NGC 7419: a young open cluster with a number of very young intermediate mass pre-MS stars

Abstract

We present a photometric and spectroscopic study of the young open cluster NGC 7419, which is known to host a large number of classical Be stars for reasons not well understood. Based on CCD photometric observations of 327 stars in UBV passbands, we estimated the cluster parameters as, reddening [E(B−V)] = 1.65 ± 0.15 mag and distance = 2900 ± 400 pc. The turn-off age of the cluster was estimated as 25 ± 5 Myr using isochrone fits. UBV data of the stars were combined with the JHK data from Two-Micron All-Sky Survey (2MASS) and were used to create the near-infrared (NIR) (J−H) versus (H−K) colour–colour diagram. A large fraction of stars (42 per cent) was found to have NIR excess and their location in the diagram was used to identify them as intermediate mass pre-main-sequence (MS) stars. The isochrone fits to pre-MS stars in the optical colour–magnitude diagram showed that the turn-on age of the cluster is 0.3–3 Myr. This indicates that there has been a recent episode of star formation in the vicinity of the cluster.

Slitless spectra were used to identify 27 stars which showed Hα in emission in the field of the cluster, of which six are new identifications. All these stars were found to show NIR excess and are located closer to the region populated by Herbig Ae/Be stars in the (J−H) versus (H−K) diagram. Slit spectra of 25 stars were obtained in the region 3700–9000 Å. The spectral features were found to be very similar to those of Herbig Be stars. These stars were found to be more reddened than the main-sequence stars by 0.4 mag, on an average. Thus, the emission-line stars found in this cluster are more similar to the Herbig Be-type stars where the circumstellar material is the remnant of the accretion disc. We conclude that the second episode of star formation has led to the formation of a large number of Herbig Be stars as well as intermediate mass pre-MS stars in the field of NGC 7419, thus explaining the presence of emission-line stars in this cluster. This could be one of the young open clusters with the largest number of Herbig Be stars.