677 research outputs found
On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars
About a quarter of all post-asymptotic giant branch (AGB) stars are
hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated
H-deficient stars by a (very) late thermal pulse scenario where the
hydrogen-rich envelope is mixed with the helium-rich intershell layer.
Depending on the particular time at which the final flash occurs, the entire
hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars
and their evolution are yet not understood. A small group of very hot,
helium-dominated stars is formed by O(He)-type stars. We performed a detailed
spectral analysis of ultraviolet and optical spectra of four O(He) stars by
means of state-of-the-art non-LTE model-atmosphere techniques. We determined
effective temperatures, surface gravities, and the abundances of H, He, C, N,
O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss
rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss.
The comparison with evolutionary models shows that the status of the O(He)
stars remains uncertain. Their abundances match predictions of a double helium
white dwarf merger scenario, suggesting that they might be the progeny of the
compact and of the luminous helium-rich sdO-type stars. The existence of
planetary nebulae that do not show helium enrichment around every other O(He)
star, precludes a merger origin for these stars. These stars must have formed
in a different way, for instance via enhanced mass-loss during their post-AGB
evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant
or AGB star. A helium-dominated stellar evolutionary sequence exists, that may
be fed by different types of mergers or CE scenarios. It appears likely, that
all these pass through the O(He) phase just before they become white dwarfs.Comment: 29 pages, 27 figures, accepted for publication in A&
Analysis of cool DO-type white dwarfs from the Sloan Digital Sky Survey Data Release 10
We report on the identification of 22 new cool DO-type white dwarfs (WD)
detected in Data Release 10 (DR10) of the Sloan Digital Sky Survey (SDSS).
Among them, we found one more member of the so-called hot-wind DO WDs, which
show ultrahigh excitation absorption lines. Our non-LTE model atmosphere
analyses of these objects and two not previously analyzed hot-wind DO WDs,
revealed effective temperatures and gravities in the ranges Teff=45-80kK and
log g= 7.50-8.75. In eight of the spectra we found traces of C (0.001-0.01, by
mass). Two of these are the coolest DO WDs ever discovered that still show a
considerable amount of C in their atmospheres. This is in strong contradiction
with diffusion calculations, and probably, similar to what is proposed for DB
WDs, a weak mass-loss is present in DO WDs. One object is the most massive DO
WD discovered so far with a mass of 1.07 M_sun if it is an ONe-WD or 1.09 M_sun
if it is a CO-WD. We furthermore present the mass distribution of all known hot
non-DA (pre-) WDs and derive the hot DA to non-DA ratio for the SDSS DR7
spectroscopic sample. The mass distribution of DO WDs beyond the wind limit
strongly deviates from the mass distribution of the objects before the wind
limit. We address this phenomenon by applying different evolutionary input
channels. We argue that the DO WD channel may be fed by about 13% by
post-extreme-horizontal branch stars and that PG1159 stars and O(He) stars may
contribute in a similar extent to the non-DA WD channel.Comment: 13 pages, accepted for publication in A&
The rapid evolution of the exciting star of the Stingray Nebula
SAO244567, the exciting star of the Stingray nebula, is rapidly evolving.
Previous analyses suggested that it has heated up from an effective temperature
of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic
giant branch evolution suggests a relatively high mass while previous analyses
indicate a low-mass star. Fitting line profiles from static and expanding
non-LTE model atmospheres to the observed UV and optical spectra, taken during
1988-2013, allowed us to study the temporal change of effective temperature,
surface gravity, mass-loss rate, and terminal wind velocity. In addition, we
determined the chemical composition of the atmosphere. We find that the central
star has steadily increased its effective temperature from 38kK in 1988 to a
peak value of 60kK in 2002. During the same time, the star was contracting, as
concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a
drop in luminosity. Simultaneously, the mass-loss rate declined from log
(dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from
1800km/s to 2800km/s. Since around 2002, the star stopped heating and has
cooled down again to 55kK by 2006. It has a largely solar surface composition
with the exception of slightly subsolar carbon, phosphorus, and sulfur. By
comparison with stellar-evolution calculations, we confirm that SAO244567 must
be a low-mass star (M < 0.55 Msun). However, the slow evolution of the
respective stellar evolutionary models is in strong contrast to the observed
fast evolution and the young planetary nebula with a kinematical age of only
about 1000 years. We speculate that the star could be a late He-shell flash
object. Alternatively, it could be the outcome of close-binary evolution. Then
SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the
common-envelope phase, during which the planetary nebula was ejected.Comment: 16 pages, 13 figures, accepted for publication in A&
Twinning in muskox and the cytogenetic investigation of a freemartin
The occurrence of twinning has been documented for muskox in both wild and captive populations. Of two known captive twin births only one set survived beyond 120 days. In both cases the twins were male-female pairs and both females showed abnormal sexual development. Two sets of stillborn twins have also been recorded. All four stillborn fetuses were female and none showed anomalies of the reproductive tract upon post-mortem examination. Blood cultures from the surviving male and female twins revealed that both were chimeric, indicating the admixture of fetal blood. Fibroblast cultures were normal for the respective sex of each individual. The freemartin heifer had anatomical abnormalities of the clitoris as well as the secondary sex characteristics of a male
Operation of Cool Thermal Energy Storage to Increase Renewable Energy Utilization
Recent international agreements on climate change aim to increase the production of electricity derived from renewable energy resources. Renewable energy generation can be pursued on both an individual building and utility scale. Due to the intermittent nature of renewables, some form of energy storage is essential to bridge diurnal mismatches between generation and demand. Air-conditioning loads associated with commercial buildings dominate peak electricity demand on the utility grid in some areas and climates. Therefore, Cool Thermal Energy Storage (CTES) is a relatively technically mature and inexpensive means of providing this “storage†and balancing supply/demand mismatches, thereby enabling the success of increased renewable energy penetration. Electrical energy generated by renewables during periods of higher availability can be used to run chillers that charge CTES systems. The stored thermal energy can subsequently be used to meet air-conditioning demand during periods of low renewable energy resource availability.  In this work, the U.S. Department of Energy Commercial Reference Building Model for a secondary school is used to obtain simulated cooling loads that are met by a combination of two chillers and a stratified chilled water thermal storage system. Control strategies are designed to charge the thermal storage system when renewable resources are available and discharge storage to meet building cooling loads during periods with low or no renewable energy resource. One optimization target is the fraction of the chiller energy consumption met by renewable power. This metric is one that may be of interest to electric utilities trying to manage a grid with increasing renewable penetration. An alternative optimization target is the net economic benefit to the building owner assuming on-site, small scale renewable generation and thermal storage. This metric is based on equipment costs, net electric demand after wind and/or solar generation offsets the chiller electric demand, and time-of-use electricity rate structures.  The results show that there is a trade-off between maximizing the use of renewable power and life-cycle cost, but a storage system designed to optimize either variable will be more cost effective and utilize the renewable resource better than a system without storage. The analysis is carried out for locations in Texas and California. These results suggest that CTES may be a technology enabling utilities to reach higher penetration of renewables while avoiding the so-called “duck curve†generation ramp caused by the time mismatch between the renewable generation and demand peaks
Age Problem in the Holographic Dark Energy Model
In this note, we test the original holographic dark energy model with some
old high redshift objects. The main idea is very simple: the universe cannot be
younger than its constituents. We find that the original holographic dark
energy model can be ruled out, unless a lower Hubble constant is taken.Comment: 12 pages, 3 tables, 3 figures, revtex4; v2: accepted for publication
in Phys. Rev. D; v3: published versio
A far-UV survey of three hot, metal-polluted white dwarf stars: WD0455-282, WD0621-376, and WD2211-495
Using newly obtained high-resolution data () from the
\textit{Hubble Space Telescope}, and archival UV data from the \textit{Far
Ultraviolet Spectroscopic Explorer} we have conducted a detailed UV survey of
the three hot, metal-polluted white dwarfs WD0455-282, WD0621-376, and
WD2211-495. Using bespoke model atmospheres we measured , log
, and photospheric abundances for these stars. In conjunction with data from
Gaia we measured masses, radii, and gravitational redshift velocities for our
sample of objects. We compared the measured photospheric abundances with those
predicted by radiative levitation theory, and found that the observed Si
abundances in all three white dwarfs, and the observed Fe abundances in
WD0621-376 and WD2211-495, were larger than those predicted by an order of
magnitude. These findings imply not only an external origin for the metals, but
also ongoing accretion, as the metals not supported by radiative levitation
would sink on extremely short timescales. We measured the radial velocities of
several absorption features along the line of sight to the three objects in our
sample, allowing us to determine the velocities of the photospheric and
interstellar components along the line of sight for each star. Interestingly,
we made detections of circumstellar absorption along the line of sight to
WD0455-282 with three velocity components. To our knowledge, this is the first
such detection of multi-component circumstellar absorption along the line of
sight to a white dwarf.Comment: 19 pages, 23 figures, 8 tables. Accepted for publication in the
Monthly Notices of the Royal Astronomical Societ
Evolved Open-Endedness in Cultural Evolution: A New Dimension in Open-Ended Evolution Research
The goal of Artificial Life research, as articulated by Chris Langton, is "to
contribute to theoretical biology by locating life-as-we-know-it within the
larger picture of life-as-it-could-be" (1989, p.1). The study and pursuit of
open-ended evolution in artificial evolutionary systems exemplifies this goal.
However, open-ended evolution research is hampered by two fundamental issues;
the struggle to replicate open-endedness in an artificial evolutionary system,
and the fact that we only have one system (genetic evolution) from which to
draw inspiration. Here we argue that cultural evolution should be seen not only
as another real-world example of an open-ended evolutionary system, but that
the unique qualities seen in cultural evolution provide us with a new
perspective from which we can assess the fundamental properties of, and ask new
questions about, open-ended evolutionary systems, especially in regard to
evolved open-endedness and transitions from bounded to unbounded evolution.
Here we provide an overview of culture as an evolutionary system, highlight the
interesting case of human cultural evolution as an open-ended evolutionary
system, and contextualise cultural evolution under the framework of (evolved)
open-ended evolution. We go on to provide a set of new questions that can be
asked once we consider cultural evolution within the framework of open-ended
evolution, and introduce new insights that we may be able to gain about evolved
open-endedness as a result of asking these questions.Comment: 26 pages, 1 figure, 1 table, submitted to Artificial Life journal
(special issue on Open-Ended Evolution
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