382 research outputs found
A spectroscopic study of the globular Cluster NGC 4147
IndexaciĂłn: Web of ScienceWe present the abundance analysis for a sample of 18 red giant branch stars in the metal-poor globular cluster NGC 4147 based on medium- and high-resolution spectra. This is the first extensive spectroscopic study of this cluster. We derive abundances of C, N, O, Na, Mg, Al, Si, Ca, Ti, Cr, Fe, Ni, Y, Ba, and Eu. We find a metallicity of [Fe/H] = -1.84 +/- 0.02 and an alpha-enhancement of +0.38 +/- 0.05 (errors on the mean), typical of halo globular clusters in this metallicity regime. A significant spread is observed in the abundances of light elements C, N, O, Na, and Al. In particular, we found an Na-O anticorrelation and Na-Al correlation. The cluster contains only similar to 15 per cent of stars that belong to the first generation (Na-poor and O-rich). This implies that it suffered a severe mass-loss during its lifetime. Its [Ca/Fe] and [Ti/Fe] mean values agree better with the Galactic halo trend than with the trend of extragalactic environments at the cluster metallicity. This possibly suggests that NGC 4147 is a genuine Galactic object at odd with what claimed by some author that proposed the cluster to be member of the Sagittarius dwarf galaxy. An antirelation between the light s-process element Y and Na may also be present.https://academic.oup.com/mnras/article-lookup/doi/10.1093/mnras/stw114
Spectroscopy of blue horizontal branch stars in NGC 6656 (M22)
Recent investigations revealed very peculiar properties of blue horizontal
branch (HB) stars in \omega Centauri, which show anomalously low surface
gravity and mass compared to other clusters and to theoretical models. \omega
Centauri, however, is a very unusual object, hosting a complex mix of multiple
stellar populations with different metallicity and chemical abundances. We
measured the fundamental parameters (temperature, gravity, and surface helium
abundance) of a sample of 71 blue HB stars in M22, with the aim of clarifying
if the peculiar results found in \omega Cen are unique to this cluster. M22
also hosts multiple sub-populations of stars with a spread in metallicity,
analogous to \omega Cen. The stellar parameters were measured on low-resolution
spectra fitting the Balmer and helium lines with a grid of synthetic spectra.
From these parameters, the mass and reddening were estimated. Our results on
the gravities and masses agree well with theoretical expectations, matching the
previous measurements in three "normal" clusters. The anomalies found in \omega
Cen are not observed among our stars. A mild mass underestimate is found for
stars hotter than 14\,000 K, but an exact analogy with \omega Cen cannot be
drawn. We measured the reddening in the direction of M22 with two independent
methods, finding E(B-V)=0.35 \pm 0.02 mag, with semi-amplitude of the maximum
variation \Delta(E(B-V))=0.06 mag, and an rms intrinsic dispersion of
\sigma(E(B-V))=0.03 mag.Comment: 11 pages, 9 Postscript figure
Spectroscopy of horizontal branch stars in Omega Centauri
We analyze the reddening, surface helium abundance and mass of 115 horizontal
branch (HB) and blue hook (BH) stars in OmegaCentauri, spanning the HB from the
blue edge of the instability strip to Teff~50000K. The mean cluster reddening
is E(B-V)=0.115+-0.004, in good agreement with previous estimates, but we
evidence a pattern of differential reddening in the cluster area. The stars in
the western half are more reddened than in the southwest quadrant by 0.03-0.04
magnitudes. We find that the helium abundances measured on low-resolution
spectra are systematically lower by ~0.25 dex than the measurements based on
higher resolution. No difference in helium abundance is detected between
OmegaCentauri and three comparison clusters, and the stars in the range
11500-20000K follow a trend with temperature, which probably reflects a
variable efficiency of the diffusion processes. There is mild evidence that two
families of extreme HB (EHB) stars (Teff>20000K) could exist, as observed in
the field, with ~15% of the objects being helium depleted by a factor of ten
with respect to the main population. The distribution of helium abundance above
30000K is bimodal, but we detect a fraction of He-poor objects lower than
previous investigations. The observations are consistent with these being stars
evolving off the HB. Their spatial distribution is not uniform, but this
asymmetric distribution is only marginally significative. We also find that EHB
stars with anomalously high spectroscopic mass could be present in
OmegaCentauri, as previously found in other clusters. The derived
temperature-color relation reveals that stars hotter than 11000K are fainter
than the expectations of the canonical models in the U band, while no anomaly
is detected in B and V. This behavior, not observed in NGC6752, is a new
peculiarity of OmegaCentauri HB stars.Comment: Accepted for publication in A&
Ghosts of Milky Way's past: the globular cluster ESO 37-1 (E 3)
Context. In the Milky Way, most globular clusters are highly conspicuous
objects that were found centuries ago. However, a few dozen of them are faint,
sparsely populated systems that were identified largely during the second half
of the past century. One of the faintest is ESO 37-1 (E 3) and as such it
remains poorly studied, with no spectroscopic observations published so far,
although it was discovered in 1976.
Aims. We investigate the globular cluster E 3 in an attempt to better
constrain its fundamental parameters. Spectroscopy of stars in the field of E 3
is shown here for the first time.
Methods. Deep, precise VI CCD photometry of E 3 down to V=26 mag is presented
and analysed. Low-resolution, medium signal-to-noise ratio spectra of nine
candidate members are studied to derive radial velocity and metallicity. Proper
motions from the UCAC4 catalogue are used to explore the kinematics of the
bright members of E 3.
Results. Isochrone fitting indicates that E 3 is probably very old, with an
age of about 13 Gyr; its distance from the Sun is nearly 10 kpc. It is also
somewhat metal rich with [Fe/H]=-0.7. Regarding its kinematics, our tentative
estimate for the proper motions is (-7.0+/-0.8, 3.5+/-0.3) mas/yr (or a
tangential velocity of 382+/-79 km/s) and for the radial velocity is 45+/-5
km/s, in the solar rest frame.
Conclusions. E 3 is one of the most intriguing globular clusters in the
Galaxy. Having an old age and being metal rich is clearly a peculiar
combination, only seen in a handful of objects like the far more conspicuous
NGC 104 (47 Tucanae). In addition, its low luminosity and sparse population
make it a unique template for the study of the final evolutionary phases in the
life of a star cluster. Unfortunately, E 3 is among the most elusive and
challenging known globular clusters because field contamination severely
hampers spectroscopic studies.Comment: 7 pages, 6+1 figures, 2 tables. Accepted for publication in Astronomy
and Astrophysics. Minor change
A hot horizontal branch star with a close K-type main-sequence companion
Dynamical interactions in binary systems are thought to play a major role in
the formation of extreme horizontal branch stars (EHBs) in the Galactic field.
However, it is still unclear if the same mechanisms are at work in globular
clusters, where EHBs are predominantly single stars. Here we report on the
discovery of a unique close binary system (period ~1.61 days) in the globular
cluster NGC6752, comprising an EHB and a main-sequence companion of 0.63+-0.05
Msun. Such a system has no counterpart among nearly two hundred known EHB
binaries in the Galactic field. Its discovery suggests that either field
studies are incomplete, missing this type of systems possibly because of
selection effects, or that a particular EHB formation mechanism is active in
clusters but not in the field
Spectroscopic search for binaries among EHB stars in globular clusters
We performed a spectroscopic search for binaries among hot Horizontal Branch
stars in globular clusters. We present final results for a sample of 51 stars
in NGC6752, and preliminary results for the first 15 stars analyzed in M80. The
observed stars are distributed along all the HBs in the range 8000 < Teff <
32000 K, and have been observed during four nights. Radial velocity variations
have been measured with the cross-correlation technique. We carefully analyzed
the statistical and systematic errors associated with the measurements in order
to evaluate the statistical significance of the observed variations. No close
binary system has been detected, neither among cooler stars nor among the
sample of hot EHB stars (18 stars with Teff > 22000 K in NGC6752). The data
corrected for instrumental effects indicate that the radial velocity variations
are always below the 3sigma level of ~15 km/s. These results are in sharp
contrast with those found for field hot subdwarfs, and open new questions about
the formation of EHB stars in globular clusters, and possibly of the field
subdwarfs.Comment: To appear in Baltic Astronomy. Proceedings of the 2nd meeting on Hot
Subdwarf Stars, La Palma, June 2005. 4 pages, 2 figure
Chemical analysis of NGC 6528: one of the most metal-rich bulge globular cluster
The Bulge Globular Clusters (GCs) are key tracers of this central ancient
component of our Galaxy. It is essential to understand their formation and
evolution to study that of the bulge, as well as their relationship with the
other Galactic GC systems (halo and disk GCs). Our main goals are to obtain
detailed abundances for a sample of seven red giant members of NGC 6528 in
order to characterize its chemical composition and study the relationship of
this GC with the bulge, and with other bulge, halo and disk GCs. Moreover, we
analyze this clusters behavior associated with the Multiple Populations
(MPs) phenomenon. We obtained the stellar parameters and chemical abundances of
light elements (Na, Al), iron-peak elements (V, Cr, Mn, Fe, Co, Ni, Cu),
{\alpha}-elements (O, Mg, Si, Ca, Ti) and heavy elements (Zr, Ba, Eu) in seven
red giant members of NGC 6528 using high resolution spectroscopy from
FLAMES-UVES. We obtained in six stars of our sample a mean iron content of
[Fe/H]=-0.14+/-0.03 dex, in good agreement with other studies. We found no
significant internal iron spread. We detected one candidate variable star,
which was excluded from the mean in iron content, we derived a metallicity in
this star of [Fe/H]=-0.55+/-0.04 dex. Moreover, we found no extended O-Na
anticorrelation but instead only an intrinsic Na spread. In addition, NGC 6528
does not exhibit a Mg-Al anticorrelation, and no significant spread in either
Mg or Al. The {\alpha} and iron-peak elements show good agreement with the
bulge field star trend. The heavy elements are slightly dominated by the
r-process. The chemical analysis suggests an origin and evolution similar to
that of typical old Bulge field stars. Finally, we find remarkable agreement in
the chemical patterns of NGC 6528 and another bulge GC, NGC 6553, suggesting a
similar origin and evolution.Comment: Accepted for publication in A&A. 12 pages, 13 figures, 4 table
Detailed abundances in stars belonging to ultra-faint dwarf spheroidal galaxies
We report preliminary results concerning the detailed chemical composition of
metal poor stars belonging to close ultra-faint dwarf galaxies (hereafter
UfDSphs). The abundances have been determined thanks to spectra obtained with
X-Shooter, a high efficiency spectrograph installed on one of the ESO VLT
units. The sample of ultra-faint dwarf spheroidal stars have abundance ratios
slightly lower to what is measured in field halo star of the same
metallicity.We did not find extreme abundances in our Hercules stars as the one
found by Koch for his 2 Hercules stars. The synthesis of the neutron capture
elements Ba and Sr seems to originate from the same nucleosynthetic process in
operation during the early stages of the galactic evolution.Comment: 3 pages, 1 figure; OMEG11 conference (Tokyo, Nov 2011
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