We present a chemical composition analysis of 36 giant stars in the mildly metal-poor ( < [ Fe/H ] > = –1.21 ) globular cluster M5 ( NGC 5904 ) .
The analysis makes use of high resolution data acquired for 25 stars at the Keck I telescope , as well as a re-analysis of the high resolution spectra for 13 stars acquired for an earlier study at Lick Observatory .
We employed two analysis techniques : one , adopting standard spectroscopic constraints , including setting the surface gravity from the ionization equilibrium of iron , and two , subsequent to investigating alternative approaches , adopting an analysis consistent with the non-LTE precepts as recently described by Thévenin & Idiart .
The abundance ratios we derive for magnesium , silicon , calcium , scandium , titanium , vanadium , nickel , barium and europium in M5 show no significant abundance variations and the ratios are comparable to those of halo field stars .
However , large variations are seen in the abundances of oxygen , sodium and aluminum , the elements that are sensitive to proton-capture nucleosynthesis .
These variations are well-correlated with the CN bandstrength index S ( 3839 ) .
Surprisingly , in M5 the dependence of the abundance variations on log g is in the opposite sense to that discovered in M13 by the Lick-Texas group where the relationship provided strong evidence in support of the evolutionary scenario .
The present analysis of M5 giants does not necessarily rule out an evolutionary scenario , but it provides no support for it either .
In comparing the abundances of M5 and M4 ( NGC 6121 ) , another mildly metal-poor ( < [ Fe/H ] > = –1.08 ) globular cluster , we find that silicon , aluminum , barium and lanthanum are overabundant in M4 with respect to what is seen in M5 , confirming and expanding the results of previous studies .
In comparing the abundances between these two clusters and others having comparable metallicities , we find that the anti-correlations observed in M5 are similar to those found in more metal-poor clusters , M3 , M10 and M13 ( < [ Fe/H ] > = –1.5 to –1.6 ) , whereas the behavior in M4 is more like that of the more metal-rich globular cluster M71 ( < [ Fe/H ] > \sim –0.8 ) .
We conclude that among stars in Galactic globular clusters , there is no definitive “ single ” value of [ el/Fe ] at a given [ Fe/H ] for at least some alpha-capture , odd-Z and slow neutron-capture process elements , in this case , silicon , aluminum , barium and lanthanum .