5 Is the $\vec r$-process universal?

Figure 11: Top: neutron-capture-element abundances of CS 31082-001 compared to the Solar System r-process scaled to match the $56 \leq Z \leq 69$ elemental abundances of CS 31082-001. Two sources are plotted for the Solar r-process: Burris et al. 2000 (dashed line) and Arlandini 1999 (full line). Note that the radioactive species (Th and U) Solar System abundances are corrected for radioactive decay since the formationof the Solar System. The dotted line show the abundances observed today for these two species (scaled in the same manner). Middle: residual abundance of CS 31082-001 after the Solar System r-process (Burris et al. 2000) has been subtracted. Bottom: residual abundance of CS 31082-001 after the Solar System r-process (Arlandini et al. 1999) has been subtracted.

From the discussion in the previous section, and from Figs. 11 and 12, it is clear that the neutron-capture elements in CS 31082-001 follow a standard pattern (i.e., they are indistinguishable from both the scaled Solar r-process pattern and the patterns observed in other metal-poor halo stars such as CS 22892-052) for elements $56 \leq Z \leq 72$. The small dispersion around the mean of the quantities ( $\log \epsilon_{{\rm CS~31082-001}} - \log \epsilon_{{\rm CS~22892-052}}$) and ( $\log \epsilon_{{\rm CS~31082-001}} - \log \epsilon_{r_{{\rm SS}}}$) in the range $56 \leq Z \leq 69$ (respectively 0.10 and 0.08 dex) reflect this level of agreement.

In CS 31082-001, the third neutron-capture peak is so far only sampled by abundance measurements of two elements (Os and Ir), and one upper limit (Pb). There may be marginal evidence for departure from the Solar r-process (Os seems overabundant), but it is premature to conclude firmly on this point (see Sect. 4.5). The third-peak abundance determinations clearly demand confirmation from better measurements and laboratory data (Os), and from new detections (Pt, Pb, Bi), which can only be done from space, as the strongest lines of these elements are too far in the UV region to be reached from the ground.

On the other hand, the actinides, although only probed by the two radioactive nuclides Th and U, do appear to be enhanced in CS 31082-001 to a higher level than observed for elements of the second r-process peak. Given the very high ratios of $\log$(Th/Eu) =-0.22 dex (where Eu is taken as a typical example of the elements $56 \leq Z \leq 69$) compared to other halo stars (for example, CS 22892-052 with $\log$(Th/Eu) =-0.66, and 115 with $\log$(Th/Eu) =-0.60) it is difficult to conceive any reasonable scenario that would account for this by an age difference: CS 22892-052 and 115 would then be 20 and 18 Gyrs older than CS 31082-001, respectively (regardless of the adopted production ratio for Th/Eu), which seems unrealistic.

We are thus left with the possibility that the actinides were enhanced ab initio by a larger factor than the elements of the second r-process peak in the matter that gave birth to CS 31082-001. This is the first time that such a large departure ($\sim$0.4 dex) from the otherwise standard Solar r-process pattern has been observed in a halo star, and the implications are important. If the actinides are not necessarily produced together with the lighter neutron-capture elements ( $56 \leq Z \leq 72$), and their initial proportions are therefore not fixed, but instead vary from star to star, then any chronometer based on ratios of an actinide to any stable element from the second r-process peak is doomed to failure. The Th/Eu ratio in CS 31082-001 is an extreme example of such a failure (see Sect. 6).

Figure 12: Neutron-capture-element abundances of CS 31082-001 compared to CS 22892-052 (Sneden et al. 2000a). The abscissa for CS 22892-052 has been artificially shifted by +0.3 for readability, and the abundances were scaled up by $<\log \epsilon_{{\rm CS~31082-001}} - \log \epsilon_{{\rm CS~22892-052}}$ > $_{56\leq Z\leq 69}$ =+0.17dex. The two open symbols are our own estimates for the Pb and U content of CS 22892-052 (see Sect. 4.5). The full line is the Solar r-process fraction from Arlandini et al. (1999).

From the r-process modeling point of view, the de-coupling of the production of actinides from the production of lighter r-process elements is in fact not unexpected. Goriely & Arnould (2001) find in their superposition of CEVs (Canonical EVents) that reproduce the Solar r-process pattern, that the CEVs that are responsible for the synthesis of the actinides do not contribute to the synthesis of nuclides lighter than Pb. This point is considered in more detail by Schatz et al. (2002).

As a final remark, we compared abundances of all elements from Na to U, to the predictions of the Qian & Wasserburg (2001b, 2002) phenomenological model which describes the chemical enrichment in the early galaxy in terms of three components: the prompt enrichment (P) is the contribution from extremely massive stars and acts on an instantaneous timescale, and SN II are divided in two classes, the high-frequeny SN II (H) and low-frequency SN II (L) are responsible respectively for the second and third r-process peak elements, and for some iron and light r-process elements. As in Qian & Wasserburg (2001a), the total number of H events that contributed to the abundances of CS 31082-001 can be computed as $n_{{\rm H}}\simeq 52$ from the observed Eu abundance in this star (predictions for the other r-process elements are made according to empirical yields determined from the observed neutron-capture elements abundances in CS 22892-052 and 115), while the P component is responsible for all elements Na-Zn (where the yields are computed in Qian & Wasserburg (2002) directly from the observed abundances of extremely metal poor stars, 115 in this case). The overall excellent agreement between the observed abundances in CS 31082-001 and the predictions of Qian & Wasserburg seen in Fig. 13 is therefore showing primarily, (i) that the abundances Na-Zn of CS 31082-001 are in very good agreement with normal extremely metal poor stars; (ii) that the neutron-capture pattern up to Z=70 is also in very good agreement with their H yields (i.e. very close to the abundances of CS 22892-052) and (iii) that the Os, Th and U abundances are above the predictions, as previously discussed in this section.

Figure 13: Abundances of CS 31082-001 compared to the predictions of Qian & Wasserburg (2001b, 2002) three component phenomenological model.