Astron. Astrophys. 343, 222-228 (1999)

2. Data

Accurate absolute magnitudes have been obtained through early release of data from the Hipparcos satellite. The errors in the derived M_v values are now well known. In general they are so small that they can safely be neglected.

In Table 1 the sample of 65 stars taken from our sample of 78 stars, for which we have IUE observations of Mg II h and k line widths and Hipparcos magnitudes is given. Earth-based absolute magnitudes and a detailed description of the data sample may be found in Elgaroy et al. (1997).

Table 1. Data for the observed stars. The estimated errors apply to as well as to .

As clearly demonstrated from Fig. 1 there is good correspondence between Hipparchos magnitudes and those deduced from Earth-based observations in most cases, but for about 20% of the stars the correspondence is less satisfactory.

Fig. 1. Absolute magnitudes as derived from Hipparcos data (HIP) plotted against absolute magnitudes given in "Sky Catalogue 2000" (Hirshfeld & Sinnott, 1982) and "Catalogue of nearby stars" (Gliese 1969; Gliese & Jahreiss, 1979). See also Elgaroy et al.(1997)

The deviations are largest for the more luminous stars. This could be expected since these are the most distant ones and consequently may have large errors in the previously derived parallaxes.

2.1. Uncertainties in the data

The standard errors in the Hipparcos parallaxes correspond to errors in absolute magnitudes less than 0.1 magnitude in all cases except for the 10 most distant stars in our sample, as shown in Table 2. In the table the possible errors in magnitude are given for the 13 stars that have parallaxes less than 28 mas. In addition three stars with parallaxes of around 50 mas , one with about 100 mas and one with about 200 mas are listed together with the uncertainties in their absolute magnitudes.

Table 2. Uncertainties in absolute magnitudes as determined from standard errors in Hipparcos parallaxes.

One may conclude that the Hipparcos observations have reduced the uncertainties in the absolute magnitudes of our sample stars to a very satisfactory low level, and most important; the possible errors are now known.

The line widths of the MgII h and k lines are the same as derived in the investigation by Elgaroy et al. (1997), where the reduction procedure and a discussion of errors may be found.

In the present contribution the analysis is restricted to the MgII k line, since the results for the k and h lines differ very little. In the conventional method the width is defined as the full width of the line at the 50% level of the peak line flux. However, since the spectrum usually is noisy and often difficult to define, the actual measurement is better done at a Gaussian fit rather than at the emission line. Besides, the spectrum may in some cases be so asymmetric that the lowest peak doesn't even reach the 50% level. Direct measurement of the FWHM width would then be meaningless, since it would give the width of the dominating peak rather than the width of the whole line. By using the Gaussian fit as an extrapolation to "fill in" the missing flux a much better estimate of the width (denoted W) is obtained. The flux is then defined as the integrated flux under the line profile (denoted F). But the width may also be defined as the full width of the fit at 50% of its peak level (denoted W´). The flux is defined as the integrated flux under the Gaussian fit (denoted F').

A further explanation and discussion of the parameters is given in Elgaroy et al. (1997).

2.2. Test of the reliability of the determination of line widths

MgII h and k line profiles have been determined by Robinson & Carpenter (1995) using high-resolution spectrographic data taken with the Goddard High Resolution Spectrograph connected to the Hubble Space Telescope and with the IUE satellite. Four of their sample stars are also present in our data sample, i.e. Dra, Cep, Ori and Cru. Of these Ori and Cru were observed both with the GHRS at the Hubble Telescope as well as with the IUE. The width of the lines were determined at 10% maximum intensity. If a Gaussian line profile is assumed, their data can be converted to FWHM values and compared with our results.

Table 3 shows that there is very close agreement between the derived widths. As might be expected (cfr. discussion in Elgaroy et al. 1997) our values for log W are particularly close to the values obtained by Robinson and Carpenter.

Table 3. Comparison of line width measurements

© European Southern Observatory (ESO) 1999

Online publication: March 1, 1999
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