Extreme ultraviolet emission lines of Ca XV in solar and laboratory spectra

New R-matrix calculations of electron impact excitation rates in Ca XV are used to derive theoretical electron density diagnostic emission line intensity ratios involving 2s(2)2p(2)- 2s2p(3) transitions, specifically R-1 = I(208.70 Angstrom)/I(200.98 Angstrom), R-2 = I(181.91 Angstrom)/I(200.98 Angstrom), and R-3 = I(215.38 Angstrom)/I(200.98 Angstrom), for a range of electron temperatures (T-e = 10(6.4)-10(6.8) K) and densities (Ne = 10(9)-10(13) cm(-3)) appropriate to solar coronal plasmas. Electron densities deduced from the observed values of R-1, R-2, and R-3 for several solar flares, measured from spectra obtained with the Naval Research Laboratory's S082A spectrograph on board Skylab, are found to be consistent. In addition, the derived electron densities are in excellent agreement with those determined from line ratios in Ca XVI, which is formed at a similar electron temperature to Ca XV. These results provide some experimental verification for the accuracy of the line ratio calculations, and hence the atomic data on which they are based. A set of eight theoretical Ca XV line ratios involving 2s(2)2p(2)-2s2p(3) transitions in the wavelength range similar to140-216 Angstrom are also found to be in good agreement with those measured from spectra of the TEXT tokamak plasma, for which the electron temperature and density have been independently determined. This provides additional support for the accuracy of the theoretical line ratios and atomic data.

Fe XIII emission lines in active region spectra obtained with the Solar Extreme-Ultraviolet Research Telescope and Spectrograph

Recent fully relativistic calculations of radiative rates and electron impact excitation cross-sections for FeXIII are used to generate emission-line ratios involving 3s23p2-3s3p3 and 3s23p2-3s23p3d transitions in the 170-225 and 235-450 Å wavelength ranges covered by the Solar Extreme-Ultraviolet Research Telescope and Spectrograph (SERTS). A comparison of these line ratios with SERTS active region observations from rocket flights in 1989 and 1995 reveals generally very good agreement between theory and experiment. Several new FeXIII emission features are identified, at wavelengths of 203.79, 259.94, 288.56 and 290.81 Å. However, major discrepancies between theory and observation remain for several FeXIII transitions, as previously found by Landi and others, which cannot be explained by blending. Errors in the adopted atomic data appear to be the most likely explanation, in particular for transitions which have 3s23p3d1D2 as their upper level. The most useful FeXIII electron-density diagnostics in the SERTS spectral regions are assessed, in terms of the line pairs involved being (i) apparently free of atomic physics problems and blends, (ii) close in wavelength to reduce the effects of possible errors in the instrumental intensity calibration, and (iii) very sensitive to changes in Ne over the range 108-1011cm-3. It is concluded that the ratios which best satisfy these conditions are 200.03/202.04 and 203.17/202.04 for the 170-225 Å wavelength region, and 348.18/320.80, 348.18/368.16, 359.64/348.18 and 359.83/368.16 for 235-450 Å.

Macroturbulent and rotational broadening in the spectra of B-type supergiants.

The absorption-line spectra of early B-type supergiants show significant broadening that implies that an additional broadening mechanism (characterized here as `macroturbulence') is present in addition to rotational broadening. Using high-resolution spectra with signal-to-noise ratios of typically 500, we have attempted to quantify the relative contributions of rotation and macroturbulence, but even with data of this quality significant problems were encountered. However, for all our targets, a model where macroturbulence dominates and rotation is negligible is acceptable; the reverse scenario leads to poor agreement between theory and observation. Additionally, there is marginal evidence for the degree of broadening increasing with line strength, possibly a result of the stronger lines being formed higher in the atmosphere. Acceptable values of the projected rotational velocity are normally less than or equal to 50 km s-1, which may also be a typical upper limit for the rotational velocity. Our best estimates for the projected rotational velocity are typically 10-20 km s-1 and hence compatible with this limit. These values are compared with those predicted by single star evolutionary models, which are initially rapidly rotating. It is concluded that either these models underestimate the rate of rotational breaking or some of the targets may be evolving through a blue loop or are binaries.

A non-LTE analysis of the spectra of two narrow lined main sequence stars in the SMC

An analysis of high-resolution VLT/UVES spectra of two B-type main sequence stars, NGC 346-11 and AV 304, in the Small Magellanic Cloud (SMC), has been undertaken, using the non-LTE tlusty model atmospheres to derive the stellar parameters and chemical compositions of each star. The chemical compositions of the two stars are in reasonable agreement. Moreover, our stellar analysis agrees well with earlier analyses of H II regions. The results derived here should be representative of the current base-line chemical composition of the SMC interstellar medium as derived from B-type stars.

Extreme ultraviolet transitions of Fe XXI in solar, stellar and laboratory spectra

Recent R-matrix calculations of electron impact excitation rates for transitions among the 2s(2)2p(2), 2s2p(3) and 2p(4) levels of Fe XXI are used to derive theoretical electron density (N-e) sensitive emission-line ratios involving 2S2(2)p(2)-2s2p(3) transitions in the similar to 98-146 Angstrom wavelength range. A comparison of these with observations from the PLT tokamak plasma, for which the electron density has been independently determined, reveals generally very good agreement between theory and experiment, and in some instances removes discrepancies found previously. The observed Fe XXI ratios for a solar flare, obtained with the OSO-5 satellite, imply electron densities which are consistent, with discrepancies that do not exceed 0.2 dex. In addition, the derived values of N-e are similar to those estimated for the high-temperature regions of other solar flares. The good agreement between theory and observation, in particular for the tokamak spectra, provides experimental support for the accuracy of the present line-ratio calculations, and hence for the atomic data on which they are based.