scholarly journals Gaia: Surveying Heavens

2018 ◽  
Vol 17 (1) ◽  
pp. 11-23
Author(s):  
Nidhi Sabu ◽  
Thomaskutty Francis ◽  
Arun Roy ◽  
Sreeja S Kartha
Keyword(s):  
Preliminary Analysis ◽  
European Space Agency ◽  
Three Dimensional ◽  
Be Stars ◽  
Distance Measurements ◽  
Gaia Dr1 ◽  
Space Agency ◽  
Data Release

In this paper we attempt to study an ongoing astrometry mission of the European Space Agency (ESA), named Gaia, whose aim is to make the largest and most precise three-dimensional map of our Galaxy. We present the scientific goals of Gaia and give a brief description of the spacecraft. We also present a preliminary analysis of comparing distance estimates of Be stars from the first Gaia data release, Gaia DR1, and Hipparcos mission. From our analysis, we confirm that Gaia stands out as a promising mission in terms of the distance measurements when compared to Hipparcos, particularly for distances greater than 1 kpc.

scholarly journals Gaia Data Release 2

2018 ◽  
Vol 616 ◽  
pp. A2 ◽  
Author(s):  
L. Lindegren ◽  
J. Hernández ◽  
A. Bombrun ◽  
S. Klioner ◽  
U. Bastian ◽  
...  
Keyword(s):  
Proper Motion ◽  
Input Data ◽  
European Space Agency ◽  
Iterative Solution ◽  
Spatial Correlations ◽  
Satellite Attitude ◽  
Space Agency ◽  
Data Release ◽  
Ccd Observations ◽  
Celestial Reference System

Context. Gaia Data Release 2 (Gaia DR2) contains results for 1693 million sources in the magnitude range 3 to 21 based on observations collected by the European Space Agency Gaia satellite during the first 22 months of its operational phase. Aims. We describe the input data, models, and processing used for the astrometric content of Gaia DR2, and the validation of these resultsperformed within the astrometry task. Methods. Some 320 billion centroid positions from the pre-processed astrometric CCD observations were used to estimate the five astrometric parameters (positions, parallaxes, and proper motions) for 1332 million sources, and approximate positions at the reference epoch J2015.5 for an additional 361 million mostly faint sources. These data were calculated in two steps. First, the satellite attitude and the astrometric calibration parameters of the CCDs were obtained in an astrometric global iterative solution for 16 million selected sources, using about 1% of the input data. This primary solution was tied to the extragalactic International Celestial Reference System (ICRS) by means of quasars. The resulting attitude and calibration were then used to calculate the astrometric parameters of all the sources. Special validation solutions were used to characterise the random and systematic errors in parallax and proper motion. Results. For the sources with five-parameter astrometric solutions, the median uncertainty in parallax and position at the reference epoch J2015.5 is about 0.04 mas for bright (G < 14 mag) sources, 0.1 mas at G = 17 mag, and 0.7 masat G = 20 mag. In the proper motion components the corresponding uncertainties are 0.05, 0.2, and 1.2 mas yr−1, respectively.The optical reference frame defined by Gaia DR2 is aligned with ICRS and is non-rotating with respect to the quasars to within 0.15 mas yr−1. From the quasars and validation solutions we estimate that systematics in the parallaxes depending on position, magnitude, and colour are generally below 0.1 mas, but the parallaxes are on the whole too small by about 0.03 mas. Significant spatial correlations of up to 0.04 mas in parallax and 0.07 mas yr−1 in proper motion are seen on small (< 1 deg) and intermediate (20 deg) angular scales. Important statistics and information for the users of the Gaia DR2 astrometry are given in the appendices.

scholarly journals Joint Discussion 6: Sun and Heliosphere - Challenges for Solar-Terrestrial Physics, Magneto-and Hydrodynamics

1995 ◽  
Vol 10 ◽  
pp. 291-293
Author(s):  
Martin C.E. Huber ◽  
Arne Pedersen ◽  
Claus Fröhlich
Keyword(s):  
Spatial Scales ◽  
Plasma Heating ◽  
European Space Agency ◽  
Three Dimensional ◽  
Plasma Boundary ◽  
Small Scale ◽  
Space Agency ◽  
Terrestrial Magnetosphere ◽  
Surrounding Space

There is one astrophysical system, where the sites of a star’s mass loss can be localised and observed in detail, and where the behaviour of the resulting stellar wind in the star’s environment and around orbiting obstacles can be investigated in situ: it is the Sun, the heliosphere and the surroundings of planets — among the latter most prominently the terrestrial magnetosphere. Indeed, within a year or so a fleet of satellites equipped with sophisticated remote-sensing and in-situ instruments will make this astronomical paradigm, or more precisely, the solar-terrestrial system accessible to intensive, multi-disciplinary study.Four identical CLUSTER spacecraft, orbiting the Earth within the magnetosphere, the surrounding space and the particularly interesting plasma boundary layers will perform a three-dimensional in-situ study of plasma-heating, particle-acceleration and other small-scale plasma processes (Schmidt and Goldstein,1988). A number of other missions — some of them already in orbit, like GEOTAIL and WIND, some to be launched within one or two years, like INTERBALL and POLAR — will provide information about the Earth’s magnetosphere and the solar wind on larger spatial scales. These missions are described in a Brochure issued jointly by the European Space Agency, NASA, the Japanese Institute of Space and Astronomical Science and the Rssian Space Agency, which can be obtained from A. Pedersen at the above address.

scholarly journals Gaia Data Release 2

2018 ◽  
Vol 616 ◽  
pp. A16 ◽  
Author(s):  
A. C. Lanzafame ◽  
E. Distefano ◽  
S. Messina ◽  
I. Pagano ◽  
A. F. Lanza ◽  
...  
Keyword(s):  
Time Series ◽  
European Space Agency ◽  
Magnetic Activity ◽  
Variable Stars ◽  
Stellar Rotation ◽  
Dynamo Action ◽  
Space Agency ◽  
Data Release ◽  
Flux Modulation ◽  
Two Populations

Context. Amongst the ≈5 × 105 sources identified as variable stars in Gaia Data Release 2 (DR2), 26% are rotational modulation variable candidates of the BY Dra class. Gaia DR2 provides their multi-band (G, GBP, and GRP) photometric time series collected by the European Space Agency spacecraft Gaia during the first 22 months of operations as well as the essential parameters related to their flux modulation induced by surface inhomogeneities and rotation. Aims. We developed methods to identify the BY Dra variable candidates and to infer their variability parameters. Methods. BY Dra candidates were pre-selected from their position in the Hertzsprung–Russel diagram, built from Gaia parallaxes, G magnitudes, and (GBP − GRP) colours. Since the time evolution of the stellar active region can disrupt the coherence of the signal, segments not much longer than their expected evolution timescale were extracted from the entire photometric time series, and period search algorithms were applied to each segment. For the Gaia DR2, we selected sources with similar periods in at least two segments as candidate BY Dra variables. Results were further filtered considering the time-series phase coverage and the expected approximate light-curve shape. Results. Gaia DR2 includes rotational periods and modulation amplitudes of 147 535 BY Dra candidates. The data unveil the existence of two populations with distinctive period and amplitude distributions. The sample covers 38% of the whole sky when divided into bins (HEALPix) of ≈0.84 square degrees, and we estimate that this represents 0.7–5% of all BY Dra stars potentially detectable with Gaia. Conclusions. The preliminary data contained in Gaia DR2 illustrate the vast and unique information that the mission is going to provide on stellar rotation and magnetic activity. This information, complemented by the exquisite Gaia parallaxes, proper motions, and astrophysical parameters, is opening new and unique perspectives for our understanding of the evolution of stellar angular momentum and dynamo action.

scholarly journals Gaia Data Release 2

2018 ◽  
Vol 616 ◽  
pp. A13 ◽  
Author(s):  
◽  
F. Spoto ◽  
P. Tanga ◽  
F. Mignard ◽  
J. Berthier ◽  
...  
Keyword(s):  
Solar System ◽  
A Priori ◽  
European Space Agency ◽  
Dramatic Improvement ◽  
Individual Values ◽  
Data Set ◽  
Position Information ◽  
Additional Information ◽  
Space Agency ◽  
Data Release

Context. The Gaia spacecraft of the European Space Agency (ESA) has been securing observations of solar system objects (SSOs) since the beginning of its operations. Data Release 2 (DR2) contains the observations of a selected sample of 14,099 SSOs. These asteroids have been already identified and have been numbered by the Minor Planet Center repository. Positions are provided for each Gaia observation at CCD level. As additional information, complementary to astrometry, the apparent brightness of SSOs in the unfiltered G band is also provided for selected observations. Aims. We explain the processing of SSO data, and describe the criteria we used to select the sample published in Gaia DR2. We then explore the data set to assess its quality. Methods. To exploit the main data product for the solar system in Gaia DR2, which is the epoch astrometry of asteroids, it is necessary to take into account the unusual properties of the uncertainty, as the position information is nearly one-dimensional. When this aspect is handled appropriately, an orbit fit can be obtained with post-fit residuals that are overall consistent with the a-priori error model that was used to define individual values of the astrometric uncertainty. The role of both random and systematic errors is described. The distribution of residuals allowed us to identify possible contaminants in the data set (such as stars). Photometry in the G band was compared to computed values from reference asteroid shapes and to the flux registered at the corresponding epochs by the red and blue photometers (RP and BP). Results. The overall astrometric performance is close to the expectations, with an optimal range of brightness G ~ 12 − 17. In this range, the typical transit-level accuracy is well below 1 mas. For fainter asteroids, the growing photon noise deteriorates the performance. Asteroids brighter than G ~ 12 are affected by a lower performance of the processing of their signals. The dramatic improvement brought by Gaia DR2 astrometry of SSOs is demonstrated by comparisons to the archive data and by preliminary tests on the detection of subtle non-gravitational effects.

scholarly journals Cross Sensor Simulation of Tomographic SAR Stacks

2019 ◽  
Vol 11 (18) ◽  
pp. 2099 ◽  
Author(s):  
Mauro Mariotti d’Alessandro ◽  
Stefano Tebaldini
Keyword(s):  
European Space Agency ◽  
Three Dimensional ◽  
Original Data ◽  
Central Frequency ◽  
Tropical Regions ◽  
Space Agency ◽  
Simulated System ◽  
Good Proxy ◽  
The Relationship ◽  
Resolution Cell

This paper presents an algorithm for simulating tomographic synthetic aperture radar (SAR) data based on another stack actually gathered by a real acquisition system. Through the procedure here proposed, the simulated system can be evaluated according to its capability to image complex natural media rather than reference point targets. This feature is particularly important whenever the biophysical properties of the target of interest must be preserved and cannot be easily modeled. The system to be simulated may be different from the original one concerning resolution, off-nadir angles, bandwidth and central frequency. The algorithm here proposed handles these differences by properly taking into account the wavenumbers of the target illuminated by the real survey and requested by the simulated one. The complex images constituting the synthetic stack are associated with the effective vertical interferometric wavenumber peculiar of the geometry to be simulated, regardless of the original data. Furthermore, the three-dimensional resolution cell of the simulated tomographic system is consistent with the simulated geometry concerning size and spatial orientation. These two latter features cannot be guaranteed by simply filtering the original stack. The simulator here proposed has been used to simulate the tomographic stack expected from the forthcoming European Space Agency (ESA) BIOMASS mission. The relationship between baseline distribution and 3D focusing capability was explored; special attention has been paid to the robustness of tomographic power at being a good proxy for the above ground biomass in tropical regions.

III. Metallurgy: The proposed French metallurgy programme for Spacelab

1978 ◽  
Vol 361 (1705) ◽  
pp. 165-174 ◽  
Keyword(s):  
Crystal Growth ◽  
Preliminary Analysis ◽  
European Space Agency ◽  
Government Agencies ◽  
Zero Gravity ◽  
Industrial Community ◽  
Space Agency ◽  
Space Experiments ◽  
The Government ◽  
High Level

The call for ideas for the first Spacelab payload issued from the European Space Agency (E.S.A.) obtained in France a fairly high level of success from the government agencies but almost no answers from the industrial community. This situation, which arose despite knowledge of some early but very promising results of the first space experiments, seems to be correlated with the absence of any guarantee about the future Law of Space, and consequently has orientated the French metallurgy experiments to more academic and less applied speculations. We shall describe the actions of the French Space Agency (C.N.E.S.) to decide which experiments would be officially supported by C.N.E.S. if accepted by E.S.A. In the particular field of metallurgy, six proposals were so selected which have been proposed to E.S.A. for the first Spacelab payload. We shall present the arguments which defended those proposals and as a function of the preliminary analysis of some similar U.S. experiments we shall try to detect some of the possible difficulties in performing them and to foresee the main results which we expect. Special emphasis will be laid on the thermodiffusion and nucleation experiments, the study of which is basic for many crystal growth or metallurgy experiments in a zero-gravity environment.

scholarly journals Revised Gaia Data Release 2 passbands

2018 ◽  
Vol 617 ◽  
pp. A138 ◽  
Author(s):  
Michael Weiler
Keyword(s):  
European Space Agency ◽  
Photometric Data ◽  
Spectral Library ◽  
Response Curves ◽  
Data Set ◽  
Space Agency ◽  
Precise Knowledge ◽  
Data Release ◽  
Photometric Measurements ◽  
Present Response

Context. The European Space Agency mission Gaia has published, with its second data release (DR2), a catalogue of photometric measurements for more than 1.3 billion astronomical objects in three passbands. The precision of the measurements in these passbands, denoted G, GBP, and GRP, reach down to the milli-magnitude level. The scientific exploitation of this data set requires precise knowledge of the response curves of the three passbands. Aims. This work aims to improve the exploitation of the photometric data by deriving an improved set of response curves for the three passbands, allowing for an accurate computation of synthetic Gaia photometry. Methods. This is achieved by formulating the problem of passband determination in a functional analytic formalism, and linking the photometric measurements with four observational, one empirical, and one theoretical spectral library. Results. We present response curves for G, GBP, and GRP that differ from the previously published curves, and which provide a better agreement between synthetic Gaia photometry and Gaia observations.

Molecular Dynamic Simulation of Thermal Confinement of Particles

2008 ◽  
Author(s):  
Maurizio Bottoni ◽  
Simone Mantovani
Keyword(s):  
Rarefied Gas ◽  
Particle System ◽  
European Space Agency ◽  
Three Dimensional ◽  
Direct Simulation Monte Carlo ◽  
Physical Parameters ◽  
Atmospheric Particle ◽  
Space Agency ◽  
Three Dimensional Representation ◽  
Thermal Confinement

The European Space Agency (ESA) in an “Invitation to Tender”, dated August 15, 2006, illustrated a feasibility study of the Interaction in Cosmic and Atmospheric Particle System (ICAPS) experiment. The experiment consists essentially of thermal elements, contained in a cylindrical box, the center of which is a trapping area for thermally confined particles floating in a rarefied gas within the chamber. Thermophoretic forces induced by thermal gradients concentrate particles carried from the gas into the trapping area. Particles are eventually collected from this area. The physical dimensions of the experiment and of its components, temperature gradients and locations the thermal elements within the chamber are free parameters that should be experimentally and numerically investigated to enhance the efficiency of the experiment in collecting particles in the trapping area. To enable numerical investigations of the proposed device a two-dimensional computer program called THERCONF-2D (THERmal CONFinement in 2D) has been build up as a first step towards a full three-dimensional representation of the experiment. This code version describes the displacement due to thermophoresis of tiny particles in a gas-filled domain subjected to a temperature gradient. The behavior of the gas molecules and their interaction with material surfaces and with the particles floating in the gas are modeled with the statistical methods of molecular dynamics, based on the “Direct Simulation Monte Carlo” (DSMC). The computational code has extensive post-processing capabilities for visualization of computational results. After verification of the code, current work is aiming at identifying the optimum combination of physical parameters allowing for the best efficiency of the thermal confinement, minimizing the loss of particles escaping from the collecting area. A variant of the code is being installed on parallel processors. The article presents the state-of-the-art of this computational endeavor.

scholarly journals A fresh look at the RR Lyrae population in the Draco dwarf spheroidal galaxy with Gaia

2020 ◽  
Vol 499 (3) ◽  
pp. 4040-4053
Author(s):  
Tatiana Muraveva ◽  
Gisella Clementini ◽  
Alessia Garofalo ◽  
Felice Cusano
Keyword(s):  
Average Distance ◽  
European Space Agency ◽  
Distance Modulus ◽  
Rr Lyrae Stars ◽  
Rr Lyrae ◽  
Space Agency ◽  
Combining Data ◽  
Data Release ◽  
Bona Fide ◽  
First Time

ABSTRACT We present a catalogue of 285 RR Lyrae stars (RRLs) in the Draco dwarf spheroidal (dSph) galaxy, obtained by combining data from a number of different surveys including the Data Release 2 (DR2) of the European Space Agency cornerstone mission Gaia. We have determined individual distances to the RRLs in our sample using for the first time a Gaia G-band luminosity–metallicity relation ($M_{G}\small{--}{\rm [Fe/H]}$) and study the structure of the Draco dSph as traced by its RRL population. We find that the RRLs located in the western/south-western region of Draco appear to be closer to us, which may be a clue of interaction between Draco and the Milky Way. The average distance modulus of Draco measured with the RRLs is $\mu =19.53\pm 0.07$ mag, corresponding to a distance of $80.5\pm 2.6$ kpc, in good agreement with previous determinations in the literature. Based on the pulsation properties of the RRLs, we confirm the Oosterhoff-intermediate nature of Draco. We present an additional sample of 41 candidate RRLs in Draco, which we selected from the Gaia DR2 catalogue based on the uncertainty of their G-band magnitudes. Additional epoch data that will become available in the Gaia Data Release 3 will help to confirm whether these candidates are bona fide Draco RRLs.

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