1,553 research outputs found
Proper Motion Study of the Magellanic Clouds using SPM material
Absolute proper motions are determined for stars and galaxies to V=17.5 over
a 450 square-degree area that encloses both Magellanic Clouds. The proper
motions are based on photographic and CCD observations of the Yale/San Juan
Southern Proper Motion program, which span over a baseline of 40 years.
Multiple, local relative proper motion measures are combined in an overlap
solution using photometrically selected Galactic Disk stars to define a global
relative system that is then transformed to absolute using external galaxies
and Hipparcos stars to tie into the ICRS. The resulting catalog of 1.4 million
objects is used to derive the mean absolute proper motions of the Large
Magellanic Cloud and the Small Magellanic Cloud;
(\mu_\alpha\cos\delta,\mu_\delta)_{LMC}=(1.89,+0.39)\pm (0.27,0.27)\;\;\{mas
yr}^{-1} and (\mu_\alpha\cos\delta,\mu_\delta)_{SMC}=(0.98,-1.01)\pm
(0.30,0.29)\;\;\{mas yr}^{-1}. These mean motions are based on best-measured
samples of 3822 LMC stars and 964 SMC stars. A dominant portion (0.25 mas
yr) of the formal errors is due to the estimated uncertainty in the
inertial system of the Hipparcos Catalog stars used to anchor the bright end of
our proper motion measures. A more precise determination can be made for the
proper motion of the SMC {\it relative} to the LMC;
(\mu_{\alpha\cos\delta},\mu_\delta)_{SMC-LMC} = (-0.91,-1.49) \pm
(0.16,0.15)\;\;\{mas yr}^{-1}. This differential value is combined with
measurements of the proper motion of the LMC taken from the literature to
produce new absolute proper-motion determinations for the SMC, as well as an
estimate of the total velocity difference of the two clouds to within 54
kms.Comment: 50 pages (referee format), 13 figures. Accepted for publication in A
Testing Theories of Gravity with Planetary Ephemerides
We describe here how planetary ephemerides are built in the framework of
General Relativity and how they can be used to test alternative theories. We
focus on the definition of the reference frame (space and time) in which the
planetary ephemeris is described, the equations of motion that govern the
orbits of solar system bodies and {electromagnetic waves}. After a review on
the existing planetary and lunar ephemerides, we summarize the results obtained
considering full modifications of the ephemeris framework with direct
comparisons with the observations of planetary systems, with a specific
attention for the PPN formalism. We then discuss other formalisms such as
Einstein-dilaton theories, the massless graviton and MOND. The paper finally
concludes on some comments and recommendations regarding misinterpreted
measurements of the advance of perihelia.Comment: Submitted at Living Review of Relativit
The Astrometric Foundation of Astrophysics
Astrophysical studies require a knowledge of very accurate positions, motions
and distances of stars. A brief overview is given of the significance and
development of astrometry by ESA's two astrometric satellites, Hipparcos and
Gaia, launched in respectively 1989 and 2013. The astrometric foundation of all
branches of astronomy from the solar system and stellar systems to compact
galaxies, quasars and dark matter is being revolutionized by the observations
from these satellites. The future of fundamental astrometry must be considered
in a time frame of 50 years, therefore science issues for a Gaia successor
mission in twenty years are discussed in an extensive report: "Absolute
astrometry in the next 50 years" available at http://arxiv.org/abs/1408.2190Comment: 2 pages, 2 figures, Abstract to the Conference Book 2014 of the
Danish Astronomical Society and abstract of a revie
Absolute astrometry in the next 50 years
With Gaia in orbit since December 2013 it is time to look at the future of
fundamental astrometry and a time frame of 50 years is needed in this matter. A
space mission with Gaia-like astrometric performance is required, but not
necessarily a Gaia-like satellite. It should be studied whether this can be
obtained within the budget of a medium-size ESA mission. A dozen science issues
for a Gaia successor mission in twenty years, with launch about 2033, are
presented and in this context also other possibilities for absolute astrometry
with milliarcsecond (mas) or sub-mas accuracies are discussed. The three
powerful techniques: VLBI, the MICADO camera on the E-ELT, and the LSST are
described and documented by literature references and by an extensive
correspondence with leading astronomers who readily responded with all the
information I needed. In brief, the two Gaia-like missions would provide an
astrometric foundation for all branches of astronomy from the solar system and
stellar systems, including exo-planet systems, to compact galaxies, quasars and
dark matter (DM) substructures by data which cannot be surpassed in the next 50
years. - In April 2017 ESA selected our proposal Hobbs et al. (2016) for study
of a detector with NIR sensitivity for a Gaia successor mission, called
GaiaNIR.Comment: 36 pages, 7 figures. Updates up to 12 June 2017 are included. Author
- [email protected]
The Atacama Cosmology Telescope: Physical Properties of Sunyaev-Zel'dovich Effect Clusters on the Celestial Equator
We present the optical and X-ray properties of 68 galaxy clusters selected
via the Sunyaev-Zel'dovich Effect at 148 GHz by the Atacama Cosmology Telescope
(ACT). Our sample, from an area of 504 square degrees centered on the celestial
equator, is divided into two regions. The main region uses 270 square degrees
of the ACT survey that overlaps with the co-added ugriz imaging from the Sloan
Digital Sky Survey (SDSS) over Stripe 82 plus additional near-infrared pointed
observations with the Apache Point Observatory 3.5-meter telescope. We confirm
a total of 49 clusters to z~1.3, of which 22 (all at z>0.55) are new
discoveries. For the second region the regular-depth SDSS imaging allows us to
confirm 19 more clusters up to z~0.7, of which 10 systems are new. We present
the optical richness, photometric redshifts, and separation between the SZ
position and the brightest cluster galaxy (BCG). We find no significant offset
between the cluster SZ centroid and BCG location and a weak correlation between
optical richness and SZ-derived mass. We also present X-ray fluxes and
luminosities from the ROSAT All Sky Survey which confirm that this is a massive
sample. One of the newly discovered clusters, ACT-CL J0044.4+0113 at z=1.1
(photometric), has an integrated XMM-Newton X-ray temperature of kT_x=7.9+/-1.0
keV and combined mass of M_200a=8.2(-2.5,+3.3)x10^14 M_sun/h70 placing it among
the most massive and X-ray-hot clusters known at redshifts beyond z=1. We also
highlight the optically-rich cluster ACT-CL J2327.4-0204 (RCS2 2327) at z=0.705
(spectroscopic) as the most significant detection of the whole equatorial
sample with a Chandra-derived mass of M_200a=1.9(-0.4,+0.6)x10^15 M_sun/h70,
comparable to some of the most massive known clusters like "El Gordo" and the
Bullet Cluster.Comment: 18 pages, 12 figures. Accepted to the Astrophysical Journal. New
version includes minor changes in the accepted pape
Mariner Mars 1971 optical navigation demonstration
The feasibility of using a combination of spacecraft-based optical data and earth-based Doppler data to perform near-real-time approach navigation was demonstrated by the Mariner Mars 71 Project. The important findings, conclusions, and recommendations are documented. A summary along with publications and papers giving additional details on the objectives of the demonstration are provided. Instrument calibration and performance as well as navigation and science results are reported
The Pioneer Anomaly
Radio-metric Doppler tracking data received from the Pioneer 10 and 11
spacecraft from heliocentric distances of 20-70 AU has consistently indicated
the presence of a small, anomalous, blue-shifted frequency drift uniformly
changing with a rate of ~6 x 10^{-9} Hz/s. Ultimately, the drift was
interpreted as a constant sunward deceleration of each particular spacecraft at
the level of a_P = (8.74 +/- 1.33) x 10^{-10} m/s^2. This apparent violation of
the Newton's gravitational inverse-square law has become known as the Pioneer
anomaly; the nature of this anomaly remains unexplained. In this review, we
summarize the current knowledge of the physical properties of the anomaly and
the conditions that led to its detection and characterization. We review
various mechanisms proposed to explain the anomaly and discuss the current
state of efforts to determine its nature. A comprehensive new investigation of
the anomalous behavior of the two Pioneers has begun recently. The new efforts
rely on the much-extended set of radio-metric Doppler data for both spacecraft
in conjunction with the newly available complete record of their telemetry
files and a large archive of original project documentation. As the new study
is yet to report its findings, this review provides the necessary background
for the new results to appear in the near future. In particular, we provide a
significant amount of information on the design, operations and behavior of the
two Pioneers during their entire missions, including descriptions of various
data formats and techniques used for their navigation and radio-science data
analysis. As most of this information was recovered relatively recently, it was
not used in the previous studies of the Pioneer anomaly, but it is critical for
the new investigation.Comment: 165 pages, 40 figures, 16 tables; accepted for publication in Living
Reviews in Relativit
The Gaia Mission and the Asteroids. A perspective from space astrometry and photometry for asteroids studies and science.
90 pagesThe Gaia space mission to be operated in early 2012 by the European Space Agency (ESA), will make a huge step in our knowledge of the Sun's neighbor-hood, up to the Magellanic clouds. Somewhat closer, Gaia will also provide ma jor improvements in the science of asteroids, and more generally to our Solar System, either directly or indirectly. Gaia is a scanning survey telescope aimed to perform high accuracy astrometry and photometry. More specifically it will provide physical and dynamical characterization of asteroids, a better knowledge of the solar system composition, formation and evolution, local test of the general relativity, and linking the dynamical reference frame to the kinematical ICRS. We develop here the general aspects of asteroid observations and the scientific harvest in perspective of what was achieved in the pre-Gaia era. In this lecture we focus on the determination of size of asteroids, shape and rotation, taxonomy, orbits and their improvements with historical highlight, and also the dynamical model in general
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