4. Atmospheric parameters and abundances
The effective temperatures were derived and averaged from the intrinsic colour indices (B-V )0 and (V-K )0 using the corresponding calibrations by Gratton et al. (1996). Colour indices have been taken from Houdashelt et al. (1992) and dereddened using =0.032 according to Nissen et al. (1987) and =3.24 according to Taylor & Joner (1988). For F266 the B-V index was taken from Coleman (1982) and V-K from Taylor & Joner (1988). The agreement between the temperatures deduced from the two colour indices is quite good, the differences do not exceed 20 K. The gravities were found by forcing Fe I and Fe II to yield the same iron abundances. The microturbulent velocities were determined by forcing Fe I line abundances to be independent of the equivalent width. The derived atmospheric parameters are listed in Table 1.
Table 4. Abundances relative to hydrogen [A/H] derived from spectra of . The quoted errors, , are the standard deviations in the mean value due to the line-to-line scatter within the species. The number of lines used is indicated by n.
Table 4. (continued)
Table 5. Abundances relative to hydrogen [A/H] and derived from spectra of . The quoted errors are the standard deviations in the mean value due to the line-to-line scatter within the species. The number of lines used is indicated by n
4.1. Estimation of uncertainties
The sources of uncertainties can be divided into two categories. The first category includes the errors which act on a single line (e.g. random errors in equivalent widths, oscillator strengths), i.e. uncertainties of the line parameters. The second category includes the errors which affect all the lines together, i.e. mainly the model errors (such as errors in the effective temperature, surface gravity, microturbulent velocity, etc.). The scatter of the deduced line abundances , presented in Table 4 and 5, gives an estimate of the uncertainty coming from the random errors in the line parameters. The mean values of =0.08 and =0.13 are for abundances derived from spectra with and , accordingly. Thus the uncertainties on the derived abundances, which are the result of random errors, amount to approximately these values. There is a small systematic difference between the equivalent widths measured with the two cameras, however the abundance effect is small. Typically 0.03 dex higher abundances are obtained from the lower resolution spectra.
Typical internal error estimates for the atmospheric parameters are: K for , dex for log g and for . The sensitivity of the abundance estimates to changes in the atmospheric parameters by the assumed errors is illustrated for the star F141 (Table 6). It is seen that possible parameter errors do not affect the abundances seriously; the element-to-iron ratios, which we use in our discussion, are even less sensitive.
Table 6. Effects on derived abundances resulting from model changes for the star F141. The table entries show the effects on the logarithmic abundances relative to hydrogen, [A/H]. Note that the effects on "relative" abundances, for example [A/Fe], are often considerably smaller than abundances relative to hydrogen, [A/H]
Since abundances of C, N and O are bound together by the molecular equilibrium in the stellar atmosphere, we have investigated also how an error in one of them effect the abundance determination of an other. The causes and , the causes and . The has no effect on either the carbon nor the oxygen abundances.
Other sources of observational errors, such as continuum placement or background subtraction problems are partly included in the equivalent width uncertainties discussed at the beginning of this section.
© European Southern Observatory (ESO) 2000
Online publication: August 17, 2000