637 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
On the Coupling between Helium Settling and Rotation-Induced Mixing in Stellar Radiative Zones: II- Application to light elements in population I main-sequence stars
In the two previous papers of this series, we have discussed the importance
of t he -gradients due to helium settling on rotation-induced mixing,
first in a n approximate analytical way, second in a 2D numerical simulation.
We have found that, for slowly rotating low mass stars, a process of ``creeping
paralysis" in which the circulation and the diffusion are nearly frozen may
take place below the convective zone. Here we apply this theory to the case of
lithium and beryll ium in galactic clusters and specially the Hyades. We take
into account the rota tional braking with rotation velocities adjusted to the
present observations. We find that two different cells of meridional
circulation appear on the hot side of the "lithium dip" and that the "creeping
paralysis" process occurs, not dir ectly below the convective zone, but deeper
inside the radiative zone, at the to p of the second cell. As a consequence,
the two cells are disconnected, which ma y be the basic reason for the lithium
increase with effective temperature on thi s side of the dip. On the cool side,
there is just one cell of circulation and t he paralysis has not yet set down
at the age of the Hyades; the same modelisatio n accounts nicely for the
beryllium observations as well as for the lithium ones .Comment: 13 printed pages, 10 figures. ApJ, in press (April 20, 2003
Implications of a Sub-Threshold Resonance for Stellar Beryllium Depletion
Abundance measurements of the light elements lithium, beryllium, and boron
are playing an increasingly important role in the study of stellar physics.
Because these elements are easily destroyed in stars at temperatures 2--4
million K, the abundances in the surface convective zone are diagnostics of the
star's internal workings. Standard stellar models cannot explain depletion
patterns observed in low mass stars, and so are not accounting for all the
relevant physical processes. These processes have important implications for
stellar evolution and primordial lithium production in big bang
nucleosynthesis. Because beryllium is destroyed at slightly higher temperatures
than lithium, observations of both light elements can differentiate between the
various proposed depletion mechanisms. Unfortunately, the reaction rate for the
main destruction channel, 9Be(p,alpha)6Li, is uncertain. A level in the
compound nucleus 10B is only 25.7 keV below the reaction's energetic threshold.
The angular momentum and parity of this level are not well known; current
estimates indicate that the resonance entrance channel is either s- or d-wave.
We show that an s-wave resonance can easily increase the reaction rate by an
order of magnitude at temperatures of approximately 4 million K. Observations
of sub-solar mass stars can constrain the strength of the resonance, as can
experimental measurements at lab energies lower than 30 keV.Comment: 9 pages, 1 ps figure, uses AASTeX macros and epsfig.sty. Reference
added, typos corrected. To appear in ApJ, 10 March 199
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
3He-Driven Mixing in Low-Mass Red Giants: Convective Instability in Radiative and Adiabatic Limits
We examine the stability and observational consequences of mixing induced by
3He burning in the envelopes of first ascent red giants. We demonstrate that
there are two unstable modes: a rapid, nearly adiabatic mode that we cannot
identify with an underlying physical mechanism, and a slow, nearly radiative
mode that can be identified with thermohaline convection. We present
observational constraints that make the operation of the rapid mode unlikely to
occur in real stars. Thermohaline convection turns out to be fast enough only
if fluid elements have finger-like structures with a length to diameter ratio
l/d > 10. We identify some potentially serious obstacles for thermohaline
convection as the predominant mixing mechanism for giants. We show that
rotation-induced horizontal turbulent diffusion may suppress the 3He-driven
thermohaline convection. Another potentially serious problem for it is to
explain observational evidence of enhanced extra mixing. The 3He exhaustion in
stars approaching the red giant branch (RGB) tip should make the 3He mixing
inefficient on the asymptotic giant branch (AGB). In spite of this, there are
observational data indicating the presence of extra mixing in low-mass AGB
stars similar to that operating on the RGB. Overmixing may also occur in
carbon-enhanced metal-poor stars.Comment: 25 pages, 6 figures, modified version, accepted by 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
Three Li-rich K giants: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952
We report on spectroscopic analyses of three K giants previously suggested to
be Li-rich: IRAS 12327-6523, IRAS 13539-4153, and IRAS 17596-3952.
High-resolution optical spectra and the LTE model atmospheres are used to
derive the stellar parameters: (, log , [Fe/H]), elemental
abundances, and the isotopic ratio C/C. IRAS 13539-4153 shows an
extremely high Li abundance of (Li) 4.2, a value ten
times more than the present Li abundance in the local interstellar medium. This
is the third highest Li abundance yet reported for a K giant. IRAS 12327-6523
shows a Li abundances of (Li) 1.4. IRAS 17596-3952 is a
rapidly rotating ( 35 km s) K giant with
(Li) 2.2. Infrared photometry which shows the presence
of an IR excess suggesting mass-loss. A comparison is made between these three
stars and previously recognized Li-rich giants.Comment: 17 pages, 6 figures, accepted for A
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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