The role of turbulent pressure as a coherent pulsational driving mechanism: the case of the delta Scuti star HD 187547

HD 187547 was the first candidate that led to the suggestion that solar-like oscillations are present in delta Scuti stars. Longer observations, however, show that the modes interpreted as solar-like oscillations have either very long mode lifetimes, longer than 960 days, or are coherent. These results are incompatible with the nature of `pure' stochastic excitation as observed in solar-like stars. Nonetheless, one point is certain: the opacity mechanism alone cannot explain the oscillation spectrum of HD 187547. Here we present new theoretical investigations showing that convection dynamics can intrinsically excite coherent pulsations in the chemically peculiar delta Scuti star HD 187547. More precisely, it is the perturbations of the mean Reynold stresses (turbulent pressure) that drives the pulsations and the excitation takes place predominantly in the hydrogen ionization zone.Comment: 8 pages, 4 figures, accepted to Ap

Effective induction heating around strongly magnetized stars

Planets that are embedded in the changing magnetic fields of their host stars can experience significant induction heating in their interiors caused by the planet's orbital motion. For induction heating to be substantial, the planetary orbit has to be inclined with respect to the stellar rotation and dipole axes. Using WX~UMa, for which the rotation and magnetic axes are aligned, as an example, we show that for close-in planets on inclined orbits, induction heating can be stronger than the tidal heating occurring inside Jupiter's satellite Io; namely, it can generate a surface heat flux exceeding 2\,W\,m$^{-2}$. An internal heating source of such magnitude can lead to extreme volcanic activity on the planet's surface, possibly also to internal local magma oceans, and to the formation of a plasma torus around the star aligned with the planetary orbit. A strongly volcanically active planet would eject into space mostly SO$_2$, which would then dissociate into oxygen and sulphur atoms. Young planets would also eject CO$_2$. Oxygen would therefore be the major component of the torus. If the O{\sc i} column density of the torus exceeds $\approx$10$^{12}$\,cm$^{-2}$, the torus could be revealed by detecting absorption signatures at the position of the strong far-ultraviolet O{\sc i} triplet at about 1304\,\AA. We estimate that this condition is satisfied if the O{\sc i} atoms in the torus escape the system at a velocity smaller than 1--10\,km\,s$^{-1}$. These estimates are valid also for a tidally heated planet.Comment: 8 pages, 6 figures, accepted for publication in Ap

Model atmospheres of chemically peculiar stars: Self-consistent empirical stratified model of HD24712

High-resolution spectra of some chemically peculiar stars clearly demonstrate the presence of strong abundance gradients in their atmospheres. However, these inhomogeneities are usually ignored in the standard scheme of model atmosphere calculations, braking the consistency between model structure and spectroscopically derived abundance pattern. In this paper we present first empirical self-consistent stellar atmosphere model of roAp star HD24712, with stratification of chemical elements included, and which is derived directly from the observed profiles of spectral lines without time-consuming simulations of physical mechanisms responsible for these anomalies. We used the LLmodels stellar model atmosphere code and DDAFIT minimization tool for analysis of chemical elements stratification and construction of self-consistent atmospheric model. Empirical determination of Pr and Nd stratification in the atmosphere of HD24712 is based on NLTE line formation for Prii/iii and Ndii/iii with the use of the DETAIL code. Based on iterative procedure of stratification analysis and subsequent re-calculation of model atmosphere structure we constructed a self-consistent model of HD24712, i.e. the model which temperature-pressure structure is consistent with results of stratification analysis. It is shown that stratification of chemical elements leads to the considerable changes in model structure as to compare with non-stratified homogeneous case. We find that accumulation of REE elements allows for the inverse temperature gradient to be present in upper atmosphere of the star with the maximum temperature increase of about 600K.Comment: Comments: Accepted by A&A, 16 pages, 10 figures, 3 table

The characteristics of solar x-class flares and CMEs: a paradigm for stellar superflares and eruptions?

This paper explores the characteristics of 42 solar X-class flares that were observed between February 2011 and November 2014, with data from the Solar Dynamics Observatory (SDO) and other sources. This flare list includes nine X-class flares that had no associated CMEs. In particular our aim was to determine whether a clear signature could be identified to differentiate powerful flares that have coronal mass ejections (CMEs) from those that do not. Part of the motivation for this study is the characterization of the solar paradigm for flare/CME occurrence as a possible guide to the stellar observations; hence we emphasize spectroscopic signatures. To do this we ask the following questions: Do all eruptive flares have long durations? Do CME-related flares stand out in terms of active-region size vs. flare duration? Do flare magnitudes correlate with sunspot areas, and, if so, are eruptive events distinguished? Is the occurrence of CMEs related to the fraction of the active-region area involved? Do X-class flares with no eruptions have weaker non-thermal signatures? Is the temperature dependence of evaporation different in eruptive and non-eruptive flares? Is EUV dimming only seen in eruptive flares? We find only one feature consistently associated with CME-related flares specifically: coronal dimming in lines characteristic of the quiet-Sun corona, i.e. 1 – 2 MK. We do not find a correlation between flare magnitude and sunspot areas. Although challenging, it will be of importance to model dimming for stellar cases and make suitable future plans for observations in the appropriate wavelength range in order to identify stellar CMEs consistently

New light on the driving mechanism in roAp stars. Part I. Effects of metallicity

Observations suggest that a relationship exists between the driving mechanism of roAp star pulsations and the heavy element distribution in these stars. We attempt to study the effects of local and global metallicity variations on the excitation mechanism of high order p-modes in A star models. We developed stellar evolutionary models to describe magnetic A stars with different global metallicity or local metal accumulation profiles. These models were computed with CLES ("Code Li\`egeois d'\'evolution stellaire"), and the stability of our models was assessed with the non-adiabatic oscillation code MAD. Our models reproduce the blue edge of the roAp star instability strip, but generate a red edge hotter than the observed one, regardless of metallicity. Surprisingly, we find that an increase in opacity inside the driving region can produce a lower amount of driving, which we refer to as the "inverse $\kappa$-mechanism".Comment: 18 pages, 23 figures. Accepted for publication in A&