79 research outputs found
The Lithium Flash - Thermal instabilities generated by lithium burning in RGB stars
We present a scenario to explain the lithium-rich phase which occurs on the red giant branch at the so-called bump in the luminosity function. The high transport coefficients required to enhance the surface lithium abundance are obtained in the framework of rotation-induced mixing thanks to the impulse of the important nuclear energy released in a lithium burning shell. Under certain conditions a lithium flash is triggered off. The enhanced mass loss rate due to the temporary increase of the stellar luminosity naturally accounts for a dust shell formation
The Abundance of Interstellar Fluorine and Its Implications
We report results from a survey of neutral fluorine (F I) in the interstellar
medium. Data from the Far Ultraviolet Spectroscopic Explorer (FUSE) were used
to analyze 26 lines of sight lying both in the galactic disk and halo,
including lines to Wolf-Rayet stars and through known supernova remnants. The
equivalent widths of fluorine resonance lines at 951.871 A and 954.827 A were
measured or assigned upper limits and combined with a nitrogen curve of growth
to obtain F I column densities. These column densities were then used to
calculate fluorine depletions. Comparisons are made to the previous study of F
I by Federman et al. (2005) and implications for F I formation and depletion
are discussed.Comment: 32 pages, 10 figures, Accepted to Ap
Fluorine abundances in planetary nebulae
We have determined fluorine abundances from the F II 4789 and F IV 4060
nebular emission lines for a sample of planetary nebulae (PNe). Our results
show that fluorine is generally overabundant in PNe, thus providing new
evidence for the synthesis of fluorine in asymptotic giant branch (AGB) stars.
[F/O] is found to be positively correlated with the C/O abundance ratio, in
agreement with the predictions of theoretical models of fluorine production in
thermally pulsing AGB stars. A large enhancement of fluorine is observed in the
Wolf-Rayet PN NGC 40, suggesting that high mass-loss rates probably favor the
survival of fluorine.Comment: 4 pages, 3 figures, accepted for publication in ApJ Letter
Possible Stellar Metallicity Enhancements from the Accretion of Planets
A number of recently discovered extrasolar planet candidates have
surprisingly small orbits, which may indicate that considerable orbital
migration takes place in protoplanetary systems. A natural consequence of
orbital migration is for a series of planets to be accreted, destroyed, and
then thoroughly mixed into the convective envelope of the central star. We
study the ramifications of planet accretion for the final main sequence
metallicity of the star. If maximum disk lifetimes are on the order of 10 Myr,
stars with masses near 1 solar mass are predicted to have virtually no
metallicity enhancement. On the other hand, early F and late A type stars with
masses of 1.5--2.0 solar masses can experience significant metallicity
enhancements due to their considerably smaller convection zones during the
first 10 Myr of pre-main-sequence evolution. We show that the metallicities of
an aggregate of unevolved F stars are consistent with an average star accreting
about 2 Jupiter-mass planets from a protoplanetary disk having a 10 Myr
dispersal time.Comment: 14 pages, AAS LaTeX, 3 figures, accepted to ApJ Letter
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
The fluorine abundance in a Galactic Bulge AGB star measured from CRIRES spectra
We present measurements of the fluorine abundance in a Galactic Bulge
Asymptotic Giant Branch (AGB) star. The measurements were performed using high
resolution K-band spectra obtained with the CRIRES spectrograph, which has been
recently installed at ESO's VLT, together with state-of-the-art model
atmospheres and synthetic spectra. This represents the first fluorine abundance
measurement in a Galactic Bulge star, and one of few measurements of this kind
in a third dredge-up oxygen-rich AGB star. The F abundance is found to be close
to the solar value scaled down to the metallicity of the star, and in agreement
with Disk giants that are comparable to the Bulge giant studied here. The
measurement is of astrophysical interest also because the star's mass can be
estimated rather accurately (1.4 \lesssim M/\mathrm{M}_{\sun} \lesssim 2.0).
AGB nucleosynthesis models predict only a very mild enrichment of F in such low
mass AGB stars. Thus, we suggest that the fluorine abundance found in the
studied star is representative for the star's natal cloud, and that fluorine
must have been produced at a similar level in the Bulge and in the Disk.Comment: 11 pages, 1 figure, accepted for publication by Ap
Fluorine Abundances in the Orion Nebula Cluster
This study uses cool dwarfs as sources with which to probe fluorine
abundances via HF. This molecule is detected for the first time in young K-M
dwarf members of an OB association. Fluorine, oxygen, and carbon abundances
were derived from the HF(1--0) R9 line along with samples of OH and CO
vibration-rotation lines present in high-resolution infrared spectra observed
with the Phoenix spectrograph on the Gemini South Telescope. The fluorine and
oxygen results obtained for these targets, still in the pre-main-sequence stage
of evolution, agree well with the general trend defined for the Milky Way disk;
the latter being deduced from observations of more evolved giant stars. In
addition, the carbon and oxygen abundances obtained for the studied stars
overlap results from previous studies of the more massive OB stars and FG dwarf
members of the Orion Nebula Cluster. We conclude from this agreement that the
fluorine abundances derived for the Orion K-M dwarfs (when there is no
conspicuous evidence of disks) can be considered a good representation of the
current fluorine abundance value for the Milky Way disk.Comment: 18 pages, including 2 tables and 3 figures. Accepted for publication
in The Astrophysical Journa
The thermonuclear production of F19 by Wolf-Rayet stars revisited
New models of rotating and non-rotating stars are computed for initial masses
between 25 and 120 Msun and for metallicities Z = 0.004, 0.008, 0.020 and 0.040
with the aim of reexamining the wind contribution of Wolf-Rayet (WR) stars to
the F19 enrichment of the interstellar medium. Models with an initial rotation
velocity vini = 300 km/s are found to globally eject less F19 than the
non-rotating models. We compare our new predictions with those of Meynet &
Arnould (2000), and demonstrate that the F19 yields are very sensitive to the
still uncertain F19(alpha,p)Ne22 rate and to the adopted mass loss rates. Using
the recommended mass loss rate values that take into account the clumping of
the WR wind and the NACRE reaction rates when available, we obtain WR F19
yields that are significantly lower than predicted by Meynet & Arnould (2000),
and that would make WR stars non-important contributors to the galactic F19
budget. In view, however, of the large nuclear and mass loss rate
uncertainties, we consider that the question of the WR contribution to the
galactic F19 remains quite largely open.Comment: 9 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
- …