תוכן
תמונות
הוסף תמונה שלך
DSS Images Other Images
מאמרים קשורים
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.
| Long-term monitoring of active stars. IX. Photometry collected in 1993
As a part of an extensive program focused on the global properties andevolution of active stars, high-precision UBV(RI)_c and UBV photometryof 31 selected stars is presented. The UBV(RI)_c observations werecollected at the European Southern Observatory over the 31 December1992-18 January 1993 and the 20 November-3 December 1993 intervals.Additional UBV photometry obtained by the ``Phoenix" and by the CataniaAstrophysical Observatory Automatic Photoelectric Telescopes from 1990to 1993 is also presented for some of the program stars. Significantevolution of the light curves, period variations and evidence forlong-term variability of the global degree of spottedness are found.Some spectral classifications are revised and the inferred photometricparallaxes are compared, whenever possible, with the values measured bythe Hipparcos satellite. These observations are finalized to theconstruction of an extended photometric database, which can giveimportant clues on topics such as the stability of spotted areas,differential rotation, solar-like cycles and the correlation betweeninhomogeneities at different atmospheric levels. Based on data collectedat the European Southern Observatory, La Silla, Chile. Tables and thecomplete data set are also available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/367/910
| The Vienna-KPNO search for Doppler-imaging candidate stars. I. A catalog of stellar-activity indicators for 1058 late-type Hipparcos stars
We present the results from a spectroscopic Ca ii H&K survey of 1058late-type stars selected from a color-limited subsample of the Hipparcoscatalog. Out of these 1058 stars, 371 stars were found to showsignificant H&K emission, most of them previously unknown; 23% withstrong emission, 36% with moderate emission, and 41% with weak emission.These spectra are used to determine absolute H&K emission-linefluxes, radial velocities, and equivalent widths of theluminosity-sensitive Sr ii line at 4077 Ä. Red-wavelengthspectroscopic and Strömgren y photometric follow-up observations ofthe 371 stars with H&K emission are used to additionally determinethe absolute Hα -core flux, the lithium abundance from the Li i6708 Å equivalent width, the rotational velocity vsin i, theradial velocity, and the light variations and its periodicity. Thelatter is interpreted as the stellar rotation period due to aninhomogeneous surface brightness distribution. 156 stars were found withphotometric periods between 0.29 and 64 days, 11 additional systemsshowed quasi-periodic variations possibly in excess of ~50 days. Further54 stars had variations but no unique period was found, and four starswere essentially constant. Altogether, 170 new variable stars werediscovered. Additionally, we found 17 new SB1 (plus 16 new candidates)and 19 new SB2 systems, as well as one definite and two possible new SB3systems. Finally, we present a list of 21 stars that we think are mostsuitable candidates for a detailed study with the Doppler-imagingtechnique. Tables A1--A3 are only available in electronic form at theCDS via anonymous 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
| On X-Ray Variability in Active Binary Stars
We have compared the X-ray emissions of active binary stars observed atvarious epochs by the Einstein and ROSAT satellites in order toinvestigate the nature of their X-ray variability. The primary aim ofthis work is to determine whether or not active binaries exhibitlong-term variations in X-ray emission, perhaps analogous to theobserved cyclic behavior of solar magnetic activity. We find that, whilethe mean level of emission of the sample remains steady, comparison ofdifferent ROSAT observations of the same stars shows significantvariation on timescales <~2 yr, with an ``effective variability''ΔI/I=0.32+/-0.04, where I and ΔI represent the mean emissionand variation from the mean emission, respectively. A comparison of theROSAT All-Sky Survey and later pointed observations with earlierobservations of the same stars carried out with Einstein yields onlymarginal evidence for a larger variation (ΔI/I=0.38+/-0.04 forEinstein vs. ROSAT All-Sky Survey and 0.46+/-0.05 for Einstein vs. ROSATpointed) at these longer timescales (~10 yr), thus indicating thepossible presence of a long-term component to the variability. Whetheror not this long-term component is due to the presence of cyclicvariability cannot be decided on the basis of existing data. However,assuming that this component is analogous to the observed cyclicvariability of the Sun, we find that the relative magnitude of thecyclic component in the ROSAT passband can, at most, be a factor of 4,i.e., I_cyc/I_min<4. This is to be compared with the correspondingbut significantly higher solar value of ~10-10^2 derived from GOES,Yohkoh, and Solrad data. These results are consistent with thesuggestions of earlier studies that a turbulent or distributive dynamomight be responsible for the observed magnetic activity on the mostactive, rapidly rotating stars.
| Radio star catalogue observed in San Juan (RSSJ95)
Using the data observed in San Juan with the photoelectric AstrolabeMark II of the Beijing Astronomical Observatory from February, 1992 toMarch, 1997, the radio star catalogue in San Juan(RSSJ95) has beencompiled. There are 69 radio stars in this catalogue. The positions ofthe radio stars are for the epoch of observation and the equinox J2000.0and a system close to that of the system FK5. The mean precisions are+/-2.2 ms and +/-0.035'' in right ascensions and declinations,respectively. The magnitudes of stars are from 0.9 to 10.7. Thedeclinations are from -2fdg 5 to -60(deg) . The mean epoch is 1995.1.Finally, the comparison results between the Hipparcos catalogue andRSSJ95 are given.
| The age-mass relation for chromospherically active binaries. III. Lithium depletion in giant components
We present a study of the lithium abundances of a sample of evolvedcomponents of Chromospherically Active Binary Systems. We show that asignificant part of them have lithium excesses, independently of theirmass and evolutionary stage. Therefore, it can be concluded that Liabundance does not depend on age for giant components of CABS. Theseoverabundances appear to be closely related to the stellar rotation, andwe interpret them as a consequence of the transfer of angular momentumfrom the orbit to the rotation as the stars evolve in and off the MainSequence, in a similar way as it happens in the dwarf components of thesame systems and in the Tidally Locked Binaries belonging to the Hyadesand M67. Based on observations collected with the 2.2\,m telescope ofthe German-Spanish Observatorio de Calar Alto (Almeria, Spain), and withthe 2.56\,m Nordic Optical Telescope in the Spanish Observatorio delRoque de los Muchachos of the Instituto de Astrof\'\i sica de Canarias(La Palma, Spain)
| Physical properties of active stars and stellar systems
We study spectra of 18 active stars and stellar systems, drawn fromlists of known active binaries or newly discovered extreme-ultravioletand soft X-ray sources. Using spectral synthesis and template modelling,we measure (often for the first time) a variety of physical propertiesof the stars, including spectral types, luminosities and vsini values.We report direct observation of previously undetected secondarycomponents in three systems, and refine the orbital elements for two ofthese (HR 7275 and HD 217344).
| San Juan radio star catalogue and comparison with HIPPARCOS catalogue.
Not Available
| Alignment of rotational axes in asynchronous late type binaries
41 RS CVn and BY Dra type binaries with reliable orbital and physicalparameters have been found to rotate asynchronously with the orbitalrevolution. For this sample inclinations of the rotational axes, i_r, ofthe primaries and the orbital inclinations, i_o, are independentlydetermined to test the alignment between the equatorial and the orbitalplanes. The observed difference Delta i=| {i_o} - {i_r}| measures adeviation from coplanarity of these planes. It turns out that mostasynchronous systems prove to be misaligned, especially when theinfluence of the position angle is taken into account. Thus, thegenerally accepted assumption that rotational axes are perpendicular toorbital planes is not valid for asynchronous RS CVn and BY Dra typebinaries. The influence of the position angle of the observer on thederived values of Delta i can be studied only by statistical methods.The distribution of the observed Delta i is compared wih the expectedvalues of Delta i() e for two cases: a) the rotational axes are randomlyinclined to the orbital planes, and b) the rotational axes areperpendicular to the orbital planes. The best fit to our observed sampleof asynchronous binaries is obtained for the case a), while for thesynchronous RS CVn and BY Dra type binaries the rotational axes areperpendicular to their orbital planes. The appendix is available onlyelectronically with the on-line publication athttp://link.springer.de/link/service/journals/00230
| Accurate Positions for Radio Stars as Determined from CCD Observations in the Extragalactic Reference Frame.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1997AJ....114.1679S&db_key=AST
| The ROSAT All-Sky Survey of Active Binary Coronae. III. Quiescent Coronal Properties for the BY Draconis--Type Binaries
We present X-ray observations of 35 active late-type BY Draconis dwarfbinary systems and 28 evolved binary systems, similar in nature to theRS Canum Venaticorum systems, obtained with the Position SensitiveProportional Counter (PSPC) during the ROSAT All-Sky Survey phase of themission. Of this sample, 52 targets were detected in exposures ofroughly 600 s or less. When these new data are combined with the earlierresults from Dempsey et al. (1993b), this survey represents the largestsample of active binary systems observed to date at any wavelength,including X-rays. We expand our investigation of how coronal properties(e.g., surface flux, luminosity, etc.) correlate with stellar parameters(e.g., rotation period, color, etc.) and confirm the conclusions ofDempsey et al. (1993b). Rotation period provides the best correlationwith X-ray surface flux with F_{{X}}~P^{-0.59+/-0.10}_{{rot}} for theentire sample. We find no evidence for a "basal" or nonmagnetic X-rayflux component. We model the low-resolution pulse-height spectra for 12systems with two-temperature thermal plasmas. The derived temperaturesfor the BY Dra systems are identical to those previously derived foractive evolved giants and subgiants in close binaries (Dempsey et al.1993c). We also show that the dependence of temperature and emissionmeasures on rotation period is the same for the dwarf, subgiant, andgiant binaries.
| On the link between rotation and lithium depletion in subgiant stars.
We present projected rotational velocity Vsini for 65 F, G and Ksubgiant stars. In this work we study the link between rotation andLithium abundance in such class of luminosity. In particular, we lookfor possible effects of rotational braking on the Lithium depletion. Weshow that a sharp decrease in Lithium abundances parallels therotational discontinuity, located at F8IV, in all its aspects. We havefound some dependence of Lithium abundance upon rotation for singlesubgiants, but there is no evident correlation between Lithium abundanceand rotational velocity for binary subgiant stars.
| Mechanical heating levels in ultra-fast rotating dwarfs.
Using available observational data we have estimated the total radiativeoutput from the chromospheric/coronal plasma as a result of magneticheating for several F, G, K & M dwarfs. When tabulated as a functionof the bolometric flux, a clear 'levelling-off/saturation' exists forthe faster rotators as found previously when using single spectrallines. However, using recent chromospheric radiative transfer modelcalculations it is shown that this 'saturation' can be due to asignificantly larger fraction of the losses being radiated by hydrogenin the ultraviolet/optical/infrared continuum regions. The implicationbeing that these ultra-fast rotators have a high density chromosphere,plus a high temperature minimum, although additional chromospheric modelcalculations are required which include better molecular line opacitiesplus and an extension of the models for stars with higher effectivetemperatures.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Ultraviolet and Radio Observations of the Young Rapidly Rotating k0 Dwarf Star HD197890
Abstract image available at:http://adsabs.harvard.edu/abs/1994MNRAS.267..918R
| Are the Rotational Axes Perpendicular to the Orbital Planes in the Binary Systems. I. Asynchronous Long-Period RS CVn Stars
We show that only 12 out of 27 analyzed binaries have data good enoughfor reliable determination of iorb and irot. In 4systems (out of these 12) assumption about coplanarity of rotationalequatorial plane and orbital plane is justified. In the remaining casesrotational axes are not perpendicular to the orbital planes(non-coplanar case). The commonly accepted assumption about thecoplanarity of rotational and orbital planes should be revised.
| The active dynamo stars: RS CVn, BY Dra, FK Com, Algol, W UMa, and T Tau
Not Available
| Longterm Monitoring of Active Stars - Part Three - UBV Ri/c Photometry of 14 Southern Hemisphere Variables
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102..655C&db_key=AST
| A catalog of chromospherically active binary stars (second edition)
The catalog contains 206 spectroscopic binary systems with at least onelate-type component that shows Ca II H and K emission in its spectrum.These systems include the classical RS CVn binaries and BY Dra binaries.The catalog summarizes information on the photometric, spectroscopic,orbital, and physical properties of the systems as well as space motionsand positions. Up to 42 'parameters' for each stellar system are listedfollowed by the appropriate reference to direct interested colleagues tothe original papers. A comprehensive selection of further informationfor each star is given in the individual notes. In addition, the catalogcontains a candidate list of 138 stars that have similar characteristicsbut are not definitely known binaries or have not had Ca II H and Kemission observed.
| Lithium in Rs-Canum Binaries and Related Chromospherically Active Stars - Part Two - Spectrum Synthesis Analysis
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&A...273..194R&db_key=AST
| Kinematics and age of RS Canum Venaticorum and by Draconis stars
Space velocities of 146 chromospherically active binary stars have beencalculated. Containing F-M spectral types on the main sequence togetherwith G and K giants and subgiants, this very heterogeneous sample hasbeen divided into groups in order to segregate stars which have similarkinematics and ages. After many trials for different criteria, thesample was divided into five groups - two groups for giants, two groupsfor main-sequence systems, and one group for main-sequence systems.Kinematics of subgiants implies a stellar age of about 2-3 Gyr. Youngand old groups of giants and main-sequence systems could becharacterized by a kinematical age of about 1 Gyr and more than 5 Gyr,respectively. These ages are estimated approximately according to spacevelocity distributions and dispersions in velocity space in each group.Inferred ages for the groups above agrees with circulation, rotationactivity relation, and stellar evolution theories.
| Lithium in RS CVn binaries and related chromospherically active stars. I - Observational results
The present survey of the Li I 6708 A line in a sample of spectral typeG and K stars with luminosity classes III, IV, and V shows that manyK-type stars in the sample, including a large number of RS CVn binaries,show an anomalously high Li abundance relative to typical inactive starsof the same spectral type. Only a few stars in the sample are likely tobe premain sequence objects of stars which have recently arrived on themain sequence. Mechanisms that could lead to the enhanced Li absorptionin chromospherically active stars are discussed; these encompass largecool spots on the stellar surface, the production of Li in stellarflares by spallation reactions, and the evolution from main-sequenceprogenitors without, or with very shallow, outer convective zones.
| An astrometric catalogue of radio stars
The first part is presented of a radio star catalog encompassing 186objects whose selection was guided by the priority criteria of theHipparchos Input Catalogue Consortium. Since these criteria are wellsuited to the need for linkage of ground-based optical systems to radioreference frames, this first selection is also considered a suitablebase for the catalog. Seventeen categories of stellar parameters arefurnished for each of the stars, including optical and radio positions,optical and radio parallax, radial velocity, type of variability, andoptical structure.
| Photometry of 50 suspected variable stars
Fifty stars have been chosen as suspected variable stars and analyzedfor variability. A large portion of this sample are stars that areeither proved active chromosphere stars or are candidates for suchactivity. The photometric data base consists of differential Vmeasurements of the Vanderbilt 16 inch (41 cm) automatic photoelectrictelescope and 25 observers at 26 observatories worldwide. Publishedphotometric data have also been utilized, with proper adjustments madeto ensure that all magnitudes are differential. Searches for photometricperiod, amplitudes, and times of minimum light showed 68 percent of thesample to be photometrically variable with periods found for 34. Twostars were deemed norvariable for the period of observation. Conclusivestatements could not be made concerning the photometric variability ofthe 14 remaining stars.
| A survey of the radio continuum emission of RS Canum Venaticorum and related active binary systems
Observations of 77 RS CVn and related active binary systems were madewith VLA at a continuum frequency of 4.86 GHz. The results are combinedwith other VLA observations to produce a total number of observedsources of 122. A search for correlations of radio properties withvarious systemic and stellar parameters is performed. Significantcorrelations are found between the normalized radio luminosityL(6)/L(bol) and the normalized soft X-ray luminosity L(x)/L(bol). It issuggested that the close correlation between low-level 6-cm emission andthe hot component of the soft X-ray emission observed by the Einsteinsolid state spectrometer is caused by both emissions being produced bythe same thermal electrons.
| The 69th Name-List of Variable Stars
Not Available
| Chromospherically active stars. VII - 39 Ceti = AY Ceti, HD 185151 = V1764 Cygni, and binary synchronization
Improved orbital elements have been determined for 39 Ceti and HD185151.39 Cet has a circular orbit with an orbital period of 56.82 days,which differs substantially from its rotational period of 75-78 days. Anobservation of the lithium region of 39 Cet shows that the G5 IIIcomponent has almost no lithium in its outer atmosphere. HD 185151 has acircular orbit with an orbital period of 40.142 days and has a nearlyidentical rotational period. The large mass function suggests that thesecondary is a late A to mid F type star whose continuum should bevisible at ultraviolet wavelengths. The orbital inclination is estimatedto be 62 + or - 12, while the distance is about 390 pc. Orbital androtational periods are compared for 114 chromospherically activebinaries.
| A catalog of chromospherically active binary stars
A catalog of 168 chromospherically active binary stars is presented,including the class of RS CVn, BY Dra binaries, and other binaries whichshow strong Ca II H and K emission in their spectra. The catalogcontains information on the photometric, spectroscopic, orbital, andphysical properties of the systems as well as the space motions andpositions. Summarized in a candidate list are in additional 37 starshaving similar characteristics but which are not definitely knownbinaries or have not had H and K emission observed. The catalog containsstars in both the northern and the southern sky.
| Magnetic structure in cool stars. XV - The evolution of rotation rates and chromospheric activity of giants
For cool giants and subgiants the observed dependence of rotationalvelocity and Ca II H and K line-core emission on color B-V isinterpreted in terms of changes in the moment of inertia by stellarevolution. Modeling of the rotational velocity during the evolution ofcool giants with masses between 2.0 and 3.0 solar masses, by taking intoaccount the change in the moment of inertia and assuming rigid-bodyrotation and conservation of angular momentum, describes the observeddecrease of v sin i with B-V. The computed evolution of the rotationalvelocity, together with the empirical relation between the Ca IIline-core emission and the rotation rate, explain the observed drop inthe Ca II line-core emission for giants at B-V = about 0.95. Forsubgiants with masses of about 1.5 solar mass, the change in the momentof inertia by itself cannot explain the observed v sin i distribution:there are indications of loss of angular momentum, presumably bymagnetic braking.
|
הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
|
תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | נחש מים |
| התרוממות ימנית: | 08h25m14.08s |
| סירוב: | -07°10'12.9" |
| גודל גלוי: | 7.355 |
| מרחק: | 49.505 פארסק |
| תנועה נכונה: | -118 |
| תנועה נכונה: | -9.9 |
| B-T magnitude: | 8.545 |
| V-T magnitude: | 7.454 |
קטלוגים וכינוים:
|
INHABITED SKY
