Flash mixing on the white dwarf cooling curve: Understanding hot horizontal branch anomalies in NGC 2808

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

Research output: Contribution to journalArticle

136 Citations (Scopus)

Abstract

We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the far-UV and near-UV cameras on the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high-temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we find a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster ω Cen. Our theoretical modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high-temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances, and leads, in turn, to a discontinuous increase in the HB effective temperatures at the transition between canonical and flash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

Original languageEnglish (US)
Pages (from-to)368-393
Number of pages26
JournalAstrophysical Journal
Volume562
Issue number1 PART I
DOIs
StatePublished - 2001
Externally publishedYes

Fingerprint

helium
flash
anomalies
cooling
horizontal branch stars
anomaly
color-magnitude diagram
stars
curves
hot stars
envelopes
diagram
hydrogen
globular clusters
dichotomies
spectral energy distribution
spectrographs
convection
cameras
telescopes

Keywords

  • Branch
  • Globular clusters: individual (NGC 2808)
  • Stars: atmospheres
  • Stars: evolution
  • Stars: horizontal
  • Ultraviolet: stars

ASJC Scopus subject areas

  • Space and Planetary Science

Cite this

Flash mixing on the white dwarf cooling curve : Understanding hot horizontal branch anomalies in NGC 2808. / Brown, Thomas M.; Sweigart, Allen V.; Lanz, Thierry; Landsman, Wayne B.; Hubeny, Ivan -.

In: Astrophysical Journal, Vol. 562, No. 1 PART I, 2001, p. 368-393.

Research output: Contribution to journalArticle

Brown, Thomas M. ; Sweigart, Allen V. ; Lanz, Thierry ; Landsman, Wayne B. ; Hubeny, Ivan -. / Flash mixing on the white dwarf cooling curve : Understanding hot horizontal branch anomalies in NGC 2808. In: Astrophysical Journal. 2001 ; Vol. 562, No. 1 PART I. pp. 368-393.
@article{189fc64c32dc4a2f9febd3734901eda9,
title = "Flash mixing on the white dwarf cooling curve: Understanding hot horizontal branch anomalies in NGC 2808",
abstract = "We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the far-UV and near-UV cameras on the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high-temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we find a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster ω Cen. Our theoretical modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high-temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the {"}born again{"} scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances, and leads, in turn, to a discontinuous increase in the HB effective temperatures at the transition between canonical and flash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.",
keywords = "Branch, Globular clusters: individual (NGC 2808), Stars: atmospheres, Stars: evolution, Stars: horizontal, Ultraviolet: stars",
author = "Brown, {Thomas M.} and Sweigart, {Allen V.} and Thierry Lanz and Landsman, {Wayne B.} and Hubeny, {Ivan -}",
year = "2001",
doi = "10.1086/323862",
language = "English (US)",
volume = "562",
pages = "368--393",
journal = "Astrophysical Journal",
issn = "0004-637X",
publisher = "IOP Publishing Ltd.",
number = "1 PART I",

}

TY - JOUR

T1 - Flash mixing on the white dwarf cooling curve

T2 - Understanding hot horizontal branch anomalies in NGC 2808

AU - Brown, Thomas M.

AU - Sweigart, Allen V.

AU - Lanz, Thierry

AU - Landsman, Wayne B.

AU - Hubeny, Ivan -

PY - 2001

Y1 - 2001

N2 - We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the far-UV and near-UV cameras on the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high-temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we find a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster ω Cen. Our theoretical modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high-temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances, and leads, in turn, to a discontinuous increase in the HB effective temperatures at the transition between canonical and flash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

AB - We present an ultraviolet color-magnitude diagram (CMD) spanning the hot horizontal branch (HB), blue straggler, and white dwarf populations of the globular cluster NGC 2808. These data were obtained with the far-UV and near-UV cameras on the Space Telescope Imaging Spectrograph (STIS). Although previous optical CMDs of NGC 2808 show a high-temperature gap within the hot HB population, no such gap is evident in our UV CMD. Instead, we find a population of hot subluminous HB stars, an anomaly only previously reported for the globular cluster ω Cen. Our theoretical modeling indicates that the location of these subluminous stars in the UV CMD, as well as the high-temperature gap along the HB in optical CMDs, can be explained if these stars underwent a late helium-core flash while descending the white dwarf cooling curve. We show that the convection zone produced by such a late helium flash will penetrate into the hydrogen envelope, thereby mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. This phenomenon is analogous to the "born again" scenario for producing hydrogen-deficient stars following a late helium-shell flash. The flash mixing of the envelope greatly enhances the envelope helium and carbon abundances, and leads, in turn, to a discontinuous increase in the HB effective temperatures at the transition between canonical and flash-mixed stars. We argue that the hot HB gap is associated with this theoretically predicted dichotomy in the HB properties. Moreover, the changes in the emergent spectral energy distribution caused by these abundance changes are primarily responsible for explaining the hot subluminous HB stars. Although further evidence is needed to confirm that a late helium-core flash can account for the subluminous HB stars and the hot HB gap, we demonstrate that an understanding of these stars requires the use of appropriate theoretical models for their evolution, atmospheres, and spectra.

KW - Branch

KW - Globular clusters: individual (NGC 2808)

KW - Stars: atmospheres

KW - Stars: evolution

KW - Stars: horizontal

KW - Ultraviolet: stars

UR - http://www.scopus.com/inward/record.url?scp=0001621192&partnerID=8YFLogxK

UR - http://www.scopus.com/inward/citedby.url?scp=0001621192&partnerID=8YFLogxK

U2 - 10.1086/323862

DO - 10.1086/323862

M3 - Article

AN - SCOPUS:0001621192

VL - 562

SP - 368

EP - 393

JO - Astrophysical Journal

JF - Astrophysical Journal

SN - 0004-637X

IS - 1 PART I

ER -