155 research outputs found
A photoionization model of the compact HII region G29.96-0.02
We present a detailed photoionization model of G29.96-0.02 (hereafter
G29.96), one of the brightest Galactic Ultra Compact HII (UCHII) regions in the
Galaxy. This source has been observed extensively at radio and infrared
wavelengths. The most recent data include a complete ISO (SWS and LWS)
spectrum, which displays a remarkable richness in atomic fine-structure lines.
The number of observables is twice as much as the number available in previous
studies. In addition, most atomic species are now observed in two ionization
stages. The radio and infrared data on G29.96 are best reproduced using a
nebular model with two density components: a diffuse (n_e~680cm-3) extended (~1
pc) component surrounding a compact (~0.1 pc) dense (n_e~57000cm-3) core. The
properties of the ionizing star were derived using state-of-the-art stellar
atmosphere models. CoStar models yield an effective temperature of
\~30^{+2}_{-1} kK whereas more recent non-LTE line blanketed atmospheres with
stellar winds indicate somewhat higher values, Teff~32--38 kK. This range in
Teff is compatible with all observational constraints, including near-infrared
photometry and bolometric luminosity. The range 33-36 kK is also compatible
with the spectral type O5-O8 determined by Watson and Hanson (97) when recent
downward revisions of the effective temperature scale of O stars are taken into
account. The age of the ionizing star of G29.96 is found to be a few 10^6 yr,
much older than the expected lifetime of UCHII regions. Accurate gas phase
abundances are derived with the most robust results being Ne/S=7.5 and N/O=0.43
(1.3 and 3.5 times the solar values, respectively).Comment: Accepted in Astronomy and Astrophysic
Physical Conditions in Barnard's Loop, Components of the Orion-Eridanus Bubble, and Implications for the WIM Component of the ISM
We have supplemented existing spectra of Barnard's Loop with high accuracy
spectrophotometry of one new position. Cloudy photoionization models were
calculated for a variety of ionization parameters and stellar temperatures and
compared with the observations. After testing the procedure with recent
observations of M43, we establish that Barnard's Loop is photoionized by four
candidate ionizing stars, but agreement between the models and observations is
only possible if Barnard's Loop is enhanced in heavy elements by about a factor
of 1.4. Barnard's Loop is very similar in properties to the brightest
components of the Orion-Eridanus Bubble and the Warm Ionized Medium (WIM). We
are able to establish models that bound the range populated in low-ionization
color-color diagrams (I([SII])/I(H{\alpha}) versus I([NII])/I(H{\alpha})) using
only a limited range of ionization parameters and stellar temperatures.
Previously established variations in the relative abundance of heavy elements
render uncertain the most common method of determining electron temperatures
for components of the Orion-Eridanus Bubble and the WIM based on only the
I([NII])/I(H{\alpha}) ratio, although we confirm that the lowest surface
brightness components of the WIM are on average of higher electron temperature.
The electron temperatures for a few high surface brightness WIM components
determined by direct methods are comparable to those of classical bright H II
regions. In contrast, the low surface brightness HII regions studied by the
Wisconsin H{\alpha} Mapper are of lower temperatures than the classical bright
HII regions
Clarifying Differences Between Reading Skills and Reading Strategies
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/88049/1/RT.61.5.1.pd
Spatial distribution of interstellar gas in the innermost 3 kpc of our Galaxy
We review the present observational knowledge on the spatial distribution and
the physical state of the different (molecular, atomic and ionized) components
of the interstellar gas in the innermost 3 kpc of our Galaxy -- a region which
we refer to as the interstellar Galactic bulge, to distinguish it from its
stellar counterpart. We try to interpret the observations in the framework of
recent dynamical models of interstellar gas flows in the gravitational
potential of a barred galaxy. Finally, relying on both the relevant
observations and their theoretical interpretation, we propose a model for the
space-averaged density of each component of the interstellar gas in the
interstellar Galactic bulge.Comment: 19 pages, 11 figure
The Nuclear Bulge of the Galaxy. III. Large-Scale Physical Characteristics of Stars and Interstellar Matter
We analyse IRAS and COBE DIRBE data at wavelengths between 2.2 and 240 mu of
the central 500pc of the Galaxy and derive the large-scale distribution of
stars and interstellar matter in the Nuclear Bulge. Models of the Galactic Disk
and Bulge are developed in order to correctly decompose the total surface
brightness maps and to apply proper extinction corrections. The Nuclear Bulge
appears as a distinct, massive disk-like complex of stars and molecular clouds
which is, on a large scale, symmetric with respect to the Galactic Centre. It
is distinguished from the Galactic Bulge by its flat disk-like morphology, very
high density of stars and molecular gas, and ongoing star formation. The
Nuclear Bulge consists of an R^-2 Nuclear Stellar Cluster at the centre, a
large Nuclear Stellar Disk with radius 230+-20 pc and scale height 45+-5 pc,
and a Nuclear Molecular Disk of same size. Its total stellar mass and
luminosity are 1.4+-0.6 10^9 M_sun and 2.5+-1 10^9 L_sun, respectively. The
total mass of interstellar hydrogen in the Nuclear Bulge is 2+-0.3 10^7 M_sun.
Interstellar matter in the Nuclear Bulge is very clumpy with ~90% of the mass
contained in dense and massive molecular clouds with a volume filling factor of
only a few per cent. This extreme clumpiness enables the strong interstellar
radiation field to penetrate the entire Nuclear Bulge and explains the
relatively low average extinction towards the Galactic Centre. In addition, we
find 4 10^7 M_sun of cold and dense material located outside the Nuclear Bulge,
which gives rise to the observed asymmetry in the distribution of interstellar
matter in the Central Molecular Zone.Comment: 28 pages, 22 figures, accepted for publication in A&
Understanding the Spectral Energy Distributions of the Galactic Star Forming Regions IRAS 18314-0720, 18355-0532 & 18316-0602
Embedded Young Stellar Objects (YSO) in dense interstellar clouds is treated
self-consistently to understand their spectral energy distributions (SED).
Radiative transfer calculations in spherical geometry involving the dust as
well as the gas component, have been carried out to explain observations
covering a wide spectral range encompassing near-infrared to radio continuum
wavelengths. Various geometric and physical details of the YSOs are determined
from this modelling scheme. In order to assess the effectiveness of this
self-consistent scheme, three young Galactic star forming regions associated
with IRAS 18314-0720, 18355-0532 and 18316-0602 have been modelled as test
cases. They cover a large range of luminosity ( 40). The modelling of
their SEDs has led to information about various details of these sources, e.g.
embedded energy source, cloud structure & size, density distribution,
composition & abundance of dust grains etc. In all three cases, the best fit
model corresponds to the uniform density distribution.Comment: AAMS style manuscript with 3 tables (in a separate file) and 4
figures. To appear in Journal of Astronophysics & Astronom
The origin of the light distribution in spiral galaxies
We analyse a high-resolution, fully cosmological, hydrodynamical disc galaxy simulation, to study the source of the double-exponential light profiles seen in many stellar discs, and the effects of stellar radial migration upon the spatiotemporal evolution of both the disc age and metallicity distributions. We find a ‘break’ in the pure exponential stellar surface brightness profile, and trace its origin to a sharp decrease in the star formation per unit surface area, itself produced by a decrease in the gas volume density due to a warping of the gas disc. Star formation in the disc continues well beyond the break. We find that the break is more pronounced in bluer wavebands. By contrast, we find little or no break in the mass density profile. This is, in part, due to the net radial migration of stars towards the external parts of the disc. Beyond the break radius, we find that ∼60 per cent of the resident stars migrated from the inner disc, while ∼25 per cent formed in situ. Our simulated galaxy also has a minimum in the age profile at the break radius but, in disagreement with some previous studies, migration is not the main mechanism producing this shape. In our simulation, the disc metallicity gradient flattens with time, consistent with an ‘inside-out’ formation scenario. We do not find any difference in the intensity or the position of the break with inclination, suggesting that perhaps the differences found in empirical studies are driven by dust extinction
Distant Voices: Learners' Stories About the Affective Side of Learning a Language at a Distance
Learning a language at a distance has its own special challenges. The remoteness of the learning context can mean isolation for the learner, communication difficulties for the teacher and problems of access for the researcher. Yet distance language learners are likely to be no more skilled in self-regulation than classroom learners, and to require high levels of support. Research tools are needed, therefore, which allow them to talk freely about their learning in order to help distance educators target support appropriately. This paper draws on data from two pilot ethnographic studies of distance language learners using think-aloud protocols to access their thought processes as they tackled two designated language tasks. They were carried out as part of a wider study in each case to investigate aspects of affect including beliefs, motivation and anxiety. The audio-taped voices provided rich insights into the advantages and disadvantages, pleasures and frustrations, comforts and anxieties of learning a language at a distance, and the strategies learners use to manage in a distance environment. The studies underlined the importance of listening to students and using their voices as a basis for discussion on improving aspects of the design and delivery of distance language courses
An application of the Rasch model to reading comprehension measurement
An effective reading comprehension measurement demands robust psychometric tools that allow teachers and researchers to evaluate the educational practices and track changes in students’ performance. In this study, we illustrate how Rasch model can be used to attend such demands and improve reading comprehension measurement. We discuss the construction of two reading comprehension tests: TRC-n, with narrative texts, and TRC-e, with expository texts. Three vertically scaled forms were generated for each test (TRC-n-2, TRC-n-3, TRC-n-4; TRC-e-2, TRC-e-3 and TRC-e-4), each meant to assess Portuguese students in second, third and fourth grade of elementary school. The
tests were constructed according to a nonequivalent groups with anchor test design and data were analyzed using the Rasch model. The results provided evidence for good psychometric qualities for each test form,
including unidimensionality and local independence and adequate reliability. A critical view of this study and future researches are discussed.CIEC – Research Centre on Child Studies, IE, UMinho (FCT R&D unit 317), PortugalThis research was supported by Grant FCOMP-01-0124-FEDER-010733 from
Fundação para a Ciência e Tecnologia (FCT) and the European Regional
Development Fund (FEDER) through the European program COMPETE
(Operational Program for Competitiveness Factors) under the National
Strategic Reference Framework (QREN).info:eu-repo/semantics/publishedVersio
Improving estimates of population status and trend with superensemble models
Fishery managers must often reconcile conflicting estimates of population status and trend. Superensemble models, commonly used in climate and weather forecasting, may provide an effective solution. This approach uses predictions from multiple models as covariates in an additional "superensemble" model fitted to known data. We evaluated the potential for ensemble averages and superensemble models (ensemble methods) to improve estimates of population status and trend for fisheries. We fit four widely applicable data-limited models that estimate stock biomass relative to equilibrium biomass at maximum sustainable yield (B/BMSY). We combined these estimates of recent fishery status and trends in B/BMSY with four ensemble methods: an ensemble average and three superensembles (a linear model, a random forest and a boosted regression tree). We trained our superensembles on 5,760 simulated stocks and tested them with cross-validation and against a global database of 249 stock assessments. Ensemble methods substantially improved estimates of population status and trend. Random forest and boosted regression trees performed the best at estimating population status: inaccuracy (median absolute proportional error) decreased from 0.42 -0.56 to 0.32 -0.33, rank-order correlation between predicted and true status improved from 0.02 - 0.32 to 0.44 - 0.48 and bias (median proportional error) declined from - 0.22 - 0.31 to - 0.12 - 0.03. We found similar improvements when predicting trend and when applying the simulation-trained superensembles to catch data for global fish stocks. Superensembles can optimally leverage multiple model predictions; however, they must be tested, formed from a diverse set of accurate models and built on a data set representative of the populations to which they are applied
- …