Bolometric luminosity variations in the luminous blue variable AFGL2298^*

¹ Department of Physics and Astronomy, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK e-mail: jsc@star.ucl.ac.uk
² Department of Physics & Astronomy, University of Sheffield, Sheffield, S3 7RH, UK
³ Astronomical Institute of St. Petersburg University, Petrodvorets, Universitetsky pr. 28, 198504 St. Petersburg, Russia
⁴ Isaac Newton Institute of Chile, St. Petersburg Branch, Russia
⁵ Astrophysics Research Institute, Liverpool JMU, Twelve Quays House, Egerton Wharf, Birkenhead, CH41, 1LD, UK
⁶ IBM United Kingdom Laboratories, Hursley Park, Winchester, Hampshire, S021 2JN, UK
⁷ Pulkovo Astronomical Observatory, 196140 St. Petersburg, Russia

Received: 21 April 2009
Accepted: 27 August 2009

Abstract

Aims. We characterise the variability in the physical properties of the luminous blue variable AFGL 2298 (IRAS 18576+0341) between 1989-2008.

Methods. In conjunction with published data from 1989-2001, we have undertaken a long term (2001-2008) near-IR spectroscopic and photometric observational campaign for this star and utilise a non-LTE model atmosphere code to interpret these data.

Results. We find AFGL 2298 to have been highly variable during the two decades covered by the observational datasets. Photometric variations of ≥1.6 mag have been observed in the JHK wavebands; however, these are not accompanied by correlated changes in near-IR colour. Non-LTE model atmosphere analysis of 4 epochs of K band spectroscopy obtained between 2001-7 suggests that the photometric changes of AFGL 2298 were driven by expansion and contraction of the stellar photosphere accompanied by comparatively small changes in the stellar temperature ( ~ 4.5 kK). Unclumped mass loss rates throughout this period were modest and directly comparable to those of other highly luminous (candidate) LBVs. However, the main finding of this analysis was that the bolometric luminosity of AFGL 2298 appears to have varied by at least a factor of ~2 between 1989-2008, with it being one of the most luminous stars in the Galaxy during maximum. Comparison to other LBVs that have undergone non bolometric luminosity conserving “eruptions” shows such events to be heterogeneous, with AFGL 2298 the least extreme example. These results – and the diverse nature of both the quiescent LBVs and associated ejecta – may offer support to the suggestion that more than one physical mechanism is responsible for such behaviour.