135 research outputs found
The use of multi-level modelling to investigate the hydrodynamic function of polyurethane prosthetic heart valves
This thesis applies multi-level statistical modelling methods, which have been developed and mainly used for analysis of educational and sociological data, to a new type of data analysis problem. There are many engineering problems that have a multi-level structure, which cannot be easily analysed by conventional methods. The particular problem investigated here is the statistical comparison of prosthetic valve hydrodynamic function data. Such data has, until now, been presented in a subjective way, with selection of valves for demonstration of the desired behaviour. Few attempts have been made at objective analysis of such data. The data presented here derive from experimental polyurethane valve designs. Of these, two design configurations have the same leaflet polyurethane material and different physical valve designs. The other design configurations use one of the physical engineering valve designs, but fabricate the leaflets out of one of four different polyurethane materials. Data were collected for seven different measures of hydrodynamic function, over a series of five different applied flow rates. The first section of the thesis examines random regression analysis of the data using summary measures to compare two designs. Data were transformed as required to approximate assumptions of normality and equal variance using a natural logarithmic transformation. It was possible to discriminate the two designs on the basis of several hydrodynamic function measures. However, the data were pooled from iterative testing into individual valve regressions (n = 6 for each design), so information about the test reproducibility was lost. The data from each individual valve regression parameter estimate was pooled to provide a mean value to represent each design. Thus information about valve-to-valve reproducibility was lost. Correlation of residuals from the same test run means that the assumption of independence that underlies ordinary regression analysis is not likely to be valid in this case. The first two designs had common regression slopes, but this is unlikely to be the case for all design configurations and would create problems with the random regression method. The data structure, therefore, has a hierarchical nature with test iterations nested within individual valves, and individual valves being members of specific design configuration groups. This data structure suggests that a multi-level modelling approach might provide useful insights, by permitting the simultaneous comparison of the regression lines, accounting for both "different intercepts" and "different slopes", as well as the data hierarchy. The ability of the multi-level modelling approach to use all the information available for a valve should give more power to detect differences between valve types. The multi-level approach is described with reference to various applications of the methodology. The method is then applied first to a single valve design, followed by comparison of the two designs used for the random regression approach initially investigated. The multi-level approach increased the power of discrimination compared with the random regression approach, so that more hydrodynamic function measures demonstrated significant differences between designs. The precision of estimation of the variances was improved in the two-design model compared with the single design model, probably as a result of the greater numbers of valves available for analysis. The analysis was then extended to include five different design configurations. The analysis was also modelled to include different slope parameters as well as different intercept parameters, with design included as a fixed parameter in the model. The outcome of the multi-level modelling is described for each hydrodynamic function measure investigated. The most important measures of valve performance (mean pressure gradient across the open valve, energy losses during forward flow and maximum effective valve orifice area) were capable of successful discrimination among all the valve designs. Variance estimates were also obtained that would enable an estimate of the acceptable degree of variation allowed for any valve within a design or for the repeatability associated with the hydrodynamic function testing itself
Discovery of new quasi-periodic oscillations in the X-ray transient source V~0332+53
We report the discovery of a new quasi-period oscillation (QPO) at 0.22 Hz,
centered on the source spin frequency of the high mass X-ray binary system
V~0332+53 when the source was observed during its November 2004/March 2005
outburst by {\em RXTE}. Besides this new QPO, we also detected the known 0.05
Hz QPO. Both the 0.22 and 0.05 Hz QPOs stand out clearly at a mid-flux level of
the outburst within January 15--19 2005, and later at an even lower flux level
as the width of 0.22 Hz QPO drops. No evolution of the centroid frequency with
the flux is seen in either QPO. The rms value below 10 keV is around 4--6% for
both QPOs and decreases at higher energies. We discuss our results in the
context of current QPO models.Comment: 5 figures, 12 pages. AASTex preprint style. (In 2005, ApJ Let., 629,
L33
The hard X-ray emission of X Per
We present an analysis of the spectral properties of the peculiar X-ray
pulsar X Per based on INTEGRAL observations. We show that the source exhibits
an unusually hard spectrum and is confidently detected by ISGRI up to more than
100 keV. We find that two distinct components may be identified in the
broadband 4-200 keV spectrum of the source. We interpret these components as
the result of thermal and bulk Comptonization in the vicinity of the neutron
star and describe them with several semi-phenomenological models. The
previously reported absorption feature at ~30 keV is not required in the
proposed scenario and therefore its physical interpretation must be taken with
caution. We also investigated the timing properties of the source in the
framework of existing torque theory, concluding that the observed phenomenology
can be consistently explained if the magnetic field of the neutron star is
~10^14 G.Comment: Published as a letter in A&A; 4 pages, 2 figure
Star formation in Cometary globule 1: the second generation
C18O spectral line observations, NIR spectrosopy, narrow and broad band NIR
imaging and stellar J,H,Ks photometry are used to analyse the structure of the
archetype cometary globule 1 (CG 1) head and the extinction of stars in its
direction. A young stellar object (YSO) associated with a bright NIR nebulosity
and a molecular hydrogen object (a probable obscured HH-object), were
discovered in the globule. Molecular hydrogen and Br_gamma line emission is
seen in the direction of the YSO. The observed maximum optical extinction in
the globule head is 9.2 magnitudes. The peak N(H2) column density and the total
mass derived from the extinction are 9.0 10^21 cm-2 and and 16.7 Msun
(d/300pc)^2. C18O emission in the globule head is detected in a 1.5'' by 4'
area with a sharp maximum SW of the YSO. Three regions can be discerned in C18O
line velocity and excitation temperature. Because of variations in the C18O
excitation temperature the integrated line emission does not follow the optical
extinction. It is argued that the variations in the C18O excitation
temperatures are caused by radiative heating by NX Pup and interaction of the
YSO with the parent cloud. No indication of a strong molecular outflow from the
YSO is evident in the molecular line data. The IRAS point source 07178-4429
located in the CG 1 head resolves into two sources in the HIRES enhanced IRAS
images. The 12 and 25 micron emission originates mainly in the star NX Puppis
and the 60 and 100 micron emission in the YSO. The IRAS FIR luminosity of the
YSO is 3.1 Lsun.Comment: Language checked v2. Accepted for publication in A&A. 16 pages, 20
figures. C18O data will be available electronicall
X-ray Variability in the Young Massive Triple theta2 Ori A
Massive stars rarely show intrinsic X-ray variability. The only O-stars
credited to be intrinsically variable are theta1 Ori C due to effects from
magnetic confinement of its wind, and theta2 Ori A suspected of similar
activity. Early Chandra observations have shown that the most massive star
system in the Orion Trapezium Cluster, theta2 Ori A, shows rapid variability on
time scales of hours. We determine X-ray fluxes and find that the star shows
very strong variability over the last 5 years. We observed a second large X-ray
outburst in November 2004 with the high resolution transmission grating
spectrometer on-board Chandra. In the low state X-ray emissivities indicate
temperatures well above 25 MK. In the high state we find an extended emissivity
distribution with high emissivities in the range from 3 MK to over 100 MK. The
outburst event in stellar terms is one of the most powerful ever observed and
the most energetic one in the ONC with a lower total energy limit of 1.5x10^37
ergs. The line diagnostics show that under the assumption that the line
emitting regions in the low states are as close as within 1 -- 2 stellar radii
from the O-star's photosphere, whereas the hard states suggest a distance of 3
-- 5 stellar radii. The two outbursts are very close to the periastron passage
of the stars. We argue that the high X-ray states are possibly the result of
reconnection events from magnetic interactions of the primary and secondary
stars of the spectroscopic binary. Effects from wind collisions seem unlikely
for this system. The low state emissivity and R-ratios strengthen the
predicament that the X-ray emission is enhanced by magnetic confinement of the
primary wind. We also detect Fe fluorescence indicative of the existence of
substantial amounts of neutral Fe in the vicinity of the X-ray emission.Comment: 11 pages, 8 figures, accepted for publication in The Astrophysical
Main Journa
First L-band Interferometric Observations of a Young Stellar Object: Probing the Circumstellar Environment of MWC 419
We present spatially-resolved K- and L-band spectra (at spectral resolution R
= 230 and R = 60, respectively) of MWC 419, a Herbig Ae/Be star. The data were
obtained simultaneously with a new configuration of the 85-m baseline Keck
Interferometer. Our observations are sensitive to the radial distribution of
temperature in the inner region of the disk of MWC 419. We fit the visibility
data with both simple geometric and more physical disk models. The geometric
models (uniform disk and Gaussian) show that the apparent size increases
linearly with wavelength in the 2-4 microns wavelength region, suggesting that
the disk is extended with a temperature gradient. A model having a power-law
temperature gradient with radius simultaneously fits our interferometric
measurements and the spectral energy distribution data from the literature. The
slope of the power-law is close to that expected from an optically thick disk.
Our spectrally dispersed interferometric measurements include the Br gamma
emission line. The measured disk size at and around Br gamma suggests that
emitting hydrogen gas is located inside (or within the inner regions) of the
dust disk.Comment: Accepted for publication in Ap
Nitrogen Enrichment in Atmospheres of A- and F- Type Supergiants
Using new accurate fundamental parameters of 30 Galactic A and F supergiants,
namely their effective temperatures Teff and surface gravities log g, we
implemented a non-LTE analysis of the nitrogen abundance in their atmospheres.
It is shown that the non-LTE corrections to the N abundances increase with
Teff. The nitrogen overabundance as a general feature of this type of stars is
confirmed. A majority of the stars has a nitrogen excess [N/Fe] between 0.2 and
0.9 dex with the maximum position of the star's distribution on [N/Fe] between
0.4 and 0.7 dex. The N excesses are discussed in light of predictions for
B-type main sequence (MS) stars with rotationally induced mixing and for their
next evolutionary phase, i.e. A- and F-type supergiants that have experienced
the first dredge-up. Rotationally induced mixing in the MS progenitors of the
supergiants may be a significant cause of the nitrogen excesses. When comparing
our results with predictions of the theory developed for stars with the mixing,
we find that the bulk of the supergiants (28 of 30) show the N enrichment that
can be expected (i) either after the MS phase for stars with the initial
rotational velocities v0 = 200-400 km s-1, (ii) or after the first dredge-up
for stars with v0 = 50-400 km s-1. The latter possibility is preferred on
account of the longer lifetime for stars on red-blue loops following the first
dredge-up. Two supergiants without a discernible N enrichment, namely HR 825
and HR 7876, may be post-MS objects with the relatively low initial rotational
velocity of about 100 km s-1. The suggested range for v0 is approximately
consistent with inferences from the observed projected rotational velocities of
B-type MS stars, progenitors of A and F supergiants.Comment: 14 pages, 13 figure
A search for p-modes and other variability in the binary system 85 Pegasi using MOST photometry
Context: Asteroseismology has great potential for the study of metal-poor
stars due to its sensitivity to determine stellar ages. Aims: Our goal was to
detect p-mode oscillations in the metal-poor sub-dwarf 85 Peg A and to search
for other variability on longer timescales. Methods: We have obtained
continuous high-precision photometry of the binary system 85 Pegasi with the
MOST space telescope in two seasons (2005 & 2007). Furthermore, we redetermined
vsini for 85 Peg A using high resolution spectra obtained through the ESO
archive, and used photometric spot modeling to interpret long periodic
variations. Results: Our frequency analysis yields no convincing evidence for
p-modes significantly above a noise level of 4 ppm. Using simulated p-mode
patterns we provide upper RMS amplitude limits for 85 Peg A. The light curve
shows evidence for variability with a period of about 11 d and this periodicity
is also seen in the follow up run in 2007; however, as different methods to
remove instrumental trends in the 2005 run yield vastly different results, the
exact shape and periodicity of the 2005 variability remain uncertain. Our
re-determined vsini value for 85 Peg A is comparable to previous studies and we
provide realistic uncertainties for this parameter. Using these values in
combination with simple photometric spot models we are able to reconstruct the
observed variations. Conclusions: The null-detection of p-modes in 85 Peg A is
consistent with theoretical values for pulsation amplitudes in this star. The
detected long-periodic variation must await confirmation by further
observations with similar or better precision and long-term stability. If the
11 d periodicity is real, rotational modulation of surface features on one of
the components is the most likely explanation.Comment: 11 pages, 9 figures, accepted for publication in A&
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