Astron. Astrophys. 360, 120-132 (2000)

5. Masses

We calculated masses for the programme stars in NGC 6752 from their values of and using the equation:

where denotes the theoretical brightness at the stellar surface as given by Kurucz (1992). We decided to use the photometry of Buonanno et al. (1986) to derive masses. As can be seen from Table 1 the photometry of Thompson et al. (1999) yields in general fainter visual magnitudes than the photometry of Buonanno et al. The effect on the masses, however, is small: On average, the masses derived from the Thompson et al. photometry are 5% lower than those derived from the Buonanno et al. photometry. We adopted and = 0.04 for the distance modulus and reddening. These are mean values derived from the determinations of Renzini et al. (1996), Reid (1997, 1998), and Gratton et al. (1997). The errors in are estimated to be about the same as obtained for the older NGC 6752 data described by Moehler et al. (1997b): 0.15 dex for stars above the gap, 0.17 dex for stars within the gap region and 0.22 dex for stars below the gap.

The masses derived from the analysis using metal-poor model atmospheres are plotted in Fig. 7 (top panel). The sdB stars hotter than 20,000 K ( = 4.3) scatter around the canonical ZAHB, whereas the stars below 16,000 K ( = 4.2) lie mainly below the canonical ZAHB. Even stronger deviations towards low masses are found between 16,000 K and 20,000 K. Comparing the masses to those predicted by the mixed ZAHB ( 0.40) we obtain similar results. To quantify the offsets we compare the masses of the stars to those they would have on the theoretical ZAHB at the same (see Table 6). We divide the stars into three groups for the further discussion (excluding stars below 11,500 K, for which diffusion should play no rôle, as well as B 2697 and B 3006): cool HBB stars ( 16,000 K, 16 stars), hot HBB stars (16,000 K 20,000 K, 9 stars), and sdB stars ( 20,000 K, 12 stars). The effective temperatures here are those derived from metal-poor model atmospheres.

Fig. 7a-c. Temperatures and masses (derived from Buonanno et al.'s photometry) of the programme stars in NGC 6752. a determined using model atmospheres with cluster metallicity ([M/H] = -1.5), b adopting a solar metallicity ([M/H] = 0) for the model atmospheres c adopting a super-solar metallicity ([M/H] = +0.5) for the model atmospheres. For more details see Sect. 4.1. The dashed resp. solid lines mark the ZAHB masses for a metallicity [M/H] = -1.54, as computed with and without mixing, respectively (see Sect. 4.2 and Fig. 4 for details).

Table 6. Mean mass ratios between spectroscopically derived masses and predicted zero-age HB masses at the same effective temperatures. B 2697, B 3006 and stars cooler than 11,500 K are excluded from this comparison. gives the Reimers' mass loss parameter for the respective ZAHB. We derived the masses using the photometry of Buonanno et al. (1986). The cited errors are standard deviations.

The results of the analyses using solar-metallicity model atmospheres are plotted in the central panel of Fig. 7 (see Table 6). The effect on the masses is similar to that on the temperatures/gravities (Fig. 4, central panel) - below 15,300 K (cool HBB stars) and above 20,000 K (sdB stars) the masses basically scatter around the canonical ZAHB, but the hot HBB stars between these two groups still show too low masses. Comparing the masses to those predicted by the mixed ZAHB ( = 0.45) gives similar results. As the stars become cooler when analysed with more metal-rich atmospheres the temperature boundaries were shifted to include the same stars as for the comparison made above.

Even the use of metal-rich model atmospheres for the analyses does not change much (see Table 6 and the bottom panel of Fig. 7). Obviously the hot HBB stars in the intermediate temperature range still show low masses, despite the use of metal-rich model atmospheres. Thus the problem of the stars in this temperature range(15,300 K 19,000 K) cannot be completely solved by the scaled-solar metal-rich atmospheres used here.

© European Southern Observatory (ESO) 2000

Online publication: July 27, 2000
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