The relations between the metallicity and orbital parameters of Galactic disk stars

The iron abundance gradient in the Galactic disk provides fundamental constraints on the chemical evolution of this important component of the Galaxy.However,there are still some arguments about the shape of the iron abundance gradient across the Galactic stellar disk.To provide quantitative constraints on these trends,we constructed an unbiased stellar sample from the common objects of two large observations.The sample consists of 4004 disk stars in the solar neighborhood with the range of metallicity [Fe/H] from ^-1.0 to ~0.5,including 3855 thin disk stars and 146 thick disk stars.The range of their mean Galactocentric distances R m is from ~4 to ~11 kpc,and the range of their maximum vertical distance from the Galactic Plane Z max is from 0 to ~3.5 kpc.The relations of the stellar metallicity [Fe/H] with the orbital eccentricity e,and [Fe/H] with R m,as well as [Fe/H] with Z max are investigated in detail.The results showed that:(1) The orbital eccentricity of disk stars decreases with increasing metallicity,while this trend becomes flat for stars with [Fe/H]】-0.2.(2) For thin disk,there is a clear radial iron gradient of-0.051±0.005 dex/kpc.However,this metallicity gradient is discontinuous once we split the sample into the inner(Rm【8kpc) and outer disk(Rm≥8 kpc):a slope of about 0 dex/kpc is present in the inner disk and a slope of-0.12 dex/kpc is in the outer disk.For the thick disk,there is no trend of [Fe/H] with R m.(3) The vertical gradient of iron abundance is-0.146 dex/kpc and-0.251 dex/kpc for the thick and thin disks,respectively.Moreover,the vertical abundance gradient shows a steeper trend with the evolution of the Galaxy.Our results confirm the collapse mechanism associated with the formation and evolution of the Galactic disk.

Ionization of protoplanetary disks by galactic cosmic rays,solar protons,and supernova remnants

Galactic cosmic rays and solar protons ionize the present terrestrial atmosphere,and the air showers are simulated by well-tested Monte-Carlo simulations,such as PHITS code.We use the latest version of PHITS to evaluate the possible ionization of protoplanetary disks by galactic cosmic rays（GCRs）,solar protons,and by supernova remnants.The attenuation length of GCR ionization is updated as 118 g cm^-2,which is approximately 20% larger than the popular value.Hard and soft possible spectra of solar protons give comparable and 20% smaller attenuation lengths compared with those from standard GCR spectra,respectively,while the attenuation length is approximately 10% larger for supernova remnants.Further,all of the attenuation lengths become 10% larger in the compound gas of cosmic abundance,e.g.128 g cm^-2 for GCRs,which can affect the minimum estimate of the size of dead zones in protoplanetary disks when the incident flux is unusually high.

Low-α metal-rich stars with sausage kinematics in the LAMOST survey: Are they from the Gaia-Sausage-Enceladus galaxy?

We search for metal-rich Sausage-kinematic(MRSK)stars with[Fe/H]>−0.8 and−100<Vϕ<50 km/s in LAMOST DR5 in order to investigate the influence of the Gaia-Sausage-Enceladus(GSE)merger event on the Galactic disk.For the first time,we find a group of low-αMRSK stars,and classify it as a metal-rich tail of the GSE galaxy based on the chemical and kinematical properties.This group has slightly larger R_(apo),Z_(max) and Etot distributions than a previously-reported high-αgroup.Its low-αratio does not allow for an origin resulting from the splash process of the GSE merger event,as is proposed to explain the high-αgroup.A hydrodynamical simulation by Amarante et al.provides a promising solution,in which the GSE galaxy is a clumpy Milky-Way analogue that develops a bimodal disk chemistry.This scenario explains the existence of MRSK stars with both high-αand low-αratios found in this work.It is further supported by another new feature that a clump of MRSK stars is located at Z_(max)=3-5 kpc,which corresponds to the widely adopted disk-halo transition at|Z|∼4 kpc.We suggest that a pile-up of MRSK stars at Zmax contributes significantly to this disk-halo transition,an interesting imprint left by the GSE merger event.These results also provide an important implication on the connection between the GSE and the Virgo Radial Merger.

Electron and positron spectra in three-dimensional spatial-dependent propagation model

The spatial-dependent propagation(SDP)model has been demonstrated to account for the spectral hardening of both primary and secondary Cosmic Rays(CRs)nuclei above about 200 GV.In this work,we further apply this model to the latest AMS-02 observations of electrons and positrons.To investigate the effect of different propagation models,both homogeneous diffusion and SDP are compared.In contrast to the homogeneous diffusion,SDP brings about harder spectra of background CRs and thus enhances background electron and positron fluxes above tens of GeV.Thereby,the SDP model could better reproduce both electron and positron energy spectra when introducing a local pulsar.The influence of the background source distribution is also investigated,where both axisymmetric and spiral distributions are compared.We find that considering the spiral distribution leads to a larger contribution of positrons for energies above multi-GeV than the axisymmetric distribution.In the SDP model,when including a spiral distribution of sources,the all-electron spectrum above TeV energies is thus naturally described.In the meantime,the estimated anisotropies in the all-electrons spectrum show that in contrary to the homogeneous diffusion model,the anisotropy under SDP is well below the observational limits set by the Fermi-LAT experiment,even when considering a local source.