A fully three-dimensional (3D) magnetohydrodynamical (MHD) model is applied to simulate the evolution of the large-scale magnetic field in cluster galaxies interacting with the intra-cluster medium (ICM). As the model input we use a time-dependent gas velocity fields resulting from 3D N-body sticky-particle simulations of a galaxy. The modeled clouds are affected by the ram pressure due to their rapid motion through the ICM in the central part of a cluster. Numerical simulations have shown that after the initial compression phase due to ram pressure, a process of gas re-accretion onto the galactic disk takes place. We find that the gas re-accretion leads to an increase of the total magnetic energy without any dynamo action. The simulated magnetic fields are used to construct the model maps of high-frequency (Faraday rotation-free) polarized radio emission. We show that the evolution of the polarized intensity shows features that are characteristic of different evolutionary stages of an ICM--ISM interaction. The comparison of polarized radio continuum emission maps with our model permits us to determine whether the galaxy is in the compression or in the re-accretion phase. It also provides an important constraint upon the dynamical modeling of ICM--ISM interactions.

I will discuss the results of our Gemini integral field spectroscopy using both CIRPASS in the near-infrared and GMOS in the optical. I will describe our demonstration science program, which highlights the many advantages of "3D" area spectroscopy over traditional techniques. I will show our first results on studies of the kinematics and star formation in z~1 galaxies, exploring galaxy evolution at these important epochs.

Jonathan Fortney (U Arizona)

Effects of Phase Separation on the Evolution of Giant Planets

We present the first models of Saturn and Jupiter to couple their evolution to both a radiative-atmosphere grid and to high-pressure phase diagrams of hydrogen with helium and other admixtures. The purpose of these models is to quantify the evolutionary effects of phase separation in the planets' deep interior. We find that prior calculated phase diagrams, in which Saturn's interior reaches a region of predicted He immiscibility, do not allow enough energy release to prolong Saturn's cooling to its known age and effective temperature. We explore modifications to published phase diagrams that would lead to greater energy release, and find a modified H-He phase diagram that is physically reasonable, leads to the correct extension of Saturn's cooling, and predicts an atmospheric He mass fraction Y_atmos = 0.185, in agreement with recent estimates. If the planet's current Y_atmos is greater than 0.21 an additional energy source other than He separation is needed to explain Saturn's current luminosity. We also explore the possibility of internal separation of elements heavier than He, and find that, alternatively, such separation could prolong Saturn's cooling under a realistic phase diagram and heavy element abundance. Incorporating He separation into Jupiter evolutionary models leads to a extension in cooling age that is inconsistent with the age of the solar system, although evidence indicates that He separation is now occurring in Jupiter's interior. A new measurement of Saturn's Y_atmos by Cassini will shed light on Saturn and Jupiter's interior processes. Future work on the coupled problem of the evolution of the planets will be discussed. Helium phase separation will have consequences for the luminosity and radii of extrasolar giant planets in the ~0.1 to 2.0 Jupiter mass range. The lowest mass giant planets, if far from their parent stars, will have luminosities ~2 times larger than homogeneous models currently predict. However, if these low mass planets are within ~5 AU of their parent stars, they will absorb enough stellar energy so that their cooling will be retarded, and their interiors will not be cold enough for He to become immiscible.

Monday, September 22nd, 3:30pm

Jack J. Lissauer (NASA Ames Research Center)

Terrestrial Planet Formation in Binary Star Systems

Most stars reside in multiple star systems; however, virtually all models of planetary growth have assumed an isolated single star. Numerical simulations of the collapse of molecular cloud cores to form binary stars suggest that disks will form within such systems. Observations indirectly suggest disk material around one or both components within young binary star systems. If planets form at the right places within such circumstellar disks, they can remain in stable orbits within the binary star systems for eons. We are simulating the late stages of growth of terrestrial planets within binary star systems, using a new, ultrafast, symplectic integrator that we have developed for this purpose. We show that the late stages of terrestrial planet formation can indeed take place in a wide variety of binary systems and we have begun to delineate the range of parameter space for which this statement is true. Results of our initial simulations of planetary growth around each star in the alpha Centauri system and other 'wide' binary systems, as well as around both stars in very close binary systems, will be presented.

Monday, October 20th, 2:30pm

Steve Soter

Early Bronze Age Helike: Discovery of a Prehistoric Coastal Town in Greece

The Corinthian Gulf is the most rapidly extending seismic rift zone in Europe. In 373 BC an earthquake and tsunami destroyed and submerged Helike, the principal city on its southwest shore. No traces of the city are now visible. We drilled 99 sediment cores on the Helike delta and located buried occupation horizons, dating from Roman to Neolithic times, at depths ranging from near the surface down to 15 m. We then opened test excavations which brought to light the foundations of Classical buildings 3 m below ground, destroyed by the historic earthquake. Microfauna in the overlying strata showed that the site was drowned by a lagoon and then silted over. Nearby we discovered an unexpected and uniquely well-preserved Early Bronze Age town, ca. 2400 BC, with cobbled streets and houses. The extensive site yielded a rich array of complete pottery vessels, some retaining their organic contents. The prehistoric town is buried under 3 to 5 m of dark anoxic clay containing marine and lagoonal microfauna. It was apparently abandoned due to sudden subsidence, like its Classical successor some 2000 years later. In July 2003 we ran a seismic reflection test which produced a striking 3-D image of the Early Bronze Age walls. We are developing a virtual model using the data from bore holes, geophysics and excavations to allow one to visualize the buried landscapes in this area.

Monday, October 27th, 3:30pm

Dimitar Sasselov (Harvard Center for Astrophysics)

OGLE-TR-56b: Small Orbit & Large Radius

Data obtained during the past summer provide accurate mass and radius for the transiting planet OGLE-TR-56b. The very small orbit and larger than expected planetary radius pose difficult questions for the theories of planet formation and thermal evolution.

Friday, October 31st, 12:00 noon

Christopher Deloye (U. C. Santa Barbara)

Low-Mass, Arbitrarily Degenerate White Dwarfs: Implications for Ultracompact Binaries

I'll present a set of new models for low-mass ($M < 0.1 M_{\odot}$) white dwarfs (WDs) with arbitrary core temperature and evolved (He or C/O) composition. In this mass range, the WD structure is influenced not only by the (semi-)degenerate electron contribution to the pressure, but by the ion ideal gas and the (negative) Coulomb contributions also. I'll show there is a continuously parameterized transition between the well known regimes of fully degenerate WDs, 'normal' stars, and Coulomb bound objects. In addition, the mass-radius relations for these objects are parameterized by both the WD mass and entropy. I'll discuss implications of this fact in the context of the ultracompact binaries, which have orbital periods $\lesssim 60-80$ min, focusing on the 3 recently discovered ultracompact accreting millisecond pulsars. From the orbital parameters, our models allow us to place constraints on the donor's composition and core temperature. Differences in donor composition and entropy affect the binary's evolution. In the case the donor responds adiabatically to mass loss, I'll highlight the impact of this through discussing the allowed ranges in mass transfer rates and the relative number distribution for these systems as a function of orbital period.

Monday, November 10th, 3:30pm

Chris Impey (U. Arizona)

Quasars as probes

This talk will cover the varied uses of quasars as cosmological probes. Unlike techniques that use galaxies as tracers, strong gravitational lensing and intervening absorption lines can be studied with nearly equal efficiency over the entire Hubble time. Strong lensing provides tests of the dark matter distribution from scales of galaxy nuclei to the scale of groups of galaxies. The lenses themselves give the unique opportunity to study the evolution of galaxies selected by mass. Well-constrained lens models offer direct tests of the parameters of the standard cosmological model, notably the Hubble constant. Quasar absorption lines can be used to trace the evolution of the IGM, using the Lyman-alpha forest, and the enrichment and clustering of galaxy halos, using metal lines. Paired and multiple quasar sightlines provide information on the topology of large scale structure and the spatial geometry of the world model.

Monday, November 17th, 3:30pm

Andrei Gruzinov (New York U.)

Dim Black Holes

Supermassive black holes are thought to exist in the centers of most galaxies. These black holes accrete gas, heat this gas up, and therefore should radiate. Yet we know that most of these black holes are not quasars at all, in fact they are very dim. I will discuss the physics of slow black hole accretion, which might explain the dimness of the supermassive black holes. I will also talk about astronomical tests of our physical picture.

Monday, November 24th, 3:30pm

Richard Larson (Yale U.)

The Role of Gas in the Merging of Black Holes in Galactic Nuclei

SPH simulations have been carried out to investigate the role of gas in driving the merging of massive black holes in galactic nuclei. During the early stages when the gravity of the gas dominates, a gravitational drag effect analogous to the dynamical friction of a stellar system operates, while during the later stages when the gravity of the black holes dominates, a decelerating torque on the black hole binary is exerted by a trailing ellipsoidal density enhancement in the gas. As a result of these effects, the separation of the black holes decays approximately exponentially with time, and in typical cases the black holes are predicted to merge within 10^7 years (the final stages of the merger being driven by gravitational radiation). These results support scenarios that postulate hierachical merging of central black holes associated with the merging of galaxies. Similar physical effects may play a role in the formation of close binary stars.

Monday, December 1st, 4:00pm

Marla Geha (Carnegie)

Internal Dynamics, Structure, and Formation of Dwarf Elliptical Galaxies

Dwarf elliptical galaxies (dEs) are the most common, yet least studied, galaxy type in the local Universe. Major-axis spectroscopy of dEs outside the Local Group have provided an unexpected picture of dE kinematics: while a small fraction of observed dE galaxies are rotationally flattened, the majority have little to no detectable rotation. These observations effectively rule out nearly all of the currently proposed models of dE galaxy formation. In this talk, I will summarize recent observational work and critically review models for the formation of dwarf elliptical galaxies.

Monday, December 8th, 3:30pm

Jeff Hester (Arizona State U.)

The Crab Nebula: The Gift that Keeps on Giving

There is no single object in all of astrophysics that has been more heavily studied than the Crab Nebula. Ask most any astronomer about the Crab, and you will likely get an immediate answer. Perhaps that answer will focus on the Crab pulsar -- the first pulsar to be detected at optical wavelengths, and the first and best observational link between pulsar birth and supernovae. Perhaps that answer will stress the historical nature of the Crab supernova, witnessed by Chinese astronomers in 1054 AD. Ask an optical or UV spectroscopist and you will hear about the chemically enriched ejecta from the explosion itself. An infrared astronomer might tell you about the peculiar IR excess from the object, and the possibility of dust emission. Ask a radio astronomer and the Crab will be painted as a plerion. Ask an X-ray astronomer and you will learn that the Crab is the flagship of pulsar wind nebulae. Everyone knows about the Crab. But when asked to describe the Crab more broadly than their area of specialization, most astronomers will probably fall back on the same basic description of the Crab that they heard in graduate school. The Crab is a free expansion supernova remnant with a pulsar and some synchrotron nebulosity thrown in for good measure. We now understand that description to be fundamentally wrong. In my talk I will discuss a wide range of observational and theoretical results on the Crab from the last few years, ranging from HST observations and MHD calculations of the filaments to STIS spectroscopy of the pulsar itself. Highlights will include recent HST and Chandra movies showing the remarkable dynamics of the inner synchrotron nebula, the only place in the sky where we can watch an ultrarelatistic magnetized pair plasma with enough spatial and temporal resolution to see how it behaves. (Bring popcorn.) Together, this work has answered many longstanding questions about the Crab, leading to a single unified description of the nebula that may be quite different from the one that you carry in your head.

Friday, December 12th, 12:00noon

Adam Burgasser (U. C. Los Angeles)

Dark Neighbors: Characterizing the Faintest Brown Dwarfs

Recent wide-field near-infrared surveys have uncovered a vast population of nearby faint brown dwarfs in the Solar Neighborhood. These objects span two new spectral classes - L dwarfs and T dwarfs - and extend to temperatures as low as 700 K. With over 300 of these ultracool dwarfs known, we are now beginning to explore the physical properties of these objects: temperature, gravity, metallicity, and atmospheric abundances. I will discuss some our efforts toward exploring the effects of these parameters on the observed spectra and photometry of cool brown dwarfs, and future methods of refining our understanding of these low mass, intrinsically faint objects.

Monday, December 15th, 12:00noon

Kelle Cruz (U. Pennsylvania)

The Luminosity Function of Ultracool Dwarfs

We have moved past the discovery phase of brown dwarfs and are now beginning to learn more about the fundamental properties of these very low-mass objects, including their luminosity and mass functions. To this end, we have compiled a statistically complete, volume-limited sample of M7-L8 dwarfs within 20 pc using the Two Micron All Sky Survey. I will describe the creation of the sample and our measured luminosity function. In addition, I will discuss how these results constrain the field mass function of brown dwarfs.

Monday, January 12th, 3:30pm

David Blank (University of Western Sydney)

A Radio Galaxy in a Spiral Host

The nearby (d=36 Mpc with h=0.5) Sa galaxy NGC 7213 has a a type 1.5 Seyfert nucleus that we have imaged in radio continuum with the Molonglo Observatory Synthesis telescope at 0.843 GHz, with the Australia Telescope Compact Array at 1.4 GHz, 2.5 GHz and HI spectral-line, and with VLBI at 8.6 GHz. NGC 7213 looks optically unremarkable, but shows in radio continuum the extended lobes (~100 Kpc across) typical of low-power active elipticals, but almost unheard of in spirals. Low-frequency variability suggests a compact core which has been confirmed by our VLBI observations. HI spectral-line observations show an HI component 200 Kpc across and a counterotating disk strongly suggesting a past merger. I will discuss the implications of our work. If time permits, I will also discuss my radio observations of the recently discovered epsilon Indi Bab, the nearest known brown dwarfs.

Tuesday, January 27th, 3:30pm

Don Neill (U. Columbia/AMNH)

Novae as Binary Population Tracers

Novae reach Mv of -10 and, as such, are the premier tracers of binary stars in external galaxies. Nova samples have been gathered for nearly a century, yet we are still unable to confirm the most basic predictions about novae from binary formation and evolution theory. The growing availability of 1m class, dedicated, or robotic observatories for comprehensive and densely time-sampled extragalactic nova surveys are finally providing the complete nova light curves, accurate spatial distributions, and luminosity specific nova rates needed to constrain the theory of nova formation and evolution. I will discuss the results from a recent survey of this kind and the impact of these data on binary star formation and evolution theory.

Monday, February 9th, 3:30pm

Jim Houck (Cornell)

Early Results from the Spitzer Space Telescope, z = 0 to 6.

The Spitzer was launched in late August 2003 and has been working very well since then. Results from the extragalactic program will be presented along with a little news from the Milky Way.

Monday, March 1st, 3:30pm

Adrian Melott (U. Kansas)

Did a Gamma-Ray Burst Initiate the Ordovician Extinction?

A GRB within our galaxy could have catastrophic consequences for the Earth. Extrapolations from the global rate suggest an average interval of 0.1 to 1 Gy for events in which the Earth is irradiated from a distance of a few kpc. Prompt emission would irradiate the surface with UV at least as intense as the present solar IR/visible/UV flux. The atmosphere would become heavily ionized, resulting in major destruction of the ozone layer. Both the prompt UV and that resulting from long-term loss of the ozone layer are destructive to living organisms. The attenuation length of UV in water is tens of meters. There is a strong candidate for a GRB based mass extinction in the late Ordovician, 440 My ago. Planktonic organisms and those animals living in shallow water seem to have been particularly hard hit during this mass extinction.

Monday, March 8th, 3:30pm

Markos Georganopoulos (GSFC)

Relativistic and slowing down: the flow in the hotspots of powerful radio galaxies and quasars

Pairs of radio emitting jets with lengths up to several hundred kiloparsecs emanate from the central region (the `core') of radio loud active galaxies. In the most powerful of them, these jets terminate in the `hotspots', compact high brightness regions, where the jet flow collides with the intergalactic medium (IGM). Although it has long been established that in their inner (parsec) regions these jet flows are relativistic, it is still not clear if they remain so at their largest (hundreds of kiloparsec) scales. We argue that the X-ray, optical and radio data of the hotspots, despite their at-first-sight disparate properties, can be unified in a scheme involving a relativistic flow upstream of the hotspot that decelerates to the sub-relativistic speed of its inferred advance through the IGM and viewed at different angles to its direction of motion. This scheme, besides providing an account of the hotspot spectral properties with jet orientation, it also suggests that the large-scale jets remain relativistic all the way to the hotspots.

Friday, March 12th, 12:00noon

Patrick Huggins (New York University)

Rings, Jets, Globules, and Other Features of the Red Giant-White Dwarf Transition

Understanding the transition from red giants to white dwarfs through the planetary nebula phase has undergone major changes in the last decade, partly due to high resolution optical imaging (especially with HST) and partly through observations at other wavelengths. This talk reviews the role of neutral gas in the transition, and its importance in the formation and evolution of planetary nebulae. Key features of planetary nebulae where neutral gas plays a role include: multiple rings, multiple jets, and globules.

Monday, March 22nd, 3:30pm

Fred C. Adams (University of Michigan)

Chaos in star formation and planet formation: Sensitive dependence on initial conditions and outcome distributions

This talk discusses the formation of molecular cloud cores through ambipolar diffusion, terrestrial planet formation, giant planet migration, planet scattering in the solar birth aggregate, and the long term stability of planets in binary systems. Chaos, including sensitive dependence on initial conditions, plays a crucial role in all of these problems. In particular, the results cannot be described by a single value - the answer is a distribution of possible outcomes.

Friday, April 9th, 12:00noon

Didier Saumon (LANL)

Partly cloudy weather on brown dwarfs

Brown dwarfs are the new kids on the block of stellar astrophysics and are remarkable hybrids of stars and planets. Their most striking planetary property is the presence of condensates and clouds in their atmospheres. The spectroscopic and photometric evidence for clouds for effective temperatures between 1800K and 1400K is very strong. This corresponds more or less to the full L spectral class. Even more interesting is rising evidence for patchy cloud cover around the transition between the L and T spectral classes. Models of brown dwarf atmospheres have reached the level where their observed characteristics in terms of effective temperature are fairly well understood. They also explain the non-LTE abundances of CO observed in some brown dwarfs and predict spectral features in the mid-IR that can be studied very well with the Spitzer Space Telescope.

Friday, April 23rd, 12:00noon

Iain Neill Reid (STScI/U.Penn)

Meeting the neighbours

The stars and brown dwarfs in the immediate vicinity of the Sun can provide crucial insight on the overall properties of the Galactic disk. Unfortunately, current studies are hampered by the fact that the best current census becomes woefully incomplete at distances of 10 parsecs or les for the lowest luminosity local inhabitants. In an effort to address this issue, I have been leading a project, under the aegis of the NASA/NSF NStars initiative, which aims to use data from the 2MASS near-infrared sky survey to fill in the missing objects out to a distance of 20 parsecs, concentrating particularly the ultracool late-M and L dwarfs which bridge the hydrogen burning limit. In this talk, I will summarise the progress so far achieved towards attaining our goal of a statistically complete census.

Monday, April 26th, 3:30pm

Ruth Daly (Penn. State U.)

A Direct Measurement of the Pressure and Energy Density of the Dark Energy as a Function of Redshift.

Understanding of the nature of dark energy, which appears to drive the expansion of the universe, is one of the central problems of physical cosmology today. A novel method to determine the expansion rate and the deceleration parameter in a largely model-independent way, directly from the data on coordinate distances, will be presented. The methodology is expanded to include measurements of the pressure of dark energy, its normalized energy density, and the equation of state parameter as functions of redshift. The methodology is applied to a new, combined data set of distances to supernovae and radio galaxies.

Monday, May 10th, 3:30pm

Dick McCray (JILA)

SN1987A: The Birth of a Supernova Remnant

During the first 10 years after its initial outburst, the radiation from SN1987A was dominated by energy deposited in the interior of the supernova debris by the decay of newly synthesized radioisotopes. Today, the blast wave from SN1987A is overtaking the inner circumstellar ring, resulting in the appearance of many optical ``hot spots'' on the ring seen in HST images and spectra. With Chandra and the Australia Compact Telescope Array, we are also observing rapidly brightening rings of X-ray and non-thermal radio emission, respectively, from the same interaction. This event marks the birth of the supernova remnant, SNR1987A, defined as the epoch when its light is dominated by the impact of the supernova debris with the circumstellar matter. The observations provide unique opportunities to understand the physics of the shock interaction and the structure of the supernova debris and the circumstellar matter.

Tuesday, May 18th, 3:30pm

Marc Freitag (Heidelberg, Germany)

The Stellar Connection: Forming Massive Black Holes in Dense Clusters

Many scenarios have been proposed for the formation of massive (MBH, >1e4 Msun) or intermediate-mass (IMBH, 100-1e4 Msun) black holes in stellar clusters, in particular (proto-)galactic nuclei, and their subsequent growth. The evolution of such clusters is driven by a potentially complex network of physical processes, including relaxation, stellar evolution, binary interactions, collisions and, possibly, interactions with a central black hole. Thank to Monte Carlo codes, we can follow the evolution of clusters in a variety of scenarios while considering most of these ingredients. In particular, we have explored the conditions under which a cluster of MS stars may undergo rapid core collapse due to mass segregation, thus entering a phase of run-away formation of a very massive star (VMS, M* > 1000Msun) through repeated collisions. Although collisional mass-loss is accounted for realistically, we find that a VMS forms even in proto-galactic nuclei models with a high velocity dispersion (many 100 km/s). Such a VMS may be a progenitor for an intermediate-mass black hole.

Friday, June 11th, 2:00pm

Dan Whalen (U. C. San Diego)

Early Cosmological Reionization 2004

The year-one WMAP satellite discovery of a large optical depth to electron scattering at redshifts greater than 10 surprised many in the cosmology community because it indicated that the universe began to be reionized much earlier than previously suspected. The observed dropoff in quasar numbers above redshifts of 4 implies that other UV sources such as protogalaxies or very massive Population III stars were the likely agents of early reionization. A core goal of numerical cosmology is to develop self-consistent computational models in which reionization properly unfolds over many redshifts and generations of ionizing sources. These models will soon be timely, given that upcoming 21 cm, NGST, and IR observations may soon be able to proble the degree of reionization as a function of redshift. I will survey the semianalytical and computational studies of early reionization done in the past year and discuss the physics we hope to incorporate in the next generation of cosmological reionization simulations.

Monday, June 14th, 3:30pm

Jenny Greene (Harvard CfA)

Intermediate-mass Black Holes in Active Galactic Nuclei

The recent discovery of a tight correlation between the stellar velocity dispersion of a galaxy bulge and the mass of the central black hole (BH) has provided unprecedented insight into the demographics of 106 - 109 solar mass BHs. Far less certain is the low-mass limit of the local BH mass function. Intermediate-mass BHs (with masses between 104 and 106 solar masses), of which only two are known in the literature, would be our best observational constraint on primordial seed BHs and are expected to contribute significantly to the integrated signal for future gravitational wave experiments. I will present our sample of 19 new intermediate-mass BHs culled from the Sloan Digital Sky Survey, and preliminary evidence that they follow the M-sigma relation.

Wednesday, June 16th, 3:30pm

Jill Knapp (Princeton)

SDSS II: Galactic Structure and the SDSS Legacy

The Sloan Digital Sky Survey is an imaging and spectroscopic survey designed to create an archive of five-band optical imaging to a depth of about 22 mag over 1/4 of the high-latitude sky and of redshifts of a million galaxies and 100,000 quasars selected from the imaging survey. The project will be about 70% complete in Summer 2005. Plans are underway to extend the operation of this still-unique facility for a further three years to carry out three interleaved large surveys: completion of the SDSS North Galactic Cap area, repeat imaging of the Celestial equator in the Southern Galactic Cap to search for supernovae in the range z = 0.2 to 0.4, and an imaging and spectroscopic survey of Galactic stars designed to disentangle the formation history of the Galaxy.

Friday, June 18th, 12:00pm

Sebastien Lepine (AMNH)

The new PMZF catalog of high proper motion stars: a fresh look at the Solar neighborhood.

The PMZF catalog is a new catalog of ~65,000 stars with large proper motions, and is the most complete and accurate compilation of high proper motion stars ever made for the northern sky. With several thousand new objects, it provides the most detailed look at the contents and distribution of stars in the neighborhood of the Sun. I will present and discuss some of the many interesting discoveries, including new very nearby stars, new white dwarfs, new halo subdwarfs, and new long-period binary systems.

Friday, June 25th, 12:00pm

Mordecai Mac Low (AMNH)

Cosmological Feedback from Dwarf Galaxies

Star formation in primordial galaxies will have strong effects on the surrounding intergalactic gas, perhaps preventing it from collapsing into galaxies by heating from ionization or turbulence. We have used high-resolution, two-dimensional gas dynamical models to predict the fraction of ionizing radiation from starbursts escaping from these galaxies, and the fraction of mass, metals, and kinetic energy escaping in the blowout of supernova-driven winds.

Friday, July 2nd, 2:00pm

Jeff Oishi, Ryan Joung (AMNH)

Modeling Protoplanetary Disks: Physics and Applications

The modeling of protoplanetary disks around T-Tauri stars allows us insight not only into these fascinating objects, but also into the early stages of our own solar system. We will discuss the use of spectral energy distributions (SEDs) to reveal aspects of disk structure and dust grain properties. These disks are thought to be magnetized, turbulent, and accreting material onto the central star. We will review the magnetorotational instability, the most promising source of this turbulent accretion, and its behavior in low-ionization disks. Finally, we will apply these physical mechanisms to the generation of thin current sheets that may explain the formation of meteoritic chondrules.

Wednesday, July 21st, 3:30pm

Ralf Klassen (AIP)

Gravoturbulent Star Formation

Stars form by gravoturbulent fragmentation of interstellar gas clouds. The supersonic turbulence ubiquitously observed in molecular gas generates strong density fluctuations with gravity taking over in the densest and most massive regions. Once gas clumps become gravitationally unstable, collapse sets in and the central density increases until a protostellar object forms and grows in mass via accretion from the infalling envelope. Turbulence plays a dual role. On global scales it provides support, while at the same time it can promote local collapse. Stellar birth is thus intimately linked to the dynamical behavior of parental gas cloud, which determines when and where protostellar cores form, and how they contract and grow in mass via accretion from the surrounding cloud material to build up stars. Slow, inefficient, isolated star formation is a hallmark of turbulent support, whereas fast, efficient, clustered star formation occurs in its absence. I will review the current progress in star formation theory and discuss results from numerical calculations of gravoturbulent cloud fragmentation. Special emphasis lies on the complex dynamical evolution of nascent star clusters, on the mass growth history of individual protostars, and on the resulting mass spectrum of stars, the IMF. The equation of state (EOS) plays a pivotal role in the fragmentation process. Under typical cloud conditions, massive stars form as part of dense clusters. However, for gas with effective polytropic index greater than unity star formation becomes biased towards isolated massive stars, which may be of relevance for understanding Pop III stars.

Friday, July 23rd, 12:00noon

Yuexing Li (Columbia/AMNH)

Star Formation in Galaxies

I will present high-resolution simulations of star formation in single disk galaxies and galaxy mergers. Our models of single galaxies quantitatively reproduce not only observed global and local Schmidt laws, but also observed star formation thresholds in disk galaxies, suggesting that the dominant physical mechanism determining the star formation rate is just the strength of gravitational instability. Galaxy mergers show significant starbursts, a lot of massive star clusters form in the tidal tails and bridges of the merging galaxies, they are identified as globular clusters. Our results show that the high specific frequency and bimodal distribution of metallicity observed in elliptical galaxies are natural products of gas-rich mergers, supporting a merger origin for the ellipticals and their globular cluster systems.

Tuesday, July 27th, 3:30pm

R. Ben Metcalf (U.C. Santa Cruz)

Small-Scale Structure, Missing Galaxies and Gravitational Lensing

I will review recent work, both theoretical and observational, on measuring the abundance of small scale structures in the dark matter distribution using gravitational lensing. The cold dark matter model predicts that there is a large number of invisible clumps of dark matter in galactic halos and intergalactic space. Strong evidence for these structures and constraints on their abundance and size are coming from the new method of spectroscopic lensing.

Friday, July 30th, 12:00noon

Kenji Bekki

Origin of ultra-compact dwarf galaxies.

A recent all-object spectroscopic survey centered on the Fornax cluster of galaxies, has discovered a population of sub-luminous and extremely compact members, called ``ultra-compact dwarf'' (UCD) galaxies. In order to clarify the origin of these objects, we have used self-consistent numerical simulations to study the dynamical evolution a nucleated dwarf galaxy would undergo if orbiting the center of the Fornax cluster and suffering from its strong tidal gravitational field. We find that the outer stellar components of a nucleated dwarf are removed by the strong tidal field of the cluster, whereas the nucleus manages to survive as a result of its initially compact nature. The developed naked nucleus is found to have physical properties (e.g., size and mass) similar to those observed for UCDs. We discuss these results in terms of (1) the physical relationships between nuclear star clusters discovered in late-type spirals and UCDs and (2) formation processes of stellar galactic nuclei.

Monday, August 23rd, 3:30pm

John Thorstensen (Dartmouth College)

Measuring Parallaxes from the Ground in the 21st Century

I will describe the CCD parallax program I have been running on the MDM 2.4m telescope, which is aimed at improving the distance scale for cataclysmic binaries and related objects. Some determinations are now complete, and in favorable cases the parallaxes are determined with precisions below 1 mas. Much of the talk will be devoted to the technicalities of how this is done. I will also describe a Bayesian technique which combines parallaxes with other information ("priors") to yield best-guess distances. If there is time I will discuss some scientific findings, but the main message is that with some luck one can obtain quite good parallaxes with general-purpose equipment.

Tuesday, September 28th, 3:30pm

Howard Bond (STScI)

Reflected Glory: Hubble Observations of the Light Echo Around V838 Monocerotis

The outburst of the unusual variable star V838 Monocerotis is being accompanied by the most spectacular display of light echoes in the history of astronomy. We have imaged the echoes on 5 occasions in 2002 with the Hubble Space Telescope and its Advanced Camera for Surveys. Analysis of the angular expansion rates and polarimetry data yields a 3D map of the dust and a direct geometric distance (6 kpc), and establishes that the star was extremely luminous at the maximum of its outburst. Unlike a classical nova, V838 Mon has remained a very cool red supergiant, and is now producing copious amounts of dust. The dust illuminated in the echoes was presumably ejected during previous similar outbursts. This event thus bears some similarities to the proto-PN ejection process from more normal AGB and post-AGB stars.

Wednesday, September 29th, 12:00noon

Howard Bond (STScI)

The Progenitor of SN 2004dj in NGC 2403

Monday, October 4th, 3:30pm

Dina Prialnik (T.A.U.)

The Evolution and activity of Comets

The structure of a comet nucleus may be modeled as a highly porous agglomeration of grains made of water ice, dust, and volatiles ices, which are mixed with the water ice or trapped within the amorphous ice matrix. While water vapor is released by sublimation at the surface, the occluded gas is released in the interior, upon crystallization of the amorphous ice. Once gases are released, they move through the pores, dragging with them small dust particles that have detached from the solid matrix. Free gases build up internal pressures which may result in cracking of the porous matrix and outbursts of gas and dust. Accumulation of the large particles on the surface of the nucleus may lead to the formation of a sealing dust mantle that may partially (or fully) quench the comet's activity. These, and other processes are taken into account in models of the evolution and activity of comet nuclei. Many of the parameters are poorly known or uncertain. Nevertheless, it turns out that cometary activity patterns are not very sensistive to parameters, and thus definite predictions and conclusions may be drawn from model calculations.

Tuesday, October 12th, 12:00noon

Bob Barber (University College London)

Lifting the veil on cool stars - 620 million water lines

Water is the third most common molecule in the universe and its spectrum is particularly rich. H2O is the principal source of opacity in M, L and T dwarfs where it may account for in excess of 60% of absorption in the IR. Only a very small proportion of the lines in the water spectrum are known experimentally and consequently all model atmospheres and much spectroscopic work depend on line lists that have been generated ab initio using quantum mechanical modelling. None of the earlier line lists has been accurate enough to allow accurate modelling of stellar atmospheres. In addition, the new generation of IR telescopes will require a spectroscopically accurate line list. We have produced a new line list which contains approx. 620 million transitions (more than twice the number in earlier lists). Moreover, the BT1 list sets new standards in accuracy as the calculations utilise superior physics.

Monday, October 25th, 3:30pm

Jay Pasachoff (Williams College)

Solar Observations during Eclipses and from Space

I summarize recent expeditions to total solar eclipses to study the heating of the solar corona and to fill in gaps in coronal coverage from space satellites, as well as observations of spicules in the solar chromosphere with the New Swedish Solar Telescope on La Palma.

Thursday, October 28th, 3:30pm

Robert O'Dell (STScI)

The Structure and Evolution of the Nearest Planetary Nebula, the Helix.

The Helix Nebula is the closest bright Planetary Nebula and has been the object of an intensive investigation of its structure and evolution, using the Hubble Space Telescope and numerous groundbased optical and radio telescopes. I will describe the results of a paper now in press which presents a new 3-D model for the nebula and the formation and evolution of the knots which characterize it and other nearby planetary nebulae. I will explain that the nebula is actually a quadrapolar structure (two axes of symmetry) and the knots seem to be a natural product of an ionization front advancing into neutral material, a phenomenon which is probably occuring in most planetary nebulae. The implications of the two results will be discussed.

Friday, October 29th, 12:00noon

Alexander Schekochihin (Cambridge)

MHD Turbulence in Galaxies and Clusters

I will review the main points of our recent work on small-scale dynamo and MHD turbulence in media with large magnetic Prandtl number --- with application to interstellar and intracluster media in mind. The properties of the magnetic fields generated by small-scale dynamo, the saturation mechanism and the nature of the fully developed state of isotropic MHD turbulence will be discussed. The key idea is that interactions in isotropic MHD turbulence may be nonlocal in wavenumber space, so Kolmogorov-style dimensional theories based on the idea of energy cascade of Alfven-wave packets may not be applicable. I will also discuss briefly the modern state of observational evidence relevant to this subject. A detailed account of this work is contained in Schekochihin et al. 2004, ApJ 612, 276.

Monday, November 15th, 12:00 noon

Bjoern Voss (Kiel Univ.)

Analysis of a large set of high resolution white dwarf spectra

About 1000 white dwarf spectra had been obtained with the ESO VLT in the course of the "SPY" project (a search for radial velocity variations, PI R. Napiwotzki). From these spectra, atmospheric parameters are determined by fitting model atmospheres to the data, yielding the so far largest sample of white dwarfs with known atmosperic parameters that were derived from high resolution spectra. The mass distribution of DA and DB white dwarfs is determined from these data, and peculiar objects of different type, e.g. variable ZZ Ceti white dwarfs, are studied.

Monday, November 22nd, 3:30pm

Margaret Turnbull (Steward Observatory)

The Search for Habitable Worlds: Habstars, Earthshine, and the Terrestrial Planet Finder

NASA's coronagraphic version of the Terrestrial Planet Finder (TPF-C) is scheduled for launch in ~2014 to directly image and take spectra of terrestrial planets orbiting within the habitable zones of nearby stars. With these stated goals, there is clearly a built-in consideration of life and the hope of finding habitable worlds. But which stars should we, or can we, observe? With the capability to search only 35 to 165 stars, and a price tag of at least $1 billion, one of the most controversial problems that scientists and engineers are now facing is that of target selection. In the first part of this talk we will discuss the challenges, both scientific and instrumental, in picking target stars. On the science side of things, we will review the concept of "habstars," stars that could set up favorable conditions for life as we know it on Earth. On the engineering side of things, we will also look at TPF's likely limitations for planet detection. But how many habstars actually meet the engineering criteria? Can we create a list of stars that is both scientifically interesting and observationally feasible? TPF's second goal is to take spectra of any detected planets, and we'll consider some of the "biosignatures" that TPF hopes to detect. We will use a real spectrum of a living terrestrial planet (the Earthshine spectrum) to determine which biosignatures TPF will and will not be able to detect, and what wavelength ranges might be worth considering for TPF-C.

Tursday, December 9th, 12:00 noon

Gregory Shields (U. Texas)

Black Holes in Quasars

Quasars afford an opportunity to measure black hole masses at high redshift. This may shed light on the striking correlation between supermassive black holes and the bulge component of the host galaxy. Available evidence suggests that luminous quasars at redshift z ~ 2 show a black hole-bulge relationship similar to that observed in nearby galaxies and AGN. At still higher redshifts, there are hints that supermassive black holes may reside in small bulges.

Friday, December 10th, 12:00 noon

John Hibbard (NRAO)

Substructure in Tidal Tails

Friday, December 10th, 3:30 noon

Mark Krumholz (U.C.

Making High Mass Stars for Fun and Profit

Monday, December 13th, 3:30pm

Ed Guinan (Villanova U.)

Extragalactic Binaries Stars - Astrophyical Laboratories and Cosmic Distance Indicators

Thursday, December 16th, 12:00 noon

Erik Muller (Arecibo Observatory)

Large-Scale Features and Enigmas of the HI in the Magellanic Bridge.

The neutral hydrogen of the Magellanic Bridge has recently been observed to high sensitivity and high spatial resolution, using the ATCA and Parkes Telescopes. The profiles of the HI line emission from the Bridge are re-exaimined with this new dataset, and it is shown that the HI in the Bridge is organised most simply into two components. The origin of the bimodality appears to be a large, tidally distorted shell, or shell conglomeration in the SMC. However, there are substantial incompatabilities between the calculated shell ages and the theoretical age of the Bridge itself by at least an order of magnitude. The large-scale morphology of the HI of the Bridge seem to be in qualitative agreement with numerical simulations. Recent findings, derived from analyses of the spatial power spectrum of the HI, also appear to support the simulated predictions. Finally, a large, rim-brightened and roughly elliptical filament loop is found off the eastern edge of the SMC. The mechanism responsible for the formation of this feature is entirely unclear, and a number of candidate scenarios, including the standard stellar-wind, or SNe, as well as infalling HVC impacts are discussed. None of the tested scenarios appear to be suitably likely, or capable of supplying the necessary energy to generate this feature however.

Tuesday, December 28th, 3:30PM

Susan Kassin (Lick Observatory)

Dark Matter and Angular Momentum in Spiral Galaxies

Monday, January 31st, 3:30pm

Itzhak Goldman (Afeka College, Tel Aviv)

Turbulent Dynamics of the Interstellar Medium of the Small Magellanic Cloud

Monday, February 21st, 3:30pm

Rosalba Perna

Gamma-Ray Bursts in the Swift Era

The launch of the Swift satellite has marked the beginning of a new era in Gamma-Ray Burst (GRB) science. I will firstly review the current status of GRB observations, and our understanding of their progenitors for the class of long bursts. I will then discuss our expectations for how GRB data during the Swift era will improve our understanding of the properties of the progenitors of long GRBs, shed light on the nature of short bursts, and impact several areas of cosmology. In particular, I will discuss how GRBs can be used to trace the evolution of the mean density of the medium with redshift and the properties of dust in high-z galaxies. Dectection of GRBs at very high redshifts can help set constraints on the small-scale power spectrum of density fluctuations.

Monday, February 28th, 3:30pm

Mike Brown (Caltech)

Sedna and the Birth of the Solar System

The discovery last year of Sedna, the coldest, most distant object ever seen in the solar system, came as a surprise to astronomers. Not only is Sedna well beyond the previously charted edge of the solar system, but its extremely elongated 12,000-year orbit takes it out to the inner regions of the long-hypothesized Oort cloud. Searching to understand how Sedna arrived in its unusual location has led astronomers on a chase that leads back 4.5 billion years -- and to the realization that Sedna is likely one of the only remaining "fossil records" of the birth of our Solar System.

Friday, March 4th, 12:30pm

Scott Gaudi (Harvard-Smithsonian Center for Astrophysics)

Transit Searches for Extrasolar Planets: Properties, Pitfalls, Payoffs, and Promises

Over the next decade, searches for extrasolar planets using the transit method will likely prove invaluable in testing theories of the formation and evolution of planetary systems. I review the landscape of transit searches for extrasolar planets, highlighting some of their basic properties, primary obstacles, important results, and future prospects. The equations that describe the observables and detectability of planetary transits are deceptively simple. However, careful consideration of these equations can elucidate nearly all of the essential properties, requirements, and difficulties of transit searches. I demonstrate how such considerations, when combined with rudimentary knowledge of the properties of stars, can reveal important, mostly unappreciated aspects of two types of transit searches: deep searches in the Galactic field, and targeted searches toward simple stellar systems. I use these arguments to interpret the results of the field transit surveys by the OGLE collaboration, drawing important conclusions regarding the period distribution of close-in planets. I conclude by speculating on the future prospects for transit searches, in particular the search for Neptune-sized planets from the ground, and habitable terrestrial planets from space with the planned mission Kepler.

Monday, March 7th, 3:30pm

Peter Allen (U. Penn.)

Star Formation via the Little Guy: The Brown Dwarf Luminosity and Mass Functions

How do stars form? This is one of our basic questions about the Universe. One way in which we understand star formation is by looking at how efficiently the process works as a function of mass. This is reflected in the much touted Initial Mass Function (IMF). Only recently have we been able to study the IMF of the lowest mass products of star formation, brown dwarfs. I will discuss my thesis work to reconstruct the brown dwarf IMF. Since brown dwarfs lack a stable energy source like stars (hydrogen fusion), they evolve significantly over time. This evolution renders traditional means of studying stellar masses, empirical mass-luminosity relations, impossible. I have developed techniques to compensate for these difficulties. Using these methods, I place constraints on the mass distribution of brown dwarfs in the Solar Neighborhood and as binary companions. The impact these constraints have on current formation theories are discussed.

Tuesday, March 8th, 3:30pm

Kazuya Saigo (National Astronomical Observatory of Japan)

The Evolution of Rotating First Core with Mass Accretion

Monday, March 21st, 3:30pm

Salman Hameed (Smith College)

Star formation properties of early-type spiral galaxies

Our perception of early-type (Sa-Sab) spiral has changed considerably over the past few decades. Dominated by a large stellar bulge, early-type spirals have largely been considered as dormant galaxies with low gas content and little star formation. However, observational results from the last few years reveal them to be one of the most dynamic systems in the local universe. I will present results from an H-alpha imaging survey of 51 nearby early-type spirals that was conducted to charaterize star formation properties of these galaxies, and a followup HI mapping study to look for past interaction signatures.

Tuesday, March 29th, 2:30PM

Paul Butler (UC San-Francisco)

Extrasolar Planets

The extraordinary growth in our knowledge of planetary systems over the past decade has been driven by the discovery of the first 130 planetary systems orbiting nearby Sun-like stars. These discoveries have yielded an unending string of undreamed surprises including planets with orbital periods of 3 days, and giant planets in markedly eccentric orbits. We are now surveying all 2,000 Sun-like stars in our immediate Galactic neighborhood. Within the next 10 years we expect to have the first empirical estimate of the number of local planetary systems similar to our own. We already know that at least 10% of Sun-like stars have planets. With another decade of increasingly sensitive data we anticipate finding a many more planets, including smaller rocky planets. Existing planet hunting programs also provide a first reconnaissance for future space-based missions like the NASA Space Interferometry Mission that will launch in 2010 and search for earth-like planets. A more distant goal is the NASA Terrestrial Planet Finder, a space-based telescope capable of directly imaging planets.

Wednesday, March 30th, 12:00noon

Peter Goldreich (Caltech)

Planet Formation

I shall briefly review modern scenarios for planet formation. The growth of the smallest gravitationally active bodies, planetesimals, is mired in controversy. Orderly growth by the merging of planetesimals is followed by runaway accretion in which a small fraction of the bodies grow much larger than all the others. When these big bodies are sparse enough, runaway growth gives way to oligarchic growth during which the big bodies grow in lockstep maintaining similar masses and uniformly spaced orbits. As oligarchs grow, their orbital spacing increases and their number decreases. My focus will be on how oligarchy ends and what happens after it does. I will address three major questions regarding solar system planets. (1) What determined their number? (2) Why are their orbits nearly circular and coplanar? (3) How long did they take to form? Answers to these will be given in terms of: stability against large scale chaos, dynamical friction by small bodies, the accretion rate at the geometrical cross section in the inner planet region, and the ejection rate at the gravitationally enhanced cross section in the outer planet region.

Wednesday, March 30th, 2:30pm

Scott Tremaine (Princeton)

The nucleus of M31

I will review recent observations and theoretical modeling of the curious double nucleus of the Andromeda galaxy, M31, which contains one of the nearest and best-resolved stellar systems surrounding a massive black hole.