With the increasing number of large stellar survey projects, the quality and quantity of excellent tracers for studying the Milky Way are rapidly growing, one of which is the classical Cepheids. Classical Cepheids are high-precision standard candles with very low typical uncertainties (<3%) available via the mid-infrared period–luminosity relation. About 3500 classical Cepheids identified from the Optical Gravitational Lensing Experiment, All-Sky Automated Survey for Supernova, Gaia, Wide-field Infrared Survey Explorer, and Zwicky Transient Facility survey data have been analyzed in this work, and their spatial distributions show a clear signature of Galactic warp. Two kinematical methods are adopted to measure the Galactic rotation curve (RC) in the Galactocentric distance range of . Gently declining RCs are derived by both the proper motion (PM) method and three-dimensional velocity vector (3DV) method. The largest sample of classical Cepheids with the most accurate 6D phase-space coordinates available to date are modeled in the 3DV method, and the resulting RC is found to decline at the relatively smaller gradient of (−1.33 ± 0.1) . Comparing to results from the PM method, a higher rotation velocity ((232.5 ± 0.83) ) is derived at the position of the Sun in the 3DV method. The virial mass and local dark matter density are estimated from the 3DV method, which is the more reliable method, and GeV, respectively.
Binary population synthesis (BPS) study provides a comprehensive way to understand evolutions of binaries and their end products. Close white dwarf (WD) binaries have crucial characteristics in examining influence of yet-unresolved physical parameters on the binary evolution. In this paper, we perform Monte Carlo BPS simulations, investigating the population of WD/main sequence (WD/MS) binaries and double WD binaries, with a publicly available binary star evolution code under 37 different assumptions on key physical processes and binary initial conditions. We considered different combinations of the binding energy parameter (λ g :considering gravitational energy only, λ b : considering both gravitational energy and internal energy, and λ e :considering gravitational energy, internal energy, and entropy of the envelope, the values of them derived with the MESA code), CE efficiency, critical mass ratio, initial primary mass function and metallicity. We find that a larger number of post-CE WD/MS binaries in tight orbits are formed when the binding energy parameters is set by λ e than the cases adopting the other prescriptions. We also find effects of the other input parameters on orbital period and mass distributions of post-CE WD/MS binaries as well. Containing at least one CO WD, the double WD system evolved from WD/MS binaries may explode as type Ia supernovae (SNe Ia) by merging. In this work, we also investigate a frequency of two WD mergers and compare it to the SNe Ia rate. The calculated Galactic SNe Ia rate with λ = λ e is comparable with observed SNe Ia rate, ∼ 8.2 × 10 −5 yr −1 -∼ 4 × 10 −3 yr −1 depending on the -2other BPS parameters, if a DD system does not require the mass ratio higher than ∼ 0.8 to become an SNe Ia. On the other hand, a scenario like a violent merger scenario, which requires a combined mass of two CO WDs ≥ 1.6M and mass ratio > 0.8, results in a much lower SNe Ia rate than observed.
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