Spectroscopic Analyses of HeliumRich Subdwarf B Stars

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Spectroscopic Analyses of Helium-Rich Subdwarf B
Stars

• Amir Ahmad
• Armagh Observatory

C.S. Jeffery, Armagh Observatory
21 July 2004
EUROWD 2004
2
Introduction

• Subdwarf B stars form the dominant population of
  faint blue stars (mB 16) in our galaxy and
  giant elliptical galaxies
• They are 0.5 MSun core helium-burning stars
• Progenitors of White Dwarfs
• Evolution has been the subject of much debate
  although binary evolution is thought to play an
  important role
• sdB stars have spectra dominated by Hydrogen
  Balmer lines
• A small subset ( 70) of hot subdwarf stars
  includes the helium-rich subdwarf B (or He-sdB)
  stars

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Outline

• A word about classification
• Atmospheric parameters
• Evolutionary models
• The FUSE/optical dilemma
• PG1544488

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He-sdB nomenclature

• Spectral classification criteria
• strong HeI lines plus weak HeII
• no detectable Balmer lines
• Helium-rich subdwarf B stars a.k.a.
• sdOD - Green et al. 1986
• He-sdB
• Moehler et al. 1990
• Beers et al. 1992
• Kilkenny et al. 1997
• sdB4 - Drilling 1996
• sdBHe4 - Jeffery et al. 1997

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Optical spectrum of PG1544488
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Atmospheric parameters

• 22 stars (10 with Teff K gt 40 000)
• 30 000 ltTeff K lt 40 000
• 5.0 lt logg cgs lt 6.0
• 0.10 lt nHe lt 0.99
• Note the range in helium abundance.
• According to the PG definition all He-sdB stars
  should be
• extremely helium rich
• He-sdBs can be divided into carbon-rich and
  carbon-poor groups.
• Carbon-rich group has nC lt 0.01

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log g - Teff diagram

• Position of He-sdB stars on the log g - Teff
  diagram with
• other subluminous stars

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Existing evolution models

• Single star evolution
• Lanz et al. (2004) have argued that stars
  evolving with high mass loss on the red giant
  branch undergo a late helium core flash on the
  white dwarf cooling track leading to convective
  flash mixing of the envelope which then forms a
  helium and carbon rich hot subdwarf
• Figure from Brown et al. 2001, ApJ 562, 368.

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Evolutionary models

• Binary merger model
• Iben Tutukov (1985, 1987) suggested that the
  merger of two degenerate white dwarfs can produce
  a hot subdwarf with depleted hydrogen atmosphere
• Saio Jeffery (2000, 2002) have more recently
  modelled WD mergers to explain the origin of EHe
  stars

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HeHe WD merger
flash-mixing model
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Previous analyses of PG1544488

• Teff logg Reference
• 31 000 5.1 Heber et al. (1988)
• optical / UV spectra
• 34 000 5.1 Ahmad Jeffery (2003)
• low resolution optical spectra
• 32 100 5.0 Ahmad Jeffery (2004)
• energy distribution
• 36 000 6.0 Lanz et al. (2004)
• far ultraviolet spectra

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Optical vs. FUV

• Lanz et al. (2004) have measured physical
  parameters of 3 He-sdB stars using FUSE spectra
• We have measured physical parameters of the same
  3 stars using optical spectra
• The effective temperatures agree but the surface
  gravities are different, our measurements are 1
  dex lower for all 3 stars (errors are 0.30 dex).
• Lanz et al. argue that this is due to enhanced
  carbon abundance which was not included in the
  models we used.
• We have tested this and found it not to be true.
  . The dilemma remains

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Optical spectrum of PG1544488
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Red spectra from WHT/ISIS
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Time-tagged spectra from FUSE
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He He subdwarf binary!

• Spectroscopic observations of PG1544488 obtained
  in 2003 May showed strange line profile in the
  neutral helium lines
• Visual examination indicated line splitting.
  Radial velocities of the 2 components were
  measured
• Archival FUSE spectra were re-extracted into
  time-resolved spectra. Radial velocity shifts
  were noted and showed large variations similar to
  those seen in the optical spectra
• Preliminary parameters of the binary indicate
  that its a system comprising two hot helium-rich
  subdwarf stars P 0.48d.

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Orbital solution
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Summary ...

• He-sdB stars form a highly inhomogenous group of
  hot subdwarf stars.
• A fraction are misclassified (or cool) He-sdO
  stars.
• Evolution for all the subgroups present is still
  under debate
• The prototype, PG1544488, is a binary with a
  period of 1/2 day containing two low-mass hot
  helium subdwarf stars
• Clear that close binary evolution plays an
  important role in formation
• We need higher resolution data for detailed
  abundance and velocity studies.