310 research outputs found
The Formation of Crystalline Dust in AGB Winds from Binary Induced Spiral Shocks
As stars evolve along the Asymptotic Giant Branch, strong winds are driven
from the outer envelope. These winds form a shell, which may ultimately become
a planetary nebula. Many planetary nebulae are highly asymmetric, hinting at
the presence of a binary companion. Some post-Asymptotic Giant Branch objects
are surrounded by torii of crystalline dust, but there is no generally accepted
mechanism for annealing the amorphous grains in the wind to crystals. In this
Letter, we show that the shaping of the wind by a binary companion is likely to
lead to the formation of crystalline dust in the orbital plane of the binary.Comment: Submitted to ApJ
Anisotropic Galactic Outflows and Enrichment of the Intergalactic Medium. I: Monte Carlo Simulations
We have developed an analytical model to describe the evolution of
anisotropic galactic outflows. With it, we investigate the impact of varying
opening angle on galaxy formation and the evolution of the IGM. We have
implemented this model in a Monte Carlo algorithm to simulate galaxy formation
and outflows in a cosmological context. Using this algorithm, we have simulated
the evolution of a comoving volume of size [12h^(-1)Mpc]^3 in the LCDM
universe. Starting from a Gaussian density field at redshift z=24, we follow
the formation of ~20,000 galaxies, and simulate the galactic outflows produced
by these galaxies. When these outflows collide with density peaks, ram pressure
stripping of the gas inside the peak may result. This occurs in around half the
cases and prevents the formation of galaxies. Anisotropic outflows follow the
path of least resistance, and thus travel preferentially into low-density
regions, away from cosmological structures (filaments and pancakes) where
galaxies form. As a result, the number of collisions is reduced, leading to the
formation of a larger number of galaxies. Anisotropic outflows can
significantly enrich low-density systems with metals. Conversely, the
cross-pollution in metals of objects located in a common cosmological
structure, like a filament, is significantly reduced. Highly anisotropic
outflows can travel across cosmological voids and deposit metals in other,
unrelated cosmological structures.Comment: 32 pages, 9 figures (2 color). Revised version accepted in Ap
The Evolution of Optical Depth in the Ly-alpha Forest: Evidence Against Reionization at z~6
We examine the evolution of the IGM Ly-alpha optical depth distribution using
the transmitted flux probability distribution function (PDF) in a sample of 63
QSOs spanning absorption redshifts 1.7 < z < 5.8. The data are compared to two
theoretical optical depth distributions: a model distribution based on the
density distribution of Miralda-Escude et al. (2000) (MHR00), and a lognormal
distribution. We assume a uniform UV background and an isothermal IGM for the
MHR00 model, as has been done in previous works. Under these assumptions, the
MHR00 model produces poor fits to the observed flux PDFs at redshifts where the
optical depth distribution is well sampled, unless large continuum corrections
are applied. However, the lognormal optical depth distribution fits the data at
all redshifts with only minor continuum adjustments. We use a simple
parametrization for the evolution of the lognormal parameters to calculate the
expected mean transmitted flux at z > 5.4. The lognormal optical depth
distribution predicts the observed Ly-alpha and Ly-beta effective optical
depths at z > 5.7 while simultaneously fitting the mean transmitted flux down
to z = 1.6. If the evolution of the lognormal distribution at z < 5 reflects a
slowly-evolving density field, temperature, and UV background, then no sudden
change in the IGM at z ~ 6 due to late reionization appears necessary. We have
used the lognormal optical depth distribution without any assumption about the
underlying density field. If the MHR00 density distribution is correct, then a
non-uniform UV background and/or IGM temperature may be required to produce the
correct flux PDF. We find that an inverse temperature-density relation greatly
improves the PDF fits, but with a large scatter in the equation of state index.
[Abridged]Comment: 45 pages, 16 figures, submitted to Ap
Do We Need Stars to Reionize the Universe at High Redshifts? Early Reionization by Decaying Heavy Sterile Neutrinos
A remarkable result of the Wilkinson Microwave Anisotropy Probe (WMAP)
observations is that the universe was significantly reionized at large
redshifts. The standard explanation is that massive stars formed early and
reionized the universe around redshift z=17. Here we explore an alternative
possibility, in which the universe was reionized in two steps. An early boost
of reionization is provided by a decaying sterile neutrino, whose decay
products, relativistic electrons, result in partial ionization of the smooth
gas. We demonstrate that a neutrino with a mass of m_nu ~ 200 MeV and a decay
time of t ~ 4 * 10^{15} s can account for the electron scattering optical depth
tau=0.16 measured by WMAP without violating existing astrophysical limits on
the cosmic microwave and gamma ray backgrounds. Reionization is then completed
by subsequent star formation at lower redshifts. This scenario alleviates
constraints on structure formation models with reduced small-scale power, such
as those with a running or tilted scalar index, or warm dark matter models.Comment: Improved the discussion of electron-CMB interaction; describe the
reionization process in more detail; conclusions unchanged; few references
added (as accepted by ApJ
Is Double Reionization Physically Plausible?
Recent observations of z~6 quasars and the cosmic microwave background imply
a complex history to cosmic reionization. Such a history requires some form of
feedback to extend reionization over a long time interval, but the nature of
the feedback and how rapidly it operates remain highly uncertain. Here we focus
on one aspect of this complexity: which physical processes can cause the global
ionized fraction to evolve non-monotonically with cosmic time? We consider a
range of mechanisms and conclude that double reionization is much less likely
than a long, but still monotonic, ionization history. We first examine how
galactic winds affect the transition from metal-free to normal star formation.
Because the transition is actually spatially inhomogeneous and temporally
extended, this mechanism cannot be responsible for double reionization given
plausible parameters for the winds. We next consider photoheating, which causes
the cosmological Jeans mass to increase in ionized regions and hence suppresses
galaxy formation there. In this case, double reionization requires that small
halos form stars efficiently, that the suppression from photoheating is strong
relative to current expectations, and that ionizing photons are preferentially
produced outside of previously ionized regions. Finally, we consider H_2
photodissociation, in which the buildup of a soft ultraviolet background
suppresses star formation in small halos. This can in principle cause the
ionized fraction to temporarily decrease, but only during the earliest stages
of reionization. Finally, we briefly consider the effects of some of these
feedback mechanisms on the topology of reionization.Comment: 13 pages, 5 figures, in press at ApJ (reorganized significantly but
major conclusions unchanged
Investigation for the puzzling abundance pattern of the neutron-capture elements in the ultra metal-poor star: CS 30322-023
The s-enhanced and very metal-poor star CS 30322-023 shows a puzzling
abundance pattern of the neutron-capture elements, i.e. several neutron-capture
elements such as Ba, Pb etc. show enhancement, but other neutron-capture
elements such as Sr, Eu etc. exhibit deficient with respect to iron. The study
to this sample star could make people gain a better understanding of s- and
r-process nucleosynthesis at low metallicity. Using a parametric model, we find
that the abundance pattern of the neutron-capture elements could be best
explained by a star that was polluted by an AGB star and the CS 30322-023
binary system formed in a molecular cloud which had never been polluted by
r-process material. The lack of r-process material also indicates that the AGB
companion cannot have undergone a type-1.5 supernova, and thus must have had an
initial mass below 4.0M, while the strong N overabundance and the
absence of a strong C overabundance indicate that the companion's initial mass
was larger than 2.0M. The smaller s-process component coefficient of
this star illustrates that there is less accreted material of this star from
the AGB companion, and the sample star should be formed in the binary system
with larger initial orbital separation where the accretion-induced collapse
(AIC) mechanism can not work.Comment: 13 pages, 2 figure
Toward the Evidence of the Accretion Disk Emission in the Symbiotic Star RR Tel
In this paper, we argue that in the symbiotic star RR Tel the existence of an
accretion disk around the hot companion is strongly implied by the
characteristic features exhibited by the Raman-scattered O VI lines around 6830
\AA and 7088 \AA. High degrees of polarization and double-peaked profiles in
the Raman-scattered lines and single-peak profiles for other emission lines are
interpreted as line-of-sight effects, where the H I scatterers near the giant
see an incident double-peaked profile and an observer with a low inclination
sees single-peak profiles. It is predicted that different mass concentrations
around the accretion disk formed by a dusty wind may lead to the disparate
ratios of the blue peak strength to the red counterpart observed in the 6830
and 7088 features. We discuss the evolutionary links between symbiotic stars
and bipolar protoplanetary nebulae and conclude that the Raman scattering
processes may play an important role in investigation of the physical
properties of these objects.Comment: 11 pages, 3 figures, accepted for publication in the ApJ Letter
Spectrophotometric assessment of the effectiveness of Opalescence PF 10%: a 14-month clinical study
Objectives:
To evaluate the effectiveness of Opalescence PF 10% just after treatment, at 6-month follow-up and at a 14-month follow-up period.
Methods:
Opalescence PF 10% was applied nightly for 14 days. The color of teeth 11 and 21 of 17 subjects were measured with a spectrophotometer (L*; a*; b*) before treatment, just after treatment (14 days), after 6 months and after 14 months. Subjects were instructed to take note of any tooth sensitivity.
Results:
For all three components (L*, a* and b*) statistical significant differences (p < 0.05) in the values between base-line, after treatment (14 days later), after 6 months and after 14 months were found (Wilcoxon Signed Rank Sum Test). The decrease in L* was about 20% after 6 months and about 50% after 14 months. The a* value decreased approximately 14% after 6 months but was worse after 14 months than at the beginning. The b* value decreased the least with about 9% after 6 months and about 8% after 14 months. The decrease in View the MathML source was âŒ20% after 14 months. Less than 20% of the subjects experienced mild tooth sensitivity just after treatment.
Conclusion:
Significantly whiter teeth were found after treatment as well as after a 6-month follow-up period. The whiteness/brightness (L*) decreased with âŒ50% after 14 months and the a* value with âŒ50% after 7 months, while the yellowness (b* value) remained even after 14 months.
Clinical implications:
The product is an effective tooth whitener resulting in only low tooth sensitivity. Re-bleaching could be done at about 14 months.Web of Scienc
- âŠ