Sistemas de referencia fundamentales

Since 1987 the International Earth Rotation Service elaborates a celestial reference frame based on VLBI positions of extragalactic radio sources precise at the miliarcsecond level. The XXI IAU General Assembly recommended to adopt the extragalactic celestial reference frame as the definition of the primary celestial system, replacing the FK primary frame.Asociación Argentina de Astronomí

The celestial reference system of the international earth rotation service (IERS)

The celestial system maintained by the International Earth Rotation Service is described in terms of physical properties of the fiducial objects, international consistency of the farm and agreement with the FK5 and dynamical systems. The celestial reference frame of IERS is based on compact extra-galactic objects observed by Very Long Baseline Interferometry (VLBI). It is maintained on the basis of several independent Earth orientation programs analysed by various analysis centres. We present hereafter its latest realization, which is a combination of individual frames obtained by four groups: the Goddard Space Flight Centre (GSFC, Ma et al., 1990), the Jet Propulsion Laboratory (JPL, Steppe et al., 1990) the National Geodetic Survey (NGC; Carter and Robertson, 1990) and the U.S. Naval Observatory (USNO, Eubanks et al., 1990). The combination model is based on a three rotation angle model applied to a selection of radio sources common to the individual frames. The initial definition of the system and the maintenance process are described by Arias and Feissel (1990); the connenction to the conventional terrestrial system of IERS at the level of 0.001" is studied by Feissel (1990). The latest realization (IERS; 1990) of the IERS celestial reference system is materialized by the J2000.0 positions of 228 extra-galactic radio sources between +85 deg and -80 deg in declination. It is well known that the use of the conventional IAU 1976 Precession and IAU 1980 Theory of Nutation in the analysis of VLBI observations would give rise to systematic errors in the source positions, and to misorientation of the axes of the frames, both at the level of a few miliarcseconds. Therefore the common practice in catalogue work is to estimate additional parameters which describe the motion of the celestial pole relative to its conventional position. In the combination performed by the Bureau Central of the IERS, only individual frames obtained by this procedure are used. The realization of the celestial reference system published in the Annual Report of IERS for 1989 contains 228 sources with different status: primary, secondary and complementary. The 51 primary sources were chosen on the basis of consistency of their estimated coordinates in the four individual frames, after removing the relative rotations: only sources which showed position differences under 0.0015" in all comparisons two by two were retained as primary. Their position uncertainties in the IERS frame, derived from this consistency, are smaller than 0.0007". The other sources common to at least two frames but with larger position discrepancies, are considered secondary; there are 40 of them in the realization described here. Finally, 137 complementary sources in the IERS frame were available from only one individual catalogue. Altogether 113 sources have a position uncertainty smaller than 0.001", 104 between 0.001" and 0.003" and 11 over 0.003". The IERS celestial reference system is barycentryc through the appropriate modelling of observations by the analysis centres which contribute individual catalogs. The Ox axis was implicitly defined in the initial realization (Arias et al, 1988), and is in agreement with the FK5 origin of right ascensions (Feissel, 1990). In addition, it is in agreement with the equinox of the JPL planetary frame DE200/LE200 within 0.02" (Dickey, 1989). The Oz axis points in the direction of the mean pole at J2000.0 as defined by the IAU conventional models for precession and nutation. As a result of the inaccuracy of the conventional models (Herring, 1990), the Oz axis of the IERS celestial system is shifted from the expected position of the mean pole by about 0.01" in longitude, sin "epsilon" and 0.001" in obliquity. New realizations of the IERS celestial reference system are produced whenever justified by the progress in the observations or in the modelling. The successive realizations produced up to now have maintained the initial definition of the axes within 0.0001".Asociación Argentina de Astronomí

Determinación del movimiento del polo terrestre en base a resultados obtenidos por estaciones del hemisferio sur

The polar coordinates during the period 1972-1965 were calculated, every five days, with the time and latitude results of Punta Indio and Mount Stromlo Photographic Zenith Tubes and San Juan and Santiago de Chile Astrolabes.Asociación Argentina de Astronomí

Analysis of consistency between extragalactic celestial reference frames elaborated with technique of VL

Quasars and galactic nuclei are compact object with angular diameters at the level of the miliarcsecond. They can only be observed with high-resolution interferometers. Most of the extragalactic radio sources are at distances of several thousands of MPc and their redshifts are cosmological. The proper motions are beyond the precision of the modern observational techniques, and therefore they can be considered as inexistent. The extragalactic radio sources are the best adapted objects to materialize a celestial reference system kinematically stable for astrometry. The technique of Very Long Baseline Interferometry (VLBI) permits to determine positions of extragalactic radio sources precise at the level of 0.001" (Hinteregger, 1972; Whitney, 1974). The equatorial coordinates of the extragalactic radio sources implicitly define the directions of the axes of the extragalactic celestial reference frame. During the last ten years the different VLBI groups have elaborated catalogues of positions of the milliarcsecond (Ma et al. 1981, Fanswloe et al. 1981, Fanselow et al. 1984, Robertson et al. 1986). However, there still exist some inconsistencies between the independent realizations of the extragalactic frame which can reach soem 0.001". These inconsistencies arise from the adopted models, the distribution of stations in the networks, the distribution of radio sources in the sky, the data acquisition systems and the methods of reduction of observations. We have compared catalogues elaborated at three VLBI laboratories in the USA: the Goddard Space Flight Centre (GSFC) (Ryan and Ma, 1985, Ma, 1988), the Jet Propulsion Laboratory (JPL) (Sovers, 1986, 1988) and the U.S. National Geodetic Survey (NGS) (Robertson, Fallon y Carter, 1986). We have analysed three different aspects: a) the relative orientation between frames, b) the calibration of formal errors, c) the regional deformations between frames. In the frames, the arbitrary origin on the equator has been fixed by the adopted right ascension of the quasar 3C273B (1226 + 023) (Hazard et al., 1971). The equatorial coordinates of the radio sources are in J2000.0 by means of the conventional models IAU 1976 of the precession and IAU 1980 of the nutation. The astrometric comparison of these frames indicates that the direction of axes of the different realizations are consistent at a level better than 0.0015" (Arias et al. 1988), and detects relative regional deformations in some zones of the sky which can reach, in mean, 0.002"( Arias and Lestrade, 1990). Concerning the calibration of errors, the analysis show that formal errors in the individual catalogues have been sub-estimated, and that this effect is larger in declination than in right ascension (Arias et al. 1988). The relative orientation between individual frames could be diminished by adopting a) a better strategy to fix the right ascension origin (it must be noted that the rotation around the polar axis, which implies a misalignment of the catalogue equinoxes, in the most significative of the three angles); b) more precise models, particularly to determine the celestial pole position (the IAU conventional models have inaccuracies which can amount a few miliarcseconds in some terms of the nutation series, as has been shown by Herring, 1986); and c) a more homogeneous distribution of radio sources in the sky. The regional deformations are most probably originated in the geometry of the networks, limiting the spatial distribution of objects and the precision of their positions. Most of the VLBI network operates stations in the Northern hemisphere, and as a consequence, the Southern sky has a few objects with precise positions. On the other hand, the lack of baselines with long components in the North-South direction (except for the baseline Australia-California of the Deep Space Network of the JPL) give a low precision determination of declinations of radio sources near the equator.Asociación Argentina de Astronomí

Mejoramiento de posiciones de estrellas del catálogo del tubo cenital fotográfico de Punta Indio

Se calcularon correcciones a posiciones de las 139 estrellas del catálogo del Tubo Cenital Fotográfico de Punta Indio con observaciones realizadas entre 1971.5 y 1982.5. Hasta el momento se han completado las siete primeras cadenas con un total de 22000 observaciones aproximadamente.Asociación Argentina de Astronomí

A Comparison of the SAO-Hipparcos reference frames

The reference systems defined by the SAO and Hipparcos catalogues are compared using vector spherical harmonic analysis. The differences between astrometric data in both catalogues have been grouped into different data sets and separate harmonic analysis performed on them. The Fourier coefficients yield estimates of systematic errors in SAO catalogue.Instituto de Astrofísica de La Plat

A new method of analysis and reduction of VLBI observations

In the classical algorithm of analysis of VLBI observations, the equatorial coordinates of the radio sources, the coordinates of the stations, the Earth Orientation Parameters (EOP) and, eventually, the corrections to the position of the celestial pole given by the IAR 1976/1980 models are simultaneously adjusted. We have explored a new method of constructing a celestial frame independently of the terrestrial frame and of the Earth's orientation in space. The new strategy of observation consists on "ideally" observing two extragalactic radio sources simultaneously with tho pairs of radiotelescopes, each on at the extremities of a VLBI baseline. On the basis of the classical VLBI observables - the delay and the the delay rate - we have constructed a new observable: the arc between a pair of sources. A set of these new observables leads to spherical coordinates of the radio sources relative to an arbitrary plane and origin. We have started with the simple case, where the delay and the delay rate are entirely due to the VLBI geometry. Then, we have considered all their components. In all the cases, the effects of polar motion, UT1-UTC, precession and nutation are neglected. We have concluded that it is possible to obtain a set of equations of observations, independently of the baseline orientation and the vector of angular rotation velocity of the Earth. Adopting a reference system attached to the baseline (s), we can construct an ew observable, the arc between a pair of radio sources simultaneously observed. The theoretical precision of the method, estimated from the terms neglected in the developments, is at the level of 0.0004" when the observations are performed with a two-element interferometer and the reference system is attached to the unique baseline and to the direction of the vector of rotation velocity of the Earth. When the reference system is defined only by two baseline vectors (case of the three-element interferometer), and only the delay "tau" is considered in the expression of the arc, the theoretical precission is "10^3" times higher.Asociación Argentina de Astronomí

Analysis of shared heritability in common disorders of the brain

ience, this issue p. eaap8757 Structured Abstract INTRODUCTION Brain disorders may exhibit shared symptoms and substantial epidemiological comorbidity, inciting debate about their etiologic overlap. However, detailed study of phenotypes with different ages of onset, severity, and presentation poses a considerable challenge. Recently developed heritability methods allow us to accurately measure correlation of genome-wide common variant risk between two phenotypes from pools of different individuals and assess how connected they, or at least their genetic risks, are on the genomic level. We used genome-wide association data for 265,218 patients and 784,643 control participants, as well as 17 phenotypes from a total of 1,191,588 individuals, to quantify the degree of overlap for genetic risk factors of 25 common brain disorders. RATIONALE Over the past century, the classification of brain disorders has evolved to reflect the medical and scientific communities' assessments of the presumed root causes of clinical phenomena such as behavioral change, loss of motor function, or alterations of consciousness. Directly observable phenomena (such as the presence of emboli, protein tangles, or unusual electrical activity patterns) generally define and separate neurological disorders from psychiatric disorders. Understanding the genetic underpinnings and categorical distinctions for brain disorders and related phenotypes may inform the search for their biological mechanisms. RESULTS Common variant risk for psychiatric disorders was shown to correlate significantly, especially among attention deficit hyperactivity disorder (ADHD), bipolar disorder, major depressive disorder (MDD), and schizophrenia. By contrast, neurological disorders appear more distinct from one another and from the psychiatric disorders, except for migraine, which was significantly correlated to ADHD, MDD, and Tourette syndrome. We demonstrate that, in the general population, the personality trait neuroticism is significantly correlated with almost every psychiatric disorder and migraine. We also identify significant genetic sharing between disorders and early life cognitive measures (e.g., years of education and college attainment) in the general population, demonstrating positive correlation with several psychiatric disorders (e.g., anorexia nervosa and bipolar disorder) and negative correlation with several neurological phenotypes (e.g., Alzheimer's disease and ischemic stroke), even though the latter are considered to result from specific processes that occur later in life. Extensive simulations were also performed to inform how statistical power, diagnostic misclassification, and phenotypic heterogeneity influence genetic correlations. CONCLUSION The high degree of genetic correlation among many of the psychiatric disorders adds further evidence that their current clinical boundaries do not reflect distinct underlying pathogenic processes, at least on the genetic level. This suggests a deeply interconnected nature for psychiatric disorders, in contrast to neurological disorders, and underscores the need to refine psychiatric diagnostics. Genetically informed analyses may provide important "scaffolding" to support such restructuring of psychiatric nosology, which likely requires incorporating many levels of information. By contrast, we find limited evidence for widespread common genetic risk sharing among neurological disorders or across neurological and psychiatric disorders. We show that both psychiatric and neurological disorders have robust correlations with cognitive and personality measures. Further study is needed to evaluate whether overlapping genetic contributions to psychiatric pathology may influence treatment choices. Ultimately, such developments may pave the way toward reduced heterogeneity and improved diagnosis and treatment of psychiatric disorders

Atomic time-keeping from 1955 to the present

International audienceThis paper summarizes the creation and technical evolution of atomic time scales, recalling the parallel development of their acceptance and the remaining problems. We consider a consequence of the accuracy of time measurement, i.e. the entry of Einstein's general relativity into metrology and its applications. We give some details about the method of calculation and the characteristics of International Atomic Time, and we show how it is disseminated at the ultimate level of precision

Maintenance of the ICRS: stability of the axes by different sets of selected radio sources

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