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New Observational Evidence of Flash Mixing on the White Dwarf Cooling CurveBlue hook stars are a class of subluminous extreme horizontal branch stars that were discovered in UV images of the massive globular clusters w Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium-core flash on the white dwarf cooling curve. This "flash mixing" produces hotter-than-normal EHB stars with atmospheres significantly enhanced in helium and carbon. The larger bolometric correction, combined with the decrease in hydrogen opacity, makes these stars appear sub luminous in the optical and UV. Flash mixing is more likely to occur in stars born with a high helium abundance, due to their lower mass at the main sequence turnoff. For this reason, the phenomenon is more common in those massive globular clusters that show evidence for secondary populations enhanced in helium. However, a high helium abundance does not, by itself, explain the presence of blue hook stars in massive globular clusters. Here, we present new observational evidence for flash mixing, using recent HST observations. These include UV color-magnitude diagrams of six massive globular clusters and far-UV spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).
Document ID
20110023560
Acquisition Source
Goddard Space Flight Center
Document Type
Preprint (Draft being sent to journal)
Authors
Brown, T. M.
(Space Telescope Science Inst. Baltimore, MD, United States)
Lanz, T.
(Maryland Univ. College Park, MD, United States)
Sweigart, A. V.
(NASA Goddard Space Flight Center Greenbelt, MD, United States)
Cracraft, Misty
(Space Telescope Science Inst. Baltimore, MD, United States)
Hubeny, Ivan
(Arizona Univ. Tucson, AZ, United States)
Landsman, W. B.
(ADNET Systems, Inc. Potomac, MD, United States)
Date Acquired
August 25, 2013
Publication Date
January 1, 2011
Subject Category
Astronomy
Report/Patent Number
GSFC.JA.5517.2011
Funding Number(s)
CONTRACT_GRANT: STScI 10815
CONTRACT_GRANT: STScI 11665
CONTRACT_GRANT: NAS5-26555
Distribution Limits
Public
Copyright
Public Use Permitted.
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