| Date | Speaker | Abstracts |
|---|---|---|
| 27 Jan | Mustafa Amin | Organizational Meeting |
| 03 Feb | Kimmy Wu Stanford Linear Accelerator | Improved constraint on primordial gravitational waves with delensing Inflation generically predicts a background of primordial gravitational waves, which generate a primordial B-mode component in the polarization of the cosmic microwave background (CMB). The measurement of such a B-mode signature would lend significant support to the paradigm of inflation. Observed B modes also contain a component from the gravitational lensing of primordial E modes, which can obscure the measurement of the primordial B modes. We reduce the sample variance in the BB spectrum contributed from this lensing component by a process called 'delensing.' In this talk, I will show results from the first demonstration in an improved constraint on primordial gravitational waves with delensing using data from BICEP/Keck, the South Pole Telescope (SPT), and Planck. In addition, I will provide an outlook of joint-analysis efforts of the BICEP/Keck and the SPT collaborations (the South Pole Observatory) and next-generation experiment CMB-S4 to constrain primordial gravitational waves. |
| 10 Feb | 1. Siyang Ling 2. Kun Hu Rice University | 1. Gravitational Particle Production of Scalar Dark Matter in Alpha-Attractor Models of Inflation Gravitational particle production, namely the creation of particles due to coupling to gravitation, is a long studied phenomenon. In the case of production of scalar particles in a background of alpha-attractor inflationary model, the produced particles can survive till today and thus serve as a candidate for dark matter. Assuming that the scalar has a minimal coupling to gravity, the abundance of gravitationally-produced particles can be calculated. Inhomogeneities in the spatial distribution of produced particles correspond to an isocurvature component, which can be probed by measurements of the cosmic microwave background anisotropies. 2. Indication of a Pulsar Wind Nebula in the hard X-ray emission from SN 1987A (Journal Club Presentation) Since the day of its explosion, SN 1987A (SN87A) was closely monitored with the aim to study its evolution and to detect its central compact relic. The detection of neutrinos from the supernova strongly supports the formation of a neutron star (NS). However, the constant and fruitless search for this object has led to different hypotheses on its nature. Up to date, the detection in the ALMA data of a feature somehow compatible with the emission arising from a proto Pulsar Wind Nebula (PWN) is the only hint of the existence of such elusive compact object. Here we tackle this 33-years old issue by analyzing archived observations of SN87A performed Chandra and NuSTAR in different years. We firmly detect nonthermal emission in the 10−20 kev energy band, due to synchrotron radiation. The possible physical mechanism powering such emission is twofold: diffusive shock acceleration (DSA) or emission arising from an absorbed PWN. By relating a state-of-the-art magneto-hydrodynamic simulation of SN87A to the actual data, we reconstruct the absorption pattern of the PWN embedded in the remnant and surrounded by cold ejecta. We found that, even though the DSA scenario cannot be firmly excluded, the most likely scenario that well explains the data is the PWN emission. |
| 17 Feb | Sprinkle Day | No Classes/Talks |
| 24 Feb | 1. Laura Flagg 2. Asa Stahl Rice University | 1. The Mysterious Affair of the H2 in AU Mic I will talk about our detection of molecular hydrogen using HST-STIS FUV spectra. While we cannot conclusively determine its origin, the most likely explanation is that its from a starspot. This is unexpected, because the temperature of the H2 is much colder (~1000 K) in comparison with previous measurements of starspots on AU Mic. 2. The Occurrence of Rocky Habitable Zone Planets Around Solar-Like Stars from Kepler Data We present occurrence rates for rocky planets in the habitable zones (HZ) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar properties. We provide the first analysis in terms of star-dependent instellation flux, which allows us to track HZ planets. We define η⊕ as the HZ occurrence of planets with radius between 0.5 and 1.5 R⊕ orbiting stars with effective temperatures between 4800 K and 6300 K. We find that η⊕ for the conservative HZ is between 0.37+0.48−0.21 (errors reflect 68% credible intervals) and 0.60+0.90−0.36 planets per star, while the optimistic HZ occurrence is between 0.58+0.73−0.33 and 0.88+1.28−0.51 planets per star. These bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where DR25 completeness data are available. The large uncertainties are due to the small number of detected small HZ planets. We find similar occurrence rates using both a Poisson likelihood Bayesian analysis and Approximate Bayesian Computation. Our results are corrected for catalog completeness and reliability. Both completeness and the planet occurrence rate are dependent on stellar effective temperature. We also present occurrence rates for various stellar populations and planet size ranges. We estimate with 95% confidence that, on average, the nearest HZ planet around G and K dwarfs is ∼6 pc away, and there are ∼ 4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun. |
| 03 Mar | Andre Izidoro Rice University | The Solar System in the context of exoplanets More than 4000 exoplanets have been confirmed so far. Observations suggest that other planetary systems have dynamical architectures strikingly different from the solar system. Most gas giant planets have very eccentric orbits, unlike Jupiter and Saturn. Statistical analysis also suggest that super-Earths – planets with sizes between 1 and 4 Earth radii; or masses between 1 and 20 Earth mass – with orbital period shorter than 100 days exist around at least 30-50% of the sun-like stars. Mercury's orbital period is about 88 days but no hot super-Earth exists inside its orbit. Given that most observed exoplanetary systems are dramatically different than our own, one question arises: how did the solar system get so weird? I will start by presenting an overview of planet formation and then use the results of N-body numerical simulations to discuss a series of events that may have happened during the solar system formation to explain why our home planetary system is so different from others. |
| 10 Mar | Oliver Roberts NASA Marshall Space Flight Center | Masquerading Magnetar GRB 200415A Magnetars are slowly-rotating neutron stars with extremely strong magnetic fields, episodically emitting 100 ms long X-ray bursts with energies of about 10^40-41 erg. Rarely, they produce extremely bright, energetic giant flares that begin with a short (200 ms) intense flash, followed by fainter emission lasting several minutes that is modulated by the magnetar spin period (typically 2-12 s). Over the last 40 years, only three such flares have been observed within our local galactic group, which all suffered from instrumental saturation due to their extreme intensity. It has been proposed that extragalactic giant flares likely constitute a subset of short gamma-ray bursts, noting that the sensitivity of current instrumentation prevents us from detecting the pulsating tail to distances slightly beyond the Magellanic clouds. However, their initial bright flash is readily observable out to distances of < 60 Mpc. In this talk, X- and gamma-ray observations of bright transient called GRB200415A will be presented. We evaluate the spectral and temporal behavior of this event, which provide tantalizing clues as to the nature of the source. |
| 17 Mar | 1. Erik Weaver 2. Jason Ling Rice University | 1. TBA 2. Not so Fast: Re-evaluating what we know about Betelgeuse and its Dimming Event The recent optical dimming event from Betelgeuse caused quite a stir, and the "dust" appears to have settled on what caused the phenomenon. However, opposing pieces of evidence from longer wavelength observations may reveal some inconsistencies in the leading explanation. Additionally, revised stellar parameters from the latest stellar models of the pulsating red supergiant also show us that the mystery is not quite solved as this star continues its inevitable march towards a supernova explosion. |
| 24 Mar | Tristan Smith Swarthmore College | A Crisis in Cosmology? I will summarize the Hubble tension — a statistically significant disagreement between several different ways of inferring the current rate of the expansion of the universe — and my work trying to search for a theoretical model to resolve this tension. Planned space and ground observations promise to shed light on this tension and either establish it as a clear disagreement within our standard cosmological model or as an indication that we have significantly underestimated the systematic uncertainties of a range of observations. |
| 31 Mar | Gavin Combs Rice University | Did Mars Absorb its Ancient Oceans? (Scheller et. al, Science, 2021) Geological evidence shows that ancient Mars had large volumes of liquid water. Models of past hydrogen escape to space, calibrated with observations of the current escape rate, cannot explain the present-day D/H isotope ratio. We simulate volcanic degassing, atmospheric escape, and crustal hydration on Mars, incorporating observational constraints from spacecraft, rovers and meteorites. We find ancient water volumes equivalent to a 100- to 1500-meter global layer are simultaneously compatible with the geological evidence, loss rate estimates, and D/H measurements. In our model, the volume of water participating in the hydrological cycle decreased by 40 to 95% over the Noachian period (~3.7 to 4.1 billion years ago), reaching present-day values by ~3.0 billion years ago. Between 30 and 99% of Martian water was sequestered by crustal hydration, demonstrating that irreversible chemical weathering can increase the aridity of terrestrial planets. |
| 07 Apr | Hui Li Los Alamos National Laboratory | A New Mechanism for the Ring and Spiral Formation in Protoplanetary Disks and Implications for Observations Discovery of dust rings, spirals, localized features has puzzled the protoplanetary disk community for several years and many models have been put forward. Understanding these features could hold the key to planet formation processes as forming planets should perturb their surrounding disks and produce observable signatures. Here, we describe a new mechanism that does not involve any forming planets, but is more directly related to waves in disks. We find that protoplanetary disks may spontaneously generate multiple, concentric gas rings through an eccentric cooling instability. Using both linear theory and non-linear hydrodynamics simulations, we show that a variety of background states may trap a slowly precessing, one-armed spiral mode that becomes unstable when a gravitationally-stable disk rapidly cools. We verify the instability evolution and ring formation mechanism from first principles with our linear theory, which shows remarkable agreement with the simulation results. Dust trapped in these rings may produce observable features consistent with observed disks. Additionally, direct detection of the eccentric gas motions may be possible when the instability saturates, and any residual eccentricity left over in the rings at later times may also provide direct observational evidence of this mechanism. |
| 14 Apr | 1. Hongyi Zhang 2. Ted Grosson Rice University | 1. Beyond Schrodinger-Poisson: nonrelativisitic effective field theory for scalar dark matter Massive scalar fields provide excellent dark matter candidates, whose dynamics are often explored analytically and numerically using nonrelativistic Schrodinger-Poisson (SP) equations in a cosmological context. In this talk, starting from the nonlinear and fully relativisitic Klein-Gordon-Einstein (KGE) equations in an expanding universe, I will provide a systematic framework for deriving the SP equations, as well as relativistic corrections to them, by integrating out ‘fast modes’ and including nonlinear metric and matter contributions. Upon including the leading-order corrections, our equations are applicable beyond the domain of validity of the SP system, and are simpler to use than the full KGE case in some contexts. As a concrete application, we calculate the mass-radius relationship of solitons in scalar dark matter and accurately capture the deviations of this relationship from the SP system towards the KGE one. 2. Magnetic Fields and their Variability in Fully Convective Stars The only way to directly probe the magnetic fields of distant stars is through the Zeeman effect. Measuring the extent to which the Zeeman effect modifies the spectrum of a star by fitting a model to the spectrum tells us not only the overall strength of the magnetic field, but also a little about the field's structure. This presentation discusses the process of measuring the magnetic fields of the M dwarfs EV Lac and AD Leo, as well as how their strengths vary over the course of a few years. |
| 21 Apr | 1. Jackson White 2. Preston Tracy Rice University | 1. Photometric Reductions and Stellar Variability In the Carina Nebula The Carina Nebula is home to a wide range of variable stars, and one of the most effective ways to study these variable stars is to observe how their brightness changes over time. In this presentation I will discuss my work with Prof. Patrick Hartigan in creating a pipeline to reduce a dataset of short cadence multi-color observations taken of the Carina Nebula into light curves. With over thirty terabytes of observations, the scale of this dataset offers an unparalleled view of Carina, but also adds to the significant challenge of obtaining optimal photometric accuracy. I will discuss the implementation of this pipeline as well as some early results of using it to characterize eclipsing binary stars. 2. Detection of an Atmosphere on a Rocky Exoplanet (Swain et al., AJ, 2021) We report the detection of an atmosphere on a rocky exoplanet, GJ 1132 b, which is similar to Earth in terms of size and density. The atmospheric transmission spectrum was detected using Hubble WFC3 measurements and shows spectral signatures of aerosol scattering, HCN, and CH4 in a low mean molecular weight atmosphere. We model the atmospheric loss process and conclude that GJ 1132 b likely lost the original H/He envelope, suggesting that the atmosphere that we detect has been reestablished. We explore the possibility of H2 mantle degassing, previously identified as a possibility for this planet by theoretical studies, and find that outgassing from ultrareduced magma could produce the observed atmosphere. In this way we use the observed exoplanet transmission spectrum to gain insights into magma composition for a terrestrial planet. The detection of an atmosphere on this rocky planet raises the possibility that the numerous powerfully irradiated super-Earth planets, believed to be the evaporated cores of sub-Neptunes, may, under favorable circumstances, host detectable atmospheres. |
| 28 Apr | Mudit Jain Rice University | CMB Birefringence from Ultralight Axion Strings Ultralight axions (ULA), whose masses can lie in a wide range of values and can be even smaller than 10^{-28} eV, are generically predicted in UV theories such as string theory. In the cosmological context, the early Universe may have gotten filled with a network of ultralight axion (ULA) cosmic strings which, depending upon the mass of the axion, can survive till very late times. If the ULA also couples to electromagnetism, and the network survives post recombination, then the interaction between the strings and the CMB photons induces a rotation of the polarization axis of the CMB photons (otherwise known as the birefringence effect). This effect is independent of the string tension, and only depends on the coupling between the ULA and the photon (which in turn is sensitive to UV physics). In this talk I will present some results for this birefringence effect on CMB, due to three different models of string network. Interestingly, this is within the reach of some current and future CMB experiments. |