93,934 research outputs found
Bifurcation analysis and phase diagram of a spin-string model with buckled states
We analyze a one-dimensional spin-string model, in which string oscillators
are linearly coupled to their two nearest neighbors and to Ising spins
representing internal degrees of freedom. String-spin coupling induces a
long-range ferromagnetic interaction among spins that competes with a spin-spin
antiferromagnetic coupling. As a consequence, the complex phase diagram of the
system exhibits different flat rippled and buckled states, with first or second
order transition lines between states. The two-dimensional version of the model
has a similar phase diagram, which has been recently used to explain the
rippled to buckled transition observed in scanning tunnelling microscopy
experiments with suspended graphene sheets. Here we describe in detail the
phase diagram of the simpler one-dimensional model and phase stability using
bifurcation theory. This gives additional insight into the physical mechanisms
underlying the different phases and the behavior observed in experiments.Comment: 15 pages, 7 figure
Calcium Triplet Synthesis
We present theoretical equivalent widths for the sum of the two strongest
lines of the Calcium Triplet, CaT index, in the near-IR, using evolutionary
techniques and the most recent models and observational data for this feature
in individual stars. We compute the CaT index for Single Stellar Populations
(instantaneous burst, standard Salpeter-type IMF) at four metallicities,
Z=0.004, 0.008, 0.02 (solar) and 0.05, and ranging in age from very young
bursts of star formation (few Myr) to old stellar populations, up to 17 gyr,
representative of globular clusters, elliptical galaxies and bulges of spirals.
The interpretation of the observed equivalent widths of CaT in different
stellar systems is discussed. Composite-population models are also computed as
a tool to interpret the CaT detections in star-forming regions, in order to
disantangle between the component due to Red Supergiants stars, RSG, and the
underlying, older, population. CaT is found to be an excellent
metallicity-indicator for populations older than 1 Gyr, practically independent
of the age. We discuss its application to remove the age- metallicity
degeneracy, characteristic of all studies of galaxy evolution based on the
usual integrated indices (both broad band colors and narrow band indices). The
application of the models computed here to the analysis of a sample of
elliptical galaxies will be discussed in a forthcoming paper (Gorgas et al.
1998).Comment: 17 pages, 7 figures, to be published in A&
Quantum Decoherence of Photons in the Presence of Hidden U(1)s
Many extensions of the standard model predict the existence of hidden sectors
that may contain unbroken abelian gauge groups. We argue that in the presence
of quantum decoherence photons may convert into hidden photons on sufficiently
long time scales and show that this effect is strongly constrained by CMB and
supernova data. In particular, Planck-scale suppressed decoherence scales D ~
E^2/M_Pl (characteristic for non-critical string theories) are incompatible
with the presence of even a single hidden U(1). The corresponding bounds on the
decoherence scale are four orders of magnitude stronger than analogous bounds
derived from solar and reactor neutrino data and complement other bounds
derived from atmospheric neutrino data.Comment: 8 pages, 9 figure
Quasi-Periodic Oscillations and energy spectra from the two brightest Ultra-Luminous X-ray sources in M82
Ultra-Luminous X-ray sources are thought to be accreting black holes that
might host Intermediate Mass Black Holes (IMBH), proposed to exist by
theoretical studies, even though a firm detection (as a class) is still
missing. The brightest ULX in M82 (M82 X-1) is probably one of the best
candidates to host an IMBH. In this work we analyzed the data of the recent
release of observations obtained from M82 X-1 taken by XMM-Newton. We performed
a study of the timing and spectral properties of the source. We report on the
detection of (46+-2) mHz Quasi-Periodic Oscillations (QPOs) in the power
density spectra of two observations. A comparison of the frequency of these
high-frequency QPOs with previous detections supports the 1:2:3 frequency
distribution as suggested in other studies. We discuss the implications if the
(46+-2) mHz QPO detected in M82 X-1 is the fundamental harmonic, in analogy
with the High-Frequency QPOs observed in black hole binaries. For one of the
observations we have detected for the first time a QPO at 8 mHz (albeit at a
low significance), that coincides with a hardening of the spectrum. We suggest
that the QPO is a milli-hertz QPO originating from the close-by transient ULX
M82 X-2, with analogies to the Low-Frequency QPOs observed in black hole
binaries.Comment: 9 pages (with 4 figures and 4 tables). Accepted for publication in
MNRAS (26/09/13
Non-analyticities in three-dimensional gauge theories
Quantum fluctuations generate in three-dimensional gauge theories not only
radiative corrections to the Chern-Simons coupling but also non-analytic terms
in the effective action. We review the role of those terms in gauge theories
with massless fermions and Chern-Simons theories. The explicit form of
non-analytic terms turns out to be dependent on the regularization scheme and
in consequence the very existence of phenomena like parity and framing
anomalies becomes regularization dependent. In particular we find
regularization regimes where both anomalies are absent. Due to the presence of
non-analytic terms the effective action becomes not only discontinuous but also
singular for some background gauge fields which include sphalerons. The
appearence of this type of singularities is linked to the existence of nodal
configurations in physical states and tunneling suppression at some classical
field configurations. In the topological field theory the number of physical
states may also become regularization dependent. Another consequence of the
peculiar behaviour of three-dimensional theories under parity odd
regularizations is the existence of a simple mechanism of generation of a mass
gap in pure Yang-Mills theory by a suitable choice of regularization scheme.
The generic value of this mass does agree with the values obtained in
Hamiltonian and numerical analysis. Finally, the existence of different
regularization regimes unveils the difficulties of establishing a Zamolodchikov
c-theorem for three-dimensional field theories in terms of the induced
gravitational Chern-Simons couplings.Comment: 21 pages; Contribution to Ian Kogan Memorial Collection, ``From
Fields to Strings: Circumnavigating Theoretical Physics'
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