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

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4. Stars # 35 and # 36

The spectra of stars # 35 and # 36 obtained using long-slit spectroscopy with EMMI at the ESO NTT during excellent seeing conditions are presented in Fig. 5. The absence of He II [FORMULA] 4686 in star #35 indicates a spectral type later than B0.5-B0.7 for this star of V = 13.45 and V - R = 0.07 (Walborn & Fitzpatrick 1990 ). The line ratio Si iii [FORMULA] 4552/Si iv [FORMULA] 4089 being [FORMULA] 1, implies a spectral type B1. The primary luminosity criterion at this type, i.e. Si iii [FORMULA] 4552/He i 4387, which shows a smooth progression along the sequence (Walborn & Fitzpatrick 1990 ), points to a supergiant class. In fact, the spectral features of # 35 are reminiscent of those of HD 86606 which is classified as B1 Ib by Walborn & Fitzpatrick (1990 ). On the basis of an interstellar reddening [FORMULA] = 0.54 mag (Sect. 5) and a distance of 50 kpc (Westerlund 1990 ), an absolute visual magnitude [FORMULA] = -5.6 can be derived for star #35, in good agreement with that expected for a B1 Ib type (Lang 1992 ). From measurement of 9 He i and hydrogen lines we find a heliocentric radial velocity of 328 [FORMULA] 12 km s-1 for this star.

[FIGURE] Fig. 5. Spectra of stars # 35 and # 36 obtained using the ESO NTT+EMMI with grating #12.

Star # 36, being 0.63 mag weaker in V than # 35, is blue with colors U - B = -0.79, B - V = -0.05, and V - R = -0.03 mag. The presence of a prominent He II [FORMULA] 4686 line and very weak lines of He II [FORMULA] [FORMULA] 4200 and 4541 indicates a very early-type B star. The ratio of Si iii [FORMULA] 4552 to Si iv [FORMULA] 4089 indicates a spectral type of B0.2. Walborn & Fitzpatrick (1990 ) noticed an extremely tight progression in the luminosity criteria at this spectral type. From the ratio of Si iv [FORMULA] 4089 to He i [FORMULA] 4121, which represents the main luminosity criterion, we deduce a main sequence class. Compared with star # 35, we note significantly larger equivalent widths for He i and broader H i Balmer lines. These facts are also indicators of a main sequence luminosity class. However, assuming an intrinsic color of (B - V) 0 = -0.30, we derive an absolute magnitude [FORMULA] = -5.1 which is too bright for a single main sequence early B type star. A medium resolution spectrum of star # 36 taken with the Boller & Chivens spectrograph at the ESO 1.5 m telescope in March 1997 shows that most of the absorption lines are double indicating that # 36 is a binary system with two stars of similar spectral types and luminosities.

Based on 10 He i, He II, and H i lines we derive a radial velocity of 351 [FORMULA] 17 km s-1 for star #36.

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© European Southern Observatory (ESO) 1997

Online publication: April 20, 1998