1,486 research outputs found
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
Homogeneous abundance analysis of dwarf, subgiant and giant FGK stars with and without giant planets
We have analyzed high-resolution and high signal-to-noise ratio optical
spectra of nearby FGK stars with and without detected giant planets in order to
homogeneously measure their photospheric parameters, mass, age, and the
abundances of volatile (C, N, and O) and refractory (Na, Mg, Si, Ca, Ti, V, Mn,
Fe, Ni, Cu, and Ba) elements. Our sample contains 309 stars from the solar
neighborhood (up to the distance of 100 pc), out of which 140 are dwarfs, 29
are subgiants, and 140 are giants. The photospheric parameters are derived from
the equivalent widths of Fe I and Fe II lines. Masses and ages come from the
interpolation in evolutionary tracks and isochrones on the HR diagram. The
abundance determination is based on the equivalent widths of selected atomic
lines of the refractory elements and on the spectral synthesis of C_2, CN, C I,
O I, and Na I features. We apply a set of statistical methods to analyze the
abundances derived for the three subsamples. Our results show that: i) giant
stars systematically exhibit underabundance in [C/Fe] and overabundance in
[N/Fe] and [Na/Fe] in comparison with dwarfs, a result that is normally
attributed to evolution-induced mixing processes in the envelope of evolved
stars; ii) for solar analogs only, the abundance trends with the condensation
temperature of the elements are correlated with age and anticorrelated with the
surface gravity, which is in agreement with recent studies; iii) as in the case
of [Fe/H], dwarf stars with giant planets are systematically enriched in [X/H]
for all the analyzed elements, except for O and Ba (the former due to
limitations of statistics), confirming previous findings in the literature that
not only iron has an important relation with the planetary formation; and iv)
giant planet hosts are also significantly overabundant for the same metallicity
when the elements from Mg to Cu are combined together.Comment: 20 pages, 16 figures, 8 table
New HST WFC3/UVIS observations augment the stellar-population complexity of omega Centauri
We used archival multi-band Hubble Space Telescope observations obtained with
the Wide-Field Camera 3 in the UV-optical channel to present new important
observational findings on the color-magnitude diagram (CMD) of the Galactic
globular cluster omega Centauri. The ultraviolet WFC3 data have been coupled
with available WFC/ACS optical-band data. The new CMDs, obtained from the
combination of colors coming from eight different bands, disclose an even more
complex stellar population than previously identified. This paper discusses the
detailed morphology of the CMDs.Comment: 17 pages, 14 figures (11 in low res), 3 tables. Accepted for
publication in AJ on June 19, 201
The Near Infrared NaI Doublet Feature in M Stars
The NaI near-infrared doublet has been used to indicate the dwarf/giant
population in composite systems, but its interpretation is still a contentious
issue. In order to understand the behaviour of this controversial feature, we
study the observed and synthetic spectra of cool stars. We conclude that the
NaI infrared feature can be used as a dwarf/giant discriminator. We propose a
modified definition of the NaI index by locating the red continuum at 8234
angstrons and by measuring the equivalent width in the range 8172-8197
angstrons, avoiding the region at lambda > 8197 angstrons, which contains VI,
ZrI, FeI and TiO lines. We also study the dependence of this feature on stellar
atmospheric parameters.Comment: 9 pages, (TeX file) + 7 Figures in Postscript format. Accepted for
publication in The Astrophysical Journa
Chemical tagging of three distinct populations of red giants in the globular cluster NGC 6752
We present aluminium, magnesium, and silicon abundances in the metal-poor
globular cluster NGC 6752 for a sample of more than 130 red giants with
homogeneous oxygen and sodium abundances. We find that [Al/Fe] shows a spread
of about 1.4 dex among giants in NGC 6752 and is anticorrelated with [Mg/Fe]
and [O/Fe] and correlated with [Na/Fe] and [Si/Fe]. These relations are not
continuous in nature, but the distribution of stars is clearly clustered around
three distinct Al values, low, intermediate, and high. These three groups
nicely correspond to the three distinct sequences previously detected using
Stromgren photometry along the red giant branch. These two independent findings
strongly indicate the existence of three distinct stellar populations in NGC
6752. Comparing the abundances of O and Mg, we find that the population with
intermediate chemical abundances cannot originate from material with the same
composition of the most O- and Mg-poor population, diluted by material with
that of the most O- and Mg-rich one. This calls for different polluters.Comment: 8 pages, 2 tables, 3 figures; uses emulateapj; accepted for
publication on Astrophysical Journal Letter
Multiple stellar populations in Magellanic Cloud clusters. II. Evidence also in the young NGC1844?
We use HST observations to study the LMC's young cluster NGC1844. We estimate
the fraction and the mass-ratio distribution of photometric binaries and report
that the main sequence presents an intrinsic breadth which can not be explained
in terms of photometric errors only, and is unlikely due to differential
reddening. We attempt some interpretation of this feature, including stellar
rotation, binary stars, and the presence of multiple stellar populations with
different age, metallicity, helium, or C+N+O abundance. Although we exclude
age, helium, and C+N+O variations to be responsible of the main-sequence spread
none of the other interpretations is conclusive.Comment: 9 Pages, 11 figures, accepted for publication in A&A
Atmospheric Parameters and Metallicities for 2191 stars in the Globular Cluster M4
We report new metallicities for stars of Galactic globular cluster M4 using
the largest number of stars ever observed at high spectral resolution in any
cluster. We analyzed 7250 spectra for 2771 cluster stars gathered with the VLT
FLAMES+GIRAFFE spectrograph at VLT. These medium resolution spectra cover by a
small wavelength range, and often have very low signal-to-noise ratios. We
attacked this dataset by reconsidering the whole method of abundance analysis
of large stellar samples from beginning to end. We developed a new algorithm
that automatically determines the atmospheric parameters of a star. Nearly all
data preparation steps for spectroscopic analyses are processed on the
syntheses, not the observed spectra. For 322 Red Giant Branch stars with we obtain a nearly constant metallicity, ( = 0.02). No difference in the metallicity at the level of
is observed between the two RGB sequences identified by
\cite{Monelli:2013us}. For 1869 Subgiant and Main Sequence Stars we
obtain ( = 0.09) after fixing the
microturbulent velocity. These values are consistent with previous studies that
have performed detailed analyses of brighter RGB stars at higher spectroscopic
resolution and wavelength coverage. It is not clear if the small mean
metallicity difference between brighter and fainter M4 members is real or is
the result of the low signal-to-noise characteristics of the fainter stars. The
strength of our approach is shown by recovering a metallicity close to a single
value for more than two thousand stars, using a dataset that is non-optimal for
atmospheric analyses. This technique is particularly suitable for noisy data
taken in difficult observing conditions.Comment: 17 pages, 20 figures, 3 tables. Accepted for publication in The
Astronomical Journa
Absolute motions of globular clusters. II. [HST astrometry and VLT radial velocities in NGC6397]
In this paper we present a new, accurate determination of the three
components of the absolute space velocity of the Galactic globular cluster
NGC6397 (l 338d, b -12d). We used three HST/WFPC2 fields with multi-epoch
observations to obtain astrometric measurements of objects in three different
fields in this cluster. The identification of 33 background galaxies with sharp
nuclei allowed us to determine an absolute reference point and measure the
absolute proper motion of the cluster. The third component has been obtained
from radial velocities measured on spectra from the multi-fiber spectrograph
FLAMES at UT2-VLT. We find [mu_alpha cos(delta), mu_delta](J2000.0) = [+3.39
+/- 0.15, -17.55 +/- 0.15] mas/yr, and V_rad = +18.36 +/- 0.09 (+/-0.10) km/s.
Assuming a Galactic potential, we calculate the cluster orbit for various
assumed distances, and briefly discuss the implications.Comment: 7 pages, 5 figures, 4 tables. Accepted for publication in A&A, on
April 27 200
Multiple stellar populations in the Galactic globular cluster NGC 6752
We have carried out high-precision photometry on a large number of archival
HST images of the Galactic globular cluster NGC 6752, to search for signs of
multiple stellar populations. We find a broadened main sequence, and
demonstrate that this broadening cannot be attributed either to binaries or to
photometric errors. There is also some indication of a main-sequence split. No
significant spread could be found along the subgiant branch, however.
Ground-based photometry reveals that in the U vs. (U-B) color-magnitude
diagram the red-giant branch exhibits a clear color spread, which we have been
able to correlate with variations in Na and O abundances. In particular the
Na-rich, O-poor stars identified by Carretta et al. (2007) define a sequence on
the red side of the red-giant branch, while Na-poor, O-rich stars populate a
bluer, more dispersed portion of the red-giant branch.Comment: 31 pages, 12 figures; Accepted for Publication in the Astrophysical
Journa
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