10,043 research outputs found
The surprising influence of late charged current weak interactions on Big Bang Nucleosynthesis
The weak interaction charged current processes (, , ) interconvert neutrons and protons in the early universe and
have significant influence on Big Bang Nucleosynthesis (BBN) light-element
abundance yields, particulary that for . We demonstrate that the
influence of these processes is still significant even when they operate well
below temperatures usually invoked for "weak freeze-out,"
and in fact down nearly into the alpha-particle formation epoch (). This physics is correctly captured in commonly used BBN
codes, though this late-time, low-temperature persistent effect of the
isospin-changing weak processes, and the sensitivity of the associated rates to
lepton energy distribution functions and blocking factors are not widely
appreciated. We quantify this late-time influence by analyzing weak interaction
rate dependence on the neutron lifetime, lepton energy distribution functions,
entropy, the proton-neutron mass difference, and Hubble expansion rate. The
effects we point out here render BBN a keen probe of any beyond-standard-model
physics that alters lepton number/energy distributions, even subtly, in epochs
of the early universe all the way down to near .Comment: 27 pages, 8 figure
The initial conditions of stellar protocluster formation. II. A catalogue of starless and protostellar clumps embedded in IRDCs in the Galactic longitude range 15<l<55
We present a catalogue of starless and protostellar clumps associated with
infrared dark clouds (IRDCs) in a 40 degrees wide region of the inner Galactic
Plane (b<1). We have extracted the far-infrared (FIR) counterparts of 3493
IRDCs with known distance in the Galactic longitude range 15<l<55 and searched
for the young clumps using Hi-GAL, the survey of the Galactic Plane carried out
with the Herschel satellite. Each clump is identified as a compact source
detected at 160, 250 and 350 mum. The clumps have been classified as
protostellar or starless, based on their emission (or lack of emission) at 70
mum. We identify 1723 clumps, 1056 (61%) of which are protostellar and 667
(39%) starless. These clumps are found within 764 different IRDCs, 375 (49%) of
which are only associated with protostellar clumps, 178 (23%) only with
starless clumps, and 211 (28%) with both categories of clumps. The clumps have
a median mass of 250 M_sun and range up to >10^4$ M_sun in mass and up to 10^5
L_sun in luminosity. The mass-radius distribution shows that almost 30% of the
starless clumps identified in this survey could form high-mass stars, however
these massive clumps are confined in only ~4% of the IRDCs. Assuming a minimum
mass surface density threshold for the formation of high-mass stars, the
comparison of the numbers of massive starless clumps and those already
containing embedded sources suggests an upper limit lifetime for the starless
phase of 10^5 years for clumps with a mass M>500 M_sun.Comment: accepted for publication in MNRAS. Online catalogues available soon,
please contact the authors if intereste
Presupernova collapse models with improved weak-interaction rates
Improved values for stellar weak interaction rates have been recently
calculated based upon a large shell model diagonalization. Using these new
rates (for both beta decay and electron capture), we have examined the
presupernova evolution of massive stars in the range 15-40 Msun. Comparing our
new models with a standard set of presupernova models by Woosley and Weaver, we
find significantly larger values for the electron-to-baryon ratio Ye at the
onset of collapse and iron core masses reduced by approximately 0.1 Msun. The
inclusion of beta-decay accounts for roughly half of the revisions, while the
other half is a consequence of the improved nuclear physics. These changes will
have important consequences for nucleosynthesis and the supernova explosion
mechanism.Comment: 4 pages, 2 figure
Majorana Neutrino Magnetic Moment and Neutrino Decoupling in Big Bang Nucleosynthesis
We examine the physics of the early universe when Majorana neutrinos
(electron neutrino, muon neutrino, tau neutrino) possess transition magnetic
moments. These extra couplings beyond the usual weak interaction couplings
alter the way neutrinos decouple from the plasma of electrons/positrons and
photons. We calculate how transition magnetic moment couplings modify neutrino
decoupling temperatures, and then use a full weak, strong, and electromagnetic
reaction network to compute corresponding changes in Big Bang Nucleosynthesis
abundance yields. We find that light element abundances and other cosmological
parameters are sensitive to magnetic couplings on the order of 10^{-10} Bohr
magnetons. Given the recent analysis of sub-MeV Borexino data which constrains
Majorana moments to the order of 10^{-11} Bohr magnetons or less, we find that
changes in cosmological parameters from magnetic contributions to neutrino
decoupling temperatures are below the level of upcoming precision observations.Comment: 19 pages, 9 figure
Probing neutrino physics with a self-consistent treatment of the weak decoupling, nucleosynthesis, and photon decoupling epochs
We show that a self-consistent and coupled treatment of the weak decoupling,
big bang nucleosynthesis, and photon decoupling epochs can be used to provide
new insights and constraints on neutrino sector physics from high-precision
measurements of light element abundances and cosmic microwave background
observables. Implications of beyond-standard-model physics in cosmology,
especially within the neutrino sector, are assessed by comparing predictions
against five observables: the baryon energy density, helium abundance,
deuterium abundance, effective number of neutrinos, and sum of the light
neutrino mass eigenstates. We give examples for constraints on dark radiation,
neutrino rest mass, lepton numbers, and scenarios for light and heavy sterile
neutrinos.Comment: 29 pages, 10 figure
On Characterizing the Data Access Complexity of Programs
Technology trends will cause data movement to account for the majority of
energy expenditure and execution time on emerging computers. Therefore,
computational complexity will no longer be a sufficient metric for comparing
algorithms, and a fundamental characterization of data access complexity will
be increasingly important. The problem of developing lower bounds for data
access complexity has been modeled using the formalism of Hong & Kung's
red/blue pebble game for computational directed acyclic graphs (CDAGs).
However, previously developed approaches to lower bounds analysis for the
red/blue pebble game are very limited in effectiveness when applied to CDAGs of
real programs, with computations comprised of multiple sub-computations with
differing DAG structure. We address this problem by developing an approach for
effectively composing lower bounds based on graph decomposition. We also
develop a static analysis algorithm to derive the asymptotic data-access lower
bounds of programs, as a function of the problem size and cache size
Neutrino energy transport in weak decoupling and big bang nucleosynthesis
We calculate the evolution of the early universe through the epochs of weak
decoupling, weak freeze-out and big bang nucleosynthesis (BBN) by
simultaneously coupling a full strong, electromagnetic, and weak nuclear
reaction network with a multi-energy group Boltzmann neutrino energy transport
scheme. The modular structure of our code provides the ability to dissect the
relative contributions of each process responsible for evolving the dynamics of
the early universe in the absence of neutrino flavor oscillations. Such an
approach allows a detailed accounting of the evolution of the ,
, , , , energy
distribution functions alongside and self-consistently with the nuclear
reactions and entropy/heat generation and flow between the neutrino and
photon/electron/positron/baryon plasma components. This calculation reveals
nonlinear feedback in the time evolution of neutrino distribution functions and
plasma thermodynamic conditions (e.g., electron-positron pair densities), with
implications for: the phasing between scale factor and plasma temperature; the
neutron-to-proton ratio; light-element abundance histories; and the
cosmological parameter \neff. We find that our approach of following the time
development of neutrino spectral distortions and concomitant entropy production
and extraction from the plasma results in changes in the computed value of the
BBN deuterium yield. For example, for particular implementations of quantum
corrections in plasma thermodynamics, our calculations show a increase
in deuterium. These changes are potentially significant in the context of
anticipated improvements in observational and nuclear physics uncertainties.Comment: 37 pages, 12 Figures, 6 Table
Neutrino-Neutrino Scattering and Matter-Enhanced Neutrino Flavor Transformation in Supernovae
We examine matter-enhanced neutrino flavor transformation
() in the region above the neutrino
sphere in Type II supernovae. Our treatment explicitly includes contributions
to the neutrino-propagation Hamiltonian from neutrino-neutrino forward
scattering. A proper inclusion of these contributions shows that they have a
completely negligible effect on the range of - vacuum
mass-squared difference, , and vacuum mixing angle, , or
equivalently , required for enhanced supernova shock re-heating.
When neutrino background effects are included, we find that -process
nucleosynthesis from neutrino-heated supernova ejecta remains a sensitive probe
of the mixing between a light and a with a
cosmologically significant mass. Neutrino-neutrino scattering contributions are
found to have a generally small effect on the
parameter region probed by -process nucleosynthesis. We point out that the
nonlinear effects of the neutrino background extend the range of sensitivity of
-process nucleosynthesis to smaller values of .Comment: 38 pages, tex, DOE/ER/40561-150-INT94-00-6
Tightening the belt: Constraining the mass and evolution in SDC335
Recent ALMA observations identified one of the most massive star-forming
cores yet observed in the Milky Way; SDC335-MM1, within the infrared dark cloud
SDC335.579-0.292. Along with an accompanying core MM2, SDC335 appears to be in
the early stages of its star formation process. In this paper we aim to
constrain the properties of the stars forming within these two massive
millimetre sources. Observations of SDC335 at 6, 8, 23 and 25GHz were made with
the ATCA. We report the results of these continuum measurements, which combined
with archival data, allow us to build and analyse the spectral energy
distributions (SEDs) of the compact sources in SDC335. Three HCHII regions
within SDC335 are identified, two within the MM1 core. For each HCHII region, a
free-free emission curve is fit to the data allowing the derivation of the
sources' emission measure, ionising photon flux and electron density. Using
these physical properties we assign each HCHII region a ZAMS spectral type,
finding two protostars with characteristics of spectral type B1.5 and one with
a lower limit of B1-B1.5. Ancillary data from infrared to mm wavelength are
used to construct free-free component subtracted SEDs for the mm-cores,
allowing calculation of the bolometric luminosities and revision of the
previous gas mass estimates. The measured luminosities for the two mm-cores are
lower than expected from accreting sources displaying characteristics of the
ZAMS spectral type assigned to them. The protostars are still actively
accreting, suggesting that a mechanism is limiting the accretion luminosity, we
present the case for two different mechanisms capable of causing this. Finally,
using the ZAMS mass values as lower limit constraints, a final stellar
population for SDC335 was synthesised finding SDC335 is likely to be in the
process of forming a stellar cluster comparable to the Trapezium Cluster and
NGC6334 I(N).Comment: 10 pages, 5 figures. Accepted for publication in A&
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