Flash mixing on the white dwarf cooling curve

Spectroscopic confirmation in NGC 2808

Thomas M. Brown, Thierry Lanz, Allen V. Sweigart, Misty Cracraft, Ivan - Hubeny, Wayne B. Landsman

Research output: Contribution to journalArticle

15 Citations (Scopus)

Abstract

We present new Hubble Space Telescope far-UV spectroscopy of two dozen hot evolved stars in NGC2808, a massive globular cluster with a large population of "blue-hook" (BHk) stars. The BHk stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the H-R diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the extreme UV while appearing subluminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000-50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly discovered pulsating subdwarfs in ω Cen. In addition to the normal hot HB and BHk stars, we also obtain spectra of five blue HB stars, a post-HB star, and three unclassified stars with unusually blue UV colors.

Original languageEnglish (US)
Article number85
JournalAstrophysical Journal
Volume748
Issue number2
DOIs
StatePublished - Apr 1 2012

Fingerprint

hooks
horizontal branch stars
flash
cooling
stars
curves
diagram
spectroscopy
temperature
globular clusters
atmospheric diffusion
color
blue stars
iron
progeny
hot stars
atmospheric models
augmentation
color-magnitude diagram
stellar evolution

Keywords

  • globular clusters: individual (NGC 2808)
  • stars: atmospheres
  • stars: evolution
  • stars: horizontal-branch
  • ultraviolet: stars

ASJC Scopus subject areas

  • Space and Planetary Science
  • Astronomy and Astrophysics

Cite this

Flash mixing on the white dwarf cooling curve : Spectroscopic confirmation in NGC 2808. / Brown, Thomas M.; Lanz, Thierry; Sweigart, Allen V.; Cracraft, Misty; Hubeny, Ivan -; Landsman, Wayne B.

In: Astrophysical Journal, Vol. 748, No. 2, 85, 01.04.2012.

Research output: Contribution to journalArticle

Brown, Thomas M. ; Lanz, Thierry ; Sweigart, Allen V. ; Cracraft, Misty ; Hubeny, Ivan - ; Landsman, Wayne B. / Flash mixing on the white dwarf cooling curve : Spectroscopic confirmation in NGC 2808. In: Astrophysical Journal. 2012 ; Vol. 748, No. 2.
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abstract = "We present new Hubble Space Telescope far-UV spectroscopy of two dozen hot evolved stars in NGC2808, a massive globular cluster with a large population of {"}blue-hook{"} (BHk) stars. The BHk stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the H-R diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the extreme UV while appearing subluminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000-50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly discovered pulsating subdwarfs in ω Cen. In addition to the normal hot HB and BHk stars, we also obtain spectra of five blue HB stars, a post-HB star, and three unclassified stars with unusually blue UV colors.",
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AB - We present new Hubble Space Telescope far-UV spectroscopy of two dozen hot evolved stars in NGC2808, a massive globular cluster with a large population of "blue-hook" (BHk) stars. The BHk stars are found in ultraviolet color-magnitude diagrams of the most massive globular clusters, where they fall at luminosities immediately below the hot end of the horizontal branch (HB), in a region of the H-R diagram unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that these subluminous HB stars are very likely the progeny of stars that undergo extensive internal mixing during a late He-core flash on the white dwarf cooling curve. This flash mixing leads to hotter temperatures and an enormous enhancement of the surface He and C abundances; these hotter temperatures, together with the decrease in H opacity shortward of the Lyman limit, make the BHk stars brighter in the extreme UV while appearing subluminous in the UV and optical. Our far-UV spectroscopy demonstrates that, relative to normal HB stars at the same color, the BHk stars of NGC2808 are hotter and greatly enhanced in He and C, thus providing unambiguous evidence of flash mixing in the subluminous population. Although the C abundance in the BHk stars is orders of magnitude larger than that in the normal HB stars, the atmospheric C abundance in both the BHk and normal HB stars appears to be affected by gravitational settling. The abundance variations seen in Si and the Fe-peak elements also indicate that atmospheric diffusion is at play in our sample, with all of our hot subdwarfs at 25,000-50,000 K exhibiting large enhancements of the iron-peak elements. The hottest subdwarfs in our BHk sample may be pulsators, given that they fall in the temperature range of newly discovered pulsating subdwarfs in ω Cen. In addition to the normal hot HB and BHk stars, we also obtain spectra of five blue HB stars, a post-HB star, and three unclassified stars with unusually blue UV colors.

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KW - ultraviolet: stars

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