23,912 research outputs found
Poisson-Lie T-plurality of three-dimensional conformally invariant sigma models II: Nondiagonal metrics and dilaton puzzle
We look for 3-dimensional Poisson-Lie dualizable sigma models that satisfy
the vanishing beta-function equations with constant dilaton field. Using the
Poisson-Lie T-plurality we then construct 3-dimensional sigma models that
correspond to various decompositions of Drinfeld double. Models with nontrivial
dilaton field may appear. It turns out that for ``traceless'' dual algebras
they satisfy the vanishing beta-function equations as well.
In certain cases the dilaton cannot be defined in some of the dual models. We
provide an explanation why this happens and give criteria predicting when it
happens.Comment: 24 pages, the published version; changes compared to v1: typos
corrected, conclusions extended, added reference
The Resolved Outer Population of NGC6822 with WFPC2
We present F336W (U), F439W (B), F555W (V), and F675W (R) Wide Field
Planetary Camera 2 (WFPC2) photometry of two outer regions of the Local Group
dwarf irregular galaxy NGC6822. The NE region is ~13 arcmin from the galaxy
centre, while the W region lies 10 arcmin out, and within the wispy low surface
brightness outer regions of the galaxy. The fields are not crowded and contain
few NGC 6822 stars. We discuss errors and uncertainties and find that the W
region contains a main sequence that extends to stars of about 2 solar masses,
with an age of about 200 Myr. The NE region has no main sequence or stars
younger than 1 Gyr, but does contain some luminous red stars that are not
matched in the W field. These stars are not clumped in the field. The results
suggest that the W region may be a trace of a tidal event that triggered the
current star-formation in this isolated galaxy.Comment: 12 pages including 2 tables, plus 4 figures (#1 omitted) To appear in
PAS
Hot and dense water in the inner 25 AU of SVS13-A
In the context of the ASAI (Astrochemical Surveys At IRAM) project, we
carried out an unbiased spectral survey in the millimeter window towards the
well known low-mass Class I source SVS13-A. The high sensitivity reached (3-12
mK) allowed us to detect at least 6 HDO broad (FWHM ~ 4-5 km/s) emission lines
with upper level energies up to Eu = 837 K. A non-LTE LVG analysis implies the
presence of very hot (150-260 K) and dense (> 3 10^7 cm-3) gas inside a small
radius ( 25 AU) around the star, supporting, for the first time, the
occurrence of a hot corino around a Class I protostar.
The temperature is higher than expected for water molecules are sublimated
from the icy dust mantles (~ 100 K). Although we cannot exclude we are observig
the effects of shocks and/or winds at such small scales, this could imply that
the observed HDO emission is tracing the water abundance jump expected at
temperatures ~ 220-250 K, when the activation barrier of the gas phase
reactions leading to the formation of water can be overcome. We derive X(HDO) ~
3 10-6, and a H2O deuteration > 1.5 10-2, suggesting that water deuteration
does not decrease as the protostar evolves from the Class 0 to the Class I
stage.Comment: MNRAS Letter
Simple model for quantum general relativity from loop quantum gravity
New progress in loop gravity has lead to a simple model of `general-covariant
quantum field theory'. I sum up the definition of the model in self-contained
form, in terms accessible to those outside the subfield. I emphasize its
formulation as a generalized topological quantum field theory with an infinite
number of degrees of freedom, and its relation to lattice theory. I list the
indications supporting the conjecture that the model is related to general
relativity and UV finite.Comment: 8 pages, 3 figure
New Ultraviolet Extinction Curves for Interstellar Dust in M31
New low-resolution UV spectra of a sample of reddened OB stars in M31 were
obtained with HST/STIS to study the wavelength dependence of interstellar
extinction and the nature of the underlying dust grain populations. Extinction
curves were constructed for four reddened sightlines in M31 paired with closely
matching stellar atmosphere models. The new curves have a much higher S/N than
previous studies. Direct measurements of N(H I) were made using the Ly
absorption lines enabling gas-to-dust ratios to be calculated. The sightlines
have a range in galactocentric distance of 5 to 14 kpc and represent dust from
regions of different metallicities and gas-to-dust ratios. The metallicities
sampled range from Solar to 1.5 Solar. The measured curves show similarity to
those seen in the Milky Way and the Large Magellanic Cloud. The Maximum Entropy
Method was used to investigate the dust composition and size distribution for
the sightlines observed in this program finding that the extinction curves can
be produced with the available carbon and silicon abundances if the metallicity
is super-Solar.Comment: ApJ, in press, 9 pages, 5 figure
The EPRL intertwiners and corrected partition function
Do the SU(2) intertwiners parametrize the space of the EPRL solutions to the
simplicity constraint? What is a complete form of the partition function
written in terms of this parametrization? We prove that the EPRL map is
injective for n-valent vertex in case when it is a map from SO(3) into
SO(3)xSO(3) representations. We find, however, that the EPRL map is not
isometric. In the consequence, in order to be written in a SU(2) amplitude
form, the formula for the partition function has to be rederived. We do it and
obtain a new, complete formula for the partition function. The result goes
beyond the SU(2) spin-foam models framework.Comment: RevTex4, 15 pages, 5 figures; theorem of injectivity of EPRL map
correcte
Asymptotics of LQG fusion coefficients
The fusion coefficients from SO(3) to SO(4) play a key role in the definition
of spin foam models for the dynamics in Loop Quantum Gravity. In this paper we
give a simple analytic formula of the EPRL fusion coefficients. We study the
large spin asymptotics and show that they map SO(3) semiclassical intertwiners
into semiclassical intertwiners. This non-trivial
property opens the possibility for an analysis of the semiclassical behavior of
the model.Comment: 14 pages, minor change
Investigating Echo-State Networks Dynamics by Means of Recurrence Analysis
This is the author accepted manuscript. The final version is available from IEEE via the DOI in this record.In this paper, we elaborate over the well-known interpretability issue in echo-state networks (ESNs). The idea is to investigate the dynamics of reservoir neurons with time-series analysis techniques developed in complex systems research. Notably, we analyze time series of neuron activations with recurrence plots (RPs) and recurrence quantification analysis (RQA), which permit to visualize and characterize high-dimensional dynamical systems. We show that this approach is useful in a number of ways. First, the 2-D representation offered by RPs provides a visualization of the high-dimensional reservoir dynamics. Our results suggest that, if the network is stable, reservoir and input generate similar line patterns in the respective RPs. Conversely, as the ESN becomes unstable, the patterns in the RP of the reservoir change. As a second result, we show that an RQA measure, called Lmax, is highly correlated with the well-established maximal local Lyapunov exponent. This suggests that complexity measures based on RP diagonal lines distribution can quantify network stability. Finally, our analysis shows that all RQA measures fluctuate on the proximity of the so-called edge of stability, where an ESN typically achieves maximum computational capability. We leverage on this property to determine the edge of stability and show that our criterion is more accurate than two well-known counterparts, both based on the Jacobian matrix of the reservoir. Therefore, we claim that RPs and RQA-based analyses are valuable tools to design an ESN, given a specific problem
Organic molecules in the protoplanetary disk of DG Tau revealed by ALMA
Planets form in protoplanetary disks and inherit their chemical compositions.
It is thus crucial to map the distribution and investigate the formation of
simple organics, such as formaldehyde and methanol, in protoplanetary disks. We
analyze ALMA observations of the nearby disk-jet system around the T Tauri star
DG Tau in the o-HCO and CHOH E,
A transitions at an unprecedented resolution of ,
i.e., au at a distance of 121 pc. The HCO emission originates from
a rotating ring extending from au with a peak at au, i.e., at
the edge of the 1.3mm dust continuum. CHOH emission is not detected down to
an r.m.s. of 3 mJy/beam in the 0.162 km/s channel. Assuming an ortho-to-para
ratio of 1.8-2.8 the ring- and disk-height-averaged HCO column density is
cm, while that of CHOH is
cm. In the inner au no o-HCO emission
is detected with an upper limit on its beam-averaged column density of
cm. The HCO ring in the disk of DG Tau is
located beyond the CO iceline (R au). This suggests that the
HCO abundance is enhanced in the outer disk due to formation on grain
surfaces by the hydrogenation of CO ice. The emission peak at the edge of the
mm dust continuum may be due to enhanced desorption of HCO in the gas phase
caused by increased UV penetration and/or temperature inversion. The
CHOH/HCO abundance ratio is , in agreement with disk chemistry
models. The inner edge of the HCO ring coincides with the radius where the
polarization of the dust continuum changes orientation, hinting at a tight link
between the HCO chemistry and the dust properties in the outer disk and at
the possible presence of substructures in the dust distribution.Comment: 8 pages, 6 figures, accepted for publication on A&A Letter
Focusing and imaging with increased numerical apertures through multimode fibers with micro-fabricated optics
The use of individual multimode optical fibers in endoscopy applications has
the potential to provide highly miniaturized and noninvasive probes for
microscopy and optical micromanipulation. A few different strategies have been
proposed recently, but they all suffer from intrinsically low resolution
related to the low numerical aperture of multimode fibers. Here, we show that
two-photon polymerization allows for direct fabrication of micro-optics
components on the fiber end, resulting in an increase of the numerical aperture
to a value that is close to 1. Coupling light into the fiber through a spatial
light modulator, we were able to optically scan a submicrometer spot (300 nm
FWHM) over an extended region, facing the opposite fiber end. Fluorescence
imaging with improved resolution is also demonstrated.Comment: 5 pages, 3 figure
- âŠ