147 research outputs found
The effect of 12C + 12C rate uncertainties on s-process yields
The slow neutron capture process in massive stars (the weak s-process)
produces most of the s-only isotopes in the mass region 60 < A < 90. The
nuclear reaction rates used in simulations of this process have a profound
effect on the final s-process yields. We generated 1D stellar models of a 25
solar mass star varying the 12C + 12C rate by a factor of 10 and calculated
full nucleosynthesis using the post-processing code PPN. Increasing or
decreasing the rate by a factor of 10 affects the convective history and
nucleosynthesis, and consequently the final yields.Comment: Conference proceedings for the Nuclear Physics in Astrophysics IV
conference, 8-12 June 2009. 4 pages, 3 figures. Accepted for publication to
the Journal of Physics: Conference Serie
NuGrid stellar data set. 1. Stellar yields from H to Bi for stars with metallicities Z=0.02 and Z=0.01
We provide a set of stellar evolution and nucleosynthesis calculations that applies established physics assumptions simultaneously to low- and intermediate-mass and massive star models. Our goal is to provide an internally consistent and comprehensive nuclear production and yield database for applications in areas such as presolar grain studies. Our non-rotating models assume convective boundary mixing (CBM) where it has been adopted before. We include 8 (12) initial masses for Z = 0.01 (0.02). Models are followed either until the end of the asymptotic giant branch phase or the end of Si burning, complemented by simple analytic core-collapse supernova (SN) models with two options for fallback and shock velocities. The explosions show which pre-SN yields will most strongly be effected by the explosive nucleosynthesis. We discuss how these two explosion parameters impact the light elements and the s and p process. For low- and intermediate-mass models, our stellar yields from H to Bi include the effect of CBM at the He-intershell boundaries and the stellar evolution feedback of the mixing process that produces the ¹³C pocket. All post-processing nucleosynthesis calculations use the same nuclear reaction rate network and nuclear physics input. We provide a discussion of the nuclear production across the entire mass range organized by element group. The entirety of our stellar nucleosynthesis profile and time evolution output are available electronically, and tools to explore the data on the NuGrid VOspace hosted by the Canadian Astronomical Data Centre are introduced
SNSPH: A Parallel 3-D Smoothed Particle Radiation Hydrodynamics Code
We provide a description of the SNSPH code--a parallel 3-dimensional
radiation hydrodynamics code implementing treecode gravity, smooth particle
hydrodynamics, and flux-limited diffusion transport schemes. We provide
descriptions of the physics and parallelization techniques for this code. We
present performance results on a suite of code tests (both standard and new),
showing the versatility of such a code, but focusing on what we believe are
important aspects of modeling core-collapse supernovae.Comment: 41 pages including 8 figures, submitted to ApJ, version with high
resolution figures and test data can be found at http://qso.lanl.gov/~clf
The effect of 12C +12C rate uncertainties on the evolution and nucleosynthesis of massive stars
Over the last 40 years, the 12C +12C fusion reaction has been the subject of considerable experimental efforts to constrain uncertainties at temperatures relevant for stellar nucleosynthesis. Recent studies have indicated that the reaction rate may be higher than that currently used in stellar models. In order to investigate the effect of an enhanced carbon-burning rate on massive star structure and nucleosynthesis, new stellar evolution models and their yields are presented exploring the impact of three different 12C +12C reaction rates. Non-rotating stellar models considering five different initial masses, 15, 20, 25, 32 and 60 M⊙, at solar metallicity, were generated using the Geneva Stellar Evolution Code (genec) and were later post-processed with the NuGrid Multi-zone Post-Processing Network tool (mppnp). A dynamic nuclear reaction network of ∼1100 isotopes was used to track the s-process nucleosynthesis. An enhanced 12C +12C reaction rate causes core carbon burning to be ignited more promptly and at lower temperature. This reduces the neutrino losses, which increases the core carbon-burning lifetime. An increased carbon-burning rate also increases the upper initial mass limit for which a star exhibits a convective carbon core (rather than a radiative one). Carbon-shell burning is also affected, with fewer convective-shell episodes and convection zones that tend to be larger in mass. Consequently, the chance of an overlap between the ashes of carbon-core burning and the following carbon shell convection zones is increased, which can cause a portion of the ashes of carbon-core burning to be included in the carbon shell. Therefore, during the supernova explosion, the ejecta will be enriched by s-process nuclides synthesized from the carbon-core s-process. The yields were used to estimate the weak s-process component in order to compare with the Solar system abundance distribution. The enhanced rate models were found to produce a significant proportion of Kr, Sr, Y, Zr, Mo, Ru, Pd and Cd in the weak component, which is primarily the signature of the carbon-core s-process. Consequently, it is shown that the production of isotopes in the Kr-Sr region can be used to constrain the 12C +12C rate using the current branching ratio for α- and p-exit channel
Physics of the Galactic Center Cloud G2, on its Way towards the Super-Massive Black Hole
The origin, structure and evolution of the small gas cloud, G2, is
investigated, that is on an orbit almost straight into the Galactic central
supermassive black hole (SMBH). G2 is a sensitive probe of the hot accretion
zone of Sgr A*, requiring gas temperatures and densities that agree well with
models of captured shock-heated stellar winds. Its mass is equal to the
critical mass below which cold clumps would be destroyed quickly by
evaporation. Its mass is also constrained by the fact that at apocenter its
sound crossing timescale was equal to its orbital timescale. Our numerical
simulations show that the observed structure and evolution of G2 can be well
reproduced if it formed in pressure equilibrium with the surrounding in 1995 at
a distance from the SMBH of 7.6e16 cm. If the cloud would have formed at
apocenter in the 'clockwise' stellar disk as expected from its orbit, it would
be torn into a very elongated spaghetti-like filament by 2011 which is not
observed. This problem can be solved if G2 is the head of a larger, shell-like
structure that formed at apocenter. Our numerical simulations show that this
scenario explains not only G2's observed kinematical and geometrical properties
but also the Br_gamma observations of a low surface brightness gas tail that
trails the cloud. In 2013, while passing the SMBH G2 will break up into a
string of droplets that within the next 30 years mix with the surrounding hot
gas and trigger cycles of AGN activity.Comment: 22 pages, 13 figures, submitted to Ap
Low angular momentum flow model of Sgr A* activity
Sgr A* is the closest massive black hole and can be observed with the highest
angular resolution. Nevertheless, our current understanding of the accretion
process in this source is very poor. The inflow is almost certainly of low
radiative efficiency and it is accompanied by a strong outflow and the flow is
strongly variable but the details of the dynamics are unknown. Even the amount
of angular momentum in the flow is an open question. Here we argue that low
angular momentum scenario is better suited to explain the flow variability. We
present a new hybrid model which describes such a flow and consists of an outer
spherically symmetric Bondi flow and an inner axially symmetric flow described
through MHD simulations. The assumed angular momentum of the matter is low,
i.e. the corresponding circularization radius in the equatorial plane of the
flow is just above the innermost stable circular orbit in pseudo-Newtonian
potential. We compare the radiation spectrum from such a flow to the broad band
observational data for Sgr A*.Comment: Proceedings of the AHAR 2008 Conference: The Universe under the
Microscope; Astrophysics at High Angular Resolution, Bad Honef
Consensuality of Peer Nominations Among Scientists
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/69069/2/10.1177_107554708200400210.pd
Traces of past activity in the Galactic Centre
The Milky Way centre hosts a supermassive Black Hole (BH) with a mass of
~4*10^6 M_Sun. Sgr A*, its electromagnetic counterpart, currently appears as an
extremely weak source with a luminosity L~10^-9 L_Edd. The lowest known
Eddington ratio BH. However, it was not always so; traces of "glorious" active
periods can be found in the surrounding medium. We review here our current view
of the X-ray emission from the Galactic Center (GC) and its environment, and
the expected signatures (e.g. X-ray reflection) of a past flare. We discuss the
history of Sgr A*'s past activity and its impact on the surrounding medium. The
structure of the Central Molecular Zone (CMZ) has not changed significantly
since the last active phase of Sgr A*. This relic torus provides us with the
opportunity to image the structure of an AGN torus in exquisite detail.Comment: Invited refereed review. Chapter of the book: "Cosmic ray induced
phenomenology in star forming environments" (eds. Olaf Reimer and Diego F.
Torres
The effect of 12C + 12C rate uncertainties on the evolution and nucleosynthesis of massive stars
[Shortened] The 12C + 12C fusion reaction has been the subject of
considerable experimental efforts to constrain uncertainties at temperatures
relevant for stellar nucleosynthesis. In order to investigate the effect of an
enhanced carbon burning rate on massive star structure and nucleosynthesis, new
stellar evolution models and their yields are presented exploring the impact of
three different 12C + 12C reaction rates. Non-rotating stellar models were
generated using the Geneva Stellar Evolution Code and were later post-processed
with the NuGrid Multi-zone Post-Processing Network tool. The enhanced rate
causes core carbon burning to be ignited more promptly and at lower
temperature. This reduces the neutrino losses, which increases the core carbon
burning lifetime. An increased carbon burning rate also increases the upper
initial mass limit for which a star exhibits a convective carbon core. Carbon
shell burning is also affected, with fewer convective-shell episodes and
convection zones that tend to be larger in mass. Consequently, the chance of an
overlap between the ashes of carbon core burning and the following carbon shell
convection zones is increased, which can cause a portion of the ashes of carbon
core burning to be included in the carbon shell. Therefore, during the
supernova explosion, the ejecta will be enriched by s-process nuclides
synthesized from the carbon core s process. The yields were used to estimate
the weak s-process component in order to compare with the solar system
abundance distribution. The enhanced rate models were found to produce a
significant proportion of Kr, Sr, Y, Zr, Mo, Ru, Pd and Cd in the weak
component, which is primarily the signature of the carbon-core s process.
Consequently, it is shown that the production of isotopes in the Kr-Sr region
can be used to constrain the 12C + 12C rate using the current branching ratio
for a- and p-exit channels.Comment: The paper contains 17 figures and 7 tables. Table 7 will be published
in full online onl
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