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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).
| The FIRST Unbiased Survey for Radio Stars
Comparison of the VLA FIRST survey with various catalogs of bright starsallows an examination of the prevalence of stellar radio emissionindependent of optical selection criteria. This FIRST unbiased surveyfor radio stars covers nearly 5000 deg^2 of the northern sky to a fluxdensity limit of 0.7 mJy at 20 cm. Using astrometric catalogs thatinclude proper-motion information, we have detected 26 stellar radiosources, doubling the number of such objects previously known in thisregion of high-latitude sky. We also show that, in the absence of goodproper motions, even the 1" precision of the FIRST positions isinsufficient to avoid crippling chance coincidence rates. We calculatethe fraction of radio detections as a function of stellar magnitude andshow that, when proper motions from the Guide Star Catalog II becomeavailable, the number of stellar radio source detections should increasefourfold.
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| Radio continuum emission from stars: a catalogue update.
An updated version of my catalogue of radio stars is presented. Somestatistics and availability are discussed.
| Microwave spectra and polarization of active stars
Almost-simultaneous observations of 24 active-chromosphere stars withthe VLA and the Parkes 64-m telescope at 4.9, 8.4 and 15 GHz over a 24-hinterval on September 14, 1985 are reported. In all, 14 stars weredetected. There is a striking difference in the power outputs andbrightness temperatures of the higher-optical-luminosity stars (RS CVn,Algol-like and CAII emitters) and the low-luminosity dMe stars; this canbe explained by differences in the surface areas and rotation velocitiesof the two groups of stars. Two three-point spectra show a broad peak inthe region near 8.4 GHz, indicating a transition from optically thick tooptically thin radio sources. For another three stars, two-pointspectral indices are negative, indicating that their spectra peakprobably well below 15 GHz; two stars with two-point positive spectralindices have higher-frequency turnovers, although perhaps below 15 GHz.
| A microwave survey of southern active stars
The results of a survey of 153 active-chromosphere stars made with theParkes 64-m telescope at 5.0/8.4 GHz from 1981 to 1987 are reported.Microwave emission was detected from 70 stars on at least one occasion,with the highest detection rate of 68 percent from the RS CVn group. TheCa II stars and Algol-type binaries yielded detection rates of 44 and 30percent, respectively. The maximum powers emitted by the stars at5.0/8.4 GHz ranged over five orders of magnitude, with a median of 2.5 x10 to the 10th W/Hz. The maximum brightness temperatures had a smallerrange of three orders of magnitude, with a median of 3.6 x 10 to the 9thK.
| Radio emission from RS CVn binaries. I - VLA survey and period-radio luminosity relationship
A VLA survey of radio emission from 36 close binary stellar systems withRS CVn properties is reported. Eight new sources were detected. Asummary of all published reports of radio emission from RS CVn systemsis presented. There appears to be a correlation between maximum radioluminosity and rotational period, with a tentative functional form L(R)varies as P to the (-0.7) power. Rapid rotators (periods approximately 2days) may be underluminous compared with the extrapolated trend fromlonger-period systems. The luminosity-period correlation probablyresults from a dynamo mechanism which produces strong magnetic fieldsand, in turn, enhances the nonthermal radio emission. The decrease inradio luminosity at short periods may be caused by a saturation ofenergy deposition in the chromosphere, possibly because the surface ofthe active star has become covered with spotted regions.
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