Members of Centaurus OB1 and NGC 4755: new spectroscopic and astrometric studies

Context. An OB association is a gravitationally unbound group of stars. The identification of the members of the association is an important aspect of its study. There are few astrometric membership studies for OB associations, but the combination of these studies with astrophysical techniques becomes a very powerful tool for this type of research. Aims. We perform new spectroscopic and astrometric studies of the members of the association Centaurus OB1 (Cen OB1) and the open cluster NGC 4755. Methods. To carry out this study we used new spectroscopic optical observations obtained at CASLEO, and proper motion and photometric data from the Tycho-2 Catalogue and other databases available via the Internet. Results. With this investigation we found 52 members of Cen OB1 (l = 303º.7, b = 0º.4) and four members of NGC 4755 (l = 303º.2, b = 2º.5), an open cluster integrated into the Cen OB1 stellar association. The distance, heliocentric radial velocity, proper motion, and age of Cen OB1 are 2.6 ± 0.5 kpc, ~-20 km s-1, μαcosδ = -4.78 ± 0.10 mas yr-1, μδ = -0.93 ± 0.10 mas yr-1, and ~20 × 106 years old, respectively. The same parameters found for NGC 4755 are 1.9 ± 0.5 kpc, -18 ± 2 km s-1, μαcosδ = -4.14 ± 0.19 mas yr-1, μδ = -1.09 ± 0.12 mas yr-1, and ~15 × 106 years old. The pulsars PSR J1253-5820 and PSR J1254-6150 could have been born as the result of supernovae explosions that occurred in the past by massive members of Cen OB1. We have seen Infrared Astronomical Satellite (IRAS) sources near Cen OB1, that show that star formation is still occurring. The age gradient observed between the different members of Cen OB1 makes it likely that the stellar association and IRAS sources represent different stages of a sequential formation process.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de RadioastronomíaInstituto de Astrofísica de La Plat

Members of Centaurus OB1 and NGC 4755: new spectroscopic and astrometric studies

Context. An OB association is a gravitationally unbound group of stars. The identification of the members of the association is an important aspect of its study. There are few astrometric membership studies for OB associations, but the combination of these studies with astrophysical techniques becomes a very powerful tool for this type of research. Aims. We perform new spectroscopic and astrometric studies of the members of the association Centaurus OB1 (Cen OB1) and the open cluster NGC 4755. Methods. To carry out this study we used new spectroscopic optical observations obtained at CASLEO, and proper motion and photometric data from the Tycho-2 Catalogue and other databases available via the Internet. Results. With this investigation we found 52 members of Cen OB1 (l = 303º.7, b = 0º.4) and four members of NGC 4755 (l = 303º.2, b = 2º.5), an open cluster integrated into the Cen OB1 stellar association. The distance, heliocentric radial velocity, proper motion, and age of Cen OB1 are 2.6 ± 0.5 kpc, ~-20 km s-1, μαcosδ = -4.78 ± 0.10 mas yr-1, μδ = -0.93 ± 0.10 mas yr-1, and ~20 × 106 years old, respectively. The same parameters found for NGC 4755 are 1.9 ± 0.5 kpc, -18 ± 2 km s-1, μαcosδ = -4.14 ± 0.19 mas yr-1, μδ = -1.09 ± 0.12 mas yr-1, and ~15 × 106 years old. The pulsars PSR J1253-5820 and PSR J1254-6150 could have been born as the result of supernovae explosions that occurred in the past by massive members of Cen OB1. We have seen Infrared Astronomical Satellite (IRAS) sources near Cen OB1, that show that star formation is still occurring. The age gradient observed between the different members of Cen OB1 makes it likely that the stellar association and IRAS sources represent different stages of a sequential formation process.Facultad de Ciencias Astronómicas y GeofísicasInstituto Argentino de RadioastronomíaInstituto de Astrofísica de La Plat

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.Fil: Arias, E. F.. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Cionco, Rodolfo Gustavo. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica la Plata; ArgentinaFil: Orellana, Rosa Beatriz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - La Plata. Instituto de Astrofísica La Plata. Universidad Nacional de La Plata. Facultad de Ciencias Astronómicas y Geofísicas. Instituto de Astrofísica la Plata; Argentina. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; ArgentinaFil: Vucetich, Hector. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentin

Sobre la órbita de Phoebe, noveno satélite de Saturno

A new orbit has been determined by means of 125 observations made between 194 and 1971.Asociación Argentina de Astronomí

Computational Taxonomy for Identification of "Moving Groups". Applied to Open Clusters

A new method is presented here to identify the members of an open cluster, based on computational taxonomy: “the spectral method”. The characters used for the analysis are position and proper motion of all the stars belonging to the cluster’s region. Taxonomy allows to groups the stars together (OTU’s) in clusters, based on its genotypic characteristic, which shows the similarity, between two or more Stars (OTU’s). The method has been applied to open cluster NGC2516. The outcome list of members agrees very well with the one obtained by applying other methods.Fil: Orellana, Rosa Beatriz. Universidad Nacional de la Plata. Facultad de Ciencias Astronómicas y Geofísicas; Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Astrofísica de La Plata; ArgentinaFil: Perichinsky, Gregorio. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico la Plata. Instituto de Astrofísica de la Plata; ArgentinaFil: Plastino, Angel Luis. Universidad Nacional de la Plata. Facultad de Ciencias Exactas; Argentin

Spectra in taxonomic evidence in databases III : Application in celestial bodies. Asteroids families

Numerical Taxonomy aims to group in clusters, using so-called structure analysis of operational taxonomic units (OTUs or taxons or taxa) through numerical methods. These clusters constitute families. Structural analysis, based on their phenotypic characteristics, exhibits the relationships, in terms of degrees of similarity, between two or more OTUs. Entities formed by dynamic domains of attributes, change according to taxonomical requirements: Classification of objects to form families or clusters. Taxonomic objects are here represented by application of the semantics of the Dynamic Relational Database Model. Families of OTUs are obtained employing as tools i) the Euclidean distance and ii) nearest neighbor techniques. Thus taxonomic evidence is gathered so as to quantify the similarity for each pair of OTUs (pair-group method) obtained from the basic data matrix. The main contribution of the present work is to introduce the concept of spectrum of the OTUs, based in the states of their characters. The concept of families’ spectra emerges, if the superposition principle is applied to the spectra of the OTUs, and the groups are delimited through the maximum of the Bienaymé-Tchebycheff relation, that determines Invariants (centroid, variance and radius). Applying the integrated, independent domain technique dynamically to compute the Matrix of Similarity, and, by recourse to an iterative algorithm, families or clusters are obtained. A new taxonomic criterion is thereby formulated. An astronomic application is worked out. The result is a new criterion for the classification of asteroids in the hyperspace of orbital proper elements (the well-known Families of Hirayama). Using an updated database of asteroids we ascertain the robustness of the method. Thus, a new approach to Computational Taxonomy is presented, that has been already employed with reference to Data Mining. The Informatics (Data Mining and Computational Taxonomy), is always the original objective of our researches.Eje: Ingeniería de Software y Base de DatosRed de Universidades con Carreras en Informática (RedUNCI

Computational Taxonomy for Identification of "Moving Groups" : Applied to Open Clusters

A new method is presented here to identify the members of an open cluster, based on computational taxonomy: “the spectral method”. The characters used for the analysis are position and proper motion of all the stars belonging to the cluster’s region. Taxonomy allows to groups the stars together (OTU’s) in clusters, based on its genotypic characteristic, which shows the similarity, between two or more Stars (OTU’s). The method has been applied to open cluster NGC2516. The outcome list of members agrees very well with the one obtained by applying other methods.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La PlataFacultad de Ciencias Exacta

Computational Taxonomy for Identification of "Moving Groups" : Applied to Open Clusters

A new method is presented here to identify the members of an open cluster, based on computational taxonomy: “the spectral method”. The characters used for the analysis are position and proper motion of all the stars belonging to the cluster’s region. Taxonomy allows to groups the stars together (OTU’s) in clusters, based on its genotypic characteristic, which shows the similarity, between two or more Stars (OTU’s). The method has been applied to open cluster NGC2516. The outcome list of members agrees very well with the one obtained by applying other methods.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La PlataFacultad de Ciencias Exacta

Computational Taxonomy for Identification of "Moving Groups" : Applied to Open Clusters

A new method is presented here to identify the members of an open cluster, based on computational taxonomy: “the spectral method”. The characters used for the analysis are position and proper motion of all the stars belonging to the cluster’s region. Taxonomy allows to groups the stars together (OTU’s) in clusters, based on its genotypic characteristic, which shows the similarity, between two or more Stars (OTU’s). The method has been applied to open cluster NGC2516. The outcome list of members agrees very well with the one obtained by applying other methods.Facultad de Ciencias Astronómicas y GeofísicasInstituto de Astrofísica de La PlataFacultad de Ciencias Exacta