Participant Support: Simulating Dense Gas in Active Galactic Nuclei, Tidal Disruption Events, and Supernovae

Overview: Astronomy is an observational, not experimental, science. We must harvest the information present in the light we receive to realize the promise of observatories like ALMA or Gemini. Most light originates in material that is far from thermodynamic equilibrium. Examples include the emission-line regions of Active Galactic Nuclei (AGN, a major emphasis of this project), debris from Tidal Disruption Events, (TDEs), and supernova ejecta. The observed spectrum depends on a wide range of coupled thermal, plasma, chemical, and condensed matter processes. The spectral simulation code Cloudy, the focus of this project, solves all of these problems simultaneously and self-consistently. It is one of more widely used theory codes in astrophysics with more than 300 papers citing its documentation per year. Intellectual Merit: Although the gas emitting radio recombination lines in H II regions, the optical / UV lines in AGN or TDEs, and the IR spectra of kilonova ejecta seem very dissimilar, the underlying microphysics is the same. Energy is deposited, perhaps following photoionization, cosmic ray penetration, or mechanical energy deposition, and is converted into the observed lines and continua. Cloudy has long solved this problem using a mixed approach. One and two electron systems such as H I, He I, and He II use a full collisional-radiative model that goes to the low-density limit and LTE over temperatures 2.7 K < T < 1e10 K. Three or more electron species are treated with a hybrid approach which is correct at low to intermediate densities but which is approximate at high densities. Following participation in denseplasma workshops, it is now possible to both quantify the accuracy of the existing methods and develop techniques to make this exact. That is the goal of this project. The improved simulations will impact models of any dense system, but especially AGN, TDEs, and SNe. This project will be felt well beyond the original research performed by this project because of the broad community use of the advanced code. Broader Impacts: The Cloudy code is an implementation of the physics described in the Osterbrock & Ferland graduate text Astrophysics of Gaseous Nebulae. The text and code are widely used in graduate research and education. As with any large code, the learning curve can be steep. One to two week long Cloudy workshops are organized in which participants study the Osterbrock & Ferland text, do hands-on problems using Cloudy, and break up into smaller teams to do joint research on topics of their choosing. Nine workshops with a total of ~300 participants took place over the course of the previous NSF award, and a similar number will be held in the next three years. The University's MacAdam Student Observatory (MSO) has a long-standing public outreach program servicing central Kentucky and Appalachia. The region also has a significant Hispanic or Latino community with some local schools having as high as a ~70% population. This community is underrepresented in STEM fields. The project Co-PI, Dr. Francisco Guzman Fulgencio, is a native Spanish speaker and is the UK representative in the Hispanic outreach program developed with in-service teachers. Local schools are visited with telescopes, visual aids, and a Spanish-language discussion. An annual MSO Spanish language "Sky-talk" and visit to the observatory involves the students and their parents and families in this STEM outreach.