Astron. Astrophys. 319, 593-606 (1997)

4. Atmospheric parameters

The appropriate model-atmosphere for the sample stars is specified by ,   and metallicity.

Ten stars out of our 14 star sample have been studied by Fuhrmann et al (1994) who determined   from the Balmer profiles of a large sample of G-dwarfs. Surface gravities and spectroscopic metallicities from Axer et al (1994) are given in column 7 and 10 of Table 2. The Axer et al gravities and metallicities are spectroscopically derived assuming the Fuhrmann et al (1994) temperature, thus forming a set of homogeneously derived stellar parameters.

Table 2. Atmospheric parameters for the program stars

A photometric estimate of the effective temperature may be derived from the index, which is almost independent of gravity between 5750 K and 6250 K. For each star we interpolated our UBV grids for an assumed metallicity and gravity to obtain the corresponding to the observed . For the reddened stars, we dereddened the observed indices by using the relation from Crawford & Mandwewala (1976), and the value derived below. These photometric temperatures may be found in column 2 of Table 2.

For a sample of seven subdwarfs, Castelli, Gratton and Kurucz (1996) showed that   derived from profiles computed with models similar to those used in this paper (namely ATLAS9 models with the overshooting option switched off) may differ from 10 K to 200 K from those derived by Axer et al. (1994). The differences between the photometric determinations of   based on our models and the effective temperatures adopted for computing the Be abundances are of the same order, except for HD 160617, HD 200654, HD 166913, and HD 106516 for which the differences are 480 K, 314 K, 258 K, and 216 K respectively. For HD 160617 and HD 166913 Magain (1989) derived   equal to 5910 K and 6030 K respectively. The above differences would reduce to 234 K and 183 K respectively.

We derived also a photometric surface gravity for the stars with available Strömgren photometry. The synthetic Strömgren indices have been computed from our grid of fluxes with enhancement and no overshooting, as described in the previous section. The use of Kurucz (1993) synthetic colours yields log g being generally 0.1 to 0.2 dex lower than our estimates.

The observed c1 indices were taken from the electronic version of the Hauck & Mermilliod (1990) catalogue supported at CDS and corrected for reddening using the intrinsic colour calibration of Schuster & Nissen (1989), which is most appropriate for late-F and G-type metal-poor stars. For each star we iterated their equation (1) until the change in was less than 0.0001 mag. The derived reddening is always very small, as expected since our stars are nearby, yet non-zero in a few cases. For those stars for which   has small negative values we imposed = 0.000. Also for the stars with 2.55, which lie outside the range of the calibration, we assumed zero reddening.

The surface gravities were derived by comparing the theoretical and observed c0 for an assumed effective temperature and metallicity. For the reddened stars c0 was dereddened by means of the relation (Crawford 1975).

Column 4 of Table 2 gives the surface gravities derived from the photometric temperatures of column 2 and photometric metallicities of column 8. In columns 5 and 6 the Fuhrmann et al temperature is assumed instead, but while in column 5 the photometric metallicity of column 8 is assumed, in column 6 the "best" literature value of column 9 is taken. To be brief, errors on the photometrically derived log g are given only in column 6, the others being comparable.

We derived photometric metallicities for the stars with available Strömgren photometry (i.e. all except HD 218502) derived from Schuster & Nissen's (1989) calibration. The values are given in column 8 of Table 2.

The result of a search for spectroscopic determinations of the metallicity in literature is reported in column 9 of Table 2. It is worth noting that when comparing the 3 different sources for metallicities only in a few cases is the disagreement larger than 0.5 dex: namely HD 128279, HD 166913, HD 200654 and HD 219617.

A direct check of the gravity is possible for HD 140283 which has a measured parallax from where Nissen et al (1994) estimate = 3.39 ; this value is consistent with all four values reported in Table 2. On the other hand, remarkable disagreements up to about 0.7 dex are found between the spectroscopic and photometric gravities for HD 166913 and HD 219617. However, this is not so uncommon, for instance, for HD 76932 the gravities in the literature show a very large spread ranging from the 3.5 of Bessel et al (1991) to 4.37 of Edvardsson et al (1993). For HD 166913 there is a considerable uncertainty in the atmospheric parameters. Laird (1985) gives = 6120 and log g = 4.43. Fuhrmann et al and Axer et al give = 5955 and = 4.45, whereas from the photometry we deduce a log g=3.8. This uncertainty in the atmospheric parameters represents a major source of uncertainty in the Be abundances.

For our analysis we adopted the effective temperature provided by Fuhrmann et al (1994) and the surface gravity of Axer et al (1994) whenever available. For these stars the metallicity of the adopted model was that of Axer et al (1994), rounded to the nearest 0.5 dex. For HD 3795, HD 25704 and HD 76932, which are not included in the paper of Fuhrmann et al (1993), we adopted the photometric gravity derived by using the literature metallicity reported in column 6 in Table 2. For these stars the effective temperature has been taken on the basis of a critical analysis of the literature. These temperatures are close to our photometric estimates. For HD 218502, which lacks the necessary photometry, we took gravity and   from Luck & Bond (1985).

© European Southern Observatory (ESO) 1997

Online publication: July 3, 1998
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