320 research outputs found
Cosmic-ray induced destruction of CO in star-forming galaxies
We explore the effects of the expected higher cosmic ray (CR) ionization
rates on the abundances of carbon monoxide (CO), atomic carbon
(C), and ionized carbon (C) in the H clouds of star-forming galaxies.
The study of Bisbas et al. (2015) is expanded by: a) using realistic
inhomogeneous Giant Molecular Cloud (GMC) structures, b) a detailed chemical
analysis behind the CR-induced destruction of CO, and c) exploring the thermal
state of CR-irradiated molecular gas. CRs permeating the interstellar medium
with (Galactic) are found to significantly
reduce the [CO]/[H] abundance ratios throughout the mass of a GMC. CO
rotational line imaging will then show much clumpier structures than the actual
ones. For (Galactic) this bias becomes
severe, limiting the utility of CO lines for recovering structural and
dynamical characteristics of H-rich galaxies throughout the Universe,
including many of the so-called Main Sequence (MS) galaxies where the bulk of
cosmic star formation occurs. Both C and C abundances increase with rising
, with C remaining the most abundant of the two throughout
H clouds, when (Galactic). C starts
to dominate for (Galactic). The thermal
state of the gas in the inner and denser regions of GMCs is invariant with
for (Galactic).
For (Galactic) this is no longer the case and
are reached. Finally we identify OH as the key
species whose sensitive abundance could mitigate the destruction
of CO at high temperatures.Comment: 17 pages, 12 figures, accepted by Ap
Cross section measurement of the astrophysically important 17O(p,gamma)18F reaction in a wide energy range
The 17O(p,g)18F reaction plays an important role in hydrogen burning
processes in different stages of stellar evolution. The rate of this reaction
must therefore be known with high accuracy in order to provide the necessary
input for astrophysical models.
The cross section of 17O(p,g)18F is characterized by a complicated resonance
structure at low energies. Experimental data, however, is scarce in a wide
energy range which increases the uncertainty of the low energy extrapolations.
The purpose of the present work is therefore to provide consistent and precise
cross section values in a wide energy range.
The cross section is measured using the activation method which provides
directly the total cross section. With this technique some typical systematic
uncertainties encountered in in-beam gamma-spectroscopy experiments can be
avoided.
The cross section was measured between 500 keV and 1.8 MeV proton energies
with a total uncertainty of typically 10%. The results are compared with
earlier measurements and it is found that the gross features of the 17O(p,g)18F
excitation function is relatively well reproduced by the present data.
Deviation of roughly a factor of 1.5 is found in the case of the total cross
section when compared with the only one high energy dataset. At the lowest
measured energy our result is in agreement with two recent datasets within one
standard deviation and deviates by roughly two standard deviations from a third
one. An R-matrix analysis of the present and previous data strengthen the
reliability of the extrapolated zero energy astrophysical S-factor.
Using an independent experimental technique, the literature cross section
data of 17O(p,g)18F is confirmed in the energy region of the resonances while
lower direct capture cross section is recommended at higher energies. The
present dataset provides a constraint for the theoretical cross sections.Comment: Accepted for publication in Phys. Rev. C. Abstract shortened in order
to comply with arxiv rule
Direct study of the alpha-nucleus optical potential at astrophysical energies using the 64Zn(p,alpha)61Cu reaction
In the model calculations of heavy element nucleosynthesis processes the
nuclear reaction rates are taken from statistical model calculations which
utilize various nuclear input parameters. It is found that in the case of
reactions involving alpha particles the calculations bear a high uncertainty
owing to the largely unknown low energy alpha-nucleus optical potential.
Experiments are typically restricted to higher energies and therefore no direct
astrophysical consequences can be drawn. In the present work a (p,alpha)
reaction is used for the first time to study the alpha-nucleus optical
potential. The measured 64Zn(p,alpha)61Cu cross section is uniquely sensitive
to the alpha-nucleus potential and the measurement covers the whole
astrophysically relevant energy range. By the comparison to model calculations,
direct evidence is provided for the incorrectness of global optical potentials
used in astrophysical models.Comment: Accepted for publication in Physical Review C as a Rapid
Communicatio
Cross section measurement of the 12C(p,gamma)13N reaction with activation in a wide energy range
The CNO cycle is one of the fundamental processes of hydrogen burning in
stars. The first reaction of the cycle is the radiative proton capture on 12C
and the rate of this 12C(p,gamma)13N reaction is related to the 12C/13C ratio
observed e.g. in the Solar System. The low-energy cross section of this
reaction was measured several times in the past, however, the experimental data
are scarce in a wide energy range especially around the resonance at 1.7 MeV.
In the present work the 12C(p,gamma)13N cross section was measured between 300
and 1900 keV using the activation method. This method was only used several
decades ago in the low-energy region. As the activation method provides the
total cross section and has uncertainties different from those of the in-beam
gamma-spectroscopy technique, the present results provide a largely independent
data set for future low-energy extrapolations and thus for astrophysical
reaction rate calculations.Comment: Accepted for publication in European Physical Journal
Cross section and reaction rate of 92Mo(p,gamma)93Tc determined from thick target yield measurements
For the better understanding of the astrophysical gamma-process the
experimental determination of low energy proton- and alpha-capture cross
sections on heavy isotopes is required. The existing data for the
92Mo(p,gamma)93Tc reaction are contradictory and strong fluctuation of the
cross section is observed which cannot be explained by the statistical model.
In this paper a new determination of the 92Mo(p,gamma)93Tc and
98Mo(p,gamma)99mTc cross sections based on thick target yield measurements are
presented and the results are compared with existing data and model
calculations. Reaction rates of 92Mo(p,gamma)93Tc at temperatures relevant for
the gamma-process are derived directly from the measured thick target yields.
The obtained rates are a factor of 2 lower than the ones used in astrophysical
network calculations. It is argued that in the case of fluctuating cross
sections the thick target yield measurement can be more suited for a reliable
reaction rate determination.Comment: Accepted for publication in Nuclear Physics
Chaotic Phase Synchronization in Bursting-neuron Models Driven by a Weak Periodic Force
We investigate the entrainment of a neuron model exhibiting a chaotic
spiking-bursting behavior in response to a weak periodic force. This model
exhibits two types of oscillations with different characteristic time scales,
namely, long and short time scales. Several types of phase synchronization are
observed, such as 1 : 1 phase locking between a single spike and one period of
the force and 1 : l phase locking between the period of slow oscillation
underlying bursts and l periods of the force. Moreover, spiking-bursting
oscillations with chaotic firing patterns can be synchronized with the periodic
force. Such a type of phase synchronization is detected from the position of a
set of points on a unit circle, which is determined by the phase of the
periodic force at each spiking time. We show that this detection method is
effective for a system with multiple time scales. Owing to the existence of
both the short and the long time scales, two characteristic phenomena are found
around the transition point to chaotic phase synchronization. One phenomenon
shows that the average time interval between successive phase slips exhibits a
power-law scaling against the driving force strength and that the scaling
exponent has an unsmooth dependence on the changes in the driving force
strength. The other phenomenon shows that Kuramoto's order parameter before the
transition exhibits stepwise behavior as a function of the driving force
strength, contrary to the smooth transition in a model with a single time
scale
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