Berkeley Astronomy Department Colloquium

Fall 2011

 

All colloquia on Thursdays at 4 pm (except where noted)

Contact:  Geoff Bower (gbower@astro.berkeley.edu)

 

Date

Speaker

Title & Abstract

Host

Location

25 August

Everyone

1 minute colloquia

Bower

3 Leconte

1 September

Alex Hayes (UCB)

Joint Astro/EPS

A Guide to Lakefront Vacationing on Titan: Hydrocarbon Lakes and their Role in the Methane Cycle.

 

SaturnÕs moon Titan is the only extraterrestrial body currently known to support standing bodies of liquid on its surface and, along with Earth and Mars, is one of only three places which we know to posses or have possessed an active hydrologic cycle. Understanding the nature of TitanÕs hydrologic system teaches us about the history of volatile compounds across the solar system and helps define EarthÕs place within it. TitanÕs hydrologic cycle represents a simpler version of EarthÕs water cycle, lacking an ocean as a global sink of liquid and heat, and acts as a model for the evolution of planets around other stars, as its methane-based system may be more common than EarthÕs water-based system.

This seminar will review several studies related to TitanÕs hydrologic system, particularly focusing on the role of lacustrine deposits. Synthetic Aperture Radar (SAR) images of TitanÕs polar regions reveal hydrocarbon lakes with morphologies and scales similar to terrestrial counterparts.  We will review the geographic distribution of these features and model their interaction with a local porous regolith. There is a hemispherical asymmetry in lake distribution, potentially explained by an asymmetry in the intensity of TitanÕs seasons caused by SaturnÕs orbital parameters. The periodic evolution of these parameters provide a mechanism for transferring not only methane, but also less volatile species such as ethane, from pole to pole over tens of thousands of years.

Lake properties will be discussed through an examination of Ontario Lacus, the largest lake in TitanÕs southern pole. The complex dielectric properties of OntarioÕs near shore region are derived through a combined analysis of radar altimetry and backscatter. The analysis is performed within multiple regions around OntarioÕs shore, producing a near shore bathymetry map. The dielectric properties inferred at Ontario can be used to measure seasonal variations in lake depth. Shoreline recession at Ontario Lacus and smaller lakes which disappeared between repeat observations suggest a net loss between southern summer solstice and autumnal equinox. These observations demonstrate that TitanÕs surface plays an active role in its methane-cycle and can be used to describe the evolution of its hydrologic system.

 

 

de Pater

141 McCone

8

Reinout van Weeren (Leiden)

Radio relics: unique probes of galaxy cluster mergers

 

Radio relics are enormous Mpc-sized synchrotron emitting sources  found in the outskirts of merging galaxy clusters. It is thought that these so-called relics trace cluster merger shock waves. 

In this talk I will present the latest observational results on several unique new relics. The properties of these relics provide unprecedented insights into the physical properties of extremely dilute cosmic plasmas, magnetic fields, and particle acceleration in shocks.  Together with simulations of cluster mergers I will demonstrate how relics can be used to probe the intracluster medium at large distances from the cluster center and reconstruct the mass ratio, impact parameter and orientation, of cluster merger events.  In addition, I will present the first high angular resolution observations of several clusters below 50 MHz from the new LOFAR telescope.  LOFAR, the Low Frequency Radio Array, is a pan-European radio telescope that is currently being commissioned. With its enormous sensitivity at the lowest frequencies it will be the breakthrough instrument for observations of diffuse radio emission in clusters.        

Parsons

3 Leconte

15

Geoffrey Bower (UCB)

Here Be Dragons:  The Dynamic Radio Sky

 

Radio variability probes a wide range of astrophysical phenomena from the solar system to the early Universe including black holes, neutron stars, gravitational wave sources, and relativistic shocks from collapsing stars.  Radio follow-up of events discovered at optical, X-ray, and gamma-ray wavelengths demonstrates a rich phenomenology but we lack a systematic and sensitive view of radio variability.  Our efforts in recent years with the Very Large Array (VLA) and the Allen Telescope Array (ATA) have provided the beginnings of an unbiased exploration on time scales from milliseconds to decades.  Powered by development of unique digital instrumentation, new algorithms, and new survey strategies, our surveys have defined the shape of next generation radio telescopes and surveys.  In particular, these surveys are shaping our understanding of radio supernovae, tidal disruption events, and gravitational-wave sources.  Planned surveys with newly commissioned national facilities such as the Expanded Very Large Array (EVLA), the Australian SKA Pathfinder (ASKAP), and the South African MeerKAT telescopes coupled with the development of new instrumentation will extend our reach into new parameter space.

de Pater

3 Leconte

22

Sean Solomon (DTM)

Joint Astro/EPS

Exploring the Planet Mercury with the MESSENGER Spacecraft

 

One of EarthÕs closest planetary neighbors, Mercury remained comparatively unexplored for the more than three decades that followed the three flybys of the innermost planet by the Mariner 10 spacecraft in 1974–75. Mariner 10 imaged 45% of MercuryÕs surface at about 1 km/pixel average resolution, confirmed MercuryÕs anomalously high bulk density and implied large fractional core size, discovered MercuryÕs internal magnetic field, documented that H and He are present in the planetÕs tenuous exosphere, and made the first exploration of MercuryÕs magnetosphere and solar wind environment. Ground-based astronomers later reported Na, K, and Ca in MercuryÕs exosphere; the presence of deposits in the floors of polar craters having radar characteristics best matched by water ice; and strong evidence from the planetÕs forced libration amplitude that Mercury has a fluid outer core. Spacecraft exploration of Mercury resumed with the selection for flight, under NASAÕs Discovery Program, of the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission. Launched in 2004, MESSENGER flew by the innermost planet three times in 2008-2009 en route to becoming the first spacecraft to orbit Mercury in March of this year. MESSENGERÕs first chemical remote sensing measurements of MercuryÕs surface indicate that the planetÕs bulk silicate fraction differs from those of the other inner planets, with a low-Fe surface composition intermediate between basalts and ultramafic rocks and best matched among terrestrial rocks by komatiites. Moreover, surface materials are richer in the volatile constituents S and K than predicted by most planetary formation models. Global image mosaics and targeted high-resolution images (to resolutions of 10 m/pixel) reveal that Mercury experienced globally extensive volcanism, including large expanses of plains emplaced as flood lavas and widespread examples of pyroclastic deposits likely emplaced during explosive eruptions of volatile-bearing magmas. Bright deposits within impact craters host fresh-appearing, rimless depressions or hollows, often displaying high-reflectance interiors and halos and likely formed through processes involving the geologically recent loss of volatiles. The tectonic history of Mercury, although dominated by near-global contractional deformation as first seen by Mariner 10, is more complex than first appreciated, with numerous examples of extensional deformation that accompanied impact crater and basin modification. MercuryÕs magnetic field is dominantly dipolar, but the field is axially symmetric and equatorially asymmetric, a geometry that poses challenges to dynamo models for field generation. The interaction between the solar wind and MercuryÕs magnetosphere, among the most dynamic in the solar system, serves both to replenish the exosphere and space weather the planetÕs surface. Plasma ions of planetary origin are seen throughout the sampled volume of MercuryÕs magnetosphere, with maxima in heavy-ion fluxes in the planetÕs magnetic-cusp regions. Bursts of energetic electrons, seen at most local times, point to an efficient acceleration mechanism operating within MercuryÕs magnetosphere on a regular basis that produces electrons with energies up to hundreds of keV on timescales of seconds.

 

 

Manga/de Pater

141 McCone

29

Jason Dexter (UCB)

Theory, Simulation, and Observation: Piecing Together Black Hole Accretion

 

Simulations based on the magnetorotational instability provide a first principles physical description of black hole accretion flows, and are starting to be used to interpret observations of astrophysical sources. Most codes neglect radiation, but in low-luminosity systems observables can be calculated from simulations after the fact via ray tracing. I will describe time-dependent, general relativistic radiative models of Sagittarius A* and M87 constructed in this fashion and their comparison to event horizon scale resolution VLBI observations at millimeter wavelengths. I will also address the prospect of detecting the black hole shadow in these sources, which would constitute direct evidence for the existence of an event horizon. In luminous objects such as AGN and the thermal (high/soft) state of black hole X-ray binaries, the comparison between simulations and observations is still necessarily indirect. I will discuss some recent progress in understanding these systems both from radiation MHD simulations and multiwavelength quasar microlensing observations.

 

Quataert

3 Leconte

6 October

Risa Wechsler (KIPAC/Stanford)

The Galaxy-Halo Connection Across Mass and Time

 

Dark matter halos are the fundamental unit of structure formation and they provide the framework for our modern understanding of galaxy formation.  I will discuss new limits on the connection between galaxy properties and their dark matter hosts over a range of masses and redshifts.     In the context of a given cosmological model, I will show how the galaxy-halo relation can be tightly constrained at low redshift, and how the galaxy-halo connection can be used to infer the full star formation histories of galaxies.   This model, based on LCDM halos, is in excellent agreement with a wide range of data, including the statistics of Magellanic Cloud mass satellites around Milky Way mass hosts.  For the faintest dwarfs, there is still considerable degeneracy between the galaxy-halo connection and the properties of dark matter.

 

Blitz

3 Leconte

13

Rolf Buehler (Stanford)

Gamma-ray flares from the Crab Nebula: A New Astrophysical Puzzle

   

Recently the Large Area Telescope on board the Fermi Gamma-ray Space Telescope detected gamma-ray (photon energy >100 MeV) flares flare from the Crab Nebula. Two major outburst occurred on the 14th and 16th of April, making the Crab nebula the brightest source in the gamma-ray sky. These surprising observations implies that the electrons are being accelerated to ~PeV energies at the radiation reaction limit on hourly time scales. Standard diffusive shock acceleration cannot explain such an efficient and rapid acceleration. I will present the observations and discuss first consequences on our understanding of Pulsars and their Nebula's as extreme cosmic particle accelerators.

Arons

3 Leconte

20

Fabio Governato (UW)

Dark Matter Cores, Bulges and SMBHs: How baryon physics shape the properties of the central regions of galaxies

 

How do dark matter cores and bulgeless galaxies form? These common properties of galaxies have been hard to explain within the Cold Dark Matter scenario and have been often referred as the 'core catastrophe' and the angular momentum problem'.  I will propose and successfully test a complete physical model based on SNe and Super Massive Black Holes feedback and resulting rapid gas outflows that simultaneously adresses these fundamental problems.  I will present results from a new set of cosmological simulations to make predictions on the dark matter and baryon distribution in galaxies over a range of halo masses going from ultra faint dwarfs to spiral galaxies.

White

3 Leconte

27

Thomas Henning (MPIA)

Protoplanetary Disks: From Dust to Gas

 

Disks around young stars are prominent cirumstellar structures in which planetary systems form. Most of our knowledge of protoplanetary disks is based on sensitive dust continuum observations. I will shortly summarize what we know about such dusty disks before I will concentrate on the next frontier in disk research, the study of the molecular content of disks. Ground-based, Spitzer and Herschel infrared observations are providing important constraints on the abundance of water and more complex organic molecules in the planet-forming regions of disks. I will discuss these results together with millimetere interferometry data, especially from our Plateau de Bure "Chemistry in Disks" program, characterizing the outer regions of protoplanetary disks.

Kalas

3 Leconte

3 November

Marc Pinsonneault (OSU)

Abundances Without Spectroscopic: Seismology of the Sun and Stars

 

The absolute solar and stellar abundance scale has broad astrophysical importance.   However, it has proven to be difficult to establish, and even the solar abundances are disputed at a significant level.  Solar and stellar oscillations provide powerful tests of stellar interiors, and in this talk I review seismic diagnostics of the solar abundances.  Four distinct tests, with differing systematic, are discussed.  The photospheric and seismic heavy metal abundances are in excellent agreement, while the seismic methods favor higher abundances of light species such as oxygen.  I trace these differences, and argue for seismology as a viable method for setting the absolute solar abundances.    I also discuss the prospects for using asteroseismology to test the absolute stellar abundance scale, which appears promising.

Quataert

3 Leconte

10

Jerry Nelson (UCO)

TMT

Bloom

3 Leconte

17

Samaya Nissanke (Caltech)

 

Bloom

3 Leconte

24 November

THANKSGIVING

 

 

 

1 December

Sean Andrews (CfA)

Radio-Wave Insights on Planet Formation in the Disks around Young Stars

 

Nearly 700 exoplanets have been discovered, suggesting that at least 10% of nearby stars host a giant planet.  A massive effort is underway to find more exoplanets, determine their key properties (masses, orbits, radii), and explain demographic trends with models of their formation.  But associating the current exoplanet properties with their formation epoch is problematic, since the early evolution processes can be dramatic and are intimately associated with the initial conditions in the progenitor circumstellar disk.  Ideally, the properties of mature exoplanet systems could be compared with those for their younger counterparts that are "caught in the act" of formation, still embedded in their natal disks.  I will present state-of-the-art high angular resolution radio observations designed to provide new insights into the spatial distribution of mass in these young protoplanetary disks.  Using these data and a sophisticated radiative transfer modeling toolkit, we can derive the temperature/density structures for these disks and use them to characterize some key disk evolution processes and estimate the likelihood of future planet formation in these systems.  In fact, a significant sample of these disks exhibits compelling evidence that the planet formation process has gotten a head start. I will focus on some exciting new observations that resolve low-density disk cavities on Solar System size-scales and discuss how these are the likely signposts of very young (1 Myr) planetary systems.

 

Chiang

3 Leconte