118 research outputs found
The ACS Survey of Galactic Globular Clusters. IX. Horizontal Branch Morphology and the Second Parameter Phenomenon
The horizontal branch (HB) morphology of globular clusters (GCs) is most
strongly influenced by metallicity. The second parameter phenomenon
acknowledges that metallicity alone is not enough to describe the HB morphology
of all GCs. In particular, the outer Galactic halo contains GCs with redder HBs
at a given metallicity than are found inside the Solar circle. Thus, at least a
second parameter is required to characterize HB morphology. Here we analyze the
median color difference between the HB and the red giant branch (RGB), d(V-I),
measured from HST ACS photometry of 60 GCs within ~20 kpc of the Galactic
Center. Analysis of this homogeneous data set reveals that, after the influence
of metallicity has been removed, the correlation between d(V-I) and age is
stronger than that of any other parameter considered. Expanding the sample to
include HST photometry of the 6 most distant Galactic GCs lends additional
support to the correlation between d(V-I) and age. This result is robust with
respect to the adopted metallicity scale and the method of age determination,
but must bear the caveat that high quality, detailed abundance information is
not available for a significant fraction of the sample. When a subset of GCs
with similar metallicities and ages are considered, a correlation between
d(V-I) and central luminosity density is exposed. With respect to the existence
of GCs with anomalously red HBs at a given metallicity, we conclude that age is
the second parameter and central density is most likely the third. Important
problems related to HB morphology in GCs, notably multi-modal distributions and
faint blue tails, remain to be explained. (Abridged)Comment: Accepted for publication in ApJ; 49 pages, 19 figure
The Gradients in the 47 Tuc Red Giant Branch Bump and Horizontal Branch are Consistent With a Centrally-Concentrated, Helium-Enriched Second Stellar Generation
We combine ground and space-based photometry of the Galactic globular cluster
47 Tuc to measure four independent lines of evidence for a helium gradient in
the cluster, whereby stars in the cluster outskirts would have a lower initial
helium abundance than stars in and near the cluster core. First and second, we
show that the red giant branch bump (RGBB) stars exhibit gradients in their
number counts and brightness. With increased separation from the cluster
center, they become more numerous relative to the other red giant (RG) stars.
They also become fainter. For our third and fourth lines of evidence, we show
that the horizontal branch (HB) of the cluster becomes both fainter and redder
for sightlines farther from the cluster center. These four results are
respectively detected at the 2.3, 3.6, 7.7 and
4.1 levels. Each of these independent lines of evidence is found to be
significant in the cluster-outskirts; closer in, the data are more compatible
with uniform mixing. Our radial profile is qualitatively consistent with but
quantitatively tighter than previous results based on CN absorption. These
observations are qualitatively consistent with a scenario wherein a second
generation of stars with modestly enhanced helium and CNO abundance formed deep
within the gravitational potential of a cluster of previous generation stars
having more canonical abundances.Comment: 20 pages, 6 figures, 1 table, submitted to The Astrophysical Journa
A Double Main Sequence in the Globular Cluster NGC 6397
High-precision multi-band HST photometry reveals that the main sequence (MS)
of the globular cluster NGC 6397 splits into two components, containing ~30%
and ~70% of the stars. This double sequence is consistent with the idea that
the cluster hosts two stellar populations: (i) a primordial population that has
a composition similar to field stars, and containing ~30% of the stars, and
(ii) a second generation with enhanced sodium and nitrogen, depleted carbon and
oxygen, and a slightly enhanced helium abundance (Delta Y~0.01). We examine the
color difference between the two sequences across a variety of color baselines
and find that the second sequence is anomalously faint in m_F336W. Theoretical
isochrones indicate that this could be due to NH depletion.Comment: 19 pages, 11 figures, accepted for pubblication in Ap
Impact of light-curing time and aging on dentin bond strength of methacrylate- and silorane-based restorative systems
AIM: To evaluate the impact of different light-curing times on dentin microtensile bond strength of two restorative systems after 24 h and 6 months of water storage. METHODS: Standardized Class II preparations were performed in 56 freshly-extracted human molars (n = 7), restored with methacrylate- or silorane-based restorative systems, and light-cured using a light-emitting diode at 1390 mW/cm2 by the recommended manufacturers' time or double this time. After storage for 24 h at 37 oC, the teeth were sectioned to yield a series of 0.8-mm thick slices. Each slab was trimmed into an hourglass shape of approximately 0.64 mm2 area at the gingival dentin-resin interface. Specimens were tested using universal testing machine at crosshead speed of 0.5 mm/min until failure, after 24 h and 6 months of storage. Data were statistically analyzed by three-way ANOVA and Tukey's test (α = 0.05). RESULTS: The highest bond strength values were recorded for the groups restored with methacrylate system (p0.05). CONCLUSIONS: Bond strength was influenced by the material and light-curing time, but the 6-month storage did not affect the bond strength of restorations133213218FUNDAĂĂO DE AMPARO Ă PESQUISA DO ESTADO DE SĂO PAULO - FAPESP2010/05666-9; 2010/15076-
Near-IR period-luminosity relations for pulsating stars in Centauri (NGC 5139)
Centauri (NGC 5139) hosts hundreds of pulsating variable stars of
different types, thus representing a treasure trove for studies of their
corresponding period-luminosity (PL) relations. Our goal in this study is to
obtain the PL relations for RR Lyrae, and SX Phoenicis stars in the field of
the cluster, based on high-quality, well-sampled light curves in the
near-infrared (IR). Centauri was observed using VIRCAM mounted on
VISTA. A total of 42 epochs in and 100 epochs in were obtained,
spanning 352 days. Point-spread function photometry was performed using DoPhot
and DAOPHOT in the outer and inner regions of the cluster, respectively. Based
on the comprehensive catalogue of near-IR light curves thus secured, PL
relations were obtained for the different types of pulsators in the cluster,
both in the and bands. This includes the first PL relations in
the near-IR for fundamental-mode SX Phoenicis stars. The near-IR magnitudes and
periods of Type II Cepheids and RR Lyrae stars were used to derive an updated
true distance modulus to the cluster, with a resulting value of mag, where the error bars correspond to the adopted
statistical and systematic errors, respectively. Adding the errors in
quadrature, this is equivalent to a heliocentric distance of
kpc.Comment: 10 pages, 8 figures, accepted for publication in A&
Influence of thickness and translucency of lithium disilicate ceramic on degree of conversion of resinous materials
In this study was assessed the degree of conversion (DC) of amine-free resin cements light cured through lithium disilicate-reinforced ceramics at different thicknesses and translucency. Specimens were divided into 21 groups (n = 5) according to luting agent used: Variolink Esthetic LC (Light shade), RelyX Ultimate (A1 shade), and Filtek Z350 XT Flow (A1 shade); the ceramic translucency: low (LT) and high (HT); and the ceramic thickness: no ceramic (control), 0.5 mm, 1 mm, and 2 mm. A Teflon mold with (5 x 5 x 0.5 mm) was used to standardize the cement and over it the ceramic block from each group was placed. Set was cured using a polywave LED light (1200 mW/cm2 - Bluephase G2) for 40 s. FTIR spectra of uncured and cured materials was obtained and DC calculated from the height of the peaks 1610 and 1640 cm-1. Data were submitted to ANOVA followed by Tukey?s test (? = 0.05). There was a significant difference for luting agents (p< 0.0001) and translucency (p = 0.025), but not for thickness (p = 0.73). Dual amine-free RelyX Ultimate showed the lowest DC values and higher translucency promoted higher DC. Dual amine-free cement showed the lowest monomer conversion and higher translucency ceramics promoted a higher DC
The Blue Hook Populations of Massive Globular Clusters
We present new HST ultraviolet color-magnitude diagrams of 5 massive Galactic
globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. These
observations were obtained to investigate the "blue hook" phenomenon previously
observed in UV images of the globular clusters omega Cen and NGC 2808. Blue
hook stars are a class of hot (approximately 35,000 K) subluminous horizontal
branch stars that occupy a region of the HR diagram that is unexplained by
canonical stellar evolution theory. By coupling new stellar evolution models to
appropriate non-LTE synthetic spectra, we investigate various theoretical
explanations for these stars. Specifically, we compare our photometry to
canonical models at standard cluster abundances, canonical models with enhanced
helium (consistent with cluster self-enrichment at early times), and
flash-mixed models formed via a late helium-core flash on the white dwarf
cooling curve. We find that flash-mixed models are required to explain the
faint luminosity of the blue hook stars, although neither the canonical models
nor the flash-mixed models can explain the range of color observed in such
stars, especially those in the most metal-rich clusters. Aside from the
variation in the color range, no clear trends emerge in the morphology of the
blue hook population with respect to metallicity.Comment: Accepted for publication in The Astrophysical Journal. Latex, 14
pages, 1 B&W and 6 color figure
Chemical composition and roughness of enamel and composite after bleaching, acidic beverages and toothbrushing
In this study was assessed the surface roughness and chemical composition of tooth enamel and composite resin after bleaching treatment, immersion in acidic beverages, and simulated toothbrushing. One hundred and twenty dental blocks (10 x 10 x 3 mm) were randomly assigned (n = 10) according to surface treatment [none (N), bleaching (B), toothbrushing (T), and B+T] and storage medium [saliva (S), whiskey (W), and orange juice (O)]: experimental groups - N+S, N+W, N+O, B+S, B+W, B+O, S+T, W+T, O+T, B+S+T, B+W+T, and B+O+T. Two bleaching sessions were conducted using 38% hydrogen peroxide (3 applications). Surface roughness was measured using a roughness tester and composition was determined by micro energy-dispersive X-ray fluorescence spectrometry (”-EDXRF) before and after treatments. Calcium/phosphorus (Ca/P) ratio in enamel and silica (Si) content in composite were evaluated. Data were statistically analyzed by ANOVA and Tukey?s test (? = 0.05). Overall, increased values of surface roughness for enamel and composite were observed mainly after immersion in orange juice and bleaching/toothbrushing association. Moreover, this association and immersion in whiskey resulted in lower Ca/P ratio and after aging methods, bleached and bleached/toothbrushed groups showed decreased in Ca/P ratio compared to initial values. All groups showed Si content decrease at the end, except the group without surface treatment and immersed in saliva, and bleaching followed by immersion in orange juice and toothbrushing caused the highest Si reduction. Bleaching and toothbrushing combination strengthened the effects caused by acidic drinks on roughness and chemical composition of enamel and composite
- âŠ