4. Effective temperatures and metallic abundances

It is well established that the Balmer lines are good temperature indicators for because of their small gravity and metallicity dependence (Smalley & Dworetsky 1993; Furhmann et al. 1994). In this work, effective temperatures for both non-variable and RR Lyrae stars have been obtained from the comparison, using a least-squares fitting technique, between the observed profile and a grid of synthetic profiles using ATLAS8 (Kurucz 1979). The gravity and metallicity were fixed to and [M/H] = -1.0 for the RR Lyrae stars and to and [M/H] = 0.0 for the sample of non-variable stars. Effective temperatures of the observed stars are given in Tables 3 (click here), 4 (click here) and 5 (click here).

It has been pointed out that for stars cooler than 8500 K, convection is starting to become efficient enough to modify the temperature gradient and hence the Balmer profiles with the exception of (van't Veer-Menneret & Megessier 1996). However, we have not found any evidence for this. Comparing our effective temperatures derived from and those obtained from the literature for the non-variable stars shows a mean difference and a standard deviation of . Moreover, previous comparisons between effective temperatures derived from from the Infrared Flux method and spectrophotometric methods (Solano & Fernley 1997) did not show systematic differences either.

The crowding of the observed spectral region made the line selection difficult and only a small number of Fe I lines were selected for the abundance calculation (Table 6 (click here)): weak lines (with associated large errors due to the moderate S/N ratio of our observations) and strong lines (sensitive to saturation effects) were not considered. The relatively low signal-to-noise together with the crowded spectral region also cause an uncertainty in the measured equivalent width. An estimation of this uncertainty was obtained by comparing the equivalent widths of lines of non-variable stars with two or more observations. This gave a typical error of 7%. For the RR Lyrae stars, which generally have lower S/N, the error in EW is higher and we estimate 10%.

Synthetic equivalent widths were calculated for a grid of temperatures and metallicities using the spectrum synthesis code XLINOP and the ATLAS8 models (Kurucz 1979). Log gf values were taken from Thévenin (1989) (Table 6 (click here)). The abundance values were calculated by computing the sum of squares of the differences between the observed equivalent widths and the synthetic equivalent widths for five values of metallicity. The sum of squares were then plotted against the metallicities and a parabola was fitted, the adopted metallicity being the minimum of the parabola. In order to check the internal consistency of the whole process we have calculated the abundances of Procyon (HR 2943) and some other standard stars. A microturbulence of 2.0 and a surface gravity of were used for all the stars. The results are given in Table 3 (click here). Comparing these values and those obtained from the literature shows good agreement (mean difference and a standard deviation of ).

The microturbulence velocity has been set to (Lambert et al. 1996) for most of the RR Lyrae stars since the scattered in the abundances derived from the lines was too large or the number of lines used too small to distinguish any systematic differences in the abundances derived from weak and strong lines. Also, following Fernley & Barnes (1996), a value of was adopted for all the RR Lyrae stars.

The derived abundances from the individual RR Lyrae spectra are listed in Tables 4 (click here), 5 (click here). For stars with two or more abundance determinations it can be seen that there is good agreement, the differences are typically . The final abundance for each RR Lyrae star is listed in the Appendix. Also listed are values from the literature. The two sets of values are compared in Fig. 1 (click here) where it can be seen that, with the exception of XZ Aps, there is reasonable agreement (rms of the difference 0.18 dex). Since the literature values have a typical error of , this rms difference implies a similar error in our work although there is some suggestion that our metallicities are systematically higher (mean difference 0.07 dex). The sensitivity of the derived abundances on the atmospheric parameters has also been studied. Errors of and have been assumed for effective temperatures and microturbulence velocities which produce errors of and respectively. The influence of the errors in on the derived abundances is negligible.

From Tables 4 (click here), 5 (click here) we can see that stars with abundances calculated at phases out of the minimum light interval show good agreement with the values calculated at minimum light even if at these phases the stellar atmosphere may suffer from fast acceleration which produces large ranges in the physical parameters in a very short time-scale. Clementini et al. (1995) also found similar results. The explanation of this is related to the fact that the steepest variations in a -phase diagram only occurs in the interval (Fernley et al. 1989). In the rest of the phase interval the variation of is such that, if the ephemeris are accurate enough, the temperature can be determined with a small error. This is also valid for the variations in gravity (e.g. Liu & Janes 1990) with the additional advantage of the little influence of on the abundance determination.

 

-1.570 28604 . 605

-0.990 32873 . 617

-3.180 7728 . 056

-0.590 27394 . 688

-1.500 27394 . 688

-1.040 22838 . 318

-0.640 27166 . 818

-0.680 27166 . 818

-0.660 19757 . 033

-1.320 27666 . 346

-2.410 29356 . 740

-0.650 19562 . 439

-0.720 26874 . 548

-1.710 24335 . 760

-0.920 27394 . 688

-2.190 23783 . 613

-1.550 22946 . 809

-0.720 28819 . 945

-1.650 24118 . 814

-0.980 19757 . 033

-0.350 19912 . 494

-1.040 12560 . 930

-2.040 24772 . 018

-2.530 31686 .

-0.250 12560 . .

Only used in metal-poor RR Lyraes (saturation effects may occur in metal-rich RR Lyraes).  

  
Figure 1: A comparison between the [Fe/H] values of the present work and previous work