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Northwestern University


Weekly Astrophysics Seminars 2013-2014

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 2013

  Date      Speaker / Seminar Host
  Oct. 1

Neal Evans
    University of Texas at Austin
    Low Mass Star Formation

Evans will review recent progress in understanding the formation of individual low mass stars and their planetary systems. In particular, results from Spitzer and Herschel, as reviewed recently at the Protostars and Planets VI meeting will be reported.

Nicholas Chapman

  Oct. 8

Elena Gallo
    University of Michigan
    X-ray constraints on the local super-massive black hole occupation fraction

Modeling of black hole growth and merger histories indicates that central supermassive black holes will be more often present even in smaller galaxies when seeds are generated from Pop III remnants rather than via direct gas collapse. Consequently, measurement of the local occupation fraction provides an observational discriminator between seed formation models. Chandraobservations of nearby early-type galaxies can directly detect even low-level supermassive black hole activity. The activefraction immediately provides a firm lower limit to the occupation fraction, and leveraging a correlation between nuclear X-rayluminosity and host galaxy stellar mass provides a statistical assessment of the proportion of X-ray upper limits arising from sensitivity restrictions versus those due to the absence of a central supermassive black hole.We use the volume-limited AMUSE surveys of ~200 optically-selected early-type galaxies to characterize simultaneously, for the first time, the occupationfraction and the scaling of LX with Mstar, accounting for intrinsic scatter, measurement uncertainties, and X-ray limits. For early-type galaxies with Mstar < 10^10 Msun, we obtain a lower limit to the occupation fraction of >20% (at 95% confidence), but fulloccupation cannot be excluded. The preferred dependence of log LX upon log Mstar has a slope of ~0.7–0.8, consistent with the “downsizing” trend previously identified from the AMUSE dataset, and a uniform Eddington efficiency is disfavored at >2sigma. We provide guidelines for the future precision with which these parameters may be refined with larger or more sensitive samples.

Daryl Haggard
  Oct. 15

Konstantin Batygin
    Harvard
    Disk-Driven Migration and Hot Jupiter Spin-Orbit Misalignments

The existence of gaseous giant planets whose orbits lie in close proximity to their host stars (“hot Jupiters") can naturally be accounted for by protoplanety disk-driven (type-II) migration, associated with viscous evolution of the nebulae. Recently, observations of the Rossiter-McLaughlin effect during planetary transits have revealed that a considerable fraction of detected hot Jupiters reside on orbits that are misaligned with respect to the spin-axes of their host stars. This observational fact has cast significant doubts on the importance of disk-driven migration as a mechanism for production of hot Jupiters, thereby reestablishing the origins of close-in planetary orbits as an open question. Here we show that spin-orbit misalignment is a natural consequence of disk-driven migration.
The abundance of binary stellar companions, whose orbital plane is uncorrelated with the spin axes of the individual stars, is enhanced in star formation environments. Accordingly, gravitational torques arising from massive distant bodies generally act to adiabatically perturb protoplanetary disks (and the embedded planets) away from their primordial planes, giving rise to orbital obliquity. Spin-orbit misalignment can be additionally amplified by non-trivial spin-axis dynamics of disk-bearing stars. Specifically, as stellar interiors evolve along pre-main-sequence tracks and protoplanetary disks loose mass, a secular resonance between the disk-torqueing frequency and the nodal recession rate of the stellar spin pole may be encountered, yielding further excitation of inclination. Collectively our results signal a consistency between disk-driven migration and the current observational aggregate while predicting that non-zero spin-orbit misalignments should be observed in systems of low-mass multi-transiting planets.

Nathan Kaib
  Oct. 22

Rosalba Perna
    University of Colorado
    Highly magnetic neutron stars: bewildering astrophysical laboratories
    and cosmological tools

Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters (SGRs) are young neutron stars characterized by high X-ray quiescent luminosities, outbursts, and, in the case of SGRs, sporadic giant flares. They are believed to be magnetars, that is neutron stars powered by ultra-strong magnetic fields. However, the diversity of their behaviours, and, especially, the observation of magnetar-like bursts from 'low-field' neutron stars, has been a theoretical puzzle. In the first part of the talk, I will discuss results of long-term MHD simulations which, by following the evolution of magnetic stresses within the neutron star crust, have allowed to relate the observed magnetar phenomenology to the physical properties of the neutron stars, and in particular to their age and magnetic field strength and topology. The dichotomy of 'high-B' field pulsars versus magnetars is naturally explained, and occasional outbursts from old, low B-field neutron stars are predicted. In the second part of the talk, I will discuss how observations of highly magnetized neutron stars can be handy tools in the cosmological domain, and in particular as a way to set constraints on the hypothetical particle 'axion'.

Vicky Kalogera
  Nov. 5

Karen Yang
    University of Michigan
    The Fermi Bubbles: Possible Nearby Laboratory for AGN Jet Activity

The two giant gamma-ray bubbles discovered by the Fermi Gamma-ray Space Telescope are nearly symmetric about the Galactic plane, suggesting some episode of energy injection from the Galactic center, such as a nuclear starburst or active galactic nucleus (AGN) jet activity. The ``Fermi bubbles” have many unique features, including their nearly flat surface brightness, sharp edges, and spatially uniform hard spectrum. They are also spatially correlated with the microwave haze emission observed by WMAP and Planck, the ROSAT X-ray arc features, and the polarized lobe emission recently revealed by NVSS. Using three-dimensional magnetohydrodynamic simulations that include relevant cosmic-ray (CR) physics, we demonstrate that the primary features of the Fermi bubbles could be successfully reproduced by recent jet activity of the central AGN. Assuming the AGN jets are leptonic, we compare the model predictions on the gamma-ray, microwave, and polarization signatures against the observational data. We identified the important physical processes required to simultaneously explain the bubble and haze emission. While the source of pressure support of extragalactic AGN bubbles is still poorly known due to observational limitations, we derived constraints on the composition of the Fermi bubbles in the leptonic jet scenario by comparing our model predictions with the spatially resolved gamma-ray bubble and microwave haze observations.

Farhad Zadeh
  Nov. 19

Linda Strubbe
    University of Toronto
    Studying Massive Black Holes Using the Tidal Disruption of Stars

A star that wanders too close to a massive black hole gets shredded by the black hole's tidal gravity. Stellar gas soon falls back to the black hole at a rate initially exceeding the Eddington rate, releasing a flare of energy as gas accretes. How often this process occurs is uncertain at present, as is the physics of super-Eddington accretion (which is relevant for black hole growth and feedback at high redshift as well). Excitingly, in just the last couple of years, new transient surveys like the Palomar Transient Factory, Pan-STARRS, and surprisingly the Swift hard X-ray satellite are, for the first time, finding and following up tidal disruption event candidates in real time. I'll describe their recent discoveries and what we're learning from them, and also look to the future at what measured rates of tidal disruption will be able to teach us about massive black holes and their surrounding galactic nuclei.

Junfeng Wang
  Dec. 3

Brian Metzger
    Princeton
    Observable Signatures of Coalescing Compact Binaries

Coalescing stellar mass compact objects (binary neutron stars and black holes) are the most promising sources for the direct detection of gravitational waves by Advanced LIGO and Virgo in the next few years. However, maximizing the scientific opportunities from such a discovery will require the identification of a coincident electromagnetic counterpart. One possible counterpart is a short duration gamma ray burst (GRB), powered by the accretion of a centrifugally supported torus onto the central black hole. Although observations of short GRBs are largely consistent with the merger model, many bursts are accompanied by delayed X-ray flaring, which does not fit current theory and may require variations on the standard model, such as the presence of a long-lived neutron star remnant. Binary NS mergers are also accompanied by a supernova-like optical/IR transient, powered by the radioactive decay of heavy neutron-rich elements synthesized in the merger ejecta (a `kilonova'). I will describe the first calculations of kilonovae including realistic nuclear physics and radiative transport and will report on the first potential discovery of kilonova emission following a short GRB earlier this year.
In the second part of my talk I will describe a model for accretion following the tidal disruption of a white dwarf by a NS or black hole. I will demonstrate that densities and temperatures in the disk are sufficiently high to burn the white dwarf material into increasingly heavier elements at sequentially smaller radii. Because the energy released by nuclear reactions is comparable to that released gravitationally, I introduce the concept of a "nuclear dominated accretion flow". Outflows from the disk power a ~ week long transient, which could be related to recently-discovered classes of subluminous supernovae.

Fred Rasio


Past Astrophysics Seminars