5 Fraction of Be stars

Maeder et al. (1999) compare fractions of Be stars in clusters of similar age in three different intervals of M_V. The widest interval, -5 < M_V < -1.4, includes all stars of type B3 and earlier. Taking V - M_V = 18.5 mag for NGC7419 (B94) and adopting average $V - R_{\rm C} \approx$ 1.0 mag for the cluster upper main-sequence estimated from photometry of Bhatt et al. (1993), we find that this interval of M_V corresponds roughly to 12.5 $< R_{\rm C} <$ 16.1. There are 100 stars in our field falling within this interval, of which 31 are Be stars. It is rather unlikely that any of the Be stars is a non-member, but the total number of stars in the interval is certainly affected by contamination by field stars.

In order to estimate the number of field stars in the above-mentioned interval of $R_{\rm C}$, four fields, situated about 7$^\prime$ north, east, south and west of the cluster were observed on June 8, 2000 through the $R_{\rm C}$ and $I_{\rm C}$ filters. In total, 52 stars having 12.5 $< R_{\rm C} <$ 16.1 were found in these fields. Hence, in the vicinity of NGC7419 there are 52/4 = 13 stars in this interval and in a 6$^\prime$ $\times$ 4$^\prime$ field of view. All of them are probably foreground and/or background stars, because at a distance of 7$^\prime$ from the centre of NGC7419 the percentage of cluster stars is negligible. The final estimate of the fraction of Be stars in NGC7419 is therefore 31/(100-13) $\approx$ 36 $\pm$ 7%, where the error was calculated assuming Poisson statistics. If we split the above interval into two parts, i.e. 12.5 $< R_{\rm C} <$ 14.3 and 14.3 $< R_{\rm C} <$ 16.1, the fractions of Be stars are equal to 61 $\pm$ 21% and 27 $\pm$ 7%, respectively.

We note that owing to the intermittency of the Be phenomenon and the fact that weak emission cannot be detected photometrically, the above estimated fraction of Be stars in NGC7419 represents a lower limit. Since the age of NGC7419 is similar to the ages of clusters investigated by Maeder et al. (1999), the fraction of Be stars we derived can be directly compared with their numbers. This comparison locates NGC7419 among the clusters with largest known fractions of Be stars, such as the galactic NGC663 (34 $\pm$ 11%), NGC330 in SMC (39 $\pm$ 7%), and NGC1818A in LMC (36 $\pm$ 7%).

Acknowledgements

We thank Prof. M.Jerzykiewicz for critical reading the manuscript and Z.Koaczkowski for his help in observations and valuable comments. This work was made with the help of Aladin Sky Atlas operating at CDS, Strasbourg.