Bolometric luminosity variations in the Luminous Blue Variable AFGL2298

TL;DR

This study investigates the long-term variability of the luminous blue variable AFGL2298, revealing significant bolometric luminosity changes driven by stellar expansion and contraction, with implications for understanding LBV behavior.

Contribution

It provides the first detailed long-term analysis of AFGL2298's physical variability, combining observational data with non-LTE modeling to reveal luminosity changes and their possible mechanisms.

Findings

AFGL2298's bolometric luminosity varied by at least a factor of 2.

Photometric variations of >1.6 mag in near-IR bands observed.

Stellar expansion and contraction drive observed photometric changes.

Abstract

We characterise the variability in the physical properties of the luminous blue variable AFGL2298 between 1989-2008. 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. We find AFGL2298 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 were driven by expansion and contraction of the stellar photosphere accompanied by comparatively small changes in the stellar temperature. Unclumped mass loss rates throughout this period were modest and directly comparable to those of other highly luminous LBVs. However, the bolometric luminosity of AFGL2298 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 AFGL2298 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. [ABRIDGED]