Flare X-ray Observations of AB Doradus: Evidence of Stellar Coronal Opacity

X-ray spectra of the late-type star AB Dor obtained with the XMM-Newton satellite are analyzed. AB Dor was particularly active during the observations. An emission measure reconstruction technique is employed to analyze flare and quiescent spectra, with emphasis on the Fe XVII 15 - 17 angstrom wavelength region. The Fe XVII 16.78 angstrom/ 15.01 angstrom line ratio increases significantly in the hotter flare plasma. This change in the ratio is opposite to the theoretical predictions and is attributed to the scattering of 15.01 angstrom line photons from the line of sight. The escape probability technique indicates an optical depth of approximate to 0.4 for the 15.01 angstrom line. During the flare, the electron density is 4.4(-1.6)(+2.7) x 10(10) cm(-3), and the fractional Fe abundance is 0.5 +/- 0.1 of the solar photospheric value Using these parameters, a path length of approximate to 8000 km is derived. There is no evidence of opacity in the quiescent X-ray spectrum of the star.

Opacity in the upper atmosphere of AU Mic

In this paper we investigate the validity of the optically thin assumption in the transition region of the late-type star AU Mic. We use Far-Ultraviolet Spectroscopic Explorer (FUSE) observations of the C III multiplet and O VI resonance lines, hence yielding information at two different levels within the atmosphere. Significant deviations from the optically thin fluxes are found for C III in both quiescent and flare spectra, where only 60% of the flux is actually observed. This could explain the apparent deviation of C III observed in emission measure distributions. We utilize escape probabilities for both homogeneous and inhomogeneous geometries and calculate optical depths as high as 10 for the C III 1175.71 Angstrom component of the multiplet. Using a lower limit to the electron density (10(11) cm(-3))we derive an effective thickness of

A detailed study of opacity in the upper atmosphere of Proxima Centauri

We present far-UV and UV spectroscopic observations of Proxima Centauri obtained as part of our continued investigation into the optically thin approximation assumed for the transition regions of late-type stars. Significant opacity is found in the C III lines during both the quiescent and flaring states of Proxima Cen, with up to 70% of the expected flux being lost in the latter. Our findings cast some doubt on the suitability of the C III lambda977 line for estimating the electron density in stellar atmospheres. However, the opacity has no significant effect on the observed line widths. We calculate optical depths for homogeneous and inhomogeneous geometries and estimate an electron density of 6 x 10(10) cm(-3) for the transition region using the O IV line ratios at 1400 Angstrom. The combination of electron density and optical depth indicates path lengths as low as approximate to 10 km, which are in excellent agreement with estimates of the small-scale structure seen in the solar transition region.

Opacity in the upper atmospheres of active stars - II. AD Leonis

We present FUV and UV spectroscopic observations of AD Leonis, with the aim of investigating opacity effects in the transition regions of late-type stars. The C III lines in FUSE spectra show significant opacity during both the quiescent and flaring states of AD Leonis, with up to 30% of the expected flux being lost during the latter. Other FUSE emission lines tested for opacity include those of O VI, while C IV, Si IV and N V transitions observed with STIS are also investigated. These lines only reveal modest amounts of opacity with losses during flaring of up to 20%. Optical depths have been calculated for homogeneous and inhomogeneous geometries, giving path lengths of approximate to 20 - 60 km and approximate to 10 - 30 km, respectively, under quiescent conditions. However path lengths derived during flaring are approximate to 2 - 3 times larger. These values are in excellent agreement with both estimates of the small-scale structure observed in the solar transition region, and path lengths derived previously for several other active late-type stars.

Line intensity enhancements in stellar coronal X-ray spectra due to opacity effects

Context. The I(15.01 Å)/I(16.78 Å) emission line intensity ratio in Fe xvii has been reported to deviate from its theoretical value
in solar and stellar X-ray spectra. This is attributed to opacity in the 15.01 Å line, leading to a reduction in its intensity, and was
interpreted in terms of a geometry in which the emitters and absorbers are spatially distinct.
Aims. We study the I(15.01 Å)/I(16.78 Å) intensity ratio for the active cool dwarf EV Lac, in both flare and quiescent spectra.
Methods. The observations were obtained with the Reflection Grating Spectrometer on the XMM-Newton satellite. The emission
measure distribution versus temperature reconstruction technique is used for our analysis.
Results. We find that the 15.01 Å line exhibits a significant enhancement in intensity over the optically thin value. To our knowledge,
this is the first time that such an enhancement has been detected on such a sound statistical basis. We interpret this enhancement
in terms of a geometry in which the emitters and absorbers are not spatially distinct, and where the geometry is such that resonant
pumping of the upper level has a greater effect on the observed line intensity than resonant absorption in the line-of-sight.

Keratoconus: an analysis of corneal asymmetry

Background: Keratoconus, a non-inflammatory corneal ectasia, is reported to have bilateral involvement in over 90% of patients. The purpose of this study was to quantify the extent of asymmetry of disease at presentation to a regional corneal clinic.

A search for line intensity enhancements in the far-UV spectra of active late-type stars arising from opacity

Context. Radiative transfer calculations have predicted intensity enhancements for optically thick emission lines, as opposed to the normal intensity reductions, for astrophysical plasmas under certain conditions. In particular, the results are predicted to be dependent both on the geometry of the emitting plasma and the orientation of the observer. Hence in principle the detection of intensity enhancement may provide a way of determining the geometry of an unresolved astronomical source.
Aims. To investigate such enhancements we have analysed a sample of active late-type stars observed in the far ultraviolet spectral region.
Methods. Emission lines of O vi in the FUSE satellite spectra of ϵ Eri, II Peg and Prox Cen were searched for intensity enhancements due to opacity.
Results. We have found strong evidence for line intensity enhancements due to opacity during active or flare-like activity for all three stars. The O vi 1032/1038 line intensity ratios, predicted to have a value of 2.0 in the optically thin case, are found to be up to ~30% larger during several orbital phases.
Conclusions. Our measurements, combined with radiative transfer models, allow us to constrain both the geometry of the O vi emitting regions in our stellar sources and the orientation of the observer. A spherical emitting plasma can be ruled out, as this would lead to no intensity enhancement. In addition, the theory tells us that the line-of-sight to the plasma must be close to perpendicular to its surface, as observations at small angles to the surface lead to either no intensity enhancement or the usual line intensity decrease over the optically thin value. For the future, we outline a laboratory experiment, that could be undertaken with current facilities, which would provide an unequivocal test of predictions of line intensity enhancement due to opacity, in particular the dependence on plasma geometry.