目录
图像
上传图像
DSS Images Other Images
相关文章
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.
| On the Use of Line Depth Ratios to Measure Starspot Properties on Magnetically Active Stars
Photometric and spectroscopic techniques have proven to be effectiveways to measure the properties of dark, cool starspots on magneticallyactive stars. Recently, a technique was introduced using atomic linedepth ratios (LDRs) to measure starspot properties. Carefullyreproducing this technique using a new set of spectroscopic observationsof active stars, we find that the LDR technique encounters difficulties,specifically by overestimating spot temperatures (because the atomiclines blend with titanium oxide absorption in cooler spots) and by nottightly constraining the filling factor of spots. While the use of LDRsfor active star studies has great promise, we believe that theseconcerns need to be addressed before the technique is more widelyapplied.This paper includes data taken at McDonald Observatory of the Universityof Texas at Austin.
| MERLIN Astrometry of 11 Radio Stars
We report accurate positions in the International Celestial ReferenceFrame (ICRF) for 11 radio stars. Observations were made using theMulti-Element Radio Linked Interferometer Network at a radio frequencyof 5 GHz. The positions are estimated to be accurate at the 5 mas level.Positions were obtained directly in the ICRF by phase referencing theradio stars to ICRF quasars whose positions are estimated to be accurateat the 0.25 mas level. We use our results together with results ofprevious observations to obtain proper-motion estimates for these stars.The average proper-motion uncertainties are 1.1 mas yr-1 inμαcosδ and 1.2 mas yr-1 inμδ, comparable to the Hipparcos values.
| Study of FK Comae Berenices. VI. Spot motions, phase jumps and a flip-flop from time-series modelling
Aims.Time-series spot modelling was used to follow the longitude changesof active regions responsible for the light variability of FK Combetween 1987-2004. Methods: .The photometric data are analysed inthe time-series mode of a spot modelling code. A scenario of one polarand two low-latitude active regions (hereafter spots, for simplicity)depicts the light variations very well. The role of the polar spotremains unclear because photometry in general does not provide directlatitudinal surface resolution, however, Doppler imaging results of FKCom also show very high latitude or even polar spots besides thelow-latitude ones. We also used a light-curve inversion method toconfirm some of the results. Results: .The two low-latitude spotsslowly migrate around 90° and 270° longitudes with quasiperiodsof 5.8 and 5.2 years. The spots prefer to stay alternately on one or theother, but on the same hemisphere of the star, with a separation oftypically 90-140°. We monitored a flip-flop in the light curve of FKComae in 1999. The two low-latitude spots, being ≈140-180° fromeach other during the season, gradually decreased until they bothpractically vanished. Shortly thereafter, two new spots appeared andstarted to grow. One of the new spots was near the location of the oldone, whereas the other turned up 90° shifted in longitude;consequently, the activity as a whole was shifted to the otherhemisphere of the star. We followed a phase jump in 1997, when the twolow-latitude spots got closer in longitude and finally merged, or elseone of them vanished. A new spot appeared soon, shifted by 100° inlongitude, but the activity remained on the same hemisphere.Conclusions: .The difference between flip-flops and phase jumps isdemonstrated. The derived longitude changes of activity centres mayallow us to better constrain the theoretical modelling on thetime-behaviour of stellar magnetic activity.
| X-ray luminosity function of faint point sources in the Milky Way
We assessed the contribution to the X-ray (above 2 keV) luminosity ofthe Milky Way by different classes of low-mass binary systems and singlestars. We began by using the RXTE Slew Survey of the sky at|b|>10° to construct an X-ray luminosity function (XLF) of nearbyX-ray sources in the range 1030 ergs-1
| 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.
| 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.
| The quasar 3C 454.3: an extragalactic reference source for the gravity Probe B mission.
Not Available
| A Sun in the Spectroscopic Binary IM Pegasi, the Guide Star for the Gravity Probe B Mission
We present the first detection of the secondary of the spectroscopicbinary system IM Pegasi (HR 8703), the guide star for the NASA-Stanfordrelativity gyroscope mission Gravity Probe B. In support of thismission, high-resolution echelle spectra of IM Peg have been obtained onan almost nightly basis. Applying the technique of least-squaresdeconvolution, we achieve very high signal-to-noise ratio line profilesand detect the orbit of the secondary of the system. Combining almost700 new radial velocity measurements of both the primary and secondaryof the system with previous measurements, we derive improved orbitalparameters of the IM Peg system. Using these estimates along with thepreviously determined range of orbital inclination angles for thesystem, we find that the primary of IM Peg is a giant of mass 1.8+/-0.2Msolar, while the secondary is a dwarf of mass 1.0+/-0.1Msolar.
| Zonal Model of Starspots. Application to RS CVn Systems
Model results on starspots in 15 chromospherically active type RS CVnbinary systems are presented. The dependences of the parameters of thespots on the principal characteristics of the stars (spectral class,luminosity class, rotation, Rossby number) are examined. Latitudinaldrift of the spots, cycles in the spot activity, and differentialrotation are found in 9 of the stars.
| Indications for anti-solar differential rotation of giant stars
Observational evidence of anti-solar differential rotation of K-typegiant stars is presented. Time-series Doppler imaging based on 70 nightsof spectroscopic data was used to derive the spot evolution of thestellar surfaces. The relative differential rotation parameters(α) of the binary stars IM Peg, HD 208472, and HK Lac wereobtained using two techniques, cross-correlation analysis and thesheared-image method. Additionally, two previously published singlegiant stars are revisited and qualitatively compared to recenttheoretical models.
| Anomalous X-ray line ratios in the cTTS TW Hydrae
The cTTS TW Hya has been observed with high-resolution X-rayspectrometers. Previously found high densities inferred from He-like f/itriplets strongly suggested the detected X-ray emission to be dominatedby an accretion shock. Because of their radiation field dependenceHe-like f/i ratios do not provide unambiguous density diagnostics. Herewe present additional evidence for high densities from ratios of Fe xviilines. Key Fe xvii line ratios in TW Hya deviate from theoreticalexpectations at low densities as well as from the same measurements in alarge sample of stellar coronae. However, a quantitative assessment ofdensities is difficult because of atomic physics uncertainties. Inaddition, estimates of low optical depth in line ratios sensitive toresonance scattering effects also support a high-density emissionscenario in the X-ray emitting regions of cTTS.
| Measuring starspot temperature from line-depth ratios. II. Simultaneous modeling of light and temperature curves
We present and apply to VY Ari, IM Peg and HK Lac a new method todetermine spot temperatures (Tsp) and areas (Arel)from the analysis of simultaneous light curves and temperaturemodulations deduced from line-depth ratios. A spot model, developed byus, has been applied to light and temperature curves. Grids of solutionswith comparable χ-square have been found for a wide range of spottemperatures. The behavior of the solution grids for temperature andlight curves in the Tsp-Arel plane is verydifferent and a rather small and unique intersection area can be found.In our spot-model we used spectral energy distributions (SEDs) based onthe Planck law and on model atmospheres to evaluate the flux ratiobetween spots and unspotted photosphere and we found higher spottemperatures with SEDs based on model atmospheres than on the Plancklaw.Based on observations collected at Catania Astrophysical Observatory,Italy. Appendix A is only available in electronic form athttp://www.edpsciences.org
| 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.
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is 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/430/165}
| Microarcsecond astrometry using the SKA
The sensitivity and versatility of SKA will provide microarcsecastrometric precision and high quality milliarcsec-resolution images bysimultaneously detecting calibrator sources near the target source. Toreach these goals, we suggest that the long-baseline component of SKAcontains at least 25% of the total collecting area in a region between1000 and 5000 km from the core SKA. We also suggest a minimum of 60elements in the long-baseline component of SKA to provide the necessary(u v) coverage. For simultaneous all-sky observations, which provideabsolute astrometric and geodetic parameters, we suggest using 10independent subarrays each composed of at least six long-baselineelements correlated with the core SKA. We discuss many anticipated SKAlong-baseline astrometric experiments: determination of distance, propermotion and orbital motion of thousands of stellar objects; planetarymotion detections; mass determination of degenerate stars using theirkinetics; calibration of the universal distance scale from 10 to107 pc; the core and inner-jet interactions of AGN. With anincrease by a factor of 10 in absolute astrometric accuracy usingsimultaneous all sky observations, the fundamental quasar referenceframe can be defined to <10 μas and tied to the solar-systemdynamic frame to this accuracy. Parameters associated with the earthrotation and orientation, nutation, and geophysical parameters, can beaccurately monitored. Tests of fundamental physics include: solar andJovian deflection experiments, the sky frame accuracy needed tointerpret the gravity wave/pulsar-timing experiment, accurate monitoringof spacecraft orbits that impact solar system dynamics.
| 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.
| 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.
| Detection of X-Ray Resonance Scattering in Active Stellar Coronae
An analysis of Lyman series lines arising from hydrogen-like oxygen andneon ions in the coronae of the active RS CVn-type binaries II Peg andIM Peg, observed using the Chandra High Resolution Transmission GratingSpectrograph, shows significant decrements in the Lyα/Lyβratios as compared with theoretical predictions and with the same ratiosobserved in similar active binaries. We interpret these decrements interms of resonance scattering of line photons out of the line of sight;these observations present the first strong evidence of this effect inactive stellar coronae. The net line photon loss implies a nonuniformand asymmetric surface distribution of emitting structures on thesestars. Escape probability arguments, together with the observed lineratios and estimates of the emitting plasma density, imply typicalline-of-sight sizes of the coronal structures that dominate the X-rayemission of 1010 cm at temperatures of 3×106K and of 108 cm at 107 K. These sizes are an orderof magnitude larger than predicted by simple quasi-static coronal loopsmodels but are still very small compared to the several 1011cm radii of the underlying stars.
| Further Results of TiO-Band Observations of Starspots
We present measurements of starspot parameters (temperature and fillingfactor) on five highly active stars, using absorption bands of TiO, fromobservations made between 1998 March and 2001 December. We determinedstarspot parameters by fitting TiO bands using spectra of inactive G andK stars as proxies for the unspotted photospheres of the active starsand spectra of M stars as proxies for the spots. For three evolved RSCVn systems, we find spot filling factors between 0.28 and 0.42 for DMUMa, 0.22 and 0.40 for IN Vir, and 0.31 and 0.35 for XX Tri; thesevalues are similar to those found by other investigators usingphotometry and Doppler imaging. Among active dwarfs, we measured a lowerspot temperature (3350 K) for EQ Vir than found in a previous study ofTiO bands, and for EK Dra a lower spot temperature (~3800 K) than foundthrough photometry. For all active stars but XX Tri, we achieved goodphase coverage through a stellar rotational period. We also present ourfinal, extensive grid of spot and nonspot proxy stars.This paper includes data taken at McDonald Observatory of the Universityof Texas at Austin.
| 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
| RXTE all-sky slew survey. Catalog of X-ray sources at |b|>10o
We report results of a serendipitous hard X-ray (3-20 keV), nearlyall-sky (|b|>10o) survey based on RXTE/PCA observationsperformed during satellite reorientations in 1996-2002. The survey is80% (90%) complete to a 4σ limiting flux of ≈ 1.8 (2.5) ×10-11 erg s-1 cm-2 in the 3-20 keVband. The achieved sensitivity in the 3-8 keV and 8-20 keV subbands issimilar to and an order of magnitude higher than that of the previouslyrecord HEAO-1 A1 and HEAO-1 A4 all-sky surveys, respectively. A combined7× 103 sq deg area of the sky is sampled to flux levelsbelow 10-11 erg s-1 cm-2 (3-20 keV). Intotal 294 sources are detected and localized to better than 1 deg. 236(80%) of these can be confidently associated with a known astrophysicalobject; another 22 likely result from the superposition of 2 or 3closely located known sources. 35 detected sources remain unidentified,although for 12 of these we report a likely soft X-ray counterpart fromthe ROSAT all-sky survey bright source catalog. Of the reliablyidentified sources, 63 have local origin (Milky Way, LMC or SMC), 64 areclusters of galaxies and 100 are active galactic nuclei (AGN). The factthat the unidentified X-ray sources have hard spectra suggests that themajority of them are AGN, including highly obscured ones(NH>1023 cm-2). For the first timewe present a log N-log S diagram for extragalactic sources above4× 10-12 erg s-1 cm-2 at 8-20keV.Table 2 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/418/927
| Photoelectric photometry.
Not Available
| Very Long Baseline Interferometry Imaging of the RS Canum Venaticorum Binary Star System HR 5110
We present VLBI images of the RS CVn binary star HR 5110 (=BH CVn; HD118216), obtained from observations made at 8.4 GHz on 1994 May 29/30 insupport of the NASA/Stanford Gravity Probe B project. Our images show anemission region with a core-halo morphology. The core was 0.39+/-0.09mas (FWHM) in size, or 66%+/-20% of the 0.6+/-0.1 mas diameter of thechromospherically active K subgiant star in the binary system. The halowas 1.95+/-0.22 mas (FWHM) in size, or 1.8+/-0.2 times the 1.1+/-0.1 masseparation of the centers of the K and F stars. (The uncertainties givenfor the diameter of the K star and its separation from the F star eachreflect the level of agreement of the two most recent publisheddeterminations.) The core increased significantly in brightness over thecourse of the observations and seems to have been the site of flareactivity that generated an increase in the total flux density of ~200%in 12 hr. The fractional circular polarization simultaneously decreasedfrom ~10% to 2.5%.
| 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.
| Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very LongBaseline Array antenna, to determine the astrometric positions of 19radio stars in the International Celestial Reference Frame (ICRF). Thepositions of these stars were directly linked to the positions ofdistant quasars through phase-referencing observations. The positions ofthe ICRF quasars are known to 0.25 mas, thus providing an absolutereference at the angular resolution of our radio observations. Averagevalues for the errors in our derived positions for all sources were 13and 16 mas in αcosδ and δ, respectively, withaccuracies approaching 1-2 mas for some of the stars observed.Differences between the ICRF positions of the 38 quasars and thosemeasured from our observations showed no systematic offsets, with meanvalues of -0.3 mas in αcosδ and -1.0 mas in δ.Standard deviations of the quasar position differences of 17 and 11 masin αcosδ and δ, respectively, are consistent with themean position errors determined for the stars. Our measured positionswere combined with previous Very Large Array measurements taken from1978 to 1995 to determine the proper motions of 15 of the stars in ourlist. With mean errors of ~1.6 mas yr-1, the accuracies ofour proper motions approach those derived from Hipparcos and, for a fewof the stars in our program, are better than the Hipparcos values.Comparing the positions of our radio stars with the Hipparcos Catalogue,we find that at the epoch of our observations, the two frames arealigned to within formal errors of approximately 3 mas. This resultconfirms that the Hipparcos frame is inertial at the expected level.
| Wing Near-Infrared, TiO-Band, and V-Band Photometry of Chromospherically Active Star λ Andromedae
As a pilot program, Wing near-IR, TiO-band, and V-band photometry isbeing conducted of the RS Canum Venaticorum type, chromosphericallyactive, G8 IV-III star λ Andromedae. The objective is toinvestigate a possible relationship between variation of the ~54 dayrotationally starspot modulated visual light curve and TiO absorptionstrength. The TiO (γ,0,0) absorption band strength at λ=719nm is very sensitive to temperature for cool stars and manifests itselfin cooler starspot regions (T<=4000 K). TiO photometry has anadvantage over conventional photometry in that it provides unambiguousmeasures of the fractional cool starspot coverage. In addition, as thestars rotate, the variation in the TiO index yields information aboutthe longitudinal distribution of the starspots. Importantly, combiningthe TiO photometry with the V-band and near-IR light curves allows thediscrimination of white-light faculae (=hot spot) and cool starspotcontributions. Initial results of this study indicate that the observedV-band and near-IR continua light variations found for λ Andprimarily arise from bright spot (plage) features rather than darkstarspots as is usually assumed. This is in contrast to current theoriesthat the visual light variation is solely due to dark spots. Modelsusing both bright and dark spot features have been developed and arebeing used to fit the light and TiO-index curves. The models account forcool/hot spot characteristics such as projected filling factor andtemperature. The long-term variation of V light and TiO index have beeninvestigated to search for any activity cycles.
| The molecular Zeeman effect and diagnostics of solar and stellar magnetic fields. II. Synthetic Stokes profiles in the Zeeman regime
Recent advances in the computation of the Zeeman splitting of molecularlines have paved the way for their use as diagnostics of solar andstellar magnetic fields. A systematic study of their diagnosticcapabilities had not been carried out so far, however. Here weinvestigate how molecular lines can be used to deduce the magnetic andthermal structure of sunspots, starspots and cool stars. First, webriefly describe the Stokes radiative transfer of Zeeman-split molecularlines. Then, we compute Stokes spectra of TiO, OH, CH and FeH lines andinvestigate their diagnostic capabilities. We also compare the syntheticprofiles with observations. Spectra of TiO, OH and FeH are found to beinteresting diagnostics of sunspot magnetic fields. This is also truefor cool stars, where, however, the OH Stokes V profiles may requirevery high S/N data to be reliably employed. Finally we investigate thepotential of various molecular bands for high-contrast imaging of thesolar surface. The violet CN and CH bands turn out to be most promisingfor imaging the photosphere, the TiO bands are excellent for imagingsunspot umbrae, while the UV OH band can be used for imaging both thephotosphere and sunspots.
| 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.
| 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.
|
提交文章
相关链接
提交链接
下列团体成员
|
观测天体数据
目录:
|
