2,790 research outputs found
Stellar Evolution Constraints on the Triple-Alpha Reaction Rate
We investigate the quantitative constraint on the triple-alpha reaction rate
based on stellar evolution theory, motivated by the recent significant revision
of the rate proposed by nuclear physics calculations. Targeted stellar models
were computed in order to investigate the impact of that rate in the mass range
of 0.8 < M / Msun < 25 and in the metallicity range between Z = 0 and Z = 0.02.
The revised rate has a significant impact on the evolution of low- and
intermediate-mass stars, while its influence on the evolution of massive stars
(M >~ 10 Msun) is minimal. We find that employing the revised rate suppresses
helium shell flashes on AGB phase for stars in the initial mass range 0.8 < M /
Msun < 6, which is contradictory to what is observed. The absence of helium
shell flashes is due to the weak temperature dependence of the revised
triple-alpha reaction cross section at the temperature involved. In our models,
it is suggested that the temperature dependence of the cross section should
have at least nu > 10 at T = 1 - 1.2 x 10^8 K where the cross section is
proportional to T^{nu}. We also derive the helium ignition curve to estimate
the maximum cross section to retain the low-mass first red giants. The
semi-analytically derived ignition curves suggest that the reaction rate should
be less than ~ 10^{-29} cm^6 s^{-1} mole^{-2} at ~ 10^{7.8} K, which
corresponds to about three orders of magnitude larger than that of the NACRE
compilation. In an effort to compromise with the revised rates, we calculate
and analyze models with enhanced CNO cycle reaction rates to increase the
maximum luminosity of the first giant branch. However, it is impossible to
reach the typical RGB tip luminosity even if all the reaction rates related to
CNO cycles are enhanced by more than ten orders of magnitude.Comment: 14 pages, 8 figures, accepted by the Ap
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Analysis of a discrete-time single-server queue with bursty imputs for traffic control in ATM networks
Due to a large number of bursty traffic sources that an ATM network is expected to support, controlling network traffic becomes essential to provide a desirable level of network performance with its users. Admission control and traffic smoothing are among the most promising control techniques for an ATM network. To evaluate the performance of an ATM network when it is subject to admission control or traffic smoothing, we build a discrete-time single-server queueing model where a new call joins the existing calls.In our model. it is assumed that the cell arrivals from a new call follow a general distribution. It is also assumed that the aggregated arrivals of cells from the existing calls form batch arrivals with a general distribution for the batch size and a geometric distribution for the interarrival times of batches. We consider both finite and infinite buffer cases, and analytically obtain the waiting time distribution and cell loss probability for a new call and for existing calls. Our analysis is an exact one. Through numerical examples, we investigate how the network performance depends on the statistics of a new call (burstiness, time that a call stays in active or inactive state, etc.). We also demonstrate the effectiveness of traffic smoothing to reduce network congestion
The IMF of Extremely Metal-Poor Stars and the Probe into the Star-Formation Process of the Milky Way
We discuss the star formation history of the Galaxy, based on the
observations of extremely metal-poor stars (EMP) in the Galactic halo, to gain
an insight into the evolution and structure formation in the early universe.
The initialmass function (IMF) of EMP stars is derived from the observed
fraction of carbon-enhanced EXP (CEMP) stars among the EMP survivors, which are
thought to originate from the evolution in the close binary systems with mass
transfer. Relying upon the theory of the evolution of EMP stars and of their
binary evolution, we find that stars of metallicity [Fe/H]<-2.5 were formed at
typical mass of ~10M_sun. The top heavy IMF thus obtained is applied to study
the early chemical evolution of the Galaxy. We construct the merging history of
our Galaxy semi-analytically and derive the metallicity distribution function
(MDF) of low-mass EMP stars that survive to date with taking into account the
contribution of binary systems. It is shown that the resultant MDF can well
reproduce the observed distribution of EMP survivors, and, in particular, that
they almost all stem from a less-mass companion in binary systems. We also
investigate how first stars affect the MDF of EMP stars.Comment: 5 pages, 4 figures, conference proceedings of First Star II
Carbon burning in intermediate mass primordial stars
The evolution of a zero metallicity 9 M_s star is computed, analyzed and
compared with that of a solar metallicity star of identical ZAMS mass. Our
computations range from the main sequence until the formation of a massive
oxygen-neon white dwarf. Special attention has been payed to carbon burning in
conditions of partial degeneracy as well as to the subsequent thermally pulsing
Super-AGB phase. The latter develops in a fashion very similar to that of a
solar metallicity 9 M_s star, as a consequence of the significant enrichment in
metals of the stellar envelope that ensues due to the so-called third dredge-up
episode. The abundances in mass of the main isotopes in the final ONe core
resulting from the evolution are X(^{16}O) approx 0.59, X(^{20}Ne) approx 0.28
and X(^{24}Mg) approx 0.05. This core is surrounded by a 0.05 M_s buffer mainly
composed of carbon and oxygen, and on top of it a He envelope of mass 10^{-4}
M_sComment: 11 pages, 11 figures, accepted for publication in A&
Evolution of Low- and Intermediate-Mass Stars with [Fe/H] <= -2.5
We present extensive sets of stellar models for 0.8-9.0Msun in mass and -5 <=
[Fe/H] <= -2 and Z = 0 in metallicity. The present work focuses on the
evolutionary characteristics of hydrogen mixing into the He-flash convective
zones during the core and shell He flashes which occurs for the models with
[Fe/H] <~ -2.5. Evolution is followed from the zero age MS to the TPAGB phase
including the hydrogen engulfment by the He-flash convection during the RGB or
AGB phase. There exist various types of mixing episodes of how the H mixing
sets in and how it affects the final abundances at the surface. In particular,
we find H ingestion events without dredge-ups that enables repeated
neutron-capture nucleosynthesis in the He flash convective zones with 13
C(a,n)16 O as neutron source. For Z = 0, the mixing and dredge-up processes
vary with the initial mass, which results in different final abundances in the
surface. We investigate the occurrence of these events for various initial mass
and metallicity to find the metallicity dependence for the He-flash driven deep
mixing (He-FDDM) and also for the third dredge-up (TDU) events. In our models,
we find He-FDDM for M <= 3Msun for Z = 0 and for M <~ 2Msun for -5 <~ [Fe/H] <~
-3. On the other hand, the occurrence of the TDU is limited to the mass range
of ~1.5Msun to ~5Msun for [Fe/H] = -3, which narrows with decreasing
metallicity. The paper also discusses the implications of the results of model
computations for observations. We compared the abundance pattern of CNO
abundances with observed metal-poor stars. The origins of most iron-deficient
stars are discussed by assuming that these stars are affected by binary mass
transfer. We also point out the existence of a blue horizontal branch for -4 <~
[Fe/H] <~ -2.5.Comment: 19 pages, 12 figures, accepted by MNRA
Formation History of Metal-Poor Halo Stars with Hierarchical Model and the Effect of ISM accretion on the Most Metal-Poor Stars
We investigate the star formation and chemical evolution in the early
universe by considering the merging history of the Galaxy in the {\Lambda}CDM
scenario according to the extended Press-Schechter theory. We give some
possible constraints from comparisons with observation of extremely metal-poor
(EMP) stars. We demonstrate that (1) The hierarchical structure formation can
explain the characteristics of the observed metallicity distribution function
(MDF) including a break around [Fe/H]~-4. (2) A high mass IMF of peak mass
~10Msun with the contribution of binaries, derived from the statistics of
carbon enhanced EMP stars (Komiya et al. 2007), predicts the frequency of
low-mass survivors consistent with the number of EMP stars observed for
-4~<[Fe/H]~<-2.5. (3) The stars formed from primordial gas before the first
supernova explosions in their host mini-halos are assigned to the HMP stars
with [Fe/H]~-5. (4) There is no indication of significant changes in the IMF
and the binary contribution at metallicity -4~<[Fe/H]~<-2.5, or even larger as
long as the field stars of Galactic halo are concerned. We further study the
effects of the surface pollution through the accretion of ISM along the
chemical and dynamical evolution of the Galaxy for low-mass Pop.III and EMP
survivors. Because of shallower potential of smaller halos, the accretion of
ISM in the mini-halos in which these stars were born dominates the surface
metal pollution. This can account for the surface iron abundances as observed
for the HMP stars if the cooling and concentration of gas in their birth
mini-halos is taken into account. We also study the feedback effect from the
very massive Pop. III stars. The metal pre-pollution by PISNe is shown to be
compatible with the observed lack of their nucleosynthetic signatures when some
positive feedback on gas cooling works and changes IMF from being very massive
to being high mass.Comment: 20 pages, 14 figures. ApJ accepte
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