next previous
Up: The evolved early-type binary revisited


Subsections

5 Line profile variability

5.1 He II $\lambda $ 4686

The He II $\lambda $ 4686 line appears as a strong emission with a nearly symmetric profile, the blue wing being only slightly steeper than the red wing. The full width at half maximum of the line amounts to about 7 Å (i.e. $\sim$450 km s-1).

This line displays no significant phase-locked line profile variations contrary to what is observed in several other interacting wind Of binary systems (e.g. Rauw et al. 1999; Sana et al. 2001). There are however some intensity variations. We have measured the equivalent width of the line integrated between 4665 and 4705 Å. There is a large scatter in the data points, without any clear dependence on the orbital phase (though the lowest EWs correspond to the two conjunction phases). The mean EW of the He II $\lambda $ 4686 emission is -5.52 Å with variations between -4.36 and -5.99 Å (see Table 2). Despite this large peak to peak variation, the $1~\sigma$ dispersion on the mean EW is only 0.32 Å, i.e. 6%. This result is in good agreement with the assertion of Massey & Conti (1977) that the EW of this line does not show variations of more than 15%.

Though the emission peak follows the orbital motion of the secondary, there could be higher order variations in the line profile reflecting an interaction with the primary. Unveiling such variations requires an analysis of the entire profile. For this purpose, we have applied a Doppler tomography technique to map the line formation region of the He II $\lambda $ 4686 emission line in velocity space (see e.g. Thaller et al. 2001 for an example of the application of this method to early-type binaries). Our method uses a Fourier filtered back projection algorithm (Horne 1991). The radial velocity of any gas flow that is stationary in the rotating frame of reference of the binary can be described by a so-called "S-wave'' relation:

 \begin{displaymath}%
v(\phi) = -v_x~\cos{(2~\pi~\phi)} + v_y~\sin{(2~\pi~\phi)} + v_z
\end{displaymath} (1)

$\phi$ stands for the orbital phase, whereas (vx, vy, vz) are the velocity coordinates of the gas flow. The S-wave relation assumes that the x-axis runs between the stars, from the primary to the secondary, whilst the positive y-axis points in the direction of the secondary's orbital motion. Finally, the z-axis is perpendicular to the orbital plane. The Doppler map consists of a projection of the S-wave relation on the (vx, vy) plane. Therefore, each pixel in a Doppler map, specified by its velocity coordinates is associated with a particular S-wave (see e.g. Horne 1991 for a detailed discussion of the method).

Our data were weighted so as to account for the uneven sampling of the orbital cycle. The resulting Doppler map (Fig. 5) reveals an extended line formation region centered on the secondary without any obvious trace of a structure that could be attributed to a wind interaction or a Roche lobe overflow. Therefore, it seems likely that the bulk of the He II $\lambda $ 4686 emission arises in a relatively unperturbed part of the stellar wind of the secondary.


  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{MS2665fig5.ps}
\end{figure} Figure 5: Doppler map of the He II $\lambda $ 4686 emission line in the spectrum of HDE 228766. The crosses correspond to the RVs of the center of mass of the primary and secondary as derived from the He II $\lambda $ 4542 absorption and N III $\lambda $ 4634 emission respectively. The shape of the Roche lobe in velocity-space (thick dashed line) has been calculated for a mass ratio of 1.67. The black dot and the black square correspond to the RV curves of the secondary's He II $\lambda $ 4542 absorption and the peak of the He II $\lambda $ 4686 emission line respectively. The Doppler map was computed with vz set to 52 km s-1.

5.2 H${\beta }$


  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{MS2665fig6.ps}
\end{figure} Figure 6: Normalized profile of the H${\beta }$ line in the spectrum of HDE 228766 as observed in July and August 1999. The various spectra have been binned into phase intervals of 0.1. The broad absorption around 4880 is due to a diffuse interstellar band.

The profile variations of the H${\beta }$ line are clearly phase-locked and stable over time scales of at least two years. The line displays a weak P-Cygni emission component between phases $\phi = 0.0$ and $\phi = 0.5$, i.e. when the lines of the secondary are red-shifted (see Fig. 6). The emission component vanishes when the secondary's lines are blue-shifted. This situation strongly suggests that the emission component is associated with the secondary star and that the bulk of the profile variations result from blending effects with the primary's absorption line.

5.3 H${\alpha }$

Figure 7 reveals H${\alpha }$ emission consisting of at least two components. The stronger component roughly follows the orbital motion of the secondary whilst the weaker component moves roughly in anti-phase. The two emission components are best seen at quadrature phases.

MC77 reported the presence of a blue-shifted absorption feature in the H${\alpha }$ line near $\phi \sim 0.3$. Although we do not observe an absorption feature that goes below the continuum level, we note that the double-peaked structure of the line around $\phi \sim 0.3$ could be interpreted as a blend of a blue-shifted absorption (associated with the primary) with a red-shifted emission (belonging to the secondary). However, in this case, we would expect the H${\alpha }$ profile to display a double-peaked profile with a roughly central absorption at conjunction phases, which is not observed. Therefore, it seems more likely that the weaker component seen near quadrature corresponds to a genuine emission component that moves independently of the stronger component.

The Doppler map of this line (Fig. 8) reveals a more complex structure than in the case of He II $\lambda $ 4686. The bulk of the emission still follows the secondary star though with a significantly larger RV amplitude. There is a secondary "peak'' in the map associated with the weaker emission component seen in the spectra. This structure does not seem to belong to any of the stars. The equivalent width of this feature usually corresponds to less than 15% of the EW of the total H${\alpha }$ emission. The remaining $\sim$85% of the flux follow the motion of the secondary star.


  \begin{figure}
\par\includegraphics[width=7cm,clip]{MS2665fig7.ps}
\end{figure} Figure 7: Montage of the H${\alpha }$ line profile variability. All data are from the September 2000 campaign apart from the spectra at $\phi = 0.418$ (Sep. 2001) and $\phi = 0.519, 0.611$ (Oct. 2001).


  \begin{figure}
\par\includegraphics[width=8.8cm,clip]{MS2665fig8.ps}
\end{figure} Figure 8: Same as Fig. 5, but for the H${\alpha }$ emission with vz set to -8.3 km s-1. The symbols have the same meaning as before except for the filled square and the filled triangle that stand for the S-waves of the dominant peak (filled square) and the secondary peak (filled triangle) of the H${\alpha }$ profile.

Finally, we have also carried out a simple S-wave analysis of the RVs of the peaks of the various emission lines in the spectrum of HDE 228766, i.e. we fitted the RVs of the peaks following Eq. (1). The results are given in Table 4. We note that the weaker H${\alpha }$ component is the only spectral feature that presents a significant vx component and has a negative vy. All other lines investigated in Table 4 display an orbital motion consistent with line formation in the wind of the secondary.

 

 
Table 4: Summary of the S-wave analysis of the RVs of the peaks of various emission features in the spectrum of HDE 228766. The absorption components of the N V P-Cygni profiles are also included. The (vx, vy, vz) velocity components and the mean ${\rm \vert O-C\vert}$ residuals are given in km s-1.
Line vx vy vz $\overline{\rm \vert O-C\vert}$
H${\alpha }$ (strong) -29.1 292.2 -8.3 23.2
H${\alpha }$ (weak) -225.3 -426.3 -51.2 60.3
He II $\lambda $ 4686 -1.7 244.5 52.0 9.1
III $\lambda $ 4511 16.1 224.6 0.4 8.3
III $\lambda $ 4515 10.3 220.1 0.0 10.9
III $\lambda $ 4634 3.8 224.9 -10.1 5.1
III $\lambda $ 4641 -2.3 222.1 -22.4 11.5
V $\lambda $ 4604 (A) 4.0 226.5 -37.6 9.1
V $\lambda $ 4604 (E) 5.8 222.8 165.8 9.9
V $\lambda $ 4620 (A) 3.2 223.3 -12.6 11.4



next previous
Up: The evolved early-type binary revisited

Copyright ESO 2002