Emma Yu bio photo

Emma Yu

Graduate Student in Astronomy

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Astronomers have recently discovered a large number of planets orbiting around other stars, suggesting that most stars have an earth-like planet. However, we still donā€™t know exactly how these planets are formed. What determines whether and what kind of planets can be formed around a star? As an astronomer, my goal is to understand the formation of planetary systems by studying the structure and evolution of protoplanetary disks ā€” accretion disks around young stars in which planets are formed.

Figure 1. A protoplanetary disk (HL Tauri) observed by Atacama Large Millimeter Array (ALMA).

Measuring the disk mass in the inner planet-forming regions

Unfortunately, protoplanetary disks are hard to observe and fundamental properties such as disk mass and temperature are very poorly constrained observationally. The main disk constituent (H2) of a protoplanetary disk is too cold to emit light in most of the disk. Commonly observed rotational emission lines from CO molecules with the most common isotopes (12C16O) are often optically thick (opaque) due to their high abundances, and therefore do not allow us to see the midplane of the disk where planet-formation happens. With numerical simulations of the structure and chemical compositions of planet-forming disks, we (Yu et al. 2016, Dodson-Robinson et al. 2014) have shown that the emission from C17O, a version of carbon monoxide with a rare oxygen isotope, is optically thin in giant planet forming regions. This result suggests that one could probe the planet-forming disk midplane directly with C17O and measure the disk mass available for planet formation with radio telescopes such as Atacama Large Millimeter Array (ALMA).

Signatures of CO depletion and the formation of complex organic molecules

Figure 2. Fractional abundances at the end of the 3 Myr disk evolution for CO and complex organic molecules (COM). The color scale is in logarithm. Red areas show where the species is abundant and the blue regions show where the species is depleted.

As the second most common molecule in molecular clouds, CO has long been used as a tracer of the molecular gas. It is thought to have simple chemistry and is used to measure the total disk mass by assuming a constant CO/H2 abundance ratio of 10āˆ’4. Unfortunately, chemical models (Yu et al. 2016) of T-Tauri disks show that CO abundance is not a constant, instead decreasing both with distance from the star and as a function of time. Our simulations have shown that at a age of 3 Myr, only 13.6% of carbons is in the form of CO in a disk similar to the one our Solar system is formed from, and as much as 40% of carbon is in ices of organic molecules.

Figure 3. Abundances of major carbon-bearing molecules as functions of time. We show the results for 40 AU on the disk midplane as an example. We can see the abundance of CO decreases over time, and the abundances of complex organic molecules increase with time, in contrast to previous work that assumes constant CO abundance.
Figure 4. Reaction network found by the model (out of more than 13,000 reactions) to be primarily responsible for CO depletion. The species with boxes drawn around them are sinks in the model, and letter G denotes species that are frozen out on grain surfaces.