The discovery of a shell-like event in the O-type star HD 120678

R. Gamen,1 J. I. Arias,2 and R. H. Barbá,3 and N. I. Morrell,4 and N. R. Walborn,5 and A. Sota,6 J. Maíz Apellániz,6 E. J. Alfaro6

1 Instituto de Astrofísica de La Plata (CONICET) and Facultad de Ciencias Astronómicas y Geofísicas, Universidad Nacional de La Plata, Paseo del Bosque s/n, B1900FWA, La Plata, Argentina
2 Departamento de Física, Universidad de La Serena, Cisternas 1200 Norte, La Serena, Chile
3 Instituto de Ciencias Astronómicas, de la Tierra y del Espacio (ICATE-CONICET), Avda España 1512 Sur, J5402DSP, San Juan, Argentina
4 Las Campanas Observatory, Carnegie Observatories, Casilla 601, La Serena, Chile
5 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218
6 Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008, Granada, Spain

Aims. We report the detection of a shell-like event in the Oe-type star HD 120678.

Methods. HD 120678 has been intensively observed as part of a high-resolution spectroscopic monitoring program of southern Galactic O stars and Wolf-Rayet stars of the nitrogen sequence.

Results. An optical spectrogram of HD 120678 obtained in June 2008 shows strong H and He I absorption lines instead of the double-peaked emission profiles observed both previously and subsequently, as well as a variety of previously undetected absorption features, mainly of O II, Si III and Fe III. Photometric data reveal that the development of the absorption spectrum coincided with a remarkable dip in the V-band lightcurve. The "shell phase" of HD 120678 did not persist for very long: the V magnitude recovered its previous average value in fewer than 120 days, whereas H and He emission lines became detectable one year later. Similar spectral variations have been observed in a few Be stars, and they are usually interpreted as changes in the circumstellar disk.

Accepted by A&A
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