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1.
Filippina Caputo 《Astronomy and Astrophysics Review》1998,9(1-2):33-61
The current knowledge of the evolution of Population II stars, as observed in galactic halos and globular clusters, is outlined.
The recent theoretical results provided by an improved physical understanding of the stars are reported, with a particular
emphasis upon those stellar evolutionary phases which are the keystones of the interpretation of globular cluster stars. Within
the up–to–date theoretical scenario, the luminosity of the turn–off stars and horizontal branch stars, which are of fundamental
importance for distance and age determinations, turns out to be fainter and brighter, respectively, in comparison with previous
theoretical computations. The predicted absolute magnitude of RR Lyrae stars is consistent with the bright values suggested from the long distance scale (Sandage 1993), but the slope of the relation between and metallicity ([Fe/H]) agrees with that suggested from the short distance scale (see, e.g., Carney et al. 1992, Clementini et al. 1995), at least with [Fe/H]-1.3. As to the globular cluster ages, the new computations provide younger ages ( 10 Gyr–13 Gyr), weakening the conflict with current cosmological estimates. The recent results derived from the fitting of
HIPPARCOS parallaxes of field subdwarfs to the fiducial main sequence of globular clusters are consistent with the up–to–date theoretical
models. However, the direct determinations of , which are based on the HIPPARCOS proper motions and trigonometric parallaxes of field RR Lyrae stars, give fainter absolute magnitudes than those obtained
from the improved evolutionary models. This seems to suggest that the problem of the globular cluster distance and age is
still open.
Received 6 April 1998 相似文献
2.
David G. Turner 《Astrophysics and Space Science》2012,337(1):303-312
A reanalysis of the (seemingly very distant) open cluster Shorlin 1, the group of stars associated with WR 38 and WR 38a,
is made on the basis of existing UBV and JHK
s observations for cluster members. The 2MASS observations, in particular, imply a mean cluster reddening of E
B−V
=1.45±0.07 and a distance of 2.94±0.12 kpc. The reddening agrees with the UBV results provided that the local reddening slope is described by E
U−B
/E
B−V
=0.64±0.01, but the distance estimates in the 2MASS and UBV systems agree only if the ratio of total-to-selective extinction for the associated dust is R=A
V
/E
B−V
=4.0±0.1. Both results are similar to what has been obtained for adjacent clusters in the Eta Carinae region by similar analyses,
which suggests that “anomalous” dust extinction is widespread through the region, particularly for groups reddened by relatively
nearby dust. Dust associated with the Eta Carinae complex itself appears to exhibit more “normal” qualities. The results have
direct implications for the interpretation of distances to optical spiral arm indicators for the Galaxy at ℓ=287°–291°, in particular the Carina arm here is probably little more than ∼2 kpc distant, rather than 2.5–3 kpc distant as
implied in previous studies. Newly-derived intrinsic parameters for the two cluster Wolf-Rayet stars WR 38 (WC4) and WR 38a
(WN5) are in good agreement with what is found for other WR stars in Galactic open clusters, which was not the case previously. 相似文献
3.
Robert W. Slawson Zoran Ninkov Elliott P. Horch 《Astrophysics and Space Science》2007,312(3-4):171-187
We have obtained and analyzed UBVRI
CCD frames of the young, 4–10 Myr, open cluster NGC 3293 and the surrounding field in order to study its stellar content and determine the cluster’s IMF. We found significantly fewer lower mass stars, M≤2.5M
⊙, than expected. This is particularly so if a single age for the cluster of 4.6 Myr is adopted as derived from fitting evolutionary
models to the upper main sequence. Some intermediate-mass stars near the main sequence in the HR diagram imply an age for the cluster of about 10 Myr. When compared with the Scalo (The stellar initial mass function. ASP
conference series, vol. 24, p. 201, 1998) IMF scaled to the cluster IMF in the intermediate mass range, 2.5≤M/M
⊙≤8.0 where there is good agreement, the high mass stars have a distinctly flatter IMF, indicating an over abundance of these stars, and there is a sharp turnover in the distribution at lower masses. The radial
density distribution of cluster stars in the massive and intermediate mass regimes indicate that these stars are more concentrated
to the cluster core whereas the lower-mass stars show little concentration. We suggest that this is evidence supporting the
formation of massive stars through accretion and/or coagulation processes in denser cluster cores at the expense of the lower
mass proto-stars.
R.W. Slawson and E.P. Horch are guest investigators at the University of Toronto Southern Observatory, Las Campanas, Chile. 相似文献
4.
The young open star cluster M25 (IC 4725) is located in the direction of the galactic center, near much irregular absorption
features on Sagittarius arm. This system is found to be at a distance of 600 pc, with a median age of 9.45 × 107 years and a distance from the galactic plane of –52.82 pc The mass data available in the literature has been gathered and
many statistical methods have been applied for this cluster. Depending only on these methods, the stellar density, the distribution
of dark matter, the luminosity and the mass functions have been estimated. The center of the cluster has been defined, it
is shifted by 45 arc sec in the northeast direction. The radius of the cluster is found to be 4 pc. More than 220 stars with
mean reddening of 0.50 mag and absorption of 1.62mag are found inside this aria. The total mass of the cluster has been estimated
with the mass of the interstellar matter (gas and dust). It is found to be 1937 M
⊙, whereas about 24% of the material mass of the cluster has remained as interstellar matter after the processes of formation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
R. A. Vardanyan 《Astrophysics》1998,41(4):377-381
About 15 wide binary systems, one component of which is a red star, have been detected in photographic and infrared observations
of the cluster NGC 6913.
Translated from Astrofizika, Vol. 41. No. 4, pp. 577–583, October–December, 1998. 相似文献
6.
We have derived the galaxy luminosity function in various regions of the cluster of galaxies Abell 496 from a wide field image
in the I band. A Schechter function fit in the 17≤ IAB ≤22(–19.5≤ MIAB ≤ –14.5) magnitude interval gives a steep power law index, which is somewhat steeper in the outer regions than in the inner
zones. This result agrees with previous findings obtained by several teams on the Coma cluster and can be interpreted as due
to the fact that faint galaxies are accreted by large ones in the central regions.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
7.
The oldest open clusters in our Galaxy set the lower limit to the age of the Galactic Disk (9–10 Gyr). Although they appear
to be very rich now, it is clear that their primordial populations were much larger. Often considered as transitional objects,
these populous open clusters show structural differences with respect to globular clusters so their dynamics and characteristic
evolutionary time scales can also be different. On the other hand, their large membership lead to different dynamical evolution
as compared with average open clusters. In this paper, the differential features of the evolution of rich open clusters are
studied using N-body simulations, including several of the largest (104 stars) published direct collisional N-body calculations so far, which were performed on a CRAY YMP. The disruption rate of rich open clusters is analysed in detail
and the effect of the initial spatial distribution of the stars in the cluster on its dynamics is studied. The results show
that cluster life-time depends on this initial distribution, decreasing when it is more concentrated. The effect of stellar
evolution on the dynamical evolution of rich clusters is an important subject that also has been considered here. We demonstrate
that the cluster's life-expectancy against evaporation increases because of mass loss by evolving high-mass stars.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
8.
An investigation of the two-dimensional distribution of flare stars in the Pleiades led to the discovery of a dip in their
surface density at a distance of 3.2 pc from the center of the cluster. The same feature is found in the three-dimensional
distribution of these stars.
Translated from Astrofizika, Vol. 42, No. 4, pp. 537–540, October–December, 1999. 相似文献
9.
We present R-band galaxy luminosity functions (GLFs) from aspectroscopic sample of six nearby rich galaxy clusters. In addition
to individual cluster GLFs, extending to, in one case, M
R=–14, we also present composite GLFs for cluster and field galaxies toM
R=–17. All six cluster samples are consistent with the composite GLF, but there is evidence that the GLF of the quiescent population
in clusters is not universal. Furthermore, the GLF of quiescent galaxies is significantly steeper in clusters than in the
field. The overall GLF in clusters is consistent with that of field galaxies, except for the luminous tip, which is enhanced
in clusters versus the field.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
The widely varying dwarf galaxy counts in different environments provide a strong challenge to standard hierarchical clustering
models. The luminosity function is not universal, but seems to be strongly dependent upon environment. In this paper we describe
an automated procedure for detecting and measuring very Low Surface Brightness (LSB) features in deep CCD data. We apply this
procedure to large area CCD survey fields of the Virgo cluster and find 105 dwarf LSB galaxies previously uncatalogued over
an area of ∼14 sq deg. We show that there are many more faint (14≤MB≤–10) LSB galaxies than would be predicted from extrapolation of the Virgo cluster catalogue luminosity function. Over our
limited range of measurement the faint end slope of the luminosity function becomesα=–1.6. Although these galaxies contribute
a small fraction of the total stellar light of the cluster, they may contribute significantly to the mass in galaxies if they
have large mass-to-light ratios similar to those recently measured for Local Group dwarf galaxies. By a comparison with different
environments, the Virgo Cluster seems to be highly `dwarf rich' if considering the dwarf to giant ratio or it's luminosity
function faint end slope.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
11.
We analyze the data presented in a previous paper by Gyulzadyan and Petrosian, and discuss the results of a statistical investigation
of the relationship between SBS galaxies and Zwicky clusters. The main results are that SBS galaxies follow the overall galaxy
distribution in clusters and they do not avoid any type of Zwicky cluster. There is a significantly higher probability of
finding SBS galaxies occurring in medium compact clusters than in open clusters. They also follow the well-established morphology-density
relation. Earlier morphological type, higher luminosity, larger linear size, and redder SBS galaxies tend to be found in clusters
with higher compactness, or in more compact regions of the clusters. The number distribution of SBS galaxies in Zwicky open
clusters probably follows the distribution of normal galaxies. The number distribution of SBS galaxies in medium compact and
compact clusters shows two-maxima structure.
Published in Astrofizika, Vol. 52, No. 2, pp. 225–236 (May 2009). 相似文献
12.
G. S. D. Babu 《Journal of Astrophysics and Astronomy》1991,12(3):187-198
Continuing the study of faint young open clusters as tracers of spiral features in our Galaxy, photoelectric and photographic
photometry of 39 stars was done in the field of the faint open cluster NGC 2236 ≡ OCl 501 in the direction of Monoceros constellation.
Out of these stars, a total of 22 down tom
v ≃ 15.4 mag have been found to be probable members. There is apparently a variable extinction across the field of the cluster
with E(B - V) ranging between 0.84 mag and 0.68 mag. The median age of this cluster is estimated to be 7.6 × 107 years and the cluster is thereby considered as belonging to the marginally old category. Thus, it cannot be specifically
used as a spiral arm tracer in the study of our Galaxy. This cluster is located at a distance of 3.72 ± 0.13 kpc, which places
it at the inner edge of the outer Perseus spiral feature of the Milky Way. 相似文献
13.
A. Fritz B.L. Ziegler R.G. Bower I. Smail R.L. Davies 《Astrophysics and Space Science》2003,285(1):61-66
We explore the evolution of the early-type galaxy population in the rich cluster Abell 2390 at z=0.23. For this purpose, we have obtained spectroscopic data of 51 elliptical and lenticular galaxies with MOSCA at the 3.5
m telescope on Calar Alto Observatory. As our investigation spans both a broad range in luminosity (–22.3≤MB≤–:19.3)and a wide field of view (10′×10′), the environmental dependence of different formation scenarios can be analysed
in detail as a function of radius from the cluster center. In this paper, we present first results on the Faber-Jackson relation
and, for a subsample of 14 galaxies with morphological and structural parameters from HST, we also investigate the evolution
of the Kormendy relation and the Fundamental Plane. We find a mild luminosity evolution of the early-type galaxies in Abell
2390: our objects are on average brighter by m
B∼0.4 mag.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
14.
Kotaro Kohno Koichiro Nakanishi Tomoka Tosaki Kazuyuki Muraoka Rie Miura Hajime Ezawa Ryohei Kawabe 《Astrophysics and Space Science》2008,313(1-3):279-285
Dense molecular medium plays essential roles in galaxies. As demonstrated by the tight and linear correlation between HCN(1–0)
and FIR luminosities among star-forming galaxies, from very nearby to high-z ones, the observation of a dense molecular component
is indispensable to understand the star formation laws in galaxies. In order to obtain a general picture of the global distributions
of dense molecular medium in normal star-forming galaxies, we have conducted an extragalactic CO(3–2) imaging survey of nearby
spiral galaxies using the Atacama Submillimeter Telescope Experiment (ASTE). From the survey (ADIoS; ASTE Dense gas Imaging
of Star-forming galaxies), CO(3–2) images of M 83 and NGC 986 are presented. Emphasis is placed on the correlation between
the CO(3–2)/CO(1–0) ratio and the star formation efficiency in galaxies. In the central regions of some active galaxies, on
the other hand, we often find enhanced or overluminous HCN(1–0) emission. The HCN(1–0)/CO(1–0) and HCN(1–0)/HCO+(1–0) intensities are often enhanced up to ∼0.2–0.3 and ∼2–3, respectively. Such elevated ratios have never been observed
in the nuclear starburst regions. One possible explanation for these high HCN(1–0)/CO(1–0) and HCN(1–0)/HCO+(1–0) ratios is X-ray induced chemistry in X-ray dominated regions (XDRs), i.e., the overabundance of the HCN molecule in
the X-ray irradiated dense molecular tori. If this view is true, the known tight correlation between HCN(1–0) and the star-formation
rate breaks in the vicinity of active nuclei. Although the interpretation of these ratios is still an open question, these
ratios have a great potential for a new diagnostic tool for the energy sources of dusty galaxies in the ALMA era because these
molecular lines are free from dust extinction. 相似文献
15.
A 6 hour patrol search in the region of the Pleiades aggregate with the 40″ and 21″ Schmidt telescopes at the Byurakan Astrophysical
Observatory provided simultaneous data in two colors (Pg and U) on a flare of the star FSP 377=HII 2411 in the Hyades cluster.
__________
Translated from Astrofizika, Vol. 49, No. 2, pp. 299–302 (May 2006). 相似文献
16.
K. S. Dwarakanath Siddharth Malu Ruta Kale 《Journal of Astrophysics and Astronomy》2011,32(4):529-532
We have discovered a giant radio halo in the massive merging cluster MACSJ0417.5-1154. This cluster, at a redshift of 0.443,
is one of the most X-ray luminous galaxy cluster in the MAssive Cluster Survey (MACS) with an X-ray luminosity in the 0.1–2.4
keV band of 2.9×1045 erg s − 1. Recent observations from GMRT at 230 and 610 MHz have revealed a radio halo of ∼ 1.2 × 0.3 Mpc2 in extent. This halo is elongated along the North-West, similar to the morphology of the X-ray emission from Chandra. The
1400 MHz radio luminosity (L
r) of the halo is ∼2 × 1025 W Hz − 1, in good agreement with the value expected from the L
x − L
r correlation for cluster halos. 相似文献
17.
Observations of the Pleiades cluster by the method of stellar tracks, carried out on the 40-inch Schmidt telescope of Byurakan
Astrophysical Observatory, have resulted in the detection of 49 flares from 38 stars, 17 of which were not previously known
to be flare stars. It is shown that for bright stars (U ≤ 16.0) the detection of flares in observations by the method of stellar
tracks is at least three times more efficient than for observations by the method of stellar chains. Another advantage of
the first method is that one can detect brief flares that last less than 6 min. The visual stellar magnitude at the minimum
for the brightest of the flare stars that we found is 11.92. This raised the upper luminosity limit of known flare stars in
the Pleiades by 0.21 magnitude. A comparison of the expected number of bright flare stars in the Pleiades with the number
of all bright members of the cluster (falling in the range from V ≈ 12.0 toV ≈16.0) suggested that all these stars evidently
must be flare stars.
Translated from Astrofizika, Vol. 42, No. 3, pp. 351–358, July–September, 1999. 相似文献
18.
R. Surendiranath N. Kameswara Rao Ram Sagar J. S. Nathan K. K. Ghosh 《Journal of Astrophysics and Astronomy》1990,11(2):151-166
bdAbstract The open cluster NGC 2818 containing a planetary nebula has been observed inVRI bands using the CCD system at prime focus of the 2.3-metre Vainu Bappu Telescope. The study extending to starsV ∼ 21 magnitude establishes the distance modulus as(m-M)
0 = 12.9 ±0.1 for the cluster. Based on the fitting of theoretical isochrones computed for solar metallicity, an age of 5(±1)
× 108 years has been assigned to the cluster. Association of the planetary nebula with the cluster indicates that the progenitor
mass of the planetary nebula on the main sequence is ≥2.5M⊙
Based on observations obtained with the Vainu Bappu Telescope. 相似文献
19.
We outline a method to determine the direction of solar open flux transport that results from the opening of magnetic clouds
(MCs) by interchange reconnection at the Sun based solely on in-situ observations. This method uses established findings about i) the locations and magnetic polarities of emerging MC footpoints, ii) the hemispheric dependence of the helicity of MCs, and iii) the occurrence of interchange reconnection at the Sun being signaled by uni-directional suprathermal electrons inside MCs.
Combining those observational facts in a statistical analysis of MCs during solar cycle 23 (period 1995 – 2007), we show that
the time of disappearance of the northern polar coronal hole (1998 – 1999), permeated by an outward-pointing magnetic field,
is associated with a peak in the number of MCs originating from the northern hemisphere and connected to the Sun by outward-pointing
magnetic field lines. A similar peak is observed in the number of MCs originating from the southern hemisphere and connected
to the Sun by inward-pointing magnetic field lines. This pattern is interpreted as the result of interchange reconnection
occurring between MCs and the open field lines of nearby polar coronal holes. This reconnection process closes down polar
coronal hole open field lines and transports these open field lines equatorward, thus contributing to the global coronal magnetic
field reversal process. These results will be further constrainable with the rising phase of solar cycle 24. 相似文献
20.
Matteo Correnti Francesco Paresce Rossella Aversa Giacomo Beccari Guido De Marchi Marcella Di Criscienzo Xiaoying Pang Loredana Spezzi Elena Valenti Paolo Ventura 《Astrophysics and Space Science》2012,340(2):263-279
We have used new, deep, visible and near infrared observations of the compact starburst cluster in the giant HII region NGC 3603 and its surroundings with the WFC3 on HST and HAWK-I on the VLT to study in detail the physical properties of its intermediate mass (∼1–3 M⊙) stellar population. We show that after correction for differential extinction and actively accreting stars, and the study of field star contamination, strong evidence remains for a continuous spread in the ages of pre-main sequence stars in the range ∼2 to ∼30 Myr within the temporal resolution available. Existing differences among presently available theoretical models account for the largest possible variation in shape of the measured age histograms within these limits. We also find that this isochronal age spread in the near infrared and visible Colour-Magnitude Diagrams cannot be reproduced by any other presently known source of astrophysical or instrumental scatter that could mimic the luminosity spread seen in our observations except, possibly, episodic accretion. The measured age spread and the stellar spatial distribution in the cluster are consistent with the hypothesis that star formation started at least 20–30 Myrs ago progressing slowly but continuously up to at least a few million years ago. All the stars in the considered mass range are distributed in a flattened oblate spheroidal pattern with the major axis oriented in an approximate South-East–North-West direction, and with the length of the equatorial axis decreasing with increasing age. This asymmetry is most likely due to the fact that star formation occurred along a filament of gas and dust in the natal molecular cloud oriented locally in this direction. 相似文献