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2.5. The optical/IR star: LS 5039 61
Since no Hα emission was seen, the star could not fill its Roche lobe even at
periastron, and considering a radius of R = 10 R⊙ for an O7V star, they directly
inferred a minimum mass of 15 M⊙ for the optical star, compatible with the expected
mass of 36 M⊙. On the other hand, in order to avoid breakup speed of the star, the
lowest inclination acceptable would be i 9 ◦ , and this would limit the maximum
mass for the compact object to be MX 8 M⊙.
We note that then it was unknown the improvement on the spectral type classifi-
cation as an O6.5V((f)) star. However, the derived values do not change significantly.
In fact, considering a mass of 40 M⊙ for an O6.5V((f)) star, as we have discussed
in Sect. 2.5.3, we obtain a maximum mass for the compact object of 9 M⊙.
Spectroscopic observations with the INT
With the aim to confirm the results obtained by McSwain et al (2001), we have
conducted new medium resolution spectroscopic observations of LS 5039 during 9
consecutive nights. We have observed 17 LS 5039 using the Intermediate Dispersion
Spectrograph (IDS) attached to the 2.5 m Isaac Newton Telescope (INT) 18 on the
nights of 2002 July 23–31. A total of 96 spectra were obtained with the combination
of the 235 mm camera and the R900V grating, which provided a useful wavelength
coverage (free from vignetting) of λλ 3900–5500. The seeing was variable (1 ′′ –2 ′′ )
during our run and we used a 1.2 ′′ slit which resulted in a resolution of 83 km s −1
(FWHM). In addition, we also obtained five spectra of LS 5039 with the holografic
grating H2400B for the purpose of measuring the rotational broadening (V sin i)
of the companion’s absorption lines. The spectral resolution of these spectra was
30 km s −1 and we varied the central wavelength in order to fully cover the range
λλ 3900–5100, where most of the prominent He i and He ii lines lie. The spectral
type standards HR 8622 (O9V spectral type) and HD 168075 (O7V((f)) spectral
type) were also observed with the same instrumental configurations. A complete log
of the observations is provided in Table 2.7.
The images were de-biased and flat-fielded, and the spectra subsequently ex-
tracted using conventional optimal extraction techniques in order to optimize the
signal-to-noise ratio of the output (Horne 1986). CuAr and CuNe comparison lamp
17 Casares, J., Ribó, M., Paredes, J. M., & Martí, J. 2002, in preparation.
18 The INT is operated on the island of La Palma by the Royal Greenwich Observatory in the
Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias.