2,616 research outputs found
Social distancing strategies against disease spreading
The recurrent infectious diseases and their increasing impact on the society
has promoted the study of strategies to slow down the epidemic spreading. In
this review we outline the applications of percolation theory to describe
strategies against epidemic spreading on complex networks. We give a general
outlook of the relation between link percolation and the
susceptible-infected-recovered model, and introduce the node void percolation
process to describe the dilution of the network composed by healthy individual,
, the network that sustain the functionality of a society. Then, we survey
two strategies: the quenched disorder strategy where an heterogeneous
distribution of contact intensities is induced in society, and the intermittent
social distancing strategy where health individuals are persuaded to avoid
contact with their neighbors for intermittent periods of time. Using
percolation tools, we show that both strategies may halt the epidemic
spreading. Finally, we discuss the role of the transmissibility, , the
effective probability to transmit a disease, on the performance of the
strategies to slow down the epidemic spreading.Comment: to be published in "Perspectives and Challenges in Statistical
Physics and Complex Systems for the Next Decade", Word Scientific Pres
A new Cepheid distance to the maser-host galaxy NGC 4258 and its implications for the Hubble Constant
We present initial results from a time-series BVI survey of two fields in NGC
4258 using the Advanced Camera for Surveys onboard the Hubble Space Telescope.
This galaxy was selected because of its accurate maser-based distance, which is
anticipated to have a total uncertainty of ~3%. The goal of the HST
observations is to provide an absolute calibration of the Cepheid Distance
Scale and to measure its dependence on chemical abundance (the so-called
"metallicity effect").
We carried out observations of two fields at different galactocentric
distances with a mean abundance difference of 0.5 dex. We discovered a total of
281 Cepheids with periods ranging from 4 to 45 days (the duration of our
observing window). We determine a Cepheid distance modulus for NGC 4258
(relative to the LMC) of 10.88 +- 0.04 (random) +- 0.05 (systematic) mag. Given
the published maser distance to the galaxy, this implies \mu (LMC)=18.41 +-
0.10 (r) +- 0.13 (s) mag or D(LMC)= 48.1 +- 2.3 (r) +- 2.9 (s) kpc. We measure
a metallicity effect of \gamma=-0.29 +- 0.09 (r) +- 0.05 (s) mag/dex. We see no
evidence for a variation in the slope of the Period-Luminosity relation as a
function of abundance.
We estimate a Hubble Constant of H_0= 74 +- 3 (r) +- 6 (s) km/s Mpc using a
recent sample of 4 well-observed type Ia SNe and our new calibration of the
Cepheid Distance Scale. It may soon be possible to measure the value of H_0
with a total uncertainty of 5%, with consequent improvement in the
determination of the equation of state of dark energy.Comment: 39 pages, 24 figures. Accepted for publication in the Astrophysical
Journal. Full-resolution version available in PS and PDF formats at
http://www.noao.edu/staff/lmacri/0608211-full.ps.gz and
http://www.noao.edu/staff/lmacri/0608211-full.pd
SONTRAC—A low background, large area solar neutron spectrometer
SONTRAC is a scintillating fiber neutron detector designed to measure solar flare neutrons from a balloon or spacecraft platform. The instrument is comprised of alternating orthogonal planes of scintillator fibers viewed by photomultiplier tubes and image intensifier/CCD camera optics. It operates by tracking the paths of recoil protons from the double scatter of 20 to 200 MeV neutrons off hydrogen in the plastic scintillator, thereby providing the necessary information to determine the incident neutron direction and energy. SONTRAC is also capable of detecting and measuring high-energy gamma rays \u3e20 MeV as a “solid-state spark chamber.” The self-triggering and track imaging features of a prototype for tracking in two dimensions have been demonstrated in calibrations with cosmic-ray muons, 14 to ∼65 MeV neutrons and ∼20 MeV protons
Monitoring luminous yellow massive stars in M33: new yellow hypergiant candidates
The evolution of massive stars surviving the red supergiant (RSG) stage
remains unexplored due to the rarity of such objects. The yellow hypergiants
(YHGs) appear to be the warm counterparts of post-RSG classes located near the
Humphreys-Davidson upper luminosity limit, which are characterized by
atmospheric instability and high mass-loss rates. We aim to increase the number
of YHGs in M33 and thus to contribute to a better understanding of the
pre-supernova evolution of massive stars. Optical spectroscopy of five
dust-enshrouded YSGs selected from mid-IR criteria was obtained with the goal
of detecting evidence of extensive atmospheres. We also analyzed BVI photometry
for 21 of the most luminous YSGs in M33 to identify changes in the spectral
type. To explore the properties of circumstellar dust, we performed SED-fitting
of multi-band photometry of the 21 YSGs. We find three luminous YSGs in our
sample to be YHG candidates, as they are surrounded by hot dust and are
enshrouded within extended, cold dusty envelopes. Our spectroscopy of star 2
shows emission of more than one H component, as well as emission of
CaII, implying an extended atmospheric structure. In addition, the long-term
monitoring of the star reveals a dimming in the visual light curve of amplitude
larger than 0.5 mag that caused an apparent drop in the temperature that
exceeded 500 K. We suggest the observed variability to be analogous to that of
the Galactic YHG Cas. Five less luminous YSGs are suggested as post-RSG
candidates showing evidence of hot or/and cool dust emission. We demonstrate
that mid-IR photometry, combined with optical spectroscopy and time-series
photometry, provide a robust method for identifying candidate YHGs. Future
discovery of YHGs in Local Group galaxies is critical for the study of the late
evolution of intermediate-mass massive stars.Comment: 24 pages, 12 figures, 7 Tables. A&A in pres
Neutrino mass constraint from CMB and its degeneracy with other cosmological parameters
We show that the cosmic microwave background (CMB) data of WMAP can give
subelectronvolt limit on the neutrino mass: m_nu < 0.63 eV (95% CL). We also
investigate its degeneracy with other cosmological parameters. In particular,
we show the Hubble constant derived from the WMAP data decreases considerably
when the neutrino mass is a few times 0.1 eV.Comment: 3 pages, 2 figures, prepared for the TAUP2007 Proceeding
Development of a Hard X-Ray Polarimeter for Astrophysics
We have been developing a Compton scatter polarimeter for measuring the linear polarization of hard X-rays (100-300 keV) from astrophysical sources. A laboratory prototype polarimeter has been used to successfully demonstrate the reliability of our Monte Carlo simulation code and to demonstrate our ability to generate a polarized photon source in the lab. Our design concept places a self-containedpolarimeter module on the front-end of a a 5-inch position sensitive PMT (PSPMT). We are currently working on the fabrication of a science model based on this PSPMT concept. Although the emphasis of our development effort is towards measuring hard X-rays from solar flares, our design has the advantage that it is sensitive over a rather large field-of-view (\u3e1 steradian), a feature that makes it especially attractive for γ-ray burst studie
Development of a hard X-ray polarimeter for astrophysics
We have been developing a Compton scatter polarimeter for measuring the linear polarization of hard X-rays (100-300 keV) from astrophysical sources. A laboratory prototype polarimeter has been used to successfully demonstrate the reliability of our Monte Carlo simulation code and to demonstrate our ability to generate a polarized photon source in the lab. Our design concept places a self-containedpolarimeter module on the front-end of a a 5-inch position sensitive PMT (PSPMT). We are currently working on the fabrication of a science model based on this PSPMT concept. Although the emphasis of our development effort is towards measuring hard X-rays from solar flares, our design has the advantage that it is sensitive over a rather large field-of-view (\u3e1 steradian), a feature that makes it especially attractive for γ-ray burst studies
A hard X-ray solar flare polarimeter design based on scintillating fibers
We have developed a design for a Compton scatter polarimeter to measure the polarization of hard X-rays (50–300 keV) from solar flares. The modular design is based on an annular array of scintillating fibers coupled to a 5-inch position-sensitive PMT. Incident photons scatter from the fiber array into a small array of NaI detectors located at the center of the annulus. The location of the interactions in both the fiber array and in the NaI array can be used to measure the linear polarization of the incident flux. This compact design may be well-suited to a variety of astrophysical applications. An extensive series of Monte Carlo simulations has been performed to characterize this design
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