2,063 research outputs found
Evolution and nucleosynthesis of extremely metal-poor and metal-free low- and intermediate-mass stars II. s-process nucleosynthesis during the core He flash
Models of primordial and hyper-metal-poor stars with masses similar to the
Sun experience an ingestion of protons into the hot core during the core helium
flash phase at the end of their red giant branch evolution. This produces a
concurrent secondary flash powered by hydrogen burning that gives rise to
further nucleosynthesis in the core. We perform post-process nucleosynthesis
calculations on a one-dimensional stellar evolution calculation of a star of 1
solar mass and metallicity [Fe/H] = -6.5 that suffers a proton ingestion
episode. Our network includes 320 nuclear species and 2,366 reactions and
treats mixing and burning simultaneously. The mixing and burning of protons
into the hot convective core leads to the production of 13C, which then burns
via the 13C(alpha,n)16O reaction releasing a large number of free neutrons.
During the first two years of neutron production the neutron poison 14N
abundance is low, allowing the prodigious production of heavy elements such as
strontium, barium, and lead via slow neutron captures (the s process). These
nucleosynthetic products are later mixed to the stellar surface and ejected via
stellar winds. We compare our results with observations of the hyper-metal-poor
halo star HE 1327-2326, which shows a strong Sr overabundance. Our model
provides the possibility of self-consistently explaining the Sr overabundance
in HE 1327-2326 together with its C, N, and O overabundances (all within a
factor of ~4) if the material were heavily diluted, for example, via mass
transfer in a wide binary system. The model produces at least 18 times too much
Ba than observed, but this may be within the large modelling uncertainties. In
this scenario, binary systems of low mass must have formed in the early
Universe. If true then this puts constraints on the primordial initial mass
function.Comment: Accepted for publication on Astronomy & Astrophysics Letter
Evolution and CNO yields of Z=10^-5 stars and possible effects on CEMP production
Our main goals are to get a deeper insight into the evolution and final fates
of intermediate-mass, extremely metal-poor (EMP) stars. We also aim to
investigate their C, N, and O yields. Using the Monash University Stellar
Evolution code we computed and analysed the evolution of stars of metallicity Z
= 10^-5 and masses between 4 and 9 M_sun, from their main sequence until the
late thermally pulsing (super) asymptotic giant branch, TP-(S)AGB phase. Our
model stars experience a strong C, N, and O envelope enrichment either due to
the second dredge-up, the dredge-out phenomenon, or the third dredge-up early
during the TP-(S)AGB phase. Their late evolution is therefore similar to that
of higher metallicity objects. When using a standard prescription for the mass
loss rates during the TP-(S)AGB phase, the computed stars lose most of their
envelopes before their cores reach the Chandrasekhar mass, so our standard
models do not predict the occurrence of SNI1/2 for Z = 10^-5 stars. However, we
find that the reduction of only one order of magnitude in the mass-loss rates,
which are particularly uncertain at this metallicity, would prevent the
complete ejection of the envelope, allowing the stars to either explode as an
SNI1/2 or become an electron-capture SN. Our calculations stop due to an
instability near the base of the convective envelope that hampers further
convergence and leaves remnant envelope masses between 0.25 M_sun for our 4
M_sun model and 1.5 M_sun for our 9 M_sun model. We present two sets of C, N,
and O yields derived from our full calculations and computed under two
different assumptions, namely, that the instability causes a practically
instant loss of the remnant envelope or that the stars recover and proceed with
further thermal pulses. Our results have implications for the early chemical
evolution of the Universe.Comment: 12 pages, 13 figures, accepted for publication in A&
Transition of the Stellar Initial Mass Function Explored with Binary Population Synthesis
The stellar initial mass function (IMF) plays a crucial role in determining
the number of surviving stars in galaxies, the chemical composition of the
interstellar medium, and the distribution of light in galaxies. A key unsolved
question is whether the IMF is universal in time and space. Here we use
state-of-the-art results of stellar evolution to show that the IMF of our
Galaxy made a transition from an IMF dominated by massive stars to the
present-day IMF at an early phase of the Galaxy formation. Updated results from
stellar evolution in a wide range of metallicities have been implemented in a
binary population synthesis code, and compared with the observations of
carbon-enhanced metal-poor (CEMP) stars in our Galaxy. We find that applying
the present-day IMF to Galactic halo stars causes serious contradictions with
four observable quantities connected with the evolution of AGB stars.
Furthermore, a comparison between our calculations and the observations of CEMP
stars may help us to constrain the transition metallicity for the IMF which we
tentatively set at [Fe/H] = -2. A novelty of the current study is the inclusion
of mass loss suppression in intermediate-mass AGB stars at low-metallicity.
This significantly reduces the overproduction of nitrogen-enhanced stars that
was a major problem in using the high-mass star dominated IMF in previous
studies. Our results also demonstrate that the use of the present day IMF for
all time in chemical evolution models results in the overproduction of Type I.5
supernovae. More data on stellar abundances will help to understand how the IMF
has changed and what caused such a transition.Comment: 8 pages, 2 figures, accepted by MNRAS Lette
Measurement of core-shifts with astrometric multi-frequency calibration
VLBI is unique, among the space geodetic techniques, in its contribution to
defining and maintaining the International Celestial Reference Frame, providing
precise measurements of coordinates of extragalactic radiosources. The quest
for increasing accuracy of VLBI geodetic products has lead to a deeper revision
of all aspects that might introduce errors in the analysis. The departure of
the observed sources from perfect, stable, compact and achromatic celestial
targets falls within this category. This paper is concerned with the impact of
unaccounted frequency-dependent position shifts of source cores in the analysis
of dual-band S/X VLBI geodesy observations, and proposes a new method to
measure them. The multi-frequency phase transfer technique developed and
demonstrated by Middelberg et al. (2005) increases the high frequency coherence
times of VLBI observations, using the observations at a lower frequency. Our
proposed SOURCE/FREQUENCY PHASE REFERENCING method endows it with astrometric
applications by adding a strategy to estimate the ionospheric contributions.
Here we report on the first successful application to measure the core shift of
the quasar 1038+528 A at S and X-bands, and validate the results by comparison
with those from standard phase referencing techniques. In this particular case,
and in general in the cm-wavelength regime, both methods are equivalent.
Moreover the proposed method opens a new horizon with targets and fields
suitable for high precision astrometric studies with VLBI, especially at high
frequencies where severe limitations imposed by the rapid fluctuations in the
troposphere prevent the use of standard phase referencing techniques.Comment: 6 pages, 2 figures. Proceedings of 17th Working Meeting on European
VLBI for Geodesy and Astrometry (April 2005
Effect of Long-lived Strongly Interacting Relic Particles on Big Bang Nucleosynthesis
It has been suggested that relic long-lived strongly interacting massive
particles (SIMPs, or particles) existed in the early universe. We study
effects of such long-lived unstable SIMPs on big bang nucleosynthesis (BBN)
assuming that such particles existed during the BBN epoch, but then decayed
long before they could be detected. The interaction strength between an
particle and a nucleon is assumed to be similar to that between nucleons. We
then calculate BBN in the presence of the unstable neutral charged
particles taking into account the capture of particles by nuclei to form
-nuclei. We also study the nuclear reactions and beta decays of -nuclei.
We find that SIMPs form bound states with normal nuclei during a relatively
early epoch of BBN. This leads to the production of heavy elements which remain
attached to them. Constraints on the abundance of particles during BBN
are derived from observationally inferred limits on the primordial light
element abundances. Particle models which predict long-lived colored particles
with lifetimes longer than 200 s are rejected based upon these
constraints.Comment: 19 pages, 4 figure
Decoherence, fluctuations and Wigner function in neutron optics
We analyze the coherence properties of neutron wave packets, after they have
interacted with a phase shifter undergoing different kinds of statistical
fluctuations. We give a quantitative (and operational) definition of
decoherence and compare it to the standard deviation of the distribution of the
phase shifts. We find that in some cases the neutron ensemble is more coherent,
even though it has interacted with a wider (i.e. more disordered) distribution
of shifts. This feature is independent of the particular definition of
decoherence: this is shown by proposing and discussing an alternative
definition, based on the Wigner function, that displays a similar behavior. We
briefly discuss the notion of entropy of the shifts and find that, in general,
it does not correspond to that of decoherence of the neutron.Comment: 18 pages, 7 figure
Examining exotic structure of proton-rich nucleus Al
The longitudinal momentum distribution (P_{//}) of fragments after one-proton
removal from ^{23} Al and reaction cross sections (\sigma_R) for
^{23,24} Al on carbon target at 74A MeV have been measured. The ^{23,24} Al
ions were produced through projectile fragmentation of 135 A MeV ^{28} Si
primary beam using RIPS fragment separator at RIKEN. P_{//} is measured by a
direct time-of-flight (TOF) technique, while \sigma_R is determined using a
transmission method. An enhancement in \sigma_R is observed for ^{23} Al
compared with ^{24} Al. The P_{//} for ^{22} Mg fragments from ^{23} Al breakup
has been obtained for the first time. FWHM of the distributions has been
determined to be 232 \pm 28 MeV/c. The experimental data are discussed by using
Few-Body Glauber model. Analysis of P_{//} demonstrates a dominant d-wave
configuration for the valence proton in ground state of ^{23} Al, indicating
that ^{23} Al is not a proton halo nucleus
- …