1,573 research outputs found
The progenitors of Type Ia supernovae with long delay times
The nature of the progenitors of Type Ia supernovae (SNe Ia) is still
unclear. In this paper, by considering the effect of the instability of
accretion disk on the evolution of white dwarf (WD) binaries, we performed
binary evolution calculations for about 2400 close WD binaries, in which a
carbon--oxygen WD accretes material from a main-sequence star or a slightly
evolved subgiant star (WD + MS channel), or a red-giant star (WD + RG channel)
to increase its mass to the Chandrasekhar (Ch) mass limit. According to these
calculations, we mapped out the initial parameters for SNe Ia in the orbital
period--secondary mass () plane for various WD
masses for these two channels, respectively. We confirm that WDs in the WD + MS
channel with a mass as low as can accrete efficiently and reach
the Ch limit, while the lowest WD mass for the WD + RG channel is . We have implemented these results in a binary population synthesis
study to obtain the SN Ia birthrates and the evolution of SN Ia birthrates with
time for both a constant star formation rate and a single starburst. We find
that the Galactic SN Ia birthrate from the WD + MS channel is according to our standard model, which is higher than
previous results. However, similar to previous studies, the birthrate from the
WD + RG channel is still low (). We also
find that about one third of SNe Ia from the WD + MS channel and all SNe Ia
from the WD + RG channel can contribute to the old populations (\ga1 Gyr) of
SN Ia progenitors.Comment: 11 pages, 9 figures, 1 table, accepted for publication in MNRA
Types of the geodesic motions in Kerr-Sen-AdS spacetime
We consider the geodesic motions in the Kerr-Sen-AdS spacetime. We obtain
the equations of motion for light rays and test particles. Using the parametric
diagrams, we shown some regions where the radial and latitudinal geodesic
motions are allowed. We analyse the impact of parameter related to dilatonic
scalar on the orbit and find that it will result in more rich and complex
orbital types.Comment: 12 pages, 14 figure
Not All Instances Contribute Equally: Instance-adaptive Class Representation Learning for Few-Shot Visual Recognition
Few-shot visual recognition refers to recognize novel visual concepts from a
few labeled instances. Many few-shot visual recognition methods adopt the
metric-based meta-learning paradigm by comparing the query representation with
class representations to predict the category of query instance. However,
current metric-based methods generally treat all instances equally and
consequently often obtain biased class representation, considering not all
instances are equally significant when summarizing the instance-level
representations for the class-level representation. For example, some instances
may contain unrepresentative information, such as too much background and
information of unrelated concepts, which skew the results. To address the above
issues, we propose a novel metric-based meta-learning framework termed
instance-adaptive class representation learning network (ICRL-Net) for few-shot
visual recognition. Specifically, we develop an adaptive instance revaluing
network with the capability to address the biased representation issue when
generating the class representation, by learning and assigning adaptive weights
for different instances according to their relative significance in the support
set of corresponding class. Additionally, we design an improved bilinear
instance representation and incorporate two novel structural losses, i.e.,
intra-class instance clustering loss and inter-class representation
distinguishing loss, to further regulate the instance revaluation process and
refine the class representation. We conduct extensive experiments on four
commonly adopted few-shot benchmarks: miniImageNet, tieredImageNet, CIFAR-FS,
and FC100 datasets. The experimental results compared with the state-of-the-art
approaches demonstrate the superiority of our ICRL-Net
The rotation of surviving companion stars after type Ia supernova explosions in the WD+MS scenario
In the SD scenario of SNe Ia the companion survives the SN explosion and thus
should be visible near the center of the SN remnant and may show some unusual
features. A promising approach to test progenitor models of SNe Ia is to search
for the companion in SNRs. Here we present the results of 3D hydrodynamics
simulations of the interaction between the SN Ia blast wave and a MS companion
taking into consideration its orbital motion and spin. The primary goal of this
work is to investigate the rotation of surviving companions after SN Ia
explosions in the WD+MS scenario. We use Eggleton's code including the
optically thick accretion wind model to obtain realistic models of companions.
The impact of the SN blast wave on these companions is followed in 3D
hydrodynamic simulations employing the SPH code GADGET3. We find that the
rotation of the companion does not significantly affect the amount of stripped
mass and the kick velocity caused by the SN impact. However, in our
simulations, the rotational velocity of the companion is significantly reduced
to about 14% to 32% of its pre-explosion value due to the expansion of the
companion and the fact that 55%-89% of the initial angular momentum is carried
away by the stripped matter. Compared with the observed rotational velocity of
the presumed companion star of Tycho's SN, Tycho G, of 6 km/s the final
rotational velocity we obtain is still higher by at least a factor of two.
Whether this difference is significant, and may cast doubts on the suggestion
that Tycho G is the companion of SN 1572, has to be investigated in future
studies. Based on binary population synthesis results we present, for the first
time, the expected distribution of rotational velocities of companions after
the explosion which may provide useful information for the identification of
the surviving companion in observational searches in other historical SNRs.Comment: 13 pages, 15 figures, accepted for publication by Astronomy and
Astrophysic
Effects of current on nanoscale ring-shaped magnetic tunnel junctions
We report the observation and micromagnetic analysis of current-driven
magnetization switching in nanoscale ring-shaped magnetic tunnel junctions.
When the electric current density exceeds a critical value of the order of
A/cm, the magnetization of the two magnetic rings can be
switched back and forth between parallel and antiparallel onion states.
Theoretical analysis and micromagnetic simulation show that the dominant
mechanism for the observed current-driven switching is the spin torque rather
than the current-induced circular Oersted field
The first symbiotic stars from the LAMOST survey
Symbiotic stars are interacting binary systems with the longest orbital
periods. They are typically formed by a white dwarf, a red giant and a nebula.
These objects are natural astrophysical laboratories for studying the evolution
of binaries. Current estimates of the population of Milky Way symbiotic stars
vary from 3000 up to 400000. However, the current census is less than 300. The
Large sky Area Multi-Object fiber Spectroscopic Telescope (LAMOST) survey can
obtain hundreds of thousands of stellar spectra per year, providing a good
opportunity to search for new symbiotic stars. In this work we detect 4 of such
binaries among 4,147,802 spectra released by the LAMOST, of which two are new
identifications. The first is LAMOST J12280490-014825.7, considered to be an
S-type halo symbiotic star. The second is LAMOST J202629.80+423652.0, a D-type
symbiotic star
- …