Of and Of/WN stars

Relative to normal O and W-R stars, Of supergiants show a composite absorption (photospheric) and emission (wind) line spectrum, with evidence for helium enrichment and CNO-cycled material at their surface (e.g. Herrero et al. 1992). Puppis, an early Of star has been quantitatively analysed (Bohannan et al. 1990; Pauldrach et al. 1994), revealing a relatively high mass-loss rate ( yr ) and helium content (H/He4, by number), while Crowther & Bohannan (1996) have recently utilised the Standard Model to analyse two late Of stars (HD151804 and HD152408), revealing yet higher levels of mass-loss and He-enrichment. The derived stellar parameters for HD151804 are shown in Table 1 while line profile fits for HD152408 are shown in Fig. 3. Crowther & Bohannan (1996) provided quantitative evidence for the surface mass flux ( /4) distinguishing between Of and WNL stars.

  
Figure 3: Profile fits (dotted line) to observations (solid line) of HD152408 (O8:Iafpe) resulting in stellar parameters of =28.5kK, log =5.8, =955 kms, H/He1.5, log yr=-4.6, the latter in excellent agreement with radio methods (Crowther & Bohannan 1996)

Of/WN stars, showing a hybrid spectral appearance between Of and WN belong to two main groups. Walborn (1986) introduced the hot O3If/WN6 stars whose spectra are dominated by photospheric absorption plus a relatively strong, broad HeII 4686 emission. Studies of such stars (e.g. Mel 42: Pauldrach et al. 1994; R136a5: de Koter et al. 1994) indicate relatively high mass-loss rates, though little, if any, chemical enrichment. Meanwhile, the cool Ofpe/WN9 stars, originally thought to show composite Of absorption and WN emission lines (Bohannan & Walborn 1989), were revealed as bona-fide WNL stars (WN9--11) by Crowther et al. (1995a) and Crowther & Smith (1996b). Derived stellar parameters for the prototype WN9 star R84 are shown in Table 1. Initially, these objects were unique to the LMC, although a newly discovered example in (a low metallicty region of) M33 has recently been analysed by Smith et al. (1995) showing very similar properties. Overall, the properties of these stars are very similar to existing WNL stars, with slightly lower levels of chemical enrichment, and lower wind densities (recall Fig. 1) -- their parameters are comparable with high excitation LBVs such as AG Car, itself given a WN11 classification at minimum by Smith et al. (1994). Conclusive evidence of a direct connection between WN9--11 stars and LBVs resulted when the LMC prototype R127 quite unexpectedly evolved to a B--type (Stahl et al. 1983) and later an A--type supergiant spectrum (Wolf et al. 1988).