Letter to the Editor

The 2006 hot phase of Romano's star (GR 290) in M 33^*

¹ INAF, Istituto di Astrofisica Spaziale e Fisica Cosmica di Roma, via Fosso del Cavaliere 100, 00133 Roma, Italy e-mail: roberto.viotti@iasf-roma.inaf.it
² INAF, Bologna Astronomical Observatory, Loiano Observing Station, Loiano, Italy
³ Dipartimento di Fisica, Università di Roma La Sapienza, Pz.le Aldo Moro 3, 00185 Roma, Italy
⁴ School of Physical Sciences and NCPST, Dublin City University, Glasnevin, Dublin 9, Ireland

Received: 21 December 2006
Accepted: 28 January 2007

Abstract

Context.Understanding the nature of the instabilities of LBVs is important to understand the late evolutionary stages of very massive stars.

Aims. We investigate the long term, S Dor-type variability of the luminous blue variable GR 290 (Romano's star) in M 33, and its 2006 minimum phase.

Methods. New spectroscopic and photometric data taken in November and December 2006 were employed in conjunction with already published data on GR 290 to derive the physical structure of GR 290 in different phases and the time scale of the variability.

Results. We find that by the end of 2006, GR 290 had reached the deepest visual minimum so far recorded. Its present spectrum resembles closely that of the Of/WN9 stars, and is the hottest so far recorded in this star (and in any LBV as well), while its visual brightness decreased by about 1.4 mag.

Conclusions. This first spectroscopic record of GR 290 during a minimum phase confirms that, similarly to AG Car and other LBVs, the star is subject to ample S Dor-type variations, being hotter at minimum, suggesting that the variations take place at constant bolometric luminosity.