1,715 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&
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
The synthesis of the light Mo and Ru isotopes: how now, no need for an exotic solution ?
The most detailed calculations of the p-process call for its development in
the O/Ne layers of Type II supernovae. In spite of their overall success in
reproducing the solar system content of p-nuclides, they suggest a significant
underproduction of the light Mo and Ru isotopes. On grounds of a model for the
explosion of a 25 solar mass star with solar metallicity, we demonstrate that
this failure might just be related to the uncertainties left in the rate of the
22Ne(alpha,n)25Mg neutron producing reaction. The latter indeed have a direct
impact on the distribution of the s-process seeds for the p-process.Comment: 4 pages, 4 figures. LaTex2e with aa.cls. A&A Letters, in pres
Dive behaviour, foraging locations and maternal-attendance patterns in Australian fur seals (Arctocephalus pusillus doriferus)
The dive behaviour, foraging locations, and colony-attendance patterns of female Australian fur seals (Arctocephalus pusillus doriferus) from Kanowna Island (39°10\u27S, 146°18\u27E) in Bass Strait, southeastern Australia, were determined throughout lactation during 1997–1999. Foraging-trip durations increased as lactation progressed, being shortest in summer (3.71 ± 0.24 days; mean ± 1 SE) and longest in winter (6.77 ± 0.57 days, P < 0.05), but maternal-attendance periods did not differ in duration (1.70 ± 0.10 days, P > 0.5). Individual mean attendance periods and trip durations were positively correlated (r2 = 0.21, P < 0.005). Diving commenced shortly after seals left the colony (2.6 ± 0.4 h), was continuous for long periods (up to 36 h), occurred mostly during daylight hours, and lacked regular diel variation in depth. The majority of dives (78%) were typically U-shaped and reached depths corresponding to the prevailing depths in Bass Strait (65–85 m), indicating that these animals forage mostly on the benthos of the shallow continental shelf in this region. Such behaviour is unusual for fur seals but is reminiscent of that of some sea lion species. Mean dive durations varied between 2.0 and 3.7 min (maximum 8.9 min) and the theoretical aerobic dive limit (3.91–4.26 min) was exceeded on 17.3% of dives. Dive frequency (8.3 ± 0.6/h) and the proportion of time at sea spent diving (40.7 ± 2.1%) were weakly negatively related to the duration of the foraging trip (r2= 0.07, P < 0.004, and r2 = 0.13, P < 0.0001, respectively). Data from at-sea locations showed that lactating females forage almost exclusively within Bass Strait during all seasons. <br /
Re(\gamm,n) cross section close to and above the neutron threshold
The neutron capture cross section of the unstable nucleus Re is
studied by investigating the inverse photodisintegration reaction
Re(,n). The special interest of the {\it s}-process branching
point Re is related to the question of possible {\it s}-process
contributions to the abundance of the {\it r}-process chronometer nucleus
^{187}^{186}\gamma^{186}$Os; the two predicted neutron-capture cross sections
differ by a factor of 2.4; this calls for future theoretical study.Comment: Phys. Rev. C, in pres
Hypoxia counteracts taxol-induced apoptosis in MDA-MB-231 breast cancer cells:Role of autophagy and JNK activation
Cancer cell resistance against chemotherapy is still a heavy burden to improve anticancer treatments. Autophagy activation and the development of hypoxic regions within the tumors are known to promote cancer cell resistance. Therefore, we sought to evaluate the role of autophagy and hypoxia on the taxol-induced apoptosis in MDA-MB-231 breast cancer cells. The results showed that taxol induced apoptosis after 16 h of incubation, and that hypoxia protected MDA-MB-231 cells from taxol-induced apoptosis. In parallel, taxol induced autophagy activation already after 2 h of incubation both under normoxia and hypoxia. Autophagy activation after taxol exposure was shown to be a protective mechanism against taxol-induced cell death both under normoxia and hypoxia. However, at longer incubation time, the autophagic process reached a saturation point under normoxia leading to cell death, whereas under hypoxia, autophagy flow still correctly took place allowing the cells to survive. Autophagy induction is induced after taxol exposure via mechanistic target of rapamycin (mTOR) inhibition, which is more important in cells exposed to hypoxia. Taxol also induced c-Jun N-terminal kinase (JNK) activation and phosphorylation of its substrates B-cell CLL/lymphoma 2 (Bcl(2)) and BCL2-like 1 (Bcl(XL)) under normoxia and hypoxia very early after taxol exposure. Bcl(2) and Bcl(XL) phosphorylation was decreased more importantly under hypoxia after long incubation time. The role of JNK in autophagy and apoptosis induction was studied using siRNAs. The results showed that JNK activation promotes resistance against taxol-induced apoptosis under normoxia and hypoxia without being involved in induction of autophagy. In conclusion, the resistance against taxol-induced cell death observed under hypoxia can be explained by a more effective autophagic flow activated via the classical mTOR pathway and by a mechanism involving JNK, which could be dependent on Bcl(2) and Bcl(XL) phosphorylation but independent of JNK-induced autophagy activation
Effects of Composite Rheology on Plate-Like Behavior in Global-Scale Mantle Convection
Earth's upper mantle rheology controls lithosphere-asthenosphere coupling and thus surface tectonics. Rock deformation experiments and seismic anisotropy measurements indicate that composite rheology (co-existing diffusion and dislocation creep) occurs in the Earth's uppermost mantle, potentially affecting convection and surface tectonics. Here, we investigate how the spatio-temporal distribution of dislocation creep in an otherwise diffusion-creep-controlled mantle impacts the planform of convection and the planetary tectonic regime as a function of the lithospheric yield strength in numerical models of mantle convection self-generating plate-like tectonics. The low upper-mantle viscosities caused by zones of substantial dislocation creep produce contrasting effects on surface dynamics. For strong lithosphere (yield strength > 35 MPa), the large lithosphere-asthenosphere viscosity contrasts promote stagnant-lid convection. In contrast, the increase of upper mantle convective vigor enhances plate mobility for lithospheric strength <35 MPa. For the here-used model assumptions, composite rheology does not facilitate the onset of plate-like behavior at large lithospheric strength
Spectroscopy of the heaviest nuclei (theory)
Recent progress in the applications of covariant density functional theory
(CDFT) to the description of the spectroscopy of the heaviest nuclei is
reviewed. The analysis of quasiparticle spectra in actinides and the heaviest A
~ 250 nuclei provides a measure of the accuracy of the description of
single-particle energies in CDFT and an additional constraint for the choice of
effective interactions for the description of superheavy nuclei. The response
of these nuclei to the rotation is rather well described by cranked
relativistic Hartree+Bogoliubov theory and it serves as a supplementary tool in
configuration assignment in odd-mass nuclei. A systematic analysis of the
fission barriers with allowance for triaxial deformation shows that covariant
density functional theory is able to describe fission barriers on a level of
accuracy comparable with the best phenomenological macroscopic+microscopic
approaches.Comment: 10 pages, 7 figures, invited talk of A.V. Afanasjev at the
International Nuclear Physics Conference (INPC 2010), Vancouver, Canada, July
4-9, 2010, to be published in Journal of Physics G: Conference Series (JPCS
- …