Abstract
Recent UV observations of the globular cluster NGC 2808 (Brown et al. 2001) show a significant population of hot stars fainter than the zero-age horizontal branch (blue hook stars), which cannot be explained by canonical stellar evolution. Their results suggest that stars which experience unusually large mass loss on the red giant branch and which subsequently undergo the helium core flash while descending the white dwarf cooling curve could populate this region. Theory predicts that these late hot flashers should show higher temperatures than the hottest canonical horizontal branch stars and should have helium- and carbon-rich atmospheres. As a test of this late hot flasher scenario, we have obtained and analysed medium resolution spectra of a sample of blue hook stars in NGC 2808 to derive their atmospheric parameters. Using the same procedures, we have also re-analyzed our earlier spectra of the blue hook stars in ω Cen (Moehler et al. 2002) for comparison with the present results for NGC 2808. The blue hook stars in these two clusters are both hotter (T eff ≥ 35 000 K) and more helium-rich than canonical extreme horizontal branch stars in agreement with the late hot flasher scenario. Moreover, we find indications for carbon enhancement in the three most helium-enriched stars in NGC 2808. However, the blue hook stars still show some hydrogen in their atmospheres, perhaps indicating that some residual hydrogen survives a late hot flash and then later diffuses to the surface during the horizontal branch phase. We note that the presence of blue hook stars apparently depends mostly on the total mass of the globular cluster and not so much on its horizontal branch morphology.
Highlights
Low-mass stars burning helium in a core of about 0.5 M and hydrogen in a shell populate a roughly horizontal region in the colour-magnitude diagrams of globular clusters, which has earned them the name “horizontal branch” (HB) stars
We note that the presence of blue hook stars apparently depends mostly on the total mass of the globular cluster and not so much on its horizontal branch morphology
The Galactic globular clusters show a great variety in horizontal branch morphology, i.e., in the temperature distribution of their HB stars
Summary
Low-mass stars burning helium in a core of about 0.5 M and hydrogen in a shell populate a roughly horizontal region in the colour-magnitude diagrams of globular clusters, which has earned them the name “horizontal branch” (HB) stars. The increase in the bolometric correction with increasing temperature turns the blue HB into a vertical blue tail in optical colour-magnitude diagrams with the faintest blue tail stars being the hottest and least massive (see Fig. 1 for a prominent example of a blue tail) It is still unclear how some stars manage to lose most of their envelope mass and still undergo the helium core flash, as indicated by the long blue tails in a number of globular clusters (e.g., NGC 6752, Moehler et al 2000). Brown et al (2001) have explored the evolution of both the early and late hot flashers through the helium core flash to the EHB in more detail Their models show that a late hot helium flash on the white dwarf cooling curve will induce substantial mixing between the hydrogen envelope and helium core, leading to helium-rich EHB stars that are much hotter than canonical ones, as found previously by Sweigart (1997).
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