The Lack of Blue Supergiants in NGC 7419, a Red Supergiant-rich Galactic Open Cluster with Rapidly Rotating Stars

Abstract

According to previous studies based on photometry alone, NGC 7419 reveals a surprisingly low ratio of blue to red supergiants: only one blue supergiant (BSG) along with a record number of five red supergiants (RSGs). However, for a cluster like NGC 7419 with solar metallicity, one expects twice as many BSGs as RSGs. To verify the small ratio of BSGs to RSGs, we have obtained spectroscopic observations of the seven most luminous blue member stars using the 1.6 m telescope of the Mount Megantic Observatory. (The RSGs have already been studied spectroscopically.) To classify the stars, we have developed a system especially adapted for these heavily reddened stars in a spectral region from 8400 to 8920 A, near the hydrogen Paschen series limit. This classification system is based on standard stars of known MK spectral type extending over O9–B5 and all luminosity classes and is linked through a grid of synthetic spectra to the atmospheric physical parameters Teff and log g. We also include Be stars. Among the seven blue stars observed in NGC 7419, four have red spectra that are dominated by absorption lines and three by emission lines. The spectral types for the former are B2.5 II–III, B2.5 III, B0 III, and B4 III (e), while those for the latter are Be, B1 III–Ve, and Be, respectively. The average heliocentric radial velocity of these stars is -66 ± 6 km s-1, compatible with the value of -74 ± 9 km s-1 measured for the five RSG members. A distance of 1.7 ± 0.4 kpc for this cluster was estimated using the blue-star spectral types, in agreement with the value of 2.3 ± 0.3 kpc found by Beauchamp and coworkers, based on isochrone fitting in the color-magnitude diagram. With no BSG stars detected spectroscopically, we confirm the low number, in this case absence, of BSGs in this cluster. The high fraction of Be stars detected by us and others among the bright blue member stars could be explained by an average rotational velocity for the stars in NGC 7419, which is significantly higher than in other clusters of similar age and metallicity. Since higher stellar rotation rates shorten the BSG phase, we suggest that this explains why the evolved stars in NGC 7419 have become RSGs. Thus, NGC 7419 is an exceptional case, since high stellar rotation normally tends to occur at lower metallicity.