Combining seismology and spectropolarimetry of hot stars

Asteroseismology and spectropolarimetry have allowed us to progress significantly in our understanding of the physics of hot stars over the last decade. It is now possible to combine these two techniques to learn even more information about hot stars and constrain their models. While only a few magnetic pulsating hot stars are known as of today and have been studied with both seismology and spectropolarimetry, new opportunities - in particular Kepler2 and BRITE - are emerging and will allow us to rapidly obtain new combined results.Comment: proceedings of the IAU Symposium 307 held in Geneva in June 201

The Potential of the Timing Method to Detect Evolved Planetary Systems

The timing method, using either stellar pulsations or eclipse timing of close binaries as a clock, is proving to be an efficient way to detect planets around stars that have evolved beyond the red giant branch. In this article we present a short review of the recent discoveries and we investigate the potential of the timing method using data both from ground-based facilities as well as from the Kepler and CoRoT space missions.Comment: Part of PlanetsbeyondMS/2010 proceedings http://arxiv.org/html/1011.6606v1, Proc. of the workshop on "Planetary Systems beyond the Main Sequence" (Bamberg, 11-14 August 2010), AIPC in press (eds. S. Schuh, H. Drechsel and U. Heber), 15 pages, 5 figure

Physics Of Eclipsing Binaries. II. Towards the Increased Model Fidelity

The precision of photometric and spectroscopic observations has been systematically improved in the last decade, mostly thanks to space-borne photometric missions and ground-based spectrographs dedicated to finding exoplanets. The field of eclipsing binary stars strongly benefited from this development. Eclipsing binaries serve as critical tools for determining fundamental stellar properties (masses, radii, temperatures and luminosities), yet the models are not capable of reproducing observed data well either because of the missing physics or because of insufficient precision. This led to a predicament where radiative and dynamical effects, insofar buried in noise, started showing up routinely in the data, but were not accounted for in the models. PHOEBE (PHysics Of Eclipsing BinariEs; http://phoebe-project.org) is an open source modeling code for computing theoretical light and radial velocity curves that addresses both problems by incorporating missing physics and by increasing the computational fidelity. In particular, we discuss triangulation as a superior surface discretization algorithm, meshing of rotating single stars, light time travel effect, advanced phase computation, volume conservation in eccentric orbits, and improved computation of local intensity across the stellar surfaces that includes photon-weighted mode, enhanced limb darkening treatment, better reflection treatment and Doppler boosting. Here we present the concepts on which PHOEBE is built on and proofs of concept that demonstrate the increased model fidelity.Comment: 60 pages, 15 figures, published in ApJS; accompanied by the release of PHOEBE 2.0 on http://phoebe-project.or

Numerical investigation of large-scale vortices in an array of cylinders in axial flow

Axial flow around an array of cylinders is commonly encountered in nuclear reactors and heat exchangers. This geometry is subject to important flow instabilities. The chaotic flow fluctuations due to turbulence are not the only source of vortex structures: large-scale vortices have also been observed, both experimentally and numerically. The periodic pressure fluctuations caused by the coherent vortex structures are possibly a source of fretting and fatigue in the aforementioned applications. In order to comprehend this phenomenon, Large-Eddy Simulations are performed on a numerical domain containing a single rigid cylinder with periodic boundary conditions, representative for a cylinder in an infinite square array. The research in this paper mainly focuses on the influence of the cylinder spacing, which is analysed by calculating the Cross Spectral Density (CSD) function of the cylinder wall pressure for different cylinder spacings. The spectral analysis shows that the amplitude of the pressure fluctuations increases up to a well-determined intercylinder gap, after which it decreases exponentially for incrementing gap size. With the weakening of the instability, the location on the cylinder circumference where the maximum pressure amplitude occurs, changes as well. Finally, it is shown that the coherent vortices are transported as a whole at a convection speed which is dependent on the cylinder spacing. An updated model for this convection speed is proposed

Ensemble Asteroseismology of the Young Open Cluster NGC 2244

Our goal is to perform in-depth ensemble asteroseismology of the young open cluster NGC2244 with the 2-wheel Kepler mission. While the nominal Kepler mission already implied a revolution in stellar physics for solar-type stars and red giants, it was not possible to perform asteroseismic studies of massive OB stars because such targets were carefully avoided in the FoV in order not to disturb the exoplanet hunting. Now is an excellent time to fill this hole in mission capacity and to focus on the metal factories of the Universe, for which stellar evolution theory is least adequate. Our white paper aims to remedy major shortcomings in the theory of stellar structure and evolution of the most massive stars by focusing on a large ensemble of stars in a carefully selected young open cluster. Cluster asteroseismology of very young stars such as those of NGC2244 has the major advantage that all cluster stars have similar age, distance and initial chemical composition, implying drastic restrictions for the stellar modeling compared to asteroseismology of single isolated stars with very different ages and metallicities. Our study requires long-term photometric measurements of stars with visual magnitude ranging from 6.5 to 15 in a large FoV with a precision better than 30 ppm for the brightest cluster members (magnitude below 9) up to 500 ppm for the fainter ones, which is well achievable with 2-Wheel Kepler, in combination with high-precision high-resolution spectroscopy and spectro-polarimetry of the brightest pulsating cluster members. These ground-based spectroscopic data will be assembled with the HERMES and CORALIE spectrographs (twin 1.2m Mercator and Euler telescopes, La Palma, Canary Islands and La Silla, Chile), as well as with the spectro-polarimetric NARVAL instrument (2m BLT at the Pic du Midi, French Pyrenees), to which we have guaranteed access.Comment: 10 pages, 3 figures, white paper submitted in response to the NASA call for community input for science investigations the Kepler 2-Wheel spacecraf

Deviations from a uniform period spacing of gravity modes in a massive star

The life of a star is dominantly determined by the physical processes in the stellar interior. Unfortunately, we still have a poor understanding of how the stellar gas mixes near the stellar core, preventing precise predictions of stellar evolution. The unknown nature of the mixing processes as well as the extent of the central mixed region is particularly problematic for massive stars. Oscillations in stars with masses a few times that of the Sun offer a unique opportunity to disentangle the nature of various mixing processes, through the distinct signature they leave on period spacings in the gravity mode spectrum. Here we report the detection of numerous gravity modes in a young star with a mass of about seven solar masses. The mean period spacing allows us to estimate the extent of the convective core, and the clear periodic deviation from the mean constrains the location of the chemical transition zone to be at about 10 per cent of the radius and rules out a clear-cut profile.Peer reviewe

Asteroseismology of OB stars with the CoRoT space mission.

The CoRoT space telescope monitors the brightness of the stars continuously and with unprecedented quality. For the first time, variability measurements of massive stars have been obtained with micromagnitude precision, allowing for detailed seismic studies of hot stars in the instability strips to uncover the internal structure.After describing a wide range of relevant observational techniques to fully exploit the acquired data, we present analyses and seismic interpretations of specific stars observed by the CoRoT space as part of the seismology program. To achieve a full exploration of the instability strips, we traverse the Hertzsprung-Russel diagram from the hot to the cool side, via an O-type binary system and a Beta Cep star, to a hybrid pulsator and a small sample of late B-type stars.The discovery of solar-like oscillations in massive stars opens up the possibility of finding a new set of seismic constraints on massive star models. Via the detection of gravity mode period spacings, we were able to describe the mixing processes around the convective core of a massive star. Finally, the late type stars showed unexpected variability, possibly attributed to rotational modulation, e.g., due to spots on the surface of these stars.nrpages: 241status: publishe