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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).
| Probing the {Na} BT I D and {K} BT I lambda 7699 resonance lines sensitivity to background opacity in late-type stars
We have measured the equivalent width WK of the K i resonanceline at 7699 Angstroms for a large sample of low activity late-typestars observed with high spectral resolution and we have verified thatthe relation WK vs. Teff is monotonicallydecreasing, for both dwarf and giant stars. This behaviour is differentfrom that of the Na I D lines for stars of the same type, which showedthat the relation WNa vs. Teff has a maximum forTeff ~ 4000 K, which is better defined for giants than fordwarfs (Tripicchio et al. 1997). The fit of the observed K I equivalentwidths by means of a NLTE spectral line synthesis using conventionalbackground opacity shows that, for dwarf stars, the adopted modelsoverestimate the observed WK for temperatures <~ 4000 K.This result is similar to that discussed for the Na I D lines in ourprevious paper. On the other hand, for giant stars with Teff<~ 3800 K these models in general underestimate WK. Thediscrepancies between observed and computed WK andWNa for cool stars are much stronger than the variations dueto uncertainties in either atmospheric model or line parameters, likeeffective temperature and surface gravity, or Van der Waals broadening.For M dwarf stars, the most convincing explanation for the disagreementis the lack of atomic and molecular line opacity in the adopted models.In fact, a NLTE spectral synthesis including an additional backgroundopacity reproduces with a good level of accuracy the equivalent widths,as well as the general shape of the profiles for both the Na I D and K Ilines, in a subsample of early-M dwarfs. Based on observations collectedat the European Southern Observatory (ESO), La Silla, Chile, and at theMcDonald Observatory, Mt. Locke, Texas, USA
| Hyades and Sirius supercluster members brighter than magnitude (V) 7.1. II - Right ascension six to twelve hours
The present star sample is contained in the Bright Star Catalogue andits Supplement, augmented with a further supplement of 788 stars foundduring various observing programs over the past 40 years. Accurate,four-color and H-beta, or (RI), photometry is available for most of thesupercluster members. The criteria for membership are the comparisons ofthe proper motion, radial velocity, and luminosity obtained from thesupercluster parameters with the observed motions and the luminosityderived from the photometric parameters. New proper motions, based onall available catalogs, have been derived for the additional 788 starsdiscussed here, as well as all supercluster members.
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