454 research outputs found
A New Approach to Systematic Uncertainties and Self-Consistency in Helium Abundance Determinations
Tests of big bang nucleosynthesis and early universe cosmology require
precision measurements for helium abundance determinations. However, efforts to
determine the primordial helium abundance via observations of metal poor H II
regions have been limited by significant uncertainties. This work builds upon
previous work by providing an updated and extended program in evaluating these
uncertainties. Procedural consistency is achieved by integrating the hydrogen
based reddening correction with the helium based abundance calculation, i.e.,
all physical parameters are solved for simultaneously. We include new atomic
data for helium recombination and collisional emission based upon recent work
by Porter et al. and wavelength dependent corrections to underlying absorption
are investigated. The set of physical parameters has been expanded here to
include the effects of neutral hydrogen collisional emission. Because of a
degeneracy between the solutions for density and temperature, the precision of
the helium abundance determinations is limited. Also, at lower temperatures (T
\lesssim 13,000 K) the neutral hydrogen fraction is poorly constrained
resulting in a larger uncertainty in the helium abundances. Thus the derived
errors on the helium abundances for individual objects are larger than those
typical of previous studies. The updated emissivities and neutral hydrogen
correction generally raise the abundance. From a regression to zero
metallicity, we find Y_p as 0.2561 \pm 0.0108, in broad agreement with the WMAP
result. Tests with synthetic data show a potential for distinct improvement,
via removal of underlying absorption, using higher resolution spectra. A small
bias in the abundance determination can be reduced significantly and the
calculated helium abundance error can be reduced by \sim 25%.Comment: 51 pages, 13 figure
The metallicities of UM151, UM408 and A1228+12 revisited
We present the results of new spectrophotometry and heavy element abundance
determinations for 3 dwarf galaxies UM151, UM408 and A1228+12 (RMB132). These
galaxies have been claimed in the literature to have very low metallicities,
corresponding to log(O/H)+12 < 7.65, that are in the metallicity range of some
candidate local young galaxies. We present higher S/N data for these three
galaxies. UM151 and UM408 have significantly larger metallicities: log(O/H)+12
= 8.5 and 7.93, respectively. For A1228+12 our new log(O/H)+12 = 7.73 is close
to that recalculated from earlier data (7.68). Thus, the rederived
metallicities allow us to remove these objects from the list of galaxies with Z
< 1/20 Z_Sun.Comment: LaTeX, 8 pages with 3 Postscript figures, A&A in pres
A HST study of the stellar populations in the cometary dwarf irregular galaxy NGC 2366
We present V and I photometry of the resolved stars in the cometary dwarf
irregular galaxy NGC 2366, using Wide Field Planetary Camera 2 images obtained
with the Hubble Space Telescope. The resulting color-magnitude diagram reaches
down to I~26.0 mag. It reveals not only a young population of blue
main-sequence stars (age <30 Myr) but also an intermediate-age population of
blue and red supergiants (20 Myr<age<100 Myr), and an older evolved populations
of asymptotic giant branch (AGB) stars (age >100 Myr) and red giant branch
(RGB) stars (age >1 Gyr). The measured magnitude I=23.65+/-0.10 mag of the RGB
tip results in a distance modulus m-M=27.67+/-0.10, which corresponds to a
distance of 3.42+/-0.15 Mpc, in agreement with previous distance
determinations. The youngest stars are associated with the bright complex of
HII regions NGC 2363=Mrk 71 in the southwest extremity of the galaxy. As a
consequence of the diffusion and relaxation processes of stellar ensembles, the
older the stellar population is, the smoother and more extended is its spatial
distribution. An underlying population of older stars is found throughout the
body of NGC 2366. The most notable feature of this older population is the
presence of numerous relatively bright AGB stars. The number ratio of AGB to
RGB stars and the average absolute brightness of AGB stars in NGC 2366 are
appreciably higher than in the BCD VII Zw 403, indicating a younger age of the
AGB stars in NGC 2366. In addition to the present burst of age <100 Myr, there
has been strong star formation activity in the past of NGC 2366, from ~100 Myr
to <3 Gyr ago.Comment: 32 pages, 15 figures, accepted for publication in the Astrophysical
Journa
High-frequency EPR study of crude oils
Four different samples of crude oil were studied by means of high-frequency W-band (94 GHz) electron paramagnetic resonance (EPR) spectroscopy with the aim to develop new methods of crude oil quality control. High spectral resolution of W-band allowed to avoid an overlap of spectra contributors. The ratio K between the integral intensity of the low-field EPR component of the vanadyl complexes to that of free radical line was chosen as an attribute of each sample. Using the K-parameters and EPR spectra simulations the crude oil leaking between adjacent horizons is shown. Pulsed EPR experiments allowed detecting free radicals signals only. It is demonstrated that the extracted transverse relaxation time could be used as an additional parameter which characterizes the origin of the crude oil and nature of the oil paramagnetic centers. © Published under licence by IOP Publishing Ltd
Protostellar Collapse with Various Metallicities
The thermal and chemical evolution of gravitationally collapsing protostellar
clouds is investigated, focusing attention on their dependence on metallicity.
Calculations are carried out for a range of metallicities spanning the local
interstellar value to zero. During the time when clouds are transparent to
continuous radiation, the temperatures are higher for those with lower
metallicity, reflecting lower radiative ability. However, once the clouds
become opaque, in the course of the adiabatic contraction of the transient
cores, their evolutionary trajectories in the density-temperature plane
converge to a unique curve that is determined by only physical constants. The
trajectories coincide with each other thereafter. Consequently, the size of the
stellar core at the formation is the same regardless of the gas composition of
the parent cloud.Comment: 30 pages. The Astrophysical Journal, 533, in pres
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