EUVE Science Highlight: October 4, 1999 ======================================== EUVE Observations of the Venus Dayglow ====================================== o Observed by EUVE 7-11 April 1998, ~20 ksec of science data o Resulted in first spectrum of Venus covering full EUV range (70-760 Å) o Spectral line measurements: 145 +/- 35 R for HeI 584 Å, 40 +/- 8 R for combined HeI 537 Å and OII 539 Å o 537/539 A modeling implies ~40% reflected solar He 537 from Venus's upper atmosphere, and ~60% O+ 539 from solar EUV photoionization and excitation of oxygen atoms in Venus's upper atmosphere Notes: This science highlight was submitted by G.R. Gladstone of the Southwest Research Institute. G. R. Gladstone, S.A. Stern, D.C. Slater, and L.J. Paxton, to be presented at the October 15,1999 meeting of the AAS Division of Plantary Sciences. During April 7-11, 1998, the EUVE satellite performed observations of Venus (not long after western elongation on March 27). About 20 ks of data were obtained in about 50 400 s pointings (one per orbit), performed while the spacecraft was in Earth's shadow. Large slews were required before and after each pointing to ensure that EUVE's look direction was >90 deg from the Sun when the satellite was in direct sunlight. These data comprise the first extreme ultraviolet spectrum of Venus in the 7-76 nm range (although a short Galileo EUVS spectrum at 55-125 nm was recorded during the February 1990 flyby, two excellent spectra were obtained by the Hopkins Ultraviolet Telescope in the 82-184 nm range, and rocket observations covering the 82.5-111 nm range have been acquired). The EUVE spectra indicate a brightness of 145 +/- 5 R for the HeI 58.4 nm emission, and 40 +/- 8 R for a combination of the HeI 53.7 nm and OII 53.9 nm emissions. For comparison, the Galileo EUVS measured a HeI 58.4 nm brightness of 200-280 R. We will present detailed simulations of these and other Venus EUV dayglow emissions. We are grateful to the EUVE project for their heroic efforts to plan and make these observations, which put the satellite at considerable risk. Time-Resolved EUVE Spectroscopy of AR UMa ========================================= [FIGURE 1] o AR Uma: very bright EUV source -- a magnetic CV with strongest known field (250 MG) o Short-wavelength spectral changes vs. phase imply change in temperature and size of accretion region on WD surface o EUVE provides spectral sampling steps at "10% of Arizona" size (~50 mile resolution) o MW spectra show presence of intrinsic He 304 A emission [FIGURE 2] o He 304 A emission evident only in phases 0.0-0.7 o Complex and sometimes multiple line profile o Unidentified emission near 301 A [FIGURE 3] o First-ever EUV radial velocity solution for any star o He 304 components from inner gas stream connecting the two stars o Source of unidentified 301 A line probably on/near WD surface Notes: This science highlight was submitted by Dr. Steve Howell of the Planetary Science Institute. The first figure shows the EUVE short- (SW) and and medium-wavelength (MW) spectra of AR UMa as a function of a full orbital phase cycle (phase decreases top to bottom). Notice the change in the SW data with phase indicating a temperature and size change of the accretion region on the white dwarf (WD) surface. The accretion region on this WD is about the size of the state of Arizona (~500 miles) sitting on a star about the size of the earth. The EUVE spectroscopy allows us to sample that region in size steps equal to "~1/10th of an Arizona" (~50 mile resolution) per spectrum. The MW spectrum reveals the presence of intrinsic He 304 emission. The second figure shows an expanded view of the He 304 emission in AR UMa. The emission appears only in binary phases of 0.0 to 0.7, being absent or very weak the remaining time. An examination of the He 304 A region in the EUVE spectra shows that the line profile is complex and at times, multiple lines appear. The third figure presents the results of detailed examination of the He 304 region shown in the second figure above. The entire he 304 complex consists of three main emission lines, two of which appear to be components of He 304 while the third is an as yet unidentified emission line near 301 A. Using the good spectral resolution of EUVE and time slices as shown in the previous figure, this third figure presents the first ever EUV radial velocity solution for any star. Interpretation of the data seem to indicate that the two He 304 A components are from the inner gas stream connecting the two stars, each having a velocity near 800 km/sec, which is typical for gas streams in magnetic systems but which has never before been confirmed in the EUV spectral region. The third (unidentified) emission line at 301 A appears to follow very precisely the radial velocity solution for the white dwarf as determined for AR UMa from the optical H beta line at 4861 A. The radial velocity phasing and semi-amplitude are nearly equal to those determined in the optical and indicate a source for this line on or near the white dwarf surface. EUVE Spectroscopy Shows Iron in G191-B2B Atmosphere has a Depth-Dependent Abundance ========================================================= Figure: Comparison of the complete EUVE count spectrum of G191-B2B and the stratified iron theoretical model that give the best match to the entire wavelength range. The envelope is best modeled with the iron in two distinct layers with an abundance (Fe/H) of 1e-6 in the outermost and 4e-5 in the deeper one. The complete EUV spectrum of G191-B2B cannot be matched by a simple homogeneous mixture of heavy elements. A complete explanation by a theoretical model where the photospheric Fe has a stratified distribution, with increasing abundance at greater depth. This may explain the discrepancies between the abundances predicted by radiative levitation calculations and the observational measurements, which are usually obtained with reference to homogeneous models. The best-fit models also give a lower inferred He ionization fraction for the local ISM, compared to the homogeneous models, more in keeping with other direct measurements. Notes: This science highlight was submitted by Dr. Martin Barstow of the Department of Physics and Astronomy at the University of Leicester. M.A. Barstow, I. Hubeny and J.B. Holberg (MNRAS in press) The presence of heavy elements in the atmospheres of the hottest H-rich DA white dwarfs has been the subject of considerable interest. While theoretical calculations can demonstrate that radiative forces, counteracting the effects of gravitational settling, can explain the detections of individual species, the predicted abundances do not accord well with observation. However, accurate abundance measurements can only be based on a thorough understanding of the physical structure of the white dwarf photospheres, which has proved elusive. Recently, the availability of new non-LTE model atmospheres with improved atomic data has allowed self-consistent analysis of the EUV, far UV and optical spectra of the prototypical object G191-B2B. Even so, the predicted and observed stellar fluxes remain in serious disagreement at the shortest wavelengths (below ~ 190 Å), while the inferred abundances remain largely unaltered. We show here that the complete spectrum of G191-B2B can be explained by a model atmosphere where Fe is stratified, with increasing abundance at greater depth. This abundance profile may explain the difficulties in matching observed photospheric abundances, usually obtained by analyses using homogeneous model atmospheres, to the detailed radiative levitation predictions. Particularly as the latter are only strictly valid for regions deeper than where the EUV/far UV lines and continua are formed. Furthermore, the relative depletion of Fe in the outer layers of the atmosphere may be evidence for radiatively driven mass loss in G191-B2B.