Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
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 relation between X-ray activity and rotation in intermediate-mass G giants
I study the relation between X-ray activity and rotation amongintermediate-mass single G giants. The results show evidence that thequiescent coronal activity of these stars, as measured by their X-raysurface flux, increases linearly with the angular rotation velocity andwith the inverse of the Rossby number. Even the most rapidly rotating Ggiants do not reach the canonical log(L_X/Lbol) ≈ -3saturation level. The effect of rapid rotation on these stars couldresult mainly in an increased coverage of their surface with magneticclose loop structures. The empirical activity-rotation relationshipaccounts for the occurrence of a maximum of magnetic activity in theatmosphere of intermediate-mass stars as they evolve off themain-sequence near the bottom of the red giant branch. Remarkably, therelation between X-ray to bolometric luminosity ratio and the Rossbynumber or rotation period for G giants differs from the power lawdependence with an index of about -2 that is observed for main-sequencestars. Possible implications for the dynamo generation of magneticfields on giants are discussed.
| The X-ray activity of the slowly rotating G giant δ CrB
δ CrB is a single G giant whose distinctive characteristicsinclude an X-ray luminosity exceptionally high for a slowly rotatingstar. δ CrB was observed in March 2003 by the {XMM{-}Newton}observatory. The X-ray spectra of δ CrB are described by a MEKALplasma model with two components at 6.5 ×106 K and107 K. Series of lines of highly ionized Fe and several linesof the Ly series are visible in RGS spectra, most notably from O and Ne.The oxygen abundance is similar to the average abundance of the otherelements but the Ne/O ratio found for δ CrB seems higher than inthe solar photosphere, reminiscent of a similar anomaly observed in asubset of solar flares and in active stellar coronae. The spectralfitting of the EPIC and RGS spectra of δ CrB suggests a coronaconfiguration with little contribution from quiet regions similar to theSun. On the contrary the temperature T ≈ 6.5 × 106 Kof the “cool” plasma component is reminiscent of solar typeactive regions, while the hot (T ≈ 107 K) component may becaused by disruptions of magnetic fields associated with a permanentflaring activity. The analysis results of the {XMM{-}Newton} observationof δ CrB were compared with those of other single G giants withsimilar spectral type, mass and evolutionary status but with higherrotation rates. The comparison suggests that rapid rotation (P < 9days) could increase the surface coverage with active regions and theflaring rate on G giants as expected from classical, helicity related,dynamo-driven activity. We argue that the X-ray emission of δ CrBand slowly rotating giants could be related to the existence of magneticfields induced by turbulent motion.
| The Indo-US Library of Coudé Feed Stellar Spectra
We have obtained spectra for 1273 stars using the 0.9 m coudéfeed telescope at Kitt Peak National Observatory. This telescope feedsthe coudé spectrograph of the 2.1 m telescope. The spectra havebeen obtained with the no. 5 camera of the coudé spectrograph anda Loral 3K×1K CCD. Two gratings have been used to provide spectralcoverage from 3460 to 9464 Å, at a resolution of ~1 Å FWHMand at an original dispersion of 0.44 Å pixel-1. For885 stars we have complete spectra over the entire 3460 to 9464 Åwavelength region (neglecting small gaps of less than 50 Å), andpartial spectral coverage for the remaining stars. The 1273 stars havebeen selected to provide broad coverage of the atmospheric parametersTeff, logg, and [Fe/H], as well as spectral type. The goal ofthe project is to provide a comprehensive library of stellar spectra foruse in the automated classification of stellar and galaxy spectra and ingalaxy population synthesis. In this paper we discuss thecharacteristics of the spectral library, viz., details of theobservations, data reduction procedures, and selection of stars. We alsopresent a few illustrations of the quality and information available inthe spectra. The first version of the complete spectral library is nowpublicly available from the National Optical Astronomy Observatory(NOAO) via ftp and http.
| 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.
| Rotation and Lithium Surface Abundances, Revisited
For giants in the Hertzsprung gap, the relations betweenTeff, vsini, and lithium surface abundances arereinvestigated and compared with the relations found for Hyadesmain-sequence F stars. For the Hyades main-sequence F stars, the vsinidecrease steeply around Teff~6450 K. At the same temperaturethe lithium surface abundances show a narrow, deep dip. For most giantsthere is also a steep decrease of vsini for Teff around 6450K. At this temperature the lithium surface abundances of the giants alsodecrease steeply and remain low for Teff<6400 K. Thechanges in rotation and Li surface abundances occur over a temperatureinterval of less than 300 K, which for a 2 Msolar giantcorresponds to an age interval of about 106 yr. Thesimultaneous steep decreases of rotation velocities and Li surfaceabundances indicate that for the giants these changes are due to thesame cause, which we suggest to be deep mixing. It then seems ratherlikely that for the Hyades main-sequence F5 V stars the decrease ofrotation and Li surface abundance is also caused by deep mixing. Wesuggest that in both cases the changes are related to the merging of thehydrogen and helium convection zones.
| 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
| The corona of HD 199178 (V 1794 Cygni)
HD 199178 (V 1794 Cygni), a chromospherically active late-type giantwith a high X-ray luminosity, was observed by the XMM-Newton spaceobservatory. Series of lines of highly ionized Fe and several Lymanlines of hydrogen-like ions are visible in the reflection gratingspectra, most notably from O and Ne. Analysis results suggest a scenariowhere the corona of HD 199178 is dominated by large magnetic structuressimilar in size to interconnecting loops between solar active regionsbut significantly hotter. The surface area coverage of these activeregions may approach up to a 90%. An hypothesis is that the interactionof these structures themselves induces a flaring activity in a smallscale not visible in the EPIC light curves that is responsible forheating HD 199178 plasma to coronal temperatures of T≥107 K. The intense X-ray activity of HD 199178 isrelated to its evolutionary position at the bottom of the red giantbranch. It is anticipated that its rotation will spin-down in the futurewith the effect of decreasing its helicity related, dynamo drivenactivity and suppressing large scale magnetic structures in its corona.
| A Far Ultraviolet Spectroscopic Explorer Survey of Coronal Forbidden Lines in Late-Type Stars
We present a survey of coronal forbidden lines detected in FarUltraviolet Spectroscopic Explorer (FUSE) spectra of nearby stars. Twostrong coronal features, Fe XVIII λ974 and Fe XIX λ1118,are observed in 10 of the 26 stars in our sample. Various other coronalforbidden lines, observed in solar flares, also were sought but notdetected. The Fe XVIII feature, formed at logT=6.8 K, appears to be freeof blends, whereas the Fe XIX line can be corrupted by a C I multiplet.FUSE observations of these forbidden iron lines at spectral resolutionλ/Δλ~15,000 provides the opportunity to studydynamics of hot coronal plasmas. We find that the velocity centroid ofthe Fe XVIII feature deviates little from the stellar rest frame,confirming that the hot coronal plasma is confined. The observed linewidths generally are consistent with thermal broadening at the hightemperatures of formation and show little indication of additionalturbulent broadening. The fastest rotating stars, 31 Com, α AurAb, and AB Dor, show evidence for excess broadening beyond the thermalcomponent and the photospheric vsini. The anomalously large widths inthese fast-rotating targets may be evidence for enhanced rotationalbroadening, consistent with emission from coronal regions extending anadditional ΔR~0.4-1.3R* above the stellar photosphere,or represent the turbulent broadening caused by flows along magneticloop structures. For the stars in which Fe XVIII is detected, there isan excellent correlation between the observed Röntgensatellit(ROSAT) 0.2-2.0 keV soft X-ray flux and the coronal forbidden line flux.As a result, Fe XVIII is a powerful new diagnostic of coronal thermalconditions and dynamics that can be utilized to study high-temperatureplasma processes in late-type stars. In particular, FUSE provides theopportunity to obtain observations of important transition region linesin the far-UV, as well as simultaneous measurements of soft X-raycoronal emission, using the Fe XVIII coronal forbidden line.
| Space Telescope Imaging Spectrograph Survey of Far-Ultraviolet Coronal Forbidden Lines in Late-Type Stars
We describe an extensive search with the Space Telescope ImagingSpectrograph (STIS) for ultraviolet coronal (T>106 K)forbidden lines in a sample of 29 F-M dwarfs, giants, and supergiants.Measuring coronal lines in the 1150-1700 Å band with STIS hasimportant advantages of superior velocity resolution and an absolutewavelength calibration compared with using the Chandra or XMM-Newtongrating spectrometers to observe permitted transitions of the same ionstages in the kilovolt X-ray region. Fe XII λλ1242, 1349(T~2×106 K) and Fe XXI λ1354 (107 K)are well known from solar studies and have been reported in previousstellar work. A search for other coronal forbidden lines in the1200-1600 Å region was largely negative. The few candidateidentifications (e.g., Ar XIII λ1330 and Ca XV λ1375) aretoo faint to be diagnostically useful. We add new dwarfs to the list ofFe XII detections, including the nearby solar twin α Cen A (G2 V).Clear detections of Fe XXI were obtained in dMe stars, active giants, ashort-period RS CVn binary, and possibly in active solar-type dwarfs. Wedeveloped a semiempirical method for removing the C I blend thatpartially affects the Fe XXI λ1354 profile. As discussed recentlyby Johnson et al., Capella (α Aur; G8 III+G1 III) displays clearFe XXI variability between Goddard High-Resolution Spectograph (GHRS)and STIS observations 4 yr apart, which is apparently due to asubstantial decline in the contribution from the G8 primary. We presentan alternative model of the GHRS and STIS era profiles using informationin the two sets of line shapes jointly, as well as knowledge of thebehavior of Fe XXI profiles of other late-G ``clump'' giants similar toCapella G8. The full survey sample also provides a context for theapparent variability: the Fe XXI flux of the G8 star in the GHRSspectrum is nearly identical (in LFeXXI/Lbol) toother clump giants of similar LX/Lbol, but it haddropped at least a factor of 6 in the STIS measurement. The He IIλ1640 Bα feature-which is thought to be responsive tocoronal irradiation-also showed significant changes between the GHRS andSTIS epochs, but the decrease in the G8 star was much smaller than FeXXI. The Fe XII flux displays a correlation with the ROSAT 0.2-2 keVX-ray flux that can be described by an α=0.5 power law. Fe XXIexhibits a steeper, perhaps linear (α=1), correlation with theROSAT flux down to an activity level ofLX/Lbol~10-5, below which detections ofthe coronal forbidden line are rare. There is no evidence of large,systematic Doppler shifts in either Fe XII λ1242 or Fe XXIλ1354. This suggests that the emissions arise dominantly inconfined structures, analogous to magnetic loops on the Sun, ratherthan, say, in a hot wind. The Fe XII and Fe XXI line widths generallyare close to thermal (FWHM~40-90 km s-1 atT~106.2-107.0 K), except for the Hertzsprung-gapgiants 31 Comae (G0 III) and Capella G1 and the K1 subgiant primary ofHR 1099, all of which show evidence for excess broadening in Fe XXI (FeXII is obscured in these objects by broad N V λ1242 features). Ifthe excess broadening is rotational, it implies that the hot coronae of``X-ray-deficient'' 31 Com and Capella G1 are highly extended, contraryto the compact structures suggested by recent density estimates in anumber of active coronal sources.
| The corona of HD 223460 (HR 9024)
HD 223460 (HR 9024), a chromospherically active late-type giant with ahigh X-ray luminosity, was observed by the XMM-Newton space observatory.Series of lines of highly ionized Fe and several Lyman lines ofhydrogen-like ions and triplet lines of helium-like ions are visible inthe reflection grating spectra, most notably from O and Ne. Analysisresults suggest a scenario where the corona of HD 223460 is dominated bylarge magnetic structures similar in size to interconnecting loopsbetween solar active regions but significantly hotter. The surface areacoverage of these active regions may approach up to 30%. A hypothesis isthat the interaction of these structures themselves induces a flaringactivity on a small scale not visible in the EPIC light curves that isresponsible for heating HD 223460 plasma to coronal temperatures of T>=107 K. The intense X-ray activity of HD 223460 isrelated to its evolutionary position at the bottom of the red giantbranch. It is anticipated that its rotation will spin down in the futurewith the effect of decreasing its helicity-related, dynamo-drivenactivity and suppressing large-scale magnetic structures in its corona.
| 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.
| Composite Spectra. XII. μ Leonis: An Evolving Am Binary
It is shown by techniques of spectrum disentangling that both componentsof the composite-spectrum binary ο Leo (V=3.52 mag) have Amcharacteristics, even though the primary is an evolving giant(logg=3.25) with Teff~6100 K. This is believed to be thefirst isolation of such a cool Am star, and the finding challenges thetheories of diffusion, which are widely accepted as the cause ofmetallicism. The primary component of ο Leo appears to bedeficient in Ca and Sc, as are classical Am stars-though the δ Delstars which are described as evolved Am stars are not-and thereforeseems to be in a class on its own. It is suggested that the unusualstate of the primary component is attributable either to its currentstate of rapid evolution or to regular Am-star evolution that isdifficult to recognize spectroscopically. This paper describes thetechnical problems that have hitherto prevented the identification ofsuch unexpected properties in this easily observed binary, determinesthe physical parameters of the component stars, examines theirevolutionary states, and debates the possible classification of thegiant component. Future directions for this work will include detailedchemical composition analyses and an observing program designed tosearch for other cases of substantially evolved Am stars.
| Optical positions of 55 radio stars from astrolabe observations from the Yunnan Observatory
The observations by the photoelectric astrolabe at Yunnan Observatoryrelative to the Hipparcos Catalogue and the optical positions of 55radio stars were obtained from observations between 1991 and 2000. Theyall resulted from processing the photon counts obtained by means of theastrolabe after the automation of the instrument. There are 46 stars incommon with the Hipparcos Catalogue. Tables \ref{t1} and \ref{t2} arealso available in electronic form at the CDS via anonymous ftpcdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/383/1062.
| Late-type members of young stellar kinematic groups - I. Single stars
This is the first paper of a series aimed at studying the properties oflate-type members of young stellar kinematic groups. We concentrate ourstudy on classical young moving groups such as the Local Association(Pleiades moving group, 20-150Myr), IC 2391 supercluster (35Myr), UrsaMajor group (Sirius supercluster, 300Myr), and Hyades supercluster(600Myr), as well as on recently identified groups such as the Castormoving group (200Myr). In this paper we compile a preliminary list ofsingle late-type possible members of some of these young stellarkinematic groups. Stars are selected from previously established membersof stellar kinematic groups based on photometric and kinematicproperties as well as from candidates based on other criteria such astheir level of chromospheric activity, rotation rate and lithiumabundance. Precise measurements of proper motions and parallaxes takenfrom the Hipparcos Catalogue, as well as from the Tycho-2 Catalogue, andpublished radial velocity measurements are used to calculate theGalactic space motions (U, V, W) and to apply Eggen's kinematic criteriain order to determine the membership of the selected stars to thedifferent groups. Additional criteria using age-dating methods forlate-type stars will be applied in forthcoming papers of this series. Afurther study of the list of stars compiled here could lead to a betterunderstanding of the chromospheric activity and their age evolution, aswell as of the star formation history in the solar neighbourhood. Inaddition, these stars are also potential search targets for directimaging detection of substellar companions.
| The Physical Basis of Luminosity Classification in the Late A-, F-, and Early G-Type Stars. I. Precise Spectral Types for 372 Stars
This is the first in a series of two papers that address the problem ofthe physical nature of luminosity classification in the late A-, F-, andearly G-type stars. In this paper, we present precise spectralclassifications of 372 stars on the MK system. For those stars in theset with Strömgren uvbyβ photometry, we derive reddenings andpresent a calibration of MK temperature types in terms of the intrinsicStrömgren (b-y)0 index. We also examine the relationshipbetween the luminosity class and the Strömgren c1 index,which measures the Balmer jump. The second paper will address thederivation of the physical parameters of these stars, and therelationships between these physical parameters and the luminosityclass. Stars classified in this paper include one new λ Bootisstar and 10 of the F- and G-type dwarfs with recently discoveredplanets.
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,354 of the stars in Part III are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives, in addition to the SI mode, the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| CaII K Emission-Line Asymmetry among Red Giants Detected by the ROSAT Satellite
Spectra of the Ca II H and K lines are reported for a number of fieldgiants from which soft X-ray emission was detected by the ROSATsatellite. Several of these stars are RS CVn systems and exhibit verystrong Ca II emission. The majority of the noninteracting giants in thesample have MV>-2.0, as determined from Hipparcosparallaxes, and spectral types earlier than K3. The Ca II Kemission-line profile for these stars is most often double-peaked andasymmetric, with the short-wavelength peak being stronger than thelongward peak. This asymmetry is in the same sense as for the integrateddisk of the Sun. The X-ray and Ca II K-line data indicate that giants ofspectral types G and early K have coronae and chromospheres seeminglyanalogous to those of the Sun. Four M giants that were detected by ROSATwere also observed. Their Ca II emission spectra show asymmetries inwhich the violet wing is weaker than the red wing, a phenomenon that istypical of M giants in general and indicative of mass outflows in theirchromospheres. The majority of these M giants, but not all, are known tobe in binary systems, so it is possible that the X-ray emission for atleast some of them may come from a companion. Alternatively, some or allof these M giants may be examples of hybrid stars.
| RBSC-NVSS Sample. I. Radio and Optical Identifications of a Complete Sample of 1556 Bright X-Ray Sources
We cross-identified the ROSAT Bright Source Catalog (RBSC) and the NRAOVLA Sky Survey (NVSS) to construct the RBSC-NVSS sample of the brightestX-ray sources (>=0.1 counts s-1~10-12 ergscm-2 s-1 in the 0.1-2.4 keV band) that are alsoradio sources (S>=2.5 mJy at 1.4 GHz) in the 7.8 sr of extragalacticsky with |b|>15deg and δ>-40deg. Thesky density of NVSS sources is low enough that they can be reliablyidentified with RBSC sources having rms positional uncertainties>=10". We used the more accurate radio positions to make reliableX-ray/radio/optical identifications down to the POSS plate limits. Weobtained optical spectra for many of the bright identifications lackingpublished redshifts. The resulting X-ray/radio sample is unique in itssize (1557 objects), composition (a mixture of nearly normal galaxies,Seyfert galaxies, quasars, and clusters), and low average redshift[~0.1].
| Rotation and lithium in single giant stars
In the present work, we study the link between rotation and lithiumabundance in giant stars of luminosity class III, on the basis of alarge sample of 309 single stars of spectral type F, G and K. We havefound a trend for a link between the discontinuity in rotation at thespectral type G0III and the behavior of lithium abundances around thesame spectral type. The present work also shows that giant starspresenting the highest lithium contents, typically stars earlier thanG0III, are those with the highest rotation rates, pointing for adependence of lithium content on rotation, as observed for otherluminosity classes. Giant stars later than G0III present, as a rule, thelowest rotation rates and lithium contents. A large spread of about fivemagnitudes in lithium abundance is observed for the slow rotators.Finally, single giant stars with masses 1.5 < M/Msun<=2.5 show a clearest trend for a correlation between rotational velocityand lithium abundance. Based on observations collected at theObservatoire de Haute -- Provence (France) and at the European SouthernObservatory, La Silla (Chile). Table 2 is only available electronicallywith the On-Line publication athttp://link.springer.de/link/service/00230/
| Optical Positions of 44 Radio Stars from Astrolabe Observations
The observations made with the photoelectric astrolabe at YunnanObservatory since 1986 have been reprocessed in the Hipparcos Catalogue,and the optical positions of 44 radio stars obtained. These are all thestars in common with the Hipparcos Catalogue.
| Starspot photometry with robotic telescopes. UBV(RI)_C and by light curves of 47 active stars in 1996/97
We present continuous multicolor photometry for 47 stars from October1996 through June 1997. Altogether, 7073 V(RI)_c, UBV, and by datapoints, each the average of three individual readings, were acquiredwith three automatic photoelectric telescopes (APTs) at FairbornObservatory in southern Arizona. Most of our targets arechromospherically active single and binary stars of spectral type G to Kbut there are also four pre-main-sequence objects and three pulsatingstars in our sample. The light variability is generally due torotational modulation of an asymmetrically spotted stellar surface andtherefore precise rotational periods and their seasonal variations aredetermined from Fourier analysis. We also report on photometricvariations of gamma CrB (A0V) with a period of 0.44534 days. All dataare available in numerical form. All data are available from CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| A catalog of rotational and radial velocities for evolved stars
Rotational and radial velocities have been measured for about 2000evolved stars of luminosity classes IV, III, II and Ib covering thespectral region F, G and K. The survey was carried out with the CORAVELspectrometer. The precision for the radial velocities is better than0.30 km s-1, whereas for the rotational velocity measurementsthe uncertainties are typically 1.0 km s-1 for subgiants andgiants and 2.0 km s-1 for class II giants and Ib supergiants.These data will add constraints to studies of the rotational behaviourof evolved stars as well as solid informations concerning the presenceof external rotational brakes, tidal interactions in evolved binarysystems and on the link between rotation, chemical abundance and stellaractivity. In this paper we present the rotational velocity v sin i andthe mean radial velocity for the stars of luminosity classes IV, III andII. Based on observations collected at the Haute--Provence Observatory,Saint--Michel, France and at the European Southern Observatory, LaSilla, Chile. Table \ref{tab5} also available in electronic form at CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Absolute declinations with the photoelectric astrolabe at Calern Observatory (OCA)
A regular observational programme with a photoelectric astrolabe havebeen performed at ``Observatoire du Calern" (Observatoire de laCôte d'Azur, OCA, phi = +43() o44′55.011″; lambda =-0() h27() m42.44() s, Calern, Caussols, France) for the last twentyyears. It has been almost fully automatized between 1984 and 1987. Since1988 the photoelectric astrolabe was used without any modification. Inaddition to determining the daily orientation of the local vertical, theyearly analysis of the residuals permits to derive corrections to theused star catalogue \cite[(Vigouroux et al. 1992)]{vig92}. A globalreduction method was applied for the ASPHO observations. The new form ofthe equations \cite[(Martin & Leister 1997)]{mar97} give us thepossibility of using the entire set of the observing program using datataken at two zenith distances (30() o and 45() o). The program containsabout 41648 stars' transits of 269 different stars taken at``Observatoire du Calern" (OCA). The reduction was based on theHIPPARCOS system. We discuss the possibility of computing absolutedeclinations through stars belonging simultaneously to the 30() o and45() o zenith distances programmes. The absolute declination correctionswere determined for 185 stars with precision of 0.027arcsec and thevalue of the determined equator correction is -0.018arcsec +/-0.005arcsec . The instrumental effects were also determined. The meanepoch is 1995.29. Catalogue only available at CDS in electronic from viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Evolution of X-ray activity and rotation on G-K giants
The recent availability of stellar parallaxes provided by the Hipparcosstar catalogue (ESA 1997) enables an accurate determination of thepositions of single field giants in a theoretical H-R diagram and areliable estimate of their masses. The present study combines these newastrometric data with previously published X-ray fluxes and rotationalvelocities. The results confirm the existence of a sharp decrease ofX-ray emission at spectral type K1 for 2.5 M_sun < M < 5 M_sungiants. The study shows that the rotational velocity of these starsreaches a minimum at the same location in the H-R diagram. However, notight relationship between X-ray luminosities and projected equatorialvelocities was found among the sample stars. I suggest that theseresults could reflect the importance of differential rotation indetermining the level of coronal emission among >= 2.5Msun G and K giants. The restoration of rigid rotation at thebottom of the red giant branch could prevent the maintenance of largescale magnetic fields, thus explaining the sharp decrease of coronalX-ray emission at spectral type K1.
| The Coronae of Moderate-Mass Giants in the Hertzsprung Gap and the Clump
We have used the Roentgensatellit (ROSAT), the Extreme UltravioletExplorer (EUVE), and the Hubble Space Telescope (HST) to measure X-rayand ultraviolet emissions of moderate-mass (~2-3 Mȯ) giants in theHertzsprung gap (spectral types early F to mid-G) and the post-heliumflash "clump" (~G8-K0). Our motivation was to document the evolution ofhot coronae (T > 106 K) along the post-main-sequence trajectoriestraveled by such stars in order to gain insight concerning the "X-raydeficiency" of the F-G0 giants and the strong braking of stellarrotation at the red edge of the Hertzsprung gap. With few exceptions,Hertzsprung gap and clump giants observed by ROSAT PSPC show hot (T ~107 K) coronal energy distributions, regardless of any X-ray deficiency.EUVE spectra of gap star 31 Com (G0 III) indicate a broad coronalemission measure hump at ~107.2 K, while the active clump giant betaCeti (K0 III) displays a sharp peak at ~106.8 K, as seen previously inthe mixed clump/gap binary Capella ( alpha Aur: G8 III + G0 III). Thegap giants upsilon Peg (F8 III) and 24 UMa (G4 III) have EUV emissionsof intermediate temperature (~107.0 K). The stars 31 Com, psi 3 Psc (G0III), and beta Cet exhibit redshifted transition zone (TZ: ~105 K) linesin HST GHRS spectra, as reported earlier in Procyon ( alpha CMi: F5IV-V) and Capella G0. Such redshifts on the Sun are thought to signifyflows in magnetic loops. beta Cas (F2 III)--a rare soft coronal sourceamong the gap stars--displays blueshifts of C IV and O IV], althoughemissions at cooler and hotter temperatures are near the photosphericvelocity. The remarkably broad line profiles of the fastest rotating gapgiants suggest that the 105 K "subcoronal" emission zones extend to h~R_{*} above the photosphere, about 50 scale heights. In contrast to theTZ line redshifts, the upper chromospheric emissions (e.g., Mg II and SiIII) of 31 Com and psi 3 Psc have blueshifted cores. Blue-asymmetricpeaks in the solar Mg II lines are thought to indicate dynamical heatingin the chromosphere. Observations of the H I Ly alpha feature of 31 Comtaken 9 months apart reveal striking profile changes, reminiscent ofthose noted previously in the Ly alpha blue peak of the Capella G0 star.We used the far-ultraviolet diagnostics, in combination with ROSAT X-rayphotometry and EUVE high-excitation line strengths, to constrainphysical models of the stellar outer atmospheres. Quasi-static magneticloops can simulate the empirical coronal emission measures of the giantstars, but the inferred pressures for sensible loop lengths conflictwith direct measurements of subcoronal densities. Furthermore, the highrate of emission at ~105 K cannot be explained by thermal conductiondown the legs of hot quasi-static loops. On the other hand, the possibleexistence of elongated (l ~ R_{*} ) emission structures on the gapgiants leads to a speculative scenario to explain the X-ray deficiency.It is based on the increased importance of the dynamical filling phase("explosive evaporation") of the loop life cycle; conductive cooling,yielding TZ emissions at the footpoints, when the heating isinterrupted; and the possibility for transitions between "hot" and"cool" energy balance solutions owing to dynamical suspension andcentrifugal trapping of the cooling gas. The long loops might representa vestigial global "magnetosphere" inherited from the main-sequencephase, which ultimately is disrupted near ~G0 by the deepeningconvective envelope and growth of a more solar-like dynamo. Coronalemissions might be boosted temporarily as the X-ray deficiency isremoved but soon would be quenched by wind braking previously inhibitedby the magnetospheric "dead zone."
| The ROSAT all-sky survey catalogue of optically bright late-type giants and supergiants
We present X-ray data for all late-type (A, F, G, K, M) giants andsupergiants (luminosity classes I to III-IV) listed in the Bright StarCatalogue that have been detected in the ROSAT all-sky survey.Altogether, our catalogue contains 450 entries of X-ray emitting evolvedlate-type stars, which corresponds to an average detection rate of about11.7 percent. The selection of the sample stars, the data analysis, thecriteria for an accepted match between star and X-ray source, and thedetermination of X-ray fluxes are described. Catalogue only available atCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Classification of Population II Stars in the Vilnius Photometric System. II. Results
The results of photometric classification of 848 true and suspectedPopulation II stars, some of which were found to belong to Population I,are presented. The stars were classified using a new calibrationdescribed in Paper I (Bartkevicius & Lazauskaite 1996). We combinethese results with our results from Paper I and discuss in greaterdetail the following groups of stars: UU Herculis-type stars and otherhigh-galactic-latitude supergiants, field red horizontal-branch stars,metal-deficient visual binaries, metal-deficient subgiants, stars fromthe Catalogue of Metal-deficient F--M Stars Classified Photometrically(MDPH; Bartkevicius 1993) and stars from one of the HIPPARCOS programs(Bartkevicius 1994a). It is confirmed that high galactic latitudesupergiants from the Bartaya (1979) catalog are giants or even dwarfs.Some stars, identified by Rose (1985) and Tautvaisiene (1996a) as fieldRHB stars, appear to be ordinary giants according to our classification.Some of the visual binaries studied can be considered as physical pairs.Quite a large fraction of stars from the MDPH catalog are found to havesolar metallicity. A number of new possible UU Herculis-type stars, RHBstars and metal-deficient subgiants are identified.
| A Search for Optical Flares in Chromospherically Active Stars
We have surveyed four years of archival UBV photometry of 69chromospherically active stars obtained with the Phoenix-10 automaticphotoelectric telescope in search of optical flares on these stars. Wedemonstrate taht, while flares can be detected in these data, the numberof events observed on evolved stars remains very small. Flares werefound only on UX Arietis, II Pegasi, and AR Piscium. (SECTION: Stars)
| A Catalogue of Correlations Between Eclipsing Binaries and Other Categories of Double Stars
Among the 9110 stars in The Bright Star Catalogue, there are 225eclipsing or ellipsoidal variables. A search has been made for these incatalogues of spectroscopic binaries, visual double or multiple stars,speckle interferometry, occulation binaries, and galatic clusters. Themajority of the photometric binaries are also members of groups ofhigher multiplicity. The variables are in systems ranging from one to 91stars, five on the average. 199 are either spectroscopic binaries (SB)or stars with variable radial velocity, with orbital periods known for160. Photometric periods are lacking for 48 while SB periods areavailable for 23 of these. Observers with photoelectric equipment areencouraged to plan observations to test if the SB periods are consistentwith photometric data. Observers are likewise encouraged to examinethose stars for which the photometric and SB periods appear to beinconsistent. Parallaxes are available for 86 of the stars, 41 of themindicating distances nearer than 50 parsecs.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
| Takýmyýldýz: | Zincirli Prenses |
| Sað Açýklýk: | 23h49m41.00s |
| Yükselim: | +36°25'31.0" |
| Görünürdeki Parlaklýk: | 5.9 |
| Uzaklýk: | 134.953 parsek |
| özdevim Sað Açýklýk: | 1.1 |
| özdevim Yükselim: | -48.7 |
| B-T magnitude: | 6.844 |
| V-T magnitude: | 5.954 |
Kataloglar ve belirtme:
|
