Exposing the Gas Braking Mechanism of the beta Pictoris Disk

TL;DR

This study investigates the gas dynamics in the beta Pictoris disk, confirming that different neutral elements reach distinct velocities before ionization, and suggests additional braking mechanisms involving ionized carbon to explain observations.

Contribution

The paper provides observational evidence supporting the gas braking model and refines the understanding of ion-neutral interactions in circumstellar disks.

Findings

Fe and Na absorption profiles are shifted, confirming different ionization velocities.

Na profile lacks an extended blue wing, indicating additional braking.

More C II is needed to match observed line profiles with the model.

Abstract

Ever since the discovery of the edge-on circumstellar disk around beta Pictoris, a standing question has been why the gas observed against the star in absorption is not rapidly expelled by the strong radiation pressure from the star. A solution to the puzzle has been suggested to be that the neutral elements that experience the radiation force also are rapidly ionized, and so are only able to accelerate to an average limiting velocity v_ion. Once ionized, the elements are rapidly braked by C II, which is observed to be at least 20x overabundant in the disk with respect to other species. A prediction from this scenario is that different neutral elements should reach different v_ion, depending on the ionization thresholds and strengths of driving line transitions. In particular, neutral Fe and Na are predicted to reach the radial velocities 0.5 and 3.3 km/s, respectively, before being ionized. In this paper we study the absorption profiles of Fe and Na from the circumstellar gas disk around beta Pic, as obtained by HARPS at the ESO 3.6m telescope. We find that the Fe and Na velocity profiles are indeed shifted with respect to each other, confirming the model. The absence of an extended blue wing in the profile of Na, however, indicates that there must be some additional braking on the neutrals. We explore the possibility that the ion gas (dominated by C II) can brake the neutrals, and conclude that about 2-5x more C than previously estimated is needed for the predicted line profile to be consistent with the observed one.