18,774 research outputs found
Full-scale aerodynamic characteristics of a propellar installed on a small twin-engine aircraft wing panel
Full-scale measurements of shaft thrust and torque were made. Wind-tunnel speeds and blade angles were set for full-scale flight conditions. Excellent quality measurements were obtained of the thrust coefficient, the power coefficient, and the propeller efficiency for various values of the advance ratio and the blade incidence angle at 3/4-blade radius. A conventional propeller theory found in the literature was applied to the present results. Although thrust, power, and efficiency were somewhat overpredicted, the advance ratio for maximum efficiency was predicted quite accurately. It was found that, for some conditions, spinner drag could be significant. A simple correction that was based on the spinner base pressure substantially accounted for the changes in efficiency that resulted from this cause
The timing and location of dust formation in the remnant of SN 1987A
The discovery with the {\it Herschel Space Observatory} of bright far
infrared and submm emission from the ejecta of the core collapse supernova
SN\,1987A has been interpreted as indicating the presence of some
0.4--0.7\,M of dust. We have constructed radiative transfer models of
the ejecta to fit optical to far-infrared observations from the literature at
epochs between 615 days and 24 years after the explosion, to determine when and
where this unexpectedly large amount of dust formed.
We find that the observations by day 1153 are consistent with the presence of
310M of dust. Although this is a larger amount than has
previously been considered possible at this epoch, it is still very small
compared to the amount present in the remnant after 24 years, and significantly
higher dust masses at the earlier epochs are firmly ruled out by the
observations, indicating that the majority of the dust must have formed at very
late times. By 8515-9200 days after the explosion, 0.6--0.8\,M of dust
is present, and dust grains with radii greater than 2\,m are required to
obtain a fit to the observed SED. This suggests that the dust mass increase at
late times was caused by accretion onto and coagulation of the dust grains
formed at earlier epochs.
These findings provide further confirmation that core collapse supernovae can
create large quantities of dust, and indicate that the reason for small dust
masses being estimated in many cases is that the vast majority of the dust
forms long after most supernovae have been detectable at mid-infrared
wavelengths.Comment: 13 pages, 16 figures. Accepted for publication in MNRA
Chemical abundances for Hf 2-2, a planetary nebula with the strongest known heavy element recombination lines
We present high quality optical spectroscopic observations of the planetary
nebula (PN) Hf 2-2. The spectrum exhibits many prominent optical recombination
lines (ORLs) from heavy element ions. Analysis of the H {\sc i} and He {\sc i}
recombination spectrum yields an electron temperature of K, a factor
of ten lower than given by the collisionally excited [O {\sc iii}] forbidden
lines. The ionic abundances of heavy elements relative to hydrogen derived from
ORLs are about a factor of 70 higher than those deduced from collisionally
excited lines (CELs) from the same ions, the largest abundance discrepancy
factor (adf) ever measured for a PN. By comparing the observed O {\sc ii}
4089/4649 ORL ratio to theoretical value as a function of
electron temperature, we show that the O {\sc ii} ORLs arise from ionized
regions with an electron temperature of only K. The current
observations thus provide the strongest evidence that the nebula contains
another previously unknown component of cold, high metallicity gas, which is
too cool to excite any significant optical or UV CELs and is thus invisible via
such lines. The existence of such a plasma component in PNe provides a natural
solution to the long-standing dichotomy between nebular plasma diagnostics and
abundance determinations using CELs on the one hand and ORLs on the other.Comment: 12 pages, 5 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Societ
The extinction and dust-to-gas structure of the planetary nebula NGC 7009 observed with MUSE
The large field and wavelength range of MUSE is well suited to mapping
Galactic planetary nebulae (PN). The bright PN NGC 7009 was observed with MUSE
on the VLT during the Science Verification of the instrument in seeing of 0.6".
Emission line maps in hydrogen Balmer and Paschen lines were formed from
analysis of the MUSE cubes. The measured electron temperature and density from
the MUSE cube were employed to predict the theoretical hydrogen line ratios and
map the extinction distribution across the nebula. After correction for the
interstellar extinction to NGC 7009, the internal dust-to-gas ratio (A_V/N_H)
has been mapped for the first time in a PN. The extinction map of NGC 7009 has
considerable structure, broadly corresponding to the morphological features of
the nebula. A large-scale feature in the extinction map, consisting of a crest
and trough, occurs at the rim of the inner shell. The nature of this feature
was investigated and instrumental and physical causes considered; no convincing
mechanisms were identified to produce this feature, other than mass loss
variations in the earlier asymptotic giant branch phase. The dust-to-gas ratio
A_V/N_H increases from 0.7 times the interstellar value to >5 times from the
centre towards the periphery of the ionized nebula. The integrated A_V/N_H is
about 2 times the mean ISM value. It is demonstrated that extinction mapping
with MUSE provides a powerful tool for studying the distribution of PN internal
dust and the dust-to-gas ratio. (Abridged.)Comment: 10 pages, 7 figures. Accepted by A&
MAN/HEP/2006/40 RARE B AND Ï„ DECAYS AND SEARCHES FOR NEW PHYSICS
New results on rare decays of B mesons and τ leptons are summarised. Measurement are generally in excellent agreement with the Standard Model predictions, the only exceptions being the polarisation of vector particles in B decays and the non-appearance of CP violation in B ± → K ± π 0
Unravelling the chemical inhomogeneity of PNe with VLT FLAMES integral-field unit spectroscopy
Recent weak emission-line long-slit surveys and modelling studies of PNe have
convincingly argued in favour of the existence of an unknown component in the
planetary nebula plasma consisting of cold, hydrogen-deficient gas, as an
explanation for the long-standing recombination-line versus forbidden-line
temperature and abundance discrepancy problems. Here we describe the rationale
and initial results from a detailed spectroscopic study of three Galactic PNe
undertaken with the VLT FLAMES integral-field unit spectrograph, which advances
our knowledge about the small-scale physical properties, chemical abundances
and velocity structure of these objects across a two-dimensional field of view,
and opens up for exploration an uncharted territory in the study and modelling
of PNe and photoionized nebulae in general.Comment: 4 pages; 3 figures; invited paper to appear in proceedings of IAU
Symp. No. 234, 2006, Planetary Nebulae in our Galaxy and Beyond (held in
Hawaii, April 2006
Atomic and molecular interstellar absorption lines toward the high galactic latitude stars HD~141569 and HD~157841 at ultra-high resolution
We present ultra-high resolution (0.32 km/s) spectra obtained with the 3.9m
Anglo-Australian Telescope (AAT) and Ultra-High-Resolution Facility (UHRF), of
interstellar NaI D1, D2, Ca II K, K I and CH absorption toward two high
galactic latitude stars HD141569 and HD157841. We have compared our data with
21-cm observations obtained from the Leiden/Dwingeloo HI survey. We derive the
velocity structure, column densities of the clouds represented by the various
components and identify the clouds with ISM structures seen in the region at
other wavelengths. We further derive abundances, linear depletions and H2
fractional abundances for these clouds, wherever possible. Toward HD141569, we
detect two components in our UHRF spectra : a weak, broad component at - 15
km/s, seen only in CaII K absorption and another component at 0 km/s, seen in
NaI D1, D2, Ca II K, KI and CH absorption. In the case of the HD157841
sightline, a total of 6 components are seen on our UHRF spectra in NaI D1, D2
Ca II K, K I and CH absorption. 2 of these 6 components are seen only in a
single species.Comment: 16 pages, Latex, 4 figures, ps files Astrophysical Journal (in press
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