2,716 research outputs found
Ultrasonic Microdissection of Rat Cerebellum for Scanning Electron Microscopy
The cerebelli of rats were initially fixed with aldehydes (modified Karnovsky\u27s fixative; 503 mOsM/L) by cardiac perfusion. Blocks of tissue were razor-cut, usually longitudinal to folia, and immersed in the same fluid for 2-4 hours. Three separate methods of treatment followed: (1) immersion in 1% aqueous boric acid, or (2) in 2% phosphate buffered OsO4 followed by boric acid or (3) in an 8/2 mixture of boric acid and OsO4. After 18-48 hours immersion the blocks were dehydrated in ascending grades of acetone. They were then exposed to ultrasound in 100% acetone at frequencies of 80 kHz or 40 kHz for 10 to 20 minutes.
Microdissection of cut surfaces (erosion) occurs after all three treatments. It is least extensive after boric acid, moderate after OsO4 and greatest after the combined mixture. All cerebellar cell types are recognizable as are numerous fibers according to morphology and position. Variable erosion accommodates analysis of different levels of neural organization. In general, structural situations not involving great depth of field are best revealed by H3BO3 or OsO4. Blood vascular relationships to other structures are best demonstrated in deeply eroded specimens
Theoretical Studies of Accretion of Matter onto White Dwarfs and the Single Degenerate Scenario for Supernovae of Type Ia
We present a brief summary of the Single Degenerate Scenario for the
progenitors of Type Ia Supernovae in which it is assumed that a low mass
carbon-oxygen white dwarf is growing in mass as a result of accretion from a
secondary star in a close binary system. Recent hydrodynamic simulations of
accretion of solar material onto white dwarfs without mixing always produce a
thermonuclear runaway and steady burning does not occur. For a broad range in
WD mass (0.4 Solar masses to 1.35 Solar Masses), the maximum ejected material
occurs for the 1.25 Solar Mass sequences and then decreases as the white dwarf
mass decreases. Therefore, the white dwarfs are growing in mass as a
consequence of the accretion of solar material and as long as there is no
mixing of accreted material with core material. In contrast, a thermonuclear
runaway in the accreted hydrogen-rich layers on the low luminosity WDs in close
binary systems where mixing of core matter with accreted material has occurred
is the outburst mechanism for Classical, Recurrent, and Symbiotic novae. The
differences in characteristics of these systems is likely the WD mass and mass
accretion rate. The high levels of enrichment of CN ejecta in elements ranging
from carbon to sulfur confirm that there is dredge-up of matter from the core
of the WD and enable them to contribute to the chemical enrichment of the
interstellar medium. Therefore, studies of CNe can lead to an improved
understanding of Galactic nucleosynthesis, some sources of pre-solar grains,
and the Extragalactic distance scale. The characteristics of the outburst
depend on the white dwarf mass, luminosity, mass accretion rate, and the
chemical composition of both the accreting material and WD material. The
properties of the outburst also depends on when, how, and if the accreted
layers are mixed with the WD core and the mixing mechanism is still unknown.Comment: 25 Pages, Bulletin of the Astronomical Society of India (BASI) in
pres
Hydrodynamic Studies of the Evolution of Recurrent, Symbiotic, and Dwarf Novae: The White Dwarf Components are Growing in Mass
Symbiotic binaries are systems containing white dwarfs (WDs) and red giants. Symbiotic novae are those systems in which thermonuclear eruptions occur on the WD components. These are to be distinguished from events driven by accretion disk instabilities analogous to dwarf novae eruptions in cataclysmic variable outbursts. Another class of symbiotic systems are those in which the WD is extremely luminous and it seems likely that quiescent nuclear burning is ongoing on the accreting WD. A fundamental question is the secular evolution of the WD. Do the repeated outbursts or quiescent burning in these accreting systems cause the WD to gain or lose mass? If it is gaining mass, can it eventually reach the Chandrasekhar Limit and become a supernova (a SN Ia if it can hide the hydrogen and helium in the system)? In order to better understand these systems, we have begun a new study of the evolution of Thermonuclear Runaways (TNRs) in the accreted envelopes of WDs using a variety of initial WD masses, luminosities and mass accretion rates. We use our 1-D hydro code, NOVA, which includes the new convective algorithm of Arnett, Meakin and Young, the Hix and Thielemann nuclear reaction solver, the Iliadis reaction rate library, the Timmes equation of state, and the OPAL opacities. We assume a solar composition (Lodders abundance distribution) and do not allow any mixing of accreted material with core material. This assumption strongly influences our results. We report here (1) that the WD grows in mass for all simulations so that canonical `steady burning' does not occur, and (2) that only a small fraction of the accreted matter is ejected in some (but not all) simulations. We also find that the accreting systems, before thermonuclear runaway, are too cool to be seen in X-ray searches for SN Ia progenitors
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
The First Events in Photosynthesis: Electronic Coupling and Energy Transfer Dynamics in the Photosynthetic Reaction Center from Rhodobacter sphaeroides
The rapid electronic state dynamics that occur prior to charge separation in the photosynthetic reaction center of Rhodobacter sphaeroides R-26 are investigated by "two-color" wavelength-resolved pump-probe and anisotropy measurements. A narrow band (40 fs duration transform limited) pump pulse is used to selectively excite reaction center pigments: the accessory bacteriochlorophyll (B), the upper excitonic state of the special pair (P y+ ), or the lower excitonic state of the special pair (P y-). Population dynamics are then measured with a 12 fs duration probe pulse across the entire Q y absorption spectral region as a function of time, wavelength, and polarization. Excitation of either P y-or B results in the formation of a distinct optical band at 825 nm exhibiting polarization characteristics consistent with those expected for P y+ ; the band appears instantaneously upon excitation of P y-with a negative anisotropy and appears somewhat delayed after excitation of B. The dynamics observed following direct excitation of the P y+ absorption band, that is identified to occur at 825 nm, suggests that internal conversion between the excitonic states of P is rapid, occurring with a 65 fs time constant. Excitation of the accessory BChl (i.e., populating the excited state, B*) provides a detailed answer for the mechanism of energy transfer within the bacterial reaction center. The process proceeds via a twostep mechanism, flowing sequentially from B* to P y+ to P y-with time constants of 120 and 65 fs, respectively. These results follow from a kinetic model analysis of several pump-wavelength-dependent and polarizationdependent differential probe transmission transients that yield the first spectrum of P y+ at room temperature. The coherent excitonic dynamics of the special pair states, P y-and P y+ , are measured and analyzed for coupling strengths and time scales for electronic dephasing and population relaxation. These results, in conjunction with a range of the transient transmission spectra, suggest that the initially excited state of the zeroth order chromophores, i.e., B* and P y+ , is delocalized at the earliest times, consistent with a supermolecular picture of the reaction center
Explosive Nucleosynthesis: What we learned and what we still do not understand
This review touches on historical aspects, going back to the early days of
nuclear astrophysics, initiated by BFH and Cameron, discusses (i) the
required nuclear input from reaction rates and decay properties up to the
nuclear equation of state, continues (ii) with the tools to perform
nucleosynthesis calculations and (iii) early parametrized nucleosynthesis
studies, before (iv) reliable stellar models became available for the late
stages of stellar evolution. It passes then through (v) explosive environments
from core-collapse supernovae to explosive events in binary systems (including
type Ia supernovae and compact binary mergers), and finally (vi) discusses the
role of all these nucleosynthesis production sites in the evolution of
galaxies. The focus is put on the comparison of early ideas and present, very
recent, understanding.Comment: 11 pages, to appear in Springer Proceedings in Physics (Proc. of
Intl. Conf. "Nuclei in the Cosmos XV", LNGS Assergi, Italy, June 2018
Turbulence and nuclear reactions in 3D hydrodynamics simulations of massive stars
Our knowledge of massive star evolution and nucleosynthesis is limited by uncertainties related to multi-dimensional processes taking place in stellar interiors. Recently, theoretical works have started to improve 1D stellar evolution codes through the implementation of results from 3D hydrodynamics models, which are used to study multi-D processes on a short time range (minutes or hours) and improve 1D prescriptions. In these proceedings, we present results coming from a new set of high-resolution hydrodynamics simulations of the neon-burning shell in a massive star, employing the PROMPI code. We focus in particular on the interplay between turbulence and nuclear reactions, discussing the impact that different boosting factors of the nuclear rates have on the results. This has important implications for supernova studies, nucleosynthesis, the physics of neutron stars and black holes.</jats:p
Nucleosynthesis in multi-dimensional SNIa explosions
We present the results of nucleosynthesis calculations based on
multidimensional (2D and 3D) hydrodynamical simulations of the thermonuclear
burning phase in SNIa. The detailed nucleosynthetic yields of our explosion
models are calculated by post-processing the ejecta, using passively advected
tracer particles. The nuclear reaction network employed in computing the
explosive nucleosynthesis contains 383 nuclear species. We analyzed two
different choices of ignition conditions (centrally ignited, in which the
spherical initial flame geometry is perturbated with toroidal rings, and
bubbles, in which multi-point ignition conditions are simulated). We show that
unburned C and O varies typically from ~40% to ~50% of the total ejected
material.The main differences between all our models and standard 1D
computations are, besides the higher mass fraction of unburned C and O, the C/O
ratio (in our case is typically a factor of 2.5 higher than in 1D
computations), and somewhat lower abundances of certain intermediate mass
nuclei such as S, Cl, Ar, K, and Ca, and of 56Ni. Because explosive C and O
burning may produce the iron-group elements and their isotopes in rather
different proportions one can get different 56Ni-fractions (and thus supernova
luminosities) without changing the kinetic energy of the explosion. Finally, we
show that we need the high resolution multi-point ignition (bubbles) model to
burn most of the material in the center (demonstrating that high resolution
coupled with a large number of ignition spots is crucial to get rid of unburned
material in a pure deflagration SNIa model).Comment: Accepted for A&A, 14 pages, 11 Figures, 2 Table
Collisional Dark Matter and the Origin of Massive Black Holes
If the cosmological dark matter is primarily in the form of an elementary
particle which has cross section and mass for self-interaction having a ratio
similar to that of ordinary nuclear matter, then seed black holes (formed in
stellar collapse) will grow in a Hubble time, due to accretion of the dark
matter, to a mass range 10^6 - 10^9 solar masses. Furthermore, the dependence
of the final black hole mass on the galaxy velocity dispersion will be
approximately as observed and the growth rate will show a time dependence
consistent with observations. Other astrophysical consequences of collisional
dark matter and tests of the idea are noted.Comment: 7 pages, no figures, LaTeX2e, Accepted for publication in Phys. Rev.
Lett. Changed conten
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