Showing papers on "Spiral galaxy published in 1973"

The space distribution of the O stars in the solar neighborhood.

Abstract: New results pertaining to the two-dimensional spectral classification system for the O stars are presented. Classifications for additional stars in both hemispheres are given, and two groups of peculiar Of-like spectra are described and illustrated. Some refinements of the absolute-magnitude calibration are discussed. The space distribution projected into the galactic plane is derived for all nonpeculiar O stars with classifications. The most striking result is the clear separation of the Carina-Sagittarius arm from the local arm along the entire longitude range of 255 to 20 deg . The good agreement with previous results for other types of young objects provides evidence in support of the new luminosity classification for the O stars. (auth)

The evolution of galaxies. III - Metal-enhanced star formation

Abstract: The problem of the paucity of low-metal-abundance low-mass stars is discussed. One alternative to the variable-initial-mass-function (VIMF) solution is proposed. It is shown that this solution - metal-enhanced star formation - satisfies the classical test which prompted the VIMF hypothesis. Furthermore, with no additional parameters it provides improved fits to other tests - e.g., inhomogeneities in the abundances in young stars, concordance of all nucleo-cosmochronologies, and a required yield of heavy-element production which is consistent with current stellar evolution theory. In this model the age of the Galaxy is 18.6 plus or minus 5.7 b.y.

Relaxation time in disk galaxy simulations

Abstract: An initially stationary and stable axisymmetric disk of stars with a mass spectrum of stars is used to determine the collisional relaxation time. The relaxation time as determined from the rate of energy equipartition was found to be 560 rotation periods for stars with 10 times the mean star mass and was 1700 rotation periods for stars with 0.55 times the mean star mass. These times are in general agreement with theoretical predictions for the relaxation time of the two mass groups. The results show that the model used for the large-scale gravitational N-body calculations is indeed 'collisionless.'

Star formation and evolution in spiral galaxies.

Abstract: Evolutionary models for regions of M31 and M33 and the solar neighborhood are based on a stellar birthrate suggested by the dynamics of spiral structure: we assume that stars are formed very efficiently until the gas content reaches equilibrium at its present value, which takes about 1 by; thereafter, the birthrate just equals the rate at which gas enters the system from stellar mass-loss or infall of intergalactic matter Each model represents an average around a cylindrical-shell-shaped region, which is homogeneous and closed except for possible infall The disk and spiral-arm populations only are considered Each star is followed in the H-R diagram from the main sequence to death as an invisible remnant Integrated magnitudes, colors, mass-to-light ratio (M/L), gas content, helium and metal abundance (Z), are computed in steps of 1 by

On the existence of subdwarfs in the (M/subbol/, log T$sub 3$)-plane. III.

Abstract: A discussion of (R,I) observationg of parallax stars indicates that (1) the earlier conclusion that F- and G-type subdwarfs populate the Hyades main sequence in the (M/sub bol/, log T/sub e/,)-plane resulted from the evolved nature of the subdwarfs and (2) the later-type subdwarfs lie about 1 mag below the old-disk main sequence. In the range of R--I between +0.28 and +0.7 mag, which avoids large nuclear-evolution effects in hotter stars and contractual- evolution effects is cooler objects, the data presents two enigma: (1) the metal- abundance variation that successfully explains the large separation of the halo and disk main sequence appears not to be as effective for the smaller abundance differences among disk stars; and (2) although the range of values of ultraviolet excess within the disk population extends to the values observed in thc halo stars, the spectroscopic analyses show that all objects with space motions indicative of halo objects are metal deficient compared to the Sun, by factors near 100, whereas some old disk-population stars with comparable ultraviolet excess are metal deficient by considerably less than a factor of 10. These results depend on the assumption that the (log T/sub e/,, R--I)-relation is unaltered by the rangemore » of chemical compositions represented in the, parallax stars. (auth)« less

Aperture synthesis study of neutral hydrogen in the galaxies ngc 6946 and ic 342.

Abstract: : Synthesis observations of netural hydrogen emission in the two spiral galaxies NGC 6946 and IC 342 are presented. The velocity resolution is 21 km/s and the angular resolution is 2 and 4 min of arc for NGC 6946 and IC 342 respectively. Results are presented in the form of contour maps of the integral and mean velocity of the HI profile at each position in the galaxy, as well as a line profile for each galaxy as a whole. Both galaxies evidence a central depression in their HI distributions. The rotation curves have steep central gradients with rather constant velocities once the maximal velocity is reached. The HI mass for NGC 6946 is 21 billion solar mass units and for IC 342 is 15 billion solar mass units, assuming distances of 10.1 and 4.5 Mpc respectively. Other properties are summarized in the table.

Cosmic Turbulence and the Origin of Galaxies

Abstract: It is shown that, contrary to previous assertions, turbulence in the Universe cannot be supported against viscous decay after the epoch t/sub eq/ when the energy densities of matter and radiation are equal. Indeed, the decay is so rapid that for cosmological models with density parameter OMEGA h/sup 2/ > OMEGA /sub m/h/sup 2/ = 0.06 the turbulent edd ies cannot develop large density variations after the epoch of recombination. This value of OMEGA /sub m/h/sup 2/ is determined by making rather conservative estimates for the rate of decay of the turbulence, and it is not unlikely that future developments will lower its value still further. The problem of galaxy formation in cosmological models with OMEGA h/sup 2/ < OMEGA /sub m/h/sup 2/ is also considered. It is shown there th at a satisfactory picture for the origin of spiral galaxies can be produced, but only if the process of condensation of material into galaxies is very efficient. The theory has difficulty in accounting for the origin of the elliptical galaxies if these are systems of low specific angular momentum. An alternative theory of galaxy formation, based on the dissipation of strong turbulence of t/sub eq/, is presented. In thismore » theory, galaxies form from density perturbations which are developed at t/sub eq/ as a resnlt of the decay of turbulence, and the problems of the turbulence theories are thereby avoided. The theory has only one arbitrary parameter, OMEGA h/sup 2/ (which is at least one less than in other theories of galaxy formation presented so far), and for agreement with observation, OMEGA h/sup 2/ ~ 0.25. With this value, it is possible to explain the typical masses and angular momenta of large galaxies, and also their present mean densities. (auth)« less

Optical and Near-Infrared Observations of the Nearby Spiral Galaxy Maffei 2

Abstract: Spectra, photographs, and photometric measurements have been used to show that Maffei 2 has a distance of 5 ± 2 Mpc and that it has a morphological type near Sbc II-in agreement with similar conclusions made earlier by radio observers. We discuss the possible relationships between Maffei 2 and the elliptical galaxy Maffei 1; there are serious inconsistencies in the existing data which bear on this question.

Magnetic Field in the Galactic Spiral Arm

Abstract: New radio source rotation measure data, combined with the galactic radio continuum and optical polarisation data, are used to derive a refined model of the magnetic field in the spiral arm. Both the radio and optical data, when separated according to distance from the Sun, are consistent with a longitudinal magnetic field which is perturbed by an anomaly centred at northern galactic latitudes.

The "rolling Motion" Phenomenon in Galactic Spiral Arms

Abstract: The 'rolling motion' in the spiral arms in the outer parts of the Galaxy are interpreted as only apparent motions caused by the combined effect of the bending of the galactic plane and the differential rotation. Theoretical contour maps (latitude vs velocity) are plotted in a number of directions on both sides of the Galaxy for direct comparison with observations. The velocity and density fields used in the calculations are based on a linear analysis of the three-dimensional gas motions in response to the spiral density-wave potential studied by Lin, Shu, and Yuan and by Vandervoort (1970). It is found that the combined effect under consideration is sufficient to cause an apparent rolling motion in a spiral arm.

Fine-structure of the next inner spiral arm

Abstract: The spiral arms of the Galaxy are not just a phenomenon of the galactic plane. Because of the inaccuracy of current distance determinations of spiral tracers, any fine structure of the spiral arms has to be investigated by an analysis of the positions on the celestial sphere. Such fine structure perpendicular to the galactic plane is revealed by wide-angle photographs and photoelectric surface photometry of the Southern Milky Way in the ultraviolet. (WDM)

Predetonation Lifetime and Mass of Supernovae from Density-Wave Theory of Galaxies

Abstract: Stars in a normal spiral galaxy are formed in a shock front on the inner edge of a spiral arm according to nonlinear density-wave theory, then drift across the arm as they age. Over half of the 19 supernovae measured occur at the innermost edge of a spiral arm, implying a lffetime of less than five million years and mass greater than 35 solar masses. Key words: supernovae - density-waves in galaxies

The kinematics of galactic spiral structure

Abstract: An understanding of the states of motion of the stellar and gaseous components of spiral structure is fundamental to the problem of the origin and maintenance of spiral structure. It is important to determine the relative ordering of the parameters characterizing the several components of a spiral arm. In our own Galaxy in many cases, this can better be done in kinematic terms rather than spatially, since for the stars the results are then less sensitive to assumptions concerning the intervening absorption, while for the gas the results are less dependent on distance derivations. The data are becoming complete enough that, with appropriate analysis procedures, quantitative measures are available of the differential effects across spiral arms. Our understanding of a few of the various problems is less tentative for external galaxies than for our own. A substantial amount of the observational material can be satisfactorily interpreted in terms of the gravitational theory of density-waves. Furthermore, it is just this kinematic information which is most crucial for the observational confrontation of this theory.

Attempt to interpret the mean properties of the velocity field of young stars in terms of Lin's theory of spiral waves

Abstract: Fitting Lin's theory for a disk galaxy affected by a spiral perturbation of the potential to the observed velocities of young stars allows one to estimate the free parameters of the spiral waves. The results are inconsistent with the ones derived from the large scale spiral structure. They suggest that Lin' s theory may be unsuitable to the kinematics of young stars in the vicinity of the Sun. Various possible explanations are suggested. (auth)

Resonant Stellar Orbits in Spiral Galaxies

Abstract: This paper reviews a series of investigations of the orbits of stars in the regions of the Lindblad resonances of a spiral galaxy. The analysis is formulated in an epicyclic approximation. Analytic solutions of the epicyclic equations of motion are obtained by the method of harmonic balance of Bogoliubov and Mitropolsky. These solutions represent the resonance phenomena exhibited by the orbits in generally excellent agreement with numerical solutions.

Giant spiral galaxy m 101 .1. high-resolution map of neutral hydrogen

Abstract: S>A preliminary map is presented of the distribution of neutral hydrogen in the Sc I galaxy M 101, with 25'' x 30'' resolution. The H I is mostly arranged in narrow elongated structures, which closely coincide with the spiral arms seen in optical photographs. Several major concentrations of H I coincide with giant H II regions and/or associations; these concentrations have sizes of 1 x 3 kpc and masses approximately 40 x 10/sup 6/ M/sub sub solar /. In the central parts of the system, no small-scale structure in the hydrogen is detected. The system used in the present observations becomes progressively less sensitive to structures on angular scales larger than about 10'. (auth)

Search for Ap stars in open clusters

Abstract: Low-dispersion spectrograms of 62 stars in 13 open clusters were observed in the intrinsic color range -- 0.13 < B -- V < +0.06 in order to search for Ap stars. The purpose was to contribute to the statistics of the occurrence of Ap stars in homogeneous age groups, which is necessary as a critical test for some theories regarding the nature of Ap stars. Three new Ap stars were positively identified (all silicon-enhanced), and six other marginal candidates were noted. (auth)

Carbon Stars in the SMC Globular Clusters NGC 121, NGC 419 and Kron 3

Abstract: Spectroscopic observations of stars with (B-- V) near 2.0 or greater in NCC 121, NGC 419, and Kron 3 have revealed carbon stars in each cluster. Statistics of very red stars show that they are unlikely to be field stars. These carbon stars have M/sub v/ = --3.0. Some observations of the galactic globular cluster NGC 362 are also reported. (auth)

Variable Stars in Dwarf Spheroidal Galaxies

Abstract: Interest in dwarf spheroidal galaxies is motivated by a number of reasons; an important one on the occasion of this colloquium is the abundance of variable stars. The theory of stellar evolution and stellar pulsations is now able to predict from theoretical considerations characteristic properties of variable stars in the colour-magnitude diagram (Iben, 1971). By observing the variable stars in the field, and in as wide a selection of objects as possible, more insight can be obtained into the history of the oldest members of our Galaxy (the globular clusters) and of the dwarf spheroidal galaxies in the Local Group. It is worthwhile to explore the spheroidal galaxies as observational tests for the theoretical predictions of conditions in space away from our Galaxy. The numbers of variable stars in the dwarf spheroidal galaxies are such that we may expect well-defined relations to emerge once reliable magnitude sequences have been set up, the variable stars found, and their periods determined. Six dwarf spheroidal galaxies are presently known in the Local Group within a distance of 250 kpc. In Table I, which lists members of the Local Group, they are at the low-luminosity end of the sequence of elliptical galaxies (van den Bergh, 1968).

Further high-resolution radio continuum observations of bright spiral galaxies at 1415 MHz

Abstract: Observations of NGC 4594 (M104), 5866, IC 342, NGC 4321 (M100), 3556 (M108), 628 (M74), 7331, 4939, 1300, 5383, 7640, (WDM)

High-resolution Radio Continuum Observations of Bright Spiral Galaxies in an Optically Complete Sample at 1415 MHz

Abstract: This paper is one in a series in which studies of the radio continuum brightness distribution in bright, nearby spiral galaxies with the Westerbork Synthesis Radio Telescope (WSRT) are reported. The observing frequency is 1415 MHz and the bandwidth is 4 MHz. The synthesized beam is elliptical and the half- power beamwidths at full resolution are 23'' (RA) and 23''/sin sigma (Dec). Observations are presented of NGC 598 (M33), 4826, 3031 (M81), published results comprise all fourteen spiral galaxies brighter than Harvard magnitude 10.5 above declination + 15 deg . The observed distributions have been used to derive, apart from individual interesting features, three important parameters. These are the flux density of the nuclear source, the spiral arm component, and the base disk. (WDM)