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X-rays vs Protons: Dawn of Creation

posted Mar 23, 2018, 10:25 AM by Jeremy Drake   [ updated Mar 23, 2018, 10:56 AM ]
Planets are formed in the remnant "protoplanetary disk" of gas surrounding a newborn star. The process is one of hierarchical merging, of dust grains to form planetesimals, and planetesimals to form protoplanets that are then large enough to accrete material from the disk by their own gravity. Planets also interact with the gas in the disk, which can pull it closer or push it further away from its host star. Natal planetary characteristics and their resulting orbits then depend critically on the properties of the disk and how long it hangs around.

If it were not for energetic radiation from the host star, the gas disk would hang around for many millions of years. Instead, X-rays and fast protons heat the very outer layers and drive off mass in a disk wind.  They also weakly ionize the gas, whose charged particles interact through electric and magnetic forces and make the gas viscous. The viscous gas slows itself down in its orbit and gradually spirals into the central star - see the posting on an observation of this at work. Within 10 million years the gas disk is gone.

Stellar X-rays had long been thought the principle driver of these processes, until an estimate of energetic proton fluxes suggested instead that they were responsible. This estimate assumed protons travel in straight lines from their acceleration sites in the star's flares and coronal mass ejections. But charged particles follow curved magnetic field lines, and young stars have strong magnetic fields - several hundred times stronger that the Sun's.  We simulated the trajectories of energetic protons within the magnetosphere and wind environment of a young "T Tauri" star.  We found most of the protons get trapped by the strong stellar magnetic field.  Those that escape follow the magnetic field in particular trajectories, hitting the protoplanetary disk in specific places in a mottled pattern. They cannot dominate the global disk ionization in this way, but disk models will need to be extended to understand the effects of the strong, localized bombardment that can dominate the ionization in specific places where the magnetic field intercepts the disk. This work was lead by SAO visiting scientist Federico Fraschetti, and was published in the 2018 February 1 edition of the Astrophysical Journal.