Cometary emission processes: fingerprints of their physical and chemical behavior

I will discuss key atomic and molecular processes in cometary atmospheres.  Like comets in our solar system, it will be difficult if not impossible to directly study the physical and chemical properties of comets around other stars. Instead, we have to infer these properties from the gas and dust surrounding them. Atomic and molecular reaction such as dissociation, ionization, and charge exchange both alter gases surrounding comets. Because many reactions result in the emission of light, they also offer insight into the composition and radiation environment exocomets are exposed to. In this presentation I will provide a broad review of radiative processes in cometary atmospheres, with a particular focus on spectral modeling, observational opportunities, and anticipated challenges in the interpretation of new observations, based on our current understanding of the atomic and molecular processes occurring in the atmospheres of small, icy bodies. Close to the surface, comet atmospheres form a thermalized atmosphere tracing the irregular shape of the nucleus. Gravity is too low to retain the gas, which flows out to form a large collisionless exosphere. As such, cometary comae represent conditions that are both familiar, as well as unattainable in laboratories on Earth, necessitating state-of-the-art theoretical treatments of the relevant microphysical processes. Radiative processes offer direct diagnostics of the local conditions, as well as the macroscopic coma properties. Finally, measurements of cometary compositions are uniquely valuable because they provide information on the formation and evolution of our solar system, but extracting chemical abundances from spectroscopic measurements of the coma requires detailed models that span a broad range of physical regimes (both macroscopic and microscopic).

Cometary emission processes: fingerprints of their physical and chemical behavior

I will discuss key atomic and molecular processes in cometary atmospheres.  Like comets in our solar system, it will be difficult if not impossible to directly study the physical and chemical properties of comets around other stars. Instead, we have to infer these properties from the gas and dust surrounding them. Atomic and molecular reaction such as dissociation, ionization, and charge exchange both alter gases surrounding comets. Because many reactions result in the emission of light, they also offer insight into the composition and radiation environment exocomets are exposed to. In this presentation I will provide a broad review of radiative processes in cometary atmospheres, with a particular focus on spectral modeling, observational opportunities, and anticipated challenges in the interpretation of new observations, based on our current understanding of the atomic and molecular processes occurring in the atmospheres of small, icy bodies. Close to the surface, comet atmospheres form a thermalized atmosphere tracing the irregular shape of the nucleus. Gravity is too low to retain the gas, which flows out to form a large collisionless exosphere. As such, cometary comae represent conditions that are both familiar, as well as unattainable in laboratories on Earth, necessitating state-of-the-art theoretical treatments of the relevant microphysical processes. Radiative processes offer direct diagnostics of the local conditions, as well as the macroscopic coma properties. Finally, measurements of cometary compositions are uniquely valuable because they provide information on the formation and evolution of our solar system, but extracting chemical abundances from spectroscopic measurements of the coma requires detailed models that span a broad range of physical regimes (both macroscopic and microscopic).

Black hole accretion flows: from nearby stellar binaries to quasars at cosmic dawn

Accretion onto black holes transforms the darkest objects in the universe into the brightest. I will review what we know about the emission from the accretion flow, starting with the stellar mass black holes in binary systems in our own galaxy. Scaling up to the supermassive black holes in active galaxies and quasars reveals both similarities and differences. One of the key similarities is the 'changing look' phenomena in active galaxies, where the UV continuum associated with an optically thick accretion flow drops over a few months/years, triggering the disappearance of the characteristic broad emission lines, analogous to the state transition in binaries. One of the key differences is the nature of the accretion flow above the transition luminosity, where the stellar mass black holes look like standard discs and have variability timescales like standard discs while the supermassive ones do not. Only at the highest luminosities (extreme narrow line Seyfert 1 galaxies and the weak line quasars) does the accretion flow match to the disc models. I will speculate on the physics underlying all the behavior, and give a united picture of the accretion flow.

Black hole accretion flows: from nearby stellar binaries to quasars at cosmic dawn

Accretion onto black holes transforms the darkest objects in the universe into the brightest. I will review what we know about the emission from the accretion flow, starting with the stellar mass black holes in binary systems in our own galaxy. Scaling up to the supermassive black holes in active galaxies and quasars reveals both similarities and differences. One of the key similarities is the 'changing look' phenomena in active galaxies, where the UV continuum associated with an optically thick accretion flow drops over a few months/years, triggering the disappearance of the characteristic broad emission lines, analogous to the state transition in binaries. One of the key differences is the nature of the accretion flow above the transition luminosity, where the stellar mass black holes look like standard discs and have variability timescales like standard discs while the supermassive ones do not. Only at the highest luminosities (extreme narrow line Seyfert 1 galaxies and the weak line quasars) does the accretion flow match to the disc models. I will speculate on the physics underlying all the behavior, and give a united picture of the accretion flow.

The gas-phase metallicity and ionization parameter are two primary parameters that drive the variations of emission line spectra in HII regions. There is an increasing amount of evidence that these two parameters are correlated. Theoretical works on the dynamical evolution of HII regions predict an anti-correlation between metallicity and ionization parameter. However, observations of HII region spectra have yielded a variety of different correlations, including anti-correlations, positive correlations, and even no correlation. All these measurements of the correlation rely on photoionization modeling. Also, different combinations of emission lines are used for calibrating metallicity and ionization parameter in different works. To solve the above problem, we examine how the derived correlation between metallicity and ionization parameter depends on the choice of models and emission lines. In this talk, I will show that it is important to constrain the model parameters using observational data before making any measurement. With the MaNGA IFU data, we compute a best-fit photoionization model for general HII regions and use a Bayesian method to fit metallicities and ionization parameters simultaneously. Our result shows that a positive correlation exists between the gas-phase metallicity and ionization parameter, regardless of which combination of emission lines is used for the fitting. This result clearly contradicts the prediction by the dynamical model. I will discuss potential mechanisms that lead to this discrepancy, and how it might be solved with future IFU surveys on HII regions.

The fifth incarnation of the Sloan Digital Sky Survey (SDSS-V) began taking data last year and is in the process of transitioning to the use of a robotic positioning system. I will described the SDSS-V Milky Way Mapper program and its goals. The first of its major goals is to understand the history and structure of the Milky Way. Following upon work done with the APOGEE-1 and 2 surveys the Milky Way Mapper will use approximately 6 million stars to trace out the detailed structure of the Galaxy. The second major goal is to understand stellar astrophysics. The Milky Way Mapper contains several smaller programs called cartons, whose goals cover a wide variety of stellar candidates including white dwarfs, binary stars, young stellar objects, planet hosts, asteroseismology targets and x-ray binaries. These cartons will allow us to explore all sorts of interesting topics that can only be done with a large-scale spectroscopic survey. I will give updates on the current progress of the survey, and lay out our plans for the future.

The deaths of massive stars seed our universe with black holes and neutron stars - the most exotic objects of the stellar graveyard. The births of these stellar remnants, as well as their mergers when paired in binaries, power explosions that can launch the most relativistic jets we know of in the universe (gamma-ray bursts) and shake the very fabric of space-time via ripples called gravitational waves. GW170817, the merger of two neutron stars witnessed through both its gravitational wave siren and its glow at all wavelengths of light, represents the first multi-messenger detection of one such extreme cosmic bang. Starting from the example of GW170817, in this talk I will discuss how radio light in particular, and gravitational waves, can be used in tandem to unveil the physics of relativistic transients. I will also highlight opportunities and challenges that lie in front of us, as improvements in detectors’ sensitivities will transform a trickle of multi-messenger discoveries into a flood.

The observations of high redshift quasars up to z~7 tell us that massive black holes (MBHs) were already in place, with masses well above 10^9 solar masses, when the Universe was less than 1 Gyr old. According to Soltan’s argument MBHs gain most of their mass via radiatively efficient accretion, hence we expect they formed early in the Universe as smaller seeds. To date, the common formation mechanism advocated to explain the most massive MBHs at high redshift is the direct collapse scenario, which leads to the formation of seed MBHs of about 10^4-5 Msun. However, because of the peculiar conditions required by this formation mechanism, its plausibility is still debated. After highlighting the main conditions required by this scenario, I will discuss whether the peculiar environment in which high-redshift massive galaxies evolve provides ideal conditions for the formation of such massive seeds, and the processes that may potentially inhibit the process. I will also discuss the subsequent evolution of these protogalaxies and their central MBHs up to the observed masses, a result that strongly depends on the interaction with its galaxy host, and how the MBH obesity found by observations is not necessarily real.

Modelling the molecular gas that is detected through CO observations of high-z galaxies constitutes a major challenge for ab initio simulations of galaxy formation. I will present recent numerical work aimed at studying the formation and evolution of the simplest and most abundant molecule, H2. Our model fully solves the out-of-equilibrium rate equations and accounts for the unresolved structure of molecular clouds. We apply our model to two types of cosmological simulations: a) the formation of a Milky Way-sized galaxy at z=2 and b) a small cosmological box in order to obtain some statistical results. The results are compared to those obtained from two different approximations commonly used in the literature and for numerical convergence. Our results indicate that independently of the model, robust results (H2 masses) can only be obtained for galaxies that are suffiiciently metal enriched in which H2 formation is fast. However, their morphology differ from model to model. Furthermore, the cosmological H2 mass function derived from the non-equilibrium model agrees well with recents observations that only sample the high-mass end. Extensions of our model towards including other molecules, such as CO, and species, in particular C and its derivatives, will also be discussed.