272 research outputs found
On the formation of oxygen-neon white dwarfs in close binary systems
The evolution of a star of initial mass 10 , and metallicity in a Close Binary System (CBS) is followed from its main sequence until
an ONe degenerate remnant forms. Restrictions have been made on the
characteristics of the companion as well as on the initial orbital parameters
in order to avoid the occurrence of reversal mass transfer before carbon is
ignited in the core. The system undergoes three mass loss episodes. The first
and second ones are a consequence of a case B Roche lobe overflow. During the
third mass loss episode stellar winds may play a role comparable to, or even
more important than Roche lobe overflow. In this paper, we extend the
previously existing calculations of stars of intermediate mass belonging to
close binary systems by following carefully the carbon burning phase of the
primary component. We also propose different possible outcomes for our scenario
and discuss the relevance of our findings. In particular, our main result is
that the resulting white dwarf component of mass more likely has
a core composed of oxygen and neon, surrounded by a mantle of carbon-oxygen
rich material. The average abundances of the oxygen-neon rich core are , , and . This result has important consequences for the Accretion
Induced Collapse scenario. The average abundances of the carbon-oxygen rich
mantle are , and . The existence of
this mantle could also play a significant role in our understanding of
cataclysmic variables.Comment: 13 pages, 12 figures, accepted for publication in A&
On the formation of Super-AGB stars in intemediate mass close binary systems
The evolution of a star of initial mass 9 M_s, and Z = 0.02 in a Close Binary
System is followed in the presence of different mass companions in order to
study their influence on the final evolutionary stages and, in particular, on
the structure and composition of the remnant components. We study two extreme
cases. In the first one the mass of the secondary is 8 M_s, whereas in the
second one the mass was assumed to be 1 M_s. For the first of those cases we
have also explored the possible outcomes of both conservative and
non-conservative mass-loss episodes. During the first mass transfer episode,
several differences arise between the models. The system with the more extreme
mass ratio is not able to survive the 1st. Roche lobe overflow, and spiral-in
of the secondary onto the envelope of the primary is most likely. The system
formed by two stars of comparable mass undergoes two mass transfer episodes in
which the primary is the donor. We have performed two sets of calculations
corresponding to this case in order to account for conservative and
non-conservative mass transfer during the first mass loss episode. One of our
main results is that for the non-conservative case the secondary becomes a
Super-AGB. Such a star undergoes a final dredge-up episode, similar to that of
a single star of comparable mass. The primary components do not undergo a
Super-AGB phase, but instead a carbon-oxygen white dwarf is formed in both
cases, before reversal mass transfer occurs. However, given the extreme mass
ratios at this stage between the components of the binary system, the
possibility of merger episodes remains likely. We also discuss the presumable
final outcomes of the system and possible observational counterparts.Comment: 10 pages, 12figures, accepted for publication in A&
The Resolved Stellar Population of the Post-Starburst Galaxy NGC 1569
We present WFPC2-HST photometry (in the F439W and F555W bands) of the
resolved stars in NGC 1569. The derived color-magnitude diagram (CMD) contains
2800 objects down to the 26th mag, and is complete down to the 23rd. Adopting
the literature distance modulus and reddening, the CMD samples stars more
massive than 4 Mo, corresponding to a look-back time of 0.15 Gyr approximately.
The data are compared to synthetic CMDs to derive the recent star formation
history in NGC 1569. It is found that the observed field has experienced a
global burst of star formation (SF) lasting at least 0.1 Gyr, and ended a few
Myr ago. During the burst, the SF rate was approximately constant, and, if
quiescent periods occurred, they lasted less than 10 Myr. The level of the SF
rate was very high: 0.5 Mo/yr for a Salpeter IMF, in stars with mass ranging
from 0.1 to 120 Mo. When scaled for the surveyed area, the SF rate is
approximately 100 times larger than found in the most active dwarf irregulars
in the Local Group. The data are consistent with a Salpeter IMF, or slightly
steeper exponents. We discuss the implications of our results in the general
context of the evolution of dwarf galaxies.Comment: 63 pages including 2 tables and 12 figures, to be published in The
Astrophysical Journa
Description of the Scenario Machine
We present here an updated description of the "Scenario Machine" code. This
tool is used to carry out a population synthesis of binary stars. Previous
version of the description can be found at
http://xray.sai.msu.ru/~mystery//articles/review/contents.htmlComment: 32 pages, 3 figures. Corrected typo
Further investigation of white dwarfs in the open clusters NGC2287 and NGC3532
We report the results of a CCD imaging survey, complimented by astrometric
and spectroscopic follow-up studies, that aims to probe the fate of
heavy-weight intermediate mass stars by unearthing new, faint, white dwarf
members of the rich, nearby, intermediate age open clusters NGC3532 and
NGC2287. We identify a total of four white dwarfs with distances, proper
motions and cooling times which can be reconciled with membership of these
populations. We find that WDJ0643-203 in NGC2287, with an estimated mass of
M=1.02-1.16Msun, is potentially the most massive white dwarf so far identified
within an open cluster. Guided by the predictions of modern theoretical models
of the late-stage evolution of heavy-weight intermediate mass stars, we
conclude that there is a distinct possibility it has a core composed of O and
Ne. We also determine that despite the cooling times of the three new white
dwarfs in NGC3532 and the previously known degenerate member NGC3532-10
spanning ~90Myr, they all have remarkably similar masses (M~0.9-1Msun). This is
fully consistent with the results from our prior work on a heterogeneous sample
of ~50 white dwarfs from 12 stellar populations, on the basis of which we
argued that the stellar initial mass-final mass relation is less steep at
Minit>4Msun than in the adjacent lower initial mass regime. This change in the
gradient of the relation could account for the secondary peak observed in the
mass distribution of the field white dwarf population and mitigate the need to
invoke close binary evolution to explain its existence. Spectroscopic
investigation of numerous additional candidate white dwarf members of NGC3532
unearthed by a recent independent study would be useful to confirm (or
otherwise) these conclusions.Comment: 8 Figures, 8 tables. Accepted for publication in MNRA
Evolutionary and pulsational properties of white dwarf stars
Abridged. White dwarf stars are the final evolutionary stage of the vast
majority of stars, including our Sun. The study of white dwarfs has potential
applications to different fields of astrophysics. In particular, they can be
used as independent reliable cosmic clocks, and can also provide valuable
information about the fundamental parameters of a wide variety of stellar
populations, like our Galaxy and open and globular clusters. In addition, the
high densities and temperatures characterizing white dwarfs allow to use these
stars as cosmic laboratories for studying physical processes under extreme
conditions that cannot be achieved in terrestrial laboratories. They can be
used to constrain fundamental properties of elementary particles such as axions
and neutrinos, and to study problems related to the variation of fundamental
constants.
In this work, we review the essentials of the physics of white dwarf stars.
Special emphasis is placed on the physical processes that lead to the formation
of white dwarfs as well as on the different energy sources and processes
responsible for chemical abundance changes that occur along their evolution.
Moreover, in the course of their lives, white dwarfs cross different
pulsational instability strips. The existence of these instability strips
provides astronomers with an unique opportunity to peer into their internal
structure that would otherwise remain hidden from observers. We will show that
this allows to measure with unprecedented precision the stellar masses and to
infer their envelope thicknesses, to probe the core chemical stratification,
and to detect rotation rates and magnetic fields. Consequently, in this work,
we also review the pulsational properties of white dwarfs and the most recent
applications of white dwarf asteroseismology.Comment: 85 pages, 28 figures. To be published in The Astronomy and
Astrophysics Revie
Evolution of the number of accreting white dwarfs with shell nuclear burning and of occurrence rate of SN Ia
We analyze temporal evolution of the number of accreting white dwarfs with
shell hydrogen burning in semidetached and detached binaries. We consider a
stellar system in which star formation lasts for 10 Gyr with a constant rate,
as well as a system in which the same amount of stars is formed in a single
burst lasting for 1 Gyr. Evolution of the number of white dwarfs is confronted
to the evolution of occurrence rate of events that usually are identified with
SN Ia or accretion-induced collapses, i.e. with accumulation of Chandrasekhar
mass by a white dwarf or a merger of a pair of CO white dwarfs with total mass
not lower than the Chandrasekhar one. In the systems with a burst of star
formation, at 10 Gyr observed supersoft X-ray sources, most probably, are
not precursors of SN Ia. The same is true for an overwhelming majority of the
sources in the systems with constant star formation rate. In the systems of
both kinds mergers of white dwarfs is the dominant SN Ia scenario. In symbiotic
binaries, accreting CO-dwarfs do not accumulate enough mass for SN Ia
explosion, while ONeMg-dwarfs finish their evolution by an accretion-induced
collapse with formation of a neutron star.Comment: 11 pages, 2 figures, accepted by Astronomy Letter
Tune in to your emotions: a robust personalized affective music player
The emotional power of music is exploited in a personalized affective music player (AMP) that selects music for mood enhancement. A biosignal approach is used to measure listeners’ personal emotional reactions to their own music as input for affective user models. Regression and kernel density estimation are applied to model the physiological changes the music elicits. Using these models, personalized music selections based on an affective goal state can be made. The AMP was validated in real-world trials over the course of several weeks. Results show that our models can cope with noisy situations and handle large inter-individual differences in the music domain. The AMP augments music listening where its techniques enable automated affect guidance. Our approach provides valuable insights for affective computing and user modeling, for which the AMP is a suitable carrier application
Differential Levels of Stress Proteins (HSPs) in Male and Female Daphnia magna in Response to Thermal Stress: A Consequence of Sex-Related Behavioral Differences?
In two independent experiments, we compared: (1) water depth selection (and accompanying temperature selection) by male and female Daphnia magna under different kinds of environmental stress, including the presence of filamentous cyanobacteria, the risk of predation from fish, and the presence of toxic compounds; and (2) sex-dependent production of heat shock proteins (HSP60, 70, and 90) in response to a sudden change in temperature. Male D. magna selected deep water strata, which offer a relatively stable environment, and thereby avoided the threat of predation and the presence of toxic compounds in surface waters. Correlated with this behavior, males reduce their molecular defenses against stress, such as the production of heat shock proteins (HSPs), and do not maintain the physiological machinery that triggers an increase in HSP levels in response to stress. In contrast, female D. magna actively select habitats that offer optimal conditions for growth and production of offspring. Consequently, females are exposed to variable environmental conditions that may be associated with increased stress. To permit survival in these different habitats, D. magna females require molecular mechanisms to protect their cells from rapid changes in stress levels. Thus, they maintain high constitutive levels of the heat shock proteins from HSP 60, 70, and 90 families, and they have the potential to further enhance the production of the majority of these proteins under stress conditions. The results of this study indicate that the separate habitats selected by male and female D. magna result in different patterns of HSP production, leading us to hypothesize that that male and female Daphnia magna adopt different strategies to maximize the fitness of the species
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