4,673 research outputs found
Na-O Anticorrelation and HB. VIII. Proton-capture elements and metallicities in 17 globular clusters from UVES spectra
We present homogeneous abundances for Fe and some of the elements involved in
the proton-capture reactions (O, Na, Mg, Al, and Si) for 202 red giants in 17
Galactic globular clusters (GCs) from the analysis of high resolution UVES
spectra obtained with FLAMES@ESO-VLT2. Our programme clusters span almost the
whole range in metallicity of GCs and were selected to sample the widest range
of global parameters (HB morphology, masses, concentration, etc). Here we focus
on the discussion of the Na-O and Mg-Al anticorrelations and related issues.
Our study finds clear Na and O star-to-star abundance variations in all GCs.
Variations in Al are present in all but a few GCs. Finally, a spread in
abundances of Mg and Si are also present in a few clusters. Mg is slightly less
overabundant and Si slightly more overabundant in the most Al-rich stars. The
correlation between Si and Al abundances is a signature of production of 28Si
leaking from the Mg-Al cycle in a few clusters. The cross sections required for
the proper reactions to take over in the cycle point to temperatures in excess
of about 65 MK for the favoured site of production. We used a dilution model to
infer the total range of Al abundances starting from the Al abundances in the
UVES spectra, and the Na abundance distributions found from analysis of the
much larger set of stars for which GIRAFFE spectra were available. We found
that the maximum amount of additional Al produced by first generation polluters
contributing to the composition of the second generation stars in each cluster
is closely correlated with the same combination of metallicity and cluster
luminosity that reproduced the minimum O abundances found from GIRAFFE spectra.
We then suggest that the high temperatures required for the Mg-Al cycle are
only reached in the most massive and most metal-poor polluters.Comment: 20 pages, 13 figures, fig. 3 degraded. Accepted for publication on
Astronomy and Astrophysic
The link between chemical anomalies along the red giant branch and the horizontal branch extension in globular clusters
We find a strong correlation between the extension of the Na-O
anticorrelation observed in red giant branch (RGB) stars and the high
temperature extension of the horizontal branch (HB) blue tails of Galactic
globular clusters (GCs). The longer is the O-depleted tail of the Na-O
anticorrelation observed in the RGB stars, the higher is the maximum
temperature reached by the bluest HB stars in the GC. This result provides a
clear, empirical evidence of a link between the extension of the HB and the
presence of star-to-star abundance variations of proton-capture elements in GC
stars. We discuss the possible interpretation of this correlation.Comment: Comments: 6 pages, 1 figure, uses emulateapj.cls; accepted for
publication in the Astrophysical Journal Letter
Spectroscopic analysis of the two subgiant branches of the globular cluster NGC1851
NGC1851 possibly shows a spread in [Fe/H], but the relation between this
spread and the division in the SGB is unknown. We obtained blue (3950-4600 A)
intermediate resolution (R~8,000) spectra for 47 stars on the bright and 30 on
the faint SGB of NGC 1851 (b-SGB and f-SGB, respectively). The determination of
the atmospheric parameters to extremely high internal accuracy leads to small
errors when comparing different stars in the cluster. We found that the b-SGB
is slightly more metal-poor than the f-SGB, with [Fe/H]=-1.227+/-0.009 and
[Fe/H]=-1.162+/- 0.012, respectively. This implies that the f-SGB is only
slightly older by ~0.6 Gyr than the b-SGB if the total CNO abundance is
constant. There are more C-normal stars in the b-SGB than in the f-SGB. This is
consistent with what is found for HB stars, if b-SGB are the progenitors of red
HB stars, and f-SGB those of blue HB ones. The abundances of the n-capture
elements Sr and Ba have a bimodal distribution, reflecting the separation
between f-SGB (Sr and Ba-rich) and b-SGB stars (Sr and Ba-poor). In both
groups, there is a clear correlation between [Sr/Fe] and [Ba/Fe], suggesting
that there is a real spread in the abundances of n-capture elements. There is
some correlation between C and Ba abundances, while the same correlation for Sr
is much more dubious. We identified six C-rich stars, which have a moderate
overabundance of Sr and Ba and rather low N abundances. This group of stars
might be the progenitors of these on the anomalous RGB in the (v, v-y) diagram.
These results are discussed within different scenarios for the formation of
NGC1851. It is possible that the two populations originated in different
regions of an inhomogeneous parent object. However, the striking similarity
with M22 calls for a similar evolution for these two clusters. Deriving
reliable CNO abundances for the two sequences would be crucial.Comment: Accepted by Astronomy and Astrophysics; 16 pages, 20 figure
Hurdles for Recent Measures in Eternal Inflation
In recent literature on eternal inflation, a number of measures have been
introduced which attempt to assign probabilities to different pocket universes
by counting the number of each type of pocket according to a specific
procedure. We give an overview of the existing measures, pointing out some
interesting connections and generic predictions. For example, pairs of vacua
that undergo fast transitions between themselves will be strongly favored. The
resultant implications for making predictions in a generic potential landscape
are discussed. We also raise a number of issues concerning the types of
transitions that observers in eternal inflation are able to experience.Comment: 15 PRD-style pages, 5 figures, expanded discussion of measures in
Sec. II, added reference
Recommended from our members
Measurements of absolute concentrations of NADH in cells using the phasor FLIM method.
We propose a graphical method using the phasor representation of the fluorescence decay to derive the absolute concentration of NADH in cells. The method requires the measurement of a solution of NADH at a known concentration. The phasor representation of the fluorescence decay accounts for the differences in quantum yield of the free and bound form of NADH, pixel by pixel of an image. The concentration of NADH in every pixel in a cell is obtained after adding to each pixel in the phasor plot a given amount of unmodulated light which causes a shift of the phasor towards the origin by an amount that depends on the intensity at the pixel and the fluorescence lifetime at the pixel. The absolute concentration of NADH is obtained by comparison of the shift obtained at each pixel of an image with the shift of the calibrated solution
Infrared photometry of Young Massive Clusters in the starburst galaxy NGC 4214
We present the results of an infrared photometric survey performed with
NICS@TNG in the nearby starburst galaxy NGC 4214. We derived accurate
integrated JK magnitudes of 10 young massive clusters and compared them with
the already available Hubble Space Telescope ultraviolet colors. These clusters
are located in the combined ultraviolet-infrared colors planes on well defined
sequences, whose shapes allow a precise determination of their age. By means of
the comparison with suitable stellar evolution models we estimated ages,
metallicities, reddening and masses of these clusters. All the analyzed
clusters appear to be younger than log(t/yr)<8.4, moderately metal-rich and
slightly less massive than present-day Galactic globular clusters. The derived
ages for clusters belonging to the secondary HII star forming complex are
significantly larger than those previously estimated in the literature. We also
discuss the possibility of using the ultraviolet-infrared color-color diagram
to select candidate young massive clusters hosting multiple stellar
populations.Comment: 12 pages, 9 figures, accepted for publication by MNRA
An Abundance Analysis for Five Red Horizontal Branch Stars in the Extremely Metal Rich Globular Cluster NGC 6553
We provide a high dispersion line-by-line abundance analysis of five red HB
stars in the extremely metal rich galactic globular cluster NGC 6553. These red
HB stars are significantly hotter than the very cool stars near the tip of the
giant branch in such a metal rich globular cluster and hence their spectra are
much more amenable to an abundance analysis than would be the case for red
giants.
We find that the mean [Fe/H] for NGC 6553 is -0.16 dex, comparable to the
mean abundance in the galactic bulge found by McWilliam & Rich (1994) and
considerably higher than that obtained from an analysis of two red giants in
this cluster by Barbuy etal (1999). The relative abundance for the best
determined alpha process element (Ca) indicates an excess of alpha process
elements of about a factor of two. The metallicity of NGC 6553 reaches the
average of the Galactic bulge and of the solar neighborhood.Comment: 29 pages, 6 figures, accepted for publication in the Ap
The chemical abundance of the very metal rich old Open Clusters NGC 6253 and NGC 6791
In the framework of a project aiming at deriving in a homogeneous way the
properties (age, distance, reddening and detailed chemical abundances) of a
large sample of old open clusters, we present here the metal abundance and the
abundance ratios of light (C, N, O, Na, Mg, Al, Si, Ca, Ti) and heavier (Cr,
Mn, Ni, Ba, Eu) elements in the galactic open clusters NGC 6253 and NGC 6791.
We performed spectrum synthesis of selected lines on high resolution spectra of
four red clump stars in NGC 6253, taken with the UVES and FEROS spectrographs.
We also determined abundances of the same elements for four red clump stars in
NGC 6791, observed with SARG, for which we had derived the atmospheric
parameters and the iron, carbon and oxygen abundances in a previous paper
(Gratton et al. 2006). The average metallicity of NGC 6253 is [Fe/H]=+0.46 (rms
= 0.03 dex, systematic error = 0.08 dex), obtained by extensive spectral
synthesis of Fe lines. This intermediate age cluster closely resembles the old
open cluster NGC 6791, as far as the chemical composition is concerned. C, N, O
do not show any significant abundance scatter; they are underabundant with
respect to the solar values both in NGC 6253 and NGC 6791. We also find no
evident star-to-star scatter in any of the elements measured in both clusters,
with the possible exception of Na in NGC 6791. The two clusters show very
similar abundances, except for Mg, overabundant in NGC 6791 and not in NGC
6253. Both have solar scaled alpha-elements abundances. We have compared our
abundance ratios with literature values for disk giants and dwarfs and bulge
giants, finding a general good agreement with the run of elemental ratios with
[Fe/H] of disk objects.Comment: Accepted for publication on A&
Abundances in Stars from the Red Giant Branch Tip to Near the Main Sequence Turn Off in M71: III. Abundance Ratios
We present abundance ratios for 23 elements with respect to Fe in a sample of
stars with a wide range in luminosity, from luminous giants to stars near the
turnoff, in the globular cluster M71. The analyzed spectra, obtained with HIRES
at the Keck Observatory, are of high dispersion (R=35,000). We find that the
neutron capture, the iron peak and the alpha-element abundance ratios show no
trend with Teff, and low scatter around the mean between the top of the RGB and
near the main sequence turnoff. The alpha-elements Mg, Ca, Si and Ti are
overabundant relative to Fe. The anti-correlation between O and Na abundances,
observed in other metal poor globular clusters, is detected in our sample and
extends to the main sequence. A statistically significant correlation between
Al and Na abundances is observed among the M71 stars in our sample, extending
to Mv = +1.8, fainter than the luminosity of the RGB bump in M5. Lithium is
varying, as expected, and Zr may be varying from star to star as well. M71
appears to have abundance ratios very similar to M5 whose bright giants were
studied by Ivans et al. (2001), but seems to have a smaller amplitude of
star-to-star variations at a given luminosity, as might be expected from its
higher metallicity. The results of our abundance analysis of 25 stars in M71
provide sufficient evidence of abundance variations at unexpectedly low
luminosities to rule out the mixing scenario. Either alone or, even more
powerfully, combined with other recent studies of C and N abundances in M71
stars, the existence of such abundance variations cannot be reproduced within
the context of our current understanding of stellar evolution.Comment: AJ, in press (June 2002), 18 figure
- …