495 research outputs found
Fluorine production in intermediate-mass stars
The 19F production during the first dozen thermal pulses of AGB stars with
(M=3,Z=0.02), (M=6,Z=0.02) and (M=3,Z=0.001) is investigated on grounds of
detailed stellar models and of revised rates for 15N(a,g)19F and 18O(a,g)22Ne.
These calculations confirm an early expectation that 19F {\it is} produced in
AGB thermal pulses. They also enlarge substantially these previous results by
showing that the variations of the level of 19F production during the evolution
is very sensitive to the maximum temperature reached at the base of the pulse.
These variations are analyzed in detail, and are shown to result from a subtle
balance between different nuclear effects (mainly 19F production or destruction
in a pulse, and 15N synthesis during the interpulse), possibly super-imposed on
dilution effects in more or less extended pulse convective tongues. Our
calculations, as most others, do not predict the third dredge-up self-
consistently. When parametrized, it appears that our models of
intermediate-mass AGB stars are able to account only for the lowest 19F
overabundances observed in solar-metallicity MS, S and C stars. That conclusion
is expected to hold true for low-mass stars when F production results from
secondary 13C only. Massive AGB stars, on the other hand, are not expected to
build up large surface F abundanc- es. Therefore, the large F overabundance
reported for the super Li-rich star WZ Cas (where HBB is supposed to be
operating) remains unexplained so far. Our results for the (M=3,Z=0.001) star
indicate that F surface overabundances can also be expected in low-metallicity
stars provided that third dredge-ups occur after the early cool pulses. The
relative increase in the surface 19F/12C ratio is, however, lower in the
low-metallicity than in the solar-metallicity star. No observations areComment: 27 pages, includes figures, postcript file (A&A format, 15 pages
including figures) can be found via anonymous ftp at
ftp://obsftp.unige.ch/pub/mowlavi/fluor.ps.gz ; accepted by A&
Stellar variability in open clusters. I. A new class of variable stars in NGC 3766
Aims. We analyze the population of periodic variable stars in the open
cluster NGC 3766 based on a 7-year multi-band monitoring campaign conducted on
the 1.2 m Swiss Euler telescope at La Silla, Chili.
Methods. The data reduction, light curve cleaning and period search
procedures, combined with the long observation time line, allow us to detect
variability amplitudes down to the milli-magnitude level. The variability
properties are complemented with the positions in the color-magnitude and
color-color diagrams to classify periodic variable stars into distinct
variability types.
Results. We find a large population (36 stars) of new variable stars between
the red edge of slowly pulsating B (SPB) stars and the blue edge of delta Sct
stars, a region in the Hertzsprung-Russell (HR) diagram where no pulsation is
predicted to occur based on standard stellar models. The bulk of their periods
ranges from 0.1 to 0.7 d, with amplitudes between 1 and 4 mmag for the majority
of them. About 20% of stars in that region of the HR diagram are found to be
variable, but the number of members of this new group is expected to be higher,
with amplitudes below our milli-magnitude detection limit.
The properties of this new group of variable stars are summarized, and
arguments set forth in favor of a pulsation origin of the variability, with
g-modes sustained by stellar rotation. Potential members of this new class of
low-amplitude periodic (most probably pulsating) A and late-B variables in the
literature are discussed.
We additionally identify 16 eclipsing binary, 13 SPB, 14 delta Sct and 12
gamma Dor candidates, as well as 72 fainter periodic variables. All are new
discoveries.
Conclusions. We encourage to search for the existence of this new class of
variables in other young open clusters, especially in those hosting a rich
population of Be stars.Comment: Accepted for publication in A&A. Size of pdf file ~7Mo. Figures 12,
13, 14 and in the Appendix are of lower quality. Full quality images
published in A&
Long Period Variable Stars: galactic populations and infrared luminosity calibrations
In this paper HIPPARCOS astrometric and kinematic data are used to calibrate
both infrared luminosities and kinematical parameters of Long Period Variable
stars (LPVs). Individual absolute K and IRAS 12 and 25 luminosities of 800 LPVs
are determined and made available in electronic form.
The estimated mean kinematics is analyzed in terms of galactic populations.
LPVs are found to belong to galactic populations ranging from the thin disk to
the extended disk. An age range and a lower limit of the initial mass is given
for stars of each population. A difference of 1.3mag in K for the upper limit
of the Asymptotic Giant Branch is found between the disk and old disk galactic
populations, confirming its dependence on the mass in the main sequence.
LPVs with a thin envelope are distinguished using the estimated mean IRAS
luminosities. The level of attraction (in the classification sense) of each
group for the usual classifying parameters of LPVs (variability and spectral
types) is examined.Comment: Accepted for publication in A&A (May 2001). 13 page
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
26Al yields from rotating Wolf--Rayet star models
We present new Al stellar yields from rotating Wolf--Rayet stellar
models which, at solar metallicity, well reproduce the observed properties of
the Wolf-Rayet populations. These new yields are enhanced with respect to
non--rotating models, even with respect to non--rotating models computed with
enhanced mass loss rates. We briefly discuss some implications of the use of
these new yields for estimating the global contribution of Wolf-Rayet stars to
the quantity of Al now present in the Milky Way.Comment: 6 pages, 2 figures, to appear in New Astronomy Review
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