Wine and Cheese Spring 2019

This page records the schedule, titles and abstracts of the JHU/STScI CAS Astrophysics Wine & Cheese Series in Spring 2019.

Wine and Cheese sessions with one talk will have a 50 minute talk with 10 minutes for questions. Sessions with two speakers will have two 25 minute talks, each with 5 minutes for questions. Sessions in the Graduate Student Series will have three 15 minute talks, each with 5 minutes for questions.

February 4th

James Owen (Imperial College)

Understanding the formation and evolution of the Kepler Planets
The observed exoplanet population unveiled by Kepler is billions of years old, distinctly separated in time from the planet formation process that only lasted ~10-100 Myr. I will argue that atmospheric escape has been one of the key evolutionary drivers shaping the exoplanet population we observed today. By understanding how these planet evolve in time, I will show we can place some intriguing constraints on how they formed.

February 11th

Dillon Brout (UPenn)

First Cosmology Results Using Type Ia Supernova from the Dark Energy Survey
Today, now 20 years after the discovery of the acceleration of the universe, the Dark Energy Survey (DES) Supernova Program has discovered thousands of Type Ia Supernovae (SNe Ia) useful for cosmological measurements. In this talk I will present the first analysis of a small subset of 207 spectroscopically confirmed SNe Ia discovered during the first 3 years of the DES Supernova Program. I will show why this state of the art dataset provides constraints competitive to measurements using aggregate samples of >1000 SNe Ia, and I will forecast the full 5 year DES photometrically classified sample.

February 18th

Matthew Petroff (JHU)

3D-printed Millimeter Wave Absorbers
Additive manufacturing in the form of 3D printing has become increasing widespread in recent years. As this technology can produce submillimeter-sized features, it has potential uses in instrumentation and optics for millimeter wave astronomy, such as in Cosmic Microwave Background experiments. This talk will discuss the application of such technology to the development of a broadband millimeter wave absorber printable via the extrusion of a carbon-loaded thermoplastic.

Jonathan Aguilar (JHU)

Discovering Benchmark Low-Mass Companions with High-Contrast Imaging
In recent decades, a number of high-contrast imaging surveys have searched for low-mass stars, brown dwarfs, and planetary-mass companions at separations that are mostly inaccessible with other techniques. In wide-field surveys, they are buried underneath the bright PSF of the primary star, and in spectroscopic surveys, they appear only as a long and typically linear radial velocity trend. The slow nature of high-contrast observing means that the distribution in mass and separation of these companions, which link the close- and far-separation populations, is only now beginning to be revealed. We discuss the discovery and characterization of one such object, our prospects for measuring a dynamical mass, and place it in context with the known population of low-mass companions at separations of tens to hundreds of AU.

Caroline Huang (JHU)

A Mira Distance to Supernova Host Galaxy NGC 1559
One of the most exciting emerging issues in extragalactic astronomy and precision cosmology is the tension between the most precise locally-measured Hubble Constant and the one inferred from the cosmic microwave background (CMB) data assuming a Lambda CDM cosmology. New distance indicators, like Mira variables, can help provide a check to local Cepheid distance, increase the number of local calibrators for Type Ia Supernovae (SNe), and allow us to match the demographics of the calibrating sample of SNe to the Hubble flow sample more closely. In my talk, I will present the first Mira-based distance to a Type-Ia SNe host galaxy, NGC 1559, and discuss the feasibility of using Mira variables as an alternative distance indicator to Cepheids.

February 25th

Paul Schechter (MIT)

Twinkling Quasars: a Strong Limit on the Contribution of LIGO-mass Primordial Black Holes to the Cosmological Dark Matter Density
On rare occasions, a galaxy acts as gravitational lens producing multiple images of a quasar directly behind it. The stars within this galaxy then act as micro-lenses, breaking up the "macro-images" into "micro-images". As the stars move, the macro-images twinkle -- the gravitational analog of atmospheric scintillation. Counterintuitively, the amplitude of the twinkling does not increase monotonically with stellar density, and instead decreases at high optical depth. A single strongly micro-lensed quasar can set a significant upper limit on the graininess of the gravitational potential. The poster-child for such a limit is SDSS0924+0219 for which at least 50% of the lens' surface mass density must be in a smooth component rather a than grainy one. A sample of ten lensed quasars gives a 10% upper limit on the contribution of LIGO-mass primordial black holes to the cosmological dark matter density after discounting the graininess due to the observed stars.

March 4th

Bridget Falck (JHU)

Testing Gravity in the Cosmic Web
The accelerated expansion of the Universe, along with the huge failure of the vacuum energy as its explanation, has motivated the development of theories that tweak Einstein’s equations, adding a new degree of freedom that produces the observed late-time acceleration. These so-called “modified gravity” theories exploit the fact that general relativity is not well-tested on cosmological scales, and working out their theoretical predictions in the nonlinear regime of structure formation requires N-body simulations of specific models. After a pedagogical introduction to modified gravity, I will discuss the prospects for testing gravity in the cosmic web of large-scale structure.

Rahul Datta (JHU/GSFC)

Extragalactic Point Sources and Their Polarization Properties at Millimeter Wavelengths
The increasing sensitivity of millimeter wavelength telescopes has enabled detections of a large number of extragalactic sources that emit brightly in the millimeter-wavelength sky. Thousands of such sources are being detected in Cosmic Microwave Background (CMB) maps as high, point-like fluctuations above the background level. Measurement of the polarization properties of these sources opens up an interesting way to study the astrophysics of the sources. The Atacama Cosmology Telescope Polarimeter (ACTPol) is a polarization sensitive millimeter-wave camera which has surveyed thousands of square degrees of sky with arc-minute level resolution and sensitivity to sources at the level of a few milli-Jansky (mJy). In this talk, I will present measurements of the polarization of extragalactic sources, predominantly Active Galactic Nuclei (AGN), at 148 GHz made during the first two seasons of the ACTPol survey. I will discuss the future prospects of such measurements and present predictions for the contribution of power from unresolved sources to the CMB polarization spectrum in the context of future CMB surveys.

March 11th

David Neufeld (JHU)

Dark Matter that Interacts with Baryons: Density Distribution within the Earth, New Constraints on the Interaction Cross-Section, and a Dark Matter Search Recently Completed in my Basement
For dark matter (DM) particles with masses in the 0.6 - 6 proton mass range, we may set stringent constraints on the interaction cross-sections for scattering with ordinary baryonic matter. These constraints follow from the recognition that such particles can be captured by - and thermalized within - the Earth, leading to a substantial accumulation and concentration of DM that interact with baryons. I will discuss the probability that DM intercepted by the Earth will be captured, the number of DM particles thereby accumulated over Earth's lifetime, the fraction of such particles retained in the face of evaporation, and the density distribution of such particles within the Earth. This analysis provides an estimate of the DM particle density at Earth's surface, which may exceed 1.E+14 cm-3, and leads to constraints on various scattering cross-sections, which are placed by: (1) the lifetime of the relativistic proton beam at the Large Hadron Collider; (2) the orbital decay of spacecraft in low Earth orbit; (3) the vaporization rate of cryogenic liquids in well-insulated storage dewars; and (4) the thermal conductivity of Earth's crust. For the scattering cross-sections that were invoked recently in Barkana's original explanation for the anomalously deep 21 cm absorption reported by EDGES, DM particle masses in the 0.6 - 4 GeV/c^2 range are excluded. Finally, I will discuss a tabletop experiment (just completed in my basement) to further constrain the interaction cross-sections for a variety of atomic nuclei.

March 25th

Philip Engelke (JHU)

OH as an Alternate Tracer for Molecular Gas: A Study in the W5 Star-Forming Region
Tracing molecular gas in the Galactic ISM is complicated by the fact that the majority of diffuse, cold molecular gas is not detectable. CO(1-0) is the usual tracer, but evidence suggests that CO is not tracing all of the molecular gas, leading to the concept of "CO-dark" molecular gas, and the need for alternate tracers of this molecular gas. We have been using OH 18 cm lines as an alternate tracer for molecular gas using the Green Bank Telescope in West Virginia. I report on a survey for OH in the W5 star-forming region. Whereas in a quiescent region, OH has been detected in many places where CO has not been detected, in W5 the OH and CO trace a similar morphology of molecular gas. The mass of molecular gas traced by OH in the portion of the survey containing OH emission is 1.7 (+ 0.6 or - 0.2) x 10^4 solar masses, whereas the corresponding CO detections trace 9.9 plus or minus 0.7 x 10^3 solar masses. I propose a volume density-based explanation for the presence or absence of CO-dark molecular gas in different regions of the ISM, and report estimates of the average volume density for three regions of the ISM: the W5 star-forming region, CO-bright gas in the quiescent region, and CO-dark gas in the quiescent region predicted using a diffuse cloud model from Neufeld and Wolfire 2016. I also discuss modeling of the line excitation temperatures resulting from different physical conditions using the molpopCEP code written by Moshe Elitzur.

Brooks Kinch (JHU)

Predicting X-ray Spectra from Simulations
X-ray spectra are the primary means by which to probe the inner accretion flow and spacetime geometry of black holes, both for stellar-mass black holes (in binary systems) and supermassive active galactic nuclei. I will review the state of the art of black hole spectral analysis and discuss new techniques for predicting spectra based on 3D general relativistic magnetohydrodynamic (GRMHD) simulation, ray-tracing, and photoionization codes. By applying the relevant physical principles to simulation output, much the otherwise-parameterized uncertainty present in standard spectral modeling can be replaced with well-understood physics.

Kirill Tchernyshyov (JHU)

The CO to H2 Ratio in Diffuse Molecular Clouds at Low(er) Metallicities
There are a number of ways of calibrating the relation between the abundance of carbon monoxide (CO) and molecular hydrogen (H2) in molecular clouds. Column density measurements of both species from their absorption features in far ultraviolet (FUV) spectra constrain this relation on the outskirts of molecular clouds, a regime which is important theoretically for understanding CO chemistry and observationally for estimating the amount of "CO-dark" molecular gas. This type of observation has mostly been done in nearby molecular clouds in the Milky Way, i.e. at a single, relatively high, metallicity. I will present a new sample of CO and H2 column density measurements in the Large and Small Magellanic Clouds and compare them with predictions from molecular cloud models.

April 1st

Kimberly Boddy (JHU)

Angular Correlations in PTAs and Astrometry from a Stochastic Gravitational-Wave Background
There is significant interest in detecting stochastic gravitational waves, which may be produced by astrophysical sources (such as coalescing binary systems) and by processes in the very early Universe (such as inflation). In particular, pulsar-timing arrays and astrometric surveys can search for gravitational waves with frequencies below ~1/yr. The gravitational waves impart a characteristic correlation pattern in pulse arrival times and in the deflection of starlight across the sky. In this talk, I will present a new calculation of the angular power spectra and correlation functions for pulsar timing and astrometry, using the total-angular-momentum formalism.

Alec Hirschauer (STScI)

Asymptotic Giant Branch Stars in the Low-Metallicity Galaxy NGC 6822
The high-redshift systems in which the earliest generations of stars were formed, produced heavy elements and dust, and subsequently ended their life cycles were vastly different from the Milky Way. Nearby galaxies with low metal abundances provide important laboratories for observationally accessing the physical conditions equivalent to what had been ubiquitous throughout the early Universe. In order to more fully understand the role of dust in metal-poor environments, it is critically important to robustly identify their evolved, dust-producing asymptotic giant branch (AGB) stars. The local (~500 kpc) metal-poor ([Fe/H] ≈ -1.2; Z ≈ 30% Z⊙) star-forming galaxy NGC 6822 is thought to be analogous to higher-redshift systems at the epoch of peak star formation. We present color-magnitude diagrams (CMDs) utilizing archival photometry from the Spitzer Space Telescope (Khan et al. 2015; IRAC 3.6, 4.5, 5.8, and 8.0 μm and MIPS 24 μm) and the United Kingdom Infrared Telescope (Sibbons et al. 2012; UKIRT J-, H-, and K-band) of NGC 6822. Isolating red-excess objects and carefully employing color cuts, we identify oxygen- and carbon-rich AGB star candidates. Subsequent work will entail spectral energy distribution (SED) fitting of these sources to quantify the dust mass and dust mass loss rate of this galaxy. This project was completed in anticipation of a James Webb Space Telescope (JWST) guaranteed time observation (GTO) program for this galaxy, which will probe NGC 6822 to a depth comparable to the Spitzer SAGE (Surveying the Agents of a Galaxy's Evolution; Meixner et al. 2006) surveys of the Large and Small Magellanic Clouds.

April 8th

Amir Jafari Alanjagh (JHU)

Topology and Stochasticity of Turbulent Magnetic Fields
We present a mathematical formulation for the topology and stochasticity of random magnetic fields based on Renormalization Group (RG) methodology. The concept of scale-split energy density, ψ, is introduced in order to quantify the notion of the field topological deformation, topology change and stochasticity level. It is shown that the evolution of the magnetic field stochasticity is directly related to the super-linear Richardson diffusion and also field-fluid slippage, which has already been linked to magnetic reconnection in previous work. Magnitude and direction of stochastic magnetic fields can be studied separately by dividing ψ into two 3 + 1 dimensional scalar fields. As an application, we define magnetic reconnection in terms of the extrema of the Lp norms of these scalar fields. This approach in fact clarifies different definitions of magnetic reconnection, which vaguely rely on the magnetic field topology, stochasticity and energy conversion. Our results support the well-founded yet partly overlooked picture in which magnetic reconnection in turbulent astrophysical fluids occurs on a wide range of scales as a result of non-linearities at large scales (turbulence inertial range) and non-idealities at small scales (dissipative range). Lagrangian particle trajectories, as well as magnetic field lines, are stochastic in turbulent magnetized media in the limit of small resistivity and viscosity. Magnetic field tends to reduce its stochasticity induced by the turbulent flow by slipping through the fluid, which may accelerate fluid particles. This suggests that reconnection is a relaxation process by which magnetic field lowers both its topological entanglements induced by turbulence and also its energy level.

Taeho Ryu (JHU)

General-Relativistic Determination of Tidal Disruption Radii of Main-Sequence Stars
Tidal disruption events occur when stars can get sufficiently close to a massive black hole (BH) due to the tidal forces of the black hole. If the debris of the disrupted star accretes rapidly onto the BH, a luminous flare will be generated. Observing TDEs can give us fruitful information about unbiased BH populations, such as their mass functions and spins. Although the so-called tidal disruption radius is widely used for estimating TDE-related quantities, its definition is approximate. In order for more accurate TDE modelling, it is important to know the physical tidal radius. In this talk, I will talk about my current project focusing on identifying the physical tidal radius using general relativity and studying partial and full TDEs based on the physical tidal radius. Unfortunately, our analysis is not complete, but I can present some preliminary results.

April 15th

Shunsaku Horiuchi (VTech)

Particle Astrophysics of the Galactic Center
The centers of galaxies provide promising targets to search for signatures of dark matter self-annihilation or decay into Standard Model particles. However, astrophysical backgrounds are complex and must be modeled with care. I will discuss detailed studies of the Milky Way galactic center in pursuit of dark matter signals. I will cover new insights on astrophysical emissions that have been gained along the way and their impacts for dark matter searches.

April 22nd

Vishal Baibhav (JHU)

Multi-mode Black Hole Spectroscopy: Detection and Parameter Estimation
As current gravitational-wave detectors undergo technological improvements, we will soon reach an era when it would be possible to identify black hole merger remnants by measuring their quasinormal mode frequencies. This idea, often called “black hole spectroscopy”, is similar to identifying atomic elements through their spectral lines. I will address the question of detectability of quasinormal modes with current and planned detectors. I will show that while second-generation detectors are incapable of observing subdominant modes, space-based detector - like LISA - will see all harmonics currently available from numerical simulations. I will also explain how the detection of multiple ringdown modes can help in parameter estimation by breaking various degeneracies. This degeneracy breaking will be very important when detectors see only the ringdown (i.e., for intermediate-mass black hole mergers observed by ground-based detectors and for the heavier supermassive black hole mergers observed by LISA).

Daniel Pfeffer (JHU)

Line Intensity Mapping with Neural Networks
In this work we present the first application of a convolution neural network (CNN) to directly determine the underlying luminosity function of a line intensity map (IM) including a full treatment of extragalactic foregrounds and instrumental noise. We apply the CNN to simulations of mock Carbon Monoxide (CO) line intensity maps of the same sky area and frequency coverage as the near-future COMAP experiment. We evaluate the trained CNN on a number of noise scenarios in order to determine how robust the network predictions are for realistic data, where one may not know the true amplitude and shape of the instrumental noise or foreground signal. We find the CNN is most accurate below L = 10^5 L_sun and in the ideal case can more accurately predict the true signal than alternative, non-machine learned methods. We then show that when the network is used to predict the signal from maps with different foregrounds and noise than it is trained on, such as the case of using it on real data, that the errors on the predicted luminosity function can increase significantly.

Sara Frederick (UMd)

A New Class of Changing-Look AGNs
The Zwicky Transient Facility (ZTF) has enabled the discovery of several AGN caught "turning on" during the first year of the survey. Classified as LINERs by weak narrow forbidden line emission in their archival spectra, they were detected by ZTF as hosting nuclear transients. We found via follow-up spectroscopy that they had transformed into broad-line AGN. In one case, follow-up UV and optical spectra revealed the transformation into a narrow-line Seyfert 1 with strong coronal lines, and Swift monitoring revealed bright UV emission that tracked the optical flare, accompanied by a luminous soft X-ray flare that peaked ~60 days later. Archival light curves of the sample revealed similar smooth, flare-like deviations from quiescence, and constrain the onset of the optical nuclear flaring. I will present the systematic selection and follow-up of this unique class of transients related to physical processes associated with the LINER accretion state, and compare their properties to previously reported changing-look Seyferts.

April 29th

Cynthia Froning (UT)

Measuring the Energetic Irradiation Environment in the Habitable Zones around Low-Mass Stars
Understanding what happens to rocky planets and their atmospheres in the habitable zones of low mass stars is an ongoing focus of current astronomical research, one that has taken on even more importance in the era of TESS and JWST, which will be discovering and observing these atmospheres in transiting systems. To interpret such observations, we must understand the high energy emission of their host stars: X-ray/EUV irradiation can erode a planet’s gaseous envelope and FUV/NUV-driven photochemistry shapes an atmosphere’s molecular abundances, including potential biomarkers like O2, O3, and CH4. The role of stellar activity in the form of flares and CMEs on shaping the exoplanet evolution is also a key question for low mass systems. At present, we do not have sufficient observations and stellar models to interpret observations of the atmospheres of potentially habitable planets in these systems. To address this, our team is using XMM, Chandra, HST, and ground-based observatories to construct panchromatic (5 A - 5 micron) SEDs of a population of low mass stars. MUSCLES is the most widely used database for early-M and K dwarf (>0.3 M sun) irradiance spectra and has supported a wide range of modeling work, from the development of semi-emperical stellar models incorporating coronal and chromospheric emission to models of the structure and photochemistry of exoplanet atmospheres. The new Mega-MUSCLES project is following on the successful survey by extending to lower stellar masses (<0.3 Msun) and to a range of rotation periods to probe XUV emission evolution over gigayear time scales. Here, I will present current results and plan for future work with Mega-MUSCLES, with a focus on what the observations are showing in the areas of stellar activity in the XUV and its ties to optical behavior, signatures of star-planet interactions in exoplanet systems, and the construction of broadband SEDs for low mass stars.