Potential effects of stellar winds on gas dynamics in debris disks leading to observable belt winds

Abstract

Context. Gas has been successfully detected in many extrasolar systems around mature stars aged between 10 Myr and ∼1 Gyr that include planetesimal belts. Gas in these mature disks is thought to be released from planetesimals and has been modeled using a viscous disk approach where the gas expands inwards and outwards from the belt where it is produced. Therefore, the gas has so far been assumed to make up the circumstellar disk orbiting the star; however, at low densities, this may not be an adequate assumption, as the gas could be blown out by the stellar wind instead. Aims. In this paper, we aim to explore the timeframe in which a gas disk transitions to such a gas wind and whether this information can be used to determine the stellar wind properties around main sequence stars, which are otherwise difficult to obtain. Methods. We developed an analytical model for A to M stars that can follow the evolution of gas outflows and target the moment of transition between a disk or a wind in order to make a comparison with current observations. The crucial criterion here is the gas density for which gas particles are no longer protected from the impact of stellar wind protons at high velocities and on radial trajectories. Results. We find that: (1) belts with a radial width, ΔR, with gas densities <7 (ΔR/50 au)−1 cm−3, would create a wind rather than a disk, which would explain the recent outflowing gas detection in NO Lup; (2) the properties of this belt wind can be used to measure stellar wind properties such as their densities and velocities; (3) very early-type stars can also form gas winds due to the star’s radiation pressure, instead of a stellar wind; (4) debris disks with low fractional luminosities, f, are more likely to create gas winds, which could be observed with current facilities. Conclusions. Systems containing low gas masses, such as Fomalhaut or TWA 7, or more generally, debris disks with fractional luminosities of f ≲ 10−5(L⋆/L⊙)−0.37 or stellar luminosity ≳20 L⊙ (A0V or earlier) are more likely to create gas outflows (or belt winds) than gas disks. Gas that is observed to be outflowing at high velocity in the young system NO Lup could be an example of such belt winds. Future observing predictions in this wind region should account for the stellar wind in the attempt to detect the gas. The detection of these gas winds is possible with ALMA (CO and CO+ could serve as good wind tracers). This would allow us to constrain the stellar wind properties of main-sequence stars, as these properties are otherwise difficult to measure, since, for example, there are no successful measures around A stars at present.