485 research outputs found
Chemical consequences of low star formation rates: stochastically sampling the IMF
When estimating the abundances which result from a given star formation
event, it is customary to treat the IMF as a series of weight factors to be
applied to the stellar yields, as a function of mass, implicitly assuming one
is dealing with an infinite population. However, when the stellar population is
small, the standard procedure would imply the inclusion of fractional numbers
of stars at certain masses. We study the effects of small number statistics on
the resulting abundances by performing an statistical sampling of the IMF to
form a stellar population out of discrete numbers of stars. A chemical
evolution code then follows the evolution of the population, and traces the
resulting abundances. The process is repeated to obtain an statistical
distribution of the resulting abundances and their evolution. We explore the
manner in which different elements are affected, and how different abundances
converge to the infinite population limit as the total mass increases. We
include a discussion of our results in the context of dwarf spheroidal galaxies
and show the recently reported internal dispersions in abundance ratios for
dSph galaxies might be partly explained through the stochastic effects
introduced by a low star formation rate, which can account for dispersions of
over 2 dex in [C/O], [N/O], [C/Fe], [N/Fe] and [O/Fe].Comment: 13 pages, 13 figures, Accepted for publication in MNRA
A Method to Study Alterations in Networks of Structural Connectivity
The global structural connectivity of the brain can be explored in vivo with a connectivity matrix derived from diffusion MRI tractography [1]. In such a matrix, M, every index i or j represents a small region of interest (ROI) at the white-gray matter (WGM) interface and every entry M(i,j) provides a measure of connectivity derived from tractography. Once the matrix computed, it is easy to obtain connectional information betwee
The effect of δ-hydride on the micromechanical deformation of Zircaloy-4 studied by in situ high angular resolution electron backscatter diffraction
Zircaloy-4(Zr-1.5%Sn-0.2%Fe-0.1%Cr wt%)is usedas nuclear fuel cladding materials and hydride embrittlementis amajor failure mechanism. To explore the effect of δ-hydrideon plastic deformation and performance of Zircaloy-4, in situhigh angular resolution electron backscatter diffraction(HR-EBSD)was used to quantify stress andgeometrically necessarydislocation(GND)density during bending tests of hydride-free and hydride-containingsingle crystalZircaloy-4 microcantilevers. Results suggest that while the stress applied was accommodated by plastic slip in the hydride-free cantilever,the hydride-containing cantilever showedprecipitation-induced GND pile-up at hydride-matrix interfacepre-deformation, andconsiderable locally-increasing GNDdensity under tensile stressupon plastic deformation
The role of β-titanium ligaments in the deformation of dual phase titanium alloys
Multiphase titanium alloys are critical materials in high value engineering components, for instance in aero engines. Microstructural complexity is exploited through interface engineering during mechanical processing to realise significant improvements in fatigue and fracture resistance and strength. In this work, we explore the role of select interfaces using in-situ micromechanical testing with concurrent observations from high angular resolution electron backscatter diffraction (HR-EBSD). Our results are supported with post mortem transmission electron microscopy (TEM). Using micro-pillar compression, we performed in-depth analysis of the role of select {\beta}-titanium (body centred cubic) ligaments which separate neighbouring {\alpha}-titanium (hexagonal close packed) regions and inhibit the dislocation motion and impact strength during mechanical deformation. These results shed light on the strengthening mechanisms and those that can lead to strain localisation during fatigue and failure
Evolution of 3-9 Mo Stars for Z=0.001 - 0.03 and Metallicity Effects on Type Ia Supernovae
Recent observations have revealed that Type Ia supernovae (SNe Ia) are not
perfect standard candles but show some variations in their absolute magnitudes,
light curve shapes, and spectra. The C/O ratio in the SNe Ia progenitors (C-O
white dwarfs) may be related to this variation. In this work, we systematically
investigate the effects of stellar mass (M) and metallicity (Z) on the C/O
ratio and its distribution in the C-O white dwarfs by calculating stellar
evolution from the main-sequence through the end of the second dredge-up for
M=3-9 Mo and Z=0.001-0.03. We find that the total carbon mass fraction just
before SN Ia explosion varies in the range 0.36 -- 0.5. We also calculate the
metallicity dependence of the main-sequence-mass range of the SN Ia progenitor
white dwarfs. Our results show that the maximum main-sequence mass to form C-O
white dwarfs decreases significantly toward lower metallicity, and the number
of SN Ia progenitors may be underestimated if metallicity effectis neglected.
We discuss the implications of these results on the variation of SNe Ia,
determination of cosmological parameters, luminosity function of white dwarfs,
and the galactic chemical evolution.Comment: Added references and corrected typos. To appear in the Astrophysical
Journal 1999 March 10 issu
Systematic effects and a new determination of the primordial abundance of 4He and dY/dZ from observations of blue compact galaxies
We use spectroscopic observations of a sample of 82 HII regions in 76 blue
compact galaxies to determine the primordial helium abundance Yp and the slope
dY/dZ from the Y-O/H linear regression. To improve the accuracy of the dY/dZ
measurement, we have included new spectrophotometric observations of 33 HII
regions which span a large metallicity range, with oxygen abundance 12+log(O/H)
varying between 7.43 and 8.30 (Zsun/30<Z<Zsun/4). For a subsample of 7 HII
regions, we derive the He mass fraction taking into account known systematic
effects, including collisional and fluorescent enhancements of HeI emission
lines, collisional excitation of hydrogen emission, underlying stellar HeI
absorption and the difference between the temperatures Te(HeII) in the He^+
zone and Te(OIII) derived from the collisionally excited [OIII] lines. We find
that the net result of all the systematic effects combined is small, changing
the He mass fraction by less than 0.6%. By extrapolating the Y vs. O/H linear
regression to O/H=0 for 7 HII regions of this subsample, we obtain
Yp=0.2421+/-0.0021 and dY/dO=5.7+/-1.8, which corresponds to dY/dZ=3.7+/-1.2,
assuming the oxygen mass fraction to be O=0.66Z. In the framework of the
standard Big Bang nucleosynthesis theory, this Yp corresponds to Omega_b h^2 =
0.012^+0.003_-0.002, where h is the Hubble constant in units of 100 km/s/Mpc.
This is smaller at the 2sigma level than the value obtained from recent
deuterium abundance and microwave background radiation measurements. The linear
regression slope dY/dO=4.3+/-0.7 (corresponding to dY/dZ=2.8+/-0.5) for the
whole sample of 82 HII regions is similar to that derived for the subsample of
7 HII regions, although it has a considerably smaller uncertainty.Comment: 53 pages, 3 Postscript figures, accepted for publication in the
Astrophysical Journa
The redshift evolution of oxygen and nitrogen abundances in emission-line SDSS galaxies
The oxygen and nitrogen abundance evolutions with redshift and galaxy stellar
mass in emission-line SDSS galaxies are investigated. This is the first such
study for nitrogen abundances, and it provides an additional constraint for the
study of the chemical evolution of galaxies. We have devised a criterion to
recognize and exclude from consideration AGNs and star-forming galaxies with
large errors in the line flux measurements. To select star-forming galaxies
with accurate line fluxes measurements, we require that, for each galaxy, the
nitrogen abundances derived with various calibrations based on different
emission lines agree. Using this selection criterion, subsamples of
star-forming galaxies have been extracted from catalogs of the MPA/JHU group.
We found that the galaxies of highest masses, those with masses > 10^11.2
M_sun, have not been enriched in both oxygen and nitrogen over the last 3 Gyr:
they have formed their stars in the so distant past that these have returned
their nucleosynthesis products to the interstellar medium before z=0.25. The
galaxies in the mass range from 10^11.0 M_sun to 10^11.2 M_sun do not show an
appreciable enrichment in oxygen, but do show some enrichment in nitrogen: they
also formed their stars before z=0.25 but later in comparison to the galaxies
of highest masses; these stars have not returned nitrogen to the interstellar
medium before z=0.25 because they have not had enough time to evolve. This
suggests that stars with lifetimes of 2-3 Gyr contribute to the nitrogen
production. Finally, galaxies with masses < 10^11 M_sun show enrichment in both
oxygen and nitrogen during the last 3 Gyr: they have undergone appreciable star
formation and have converted up to 20% of their mass into stars over this
period.Comment: 43 pages, 15 figures, accepted for publication in the Astrophysical
Journa
Neutral material around the B[e] supergiant star LHA 115-S 65: An outflowing disk or a detached Keplerian rotating disk?
B[e] supergiants are surrounded by large amounts of hydrogen neutral
material, traced by the emission in the optical [OI] lines. This neutral
material is most plausibly located within their dense, cool circumstellar
disks, which are formed from the (probably non-spherically symmetric) wind
material released by the star. Neither the formation mechanism nor the
resulting structure and internal kinematics of these disks (or disk-like
outflows) are well known. However, rapid rotation, lifting the material from
the equatorial surface region, seems to play a fundamental role. The B[e]
supergiant LHA 115-S 65 (S65) in the SMC is one of the two most rapidly
rotating B[e] stars known. Its almost edge-on orientation allows a detailed
kinematical study of its optically thin forbidden emission lines. With a focus
on the [OI] lines, we test the two plausible disk scenarios: the outflowing and
the Keplerian rotating disk. Based on high- and low-resolution optical spectra,
we investigate the density and temperature structure in those disk regions that
are traced by the [OI] emission to constrain the disk sizes and mass fluxes
needed to explain the observed [OI] line luminosities. In addition, we compute
the emerging line profiles expected for either an outflowing disk or a
Keplerian rotating disk, which can directly be compared to the observed
profiles. Both disk scenarios deliver reasonably good fits to the line
luminosities and profiles of the [OI] lines. Nevertheless, the Keplerian disk
model seems to be the more realistic one, because it also agrees with the
kinematics derived from the large number of additional lines in the spectrum.
As additional support for the presence of a high-density, gaseous disk, the
spectrum shows two very intense and clearly double-peaked [CaII] lines. We
discuss a possible disk-formation mechanism, and similarities between S65 and
the group of LBVs.Comment: 13 pages, 12 figures, accepted for publication in A&
Helium abundance in the most metal-deficient blue compact galaxies: I Zw 18 and SBS 0335-052
We present high-quality spectroscopic observations of the two most-metal
deficient blue compact galaxies known, I Zw 18 and SBS 0335-052 to determine
the helium abundance. The underlying stellar absorption strongly influences the
observed intensities of He I emission lines in the brightest NW component of I
Zw 18, and hence this component should not be used for primordial He abundance
determination. The effect of underlying stellar absorption, though present, is
much smaller in the SE component. Assuming all systematic uncertainties are
negligible, the He mass fraction derived in this component is Y =
0.243+/-0.007. The high signal-to-noise ratio spectrum (> 100 in the continuum)
of SBS 0335-052 allows us to measure the helium mass fraction with a precision
better than 2% -- 5% in nine different regions along the slit. Assuming all
systematic uncertainties are negligible, the weighted mean He mass fraction in
SBS 0335-052 is Y = 0.2437+/-0.0014 when the three He I 4471, 5876 and 6678
emission lines are used, and is 0.2463+/-0.0015 when the He I 4471 emission
line is excluded. The weighted mean helium mass fraction in the two most
metal-deficient BCGs I Zw 18 and SBS 0335-052, Y=0.2462+/-0.0015, after
correction for the stellar He production results in a primordial He mass
fraction Yp = 0.2452+/-0.0015. The derived Yp leads to a baryon-to-photon ratio
of (4.7+/-1.0) 10^{-10}, consistent with the values derived from the primordial
D and 7Li abundances, and supporting the standard big bang nucleosynthesis
theory. For the most consistent set of primordial D, 4He, and 7Li abundances we
derive an equivalent number of light neutrino species 3.0+/-0.3 (95% C.L.).Comment: 28 pages, 10 figures. To appear in Ap
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