2,851 research outputs found
The Chemical Evolution of the Galaxy: the two-infall model
In this paper we present a new chemical evolution model for the Galaxy which
assumes two main infall episodes for the formation of halo-thick disk and thin
disk, respectively. We do not try to take into account explicitly the evolution
of the halo but we implicitly assume that the timescale for the formation of
the halo was of the same order as the timescale for the formation of the thick
disk. The formation of the thin-disk is much longer than that of the thick
disk, implying that the infalling gas forming the thin-disk comes not only from
the thick disk but mainly from the intergalactic medium. The timescale for the
formation of the thin-disk is assumed to be a function of the galactocentric
distance, leading to an inside-out picture for the Galaxy building. The model
takes into account the most up to date nucleosynthesis prescriptions and adopts
a threshold in the star formation process which naturally produces a hiatus in
the star formation rate at the end of the thick disk phase, as suggested by
recent observations. The model results are compared with an extended set of
observational constraints. Among these constraints, the tightest one is the
metallicity distribution of the G-dwarf stars for which new data are now
available. Our model fits very well these new data. We show that in order to
reproduce most of these constraints a timescale Gyr for the
(halo)-thick-disk and of 8 Gyr for the thin-disk formation in the solar
vicinity are required. We predict that the radial abundance gradients in the
inner regions of the disk () are steeper than in the outer
regions, a result confirmed by recent abundance determinations, and that the
inner ones steepen in time during the Galactic lifetime.Comment: 48 pages, 20 Postscript figures, AASTex v.4.0, to be published in
Astrophysical Journa
Na-O Anticorrelation and HB. IV. Detection of He-rich and He-poor stellar populations in the globular cluster NGC 6218
We used the multifiber spectrograph FLAMES on the ESO Very Large Telescope
UT2 to derive atmospheric parameters, metallicities and abundances of O and Na
for 79 red giant stars in the Galactic globular cluster NGC 6218 (M 12). We
analyzed stars in the magnitude range from about 1 mag below the bump to the
tip of the Red Giant Branch. The average metallicity we derive is
[Fe/H]=-1.31+/-0.004+/-0.028 dex (random and systematic errors, respectively),
with a very small star-to-star scatter (rms=0.033 dex), from moderately
high-resolution Giraffe spectra. This is the first extensive spectroscopic
abundance analysis in this cluster. Our results indicate that NGC 6218 is very
homogeneous as far as heavy elements are concerned. On the other hand, light
elements involved in the well known proton-capture reactions of H-burning at
high temperature, such as O and Na, show large variations, anticorrelated with
each other, at all luminosities along the red giant branch. The conclusion is
that the Na-O anticorrelation must be established in early times at the cluster
formation. We interpret the variation of Na found near the RGB-bump as the
effect of two distinct populations having different bump luminosities, as
predicted for different He content. To our knowledge, NGC 6218 is the first GC
where such a signature has been spectroscopically detected, when combined with
consistent and homogeneous data obtained for NGC 6752 to gain in statistical
significance.Comment: 15 pages, 10 figures; fig.5 degraded in resolution; tables 2,3,5
available at CDS. 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
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
Mixing along the Red Giant Branch in Metal-poor Field Stars
We have determined Li, C, N, O, Na, and Fe abundances, and 12C/13C isotopic
ratios for a sample of 62 field metal-poor stars (plus 43 taken from the
literature). This large sample was used to show that small mass lower-RGB stars
(i.e., fainter than the RGB bump) have abundances of light elements in
agreement with theoretical predictions from classical evolutionary models. A
second, distinct mixing episode occurs just after the RGB bump, reaching
regions of incomplete CNO burning. No O-Na anticorrelation, as observed in
globular cluster stars, is found in field stars. This means that the mixing
episode is not deep enough to reach regions where ON-burning occurs.Comment: 6 pages, 3 encapsulated figures, LateX, uses crckapb.sty; invited
talk, in "The Chemical Evolution of the Milky Way: Stars vs Clusters, Vulcano
(Italy), 20-24 September 1999, F. Matteucci and F. Giovannelli eds, Kluwer,
in pres
Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars
We explore the effects on nucleosynthesis in Type II supernovae of various
parameters (mass cut, neutron excess, explosion energy, progenitor mass) in
order to explain the observed trends of the iron-peak element abundance ratios
([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of
metallicity for the range [Fe/H] . [Cr/Fe] and [Mn/Fe]
decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and
increases. We show that such a behavior can be explained by a variation of mass
cuts in Type II supernovae as a function of progenitor mass, which provides a
changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and
incomplete Si-burning. This explanation is consistent with the amount of
ejected Ni determined from modeling the early light curves of individual
supernovae. We also suggest that the ratio [H/Fe] of halo stars is mainly
determined by the mass of interstellar hydrogen mixed with the ejecta of a
single supernova which is larger for larger explosion energy and the larger
Str\"omgren radius of the progenitor.Comment: 17 pages, LaTeX, Accepted for publication in the Astrophysical
Journal, more discussion on the Galactic chemical evolutio
Na-O anticorrelation and HB. IX. Kinematics of the program clusters. A link between systemic rotation and HB morphology?
We use accurate radial velocities for 1981 member stars in 20 Galactic
globular clusters, collected within our large survey aimed at the analysis of
the Na-O anti-correlation, to study the internal kinematics of the clusters. We
performed the first systematic exploration of the possible connections between
cluster kinematics and the multiple populations phenomenon in GCs. We did not
find any significant correlation between Na abundance and either velocity
dispersion or systemic rotation. We searched for systemic rotation in the eight
clusters of our sample that lack such analysis from previous works in the
literature (NGC2808, NGC5904, NGC6171, NGC6254, NGC6397, NGC6388, NGC6441, and
NGC6838). These clusters are found to span a large range of rotational
amplitudes, from ~0.0 km/s (NGC6397) to ~13.0 km/s (NGC6441). We found a
significant correlation between the ratio of rotational velocity to central
velocity dispersion (V_{rot}/sigma_0) and the Horizontal Branch Morphology
parameter (B-R)/(B+R+V). V_{rot}/sigma_0 is found to correlate also with
metallicity, possibly hinting to a significant role of dissipation in the
process of formation of globular clusters. V_{rot} is found to correlate well
with (B-R)/(B+R+V), M_V, sigma_0 and [Fe/H]. All these correlations strongly
suggest that systemic rotation may be intimately linked with the processes that
led to the formation of globular clusters and the stellar populations they
host.Comment: Accepted for publication on Astronomy & Astrophysics. Pdflatex, 16
pages, 16 pdf figures. The position angles of the rotation axes have been
corrected, since the values reported in the previous version were erroneous.
The results of the analysis are unchanged. The manuscript has also been
processed by a language edito
Detailed abundances of a large sample of giant stars in M 54 and in the Sagittarius nucleus
Homogeneous abundances of light elements, alpha and Fe-group elements from
high-resolution FLAMES spectra are presented for 76 red giant stars in M54, a
massive globular cluster (GC) lying in the nucleus of the Sagittarius dwarf
galaxy. We also derived detailed abundances for 27 red giants belonging to the
Sgr nucleus. Our abundances assess the intrinsic metallicity dispersion (~0.19
dex, rms scatter) of M54, with the bulk of stars peaking at [Fe/H]~-1.6 and a
long tail extending to higher metallicities, similar to omega Cen. The spread
in these probable nuclear star clusters exceeds those of most GCs: these
massive clusters are located in a region intermediate between normal GCs and
dwarf galaxies. M54 shows the Na-O anticorrelation, typical signature of GCs,
which is instead absent in the Sgr nucleus. The light elements (Mg, Al, Si)
participating to the high temperature Mg-Al cycle show that the pattern of
(anti)correlations produced by proton-capture reactions in H-burning is clearly
different between the most metal-rich and most metal-poor components in the two
most massive GCs in the Galaxy, confirming early result based on the Na-O
anticorrelation. As in omega Cen, stars affected by most extreme processing,
i.e. showing the signature of more massive polluters, are those of the
metal-rich component. This can be understood if the burst of star formation
giving birth to the metal-rich component was delayed by as much as 10-30 Myr
with respect to the metal-poor one. The evolution of these massive GCs can be
reconciled in the general scenario for the formation of GCs sketched in
Carretta et al.(2010a) taking into account that omega Cen could have already
incorporated the surrounding nucleus of its progenitor and lost the rest of the
hosting galaxy while the two are still observable as distinct components in M54
and the surrounding field.Comment: 22 pages (3 pages of appendix), 25 figures. Tables 2, 3, 5, 6, and 7
are only available in electronic form at the CDS Accepted for publication on
Astronomy and Astrophysic
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