Astron. Astrophys. 344, 277-281 (1999)

3. Results

Tables 2 and 3 report the Balmer lines parameters measured on our spectra from 1985 to 1997. The statistical uncertainties on the lines equivalent widths are evaluated following Chalabaev & Maillard (1983). We are confident about the measured flux values since the flux integral over the V filter band gives a value V=12.1 in the 1985 spectra that is in agreement with the V magnitude reported in literature. To our knowledge, no V photometry is available in other epochs. In any case all our spectra taken from 1994 to 1997 give V values between 12.2 and 12.3. These variation in magnitude with respect to the 1985 value are compatible with the classical behaviour of Be stars (e.g. Piccioni et al., 1985). Fig. 1 shows the H equivalent width (EW) from 1979 to 1997; in addition to our data, the whole set of observations reported by Coe et al. (1994) is also shown.

Fig. 1. Hen 3-640 H equivalent width from 1979 to 1997; in addition to our data, the whole set of observations reported by Coe et al. (1994) is also shown.

Table 2. Hen 3-640 - H and H parameters. Fluxes and their errors are in erg s^-1 cm^-2

Table 3. Hen 3-640 - H and H parameters. Fluxes and their errors are in erg s^-1 cm^-2

We can see that the H EW is of order of 50-60 Å when the system is in a quiescent state, while it rose to EW100 Å during the outburst. However, the figure shows also that the enhancement of H EW is consistent with a gradual rise, while a sharp decrease followed the 1991-1992 outburst. At the same time, minor, but statistically significant short term variability is evident in all epochs. A similar behaviour is seen in H.

In 1985, H/H flux ratio was 6.0 (after dereddenning with =1.2), while / 7. After the outburst, we can evaluate an approximate H/H flux ratio 3 in 1994 and 1995 and 5 in 1996 and 1997, while the / is always 10, which is roughly in agreement with the 1985 value and also in agreement with the ratios found in in other Be/X systems (e.g. Giovannelli & Sabau-Graziati, 1992).

Higher lines of the Balmer series are more difficult to measure because of the uncertainties on the continuum, that is strongly affected by a crowd of metal lines. However, Tables 2 and 3 clearly demonstrate a significant variability in all the measured Balmer lines.

We discussed elsewhere (Villada et al., 1992; Polcaro et al., 1993; Coe et al., 1994 and referneces therein) the H and H profiles during and immediately after the 1991-1992 X-ray outburst. The following year's spectra never showed the line splitting that was present during the outburst. However, H is always asymmetric, suggesting that a double component is still present, albeit at a velocity separation which is now lower than our spectral resolving power. A similar asymmetry was also present in 1985 spectra (see Fig. 2).

Fig. 2. Detail of the H region of the averaged spectrum of Hen 3-640 at different epoch. The spectra are rectified and vertically shifted for clarity.

The post-outburst spectra of Hen 3-640 (Figs. 3 and 4) show the general decrease of the ionization level: the He II 4686 line, that was clearly visible from 1990 to 1992 (see Fig. 1 in Villada et al., 1992, is undetectable. The He I 4471 line, that presented in 1992 a narrow but evident re-emission peak (see Fig. 3 in Polcaro et al., 1993), is now always in absorption.

Fig. 3. Combined flux calibrated spectrum of Hen 3-640 on 5/6 March 1994; the spectra are dereddened with =1.2. Flux is in erg s-1 cm- 2 Å-1.

Fig. 4. Combined flux calibrated spectrum of Hen 3-640 on 23/25 February 1997; the spectra are dereddened with =1.2. Flux is in erg s-1 cm-2 Å- 1.

The Fe II emissions disappeared, while the Fe I lines are usually quite strong. In 1996, two strong emission lines, that were never previously detected, appeared at 5569 and 5580 Å: we tentatively identified these lines as blends Fe I 5569.6,5572.1 and Ti I 5579.0,5583.0. The two lines are clearly visible in the 1997 spectrum shown in Fig. 4 and are clearly separated from the 5577 Å night-sky feature, that is completely removed by the sky subtraction procedure (as well as all the other sky lines).

© European Southern Observatory (ESO) 1999

Online publication: March 10, 1999
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