2,964 research outputs found
Spitzer Mapping of PAHs and H2 in Photodissociation Regions
The mid-infrared (MIR) spectra of dense photodissociation regions (PDRs) are
typically dominated by emission from polycyclic aromatic hydrocarbons (PAHs)
and the lowest pure rotational states of molecular hydrogen (H2); two species
which are probes of the physical properties of gas and dust in intense UV
radiation fields. We utilize the high angular resolution of the Infrared
Spectrograph on the Spitzer Space Telescope to construct spectral maps of the
PAH and H2 features for three of the best studied PDRs in the galaxy, NGC 7023,
NGC 2023 and IC 63. We present spatially resolved maps of the physical
properties, including the H2 ortho-to-para ratio, temperature, and G_o/n_H. We
also present evidence for PAH dehydrogenation, which may support theories of H2
formation on PAH surfaces, and a detection of preferential self-shielding of
ortho-H2. All PDRs studied exhibit average temperatures of ~500 - 800K, warm H2
column densities of ~10^20 cm^-2, G_o/n_H ~ 0.1 - 0.8, and ortho-to-para ratios
of ~ 1.8. We find that while the average of each of these properties is
consistent with previous single value measurements of these PDRs, when
available, the addition of spatial resolution yields a diversity of values with
gas temperatures as high as 1500 K, column densities spanning ~ 2 orders of
magnitude, and extreme ortho-to-para ratios of 3.Comment: 14 figure
Phase-Dependent Properties of Extrasolar Planet Atmospheres
Recently the Spitzer Space Telescope observed the transiting extrasolar
planets, TrES-1 and HD209458b. These observations have provided the first
estimates of the day side thermal flux from two extrasolar planets orbiting
Sun-like stars. In this paper, synthetic spectra from atmospheric models are
compared to these observations. The day-night temperature difference is
explored and phase-dependent flux densities are predicted for both planets. For
HD209458b and TrES-1, models with significant day-to-night energy
redistribution are required to reproduce the observations. However, the
observational error bars are large and a range of models remains viable.Comment: 8 pages, 7 figures, accepted for publication in the Astrophysical
Journa
The NextGen Model Atmosphere grid: II. Spherically symmetric model atmospheres for giant stars with effective temperatures between 3000 and 6800~K
We present the extension of our NextGen model atmosphere grid to the regime
of giant stars. The input physics of the models presented here is nearly
identical to the NextGen dwarf atmosphere models, however spherical geometry is
used self-consistently in the model calculations (including the radiative
transfer). We re-visit the discussion of the effects of spherical geometry on
the structure of the atmospheres and the emitted spectra and discuss the
results of NLTE calculations for a few selected models.Comment: ApJ, in press (November 1999), 13 pages, also available at
http://dilbert.physast.uga.edu/~yeti/PAPERS and at
ftp://calvin.physast.uga.edu/pub/preprints/NG-giants.ps.g
Towards the model driven organization
Todays modern organization is faced with a need for rapid response to changes from external business pressures by updating systems and operational procedures. The effect of such continuous evolution eventually leads to sub- optimal configurations of its underlying systems. The management of continuous business change is compromised by uncertainty due to the inadequacy of existing mechanisms for responding to multiple change drivers thus leading to signifi- cant organizational costs. This represents a major opportunity for seeking greater efficiencies. To date, there has been little or no attempt to apply model driven principles or approaches to addressing these issues. We present a new vision of a Model Driven Organisation (MDO) that has the potential to increase produc- tivity by promoting integration of business processes and collaborations across the organisation whilst supporting safe and convenient adaptations that enable rapid response to change whilst maintaining integrity of the systems within the organisation. The approach proposed is based on the use of modelling languages and simulation technologies that support abstractions for understanding business goals through to specification of IT systems and ultimately to deployed systems. The paper motivates the problem and proposes a definition of the MDO. We val- idate the proposal through an illustrative case and conclude with a review of the state of the art leading to a roadmap of research and emergent grand challenges towards achieving the MDO vision
The Nature of the Secondary Star in the Black Hole X-Ray Transient V616 Mon (=A0620-00)
We have used NIRSPEC on Keck II to obtain -band spectroscopy of the low
mass X-ray binary V616 Mon (= A062000). V616 Mon is the proto-typical soft
x-ray transient containing a black hole primary. As such it is important to
constrain the masses of the binary components. The modeling of the infrared
observations of ellipsoidal variations in this system lead to a derived mass of
11.0 M_{\sun} for the black hole. The validity of this derivation has been
called into question due to the possiblity that the secondary star's spectral
energy distribution is contaminated by accretion disk emission (acting to
dilute the variations). Our new -band spectrum of V616 Mon reveals a
late-type K dwarf secondary star, but one that has very weak CO
absorption features. Comparison of V616 Mon with SS Cyg leads us to estimate
that the accretion disk supplies only a small amount of -band flux, and the
ellipsoidal variations are not seriously contaminated. If true, the derived
orbital inclination of V616 Mon is not greatly altered, and the mass of the
black hole remains large. A preliminary stellar atmosphere model for the
-band spectrum of V616 Mon reveals that the carbon abundance is
approximately 50% of the solar value. We conclude that the secondary star in
V616 Mon has either suffered serious contamination from the accretion of
supernova ejecta that created the black hole primary, or it is the stripped
remains of a formerly more massive secondary star, one in which the CNO cycle
had been active.Comment: 20 pages, 5 figure
Mitigating impacts of the COVID-19 pandemic on primary and lower secondary children during school closures: a rapid evidence review
BACKGROUND:
To control the spread of the SARS-CoV-2 virus during the COVID-19 pandemic, UK schools were closed and education activity was undertaken at home resulting in considerable disruption to children’s education.
AIMS:
To identify and assess evidence of harms caused to primary and lower secondary pupils during this time and identify mitigation strategies relevant to those harms.
METHODS:
A rapid evidence review tailored to delivery at pace, drawing on UK evidence for harms and relevant mitigation strategies.
FINDINGS ON HARM:
There is evidence that the patterns of disruption to education during the pandemic have impacted on children’s learning and attainment, mental health and wellbeing, physical health and nutrition and increased exposure to risk especially for those children living in potentially dangerous domestic settings. Although the quality of the evidence is uneven, it is clear that children living in poverty have been most affected, in particular through food insecurity and conditions triggering stress and anxiety in the home, alongside their more limited opportunities to access digital resources for learning, or indeed outside space for physical activity. Attempts to distinguish harms that impact in the short term from longer lasting harms may take time. It also requires schools to have access to contextually relevant diagnostic tools they can use to assess the range of harms in need of redress in their local context.
FINDINGS ON MITIGATION STRATEGIES:
We found no evidence for mitigation strategies directly relevant to the harms experienced by children due to school closures under COVID-19. Mitigation strategies suggested in the UK often derived their evidence of efficacy from circumstances quite unlike the prolonged patterns of disruption to education that COVID has caused. Most were designed to address the needs of a few pupils struggling under normal circumstances and were not able to demonstrate their relevance at scale. We therefore examined the primary literature on recovery from unplanned school closures in other countries focused on school-based strategies that had been evaluated as effective under similar conditions.
CONCLUSION:
We found some evidence of a range of harms but little research evidence on relevant mitigation strategies and an absence of evidence on those strategies that schools themselves have adopted since re-opening, tailored to local needs. Such mitigation strategies may be highly relevant for system learning, and it is important to document and evaluate their efficacy, and indeed learn from them. Closing schools during the pandemic has revealed the importance of schools in safeguarding children. School staff should be given the training and resources to be able to identify children at risk and refer pupils to appropriate services if necessary
Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
We have used archival far-ultraviolet spectra from observations made by
HST/STIS and FUSE to determine the column densities and rotational excitation
temperatures for CO and H2, respectively, along the lines of sight to 23
Galactic O and B stars. The sightlines have reddening values in the range
E(B-V)= 0.07-0.62, sampling the diffuse to translucent interstellar medium. We
find that the H2 column densities range from 5x10^18-8x10^20 cm^-2 and the CO
from upper limits around 2x10^12 cm^-2 to detections as high as 1.4x10^16
cm^-2. CO increases with increasing H2, roughly following a power law of factor
\~2. The CO/H2 column density ratio is thus not constant, and ranges from 10^-7
- 10^-5, with a mean value of 3x10^-6. The sample segregates into "diffuse" and
"translucent" regimes, the former having a molecular fraction less than ~0.25
and A_V/d<1 mag kpc^-1. The mean CO/H2 for these two regimes are 3.6x10^-7 and
9.3x10^-6, respectively, significantly lower than the canonical dark cloud
value of 10^-4. In six of the sightlines, 13CO is observed, and the isotopic
ratio we observe (~50-70) is consistent with, if perhaps a little below, the
average 12C/13C for the ISM at large. The average H2 rotational excitation
temperature is 74+/-24 K, in good agreement with previous studies, and the
average CO temperature is 4.1 K, with some sightlines as high as 6.4 K. The
higher excitation CO is observed with higher column densities, consistent with
the effects of photon trapping in clouds with densities in the 20-100 cm^-3
range. We discuss the implications for the structure of the diffuse/translucent
regimes of the interstellar medium and the estimation of molecular mass in
galaxies.Comment: emualateapj style, 6 figures, 3 tables, accepted on 21 Nov 2006 for
publication in The Astrophysical Journa
Suppressed Far-UV stellar activity and low planetary mass-loss in the WASP-18 system
WASP-18 hosts a massive, very close-in Jupiter-like planet. Despite its young age (R′HK activity parameter lies slightly below the basal level; there is no significant time-variability in the log R′HK value; there is no detection of the star in the X-rays. We present results of far-UV observations of WASP-18 obtained with COS on board of HST aimed at explaining this anomaly. From the star’s spectral energy distribution, we infer the extinction (E(B − V) ≈ 0.01mag) and then the ISM column density for a number of ions, concluding that ISM absorption is not the origin of the anomaly. We measure the flux of the four stellar emission features detected in the COS spectrum (C II, C III, C IV, Si IV). Comparing the C II/C IV flux ratio measured for WASP-18 with that derived from spectra of nearby stars with known age, we see that the far-UV spectrum of WASP-18 resembles that of old (>5Gyr), inactive stars, in stark contrast with its young age. We conclude that WASP-18 has an intrinsically low activity level, possibly caused by star-planet tidal interaction, as suggested by previous studies. Re-scaling the solar irradiance reference spectrum to match the flux of the Si IV line, yields an XUV integrated flux at the planet orbit of 10.2 erg s−1 cm−2. We employ the rescaled XUV solar fluxes to model of the planetary upper atmosphere, deriving an extremely low thermal mass-loss rate of 10−20MJ Gyr−1. For such high-mass planets, thermal escape is not energy limited, but driven by Jeans escape
Challenges and Directions in Formalizing the Semantics of Modeling Languages
Developing software from models is a growing practice and there exist many model-based tools (e.g., editors, interpreters, debuggers, and simulators) for supporting model-driven engineering. Even though these tools facilitate the automation of software engineering tasks and activities, such tools are typically engineered manually. However, many of these tools have a common semantic foundation centered around an underlying modeling language, which would make it possible to automate their development if the modeling language specification were formalized. Even though there has been much work in formalizing programming languages, with many successful tools constructed using such formalisms, there has been little work in formalizing modeling languages for the purpose of automation. This paper discusses possible semantics-based approaches for the formalization of modeling languages and describes how this formalism may be used to automate the construction of modeling tools
Three-dimensional distribution of ejecta in Supernova 1987A at 10 000 days
Due to its proximity, SN 1987A offers a unique opportunity to directly
observe the geometry of a stellar explosion as it unfolds. Here we present
spectral and imaging observations of SN 1987A obtained ~10,000 days after the
explosion with HST/STIS and VLT/SINFONI at optical and near-infrared
wavelengths. These observations allow us to produce the most detailed 3D map of
H-alpha to date, the first 3D maps for [Ca II] \lambda \lambda 7292, 7324, [O
I] \lambda \lambda 6300, 6364 and Mg II \lambda \lambda 9218, 9244, as well as
new maps for [Si I]+[Fe II] 1.644 \mu m and He I 2.058 \mu m. A comparison with
previous observations shows that the [Si I]+[Fe II] flux and morphology have
not changed significantly during the past ten years, providing evidence that it
is powered by 44Ti. The time-evolution of H-alpha shows that it is
predominantly powered by X-rays from the ring, in agreement with previous
findings. All lines that have sufficient signal show a similar large-scale 3D
structure, with a north-south asymmetry that resembles a broken dipole. This
structure correlates with early observations of asymmetries, showing that there
is a global asymmetry that extends from the inner core to the outer envelope.
On smaller scales, the two brightest lines, H-alpha and [Si I]+[Fe II] 1.644
\mu m, show substructures at the level of ~ 200 - 1000 km/s and clear
differences in their 3D geometries. We discuss these results in the context of
explosion models and the properties of dust in the ejecta.Comment: Accepted for publication in Ap
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