On the Determination of the Rotational Oblateness of Achernar

TL;DR

This paper reinterprets interferometric data of Achernar, proposing a model with a critically rotating star and a small equatorial disk, advancing understanding of stellar oblateness and the Be phenomenon through multi-technique observations.

Contribution

It introduces a new model combining a critically rotating star with a small circumstellar disk, fitting multiple observational data types and challenging previous Roche approximation explanations.

Findings

Achernar's oblateness explained by a critically rotating star.

Identified two disk models fitting spectroscopic, polarimetric, and interferometric data.

Small disks likely represent the transition between photosphere and circumstellar environment.

Abstract

The recent interferometric study of Achernar, leading to the conclusion that its geometrical oblateness cannot be explained in the Roche approximation, has stirred substantial interest in the community, in view of its potential impact in many fields of stellar astrophysics. It is the purpose of this paper to reinterpret the interferometric observations with a fast rotating, gravity darkened central star surrounded by a small equatorial disk, whose presence is consistent with contemporaneous spectroscopic data. We find that we can only fit the available data assuming a critically rotating central star. We identified two different disk models that simultaneously fit the spectroscopic, polarimetric, and interferometric observational constraints: a tenuous disk in hydrostatic equilibrium (i.e., with small scaleheight) and a smaller, scaleheight enhanced disk. We believe that these relatively small disks correspond to the transition region between the photosphere and the circumstellar environment, and that they are probably perturbed by some photospheric mechanism. The study of this interface between photosphere and circumstellar disk for near-critical rotators is crucial to our understanding of the Be phenomenon, and the mass and angular momentum loss of stars in general. This work shows that it is nowadays possible to directly study this transition region from simultaneous multi-technique observations.