11,034 research outputs found
The Hubble Deep Field Reveals a Supernova at z~0.95
We report the discovery of a variable object in the Hubble Deep Field North
(HDF-N) which has brightened, during the 8.5 days sampled by the data, by more
than 0.9 mag in I and about 0.7 mag in V, remaining stable in B. Subsequent
observations of the HDF-N show that two years later this object has dimmed back
to about its original brightness in I. The colors of this object, its
brightness, its time behavior in the various filters and the evolution of its
morphology are consistent with being a Type Ib supernova in a faint galaxy at
z~0.95.Comment: 5 pages including 2 figures. Accepted for publication in MNRA
Mass formulae and natural hierarchy in string effective supergravities
We study some conditions for the hierarchy to occur
naturally in a generic effective supergravity theory. Absence of fine-tuning
and perturbative calculability require that the effective potential has a
sliding gravitino mass and vanishing cosmological constant, up to corrections. In particular, cancellation of quadratically
divergent contributions to the one-loop effective potential should take place,
including the `hidden sector' of the theory. We show that these conditions can
be met in the effective supergravities derived from four-dimensional
superstrings, with supersymmetry broken either at the string tree level via
compactification, or by non-perturbative effects such as gaugino condensation.
A crucial role is played by some approximate scaling symmetries, which are
remnants of discrete target-space dualities in the large moduli limit. We
derive explicit formulae for the soft breaking terms arising from this class of
`large hierarchy compatible' (LHC) supergravities.Comment: 40 pages, no figures, CERN-TH.7192/94, LPTENS -94/12, UCLA/94/TEP13
(plain LATEX, to be run twice). Some trivial misprints and references have
been correcte
Inhomogeneous Reionization Regulated by Radiative and Stellar Feedbacks
We study the inhomogeneous reionization in a critical density CDM universe
due to stellar sources, including Population III objects. The spatial
distribution of the sources is obtained from high resolution numerical N-body
simulations. We calculate the source properties taking into account a
self-consistent treatment of both radiative (ie ionizing and H2
-photodissociating photons) and stellar (ie SN explosions) feedbacks regulated
by massive stars. This allows us to describe the topology of the ionized and
dissociated regions at various cosmic epochs and derive the evolution of H, He,
and H2 filling factors, soft UV background, cosmic star formation rate and the
final fate of ionizing objects. The main results are: (i) galaxies reionize the
IGM by z~10 (with some uncertainty related to the gas clumping factor), whereas
H2 is completely dissociated already by z~25; (ii) reionization is mostly due
to the relatively massive objects which collapse via H line cooling, while
objects whose formation relies on H2 cooling alone are insufficient to this
aim; (iii) the diffuse soft UV background is the major source of radiative
feedback effects for z<15; at higher z direct flux from neighboring objects
dominates; (iv) the match of the calculated cosmic star formation history with
the one observed at lower redshifts suggests that the conversion efficiency of
baryons into stars is ~1%; (v) we find that a very large population of dark
objects which failed to form stars is present by z~8. We discuss and compare
our results with similar previous studies.Comment: 34 pages, emulateapj.sty, LaTeX, 13 figures. MNRAS, submitte
The European way out of recession
This paper proposes a two-regime Bounce-Back Function augmented Self-Exciting Threshold AutoRegression (SETAR) which allows for various shapes of recoveries from the recession regime. It relies on the bounce-back effects first analyzed in a Markov-Switching setup by Kim, Morley and Piger [2005] and recently extended by Bec, Bouabdallah and Ferrara [2011a]. This approach is then applied to post-1973 quarterly growth rates of French, German, Italian, Spanish and Euro area real GDPs. Both the linear autoregression and the standard SETAR without bounce-back effect null hypotheses are strongly rejected against the Bounce-Back augmented SETAR alternative in all cases but Italy. The relevance of our proposed model is further assessed by the comparison of its short-term forecasting performances with the ones obtained from a linear autoregression and a standard SETAR. It turns out that the bounce-back models one-step ahead forecasts generally outperform the other ones, and particularly so during the last recovery period in 2009Q3-2010Q4.Threshold autoregression, bounce-back effects, asymmetric business cycles.
Two-Centered Magical Charge Orbits
We determine the two-centered generic charge orbits of magical N = 2 and
maximal N = 8 supergravity theories in four dimensions. These orbits are
classified by seven U-duality invariant polynomials, which group together into
four invariants under the horizontal symmetry group SL(2,R). These latter are
expected to disentangle different physical properties of the two-centered
black-hole system. The invariant with the lowest degree in charges is the
symplectic product (Q1,Q2), known to control the mutual non-locality of the two
centers.Comment: 1+17 pages, 1 Table; v2: Eq. (3.23) corrected; v3: various
refinements in text and formulae, caption of Table 1 expanded, Footnote and
Refs. added. To appear on JHE
On Black Attractors in 8D and Heterotic/Type IIA Duality
Motivated by the study of black attractors in 8D supergravity with 16
supersymmetries, we use the field theory approach and 8D supersymmetry with non
trivial central charges to shed light on the exact duality between heterotic
string on T^2 and type IIA on real connected and compact surfaces {\Sigma}2. We
investigate the two constraints that should be obeyed by {\Sigma}2 and give
their solutions in terms of intersecting 2-cycles as well their classification
using Dynkin diagrams of affine Kac-Moody algebras. It is shown as well that
the moduli space of these dual theories is given by
SO(1,1)x((SO(2,r+2))/(SO(2)xSO(r+2))) where r stands for the rank of the gauge
symmetry G_{r} of the 10D heterotic string on T^2. The remarkable cases
r=-2,-1,0 as well as other features are also investigated.Comment: LaTex, 18 pages, 2 figures, To appear in JHE
Effects of dark matter annihilation on the first stars
We study the evolution of the first stars in the universe (Population III)
from the early pre-Main Sequence until the end of helium burning in the
presence of WIMP dark matter annihilation inside the stellar structure. The two
different mechanisms that can provide this energy source are the contemporary
contraction of baryons and dark matter, and the capture of WIMPs by scattering
off the gas with subsequent accumulation inside the star. We find that the
first mechanism can generate an equilibrium phase, previously known as a "dark
star", which is transient and present in the very early stages of pre-MS
evolution. The mechanism of scattering and capture acts later, and can support
the star virtually forever, depending on environmental characteristic of the
dark matter halo and on the specific WIMP model.Comment: Proceedings of the IAU Symposium 255, "Low-Metallicity Star
Formation: From the First Stars to Dwarf Galaxies"; L.K. Hunt, S. Madden and
R. Schneider ed
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