26,689 research outputs found
PO and PN in the wind of the oxygen-rich AGB star IK Tau
Phosphorus-bearing compounds have only been studied in the circumstellar
environments (CSEs) of the asymptotic giant branch (AGB) star IRC +10216 and
the protoplanetary nebula CRL 2688, both C-rich objects, and the O-rich red
supergiant VY CMa. The current chemical models cannot reproduce the high
abundances of PO and PN derived from observations of VY CMa. No observations
have been reported of phosphorus in the CSEs of O-rich AGB stars. We aim to set
observational constraints on the phosphorous chemistry in the CSEs of O-rich
AGB stars, by focussing on the Mira-type variable star IK Tau. Using the IRAM
30m telescope and the Submillimeter Array (SMA), we observed four rotational
transitions of PN (J=2-1,3-2,6-5,7-6) and four of PO
(J=5/2-3/2,7/2-5/2,13/2-11/2,15/2-13/2). The IRAM 30m observations were
dedicated line observations, while the SMA data come from an unbiased spectral
survey in the frequency range 279-355 GHz.
We present the first detections of PN and PO in an O-rich AGB star and
estimate abundances X(PN/H2) of about 3x10^-7 and X(PO/H2) in the range
0.5-6.0x10^-7. This is several orders of magnitude higher than what is found
for the C-rich AGB star IRC +10216. The diameter (<=0.7") of the PN and PO
emission distributions measured in the interferometric data corresponds to a
maximum radial extent of about 40 stellar radii. The abundances and the spatial
occurrence of the molecules are in very good agreement with the results
reported for VY CMa. We did not detect PS or PH3 in the survey. We suggest that
PN and PO are the main carriers of phosphorus in the gas phase, with abundances
possibly up to several 10^-7. The current chemical models cannot account for
this, underlining the strong need for updated chemical models that include
phosphorous compounds.Comment: Accepted for publication in Astronomy & Astrophysics, 10 pages, 8
figure
Models and Strategies for Variants of the Job Shop Scheduling Problem
Recently, a variety of constraint programming and Boolean satisfiability
approaches to scheduling problems have been introduced. They have in common the
use of relatively simple propagation mechanisms and an adaptive way to focus on
the most constrained part of the problem. In some cases, these methods compare
favorably to more classical constraint programming methods relying on
propagation algorithms for global unary or cumulative resource constraints and
dedicated search heuristics. In particular, we described an approach that
combines restarting, with a generic adaptive heuristic and solution guided
branching on a simple model based on a decomposition of disjunctive
constraints. In this paper, we introduce an adaptation of this technique for an
important subclass of job shop scheduling problems (JSPs), where the objective
function involves minimization of earliness/tardiness costs. We further show
that our technique can be improved by adding domain specific information for
one variant of the JSP (involving time lag constraints). In particular we
introduce a dedicated greedy heuristic, and an improved model for the case
where the maximal time lag is 0 (also referred to as no-wait JSPs).Comment: Principles and Practice of Constraint Programming - CP 2011, Perugia
: Italy (2011
What measurable zero point fluctuations can(not) tell us about dark energy
We show that laboratory experiments cannot measure the absolute value of dark
energy. All known experiments rely on electromagnetic interactions. They are
thus insensitive to particles and fields that interact only weakly with
ordinary matter. In addition, Josephson junction experiments only measure
differences in vacuum energy similar to Casimir force measurements. Gravity,
however, couples to the absolute value. Finally we note that Casimir force
measurements have tested zero point fluctuations up to energies of ~10 eV, well
above the dark energy scale of ~0.01 eV. Hence, the proposed cut-off in the
fluctuation spectrum is ruled out experimentally.Comment: 4 page
Status of KLOE-2
In a few months the KLOE-2 detector is expected to start data taking at the
upgraded DANE -factory of INFN Laboratori Nazionali di
Frascati. It aims to collect 25 fb at the peak, and about 5
fb in the energy region between 1 and 2.5 GeV. We review the status and
physics program of the projectComment: 6 pages, 5 figures, to appear in the Proceedings of the PHIPSI09
Workshop, Oct 13-16, 2009, Beijing, Chin
Search for aluminium monoxide in the winds of oxygen-rich AGB stars
Aluminium monoxide, AlO, is likely efficiently depleted from the gas around
oxygen-rich evolved stars to form alumina clusters and dust seeds. Its presence
in the extended atmospheres of evolved stars has been derived from optical
spectroscopy. More recently, AlO gas was also detected at long wavelengths
around the supergiant VY CMa and the oxygen-rich asymptotic giant branch (AGB)
star o Cet (Mira A). In search of AlO, we mined data obtained with APEX, the
IRAM 30m telescope, Herschel/HIFI, SMA, and ALMA, which were primarily aimed at
studying other molecular species. We report here on observations of AlO towards
a sample of eight oxygen-rich AGB stars in different rotational transitions, up
to seven for some stars. We present definite detections of one rotational
transition of AlO for o Cet and R Aqr, and tentative detections of one
transition for R Dor and o Cet, and two for IK Tau and W Hya. The presented
spectra of WX Psc, R Cas, and TX Cam show no signature of AlO. For o Cet, R
Aqr, and IK Tau, we find that the AlO(N=9-8) emission likely traces the inner
parts of the wind, out to only a few tens of AU, where the gas has not yet
reached its terminal velocity. The conclusive detections of AlO emission in the
case of o Cet and R Aqr confirm the presence of AlO gas in outflows of AGB
stars. The tentative detections further support this. Since most of the
observations presented in this study were obtained with stronger emission from
other species than AlO in mind, observations with higher sensitivity in
combination with high angular resolution will improve our understanding of the
presence and behaviour of AlO. From the current data sets we cannot firmly
conclude whether there is a direct correlation between the wind properties and
the detection rate of AlO emission. We hope that this study can serve as a
stimulus to perform sample studies in search of AlO in oxygen-rich outflows.Comment: 9 pages, 9 figures, Accepted for publication in Astronomy &
Astrophysic
Acceleration and vortex filaments in turbulence
We report recent results from a high resolution numerical study of fluid
particles transported by a fully developed turbulent flow. Single particle
trajectories were followed for a time range spanning more than three decades,
from less than a tenth of the Kolmogorov time-scale up to one large-eddy
turnover time. We present some results concerning acceleration statistics and
the statistics of trapping by vortex filaments.Comment: 10 pages, 5 figure
A comparison between Pa alpha and H alpha emission: The relation between HII region mean reddening, local gas density and metallicity
We measure reddenings to HII regions in NGC 2903, NGC 1512, M51, NGC 4449 and
NGC 6946 from Hubble Space Telescope Pa alpha and H alpha images. Extinctions
range from A_V ~ 5 - 0 depending upon the galaxy. For the galaxies with HST
images in both lines, NGC 2903, NGC 1512 and M51, the Pa alpha and H alpha
emission are almost identical in morphology which implies that little emission
from bright HII regions is hidden from view by regions of comparatively high
extinction. The scatter in the measured extinctions is only +- 0.5 mag.
We compare the reddenings we measure in five galaxies using the Pa alpha to H
alpha ratios to those measured previously from the Balmer decrement in the LMC
and as a function of radius in M101 and M51. We find that luminosity weighted
mean extinctions of these ensembles of HI regions are correlated with gas
surface density and metallicity. The correlation is consistent with the mean
extinction depending on dust density where the dust to gas mass ratio scales
with the metallicity. This trend is expected if HII regions tend to be located
near the mid-plane of a gas disk and emerge from their parent molecular clouds
soon after birth. In environments with gas densities below a few hundred
Msol/pc^2 star formation rates estimated from integrated line fluxes and mean
extinctions are likely to be fairly accurate.Comment: accepted for publication in A
Fermi gamma-ray `bubbles' from stochastic acceleration of electrons
Gamma-ray data from Fermi-LAT reveal a bi-lobular structure extending up to
50 degrees above and below the galactic centre, which presumably originated in
some form of energy release there less than a few million years ago. It has
been argued that the gamma-rays arise from hadronic interactions of high energy
cosmic rays which are advected out by a strong wind, or from inverse-Compton
scattering of relativistic electrons accelerated at plasma shocks present in
the bubbles. We explore the alternative possibility that the relativistic
electrons are undergoing stochastic 2nd-order Fermi acceleration by plasma wave
turbulence through the entire volume of the bubbles. The observed gamma-ray
spectral shape is then explained naturally by the resulting hard electron
spectrum and inverse Compton losses. Rather than a constant volume emissivity
as in other models, we predict a nearly constant surface brightness, and
reproduce the observed sharp edges of the bubbles.Comment: 4 pages, 4 figures; REVTeX4-1; discussion amended and one figure
added; to appear in PR
Quantum dot admittance probed at microwave frequencies with an on-chip resonator
We present microwave frequency measurements of the dynamic admittance of a
quantum dot tunnel coupled to a two-dimensional electron gas. The measurements
are made via a high-quality 6.75 GHz on-chip resonator capacitively coupled to
the dot. The resonator frequency is found to shift both down and up close to
conductance resonance of the dot corresponding to a change of sign of the
reactance of the system from capacitive to inductive. The observations are
consistent with a scattering matrix model. The sign of the reactance depends on
the detuning of the dot from conductance resonance and on the magnitude of the
tunnel rate to the lead with respect to the resonator frequency. Inductive
response is observed on a conductance resonance, when tunnel coupling and
temperature are sufficiently small compared to the resonator frequency.Comment: 8 pages, 4 figure
A "superstorm": When moral panic and new risk discourses converge in the media
This is an Author's Accepted Manuscript of an article published in Health, Risk and Society, 15(6), 681-698, 2013, copyright Taylor & Francis, available online at: http://www.tandfonline.com/10.1080/13698575.2013.851180.There has been a proliferation of risk discourses in recent decades but studies of these have been polarised, drawing either on moral panic or new risk frameworks to analyse journalistic discourses. This article opens the theoretical possibility that the two may co-exist and converge in the same scare. I do this by bringing together more recent developments in moral panic thesis, with new risk theory and the concept of media logic. I then apply this theoretical approach to an empirical analysis of how and with what consequences moral panic and new risk type discourses converged in the editorials of four newspaper campaigns against GM food policy in Britain in the late 1990s. The article analyses 112 editorials published between January 1998 and December 2000, supplemented with news stories where these were needed for contextual clarity. This analysis shows that not only did this novel food generate intense media and public reactions; these developed in the absence of the type of concrete details journalists usually look for in risk stories. Media logic is important in understanding how journalists were able to engage and hence how a major scare could be constructed around convergent moral panic and new risk type discourses. The result was a media ‘superstorm’ of sustained coverage in which both types of discourse converged in highly emotive mutually reinforcing ways that resonated in a highly sensitised context. The consequence was acute anxiety, social volatility and the potential for the disruption of policy and social change
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