4 Be stars in NGC7419

The first two Be stars in NGC7419 were discovered by González & González (1956) and designated in their list as stars Nos. 112 and 113. Dolidze (1959) increased this number to seven. These seven stars appear also in his later compilation (Dolidze 1975). Unfortunately, finding charts in the above-mentioned papers are not detailed enough, so that only two out of the seven stars, D22-124 and D22-125 (Dolidze 1975), can be unambiguously identified. B94 found emission in one of the five brightest blue members, BMD389 (we shall use the numbering system of B94 preceding their number by "BMD''). Recently, Kohoutek & Wehmeyer (1997) detected 12 Be stars in the cluster, but list also two others. Altogether, 14 Be stars were known in NGC7419 prior to our study.

Our discovery of stars showing emission in H$\alpha$ line was made by means of the $\alpha$ index defined by two H$\alpha$ filters described in Sect. 2, in analogy to the well-known photometric $\beta$ index, namely:

$$\alpha = m_{\rm narrow} - m_{\rm wide} + C,$$ (1)

where $m_{\rm narrow}$ and $m_{\rm wide}$ denote the magnitude of a star as seen through the narrow and wide H$\alpha$ filter, respectively. The constant C takes into account the differences in the exposure times and differences in zero points between frames.

Figure 3: The same as in Fig. 2 but plotted as a function of $(R-I)_{\rm C}$ colour. Symbols and the dashed line have the same meaning as in Fig. 2. For clarity, the error bars were omitted here

The $\alpha$ index is shown in Fig. 2 for 107 brightest stars in the field down to magnitude $R_{\rm C}$ = 16.1. Only one of the previously known 14 Be stars, BMD967 = HBH6206-16, shows no or only weak emission. A weakening of the emission was observed in many Be stars, and this could be such a case. Remaining 13 stars stand clearly above the sequence of non-emission stars (see Fig. 2). In order to be able better recognize emission stars, especially those of late type, the $\alpha$ index was also plotted as a function of colour index $(R-I)_{\rm C}$ (Fig. 3).

In addition to the 14 emission-line objects in NGC7419 listed by Kohoutek & Wehmeyer (1997), 19 stars showing emission were found. Out of them, two are of late type. The first one is MY Cep = BMD950 mentioned in Sect. 3. The other one, BMD350, probably also of late type, is not a member as can be judged from its position in the cluster CMD (Fig. 4). Thus, there are now 31 stars in the observed field which are very likely cluster Be stars.

The data for all 33 stars in the observed field showing H$\alpha$ emission are given in Table 1. The typical error of $R_{\rm C}$ magnitude given in Table 1 equals to 0.004 and 0.011 mag for a star with $R_{\rm C}$ = 13 and 16 mag, respectively. The corresponding errors of colour indexes for such stars are 0.007 and 0.035 mag. The coordinates of stars were derived using the positions of 10 stars in the field from the NASA Guide Star Catalog, v. 1.1. They are accurate to within 0.5 $^{\prime\prime}$.

As can be seen in the cluster CMD (Fig. 4), the Be stars are 0.1-0.2 mag redder in $(R-I)_{\rm C}$ than non-emission stars of the same magnitude. This is a well-known effect observed in many open clusters with Be stars.

Figure 4: CMD for NGC7419. Open circles denote stars brighter than $R_{\rm C}$ = 16.1 mag which are too red or too blue to be members. Diamonds are used to indicate the emission-line objects. The solid line shows the limit of our $(RI)_{\rm C}$ photometry while the dashed one, that of the H$\alpha$ photometry