Appendix A: Details on the derivation of photospheric parameters for the main target Star 6

In this Appendix, we describe the steps undertaken to derive the photospheric parameters for Star 6 from its UVES spectrum. Initially, the equivalent widths of 228 lines of Fe  I were measured throughout a wavelength range of 4786 - 6752 Å, corresponding to the highest signal-to-noise ratio spectral segments which were relatively unaffected by telluric lines. These lines yielded a heliocentric radial velocity for Star 6 of $+52.4 \pm 0.6$ km s^-1.

   

Table A.1: Elemental abundances for the Star 6 photosphere, relative to the solar abundance. Adopted solar values are given in the last column. N is the number of lines used for each species. For clarity, elements are divided into $\alpha$elements, Fe group elements, and others. Sources for gf-values are indicated.

Element	[X/H]/dex	N	[X/H]$_{\odot}$a
Tib	-0.25	50	4.99
Sic	-0.12	12	7.55
Mgd	-0.19	4	7.58
Cae	-0.38	31	6.36
Of	-0.01	4	8.93
Feg	-0.39	214	7.55
Coh	-0.17	21	4.92
Cri	-0.47	45	5.68
Nic	-0.39	21	6.25
Nac	-0.24	3	6.32
Cj	-0.22	7	8.57

^a Anders & Grevesse (1989), ^b Grevesse et al. (1989), ^c gf-values derived from solar equivalent widths using abundances of (a), ^d Thévenin (1989), Thévenin (1990), ^e Smith & Raggett (1981), ^f Biémont et al. (1991b), ^g see Refs. in text, ^h Cardon et al. (1982), ⁱ Blackwell et al. (1984), Blackwell et al. (1986b), ^j Biémont et al. (1993).

The equivalent widths of 88 relatively weak ($\leq$60 mÅ) lines were used to obtain a first estimate of $T_{\rm eff}$ and simultaneously, metallicity. By minimising the gradient of a plot of log [Fe] vs. the excitation potential $\chi$ (using the log gf values of O'Brian et al. 1991; Bard et al. 1991; Blackwell et al. 1986a and references therein), values of $T_{\rm eff} = 5650 \pm 50$ K and $\rm [Fe/H] = -0.36$ dex relative to solar, were obtained.

Using these values of $T_{\rm eff}$ and [Fe/H], the equivalent widths of 214 Fe  I lines (including the 88 above) were used to derive the microturbulent velocity $V_{\rm turb}$, by minimising the gradient of a plot of log [Fe] vs. log ( $W_{\lambda}/\lambda$). A best value of 1.3 km s^-1was obtained, together with a second metallicity estimate of $\rm [Fe/H] = -0.41$ dex, relative to solar.

The third step was to use the equivalent widths of Fe  II lines, and the log gf values of Biémont et al. (1991a), to derive log g by varying log g until [Fe/H] converged on the value yielded by the Fe  I lines. Using 28 lines, a value of $\log~g = 4.3 \pm 0.1$ was found, with $\rm [Fe/H] = -0.39$ dex.

With metallicity values converging on $\rm [Fe/H] = -0.4$ dex relative to solar, a final model of $T_{\rm eff} = 5650$ K, $\log~g=4.3$ (cgs), $V_{\rm turb} = 1.3$ km s^-1 and $\rm [Fe/H] = -0.4$ dex has been adopted. Using this model, abundances for other elements were derived and are given in Table A.1. With the exception of oxygen, all derived abundances lie in the range -0.1 -0.5 dex relative to solar, with the $\alpha$ elements perhaps being less underabundant than the Fe group elements. The oxygen abundance is obtained from only four lines, three of which are the O  I triplet at 7772, 7774 and 7775 Å, which are known to yield higher abundances than other O  I lines (Nissen & Edvardsson 1992).

A similar analysis was performed to derive the photospheric parameters for Star C, to obtain a spectroscopic distance estimate. Owing to the poorer quality of the spectrum, no individual elemental abundances were derived. From 82 Fe  I lines, $T_{\rm eff}$ in the range 6200-6250 K was derived, with $V_{\rm turb} = 2.1 {-} 2.2$ km s^-1 obtained using a further 78 stronger lines. The surface gravity was not well constrained (using 21 Fe  II lines) and is certainly >4.5 and possibly as high as 5.0. A metallicity [Fe/H] of $-0.4\pm0.1$ dex (relative to solar) has been adopted.