525 research outputs found
Globular Clusters in the Magellanic Clouds.I:BV CCD-Photometry for 11 Clusters
We present BV CCD-data for 11 intermediate-age LMC clusters; the main
conclusions are: 1. in the (V_to, V_cl,m) and (V-to, (V_to-V_cl,m)) planes the
models yield a good overall description of the data; 2. with the current
sample, it is impossible to firmly choose between "classical" and
"overshooting" models; 3. the separation in colour between the MS band and the
Red He-burning Clump is smaller than predicted by theoretical tracks; 4. the
existence of the so-called "RGB phase-transition (Renzini and Buzzoni 1986)
seems to be confirmed.Comment: 62 pages, 37 figures and tables 6 to 16 available on request,
uuencoded compressed postscript file with tables 1-5 and 17-18 included, BAP
08-1994-020-OA
Theoretical Uncertainties in Red Giant Branch Evolution: The Red Giant Branch Bump
A Monte Carlo simulation exploring uncertainties in standard stellar
evolution theory on the red giant branch of metal-poor globular clusters has
been conducted. Confidence limits are derived on the absolute V-band magnitude
of the bump in the red giant branch luminosity function (M_v,b) and the excess
number of stars in thebump, R_b. The analysis takes into account uncertainties
in the primordial helium abundance, abundance of alpha-capture elements,
radiative and conductive opacities, nuclear reaction rates, neutrino energy
losses, the treatments of diffusion and convection, the surface boundary
conditions, and color transformations.
The uncertainty in theoretical values for the red giant bump magnitude varies
with metallicity between +0.13/-0.12 mag at [Fe/H] = -2.4 and +0.23/-0.21 mag
at [Fe/H] = -1.0 to 0.50 at [Fe/H] =
-1.0. These theoretical values for R_b are in agreement with observations.Comment: 30 pages, 6 figures. To appear in Ap
Photographic detection of Cadmium(II) and Zinc(II) ions
The binding properties of two 2,8-dithia-5-aza-2,6-pyridinophane-based ligands, bearing respectively coumarin (HNCum) and naphthol-benzoxaazole (HNBO) optically active units as pendant arms, were studied with the aim of transition metal cations selective detection. The novel all-solid-state optodes were obtained by inclusion of fluorophores inside PVC-based polymeric films. The influence of lipophilic sites incorporation and plasticizer nature on pyridinophane-based optodes response was investigated. An enhanced selectivity of the ligands towards Cadmium(II) and Zinc(II) ions was detected. Light Emitting Diode, LED 380 nm, was used as light source, and a digital camera as a signal detector. The results obtained demonstrated the suitability of the developed optodes to perform fast and low cost simultaneous monitoring of Zn2+and Cd2+ ions by means of familiar devices and chemometrics. © 2016 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
The Accretion of Lyman Alplha Clouds onto Gas-Rech Protogalaxies; A Scenario for the Formation of Globular Star Clusters
A satisfactory theory for the formation of globular star clusters (GCs) has
long been elusive, perhaps because their true progenitors had not yet been
guessed. In this paper I propose a causal relationship between the strongly
decreasing densities of Lyman alpha (LyA) clouds at high redshift and the
formation of GCs - namely that GCs were created by the accretion of LyA clouds
onto protogalaxies. I describe a scenario which involves an inherently stable
and orderly cycling of compression and cooling in the central cores of clouds
during the extended period of dissipation in the outer regins of gas-rich proto
galaxies, culminating in a burst of efficient star formation. I demonstrate
that the comoving density of GCs is comparable to that of LyA clouds at high
redshift, that the energetic requirements for compression to core GC densities
can be met, and that the time-scale for cooling is within obvious limits
imposed by dynamical stability.
This dissipative process requires there to be a large column of dissipated
gas about the attractor in order to form GCs. In addition, the energy
requirements for compression requires attractor masses greater than that
capable of sustaining circular velocities of ~40 km/s. If this scenario is
supported by numerical simulations, then by implication, the GCs were formed at
modest redshifts of z~1-3. This knowledge could help to break the degeneracy
between lookback time and redshift. The model is consistent with a picture of
hierarchical galaxy growth over time scales of many billions of years.Comment: 7 pages. Accepted, 10 June 1999 Astrophysical Journa
High Performances Corrugated Feed Horns for Space Applications at Millimetre Wavelengths
We report on the design, fabrication and testing of a set of high performance
corrugated feed horns at 30 GHz, 70 GHz and 100 GHz, built as advanced
prototypes for the Low Frequency Instrument (LFI) of the ESA Planck mission.
The electromagnetic designs include linear (100 GHz) and dual shaped (30 and 70
GHz) profiles. Fabrication has been achieved by direct machining at 30 GHz, and
by electro-formation at higher frequencies. The measured performances on side
lobes and return loss meet the stringent Planck requirements over the large
(20%) instrument bandwidth. Moreover, the advantage in terms of main lobe shape
and side lobes levels of the dual profiled designs has been demonstrated.Comment: 16 pages, 7 figures, accepted for publication in Experimental
Astronom
The Age Dependent Luminosities of the Red Giant Branch Bump, Asymptotic Giant Branch Bump, and Horizontal Branch Red Clump
Color-magnitude diagrams of globular clusters often exhibit a prominent
horizontal branch (HB) and may also show features such as the red giant branch
(RGB) bump and the asymptotic giant branch (AGB) bump. Stellar evolution theory
predicts that the luminosities of these features will depend on the metallicity
and age of the cluster. We calculate theoretical lines of 2 to 12 Gyr constant
age RGB-bumps and AGB-bumps in the V(HB-Bump)--[Fe/H] diagram, which shows the
brightness difference between the bump and the HB as a function of metallicity.
In order to test the predictions, we identify giant branch bumps in new Hubble
Space Telescope color-magnitude diagrams for 8 SMC clusters. First, we conclude
that the SMC cluster bumps are RGB-bumps. The data for clusters younger than ~6
Gyr are in fair agreement the relative age dependent luminosities of the HB and
RGB-bump. The V(HB-Bump)--[Fe/H] data for clusters older then ~6 Gyr
demonstrate a less satisfactory agreement with our calculations. We conclude
that ~6 Gyr is a lower bound to the age of clusters for which the Galactic
globular cluster, age independent V(HB-Bump)--[Fe/H] calibration is valid.
Application of the V(HB-bump)--[Fe/H] diagram to stellar population studies is
discussed.Comment: Accepted for publication in the Astrophysical Journal, 30 pages,
Latex aaspp4.sty, including 7 postscript figure
The Second-Parameter Effect in Metal-Rich Globular Clusters
Recent Hubble Space Telescope observations have found that the horizontal
branches (HB's) in the metal-rich globular clusters NGC 6388 and NGC 6441 slope
upward with decreasing B-V. Such a slope is not predicted by canonical HB
models and cannot be produced by either a greater cluster age or enhanced mass
loss along the red-giant branch (RGB). The peculiar HB morphology in these
clusters may provide an important clue for understanding the second-parameter
effect.
We have carried out extensive evolutionary calculations and numerical
simulations in order to explore three non-canonical scenarios for explaining
the sloped HB's in NGC 6388 and NGC 6441: i) A high cluster helium abundance
scenario, where the HB evolution is characterized by long blue loops; ii) A
rotation scenario, where internal rotation during the RGB phase increases the
HB core mass; iii) A helium-mixing scenario, where deep mixing on the RGB
enhances the envelope helium abundance. All three of these scenarios predict
sloped HB's with anomalously bright RR Lyrae variables. We compare this
prediction with the properties of the two known RR Lyrae variables in NGC 6388.
Other possible observational tests are suggested.Comment: 12 pages, including three embedded figures. ApJ (Letters), in pres
Variable Stars in the Unusual, Metal-Rich Globular Cluster NGC 6388
We have undertaken a search for variable stars in the metal-rich globular
cluster NGC 6388 using time-series BV photometry. Twenty-eight new variables
were found in this survey, increasing the total number of variables found near
NGC 6388 to ~57. A significant number of the variables are RR Lyrae (~14), most
of which are probable cluster members. The periods of the fundamental mode RR
Lyrae are shown to be unusually long compared to metal-rich field stars. The
existence of these long period RRab stars suggests that the horizontal branch
of NGC 6388 is unusually bright. This implies that the metallicity-luminosity
relationship for RR Lyrae stars is not universal if the RR Lyrae in NGC 6388
are indeed metal-rich. We consider the alternative possibility that the stars
in NGC 6388 may span a range in [Fe/H]. Four candidate Population II Cepheids
were also found. If they are members of the cluster, NGC 6388 would be the most
metal-rich globular cluster to contain Population II Cepheids. The mean V
magnitude of the RR Lyrae is found to be 16.85+/-0.05 resulting in a distance
of 9.0 to 10.3 kpc, for a range of assumed values of for RR Lyrae. We
determine the reddening of the cluster to be E(B-V)=0.40+/-0.03 mag, with
differential reddening across the face of the cluster. We discuss the
difficulty in determining the Oosterhoff classification of NGC 6388 and NGC
6441 due to the unusual nature of their RR Lyrae, and address evolutionary
constraints on a recent suggestion that they are of Oosterhoff type II.Comment: 35 pages, 16 figures, emulateapj5/apjfonts style. Astronomical
Journal, in press. We recommend the interested reader to download instead the
preprint with full-resolution figures, which can be found at
http://www.noao.edu/noao/staff/pritzl/clusters.htm
Design, Synthesis and Catalytic Activity of (Cyclopentadienone)iron Complexes Containing a Stereogenic Plane and a Stereogenic Axis
Herein, we report the synthesis and characterization of several chiral (cyclopentadienone)iron complexes (CICs) featuring either two (R)-BINOL-derived stereoaxes or a combination of one (R)-BINOL-derived stereoaxis and a stereogenic plane. The stereoplane-containing CICs were obtained as epimer mixtures, which were separated by flash column chromatography and assigned an absolute configuration based on XRD analysis, NMR and order of elution. The library was tested in the asymmetric hydrogenation of ketones showing good catalytic activity and a moderate stereoselectivity which, notably, is mostly imparted by the stereogenic plane. Indeed, the two epimers of each CIC possessing a stereoplane show opposite and equally strong stereochemical preference.Winning plane: chiral (cyclopentadienone)iron complexes (CICs) featuring a combination of an (R)-BINOL-derived stereoaxis and a stereogenic plane were synthesized in diastereoisomerically pure form, characterized and tested in the asymmetric hydrogenation of ketones. The new CICs showed good catalytic activity and, remarkably, the stereoplane epimers displayed similar and opposite stereochemical preference
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