C. Flynn and J. Holmberg (Lund Observatory) have completed ananalysis of the kinematics of early type stars in the Hipparcos catalog, launched by the European Space Agency in 1989. They have measured the height to which these stars on average are able to rise out of the central plane of the Milky Way, and comparing this to the velocity dispersion of the stars made an estimate of the total gravitational pull of the Milky Way near the Sun. This estimate is in good accord with the amount of gravity expected from a total census of known stars and gas in the Milky Way, and is in good accord with other studies of Hipparcos and other stars made in the last few years. This leads to the conclusion that most of the material making up the disk of the Galaxy around the Sun has been accounted for. The gravitational field of the disk allows the oscillation period of the Sun up and down about the disk mid-plane to be computed as about 90 million years. This is to be contrasted with a predicted oscillation period of about 60 million years if the apparent periodicities in the craterisation record of the Earth are related to tidal effects on the comet cloud in the outer solar system (the Oort cloud). It seems less likely that this is the case in view of the Hipparcos results.

C. Flynn has with A. Gould (Ohio State University) and J. Bahcall (Princeton) embarked on an expanded program of star counting with the Hubble Space Telescope in order to determine the luminosity and mass functions (amount and total mass of stars as a function of their intrinsic brightness) in the disk and spheroid of the Galaxy. These determinations happen to be of particular topical interest because of the generation of microlensing surveys which have discovered a candidate for the "dark matter" in the Galaxy which seems to make up some 90 to 95% of the Galaxy's mass but has never been seen directly. The star counts with the Hubble telescope strongly exclude low mass stars as the cause of the microlensing, and also come close (but not quite) to ruling out old white dwarfs. With W. Evans (Oxford) and C. Gyuk (Trieste) the Space Telescope has been used to rule out the idea that the microlensing might be a "false alarm" caused by an intervening stellar population in the "warp/flare" of the outer disk.

C. Flynn has with H. Rocha-Pinto, W. Maciel (Sao Paolo) and J. Scalo (Texas) worked on the determination of the Galaxy's star formation rate over the life time of the disk. The relatively flat star formation history found for the local disk is quite different from the global star formation rate for the cosmos, recently determined using Space Telescope and Keck high redshift morphology and galaxy counts. The age of the disk has been found to be at least 8 Gyr and probably a few Gyr older using the colour magnitude diagram of local stars observed by Hipparcos by C. Flynn, R. Jimenez (Edinburgh) and E. Kotoneva.

Groups of galaxies

S. Wiren, B. Liu (Beijing Observatory), and M. Valtonen have developed an N-body code which mimics dynamical friction and mergers between galaxies, each represented by a single particle. The properties of the particle - particle interaction depend on the mass, radius and spin of each particle (galaxy). The code has been used to follow the evolution of small compact groups of galaxies until only two galaxies remain. The statistical properties of the remnant binaries are studied. The most interesting finding is that even though the distribution of relative spin orientations is wide there is a bias in favour of parallel spins.

A. Chernin, in collaboration with V. Dolgachev and L. Domozhilova (Sternberg Astronomical Institute, Moscow University), developed a set of more than 5 000 computer models for the wide triple systems of galaxies with a typical size 600-700 kpc. The free-fall models, which are dynamically similar to the Kahn-Woltjer model for the Local Group, were shown to simulate the major dynamical characteristics of the observed ensemble of the systems sampled earlier by Trofimov and Chernin. The models enable one to make a statistical estimate of the total mass of a typical wide triple galaxy. The mass proved to be about 10¹³ solar masses which is 5-7 times the typical mass of compact (with size 40-50 kpc) triplets of galaxies. The mass-to-light ratio was found to be 100-300, in solar units, and this indicates that a considerable amount of dark matter (10-30 times the luminous mass) is contained in these systems.

A. Chernin and M. Valtonen have reviewed the work on the Triple systems of galaxies together with I. Karachentsev (Caucasus) and G. Byrd (Alabama). Triple systems are used, among other things, to evaluate the contribution of dark halos in galaxies.

R. Rekola, P. Heinämäki, L. Takalo and M. Valtonen together with I. Karachentsev et al. from Caucasus and M. McCall from York University (Toronto) have embarked on a project of determining distances of galaxies in the vicinity of the Local Group. The NOT-telescope has been used to image the galaxies in different colours. The images of brightest red and blue supergiants, and more recently, of Cepheids have been identified in the images. These stars are used as distance indicators to the galaxies. The long term goal of this project is to determine the maximum extent of the Local Group as a dynamical unit.

Together with V. Dolgachev and E. Kalinina (Sternberg Astronomical Institute, Moscow University), A. Chernin worked out a computer model for the observed phenomenon of intergalactic gas infall onto the disk of the Galaxy. The model describes the dynamics of gas clouds in the non-stationary gravitational field of two major galaxies of the Local Group. About 500 trajectories were integrated for the restricted three-body problem, and local dynamical instability was demonstrated along the quasisymmetry axis of the system; the set of initial conditions appropriate for the variety of infall trajectories was found. The total amount of gas clouds that could be accreted by the Galactic disk from intergalactic space was estimated which proved to be in general agreement with the observational limits.

Cosmic gasdynamics

In collaboration with V. Artemiev and coworkers (Institute of Geosphere Dynamics, Russian Academy of Sciences), A.Chernin studied the physics of collision and reflection of spherical shock waves in a set of laboratory experiments with laser- induced explosions. The results revealed a high degree of similarity with the analytical theory by Courant and Friedrichs (initial stages of the process) and recent computer simulations by Voinoich and Chernin (advanced stages). The stability of the nonlinearly evolving supersonic pattern was observed against non-axisymmetric perturbations. On this basis, Chernin suggested a quantitative model of supersonic bubble-bubble collision and merging in the interstellar medium. Processes of this kind occur in starforming regions where the stellar winds from massive stars and supernova explosions produce multiple expanding quasispherical shocks that come into contact with each other. The model describes the formation of a common expanding shell of two colliding bubbles and predicts the development of a complex internal dynamical structure of the merger product which includes a stable collimated binary outflow along the axis of symmetry of the structure.

Chernin studied the phenomenon of large-scale straight segments in spiral arms of grand-design galaxies. Optical, H-alpha, UV, far-UV, CO, 21-cm and synchrotron emission data were used to recognize multiple rows in the archetype galaxy M 51. It was found that: (1) the spiral pattern of M 51 is presented by two polygonal lines almost entirely; (2) the polygons are almost identical; (3) the lengths of the segments increase almost lineally with the distance from the center; (4) the segments intersect each other at an angle of about 120°. A similar structure is found also in M 101 and a dozen other nearby giant spirals seen nearly face-on. A gas-dynamical interpretation of the phenomenon was suggested on the basis of Roberts-Pikelner theory of spiral shocks. The formation of straight segments is treated as a result of generic stability of flat shock fronts and the tendency of slightly curved shocks to become flat. A quantitative criterion of local (on the scale of the local radius of curvature) flattening for spiral shocks was demonstrated to explain the geometrical properties found in the observed polygonal patterns.

Interstellar polarization at high galactic latitudes

Together with A. Berdyugin (Crimean Astrophysical Observatory) P. Teerikorpi and M. Hanski have continued interstellar polarization measurements at the Nordic Optical Telescope. The aim is to get a good picture of the distribution of polarizing dust and magnetic fields at high galactic latitudes up to a distance of 1000 pc. The second article on these studies was published in 1997. The work has much benefitted from a new sample of distant high-latitude A stars which were received in 1998 from J. Knude (Copenhagen). This work also bears on the question of the galactic extinction correction which is needed for extragalactic photometric distance indicators, influencing the derived value of the Hubble constant.

The distribution of interstellar polarization around the North Galactic Pole. Lengths of the bars give magnitude of polarization, their directions show directions of the polarization plane (magnetic field). These stars have been measured at the NOT, at the Crimean AO, and at Tuorla Observatory. The results from the on-going programme at the NOT were reported by Berdyugin & Teerikorpi (1997).