Analysis of webs of partial-tension-field beams subjected to lateral pressure loadings

Webs of partial-tension-field beams subjected to lateral pressure loading

Astrometric Positions and Proper Motions of 19 Radio Stars

We have used the Very Large Array, linked with the Pie Town Very Long Baseline Array antenna, to determine astrometric positions of 19 radio stars in the International Celestial Reference Frame (ICRF). The positions of these stars were directly linked to the positions of distant quasars through phase referencing observations. The positions of the ICRF quasars are known to 0.25 mas, thus providing an absolute reference at the angular resolution of our radio observations. Average values for the errors in our derived positions for all sources were 13 mas and 16 mas in R.A. and declination respectively, with accuracies approaching 1-2 mas for some of the stars observed. Differences between the ICRF positions of the 38 quasars, and those measured from our observations showed no systematic offsets, with mean values of -0.3 mas in R.A. and -1.0 mas in declination. Standard deviations of the quasar position differences of 17 mas and 11 mas in R.A. and declination respectively, are consistent with the mean position errors determined for the stars. Our measured positions were combined with previous Very Large Array measurements taken from 1978-1995 to determine the proper motions of 15 of the stars in our list. With mean errors of approximately 1.6 mas/yr, the accuracies of our proper motions approach those derived from Hipparcos, and for a few of the stars in our program, are better than the Hipparcos values. Comparing the positions of our radio stars with the Hipparcos catalog, we find that at the epoch of our observations, the two frames are aligned to within formal errors of approximately 3 mas. This result confirms that the Hipparcos frame is inertial at the expected level.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March 1

Astrometric Control of the Inertiality of the Hipparcos Catalog

Based on the most complete list of the results of an individual comparison of the proper motions for stars of various programs common to the Hipparcos catalog, each of which is an independent realization of the inertial reference frame with regard to stellar proper motions, we redetermined the vector $\omega$ of residual rotation of the ICRS system relative to the extragalactic reference frame. The equatorial components of this vector were found to be the following: $\omega_x = +0.04\pm 0.15$ mas yr$^{-1}$, $\omega_y = +0.18\pm 0.12$ mas yr$^{-1}$, and $\omega_z = -0.35\pm 0.09$ mas yr$^{-1}$.Comment: 8 pages, 1 figur

VIBROSEISMIC INVESTIGATIONS OF THE BAIKAL RIFT ZONE WITH A POWERFUL CVO-100 VIBRATOR

The article provides an overview of vibroseismic studies carried out in the Baikal rift zone using LargeScale Research Facilities – a powerful CVO-100 seismic vibrator, installed at the South Baikal geodynamic test site SB RAS. Research is carried out according to several methods focused on different tasks: study of the structure of the Earth’s crust and upper mantle in the BRZ, active vibroseismic monitoring, and verification of velocity models of the Earth’s crust. To study the structure of the Earth’s crust and the upper mantle, there were done the vibrator-generated wavefield recordings at the stationary regional network of seismic stations in the Buryat and Baikal branches of the Federal Research Center of the GS RAS, as well as the experimental studies involving the mobile networks deployment (ICMMG SB RAS, SIPE RAS, GIN SB RAS). The aim of the work is to carry out deep vibroseismic sounding of the Earth’s crust (vibro-DSS) at the junction of the Siberian platform, the BRZ and the Sayan-Baikal folded area. The methodology is based on the study of vibration seismograms with the determination of arrival times of the main groups of waves and their correlation with the velocity models of the Earth’s crust in the BRZ. A CVO-100 vibrator and a regional network of seismic stations are used to carry out active vibroseismic monitoring of the southern part of the BRZ. The active monitoring area is about 500×200 km. During vibroseismic monitoring, there were done thorough studies of seasonal variations of the vibrator-generated wavefield and the development of techniques for spectral correction of seismograms. A seismic vibrator CVO-100 was used to carry out experimental verification of the velocity models of the Earth’s crust, developed based on the BEST and PASSCAL experimental data. The vibrational deep seismic sounding (vibro-DSS) on the Baikal – Ulan Bator profile was carried out by the ICMMG SB RAS, GIN SB RAS and BB FRC GS RAS (Russia) in cooperation with IAG MAN (Mongolia)

Sufficient conditions for topological invariance of 2D images under rigid transformations

International audienceIn ℝ^2, rigid transformations are topology-preserving operations. However, this property is generally no longer true when considering digital images instead of continuous ones, due to digitization effects. In this article, we investigate this issue by studying discrete rigid transformations (DRTs) on ℤ^2. More precisely, we define conditions under which digital images preserve their topological properties under any arbitrary DRTs. Based on the recently introduced notion of DRT graph and the classical notion of simple point, we first identify a family of local patterns that authorize topological invariance under DRTs. These patterns are then involved in a local analysis process that guarantees topological invariance of whole digital images in linear time

Correlated errors in Hipparcos parallaxes towards the Pleiades and the Hyades

We show that the errors in the Hipparcos parallaxes towards the Pleiades and the Hyades open clusters are spatially correlated over angular scales of 2 to 3 deg, with an amplitude of up to 2 mas. This correlation is stronger than expected based on the analysis of the Hipparcos catalog. We predict the parallaxes of individual cluster members, pi_pm, from their Hipparcos proper motions, assuming that all cluster members have the same space velocity. We compare pi_pm with their Hipparcos parallaxes, pi_Hip, and find that there are significant spatial correlations in pi_Hip. We derive a distance modulus to the Pleiades of 5.58 +- 0.18 mag using the radial-velocity gradient method. This value, agrees very well with the distance modulus of 5.60 +- 0.04 mag determined using the main-sequence fitting technique, compared with the value of 5.33 +- 0.06 inferred from the average of the Hipparcos parallaxes of the Pleiades members. We show that the difference between the main-sequence fitting distance and the Hipparcos parallax distance can arise from spatially correlated errors in the Hipparcos parallaxes of individual Pleiades members. Although the Hipparcos parallax errors towards the Hyades are spatially correlated in a manner similar to those of the Pleiades, the center of the Hyades is located on a node of this spatial structure. Therefore, the parallax errors cancel out when the average distance is estimated, leading to a mean Hyades distance modulus that agrees with the pre-Hipparcos value. We speculate that these spatial correlations are also responsible for the discrepant distances that are inferred using the mean Hipparcos parallaxes to some open clusters. Finally, we note that our conclusions are based on a purely geometric method and do not rely on any models of stellar isochrones.Comment: 33 pages including 10 Figures, revised version accepted for publication in Ap