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HD 174005: another binary classified as lambda Boo

We demonstrate that HD 174005, a star recently classified as belonging to the lambda Boo group, is in reality a double lined spectroscopic binary; at some phases, the observed composite spectrum may be similar to that of a single star with weak metal lines.


Introduction
In a recent paper Gray et al (2001) give a precise spectral classification of 372 late-A to early-G stars and announce the discovery of a new λ Boo star: HD 174005. The spectrum is described as that of a star with the temperature of an A7-type star but with the K line and metallic line strengths of an A2-type star.
The spectra of the A-F type stars classified as λ Boo are characterized by weak lines of most metallic species (with the possible exception of C,N,O and S); the interpretation of the phenomenon responsible for these anomalous atmospheric abundances is still controversial in spite of the large efforts made to define the spectral peculiarities common in these stars and to interpret the position of these stars in the HR diagram in terms of their evolutionary stage.
The non homogeneous properties of λ Boo stars have been confirmed by the analyses extended to spectral ranges other than the classical optical region. For example the IRAS data made it possible to detect the presence of an IR excess, interpreted as the signature of the presence of circumstellar matter around some of these stars. A broad absorption feature centred at 1600Å has been detected in some λ Boo stars by studying their IUE spectra; this feature, due to a Lyman α satellite, is present in white dwarfs and FHB stars too and its detectability in λ Boo stars is made possible, in the current opinion, by the lower opacity sources of λ Boo stars compared to "normal" A-type stars. These are the main, but not the only, heterogeneous properties of this class of stars.
A detailed inspection of the global situation prompted us to inspect an alternative explanation of the so called λ Boo phenomenon, that of the duplicity of these stars and we found several observational facts to support this new hypothesis as described in Faraggiana & Bonifacio (1999) and in Faraggiana et al (2001). In fact a composite spectrum, not detected as such, can be easily confused with that of a single metal deficient star. Several catalogues which provide λ Boo classifications are in fact contaminated by unrecognized binaries and the abundance analyses of such objects, treated as single sources, are meaningless. The purpose of our study is to detect the binary stars considered up to now as single peculiar objects.
HD 174005 represents a good example to confirm our hypothesis. We discuss in the next sections the photometric and spectroscopic characteristics of this object.  Gerbaldi et al (1999). These spectra have been been already analysed for the measure of stellar radial velocity by Grenier et al (1999) and the duplicity of this star was then detected.

Observations
The two spectra have been obtained at significantly different phases; the duplicity is obvious in the first one, taken in 1992, while it is not so obvious in the 1993 observation (see Fig. 1). A large difference in the metal line profiles is easily noticed when these two spectra are compared to each other and can be better quantified by comparing the observations with theoretical predictions.
We looked for more observations from the bibliography. We were able to find two contradictory stellar classifications: the recent one by Gray et al. (2001) referring to a metal weak object and the one by Abt (1988) who classifies HD 174005 as an Am star.
The spectra by Gray et al (2001) on which the classification of HD 174005 as a classic λ Boo star is based, were obtained with a resolution of 1.8Å and cover the wavelength region from 3800 to 4600Å; the number of spectra collected by these authors is not specified. The spectra by Abt (1988) have a resolution of 1.0Å . In a previous paper, Abt (1985) classified this star as an A2 II, with spectra of 2.6Å resolution.
According to our interpretation, these different classifications are likely to be the consequence of the spectroscopic variability of this object.

Photometry
The information we could retrieve from the literature on this star are quite scanty; the star has been observed photometrically only by Oblak E. (1978) in uvbyβ (two observations) and by Hall & Mallama (1974) and by Eggen (1968)  physically linked. However since the parallax has not been measured for HD 174005B a doubt remains on the physical association of these two objects.
From the uvbyβ colour indices we derived the interstellar absorption by using the Moon (1985) code. The resulting E(b-y)=0.050 is slightly larger than that obtained for HD 174005 B by the same procedure: E(b-y)=0.030. By assuming the hypothesis that the two stars are physically associated, i.e. have the same distance, the latter E(b-y) value is coherent with the results of the analysis by Vergely et al (1998) of the interstellar extinction in the solar neighbourhood. has been used to derive an independent reddening value.

UV data
The extinction E(B-V)=0.152 so obtained is much higher than that derived from the visual uvbyβ photometry, E(B-V)=0.069. We shall discuss these UV data in the next section with theoretical computations of the UV fluxes. HD174005a and HD 174005b refer to spectra taken on June 9, 1992 and on Sept. 7, 1993 respectively.

Atmospheric parameters
The atmospheric parameters derived from the uvbyβ colour indices by using the Moon and Dworetsky (1985) code and adopting the previously derived reddening, E(by)=0.050, are T eff =7910 K and log=3.79.
We have also used the Geneva photometry indices to compute T eff and log g, by adopting the same reddening value and by using the Künzli et al (1997)

HR diagram
These data allow us to put the star in the HR diagram. For that purpose we used the evolutionary tracks computed by Schaller et al (1992)

Computed spectra
We have computed a synthetic spectrum by using the Kurucz model with T eff =8000 K, log g=4.0 and solar abundances to investigate the correlation with the observations. The normalized cross correlation indices between the two observed spectra and the theoretical one are displayed in Fig. 1. The two peaks obtained for the 1992 spectrum are evident, while only a broad and asymmetric cross correlation curve results from the 1993 data; however we note the low value of the normalized correlation peak even at this date.
A further correlation (Fig. 2) of only the metallic lines with as template the spectrum computed by using the model with the Geneva atmospheric parameters shows even better the two peaks for the 1992 spectrum and allows to measure the differential radial velocity As a further sign of duplicity we recall that the observed core of Balmer lines should be deeper than that in the spectrum computed by using Kurucz SYNTHE code which does not include NLTE effects. In the contrary, the spectrum taken in 1992 has the H γ core shallower than or as deep as the computed ones. In Fig. 4 this observed profile is compared with those computed by using the models with the parameters derived in section 3.1. The best fit of H γ is obtained by using the parameters T eff =7673 K, log g=3.98 and solar abundances; however this choice does not reproduce the metallic lines as may be seen in Fig. 5.
The broad and apparently weak metal lines in the 1992 spectrum are given in Fig.   5 where are compared with two computed spectra (T eff =7673 K, log g=3.98), one with solar abundances and the other one with these reduced by 10.; we note in particular the non-rotationally broadened profile of Mg II 4481 which is characterized by a very flat and square core in the 1992 spectrum.
Having only two spectra at our disposal and taken at more than one year of interval, we cannot make any guess on the possible period of this system.

UV photometry
We have compared the S2/68 TD1 observations, dereddened by using E (B-V)

Conclusions
We have shown that HD 174005 is a spectroscopic binary and the observed spectrum is highly variable with the phase. The confusion between the spectrum of a single weak lined star and the composite spectrum of a binary composed by two similar, but not equal sources, is demonstrated.
A careful check of high dispersion data and the analysis based on more than a single spectrum appear to be necessary to obtain reliable abundance analyses of λ Boo candidates; as a first step it is necessary to distinguish composite spectra from those of single weak-lined stars and this can require a quite subtle analysis of mean-high resolution spectra.