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Effective temperature scale and bolometric corrections from 2MASS photometry
We present a method to determine effective temperatures, angularsemi-diameters and bolometric corrections for population I and II FGKtype stars based on V and 2MASS IR photometry. Accurate calibration isaccomplished by using a sample of solar analogues, whose averagetemperature is assumed to be equal to the solar effective temperature of5777 K. By taking into account all possible sources of error we estimateassociated uncertainties to better than 1% in effective temperature andin the range 1.0-2.5% in angular semi-diameter for unreddened stars.Comparison of our new temperatures with other determinations extractedfrom the literature indicates, in general, remarkably good agreement.These results suggest that the effective temperaure scale of FGK starsis currently established with an accuracy better than 0.5%-1%. Theapplication of the method to a sample of 10 999 dwarfs in the Hipparcoscatalogue allows us to define temperature and bolometric correction (Kband) calibrations as a function of (V-K), [m/H] and log g. Bolometriccorrections in the V and K bands as a function of T_eff, [m/H] and log gare also given. We provide effective temperatures, angularsemi-diameters, radii and bolometric corrections in the V and K bandsfor the 10 999 FGK stars in our sample with the correspondinguncertainties.
| 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
| Astrometric radial velocities. III. Hipparcos measurements of nearby star clusters and associations
Radial motions of stars in nearby moving clusters are determined fromaccurate proper motions and trigonometric parallaxes, without any use ofspectroscopy. Assuming that cluster members share the same velocityvector (apart from a random dispersion), we apply a maximum-likelihoodmethod on astrometric data from Hipparcos to compute radial and spacevelocities (and their dispersions) in the Ursa Major, Hyades, ComaBerenices, Pleiades, and Praesepe clusters, and for theScorpius-Centaurus, alpha Persei, and ``HIP 98321'' associations. Theradial motion of the Hyades cluster is determined to within 0.4 kms-1 (standard error), and that of its individual stars towithin 0.6 km s-1. For other clusters, Hipparcos data yieldastrometric radial velocities with typical accuracies of a few kms-1. A comparison of these astrometric values withspectroscopic radial velocities in the literature shows a good generalagreement and, in the case of the best-determined Hyades cluster, alsopermits searches for subtle astrophysical differences, such as evidencefor enhanced convective blueshifts of F-dwarf spectra, and decreasedgravitational redshifts in giants. Similar comparisons for the ScorpiusOB2 complex indicate some expansion of its associations, albeit slowerthan expected from their ages. As a by-product from the radial-velocitysolutions, kinematically improved parallaxes for individual stars areobtained, enabling Hertzsprung-Russell diagrams with unprecedentedaccuracy in luminosity. For the Hyades (parallax accuracy 0.3 mas), itsmain sequence resembles a thin line, possibly with wiggles in it.Although this main sequence has underpopulated regions at certaincolours (previously suggested to be ``Böhm-Vitense gaps''), suchare not visible for other clusters, and are probably spurious. Futurespace astrometry missions carry a great potential for absoluteradial-velocity determinations, insensitive to the complexities ofstellar spectra. Based on observations by the ESA Hipparcos satellite.Extended versions of Tables \ref{tab1} and \ref{tab2} are available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.125.8) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/446
| A Hipparcos study of the Hyades open cluster. Improved colour-absolute magnitude and Hertzsprung-Russell diagrams
Hipparcos parallaxes fix distances to individual stars in the Hyadescluster with an accuracy of ~ 6 percent. We use the Hipparcos propermotions, which have a larger relative precision than the trigonometricparallaxes, to derive ~ 3 times more precise distance estimates, byassuming that all members share the same space motion. An investigationof the available kinematic data confirms that the Hyades velocity fielddoes not contain significant structure in the form of rotation and/orshear, but is fully consistent with a common space motion plus a(one-dimensional) internal velocity dispersion of ~ 0.30 kms-1. The improved parallaxes as a set are statisticallyconsistent with the Hipparcos parallaxes. The maximum expectedsystematic error in the proper motion-based parallaxes for stars in theouter regions of the cluster (i.e., beyond ~ 2 tidal radii ~ 20 pc) isla 0.30 mas. The new parallaxes confirm that the Hipparcos measurementsare correlated on small angular scales, consistent with the limitsspecified in the Hipparcos Catalogue, though with significantly smaller``amplitudes'' than claimed by Narayanan & Gould. We use the Tycho-2long time-baseline astrometric catalogue to derive a set of independentproper motion-based parallaxes for the Hipparcos members. The newparallaxes provide a uniquely sharp view of the three-dimensionalstructure of the Hyades. The colour-absolute magnitude diagram of thecluster based on the new parallaxes shows a well-defined main sequencewith two ``gaps''/``turn-offs''. These features provide the first directobservational support of Böhm-Vitense's prediction that (the onsetof) surface convection in stars significantly affects their (B-V)colours. We present and discuss the theoretical Hertzsprung-Russelldiagram (log L versus log T_eff) for an objectively defined set of 88high-fidelity members of the cluster as well as the delta Scuti startheta 2 Tau, the giants delta 1, theta1, epsilon , and gamma Tau, and the white dwarfs V471 Tau andHD 27483 (all of which are also members). The precision with which thenew parallaxes place individual Hyades in the Hertzsprung-Russelldiagram is limited by (systematic) uncertainties related to thetransformations from observed colours and absolute magnitudes toeffective temperatures and luminosities. The new parallaxes providestringent constraints on the calibration of such transformations whencombined with detailed theoretical stellar evolutionary modelling,tailored to the chemical composition and age of the Hyades, over thelarge stellar mass range of the cluster probed by Hipparcos.
| Internal velocity dispersion in the Hyades as a test for Tycho-2 proper motions
The Hyades stars have highly coherent motions in space which reflect inthe convergence of their astrometric proper motions in or about a commonconvergent point. The internal dispersion of the Hyades velocities is aslow as about 0.3 km s-1. This is comparable with the expecteduncertainty of the Tycho-2 proper motion at the distance of the Hyades.The classical approach of using the proper motion components normal tothe convergent point direction is employed for some 200 bona fidecluster members located in an area of some 2 000 deg2 andranging in V magnitude 3.4 to 12.1. It is demonstrated that with themost precise Tycho-2 proper motions for some 50 stars, the intrinsicdispersion is measurable at the level of 0.32 km s-1, inagreement with previous estimations obtained from the Hipparcos data andradial velocities. When only 30 stars out of the 50 are considered,which are not known to be spectroscopic binaries, the observed scattersuggests a dispersion of only 0.22 km s-1. The approach inuse is model independent, and sets an upper limit on the intrinsicdispersion of velocities. It is shown for some 180 stars in common, thatthe Tycho-2 proper motions have a very similar precision to that ofHipparcos, but reveal fewer extreme deviants from the common convergentpoint direction. Tycho-2 proper motions are based on long series ofobservations, up to a 100 years, and are therefore less subject to theorbital motion effects in binaries. The ratio of the external error tothe formal error of the Tycho-2 proper motions may be up to 1.3 forstars fainter than V_T=8.5 mag. An alternative explanation for the extrascatter in low-precision subsets is discussed, that less massive starsmay indeed have larger observed velocity dispersion due to the binarityeffect. Based on observations made with the ESA Hipparcos satellite.
| 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.
| The Hyades: distance, structure, dynamics, and age
{We use absolute trigonometric parallaxes from the Hipparcos Catalogueto determine individual distances to members of the Hyades cluster, fromwhich the 3-dimensional structure of the cluster can be derived.Inertially-referenced proper motions are used to rediscuss distancedeterminations based on convergent-point analyses. A combination ofparallaxes and proper motions from Hipparcos, and radial velocities fromground-based observations, are used to determine the position andvelocity components of candidate members with respect to the clustercentre, providing new information on cluster membership: 13 newcandidate members within 20 pc of the cluster centre have beenidentified. Farther from the cluster centre there is a gradual mergingbetween certain cluster members and field stars, both spatially andkinematically. Within the cluster, the kinematical structure is fullyconsistent with parallel space motion of the component stars with aninternal velocity dispersion of about 0.3 km s(-1) . The spatialstructure and mass segregation are consistent with N-body simulationresults, without the need to invoke expansion, contraction, rotation, orother significant perturbations of the cluster. The quality of theindividual distance determinations permits the cluster zero-age mainsequence to be accurately modelled. The helium abundance for the clusteris determined to be Y =3D 0.26+/-0.02 which, combined with isochronemodelling including convective overshooting, yields a cluster age of625+/-50 Myr. The distance to the observed centre of mass (a conceptmeaningful only in the restricted context of the cluster memberscontained in the Hipparcos Catalogue) is 46.34+/-0.27 pc, correspondingto a distance modulus m-M=3D3.33+/-0.01 mag for the objects within 10 pcof the cluster centre (roughly corresponding to the tidal radius). Thisdistance modulus is close to, but significantly better determined than,that derived from recent high-precision radial velocity studies,somewhat larger than that indicated by recent ground-based trigonometricparallax determinations, and smaller than those found from recentstudies of the cluster convergent point. These discrepancies areinvestigated and explained. } Based on observations made with the ESAHipparcos astrometry satellite. Table~2 is also available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/Abstract.html
| Determination of atmospheric parameters of T Tauri stars.
The inferred effective temperatures (T_eff_) and surface gravities(logg) of T Tauri stars (TTS) are usually contaminated by the presenceof a non stellar continuum emission (veiling) and the strongchromospheric activity characteristic of these objects. In this work, wedevelop a method to determine T_eff_'s and surface gravities of a groupof TTS. This method is based on the comparison between observed andtheoretical molecular and atomic line depth ratios being thereforeinsensitive to the influence of veiling. We show the strong dependenceof our line depth ratios upon gravity and temperature. The resultinggravities, as expected for TTS, average between the values of dwarf andgiant stars. Previously published gravities for each of our stars varywidely due in part to the differences in the adopted visual extinction,veiling (if ever considered) and methods of assessing the stellarluminosity. Our values of T_eff_ and logg have the uniqueness of beingentirely derived from high resolution data and are not affected bycircumstellar extinction or veiling. They therefore serve as morereliable input parameters for future spectral synthesis analyses of TTauri stars requiring model atmospheres. We provide a table relatingtheoretical line depth ratios with T_eff_ and log g for easy assessmentof TTS fundamental parameters.
| 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 Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
| Photometric discriminate for GK dwarfs of disk populations
The young and old disk populations, with ages respectively less than andgreater than about 2 x 109 yr, are separated in the (R - I,42 - 45) plane, where the 42 - 45 index of the DDO system is based onbandpasses of 75 A half-width and effective wavelengths of 4257 and 4571A. A similar, but smaller separation is seen in the (g,B2 - VI) planewhere g is the Geneva system alternative to 42 - 45 but with bandpassesof 175 A half-width and effective wavelengths of 4015 and 4476 A.Although models, based on the assumption that this separation resultsfrom a veil of weak metal lines (Magain 1983), may explain thesephenomena in the (g,B2 - VI) plane for the hotter stars, more models arerequired to test it for the DDO system and for cooler objects. Theavailable evidence suggests that an additional causation, probablyconnected with the CN abundance, also exists.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| 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.
| 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
| The low mass Hyades and the evaporation of clusters
The 135 single stars and 85 binary systems, redder than R-I = +0.34 magand brighter than V = 17 mag, between alpha = 3.75 h and 5.0 h and delta= +5 deg and + 25 deg show a luminosity function that differsconsiderably from that of the general field stars within 20 pc of theSun. The ratio of double star components to single cluster membersincreases markedly with decreasing luminosity. Forty-three single starsand 16 binary systems that are members of the Hyades supercluster within20 pc of the Sun show the same luminosity function as the field stars inthat region. Fifty percent of the cluster members and 40 percent of thesupercluster members are components of binary stars. The equivalentwidths of H-alpha appear to support a range of ages (approximately 8 to16 x 108 yr) for the cluster stars and demonstrate that theoldest objects are in the supercluster. A list of cluster members, whichmay include the end of the stable main sequence, but for which accurate(R-I) photometry is not available, is included. The half-dozen knownparallax stars of the faintest luminosity contain at least onesupercluster member, TVLM 868-110639, which is probably beyond thestable, nuclear burning main sequence as a 'transitional' or 'brown'dwarf.
| 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
| Four colour and H-beta photometry for low mass members in open clusters. II - The Hyades
Photoelectric uvby and H-beta photometry has been obtained for 28low-mass members in the Hyades cluster. The stars observed are G and Ktype stars of the lower main sequence with colors up to (b-y) = 0.8 mag,selected from the proper motion study of Van Altena (1969) with highprobability of membership. Their position in the related four color andH-beta photometric diagrams is discussed. The derived values for thedistance modulus and metallicity are compatible with the common acceptedvalues for this cluster. The c1-(b-y) plane shows the persistence of theHyades anomaly in the G star range, disappearing for late G and Kspectral types. This result is in good agreement with previous resultsobtained by the authors for the Praesepe star cluster.
| Hamilton echelle spectra of young stars. I - Optical veiling
An extensive set of veiling measurements of T Tauri stars coveringalmost the entire optical spectrum is presented. These are based onHamilton echelle spectra obtained during 1987-1989. A full range of TTauri stars, from 'weak' to 'continuum' stars is studied; in some cases,several spectra of a given target are seen in which the veiling haschanged, occasionally dramatically. Veilings are determined using twonew methods: one which concentrates on the residual intensities ofselected spectral lines, and the other which compares theautocorrelation functions for a T Tauri star and a veiled standard. Thegeneral shape of the veiling is often as predicted by accretion diskmodels: almost flat in the red and increasing in the blue. Cooler starshave generally higher veiling, implying that the veiling power isrelatively independent of stellar mass. The relation between veiling andthe narrow and broad emission lines supports a nonstellar origin for theveiling and broad line components, and a stellar origin for the narrowlines.
| Very active chromosphere stars and the age, chromosphere activity relation
The wide range of objects in stellar superclusters and groups near thesun is used in an attempt to define normal stellar chromosphericbehavior, with which 'very active chromosphere' (VAC) stars can beisolated. Two chromosphere signatures, soft X-ray flux, and Mg II h andk line flux, are discussed. The X-ray flux is of limited use in thisregard because of a wide range of variation with time, although themaximum values are well defined for stars of a given age. The VAC stars,which have an excessive X-ray flux, also have an excessive bolometricflux. The ratio of the Mg II h and k (and Ca II H and K) line emissionand the total bolometric emission, R(hk) decreases with bolometricluminosity. The rate of decrease in log R(hk), 0.175 (35.00 - logLBOL), is not very dependent on stellar age, but the zeropoint at log LBOL = 35.00 decades varies from log R(hk) =-4.45 decades for main-sequence stars in the Pleiades supercluster(200-300 million yr) to - 4.91 decades for those in the HR 1614 group (8billion yr). The age dependence of R(hk) is clearly demonstrated notonly by the supercluster and group stars but by the 'Mount Wilsonsample' of objects that have been observed in Ca II H and K for over 15years (e.g., Noyes et al., 1984). Several determinations (e.g.,Soderblom and Clements, 1987) have established a linear relation betweenR(hk) and R(HK). Some 50 stars in the Mount Wilson sample havewell-established luminosities and space motions (e.g., Eggen, 1989) thatallow the separation of young and old disk stars in velocity space(Eggen, 1989). The young disk stars all have values of R(hk) thatidentify them as having ages less than the youngest Hyades superclustermembers (about 600 million yr).
| Further Cousins VRI photometry of the Hyades and M67
New V magnitudes and/or (VRI)C colors are reported for 17 Hyads and 11bright M67 stars. It is found that a source of variation in the Hyads(probably chromospheric variation) can change R-I while leaving V-Runaffected. The V magnitude can also vary, but is uncorrelated with R-I.The time scale for color variation is roughly a week; V magnitudes canvary on time scales from weeks to years.
| A radial-velocity survey of the Hyades
Photoelectric radial-velocity measurements, obtained with external erroras small as 0.1 km/s using the 200-inch Hale telescope at PalomarObservatory during the period 1971-1986, are reported for over 400candidate members (with V magnitude between 6 and 14) of the Hyadescluster. The history of Hyades observations is recalled; the Palomarinstrumentation and observing program are described; the data-reductionand standardization procedures are discussed in detail; and the data arepresented in extensive tables and graphs. About 200 of the stars areclassified as cluster members, including 60 spectroscopic binaries.
| The Einstein Observatory survey of stars in the Hyades cluster region
The paper reports the results of an extensive X-ray study of the Hyadesregion and improves on previous studies by using refined X-ray sourcedetection algorithms and the complete set of Einstein Observatory IPCexposures covering the Hyades cluster region (a total of 63 1 x 1 degimages). Maximum likelihood integral X-ray luminosity functions werecomputed for the Hyades stars for given color index (B-V) ranges. Thepresent results agree substantially with a previous survey about theubiquity of the stellar activity in the Hyades cluster, especially amongsolar-type stars. The result of Stern et al. (1981) that stellar X-rayemission is dependent on stellar age is confrimed.
| Positions of 127 Hyads and 6-cm observations of 320 Hyads
Accurate positions are given for 127 Hyades cluster stars whichpreviously had only approximate positions known. The fields of 320Hyads, including 55 X-ray stars, were observed with the VLA to a 6-cmdetection level of approximately 0.5 mJy. There were no detections.
| Photometry of possible members of the Hyades cluster. VI
Photoelectric observations in BVRI colors have been obtained for about200 stars suspected of being members of the Hyades cluster, mostly onthe basis of their proper motions. More than half of this total havebeen observed photometrically for the first time; the rest are mostlystars covered in the early papers of this series for which the V - Rcolors had not been determined and for which the accuracy of thephotometry could be improved by reobservation. Based on the photometry,about half of these stars appear to be probable cluster members. Most ofthe stars included here are faint and have appeared in proper-motionlists for which identification charts have not been available. Theiridentifications were confirmed from their motions measured on platestaken in the course of the Lick Proper Motion Survey. This list raisesto about 810 the total number of stars in the Hyades region withbroadband photometry. Of these, 365 appear to be probable or possiblemembers of the cluster.
| Color anomalies and starspots in Hyades dwarfs
Hyades dwarfs later than about F7 show color anomalies which are foundto correlate well with various indicators of chromospheric activity. Itis proposed that the color anomalies are due to starspots, whichapparently also cause the low level photometric variations seen in somestars. That some stars with small color anomaly are variable, plus thefinding that Hyades dwarfs show a mean color anomaly relative to fieldstars, implies that virtually all late-type Hyades are spotted.Intermediate band photometry of spotted stars could give an apparentoverestimate of the metallicity. This may explain why many Hyadesabundance estimates from photometry are larger than from spectroscopicanalyses. Similarly, starspots could explain the 'Hyades anomaly', thec1 excess of unevolved Hyades dwarfs relative to field stars. Starspotscould lead to a displacement of the Hyades from the standard mainsequence in the color-magnitude diagram, but detailed models of spottedstars are required to test this. Tentative evidence is found that Hyadesdwarfs are roughly 0.1 mag brighter in V than field stars of the same B- V and metallicity. A new value of the solar B - V color is derived inthe Appendix.
| The Hyades main sequence
Intermediate band, H-beta and RI observations of 72 Hyades cluster starsto V = 11 mag are reported and discussed. A modulus of 3.2 mag isderived on the basis of a comparison with field stars of large parallax.Also presented are observations of 98 main-sequence stars of the Hyadesgroup that were previously found to be group members from kinematicalconsiderations. Parallaxes of the group stars, computed on theassumption that they are members of an extended Hyades cluster, yieldmean values of (U, V, W) = (+40.5, -18.4, -4.9) km/s, with dispersionsof (2.3, 2.3, 6.0) km/s, compared with (+41.7, -18.4, -2.0) and (2.6,1.3, 1.9) km/s for the brightest cluster members. It is noted that allthe stars discussed can be considered as members of a supercluster inwhich only a slight relaxation control of the W velocities is presentfor stars far from the nucleus. Evidence is found, including that of thePraesepe cluster at Z = +80 pc, for some interchange between the U, V,and W velocities in stars farthest from the galactic plane, with thetotal cluster velocity being maintained.
| Photoelectric observations of lunar occultations. XIII
Occultation observations made with the 0.76 m reflector at McDonaldObservatory are presented, for 319 events occurring between Jan. 15,1981 and Feb. 7, 1982. One table reports timings, and a second recordsobservations of 16 double stars, in particular, Nu Gem observationsreveal that the two close components are similar in color. This allowsan estimation of individual visible magnitudes (near 4.60 and 5.32)suggesting that the secondary is a late B-type main-sequence star. SAO158929 observations reveal systematic differences in angular diametervalues. Of the 16 observed double stars, 9 appear to be new discoveries,including SAO 93067, SAO 94595, and SAO 98770. A third table gives fullydarkened angular diameters for the stars SAO 158929, SAO 77516 Y Tau,and SAO 119035 Nu Vir.
| Infrared photometry of Hyades dwarfs
JHK infrared photometry is presented for 76 main-sequence proper motionmembers brighter than V = 12 in the Hyades. Known binaries are confirmedand new candidates proposed using color-magnitude and color-colordiagrams. The cluster binary frequency (about 40%) is discussed, as wellas its possible effects on the cluster distance modulus and stellarchromospheric/coronal activity.
| Stellar coronae in the Hyades - A soft X-ray survey with the Einstein Observatory
An X-ray survey of the central region of the Hyades cluster demonstratesthat soft X-ray emission is a common property of the stars in thecluster. Half of the 85 stars surveyed are detected above a sensitivitythreshold of 10 to the 28.5th ergs/s at the Hyades distance of 45 pc.The high incidence of X-ray emission and range of observed X-rayluminosities indicate that stellar coronas produce the observed X-rayemission, with a typical X-ray luminosity for solar-type Hyades of 10 tothe 29th ergs/s. The use of coronal scaling laws is found to yieldreasonable values of maximum coronal temperatures and the fraction ofstellar surface covered for the Hyades coronas.
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