For the past six months, F. Boffi has worked on her Ph.D. Thesis with advisor A. Renzini (ESO, Garching, Germany and Univ. of Bologna, Italy). During this period she has mostly focussed on the search for light echoes from historical supernovae. She works on this project, which is part of her Ph.D. Thesis, with W. Sparks, N. Panagia and D. Macchetto (ST ScI). Light echoes could be used as a new geometrical method of distance determination. She finds a number of candidate light echoes on the basis of the photometric data collected from ground based observations. Boffi has also co-authored a paper on the radio emission from two type II supernovae together with D. Branch, J. Cowan and C. Eck (U. OK).
Research activities of C. Burrows have centered upon HST Observations of the Beta Pictoris circumstellar disk, which have revealed the presence of a warp that can most naturally be interpreted as due to the perturbations induced by a Jupiter sized planet. Detailed models of the dynamics and evolution of the disk have been developed and reported at the AAS meeting in San Antonio.
Burrows' HST observations of circumstellar nebulosity have shown the environments of a number of newly forming stars at hitherto unrealized resolution. HL Tau has been shown to be purely a reflection nebula with the central protostar totally obscured in the optical, contrary to previous assumptions. HH30 has been shown to be a disk-like structure with an orthogonal bipolar outflowing jet---A first and excellent direct picture of the standard model for star formation. T Tauri has been shown to have an inner reflection nebula which we interpret as probably a circumbinary elliptical accretion disk. XZ Tau shows a unique bubble of outflowing nebulosity in addition to a bipolar jet. All of these four observations are published or submitted (as first or second author).
Burrows' HST observations of GL229B have confirmed that it is a proper motion companion to GL229A, detected it for the first time in the R band, and provided the first accurate visible astrometry and photometry on this unique brown Dwarf (publication in preparation, pending a third spacecraft visit).
C. Charbonnel has worked on the physics of microscopic diffusion and rotation-induced mixing in stars. She introduced the most recent physical descriptions of these transport processes in the Toulouse-Geneva stellar evolution code. Detailed numerical simulations were performed in order to test the effects of these mechanisms on the stellar structure and on the nucleosynthesis at different phases of the evolution. With her collaborators, she proposed the first solar model which is in agreement with helioseismic data and which simultaneously reproduces solar Li and Be abundances. She showed how mass loss- and rotation-induced mixing accounts for chemical anomalies in giant stars, and strongly destroys helium 3 on the red giant branch of low-mass stars. This led her to propose a new scenario for the galactic evolution of helium 3. Charbonnel also examined the sensitivity of stellar structure to the microphysics, and studied in particular the influence of the equation of state on the evolution of low mass stars at different metallicities.
R. Dempsey continued work in studying the coronae and chromospheres of active binary stars. In particular, he organized and executed a coordinated observing campaign for the RS CVn HD 155555. The main point of this program consisted of 26 CVZ orbits of HST observing with the GHRS that produced almost 49,000 spectra. In addition, there were simultaneous observations with the EUVE, AAT, AT, and ESO observatories covering 4 days. He also continued working on the ROSAT All-Sky Survey project.
R. Downes, Szkody and Silber (U. WA), Sion (Villanova U.), Howell
(PSI), and Costa and Moreno (U. Chile) obtained spectral of the
cataclysmic variable AL Com in a rare superoutburst. The optical data
were obtained from 3 to 21 days after outburst, while IUE data were
obtain at 10 and 19 days into the outburst. The IUE spectrum show no
evidence of P Cygni profiles and do show an early presence of Lyman
,
which may be indicative of a 25,000 K white dwarf. The optical spectra
show weak Balmer absorption, which remained fairly constant over the three
week interval. The spectra of AL Com during outburst are considerably
different than that of WZ Sge, despite the similar outburst properties
of the two stars.
Downes, in collaboration with Anderson and Margon (U. WA) obtained FOS spectra of the globular cluster x-ray sources in NGC 7078 and NGC 6712. For AC 211 in NGC 7078, the FOS data revealed both strong absorption and emission features, and are much less contaminated them previous IUE data. The energy distribution is consistent with a 15,000 K blackbody, although there is a clear flux deficit form 1600 to 2500 Å. The spectrum of Star S in NGC 6712 revealed a featureless continuum. The optical energy distribution is consistent with a 9000 K blackbody, while the ultraviolet distribution could not be fit with a blackbody of T<100,000K.
T. Gäng and collaborators presented new results from their on-going long-term monitoring program of early-type stars at the AAS meeting in San Antonio and in various articles. They also started a new ultra-high resolution optical survey of Luminous Blue Variables (LBVs) in order to scrutinize their atmospheric conditions for inhomogeneities. For this program they have obtained high-resolution (R = 60,000--230,000) spectra of a sample of 20 LBVs and LBV-candidates in the Milky Way and the Magellanic Clouds. The observations were carried out at AAT with UCLES in October 1995 and January 1996, at ESO/CAT with CES in April 1996 and at KPNO with the Coude Feed Telescope in May 1996.
Due to the exceptional data quality, previously undetected features and extreme spectral morphologies are found in a significant number of the spectra. First results of this project were presented at the LBV-workshop in Hawaii in October 1996.
J. Kingdon, in collaboration with Williams, Panagia, and Livio, has continued the examination of the post-outburst nova V1974 Cyg/92. This analysis involves devising a method of deriving abundances from emission lines which are formed in various regions of the nebulae under differing physical conditions. The individual line profiles are divided into distinct velocity and spatial regions for comparison. In conjunction with this project, observations are being planned to study a wide variety of emission-line objects at different spectral and spatial resolutions to compare how the derived abundances differ.
M. Livio, in collaboration with Southwell, Charles, and Sutherland (Oxford), examined the nature of the supersoft x-ray source J0513--69. They showed that the object produces collimated jets, and proposed a model which involves variable accretion onto a white dwarf which undergoes steady nuclear burning at its surface.
Livio, working with Rasio (MIT), simulated the common envelope phase in the evolution of binary systems. They performed the first three-dimensional calculation which started from an exact equilibrium configuration and showed that at the end of the dynamical phase, a disk which corotates with the binary is formed.
Livio, along with Della Valle (Padova), examined the question of whether some of the observed microlensing events are contaminated by dwarf nova eruptions. They concluded that dwarf nova eruptions of some old nova systems could produce non-negligible contamination.
Livio, collaborating with Yungelson (Moscow), Tutukov (Moscow), Fedorova (Moscow) and Truran (Chicago), constructed population synthesis models for the Galactic population of supersoft x-ray sources. They showed that these sources belong to at least five classes of objects.
Livio and Armitage (Cambridge) performed three-dimensional numerical simulations of the interaction between the stream from the L1 point and an accretion disk. They showed that the observed dips in the light curves of x-ray binaries can be explained by the stream-disk interaction.
Livio, in collaboration with Nota, Clampin, Pasquali (ST ScI), and Pollacco (Cambridge), examined the nebula around the LBV HD 168625, and proposed models for the generation of the observed morphology.
Livio and Pringle (Cambridge) showed that the disks formed around the central stars of planetary nebulae from the dissipation of a low mass secondary companion, are unstable to radiation-induced warping. These warped disks can precess, together with the jets which they produce. This can explain the formation of ``point symmetric" nebulae.
K. Long pursues research topics on the ultraviolet characteristics of cataclysmic variables, supernova remnants, and the properties of the interstellar medium in nearby galaxies. He remains an active co-investigator in the Hopkins Ultraviolet Telescope project, analyzing data from Astro-1, which was flown in 1990 December and Astro-2, which was flown in 1995 March.
Long, in collaboration with Mauche (LLNL), Szkody (U. WA), Raymond (SAO), and Mattei (AAVSO) obtained the first spectra of U Gem in the extreme ultraviolet. The observations, which were made with EUVE, cover the peak and decline of a normal outburst. The (75--225 Å) spectra are complex, and there is an eclipse of the continuum source near orbital phase 0.7. At outburst maximum, the effective temperature of the continuum, is about 140,000 K and the effective size of the emitting region is about that of the white dwarf in the system. The eclipse spectrum, which is most likely associated with a wind emerging from the vicinity of the whited dwarf, is dominated by emission lines, expected in a relatively cool (T<160,000 K) photoionized plasma. The EUV lines arise from the dominant state of the wind, and their strengths suggest that the wind mass loss rate, at least in U Gem, is a substantial fraction of the white dwarf accretion rate. In other studies of U Gem, Szkody, Long, Sion (Villanova), and Raymond used ASCA to show that there was also a partial eclipse of the X-ray source in U Gem near orbital phase 0.7 during optical quiescence, and Long, Blair (JHU) and Raymond showed that the FUV (820--1840 Å) spectra of U Gem far from outburst were well-modelled in terms of a white dwarf with a T of 30,000 degrees, with the possible exception of the region below 950 Å, where there is excess emission.
Long, Blair, and Raymond observed a number of quiescent dwarf novae, including SS Cyg, YZ Cnc, WX Hyi, and VW Hyi, on Astro-2. Only U Gem (mentioned above) and VW Hyi show the broad Lyman profiles and narrow metal lines expected from a white dwarf salted with material accreted from the secondary star. And Long, Blair, Hubeny (NASA/GSFC) and Raymond found that although the spectrum of VW Hyi could be modelled as an approx. 17,000 K white dwarf with subsolar abundances, a better fit could be obtained from the combination of a white dwarf with near-solar abundances and an accretion disk. In the other systems, it seems likely that the disk is dominant, a suggestion which Long, Mauche, Ko (U. MI) and Kallman (NASA/GSFC) are following up in an ongoing HST investigations of WX Hyi and SS Cyg.
Long and postdoc Knigge have also continued earlier efforts to analyze the
outburst spectra of dwarf novae observed with HUT. Specifically,
Knigge, Long, Blair and Wade (PSU) have recently completed an analysis of the
spectrum of Z Cam as observed on Astro-2. They were able to
approximate the overall spectrum in terms of a steady state accretion disk with
of 
g/s. They also succeeded in reproducing the
profiles of 5 high ionization resonance lines in the Z Cam spectrum using a
Monte Carlo radio transfer code and a description of the geometry, density and
velocity profile for the wind which had been used earlier by Knigge to model
CIV in the novalike variable UX UMa.
Long, Raymond, and Blair also used HUT to obtain FUV spectra of a
number of supernova remnants, including SN1006, Vela, Puppis and the Cygnus
Loop. Due to the availability of slits as large as 
and due to
the fact that the HUT wavelength range includes O VI 1035 which is not
accessible to HST or IUE, HUT is extremely useful for
such studies. With HUT, Raymond, Blair and Long were able to detect
the non-radiative filament on the NW rim on SN1006 for the first time and to
show that in this 2300 km/s shock the velocities of different ions are
independently randomized in the shock. Plasma turbulence is not effective in
equilibrating temperatures among different ion species and is unlikely to be
effective in equilibrating electron-ion temperatures. This result is
consistent with conclusions being obtained by Winkler (Middlebury) and Long
from ROSAT HRI imagery showing that X-ray emission peaks about 7 arcsec
inside the optical rim on SN1006 in the NW. Blair, Long, and Raymond have
also completed an analysis of the spectrum obtained of the
Schweitzer-Middleditch star which lies behind SN1006, in which they confirm
earlier detections of absorption from Fe II due to SN ejecta, and constrain the
total amount of Fe III, in an attempt to inventory the total amount of iron
ejected by the SN explosion.
S. Lubow together with P. Artymowicz (Stockholm) have continued investigations of the nature of disks around young binary stars. They have also found that circumbinary disks can transfer mass via gas streams to the binary, under certain circumstances. A semianalytic description of the gas stream was found. The gas stream mass flux is modulated on an orbital timescale. They are also investigating the role of this mass flow on the evolution of young planets. They are investigating whether superplanets could have gained their observed eccentricity by their interactions with nebulae during their early stages.
Lubow and Pringle (IoA) have continued investigations of wave propagation in accretion disks. They are attempting the understand how resonant excited waves can propagate in thermally stratified disks.
Lubow, Tout (IoA), and Livio are investigating the role of resonant torques in despinning close orbiting planets and circularizing their orbits.
Lubow and Murray (CITA) are investigating models of superoutbursts in CVs. The
current view is that superoutburst light is the product of enhanced dissipation
once the disk meets the 3:1 resonance. They are investigating the viability of
that model. They are also determining the permitted range of 
value that
is consistent with forming an eccentric disk during superoutburst.
S. Moehler worked for most of the year with M. Dahlem on a project to use correlation techniques for the analysis of low S/N spectra of hot compact stars (e.g., white dwarfs). The idea is to be able to get some idea of what the star is from spectra with S/N of 1--5, which may be the expexcted S/N of spectra taken of white dwarfs in globular clusters. It will be possible to distinguish between white dwarfs and hot subdwarfs with this method and also determine whether the white dwarf is helium- or hydrogen-rich. It will not be possible to derive temperatures or surface gravities that way.
In addition, Moehler continued her work with Heber (Bamberg) and others on hot subluminous stars in globular clusters. They could derive a mean mass for the sdB stars in NGC 6752 and verify that these objects are most probably formed as extended horizontal branch stars. In addition, for the first time they found sdB stars in another globular cluster (M 15), one of them being helium-rich. Helium-rich sdB stars are very rare in the field (compared to the helium-poor ones; 48 helium-rich are known vs. more than 1000 helium-poor) and have never before been found in globular clusters.
M. Romaniello worked on his Ph.D. thesis with his advisor Panagia (ESA/ST ScI). The main topic of his thesis is the study of the young and intermediate stellar populations in different astrophysical environments, in order to shed light on the properties of starbursts such as the IMF, the pre-main sequence evolution and the mechanisms that trigger star formation.
In collaboration with Panagia, Scuderi (Oss. Catania) and Kirshner (CfA),
Romaniello studied the stellar population around Supernova 1987A in the LMC.
Using the HST-WFPC2 camera, they observed the field around the
supernova in 6 wide bands and in two narrow band filters (O III and H
)
and they determined the luminosity, the temperature and the reddening for each
star. Comparing the observed quantities with theoretical models, they were able
to identify several different generations of stars, ranging from a few Myrs to
several Gyrs. In particular, the youngest population was identified using
H
emission as a diagnostic for detecting pre-main sequence stars.
Scuderi, Capetti (SISSA, Trieste), Panagia, Romaniello, Lamers (Utrecht, Holland) and Kirshner have further developed this topic by studying the unresolved stellar populations in the core of the spiral galaxy M51. By studying the color-color and the color-magnitude diagrams, they were able to deredden the images and to study the spatial distribution of the dust and the intrinsic properties of starlight. In particular, they found that the nuclear region is dominated by a starburst that occurred about 450 Myrs ago, i.e., almost coeval to the closest passage of the companion dwarf galaxy NGC 5195, whereas the bulge is composed of stars older than 8 Gyrs.
K. Sahu, in collaboration with Van de Steene (ESO) and Pottasch (Kapteyn Lab, Groningen), completed a project to study the optical spectra of a sample of IRAS-selected, radio-detected, planetary nebula candidates. The sample of PN-candidates was selected based on their position in the IRAS color-color diagram. They were then observed in the 6-cm radio continuum with positive detections, suggestive of nebular emission. The optical spectra, obtained with ESO telescopes, confirmed them to be low-excitation PNs, with central star temperatures of 60,000 K or less.
Sahu, in collaboration with R. Oudmaijer (Imperial College) and others, studied the spectral energy distribution and mass-loss history of the peculiar hyper-giant IRC+10420. It was shown that the object has increased in temperature by more than 1000 K during the past 20 years. The spectral energy distribution cannot be fitted with a single-shell model, and needs a hot circumstellar disc component. It is also shown that IRC+10420 is not a post-AGB star.
In collaboration with G. Stasinska (IAP), D. Schaerer studied the impact of new theoretical ionizing fluxes obtained from his ``combined stellar structure and atmosphere" (CoStar) models on single star H II regions. These results have been included in a large database for the modeling of Galaxy Evolution. Schaerer studied the IMF and the origin of He II emission in young starburst galaxies. New evolutionary synthesis models for WR galaxies were developed in collaboration with W. Vacca (IfA).
H. Stockman, working with G. D. Schmidt (U. AZ), completed the analysis of time-resolved HST data obtained for two eclipsing magnetic Cataclysmic Variables (mCV). By comparing the UV spectra to that predicted for the photoionized accretion column in such systems, he was able to find a good match of line strengths and total UV line emission for clumpy accretion, i.e., low filling factor gas, in a wide, extended column. These results are consistent with models invoking penetrating blobby accretion to hide the expected hard X-ray fluxes in these systems. Further studies of eclipsing mCVs are underway using data obtained during Cycle 6.
N. Walborn and R. Bohlin prepared an atlas of Copernicus data displaying the systematics of OB spectra in the 1000--1200 Å range. It extends the correlations between the stellar-wind phenomenology and the optical spectral types found previously in the IUE data for the OB stars at longer wavelengths. Perhaps the most important new contribution is the demonstration of a pronounced luminosity effect in the S IV 1063, 1073 Å wind profiles, identical to those previously recognized in Si IV 1394, 1403 Å and C III 1176 Å, which share essentially the same ionization potential.
In Walborn's HST Cycle 5 program, a compact cluster in the 30 Doradus periphery, previously classified as WN + OB in integrated light from the ground, has been resolved spectroscopically into 11 components, including in addition to the WN a peculiar Be star and a peculiar Of star with twice the mass of any other members---a good candidate for a stellar merger. HST spatially resolved optical spectroscopy of 3 further compact groups in the 30 Dor periphery and the LMC giant shell H II region Henize N11 is being carried out in Cycle 6.
Currently, Walborn, in collaboration with C. Blades is completing a major classification study of 100 stars in the 30 Doradus Nebula, based on it AAT multiple-fiber and CTIO 4m data, which reveals new spatial and temporal structures among the associated clusters, as well as numerous individual objects of special astrophysical interest. The average difference between the derived and calibration absolute visual magnitudes is 0.05, indicating that the classification, calibration, and adopted distance modulus (18.6) are accurate. The most massive star known in the complex has been identified. Further high-quality classifications in 30 Dor will become available from a current program led by A. Moffat, in which repeated observations with the CTIO 4m Argus fiber system are being made to search for spectroscopic binaries; Walborn is deriving spectral types from the high-S/N sums of the individual observations. A report on this work was presented at the Madison AAS meeting.
R. E. Williams has continued his research into the analysis of nova shells and the interpretation of emission-line spectra in the presence of density inhomogeneities. With M. Shara and D. Zurek, WFPC2 images of the shells around the novae T Pyx and GK Per have been obtained in the lines of various ions to determine the characteristics and structure of condensations. Together with N. Panagia, M. Livio, and J. Kingdon, a procedure is being developed and applied to HST spectra of Nova V1974 Cyg/92 by which emission line profiles are deconvolved into their major individual components based upon a kinematical model of the ejecta. Physical conditions will then be derived for the kinematically distinct regions from selected line ratios rather than attempting to calculate a detailed photoionization model with an ad hoc assumed geometry. Those emission ratios which are most invariant to conditions in the gas are being identified from the Emission Line Chart, where the ultimate goal is a process by which abundances can be determined reliably independent of a detailed knowledge of the structure of the emitting gas.
R. Williamson, in collaboration with P. Thejll and C. Flynn (Nordita), and R. Saffer (ST ScI), determined proper motions for a large group of subdwarf B (sdB) and subdwarf O (sdO) stars for kinematical analysis and intercomparison with each other. Good agreement was found of sdB star space velocities and absolute magnitude for those with previously published radial velocities. Reduced proper motions for sdO and sdB stars which had proper motion data only seemed to indicate that if the sdB stars evolve from sdO stars then the sdO star absolute magnitude is 1 magnitude brighter and 1 magnitude broader than sdB distributions.
Williamson, with D. J. MacConnell as advisor, is deriving proper motions for all known carbon stars to search for carbon dwarf stars. Approximately 700 stars to date have been reduced, with no definitive motions for carbon stars yet.