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John J. Cowan

Publications and source records attributed to John J. Cowan.

48 records · Page 3Linked to original sources
arXiv2002

The Chemical Composition and Age of the Metal-Poor Halo Star BD +17^\circ 3248

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.

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arXiv2002

Europium Isotopic Abundances in Very Metal-poor Stars

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.

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arXiv2001

The Radio Recovery of SN 1970G: The Continuing Radio Evolution of SN 1970G

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.

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arXiv2001

The Fading Radio Emission from SN 1961V: Evidence for a Type II Peculiar Supernova?

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.

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arXiv2001

A Simple Model for r-Process Scatter and Halo Evolution

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.

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arXiv2000

Neutron-Capture Elements in the Early Galaxy: Insights from a Large Sample of Metal-Poor Giants

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.

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arXiv2000

Evidence of Multiple r-Process Sites in the Early Galaxy: New Observations of CS 22892-052

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.

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arXiv1999

The r-Process Enriched Low Metallicity Giant HD 115444

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.

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arXiv1999

R-Process Abundances and Chronometers in Metal-Poor Stars

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.

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arXiv1998

Discovery of a Nuclear X-Ray Source in NGC 7331: Evidence for a Massive Black Hole

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.

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arXiv1997

R-Process in Collapsing O/Ne/Mg Cores

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.

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