CfA Colloquium Schedule Spring 1997

January 9
Dr. Geoffrey Marcy
San Francisco State University
and U.C. Berkeley, California

Title: Planets Orbiting Sun-Like Stars

Eight planetary companions have been detected by the accelerations of the host stars. Most were detected by stellar doppler measurements with a precision of 3 m/s. Some of the new planets have properties similar to planets in our Solar System. However, several are more massive than Jupiter, and four orbit their host star closer than 0.1 AU. Three planets reside in non-circular orbits. Augmentation of existing theories of planet formation may account for their diverse properties. The occurrence rate of true Solar-System analogs remains a mystery. During the upcoming 5 years, several new techniques for detection of extrasolar planets will be tested, including use of the Keck 10-meter telescope, MMT adaptive optics, and interferometry.

January 16
Dr. Brian W. Kernigan
Bell Laboratories
Murray Hill, New Jersey

Title: The C Programming Language -- Past, Present and Future

Since its creation by Dennis Ritchie in 1973, the C programming language has spread from its original home on PDP-11's and Unix to all machines and all operating systems. For many years and in most environments, C was the language of choice for systems programming, and indeed for programming in general. Today, C remains among the most popular of programming languages, and its main competition comes from its children, C++ and Java.

This talk will trace the history of C, study some of its strengths and weaknesses, and explain some of the issues that led to C++ and, more recently, Java.

January 23
Dr. Antoinette B. Galvin
Space Physics Group
University of Maryland
College Park, MD

Title: Solar Wind Composition Measurements: Guideposts to Solar and Heliospheric Processes

The solar wind is an extension of the solar corona (the sun's "atmosphere") into interplanetary space and beyond. The first measurement of the composition of the solar wind was accomplished 35 years ago. It was determined that the ion components typically consist of 95% singly ionized hydrogen, 4% doubly ionized helium, and less than 0.5% other ions. These "other ions", usually designated as "minor ions", nonetheless hold a wealth of information on solar and heliospheric processes. Beginning with the launch of the Ulysses spacecraft in 1990, and continuing with the launch the WIND spacecraft in 1994 and of the Solar Heliospheric Observatory (SoHO) in 1995, new instrumental techniques for advanced solar wind composition studies have become available, yielding exciting new results, and sometimes new questions.

January 30
Dr. James Buckley
F.L. Whipple Observatory
Tucson, Arizona

Title: TeV Gamma-Rays from Blazars and Supernova Remnants

The Whipple 10m atmospheric Cerenkov telescope has now detected two extragalactic sources of TeV gamma-rays; Mkn~421 and Mkn~501, both of which belong to the BL Lac class of AGNs. I'll report on observations of these objects during the 1995 and 1996 seasons as well as the results of multiwavelength observations of Mkn~421 which indicate correlations in the gamma-ray, X-ray, extreme ultraviolet, and optical wavelengths. Dramatic outbursts of TeV gamma-rays were detected on two occasions in May of 1996 where the gamma-ray flux reached the highest levels ever recorded (a factor of $\sim$20 greater than typical levels) and with incredibly short variability timescales $\sim$15min to 1hr. Together with the multiwavelength correlations these provide serious constraints on the Doppler factor of the jet, the size and location of the emission region and the mechanisms responsible for the nonthermal emission in BL Lac objects.

Nondetections are also important. The failure of the Whipple 10m telescope to observe the predicted pion-decay gamma-ray flux from nearby shell-type SNRs is producing serious problems for the standard models of the origin of the galactic cosmic rays. The implications of these observations on the diffusive shock acceleration model, as well as the implications on the nature of the low galactic latitude unidentified EGRET sources will also be discussed.

February 6
Prof. Christopher Kochanek
Harvard University

Title: Rebuilding the Cepheid Distance Scale

Cepheid variables are the keystone for much of the extragalactic distance scale. The advent of HST has lead to an enormous increase in the number of extragalactic Cepheids, but the interpretation of the data has not kept pace. The standard analyses gloss over several known physical effects such as differential extinction, the positivity of the extinction, temperature distributions at fixed period, and metallicity. We examine the consequences of these effects on the Cepheid distance scale and the Hubble constant, and discuss how to control and reduce the remaining systematic errors.

February 13
Dr. Fabrizio Fiore
Osservatorio Astronomico di Roma
Monteporzio, Italy

Title: Observations with the BeppoSAX Satellite

The X-ray astronomy satellite BeppoSAX (Satellite per Astronomia X "Beppo" in honor of Giuseppe Occhialini) was launched on April 30 1996, and since the end of June 1996 it is performing observations of cosmic X-ray sources. In the framework of past and future X-ray missions BeppoSAX stands out for its wide spectral coverage, covering more than three decades of energy (from 0.1 to over 200 keV) with relatively large collecting area, good energy resolution and imaging capabilities (below 10 keV). The sensitivity of the scientific payload allows the detailed study over the entire energy band of sources as weak as about 1/10 of 3C273.

BeppoSAX comes at a peculiar time, after missions like Einstein, EXOSAT, ROSAT and ASCA, which established X-ray astronomy as a discipline accessible to the whole astronomical community, and just before large next generation missions like AXAF and XMM. BeppoSAX data anticipate some of the problems and challanges that X-ray observers and data-centers will face with AXAF and XMM. The current BeppoSAX experience can then be viewed also as a laboratory to propose and test new ideas and services, and can be used to prepare astronomers for these upcoming missions.

I will present preliminary results from observations performed in the Science Verification Phase and as part of the BeppoSAX AO1. I will mainly foucus my attention on Active Galactic Nuclei observations.

February 20
Dr. John Bally
Center for Astrophysics and Space Astronomy
University of Colorado
Boulder, Colorado

Title: Parsec-Scale Herbig-Haro Outflows from Young Stars

Herbig-Haro (HH) objects are collisionally excited nebulae powered by outflows from young stars. They have become ``Rosetta stones'' in the study of astrophysical jets and in the investigation of star formation. Large format CCDs and IR arrays used on small telescopes are revolutionizing our understanding of how stars form, the outflows they produce, and the impact of these flows on the ISM. Many HH flows have punched out of their host molecular clouds and are exciting shocks many parsecs from their driving sources. HH objects provide a fossil record of recent YSO jet ejection velocity variations, may power the lower velocity bipolar molecular outflows, may be a major source of turbulence, chemical rejuvenation, and cloud disruption in the ISM, and an important agent in the regulation of the gravitational collapse of clouds.

February 27
Dr. Ronald Remillard
Center for Space Research
Massachusetts Institute of Technology

Title: Probing the Pandemonium: RXTE Observations of the Galactic Microquasars

The two galactic `microquasars' with superluminal radio jets have been quite active during 1996. New results are reviewed with emphasis on observations with the Rossi X-ray Timing Explorer. GRS1915+105 has displayed dramatic light curves and emission states unlike anything previously seen, while GROJ1655-40 has displayed canonical X-ray states. Both have shown QPOs (0.1 to 22 Hz) that have been linked to the origin of the X-ray power-law component. In addition, both sources exhibit transient yet stationary high-frequency QPOs that may eventually constrain the mass and rotation of the accreting black holes in these systems.

March 6
Dr. Hans-Walter Rix
Steward Observatory
Universityof Arizona
Tucson, Arizona

Title: Dark Matter in Elliptsicals at Small and at Very Small Radii

The observable kinematics of stars in galaxies depend both on the gravitational potential in which they orbit and on the distribution of orbital parameters. If the orbital `anisotropy' is unknown, a wide range of potentials is often compatible with the observed velocities and velocity dispersions. I will report on recent progress in measuring and modeling the detailed velocity distribution of stars in elliptical galaxies, which permits to constrain the orbital anisotropies observationally. By combining these new techniques with improved data, one can now measure robustly the masses of central black holes. It is now also possible to assess the role of dark matter in the inner parts of elliptical galaxies.

March 13
Dr. Charles Lada
Radio and Geoastronomy Division
Center for Astrophysics

Title: Deciphering the Nature of the Youngest Stellar Objects;
And the Need for an Infrared Spectrograph for the MMT

Stars are the fundamental objects of astronomy. They are responsible for transforming hydrogen, the primary product of the Big Bang, into the varied elements of the Periodic Table and in doing so they control the chemical evolution of the Cosmos. Stars mitigate the appearance and evolution of galaxies and, perhaps most significantly, are necessary for the existence of habitable planetary systems. Understanding the formation of stars has far-reaching astrophysical implications, yet the problem of star formation remains largely unsolved. However, significant inroads toward its solution and the development of a theory of star formation continue to be made. This is largely due to improving observational technology across thelectromagnetic spectrum but particularly at infrared, submillimeter and millimeter wavelengths. In this lecture I will discuss the status of our knowledge of the earliest phases stellar evolution and of the physical natures of young stellar objects, including those mysterious things called protostars. I will detail recent findings resulting from the application of infrared spectroscopy to investigation of the evolutionary states of young stellar objects. I will examine the link between accretion and the physical appearance of extremely young stars and will present observations of what appear to be the youngest objects yet found to exhibit features characteristic of stellar photospheres. If time perimts I will also summarize results from infrared imaging experiments which have enabled the investigation of the Initial Mass Function down to and below the hydrogen burning limit in nearby embedded protoclusters and will review the implication of these results for star and cluster formation.

March 20
Dr. Jane Charlton
Dept. of Astronomy and Astrophysics
Pennsylvania State University
University Park, PA 16802

Title: Quasar Absorption Lines: The Evolution of Galactic Gas
Over Cosmic Time

Abstract: A view of the formation and evolution of galaxies and structure over the whole history of the Universe requires observations both of stars and of gas. This talk will present an overview of progress toward a comprehensive picture of the formation and evolution of galaxies through quasar absorption line studies. The absorption profiles that are observed due to the passage of quasar light through a given structure are a convolution of several properties of the gas, including its spatial and kinematic distribution, its chemical composition, and its state of ionization. Illustrative models will be used to show how these various factors affect the appearance of synthetic spectra. Finally, the talk will review how ultraviolet, visible, and near-IR spectroscopy can combine to provide information on the variety of chemical and ionization species from the present back to the epochs of the most distant quasars.

March 27
Dr. Leo Blitz
Department of Astronomy
University of California, Berkeley
Berkeley, CA

Title: What the High Velocity Clouds Are

The nature of the High Velocity Clouds (HVCs), clouds of hydrogen gas that cannot be in circular rotation about the center of the Milky Way, is one of the oldest unsolved problems of the interstellar medium. There is not even a consensus whether the clouds are galactic or extragalactic. Using several recently completed surveys of neutral hydrogen, it will be shown that some of the HVCs must, however, be extragalactic. The kinematics, linewidths and angular sizes of the entire ensemble will be shown to be consistent with clouds being in orbit in the gravitational potential of the Local Group of galaxies. A simulation of the dynamical history of the cloud ensemble will be presented which reproduces both the spatial distribution and velocity distribution of the HVCs on the sky. The implication of this work is that the clouds are the detritus of the formation of the Local Group of Galaxies. The HVCs are also consistent with being local examples of the Lyman Alpha clouds seen in absorption toward distant galaxies and quasars.

April 3
Dr. Charles C. Steidel
Department of Astronomy
California Institute of Technology
Pasadena, CA

Title: Galaxies and Large Scale Structure at Redshift Z~3

I will discuss the current state of a large survey of star-forming galaxies at redshifts well beyond those of current field galaxy redshift surveys. The samples of z~3 galaxies, selected by the presence of continuum breaks at the rest--frame Lyman limit, are now large enough that we can begin to trace the large-scale distribution of galaxies at these early times, with interesting implications for the progress of the growth of structure and the cosmological world model. I will also discuss the implications of these techniques and other complementary techniques for the history of galaxies and star formation since z~4.

April 10
Dr. David DeVorkin
Curator of Astronomy
National Air and Space Museum
Smithsonian Institution
Washington, D.C.

Title: The Astrophysical Observatory of the Smithsonian Institution under Charles Greeley Abbot: A Time of Expansive Dreams and Promise.

Abbot was the second director (1906 - 1944) of the Astrophysical Observatory of the Smithsonian Institution, succeeding its founder Samuel Pierpont Langley, and Abbot ultimately became the fifth secretary of the Smithsonian Institution (1928 - 1944). He was a deft instrument designer and observer, whose lifelong goal soon became the refinement of Langley's determination of the solar constant. His preoccupation became, again following Langley, confirming his conviction that the solar constant varied and influenced terrestrial weather. Following a well-established Smithsonian tradition, Abbot established observing stations at the four corners of the earth to keep constant watch on the sun. As we take a visual tour of the many exotic observing stations Abbot created during his tenure, we will review how the SAO prospered under Abbot at a time when the Smithsonian itself was undergoing retrenchment, and examine Abbot's tactics for keeping SAO alive.

April 17
Dr. Simon D.M. White
Max Planck Institut fur Astrophysik
Munich, Germany

Title: The Formation of Galaxies

A flood of data have recently been obtained on galaxies at high redshift. These come primarily from the Hubble Space Telescope and the Keck Observatory but also from extensive surveys on 4m telescopes. The population of normal bright galaxies has been surveyed to a redshift of one, while unobscured star-forming galaxies have been surveyed to redshift five. It is clear that the observed populations account for a significant fraction of the formation of all the stars in galaxies, and that most of this star formation occurs at redshifts of one or less. More than a decade of analytic and simulation work has produced a quantitative understanding of how nonlinear structures grow as collisionless dark matter clusters under the influence of gravity. This provides the basis for detailed modelling of galaxy formation in any of the currently popular Cold Dark Matter cosmogonies. Such models permit a calculation of the observable properties of the galaxy population (luminosities, colours morphologies, sizes, star formation rates, clustering, etc.) at all epochs, based on simple physical assumptions about the critical processes. Early versions of these models predicted many of the ``surprising'' properties of recent data well before the observations were taken. While this success must be in part fortuitous given the crudeness of the modelling, recent refinements reproduce in detail many aspects of the galaxy population both at high and at low redshift. Galaxy formation is a process, not an event, and it seems likely that most of the formation activity associated with all the major types of bright galaxy has already been directly observed.

April 24
Dr. Jonathan W. Arons
Astronomy Department
University of California

Title: Pulsar Death At An Advanced Age

I briefly review the phenomenology of rotation powered pulsars' (RPPs') photon emission, with emphasis on the evidence that the magnetic field has a locally dipolar form at low altitude. I use the observations to motivate the ``classical'' picture of pulsar radio radiation being a searchlight beam emitted by relativstic particles moving parallel to dipolar magnetic field lines. I outline the basic electrodynamics and particle acceleration characteristics of these stars' magnetic polar regions. I review the result that the general relativistic dragging of inertial frames increases the available accelerating voltage by roughly an order of magnitude, compared to previous estimates. This surprising result occurs because the polarization electric field caused by the accelerated particles cancels the vacuum electric field, to which the general relativistic effects are a small correction. I then review the association of pair creation by the gamma rays emitted by the accelerated charges with the occurrence of radio emission, as evidenced by whether or not the theory of pair creation encompasses all the known pulsars, with the conclusion that previous theories have associated pair creation with radio emission only through hypothesizing artificial complexity to the surface magnetic field which is not in accord with the evidence for the dipolar character of the low altitude magnetic field. I then show that mild offsets of the dipole's center from the stellar, which ARE consistent with the observations of pulse width and polarization structure, causes substantial general relativistic bending of gamma rays' orbits, which in turn increases the opacity for pair creation. This effect, combined with the general relativistically induced increase in the accelerating potential, allows polar cap pair creation theory to cover all the known pulsars for the first time.

May 1
Dr. Bohdan Paczynski
Princeton University Observatory
Princeton, NJ

Title: Present and Future of Microlensing Searches

At least three groups of observers: DUO, MACHO, and OGLE, detected dozens of gravitational microlensing events monitoring millions of stars in the Galactic Bulge and in the Large Magellanic Cloud. The reality of detection is beyond any doubt, with the most spectacular case provided by the MACHO 95-30 event. Preliminary estimates of the optical depth, which is proportional to the probability of microlensing, towards the Galactic Bulge and the LMC are surprizingly high, possibly indicating some unrecognized systematic problem with the calibration of the detection efficiency. A variety of simple theoretical predictions was verified observationally. These include: binary lenses with caustic crossings, resolution of sources in the very high magnification events, the effect of Earth orbital motion, chromaticity as a consequence of image blending, The complexity of the problem became apparent, and the next challenge is to make the quantitative analysis of microlensing results more reliable. In a few years the data rate generated by various observing projects will increase substantially, and the rate of detection of new microlensing events will increase accordingly. This will be accompanied with the corresponding improvements of software, and will lead to robust results for the mass distribution and space distribution of the lensing objects, and in the long run will lead to detection of earth-mass planets, and will provide the answer to the important question: are there dark objects among the lenses.

May 8
Dr. Arthur M. Wolfe
University of California at San Diego
La Jolla, California

Title: On the Nature of the Damped Lyman Alpha Protogalaxies

The damped Lyman alpha absorption systems are a class of highly redshifted layers of neutral gas with properties indicating they are the progenitors of current galactic disks. I will discuss recent evidence obtained with the Keck 10 m telescope that both supports this hypothesis and adds new insights into the evolution of protogalactic disks. Specifically, I will discuss evidence that the damped systems formed at z > 4, and that significant gas consumption by star formation did not occur until z < 1.8.

However, most of my talk will focus on kinematics. Kinematic information is crucial, since velocity fields in protogalaxies carry important information about the process of galaxy formation. In particular, the velocity structure of protogalactic gas is a sensitive discriminator among competing theories of galaxy formation. Using the HIRES echelle spectrograph on the keck telescope we have obtained absorption line profiles for weak low-ions in 29 damped systems. The velocity profiles (1) comprise multiple narrow components, (2) are asymmetric in that the component with the strongest absorption tends to be at one edge of the profile, and (3) the velocity intervals over which absorption occurs are nearly uniformly distributed between 20 and 200 km/s. By constructing statistical measures characterizing the velocity structure we show the asymmetry rules out models dominated by random motions or spherically symmetric radial motions. Rather the asymmetry is naturally explained by the rotation of ``cold'' gaseous disks with significant vertical scale heights and rotation speeds v_{rot} = 225 km/s. Significantly, hierarchical models for galaxy formation predict the progenitors of current spirals to be low mass subunits with v_{rot} < 100 km/s. Such models are ruled out by the kinematic data at high confidence levels. We combine the kinematic data with length scales recently inferred for a damped system detected in emission to obtain mass estimates exceeding 10^{11} solar masses; i.e., the current masses of spirals may have been in place since z > 3.

Finally, we discuss some dilemmas stimulated by this research. In particular the metallicities and abundance patterns found in damped systems bring to mind population II spheroids dominated by random motions rather than the systematic motions dominating rotating disks.

May 15
Dr. Robert E. Williams
Space Telescope Science Institute
Baltimore, Maryland

Abstract: Some of the recent results from HST will be discussed, including CM diagrams of extragalactic clusters, inhomogeneities in nebulae, and the results that are emerging from the early studies of the Hubble Deep Field.

May 22
Dr. Mitchell Begelman
JILA, Universityo f Colorado
Boulder, Colorado

Title: The Emancipation of Slaved Accretion Disks: Why Disks Warp and Precess

Accretion disks are frequently misaligned with respect to their apparent sources of angular momentum. X-ray binaries like SS 433 and Hercules X-1 have tilted, precessing disks; water maser observations of AGNs like NGC 4258 and NGC 1068 reveal warped disks in profile.

Previous models of warped and precessing disks have proposed that the disks are "slaved" to boundary conditions, or are responding to gravitational torques. But these beg the question of why disks become misaligned in the first place.

I will discuss a new instability (discovered by Pringle) that may solve this longstanding problem. Radiation from the nucleus is reprocessed nonaxisymmetrically by a slightly warped disk. The pattern of radiation pressure (and/or radiative heating) leads to torques that can enhance the warp and drive precession. Thus, disks apparently become warped spontaneously.

I will review the theoretical progress we have made in understanding this instability and in applying it to X-ray binaries and AGNs.

May 29
Bok Prize Lecture

Cfa Colloquium organizers for 1995-1997 are Martin Elvis and Bob Noyes.

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