6,746 research outputs found
R chart control limits based on a small number of subgroups technical report no. 83
Statistical analysis - R chart control limit based on small number of subgroup
Radial dependence of line profile variability in seven O9--B0.5 stars
Massive stars show a variety of spectral variability: presence of discrete
absorption components in UV P-Cygni profiles, optical line profile variability,
X-ray variability, radial velocity modulations. Our goal is to study the
spectral variability of single OB stars to better understand the relation
between photospheric and wind variability. For that, we rely on high spectral
resolution, high signal-to-noise ratio optical spectra collected with the
spectrograph NARVAL on the Telescope Bernard Lyot at Pic du Midi. We
investigate the variability of twelve spectral lines by means of the Temporal
Variance Spectrum (TVS). The selected lines probe the radial structure of the
atmosphere, from the photosphere to the outer wind. We also perform a
spectroscopic analysis with atmosphere models to derive the stellar and wind
properties, and to constrain the formation region of the selected lines. We
show that variability is observed in the wind lines of all bright giants and
supergiants, on a daily timescale. Lines formed in the photosphere are
sometimes variable, sometimes not. The dwarf stars do not show any sign of
variability. If variability is observed on a daily timescale, it can also (but
not always) be observed on hourly timescales, albeit with lower amplitude.
There is a very clear correlation between amplitude of the variability and
fraction of the line formed in the wind. Strong anti-correlations between the
different part of the temporal variance spectrum are observed. Our results
indicate that variability is stronger in lines formed in the wind. A link
between photospheric and wind variability is not obvious from our study, since
wind variability is observed whatever the level of photospheric variability.
Different photospheric lines also show different degrees of variability.Comment: 13 pages, 9 figures + appendix. A&A accepted. Figures degraded for
arxiv submissio
Modern building services for listed historic buildings: problems and risk
The English Heritage Press Office has estimated that listed buildings represent a finite
cultural resource of approximately 6% of England's building stock. Many of these listed
buildings are historic. It is often necessary to find an appropriate use for them, in order to
preserve them for future generations. Many, not threatened by obsolescence, require
upgrading to comply with statutory regulations. Others are improved to provide the
occupants with twenty-first century standards of environmental comfort, lighting and
power, sanitation, security and communications systems.
The revitalisation or upgrading of property is commonly accepted as refurbishment. The
study initially focused on the refurbishment of listed historic buildings - the enquiry was
then narrowed to investigate the integration of modern building services into listed historic
buildings. This element of the refurbishment process was identified as an area that
frequently constitutes a considerable proportion of the contract sum. A naturalistic form of
enquiry was used to elicit information that would contribute to the body of knowledge on
this area of building conservation work.
Refurbishment characteristics pertaining to the process of integrating modern building
services into listed historic buildings were identified and rated for frequency of occurrence
and degree of difficulty. An evaluation of perceptions and attitudes to adverse events, that
posed risk in the process, was also undertaken. Practitioners from the Construction
Industry then described both success and problem factors encountered in this element of
building conservation. Having built up a picture of the process, based on the experiences of
the constituent members of the project team, three individual projects were studied to gain
further insight into the project and what might be considered as 'repetitive essentials'. To
facilitate analysis the elicited data was assigned to the most appropriate choice from eight
different project categories. These categories were employed as a way of recording the data
throughout all stages in the research enquiry. The study concludes by outlining a model,
based on these project categories, which could be developed to provide a common project
framework. A key function of such a project framework would be to promote
understanding through improved communication and integrated teamwork. The study
concludes by recommending areas worthy of further research relating to the project
framework, the project team and risk in listed historic buildings
On the sensitivity of HeI singlet lines to the FeIV model atom in O stars
Recent calculations and analyses of O star spectra have revealed
discrepancies between theory and observations, and between different
theoretical calculations, for the strength of optical HeI singlet
transitions.We investigate the source of these discrepancies. Using a non-LTE
radiative transfer code we have undertaken detailed test calculations for a
range of O star properties. Our principal test model has parameters similar to
those of the O9V star, 10 Lac. We show that the discrepancies arise from
uncertainties in the radiation field in the HeI resonance transition near
584Angs. The radiation field at 584Angs. is influenced by model assumptions,
such as the treatment of line-blanketing and the adopted turbulent velocity,
and by the FeIV atomic data. It isshown that two FeIV transitions near 584Angs
can have a substantial influence on the strength of the HeI singlet
transitions. Because of the difficulty of modeling the HeI singlet lines,
particularly in stars with solar metalicity, the HeI triplet lines should be
preferred in spectral analyses. These lines are much less sensitive to model
assumptions.Comment: 7 pages, 9 figures, accepted for publication in A&
Properties of WNh stars in the Small Magellanic Cloud: evidence for homogeneous evolution
We derive the physical properties of three WNh stars in the SMC to constrain
stellar evolution beyond the main sequence at low metallicity and to
investigate the metallicity dependence of the clumping properties of massive
stars. We compute atmosphere models to derive the stellar and wind properties
of the three WNh targets. A FUV/UV/optical/near-infrared analysis gives access
to temperatures, luminosities, mass loss rates, terminal velocities and stellar
abundances. All stars still have a large hydrogen mass fraction in their
atmosphere, and show clear signs of CNO processing in their surface abundances.
One of the targets can be accounted for by normal stellar evolution. It is a
star with initial mass around 40-50 Msun in, or close to, the core He burning
phase. The other two objects must follow a peculiar evolution, governed by fast
rotation. In particular, one object is likely evolving homogeneously due to its
position blue-ward of the main sequence and its high H mass fraction. The
clumping factor of one star is found to be 0.15+/-0.05. This is comparable to
values found for Galactic Wolf-Rayet stars, indicating that within the
uncertainties, the clumping factor does not seem to depend on metallicity.Comment: 16 pages. A&A accepte
The Atomic Physics Underlying the Spectroscopic Analysis of Massive Stars and Supernovae
We have developed a radiative transfer code, CMFGEN, which allows us to model
the spectra of massive stars and supernovae. Using CMFGEN we can derive
fundamental parameters such as effective temperatures and surface gravities,
derive abundances, and place constraints on stellar wind properties. The last
of these is important since all massive stars are losing mass via a stellar
wind that is driven from the star by radiation pressure, and this mass loss can
substantially influence the spectral appearance and evolution of the star.
Recently we have extended CMFGEN to allow us to undertake time-dependent
radiative transfer calculations of supernovae. Such calculations will be used
to place constraints on the supernova progenitor, to place constraints on the
supernova explosion and nucleosynthesis, and to derive distances using a
physical approach called the "Expanding Photosphere Method". We describe the
assumptions underlying the code and the atomic processes involved. A crucial
ingredient in the code is the atomic data. For the modeling we require accurate
transition wavelengths, oscillator strengths, photoionization cross-sections,
collision strengths, autoionization rates, and charge exchange rates for
virtually all species up to, and including, cobalt. Presently, the available
atomic data varies substantially in both quantity and quality.Comment: 8 pages, 2 figures, Accepted for publication in Astrophysics & Space
Scienc
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