177,440 research outputs found
Behavior of carbonyl ylide generated from 3-chloro-3-(p-nitrophenyl)diazirine and acetone 1,3-dipolar cycloaddition to benzaldehyde and epoxide formation
No full textPT: J; CR: DEMARCH P, 1982, J AM CHEM SOC, V104, P4952 DEMARCH P, 1982, J AM CHEM SOC, V104, P4953 GILL HS, 1983, J ORG CHEM, V48, P1051 HOUK KN, 1973, J AM CHEM SOC, V95, P7302 HUISGEN R, 1977, ANGEW CHEM INT EDIT, V16, P572 IBATA T, 1986, TETRAHEDRON LETT, V27, P4383 LIU MTH, 1974, TETRAHEDRON LETT, P1329 LIU MTH, 1987, TETRAHEDRON LETT, V28, P1011 MARTIN CW, 1971, J CHEM SOC CHEM COMM, P1438 MARTIN CW, 1971, J CHEM SOC CHEM COMM, P15 MARTIN CW, 1985, J ORG CHEM, V50, P2050 PADWA A, 1969, J ORG CHEM, V34, P2728 SEYFERTH D, 1974, J ORGANOMET CHEM, V67, P341 UEDA K, 1972, B CHEM SOC JPN, V45, P2779; NR: 14; TC: 8; J9: CHEM LETT; PG: 4; GA: L0892Source type: Electronic(1
A laser flash photolysis study of carbonyl ylides of arylchlorocarbenes: kinetics and reversibility of the formation, cyclization, and cycloaddition
No full textPT: J; CR: BARCUS RL, 1986, J AM CHEM SOC, V108, P3928 BEKHAZI M, 1981, J AM CHEM SOC, V103, P2473 BONNEAU R, 1989, J CHEM SOC CHEM COMM, P510 DEMARCH P, 1982, J AM CHEM SOC, V104, P4952 GILL HS, 1983, J ORG CHEM, V48, P1051 GRAHAM WH, 1965, J AM CHEM SOC, V87, P4396 HUISGEN R, 1977, ANGEW CHEM INT EDIT, V16, P572 HUISGEN R, 1982, J AM CHEM SOC, V104, P4953 IBATA T, 1986, TETRAHEDRON LETT, V27, P4383 IBATA T, 1987, CHEM LETT, P2135 LIU MTH, 1987, TETRAHEDRON LETT, V28, P1011 MARTIN CW, 1985, J ORG CHEM, V50, P2050 SHIMIZU N, 1978, J AM CHEM SOC, V100, P4260 SOUNDARARAJAN N, 1988, TETRAHEDRON LETT, V29, P3419 TROZZOLO AM, 1979, PURE APPL CHEM, V51, P261 WONG PC, 1982, J AM CHEM SOC, V104, P6631; NR: 16; TC: 19; J9: J AMER CHEM SOC; PG: 4; GA: CJ665Source type: Electronic(1
The formation history of the Galactic bulge
No full textThe distributions of the stellar metallicities of K giant stars in several fields of the Galactic bulge, taken from the literature and probing projected Galactocentric distances of similar to500 pc to similar to3 kpc, are compared with a simple model of star formation and chemical evolution. Our model assumes a Schmidt law of star formation and is described by only a few parameters that control the infall and outflow of gas and the star formation efficiency. Exploring a large volume of parameter space, we find that very short infall time-scales are needed (less than or similar to0.5 Gyr), with durations of infall and star formation greater than 1 Gyr being ruled out at the 90 per cent confidence level. The metallicity distributions are compatible with an important amount of gas and metals being ejected in outflows, although a detailed quantification of the ejected gas fraction is strongly dependent on a precise determination of the absolute stellar metallicities. We find a systematic difference between the samples of Ibata & Gilmore, at projected distances of 1-3 kpc, and the sample in Baade's window (Sadler et al.). This could be caused either by a true metallicity gradient in the bulge or by a systematic offset in the calibration of [Fe/H] between these two samples. This offset does not play an important role in the estimate of infall and formation time-scales, which are mostly dependent on the width of the distributions. The recent bulge data from Zoccali et al. are also analysed, and the subsample with subsolar metallicities still rules out infall time-scales greater than or similar to1 Gyr at the 90 per cent confidence level. Hence, the short time-scales we derive based on the observed distribution of metallicities are robust and should be taken as stringent constraints on bulge formation models
The luminosity function and stellar mass-to-light ratio of the massive globular cluster NGC 2419★
No full textWe used archival Hubble Space Telescope Wide Field Camera 3 (WFC3) images to obtain the luminosity function of the remote globular cluster NGC 2419 from 2 mag above the horizontal branch level down to ≃ 3.0 mag below the turn-off point (to M I≃ 6.4), approximately covering the range of initial stellar masses The completeness-corrected luminosity function does not display any change of shape over the radial range covered by the WFC3 data, out to ≃ 6 core radii (r c), or, equivalently, to ≃ 2 half-light radii. The luminosity function in this radial range is also identical to that obtained from ground-based data at much larger distances from the cluster centre (), in the magnitude range in which the two distributions overlap (M I≤ 4.0). These results support the conclusion by Dalessandro et al. that there is no significant mass segregation among cluster stars; hence, the stellar mass-to-light ratio remains constant with distance from the cluster centre. We fitted the observed luminosity function with theoretical counterparts with the proper age and metallicity from different sets of stellar evolution models, and we consistently derive a total V-band mass-to-light ratio by extrapolating to the hydrogen-burning limit, with a best-fitting value M/L V= 1.5 ± 0.1. On the other hand, assuming that there are no cluster stars with m≤ 0.3M ⊙, we establish a robust lower limit M/L V > 0.8. These estimates provide useful constraints for dynamical models of the cluster that were forced to consider the stellar mass-to-light ratio as a (nearly) free parameter
Appropriate Similarity Measures for Author Cocitation Analysis
No full textWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
"Closing the R&D Gap, Evaluating the Sources of R&D Spending"
No full textBoth spending and tax policies have been implemented in the United States with the goal of stimulating private sector research and development (R&D). Karier questions whether current R&D policy, especially the research and experimentation tax credit, can contribute to closing the gap between nondefense expenditures on R&D in the United States and such expenditures in other countries, such as Japan and Germany. He also explores possible changes to our current R&D policy to make it more effective.
News from the Galactic suburbia: the chemical composition of the remote globular cluster NGC 2419
No full textinfo:eu-repo/semantics/publishe
The Northern Wraps of the Sagittarius Stream as Traced by Red Clump Stars: Distances, Intrinsic Widths, and Stellar Densities
No full texta
Going Beyond Counting First Authors in Author Co-citation Analysis
No full textThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The metallicity distribution in the core of the Sagittarus dwarf spheroidal: Minimising the metallicity biases
No full textWe present the metallicity and radial velocity for 450 bona fide members of the Sagittarius dwarf spheroidal (Sgr dSph) galaxy, measured from high-resolution spectra (R ' 18 000) obtained with FLAMES at the VLT. The targets were carefully selected (a) to sample the core of the main body of Sgr dSph while avoiding contamination from the central stellar nucleus, and (b) to prevent any bias on the metallicity distribution by selecting targets based on their Gaia parallax and proper motions. All the targets selected in this way were confirmed as radial velocity members. We used this sample to derive the first metallicity distribution of the core of Sgr dSph, which is virtually unaffected by metallicity biases. The observed distribution ranges from [Fe=H] ≃-2:3 to [Fe=H] ≃0:0, with a strong, symmetric, and relatively narrow peak around [Fe=H] ' -0:5 and a weak and extended metal-poor tail, in which only 13:8 ±1:9% of the stars have [Fe=H] < -1:0. We confirm previous evidence of correlations between chemical and kinematical properties of stars in the core of Sgr. In our sample, stars with [Fe=H] ≥ -0:6 display a lower velocity dispersion and a higher rotation amplitude than those with [Fe=H] < -0:6, confirming previous suggestions of disk/halo structure for the progenitor of the system
- …
