5 Surface gravities

The method of calculating spectroscopic gravities using the standard technique of making abundances of both neutral and ionized iron lines agree was not used here due to the scarcity of usable Fe II lines in the observed wavelength range. Alternatively, surface gravities were derived for those stars with accurate parallaxes in the HIPPARCOS catalogue following a method similar to that described in Nissen et al. (1997). In short, the method relies on the basic relations

$$M_{\rm bol} = m_{\rm v} + 5 \log \pi + 5 +{\rm BC}$$ (1)

$$L = L_{\odot} \times 10^{0.4(M_{{\rm bol},\odot}-M_{\rm bol})}$$ (2)

where $M_{\rm bol}$ is the bolometric absolute magnitude, $m_{\rm v}$ is the apparent visual magnitude, $\pi$ is the trigonometrical parallax determined by HIPPARCOS (in arcseconds), BC is the bolometric correction and L is the luminosity. $L_{\odot}$ and $M_{{\rm bol},\odot}$ refer to the solar luminosity and absolute bolometric magnitude, respectively. Visual magnitudes are from SIMBAD whereas the bolometric corrections were taken from Lang (1980). A value of $M_{{\rm bol},\odot} = 4.71$ was adopted. Bolometric corrections from other authors (e.g. Bessell 1998) have been found to produce negligible differences in the derived quantities (luminosities and masses). Only stars with relative errors in parallax less than about 10% were considered to neglect bias in the estimation of the absolute magnitude (Brown et al. 1997).

The mass of the stars were calculated from its position in the $\log ~L$-log $T_{\rm eff}$diagram by interpolating in the isochrones given in Claret (1995). The mass so calculated was used as input value in the log $T_{\rm eff}$-$\log\,g$  diagram to obtain the surface gravity. A chemical composition X = 0.70, Z = 0.02 (solar abundance) was adopted. This is based on the fact that the main contribution to the overall metallicity is due to C, N, O (solar abundant in $\lambda$ Bootis stars). Furthermore, there are strong indications that the $\lambda$ Bootis phenomenon is restricted to the stellar surface (Holweger & Rentzsch-Holm 1995). The mixing length and core overshooting parameters were fixed to 1.52 and 2.0 respectively. Surfaces gravities are given in Table 2. Parallaxes, bolometric corrections, magnitudes, luminosities, masses and ages are displayed in Table 3. Age estimations for the sample stars indicate that they cover an area slightly above the Main Sequence. Surface gravities calculated using HIPPARCOS are only slightly higher than those derived using Moon & Dworetsky (1985) ( $\rm HIPP - MD85 = 0.07 \pm 0.14$). A conservative value of $\Delta$ $\log\,g$ = 0.15 dex has been estimated based on errors in the bolometric correction and in the calculated mass.

Figure 2: Comparison between observed and synthetic spectra for two stars of our sample.