The evolution of heliumrich subdwarf B stars

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The evolution of helium-rich subdwarf B stars

• Amir Ahmad
• Armagh Observatory

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Introduction

• Subdwarf stars form the dominant population of
  faint blue blue stars in our galaxy and giant
  elliptical galaxies
• sdB stars are 0.5Mo core helium-burning stars
  with a thin hydrogen envelope
• Early 1980s helium-rich and helium-poor objects
  thought to be separated by a temperature boundary
  at 40000 K
• PG survey reports a small fraction of subdwarfs
  with strong HeI lines
• Heber et al. 1988 present spectral analysis of
  helium-rich subdwarf B star - 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 names
• sdOD - Green et al. 1986
• He-sdB
• Moehler et al. 1990
• Beers et al. 1992
• Kilkenny et al. 1997
• sdB4 - Drilling 1996 (preliminary classification)
• sdBHe4 - Jeffery et al. 1997 (preliminary
  classification)

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Spectrum of He-sdB stars
High resolution blue spectrum of PG1544488
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Spectroscopic observations
Isaac Newton Telescope
William Herschel Telescope
Angelo-Australian Telescope
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Model atmospheres

• Model spectra computed with the line blanketed
  plane-parallel LTE codes STERNE and SPECTRUM
  (Jeffery et al. 2001)
• 3D rectangular grid defined by
• helium rich grid
• Teff 25000 (5000) 40000
• log g 4.0 (0.5) 6.0
• nHe 0.9480, 0.9880, 0.9970
• helium poor grid
• Teff 15000 (5000) 40000
• log g 4.0 (0.5) 6.0
• nHe 0.1000, 0.5000, 0.8990

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Reduction / Analysis

• Spectra reduced using standard IRAF routines
• Spectral analysis done using SFIT2 - the latest
  version of the spectral fitting software SFIT
  (Jeffery et al. 2001)

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Spectral fitting

• Blue spectrum model fit of PG 1544488 (SFIT2)
• Teff 34 000300 K, log g 5.10.1, nHe
  0.990.01
• (Ahmad Jeffery 2003)

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Optical spectra
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Spectra analysis

• Spectra with model fits for He-sdB stars
• (Ahmad Jeffery 2003)

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Hot He-sdB stars

• Significant number of He-sdB stars appear to be
  hotter than 40000K
• nLTE effects become significant for temperatures
  gt 35000K (Kudritzki 1979)
• nLTE model grid being computed using TMAP
  (Dreizler Werner 1993)

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HOT He-sdB stars
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Cluster He-sdB stars
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Parameters of cluster He-sdB

• Star Teff log g nHe Reference
• K cgs
• F2-2 360004000 5.900.5 0.87 Moehler et
  al.1997
• (M15) 35600650 5.500.2 0.870.01 SFIT2
• D10763 352001500 4.350.19 0.90 Moehler et
  al.2002
• (? Cen) 36100650 4.600.2 0.870.01 SFIT2

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Another interesting He-sdB

• JL87 has been called a helium-rich star by Schulz
  et al. 1991
• Based on the criteria for spectral classification
  set by various authors this star would clearly
  not qualify as a He-sdB star as it shows strong
  Balmer lines

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Parameters for JL87

• Teff log g nHe Reference
• K cgs
• 280001000 5.200.3 0.170.05 Schulz et al.
  1991
• 30000 5.00 Magee et al. 1998

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SED of He-sdB stars
IUE spectra of He-sdB stars
SED fitting He-sdB stars
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He-sdB or He-sdO?

• EC 14316-1908
• He-sdB (Kilkenny et al. 1997)
• RA(J2000) 14 34 29.9
• dec(J2000) -19 21 30
• V 13.21
• CS 22871-0019
• He-sdO (Beers et al. 1992)
• RA(J2000) 14 34 30.2
• dec(J2000) -19 21 27
• V 13.25

DSS1/STScI J image (Aladin)
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Spectrum (IUE Optical)

• Teff 33 800 K (Beers et al. 1992)
• Teff 77 000 K (Drilling Beers 1995)
• Teff gt 40 000 K (SFIT2)

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log g - Teff diagram

• Position of He-sdB stars on the log g - Teff
  diagram with other subluminous stars (Ahmad
  Jeffery 2003)

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log g - Teff diagram

• Helium-rich stars on the log g - Teff diagram.
  Single star evolutionary tracks (dashed lines)
  taken from Dorman et al. 1993. Merged He-He WD
  tracks taken from Saio Jeffery 2000. (Ahmad
  Jeffery 2003)

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Subclasses of He-sdB stars

• From optical and IUE spectra of He-sdB it is
  clear that there are two distinct subclasses of
  He-sdB stars
• Carbon-rich, eg. PG1544488
• Carbon-poor, eg. PG0914-037

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Evolution

• It was realised early on that helium-rich
  subdwarfs do not evolve from helium-poor sdB
  stars (Groth et al. 1985)
• sdB have nHe lt 0.01 (Heber 1986)
• sdB evolution takes 108 yr (Caloi 1989)
• Gravitational settling takes 105 yr (Wesemael et
  al. 1982)
• Hence objects evolving from sdB stars should have
  depleted helium abundance

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Existing evolution models

• Single star evolution
• Brown et al. (2001) suggest 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 helium
  and carbon rich hot subdwarf

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Evolutionary models

• Binary merger model
• Iben Tutukov (1985, 1987) suggested that merger
  of two degenerate white dwarf can produce 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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Binary merger
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Chemical abundance

• Single star evolution model and white dwarf
  merger models both can predict enriched carbon in
  the atmospheres of He-sdB stars
• Carbon poor He-sdB stars must be products of
  HeHe WD merge while carbon-rich He-sdB stars
  might result from COHe WD merger

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Further investigation?

• Binary fraction
• Luminosity
• Kinematics
• Chemical abundance

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Odd He-sdB stars