scholarly journals Summary-Introduction: Velocity Fields and Associated Thermodynamic Variations in the External Layers of Intrinsic Variable Stars

1960 ◽  
Vol 12 ◽  
pp. 131-193 ◽  
Author(s):  
P. Ledoux ◽  
C. A. Whitney
Keyword(s):  
Radial Velocity ◽  
Variable Stars ◽  
Velocity Fields ◽  
Light Spectrum ◽  
Physical Modification ◽  
Intrinsic Variable

By intrinsic variable stars, we mean those stars which present variations in light, spectrum and radial velocity which cannot be accounted for in terms of purely geometrical or orbital factors; so that we have to appeal to some kind of periodic physical modification of the star.

C. The Continuous Spectrum of Pulsating Variable Stars

1967 ◽  
Vol 28 ◽  
pp. 177-206
Author(s):  
J. B. Oke ◽  
C. A. Whitney
Keyword(s):  
Continuous Spectrum ◽  
Variable Stars ◽  
Velocity Fields ◽  
The Other ◽  
Line Spectrum ◽  
Direct Information ◽  
Thermodynamic Structure ◽  
Diagnostic Interpretation ◽  
Pulsating Variable Stars ◽  
The Continuum

Pecker:The topic to be considered today is the continuous spectrum of certain stars, whose variability we attribute to a pulsation of some part of their structure. Obviously, this continuous spectrum provides a test of the pulsation theory to the extent that the continuum is completely and accurately observed and that we can analyse it to infer the structure of the star producing it. The continuum is one of the two possible spectral observations; the other is the line spectrum. It is obvious that from studies of the continuum alone, we obtain no direct information on the velocity fields in the star. We obtain information only on the thermodynamic structure of the photospheric layers of these stars–the photospheric layers being defined as those from which the observed continuum directly arises. So the problems arising in a study of the continuum are of two general kinds: completeness of observation, and adequacy of diagnostic interpretation. I will make a few comments on these, then turn the meeting over to Oke and Whitney.

The Flow of an Incompressible Fluid through an Axial Turbo-Machine with any Number of Rows

1951 ◽  
Vol 3 (2) ◽  
pp. 133-144 ◽  
Author(s):  
J. W. Railly
Keyword(s):  
Aspect Ratio ◽  
Incompressible Fluid ◽  
Radial Velocity ◽  
Simple Form ◽  
High Aspect Ratio ◽  
Unknown Function ◽  
Axial Velocity ◽  
Velocity Fields ◽  
Single Row ◽  
Undetermined Constant

SummaryA method is described whereby, at any point in an infinite parallel annulus, the approximate axial velocity due to a single row of high aspect ratio blades may be calculated from a knowledge of the conditions of flow adjacent to the blades. The method is based on the assumption of a simplified expression for the radial velocity, being the product of an unknown function of the radius and an exponential term independent of the radius containing an undetermined constant; the function and the undetermined constant are calculated by reference to the conditions of flow in the plane of the row considered. The flow due to any number of rows is then obtained by summing the radial velocity fields due to each row and obtaining the axial velocities by integration of the equation of continuity.The solution of the problem with infinitely many rows is shown to have a simple form by virtue of the fact that the flow (provided that the velocities remain finite) settles down to a pattern which is periodic by one stage pitch.

scholarly journals The relative calibration of radial velocity for LAMOST medium resolution stellar spectra

2021 ◽  
Vol 21 (10) ◽  
pp. 265
Author(s):  
Jian-Ping Xiong ◽  
Bo Zhang ◽  
Chao Liu ◽  
Jiao Li ◽  
Yong-Heng Zhao ◽  
...  
Keyword(s):  
Radial Velocity ◽  
Time Domain ◽  
Binary Stars ◽  
Zero Point ◽  
Systematic Difference ◽  
Variable Stars ◽  
Systematic Bias ◽  
Wavelength Calibration ◽  
Stellar Spectra ◽  
Medium Resolution

Abstract The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) started a median-resolution spectroscopic (MRS, R ∼7500) survey since October 2018. The main scientific goals of MRS, including binary stars, pulsators and other variable stars, were launched with a time-domain spectroscopic survey. However, the systematic errors, including the bias induced from wavelength calibration and the systematic difference between different spectrographs, have to be carefully considered during radial velocity measurement. In this work, we provide a technique to correct the systematics in the wavelength calibration based on the relative radial velocity measurements from LAMOST MRS spectra. We show that, for the stars with multi-epoch spectra, the systematic bias which is induced from the exposures on different nights can be corrected well for LAMOST MRS in each spectrograph. In addition, the precision of radial velocity zero-point of multi-epoch time-domain observations reaches below 0.5 km s−1. As a by-product, we also give the constant star candidates**, which can be the secondary radial-velocity standard star candidates of LAMOST MRS time-domain surveys.

scholarly journals The empirical period-radius relation for pulsating stars: a synthesis based on photometric and radial velocity measurements

1986 ◽  
Vol 118 ◽  
pp. 273-274
Author(s):  
G. Burki
Keyword(s):  
Radial Velocity ◽  
Stellar Evolution ◽  
Variable Stars ◽  
Radial Velocities ◽  
Velocity Measurements ◽  
Pulsating Stars ◽  
Powerful Test ◽  
The Mean

The relation existing between the radius and the period for the pulsating stars of a given class constitutes a powerful test for the theory of stellar evolution and for the identification of the pulsation modes. In recent years, several authors have determined the mean radius of a lot of pulsating stars of various classes by applying the Baade-Wesselink method. Fig. 1 presents the resulting general logP - logR diagram grouping these determinations. The sources for the radii are given by Burki and Meylan (1986). The variable stars in known binaries have been excluded since the presence of a companion biases the radius calculation (Burki, 1984). The determinations marked by arrows are based on the radial velocities by CORAVEL (1m telescope at the Haute-Provence Observatory, France) or/and on the photometry in the Geneva system (40cm and 70cm telescopes at La Silla Observatory, Chile).

scholarly journals Retrieval of Urban Boundary Layer Structures from Doppler Lidar Data. Part I: Accuracy Assessment

2008 ◽  
Vol 65 (1) ◽  
pp. 3-20 ◽  
Author(s):  
Quanxin Xia ◽  
Ching-Long Lin ◽  
Ronald Calhoun ◽  
Rob K. Newsom
Keyword(s):  
Boundary Layer ◽  
Radial Velocity ◽  
Eddy Viscosity ◽  
Velocity Fields ◽  
Urban Boundary Layer ◽  
Flow Structures ◽  
Lidar Data ◽  
Velocity Data ◽  
Beam Direction ◽  
The Cross

Abstract Two coherent Doppler lidars from the U.S. Army Research Laboratory (ARL) and Arizona State University (ASU) were deployed in the Joint Urban 2003 atmospheric dispersion field experiment (JU2003) held in Oklahoma City, Oklahoma. The dual-lidar data were used to evaluate the accuracy of a four-dimensional variational data assimilation (4DVAR) method and to identify the coherent flow structures in the urban boundary layer. The objectives of the study are threefold. The first objective is to examine the effect of eddy viscosity models on the quality of retrieved velocity data. The second objective is to determine the fidelity of single-lidar 4DVAR and evaluate the difference between single- and dual-lidar retrievals. The third objective is to inspect flow structures above some geospatial features on the land surface. It is found that the approach of treating eddy viscosity as part of the control variables yields better results than the approach of prescribing viscosity. The ARL single-lidar 4DVAR is able to retrieve radial velocity fields with an accuracy of 98% in the along-beam direction and 80%–90% in the cross-beam direction. For the dual-lidar 4DVAR, the accuracy of retrieved radial velocity in the ARL cross-beam direction improves to 90%–94% of the ASU radial velocity data. By using the dual-lidar-retrieved data as a reference, the single-lidar 4DVAR is able to recover fluctuating velocity fields with 70%–80% accuracy in the along-beam direction and 60%–70% accuracy in the cross-beam direction. Large-scale convective roll structures are found in the vicinity of the downtown airport and parks. Vortical structures are identified near the business district. Strong up- and downdrafts are also found above a cluster of restaurants.

scholarly journals Precise Velocity Observation of K-giants: Evidence for Solar-Like Oscillations in Arcturus

1999 ◽  
Vol 170 ◽  
pp. 187-192 ◽  
Author(s):  
W. J. Merline
Keyword(s):  
Radial Velocity ◽  
Extrasolar Planets ◽  
Average Power ◽  
Power Spectra ◽  
Variable Stars ◽  
Data Sets ◽  
The Sun ◽  
Acoustic Oscillations ◽  
Point To Point ◽  
K Giants

AbstractHigh accuracy measurements of variations in the radial velocity of the K1 giant star Arcturus have been obtained. The observations span 5 years and have a point-to-point repeatability of 5 ms−1 and night-to-night stability of better than 20 ms−1. Velocity oscillations of Arcturus were discovered during the course of this work in 1986. Subsequent, extensive additional data, indicate that Arcturus is exhibiting global nonradial acoustic oscillations with characteristics similar to those occurring in the Sun.All observations were done using a radial velocity spectrometer, designed to search for extrasolar planets, at a dedicated facility of the University of Arizona on Kitt Peak. A dedicated facility was crucial to this work — because of the changing nature of the oscillations, many observing runs, over several years, were required to understand the star’s behavior. Continuous data sets as long as 30 days were acquired. Preliminary pulsation models were performed in collaboration of Art Cox at Los Alamos National Lab.The velocity power spectra are complicated and variable. There is substantial evidence that the variations are solar-like p-mode oscillations. At least 10 frequencies have been identified, over the range 8.3 to 1.7 days. A spectrum of evenly spaced modes is apparent, yielding a value for Δν0 ≈ 1.2 μ Hz. The average power spectrum peaks near 3 days. There is a broad envelope of power with a distribution reminiscent of that seen in the Sun. Both the mode spacing and the period of peak power are consistent with scaling from the Sun (Kjeldsen & Bedding 1995). The oscillations appear to undergo abrupt discontinuities and have phase coherence times of a few weeks. We interpret the driving to be due to stochastic excitation by convection.Recent observations of the G5 IV star η Boo by Brown et al. (1997) have failed to confirm the detection of p-mode oscillations reported by Kjeldsen et al. (1995) Thus, Arcturus may be one of the first stars known to exhibit solar-like oscillations. If other K-giant variables can be shown to exhibit similar oscillations, Arcturus may represent the prototype for a new class of variable stars. We know some other K-giants are variable on short time scales (Hatzes and Cochran 1994b; Edmonds & Gilliland 1996), but many are not (Horner 1996).

Sign in / Sign up

Export Citation Format

Share Document