173 research outputs found
Ci-gît la vie : dans la chair du papier
Catalogue, texte de Paul Ripoche, Filigranes éditions, 2013. ISBN 9782350462837 Des cyclistes, des motards qui filent, la rencontre d’un homme et d’une femme, un couple se tenant par la main et avançant de face, un clarinettiste, un homme saluant, les mains jointes en l’air, un autre levant un verre à votre santé, des amoureux enlacés soudés… Des figures sans leur environnement propre, sans ancrage terrestre, flottant en apesanteur en apparente dissidence sociale, comme si le monde s’était re..
A photometric mode identification method, including an improved non-adiabatic treatment of the atmosphere
We present an improved version of the method of photometric mode
identification of Heynderickx et al. (1994). Our new version is based on the
inclusion of precise non-adiabatic eigenfunctions determined in the outer
stellar atmosphere according to the formalism recently proposed by Dupret et
al.(2002). Our improved photometric mode identification technique is therefore
no longer dependent on ad hoc parameters for the non-adiabatic effects. It
contains the complete physical conditions of the outer atmosphere of the star,
provided that rotation does not play a key role. We apply our improved method
to the two slowly pulsating B stars HD 74560 and HD 138764 and to the beta
Cephei star EN (16) Lac. Besides identifying the degree l of the pulsating
stars, our method is also a tool for improving the knowledge of stellar
interiors and atmospheres, by imposing constraints on parameters such as the
metallicity and the mixing-length parameter alpha (a procedure we label
non-adiabatic asteroseismology).Comment: 10 pages, 9 figures Accepted for publication in Astronomy and
Astrophysic
BRITE view of Scorpii, Cephei-type star studied for over a century
Preliminary results of the analysis of the combined space-based BRITE and
SMEI, and ground-based Stroemgren photometry are presented. The BRITE data
allowed to find seven p and three g modes in the frequency spectrum of this
star; only four p modes were known in this star prior to this study. The first
results of seismic modelling are also presented.Comment: 3 pages, 1 figure, to appear in the Proceedings of the 3rd BRITE
Science Conference, Auberge du Lac Taureau, Canad
Photometric amplitudes and phases of nonradial oscillation in rotating stars
Effects of rotational mode coupling on photometric parameters of stellar
oscillations are studied. At moderate rotation rates, a strong coupling between
modes of spherical harmonic degree, , differing by 2 and of the same
azimuthal order, , takes place if the frequencies are close. This is a
common situation amongst main sequence pulsators. Numerical results for a
sequence of Cephei star models are reported for the two- and three-mode
couplings.
One consequence of mode coupling is that modes of higher degree should be
considered in photometric mode identification. Modes with nominal degree
acquire substantial components and therefore are more
likely to reach detectable amplitudes. Coupled mode positions in the amplitude
ratio - phase difference diagrams, based on multicolour photometry, become both
aspect- and -dependent. Examples of the mode path in the diagram with
varying aspect are given. The diagrams remain a useful tool for mode
identification in rotating stars but the tool must be used with care.Comment: 9 pages, 8 figures, accepted for publication in A&
A Quantitative Analysis of the Available Multicolor Photometry for Rapidly Pulsating Hot B Subdwarfs
We present a quantitative and homogeneous analysis of the broadband
multicolor photometric data sets gathered so far on rapidly pulsating hot B
subdwarf stars. This concerns seven distinct data sets related to six different
stars. Our analysis is carried out within the theoretical framework developed
by Randall et al., which includes full nonadiabatic effects. The goal of this
analysis is partial mode identification, i.e., the determination of the degree
index l of each of the observed pulsation modes. We assume possible values of l
from 0 to 5 in our calculations. For each target star, we compute a specific
model atmosphere and a specific pulsation model using estimates of the
atmospheric parameters coming from time-averaged optical spectroscopy. For
every assumed value of l, we use a formal chi-squared approach to model the
observed amplitude-wavelength distribution of each mode, and we compute a
quality-of-fit Q probability to quantify the derived fit and to discriminate
objectively between the various solutions. We find that no completely
convincing and unambiguous l identification is possible on the basis of the
available data, although partial mode discrimination has been reached for 25
out of the 41 modes studied. A brief statistical study of these results
suggests that a majority of the modes must have l values of 0, 1, and 2, but
also that modes with l = 4 could very well be present while modes with l = 3
appear to be rarer. This is in line with recent results showing that l = 4
modes in rapidly pulsating B subdwarfs have a higher visibility in the optical
domain than modes with l = 3. Although somewhat disappointing in terms of mode
discrimination, our results still suggest that the full potential of multicolor
photometry for l identification in pulsating subdwarfs is within reach.Comment: 59 pages, 18 figures, accepted for publication in the Astrophysical
Journal Supplement Serie
Effect of NLTE model atmospheres on photometric amplitudes and phases of early B-type pulsating stars
We study all possible sources of inaccuracy in theoretical values of the
photometric observables, i.e. amplitude ratios and phase differences, of early
B-type main sequence pulsators. Here, we discuss effects of parameters coming
from both models of stellar atmospheres and linear nonadiabatic theory of
stellar pulsation. In particular, we evaluate for the first time the effect of
the departure from the LTE approximation. The atmospheric input comes from
line-blanketed, LTE and NLTE plane-parallel, hydrostatic models. To compute the
limb-darkening coefficients for NLTE models, we use the Least-Square Method
taking into account the accuracy of the flux conservation. We present effects
of NLTE atmospheres, chemical composition and opacities on theoretical values
of the photometric observables of early B-type pulsators. To this end, we
compute tables with the passband fluxes, flux derivatives over effective
temperature and gravity as well as the non-linear limb-darkening coefficients
in 12 most often used passbands, i.e. in the Str\"omgern system, , and in
the Johnson-Cousins-Glass system, . We make these tables public
available at the Wroc{\l}aw HELAS Web page, http://helas.astro.uni.wroc.pl.Comment: 13 pages, 2 tables, 17 figues submitted to A&
Spatial, Ecological and Social Dimensions of Assessments for Bivalve Farming Management
The general purpose of assessment is to provide decision-makers with the best valuable data, information, and predictions with which management decisions will be supported. Using case studies taken from four scientific projects and dealing with the management of marine bivalve resources, lessons learned allowed identifying some issues regarding assessment approaches. The selected projects also introduced methodological or institutional frameworks: ecosystem approach to aquaculture (EAA), system approach framework (SAF), marine spatial planning (MSP), and valuation of ecosystem services (ES).
The study on ecosystem services linked ES to marine habitats and identified ES availability and vulnerability to pressures. The results were displayed as maps of resulting potential services with qualitative metrics. The vulnerability value is an alternative to monetary valuation and, in addition to identifying the most suitable areas for each type of ES, this metric allows identifying the management strategies that will most probably maintain or affect each individual ES.
The MSP example focused on bivalve farming activity and accounted for several criteria: habitat suitability, growth performance, environmental and regulation constraints and presence of other activities. The ultimate endpoint of such an approach is a map with qualitative values stating whether a location is suitable or not, depending on the weight given to each criterion.
In the EAA case study, the indicator was defined by the growth performance of cultivated bivalves in different locations. This indicator is affected by distant factors – e.g. populations of marine organisms competing for the same food resource, nutrient inputs from rivers, time to renew water bodies under the action of tidal currents. The role and interactions of these factors were assessed with a dynamical ecosystem model.
Examples illustrate that the assessment is often multi-dimensional, and that multiple variables would interact and affect the response to management options. Therefore, the existence of trade-offs, the definition of the appropriate spatial scale and resolution, the temporal dynamics and the distant effects of factors are keys to a policy-relevant assessment. EA and SAF examples show the interest of developing models relating response to input variables and testing scenarios. Dynamic models would be preferred when the relationship between input and output variables may be masked by non-linear effects, delay of responses or differences of scales.
When decision-making requires economic methods, monetary values are often of poor significance, especially for those ecosystem services whose loss could mean the end of life, and appear to be a comfortable oversimplification of reality of socio-ecological systems which cannot be summarized in single numbers. Alternative methods, such as the ones proposed in the SAF and ES examples, would preferably consider institutional analysis or multicriteria assessment rather than single monetary values.
Case studies also highlighted that credibility of assessment tools benefit from the association of stakeholders at different stages, among which: identification of the most critical policy issues; definition of system characteristics including ecological, economical and regulation dimensions; definition of modelling scenarios to sort out the most effective management options; assessment of models and indicators outputs.publishedVersio
Constraints on parameters of B-type pulsators from combined multicolour photometry and radial velocity data. I. Cephei stars
We analyze data on pulsation amplitudes and phases for two Cephei
stars, Cet and Eri. Str\"omgren photometry and radial velocity
measurements are used simultaneously to obtain constraints on mean parameters
of the stars and identification of the excited modes. The inference about the
radial mode order and mean star parameters is based on comparison of certain
complex parameter, , determined from data, with its counterpart derived from
linear nonadiabatic modelling of stellar oscillations. The theoretical
values are very sensitive to the adopted opacity data. In our modelling we rely
on the data from OPAL and OP projects. Significant differences were found. New
seismic models of Eri were constructed with both the OPAL and OP
opacities.Comment: 11 pages, 13 figures, A&A in pres
Constraints on stellar convection from multi-colour photometry of Delta Scuti stars
In Delta Scuti star models, the calculated amplitude ratios and phase
differences for multi-colour photometry exhibit a strong dependence on
convection. These observables are tools for determination of the spherical
harmonic degree of the excited modes. The dependence on convection enters
through the complex parameter f, which describes bolometric flux perturbation.
We present a method of simultaneous determination of f and spherical harmonic
degree from multi-colour data and apply it to three Delta Scuti stars. The
method indeed works. Determination of the degree appears unique and the
inferred f's are sufficiently accurate to yield a useful constraint on models
of stellar convection. Furthermore, the method helps to refine stellar
parameters, especially if the identified mode is radial.Comment: 9 pages, 12 figures, to appear in Astronomy and Astrophysic
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