共查询到20条相似文献,搜索用时 31 毫秒
1.
S. Ortolani 《Astrophysics and Space Science》1999,265(1-4):355-359
In this paper we discuss the characteristics of the stellar content of the galactic bulge excluding the stars within a few
parsec from the galactic center. The bulge clusters and the field stars are comparedto the disk population. A scenario with
a flattened bulge extending toabout 3–4 Kpc from the galactic center is presented. There is evidencefor an old bulge stellar
population, decoupled from the disk.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
F. Delplancke F. Derie F. Paresce A. Glindemann F. Lévy S. Lévêque S. Ménardi 《Astrophysics and Space Science》2003,286(1-2):99-104
The four main scientific objectives of PRIMA – the Phase-Referenced Imaging and Micro-arc second Astrometry facility for the
VLTI – will be described:– extra-solar system characterization with astrometry, to detect planets and evaluate their mass,
and imaging of the dust accretion disk,– galactic center study with astrometry(dynamics of the bulge stars) and imaging at
10μm (piercing the gas and dust clouds surrounding the galactic center),– observations of AGNs and other extra-galactic objects,
too faint to be observed without PRIMA, for which partial imaging is needed to constrain their structuremodels,– micro-gravitational
lensing event resolution (imaging and astrometry of their photo-center) in the Galactic Bulge and Magellanic Clouds, helping
to determine directly the lens mass and distance.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
3.
A sample of subdwarfs with accurate space velocities and standarized metallicities is presented. This was constructed by combining
Hipparcos parallaxes and proper motions with radial velocities and metallicities from Carney et al. (1994; CLLA). The accurate Hipparcos parallaxes lead to an – upward – correction factor of 11% of the photometric distance
scale of CLLA. The kinematical behaviour of the subdwarfs is discussed in particular in relation to their metallicities. Most
of the stars turn out to be thick disk stars, but the sample contains also many genuine halo stars. While the extreme metal
poor halo does not rotate, a population of subdwarfs with metallicities in the range −1.6≤ [Fe/H] ≤ −1.0 dex appears to rotate
around the galactic center with a mean rotation speed of about 100 km s-1.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
R. D. Blandford 《Journal of Astrophysics and Astronomy》1995,16(2):191-206
It is shown that if neutron stars contain dynamically de-coupled components, most plausibly discrete superfluid zones, then
it is possible for the spin axes of these components o become slightly misaligned with respect to the crustal spin following
a series of glitches. The crust will then undergo Lense-Thirring precession about the total angular moment with a period of
∼ 6–7.5P (assuming a crustal superfluid) and ∼ 3–6P (if the core superfluid is not tightly coupled to the crust). The precise precessional period is diagnostic of the mass distribution
within each component. The implications of recent observational inferences concerning glitching pulsars are discussed. The
conditions necessary for precession to be observable are analysed phenomenologically and a search of pulse-timing data for
evidence of a Lense-Thirring modulation within the period range ∼ 3–8P is proposed. 相似文献
5.
D. K. Ojha A. Omont S. Ganesh G. Simon M. Schultheis 《Journal of Astrophysics and Astronomy》2000,21(1-2):77-90
ISOGAL is a survey at 7 and 15 μm with ISOCAM of the inner galactic disk and bulge of our Galaxy. The survey covers ∼ 22 deg2 in selected areas of the centrall = ±30 degree of the inner Galaxy. In this paper, we report the study of a small ISOGAL field in the inner galactic bulge
(l = 0°,b = −1°, area = 0.033deg2). Using the multicolor nearinfrared data (IJKs) of DENIS (DEep Near Infrared Southern Sky Survey) and mid-infrared ISOGAL data, we discuss the nature of the ISOGAL sources.
The various color-color and color-magnitude diagrams are discussed in the paper. While most of the detected sources are red
giants (RGB tip stars), a few of them show an excess in J-Ks and Ks-[15] colors with respect to the red giant sequence. Most of them are probably AGB stars with large mass-loss rates. 相似文献
6.
K. A. Innanen 《Celestial Mechanics and Dynamical Astronomy》2007,99(3):161-168
The classical assumption that most globular clusters (hereafter GC) formed in situ in galactic halos is examined in an approximate,
empirical way. Although this viewpoint is not rejected per se, an alternative possibility is investigated: the presence of
multiple resonances in the galactic disk, together with the concurrent action of a resonant internal bar or distortion, may
stir these resonances. This may lead to chaotic motion which breaks the action of the third integral for moderately eccentric
orbits. These circumstances may consequently allow the formation of some GC’s in the disk with moderate to highly eccentric
orbits, with the action of the resonant bar subsequently gradually driving them (as well as other stars with similar orbits)
to spend most of their time in the Galaxy’s halo. The size of the resonant region and the probable effectiveness of the various
agents in the associated phase space in the axisymmetric model are listed. An n-body simulation would be required to establish this proposal in a fully self-consistent way.
Paper presented at the Division of Dynamical Astronomy Meetings in Halifax, N.S., Canada, June 2006. 相似文献
7.
Using transmittance data appropriate for grain material which is predominantly comprised of polysaccharides we have computed
infrared fluxes from several types of galactic infrared source. The model used in these computations involves polysaccharide
condensation in material flowing out from O-type stars. With the exception of rather minor discrepancies we show that it is
possible to match the2.1–13 μ observations of a wide range of galactic infrared sources.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
M. Grenon 《Astrophysics and Space Science》1989,156(1-2):29-37
Members of the Galaxy components are identified according to stellar ages, metallicities and galactic orbits. The local thin disk is found to have a maximum age of 11 billion years and a small abundance scatter partially controlled by the radial gradient of abundances. Metal-rich and old metal-poor stars belong to inner galactic populations and SMRs represent the ultimate star generation in the bulge. The thick disk forms a smooth transition between the halo and thin disk. 相似文献
9.
Using the method of two-dimensional spectroscopy, we have investigated the kinematics and distribution of the gas and stars
at the center of the early-type spiral galaxy NGC 7177 with a mediumscale bar as well as the change in the mean age of the
stellar population along the radius. A classical picture of radial gas inflow to the galactic center along the shock fronts
delineated by dust concentration at the leading edges of the bar has been revealed. The gas inflow is observed down to a radius
R = 1″.5−2″, where the gas flows at the inner Lindblad resonance concentrate in an azimuthally highly inhomogeneous nuclear
star formation ring. The bar in NGC 7177 is shown to be thick in z coordinate—basically, it has already turned into a pseudo-bulge as a result of secular dynamical evolution. The mean stellar
age inside the star formation ring, in the galactic nucleus, is old, ∼10 Gyr.Outside, at a distance R = 6″−8″ from the nucleus, the mean age of the stellar population is ∼2 Gyr. If we agree that the bar in NGC 7177 is old,
then, obviously, the star formation ring has migrated radially inward in the last 1–2 Gyr, in accordance with the predictions
of some dynamical models. 相似文献
10.
Based on archival Hubble Space Telescope images, we have performed stellar photometry for eight edge-on spiral and irregular
galaxies. We have identified stars of three ages in the derived Hertzsprung-Russell diagrams and constructed their number
density distribution perpendicularly to the plane of the galactic disk. The sizes of the stellar subsystems of young (up to
100 Myr), middle (0.1–1.0 Gyr), and old (up to 12 Gyr) ages have been determined. A relationship between the age of a subsystem
and its size has been found in all of the galaxies studied. Our results can be explained by the model of galactic thick-disk
formation through thin-disk expansion. In this case, the middle-age stellar subsystem is a transitional stage from the thin
disk to the thick one. 相似文献
11.
Yu-Qin Chen Gang Zhao Jian-Rong ShiNational Astronomical Observatories Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2002,2(5):419-428
We have calculated the orbital parameters for 90 stars in Chen et al. and updated the kinematic data for stars in Edvardsson et al. by using the accurate Hipparcos parallaxes and proper motions, and recalculated the \\\\\\\\\\\\-element abundances in Edvardsson et al. in a way consistent with Chen et al. The two sets of data are combined in a study of stellar populations and characteristics of F & G stars in the solar neighborhood. We confirm the result of Chen et al. that a distinguishable group of stars may belong to the thick disk rather than the thin disk. The ages for the stars are determined using the theoretical isochrones of VandenBerg et al. The age-metallicity relation is investigated for different subgroups according to distance from the sun and galactic orbital parameters. It is found that a mixing of stars with different orbital parameters significantly affect the age-metallicity relation for the disk. Stars with orbits confined to the solar circle all have metallicities [Fe/H] > -0.3 irresp 相似文献
12.
S. Chatterjee 《Journal of Astrophysics and Astronomy》1991,12(4):269-280
We present here rigorous analytical solutions for the Boltzmann-Poisson equation concerning the distribution of stars above
the galactic plane. The number density of stars is considered to follow a behaviour n(m,0) ∼H(m - m0)m−x, wherem is the mass of a star andx an arbitrary exponent greater than 2 and also the velocity dispersion of the stars is assumed to behave as < v2(m)> ∼ m−θ the exponent θ being arbitrary and positive. It is shown that an analytic expression can be found for the gravitational field
Kz, in terms of confluent hypergeometric functions, the limiting trends being Kz∼z for z →0, while Kz
→ constant for z → infinity. We also study the behaviour of < |z(m)|2>,i.e. the dispersion of the distance from the galactic disc for the stars of massm. It is seen that the quantity < |z(m)|2>∼ mt-θ, for m→ t, while it departs significantly from this harmonic oscillator behaviour for stars of lighter masses. It is suggested
that observation of < |z(m)|2> can be used as a probe to findx and hence obtain information about the mass spectrum. 相似文献
13.
M. Jura 《Astrophysics and Space Science》1997,251(1-2):81-88
We propose that at least two stars on or near the AGB have long-lived orbiting disks: HD 44179, the central star in the Red
Rectangle, and BM Gem, a carbon-rich star with an oxygen-rich circumstellar envelope. The CO emission from both of these disks
has a spike with a width near ∼2 km s−1, indicating disk radii of ∼1016 cm. The dust in such disks is therefore quite cold (near T ∼ 50 K for the Red Rectangle) and may emit primarily at submillimeter
wavelengths. The disks around stars where there is also substantial mass loss may not be easily observable; there could be
many as yet undiscovered disks around AGB stars
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
14.
We report first evidence for a new unidentified and variable MeV source, located near the galactic plane at (l,b)∼(284.5°,
2.5°). The source, GRO J1036-55, is found at a significance level of ∼5.6σ by COMPTEL in its 3–10 MeV band. The energy spectrum indicates a spectral maximum at 3–4.3 MeV with a steep slope at higher
energies. Since the COMPTEL 3–4.3 MeV data contain contamination by an instrumental background line, we performed several
consistency checks, which all are consistent with an astrophysical nature of this emission feature. 相似文献
15.
We consider disk and spherical subsystems of stars with nearly radial orbits under conditions when the well-known radial orbit instability is not possible. This requires that the precession of stellar orbits be retrograde, i.e., in the direction opposite to the orbital rotation of stars. We show that an instability that is an analogue of the loss-cone instability known in plasma physics can then develop in the presence of a “loss cone” in the angular momentum distribution of stars, which ensures a deficit or even absence of stars with low angular momenta. Examples of systems with a loss cone are the centers of galaxies or star clusters with massive black holes. The instability can produce a flux of stars onto the galactic center, i.e., it can serve as a mechanism of fueling the nuclear activity of galaxies. Mathematically, the problem is reduced to analyzing simple characteristic equations that describe small perturbations in a disk and a sphere of radially highly elongated stellar orbits. In turn, these characteristics equations are derived through a number of successive simplifications of the general linearized Vlasov equations (i.e., the system that includes the collisionless Boltzmann kinetic equation and the Poisson equation) in action—angle variables. 相似文献
16.
A sample containing 1 026 stars of spectral types F, G, and K, mainly dwarfs, from the solar neighbourhood with available space velocities and metallicities is treated. The treatment comprises a statistical analysis of the metallicity and velocity data and calculation of galactocentric orbits. Sample stars identified as members of the galactic halo are detached from the rest of the sample based on the values of their metallicities, velocity components and galactocentric orbits. In identifying halo stars a new, kinematical, criterion is proposed. Except one, these halo stars are the metal‐poorest ones in the sample. Besides, they have very high velocities with respect to LSR. On the other hand, the separation between the thin disc and thick one is done statistically based on LSR space velocities, membership probability (Schwarzschild distribution with assumed parameters) and galactocentric orbits. In the metallicity these two groups are not much different. For each of the three subsamples the mean motion and velocity ellipsoid are calculated. The elements of the velocity ellipsoids agree well with the values found in the literature, especially for the thin disc. The fractions of the subsystems found for the present sample are: thin disc 93%, thick disc 6%, halo 1%. The sample stars established to be members of the thin disc are examined for existence of star streams. Traces of both, known and unknown, star streams are not found (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
17.
Johan Holmberg Birgitta NordstrÖm Bjarne Rosenkilde Jørgensen Johannes Andersen 《Astrophysics and Space Science》2003,284(2):685-688
The evolution of the Solar neighbourhood is followed using a unique, magnitude complete and kinematically unbiased sample
of 14,000 F, G, and K dwarfs. Metallicity, age, space motion and galactic orbits have been determined for all stars. The result
is a detailed view of the complex evolution of the local Milky Way, which must be matched by any model for the chemical and
dynamical evolution of the Galactic disk. E.g., such models must explain the shape and large scatter in the age-metallity
relation as well as the overall metallicity distribution; the evolution of stellar kinematics with age; the distribution of
stars in velocity space; and the contributions from the thick and thin disks to all these relations.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
18.
A. Quirrenbach 《Astrophysics and Space Science》2003,286(1-2):277-289
The scientific capabilities of the VLT Interferometer can be substantially enhanced through new focal-plane instruments. Many
interferometric techniques– astrometry, phase-referenced imaging, nulling, and differential phase measurements – require control
of the phase to ≲ 1 rad; this capability will be provided at the VLTI by the PRIMA facility. Phase-coherent operation of the
VLTI will also make it possible to perform interferometry with spectral resolution up to R ∼ 100,000 by building fiber links
to the high-resolution spectrographs UVES and CRIRES. These developments will open new approaches to fundamental problems
in fields as diverse as extra solar planets,stellar atmospheres, circumstellar matter, and active galactic nuclei.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
We present preliminary results of an optical-UV survey of the North Celestial Cap (NCCS) based on ∼5% areal coverage. The
NCCS will provide good photometric and astrometric data for the North Celestial Cap region (80°≤δ≤90°). This region, at galactic latitudes from 17°≲b≲37°, is poorly covered by modern CCD-based surveys. The expected number of detected objects in NCCS is ∼1,500,000. We discuss
issues of galactic structure, extinction, and the galaxy clustering in the colour-colour diagrams. 相似文献
20.
Arūnas Kučinskas Audrius Bridžius Vladas Vansevičius 《Astrophysics and Space Science》2002,280(1-2):159-163
The goal of this work is to assess the expected scientific output from the photometric studies of globular clusters in the
Large Magellanic Cloud with ESA's astrometric space mission GAIA. For this purpose we simulate GAIA photometry of individual
stars in synthetic cluster populations, covering a large range of cluster ages and metallicities. We find that accurate effective
temperatures (Δ T
eff<10%) can be obtained from GAIA photometry down to V ∼ 18 for stars in populations within the studied metallicity range ([M/H] = -0.4 ... -1.7). GAIA will also provide photometric
metallicities (Δ [M/H] ≲ 0.3 dex) for the cluster giants brighter than V ∼ 17.5. The knowledge of the effective temperature sand metallicities will allow to obtain accurate ages of stellar populations
younger than about 1 Gyr using the usual procedure of main sequence turn-off point fitting. Ages of older stellar populations
(≳ 1 Gyr) may be constrained from the isochrone fits to the giant branches in the observed CMDs. We conclude that GAIA will
provide excellent opportunities for studying star formation histories far beyond the Milky Way, providing means for better
understanding of stellar and galactic evolution in different astrophysical environments.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献