December 3, 2015 3:30PM BPB-207. Phil Armitage. New directions in astrophysical accretion.

Accretion disks play a critical role in planet formation, and in compact object and black hole astrophysics. I will discuss the implications of recent numerical simulations of astrophysical disks for both classes of system, which challenge long-held assumptions as to how disks work. In protoplanetary disks, the non-ideal physics of the Hall effect appears to dominate disk evolution on planet-forming scales, and this may lead to a bimodal distribution of planetary system properties. In black hole disks I will argue that strongly magnetized disks provide an appealing framework for addressing a range of unexplained observations. I will discuss the obstacles and prospects for developing a theory for the long-term evolution of both planet forming and black hole accretion flows.

November 20, 2015 3:30PM BPB-207. Bharat Ratra. Dark Energy: constant or time variable? (... and other open questions).

Experiments and observations over the last decade and a half have persuaded cosmologists that (as yet undetected) dark energy is by far the main component of the energy budget of the universe. I review a few simple dark energy models and compare their predictions to observational data, to derive dark energy model-parameter constraints and to test consistency of different data sets. I conclude with a list of open cosmological questions.

November 13, 2015 3:30PM BPB-207. Kailash Sahu. Detecting Exoplanets, and Isolated, Stellar-Mass Black Holes through Microlensing.

This talk consists of two parts. In the first part, I will review how almost all the extra-solar planets discovered so far have been relatively nearby --- within ~500 parsecs from the Sun, and within about 2 AU from their parent stars. I will discuss how microlensing provides insights into the frequency of planets across the Milky Way, and up to several AU from the host stars. In the second part, I will discuss the technique of astrometric microlensing, and two HST programs underway aimed at the first detections of isolated, stellar-mass black holes through this technique.

November 13, 2015 10:30AM BPB-248. Daniel Antonio Phd. Defense. High Pressure X-Ray Absorption Spectroscopy Studies of Heavy-Fermion Cerium and Uranium Compounds.

Investigations into f- electron heavy-fermion materials have revealed a wide range of novel behavior. Hydrostatic pressure is a valuable "clean" non-thermal parameter that can be used to systematically study them by tuning their ground state properties. The rare earth compound CeCu2Ge2 shows an unusual two-domed region of unconventional superconductivity under pressure, similar to its isostructural counterpart CeCu2Si2. While the lower pressure dome at about 10 GPa is caused by a magnetic quantum critical point, the higher one at about 16 GPa is less well understood. Previous structural measurements have indicated that it may be caused by critical valence fluctuations, so in this study the valence of CeCu2Ge2 is directly measured using X-ray Absorption Near Edge Spectroscopy (XANES) under pressure in a diamond anvil cell up to 20 GPa. An expected valence discontinuity is not seen, but comparisons to CeCu2Si2 show interesting similarities. Uranium's 5f electrons are intermediate between localized and delocalized. Studying the degree of localization is vital to completely understanding properties of actinides. Using XANES and partial Florescence Yield (PFY) in a diamond anvil cell to tune the distance between uranium atoms, I have measured the energy shift in the white line of UCu2Si2, U3Ni5Al19, and UCd11 with pressure. A positive shift in energy indicated a delocalization of 5f electrons, a change in 5f configurations, or a combination of both.

November 6, 2015 3:30PM BPB-207. Duncan Lorimer. Fast Radio Bursts: The Story So Far.

I will describe a brief history of discovery and some exciting recent developments in the world of pulsars and fast radio bursts. Pulsars, rapidly rotating highly magnetized neutron stars, were discovered in 1967 and continue to surprise and delight astronomers as powerful probes of fundamental physics and astrophysics. Fast radio bursts are millisecond-duration pulses of currently unknown origin that were discovered in 2007. Both pulsars and fast radio bursts have great promise at probing the universe on large scales and in fundamental ways. I will describe the science opportunities these phenomena present, and discuss the challenges and opportunities presented in their discovery.

October 26, 2015 12:00PM BPB-207. Yan-Fei Jiang. Iron Opacity Bump Changes the Structure and Stability of Black Hole Accretion Disks in AGNs.

In the standard thin accretion disk model, electron scattering is usually thought to be the dominant opacity in the radiation pressure dominated inner region of black hole accretion disks. I will show that this is not the case for most AGNs, where the density and temperature ranges in the accretion disks are similar as in the envelopes of massive stars and opacity bump caused by Irons are much more important. Through a series of self-consistent radiation MHD simulations, I will show that the iron opacity bump can significant decrease the growth rate of thermal instability in the standard thin accretion disk model compared to case with dominated electron scattering opacity. It can even stabilize the disk. Viscous instability also does not exist for AGN disks with iron opacity bump. I will also discuss implications of the iron opacity bump on resolving various puzzles between standard thin disk models and observations.

October 16, 2015 3:30PM BPB-207. Jason Steffen. Laser Probes of the Dark Sector.

The majority of the universe is composed of exotic matter and energy of which we know very little. This "dark sector" comprising dark matter, dark energy, and gravity, is likely to be one of the primary scientific challenges of the 21st century. I report on a series of three laboratory experiments at Fermilab to probe this dark sector. These experiments include searches for dark matter axions, dark energy chameleons, and tests of the very nature of space time.

Jason Steffen featured in online UNLV news "New Faces".

New faces introduces new faculty to the university community. The article on Jason is here.

Michael Pravica receives 2015 Stewardship Science Academic Alliances Award.

Michael Pravica recently received a 2015 Stewardship Science Academic Alliances award through the U.S. Department of Energy's National Nuclear Security Administration. The three-year, $780,000 grant is to develop a novel field of science Pravica has been developing, called "useful hard X-ray photochemistry." He takes advantage of the highly focused, highly penetrating, and highly ionizing properties of hard X-rays (>7keV) to initiate novel decomposition and synthetic chemistry under isolated or extreme conditions.

October 9, 2015 3:30PM BPB-207. Kazumi Kashiyama. Searching for Newborn Black Holes and Magnetars

Stellar-mass black holes and magentars have been identified by their bright non-thermal emissions. It is highly uncertain how these objects are formed, which is a missing link of massive stellar evolution. Newborn black holes and magnetars in collapsing massive stars have been considered as the central engine of e.g., gamma-ray bursts and superluminous supernovae. However, the event rates of such luminous transients are extremely small, say ~ 0.01 % of core-collapse supernovae. On the other hand, the formation rates of black holes and magnetars has been estimated to be possibly as high as ~10% of core-collapse supernovae. Thus, a large fraction of black-hole and magnetar formation would occur with ordinary supernovae or less prominent new transients. The key questions are "What are possible smoking guns of black-hole and mangetar formation?", "Whether they can be detectable by multi-messenger time-domain astronomy in the coming years?", and "What is the optimal observational strategy?". I will present some theoretical modeling of multi-messenger signals from newborn black holes and magnetars in collapsing massive stars and discussing the detectability by using current and future observational facilities.

October 2, 2015 3:30PM BPB-207. Drew Clausen. Using Tidal Disruption Events to Study Massive Black Holes and Their Environments.

When a black hole tidally disrupts a star, accretion of the debris will produce a luminous flare and reveal the presence of a dormant black hole. Emission lines produced when the stellar debris and/or other gas in the black hole's vicinity are photoionized by the accretion flare have considerable diagnostic power. I will discuss models of the emission line spectrum produced in the debris released when evolved stars are tidally disrupted by an intermediate-mass black hole (100-10000 solar masses), and discuss the possibility of using the emission lines to identify such events and constrain the properties of the black hole. While there is some agreement between these models and observations of white dwarf tidal disruption candidates in globular clusters associated with NGC 4472 and NGC 1399, there are also drawbacks to interpreting these sources as tidally disrupted white dwarfs. I will also present results from time-dependent photoionization calculations that model the emission line spectrum produced when ambient, circumnuclear gas is illuminated by a tidal disruption flare. The emission line light curves are consistent with the transient extreme coronal line emitters recently identified in SDSS. These tidal disruption event light echoes can be used to probe the circumnuclear environments of quiescent galaxies and to constrain the extreme UV component of tidal disruption flares.

UNLV Physics & Astronomy Forum Schedule

Public Talk Thursday 9/17: Mario Livio, Brilliant Blunders.

The Russell Frank Astronomy Lecture Series

Brilliant Blunders

internationally known astrophysicist and best-selling author Dr. Mario Livio

This talk is intended for a general audience including enthusiasts of all backgrounds and ages.

Dr. Livio is the author of acclaimed popular level books including The Golden Ratio, The Equation That Couldn't Be Solved, Brilliant Blunders, and Is God a Mathematician?. He has authored many scientific publications as well. You can listen to a recent KNPR interview and see other talks he has given here.

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September 18, 2015 3:30PM BPB-102. Mario Livio, Type Ia Supernovae: Progenitors and Cosmology.

I will review the current status concerning the identification of the progenitor systems of Type Ia supernovae, in light of recent observations and theoretical developments. I will also show how current and future observations can be used to tame evolutionary effects, on the road to better constraints on the nature of dark energy.

September 11, 2015 3:30PM BPB-217. Fan Guo, Magnetic Reconnection: A Powerful Cosmic Particle Accelerator.

Astrophysical magnetic reconnection sites have long been expected to be sources of high-energy particles. Recent observations of high-energy gamma-ray flares from the Crab nebula and models of gamma-ray bursts and TeV blazars have motivated us to better understand magnetic reconnection and its associated particle acceleration in plasma conditions where the magnetic energy is dominant. We will present fully kinetic particle-in-cell simulations of anti-parallel magnetic reconnection in the highly magnetized regime (the magnetization parameter sigma >> 1). The magnetic energy is converted efficiently into kinetic energy of nonthermal relativistic particles in a power-law spectrum. For a sufficiently large system and strong magnetic field, the power-law index approaches "p=1". The dominant acceleration mechanism is a first-order Fermi process accomplished through the curvature drift motion of particles in magnetic flux tubes along the electric field induced by fast plasma flows. We will also present an analytical model for the formation of power-law distribution and show the nonthermal distribution may be a common feature of magnetically dominated reconnection.

September 4, 2015 3:30PM BPB-217. Daniel Whalen, Finding the First Cosmic Explosions.

Primordial stars formed about 200 Myr after the big bang, ending the cosmic dark ages. They were the first great nucelosynthetic engines of the universe and may be the origins of the supermassive black holes found in most massive galaxies today. In spite of their importance to the evolution of the early universe not much is known for certain about the properties of Pop III stars. Until now, observers have looked for the nucleosynthetic imprint of Pop III SNe in the chemical abundances in ancient, dim metal-poor stars to constrain the masses of the SN progenitors. But with the advent of JWST, WFIRST and the 30 m telescopes it may soon be possible to directly observe the explosions themselves in the NIR and thus unambiguously constrain the properties of the first stars. I will present radiation hydrodynamical calculations of the light curves of the first SNe in the universe and discuss strategies for their detection. I will also describe how some may already have been captured in surveys of galaxy cluster lenses such as CLASH, Frontier Fields and GLASS.

Lon Spight rememberance Friday August 28, 2015; 3:30-6:30 BPB-102

The Department of Physics & Astronomy will host an informal memorial for our recently departed friend and colleague Dr. Lon Spight -- one of the founders of our department.

August 21, 2015 10:00AM BPB-217. Wei-Hua Lei, On-beam and Off-beam Jetted TDEs.

A star or sub-stellar object will be destroyed by tidal forces when it passes close enough by a supermassive black hole (SMBH). These events, known as tidal disruption events (TDEs), are expected to produce luminous flare emission in the UV to X-ray band. Recent observations of Sw J1644+57, in particular, suggest that at least some TDEs can launch a relativistic jet. A common speculation is that these rare events are related to rapidly spinning BHs. We constrained the BH spin parameter by using the available data, and found that the BH indeed carries a moderate to high spin, suggesting that BH spin is likely the crucial factor behind the Sw J1644+57-like events. Other observational properties include the rough 2.7 day periodicity in X-ray dips and 200s QPO, which we interpret as due to precession of the jet. In addition, Sw J2058+05 and Sw J1112.2-8238 are also thought to be a TDE with an on-beam relativistic jet. It is natural to expect that there should be some events with off-beam ones. We will present the first candidate for such off-beam jetted TDEs

Ye Li's work on life in the early universe discussed in New Scientist

Yi Li and Bing Zhang's paper due to be published in The Astrophysical Journal: Can life survive Gamma-Ray Bursts in the high-redshift universe?, is discussed in a daily news article in The New Scientist. That article is titled: Giant old galaxies, not Milky Ways, are best for life to thrive.

Prof. Bing Zhang receives NASA Astrophysics Theory Program Award.

Prof. Bing Zhang received a near $400,000 research grant from NASA's Astrophysics Theory Program. The grant is to support his research group in developing novel theoretical models of relativistic astrophysical jets in the magnetically dominated regime. The models will use a relativistic magnetohydrodynamic (MHD) code and conduct global and local simulations of colliding magnetically dominated shells. They also will investigate the detailed physical processes of collision-triggered magnetic reconnection and dissipation, particle acceleration and radiation, as well as the light curves, spectra, and polarization properties of the emission. The results from the proposal will be applied to understand the physical mechanisms of gamma-ray bursts and other relativistic jets in the universe.

May 14, 2015 1:00PM BPB-217. PhD. Defense: Wei Deng, Study the Mechanism of the Prompt Emission of Gamma-Ray Bursts.

PhD Defense

The prompt emission of Gamma-Ray Bursts (GRBs) has been detected over 40 years, but the mechanism of the prompt emission is still a mystery. The problem can be summarized as several debatable questions: What is the energy composition of the jets/outflows? What is the energy dissipation mechanism? How are the particles accelerated during the energy dissipation process? What is the radiation mechanism to produce the observed prompt emission? In order to solve these problems, several theoretical models have been proposed, including the classical "internal shock (IS)" model, the "dissipative photosphere" model, and the "Internal-Collision-induced MAgnetic Reconnection and Turbulence (ICMART)" model. This dissertation includes two parts. The first part is related to the photosphere model of prompt emission. The second part presents a global special relativistic MHD simulations based on the ICAMRT scenario.

May 1, 2015 10:30AM TBE A-120. (HiPSEC thesis defense) Niup Bandaru, STRUCTURE AND OPTICAL PROPERTIES OF TRANSITION METAL DICHALCOGENIDES (TMDs) – MX2 (M = MO, W & X = S, Se) UNDER HIGH PRESSURE AND HIGH TEMPERATURE CONDITIONS.

Layered structured materials such as transition metal dichalcogenides (TMDs) have gained immense interest in recent times due to their exceptional structural, electrical and optical properties. Recent studies show semiconducting TMDs such as MX2 (M= Mo, W & X = S, Se) could be used as potential shock absorbing material, which has resulted in extensive studies on structural stability of these materials under the influence of high pressure. Understanding the structural stability of transition metal dichalcogenides (TMDs) such as MoS2, MoSe2, WS2, and WSe2 under high pressure has been very challenging due to contradicting observations and interpretations reported in the past. Hence, the main objective of this work is to study the crystal structure and optical properties of bulk MX2 at high hydrostatic pressures up to 51 GPa using a diamond anvil cell with synchrotron radiation in addition to high pressure Raman spectroscopic and high temperature X-ray diffraction (XRD) experiments. Crystal structures of MX2 materials are observed to be stable up to 500C with nonlinear thermal coefficients of expansion. Results of high pressure experiments show a pressure induced isostructural hexagonal distortion to a 2Ha-hexagonal P63/mmc phase in MoS2 around 26 GPa as predicted by theoretical calculations reported earlier. No pressure induced phase transformation is observed in other MX2 (MoSe2, WS2, WSe2) compounds. A semi empirical model based on the energy of interaction of bond electrons is proposed to explain the observed inconsistency between MoS2 and other TMDs studied. Using this model, it is shown that except MoS2, no other MX2 within the scope of this study undergoes pressure induced phase transition in the pressure range 0 - 50 GPa. High pressure Raman results show continuous red shifts in dominant vibrational modes with increase in pressure in MX2. Additionally, emergence of a new peak, namely 'd-band' associated with 2Ha structure in MoS2 supports the observation of a isostructural phase transition in high pressure X-ray diffraction experiments. In addition to the studies on bulk MoS2 material, thin film (approximately 100 nm thicknesses) is successfully fabricated via DC magnetron sputtering system and sulfurization technique.

April 24, 2015 3:30PM BPB-217. Jason Rhodes, Exploring The Dark Sector with Euclid and WFIRST-AFTA.

Dark energy, the name given to the cause of the accelerating expansion of the Universe, is one of the most profound mysteries in modern science. Current cosmological models hold that dark energy is currently the dominant component of the Universe, but the exact nature of dark energy remains poorly understood. There are ambitious ground-based surveys underway that seek to understand dark energy and NASA is participating in the development of significantly more ambitious space-based surveys planned for the next decade. NASA is providing mission-enabling technology to the European Space Agency's (ESA) Euclid mission in exchange for US scientists to participate in the Euclid mission. NASA is also developing the Wide Field Infrared Survey Telescope-Astrophysics Focused Telescope Asset (WFIRST-AFTA) mission for possible launch in 2023. WFIRST was the highest ranked space mission in the Astro2010 Decadal Survey and the AFTA incarnation of the WFIRST design uses a 2.4m space telescope to go beyond what the Decadal Survey envisioned for WFIRST. Understanding dark energy is one of the primary science goals of WFIRST-AFTA. I'll discuss the status of Euclid and WFIRST and comment on the complementarity of the two missions. I'll also briefly discuss other, exciting science goals for WFIRST, including a search for exoplanets using both microlensing and a dedicated coronagraph for exoplanet imaging.

April 10, 2015 3:30PM BPB-217. Joshua C. H. Lui, Shedding light on two-dimensional electrons in graphene and beyond.

Graphene, a single layer of carbon atoms, has stimulated intense scientific interest due to its distinctive electronic and mechanical properties. Graphene exhibits strong interactions with light over a broad spectral range. This enables us to examine its electronic and vibrational properties through optical spectroscopy. In addition to gaining understanding of the properties of single-layer graphene, we can also probe the behavior of electrons in few-layer graphene. This reveals the unique electronic and vibrational properties for graphene of each layer thickness and stacking order, as well as their distinct capability to induce an electrically tunable band gap. I will also highlight recent development of 2D materials beyond graphene.

April 9, 2015 9:00AM BPB-217. PhD. Defense: Houjun Lu, Constraining the Progenitor and Central Engine of Gamma-Ray Bursts with Observational Data.

Gamma-ray bursts (GRBs) are extremely energetic explosions at cosmological distances. We have made great progress in understanding the mysteries of these events since they were discovered more than forty years ago. However, some open questions still remain, e.g. how many classes of GRBs are there, what are the progenitors of these classes, and what is the central engine powering these huge explosions? Thanks to the NASA missions Swift and Fermi, which are used to detect the multi-wavelength emission from these transients, our understanding of GRBs has been greatly advanced. In this dissertation, I use multi-wavelength data to constrain the progenitor and central engine of GRBs. My dissertation consists of three parts: (1) By adding the third dimension ``amplitude'' as a complementary criterion in classifying GRBs, we test whether some short GRBs are ``tip-of-iceberg'' of long GRBs, and explain why some high redshift long GRBs have short durations in the rest frame. (2) Using Swift data, we investigate whether the data are consistent with the hypothesis that there exist millisecond magnetar central engines in some long GRBs. We reach the conclusion that at least some long GRBs have a magnetar central engine. (3) We test how well the data are consistent with the magnetar central engine model for short GRBs. We identify that a good fraction of short GRBs have a supra-massive magnetar central engine, which collapses to a black hole after hundreds of seconds. We use the data to constrain the neutron star equation of state.

April 7, 2015 3:30 BPB-217. LiDong Pan, How to weigh a monopole – studying complex material response with THz spectroscopy.

An important theme of modern condensed matter physics is the realization of novel excitations in materials (e.g. quasiparticles). Although they are not fundamental particles, such quasiparticles do constitute the most basic description of the excited states of the "vacuum" they reside in. In this regard the magnetic textures of the excited states of spin ices, magnetic pyrochlore oxides with dominant Ising interactions, can be modeled as effective magnetic monopoles. Utilizing the unique phase sensitive capabilities of time domain terahertz spectroscopy and microwave cavity techniques, we study the complex dynamic magnetic susceptibility of quantum spin ice Yb2Ti2O7. We find strong evidence of inertial effects in the monopoles dynamics. From the spectral weight, an effective mass of the monopoles is also obtained. Our results establish the magnetic monopoles as true coherently propagating quasiparticles in quantum spin ice.

March 27, 2015, 3:30 BPB-217. Kentaro Nagamine, Accelerated Structure Formation at High Redshift.

I will present two topics that suggest an accelerated growth of structures in high-density regions of the early universe. First is the birth of supermassive black hole (SMBH), and second is the formation of massive disk galaxies, both at redshifts z>6. Recent discoveries of billion solar mass SMBH at z=6-7 suggests that the gas accretion was quite rapid in the early universe with a super-Eddington rate. I will discuss a scenario called the 'Direct Collapse' of BH seed at high redshift, which has been attracting significant attention lately. I will also present our cosmological SPH simulation results of high-redshift galaxies at z=6-12, and discuss their observability with ALMA.

March 24, 2015 3:30 BPB-217. Kayhan Gultekin, Supermassive Black Holes

Supermassive black holes are some of the most fascinating energetic objects in the Universe, and they play a key role in what we can see across cosmic time and a large range of critical astrophysical phenomena. Despite their importance, much is unknown about their basic physics including how they were formed, how they grow, how they appear in different wavelengths, and what kind of galaxies they live in. The answers to many of these basic questions are within reach. I will review my recent, current, and future research plans to find their solutions.

March 19, 2015 3:30 BPB-217. Ashkan Salamat, A liquid-liquid phase transition to Metallic Hydrogen

Hydrogen is the simplest and most abundant element in the Universe. It is estimated that more than 70% of the planetary mass in our solar system is in the form of dense fluid hydrogen, with Jupiter and Saturn being the largest reservoirs. These planets contain extreme pressure and temperature conditions which are predicted to lead to the formation of metallic fluid hydrogen. The pressure-induced transition from insulator to metal in solid hydrogen was predicted as early as 1935 by Wigner and Huntington, but to date has not been experimentally confirmed. Metallic hydrogen is predicted to have spectacular properties such as room temperature superconductivity and metastability (i.e. it remains metallic when the pressure is released). If metallic hydrogen is found to be metastable, its application could revolutionize rocketry and fusion technology. There are two thermodynamic pathways to metallic hydrogen: direct pressurization at low or modest temperatures to a solid metallic phase, and in the megabar pressure region, heating into the liquid metallic phase. In this talk, I will present my recent work in which the insulator to metal transition in dense liquid hydrogen was observed experimentally for the first time.

March 18, 2015 10:45 BPB-217. Wei Bao, High-Pressure Single Crystal Neutron Scattering Study of the 245 Superconductor.

The iron vacancy order and the block antiferromagnetic order exist in the new iron 245 superconductors [1,2]. The appearance of the superconductivity crucially depends on the perfectness of the vacancy order [3]. The magnetic and vacancy orders in superconducting (Tl,Rb)2Fe4Se5 (245) single-crystals were investigated using high-pressure neutron diffraction technique [4]. Similar to the temperature effect, the block antiferromagnetic order gradually decreases upon increasing pressure while the Fe vacancy superstructural order remains intact before its precipitous drop at the critical pressure Pc =8.3 Gpa. Combining with previously determined Pc for superconductivity, our phase diagram under pressure reveals an intimate connection among the block antiferromagnetic order, the Fe vacancy order and superconductivity for the 245 superconductor. Similar connection between the perfectness of crystalline order and superconductivity has been previously demonstrated in our neutron scattering study on related Fe based superconductors [5,6].

March 17, 2015 3:30 BPB-217. Jake Simon, Probing the Nature of Accretion and Planet Formation in Protoplanetary disks: Connecting Theory with ALMA Observations.

Protoplanetary disks play a key role in star and planet formation processes. Turbulence in these disks, which arises from the magnetorotational instability (MRI), not only causes accretion of mass onto the central star, but also sets the conditions for processes such as dust settling, planetesimal formation, and planet migration. However, the exact nature of this turbulence is still not very well constrained in these systems. In this talk, I will first present recent numerical simulations of magnetohydrodynamic (MHD) turbulence in protoplanetary disks that point to the importance of large scale, vertical magnetic fields in driving disk accretion through both turbulent processes and magnetic winds. I will then describe new work, utilizing both state-of-the-art numerical simulations and powerful new radio observations, to directly link numerical predictions for the turbulent velocity structure of protoplanetary disks to observations by the Atacama Large Millimeter Array (ALMA). ALMA’s unprecedented resolution and sensitivity will allow us to generate a three-dimensional map of disk turbulence by measuring the turbulent broadening component of molecular lines at different disk heights (i.e., optical depths) and radii. A direct comparison between the observed turbulence values and those obtained from simulations will strongly constrain our theoretical understanding of these disks. I will conclude with an outlook for protoplanetary disk studies, and in particular how our current results may influence studies of planet formation processes and the construction of exoplanetary systems.

March 13, 2015 3:30 BPB-217. Florian Rodler, High-resolution spectroscopy of exoplanet atmospheres is cool!

High-resolution spectroscopy is a powerful tool to investigate the atmospheres of transiting exoplanets as well as of non-transiting ones. I will briefly review the basics underlying that technique and present the most important results up to now. In the light of the three upcoming Extremely Large Telescopes (ELTs), i.e. the GMT, TMT and E-ELT, I will further present feasibility studies dedicated on the detection of oxygen in the atmospheres of Earth-like planets orbiting M-dwarfs.

March 12, 2015 3:30 BPB-217. Norbert Werner, How supermassive black holes and star-formation sculpt the visible Universe

In the course of structure formation, only a small fraction of the baryons turned into stars - most remain in a diffuse intergalactic medium. The growth of galaxies is regulated by feedback processes, such as energy and momentum input from supernovae, and jets and winds of accreting supermassive black holes. These processes, collectively called galactic feedback, can limit or even inhibit star formation, and thus a detailed knowledge of how they work is essential for our understanding of galaxy formation and evolution. I will start my talk by presenting recent observational results on the role of supermassive black holes in keeping the most massive galaxies 'red and dead'. Then, I will 'zoom out' to the outskirts of galaxy clusters where we also find hints that supermassive black holes played an important role in the distant past. X-ray observations with the Suzaku satellite reveal a remarkably homogeneous distribution of iron out to the virial radius of the nearby Perseus Cluster, requiring that most of the metal enrichment of the intergalactic medium occurred before the cluster formed, probably more than ten billion years ago, during the period of maximal star formation and black hole activity. Finally, I will talk about the upcoming ASTRO-H satellite which will revolutionize X-ray spectroscopy and our understanding of how feedback processes couple to the intergalactic medium.

March 10, 2015 3:30 BPB-217. Jason Steffen, Properties of exoplanets and systems with Kepler

NASA's Kepler mission has revolutionized the field of exoplanets and its discoveries give new insights into our theories of planet formation and dynamical evolution. With over 4000 planet candidates and 1000 confirmed planets, the variety of systems and planets shows the breadth of properties that planet formation models must encompass. I present some of the landmark results of the Kepler mission, especially relating to the planet masses and orbital architectures of the planetary systems. I discuss how these results affect our understanding of the solar system and of planets in general.

March 6, 2015 3:30PM BPB-217. Samaya Nissanke, Follow the chirp: seeing and listening to the transient Universe

The mergers of binary compact objects (black holes, neutron stars, white dwarfs) are amongst some of the most violent events in the Universe. The physics driving these events in strong field gravity are extremely complex, rich but still remain elusive. These cosmic laboratories present us now with both a challenge and an opportunity. The challenge is to explain the physics at play in strong-field gravity in Universe. The opportunity is to detect the accompanying radiation and panoply of multi-messenger particles (high energy neutrinos, cosmic rays and gravitational waves) for the first time with a suite of time-domain telescopes and experiments. In this pivotal new era of multi-messenger astronomy, the most compelling astrophysical sources are neutron star binary mergers, which should emit both in electromagnetic (EM) and gravitational waves (GW). I will first review the most recent advances in this blossoming field of EM+GW astronomy, which combines two active disciplines: time-domain astronomy and general relativity. I will discuss the promises of this new convergence by illustrating the wealth of astrophysical information that a combined EM+GW measurement would immediately bring. I will then outline the main challenge that lies ahead for this new field in pinpointing the sky location of neutron star mergers using GW detectors and EM wide-field synoptic surveys.

March 6, 2015 10:00AM BPB-217. Jean-Michel Desert, New Frontiers in Exoplanetary Science

The field of exoplanetology has recently transitioned from the investigation of individual objects to statistical studies. However, answers to key questions in exoplanetary science come not only from the statistics of discovery surveys, but also from the detailed characterisation of individual systems. I argue that the study of exoplanet atmospheres and their diversity is the next step in leveraging their detections. This is because a planet's atmosphere provides a fossil record of its primordial origins and controls its fate, size and appearance. The study of exoplanet atmospheres thus is crucial to answer fundamental questions in planetary formation and exoplanetary physics. In this context, I present new results from ongoing comparative exoplanetology programs that aim to characterise planetary systems transiting nearby stars through the observations of their atmospheres. This is achieved by combining ground- and space-based multi-wavelength observations secured over wide spectral regions. The results on the atmospheric composition and physical properties provide insights into the formation and evolution of planetary systems and enhance our understanding of our own Solar System's formation. Finally, I also present strategies for probing rocky exoplanet atmospheres orbiting in the habitable zone of their parent stars, and for searching for bio-signatures with future facilities.

February 20, 2015, 9:30AM BPB-217, Patricia Kalita, Ph.D. Defense

High Pressure Behavior of Mullite-Type Materials: Phase Transitions, Negative Linear Compressibility, Amorphization and Other Microstructural Implications

Even though mullite occurs rarely in natural rocks, it is perhaps one of the most important phases in both traditional and advanced ceramics and thus one of the most widely studied ceramic phases. Existing and emerging applications include: high temperature materials, aerospace materials, ballistic shielding components and even non-linear optical materials. There are many uncertainties regarding the basic physical properties of mullite-type materials, particularly in terms of their high-pressure phase stability and mechanical behavior that are important to address for new applications of mullites as an engineering materials. This work presents results of structural investigations of mullites and mullite-type materials at extreme pressures using synchrotron x-ray diffraction and laser Raman spectroscopy. The two experimental techniques used in this work are ideally suited to provide a synergical interplay in the study of mullites under high-pressure conditions: Raman spectroscopy is a technique for investigating short range order phenomena while x-ray diffraction accesses phenomena occurring at the long range order. Outcomes discussed include pressure-driven amorphization, phase transitions and negative linear compressibility.

February 27, 2015: Shri Kulkarni, Russell Frank Lecture Series

Professor Shri Kulkarni Director of the Caltech Optical Observatories will present a lecture entitled "The Restless Universe and the Palomar Transient Factory”

7:30pm Friday February 27 Bigelow Physics Building

Cosmic explosions were first noted nearly two thousand years ago Together, supernovae and variable stars have contributed richly to key problems in modern astrophysics: distances to galaxies, cosmography and the build up of elements in the Universe.

The Palomar Transient Factory (PTF), an innovative 2-telescope system, was designed to explicitly to chart the transient sky with a particular focus on events which lie in the nova-supernova gap. The PTF is now finding an extragalactic transient every 20 minutes and a Galactic (strong) variable every 10 minutes. The results so far: ultra-luminous supernovae as the end of the most massive stars in the Universe, progress in understanding the origin of Ia supernovae (which were used by astronomers to discover dark energy), and the identification of curious events of value to future gravitational wave observatories in space.

Admission is free. This talk is intended for a general audience including enthusiasts of all backgrounds and ages.

best regards
George Rhee

Jan 30, 2015 Forum: Fulvio Melia, The Zero Active Mass Condition in FRW Cosmologies

Abstract: The standard model of cosmology is based on the Friedmann-Robertson-Walker (FRW) metric. Often written in terms of co-moving coordinates, this elegant and highly practical solution to Einstein's equations is based on the Cosmological principal and Weyl's postulate. But not all of the physics behind such symmetries has yet been recognized. We invoke the fact that the co-moving frame also happens to be in free fall to demonstrate that the FRW metric is valid only for a medium with zero active mass. In other words, the application of FRW appears to require an equation-of-state rho+3p = 0, in terms of the total energy density rho and total pressure p. Though the standard model is not framed in these terms, the optimization of its parameters brings it ever closer to this constraint as the precision of the observations continues to improve.

UNLV Physics & Astronomy Forum is held in the conference room of the Bigelow Physics Building, BPB-217 at 3:45PM.  Refreshments at 3:30.