Variable rotational line broadening in the Be star Achernar^⋆,⋆⋆

¹ ESO – European Organisation for Astron. Research in the Southern Hemisphere, Casilla 19001, Santiago, Chile
e-mail: triviniu@eso.org
² ESO – European Organisation for Astron. Research in the Southern Hemisphere, Karl-Schwarzschild-Str. 2, Garching, Germany
³ Department of Astronomy, Univ. of Wisconsin-Madison, 2535 Sterling Hall, 475 N. Charter Street, Madison, WI 53706, USA
⁴ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, 05508-900 São Paulo SP, Brazil
⁵ ESO/ALMA – The Atacama Large Millimeter/Submillimeter Array, Alonso de Córdova 3107, Vitacura, Santiago, Chile

Received: 20 August 2013
Accepted: 26 September 2013

Abstract

Aims. The main theoretical problem for the formation of a Keplerian disk around Be stars is how angular momentum is supplied from the star to the disk, even more so since Be stars probably rotate somewhat subcritically. For instance, nonradial pulsation may transport angular momentum to the stellar surface until (part of) this excess supports the disk-formation/replenishment. The nearby Be star Achernar is presently building a new disk and offers an excellent opportunity to observe this process from relatively close-up.

Methods. Spectra from various sources and epochs are scrutinized to identify the salient stellar parameters characterizing the disk life cycle as defined by Hα emission. The variable strength of the non-radial pulsation is confirmed, but does not affect the other results.

Results. For the first time it is demonstrated that the photospheric line width does vary in a Be star, by as much as Δv sini ≲ 35 km s^-1. However, unlike assumptions in which a photospheric spin-up accumulates during the diskless phase and then is released into the disk as it is fed, the apparent photospheric spin-up is positively correlated with the appearance of Hα line emission. The photospheric line widths and circumstellar emission increase together, and the apparent stellar rotation declines to the value at quiescence after the Hα line emission becomes undetectable.