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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 High 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., Comment: 29 pages, 8 figures. Accepted for publication in ApJ. A full version of table 3 is available from the first author upon request

We have derived isotopic fractions of europium, samarium, and neodymium in two metal-poor giants with differing neutron-capture nucleosynthetic histories. These isotopic fractions were measured from new high resolution (R ~ 120,000), high signal-to-noise (S/N ~ 160-1000) spectra obtained with the 2dCoude spectrograph of McDonald Observatory's 2.7m Smith telescope. Synthetic spectra were generated using recent high-precision laboratory measurements of hyperfine and isotopic subcomponents of several transitions of these elements and matched quantitatively to the observed spectra. We interpret our isotopic fractions by the nucleosynthesis predictions of the stellar model, which reproduces s-process nucleosynthesis from the physical conditions expected in low-mass, thermally-pulsing stars on the AGB, and the classical method, which approximates s-process nucleosynthesis by a steady neutron flux impinging upon Fe-peak seed nuclei. Our Eu isotopic fraction in HD 175305 is consistent with an r-process origin by the classical method and is consistent with either an r- or an s-process origin by the stellar model. Our Sm isotopic fraction in HD 175305 suggests a predominantly r-process origin, and our Sm isotopic fraction in HD 196944 is consistent with an s-process origin. The Nd isotopic fractions, while consistent with either r-process or s-process origins, have very little ability to distinguish between any physical values for the isotopic fraction in either star. This study for the first time extends the n-capture origin of multiple rare earths in metal-poor stars from elemental abundances to the isotopic level, strengthening the r-process interpretation for HD 175305 and the s-process interpretation for HD196944., Comment: 40 pages, 16 figures. Accepted for publication in ApJ. Full versions of tables 4 and 5 are available from the first author upon request

Abundances of heavier elements (barium and beyond) in many neutron-capture-element-rich halo stars accurately replicate the solar system r-process pattern. However, abundances of lighter neutron-capture elements in these stars are not consistent with the solar system pattern. These comparisons suggest contributions from two distinct types of r-process synthesis events -- a so called main r-process for the elements above the 2nd r-process peak and a weak r-process for the lighter neutron-capture elements. We have performed r-process theoretical predictions to further explore the implications of the solar and stellar observations. We find that the isotopic composition of barium and the elemental Ba/Eu abundance ratios in r-process-rich low metallicity stars can only be matched by computations in which the neutron densities are in the range 23< log n_n < 28, values typical of the main r-process. For r-process conditions that successfully generate the heavy element pattern extending down to A=135, the relative abundance of I129 produced in this mass region appears to be at least 90% of the observed solar value. Finally, in the neutron number density ranges required for production of the observed solar/stellar 3rd r-process-peak (A~200), the predicted abundances of inter-peak element hafnium (Z=72, A~180) follow closely those of 3rd-peak elements and lead. Hf, observable from the ground and close in mass number to the 3rd r-process-peak elements, might also be utilized as part of a new nuclear chronometer pair, Th/Hf, for stellar age determinations., Comment: 42 pages, 2 tables, 12 figures; To appear in the Astrophysical Journal

Al Cameron, who died recently (October 3, 2005) at 80, was one of the giants in astrophysics. His insights were profound and his interests were wide-ranging. Originally trained as a nuclear physicist, he made major contributions in a number of fields, including nuclear reactions in stars, nucleosynthesis, the abundances of the elements in the Solar System, and the origin of the Solar System and the Moon. In 1957, Cameron and, independently, Burbidge, Burbidge, Fowler and Hoyle, wrote seminal papers on nuclear astrophysics. Most of our current ideas concerning element formation in stars have followed from those two pioneering and historical works. Al also made many contributions in the field of Solar System physics. Particularly noteworthy in this regard was Cameron's work on the formation of the Moon. Al was also a good friend and mentor of young people. Al Cameron will be missed by many in the community both for his scientific contributions and for his friendship., Comment: 8 pages, 6 figures; To appear in the Proceedings of the International Symposium on Nuclear Astrophysics - Nuclei in the Cosmos - IX (2006)

Abundance observations indicate the presence of often surprisingly large amounts of neutron capture (i.e., s- and r-process) elements in old Galactic halo and globular cluster stars. These observations provide insight into the nature of the earliest generations of stars in the Galaxy -- the progenitors of the halo stars -- responsible for neutron-capture synthesis. Comparisons of abundance trends can be used to understand the chemical evolution of the Galaxy and the nature of heavy element nucleosynthesis. In addition age determinations, based upon long-lived radioactive nuclei abundances, can now be obtained. These stellar abundance determinations depend critically upon atomic data. Improved laboratory transition probabilities have been recently obtained for a number of elements. These new gf values have been used to greatly refine the abundances of neutron-capture elemental abundances in the solar photosphere and in very metal-poor Galactic halo stars. The newly determined stellar abundances are surprisingly consistent with a (relative) Solar System r-process pattern, and are also consistent with abundance predictions expected from such neutron-capture nucleosynthesis., Comment: 8 pages, 2 figures, 1 table. To appear in the Proceedings of the NASA Laboratory Astrophysics Workshop in Las Vegas, NV (February 2006)

Employing high resolution spectra obtained with the near-UV sensitive detector on the Keck I HIRES, supplemented by data obtained with the McDonald Observatory 2-d coude, we have performed a comprehensive chemical composition analysis of the bright r-process-rich metal-poor red giant star HD221170. Analysis of 57 individual neutral and ionized species yielded abundances for a total of 46 elements and significant upper limits for an additional five. Model stellar atmosphere parameters were derived with the aid of ~200 Fe-peak transitions. From more than 350 transitions of 35 neutron-capture (Z > 30) species, abundances for 30 neutron-capture elements and upper limits for three others were derived. Utilizing 36 transitions of La, 16 of Eu, and seven of Th, we derive ratios of log epsilon(Th/La) = -0.73 (sigma = 0.06) and log epsilon(Th/Eu) = -0.60 (sigma = 0.05), values in excellent agreement with those previously derived for other r-process-rich metal-poor stars such as CS22892-052, BD+17 3248, and HD115444. Based upon the Th/Eu chronometer, the inferred age is 11.7 +/- 2.8 Gyr. The abundance distribution of the heavier neutron-capture elements (Z >= 56) is fit well by the predicted scaled solar system r-process abundances, as also seen in other r-process-rich stars. Unlike other r-process-rich stars, however, we find that the abundances of the lighter neutron-capture elements (37 < Z < 56) in HD221170 are also statistically in better agreement with the abundances predicted for the scaled solar r-process pattern., Comment: 22 pages, 12 figures (10 colour), 6 tables, accepted by ApJ

Employing spectra obtained with the new Keck I HIRES near-UV sensitive detector, we have performed a comprehensive chemical composition analysis of the binary blue metal-poor star CS29497-030. Abundances for 29 elements and upper limits for an additional seven have been derived, concentrating on elements largely produced via neutron-capture nucleosynthesis. Included in our analysis are the two elements that define the termination point of the slow neutron-capture process, lead and bismuth. We determine an extremely high value of [Pb/Fe] = +3.65 +/- 0.07 (sigma = 0.13) from three features, supporting the single-feature result obtained in previous studies. We also detect Bi for the first time in a metal-poor star. Our derived Bi/Pb ratio is in accord with those predicted from the most recent FRANEC calculations of the slow neutron-capture process in low-mass AGB stars. We find that the neutron-capture elemental abundances of CS29497-030 are best explained by an AGB model that also includes very significant amounts of pre-enrichment of rapid neutron-capture process material in the protostellar cloud out of which the CS29497-030 binary system formed. Thus, CS29497-030 is both an ``r+s'' and ``extrinsic AGB'' star. Furthermore, we find that the mass of the AGB model can be further constrained by the abundance of the light odd-element [Na/Fe] which is sensitive to the neutron excess., Comment: 7 pages = 4 + 2 colour encapsulated postscript figures + 1 table; to appear in ApJ Letters; additional jpeg figure available at ftp://www.astro.caltech.edu/users/iii/cs29497030

We present new abundance determinations of neutron-capture elements Ge, Zr, Os, Ir, and Pt in a sample of 11 metal-poor (-3.1 <= [Fe/H] <= -1.6) Galactic halo giant stars, based on Hubble Space Telescope UV and Keck I optical high-resolution spectroscopy. The stellar sample is dominated by r-process-rich stars such as the well-studied CS 22892-052 and bd+173248, but also includes the r-process-poor, bright giant HD 122563. Our results demonstrate that abundances of the 3rd r-process peak elements Os, Ir and Pt in these metal-poor halo stars are very well-correlated among themselves, and with the abundances of the canonical r-process element Eu (determined in other studies), thus arguing for a common origin or site for r-process nucleosynthesis of heavier (Z>56) elements. However, the large (and correlated) scatters of [Eu,Os,Ir,Pt/Fe] suggests that the heaviest neutron-capture r-process elements are not formed in all supernovae. In contrast, the Ge abundances of all program stars track their Fe abundances, very well. An explosive process on iron-peak nuclei (e.g., the alpha-rich freeze-out in supernovae), rather than neutron capture, appears to have been the dominant synthesis mechanism for this element at low metallicities -- Ge abundances seem completely uncorrelated with Eu., Comment: 35 pages, 5 tables, 7 figures; To appear in the Astrophysical Journal

We present new abundance observations of neutron-capture elements in Galactic stars. These include new Hubble Space Telescope (HST) detections of the elements Ge, Zr and Pt in a group of 11 halo stars. Correlations between these elements and Eu (obtained with ground-based observations), and with respect to metallicity, are also presented., Comment: 4 pages, 5 figures; To appear in the Proceedings of Cosmic Abundances as Records of Stellar Evolution and Nucleosynthesis

We present new Hubble Space Telescope (HST) abundance observations of the neutron-capture elements Ge, Zr, Os and Pt in a group of 11 Galactic halo stars, including the well studied r-process rich CS 22892-052 and the r-process poor, bright giant HD 122563. Abundance comparisons indicate that the third r-process peak elements Pt and Os seem to be correlated with Eu (obtained with ground-based observations) in these metal-poor halo stars, suggesting a common origin or site for these three n-capture elements., Comment: 4 pages, 3 figures; To appear in the Proceedings of the 12th Workshop on Nuclear Astrophysics, eds. W. Hillebrandt and E. Muller

The nature of SN 1961V has been uncertain. Its peculiar optical light curve and slow expansion velocity are similar to those of super-outbursts of luminous blue variables (LBVs), but its nonthermal radio spectral index and declining radio luminosity are consistent with decades-old supernovae (SNe). We have obtained Hubble Space Telescope STIS images and spectra of the stars in the vicinity of SN 1961V, and find Object 7 identified by Filippenko et al. to be the closest to the optical and radio positions of SN 1961V. Object 7 is the only point source detected in our STIS spectra and only its H-alpha emission is detected; it cannot be the SN or its remnant because of the absence of forbidden lines. While the H-alpha line profile of Object 7 is remarkably similar to that of eta Car, the blue color (similar to an A2Ib supergiant) and lack of appreciable variability are unlike known post-outburst LBVs. We have also obtained Very Long Baseline Array (VLBA) observations of SN 1961V at 18 cm. The non-detection of SN 1961V places a lower limit on the size of the radio-emitting region, 7.6 mas or 0.34 pc, which implies an average expansion velocity in excess of 4,400 km/s, much higher than the optical expansion velocity measured in 1961. We conclude the following: (1) A SN occurred in the vicinity of SN 1961V a few decades ago. (2) If the SN 1961V light maximum originates from a giant eruption of a massive star, Object 7 is the most probable candidate for the survivor, but its blue color and lack of significant variability are different from a post-outburst eta Car. (3) The radio SN and Object 7 could be physically associated with each other through a binary system. (4) Object 7 needs to be monitored to determine its nature and relationship to SN 1961V., Comment: 16 pages, 3 figures, accepted by the Astronomical Journal for the 2004 May issue

We follow the Galactic enrichment of three easily observed light n-capture elements Sr,Y,and Zr.Input stellar yields have been first separated into their respective main and weak s-process,and r-process components.The s-process yields from AGB stars are computed,exploring a wide range of efficiencies of the major neutron source,13C,and covering both disk and halo metallicities.AGBs have been shown to reproduce the main s-component in the solar system.The concurrent weak s-process,which accounts for the major fraction of the light s-process isotopes in the solar system and occurs in massive stars by the operation of the 22Ne n-source,is discussed in detail.Neither the main s-,nor the weak s-components are shown to contribute significantly to the n-capture element abundances observed in unevolved halo stars.We present a detailed analysis of a large database of spectroscopic observations of Sr,Y,Zr, Ba,and Eu for Galactic stars at various metallicities.Spectroscopic observations of Sr,Y,and Zr to Ba and Eu abundance ratios versus metallicity provide useful diagnostics of the types of n-capture processes forming Sr,Y and Zr.The observed [Sr,Y,Zr/Ba,Eu] ratio is clearly not flat at low metallicities,as we would expect if Ba,Eu and Sr,Y,Zr all had the same r-process origin.We discuss our chemical evolution predictions, taking into account the interplay between different processes to produce Sr-Y-Zr.We find hints for a primary process in low-metallicity massive stars, different from the 'classical s-process' and from the 'classical r-process',that we tentatively define LEPP (Lighter Element Primary Process).This allows us to revise the estimates of the r-process contributions to the solar Sr,Y and Zr abundances,as well as of the contribution to the s-only isotopes 86Sr,87Sr,96Mo., Comment: Accepted for ApJ, 52 pages, 12 figures

During the last several decades, there have been a number of advances in understanding the rapid neutron-capture process (i.e., the r-process). These advances include large quantities of high-resolution spectroscopic abundance data of neutron-capture elements, improved astrophysical models, and increasingly more precise nuclear and atomic physics data. The elemental abundances of the heavy neutron-capture elements, from Ba through the third r-process peak, in low-metallicity ([Fe/H] <~ -2.5) Galactic halo stars are consistent with the scaled (i.e., relative) solar system r-process abundance distribution. These abundance comparisons suggest that for elements with Z >= 56 the r-process is robust--appearing to operate in a relatively consistent manner over the history of the Galaxy--and place stringent constraints on r-process models. While not yet identified, neutron-rich ejecta outside of the core in a collapsing (Type II, Ib) supernova continues to be a promising site for the r-process. Neutron star binary mergers might also be a possible alternative site. Abundance comparisons of lighter n-capture elements in halo stars show variations with the scaled solar r-process curve and might suggest either multiple r-process sites, or, at least, different synthesis conditions in the same astrophysical site. Constraints on r-process models and clues to the progenitors of the halo stars--the earliest generations of Galactic stars--are also provided by the star-to-star abundance scatter of [Eu/Fe] at low metallicities in the early Galaxy. Finally, abundance observations of long-lived radioactive elements (such as Th and U) produced in the r-process can be used to determine the chronometric ages of the oldest stars, placing constraints on the lower limit age estimates of the Galaxy and the Universe., Comment: 17 pages, 8 figures; To appear in Carnegie Observatories Astrophysics Series, Vol. 4: Origin and Evolution of the Elements, ed. A. McWilliam & M. Rauch (Cambridge: Cambridge Univ. Press)

We report VLA radio observations of 29 SNe with ages ranging from 10 days to about 90 years past explosion. These observations significantly contribute to the existing data pool on such objects. Included are detections of known radio SNe~1950B, 1957D, 1970G, 1983N, the suspected radio SN 1923A, and the possible radio SN 1961V. None of the remaining 23 observations resulted in detections, providing further evidence to support the observed trend that most SNe are not detectable radio emitters. To investigate the apparent lack of radio emission from the SNe reported here, we have followed standard practice and used Chevalier's ``standard model'' to derive (upper limits to) the mass-loss rates for the super nova progenitors. These upper limits to the fluxes are consistent with a lack of circumstellar material needed to provide detectable radio emission for SNe at these ages and distances. Comparison of the radio luminosities of these supernovae as a function of age past explosion to other well-observed radio SNe indicates that the Type II SNe upper limits are more consistent with the extrapolated light curves of SN 1980K than of SN 1979C, suggesting that SN 1980K may be a more typical radio emitter than SN 1979C. For completeness, we have included an appendix where the results of analyses of the non-SN radio sources are presented. Where possible, we make (tentative) identifications of these sources using various methods., Comment: 42 pages, 9 fiugres, 5 tables; To appear in ApJ

We present detailed abundance observations of the metal-poor ([Fe/H] = --2.0) Galactic halo star BD+17 3248. We have detected gold in this star, for the first time in any halo star. We also detected a weak line at 3859.60 A in the spectrum of BD+17 3248 that we tentatively identify with uranium. This would be only the second such detection of this radioactive element in any metal-poor halo stars. Similarly to the case of CS 22892--052, the heavier n-capture elements (Z >= 56) fall on the scaled solar system r-process abundance curve, while the elements with Z = 40--50 in BD+17 3248 fall below that same solar curve. Employing the newly detected Th, U and third r-process peak element (e.g., Pt, Ir) abundances, chronometric age estimates have been made for BD+17 3248. The average value of the various chronometric pairs suggests an age of 13.8 +/- 4 Gyr for this star, consistent, within error limits, with other chronometric age determinations for metal-poor Galactic halo stars., Comment: 4 pages, 2 figures; In Proceedings of the 11th Workshop on Nuclear Astrophysics, eds. W. Hillebrandt and E. Muller, p. 176

We have combined new high-resolution spectra obtained with the Hubble Space Telescope (HST) and ground-based facilities to make a comprehensive new abundance analysis of the metal-poor, halo star BD +17^\circ 3248. We have detected the third r-process peak elements osmium, platinum, and (for the first time in a metal-poor star) gold, elements whose abundances can only be reliably determined using HST. Our observations illustrate a pattern seen in other similar halo stars with the abundances of the heavier neutron-capture elements, including the third r-process peak elements, consistent with a scaled solar system r-process distribution. The abundances of the lighter neutron-capture elements, including germanium and silver, fall below that same scaled solar r-process curve, a result similar to that seen in the ultra-metal-poor star CS 22892--052. A single site with two regimes or sets of conditions, or perhaps two different sites for the lighter and heavier neutron-capture elements, might explain the abundance pattern seen in this star. In addition we have derived a reliable abundance for the radioactive element thorium. We tentatively identify U II at 3859 A in the spectrum of BD +17^\circ 3248, which makes this the second detection of uranium in a very metal-poor halo star. Our combined observations cover the widest range in proton number (from germanium to uranium) thus far of neutron-capture elements in metal-poor Galactic halo stars. Employing the thorium and uranium abundances in comparison with each other and with several stable elements, we determine an average cosmochronological age for BD +17^\circ 3248 of 13.8 +/- 4 Gyr, consistent with that found for other similar metal-poor halo stars., Comment: 58 pages, 4 tables, 11 figures; To appear in ApJ Typo corrected

Europium isotopic abundance fractions are reported for the very metal-poor, neutron-capture-rich giant stars CS 22892-052, HD 115444, and BD +17 3248. The abundance fractions, derived from analysis of several strong Eu II lines appearing in high-resolution spectra of these stars, are in excellent agreement with each other and with their values in the Solar System: fraction(\iso{Eu}{151}) ~= fraction(\iso{Eu}{153}) ~= 0.5. Detailed abundance studies of very metal-poor stars have previously shown that the total elemental abundances of stable atoms with atomic numbers z >= 56 typically match very closely those of a scaled solar-system r-process abundance distribution. The present results for the first time extend this agreement to the isotopic level., Comment: 13 pages, 3 figures; To appear in ApJ Letters

Using the Very Large Array, we have detected radio emission from the site of SN 1970G in the Sc galaxy M101. These observations are 31 years after the supernova event, making SN 1970G the longest monitored radio supernova. With flux densities of 0.12 +/- 0.020 mJy at 6 cm and 0.16 +/- 0.015 mJy at 20 cm, the spectral index of -0.24 +/- 0.20 appears to have flattened somewhat when compared with the previously reported value of -0.56 +/- 0.11, taken in 1990. The radio emission at 20 cm has decayed since the 1990 observations with a power-law index of beta_20cm = -0.28 +/- 0.13. We discuss the radio properties of this source and compare them to those of other Type II radio supernovae., Comment: 11 pages, 1 table and 2 figures; To appear in Astrophysical Journal Letters

Using the Very Large Array (VLA), we have detected radio emission from the site of SN 1961V in the Sc galaxy NGC 1058. With a peak flux density of 0.063 +/- 0.008 mJy/beam at 6 cm and 0.147 +/- 0.026 mJy/beam at 18 cm, the source is non-thermal, with a spectral index of -0.79 +/- 0.23. Within errors, this spectral index is the same value reported for previous VLA observations taken in 1984 and 1986. The radio emission at both wavelengths has decayed since the mid 1980's observations with power-law indices of beta(20cm) = -0.69 +/- 0.23 and beta(6cm) = -1.75 +/- 0.16. We discuss the radio properties of this source and compare them with those of Type II radio supernovae and luminous blue variables., Comment: 19 pages, 3 figures; To appear in the Astronomical Journal

Recent observations of heavy elements produced by rapid neutron capture (r-process) in the halo have shown a striking and unexpected behavior: within a single star, the relative abundances of r-process elements heavier than Eu are the same as the same as those of solar system matter, while across stars with similar metallicity Fe/H, the r/Fe ratio varies over two orders of magnitude. In this paper we present a simple analytic model which describes a star's abundances in terms of its ``ancestry,'' i.e., the number of nucleosynthesis events (e.g., supernova explosions) which contributed to the star's composition. This model leads to a very simple analytic expression for the abundance scatter versus Fe/H, which is in good agreement with the data and with more sophisticated numerical models. We investigate two classes of scenarios for r-process nucleosynthesis, one in which r-process synthesis events occur in only \sim 4% of supernovae but iron synthesis is ubiquitous, and one in which iron nucleosynthesis occurs in only about 9% of supernovae. (the Wasserburg- Qian model). We find that the predictions in these scenarios are similar for [Fe/H] \ga -2.5, but that these models can be readily distinguished observationally by measuring the dispersion in r/Fe at [Fe/H] \la -3., Comment: AASTeX, 21 pages, includes 4 figures

New abundances for neutron-capture (n-capture) elements in a large sample of metal-poor giants from the Bond survey are presented. The spectra were acquired with the KPNO 4-m echelle and coude feed spectrographs, and have been analyzed using LTE fine-analysis techniques with both line analysis and spectral synthesis. Abundances of eight n-capture elements (Sr, Y, Zr, Ba, La, Nd, Eu, Dy) in 43 stars have been derived from blue (lambda = 4070--4710, R~20,000, S/N ratio~100-200) echelle spectra and red (lambda = 6100--6180, R~22,000, S/N ratio~100-200) coude spectra, and the abundance of Ba only has been derived from the red spectra for an additional 27 stars. Overall, the abundances show clear evidence for a large star-to-star dispersion in the heavy element-to-iron ratios. The new data also confirm that at metallicities [Fe/H] <~ --2.4, the abundance pattern of the heavy (Z >= 56) n-capture elements in most giants is well-matched to a scaled Solar System r-process nucleosynthesis pattern. The onset of the main r-process can be seen at [Fe/H] ~ --2.9. Contributions from the s-process can first be seen in some stars with metallicities as low as [Fe/H] ~ --2.75, and are present in most stars with metallicities [Fe/H] > --2.3. The lighter n-capture elements (Sr-Y-Zr) are enhanced relative to the heavier r-process element abundances. Their production cannot be attributed solely to any combination of the Solar System r- and main s-processes, but requires a mixture of material from the r-process and from an additional n-capture process which can operate at early Galactic time., Comment: Text + 5 Tables and 11 Figures: Submitted to the Astrophysical Journal

First results are reported of a new abundance study of neutron-capture elements in the ultra-metal-poor (UMP; [Fe/H] = -3.1) halo field giant star CS 22892-052. Using new high resolution, high signal-to-noise spectra, abundances of more than 30 neutron-capture elements (Z>30) have been determined. Six elements in the 40=56) stable neutron-capture elements in CS 22892-052 match well the scaled solar system r-process abundance distribution. From the observed Th abundance, an average age of ~= 16 +/- 4 Gyr is derived for cs22892-052, consistent with the lower age limit of ~= 11 Gyr derived from the upper limit on the U abundance. The concordance of scaled solar r-process and CS 22892-052 abundances breaks down for the lighter neutron-capture elements, supporting previous suggestions that different r-process production sites are responsible for lighter and heavier neutron-capture elements., Comment: To be published in the Astrophysical Journal Letters

New high resolution, very high signal-to-noise spectra of ultra-metal-poor (UMP) giant stars HD 115444 and HD 122563 have been gathered with the High-Resolution Echelle Spectrometer of the McDonald Observatory 2.7m Telescope. With these spectra, line identification and model atmosphere analyses have been conducted, emphasizing the neutron-capture elements. Twenty elements with Z > 30 have been identified in the spectrum of HD 115444. This star is known to have overabundances of the neutron-capture elements, but it has lacked a detailed analysis necessary to compare with nucleosynthesis predictions. The new study features a line-by-line differential abundance comparison of HD 115444 with the bright, well-studied halo giant HD 122563. For HD 115444, the overall metallicity is [Fe/H]~ -3.0. The abundances of the light and iron-peak elements generally show the same pattern as other UMP stars (e.g. overdeficiencies of manganese and chromium, overabundances of cobalt), but the differential analysis indicates several nucleosynthesis signatures that are unique to each star., Comment: To Appear in the Astrophysical Journal

Rapid neutron-capture (i.e., r-process) nucleosynthesis calculations, employing internally consistent and physically realistic nuclear physics input (QRPA beta-decay rates and the ETFSI-Q nuclear mass model), have been made. These calculations are compared with ground-based and HST observations of neutron-capture elements in the metal poor halo stars CS 22892--052, HD 115444, HD 122563 and HD 126238. The elemental abundances in all four metal-poor stars are consistent with the solar r-process elemental distribution for the elements Z >/= 56. These results strongly suggest, at least for those elements, that the relative elemental r-process abundances have not changed over the history of the Galaxy. This further suggests that there is one r-process site in the Galaxy, at least for elements Z >/= 56. Employing the observed stellar abundances of stable elements, in conjunction with the solar r-process abundances to constrain the calculations, predictions for the zero decay-age abundances of the radioactive elements Th and U are made. We compare these (least-squares-fit ETFSI-Q) predictions with newly derived observational values in three very metal-poor halo stars: HD 115444, CS 22892--052 and HD 122563. Within the observational errors the observed ratio of [Th/Eu] is the same in both CS 22892--052 and HD 115444. Comparing with the theoretical ratio suggests an average age of these two very metal poor stars to be approximately 15.6 +/- 4.6 Gyr, consistent with earlier radioactive age estimates and recent globular and cosmological age estimates., Comment: Astrophysical Journal, in press

Using ROSAT, we have made the first deep X-ray observations of NGC 7331, discovering a nuclear X-ray source coincident with the previously determined radio and optical nuclear centers. Positions, luminosities, and fluxes of X-ray sources in the field are compared to previously identified radio and optical sources. The nucleus of NGC 7331 has been analyzed to discern any new evidence supporting the presence of a massive black hole (MBH), and comparisons are made with other Low-Ionization Nuclear Emission-line Region (LINER) galaxies. The flux ratio of core radio to soft X-ray emission in NGC 7331 is lower than in other LINERs included in our sample., Comment: ApJ Letters in press

Ground- and space-based observations of stellar heavy element abundances are providing a clearer picture of the chemical evolution of the Galaxy. A large number of (r)apid and (s)low neutron capture process elements, including the first Hubble Space Telescope observations of Pt, Os, Pb and Ge, have been identified in metal-poor, galactic halo stars. In the very low metallicity (i.e. [Fe/H] $<$ --2.0) stars, the abundance pattern of the elements from Ba through the third neutron-capture peak (Os-Pt) is consistent with a scaled solar {\it r}-process distribution. These results support previous observations that demonstrate the operation of the {\it r}-process, including the synthesis of the heaviest such elements, early in the history of the Galaxy. New ground-based observations further confirm that the {\it s}-process element Ba and the {\it r}-process element Eu were both synthesized solely by the r-process at low metallicities, and indicate the onset of the {\it s}-process occurred near [Fe/H] = --2. Over a range of metallicity (--2.90 $<$ [Fe/H] $<$ --0.86) the data indicate that there exist real star-to-star differences in the ratios of the [n-capture/Fe] abundances as well as in the actual spectra of the stars., Comment: 10 pages, 6 figures To appear in the Proceedings of the Second Oak Ridge Symposium on Atomic & Nuclear Astrophysics

Until we can more carefully assess the validity of the physical assumptions made we withdraw this paper., Comment: 10 pages, LaTeX, AASTeXv4 macros, natbib macros. Withdrawn

Several circumstantial arguments point to the formation of the third r-process peak at A about 190, near platinum, in stars of mass of about 8-10 solar masses: 1) The delayed production of europium with respect to iron imposes a time scale that restricts the progenitor stars to less than about 10 solar masses; 2) the r-process demands a dominant robust mechanism at least for barium and above, since the relative abundance pattern of those r-process elements in low-metallicity stars is consistent with the solar pattern; 3) stars of about 8-10 solar masses produce nearly identical degenerate O/Ne/Mg cores that collapse due to electron capture; and 4) the resulting low-mass cores may produce both an r-process in a prompt explosion and a subsequent r-process in a neutrino driven wind. The prompt explosion of an O/Ne/Mg core yields low entropy and low electron fraction, and hence may produce a reasonable r-process peak at A about 190 as well as all of the r-process elements with Z greater than 56. The possible differences in the neutrino-driven wind and associated r-process due to the low-mass neutron stars expected in this mass range are also discussed., Comment: 16 pages, LaTeX aasms4; to be published in ApJ Letters

Abundance observations indicate the presence of often surprisingly large amounts of neutron capture (i.e., s- and r-process) elements in old Galactic halo and globular cluster stars. These observations provide insight into the nature of the earliest generations of stars in the Galaxy the progenitors of the halo stars responsible for neutron-capture synthesis. Comparisons of abundance trends can be used to understand the chemical evolution of the Galaxy and the nature of heavy element nucleosynthesis. In addition age determinations, based upon long-lived radioactive nuclei abundances, can now be obtained. These stellar abundance determinations depend critically upon atomic data. Improved laboratory transition probabilities have been recently obtained for a number of elements. These new gf values have been used to greatly refine the abundances of neutron-capture elemental abundances in the solar photosphere and in very metal-poor Galactic halo stars. The newly determined stellar abundances are surprisingly consistent with a (relative) Solar System r-process pattern, and are also consistent with abundance predictions expected from such neutron-capture nucleosynthesis.

Radio emission has been detected from a compact source which satisfies the criteria for a very young galactic supernova remnant. The source, G25.5 + 0.2 has a partially-filled shell structure, a total integrated flux density at 20 cm of 315 mJy, and a flat spectrum between 2 and 20 cm. Observations at 843 and 327 MHz indicate thermal absorption at low frequencies with a turnover in the spectrum near 1 GHz. It is suggested that the lower limit for the age of the supernova remnant is 25 yr, while the upper limit is about 100 yr. It is concluded that G25.5 + 0.2 could be the youngest known supernova remnant in the Galaxy.

Radio observations of the Sc galaxy NGC 1058 at 20 cm and 6 cm reveal two radio sources. One of the sources is coincident with supernova 1961V and has a spectral index of -0.4 + or - 0.3. This is the second (and probably the third) detection of radio emission from a supernova decades after optical maximum. The absolute luminosity of SN 1961V at 20 cm is comparable to Cas A. A second source, about 2.3 arcsec to the west, has a spectral index of -0.3 + or - 0.3 and is likely to be the remnant of a supernova that was not optically detected. Optical images of NGC 1058 show two faint H II regions associated with the radio sources. Filter photometry in several bands indicates that these two H II regions are normal with respect to the many optically brighter, albeit radio-quiet, H II regions in NGC 1058.

A number of sites have been suggested for the r-process, including neutronized cores of exploding supernovae, jets of neutronized matter ejected from the collapse of rotating magnetized stellar cores, the helium and carbon zones of stars undergoing supernova explosions, and helium core flashes in low-mass stars. Despite much work and many advances in nuclear physics, the site or sites of the r-process is still unknown. Observations of metal-poor stars in the halo of the Galaxy indicate r-process production early in the history of the Galaxy and provide important constraints on galactic nucleosynthesis. Further observations of metal-poor stars, along with advances in understanding the nuclear properties of neutron-rich nuclei and improved astrophysical models of stars in the late stages of evolution, should help to identify the site of the r-process.