Stellar Astrophysics

Bruhweiler and Lyu, have further pursued a model suggesting that tidal interactions produce spiral shocks in young planetary systems. The signature of these shocks can be seen in a temperature peaks in gaseous infall in $\beta$ Pic, and possibly in other similar stars, and even in Herbig Ae/Be stars. This is an outgrowth of recent work (Bruhweiler, Lyu, Kondo, and Boggess), which detected a double peaked temperature profile from HST data of $\beta$ Pic. A new model is proposed that does not directly require infalling cometary bodies to explain the variable infall.

Bruhweiler, with Holberg (Arizona), Barstow (Leichester), and Ivan Hubeny (AURA/GSFC) were selected to obtain early observations using the recently launched Far Ultraviolet Spectral Explorer (FUSE). Time-resolved spectra will be obtained of very hot white dwarfs displaying hydrogen-like O VIII emission.

Bruhweiler, with Sahu, Holberg and Barstow is also studying the very high resolution STIS echelle photospheric spectrum of the DA white dwarf, G191-B2B. These observations reveal very sharp "interstellar-like C~IV features at $\lambda\lambda$1548,1550 in addition to the corresponding photospheric features. However, these features cannot be produced in the local ISM. Modeling by Bruhweiler and the absence of the Si~IV resonance doublet at the same velocities indicate that these features must arise in the immediate vicinity of G191-B2B, most likely in the gravitational well of the star. The redward- displaced photospheric C~IV features are then displaced due to the gravitational redshift of the white dwarf.

Bruhweiler and Walter Feibelman (GSFC), are studying the recently acquired HST/STIS spectra of the ultra-hot pre-white dwarfs, Sand~3 and NGC~6905. The data reveal spectra rich in very high ionization emission lines characteristic of temperatures far in excess of the stellar effective temperature (T$\approx$ 100-150x10$^{3}$ K). Results are only preliminary and detailed analyses are planned.

Proffitt in collaboration with P. J{\"o}nsson (Lund), U. Litz{\'e}n (Lund), J.~C. Pickering (Imperial College London), and Wahlgren completed a study of the B~III resonance doublet in the early-B stars HD~35299 and HD~886, finding $^{11}$B/$^{10}$B isotope ratios consistent with the 4:1 ratio seen in the solar system. Work continues on a study of the B~III lines in IUE spectra of early-B stars. This latter work demonstrates that a substantial fraction of early-B main-sequence stars are depleted in boron.

Proffitt, together with David Leckrone (GSFC), Glenn M. Wahlgren, Jack Brandt (Colorado), and others continued working on radiative acceleration and non-LTE calculations for HgMn stars, especially the stars $\chi$~Lupi and HR~7775 for which high resolution and S/N Goddard High Resolution Spectrograph (GHRS) data exist. A annotated UV spectral atlas of the GHRS observations of $\chi$~Lupi (Brandt et al.\ 1999) and a detailed summary of the scientific results derived from these data (Leckrone et al.\ 1999) were published. Proffitt has prepared an on-line version of the Brandt et al.\ (1999) $\chi$~Lupi atlas, which is now available through the HST archive page at http://archive.stsci.edu/. Proffitt also initiated a new project on the radiative acceleration of very heavy elements in stellar envelopes in collaboration with F. Rogers and C. Iglesias (LLNL).

Robinson, in collaboration with Carpenter (GSFC) and Percival (U. Wisconsin) completed work on a study of microflare activity from the classical dMe flare star, YZ CMi. This work used high time resolution UV photometric time sequences obtained with the High Speed Photometer aboard the Hubble Space Telescope using the F240W filter, which is centered near 240 nm and has a 80 nm bandwidth. During 2.5 hours of observations a total of 54 flare and microflare events were detected, ranging in integrated flux from $2.0 \times 10^{28}$ to $3.0 \times 10^{30}$ ergs. These events are superimposed on a relatively strong background which shows non-flare like variations in flux with amplitudes up to 50\% on timescales ranging from 10 minutes to several hours. The occurrence distribution of microflare events is very similar to that determined for flare events observed from the ground. However, a statistical analysis of the background suggests that it may be composed of unresolved nano-flare events with a much steeper distribution than for the resolved events. This is consistent with previous results.

Robinson worked on the analysis of high quality STIS spectra of the dMe flare star AU Mic, taken as part of the GTO program of Linsky (JILA). In collaboration with Pagano (Catania Astrophysical Observatory) a study was carried out to investigate a high quality, medium resolution (R=20,000) quiescent spectrum of the star in the 1150-1730 \AA\ region. A total of 142 emission lines from 28 atomic species were identified in this spectrum, covering a temperature range from the chromospheric C I and O I lines to the coronal Fe XXI line at 1354 \AA. Emission measures were determined from the line fluxes and the transition region line profiles were found to have extremely broad winds with a centroid which is offset slightly from a much narrower core, suggesting a multi-component atmosphere. The atmospheric density was derived from flux ratios of the density sensitive O IV] intercombination lines. It was difficult to reconcile these densities with emission measures derived from other atomic species and a model was proposed which involved emission from both dense, flare related plasma and relatively low density plages.

The STIS data set which was used to study the quiescent properties of AU Mic was obtained using the time-tagged mode of observation, which tagged each detected photon with its time and location. Using this data it was possible to develop a spectral time sequence and thereby study the flaring activity on AU Mic. Robinson examined these data and found four relatively large flares during a total of $\sim$3 hours of on-source observing. These flares had overall durations lasting from 3 to 7 minutes and were primarily detected in the FUV continuum and the Si IV and C IV resonance lines. An emission measure analysis of the time averaged flare spectrum showed a pronounced peak near 10$^5$ K, with emission measures falling off towards both high and low temperatures. Enhancements of the Fe XXI line formed at 10$^7$ K were seen during the initial phases of two of the strongest flares, but only lasted for a few seconds. There was no evidence of proton beams associated with any of the major flare events.

Myron Smith and Rich Robinson carried out investigations of several aspects of activity seen in the classical Be star $\gamma$ Cas. In one study they investigated variations in the UV spectrum near the Si IV resonance lines at 1400 \AA. These observations consisted of a 22 hour spectral time series obtained using the GHRS and having a spectral resolution of 10,000 and a time resolution of 1 second. The data indicate the presence of clouds of plasma having temperatures ranging from 10,000 to more than 35,000 K within the circumstellar environment. In some cases these clouds were tied to the surface of the star, presumably by magnetic fields, and forced to co-rotate with the star. In other cases the clouds moved away from or towards the star with velocities up to 1500 km/s. It was suggested that this activity points to an interaction between magnetic loops from the surface of the star and the dense, equatorial disc. $\gamma$ Cas also shows strong, blueshifted absorption features seen in the wing of the Si IV line. These are normally referred to as Discrete Absorption Components (DACs). These were investigated by Smith and Robinson in collaboration with Cranmer (CfA) using the data set described above. In addition to the `time-averaged' DAC feature, which was found to be optically thick in Si IV, they also found a substantial amount of fine structure which varied on a time scale of several hours to the full 27 hour rotation period of the star. It is speculated that this variable fine structure is associated with sources of X-ray emission which are tied to the surface of the star and rotate with respect to the stationary DAC structures. Finally, Smith and Robinson completed a study of an X-ray time sequence of $\gamma$ Cas which was obtained with the RXTE satellite. This time sequence shows short duration bursts superimposed on a slowly varying background and the object of the program was to search for rotational modulation of the background, thereby proving that the X-rays originate from the Be star rather than from a hypothetical degenerate companion. While earlier observations showed variations in the background on time scales of 10 hours, this data set had variations on time scales of 1 hour or less, which is much too rapid to be caused by rotation (the period for the star is 27 hours). These rapid variations masked any rotation effects which might have been present.

Physics of the Interstellar Medium