391 research outputs found
Pulsed Doppler lidar for the detection of turbulence in clear air
A pulsed C02 Doppler lidar system is described and demonstration tests in ground-based and airborne flight operations are discussed. As a ground-based system, it can detect wind shears in thunderstorm gust fronts to a range of 6 km. When in the airborne configuration, the lidar can detect clear air turbulence in advance of the aircraft encountering clear air turbulence. The data provided by the lidar included turbulence location and intensity with intensity being indicated by the measured spectral width which is proportional to the wind gust velocity
A Keplerian Disk around the Herbig Ae star HD169142
We present Submillimeter Array observations of the Herbig Ae star HD169142 in
1.3 millimeter continuum emission and 12CO J=2-1 line emission at 1.5 arcsecond
resolution that reveal a circumstellar disk. The continuum emission is centered
on the star position and resolved, and provides a mass estimate of about 0.02
solar masses for the disk. The CO images show patterns in position and velocity
that are well matched by a disk in Keplerian rotation with low inclination to
the line-of-sight. We use radiative transfer calculations based on a flared,
passive disk model to constrain the disk parameters by comparison to the
spectral line emission. The derived disk radius is 235 AU, and the inclination
is 13 degrees. The model also necessitates modest depletion of the CO
molecules, similar to that found in Keplerian disks around T Tauri stars.Comment: 10 pages, 2 figures, accepted by A
Attitudes, Influences and Perceptions towards Plastic Surgery amongst Medical Students
Introduction: Plastic surgery is a dynamic and evolving field but remains poorly understood due to lack of knowledge, media misconceptions and recent changes to medical undergraduate curricula. To address issues around student interest and recruitment into the speciality, it is imperative to understand the factors influencing medical students and future clinicians. Aims: To examine influences, interest and perceptions of plastic surgery amongst Scottish medical students and explore methods to increase undergraduate engagement. Method: Cross-sectional survey distributed online via Scottish undergraduate medical school offices comprising 6 domains: demographics; career interest; perceptions, interests and influences in plastic surgery; curriculum and trainer views; understanding the role of a plastic surgeon; and undergraduate engagement. Results: A total of 193 students responded with no statistically significant relationship between year group, gender, and interest in plastic surgery. Phrases most strongly identified with plastic surgery included private practice, reconstruction and cosmetics. Placements, teaching staff and workshops/courses were found to influence perception of plastic surgery. Fortunately, only 6% of students encountered antagonism towards plastic surgery encompassing themes of negative stereotypes of surgeons and connotations surrounding cosmetic surgery. Importantly, many students were largely unaware of the range of common procedures undertaken by plastic surgeons. To overcome this lack of awareness and generate greater interest, students suggested greater plastics exposure, consultant-led teaching and workshops showcasing the specialty. Conclusion: Medical students want varied, stimulating and flexible careers – something which plastic surgery can provide. However it seems the understanding of the scope of plastic surgery is poorly understood amongs future trainees. To increase uptake and interest, negative perceptions need to be addressed and greater engagement is required from medical school upwards
Could the Ultra Metal-poor Stars be Chemically Peculiar and Not Related to the First Stars?
Chemically peculiar stars define a class of stars that show unusual elemental
abundances due to stellar photospheric effects and not due to natal variations.
In this paper, we compare the elemental abundance patterns of the ultra
metal-poor stars with metallicities [Fe/H] to those of a subclass of
chemically peculiar stars. These include post-AGB stars, RV Tauri variable
stars, and the Lambda Bootis stars, which range in mass, age, binarity, and
evolutionary status, yet can have iron abundance determinations as low as
[Fe/H] . These chemical peculiarities are interpreted as due to the
separation of gas and dust beyond the stellar surface, followed by the
accretion of dust depleted-gas. Contrary to this, the elemental abundances in
the ultra metal-poor stars are thought to represent yields of the most
metal-poor supernova and, therefore, observationally constrain the earliest
stages of chemical evolution in the Universe. The abundance of the elements in
the photospheres of the ultra metal-poor stars appear to be related to the
condensation temperature of that element; if so, then their CNO abundances
suggest true metallicities of [X/H]~ -2 to -4, rather than their present
metallicities of [Fe/H] < -5.Comment: Accepted for ApJ. 17 pages, 10 figure
PAH emission from Herbig AeBe stars
We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained
with the Infrared Spectrograph on the Spitzer Space Telescope. All but one of
the Herbig stars show emission from polycyclic aromatic hydrocarbons (PAHs) and
seven of the spectra show PAH emission, but no silicate emission at 10 microns.
The central wavelengths of the 6.2, 7.7--8.2, and 11.3 micron emission features
decrease with stellar temperature, indicating that the PAHs are less
photo-processed in cooler radiation fields. The apparent low level of photo
processing in HAeBe stars, relative to other PAH emission sources, implies that
the PAHs are newly exposed to the UV-optical radiation fields from their host
stars. HAeBe stars show a variety of PAH emission intensities and ionization
fractions, but a narrow range of PAH spectral classifications based on
positions of major PAH feature centers. This may indicate that, regardless of
their locations relative to the stars, the PAH molecules are altered by the
same physical processes in the proto-planetary disks of intermediate-mass
stars. Analysis of the mid-IR spectral energy distributions indicates that our
sample likely includes both radially flared and more flattened/settled disk
systems, but we do not see the expected correlation of overall PAH emission
with disk geometry. We suggest that the strength of PAH emission from HAeBe
stars may depend not only on the degree of radial flaring, but also on the
abundance of PAHs in illuminated regions of the disks and possibly on the
vertical structure of the inner disk as well.Comment: 52 pages, 12 figure
Warm Gas in the Inner Disks around Young Intermediate Mass Stars
The characterization of gas in the inner disks around young stars is of particular interest because of its connection to planet formation. In order to study the gas in inner disks, we have obtained high-resolution K-band and M-band spectroscopy of 14 intermediate mass young stars. In sources that have optically thick inner disks, i.e. E(K-L)>1, our detection rate of the ro-vibrational CO transitions is 100% and the gas is thermally excited. Of the five sources that do not have optically thick inner disks, we only detect the ro-vibrational CO transitions from HD 141569. In this case, we show that the gas is excited by UV fluorescence and that the inner disk is devoid of gas and dust. We discuss the plausibility of the various scenarios for forming this inner hole. Our modeling of the UV fluoresced gas suggests an additional method by which to search for and/or place stringent limits on gas in dust depleted regions in disks around Herbig Ae/Be stars
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
Discovery of Reflection Nebulosity Around Five Vega-like Stars
Coronagraphic optical observations of six Vega-like stars reveal reflection
nebulosities, five of which were previously unknown. The nebulosities
illuminated by HD 4881, HD 23362, HD 23680, HD 26676, and HD 49662 resemble
that of the Pleiades, indicating an interstellar origin for dust grains. The
reflection nebulosity around HD 123160 has a double-arm morphology, but no
disk-like feature is seen as close as 2.5 arcsec from the star in K-band
adaptive optics data. We demonstrate that uniform density dust clouds
surrounding HD 23362, HD 23680 and HD 123160 can account for the observed
12-100 micron spectral energy distributions. For HD 4881, HD 26676, and HD
49662 an additional emission source, such as from a circumstellar disk or
non-equilibrium grain heating, is required to fit the 12-25 micron data. These
results indicate that in some cases, particularly for Vega-like stars located
beyond the Local Bubble (>100 pc), the dust responsible for excess thermal
emission may originate from the interstellar medium rather than from a
planetary debris system.Comment: The Astrophysical Journal, in press for March, 2002 (32 pages, 13
figures
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