References of "Salmon, Sébastien"
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See detailThe puzzling new class of variable stars in NGC 3766 : old friend pulsators?
Salmon, Sébastien ULg; Montalban Iglesias, Josefa ULg; Reese, Daniel et al

in Astronomy and Astrophysics (2014)

The recent variability survey of the NGC 3766 cluster revealed a considerable number of periodic variable stars in a region of the H-R diagram where no pulsation is expected. This region lies between the ... [more ▼]

The recent variability survey of the NGC 3766 cluster revealed a considerable number of periodic variable stars in a region of the H-R diagram where no pulsation is expected. This region lies between the instability strips of the delta Scuti and SPB stars. Moreover the periods of the new phenomenon, P~0.1-0.7 d, do not allow to associate it a priori to either of these two types of pulsations. Stars in the NGC 3766 cluster are known as fast rotators with rotational velocities typically larger than half of their critical velocity. Rotation can affect both the geometrical properties and period domain of pulsations. It also alters the apparent stellar luminosity through gravity darkening, effect seldom taken considered in theoretical studies of the rotation-pulsation interaction. We explore if both of these effects are able to deliver a consistent interpretation for the observed properties of the "new variables" in NGC 3766: explaining their presence outside the known instability strips and their variability periods. We carry out an instability analysis of SPB models within the framework of the Traditional Approximation of Rotation and study the visibility of modes according to the angle of view and rotation. We also check how gravity darkening affects the effective temperature and luminosity of stellar models for different angles of view and rotation velocities. At the red (cold) border of the instability strip, prograde sectoral modes are preferentially excited and their visibilities are maximum when seen equator-on. Furthermore low-mass SPB models seen equator-on can appear in the gap between non-rotating SPB and delta Scuti stars due to gravity darkening. In that case, periods of these most visible modes are shifted to the 0.2-0.5 d range due to the effects of the Coriolis force. We hence suggest that the new variable stars observed in NGC 3766 are actually fast rotating SPB pulsators. [less ▲]

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See detailImpact of micro-physics and macro-physics on the seismic properties of main-sequence B-type stars
Salmon, Sébastien ULg

Doctoral thesis (2014)

L’étude des pulsations stellaires, tout comme les ondes sismiques sur Terre, nous donne une méthode puissante afin de découvrir l’intérieur des étoiles. Cette discipline scientifique est aussi connue sous ... [more ▼]

L’étude des pulsations stellaires, tout comme les ondes sismiques sur Terre, nous donne une méthode puissante afin de découvrir l’intérieur des étoiles. Cette discipline scientifique est aussi connue sous le nom d’astérosismologie. Grâce aux missions spatiales CoRoT et Kepler, l’astérosismologie est entrée depuis dans une période faste. Par ailleurs, une frac- tion significative des étoiles présente des pulsations, ce qui nous permet de sonder leur intérieur pour la plupart de leurs stades d’évolution. Cette thèse se concentre sur les étoiles de type B de la séquence principale. Via l’inter- action de leurs vents stellaires avec leur milieu environnant et la production de nouveaux éléments chimiques en leurs intérieurs, elles contribuent aux processus dynamiques et d’enrichissement chimique du milieu interstellaire. Heureusement, deux types de pulsa- tion stellaire affectent ces étoiles: d’une part les pulsations dites de type β Cephei et d’autre part celles de type SPB. La nature de ces pulsations est sensible aux processus physiques internes de ces étoiles. Ainsi, l’observation de ces pulsations nous offre de sonder ces différents processus. Dans cette thèse, nous étudions tout particulièrement la sensibilité de ces étoiles aux opacités ra- diatives, et nous appuyant sur ces résultats, contraignons l’opacité radiative des éléments du groupe du fer à l’aide des étoiles pulsantes repérées dans les Nuages de Magellan. Dans une seconde étape, nous évaluons la possibilité de contraindre la nature du mélan- ge (diffusif ou overshooting instantané) à l’aide de la modélisation des β Cephei par l’ap- proche directe. Nous testons également les facteurs influençant la détermination de l’exten- sion de la zone mélangée. Enfin, nous proposons un scénario explicatif de la récente découverte d’une nouvelle classe d’étoiles variables dans un amas stellaire connu pour comporter de nombreuses étoiles en rotation rapide. [less ▲]

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See detailThe PLATO Simulator: modelling of high-precision high-cadence space-based imaging
Marcos-Arenal, P.; Zima, W.; De Ridder, J. et al

in Astronomy and Astrophysics (2014)

Many aspects of the design trade-off of a space-based instrument and its performance can best be tackled through simulations of the expected observations. The complex interplay of various noise sources in ... [more ▼]

Many aspects of the design trade-off of a space-based instrument and its performance can best be tackled through simulations of the expected observations. The complex interplay of various noise sources in the course of the observations make such simulations an indispensable part of the assessment and design study of any space-based mission. We present a formalism to model and simulate photometric time series of CCD images by including models of the CCD and its electronics, the telescope optics, the stellar field, the jitter movements of the spacecraft, and all important natural noise sources. This formalism has been implemented in a versatile end-to-end simulation software tool, called PLATO Simulator, specifically designed for the PLATO space mission to be operated from L2, but easily adaptable to similar types of missions. We provide a detailed description of several noise sources and discuss their properties, in connection with the optical design, the allowable level of jitter, the quantum efficiency of the detectors, etc. The expected overall noise budget of generated light curves is computed as a function of the stellar magnitude, for different sets of input parameters describing the instrument properties. The simulator is offered to the scientific community for future use. [less ▲]

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See detailEffects of the Coriolis force on high-order g modes in γ Doradus stars
Bouabid, M.-P.; Dupret, Marc-Antoine ULg; Salmon, Sébastien ULg et al

in Monthly Notices of the Royal Astronomical Society (2013), 429(3), 2500

γ Doradus stars pulsate with high-order gravity modes having typical frequencies which can be comparable to or higher than their rotation frequencies. Therefore, rotation has a non-negligible effect on ... [more ▼]

γ Doradus stars pulsate with high-order gravity modes having typical frequencies which can be comparable to or higher than their rotation frequencies. Therefore, rotation has a non-negligible effect on their oscillation properties. To explore the rotation-pulsation coupling in γ Dor stars, we perform a non-adiabatic study including the traditional approximation of rotation on a grid of spherical stellar models covering the mass range 1.4 < M[SUB]*[/SUB] < 2.1 M[SUB]&sun;[/SUB]. This approximation allows us to treat the effect of the Coriolis force on the frequencies and the stability of high-order g modes. The effect of the Coriolis force depends on the kind of mode considered (prograde sectoral or not) and increases with their periods. As a consequence, we first find that the period spacing between modes is no longer periodically oscillating around a constant value. Secondly, we show that the frequency gap (5-15 cycles day[SUP]-1[/SUP]) arising from stable modes between γ Dor-type high-order g modes and δ Scuti-type modes can be easily filled by g-mode frequencies shifted to higher values by the rotation. Thirdly, we analyse the combined effect of diffusive mixing and the Coriolis force on the period spacings. And finally, we predict a slight broadening of the γ Dor instability strip. [less ▲]

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See detailTesting the effects of opacity and the chemical mixture on the excitation of pulsations in B stars of the Magellanic Clouds
Salmon, Sébastien ULg; Montalban Iglesias, Josefa ULg; Morel, Thierry ULg et al

in Monthly Notices of the Royal Astronomical Society (2012), 422

The B-type pulsators known as β Cephei and slowly pulsating B (SPB) stars present pulsations driven by the κ mechanism, which operates thanks to an opacity bump due to the iron-group elements. In low ... [more ▼]

The B-type pulsators known as β Cephei and slowly pulsating B (SPB) stars present pulsations driven by the κ mechanism, which operates thanks to an opacity bump due to the iron-group elements. In low-metallicity environments such as the Magellanic Clouds, β Cep and SPB pulsations are not expected. Nevertheless, recent observations show evidence for the presence of B-type pulsator candidates in both galaxies. We seek an explanation for the excitation of β Cep and SPB modes in those galaxies by examining basic input physics in stellar modelling: (i) the specific metal mixture of B-type stars in the Magellanic Clouds and (ii) the role of a potential underestimation of stellar opacities. We first derive the present-day chemical mixtures of B-type stars in the Magellanic Clouds. Then, we compute stellar models for that metal mixture and perform a non-adiabatic analysis of these models. In the second approach, we simulate parametric enhancements of stellar opacities due to different iron-group elements. We then study their effects in models of B stars and their stability. We find that adopting a representative chemical mixture of B stars in the Small Magellanic Cloud cannot explain the presence of B-type pulsators there. An increase of the opacity in the region of the iron-group bump could drive B-type pulsations, but only if this increase occurs at the temperature corresponding to the maximum contribution of Ni to this opacity bump. We recommend an accurate computation of the Ni opacity to understand B-type pulsators in the Small Magellanic Cloud, as well as the frequency domain observed in some Galactic hybrid β Cep–SPB stars. [less ▲]

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See detailKepler observations of the variability in B-type stars
Balona, L. A.; Pigulski, A.; Cat, P De et al

in Monthly Notices of the Royal Astronomical Society (2011), 413

The analysis of the light curves of 48 B-type stars observed by Kepler is presented. Among these are 15 pulsating stars, all of which show low frequencies, characteristic of slowly pulsating B (SPB) stars ... [more ▼]

The analysis of the light curves of 48 B-type stars observed by Kepler is presented. Among these are 15 pulsating stars, all of which show low frequencies, characteristic of slowly pulsating B (SPB) stars. Seven of these stars also show a few weak, isolated high frequencies and they could be considered as SPB/β Cephei (β Cep) hybrids. In all cases, the frequency spectra are quite different from what is seen from ground-based observations. We suggest that this is because most of the low frequencies are modes of high degree which are predicted to be unstable in models of mid-B stars. We find that there are non-pulsating stars within the β Cep and SPB instability strips. Apart from the pulsating stars, we can identify stars with frequency groupings similar to what is seen in Be stars but which are not Be stars. The origin of the groupings is not clear, but may be related to rotation. We find periodic variations in other stars which we attribute to proximity effects in binary systems or possibly rotational modulation. We find no evidence for pulsating stars between the cool edge of the SPB and the hot edge of the δ Sct instability strips. None of the stars shows the broad features which can be attributed to stochastically excited modes as recently proposed. Among our sample of B stars are two chemically peculiar stars, one of which is a HgMn star showing rotational modulation in the light curve. [less ▲]

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See detailHow asteroseismology can help to precisely constrain properties of planet-host stars
Salmon, Sébastien ULg; Montalban Iglesias, Josefa ULg; Lanotte, Audrey ULg et al

Poster (2011, January)

Nowadays more than 500 exoplanets have been discovered, mainly studied by radial velocity and transit measurements. Precise knowledge on their characteristics is crucial to develop theories of planetary ... [more ▼]

Nowadays more than 500 exoplanets have been discovered, mainly studied by radial velocity and transit measurements. Precise knowledge on their characteristics is crucial to develop theories of planetary formation and evolution. In that aim, not only star and planet(s) masses but also the evolutionary stage of systems are needed. From radial velocity measurements one has to assume the inclination and the stellar mass of the system to disentangle the mass of the planet. When transit is observable, one can measure the ratio of planetary and stellar radii. Finally, the degree of evolution of the system is determined by the one of the star. Thus the host star must be well known to obtain a full set of system properties. However, determination of stellar parameters such as the mass, radius and its evolution from classical observables (Teff, log g, [Fe/H]) suffers of large uncertainties. This is particularly true for dwarf stars on the Main Sequence. Fortunately we can obtain better constrains with the help of asteroseismology. That latter approach probes the stellar properties through observation of oscillations present in stars. With the launches of high-precision photometry space missions, CoRoT and Kepler, we are now able to detect oscillations in a huge number of stars. In particular Kepler photometry, primarily intended to detect transits of planet, can give accurate stellar parameters of planetary systems as it also affords to make [less ▲]

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See detailCan an underestimation of opacity explain B-type pulsators in the SMC?
Salmon, Sébastien ULg; Montalban Iglesias, Josefa ULg; Miglio, Andrea ULg et al

in Astronomische Nachrichten (2010, October 09), 331(9-10),

Slowly Pulsating B and ß Cephei are ĸ mechanism driven pulsating B stars. That ĸ mechanism works since a peak in the opacity due to a high number of atomic transitions from iron-group elements occurs in ... [more ▼]

Slowly Pulsating B and ß Cephei are ĸ mechanism driven pulsating B stars. That ĸ mechanism works since a peak in the opacity due to a high number of atomic transitions from iron-group elements occurs in the area of log T ~ 5.3. Theoretical results predict very few SPBs and no ß Cep to be encountered in low metallicity environments such as the Small Magellanic Cloud. However recent variability surveys of B stars in the SMC reported the detection of a significant number of SPB and ß Cep candidates. Though the iron content plays a major role in the excitation of ß Cep and SPB pulsations, the chemical mixture representative of the SMC B stars such as recently derived does not leave room for a significant increase of the iron abundance in these stars. Whilst abundance of iron-group elements seems reliable, is the opacity in the iron-group elements bump underestimated? We determine how the opacity profile in B-type stars should change to excite SPB and ß Cep pulsations in early-type stars of the SMC. [less ▲]

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See detailThe PLATO End-to-End CCD Simulator -- Modelling space-based ultra-high precision CCD photometry for the assessment study of the PLATO Mission
Zima, W.; Arentoft, T.; De Ridder, J. et al

in Astronomische Nachrichten (2010, October 09), 331(9-10),

The PLATO satellite mission project is a next generation ESA Cosmic Vision satellite project dedicated to the detection of exo-planets and to asteroseismology of their host-stars using ultra-high ... [more ▼]

The PLATO satellite mission project is a next generation ESA Cosmic Vision satellite project dedicated to the detection of exo-planets and to asteroseismology of their host-stars using ultra-high precision photometry. The main goal of the PLATO mission is to provide a full statistical analysis of exo-planetary systems around stars that are bright and close enough for detailed follow-up studies. Many aspects concerning the design trade-off of a space-based instrument and its performance can best be tackled through realistic simulations of the expected observations. The complex interplay of various noise sources in the course of the observations made such simulations an indispensable part of the assessment study of the PLATO Payload Consortium. We created an end-to-end CCD simulation software-tool, dubbed PLATOSim, which simulates photometric time-series of CCD images by including realistic models of the CCD and its electronics, the telescope optics, the stellar field, the pointing uncertainty of the satellite (or Attitude Control System [ACS] jitter), and all important natural noise sources. The main questions that were addressed with this simulator were the noise properties of different photometric algorithms, the selection of the optical design, the allowable jitter amplitude, and the expected noise budget of light-curves as a function of the stellar magnitude for different parameter conditions. The results of our simulations showed that the proposed multi-telescope concept of PLATO can fulfil the defined scientific goal of measuring more than 20000 cool dwarfs brighter than mV =11 with a precision better than 27 ppm/h which is essential for the study of earth-like exo-planetary systems using the transit method. [less ▲]

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See detailThe Enigma of B-type Pulsators in the SMC
Salmon, Sébastien ULg; Montalban Iglesias, Josefa ULg; Miglio, Andrea ULg et al

in AIP Conference Proceedings (2009, September 01), 1170

Since the early nineties it is accepted that the excitation mechanism of B-type pulsators on the main sequence is due to the opacity peak in the iron-group elements at T~200,000 K. The Fe content plays ... [more ▼]

Since the early nineties it is accepted that the excitation mechanism of B-type pulsators on the main sequence is due to the opacity peak in the iron-group elements at T~200,000 K. The Fe content plays then a major role in the excitation of β Cep and SPB pulsations. While theoretical non-adiabatic computations predict no β Cep pulsators and only a small number of SPBs for low metallicity environments such as that of the Magellanic Clouds (MCs), recent variability surveys of B stars in the SMC have reported the detection of a significant number of SPB and β Cep candidates. Since the SMC is the metal poorest (Z~0001-0.004) of the MCs, it constitutes an interesting object for investigating the disagreement between theory and observations. We approach the problem by calling into question some of the hypotheses made in previous studies: given the different chemical evolution of the SMC compared with our local galactic environment, is it appropriate to describe the chemical composition of SMC B stars by scaling the solar mixture to lower Z? Is that composition uniform in space and time? In this paper we present the results of a stability analysis of B-type stellar models computed with a revised chemical composition and metallicity specific to the SMC. [less ▲]

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