HD 152248: Evidence for interaction

6 The emission lines

As mentioned earlier in this paper, the HD152248 spectrum presents several emission features. These include the He II$\lambda$4686 and H$\alpha$ lines that consist of a mixture of absorption and emission lines, the heavily blended N III$\lambda$$\lambda$4634-4641 features, the C III$\lambda$5696 line and the faint unidentified of emissions at $\lambda$4486 and $\lambda$4504.

6.1 The C III$\lambda$5696 line

As it clearly appears from Fig. 5, the C III$\lambda$5696 emission line turns out to be double peaked around quadrature and the peaks seem to move in phase with the RV curves of the absorption lines. In order to measure the RVs associated with these peaks, we first modelled the diffuse interstellar band (DIB) at $\lambda=5704.97$ Å ( ${\it EW}\approx0.064$ Å) on a spectrum where it was well separated from the C III line (HJD2451669.853). We then subtracted the modelled DIB from all spectra and fitted two Gaussians to the C III emission line whenever the separation between the components was large enough; at other phases, we used the cross-correlation like method. The measured RVs are plotted together with the He I$\lambda$4471 RV curve on the right panel of Fig. 5. The left panel presents a grey scale image around the C III line; the He I$\lambda$4471 line orbital solution has been overplotted. It is clear from the two panels of Fig. 5 that the C III$\lambda$5696 lines closely follow the orbital motion. As we already pointed out in the case of HD93403 (Rauw et al. 2000), this is a strong argument in favour of a photospheric origin of the C III $\lambda5696$ line as suggested by Nussbaumer (1971) and Cardona-Núnez (1978). Finally, unlike the case of HD93403 (see Rauw et al. 2001), the C III$\lambda$5696 line in the spectrum of HD152248 does not seem to display a reversal of the intensity ratio due to the Struve-Sahade effect.

6.2 The N III $\lambda$$\lambda$4634-4641 structure

The HD152248 spectrum presents a highly complex emission structure between about $\lambda = 4625$ Å and $\lambda = 4645$ Å, which displays important phase-locked profile variations. In this region of the spectrum, the N III$\lambda$$\lambda$4634 and 4641 lines are heavily blended with each other as well as with the neighbouring Si IV$\lambda$4631 and C III$\lambda$$\lambda$4647-4650 lines at all phases. Due to their lower resolution, we excluded the B&C spectra from our study. Similarly we did not consider the CTIO spectra because of their relatively lower S/N ratio. A complementary journal of the CAT N III$\lambda$$\lambda$4634-4641 observations is given in Table 6. A careful comparison of the CAT and FEROS N III$\lambda$$\lambda$4634-4641 spectra with the predicted line positions computed from our ephemerides and our orbital solution allows us to identify the components of the blend. Both stars of the HD152248 system present the N III$\lambda$$\lambda$4634-4641 lines in emission in their spectra. Following e.g. Mathys (1988), an (f) tag should then be added to their spectral classification. The Si IV$\lambda$4631 and C III$\lambda$$\lambda$4647-4650 lines are in absorption in both spectra.

6.3 The unidentified $\lambda$$\lambda$ 4486, 4504 lines

The two $\lambda$$\lambda$4486, 4504 emission lines are related to the Of phenomenon and still remain unidentified, although they have been known for several decades now. These two lines are rather faint in the spectra of HD152248 and their peak intensities reach maxima of respectively 2.5% and 2% of the continuum level at conjunction phases. The stellar Mg II$\lambda$4481 lines, the DIB near $\lambda$4502 and the probable presence of the N III$\lambda$$\lambda$4510-4514 absorption lines render this region of the spectrum very crowded and prevent us from performing a detailed analysis. It seems however clear that the $\lambda$4486 line is double peaked with the two components closely following the orbital motion. The situation is less clear concerning the $\lambda$4504 line, because of the DIB that affects the blue side of the line. A careful inspection of the spectra however suggests that the $\lambda$4504 line might also be double peaked according to the orbital motion.

  

Table 6: Complementary journal of the CAT observations of the N III $\lambda \lambda$ 4634-4641 and He II$\lambda$4686 lines. The notations used are identical to those of Table 1

$$\begin{tabular}{c c l l} \hline Line & \begin{tabular}{c}HJD\... ... & 1000.727& 0.520 & CAT + CES + VLC \\ \hline \end{tabular}$$

6.4 The He II $\lambda$4686 and H $\alpha$ lines

 

 

Table 7: Measured equivalent widths of the three components of the He II$\lambda$4686 and H$\alpha$ lines as measured on the spectrum taken on HJD2451673.809 ( $\phi =0.248$)

Line	EW (He II$\lambda$4686)	EW (H $\alpha$)
component	(Å)	(Å)
Primary absorption	0.245	0.590
Secondary absorption	0.178	0.400
Emission	-0.630	-2.588

Figure 6: Two superimposed spectra of the He II$\lambda$4686 line: the full line corresponds to $\phi =0.910$; the dashed line, to $\phi =0.568$

We have a total of 51 He II$\lambda$4686 line spectra (17 B&C, 16 CES, 5 BME and 13 FEROS) at our disposal that provide a very good phase coverage. The journal of the CAT observations of He II$\lambda$4686 is given in Table 6. Our H$\alpha$ line data set is much more limited and consists of only 13 FEROS spectra. The normalization of the FEROS spectra around these two lines turned out to be a rather difficult task. Indeed, both lines are situated at the junction of two echelle orders and we had to normalize the spectra locally. To compare the B&C spectra with the echelle spectra, we also had to apply a local normalization around He II$\lambda$4686 on the B&C spectra, in order to account for the broad underlying emission extending from 4600 to 4720 Å.

Figure 7: Dynamical spectra of the He II$\lambda$4686 (left) and H$\alpha$ (right) lines. The cuts are respectively set to [0.95-1.05] and [0.95-1.1]. The He I$\lambda$4471 RV curve has been overplotted on both panels

Figure 8: Equivalent widths of the He II$\lambda$4686 (left) and H$\alpha$ (right) lines. Different symbols refer to different instruments: ${\rm triangle} = {\rm BME}$, ${\rm square} = {\rm B\&C}$, ${\rm diamond} = {\rm CES}$, ${\rm circle} = {\rm FEROS}$

Figure 9: Full widths of the base of the emission component of the He II$\lambda$4686 (left) and H$\alpha$ (right) lines. Different symbols refer to different instruments: ${\rm triangle} = {\rm BME}$, ${\rm square} = {\rm B\&C}$, ${\rm diamond} = {\rm CES}$, ${\rm circle} = {\rm FEROS}$

Both the He II$\lambda$4686 and H$\alpha$ lines consist of a mixture of absorptions and emissions. Dynamical spectra of these two lines are displayed as a function of orbital phase on the grey-scale images in Fig.7. These were built by direct linear interpolation along the time axis of the observed spectra.

It is clear from these two diagrams and from Fig. 6 that the He II$\lambda$4686 and H$\alpha$ line profiles are strongly variable. For both lines, we can identify two absorption components at RVs that closely match the orbital motion. These absorptions are thus most probably formed in the atmosphere of the stars of the system. These two absorption features are superimposed on a broader emission component. The width of this latter emission varies with orbital phase (see e.g. Fig. 6).

To quantify these variations, we first measured the total EW of the lines. The results for both lines are plotted against the phase in Fig. 8. Concerning the He II$\lambda$4686 line, we notice that the absorption is clearly dominating near phase $\phi=0.5$ and in a less prominent way near $\phi=0.0$. On the other side, the emission is overwhelming at phase $\phi \approx 0.8$ (and to some extent at $\phi \approx 0.25$).

We next attempted to restore the emission component. To this aim, in order to model the three line components, we fitted three Gaussians to the line profile on a spectrum where the two absorption contributions are well separated. The EWs of all three components as obtained from the fit are reported in Table 7. These results should however be considered with caution since the EWs of the three components are most probably undergoing some variation with phase. For each observed spectrum, we then shifted the template of the absorption components according to our He I$\lambda$4471 orbital solution and the observation phase, and we subtracted the shifted fake lines from the observed profile. This ``restoration'' process is a very crude method. In fact, the apparent systemic velocities of the He II$\lambda$4686 and H$\alpha$ absorption components are unknown and could be different from that of the He I$\lambda$4471 line, though Fig. 7 suggests that this difference should be small. In addition, HD152248 displays both ellipsoidal variations and photometric eclipses. As a consequence, the EWs of the lines are most probably varying with phase, and it is almost impossible to correctly account for this effect without the exact knowledge of the amount of light originating from the primary and from the secondary at each orbital phase. However, we used the eclipse depths from PGB to derive a first order correction for the line intensities of the absorbing components. We then measured the full width at the base of the restored emission components. To this aim, we defined the limits of the emission at a normalized intensity of 1.01, i.e. we considered the emission above 1% of the continuum. Results are displayed in Fig. 9 and show a strong phase-locked variation of the full width at the base of the emission components. Similar measurements have been carried out on the raw observed spectra to check that our restoration method does not bias the results. Only slight differences of the order of 20 kms^-1 for the start and end positions of the He II$\lambda$4686 emission component were detected. The H$\alpha$ emission component being much more intense, no significant difference could be detected for this line.

In summary, the He II$\lambda$4686 and H$\alpha$ lines behave similarly in many points. A common interpretation of the observed phase-locked modulation of both lines will be provided in Sect. 8, within the framework of a colliding wind model. However, we first need to discuss the evolutionary status of the components of HD 152248.

HD 152248: Evidence for interaction