List of Past CTC Theory Lunches : 01-Sep-2010 to 31-Dec-2010

Date:   Monday 12-July-2010
Speaker:   Sebastien Besse (UMD)
Title:  Resurfacing of Asteroids with Planetary Flyby

A review/discussion of two papers talking about the above subject.

1. Earth encounters as the origin of fresh surfaces on near-Earth asteroids Binzel, Morbidelli, & Merouane et al, Nature 2010 ( )

2. Do Planetary Encounters Reset Surfaces of Near Earth Asteroids? Nesvorny, Bottke, & Vokrouhlicky et al 2010, mainly discusses the result of the first paper.( )

Date:   Monday 13-September-2010
Speaker:   Mia Bovill (UMD)
Title:  What you've wanted to know about HD simulations and galaxy formation

A summary from a summer school and a workshop Mia attended during the summer.

(1) HIPACC Summer School on cosmological HD and MHD simulations:

(2) 2010 Galaxy Formation Workshop at UCSC:

Some key websites from Mia's summary.

* ENZO 2.0 on Google codes :

* ENZO documentation :

* YT :


Date:   Monday 20-September-2010
Speaker:   Bruno Giacomazzo (UMD/GSFC)
Title:  Accurate evolutions of inspiralling and magnetized neutron-stars: equal-mass binaries

Bruno is going to talk about a paper which was just submitted to Phys. Rev. D ( ).

By performing new, long and numerically accurate general-relativistic simulations of magnetized, equal-mass neutron-star binaries, we investigate the role that realistic magnetic fields may have in the evolution of these systems. In particular, we study the evolution of the magnetic fields and show that they can influence the survival of the hypermassive-neutron star produced at the merger by accelerating its collapse to a black hole. We also provide evidence that even if purely poloidal initially, the magnetic fields produced in the tori surrounding the black hole have toroidal and poloidal components of equivalent strength. When estimating the possibility that magnetic fields could have an impact on the gravitational-wave signals emitted by these systems either during the inspiral or after the merger we conclude that for realistic magnetic-field strengths B<~1e12 G such effects could be detected, but only marginally, by detectors such as advanced LIGO or advanced Virgo. However, magnetically induced modifications could become detectable in the case of small-mass binaries and with the development of gravitational-wave detectors, such as the Einstein Telescope, with much higher sensitivities at frequencies larger than ~2 kHz.

Date:   Monday 27-September-2010
Speaker:   Tamara Bogdanovic & Cole Miller (UMD)
Title:  LISA Astro_GR workshop

Tamara and Cole will summarize their impressions from the LISA Astro_GR workshop held from 13-17 Sep in Paris ( ).

This annual workshop gathers astrophysicists, numerical relativists and gravitational wave data analysts to discuss electromagnetic and gravitational wave sources relevant for the future gravitational wave observatory, LISA.

Date:   Monday 11-October-2010
Speaker:   Eve Ostriker (UMD), Moved to October 25

Date:   Friday 15-October-2010
Speaker:   Eva Sagi (Hebrew University)
Title:  Observational Signatures of TeVeS

Observations on all scales point to a gap in our understanding of gravitation; from the Pioneer anomaly in the solar system, through the shape of galaxy rotation curves and the amount of gravitational lensing by galaxy clusters, to the accelerated expansion of the universe. This observed discrepancy between the dynamics and the distribution of the visible matter in the universe is usually ascribed to dark matter and dark energy. However, it is possible that both dark matter and dark energy are manifestations of a theory of gravity different than General Relativity. One such theory is TeVeS, suggested by Bekenstein in 2004. I will present several results on the match up of TeVeS with observations. Surprisingly, its PPN parameters show it to be indiscernible from GR in the solar system; however, its gravitational waves exhibit an unusual behavior, which can be traced back to the theory's MOND origin.

Date:   Monday 18-October-2010
Speaker:   Rory Barnes (University of Washington)

Date:   Monday 25-October-2010
Speaker:   Eve Ostriker (UMD)
Title:  Self-regulation of star formation rates in disk galaxies

Star formation rates depend on both the total available interstellar gas mass, and on the physical state of that gas and the local galactic environment -- including the stellar and dark matter gravitational potentials. In a multiphase disk, the relative proportions of mass in gravitationally-bound clouds vs. the diffuse ISM depends on energy injected by star formation. This energetic feedback both heats gas and drives turbulence, and can lead to a self-regulated star-forming state. I will introduce a new theoretical model that predicts the star formation rate as a function of the total gaseous surface density and the midplane density of stars + dark matter. This prediction derives from requirements for maintaining thermal and dynamical equilibrium in the diffuse gas. In HI-dominated outer-disk regions, star formation rates increase until the thermal pressure in the two-phase ISM matches the dynamic pressure. In the central regions of galaxies, the total surface density of HI is limited due to the high cooling rate of vertically-confined, high-pressure gas. Cooling cannot exceed the heating provided by UV from young stars; this leads to a saturation of the HI surface density, consistent with observations. Application of this thermal/dynamical equilibrium theory to a set of spiral galaxies shows excellent agreement between predicted and observed star formation rates, especially for flocculent galaxies.

Date:   Monday 1-November-2010
Speaker:   Roman Shcherbakov (Harvard University)
Title:  Dynamical and Radiative Modeling of Sgr A*

The Galactic Center supermassive black hole Sgr A* is a unique laboratory to test the behavior of matter under extreme conditions and measure the BH spin. I discuss two models deciphering the properties of the accretion flow and the space-time far from and close to the compact object. First, I compute an inflow-outflow solution with conduction, which fits Chandra X-ray brightness profile. Powerful energy transfer by conduction limits the accretion rate to <1% of Bondi value. Second, I devise a dynamical model based on 3D GRMHD simulations, pair it with the original polarized GR radiative transfer of cyclo-synchrotron, compare to observations of sub-mm fluxes, linear and circular polarization fractions, and provide constraints on Sgr A* spin and inner flow. The analysis suggests the best combination of spin a*=0.9, inclination angle i=59deg, and accretion rate Mdot=1.3*10^(-8)Msun/yr in agreement with the conduction solution. Movies will be shown during the talk.

Date:   Monday 8-November-2010
Speaker:   Francesco Tombesi (UMD and NASA/GSFC)
Title:  X-ray evidence for ultra-fast outflows in radio-quiet and radio-loud AGNs

I present the results of an extensive search for blue-shifted Fe K absorption lines in the X-ray spectra of a large sample of radio-quiet and radio-loud AGNs observed with XMM-Newton and Suzaku. Their interpretation as Fe XXV/XXVI K-shell resonances implies mildly relativistic blue-shifted velocities, up to ~0.3c. If we define Ultra-fast Outflows (UFOs) those absorbers with velocities >10,000 km/s, then we find a significant incidence of ~40-60%. This suggests a large global covering fraction, thereby implying wide opening angles. We detect variability even on ~days, indicating also somewhat compact absorbers. A fit with specific photo-ionization models gives extreme ionization parameters and large column densities N_H~10^22-10^24 cm^-2. Consequently, the location of UFOs is estimated to be very close to the central SMBH. These characteristics are in overall agreement with what expected from simulations of accretion disk winds/ejecta. Moreover, we find that the instantaneous mass outflow rate of UFOs can be comparable to the accretion rate and their kinetic power can be similar to that of jets in radio-loud objects. Therefore, the UFOs can play a new significant role in the expected AGN cosmological feedback besides jets and provide further clues on the accretion disk-winds/jets connection in AGNs.

Date:   Monday 22-November-2010
Speaker:   Nurur Rahman (UMD)
Title:  Constraining the Functional Form of the Molecular Gas Star Formation Law

What is the power law index in the spatially resolved molecular gas Kennicutt-Schmidt relation ? Whether the relation between molecular gas and star formation is linear or not has important implications on the physics regulating star formation activity on galactic scales. Observational studies, however, frequently arrive at inconsistent results even on the same object, mostly because of methodological differences. We discuss a systematic study carried out on a nearby galactic disk (M99), which assesses the effects of the choice of SFR tracer, the removal of diffuse emission, and the sampling and fitting methodologies. We find that while the result for the power law index in the faint emission is very sensitive to the methodology, the result of the measurement is well defined in the bright regions of galaxies. There we obtain an approximately linear relation independent of the choice of SFR tracer if the fraction of diffuse emission is smaller than 30-40%.

Date:   Monday 22-November-2010
Speaker:   Kaveh Pahlevan (Yale University)
Title:   When the Moon was a Cloud: A Creation Myth

Date:   Monday 06-December-2010
Speaker:   Bruno Giacomazzo (GSFC/UMD)
Title:  Analytic modelling of tidal effects in the relativistic inspiral of binary neutron stars

Luca Baiotti, Thibault Damour, Bruno Giacomazzo, Alessandro Nagar, Luciano Rezzolla

To detect the gravitational-wave signal from binary neutron stars and extract information about the equation of state of matter at nuclear density, it is necessary to match the signal with a bank of accurate templates. We have performed the two longest (to date) general-relativistic simulations of (equal-mass) binary neutron stars with different compactnesses, C = 0.12 and C = 0.14, and compared them to a tidal extension of the effective-one-body (EOB) model. The typical numerical phasing errors over the ~22 gravitational-wave cycles are \Delta \phi ~ \pm 24 rad, thus with relative phase errors \Delta \phi /\phi ~0.2%. After calibrating only one parameter (representing a higher-order amplification of tidal effects) we found that the EOB model can reproduce, well within the numerical error, the two numerical waveforms essentially up to the merger. On the other hand, the third post-Newtonian Taylor-T4 approximant augmented by leading-order tidal corrections dephases with respect to the numerical waveforms by several radians.

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