List of Past CTC Theory Lunches : 01-Sep-2015 to 31-Dec-2015

Date:   Monday 13-Jul-2015
Speaker:   Joonas Nättilä (University of Turku)
Title:  "Equation of state constraints for the cold dense matter inside neutron stars"

In my talk I will describe how observations of thermonuclear explosions on top of neutron stars ends up constraining the size of these ultra-compact stars. I will also show how we can model these explosions and how the atmosphere of the star modifies the emerging spectrum. As it turns out, good understanding of the physics behind these powerful bursts is also crucial for accurate mass and radius measurements. From the size measurements we can then obtain a parameterized equation of state (EoS) of the cold dense matter by using Bayesian methods. This allows us to set limits on various nuclear parameters and to constrain an empirical pressure-density relation for the dense neutron matter.

Date:   Monday 03-Sep-2015
Speaker:   Andrea Morandi (University of Alabama)
Title:  "Understanding the physics of galaxy clusters"

Recent years have witnessed the emergence of galaxy clusters as a fundamental cosmological and astrophysical tool. In this talk I will review recent advances in the physics of clusters. I will focus on what can be learned from X-ray, Sunyaev Zel'dovich and lensing observations, and hydrodynamical numerical simulations. I will discuss some recent developments in the modeling of both dark matter and the intracluster medium, in particular with respect to the first determination of the intrinsic shape and physical parameters of both the dark matter and intracluster medium in triaxial galaxy clusters via a multiwavelength analysis. Next, I will discuss a method to determine the physical parameters in clusters, including gas temperature and peculiar velocities, via multi-frequency Sunyaev Zel'dovich observations and independent of X-ray observations. Finally, I will review recent developments in the virialization region of clusters, which has recently gained a lot of attention in the scientific community in offering a direct view of structure formation. In particular, I will present novel results on the physical properties of the intracluster medium at the cluster outskirts, including average emission measure, gas density and gas fraction, temperature, entropy and gas inhomogeneities.

Date:   Monday 05-Oct-2015
Speaker:   Yi Cao (Caltech)
Title:  "Fast and Furious: Young Supernovae and Their Progenitors"

Observations of supernovae within a few days of explosion provide unique constraints to the nature of their progenitors. Since 2013, as a part of my thesis, I used the intermediate Palomar Transient Factory (iPTF) to systematically study extraordinarily young supernovae. I will first provide an overview of iPTF survey design, summarize the fast turnaround discovery pipeline and then outline the rapid response follow-up. The highlights of my work are as follows: 1) We observed a strong declining UV emission from a low-velocity Type Ia supernova which is consistent with the expected emission from a supernova slamming into a companion star. Our observation evidently suggests that some Type Ia supernovae arise from the so-called "single degenerate" channel. 2) We identified the first progenitor candidate of a Type Ib supernova in the pre-explosion HST images. Our multi-wavelength observations of this young Type Ib supernova constrain its progenitor to be smaller than several solar radii and with strong mass loss. 3) We also found a peculiar hydrogen-deficient supernova which shows a double-peaked lightcurve, low peak luminosity, slow expansion velocity and fast declining. This supernova is consistent with formation of a black hole from core-collapse of a massive star. I will end my talk with prospects for this field with the upcoming Zwicky Transient Facility.

Date:   Monday 19-Oct-2015
Speaker:   John Chisholm (University of Wisconsin)
Title:  "Shining a light on star formation driven outflows: the physical conditions within galactic outflows"

Stellar feedback drives energy and momentum into the surrounding gas, which drives gas and metals out of galaxies through a galactic outflow. Unfortunately, galactic outflows are difficult to observe and characterize because they are extremely diffuse, and contain gas at many different temperatures. Here we present results from a sample of 37 nearby (z < 0.27) star forming galaxies observed in the ultraviolet with the Cosmic Origins Spectrograph on the Hubble Space Telescope. The sample covers over three decades in stellar mass and star formation rate, probing different morphologies such as dwarf irregulars and high-mass merging systems. Using four different UV absorption lines (O I, Si II, Si III and Si IV) that trace a wide range of temperatures (ionization potentials between 13.6eV and 45eV), we find shallow correlations between the outflow velocity or the equivalent width of absorption lines with stellar mass or star formation rate. Absorption lines probing different temperature phases have similar centroid velocities and line widths, indicating that they are comoving. Using the equivalent width ratios of the four different transitions, we find the ratios to be consistent with photo-ionized outflows, with moderately strong ionization parameters. By constraining the ionization mechanism we model the ionization fractions for each transition, but find the ionization fractions depend crucially on input model parameters.

Date:   Monday 26-Oct-2015
Speaker:   Kouichi Hagino (ISAS)
Title:  "An Observational Study of Ultra-fast Outflows in Active Galactic Nuclei with 3-dimensional Radiative Transfer Calculation"

The ultra fast outflows (UFOs) in AGN are potentially the most effective process for transporting energy to their host galaxies, so that they are considered to play an important role in the coevolution of supermassive black holes and galaxies. However, the physical properties of UFOs are still unknown. We have constructed a new X-ray spectral model for the AGN outflows, and applied it to the multiple observational data. Our new code can calculate the radiation transfer in the realistic 3-dimensional geometry of the accretion disk winds by utilizing a Monte Carlo method. Our spectral model successfully reproduces the emission lines as well as the absorption lines of H/He-like Fe ions. I will present our recent results, and discuss the launching mechanism of the UFOs.

Date:   Monday 02-Nov-2015
Speaker:   Laurens Keek (GSFC/UMD)
Title:  "X-ray reflection spectra from accreting neutron stars and AGN: a thermonuclear superburst and an evolving disk corona"

When X-rays scatter off an accretion disk, "reflection" features are imprinted on the spectrum. These features encode properties of the disk, such as its ionization state. Furthermore, they improve our estimates of the intrinsic brightness of the X-ray source, as we can separate direct emission from reflection. For AGN, the X-ray source is an accretion disk corona. We present our multi-epoch analysis of spectra from Mrk 335, where reflection spectra reveal how the corona changes as a function of accretion rate. Alternatively, for accreting neutron stars, the neutron star itself can be the X-ray source, especially when its surface is heated by runaway thermonuclear burning. We studied a thermonuclear event that lasted several hours: a superburst. We discuss how the burst impacted the accretion disk, as well as how this improves our understanding of the superburst itself.

Date:   Monday 09-Nov-2015
Speaker:   Alberto Bolatto (UMD)
Title:  "What Makes a Starburst Tick?"

Abstract: In the context of galaxy evolution it is particularly interesting to understand better the mechanisms that regulate star formation activity in galaxies. ALMA observed the prototypical nearby starburst galaxy NGC 253 in cycles 0 and 1 in the 3mm band that contains the ground rotational transitions of several key molecules. In this talk I will present an analysis of the molecular ISM properties in the nuclear starburst region derived from the ALMA data. I will show that the molecular wind is very significant at determining the duration of the starburst phase, and I will discuss the possible gas entraining mechanisms. These data also allow us to obtain for the first time a detailed view of giant molecular clouds in a starburst environment. I will show that the clouds we are able to identify have very large surface and volume densities, and consequently short free-fall and sound crossing times, which are likely connected to the large star formation efficiencies found in these environments. Finally, I will present and discuss some of the chemical complexity we see in the data. The extremely rich spectroscopy, a common feature in many ALMA datasets, opens new windows for the study of physical conditions in extragalactic systems.

Date:   Monday 23-Nov-2015
Speaker:   Lauren Woolsey (CfA)
Title:  "Magnetic influences on the solar wind: turbulent heating in the chromosphere and corona"

The physical mechanism(s) that generate and accelerate the solar wind have not been conclusively determined after decades of study, though not for lack of possibilities. The long list of proposed processes can be grouped into two main paradigms: 1) models that require the rearranging of magnetic topology through magnetic reconnection in order to release energy and accelerate the wind and 2) models that require the launching of magnetoacoustic and Alfvén waves to propagate along the magnetic field and generate turbulence to heat the corona and accelerate the emanating wind. I will present my ongoing dissertation work that seeks to investigate the latter category of theoretical models and the role that different magnetic field profiles play in the resulting solar wind properties with Alfvén-wave-driven turbulent heating. I will describe the computer modeling in 1D and 3D that I have done of bundles of magnetic field (flux tubes) that are open to the heliosphere, and what our results can tell us about the influences of magnetic field on the solar wind in these flux tubes, including the latest time-dependent modeling that produces bursty, nanoflare-like heating.

Date:   Monday 07-Dec-2015
Speaker:   Nathan Roth (UC Berkeley)
Title:  "The X-ray through Optical Fluxes of Tidal Disruption Event"

One of the most violent ways a supermassive black hole can interact with its surroundings is by tearing apart stars that pass too close to it. The resulting debris streams can collide, shock, and accrete onto the black hole, giving rise to a flare of light that tends to peak on a timescale of weeks to months. Such flares, labeled tidal disruption events (TDEs), were first predicted decades ago, but it is only in the last few years that they have been observed systematically at a variety of wavelengths. These observations have led to a number of puzzles, including the need to explain why some TDEs are bright at optical/UV wavelengths, why are hydrogen emission lines in the optical spectrum often so weak compared to those of helium, and why the color of the optical/UV emission does not appear to evolve significantly over time. In this talk I will present radiative transfer calculations to address these questions. We track the reprocessing of accretion luminosity from a supermassive black hole as the light travels through an extended, optically thick, spherical envelope composed of hydrogen, helium, and oxygen from the disrupted star. The steady-state radiative transfer equation is coupled to a solver for the atomic level populations and ionization states that does not assume local thermodynamic equilibrium. Our calculations show how the hydrogen optical emission lines can become more effectively optically thick than their helium counterparts, causing them to remain hidden even in the disruption of a hydrogen-rich star. More generally, variations in the structure of the reprocessing material can give rise to a variety of hydrogen-to-helium line ratios, as has been seen in recent observations. We also determine the amount of material necessary to transfer enough radiative energy from x-ray to optical wavelengths to match what is observed, and we demonstrate how the partial absorption of ionizing radiation can give rise to events simultaneously observed in x-rays and in the optical.

Date:   Monday 21-Dec-2015
Speaker:   Avi Mandell (GSFC)
Title:  "Extreme Air: Investigating Hot Exoplanet Atmospheres with HST and JWST"

Hot transiting exoplanets represent a one endpoint of the exoplanet distribution, and therefore provide key tests of theories on planet formation and structure. They orbit very close to their host stars, which subjects them to intense heating from stellar radiation. However, they almost certainly did not form in their current short orbits, and their complex formation histories provide both an opportunity and an impediment for learning about their formation and migration. Investigating their structure and composition through atmospheric characterization can help us break model degeneracies and develop a more holistic picture of the evolution of planetary systems in general. In this talk I will discuss the current state of atmospheric characterization of hot transiting exoplanets, focusing primarily on results from the Hubble Space Telescope and future observations with the James Webb Space Telescope. HST has provided the first opportunity for spectroscopy of a significant number of transiting exoplanets, and results over the last several years give us the first indication of trends in composition and atmospheric structure. However, HST only barely provides the needed wavelength coverage and sensitivity for measuring key atmospheric absorption features across the near- and mid-infrared. With an order of magnitude improvement in both IR wavelength coverage and sensitivity, JWST will revolutionize our ability to constrain the atmospheric composition of a wide array of planets and break degeneracies between different absorbers, leading to new breakthroughs in our understanding of the atmospheric temperature structure, the role of clouds and hazes, and the overall classification of planetary atmospheres and what this tells us about how planets form.

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