6 Search for reference stars

Using the criteria listed above, we extend a similar analysis to all the sample stars. The results are summarized in Table 3.

  

Table 3: Summary of the spectral analysis

From this analysis it appears that a large percentage of our sample stars (being either binaries or peculiar) cannot be used to build a set of reference stars.

6.1 Newly detected binaries

For 13 stars the cross correlation curve has a highly distorted shape (see, for example, the cross correlations of HD 36473 and HD 56341 in Figs. 7a and b). In all the spectra of these stars the line profiles are distorted; details on each star are summarized in Table 3 and in the Appendix.

These stars are:

HD 16152, HD 21473, HD 22789, HD 30397, HD 36473, HD 56341, HD 104430, HD 106797, HD 111519, HD 111786, HD 113852, HD 139129, HD 151527.

  

Figure 7: The distorted cross correlations of HD 36473 a) and of HD 56341 b) indicate the presence of a secondary shifted source. (1 pixel = 0.05 Å)

HD 111786 has been discussed by Faraggiana et al. (1997) and its highly distorted UV colours are plotted in Fig. 8b.

  

Figure 8: The observed UV colour indices $m_{\rm UV}-V$ dereddened according to the E(b-y) given in Table 2 and compared with those computed with the MD atmospheric parameters for a) HD 109573, b) HD 111786 and c) HD 151527 where the filled triangle refer to E(b-y)=0.05 and filled square to E(b-y)=0.19. The computed values are indicated with (+). (see comments on HD 109573 in the Appendix)

HD 139129 is the only star for which spectroscopic informations are found in the literature (see Appendix); the peculiar abundances, derived by Lemke (1989, 1990), strengthen our binary interpretation.

HD 151527 is an abnormally reddened star; the choice of the Strömgren group according to the lowest value of the colour excess produces atmospheric parameters in conflict with observations; from E(b-y)=0.19 the derived atmospheric parameters produce a synthetic spectrum which agrees with observed H$_\gamma$, but not with the metal lines spectrum. We note also the low value of the derived logg value, in disagreement with its MK classification. The distorted UV colours of this star are plotted Fig. 8c; this star and HD 111786 are the only two objects for which a large discrepancy between observed and computed UV colours is found.

According to the Hipparcos and Tycho Catalogues (ESA, 1997) HD 111519, HD 113852 and HD 151527 are probable astrometric binaries, due to the flag in field 59 of the Hipparcos Catalogue.

6.2 Suspected binaries

For other stars, even if the cross correlation is not as distorted as for the previous ones, the spectral peculiarities and the distorted line profiles suggest a binary nature of the observed objects or some peculiarity.

The 9 stars suspected to be binaries on the basis of the visual and UV data are: HD 15646, HD 20980, HD 71043, HD 79108 HD 84461, HD 114570, HD 129791 HD 188228 and HD 193571. Details on them can be found in Table 3 and in the Appendix.

We note that HD 188228 belongs to the system of standard stars for rotational velocity determinations defined by Slettebak et al. (1975); the $v\,\sin\,i$ value derived by Levato (1972) differs considerably from that by Slettebak et al.

For HD 129791, in the Hipparcos Catalogue (ESA, 1997), a note in the field 59 indicates that it is probably an astrometric binary.

6.3 Remarks from Hipparcos astrometric data

In the Hipparcos Catalogue (ESA, 1997) besides the stars for which a companion is observed and its motion described, there are several stars from Table 1 considered as probable astrometric binaries on the basis of their astrometric solution.

They are: HD 4150, HD 42301, HD 63584, HD 87887, HD 92845, HD 109573 HD 111519, HD 113852, HD 129791, HD 151527, HD 223352.

Among these 11 stars only 4 present spectral peculiarities indicating a possible binarity. We cannot expect to have a one to one coincidence because the detection of binarity indices in the spectra at our disposal requires a small difference in magnitude and separation, and a difference in radial velocity if the two components have similar rotational velocities.

The other peculiar objects are:

6.4 Shell stars

HD 225200 is a shell star; its spectrum is similar to that of the already known shell star HD 15004. The hypothesis of binarity for HD 225200, as suggested by AM, must be rejected because the sharp components are only present on the lines expected in shell stars: the core of H$_\gamma$, all the strong lines of the Fe II Multiplet 27 and the Ti II 4468.

6.5 A misclassified star

HD 216931 has been misclassified as A0, according to our two spectra; these two spectra refer to an object much cooler than an A0 star. The parameters given in Table 2 have been derived from the Geneva photometry, neglecting any colour excess, in spite of the E(B-V)=0.06. The whole spectrum (and in particular the H$_\gamma$ profile) is not reproduced by that computed with the above parameters. The H$_\gamma$ profile is fitted by a spectrum computed with a temperature 500 K higher than the predicted one, (so that the neglected reddening can be at the origin) i.e. by the spectrum computed with $T_{\mathrm{eff}} =8000$ K and logg =4.0, values which produce a fairly good agreement also between observed and computed UV colours. However several metal lines are poorly fitted by the synthetic spectrum computed with these parameters; the ratio SrII4215/ScII4247 of the observed spectrum is much higher than the computed one, as in Am stars; several stronger than computed features are present on the two observed spectra. The Mg II 4481 is distorted and very broad compared to other metal lines.

The presence of a companion which cannot be identified with the known B and C components of this visual triple system (ADS 16392) is reported by Richichi et al. (1997) from lunar occultations observed in the near infrared and must be at the origin of the observed peculiarities and of the discrepancy between our two spectra and the MK classification.