Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is 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/430/165}
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BVRI photometry and light curve analysis of VW CEP
Synthetic light curve solutions for the W UMa-system VW Cep have beendetermined by applying the most new version of Wilson (1998) approach toB-observations of 1996 and VRI-observations of 1999. From aconsideration of the possible evolution of this system, it is found thatthe system is a partial eclipsing contact one, and its primary to be anearly Main-Sequence F5 star of mass 0.85 Msolar and thesecondary to be a dwarf of spectral type G0 of mass 0.34Msolar.
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Period study of the contact system VW Cep
New photoelectric U,B and V observations of the eclipsing contact systemVW Cep were taken in 1998-2000 and 23 new minima times were determined.The (O-C) diagram constructed using photographic and photoelectricminima times can be explained by the light-time effect caused by thepresence of a third and fourth body in the system and the long-termperiod decrease interpreted by the mass transfer from the more to theless massive component or/and the magnetic-braking process. The suddenperiod increase detected in 1999 is probably a consequence of theepisodic mass transfer from the less to the more massive component.Enhanced surface activity causes short-term apparent variations of theorbital period. The differences of times of the subsequent primary andsecondary minima show two significant periodicities P_1 = 2.94 +/- 0.07years and P_2 = 2.36 +/- 0.05 years. The latter periodicity is probablythe beat period of the orbital and rotational period at the latitudes ofmost frequent occurrence of the spots.
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Period variation and surface activity of the contact binary VW Cephei
Long-term behavior of the well-known contact binary VW Cephei isdiscussed based on new multicolor (BV) photometry made over 8 years andhigh-resolution spectroscopy. The existence of an at least 7 year-longactivity cycle is confirmed using light curve disturbances as theindicator of surface brightness variations. The orbital period of thesystem has been decreased further in accordance with previouspredictions. It is shown that the cyclic component of the periodvariation (O-C) curve has an amplitude too large to be fully due to thelight-time effect of the astrometric 3rd companion. It is suspected thatthe magnetic activity cycle of the primary component affects the orbitalperiod significantly. Simultaneous solution of light- and velocitycurves (obtained during the ``quiet'' stage) resulted in new parametersof the system. The mass ratio (q=3D0.35) is considerably larger than theprevious spectroscopic value given by Hill (1989). High resolutionoptical spectroscopy made at the beginning of the new active cyclereveals Hα emission arising from the chromosphere of the primarywhereas the secondary lacks such emission. This fact gives furthersupport to the theoretical prediction that the primary component drivesthe activity of contact binaries. Fitting cool spots(ΔTeff=3D2000 K) to the surface of the primary, theminimum spot coverage was found to be 12 % at the active stage, while itis about 1-2 % at the quiet stage. An anticorrelation of Hαemission and spot position has been pointed out.
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Photoelectric and CCD times of minima of 19 eclipsing binary systems
We present several photoelectric and unfiltered CCD minima observationsof 19 eclipsing binaries. Most of them are apsidal motion stars (or atleast candidate ones).
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Photoelectric minima of 30 eclipsing binary systems
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Photometric Observations of VW Cephei
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Visual polarization measurements in the Cepheus flare
We present polarization measurements on a stellar sample located in alarge area in Cepheus. The observations were made with the 2-m telescopeat the Pic du Midi (France), around 4350 A and with an accuracy of thepolarization degree of the order of 0.0001. At large scales, we findthat the galactic magnetic field as given by the direction of thepolarization (alignment of the dust) is, roughly speaking, parallel tothe galactic plane in the Cassiopeia cloud, with a scale size of theorder of 100 pc, while it is turned by about 90 in the Cepheus cloud. Atsmall scales, in both the Cepheus and the Cassiopeia clouds, the COcontent has little correlation with the degree of polarization.
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Times of Minima and the Light Curve of VW Cep
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A New B V Lightcurve of the Eclipsing Binary VW Cephei
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Untersuchungen an W Ursae Majoris Sternen. I. Mit 6 Textabbildungen
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3 W UMa-type binaries with changing periods.
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Photographic magnitudes of stars brighter than 7m.75 between +75° and +80° declination (Errata: 11 270)
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Yerkes actinometry. Zone +73deg to +90deg.
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