1,109 research outputs found
Mg I emission lines at 12 and 18 micrometer in K giants
The solar Mg I emission lines at 12 micrometer have already been observed and
analyzed well. Previous modeling attempts for other stars have, however, been
made only for Procyon and two cool evolved stars, with unsatisfactory results
for the latter. We present high-resolution observational spectra for the K
giants Pollux, Arcturus, and Aldebaran, which show strong Mg I emission lines
at 12 micrometer as compared to the Sun. We also present the first observed
stellar emission lines from Mg I at 18 micrometer and from Al I, Si I, and
presumably Ca I at 12 micrometer. To produce synthetic line spectra, we employ
standard non-LTE modeling for trace elements in cool stellar photospheres. We
compute model atmospheres with the MARCS code, apply a comprehensive magnesium
model atom, and use the radiative transfer code MULTI to solve for the
magnesium occupation numbers in statistical equilibrium. We successfully
reproduce the observed Mg I emission lines simultaneously in the giants and in
the Sun, but show how the computed line profiles depend critically on atomic
input data and how the inclusion of energy levels with n > 9 and collisions
with neutral hydrogen are necessary to obtain reasonable fits.Comment: 9 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
Investigating the origin of cyclical wind variability in hot, massive stars - II. Hydrodynamical simulations of co-rotating interaction regions using realistic spot parameters for the O giant Persei
OB stars exhibit various types of spectral variability historically
associated with wind structures, including the apparently ubiquitous discrete
absorption components (DACs). These features have been proposed to be caused
either by magnetic fields or non-radial pulsations. In this second paper of
this series, we revisit the canonical phenomenological hydrodynamical modelling
used to explain the formation of DACs by taking into account modern
observations and more realistic theoretical predictions. Using constraints on
putative bright spots located on the surface of the O giant Persei
derived from high precision space-based broadband optical photometry obtained
with the Microvariability and Oscillations of STars (MOST) space telescope, we
generate two-dimensional hydrodynamical simulations of co-rotating interaction
regions in its wind. We then compute synthetic ultraviolet (UV) resonance line
profiles using Sobolev Exact Integration and compare them with historical
timeseries obtained by the International Ultraviolet Explorer (IUE) to evaluate
if the observed behaviour of Persei's DACs is reproduced. Testing three
different models of spot size and strength, we find that the classical pattern
of variability can be successfully reproduced for two of them: the model with
the smallest spots yields absorption features that are incompatible with
observations. Furthermore, we test the effect of the radial dependence of
ionization levels on line driving, but cannot conclusively assess the
importance of this factor. In conclusion, this study self-consistently links
optical photometry and UV spectroscopy, paving the way to a better
understanding of cyclical wind variability in massive stars in the context of
the bright spot paradigm.Comment: 16 pages, 10 figures, accepted for publication by MNRA
Public R&D Innovation: The Case of Wind Energy in Denmark, Germany and the United Kingdom
This paper examines the impact of public research and development (R&D) support on cost reducing innovation for wind turbine farms in Denmark, Germany and the United Kingdom (UK). First we survey the literature in this field. The literature indicates that in Denmark R&D policy has been more successful than in Germany or the UK in promoting innovation of wind turbines. Furthermore, such studies point out that (subsidy-induced) capacity expansions were more effective in the UK and Denmark in promoting cost-reducing innovation than in Germany. The second part of the paper describes the quantitative analysis of the impact of R&D and the capacity expansion on innovation. This is calculated using the two-factor learning curve (2FLC) model, in which investment cost reductions are explained by cumulative capacity and the R&D based knowledge stock. Time-series data were collected for the three countries and organized as a panel data set. The parameters of the 2FLC model were estimated, focusing on the heterogeneity of the parameters across countries. We arrive at robust estimations of a learning-by-doing rate of 5.4% and a learning-by-searching rate of 12.6%. The analysis underlies the homogeneity of the learning parameters, enhancing the validity of the 2FLC formulation
Perspectives on Radioactive Waste Repository Monitoring: Confirmation, Compliance, Confidence Building, and Societal Vigilance
The effect of O2 impurities on the low temperature radial thermal expansion of bundles of closed single-walled carbon nanotubes
The effect of oxygen impurities upon the radial thermal expansion (ar) of
bundles of closed single-walled carbon nanotubes has been investigated in the
temperature interval 2.2-48 K by the dilatometric method. Saturation of bundles
of nanotubes with oxygen caused an increase in the positive ar-values in the
whole interval of temperatures used. Also, several peaks appeared in the
temperature dependence ar(T) above 20 K. The low temperature desorption of
oxygen from powders consisting of bundles of single-walled nanotubes with open
and closed ends has been investigatedComment: 7 pages, 3 figure
X-ray emission from the giant magnetosphere of the magnetic O-type star NGC 1624-2
We observed NGC 1624-2, the O-type star with the largest known magnetic field
Bp~20 kG), in X-rays with the ACIS-S camera onboard the Chandra X-ray
Observatory. Our two observations were obtained at the minimum and maximum of
the periodic Halpha emission cycle, corresponding to the rotational phases
where the magnetic field is the closest to equator-on and pole-on,
respectively. With these observations, we aim to characterise the star's
magnetosphere via the X-ray emission produced by magnetically confined wind
shocks. Our main findings are:
(i) The observed spectrum of NGC 1624-2 is hard, similar to the magnetic
O-type star Theta 1 Ori C, with only a few photons detected below 0.8 keV. The
emergent X-ray flux is 30% lower at the Halpha minimum phase.
(ii) Our modelling indicated that this seemingly hard spectrum is in fact a
consequence of relatively soft intrinsic emission, similar to other magnetic
Of?p stars, combined with a large amount of local absorption (~1-3 x 10^22
cm^-2). This combination is necessary to reproduce both the prominent Mg and Si
spectral features, and the lack of flux at low energies. NGC 1624-2 is
intrinsically luminous in X-rays (log LX emission ~ 33.4) but 70-95% of the
X-ray emission produced by magnetically confined wind shocks is absorbed before
it escapes the magnetosphere (log LX ISM corrected ~ 32.5).
(iii) The high X-ray luminosity, its variation with stellar rotation, and its
large attenuation are all consistent with a large dynamical magnetosphere with
magnetically confined wind shocks.Comment: Accepted in MNRAS 13 pages, 10 figures, 4 table
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