Astronomy and Astrophysics – Astrophysics
Scientific paper
2003-08-28
Astrophys.Space Sci.291:367-370,2004
Astronomy and Astrophysics
Astrophysics
4 pages, 2 figures, to appear in "Extreme Horizontal Branch Stars and Related Objects", Astrophysics and Space Science, Kluwer
Scientific paper
10.1023/B:ASTR.0000044345.85262.
We present FUSE spectra of three He-rich sdB stars. Two of these stars, PG1544+488 and JL87, reveal extremely strong C III lines, suggesting that they have mixed triple-alpha carbon from the deep interior out to their surfaces. Using TLUSTY NLTE line-blanketed model atmospheres, we find that PG1544+488 has a surface composition of 96% He, 2% C, and 1% N. JL87 shows a similar surface enrichment of C and N but still retains a significant amount of hydrogen. In contrast, the third star, LB1766, is devoid of hydrogen and strongly depleted of carbon, indicating that its surface material has undergone CN-cycle processing. We interpret these observations with new evolutionary calculations which suggest that He-rich sdB stars with C-rich compositions arise from a delayed helium-core flash on the white-dwarf cooling curve. During such a flash the interior convection zone will penetrate into the stellar envelope, thereby mixing the envelope with the He- and C-rich core. Such "flash-mixed" stars will arrive on the extreme horizontal branch (EHB) with He- and C-rich surface compositions and will be hotter than the hottest canonical EHB stars. Two types of flash mixing are possible: "deep" and "shallow", depending on whether the hydrogen envelope is mixed deeply into the site of the helium flash or only with the outer layers of the core. Based on both their stellar parameters and surface compositions, we suggest that PG1544+488 and JL87 are examples of "deep" and "shallow" flash mixing, respectively.
Brown Thomas M.
Hubeny Ivan
Landsman Wayne B.
Lanz Thierry
Sweigart Allen V.
No associations
LandOfFree
Evidence for Flash Mixing in He-rich sdB Stars does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Evidence for Flash Mixing in He-rich sdB Stars, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Evidence for Flash Mixing in He-rich sdB Stars will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-341465