189 research outputs found
The Primordial Helium Abundance: Towards Understanding and Removing the Cosmic Scatter in the dY/dZ Relation
We present results from photoionization models of low-metallicity HII
regions. These nebulae form the basis for measuring the primordial helium
abundance. Our models show that the helium ionization correction factor (ICF)
can be non-negligible for nebulae excited by stars with effective temperatures
larger than 40,000 K. Furthermore, we find that when the effective temperature
rises to above 45,000 K, the ICF can be significantly negative. This result is
independent of the choice of stellar atmosphere. However, if an HII region has
an [O III] 5007/[O I] 6300 ratio greater than 300, then our models show that,
regardless of its metallicity, it will have a negligibly small ICF. A similar,
but metallicity dependent, result was found using the [O III] 5007/H
ratio. These two results can be used as selection criteria to remove nebulae
with potentially non-negligible ICFs. Using our metallicity independent
criterion on the data of Izotov & Thuan (1998) results in a 20% reduction of
the rms scatter about the best fit line. A fit to the selected data
results in a slight increase of the value of the primordial helium abundance.Comment: 10 pages, 5 figures, accepted by the Ap
The kinematics of the most oxygen-poor planetary nebula PN G135.9+55.9
PN G135.9+55.9 is a compact, high excitation nebula that has been identified
recently as the most oxygen-poor halo planetary nebula. Given its very peculiar
characteristics and potential implications in the realms of stellar and
Galactic evolution, additional data are needed to firmly establish its true
nature and evolutionary history. Here we present the first long-slit, high
spectral resolution observations of this object in the lines of H and
He II 4686. The position-velocity data are shown to be compatible with the
interpretation of PN G135.9+55.9 being a halo planetary nebula. In both
emission lines, we find the same two velocity components that characterize the
kinematics as that of an expanding elliptical envelope. The kinematics is
consistent with a prolate ellipsoidal model with axis ratio about 2:1, a
radially decreasing emissivity distribution, a velocity distribution that is
radial, and an expansion velocity of 30 km/s for the bulk of the material. To
fit the observed line profiles, this model requires an asymmetric matter
distribution, with the blue-shifted emission considerably stronger than the
red-shifted emission. We find that the widths of the two velocity components
are substantially wider than those expected due to thermal motions, but
kinematic structure in the projected area covered by the slit appears to be
sufficient to explain the line widths. The present data also rule out the
possible presence of an accretion disk in the system that could have been
responsible for a fraction of the H flux, further supporting the
planetary nebula nature of PN G135.9+55.9.Comment: accepted by Astronomy & Astrophysic
The effect of collisional enhancement of Balmer lines on the determination of the primordial helium abundance
This paper describes a new determination of the primordial helium abundance
(Y_P), based on the abundance analysis of five metal-poor extragalactic HII
regions. For three regions of the sample (SBS 0335-052, I Zw 18, and H29) we
present tailored photoionization models based on improved calculations with
respect to previous models. In particular, we use the photoionization models to
study quantitatively the effect of collisional excitation of Balmer lines on
the determination of the helium abundance (Y) in the individual regions. This
effect is twofold: first, the intensities of the Balmer lines are enhanced with
respect to the pure recombination value, mimicking a higher hydrogen abundance;
second, the observed reddening is larger than the true extinction, due to the
differential effect of collisions on different Balmer lines. In addition to
these effects, our analysis takes into account the following features of HII
regions: (i) the temperature structure, (ii) the density structure, (iii) the
presence of neutral helium, (iv) the collisional excitation of the HeI lines,
(v) the underlying absorption of the HeI lines, and (vi) the optical thickness
of the HeI lines. The object that shows the highest increase in Y after the
inclusion of collisional effects in the analysis is SBS 0335-052, whose helium
abundance has been revised by Delta Y = +0.0107. The revised Y values for the
five objects in our sample yield an increase of +0.0035 in Y_P, giving Y_P =
0.2391 +/- 0.0020.Comment: 59 pages, 8 figures. AAS Latex. Accepted for publication in the
Astrophysical Journa
Ionization Corrections For Low-Metallicity H II Regions and the Primordial Helium Abundance
Helium and hydrogen recombination lines observed in low-metallicity,
extragalactic H II regions provide the data used to infer the primordial helium
mass fraction, Y_P. The ionization corrections for unseen neutral helium (or
hydrogen) are usually assumed to be absent; i.e., the ionization correction
factor is taken to be unity (icf = 1). In this paper we revisit the question of
the icf for H II regions ionized by clusters of young, hot, metal-poor stars.
Our key result is that for the H II regions used in the determination of Y_P,
there is a ``reverse'' ionization correction: icf < 1. We explore the effect on
the icf of more realistic inhomogeneous H II region models and find that for
those regions ionized by young stars, with ``hard'' radiation spectra, the icf
is reduced further below unity. In Monte Carlos using H II region data from the
literature (Izotov and Thuan 1998) we estimate a reduction in the published
value of Y_P of order 0.003, which is roughly twice as large as the quoted
statistical error in the Y_P determination.Comment: 23 pages, 2 postscript figures; ApJ accepted; minor change
Disordered Bose Einstein Condensates with Interaction in One Dimension
We study the effects of random scatterers on the ground state of the
one-dimensional Lieb-Liniger model of interacting bosons on the unit interval
in the Gross-Pitaevskii regime. We prove that Bose Einstein condensation
survives even a strong random potential with a high density of scatterers. The
character of the wave function of the condensate, however, depends in an
essential way on the interplay between randomness and the strength of the
two-body interaction. For low density of scatterers or strong interactions the
wave function extends over the whole interval. High density of scatterers and
weak interaction, on the other hand, leads to localization of the wave function
in a fragmented subset of the interval
SBS 0335-052W: The Lowest-Metallicity Star-Forming Galaxy Known
We present 4-meter Kitt Peak telescope and 6.5-meter MMT spectrophotometry of
the extremely low-metallicity galaxy SBS 0335-052W, the western companion of
the blue compact dwarf galaxy SBS 0335-052E. These observations have been
combined with published 10-meter Keck data to derive for the brightest region
of SBS 0335-052W an oxygen abundance 12+logO/H=7.12+/-0.03. This makes SBS
0335-052W the lowest metallicity star-forming galaxy known in the local
universe. Using a Monte Carlo technique, we fit the spectral energy
distribution of SBS 0335-052W to derive the age of the oldest stars
contributing to its optical light. We find that star formation in SBS 0335-052W
began less than 500 Myr ago, making it a likely nearby young dwarf galaxy.Comment: 13 pages, 3 figures, accepted for publication in the Astrophysical
Journa
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