共查询到20条相似文献，搜索用时 31 毫秒
1.
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
2.
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
3.
P.E. Nissen 《Astrophysics and Space Science》1999,267(1-4):119-128
Observational studies of the relations between ages, metalicities and kinematics of disk stars in the solar neighbourhood
are discussed with emphasis on the recent survey by Edvardsson et al. (1993), and galactic metallicity gradients inferred from these nearby stars are compared with gradients determined from distant
B stars and open clusters.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
4.
5.
Krzysztof Rochowicz 《Astrophysics and Space Science》1996,238(1):71-74
Ultraviolet spectra of population I WR stars obtained from IUE archive are used to determine fundamental stellar parameters. Terminal velocities for 85 galactic and LMC Wolf-Rayet stars were obtained by means of the empirical relation between spectral quantities easily measured in low resolution and high-resolution terminal velocity measurements. Temperatures and so-called transformed radii were derived based on available contour plots of spectral characteristics for a grid of NLTE models. The effect of the reddening law on stellar far ultraviolet continua is emphasized and the revised extinction curve towards WR stars is used for dereddening. For the sample of stars attributed to open clusters or associations we construct the stellar distance scale and adopt it for the other WR stars. The remaining fundamental parameters are derived and HR diagram for population I WR stars is presented. 相似文献
6.
Karel A. Van Der Hucht 《Astrophysics and Space Science》1997,251(1-2):311-320
We review aspects of Population I Wolf-Rayet stars, like classification, inventory, chemical composition, temperature, stellar
wind, mass loss rate, ring nebulae, and galactic enrichment in specific isotopes.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
7.
Annette M.N. Ferguson 《Astrophysics and Space Science》2002,281(1-2):119-122
The far outer regions of galactic disks allow an important probe of both star formation and galaxy formation. I discuss how
observations of HII regions in these low gas density, low metallicity environments can shed light on the physical processes
which drive galactic star formation. The history of past star formation at large radii, as traced by observations of old and
intermediate-age stars, constrains the epoch at which the highest angular momentum regions of disks were in place; first results
for the M31 disk suggest this occured a significant (≳ 8 Gyr) time ago.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
8.
The abundance ratio C/O in the atmospheres of 89 faint carbon stars are detected on the basis of spectra obtained on 2.6 m
telescope of Byurakan AO. The frequency distribution of C/O in the Perseus galactic arm differs from that in the Orion arm.
The conclusion is made that the reason of the difference may be the heavy element abundance differences in these galactic
arms.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
9.
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
10.
B.J. Taylor 《Monthly notices of the Royal Astronomical Society》2002,329(4):839-847
High-dispersion and low-resolution data are combined to search for super-metal-rich (SMR) FGK stars in the solar neighbourhood and Baade's Window. The data are assessed by using statistical analysis, with their rms errors playing a key role. A star is considered to be SMR if its value of , while 'borderline' SMR status may be assigned if . Borderline SMR status is assigned to μ Leo and three other giants, but no full-fledged SMR giants are found in either Baade's Window or the solar neighbourhood. By contrast, the existence of SMR class stars turns out to be well established, with values found for [Fe/H] that are as large as ∼+0.4 dex. It is concluded that this apparent contrast between class stars and giants should not be interpreted in astrophysical terms at present because of marked shortcomings in the available data base for giants. Recommendations are made about future research that may cure this problem and extend present knowledge about SMR dwarfs. 相似文献
11.
B. Nordström J. Andersen E.H. Olsen R. Fux M. Mayor N. Mowlavi F. Pont 《Astrophysics and Space Science》1999,265(1-4):235-241
We describe the overall properties of a new catalogue of metallicities, ages, and galactic orbits for a large, complete sample
of F and G dwarfs in the solar neighbourhood. Based on a magnitude-limited sample of ∼ 14000 stars, it is volume-complete
to ∼ 40 pc. Together with the astrophysical parameters of direct relevance to models of the evolution of the disk, it will
contain the basic photometric, astrometric, and radial velocity data from which they are derived. Information on duplicity
is also included. The full exploitation of the data will require a lengthy analysis, in particular to assess the degree of
completeness of subsamples of stars of different population types. An early result on the effects of diffusion of galactic
orbits in the disk – essential for understanding the scatter in the age-metallicity diagram and estimating the birth radius
of stars – is briefly illustrated.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
12.
R.G. Gratton 《Astrophysics and Space Science》1999,265(1-4):157-164
Since about thirty years it is known that Oxygen and other α-elementsare overabundant in metal-poor stars. In this talk I
briefly review thehistorical and theoretical background, discuss reliability of presentabundance determinations for O, and
finally comment about the implicationsrelevant to galactic chemical evolution.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
13.
Peter Berczik Gerhard Hensler Christian Theis Rainer Spurzem 《Astrophysics and Space Science》2002,281(1-2):297-300
We present our recently developed 3-dimensional chemodynamical code for galaxy evolution. This code follows the evolution
of different galactic components like stars, dark matter and different components of the interstellar medium (ISM), i.e. a
diffuse gaseous phase and the molecular clouds. Stars and dark matter are treated as collisionless N-body systems. The ISM
is numerically described by a smoothed particle hydrodynamics (SPH) approach for the diffuse gas and a sticky particle scheme
for the molecular clouds. Additionally, the galactic components are coupled by several phase transitions like star formation,
stellar death or condensation and evaporation processes within the ISM. As an example we show the dynamical and chemical evolution
of a star forming dwarf galaxy with a total baryonic mass of 2 ċ 109 M⊙. After a moderate collapse phase the stars and the molecular clouds follow an exponential radial distribution, whereas the
diffuse gas shows a central depression as a result of stellar feedback. The metallicities of the galactic components behave
quite differently with respect to their temporal evolution as well as their radial distribution. Especially, the ISM is at
no stage well mixed.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
14.
L. Meillon 《Astrophysics and Space Science》1999,265(1-4):179-180
A large number of high-velocity stars was included in the Hipparcos Input Catalogue for the purpose of determining the galactic
escape velocity in the solar vicinity. However, the `fastest' stars known, listed by Carney et al. (1994, CLLA) were not included because they are too faint. In the intersection between the CLLA list and the Hipparcos Catalogue
(770 common stars), the metal-deficient stars with the most reliable parallaxes (fracσππ)HIP ≤ 0.15) are used for recalibrating CLLA absolute magnitudes and photometric parallaxes, using metallicities and VandenBerg
et al. (1998) isochrones. In this way, about twenty non-Hipparcos stars get improved parallaxes and are added to our primary sample
of Hipparcos high-velocity stars, for a better determination of the escape velocity from the Galaxy.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
15.
G. Cayrel de Strobel Y. Lebreton C. Soubiran E.D. Friel 《Astrophysics and Space Science》1999,265(1-4):345-352
The authors of this paper try to disentangle the many problems arisen from a new enlarged sample of nearby low-mass,metal-rich-stars.
These stars have reliable absolute magnitudes, deduced from Hipparcos parallaxes, precise bolometric corrections, effective
temperatures and metal abundances from high resolution detailed spectroscopic analyses. Their ages have been derived from
a grid of isochrones calculated with up to date physics. The main goal of this paper is to determine the ages of the slightly
evolved SMR stars. Among those with well determinated ages about 80% of them have intermediate ages of (2 to 5 Gyr), but only
20% have ages of 8 Gyr or more. Nevertheless, the existence of very old metal-rich stars is confirmed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
16.
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
17.
K.C. Freeman 《Astrophysics and Space Science》1999,267(1-4):129-138
Away from the young disk, several classes of early type stars are found. They include (i) the old, metal-poor blue horizontal branch stars of the halo and the metal-poor tail of the thick disk; (ii) metal-rich young A stars in a rapidly rotating subsystem but with a much higher velocity dispersion than the A stars of the
young disk, and (iii) a newly discovered class of metal-poor young main sequence A stars in a subsystem of intermediate galactic rotation (Vrot ≈ 120 km s−1). The existence and kinematics of these various classes of early type stars provide insight into the formation of the metal-poor
stellar halo of the Galaxy and into the continuing accretion events suffered by our Galaxy.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
18.
Karel A. van der Hucht 《Astrophysics and Space Science》2002,281(1-2):199-202
The recent VIIth Catalogue of Galactic Wolf-Rayet Stars lists 227 Population I WR stars, comprising 127 WN, 87 WC, 10 WN/WC and 3 WO stars. Additional discoveries bring the census
to 234 WR stars. A re-determination of the optical photometric distances and the galactic distribution of WR stars shows in
the solar neighbourhood a projected surface density of 2.7 WR stars per kpc2, a N
WC/N
WN number ratio of 1.3, and a WR binary frequency of 40 %.The galactocentric distance (R
WR) distribution per subtype showsR
WN and R
WC decreasing with WN and WC subtypes.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
19.
O. Suárez M. Manteiga A. Ulla P. García-Lario A. Manchado Torres 《Astrophysics and Space Science》1998,263(1-4):283-286
We present the first results of a survey carried out on a sample of optically bright post-AGB candidates selected from the
IRAS Point Source Catalogue, many of them previously unidentified in the literature. The selection criteria are based on their
characteristic far infrared colours. Low resolution spectroscopy of more than 150 sources has been obtained during 6 years.
Our preliminary analysis confirms that most of the candidates observed are true post-AGB stars, some of them still strongly
reddened. Some young planetary nebulae, a few young stellar objects and active galactic nuclei are also present in the sample.
The distribution of post-AGB stars with different spectral types in the IRAS two-colour diagram and its evolutionary implications
are briefly discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
20.
A.E. Lynas-Gray 《Astrophysics and Space Science》2004,291(3-4):197-203
Studies in extragalactic astronomy, galactic structure and the late stages of stellar evolution provide ample motivation for surveys of fields in the Galactic Halo. Apart from white dwarfs, blue stars had been regarded as luminous objects confined to star-forming regions in the Galactic Plane; finding them at high galactic latitudes attracted immediate interest, because their luminosities were intermediate between those of white dwarfs and blue Main Sequence stars. The study of blue stars away from the Galactic Plane was initiated by Greenstein; in due course effective temperatures (T e ff), surface gravities (log g) and abundances showed these stars form what appeared to be a blue extension of the known Horizontal Branch (HB) in the Hertzsprung–Russell Diagram. Extended Horizontal Branch (EHB) stars were identified with Extreme Horizontal Branch stars in globular clusters. It was realised that HB and EHB stars must have formed as a consequence of mass-loss on the Giant Branch, either at or before the helium flash. Mass-loss on the Giant Branch leading to the formation of EHB stars was considered more likely for stars in binary systems. The scene was then set for three decades of EHB star research. 相似文献