A. H. Andrei

Charon's size and an upper limit on its atmosphere from a stellar occultation

Pluto and its satellite, Charon (discovered in 1978; ref. 1), appear to form a double planet, rather than a hierarchical planet/satellite couple. Charon is about half Plutos size and about one-eighth its mass. The precise radii of Pluto and Charon have remained uncertain, leading to large uncertainties on their densities. Although stellar occultations by Charon are in principle a powerful way of measuring its size, they are rare, as the satellite subtends less than 0.3 microradians (0.06 arcsec) on the sky. One occultation (in 1980) yielded a lower limit of 600u2009km for the satellites radius, which was later refined to 601.5u2009km (ref. 4). Here we report observations from a multi-station stellar occultation by Charon, which we use to derive a radius, RC = 603.6 ± 1.4u2009km (1σ), and a density of ρ = 1.71 ± 0.08u2009gu2009cm-3. This occultation also provides upper limits of 110 and 15 (3σ) nanobar for an atmosphere around Charon, assuming respectively a pure nitrogen or pure methane atmosphere.

76 T dwarfs from the UKIDSS LAS: benchmarks, kinematics and an updated space density

We report the discovery of 76 new T dwarfs from the UKIRT Infrared Deep Sky Survey (UKIDSS) Large Area Survey (LAS). Near-infrared broad- and narrow-band photometry and spectroscopy are presented for the new objects, along with Wide-field Infrared Survey Explorer (WISE) and warm-Spitzer photometry. Proper motions for 128 UKIDSS T dwarfs are presented from a new two epoch LAS proper motion catalogue. We use these motions to identify two new benchmark systems: LHS 6176AB, a T8p+M4 pair and HD 118865AB, a T5.5+F8 pair. Using age constraints from the primaries and evolutionary models to constrain the radii, we have estimated their physical properties from their bolometric luminosity. We compare the colours and properties of known benchmark T dwarfs to the latest model atmospheres and draw two principal conclusions. First, it appears that the H - [4.5] and J - W2 colours are more sensitive to metallicity than has previously been recognized, such that differences in metallicity may dominate over differences in T-eff when considering relative properties of cool objects using these colours. Secondly, the previously noted apparent dominance of young objects in the late-T dwarf sample is no longer apparent when using the new model grids and the expanded sample of late-T dwarfs and benchmarks. This is supported by the apparently similar distribution of late-T dwarfs and earlier type T dwarfs on reduced proper motion diagrams that we present. Finally, we present updated space densities for the late-T dwarfs, and compare our values to simulation predictions and those from WISE.

Investigation of USNO-A2.0 Catalog Positions

We present an investigation of the USNO-A2.0 Catalog positions. We have compared USNO-A2.0 positions with the Astrographic Catalogue-Tycho, with improved HST Guide Star Catalog positions in the ACT frame, and with International Celestial Reference Frame source positions and observational data from Valinhos CCD Meridian Circle, covering all the USNO-A2.0 magnitude range within 7 ≤ V ≤ 22. We report striking, peculiar features first found for the USNO-A2.0 catalog positions, which are different for its northern and southern parts.

PARALLAXES OF SOUTHERN EXTREMELY COOL OBJECTS. I. TARGETS, PROPER MOTIONS, AND FIRST RESULTS

We present results from the PARallaxes of Southern Extremely Cool objects (PARSEC) program, an observational program begun in April 2007 to determine parallaxes for 122 L and 28 T southern hemisphere dwarfs using the Wide Field Imager on the ESO 2.2m telescope. The results presented here include parallaxes of 10 targets from observations over 18 months and a first version proper motion catalog. The proper motions were obtained by combining PARSEC observations astrometrically reduced with respect to the UCAC2 Catalog, and the 2MASS Catalog. The resulting median proper motion precision is 5mas/yr for 195,700 sources. The 140 0.3deg2 fields sample the southern hemisphere in an unbiased fashion with the exception of the galactic plane due to the small number of targets in that region. We present preliminary parallaxes with a 4.2 mas median precision for 10 brown dwarfs, 2 of which are within 10pc. These increase by 20% the present number of L dwarfs with published parallaxes. Of the 10 targets, 7 have been previously discussed in the literature: two were thought to be binary but the PARSEC observations show them to be single, one has been confirmed as a binary companion and another has been found to be part of a binary system, both of which will make good benchmark systems. Observations for the PARSEC program will end in early 2011 providing 3-4 years of coverage for all targets. The main expected outputs are: more than a 100% increase of the number of L dwarfs with parallaxes; to increment - in conjuction with published results - to at least 10 the number of objects per spectral subclass up to L9, and; to put sensible limits on the general binary fraction of brown dwarfs. We aim to contribute significantly to the understanding of the faint end of the H-R diagram and of the L/T transition region.

Precise predictions of stellar occultations by Pluto, Charon, Nix, and Hydra for 2008–2015

Context. We investigate transneptunian objects, including Pluto and its satellites, byxa0stellar occultations.Aims. Our aim is to derive precise, astrometric predictions for stellar occultations by Pluto and its satellites Charon, Hydra and Nix for 2008-2015. We construct an astrometric star catalog in the UCAC2xa0system covering Plutos sky path.Methods. We carried out in 2007 an observational program at the ESO2p2/WFI instrument covering the sky path of Pluto from 2008 to 2015. We made the astrometry of 110xa0GB of images with the Platform for Reduction of Astronomical Images Automatically (PRAIA). By relatively simple astrometric techniques, we treated the overlapping observations and derived a field distortion pattern for the WFI mosaic of CCDs to within 50xa0mas precision.Results. Positions were obtained in the UCAC2 frame with errors of 50xa0mas for stars up to magnitude R = 19, and 25xa0mas up to R = 17. New stellar proper motions were also determined with 2MASS and the USNO B1.0xa0catalog positions as first epoch. We generated 2252 predictions of stellar occultations by Pluto, Charon, Hydra and Nix for 2008-2015. An astrometric catalog with proper motions was produced, containing 2.24xa0million stars covering Plutos sky path with xa0width. Its magnitude completeness is about R = 18–19 with a limit about R = 21. Based on the past 2005–2008 occultations successfully predicted, recorded and fitted, axa0linear drift with time in declination with regard to DE418/plu017xa0ephemerides was determined for Pluto and used in the current predictions. For offset (mas) xa0= A * (t (yr) - 2005.0) + B , we find A = +30.5xa0± 4.3xa0masxa0yr-1 and B = -31.5 ± 11.3xa0mas, with standard deviation of 14.4xa0mas for the offsets. For these past occultations, predictions and follow-up observations were made with the 0.6xa0m and 1.6xa0m telescopes at the Laboratorio Nacional de Astrofisica/Brazil.Conclusions. Recurrent issues in stellar occultation predictions were addressed and properly overcome: body ephemeris offsets, catalog zero-point position errors and field-of-view size, long-term predictions and stellar proper motions, faint-visual versus bright-infrared stars and star/body astrometric follow-up. In particular, we highlight the usefulness of the obtained astrometric catalog as a reference frame for star/body astrometric follow-up before and after future events involving the Pluto system. Besides, itxa0also furnishes useful photometric information for field stars in the flux calibration of observed light curves. Updates on the ephemeris offsets and candidate star positions (geometric conditions of predictions and finding charts) are made available by the group at www.lesia.obspm.fr/perso/bruno-sicardy/.

The large quasar reference frame (LQRF) An optical representation of the ICRS

Context. The large number and all-sky distribution of quasars from different surveys, along with their presence in large, deep astrometric catalogs, enables us to build of an optical materialization of the International Celestial Reference System (ICRS) following its defining principles. Namely: that it is kinematically non-rotating with respect to the ensemble of distant extragalactic objects; aligned with the mean equator and dynamical equinox of J2000; and realized by a list of adopted coordinates of extragalatic sources. Aims. The Large Quasar Reference Frame (LQRF) was built with the care of avoiding incorrect matches of its constituents quasars, homogenizing the astrometry from the different catalogs and lists in which the constituent quasars are gathered, and attaining a milli-arcsec global alignment with the International Celestial Reference Frame (ICRF), as well as typical individual source position accuracies higher than 100 milli-arcsec � . Methods. Starting from the updated and presumably complete Large Quasar Astrometric Catalog (LQAC) list of QSOs, the initial optical positions of those quasars are found in the USNO B1.0 and GSC2.3 catalogs, and from the SDSS Data Release 5. The initial positions are next placed onto UCAC2-based reference frames, This is followed by an alignment with the ICRF, to which were added the most precise sources from the VLBA calibrator list and the VLA calibrator list – when reliable optical counterparts exist. Finally, the LQRF axes are inspected through spherical harmonics, to define right ascension, declination and magnitude terms. Results. The LQRF contains 100,165 quasars, well represented accross the sky, from −83.5 to +88.5 ◦ in declination, being 10 arcmin the average distance between adjacent elements. The global alignment with the ICRF is 1.5 mas, and the individual position accuracies are represented by a Poisson distribution that peaks at 139 mas in right ascension and 130 mas in declination. As a by-product, significant equatorial corrections are found for all the catalogs used (apart from the SDSS DR5), an empirical magnitude correction can be discussed for the GSC2.3 intermediate and faint regimes, both the 2MASS and the preliminary northernmost UCAC2 positions are shown of astrometry consistent with the UCAC2 main catalog, and the harmonic terms are found to be always small. Conclusions. The LQRF contains J2000 referred equatorial coordinates, and is complemented by redshift and photometry information from the LQAC. It is designed to be an astrometric frame, but it is also the basis for the GAIA mission initial quasars’ list, and can be used as a test bench for quasars’ space distribution and luminosity function studies. The LQRF is meant to be updated when new quasar identifications and newer versions of the astrometric frames used are realized. In the later case, it can itself be used to examine the relations between those frames.

PARALLAXES OF SOUTHERN EXTREMELY COOL OBJECTS (PARSEC). II. SPECTROSCOPIC FOLLOW-UP AND PARALLAXES OF 52 TARGETS*

We present near-infrared spectroscopy for 52 ultracool dwarfs, including two newly discovered late-M dwarfs, one new late-M subdwarf candidate, three new L, and four new T dwarfs. We also present parallaxes and proper motions for 21 of them. Four of the targets presented here have previous parallax measurements, while all the others are new values. This allow us to populate further the spectral sequence at early types (L0-L4). Combining the astrometric parameters with the new near-infrared spectroscopy presented here, we are able to investigate further the nature of some of the objects. In particular, we find that the peculiar blue L1 dwarf SDSS J133148.92–011651.4 is a metal-poor object, likely a member of the galactic thick disk. We discover a new M subdwarf candidate, 2MASS J20115649–6201127. We confirm the low-gravity nature of EROS-MP J0032–4405, DENIS-P J035726.9–441730, and 2MASS J22134491–2136079. We present two new metal-poor dwarfs: the L4pec 2MASS J19285196–4356256 and the M7pec SIPS2346–5928. We also determine the effective temperature and bolometric luminosity of the 21 targets with astrometric measurements, and we obtain a new polynomial relation between effective temperature and near-infrared spectral type. The new fit suggests a flattening of the sequence at the transition between M and L spectral types. This could be an effect of dust formation, which causes a more rapid evolution of the spectral features as a function of the effective temperature.

The second release of the Large Quasar Astrometric Catalog (LQAC-2)

Context. Since the first release of the LQAC (Large Quasar Astrometric Catalogue) a large number of quasars have been discovered through very dense observational surveys. As these objects constitute the cornerstones of modern astrometry by indicating quasi inertial directions, their spatial density and their astrometric quality must be studied in detail. Aims. Following the same procedure as in this first release of the LQAC, our aim is to compile all the quasars recorded until the present date, with the best determination of their equatorial coordinates in the ICRS, i.e. with respect to the newly established ICRF2 and with the maximum of information concerning their physical properties (redshift, photometry, absolute magnitudes). Methods. First of all we made a substantial review of the definitions and properties of quasars and AGN (active galactic nuclei), because the differenciation of these objects is unclear in the literature, even for specialists. Then we carried out the cross-identification between the nine catalogs of quasars chosen for their accuracy and their huge number of objects, including all the available data related to magnitudes, radiofluxes, and redshifts. Moreover, we computed the absolute magnitude of our extragalactic objects by taking the recent studies concerning the galactic absorption into account. In addition, substantial improvements were made with respect to the first release of the LQAC. First, an LQAC name is given for each object based on its equatorial coordinates with respect to the ICRS, following a procedure that creates no ambiguity in the identification. Second the equatorial coordinates of the objects were recomputed more accurately according to the algorithms used for the elaboration of the Large Quasar Reference Frame (LQRF). Third we introduce a morphological classification for the objects that in particular clearly defines if the object is point-like or extended. Results. Our final catalog, called LQAC-2, contains 187 504 quasars. This is roughly 65% more than the 113 666 quasars recorded in the first version of the LQAC and a little more than the number of quasars recorded in the updated version of the Veron-Cetty & Veron (2010, A&A, 518, A10) catalog, which was the densest compilation of quasars up to now. In addition to the quantitative and qualitative improvements brought by our compilation, we discuss the homogeneity of the data and carry out statistical analysis of the spatial density and the distance to the closest neighbor. Conclusions. The LQAC-2 will be useful for the astronomical community since it gives the most complete information available about the whole set of already recorded quasars, insisting on the precision and accuracy of their coordinates with respect to the ICRF-2.

The construction of the large quasar astrometric catalogue (LQAC)

Context. The very large and increasing number of quasars reckoned from various sky surveys leads to a large quantity of data which brings various and inhomogeneous information in the fields of astrometry, photometry, radioastronomy and spectroscopy. Aims. In this paper, we describe our work that aims to make available a general compilation of the largest number of recorded quasars obtained from all the available catalogues, with their best position estimates, and providing physical information at both optical and radio wavelengths. Thus, we construct a catalogue compilation designated Large Quasar Astrometric Catalogue (LQAC) giving coordinates, multiband photometry, radio fluxes, redshift, luminosity distances and absolute magnitudes. Methods. We gather the 12 largest quasar catalogues (4 from radio interferometry programs, 8 from optical surveys), and we carry out systematic cross-identifications of the objects. Information concerning u, b, v, g, r, i, z, J, K photometry as well as redshift and radio fluxes at 1.4 GHz, 2.3 GHz, 5.0 GHz, 8.4 GHz and 24 GH are given when available. A small proportion of remaining objects, not present in the 12 catalogues and included in the Veron-Cetty & Veron quasar catalogues, are added to the compilation. Results. The LQAC contains 113 666 quasars. We discuss the external homogeneity of the data by comparing the coordinates, the redshifts and the magnitudes of objects belonging to different catalogues. We use up-to-date cosmological parameters as well as recent models for galactic extinction and K-correction in order to evaluate the absolute magnitudes of the objects.

NPARSEC : NTT Parallaxes of Southern Extremely Cool objects. Goals, targets, procedures and first results

The discovery and subsequent detailed study of T dwarfs has provided many surprises and pushed the physics and modeling of cool atmospheres in unpredicted directions. Distance is a critical parameter for studies of these objects to determine intrinsic luminosities, test binarity and measure their motion in the Galaxy. We describe a new observational program to determine distances across the full range of T dwarf sub-types using the NTT/SOFI telescope/instrument combination. We present preliminary results for ten objects, five of which represent new distances.