ABSTRACT: 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 p(V) = 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.
Nature 11/2012; 491(7425):566-9. · 36.28 Impact Factor
ABSTRACT: We study large trans-neptunian objects (TNOs) using stellar occultations. We
derive precise astrometric predictions for stellar occultations by Eris,
Haumea, Ixion, Makemake, Orcus, Quaoar, Sedna, Varuna, 2002 TX300, and 2003
AZ84 for 2011-2015. We construct local astrometric catalogs of stars in the
UCAC2 (Second US Naval Observatory CCD Astrograph Catalog) frame covering the
sky path of these objects. For that purpose, during 2007-2009, we carried out
an observational program at the ESO2p2/WFI (2.2 m Max-Planck ESO telescope with
the Wide Field Imager) instrument. Astrometric catalogs with proper motions
were produced for each TNO, containing more than 5.35 million stars covering
the sky paths with 30' width in declination. The magnitude completeness is
about R = 19 with a limit of about R = 21. We predicted 2717 stellar
occultation candidates for all targets. Ephemeris offsets with about from 50
mas to 100 mas precision were applied to each TNO orbit to improve the
predictions. They were obtained during 2007-2010 from a parallel observational
campaign carried out with from 0.6 m to 2.2 m in size telescopes. This extends
our previous work for the Pluto system to large TNOs, using the same
observational and astrometric procedures. The obtained astrometric catalogs are
useful for follow-up programs at small to large telescopes used to improve the
candidate star positions and TNO ephemeris. They also furnish valuable
photometric information for the field stars. For each TNO, updates on the
ephemeris offsets and candidate star positions (geometric conditions of
predictions and finding charts) are made available in the web by the group.
ABSTRACT: 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 cm(3) and a high visible geometric albedo, Pv = 0.96(+0.09)(-0.04). No nitrogen, argon or methane atmospheres are detected with surface pressure larger than ∼1 nanobar, about 10,000 times more tenuous than Pluto's present atmosphere. As Pluto's 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.
Nature 10/2011; 478(7370):493-6. · 36.28 Impact Factor
ABSTRACT: Context: To investigate the differences between positions, as determined by
optical (direct imaging) and Very Long Baseline Interferometry (VLBI)
techniques, of extragalactic sources listed in the second realization of the
International Celestial Reference Frame (ICRF2). Aims: To verify the influence
of the source's intrinsic structure on these differences. Methods: Instruments
with mosaics of CCDs were used to acquire the optical positions presented here,
leading us to opt for overlapping techniques to build a virtual, continuous CCD
over the whole angular size of the respective fields of view, whose translation
of the resulting intrumental positions into positions that are consistent with
those in the ICRF2 was made with the help of the UCAC2. Results: The
differences obtained between the optical and VLBI positions of the observed
sources may reach more than 80 milliarcseconds and, taking into consideration
that they are hardly explained only by statistical fluctuations or systematic
errors in the optical reference frame used here, we argue that these
differences can be related to the sources' X-band structure index (8.4 GHz).
Conclusions: In this context, the presence of the intrinsic structure should be
taken into consideration when comparing the optical and VLBI positions of ICRF2
sources in the future.
ABSTRACT: Pluto and its main satellite, Charon, occulted the same star on 2008 June 22. This event was observed from Australia and La Réunion Island, providing the east and north Charon Plutocentric offset in the sky plane (J2000): X= + 12,070.5 ± 4 km (+ 546.2 ± 0.2 mas), Y= + 4,576.3 ± 24 km (+ 207.1 ± 1.1 mas) at 19:20:33.82 UT on Earth, corresponding to JD 2454640.129964 at Pluto. This yields Charon's true longitude L= 153.483 ± 0071 in the satellite orbital plane (counted from the ascending node on J2000 mean equator) and orbital radius r= 19,564 ± 14 km at that time. We compare this position to that predicted by (1) the orbital solution of Tholen & Buie (the "TB97" solution), (2) the PLU017 Charon ephemeris, and (3) the solution of Tholen et al. (the "T08" solution). We conclude that (1) our result rules out solution TB97, (2) our position agrees with PLU017, with differences of ΔL= + 0.073 ± 0071 in longitude, and Δr= + 0.6 ± 14 km in radius, and (3) while the difference with the T08 ephemeris amounts to only ΔL= 0.033 ± 0071 in longitude, it exhibits a significant radial discrepancy of Δr= 61.3 ± 14 km. We discuss this difference in terms of a possible image scale relative error of 3.35 × 10–3in the 2002-2003 Hubble Space Telescope images upon which the T08 solution is mostly based. Rescaling the T08 Charon semi-major axis, a = 19, 570.45 km, to the TB97 value, a = 19636 km, all other orbital elements remaining the same ("T08/TB97" solution), we reconcile our position with the re-scaled solution by better than 12 km (or 0.55 mas) for Charon's position in its orbital plane, thus making T08/TB97 our preferred solution.
The Astronomical Journal 01/2011; 141(2):67. · 4.03 Impact Factor
ABSTRACT: We present results from the PARallaxes of Southern Extremely Cool objects (PARSEC) program, an observational program begun in 2007 April to determine parallaxes for 122 L and 28 T southern hemisphere dwarfs using the Wide Field Imager on the ESO 2.2 m 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 Second US Naval Observatory CCD Astrograph Catalog, and the Two Micron All Sky Survey Point Source Catalog. The resulting median proper motion precision is 5 mas yr–1 for 195,700 sources. The 140 0.3 deg2 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. The proper motion distributions are shown to be statistically well behaved. External comparisons are also fully consistent. We will continue to update this catalog until the end of the program, and we plan to improve it including also observations from the GSC2.3 database. We present preliminary parallaxes with a 4.2 mas median precision for 10 brown dwarfs, two of which are within 10 pc. These increase the present number of L dwarfs by 20% with published parallaxes. Of the 10 targets, seven 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. These results confirm that the foreseen precision of PARSEC can be achieved and that the large field of view will allow us to identify wide binary systems. Observations for the PARSEC program will end in early 2011 providing three to four years of coverage for all targets. The main expected outputs are: more than a 100% increase in the number of L dwarfs with parallaxes, increment in the number of objects per spectral subclass up to L9—in conjunction with published results—to at least 10, 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.
The Astronomical Journal 01/2011; 141(2):54. · 4.03 Impact Factor
ABSTRACT: The large number and all-sky distribution of quasars from different surveys, along with their presence in large, deep astrometric catalogs,enables the building of an optical materialization of the 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. Starting from the updated and presumably complete 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 DR5. The initial positions are next placed onto UCAC2-based reference frames, following 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, contemplating to define right ascension, declination and magnitude terms. The LQRF contains J2000 referred equatorial coordinates for 100,165 quasars, well represented across the sky, from -83.5 to +88.5 degrees in declination, and with 10 arcmin being 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. It is complemented by redshift and photometry information from the LQAC. The LQRF 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. Comment: 23 pages, 23 figures, 6 tables Accepted for publication by Astronomy & Astrophysics, on 25 May 2009
ABSTRACT: Every 42 years, the Earth and the Sun pass through the plane of the orbits of the main satellites of Uranus. In these occasions, mutual occultations and eclipses between these bodies can be seen from the Earth. The current Uranus equinox from 2007 to 2009 offers a precious opportunity to observe these events. Here, we present the analysis of five occultations and two eclipses observed from Brazil during 2007. For the reduction of the CCD images, we developed a digital coronagraphic method that removed the planet's scattered light around the satellites. A simple geometric model of the occultation/eclipse was used to fit the observed light curves. Dynamical quantities such as the impact parameter, the relative speed, and the central time of the event were then obtained with precisions of 7.6 km, 0.18 km s–1, and 2.9 s, respectively. These results can be further used to improve the parameters of the dynamical theories of the main Uranus satellites.
The Astronomical Journal 03/2009; 137(4):4046. · 4.03 Impact Factor
ABSTRACT: 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.
The Astrophysical Journal 12/2008; 552(1):380. · 6.02 Impact Factor
ABSTRACT: 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 Laboratório Nacional de Astrofísica, 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.
The Astronomical Journal 12/2007; 125(5):2728. · 4.03 Impact Factor
ABSTRACT: 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.
The Astronomical Journal 12/2007; 124(1):612. · 4.03 Impact Factor
ABSTRACT: We present results of a pilot investigation on the astrometry of International Celestial Reference Frame (ICRF) sources using small- to medium-sized telescopes and the second US Naval Observatory CCD Astrograph Catalog (UCAC2). For this purpose, 31 ICRF sources were observed, mostly south of the equator, during 1997–2000. We used the automated 0.6 and 1.6 m Cassegrain telescopes equipped with CCD detectors located at Laboratório Nacional de Astrofísica, Brazil. The source positions were referred to UCAC2, with fainter 0.6 m telescope stars serving as a reference frame to the reductions of the 1.6 m telescope CCD fields. Observations were made in the V band in a compromise between the 579–643 nm bandpass (between V and R) of UCAC2 and the bluer ICRF sources. To ensure that UCAC2, with its magnitude bandpass system, is a reliable reference catalog for our V-band CCD frame reductions, we have also compared it against an independent set of star positions with similar characteristics obtained in the V band with the Valinhos CCD Meridian Circle, Brazil. Average values and errors for the optical - radio position offsets using the 0.6 m telescope were +4 ± 8 mas (41 mas) and +1 ± 8 mas (42 mas) for right ascension and declination, respectively. (Parenthetical values refer to standard deviation, i.e., to the typical error of a single measurement given the quantity of sources.) For the 1.6 m telescope, offsets were -12 ± 9 mas (45 mas) and +8 ± 9 mas (46 mas). An expected random error that increases with magnitude and affects the positions of the fainter 0.6 m telescope secondary stars is verified. No systematic errors were found within the attained position precision, including differential color refraction. External comparisons with independent telescope/catalog sets of precise source positions were also made, showing consistent results within the respective errors.
The Astronomical Journal 12/2007; 129(6):2907. · 4.03 Impact Factor
ABSTRACT: 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 Pluto's 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 satellite's 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, R(C) = 603.6 +/- 1.4 km (1sigma), and a density of rho = 1.71 +/- 0.08 g cm(-3). This occultation also provides upper limits of 110 and 15 (3sigma) nanobar for an atmosphere around Charon, assuming respectively a pure nitrogen or pure methane atmosphere.
Nature 02/2006; 439(7072):52-4. · 36.28 Impact Factor
ABSTRACT: 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 Pluto's size and about one-eighth its mass. The precise radii of Pluto and Charon have remained uncertain, leading to large uncertainties on their densities
Nature 01/2006; 439(7072):52-54. · 36.28 Impact Factor
ABSTRACT: The Gaia Extragalactic Celestial Reference Frame (GCRF) will be formed
by about 500 000 quasars, up to magnitude G = 20, defined to typical
precision of 50 µas. The GCRF is pivotal for many of the mission
objectives, starting with the astrometry catalogue. Yet the pre-mission
representation of the GCRF is complicated because of the comparatively
small number of observed quasars. Here, we present a restricted
representation of the GCRF, based on the V´eron-Cetty &
V´eron list of 48 921 quasars. This representation brings the
original list to a fully coherent placement on the ICRS. The source
positions have been collected from the USNO B1.0 catalogue, which is
complete to V = 20. Around each of them, fields of size as small as 6
were detailed, in which were picked up B1.0 stars and their
corresponding positions from catalogs extending the HCRS to dimmer
magnitudes. The UCAC2 (48 million stars, to RV = 16, precise to 30 mas)
and the 2MASS (470 million objects, complete to J = 16, precise to 100
mas) acted as the astrometric reference catalogs. Taking as paradigm the
B1.0 positions corrected by the UCAC2, a reference frame is obtained
containing 37 513 quasars, globally aligned to 1mas with the ICRF. The
optical minus radio standard deviation is at 150 mas, much smaller
therefore than the nominal 200 mas B1.0 accuracy (the o-r standard
deviation is above 300 mas for the original V&V entries). The
extragalactic reference frame obtained in this manner enables us to
gather insights on the distribution and luminosity of the GCRF. At the
same time it provides a useful frame for all-purpose observations.
ABSTRACT: The Gaia extragalactic reference frame (GCRF) will be formed by about
500 000 quasars, to a positional precision reaching 50 µas. The
GCRF is a key factor for several of the Gaia science goals, and it is
important to access its sky and magnitude distribution before the
mission launch. Despite recent in-depth investigations, such as the 2dF
and SLOAN, there is no all-sky account to such. Hitherto, to the M = 20
limit, the USNO B1.0 and GSC2 catalogues attain completeness. They can
thus be used to generate an accurate probabilistic description of the
GCRF. Starting from those catalogues, the main strategies that are
brought into play for the task involve determination of the quasars loci
in the proper motions space; in the colour/colour space; in the
broad-band spectroscopy space; and in the probability space as given by
the stars and extragalactic objects distribution models. Here we discuss
such strategies and some of the obtained results.
ABSTRACT: This work brings an approach intending to improve the connection between
the Dynamical Reference Frame and the Extragalactic Reference Frame. For
that, close encounters of outer Solar System objects and quasars are
used. With this goal, Uranus, Neptune and two quasars were observed at
Laborat´orio Nacional de Astrof´ısica (LNA), Brazil.
The optical reference frame is the HCRF, as given by the UCAC2
catalogue. The first results show an accuracy of 45 mas - 50 mas in the
optical positions. The optical minus radio offsets give the local
orientation between the catalogue and radio frame. From this, it is
possible to place the optical planet coordinates on the extragalactic
frame. A comparison between the new corrected optical coordinates and
the respective DE ephemeris to these planets can give the instant
orientations of the Dynamical Reference Frame with regard to the ICRS,
for this zone of outer Solar System.
ABSTRACT: In the context of a joint collaboration involving the United States
Naval Observatory (USNO) and 3 Brazilian research institutions: the
Valongo Observatory (OV/UFRJ), the National Observatory (ON) and the
National Laboratory for Astrophysics (LNA), we report new optical
positions on the 30 mas precision level for 172 extragalactic,
International Celestial Reference Frame (ICRF) sources mainly between
-30< = d = >+25. Results are from a pilot investigation including
four CTIO 0.9-meter runs (1999-2001). Reference stars in the R ~ 10 to
16.5 magnitude range from a preliminary USNO 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
1.60m Cassegrain telescope of the (LNA). A significant improvement in
the optical positions was achieved over a previous determination of
source positions (Zacharias et al. 1999). The mean optical positions are
compared with the ICRF radio positions. The overall optical minus radio
offsets are -6 mas and -15 mas for RA and Dec, 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 years after the Hipparcos mean epoch and our
result set an upper limit for a possible Hipparcos system rotation
w.r.t. the ICRS for the z-axis of about 0.7 mas/year.
ABSTRACT: Ground-based observations of faint satellites nearby their planets with 1–2 m class telescopes usually do not allow for high quality astrometry and photometry, due to saturation by the primary or to poor S/N ratio on the satellite images, generally embedded on the scattered light of the planet. Earth-based observations of the Uranus satellites’ upcoming mutual events in 2007–2008 are no exception. In most cases, the event will take place at 4 arcs or less from the planet, with “planet minus satellite” brightness differences of 10 magnitudes. So as to make feasible the observations of these important phenomena, we have developed a prototype of a coronagraph of simple design making use of good quality commercial optical systems. Pilot tests made with this coronagraph with a telescope of show that it is possible to obtain S/N ratios of 50 or higher for 10 s exposures, for satellites as close as 2 Uranus radii from the planet center. We have also developed numerical algorithms which perform digital coronagraphy in the images, with the elimination of the influence of the planets’ scattered light. This procedure considerably improves the S/N ratio of the satellite images (with or without a coronagraph instrument) and shall be applied in the reduction of the observations of the Uranus events so as to achieve the highest possible photometric and astrometric quality.
Planetary and Space Science 56(14):1882-1887. · 2.22 Impact Factor
ABSTRACT: Context.To investigate the link between the International Celestial Reference Frame (ICRF) and its optical Hipparcos-based representation on the northern hemisphere.Aims.To present results of a pilot investigation on the astrometry of 59 northern ICRF sources.Methods.We used the 0.6 m Zeiss telescope at Belogradchik Observatory, Bulgaria. The optical CCD source positions were referred to the UCAC2 catalog. Improved astrometric methods were applied including telescope shifting between exposures, ($x,\,y$) Gaussian measurements within 1 full width at half maximum, and the observation of a large number of frames per source. The huge amount of data generated was treated with a new astrometric package, PRAIA (Platform for Reduction of Astronomical Images Astrometrically).Results.Average and standard deviation for the optical minus radio position offsets were +6 mas (51 mas) and +7 mas (57 mas) for RA and Dec, respectively. The errors of all ($x,\,y$) measurements from Gaussian fits displayed a typical distribution with respect to magnitude, dependent on the sky transparency. For the ICRF objects, the ($x,\,y$) errors per source ranged from 50 mas to 100 mas. The RA and Dec reduction mean errors were 46 mas.Conclusions.No large scale systematic errors with respect to RA or Dec were found within the attained position precision. Comparison with three independent telescope$/$catalog datasets shows that the precision obtained here represents an important improvement on previous works. In comparison with the southern hemisphere, more astrometry on ICRF sources are needed in the north, and it is shown here that the continuation of this program can fill this need.