Mean Rotational Velocity Research Articles

A representative sample of Be stars

We present K band (2.05 m 2.22m) spec- tra of 66 isolated Be stars of spectral types O9-B9 and luminosity classes III, IV & V. We nd that objects with He i features either in emission or absorption are B3 or earlier. Objects with Mg ii emission but no He i are B2 to B4, while objects with Br emission but no evidence of He i or Mg ii are B5 or later. Na i emission in the spec- tra of 4 objects appears to indicate that regions of the circumstellar envelopes of these stars must be shielded from direct stellar radiation. Systematic trends in the line strength and prole of Br are seen from early to late spectral types which can be understood in terms of dif- ferences in the disc temperature and density. 30 per-cent of the stars do not currently show evidence for line emis- sion. Compared to the emission line stars these objects have a signicantly lower mean rotational velocity and a distribution of spectral types that is signicantly earlier. This can be explained either as the original misidentica- tion of these objects as Be stars (i.e. they never had line emission), or as evidence that stars with lower rotational velocities may be more prone to changes between the Be and B phases.

Spectroscopic binaries in the Alpha Persei cluster

An average of 16 radial velocity measures for each of the 28 brightest (B3-A2) cluster members and found four binaries was obtained. The resulting binary frequency of 14 percent is, like the previous 20 percent for the B6-A1 stars in the Pleiades, unusually low compared with a typical 30 percent for early-type field stars or with 30 percent or more in other open clusters. These two clusters are the only known ones with unusually high mean rotational velocities. It is suspected that the mean rotational velocities are high either because these clusters lack short-period binaries or are not old enough for synchronization of rotational and orbital velocities to have occurred. The four Alpha Persei binaries are all relatively wide ones (greater than 20 d) and with small mass ratios (0.1-0.5). The same results apply to the young Orion Nebula cluster. These results can be explained in terms of the formation of binaries by capture in that during the first free-fall time, capture will produce wide binaries with small mass ratios, as in the Orion Nebula and Alpha Persei clusters, but repeated captures and disruptions will produce more closely spaced binaries with many mass ratios near 1.0, as in IC 4665.

Spectroscopic binaries in the Orion Nebula cluster

A search was made for spectroscopic binaries among the 26 brightest members of this very young cluster. Orbital elements are given for four binaries (three known binaries, one newly discovered), and one star is a probable binary. In agreement with published predictions from n-body simulations for binary formation by capture, it is found that the spectroscopic binaries have long periods (median of 20 days), have low-mass companions, are not rotating synchronously with their orbital motions even for a period of 6.5 days, and are probably not concentrated toward the cluster center more than are the massive single stars. The binary frequency is probably normal for a cluster with a high mean rotational velocity. 38 refs.

A survey of proper-motion stars. X - The early evolution of the Galaxy's halo

view Abstract Citations (158) References (80) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Survey of Proper-Motion Stars. X. The Early Evolution of the Galaxy's Halo Carney, Bruce W. ; Latham, David W. ; Laird, John B. Abstract The recent history of work bearing on the early chemical/dynamical evolution of the Galaxy, and the relation of the halo population to the disk population(s) are discussed. In particular, the mean rotational velocity v(rot), and planar orbital eccentricity (e) are considered as functions of time, inferred from mean metallicity. For m/H less than about -1, signs of dynamicl evolution in mean v(rot) or mean e are not detected. It is argued that (e) for metal-poor stars is about 0.6, not 0.8 as previously believed, and is even lower in the three-dimensional case. It is pointed out that all the data are consistent with the high-velocity, low metallicity halo population having had a chemical and dynamical history almost independent of the Galactic disk. It is proposed that the halo was assembled from mergers of small satellites with the Galaxy along the lines discussed by Searle and Zinn (1978). It is further proposed that the Galactic bulge originated from the gaseous outflow from the Galaxy's halo and these fragments. Publication: The Astronomical Journal Pub Date: February 1990 DOI: 10.1086/115351 Bibcode: 1990AJ.....99..572C Keywords: Galactic Evolution; Halos; Sky Surveys (Astronomy); Stellar Motions; Angular Velocity; Chemical Evolution; Milky Way Galaxy; Astronomy; GALAXIES: THE GALAXY; STARS: POPULATION II; STARS: SUBDWARFS; STARS: HIGH-VELOCITY full text sources ADS | data products SIMBAD (86) Related Materials (12) Part 1: 1987AJ.....93..116C Part 2: 1987AJ.....94.1066C Part 3: 1988AJ.....95.1843L Part 4: 1988AJ.....96..378C Part 5: 1988AJ.....96..560C Part 6: 1988AJ.....96..567L Part 7: 1988AJ.....96.1908L Part 8: 1989AJ.....97..423C Part 10: 2001AJ....122.3419C Part 11: 2002AJ....124.1132G Part 12: 2002AJ....124.1144L Part 13: 2005AJ....129.1886C

Structure of the turbulent flow field under breaking waves in the surf zone

The structure of turbulence and its role in the breaking wave dynamics within the surf zone have been investigated through laboratory experiments using several flow visualization techniques and a fibre-optic LDV system. The results indicate that there exists a characteristic structure of large-scale eddies referred to here as ‘horizontal eddies’ and ‘obliquely descending eddies’, which has a significant role in the generation of Reynolds stress and thus affects the deformation of the mean flow field. The experiments also reveal that these eddies caused by the wave breaking bring a large amount of vorticity (with non-zero average) into otherwise almost irrotational velocity fields, resulting in the generation of vorticity-related mean flow fields as well as turbulence (vorticity-containing velocity fluctuation). This means that the breaking waves in the surf zone can be regarded as pseudowaves which consist of irrotational velocity components as ‘wave motion’ and appreciable amounts of rotational mean velocity components as ‘eddying motion’ (with non-zero mean vorticity) together with turbulence. It is found that the generation of the mean rotational velocity component due to wave breaking causes considerable increase in mass and momentum transport, as compared with ordinary non-breaking waves, and thus a decrease in wave height.

ANGULAR MOMENTUM IN STARS - THE KRAFT CURVE REVISITED

ABSTRACT The author has rederived the relationship between angular momentum and stellar mass for stars earlier than F0 using current stellar models and rotational velocities. The form of the relationship is the same as found by Kraft (1970), but that the exponent of the power-law fit is higher. He shows that the derived relation is consistent with the mean rotational velocity being the same fraction of the equatorial breakup velocity for all masses. By extending this correlation to low masses, he estimates the mean initial angular momentum for low-mass stars.

A SPECTROSCOPIC STUDY OF THE OPEN CLUSTER NGC 2281.

ABSTRACT Photographic spectra of 16 stars in NGC 2281 (= C0645+411) have been used to derive a mean E(B-V) = 0.11 mag and a true distance modulus of 8.3, in agreement with earlier studies. Radial velocities of most of the same stars were derived from low-resolution CCD spectra to yield a mean cluster velocity of +5 km/s. Rotational velocities estimated from low-dispersion photographic spectra yield a distribution of the mean v sin i's as a function of absolute magnitude typical of intermediate-age open clusters. The latter result suggests that stellar evolutionary effects largely eliminate differences in the cluster mean rotational velocity distributions by the time clusters are between 50-100 x 10 to the 6th years old.

The equilibria of rotating isothermal clouds

view Abstract Citations (39) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Equilibria of Rotating Isothermal Clouds Kiguchi, Masayoshi ; Narita, Shinji ; Miyama, Shoken M. ; Hayashi, Chushiro Abstract The effect of rotation on the equilibrium structure of an axisymmetric isothermal gas cloud embedded in a stationary external medium is investigated by means of numerical simulations. The underlying assumptions, basic equations, input parameters, and numerical approach are explained, and the results are presented in extensive tables and graphs and characterized in detail. The critical central density and rotation energy, beyond which clouds become unstable to global contraction/expansion and ring formation, respectively, are found to be 800 times the boundary-surface density (BSD) and 0.44 times the gravitational energy. Stable rotating clouds are shown to have maximum mass 31 times that of nonrotating clouds, maximum mean rotation velocity 2.7 times the sound speed, and maximum mean density 6 times BSD. An expression for the maximum height of the boundary surface above the equatorial plane is derived. Publication: The Astrophysical Journal Pub Date: June 1987 DOI: 10.1086/165332 Bibcode: 1987ApJ...317..830K Keywords: Interstellar Gas; Isothermal Processes; Molecular Clouds; Nebulae; Rotating Fluids; Angular Momentum; Angular Velocity; Gas Density; Velocity Distribution; Astrophysics; INTERSTELLAR: MOLECULES; NEBULAE: GENERAL; ROTATION full text sources ADS |

Measurements of Apparent Mass Torque Coefficients of Two-Dimensional Centrifugal Impellers

Apparent mass torque coefficients for fluctuations of flow rate and angular velocity are determined experimentally for two-dimensional centrifugal impellers. Nearly sinusoidal fluctuations of flow rate and angular velocity are produced by using crank mechanisms, and the resulting unsteady torque on the impeller is measured. The torque is divided into components in-phase and out-of-phase with the displacements. The in-phase components are used to determine the apparent mass coefficients. Drag torque coefficients are defined and used to represent the out-of-phase components. The tests are conducted under various frequencies and amplitudes of the fluctuations with zero mean flow rate and rotational velocity. The apparent mass torque coefficients are compared with theoretical values obtained under the assumption of a two-dimensional potential flow. The experimental values are 5 to 20 percent larger than the theoretical ones and no appreciable effects of the frequency and the amplitude are observed within the range of the experiments.

Viscosity, granular‐temperature, and stress calculations for shearing assemblies of inelastic, frictional disks

Employing nonequilibrium molecular‐dynamics methods the effects of two energy loss mechanisms on viscosity, stress, and granular‐temperature in assemblies of nearly rigid, inelastic frictional disks undergoing steady‐state shearing are calculated. Energy introduced into the system through forced shearing is dissipated by inelastic normal forces or through frictional sliding during collisions resulting in a natural steady‐state kinetic energy density (granular‐temperature) that depends on the density and shear rate of the assembly and on the friction and inelasticity properties of the disks. The calculations show that both the mean deviatoric particle velocity and the effective viscosity of a system of particles with fixed friction and restitution coefficients increase almost linearly with strain rate. Particles with a velocity‐dependent coefficient of restitution show a less rapid increase in both deviatoric velocity and viscosity as strain rate increases. Particles with highly dissipative interactions result in anisotropicpressure and velocity distributions in the assembly, particularly at low densities. At very high densities the pressure also becomes anisotropic due to high contact forces perpendicular to the shearing direction. The mean rotational velocity of the frictional disks is nearly equal to one‐half the shear rate. The calculated ratio of shear stress to normal stress varies significantly with density while the ratio of shear stress to total pressure shows much less variation. The inclusion of surface friction (and thus particle rotation) decreases shear stress at low density but increases shear stress under steady shearing at higher densities.

A photometric survey of the Hyades for Delta Scuti variables

view Abstract Citations (34) References (21) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A photometric survey of the Hyades for delta Scuti variables. Horan, S. Abstract A photoelectric survey of fourteen normal Hyads of spectral types A-F within the Delta Scuti instability strip has revealed that 36% of these stars are variable. With previously discovered variables taken into account, the minimum incidence of Delta Scuti variability in the cluster is 37%. This is consistent with the results found in other galactic clusters. An additional variable, 60 Tauri, with spectral classification Am was detected during this survey. 60 Tauri lies within the predicted instability strip for Am stars as given by Cox, King, and Hodson (1979). No significant differences in the mean rotational velocities were found between the variable and constant stars of normal A-F spectral types. Publication: The Astronomical Journal Pub Date: November 1979 DOI: 10.1086/112607 Bibcode: 1979AJ.....84.1770H Keywords: Electrophotometry; Galactic Clusters; Stellar Spectrophotometry; Variable Stars; Angular Velocity; Classifications; Spectral Energy Distribution; Stellar Rotation; Astronomy; Delta Scuti Stars:Hyades; Delta Scuti Stars:Open Clusters; Delta Scuti Stars:Rotation full text sources ADS | data products SIMBAD (27)

The frequencies of binaries on the main sequence

Solar-type dwarfs and their evidence are discussed for two types of binaries: (1) the bifurcation doubles with periods less than about 100 y and frequencies of secondary masses that vary as M/sub 2//sup 0.4/ and (2) the independent-condensation doubles of longer periods having secondaries that follow the van Rhijn frequency distribution. Integrations of these distributions of secondaries indicate that more than half of the primaries have nearby stellar companions and more than half have distant stellar companions. The high-velocity stars have few nearby companions but a normal frequency of distant companions. The most likely explanation for these properties are that metal-poor protostars are slow in radiating away excess energy and therefore slow in contracting; they may be able to lose angular momentum fast enough so that they need not bifurcate to dispose of the initial excess. The properties of binaries among B2--B5 dwarfs are very similar to those of the solar-type dwarfs in (1) their distribution of orbital periods, (2) the occurrence of two types of binaries, (3) the break period between the two types, and (4) the frequency distribution of secondary masses for bifurcation doubles. These results imply that the binary characteristics of normal stars may not depend sensitivelymore » upon primary mass. The published data on the binary frequencies in clusters of rapidly-rotating and slowly-rotating stars are reviewed in a different way and the evidence implies the existence of an inverse correlation between duplicity and mean rotational velocity. Whether clusters are rich or poor in binaries compared with field stars yields the provisional answer that they are somewhat deficient in binaries.« less

THE PLEIADES AND THE ZERO-AGE MAIN SEQUENCE.

ABSTRACT New four-color uvby observations of the Pleiades are used to calibrate the ZAMS in the spectral range A2 to F5. Although the Pleiades Co versus (b - y)o calibration is in good agreement with the ZAMS calibration presented by , its versus (b - y)o relation shows a systematic difference from his values. This difference is ?o be caused by the higher mean rotational velocity of the Pleiades stars in comparison to field stars. Key words: zero-age main sequence (ZAMS) - uvby photometry

The binary frequency of IC 4665.

New radial velocity observations of 13 stars in IC 4665 indicate that the spectroscopic binary frequency in this cluster is 50 percent. It is suggested that the high frequency (95Th) previously found by Abt, Bolton, and Levy is a spurious one. The methods of detecting variable velocities and comparing binary frequencies are discussed and applied to IC 4665 and other clusters. On this basis, we conclude that the correlation between mean rotational velocity and binary frequency in clusters is marginal. Subject headings: clusters: open - stars: binaries - stars: rotation

A spectroscopic study of the open cluster M39.

view Abstract Citations (38) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A spectroscopic study of the open cluster M39. Abt, H. A. ; Sanders, W. L. Abstract M39 is found to be intermediate among open clusters in its mean stellar rotational velocity and its frequency of binaries. A study of the 15 brightest members yielded orbital elements for four spectroscopic binaries; one other star has double lines. For six well-studied clusters there is a welldefined inverse correlation between mean rotational velocity and frequency of binaries with periods less than about 10 days; if longer-period binary motion is also effective in reducing rotational velocities, then the results imply preferential inclinations for some clusters. Subject headings: binaries - open clusters - rotation, stellar Publication: The Astrophysical Journal Pub Date: November 1973 DOI: 10.1086/152486 Bibcode: 1973ApJ...186..177A full text sources ADS | data products SIMBAD (16)

An experimental investigation of a magnetically driven rotating liquid-metal flow

Flow and turbulence in a 50 Hz rotating-field MHD system are investigated using the hot-film constant-temperature anemometer. Factors affecting anemometer disturbances and response time are discussed. From measurements of the magnetic field at points within the liquid the distribution of MHD forces is estimated. The mean rotational velocity of the flow is of the expected order of magnitude but much less dependent on the axial co-ordinate than the corresponding MHD force. With the aid of a thermal transit-time anemometer, a weak secondary flow is detected. A note on scale-model studies of MHD systems envisaged in metallurgical applications of magnetohydrodynamics points out some basic difficulties in modelling large high-powered systems on a small scale.

Spectroscopic Binaries in the Open Cluster NGC 2516.

view Abstract Citations (41) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Spectroscopic Binaries in the Open Cluster NGC 2516. Abt, Helmut A. ; Levy, Saul G. Abstract A limited study of the radial velocities of the 16 brightest early-type stars in NGC 2516 yields approximate periods for six binaries. Tidal interaction in that number of binaries is insufficient to explain the low mean rotational velocity obtained for the cluster stars, but allowance for the additional Ap stars, in which decreased rotation by magnetic braking may have occurred, does account for the low rotational velocities. Publication: The Astrophysical Journal Pub Date: March 1972 DOI: 10.1086/151353 Bibcode: 1972ApJ...172..355A full text sources ADS | data products SIMBAD (30)

Rotational Velocities in the Orion Nebula Cluster

view Abstract Citations (18) References (15) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Rotational Velocities in the Orion Nebula Cluster Abt, Helmut A. ; Muncaster, George W. ; Thompson, Laird A. Abstract Rotational velocities for 26 stars in the Orion Nebula cluster show: (i) the unusually low mean rotational velocity for the early B stars that is a characteristic of the Orion association, (11) higher rotational velocities for the middle and late B stars than in the association, and (ill) very high rotational velocities among A stars relative to similar field stars. it is not known whether the high rotational velocities are a peculiarity of the Orion stars or a characteristic of all B- and A-type stars that have recently arrived on the main sequence. Publication: The Astronomical Journal Pub Date: December 1970 DOI: 10.1086/111062 Bibcode: 1970AJ.....75.1095A full text sources ADS | data products SIMBAD (29)

The Rotation of the Ap Stars From the Point of View of the Rigid Rotator Model

AbstractDeutsch’s period vs. line-width relation for the periodic Ap stars is re-examined from the point of view of the rigid rotator with the aid of recently determined values of v sin i for these objects. The agreement between computed rotational velocities and observed values of v sin i is satisfactory if the radii of the Ap stars are twice those of zero-age main sequence stars of the same color. The computed rotational velocities also agree with the mean rotational velocities of all Ap stars if suitable allowances are made for observational limitations. However, a difficulty arises in connection with the number of ‘long-period’ stars that have been discovered. Their small computed rotational velocities and their frequency are such that they cannot be regarded as part of any rotational distribution function that describes normal stars or even most Ap stars. It is concluded that if the periods of these objects are rotational periods, then a powerful rotational deceleration mechanism must be operative in at least some Ap stars, primarily those of the SrCrEu group.

The theory of spiral structure of galaxies

A Landau-type instability mechanism for generating spiral waves is suggested.Two populations of stars, Populations I and II, are considered, the second one with mean rotational velocity zero. Then a dispersion relation is derived which is reduced to the Lin-Shu dispersion relation in the case of vanishing Population II. The amplification of the wave is of the same type as the two-stream instability. It occurs if the angular velocity of the spiral pattern Ωs is smaller than the angular velocity of the Population I stars. A value of Ωs = 22–25 km s−1 kpc−1 was found, as well as the growth parameter. Spiral arms are formed in 108–109 yr, while trailing and leading waves grow at the same rate.A quasi-linear theory is developed to account for the limited growth of the spiral waves.Detailed accounts of the theory and of its implications are contained in recent publications (Marochnik, 1969; Marochnik and Suchkov, 1969a; 1969b; Marochnik and Ptitzina, 1969; Marochnik et al., 1969).