The continuous period search method and its application to the young solar analogue HD 116956

We formulate an improved time series analysis method for the analysis of photometry of active stars. This new Continuous Period Search (CPS) method is applied to 12 years of V band photometry of the young solar analogue HD 116956 (NQ UMa). The new method is developed from the previous Three Stage Period Analysis (TSPA) method. Our improvements are the use of a sliding window in choosing the modelled datasets, a criterion applied to select the best model for each dataset and the computation of the time scale of change of the light curve. We test the performance of CPS with simulated and real data. The CPS has a much improved time resolution which allows us to better investigate fast evolution of stellar light curves. We can also separate between the cases when the data is best described by periodic and aperiodic (e.g. constant brightness) models. We find, however, that the performance of the CPS has certain limitations. It does not determine the correct model complexity in all cases, especially when the underlying light curve is constant and the number of observations too small. Also the sensitivity in detecting two close light curve minima is limited and it has a certain amount of intrinsic instability in its period estimation. Using the CPS, we find persistent active longitudes in the star HD 116956 and a "flip-flop" event that occurred during the year 1999. Assuming that the surface differential rotation of the star causes observable period variations in the stellar light curve, we determine the differential rotation coefficient to be |k|>0.11. The mean timescale of change of the light curve during the whole 12 year observing period was T_C=44.1 d, which is of the same order as the predicted convective turnover time of the star. We also investigate the presence of activity cycles on the star, but do not find any conclusive evidence supporting them.Comment: 14 pages, 11 figures, 3 table

Did the ancient egyptians record the period of the eclipsing binary Algol - the Raging one?

The eclipses in binary stars give precise information of orbital period changes. Goodricke discovered the 2.867 days period in the eclipses of Algol in the year 1783. The irregular orbital period changes of this longest known eclipsing binary continue to puzzle astronomers. The mass transfer between the two members of this binary should cause a long-term increase of the orbital period, but observations over two centuries have not confirmed this effect. Here, we present evidence indicating that the period of Algol was 2.850 days three millenia ago. For religious reasons, the ancient Egyptians have recorded this period into the Cairo Calendar, which describes the repetitive changes of the Raging one. Cairo Calendar may be the oldest preserved historical document of the discovery of a variable star.Comment: 26 pages, 5 figures, 11 table

Flip-flop phenomenon: observations and theory

In many active stars the spots concentrate on two permanent active longitudes which are 180 degrees apart. In some of these stars the dominant part of the spot activity changes the longitude every few years. This so-called flip-flop phenomenon has up to now been reported in 11 stars, both single and binary alike, and including also the Sun. To explain this phenomenon, a non-axisymmetric dynamo mode, giving rise to two permanent active longitudes at opposite stellar hemispheres, is needed together with an oscillating axisymmetric magnetic field. Here we discuss the observed characteristics of the flip-flop phenomenon and present a dynamo solution to explain them.Comment: 4 pages, 5 figures, contribution to the conference "Dynamos of the Sun, Stars and Planets", to be published in AN Volume 32

The first close-up of the "flip-flop" phenomenon in a single star

We present temperature maps of the active late-type giant FK Com which exhibit the first imagining record of the ``flip-flop'' phenomenon in a single star. The phenomenon, in which the main part of the spot activity shifts 180 degrees in longitude, discovered a decade ago in FK Com, was reported later also in a number of RS CVn binaries and a single young dwarf. With the surface images obtained right before and after the ``flip-flop'', we clearly show that the ``flip-flop'' phenomenon in FK Com is caused by changing the relative strengths of the spot groups at the two active longitudes, with no actual spot movements across the stellar surface, i.e. exactly as it happens in other active stars.Comment: 4 pages, accepted by A&A Letter

Spot activity of the RS CVn star {\sigma} Geminorum

We model the photometry of RS CVn star $\sigma$ Geminorum to obtain new information on the changes of the surface starspot distribution, i.e., activity cycles, differential rotation and active longitudes. We use the previously published Continuous Periods Search-method (CPS) to analyse V-band differential photometry obtained between the years 1987 and 2010 with the T3 0.4 m Automated Telescope at the Fairborn Observatory. The CPS-method divides data into short subsets and then models the light curves with Fourier-models of variable orders and provides estimates of the mean magnitude, amplitude, period and light curve minima. These light curve parameters are then analysed for signs of activity cycles, differential rotation and active longitudes. We confirm the presence of two previously found stable active longitudes, synchronised with the orbital period $P_{\rm{orb}}=19.60$d and find eight events where the active longitudes are disrupted. The epochs of the primary light curve minima rotate with a shorter period $P_{\rm{min,1}}=19.47$d than the orbital motion. If the variations in the photometric rotation period were to be caused by differential rotation, this would give a differential rotation coefficient of $\alpha \ge 0.103$. The presence of two slightly different periods of active regions may indicate a superposition of two dynamo modes, one stationary in the orbital frame and the other one propagating in the azimuthal direction. Our estimate of the differential rotation is much higher than previous results. However, simulations show that this can be caused by insufficient sampling in our data.Comment: 10 pages, 6 figures. Submitted to A&

Matériaux isolants: Développement d'un produit expansé à base de cellulose

National audienceInsulation board market is largely dominated by mineral and iol based sources and presente a constant increase. Foamed products are expected to rise in the near future thanks to high performance products. However, the environmental impact is bad. As a consequence, WoTIM European project, funded by WoodWisdom–Net Research Program 2014, aims at developing insulation products based on cellulose obtained from wood. Two products are proposed: expending cellulose foam competing with polyurethane foams to be sprayed and boards of various dimensions. Thanks to highly porous and low density network, insulation performances are high. As an exemple, thermal conductivity as low as 0.035 W/K.m is obtained on under optimization products. During project, economic and environmental performance of products will be evaluated to propose competitive solutions. Partners are VTT in Finland; Holmen and Innventia in Sweden; Soprema and FCBA in France.Les matériaux d'isolation à base minérales ou issus de pétroressources sont actuellement les plus largement utilisés dans le monde et en croissance constante. Une augmentation est attendue dans le secteur de l'isolation à base de mousse plastique à cause de leurs hautes performances. Toutefois, d'un point de vue environnemental et durabilité, cette tendance est problématique. WoTIM, projet européen WoodWisdom–Net Research Programme 2014, propose de développer des matériaux d'isolation, à base de cellulose issue du bois, sous forme de mousses expansives ou panneaux. Grace à cette structure de très faible densité, les propriétés d'isolation sont augmentées en créant des espaces d'air dans une matrice. Le meilleur de ces démonstrateurs, en cours d'optimisation, présente une conductivité thermique de 0.035 W/K.m, illustrant ainsi la bonne performance de ces nouveaux produits d'isolation cellulosiques qui rivalise avec les produits courants issus de laine minérale

General Model for Light Curves of Chromospherically Active Binary Stars

The starspots on the surface of many chromospherically active binary stars concentrate on long-lived active longitudes separated by 180°. Shifts in activity between these two longitudes, the “flip-flop” events, have been observed in single stars like FK Comae and binary stars like σ Geminorum. Recently, interferometry has revealed that ellipticity may at least partly explain the flip-flop events in σ Geminorum. This idea was supported by the double-peaked shape of the long-term mean light curve of this star. Here we show that the long-term mean light curves of 14 chromospherically active binaries follow a general model that explains the connection between orbital motion, changes in starspot distribution, ellipticity, and flip-flop events. Surface differential rotation is probably weak in these stars, because the interference of two constant period waves may explain the observed light curve changes. These two constant periods are the active longitude period ##IMG## [http://ej.iop.org/images/0004-637X/838/2/122/apjaa65cbieqn1.gif] $(P_act)$ and the orbital period ##IMG## [http://ej.iop.org/images/0004-637X/838/2/122/apjaa65cbieqn2.gif] $(P_orb)$ . We also show how to apply the same model to single stars, where only the value of P act is known. Finally, we present a tentative interference hypothesis about the origin of magnetic fields in all spectral types of stars.Peer reviewe