5,549 research outputs found
Isotopic Titanium Abundances In Local M Dwarfs
No full textRelative abundances of the five stable isotopes of titanium (Ti-46 to Ti-50) are measured for 11 M dwarfs belonging to the thin disk (four stars), thick disk (three stars), the halo (one star), and either the thick or the thin disk (three stars). Over the metallicity range of the sample (-1 < [Fe/H] < 0), the isotopic ratios are approximately constant at the solar system ratios. There is no discernible difference between the isotopic ratios for thin and thick disk stars. Isotopic ratios are in fair accord with recent calculations of Galactic chemical evolution despite the fact that such calculations underpredict [Ti/Fe] by about 0.4 dex at all metallicities.Robert A. Welch Foundation of Houston, TexasAstronom
Carbon And Oxygen Abundances Across The Hertzsprung Gap
No full textWe derived atmospheric parameters and spectroscopic abundances for C and O for a large sample of stars located in the Hertzsprung gap in the Hertzsprung-Russell diagram in order to detect chemical peculiarities and get a comprehensive overview of the population of stars in this evolutionary state. We have observed and analyzed high-resolution spectra (R = 60,000) of 188 stars in the mass range 2-5M(circle dot) with the 2.7 m Harlan J. Smith Telescope at the McDonald Observatory including 28 stars previously identified as Am/Ap stars. We find that the C and O abundances of the majority of stars in the Hertzsprung gap are in accordance with abundances derived for local lower-mass dwarfs but detect expected peculiarities for the Am/Ap stars. The C and O abundances of stars with T-eff < 6500 K are slightly lower than for the hotter objects but the C/O ratio is constant in the analyzed temperature domain. No indication of an alteration of the C and O abundances of the stars by mixing during the evolution across the Hertzsprung gap could be found before the homogenization of their atmospheres by the first dredge-up.Robert A. Welch Foundation of Houston, Texas F-634McDonald Observator
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Atmospheric Composition Of Weak G Band Stars: CNO And Li Abundances
No full textWe 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.Robert A. Welch Foundation F-634Astronom
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Dust Around R Coronae Borealis Stars. I. Spitzer/Infrared Spectrograph Observations
No full textSpitzer/infrared spectrograph (IRS) spectra from 5 to 37 mu m for a complete sample of 31 R Coronae Borealis stars (RCBs) are presented. These spectra are combined with optical and near-infrared photometry of each RCB at maximum light to compile a spectral energy distribution (SED). The SEDs are fitted with blackbody flux distributions and estimates are made of the ratio of the infrared flux from circumstellar dust to the flux emitted by the star. Comparisons for 29 of the 31 stars are made with the Infrared Astronomical Satellite (IRAS) fluxes from three decades earlier: Spitzer and IRAS fluxes at 12 mu m and 25 mu m are essentially equal for all but a minority of the sample. For this minority, the IRAS to Spitzer flux ratio exceeds a factor of three. The outliers are suggested to be stars where formation of a dust cloud or dust puff is a rare event. A single puff ejected prior to the IRAS observations may have been reobserved by Spitzer as a cooler puff at a greater distance from the RCB. RCBs which experience more frequent optical declines have, in general, a circumstellar environment containing puffs subtending a larger solid angle at the star and a quasi-constant infrared flux. Yet, the estimated subtended solid angles and the blackbody temperatures of the dust show a systematic evolution to lower solid angles and cooler temperatures in the interval between IRAS and Spitzer. Dust emission by these RCBs and those in the LMC is similar in terms of total 24 mu m luminosity and [8.0]-[24.0] color index.Spanish Ministry of Science and Innovation (MICINN) AYA-2007-64748NASA GO 50212, 1407Robert A. Welch Foundation of Houston, Texas F-634McDonald Observator
ORIGIN OF LITHIUM ENRICHMENT IN K GIANTS
No full textIn this Letter, we report on a low-resolution spectroscopic survey for Li-rich K giants among 2000 low-mass (M <= 3 M-circle dot) giants spanning the luminosity range from below to above the luminosity of the clump. Fifteen new Li-rich giants including four super Li-rich K giants (log epsilon(Li) >= 3.2) were discovered. A significant finding is that there is a concentration of Li-rich K giants at the luminosity of the clump or red horizontal branch. This new finding is partly a consequence of the fact that our low-resolution survey is the first large survey to include giants well below and above the red giant branch (RGB) bump and clump locations in the H-R diagram. Origin of the lithium enrichment may be plausibly attributed to the conversion of He-3 via Be-7 to Li-7 by the Cameron-Fowler mechanism but the location for the onset of the conversion is uncertain. Two possible opportunities to effect this conversion are discussed: the bump in the first ascent of the RGB and the He-core flash at the tip of the RGB. The finite luminosity spread of the Li-rich giants serves to reject the idea that Li enhancement is, in general, a consequence of a giant swallowing a large planet.Robert A. Welch Foundation of Houston, Texas F-634McDonald Observator
MANGANESE ABUNDANCES IN THE GLOBULAR CLUSTER ω CENTAURI
No full textWe present manganese abundances in 10 red giant members of the globular cluster omega Centauri; eight stars are from the most metal-poor population (RGB MP and RGB MInt1) while two targets are members of the more metal-rich groups (RGB MInt2 and MInt3). This is the first time Mn abundances have been studied in this peculiar stellar system. The LTE values of [Mn/Fe] in omega Cen overlap those of Milky Way stars in the metal-poor. Cen populations ([Fe/H] similar to -1.5 to -1.8), however unlike what is observed in Milky Way halo and disk stars, [Mn/Fe] declines in the two more metal-rich RGB MInt2 and MInt3 targets. Non-LTE calculations were carried out in order to derive corrections to the LTE Mn abundances. The non-LTE results for omega Cen in comparison with the non-LTE [Mn/Fe] versus [Fe/H] trend obtained for the Milky Way confirm and strengthen the conclusion that the manganese behavior in omega Cen is distinct. These results suggest that low-metallicity supernovae (with metallicities <= -2) of either Type II or Type Ia dominated the enrichment of the more metal-rich stars in omega Cen. The dominance of low-metallicity stars in the chemical evolution of omega Cen has been noted previously in the s-process elements where enrichment from metal-poor asymptotic giant branch stars is indicated. In addition, copper, which also has metallicity-dependent yields, exhibits lower values of [Cu/Fe] in the RGB MInt2 and MInt3 omega Cen populations.National Science Foundation AST 06-46790Robert A.Welch Foundation F-634Astronom
Dust Around R Coronae Borealis Stars. II. Infrared Emission Features In An H-Poor Environment
No full textResidual Spitzer/Infrared Spectrograph spectra for a sample of 31 R Coronae Borealis (RCB) stars are presented and discussed in terms of narrow emission features superimposed on the quasi-blackbody continuous infrared emission. A broad similar to 6-10 mu m dust emission complex is seen in the RCBs showing an extreme H-deficiency. A secondary and much weaker similar to 11.5-15 mu m broad emission feature is detected in a few RCBs with the strongest similar to 6-10 mu m dust complex. The Spitzer infrared spectra reveal for the first time the structure within the similar to 6-10 mu m dust complex, showing the presence of strong C-C stretching modes at similar to 6.3 and 8.1 mu m as well as of other dust features at similar to 5.9, 6.9, and 7.3 mu m, which are attributable to amorphous carbonaceous solids with little or no hydrogen. The few RCBs with only moderate H-deficiencies display the classical "unidentified infrared bands (UIRs)" and mid-infrared features from fullerene-related molecules. In general, the characteristics of the RCB infrared emission features are not correlated with the stellar and circumstellar properties, suggesting that the RCB dust features may not be dependent on the present physical conditions around RCB stars. The only exception seems to be the central wavelength of the 6.3 mu m feature, which is blueshifted in those RCBs showing also the UIRs, i.e., the RCBs with the smallest H deficiency.NASA 1407, GO 50212Spanish Ministry of Economy and Competitiveness AYA-2011-27754Robert A. Welch Foundation of Houston, Texas F-634McDonald Observator
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No full textIn any domain, a key activity of researchers is to search for and synthesize data from multiple sources in order to create new knowledge. In many cases this process is laborious, to the point of making certain questions effectively intractable because the cost of the searches outstrip the time available to complete the research. As more resources are published as Linked Data, and with the development of appropriate tools, data from multiple heterogeneous sources should be more rapidly discoverable and automatically integrable, enabling previously intractable queries to be explored, and standard queries to be significantly accelerated for more rapid knowledge discovery. But Linked Data is not of itself a complete solution. One of the key challenges of Linked Data is that its strength is also a weakness: anyone can publish anything. So in classical music, for instance, 17 sources may publish data about „Schubert?, but there is no de facto way to know that any of these Schuberts are one and the same, because the sources are not aligned. Without alignment, much of the benefit of Linked Data is diminished: resources can effectively be stranded rather than discovered, or tangled nets of only guessed at associations can cost more time than they are worth to determine whether a particular dataset is relevant or not
Breakwater morphological modelling: predicting equilibrium morphologies using entropy based techniques
No full textIn order to design suitable breakwaters for beach protection, morphological modelling is required. This modelling can be utilised to determine the effectiveness of different structures and the position that sand is likely to move to when a stable, equilibrium morphology develops. The majority of morphological models currently used are time-step based. They involve the interaction of wave, current, sediment transport and sediment balance modules. Sediment is transported over a small amount of time, after which the modules are updated and sediment transport calculations begin again. In this paper a new method is put forward, where the need to time step is avoided. Different morphologies are compared directly, based on an objective function, where morphologies closer to the desired equilibrium have a smaller net sediment transport. Morphologies are modified using global optimisation techniques. This new method is able to predict sediment deposition in the lee of detached shore parallel breakwaters
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