516 research outputs found
Calibration of the CH and CN Variations Among Main Sequence Stars in M71 and in M13
An analysis of the CN and CH band strengths measured in a large sample of M71
and M13 main sequence stars by Cohen (1999a,b) is undertaken using synthetic
spectra to quantify the underlying C and N abundances. In the case of M71 it is
found that the observed CN and CH band strengths are best matched by the
{\it{identical}} C/N/O abundances which fit the bright giants, implying: 1)
little if any mixing is taking place during red giant branch ascent in M71, and
2) a substantial component of the C and N abundance inhomogeneities is in place
before the main sequence turn-off. The unlikelihood of mixing while on the main
sequence requires an explanation for the abundance variations which lies
outside the present stars (primordial inhomogeneities or intra-cluster self
enrichment). For M13 it is shown that the 3883\AA CN bands are too weak to be
measured in the spectra for any reasonable set of expected compositions. A
similar situation exists for CH as well. However, two of the more luminous
program stars do appear to have C abundances considerably greater than those
found among the bright giants thereby suggesting deep mixing has taken place on
the M13 red giant branch.Comment: 14 pages, 4 figures, accepted for publication by A
CNONa and 12C/13C in giants of 10 open clusters
Evolved low-mass stars of a wide range of metallicity bear signatures of a
non-standard mixing event in their surface abundances of Li, C, and N, and in
their 12C/13C ratio. A Na overabundance has also been reported in some giants
of open clusters but remains controversial. The cause of the extra-mixing has
been attributed to thermohaline convection that should take place after the RGB
bump for low-mass stars and on the early-AGB for more massive objects. To track
the occurrence of this process over a wide mass range, we derive in a
homogeneous way the abundances of C, N, O, and Na, as well as the 12C/13C ratio
in a sample of 31 giants of 10 open clusters with turn-off masses from 1.7 to
3.1 Msun. A group of first ascent red giants with M/Msun \leq 2.5 exhibits
lower [N/C] ratios than those measured in clump giants of the same mass range,
suggesting an additional increase in the [N/C] ratio after the first dredge-up.
The sodium abundances corrected from NLTE are found to be about solar. [Na/Fe]
shows a slight increase of 0.10 dex as a function of stellar mass in the 1.8 to
3.2 Msun range covered by our sample, in agreement with standard first
dredge-up predictions. Our results do not support previous claims of sodium
overabundances as high as +0.60 dex. An anti-correlation between 12C/13C and
turn-off mass is identified and interpreted as being caused by a post-bump
thermohaline mixing. Moreover, we find low 12C/13C ratios in a few
intermediate-mass early-AGB stars, confirming that an extra-mixing process also
operates in stars that do not experienced the RGB bump. In this case, the
extra-mixing possibly acts on the early-AGB, in agreement with theoretical
expectations for thermohaline mixing. [abridged]Comment: A&A accepted, revised versio
WIYN/Hydra Detection of Lithium Depletion in F Stars of the Young Open Cluster M35 and Implications for the Development of the Lithium Gap
We report discovery of significant depletion of Li on the surfaces of F dwarf
stars in the 150-Myr-old open cluster M35, analagous to a feature in the
700-Myr-old Hyades cluster that has been referred to as the ``Li gap.'' We have
caught the gap in the act of forming: using high resolution, high S/N,
WIYN/Hydra observations, we detect Li in all but a few M35 F stars; the maximum
depletion lies at least 0.6-0.8 dex below minimally depleted (or undepleted)
stars. The M35 Li depletion region, a) is quite wide, with clear depletion seen
from 6000K to 6700K or hotter; b) shows a significant dispersion in Li
abundance at all T_eff, even with stars of the same T_eff; and c) contains
undepleted stars (as well as depleted ones) in the (narrow) classical Hyades
gap region, which itself shows no undepleted stars. All of these M35 Li
depletion properties support rotationally-induced slow mixing as the primary
physical mechanism that forms the gap, and argues against other proposed
mechanisms, particularly diffusion and steady main sequence mass loss. When
viewed in the context of the M35 Li depletion properties, the Hyades Li gap may
well be wider than is usually recognized.Comment: 14 Pages, 3 figures. Accepted to ApJ Letter
Low-mass lithium-rich AGB stars in the Galactic bulge: evidence for Cool Bottom Processing?
Context: The stellar production of the light element lithium is still a
matter of debate.
Aims: We report the detection of low-mass, Li-rich Asymptotic Giant Branch
(AGB) stars located in the Galactic bulge.
Methods: A homogeneous and well-selected sample of low mass, oxygen-rich AGB
stars in the Galactic bulge has been searched for the absorption lines of Li.
Using spectral synthesis techniques, we determine from high resolution UVES/VLT
spectra the Li abundance in four out of 27 sample stars, and an upper limit for
the remaining stars.
Results: Two stars in our sample have a solar Li abundance or above; these
stars seem to be a novelty, since they do not show any s-element enhancement.
Two more stars have a Li abundance slightly below solar; these stars do show
s-element enhancement in their spectra. Different scenarios which lead to an
increased Li surface abundance in AGB stars are discussed.
Conclusions: Of the different enrichment scenarios presented, Cool Bottom
Processing (CBP) is the most likely one for the Li-rich objects identified
here. Self-enrichment by Hot Bottom Burning (HBB) seems very unlikely as all
Li-rich stars are below the HBB mass limit. Also, the ingestion of a low mass
companion into the stars' envelope is unlikely because the associated
additional effects are lacking. Mass transfer from a former massive binary
companion is a possible scenario, if the companion produced little s-process
elements. A simple theoretical estimation for the Li abundance due to CBP is
presented and compared to the observed values.Comment: 5 pages, 3 figures, accepted by A&A Letter
Abundances in Stars from the Red Giant Branch Tip to the Near the Main Sequence in M71: I. Sample Selection, Observing Strategy and Stellar Parameters
We present the sample for an abundance analysis of 25 members of M71 with
luminosities ranging from the red giant branch tip to the upper main sequence.
The spectra are of high dispersion and of high precision. We describe the
observing strategy and determine the stellar parameters for the sample stars
using both broad band colors and fits of H profiles. The derived
stellar parameters agree with those from the Yale stellar evolutionary
tracks to within 50 -- 100K for a fixed log g, which is within the level of the
uncertainties.Comment: Minor changes to conform to version accepted for publication, with
several new figures (Paper 1 of a pair
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