251 research outputs found
Inversion of HIPPARCOS and Gaia photometric data for asteroids Asteroid rotational properties from sparse photometric data
Context. Sparse photometric data can be used to determine the spin properties and infer information about the shapes of asteroids. The algorithm adopted for the inversion of Gaia photometric data assumes, for the sake of simplicity and to minimize CPU execution time, that the objects have triaxial ellipsoid shapes. In the past, this algorithm was tested against large sets of simulated data and small numbers of sparse photometric measurements obtained by HIPPARCOS. Aims. After the second Gaia data release, it is now possible to test the inversion algorithm against small samples of actual Gaia data for the first time. At the same time, we can attempt a new inversion of older HIPPARCOS measurements, using an updated version of the photometric inversion algorithm. Methods. The new version of our inversion algorithm includes the treatment of a Lommel-Seeliger scattering relation especially developed for the case of triaxial ellipsoid shapes. In addition, we also performed inversion attempts using a more refined shape model, based on the so-called cellinoid shapes. Results. With respect to the old inversion of HIPPARCOS data carried out in the past, we obtain only marginal improvements. In the case of Gaia data, however, we obtain very encouraging results. A successful determination of the rotation period is possible in most cases, in spite of the limited time span covered by data published in the second Gaia data release (GDR2), which makes the determination of the spin axis direction still uncertain. Even a small number of measurements, less than 30 in many cases, are sufficient to obtain a satisfactory inversion solution. Using the more realistic cellinoid shape model, we find further improvement in the determination of the spin period. Conclusions. This is a relevant validation of GDR2 photometry of asteroids, and proof of the satisfactory performances of the adopted inversion algorithm.Peer reviewe
A low density of 0.8 g/cc for the Trojan binary asteroid 617 Patroclus
The Trojan population consists of two swarms of asteroids following the same
orbit as Jupiter and located at the L4 and L5 Lagrange points of the
Jupiter-Sun system (leading and following Jupiter by 60 degrees). The asteroid
617 Patroclus is the only known binary Trojan (Merline et al. 2001). The orbit
of this double system was hitherto unknown. Here we report that the components,
separated by 680 km, move around the system centre of mass, describing roughly
a circular orbit. Using the orbital parameters, combined with thermal
measurements to estimate the size of the components, we derive a very low
density of 0.8 g/cc. The components of Patroclus are therefore very porous or
composed mostly of water ice, suggesting that they could have been formed in
the outer part of the solar system.Comment: 10 pages, 3 figures, 1 tabl
Imaging the dynamical atmosphere of the red supergiant Betelgeuse in the CO first overtone lines with VLTI/AMBER
We present the first 1-D aperture synthesis imaging of the red supergiant
Betelgeuse in the individual CO first overtone lines with VLTI/AMBER. The
reconstructed 1-D projection images reveal that the star appears differently in
the blue wing, line center, and red wing of the individual CO lines. The 1-D
projection images in the blue wing and line center show a pronounced,
asymmetrically extended component up to ~1.3 stellar radii, while those in the
red wing do not show such a component. The observed 1-D projection images in
the lines can be reasonably explained by a model in which the CO gas within a
region more than half as large as the stellar size is moving slightly outward
with 0--5 km s^-1, while the gas in the remaining region is infalling fast with
20--30 km s^-1. A comparison between the CO line AMBER data taken in 2008 and
2009 shows a significant time variation in the dynamics of the CO line-forming
region in the photosphere and the outer atmosphere. In contrast to the line
data, the reconstructed 1-D projection images in the continuum show only a
slight deviation from a uniform disk or limb-darkened disk. We derive a
uniform-disk diameter of 42.05 +/- 0.05 mas and a power-law-type limb-darkened
disk diameter of 42.49 +/- 0.06 mas and a limb-darkening parameter of (9.7 +/-
0.5) x 10^{-2}. This latter angular diameter leads to an effective temperature
of 3690 +/- 54 K for the continuum-forming layer. These diameters confirm that
the near-IR size of Betelgeuse was nearly constant over the last 18 years, in
marked contrast to the recently reported noticeable decrease in the mid-IR
size. The continuum data taken in 2008 and 2009 reveal no or only marginal time
variations, much smaller than the maximum variation predicted by the current
3-D convection simulations.Comment: 21 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
Characterization of the near-Earth Asteroid 2002NY40
In August 2002, the near-Earth asteroid 2002 NY40, made its closest approach
to the Earth. This provided an opportunity to study a near-Earth asteroid with
a variety of instruments. Several of the telescopes at the Maui Space
Surveillance System were trained at the asteroid and collected adaptive optics
images, photometry and spectroscopy. Analysis of the imagery reveals the
asteroid is triangular shaped with significant self-shadowing. The photometry
reveals a 20-hour period and the spectroscopy shows that the asteroid is a
Q-type
Toward Direct Detection of Hot Jupiters with Precision Closure Phase: Calibration Studies and First Results from the CHARA Array
Direct detection of thermal emission from nearby hot Jupiters has greatly
advanced our knowledge of extrasolar planets in recent years. Since hot Jupiter
systems can be regarded as analogs of high contrast binaries, ground-based
infrared long baseline interferometers have the potential to resolve them and
detect their thermal emission with precision closure phase - a method that is
immune to the systematic errors induced by the Earth's atmosphere. In this
work, we present closure phase studies toward direct detection of nearby hot
Jupiters using the CHARA interferometer array outfitted with the MIRC
instrument. We carry out closure phase simulations and conduct a large number
of observations for the best candidate {\upsion} And. Our experiments suggest
the method is feasible with highly stable and precise closure phases. However,
we also find much larger systematic errors than expected in the observations,
most likely caused by dispersion across different wavelengths. We find that
using higher spectral resolution modes (e.g., R=150) can significantly reduce
the systematics. By combining all calibrators in an observing run together, we
are able to roughly recalibrate the lower spectral resolution data, allowing us
to obtain upper limits of the star-planet contrast ratios of {\upsion} And b
across the H band. The data also allow us to get a refined stellar radius of
1.625\pm0.011 R\odot. Our best upper limit corresponds to a contrast ratio of
2.1\times10^3:1 with 90% confidence level at 1.52{\mu}m, suggesting that we are
starting to have the capability of constraining atmospheric models of hot
Jupiters with interferometry. With recent and upcoming improvements of
CHARA/MIRC, the prospect of detecting emission from hot Jupiters with closure
phases is promising.Comment: 30 pages, including 9 figures and 4 tables. Published in PASP in
August 201
The UT 7/8 February 2013 Sila-Nunam Mutual Event and Future Predictions
A superior mutual event of the Kuiper Belt binary system (79360) Sila-Nunam was observed over 15.47 h on UT 7/8 February 2013 by a coordinated effort at four different telescope facilities; it started approximately 1.5 h earlier than anticipated, the duration was approximately 9.5 h (about 10% longer than predicted), and was slightly less deep than predicted. It is the first full event observed for a comparably sized binary Kuiper Belt object. We provide predictions for future events refined by this and other partial mutual event observations obtained since the mutual event season began
Polarity in GaN and ZnO: Theory, measurement, growth, and devices
This article may be downloaded for personal use only. Any other use requires prior permission of the author and AIP Publishing. This article appeared in Appl. Phys. Rev. 3, 041303 (2016) and may be found at https://doi.org/10.1063/1.4963919.The polar nature of the wurtzite crystalline structure of GaN and ZnO results in the existence of a spontaneous electric polarization within these materials and their associated alloys (Ga,Al,In)N and (Zn,Mg,Cd)O. The polarity has also important consequences on the stability of the different crystallographic surfaces, and this becomes especially important when considering epitaxial growth. Furthermore, the internal polarization fields may adversely affect the properties of optoelectronic devices but is also used as a potential advantage for advanced electronic devices. In this article, polarity-related issues in GaN and ZnO are reviewed, going from theoretical considerations to electronic and optoelectronic devices, through thin film, and nanostructure growth. The necessary theoretical background is first introduced and the stability of the cation and anion polarity surfaces is discussed. For assessing the polarity, one has to make use of specific characterization methods, which are described in detail. Subsequently, the nucleation and growth mechanisms of thin films and nanostructures, including nanowires, are presented, reviewing the specific growth conditions that allow controlling the polarity of such objects. Eventually, the demonstrated and/or expected effects of polarity on the properties and performances of optoelectronic and electronic devices are reported. The present review is intended to yield an in-depth view of some of the hot topics related to polarity in GaN and ZnO, a fast growing subject over the last decade
Asteroids. From Observations to Models
We will discuss some specific applications to the rotation state and the
shapes of moderately large asteroids, and techniques of observations putting
some emphasis on the HST/FGS instrument.Comment: to appear in LNP; 28pages; written in 2003; Winter School "Dynamique
des Corps Celestes Non Ponctuels et des Anneaux", Lanslevillard (FRANCE
An Automated Algorithm to Distinguish and Characterize Solar Flares and Associated Sequential Chromospheric Brightenings
We present a new automated algorithm to identify, track, and characterize
small-scale brightening associated with solar eruptive phenomena observed in
H{\alpha}. The temporal spatially-localized changes in chromospheric
intensities can be separated into two categories: flare ribbons and sequential
chromospheric brightenings (SCBs). Within each category of brightening we
determine the smallest resolvable locus of pixels, a kernel, and track the
temporal evolution of the position and intensity of each kernel. This tracking
is accomplished by isolating the eruptive features, identifying kernels, and
linking detections between frames into trajectories of kernels. We fully
characterize the evolving intensity and morphology of the flare ribbons by
observing the tracked flare kernels in aggregate. With the location of SCB and
flare kernels identified, they can easily be overlaid on top of complementary
data sets to extract Doppler velocities and magnetic field intensities
underlying the kernels. This algorithm is adaptable to any dataset to identify
and track solar features.Comment: 22 pages, 9 figure
The astrometric Gaia-FUN-SSO observation campaign of 99 942 Apophis
Astrometric observations performed by the Gaia Follow-Up Network for Solar
System Objects (Gaia-FUN-SSO) play a key role in ensuring that moving objects
first detected by ESA's Gaia mission remain recoverable after their discovery.
An observation campaign on the potentially hazardous asteroid (99 942) Apophis
was conducted during the asteroid's latest period of visibility, from
12/21/2012 to 5/2/2013, to test the coordination and evaluate the overall
performance of the Gaia-FUN-SSO . The 2732 high quality astrometric
observations acquired during the Gaia-FUN-SSO campaign were reduced with the
Platform for Reduction of Astronomical Images Automatically (PRAIA), using the
USNO CCD Astrograph Catalogue 4 (UCAC4) as a reference. The astrometric
reduction process and the precision of the newly obtained measurements are
discussed. We compare the residuals of astrometric observations that we
obtained using this reduction process to data sets that were individually
reduced by observers and accepted by the Minor Planet Center. We obtained 2103
previously unpublished astrometric positions and provide these to the
scientific community. Using these data we show that our reduction of this
astrometric campaign with a reliable stellar catalog substantially improves the
quality of the astrometric results. We present evidence that the new data will
help to reduce the orbit uncertainty of Apophis during its close approach in
2029. We show that uncertainties due to geolocations of observing stations, as
well as rounding of astrometric data can introduce an unnecessary degradation
in the quality of the resulting astrometric positions. Finally, we discuss the
impact of our campaign reduction on the recovery process of newly discovered
asteroids.Comment: Accepted for publication in A&
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