Wine and Cheese Spring 2016

This page records the schedule, titles and abstracts of the JHU/STScI CAS Astrophysics Wine & Cheese Series in Spring 2016.

If not specified otherwise, the talks are a 25-min presentation plus a 5-min Q/A session.

February 1st

Ilias Cholis

Towards a predictive analytic model for the solar modulation of cosmic rays
An important factor limiting our ability to understand the production and propagation of cosmic rays pertains to the effects of heliospheric forces, commonly known as solar modulation. The solar wind is capable of generating time and charge-dependent effects on the spectrum and intensity of low energy (~10 GeV) cosmic rays reaching Earth. Previous analytic treatments of solar modulation have utilized the force-field approximation, in which a simple potential is adopted whose amplitude is selected to best fit the cosmic-ray data taken over a given period of time. Making use of recently available cosmic-ray data from the Voyager 1 spacecraft, along with measurements of the heliospheric magnetic field and solar wind, I will show a time, charge and rigidity-dependent model of solar modulation that can be directly compared to data from a variety of cosmic-ray experiments. This is a simple analytic formula that can be easily utilized in a variety of applications, allowing us to better predict the effects of solar modulation and reduce the number of free parameters involved in cosmic ray propagation models.

William Blair

Understanding the Curious Young Supernova Remnant Population in M83
The nearby starburst galaxy M83 has been host to at least six (and likely seven!) supernovae in the last century, many of the core-collapse type. Hence, one might expect dozens of young SN remnants similar to, say, Cas A in our Galaxy or E0102-7219 in the SMC. We have used deep Chandra observations in conjunction with HST WFC3 imaging to find and diagnose the young SN remnants in M83 and, by in large, they do not look like Cas A (that is to say, dominated by emission from SN ejecta). Rather, they appear to be bright radiative remnants like the Cygnus Loop even though they are much smaller and younger. This rapid evolution into the radiative phase may be unique to the M83 population, due to high metal abundances and a high pressure ISM. Our investigation is ongoing, with Gemini GMOS spectroscopy of many of these young SN remnants providing additional clues.

February 8th

David Hogg

Postponed
Abstract

February 15th

Saleem Zaroubi

Probing the Epoch of Reionization from LOFAR
The Epoch of Reionization is one of the least explored epochs in the history of the Universe. The redshifted 21 cm line from neutral hydrogen emitted during this epoch is the most promising probe for exploring it. To date there are a number of low frequency radio telescope that are aiming at detecting this radiation. The LOw Frequency ARray, LOFAR, which a European telescope centred in the Netherlands, has started collecting data on December 2012. I will discuss the current status of the experiment and the main results coming out of it.

February 22nd

Mubdi Rahman

Early-time Feedback in the Milky Way
Understanding the process of energetic feedback from star formation requires comprehensive exploration of both the stellar and gaseous components of star forming complexes. The diversity of scales probed make conducting such studies difficult in extragalactic systems. The Milky Way provides an excellent opportunity to explore feedback processes in detail, albeit with a unique set of challenges. In this talk, I will be discussing our current strides in measuring and analyzing feedback from massive star formation in the Milky Way.

K.D. Kuntz

Solar Wind Charge Exchange, from Annoying Background to Interesting Physics
Solar Wind Charge Exchange (SWCX) in the Earth's magnetosheath produces a very poorly characterized, directionally dependent, highly time variable foreground component to all X-ray observations. However, the X-ray emission from the magnetosheath will also allow one to make global images of the magnetosheath. Tradition methods of studying the magnetosheath have relied on very local measurements of the magnetic field and the particle distribution. As a result, some of the physical processes shaping the magnetosheath are poorly understood. It would appear that different mechanisms dominate under different solar wind conditions. Global imaging of the magnetosheath in the X-ray will allow measurement of key physical properties of the magnetosheath, and will severely test existing MHD models.

February 29th

Ethan Vishniac

The Role of Helicity Conservation in Turbulent Dynamos
The analytic theory of large scale magnetic field generation is usually conceived as a process driven by the fluid kinetic helicity and poisoned by the accumulation of eddy scale magnetic helicity. I will discuss a better approach, in which turbulence in a a rotating and/or shearing flow leads to a spontaneous flux of magnetic helicity. Its accumulation in separate domains drives the magnetic dynamo. The kinetic helicity is typically subdominant at all times. Balancing this process against turbulent mixing and buoyant loss leads to a prediction for the saturated large scale magnetic field strength in rotating stars and disks. This prediction is consistent with observations of stars. I will briefly discuss the application of this model to field growth in young galaxies and typical field strengths in accretion disks. If time allows I will comment on the implications of this work for numerical modeling of dynamo processes.

March 7th

Nathan Miller

Recovery of Large Angular Scale CMB Polarization for Instruments Employing Variable-delay Polarization Modulators
The Cosmology Large Angular Scale Surveyor (CLASS) will attempt to measure the inflationary CMB B-modes on the largest angular scales. In order to reach the largest scales from the ground, rapid polarization modulation is required. For CLASS, this will be done using a variable-delay polarization modulator (VPM). It is important to identify and mitigate any time-varying effects generated by the VPM. In this talk, I will go over the different time-varying effects that the VPM can introduce and discuss a strategy for removing these spurious signals from the data. I will show that systematic effects introduced by the VPM will not limit the sensitivity of CLASS.

Duncan Watts

Measuring the Largest Angular Scale CMB B-mode Polarization with Galactic Foregrounds on a Cut Sky
The Cosmology Large Angular Scale Surveyor (CLASS) is a multifrequency polarization experiment designed to target CMB B-mode polarization sourced by gravitational waves (tensor modes) generated during inflation. CLASS also measures the E-mode polarization on the largest scales, providing a cosmic variance limited estimate of the optical depth to reionization. Accurately measuring CMB polarization on the largest angular scales requires accurate removal of foregrounds including Galactic thermal dust and synchrotron emission. In my talk, I will discuss development of an exact pixelized likelihood code in combination with a power spectrum analysis for estimating the amplitudes of the tensor modes from large angular scale polarization data with Galactic foregrounds. Applying this to simulated CLASS data, we can constrain a primordial B-mode signal with input tensor-to-scalar ratio r = 0.01 to r = 0.01 +/- 0.004. I will also present updates on the deployment of our 40 GHz telescope.

March 21st

Tony Sohn

HST Proper Motions along Stellar Streams: Constraining Dark Halo Properties of the Milky Way
Stellar streams in the Milky Way are unique dynamical tracers of the dark matter halo, and provide strong tests of galaxy formation models. However, lack of proper motion data limits our understanding of stream orbits and dark halo properties. Thanks to the HST's excellent astrometric accuracy, we are now able to obtain high quality proper motions along stellar streams in the Milky Way. In this talk, I will present our results on HST projects for measuring proper motions along two stellar streams in the halo: the Sagittarius Stream and the Orphan Stream.

Max Gronke

Lyman-alpha observables of the high-z Universe
Lyman alpha (Lya) emission provides a unique window into the high redshift Universe and can further our understanding of the physical processes involved. In this talk I will discuss two particular Lya observables: The Lya luminosity function and Lya spectra of galaxies. First, I will present a technique to compute the Lya luminosity function (LF) from the much better constrained UV continuum LF. This allows us to predict the evolution of the Lya LF with cosmic time, which can be relevant for understanding the Epoch of Reionization. Interestingly, our model predicts a very steep faint-end slope, which appears to have been confirmed by recent observations. Secondly, I will show results from our Lya radiative transfer calculations. In particular, I will discuss the "shell model", which is an only six-parameter subgrid-model representing a Lya emitting galaxy by a central luminous source surrounded by an outflowing shell of hydrogen and dust. In spite of its simplicity, the shell-model is highly successful in reproducing observed Lya spectra. Why this model can reproduce the complex scattering process through the multiphase interstellar medium is still unclear. After reviewing the free parameters of the "shell-model", I will describe common problems arising when shell-model parameters are extracted from observed Lya spectra. In order to overcome this, we propose a fully automated procedure, which allows for quantification of uncertainties and potential degeneracies. I will conclude with a brief comparison between the shell-model parameters and the ones of more complex multiphase medium. Time permitting, I will also discuss the correlation between Lya and ionizing photon escape fraction in clumpy models, and discuss implications this may have for understanding cosmic reionization.

March 28th

Ravi Sankrit

The Stratospheric Observatory for Infrared Astronomy (SOFIA)
SOFIA is an airborne observatory, with a 2.7 meter telescope mounted on board a Boeing 747SP aircraft. SOFIA operates at altitudes of up to 45,000 feet, above 99% of the water vapor in the Earth's atmosphere, and provides access to the mid and far-infrared wavelength regions, which are not available from ground-based observatories.

The observing programs undertaken by SOFIA are driven by peer-reviewed General Investigator proposals from the world-wide astronomy community. SOFIA started its 4th annual cycle of observations in February 2016; the Cycle 5 Call for Proposals is scheduled for release at the end of April 2016.

In this talk, I will describe the observatory, the instruments and their capabilities, present some of the exciting science accomplished using SOFIA, give an overview of the proposal process, and conclude with a brief look ahead to the future of the mission.

Zhilei Xu

Measuring CMB polarization with Cosmology Large Angular Scale Surveyor (CLASS)
During the past decades, CMB temperature anisotropy measurement brought cosmology to a precision era, strongly supporting the 6-parameter LCDM model. However, hints beyond LCDM emerged with increasingly precise measurements.While not much more information still remains in CMB temperature anisotropy and CMB polarization anisotropy provides another rich source of information, the polarization measurement is the crucial way to boost our understanding on our universe. In my talk, I will first talk about the standing issues (say cosmic inflation, parameter degeneracy and neutrino mass etc.) from current CMB anisotropy measurement and how they can be solved by CMB polarization anisotropy measurement, especially at large angular scales. Then I will introduce our in-house project CLASS which was uniquely designed to answer these questions. At last, I will give an update on the deployment of our first telescope in Chile.

April 4th

Michael Fall

TBD
Abstract

April 11th

Leo Singer

Advanced LIGO First Light: Astrophysics with a Gravitational-Wave Observatory
Advanced LIGO's direct observation of gravitational radiation from a binary black hole merger has sent quakes through the physics and astronomy community, not to mention popular culture. In a few short years, the search for gravitational waves will complete its transformation from an experimental effort into a new discipline of observational astronomy as we rapidly build a sample of merging compact binaries. However, the greatest prize of all may come from combining our new GW observatories with existing electromagnetic ones---uncovering the host environments and formation channels of compact binaries, explaining the cosmic inventory of r-process elements, and even testing if stellar-mass black hole binaries are truly barren of matter or magnetic fields. I will describe the LIGO/Virgo EM follow-up program in detail. It brings together groundbreaking real-time GW data analysis techniques, a score of on-duty GW astronomers, and 63 groups who are searching for counterparts of LIGO sources using ground- and space-based partner facilities spanning gamma ray, x-ray, optical, infrared, and radio wavelengths. During LIGO's first observing run, these facilities embarked upon heroic broadband observing campaigns of LIGO/Virgo triggers including GW150914. In recounting this campaign, I will focus especially on my own role in leading follow-up with the Palomar Transient Factory. I will then describe how our GW analyses, human vetting, and follow-up consortium should evolve in the coming months to cope with an anticipated detection every few weeks by O2 and O3. I will introduce some ideas for the future to leverage GW distance estimates, large scale cosmography, and targeted infrared follow-up to measure the host properties of GW events, both with and without an electromagnetic counterpart.

April 18th

Joel Green

Anatomy of a Burst: The Evolution of FU Orionis Disks
Most low mass young stars accrete at modest rates that gradually diminish as the system evolves, the circumstellar envelope thins, and the accretion disk accretes onto the central star, agglomerates in disk regions, or forms protoplanetary objects. FU Orionis objects (FUors, named after the class archetype) offer a potential counterpoint to the recent discovery of the “Luminosity Problem”, the observation that most nearby protostars are underluminous compared with models of core formation in molecular clouds. The rare dramatic outbursts of FUors, in which their accretion rates increase up to a thousandfold for short (10–100 yr) bursts, could account for the missing mass accretion if they represent a stage that all protostars occupy for a small fraction of their lifetime. These bursts also will modify the protoplanetary disk chemistry and require a very different model than simple magnetospheric accretion. Modern telescopes can track these bursts in progress when they occur, to address key questions: (1) what triggers a burst and what are the lasting effects on circumstellar disks and (2) are FUor bursts common to all stars? And did it happen here?

Chi Ho Chan

Radiative Hydrodynamics Simulations of IR and UV Radiative Pressure on Dusty AGN Tori
Substantial evidence points to dusty, geometrically thick tori obscuring the central engines of AGNs, but so far no mechanism satisfactorily explains why cool dust in the torus remains in a puffy geometry. IR radiation pressure on dust can play a significant role in shaping the torus, yet the separation of hydrodynamic evolution from radiative transfer in previous work on radiation-supported tori precluded a self-consistent picture. Here we present the first three-dimensional radiative hydrodynamics simulations of dusty AGN tori subject to point-source UV and diffuse IR radiative acceleration. We discuss the implications of our results on torus morphology and dynamics, as well as the possible role the torus plays in galactic feedback.