We report the discovery of nine metal-poor stars with high levels of r-process enhancement (+0.81<[Eu/Fe]<+1.13), including six subgiants and three stars on the red horizontal branch. We also analyze four previously-known r-process-enhanced metal-poo
r red giants. From this sample of 13 stars, we draw the following conclusions. (1) High levels of r-process enhancement are found in a broad range of stellar evolutionary states, reaffirming that this phenomenon is not associated with a chemical peculiarity of red giant atmospheres. (2) Only 1 of 10 stars observed at multiple epochs shows radial velocity variations, reaffirming that stars with high levels of r-process enhancement are not preferentially found among binaries. (3) Only 2 of the 13 stars are highly-enhanced in C and N, indicating that there is no connection between high levels of r-process enhancement and high levels of C and N. (4) The dispersions in [Sr/Ba] and [Sr/Eu] are larger than the dispersions in [Ba/Eu] and [Yb/Eu], suggesting that the elements below the second r-process peak do not always scale with those in the rare earth domain, even within the class of highly-r-process-enhanced stars. (5) The light-element (12<Z<30) abundances of highly-r-process-enhanced stars are indistinguishable from those with normal levels of r-process material at the limit of our data, 3.5 per cent (0.015 dex) on average. The nucleosynthetic sites responsible for the large r-process enhancements did not produce any detectable light-element abundance signatures distinct from normal core-collapse supernovae.
We use the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope to obtain new high-quality spectra covering the 1900 to 2360 Angstrom wavelength range for two metal-poor stars, HD 108317 and HD 128279. We derive abundances of Cu I
I, Zn II, As I, Se I, Mo II, and Cd II, which have not been detected previously in either star. Abundances derived for Ge I, Te I, Os II, and Pt I confirm those derived from lines at longer wavelengths. We also derive upper limits from the non-detection of W II, Hg II, Pb II, and Bi I. The mean [As/Fe] ratio derived from these two stars and five others in the literature is unchanged over the metallicity range -2.8 < [Fe/H] < -0.6, <[As/Fe]> = +0.28 +/- 0.14 (std. dev. = 0.36 dex). The mean [Se/Fe] ratio derived from these two stars and six others in the literature is also constant, <[Se/Fe]> = +0.16 +/- 0.09 (std. dev. = 0.26 dex). The As and Se abundances are enhanced relative to a simple extrapolation of the iron-peak abundances to higher masses, suggesting that this mass region (75 < A < 82) may be the point at which a different nucleosynthetic mechanism begins to dominate the quasi-equilibrium alpha-rich freezeout of the iron peak. <[CuII/CuI]> = +0.56 +/- 0.23 in HD 108317 and HD 128279, and we infer that lines of Cu I may not be formed in local thermodynamic equilibrium in these stars. The [Zn/Fe], [Mo/Fe], [Cd/Fe], and [Os/Fe] ratios are also derived from neutral and ionized species, and each ratio pair agrees within the mutual uncertainties, which range from 0.15 to 0.52 dex.
The elements germanium (Ge, Z=32), arsenic (As, Z=33), and selenium (Se, Z=34) span the transition from charged-particle or explosive synthesis of the iron-group elements to neutron-capture synthesis of heavier elements. Among these three elements, o
nly the chemical evolution of germanium has been studied previously. Here we use archive observations made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope and observations from several ground-based facilities to study the chemical enrichment histories of seven stars with metallicities -2.6 < [Fe/H] < -0.4. We perform a standard abundance analysis of germanium, arsenic, selenium, and several other elements produced by neutron-capture reactions. When combined with previous derivations of germanium abundances in metal-poor stars, our sample reveals an increase in the [Ge/Fe] ratios at higher metallicities. This could mark the onset of the weak s-process contribution to germanium. In contrast, the [As/Fe] and [Se/Fe] ratios remain roughly constant. These data do not directly indicate the origin of germanium, arsenic, and selenium at low metallicity, but they suggest that the weak and main components of the s-process are not likely sources.
Using near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope, we detect neutral tellurium in three metal-poor stars enriched by products of r-process nucleosynthesis, BD+17 3248, HD 108317,
and HD 128279. Tellurium (Te, Z=52) is found at the second r-process peak (A=130) associated with the N=82 neutron shell closure, and it has not been detected previously in Galactic halo stars. The derived tellurium abundances match the scaled solar system r-process distribution within the uncertainties, confirming the predicted second peak r-process residuals. These results suggest that tellurium is predominantly produced in the main component of the r-process, along with the rare earth elements.
Abundance observations indicate the presence of rapid-neutron capture (i.e., r-process) elements in old Galactic halo and globular cluster stars. Recent observations of the r-process-enriched star BD +17 3248 include new abundance determinations for
the neutron-capture elements Cd I (Z=48), Lu II (Z = 71) and Os II (Z = 76), the first detections of these elements in metal-poor r-process-enriched halo stars. Combining these and previous observations, we have now detected 32 n-capture elements in BD +17 3248. This is the most of any metal-poor halo star to date. For the most r-process-rich (i.e. [Eu/Fe] ~= 1) halo stars, such as CS 22892-052 and BD +17 3248, abundance comparisons show that the heaviest stable n-capture elements (i.e., Ba and above, Z >= 56) are consistent with a scaled solar system r-process abundance distribution. The lighter n-capture element abundances in these stars, however, do not conform to the solar pattern. These comparisons, as well as recent observations of heavy elements in metal-poor globular clusters, suggest the possibility of multiple synthesis mechanisms for the n-capture elements. The heavy element abundance patterns in most metal-poor halo stars do not resemble that of CS 22892-052, but the presence of heavy elements such as Ba in nearly all metal-poor stars without s-process enrichment indicates that r-process enrichment in the early Galaxy is common.
Heavy elements, those produced by neutron-capture reactions, have traditionally shown no star-to-star dispersion in all but a handful of metal-poor globular clusters (GCs). Recent detections of low [Pb/Eu] ratios or upper limits in several metal-poor
GCs indicate that the heavy elements in these GCs were produced exclusively by an r-process. Reexamining GC heavy element abundances from the literature, we find unmistakable correlations between the [La/Fe] and [Eu/Fe] ratios in 4 metal-poor GCs (M5, M15, M92, and NGC 3201), only 2 of which were known previously. This indicates that the total r-process abundances vary star-to-star (by factors of 2-6) relative to Fe within each GC. We also identify potential dispersion in two other GCs (M3 and M13). Several GCs (M12, M80, and NGC 6752) show no evidence of r-process dispersion. The r-process dispersion is not correlated with the well-known light element dispersion, indicating it was present in the gas throughout the duration of star formation. The observations available at present suggest that star-to-star r-process dispersion within metal-poor GCs may be a common but not ubiquitous phenomenon that is neither predicted by nor accounted for in current models of GC formation and evolution.
Dispersion among the light elements is common in globular clusters (GCs), while dispersion among heavier elements is less common. We present detection of r-process dispersion relative to Fe in 19 red giants of the metal-poor GC M92. Using spectra obt
ained with the Hydra multi-object spectrograph on the WIYN Telescope at Kitt Peak National Observatory, we derive differential abundances for 21 species of 19 elements. The Fe-group elements, plus Y and Zr, are homogeneous at a level of 0.07-0.16 dex. The heavy elements La, Eu, and Ho exhibit clear star-to-star dispersion spanning 0.5-0.8 dex. The abundances of these elements are correlated with one another, and we demonstrate that they were produced by r-process nucleosynthesis. This r-process dispersion is not correlated with the dispersion in C, N, or Na in M92, indicating that r-process inhomogeneities were present in the gas throughout star formation. The r-process dispersion is similar to that previously observed in the metal-poor GC M15, but its origin in M15 or M92 is unknown at present.
We examine the Pb and Th abundances in 27 metal-poor stars (-3.1 < [Fe/H] < -1.4) whose very heavy metal (Z > 56) enrichment was produced only by the rapid (r-) nucleosynthesis process. New abundances are derived from HST/STIS, Keck/HIRES, and VLT/UV
ES spectra and combined with other measurements from the literature to form a more complete picture of nucleosynthesis of the heaviest elements produced in the r-process. In all cases, the abundance ratios among the rare earth elements and the 3rd r-process peak elements considered (La, Eu, Er, Hf, and Ir) are constant and equivalent to the scaled solar system r-process abundance distribution. We compare the stellar observations with r-process calculations within the classical waiting-point approximation. In these computations a superposition of 15 weighted neutron-density components in the range 23 < log(n_n) < 30 is fit to the r-process abundance peaks to successfully reproduce both the stable solar system isotopic distribution and the stable heavy element abundance pattern between Ba and U in low-metallicity stars. Under these astrophysical conditions, which are typical of the main r-process, we find very good agreement between the stellar Pb r-process abundances and those predicted by our model. For stars with anomalously high Th/Eu ratios (the so-called actinide boost), our observations demonstrate that any nucleosynthetic deviations from the main r-process affect--at most--only the elements beyond the 3rd r-process peak, namely Pb, Th, and U. Our theoretical calculations also indicate that possible r-process abundance losses by nuclear fission are negligible for isotopes along the r-process path between Pb and the long-lived radioactive isotopes of Th and U.
We present the first detailed abundance analysis of the metal-poor giant HKII 17435-00532. This star was observed as part of the University of Texas long-term project Chemical Abundances of Stars in the Halo (CASH). A spectrum was obtained with the H
igh Resolution Spectrograph (HRS) on the Hobby-Eberly Telescope with a resolving power of R~15,000. Our analysis reveals that this star may be located on the red giant branch, red horizontal branch, or early asymptotic giant branch. We find that this metal-poor ([Fe/H]=-2.2) star has an unusually high lithium abundance (log eps (Li)=+2.1), mild carbon ([C/Fe]=+0.7) and sodium ([Na/Fe]=+0.6) enhancement, as well as enhancement of both s-process ([Ba/Fe]=+0.8) and r-process ([Eu/Fe]=+0.5) material. The high Li abundance can be explained by self-enrichment through extra mixing that connects the convective envelope with the outer regions of the H-burning shell. If so, HKII 17435-00532 is the most metal-poor star in which this short-lived phase of Li enrichment has been observed. The Na and n-capture enrichment can be explained by mass transfer from a companion that passed through the thermally-pulsing AGB phase of evolution with only a small initial enrichment of r-process material present in the birth cloud. Despite the current non-detection of radial velocity variations (over ~180 days), it is possible that HKII 17435-00532 is in a long-period or highly-inclined binary system, similar to other stars with similar n-capture enrichment patterns.
