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Northwestern University

Weekly Astrophysics Seminars 2014-2015

Seminars are held at 4:00 PM on Tuesdays in Room F160
on the first floor of the Technological Institute (2145 Sheridan Road) unless otherwise noted


Fall Quarter 2014

  Date      Speaker / Seminar Host
  Sept. 23

Mike McCourt
   UC Berkeley
    Do Galaxy Clusters Boil?

The hot plasma filling galaxy clusters is susceptible to a convective MHD instability known as the MTI. Understanding the implications of this convection has been surprisingly difficult, however; it depends on both the large-scale evolution of the cluster and on the plasma physics of the gas. I will describe a new set of "semi-cosmological" simulations which capture enough of the cosmic evolution of galaxy clusters to reproduce their large-scale properties but still provide an idealized environment in which to study cluster convection and its implications. I will also describe some future applications of these simulations to other problems in galaxy formation.

Claude-André Faucher-Giguère

  Sept. 30

Francesca Valsecchi
   Northwestern University
    Investigating the Role of Stellar Tides in Hot Jupiters’ Origin and Fate

Two formation models have been proposed to explain hot Jupiters’ tight orbits. These could have migrated inward in a disk (disk migration), or they could have formed via tidal circularization of an orbit made highly eccentric following gravitational interactions with a companion (high-eccentricity migration). Disk migration drives hot Jupiters down to their Roche limit separations a_R, in orbits where the stellar spin and orbital angular momentum vectors are nearly aligned. High-eccentricity migration results in an inner cutoff at 2a_R and in a broad range of misalignments. Using state-of-the-art stellar models and a detailed treatment of tidal dissipation, we show that currently observed systems are consistent with high-eccentricity migration. In this scenario, stellar tides shaped the observed distribution of misalignments, and brought inward from beyond 2a_R the currently known hot Jupiters that lie within 2a_R. Interestingly, this population potentially provides direct empirical constraints on tidal dissipation theories.
Eventually, stellar tides will cause the orbits of many hot Jupiters to decay down to a_R. Using a standard binary mass transfer model we show how a hot Jupiter undergoing a phase of Roche-lobe overflow (RLO) leads to a hot super-Earth in an orbit of few hours to several days. This model predicts planets with intermediate masses (``hot Neptunes'') that should be found in the process of losing mass through RLO. The observed excess of small single-planet candidate systems observed by Kepler may also be the result of this process. If so, the number of systems that produced hot Jupiters could be 2-3 times larger than one would infer from contemporary observations.


  Oct. 7

Andrew MacFadyen
   New York University
    The Dynamics, Stability and Radiation of GRB Jets

Gamma-rays bursts and their afterglows involve the dynamics of highly relativistic plasma as it is accelerated at the central engine and expands over more than ten orders of magnitude in length scale. I will discuss high resolution studies of GRB jets using a powerful new code (JET) which employs a moving numerical mesh, thus allowing for highly accurate Lagrangian multi-dimensional simulations of relativistic jet dynamics. I will present the first multidimensional simulations of a collapsar jet starting from the center of a massive star, breaking out of the stellar surface, coasting and producing internal then external shocks. Synchrotron light curves computed from the simulation data naturally produce the early steep decay and extended plateau observed in early X-ray light curves. During the deceleration phase I will demonstrate that GRB jets are unstable to the Rayleigh-Taylor instability. This makes the jets turbulent, thus amplifying magnetic fields via small-scale dynamo to values sufficient to explain the synchrotron emission. The Rayleigh-Taylor fingers can also impact the forward shock thus corrugating it with possible implications for the afterglow emission. I will demonstrate fits of theoretical light curves computed from numerical simulations to the observational data.

Shane Larson

  Oct. 14

Ryan Foley
    University of Illinois at Urbana-Champaign
    The Most Common "Peculiar" Supernova

In the last decade, transient surveys have identified several new types of supernovae (SNe). These new events represent astrophysical phenomena that are either less luminous or rarer than the more prevalent classes of SNe Ia, II, Ib, and Ic. I will discuss a relatively new class, Type Iax supernovae (SNe Iax). These events are observationally similar to SNe Ia, but are physically distinct being less luminous and having lower kinetic energy. To date, ~40 clear members of the class have been identified, making them the most common (by number) peculiar class of supernova. After accounting for their luminosity, there are roughly 30 SNe Iax for every 100 SNe Ia in a given volume, also making SNe Iax the most common peculiar SN by rate and more common than SNe Ib. I will describe observations of individual members of the class and those of the entire class. Taken together, we can constrain their progenitor systems much better than we have for normal SNe Ia. The progenitors are likely a C/O white dwarf that accretes material from a non-degenerate helium star. The explosion is likely a sub-sonic deflagration, and at least some of the time the white dwarf does not completely disrupt, leaving a remnant with particular observational signatures.
I will also present Hubble Space Telescope observations of two SNe Iax. For one SN, in pre-explosion images, we have detected its progenitor system, which is most consistent with being the predicted C/O WD-He-star system. This is the first detection of a thermonuclear SN progenitor system. For the other SN, in images taken 4 years after explosion, we detect a source consistent with being a puffed up remnant star. If true, this would represent a new class of objects, of which there may be a handful in the Milky Way.

Vicky Kalogera

  Oct. 21

Rachel Friesen
   Dunlap Institute, University of Toronto
    Tracing the Mass Flow in Clustered Star Forming Regions

Most stars in our galaxy do not form in isolation. Instead, stars are born in groups and clusters embedded within dense filaments and clumps in molecular clouds. Many clustered star-forming regions share similar morphologies, where the greatest star formation rates are found within a central ‘hub’ of dense molecular gas, that is connected to streams or filaments of additional material. To understand how stars form in clusters, we need to understand how these filaments accrete mass from the surrounding environment, funnel mass to star-forming ‘hubs’, and fragment to form dense star-forming cores. I will present observational evidence of ongoing accretion of material onto dense filaments in a nearby young cluster, with derived mass accretion rates that are sufficient to trigger additional fragmentation and gravitational collapse. In particular, I will show how combining observations of gas dynamics and chemistry in star forming regions can be used to answer these questions.

Laura Fissel

  Nov. 4

Gwen Rudie
    The Gaseous Environments of High-Redshift Galaxies:
    The Circumgalactic and Interstellar Medium of Star-Forming Systems at 2<z<3

The gaseous component of galaxies is a crucial but poorly-constrained aspect of galaxy formation and evolution. I will present results from the Keck Baryonic Structure Survey (KBSS), a unique spectroscopic survey designed to explore both the physical properties of high-redshift galaxies and the connection between these galaxies and their surrounding intergalactic baryons. The KBSS is optimized to trace the cosmic peak of star formation (z~2-3), combining high-resolution spectra of 15 hyperluminous QSOs with densely-sampled galaxy redshift surveys surrounding each QSO sightline. I will characterize the physical properties of the gas within the circumgalactic medium (CGM) through the spatial distribution, column densities, and kinematics of ~6000 HI absorbers surrounding ~900 foreground star-forming galaxies within 50 kpc to 3 Mpc of a sightline. These measurements provide clear evidence of gas inflow and outflow as well as accretion shocks or hot outflows from these forming galaxies. I will also discuss KBSS-MOSFIRE, a rest-frame optical spectroscopic survey of more than 800 galaxies in these same QSO fields. These data provide new insight into the physical properties of HII regions at high redshift and show remarkable discrepancies between the ionization and excitation conditions in low and high-redshift star-forming regions. These results have significant implications for both measurements of the chemical abundances of high-z galaxies as well as our understanding of massive stars during the peak of cosmic star formation.

Daniel Anglés-Alcázar

  Nov. 11

Saul Rappaport
    More Unexpected Results from the Kepler Mission

I will present some of the impressive science that emerges from Kepler's four-year nearly continuous monitoring of the flux from 200,000 otherwise anonymous stars in Cygnus. The Kepler mission features unprecedented photometric flux measurements at the ~100 parts per million (ppm) level, and sensitivity to periodic events at the few ppm level. The Kepler results on thousands of newly discovered exoplanets will be reviewed only very briefly. I will focus on four topics that have occupied the attention of a group of my colleagues. These include the discovery, analysis, and study of (1) thermally highly bloated hot white dwarfs in close binary star systems; (2) several dozen hierarchical triple-star systems; (3) ultra-short period planets (with orbital periods between 4 hours and 1 day); and (4) two close-in rocky planets that are leaving a comet-like tail of dust behind them in their orbits.

Fred Rasio and Francesca Valsecchi

  Nov. 18

Jacob Bean
   University of Chicago
    Exoplanets in HD

Exoplanet surveys have revealed an amazing diversity of planets orbiting other stars in the last two decades. Studying the atmospheres of representative exoplanets is the key next step in leveraging these detections to further transform our understanding of planet formation and planetary physics. Additionally, atmospheric studies are critical for determining if any of the small habitable zone exoplanets that are now being detected are truly habitable, and even inhabited. In this talk I will describe a vision for how we can pursue the compelling opportunities in exoplanet atmospheres today and in the future. One crucial need in this area is spectroscopy to reveal planets in high definition. I will present new results from intensive observational campaigns with the Hubble Space Telescope that serve as a model for the proposed program, including a definitive constraint on the atmosphere of the super-Earth archetype GJ1214b, a precise measurement of the water abundance in a giant planet, and the inference of the thermal structure of an exoplanet atmosphere as a function of longitude. A fundamental component of the envisioned approach for the future is the need for a strategic program combining observations with multiple ground- and space-based telescopes using a suite of techniques to investigate the question of habitability. I will conclude by discussing how future facilities are poised to play a crucial role in the identification of the first Earth twin as part of this plan.

Francesca Valsecchi

Past Astrophysics Seminars