Natural scenes sparsely activate neurons in the primary visual cortex (V1). However, how sparsely active neurons reliably represent complex natural images and how the information is optimally decoded ...from these representations have not been revealed. Using two-photon calcium imaging, we recorded visual responses to natural images from several hundred V1 neurons and reconstructed the images from neural activity in anesthetized and awake mice. A single natural image is linearly decodable from a surprisingly small number of highly responsive neurons, and the remaining neurons even degrade the decoding. Furthermore, these neurons reliably represent the image across trials, regardless of trial-to-trial response variability. Based on our results, diverse, partially overlapping receptive fields ensure sparse and reliable representation. We suggest that information is reliably represented while the corresponding neuronal patterns change across trials and collecting only the activity of highly responsive neurons is an optimal decoding strategy for the downstream neurons.
ABSTRACT
GW190521 is a merger of two black holes (BHs), wherein at least one BH lies within the pair-instability (PI) mass gap, and it is difficult to form because of the effects of PI supernovae and ...pulsational PI (PPI). In this study, we examined the formation of GW190521-like BH-BHs under Population (Pop) III environments by binary population synthesis calculations. We reveal that convective overshooting in stellar evolution strongly affects the formation of GW190521-like BH-BHs. A model with a small overshoot parameter (similar to GENEC) can form GW190521-like BH-BHs. The derived merger rate is 4 × 10−2 yr−1 Gpc−3 at a redshift of ∼0.82, which is comparable to the merger rate of GW190521-like BH-BHs inferred by gravitational wave (GW) observations. In this model, a ∼90 M⊙ star collapses to form a ∼90 M⊙ BH by avoiding PPI and PISN even if it is a member of a binary star. This is because it expands up to 102 R⊙, and lose only little mass through binary evolution. However, a model with a large overshoot parameter (similar to Stern) cannot form GW190521-like BH-BHs at all. Thus, we cannot conclude that a Pop III binary system is the origin of GW190521 because determination of the overshoot parameter involves highly uncertain. If a Pop III binary system is the origin of GW190521, the merger rate of BH-BHs including a 100–135 M⊙ BH is substantially smaller than that of GW190521-like BH-BHs. This will be assessed by GW observations in the near future.
Implantable neural microelectrodes that can record extracellular biopotentials from small, targeted groups of neurons are critical for neuroscience research and emerging clinical applications ...including brain-controlled prosthetic devices. The crucial material-dependent problem is developing microelectrodes that record neural activity from the same neurons for years with high fidelity and reliability. Here, we report the development of an integrated composite electrode consisting of a carbon-fibre core, a poly(p-xylylene)-based thin-film coating that acts as a dielectric barrier and that is functionalized to control intrinsic biological processes, and a poly(thiophene)-based recording pad. The resulting implants are an order of magnitude smaller than traditional recording electrodes, and more mechanically compliant with brain tissue. They were found to elicit much reduced chronic reactive tissue responses and enabled single-neuron recording in acute and early chronic experiments in rats. This technology, taking advantage of new composites, makes possible highly selective and stealthy neural interface devices towards realizing long-lasting implants.
Abstract
We present the merger rate density of Population III binary black holes (BHs) by means of a widely used binary population synthesis code
BSE
with extensions to very massive and extreme ...metal-poor stars. We consider not only low-mass BHs (lBHs: 5–50
M
⊙
) but also high-mass BHs (hBHs: 130–200
M
⊙
), where lBHs and hBHs are below and above the pair-instability mass gap (50–130
M
⊙
), respectively. Population III BH–BHs can be categorized into three subpopulations: BH–BHs without hBHs (hBH0s:
m
tot
≲ 100
M
⊙
), with one hBH (hBH1s:
m
tot
∼ 130–260
M
⊙
), and with two hBHs (hBH2s:
m
tot
∼ 270–400
M
⊙
), where
m
tot
is the total mass of a BH–BH. Their merger rate densities at the current universe are ∼0.1 yr
−1
Gpc
−3
for hBH0s, and ∼0.01 yr
−1
Gpc
−3
for the sum of hBH1s and hBH2s, provided that the mass density of Population III stars is ∼10
13
M
⊙
Gpc
−3
. These rates are modestly insensitive to initial conditions and single star models. The hBH1 and hBH2 mergers can dominate BH–BHs with hBHs discovered in the near future. They have low effective spins ≲0.2 in the current universe. The number ratio of hBH2s to hBH1s is high, ≳0.1. We also find that BHs in the mass gap (up to ∼85
M
⊙
) merge. These merger rates can be reduced to nearly zero if Population III binaries are always wide (≳100
R
⊙
), and if Population III stars always enter into chemically homogeneous evolution. The presence of close Population III binaries (∼10
R
⊙
) is crucial for avoiding the worst scenario.
Massive stars having a CO core of ∼40–60 M⊙ experience pulsational pair-instability (PPI) after carbon-burning. This instability induces strong pulsations of the whole star and a part of outer ...envelope is ejected. We investigate the evolution and mass ejection of metal-poor very massive stars which experience PPI. We use stellar models with initial masses of 140, 200, and 250 M⊙ and the metallicity Z = 0.004. Their masses decrease to 54.09, 58.65, and 61.03 M⊙ before the neon-burning owing to mass-loss and He mass fraction at the surface becomes about 20 per cent. During the PPI period of ∼1–2000 yr, they experience six, four, and three pulsations, respectively. The larger CO-core model has the longer PPI period and ejects the larger amount of mass. Since almost all surface He has been lost by the pulsations, these stars become Type Ic supernovae if they explode. Light curves during the PPI stage and supernovae are investigated and are implicated in luminous supernovae. The luminosity created by the interaction of different PPI ejecta becomes M
bol ∼ −16 to −20. The interaction between the circumstellar shell ejected by PPI and the supernova ejecta can be more luminous. These luminous transients could be an origin of Type I superluminous supernovae and supernovae with precursor.
We perform two- (2D) and three-dimensional (3D) hydrodynamics simulations of convective oxygen-shell burning that takes place deep inside a massive progenitor star of a core-collapse supernova. Using ...a one-dimensional (1D) stellar evolution code, we first calculate the evolution of massive stars with an initial mass of 9-40 M . Four different overshoot parameters are applied, and a CO-core mass trend similar to previous works is obtained in the 1D models. Selecting eleven 1D models that have a coexisting silicon and oxygen layer, we perform 2D hydrodynamics simulations of the evolution for ∼100 s until the onset of core collapse. We find that convection with large-scale eddies and the turbulent Mach number of ∼0.1 is obtained in the models having a Si/O layer with a scale of 108 cm, whereas most models that have an extended O/Si layer up to a few ×109 cm exhibit lower turbulent velocity. Our results indicate that the supernova progenitors that possess a thick Si/O layer could provide the preferred condition for perturbation-aided explosions. We perform the 3D simulation of a 25 M model, which exhibits large-scale convection in the 2D models. The 3D model develops large-scale ( = 2) convection similar to the 2D model; however, the turbulent velocity is lower. By estimating the neutrino emission properties of the 3D model, we point out that a time modulation of the event rates, if observed in KamLAND and Hyper-Kamiokande, could provide important information about structural changes in the presupernova convective layer.
Abstract
We investigate the formation of merging binary black holes (BHs) through isolated binary evolution, performing binary population synthesis calculations covering an unprecedentedly wide ...metallicity range of Population (Pop) I, II, III, and extremely metal-poor (EMP) binary stars. We find that the predicted merger rate density and primary BH mass (
m
1
) distribution are consistent with the gravitational wave (GW) observations. Notably, Population III and EMP (<10
−2
Z
⊙
) binary stars yield most of the pair instability (PI) mass gap events with
m
1
= 65–130
M
⊙
. Population III binary stars contribute more to the PI mass gap events with increasing redshift, and all the PI mass gap events have the Population III origin at redshifts ≳8. Our result can be assessed by future GW observations in the following two points. First, there are no binary BHs with
m
1
= 100–130
M
⊙
in our result, and thus the
m
1
distribution should suddenly drop in the range of
m
1
= 100–130
M
⊙
. Second, the PI mass gap event rate should increase toward higher redshift up to ∼11, since those events mainly originate from the Population III binary stars. We find that the following three assumptions are needed to reproduce the current GW observations: a top-heavy stellar initial mass function and the presence of close binary stars for Population III and EMP binary stars, and inefficient convective overshoot in the main-sequence phase of stellar evolution. Without any of the above, the number of PI mass gap events becomes too low to reproduce current GW observations.
ABSTRACT
We have devised fitting formulae for evolution tracks of massive stars with 8 ≲ M/M⊙ ≲ 160 under extreme metal-poor (EMP) environments for log (Z/Z⊙) = −2, −4, −5, −6, and −8, where M⊙ and ...Z⊙ are the solar mass and metallicity, respectively. Our fitting formulae are based on reference stellar models which we have newly obtained by simulating the time evolutions of EMP stars. Our fitting formulae take into account stars ending with blue supergiant (BSG) stars, and stars skipping Hertzsprung gap phases and blue loops, which are characteristics of massive EMP stars. In our fitting formulae, stars may remain BSG stars when they finish their core Helium burning phases. Our fitting formulae are in good agreement with our stellar evolution models. We can use these fitting formulae on the sse, bse, nbody4, and nbody6 codes, which are widely used for population synthesis calculations and star cluster simulations. These fitting formulae should be useful to make theoretical templates of binary black holes formed under EMP environments.
Monotonicity of the Cores of Massive Stars Takahashi, Koh; Takiwaki, Tomoya; Yoshida, Takashi
The Astrophysical journal,
03/2023, Letnik:
945, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
Massive stars are linked to diverse astronomical processes and objects including star formation, supernovae and their remnants, cosmic rays, interstellar media, and galaxy evolution. ...Understanding their properties is of primary importance for modern astronomy, and finding simple rules that characterize them is especially useful. However, theoretical simulations have not yet realized such relations, instead finding that the late evolutionary phases are significantly affected by a complicated interplay between nuclear reactions, chemical mixing, and neutrino radiation, leading to nonmonotonic initial-mass dependencies of the iron core mass and the compactness parameter. We conduct a set of stellar evolution simulations, in which evolutions of He star models are followed until their central densities uniformly reach 10
10
g cm
−3
, and analyze their final structures as well as their evolutionary properties, including the lifetime, surface radius change, and presumable fates after core collapse. Based on the homogeneous data set, we have found that monotonicity is inherent in the cores of massive stars. We show that not only the density, entropy, and chemical distributions, but also their lifetimes and explosion properties such as the proto-neutron-star mass and the explosion energy can be simultaneously ordered into a monotonic sequence. This monotonicity can be regarded as an empirical principle that characterizes the cores of massive stars.
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