First Detection of Hydrogen in the β Pictoris Gas Disk

Abstract

The young and nearby star β Pictoris (β 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 α (Ly-α\) line. We present a new technique to decrease the contamination of the Ly-α line by geocoronal airglow in COS spectra. This Airglow Virtual Motion (AVM) technique allows us to shift the Ly-α 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 β 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-α 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 $41\pm 6$ km/s relative to β Pic and a column density of $\log (N_{\mathrm{H}}/1{\mathrm{cm}}^2)=18.6\pm 0.1$. 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.

arXiv comment11 pages including 10 figures

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Cited by in corpus (18)

• Debris Disks: Structure, Composition, and Variability
• Predictions for the secondary CO, C and O gas content of debris discs from the destruction of volatile-rich planetesimals
• Peter Pan Disks: Long-lived Accretion Disks Around Young M Stars
• ALMA resolves CI emission from the $\beta$ Pictoris debris disk
• The Hubble PanCET program: An extensive search for metallic ions in the exosphere of GJ 436 b
• Exocomets from a Solar System Perspective
• Survey of planetesimal belts with ALMA: gas detected around the Sun-like star HD 129590
• Primordial or Secondary? Testing models of debris disk gas with ALMA
• A Characterization of the Circumstellar Gas around WD 1124-293 using Cloudy
• The $\delta$ Scuti pulsations of $\beta$ Pictoris as observed by ASTEP from Antarctica
• bRing: An observatory dedicated to monitoring the $\beta$ Pictoris b Hill sphere transit
• Detection of Nitrogen gas in the $\beta$ Pictoris circumstellar disk
• Observability of hydrogen-rich exospheres in Earth-like exoplanets
• HST PanCET program: Non-detection of atmospheric escape in the warm Saturn-sized planet WASP-29 b
• The Hubble PanCET program: Long-term chromospheric evolution and flaring activity of the M dwarf host GJ 3470
• Upper limits on the water vapour content of the $\beta$ Pictoris debris disk
• High CO/H2 ratios supports an exocometary origin for a CO-rich debris disk
• Transit distances and composition of low-velocity exocomets in the $\beta$ Pic system