Elisha F. Polomski

An Unusual Brightening Of Eta Carinae

HST/Space Telescope Imaging Spectrograph data show that the apparent near-UV, visual-wavelength, and near-IR brightness of η Car increased by a factor of two during 1998. Meanwhile its Homunculus ejecta nebula brightened by about 30%, the largest fluctuation of this type in the past 40 years. These developments were quite unexpected and are not easy to explain. Some dust has probably been destroyed, while the stars luminosity may have increased even though it was already close to the Eddington limit. Such a rapid luminosity change would be a truly remarkable phenomenon, not predicted by existing models.

Stellar Masses, Kinematics, and White Dwarf Composition for Three Close DA+dMe Binaries

We determine the mass functions and mass ratios for three close white dwarf plus red dwarf binaries (EUVE J0720-317, 1016-053, and 2013+400). Hubble Space Telescope Goddard High-Resolution Spectrograph spectra of the He II ?1640 and C IV ?1550 spectral lines trace the white dwarf orbital motion, and Hamilton Spectrograph echelle spectra (Lick Observatory) and lower dispersion spectra trace the red dwarf orbital motion. The data sets allow us to measure orbital periods and velocities, as well as the white dwarf gravitational redshifts. The red dwarf and white dwarf mass estimates obtained from a combination of independent mass measurements for the white dwarf stars and our new orbital elements help constrain probable evolutionary outcomes. We find that EUVE J0720-317 will probably come into contact within a Hubble time and that the mass transfer will be unstable on a dynamical time. We also conclude that the much lower secondary masses in EUVE J1016-053 and EUVE J2013+400 exclude the possibility of significant interaction in these systems. We also present new helium and carbon photospheric abundance measurements in the three white dwarfs. The white dwarf atmospheric composition may show the effects of accretion of red dwarf mass-loss material onto its surface. Finally, we study the kinematics of the systems, and we also show that the white dwarf in EUVE?J1016-053 is part of a quadruple system.

The Complete Spectral Energy Distribution and the Atmospheric Properties of the Helium-Rich White Dwarf MCT 0501–2858*

We analyze high-dispersion ultraviolet spectroscopic observations of the peculiar helium-rich white dwarf MCT 050122858. Ultraviolet spectra obtained during the 1996 November ORFEUS-SPAS IImission show the complete He ii ( ) line series as well as narrow heavy-element lines; the spectrum constrains the effective temperature n 5 2 (68,600 5 1800 K) and surface gravity (log ), imposing a low mass (0.4 ) and short cooling g 5 7.20 5 0.07 M, age (10 5 yr) for the star. A study of the complete spectral energy distribution, based on EUVE, ORFEUS, and IUE spectra, shows that carbon is the second most abundant element [log (C/He) 52 1.9] and that lower abundances are measured for other elements [log (N/He) 52 4.0, log (O/He) 52 3.3, log (Si/He) 52 4.6, and log (P/He) 52 6.2]. The new atmospheric parameters (temperature, gravity, and chemical composition) suggest a possible evolutionary filiation with the class of PG 1159 stars. Subject headings: stars: abundances — stars: individual (MCT 0501 22858) — ultraviolet: stars — white dwarfs

Faint Sources in the EUVE Survey: Identification of White Dwarfs, Active Late-Type Stars, and Galactic Nuclei

We report the classification of 21 new extreme-ultraviolet sources from the recent catalog of Lampton et al. The optical spectra presented identify the objects as 14 active late-type stars (including two double active stars and a possible T Tauri star), three white dwarfs, and six active galactic nuclei (a Seyfert galaxy, the BL Lac object 1ES 1028+511 [=EUVE J1031+508], and four quasi-stellar objects). We have detected Ca II absorption lines in the BL Lac object and measured its redshift. Two of the white dwarfs are unusually massive (M > 1.1 M☉). Our sample of late-type stars includes five previously known high proper motion objects (EUVE J1004+503, J2244-332A,B, J1802+642, and J1131-346), of which one is the well-known flare star TX PsA (EUVE J2244-332B). We report an unusually high level of activity for the primary component of the TX PsA system (EUVE J2244-332A), which may indicate flare activity. The group of late-type stars is on average almost 3 mag fainter (m ≈ 13) than the typical member of the Extreme Ultraviolet Explorer (EUVE) all-sky survey catalog. All Galactic and extragalactic objects were also detected in the ROSAT Position Sensitive Proportional Counter survey, and most are at the faint limit of the EUVE detectors. These new identifications substantially increase the total number of EUV-selected extragalactic sources.

The Dusty Circumstellar Environments of Ae/Be Protoplanetary Disk Candidates

We have obtained high spatial resolution mid-IR observations of four intermediate-mass pre–main-sequence stars: HD 259431, AS 310, LkHα 234, and MWC 1080. Our observations reveal the morphology of the circumstellar (500–10,000 AU) environments of these young stars. We resolve companion stars and complex diffuse emission from dust that can account for on average 50% of the IR emission detected in previous low-resolution observations. We calculate new spectral indices for these objects and show that they differ from those expected from simple geometrically flat disk models. We also show mid-IR spectral energy distributions that reveal the presence of silicate emission and absorption features for several objects. Finally, temperature maps constructed from our observations suggest the presence of transiently heated grains, and optical depth maps point to the locations of the most embedded objects.

Dust Morphology and Composition in FU Orionis Systems

FU Orionis stars are a small group of pre–main-sequence stars known for large-amplitude optical variability. These objects also exhibit multiwavelength phenomena suggestive of active accretion from a circumstellar disk. We present high spatial resolution mid-IR imaging and spectroscopy, submillimeter photometry, and 3–4 μm photometry of four FU Ori–class objects, RNO 1B and C, Z CMa, and Par 21, and one object classified as a pre–FU Ori star, V380 Ori. We resolve multiple IR sources and extended emission in the RNO 1B/C system, and we discuss in detail their association with disk activity and the source of the Infrared Astronomical Satellite far-IR and radio maser emission in this field. We derive dust temperatures and masses for all sources and discuss how dust composition and morphology is related to the evolutionary stage of these objects.