4 Early type emission-line stars

Quite a few stars showed at least H$\alpha$ emission in our spectra. In IC 1805, the star No. 160 was confirmed as an Of star, and No. 113 an Be star (Ishida 1970; Massey et al. 1995). No. 70 is classified as an early type supergiant(B0.5Ia) with H$\alpha$ emission. In NGC 654, four stars were found to have H$\alpha$ emission. These include stars Nos. 9, 52, 104 and 135. No. 52 is classified as a supergiant, while the other three appear to be new Be stars from our spectral classification. In NGC 6823, The stars No. 4 and No. 88 were once classified as B0IVe and B0Vpe by Turner (1979). Both of them, however, did not show any distinct H$\alpha$ emission in our spectra.

The frequency of Be stars has been discussed by many authors in the literature (Schild & Romanishin 1976; Mermilliod 1982; Jaschek & Jaschek 1983). Open clusters provide the desirable B-type parent population to which the Be stars can be compared. By definition, Be stars are non-supergiant B-type stars having at least once shown emission in one of the Balmer lines (Jaschek et al. 1981). Therefore, in IC 1805, NGC 6823 and NGC 654 respectively, one,two and three Be stars have been found as mentioned above. In Table 5, the number of all observed stars earlier than B2 (in Col. 2) and the number of Be stars ever found (in Col. 3) are given. All these Be stars also have types earlier than B2. According to both the proper motion surveys (mentioned in the previous section) and our spectral classification surveys over the three clusters, all of the cluster members earlier than B2 may well have been spectroscopically observed by us, on a statistical stand. The frequency of Be stars have been computed and given in Table 5. The averaged frequency of Be stars in these three clusters is 7%. For the sake of comparison, we just list the results of similar work from two other sources below. A frequency of about 7% was also obtained by Schild & Romanishin (1976) from a survey of 566 stars in 29 young open clusters, of which 41 Be stars were detected. By using the data of bright stars ($V\leq 6\hbox{$.\!\!^{\rm m}$}5$), Jaschek & Jaschek (1983) derived a frequency of about 17% and 11% of Be stars among B0 through B2 stars and among all B-type stars respectively. It should be pointed out that a Be star may not exhibit emissions at the time the observation was carried out, and the percentage of Be stars among B-type stars at a given moment is smaller than that when all stars which were once Be are included (Jaschek & Jaschek 1983). For a more reliable determination of the frequency of Be stars in open clusters, systematic detection of Be stars in more clusters should be made.

Detailed study on individual Be stars in open clusters such as those found in this work is of special interest, since the distances, ages and intrinsic colours can be inferred from the well-determined cluster parameters (Mermilliod 1982; Slettebak 1985). It will help in understanding the evolutionary status of Be stars and the origin of the envelops around them.