435 research outputs found
Multiple scattering of polarized radiation by non-spherical grains: first results
We present the first numerical radiative transfer simulation of multiple
light scattering in dust configurations containing aligned non-spherical
(spheroidal) dust grains. Such models are especially important if one wants to
explain the circular polarization of light, observed in a variety of
astronomical objects. The radiative transfer problem is solved on the basis of
the Monte Carlo method. Test simulations, confirming the correct numerical
implementation of the scattering mechanism, are presented. As a first
application, we investigate the linear and circular polarization of light
coming from a spherical circumstellar shell. This shell contains perfectly
aligned prolate or oblate spheroidal grains. The most remarkable features of
the simulated linear polarization maps are so-called polarization null points
where the reversal of polarization occurs. They appear in the case when the
grain alignment axis is perpendicular to the line of sight. The maps of
circular polarization have a sector-like structure with maxima at the ends of
lines inclined to the grain alignment axis by \pm 45\degr.Comment: 13 pages, 14 figures, accepted by A&
A culturally diverse learnersâ conceptual physics progression: A COVID transition in 2020 and onwards
In this presentation we discuss the conceptual progression of a mechanics physics course taught at the Physics Department at the University of Johannesburg (UJ) in 2020 (Carleschi et al, 2022). The number of students enrolled is typically of the order of 400 each year and this has led to challenges in transitions between online learning and face-to-face. The force concept inventory (FCI) has been extended at the Physics Department using a conceptions/'misconceptions' lens for a further two years, to include 2021 and 2022. We present and explore the effects that switching mid-semester from traditional classes to online classes, as imposed by the COVID-19 lockdown in South Africa, has had on the teaching andragogy at UJ.
We discuss the initial stages of how using `dominant misconceptions', as derived from the pre-test part of the Force Concept Inventory (FCI) assessment tool, can give informative data on how to proceed with teaching practices on a classical mechanics course, as taught in the Physics Department of the University of Johannesburg. We also briefly discuss some details of language effects between high school matriculation marks and FCI scores, and how this may prepare faculty members when considering their teaching andragogy. South Africa is a diaspora of languages with over 10 official languages spoken, besides the multitudes of other dialects.
Specifically, we used a question-by-question comparison of different cohorts of the 30 question FCI and we also looked at correlation coefficients between high school matriculation scores including language spoken at home and performance on the FCI.
REFERENCE
Carleschi. E., Chrysostomou A., Cornell A. S., & Naylor W. (2022). Probing the effect on student conceptual understanding due to a forced mid-semester transition to online teaching. European Journal of Physics, 43, 035702. https://doi.org/10.1088/1361-6404/ac41d
The Ratio of Ortho- to Para-H2 in Photodissociation Regions
We discuss the ratio of ortho- to para-H2 in photodissociation regions
(PDRs). We draw attention to an apparent confusion in the literature between
the ortho-to-para ratio of molecules in FUV-pumped vibrationally excited
states, and the H2 ortho-to-para abundance ratio. These ratios are not the same
because the process of FUV-pumping of fluorescent H2 emission in PDRs occurs
via optically thick absorption lines. Thus, gas with an equilibrium ratio of
ortho- to para-H2 equal to 3 will yield FUV-pumped vibrationally excited
ortho-to-para ratios smaller than 3, because the ortho-H2 pumping rates are
preferentially reduced by optical depth effects. Indeed, if the ortho and para
pumping lines are on the ``square root'' part of the curve-of-growth, then the
expected ratio of ortho and para vibrational line strengths is the square root
of 3, ~ 1.7, close to the typically observed value. Thus, contrary to what has
sometimes been stated in the literature, most previous measurements of the
ratio of ortho- to para-H2 in vibrationally excited states are entirely
consistent with a total ortho-to-para ratio of 3, the equilibrium value for
temperatures greater than 200 K. We present an analysis and several detailed
models which illustrate the relationship between the total ratios of ortho- to
para-H2 and the vibrationally excited ortho-to-para ratios in PDRs. Recent
Infrared Space Observatory (ISO) measurements of pure rotational and
vibrational H2 emissions from the PDR in the star-forming region S140 provide
strong observational support for our conclusions.Comment: 23 pages (including 5 figures), LaTeX, uses aaspp4.sty, accepted for
publication in Ap
Observatory/data centre partnerships and the VO-centric archive: The JCMT Science Archive experience
We present, as a case study, a description of the partnership between an
observatory (JCMT) and a data centre (CADC) that led to the development of the
JCMT Science Archive (JSA). The JSA is a successful example of a service
designed to use Virtual Observatory (VO) technologies from the start. We
describe the motivation, process and lessons learned from this approach.Comment: Accepted for publication in the second Astronomy & Computing Special
Issue on the Virtual Observatory; 10 pages, 5 figure
The role of classifiers in feature selection : number vs nature
Wrapper feature selection approaches are widely used to select a small subset of relevant features from a dataset. However, Wrappers suffer from the fact that they only use a single classifier when selecting the features. The problem of using a single classifier is that each classifier is of a different nature and will have its own biases. This means that each classifier will select different feature subsets. To address this problem, this thesis aims to investigate the effects of using different classifiers for Wrapper feature selection. More specifically, it aims to investigate the effects of using different number of classifiers and classifiers of different nature. This aim is achieved by proposing a new data mining method called Wrapper-based Decision Trees (WDT). The WDT method has the ability to combine multiple classifiers from four different families, including Bayesian Network, Decision Tree, Nearest Neighbour and Support Vector Machine, to select relevant features and visualise the relationships among the selected features using decision trees. Specifically, the WDT method is applied to investigate three research questions of this thesis: (1) the effects of number of classifiers on feature selection results; (2) the effects of nature of classifiers on feature selection results; and (3) which of the two (i.e., number or nature of classifiers) has more of an effect on feature selection results. Two types of user preference datasets derived from Human-Computer Interaction (HCI) are used with WDT to assist in answering these three research questions. The results from the investigation revealed that the number of classifiers and nature of classifiers greatly affect feature selection results. In terms of number of classifiers, the results showed that few classifiers selected many relevant features whereas many classifiers selected few relevant features. In addition, it was found that using three classifiers resulted in highly accurate feature subsets. In terms of nature of classifiers, it was showed that Decision Tree, Bayesian Network and Nearest Neighbour classifiers caused signficant differences in both the number of features selected and the accuracy levels of the features. A comparison of results regarding number of classifiers and nature of classifiers revealed that the former has more of an effect on feature selection than the latter. The thesis makes contributions to three communities: data mining, feature selection, and HCI. For the data mining community, this thesis proposes a new method called WDT which integrates the use of multiple classifiers for feature selection and decision trees to effectively select and visualise the most relevant features within a dataset. For the feature selection community, the results of this thesis have showed that the number of classifiers and nature of classifiers can truly affect the feature selection process. The results and suggestions based on the results can provide useful insight about classifiers when performing feature selection. For the HCI community, this thesis has showed the usefulness of feature selection for identifying a small number of highly relevant features for determining the preferences of different users.EThOS - Electronic Theses Online ServiceGBUnited Kingdo
Near-Infrared Spectroscopy of Molecular Filaments in the Reflection Nebula NGC 7023
We present near-infrared spectroscopy of fluorescent molecular hydrogen (H_2)
emission from molecular filaments in the reflection nebula NGC 7023. We derive
the relative column densities of H_2 rotational-vibrational states from the
measured line emission and compare these results with several model
photodissociation regions covering a range of densities, incident UV-fields,
and excitation mechanisms. Our best-fit models for one filament suggest, but do
not require, either a combination of different densities, suggesting clumps of
10^6 cm^{-3} in a 10^4 - 10^5 cm^{-3} filament, or a combination of fluorescent
excitation and thermally-excited gas, perhaps due to a shock from a bipolar
outflow. We derive densities and UV fields for these molecular filaments that
are in agreement with previous determinations.Comment: ApJ accepted, 26 pages including 5 embedded figures, uses AASTEX.
Also available at http://www-astronomy.mps.ohio-state.edu/~martini/pubs.htm
A new era of wide-field submillimetre imaging: on-sky performance of SCUBA-2
SCUBA-2 is the largest submillimetre wide-field bolometric camera ever built.
This 43 square arc-minute field-of-view instrument operates at two wavelengths
(850 and 450 microns) and has been installed on the James Clerk Maxwell
Telescope on Mauna Kea, Hawaii. SCUBA-2 has been successfully commissioned and
operational for general science since October 2011. This paper presents an
overview of the on-sky performance of the instrument during and since
commissioning in mid-2011. The on-sky noise characteristics and NEPs of the 450
and 850 micron arrays, with average yields of approximately 3400 bolometers at
each wavelength, will be shown. The observing modes of the instrument and the
on-sky calibration techniques are described. The culmination of these efforts
has resulted in a scientifically powerful mapping camera with sensitivities
that allow a square degree of sky to be mapped to 10 mJy/beam rms at 850 micron
in 2 hours and 60 mJy/beam rms at 450 micron in 5 hours in the best weather.Comment: 18 pages, 15 figures.SPIE Conference series 8452, Millimetre,
Submillimetre and Far-infrared Detectors and Instrumentation for Astronomy VI
201
UV Circular Polarisation in Star Formation Regions : The Origin of Homochirality?
Ultraviolet circularly polarised light has been suggested as the initial cause of the homochirality of organic molecules in terrestrial organisms, via enantiomeric selection of prebiotic molecules by asymmetric photolysis. We present a theoretical investigation of mechanisms by which ultraviolet circular polarisation may be produced in star formation regions. In the scenarios considered here, light scattering produces only a small percentage of net circular polarisation at any point in space, due to the forward throwing nature of the phase function in the ultraviolet. By contrast, dichroic extinction can produce a fairly high percentage of net circular polarisation (âŒ10%) and may therefore play a key role in producing an enantiomeric excessPeer reviewe
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