Probing the composition of a young debris disk

The young and nearby star β Pictoris is surrounded by a debris disk composed of dust and gas known to host a myriad of evaporating exocomets, planetesimals and at least one planet. In Cycle 23 of the Hubble Space Telescope (HST) proposals I was awarded time (as PI) for writing a top ranked proposal aimed at determining the abundance of key species (H, C, N and O) in the debris disk.

Two papers have been published so far: First detection of hydrogen in the β Pictoris gas disk and the Detection of Nitrogen gas in the β Pictoris circumstellar disk. (the latter of which I have summarised in a blog post).

First detection of hydrogen in the β Pictoris gas disk:

Through the development of a new technique (Airglow Virtual Motion), which I mathematically derive in the Appendix of Wilson et al. 2017, I was able to sufficiently suppress the contaminating geocoronal airglow emission. This revealed, for the first time, the shape of the β Pic Ly-α line and lead to the first unequivocal detection of the most abundant element in the the debris disk: hydrogen.

My subsequent analysis of the column density of hydrogen lead to a series of important conclusions: The high column density of hydrogen relative to the hydrogen content of CI chondrite meteorites indicated that the bulk of the detected hydrogen gas does not originate from the dust in the disk. The column density of hydrogen was also found to be much lower than the solar value, which revealed that the hydrogen can not be a remnant of the protoplanetary disk or gas expelled by the star. I presented the new hypothesis that the hydrogen gas observed falling towards the star arises from the dissociation of water originating from evaporating exocomets.

Image credit: NASA/JPL-Caltech