16,047 research outputs found
Study of extremely reddened AGB stars in the Galactic bulge
Context. Extremely reddened AGB stars lose mass at high rates of >10^-5
Msun/yr. This is the very last stage of AGB evolution, in which stars in the
mass range 2.0--4.0 Msun (for solar metallicity) should have been converted to
C stars already. The extremely reddened AGB stars in the Galactic bulge are
however predominantly O-rich, implying that they might be either low-mass stars
or stars at the upper end of the AGB mass range. Aims. To determine the mass
range of the most reddened AGB stars in the Galactic bulge. Methods. Using
Virtual Observatory tools, we constructed spectral energy distributions of a
sample of 37 evolved stars in the Galactic bulge with extremely red IRAS
colours. We fitted DUSTY models to the observational data to infer the
bolometric fluxes. Applying individual corrections for interstellar extinction
and adopting a common distance, we determined luminosities and mass-loss rates,
and inferred the progenitor mass range from comparisons with AGB evolutionary
models. Results. The observed spectral energy distributions are consistent with
a classification as reddened AGB stars, except for two stars, which are
proto-planetary nebula candidates. For the AGB stars, we found luminosities in
the range 3000--30,000 Lsun and mass-loss rates 10^-5--3x10^-4 Msun/yr. The
corresponding mass range is 1.1--6.0 Msun assuming solar metallicity.
Conclusions. Contrary to the predictions of the evolutionary models, the
luminosity distribution is continuous, with many O-rich AGB stars in the mass
range in which they should have been converted into C stars already. We suspect
that bulge AGB stars have higher than solar metallicity and therefore may avoid
the conversion to C-rich. The presence of low-mass stars in the sample shows
that their termination of the AGB evolution also occurs during a final phase of
very high mass-loss rate, leading to optically thick circumstellar shells
Radiative decays of dynamically generated charmed baryons
In this work we study the radiative decay of dynamically generated
J^P=\oh^- charm baryons into the ground state J^P=\oh^+ baryons. Since
different theoretical interpretations of these baryonic resonances, and in
particular of the , give different predictions, a precise
experimental measurement of these decays would be an important step for
understanding their nature.Comment: 10 pages, 1 figur
Nanoscale austenite reversion through partitioning, segregation, and kinetic freezing: Example of a ductile 2 GPa Fe-Cr-C steel
Austenite reversion during tempering of a Fe-13.6Cr-0.44C (wt.%) martensite
results in an ultrahigh strength ferritic stainless steel with excellent
ductility. The austenite reversion mechanism is coupled to the kinetic freezing
of carbon during low-temperature partitioning at the interfaces between
martensite and retained austenite and to carbon segregation at
martensite-martensite grain boundaries. An advantage of austenite reversion is
its scalability, i.e., changing tempering time and temperature tailors the
desired strength-ductility profiles (e.g. tempering at 400{\deg}C for 1 min.
produces a 2 GPa ultimate tensile strength (UTS) and 14% elongation while 30
min. at 400{\deg}C results in a UTS of ~ 1.75 GPa with an elongation of 23%).
The austenite reversion process, carbide precipitation, and carbon segregation
have been characterized by XRD, EBSD, TEM, and atom probe tomography (APT) in
order to develop the structure-property relationships that control the
material's strength and ductility.Comment: in press Acta Materialia 201
The role of slip transfer at grain boundaries in the propagation of microstructurally short fatigue cracks in Ni-based superalloys
Crack initiation and propagation under high-cycle fatigue conditions have
been investigated for a polycrystalline Ni-based superalloy by in-situ
synchrotron assisted diffraction and phase contrast tomography. The cracks
nucleated along the longest coherent twin boundaries pre-existing on the
specimen surface, that were well oriented for slip and that presented a large
elastic incompatibility across them. Moreover, the propagation of
microstructurally short cracks was found to be determined by the easy slip
transfer paths across the pre-existing grain boundaries. This information can
only be obtained by characterization techniques like the ones presented here
that provide the full set of 3D microstructural information
The onset of solar cycle 24: What global acoustic modes are telling us
We study the response of the low-degree, solar p-mode frequencies to the
unusually extended minimum of solar surface activity since 2007. A total of
4768 days of observations collected by the space-based, Sun-as-a-star
helioseismic GOLF instrument are analyzed. A multi-step iterative
maximum-likelihood fitting method is applied to subseries of 365 days and 91.25
days to extract the p-mode parameters. Temporal variations of the l=0, 1, and 2
p-mode frequencies are then obtained from April 1996 to May 2009. While the
p-mode frequency shifts are closely correlated with solar surface activity
proxies during the past solar cycles, the frequency shifts of the l=0 and l=2
modes show an increase from the second half of 2007, when no significant
surface activity is observable. On the other hand, the l=1 modes follow the
general decreasing trend of the solar surface activity. The different
behaviours between the l=0 and l=2 modes and the l=1 modes can be interpreted
as different geometrical responses to the spatial distribution of the solar
magnetic field beneath the surface of the Sun. The analysis of the low-degree,
solar p-mode frequency shifts indicates that the solar activity cycle 24
started late 2007, despite the absence of activity on the solar surface.Comment: To be accepted by A&A (with minor revisions), 4 pages, 3 figures, 1
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