Stellar ejecta from falling comet-like bodies: young stars

High-resolution spectral observations of young stars with dense protoplanetary discs like Beta Pictoris led to the discovery of variable emission lines of metal atoms, Na, Fe etc., that indicate the presence of fluxes of comet-like evaporating bodies falling onto the stars, FEBs. Assuming the presence of stellar atmospheres similar to the solar one, we show that passages of the FEBs through the stellar chromosphere and photosphere with velocities around 600 km/s will be accompanied by aerodynamic crushing of the nuclei, transverse expansion of the crushed matter, “explosion” of the flattened nuclei in a relatively very thin sub-photosphere layer due to sharp deceleration, and impulse production of a hot plasma. The impulsive rise of the layer's temperature and density lead to the generation of a strong “blast” shock wave and shock wave-induced ejection/eruption of hot plasma into space above the chromosphere. Observations of such impact-induced high-temperature phenomena are of interest for the physics/prognosis of stellar/solar flares as well as physics of comets.

Search for the Star-Planet Interaction

We analyse the chromospherical activity of stars with extrasolar planets and search for a possible correlation between the equivalent width of the core of the Ca II K line and orbital parameters of the planet. We found statistically significant evidence that the equivalent width of the Ca II K line reversal, which originates in the stellar chromosphere, depends on the orbital period Porb of the exoplanet. Planets orbiting stars with Teff < 5 500 K and with Porb < 20 days generally have much stronger emission than planets at similar temperatures but at longer orbital periods. Porb = 20 days marks a sudden change in behaviour, which might be associated with a qualitative change in the star-planet interaction.

Only Binary Stars Can Help Us Actually SEE a Stellar Chromosphere

Binary stars of the ζ Aurigae type (eclipsing systems containing a cool giant plus a hot main-sequence star) offer a unique and highly effective method of probing a stellar chromosphere. Close to occultation the main-sequence star acts as a light probe behind the giant's chromosphere, enabling an observer to detect changing conditions in that chromosphere along the line of sight. The technique is powerful, the effects dramatic. However, presently known eclipsing systems number only about 10, and a much greater sample is required for meaningful statistics of the properties of stellar chromospheres. New surveys of fainter binaries should be investigated for eclipses in order to gain more information on chromospheres in general. Such information is vital for modelling stellar photospheres, from which abundances are derived. This paper describes the very different behaviour of chromospheric material in three 3rd-magnitude giants.

EUV Emission Sources in Gas-Dynamic Models of Stellar Flares

A stellar flare model in which the main energy release is located above the chromosphere based on a set of elementary acts of the electron acceleration or impulsive heating of plasma is discussed. The response of the chromosphere to impulsive heating for both a single burst and the simultaneous effect of a set of the bursts is considered.The results of numerical modeling of the process of explosive evaporation of the stellar chromosphere allow us to select 3 classes of EUV emission source: (1) the bursts of the EUV emission in the temperature range of 3 · 104 – 3 · 105 K at the beginning of each of elementary act of the energy release; (2) the bursts of the EUV emission at temperatures T ≈ 106 K, accompanied by outflow of heated plasma; (3) the EUV emission, caused by new features of the process, namely, when the maxima of the distribution of the pressure are forming in the region of the downward-moving thermal front.The properties of the EUV sources, velocities of the plasma motions therein, and possible behaviour of the light curve for an elementary burst are discussed.

Flare in Late-type Stars: UV

Early studies of stellar flares were made entirely in the optical regime. It was recognised that flares arose from the generation of hot plasma within the stellar chromosphere at whose temperature (indicated, for instance, by the presence of a strong, blue optical continuum) a substantial emission in the ultraviolet would be expected. It was not until the advent of space-borne instruments of adequate sensitivity, however, that direct confirmation of this prediction was forthcoming. In this review I examine some results of more than a decade of observation of stellar flares in the UV.The major source of ultraviolet data on stellar flares has been the International Ultraviolet Explorer (IUE) satellite (Boggess et al. 1979). In order to understand the limitations of our current understanding in this area it is important to appreciate some of the characteristics of its instrumentation. IUE’s telescope is of 40 cm aperture and it is equipped with a spectrograph which can operate at two resolutions, i.e. Δλ/λ ∼350 (LORES) and ∼17000 (HIRES). Its detectors are optimised for operation in two wavebands, i.e. ∼1150-1950Å (SW) and ∼1950-3200 Å (LW). IUE’s small apert ure results in a limited sensitivity, a consequence of which is a modest time resolution when studying stellar flares (a long exposure time is needed to gain adequate signal-to-noise). IUE’s elliptical 24-hour quasi-geosynchronous orbit and its resulting interactive mode of operation make continuous monitoring feasible, a feature suiting flare star work.

The Chromospheric Spectrum of the ζ Aur Binary HR 6902

Observations of the composite-spectrum binary HR 6902 around the time of total eclipse reveal absorption features that are due to the chromosphere of the G9 II primary star. By the application of o method of digital subtraction we have succeeded in isolating the spectrum of the stellar chromosphere.