摘要：Context.The debris disk surroundingβPictoris 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 ofβPictoris.Methods.We used archival data from the Heterodyne Instrument for the Far-Infrared (HIFI) aboard theHerschelSpace Observatory to study the ortho-H2O 110–101emission line. The line is undetected. Using a python implementation of the radiative transfer codeRADEX, 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 × 10−20W m−2or 1.2 × 10−19W m−2. These translate into an upper limit on the H2O mass of 7.4 × 1016–1.1 × 1018kg 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.
