D. N. da Silva Neto

A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation

The dwarf planet Eris is a trans-Neptunian object with an orbital eccentricity of 0.44, an inclination of 44 degrees and a surface composition very similar to that of Pluto. It resides at present at 95.7 astronomical units (1 au is the Earth-Sun distance) from Earth, near its aphelion and more than three times farther than Pluto. Owing to this great distance, measuring its size or detecting a putative atmosphere is difficult. Here we report the observation of a multi-chord stellar occultation by Eris on 6 November 2010 ut. The event is consistent with a spherical shape for Eris, with radius 1,163 ± 6 kilometres, density 2.52 ± 0.05 grams per cm3 and a high visible geometric albedo, . No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ∼1 nanobar, about 10,000 times more tenuous than Plutos present atmosphere. As Plutos radius is estimated to be between 1,150 and 1,200 kilometres, Eris appears as a Pluto twin, with a bright surface possibly caused by a collapsed atmosphere, owing to its cold environment. We anticipate that this atmosphere may periodically sublimate as Eris approaches its perihelion, at 37.8 astronomical units from the Sun.

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 600 km for the satellites radius, which was later refined to 601.5 km (ref. 4). Here we report observations from a multi-station stellar occultation by Charon, which we use to derive a radius, RC = 603.6 ± 1.4 km (1σ), and a density of ρ = 1.71 ± 0.08 g cm-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.

Albedo and atmospheric constraints of dwarf planet Makemake from a stellar occultation

Pluto and Eris are icy dwarf planets with nearly identical sizes, comparable densities and similar surface compositions as revealed by spectroscopic studies. Pluto possesses an atmosphere whereas Eris does not; the difference probably arises from their differing distances from the Sun, and explains their different albedos. Makemake is another icy dwarf planet with a spectrum similar to Eris and Pluto, and is currently at a distance to the Sun intermediate between the two. Although Makemake’s size (1,420 ± 60 km) and albedo are roughly known, there has been no constraint on its density and there were expectations that it could have a Pluto-like atmosphere. Here we report the results from a stellar occultation by Makemake on 2011 April 23. Our preferred solution that fits the occultation chords corresponds to a body with projected axes of 1,430 ± 9 km (1σ) and 1,502 ± 45 km, implying a V-band geometric albedo pV = 0.77 ± 0.03. This albedo is larger than that of Pluto, but smaller than that of Eris. The disappearances and reappearances of the star were abrupt, showing that Makemake has no global Pluto-like atmosphere at an upper limit of 4–12 nanobar (1σ) for the surface pressure, although a localized atmosphere is possible. A density of 1.7 ± 0.3 g cm−3 is inferred from the data.

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.

Optical Positions of ICRF Sources Using UCAC Reference Stars

New optical positions on the 30 mas precision level have been obtained for 172 extragalactic International Celestial Reference Frame (ICRF) sources mainly in the range -30° ≤ δ ≤ +25°. Results are presented from a pilot investigation including four Cerro Tololo (CTIO) 0.9 m runs (1999–2001). Reference stars in the R ≈ 10–16.5 mag range from a preliminary US Naval Observatory CCD Astrograph Catalog (UCAC) are used. Systematic errors have been investigated, and a field distortion pattern based on the residuals has been removed. The errors of the fainter stars in the CTIO data were assessed by evaluating an auxiliary set of CCD observations of common ICRF sources, taken at the 1.60 m Cassegrain telescope of the Laboratorio Nacional de Astrofisica, Brazil. A significant improvement in the optical positions was achieved over a previous determination of source positions. The mean optical positions are compared with the ICRF radio positions. The overall optical minus radio offsets are -6 and -15 mas for right ascension and declination, respectively. The formal internal error of these mean offsets is ≈2.3 mas. This indicates a possible systematic error in the UCAC declinations of ≈10 to 15 mas. Both the optical counterpart observations and the optical reference stars are observed about 9 yr after the Hipparcos mean epoch, and our results set an upper limit for a possible Hipparcos system rotation with respect to the International Celestial Reference System for the z-axis of about 0.7 mas yr-1.

A PATTERN OF NONCOINCIDENCE BETWEEN RADIO AND OPTICAL POSITIONS OF INTERNATIONAL CELESTIAL REFERENCE FRAME SOURCES

Many of the International Celestial Reference Frame (ICRF) sources are not pointlike, as shown by the 2 GHz and 8 GHz radio maps. The size scale reaches up to a few tens of milliarcseconds for extended sources. Also, although the optical images are not resolved, the centers of emission are not necessarily coincident with the radio centroids. Here we search for indications of such noncoincidence. We divide the sources into two sets, extended and compact, according to the radio structure index given in the ICRF extension. The optical positions are from recent determinations, to obtain the highest precision and evenness of accuracy. The ICRF radio positions are of milliarcsecond precision or better. The average of the absolute values of the differences between the lengths of the optical and radio arcs joining pairs of sources taken within each of the sets is found to be about 7.9 mas larger for the extended sources than for the compact sources. This is interpreted as evidence of noncoincidence between the radio and optical centers, at least for the extended sources. Additional checks made with larger, different sets using the optical source positions from the USNO-A2.0 catalog support this conclusion.

An exploration of Pluto’s environment through stellar occultations

Context. Pluto has five known satellites with diameters ranging from ~1200 km down to ~40 km, a possible outcome of a collisional origin. Smaller objects probably exist and may maintain tenuous rings, thus representing hazards during the New Horizons flyby of July 2015. Aims. The goal is to provide an upper limit for the numbers of unseen small bodies and/or equivalent widths of putative Pluto rings. Methods. We use a Pluto stellar appulse on April 10, 2006, and a stellar occultation by the dwarf planet on June 14, 2007, to scan Pluto’s surroundings. Results. Our best data set places a 3σ upper limit of 0.3 km for the radius of isolated moonlets that we can detect. In the absence of detection, we derive an upper limit of 15 000 for the number of such bodies at distances smaller than ~70 000 km from Pluto’s system barycenter. We place a 3σ upper limit of typically 30−100 m for the equivalent width of ring material at barycentric distances ranging from 13 000 to 70 000 km. This limit applies for narrow rings only, i.e. less than about 10 km in width

Astrometry of the main satellites of Uranus: 18 years of observations ? ??

Context. We contribute to developing dynamical models of the motions of Uranus’ main satellites. Aims. We determine accurate positions of the main satellites of Uranus: Miranda, Ariel, Umbriel, Titania, and Oberon. Positions of Uranus, as derived from those of these satellites, are also determined. The observational period spans from 1992 to 2011. All runs were made at the Pico dos Dias Observatory, Brazil. Methods. We used the software called Platform for Reduction of Astronomical Images Automatically (PRAIA) to perform a digital coronography to minimise the influence of the scattered light of Uranus on the astrometric measurements and to determine accurate positions of the main satellites. The positions of Uranus were then indirectly determined by computing the mean di erences between the observed and ephemeris positions of these satellites. A series of numerical filters was applied to filter out spurious data. These filters are mostly based on (a) the comparison between the positions of Oberon with those of the other satellites and on (b) the o sets as given by the di erences between the observed and ephemeris positions of all satellites. Results. We have, for the overall o sets of the five satellites, 29 mas ( 63 mas) in right ascension and 27 mas ( 46 mas) in declination. For the overall di erence between the o sets of Oberon and those of the other satellites, we have +3 mas ( 30 mas) in right ascension and 2 mas ( 28 mas) in declination. Ephemeris positions for the satellites were determined from DE432+ura111. Comparisons using other modern ephemerides for the solar system ‐ INPOP13c ‐ and for the motion of the satellites ‐ NOE-7-2013 ‐ were also made. They confirm that the largest contribution to the o sets we find comes from the motion of the barycenter of the Uranus system around the barycenter of the solar system, as given by the planetary ephemerides. For the period from 1992 to 2011, our final catalogues contain 584 observed positions of Miranda, 1,710 of Ariel, 1,987 of Umbriel, 2,588 of Titania, 2,928 of Oberon, and 3,516 of Uranus.

Astrometric detection of faint companions – the Pluto/Charon case study

The resolution of pairs of objects closer than the scale of seeing, and of difference of magnitude larger than ten percent is unreliable by direct imaging. The resulting image FWHM differs from a true PSF by no more than four percent. Yet, the peak of the associated Gaussian is shifted to a larger proportion. The main results are the description of the FWHM and peak location shifts as function of the seeing scale, the centers separation, and of the magnitudes difference. Analytically, the estimators of variation were the resulting Gaussian amplitude, mean value, and standard deviation. The later is shown to be the most reliable estimator.

Astrometric and Photometric Variability in Quasars

A. H. Andrei1,2, S. Bouquillon3, J. L. Penna1, F. Taris3, S. Anton4, J. Souchay3, J. I. B. Camargo1, D. N. da Silva Neto5, R. Vieira Martins1, M. Assafin2, and S. dos Reis Carvalho Pinto1,6 Observatório Nacional/MCT, R. Gal. Jose Cristino 77, Rio de Janeiro, Brazil Email: oat1@on.br Observatório do Valongo, UFRJ, Brazil SYRTE/Observatoire de Paris, France Centro de Investigação em Ciencias Geo-Espaciais/FCUP, Portugal Universidade Estadual da Zona Oeste-BR, Universidade Gama Filho, Brazil