Astron. Astrophys. 326, 143-154 (1997)

3. Spectroscopy of R 84

The fact that Ofpe/WN9 stars are transition objects linked to LBVs and so susceptible of variability, motivates monitoring of R 84 more especially as this star has shown photometric and spectroscopic variations. The CASPEC spectrum of R 84 obtained in 1989 September (JD 2447784.84) is displayed in Fig. 4. Besides the characteristic Of He II 4686 and N iii 4634-40 emission lines, the spectrum is dominated by strong emission lines of H i, He i, N ii, N iii, Si iii and Si iv, some of these lines displaying P Cyg absorption components.

Fig. 4. Echelle spectra of R 84 obtained using CASPEC at the ESO 3.6 m telescope in 1989. The normalized intensities of the strongest lines are listed in Table 4.

Detailed information on our spectrum is listed in Table 4. Columns 3 and 4 provide the heliocentric velocities of the absorption trough and emission peak respectively. The equivalent widths (EW) of the absorption components are listed in column 6, whereas columns 7 and 8 respectively provide the EWs and normalized intensities of the emission components. The accuracies on the equivalent widths are for the stronger, non-blended lines and for weaker or blended features.

Table 4. Lines measured in the spectrum of R 84

We notice some differences in strength between the lines in our spectrum and the published data. Nota et al. (1996 ) report an EW of -9.7Å for the H + N iii emission line while we find a much lower value of -5.8 Å similar to the value of -5.5Å measured by Crowther et al. (1995 ) on the spectrum of Stahl et al. (1985 ). The variability of this line was already noticed by Stahl et al. (1985 ). Similar differences exist for the He i 4471 and 4713 lines, that appear slightly stronger in Crowther et al.'s data and show EWs enhanced by nearly a factor of 2 in the Nota et al.'s spectra. In contrast, the absorption line He II 4542 has comparable intensities in the spectra obtained at different epochs. Although some differences in EW and intensity also exist for the Of-type emission lines, they are usually less severe than for the H i and He i lines. In general, the strongest variability is seen for those lines that are formed in the outer layers of the stellar wind. Note that IUE observations also indicate strong variations of the N iii 1750 emission with EW between 1.5 and 5.3 Å (Hutchings 1980 ).

Besides the well-known unidentified Of features at 4486 and 4504, we find two unidentified weak and broad features at 4080 and 4085 Å. The latter lines do not show up in previous observations reported by Stahl et al. (1985 ), Wolf et al. (1987 ), and Smith et al. (1995 ) and are out of the spectral range of Nota et al. (1996 ). These features are, however, seen on the spectra of Moffat (1989 ) and could be marginally present in the spectra of Cowley & Hutchings (1978 ) and Bohannan & Walborn (1989 ). Smith et al. (1995 ) find a very broad emission feature in the region around the He II 4686, N iii 4634-40 lines that they tentatively attribute to N ii. We find no such feature in our spectrum, but even if such a broad structure was present, it would cover the whole width of an order and would therefore be masked by the normalization procedure. As a consequence, depending on the adopted normalization, any measurement of the EWs of the N iii 4634-40 and/or He II 4686 lines is more or less affected by this broad feature and any comparison of the strengths of these lines with data taken from the literature would be rather hazardous.

Concerning the radial velocities of the H i and He i lines, the most important differences between our data and those of Nota et al. (1996 ) exist again for the He i 4471 (60 km s^-1) and the He i 4713 (30 km s^-1) lines. For most of the other lines, the differences are of the order of 10-15 km s^-1 which is about the estimated error of our measurements.

Narrow nebular emission lines of [Fe iii] are detected at 4658, 4701. A narrow emission component is also seen on top of the broad stellar H emission component. These nebular features have widths of km s^-1 (FWHM), in good agreement with the widths of the nebular H , H and [N ii] 6548,6583 lines (Nota et al. 1996 ). The mean heliocentric radial velocity of the nebular lines in our spectrum is km s^-1, in agreement with Cowley & Hutchings's (1978 ) velocity of km s^-1 and Wolf et al.'s (1987 ) value of 250 km s^-1 both derived from the [N ii] lines.

© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998
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