452 research outputs found
A Polarization Sequence for Type Ia Supernovae?
Early polarization observations on Type Ia supernovae (SNe Ia) may reveal the
geometry of supernova ejecta, and then put constraints on their explosion
mechanism and their progenitor model. We performed a literature search of SNe
Ia with polarization measurements and determined the polarization and relative
equivalent width (REW) of Si II 635.5-nm absorption feature at -5 days after
the maximum light. We found that either the distribution of observed
polarization degree is bimodal, i.e. the broad line SNe Ia have systematically
higher polarization than all other SNe Ia, or all kind of SNe Ia share the same
polarization sequence, i.e. the polarization of Si II 635.5-nm absorption
feature increases with the REW. We also discussed the potential meaning of the
discovery on the explosion mechanism and progenitor model of SNe Ia.Comment: 4 figures, accepted for publication in Ap
Structure and evolution of low mass W UMa type systems
The structure and evolution of low mass W UMa type contact binaries are
discussed by employing Eggleton's stellar evolution code. Assuming that these
systems completely satisfy Roche geometry, we calculate the relative radii of
both components of contact binaries in different contact depth between inner
and outer Roche lobes. We obtain a radius grid of contact binaries, and can
ensure the surfaces of two components lying on an equipotential surface by
interpolation using this radius grid when we follow the evolution of the
contact binaries. We assume that the energy transfer takes place in the
different regions of the common envelope to investigate the effects of the
region of energy transfer on the structure and evolution of contact binaries.
We find that the region of energy transfer has significant influence on the
structure and evolution of contact binaries, and conclude that the energy
transfer may occur in the outermost layers of the common convective envelope
for W-type systems, and this transfer takes place in the deeper layers of the
common envelope for A-type systems. Meanwhile, if we assume that the energy
transfer takes place in the outermost layers for our model with low total mass,
and find that our model steadily evolves towards a system with a smaller mass
ratio and a deeper envelope, suggesting that some A-type W UMa systems with low
total mass could be considered as the later evolutionary stages of W-subtype
systems, and that the surface temperature of the secondary excesses that of the
primary during the time when the primary expands rapidly, or the secondary
contracts rapidly, suggesting that W-subtype systems may be caused by expansion
of the primary, or by the contraction of the secondary.Comment: 12 pages, 10 figures, to be published in MNRA
Probability Distribution of Terrestrial Planets in Habitable Zones around Host Stars
With more and more exoplanets being detected, it is paid closer attention to
whether there are lives outside solar system. We try to obtain habitable zones
and the probability distribution of terrestrial planets in habitable zones
around host stars. Using Eggleton's code, we calculate the evolution of stars
with masses less than 4.00 \mo. We also use the fitting formulae of stellar
luminosity and radius, the boundary flux of habitable zones, the distribution
of semimajor axis and mass of planets and the initial mass function of stars.
We obtain the luminosity and radius of stars with masses from 0.08 to 4.00 \mo,
and calculate the habitable zones of host stars, affected by stellar effective
temperature. We achieve the probability distribution of terrestrial planets in
habitable zones around host stars. We also calculate that the number of
terrestrial planets in habitable zones of host stars is 45.5 billion, and the
number of terrestrial planets in habitable zones around K type stars is the
most, in the Milky Way.Comment: 6 pages, 5 figure
Post-merger evolution of carbon-oxygen + helium white dwarf binaries and the origin of R Coronae Borealis and extreme helium stars
Orbital decay by gravitational-wave radiation will cause some close-binary
white dwarfs (WDs) to merge within a Hubble time. The results from previous
hydrodynamical WD-merger simulations have been used to guide calculations of
the post-merger evolution of carbon-oxygen + helium (CO+He) WD binaries. Our
models include the formation of a hot corona in addition to a Keplerian disk.
We introduce a 'destroyeddisk' model to simulate the effect of direct disk
ingestion into the expanding envelope. These calculations indicate significant
lifetimes in the domain of the rare R Coronae Borealis (RCB) stars, before a
fast evolution through the domain of the hotter extreme helium (EHe) stars.
Surface chemistries of the resulting giants are in partial agreement with the
observed abundances of RCB and EHe stars. The production of 3He, 18O and 19F
are discussed. Evolutionary timescales combined with binary white-dwarf merger
rates from binary-star population synthesis are consistent with present-day
numbers of RCBs and EHes, provided that the majority come from relatively
recent (< 2Gyr) star formation. However, most RCBs should be produced by CO-WD
+ low-mass HeWD mergers, with the He-WD having a mass in the range 0.20-0.35
Msolar. Whilst, previously, a high He-WD mass (> 0.40 Msolar) was required to
match the carbon-rich abundances of RCB stars, the 'destroyed-disk' model
yields a high-carbon product with He-WD mass > 0.30 Msolar, in better agreement
with population synthesis results.Comment: 18 pages, 26 figures, accepted for publication in MNRA
The evolutionary status of W Ursae Majoris-type systems
Well-determined physical parameters of 130 W UMa systems have been collected
from the literature. Based on these data, the evolutionary status and dynamical
evolution of W UMa systems are investigated. It is found that there is no
evolutionary difference between W- and A-type systems in diagram which is
consistent with the results derived from the analysis of observed spectral
type, and diagrams of W UMa systems. and diagrams of W-
and A-type systems indicate that a large amount of energy should be transferred
from the more massive to the less massive component so that they are not in
thermal equilibrium and undergo thermal relaxation oscillation (TRO).
Meanwhile, the distribution of angular momentum, together with the distribution
of mass ratio, suggests that the mass ratio of the observed W UMa systems is
decreased with the decrease of their total mass. This could be the result of
the dynamical evolution of W UMa systems which suffer angular momentum loss
(AML) and mass loss due to magnetic stellar wind (MSW). Consequently, the tidal
instability forces these systems towards the lower q values and finally to fast
rotating single stars.Comment: 8 pages, 5 figures, accepted for publication in MNRA
Evolution of luminosity function and obscuration of AGN: connecting X-ray and Infrared
We present a detailed comparison between the 2-10 keV hard X-ray and infrared
(IR) luminosity function (LF) of active galactic nuclei (AGN). The composite
X-ray to IR spectral energy distributions (SEDs) of AGN used for connecting the
hard X-ray LF (HXLF) and IR LF (IRLF) are modeled with a simple but well tested
torus model based on the radiative transfer and photoionization code CLOUDY.
Four observational determinations of the evolution of 2-10 keV HXLF and six
evolution models of the obscured type-2 AGN fraction () have been
considered. The 8.0 and 15 \micron LFs for the total, unobscured type-1 and
obscured type-2 AGN are predicted from the HXLFs, and then compared with the
measurements currently available. We find that the IRLFs predicted from HXLFs
tend to underestimate the number of the most IR-luminous AGN. This is
independent of the choices of HXLF and , and even more obvious for the
HXLFs recently measured. We show that the discrepancy between the HXLFs and
IRLFs can be largely resolved when the anticorrelation between the UV to X-ray
slope and UV luminosity is appropriately
considered. We also discuss other possible explanations for the discrepancy,
such as the missing population of Compton-thick AGN and possible contribution
of star-formation in the host to the mid-IR. Meanwhile, we find that the HXLFs
and IRLFs of AGN can be more consistent with each other if the obscuration
mechanisms of quasars and Seyferts are assumed to be different, corresponding
to their different triggering and fueling mechanisms. More accurate
measurements of the IRLFs of AGN, especially that determined at smaller
redshift bins and more accurately separated to that for type-1 and type-2, are
very helpful for clarifying these interesting issues.Comment: 15 pages, 11 figures, accepted for publication in MNRA
Uncertainties in the calibrations of star formation rate
The calibrations of star formation rate (SFR) are prone to be affected by
many factors, such as metallicity, initial mass function (IMF), evolutionary
population synthesis (EPS) models and so on. In this paper we will discuss the
effects of binary interactions, metallicity, EPS models and IMF on several
widely used SFR calibrations based on the EPS models of Yunnan with and without
binary interactions, BC03, SB99, PEGASE and POPSTAR. The inclusion of binary
interactions makes these SFR conversion coefficients smaller (less than
0.2dex), and these differences increase with metallicity. The differences in
the calibration coefficient between SFR and the luminosity of
recombination line (C) and that between SFR and the ultraviolet
(UV) fluxes at 1500 and 2800\, (C), caused by IMF, are
independent of metallicity (0.03-0.33\,dex) except C when using the POPSTAR and C when using the
PEGASE models. Moreover, we find that is not suitable to the
linear calibration of SFR at low metallicities.
At last, we compare the effects of these several factors on the SFR
calibrations considered in this paper. The effects of metallicity/IMF and EPS
models on the C and C (the conversion coefficient
between SFR and the far-infrared flux) are the largest among these factors,
respectively. For the calibration between SFR and C, the
effects of these several factors are comparable.Comment: 17 pages, 7 figures, 9 tables, accepted by MNRA
Effect of Binary Fraction on Horizontal Branch Morphology under Tidally Enhanced Stellar Wind
Tidally enhanced stellar wind may affect horizontal branch (HB) morphology in
globular clusters (GCs) by enhancing the mass loss of primary star during
binary evolution. Lei et al. (2013a, 2013b) studied the effect of this kind of
wind on HB morphology in details, and their results indicated that binary is a
possible secondparameter (2P) candidate in GCs. Binary fraction is an very
important fact in the tidally-enhanced-stellar-wind model. In this paper, we
studied the effect of binary fraction on HB morphology by removing the effects
of metallicity and age. Five different binary fractions (i.e., 10%, 15%, 20%,
30% and 50%) are adopted in our model calculations. The synthetic HB
morphologies with different binary fractions are obtained at different
metallicities and ages. We found that, due to the great influence of
metallicity and age, the effect of binary fraction on HB morphology may be
masked by these two parameters. However, when the effects of metallicity and
age are removed, the tendency that HB morphologies become bluer with increasing
of binary fractions is clearly presented. Furthermore, we compared our results
with the observation by Milone et al. (2012). Our results are consistent well
with the observation at metalrich and metal-poor GCs. For the GCs with
intermediate metallicity, when the effect of age on HB morphology is removed, a
weak tendency that HB morphologies become bluer with increasing of binary
fractions is presented in all regions of GCs, which is consistent with our
results obtained in this metallicity range.Comment: 14 pages, 9 figures, 1 tables, accepted for publication in PAS
On the evolution of rotating accreting white dwarfs and type Ia supernovae
The potential importance of the angular momentum which is gained by accreting
white dwarfs (WDs) has been increasingly recognized in the context of type Ia
supernova (SN Ia) single-degenerate model. The expectation that the spin of the
WD can delay the explosion should help the single-degenerate model to be
consistent with the observed properties of most SNe Ia, in particular by
avoiding hydrogen contamination. In this article, we attempt to study the most
prominent single-degenerate supersoft (WD + MS) channel when the rotation of
accreting WDs is considered. We present a detailed binary population synthesis
study to examine the predicted population of SNe Ia for this channel. For our
standard model, we find that 77% of these SNe Ia explode with WD masses which
are low enough to be supported by solid-body rotation (1.378-1.5Msun); this is
a substantially higher proportion than found by previous work. Only 2% have WD
explosion masses >2.0Msun; these require the initial WD mass to be larger than
1.0 Msun. We further discuss the possible origin of the diversity of SNe Ia
from the pre- and post- accretion properties of the WDs in this population. We
also suggest that some SN Ia progenitors with substantial circumstellar
hydrogen, including some apparent type IIn SNe, might be related to WDs which
required support from differential rotation to avoid explosion, since these can
still be accreting from hydrogen-rich donors with a relatively high
mass-transfer rate at the time of the SN explosion.Comment: 22 pages, 15 figures, accepted for publication in MNRA
Binary Stellar Population Synthesis Study of Elliptical Galaxies
We determine relative stellar ages and metallicities mainly for about 80
elliptical galaxies in low and high density environments via the latest binary
stellar population (BSP) synthesis model and test a latest hierarchical
formation model of elliptical galaxies which adopted the new CDM
cosmology for the first time. The stellar ages and metallicities of galaxies
are estimated from two high-quality published spectra line indices, i.e.
H and [MgFe]. The results show that elliptical galaxies have stellar
populations older than 3.9 Gyr and more metal rich than 0.02. Most of our
results are in agreement with predictions of the model: First, elliptical
galaxies in denser environment are redder and have older populations than field
galaxies. Second, elliptical galaxies with more massive stellar components are
redder while have older and more metal rich populations than less massive ones.
Third, the most massive galaxies are shown to have the oldest and most metal
rich stars. However, some of our results are found to be different with
predictions of the galaxy formation model, i.e. the metallicity distributions
of low- and high-density elliptical galaxies and the relations relating to
cluster-centric distance.Comment: 15 pages, 10 figures, accepted for publication to ChJA
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