13,116 research outputs found
Systematic Uncertainties In Constraining Dark Matter Annihilation From The Cosmic Microwave Background
Anisotropies of the cosmic microwave background (CMB) have proven to be a
very powerful tool to constrain dark matter annihilation at the epoch of
recombination. However, CMB constraints are currently derived using a number of
reasonable but yet un-tested assumptions that could potentially lead to a
misestimation of the true bounds. In this paper we examine the potential impact
of these systematic effects. In particular, we separately study the propagation
of the secondary particles produced by annihilation in two energy regimes;
first following the shower from the initial particle energy to the keV scale,
and then tracking the resulting secondary particles from this scale to the
absorption of their energy as heat, ionization, or excitation of the medium. We
improve both the high and low energy parts of the calculation, in particular
finding that our more accurate treatment of losses to sub-10.2 eV photons
produced by scattering of high-energy electrons weakens the constraints on
particular DM annihilation models by up to a factor of two. On the other hand,
we find that the uncertainties we examine for the low energy propagation do not
significantly affect the results for current and upcoming CMB data. We include
the evaluation of the precise amount of excitation energy, in the form of
Lyman-alpha photons, produced by the propagation of the shower, and examine the
effects of varying the Helium fraction and Helium ionization fraction. In the
recent literature, simple approximations for the fraction of energy absorbed in
different channels have often been used to derive CMB constraints: we assess
the impact of using accurate versus approximate energy fractions. Finally we
check that the choice of recombination code (between RECFAST v1.5 and
COSMOREC), to calculate the evolution of the free electron fraction in the
presence of dark matter annihilation, introduces negligible differences.Comment: 29 pages, 21 figure
A study of periodicities and recurrences in solar activity and cosmic ray modulation
The 154d periodicity was found in the cosmic ray intensity (RE) vs Flares, and some other peaks of coherency in the RC vs aa sub I, that when interpreted as aliased values, might correspond to recurring interplanetary magnetic field structures and solar wind streams. It cannot be excluded, however, that some of the correspondence with aa are of terrestrial origin. This study cannot be considered exhaustive due to the fact that other solar variables, such as polar hole size, are possibly correlated to cosmic ray intensities. However, the number of observations is small so that the interpretation of the results is very difficult
Beryllium in turnoff stars of NGC6397: early Galaxy spallation, cosmochronology and cluster formation
We present the first detection of beryllium in two turnoff stars of the old,
metal-poor globular cluster NGC 6397. The beryllium lines are clearly detected
and we determine a mean beryllium abundance of log(Be/H)=-12.35 +/- 0.2. The
beryllium abundance is very similar to that of field stars of similar Fe
content. We interpret the beryllium abundance observed as the result of primary
spallation of cosmic rays acting on a Galactic scale, showing that beryllium
can be used as a powerful cosmochronometer for the first stellar generations.
With this method, we estimate that the cluster formed 0.2-0.3 Gyr after the
onset of star formation in the Galaxy, in excellent agreement with the age
derived from main sequence fitting. From the same spectra we also find low O
(noticeably different for the two stars) and high N abundances, suggesting that
the original gas was enriched in CNO processed material. Our beryllium results,
together with the N, O, and Li abundances, provide insights on the formation of
this globular cluster, showing that any CNO processing of the gas must have
occurred in the protocluster cloud before the formation of the stars we observe
now. We encounter, however, difficulties in giving a fully consistent picture
of the cluster formation, able to explain the complex overall abundance
pattern.Comment: to appear in A&
Cosmic ray biannual variation
The study of the cosmic ray (CR) power spectrum has revealed a significant variation with a period around 2 yr that cannot be explained as a high order harmonic of the 11 yr solar cycle. Comparative study of the correlation on different time scales between CR intensity and Rz, aa, high speed streams and polar hole size has put in evidence that a high degree of coherency exists between each couple of variables at 1.58 to 1.64 yr, except between CR and Rz. On the other hand cyclic variation on a short time scale, around 26 months, has been claimed to be present in the neutrino flux. Critical tests of this hypothesis are considered and a preliminary result seems to indicate that the hypothesis of the existence of a 1.6 yr periodicity in the neutrino data during the measurement time interval, has a significance or = 99.9%. The possible origin of this variation as due to a contribution either of CR interactions in the upper atmosphere or to the solar dynamics, are discussed
Massive black hole factories: Supermassive and quasi-star formation in primordial halos
Supermassive stars and quasi-stars (massive stars with a central black hole)
are both considered as potential progenitors for the formation of supermassive
black holes. They are expected to form from rapidly accreting protostars in
massive primordial halos. We explore how long rapidly accreting protostars
remain on the Hayashi track, implying large protostellar radii and weak
accretion luminosity feedback. We assess the potential role of energy
production in the nuclear core, and determine what regulates the evolution of
such protostars into quasi-stars or supermassive stars. We follow the
contraction of characteristic mass scales in rapidly accreting protostars, and
infer the timescales for them to reach nuclear densities. We compare the
characteristic timescales for nuclear burning with those for which the extended
protostellar envelope can be maintained. We find that the extended envelope can
be maintained up to protostellar masses of 3.6x10^8 \dot{m}^3 solar, where
\dot{m} denotes the accretion rate in solar masses per year. We expect the
nuclear core to exhaust its hydrogen content in 7x10^6 yrs. If accretion rates
\dot{m}>>0.14 can still be maintained at this point, a black hole may form
within the accreting envelope, leading to a quasi-star. Alternatively, the
accreting object will gravitationally contract to become a main-sequence
supermassive star. Due to the limited gas reservoir in dark matter halos with
10^7 solar masses, the accretion rate onto the central object may drop at late
times, implying the formation of supermassive stars as the typical outcome of
direct collapse. However, if high accretion rates are maintained, a quasi-star
with an interior black hole may form.Comment: 9 pages, 4 figures, submitted to A&A. Comments are welcom
Formation and Disruption of Cosmological Low Mass Objects
We investigate the evolution of cosmological low mass (low virial
temperature) objects and the formation of the first luminous objects. First,
the `cooling diagram' for low mass objects is shown. We assess the cooling rate
taking into account the contribution of H_2, which is not in chemical
equilibrium generally, with a simple argument of time scales. The reaction
rates and the cooling rate of H_2 are taken from the recent results by Galli &
Palla (1998). Using this cooling diagram, we also estimate the formation
condition of luminous objects taking into account the supernova (SN) disruption
of virialized clouds. We find that the mass of the first luminous object is
several times 10^7 solar mass, because smaller objects may be disrupted by the
SNe before they become luminous. Metal pollution of low mass (Ly-alpha) clouds
also discussed. The resultant metallicity of the clouds is about 1/1000 of the
solar metallicity.Comment: 11 pages, 2 figures, To appear in ApJ
Electron Emission from Diamondoids: A Diffusion Quantum Monte Carlo Study
We present density-functional theory (DFT) and quantum Monte Carlo (QMC)
calculations designed to resolve experimental and theoretical controversies
over the optical properties of H-terminated C nanoparticles (diamondoids). The
QMC results follow the trends of well-converged plane-wave DFT calculations for
the size dependence of the optical gap, but they predict gaps that are 1-2 eV
higher. They confirm that quantum confinement effects disappear in diamondoids
larger than 1 nm, which have gaps below that of bulk diamond. Our QMC
calculations predict a small exciton binding energy and a negative electron
affinity (NEA) for diamondoids up to 1 nm, resulting from the delocalized
nature of the lowest unoccupied molecular orbital. The NEA suggests a range of
possible applications of diamondoids as low-voltage electron emitters
Study of solid 4He in two dimensions. The issue of zero-point defects and study of confined crystal
Defects are believed to play a fundamental role in the supersolid state of
4He. We report on studies by exact Quantum Monte Carlo (QMC) simulations at
zero temperature of the properties of solid 4He in presence of many vacancies,
up to 30 in two dimensions (2D). In all studied cases the crystalline order is
stable at least as long as the concentration of vacancies is below 2.5%. In the
2D system for a small number, n_v, of vacancies such defects can be identified
in the crystalline lattice and are strongly correlated with an attractive
interaction. On the contrary when n_v~10 vacancies in the relaxed system
disappear and in their place one finds dislocations and a revival of the
Bose-Einstein condensation. Thus, should zero-point motion defects be present
in solid 4He, such defects would be dislocations and not vacancies, at least in
2D. In order to avoid using periodic boundary conditions we have studied the
exact ground state of solid 4He confined in a circular region by an external
potential. We find that defects tend to be localized in an interfacial region
of width of about 15 A. Our computation allows to put as upper bound limit to
zero--point defects the concentration 0.003 in the 2D system close to melting
density.Comment: 17 pages, accepted for publication in J. Low Temp. Phys., Special
Issue on Supersolid
Collective dipole excitations in sodium clusters
Some properties of small and medium sodium clusters are described within the
RPA approach using a projected spherical single particle basis. The oscillator
strengths calculated with a Schiff-like dipole transition operator and folded
with Lorentzian functions are used to calculate the photoabsorbtion cross
section spectra. The results are further employed to establish the dependence
of the plasmon frequency on the number of cluster components. Static electric
polarizabilities of the clusters excited in a RPA dipole state are also
calculated.
Comparison of our results with the corresponding experimental data show an
overall good agreement.Comment: 23 pages, 5 figure
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