Wilton S. Dias

New catalogue of optically visible open clusters and candidates

We have compiled a new catalogue of open clusters in the Galaxy which updates the previous cat- alogues of Lyng a (1987) and of Mermilliod (1995) (included in the WEBDA database). New objects and new data, in particular, data on kinematics (proper motions) that were not present in the old catalogues, have been included. Virtually all the clusters (1537) presently known were included, which represents an increment of about 347 objects relative to the Lyng a (1987) catalogue. The catalogue is presented in a single table con- taining all the important data, which makes it easy to use. The catalogue can be accessed on line either at http://www.astro.iag.usp.br/~wilton/ or as an electronic table which will be made available at the CDS.

DIRECT DETERMINATION OF THE SPIRAL PATTERN ROTATION SPEED OF THE GALAXY

The rotation velocity of the spiral pattern of the Galaxy is determined by direct observation of the birthplaces of open clusters of stars in the Galactic disk as a function of their age. Our measurement does not depend on any specific model of the spiral structure, such as the existence of a given number of spiral arms, or the presence of a bar in the central regions. This study became possible due to the recent completion of a large database on open clusters by our group. The birthplaces of the clusters are determined by two methods: one that assumes that the orbits are circular, and the other by integrating the orbits in the Galactic potential for a time equal to the age of the clusters. We selected in the database a sample of 212 clusters for which proper motions, radial velocities, distances, and ages are available, or of 612 clusters that have ages and distances available. We tested different assumptions concerning the rotation curve and the radius R0 of the solar orbit. Our results confirm that a dominant fraction of the open clusters are formed in spiral arms, and that the spiral arms rotate like a rigid body, as predicted by the classical theory of spiral waves. We find that the corotation radius, Rc, is close to the solar Galactic orbit (Rc/R0 = 1.06 ± 0.08). This proximity has many potentially interesting consequences, such as a better preservation of life on the Earth and a new understanding of the history of star formation in the solar neighborhood, and of the evolution of the abundance of elements in the Galactic disk.

Proper motion determination of open clusters based on the UCAC2 catalogue

We present the kinematics of hundreds of open clusters, based on the UCAC2 Catalogue positions and proper motions. Membership probabilities were obtained for the stars in the cluster fields by applying a statistical method uses stellar proper motions. All open clusters with known distance were investigated, and for 75 clusters this is the first determination of the mean proper motion. The results, including the DSS images of the clusters fields with the kinematic members marked, are incorporated in the Open Clusters Catalogue supported on line by our group.

Proper motions of the optically visible open clusters based on the UCAC4 catalog

We present a catalog of mean proper motions and membership probabilities of individual stars for optically visible open clusters, which have been determined using data from the UCAC4 catalog in a homogeneous way. The mean proper motion of the cluster and the membership probabilities of the stars in the region of each cluster were determined by applying the statistical method in a modified fashion. In this study, we applied a global optimization procedure to fit the observed distribution of proper motions with two overlapping normal bivariate frequency functions, which also take the individual proper motion errors into account. For 724 clusters, this is the first determination of proper motion, and for the whole sample, we present results with a much larger number of identified astrometric member stars. Furthermore, it was possible to estimate the mean radial velocity of 364 clusters (102 unpublished so far) with the stellar membership using published radial velocity catalogs. These results provide an increase of 30% and 19% in the sample of open clusters with a determined mean absolute proper motion and mean radial velocity, respectively.

Proper motions of open clusters within 1 kpc based on the TYCHO2 Catalogue

We present mean absolute proper motions of 112 open clusters, determined using the data from the Tycho2 Catalogue. For 28 clusters, this is the rst determination of proper motion. The measurements made use of a large number of stars (usually several tens) for each cluster. The total number of stars studied in the elds of the 164 open clusters is 5016, of which 4006 were considered members. The mean proper motions of the clusters and membership probability of individual stars were obtained from the proper motion data by applying the statistical method proposed by Sanders (1971).

Searching for unknown open clusters in the Tycho-2 catalog ,

We present 11 new open cluster candidates found in a systematic search for unknown star clusters using the astro- metric and photometric data included in the Tycho 2 catalog. The possible existence of these stellar aggregates is supported by the analysis of proper motions, color-magnitude diagrams, stellar density distributions, and by the visual inspection of the Digitized Sky Survey (DSS) plates. With these tools we were able to determine mean absolute proper motions as well as pre- liminary reddenings, distances and ages for the majority of the candidates. We found that most of them are possibly nearby (closer than ∼600 pc) open clusters never studied before.

A velocidade de rotação dos braços espirais da Via Láctea

We determine the rotation velocity of the spiral pattern of the Galaxy, by direct observation of the birthplaces of open clusters of stars in the galactic disk as a function of their age. Our measurement does not depend on any specific model of the spiral structure, like the existence of a given number of spiral arms, or the presence of a bar in the central regions. The results first confirm that a dominant fraction of the open clusters are formed in spiral arms, a question which has been a subject of controversy. We find that the spiral arms rotate like a rigid body, as predicted by the classical theory of spiral waves, and show that the corotation radius is close to the solar galactic orbit. This proximity has many interesting consequences, like the preservation of life on the Earth, and a new understanding of the history of star formation in the solar neighborhood, and of the evolution of the abundance of elements in the galactic disk.

Star formation in local spiral arms

We analyze the dynamics of the young stellar clusters belonging to the local spiral arms, based on the proper motions, radial velocities and ages of the clusters, that are compiled in our Catalog of Open Clusters. We find the bitrthplaces of the clusters, and as a first result, we confirm that almost all the clusters are born the spiral arms. This allows us to measure the velocity of the spiral pattern, and to determine the position of the corotation radius, where the spiral pattern and the galactic marerial rotate at the same velocity. The corotation radius is Rc = 1.08 ± 0.08 R0 , that is, very close to the Sun at R0. The direction of the velocity perturbation of the clusters (velocity excess with respect to circular rotation) shows a linear variatin as a function of age, at least for a fraction of the clusters. This is interpreted in terms of a common initial direction dictated by the star‐formation process in the spiral arms, followed by rotation of the velocity vector, according to the epicycle approximation.We analyze the dynamics of the young stellar clusters belonging to the local spiral arms, based on the proper motions, radial velocities and ages of the clusters, that are compiled in our Catalog of Open Clusters. We find the bitrthplaces of the clusters, and as a first result, we confirm that almost all the clusters are born the spiral arms. This allows us to measure the velocity of the spiral pattern, and to determine the position of the corotation radius, where the spiral pattern and the galactic marerial rotate at the same velocity. The corotation radius is Rc = 1.08 ± 0.08 R0 , that is, very close to the Sun at R0. The direction of the velocity perturbation of the clusters (velocity excess with respect to circular rotation) shows a linear variatin as a function of age, at least for a fraction of the clusters. This is interpreted in terms of a common initial direction dictated by the star‐formation process in the spiral arms, followed by rotation of the velocity vector, according to the epicycle appro...