Browsing by Subject "star formation"

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AGB stars in the early universe I: Towards a portable chemical and thermal evolution model
(2012) Liao, Andy Sha; Milosavljevic, Milos
Stars in the asymptotic giant branch (AGB) are in the penultimate stage of their lives as low- mass dwarfs. During this stage, they produce in their cool, bloated envelopes vast quantities of dust composed from metals forged deep in their interiors from nuclear reactions. They expel these dust grains in shells blown out from the star by strong winds powered by radiation pressure. In order to better understand the role of asymptotic giants as sources of metal and dust enrichment in the early, metal-poor universe, we create a portable chemical and thermal network considering physics on all the major primordial species including H; H+; H−; H2; H+2 ; He; He+; He++; D; D+; HD; e− as well as the most important metal coolants C, Si, O and their singly ionized state C+, Si+, O+. We involve in our code all of the important heating and cooling processes including atomic lines, molecular lines, continuum radiation and absorption, metal fine structure lines, and collisional heating and cooling including that between dust grains and gas particles. We test our reaction network by numerically solving chemical and thermal rate equations in a one-zone freefall model. Our results show that the presence of dust and metals in a cloud creates at lower densities a significant excess of molecular hydrogen, an important coolant, compared with dust and metal-free clouds.
Investigating the Role of Magnetic Fields in Binary Star Formation
(2019-05) Kang, Justin; Offner, Stella
The initial conditions in prestellar cores that lead to single versus binary star systems are debated. The role of magnetic fields, in particular, is uncertain. Observations from the Atacama Large Millimeter/submillimeter Array telescope (ALMA), which can probe the polarization of thermal emission from dust within star-forming dense cores, suggests that the environments single star systems form in are most similar to those of weakly-magnetized simulations. In this weak-field case, the magnetic field is weak relative to the turbulent pressure and gas pressure; turbulence shapes the material that forms the protostar. However, no such comparison has been conducted for the environments of binary star systems. We expect these systems to be different as the binary companion will interact with the system through mechanisms such as radiation, accretion, and dynamics. In order to compare the outcomes of these two systems, we analyze a simulation of a turbulent, star-forming molecular cloud with moderate-strength magnetic fields. To remain comparable to observations, we model the propagation of the simulation's emission. We also introduce various physics, such as scattering, absorption, and emission processes of the dust and gas. By accounting for effects such as noise and instrumental resolution, we generate "synthetic observations" of the dust polarization that are comparable to ALMA's real observations. We then compare the polarization properties between cores forming single and multiple stars to predict what real observable differences may exist. Comparing the synthetic observations with visualizations of the simulation, we gain physical insight into the role of magnetic fields in star-forming regions.