45 research outputs found
Formation and Destiny of White Dwarf and Be Star Binaries
The binary systems consisting of a Be star and a white dwarf (BeWDs) are very
interesting.They can originate from the binaries composed of a Be star and a
subdwarf O or B star (BesdOBs), and they can merge into red giants via luminous
red nova or can evolve into double WD potentially detected by mission.
Using the method of population synthesis, we investigate the formation and the
destiny of BeWDs,and discuss the effects of the metallicity () and the
common envelope evolution parameters. We find that BesdOBs are significant
progenitors of BeWDs. About 30\% ()-50\% () of BeWDs come
from BesdOBs. About 60\% () -70\% () of BeWDs turn into red
giants via a merger between a WD and a non-degenerated star. About 30\%
() -40\% () of BeWDs evolve into double WDs which are
potential gravitational waves of mission at a frequency band between
about and Hz. The common envelope evolution
parameter introduces an uncertainty with a factor of about 1.3 on BeWD
populations in our simulations.Comment: 17 pages, 12 figures, 2 table, accepted for publication in RA
An Alternative Formation Scenario for Uranium-rich Giants: Engulfing a Earth-like Planet
The actinides, such as the uranium (U) element, are typically synthesized
through the rapid neutron-capture process (r-process), which can occur in
core-collapse supernovae or double neutron star mergers. There exist nine
r-process giant stars exhibiting conspicuousUabundances, commonly referred to
as U-rich giants. However, the origins of these U-rich giants remain ambiguous.
We propose an alternative formation scenario for these U-rich giants whereby a
red giant (RG) engulfs an Earth-like planet. To approximate the process of a RG
engulfing an Earth-like planet, we employ an accretion model wherein the RG
assimilates materials from said planet. Our findings demonstrate that this
engulfment event can considerably enhance the presence of heavy elements
originating from Earth-like planets on the surfaces of very metal-poor stars (Z
= 0.00001), while its impact on solar-metallicity stars is comparatively
modest. Importantly, the structural and evolutionary properties of both very
metalpoor and solar-metallicity stars remain largely unaffected. Notably, our
engulfment model effectively accounts for the observed U abundances in known
U-rich giants. Furthermore, the evolutionary trajectories of U abundances on
the surfaces of RGs subsequent to the engulfment of Earth-like planets
encompass all known U-rich giants. Therefore, it is plausible that U-rich
giants are formed when a RG engulfs an Earth-like planet.Comment: 9 pages, 8 figures, accepted 2023 July 10 by MNRA
Hydrogen-free Wolf-Rayet stars: Helium stars with envelope-inflation structure and rotation
Observations have shown that the effective temperature of hydrogen-free
Wolf-Rayet (WR) stars is considerably lower than that of the standard model,
which means that the radius of the observed H-free WR stars is several times
larger than that estimated by the standard model. The envelope inflation
structure (EIS) caused by the radiation luminosity being close to the Eddington
luminosity in the iron opacity peak region of H-free WR stars may be the key to
resolve the radius problem of H-free WR stars. We try to explain the H-free WR
stars observed in the Milk Way (MW) and the Large Magellanic Cloud (LMC) by the
He stars. Using the Modules for Experiments in Stellar Astrophysics code, we
compute the evolution of He stars with and without MLT++ prescriptions and
discuss their effects on the EIS. We have calculated the evolution of He stars
using a new mass-loss rate formula and three different relative rotational
velocity and compared our results with observations on Hertzsprung-Russell
diagrams. The low luminosity (log) H-free WR stars in
the MW and the LMC can be explained by the helium giant phase in low-mass He
stars, the high and in WC stars can only evolve through
low-mass He stars with a rapid rotation. High-mass He stars with the EIS can
explain H-free WR stars with a luminosity exceeding and
an effective temperature above K in the MW. They can also explain
H-free WR stars on the right-hand side of the He zero-age main sequence in the
LMC. High-mass stars with the EIS evolve into WO stars at the final evolution
stage, and the shorter lifetime fraction is consistent with the small number of
observed WO stars.Comment: 9 pages, 7 figures 1 tables, Accepted to A&
First Detailed Analysis of a Relatively Deep, Low Mass-ratio Contact Binary: ATO J108.6991+27.8306
We present the first detailed photometric analysis of ATO J108.6991+27.8306
(hereinafter as J108). The short-period close binary J108 was observed by the
Nanshan 1 m Wide Field Telescope of the Xinjiang Astronomical Observatory. The
obtained BVRI-band light curves were used to determine the photometric solution
by using the 2003 version of the Wilson-Devinney code. J108 is a typical deep (
f > 50%), low mass ratio (q < 0.25) overcontact binary system with a mass ratio
of q = 0.1501 and a fill-out factor of f = 50.1 %, suggesting that it is in the
late evolutionary stage of contact binary systems. We found the target to be a
W-type W UMa binary and provided evidence for the presence of starspots on both
components. From the temperature-luminosity diagram, the main component is the
evolved main sequence star with an evolutionary age of about 7.94 Gyr.Comment: 7 pages, 6 figure
Li-rich and super Li-rich giants produced by element diffusion
Context. About 0.2-2% of giant stars are Li-rich, whose lithium abundance
(A(Li)) is higher than 1.5 dex. Among them, near 6% are super Li-rich with
A(Li) exceeding 3.2 dex. Meanwhile, the formation mechanism of these Li-rich
and super Li-rich giants is still under debate. Aims. Considering the compact
He core of red giants, attention is paid to the effect of element diffusion on
A(Li). In particular, when the He core flash occurs, the element diffusion
makes the thermohaline mixing zone extend inward and connect to the inner
convection region of stars. Then, a large amount of 7Be produced by the He
flash can be transferred to stellar surface, finally turning into 7Li. Thus,
the goal of this work is to propose the mechanism of A(Li) enrichment and
achieve the consistency between the theoretical and observation data. Methods.
Using the Modules for Experiments in Stellar Astrophysics (MESA), we simulate
the evolution of low-mass stars, with considering the effects of element
diffusion on the Li abundances. The timescale ratio of Li-rich giants to normal
giants is estimated by population synthesis method. Then we get the theoretical
value of A(Li) and make a comparison with observations. Results. Considering
the influence of element diffusion in the model results in the increase of
lithium abundance up to about 1.8dex, which can reveal Li-rich giants.
Simultaneously, introducing high constant diffusive mixing coefficients (Dmix)
with the values from 10e11 to 10e15in the model allows A(Li) to increase from
2.4 to 4.5dex, which can explain the most of Li-rich and super Li-rich giant
stars. The population synthesis method reveals that the amount of Li-rich
giants among giants is about 0.2-2%, which is consistent with observation
estimated levels