scholarly journals Interaction between the Galactic Disk and Halo Components

1977 ◽  
Vol 45 ◽  
pp. 241-246 ◽  
Author(s):  
Jeremiah P. Ostriker
Keyword(s):  
Rotational Motion ◽  
Galactic Disk ◽  
Stellar Mass ◽  
Point Of View ◽  
The Sun ◽  
Disk System ◽  
The Galaxy ◽  
Metal Abundance ◽  
Random Motions ◽  
Disk Component

At least three component parts of the galaxy must be recognized. TheDisk Componentof the galaxy might be defined as follows. Spatially it is largely confined between the planes ± 1 kpc from the plane of symmetry. With regard to velocities, it is acoldsubsystem in that the random motions within it (~ 20 km/s) are small compared to the systematic flow of rotational motion (~ 200 km/s). Finally, its composition is largely stellar with stars of all ages and masses being present. Few galaxies are known where the ratio of (gas/stellar) mass is > 10% (cf. Roberts 1975a), and the metal abundance is typically high with at most one percent of the stars having metallicity less than 1/4 that of the Sun (cf. Schmidt 1963). From this point of view the spiral parts are a relatively unimportant (in terms of mass and composition) sub-part of the disk system.

scholarly journals The Evolution of 3He, 4He and D in the Galaxy

2000 ◽  
Vol 198 ◽  
pp. 540-546 ◽  
Author(s):  
Cristina Chiappini ◽  
Francesca Matteucci
Keyword(s):  
Chemical Evolution ◽  
Present Model ◽  
Galactic Disk ◽  
The Sun ◽  
Mixing Process ◽  
Low Mass Stars ◽  
A Value ◽  
The Galaxy ◽  
Low Mass ◽  
Standing Problem

In this work we present the predictions of a modified version of the ‘two-infall model’ (Chiappini et al. 1997 - CMG) for the evolution of 3He, 4He and D in the solar vicinity, as well as their distributions along the Galactic disk. In particular, we show that when allowing for extra-mixing process in low mass stars (M < 2.5 M⊙), as predicted by Charbonnel and do Nascimento (1998), a long standing problem in chemical evolution is solved, namely: the overproduction of 3He by the chemical evolution models as compared to the observed values in the sun and in the interstellar medium. Moreover, we show that chemical evolution models can constrain the primordial value of the deuterium abundance and that a value of (D/H)p < 3 × 10—5 is suggested by the present model. Finally, adopting the primordial 4He abundance suggested by Viegas et al. (1999), we obtain a value for ΔY/ΔZ ≃ 2 and a better agreement with the solar 4He abundance.

scholarly journals Determination of the C/M5+ Ratio in the Galactic Disk

1995 ◽  
Vol 148 ◽  
pp. 276-279
Author(s):  
Francisco J. Fuenmayor
Keyword(s):  
Near Infrared ◽  
Galactic Center ◽  
Galactic Disk ◽  
Mean Value ◽  
Galactocentric Distance ◽  
The Galaxy ◽  
Metal Abundance ◽  
Objective Prism ◽  
M Stars

AbstractA determination of the C/M5+ ratio, as a function of the galactocentric distance, in the galactic disk is presented. These results are based upon previous determinations of the space density for cool carbon stars and for late giant M stars in the Milky Way. Most of these results were obtained from objective-prism surveys in the near infrared using mainly Schmidt-type telescopes. The ratio C/M5+ appears to increase from 0.05 to 0.25 in the galactic disk, from the galactic center outwards. A mean value of 0.15 of this ratio for the Galaxy is suggested. Correlations between the C/M5+ ratio and currently known metal abundance gradients in the galactic disk are discussed.

scholarly journals Dark Matter Near the Sun: Simulated Star Counts and the Oort Limit

1984 ◽  
Vol 81 ◽  
pp. 326-329
Author(s):  
David Gilden ◽  
John N. Bahcall
Keyword(s):  
Dark Matter ◽  
Galactic Disk ◽  
The Sun ◽  
Sampling Errors ◽  
Specific Mass ◽  
Mass Model ◽  
Density Distributions ◽  
Self Consistent ◽  
The Galaxy ◽  
Solar Position

AbstractAn ensemble of orbits passing through the solar position have been generated for a specific mass model of the galaxy. These orbits are randomly sampled to form simulated density distributions of tracer stars perpendicular to the galactic disk. The simulated distributions are analyzed in order to determine the sampling errors in a self-consistent derivation of the total amount of matter near the sun (the Oort limit).

Approaches of stars to the Sun

1999 ◽  
Vol 173 ◽  
pp. 345-352 ◽  
Author(s):  
P.A. Dybczyński ◽  
P. Kankiewicz
Keyword(s):  
Minimum Distance ◽  
Galactic Disk ◽  
Equations Of Motion ◽  
Oort Cloud ◽  
The Sun ◽  
The Past ◽  
Straight Line ◽  
The Galaxy ◽  
Hipparcos Catalogue ◽  
Nearby Stars

AbstractClose approaches of stars to the Solar System perturb comets from the Oort cloud so that they pass into the planetary system − the gravitational impulse changes the distribution of observable comets. This paper presents the results of calculations of the motion of stars in the solar neighbourhood in the past and future. The main results for each star are: the time of the encounter and the minimum distance between the Sun and the star. They are calculated using three different methods: a straight line motion model, a model with a Sun − star Keplerian interaction, and the numerical integration of the equations of motion with galactic perturbations included. In the last case, two models of the Galactic potential are used: a simplified potential of the Galactic disk and the more complex potential of the Galaxy by Dauphole and Colin. Coordinates and velocities of nearby stars are taken from several different catalogues: the Gliese catalogue, the Hipparcos catalogue, and the Barbier-Brossat catalogue of Radial Velocities.

scholarly journals Global Instabilities of Disks

1975 ◽  
Vol 69 ◽  
pp. 297-320 ◽  
Author(s):  
J. M. Bardeen
Keyword(s):  
Kinetic Energy ◽  
Local Stability ◽  
Galactic Disk ◽  
The Sun ◽  
Global Instability ◽  
Precise Form ◽  
The Galaxy ◽  
Central Bulge ◽  
Theoretical Evidence ◽  
The Stability

Current understanding of the stability of gas and stellar disks suggests very strongly that local stability to axisymmetric modes is not sufficient for global stability. A global instability to a bar mode will develop unless the rotational kinetic energy is sufficiently small compared with the random kinetic energy for the system as a whole. A disk as cool as the galactic disk near the Sun can survive only if most of the mass of the Galaxy is in a ‘hot’ component, such as a central bulge and/or an extended halo. We review the theoretical evidence for this conclusion coming from analytic results for simple gas and stellar disks, from numerical simulations of stellar disks, and from numerical calculations of the stability of gas disks. Some new results on the precise form of dynamic bar instabilities of gas disks with and without halos are reported.

scholarly journals The Hot Galactic Corona and the Soft X-ray Background

1997 ◽  
Vol 166 ◽  
pp. 503-512
Author(s):  
Q. Daniel Wang
Keyword(s):  
Galactic Disk ◽  
Relative Dispersion ◽  
Polytropic Index ◽  
The Sun ◽  
X Ray ◽  
Large Intensity ◽  
The Galaxy ◽  
Electron Density And Temperature ◽  
X Ray Background ◽  
X Ray Absorption

AbstractI characterize the global distribution of the ¾ keV band background with a simple model of the hot Galactic corona, plus an isotropic extragalactic background. The corona is assumed to be approximately polytropic (index = 5/3) and hydrostatic in the gravitational potential of the Galaxy. The model accounts for X-ray absorption, and is constrained iteratively with the ROSAT all-sky X-ray survey data. Regions where the data deviate significantly from the model represent predominantly the Galactic disk and individual nearby hot superbubbles. The global distribution of the background, outside these regions, is well characterized by the model; the 1σ relative dispersion of the data from the model is ~ 15%. The electron density and temperature of the corona near the Sun are ~ 1.1 × 10−3 cm−3 and ~ 1.7 × 106 K. The same model also explains well the 1.5 keV band background. The model prediction in the ¼ keV band, though largely uncertain, qualitatively shows large intensity and spectral variations of the corona contribution across the sky.

scholarly journals Dark Matter in the Galactic Disk

1987 ◽  
Vol 117 ◽  
pp. 17-31 ◽  
Author(s):  
John N. Bahcall
Keyword(s):  
Dark Matter ◽  
Proper Motion ◽  
Column Density ◽  
Galactic Disk ◽  
Volume Density ◽  
Scale Height ◽  
The Sun ◽  
Vlasov Equations ◽  
Disk Mass ◽  
The Galaxy

The Poisson and Vlasov equations are solved self-consistently for realistic Galaxy models which include multiple disk components, a Population II spheroid, and an unseen massive halo. The total amount of matter in the vicinity of the Sun is determined by comparing the observed distributions of tracer stars, samples of F dwarfs and of K giants, with the predictions of the Galaxy models. Results are obtained for a number of different assumed distributions of the unseen disk mass. The major uncertainties, observational and theoretical, are estimated. For all the observed samples, typical models imply that about half of the mass in the solar vicinity must be in the form of unobserved matter. The volume density of unobserved material near the Sun is about 0.1M⊙pc−3; the corresponding column density is about 30M⊙pc−2. This so far unseen material must be in a disk with an exponential scale height of less than 0.7 kpc. If the unseen material is in the form of stars with masses less than 0.1M⊙, then the nearest such object is about 1 pc away and has a proper motion of more than 1 arcsecond per year.

scholarly journals A Radio Survey of the Southern Milky Way at a Frequency of 1440 Mc/s. II. The Continuum Emission from the Galactic Disk

1962 ◽  
Vol 15 (3) ◽  
pp. 369 ◽  
Author(s):  
DS Mathewson ◽  
JR Healey ◽  
JM Rome
Keyword(s):  
Milky Way ◽  
Spiral Structure ◽  
Galactic Disk ◽  
Neutral Hydrogen ◽  
Continuum Emission ◽  
Nonthermal Radiation ◽  
The Galaxy ◽  
Disk Component ◽  
Good Agreement ◽  
The Continuum

The 1440 Mcls survey (Part I of this series) has been used in conjunction with the 85�5Mc/s survey of Hill, Slee, and Mills (1958) to delineate the distribution of the thermal and nonthermal radiation from the disk component of the Southern Milky Way and so complete an investigation commenced by the Northern Hemisphere observers Westerhout (Leiden) and Large, Mathewson, and Haslam (Jodrell Bank). Results of the analysis show an intense concent,ration of ionized hydrogen in an irregular spiral structure in the inner regions of the Galaxy. From lII=256� to 88�, good agreement was obtained between the longitudes at which concentrations of neutral hydrogen were found to occur from H-line studies and the longitudes at which the ionized hydrogen was concentrated. The steps in the longitude distribution of the 85�5 Mcls radiation which Mills used to delineate the spiral arms of the Galaxy were not all visible in the longitude distribution of the nonthermal component obtained from this present analysis. It is believed that three of Mills's steps are thermal in origin.

scholarly journals Absolute Magnitudes of RR Lyrae Stars

1971 ◽  
Vol 2 ◽  
pp. 777-780 ◽  
Author(s):  
Robert F. Christy
Keyword(s):  
Fixed Points ◽  
Point Of View ◽  
Stellar Structure ◽  
Neutrino Experiment ◽  
The Sun ◽  
Rr Lyrae Stars ◽  
Rr Lyrae ◽  
The Galaxy ◽  
Absolute Magnitudes ◽  
Lyrae Stars

In discussing the absolute magnitudes of RR Lyrae stars, I concur in the importance of the question but I would, at the outset, like to insert a word of caution: It seems to me most likely that the Mb of RR Lyrae stars is not an immutable constant but depends on the original composition of the star. I will come back to this point later.The principal point of view which I shall emphasize is rather different from that of most observers. The observer is usually interested in using the RR Lyrae stars as a means to calibrate distances and thereby looks out from the star to the Galaxy and then to the Cosmos. In contrast, the interests of the theorist look inward and he views the RR Lyrae stars as convenient fixed points for the comparison of theory with observation.At present, the theory of stellar structure is tied to fitting the Sun. We all know the difficulties in the solar neutrino experiment which casts some doubt on whether we even understand the Sun. But we have few if any cases of evolved stars where we know M, L, R, to check our stellar evolution calculations.We are now in desperate need of new fixed points where we can compare observation and the theory of stellar structure and evolution. The RR Lyrae or Cepheid type variables provide excellent examples for this comparison of observation and calculation. They are in late stages of evolution where tests are needed, they are readily identifiable, and well observed, and already the knowledge of the period provides a very precise mass-radius relationship so that a complete determination of the model will be provided by only a few additional measures.

Galactic orbits of stars

1966 ◽  
Vol 25 ◽  
pp. 93-97
Author(s):  
Richard Woolley
Keyword(s):  
Globular Cluster ◽  
Potential Field ◽  
High Velocity ◽  
Velocity Vector ◽  
Variable Stars ◽  
The Sun ◽  
Proper Motions ◽  
Rr Lyrae ◽  
The Galaxy

It is now possible to determine proper motions of high-velocity objects in such a way as to obtain with some accuracy the velocity vector relevant to the Sun. If a potential field of the Galaxy is assumed, one can compute an actual orbit. A determination of the velocity of the globular clusterωCentauri has recently been completed at Greenwich, and it is found that the orbit is strongly retrograde in the Galaxy. Similar calculations may be made, though with less certainty, in the case of RR Lyrae variable stars.

Sign in / Sign up

Export Citation Format

Share Document