168 research outputs found
The evolution of low-metallicity asymptotic giant branch stars and the formation of carbon-enhanced metal-poor stars
We investigate the behaviour of asymptotic giant branch (AGB) stars between
metallicities Z = 10-4 and Z = 10-8 . We determine which stars undergo an
episode of flash-driven mixing, where protons are ingested into the intershell
convection zone, as they enter the thermally pulsing AGB phase and which
undergo third dredge-up. We find that flash-driven mixing does not occur above
a metallicity of Z = 10-5 for any mass of star and that stars above 2 M do not
experience this phenomenon at any metallicity. We find carbon ingestion (CI),
the mixing of carbon into the tail of hydrogen burning region, occurs in the
mass range 2 M to around 4 M . We suggest that CI may be a weak version of the
flash-driven mechanism. We also investigate the effects of convective
overshooting on the behaviour of these objects. Our models struggle to explain
the frequency of CEMP stars that have both significant carbon and nitrogen
enhancement. Carbon can be enhanced through flash-driven mixing, CI or just
third dredge up. Nitrogen can be enhanced through hot bottom burning and the
occurrence of hot dredge-up also converts carbon into nitrogen. The C/N ratio
may be a good indicator of the mass of the primary AGB stars.Comment: 15 pages, 13 figures, 1 table, accepted by MNRA
Carbon and Strontium Abundances of Metal-Poor Stars
We present carbon and strontium abundances for 100 metal-poor stars measured
from R7000 spectra obtained with the Echellette Spectrograph and Imager
at the Keck Observatory. Using spectral synthesis of the G-band region, we have
derived carbon abundances for stars ranging from [Fe/H] to
[Fe/H]. The formal errors are dex in [C/Fe]. The strontium
abundance in these stars was measured using spectral synthesis of the resonance
line at 4215 {\AA}. Using these two abundance measurments along with the barium
abundances from our previous study of these stars, we show it is possible to
identify neutron-capture-rich stars with our spectra. We find, as in other
studies, a large scatter in [C/Fe] below [Fe/H]. Of the stars with
[Fe/H], 94% can be classified as carbon-rich metal-poor stars. The Sr
and Ba abundances show that three of the carbon-rich stars are
neutron-capture-rich, while two have normal Ba and Sr. This fraction of carbon
enhanced stars is consistent with other studies that include this metallicity
range.Comment: ApJ, Accepte
Clear evidence for the presence of second-generation asymptotic giant branch stars in metal-poor Galactic globular clusters
Galactic globular clusters (GCs) are known to host multiple stellar
populations: a first generation with a chemical pattern typical of halo field
stars and a second generation (SG) enriched in Na and Al and depleted in O and
Mg. Both stellar generations are found at different evolutionary stages (e.g.,
the main-sequence turnoff, the subgiant branch, and the red giant branch). The
non detection of SG asymptotic giant branch (AGB) stars in several metal-poor
([Fe/H] < -1) GCs suggests that not all SG stars ascend the AGB phase, and that
failed AGB stars may be very common in metal-poor GCs. This observation
represents a serious problem for stellar evolution and GC formation/evolution
theories. We report fourteen SG-AGB stars in four metal-poor GCs (M 13, M 5, M
3, and M 2) with different observational properties: horizontal branch (HB)
morphology, metallicity, and age. By combining the H-band Al abundances
obtained by the APOGEE survey with ground-based optical photometry, we identify
SG Al-rich AGB stars in these four GCs and show that Al-rich RGB/AGB GC stars
should be Na-rich. Our observations provide strong support for present,
standard stellar models, i.e., without including a strong mass-loss efficiency,
for low-mass HB stars. In fact, current empirical evidence is in agreement with
the predicted distribution of FG and and SG stars during the He-burning stages
based on these standard stellar models.Comment: Accepted for publication in The Astrophysical Journal Letters (16
pages, 4 figures, and 1 table
Observational evidence for a different IMF in the early Galaxy
The unexpected high incidence of carbon-enhanced, s-process enriched
unevolved stars amongst extremely metal-poor stars in the halo provides a
significant constraint on the Initial Mass Function (IMF) in the early Galaxy.
We argue that these objects are evidence for the past existence of a large
population of intermediate-mass stars, and conclude that the IMF in the early
Galaxy was different from the present, and shifted toward higher masses.Comment: 14 pages, 1 color figure, accepted for publication on Ap
The Na-O anticorrelation in horizontal branch stars. IV. M22
We obtained high-resolution spectra for 94 candidate stars belonging to the
HB of M22 with FLAMES. The HB stars we observed span a restricted temperature
range (7,800<Teff<11,000 K), where about 60% of the HB stars of M22 are. Within
our sample, we can distinguish three groups of stars segregated (though
contiguous) in colours: Group 1 (49 stars) is metal-poor, N-normal, Na-poor and
O-rich with abundances that match those determined for the primordial group of
RGB stars from previous studies. Group 2 (23 stars) is still metal-poor, but it
is N- and Na-rich, though only very mildly depleted in O. We can identify this
intermediate group as the progeny of the metal-poor RGB stars that occupy an
intermediate location along the Na-O anti-correlation. The third group (20
stars) is metal-rich, Na-rich, and O-rich and likely corresponds to the most
O-rich component of the previously found metal-rich RGB population. We did not
observe any severely O-depleted stars and we think that the progeny of these
stars falls on the hotter part of the HB. The metal-rich population is also
over-abundant in Sr, in agreement with results for corresponding RGB and SGB
stars. However, we do not find any significant variation in the ratio between
the sum of N and O abundances to Fe. There is some evidence of an enhancement
of He content for Groups 2 and 3 stars (Y=0.338\pm 0.014\pm 0.05). Our results
agree with the proposition that chemical composition drives the location of
stars along the HB of a GC. Furthermore, we found a number of fast rotators.
They are concentrated in a restricted temperature range along the HB of M22.Comment: Accepted for publication on Astronomy and Astrophysics. 23 pages, 21
figure
The Na-O anticorrelation in horizontal branch stars. V. NGC 6723
We used FLAMES+GIRAFFE (Medusa mode) at the VLT to obtain moderately high
resolution spectra for 30 red horizontal branch (RHB) stars, 4 RR Lyrae
variables, and 17 blue horizontal branch (BHB) stars in the low-concentration,
moderately metal-rich globular cluster NGC6723 ([Fe/H]=-1.22+/-0.08 from our
present sample). The spectra were optimized to derive O and Na abundances. In
addition, we obtained abundances for other elements, including N, Fe, Mg, Ca,
Ni, and Ba. We used these data to discuss the evidence of a connection between
the distribution of stars along the horizontal branch (HB) and the multiple
populations that are typically present in globular clusters. We found that all
RHB and most (13 out of 17) BHB stars are O-rich, Na-poor, and N-poor; these
stars probably belong to the first stellar generation in this cluster. Only the
four warmest observed stars are (moderately) O-poor, Na-rich, and N-rich, and
they probably belong to the second generation. While our sample is not fully
representative of the whole HB population in NGC6723, our data suggest that in
this cluster only HB stars warmer than ~9000 K, that is one fourth of the
total, belong to the second generation, if at all. Since in many other clusters
this fraction is about two thirds, we conclude that the fraction of
first/second generation in globular clusters may be strongly variable. In
addition, the wide range in colour of chemically homogeneous first-generation
HB stars requires a considerable spread in mass loss (>0.10 Mo). The reason for
this spread is yet to be understood. Finally, we found a high Ba abundance,
with a statistically significant radial abundance gradient.Comment: Astronomy and Astrophysics, in press; 15 pages; 11 figure
A Puzzle Involving Galactic Bulge Microlensing Events
We study a sample of 16 microlensed Galactic bulge main sequence turnoff
region stars for which high dispersion spectra have been obtained with detailed
abundance analyses. We demonstrate that there is a very strong and highly
statistically significant correlation between the maximum magnification of the
microlensed bulge star and the value of the [Fe/H] deduced from the high
resolution spectrum of each object. Physics demands that this correlation,
assuming it to be real, be the result of some sample bias. We suggest several
possible explanations, but are forced to reject them all,and are left puzzled.
To obtain a reliable metallicity distribution in the Galactic bulge based on
microlensed dwarf stars it will be necessary to resolve this issue through the
course of additional observations.Comment: Submitted to ApJL, table 2 (quite long) will only appear in the
on-line version of ApJ
Evolution of computed tomography-detectable adrenal nodules in patients with bilateral primary aldosteronism
Discovery of Carbon/Oxygen depleted Blue Straggler Stars in 47 Tucanae: the chemical signature of a mass-transfer formation process
We use high-resolution spectra obtained with the ESO Very Large Telescope to
measure surface abundance patterns of 43 Blue Stragglers stars (BSS) in 47 Tuc.
We discovered that a sub-population of BSS shows a significant depletion of
Carbon and Oxygen with respect to the dominant population. This evidence would
suggest the presence of CNO burning products on the BSS surface coming from a
deeply peeled parent star, as expected in the case of mass-transfer process.
This is the first detection of a chemical signature clearly pointing to a
specific BSS formation process in a globular cluster.Comment: Published on 2006, August 10, in ApJ 647, L5
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