6,659 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
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
On the formation of CIII 4647-50-51 and CIII 5696 in O star atmospheres
We investigate the formation of CIII 4647-51-50 and CIII 5696 in the
atmosphere of O stars to see if they can be reliably used for abundance
determinations. We use atmosphere models computed with the code CMFGEN. The key
physical ingredients explaining the formation of the CIII lines are extracted
from comparisons of models with different stellar parameters and through
examining rates controlling the level populations. The strength of CIII 5696
critically depends on UV CIII lines at 386, 574 and 884 A. These lines control
the CIII 5696 upper and lower level population. CIII 884 plays a key role in
late O stars where it drains the lower level of CIII 5696. CIII 386 and CIII
574 are more important at early spectral types. The overlap of these UV lines
with FeIV 386.262, FeIV 574.232 and SV 884.531 influences the radiative
transfer at 386, 574 and 884 A, and consequently affects the strength of CIII
5696. CIII 4650 is mainly controlled by the CIII 538 line which acts as a drain
on its lower level. FeIV 538.057 interacts with CIII 538 and has an impact on
the CIII 4650 profile. Low temperature dielectronic recombinations have a
negligible effect on the line profiles. Given our current understanding of the
stellar and wind properties of O stars, and in view of the present results, the
determination of accurate carbon abundances from CIII 4647-51-50 and CIII 5696
is an extremely challenging task. Uncertainties lower than a factor of two on
C/H determinations based only on these two sets of lines should be regarder as
highly doubtful. Our results provide a possible explanation of the variability
of CIII 4650 in Of?p stars.Comment: 17 pages. A&A accepte
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&
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