Keywords

The origin of intermediate helium (He)-rich hot subdwarfs is still unclear. Previous studies have suggested that some surviving Type Ia supernovae (SNe Ia) companions from the white dwarf + main-sequence (WD+MS) channel may contribute to the intermediate He-rich hot subdwarfs. However, previous studies ignored the impact of atomic diffusion on the post-explosion evolution of surviving companion stars of SNe Ia, leading to the aspect that they could not explain the observed surface He abundance of intermediate He-rich hot subdwarfs. In this work, by taking the atomic diffusion and stellar wind into account, we trace the surviving companions of SNe Ia from the WD+MS channel using the one-dimensional stellar evolution code MESA until they evolve into hot subdwarfs. We find that the surface He-abundances of our surviving companion models during their core He-burning phases are in a range of $-1\lesssim \mathrm{log}({N}_{\mathrm{He}}/{N}_{{\rm{H}}})\lesssim 0$, which are consistent with those observed in intermediate He-rich hot subdwarfs. This seems to further support the notion that it is possible for surviving companions of SNe Ia in the WD+MS channel to form some intermediate He-rich hot subdwarfs.

Element diffusion has small but significant effects on the structure of the stellar interior. It is interesting to investigate the effects of element diffusion using asteroseismology. We have constructed two grids of stellar models, one with diffusion and one without, for solar-like stars with masses between 0.9 and 1.4 solar masses, and varied initial helium abundance and metallicity. The oscillation frequencies of all stellar models have also been calculated. Piecewise Hermite cubic polynomials are adopted to interpolate stellar p-mode frequencies at an arbitrary age on a stellar evolutionary track. We have investigated 16 Kepler solar-like stars by comparing the model frequencies with observations. The suggested ranges of stellar parameters and some global variables are obtained. For all stars, the best model reproduces the observational frequencies with a χ² of the order of unity. It has been found that element diffusion is important in modeling solar-like stars. Without diffusion, the best value of the initial helium abundance is below the primordial helium abundance from Big Bang nucleosynthesis. When diffusion is taken into account, the required initial helium abundance increases to be higher than the primordial abundance. Diffusion also generally improves the frequency fitting results by reducing the minimum of χ². Investigation of the second difference of the oscillation frequencies on KIC 8694723 and KIC 10454113 indicates that the current model of element diffusion may underestimate the strength of settling.

In this work, we analyze the long-term cosmic-ray modulation observed by the Hermanus neutron monitor, which is the detector with the longest cosmic-ray record, from 1957 July. For our study we use the force-field approximation to the cosmic-ray transport equation, and the newest results on the mean free paths from the scattering theory. We compare the modulation parameter (ϕ) with different rigidity (P) dependences: P, P², and P^2/3. We correlate them with solar and interplanetary parameters. We found that (1) these rigidity dependences properly describe the modulation, (2) long-term cosmic-ray variations are better correlated with the magnitude of the heliospheric magnetic field (HMF) than the sunspot number, solar wind speed, and tilt angle of the HMF, and (3) the theoretical dependence of the parallel mean free path on the magnetic field variance is in agreement with the modulation parameter and therefore with the neutron monitor record. We also found that the force-field approximation is not able to take into account the effects of three-dimensional particle transport, showing a poor correlation with the perpendicular mean free path.

The Vela X pulsar wind nebula (PWN) is characterized by the extended radio nebula (ERN) and the central X-ray "cocoon." We have interpreted the γ-ray spectral properties of the cocoon in the first paper; here, we account for the broadband photon spectrum of the ERN. Since the diffusive escape of the electrons from the TeV emitting region is expected to play an insignificant role in shaping the spectrum of the ERN, we attribute the GeV cutoff of the ERN to the reverse shock–PWN interaction. Due to the disruption of the reverse shock, most of the plasma of the PWN is driven into the ERN. During the subsequent reverberation phase, the ERN could be compressed by a large factor in radius, and the magnetic field in the ERN is thus significantly enhanced, burning off the high-energy electrons. We thus obtain the electron spectrum of the ERN, and the broadband spectrum of the ERN is explained satisfactorily.

Various instabilities have been proposed as a promising mechanism for accumulating dust. Moreover, some of them are expected to lead to multiple-ring structure formation and planetesimal formation in protoplanetary disks. In a turbulent gaseous disk, the growth of the instabilities and the dust accumulation are quenched by the turbulent diffusion of dust grains. The diffusion process has often been modeled by a diffusion term in the continuity equation for the dust density. The dust diffusion model, however, does not guarantee conservation of angular momentum in a disk. In this study, we first formulate equations that describe dust diffusion and also conserve the total angular momentum of a disk. Second, we perform a linear perturbation analysis on the secular gravitational instability (GI) using the equations. The results show that the secular GI is a monotonically growing mode, contrary to the result of previous analyses that found it overstable. We find that the overstability is caused by the nonconservation of the angular momentum. Third, we find a new axisymmetric instability due to the combination of dust–gas friction and turbulent gas viscosity, which we refer to as two-component viscous gravitational instability (TVGI). The most unstable wavelength of TVGI is comparable to or smaller than the gas scale height. TVGI accumulates dust grains efficiently, which indicates that TVGI is a promising mechanism for the formation of multiple-ring-like structures and planetesimals. Finally, we examine the validity of the ring formation via the secular GI and TVGI in the HL Tau disk and find both instabilities can create multiple rings whose width is about 10 au at orbital radii larger than 50 au.

Various numerical solar energetic particle (SEP) transport studies have shown that perpendicular diffusion plays a significant role in the propagation of these particles in the heliosphere. In particular, computed SEP intensities and anisotropies have been shown to be sensitive to the pitch-angle dependence of the perpendicular diffusion coefficient as well as its magnitude. This study proposes a novel approach to the calculation of this quantity and compares this to the results of previous theoretical approaches. These various perpendicular diffusion coefficient expressions are demonstrated for turbulence conditions prevalent at Earth and closer to the Sun.

The High Altitude Water Cherenkov (HAWC) telescope recently observed extended emission around the Geminga and PSR B0656+14 pulsar wind nebulae (PWNe). These observations have been used to estimate cosmic-ray (CR) diffusion coefficients near the PWNe that appear to be more than two orders of magnitude smaller than the diffusion coefficients typically derived for the interstellar medium from measured abundances of secondary species in CRs. Two-zone diffusion models have been proposed as a solution to this discrepancy, where the slower diffusion zone (SDZ) is confined to a small region around the PWN. Such models are shown to successfully reproduce the HAWC observations of the Geminga PWN while retaining consistency with other CR data. It is found that the size of the SDZ influences the predicted positron flux and the spectral shape of the extended γ-ray emission at lower energies that can be observed with the Fermi Large Area Telescope. If the two observed PWNe are not unique, then it is likely that there are similar pockets of slow diffusion around many CR sources elsewhere in the Milky Way. The consequences of such a picture for Galactic CR propagation is explored.

A global MHD–neutrals–cosmic-rays simulation is conducted to investigate the effects of anomalous cosmic-rays (ACRs) on the large-scale structure of the outer heliosphere. In the model, the cosmic-rays are treated as a massless fluid that only contribute their pressure to the dynamics of the system. The diffusion of cosmic-rays in the interstellar medium is assumed to be much faster than inside the heliosphere, where it depends on the strength of the magnetic field. The results show that the influence of the cosmic-rays on the structure of the outer heliosphere depends on momentum and energy transfer from the solar wind plasma to the ACRs, accomplished through diffusive shock acceleration at the termination shock, and the subsequent loss of ACRs across the heliopause and their rapid escape into the interstellar medium. Under favorable conditions characterized by a large fraction of energy conversion and a high enough diffusion coefficient in the solar wind, the ACRs were found to reduce the width of the heliosheath by up to ∼18 au. Consequently, these results indicate that the effect of cosmic-rays is a potential key factor for the global structure of the outer heliosphere in a computer model that could partially explain the timing of the heliopause encounters of the two Voyager probes.

We present a test-particle simulation describing the interstellar pickup ion (PUI) velocity distribution in the turbulent solar wind (SW). The classical Vasyliunas and Siscoe (V&S) model assumes instantaneous pitch angle scattering that leads to an isotropic distribution in the SW frame, and considers only convection and adiabatic cooling as PUIs propagate in the expanding SW. In this paper, the nearly isotropic PUI transport equation, including the effect of spatial diffusion due to the fluctuating magnetic field, is solved at different heliospheric distances. The creation of PUIs due to the ionization of interstellar neutral hydrogen (H) and charge exchange between SW protons and neutral H are considered separately. The varying SW velocity, density, and temperature with heliocentric distance from a comprehensive fluid model have been incorporated into our simulations. Specifically, we find (1) the spatial diffusion augments adiabatic cooling effects by extending the transport time and distance, which leads to an enhanced production of low-energy PUIs, especially at small heliospheric distances; (2) spatial diffusion is unimportant at large distances (≥15 au), because the particles have had a sufficiently long time to undergo adiabatic cooling; (3) moments of the simulated velocity distribution function are consistent with PUI hydrogen properties measured by the New Horizons' SW Around Pluto instrument; and (4) the simulated PUI distribution is of potential importance for the PUI measurements to be carried out by IMAP at 1 au.

In the interstellar medium at rest, containing low-frequency magnetohydrodynamic linearly polarized slab Alfvén waves, the anisotropy of relativistic galactic cosmic rays consists of two parts: the streaming anisotropy g_s (z, p,μ), caused by the spatial gradient of the isotropic part of the cosmic ray distribution function, and the interstellar Compton–Getting anisotropy ${g}_{c}(z,p,\mu )$, caused by the momentum gradient of the isotropic part of the cosmic ray distribution function. Both anisotropies depend differently on the cosmic ray pitch-angle cosine μ, cosmic ray momentum p, and cross-helicity state H_c of the Alfvenic slab turbulence. First, the streaming anisotropy is independent from H_c and varies as ${g}_{s}{(z,p,\mu )\propto (p| \mu | )}^{\eta }\mathrm{sgn}(\mu )$ with η = 2 − s, where s denotes the power-law spectral index of interstellar turbulence. Second, the interstellar Compton–Getting anisotropy ${g}_{c}(z,p,\mu )\propto {H}_{c}\mu$ is independent of momentum and linearly proportional to ${H}_{c}\mu$. These different pitch-angle dependencies can be tested by the Liouville mapping technique to infer the pristine interstellar cosmic ray anisotropy from measurements inside the solar system. For cosmic rays with energy of 4 TeV the derived pristine interstellar cosmic ray anisotropy suggest the linear ($g\propto | \mu | \mathrm{sgn}(\mu )$) pitch-angle dependence. This is well explained by the interstellar Compton–Getting anisotropy, provided the Alfvén speed in the local interstellar medium is about 62 km s⁻¹.