3,154 research outputs found
Locally linear embedding: dimension reduction of massive protostellar spectra
We present the results of the application of locally linear embedding (LLE)
to reduce the dimensionality of dereddened and continuum subtracted
near-infrared spectra using a combination of models and real spectra of massive
protostars selected from the Red MSX Source survey database. A brief comparison
is also made with two other dimension reduction techniques; Principal Component
Analysis (PCA) and Isomap using the same set of spectra as well as a more
advanced form of LLE, Hessian locally linear embedding. We find that whilst LLE
certainly has its limitations, it significantly outperforms both PCA and Isomap
in classification of spectra based on the presence/absence of emission lines
and provides a valuable tool for classification and analysis of large spectral
data sets.Comment: 8 pages, 7 figures. Accepted for publication in MNRAS 2016 June 2
Determination of the SSME high pressure oxidizer turbopump bearing temperature
The SSME high pressure liquid oxygen turbopump (HPOTP) bearings sometimes wear and experience heating and oxidation of the ball and raceway surfaces. So far it has been impossible to measure the temperature of the bearings directly during operation of the turbopumps. However, a method was developed for determining the surface temperature of the bearings from the composition of the oxides using oxidation samples for calibration and Auger Electron Spectroscopy (AES) for chemical analysis
Near Infrared Spectra of Compact Planetary Nebulae
This paper continues our study of the behaviour of near infrared helium
recombination lines in planetary nebula. We find that the 1.7007um 4^3D-3^3P
HeI line is a good measure of the HeI recombination rate, since it varies
smoothly with the effective temperature of the central star. We were unable to
reproduce the observed data using detailed photoionisation models at both low
and high effective temperatures, but plausible explanations for the difference
exist for both. We therefore conclude that this line could be used as an
indicator of the effective temperature in obscured nebula. We also
characterised the nature of the molecular hydrogen emission present in a
smaller subset of our sample. The results are consistent with previous data
indicating that ultraviolet excitation rather than shocks is the main cause of
the molecular hydrogen emission in planetary nebulae.Comment: Accepted for publication in MNRA
Infrared Helium-Hydrogen Line Ratios as a Measure of Stellar Effective Temperature
We have observed a large sample of compact planetary nebulae in the
near-infrared to determine how the 2^1P-2^1S HeI line at 2.058um varies as a
function of stellar effective temperature, Teff. The ratio of this line with HI
Br g at 2.166um has often been used as a measure of the highest Teff present in
a stellar cluster, and hence on whether there is a cut-off in the stellar
initial mass function at high masses. However, recent photoionisation modelling
has revealed that the behaviour of this line is more complex than previously
anticipated. Our work shows that in most aspects the photoionisation models are
correct. In particular, we confirm the weakening of the 2^1P-2^1S as Teff
increases beyond 40000K. However, in many cases the model underpredicts the
observed ratio when we consider the detailed physical conditions in the
individual planetary nebulae. Furthermore, there is evidence that there is
still significant 2^1P-2^1S HeI line emission even in the planetary nebulae
with very hot (Teff>100000K) central stars. It is clear from our work that this
ratio cannot be considered as a reliable measure of effective temperature on
its own.Comment: 24 pages 11 figures (in 62 separate postscript files) Accepted for
publication in Monthly Notices of the Royal Astronomical Societ
How independent? an analysis of GE13 coverage by Malaysia’s online news portal coverage
The May 5, 2013, general election in Malaysia may have marked the move of the nation’s alternative news media—online, unlicensed news portals that challenge the hegemony of the party that has ruled the nation since 1957—into the mainstream. Malaysiakini, The Malaysian Insider, and Free Malaysia Today editors pride themselves on their independence and journalistic role as government watchdogs, a role they claim has been abdicated by newspapers and broadcast outlets. This article critiques the self-professed editorial independence of these three online news portals through analysis of their GE13 campaign coverage. This research will expand knowledge about Malaysia’s
evolving news environment and will add to research on media
coverage of previous Malaysian elections. The study employs two research methods: content analysis and personal interviews with news portal editors and media scholars. Following an overview of Malaysia’s historically hegemonic mainstream news media, the article traces how the Internet facilitated the introduction of Malaysia’s counter-hegemonic online news portals. Its basic research question is: Did Malaysia’s leading independent news portals provide unbiased news coverage of GE13? The hypothesis was that the sites would be negatively biased against the ruling Barisan Nasional coalition and positively biased toward opposition
coalition candidates, However, the study found 8.2 percent more positive BN stories than negative BN stories in overall coverage. A greater disparity appeared in opposition coverage, where positive stories as predicted outnumbered negative stories by 56.1 percent. News portals criticized both parties, leading to the conclusion that they add to the political discourse that is requisite for successful
participatory democracy
The Near-Infrared Extinction Law in Regions of High Av
We present a spectroscopic study of the shape of the dust-extinction law
between 1.0 and 2.2um towards a set of nine ultracompact HII regions with Av >
15 mag. We find some evidence that the reddening curve may tend to flatten at
higher extinctions, but just over half of the sample has extinction consistent
with or close to the average for the interstellar medium. There is no evidence
of extinction curves significantly steeper than the standard law, even where
water ice is present. Comparing the results to the predictions of a simple
extinction model, we suggest that a standard extinction law implies a robust
upper limit to the grain-size distribution at around 0.1 - 0.3um. Flatter
curves are most likely due to changes in this upper limit, although the effects
of flattening due to unresolved clumpy extinction cannot be ruled out.Comment: 9 pages, 7 figure
Helium and Hydrogen Line Ratios and The Stellar Content of Compact HII Regions
We present observations and models of the behaviour of the HI and HeI lines
between 1.6 and 2.2um in a small sample of compact HII regions. As in our
previous papers on planetary nebulae, we find that the `pure' 1.7007um
4^3D-3^3P and 2.16475um 7^(3,1)G-4^(3,1)F HeI recombination lines behave
approximately as expected as the effective temperature of the central exciting
star(s) increases. However, the 2.058um 2^1P-2^1S HeI line does not behave as
the model predicts, or as seen in planetary nebulae. Both models and planetary
nebulae showed a decrease in the HeI 2^1P-2^1S/HI Br gamma ratio above an
effective temperature of 40000K. The compact HII regions do not show any such
decrease. The problem with this line ratio is probably due to the fact that the
photoionisation model does not account correctly for the high densities seen in
these HII regions, and that we are therefore seeing more collisional excitation
of the 2^1P level than the model predicts. It may also reflect some deeper
problem in the assumed model stellar atmospheres. In any event, although the
normal HeI recombination lines can be used to place constraints on the
temperature of the hottest star present, the HeI 2^1P-2^1S/HI Br gamma ratio
should not be used for this purpose in either Galactic HII regions or in
starburst galaxies, and conclusions from previous work using this ratio should
be regarded with extreme caution. We also show that the combination of the near
infrared `pure' recombination line ratios with mid-infrared forbidden line data
provides a good discriminant of the form of the far ultraviolet spectral energy
distribution of the exciting star(s). From this we conclude that CoStar models
are a poor match to the available data for our sources, though the more recent
WM-basic models are a better fit.Comment: Accepted for publication in MNRA
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