scholarly journals Linking Hipparcos to the galaxies: the Bonn and Potsdam programmes

1992 ◽  
Vol 9 ◽  
pp. 439-439
Author(s):  
P. Brosche ◽  
W. R. Dick ◽  
R. Galas ◽  
M. Geffert ◽  
S. Hirte ◽  
...  
Keyword(s):  
Numerical Simulations ◽  
Reference Frame ◽  
Inertial System ◽  
Photographic Method ◽  
Proper Motions ◽  
Radio Stars

Besides the link of the HIPPARCOS reference frame to extragalactic objects via radio stars or by the HST, also photographic astrometry is able to calibrate the HIPPARCOS proper motions with regard to an inertial system. Numerical simulations have shown that even with a very small number of well-distributed link fields (3 to 5) the photographic method is competitive with other techniques.

scholarly journals Uses and Required Characteristics of Reference Frames for Galactic Astronomy

1986 ◽  
Vol 7 ◽  
pp. 51-54
Author(s):  
Roland Wielen
Keyword(s):  
Reference Frame ◽  
Reference Frames ◽  
Inertial System ◽  
Proper Motions ◽  
Upper Limits ◽  
Galactic Astronomy

AbstractGalactic astronomy requires mainly accurate proper motions of stars, referred to a system which represents an inertial system as closely as possible. We discuss the required accuracy, give an outlook on forthcoming reference frames of use for galactic astronomy, and discuss observationally determined upper limits on the rotation of the extra-galactic reference frame.

scholarly journals Accuracy Assessment of Hipparcos Optical and VLA Radio Positions of Stars

1993 ◽  
Vol 156 ◽  
pp. 377-380
Author(s):  
H. G. Walter ◽  
R. Hering ◽  
H. Lenhardt ◽  
Chr. deVegt ◽  
D.R. Florkowski ◽  
...  
Keyword(s):  
Radio Emission ◽  
Reference Frame ◽  
Accuracy Assessment ◽  
Optical Emission ◽  
Relative Orientation ◽  
Large Array ◽  
Proper Motions ◽  
Data Noise ◽  
Very Large Array ◽  
Radio Stars

Optical positions of some 30 radio stars derived from 12 months of HIPPARCOS measurements are compared with their radio positions obtained with the Very Large Array (VLA). — Once the lengths of arcs between optical and radio positions of pairs of stars are calculated the differences of the arcs are formed. They provide an estimate of the coincidence of the optical and radio emission centres. — From the comparison of optical and radio positions infinitesimal rotation angles of the HIPPARCOS frame with respect to the VLA extragalactic reference frame are determined by rigid rotations. After taking account of the relative orientation of the frames the standard deviations of the remaining residuals are approximately of the order of the VLA observation errors, thus demonstrating the reliability of the HIPPARCOS results. However, they also indicate some data noise very likely caused by the low accuracy of optical proper motions used to bridge the HIPPARCOS-radio epoch differences up to 9 years, and possible discrepancies of radio-optical emission centres of some stars.

scholarly journals Results of VLBI Observations of Radio Stars and Their Potential for Linking the HIPPARCOS and Extragalactic Reference Frames

1986 ◽  
Vol 109 ◽  
pp. 779-788 ◽  
Author(s):  
Jean-Francois Lestrade ◽  
Robert A. Preston ◽  
Arthur E. Niell ◽  
Robert L. Mutel ◽  
Robert B. Phillips
Keyword(s):  
Reference Frame ◽  
Binary Systems ◽  
Reference Frames ◽  
Proper Motions ◽  
Vlbi Observations ◽  
Angular Extent ◽  
The Future ◽  
Radio Stars

VLBI observations of bright radio stars have been initiated in an attempt to measure the positions and proper motions of their radio components in order to tie the future HIPPARCOS stellar frame to a VLBI extragalactic reference frame. Through VLBI observations of a sample of 20 known radio stars we have identified 11 stars that should be appropriate for both astrometric VLBI and HIPPARCOS observations. Our measurements indicate that the angular extent of their radio emitting regions is small, i.e. < 3 milliarcseconds for 7 of them. Most of these radio stars belong to the RS Canum Venaticorum class of binary systems.

scholarly journals The Gaia reference frame for bright sources examined using VLBI observations of radio stars

2019 ◽  
Vol 633 ◽  
pp. A1 ◽  
Author(s):  
Lennart Lindegren
Keyword(s):  
Reference Frame ◽  
Positional Information ◽  
Simultaneous Estimation ◽  
Proper Motions ◽  
Long Baseline ◽  
Vlbi Observations ◽  
Independent Observations ◽  
Vlbi Data ◽  
Single Epoch ◽  
Radio Stars

Context. Positions and proper motions of Gaia sources are expressed in a reference frame that ideally should be non-rotating relative to distant extragalactic objects, coincident with the International Celestial Reference System (ICRS), and consistent across all magnitudes. For sources fainter than 16th magnitude, this is achieved through Gaia’s direct observations of quasars. At brighter magnitudes, it is difficult to validate the quality of the reference frame because comparison data are scarce. Aims. The aim of this paper is to examine the use of very long baseline interferometry (VLBI) observations of radio stars to determine the spin and orientation of the bright reference frame of current and future Gaia data releases. Methods. Simultaneous estimation of the six spin and orientation parameters makes optimal use of VLBI data and makes it possible to include even single-epoch VLBI observations in the solution. The method is applied to Gaia Data Release 2 (DR2) using published VLBI data for 41 radio stars. Results. The VLBI data for the best-fitting 26 sources indicate that the bright reference frame of Gaia DR2 rotates relative to the faint quasars at a rate of about 0.1 mas yr−1, which is significant at the 2σ level. This supports a similar conclusion based on a comparison with stellar positions in the HIPPARCOS frame. The accuracy is currently limited because only a few radio sources are included in the solution, by uncertainties in the Gaia DR2 proper motions, and by issues related to the astrophysical nature of the radio stars. Conclusions. While the origin of the indicated rotation is understood and can be avoided in future data releases, it remains important to validate the bright reference frame of Gaia by independent observations. This can be achieved using VLBI astrometry, which may require re-observing the old sample of radio stars as well as measuring new objects. The unique historical value of positional measurements is stressed and VLBI observers are urged to ensure that relevant positional information is preserved for the future.

Flow Structures in the Tip Region for a Transonic Compressor Rotor

2013 ◽  
Vol 135 (3) ◽  
Author(s):  
Juan Du ◽  
Feng Lin ◽  
Jingyi Chen ◽  
Chaoqun Nie ◽  
Christoph Biela
Keyword(s):  
Numerical Simulations ◽  
Reference Frame ◽  
Three Dimensional ◽  
Flow Structures ◽  
Leakage Flow ◽  
Tip Leakage Flow ◽  
Three Dimensional Flow ◽  
Tip Leakage ◽  
Leakage Flows ◽  
Tip Leakage Flows

Numerical simulations are carried out to investigate flow structures in the tip region for an axial transonic rotor, with careful comparisons with the experimental results. The calculated performance curve and two-dimensional (2D) flow structures observed at casing, such as the shock wave, the expansion wave around the leading edge, and the tip leakage flow at peak efficiency and near-stall points, are all captured by simulation results, which agree with the experimental data well. An in-depth analysis of three-dimensional flow structures reveals three features: (1) there exists an interface between the incoming main flow and the tip leakage flow, (2) in this rotor the tip leakage flows along the blade chord can be divided into at least two parts according to the blade loading distribution, and (3) each part plays a different role on the stall inception mechanism in the leakage flow dominated region. A model of three-dimensional flow structures of tip leakage flow is thus proposed accordingly. In the second half of this paper, the unsteady features of the tip leakage flows, which emerge at the operating points close to stall, are presented and validated with experiment observations. The numerical results in the rotor relative reference frame are first converted to the casing absolute reference frame before compared with the measurements in experiments. It is found that the main frequency components of simulation at absolute reference frame match well with those measured in the experiments. The mechanism of the unsteadiness and its significance to stability enhancement design are then discussed based on the details of the flow field obtained through numerical simulations.

scholarly journals The Relationships Between Various Techniques for Obtaining Proper Motions

1968 ◽  
Vol 1 ◽  
pp. 311-315
Author(s):  
C. A. Murray
Keyword(s):  
Reference Frame ◽  
Apparent Magnitude ◽  
Galactic Kinematics ◽  
Proper Motions

In many problems of galactic kinematics we need to know the proper motions of faint stars, as well as bright stars, within some well-defined reference frame. Generally speaking, the accuracy required is greater for faint stars on account of their larger distances. Techniques at present used for determining proper motions vary according to apparent magnitude, and it is clearly desirable that all methods should give results which are capable of reduction to a common frame.

scholarly journals Celestial Coordinate System and the FK5 Catalogue

1990 ◽  
Vol 141 ◽  
pp. 383-384
Author(s):  
A. P. Gulyaev
Keyword(s):  
Coordinate System ◽  
Reference Frame ◽  
Proper Motions

To study the motion of the celestial objects in a reference frame one should Know the evolution of the frame itself, In our case it the motion of the origin of coordinates and the proper motions of the objects, the system is based upon.

scholarly journals Astrometry With Schmidt Telescopes

1984 ◽  
Vol 78 ◽  
pp. 217-224
Author(s):  
C A Murray
Keyword(s):  
Reference Frame ◽  
Proper Motions ◽  
Large Area ◽  
Full Field ◽  
Potential Value ◽  
Focal Surface ◽  
Single Plate ◽  
Trigonometric Parallaxes ◽  
The Absolute

Photographic astrometry, including work with Schmidt telescopes, can be divided into two main fields, (i) the measurement of positions of objects relative to a reference frame of stars with known celestial coordinates, and (ii) the measurement of relative proper motions and trigonometric parallaxes from a series of plates taken on the same field. The former demands a knowledge of the absolute transformation between angles on the sky and measurements on a plate, whereas in the latter we are only interested in differential transformations from plate to plate. The potential value of Schmidt telescopes for both these fields of astrometry lies in the large area of sky and range of magnitude which can be imaged on a single plate. The former advantage is however, to some extent offset by the curvature of the focal surface which means that, in order to utilize the full field the plates must be constrained to the form of the focal surface during exposure.

scholarly journals Proper Motions With Respect to the Extragalactic Reference Frame

1990 ◽  
Vol 141 ◽  
pp. 407-417
Author(s):  
A. R. Klemola
Keyword(s):  
Reference Frame ◽  
Proper Motion ◽  
Milky Way ◽  
Proper Motions ◽  
Stellar Proper Motions

The Lick proper motion program, one of several using galaxies as a reference frame, is summarized with a statement of work accomplished for the non-Milky Way sky. The problem of identifying relatively transparent regions at low galactic latitudes is discussed, with tabular results presented for 41 windows from the literature having observable galaxies. These fields may be helpful for attaching stellar proper motions directly to the extragalactic frame.

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