List of Past CTC Theory Lunches : 01-Jan-2022 to 01-Jun-2022

Date:   Wednesday 26-Jan-2022
Speaker:   Arshia Jacob (JHU)
Title:  "Investigating the interstellar medium using hydrides - A tale of hydrides past, present and future"

One of the fundamental questions in modern astronomy concerns the life cycle of molecular material in the universe and addressing how molecular clouds are formed. Given, that atomic and molecular hydrogen gases form the major components of the neutral interstellar medium (ISM), cloud formation must involve the transition from regions dominated by HI gas to those dominated by H2 gas. Therefore, understanding the processes responsible for this phase transition are crucial. While this topic is highly complex, absorption spectroscopy of hydrides (molecules or molecular ions with a heavy atom covalently bonded to one or more hydrogen atoms) performed over the past decade have provided a wealth of new information about their use as sensitive tracers of the different phases of the ISM. Here, I’ll present a journey, starting from the earliest detections of hydrides in the ISM to their most recent observations carried out as a part of an ongoing SOFIA legacy program, HyGAL. I will highlight along the way the use of hydrides as powerful diagnostic tools for the different phases of ISM with particular emphasis on the methylidyne radical, CH.

Date:   Wednesday 09-Feb-2022
Speaker:   Yun Zhang (UMD)
Title:  "LPC analogs as the origin of circumstellar debris and pollution around white dwarfs"

A significant fraction of white dwarfs (WDs) exhibit signs of ongoing accretion of refractory elements at rates ∼10^3–10^7 kg/s, among which, about 40 WDs were detected to harbor dusty debris disks. It has been commonly suggested that this material could be supplied by solar system analogs of Main Belt asteroids or Kuiper Belt objects. These remnant planetary bodies are grounded to micrometer-sized dust by WDs’ tidal effect and by collisional cascades when they approach the WDs’ tidal radii. However, these bodies may have migrated to distances comparable to the Oort Cloud during stars’ post-main-sequence evolution, and substantial orbital migration is required to drive these bodies to the proximity of WDs. In this talk, I will show that WDs’ magnetic fields can rapidly circularize remnant bodies with orbits equivalent to the solar system’s long-period comets (LPCs). This process could account for the observed accretion rates of heavy elements and the generation of circus-WD debris disks.

Date:   Wednesday 02-Mar-2022
Speaker:   Catherine Zucker (STScI)
Title:  "Revealing the Star Formation History of our Solar Neighborhood: Supernova-Driven Star Formation on the Surface of the Local Bubble"

Given its proximity, the solar neighborhood has long been considered a fundamental laboratory for understanding how stars form. However, until very recently, this understanding was largely based on static “plane of the sky” views, making it extraordinarily challenging to build a 3D physical picture of dense gas and young stars using 2D integrated quantities. In this talk, I will discuss how new 3D spatial and dynamical constraints from Gaia have transformed our understanding of star formation near the Sun. I will show how essentially all nearby star-forming complexes lie on the surface of the Local Bubble, and that their young stars show outward expansion perpendicular to the bubble’s surface. Using these young stars’ motions to reconstruct the star formation history of our solar neighborhood, I will explain how a set of supernova explosions beginning 14 Myr ago powered the expansion of the Local Bubble. This expansion swept up the ambient interstellar medium into an extended shell that has now fragmented and collapsed into the most prominent nearby star-forming complexes, in turn providing strong observational evidence for supernova-driven star formation. I will conclude by discussing the relationship between the Local Bubble and other recently discovered structures (the Radcliffe Wave and the Perseus-Taurus Supershell), which together provide new context for understanding triggered star formation in the larger galactic ecosystem.

Date:   Wednesday 30-Mar-2022
Speaker:   Anjali Piette (Carnegie Institution of Washington)
Title:  "Atmospheric Properties of sub-Neptune Exoplanets"

Among the thousands of exoplanets discovered to date, planets between the sizes of Earth and Neptune (sub-Neptunes) are the most common. Spanning rocky, Earth-like planets to possible water worlds with hydrogen-rich atmospheres, this regime encompasses a diverse range of planetary conditions and processes. In particular, the atmospheres of such exoplanets provide an observational window into their properties and affect the surface conditions which may lie beneath them. In this talk, I will begin by exploring the atmospheric conditions in mini-Neptunes, including physically-motivated atmospheric conditions that allow for liquid water under the H2-rich atmospheres of planets such as K2-18b. Following this, I will discuss the characterisation of rocky exoplanet atmospheres using thermal emission spectroscopy. I will present HyDRo, a new retrieval framework tailored for thermal emission observations of rocky exoplanet atmospheres, and assess the observability of the atmospheres of three promising rocky exoplanets: Trappist-1b, GJ1132b, and LHS3844b.

Date:   Wednesday 06-Apr-2022
Speaker:   Ryan Tanner (NASA Goddard)
Title:  "Weird Suppressed Jets"

The typical image of an active galactic nuclei (AGN) jet is a grand, symmetric, large scale structure stretching hundreds or even thousands of kiloparsecs beyond its host galaxy. But the majority of AGN jets are not these grand, symmetric structures. Many are asymmetric, bent, split, or stunted by the ISM of the host galaxy. Using a series of 3D relativistic hydrodynamical simulations of AGN we investigate how AGN power, a clumpy ISM structure, and AGN jet angle with respect to the galactic disk affect the morphology and content of the resulting galactic outflow. For low power AGN across three orders of magnitude of AGN luminosities (10^41-10^43 erg/s) our simulations did not show significant changes to the morphology of the outflow. Jets perpendicular to the galactic disk created hot single-phase outflows, while jets close to parallel with the disk the outflows were multi-phase with equal parts warm and hot, and significant cold gas. Overall, the final morphology of low power AGN outflows depends primarily on how the jet impacts and interacts with large, dense clouds in the clumpy ISM. These clouds can disrupt, deflect, split, or suppress the jet preventing it from leaving the galactic disk as a coherent structure. But for simulations with AGN luminosities > 10^44 erg/s the ISM played a minor role in determining the morphology of the outflow with an undisrupted jet leaving the disk. The final morphology of AGN outflows is different for low power AGNs vs. high power AGNs with the final morphology of low power AGN outflows dependent on the ISM structure within the first kpc surrounding the AGN.

Date:   Wednesday 13-Apr-2022
Speaker:   Victor Guedes (U. Virginia)
Title:  "Universal relations for proto-neutron stars: the time evolution of I-Love-Q"

Neutron stars are powerful astrophysical laboratories for probing strong gravity and dense matter. The internal structure of these objects is strongly dependent on their (still unknown) equation of state. However, neutron stars have a very tight relation between their moment of inertia ($I$), spin-induced quadrupole moment ($Q$), and tidal deformability ($\Lambda$, related to the Love number). These (and other) relations do not depend on the equation of state and are called "universal"; in particular, the relations between $I$, $Q$ and $\Lambda$ are known as the I-Love-Q relations. These relations have several applications, e.g., in the context of gravitational wave data analysis. In this seminar, I am going to talk about some universal relations for proto-neutron stars in the post-bounce phase of core-collapse supernovae. Using results from three-dimensional core-collapse supernovae simulations, we extract the equation of state of the system and calculate the structure of spherically symmetric proto-neutron stars. We then calculate $I$, $Q$ and $\Lambda$ using a perturbative approach. We verify that the relations remain universal for proto-neutron stars in their late evolutionary stage (nearly 1 second after the bounce) and report how they connect to those for neutron stars. Besides that, we obtain nearly universal relations for early post-bounce times (around 0.1 s). The simplicity of these relations could help us make more robust predictions related to the highly complex physics of supernovae.

Date:   Wednesday 27-Apr-2022
Speaker:   Joost Wardenier (Oxford)
Title:   "TBD"

Date:   Wednesday 05-May-2022
Speaker:   Yuan Li (UNT)
Title:   "TBD"

Date:   Wednesday 11-May-2022
Speaker:   Matt Nixon (Cambridge)
Title:   "TBD"

Date:   Wednesday 06-Jul-2022
Speaker:   Kartick Sarkar (HUJI)
Title:  "Non-equilibrium ionization in galactic bubbles and winds"

Supernovae-driven bubbles and winds form a major part of the feedback mechanism that controls the flow of baryonic matter in galaxies. Observational estimates of the momentum deposition by supernovae, and mass and energy outflow in galactic winds depend on theoretical modeling of the emission/absorption properties of these systems. Such emission/absorption properties are highly dependent on the ionization state of the plasma which is often assumed to be in equilibrium. The assumption, however, becomes invalid for rapidly evolving plasma. In the talk, I will describe our recent efforts to model some of these systems by considering a self-consistent non-equilibrium ionization network and frequency-dependent radiative transport physics. I will show how the addition of the new physics changes our basic understanding of these systems and what it implies for the observations.

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