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Browsing by Author "Ryan, Sean G."
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6Li/7Li estimates for metal-poor stars
Garcia Perez, A.E.; Aoki, W.; Inoue, S.; Ryan, Sean G.; Suzuki, T.K.; Chiba, M. (2009-09)Context. The presence of the lithium-6 isotope in some metal-poor stars is a matter of surprise because of the high values observed. Non-standard models of Big Bang nucleosynthesis and pre-Galactic cosmic ray fusion and ... -
Abundances and Evolution of Lithium in the Galactic Halo and Disk
Ryan, Sean G.; Kajino, T.; Beers, T.C.; Suzuki, T.K.; Romano, D.; Matteucci, F.; Rosolankova, K. (2001-03)We have measured the Li abundance of 18 stars with [2[[Fe/H][[1 and 6000[Teff[6400 K, a parameter range that was poorly represented in previous studies. We examine the Galactic chemical evolution (GCE) of this element, ... -
Abundances and Kinematics of Extremely Metal-Deficient, Carbon-Rich Halo Stars
Tsangarides, S.; Ryan, Sean G.; Beers, T.C. (Astronomical Society of the Pacific, 2003)To study the possible nucleosynthetic backgrounds of extremely metal-deficient, carbon-rich (EMDC) stars, we have conducted high-resolution spectroscopy of 26 candidate objects selected from HK-survey stars with [Fe/H]? ... -
Abundances in very metal-poor stars
Ryan, Sean G. (Institut d'Astrophysique et de Geophysique, 2000)Metal-poor stars provide information on the characteristics and chemical evolution of the halo population of the Galaxy, the first epoch of star formation and Galaxy formation (not just locally but with relevance to ... -
The AGB population in IC 1613 using JHK photometry
Sibbons, L. F.; Ryan, Sean G.; Irwin, M.; Napiwotzki, R. (2015-01-01)Context. A member of the Local Group, IC 1613 is a gas rich irregular dwarf galaxy that appears to have formed stars continuously over the last 10 Gyr and is relatively independent of external influences from other galaxies. ... -
The AGB population of NGC 6822 : Distribution and the C/M ratio from JHK photometry.
Sibbons, L. F.; Ryan, Sean G.; Cioni, M-R.L.; Irwin, M.; Napiwotzki, R. (2012-04)Context. NGC 6822 is an irregular dwarf galaxy and part of the Local Group. Its close proximity and apparent isolation provide a unique opportunity to study galactic evolution without any obvious strong external influences. ... -
The barium isotopic fractions in five metal-poor stars
Gallagher, Andrew; Ryan, Sean G.; Hosford, Adam; Garcia Perez, A E; Aoki, W; Honda, S (2012)Context. Theory and observations of heavy element nucleosynthesis are in conflict with one-another. Theory states that in the most metal-poor stars, the rapid (r-) neutron-capture nucleosynthetic process would be dominant ... -
The barium isotopic mixture for the metal-poor subgiant star HD 140283
Gallagher, A.J.; Ryan, Sean G.; Garcia Perez, A.E.; Aoki, W. (2010)Context: Current theory regarding heavy element nucleosynthesis in metal-poor environments states that the r-process would be dominant. The star HD 140283 has been the subject of debate after it appeared in some studies ... -
Beryllium Abundances in Halo Stars from Keck/HIRES Observations
Boesgaard, A.M.; Deliyannis, C.P.; King, J.R.; Ryan, Sean G.; Vogt, S.; Beers, T.C. (1999)We have determined the abundance of Be in stars with an array of metal abundances in order to enhance our understanding of the chemical evolution of the Galaxy, cosmic-ray theory, and cosmology. Observations of the Be II ... -
Big Bang Nucleosynthesis, Population III, and Stellar Genetics in the Galactic Halo
Ryan, Sean G. (2002)The diverse isotopic and elemental signatures produced in different nucle- osynthetic sites are passed on to successive generations of stars. By tracing these chemical signatures back through the stellar populations of the ... -
The Binary Frequency Among Carbon-enhanced, s-Process-rich, Metal-poor Stars
Lucatello, S.; Tsangarides, S.; Beers, T.C.; Carretta, E.; Gratton, R.G.; Ryan, Sean G. (2005)We discuss radial velocities for a sample of carbon-enhanced, s-process–rich, very metal-poor (CEMP-s) stars, analyzed with high-resolution spectroscopy obtained over multiple epochs. We find that ~68% of the stars in the ... -
Boron in the very metal-poor star BD-13 3442
Duncan, D.K.; Rebull, L.M.; Primas, F.; Boesgaard, A.M.; Deliyannis, C.P.; Hobbs, L.M.; King, J.R.; Ryan, Sean G. (1998)The Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) has been used to observe the boron 2500 °A region of BD −13 3442. At a metallicity of [Fe/H]=−3.00 this is the most metal- poor star ever ... -
Carbon enhanced metal-poor stars I : Chemical compositions of 26 stars
Aoki, W.; Beers, T.C.; Christlieb, N.; Norris, J.E.; Ryan, Sean G.; Tsangarides, S. (2007) -
Carbon-Enhanced Metal-Poor Stars. III : Main-Sequence Turn-Off Stars from the SDSS/SEGUE Sample
Aoki, W.; Beers, T.C.; Sivarani, T.; Marsteller, B.; Lee, Y.S.; Honda, S.; Norris, J.E.; Ryan, Sean G.; Carollo, D. (2008-05-10)The chemical compositions of seven CEMP turnoff stars are determined from high-resolution spectroscopy. Five are selected from the SDSS SEGUE sample of metal-poor stars. Another star chosen from the SDSS SEGUE sample has ... -
Carbon-enhanced metal-poor stars. Osmium and iridium abundances in the neutron-capture-enhanced subgiants CS31062-050 and LP625-44
Aoki, W.; Bisterzo, S.; Gallino, R.; Beers, T.C.; Norris, J.E.; Ryan, Sean G.; Tsangarides, S. (2006) -
Carbon-rich, Extremely Metal-Poor Population II stars
Ryan, Sean G. (Astronomical Society of the Pacific, 2003)A significant fraction of very metal-poor stars have unexpectedly high carbon abundances. I recap on their discovery, describe the results of high-resolution spectroscopic studies of their composition, and consider possible ... -
Chemical Composition fo the Carbon-rich, Extremely Metal Poor Star CS 29498-043
Aoki, W.; Norris, J.E.; Ryan, Sean G.; Beers, T.C.; Ando, H. (2002-09) -
The Chemical Composition of Carbon-rich, very Metal Poor Stars : A New Class of Mildly Carbon Rich Objects without Excess of Neutron-Capture Elements
Aoki, W.; Norris, J.E.; Ryan, Sean G.; Beers, T.C.; Ando, H. (2002-03) -
The Detection of Low Eu Abundances in Extremely Metal-poor Stars and the Origin of r-Process Elements
Ishimaru, Y.; Wanajo, S.; Aoki, W.; Ryan, Sean G. (2004-01) -
Estimation of Stellar Metal Abundance. II : A Recalibration of the Ca II K Technique, and the Autocorrelation Function Method
Beers, T.C.; Rossi, S.; Norris, J.E.; Ryan, Sean G.; Shefler, T. (1999)We have recalibrated a method for the estimation of stellar metal abundance, parameterized as [Fe/H], based on medium-resolution (1–2 Å) optical spectra (the majority of which cover the wavelength range 3700–4500 Å). The ...