No abstract

▪ Abstract  Radio astronomy has provided evidence for the presence of ionized atmospheres around almost all classes of nondegenerate stars. Magnetically confined coronae dominate in the cool half of the Hertzsprung-Russell diagram. Their radio emission is predominantly of nonthermal origin and has been identified as gyrosynchrotron radiation from mildly relativistic electrons, apart from some coherent emission mechanisms. Ionized winds are found in hot stars and in red giants. They are detected through their thermal, optically thick radiation, but synchrotron emission has been found in many systems as well. The latter is emitted presumably by shock-accelerated electrons in weak magnetic fields in the outer wind regions. Radio emission is also frequently detected in pre–main sequence stars and protostars and has recently been discovered in brown dwarfs. This review summarizes the radio view of the atmospheres of nondegenerate stars, focusing on energy release physics in cool coronal stars, wind phenomenology in hot stars and cool giants, and emission observed from young and forming stars. Eines habe ich in einem langen Leben gelernt, nämlich, dass unsere ganze Wissenschaft, an den Dingen gemessen, von kindlicher Primitivität ist—und doch ist es das Köstlichste, was wir haben. One thing I have learned in a long life: that all our science, measured against reality, is primitive and childlike—and yet it is the most precious thing we have.    A. Einstein 1951, in a letter to H. Mühsam, Einstein Archive 36-610

No abstract

Thousands of exoplanets have now been discovered with a huge range of masses, sizes and orbits: from rocky Earth-like planets to large gas giants grazing the surface of their host star. However, the essential nature of these exoplanets remains largely mysterious: there is no known, discernible pattern linking the presence, size, or orbital parameters of a planet to the nature of its parent star. We have little idea whether the chemistry of a planet is linked to its formation environment, or whether the type of host star drives the physics and chemistry of the planet's birth, and evolution. ARIEL was conceived to observe a large number (~1000) of transiting planets for statistical understanding, including gas giants, Neptunes, super-Earths and Earth-size planets around a range of host star types using transit spectroscopy in the 1.25-7.8 μm spectral range and multiple narrow-band photometry in the optical. ARIEL will focus on warm and hot planets to take advantage of their well-mixed atmospheres which should show minimal condensation and sequestration of high-Z materials compared to their colder Solar System siblings. Said warm and hot atmospheres are expected to be more representative of the planetary bulk composition. Observations of these warm/hot exoplanets, and in particular of their elemental composition (especially C, O, N, S, Si), will allow the understanding of the early stages of planetary and atmospheric formation during the nebular phase and the following few million years. ARIEL will thus provide a representative picture of the chemical nature of the exoplanets and relate this directly to the type and chemical environment of the host star. ARIEL is designed as a dedicated survey mission for combined-light spectroscopy, capable of observing a large and welldefined planet sample within its 4-year mission lifetime. Transit, eclipse and phasecurve spectroscopy methods, whereby the signal from the star and planet are differentiated using knowledge of the planetary ephemerides, allow us to measure atmospheric signals from the planet at levels of 10-100 part per million (ppm) relative to the star and, given the bright nature of targets, also allows more sophisticated techniques, such as eclipse mapping, to give a deeper insight into the nature of the atmosphere. These types of observations require a stable payload and satellite platform with broad, instantaneous wavelength coverage to detect many molecular species, probe the thermal structure, identify clouds and monitor the stellar activity. The wavelength range proposed covers all the expected major atmospheric gases from e.g. H 2 O, CO 2 , CH 4 NH 3 , HCN, H 2 S through to the more exotic metallic compounds, such as TiO, VO, and condensed species. Simulations of ARIEL performance in conducting exoplanet surveys have been performedusing conservative estimates of mission performance and a

Context. This work is part of a systematic X-ray survey of the Taurus star-forming complex with XMM-Newton. Aims. We study the time series of all X-ray sources associated with Taurus members, to statistically characterize their X-ray variability, and compare the results to those for pre-main sequence stars in the Orion Nebula Cluster and to expectations arising from a model where all the X-ray emission is the result of a large number of stochastically occurring flares. Methods. The analysis of the light curves is based on a maximum likelihood algorithm that segments the time series in intervals of constant signal without the need of binning. Flares are defined with criteria that take into account the amplitude and the derivative of the segmented light curves. Variability statistics are evaluated for different classes of pre-main sequence stars (protostars, cTTS, wTTS, brown dwarfs), and for different spectral type ranges. Flare frequency and energy distribution are computed. Results. We find that roughly half of the detected X-ray sources show variability above our sensitivity limit, and in ∼26% of the cases this variability is recognized as flares. Variability is more frequently detected at hard than at soft energies. The variability statistics of cTTS and wTTS are indistinguishable, suggesting a common (coronal) origin for their X-ray emission. The frequency of large flares (E > 10 35 erg) on Taurus members is 1 event per star in 800 ks. The typical duration of these flares -probably biased by the finite observing time -is about 10 ks. We have applied a rigorous maximum likelihood method in the analysis of the number distribution of flare energies on pre-main sequence stars for the first time. In its differential form this distribution follows a power law with index α = 2.4 ± 0.5, in the range typically observed for late-type stars and the Sun. Conclusions. The signature of the X-ray variability in the pre-main sequence stars in Taurus and Orion provides twofold support for coronal heating by flares: (i) the correlation between the maximum variability amplitude and the minimum emission level indicates that both flare and quiescent emissions are closely related to the coronal heating process; (ii) the power law index α derived for the flare energy distribution is large enough to explain the heating of stellar coronae by nano-flares (α > 2), albeit associated with a rather large uncertainty that leaves some doubt as to this conclusion.