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Astron. Astrophys. 322, 554-564 (1997)

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5. Discussions and concluding remarks

In order to get an insight into the behavior of HD 5980, we should first compare it with other LBVs and related objects. Nota et al. (1996 ) study a sample of eight Ofpe/WN9 objects in the LMC. Comparison with these objects indicates that HD 5980 has an eruption spectrum quite similar to BE 294 (HDE 269582), mainly on what concerns the P Cyg profiles of hydrogen and He i lines. Moreover, the EWs of He II [FORMULA] 4686 are comparable in both cases given the accuracies (-1.31 and -1.5 Å for HD 5980 in September 1994, and BE 294 respectively). There are however some slight differences. For instance, the emission and absorption components of the He i P Cyg profiles (i.e. He i [FORMULA] [FORMULA] 4713, 5876, 7065) in BE 294 are generally stronger than the corresponding components in HD 5980.

More remarkable differences concern the absence of some higher ionization spectral features in HD 5980 as following. The He II [FORMULA] 5412 line which is in weak absorption in BE 294, does not show up in HD 5980. Regarding other ions, the lines N iii [FORMULA] [FORMULA] 4634-41, C iv [FORMULA] 4658 and Si iv [FORMULA] [FORMULA] 4089, 4101, and 6702 do not appear in HD 5980. C ii [FORMULA] [FORMULA] 7231-36 is visible in HD 5980 but due to the strong telluric absorptions, it is difficult to compare the strength of the line with that in BE 294. Nota et al. (1996 ) report a relatively strong C iii [FORMULA] 5696 emission blended with Al III [FORMULA] [FORMULA] 5697, 5723 and Si iii [FORMULA] 5740 in the spectrum of BE 294. In the case of HD 5980 we have attributed the blend between [FORMULA] [FORMULA] 5650 and 5750 to the same Al III and Si iii ions but to N ii rather than to C iii. The weak emission at [FORMULA] 4507, visible in BE 294, which is possibly due to the "unidentified" [FORMULA] 4504 feature (Crowther & Bohannan 1996 ) is not detected in the spectrum of HD 5980.

Barbá et al. (1995 ) compare the eruption spectrum of HD 5980 in the visible to the spectrum of the Galactic peculiar object He 3-519 discussed by Davidson et al. (1993 ). The latter authors suggest that He 3-519 could be a post-LBV object and notice the similarity of its spectrum with that of BE 294 as observed by Bohannan & Walborn (1989 ). However, using high dispersion spectra, Smith et al. (1994 ) notice important differences between He 3-519 and the Ofpe/WN9 stars. These differences mainly concern the absence of the same N iii and Si iv Of spectral features as discussed above for HD 5980. These authors therefore introduce the WN 11 spectral-type as an extension of the WN classification towards lower ionization. They suggest to classify He 3-519 and AG Car at minimum as WN 11 stars. Given the similarity between HD 5980 and He 3-519 (Smith et al. 1994 ), HD 5980 could also be classified as WN 11 with some of the Balmer lines displaying P Cyg profiles instead of pure emission lines. These morphological similarities with WN 11 and/or quiescent LBV stars are also noticed in the infrared (Eenens & Morris 1996 ).

A comparison between the eruption spectrum of HD 5980 and blue spectra of He 3-519 and AG Car at minimum, kindly provided by D. Hutsemékers, shows that He II [FORMULA] 4686 has nearly the same EW in all the spectra, whereas the most important lines (H [FORMULA], He i, N ii, Si iii) are generally stronger in AG Car and He 3-519. We also note that neither HD 5980 nor He 3-519 display the [Fe ii] emissions seen in the spectrum of AG Car at minimum.

Assuming a relative velocity of the SMC with respect to the Galaxy of [FORMULA] 150 km s-1, we find a mean corrected velocity of [FORMULA] 350 km s-1 for those lines that display a high velocity absorption component (Table 2). This value is again very similar to the value of [FORMULA] = 365 km s-1 found by Smith et al. (1994 ) in the case of He 3-519, but slightly higher than the terminal velocities of the LMC WN 11 stars investigated by Crowther & Smith (1996 ).

It is also interesting to compare the 1994 spectrum of HD 5980 with that of the star P Cygni (Stahl et al. 1993 ). Almost all the lines characterizing HD 5980 are present in P Cygni (B1Ia); the most important exception being of course He II [FORMULA] 4686 which does not show up in P Cygni. Also, some ions present in P Cygni are missing or perhaps very weak in HD 5980 (i.e. O II, [Fe II ], Mg II, Ni II). This may be due to the fact that our spectral resolution is half that used by Stahl et al. (1993 ). Another remarkable difference is that while in star P Cygni all the hydrogen Balmer and He i lines show P Cyg profiles, in HD 5980 some of them (i.e. H [FORMULA], H [FORMULA], He i [FORMULA] 7065) are almost pure emission lines. On the other hand, the Fe iii lines in the spectrum of HD 5980 indicate the same dichotomy as reported by Stahl et al. (1993 ) for the star P Cygni: transitions with low multiplet numbers display P Cyg profiles, while those with high multiplet numbers show pure emission lines. The resemblance between HD 5980 and the star P Cygni is somewhat expected since Koenigsberger et al. (1996 ) assign a spectral type B1.5Ia [FORMULA] to the ultraviolet spectrum of HD 5980 during the eruption.

HD 5980 is not the only LBV candidate in the SMC. In fact, the first LBV-like event in the SMC was reported for R 40 (Szeifert et al. 1993 ). A noteworthy difference between HD 5980 and R 40 is that contrarily to HD 5980 and other well-studied LBVs, R 40 has no P Cyg-type profiles of singly ionized metallic lines. In reality, the spectrum of R 40 strikingly resembles the spectra of normal hypergiants. An expanding envelope and strong wind in R 40 are only inferred from extended blue wings of the metallic lines and the P Cyg profile of H [FORMULA].

While some LBVs at visual minimum resemble Ofpe/WN9 stars (see Bohannan & Walborn 1989 ; Humphreys & Davidson 1994 and references therein), or WN 11 stars (Smith et al. 1994 ), HD 5980 is unique in the sense that it displays an WN 11-like spectrum near maximum visual brightness! Since mass loss, which is a key parameter of the LBV phenomenon, is now believed to be metallicity dependent (Abbott 1982 , Leitherer et al. 1992 and references therein), it is necessary to compare LBVs in environments with different metallicities. The difference between HD 5980 and R 40 on the one side and the similarity between spectral features of HD 5980, BE 294, and He 3-519 which lie in three galaxies with significantly different metallicities on the other side may hint that other physical factors play a role as important as metallicity.

Several Galactic shell nebulae are known to be ejected mainly by W-R stars (see Esteban et al. 1992 , Chu 1991 and references therein). Also, circumstellar nebulae are found to be associated with Galactic LBVs and related objects, in particular He 3-519 (Nota et al. 1995 and references therein). These nebulae have abnormal chemical abundances, in the sense that helium and nitrogen are enhanced and oxygen is depleted with respect to the Solar values, in agreement with predictions of stellar evolutionary models (Maeder 1990 ). As to the Magellanic Clouds, low-excitation nebular lines have been detected towards a number of LMC transition stars of category Ofpe/WN9 (Nota et al. 1995 ). The first case of abundance enhancements in a high-excitation nebula in the Magellanic Clouds was reported by Heydari-Malayeri et al. (1990 ) for the LMC N 82, a very compact H II region of radius 1 [FORMULA].3. The central star that has undergone a violent mass loss is probably a unique transition star that has no known counterpart (Heydari-Malayeri & Melnick 1992 ).

On what concerns HD 5980, in view of the tight similarity to He 3-519, we may expect the presence of a surrounding nebular envelope. However, our high-resolution imaging attempts have failed to reveal such structure. This is not astonishing should the envelope be due to the 1994 eruption. In fact, some 700 years are necessary before a detectable shell shows up at a 0 [FORMULA].1 separation from the star, corresponding to a linear distance of [FORMULA] 0.03 pc, if we assume a mean velocity of 40 km s-1 for the ejecta, comparable to that found for He 3-519 (Davidson et al. 1993 ). However, if future observations bring out an associated circumstellar ejecta, this means that HD 5980 has undergone at least another major mass loss in the past. If the 1994 event was not the only eruption of HD 5980, the detection of a nebula around the system and its detailed study should provide valuable information on the mass-loss history of HD 5980 and help to better understand the models of mass ejection during the evolution of massive stars. It is noteworthy to underline that HD 5980 has now returned to its pre-eruption state (Koenigsberger et al. 1996 , Barbá et al 1996 ), showing a WN 6 type spectrum, as if nothing has happened.

The true nature of HD 5980 is far from being understood. Barbá et al. (1996 ) suggest that the main component of the eclipsing binary, a WN3 type, has undergone the outburst. They argue that there is no obvious indication of an O type component in the system, since the absorption lines, present in the pre-outburst spectrum, did not follow the orbital motion of the system. These lines may have been due to a line-of-sight O star (a resolution of 0 [FORMULA].1 at the SMC corresponds to a linear separation of some 6200 AU, enough room to contain other components). If the WN3 component is indeed confirmed to be the one that has undergone the eruption, this raises serious problems for the conventional scenarios of the evolution of massive stars. For instance, the LBV phenomenon is commonly admitted to be a transitional step in the evolution of O type stars into W-R stars and the evolution of WN stars is usually believed to proceed from late spectral types (WNL) to early types (WNE). In the case of HD 5980 we are dealing with a system of W-R stars that has changed its spectral type from WNE to WNL in less than twenty years!

On the other hand, Moffat et al. (1996 ) suggest that HD 5980 likely consisted of two hot, luminous Of or hybrid Of/WNE stars in which the primary went into eruption. They ascribe the emission lines and the important spectral variability observed before the event to the action of two nearly equal colliding winds according to Usov's (1995 ) model. However, it is not clear how this scheme can explain the observed behavior of the emission spectrum and in particular the motionlessness of the absorption lines.

Further studies, observational as well as theoretical, are necessary in order to better understand this peculiar object. HD 5980 may represent the prototype of a new class of LBV phenomena occuring in close massive binaries in metal-poor environments.

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998