List of Past Astronomy Colloquia : 01-Jan-1996 to 01-Jun-1996


Date:   Thursday 1-Feb-96 **Note special date**
Speaker:   Pete Schloerb (FCRAO/UMass)
Title:  Optical Interferometry with the IOTA

The largest earth based telescopes and the Hubble Space Telescope are remarkable instruments which can resolve features with an angular size scale measured in tenths of an arc second. However, since almost all stars in the sky are much smaller than this, even these amazing (and expensive) devices can only view them as points of light. To actually resolve a typical star and obtain an image of it requires either a telescope with an aperture of hundreds of meters or a new approach to the problem. In this talk, we discuss such a new approach, the use of interferometry at optical and infrared wavelengths to achieve milli-arcsecond resolution on stars and stellar systems.

The interferometric approach was exploited early in this century by Michelson to measure the diameter of one of the largest stars in the sky. It then languished for over 50 years waiting for technology to catch up to Michelson's original idea. Today, there are several projects around the world attempting to build interferometers for optical work, and our group at UMass has been participating in one such project: the Infrared-Optical Telescope Array (IOTA) project. IOTA has been completed initially with two telescopes for infrared and visible wavelength observations and is just now making its first scientific observations. I will describe how IOTA works and share some of our initial results and future plans.


Date:   Wednesday 7-Feb-96
Speaker:   Bill Langer (JPL)
Title:  How Do Protostars Form?

Abstract not yet available.


Date:   Wednesday 14-Feb-96
Speaker:   Eli Dwek (GSFC)
Title:  In Pursuit of the Cosmic Infrared Background

The cosmic infrared background (CIB) consists of the cumulative emissions from pregalactic objects, protogalaxies, and evolving galaxies throughout the historyof the universe. The light from these objects is partially redshifted and partially absorbed and reradiated by dust into the infrared (IR) wavelength region. The Diffuse Infrared Background Experiment (DIRBE) on board the Cosmic Background Explorer (COBE*) satellite is the first space experiment designed to conduct a systematic search for the CIB in the 1.25 to 240 micron region.

The detection of the CIB is the most challenging of the three experiment on the COBE satellite. Its signal is hidden behind a veil of infrared emission from interplanetary dust, and from galactic stellar and dust emission. Peeling off these layers of foreground emissions, while leaving the cosmic signal unscathed is a formidable task.

In this talk I will describe the efforts of the DIRBE team to unveil the CIB. Additional clues to the energy density of the CIB is provided from the observations of TeV-gamma rays from Mrk421, and I will describe our analysis of its spectrum, and the resulting limits we derived on the CIB intensity. Finally, I will describe the theoretical efforts underway for modeling the IR emission from evolving galaxies, in order to estimate their contribution to the CIB. --------------------------------------------------------------------------- * COBE is supported by NASA's Astrophysics Division. Goddard Space Flight Center (GSFC), under the scientific guidance of the COBE Science Working Group, is responsible for the development and operation of COBE.


Date:   Wednesday 21-Feb-96
Speaker:   Fred Lo (U. Illinois)
Title:  Circumnuclear Disk and Jet Confinement in NGC 4258

Abstract not yet available.


Date:   Wednesday 28-Feb-96
Speaker:   Neal Evans (U. Texas)
Title:  The Quest for Collapsing Protostars

Theoreticians assure us that stars form in a molecular cloud through a process of gravitational collapse. Observers, however, have had a very difficult time finding hard evidence for this process. Instead, outflow motions are the most evident kinematic signature of star formation. I will review recent progress in identifying candidates for collapsing protostars. One approach is to use the broad-band spectral energy distributions to infer the youth of an object, and I will consider recent developments in this area. A more conclusive method is to look for specific spectral signatures of collapse in molecular lines. The best candidates identified on the basis of these considerations will be discussed, with a view toward identifying the areas where further progress can be made.


Date:   Friday 1-Mar-96 **NOTE SPECIAL DATE**
Speaker:   Felix Mirabel (CESR)
Title:  Superluminal Sources in the Galaxy

I will review the recent discovery of galactic sources of relativistic jets with apparent motions greater than the speed of light. The study of these `microquasars' has an impact on: 1) the arguments against an expanding Universe, 2) the physical interpretation of superluminal motions in quasars, 3) the models of gamma-ray bursts, and 4) a new method to determine distances in astronomy.


Date:   Wednesday 6-Mar-96
Speaker:   Reuven Ramaty (NASA/GSFC)
Title:  Low Energy Accelerated Particle Interactions in Astrophysics and Solar Physics

I shall discuss the role of accelerated particles of energies from a few to tens of MeV/nucleon in Astrophysics and Solar Physics. Such particles produce gamma ray lines which have been observed from many solar flares, and recently also from the Orion star formation region. While other processes, e.g. nucleosynthesis, can produce a variety of gamma ray lines, there are some lines, such as the 4.44 MeV line of C-12, which can only be produced by accelerated particle interactions. Low energy accelerated particles may also have a very important, and perhaps even dominant role in the production of the Galactic inventories of Li-6, Be and B. These light elements can only be produced in spallation reactions of heavier nuclei (C, N, O) or in interactions amongst He nuclei. The current debate centers around the question of whether the the bulk of the production is due to the standard Galactic cosmic rays or the new low energy component implied by the Orion observations. Finally, low energy accelerated particles may also play a role in the early solar system, where the now extinct radioisotopes Ca-41, Al-26 and Mn-53 may have had a spallogenic origin. I will also try to discuss the origin of the accelerated particles at the various site. For example, in solar flares stochastic acceleration due to gyroresonant interactions with plasma waves seems to be the dominant process. In Orion, on the other hand, shock acceleration is favored.


Date:   Tuesday 12-Mar-96 **NOTE SPECIAL DATE**
Speaker:   Jim Peebles (Princeton)
Title:  Is the Universe Open or Closed? A Survey of Opinions and the Evidence

Opinions on whether the mass of the universe is large enough to be capable of reversing the present general expansion have been driven by theory rather than the empirical evidence, but that is changing. The observational indications still are mixed but improving at an impressive rate; a convincing test may be within reach.


Date:   Wednesday 13-Mar-96
Speaker:   Tom Wilson (MPI-Bonn)
Title:  The Search for `Protostars' in NGC 2024

The structure of molecular clouds, and classification of such clouds are briefly discussed. The searches for collapsing molecular clouds which will form stars is reviewed. As one example, the results of a number of measurements with the BIMA array, the IRAM 30 meter telescope and the IRAM interferometer are presented for the NGC 2024 region, where molecular hydrogen densities of order 10E8 per cubic centimeter are thought to be present. The physics and chemistry of such high density regions is discussed in the context of star formation.


Date:   Wednesday 27-Mar-96
Speaker:   David Sanders (U. Hawaii)
Title:  Luminous Infrared Galaxies

At luminosities above 10^11 Lsun, infrared galaxies become the dominant population of extragalactic objects in the local Universe (z < 0.5), being more numerous than optically selected starburst and Seyfert galaxies at comparable bolometric luminosity, and as much as 3-5 times more numerous than quasi-stellar objects at the highest luminosities. Multiwavelength observations of luminous infrared galaxies clearly show that the trigger for the intense infrared emission is the strong interaction/merger of molecular gas-rich spirals, and that the bulk of the infrared luminosity for all but the most luminous objects is due to dust heating from an intense starburst within giant molecular clouds. At the highest luminosities (Lir > 10^12 Lsun), nearly all objects appear to be advanced mergers that are powered by a mixture of circumnuclear starburst and active galactic nucleus energy sources, both of which are fueled by an enormous concentration of molecular gas that has been funneled into the merger nucleus. I will review the evidence that ultraluminous infrared galaxies represent an important transition stage in the formation of QSOs and powerful radio galaxies, and that the intense circumnuclear starburst that accompanies the luminous infrared phase represents a primary stage in the formation of elliptical galaxy cores, the formation of globular clusters, and the metal enrichment of the intergalactic medium by gas and dust expelled from the nucleus due to the combined forces of supernova explosions and powerful stellar winds.


Date:   Wednesday 3-Apr-96
Speaker:   Tim Heckman (JHU)
Title:  Starbursts and Cosmogony

Starbursts are energetically significant phenomena in the local universe. They are interesting objects in their own right, but are even more important from a broader perspective. They offer the opportunity to test current ideas about star-formation and the interstellar medium under extreme conditions and provide a unique test-bed for the evolution of high-mass stars over a broad range in metallicity. They also provide us with local laboratories for the investigation of the processes that are believed to be important in the formation and early evolution of galaxies and in the heating and chemical enrichment of the inter-galactic medium.


Date:   Monday 8-Apr-96 **Note special date**
Speaker:   Andrew Harris (UMass)
Title:  Submillimeter line observations of galactic nuclei

Submillimeter molecular lines are selective probes of the warm and dense molecular gas in galactic nuclei. This nuclear gas, heated and compressed by gravitational forces and winds and radiation from massive stars, has densities and temperatures well above those typical of molecular clouds in galactic disks. Submillimeter lines are particularly suitable tracers of the molecular material in active regions, as the lines are relatively rapid transitions between energy levels of order 100 K above ground. In addition, the comparatively quiescent galactic disk material is not significantly excited in submillimeter transitions, permitting an uncluttered view of nuclei.

Establishing accurate values for gas temperature, particle density, column density, physical distribution, and composition are necessary to make good estimates of nuclear gas masses, star formation efficiencies, and physical and chemical evolution. In this talk I will review recent submillimeter observations that establish gas physical conditions within the nuclei of external galaxies and in our own Galactic center. I will also describe a new measurement of the very low [12CO]/[13CO] abundance ratio in the Galactic center's circum-nuclear disk (galactocentric radius ~1.5 pc) and discuss the implications for the evolutionary path within the very center of the Galaxy.


Date:   Wednesday 10-Apr-96
Speaker:   Tom Megeath (MIT Haystack Observatory)
Title:  On The Evolution of Giant Molecular Clouds: Multiwavelength Studies of Dense Cores and Stellar Populations

I address two key problem in understanding the evolution of giant molecular clouds (GMCs): 1.) what are the properties of molecular clouds before the onset of OB star formation? 2.) what effect OB stars have on star formation in nearby molecular clouds?

These problems are approached through multiwavelength observations of three molecular clouds. Two of the clouds, the NGC 281 West and DR22 molecular clouds, are clearly evolved and are associated with young OB associations. The third, the Maddalena Thaddeus cloud, may be a rare example of a massive giant molecular cloud before the onset of massive star formation. Using a combination of near--infrared imaging, CO and CS mapping, and VLA continuum imaging, we will discuss the 1.) the distribution of dense gas, 2.) the number and distribution of embedded stars, 3.) and the detection of associated ionized gas.

For the NGC 281 West and DR 22 molecular clouds, we find evidence that the dense gas is concentrated on the edge of the molecular cloud facing an OB association. In NGC 281, a cluster appears to be forming in the interface of the HII region and the molecular cloud. We argue that OB stars have a strong effect on the density structure and star formation in these molecular clouds.

The Maddalena Thaddeus cloud is an unusual GMC which shows no evidence of recent OB star formation. We have mapped the region of peak CO intensity in the CS (2-1) line from SEST telescope and detected a number of dense cores. The cores seem to be density peaks in clumps identified in previous 13CO observations by Williams et al. (1995). Although dense gas is present, NIR observations of the dense cores show little, if any, evidence for low mass star formation.

Finally, I present a brief comparison of the observed regions to other "well studied" regions such as Orion, Taurus, and rho Ophiucus in respect to their CS emission, IRAS emission, and the distribution of young stars.


Date:   Friday 12-Apr-96 **NOTE SPECIAL DATE**
Speaker:   Paul Butler (S.F. State University & U.C. Berkeley)
Title:  The Discovery of Extrasolar Planets: Present and Future

For the past 9 years we have monitored 120 stars for periodic Doppler shifts that would indicate the presence of an unseen planet. Observations from the first 7 years have a precision of 10 m/s. Improvements in the spectrometer have yielded a precision of 3 m/s over the last 2 years. In comparison, Jupiter causes the Sun to "wobble" with an amplitude of 13 m/s.

In October 1995 we confirmed the the orbital parameters for the planet around 51 Peg announced by Mayor & Queloz on October 6, 1995. The orbital period is 4.229 days, the minimum mass is 0.46 Jupiter masses, and the eccentricity is 0.0 +-0.01. We do not find evidence for the second companion to 51 Peg suggested by Mayor & Queloz. In January 1996 we announced the discovery of planets orbiting the solar-type stars 47 Ursa Majoris and 70 Virginis. These planets have minimum masses of 2.5 and 6.6 Jupiter masses, and orbit at a distance of 2.1 and 0.43 AU from their respective stars. The planet orbiting 70 Vir has a surprisingly large eccentricity of 0.40.

The discoveries to date suggest that approximately 3% of solar-type stars have companions of 2 to 10 Jupiter masses. In contrast, we find no objects of 10 to 80 Jupiter masses (brown dwarfs) in our sample. Our survey suffers from incompleteness for objects of less than 2 Jupiter masses, though there is a suggestion that such low mass planets are more common than the "Super-Jupiters". We continue to improve the technique. In July 1996 we will begin surveying 300 stars with the Keck telescope.

We expect to announced the discovery of more planets in the next few months.


Date:   Wednesday 17-Apr-96
Speaker:   David Neufeld (JHU/UMD)
Title:  The Chemistry of Oxygen Bearing Interstellar Molecules and The Submillimeter Wave Astronomy Satellite

Molecules are a ubiquitious constituent of the interstellar medium, and molecular line emissions are important not only as diagnostic probes but also as the dominant coolant of dense interstellar gas. Despite the fact that oxygen is the most abundant heavy element in the Universe, the chemistry of oxygen-bearing interstellar molecules is not well understood. While theoretical models suggest that water and molecular oxygen are likely to be significant reservoirs of interstellar oxygen, such models remain largely untested, in large part because of the difficulties inherent in observing water and molecular oxygen from the surface of a planet which has an atmosphere containing large concentrations of both species.

I will review the chemistry of oxygen-bearing interstellar molecules, and describe the Submillimeter Wave Astronomy Satellite (SWAS), a NASA Small Explorer Mission now scheduled for launch in Fall 1996. SWAS will carry submillimeter receivers capable of detecting line emissions from the species estimates of the water and molecular oxygen abundances in the interstellar gas.


Date:   Wednesday 24-Apr-96
Speaker:   Holland Ford (JHU)
Title:  HST Observations of Elliptical Galaxies

Hubble Space Telescope observations of the centers of elliptical galaxies reveal that there are two types of ellipticals, divided by luminosity and morphology. The first type, denoted Type I (or core galaxies), have luminosity profiles which can be fitted accurately with a double power law for radii smaller than ~1 Kpc, and are characterized by high luminosity, relatively low central surface brightness, and low ellipticity. Type II galaxies (power law galaxies) have power law brightness profiles that rise steeply into the very center, and are characterized by high central surface brightness, high ellipticity, and relatively low luminosity. The Type II galaxies often have a distinct, bright, thin (r < 25 pc) nuclear disk. There are no elliptical galaxies with isothermal cores. Dust is the rule rather than the exception in Virgo ellipticals. Elliptical galaxies often have gaseous disks, dusty disks being the most common. Recent COSTAR plus FOS observations show that the small nuclear disks in two luminous radio ellipticals, M87 and NGC 4261, are rotating around central masses greater than ~10^9 M_solar. The respective mass to light ratios are {M/L}_V ~3000 and ~2000, leading to the conclusion that these galaxies host massive black holes.


Date:   Wednesday 1-May-96
Speaker:   Al Cameron (Harvard University)
Title:  Extinct Radioactivities in the Primitive Solar Nebula and Triggered Star Formation

In the early solar nebula there were a number of relatively short-lived radioactivities that were incorporated into meteoritic material and then decayed away. Of particular importance was 41Ca, with a mean life of 150,000 years. There are several stellar sources that can make some of these, but the only one that makes all five of the shortest-lived radioactivities is the implosion type of supernova. New stars are commonly formed through the collapse of a molecular cloud core. The unassisted free fall collapse time of such a core is about 100,000 years or longer. Actual collapses are opposed by pressure gradients and are about an order of magnitude slower. Thus 41Ca can be live in the solar nebula only as a result of assisted collapse in which a shock wave, probably from the radioactivity-producing supernova, triggers the collapse of the core. I shall discuss some thermodynamic constraints on the collapse using as a toy model a uniform density sphere, and I shall show the results of simulations using smooth particle hydrodynamics. I shall also discuss the problem of injecting the radioactivities into the cloud core as it collapses.


Date:   Wednesday 8-May-96
Speaker:   Joe Burns (Cornell U.)
Title:  A Renaissance in Celestial Mechanics

When the space age began, the solar system was viewed as static. This talk will show why and how that view changed. Major advances have been made in understanding the solar system's structure and its cause owing to provocative data provided by spacecraft and ground-based observatories, improved and widely accessible computers that have allowed extraordinarily long orbit integrations, and new paradigms from non-linear dynamics. Precise tracking of spacecraft/celestial bodies has measured the solar system's dimensions and has significantly constrained geophysical parameters of various planets.

No longer is the solar system thought to be a fixed, deterministic entity. Dissipation (by tides, drags and anelasticity) is now known to have profoundly modified the solar system: rotations damp into pure spins while orbits evolve significantly, occasionally leading to the loss of past objects through collisions. Objects that drift into resonant orbits may become trapped at these positions, sometimes protecting themselves, but may also suffer substantial jumps in orbital eccentricity and inclinations, which allow orbits to cross at high speeds. Many orbits were--and are--chaotic. In total, dissipation, close-interactions and chaos have reconfigured the solar system, ejecting many objects to interstellar space, causing some to collide and transmuting yet others from "comets" to "asteroids".

Rotations have also evolved over the eons: Mercury is locked in a 3:2 spin-orbit resonance while Venus' spin does not appear to be tidally damped; Mars undergoes substantial obliquity oscillations that may have markedly influenced its climatic history; Saturn's satellite Hyperion tumbles chaotically and other irregular moons may have, with important thermal consequences. The rotations of asteroids are intimately tied to the collisional histories of the minor planets; some spin extraordinarily slowly, very few others wobble like comet Halley.

Planetary rings exhibit a bewildering array of dynamical phenomena: bending and spiral density waves at resonances; satellite perturbations, including shepherding and clumping; angular momentum transfer through collisions; spokes; and electromagnetic interactions.


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