Browsing by Subject "abundance analysis"
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Item Chemical Similarities Between Galactic Bulge And Local Thick Disk Red Giant Stars(2008-06) Melendez, J.; Asplund, M.; Alves-Brito, A.; Cunha, K.; Barbuy, B.; Bessell, M. S.; Chiappini, C.; Freeman, K. C.; Ramirez, I.; Smith, V. V.; Yong, D.; Ramirez, I.Context. The evolution of the Milky Way bulge and its relationship with the other Galactic populations is still poorly understood. The bulge has been suggested to be either a merger-driven classical bulge or the product of a dynamical instability of the inner disk. Aims. To probe the star formation history, the initial mass function and stellar nucleosynthesis of the bulge, we performed an elemental abundance analysis of bulge red giant stars. We also completed an identical study of local thin disk, thick disk and halo giants to establish the chemical differences and similarities between the various populations. Methods. High-resolution infrared spectra of 19 bulge giants and 49 comparison giants in the solar neighborhood were acquired with Gemini/Phoenix. All stars have similar stellar parameters but cover a broad range in metallicity. A standard 1D local thermodynamic equilibrium analysis yielded the abundances of C, N, O and Fe. A homogeneous and differential analysis of the bulge, halo, thin disk and thick disk stars ensured that systematic errors were minimized. Results. We confirm the well-established differences for [O/Fe] (at a given metallicity) between the local thin and thick disks. For the elements investigated, we find no chemical distinction between the bulge and the local thick disk, which is in contrast to previous studies relying on literature values for disk dwarf stars in the solar neighborhood. Conclusions. Our findings suggest that the bulge and local thick disk experienced similar, but not necessarily shared, chemical evolution histories. We argue that their formation timescales, star formation rates and initial mass functions were similar.Item Could The Ultra-Metal-Poor Stars Be Chemically Peculiar And Not Related To The First Stars?(2008-04) Venn, Kim A.; Lambert, David L.; Lambert, David L.Chemically peculiar stars define a class of stars that show unusual elemental abundances due to stellar photospheric effects and not due to natal variations. In this paper, we compare the elemental abundance patterns of the ultra-metal-poor stars with metallicities [Fe/H]similar to-5 to those of a subclass of chemically peculiar stars. These include post-AGB stars, RV Tauri variable stars, and the Lambda Bootis stars, which range in mass, age, binarity, and evolutionary status, yet can have iron abundance determinations as low as [Fe/H]similar to-5. These chemical peculiarities are interpreted as due to the separation of gas and dust beyond the stellar surface, followed by the accretion of dust-depleted gas. Contrary to this, the elemental abundances in the ultra-metal-poor stars are thought to represent yields of the most metal-poor supernovae and, therefore, observationally constrain the earliest stages of chemical evolution in the universe. Detailed chemical abundances are now available for HE 1327-2326 and HE 0107-5240, the two extreme ultra-metal-poor stars in our Galaxy, and for HE 0557-4840, another ultra-metal-poor star found by the Hamburg/ESO survey. There are interesting similarities in their abundance ratios to those of the chemically peculiar stars; e. g., the abundances of the elements in their photospheres are related to the condensation temperature of that element. If these three stars are chemically peculiar, then their CNO abundances suggest true metallicities of [X/H] similar to-2 to -4. It is important to establish the nature of these stars, since they are used as tests of the early chemical evolution of the Galaxy.Item An Observational Perspective On Some Aspects Of Early Stellar Nucleosynthesis(2008-03) Sneden, C.; Lawler, J. E.; Sneden, ChristopherSome basic abundance results for low metallicity stars that were formed in the early days of the Milky Way Galaxy are summarized. Discussion is centered on two nucleosynthetic groups: the light a elements (Mg, Si, and Ca), and the neutron-capture elements (those heavier than the Fe group, atomic numbers greater than 30). Emphasis is placed on the present state of stellar spectroscopic and atomic transition data.Item Our Milky Way As A Pure-Disk Galaxy-A Challenge for Galaxy Formation(2010-09) Shen, Juntai T.; Rich, R. Michael; Kormendy, John; Howard, Christian D.; De Propris, Roberto; Kunder, Andrea; Shen, Juntai T.; Kormendy, JohnBulges are commonly believed to form in the dynamical violence of galaxy collisions and mergers. Here, we model the stellar kinematics of the Bulge Radial Velocity Assay ( BRAVA) and find no sign that the Milky Way contains a classical bulge formed by scrambling pre-existing disks of stars in major mergers. Rather, the bulge appears to be a bar seen somewhat end-on, as hinted from its asymmetric boxy shape. We construct a simple but realistic N-body model of the Galaxy that self-consistently develops a bar. The bar immediately buckles and thickens in the vertical direction. As seen from the Sun, the result resembles the boxy bulge of our Galaxy. The model fits the BRAVA stellar kinematic data covering the whole bulge strikingly well with no need for a merger-made classical bulge. The bar in our best-fit model has a half-length of similar to 4 kpc and extends 20 degrees from the Sun-Galactic center line. We use the new kinematic constraints to show that any classical bulge contribution cannot be larger than similar to 8% of the disk mass. Thus, the Galactic bulge is a part of the disk and not a separate component made in a prior merger. Giant, pure-disk galaxies like our own present a major challenge to the standard picture in which galaxy Formation is dominated by hierarchical clustering and galaxy mergers.Item Searching For Dust Around Hyper Metal Poor Stars(2014-08) Venn, Kim A.; Puzia, Thomas H.; Divell, Mike; Cote, Stephanie; Lambert, David L.; Starkenburg, Else; Lambert, David L.We examine the mid-infrared fluxes and spectral energy distributions for stars with iron abundances [Fe/H] < -5, and other metal-poor stars, to eliminate the possibility that their low metallicities are related to the depletion of elements onto dust grains in the formation of a debris disk. Six out of seven stars examined here show no mid-IR excesses. These non-detections rule out many types of circumstellar disks, e. g., a warm debris disk (T <= 290 K), or debris disks with inner radii <= 1 AU, such as those associated with the chemically peculiar post-asymptotic giant branch spectroscopic binaries and RV Tau variables. However, we cannot rule out cooler debris disks, nor those with lower flux ratios to their host stars due to, e. g., a smaller disk mass, a larger inner disk radius, an absence of small grains, or even a multicomponent structure, as often found with the chemically peculiar Lambda Bootis stars. The only exception is HE0107-5240, for which a small mid-IR excess near 10 m is detected at the 2s level; if the excess is real and associated with this star, it may indicate the presence of (recent) dust-gas winnowing or a binary system.Item The Stellar Content Of The Hamburg/ESO Survey - IV. Selection Of Candidate Metal-Poor Stars(2008-06) Christlieb, N.; Schorck, T.; Frebel, A.; Beers, T. C.; Wisotzki, L.; Reimers, D.; Frebel, A.We present the quantitative methods used for selecting candidate metal-poor stars in the Hamburg/ESO objective-prism survey (HES). The selection is based on the strength of the Ca II K line, B - V colors (both measured directly from the digital HES spectra), as well as J - K colors from the 2 Micron All Sky Survey. The KP index for Ca II K can be measured from the HES spectra with an accuracy of 1.0 angstrom, and a calibration of the HES B - V colors, using CCD photometry, yields a 1-sigma uncertainty of 0.07 mag for stars in the color range 0.3 < B - V < 1.4. These accuracies make it possible to reliably reject stars with [Fe/H] > -2.0 without sacrificing completeness at the lowest metallicities. A test of the selection using 1121 stars of the HK survey of Beers, Preston, and Shectman present on HES plates suggests that the completeness at [Fe/H] < -3.5 is close to 100% and that, at the same time, the contamination of the candidate sample with false positives is low: 50% of all stars with [Fe/H] > -2.5 and 97% of all stars with [Fe/H] > -2.0 are rejected. The selection was applied to 379 HES fields, covering a nominal area of 8853 deg(2) of the southern high Galactic latitude sky. The candidate sample consists of 20 271 stars in the magnitude range 10 less than or similar to B less than or similar to 18. A comparison of the magnitude distribution with that of the HK survey shows that the magnitude limit of the HES sample is about 2mag fainter. Taking the overlap of the sky areas covered by both surveys into account, it follows that the survey volume for metal-poor stars has been increased by the HES by about a factor of 10 with respect to the HK survey. We have already identified several very rare objects with the HES, including, e. g., the three most heavy-element deficient stars currently known.Item The Stellar Content Of The Hamburg/ESO Survey V. The Metallicity Distribution Function Of The Galactic Halo(2009-11) Schorck, T.; Christlieb, N.; Cohen, J. G.; Beers, T. C.; Shectman, S.; Thompson, I.; McWilliam, A.; Bessell, M. S.; Norris, J. E.; Melendez, J.; Ramirez, S.; Haynes, D.; Cass, P.; Hartley, M.; Russell, K.; Watson, F.; Zickgraf, F. J.; Behnke, B.; Fechner, C.; Fuhrmeister, B.; Barklem, P. S.; Edvardsson, B.; Frebel, A.; Wisotzki, L.; Reimers, D.; Frebel, A.We determine the metallicity distribution function (MDF) of the Galactic halo by means of a sample of 1638 metal-poor stars selected from the Hamburg/ESO objective-prism survey (HES). The sample was corrected for minor biases introduced by the strategy for spectroscopic follow-up observations of the metal-poor candidates, namely >best and brightest stars first>. Comparison of the metallicities [Fe/H] of the stars determined from moderate-resolution (i.e., R similar to 2000) follow-up spectra with results derived from abundance analyses based on high-resolution spectra (i.e., R > 20 000) shows that the [Fe/H] estimates used for the determination of the halo MDF are accurate to within 0.3 dex, once highly C-rich stars are eliminated. We determined the selection function of the HES, which must be taken into account for a proper comparison between the HES MDF with MDFs of other stellar populations or those predicted by models of Galactic chemical evolution. The latter show a reasonable agreement with the overall shape of the HES MDF for [Fe/H] > -3.6, but only a model of Salvadori et al. (2007) with a critical metallicity for low-mass star formation of Z(cr) = 10(-3.4) Z(circle dot) reproduces the sharp drop at [Fe/H] similar to -3.6 present in the HES MDF. Although currently about ten stars at [Fe/H] < -3.6 are known, the evidence for the existence of a tail of the halo MDF extending to [Fe/H] similar to -5.5 is weak from the sample considered in this paper, because it only includes two stars [Fe/H] < -3.6. Therefore, a comparison with theoretical models has to await larger statistically complete and unbiased samples. A comparison of the MDF of Galactic globular clusters and of dSph satellites to the Galaxy shows qualitative agreement with the halo MDF, derived from the HES, once the selection function of the latter is included. However, statistical tests show that the differences between these are still highly significant.Item Transiting Exoplanets From The CoRoT Space Mission XII. CoRoT-12B: A Short-Period Low-Density Planet Transiting A Solar Analog Star(2010) Gillon, M.; Hatzes, A.; Csizmadia, S.; Fridlund, M.; Deleuil, M.; Aigrain, S.; Alonso, R.; Auvergne, M.; Baglin, A.; Barge, P.; Barnes, S. I.; Bonomo, A. S.; Borde, P.; Bouchy, F.; Bruntt, H.; Cabrera, J.; Carone, L.; Carpano, S.; Cochran, W. D.; Deeg, H. J.; Dvorak, R.; Endl, M.; Erikson, A.; Ferraz-Mello, S.; Gandolfi, D.; Gazzano, J. C.; Guenther, E.; Guillot, T.; Havel, M.; Hebrard, G.; Jorda, L.; Leger, A.; Llebaria, A.; Lammer, H.; Lovis, C.; Mayor, M.; Mazeh, T.; Montalban, J.; Moutou, C.; Ofir, A.; Ollivier, M.; Patzold, M.; Pepe, F.; Queloz, D.; Rauer, H.; Rouan, D.; Samuel, B.; Santerne, A.; Schneider, J.; Tingley, B.; Udry, S.; Weingrill, J.; Wuchterl, G.; Cochran, W. D.; Endl, M.We report the discovery by the CoRoT satellite of a new transiting giant planet in a 2.83 days orbit about a V = 15.5 solar analog star (M(*) = 1.08 +/- 0.08 M(circle dot), R(*) = 1.1 +/- 0.1 R(circle dot), T(eff) = 5675 +/- 80 K). This new planet, CoRoT-12b, has a mass of 0.92 +/- 0.07 M(Jup) and a radius of 1.44 +/- 0.13 R(Jup). Its low density can be explained by standard models for irradiated planets.Item Transiting Exoplanets From The CoRoT Space Mission XVII. The Hot Jupiter CoRoT-17B: A Very Old Planet(2011-07) Csizmadia, S.; Moutou, C.; Deleuil, M.; Cabrera, J.; Fridlund, M.; Gandolfi, D.; Aigrain, S.; Alonso, R.; Almenara, J. M.; Auvergne, M.; Baglin, A.; Barge, P.; Bonomo, A. S.; Borde, P.; Bouchy, F.; Bruntt, H.; Carone, L.; Carpano, S.; Cavarroc, C.; Cochran, W.; Deeg, H. J.; Diaz, R. F.; Dvorak, R.; Endl, M.; Erikson, A.; Ferraz-Mello, S.; Fruth, T.; Gazzano, J. C.; Gillon, M.; Guenther, E. W.; Guillot, T.; Hatzes, A.; Havel, M.; Hebrard, G.; Jehin, E.; Jorda, L.; Leger, A.; Llebaria, A.; Lammer, H.; Lovis, C.; MacQueen, P. J.; Mazeh, T.; Ollivier, M.; Patzold, M.; Queloz, D.; Rauer, H.; Rouan, D.; Santerne, A.; Schneider, J.; Tingley, B.; Titz-Weider, R.; Wuchterl, G.; Cochran, W. D.; Endl, M.; MacQueen, P.J.We report on the discovery of a hot Jupiter-type exoplanet, CoRoT-17b, detected by the CoRoT satellite. It has a mass of 2.43 +/- 0.30 M-Jup and a radius of 1.02 +/- 0.07 R-Jup, while its mean density is 2.82 +/- 0.38 g/cm(3). CoRoT-17b is in a circular orbit with a period of 3.7681 +/- 0.0003 days. The host star is an old (10.7 +/- 1.0 Gyr) main-sequence star, which makes it an intriguing object for planetary evolution studies. The planet's internal composition is not well constrained and can range from pure H/He to one that can contain similar to 380 earth masses of heavier elements.Item Transiting Exoplanets From The CoRoT Space Mission XVII. The Hot Jupiter CoRoT-17B: A Very Old Planet(2011-07) Csizmadia, S.; Moutou, C.; Deleuil, M.; Cabrera, J.; Fridlund, M.; Gandolfi, D.; Aigrain, S.; Alonso, R.; Almenara, J. M.; Auvergne, M.; Baglin, A.; Barge, P.; Bonomo, A. S.; Borde, P.; Bouchy, F.; Bruntt, H.; Carone, L.; Carpano, S.; Cavarroc, C.; Cochran, W.; Deeg, H. J.; Diaz, R. F.; Dvorak, R.; Endl, M.; Erikson, A.; Ferraz-Mello, S.; Fruth, T.; Gazzano, J. C.; Gillon, M.; Guenther, E. W.; Guillot, T.; Hatzes, A.; Havel, M.; Hebrard, G.; Jehin, E.; Jorda, L.; Leger, A.; Llebaria, A.; Lammer, H.; Lovis, C.; MacQueen, P. J.; Mazeh, T.; Ollivier, M.; Patzold, M.; Queloz, D.; Rauer, H.; Rouan, D.; Santerne, A.; Schneider, J.; Tingley, B.; Titz-Weider, R.; Wuchterl, G.; Cochran, W. D.; Endl, M.; MacQueen, P.J.We report on the discovery of a hot Jupiter-type exoplanet, CoRoT-17b, detected by the CoRoT satellite. It has a mass of 2.43 +/- 0.30 M-Jup and a radius of 1.02 +/- 0.07 R-Jup, while its mean density is 2.82 +/- 0.38 g/cm(3). CoRoT-17b is in a circular orbit with a period of 3.7681 +/- 0.0003 days. The host star is an old (10.7 +/- 1.0 Gyr) main-sequence star, which makes it an intriguing object for planetary evolution studies. The planet's internal composition is not well constrained and can range from pure H/He to one that can contain similar to 380 earth masses of heavier elements.Item Transiting Exoplanets From The CoRoT Space Mission XVII. The Hot Jupiter CoRoT-17B: A Very Old Planet(2011-07) Csizmadia, S.; Moutou, C.; Deleuil, M.; Cabrera, J.; Fridlund, M.; Gandolfi, D.; Aigrain, S.; Alonso, R.; Almenara, J. M.; Auvergne, M.; Baglin, A.; Barge, P.; Bonomo, A. S.; Borde, P.; Bouchy, F.; Bruntt, H.; Carone, L.; Carpano, S.; Cavarroc, C.; Cochran, W.; Deeg, H. J.; Diaz, R. F.; Dvorak, R.; Endl, M.; Erikson, A.; Ferraz-Mello, S.; Fruth, T.; Gazzano, J. C.; Gillon, M.; Guenther, E. W.; Guillot, T.; Hatzes, A.; Havel, M.; Hebrard, G.; Jehin, E.; Jorda, L.; Leger, A.; Llebaria, A.; Lammer, H.; Lovis, C.; MacQueen, P. J.; Mazeh, T.; Ollivier, M.; Patzold, M.; Queloz, D.; Rauer, H.; Rouan, D.; Santerne, A.; Schneider, J.; Tingley, B.; Titz-Weider, R.; Wuchterl, G.; Cochran, W. D.; Endl, M.; MacQueen, P.J.We report on the discovery of a hot Jupiter-type exoplanet, CoRoT-17b, detected by the CoRoT satellite. It has a mass of 2.43 +/- 0.30 M-Jup and a radius of 1.02 +/- 0.07 R-Jup, while its mean density is 2.82 +/- 0.38 g/cm(3). CoRoT-17b is in a circular orbit with a period of 3.7681 +/- 0.0003 days. The host star is an old (10.7 +/- 1.0 Gyr) main-sequence star, which makes it an intriguing object for planetary evolution studies. The planet's internal composition is not well constrained and can range from pure H/He to one that can contain similar to 380 earth masses of heavier elements.Item Two Spotted And Magnetic Early B-Type Stars In The Young Open Cluster NGC 2264 Discovered By Most And ESPaDOnS(2014-02) Fossati, L.; Zwintz, K.; Castro, N.; Langer, N.; Lorenz, D.; Schneider, F. R. N.; Kuschnig, R.; Matthews, J. M.; Alecian, E.; Wade, G. A.; Barnes, T. G.; Thoul, A. A.; Matthews, J. MStar clusters are known as superb tools for understanding stellar evolution. In a quest for understanding the physical origin of magnetism and chemical peculiarity in about 7% of the massive main-sequence stars, we analysed two of the ten brightest members of the similar to 10 Myr old Galactic open cluster NGC 2264, the early B-dwarfs HD47887 and HD47777. We find accurate rotation periods of 1.95 and 2.64 days, respectively, from MOST photometry. We obtained ESPaDOnS spectropolarimetric observations, through which we determined stellar parameters, detailed chemical surface abundances, projected rotational velocities, and the inclination angles of the rotation axis. Because we found only small (<5 km s(-1)) radial velocity variations, most likely caused by spots, we can rule out that HD47887 and HD47777 are close binaries. Finally, using the least-squares deconvolution technique, we found that both stars possess a large-scale magnetic field with an average longitudinal field strength of about 400 G. From a simultaneous fit of the stellar parameters we determine the evolutionary masses of HD47887 and HD47777 to be 9.4(-0.7)(+0.6) M-circle dot and 7.6(-0.5)(+0.5) M-circle dot. Interestingly, HD47777 shows a remarkable helium underabundance, typical of helium-weak chemically peculiar stars, while the abundances of HD47887 are normal, which might imply that diffusion is operating in the lower mass star but not in the slightly more massive one. Furthermore, we argue that the rather slow rotation, as well as the lack of nitrogen enrichment in both stars, can be consistent with both the fossil and the binary hypothesis for the origin of the magnetic field. However, the presence of two magnetic and apparently single stars near the top of the cluster mass-function may speak in favour of the latter.Item Very Metal-Poor Stars in the Outer Galactic Bulge Found By the APOGEE Survey(2013-04) Garcia Perez, Ana E.; Cunha, Katia; Shetrone, Matthew; Majewski, Steven R.; Johnson, Jennifer A.; Smith, Verne V.; Schiavon, Ricardo P.; Holtzman, Jon; Nidever, David; Zasowski, Gail; Allende Prieto, Carlos; Beers, Timothy C.; Bizyaev, Dmitry; Ebelke, Garrett; Eisenstein, Daniel J.; Frinchaboy, Peter M.; Girardi, Leo; Hearty, Fred R.; Malanushenko, Elena; Malanushenko, Viktor; Meszaros, Szabolcs; O'Connell, Robert W.; Oravetz, Daniel; Pan, Kaike; Robin, Annie C.; Schneider, Donald P.; Schultheis, Mathias; Skrutskie, Michael F.; Simmonsand, Audrey; Wilson, John C.; Shetrone, MatthewDespite its importance for understanding the nature of early stellar generations and for constraining Galactic bulge Formation models, at present little is known about the metal-poor stellar content of the central Milky Way. This is a consequence of the great distances involved and intervening dust obscuration, which challenge optical studies. However, the Apache Point Observatory Galactic Evolution Experiment (APOGEE), a wide-area, multifiber, high-resolution spectroscopic survey within Sloan Digital Sky Survey III, is exploring the chemistry of all Galactic stellar populations at infrared wavelengths, with particular emphasis on the disk and the bulge. An automated spectral analysis of data on 2403 giant stars in 12 fields in the bulge obtained during APOGEE commissioning yielded five stars with low metallicity ([Fe/H] <= -1.7), including two that are very metal-poor [Fe/H] similar to -2.1 by bulge standards. Luminosity-based distance estimates place the 5 stars within the outer bulge, where 1246 of the other analyzed stars may reside. A manual reanalysis of the spectra verifies the low metallicities, and finds these stars to be enhanced in the alpha-elements O, Mg, and Si without significant alpha-pattern differences with other local halo or metal-weak thick-disk stars of similar metallicity, or even with other more metal-rich bulge stars. While neither the kinematics nor chemistry of these stars can yet definitively determine which, if any, are truly bulge members, rather than denizens of other populations co-located with the bulge, the newly identified stars reveal that the chemistry of metal-poor stars in the central Galaxy resembles that of metal-weak thick-disk stars at similar metallicity.