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Molecular Clouds and Chemistry
Molecular Clouds and Chemistry
"Darkness cannot drive out darkness; only light can do that." - Martin Luther King, Jr.
From numerical simulation to mock observation, Gong, Ostriker and Kim (2018)
Stars like our own Sun are born by the gravitational collapse of molecular clouds. These dark and cold clouds of gas are also the sites of a wide variety of chemical reactions, which change the thermodynamics, and form observable species. To study chemistry in molecular clouds, we developed a new chemical network that is simple and accurate for numerical simulations (with an accompanying public PDR code ). Recently, we implemented chemistry in the MHD code Athena++ and made it public. We used chemistry to accurately model thermodynamics in the galaxy simulation TIGRESS-NCR, and to understand the observational parameters such as the X_CO conversion factor around our solar neighborhood and across different galactic environments.
Core Mass Function (CMF) and the Initial Mass Function (IMF)
Core Mass Function (CMF) and the Initial Mass Function (IMF)
“If you love a flower that lives on a star, it is sweet to look at the sky at night. All the stars are a-bloom with flowers.” - Antoine de Saint-Exupéry, The Little Prince
Simulated prestellar cores, Gong & Ostriker (2016)
What determines the stellar IMF? Observations suggest that the prestellar CMF has a similar shape to the IMF: is CMF the answer? Our simulations suggest not, or at least not entirely. We found that a large fraction of low mass prestellar cores are unbound transient structures, and will never collapse to form stars. Moreover, the masses of stars are not limited by their core masses, because they are embedded in dense filaments, and the critical Larson-Penston profile at the time of core collapse extends beyond the boundary of tidally bound cores. Without feedback, stars gain one core mass per freefall time, even after the initial core mass is accreted. Some feedback mechanism must be involved to determine the final IMF. We also found that the peak of the CMF is roughly the Bonnor-Ebert mass, and is numerically converged in our isothermal simulations with increasing resolution.
Dust in Protoplanetary Disks
Dust in Protoplanetary Disks
“Yes, we too are stardust.” ― Jostein Gaarder, Sophie's World
Grain growth in protoplanetary disks is the first step towards planet formation. One of the most important pieces in the grain growth model is calculating the collisional velocity between two grains in turbulent gas. The collisional velocities in previous works are obtained based on the assumption that the turbulence is hydrodynamic with the Kolmogorov power spectrum. However, realistic protoplanetary disks are magnetized, and turbulent motions can be induced by the magneto-rotational instabilities (MRI). Using magneto-hydrodynamic (MHD) simulations of MRI, we observe a persistent kinetic energy spectrum of 4/3, shallower than the Kolmogorov spectrum of 5/3. We investigate the impact of turbulence properties on grain collisional velocities, and find that for the modeled cases of the Iroshnikov-Kraichnan turbulence and the turbulence induced by the magneto-rotational instabilities, collisional velocities of small grains are much larger than those for the standard Kolmogorov turbulence. This leads to faster grain coagulation in the outer regions of protoplanetary disks, resulting in rapid increase of dust opacity in mm-wavelength and possibly promoting planet formation in very young disks.
Turbulent magnetic field structure in the midplane, of protoplanetary disks, Gong et al (2020)
Chondrule formation and the Early Solar System
Chondrule formation and the Early Solar System
“Who is it that can tell me who I am?” - William Shakespeare, King Lear
Schematic picture of chondrule formation and disk eovlution, Gong et al. (2019)
Chondrules are these little glassy pebbles found in meteoroids. There is a mystery about chondrules: they are heated to white-hot temperatures, temperatures high enough to melt glass, in the early solar nebula that is considered ice-cold in typical conditions. We propose a new model that chondrules are formed in strong shocks created by highly eccentric planetesimals, which gain their eccentricity by undergoing the sweeping secular resonance with Jupiter. Our model implies that the disk depletion timescale is ~ 1 Myr, comparable to the age spread of chondrules; and that Jupiter formed before chondrules, not more than 0.7 Myr after CAIs.
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