NGC 6388 and NGC 6441 Discussion

Here is the full-resolution postscript version of our recent paper on the HST snapshot survey of NGC 6441 paper. photometry of the variable stars.

The Science:

It has been known that the horizontal branch (HB) morphology of globular clusters is not governed solely by the metallicity of the cluster. While there has been no consensus as to what this second parameter is, the most likely explanations are age, helium abundance, CNO abundance, and mass loss. An understanding of the second parameter phenomenon is essential to any understanding of the formation and evolution of the globular cluster system and the formation history of the Galaxy.

It was discovered by Rich et al. (1997) that the metal-rich globular clusters NGC 6388 and NGC 6441 have prominent blue extensions to their HB's not typically associated with a cluster of this metallicity (e.g. 47 Tuc). It was also shown that the HB's also slope upward in V with decreasing (B-V), possibly providing an added constraint on second parameter models. In an attempt to model the HB's of NGC 6388 and NGC 6441, Sweigart & Catelan (1998) found that these unusual HB's could not be explained by age differences, mass loss, or differential reddening. They could be explained by high helium abundances in the envelope or by increased core masses caused by rotation. These models also predicted that the HB's of NGC 6388 and NGC 6441 should be unusually bright. As a result, any RR Lyrae found in these clusters should have unusually long periods for their metallicity. This would have consequences on the use of RR Lyrae as standard candles for determining globular cluster distances and ages. More recently, Sweigart (2002) showed that the morphology of the HB's in NGC 6388 and NGC 6441 could also be explained by a metallicity spread within each cluster. Although this would remove any second parameter effect in these clusters, NGC 6388 and NGC 6441 would become metal-rich analogies to Centauri.

The Research:

We obtained V,B observations at the 0.9m telescope at the Cerro Tololo Inter-American Observatory of NGC 6388 and NGC 6441 with the goal of detecting the RR Lyrae. Periods and light curves were determined for known and newly discovered variables. The RR Lyrae data is used to help in determining the unusual nature of these clusters and test the models of Sweigart & Catelan (1998).

The Results:

NGC 6441:
We were able to increase the number of variables found near NGC 6441 to about 104, with 48 new variables being found in this survey. Forty-six of these variables are RR Lyrae stars, most of which are probable cluster members. As was noted in Layden et al. (1999) and Pritzl et al. (2000), the periods of the RRab stars are unusually long when compared to field RRab of similar metallicity. This is consistent with the prediction of Sweigart & Catelan (1998) that the HB of NGC 6441 would be unusually bright. We were able to show through the use of a period-amplitude diagram that the RRab in NGC 6441 fall at periods as long as, if not longer than, Oosterhoff type II clusters such as M15. The RRab of NGC 6441 also fall in the region predicted by Sweigart & Catelan according to a helium-mixing scenario.

Due to the unusual nature of the HB morphology, it is difficult to place NGC 6441 in either of the Oosterhoff groups. According to its metallicity, [Fe/H]=-0.53, NGC 6441 should be an Oosterhoff type I cluster. Yet, the mean period of the RRab, <P_ab>=0.75d, is even higher than typically found in Oosterhoff type II clusters. The mean period of the RRc stars, <P_c>=0.38d, and the ratio of the number of RRc stars to the total number of RRL, N_c/N_RR=0.31, are similar to the values found for Oosterhoff type II clusters.

An interesting feature of NGC 6441, a property shared with NGC 6388, is the presence of longer period RRab and RRc stars. Of the RRab considered to be members of NGC 6441, 38% (10 of 26) have periods greater than 0.8d. To our knowledge, there is only one other cluster, NGC 5897, that has such a large proportion of long period RRab. NGC 6441 also contains two RRc stars with periods longer than 0.45d. Other than Centauri, which contains a small number of these RRc stars, we were able to only find two other examples of these types of stars in globular clusters, V70 in M3 and possibly V76 in M5.

Unfortunately, we were not able to ascertain if there is a metallicity spread in NGC 6441 with this study. Clearly, spectra will have to be taken of this cluster to determine if this effect exists. If the RR Lyrae in NGC 6441 are shown to have the same metallicity as their parent cluster, [Fe/H]=-0.53, NGC 6441, along with NGC 6388, would provide a unique metal-rich environment in which to study the second parameter effect. The unusually bright RRab in NGC 6441 would also prove to be a contradiction to the metallicity-luminosity relation for RR Lyrae which states that the more metal-poor the RR Lyrae, the brighter they are.

Please see Pritzl et al. (2001) and Layden et al. (1999) for more in-depth analyses of NGC 6441.

NGC 6388:
Unlike NGC 6441, which had only been more recently investigated (Layden et al. 1999), NGC 6388 was shown to probably contain RR Lyrae (Hazen & Hesser 1986; Silbermann et al. 1994). Through this study, we were able to refine the periods and improve upon the light curves of the known RR Lyrae in addition to discovering new variables. We found 28 new variables in our survey, increasing the total number of variables found near NGC 6388 to approximately 57. Fourteen of these variables are likely cluster member RR Lyrae and a number of the variables found are likely RR Lyrae. We also found 4 candidate Population II Cepheids. If they are members of the cluster, NGC 6388 would be the most metal-rich globular cluster to contain this type of variable star.

As with NGC 6441, the RRab in NGC 6388 were shown to have unusually long periods, confirming the predictions made by Sweigart & Catelan (1998). NGC 6388 is also difficult to place in the typical Oosterhoff dichotomy. While its metallicity ([Fe/H]=-0.60) would indicate that it should be among the Oosterhoff type I clusters, the mean periods of the RR Lyrae, <P_ab>=0.71d and <P_c>=0.36d, and the ratio of RRc stars to the total number of RR Lyrae, N_c/N_RR=0.71, imply that NGC 6388 is an Oosterhoff type II cluster. A similar effect is seen in the period-amplitude diagram where the NGC 6388 RRab stars fall among the RRab from metal-poor globular clusters.

We also continue our discussion on whether or not NGC 6388, and NGC 6441, have a metallicity spread among their stars. Recently, there have been other papers on this topic as well. Raimondo et al. 2002 argued that there can be no metallicity spread in these two clusters since any significant spread in metallicity would lead to a broader red giant branch than is seen in these clusters. On the other hand, Ree et al. 2002 used models to show that the horizontal branches of NGC 6388 and NGC 6441 can be reproduced through a spread in metallicity and differences in age among the cluster stars. It is the belief of Ree et al. that these two clusters are in fact remnants from dwarf galaxies similar to what is believed for Centauri. Fortunately, a collaboration that we have taken up with other astronomers has obtained spectra of the stars in NGC 6388 and we should be able to (hopefully) resolve the metallicity spread issue.

It has been thought that the Oosterhoff classification of a globular cluster is more dependent on evolutionary effects rather than metallicity. In this scheme, NGC 6388 and NGC 6441 would be clearly classified as Oosterhoff type II clusters. In our investigation of the RR Lyrae in NGC 6388, we discussed this idea and whether or not the the RR Lyrae evolved from horizontal branch stars found blueward of the instability strip. Through modelling, we found that due to the rapid evolution through the instability strip there needs to be a significantly larger number of blue horizontal branch stars to account for the number of RR Lyrae found in most Oosterhoff type II clusters. As a result, the RR Lyrae found in NGC 6388 and NGC 6441 can not be totally accounted for by evolutionary effects.

Please see Pritzl et al. (2002) for a more in-depth analysis of NGC 6388.

References and Relevant Links

Hazen, M.L., & Hesser, B.H. 1986
" The variable stars in the globular cluster NGC 6388"
AJ, 92, 1094

Layden, A.C., Ritter, L.A., Welch, D.L., & Webb, T.M.A. 1999
" The Variable Stars and Blue Horizontal Branch of the Metal-rich Globular Cluster NGC 6441"
AJ, 117, 1313

Pritzl, B., Smith, H.A., Catelan, M., & Sweigart, A.V. 1999
" RR Lyrae Stars in NGC 6388 and NGC 6441: A New Oosterhoff Group?"
ApJ, 530, L41

Pritzl, B.J., Smith, H.A., Catelan, M., Sweigart, A.V. 2001
" Variable Stars in the Unusual, Metal-Rich Globular Cluster NGC 6441"
AJ, 122, 2600

Pritzl, B.J., Smith, H.A., Catelan, M., Sweigart, A.V. 2002
"Variable Stars in the Unusual, Metal-Rich Globular Cluster NGC 6388"
AJ, in press (astro-ph/0204296)

Raimondo, G., Castellani, V., Cassisi, S., Brocato, E., & Piotto, G. 2002
" On the Observational Properties of He-burning Stars: Some clues on the Tilt of the HB in Metal Rich Clusters"
ApJ, in press (astro-ph/0201123)

Ree, C.H., Yoos, S.-J., Rey, S.-C., Lee, Y.-W. 2002
" Synthetic Color-Magnitude Diagrams for omega Centauri and Other Massive Globular Clusters with Multiple Populations"
To appear in Omega Centauri: A Unique Window Into Astrophysics, in press (astro-ph/0110689)

Rich, R.M., Sosin, C., Pjorgovski, S.G., Piotto, G., King, I.R., Renzini, A.,Phinney, E.S., Dorman, B., Liebert, J., & Meylan, G. 1997
" Discovery of Extended Blue Horizontal Branches in Two Metal-rich Globular Clusters"
ApJ, 484, L25

Silbermann, N.A., Smith, H.A., Bolte, M., & Hazen, M.L. 1994
" RR Lyrae stars and color-magnitude diagram of the globular cluster NGC 6388"
AJ, 107, 1764

Sweigart, A.V. 2002
" Horizontal Branch Models as a Test of Mixing on the RGB"
To appear in Highlights of Astronomy, Vol. 12, in press (astro-ph/0103133)

Sweigart, A.V., & Catelan, M. 1998
" The Second-Parameter Effect in Metal-rich Globular Clusters"
ApJ, 501, L63

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