterparts in the SIMBAD, theUSNO-A2.0 and NED data bases, but a significant part of the X-raysources remains without cataloged optical counterparts. Out of the 1207sources classified as variable 767 (63.5%) were identified with stars,118 (9.8%) are of extragalactic origin, 10 (0.8%) are identified withother sources and 312 (25.8%) could not uniquely be identified withentries in optical catalogs. We give a statistical analysis of thevariable X-ray population and present some outstanding examples of X-rayvariability detected in the ROSAT all-sky survey. Most prominent amongthese sources are white dwarfs, apparently single, yet neverthelessshowing periodic variability. Many flares from hitherto unrecognisedflare stars have been detected as well as long term variability in theBL Lac 1E1757.7+7034.The complete version of Table 7 is only available in electronic form atthe 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/403/247
| BeppoSAX observations of CF Tucanae and TY Pyxidis
We present the results of BeppoSAX observations of the RS CVn binarysystems CF Tuc and TY Pyx. A long-duration flare was detected on CF Tucjust at the beginning of the observation; a second smaller flareoccurred about half a period later. The light curve of TY Pyx shows asignificant orbital modulation of the quiescent emission with possibleevidence for eclipses, and strong flares. We find that the coronae ofthe two stars are very similar: both systems have quiescent temperaturesof ~ 10 and 20-25 MK, with only a factor of two difference in theemission measures. During the flares, temperatures of ~ 30-40 MK arereached; these temperatures are not high enough to give detectable hardX-ray emission in the PDS. Both stars have subsolar coronalmetallicities: we confirm the very low coronal abundance of CF Tuc (Z ~0.1-0.2 Zsun), while for TY Pyx we find Z ~ 0.5-0.6Zsun. For both stars the coronal metallicities are in goodagreement with their measured photospheric abundances.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| An ATCA radio-continuum study of the Small Magellanic Cloud - I. Source catalogues at 1.42, 2.37, 4.80 and 8.64 GHz
We have analysed radio-continuum mosaics of the Small Magellanic Cloud(SMC) obtained using both the Australia Telescope Compact Array (ATCA)and the Parkes telescope and present a new catalogue of sources in theSMC at 1.42, 2.37, 4.80 and 8.64 GHz (λ= 20, 13, 6 and 3 cm). Wefind a total of 717 radio-continuum sources at these frequencies in thefield of the SMC, which is three times more than previous Parkessurveys. From these 717 radio-continuum sources, some 534 were detectedat 1.42 GHz, 697 at 2.37 GHz, 75 at 4.80 GHz and 54 at 8.64 GHz. Theintegrated flux density was measured for each of these sources. We haveassessed the accuracy of the position and flux measurements of ourcatalogue and found no significant discrepancy with previous catalogues.Our new data show a significant improvement in sensitivity (over 10times) and positional accuracy (<1 arcsec) over previous catalogues.
| 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.
| Gravitational quadrupole-moment variations in active binaries
The principal characteristics of the observed orbital period variationin magnetically active close binaries are briefly reviewed and thetheoretical models proposed to interpret them are presented. Inparticular, we focus on the models proposed by Applegate (1992) andLanza, Rodonò& Rosner (1998a) to explain the short-termmodulation of the orbital period, as a consequence of the changes of thegravitational quadrupole moment of the active component driven by acyclic hydromagnetic dynamo. Recent observational results support ourinterpretation and the constraints on the intensity of the internalmagnetic fields that are required by the proposed mechanism, arediscussed. A novel, stringent test of our model based on futureasteroseismic space observations of oscillation mode splitting is alsobriefly presented.
| Doppler images of starspots
I present a literature survey of the currently available Doppler imagesof cool stars. The 65 individual stars with Doppler images consist of 29single stars and 36 components in close binaries. Out of the total, 31were observed only once but 12 stars are (or were) being monitored foryears. Each image for each star is identified with the time when it wasobserved, whether photometry was used in the imaging, the inclination ofthe stellar rotation axis, the vsin i, the stellar rotation period, andwhether a polar spot and/or a high-latitude or low-latitude spot wasseen. The type of variable star and its M-K spectral classification isalso listed to identify the evolutionary status. The sample consists of3 classical T Tauri stars, 8 weak-lined T Tauri's, 27 main-sequencestars, 9 subgiants, and 18 giants. The total number of Doppler images is245 as of June 2002.
| Measuring starspots on magnetically active stars with the VLTI
We present feasibility studies to directly image stellar surfacefeatures, which are caused by magnetic activity, with the Very LargeTelescope Interferometer (VLTI). We concentrate on late typemagnetically active stars, for which the distribution of starspots onthe surface has been inferred from photometric and spectroscopic imaginganalysis. The study of the surface spot evolution during consecutiverotation cycles will allow first direct measurements (apart from theSun) of differential rotation which is the central ingredient ofmagnetic dynamo processes. The VLTI will provide baselines of up to 200m, and two scientific instruments for interferometric studies at near-and mid-infrared wavelengths. Imaging capabilities will be made possibleby closure-phase techniques. We conclude that a realistically modeledcool surface spot can be detected on stars with angular diametersexceeding ~ 2 mas using the VLTI with the first generation instrumentAMBER. The spot parameters can then be derived with reasonable accuracy.We discuss that the lack of knowledge of magnetically active stars ofthe required angular size, especially in the southern hemisphere, is acurrent limitation for VLTI observations of these surface features.
| Chromospherically young, kinematically old stars
We have investigated a group of stars known to have low chromosphericages, but high kinematical ages. Isochrone, chemical and lithium agesare estimated for them. The majority of stars in this group show lithiumabundances much smaller than expected for their chromospheric ages,which is interpreted as an indication of their old age. Radial velocitymeasurements in the literature also show that they are not closebinaries. The results suggest that they can be formed from thecoalescence of short-period binaries. Coalescence rates, calculatedtaking into account several observational data and a maximum theoreticaltime scale for contact, in a short-period pair, predict a number ofcoalesced stars similar to what we have found in the solarneighbourhood.
| Correlation effects in microwave observations of selected RS CVn-like stars
Sets of dual frequency microwave data on selected chromosphericallyactive stars, from the Australia Telescope Compact Array, have beeninvestigated for their auto and cross-correlation effects. Comparison ofcross-correlation peak values with theoretical expectation indicates ahigh degree of real physical connection between the emission at thepairs of frequencies (4.8 and 8.64 GHz) compared. This fact should helpconstrain models for the emission mechanism. The timescale of observedtime-shifts between the emissions at the two frequencies is consistent,in general, with the underlying energization being propagated bymagnetohydrodynamic waves in a compact turbulent medium.
| A Radio and Optical Study of the Active Young F Star HR1817 (=HD35850)
This paper presents the results of a multiwavelength observational studyof the active young F-type star HR1817. The star was monitored at 4.80and 8.64GHz over 2×12h allocations with the Australia TelescopeCompact Array on 8 and 9 December, 2000. The Anglo-Australian Telescopewas used for simultaneous optical spectropolarimetry during a 2h periodon 9 December. The low levels of observed radio emission havecharacteristics that are similar to those seen in other active stars,and a gyrosynchrotron mechanism is proposed to explain them; this issupported by the relatively low fractions of circular polarisationmeasured in HR1817. Comparison of the emissions from 4.80 and 8.64GHzshows a very strong cross-correlation peak, indicative of a commonorigin, although the shift of this peak indicates that 8.64GHzvariations tend to precede those at 4.80GHz by, typically, ~20min. Theoptical spectropolarimetry reveals polarisation signals characteristicof surface magnetic fields, with profile changes indicating a complexdynamo-type magnetic topology is present on the star. This result makesHR1817 the star with the earliest spectral type on which dynamo magneticfields have been detected directly up to now.
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Datos observacionales y astrométricos
| Constelación: | Tucán |
| Ascensión Recta: | 00h53m07.77s |
| Declinación: | -74°39'05.6" |
| Magnitud Aparente: | 7.633 |
| Distancia: | 86.207 parsecs |
| Movimiento Propio en Ascensión Recta: | 243.3 |
| Movimiento Propio en Declinación: | 20.9 |
| B-T magnitude: | 8.482 |
| V-T magnitude: | 7.704 |
Catálogos y designaciones:
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