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
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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

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Zhilei Xu

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April 4th

Michael Fall

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April 11th

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April 18th

Joel Green

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Chi Ho Chan

Self-consistent radiative hydrodynamics simulations of dusty AGN Tori
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April 25th

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May 2nd

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