B. Paczynski, and his associates worked mostly on a major observing program aimed at detecting gravitational microlensing in our Galaxy (the Optical Gravitational Lensing Experiment - OGLE), and various projects that arose serendipitously from the OGLE data. The most important new result was the discovery of a number of detached eclipsing binaries in the globular cluster Omega Centauri, and the presentation of theoretical reasons for those systems to be excellent distance and age indicators. It is expected that within the next few years, studies of detached eclipsing binaries will provide an accurate distance scale within the Local Group of Galaxies, and reliable ages for the globular clusters.
Paczynski wrote a review article about the current status of microlensing searches towards the galactic bulge and the Large Magellanic Cloud, and the prospects for the future developments.
H-S. Zhao (MPI/Munich), M. Rich (Columbia) and Spergel constructed a
consistent microlensing model of
the Galactic bar. The predicted event rates and event durations are
consistent with the 55
events reported recently by the MACHO and OGLE collaborations. Most of the
events are due to lensing
by stars in the near end of the bar. Lens mass functions with about
30-50% of lens mass as brown
dwarfs are rejected at 2-6
levels. To make the standard model
useful for other workers, the
bar's optical depth and average event duration (scaled to 
lenses) was tabulated on a grid
of Galactic coordinates. The distance and the proper motions of the lens
and the source were derived
from a consistent dynamical model of the stellar bar, which has
originally been built to fit data on
the stellar light and stellar/gas kinematics of the bar. Several
alternative models were explored
and the standard model was found to best match observations.
B.T. Draine has continued to work on theoretical astrophysics of the interstellar medium with particular attention to (1) dust grains; (2) photodissociation fronts; and (3) shock waves.
Draine and J. Weingartner (graduate student) are studying the effects of radiative torques exerted on irregular dust grains when they are illuminated by starlight. The theory underlying calculation of these torques was developed, and torques were calculated for examples of irregular grains using a modified version of the DDSCAT code for calculation of scattering and absorption of light using the discrete dipole approximation. Radiative torques exerted on grains by the interstellar radiation field can result in spinup of grains to superthermal rotation rates. Even isotropic starlight can produce superthermal rotation, but modest anisotropies in the radiation field -- such as are expected in the interstellar medium -- can be very important. Work now in progress investigates the role which anisotropic starlight may play in the process of alignment of dust grains with the magnetic field.
Draine reviewed the electromagnetic properties of grains important for the physics of dust grain alignment. In particular, the frequency-dependence of the imaginary part of the magnetic susceptibility was examined for paramagnetic, superparamagnetic, and ferromagnetic materials. Since dust grains can in some cases be spinning very rapidly, it is important to understand the frequency-dependence of the magnetic dissipation.
Draine, in collaboration with F. Bertoldi (MPI, Garching), continued to work on the theory of time-dependent photodissociation fronts associated with ionization fronts. The theory of coupled ionization-dissociation fronts was developed to determine under what circumstances the dissociation fronts will not be well-approximated by ``classical'' models of stationary, steady-state photodissociation fronts.
The theory of stationary photodissociation fronts was investigated by
Draine and Bertoldi. It was
shown that overlap of the ultraviolet absorption lines of H
(neglected in much previous work)
can often be important. New approximations were developed for the
treatment of self-shielding when
line overlap becomes important, and photodissociation front models
including these effects were
computed. The models were applied to the bright photodissociation region
in NGC 2023, for which there
are now observations of H fluorescent emission in both the K band and
the far-red. It is shown
that the photodissociation region in NGC 2023 must be considerably denser
and warmer than previously
believed in order to reproduce the observed H emission spectrum.
Grain destruction in shock waves is important because of its role in the evolution of the interstellar grain population and because it can alter the gas-phase abundance of atoms, ions, and molecules in a shock wave. The destruction of grains in interstellar shock waves was reviewed by Draine for the Manchester conference ``Shocks in Astrophysics''.
E. Jenkins collaborated with G. Wallerstein (U. Washington), A. Vanture (U. Washington) and G. Fuller (U.C., San Diego) in a search for an enhancement of some r-process elements in the interstellar material within the Vela supernova remnant. The GHRS on HST was used to observe wavelength regions centered on the absorption lines of Ge II, Kr I, Yb II, Os II and Hg I in the uv spectra of 5 stars whose locations were within about 1 degree of the position of the Vela pulsar. The only features that were detected were those of Ge II. The column density of Ge II and upper limits for other species indicated that the abundances of these elements were consistent with the usual cosmic abundances, with no measurable enrichment arising from gas ejected by the supernova.
In another observing program with the GHRS on HST, Jenkins, Wallerstein and Vanture compared interstellar absorption lines appearing in the uv spectra of two components of the visual binary HD72127A,B behind the Vela supernova remnant. The objective of their study was to observe some important uv transitions to obtain a better understanding of differences in the interstellar material within the remnant over a length scale as small as 0.013 pc. Previous ground-based observations of the Ca II lines showed that there were differences in the interstellar features for these two stars. The fine-structure excitation of C I indicates that some of the material in front of the stars is at an elevated pressure [log(p/k) > 5.5] and thus must have been shocked and compressed by the supernova blast wave. Lines of Ge II and P II are seen in star A but seem to be significantly weaker in star B, while the weak lines of Mg II appear to be identical in both stars. The data recorded in this program will also be useful for future studies that will search for changes in the features with time.
Jenkins and Wallerstein measured the abundances of various elements in the interstellar gas in front of 3 stars in the galactic halo, again using the GHRS. It is known from previous investigations that certain elements show much less depletion in the halo than in the disk. By comparing the results for the halo stars with shocked gas in the Vela remnant (in front of HD72089), Jenkins and Wallerstein found that the pattern of depletions from one element to another were consistent with that of reduced depletions caused by the destruction of dust grains. These results seem to rule out the proposition that such changes are caused principally by the injection of iron-peak elements by Type Ia supernovae in the halo. The lack of a significant increase in the concentrations of these elements is probably due to the exchange of matter between the halo and disk of our galaxy.
A. Lazarian has continued to work on developing the theory of grain
alignment. For paramagnetic
alignment of grains rotating under the influence of torques arising from
H formation (Purcell
alignment), the major process which limits the life-time of catalytic
sites of H formation was
shown to be oxygen poisoning, rather than the customarily accepted grain
resurfacing. Grains smaller
than a calculated critical size were shown to have short-lived spin-up,
and the preferential
alignment of large grains was established. Lazarian also elaborated on the
way photodesorption
enhances the alignment.
Lazarian studied paramagnetic alignment of thermally rotating oblate grains (Davis-Greenstein alignment). A perturbative approach to solving the corresponding Fokker-Planck equation was used, and analytical solutions were obtained. The accuracy of these results was confirmed by independent numerical simulations. Working on the alignment of thermally rotating grains Lazarian showed that thermal fluctuations within the grain material limit the degree of alignment of the grain angular momentum with the grain principal axis of maximal inertia. This effect was shown to decrease the Rayleigh reduction factor for the Gold and Davis-Greenstein alignments.
Lazarian analyzed the mechanical alignment of grains rotating suprathermally, i.e,. with kinetic energy substantially exceeding the energy of Brownian rotation and proposed two new mechanisms of alignment. So-called ``crossover alignment'' happens due to angular momentum delivered to the grain by a gaseous flow during ``crossovers'', the short intervals between successive spin-ups. Although crossovers are short, grains are susceptible to alignment because their angular momenta are small during these intervals. ``Cross-section alignment'' happens due to variation of the grain-gas cross-section for different angles of grain orientation. This difference influences the time back to crossover. Analytical expressions for the measure of alignment when both mechanisms were in action were derived. This study indicates that mechanical alignment can be more widely spread than it is usually believed. In collaboration with M. Efroimsky (Tufts University), A. Lazarian elaborated the theory of the cross-sectional alignment for oblate grains, and showed that grain helicity is an important factor which must be accounted for.
Lazarian, in collaboration with P. Gerakines (RPI) and D. Whittet (RPI),
studied grain alignment in the
Taurus dark cloud. The variations of the polarization efficiency (p/A)
for this cloud were shown to
obey a power law 
. The effects of small-scale
magnetic field structure, coupling
of the dust-gas temperature and depletion of the atomic hydrogen with
optical depth were examined.
Together with A. Chrysostomou, J. Hough, D. Aitken (Hertfortshire), D. Whittet (RPI) and P. Roche (Oxford) A. Lazarian has studied interstellar polarization arising from CO mantled grains. The polarization in both broad and narrow CO components was detected and the consequences for grain alignment theory were discussed.
A. Lazarian and E. Vishniac (UT, Austin) have studied the structure of magnetic field embedded in a turbulent plasma and applied the results to the ISM. They showed that the field can form isolated flux tubes that are confined both by the pressure of the external gas and by shocks in the surrounding media. This fibralization alters the conventional dynamo theory and invalidates a number of anti-dynamo arguments.
A. Howard (recent Ph.D. student) and Kulsrud continued their research on the evolution of a primordial magnetic field. They showed that the results of their model produced a magnetic field that is not subject to the main objections advanced against a primordial magnetic field. The wind up of the magnetic field by galactic rotation leads to a tightly wound azimuthal field, but this field can be shown to be in agreement with observations. Compression in the spiral density wave makes the magnetic field nearly parallel to the spiral arms in the region of the arms, so that observations of such a field in other galaxies would detect a magnetic field that would appear to trace out the spiral arms. The field would be azimuthal in between the arms. The rapid reversal of the field on the scale of about one hundred parsecs need not average out when the magnetic field is observed at any reasonable resolution. Further, because of its topology the magnetic field can not be expelled from the galactic disk in a finite time.
D. Uzdensky (graduate student), R. Kulsrud and M. Yamada (PPPL), developed a general theory of magnetic reconnection at large magnetic Reynold's numbers. They showed that any magnetic reconnection problem breaks naturally into a global magnetostatic problem, which can be solved independently of the magnetic reconnection process, and a local problem consisting of the determination of the physics of the magnetic reconnection in the very thin resistivity layer. The global problem connects the general boundary conditions (far away), to the reconnection. In the bulk of the region outside of the reconnection layer motions are very small, and resistivity is negligible. The parameters which uniquely determine the solution of the global problem, are determined by demanding that the entropy on the freshly reconnected surface satisfy total energy conservation. This enables one to properly take into account the conversion of magnetic energy into kinetic and thermal energy. The change of topology of the magnetic lines during reconnection must also be properly taken into account. The solution of the global problem uniquely determines the position of the reconnection layer and also most of the conditions outside of the layer. Using these conditions it is believed possible to solve for the physics of the reconnection in the layer depending on the relevant physical properties of the plasma. This breakup leads to a considerable simplification of the reconnection problem and should make possible a nearly general solution of the complete reconnection problem in the limit in which the magnetic Reynold's number is large compared to one. The ambiguity in the solution depends only on whether the plasma in the reconnection layer acts as a resistive fluid or a collisionless fluid and whether there are anomalous resistivity processes present in the layer. This ambiguity, in general, does not affect the global solution.
B. Chandran (graduate student) completed his study extending the quasilinear theory for the amplification of small scale magnetic fields by turbulence, taking into account the effects of realistic velocity correlation times. A new type of numerical simulation was used to evolve the magnetic field at a point moving with the flow in a random velocity field. The ideas of Kraichnan were used to model the random velocity field. The numerical simulations indicated that the quasilinear theory overestimates the growth rate of the magnetic energy by a factor of about 2. This result is important, because the quasilinear theory is not applicable to Kolmogoroff turbulence, and thus might have been off by orders of magnitude, leading to spurious conclusions about the origin of cosmic fields. An analytic theory based on the work of Van Kampen was also used to corroborate and explain the results of the simulations.
In a separate project, Chandran investigated four stages in the turbulent amplification of small scale magnetic fields in the early Galaxy assuming the Galaxy is born with a very weak magnetic field. In the first stage, the magnetic field is dynamically unimportant, and magnetic energy builds up very rapidly on scales much smaller than the smallest turbulent eddies. In the second stage, this small scale magnetic field is strong enough to influence the smallest turbulent eddies. The small scale field acts like a network of tangled rubber bands, making the plasma elastic to large scale motions. This leads to the conversion of some of the hydrodynamic modes into elastic shear waves. In the third stage, a new type of damping of magnetic fields places a ceiling on the energy of the very small scale magnetic field, allowing fields to build up only on the scales of the turbulent eddies. This new type of damping is analogous to ambipolar damping, only ions and neutrals move together to straighten out field lines. In the fourth stage, the magnetic field becomes strong on the scales of the velocity eddies. Strong MHD turbulence develops, and is treated using a numerical integration of the direct interaction approximation (DIA) equations. It is shown that the magnetic energy builds up to equipartition with the turbulent energy on all scales.
The source of the highly ionized interstellar atoms has been considered by
Spitzer, based on a
comparison of the column density ratio N(C
) / N(O
between
the Galactic halo and disks.
The C observations were mostly obtained with the Goddard
High-Resolution Spectrograph on HST,
partly by Fitzpatrick & Spitzer and partly by Huang, Songaila, Cowie and
Jenkins. The O
observations were obtained by Jenkins with Copernicus. Toward six
disk stars this ratio
averages 0.15, toward five halo stars, 0.9. Individual values scatter
about these means by about a
factor two. The low value toward disk stars agrees with models of
conductive heating of a cool gas in
contact with a hot one, as in theories of cloud evaporation, stellar winds,
and young supernova
remnants. The higher ratio toward halo stars is in the general range
covered by different theories of
downfalling, initially hot gas, as in a Galactic fountain. These data,
while limited, tend to confirm
theoretical expectations as to the major dynamical processes in the
interstellar halo and disk.
E.L. Fitzpatrick and Spitzer have continued their multi-year study of the
physical properties of
individual interstellar clouds with a detailed examination of the line of
sight towards the star HD
215733. This star is located at a distance of 
3000 pc, some 1700 pc
below the Galactic plane, in
the direction (l,b) = (
,
). Analysis of data from
the Goddard High
Resolution Spectrograph (
) and the
Kitt Peak Coudé Feed
spectrograph (
) reveals an exceedingly
complex line of
sight, with more than 20 individual absorption components identified in the
low-ionization species.
The determination of kinetic temperatures 
and electron densities
for the individual
components (clouds) is a primary goal of this study. In strong contrast to
another halo star (HD
93521), most of the gas seen towards HD 215733 is cold, with temperatures
of order 100 K. Five
different electron density diagnostics are available, based on collisional excitation equilibrium of
C
fine structure levels, and ionization equilibrium of C
/C,
Mg/Mg, S/S, and
Ca/Ca
. The various ionization equilibrium diagnostics are found
to have systematic
discrepancies of up to 1 dex, in the sense that the values involving the
neutral species tend to
be larger than those derived from the Ca/Ca ratio. The values
derived from Ca are
consistent with the observed C excitation in the cold clouds if the
free electrons come primarily
from ionization of the metals. The gas pressures P/k implied by this
condition are reasonable,
in the range 1000-5000 cm
K. The reason for the discrepancy among
the ionization equilibrium
diagnostics is not known. Future studies will seek to establish the
systematic behavior of these
discrepancies.
Analysis of the highly ionized species C
, Si, and N
towards HD 215733 reveals an
additional 7 absorption components, found in one or more of the ``high
ions.'' For two of the high ion
components, both Si and C are observed with a ratio of line
widths which equals
the square-root of the atomic mass ratio, indicating thermal broadening at
temperatures of
K and 
K. The column density ratios of C
to Si in these two
components would require a temperature of about 
K in
collisional equilibrium. This
result appears to give direct observational confirmation that transient
phenomena must be assumed if
collisional ionization is to explain the high ion data.
Fitzpatrick has analyzed the pattern of interstellar gas-phase abundances
of the elements Si, S, Mn,
Cr, Fe, and Zn derived for about 30 individual interstellar clouds along
the sightlines toward
the Galactic disk star HD 68273 and the halo stars HD 93521, HD 149881, and
HD 215733. The gas-phase
abundance of S relative to H in these clouds appears indistinguishable from
the solar value. For
the other elements, well-defined upper limits are found in the gas-phase
abundances at significantly
subsolar values. For Fe, Mn, and Cr (and probably Ti) there are no
convincing cases where
the relative gas-phase abundances exceed -0.5 dex, i.e., these
elements have not been seen in
interstellar gas with an abundance greater than about 1/3 solar. For Si
the limit is -0.15
dex, and for Zn a constant abundance of -0.13 dex is found from seven
clouds along one halo
sightline. These subsolar maximum abundances have two possible
interpretations: (1) they indicate the
presence of an essentially indestructible component of interstellar dust,
which contains about 2/3 of
the Ti, Mn, Cr, and Fe and about 1/3 of the Si (assuming that the intrinsic
interstellar abundances are
solar) or (2) they indicate that the true total abundances of these
elements are substantially less
than in the Sun. The subsolar abundance hypothesis is qualitatively
consistent with recent
suggestions -- based primarily on measurements of O in the ISM and of a
number of elements in nearby
B-type stars -- that the general metallicity of the ISM could be as much
as 0.2 dex below that of the
Sun. The magnitude of the departure from solar abundances for Fe, Cr, Mn,
and Ti is however much
greater than has been suggested for other elements and may be difficult to
reconcile with the stellar
results.
E.L. Turner and Y. Wang (Fermilab) point out that for a given source and gravitational lens pair, there is a thin on-axis tube-like volume behind the lens in which the radiation flux from the source is greatly increased due to gravitational lensing. Any objects (such as dust grains) which pass through such a thin tube will experience strong bursts of radiation, i.e., Extreme Gravitational Lensing Events (EGLE). They study the physics and statistics of EGLE for the case in which finite source size is more important than shear. As an illustration of one of the several possible significant astrophysical effects, they investigate the evaporation of dust grains by EGLE in an idealized globular cluster system.