24,250 research outputs found
Slicing the Ising model: critical equilibrium and coarsening dynamics
We study the evolution of spin clusters on two dimensional slices of the
Ising model in contact with a heat bath after a sudden quench to a subcritical
temperature. We analyze the evolution of some simple initial configurations,
such as a sphere and a torus, of one phase embedded into the other, to confirm
that their area disappears linearly in time and to establish the temperature
dependence of the prefactor in each case. Two generic kinds of initial states
are later used: equilibrium configurations either at infinite temperature or at
the paramagnetic-ferromagnetic phase transition. We investigate the
morphological domain structure of the coarsening configurations on slices
of the system, comparing with the behavior of the bidimensional model.Comment: 12 page
On the Star Formation Rates in Molecular Clouds
In this paper we investigate the level of star formation activity within
nearby molecular clouds. We employ a uniform set of infrared extinction maps to
provide accurate assessments of cloud mass and structure and compare these with
inventories of young stellar objects within the clouds. We present evidence
indicating that both the yield and rate of star formation can vary considerably
in local clouds, independent of their mass and size. We find that the surface
density structure of such clouds appears to be important in controlling both
these factors. In particular, we find that the star formation rate (SFR) in
molecular clouds is linearly proportional to the cloud mass (M_{0.8}) above an
extinction threshold of A_K approximately equal to 0.8 magnitudes,
corresponding to a gas surface density threshold of approximaely 116 solar
masses per square pc. We argue that this surface density threshold corresponds
to a gas volume density threshold which we estimate to be n(H_2) approximately
equal to 10^4\cc. Specifically we find SFR (solar masses per yr) = 4.6 +/- 2.6
x 10^{-8} M_{0.8} (solar masses) for the clouds in our sample. This relation
between the rate of star formation and the amount of dense gas in molecular
clouds appears to be in excellent agreement with previous observations of both
galactic and extragalactic star forming activity. It is likely the underlying
physical relationship or empirical law that most directly connects star
formation activity with interstellar gas over many spatial scales within and
between individual galaxies. These results suggest that the key to obtaining a
predictive understanding of the star formation rates in molecular clouds and
galaxies is to understand those physical factors which give rise to the dense
components of these clouds.Comment: accepted for publicaton in the Astrophysical Journal; 22 pages, 4
figure
Statistics of the fractional polarisation of extragalactic dusty sources in Planck HFI maps
We estimate the average fractional polarisation at 143, 217 and 353 GHz of a
sample of 4697 extragalactic dusty sources by applying stacking technique. The
sample is selected from the second version of the Planck Catalogue of Compact
Sources at 857 GHz, avoiding the region inside the Planck Galactic mask (fsky ~
60 per cent). We recover values for the mean fractional polarisation at 217 and
353 GHz of (3.10 \pm 0.75) per cent and (3.65 \pm 0.66) per cent, respectively,
whereas at 143 GHz we give a tentative value of (3.52 \pm 2.48) per cent. We
discuss the possible origin of the measured polarisation, comparing our new
estimates with those previously obtained from a sample of radio sources. We
test different distribution functions and we conclude that the fractional
polarisation of dusty sources is well described by a log-normal distribution,
as determined in the radio band studies. For this distribution we estimate
{\mu}_{217GHz} = 0.3 \pm 0.5 (that would correspond to a median fractional
polarisation of {\Pi}_{med} = (1.3 \pm 0.7) per cent) and {\mu}_{353GHz} = 0.7
\pm 0.4 ({\Pi}_{med} = (2.0 \pm 0.8) per cent), {\sigma}_{217GHz} = 1.3 \pm 0.2
and {\sigma}_{353GHz} = 1.1 \pm 0.2. With these values we estimate the source
number counts in polarisation and the contribution given by these sources to
the CMB B-mode angular power spectrum at 217, 353, 600 and 800 GHz. We conclude
that extragalactic dusty sources might be an important contaminant for the
primordial B-mode at frequencies > 217 GHz.Comment: arXiv admin note: text overlap with arXiv:1703.0995
On compressive radial tidal forces
Radial tidal forces can be compressive instead of disruptive, a possibility
that is frequently overlooked in high level physics courses. For example,
radial tidal compression can emerge in extended stellar systems containing a
smaller stellar cluster. For particular conditions the tidal field produced by
this extended mass distribution can exert on the cluster it contains
compressive effects instead of the common disruptive forces. This interesting
aspect of gravity can be derived from standard relations given in many
textbooks and introductory courses in astronomy and can serve as an opportunity
to look closer at some aspects of gravitational physics, stellar dynamics, and
differential geometry. The existence of compressive tides at the center of huge
stellar systems might suggest new evolutionary scenarios for the formation of
stars and primordial galactic formation processes.Comment: 22 pages, 2 figure
Distinguishing low frequency mutations from RT-PCR and sequence errors in viral deep sequencing data
There is a high prevalence of coronary artery disease (CAD) in patients with left bundle branch block (LBBB); however there are many other causes for this electrocardiographic abnormality. Non-invasive assessment of these patients remains difficult, and all commonly used modalities exhibit several drawbacks. This often leads to these patients undergoing invasive coronary angiography which may not have been necessary. In this review, we examine the uses and limitations of commonly performed non-invasive tests for diagnosis of CAD in patients with LBBB
Double fermiophobic Higgs boson production at the LHC and LC
We consider the phenomenology of a fermiophobic Higgs boson (h_f) at the
Large Hadron Collider (LHC) and a e+e- Linear Collider (LC). At both machines
the standard production mechanisms which rely on the coupling h_fVV (V=W,Z) can
be very suppressed at large tan beta. In such cases the complementary channels
pp to H^\pm h_f, A^0 h_f and e+e- to A^0 h_f offer promising cross-sections.
Together with the potentially large branching ratios for H^\pm to h_fW* and A^0
to h_fZ*, these mechanisms would give rise to double h_f production, leading to
signatures of gamma gamma gamma gamma, gamma gamma VV and VVVV.Comment: 19 pages, 9 figures, expanded discussion, fig.1 changed slightly,
version to appear in Phys.Rev.
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