共查询到20条相似文献,搜索用时 31 毫秒
1.
Arūnas Kučinskas Vladas Vansevičius Toshihiko Tanabé 《Astrophysics and Space Science》2002,280(1-2):151-157
We discuss a possible use of the asymptotic giant branch (AGB) stars for tracing star formation histories on the Galactic
and extragalactic distance scales with the ESA's astrometric space mission GAIA. Extensive numerical simulations demonstrate
that metallicities (Δ [M/H] ≲ 0.3) can be obtained for the AGB stars with GAIA up to the distances of ∼ 200 kpc, if no interstellar
extinction is present. Reliable population ages can be also obtained from the AGB stars if their T
eff are constrained precisely. We show that precise effective temperatures can be obtained by fitting observed spectral energy
distributions of the AGB stars with theoretical fluxes calculated from the synthetic spectra. A combination of the derived
effective temperatures with the bolometric luminosities allows to derive precise population ages for a wide range of ages
and metallicities over the large distance scales. This demonstrates that AGB stars can be employed very effectively for tracing
star formation histories with GAIA, allowing to refine the global evolutionary scenarios of stellar populations in the Milky
Way and the galaxies beyond.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
2.
Claudia Maraston Markus Kissler-Patig Jean Brodie Pauline Barmby John Huchra 《Astrophysics and Space Science》2002,281(1-2):137-138
A new method to detect and study young star clusters is presented. This is based on the knowledge that the light of stellar
populations with ages between ∼ 200 Myr and ∼ 1/2 Gyr is dominated by very red, bright AGB stars. Star clusters undergoing this so-called ‘AGB phase transition’ are featured by very red V-K colours, like those of Magellanic
Clouds clusters, while optical colours like B-V remain blue typical for young populations. The best channel for detecting
star clusters in this age range is therefore the near IR. From the theoretical side, SSP models including properly the contribution
of the bright AGB are required (Maraston, 1998). Using this strategy, we succesfully detected the AGB phase transition among
the clusters of the merger remnant galaxy NGC 7252 (see Maraston etal., 2001).
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
3.
I. S. Shklovskii 《Journal of Astrophysics and Astronomy》1984,5(1):13-18
It is argued that the iron nucleosynthesis rate in the universe due to SNI outbursts is dependent on the mass function of
star formation. Since the mass function depends on the chemical composition and since the masses of SNI precursors have upper
limits, the iron nucleosynthesis rate was low at an earlier evolutionary epoch of the universe when mainly massive stars were
formed. The iron nucleosynthesis rate should reach a maximum near z ∼ 0.5. At such or similar value of z the well-known ‘step’
in the cosmic γ-ray background spectrum may be explained by the presence of γ-gray quanta accompanying the radioactive56Co →56Fe decay. An argument is presented against the identification of the hidden mass of the universe with black-hole remnants
of ‘type III’ stars. 相似文献
4.
A. Niedzielski 《Astrophysics and Space Science》2003,286(1-2):237-244
The Wolf-Rayet stars represent an advanced stage of evolution of the most massive stars. Their next immediate stadium will
be supernova explosion. The most striking property of this very rare but exceptionally hot and bright objects is their extreme
mass loss, of the order of 10-5 solar mass per year. In turn of evolution before and during the Wolf-Rayet phase such stars eject a lot of matter (∼ 10 M⊙) with velocity up to 3000 km/s that surrounds the min the form of gas and dust. In the case of binary Wolf-Rayet star such
expanding envelope may interact with a companion (usually hot OB star) wind forming a tail extended for ∼ 100 AU. This spectacular
phenomenon as well as some other connected with Wolf-Rayet stars that can be studied with high spatial resolution instruments
(both astrometric and imaging) are reviewed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
5.
Jiang Zhang Fang Zhao Yanping Chen Wenyuan Cui Bo Zhang 《Journal of Astrophysics and Astronomy》2013,34(4):373-391
CEMP-r/s stars at low metallicity are known as double-enhanced stars that show enhancements of both r-process and s-process elements. The chemical abundances of these very metal-poor stars provide us a lot of information for putting new restraints on models of neutron-capture processes. In this article, we put forward an accreted scenario in which the double enrichment of r-process and s-process elements is caused by a former intermediate-mass Asymptotic Giant Branch (AGB) companion in a detached binary system. As the AGB superwind is only present at the ultimate phase of AGB stars, there is thus a lot of potential that the degenerate-core mass of an intermediate-mass AGB star reaches the Chandrasekhar limit before the AGB superwind. In these circumstances, both s-process elements produced in the AGB shell and r-process elements synthesized in the subsequent explosion would be sprayed contemporaneously and accreted by its companion. Despite similarity to physical conditions of a core-collapse supernova, a major focus in this scenario is the degenerate C–O core surrounded by an envelope of a former intermediate-mass AGB donor that may collapse and explode. Due to the existence of an outer envelope, r-process nucleosynthesis is expected to occur. Hypothesizing the material-rich europium (Eu) accreted by the secondary via the wind from the supernova to be in proportion to the geometric fraction of the companion with respect to the exploding donor star, we find that the estimated yield of Eu (as representative of r-process elements) per AGB supernova event is about 1 × 10?9 M ⊙ ~ 5 × 10?9 M ⊙. Using the yields of Eu, the overabundance of r-process elements in CEMP-r/s stars can be accounted for. The calculated results show that the value of parameter f , standing for efficiency of wind pollution from the AGB supernova, will reach about 104, which means that the enhanced factor is much larger than unity due to the impact of gravity of the donor and the result of the gravitational focusing effect of the companion. 相似文献
6.
C. Travaglio S. Randich D. Galli C. Abia J. Lattanzio 《Astrophysics and Space Science》2002,281(1-2):219-220
Different stellar sources may have contributed to the 7Li enrichmentof the Galaxy: type-II supernovae, novae, and AGB stars. In the latter case, the interplay between the Hot Bottom
Burning (HBB) process (via the Cameron-Fowler mechanism) and a very high mass-loss rate before the evolution off the AGB (the
so-called ‘superwind’ phase), can lead to a significant production of 7Li from low- and intermediate-mass AGB stars (Travaglio et al., 2001). We have now undertaken an observational campaign aimed
at constraining our stellar and Galactic models, with a twofold goal: (i) to assemble a compilation of high-resolution spectra of Galactic, unevolved (i.e. dwarfs), warm(spectral type F) stars,
in a selected metallicity range (-1.0 ≤>[Fe/H] ≤ -0.3), using the ESO 1.5m telescope and the FEROS spectrograph; (ii) to carry out a Li survey among a sample of selected AGB stars, to investigate the possible correlation between7Li abundance (when detected) and mass-loss rate.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
7.
The abundances of 22 heavy elements from Sr to Pb have been determined for the halo star HD 29907 (T
eff = 5500 K, log g = 4.64) with [Fe/H] = −1.55 using high-quality VLT/UVES spectra (ESO, Chile). The star has a moderate enhancement of r-process elements (Eu-Tm) with [r/Fe] = 0.63. In the range from Ba to Yb, the derived abundance pattern agrees well with those for strongly r-process enhanced stars (r-II stars with [Eu/Fe] > 1 and [Ba/Eu] < 0), such as CS 22892-052 and CS 31082-001, as well as with
the scaled solar r-process curve and the r-process model HEW. Thus, Ba-Yb in HD 29907 originate in the r-process. Just as other moderately r-process enhanced stars studied in the literature, HD 29907 exhibits higher Sr, Y, and Zr abundances than those for r-II stars.
These results confirm the assumption by other authors about the existence of an additional Sr-Zr synthesis mechanism in the
early Galaxy before the onset of nucleosynthesis in asymptotic giant branch (AGB) stars. The same mechanism can be responsible
for the enhancement of Mo-Ag in the star being investigated compared to r-II stars. There are no grounds to suggest the presence
of s-nuclei of lead in the material of the star being investigated, because its measured abundance ratio log ɛ(Pb/Eu) = 1.20 lies within the range for the comparison stars: from log ɛ(Pb/Eu) = 0.17 (CS 31082-001) to < 1.55 (HE 1219-0312). Thus, even if there was a contribution of AGB stars to the heavy-element
enrichment of the interstellar medium at the epoch with [Fe/H] = −1.55, it was small, at the level of the abundance error. 相似文献
8.
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
9.
Stellar abundance pattern of n-capture elements such as barium is used as a powerful tool to infer how the star formation proceeded in dwarf spheroidal
(dSph) galaxies. It is found that the abundance correlation of barium with iron in stars belonging to dSph galaxies orbiting
the Milky Way, i.e., Draco, Sextans, and Ursa Minor have a feature similar to that in Galactic metal-poor stars. The common
feature of these two correlations can be realized by our in homogeneous chemical evolution model based on the supernova-driven
star formation scenario if dSph stars formed from gas with a velocity dispersion of ∼ 26 km s-1. This velocity dispersion together with the stellar luminosities strongly suggest that dark matter dominated dSph galaxies.
The tidal force of the Milky Way links this velocity dispersion with the currently observed value ≲ 10 km s-1 by stripping the dark matter in dSph galaxies. As a result, the total mass of each dSph galaxy is found to have been originally
∼ 25 times larger than at present. In this model, supernovae immediately after the end of the star formation can expel the
remaining gas over the gravitational potential of the dSph galaxy.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
10.
DDO 68 (UGC 5340) is the second most metal-poor star-forming galaxy (12 + log(O/H) = 7.14). Its peculiar optical morphology
and its HI distribution and kinematics are indicative of a merger origin. We use the u, g, r, and i photometry based on the SDSS images of DDO 68 to estimate its stellar population ages. The Hα images of DDO 68 were used to select several representative regions without nebular emission. The derived colors were analyzed
by comparison with the PEGASE2 evolutionary tracks for various star formation (SF) scenarios, including the two extreme cases:
(i) an instantaneous starburst and (ii) continuous SF with a constant rate. The (u − g) and (g − r) colors for all of the selected regions are consistent with the scenario of several “instantaneous” SF episodes with ages
between ∼0.05 and ∼1 Gyr. The total mass of the visible stars in DDO 68 was estimated by comparing the colors and fluxes of
the observed stellar subsystems with PEGASE2 models to be ∼2.4 × 107
M
⊙. This accounts for ∼6% of the total baryonic mass of the galaxy. All of the available data are consistent with the fact that
DDO 68 is a very rare candidate for young galaxies. The bulk of its stars were formed during the recent (with the first encounter
∼1 Gyr ago) merger of two very gas-rich disks. DDO 68 is closest in its properties to cosmologically young low-mass galaxies.
This article was submitted by the authors in English. 相似文献
11.
《New Astronomy》2003,8(5):401-414
Recently, Brittain and Rettig, using the cryogenic echelle spectrograph at the Infrared Telescope Facility to study the infrared emission from the inner preplanetary disk of the Herbig Ae/Be star HD141569, detected CO and H3+ ion emission. This emission has been tentatively interpreted as due to the existence of a forming gas giant planet. The suggested protoplanetary blob appears to be orbiting its host star at about 7 AU being perhaps 2 AU across and roughly five times the mass of Jupiter. Based on numerical modeling of the evolution of the dust disk we show that their observational results are compatible with the presence of an evolved giant vortex in the disk. Our calculations suggest that vortices formed in disks similar to the one found around HD141569 are more effective at capturing solid material than equivalent structures around solar-like stars. On the other hand, we investigate the possibility to find evidence for large-scale vortices in preplanetary disks by submillimeter interferometry. Disks around Herbig Ae/Be stars may be primary targets for giant vortex detection using this technique. 相似文献
12.
N. R. Ikhsanov 《Astrophysics and Space Science》2007,308(1-4):137-140
Accretion of interstellar material by an isolated neutron star is discussed. The point I address here is the interaction between
the accretion flow and the stellar magnetosphere. I show that the interchange instabilities of the magnetospheric boundary
under the conditions of interest are basically suppressed. The entry of the material into the magnetosphere is governed by
diffusion. Due to this reason the persistent accretion luminosity of isolated neutron stars is limited to <4×1026 erg s−1. These objects can also appear as X-ray bursters with the burst durations of ∼30 min and repetition time of ∼105 yr. This indicates that the number of the accreting isolated neutron stars which could be observed with recent and modern
X-ray missions is a few orders of magnitude smaller than that previously estimated.
相似文献
13.
Zhi-Yao Yu 《Astrophysics and Space Science》1993,210(1-2):167-168
The life-time of the star on AGB is approximately 6 × 104 yr. We divide it into front half and back half of AGB (including to optical Mira variable and OH/IR star) according to their evolution character. The observations show that the star has non-pulsation, but constant mass loss rate ( 5 × 10–7
M
yr–1) on front half of AGB. Its circumstellar envelope is formed. When the star has pulsation on back half of AGB, its mass loss rate is relative with time, and increases gradually. In this time the star is on the stage of optical Mira variable. When the mass loss rate reaches the value of 3 × 10–6
M
yr–1, the star evoluted from the stage of optical variable into the stage of radio bright OH/IR star. On the end of AGB the mass loss rate reaches 10–4
M
yr–1. (Band and Habing 1983, Hermen and Habing 1985). 相似文献
14.
Josep M. TRIGO‐RODRÍGUEZ Domingo Aníbal GARCÍA‐HERNÁNDEZ Maria LUGARO Amanda I. KARAKAS M. van RAAI Pedro García LARIO Arturo MANCHADO 《Meteoritics & planetary science》2009,44(5):627-639
Abstract— We demonstrate that a massive asymptotic giant branch (AGB) star is a good candidate as the main source of short‐lived radionuclides in the early solar system. Recent identification of massive (4–8 M⊙) AGB stars in the galaxy, which are both lithium‐ and rubidium‐rich, demonstrates that these stars experience proton captures at the base of the convective envelope (hot bottom burning), together with high‐neutron density nucleosynthesis with 22Ne as a neutron source in the He shell and efficient dredge‐up of the processed material. A model of a 6.5 M⊙ star of solar metallicity can simultaneously match the abundances of 26Al, 41Ca, 60Fe, and 107Pd inferred to have been present in the solar nebula by using a dilution factor of 1 part of AGB material per 300 parts of original solar nebula material, and taking into account a time interval between injection of the short‐lived nuclides and consolidation of the first meteorites equal to 0.53 Myr. Such a polluting source does not overproduce 53Mn, as supernova models do, and only marginally affects isotopic ratios of stable elements. It is usually argued that it is unlikely that the short‐lived radionuclides in the early solar system came from an AGB star because these stars are rarely found in star forming regions, however, we think that further interdisciplinary studies are needed to address the fundamental problem of the birth of our solar system. 相似文献
15.
We investigate the instability driven by viscosity in rotating relativistic stars by means of an iterative approach. We focus
on polytropic rotating equilibrium stars and impose an m=2 perturbation in the lapse. We vary both the stiffness of the equation of state and the compactness of the star to study
these factors on the critical value T/W for the instability. For a rigidly rotating star, the criterion T/W, where T is the rotational kinetic energy and W the gravitational binding energy, mainly depends on the compactness of the star and takes values around 0.13–0.16, which
slightly differ from that of Newtonian incompressible stars (∼0.14). For differentially rotating stars, the critical value
of T/W is found to span the range 0.17–0.25. The value is significantly larger than in the rigidly rotating case with the same compactness
of the star. Finally we discuss the possibility of detecting gravitational waves from viscosity-driven instabilities using
ground-based interferometers.
相似文献
16.
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
17.
We present two new luminous blue variable (LBV) candidate stars discovered in the M33 galaxy. We identified these stars as
massive star candidates at the final stages of evolution, presumably with a notable interstellar extinction. The candidates
were selected from the Massey et al. catalog based on the following criteria: emission in H
α
, V<18./m 5 and 0.m 35 < (B - V) < 1.m 2. The spectra of both stars reveal a broad and strong H
α
emission with extended wings (770 and 1000 kms−1). Based on the spectra we estimated the main parameters of the stars. Object N45901 has a bolometric luminosity log(L/L⊙) = 6.0–6.2 with the value of interstellar extinction A
V
= 2.3 ± 0.1. The temperature of the star’s photosphere is estimated as T⋆ ∼ 13000–15000 K, its probable mass on the Zero
Age Main Sequence is M∼ 60–80 M⊙. The infrared excess in N 45901 corresponds to the emission of warm dust with the temperature Twarm ∼ 1000 K, and amounts
to 0.1%of the bolometric luminosity. A comparison of stellar magnitude estimates from different catalogs points to the probable
variability of the object N45901. Bolometric luminosity of the second object, N125093, is log(L/L⊙) = 6.3 − 6.6, the value of interstellar extinction is A
V
= 2.75 ± 0.15. We estimate its photosphere’s temperature as T⋆∼ 13000–16000K, the initial mass as M ∼ 90–120 M⊙. The infrared excess in N125093 amounts to 5–6% of the bolometric luminosity. Its spectral energy distribution reveals two
thermal components with the temperatures Twarm ∼ 1000K and Tcold ∼ 480 K. The [Ca II] λλ7291, 7323 lines, observed in LBV-like stars Var A and N93351 in M33 are also present in the spectrum of N 125093. These lines
indicate relatively recent gas eruptions and dust activity linked with them. High bolometric luminosity of these stars and
broad H
α emissions allow classifying the studied objects as LBV candidates. 相似文献
18.
We present some insight on the ionization structure of the circumstellar disks around Be type stars based on the study of
Balmer decrements for absorption-corrected emission lines. Owing to this analysis, three different ionization zones in the
envelope are proposed: the inner one, completely ionized, of about ∼ 5R*, an intermediate recombination zone extending up to ∼ [10 - 20]R*, and an outer extended zone of neutral H.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
19.
O. N. Sholukhova S. N. Fabrika A. V. Zharova A. F. Valeev V. P. Goranskij 《Astrophysical Bulletin》2011,66(2):123-143
We present the results of studying the spectral and photometric variability of the luminous blue variable star V532 in M33.
The photometric variations are traced from 1960 to 2010, spectral variations—from 1992 to 2009. The star has revealed an absolute
maximum of visual brightness (1992–1994, high/cold state) and an absolute minimum (2007–2008, low/hot state) with a brightness
difference of ΔB ≈ 2.3
m
. The temperature estimates in the absolute maximum and absolute minimum were found to be T ∼ 22000 K and T ∼ 42000 K, respectively. The variability of the spectrum of V532 is fully consistent with the temperature variations in its
photosphere, while both permitted and forbidden lines are formed in an extended stellar atmosphere. Broad components of the
brightest lines were found, the broadening of these components is due to electron scattering in the wind parts closest to
the photosphere. We measured the wind velocity as a difference between the emission and absorption peaks in the PCyg type
profiles. The wind velocity clearly depends on the size of the stellar photosphere or on the visual brightness, when brightness
declines, the wind velocity increases. In the absolute minimum a kinematic profile of the V532 atmosphere was detected. The
wind velocity increases and its temperature declines with distance from the star. In the low/hot state, the spectral type
of the star corresponds to WN8.5h, in the high/cold state—to WN11. We studied the evolution of V532 along with the evolution
of AGCar and the massive WR binary HD5980 in SMC. During their visual minima, all the three stars perfectly fit with the WNL
star sequence by Crowther and Smith (1997). However, when visual brightness increases, all the three stars form a separate
sequence. It is possible that this reflects a new property of LBV stars, namely, in the high/cold states they do not pertain
to the bona fide WNL stars. 相似文献
20.
By systematically searching regions around planetary nebulae (PNe) for signs of interactions of their precursors’ wind with
ambient matter we found a number of huge IRAS dust structures. Some of them may be chance projections, but a few appear to
be real, like those around NGC 6826 and NGC 2899. In the case of NGC 6826 we noticed a giant (∼2°) bipolar dust emission,
whose axis is along the proper motion of the central star. The PN itself is offset in the direction of motion both as to the
center of this ∼30 pc large dust structure and to the center of a similarly large new Hα nebula. NGC 2899 was found in the center of a 14×11 pc quadrupolar cavity, whose directions of axes coincide with the directions
of the main axes of the optical PN. In both cases, the formation of these structures appears to have commenced in the asymptotic
giant branch (AGB) phase. 相似文献