134 research outputs found
Data reduction methods for single-mode optical interferometry - Application to the VLTI two-telescopes beam combiner VINCI
The interferometric data processing methods that we describe in this paper
use a number of innovative techniques. In particular, the implementation of the
wavelet transform allows us to obtain a good immunity of the fringe processing
to false detections and large amplitude perturbations by the atmospheric piston
effect, through a careful, automated selection of the interferograms. To
demonstrate the data reduction procedure, we describe the processing and
calibration of a sample of stellar data from the VINCI beam combiner. Starting
from the raw data, we derive the angular diameter of the dwarf star Alpha Cen
A. Although these methods have been developed specifically for VINCI, they are
easily applicable to other single-mode beam combiners, and to spectrally
dispersed fringes.Comment: Accepted for publication in Astronomy & Astrophysics, 17 pages, 19
figure
Ground-Based Coronagraphy with High Order Adaptive Optics
We summarize the theory of coronagraphic optics, and identify a dimensionless
fine-tuning parameter, F, which we use to describe the Lyot stop size in the
natural units of the coronagraphic optical train and the observing wavelength.
We then present simulations of coronagraphs matched to adaptive optics (AO)
systems on the Calypso 1.2m, Palomar Hale 5m and Gemini 8m telescopes under
various atmospheric conditions, and identify useful parameter ranges for AO
coronagraphy on these telescopes. Our simulations employ a tapered, high-pass
filter in spatial frequency space to mimic the action of adaptive wavefront
correction. We test the validity of this representation of AO correction by
comparing our simulations with recent K-band data from the 241-channel Palomar
Hale AO system and its dedicated PHARO science camera in coronagraphic mode.Comment: To appear in ApJ, May 2001 (28 pages, 10 figs
First radius measurements of very low mass stars with the VLTI
e present 4 very low mass stars radii measured with the VLTI using the 2.2
microns VINCI test instrument. The observations were carried out during the
commissioning of the 104-meter-baseline with two 8-meter-telescopes. We measure
angular diameters of 0.7-1.5 mas with accuracies of 0.04-0.11 mas, and for
spectral type ranging from M0V to M5.5V. We determine an empirical mass-radius
relation for M dwarfs based on all available radius measurements. The observed
relation agrees well with theoretical models at the present accuracy level,
with possible discrepancy around 0.5-0.8 Msolar that needs to be confirmed. In
the near future, dozens of M dwarfs radii will be measured with 0.1-1%
accuracy, with the VLTI, thanks to the improvements expected from the near
infrared instrument AMBER. This will bring strong observational constraints on
both atmosphere and interior physics.Comment: Accepted for publication in Astronomy and Astrophysics Letters, 4
pages, 3 figure
An Analysis of Fundamental Waffle Mode in Early AEOS Adaptive Optics Images
Adaptive optics (AO) systems have significantly improved astronomical imaging
capabilities over the last decade, and are revolutionizing the kinds of science
possible with 4-5m class ground-based telescopes. A thorough understanding of
AO system performance at the telescope can enable new frontiers of science as
observations push AO systems to their performance limits. We look at recent
advances with wave front reconstruction (WFR) on the Advanced Electro-Optical
System (AEOS) 3.6 m telescope to show how progress made in improving WFR can be
measured directly in improved science images. We describe how a "waffle mode"
wave front error (which is not sensed by a Fried geometry Shack-Hartmann wave
front sensor) affects the AO point-spread function (PSF). We model details of
AEOS AO to simulate a PSF which matches the actual AO PSF in the I-band, and
show that while the older observed AEOS PSF contained several times more waffle
error than expected, improved WFR techniques noticeably improve AEOS AO
performance. We estimate the impact of these improved WFRs on H-band imaging at
AEOS, chosen based on the optimization of the Lyot Project near-infrared
coronagraph at this bandpass.Comment: 15 pages, 11 figures, 1 table; to appear in PASP, August 200
Trophic State in Canterbury Waterways
Aquatic eutrophication is a serious global problem, associated with phytoplankton blooms, hypoxia, and loss of species. The objective of this thesis was to advance understanding of stream and lake eutrophication within Canterbury (South Island, New Zealand). I investigated three key questions: 1) How do riparian characteristics control stream trophic state, 2) how does stream trophic state in the Canterbury region compare to stream trophic state nationally and internationally, and 3) what factors control trophic state in Te Wairewa/Lake Forsyth. I measured rates of stream community metabolism in 21 Canterbury streams over a gradient of riparian canopy cover, and conducted a literature review of national and international studies of stream metabolism. I also examined the occurrence of cyanobacterial blooms in Te Wairewa in relation to water quality and weather from 17 years of measurements, and performed series of nutrient addition assays on the lake to assess nutrient limitation. I found that riparian characteristics strongly controlled stream trophic state by shading, thereby reducing photosynthetic productivity. This overwhelmed the effects of high nitrate concentrations, which increased primary production. Compared to national and international rates of stream metabolism, Canterbury streams were strongly heterotrophic, with low rates of autotrophic production. Catchment streams draining into Te Wairewa were unlikely to be the main source of nutrients supporting large cyanobacterial blooms. Instead, internal lake nutrient loading mechanisms associated with calm weather were likely to supply blooms. My results emphasize the importance of light limitation, nitrogen and heterotrophy in controlling stream trophic state, and nutrient supply and weather in controlling lake trophic state
New insights on the AU-scale circumstellar structure of FU Orionis
We report new near-infrared, long-baseline interferometric observations at
the AU scale of the pre-main-sequence star FU Orionis with the PTI, IOTA and
VLTI interferometers. This young stellar object has been observed on 42 nights
over a period of 6 years from 1998 to 2003. We have obtained 287 independent
measurements of the fringe visibility with 6 different baselines ranging from
20 to 110 meters in length, in the H and K bands. Our extensive (u,v)-plane
coverage, coupled with the published spectral energy distribution data, allows
us to test the accretion disk scenario. We find that the most probable
explanation for these observations is that FU Ori hosts an active accretion
disk whose temperature law is consistent with standard models. We are able to
constrain the geometry of the disk, including an inclination of 55 deg and a
position angle of 47 deg. In addition, a 10 percent peak-to-peak oscillation is
detected in the data (at the two-sigma level) from the longest baselines, which
we interpret as a possible disk hot-spot or companion. However, the oscillation
in our best data set is best explained with an unresolved spot located at a
projected distance of 10 AU at the 130 deg position angle and with a magnitude
difference of DeltaK = 3.9 and DeltaH = 3.6 mag moving away from the center at
a rate of 1.2 AU/yr. we propose to interpret this spot as the signature of a
companion of the central FU Ori system on an extremely eccentric orbit. We
speculate that the close encounter of this putative companion and the central
star could be the explanation of the initial photometric rise of the luminosity
of this object
The Mira variable S Ori: Relationships between the photosphere, molecular layer, dust shell, and SiO maser shell at 4 epochs
We present the first multi-epoch study that includes concurrent mid-infrared
and radio interferometry of an oxygen-rich Mira star. We obtained mid-infrared
interferometry of S Ori with VLTI/MIDI at four epochs between December 2004 and
December 2005. We concurrently observed v=1, J=1-0 (43.1 GHz), and v=2, J=1-0
(42.8 GHz) SiO maser emission toward S Ori with the VLBA at three epochs. The
MIDI data are analyzed using self-excited dynamic model atmospheres including
molecular layers, complemented by a radiative transfer model of the
circumstellar dust shell. The VLBA data are reduced to the spatial structure
and kinematics of the maser spots. The modeling of our MIDI data results in
phase-dependent continuum photospheric angular diameters between about 7.9 mas
(Phase 0.55) and 9.7 mas (Phase 1.16). The dust shell can best be modeled with
Al2O3 grains using phase-dependent inner boundary radii between 1.8 and 2.4
photospheric radii. The dust shell appears to be more compact with greater
optical depth near visual minimum, and more extended with lower optical depth
after visual maximum. The ratios of the SiO maser ring radii to the
photospheric radii are between about 1.9 and 2.4. The maser spots mark the
region of the molecular atmospheric layers just beyond the steepest decrease in
the mid-infrared model intensity profile. Their velocity structure indicates a
radial gas expansion. Al2O3 dust grains and SiO maser spots form at relatively
small radii of 1.8-2.4 photospheric radii. Our results suggest increased mass
loss and dust formation close to the surface near the minimum visual phase,
when Al2O3 dust grains are co-located with the molecular gas and the SiO maser
shells, and a more expanded dust shell after visual maximum. Silicon does not
appear to be bound in dust, as our data show no sign of silicate grains.Comment: Accepted for publication in A&A. See ESO press release 25/07 at
http://www.eso.org/public/outreach/press-rel/pr-2007/pr-25-07.htm
Targeted prevention of common mental health disorders in university students: randomised controlled trial of a transdiagnostic trait-focused web-based intervention
Background:
A large proportion of university students show symptoms of common mental disorders, such as depression, anxiety, substance use disorders and eating disorders. Novel interventions are required that target underlying factors of multiple disorders.<p></p>
Aims:
To evaluate the efficacy of a transdiagnostic trait-focused web-based intervention aimed at reducing symptoms of common mental disorders in university students.<p></p>
Method:
Students were recruited online (n = 1047, age: M = 21.8, SD = 4.2) and categorised into being at high or low risk for mental disorders based on their personality traits. Participants were allocated to a cognitive-behavioural trait-focused (n = 519) or a control intervention (n = 528) using computerised simple randomisation. Both interventions were fully automated and delivered online (trial registration: ISRCTN14342225). Participants were blinded and outcomes were self-assessed at baseline, at 6 weeks and at 12 weeks after registration. Primary outcomes were current depression and anxiety, assessed on the Patient Health Questionnaire (PHQ9) and Generalised Anxiety Disorder Scale (GAD7). Secondary outcome measures focused on alcohol use, disordered eating, and other outcomes.<p></p>
Results:
Students at high risk were successfully identified using personality indicators and reported poorer mental health. A total of 520 students completed the 6-week follow-up and 401 students completed the 12-week follow-up. Attrition was high across intervention groups, but comparable to other web-based interventions. Mixed effects analyses revealed that at 12-week follow up the trait-focused intervention reduced depression scores by 3.58 (p<.001, 95%CI [5.19, 1.98]) and anxiety scores by 2.87 (p = .018, 95%CI [1.31, 4.43]) in students at high risk. In high-risk students, between group effect sizes were 0.58 (depression) and 0.42 (anxiety). In addition, self-esteem was improved. No changes were observed regarding the use of alcohol or disordered eating.<p></p>
Conclusions
This study suggests that a transdiagnostic web-based intervention for university students targeting underlying personality risk factors may be a promising way of preventing common mental disorders with a low-intensity intervention
Optical Intensity Interferometry with the Cherenkov Telescope Array
With its unprecedented light-collecting area for night-sky observations, the
Cherenkov Telescope Array (CTA) holds great potential for also optical stellar
astronomy, in particular as a multi-element intensity interferometer for
realizing imaging with sub-milliarcsecond angular resolution. Such an
order-of-magnitude increase of the spatial resolution achieved in optical
astronomy will reveal the surfaces of rotationally flattened stars with
structures in their circumstellar disks and winds, or the gas flows between
close binaries. Image reconstruction is feasible from the second-order
coherence of light, measured as the temporal correlations of arrival times
between photons recorded in different telescopes. This technique (once
pioneered by Hanbury Brown and Twiss) connects telescopes only with electronic
signals and is practically insensitive to atmospheric turbulence and to
imperfections in telescope optics. Detector and telescope requirements are very
similar to those for imaging air Cherenkov observatories, the main difference
being the signal processing (calculating cross correlations between single
camera pixels in pairs of telescopes). Observations of brighter stars are not
limited by sky brightness, permitting efficient CTA use during also bright-Moon
periods. While other concepts have been proposed to realize kilometer-scale
optical interferometers of conventional amplitude (phase-) type, both in space
and on the ground, their complexity places them much further into the future
than CTA, which thus could become the first kilometer-scale optical imager in
astronomy.Comment: Astroparticle Physics, in press; 47 pages, 10 figures, 124 reference
Modern optical astronomy: technology and impact of interferometry
The present `state of the art' and the path to future progress in high
spatial resolution imaging interferometry is reviewed. The review begins with a
treatment of the fundamentals of stellar optical interferometry, the origin,
properties, optical effects of turbulence in the Earth's atmosphere, the
passive methods that are applied on a single telescope to overcome atmospheric
image degradation such as speckle interferometry, and various other techniques.
These topics include differential speckle interferometry, speckle spectroscopy
and polarimetry, phase diversity, wavefront shearing interferometry,
phase-closure methods, dark speckle imaging, as well as the limitations imposed
by the detectors on the performance of speckle imaging. A brief account is
given of the technological innovation of adaptive-optics (AO) to compensate
such atmospheric effects on the image in real time. A major advancement
involves the transition from single-aperture to the dilute-aperture
interferometry using multiple telescopes. Therefore, the review deals with
recent developments involving ground-based, and space-based optical arrays.
Emphasis is placed on the problems specific to delay-lines, beam recombination,
polarization, dispersion, fringe-tracking, bootstrapping, coherencing and
cophasing, and recovery of the visibility functions. The role of AO in
enhancing visibilities is also discussed. The applications of interferometry,
such as imaging, astrometry, and nulling are described. The mathematical
intricacies of the various `post-detection' image-processing techniques are
examined critically. The review concludes with a discussion of the
astrophysical importance and the perspectives of interferometry.Comment: 65 pages LaTeX file including 23 figures. Reviews of Modern Physics,
2002, to appear in April issu
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