Abstract: CASS All-Hands Meet & Greet this Friday, October 9, from 12:00 to 1:00 pm via Zoom (link above - Meeting ID: 921 9114 3473). This meeting will be an opportunity for new members of CASS to introduce themselves and present a quick overview of their research. It will also give established members of CASS a chance to summarize their recent work. New graduate students are especially encouraged to attend. Journal Club is an informal environment in which graduate students can give short talks on any subject of interest to them, be it a recent paper, their own research, or any other topic. All are welcome to attend.

 Abstract: Meeting ID: 997 8928 7083

 Abstract: Meeting ID: 999 4201 6214

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Meeting ID: 999 4201 6214
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ABSTRACT: Rossby/Drift-zonal flow turbulence frequently occurs in a background of a static stochastic (tangled) magnetic field. Tangled fields that coexist with an ordered mean field play a key role in turbulence in the solar tachocline, and in magnetic confinement devices. Recent experiments of confinement devices indicate that RMP fields can reduce fluctuation-driven Reynolds forces and so inhibit the initiation of the L-H transition. We present a theory of vorticity flux decoherence and its implications for zonal flow evolution. This theory builds upon recent fundamental work on vorticity mixing in a tangled magnetic field.

We calculate the decoherence of the vorticity flux due to stochastic magnetic field scattering in presence of a strong toroidal field. The three relevant rates are: (1) the bandwidth of the ambient electrostatic micro-instabilities, (2) the bandwidth of Alfvén waves excited by Drift-Alfvén coupling, and (3) the stochasticity-induced decorrelation rate, where D accounts for scattering by the stochastic field). Decoherence requires (3) > (1) , as well as (3) ≥ (2) (i.e. large Kubo number). These inequalities define the critical value of stochastic magnetic intensity for an effect on the transition. Using an L-H transition model, the incremental relative increase in critical power for transition is obtained. The analysis proceeds by considering the Elsässer population responses.

The foundation of the theory of stochastic decoherence lies in the basic theory of vorticity and pressure mixing in a disordered magnetic field. The physically relevant case is that of a strong but disordered field.

 Abstract: Meeting ID: 954 7128 7572

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Meeting ID: 954 7128 7572

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ABSTRACT: Ultracool dwarfs (UCDs, mass M < 0.1 Msun, effective temperature Teff < 3000 K) are the lowest-mass stars and brown dwarfs. They trace the structure, star-formation history and chemical evolution of the Milky Way, due in part to the cooling evolution of non-fusing brown dwarfs. The wide-field optical and infrared spectroscopic and photometric surveys that have uncovered the majority of ultracool dwarfs now known are generally limited to the local volume (distances < 100 pc) due to these objects' faint luminosities. To expand this sample, we have searched for distant ultracool dwarfs in 0.5 square degrees of low-resolution near-infrared spectral survey data in the WFC3 Infrared Spectroscopic Parallel Survey (WISPS) and the 3D-HST parallel survey . We report the discovery of 182 M7-T9 dwarfs in these samples, with spectro-photometric distances up to 2 kpc for L dwarfs and 400 pc for T dwarfs. We model the number density distribution with population simulations incorporating various assumptions of the initial mass function, star formation history, binary fraction, Galactic scaleheight, and different ultracool dwarf evolutionary models. Our number counts are consistent with a scaleheight that varies with spectral type, ranging from 350 pc for late M dwarfs, 150--200 pc for L dwarfs and >300 pc for T dwarfs, generally consistent with population simulations.

 Abstract: Meeting ID: 958 5800 8375

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Meeting ID: 958 5800 8375

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 Abstract: Meeting ID: 932 1078 2533

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ABSTRACT: Ultracool dwarfs are the lowest mass stars and brown dwarfs (Teff <= 3000 K, M <= 0.1 Msun). They are ideal tracers of Galactic star formation history because of their long lifetimes, weak or absent fusion, and fully convective interiors. While the ages of individual ultracool dwarfs are difficult to measure, the ages of ultracool dwarf populations can be estimated through their kinematics and velocity dispersions. I present an analysis of a local (d <= 20 pc) sample of 166 UCDs with full 6D spatial and velocity coordinate measurements based on high-precision astrometry and radial velocities, the latter based on high-resolution spectroscopy with Keck/NIRSPEC. I find that L-type ultracool dwarfs are significantly more dispersed and hence older than expected from population simulations, suggesting problems in current evolutionary models or a feature unique to the local ultracool dwarf population.

 Abstract: Meeting ID: 923 3958 0517

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 Abstract: Meeting ID: 943 8386 9575

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Meeting ID: 943 8386 9575
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ABSTRACT: We will hold our annual discussion on sexual harassment and work place climate issues in this Journal Club. We will have a speaker from OPHD, Helen Kaiser. In past years we have had discussions on these topics involving faculty, researchers, graduate and undergraduate students and administrative staff. All in CASS and affiliated departments (e.g., Physics, Chemistry, MAE, Math, SIO, etc.) are welcome to attend.

 Abstract: ABSTRACT: A recently released paper reports a 2.4 sigma detection of static cosmic birefringence (!) in Planck 2018 data. They accomplish this by fitting out a rotation angle miscalibration at each frequency based on galactic foreground information (one of the few times foregrounds are useful rather than harmful for CMB analysis). This allows them to break the degeneracy between hardware angle miscalibration and cosmic birefringence angle. Obviously if this claim strengthens with further datasets it would be a huge breakthrough in the search for physics beyond the standard model, so I'll also attempt to go over which models of birefringence this would provide support for.

 Abstract: Meeting ID: 955 1230 2906

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Meeting ID: 955 1230 2906
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ABSTRACT: The abundance of metals (i.e metallicity), as well as its spatial variation, traces a galaxy's chemical enrichment history. A direct method to constrain the metallicity in a galaxy is to measure the gas-phase oxygen abundance, 12+Log(O/H), via the emission from a suite of Oxygen strong lines as well as the fainter temperature sensitive auroral line, [O III](4363).

In the era of Integral Field Spectrographs (IFS) it is possible to create kiloparsec-parsec scale 2-dimensional maps of the emission line structure of local galaxies. As part of a program to measure the spatial variation of metallicity in nearby galaxies, we observed the blue compact dwarf galaxy I Zw 18 using the IFS Keck Cosmic Web Imager (KCWI). In addition to kiloparsec-parsec scale mapping of sensitive metallicity diagnostics, these KCWI observations reveal two, previously unresolved, He III regions. The two He III regions, characterized by emission from the recombination line He II(4686), lie along an axis which not only intercepts a third He III region but also the position of I Zw 18's Ultra-luminous X-ray (ULX) source. Conventional sources capable of doubly ionizing Helium are Wolf-Rayet and O stars. However, Wolf-Rayet stars are not detected in all He II emitting galaxies, and modeling of the most massive O stars underpredicts He II/Hb by an order of magnitude. Each He III region exhibits He II/Hb greater than 2%. The largest HeII/Hb enhancement occurs in a region with no detected stellar population. These observations suggest that HeII emission in I Zw 18 may be consistent with X-ray or shock ionized nebulae powered by I Zw 18’s ULX. I will present the results from the analysis of this He II emission in the context that all of the He II ionizing radiation originates from the ULX in I Zw 18. I will also present the preliminary results of the spatial distribution of metallicity across I Zw 18.

 Abstract: Meeting ID: 937 7425 4162
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 Abstract: Meeting ID: 975 1185 0624

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Meeting ID: 936 8966 8431
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ABSTRACT: Simulations studying galaxy evolution typically focus on incorporating good initial conditions from the CMB, galactic processes such as star formation and supernova feedback from first principles. They track observables such as metallicity, star formation rates and the morphological profiles to give extremely detailed simulations and predictions inside the disk and bulge of galaxies. Such an approach runs into a computational resource intensive problem as we probe larger radii and try to analyze the circumgalactic medium. As a result, the simulated circumgalactic medium tends to suffer from problems of numerical resolution, which has a noticeable impact on observables since numerical dilution of gas profiles can impact predicted line intensities in these regions. For this talk, I will primarily review the first article (FOGGIE I) in a series published by the Figuring Out Gas & Galaxies in Enzo (FOGGIE) project. I will discuss how FOGGIE tackles some of the aforementioned problems and show some initial results.

Links to the FOGGIE articles (published as of January 2021) -
1. Figuring Out Gas & Galaxies in Enzo (FOGGIE). I. Resolving Simulated Circumgalactic Absorption at 2 ≤ z ≤ 2.5
2. Figuring Out Gas & Galaxies in Enzo (FOGGIE). II. Emission from the z = 3 Circumgalactic Medium
3. Figuring Out Gas & Galaxies in Enzo (FOGGIE). III. The Mocky Way: Investigating Biases in Observing the Milky Way's Circumgalactic Medium
4. Figuring Out Gas & Galaxies in Enzo (FOGGIE). IV. The Stochasticity of Ram Pressure Stripping in Galactic Halos

 Abstract: Meeting ID: 999 4668 8639
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Meeting ID: 999 4668 8639

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 Abstract: Meeting ID: 932 7627 4909

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Meeting ID: 932 7627 4909

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ABSTRACT: In this talk, I will explore a surprising connection between speculative issues in elementary particle physics and the Sun. I will show how the extreme sensitivity of nuclear reaction Coulomb barrier penetration at the low energies of the solar core and the recent observation of the CNO neutrinos from the Sun by the Borexino collaboration could be leveraged to probe aspects of the non-standard interactions involving light mediators recently invoked to explain anomalies in short baseline neutrino experiments.

I will also discuss how our work highlights the necessity for a better sensitivity to probe the pp and CNO reactions in laboratory experiments.

Moreover, with an improved determination of solar metallicity, and a more precise measurement of the CNO flux, our perspective limits on the mass and the coupling of the non-standard mediators will likely improve.

 Abstract: Meeting ID: 935 9740 1812

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Meeting ID: 935 9740 1812

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 Abstract: Meeting ID: 999 8603 3545

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Meeting ID: 999 8603 3545

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 Abstract: Meeting ID: 991 1549 9521

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Meeting ID: 991 1549 9521

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ABSTRACT: Einstein's theory of general relativity provides an excellent classical model of gravitation, and the Standard Model of particle physics is a well-established quantum theoretical model of particles and all forces except gravity. However, at the Planck scale, a quantum-consistent theory of gravity is required. Several theories that attempt to unify gravity and the Standard Model, including String Theory and loop quantum gravity, suggest that there may be deviations from Lorentz invariance at this energy scale. If Lorentz symmetry is broken, the phase velocity of light in vacuum may depend on photon energy, polarization, and direction of propagation. The Standard-Model Extension (SME) is an effective field theory framework, which extends the Standard Model of particle physics by introducing new, Lorentz and CPT violating terms in the Lagrangian, while conserving charge, energy, and momentum. We have developed a method to combine astronomical polarization measurements from multiple extragalactic sources in order to individually constrain Lorentz invariance violation within the SME framework. By applying this method to a large number of optical polarization measurements, we have obtained the strongest constraints on individual SME coefficients to date.

 Abstract: Meeting ID: 941 3339 8123

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Meeting ID: 941 3339 8123

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ABSTRACT: The CO-to-H2 conversion factor (αco) is critical to studying molecular gas and star formation in galaxies. The value of αco has been observed to vary in different regions of a galaxy, and it is dependent on environmental parameters such as gas densities and temperatures. Previous observations on kpc scales revealed lower αco values in the centers of some nearby star-forming galaxies, including NGC 3351. We present new ALMA Band 3, 6, and 7 observations of 12CO, 13CO and C18O rotational lines on 50 pc scales in the center of NGC 3351. Such resolution is sufficient to resolve giant molecular clouds and reveal variations of αco in galaxy centers. Using multi-line modeling and a Bayesian likelihood analysis, we constrain possible values of the H2 density, kinetic temperature, CO column density, and CO isotopologue abundances at each pixel. The αco distribution can be derived from the CO column densities and then compared with other parameters.

 Abstract: Meeting ID: 985 3390 0485

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Meeting ID: 985 3390 0485

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ABSTRACT: Cosmological models and data show that a cosmic neutrino background (CnuB) existed at the time of last scattering, when the cosmic microwave background was formed. However, this CnuB has never been directly detected, because ofthe weakness of neutrino interactions. Experiments already being built, such as Ptolemy, might reach the sensitivity needed for a direct detection of the CnuB. This would provide valuable validation of cosmological models, aspects of the Standard Model of particles, and possibly new information about the Dirac or Majorana nature of neutrinos, and even their stability as particles. We present some of the physics behind such an intriguing measurement.

 Abstract: Meeting ID: 948 2567 6395

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Meeting ID: 948 2567 6395

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ABSTRACT: HR2562 and its brown dwarf companion, HR2562B, were the first discovered system where a BD companion resided in a cleared inner hole of a debris disk, and hence they provide invaluable opportunities to study the direct interaction between the companion and the disk. In order to study this dynamical interaction, we present our results on determining the properties of the debris disk based on the first ALMA image of the system HR2562 and the coplanarity between the disk and the orbit of the companion. We determined the disk properties by fitting the ALMA image with an MCMC routine that uses the MCFOST radiative transfer code. We find the disk to be well resolved and nearly edge on, consistent with the previously marginally resolved Hershel image. We also find that the full extent of the disk is ~200 AU and place an upper limit on the radius of the inner hole, which remains unresolved in our ALMA images. An inner radius comparable to the distance between HR2562B and its host star may be an evidence for dynamical sculpting. We also use the derived properties of the disk to shed light on both the formation history and true mass of HR2562B.

 Abstract: Meeting ID: 987 3708 7503

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Meeting ID: 987 3708 7503

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ABSTRACT: I will discuss the Pen Pals Program, started by Lindsay Lowry, where we wrote letters to elementary school kids this year and go over some of the highlight questions of the year!