1,719 research outputs found
The stability of a rising droplet: an inertialess nonmodal growth mechanism
Prior modal stability analysis (Kojima et al., Phys. Fluids, vol. 27, 1984)
predicted that a rising or sedimenting droplet in a viscous fluid is stable in
the presence of surface tension no matter how small, in contrast to
experimental and numerical results. By performing a non-modal stability
analysis, we demonstrate the potential for transient growth of the interfacial
energy of a rising droplet in the limit of inertialess Stokes equations. The
predicted critical capillary numbers for transient growth agree well with those
for unstable shape evolution of droplets found in the direct numerical
simulations of Koh & Leal (Phys. Fluids, vol. 1, 1989). Boundary integral
simulations are used to delineate the critical amplitude of the most
destabilizing perturbations. The critical amplitude is negatively correlated
with the linear optimal energy growth, implying that the transient growth is
responsible for reducing the necessary perturbation amplitude required to
escape the basin of attraction of the spherical solution.Comment: 11pages, 7 figure
Nucleosynthesis in asymptotic giant branch stars: Relevance for galactic enrichment and solar system formation
We present a review of nucleosynthesis in AGB stars outlining the development of theoretical models and their relationship to observations. We focus on the new high resolution codes with improved opacities, which recently succeeded in accounting for the third dredge-up. This opens the possibility of understanding low luminosity C stars (enriched in s-elements) as the normal outcome of AGB evolution, characterized by production of 12C and neutron-rich nuclei in the He intershell and by mass loss from strong stellar winds. Neutron captures in AGB stars are driven by two reactions: 13C(α,n)16O, which provides the bulk of the neutron flux at low neutron densities (Nn ≤ 107 n/cm3), and 22Ne(α,n)25Mg, which is mildly activated at higher temperatures and mainly affects the production of s-nuclei depending on reaction branchings. The first reaction is now known to occur in the radiative interpulse phase, immediately below the region previously homogenized by third dredge-up. The second reaction occurs during the convective thermal pulses. The resulting nucleosynthesis phenomena are rather complex and rule out any analytical approximation (exponential distribution of neutron fluences). Nucleosynthesis in AGB stars, modeled at different metallicities, account for several observational constraints, coming from a wide spectrum of sources: evolved red giants rich in s-elements, unevolved stars at different metallicities, presolar grains recovered from meteorites, and the abundances of s-process isotopes in the solar system. In particular, a good reproduction of the solar system main component is obtained as a result of Galactic chemical evolution that mixes the outputs of AGB stars of different stellar generations, born with different metallicities and producing different patterns of s-process nuclei. The main solar s-process pattern is thus not considered to be the result of a standard archetypal s-process occurring in all stars. Concerning the 13C neutron source, its synthesis requires penetration of small amounts of protons below the convective envelope, where they are captured by the abundant 12C forming a 13C-rich pocket. This penetration cannot be modeled in current evolutionary codes, but is treated as a free parameter. Future hydrodynamical studies of time dependent mixing will be required to attack this problem. Evidence of other insufficiencies in the current mixing algorithms is common throughout the evolution of low and intermediate mass stars, as is shown by the inadequacy of stellar models in reproducing the observations of CNO isotopes in red giants and in circumstellar dust grains. These observations require some circulation of matter between the bottom of convective envelopes and regions close to the H-burning shell (cool bottom processing). AGB stars are also discussed in the light of their possible contribution to the inventory of short-lived radioactivities that were found to be alive in the early solar system. We show that the pollution of the protosolar nebula by a close-by AGB star may account for concordant abundances of 26Al, 41Ca, 60Fe, and 107Pd. The AGB star must have undergone a very small neutron exposure, and be of small initial mass (M <= 1.5 [sols]). There is a shortage of 26Al in such models, that however remains within the large uncertainties of crucial reaction rates. The net 26Al production problem requires further investigation
Edge states control droplet break-up in sub-critical extensional flows
A fluid droplet suspended in an extensional flow of moderate intensity may
break into pieces, depending on the amplitude of the initial droplet
deformation. In subcritical uniaxial extensional flow the non-breaking base
state is linearly stable, implying that only a finite amplitude perturbation
can trigger break-up. Consequently, the stable base solution is surrounded by
its finite basin of attraction. The basin boundary, which separates initial
droplet shapes returning to the non-breaking base state from those becoming
unstable and breaking up, is characterized using edge tracking techniques. We
numerically construct the edge state, a dynamically unstable equilibrium whose
stable manifold forms the basin boundary. The edge state equilibrium controls
if the droplet breaks and selects a unique path towards break-up. This path
physically corresponds to the well-known end-pinching mechanism. Our results
thereby rationalize the dynamics observed experimentally [Stone & Leal, J.
Fluid Mech. 206, 223 (1989)
Si Isotopic Ratios in Mainstream Presolar SiC Grains Revisited
Although mainstream SiC grains, the major group of presolar SiC grains found
in meteorites, are believed to have originated in the expanding envelope of
asymptotic giant branch (AGB) stars during their late carbon-rich phases, their
Si isotopic ratios show a distribution that cannot be explained by
nucleosynthesis in this kind of stars. Previously, this distribution has been
interpreted to be the result of contributions from many AGB stars of different
ages whose initial Si isotopic ratios vary due to the Galactic chemical
evolution of the Si isotopes. This paper presents a new interpretation based on
local heterogeneities of the Si isotopes in the interstellar medium at the time
the parent stars of the mainstream grains were born. Recently, several authors
have presented inhomogeneous chemical evolution models of the Galactic disk in
order to account for the well known evidence that F and G dwarfs of similar age
show an intrinsic scatter in their elemental abundances.Comment: Accepted for publication by ApJ. 19 pages of text + 17 figures and 4
table
s-Process in Low Metallicity Stars. I. Theoretical Predictions
A large sample of carbon enhanced metal-poor stars enriched in s-process
elements (CEMP-s) have been observed in the Galactic halo. These stars of low
mass (M ~ 0.9 Msun) are located on the main-sequence or the red giant phase,
and do not undergo third dredge-up (TDU) episodes. The s-process enhancement is
most plausibly due to accretion in a binary system from a more massive
companion when on the asymptotic giant branch (AGB) phase (now a white dwarf).
In order to interpret the spectroscopic observations, updated AGB models are
needed to follow in detail the s-process nucleosynthesis. We present
nucleosynthesis calculations based on AGB stellar models obtained with FRANEC
(Frascati Raphson-Newton Evolutionary Code) for low initial stellar masses and
low metallicities. For a given metallicity, a wide spread in the abundances of
the s-process elements is obtained by varying the amount of 13C and its profile
in the pocket, where the 13C(a, n)16O reaction is the major neutron source,
releasing neutrons in radiative conditions during the interpulse phase. We
account also for the second neutron source 22Ne(a, n)25Mg, partially activated
during convective thermal pulses. We discuss the surface abundance of elements
from carbon to bismuth, for AGB models of initial masses M = 1.3 -- 2 Msun, low
metallicities ([Fe/H] from -1 down to -3.6) and for different 13C-pockets
efficiencies. In particular we analyse the relative behaviour of the three
s-process peaks: light-s (ls at magic neutron number N = 50), heavy-s (hs at N
= 82) and lead (N = 126). Two s-process indicators, [hs/ls] and [Pb/hs], are
needed in order to characterise the s-process distribution. In the online
material, we provide a set of data tables with surface predictions. ...Comment: 31 pages, 15 figures + 6 online material, 10 table
- …