160 research outputs found
A sequence of nitrogen-rich very red giants in the globular cluster NGC 1851
We present the abundances of N in a sample of 62 stars on the red giant
branch (RGB) in the peculiar globular cluster NGC 1851. The values of [N/Fe]
ratio were obtained by comparing the flux measured in the observed spectra with
that from synthetic spectra for up to about 15 features of CN. This is the
first time that N abundances are obtained for such a large sample of RGB stars
from medium-resolution spectroscopy in this cluster. With these abundances we
provide a chemical tagging of the split red giant branch found from several
studies in NGC 1851. The secondary, reddest sequence on the RGB is populated
almost exclusively by N-rich stars, confirming our previous suggestion based on
Stromgren magnitudes and colours. These giants are also, on average, enriched
in s-process elements such as Ba, and are likely the results of pollution from
low mass stars that experienced episodes of third dredge-up in the asymptotic
giant branch phase.Comment: Version to match the one in press on Astronomy and Astrophysic
An aluminium tool for multiple stellar generations in the globular clusters 47 Tuc and M 4
We present aluminium abundances for a sample of about 100 red giant stars in
each of the Galactic globular clusters 47 Tuc (NGC 104) and M 4 (NGC 6121). We
have derived homogeneous abundances from intermediate-resolution FLAMES/GIRAFFE
spectra. Aluminium abundances are from the strong doublet Al I at 8772-8773 A
as in previous works done for giants in NGC 6752 and NGC 1851, and nitrogen
abundances are extracted from a large number of features of the CN molecules,
by assuming a suitable carbon abundance. We added previous homogeneous
abundances of O and Na and newly derived abundances of Mg and Si for our
samples of 83 stars in M 4 and 116 stars in 47 Tuc to obtain the full set of
elements from proton-capture reactions produced by different stellar
generations in these clusters. By simultaneously studying the Ne-Na and Mg-Al
cycles of H-burning at high temperature our main aims are to understand the
nature of the polluters at work in the first generation and to ascertain
whether the second generation of cluster stars was formed in one or, rather,
several episodes of star formation. Our data confirm that in M 4 only two
stellar populations are visible. On the other hand, for 47 Tuc a cluster
analysis performed on our full dataset suggests that at least three distinct
groups of stars are present on the giant branch. The abundances of O, Na, Mg
and Al in the intermediate group can be produced within a pollution scenario;
results for N are ambiguous, depending on the C abundance we adopt for the
three groups.Comment: 11 pages, 9 figures, 2 on-line tables: accepted for publication on
Astronomy and Astrophysic
Reading the book: from "chemical anomalies" to "standard composition" of globular clusters
It is now commonly accepted that globular clusters (GCs) have undergone a
complex formation and that they host at least two stellar generations. This is
a recent paradigm and is founded on both photometric and spectroscopic
evidence. We concentrate on results based on high-resolution spectroscopy and
on how we moved from single to multiple stellar populations concept for GCs. We
underline that the peculiar chemical composition of GC stars is fundamental in
establishing the multiple populations scenario and briefly outline what can be
learned from observations. Finally, recent observational results on large
samples of stars in different evolutionary phases are discussed.Comment: 5 pages, 1 figure. To appear in the proceedings of "Reading the book
of globular clusters with the lens of stellar evolution", in the Memorie
della Societa Astronomica Italian
Calcium and light-elements abundance variations from high resolution spectroscopy in globular clusters
We use abundances of Ca, O, Na, Al from high resolution UVES spectra of 200
red giants in 17 globular clusters (GCs) to investigate the correlation found
by Lee et al. (2009) between chemical enrichment from SN II and star-to-star
variations in light elements in GC stars. We find that (i) the [Ca/H]
variations between first and second generation stars are tiny in most GCs
(~0.02-0.03 dex, comparable with typical observational errors). In addition,
(ii) using a large sample of red giants in M 4 with abundances from UVES
spectra from Marino et al. (2008), we find that Ca and Fe abundances in the two
populations of Na-poor and Na-rich stars are identical. These facts suggest
that the separation seen in color-magnitude diagrams using the U band or hk
index (as observed in NGC 1851 by Han et al. 2009) are not due to Ca
variations. Small differences in [Ca/H] as associated to hk variations might be
due to a small systematic effect in abundance analysis, because most
O-poor/Na-rich (He-rich) stars have slightly larger [Fe/H] (by 0.027 dex on
average, due to decreased H in the ratio) than first generation stars and are
then located at redder positions in the V,hk plane. While a few GCs (M 54,
omega Cen, M 22, maybe even NGC 1851) do actually show various degree of
metallicity spread, our findings eliminate the need of a close link between the
enrichment by core-collapse SNe with the mechanism responsible for the Na-O
anticorrelation.Comment: Uses emulateapj, 3 figures, 2 tables (1 only available in electronic
form), accepted for publication on ApJ Letter
Na-O Anticorrelation And HB I. The Na-O anticorrelation in NGC 2808
We derived atmospheric parameters and elemental abundances of Fe, O and Na
for about 120 red giant stars in the Galactic globular cluster NGC 2808. Our
results are based on the analysis of medium-high resolution (R=22000-24000)
GIRAFFE spectra acquired with the FLAMES spectrograph at VLT-UT2 as a part of a
project aimed at studying the Na-O anticorrelation as a function of physical
parameters in globular clusters. We present here the anticorrelation of Na and
O abundances in NGC 2808, and we discuss the distribution function of stars
along this relation. Besides a bulk of O-normal stars, with composition typical
of field halo stars, NGC 2808 seems to host two other groups of O-poor and
super O-poor stars. In this regard, NGC 2808 is similar to M 13, the template
cluster for the Na-O anticorrelation. However, at variance with M 13, most
stars in NGC 2808 are O-rich. This might be related to the horizontal branch
morphologies which are very different in these two clusters. The average
metallicity we found for NGC 2808 is [Fe/H]=-1.10 (rms=0.065 dex, from 123
stars). We also found some evidence of a small intrinsic spread in metallicity,
but more definitive conclusions are hampered by the presence of a small
differential reddening.Comment: 12 pages, 6 tables, 7 figures. Accepted for publication on A&
A Search for Binary Stars at Low Metallicity
We present initial results measuring the companion fraction of metal-poor
stars ([Fe/H]2.0). We are employing the Lick Observatory planet-finding
system to make high-precision Doppler observations of these objects. The binary
fraction of metal-poor stars provides important constraints on star formation
in the early Galaxy (Carney et al. 2003). Although it has been shown that a
majority of solar metallicity stars are in binaries, it is not clear if this is
the case for metal-poor stars. Is there a metallicity floor below which binary
systems do not form or become rare? To test this we are determining binary
fractions at metallicities below [Fe/H]. Our measurments are not as
precise as the planet finders', but we are still finding errors of only 50 to
300 m/s, depending on the signal-to-noise of a spectrum and stellar atmosphere
of the star. At this precision we can be much more complete than previous
studies in our search for stellar companions.Comment: To appear in conference proceedings,"First Stars III", eds. B.
O'Shea, A. Heger & T. Abel. 3 pages, 5 figure
Line lists for the A2PI-X2Sigma+ (red) and {B2Sigma+-X2Sigma} (violet) Systems of CN, 13C14N, and 12C15N, and Application to Astronomical Spectra
New red and violet system line lists for the CN isotopologues 13C14N and
12C15N have been generated. These new transition data are combined with those
previously derived for 12C14N, and applied to the determination of CNO
abundances in the solar photosphere and in four red giant stars: Arcturus, the
bright very low-metallicity star HD 122563, and carbon-enhanced metal-poor
stars HD 196944 and HD 201626. When lines of both red and violet system lines
are detectable in a star, their derived N abundances are in good agreement. The
mean N abundances determined in this work generally are also in accord with
published values.Comment: ApJS, in press, 37 pages, 7 figures, 3 table
Terzan 8: a Sagittarius-flavoured globular cluster
Massive globular clusters (GCs) contain at least two generations of stars
with slightly different ages and clearly distinct light elements abundances.
The Na-O anticorrelation is the best studied chemical signature of multiple
stellar generations. Low-mass clusters appear instead to be usually chemically
homogeneous. We are investigating low-mass GCs to understand what is the lower
mass limit where multiple populations can form, mainly using the Na and O
abundance distribution. We used VLT/FLAMES spectra of giants in the low-mass,
metal-poor GC Terzan 8, belonging to the Sagittarius dwarf galaxy, to determine
abundances of Fe, O, Na, alpha-, Fe-peak, and neutron-capture elements in six
stars observed with UVES and 14 observed with GIRAFFE. The average metallicity
is [Fe/H]=-2.27+/-0.03 (rms=0.08), based on the six high-resolution UVES
spectra. Only one star, observed with GIRAFFE, shows an enhanced abundance of
Na and we tentatively assign it to the second generation. In this cluster, at
variance with what happens in more massive GCs, the second generation seems to
represent at most a small minority fraction. We discuss the implications of our
findings, comparing Terzan 8 with the other Sgr dSph GCs, to GCs and field
stars in the Large Magellanic Cloud, Fornax, and in other dwarfs galaxies.Comment: 15 pages, 12 figures, 10 tables; accepted for publication on
Astronomy and Astrophysic
- …