18 research outputs found
Stellar Parameters of Main Sequence Turn-off Star Candidates Observed with the LAMOST and Kepler
Main sequence turn-off (MSTO) stars have advantages as indicators of Galactic
evolution since their ages could be robustly estimated from atmospheric
parameters. Hundreds of thousands of MSTO stars have been selected from the
LAMOST Galactic sur- vey to study the evolution of the Galaxy, and it is vital
to derive accurate stellar parameters. In this work, we select 150 MSTO star
candidates from the MSTO stars sample of Xiang that have asteroseismic
parameters and determine accurate stellar parameters for these stars combing
the asteroseismic parameters deduced from the Kepler photometry and atmospheric
parameters deduced from the LAMOST spectra.With this sample, we examine the age
deter- mination as well as the contamination rate of the MSTO stars sample. A
comparison of age between this work and Xiang shows a mean difference of 0.53
Gyr (7%) and a dispersion of 2.71 Gyr (28%). The results show that 79 of the
candidates are MSTO stars, while the others are contaminations from either main
sequence or sub-giant stars. The contamination rate for the oldest stars is
much higher than that for the younger stars. The main cause for the high
contamination rate is found to be the relatively large systematic bias in the
LAMOST surface gravity estimates.Comment: accepted by RA
Imprints of Sagittarius accretion event: Young O-rich stars and discontinuous chemical evolution in Milky Way disc
The Milky Way has undergone significant transformations in its early history,
characterised by violent mergers and the accretion of satellite galaxies. Among
these events, the infall of the satellite galaxy Gaia-Enceladus/Sausage is
recognised as the last major merger event, fundamentally altering the evolution
of the Milky Way and shaping its chemo-dynamical structure. However, recent
observational evidence suggests that the Milky Way remains undergone notable
events of star formation in the past 4 Gyr, which is thought to be triggered by
the perturbations from Sagittarius dwarf galaxy (Sgr). Here we report chemical
signatures of the Sgr accretion event in the past 4 Gyr, using the [Fe/H] and
[O/Fe] ratios in the thin disc, which is reported for the first time. It
reveals that the previously discovered V-shape structure of age-[Fe/H] relation
varies across different Galactic locations and has rich substructures.
Interestingly, we discover a discontinuous structure at z 0.3
kpc, interrupted by a recent burst of star formation from 4 Gyr to 2 Gyr ago.
In this episode, we find a significant rise in oxygen abundance leading to a
distinct [O/Fe] gradient, contributing to the formation of young O-rich stars.
Combined with the simulated star formation history and chemical abundance of
Sgr, we suggest that the Sgr is an important actor in the discontinuous
chemical evolution of the Milky Way disc.Comment: 17 pages, 15 figures. Under review at Nature Communication
Characterising abundance-age relations of GALAH stars using oxygen-enhanced stellar models
Main Sequence Turn-off stars (MSTO) and subgiant stars are good tracers of
galactic populations. We present a study of 41,034 MSTO and subgiant stars from
the GALAH survey. Using a grid of stellar models that accounts for the
variation of O abundances, we determine their ages with a median age
uncertainty of 9.4 per cent. Our analysis reveals that the ages of high-O
stars based on O-enhanced models (OEM models) are smaller than those determined
with -enhanced models, resulting in a mean fractional age difference of
-5.3 per cent at [O/] = 0.2 and -11.0 per cent at [O/] = 0.4.
This age difference significantly impacts the age distribution of thick disc
and halo stars, leading to a steeper downward trend in the [Fe/H]-age plane
from 8 Gyr to 14 Gyr, indicating a shorter formation time-scale and a faster
chemical-enhanced history for these populations. We confirm the V-shape of the
normalized age-metallicity distribution ([Fe/H]) of thin disc
stars, which is presumably a consequence of the second gas infall.
Additionally, we find that the halo stars in our sample can be divided into two
sequences, a metal-rich sequence (Splash stars) and a metal-poor sequence
(accreted stars), with the Splash stars predominantly older than 9 Gyr and the
accreted halo stars older than 10 Gyr. Finally, we observe two distinct
sequences in the relations between various chemical abundances and age for disc
stars, namely a young sequence with ages 8 Gyr and an old sequence
with ages 8 Gyr.Comment: 11 pages, 12 figure
Theoretical predictions for
The -decay half-lives of synthesized superheavy nuclei (SHN) from seaborgium to oganesson are calculated by employing the generalized liquid-drop model (GLDM), the Royer formula and the universal decay law (UDL) with experimental -decay energies . For the GLDM, we consider the shell correction. The agreement between the experimental data and the calculations indicates that all the methods we used are successful to reproduce -decay half-lives of known SHN. The decay-modes of known nuclei on the 294Og decay-chain are also consistent with the experiments. For the unknown nuclei, the -decay half-lives have been predicted by inputting values extracted from the newest Weizsäcker-Skyrme-4 (WS4) model. In the GLDM with shell correction, we adopt the constant -preformation factor as well as extracted by Cluster Formation Model (CFM). To calculate CFM values, we use FRDM binding energies and WS4 mass excess values. The relationship of and shows that 294, 296, 314, 316, 320Og isotopes are relatively stable. The competition between -decay and spontaneous fission is discussed in detail for 283-339Og isotopes. The decay-chains of 290-300Og have also been presented. Since the -decay half-lives of 283-303Og isotopes are obviously lower than their spontaneous fission half-lives by more than 6 orders, these isotopes would mainly have -decay. The 306-334Og isotopes may undergo spontaneous fission. The nuclei 304, 305Og would have both -decay and spontaneous fission. By the shell-effect included GLDM with CFM , we predict 295Og undergoes -decay and ms. The 296Og is also -decay and has ms