1,303 research outputs found
Atomic and Molecular Data for Stellar Physics: Former Successes and Future Challenges
This review highlights current (and future) hot topics in astrophysics where
atomic or molecular input data are (or will be) essential, with special
emphasis on topics relating to nucleosynthesis and cosmochemistry.
We first discuss issues (like the abundances of oxygen and iron in the Sun,
and that of lithium in post-AGB stars) where the use of poor-quality atomic or
molecular data have led to spurious astrophysical puzzles which sparked fancy
astrophysical models or theories. We then address issues where the advent of
new instruments (like the ultraviolet high-resolution spectrographs--GHRS
onboard HST, Keck-HRS or VLT-UVES--or future infrared satellites) calls for new
and accurate atomic or molecular data.Comment: 20 pages, 12 figures, to appear in Physica Scripta, Topical Issue
(Proceedings of the 35th EGAS conference -- European Group for Atomic
Spectroscopy
AGB stars in binaries and their progeny
It is currently admitted that an AGB star in a binary system is likely to
pollute its companion with carbon- and s-process-rich matter. After the AGB
star has faded into an unconspicuous white dwarf, the polluted companion enters
the zoo of stars with chemical peculiarities. In this paper, the progeny of AGB
stars in binary systems are identified among existing spectroscopic classes
(Abell 35-like, binary post-AGB, WIRRing, dwarf Ba and C, subgiant CH, Ba, CH,
S, yellow symbiotics) and their filiation is discussed from the properties of
their eccentricity -- period diagrams.Comment: Invited review at IAU Symp. 191 (AGB Stars, eds. T. Le Bertre, A.
Lebre, C. Waelkens), 10 pages, 2 figures (LaTeX/paspconf.sty). Also available
at: http://astro.ulb.ac.be/Htm/ps.ht
The link between symbiotic stars and chemically-peculiar red giants
Barium stars and technetium-poor, extrinsic S stars are binary systems with a
white dwarf companion, and with orbital elements similar to those of symbiotic
systems. One may thus wonder whether these various families of binary systems
involving red giant stars are somehow related. This question is actually
twofold: (i) Do barium and binary S stars exhibit some symbiotic activity? (ii)
Do symbiotic systems exhibit overabundances of s-process elements like barium
and S stars? This paper reviews the current situation regarding these two
questions.Comment: 16 pages, 5 figures, to be published in 'Symbiotic Stars Probing
Stellar Evolution' (R.L.M. Corradi, J. Mikolajewska, T.J. Mahoney eds.), ASP
Conf. Ser. Also available at http://www-astro.ulb.ac.be/Html/ps.html#symbi
Is Amino-Acid Homochirality Due To Asymmetric Photolysis In Space?
Amino acids occurring in proteins are, with rare exceptions, exclusively of
the L-configuration. Among the many scenarios put forward to explain the origin
of this chiral homogeneity (i.e., homochirality), one involves the asymmetric
photolysis of amino acids present in space, triggered by circularly polarized
UV radiation. The recent observation of circularly polarized light (CPL) in the
Orion OMC-1 star-forming region (Bailey et al. 1998, Science 281, 672) has been
presented as providing a strong validation of this scenario. The present paper
reviews the situation. It is stressed for example that one important condition
for the asymmetric photolysis by CPL to be at the origin of the terrestrial
homochirality of natural amino acids is generally overlooked, namely, the
asymmetric photolysis should favour the L-enantiomer for ALL the primordial
amino acids involved in the genesis of life (i.e., biogenic amino acids).
Although this condition is probably satisfied for aliphatic amino acids, some
non-aliphatic amino acids like tryptophan and proline may violate the condition
and thus invalidate the asymmetric photolysis scenario, assuming they were
among the primordial amino acids. Alternatively, if CPL photolysis in space is
indeed the source of homochirality of amino acids, then tryptophan and proline
may be crossed out from the list of biogenic amino acids.Comment: To appear in Space Science Reviews, 11 pages, 1 figure (LaTeX
Cosmic abundances: The impact of stellar duplicity
The mass-transfer scenario links chemical peculiarities with stellar
duplicity for an increasing number of stellar classes (classical and dwarf
barium stars, subgiant and giant CH stars, S stars without technetium, yellow
symbiotic stars, WIRRING stars, Abell-35-like nuclei of planetary nebulae...).
Despite these successes, the mass-transfer scenario still faces several
problems: What is the mass-transfer mode? Why orbital elements of dwarf barium
stars do not fully match those of the classical barium stars? What is the
origin of the few non-binary stars among dwarf barium stars? The paper reviews
these open questions.Comment: 14 pages, 4 figures, to appear in `Cosmic Abundances as Records of
Stellar Evolution and Nucleosynthesis', edited by F.N. Bash, T.G. Barnes, ASP
Conf. Ser., in pres
Circumstellar shells and mass loss rates: Clues to the evolution of S stars
It is the purpose of this paper to rediscuss the circumstellar properties of
S stars and to put these properties in perspective with our current
understanding of the evolutionary status of S stars, in particular the
intrinsic/extrinsic dichotomy. Accordingly, an extensive data set probing the
circumstellar environment of S stars (IRAS flux densities, maser emission, CO
rotational lines) has been collected and critically evaluated. This data set
combines new observations (9 stars have been observed in the CO J=2-1 line and
3 in the CO J=3-2 line, with four new detections) with existing material (all
CO and maser observations of S stars published in the literature). The IRAS
flux densities of S stars have been re-evaluated by co-adding the individual
scans, in order to better handle the intrinsic variability of these stars in
the IRAS bands, and possible contamination by Galactic cirrus.
Mass loss rates or upper limits have been derived for all S stars observed in
the CO rotational lines, and range from < 2 10^{-8} Msun y^{-1} for extrinsic S
stars to 10^{-5} Msun y^{-1}. These mass-loss rates correlate well with the K -
[12] color index, which probes the dust loss rate, provided that the mass loss
rate be larger than 10^{-8} Msun~y^{-1}. Small mass-loss rates are found for
extrinsic S stars, consistent with their not being so evolved (RGB or
Early-AGB) as the Tc-rich S stars. This result does not support the claim often
made in relation with symbiotic stars that binarity strongly enhances the
mass-loss rate.Comment: Astronomy & Astrophysics Suppl., 40 pages, 22 figures, 6 tables
(LaTeX). Also available at: http://astro.ulb.ac.be/Htm/ps.ht
The Henize sample of S stars. I. The technetium dichotomy
This paper is the first one in a series investigating the properties of the S
stars belonging to the Henize sample (205 S stars with delta<-25 deg. and
R<10.5) in order to derive the respective properties (like galactic
distribution and relative frequencies) of intrinsic (i.e. genuine asymptotic
giant branch) S stars and extrinsic (i.e. post mass-transfer binary) S stars.
High-resolution (R=30 000 to 60 000) spectra covering the range 4230-4270AA
have been obtained for 76 S stars, 8 M stars and 2 symbiotic stars. The 4262AA
and 4238AA blends involving a Tc I line were analysed separately and yield
consistent conclusions regarding the presence or absence of technetium. Only
one `transition' case (Hen 140 = HD 120179, a star where only weak lines of
technetium are detectable) is found in our sample. A resolution greater than R
=30 000 is clearly required in order to derive unambiguous conclusions
concerning the presence or absence of technetium. The Tc/no Tc dichotomy will
be correlated with radial velocity and photometric data in a forthcoming paper.Comment: 10 pages, 5 figures, Latex, accepted for publication in Astronomy and
Astrophysics main journal. Also available at http://astro.ulb.ac.be
Management control in the transfer pricing tax compliant multinational enterprise
This paper studies the impact of transfer pricing tax compliance on management control system (MCS) design and use within one multinational enterprise (MNE) which employed the same transfer prices for tax compliance and internal management purposes. Our analysis shows immediate effects of tax compliance on the design of organising controls with subsequent effects on planning, evaluating and rewarding controls which reveal a more coercive use of the MCS overall. We argue that modifications to the MCS cannot be understood without an appreciation of the MNEs’ fiscal transfer pricing compliance process
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&
Non-explosive hydrogen and helium burnings: Abundance predictions from the NACRE reaction rate compilation
The abundances of the isotopes of the elements from C to Al produced by the
non-explosive CNO, NeNa and MgAl modes of hydrogen burning, as well as by
helium burning, are calculated with the thermonuclear rates recommended by the
European compilation of reaction rates for astrophysics (NACRE: details about
NACRE may be found at http://astro.ulb.ac.be. This electronic address provides
many data of nuclear astrophysics interest and also offers the possibility of
generating interactively tables of reaction rates for networks and temperature
grids selected by the user). The impact of nuclear physics uncertainties on the
derived abundances is discussed in the framework of a simple parametric
astrophysical model. These calculations have the virtue of being a guide in the
selection of the nuclear uncertainties that have to be duly analyzed in
detailed model stars, particularly in order to perform meaningful
confrontations between abundance observations and predictions. They are also
hoped to help nuclear astrophysicists pinpointing the rate uncertainties that
have to be reduced most urgently.Comment: 13 pages, 13 figures, Latex, accepted for publication in Astronomy
and Astrophysics main journal. Also available at
http://astro.ulb.ac.be/Htm/iaa0.ht
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