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The Formation and Evolution of Planetary Systems: Placing Our Solar System in Context with Spitzer
We provide an overview of the Spitzer Legacy Program, Formation andEvolution of Planetary Systems, that was proposed in 2000, begun in2001, and executed aboard the Spitzer Space Telescope between 2003 and2006. This program exploits the sensitivity of Spitzer to carry outmid-infrared spectrophotometric observations of solar-type stars. With asample of ~328 stars ranging in age from ~3 Myr to ~3 Gyr, we trace theevolution of circumstellar gas and dust from primordial planet-buildingstages in young circumstellar disks through to older collisionallygenerated debris disks. When completed, our program will help define thetimescales over which terrestrial and gas giant planets are built,constrain the frequency of planetesimal collisions as a function oftime, and establish the diversity of mature planetary architectures. Inaddition to the observational program, we have coordinated a concomitanttheoretical effort aimed at understanding the dynamics of circumstellardust with and without the effects of embedded planets, dust spectralenergy distributions, and atomic and molecular gas line emission.Together with the observations, these efforts will provide anastronomical context for understanding whether our solar system-and itshabitable planet-is a common or a rare circumstance. Additionalinformation about the FEPS project can be found on the team Web site.
| Zero-Age Main Sequence in the HR Diagram of the Vilnius Photometric System
The zero-age main sequence for solar metallicity stars in the absolutemagnitude vs. color diagram of the Vilnius seven-color photometricsystem is determined. The calibration is based on the results ofphotoelectric photometry of stars in the Hyades, Pleiades and Praesepeopen clusters and the Ori OB1 association. A theoretical Victoria-Reginaisochrone, corresponding to an age of 10 million years, coincides wellwith the lower envelope of the unevolved main sequence.
| A Catalog of Temperatures and Red Cousins Photometry for the Hyades
Using Hyades photometry published by Mendoza and other authors,Pinsonneault et al. have recently concluded that Cousins V-I photometrypublished by Taylor & Joner is not on the Cousins system. Extensivetests of the Taylor-Joner photometry and other pertinent results aretherefore performed in this paper. It is found that in part, thePinsonneault et al. conclusion rests on (1) a systematic error inMendoza's (R-I)J photometry and (2) a small error in anapproximate Johnson-to-Cousins transformation published by Bessell. Forthe Taylor-Joner values of (V-R)C, it is found that there arepossible (though not definite) differences of several mmag with otherresults. However, the Taylor-Joner values of (R-I)C data aresupported at the 1 mmag level. Using the (R-I)C data andother published results, an (R-I)C catalog is assembled for146 Hyades stars with spectral types earlier than about K5. For singlestars with multiple contributing data, the rms errors of the catalogentries are less than 4.4 mmag. Temperatures on the Di Benedettoangular-diameter scale are also given in the catalog and are used tohelp update published analyses of high-dispersion values of [Fe/H] forthe Hyades. The best current mean Hyades value of [Fe/H] is found to be+0.103+/-0.008 dex and is essentially unchanged from its previous value.In addition to these numerical results, recommendations are made aboutimproving attitudes and practices that are pertinent to issues likethose raised by Pinsonneault et al.
| 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
| Dependence of coronal X-ray emission on spot-induced brightness variations in cool main sequence stars
The maximum amplitude (Amax) of spot-induced brightnessvariations from long-term V-band photometry and the ratioLX/Lbol between X-ray and bolometric luminositiesare suitable indicators of the level of magnetic activity in thephotosphere and in the corona of late-type stars, respectively. By usingthese activity indicators we investigate the dependence of coronal X-rayemission on the level of photospheric starspot activity in a homogeneoussample of low mass main sequence field and cluster stars of differentages (IC 2602, IC 4665,IC 2391, alpha Persei,Pleiades and Hyades). First, theactivity-rotation connection at the photospheric level is re-analysed,as well as its dependence on spectral type and age. The upper envelopeof Amax increases monotonically with decreasing rotationalperiod (P) and Rossby number (R0) showing a break around 1.1d that separates two rotation regimes where the starspot activity showsdifferent behaviours. The Amax-P andAmax-R0 relations are fitted with linear,exponential and power laws to look for the function which bestrepresents the trend of the data. The highest values of Amaxare found among K-type stars and at the ages of alphaPersei and Pleiades. We also analyse theactivity-rotation connection at the coronal level as well as itsdependence on spectral type. The level of X-ray emission increases withincreasing rotation rate up to a saturation level. The rotational periodat which saturation occurs is colour-dependent and increases withadvancing spectral type. Also the LX/Lbol-P andLX/Lbol-R0 relations are fitted withlinear, exponential and power laws to look for the best fittingfunction. Among the fastest rotating stars (P<=0.3 d) there isevidence of super-saturation. Also the highest values ofLXLbol are found among K-type stars. Finally, thephotospheric-coronal activity connection is investigated by using forthe first time the largest ever sample of light curve amplitudes asindicators of the magnetic filling factor. The activity parametersLX/Lbol and Amax are found to becorrelated with each other, thus confirming the dependence of coronalactivity on photospheric magnetic fields. More precisely, theLX/Lbol-Amax distribution shows thepresence of an upper envelope, which is constant at theLX/Lbol =~ -3.0 saturation level, and of a lowerenvelope. The best fit to the lower envelope is given by a power lawwith steepness decreasing from F-G to M spectral types. However, it isconsidered a tentative result, since the fit reduced chi-squares arelarge. Such spectral-type dependence may be related to a colourdependence of Amax on the total starspot filling factor, aswell as to the coronal emission being possibly more sensitive tostarspot activity variations in F- and G-type than in M-type stars. TheLX/Lbol-Amax mean values for eachcluster in our sample decrease monotonically with increasing age,showing that the levels of photospheric and coronal activity evolve intime according to a single power law till the Sun's age.Tables of the photometric and X-ray data sets are only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/410/671
| The stellar activity-rotation relationship revisited: Dependence of saturated and non-saturated X-ray emission regimes on stellar mass for late-type dwarfs
We present the results of a new study on the relationship betweencoronal X-ray emission and stellar rotation in late-type main-sequencestars. We have selected a sample of 259 dwarfs in the B-V range 0.5-2.0,including 110 field stars and 149 members of the Pleiades, Hyades, alphaPersei, IC 2602 and IC 2391 open clusters. All the stars have beenobserved with ROSAT, and most of them have photometrically-measuredrotation periods available. Our results confirm that two emissionregimes exist, one in which the rotation period is a good predictor ofthe total X-ray luminosity, and the other in which a constant saturatedX-ray to bolometric luminosity ratio is attained; we present aquantitative estimate of the critical rotation periods below which starsof different masses (or spectral types) enter the saturated regime. Inthis work we have also empirically derived a characteristic time scale,taue , which we have used to investigate the relationshipbetween the X-ray emission level and an X-ray-based Rossby numberRe = Prot/taue: we show that ourempirical time scale taue resembles the theoreticalconvective turnover time for 0.4 <~ M/Msun <~ 1.2, butit also has the same functional dependence on B-V asLbol-1/2 in the color range 0.5 <~ B-V <~1.5. Our results imply that - for non-saturated coronae - theLx - Prot relation is equivalent to theLx/Lbol vs. Re relation. Tables 1 and 2are only available in electronic form at \ http://www.edpsciences.org
| 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 ``rotation-activity connection'': Its extension to photospheric activity diagnostics
In this paper we present the results of a different approach in thestudy of the so-called rotation-activity connection, which is a wellestablished correlation between rotation and magnetic activity atchromospheric and outer atmospheric levels. The present study concernsthe photospheric level and was carried out by using V-band photometriclight curve amplitudes as indicators of starspot coverage and ofmagnetic activity. A high degree of correlation between the envelope ofmaximum V-band light curve amplitudes and the rotation period is foundfor the active star members of young open clusters (IC 2602, IC 2391,Alpha Persei, Pleiades and Hyades), as well as for active field stars.This correlation shows a different behaviour in two different rotationperiod ranges. Moreover, some evidence of a possible activity``saturation'' is found among the most rapidly rotating stars of thesample. Additional correlations between photospheric and other magneticactivity indicators in the chromosphere, transition region and coronaare also investigated. The results presented here can be considered asan extension of the well established rotation-activity connection validfrom the corona, transition region and chromosphere, down to thephotosphere.
| Time Evolution of the Magnetic Activity Cycle Period. II. Results for an Expanded Stellar Sample
We further explore nondimensional relationships between the magneticdynamo cycle period P_cyc, the rotational period P_rot, the activitylevel (as observed in Ca II HK), and other stellar properties byexpanding the stellar sample studied in the first paper in this series.We do this by adding photometric and other cycles seen in active starsand the secondaries of CV systems and by selectively adding less certaincycles from the Mount Wilson HK survey; evolved stars, long-term HKtrends and secondary P_cyc are also considered. We confirm that moststars with age t>~0.1 Gyr occupy two roughly parallel branches,separated by a factor of ~6 in P_cyc, with the ratio of cycle androtational frequencies ω_cyc/Ω~Ro^-0.5, where Ro is theRossby number. Using the model of the first paper in this series, thisresult implies that the α effect increases with mean magneticfield (contrary to the traditional α-quenching concept) and thatα and ω_cyc decrease with t. Stars are not strictlysegregated onto one or the other branch by activity level, though thehigh-ω_cyc/Ω branch is primarily composed of inactive stars.The expanded data set suggests that for t>~1 Gyr, stars can havecycles on one or both branches, though among older stars, those withhigher (lower) mass tend to have their primary P_cyc on the lower(upper) ω_cyc/Ω branch. The Sun's ~80 yr Gleissberg cycleagrees with this scenario, suggesting that long-term activity ``trends''in many stars may be segments of long (P_cyc~50-100 yr) cycles not yetresolved by the data. Most very active stars (P_rot<3 days) appear tooccupy a new, third branch with ω_cyc/Ω~Ro^0.4. Many RS CVnvariables lie in a transition region between the two most activebranches. We compare our results with various models, discuss theirimplications for dynamo theory and evolution, and use them to predictP_cyc for three groups: stars with long-term HK trends, stars in youngopen clusters, and stars that may be in Maunder-like magnetic minima.
| Color indices of the Sun and Hyades stars in the WBVR system
Using an original setup at high altitudes, we measured the color indicesof the Sun in the WBVR photometric system relative to standard stars: (W- B)_solar = -0.05, (B - V)_solar = +0.67, and (V - R)_solar = +0.53. Wepresent the WBVR photometry for the Hyades members selected by vanBueren by their space velocities. The solar position is shown intwo-color diagrams relative to Hyades stars and bright G2 V stars. Acomparison of our results with Kurucz's models reveals a discrepancybetween the metallicities of the models and the Hyades members.
| The Multiplicity of the Hyades and Its Implications for Binary Star Formation and Evolution
A 2.2 μm speckle imaging survey of 167 bright (K < 8.5 mag) Hyadesmembers reveals a total of 33 binaries with separations spanning 0.044"to 1.34" and magnitude differences as large as 5.5 mag. Of thesebinaries, 9 are new detections and an additional 20 are now spatiallyresolved spectroscopic binaries, providing a sample from which dynamicalmasses and distances can be obtained. The closest three systems,marginally resolved at Palomar Observatory, were reobserved with the 10m Keck Telescope in order to determine accurate binary star parameters.Combining the results of this survey with previous radial velocity,optical speckle, and direct-imaging Hyades surveys, the detectedmultiplicity of the sample is 98 singles, 59 binaries, and 10 triples. Astatistical analysis of this sample investigates a variety of multiplestar formation and evolution theories. Over the binary separation range0.1"-1.07" (5-50 AU), the sensitivity to companion stars is relativelyuniform, with = 4 mag, equivalent to a mass ratio = 0.23. Accounting for the inability to detect high fluxratio binaries results in an implied companion star fraction (CSF) of0.30 +/- 0.06 in this separation range. The Hyades CSF is intermediatebetween the values derived from observations of T Tauri stars (CSF_TTS =0.40 +/- 0.08) and solar neighborhood G dwarfs (CSF_SN = 0.14 +/- 0.03).This result allows for an evolution of the CSF from an initially highvalue for the pre-main sequence to that found for main-sequence stars.Within the Hyades, the CSF and the mass ratio distribution provideobservational tests of binary formation mechanisms. The CSF isindependent of the radial distance from the cluster center and theprimary star mass. The distribution of mass ratios is best fitted by apower law q^-1.3+/-0.3 and shows no dependence on the primary mass,binary separation, or radial distance from the cluster center. Overall,the Hyades data are consistent with scale-free fragmentation, butinconsistent with capture and disk-assisted capture in small clusters.Without testable predictions, scale-dependent fragmentation and diskfragmentation cannot be assessed with the Hyades data.
| A New and Comprehensive Determination of the Distance to Member Stars of the Hyades
This paper critiques the methods used in the past for estimating thedistances to the stars that make up the Hyades moving cluster, anddevelops one which is new and comprehensive. We develop a formalism forthe application of stochastic restrictions to the adjustment parameters(velocity components and distances) and test it on constructed clustermodels. We perform model calculations with fictitious model starclusters in which the `true' values of the input parameters (ie.positions, distances and velocity components) are therefore known. Thecomparison of the distances recovered by different methods with the trueinput distances shows our approach to be very superior to thetraditional ones that estimate distances by the method of streamparallaxes without restricting the adjustment parameters. Applying ouralgorithm to the Hyades, we assume a dispersion of 0.25 kps in eachcoordinate about a common value for the velocity components of thecluster members and a dispersion of 5pc about the distance to thecluster's centre for the distances to the stars in the cluster. Underthese stochastic restrictions, we analyse the known estimates (ie.measurements) of trigonometric parallaxes, sets of high-precision propermotion pairs and high-precision radial velocities subject to theabove-stated stochastic constraints, and as a result estimate thecluster's centre to be at a distance of 45.8+/-1.25 (standard error) pc.
| The Lithium Abundance Dispersion in the Pleiades?
Abstract image available at:http://adsabs.harvard.edu/abs/1996ApJ...463..593R
| Lithium abundance in binaries of the Hyades open cluster.
We have derived accurate and homogeneous lithium abundances in 49 MainSequence binary systems belonging to the Hyades open cluster by using adeconvolution method to determine individual magnitudes and colors forthe primary and secondary components of the binary. The input parametersof the model are the observed Li equivalent width, the actual distanceto the binary, the integrated apparent magnitude and the integratedcolors of the binaries -BV(RI)_K_. We show that the general behavior isthe same in binaries and in single stars (Li is depleted faster in Kstars than in G stars and there is a deep dip for mid-F stars). However,there is a larger scatter in the abundances of binary systems than insingle stars. Moreover, in general, binary systems have anoverabundance, which is more conspicuous in close binaries. In fact,there is a cut-off period, which can be estimated as P_orb_~9d. Thisvalue is in excellent agreement with the theoretical prediction of Zahn(1994).
| Classification of Population II Stars in the Vilnius Photometric System. I. Methods
The methods used for classification of Population II stars in theVilnius photometric system are described. An extensive set of standardswith known astrophysical parameters compiled from the literature sourcesis given. These standard stars are classified in the Vilnius photometricsystem using the methods described. The accuracy of classification isevaluated by a comparison of the astrophysical parameters derived fromthe Vilnius photometric system with those estimated from spectroscopicstudies as well as from photometric data in other systems. For dwarfsand subdwarfs, we find a satisfactory agreement between our reddeningsand those estimated in the uvbyscriptstyle beta system. The standarddeviation of [Fe/H] deter mined in the Vilnius system is about 0.2 dex.The absolute magnitude for dwarfs and subdwarfs is estimated with anaccuracy of scriptstyle <=0.5 mag.
| Vitesses radiales. Catalogue WEB: Wilson Evans Batten. Subtittle: Radial velocities: The Wilson-Evans-Batten catalogue.
We give a common version of the two catalogues of Mean Radial Velocitiesby Wilson (1963) and Evans (1978) to which we have added the catalogueof spectroscopic binary systems (Batten et al. 1989). For each star,when possible, we give: 1) an acronym to enter SIMBAD (Set ofIdentifications Measurements and Bibliography for Astronomical Data) ofthe CDS (Centre de Donnees Astronomiques de Strasbourg). 2) the numberHIC of the HIPPARCOS catalogue (Turon 1992). 3) the CCDM number(Catalogue des Composantes des etoiles Doubles et Multiples) byDommanget & Nys (1994). For the cluster stars, a precise study hasbeen done, on the identificator numbers. Numerous remarks point out theproblems we have had to deal with.
| ROSAT All-Sky Survey Observations of the Hyades Cluster
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...448..683S&db_key=AST
| The Lithium Dip in M67: Comparison with the Hyades, Praesepe, and NGC 752 Clusters
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1995ApJ...446..203B&db_key=AST
| Photometric Survey Near the Main Galactic Meridian - Part One - Photoelectric Stellar Magnitudes and Colours in the UBVR System
Not Available
| Chromospheric and coronal activity in low-mass Hyades dwarfs
We present an extensive survey of the chromospheric and coronal activityamong low-mass members of the Hyades open cluster. We have obtained Halpha spectroscopy of 91 probable cluster members, including 23 that liewithin three fields for which we have deep ROSAT X-ray observations. 20of the 23 stars are detected in X-rays; the non-detections are probablynot Hyades members. Combining these observations with data from theliterature, we find that the mean H alpha to bolometric luminosity ratiolog (L_H alpha/L_bol) is constant at a value of ~-3.9 for M_bol>8,with a substantial dispersion and an upper limit of -3.5. The X-ray tobolometric luminosity ratio log (L_x/L_bol) first increases withincreasing M_bol, then flattens at a value of ~-3.3 for M_bol>8.Comparing this behaviour with stars in the Pleiades cluster, there aresignificant differences: first, the log (L_H alpha/L_bol) ratioincreases with increasing M_bol while M_bol<10, but appears to turnover sharply at fainter magnitudes, with the lower luminosity starsbeing nearly an order of magnitude lower in the ratio; secondly, we findno evidence for saturation in the log (L_x/L_bol) ratio; L_x increasesmonotonically with M_bol, reaching a value of ~-2.5 at M_bol~10. Lowerluminosity stars have not yet been detected in X-ray observations. Thenearly constant H alpha to H beta ratio in the Hyades stars, coupledwith the small range in coronal temperature seen in the X-ray data,suggest that the (presumably magnetically generated) heating rate isnearly constant in the outer atmospheres of these stars. Differences inthe relative activity levels are then ascribed to differences in thefilling factor of active regions, and in the sizes of the coronalstructures.
| Coronal X-ray emission and rotation of cool main-sequence stars
We analyze the coronal X-ray emission of single main sequence stars ofspectral type F through M with photometrically (CaII H+K or broad-bandphotometry) determined rotation periods, using X-ray data from the ROSATall-sky-survey. Our sample contains both field stars in the solarneighborhood and members of the Pleiades and Hyades open clusters. Fieldstars and members of the two young open clusters follow the samerotation-activity relation, i.e., we find no intrinsic dependence ofcoronal activity on age. Assuming a power law relationship betweencoronal X-ray emission and stellar rotation, we estimate a power lawindex close to unity. With a high level of confidence (alpha = 0.99), wefind a qualitative change in behavior around Rossby number valuesR0 approximately equals 1. For R0 greater than 1,coronal activity drops more rapidly with increasing Rossby number as forR0 less than 1. Assuming an exponential relation between theLx/Lbol ratios and Rossby number, R0approximately equals 1/3 is the characteristic Rossby number for a dropof X-ray activity.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Far-infrared observations of chromospherically active stars in stellar clusters.
Using the IRAS database we have searched for far-infrared emission froma large number of chromospherically active dwarf stars (including flarestars) in stellar clusters of different ages and distances. Only starsin the nearest clusters (<50pc) were detected by IRAS. Their 12μmluminosity (L_12_) is in the range 0.2-5x10^31^erg/s. We find that thedetected emission is photospheric in origin and that there is nocorrelation of flux either with age or with stellar-activity levels. Thecluster stars' far-infrared radiation characteristics are very similarto those of solar-vicinity field stars of similar spectral type andchromospheric activity.
| Empirical Calibration of Absolute Magnitudes for G-K Dwarfs and Subdwarfs in the Vilnius Photometric System
Calibrations giving l M_V as functions of [Fe/H] and various intrinsiccolor indices of the Vilnius photometric system are derived for G--Kdwarfs and subdwarfs. The calibrations are based only on the stars withknown trigonometric parallaxes and allow one to estimate the absolutemagnitudes with a standard deviation of +/- 0.6 mag. Comparisons of ourcalibrations with the absolute magnitude estimates in the literatureshow a satisfactory agreement.
| Applicability of the Rossby number in activity-rotation relations for dwarfs and giants
Empirical arguments for using the Rossby number as a rotation measure ofactive stars are reviewed using the log activity - rotation perioddiagrams. It is shown that when giant stars are considered the scatteron such diagrams cannot be appreciably diminished by replacing rotationperiod with the Rossby number. The observed scatter is, however,substantially reduced in activity-rotation diagrams of the solar typestars, i.e. dwarfs with 0.5 less than or approximately = B-V less thanor approximately = 0.8, when the rotation period is scaled withappropriate factors, in other words, when the Rossby number is used. Incase of active dwarfs lying outside of this spectral range, the using ofthe Rossby number does not improve the scatter. It is shown that theempirical turnover times determined from the calcium emission flux,magnesium emission flux and X-ray flux are identical with one anotherapart from constant factors: after a rapid increase with increasing Fand early G spectral type, the turnover time levels off for late G and Ktypes. Numerical relations between the calcium emission flux, X-ray fluxor the surface magnetic field of active dwarfs on one side with thestellar color index and the Rossby number on the other side aredetermined from the observational data. They indicate that thechromospheric losses are proportional to Teff2.9times (the mean value of Bsurf)exp(0.6) whereas the coronallosses are more sensitive to the both parameters and are proportional toTeff8.3 times (the mean value ofBsurf)exp(1.9).
| Stroemgren photometry of F- and G-type stars brighter than V = 9.6. I. UVBY photometry
Within the framework of a large photometric observing program, designedto investigate the Galaxy's structure and evolution, Hβ photometryis being made for about 9000 stars. As a by-product, supplementary uvbyphotometry has been made. The results are presented in a cataloguecontaining 6924 uvby observations of 6190 stars, all south ofδ=+38deg. The overall internal rms errors of one observation(transformed to the standard system) of a program star in the interval6.5
| ROSAT X-ray luminosity functions of the Hyades dK and dM stars
Long-duration ROSAT PSPC pointed observations of the Hyades open starcluster are performed. The Hyades dK and XLFs from the presentobservations are compared with published Einstein dK/dM XLFs. The HyadesdK binaries have significantly higher L(X) than the Hyades dK stars.However, all these binaries have relatively long periods (greater thanabout 1 yr), and hence the L(X) levels cannot be attributed to theenhanced activity expected in short-period, 'BY Dra-type' systems. It isalso shown that the effect cannot be due simply to the summedluminosities of the component stars.
| Seven-Color Photoelectric Photometry of Stars in the Hyades Clusters
Not Available
| Low-Mass Stars in the Hyades
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993MNRAS.265..785R&db_key=AST
| Lithium in the Hyades. I - New observations
Results are presented of new observations, carried out at Li 6707 A, on68 main-sequence members of the Hyades cluster with spectral typesranging from late F to and early K, and their surface lithium abundanceLi/H is related to their mass, age, and initial composition. It is foundthat, for the sample as a whole, the observed distribution of lithiumabundances is inconsistent with the hypothesis of a unique relationshipbetween lithium abundance and mass. Moreover, is strong observationalevidence for main-sequence lithium depletion when the data are comparedwith observations in younger clusters, even for masses that are notpredicted to experience such depletion in standard stellar models.
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Pozíciós és asztrometriai adatok
| Csillagkép: | Bika |
| Rektaszcenzió: | 04h32m50.11s |
| Deklináció: | +16°00'20.9" |
| Vizuális fényesség: | 8.906 |
| RA sajátmozgás: | 102.5 |
| Dec sajátmozgás: | -28.5 |
| B-T magnitude: | 10.094 |
| V-T magnitude: | 9.005 |
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