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J. J. Cowan

Publications and source records attributed to J. J. Cowan.

36 records · Page 2Linked to original sources
arXiv2006

A Study of Compact Radio Sources in Nearby Face-on Spiral Galaxies. I. Long Term Evolution of M83

We present analyses of deep radio observations of M83 taken with the Very Large Array spanning fifteen years, including never before published observations from 1990 and 1998. We report on the evolution of 55 individual point sources, which include four of the six known historical supernovae in this galaxy. A total of 10 sources have X-ray counterparts from a {\em Chandra} survey. Each of these sources show non-thermal spectral indices, and most appear to be X-ray supernova remnants. Comparing the radio source list to surveys in optical and X-ray, we identify three optical/X-ray supernova remnants. Nearly half of the detected radio sources in these observations are coincident with known H II regions lying in the spiral arm structures of the galaxy. We also report on changes in emission from the complex nuclear region, which has shown variability at 20cm wavelengths. We confirm that the peak radio emission from the nucleus is not coincident with the known optical center. One lesser nuclear peak is consistent with the optical/IR nucleus. Previous dynamical studies of a ``dark'' nuclear mass indicate a possible match to other radio nuclear emission regions in M83.

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arXiv2005

A Radio and X-Ray Study of Historical Supernovae in M83

We report the results of 15 years of radio observations of the six historical supernovae (SNe) in M83 using the Very Large Array. We note the near linear decline in radio emission from SN 1957D, a type II SN, which remains a non-thermal radio emitter. The measured flux densities from SNe 1923A and 1950B have flattened as they begin to fade below detectable limits, also type II SNe. The luminosities for these three SNe are comparable with the radio luminosities of other decades-old SNe at similar epochs. SNe 1945B, 1968L, and 1983N were not detected in the most recent observations and these non-detections are consistent with previous studies. We report the X-ray non-detections of all six historical SNe using the Chandra X-ray Observatory, consistent with previous X-ray searches of other decades-old SNe, and low inferred mass loss rates of the progenitors.

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arXiv2005

Sm Transition Probabilities and Abundances

Radiative lifetimes, accurate to +/- 5%, have been measured for 212 odd-parity levels of Sm II using laser-induced fluorescence. The lifetimes are combined with branching fractions measured using Fourier-transform spectrometry to determine transition probabilities for more than 900 lines of Sm II. This work is the largest-scale laboratory study to date of Sm II transition probabilities using modern methods. This improved data set has been used to determine a new solar photospheric Sm abundance, log epsilon = 1.00 +/- 0.03, from 26 lines. The spectra of three very metal-poor, neutron-capture-rich stars also have been analyzed, employing between 55 and 72 Sm II lines per star. The abundance ratios of Sm relative to other rare earth elements in these stars are in agreement, and are consistent with ratios expected from rapid neutron-capture nucleosynthesis (the r-process).

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arXiv2004

The Rise of the s-Process in the Galaxy

From newly-obtained high-resolution, high signal-to-noise ratio spectra the abundances of the elements La and Eu have been determined over the stellar metallicity range -3<[Fe/H]<+0.3 in 159 giant and dwarf stars. Lanthanum is predominantly made by the s-process in the solar system, while Eu owes most of its solar system abundance to the r-process. The changing ratio of these elements in stars over a wide metallicity range traces the changing contributions of these two processes to the Galactic abundance mix. Large s-process abundances can be the result of mass transfer from very evolved stars, so to identify these cases, we also report carbon abundances in our metal-poor stars. Results indicate that the s-process may be active as early as [Fe/H]=-2.6, alalthough we also find that some stars as metal-rich as [Fe/H]=-1 show no strong indication of s-process enrichment. There is a significant spread in the level of s-process enrichment even at solar metallicity.

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arXiv2003

Binary Blue Metal-Poor Stars: Evidence for AGB Mass Transfer

We present new abundance analyses of six blue metal-poor (BMP) stars with very low iron abundances ([Fe/H] < -2), based on new high resolution echelle spectra. Three are spectroscopic binaries and three have constant radial velocities. The chemical compositions of these two groups are very different, as the binary BMP stars have large enhancements of carbon and neutron-capture elements that are products of s-process nucleosynthesis. One star, CS 29497-030, has an extreme enhancement of lead, [Pb/Fe] = +3.7, the largest abundance in any star yet discovered. It probably also has an oxygen overabundance compared to the other BMP stars of our sample. The binary BMP stars must have attained their status via mass transfer during the asymptotic giant branch (AGB) evolutions of their companion stars, which are now unseen and most likely are compact objects. We have not found any examples of AGB mass transfer among BMP binaries with [Fe/H] > -2.

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arXiv2003

The Extremely Metal-Poor, Neutron-Capture-Rich Star CS 22892-052: A Comprehensive Abundance Analysis

High-resolution spectra obtained with three ground-based facilities and the Hubble Space Telescope (HST) have been combined to produce a new abundance analysis of CS 22892-052, an extremely metal-poor giant with large relative enhancements of neutron-capture elements. A revised model stellar atmosphere has been derived with the aid of a large number of Fe-peak transitions, including both neutral and ionized species of six elements.Several elements, including Mo, Lu, Au, Pt and Pb, have been detected for the first time in CS 22892-052, and significant upper limits have been placed on the abundances of Ga, Ge, Cd, Sn, and U in this star. In total, abundance measurements or upper limits have been determined for 57 elements, far more than previously possible. New Be and Li detections in CS 22892-052 indicate that the abundances of both these elements are significantly depleted compared to unevolved main-sequence turnoff stars of similar metallicity. Abundance comparisons show an excellent agreement between the heaviest n-capture elements (Z >= 56) and scaled solar system r-process abundances, confirming earlier results for CS 22892-052 and other metal-poor stars. New theoretical r-process calculations also show good agreement with CS 22892-052 abundances as well as the solar r-process abundance components.The abundances of lighter elements (40<= Z <= 50), however, deviate from the same scaled abundance curves that match the heavier elements, suggesting different synthesis conditions or sites for the low-mass and high-mass ends of the abundance distribution. The detection of Th and the upper limit on the U abundance together imply a lower limit of 10.4 Gyr on the age of CS 22892-052, quite consistent with the Th/Eu age estimate of 12.8 +/- ~= 3 Gyr. An average of several chronometric ratios yields an age 14.2 +/- ~= 3 Gyr.

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arXiv2002

r-Process Abundance Signatures

Abundance observations indicate the presence of rapid-neutron capture (i.e., r-process) elements in old Galactic halo and globular cluster stars. These observations demonstrate that the earliest generations of stars in the Galaxy, responsible for neutron-capture synthesis and the progenitors of the halo stars, were rapidly evolving. Abundance comparisons among large numbers of stars provide clues about the nature of neutron-capture element synthesis both during the earliest times and throughout the history of the Galaxy. In particular, these comparisons suggest differences in the way the heavier (including Ba and above) and lighter neutron capture elements are synthesized in nature. Understanding these differences will help to identify the astrophysical site (or sites) of and conditions in the r-process. The abundance comparisons also demonstrate a large star-to-star scatter in the neutron-capture/iron ratios at low metallicities- which disappears with increasing [Fe/H]- and suggests an early, chemically unmixed and inhomogeneous Galaxy. The very recent neutron-capture element observations indicate that the early phases of Galactic nucleosynthesis, and the associated chemical evolution, are quite complex, with the yields from different (progenitor) mass-range stars contributing to different chemical mixes. Stellar abundance comparisons suggest a change from the r-process to the slow neutron capture (i.e., s-) process at higher metallicities (and later times) in the Galaxy. Finally, the detection of thorium and uranium in halo and globular cluster stars offers a promising, independent age-dating technique that can put lower limits on the age of the Galaxy and thus the Universe.

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arXiv2002

Observations of Neutron-Capture Elements in the Early Galaxy

Neutron-capture elements in low metallicity Galactic halo stars vary widely both in overall contents and detailed abundance patterns. This review discusses recent observational results on the n-capture elements, discussing the implications for early Galactic nucleosynthesis of: (a) the star-to-star ``bulk'' variations in the n-capture/Fe abundance ratios; (b) the distinct signature of rapid n-capture synthesis events in many (most?) of the lowest metallicity stars; (c) the existence of metal-poor stars heavily enriched in the products of slow n-capture synthesis reactions; and (d) the now-routine detection of radioactive thorium (and even uranium in one and possibly two cases) in the spectra of metal-poor stars.

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arXiv2002

Probing the Neutron-Capture Nucleosynthesis History of Galactic Matter

The heavy elements formed by neutron capture processes have an interesting history from which we can extract useful clues to and constraints upon both the characteristics of the processes themselves and the star formation and nucleosynthesis history of Galactic matter. Of particular interest in this regard are the heavy element compositions of extremely metal-deficient stars. At metallicities [Fe/H] <= -2.5, the elements in the mass region past barium (A >= 130-140 have been found (in non carbon-rich stars) to be pure r-process products. The identification of an environment provided by massive stars and associated Type II supernovae as an r-process site seems compelling. Increasing levels of heavy s-process (e.g., barium) enrichment with increasing metallicity, evident in the abundances of more metal-rich halo stars and disk stars, reflect the delayed contributions from the low- and intermediate-mass (M \~ 1-3 Msol) stars that provide the site for the main s-process nucleosynthesis component during the AGB phase of their evolution. New abundance data in the mass region 60 <~ A <~ 130 is providing insight into the identity of possible alternative r-process sites. We review recent observational studies of heavy element abundances both in low metallicity halo stars and in disk stars, discuss the observed trends in light of nucleosynthesis theory, and explore some implications of these results for Galactic chemical evolution, nucleosynthesis, and nucleocosmochronology.

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arXiv2001

Nucleosynthesis Clocks and the Age of the Galaxy

Nucleocosmochronology involves the use of the abundances of radioactive nuclear species and their radiogenic decay daughters to establish the finite age of the elements and the time scale for their formation. While there exist radioactive products of several specific nucleosynthesis mechanisms that can reveal the histories of these mechanisms, it is the long lived actinide isotopes {232}Th, {235}U, and {238}U, formed in the r-process, that currently play the major role in setting the time scale of Galactic nucleosynthesis. Age determinations in the context of Galactic chemical evolution studies are constrained by intrinsic model uncertainties. Recent studies have rather taken the alternative approach of dating individual stars. Thorium/europium dating of field halo stars and globular cluster stars yields ages on the order of 15 +/- 4 Gyr. A solid lower limit on stellar ages is available as well, for stars for which one both knows the thorium abundance and has an upper limit on the uranium abundance. For the cases of the two extremely metal deficient field halo stars CS 22892-052 and HD 115444, lower limits on the nuclear age of Galactic matter lie in the range 10-11 Gyr. Th/U dating of the star CS 31082-00 gives an age of 12.5 +/- 3 Gyr. Observations of thorium and uranium abundances in globular cluster stars should make possible nuclear determinations of the ages of clusters that can be compared with ages from conventional stellar evolution considerations.

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arXiv2001

Galactic Cosmochronometry from Radioactive Elements in the Spectra of Very Old Metal-Poor Stars

In a short review of neutron-capture elemental abundances in Galactic halo stars, emphasis is placed on the use of these elements to estimate the age of the Galactic halo. Two prominent characteristics of neutron-capture elements in halo stars are their large star-to-star scatter in the overall abundance level with respect to lighter elements, and the dominance of r-process abundance patterns at lowest stellar metallicities. The r-process abundance signature potentially allows the direct determination of the age of the earliest Galactic halo nucleosynthesis events, but further developments in r-process theory, high resolution spectroscopy of very metal-poor stars, and in basic atomic data are needed to narrow the uncertainties in age estimates. Attention is brought to the importance of accurate transition probabilities in neutron-capture element cosmochronometry. Recent progress in the transition probabilities of rare earth elements is discussed, along with suggestions for future work on other species.

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arXiv2001

The Early Formation, Evolution and Age of the Neutron-Capture Elements in the Early Galaxy

Abundance observations indicate the presence of rapid-neutron capture (i.e., r-process) elements in old Galactic halo and globular cluster stars. These observations demonstrate that the earliest generations of stars in the Galaxy, responsible for neutron-capture synthesis and the progenitors of the halo stars, were rapidly evolving. Abundance comparisons among several halo stars show that the heaviest neutron-capture elements (including Ba and heavier) are consistent with a scaled solar system r-process abundance distribution, while the lighter such elements do not conform to the solar pattern. These comparisons suggest two r-process sites or at least two different sets of astrophysical conditions. The large star-to-star scatter observed in the neutron-capture/iron ratios at low metallicities -- which disappears with increasing [Fe/H] -- suggests an early, chemically unmixed and inhomogeneous Galaxy. The stellar abundances indicate a change from the r-process to the slow neutron capture (i.e., s-) process at higher metallicities in the Galaxy. The detection of thorium in halo and globular cluster stars offers a promising, independent age-dating technique that can put lower limits on the age of the Galaxy.

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arXiv2001

Halo Star Abundances and r-Process Synthesis

We review recent observational studies of heavy element abundances in low metallicity stars and explore some implications of these results for nucleosynthesis and early Galactic chemical evolution.

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arXiv2001

Neutron-Capture Element Trends in the Halo

In a brief review of abundances neutron-capture elements (Z > ~30) in metal-poor halo stars, attention is called to their star-to-star scatter, the dominance of r-process synthesis at lowest metallicities, the puzzle of the lighter members of this element group, and the possibility of a better r-/s-process discriminant.

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arXiv2001

Nuclear Chronometers

Observations of metal-poor Galactic halo stars indicate that the abundance pattern of the (heaviest) neutron-capture elements is consistent with the scaled solar system r-process abundances. Utilizing the radioactive (r-process) element thorium, age determinations have been made for several of these same stars, placing constraints on both Galactic and cosmological age estimates.

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arXiv2000

The Age of the Galaxy from Thorium Cosmochronometry

The long-lived radioactive element thorium can potentially provide a simple and direct clock to determine the age of our Galaxy. Spectroscopic investigations of thorium in metal-poor stars have yielded some promising initial results. We discuss the major observational and theoretical limitations in thorium cosmochronometry, and point out the ways in which the implied Galactic ages from thorium abundances can be made more accurate.

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arXiv1998

The Probable Detection of SN 1923A: The Oldest Radio Supernova?

Based upon the results of VLA observations, we report the detection of two unresolved radio sources that are coincident with the reported optical position of SN 1923A in M83. For the source closest to the SN position, the flux density was determined to be 0.30 +/- 0.05 mJy at 20 cm and 0.093 +/- 0.028 mJy at 6 cm. The flux density of the second nearby source was determined to be 0.29 +/- 0.05 at 20 cm and 0.13 +/- 0.028 at 6 cm. Both sources are non-thermal with spectral indices of alpha = -1.0 +/- 0.30 and -0.69 +/- 0.24, respectively. SN 1923A has been designated as a Type II-P. No Type II-P (other than SN 1987A) has been detected previously in the radio. The radio emission from both sources appears to be fading with time. At an age of approximately 68 years when we observed it, this would be the oldest radio supernova (of known age) yet detected.

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