Fluctuacions de la rotació de la Terra del 1800 al 1955.5: correccions al sistema FK5, elements orbitals de la lluna i datum de Watts

[cat] Hom va considerar uniforme la velocitat de rotació de la Terra fins el segle XIX. A finals del segle, alguns autors van començar a assenyalar que les fluctuacions observades del moviment de la Lluna podien ser degudes a variacions de la rotació terrestre. Per corroborar això calia una bona teoria del moviment lunar. Per altra banda, si existien fluctuacions en la rotació de la Terra, s'havien d'observar irregularitats no només en la Lluna, sinó en tots els altres cossos del sistema solar, en funció de la raó de moviments propis. Observacions meridianes, eclipsis i trànsits de Mercuri van confirmar la suposició. L'existència de fluctuacions en la velocitat de rotació de la Terra fa que l'escala de temps universal, lligada a la rotació diürna, no sigui una escala uniforme del temps. Per tal d'establir una escala uniforme (Temps Dinàmic) que serveixi com a argument de temps a les equacions de moviment, diferents autors han comparat posicions observades i calculades de cossos del sistema solar com a via per a determinar les irregularitats existents. La comparació d'ambdues posicions permet determinar la diferència T = TD - TU (Temps Dinàmic - Temps Universal). El 1955.5 es va establir la nova escala de Temps Atòmic Internacional (TAI). La seva relació amb el temps dinàmic i amb el temps universal es coneix i, per tant, l'escala uniforme TD està definida des del 1955.5. El període anterior ha estat estudiat per diversos autors que observen variacions molt elevades al voltant del 1900, variacions que no s’expliquen per la transferència de moment entre el nucli i el mantell terrestres, possible causa de les irregularitats. El present treball pretén millorar els seus resultats, incorporant els darrers acords de la UAI(establiment d'un nou sistema fonamental, nova escala de temps, nou sistema de constants, nova època de referencia, etc.), i utilitzant les efemèrides DE200/LE200 calculades per integració numèrica de les equacions de moviment dels cossos del Sistema solar. Aquesta millora pot contribuir a clarificar els moments que actuen i trobar els fenòmens que poden produir-los. Les ocultacions d'estrelles per la Lluna és un fenomen observat sistemàticament, des de fa anys, per observadors afeccionats i professionals. Podem disposar, doncs, de gran quantitat d'aquestes observacions que cobreixen un període molt ampli. Les dues raons, quantitat i extensió en el temps, fan que l'anàlisi i reducció d'aquest tipus d'observacions sigui un bon mètode per a la determinació de canvis en la rotació de la Terra. Per tal de separar els errors deguts a les diferències d’escales de temps, dels deguts a les efemèrides, equacions personals i altres, hem dividit les observacions de que disposem en dos períodes: a) observacions del 1955.5 al 1980, i b) observacions del 1800 al 1955.5. Hem analitzat les observacions del primer cas per determinar correccions a les constants utilitzades a les efemèrides, diferències entre els sistemes de referència dinàmic i estel.lar (FKS), correccions al datum de Watts i equacions personals. Hem aplicat aquestes correccions a les observacions del periode 1800-1955.5 i considerat que l’única font d’error que resta és la diferència entre les escales de temps provocada per les fluctuacions de la velocitat de la rotació terrestre. D'aquesta manera aquest treball cobreix dos objectius importants. Un, analitzar el nou sistema de referència FK5 establert per la UAI (1976) i UAI (1979),comparant-lo amb les efemèrides DE200/LE200, a la vegada que s'estudien les pròpies efemèrides, i l'altre, la redeterminació de les irregularitats en la velocitat de rotació de la Terra. L'anàlisi de les causes geofísiques d'aquestes fluctuacions queda fora dels objectius d’aquest treball. S'inclouen correccions al datum de Watts, millorant resultats anteriors ja que aquí es tracta amb un nombre més elevat d'observacions. Al capítol l descrivim el mètode seguit per a la reducció i al capítol 2, l'equació de condició establerta. Al capítol 3 fem una anàlisi de les observacions de què disposem i descrivim el procés de tractament de les dades i la resolució de l'equació de condició. Al capítol 4 presentem els resultats obtinguts i les conclusions que se'n deriven

Professor Maria Assumpció Català Poch (1925-2009)

Maria Assumpció Català Poch was born in Barcelona on July 14, 1925. In the early years of her childhood, the family lived in Montblanc, Tarragona, where her father worked. She began primary school in Montblanc but finished in Barcelona, at the Catholic school of the Sacred Heart and Mary Immaculate, operated by the sisters of St Paul. After the Spanish civil war, she attended high school at the Institut Maragall in Barcelona..

An analysis of the currently available calibrations in Stromgren photometry by using open clusters

In recent years, several authors have revised the calibrations used to compute physical parameters (tex2html_wrap_inline498, tex2html_wrap_inline500, log g, [Fe/H]) from intrinsic colours in the tex2html_wrap_inline504 photometric system. For reddened stars, these intrinsic colours can be computed through the standard relations among colour indices for each of the regions defined by Strömgren (1966) on the HR diagram. We present a discussion of the coherence of these calibrations for main-sequence stars. Stars from open clusters are used to carry out this analysis. Assuming that individual reddening values and distances should be similar for all the members of a given open cluster, systematic differences among the calibrations used in each of the photometric regions might arise when comparing mean reddening values and distances for the members of each region. To classify the stars into Strömgren's regions we extended the algorithm presented by Figueras et al. (1991) to a wider range of spectral types and luminosity classes. The observational ZAMS are compared with the theoretical ZAMS from stellar evolutionary models, in the range tex2html_wrap_inline506 K. The discrepancies are also discussed

Modelling the photosphere of active stars for planet detection and characterizaton

Context. Stellar activity patterns are responsible for jitter effects that are observed at different timescales and amplitudes in the measurements obtained from photometric and spectroscopic time series observations. These effects are currently in the focus of many exoplanet search projects, since the lack of a well-defined characterization and correction strategy hampers the detection of the signals associated with small exoplanets. Aims. Accurate simulations of the stellar photosphere based on the most recent available models for main-sequence stars can provide synthetic photometric and spectroscopic time series data. These may help to investigate the relation between activity jitter and stellar parameters when considering different active region patterns. Moreover, jitters can be analysed at different wavelength scales (defined by the passbands of given instruments or space missions) to design strategies to remove or minimize them. Methods. We present the StarSim tool, which is based on a model for a spotted rotating photosphere built from the integration of the spectral contribution of a fine grid of surface elements. The model includes all significant effects affecting the flux intensities and the wavelength of spectral features produced by active regions and planets. The resulting synthetic time series data generated with this simulator were used to characterize the effects of activity jitter in extrasolar planet measurements from photometric and spectroscopic observations. Results. Several cases of synthetic data series for Sun-like stars are presented to illustrate the capabilities of the methodology. A specific application for characterizing and modelling the spectral signature of active regions is considered, showing that the chromatic effects of faculae are dominant for low-temperature contrasts of spots. Synthetic multi-band photometry and radial velocity time series are modelled for HD 189733 by adopting the known system parameters and fitting for the map of active regions with StarSim. Our algorithm reproduces both the photometry and the radial velocity (RV) curves to good precision, generally better than the studies published to date. We evaluate the RV signature of the activity in HD 189733 by exploring a grid of solutions from the photometry. We find that the use of RV data in the inverse problem could break degeneracies and allow for a better determination of some stellar and activity parameters, for example, the configuration of active regions, the temperature contrast of spots, and the amount of faculae. In addition, the effects of spots are studied for a set of simulated transit photometry, showing that these can introduce variations in Rp/R∗ measurements with a spectral signature and amplitude that are very similar to the signal of an atmosphere dominated by dust

Constraining galactic structure parameters from a new extinction model and four star count samples

We propose a new 3-dimensional extinction model based on the COBE/IRAS all sky reddening map. Its application to globular and open cluster data evidences that the COBE/IRAS reddening map has an accuracy of 18%, but overestimates visual absorption by a factor of 1.16. This systematic error does not change with galactic latitude and opacity significantly. The implementation of the new extinction model has optimized our galactic structure and kinematic model to low-galactic latitudes. Four star count samples distributed in different galactic directions have been compared with galactic model simulations. Numerical experiments allow us to constrain the radial distribution of the galactic disk. The disk scale length is found to be 2250 +/- 50 pc and the displacement of the Sun from the galactic plane ZSun = 27.5 +/- 6.0 pc

A ring in a shell: the large-scale 6D structure of the Vela OB2 complex

The Vela OB2 association is a group of 10 Myr stars exhibiting a complex spatial and kinematic substructure. The all-sky Gaia DR2 catalogue contains proper motions, parallaxes (a proxy for distance) and photometry that allow us to separate the various components of Vela OB2. We characterise the distribution of the Vela OB2 stars on a large spatial scale, and study its internal kinematics and dynamic history. We make use of Gaia DR2 astrometry and published Gaia-ESO Survey data. We apply an unsupervised classification algorithm to determine groups of stars with common proper motions and parallaxes. We find that the association is made up of a number of small groups, with a total current mass over 2330 Msun. The three-dimensional distribution of these young stars trace the edge of the gas and dust structure known as the IRAS Vela Shell across 180 pc and shows clear signs of expansion. We propose a common history for Vela OB2 and the IRAS Vela Shell. The event that caused the expansion of the shell happened before the Vela OB2 stars formed, imprinted the expansion in the gas the stars formed from, and most likely triggered star formation

Hunting for open clusters in Gaia DR2: the Galactic anticentre

The Gaia Data Release 2 (DR2) provided an unprecedented volume of precise astrometric and excellent photometric data. In terms of data mining the Gaia catalogue, machine learning methods have shown to be a powerful tool, for instance in the search for unknown stellar structures. Particularly, supervised and unsupervised learning methods combined together significantly improves the detection rate of open clusters. We systematically scan Gaia DR2 in a region covering the Galactic anticentre and the Perseus arm (120≤l≤205 and −10≤b≤10), with the goal of finding any open clusters that may exist in this region, and fine tuning a previously proposed methodology successfully applied to TGAS data, adapting it to different density regions. Our methodology uses an unsupervised, density-based, clustering algorithm, DBSCAN, that identifies overdensities in the five-dimensional astrometric parameter space (l,b,ϖ,μα∗,μδ) that may correspond to physical clusters. The overdensities are separated into physical clusters (open clusters) or random statistical clusters using an artificial neural network to recognise the isochrone pattern that open clusters show in a colour magnitude diagram. The method is able to recover more than 75% of the open clusters confirmed in the search area. Moreover, we detected 53 open clusters unknown previous to Gaia DR2, which represents an increase of more than 22% with respect to the already catalogued clusters in this region. We find that the census of nearby open clusters is not complete. Different machine learning methodologies for a blind search of open clusters are complementary to each other; no single method is able to detect 100% of the existing groups. Our methodology has shown to be a reliable tool for the automatic detection of open clusters, designed to be applied to the full Gaia DR2 catalogue

A new method for unveiling Open Clusters in Gaia: new nearby Open Clusters confirmed by DR2

Context. The publication of the Gaia Data Release 2 (Gaia DR2) opens a new era in astronomy. It includes precise astrometric data (positions, proper motions, and parallaxes) for more than 1.3 billion sources, mostly stars. To analyse such a vast amount of new data, the use of data-mining techniques and machine-learning algorithms is mandatory. Aims. A great example of the application of such techniques and algorithms is the search for open clusters (OCs), groups of stars that were born and move together, located in the disc. Our aim is to develop a method to automatically explore the data space, requiring minimal manual intervention. Methods. We explore the performance of a density-based clustering algorithm, DBSCAN, to find clusters in the data together with a supervised learning method such as an artificial neural network (ANN) to automatically distinguish between real OCs and statistical clusters. Results. The development and implementation of this method in a five-dimensional space (l, b, ϖ, μα*, μδ) with the Tycho-Gaia Astrometric Solution (TGAS) data, and a posterior validation using Gaia DR2 data, lead to the proposal of a set of new nearby OCs. Conclusions. We have developed a method to find OCs in astrometric data, designed to be applied to the full Gaia DR2 archive

The open cluster King 1 in the second quadrant

We analyse the poorly studied open cluster King 1 in the second Galactic quadrant. From wide-field photometry, we have studied the spatial distribution of this cluster. We determined that the centre of King 1 is located at α2000 = 00h22m and δ2000 = +64°23΄. By parameterizing the stellar density with a King profile, we have obtained a central density of ρ0 = 6.5 ± 0.2 star arcmin-2 and a core radius of rcore = 1.9 ± 0.2 arcmin. By comparing the observed colour-magnitude diagram of King 1 with those of similar open clusters and with different sets of isochrones, we have estimated an age of 2.8 ± 0.3 Gyr, a distance modulus of (m - M)o = 10.6 ± 0.1 mag and a reddening of E(B - V) = 0.80 ± 0.05 mag. To complete our analysis, we acquired medium resolution spectra for 189 stars in the area of King 1. From their derived radial velocities, we determined an average velocity = -53.1 ± 3.1 km s-1. From the strength of the infrared Ca II lines in red giants we have determined an average metallicity of = +0.07 ± 0.08 dex. From spectral synthesis, we have also estimated an α-elements abundance of = -0.10 ± 0.08 dex

Gaia Focused Product Release: Asteroid orbital solution

Context. We report the exploitation of a sample of Solar System observations based on data from the third Gaia Data Release (Gaia DR3) of nearly 157 000 asteroids. It extends the epoch astrometric solution over the time coverage planned for the Gaia DR4, which is not expected before the end of 2025. This data set covers more than one full orbital period for the vast majority of these asteroids. The orbital solutions are derived from the Gaia data alone over a relatively short arc compared to the observation history of many of these asteroids. Aims. The work aims to produce orbital elements for a large set of asteroids based on 66 months of accurate astrometry provided by Gaia and to assess the accuracy of these orbital solutions with a comparison to the best available orbits derived from independent observations. A second validation is performed with accurate occultation timings. Methods. We processed the raw astrometric measurements of Gaia to obtain astrometric positions of moving objects with 1D sub-mas accuracy at the bright end. For each asteroid that we matched to the data, an orbit fitting was attempted in the form of the best fit of the initial conditions at the median epoch. The force model included Newtonian and relativistic accelerations to derive the observation equations, which were solved with a linear least-squares fit. Results. Orbits are provided in the form of state vectors in the International Celestial Reference Frame for 156 764 asteroids, including near-Earth objects, main-belt asteroids, and Trojans. For the asteroids with the best observations, the (formal) relative uncertainty σa/a is better than 10−10. Results are compared to orbits available from the Jet Propulsion Laboratory and MPC. Their orbits are based on much longer data arcs, but from positions of lower quality. The relative differences in semi-major axes have a mean of 5 × 10−10 and a scatter of 5 × 10−9