Abstract
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, which is the same physics that was used in the construction of the new Yale Isochrones, 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 that is analogous to the early asymptotic giant branch phase. The SBP of a more metal-rich star is slower than a metal-poor counterpart if they have the same T eff 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 μ, whereas the luminosity from the helium-burning core is not. Under the assumption of a positive ΔY/ΔZ, helium abundance plays the most important role in governing μ, and thus Dorman et al. found that the SBP occurs only when Y ≳ 0.4 when ΔY/ΔZ ≳ 0. 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.
| Original language | English |
|---|---|
| Pages (from-to) | 677-684 |
| Number of pages | 8 |
| Journal | Astrophysical Journal |
| Volume | 482 |
| Issue number | 2 PART I |
| DOIs | |
| Publication status | Published - 1997 Jan 1 |
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All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science
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On the ultraviolet-bright phase of metal-rich horizontal-branch stars. / Yi, Sukyoung; Demarque, Pierre; Kim, Yong Cheol.
In: Astrophysical Journal, Vol. 482, No. 2 PART I, 01.01.1997, p. 677-684.Research output: Contribution to journal › Article
TY - JOUR
T1 - On the ultraviolet-bright phase of metal-rich horizontal-branch stars
AU - Yi, Sukyoung
AU - Demarque, Pierre
AU - Kim, Yong Cheol
PY - 1997/1/1
Y1 - 1997/1/1
N2 - 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, which is the same physics that was used in the construction of the new Yale Isochrones, 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 that is analogous to the early asymptotic giant branch phase. The SBP of a more metal-rich star is slower than a metal-poor counterpart if they have the same T eff 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 μ, whereas the luminosity from the helium-burning core is not. Under the assumption of a positive ΔY/ΔZ, helium abundance plays the most important role in governing μ, and thus Dorman et al. found that the SBP occurs only when Y ≳ 0.4 when ΔY/ΔZ ≳ 0. 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.
AB - 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, which is the same physics that was used in the construction of the new Yale Isochrones, 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 that is analogous to the early asymptotic giant branch phase. The SBP of a more metal-rich star is slower than a metal-poor counterpart if they have the same T eff 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 μ, whereas the luminosity from the helium-burning core is not. Under the assumption of a positive ΔY/ΔZ, helium abundance plays the most important role in governing μ, and thus Dorman et al. found that the SBP occurs only when Y ≳ 0.4 when ΔY/ΔZ ≳ 0. 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.
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U2 - 10.1086/304192
DO - 10.1086/304192
M3 - Article
VL - 482
SP - 677
EP - 684
JO - Astrophysical Journal
JF - Astrophysical Journal
SN - 0004-637X
IS - 2 PART I
ER -