Publications ORBi OA
Bookmark and Share    
Full Text
See detailModeling pulsations in hot stars with winds
Noels-Grötsch, Arlette ULg; Dupret, Marc-Antoine ULg; Godart, Mélanie ULg

in Journal of Physics: Conference Series (2008), 118

The interaction pulsation/mass loss takes different aspects. Pulsations can trigger mass loss as in LBVs and Miras; on the other hand, mass loss can modify the driving conditions within the stars. But the ... [more ▼]

The interaction pulsation/mass loss takes different aspects. Pulsations can trigger mass loss as in LBVs and Miras; on the other hand, mass loss can modify the driving conditions within the stars. But the most spectacular aspect is the effect on stellar models which, in turn, opens a royal way to asteroseismology to test physical conditions inside massive stars, such as the extent of convective cores or the appearance of new driving mechanisms. We start with a discussion on MS stars and their strange mode instabilities. We then move on to the excitation of the LBV phenomenon. WR stars and the newly observed MOST period in WR123 are discussed in view of the power of asteroseismology. We then turn to B supergiants, in particular HD163899, and show how asteroseismology can really probe convection, semiconvection and mass loss. [less ▲]

Detailed reference viewed: 11 (3 ULg)
Full Text
See detailEffect of mass loss on the driving of g-modes in B supergiant stars
Godart, Mélanie ULg; Noels-Grötsch, Arlette ULg; Dupret, Marc-Antoine ULg

in Journal of Physics: Conference Series (2008), 118

MOST has detected p and g-modes in the B supergiant star HD163899. Saio et al. (2006) have explained the driving of g-modes in a post main sequence star by the presence of a convective shell which ... [more ▼]

MOST has detected p and g-modes in the B supergiant star HD163899. Saio et al. (2006) have explained the driving of g-modes in a post main sequence star by the presence of a convective shell which prevents some modes from entering the damping radiative core. We show that this scenario depends on the evolution of the star, with or without mass loss. If the mass loss rate is high enough, the convective shell disappears and all the g-modes are stable. [less ▲]

Detailed reference viewed: 10 (2 ULg)
Full Text
Peer Reviewed
See detailA seismic approach to testing different formation channels of subdwarf B stars
Hu, Haili; Dupret, Marc-Antoine ULg; Aerts, C. et al

in Astronomy and Astrophysics (2008), 490

Context: There are many unknowns in the formation of subdwarf B stars. Different formation channels are considered to be possible and to lead to a variety of helium-burning subdwarfs. All seismic models ... [more ▼]

Context: There are many unknowns in the formation of subdwarf B stars. Different formation channels are considered to be possible and to lead to a variety of helium-burning subdwarfs. All seismic models to date, however, assume that a subdwarf B star is a post-helium-flash-core surrounded by a thin inert layer of hydrogen. Aims: We examine an alternative formation channel, in which the subdwarf B star originates from a massive (>~2 M[SUB]o[/SUB]) red giant with a non-degenerate helium-core. Although these subdwarfs may evolve through the same region of the log g-T_eff diagram as the canonical post-flash subdwarfs, their interior structure is rather different. We examine how this difference affects their pulsation modes and whether it can be observed. Methods: Using detailed stellar evolution calculations we construct subdwarf B models from both formation channels. The iron accumulation in the driving region due to diffusion, which causes the excitation of the modes, is approximated by a Gaussian function. The pulsation modes and frequencies are calculated with a non-adiabatic pulsation code. Results: A detailed comparison of two subdwarf B models from different channels, but with the same log g and T_eff, shows that their mode excitation is different. The excited frequencies are lower for the post-flash than for the post-non-degenerate subdwarf B star. This is mainly due to the differing chemical composition of the stellar envelope. A more general comparison between two grids of models shows that the excited frequencies of most post-non-degenerate subdwarfs cannot be well-matched with the frequencies of post-flash subdwarfs. In the rare event that an acceptable seismic match is found, additional information, such as mode identification and log g and T_eff determinations, allows us to distinguish between the two formation channels. [less ▲]

Detailed reference viewed: 22 (2 ULg)
Full Text
See detailTime-dependent convection study of the driving mechanism in the DBV white dwarfs
Dupret, Marc-Antoine ULg; Quirion, P. O.; Fontaine, G. et al

in Journal of Physics: Conference Series (2008), 118

We apply for the first time time-dependent convection (TDC) models to the study of the driving mechanism of the Pulsating DB (V777 Herculis) white dwarfs. From the blue to the red edge of the instability ... [more ▼]

We apply for the first time time-dependent convection (TDC) models to the study of the driving mechanism of the Pulsating DB (V777 Herculis) white dwarfs. From the blue to the red edge of the instability strip of these stars, TDC appears to play a central role in the driving. Around the blue edge, the convection adapts quasi-instantaneously to the oscillations, so that TDC must be included in the models. For the first time, we show that the red edge of the DB instability strip is successfully obtained with a TDC treatment, especially thanks to the terms due to the turbulent pressure variations, while it is not reproduced with frozen convection models. [less ▲]

Detailed reference viewed: 20 (2 ULg)
Full Text
See detailThe driving mechanism of roAp stars
Dupret, Marc-Antoine ULg; Théado, S.; Noels-Grötsch, Arlette ULg

in Journal of Physics: Conference Series (2008), 118

We analyse in detail the driving mechanism of roAp stars and present the theoretical instability strip predicted by our models with solar metallicity. A particular attention is given to the interpretation ... [more ▼]

We analyse in detail the driving mechanism of roAp stars and present the theoretical instability strip predicted by our models with solar metallicity. A particular attention is given to the interpretation of the role played by the different eigenfunctions in the stabilization of the modes at the red edge of the instability strip. The gradient of temperature in the H[SUB]I[/SUB] opacity bump appears to play a major role in this context. We also consider the particular and complex role played by the shape of the eigenfunctions (location of the nodes, ...). [less ▲]

Detailed reference viewed: 7 (3 ULg)
Full Text
Peer Reviewed
See detailModeling the excitation of acoustic modes in alpha Centauri A
Samadi, R.; Belkacem, Kevin ULg; Goupil, M. J. et al

in Astronomy and Astrophysics (2008), 489

From different seismic observations we infer the energy supplied per unit of time by turbulent convection to the acoustic modes of alpha Centauri A (HD 128620), a star that is similar but not identical to ... [more ▼]

From different seismic observations we infer the energy supplied per unit of time by turbulent convection to the acoustic modes of alpha Centauri A (HD 128620), a star that is similar but not identical to the Sun. The inferred rates of energy supplied to the modes (i.e. mode excitation rates) are found to be significantly higher than in the Sun. They are compared with those computed with an excitation model that includes two sources of driving, the Reynolds stress contribution and the advection of entropy fluctuations. The model also uses a closure model, the Closure Model with Plumes (CMP hereafter), that takes the asymmetry between the up- and down-flows (i.e. the granules and plumes, respectively) into account. Different prescriptions for the eddy-time correlation function are also compared to observational data. Calculations based on a Gaussian eddy-time correlation underestimate excitation rates compared with the values derived from observations for alpha Centauri A. On the other hand, calculations based on a Lorentzian eddy-time correlation lie within the observational error bars. This confirms results in the solar case. Compared to the helioseismic data, those obtained for alpha Centauri A constitute an additional support for our model of excitation. We show that mode masses must be computed taking turbulent pressure into account. Finally, we emphasize the need for more accurate seismic measurements in order to distinguish between the CMP closure model and the quasi-normal approximation in the case of alpha Centauri A, as well as to confirm or not the need to include the excitation by the entropy fluctuations. [less ▲]

Detailed reference viewed: 10 (1 ULg)
Full Text
Peer Reviewed
See detailNulling interferometry: performance comparison between space and ground-based sites for exozodiacal disc detection
Defrere, Denis ULg; Absil, Olivier ULg; Coudé Du Foresto, V. et al

in Astronomy and Astrophysics (2008), 490

Context: Characterising the circumstellar dust around nearby main sequence stars is a necessary step in understanding the planetary formation process and is crucial for future life-finding space missions ... [more ▼]

Context: Characterising the circumstellar dust around nearby main sequence stars is a necessary step in understanding the planetary formation process and is crucial for future life-finding space missions such as ESA's Darwin or NASA's terrestrial planet finder (TPF). Besides paving the technological way to Darwin/TPF, the space-based infrared interferometers Pegase and FKSI (Fourier-Kelvin Stellar Interferometer) will be valuable scientific precursors. Aims: We investigate the performance of Pegase and FKSI for exozodiacal disc detection and compare the results with ground-based nulling interferometers. Methods: We used the GENIEsim software (Absil et al. 2006, A&A, 448, 787) which was designed and validated to study the performance of ground-based nulling interferometers. The software has been adapted to simulate the performance of space-based nulling interferometers by disabling all atmospheric effects and by thoroughly implementing the perturbations induced by payload vibrations in the ambient space environment. Results: Despite using relatively small telescopes (<=0.5 m), Pegase and FKSI are very efficient for exozodiacal disc detection. They are capable of detecting exozodiacal discs 5 and 1 time respectively, as dense as the solar zodiacal cloud, and they outperform any ground-based instrument. Unlike Pegase, FKSI can achieve this sensitivity for most targets of the Darwin/TPF catalogue thanks to an appropriate combination of baseline length and observing wavelength. The sensitivity of Pegase could, however, be significantly boosted by considering a shorter interferometric baseline length. Conclusions: Besides their main scientific goal (characterising hot giant extrasolar planets), the space-based nulling interferometers Pegase and FKSI will be very efficient in assessing within a few minutes the level of circumstellar dust in the habitable zone around nearby main sequence stars down to the density of the solar zodiacal cloud. These space-based interferometers would be complementary to Antarctica-based instruments in terms of sky coverage and would be ideal instruments for preparing future life-finding space missions. [less ▲]

Detailed reference viewed: 24 (3 ULg)
Full Text
Peer Reviewed
See detailErratum to "Accurate Spitzer infrared radius measurement for the hot Neptune GJ 436b"
Gillon, Michaël ULg; Demory, B*-O; Barman, T. et al

in Astronomy and Astrophysics (2008), 490

Detailed reference viewed: 24 (1 ULg)
Full Text
Peer Reviewed
See detailTW ;Hydrae: evidence of stellar spots instead of a Hot Jupiter
Huélamo, N.; Figueira, P.; Bonfils, X. et al

in Astronomy and Astrophysics (2008), 489

Context: TW Hya is a classical T Tauri star that shows significant radial-velocity variations in the optical regime. These variations have been attributed to a 10 M_Jup planet orbiting the star at 0.04 AU ... [more ▼]

Context: TW Hya is a classical T Tauri star that shows significant radial-velocity variations in the optical regime. These variations have been attributed to a 10 M_Jup planet orbiting the star at 0.04 AU. Aims: The aim of this letter is to confirm the presence of the giant planet around TW Hya by (i) testing whether the observed RV variations can be caused by stellar spots and (ii) analyzing new optical and infrared data to detect the signal of the planet companion. Methods: We fitted the RV variations of TW Hya using a cool spot model. In addition, we obtained new high-resolution optical & infrared spectra, together with optical photometry of TW Hya and compared them with previous data. Results: Our model shows that a cold spot covering 7% of the stellar surface and located at a latitude of 54° can reproduce the reported RV variations. The model also predicts a bisector semi-amplitude variation <10 m s[SUP]-1[/SUP], which is less than the errors of the RV measurements discussed in Setiawan et al. (2008, Nature, 451, 38). The analysis of our new optical RV data, with typical errors of 10 m s[SUP]-1[/SUP], shows a larger RV amplitude that varies depending on the correlation mask used. A slight correlation between the RV variation and the bisector is also observed although not at a very significant level. The infrared H-band RV curve is almost flat, showing a small variation (<35 m s[SUP]-1[/SUP]) that is not consistent with the published optical orbit. All these results support the spot scenario rather than the presence of a hot Jupiter. Finally, the photometric data shows a 20% (peak to peak) variability, which is much larger than the 4% variation expected for the modeled cool spot. The fact that the optical data are correlated with the surface of the cross-correlation function points towards hot spots as being responsible for the photometric variability. Conclusions: We conclude that the best explanation for the RV signal observed in TW Hya is the presence of a cool stellar spot and not an orbiting hot Jupiter. Based on observations taken at the VLT (Paranal), under programs 280.C-5064(A) and 075.C-0202(A), and with the CORALIE spectrograph and EulerCAM both at the Euler Swiss telescope (La Silla). [less ▲]

Detailed reference viewed: 53 (0 ULg)
Full Text
Peer Reviewed
See detailErratum: "the Transit Light Curve Project. VII. The Not-So Exoplanet HAT-P-1B" (2007, AJ, 134, 1707)
Winn, Joshua N; Holman, Matthew J; Bakos, Gaspar Á et al

in Astronomical Journal (The) (2008), 136

Two of the midtransit times that were given in Table 3 of Winn et al. 2007 (AJ, 134, 1707-1712) are incorrect. The first entry was incorrect because a trend in the out-of-transit flux had not been removed ... [more ▼]

Two of the midtransit times that were given in Table 3 of Winn et al. 2007 (AJ, 134, 1707-1712) are incorrect. The first entry was incorrect because a trend in the out-of-transit flux had not been removed as described in the text. The sixth entry gave an incorrect epoch, and a midtransit time that was too small by one orbital period, because of a rounding error in the computer code that generated the table. The corrected times are given below in a revised version of Table 3. With these revisions, the ephemeris parameters given in the text following Equation (4) are changed slightly, to T[SUB]c[/SUB] (0) = 2453997.79252(32) (HJD) and P = 4.46543(13) days, where the numbers in parentheses indicate the 1sigma uncertainty in the last two digits. We note that a more precise ephemeris has been calculated by Johnson et al. (2008), based on a combination of the data presented here and more recent transit observations. [less ▲]

Detailed reference viewed: 17 (1 ULg)
Full Text
See detailL'astronomie dans le monde
Manfroid, Jean ULg

Article for general public (2008)

L'énigme magnétique solaire; V1280 Sco : la nova fumante; Spirales barrées; Galaxies en fusion; L'étoile pivoine; Fermi-GLAST; Nuages martiens; Chaînon manquant; 2006 SQ372; Rosetta près de Steins; Nuages ... [more ▼]

L'énigme magnétique solaire; V1280 Sco : la nova fumante; Spirales barrées; Galaxies en fusion; L'étoile pivoine; Fermi-GLAST; Nuages martiens; Chaînon manquant; 2006 SQ372; Rosetta près de Steins; Nuages noctiluques; Une découverte massive de XMM-Newton; [less ▲]

Detailed reference viewed: 7 (0 ULg)
Full Text
See detailPoints de repère pour prévenir la maltraitance
Bullens, Quentin ULg; Debluts, Danielle; Dubois, Françoise

Learning material (2008)

Detailed reference viewed: 95 (13 ULg)
Full Text
Peer Reviewed
See detailA structural, magnetic and Mössbauer spectral study of the magnetocaloric Mn1.1Fe0.9P1-xGex compounds
Sougrati, Moulay Tahar ULg; Hermann, Raphaël ULg; Grandjean, Fernande ULg et al

in Journal of Physics : Condensed Matter (2008), 20

The structural, magnetic and Mössbauer spectral properties of the magnetocaloric Mn1.1Fe0.9P1−xGex compounds, with 0.19 < x < 0.26, have been measured between 4.2 and 295 K. The 295 K unit-cell volume ... [more ▼]

The structural, magnetic and Mössbauer spectral properties of the magnetocaloric Mn1.1Fe0.9P1−xGex compounds, with 0.19 < x < 0.26, have been measured between 4.2 and 295 K. The 295 K unit-cell volume increases from x = 0.19 to 0.22 and is substantially smaller in the ferromagnetic Mn1.1Fe0.9P0.74Ge0.26. The temperature dependence of the magnetization reveals a ferromagnetic to paramagnetic transition with a Curie temperature between approximately 250 and 330 K and hysteresis width of 10 to 4 K, for 0.19 < x < 0.25. The composition Mn1.1Fe0.9P0.78Ge0.22 shows the largest isothermal entropy change of approximately 10 J/(kgKT) at 290 K. The M¨ossbauer spectra have been analysed with a binomial distribution of hyperfine fields correlated with a change in isomer shift and quadrupole shift, a distribution that results from the distribution of phosphorus and germanium among the near neighbours of the iron. The coexistence of paramagnetic and magnetically ordered phases in ranges of temperature of up to 50 K around the Curie temperature is observed in the Mössbauer spectra and is associated with the first-order character of the ferromagnetic to paramagnetic transition. The temperature dependence of the weighted average hyperfine field is well fitted within the magnetostrictive model of Bean and Rodbell. Good fits of the Mössbauer spectra could only be achieved by introducing a difference between the isomer shifts in the paramagnetic and ferromagnetic phases, a difference that is related to the magnetostriction and electronic structure change. [less ▲]

Detailed reference viewed: 63 (15 ULg)
Full Text
Peer Reviewed
See detailPhilosophie politique et approche pragmatique du politique en contexte islamique. Lecture d’al-Mâwardi
Nachi, Mohamed ULg

in Gandolfi et Levi, Paola et Giovani (Ed.) Entre théologie et Politique: Les origines théologiques cachées de la pensée politique contemporaine dans les pays de la Méditerranée (2008, October)

Montrer l'apport d'al-Mâwardi pour le développement d'une approche pragmatique du politique en contexte islamique

Detailed reference viewed: 48 (2 ULg)
Full Text
Peer Reviewed
See detailFire Performance of undamaged and pre-damaged welded steel-concrete composite beam-to-column joints with concrete filled tubes
Alderighi, Elisabetta; Bursi, Oreste; Franssen, Jean-Marc ULg et al

Conference (2008, October)

Major earthquakes in urban areas have often been followed by significant conflagrations that have been difficult to control and have resulted in extensive damage to property. Earthquakes, then, increase ... [more ▼]

Major earthquakes in urban areas have often been followed by significant conflagrations that have been difficult to control and have resulted in extensive damage to property. Earthquakes, then, increase the risk of loss of life if a fire occurs within a building. It is obvious therefore that a fire after an earthquake is a design scenario that should be properly addressed in any performance-based design, in locations where significant earthquakes can occur. In this paper both experimental and numerical results of undamaged and pre-damaged welded steel-concrete composite beam-to-column joints with concrete filled tubes are described as part of a European project aimed at developing fundamental data, design guidelines and prequalification of ductile and fire resistant composite beam-to-column joints. In detail, both the experimental program and the fire experimental results are presented and discussed in this paper together with thermal numerical simulations on frames and joints. Both the experimental activity and the numerical work demonstrated the adequacy of the seismic and joint fire design. [less ▲]

Detailed reference viewed: 68 (2 ULg)
Full Text
See detailFractionation process to produce a new milk ingredient enriched in phospholipids and sphingolipids
Bodson, P.; Dalemans, D.; Danthine, Sabine ULg et al

Poster (2008, October)

Detailed reference viewed: 17 (2 ULg)
Full Text
See detailULG FACULTE D'ARCHITECTURE (ex LAMBERT LOMBARD ).SITE BOTANIQUEISA
Frisenna, Marina ULg

in PROVINCE DE LIEGE-CULTURE (Ed.) BIENNALE INTERNATIONALE -DESIGN LIEGE 2008 (2008)

Detailed reference viewed: 17 (4 ULg)
Full Text
See detailDes chiffres et des Lettres
Habrand, Tanguy ULg

Article for general public (2008)

Detailed reference viewed: 35 (2 ULg)
Full Text
See detailUnderwater Imagery, a Measuring Tool to Extend the Spatio-Temporal Understanding of Benthic Organisms Dynamics: Case Study of Codium elisabethae in the Azores.
Sirjacobs, Damien ULg; Tempera, Fernando; Cardigos, Frederico et al

Poster (2008, October)

Benthic habitat mapping studies have been increasingly exploiting the use of underwater images to collect information on substrate nature and biological coverage. Concurrently, research has been ongoing ... [more ▼]

Benthic habitat mapping studies have been increasingly exploiting the use of underwater images to collect information on substrate nature and biological coverage. Concurrently, research has been ongoing to develop methods that use the imagery collected to conduct regular quantitative monitoring studies of biological resources distributed over large areas. This study provides the first multi-annual monitoring information on the dynamics of a benthic macroalgae population derived from underwater imagery collected by scuba divers in the Monte da Guia Site of Community Importance /Natura 2000 network (Faial isl. Azores, NE Atlantic). The green alga Codium elisabethae - a long-living green alga that represents a potential good indicator of coastal environmental change - was chosen for the study. The analyses focus on using the underwater imagery to quantify seasonal fluctuations of density, percentage cover, biomass, growth rate and primary production of the species. Two study sites were investigated: one was located in a sheltered no-go reserve exhibiting a dense C. elisabethae population, and the other in a location experiencing more exposed conditions and holding a sparser population. Between August 2003 and November 2005, fifteen (15) photo coverages were collected by scuba-divers. Subsequent processing consisted of producing image mosaics and using automated and interactive change detection methods that recognized, measured and counted individuals present in photos of fixed quadrats and yielded dynamical parameters such as population structures, growth, recruitment, and mortality. Chi-square tests of image-derived estimates and in situ measurements confirmed the validity of a centimeter precision estimation of population structure for individuals above 4 cm diameter. Important variability of population structure and density were observed at small spatial scales. Population density showed a sharp reduction in autumn 2003 and did not show a full recovery in spring and summer 2004. During the following year, population of the protected site maintained density and biomass, while at the exposed site population density dropped. The production of information based on observations of thousands of individuals is mandatory in biological population statistics. The presented imagery approach made it possible, avoiding the need to collect all the measurements and quantitative information during time-constrained SCUBA diving operations. [less ▲]

Detailed reference viewed: 68 (15 ULg)