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Astron. Astrophys. 355, 1003-1008 (2000)

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3. Absolute magnitudes of our program stars

Using the observed H[FORMULA] values, we have applied the standard calibrations in the [FORMULA] system to derive absolute magnitudes of our program stars as well as of a sample of apparent "normal" type stars in the relevant spectral range. These values were compared to absolute magnitudes derived by the parallaxes measured with the Hipparcos satellite.

3.1. Calibration in the [FORMULA] system

The program stars were divided in three groups (early, intermediate and late) as suggested by Crawford (1975, 1978, 1979) before applying the relevant calibrations. The uvby photometry was taken from Vogt & Faúndez (1979).

The absolute magnitudes were then derived using the calibrations given in Crawford (1975, 1978, 1979) and Hilditch et al. (1983). We have considered the individual reddening (mainly based on the H[FORMULA] index) of the stars. For most of the stars the reddening can be neglected because of their neighborhood to the Sun. For the remaining objects, the derived reddening was compared with that of the Johnson UBV system (taking the UBV colors from Mermilliod et al. 1997 and the calibration given by Schmidt-Kaler 1982).

In order to avoid effects by selecting the wrong calibration (e.g. stars which are just on the edge of the validity for a selected group), "critical" stars were also calibrated within the adjacent group. Since none of the program stars exhibit a large deviation within this procedure we are confident to believe that a wrongly selected group does not affect the derived absolute magnitudes significantly.

Another important issue is the error estimation. There are two main sources which have to be considered: 1) the error propagation of the observational uncertainties; 2) the intrinsic error of the calibration itself. The latter is always given as [FORMULA]0.3 mag. This value takes into account the apparent width of the main sequence (due to rotation and evolutionary effects) as well as the observational errors from [FORMULA] data of galactic clusters used for the calibration. Since we are not able to independently test the intrinsic validity of the calibration, we have to assume the correctness of the given error. The estimation of the error propagation of the observational uncertainties on the absolute magnitude is not straightforward. But we are able to make the following assumption: taking the changes of [FORMULA] with [FORMULA] from Crawford (1975, 1978, 1979) we find that an error of 0.02 mag in [FORMULA] results in an error of 0.15 mag in [FORMULA]. Fig. 1 shows that about 95 % of our program stars fall below [FORMULA] = 0.02 mag. We therefore have adopted an overall error of the photometrically calibrated absolute magnitudes of [FORMULA]0.4 mag taking into account the most deviant H[FORMULA] measurements.

3.2. Calibration in the Hipparcos system

The Hipparcos database (Perryman et al. 1997) was searched for entries of our program stars. In total, 152 stars with a measured parallax were found. The V magnitudes from Vogt & Faúndez (1979) as well as the reddening values from the [FORMULA] photometry were used to calculate absolute magnitudes. We have not applied the Lutz-Kelker correction (Koen 1992). This seems justified because the relative error of the parallaxes (Fig. 2) is very small for almost all stars implying that these objects are in the vicinity of the Sun.

[FIGURE] Fig. 2. The error distribution of the relative parallax errors for 152 program CP2 stars.

We have to emphasize that there is an unknown error introduced by not correcting for apparent variability (e.g. rotational light variations or binarity). The amplitudes are typically a few hundredth magnitudes but can go up to one magnitude (Paunzen & Maitzen 1998). This might effect individual stars but will not change the conclusions drawn in the next section.

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© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000