*Tech = Technological Institute (2145 Sheridan Road)
**Db = Dearborn Observatory (2131 Tech Drive)
Summer Quarter 2014
| Date/Time | Visitor | Host | |
| July 23 Tech F160 4:00pm |
 |
Kostas Tassis We are currently entering a new, precision-measurement era of
interstellar medium (ISM) studies, as a result of both greatly
improved capabilities in instrumentation, as well as necessity. On the
one hand, Herschel has revealed an unprecedented-accuracy picture of
the structure of the ISM, with molecular clouds appearing as a network
of filamentary structures, where high-density cores preferentially
appear. |
Vicky Kalogera |
| Aug. 22 Tech F160 2:00pm |
 |
Tassos Fragos Galactic field black hole (BH) low-mass X-ray binaries (LMXBs) are believed to form in situ via the evolution of isolated binaries. In the standard formation channel, these systems survived a common envelope phase, after which the remaining helium core of the primary star and the subsequently formed BH are not expected to be highly spinning. However, the measured spins of BHs in LMXBs cover the whole range of spin parameters from a*~0 to a*~1. In this talk, I propose that the BH spin in LMXBs is acquired through accretion onto the BH during their long and stable accretion phase. In order to test this hypothesis, we calculated extensive grids of binary evolutionary sequences in which a BH accretes matter from a close companion. For each evolutionary sequence, we examined whether, at any point in time, the calculated binary properties are in agreement with their observationally inferred counterparts of 16 observed Galactic LMXBs. Mass-transfer sequences that simultaneously satisfy all observational constraints represent possible progenitors of the considered LMXBs and thus give estimates of the mass that the BH has accreted since the onset of Roche-Lobe overflow. We find that in all Galactic LMXBs with measured BH spin, the origin of the spin can be accounted by the accreted matter, and make predictions about the maximum BH spin in LMXBs where no measurement is yet available. Furthermore, based on this hypothesis, we derive limits on the maximum spin that a BH can have depending on the orbital period of the binary it resides in and the effective temperature of the companion star. Finally we discuss the implication that our findings have on the BH birth-mass distribution, which is shifted by ~1.3 Msolar towards lower masses, compared to the currently observed one. |
Vicky Kalogera |
Spring Quarter 2014
| Date/Time | Visitor | Host | |
| April 3 Tech F160 12:00pm |
|
Albert Stebbins |
Jason Steffen |
| April 9 Tech F160 1:00pm |
 |
Richard de Grijs Until about a decade ago, star clusters were considered "simple" stellar populations: all stars in a cluster were thought to have similar ages and the same metallicity. Only the individual stellar masses were thought to vary, in essence conforming to a "universal" initial mass function. Over the past decade, this situation has changed dramatically. Yet, at the same time, star clusters are among the brightest stellar population components and, as such, they are visible out to much greater distances than individual stars, even the brightest, so that understanding the intricacies of star cluster composition and their evolution is imperative for understanding stellar populations and the evolution of galaxies as a whole. I will discuss my group's recent progress in this context, with particular emphasis on the properties and importance of binary systems, the effects of rapid stellar rotation, and the presence of multiple populations in Local Group star clusters across the full age range. |
Aaron Geller |
| April 21 Tech F160 2:00pm |
 |
Abigail Vieregg Inflation, the superluminal expansion of the universe during the first moments after the Big Bang, predicts a Cosmic Gravitational-Wave Background, which in turn imprints a faint but unique signature of “B-mode” polarization of the Cosmic Microwave Background (CMB) at degree angular scales. Detection of the B-mode signature from inflation would constitute strong evidence for inflation and a test of inflationary models at the GUT scale, the first direct image of gravitational waves, and direct empirical evidence of quantum gravity. BICEP2, which observed from the South Pole from 2010-2012, is a microwave polarimeter that uses antenna-coupled Transition Edge Sensor arrays to observe the CMB at degree angular scales and is specifically designed to search for this signature of inflation. BICEP2 is the second experiment in a four-stage line of degree-scale polarimeters at the South Pole. I will discuss the recent detection of B-mode polarization at degree angular scales with BICEP2 ( > 5 sigma and consistent with tensor modes from inflation), and the promise for follow-up to this measurement with the Keck Array (currently observing) and BICEP3 (deploying this year). |
Claude-André Faucher-Giguère |
| May 1 Tech F210 4:00pm |
 |
Astrid Lamberts The very high energy gamma-ray radiation from gamma-ray binaries is thought to arise from the interaction between a relativistic pulsar wind and the stellar wind of its massive companion. Multiwavelength observations display orbital variability in all wavebands and suggest that at least two distinct populations of high-energy particles are present. I present hydrodynamical simulations of such systems, using a recently developed relativistic extension to the RAMSES code. I explain how the relativistic nature of the pulsar wind affects the colliding wind structure. I show how these simulations have been coupled to a radiative code in order to model the emission properties of gamma-ray binaries. |
John Everett |
| May 8 Tech F160 2:00pm |
 |
Alex Lazarian The theory of star formation was developed assuming magnetic flux conservation in highly conductive fluids (Alfven theorem). Thus it usually assumed that to form a star either collecting of matter along magnetic field lines or non-ideal effects of ambipolar diffusion are necessary. However, the above assumption is not true. I shall show that magnetic flux freezing is violated in the perfectly conducting turbulent fluid. My conclusion is based on our analytical model of magnetic reconnection in the presence of weak turbulence. The predictions of this model have been successfully tested numerically and, in a separate development, the deep relation of our reconnection model with the recent developments in the Lagrangian theory of MHD turbulence has been established. On the basis of this I shall show the existence of a new process termed "reconnection diffusion". In my talk I shall show how reconnection diffusion induces flux loss in molecular clouds and accretion disks and provides a comparison of the observational data and the theoretical predictions. In particular, I shall show that the reconnection diffusion can solve the so-called the problem of "magnetic braking catastrophe" for the circumstellar accretion disks. |
Giles Novak |
| May 9 Tech F160 11:00am |
 |
Eleni Kalfountzou Star formation and AGN activity play primary roles in the formation and
evolution of galaxies. A significant bank of evidence has developed
demonstrating the close connection between AGN and their hosts.
Theoretical
models suggest that these connections arise through feedback processes
between
the galaxy and its accreting black hole. However, it is still poorly
understood whether the AGN activity impacts star formation or vice
versa.
The negative AGN feedback is necessary for these models in order to
explain
the strong suppression of star formation. The feedback process
becomes more
complicated in the case of powerful radio sources where there are
results
that suggest a positive feedback.
|
Daryl Haggard |
| May 23 Tech F160 1:00pm |
 |
Sweta Shah Millions of short period binaries in our Galaxy will be observed by the future space-based Gravitational wave (GW) detector, eLISA. It is expected that thousands of these binaries will be individually resolved for which we can measure their GW parameters. From GW observations we expect to measure their GW frequencies to nano-Hertz accuracies. However most other parameters greatly vary in their accuracies owing to signal strengths and the relative geometry of the detector with the source. I will address to what extent will combining the GW observations (for systems mostly made of white-dwarfs) with their potential electro-magnetic (EM) data can improve our knowledge of the binary parameters further. I will also address the issue of measuring tidal deviations in detached short-period binaries using both GW and EM observations. |
Vicky Kalogera |
Winter Quarter 2014
| Date/Time | Visitor | Host | |
| Jan. 8 Tech F210 11:00am |
 |
Eliot Quataert In the next ~ 5 years, ground-based interferometers such as Advanced LIGO are likely to provide the first direct detections of gravitational waves, with the most promising sources being the mergers of two neutron stars or a neutron star and a black hole. Maximizing the scientific return of this new window into the universe requires connecting gravitational wave detections to the wealth of electromagnetic data on similar sources. In this talk I will describe the motivation for combining electromagnetic and gravitational views of the universe and the physics that determines the electromagnetic counterparts to compact object mergers. |
Claude-André Faucher-Giguère |
| Jan. 27 Tech F160 11:00am |
 |
Simeon Bird I will discuss recent work modelling the distribution of cold gas and metals around DLAs, using the moving mesh code Arepo. DLAs are high column density HI absorption systems and probe directly the gas distribution in halos. Using synthetic metal and HI spectra extracted from recent Arepo simulations incorporating metal enrichment and feedback models, I will compare the properties of DLAs to high-redshift observations, focussing on the column density distribution function, the bias of DLAs to the linear matter power spectrum, and the metallicity distribution, showing that these are (mostly) in good agreement with reality. I will then discuss a solution to the historical disagreement of simulations with observed DLA velocity widths. |
Claude-André Faucher-Giguère |
| Jan. 29 Tech L324 2:00pm |
 |
Katherine Deck I will discuss recent work on the long-term stability of compact two planet systems which was motivated by Kepler 36 and the large number of observed systems with period ratios 1< P2/P1 < 2. For these types of systems, resonance overlap can lead to instability even for orbits which satisfy the Hill criterion (an analytic stability criterion for two planet systems). I will explain why the Hill criterion falls short of being a complete stability criterion, what resonance overlap is, and why resonance overlap can be used as a criterion for chaos and instability.
|
Jason Steffen |
| Feb. 26 Tech F160 11:00am |
 |
Xuening Bai The structure and evolution of protoplanetary disks (PPDs) are largely determined by the process of angular momentum transport, and are key to understanding many aspects of planet formation. Due to extremely weak levels of ionization, the gas dynamics of PPDs is largely controlled by non-ideal magnetohydrodynamical effects, including Ohmic resistivity, Hall effect and ambipolar diffusion (AD). Most previous studies considered only the effect of Ohmic resistivity under the framework of magnetorotational instability (MRI) driven accretion with dead zones. We show via self-consistent local numerical simulations that the inclusion of ambipolar diffusion (AD) qualitatively changes the conventional picture. In the inner disk around 1 AU, the MRI is suppressed due to the inclusion of AD, and a strong magnetocentrifugal wind can be launched that efficiently drives disk accretion. With a parameter survey, we find that wind-dominated accretion with laminar disk is likely to extend to about 5-10 AU. Beyond this radius, angular momentum transport is likely to proceed through a combination of wind and the MRI, and eventually dominated by the MRI in the outer disk. Ongoing work to include the Hall effect further introduces modest changes to the wind scenario, and also makes the gas dynamics depend on the polarity of the external magnetic field. Our simulation results provide key ingredients for a new paradigm on the transport and accretion processes in PPDs. |
Yoram Lithwick |
| Feb. 27 Tech F210 4:00pm |
 |
Kat Barger Galaxy evolution is governed by an intricate ballet of gaseous inflows and outflows and galaxy interactions. The nearby Magellanic Clouds provide an advantageous opportunity to study these processes in detail. In this talk, we will explore the gas flows associated with these galaxies through H-alpha emission- and UV absorption-line observations. Galaxy interactions have greatly disturbed the Magellanic Clouds and have tidally displaced massive amounts of gas from these galaxies. These interactions have also triggered intense star formation throughout their disks; we provide compelling evidence that this elevated star formation drives even more material from these galaxies in wide spread, feedback driven winds. Through these observations, we determine the properties of these gas flows and explore their fates. We find that over a billion solar masses of neutral and ionized gas surrounds these galaxies. Much of this dislodged gas is transferring between the Magellanic Clouds and the Milky Way, depleting and replenishing their star formation gas reservoir respectively. |
Aaron Geller |
Fall Quarter 2013
| Date/Time | Visitor | Host | |
| Nov. 7 Tech F160 11:00am |
 |
Chris Hayward Galaxy formation has been studied using idealized numerical simulations of isolated disk galaxies and galaxy mergers for decades, but most simulations performed to date have suffered from two potentially significant limitations: First, when comparing simulations with observations, physical quantities - rather than observables - from the simulations are used. Second, the most-commonly used techniques, smoothed-particle hydrodynamics (SPH) and adaptive mesh refinement, suffer from numerical inaccuracies that can potentially jeopardize the results of simulations performed with those techniques.
|
Claude-André Faucher-Giguère |
| Nov. 20 Tech F160 12:00pm |
 |
Natalie Gosnell The formation mechanism of blue straggler stars, defined to be brighter and bluer than the main sequence turnoff in a star cluster, has been a question for almost six decades. The blue straggler population of the old (7 Gyr) open cluster NGC 188 provides a unique opportunity to probe the formation histories of blue straggler stars in open clusters. In comparison to the blue straggler populations in younger open clusters and in globular clusters, the cooler temperatures (6,000 to 6,750 K) and close proximity (2.5 kpc) of the blue stragglers in NGC 188 allow for in-depth high-resolution spectroscopic investigation. Long-term radial velocity studies revealed that over 75% of the NGC 188 blue stragglers exist in binaries with a prevalence of 1000-day periods and a statistical secondary mass distribution that peaks at 0.5 Msolar. Using HST/ACS/SBC far-UV photometry I will present direct observational detections of young (<300 Myr), hot white dwarf companions in three blue straggler binaries. Given the age distribution predicted in full N-body models, which translates into a white dwarf temperature distribution, three detections is consistent with the entire NGC 188 blue straggler population being formed via mass transfer. These detections affirm the prediction made by previous studies that the blue straggler population of NGC 188 is dominated by mass transfer formation. |
Aaron Geller |
| Nov. 26 Tech F160 1:00pm |
 |
Eric Gaidos M dwarf stars are especially attractive targets in the search for Earth-like planets because small planets are easier to detect around such stars, and the "habitable zone" is closer to the star where planets are more readily detected. The Kepler mission has discovered Earth-size planets transiting M dwarfs, including some that may orbit in the habitable zone. But the properties of these stars, and hence those of their planets, are poorly constrained. We have refined the temperatures of nearby M dwarfs whose angular diameters have been measured by interferometry. We find excellent agreement between certain model spectra and the observed spectra of these stars and use "spectrothermometry" to estimate the temperatures of distant Kepler M dwarfs with a precision of 60K. We then apply model-independent empirical relations to estimate stellar radius, luminosity, and mass, and use these in turn to revise the radii and stellar irradiances of M dwarf planets. We find that a recent estimate of the occurrence of Earth-size planets in the habitable zone of M dwarfs may need to be revised downwards by a factor of four. |
Nick Cowan |
Past CIERA Special Seminars