First Detection of Hydrogen in the \beta\ Pictoris Gas Disk
  The young and nearby star \beta\ Pictoris (\beta\ Pic) is surrounded by a
debris disk composed of dust and gas known to host a myriad evaporating
exocomets, planetesimals and at least one planet. At an edge-on inclination, as
seen from Earth, this system is ideal for debris disk studies providing an
excellent opportunity to use absorption spectroscopy to study the planet
forming environment. Using the Cosmic Origins Spectrograph (COS) instrument on
the Hubble Space Telescope (HST) we observe the most abundant element in the
disk, hydrogen, through the HI Lyman \alpha\ (Ly-\alpha\) line. We present a
new technique to decrease the contamination of the Ly-\alpha\ line by
geocoronal airglow in COS spectra. This Airglow Virtual Motion (AVM) technique
allows us to shift the Ly-\alpha\ line of the astrophysical target away from
the contaminating airglow emission revealing more of the astrophysical line
profile. The column density of hydrogen in the \beta\ Pic stable gas disk at
the stellar radial velocity is measured to be $\log(N_{\mathrm{H}}/1
\mathrm{cm}^2) \ll 18.5$. The Ly-\alpha\ emission line profile is found to be
asymmetric and we propose that this is caused by HI falling in towards the star
with a bulk radial velocity of $41\pm6$ km/s relative to \beta\ Pic and a
column density of $\log(N_{\mathrm{H}}/1 \mathrm{cm}^2) = 18.6\pm0.1$. The high
column density of hydrogen relative to the hydrogen content of CI chondrite
meteorites indicates that the bulk of the hydrogen gas does not come from the
dust in the disk. This column density reveals a hydrogen abundance much lower
than solar, which excludes the possibility that the detected hydrogen could be
a remnant of the protoplanetary disk or gas expelled by the star. We
hypothesise that the hydrogen gas observed falling towards the star arises from
the dissociation of water originating from evaporating exocomets.
