A chemical signature from fast-rotating low-metallicity massive stars: ROA 276 in ω Centauri

© 2017. The American Astronomical Society. All rights reserved. We present a chemical abundance analysis of a metal-poor star, ROA 276, in the stellar system ω Centauri. We confirm that this star has an unusually high [Sr/Ba] abundance ratio. Additionally, ROA 276 exhibits remarkably high abundance ratios, [X/Fe] , for all elements from Cu to Mo along with normal abundance ratios for the elements from Ba to Pb. The chemical abundance pattern of ROA 276, relative to a primordial ω Cen star ROA 46, is best fit by a fast-rotating low-metallicity massive stellar model of 20 , [Fe/H] = -1.8, and an initial rotation 0.4 times the critical value; no other nucleosynthetic source can match the neutron-capture element distribution. ROA 276 arguably offers the most definitive proof to date that fast-rotating massive stars contributed to the production of heavy elements in the early universe

A Sr-Rich Star on the Main Sequence of Omega Centauri

Abundance ratios relative to iron for carbon, nitrogen, strontium and barium are presented for a metal-rich main sequence star ([Fe/H]=--0.74) in the globular cluster omega Centauri. This star, designated 2015448, shows depleted carbon and solar nitrogen, but more interestingly, shows an enhanced abundance ratio of strontium [Sr/Fe] ~ 1.6 dex, while the barium abundance ratio is [Ba/Fe]<0.6 dex. At this metallicity one usually sees strontium and barium abundance ratios that are roughly equal. Possible formation scenarios of this peculiar object are considered.Comment: 13 pages, 3 figures. Accepted to ApJ

M22: A [Fe/H] Abundance Range Revealed

Intermediate resolution spectra at the Ca II triplet have been obtained for 55 candidate red giants in the field of the globular cluster M22 with the VLT/FORS instrument. Spectra were also obtained for a number of red giants in standard globular clusters to provide a calibration of the observed line strengths with overall abundance [Fe/H]. For the 41 M22 member stars that lie within the V-V_HB bounds of the calibration, we find an abundance distribution that is substantially broader than that expected from the observed errors alone. We argue that this broad distribution cannot be the result of differential reddening. Instead we conclude that, as has long been suspected, M22 is similar to omega Cen in having an intrinsic dispersion in heavy element abundance. The observed M22 abundance distribution rises sharply to a peak at [Fe/H] = -1.9 with a broad tail to higher abundances: the highest abundance star in our sample has [Fe/H] = -1.45 dex. If the unusual properties of omega Cen have their origin in a scenario in which the cluster is the remnant nucleus of a disrupted dwarf galaxy, then such a scenario likely applies also to M22.Comment: 29 pages, 9 figures, accepted for publication in the Astrophysical Journa

The Age and Metallicity Relation of Omega Centauri

We present a metallicity distribution based on photometry and spectra for 442 omega Cen cluster members that lie at the main sequence turnoff region of the color-magnitude diagram. This distribution is similar to that found for the red giant branch. The distribution shows a sharp rise to a mean of [Fe/H] = -1.7 with a long tail to higher metallicities. Ages have then been determined for the stars using theoretical isochrones enabling the construction of an age-metallicity diagram. Interpretation of this diagram is complicated by the correlation of the errors in the metallicities and ages. Nevertheless, after extensive Monte-Carlo simulations, we conclude that our data show that the formation of the cluster took place over an extended period of time: the most metal-rich stars in our sample ([Fe/H] ~ -0.6) are younger by 2-4 Gyrs than the most metal-poor population.Comment: 53 pages, 20 figures, 5 tables, accepted for publication in Ap

Abundances on the Main Sequence of Omega Centauri

Abundance ratios of carbon, nitrogen and strontium relative to iron, calculated using spectrum synthesis techniques, are given for a sample of main sequence and turnoff stars that belong to the globular cluster omega Centauri. The variations of carbon, nitrogen and/or strontium show several different abundance patterns as a function of [Fe/H]. The source of the enhancements/depletions in carbon, nitrogen and/or strontium may be enrichment from asymptotic giant branch stars of low (1--3 solar masses) and intermediate (3--8 solar masses) mass. Massive rotating stars which produce excess nitrogen without carbon and oxygen overabundances may also play a role. These abundances enable different contributors to be considered and incorporated into the evolutionary picture of omega Cen.Comment: 43 Pages, 13 Figures. Accepted for publication in Ap

On the Nature of the EIS Candidate Clusters: Confirmation of z<0.6 candidates

We use public V-band imaging data from the wide-angle surveys conducted by the ESO Imaging Survey project (EIS) to further investigate the nature of the EIS galaxy cluster candidates. These were originally identified by applying a matched-filter algorithm which used positional and photometric data of the galaxy sample extracted from the I-band survey images. In this paper, we apply the same technique to the galaxy sample extracted from V-band data and compare the new cluster detections with the original ones. We find that ~75% of the low-redshift cluster candidates (z<0.6) are detected in both passbands and their estimated redshifts show good agreement with the scatter in the redshift differences being consistent with the estimated errors of the method. For the ``robust'' I-band detections the matching frequency approaches ~85%. We also use the available (V-I) color to search for the red sequence of early-type galaxies observed in rich clusters over a broad range of redshifts. This is done by searching for a simultaneous overdensity in the three-dimensional color-projected distance space. We find significant overdensities for ~75% of the ``robust'' candidates with z_I<0.6. We find good agreement between the characteristic color associated to the detected "red sequence" and that predicted by passive evolution galaxy models for ellipticals at the redshift estimated by the matched-filter. The results presented in this paper show the usefulness of color data, even of two-band data, to both tentatively confirm cluster candidates and to select possible cluster members for spectroscopic observations. Based on the present results, we estimate that ~150 EIS clusters with z_I<0.6 are real, making it one of the largest samples of galaxy clusters in this redshift range currently available in the southern hemisphere.Comment: 11 pages, 10 figures, accepted for publication in A&

Simple strategy for simulation of large area of axially symmetric metasurfaces

Metalenses are composed of nanostructures for focusing light and have been widely explored in many exciting applications. However, their expanding dimensions pose simulation challenges. We propose a method to simulate metalenses in a timely manner using vectorial wave and ray tracing models. We sample the metalens' radial phase gradient and locally approximate it by a linear phase response. Each sampling point is modeled as a binary blazed grating, employing the chosen nanostructure, to build a transfer function set. The metalens transmission or reflection is then obtained by applying the corresponding transfer function to the incoming field on the regions surrounding each sampling point. Fourier optics is used to calculate the scattered fields under arbitrary illumination for the vectorial wave method and a Monte Carlo algorithm is used in the ray tracing formalism. We validated our method against finite difference time domain simulations at 632 nm and we were able to simulate metalenses larger than 3000lambda0 in diameter on a personal computer.Comment: 10 page

Historical inductions, Old and New

I review prominent historical arguments against scientific realism to indicate how they display a systematic overshooting in the conclusions drawn from the historical evidence. The root of the overshooting can be located in some critical, undue presuppositions regarding realism. I will highlight these presuppositions in connection with both Laudan’s ‘Old induction’ and Stanford’s New induction, and then delineate a minimal realist view that does without the problematic presuppositions