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1.
We discuss the behavior of density fluctuations in an expanding universe and show that these should lead to the early formation
of pregalactic hydrogen-helium stars of several hundred to several thousand solar masses. These stars flood the universe with
radiation having a color temperature ≳105 K; this terminates star formation but permits galaxy formation to continue. About 10−2 of the mass of the galaxies is converted into heavy elements by pregalactic nucleosynthesis, with an error factor of a few. 相似文献
2.
Nucleosynthetic yields and production rates of helium and heavy elements are derived using new initial mass functions which
take into account the recent revisions in O star counts and the stellar models of Maeder (1981a, b) which incorporate the
effects of massloss on evolution. The current production rates are significantly higher than the earlier results due to Chiosi
& Caimmi (1979) and Chiosi (1979), and a near-uniform birthrate operating over the history of the galactic disc explains the
currently observed abundances. However, the yields are incompatibly high, and to obtain agreement it is necessary to assume
that stars above a certain mass do not explode but proceed to total collapse. Further confirmation of this idea comes from
the consideration of the specific yields and production rates of oxygen, carbon and iron and the constraints imposed by the
observational enrichment history in the disc as discussed by Twarog & Wheeler (1982). Substantial amounts of4He and14C, amongst the primary synthesis species, are contributed by the intermediate mass stars in their wind phases. If substantial
numbers of them exploded as Type I SN, their contribution to the yields of12C and56Fe would be far in excess of the requirements of galactic nucleosynthesis. Either efficient massloss precludes such catastrophic
ends for these stars, or the current stellar models are sufficiently in error to leave room for substantial revisions in the
specific yields. The proposed upward revision of the12C (α,γ)16O rate may produce the necessary changes in stellar yields to provide a solution to this problem. Stars that produce most
of the metals in the Galaxy are the same ones that contribute most to the observed supernova rate. 相似文献
3.
《Experimental Astronomy》2009,23(1):91-120
The primary scientific goal of the GRIPS mission is to revolutionize our understanding of the early universe using γ-ray bursts. We propose a new generation gamma-ray observatory capable of unprecedented spectroscopy over a wide range of
γ-ray energies (200 keV–50 MeV) and of polarimetry (200–1000 keV). The γ-ray sensitivity to nuclear absorption features enables the measurement of column densities as high as 1028cm − 2. Secondary goals achievable by this mission include direct measurements of all types of supernova interiors through γ-rays from radioactive decays, nuclear astrophysics with massive stars and novae, and studies of particle acceleration near
compact stars, interstellar shocks, and clusters of galaxies.
See for the authors’ affiliations. 相似文献
4.
Matthew R. Edwards 《Astrophysics and Space Science》2012,339(1):13-17
Due to the Hubble redshift, photon energy, chiefly in the form of CMBR photons, is currently disappearing from the universe
at the rate of nearly 1055 erg s−1. An ongoing problem in cosmology concerns the fate of this energy. In one interpretation it is irretrievably lost, i.e., energy is not conserved on the cosmic scale. Here we consider a different possibility which retains universal energy conservation.
Treating gravitational potential energy conventionally as ‘negative’, it has earlier been proposed that the Hubble shift flips
positive energy (photons) to negative energy (gravitons) and vice versa. The lost photon energy would thus be directed towards gravitation, making gravitational energy wells more negative. Conversely, within astrophysical bodies, the flipping of gravitons
to photons would give rise to a ‘Hubble luminosity’ of magnitude −UH
0, where U is the internal gravitational potential energy of the object. Preliminary evidence of such an energy release is presented
in bodies ranging from planets, white dwarfs and neutron stars to supermassive black holes and the visible universe. 相似文献
5.
The effect of time dependent bulk viscosity on the evolution of Friedmann models with zero curvature in Brans-Dicke theory
is studied. The solutions of the field equations with ‘gamma-law’ equation of state p = (γ-1) ρ, where γ varies continuously
as the Universe expands, are obtained by using the power-law relation φ = bR
n
, which lead to models with constant deceleration parameter. We obtain solutions for the inflationary period and radiation
dominated era of the universe. The physical properties of cosmological solutions are also discussed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
6.
D. C. V. Mallik 《Journal of Astrophysics and Astronomy》1981,2(2):171-185
The current rate of nucleosynthesis in the solar neighbourhood is re-evaluated on the basis of Arnett’s (1978) stellar yields,
the mass loss models of Chiosi, Nasi and Sreenivasan (1978) and the initial mass function determined by Lequeux (1978). If
massive stars are held responsible for most of the metals we observe, a higher birthrate of these stars in the past is indicated
in view of the low current rate of nucleosynthesis. The intermediate mass stars may not supply the bulk of the metals unless
total disruption of their carbon core takes place.
While a declining birthrate is in conflict with the result obtained from the age-metallicity relation of stars, it is supported
by some galactic evolution models which interpret successfully the white dwarf mass distribution data. If the constraint of
a nearly time-invariant birthrate were strictly accepted, then models of the prompt initial enrichment type are required to
explain the observed abundances in terms of nucleosynthesis in massive stars. 相似文献
7.
Donald d. Clayton 《Meteoritics & planetary science》1999,34(Z4):A145-A160
Abstract— This historical review focuses on the idea that this very abundant chemical element was overwhelmingly created and ejected from the stars not in its own chemical form but that of radioactive Ni progenitors. Iron in the universe outnumbers all of the common metals. Its thermonuclear origin provided the beginnings for the theory of nucleosynthesis in stars. Three of its isotopes (masses 54, 56, and 57) are counted among the most prominent isotopes of any element. Two of these isotopes (56Fe and 57Fe) are now known to have derived naturally from the radioactive decay of Ni isobars outside of exploding stars. Tension and numerous mistakes surrounding the discovery of its radiogenic origin are analyzed with historical accuracy. The radioactive origin is described as having been first overlooked and later resisted to considerable degree. But incontrovertible evidence, especially from supernova light curves and gamma-ray-line astronomy, has established its correctness. Radiogenic Fe thus remains the centerpiece for both the theory and the observations of nucleosynthesis in stars. 相似文献
8.
In this paper it is suggested that inclusion of mutual gravitational interactions among the particles in the early dense universe
can lead to a ‘pre-big bang’ scenario, with particle masses greater than the Planck mass implying an accelerating phase of
the universe, which then goes into the radiation phase when the masses fall below the Planck mass. The existence of towers
of states of such massive particles (i.e. multiples of Planck mass) as implied in various unified theories, provides rapid
acceleration in the early universe, similar to the usual inflation scenario, but here the expansion rate goes over ‘smoothly’
to the radiation dominated universe when temperature becomes lower than the Planck temperature. 相似文献
9.
String cosmological models with bulk viscosity are investigated in Kantowski-Sachs space-time. To obtain a determinate solution,
it is assumed that the coefficient of bulk viscosity is a power function of the scalar of expansion ζ = kθm and the scalar of expansion is proportional to the shear scalar θ ∝ σ, which leads to a relation between metric potentials
R = AS
n
. The physical and geometrical aspects of the model are also discussed. It is shown that the bulk viscosity has significant
influence on the evolution of the universe. There is a ‘big bang’ start in the model when m ≤ 1 but there is no ‘big bang’
start when m > 1. 相似文献
10.
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
11.
Questions of the equilibrium, stability, and observational manifestations of strange stars are considered, in which electrical
neutralization of the quark matter is provided by positrons, as occurs for some sets of bag parameters resulting in a stiffer
equation of state. Such models consist entirely of self-contained, strange quark matter and their maximum mass reaches 2.4–2.5
M⊙ with a radius of 13–14 km. The cooling of such strange quark stars both in the absence and in the presence of mass accretion
is investigated. It is shown that in the absence of mass accretion onto the strange star, the dependence of temperature (T,
K) on age (t, yr) depends very little on the mass of the configuration and has the form T ≈ 2.3·108r−1/5. If the star’s initial temperature is sufficiently high (T0≥2·1010K), then the total number of electron-positron pairs emitted does not depend on it and is determined only by the total mass
of the configuration. In the case of accretion, the annihilation of electrons of the infalling fatter with positrons of the
strange quark matter results in the emission of γ-rays with an energy of∼0.5 MeV, by observing which one can distinguish candidates
for strange stars. The maximum temperature of strange stars with mass accretion is calculated.
Translated from Astrofizika, Vol. 42, No. 4, pp. 617–630, October–December, 1999. 相似文献
12.
In understanding the nucleosynthesis of the elements in stars, one of the most important quantities is the reaction rate and
it must be evaluated in terms of the stellar temperature T, and its determination involves the knowledge of the excitation function σ(E) of the specific nuclear reaction leading to the final nucleus. In this paper, the effect of thermonuclear reaction rates
to the pre-main sequence evolution of low mass stars having masses 0.7, 0.8, 0.9 and 1M⊙ are studied by using our modified Stellar Evolutionary Program. 相似文献
13.
E. P. J. van den Heuvel 《Journal of Astrophysics and Astronomy》1995,16(2):255-288
The evolution of high-and low-mass X-ray binaries (HMXB and LMXB) into different types of binary radio pulsars, the ‘high-mass
binary pulsars’(HMBP) and ‘low-mass binary pulsars’ (LMBP) is discussed. The HMXB evolve either into Thorne-Zytkow objects
or into short-period binaries consisting of a helium star plus a neutron star (or a black hole), resembling Cygnus X-3. The
latter systems evolve (with or without a second common-envelope phase) into close binary pulsars, in which the companion of
the pulsar may be a massive white dwarf, a neutron star or a black hole ( some final systems may also consist of two black
holes). A considerable fraction of the systems may also be disrupted in the second supernova explosion. We discuss the possible
reasons why the observed numbers of double neutron stars and of systems like Cyg X-3 are several orders of magnitude lower
than theoretically predicted. It is argued that the observed systems form the tip of an iceberg of much larger populations
of unobserved systems, some of which may become observable in the future. As to the LMBP, we consider in some detail the origins
of systems with orbital periods in the range 1–20 days. We show that to explain their existence, losses of orbital angular
momentum (e.g., by magnetic braking) and in a number of cases: also of mass, have to be taken into account. The masses of
the low-mass white dwarf companions in these systems can be predicted accurately. We notice a clear correlation between spin
period and orbital period for these systems, as well as a clear correlation between pulsar magnetic field strength and orbital
period. These relations strongly suggest that increased amounts of mass accreted by the neutron stars lead to increased decay
of their magnetic fields: we suggest a simple way to understand the observed value of the ‘bottom’ field strengths of a few
times 108 G. Furthermore, we find that the LMBP-systems in which the pulsar has a strong magnetic field (> 1011 G) have an about two orders of magnitude larger birth rate (i.e., about 4 × 10-4 yr-1 in the Galaxy) than the systems with millisecond pulsars (which have B < 109 G). Using the observational fact that neutron stars receive a velocity kick of ∼450 km/s at birth, we find that some 90%
of the potential progenitor systems of the strong-field LMBP must have been disrupted in the Supernovae in which their neutron
stars were formed. Hence, the formation rate of the progenitors of the strong-field LMBP is of the same order as the galactic
supernova rate (4 × 10-3 yr-1). This implies that a large fraction of all Supernovae take place in binaries with a close low-mass (< 2.3 M⊙) companion. 相似文献
14.
Zhe Chen Jiang Zhang YanPing Chen WenYuan Cui Bo Zhang 《Astrophysics and Space Science》2006,306(1-2):33-39
The supernova yields of r-process elements are obtained as a function of the mass of their progenitor stars from the abundance patterns of extremely metal-poor stars on the left-side [{Ba/Mg}]--[{Mg/H}] boundary with a procedure proposed by Tsujimoto and Shigeyama. The ejected masses of r-process elements associated with stars of progenitor mass M
ms ≤ 18 M
⊙ are infertile sources and the SNe II with 20 M
⊙ ≤ M
ms ≤ 40 M
⊙ are the dominant source of r-process nucleosynthesis in the Galaxy. The ratio of these stars 20 M
⊙ ≤ M
ms ≤ 40 M
⊙
with compared to the all massive stars is about∼ 18%. In this paper, we present a simple model that describes a star's [r/Fe] in terms of the nucleosynthesis yields of r-process elements and the number of SN II explosions. Combined the r-process yields obtained by our procedure with the scatter model of the Galactic halo, the observed abundance patterns of the metal-poor stars can be well reproduced. 相似文献
15.
This review presents a selection of recent highlights of observations of R Coronae Borealis variables. Emphasis is placed
on an abundance analysis of a complete sample (18 stars) of the warm galactic RCBs. It is shown that 14 of the 18 have very
similar compositions: the iron mass fraction ranges about a factor of 3 around the solar value (assuming C/He = 3%) but abundance
ratios X/Fe for elements from Na to Ba show little variation. By contrast, the other 4 stars are deficient in iron but not
in Na, Si, S and some other elements. With for example, [Si/Fe] ≃ 2, the quartet is indeed ‘peculiar’. One of the quartet,
V854 Cen shows depletions of elements (other than CNO) similar to the depletions seen in interstellar medium corresponding
to average logn(H
tot) = − 1.5. Scenarios for creating RCB from normal single and double stars are summarised.
Invited review talk presented at the Asia-Pacific regional IAU meeting held at Pune from 16–20 August 1993. 相似文献
16.
Hasmukh K. Tank 《Astrophysics and Space Science》2011,336(2):341-343
This letter points out that the values of ‘critical-acceleration’ of MOND, and the ‘accelerated-expansion’ of the universe
are just two of the fourteen strikingly equal values of accelerations recurring in different physical situations. Some of
them could be explained by a new law of equality of potential-energy and energy-of-mass of reasonably-independent systems
(Tank in Astrophys. Space Sci. 330:203–205, 2010; Tank in Adv. Stud. Theor. Phys. 5:45–55, 2011). This new conservation-law, of equality of potential-energy, energy-of-mass and ‘kinetic-energy’ may be a clue to understand
MOND, and the ‘accelerated-expansion’ of the universe. Alternative expressions for the cosmological red-shift, the ‘critical-acceleration’
of MOND and Newton’s law of universal gravitation are also presented for comparison of three different accelerations. 相似文献
17.
The probabilities of discovering detached close binary (type DM) stars as eclipsing variables are calculated as a function
of the mass of the main component, mass ratio, major semiaxis, and angle of inclination of the orbit. The case of total limb
darkening (hypothesis “D”) is examined. This is compared with earlier results for uniformly bright stellar disks (hypothesis
“U”). Based on data from Svechnikov and Kuznetsova’s Catalog of Approximate Photometric and Absolute Elements of Eclipsing
Variables, the spatial density of stars of this type in the neighborhood of the sun is estimated to be ≈ 460 · 10
−6
pc−3.
__________
Translated from Astrofizika, Vol. 49, No. 1, pp. 151–169 (February 2006). 相似文献
18.
It is surprising that we hardly know only 4% of the universe. Rest of the universe is made up of 73% of dark-energy and 23%
of dark-matter. Dark-energy is responsible for acceleration of the expanding universe; whereas dark-matter is said to be necessary
as extra-mass of bizarre-properties to explain the anomalous rotational-velocity of galaxy. Though the existence of dark-energy
has gradually been accepted in scientific community, but the candidates for dark-matter have not been found as yet and are
too crazy to be accepted. Thus, it is obvious to look for an alternative theory in place of dark-matter. Milgrom (Astrophys.
J. 270:365, 1983a; 270:371, 1983b) has suggested a ‘Modified Newtonian Dynamics (MOND)’ which appears to be highly successful for explaining the anomalous
rotational-velocity. But unfortunately MOND lacks theoretical support. The MOND, in-fact, is (empirical) modification of Newtonian-Dynamics
through modification in the kinematical acceleration term ‘a’ (which is normally taken as
a=\fracv2ra=\frac{v^{2}}{r}) as effective kinematic acceleration
aeffective = a m(\fracaa0)a_{\mathit{effective}} = a \mu(\frac{a}{a_{0}}), wherein the μ-function is 1 for usual-values of accelerations but equals to
\fracaa0 ( << 1)\frac{a}{a_{0}} (\ll1) if the acceleration ‘a’ is extremely-low lower than a critical value a
0(10−10 m/s2). In the present paper, a novel variant of MOND is proposed with theoretical backing; wherein with the consideration of universe’s
acceleration a
d
due to dark-energy, a new type of μ-function on theoretical-basis emerges out leading to
aeffective = a(1 -K \fraca0a)a_{\mathit{effective}} = a(1 -K \frac{a_{0}}{a}). The proposed theoretical-MOND model too is able to fairly explain ‘qualitatively’ the more-or-less ‘flat’ velocity-curve
of galaxy-rotation, and is also able to predict a dip (minimum) on the curve. 相似文献
19.
R. A. W. Bradford 《Journal of Astrophysics and Astronomy》2009,30(2):119-131
By calculation of the proton-proton capture cross-section, it is shown that the existence of a bound diproton state would
not lead to significant production of diprotons during big bang nucleosynthesis, contrary to popular belief. In typical stellar
interiors, the stability of diprotons would lead to a reaction pathway for converting protons to deuterons perhaps ∼1010 times faster than the usual weak capture reaction. This would prevent stars of the familiar hot, dense type from occurring
in the universe. However, if diproton stability is achieved by an increase in the low-energy strong coupling, gs, then stars with temperatures and densities sufficiently reduced so as to offset the faster reaction pathway to deuterium
will appear to meet elementary stability criteria. The claim that there is a fine-tuned, anthropic upper bound to the strong
force which ensures diproton instability therefore appears to be unfounded. 相似文献
20.
Martin J. Rees 《Journal of Astrophysics and Astronomy》1984,5(4):331-348
The evidence for unseen mass (which is briefly reviewed) suggests that the cosmological density parameter Ω is at least 0.1–0.2.
An Einstein-de-Sitter ‘flat’ universe with Ω = 1 — which is appealing for theoretical reasons — can only be reconciled with
the data if the galaxies are more ‘clumped’ than the overall mass distribution, and are poor tracers of the unseen mass even
on scales of several Mpc. Possible forms for the unseen mass are discussed; and feedback processes are outlined whereby galaxy
formation can be suppressed in underdense regions. 相似文献