A study of abundances of four giants in the low-mass globular cluster Palomar 5

摘要

The low-mass globular cluster Pal 5 is found to contain a population of red giants that exhibits chemical abundance inhomogeneities similar to those seen in more massive globular clusters of comparable overall metallicity, such as M5. Previous observations had shown that CN and CH inhomogeneities exist among the brighter giants in this cluster. High-resolution spectra have been obtained of four giants of known CN-band strength in Pal 5 using the HIRES instrument on the Keck I telescope. The data have been analyzed to derive the abundances of the elements Fe, O, Na, Al, Si, Ca, Sc, Ti, V, Mn, Ni, Ba, La, and Eu. The Fe/H abundance is found to be -1.3 dex, similar to that of the globular cluster M5 (Fe/H = 1.2.) Among the proton-capture elements Na and Al, the Pal 5 stars show a very significant range of 0.6 dex in Na/Fe, with a mean Na/Fe of +0.08, very similar to that of M5. The mean Al abundance of the Pal 5 sample is 0.2 dex lower than the mean for M5, and the Pal 5 giants show less spread in Al/Fe (similar to0.4 dex) than the M5 giants, although, with only four giants, our Pal 5 sample is unlikely to reflect the extremes in the Al abundance distribution of the cluster. The CN-band strengths of Pal 5 giants show correlations with Na/Fe and Al/Fe, and an anticorrelation with O/Fe. Such abundance relations suggest that material has been brought to the surface of some Pal 5 giants in which Na and Al has been synthesised and oxygen depleted by proton-addition reactions. Within the observational uncertainties the Fe-peak and neutron-capture elements have similar X/Fe ratios as M5 giants and field subdwarfs. In the case of the alpha-capture elements the observed Pal 5 giants have alpha/Fe lower by 0.12 dex than field subdwarfs and M5 giants. This measured difference is only a 2 sigma result, however, and, given the smallness of our Pal 5 sample, it warrants confirmation by further observations. In terms of its element abundance ratios and abundance inhomogeneities Pal 5 appears to be quite typical of other higher mass globular clusters. Whatever process causes abundance inhomogeneities in globular clusters appears not to be sensitive either to the current mass of these objects or to disruptive processes, such as tidal shocking, that can lead to the loss of a significant fraction of the initial stellar content of a cluster. References: 86