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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).
| Photospheric-Phase Spectropolarimetry and Nebular-Phase Spectroscopy of the Peculiar Type Ic Supernova 2002ap
The early-time optical spectrum of the Type Ic supernova (SN) 2002ap wascharacterized by unusually broad features, leading some authors todesignate it a ``hypernova.'' We present optical spectropolarimetry ofthis object 16 and 37 days after the estimated date of explosion. Aftercorrecting for interstellar polarization, we find evidence for a highlevel of intrinsic continuum polarization at both epochs: p>~1.3% onday 16 and p>~1.0% on day 37. Prominent line polarization is alsoseen, especially in the trough of the Ca II near-infrared triplet duringthe second epoch. When interpreted in terms of the oblate,electron-scattering model atmospheres of Höflich, our results implyan asymmetry of at least 20% (day 16) and 17% (day 37). The data suggesta fair degree of axisymmetry, although the polarization angle of thedominant polarization axes are different by about 55° between thetwo epochs, implying a complex morphology for the thinning ejecta. Inparticular, there exists some spectropolarimetric evidence for adifferent distribution of Ca relative to iron-group elements. We alsopresent flux spectra of SN 2002ap taken 131 and 140 days after theexplosion. The spectra are characterized by a very weak continuum andbroad emission lines, indicating that SN 2002ap has entered the nebularphase. The spectral features are substantially similar to those of``normal'' SNe Ic in the nebular phase, and the emission lines are notsignificantly broader. However, some of the broad lines are very sharplypeaked and may possess a narrow component (probably unresolved by ourspectra, FWHM<~400 km s-1) that is redshifted by ~580 kms-1 with respect to the systemic velocity of the hostgalaxy.
| A catalog of precise reference star positions for the astrometry network of the international comet P/Halley compaign
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1987A&AS...71..525D&db_key=AST
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