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Yong-Cheol Kim

Publications and source records attributed to Yong-Cheol Kim.

26 records · Page 2Linked to original sources
arXiv2007

Pre-Main Sequence Evolutions of Solar Abundance Lowmass Stars

We present the Pre-Main-Sequence (PMS) evolutionary tracks of stars with 0.0065~5.0Ms. The models were evolved from the PMS stellar birthline to the onset of hydrogen burning in the core. The convective turnover timescales which enables an observational test of theoretical model, particulary in the stellar dynamic activity, are also calculated. All models have Sun-like metal abundances, typically considered as the stars in the Galactic disk and the star formation region of Population I star. The convection phenomenon is treated by the usual mixing length approximation. All evolutionary tracks are available upon request.

astro-ph↗
arXiv2005

Weak type estimates on certain Hardy spaces for smooth cone type multipliers

Let $\varrho\in C^{\infty} ({\Bbb R}^d\setminus\{0\})$ be a non-radial homogeneous distance function satisfying $\varrho(tξ)=t\varrho(ξ)$. For $f\in\frak S ({\Bbb R}^{d+1})$ and $δ>0$, we consider convolution operator ${\Cal T}^δ$ associated with the smooth cone type multipliers defined by $$\hat {{\Cal T}^δ f}(ξ,τ)= (1-\frac{\varrho(ξ)}{|τ|} )^δ_+\hat f (ξ,τ), (ξ,τ)\in {\Bbb R}^d \times \Bbb R.$$ If the unit sphere $Σ_{\varrho}\fallingdotseq\{ξ\in {\Bbb R}^d : \varrho(ξ)=1\}$ is a convex hypersurface of finite type and $\varrho$ is not radial, then we prove that ${\Cal T}^{δ(p)}$ maps from $H^p({\Bbb R}^{d+1})$, $0 0} λ^p|\{(x,t)\in \bar{{\Bbb R}^{d+1}\setminusΓ_γ} : |{\Cal T}_{\varrho}^{δ(p)}f(x,t)|>λ\}|=\infty.$$

math.CA↗
arXiv2005

Super Helium-Rich Population and the Origin of Extreme Horizontal-Branch Stars in Globular Clusters

Recent observations for the color-magnitude diagrams (CMDs) of the massive globular cluster Omega Centauri have shown that it has a striking double main sequence (MS), with a minority population of bluer and fainter MS well separated from a majority population of MS stars. Here we confirm, with the most up-to-date Y2 isochrones, that this special feature can only be reproduced by assuming a large variation (Delta Y = 0.15) of primordial helium abundance among several distinct populations in this cluster. We further show that the same helium enhancement required for this special feature on the MS can by itself reproduce the extreme horizontal-branch (HB) stars observed in Omega Cen, which are hotter than normal HB stars. Similarly, the complex features on the HBs of other globular clusters, such as NGC 2808, are explained by large internal variations of helium abundance. Supporting evidence for the helium-rich population is also provided by the far-UV (FUV) observations of extreme HB stars in these clusters, where the enhancement of helium can naturally explain the observed fainter FUV luminosity for these stars. The presence of super helium-rich populations in some globular clusters suggests that the third parameter, other than metallicity and age, also influences CMD morphology of these clusters.

astro-ph↗
arXiv2004

Y2 Isochrones with an Improved Core Overshoot

Convective core overshoot affects stellar evolution rates and the dating of stellar populations. In this paper, we provide a patch to the $Y^2$ isochrones with an improved treatment of convective core overshoot. The new tracks cover the transition mass range from no convective core to a fully developed convective core. We compare the improved isochrones to CMDs of a few well observed open star clusters in the Galaxy and the Large Magellanic Cloud. Finally we discuss future prospects for improving the treatment of core overshoot with the help of asteroseismology.

astro-ph↗
arXiv2003

Weak type estimates of the maximal quasiradial Bochner-Riesz operator on certain Hardy spaces

Let $\{A_t\}_{t>0}$ be the dilation group in ${\Bbb R}^n$ generated by the infinitesimal generator $M$ where $A_t=\exp(M\log t)$, and let $\varrho\in C^{\infty}({\Bbb R}^n\setminus\{0\})$ be a $A_t$-homogeneous distance function defined on ${\Bbb R}^n$. For $f\in {\frak S}({\Bbb R}^n)$, we define the maximal quasiradial Bochner-Riesz operator ${\frak M}^δ_{\varrho}$ of index $δ>0$ by $${\frak M}^δ_{\varrho} f(x)=\sup_{t>0}|{\Cal F}^{-1}[(1-\varrho/t)_+^δ\hat f ](x)|.$$ If $A_t=t I$ and $\{ξ\in {\Bbb R}^n| \varrho(ξ)=1\}$ is a smooth convex hypersurface of finite type, then we prove in an extremely easy way that ${\frak M}^δ_{\varrho}$ is well defined on $H^p({\Bbb R}^n)$ when $δ=n(1/p-1/2)-1/2$ and $0 n(1/p-1/2)-1/2$ and $0<p<1$.

math.CA↗
arXiv1997

On the UV Bright Phase of Metal-Rich Horizontal-Branch Stars

We consider the origin of the UV bright phase of metal-rich helium-burning stars, the slow blue phase (SBP), that was predicted by various earlier works. Based on improved physics including OPAL opacities, we confirm the existence of the SBP. In addition to our grid of evolutionary tracks, we provide an analytical understanding of the main characteristics of the SBP phenomenon. The SBP is slow because it is a slow evolving helium-shell-burning phase which is analogous to the early AGB phase. The SBP of a more metal-rich star is slower than a metal-poor counterpart if their Teff's are the same because a more metal-rich helium-burning star has a smaller mass than a metal-poor one and because lifetime increases as mass decreases. Metal-rich helium-burning stars easily become hot because the luminosity from the hydrogen-burning shell is extremely sensitive to the mean molecular weight whereas the luminosity from the helium-burning core is not. We suggest that the SBP phenomenon is a major cause of the UV upturn phenomenon in giant elliptical galaxies as will be shown in subsequent papers. The new HB tracks can be retrieved from S.Y.'s web site http://shemesh.gsfc.nasa.gov/astronomy.html.

astro-ph↗