Astron. Astrophys. 343, 536-544 (1999)

1. Introduction

Ofpe/WN9 stars have only recently received wide-spread attention, but are now central to understanding the evolution of massive stars. The Ofpe/WN9 class was first introduced by Walborn (1982), who discovered several Of-like objects with very extended envelopes and, in order to indicate that these stars could neither be classified as pure WNL stars, nor as pure Of stars, introduced the Ofpe/WN9 classification. The presence of nebular lines indicated enhanced nitrogen abundances, suggesting that the circumstellar gas had been ejected by the stars during their post main sequence evolution. Bohannan & Walborn (1989) subsequently summarized the properties of all known Ofpe/WN9 stars in the Large Magellanic Cloud (LMC).

The significance of Ofpe/WN9 stars was first recognized when Stahl et al. (1983) discovered that the prototype of the Ofpe/WN9 class (R127) had turned into a Luminous Blue Variable (LBV) on a timescale of less than a few years. Stahl (1986) also found that when the bright LBV AG Carinae is in a high-temperature state, it displays the spectral characteristics of Ofpe/WN9 stars. LBVs as a class were introduced by Conti (1984), although some of them had already been studied individually four decades ago (Hubble & Sandage 1953). Together with the Ofpe/WN9 stars, LBVs populate the uppermost part of the HR Diagram, and are believed to be in evolutionary transition to Wolf-Rayet stars (Maeder 1989). LBV evolution is characterized by enhanced mass-loss rates, outbursts and shell ejections. Observationally, LBVs display irregular photometric (0.5-2 mag) and spectral variations over timescales of decades. In addition, they sometimes undergo "giant outbursts" , in which they eject a significant fraction of their mass. The ejected mass is often observed in the form of a spectacular circumstellar nebula (e.g. Car, Davidson 1987).

The observational evidence indicates that there is a close relationship between the class of LBVs and the Ofpe/WN9 stars. The possibility of such a relationship has been investigated by Smith, Crowther & Prinja (1994), who proposed that some LBVs show spectral morphologies that make them appear as an extension of the WN sequence towards later spectral types, hence unifying the classes of WNL, LBV and Ofpe/WN9 stars. Recently, Nota et al. (1996b) presented a new dataset comprising ground-based, high S/N echelle spectra of the complete sample of Ofpe/WN9, with two Of and B[e] stars taken for comparison. They concluded, on the basis of the spectral morphologies alone, that the Ofpe/WN9 type formed a very homogeneous group, with wind characteristics in between the Of and the B[e] stars. At the same time, Morris et al. (1996) independently reached the same conclusion on the transitional nature of Ofpe/WN9 and LBVs, by comparing IR spectra of Ofpe/WN9, LBVs late-type WN stars and B[e]. Pasquali et al. (1997) and Crowther et al. (1995) finally placed these statements on a firm quantitative footing by deriving the fundamental parameters of these stars. Pasquali et al. (1997) used ultraviolet HST/FOS spectra for the same sample of Ofpe/WN9 stars studied by Nota et al. (1996b), and investigated the evolutionary status of these stars, concluding that most likely the Ofpe/WN9 stars are quiescent LBVs.

This concept had already been proposed by Nota et al. (1996), on the basis that, although Ofpe/WN9 stars do not display any variability, in some cases they show an associated circumstellar nebula which is most likely the relic of a LBV-type outburst which occurred some time in their post main sequence evolutionary history. Out of the seven Ofpe/WN9 stars they considered, they found that five showed evidence of nebular emission lines in their spectra, confirming the original findings by Walborn (1982). Unfortunately, the high resolution echelle spectra used for their investigation did not provide sufficient spatial information to study in detail the characteristics of the detected nebular emission, except for S119. The nebula around S119 was subsequently imaged (Nota et al. 1994; Smith et al. 1998), and was found to have morphological and physical properties very similar to most LBV nebulae. The nebula was most likely ejected by the central star 5 10⁴ yrs ago.

In order to further investigate the relationship between LBVs and Ofpe/WN9 stars, we have initiated a program of high resolution coronographic imaging and medium resolution longslit spectroscopy of the circumstellar environment of all Ofpe/WN9 stars displaying presence of nebular lines in their spectra. Our objective is threefold: 1) to establish the presence of a circumstellar nebula, 2) to assess whether the nebula is physically associated with the star, 3) to study in detail the kinematical and chemical properties of the resolved Ofpe/WN9 nebulae and compare them to known LBV nebulae, to strengthen the suspected connection between the two classes of objects.

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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