Periodic Reporting for period 1 - ICED (Initial conditions of exoplanet formation in protoplanetary disks)
Reporting period: 2017-10-01 to 2019-09-30
Astronomers are now beginning the process of measuring the compositions of extrasolar planets. The precise combination of elements in the atmospheres of these planets should reflect their formation location and time. To this end, the distribution of elements as a function of radius in protoplanetary disks (where planets form) can provide a map with which to compare extrasolar planet compositions. Ultimately this knowledge will help us to understand how certain elements that are critical to life, e.g. carbon, hydrogen, oxygen, and nitrogen, came to be incorporated into Earth and which types of extrasolar planets should have the necessary bulk elemental ingredients to support life. This issue is important for our society to understand our place in the universe and to motivate future exploration and potentially resource management of the bodies in our own solar system.
The first overall objective of the program was to measure the gas phase composition in a range of protoplanetary disks at the radial distance from the central star analogous to the location of most observed extrasolar hot gas giant planet, to determine what these planets' atmospheric composition would be if they formed in situ. A second goal was to use the deficit of certain elements in the gas in this region of protoplanetary disks to infer the solid rocky and icy composition of forming planetesimals in these disks. We met these objectives successfully. We have concluded that the gas in the inner regions of some protoplanetary disks is depleted in ice-forming elements, suggesting that hot gas giants that form in situ should have relatively little of these elements in their atmospheres (McClure 2019; McClure & Dominik, submitted & under revision). We also found that rock-bearing elements like iron, silicon, and calcium were depleted in one disk by ~100 times more than the ice-forming elements, suggesting that asteroid-like planetesimal formation was underway in these disks. We also determined that the first steps of forming the planetesimal cores must happen already by ~100,000 years after the protostar forms (McClure, Dominik, & Kama, drafting).
The first overall objective of the program was to measure the gas phase composition in a range of protoplanetary disks at the radial distance from the central star analogous to the location of most observed extrasolar hot gas giant planet, to determine what these planets' atmospheric composition would be if they formed in situ. A second goal was to use the deficit of certain elements in the gas in this region of protoplanetary disks to infer the solid rocky and icy composition of forming planetesimals in these disks. We met these objectives successfully. We have concluded that the gas in the inner regions of some protoplanetary disks is depleted in ice-forming elements, suggesting that hot gas giants that form in situ should have relatively little of these elements in their atmospheres (McClure 2019; McClure & Dominik, submitted & under revision). We also found that rock-bearing elements like iron, silicon, and calcium were depleted in one disk by ~100 times more than the ice-forming elements, suggesting that asteroid-like planetesimal formation was underway in these disks. We also determined that the first steps of forming the planetesimal cores must happen already by ~100,000 years after the protostar forms (McClure, Dominik, & Kama, drafting).
I began the project by analyzing near-infrared spectra for a sample of 26 young, low-mass stars surrounded by protoplanetary disks. I identified and measured line fluxes for emission lines produced by elements of interest to ice and rock-formation during planet formation, as discussed above: carbon, hydrogen, oxygen, nitrogen, magnesium, sodium, silicon, aluminum, calcium, and iron. I used the publicly available photoionization and chemistry modeling code, Cloudy, and plasma code CHIANTI to make models of the 5 disks with the best quality spectra to measure the carbon abundance in their inner disks. This resulted in one paper, McClure (2019). Next we analyzed the near-infrared spectrum of the most nearby disk, TW Hya, for which multiple epochs of spectroscopy are archivally available. By subtracting the spectrum from a low-accretion epoch from that of a higher accretion epoch, we were able to separate out the emission contribution from the gas being accreted from the inner most disk. By constructing a Cloudy model to include all of the emission lines we discovered in the residual spectrum, we were able to obtain abundances for all of the elements discussed above. The results showed that all of the elements were depleted in abundance relative to hydrogen, suggesting that extrasolar gas giant planets formed in this region would have depleted atmospheres (McClure & Dominik, 2020, under review). The depletion of these elements suggests that they have been left behind in solids in the disk. We also determined at what time these solids began to be retained in this disk, i.e. at what time planetesimal formation started (McClure, Dominik, & Kama, in prep.), finding that it must happen at <100,000 years. We disseminated these results at 1) a number of conferences for astronomers, as well as separate conferences for the fields of planetary science and chemistry, 2) events for undergraduate and masters students, 3) a public outreach evening at the university.
This is the first time that absolute inner disk abundances have been directly measured in young, solar precursor stars. It is also the first time that we have constrained the starting timescale for planet formation in a protoplanetary disk. These results impact our understanding of the timeframe for planet formation, which, combined with the formation location in the disk, will determine what the initial composition of the planet is. Eventually this could improve our understanding of how planets come to have the bulk ingredients for life available to them, which is of strong public interest.