Alexander Brown

We present far-ultraviolet spectra of the classical T Tauri star RU Lupi covering the 912-1710 A spectral range, as observed by the HST/STIS and FUSE satellites. We use these spectra, which are rich in emission and absorption lines, to probe both the accreting and outflowing gas. Absorption in the Ly-alpha profile constrains the extinction to A_V=0.07 mag, which we confirm with other diagnostics. We estimate a mass accretion rate of (5\pm2)\times10^{-8}$ M_\odot/yr using the optical-NUV accretion continuum. The accreting gas is also detected in bright, broad lines of C IV, Si IV, and N V, which all show complex structures across the line profile. Many other emission lines, including those of H_2 and Fe II, are pumped by Ly-alpha. RU Lupi's spectrum varies significantly in the FUV; our STIS observations occurred when RU Lupi was brighter than several other observations in the FUV, possibly due to a high mass accretion rate.

We have obtained three long-slit, far UV spectra of the pre-main sequence system T Tauri. These HST/STIS spectra show a strong and variable on-source spectrum composed of both fluoresced H_2 and stellar chromospheric lines. Extended H_2 emission is seen up to 10" from the T Tau system. The on-source and extended H_2 are both pumped by H I Lyman alpha. The on-source H_2 is pumped by the red wing of a broad, self-absorbed Ly-alpha line, while the progressions seen in the extended gas are pumped from near line center. This suggests that the extended H_2 is pumped locally, and not by the stellar Ly-alpha line. The H_2 to the north and west coincides with the evacuated cavity bounded by the optical reflection nebulosity; to the south the extended H_2 coincides with the HH 255 outflow from the embedded infrared companion T Tau S. The spatial profile of the extended gas shows a prominent dip coincident with the position of T Tau S. This may be absorption by a disk associated with T Tau S. There is no evidence for absorption by a disk surrounding T Tau N large enough to obscure T Tau S.

A 34 day Extreme Ultraviolet Explorer (EUVE) pointing on the "clump" giant b Ceti (HD 4128; K0 III) recorded a series of striking coronal flare events, reminiscent of a singular outburst seen previously from m Velorum (HD 93497; G6 III ϩ dF). The recent flaring episode contrasts with a more placid behavior in a 6 day EUVE observation of b Cet 6 years earlier. The average 70-180 A ˚Deep Survey count rate in the new observation is twice as high, and the 75-150 A ˚spectrum displays a distinct hardening. The discovery of sustained flare activity on b Cet raises the possibility that such episodes are more common than suspected among the core helium-burning giants and sharpens the puzzle of the survival of magnetic activity beyond helium flash.

HR 2554 is an eclipsing binary system that contains a G6 II primary star and an A dwarf secondary. We have obtained ultraviolet spectra during two eclipses of HR 2554 in 1994 MarchÈApril and 1995 May using the HST Goddard High Resolution Spectrograph (GHRS) to study the atmospheric structure of the primary and to better establish the properties of the binary system. We have determined accurate orbital elements for both components, using new and existing optical spectra for the primary and the GHRS spectra for the secondary, in conjunction with eclipse photometry presented in 1992 by Schro der & The derived orbital period is 195.258 ^0.003 days, the derived radii are 31.3 ^0.9 and Hu nsch. R _ 1.9 ^0.1 and the derived masses are 3.14 ^0.17 and 1.98 ^0.11 respectively. The R _ , M _ M _ , resulting positions of HR 2554 in the Hertzsprung-Russell (H-R) diagram are somewhat inconsistent with the independent evolution of the component stars ; in particular, the G star primary is both overluminous and too blue for its mass. The difficulties might be overcome if the metallicity of the system were unusually low ([Fe/H] D [1), but this possibility appears to be ruled out by the [Fe/H] \ [0.04 determination of Eggen. We suspect HR 2554 has undergone a more complicated evolutionary history involving mass transfer than has been considered previously.

We have constructed a detailed mean density and temperature model for the extended outer atmosphere of the O-rich supergiant Betelgeuse (α Ori [M2 Iab]), which extends from 1.0 to 10.0 stellar radii. A one-dimensional model is based on fitting NRAO1 VLA centimeter visibility data, and two-dimensional models are constructed using the intensity contours of the 0.7 cm observations of Lim et al. As one moves in toward the star from about 10 R* the mean electron temperature increases to a value of ≃3800 K, then declines down below Teff, and then rises to photospheric values. The peak mean model temperature is less than the typical chromospheric temperatures found in previous models. Observations of Hα and chromospheric ultraviolet (UV) emission show that higher temperature components must also exist, but they do not dominate the weighted mean temperature structure. We tentatively identify the radius where the temperature distribution peaks (R ≃ 1.45 R*) with the dominant chromospheric UV emission region and find an areal filling factor of ≤1/4. In the extended semiempirical model the dominant source of electrons is from photoionized metals and is dominated by carbon. The low ionization of hydrogen leads to a dominance by H- (free-free) opacity at centimeter wavelengths. We derive simple estimates of the radio spectral indices for other similar M supergiants. We have constructed two-dimensional models to examine whether the intensity asymmetry observed at 0.7 cm is most likely to result from density or temperature variations. Adopting an elliptical two-dimensional model, a density asymmetry along the axes of symmetry would need to be 20:1. If we assume the radial wind velocity is independent of angle the integrated mass-loss rate is only a factor of ∼ 2 greater than that derived from the one-dimensional model. However, previous Hα speckle observations that sample the same spatial regions suggest the asymmetry observed at 0.7 cm is not due to such a large-scale density asymmetry. A modest change in temperature can more easily provide the asymmetry, increasing both the opacity and the thermal source term. If the radial density structure is assumed to be the same as in 1992 September, when HST/GHRS spectra were obtained, then the Fe 11 wind absorption features provide an estimate of the mass-loss rate of 3.1(±1.3) × 10-6 M⊙ yr-1. This further implies that the cool material dominates the mass of the extended atmosphere and that the radio-emitting region is within the base of the outflow observed in the circumstellar layers. A simple silicate dust model is constructed and the semiempirical model suggests an onset of dust formation at R ≃ 33 R* where Tdust ∼ 360 K. This region lies outside the semiempirical model but simple extrapolations suggest that at this radius Te ∼ 220 K, and the mean hydrogen density nH ∼ 3 × 106 cm-3. We address the difficult question of whether the mean thermal model based on the radio data can be consistent with the observed off-limb Hα scattering emission if inhomogeneities are present.link_to_subscribed_fulltex