On the Determination of the Rotational Oblateness of Achernar

Abstract

The recent interferometric study of Achernar, leading to the conclusionthat its geometrical oblateness cannot be explained by the Rocheapproximation, has stirred substantial interest in the community, inview of its potential impact on many fields of stellar astrophysics. Itis the purpose of this Letter to reinterpret the interferometricobservations with a fast-rotating, gravity-darkened central starsurrounded by a small equatorial disk, whose presence is consistent withcontemporaneous spectroscopic data. We find that we can fit theavailable data only assuming a critically rotating central star. Weidentified two different disk models that simultaneously fit thespectroscopic, polarimetric, and interferometric observationalconstraints: a tenuous disk in hydrostatic equilibrium (i.e., with smallscale height) and a smaller, scale height enhanced disk. We believe thatthese relatively small disks correspond to the transition region betweenthe photosphere and the circumstellar environment and that they areprobably perturbed by some photospheric mechanism. The study of thisinterface between photosphere and circumstellar disk for near-criticalrotators is crucial to our understanding of the Be phenomenon and themass and angular momentum loss of stars in general. This work shows thatit is nowadays possible to directly study this transition region fromsimultaneous multitechnique observations.