2 Spectroscopic observations

Optical spectroscopic observations were made with the 1.9 m telescope at the South African Astronomical Observatory (SAAO) on 1997 June 23, the 1.3 m telescope at Mount Skinakas (Crete, Greece) on 1999 July 27 and 2000 July 20-22 and the ESO 1.52-m telescope at La Silla Observatory, Chile on 2000 September 13. The spectrum at SAAO was taken with the ITS spectrograph + SITe CCD + 1200 lines mm^-1 (grating No 4) + 250 $\mu$m slit. With this set-up the dispersion was ${\sim} 0.5$ Å/pixel and the wavelength coverage 4100-4900 Å. The f/7.7 Ritchey-Chretien telescope at Skinakas observatory was equipped with a $2000\times800$ ISA SITe CCD + 1302 lines mm^-1 grating, giving a dispersion of ${\sim} 1$ Å/pixel. In the 1999 July spectrum the slit width was 320 $\mu$m, whereas in the 2000 July spectra it was 80 $\mu$m. Finally the 1.52 m La Silla Observatory telescope was equipped with the Boller and Chivens spectrograph + #33 holographic grating and the Loral 38 camera. This configuration gives a dispersion of ${\sim} 1.0$ Å/pixel (resolution of ${\approx} 3$ Å at ${\sim} 4500$ Å).

Near-IR observations of LS 5039 were made with the United Kingdom Infrared Telescope (UKIRT) on 1999 July 22-24, using the Cooled Grating Spectrometer (CGS4). Observations between 1.5-2.2 microns were made using the long focal length camera plus the 150 line/mm grating, giving a velocity resolution of ${\sim} 50$ km s^-1. Note that at this resolution the wavelength range of a single spectrum is ${\sim}0.08~\mu$m - hence breaks in the spectra presented in Fig. 3 are due to slight missmatches in wavelength coverage between individual spectra. Initial data reduction was carried out at the telescope using the CGS4DR software (Puxley et al. 1992). This removes bad pixels, debiases, flat-fields, linearity corrects and interleaves oversampled scan positions. The subsequent stages of data reduction, comprising of sky subtraction, extraction, derippling and wavelength calibration using observations of a CuAr lamp, were carried out using the Starlink-supported package FIGARO. Removal of telluric features was accomplished via the procedure described by Clark & Steele (2000).

Figure 1:  Blue end (4050-4950 Å) spectra of LS 5039 with line identifications indicated (Diffuse Interstellar bands=DIB). The comparison stars are the O6V((f)) and O7V((f)) standards from the digital atlas of Walborn & Fitzpatrick (1990; HD 101190 and 15 Mon respectively).

Figure 2: Red end (5650-7500 Å) spectrum of LS 5039.

Figure 3: Composite K band (2.05-2.20 $\mu$m) and H band (1.49-1.72 $\mu$m) spectra of LS 5039; wavelength given in $\mu$m. Note the break between the 2 spectra at 2.125 $\mu$m in the K band and at 1.64 $\mu$m in the H band; unfortunately the latter break obscures the H I (12-4) 1.6412 $\mu$m transition.

2.1 Spectral classification

The optical Balmer lines are in absorption, indicating the absence of a dense circumstellar envelope around the star, and precluding an Oe/Be classification. The red-end spectrum is characterised by the presence of numerous interstellar lines (we derive reddening estimates from these in Sect. 4) and neutral helium lines in absorption ($\lambda$ 5876, 6406, 6679 and 7065). The blue-end of the optical spectrum is dominated by the hydrogen Balmer series and by strong He II lines. The ratio between He II 4541 and He I 4471 corresponds to a spectral type O6.5, according to the relations of Mathys (1988). The fact that He II 4686 is strongly in absorption identifies the star as main-sequence. This can be checked by direct comparison with MK standards. As seen in Fig. 1, the strength of the He I and He II lines in LS 5039 falls between those of the O6V((f)) and O7V((f)) standards. Very weak N III $\lambda$4634-40-42 emission is present. The combination of strong He II absorption and weak N III emission is represented by ((f)) - suggesting an O6.5V((f)) classification - slightly earlier than the classification of M97, made with a single low resolution $\lambda$3800-4600 spectrum.

 

 

Table 1: Equivalent widths of the identified lines shown in Figs. 1 and 2. Errors are <10%.

Line	EW (Å)
	1997	1999	2000
H$\delta$	2.3		2.2
HeII $\lambda$4200	0.50		0.55
H$\gamma$	2.4		2.4
HeI $\lambda$4471	0.50		0.50
HeII $\lambda$4541	0.92		0.90
HeII $\lambda$4686	0.95		1.0
H$\beta$		2.4	2.5
HeI $\lambda$6406		0.24	0.26
H$\alpha$		3.6	3.9
HeI $\lambda$6678		0.90	0.85
HeI $\lambda$7065		0.50	0.55

The spectral classification of hot, luminous stars via near-IR spectroscopy alone is far less precise than via conventional optical classification. Nevertheless, it provides an independent test of the optical classification. The comprehensive spectral atlas of Hanson et al. (1997) was used for classification of the K band spectrum of LS 5039. The strength of H I 2.166 $\mu$m and He II 2.1885 $\mu$m lines (Table 2) restrict the spectral type to later than O6, and earlier than O8, while the He I 2.113 $\mu$m absorption feature further constrains the spectral type to earlier than O7.5. Finally, the N III emission line is absent in normal Main Sequence O6-O8 spectra and confirms the O((f)) classification, fully consistent with the optical classification.

Unfortunately, no such comprehensive reference work exists for the classification of OB stars via the H band; we use the preliminary work of Blum et al. (1997) and the more extensive work of Hanson et al. (1998), noting that this atlas is restricted to a narrow waveband (1.66-1.72 $\mu$m) and complete only for spectral types between O7-B9. Comparison of the strengths of the H I brackett series to the spectra presented by Blum et al. (1997) confirm an O star classification, while the strength of the He  II transition is consistent with a spectral type of O8 or earlier (Hanson et al. 1998). Direct comparison to the spectrum of the similar O7V((f)) star HD 47839 presented in Hanson et al. (1998) shows the He II feature to be stronger for LS 5039, while the H I Brackett 11 line is somewhat weaker. However He I absorption is of comparable strength in both spectra.

In conclusion we find that both H and K band spectra are consistent with the optical classification of LS 5039 as O6.5V((f)), with the K band spectrum providing the more rigorous constraints to spectral type (i.e. no earlier than O6.5 and no later than O7) We note that we find no evidence for a luminous stellar companion for LS 5039 in any of the spectra, nor any additional emission features that might be attributed to emission from a jet or accretion disc.

 

 

Table 2: Line identifications for LS 5039 in the range 1.49-1.72 and 2.05-2.20 $\mu$m. Note the EW quoted for the line identified with He II 1.693 $\mu$m also includes a contribution from the feature visible in the blue wing of the profile. Errors are estimated at 20% of the quoted EW.

Wavelength	Feature	EW
(microns)	(microns)	(Å)
1.573	He II $\lambda$1.572	0.5
1.612	H I $\lambda$1.611	0.8
1.681	H I $\lambda$1.681	1.6
1.690	?	Blend
1.692	He II $\lambda$1.693	1.4
1.701	He I $\lambda$1.701	1.1
2.059	He I $\lambda$2.058	0.5
2.113	He I $\lambda$2.113	0.1
2.116	N III $\lambda$2.116	-1.1
2.167	H I $\lambda$2.166	2.6
2.189	He II $\lambda$2.189	2.0