Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
Exploring the galaxy cluster-group transition regime at high redshifts. Physical properties of two newly detected z > 1 systems
Context. Multi-wavelength surveys for clusters of galaxies are opening awindow on the elusive high-redshift (z > 1) cluster population. Wellcontrolled statistical samples of distant clusters will enable us toanswer questions about their cosmological context, early assembly phasesand the thermodynamical evolution of the intracluster medium. Aims: We report on the detection of two z > 1 systems, XMMUJ0302.2-0001 and XMMU J1532.2-0836, as part of the XMM-Newton DistantCluster Project (XDCP) sample. We investigate the nature of the sources,measure their spectroscopic redshift and determine their basic physicalparameters. Methods: The results of the present paper are basedon the analysis of XMM-Newton archival data, optical/near-infraredimaging and deep optical follow-up spectroscopy of the clusters. Results: We confirm the X-ray source XMMU J0302.2-0001 as agravitationally bound, bona fide cluster of galaxies at spectroscopicredshift z = 1.185. We estimate its M500 mass to (1.6± 0.3) × 1014 M&sun; from itsmeasured X-ray luminosity. This ranks the cluster among intermediatemass system. In the case of XMMU J1532.2-0836 we find the X-raydetection to be coincident with a dynamically bound system of galaxiesat z = 1.358. Optical spectroscopy reveals the presence of a centralactive galactic nucleus, which can be a dominant source of the detectedX-ray emission from this system. We provide upper limits of X-rayparameters for the system and discuss cluster identification challengesin the high-redshift low-mass cluster regime. A third, intermediateredshift (z = 0.647) cluster, XMMU J0302.1-0000, is serendipitouslydetected in the same field as XMMU J0302.2-0001. We provide its analysisas well.Based on observations obtained with ESO Telescopes at the ParanalObservatory under program ID 080.A-0659 and 081.A-0312, observationscollected at the Centro Astrnómico Hispano Alemán (CAHA)at Calar Alto, Spain operated jointly by the Max-Planck Institutfür Astronomie and the Instituto de Astrofísica deAndalucía (CSIC). X-ray observations were obtained by XMM-Newton.
| XID II: Statistical Cross-Association of ROSAT Bright Source Catalog X-ray Sources with 2MASS Point Source Catalog Near-Infrared Sources
The 18,806 ROSAT All Sky Survey Bright Source Catalog (RASS/BSC) X-raysources are quantitatively cross-associated with near-infrared (NIR)sources from the Two Micron All Sky Survey Point Source Catalog(2MASS/PSC). An association catalog is presented, listing the mostlikely counterpart for each RASS/BSC source, the probability Pid that the NIR source and X-ray source are uniquelyassociated, and the probability P no-id that none of the2MASS/PSC sources are associated with the X-ray source. The catalogincludes 3853 high quality (P id>0.98) X-ray-NIR matches,2280 medium quality (0.98 >= P id>0.9) matches, and4153 low quality (0.9 >= P id>0.5) matches. Of the highquality matches, 1418 are associations that are not listed in the SIMBADdatabase, and for which no high quality match with a USNO-A2 opticalsource was presented for the RASS/BSC source in previous work. Thepresent work offers a significant number of new associations withRASS/BSC objects that will require optical/NIR spectroscopy forclassification. For example, of the 6133 P id>0.92MASS/PSC counterparts presented in the association catalog, 2411 haveno classification listed in the SIMBAD database. These 2MASS/PSC sourceswill likely include scientifically useful examples of known sourceclasses of X-ray emitters (white dwarfs, coronally active stars, activegalactic nuclei), but may also contain previously unknown sourceclasses. It is determined that all coronally active stars in theRASS/BSC should have a counterpart in the 2MASS/PSC, and that the uniqueassociation of these RASS/BSC sources with their NIR counterparts thusis confusion limited.
| Starspots
Starspots are created by local magnetic fields on the surfaces of stars,just as sunspots. Their fields are strong enough to suppress theoverturning convective motion and thus block or redirect the flow ofenergy from the stellar interior outwards to the surface andconsequently appear as locally cool and therefore dark regions againstan otherwise bright photosphere (Biermann in Astronomische Nachrichten264:361, 1938; Z Astrophysik 25:135, 1948). As such, starspots areobservable tracers of the yet unknown internal dynamo activity and allowa glimpse into the complex internal stellar magnetic field structure.Starspots also enable the precise measurement of stellar rotation whichis among the key ingredients for the expected internal magnetictopology. But whether starspots are just blown-up sunspot analogs, we donot know yet. This article is an attempt to review our current knowledgeof starspots. A comparison of a white-light image of the Sun (G2V, 5Gyr) with a Doppler image of a young solar-like star (EK Draconis;G1.5V, age 100 Myr, rotation 10 × ? Sun) and witha mean-field dynamo simulation suggests that starspots can be ofsignificantly different appearance and cannot be explained with ascaling of the solar model, even for a star of same mass and effectivetemperature. Starspots, their surface location and migration pattern,and their link with the stellar dynamo and its internal energytransport, may have far reaching impact also for our understanding oflow-mass stellar evolution and formation. Emphasis is given in thisreview to their importance as activity tracers in particular in thelight of more and more precise exoplanet detections around solar-like,and therefore likely spotted, host stars.
| Multiple and changing cycles of active stars. II. Results
Aims. We study the time variations in the cycles of 20 active starsbased on decade-long photometric or spectroscopic observations. Methods:A method of time-frequency analysis, as discussed in a companion paper,is applied to the data. Results: Fifteen stars definitely show multiplecycles, but the records of the rest are too short to verify a timescalefor a second cycle. The cycles typically show systematic changes. Forthree stars, we found two cycles in each of them that are not harmonicsand vary in parallel, indicating a common physical mechanism arisingfrom a dynamo construct. The positive relation between the rotationaland cycle periods is confirmed for the inhomogeneous set of activestars. Conclusions: Stellar activity cycles are generally multiple andvariable.
| Absolute Properties of the Spotted Eclipsing Binary Star CV Boötis
We present new V-band differential brightness measurements as well asnew radial-velocity measurements of the detached, circular, 0.84 dayperiod, double-lined eclipsing binary system, CV Boo. These data, alongwith other observations from the literature, are combined to deriveimproved absolute dimensions of the stars for the purpose of testingvarious aspects of theoretical modeling. Despite complications fromintrinsic variability that we detect in the system, and despite therapid rotation of the components, we are able to determine the absolutemasses and radii to better than 1.3% and 2%, respectively. We obtain MA = 1.032 ± 0.013 M sun and R A= 1.262 ± 0.023 R sun for the hotter, larger, andmore-massive primary (star A), and M B = 0.968 ± 0.012M sun and R B = 1.173 ± 0.023 Rsun for the secondary. The estimated effective temperaturesare 5760 ± 150 K and 5670 ± 150 K, respectively. Theintrinsic variability with a period ~1% shorter than the orbital periodis interpreted as being due to modulation by spots on one or bothcomponents. This implies that the spotted star(s) must be rotatingfaster than the synchronous rate, which disagrees with predictions fromcurrent tidal evolution models according to which both stars should besynchronized. We also find that the radius of the secondary is largerthan expected from stellar evolution calculations by ~10%, a discrepancyalso seen in other (mostly lower-mass and active) eclipsing binaries. Weestimate the age of the system to be approximately 9 Gyr. Bothcomponents are near the end of their main-sequence phase, and theprimary may have started the shell hydrogen-burning stage.
| A catalogue of chromospherically active binary stars (third edition)
The catalogue of chromospherically active binaries (CABs) has beenrevised and updated. With 203 new identifications, the number of CABstars is increased to 409. The catalogue is available in electronicformat where each system has a number of lines (suborders) with a uniqueorder number. The columns contain data of limited numbers of selectedcross references, comments to explain peculiarities and the position ofthe binarity in case it belongs to a multiple system, classicalidentifications (RS Canum Venaticorum, BY Draconis), brightness andcolours, photometric and spectroscopic data, a description of emissionfeatures (CaII H and K, Hα, ultraviolet, infrared),X-ray luminosity, radio flux, physical quantities and orbitalinformation, where each basic entry is referenced so users can go to theoriginal sources.
| Anti-solar differential rotation and surface flow pattern on UZ Libræ
We re-investigate UZ Libræ spectra obtained at KPNO in 1998 and 2000.From the 1998 data we compose 11 consecutive Doppler images using the CaI-6439, Fe I-6393 and Fe I-6411 lines. Applying the method of averagecross-correlation of contiguous Doppler images we find anti-solardifferential rotation with a surface shear of α≈-0.03. The pilotapplication of the local correlation tracking technique for the samedata qualitatively confirms this result and indicates complex flowpattern on the stellar surface. From the cross-correlation of the twoavailable Doppler images in 2000 we also get anti-solar differentialrotation but with a much weaker shear of α≈-0.004.
| Changing stellar activity cycles
We investigated continuous long-term photometric datasets of thirteenactive stars, Ca II variability of one single main-sequence star, and10.7cm radio data of the Sun, with simple Fourier- and time-frequencyanalysis. The data reflect the strength of the activity manifested inmagnetic spots. All studied stars show multiple (2 to 4) cycles ofdifferent lengths. The time-frequency analysis reveals, that in severalcases of the sample one or two of the cycles exhibit continuous changes(increase or decrease). For four stars (V711 Tau, IL Hya, HK Lac, HD100180) and for the Sun we find that the cycle length changes arestrong, amounting to 10-50% during the observed time intervals. Thecycle lengths are generally longer for stars with longer rotationalperiods.
| The XMM-Newton serendipitous survey. IV. Optical identification of the XMM-Newton medium sensitivity survey (XMS)
Aims:X-ray sources at intermediate fluxes (a few ×10-14 erg cm-2 s-1) with a sky densityof ~100 deg-2 are responsible for a significant fraction ofthe cosmic X-ray background at various energies below 10 keV. The aim ofthis paper is to provide an unbiased and quantitative description of theX-ray source population at these fluxes and in various X-ray energybands. Methods: We present the XMM-Newton Medium sensitivitySurvey (XMS), including a total of 318 X-ray sources found among theserendipitous content of 25 XMM-Newton target fields. The XMS comprisesfour largely overlapping source samples selected at soft (0.5-2 keV),intermediate (0.5-4.5 keV), hard (2-10 keV) and ultra-hard (4.5-7.5 keV)bands, the first three of them being flux-limited. Results: Wereport on the optical identification of the XMS samples, complete to85-95%. At the flux levels sampled by the XMS we find that the X-ray skyis largely dominated by Active Galactic Nuclei. The fraction of stars insoft X-ray selected samples is below 10%, and only a few per cent forhard selected samples. We find that the fraction of optically obscuredobjects in the AGN population stays constant at around 15-20% for softand intermediate band selected X-ray sources, over 2 decades of flux.The fraction of obscured objects amongst the AGN population is larger(~35-45%) in the hard or ultra-hard selected samples, and constantacross a similarly wide flux range. The distribution in X-ray-to-opticalflux ratio is a strong function of the selection band, with a largerfraction of sources with high values in hard selected samples. Sourceswith X-ray-to-optical flux ratios in excess of 10 are dominated byobscured AGN, but with a significant contribution from unobscured AGN.Based on observations obtained with XMM-Newton, an ESA science missionwith instruments and contributions directly funded by ESA Member Statesand the USA (NASA). Based on observations made with the INT/WHT, TNG andNOT operated on the island of La Palma by the Isaac Newton Group, theCentro Galileo Galilei and the Nordic Optical Telescope ScienceAssociation respectively, in the Spanish Observatorio del Roque de losMuchachos. Based on observations collected at the CentroAstronómico Hispano Alemán (CAHA) at Calar Alto, operatedjointly by the Max-Planck Institut für Astronomie and the Institutode Astrofísica de Andalucía (CSIC). Based on observationscollected at the European Southern Observatory, Paranal, Chile, as partof programme 75.A-0336.
| Anti-solar differential rotation on the active K-giant σ Geminorum
The active K1 giant σ Gem and its differential surface rotation isrevisited. We refine our previous inconclusive result by recovering thespot migration pattern of this long-period RS CVn-type binary throughapplication of the technique of “average cross-correlation ofcontiguous Doppler images” to a set of six Doppler images from 3.6consecutive rotation cycles. We find an anti-solar differential rotationlaw with a shear of α≈ -0.022±0.006. We also findevidence of a poleward migration trend of spots with an average velocityof ≈300 m s-1.
| The XMM-Newton serendipitous survey. III. The AXIS X-ray source counts and angular clustering
Context: Recent results have revised upwards the total X-ray background(XRB) intensity below ~10 keV, therefore an accurate determination ofthe source counts is needed. There are also contradictory results on theclustering of X-ray selected sources. Aims: We have studied theX-ray source counts in four energy bands: soft (0.5-2 keV), hard (2-10keV), XID (0.5-4.5 keV) and ultra-hard (4.5-7.5 keV) in order toevaluate the contribution of sources at different fluxes to the X-raybackground. We have also studied the angular clustering of X-ray sourcesin those bands. Methods: AXIS (An XMM-NewtonInternational Survey) is a survey of 36 high Galactic latitudeXMM-Newton observations covering 4.8deg2 in the Northern sky and containing 1433 serendipitousX-ray sources detected with 5-? significance. This survey hassimilar depth to the XMM-Newton catalogues andtherefore serves as a pathfinder to explore their possibilities. We havecombined this survey with shallower and deeper surveys, and fitted thesource counts with a Maximum Likelihood technique. Using only AXISsources we have studied the angular correlation using a novel robusttechnique. Results: Our source counts results are compatible withmost previous samples in the soft, XID, ultra-hard and hard bands. Wehave improved on previous results in the hard band. The fractions of theXRB resolved in the surveys used in this work are 87%, 85%, 60% and 25%in the soft, hard, XID and ultra-hard bands, respectively. Extrapolationof our source counts to zero flux is not sufficient to saturate the XRBintensity. Only galaxies and/or absorbed AGN could contribute theremaining unresolved XRB intensity. Our results are compatible, withinthe errors, with recent revisions of the XRB intensity in the soft andhard bands. The maximum fractional contribution to the XRB comes fromfluxes within about a decade of the break in the source counts(~10-14 cgs), reaching ~50% of the total in the soft and hardbands. Angular clustering (widely distributed over the sky and notconfined to a few deep fields) is detected at 99-99.9% significance inthe soft and XID bands, with no detection in the hard and ultra-hardband (probably due to the smaller number of sources). We cannot confirmthe detection of significantly stronger clustering in the hard-spectrumhard sources. Conclusions: Medium depth surveys such as AXIS areessential to determine the evolution of the X-ray emission in theUniverse below 10 keV.Table 1 and Appendices are only available in electronic form athttp://www.aanda.org Based on observations obtained with XMM-newton, anESA science mission with instruments and contributions funded by ESAMember States and the USA (NASA). Tables 2 and 3 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/469/27
| Doppler imaging of stellar surface structure. XXIII. The ellipsoidal K giant binary ζ Andromedae
Aims.We present the first Doppler images of the bright RS CVn-typebinary ζ And. The star is a magnetically active K1 giant with itsrotation synchronized to the 17.8-day orbital period. Our revisedlithium abundance of log n=1.2 places ζ And in the vicinity ofLi-rich RGB stars but it is nevertheless a Li-normal chromosphericallyactive binary star. The star seems to undergo its first standarddredge-up dilution. Methods: Four consecutive Doppler images wereobtained from a continuous 67-night observing run at NSO-McMath in1996/97. An additional single image was obtained from a continuous19-night run at KPNO in 1997/98. These unique data allow to compute asmall time series of the evolution of the star's surface structure. Allline-profile inversions are done with a modified TempMap version thattakes into account the non-spherical shape of the star. Representativetest reconstructions are performed and demonstrate the code'sreliability and robustness. Results: High and low-latitude spotactivity was recovered together with an asymmetric polar cap-likefeature. The latter dominated the first half of the two-month timeseries in 1996/97. The second half showed mostly medium-to-high latitudeactivity and only a fainter polar spot. The coolest areas were restoredwith a temperature contrast of about 1000±200 K. Some weakerfeatures at equatorial latitudes were also recovered but these could bepartially spurious and appear blurred due to imperfect phase coverage.We use our line profiles to reconstruct an average non-sphericity ofR_pole/R_point=0.96 which would, if not taken into account, mimic atemperature difference pole-to-equator of ≈220 K, especially at thephases of quadrature. Finally, we apply two different methods forrestoring surface differential rotation and found a weak solar-typerotation law with a shear ΔΩ≈0.95°/day(α=ΔΩ/Ω_eq=+0.049±0.003), i.e. roughly afactor of four weaker at a rotation rate roughly 1.5 times faster thanthe Sun's.Figures A.1-A.3 and Table A.1 are only available in electronic form athttp://www.aanda.org
| Dynamical evolution of active detached binaries on the logJo-logM diagram and contact binary formation
Orbital angular momentum (OAM, Jo), systemic mass (M) andorbital period (P) distributions of chromospherically active binaries(CAB) and W Ursae Majoris (W UMa) systems were investigated. Thediagrams of and logJo-logM were formed from 119 CAB and 102 WUMa stars. The logJo-logM diagram is found to be mostmeaningful in demonstrating dynamical evolution of binary star orbits. Aslightly curved borderline (contact border) separating the detached andthe contact systems was discovered on the logJo-logM diagram.Since the orbital size (a) and period (P) of binaries are determined bytheir current Jo, M and mass ratio, q, the rates of OAM loss(dlogJo/dt) and mass loss (dlogM/dt) are primary parametersto determine the direction and the speed of the dynamical evolution. Adetached system becomes a contact system if its own dynamical evolutionenables it to pass the contact border on the logJo-logMdiagram. The evolution of q for a mass-losing detached system is unknownunless the mass-loss rate for each component is known. Assuming q isconstant in the first approximation and using the mean decreasing ratesof Jo and M from the kinematical ages of CAB stars, it hasbeen predicted that 11, 23 and 39 per cent of current CAB stars wouldtransform to W UMa systems if their nuclear evolution permits them tolive 2, 4 and 6 Gyr, respectively.
| Stellar activity and the Konkoly Observatory: the beginnings
The early observational facts on stellar activity are discussed withspecial emphasis on L. Detre's interest in those results.
| Starspot activity in late stars: Methods and results
Three types of methods for studying the surface inhomogeneities of coolstars and the results of their use on type BY Dra, RS CVn, FK Com, and TTau variables are discussed. The current relevance of traditionalphotometric methods and the advantages of the zonal spottedness modelare pointed out. Dependences of the maximum total areas, averagelatitudes, and temperatures of spots on the global parameters of thestars are given. Analogs of the solar cycle in the variations of theareas and latitudes of starspots are examined, as well as the effects ofdifferential rotation and active longitudes.
| 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.
| Photospheric and chromospheric active regions on three single-lined RS CVn binaries
A monitoring of three active RS CVn binaries has been performed withmedium resolution spectroscopy with the aim of investigating thebehavior of chromospheric and photospheric inhomogeneities. Surfacetemperature, as recovered from line-depth ratios (LDRs), allowed us tomap the photospheric spots, while the Hα emission has been used asan indicator of chromospheric inhomogeneities. We have found that therotational modulation of the Hα emission is always in anti-phasewith the temperature wave, i.e. at the time of our observations activeregions at chromospheric and photospheric levels are closely spatiallyassociated in these active stars. The residual Hα profiles,obtained as the difference between the observed spectra and non-activetemplates, are well reproduced by a two Gaussian fitting. The broademission component, responsible for the wide emission wings in near allthe spectra, is often blue-shifted with respect to the center of thestellar disk. The narrow Hα emission displays a phase-dependentvariation in all stars and is anti-correlated with the photosphericdiagnostics, while the broad one displays no or little rotationalmodulation. We suggest that the broad emission component is mainlyrelated to physical phenomena, like micro-flaring or strongchromospheric velocity fields, occurring all over the star disk, whilethe central narrow emission is more affected by chromospheric plages. Wehave also detected a modulation of the intensity of the He I D3 linewith the star rotation, suggesting surface features also in the upperchromosphere of these stars.
| On the Binary Nature of 1RXS J162848.1-415241
We present spectroscopy of the optical counterpart to 1RXSJ162848.1-41524, also known as the microquasar candidate MCQCJ162847-4152. All the data indicate that this X-ray source is not amicroquasar and that it is a single-lined chromospherically activebinary system with a likely orbital period of 4.9 days. Our analysissupports a K3 IV spectral classification for the star, which is dominantat optical wavelengths. The unseen binary component is most likely alate-type (K7-M) dwarf or a white dwarf. Using the high-resolutionspectra, we have measured the K3 star's rotational broadening to bevsini=43+/-3 km s-1 and determined a lower limit to thebinary mass ratio of q(=M2/M1)>2.0. The highrotational broadening together with the strong Ca II H and K/Hαemission and high-amplitude photometric variations indicate that theevolved star is very chromospherically active and responsible for theX-ray/radio emission.
| X-ray spectra of XMM-Newton serendipitous medium flux sources
We report on the results of a detailed analysis of the X-ray spectralproperties of a large sample of sources detected serendipitously withthe XMM-Newton observatory in 25 selected fields, for which opticalidentification is in progress. The survey covers a total solid angle of~3.5 deg2 and contains 1137 sources with~10-151043~erg s-1, and thereforeclassified as type 2 AGNs) is significantly higher (40%), with a hint ofmoderately higher columns. After correcting for absorption, we do notfind evidence for a redshift evolution of the underlying power law indexof BLAGNs, which stays roughly constant at Γ 1.9, withintrinsic dispersion of 0.4. A small fraction (~7%) of BLAGNs and NELGsrequire the presence of a soft excess, that we model as a black bodywith temperature ranging from 0.1 to 0.3 keV. Comparing our results onabsorption to popular X-ray background synthesis models, we findabsorption in only ~40% of the sources expected. This is due to adeficiency of heavily absorbed sources (with NH1022-1024~cm-2) in our sample incomparison with the models. We therefore conclude that the synthesismodels require some revision in their specific parameters.
| Direct Evidence for a Polar Spot on SV Camelopardalis
We have used spectrophotometric data from the Hubble Space Telescope(HST) to eclipse-map the primary component of the RS CVn binary SV Camover nine HST orbits. We find from these observations and the Hipparcosparallax that the surface flux in the eclipsed low-latitude region ofthe primary is about 30% lower than that computed from a PHOENIX modelatmosphere at the effective temperature that best fits the spectralenergy distribution of the eclipsed flux. This can only be accounted forif about a third of the primary's surface is covered with unresolveddark starspots. Extending this to the full surface of the primary, wefind that, even taking into account this spot-filling factor, there isan additional flux deficit on the primary star. This can only beexplained if there is a large polar spot on the primary star extendingfrom the pole to latitude 48deg+/-6deg.
| SB9: The ninth catalogue of spectroscopic binary orbits
The Ninth Catalogue of Spectroscopic Binary Orbits(http://sb9.astro.ulb.ac.be) continues the series of compilations ofspectroscopic orbits carried out over the past 35 years by Batten andcollaborators. As of 2004 May 1st, the new Catalogue holds orbits for2386 systems. Some essential differences between this catalogue and itspredecessors are outlined and three straightforward applications arepresented: (1) completeness assessment: period distribution of SB1s andSB2s; (2) shortest periods across the H-R diagram; (3)period-eccentricity relation.
| An atmospheric model for UZ Librae from mean H?-line profiles
We present the results from fitting a semi-empirical atmospheric modelincluding a chromosphere and a transition region to the mean (seasonal)Balmer H? line profiles of the RS CVn-type K0-giant UZ Librae. Asa first step, a static 1D spherical model was applied to the meancomponent of the H?-emission core and its profile. The main resultof the fitting is that the transition region begins at a log mass depthof -1.8 g cm-2 at a temperature of 7400 K (approximately 3000K warmer than the photosphere) and then has a steep increase to the peaktemperature of ?106 K. A stellar model in plane-parallelmode with ``partial-frequency redistribution'' option in the linetransfer gave roughly the best fit. Subsequently, two-Gaussian fittingof the phase-dependent H? -line profiles yields a complex velocityfield. The radial velocities, from both the absorption reversal and themain emission component, display rotational and/or orbital modulation.The largest differences between consecutive line profiles occur mostlyin the red line wings, suggesting the existence of both aninward-pointed velocity field and sporadic radiation events possiblyrelated to flares. The total H? emission, as derived from theequivalent width, possibly also displays rotational modulation. Indirectevidence is presented for the existence of circumstellar matter in formof an H? absorption shell. Finally, a preliminary elementalabundance analysis suggests sub-solar metallicity of [M/H]? -0.5 dexbased on ATLAS models, or -0.7 dex based on PHOENIX models.
| 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.
| Differential rotation on UZ Librae
We analysed a time series of nine consecutive years of high-precisionphotometry of the spotted RS CVn star UZ Lib by using a discreteFourier-transform technique and a non-linear least-squares minimization.The main period of 4.77 days due to stellar rotation was resolved intothree individual periods separated by -0.2% and +0.4% around the mainperiod. The stability of the spot pattern over many years, as derivedfrom our contemporaneous Doppler images, allowed us to relate thedifferent periods to co-existing spots at different latitudes, and thusto the direct determination of the strength and the sign of thedifferential rotation. The main period originates from the equatorialsurface regions and is practically the same as the orbital period fromindependent radial-velocity measurements, suggesting that the stellarequator is tidally locked to the orbital motion. The higher latitudesrotate slightly faster than the equator, suggesting non-solardifferential rotation with a parameter of alpha =Delta Omega /Omega=-0.0026, 80 times weaker than on the solar surface, and a lap time ofPequator/alpha ~ -1800 days, i.e. 14 times longer than forthe Sun.
| Doppler imaging of stellar surface structure. XX. The rapidly-rotating single K2-giant HD 31993 = V1192 Orionis
We present two Doppler images from two consecutive stellar rotations ofthe single K2-giant HD 31993. Each Doppler image is reconstructed fromspectra obtained within a single stellar rotation. With its 25-dayrotational period and a radius of ~18 solar radii, HD 31993 isconsidered a very rapidly rotating star and thus allows the applicationof the Doppler-imaging technique, despite the unusually long period. Allmaps reveal 7 isolated, predominantly low-latitude spots with atemperature difference, photosphere minus spot, of just ~200 K. No polarspot or high-latitude activity above, say, +60° is seen. A largewarm feature is detected at high latitude and is believed to be real.These spots act as tracers for a cross correlation analysis and yield aclear signature of anti-solar differential surface rotation, i.e. thepolar regions rotating faster than the equator, with alpha =0.125+/-0.05 corresponding to a lap time of ~200 days. A detailed parameterstudy is carried out to verify the reality of the HD 31993 maps.
| X-ray spectroscopy of UZ Librae
UZ Librae is the giant primary of a close binary system with extremelyfast rotation and revolution. Its distinctive characteristics include anoptical light curve with a large amplitude and a high X-ray luminosity.UZ Librae was observed twice at five months interval in August 2000 andJanuary 2001 by the XMM-Newton space observatory. Serie of lines ofhighly ionized Fe and several lines of the Ly and He series are visiblein the reflection grating spectra, most notably from C, O and Ne. Thecorresponding ions are associated with plasma components withtemperature in the range 3-8 MK. Bremstrahlung emission from plasma athigh temperature (T > 3x 107 K) indicates an intenseflaring activity on UZ Librae. This is also supported by a Ne abundanceenhancement relative to oxygen reminiscent of abundance anomaliesobserved during stellar and solar flares. In contrast to oxygen andneon, the abundance of Fe in the corona of UZ Librae is determined to bea tenth of the solar photospheric value. We conjecture that the largefluid kinetic helicity induced by the rapid rotation of the starcurrently generates magnetic fields with two characteristic scalescomparable with compact loops within solar active regions and withlarger cooler loops interconnecting solar active regions. The X-rayemission of UZ Librae would be strongly enhanced not only due to theoccurrence of these magnetic structures, but mainly due to their largesurface coverage and to their permanent interactions responsible forheating UZ Librae plasma to high temperatures. A rotational modulationeffect could have contributed to the luminosity variation between thetwo XMM-Newton observations since a large fraction of the X-ray emittingmaterial might be located above large photospheric spots detected onDoppler images of the star.
| Time-series photometric spot modelling VI. A new computer code and its application to 23 years of photometry of the active giant IM Pegasi
We present and apply a new computer program named SpotModeL to analyzesingle and multiple bandpass photometric data of spotted stars. It isbased on the standard analytical formulae from Budding and Dorren. Theprogram determines the position, size, and temperature of up to threespots by minimizing the fit residuals with the help of theMarquardt-Levenberg non-linear least-squares algorithm. We also expandthis procedure to full time-series analysis of differential data, justas real observations would deliver. If multi-bandpass data areavailable, all bandpasses can be treated simultaneously and thus thespot temperature is solved for implicitly. The program may be downloadedand used by anyone. In this paper, we apply our code to an ~23 year longphotometric dataset of the spotted RS CVn giant IM Peg. We extracted andmodelled 33 individual light curves, additionally, we fitted the entireV dataset in one run. The resulting spot parameters reflect the longterm light variability and reveal two active longitudes on thesubstellar point and on the antipode. The radius and longitude of thedominant spot show variations with 29.8 and 10.4 years period,respectively. Our multicolour data suggests that the spot temperature isincreasing with the brightening of the star. The average spottemperature from V,I_C is 3550+/- 150 K or approximately 900 K below theeffective temperature of the star.
| Time series photometric spot modelling V. Phase coherence of spots on UZ Librae
We present spot models for nine years of continuous VI_C photometry ofUZ Lib from 1993-2001. The relatively stable double-humped light curveshape suggests extreme phase coherence. From the spot-modellinganalysis, we found that the major spots or spot groups are alwayslocated on the hemisphere facing the secondary star and exactly in theopposite hemisphere anti-facing the secondary. Several single-humpedlight curves and our suggested binary scenario rule out a pureellipsoidal variability as the cause of the double-humped light curveshape. We try to explain this preferred spot pattern with amagnetic-field structure that connects the two components, as suggestedearlier for RS CVn stars in general. A possible 4.8 years spot cycle isfound from the long-term brightness variations but needs confirmation.We rediscuss the basic astrophysical data of UZ Lib. The Hipparcosparallax is likely wrong, a possible reason could be that UZ Lib ise.g., a triple system.
| Starspot cycles from long-term photometry
We continue investigating the photometric cycle lengths of some activestars. Using datasets now up to 34 years long we confirm previouslyderived activity cycles for the shorter period systems of our sample (LQHya, V833 Tau, EI Eri, V711 Tau, HU Vir and IL Hya), but find differentcycle lengths for the two long-period giants HK Lac and IM Peg. We addUZ Lib to our previous sample. The connection between the rotationalperiod and cycle length seems evident for the shortest derived cyclelengths. A similar connection between the rotational period and thelonger cycle lengths is not as clear, taking into account recent resultson the time variability of the longer term solar cycles. While the timebase of the observations puts an upper limit to the detectable lengthsof the longer cycles, a general dependence of the cycle period on therotation rate is maintained. The length of the shortest cycle that wefound for LQ Hya agrees with the cycle period derived from dynamomodelling of this star.
| 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.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Terazi |
| Sað Açýklýk: | 15h32m23.21s |
| Yükselim: | -08°32'00.9" |
| Görünürdeki Parlaklýk: | 9.142 |
| özdevim Sað Açýklýk: | 25.4 |
| özdevim Yükselim: | 0.4 |
| B-T magnitude: | 10.444 |
| V-T magnitude: | 9.25 |
Kataloglar ve belirtme:
|
