5 Spectroscopic behaviour of double stars

According to the values given Table 1 Cols. 5 and 6 we can suspect that the photometric indices and consequently the derived atmospheric parameters, are affected by a companion for 10 stars. The $\Delta m$ and the angular separation is such that the spectra of five of these binaries must be composite: HD 3003, HD 4065, HD 47827, HD 104039, HD 152849.

According to our analysis of the spectra we note:

$\bullet$ HD 3003: The correlation curve is not symmetric and the Mg II 4481 is not rotationally broadened (Figs. 2a and b).

  

Figure 2: a) the cross correlation between the observed spectrum of HD 3003 and the computed one (1 pixel = 0.05 Å). b) the Mg II 4481 line (thick line) overplotted on the computed one (thin line)

$\bullet$ HD 4065: The profile of H$_\gamma$ is not fitted by the computed ones with MD or Geneva parameters and the core of the line is considerably shallower than the computed ones (Figs. 3a and b); the Mg II 4481 has a very peculiar profile and the metal lines are poorly reproduced by computations (Fig. 3c).

  

Figure 3: a) the observed H$_\gamma$ (thick line) of HD 4065 overplotted on that computed with MD parameters. b) the same, but with Geneva parameters. c) the Mg II 4481 region compared with the computed spectrum (at $\lambda$4470 a defect of the CCD is present)

$\bullet$ HD 47827: The Mg II profile has broad wings as in the previous star; H$_\gamma$ is slightly asymmetric and flatter than the computed one and the metallic lines are too weak for a non peculiar star.

$\bullet$ HD 104039: The duplicity is dubious according to the Remarks in the BSC. This star has E(b-y)=0.03 as HD 7916 (see below). H$_\gamma$ does not fit the synthetic spectrum computed with the MD parameters taking into account the reddening; the metal lines are stronger than the computed ones. A better fit is obtained with the spectrum computed with the parameters corresponding to no colour excess, but the metal lines intensities do not agree even with these parameters; also the TD1 colour indices fit better those computed with no colour excess.

$\bullet$ HD 152849: The spectrum is of low S/N quality and the line profiles cannot be as well defined as in most of the other spectra but the profile of Mg II 4481 is non rotationally broadened (Fig. 4) and we have checked that this conclusion does not rely upon the position of the continuum.

  

Figure 4: The Mg II 4481 region of HD 152849 (thick line) compared with the computed spectrum (thin line); the narrow features at $\lambda$$\lambda$4452, 4460 and 4473 correspond to defects of the CCD

We conclude that, at the resolution of our spectra, the contamination by a companion star can be detected by at least one of the criteria we have chosen:

a) non symmetry of the cross correlation curve (see Fig. 2a)

b) peculiar profile of Mg II 4481 line (see Figs. 2b, 3c, 4)

c) distorted line profiles and/or flat H$_\gamma$ core (see Fig. 3a).

In this temperature range the H$_\gamma$ line is so strong that the cross correlation is dominated by its profile; if the combined profile of a binary star can be reproduced by a model, the duplicity detection is confined to the behaviour of the metal lines.

For the other 5 binaries, HD 7916, HD 17864, HD 34968, HD 99922, HD 213320, the $\Delta m$ and/or their separation are such that, a priori, their spectra will not be affected by the companion. Nevertheless, we found some peculiarities in their spectra.

$\bullet$ For HD 7916 the Mg II 4481 is broader than the other metal lines and we found indications of duplicity in the line profiles; the profiles of H$_\gamma$and Mg II 4481 (Figs. 5a and b) indicate the presence of a secondary companion on the short wavelength side and these peculiarities cannot be due to the visual companion which has a too faint magnitude according to the $\Delta m$ given in Table 1; however a lower magnitude difference, $\Delta V=1.95$ has been measured by the Hipparcos experiment in good agreement with the value (1.84), estimated by Corbally (1984). We note that HD 7916 is one of the most reddened stars; we interpret this colour excess as an ad hoc quantity needed to reproduce the colours of a single normal star.

$\bullet$ For HD 17864 The Mg II profile is peculiar: flat and its broadening does not correspond to a rotational broadening. The H$_\gamma$ profile is asymmetric with a too flat core.

  

Figure 5: The observed and computed spectra of HD 7916 in the regions of H$_\gamma$ (Fig. 5a) and Mg II 4481 (Fig. 5b); sharp and narrow absorptions are due to bad CCD columns

$\bullet$ For HD 99922 the magnitude difference and the separation are too large to affect our spectroscopic data; unfortunately, the Mg II 4481 region is not available on our spectrum. Neverthless we note that the other metal lines are abnormally shallow and weak and strongly suggest a "veiling" effect (Fig. 6), according to the definition introduced by Corbally (1987). We interpret these peculiarities as due to a source which cannot be identified with the known companion.

  

Figure 6: The observed (thick line) and the computed (thin line) spectra of HD 99922 shown a pronounced "veiling" effect on the metal lines

$\bullet$ In the spectrum of HD 34968 only a slight asymmetry of the MgII line profile is detected.

$\bullet$ For HD 213320 the H$_\gamma$ profile is very well reproduced by computations; in spite of this, the metallic lines present distorted profiles and their intensity is poorly reproduced by the synthetic spectrum.

So we note that unexpectedly distorted line profiles are found also among these 5 stars.