Autumn Quarter 2018 (current)
An overview of UW Astronomy, announcements, and a welcome to new members of the department.
Asteroid families are the remnant fragments of asteroids broken apart by collisions. There are only a few known Main Belt (MB) asteroid families with ages greater than 2 Ga (Brož et al., 2013; Spoto et al.,2015). Estimates based on the family producing collision rate suggest that the lack of >2 Ga-old families may be due to a selection bias in classic techniques used to identify families. Family fragments disperse in their orbital elements, semi-major axis, a, eccentricity,e, and inclination, i, due to secular resonances and the non gravitational Yarkovsky force. This causes the family fragments to be more difficult to identify with the hierarchical clustering method (HCM), which attempts to find cluster in orbital element space, when applied to family fragments’ elements as the fragments age. We have developed a new technique that is insensitive to the spreading of fragments in e and i by searching for V-shaped correlations of family members in an asteroid diameter, D. A group of asteroids is identified as a collisional family if its boundary in the a vs. 1/D plane has a characteristic V-shape which is due to the size dependent Yarkovsky effect. The V shape technique is demonstrated on the known families and families difficult to identify by HCM, and used to discover a 4 Ga-old family linking most dark asteroids in the inner MB not included in any known family (Delbo’ et al., 2017). The 4 Ga-old family reveals asteroids with D > 35 km that do not belong to any asteroid family implying that they originally accreted from the protoplanetary disk and support recent theories on the formation of asteroids (Morbidelli et al., 2009).
As science educators, researchers, communicators, and/or supporters, we cannot deny the connection between science and government. The ability to send missions to Mars, to study star formation in a galaxy, and to model the early universe primarily depends on both government — i.e., taxpayer — money and public — i.e., not just scientist — support. Scientists can and do engage in work to determine the direction of our field and how society prioritizes science. As we approach the 2018 midterms, two years in to a new administration, how is science faring on a national stage? In this talk, I will discuss the current environment for science and space policy in general, and current events dominating policy discussions in the astronomical sciences in particular, including NASA flagship mission development, sexual harassment in the sciences, NSF facilities support, and how Congress has responded to the Trump administration’s science priorities. I will discuss the role of the American Astronomical Society (AAS) in such discussions and in advocating for the astronomical sciences in Washington and what individual scientists can do to effectively engage in the political process.
Spring Quarter 2018
Following the Big Bang the Universe was homogeneous in matter, energy and barren of chemistry. It is the stars which built up the periodic table. Astronomers have now identified several classes of cosmic explosions of which supernovae constitute the largest group. The Palomar Transient Factory (PTF) consisting of the 48-inch Oschin Schmidt-optics telescope (hosting a large field-of-view mosaic detector) and the Palomar 60-inch robotic telescope (initially equipped with an imaging CCD photometer) was designed to explicitly undertake a systematic survey of the optical night sky. The speaker will talk about the returns and surprises from this project: super-luminous supernovae, new classes of transients, new light on progenitors of supernovae, detection of gamma-ray bursts by purely optical techniques and troves of pulsating stars and binary stars. The successor to PTF is the Zwicky Transient Factory (ZTF) with a wide field-of-view imager (47 square degrees) and an IFU spectrograph aimed squarely at classification on the 60-inch. ZTF is poised to become the stepping stone for the Large Synoptic Survey Telescope.
A significant fraction of heavy elements in the universe spend some time in the interstellar medium as dust grains. But how much do we really know about the origin and evolution of dust in the Universe? I will review the current state of astromineralogy and describe how observations of X-ray bright compact objects can yield key insights into the evolution of Milky Way dust. With high resolution X-ray spectroscopy, we directly measure the abundance of gas-state metals and mineral composition of dust in the interstellar medium (ISM). With imaging, we timing and imaging, we obtain dust grain size and spatial distributions from X-ray scattering halos around bright point sources. Understanding the scattering component of X-ray extinction is also important for interpreting accretion by compact objects, including the supermassive black hole at the center of our galaxy. I will review the most recent exciting dust scattering discoveries, many of which draw on multi-wavelength observations. Finally, I will explain how future X-ray observatories can tackle current open questions about the dusty Universe.
Estimating the mass of complex dynamical systems such as star clusters, galaxies, and galaxy clusters is important for testing astrophysical theories at a variety of scales, but is difficult in practice. The Milky Way Galaxy’s mass in particular is not well known within a factor of two. Popular techniques use position and velocity measurements of tracer objects, such as globular clusters and halo stars, to constrain the total gravitational potential of the Galaxy. This approach presents many challenges, including: different degrees of measurement uncertainties, incomplete data, and how best to use (or not use) multiple tracer populations in the analysis. I will discuss a hierarchical Bayesian method that estimates the mass of the Milky Way while attempting to overcome these challenges. We hope the method provides a concrete step forward in our understanding of the Milky Way’s total mass and mass distribution. I will describe the results as it applies to the Milky Way, as well as results from a series of blind tests on simulated data from galaxies made in cosmological simulations.
Supermassive black holes are in place by the first billion years of the universe’s existence. Several promising channels have be proposed for their formation, but forming supermassive black holes in the requisite time-frame remains a theoretical puzzle. One promising channel is that of direct collapse, in which a cloud of gas collapses to a massive seed black hole that then grows to supermassive size via accretion. In this talk I will give an overview of potential supermassive black hole formation channels and outline a suite of 3D hydrodynamical simulations probing conditions conducive to formation of a massive seed black hole via direct collapse.
Does the star and planet forming environment matter for protoplanetary disks and stellar properties? What are the observational signatures and theoretical implications? UV radiation from massive stars and stellar density may affect the protoplanetary disk properties including disk mass, disk size, accretion rate, and disk lifetime. The final mass of young stellar objects (YSOs) in high UV radiation and rich cluster environment may also be influenced due to early loss of circumstellar materials. In this talk I will present our on-going multi-wavelength studies of the Orion A region to probe these questions. I will focus on protoplanetary disks in three star forming regions within the Orion A cloud to probe the role of UV radiation and stellar density. The Orion Nebula Cluster, NGC 1977, and Lynds 1641 are all located in the Orion A cloud having similar ages, yet their environments are different. I will discuss our findings from our current surveys of these three regions in Orion A, and compare our findings to some low density, weak radiation environments, such as Taurus star forming region. This study is also a part of one of the NASA NExSS program, Earth in Solar System (EOS, PI Apai). I will also briefly talk about the EOS program.
Cosmological simulations can now make specific and detailed predictions for the shapes, masses, and substructure fractions in galactic dark matter halos that depend on the dark matter model assumed. Comparing these predictions to the observed mass distributions of galaxies should in principle lead to constraints on the nature of dark matter, but observable dynamical tracers can be scarce in regions where the dark matter distribution is best able to discriminate between models. One such region is the distant outskirts of galaxies, where the influence of baryonic matter on the dark matter halo is limited and the effect of dark substructures most prominent. New surveys of Milky Way stars like Gaia, alongside next-generation instruments and giant telescopes, are for the first time providing accurate positions, velocities, and chemical abundances for large numbers of stars in faint tidal streams: remnants of tidally-disrupted satellite galaxies that trace out the mass distribution in the distant reaches of galaxy halos. I will show how state-of-the-art simulations play a crucial role in interpreting and analyzing this wealth of new information about stellar halos, and how stellar halo observations over the next decade will characterize the dark matter distribution in galaxies, test theories of the nature of dark matter, and illuminate the role of dark matter in galaxy formation.
Observations from the WISE and NEOWISE missions have provided invaluable information about the diameter, visible and IR albedos, and other properties of ~164,000 asteroids—more than 16 times as many as all previous infrared asteroid observations combined. No other four-band space observing mission is currently planned. So it is imperative that we understand the strengths and weaknesses of this data and have clearly documented, reproducible methods of analyzing them. Dr. Nathan Myhrvold will discuss what he has learned from his independent, two-year empirical examination of the NEOWISE methods and results. His investigation has found that despite numerous preliminary papers published by the NEOWISE group, analysis of these data is far from a solved problem. Much work remains to be done to address systematic errors and inconsistencies in the results, to correct poorly fit properties, and to develop open, transparent analytical methods that can be reproduced for all of the data sets.
The circumgalactic medium (CGM; non-ISM gas within a galaxy virial radius) regulates the gas flows that shape galaxies. Owing to the vastly improved capabilities in space-based UV spectroscopy with the installation of HST/COS in 2009, observations and simulations of the CGM have emerged as the new frontier of galaxy evolution studies. In the last decade, we have learned that the CGM of Milky Way mass galaxies likely contains enough material to harbor most of the metals lost in galaxy winds and to sustain star-formation for billions of years. Remarkably, this implies that most of the heavy elements on earth cycled back and forth multiple times through the Milky Way’s own CGM before the formation of the solar system. I will describe constraints we have placed on the origin and fate of this material by studying the gas kinematics, metallicity and ionization state. I will conclude by posing several unanswered questions about the CGM that will be addressed with future survey data and hydrodynamic simulations in a cosmological context.
Winter Quarter 2018
The intergalactic medium (IGM) plays a unique role in constraining the (small scale) matter power spectrum, since the low-density, high redshift IGM filaments are particularly sensitive to the small scale properties of dark matter. The main observable manifestation of the IGM, the Lyman-alpha forest, has provided important constraints on the linear matter power spectrum, especially when combined with cosmic microwave background data. This includes, most notably, the tightest constraints on warm dark matter (WDM) and fuzzy dark matter (FDM) models, that I will present in this talk.
Galaxies are complicated beasts – many physical processes operate simultaneously, and over a huge range of scales in space and time. As a result, creating accurate models of the formation and evolution of galaxies over the lifetime of the universe presents tremendous challenges. In this talk I will discuss these challenges and their solutions, and will explain how large-scale computational models can be used to gain insights into the very first galaxies that formed in the universe (over 13 billion years ago!), and how we can use both these computational models and observations of the Milky Way and its neighbors to infer how galaxies have grown and evolved in the intervening time.
Although thermonuclear (Type Ia) supernovae and neutron star mergers are some of the most important astrophysical events, our understanding of these explosions is vague. I will present abundance measurements of elements across the periodic table (Mg, Fe, Co, Ba, and others) that address the nature of both types of explosions. The measurements are based on Keck/DEIMOS spectroscopy of red giants in dwarf galaxies, which experienced a large number of Type Ia supernovae. The iron-peak elemental abundances strongly suggest that the majority of Type Ia supernovae in dwarf galaxies exploded below the Chandrasekhar mass, i.e., the double-degenerate model or the single-degenerate, double-detonation model. The DEIMOS spectra also reveal that barium comes from the r-process and appears in the dwarf galaxies on a timescale similar to iron (at least 100 Myr). Therefore, the mostly likely origin is not supernovae but neutron star mergers. The evolution of the [Ba/Fe] ratio indicates a neutron star merger rate consistent with results from LIGO.
What happened after inflation? Pretty much everything. But if we make this statement more precise and ask, “What happened immediately after inflation?” the answer is that we actually don’t know. The era immediately following the universe’s first period of accelerated expansion is known as reheating, and it is a part of the cosmic timeline for which we have no observables. In this talk, I will give some background on inflation, explain what reheating is, and describe my work on related questions at the non-dark matter intersection of particle physics and astrophysics.
One of the key objectives of modern astrophysics is to understand the formation and evolution galaxies. In this regard, the Milky Way is a fantastic testing ground for our theories of galaxy formation. However, dissecting the assembly history of the Galaxy, requires a detailed mapping of the structural, dynamical, chemical, and age distributions of its stellar populations. Recently, we have entered an era of large spectroscopic and astrometric surveys, which has begun to pave the way for the exciting advancements in this field. Combining data from the many multi-object spectroscopic surveys, which are already underway, and the rich dataset from Gaia will undoubtedly be the way forward in order to disentangle the full chemo-dynamical history of our Galaxy. In this talk, I will discuss my current work in Galactic archaeology and how large spectroscopic surveys have been used to dissect the structure of our Galaxy. I will also explore the future of Galactic archaeology through chemical cartography.
The NASA Juno Spacecraft is the ninth spacecraft launched from Earth to reach Jupiter and only the second to orbit the largest planet in the Solar System. It is also the first solar-powered spacecraft to orbit Jupiter. Launched in 2011 and currently in polar orbit around Jupiter, Juno’s primary goal is to improve our understanding of the formation and evolution of Jupiter. Juno has now completed 11 perijove passes over a wide range of Jovian longitudes. This talk will provide an overview of the Juno mission and present a few early results.
Precise and accurate star formation histories from galaxy spectra and the path to life finding NIR detectors
Recent discoveries of black hole mergers and the collision of neutron stars by the Laser Interferometer Gravitational wave Observatory (LIGO) have opened up the era of gravitational wave astronomy and mutimessenger astronomy. The Advanced LIGO detectors are currently being upgraded in preparation for their third observing run (O3), looking forward to more gravitational wave detections including the possibility of the first un-modeled gravitational wave signals. Beyond Advanced LIGO, the future of gravitational wave astronomy includes the Laser Interferometer Space Antenna (LISA) mission led by the European Space Agency (ESA). Meanwhile, Pulsar Timing Arrays (PTAs) such as the North American Nanohertz Observatory for Gravitational waves (NANOGrav) are poised to make detections of gravitational waves in the nanohertz range. The historic first detections of gravitational waves have opened a new way to study our universe, with discovery potential across the entire gravitational wave spectrum.
Autumn Quarter 2017
I will describe our development of a convolutional neural network (CNN) to learn to search for and characterize absorption lines in quasar spectra. Specifically, the algorithm discovers and measures the redshift and Hydrogen column density of damped Lya systems (DLAs). These systems dominate the neutral hydrogen gas of the universe, trace the interstellar medium of distant galaxies, and offer cosmological constraints on the build up of gas and heavy elements across cosmic time. I will discuss the lessons learned employing CNN techniques on large spectral datasets and the prospects for future analysis.
Line Intensity Mapping has emerged as a powerful tool to probe the large-scale structure across redshifts, with the potential to shed light on dark energy at low redshifts and the cosmic reionization process at high redshift. Multiple spectral lines, including the redshifted 21cm, CO, [CII], and Lyman-alpha transitions, are promising tracers with several experiments on-going or in the planning. I will describe current pilot programs and cross-correlation sciences, and present future prospects.
A prediction of axion dark matter models is they can form Bose-Einstein condensates and rigid caustic rings as a halo collapses in the non-linear regime. In this talk, I will present results from the first study of the caustic ring model for the Milky Way halo (Duffy & Sikivie 2008), focusing on observational consequences. I will describe the formalism for calculating the gravitational acceleration of a caustic ring halo. The caustic ring dark matter theory reproduces a roughly logarithmic halo, with large perturbations near the rings. I will show that this halo can reasonably match the known Galactic rotation curve. We explored the effects of dark matter caustic rings on dwarf galaxy tidal disruption with N-body simulations. N-body simulations of the Sagittarius (Sgr) dwarf galaxy in a caustic ring halo potential, with disk and bulge parameters that are tuned to match the Galactic rotation curve, match observations of the Sgr trailing tidal tails as far as 90 kpc from the Galactic center. Like the Navarro-Frenk-White (NFW) halo, they are, however, unable to match the leading tidal tail. None of the caustic, NFW, or triaxial logarithmic halos are able to simultaneously match observations of the leading and trailing arms of the Sagittarius stream. I will further show that simulations of dwarf galaxies that move through caustic rings are qualitatively similar to those moving in a logarithmic halo.
The sky in X-rays is incredibly dynamic. Black holes and neutron stars vary on time scales ranging from milli-seconds to decades, their brightness occasionally changing by several orders of magnitude within seconds or minutes. Studying this variability is one of the best ways to understand key physical processes that are unobservable on Earth: general relativity in strong gravity, extremely dense matter and the strongest magnetic fields known to us are just a few examples. In this talk, I will give an overview of the state-of-the-art of time series analysis in high-energy astronomy. I will present key statistical methods and machine learning models we have been developing recently as well as point out the opportunities and challenges of the spectral-timing revolution we are moving toward with data from current and future space missions. At the same time, this talk also chronicles my path from conventional astronomical research into data science and data-intensive astronomy, and thus touches upon some of the occasionally surprising turns that path has taken.
The Dawn of Multi-Messenger Astrophysics with Gravitational Waves: the first joint detection of GWs and Light
I present the results from the first detection of gravitational waves and light from the same cosmic source, GW170817. GW170817 is the merger of two neutron stars, detected in Gravitational Waves (GWs) by Advanced LIGO/Virgo on August 17, 2017. The GW detection was followed by the discovery of electromagnetic emission across ten orders of magnitude in wavelength, which allowed the localization of GW170817 to a galaxy in our own cosmic neighborhood. In this colloquium I will present the results from the first successful hunt for electromagnetic radiation from a GW source across the electromagnetic spectrum, from the X-rays to the radio band. I discuss the open questions raised by GW170817 and the bright future ahead for multi-messenger Astrophysics.
How well you communicate your research to the media and the public can make or break your career in astronomy. But the imperatives that drive science media can seem mysterious. What makes research projects and results newsworthy? What are the ingredients of a compelling science story? How can you help journalists get it right? When is it better to bypass the media and tell your story yourself? Two veteran science writers will pull back the curtain on the inner workings of science publications and share tips on how to convey your work compellingly without sacrificing clarity or accuracy. W. Wayt Gibbs, a contributing editor for Scientific American and editorial director at Intellectual Ventures, will dissect the anatomy of an astronomy cover feature and explain how he successfully pitched an astronomy story to the New York Times, Science, Scientific American, and other major outlets, while simultaneously exploiting social media channels. And Alan Boyle, an award-winning space journalist for MSNBC and currently aerospace and science editor at Seattle’s GeekWire tech news site, will illuminate how science news works and the challenges of writing short and fast for a general audience.
Since Hubble’s early classification of galaxies a century ago, it has been clear that galaxies fall into 2 broad categories – those that are blue and characterized by spiral disks, and those that are red and elliptical in shape. With the advent of modern spectroscopic surveys, this galaxy ‘bimodality’ has been understood in the context of star formation, in which galaxies can be broadly classified as either star forming or passive. However, the physical processes that regulate star formation, transforming galaxies between these populations, remain hotly debated. In this talk, I will review recent results that investigate some of the mechanisms proposed to play a role in the enhancement and suppression of star formation, including galaxy mergers, bars and the role of active galactic nuclei (AGN). I will also present new results from the SDSS MaNGA survey that uses spatially resolved spectroscopy to map where in the galaxy these processes are occurring. Taken together, I will show that whilst a wide variety of mechanisms can alter a galaxy’s global star formation, these processes seem to generally both boost and quench star formation from the inside-out.
The local expansion rate of the Universe, the Hubble constant, is one of the fundamental parameters in our current concordance cosmology and one that anchors the expansion history of the Universe. The resolution of the historical factor-of-two controversy in the Hubble constant nearly two decades ago (e.g., the Hubble Space Telescope Key Project; Freedman et al. 2001) has evolved into a 3.4-sigma tension between the traditional Cepheid-distance ladder measurements (Riess et al. 2016, Freedman et al. 2012, Freedman et al. 2001) and that determined from modelling anisotropies in the cosmic microwave background (CMB; Planck Collaboration et al. 2016). At the heart of the tension, is not only a difference in method, but also a fundamental difference in the state of the observed Universe: the distance ladder measures the local rate in the nearby universe (e.g., z~0), whereas the CMB anisotropy measurements uses the very young Universe (z ~1100). Resolution of the tension requires (i) a full scale evaluation of the systematic effects in either technique or (ii) “new physics” added to the standard cosmological model. The trigonometric parallaxes provided by Gaia in the near term permit an unprecedented opportunity to use alternative standard candles and construct a full end-to-end distance ladder without Cepheids. The Carnegie-Chicago Hubble Program is doing just that; we are in the middle of building a new distance ladder that relies on the tip of the red giant branch (TRGB; Beaton et al. 2016). As I will demonstrate, this not only provides a direct cross-check on the Cepheid path, but there are numerous advantages to using a distance indicator that, as a standard candle from old stellar populations, is nearly ubiquitously present low-crowding and low-extinction components of galaxies. More specifically, by being able to calibrate every ‘local’ SNe Ia and easily probing ever-larger volumes with JWST and WFIRST, the TRGB-based distance ladder paves a clear path to a 1% measurement within the foreseeable future.
The Zeeman effect is the only observational tool that allows us to directly measure the magnetic field strength and direction in the interstellar medium. We provide an overview of ongoing projects in which we are using Zeeman splitting of the 21-cm hydrogen line and the 18-cm hydroxyl (OH) transitions in order to probe astrophysical magnetic fields. We will highlight the first detection of extragalactic Zeeman splitting in the OH megamaser emission from starburst galaxies. Results will be shown from a survey of Zeeman splitting of OH masers in our Milky Way’s spiral arms that show large-scale field directions conflicting with those probed by Faraday rotation. We will also discuss previous measurements of the Zeeman effect in Galactic 21-cm radio emission, some of the instrumental challenges involved in such measurements, and plans for a large-scale survey using the 26-m John A. Galt Telescope at the Dominion Radio Astrophysical Observatory.
Spring Quarter 2017
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Future constraints on the cosmological initial conditions and the different constituents of the cosmic energy budget will be driven by large-scale structure surveys, as we have extracted much of what is possible from our workhorse, the cosmic microwave background. I will discuss some challenges with using these surveys to understand how to relate what we observed to the cosmic initial conditions. I will mainly focus on a mini-revolution in how we understand the late time cosmos perturbatively, which potentially enables us to extract more information from the large-scale matter distribution. I will also briefly comment on some work my group is doing that is relevant to large weak lensing surveys (the era of large weak lensing surveys is dawning!) and the Lyman-alpha forest (a large-scale structure probe that has never been as successful as one might hope – we are working to understand why).
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Karl-Heinz Bohm and Erika Bohm-Vitense arrived at the University of Washington Astronomy Department in 1968. Over the next decades their research programs made important contributions to various aspects of stellar evolution. Karl-Heinz worked on Herbig-Haro objects and other types of pre-main sequence stars. Erika made major contributions to stellar abundances, convection and rotation. Both of them served as advisors for the Ph.D. research of many graduate students and were known particularly for their teaching of graduate courses. In this colloquium we will highlight the backgrounds and some of the research contributions of this UW Astronomy Dynamic Duo.
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Gravitational lensing represents a unique tool to study the dark Universe. In the weak lensing regime small distortions in the images of galaxies caused by the large-scale structure can be detected over the whole sky. Measuring these coherent distortions yields cosmological insights complementary to other probes like the cosmic microwave background (CMB). Ongoing wide-field imaging surveys exploit this to come up with competitive constraints on important cosmological parameters. In this colloquium I will concentrate on recent results from the ongoing European Kilo Degree Survey (KiDS) and show a mild tension of these results with CMB measurements from the Planck mission. Possible future developments will be discussed that could help make cosmic shear measurements even more robust and lead to an answer to the question whether this tension is real or not. I will conclude with an outlook towards future missions like Euclid, LSST, and WFIRST.
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At the outskirts of the solar system, beyond the orbit of Neptune, lies an expansive field of icy debris known as the Kuiper belt. The orbits of the individual asteroid-like bodies within the Kuiper belt trace out highly elongated elliptical paths, and require hundreds to thousands of years to complete a single revolution around the Sun. Although the majority of the Kuiper belt’s dynamical structure can be understood within the framework of the known eight-planet solar system, bodies with orbital periods longer than 4,000 years exhibit a peculiar orbital alignment that eludes explanation. What sculpts this alignment and how is it preserved? In this talk, I will argue that the observed clustering of Kuiper belt orbits can be maintained by a distant, eccentric, Neptune-like planet, whose orbit lies in approximately the same plane as those of the distant Kuiper belt objects. In addition to accounting for the observed grouping of trajectories, the existence of such a planet naturally explains other, seemingly unrelated dynamical features of the solar system.
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Massive stars have a profound astrophysical influence throughout their tumultuous lives and deaths. Stellar feedback – the injection of energy and momentum by stars to the interstellar medium (ISM) – occurs through a variety of mechanisms: radiation, photoionization heating, winds, jets/outflows, supernovae, and cosmic-ray acceleration. Despite its importance, stellar feedback is cited as one of the biggest uncertainties in astrophysics today, stemming from a dearth of observational constraints and the challenges of considering many feedback modes simultaneously. In this talk, I will discuss how a systematic approach to multiwavelength observations can be used to overcome these issues. I will summarize results from application of these methods to massive-star regions in the Milky Way and nearby galaxies, where feedback processes are best resolved. Finally, I will highlight exciting prospects of using current and upcoming facilities to explore feedback in diverse conditions.
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Polarimetry provides unique information about planetary atmospheres and surface properties complementing more conventional observations. However, polarimetry is currently an under- utilized technique for exoplanet characterization, especially for smaller planets, where its utility is poorly understood. Models of polarised light from terrestrial planets can allow for the detection of biosignatures and habitability markers. Combining the abilities of the Virtual Planetary Laboratory’s SMART radiative transfer code (glint capabilities from a Cox-Munk Formalism ) and the University of New South Wales’ VSTAR radiative transfer code (polarimetric capabilities) we explore the detectability of ocean glint for an Earth-like planet in polarised light. This is compared to theory (e.g.   ) and assessed in observational contexts.
The practical utility of polarimetry in determining cloud species, identifying biomarkers, and detecting ocean glint is assessed with a first order example in the form of the Earth as an exoplanet. In a similar vein to photometric surface mapping (see ), polarimetric surface mapping can provide detailed information about terrestrial exoplanets which may be crucial to exotic worlds such as super Earths and planets around red dwarfs. We explore the capabilities of polarimetry in the context of state-of-the-art Earth-based imaging and aperture polarimeters (e.g. SPHERE  or HiPPI ) and next era space telescopes (e.g. HabEx or LUVOIR). This research is relevant to upcoming large ground-based and future NASA exoplanet characterization missions, such as the proposed HabEx and LUVOIR telescope concepts, particularly in the context of coronography.
References:  Cox, C. and Munk, W. (1954) JOSA, 44, 11  Bailey, J. (2007) Astrobiology, 7, 2  Seager, S. et al. (2000) ApJ 540, 1, 504-520  Zugger, M.E. et al. (2010) ApJ, 723, 1168-1179  Cowan, N. et al. (2009) ApJ, 700, 2, 915-923  Thalmann et al. (2008) SPIE, 7014  Bailey, J. (2015) MNRAS, 449, 3, 3064-3073
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My collaboration recently argued that Kepler space telescope data includes “golden” stars whose luminosities vary quasiperiodically, with two frequencies nearly in the golden ratio, and whose secondary frequencies exhibit power-law scaling with an exponent near -1.5, suggesting strange nonchaotic dynamics. Fractal state space structure without sensitive dependence on initial conditions is a distinctive but underappreciated dynamics between order and chaos. While familiar strange chaotic attractors famously exhibit complicated and unrepeatable dynamics, strange nonchaotic attractors exhibit complicated but predictable dynamics. In this talk, I describe how spectral scaling can distinguish between different sub classes of RR Lyrae variable stars in both Kepler and Optical Gravitational Lensing Experiment (OGLE) photometry. I then use a series of phenomenological models to make plausible the connection between golden stars and fractal spectra. I thereby suggest that some features of variable star dynamics may reflect universal nonlinear phenomena common to even simple systems.
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The James Webb Space Telescope has just passed two huge milestones; completion of: 1.) the telescope itself; and 2.) its entire instrument package. The project has held schedule (and budget) since being replanned in 2011 and is heading toward launch in October, 2018. Although there are many more steps to take before then, most notably a full system test in the mid-2017, it is urgent to plan how to use our new flagship space telescope. The guaranteed time observational programs will have been defined, an Early Release Science program will allocate 500 hours roughly in July, 2017, and the call for General Observer proposals will be released in November 2017. This talk will focus on the capabilities of the observatory, emphasizing the advances over present (and even some future) facilities, with examples of the science it will enable. JWST will redefine our views of topics ranging from exoplanets to the first galaxies and super-massive black holes.
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In the last several years, the disruption and accretion of stars by super-massive black holes (SMBHs) has been linked to tens of luminous flares observed in the cores of nearby galaxies. Our theoretical understanding of these tidal disruption events (TDEs), however, remains incomplete. While recent simulations have provided unprecedented detail on the dynamics of the disruption, we still do not have a good understanding of how infalling gas circularizes and accretes onto the SMBH, or how or where the thermal emission we observe is generated. The art of modeling the tidal disruption of stars by massive black holes forms the main theme of my talk. Detailed simulations should tell us what happen when stars of different types get tidally disrupted, and what radiation a distant observer might detect as the observational signature of such events.
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Disk initial conditions combined with formation processes determine the bulk composition of a planet. I will discuss new work showing that the masses of observed protoplanetary disks may be larger than previously thought and suggest that the features of high-mass disks, combined with the efficacy of pebble accretion in turbulent gas, may explain the observed distribution of giant planets. Within this context, I will suggest a possible source for the observed correlation between planet mass and metallicity among giants, discuss implications for planetary system architectures, and provide predictions for correlations between the compositions of low-mass planets discovered by Kepler/K2 and the likely presence and properties of giant planets orbiting the same stars on more distant orbits.
Winter Quarter 2017
Galaxy mergers drive central inflows of gas, which are an important triggering mechanism for AGN. Kpc-scale separation supermassive black hole pairs in ongoing galaxy mergers are ideal tracers of this link between galaxy mergers and AGN. In dual AGN systems both black holes are fueled as AGN, whereas in offset AGN systems only one of the black holes is an AGN. I will present multiwavelength approaches to building catalogs of dual AGN and offset AGN, and show the results of our observing campaigns with HST, Chandra, VLA, and Keck. Finally, I will discuss what our results show about whether galaxy mergers preferentially fuel the most luminous AGN, which black hole in a merger is more efficient at accreting gas, and where in a merger the AGN fueling occurs.
Galaxy clusters provided the first observational evidence of dark matter. They remain important cosmological probes today, as evidenced by recent controversy about the apparent deficit of clusters observed in Planck studies of the Sunyaev-Zeldovich (SZ) effect compared to the number of clusters expected based on observations of the microwave background. One possible solution would be that multiple cluster mass estimators are systematically underestimated by ~60%. There are now three methods to identify large samples of clusters for cosmological studies: optical richness, X-ray flux, and SZ decrement. I will describe a series of spectroscopic surveys we have conducted of cluster samples identified with each of the three methods using MMT/Hectospec. These surveys show that dynamical mass estimators such as velocity dispersion agree well with most mass proxies, albeit with substantial scatter. In particular, Planck-SZ mass estimates (calibrated from hydrostatic X-ray masses) agree well with measured velocity dispersions. This agreement suggests that SZ mass bias is too small to resolve the Planck SZ-CMB tension. I will discuss possible explanations for this tension and how we can test some of them in the near future.
The stellar disks of galaxies are enormous, but they are only the tip of the iceberg: galaxies are embedded in much larger structures known as halos, roughly ten times as big and ten times as massive as the galaxy disk. Galaxy halos are expected to be filled with hot, diffuse plasma; this hot plasma plays a crucial role galaxy formation, and fundamentally determines the mass-scale of galaxies. As new observational techniques begin to probe halo gas for the first time, however, they have found that their halos are also full of cold gas. This cold gas is unexpected because it cannot be hydrostatically supported against gravity. Moreover, these observations typically indicate a relatively modest total fraction of cold gas (~0.01% by volume), yet find it in essentially every sightline through the galaxy. It’s hard to understand how so little gas manages to be present everywhere. I will show that cold gas clouds are prone to “shattering” into fragments, producing a mist of tiny, distributed cloudlets which naturally reproduce these unexpected observations. This same effect dramatically enhances the drag force coupling the dynamics of cold and hot gasses; I will also discuss potential applications to entrainment of cold gas in galaxy winds, and the possibility of using cold gas to constrain the kinematics of the hot halo gas.
Our current story for the origin of the heavy elements has at its core a recycling program on cosmological scales. Using telescopes on the ground and in space, we have traced baryons as they flow from the intergalactic medium into galaxies, where they are incorporated into stars, undergo fusion, and are expelled in supernovae and other types of stellar death. The process can repeat many times, raising the cosmological heavy element contribution to the baryon budget. In principle, if we run the clock backwards far enough, we could observe the very first stars, aka ‘Population III stars’: those formed from only those elements created in nucleosynthesis shortly after the big bang. As of yet, however, the Population III stars have eluded direct detection. In this talk, I will present a new suite of tools and techniques used in the hunt for Population III stars, and will discuss how the future generation of telescopes may finally reveal these first stars in the universe.
It’s Alive, Alive! The Commissioning of APOGEE-2 South Instrument and the Use of Data-Driven Techniques for Analysis of APOGEE Results
The Apache Point Observatory Galactic Evolution Experiment (APOGEE) is one of the cornerstone projects of the Sloan Digital Sky Survey (SDSS). APOGEE is a large-scale (> 10^5), near-infrared (1.51-1.69 μm), high-resolution (R~22,500) spectroscopic survey that generates highly accurate radial velocity and element abundance values for stars of the Milky Way Galaxy. The APOGEE data are used to generate metallicity gradient, metallicity distribution function, and star formation rate information that in turn can be used to shed light upon Galactic assembly and growth.
Over the 2008-2011 period, APOGEE-1 observations were taken at the 2.5m Sloan Foundation Telescope of the Apache Point Observatory (APO). In 2014, APOGEE-2 continued data acquisition at APO (APOGEE-2N). Additionally, the build of the second APOGEE instrument began in earnest. This second spectrograph is located at the 2.5m du Pont Telescope of the Las Campanas Observatory (APOGEE-2S). First light of the APOGEE-2S instrument is set for February 15, 2017. The “all sky” view of APOGEE-2N and APOGEE-2S extends the APOGEE-1 chemodynamical examination of the Milky Way to include: the four disk quadrants, the inner and outer halo, the full expanse of the bulge, tidal streams, and local satellite galaxies.
I will provide an overview of the APOGEE-2 Project. I will describe the (herculean) set-up process of SDSS operations in the Southern Hemisphere and relay details of the instrument build, infrastructure modifications, instrument reassembly and reintegration at LCO. Furthermore, I will highlight a few APOGEE scientific efforts including those which harness APOGEE data via “data-driven” techniques (e.g. The Cannon).
Over the past two decades ongoing surveys have detected thousands of new planetary systems around nearby stars. These systems include apparently single gas giant planets on short period orbits, closely packed systems of up to 5-6 “mini-Neptunes”, and solar-system-like architectures with either one small planet or no planets interior to 0.5 AU. Despite our success in cataloguing the diverse properties of these systems, we are still struggling to develop narratives that can explain their divergent evolutionary paths. In my talk I will describe two promising new avenues of investigation, including constraints on the compositions of short-period planets and statistical studies of the frequency of outer gas giant and stellar companions in these systems. Taken together, these observations provide important clues that can be used to determine whether or not the observed population of short period exoplanets formed in situ or migrated in from farther out in the disk.
Galaxy clusters contain large amounts of cold dark matter, hot ionized gas, and tens to hundreds of visible galaxies. The abundance of clusters as a function of mass and redshift can be used to probe the growth of structure and constrain cosmological parameters. Dynamical measurements probe the entire mass distribution, but standard analyses yield unwanted high mass errors. First we show that modern machine learning algorithms can improve mass measurements by more than a factor of two compared to using standard scaling relations. Support Distribution Machines are used to train and test on the entire distribution of galaxy velocities to maximally use available information. Second we show that cluster abundance can be quantified with the distribution of direct observables rather than inferred mass to avoid uncertainties in the mass-observable relation. A novel statistic called the velocity distribution function (VDF) is constructed by stacking the probability distribution of galaxy velocities for a select number of clusters in a given volume. The VDF can be measured directly and precisely, and produces unbiased constraints on cosmological parameters. Finally we discuss how our approaches can be generalized to multi-wavelength observations of gravitational lensing, SZ effect, and X-ray emissions.
As a result of the high precision and cadence of surveys like MOST, CoRoT, and Kepler, we may now directly observe the very low-level light variations arising from stellar granulation in cool stars. Here, we discuss how this enables us to more accurately determine the physical properties of Sun-like stars, to understand the nature of surface convection and its connection to magnetic activity, and to better determine the properties of planets around cool stars. Indeed, such sensitive photometric “flicker” variations are now within reach for thousands of stars, and we estimate that upcoming missions like TESS will enable such measurements for ~100,000 stars. We present recent results that tie “flicker” to granulation and enable a simple measurement of stellar surface gravity with a precision of ~0.1 dex. We use this, together and solely with two other simple ways of characterizing the stellar photometric variations in a high quality light curve, to construct an evolutionary diagram for Sun-like stars from the Main Sequence on towards the red giant branch. We discuss further work that correlates “flicker” with stellar density, allowing the application of astrodensity profiling techniques used in exoplanet characterization to many more stars. We also present results suggesting that the granulation of F stars must be magnetically suppressed in order to fit observations. Finally, we show that we may quantitatively predict a star’s radial velocity jitter from its brightness variations, permitting the use of discovery light curves to help prioritize follow-up observations of transiting exoplanets.
Autumn Quarter 2016
Join us as we welcome new folks, address the state of the department, and highlight plans for another year of UW astronomy.
The Latte Project is a new suite of cosmological zoom-in baryonic simulations that model the formation of Milky Way-like galaxies at parsec-scale resolution, using the FIRE (Feedback in Realistic Environments) model for star formation and stellar feedback. Using these simulations, I examine the roles of accretion and stellar feedback in driving the formation and structure of disk galaxies like the Milky Way. The Latte simulations also self-consistently resolve the dwarf galaxies that form around each host, and I will discuss the relative impacts of internal stellar feedback and external environment on their star formation histories, stellar kinematics, and chemical enrichment histories, including progress in addressing the long-standing “missing satellites” and “too-big-to-fail” problems of LCDM cosmology. Finally, I discuss promising new avenues for using upcoming observations to test the role of feedback in dwarf galaxies.
Over the past decade it has become increasingly clear that supermassive black holes are essential components of galaxies, as demonstrated by the correlations connecting black hole masses and galaxy bulge properties. Although ~100 dynamical black hole mass measurements have been made to date, the local black hole mass census is highly incomplete. Gaining a more complete picture of black hole demographics and a deeper understanding of the mechanisms that drive black hole/galaxy evolution requires the measurement of black holes in a wider range of galaxy types with diverse evolutionary histories. In this talk, I will discuss the observation and measurement of black holes in an exciting population of local compact galaxies that appear to be relics of the z~2 quiescent galaxies. I will also describe our new Gemini Large and Long Program aimed at addressing a bias in the types of galaxies for which dynamical black hole mass measurements have been made.
The environments extending several hundred kiloparsecs from galaxies contain the fuel that feeds galactic star formation, and act as the reservoir into which ejecta from stellar and AGN feedback are driven. Observations of the cool hydrogen and metal content, kinematics, and morphology in these regions (i.e., the circumgalactic medium, or CGM) can therefore provide incisive tests of our understanding of these processes. Background quasar spectroscopy has long been the tool of choice for such observations; however, spectroscopy of bright galaxies can also be used to study the halos of galaxies in the foreground (as well as their own). Moreover, galaxies offer background beams many orders of magnitude larger than those provided by QSOs and may be spatially resolved with modern IFUs. I will discuss the first systematic survey to use this technique to characterize cool gas in individual foreground galaxy halos, reporting a unique constraint on the morphology of MgII absorption in the z~0.5 CGM. I will then describe new measurements of gas flows made possible with the spatially-resolved galaxy spectroscopy being obtained by the SDSS-IV/MaNGA survey, demonstrating MaNGA’s potential for establishing the frequency and cross section of gas accretion onto galaxies. Such three-dimensional study is crucial to understanding the thermodynamics of the gas flows which regulate galaxy growth.
Massive stars are the “cosmic engines” of the Universe, proving most of the UV ionizing radiation of galaxies, while also powering their far-IR luminosities. And, massive stars serve as the primary source of carbon and oxygen enrichment of the ISM, as well as manufacturing the elements heavier than Fe during their core-collapse deaths as SNe. Population III massive stars likely played a key role in the re-ionization of the Universe, and their black-hole remnants may have formed the seeds of the super-massive black holes found in the centers of many galaxies today. However, the physics of massive star is complicated, and feedback from observations is crucial to advancing stellar evolution modeling. Massive star evolution depends heavily on the metallicity of the gas out of which these stars form owing to the importance of mass loss via their strong stellar winds. Thus we can use the galaxies of the Local Group as our laboratories for closing the loop in our understanding of massive star evolution. In this talk I will discuss how we identify OB stars, Luminous Blue Variables, yellow and red supergiants, and Wolf-Rayet stars, and what we have so far learned from these studies.
Discussion of the scientific as well as public policy challenges related to potential asteroid impact scenarios we will be faced with when the LSST comes online. At that point, the discovery rate of Near Earth Asteroids will increase by more than a factor of 20 over the current rate, and the list of asteroids with worrisome probability of hitting the Earth will also become much larger.
For most of the history of the universe, quasars have been the dominant source of ionizing photons. Most of the baryons in the universe reside in the intergalactic medium (IGM), even at the present. The most dramatic interaction between these two was the last major phase transition of the universe, when quasars fully reionized helium at z~3. I will discuss our current observational understanding of the helium reionization epoch, as well as what these observations reveal about quasars themselves in the extreme UV (EUV). The EUV quasar spectrum can be modeled as a power law, but the average spectral index and its dispersion are poorly understood, leading to large uncertainty in theoretical models of IGM ionization. Restframe EUV observations with HST are allowing us to finally constrain quasar continuum emission in the EUV, and disentangle it from weak emission lines.
The Near-Earth Object Wide-field Infrared Survey Explorer (NEOWISE) is undertaking an all-sky thermal infrared survey to both discover new near-Earth asteroids and comets, and characterize previously known NEOs. NEOWISE provides simultaneous imaging at 3.4 and 4.6 microns, measuring the thermal emission from NEOs and allowing their diameters
to be computed. I will give an overview of the NEOWISE mission, and present some recent results from this dataset.
We are currently in the midst of several sky surveys and on the cusp of more in the future. Both targeted and wide surveys have provided much needed information on the numbers and origin of cataclysmic variables. I will review some of the results from the past and ongoing surveys, with emphasis on UV and optical wavelengths, and point out what is needed to make future progress in this field.
In this talk I will discuss the observational techniques I use to understand the progenitor systems of Type Ia supernovae (SN Ia), a powerful cosmological standard candle. The characteristics of a SN Ia explosion strongly indicates the thermonuclear detonation of a carbon-oxygen white dwarf star, but how and why it explodes, and the role of a binary companion, are not well constrained. We will explore several unique case scenarios and investigate the evidence for and against non-degenerate companion stars, such as the presence of circumstellar material and emission from the impacted secondary star. We may also delve into the reverse problem for candidate Type IIn SN 2009ip, a transient for which the progenitor system was well characterized but the true nature of the explosion remains shrouded in mystery.
Spring Quarter 2016
Where do new telescopes come from? To start this process NASA has recently convened a group of Science and Technology Definition Teams. These teams will explore the science yield and engineering challenges for exciting new astronomy flagship mission concepts to follow the upcoming James Webb Space Telescope. UW Astronomy professors Julianne, Vikki and Eric will describe aspects of planning, designing and campaigning for these missions, focusing primarily on the LUVOIR concept – a 12 meter-class Large UV Optical Infrared Telescope. They will also engage the audience in a discussion of the political climate, science case, engineering challenges and potential science yield for these missions, which could revolutionize our understanding of habitable zone exoplanets, as well as Galactic and extragalactic science.
Mass loss is important for our understanding of the late evolutionary phases of massive stars, including the formation of Wolf-Rayet (WR) stars and determining what type of supernovae various systems will become. However, small changes in mass loss rates cause very different theoretical predictions for the evolutionary path and supernovae end state for a massive star. For example, massive star winds are expected to be clumped, but this lowers overall mass loss rate estimates to a point where theoretical models cannot explain the observed number of WR stars. Additionally, massive binaries further complicate the mass loss picture because they lose material and interact with their local environments in several ways that single stars cannot: through mass streams, accretion, and mass loss via outflows in Roche-lobe overflow systems; and through stellar winds in colliding-wind systems. In this talk I will focus on presenting results from studies of a small number of systems that provide an in depth look at the circumstellar structures that can form during periods of heavy mass loss. In particular, I will show data from many observing techniques and wavelength regimes that, when viewed all together, present a three dimensional view of where circumstellar material is located in massive systems.
Kepler revealed the common existence of tightly packed super Earth systems around solar type stars, existing entirely inside the orbit of our Venus. Those systems must be stable for the ages of their host stars (~109 years); their formation mechanism must provide interplanet spacings that permit long term stability. If one postulates that most planetary systems form with tightly packed inner planets, their current absence in some systems could be explained by the collisional destruction of the inner system after a period of metastability. We posit that our Solar System also originally had a system of multiple planets interior to the orbit of Venus. This would resolve a known issues with extent of and the energy/angular momentum of our inner planet system; in our picture the disk material closer to the Sun also formed planets, but they have since been destroyed. By studying the orbital stability of systems like the known Kepler systems, we demonstrate that orbital excitation and collisional destruction could be confined to just the inner parts of the system. In this scenario, Mercury is the final remnant of the inner system’s destruction.
Primitive meteorites and cometary dust particles contain preserved interstellar dust grains that predate the origin of the Solar System. These grains are recognized as pristine presolar stardust by large isotopic anomalies indicating that they originated in the outflows or explosions of evolved stars. They were witnesses to stellar, interstellar and protosolar environments and thus serve as probes of a wide array of astrophysical and cosmochemical processes. This talk will provide numerous examples of how presolar grains are identified, studied in the laboratory and used to gain important information about astrophysics with a focus on stellar nucleosynthesis and galactic chemical evolution.
From its flight through the Pluto system in July, a spectacular trove of data was recorded on the solid state data recorders aboard the New Horizons spacecraft. It will be late 2016 before all is transmitted back to Earth, but already there have been many remarkable discoveries giving a sense of the complexities in the system. This talk will show some of the scientific highlights and puzzles that the New Horizons science team is investigating. It will also briefly touch on plans for January 2019 when New Horizons will get the first up-close look at a small Kuiper belt object.
Supernova impostors are optical transients that, despite being assigned a supernova designation, do not signal the death of a massive star or an accreting white dwarf. Instead, most impostors are likely to be the result of a major eruption from a massive star. Although the physical cause of these eruptions is still debated, tidal interactions from a binary companion has recently gained traction as a possible explanation for observations of some supernova impostors. In this talk, I will discuss the particularly interesting impostor SN 2010da, which is (so far) unique among this class of objects due to its high-luminosity, variable X-ray emission. The X-ray emission is consistent with accretion onto a neutron star, making SN 2010da both a supernova impostor and likely high mass X-ray binary. If it is indeed a high mass X-ray binary, it is extremely young: CMD modeling of the coeval stellar population revealed by HST imaging constrains the age of SN 2010da system to <5 Myr. This age is consistent with the theoretically predicted first onset of X-ray production in X-ray binaries, making SN 2010da an important object for understanding massive binaries and high mass X-ray binary formation.
Massive early-type galaxies are central to our understanding of large-scale structure, stellar populations, and supermassive black holes, but their detailed formation histories remain poorly understood. The recently initiated MASSIVE Galaxy Survey is a volume-limited, integral-field spectroscopic and photometric survey of the structure and internal dynamics of the ~100 most massive early-type galaxies within a distance of 100 Mpc. The combination of integral-field spectroscopy on sub-arcsecond and large scales (out to two effective radii) allows us to perform simultaneous dynamical modeling of the supermassive black holes, stars, and dark matter. We also have an ongoing Hubble program to image a high-priority subsample of the MASSIVE galaxies. The ultimate goals of the survey include understanding variations in dark matter fraction and stellar IMF, the connection between black hole accretion and galaxy growth, and the late-time assembly of galaxy outskirts. I will describe the survey design and observational strategy and present first results on black hole mass measurements, stellar populations, and molecular gas detections in MASSIVE Survey galaxies.
Understanding the origin of the elements is one of the major challenges of modern astrophysics. Elements along the bottom two-thirds of the periodic table — including arsenic, selenium, barium, europium, lead, thorium, uranium, and others — are mainly produced by neutron-capture reactions. Some had not been detected previously in late-type stars, and the origins of all are not fully understood at present. My work focuses on abundances derived from ultraviolet and optical high-resolution spectroscopic data of dwarf galaxies, globular clusters, and field stars in the stellar halo. I will present recent observations of these elements that successfully muddy our understanding of when and how they were first produced in the early Universe.
Sniffing Alien Atmospheres: Exoplanet spectrophotometry (from ground-, airborne- and space-based observatories)
In my presentation I will give a short introduction to the science of extrasolar planets, in particular the technique of transit, eclipse and phasecurve spectro-photometry. I will describe my various projects in this emerging field using state of the art spectroscopic and photometric instruments on the largest ground based telescopes, the ‘flying telescope’ SOFIA (Stratospheric Observatory for Infrared Astronomy) and the Kepler and Hubble space telescopes.
NOTE: This colloquium occurs on a Friday.
Winter Quarter 2016
The initial mass function (IMF) for stars above ~1 Msun is essential to testing and validating theories of star formation, constraining chemical enrichment models, the frequency of core-collapse supernovae, and interpreting the stellar populations of galaxies across cosmic time. Yet, despite more than 60 years of research, observational constraints on the high-mass IMF remain remarkably uncertain. Widely used high-mass IMFs (e.g., Kroupa) have associated uncertainties approaching an order-of-magnitude, making it virtually impossible to determine if the high-mass IMF varies with respect to environment (e.g., metallicity or star formation intensity) or is “universal”. In this talk, I will present the most precise measurement of the high-mass IMF to date. Using ~100 young, resolved star clusters imaged as part of the Panchromatic Hubble Andromeda Treasury (PHAT) survey, we find the high-mass IMF slope in M31 to be Gamma=1.45+/-0.03. Compared to the canonical Kroupa IMF (Gamma=1.3+/-0.7), the high-mass IMF in M31 is 0.15 dex steeper (i.e., fewer massive stars) and represents a factor of ~20 improvement in precision. There are no significant trends between the cluster IMF slopes and their ages, masses, and sizes, indicating that the IMF is remarkably “universal” in this sample of ~100 clusters. I will illustrate some of the broader implications of a steeper IMF slope (e.g., on star formation rate indicators, core-collapse supernovae rates) and will conclude by discussing the prospects for precision IMF measurements in other environments.
A radical new scenario has recently been suggested for the formation of giant planet cores that reports to solve this long-standing problem. This scenario, known as pebble accretion, envisions: 1) Planetesimals form directly from millimeter- to meter-sized objects (the pebbles) that are concentrated by hydrodynamic forces and then gravitationally collapse to form 100 – 1000 km objects (Cuzzi+ 2008, AJ 687, 1432; Johansen+ 2007, Nature 448, 1022). 2) These planetesimals quickly sweep up the remaining pebbles because their capture cross sections are significantly enhanced by aerodynamic drag (Lambrechts & Johansen 2012, A&A 544, A32; Ormel & Klahr (2010) A&A Volume 520, id.A43). Calculations show that a single 1000 km object embedded in a swarm of pebbles can grow to ~10 Earth-masses in less than 10,000 years. However, recent full-scale simulations of core formation with this process have failed to reproduce the giant planets in the Solar System (Kretke & Levison 2014, AJ 148, 19). I will discuss a new modification to the basic pebble accretion picture that appears to solve this problem and apply these new ideas to the terrestrial planet region.
When the first galaxies emerged, ~100 – 500 million years after the Big Bang, their starlight likely reionized and heated the intergalactic hydrogen that had existed since cosmological recombination. Much is currently unknown about this process, including what spatial structure it had, when it started and completed, and even which sources drove it. Recent observations of high-redshift quasars show large-scale spatial variations in the opacity of the z~5.5 intergalactic medium to Lyman-alpha photons. These spatial variations grow rapidly with redshift, far in excess of expectations from previous empirically motivated models. I will discuss possible explanations for the excess, as well as what they imply about the reionization process.
From Imaging Disks and Gas Giant Planets with the Gemini Planet Imager to Habitable Earth-like planets with the ELTs
On Thursday, February 11, 10:30 EST, scientists from Caltech, MIT and the LIGO Scientific Collaboration are scheduled to provide “a status report on the effort to detect gravitational waves – or ripples in the fabric of spacetime – using the Laser Interferometer Gravitational-wave Observatory (LIGO).” We will review astronomical sources of gravitational waves, and the key science questions that can be addressed by observing them. The implications for Astronomy of the results from current and future gravitational wave observatories will be discussed.
About half of observed exoplanets are estimated to be in binary systems. Thus, understanding planet formation and evolution in binaries is essential for explaining observed exoplanet properties. I will show how planet—disc interactions in a mildly inclined disc around one component of a binary can lead to the formation of highly eccentric and highly inclined planets.
The dust extinction curve is a critical component of many
observational programs and an important diagnostic of the physics of
the interstellar medium. Here we present new measurements of the dust
extinction curve and its variation towards tens of thousands of stars,
a hundred-fold larger sample than in existing detailed studies. We
use data from the APOGEE spectroscopic survey in combination with
ten-band photometry from Pan-STARRS1, 2MASS, and WISE. We find that
the extinction curve in the optical through infrared is well characterized by a one-parameter family of curves described by R(V). The extinction curve is more uniform than suggested in past works, with sigma(R(V)) = 0.18, and with less than one percent of sight lines having R(V) > 4. Our data and analysis have revealed two new aspects of Galactic extinction: first, we find significant, wide-area variations in R(V) throughout the Galactic plane. These variations are on scales much larger than individual molecular clouds, posing a challenge to existing paradigms of dust formation and processing: R(V) must be tracing much more than just grain growth in dense molecular regions. Indeed, we find no correlation between R(V) and dust column density up to E(B-V) ~ 2. Second, we discover a strong relationship between R(V) and the far-infrared dust emissivity.
Dusty star-forming galaxies host the most intense stellar nurseries in the Universe. Their unusual characteristics (SFRs=200-2000Msun/yr) pose a unique challenge for cosmological simulations and galaxy formation theory, particularly at early times. Although rare today, they were factors of 1000 times more prevalent at z~2-5, contributing significantly to the buildup of the Universe’s stellar mass and the formation of high-mass galaxies. However, an ongoing debate lingers as to their evolutionary origins at high-redshift, whether or not they are triggered by major mergers of gas-rich disk galaxies, or if they are solitary galaxies continually fed pristine gas from the intergalactic medium. Observational evidence has been mixed over recent years; some studies clearly point to chaotic kinematic histories and fast gas depletion times (~<100Myr), while other work may demonstrate secular (though active) disks can sustain high star-formation rates over long periods of time. Similarly, some works argue such extreme star-formers contribute very little to cosmic star-formation, while others find quite the opposite. Furthermore, their presence in early protoclusters, only revealed quite recently, pose intriguing questions regarding the collapse of large scale structure. I will discuss some of the latest observational programs dedicated to understanding their origins and frequency at early times, their context in the cosmic web, and future long-term observing campaigns that will reveal their relationship to “normal” galaxies, thus teaching us valuable lessons on the physical mechanisms of galaxy growth and the collapse of large scale structure in an evolving Universe.
Because faint, low mass galaxies are numerous at high redshifts, their impact on the Universe is expected to be significant. They may host a substantial fraction of the Universe’s star formation, provide many of the energetic photons needed to reionize the hydrogen gas surrounding galaxies, and affect their surroundings via powerful, starburst-driven galactic outflows. Because of their faintness, however, the properties of these galaxies are difficult to determine. I will discuss a variety of observations aimed at characterizing the physical conditions in low mass galaxies during the peak epoch of star formation, when the Universe was ~20% of its current age, with particular emphasis on the study of galactic outflows in faint galaxies.