3. Highlights of the abundance correlations
We find a close correlation between [Mg/Fe], [Ca/Fe] and [Ti/Fe], as illustrated in Fig. 2. This indicates that the so-called -elements were synthesized by the same process in the same objects. We find a similar correlation between the abundances of Cr, Ni and Fe relative to Ti, indicating a common origin for these iron-peak elements.
We have also carried out this analysis for the neutron-capture elements, because we wanted to identify the sites and mechanisms for the synthesis of these elements in a relatively early phase of the galactic evolution. A first hint was put forward by Zhao and Magain (1991 ) who found that the elements Y and Zr are better correlated with Ti than with Fe. They suggested that this indicates that massive stars played a dominant role in the early nucleosynthesis of Y and Zr. Our results confirm their findings. For example, while the scatter of [Y/Fe] amounts to 0.12 dex (30 ), the scatter of [Y/Ti] is only 0.07 dex (18 ). Our new data allow us to go further than just compare the scatters, as shown in Fig. 3 where the values of [Y/Fe] versus [Ti/Fe] are plotted for each star in our sample. We see that [Y/Fe] is indeed correlated with [Ti/Fe], but this correlation is not simple. We can see two separate behaviours. For one subsample of the stars, the value of [Ti/Fe] increases with increasing [Y/Fe], while for the other subsample [Ti/Fe] is constant (and maximum) while [Y/Fe] increases. We find similar results when any of the elements Sr, Y and Zr is compared to any of the -elements.
A very clean result is presented in Fig. 4, where the abundance of the prototypical r -process element Eu is compared to the Ti abundance. The correlation is nearly perfect. It allows us to conclude that, in general, the r -process element Eu is synthesized in the same objects as the -elements, i.e. most probably in the supernova explosion of massive stars, confirming the generally accepted scenario. We notice the absence of a vertical feature similar to the one obtained for [Y/Fe]. Instead, it is replaced by a clumping of the points at the maximum value of [Ti/Fe] and [Eu/Fe].
© European Southern Observatory (ESO) 1998
Online publication: January 16, 1998