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The internal properties of stars in the red-giant phase undergo significant changes on relatively short timescales. Long near-uninterrupted high-precision photometric timeseries observations from dedicated space missions such as CoRoT and Kepler have provided seismic inferences of the global and internal properties of a large number of evolved stars, including red giants. These inferences are confronted with predictions from theoretical models to improve our understanding of stellar structure and evolution. Our knowledge and understanding of red giants have indeed increased tremendously using these seismic inferences, and we anticipate that more information is still hidden in the data. Unraveling this will further improve our understanding of stellar evolution. This will also have significant impact on our knowledge of the Milky Way Galaxy as well as on exo-planet host stars. The latter is important for our understanding of the formation and structure of planetary systems.
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The space-borne missions CoRoT and Kepler are indiscreet. With their asteroseismic programs, they tell us what is hidden deep inside the stars. Waves excited just below the stellar surface travel throughout the stellar interior and unveil many secrets: how old is the star, how big, how massive, how fast (or slow) its core is dancing. This paper intends to emph{paparazze} the red giants according to the seismic pictures we have from their interiors.
We perform a Bayesian grid-based analysis of the solar l=0,1,2 and 3 p modes obtained via BiSON in order to deliver the first Bayesian asteroseismic analysis of the solar composition problem. We do not find decisive evidence to prefer either of the contending chemical compositions, although the revised solar abundances (AGSS09) are more probable in general. We do find indications for systematic problems in standard stellar evolution models, unrelated to the consequences of inadequate modelling of the outer layers on the higher-order modes. The seismic observables are best fit by solar models that are several hundred million years older than the meteoritic age of the Sun. Similarly, meteoritic age calibrated models do not adequately reproduce the observed seismic observables. Our results suggest that these problems will affect any asteroseismic inference that relies on a calibration to the Sun.
Altair is the fastest rotating star at less than 10 parsecs from the Sun. Its precise modelling is a landmark for our understanding of stellar evolution with fast rotation, and all observational constraints are most welcome to better determine the fundamental parameters of this star. We wish to improve the seismic spectrum of Altair and confirm the $delta$-Scuti nature of this star. We used the photometric data collected by the Microvariability and Oscillations of STars (MOST) satellite in the form of a series of Fabry images to derive Altair light curves at four epochs, namely in 2007, 2011, 2012, and 2013. We first confirm the presence of $delta$-Scuti oscillations in the light curves of Altair. We extend the precision of some eigenfrequencies and add new ones to the spectrum of Altair, which now has 15 detected eigenmodes. The rotation period, which is expected at $sim$7h46min from models reproducing interferometric data, seems to appear in the 2012 data set, but it still needs confirmation. Finally, Altair modal oscillations show noticeable amplitude variations on a timescale of 10 to 15 days, which may be the signature of a coupling between oscillations and thermal convection in the layer where the kappa-mechanism is operating.The Altair oscillation spectrum does not contain a large number of excited eigenmodes, which is similar to the fast rotating star HD220811. This supports the idea that fast rotation hinders the excitation of eigenmodes as already pointed out by theoretical investigations.
We combine all the reliably-measured eigenperiods for hot, short-period ZZ Ceti stars onto one diagram and show that it has the features expected from evolutionary and pulsation theory. To make a more detailed comparison with theory we concentrate on a subset of 16 stars for which rotational splitting or other evidence gives clues to the spherical harmonic index (l) of the modes. The suspected l=1 periods in this subset of stars form a pattern of consecutive radial overtones that allow us to conduct ensemble seismology using published theoretical model grids. We find that the best-matching models have hydrogen layer masses most consistent with the canonically thick limit calculated from nuclear burning. We also find that the evolutionary models with masses and temperatures from spectroscopic fits cannot correctly reproduce the periods of the k=1 to 4 mode groups in these stars, and speculate that the mass of the helium layer in the models is too large.
We report the serendipitous discovery of a late-type giant star that exhibited a smooth, eclipse-like drop in flux to a depth of 97 per cent. Minimum flux occurred in April 2012 and the total event duration was a few hundred days. Light curves in V, I and K$_s$ from the Optical Gravitational Lensing Experiment and VISTA Variables in the Via Lactea surveys show a remarkably achromatic event. During 17 years of observational coverage of this source only one such event was detected. The physical properties of the giant star itself appear somewhat unusual, which may ultimately provide a clue towards the nature of the system. By modelling the event as an occultation by an object that is elliptical in projection with uniform transparency, we place limits on its physical size and velocity. We find that the occultation is unlikely to be due to a chance alignment with a foreground object. We consider a number of possible candidates for the occulter, which must be optically thick and possess a radius or thickness in excess of 0.25 au. None are completely satisfactory matches to all the data. The duration, depth and relative achromaticity of the dip mark this out as an exceptionally unusual event, whose secret has still not been fully revealed. We find two further candidates in the VVV survey and we suggest that these systems, and two previously known examples, may point to a broad class of long period eclipsing binaries wherein a giant star is occulted by a circumsecondary disc.
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