Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Direct detection of a magnetic field at the surface of V390 Aurigae - an effectively single active giant
Aims.We have studied the active giant V390 Aur using spectropolarimetryto obtain direct and simultaneous measurements of the magnetic field andthe activity indicators in order to infer the origin of the activity. Methods: We used the new spectropolarimeter NARVAL at the Bernard LyotTelescope (Observatoire du Pic du Midi, France) to obtain a series ofStokes I and Stokes V profiles. Using the LSD technique we were able todetect the Zeeman signature of the magnetic field in each of our 5observations and to measure its longitudinal component. Using the widewavelength range of the spectra we could monitor the CaII K&H and IRtriplet, as well as the Hα lines which are activity indicators.The Stokes I LSD profiles enabled us to detect and measure the profilesof two weak stellar companions. Results: From five observationsobtained from November 2006 to March 2007, we deduce that the magneticfield has a complex structure which evolves with time and is reminiscentof a dynamo-induced magnetic field. The activity indicators also presentday to day variations, but their behaviour does not completely followthe magnetic field variations, because their longitudinal component cancancel the contribution of complex magnetic features. There is asignificant difference between the magnetic field observed on November27, 2006 and on March 15, 2007, at the same rotational phase, but withan interval of 10 rotations. The behaviour of the activity indicatorstogether with the measured enhanced magnetic field on March 15, 2007support the idea of a change in the field topology. Analysis (RV and EW)of the absorption components of the Stokes I LSD profile shows that thesecondary of the visual wide orbit binary ADS 3812 is itself aspectroscopic binary, and suggests that the synchronization effect doesnot play role for V390 Aur (the primary), and that the giant should beconsidered as effectively single with regard to its fast rotation andactivity.Based on data obtained using the Télescope Bernard Lyot atObservatoire du Pic du Midi, CNRS and Université Paul Sabatier,France.
| Pulkovo compilation of radial velocities for 35495 stars in a common system.
Not Available
| 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 flare activity of V390 Aur: First results of an international collaboration
V390 Aur is classified as a G8 spectral class active single giant byFekel and Marshall (1991). Multi-site cooperative high-speedphotoelectric monitoring was carried out for it in the period 1998-2003as part of a program developed in Bulgaria recently to studychromospherically active evolved stars. Astronomers from Greece andUkraine joined the flare activity study of V390 Aur in 2000. As aresult, several flares with a duration from seconds to minutes weredetected on this star at 3 observatories. Taking into account all theevents detected, the flare rate of the star is estimated to 0.205 flaresper hour. Comparison with the active dwarfs AD Leo and EV Lac indicatesthat the flare rate of V390 Aur is lower than that of AD Leo butcomparable with the well-known flare star EV Lac.
| 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.
| The Geneva-Copenhagen survey of the Solar neighbourhood. Ages, metallicities, and kinematic properties of 14 000 F and G dwarfs
We present and discuss new determinations of metallicity, rotation, age,kinematics, and Galactic orbits for a complete, magnitude-limited, andkinematically unbiased sample of 16 682 nearby F and G dwarf stars. Our63 000 new, accurate radial-velocity observations for nearly 13 500stars allow identification of most of the binary stars in the sampleand, together with published uvbyβ photometry, Hipparcosparallaxes, Tycho-2 proper motions, and a few earlier radial velocities,complete the kinematic information for 14 139 stars. These high-qualityvelocity data are supplemented by effective temperatures andmetallicities newly derived from recent and/or revised calibrations. Theremaining stars either lack Hipparcos data or have fast rotation. Amajor effort has been devoted to the determination of new isochrone agesfor all stars for which this is possible. Particular attention has beengiven to a realistic treatment of statistical biases and errorestimates, as standard techniques tend to underestimate these effectsand introduce spurious features in the age distributions. Our ages agreewell with those by Edvardsson et al. (\cite{edv93}), despite severalastrophysical and computational improvements since then. We demonstrate,however, how strong observational and theoretical biases cause thedistribution of the observed ages to be very different from that of thetrue age distribution of the sample. Among the many basic relations ofthe Galactic disk that can be reinvestigated from the data presentedhere, we revisit the metallicity distribution of the G dwarfs and theage-metallicity, age-velocity, and metallicity-velocity relations of theSolar neighbourhood. Our first results confirm the lack of metal-poor Gdwarfs relative to closed-box model predictions (the ``G dwarfproblem''), the existence of radial metallicity gradients in the disk,the small change in mean metallicity of the thin disk since itsformation and the substantial scatter in metallicity at all ages, andthe continuing kinematic heating of the thin disk with an efficiencyconsistent with that expected for a combination of spiral arms and giantmolecular clouds. Distinct features in the distribution of the Vcomponent of the space motion are extended in age and metallicity,corresponding to the effects of stochastic spiral waves rather thanclassical moving groups, and may complicate the identification ofthick-disk stars from kinematic criteria. More advanced analyses of thisrich material will require careful simulations of the selection criteriafor the sample and the distribution of observational errors.Based on observations made with the Danish 1.5-m telescope at ESO, LaSilla, Chile, and with the Swiss 1-m telescope at Observatoire deHaute-Provence, France.Complete Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/989
| 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.
| 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.
| The corona of V390 Aurigae (HD 33798)
V390 Aurigae (HD 33798) is a rapidly rotating, lithium rich, late-typegiant whose distinctive characteristics include a high X-ray luminosityobserved by the XMM-Newton space observatory. Series of lines of highlyionized Fe and several Lyman lines of hydrogen-like ions and tripletlines of helium-like ions are visible in the reflection grating spectra,most notably from O and Ne. X-ray emission from plasma at hightemperature (T> 107 K) indicates intense flaring activityon this star. Analysis results suggest a scenario where the corona ofV390 Aurigae is dominated by large magnetic structures similar in sizeto interconnecting loops between solar active regions but significantlyhotter. The interaction of these structures could explain the permanentflaring activity on large scales that is responsible for heating plasmato high temperatures. The intense activity on V390 Aurigae is related toits evolutionary position at the bottom of the red giant branch. It isanticipated that the rotation of the star will spin-down in the future,thus decreasing the efficiency of its alpha -Omega dynamo with thesuppressing of 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.
| Rapidly Rotating Lithium-rich K Giants: The New Case of the Giant PDS 365
PDS 365 is a newly detected, rapidly rotating (vsini=20 kms-1), single, low-mass giant star that with HD 233517 and HD219025 forms a remarkable ensemble of single K giants with the uniqueproperties of rapid rotation, very strong Li lines, an asymmetricalHα profile, and a large far-infrared excess. Their vsini valuesare between 18 and 23 km s-1, and their LTE Li abundances,logɛ(Li), are between 2.9 and 3.9. Detailed analysis of PDS 365reveals it to be a ~1 Msolar giant with a value of12C/13C approximately equal to 12. A clearrelation between high rotational velocities and very high Li abundancesfor K giant stars is found only when asymmetrical Hα profiles andlarge far-infrared excesses are present. If we consider single K giants,we find that among rapid (vsini>=8 km s-1) rotators, avery large proportion (~50%) are Li-rich giants. This proportion is incontrast with a very low proportion (~2%) of Li-rich stars among themuch more common slowly rotating K giants. This striking difference isdiscussed in terms of proposed mechanisms for Li enrichment.
| Speckle Interferometry of New and Problem Hipparcos Binaries. II. Observations Obtained in 1998-1999 from McDonald Observatory
The Hipparcos satellite made measurements of over 9734 known doublestars, 3406 new double stars, and 11,687 unresolved but possible doublestars. The high angular resolution afforded by speckle interferometrymakes it an efficient means to confirm these systems from the ground,which were first discovered from space. Because of its coverage of adifferent region of angular separation-magnitude difference(ρ-Δm) space, speckle interferometry also holds promise toascertain the duplicity of the unresolved Hipparcos ``problem'' stars.Presented are observations of 116 new Hipparcos double stars and 469Hipparcos ``problem stars,'' as well as 238 measures of other doublestars and 246 other high-quality nondetections. Included in these areobservations of double stars listed in the Tycho-2 Catalogue andpossible grid stars for the Space Interferometry Mission.
| The Nature of the lithium rich giants. Mixing episodes on the RGB and early-AGB
We present a critical analysis of the nature of the so-called Li-richRGB stars. For a majority of the stars, we have used Hipparcosparallaxes to determine masses and evolutionary states by comparingtheir position on the Hertzsprung-Russell diagram with theoreticalevolutionary tracks. Among the twenty Li-rich giants whose location onthe HR diagram we were able to determine precisely, five appear to beLi-rich because they have not completed the standard first dredge-updilution, and three have abundances compatible with the maximum allowedby standard dilution. Thus, these should be re-classified as Li-normal.For the remaining stars, the high Li abundance must be a result of freshsynthesis of this fragile element. We identify two distinct episodes ofLi production which occur in advanced evolutionary phases depending uponthe mass of the star. Low-mass RGB stars, which later undergo the heliumflash, produce Li at the phase referred to as the bump in the luminosityfunction. At this evolutionary phase, the outwardly-moving hydrogenshell burns through the mean molecular weight discontinuity created bythe first dredge-up. Any extra-mixing process can now easily connect the3He-rich envelope material to the outer regions of thehydrogen-burning shell, enabling Li production by the Cameron &Fowler (1971) process. While very high Li abundances are then reached,this Li-rich phase is extremely short lived because once the mixingextends deep enough to lower the carbon isotopic ratio below thestandard dilution value, the freshly synthesized Li is quicklydestroyed. In intermediate-mass stars, the mean molecular weightgradient due to the first dredge-up is not erased until after the starhas begun to burn helium in its core. The Li-rich phase in these starsoccurs when the convective envelope deepens at the base of the AGB,permitting extra-mixing to play an effective role. Li production ceaseswhen a strong mean molecular weight gradient is built up between thedeepening convective envelope and the shell of nuclear burning thatsurrounds the inert CO core. This episode is also very short lived.Low-mass stars may undergo additional mixing at this phase. The compileddata provide constraints on the time scales for extra mixing and someinsight on processes suggested in the literature. However, our resultsdo not suggest any specific trigger mechanism. Since the Li-rich phasesare extremely short, enrichment of the Li content of the ISM as a resultof these episodes is negligible.
| Two-colour photometry for 9473 components of close Hipparcos double and multiple stars
Using observations obtained with the Tycho instrument of the ESAHipparcos satellite, a two-colour photometry is produced for componentsof more than 7 000 Hipparcos double and multiple stars with angularseparations 0.1 to 2.5 arcsec. We publish 9473 components of 5173systems with separations above 0.3 arcsec. The majority of them did nothave Tycho photometry in the Hipparcos catalogue. The magnitudes arederived in the Tycho B_T and V_T passbands, similar to the Johnsonpassbands. Photometrically resolved components of the binaries withstatistically significant trigonometric parallaxes can be put on an HRdiagram, the majority of them for the first time. Based on observationsmade with the ESA Hipparcos satellite.
| Flares in the Active Giant V390 Aur
Not Available
| 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.
| 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
| The accretion of brown dwarfs and planets by giant stars - II. Solar-mass stars on the red giant branch
This paper extends our previous study of planet/brown dwarf accretion bygiant stars to solar-mass stars located on the red giant branch. Themodel assumes that the planet is dissipated at the bottom of theconvective envelope of the giant star. The evolution of the giant isthen followed in detail. We analyse the effects of different accretionrates and different initial conditions. The computations indicate thatthe accretion process is accompanied by a substantial expansion of thestar, and, in the case of high accretion rates, hot bottom burning canbe activated. The possible observational signatures that accompany theengulfing of a planet are also extensively investigated. They includethe ejection of a shell and a subsequent phase of IR emission, anincrease in the ^7Li surface abundance and a potential stellarmetallicity enrichment, spin-up of the star because of the deposition oforbital angular momentum, the possible generation of magnetic fields andthe related X-ray activity caused by the development of shear at thebase of the convective envelope, and the effects on the morphology ofthe horizontal branch in globular clusters. We propose that the IRexcess and high Li abundance observed in 4-8per cent of the G and Kgiants originate from the accretion of a giant planet, a brown dwarf ora very low-mass star.
| The peculiar single giant HD 112989: rotation, activity and evolution
A spectroscopic, photometric and evolutionary study is presented for HD112989, an active peculiar giant star. The present spectroscopicobservations show a significant CaII K&H emission core variability,whereas the UBV photometry shows that HD 112989 has a variablebrightness. The rotational velocity for this star, 11.0 kms(-1) , isabout five times larger than the mean rotational velocity for giantswith the same spectral type. In addition, we have found no sign ofbinarity for this star. Based on observations collected at theObservatoire de Haute-Provence (France) and at the Bulgarian NationalAstronomical Observatory-Rozhen (Bulgaria)
| The Chromospheric Radiation on the Circumstellar Shell of delta Andromedae
We have calculated the ionization of the elements O, C, Mg, Si, S, andFe and the dissociation of the molecules CO, OH, NH, CH, SiO, CN, C2,and H2O in the circumstellar envelope (CSE) of the red giant deltaAndromedae (K3 III) due to the ultraviolet radiation of thechromosphere. This calculation is based on the penetration of thechromospheric UV radiation into the CSE and its effect on thewavelengths of absorption of the species listed. We show that O, C, Mg,Si, Fe, and SiO, which are important for dust formation, survive theharsh non-LTE environment close to the central star of delta And. Someimplications for dust formation in the inner region of the CSE of deltaAnd are presented.
| Rotational Velocities of Late-Type Stars
A calibration based on the results of Gray has been used to determineprojected rotational velocities for 133 bright stars with spectral typesof F, G, or K, most of which appear in {\it The Bright Star Catalogue}.The vast majority have {\it v} sin {\it i} $\leq$ 10 km s$^{-1}$ and,thus, are slow rotators. With the new calibration, projected rotationalvelocities have been determined for a sample of 111 late-type stars,most of which are chromospherically active. Some of the stars have hadtheir rotational velocities measured for the first time. (SECTION:Stars)
| The Extremely Active Single Giant 1E 1751+7046 = ET Draconis: Revised Properties and a Reevaluation of Its Evolutionary Status
New spectroscopic observations have resulted in significant revisions toseveral of the orginally published properties of the X-ray bright,chromospherically active star 1E 1751+7046: the spectral classificationhas been revised from K5 IV to K0 III, v sin i from 30--40 km s-1 to 23km s-1, and the reported nondetection of the Li I 6707 A line to asignificant log epsilon (Li) = 1.8. Chromospheric and transition regionsurface fluxes from IUE observations and the coronal surface flux fromearlier Einstein data are close to saturation levels, and comparableonly to very active binaries, pre--main-sequence stars, and FK Comaeitself. IUE observations also recorded a flare. Fifteen new radialvelocity measurements show no evidence for a companion and areconsistent with intermediate or young disk membership. On the otherhand, we show the star to be located about 250 pc above the galacticplane, suggesting an intermediate or old disk object. The new spectralclass (Teff) and limits on the luminosity indicate that 1E 1751+7046 isa low-mass star on its first ascent of the giant branch. Photometry fromthe Four College Consortium Automatic Photometry Telescope is consistentwith the recently published photometric period of 13.98 days, and thelight curve is well fitted by a model consisting of two large spots atlatitudes of ~30 deg and ~50 deg. There are currently only two possibleevolutionary scenarios for this anomalous star: (a) coalescence from aprogenitor W UMa--type contact binary; or (b) the dredge-up of bothangular momentum and nuclear processed material in a low-mass (~1--2.5Mȯ) giant. A space motion, obtainable once a parallax and propermotion are available from Hipparcos, may resolve the age (old disk-youngdisk) uncertainty: young disk motions would favor the angular momentumtransfer scenario. There is no current theory that can account for theobserved lithium abundance.
| The lithium content and other properties of F2-G5 giants in the Hertzsprung Gap
As stars of 2-5 solar mass evolve across the Hertzsprung Gap they shouldfirst deplete their surface lithium by convective dilution and then,when convection penetrates deeper, begin to bring CN processed materialto their surfaces. To investigate this process we have observed 52giants, 25 of which have known C/N ratios, for their Li abundances.After eliminating four stars that may actually be dwarfs and includingthe two components of Capella analyzed by Pilachowski and Sowell we havecompared our (Li/Fe) ratios with models of Swenson. For stars showing vsin i greater than 50 km/s we find (Li/Fe) to be unaffected by mixingfor B - V less than 0.7 as predicted. For stars cooler than B - V = 0.7both v sin i and (Li/Fe) drop to smaller values. For the sharp linedstars (v sin i less than 50 km/s) we find a drop in Li between B - V =0.45 and 0.60 which cannot be understood in terms of dilution byconvection. Various possible causes of such an early depletion ordilution of surface Li are discussed including diffusion at the base ofthe convection zone, mass loss possibly enhanced by pulsation, andmagnetic activity as in the magnetic A and B type stars. The models ofRicher & Michaud (1993) with diffusion point toward a satisfactorysolution. A few giants with low v sin i values stand out with muchhigher than expected (Li/Fe) values despite their cool effectivetemperatures. We do not understand why those stars have not depletedtheir lithium as have most giants of similar color. The correlation of(N/C) with (Li/Fe) follows expectations in so far as almost all starswith enhanced (N/C) have depleted their Li as well.
| The active dynamo stars: RS CVn, BY Dra, FK Com, Algol, W UMa, and T Tau
Not Available
| Extremely active long-period RS CVn binary HD 12545
The active-chromosphere giant HD 12545 is noteworthy for its remarkable1990 photmetric amplitude of 0.6 mag in V, which implies that nearlyhalf the visible hemisphere of the star was covered by cool spots. Wereport the results of a spectroscopic study of HD 12545, showing it tobe an active-chromosphere binary with an orbital period of 23.97 days.We establish the spectral type as KO III, and measure v sin i = 17 +/- 2km/s. The various indicators of activity in the optical and ultravioletsuggest that HD 12545 is one of the most active RS CVn systems yetobserved. H alpha is a broad, variable emission feature, with a strengthcomparable to what is observed in very active RS CVn systems such asV711 Tau (HR 1099) or II Peg. The surface fluxes of chromospheric andtransition-regions lines in the UV range from 10 to nearly 400 times thesolar values. With a moderately strong Li I lambda 6707 feature and alarge space motion, HD 12545 is an unusual but not unique giant, sincethese properties are similar to those of the single active-chromospheregiant HD 33798.
| Stromgren Four-Colour UVBY Photometry of G5-TYPE Hd-Stars Brighter than MV=8.6
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993A&AS..102...89O&db_key=AST
| Lithium and rapid rotation in chromospherically active single giants
The rotational velocities presently obtained via spectroscopicobservations of a group of moderately rapidly rotating, chomosphericallyactive single giants indicate that Gray's (1989) rotostat hypothesisrequires modification. Their rapid rotation appears to be due to high Liabundances, and results in increased chromospheric activity. A scenariois projected in which the surface convection zone reaches the rapidlyrotating core just as a star begins its first ascent of the giantbranch, and dredges both high angular momentum material and freshlysynthesized Li to the surface.
| Chromospherically active stars. IX - HD 33798 = V390 Auirigae - A lithium-rich rapidly rotating single giant
Results are presented of spectroscopic observations of HD 33798 obtainedto determine if this star is a short-period binary and to examine itsevolutionary status. Analysis of 40 radial velocities indicates noperiodic velocity variations, suggesting that the star is single, so itsrapid rotation is highly unusual. This rotation is inconsistent with therotational brake hypothesis advanced by Gray (1989). It is proposed thatHD 33798 is in a post-main-sequence phase of evolution. Its space motionis similar to FK Com, suggesting that it is a coalesced binary in theprocess of spinning down. Scenarios are presented to explain its largelithium abundance. A version in which material is transferred from arapidly rotating core is suggested as the most likely.
| 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.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Кочијаш |
| Ректацензија: | 05h15m15.46s |
| Deклинација: | +47°10'14.6" |
| Apparent магнитуда: | 6.923 |
| Даљина: | 111.857 parsecs |
| Proper motion RA: | 89.6 |
| Proper motion Dec: | -107.1 |
| B-T magnitude: | 8.153 |
| V-T magnitude: | 7.025 |
Каталог и designations:
|
