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A portrait of Centaur 10199 Chariklo
Context: An ESO Large Program was undertaken in October 2006 (P.I.: M.A. Barucci) to provide as complete observations as possible of about 40Trans-Neptunian Objects and Centaurs, to investigate their surfaceproperties. Hence, new visible and near-infrared observations of Centaur10199 Chariklo (1997 CU{26})) were performed. Aims: Weinvestigate Chariklo's surface composition. It has already beensuspected of being inhomogeneous. We try to confirm this assumption bycomparing our results with previously published works, and find anexplanation related the observed variations. Methods: A spectralmodeling is applied to the spectra, using different types of mixtures,to place constraints on the amount of water ice present in our newspectrum. Several spectra, obtained at different moments by differentgroups, are compared by studying the variations in the depth ofabsorption bands attributable to water ice. The irradiation dosesreceived by Chariklo's surface are also considered to interpret theobserved variations. Results: The presence of water ice is notconfirmed by our featureless near-infrared spectra. The main componenton the surface, identified by our spectral modeling, is amorphouscarbon, which may have been produced by irradiation if Chariklooriginated in the transneptunian region. The suspected surfaceheterogeneity is also confirmed. We show that the variations inChariklo's spectral behaviour could be explained by a variation in thenumber of craters across the surface. Comet-like activity is notdetected in our data, though it cannot be excluded.Based on observations obtained at the VLT Observatory, Cerro Paranal ofEuropean Southern Observatory, ESO, Chile, in the framework of program178.C-0036.
| Discovery of a T Dwarf Binary with the Largest Known J-Band Flux Reversal
We present Keck laser guide star observations of two T2.5 dwarfs-2MASSJ11061197+2754225 and 2MASS J14044941-3159329-using NIRC2 on Keck II andfind 2MASS J14044941-3159329 to be a 0.13" binary. This system has asecondary that is 0.45 mag brighter than the primary in J band, but 0.49mag fainter in H band and 1.13 mag fainter in Ks band. We usethis relative photometry along with near-infrared synthetic modelingperformed on the integrated light spectrum to derive component types ofT1+/-1 for the primary and T5+/-1 for the secondary. Opticalspectroscopy of this system obtained with Magellan/LDSS-3 is alsopresented. This is the fourth L/T transition binary to show a fluxreversal in the 1-1.2 ?m regime, and this one has the largest fluxreversal. Unless the secondary is itself an unresolved binary, theJ-band magnitude difference between the secondary and primary shows thatthe J-band ``bump'' is indeed a real feature and not an artifact causedby unresolved binarity.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation. This paper includes data gathered with the 6.5 m MagellanTelescopes located at Las Campanas Observatory, Chile.
| Proper-motion binaries in the Hipparcos catalogue. Comparison with radial velocity data
Context: .This paper is the last in a series devoted to the analysis ofthe binary content of the Hipparcos Catalogue. Aims: .Thecomparison of the proper motions constructed from positions spanning ashort (Hipparcos) or long time (Tycho-2) makes it possible to uncoverbinaries with periods of the order of or somewhat larger than the shorttime span (in this case, the 3 yr duration of the Hipparcos mission),since the unrecognised orbital motion will then add to the propermotion. Methods: .A list of candidate proper motion binaries isconstructed from a carefully designed χ2 test evaluatingthe statistical significance of the difference between the Tycho-2 andHipparcos proper motions for 103 134 stars in common between the twocatalogues (excluding components of visual systems). Since similar listsof proper-motion binaries have already been constructed, the presentpaper focuses on the evaluation of the detection efficiency ofproper-motion binaries, using different kinds of control data (mostlyradial velocities). The detection rate for entries from the NinthCatalogue of Spectroscopic Binary Orbits (S_B^9) is evaluated, as wellas for stars like barium stars, which are known to be all binaries, andfinally for spectroscopic binaries identified from radial velocity datain the Geneva-Copenhagen survey of F and G dwarfs in the solarneighbourhood. Results: .Proper motion binaries are efficientlydetected for systems with parallaxes in excess of ~20 mas, and periodsin the range 1000-30 000 d. The shortest periods in this range(1000-2000 d, i.e., once to twice the duration of the Hipparcos mission)may appear only as DMSA/G binaries (accelerated proper motion in theHipparcos Double and Multiple System Annex). Proper motion binariesdetected among S_B9 systems having periods shorter than about400 d hint at triple systems, the proper-motion binary involving acomponent with a longer orbital period. A list of 19 candidate triplesystems is provided. Binaries suspected of having low-mass(brown-dwarf-like) companions are listed as well. Among the 37 bariumstars with parallaxes larger than 5 mas, only 7 exhibit no evidence forduplicity whatsoever (be it spectroscopic or astrometric). Finally, thefraction of proper-motion binaries shows no significant variation amongthe various (regular) spectral classes, when due account is taken forthe detection biases.Full Table [see full textsee full text] is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/464/377
| 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).
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