Browsing by Subject "chemical-composition"

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The Abundances Of Neutron-Capture Species In The Very Metal-Poor Globular Cluster M15: A Uniform Analysis Of Red Giant Branch And Red Horizontal Branch Stars
(2011-06) Sobeck, Jennifer S.; Kraft, Robert P.; Sneden, Christopher; Preston, George W.; Cowan, John J.; Smith, Graeme H.; Thompson, Ian B.; Shectman, Stephen A.; Burley, Gregory S.; Sneden, Christopher
The globular cluster M15 is unique in its display of star-to-star variations in the neutron-capture elements. Comprehensive abundance surveys have been previously conducted for handfuls of M15 red giant branch (RGB) and red horizontal branch (RHB) stars. No attempt has been made to perform a single, self-consistent analysis of these stars, which exhibit a wide range in atmospheric parameters. In the current effort, a new comparative abundance derivation is presented for three RGB and six RHB members of the cluster. The analysis employs an updated version of the line transfer code MOOG, which now appropriately treats coherent, isotropic scattering. The apparent discrepancy in the previously reported values for the metallicity of M15 RGB and RHB stars is addressed and a resolute disparity of Delta(RHB-RGB) approximate to 0.1 dex in the iron abundance was found. The anti-correlative behavior of the light neutron-capture elements (Sr, Y, Zr) is clearly demonstrated with both Ba and Eu, standard markers of the s- and r-process, respectively. No conclusive detection of Pb was made in the RGB targets. Consequently for the M15 cluster, this suggests that the main component of the s-process has made a negligible contribution to those elements normally dominated by this process in solar system material. Additionally for the M15 sample, a large Eu abundance spread is confirmed, which is comparable to that of the halo field at the same metallicity. These abundance results are considered in the discussion of the chemical inhomogeneity and nucleosynthetic history of M15.
Atmospheric Composition Of Weak G Band Stars: CNO And Li Abundances
(2013-03) Adamczak, Jens; Lambert, David L.; Adamczak, Jens; Lambert, David L.
We determined the chemical composition of a large sample of weak G band stars-a rare class of G and K giants of intermediate mass with unusual abundances of C, N, and Li. We have observed 24 weak G band stars with the 2.7 m Harlan J. Smith Telescope at the McDonald Observatory and derived spectroscopic abundances for C, N, O, and Li, as well as for selected elements from Na-Eu. The results show that the atmospheres of weak G band stars are highly contaminated with CN-cycle products. The C underabundance is about a factor of 20 larger than for normal giants and the C-12/C-13 ratio approaches the CN-cycle equilibrium value. In addition to the striking CN-cycle signature the strong N overabundance may indicate the presence of partially ON-cycled material in the atmospheres of the weak G band stars. The exact mechanism responsible for the transport of the elements to the surface has yet to be identified but could be induced by rapid rotation of the main sequence progenitors of the stars. The unusually high Li abundances in some of the stars are an indicator for Li production by the Cameron-Fowler mechanism. A quantitative prediction of a weak G band star's Li abundance is complicated by the strong temperature sensitivity of the mechanism and its participants. In addition to the unusual abundances of CN-cycle elements and Li, we find an overabundance of Na that is in accordance with the NeNa chain running in parallel with the CN cycle. Apart from these peculiarities, the element abundances in a weak G band star's atmosphere are consistent with those of normal giants.
Boron Abundances In B-Type Stars: A Test Of Rotational Depletion During Main-Sequence Evolution
(2002-01) Venn, Kim A.; Brooks, A. M.; Lambert, David L.; Lemke, M.; Langer, N.; Lennon, D. J.; Keenan, F. P.; Lambert, David L.
Boron abundances have been derived for seven main-sequence B-type stars from Hubble Space Telescope STIS spectra around the B III lambda2066 line. In two stars, boron appears to be undepleted with respect to the presumed initial abundance. In one star, boron is detectable but is clearly depleted. In the other four stars, boron is undetectable, implying depletions of 1-2 dex. Three of these four stars are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only rotationally induced mixing predicts that boron depletions are unaccompanied by nitrogen enrichments. The inferred rate of boron depletion from our observations is in good agreement with these predictions. Other boron-depleted nitrogen-normal stars are identified from the literature. In addition, several boron-depleted nitrogen-rich stars are identified, and while all fall on the boron-nitrogen trend predicted by rotationally induced mixing, a majority have nitrogen enrichments that are not uniquely explained by rotation. The spectra have also been used to determine iron group (Cr, Mn, Fe, and Ni) abundances. The seven B-type stars have near-solar iron group abundances, as expected for young stars in the solar neighborhood. We have also analyzed the halo B-type star PG 0832 + 676. We find [Fe/H] = -0.88 +/- 0.10, and the absence of the B III line gives the upper limit [B/H] < -2.5. These and other published abundances are used to infer the star's evolutionary status as a post-asymptotic giant branch star.
Center-To-Limb Variation Of Solar Three-Dimensional Hydrodynamical Simulations
(2008-06) Koesterke, Lars; Prieto, Carlos Allende; Lambert, David L.; Koesterke, Lars; Prieto, Carlos Allende; Lambert, David L.
We examine closely the solar center-to-limb variation of continua and lines and compare observations with predictions from both a three-dimensional (3D) hydrodynamic simulation of the solar surface ( provided by M. Asplund and collaborators) and one-dimensional (1D) model atmospheres. Intensities from the 3D time series are derived by means of the new synthesis code ASS epsilon T, which overcomes limitations of previously available codes by including a consistent treatment of scattering and allowing for arbitrarily complex line and continuum opacities. In the continuum, we find very similar discrepancies between synthesis and observation for both types of model atmospheres. This is in contrast to previous studies that used a "horizontal'' and time-averaged representation of the 3D model and found a significantly larger disagreement with observations. The presence of temperature and velocity fields in the 3D simulation provides a significant advantage when it comes to reproducing solar spectral line shapes. Nonetheless, a comparison of observed and synthetic equivalent widths reveals that the 3D model also predicts more uniform abundances as a function of position angle on the disk. We conclude that the 3D simulation provides not only a more realistic description of the gas dynamics, but despite its simplified treatment of the radiation transport, it also predicts reasonably well the observed center-to-limb variation, which is indicative of a thermal structure free from significant systematic errors.
Clear Evidence for the Presence of Second-Generation Asymptotic Giant Branch Stars in Metal-Poor Galactic Globular Clusters
(2015-12) Garcia-Hernandez, D. A.; Meszaros, S.; Monelli, M.; Cassisi, S.; Stetson, P. B.; Zamora, O.; Shetrone, Matthew; Lucatello, Sara; Shetrone, Matthew
Galactic globular clusters (GCs) are known to host multiple stellar populations: a first generation (FG) with a chemical pattern typical of halo field stars and a second generation (SG) enriched in Na and Al and depleted in O and Mg. Both stellar generations are found at different evolutionary stages (e.g., the main-sequence turnoff, the subgiant branch, and the red giant branch (RGB)). The non detection of SG asymptotic giant branch (AGB) stars in several metal-poor ([Fe/H] < -1) GCs suggests that not all SG stars ascend the AGB phase, and that failed AGB stars may be very common in metal-poor GCs. This observation represents a serious problem for stellar evolution and GC Formation/evolution theories. We report fourteen SG-AGB stars in four metal-poor GCs (M13, M5, M3, and M2) with different observational properties: horizontal branch (HB) morphology, metallicity, and age. By combining the H-band Al abundances obtained by the Apache Point Observatory Galactic Evolution Experiment survey with ground-based optical photometry, we identify SG Al-rich AGB stars in these four GCs and show that Al-rich RGB/AGB GC stars should be Na-rich. Our observations provide strong support for present, standard stellar models, i.e., without including a strong mass-loss efficiency, for low-mass HB stars. In fact, current empirical evidence is in agreement with the predicted distribution of FG and SG stars during the He-burning stages based on these standard stellar models.
Convective Line Shifts For The Gaia RVS From The CIFIST 3D Model Atmosphere Grid
(2013-02) Prieto, C. Allende; Koesterke, L.; Ludwig, H. G.; Freytag, B.; Caffau, E.; Koesterke, L.
Context. To derive space velocities of stars along the line of sight from wavelength shifts in stellar spectra requires accounting for a number of second-order effects. For most stars, gravitational redshifts, convective blueshifts, and transverse stellar motion are the dominant contributors. Aims. We provide theoretical corrections for the net velocity shifts due to convection expected for the measurements from the Gaia Radial Velocity Spectrometer (RVS). Methods. We used a set of three-dimensional time-dependent simulations of stellar surface convection computed with CO5BOLD to calculate spectra of late-type stars in the Gaia RVS range and to infer the net velocity offset that convective motions will induce in radial velocities derived by cross-correlation. Results. The net velocity shifts derived by cross-correlation depend both on the wavelength range and spectral resolution of the observations. Convective shifts for Gaia RVS observations are less than 0.1 km s(-1) for late-K-type stars, and they increase with stellar mass, reaching about 0.3 k ms(-1) or more for early F-type dwarfs. This tendency is the result of an increase with effective temperature in both temperature and velocity fluctuations in the line-forming region. Our simulations also indicate that the net RVS convective shifts can be positive (i.e. redshifts) in some cases. Overall, the blueshifts weaken slightly with increasing surface gravity, and are enhanced at low metallicity. Gravitational redshifts amount to 0.7 km s(-1) and dominate convective blueshifts for dwarfs, but become much weaker for giants.
Detection of the Second r-Process Peak Element Tellurium in Metal-Poor Stars
(2012-03) Roederer, Ian U.; Lawler, James E.; Cowan, John J.; Beers, Timothy C.; Frebel, Anna; Ivans,, Inese I.; Schatz, Hendrik; Sobeck, Jennifer S.; Sneden, Christopher; Sneden, Christopher
Using near-ultraviolet spectra obtained with the Space Telescope Imaging Spectrograph on board 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 approximate to 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.
The Distribution Of The Elements In The Galactic Disk. III. A Reconsideration Of Cepheids From L=30 Degrees To 250 Degrees
(2011-10) Luck, R. Earle; Lambert, David L.; Lambert, David L.
This paper reports on the spectroscopic investigation of 238 Cepheids in the northern sky. Of these stars, about 150 are new to the study of the galactic abundance gradient. These new Cepheids bring the total number of Cepheids involved in abundance distribution studies to over 400. In this work, we also consider systematics between various studies and also those which result from the choice of models. We find that systematic variations exist at the 0.06 dex level both between studies and model atmospheres. In order to control the systematic effects our final gradients depend only on abundances derived herein. A simple linear fit to the Cepheid data from 398 stars yields a gradient d[Fe/H]/dR(G) = -0.062 +/- 0.002 dex kpc(-1) which is in good agreement with previously determined values. We have also re-examined the region of the >metallicity island> of Luck et al. With the doubling of the sample in that region and our internally consistent abundances, we find that there is scant evidence for a distinct island. We also find in our sample the first reported Cepheid (V1033 Cyg) with a pronounced Li feature. The Li abundance is consistent with the star being on its redward pass toward the first giant branch.
The Evolving Activity Of The Dynamically Young Comet C/2009 P1 (Garradd)
(2014-05) Bodewits, D.; Farnham, T. L.; A'Hearn, M. F.; Feaga, L. M.; McKay, A.; Schleicher, D. G.; Sunshine, J. M.; McKay, A.
We used the Ultraviolet-Optical Telescope on board Swift to observe the dynamically young comet C/2009 P1 (Garradd) from a heliocentric distance of 3.5 AU pre-perihelion until 4.0 AU outbound. At 3.5 AU pre-perihelion, comet Garradd had one of the highest dust-to-gas ratios ever observed, matched only by comet Hale-Bopp. The evolving morphology of the dust in its coma suggests an outburst that ended around 2.2 AU pre-perihelion. Comparing slit-based measurements and observations acquired with larger fields of view indicated that between 3 AU and 2 AU pre-perihelion a significant extended source started producing water in the coma. We demonstrate that this source, which could be due to icy grains, disappeared quickly around perihelion. Water production by the nucleus may be attributed to a constantly active source of at least 75 km(2), estimated to be > 20% of the surface. Based on our measurements, the comet lost 4 x 10(11) kg of ice and dust during this apparition, corresponding to at most a few meters of its surface. Even though this was likely not the comet's first passage through the inner solar system, the activity of Garradd was complex and changed significantly during the time it was observed.
Galactic R Coronae Borealis Stars: The C-2 Swan Bands, The Carbon Problem, And The C-12/C-13 Ratio
(2012-03) Hema, B. P.; Pandey, Gajendra; Lambert, David L.; Lambert, David L.
Observed spectra of R Coronae Borealis (RCB) and hydrogen-deficient carbon (HdC) stars are analyzed by synthesizing the C-2 Swan bands (1, 0), (0, 0), and (0, 1) using our detailed line list and the Uppsala model atmospheres. The (0, 1) and (0, 0) C-2 bands are used to derive the C-12 abundance, and the (1, 0) (CC)-C-12-C-13 band to determine the C-12/C-13 ratios. The carbon abundance derived from the C-2 Swan bands is about the same for the adopted models constructed with different carbon abundances over the range 8.5 (C/He = 0.1%) to 10.5 (C/He = 10%). Carbon abundances derived from C I lines are about a factor of four lower than the carbon abundance of the adopted model atmosphere over the same C/He interval, as reported by Asplund et al., who dubbed the mismatch between adopted and derived C abundance as the "carbon problem." In principle, the carbon abundances obtained from C-2 Swan bands and that assumed for the model atmosphere can be equated for a particular choice of C/He that varies from star to star. Then, the carbon problem for C-2 bands is eliminated. However, such C/He ratios are in general less than those of the extreme helium stars, the seemingly natural relatives to the RCB and HdC stars. A more likely solution to the C-2 carbon problem may lie in a modification of the model atmosphere's temperature structure. The derived carbon abundances and the C-12/C-13 ratios are discussed in light of the double degenerate and the final flash scenarios.
HE 1327-2326, An Unevolved Star With Fe/H < -5.0. II. New 3D-1D Corrected Abundances From A Very Large Telescope UVES Spectrum
(2008-09) Frebel, Anna; Collet, Remo; Eriksson, Kiell; Christlieb, Norbert; Aoki, Wako; Frebel, Anna
We present a new abundance analysis of HE 1327-2326, which is currently the most iron-poor star, based on observational data obtained with the VLT Ultraviolet and Visual Echelle Spectrograph (UVES). We correct the one-dimensional (1D) LTE abundances for three-dimensional (3D) effects to provide an abundance pattern that supersedes previous works and should be used to observationally test current models of the chemical yields of the first-generation supernovae (SNe). Apart from confirming the 1D LTE abundances found in previous studies before accounting for 3D effects, we make use of a novel technique to apply the 3D 1D corrections for CNO which are a function of excitation potential and line strength for the molecular lines that comprise the observable CH, NH, and OH features. We find that the fit to the NH band at 33608 is greatly improved due to the application of the 3D-1D corrections. This may indicate that 3D effects are actually observable in this star. We also report the first detection of several weak Ni lines. The cosmologically important element Li is still not detected; the new Li upper limit is extremely low, A(Li) < 0: 62, and in stark contrast with results not only from the Wilkinson Microwave Anisotropy Probe (WMAP) but also from other metal-poor stars. We also discuss how the new corrected abundance pattern of HE 1327-2326 is being reproduced by individual and integrated yields of SNe.
High Precision Abundances of the Old Solar Twin HIP 102152: Insights On Li Depletion from the Oldest Sun
(2013-09) Monroe, TalaWanda R.; Melendez, Jorge; Ramirez, Ivan; Yong, David; Bergemann, Maria; Asplund, Martin; Bedell, Megan; Maia, Marcelo Tucci; Bean, Jacob; Lind, Karin; Alves-Brito, Alan; Casagrande, Luca; Castro, Matthieu; do Nascimento, Jose-Dias; Bazot, Michael; Freitas, Fabricio C.; Ramirez, Ivan
We present the first detailed chemical abundance analysis of the old 8.2 Gyr solar twin, HIP 102152. We derive differential abundances of 21 elements relative to the Sun with precisions as high as 0.004 dex (<= 1%), using ultra high-resolution (R = 110,000), high S/N UVES spectra obtained on the 8.2 m Very Large Telescope. Our determined metallicity of HIP 102152 is [Fe/H] = -0.013 +/- 0.004. The atmospheric parameters of the star were determined to be 54 K cooler than the Sun, 0.09 dex lower in surface gravity, and a microturbulence identical to our derived solar value. Elemental abundance ratios examined versus dust condensation temperature reveal a solar abundance pattern for this star, in contrast to most solar twins. The abundance pattern of HIP 102152 appears to be the most similar to solar of any known solar twin. Abundances of the younger, 2.9 Gyr solar twin, 18 Sco, were also determined from UVES spectra to serve as a comparison for HIP 102152. The solar chemical pattern of HIP 102152 makes it a potential candidate to host terrestrial planets, which is reinforced by the lack of giant planets in its terrestrial planet region. The following non-local thermodynamic equilibrium Li abundances were obtained for HIP 102152, 18 Sco, and the Sun: log epsilon (Li) = 0.48 +/- 0.07, 1.62 +/- 0.02, and 1.07 +/- 0.02, respectively. The Li abundance of HIP 102152 is the lowest reported to date for a solar twin, and allows us to consider an emerging, tightly constrained Li-age trend for solar twin stars.
High-Dispersion Infrared Spectroscopic Observations Of Comet 8P/Tuttle With VLT/CRIRES
(2010-01) Kobayashi, H.; Bockelee-Morvan, D.; Kawakita, H.; Dello Russo, N.; Jehin, E.; Manfroid, J.; Smette, A.; Hutsemekers, D.; Stuwe, J.; Weiler, M.; Arpigny, C.; Biver, N.; Cochran, A.; Crovisier, J.; Magain, P.; Sana, H.; Schulz, R.; Vervack, R. J.; Weaver, H.; Zucconi, J. M.; Cochran, A.
We report on the composition of the Halley-family comet (HFC) 8P/Tuttle investigated with high-dispersion near-infrared spectroscopic observations. The observations were carried out at the ESO VLT (Very Large Telescope) with the CRIRES instrument as part of a multi-wavelength observation campaign of 8P/Tuttle performed in late January and early February 2008. Radar observations suggested that 8P/Tuttle is a contact binary, and it was proposed that these components might be heterogeneous in chemistry. We determined mixing ratios of organic volatiles with respect to H(2)O and found that mixing ratios were consistent with previous near infrared spectroscopic observations obtained in late December 2007 and in late January 2008. It has been suggested that because 8P/Tuttle is a contact binary, it might be chemically heterogeneous. However, we find no evidence for chemical heterogeneity within the nucleus of 8P/Tuttle. We also compared the mixing ratios of organic molecules in 8P/Tuttle with those of both other HFCs and long period comets (LPCs) and found that HCN, C(2)H(2), and C(2)H(6) are depleted whereas CH(4) and CH(3)OH have normal abundances. This may indicate that 8P/Tuttle was formed in a different region of the early solar nebula than other HFCs and LPCs. We estimated the conversion efficiency from C(2)H(2) to C(2)H(6) by hydrogen addition reactions on cold grains by employing the C(2)H(6)/(C(2)H(6)+C(2)H(2)) ratio. The C(2)H(6)/(C(2)H(6)+C(2)H(2)) ratio in 8P/Tuttle is consistent with the ratios found in other HFCs and LPCs within the error bars. We also discuss the source of C(2) and CN based on our observations and conclude that the abundances in the comet, so at least one additional parent is needed for each species, as pointed out in previous study.
Identification and Quantification of Particle Growth Channels During New Particle Formation
(2013) Pennington, M. R.; Bzdek, B. R.; DePalma, J. W.; Smith, J. N.; Kortelainen, A. M.; Ruiz, L. H.; Petaja, T.; Kulmala, M.; Worsnop, D. R.; Johnston, M. V.; Petaja, T.; Kulmala, M.; Worsnop, D. R.
Atmospheric new particle formation (NPF) is a key source of ambient ultrafine particles that may contribute substantially to the global production of cloud condensation nuclei (CCN). While NPF is driven by atmospheric nucleation, its impact on CCN concentration depends strongly on atmospheric growth mechanisms since the growth rate must exceed the loss rate due to scavenging in order for the particles to reach the CCN size range. In this work, chemical composition measurements of 20 nm diameter particles during NPF in Hyytiala, Finland, in March-April 2011 permit identification and quantitative assessment of important growth channels. In this work we show the following: (A) sulfuric acid, a key species associated with atmospheric nucleation, accounts for less than half of particle mass growth during this time period; (B) the sulfate content of a growing particle during NPF is quantitatively explained by condensation of gas-phase sulfuric acid molecules (i.e., sulfuric acid uptake is collision-limited); (C) sulfuric acid condensation substantially impacts the chemical composition of preexisting nanoparticles before new particles have grown to a size sufficient to be measured; (D) ammonium and sulfate concentrations are highly correlated, indicating that ammonia uptake is driven by sulfuric acid uptake; (E) sulfate neutralization by ammonium does not reach the predicted thermodynamic end point, suggesting that a barrier exists for ammonia uptake; (F) carbonaceous matter accounts for more than half of the particle mass growth, and its oxygen-to-carbon ratio (similar to 0.5) is characteristic of freshly formed secondary organic aerosol; and (G) differences in the overall growth rate from one formation event to another are caused by variations in the growth rates of all major chemical species, not just one individual species.
Improved Laboratory Transition Probabilities for Ce II, Application to the Cerium Abundances of the Sun and Five R-Process-Rich, Metal-Poor Stars, and Rare Earth Lab Data Summary
(2009-05) Lawler, James E.; Sneden, Christopher; Cowan, John J.; Ivans,, Inese I.; Den Hartog, E. A.; Sneden, Christopher
Recent radiative lifetime measurements accurate to +/- 5% using laser-induced fluorescence (LIF) on 43 even-parity and 15 odd-parity levels of Ce II have been combined with new branching fractions measured using a Fourier transform spectrometer (FTS) to determine transition probabilities for 921 lines of Ce II. This improved laboratory data set has been used to determine a new solar photospheric Ce abundance, log epsilon = 1.61 +/- 0.01 (sigma = 0.06 from 45 lines), a value in excellent agreement with the recommended meteoritic abundance, log epsilon = 1.61 +/- 0.02. Revised Ce abundances have also been derived for the r-process-rich metal-poor giant stars BD+17 degrees 3248, CS 22892-052, CS 31082-001, HD 115444, and HD 221170. Between 26 and 40 lines were used for determining the Ce abundance in these five stars, yielding a small statistical uncertainty of +/- 0.01 dex similar to the solar result. The relative abundances in the metal-poor stars of Ce and Eu, a nearly pure r-process element in the Sun, matches r-process-only model predictions for solar system material. This consistent match with small scatter over a wide range of stellar metallicities lends support to these predictions of elemental fractions. A companion paper includes an interpretation of these new precision abundance results for Ce as well as new abundance results and interpretation for Pr, Dy, and Tm.
Improved Laboratory Transition Probabilities for Er II and Application to the Erbium Abundances of the Sun and Five R-Process-Rich, Metal-Poor Stars
(2008-09) Lawler, James E.; Sneden, Christopher; Cowan, John J.; Wyart, J. F.; Ivans,, Inese I.; Sobeck, Jennifer S.; Stockett, M. H.; Den Hartog, E. A.; Sneden, Christopher
Recent radiative lifetime measurements accurate to +/- 5% (Stockett et al. 2007, J. Phys. B 40, 4529) using laser-induced fluorescence (LIF) on 7 even-parity and 63 odd-parity levels of Er II have been combined with new branching fractions measured using a Fourier transform spectrometer (FTS) to determine transition probabilities for 418 lines of Er II. This work moves Er II onto the growing list of rare-earth spectra with extensive and accurate modern transition probability measurements using LIF plus FTS data. This improved laboratory data set has been used to determine a new solar photospheric Er abundance, log epsilon = 0.96 +/- 0.03 (sigma = 0.06 from 8 lines), a value in excellent agreement with the recommended meteoritic abundance, log epsilon = 0.95 +/- 0.03. Revised Er abundances have also been derived for the r-process-richmetal-poor giant stars CS 22892-052, BD + 17 3248, HD 221170, HD 115444, and CS 31082-001. For these five stars the average Er/Eu abundance ratio, < log epsilon(Er/Eu)> = 0.42, is in very good agreement with the solar-system r-process ratio. This study has further strengthened the finding that r-process nucleosynthesis in the early Galaxy, which enriched these metal-poor stars, yielded a very similar pattern to the r-process, which enriched later stars including the Sun.
Improved Ti II Log(gf) Values and Abundance Determinations in the Photospheres of the Sun and Metal-Poor Star HD 84937
(2013-10) Wood, M. P.; Lawler, James E.; Sneden, Christopher; Cowan, John J.; Sneden, Christopher
Atomic transition probability measurements for 364 lines of Ti II in the UV through near-IR are reported. Branching fractions from data recorded using a Fourier transform spectrometer (FTS) and a new echelle spectrometer are combined with published radiative lifetimes to determine these transition probabilities. The new results are in generally good agreement with previously reported FTS measurements. Use of the new echelle spectrometer, independent radiometric calibration methods, and independent data analysis routines enables a reduction of systematic errors and overall improvement in transition probability accuracy over previous measurements. The new Ti II data are applied to high-resolution visible and UV spectra of the Sun and metal-poor star HD 84937 to derive new, more accurate Ti abundances. Lines covering a range of wavelength and excitation potential are used to search for non-LTE effects. The Ti abundances derived using Ti II for these two stars match those derived using Ti I and support the relative Ti/Fe abundance ratio versus metallicity seen in previous studies.
An In-Depth Spectroscopic Analysis Of The Blazhko Star Rr Lyrae I. Characterisation Of The Star: Abundance Analysis And Fundamental Parameters
(2010-09) Kolenberg, K.; Fossati, L.; Shulyak, D.; Pikall, H.; Barnes, T. G.; Kochukhov, O.; Tsymbal, V.; Barnes, T. G.
Context. The knowledge of accurate stellar parameters is a keystone in several fields of stellar astrophysics, such as asteroseismology and stellar evolution. Although the fundamental parameters can be derived from both spectroscopy and multicolour photometry, the results obtained are sometimes affected by systematic uncertainties. Stellar pulsation reaches high amplitudes in RR Lyrae stars, and as a consequence the stellar parameters vary significantly over the pulsation cycle. The abundances of the star, however, are not expected to change. Aims. We present a self-consistent spectral analysis of the pulsating star RR Lyr, which is the primary target of our study of the Blazhko effect. Methods. We used high-resolution and high signal-to-noise ratio spectra to carry out a consistent parameter determination and abundance analysis for RR Lyr. The LLmodels code was employed for model atmosphere calculations, while the SYNTH3 and WIDTH9 codes were used for line profile calculations and LTE abundance analysis. We describe in detail the methodology adopted to derive the fundamental parameters and the abundances. From a set of available high-resolution spectra of RR Lyr, we selected the phase of maximum radius at which the spectra are least disturbed by the pulsation. Using the abundances determined at this phase as a starting point, we expect to be able to determine the fundamental parameters determined at other phases more accurately. Results. The set of fundamental parameters obtained in this work fits the observed spectrum accurately. From the abundance analysis, we find clear indications of a depth-dependent microturbulent velocity, that we quantify. Conclusions. We confirm the importance of a consistent analysis of relevant spectroscopic features, the application of advanced model atmospheres, and the use of up-to-date atomic line data for determining stellar parameters. These results are crucial for further studies, e. g., detailed theoretical modelling of the observed pulsations.
Line Lists for the A(2)Pi-X-2 Sigma(+) (Red) and B-2 Sigma(+)-X-2 Sigma(+) (Violet) Systems of Cn, (Cn)-C-13-N-14, and (Cn)-C-12-N-15, and Application To Astronomical Spectra
(2014-10) Sneden, Christopher; Lucatello, Sara; Ram, Ram S.; Brooke, James S. A.; Bernath, Peter; Sneden, Christopher
New red and violet system line lists for the CN isotopologues (CN)-C-13-N-14 and (CN)-C-12-N-15 have been generated. These new transition data are combined with those previously derived for (CN)-C-12-N-14, and applied to the determination of CNO abundances in the solar photosphere and in four red giant stars: Arcturus, the bright, very low-metallicity star HD 122563, and the carbon-enhanced metal-poor stars HD 196944 and HD 201626. When both red and violet system lines are detectable in a star, their derived N abundances are in good agreement. The mean N abundances determined in this work are also generally in accord with published values.
Metallicity And Alpha-Element Abundance Measurement In Red Giant Stars From Medium-Resolution Spectra
(2008-08) Kirby, Evan N.; Guhathakurta, Puragra; Sneden, Christopher; Sneden, Christopher
We present a technique that applies spectral synthesis to medium-resolution spectroscopy (MRS; R similar to 6000) in the red (6300 angstrom < lambda < 9100 angstrom) to measure [Fe/H] and [alpha/Fe] of individual red giant stars over a wide metallicity range. We apply our technique to 264 red giant stars in seven Galactic globular clusters and demonstrate that it reproduces the metallicities and alpha-enhancements derived from high-resolution spectroscopy (HRS). The MRS technique excludes the three Ca II triplet lines and instead relies on a plethora of weaker lines. Unlike empirical metallicity estimators, such as the equivalent width of the Ca II triplet, the synthetic method presented here is applicable over an arbitrarily wide metallicity range and is independent of assumptions about the alpha-enhancement. Estimates of cluster mean [Fe/H] from different HRS studies show typical scatter of similar to 0.1 dex but can be larger than 0.2 dex for metal-rich clusters. The scatter in HRS abundance estimates among individual stars in a given cluster is also comparable to 0.1 dex. By comparison, the scatter among MRS [Fe/H] estimates of individual stars in a given cluster is similar to 0.1 dex for most clusters but 0.17 dex for the most metal-rich cluster, M71 (<[Fe/H]> = - 0.8). A star-by-star comparison of HRS versus MRS [alpha/Fe] estimates indicates that the precision in [alpha/Fe](MRS) is 0.05 dex. The errors in [Fe/H](MRS) and [alpha/Fe](MRS) increase beyond 0.25 dex only below signal-to-noise ratios of 20 angstrom(-1), which is typical for existing MRS of the red giant stars in Leo I, one of the most distant Milky Way satellites (250 kpc).