RR Lyrae Stars in NGC 6388 and NGC 6441: A New Oosterhoff Group?

Details RR Lyrae Stars in NGC 6388 and NGC 6441: A New Oosterhoff Group? RR Lyrae Stars in NGC 6388 and NGC 6441: A New Oosterhoff Group?

1999-12-10

arxiv.org/abs/astro-ph/9912217v1

Astronomy and Astrophysics

Astrophysics

Four pages, three figures. ApJ (Letters), in press

Scientific paper

10.1086/312482

NGC 6388 and NGC 6441 are anomalies among Galactic globular clusters in that they cannot be readily place into either Oosterhoff group I or Oosterhoff group II despite their significant numbers of RR Lyrae variables. The mean pulsation periods, , of their RRab variables, at 0.71 d and 0.76 d, respectively, are even larger than for Oosterhoff II clusters. Moreover, Oosterhoff II clusters are very metal-poor, whereas NGC 6388 and NGC 6441 are the most metal-rich globular clusters known to contain RR Lyrae stars. The location of the NGC 6388 and NGC 6441 RRab variables in the period-amplitude diagram implies that the RR Lyrae stars in those two clusters are brighter than expected for their metallicities. Our results therefore indicate that a universal relationship may not exist between the luminosity and the metallicity of RR Lyrae variables.

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