Radiation-pressure-driven sub-Keplerian rotation of the disc around the AGB star L2 Pup

TL;DR

This study demonstrates that radiation pressure on dust can explain the sub-Keplerian rotation observed around the AGB star L2 Pup, constraining dust properties and dynamics in its circumstellar environment.

Contribution

It introduces a model where radiation pressure on dust drives the observed rotation profile, constrained by spectral energy distribution and dust dynamics, providing new insights into AGB star circumstellar material.

Findings

Radiation pressure can drive sub-Keplerian rotation in L2 Pup's disc.

Dust-to-gas ratio of ~10^{-3} fits both rotation and SED.

Dust grains larger than 0.5μm are expelled outward at ~5 km/s.

Abstract

We study the sub-Keplerian rotation and dust content of the circumstellar material around the asymptotic giant branch (AGB) star L$_2$ Puppis. We find that the thermal pressure gradient alone cannot explain the observed rotation profile. We find that there is a family of possible dust populations for which radiation pressure can drive the observed sub-Keplerian rotation. This set of solutions is further constrained by the spectral energy distribution (SED) of the system, and we find that a dust-to-gas mass ratio of $\sim10^{-3}$ and a maximum grain size that decreases radially outwards can satisfy both the rotation curve and SED. These dust populations are dynamically tightly coupled to the gas azimuthally. However grains larger than $\sim0.5\mu$m are driven outward radially by radiation pressure at velocities $\sim5$kms$^{-1}$, which implies a dust replenishment rate of $\sim3\times10^{-9}$M$_\odot$yr$^{-1}$. This replenishment rate is consistent with observational estimates to within uncertainties. Coupling between the radial motion of the dust and gas is weak and hence the gas does not share in this rapid outward motion. Overall we conclude that radiation pressure is a capable and necessary mechanism to explain the observed rotation profile of L$_2$ Pup, and offers other additional constraints on the dust properties.