To check the impact of the multiple population scenario for globular clusters on their horizontal branch ( HB ) , we present an analysis of the composition of 110 red HB ( RHB ) stars in 47Â Tucanae and of 61 blue HB ( BHB ) and 30 RHB stars in MÂ 5 .
In 47Â Tuc we found tight relations between the colours of the stars and their abundances of p - capture elements .
This strongly supports the idea that the He content - which is expected to be closely correlated with the abundances of p - capture elements - is the third parameter ( after overall metallicity and age ) that determines the colour of HB stars .
However , the range in He abundance must be small ( \Delta Y < 0.03 ) in 47Â Tuc to reproduce our observations ; this agrees with previous analyses .
There is possibly a correlation between the abundances of p - Â and n - capture elements in 47Â Tuc .
If confirmed , this might suggest that asymptotic giant branch stars of moderate mass contributed to the gas from which second-generation stars formed .
Considering the selection effects in our sample ( which does not include stars warmer than 11000Â K and RR Lyrae variables , which were excluded because we could not obtain accurate abundances with the adopted observing procedure ) is important to understand our results for MÂ 5 .
In this case , we find that , as expected , RHB stars are Na-poor and O-rich , and likely belong to the primordial population .
There is a clear correlation of the [ Na/O ] ratio and N abundance with colour along the BHB .
A derivation of the He abundance for these stars yields a low value of Y = 0.22 \pm 0.03 .
This is expected because HB stars of a putative He-rich population in this cluster should be warmer than 11000Â K , and would accordingly not have been sampled by our analysis .
However , we need some additional source of scatter in the total mass loss of stars climbing up the red giant branch to reproduce our results for MÂ 5 .
Finally , we found a C-star on the HB of 47Â Tuc and a Ba-rich , fast-rotating , likely binary star on the HB of MÂ 5 .
These stars are among the brightest and coolest HB stars .