Upper limits on the water vapour content of the β Pictoris debris disk^★

¹ Department of Astronomy, AlbaNova University Center, Stockholm University, 106 91 Stockholm, Sweden
e-mail: maria.cavallius@astro.su.se; alexis@astro.su.se; olofsson@astro.su.se
² Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Hungarian Academy of Sciences, Konkoly-Thege Miklós út 15–17, 1121 Budapest, Hungary
³ Subaru Telescope, National Astronomical Observatory of Japan, 650 North A’ohoku Place, Hilo, HI 96720, USA
⁴ Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada
⁵ Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, 439 92 Onsala, Sweden

Received: 10 April 2019
Accepted: 23 June 2019

Abstract

Context. The debris disk surrounding β Pictoris has been observed with ALMA to contain a belt of CO gas with a distinct peak at ~85 au. This CO clump is thought to be the result of a region of enhanced density of solids that collide and release CO through vaporisation. The parent bodies are thought to be comparable to solar system comets, in which CO is trapped inside a water ice matrix.

Aims. Since H₂O should be released along with CO, we aim to put an upper limit on the H₂O gas mass in the disk of β Pictoris.

Methods. We used archival data from the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory to study the ortho-H₂O 1₁₀–1₀₁ emission line. The line is undetected. Using a python implementation of the radiative transfer code RADEX, we converted upper limits on the line flux to H₂O gas masses. The resulting lower limits on the CO/H₂O mass ratio are compared to the composition of solar system comets.

Results. Depending on the assumed gas spatial distribution, we find a 95% upper limit on the ortho-H₂O line flux of 7.5 × 10⁻²⁰ W m⁻² or 1.2 × 10⁻¹⁹ W m⁻². These translate into an upper limit on the H₂O mass of 7.4 × 10¹⁶–1.1 × 10¹⁸ kg depending on both the electron density and gas kinetic temperature. The range of derived gas-phase CO/H₂O ratios is marginally consistent with low-ratio solar system comets.