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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).
| ROSAT PSPC observations of globular clusters
We present deep pointed observations of nine globular clusters obtainedwith the Rosat position-sensitive proportional counter (PSPC). Theclusters observed were selected to be nearby, have low interstellarabsorption towards them, and to have high collision number. X-raysources are detected in the cores of seven of them, at luminosities ofapproximately 1 - 6 x 1032 erg/s. At least two of these aremultiple. The sources are too dim to obtain spectra of them, butanalysis of the X-ray colors indicates that their spectra are not allidentical. Four of the seven have colors indicating soft spectra, withblackbody temperature kappa T less than or approximately = 0.3 keV. Softspectra can be excluded for the other three sources.
| The interstellar reddening in front of globular clusters.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1975PASP...87...65C&db_key=AST
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