will present the Raymond and Beverly Sackler Distinguished Lecture in Astronomy.

Title: How to Look for Primitive Life on Planets Orbiting Other Stars

Life on Earth is more than a scum on the surface. It has transformed the composition of Earth's atmosphere by breaking down carbon dioxide and water to make organic molecules. Oxygen released in this way, having oxidized the rocks, has for the past billion years been established as a major atmospheric constituent.

If similar biological processes have taken place on temperate planets of other stars, then ozone, which is a good tracer of oxygen, may be detectable in their atmospheres. Interferometric techniques in the infrared look most promising for both exoplanet detection and spectroscopic analysis. The conflicting requirements for destructive interference to null the stellar emsission and for high resolution imaging to distinguish planets from dust are reconciled in a new linear interferometer configuration. If stars at 10 pc distance have zodiacal clouds no stronger than the sun's, a 50 m long space interferometer with four 1 m elements should see individual planets like the Earth in images taken over 10 hours, and could obtain useful spectra in 3 months.

Title: Learning How To Count: The Statistics of Gamma Ray Bursts

Satellite observations of the enigmatic Gamma Ray Burst (GRB) sources present astronomers with a number of challenging problems in statistical inference with discrete data. Adopting the Bayesian approach to statistical inference produces methods that differ in methodology---and sometimes in results---from methods using the more traditional "frequentist" approach. I will summarize some of the most crucial differences between Bayesian and frequentist analyses, and show how these differences are manifested in a variety of problems arising in the analysis of GRB data. Among the problems I will discuss are the analysis of GRB spectra, and the analysis of the joint distribution of burst intensities and directions (including a search for evidence that the sources of GRBs repeat). These problems all involve the analysis of Poisson counting processes and point processes; I will argue that the Bayesian approach is thus helping us learn better "how to count."

Title:Diversity of Supernovae: SN 1987A and SN 1993J

Recent radio and X-ray observations of SN 1987A give evidence for supernova interaction with an H II region created in the swept up dense wind from a previous evolutionary phase. The spectrum of the ejecta can be explained with radioactive power input and does not show any signs of power from a central compact source. The presence of the H II region will delay the arrival of the shock front at the dense ring, when bright optical/uv emission is expected. In the case of SN 1993J in M81, dense gas was present close to the explosion, giving rise to radiative phenomena at an early phase.

Title: Deviations from Hubble Flow in the Local Universe

Large-scale deviations from Hubble flow, thought to be driven by mass density inhomogeneities, can now be mapped out to distances in excess of 100 Megaparsec. They can be used not only to characterize the details of the local mass distribution and to identify the main drivers of the motion of the Local Group with respect to the comoving reference frame, but also to set important constraints to the values of parameters of cosmological interest, such as the Hubble constant and the density parameter.

The results of an all-sky survey of peculiar motions will be presented, based on the application of the Tully--Fisher technique for the determination of individual galaxy distances. The importance of the quality of a template relation and the treatment of bias will be underscored, as well as the potential of the technique for future applications. Maps of the mass distribution, estimates of cosmological parameters and inferences on the convergence depth will be discussed.

Title: Intergalactic hydrogen and helium and the baryon density of the Universe

It has been known for 30 years that any appreciable amount of intergalactic hydrogen must have been highly ionized at z > 2. Numerous studies have revealed H I absorption in quasar spectra due to the Lyman alpha forest clouds, but there have generally been only upper limits placed on any absorption by smoothly distributed intergalactic hydrogen, the Gunn-Peterson effect. More recently the analogous absorption by singly ionized intergalactic helium has been detected and measured at low resolution. I will argue that only part of the measured He II absorption can be attributed to the well-known Lyman alpha forest clouds, while a significant part must arise in more diffusely distributed intergalactic gas. I will describe a simple analytical model for a continuous IGM with density variations associated with primordial fluctuations in a universe dominated by cold dark matter. Photoionization of such an IGM by quasar radiation can explain many features of the Lyman alpha forest as well as the recent helium data if the mean baryon density is at the upper end of the range allowed by observations of the light element abundances in the standard big bang theory.

Title: The Frequency and Effect of Young Companion Stars

Understanding the formation of stars like the Sun remains one of the unsolved problems of astrophysics. Most of the work on this topic has been driven by the desire to explain the origins of our solar system and thus has resulted in scenarios for the production of single low mass stars that, at a young age, are surrounded by a disk of gas and dust from which planets may form.

I will report on high spatial resolution imaging studies of T Tauri stars, a particular class of young low mass stars. This work suggests that most, if not all, low mass stars have companions stars. Futhermore these close companions star appear to have a disprutive influence on circumstellar disks, which most likely suppresses the potential for planetary formation in these systems.

Title: The Ubiquity of Planets and the Diversity of Planetary Systems

Recent discoveries of planets around solar type stars suggest that planets are ubiquitous and their dynamical properties are diverse. We present formation mechanisms for protoplanets and discuss post-formation planet-disk tidal interaction which may have led the short-period planets around 51 Peg and 55 Cnc to their present configuration. We show evidences which indicate that a populations of similar planets may have plunged into and contaminated the stellar convection zone. We argue that massive, eccentric planets such as HD 114762 and 70 Vir were formed as the merger products of planetary systems which became dynamically unstable. Finally, we speculate the giant planets in the solar systems may be the survivors of protoplanetary evolution in the primordial solar nebula.

Title: X-ray Emission from Clusters of Galaxies--A Cosmological Laboratory

In the last 5 years our knowledge of the x-ray emission from clusters has exploded with high quality Rosat image and ASCA spectra. We are now able to obtain detailed measurements of the mass and mass distribution of clusters out to radii of ~1 Mpc for relaxed systems.over a a mass range from 10^13-4^15 solar masses. These data confirm that the baryonic mass fraction of clusters is more than that implied by big bang nucleosynthesis and omega=1, the so-called baryon catastrophe. However we now have direct evidence that the baryon fraction can vary from cluster to cluster.

In addtion the abundances of O,Si,S,and Fe can be reliably obtained over the same mass range. For massive clusters the elemental ratios strongly indicate that type II supernova were responsible for most of the metals. However less massive systems may have a substantial contribution from type Is, The total mass of alpha burning elements strongly indicates that most galaxies went through a extremely luminous early phase which resulted in the ejection of ~1/2 of their total mass. The total energy involved was on the order of 1/4-1/2 of the present day binding mass of the cluster, showing the importance of non-gravitational energy sources for structure formation. The Fe abundance in clusters shows little if any change out to redshifts of ~0.5 confirming the early enrichment of the intergalactic medium in clusters.

Title: Nine New Carbon Chains

Some of the most exciting, current problems in science, including the discovery of the soccer ball molecule C$_{60}$ (this year's Nobel Prize in Chemistry), have come directly from laboratory studies of molecules known or thought to occur in space. With the radio discovery of OH some 30 years ago and the subsequent identification of more than 110 interstellar molecules, we now know that the interstellar medium is a fascinatingly rich source for many familiar and exotic molecules. During the past few months we have detected in the laboratory nine new long carbon chains, all of astrophysical interest, using a high sensitivity microwave spectrometer. These highly reactive molecules, which include cyanopolyynes, free radicals, and carbenes generally unfamiliar or unknown to the laboratory physicist or chemist, are significant because carbon chains are the dominant structural theme in space. Two of our new chains -- possibly three -- have already been detected astronomically. In this talk our recent findings, the experimental advances which made these discoveries possible, and future work will be discussed.