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Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| Distances and ages of NGC 6397, NGC 6752 and 47 Tuc
New improved distances and absolute ages for the Galactic globularclusters NGC 6397, NGC 6752, and 47 Tuc are obtained using the MainSequence Fitting Method. We derived accurate estimates of reddening andmetal abundance for these three clusters using a strictly differentialprocedure, where the Johnson B-V and Strömgren b-y colours and UVEShigh resolution spectra of turn-off stars and early subgiants belongingto the clusters were compared to similar data for field subdwarfs withaccurate parallaxes measured by Hipparcos. The use of a reddening freetemperature indicator (the profile of Hα ) allowed us to reducethe error bars in reddening determinations to about 0.005 mag, and inmetal abundances to 0.04 dex, in the scales defined by the localsubdwarfs. Error bars in distances are then reduced to about 0.07 magfor each cluster, yielding ages with typical random errors of about 1Gyr. We find that NGC 6397 and NGC 6752 have ages of 13.9+/- 1.1 and13.8+/- 1.1 Gyr respectively, when standard isochrones withoutmicroscopic diffusion are used, while 47 Tuc is probably about 2.6 Gyryounger, in agreement with results obtained by other techniquessensitive to relative ages. If we use models that include the effects ofsedimentation due to microscopic diffusion in agreement with ourobservations of NGC 6397, and take into account various sources ofpossible systematic errors with a statistical approach, we conclude thatthe age of the oldest globular clusters in the Galaxy is 13.4+/- 0.8+/-0.6 Gyr, where the first error bar accounts for random effects, and thesecond one for systematic errors. This age estimate is fully compatiblewith the very recent results from WMAP, and indicates that the oldestGalactic globular clusters formed within the first 1.7 Gyr after the BigBang, corresponding to a redshift of z>= 2.5, in a standard LambdaCDM model. The epoch of formation of the (inner halo) globular clusterslasted about 2.6 Gyr, ending at a time corresponding to a redshift ofz>= 1.3. On the other hand, our new age estimate once combined withvalues of H_0 given by WMAP and by the HST Key Project, provides arobust upper limit at 95% level of confidence of Omega_M <0.57,independently of type Ia SNe, and strongly supports the need for a darkenergy. The new cluster distances lead to new estimates of thehorizontal branch luminosity, that may be used to derive the zero pointof the relation between the horizontal branch absolute magnitude andmetallicity: we obtain M_V(HB)=(0.22+/- 0.05)([Fe/H]+1.5)+(0.56+/-0.07). This zero point is 0.03 mag shorter than obtained by Carretta etal. (\cite{Carretta2000}) and within the error bar it agrees with, butit is more precise than most of the previous individual determinationsof the RR Lyrae absolute magnitude. When combined with the apparentaverage luminosity of the RR Lyrae stars in the LMC by Clementini et al.(\cite{Clementini2003}), this zero point provides a new estimate of thedistance modulus to the LMC: (m-M)_0=18.50+/- 0.09.Based on data collected at the European Southern Observatory, Chile,telescopes (program 165.L-0263).
| Stars with the Largest Hipparcos Photometric Amplitudes
A list of the 2027 stars that have the largest photometric amplitudes inHipparcos Photometry shows that most variable stars are all Miras. Thepercentage of variable types change as a function of amplitude. Thiscompilation should also be of value to photometrists looking forrelatively unstudied, but large amplitude stars.
| Kinematical and orbital properties for selected southern high-velocity stars
Using the model of the Galaxy presented by Eggen, Lynden-Bell, andSandage (1962), plane galactic orbits have been calculated for severalsouthern high-velocity stars which possess parallax, proper motion, andradial velocity data. Extensive lists of both raw and computed data forthese stars are included. Published values of U-B and B-V for some ofthese stars were used in plots of each of the orbital parameters versusU-B, B-V, or the ultraviolet excess. Also, a comparison is made betweenthe H-R diagrams for the southern high-velocity star group and that ofM3, a globular cluster, and again for M67, an old open cluster. Thehigh-velocity star group is found to resemble an old open cluster morethan a globular cluster.
| On the relationship between the apparent magnitudes given in several catalogues and the UBV system.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974A&AS...15..215O&db_key=AST
| The Local Velocity of Rotation in the Galaxy
Abstract image available at:http://adsabs.harvard.edu/abs/1974A&A....36..327I
| Stars with Motions of More Than 100 km/sec Perpendicular to the Galactic Plane
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