Talks & Events
Astronomy Colloquia: 2010
The Early Stages of Planet Formation: Astrophysics Meets Cosmochemistry
Protoplanetary disks are dynamic objects through which mass and angular momentum are transported as part of the final stages of pre-main sequence evolution for a star. Chondritic meteorites record a dynamic history for our own solar system as they contain a variety of objects that formed in distinct physical and chemical environments, yet are intimately mixed on fine-scales. To date it remains to be determined whether models of protoplanetary disks can explain the variety of primitive materials found in our own solar system and how they came to be accreted into common meteorite parent bodies. Further, it remains unclear what stages of disk evolution identified by astrophysical models are recorded within meteorites. I will discuss these issues and argue that the earliest stages of solar nebula evolution recorded by meteorites coincide with no later than the first few hundred thousand years of our sun's formation.
Understanding the Star Formation Rate
Stars are the engines of the Universe: nuclear reactions within them are the only significant source of non-gravitational power in the cosmos and the source of all heavy elements. However, the process by which stars form remains poorly understood, and one mystery in particular stands out: why is star formation so slow? In many galaxies the bulk of the interstellar medium does not participate in star formation, and in all galaxies even those clouds that are active form stars at a rate of only ~1% of their mass per dynamical time. Any successful theory of cosmic evolution must be able to explain these facts, and be able to predict how the star formation process changes with galactic environment and over cosmological time. In this talk I discuss progress toward a physical theory of star formation capable of meeting these requirements.
Waves, Winds and Jets from Coalescing Compact Binaries
Merging compact binaries are the one source of gravitational radiation so far identified. Because short-period systems that will merge in less than a Hubble time have already been observed as binary pulsars, they are important both as gravitational wave sources for observatories such as LIGO, but also as progenitors for short gamma-ray bursts. Recent progress in our understanding of these systems is outlined, emphasizing the breadth of the subject and the links with fundamental physics. An effort is made to distinguish between ideas that are already well established and those that still lie on the speculative frontiers. There are, fortunately, several feasible types of observation that could soon clarify the issues.
First Science Results from Kepler
The Kepler spacecraft, launched in March 2009, is designed to detect potentially habitable Earths around other stars by detecting the transits of these planets across the disks of their parent stars. This requires performing differential photometry to a precision of 20ppm on a sample of 170,000 stars for a period of 3.5 years. We will discuss the on-orbit performance of the Kepler photometer, and then present the first scientific results from Kepler. Five new transiting planets around solar-type stars have been discovered so far, along with several other interesting objects.
Insights from the Galactic Planetary Census
The past year has seen enormous advances in our understanding of extrasolar planets. In this talk, I'll focus on some of the most exciting recent highlights. These include (i) the discovery and characterization of remarkable new transiting planets, (ii) a complete upending of the conventional wisdom regarding the statistics of the galactic planetary census, and (iii) a new method for actually looking inside certain transiting planets.
Microlensing Planets: A Controlled Scientific Experiment Drawn From Absolute Chaos
Microlensing planet searches have discovered a total of 17 planets, including the first Jupiter-Saturn like system and the only 4 "cold Neptunes" yet detected. The discovery process is almost unbelievably chaotic, with the so-called "high-magnification events" being the most chaotic. I show, nevertheless, that the high-magnification subsample constitutes a "controlled experiment",
which enables rigorous statistical analysis, yielding important new clues to planetary architecture. I also discuss the future potential of microlensing to explore domains of planet parameter space not probed by any other method.
The North American Nanohertz Observatory of Gravitational Waves (NANOGrav)
NANOGrav is a consortium of radio astronomers and gravitational wave physicists whose goal is to detect gravitational waves using an array of millisecond pulsars as clocks. Whereas interferometric gravitational wave experiments use lasers to create the long arms of the detector, NANOGrav uses earth-pulsar pairs. The limits that pulsar timing places on the energy density of gravitational waves in the universe are on the brink of limiting models of galaxy formation and have already placed limits on the tension of cosmic strings. Pulsar timing has traditionally focused on stochastic sources, but most recently I have been investigating the idea of detecting individual gravitational wave bursts wherein there are some interesting advantages. I will also demonstrate how the array can be used to reconstruct the waveform and obtain its direction.
Searching for the Dark Matter
There is a large body of evidence that ~85% of the matter in the Universe is in the form of cold, non-baryonic dark matter. I describe how terrestrial particle detectors are searching for clues on the nature of the dark matter. I focus on two leading experiments, current results, and challenges in searching for an unknown form of matter.
Reaching for the sky: from SDSS to LSST
Despite a several thousand years long history, sky surveying is experiencing a bonanza as detectors, telescopes and computers become ever more powerful. I will discuss how the unprecedentedly accurate and diverse data from the optical Sloan Digital Sky Survey have recently enabled numerous exciting discoveries. I will use three specific examples (asteroids, quasar variability, and mapping of the Milky Way stellar distribution) to give a preview of what to expect from the upcoming next-generation surveys, such as the Dark Energy Survey and the Large Synoptic Survey Telescope.
How to Find a Habitable Planet
Over 400 planets have been found around nearby stars, but none of them is thought to be at all like Earth. The goal now is to identify rocky planets within the habitable zones of their stars and to search their atmospheres spectroscopically for signs of life. To do this, we need new space-based telescopes such as NASA’s proposed Terrestrial Planet Finders or ESA’s Darwin mission (all of which are indefinitely postponed at the moment). If spectra of extrasolar planet atmospheres can be obtained, the presence of O2, which is produced from photosynthesis, or O3, which is produced photochemically from O2, would under most circumstances provide strong evidence for life beyond Earth. But “false positives” for life may also exist, and these need to be clearly delineated in advance of such missions, if at all possible. I will also contrast my optimism about the search for complex life with the more pessimistic view expressed by Ward and Brownlee in their book, Rare Earth.
The gravitational two-body problem in general relativity
A simple problem in Newtonian gravity, the motion of two bodies about one another is far more challenging in general relativity (GR). Motivated largely by the anticipated importance of compact binaries as gravitational-wave sources, many years of effort have produced a suite of tools for modeling binaries with GR. In this talk, I will present an overview of how we model these sources in GR and what we have learned from the relativistic two-body problem. I will focus in particular on how unique aspects of relativistic gravity flavor the gravitational waves which binaries generate, and how these flavorings can be exploited to learn about compact bodies, especially black holes. I will emphasize analogs between the GR analysis and electromagnetic theory, hopefully demonstrating that the rich features of these models are in fact surprisingly intuitive.
When Stars Attack! Live Radioactivities as Signatures of Near-Earth Supernovae
The lifespans of the most massive stars are a symphony of the fundamental forces, culminating in a spectacular and violent supernova explosion. While these events are awesome to observe, they can take a more sinister shade when they occur closer to home, because an explosion inside a certain "minimum safe distance" would pose a grave threat to the biosphere on Earth or elsewhere. We will discuss these cosmic insults to life, and ways to determine whether a supernova occurred nearby over the course of the Earth's existence. We will then present recent evidence that a star exploded near the Earth about 3 million years ago. Radioactive iron-60 atoms have been found in ancient samples of deep-ocean material, and are likely to be debris from this explosion. Recent data confirm this radioactive signal, and for the first time allow sea sediments to be used as a telescope, probing the nuclear reactions that power exploding stars. Furthermore, an explosion so close to Earth was probably a "near-miss," which emitted intense and possibly harmful radiation. The resulting environmental damage may even have led to extinction of species which were the most vulnerable to this radiation.
The Past, Present, and Future of Supernova Cosmology
Supernova have been developed into a powerful tool for cosmological distance measurement. In the (recent) past, supernovae showed that we live in an accelerating universe. In the present supernovae are a key element in constraining the properties of dark energy. While the present data are consistent with a cosmological constant, today's constraints are not very rigorous. As a community, we are beginning to learn where the systematic problems arise in tightening the noose and improving our knowledge. I'll review some of the problems we have encountered with dust absorption and supernova environments and I will show some promising developments using thermonuclear supernovae in the near-infrared that may mitigate these difficulties. The future will not be as easy as the past, but the conclusion of programs like ESSENCE, Supernova Legacy Survey and the Sloan Supernova Survey plus the Palomar Transient Factory, Pan-STARRS, and the Dark Energy Survey all promise real progress in the years just ahead.
X-rays and Planet Formation
High-resolution X-ray observations of star forming regions show that magnetic reconnection flares are powerful and frequent in pre-main sequence solar-type stars. Well-defined samples in the Orion Nebula Cluster and Taurus clouds exhibit flares with peak X-ray luminosities L_x ~ 1e29-1e32 erg/s, orders of magnitude stronger and more frequent than contemporary solar flares. X-rays are emitted in magnetic loops extending 0.1-10 stellar radii above the stellar surface and thus have a favorable geometry to irradiate the protoplanetary disk. The fluorescent FeK 6.4 keV emission line and X-ray absorption directly indicates X-ray irradiation of cold material in some young systems; this is supported by observations of infrared lines from ionized neon and excited molecules from the outer disk layers. A tail of penetrating hard X-rays with energies ~10-30 keV is sometimes present.
There is thus considerable empirical evidence that X-rays from the host star irradiate protoplanetary disks, heating and ionizing their outer layers, and likely penetrating to the midplane in some disk regions. As ionization fractions need only reach ~ 1e-12 to induce the magnetorotational instability and associated turbulence, X-rays may be the principal determinant of the extent of the viscous `active zone' and laminar `dead zone' in the layered accretion disk. It may be important for the dissipation of gas in older disks via photoevaporation. X-ray irradiation may thus play a major role in planet formation processes: whether particle growth occurs by settling or in turbulent eddies; whether turbulence inhibits rapid inspiral from headwinds; whether protoplanets suffer secular migration or random walk interactions; whether transition disks are quickly dissipated. The violent magnetic flares from young stars may also explain shock melting of chondrules and a spallogenic origin of some anomalous short-lived radioisotopes found in ancient meteorites.
Asymmetric Planetary Nebulae: Emerging Paradigms and Related Frontiers of Magnetohydrodynamics
Many, if not all, post-AGB stars rapidly transform from spherical to a powerful aspherical pre-planetary nebula (pPN) outflow phase before presumably fading into a less powerful planetary nebula (PN). The pPNe outflows require engine rotational energy and a mechanism to extract this energy into collimated outflows. Just radiation and rotation are insufficient, but an interplay between rotation, differential rotation and magnetic fields seems promising, not unlike the presumed symbiosis of these ingredients in other jetted sources. Present observational evidence for magnetic fields in evolved stars is suggestive of dynamically important magnetic fields, but both theory and observation are rife with opportunity. I will discuss why magnetohydrodynamic (MHD) shaping and launch might arise in pPNe and PNe. Scenarios involving binary driven dynamos and accretion engines cannot yet be ruled out. One noteworthy paradigm involves accretion onto the primary post-AGB white dwarf core from a low mass companion whose decaying accretion supply rate powers the asymmetric pPN. Strategies for distinguishing different engine mechanisms is a topic of active research. The related physics of dynamos and accretion disks underpins MHD launch and shaping scenarios throughout astrophysics and I will summarize some progress and challenges in our evolving understanding of the underlying principles.
Probing Star and Planet Formation: Gas and Dust Within 1 AU of Pre-Main-Sequence Stars
Planetary systems form out of disks of dust and gas that are remnants of the star formation process. The structure of these protoplanetary disks within 1 AU of their central stars has important implications for terrestrial planet formation, giant planet migration, and disk accretion. I will present spatially and spectrally resolved observations of gas and dust within 1 AU of young stars, with a focus on hydrogen gas at stellocentric radii smaller than a tenth of an AU. I will describe the new instrumentation and techniques that enabled these measurements, and discuss the resultant constraints on star and planet formation processes.
Climate of Gliese 581g -- The First Potentially Habitable Extrasolar World
On 2010 September 30, Vogt et al. reported the detection of two new planets orbiting the M-dwarf star Gliese 581. One of these two planets, 581g, is in what is loosely known as the "habitable zone," where a planet can support an Earth-like climate given a suitable atmosphere. I will discuss the factors governing whether this potentially habitable planet is indeed habitable, and the prospects for detecting whether the planet actually has an atmosphere which would render it habitable.
Self-Regulation of Star Formation Rates in Disk Galaxies
Star formation rates depend on both the total available interstellar gas mass and the physical state of that gas and the local galactic environment -- including the stellar and dark matter gravitational potentials. In a multiphase disk, the relative proportion of mass in gravitationally bound clouds vs. the diffuse ISM depends on energy injected by star formation. This energetic feedback both heats gas and drives turbulence, and it can lead to a self-regulated star-forming state. I will discuss recent numerical work showing that multiphase, turbulent ISM simulations are able to reproduce observed star formation rates, provided that the disks' vertical structure is resolved down to ~pc scales. These results are also consistent with empirical relationships that have been found between midplane pressure and the star formation rate (or molecular gas fraction). I will also introduce a new theoretical model that predicts the star formation rate as a function of the total gaseous surface density and the midplane density of stars + dark matter. This prediction derives from requirements for maintaining thermal and dynamical equilibrium in the diffuse gas. In HI-dominated outer-disk regions, star formation rates increase until the thermal pressure in the two-phase ISM matches the dynamic pressure. In the central regions of galaxies, the total surface density of HI is limited due to the high cooling rate of vertically confined, high-pressure gas. Cooling cannot exceed the heating provided by UV from young stars; this leads to a saturation of the HI surface density, consistent with observations. Application of this thermal/dynamical equilibrium theory to a set of spiral galaxies shows excellent agreement between predicted and observed star formation rates, especially for flocculent galaxies.