Astron. Astrophys. 362, 666-672 (2000)

3. Stellar parameters

Neither star has been widely studied in the literature. Ryan et al. (1991, 1999) performed an extensive chemical abundance analysis and derived the following stellar parameters:  =   (an average value of several colour-temperature calibrations),  = 4.0 dex (based on the ionization balance of FeI and FeII lines), [Fe/H] = -3.05 dex (assuming = 7.50),  = 2.0 km s^-1 for LP 815-43, and  =  ,  = 4.0 dex (which in this case was assumed ), [Fe/H] = -3.55 dex,  = 1.5 km s^-1 for CD-24^o17504. From the fitting of the H wings, Spite et al. (1996) determined  = 6300  for both objects.

In order to finalize our choice of stellar parameters, we decided to take advantage of the colour information available from Ryan et al. (1999; cf. Table 2, this work). We used both Carney (1983) and King (1993)  vs. (b-y) calibrations, and derived respectively  = 6501.52  and 6527.34  in the case of LP 815-43 and  = 6187.46  and 6287.31  for CD-24^o17504. We note that both stars have recent (unpublished) JHK photometry, to which a direct application of the InfraRed Flux Method (IRFM) provides  = 6557  for LP 815-43 and  = 6373  for CD-24^o17504 (Alonso, private communication ).

Table 2. Colour information and adopted stellar parameters.

As far as the gravity is concerned, we initially assumed  = 4.0, as suggested by Ryan and collaborators. From a quick inspection of the position of our targets in the evolutionary diagram c₁ vs (b-y) (which gives information on the evolutionary status of the object) compared to the Schuster & Nissen (1989) loci used as reference, we found that gravities lower than 4.0 could be excluded (the stars fall very close to the turn-off, if not still on the main sequence). A cross-check with the isochrones of Bergbusch & VandenBerg (1992) and VandenBerg & Bell (1985) provided consistent information: gravities slightly higher than 4.0 (4.35 and 4.45 respectively) were derived when an age of 14 Gyr (although no difference was detected between 12, 14, and 16 Gyr) and the most metal-poor isochrone (which corresponds to [Fe/H] = -2.26) are assumed. These gravity values correspond to  = 6560 K (LP 815-43) and  = 6300 K (CD-24^o17504). Because beryllium is strongly dependent on the choice of gravity, we decided to further check  via the ionization balance. For this purpose, several lines of both titanium and iron in two different ionization stages (neutral and ionized) were selected between 3100 and 3800 Å. Their oscillator strengths were taken from the latest works of Martin et al. (1988) and Fuhr et al. (1988), and in the case of neutral iron were further cross-checked with the compilation of Nave et al. (1994). The accuracy given in these compilations (from A to D, i.e. from 10 to 50%) drove the final selection of the subsample of lines that were then used to check the ionization balances (no "D" line was used, and only few "C"). The first run of WIDTH9 (Kurucz 1993) was performed assuming  = 6500 ,  = 4.0, [Fe/H] = -3.0 for LP 815-43, and  = 6250 ,  = 4.0, [Fe/H] = -3.5 for CD-24^o17504 respectively. First, by requiring no dependence of the abundance on the equivalent width, the microturbulence was constrained to 1 km s^-1 and  km s^-1 for LP 815-43 and CD-24^o17504 respectively. Then, the same code was run for different values of gravity ( 0.25,  0.5) and temperature ( 250 K). The ionization balance was checked by using (10FeI , 8FeII ) and (5TiI , 13TiII ) lines for LP 815-43, and (6FeI , 7 FeII ) plus (2TiI , 13TiII ) for CD-24^o17504 (cf. Table 3, where LP and CD stands for LP 815-43 and CD-24^o17504 respectively); slightly higher values (4.25 and 4.5) were found confirming what we had derived from the isochrones. These are in good agreement, within the errors, with the values determined by Thevenin & Idiart (1999), who studied non Local Thermodynamic Equilibrium (NLTE) corrections for iron abundances, and found  = 4.39 for both stars.

Table 3. Atomic data.

Considering all the estimates of  and  thus obtained and their quite good agreement, we adopted  = 6500 K,  = 4.25, and  = 1.75 km s^-1 in the case of LP 815-43, and  = 6300 K,  = 4.5, and  = 1.0 km s^-1 for CD-24^o17504 as our final stellar parameters.

Metallicities, on the contrary, were taken from the work of Ryan et al. (1999), but corrected for the different temperature and gravity we adopted. The values thus found, [Fe/H] = -2.90 and -3.32 for LP 815-43 and CD-24^o17504 respectively, are in very good agreement with the metallicity inferred from our spectrum synthesis analysis (see next section).

By evaluating the uncertainties in the different methods followed to determine the stellar parameters, we find that 100  in ,  0.25 in , 0.15 dex in metallicity, and  0.2km s^-1 in the microturbulent velocity are representative of the uncertainty associated to each single parameter.

© European Southern Observatory (ESO) 2000

Online publication: October 24, 2000
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