Date: Wednesday 01-Feb-2023
Speaker: ChongChong He (UMD)
Title: "Zooming in 18 orders of magnitude: formation of large circumstellar disks in strongly magnetized cores"
Using a novel zoom-in adaptive-mesh-refinement method, I simulate the formation and fragmentation of prestellar cores in strongly magnetized giant molecular clouds (GMCs). The simulation simultaneously follow the evolution of a GMC, ~80 pc in size, and the formation of circumstellar disks with ~ AU resolution, achieving an unprecedented dynamic range of 18 orders of magnitude in volume, bridging the gap between clouds and individual protostars. I have shown that massive stars form from the filamentary collapse of dense cores and grow to masses several times bigger than the core mass due to accretion from larger scales via circumstellar disks, suggesting a competitive accretion scenario for high-mass star formation, a problem that is not well understood. Magnetic braking fails to prevent the formation of Keplerian disks even in cores with mass-to-flux ratios close to critical. I have shown that this is due to the extremely turbulent nature of the magnetic field which is many times weaker at braking disk rotation than in the perfectly aligned case, proposing a solution to the “magnetic braking catastrophe
Date: Wednesday 08-Feb-2023
Speaker: Chia-Yu Hu (University of Florida)
Title: "Co-evolution of dust and chemistry in simulated dwarf galaxies with a resolved interstellar medium"
The cycle of gas in and around galaxies plays a fundamental role in galaxy formation. Current hydrodynamical simulations have managed to reach parsec-scale resolutions to follow the small-scale physics of interstellar medium (ISM). As our knowledge of the gas cycle comes from multi-wavelength observations, detailed modeling of the ISM chemistry is necessary in order to predict observables. However, interstellar dust, the major catalyst of ISM chemistry, is often regarded as a non-evolving species. In this talk, I will present recent progress on resolved hydrodynamical simulations of low-metallicity dwarf galaxies coupled with chemistry and dust evolution. I will show that the observed CO(1-0) luminosity can only be reproduced if dust growth in the ISM is included as otherwise CO would be completely photo-dissociated. Sputtering from supernova explosions is not efficient enough to destroy all the newly-created dust in dense gas, which leads to galactic outflows 20 - 50% dustier than the ISM traveling to large distances, providing a possible source of the intergalactic dust.
Date: Wednesday 22-Feb-2023
Speaker: Alisa Galishnikova (Princeton)
Title: "Collisionless Black Hole Accretion"
The accretion flows around the black holes in Sgr A*, M87, and other systems are strongly magnetized and collisionless. This, in fact, makes the usually employed general relativistic (GR) magnetohydrodynamic (MHD) method formally inapplicable. Thus, addressing the BH accretion problem, in principle, requires a fully kinetic approach. In this talk, I will show a study of axisymmetric accretion of collisionless plasma around the black holes from first principles using GR particle-in-cell simulations (GRPIC). By doing so, I carry out a side-by-side comparison of global dynamics in GRMHD simulations and GRPIC for the same black hole accretion problem. Magnetic reconnection, which is believed to be responsible for particle acceleration and subsequent flares, is accurately captured in the kinetic approach. I directly examine the production of non-thermal particles due to magnetic reconnection. I will also discuss the implications of our results for modeling event-horizon scale observations of Sgr A* and M87 by GRAVITY and the Event Horizon Telescope.
Date: Wednesday 15-Mar-2023
Speaker: Megan Mansfield (University of Arizona)
Title: "Spectroscopic eclipse mapping of the ultra-hot Jupiter WASP-18b"
Exoplanet eclipse and phase curve observations have revealed information about energy transport and thermal structures of planetary atmospheres. However, this information has been fundamentally limited by these observational techniques, which only probe spatially-integrated fluxes. The launch of JWST in 2021 enabled the new technique of spectroscopic eclipse mapping, which uses measurements during secondary eclipse ingress and egress to construct a brightness map resolved in latitude, longitude, and altitude. No other observational technique can simultaneously resolve exoplanet atmospheres in all three dimensions.
In this talk, I will present early results of a spectroscopic eclipse map of the ultra-hot Jupiter WASP-18b, the first such map produced from JWST data. I will describe the methods used to extract spatially-resolved information from secondary eclipse observations and what this information reveals about compositional and thermal gradients across the dayside of the planet. Finally, I will discuss the prospects for eclipse mapping other exoplanets with JWST.
Date: Wednesday 29-Mar-2023
Speaker: Scott Lucchini (UW–Madison)
Title: "The Magellanic Corona: The Missing Piece of the Magellanic System"
While previous models of the formation of the Magellanic Stream have been able to reproduce many of the observations, two puzzles remain - the large amount of ionized gas associated with the Magellanic System, and the high mass of the Large Magellanic Cloud (LMC). To solve both these outstanding discrepancies simultaneously, we introduce the Magellanic Corona. This warm, ionized circumgalactic medium should surround the LMC as it interacts with the Small Magellanic Cloud (SMC) and the Milky Way. In this talk, I will present simulations of the formation of the Magellanic Stream including the Magellanic Corona in which we are able to account for both the neutral and ionized components of the Stream while also reproducing the present-day positions and velocities of the LMC and the SMC in their motion around the Milky Way. Upon inclusion of the Magellanic Corona, we find a new family of orbital histories for the Clouds that result in the neutral Stream being significantly closer than previous models predicted. This nearby Stream implies lower observational gas masses, explains the turbulent structure in the observed HI, and predicts higher Halpha intensities in agreement with observations.
Date: Wednesday 05-Apr-2023
Speaker: Vanessa López-Barquero and Greg Marcel (University of Cambridge)
Title: "Chaos, Magnetic Fields, and the Cosmic Ray Anisotropy" and "A unified accretion ejection paradigm for X-ray binaries : the JED-SAD paradigm"
"Chaos, Magnetic Fields, and the Cosmic Ray Anisotropy"
Over the past decades, various experiments, such as IceCube and HAWC, have observed cosmic-ray anisotropy at different angular scales in a wide energy range. However, no comprehensive or satisfactory explanation has been put forth to date. One of the difficulties is that these particles interact with magnetic fields; therefore, their directional information is distorted as they travel. In addition, as cosmic rays (CRs) propagate in the Galaxy, they can be affected by magnetic structures that temporarily trap them and cause their trajectories to display chaotic behavior, therefore modifying the simple diffusion scenario.
Here, we examine the effects of chaos and trapping on the TeV CR anisotropy. Concretely, we apply this method to study the behavior of CRs in the heliosphere since its effects can be remarkably significant for this anisotropy. Specifically, how the distinct heliospheric structures can affect chaos levels. We model the heliosphere as a coherent magnetic structure given by a static magnetic bottle and the presence of temporal magnetic perturbations. This configuration is used to describe the draping of the local interstellar magnetic field lines around the heliosphere and the effects of magnetic field reversals induced by the solar cycles.
In this work, we explore the possibility that particle trajectories may develop chaotic behavior while traversing and being temporarily trapped in this heliospheric-inspired toy model and the potential consequences that it can have on the cosmic ray arrival distribution. It was found that the level of chaos in a trajectory is linked to the time the particles remain trapped in the system. This relation is described by a power law that could prove inherently characteristic of the system. Also, the arrival distribution maps show areas where the different chaotic behaviors are present, which can constitute a source of time-variability in the CR maps and can prove critical in understanding the anisotropy on Earth.
--
"A unified accretion ejection paradigm for X-ray binaries : the JED-SAD paradigm"
The hysteresis behavior of X-ray binaries during their outbursts remains a mystery. In this work, we developed a paradigm where the disk material accretes in two possible, mutually exclusive, ways (Ferreira et al. 2006). In the usual alpha-disk mode (SAD, Shakura & Sunayev 73), the dominant local torque is due to a radial transport of the disk angular momentum. In the jet-emitting disk mode (JED), magnetically-driven jets carry away mass, energy, and all the angular momentum vertically. Within this framework, the transition from one mode to another is related to the magnetic field distribution, an unknown.
We have shown that typical hard states of X-ray binaries can be reproduced up to unprecedented X-ray luminosities in this paradigm (Marcel+18a). In addition, we used a simple physical model to estimate the radio fluxes (at 8.6-9 GHz) radiated by the jets for any given set of parameter. Strikingly, both spectral features fit extremely well. We have shown that the X-ray spectral behavior of the X-ray binary GX 339-4 can be covered, both in X-ray and radio (Marcel+18b, A&A 617, A46). We then extended this work to 3 more outbursts from GX 339-4 Marcel et al., 2019, A&A 626, A115). This is, to our knowledge, the first time that accretion-ejection cycles are being reproduced, using both accretion (X-rays) and ejection (radio) constraints.
We have since improved the fitting procedure with direct spectral fits on XMM-Newton+NuSTAR data from the AGN HE 1143-1810 (Ursini et al,. 2020, A&A 634, A92), XRT+NICER+NuSTAR+BAT data from the X-ray binary MAXI J1820+070 (Marino et al. 2021, A&A 656, A63), and RXTE data from the X-ray binary GX 339-4 (Barnier et al. 2022, A&A 657, A11). Moreover, we have addressed the production of low frequency quasi-periodic oscillations during the outbursts (Marcel et al. 2020, A&A 640, A18), as well as the radiative efficiency of the accretion flow and the associated radio--X-ray correlation (Marcel et al. 2022, A&A 659, A194). The timing properties have also since been addressed (Malzac et al., to be submitted, Marcel et al, in prep.).
I will introduce the model and present some of its associated results.
Date: Wednesday 12-Apr-2023
Speaker: Sam Grunblatt (Johns Hopkins)
Title: Giants Transiting Giants: Revealing planet demographics of post-main sequence systems
Despite the thousands of planets discovered orbiting stars on the main sequence, the planetary systems of more evolved stars remain poorly understood. Studying planetary systems around subgiant and red giant stars can reveal how main sequence planetary systems respond to stellar evolution over time. The 'Giants Transiting Giants' survey has increased the number of planets known transiting evolved stars by a factor of three. These planetary systems provide testbeds for understanding planet inflation, orbital alignment, decay, and chaotic disruption, planet engulfment and related chemical enhancement of host stars. Recent studies have revealed that the orbital configurations of these systems differ from those of main sequence systems--planets around evolved stars seem to follow unique period-eccentricity and orbital alignment trends that are not seen in main sequence systems. Furthermore, the discovery of a low-density hot Neptune around a red giant star suggests that these planets are more resilient to atmospheric erosion than previously thought. Future studies of these and similar systems with JWST and other upcoming missions will reveal how planet atmospheres respond to the evolution of their host stars, and will eventually bridge the gap between main sequence and white dwarf planetary systems, teaching us about the eventual fate of Solar Systems such as our own.
Date: Wednesday 19-Apr-2023
Speaker: Jake Bennett (Harvard CfA)
Title: The impact of the first gargantuan black holes on galaxies and their surroundings in the early Universe - insights from cosmological simulations
The characterisation of high-redshift (z >= 6) galaxies with JWST is revolutionising our understanding of the formation of the earliest galaxies. At this epoch, the co-evolution of massive galaxies and their black holes can begin in earnest, and JWST can allow us to study the role that the first quasars have in the formation of their host galaxies and their surroundings.
The largest black hole discovered at z > 6 to date has an inferred mass in excess of 10^10 solar masses, raising questions about both how they reach such a size, and the impact these monsters have on their surroundings at early times. Feedback from early quasars can eject gas from their hosts and heat their surroundings, cutting off inflows and interrupting star formation, which can lead to changes in the structure of the galaxies themselves. Additionally, as the masses inferred from observations have a significant associated uncertainty, it is important to explore how the use of probes of the primordial circumgalactic medium (CGM) could also be used to further constrain early BH growth.
With zoom-in simulations of a massive high-redshift galaxy, we investigate how galaxies are impacted by feedback from the first supermassive black holes. We show how these quasars can drive strong outflows that expel gas far into the CGM, how many early quasars are likely to be obscured for a significant fraction of their history, and how AGN feedback plays a role in clearing the surroundings of BHs for them to be detected in the UV. Furthermore, we investigate the impact of such feedback on the host galaxy itself and find notable differences in the structure and metal content of the stellar population. Many of these results have the potential to be tested with JWST, ALMA, and future X-ray and SZ observations, allowing us to constrain the early growth of high-redshift galaxies.
Date: Wednesday 26-Apr-2023
Speaker: Michael Foley (Harvard CfA)
Title: Connecting 3D Observations and Simulations of Star Formation in Our Local ISM
Our understanding of our local galaxy has been completely transformed by Gaia. In this talk, I will explain how we have used 3D data products derived from Gaia to build the first high-resolution 3D model of the Orion star-forming region featuring gas, dust, and stars. Building on this model and others we have constructed in 3D (Perseus, Taurus, the Radcliffe Wave, and the Local Bubble), we have been able to construct new theories about the history of stellar feedback in the local ISM. I will discuss how we are beginning to test some of these theories with a simulation suite of isolated shock-turbulence interactions. Finally, I will conclude with how these observations and simulations can be coupled to extragalactic observations, particularly those conducted by the PHANGS survey.
Date: Wednesday 03-May-2023
Speaker: Sonny Harman (NASA Ames)
Title: A Slice of PIE: The Photochemical Intercomparison for Exoplanets
There is a rich scientific legacy in comparing simulations of planetary atmospheres against prior efforts and observations. For modern Earth, for example, the Atmospheric Chemistry and Model Intercomparison Project (ACCMIP) has been working for over a decade to evaluate interactions between chemistry and climate. Exoplanets, however, lack the same breadth and quality of data, forcing the community to examine other options in order to make stronger inferences and avoid a long list of potential pitfalls. I will describe some examples of how 1-on-1 and smaller party intercomparisons have made advances and then walk through our evolving ideas on the more ambitious work of developing a framework for not just one-off comparisons but for robust, long-term community engagement in addressing knowledge, input data, and validation gaps. Many of these ideas are based on efforts such as THAI (the TRAPPIST Habitable Atmospheres Intercomparison led by Thomas Fauchez) and Benjamin Drummond’s Hot Jupiters Intercomparison work, but we are working to refine them for the widely disparate compositional and temperature regimes we see in our own solar system and expect to find in extrasolar planetary systems.
