Spectroscopy and the evolution
of hot subdwarf stars
Peter Nemeth
Astronomical Institute of the Czech Republic
Pannon Observatory and Visitor Center
Bakonybél
K. U. Leuven - Nov. 9., 2012.
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Subdwarf stars?
• The Hertzsprung-Russell diagram
• Red Giants, White dwarfs.
• Stellar evolution
• Stellar populations
• Cool/hot subdwarfs
• Globular cluster CMD
• EHB stars.
• Heavy traffic of evolved stars
around the EHB
K. U. Leuven - Nov. 9., 2012.
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Globular cluster CMD
NGC 2880
Heber, U., 2009, ARA&A, 47, 211
Yi, S.K., 2008, ASPC, 392, 3
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What we know
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Progenitor MS mass between 1 and ~5 Mʘ
Evolved, core helium burning stars
Thin hydrogen layer
Many in binaries with MS or WD companions
Direct evolution towards white dwarfs
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Structure of subdwarfs
sdB
sdO
From Wikipedia
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Spectral classification
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sdO – dominant H and He II absorption lines
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sdB – dominant H lines, weak He I absorption lines
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A GALEX sample
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The sample
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694 UV-excess objects, NUVV < 0.5
7 observing runs, 2007-2011
~200 targets
Low-resolution, optical
spectroscopy
Modeling with TLUSTYSYNSPEC
Paper I: 52 stars,
interpolation in 3 grids,
H, He
Paper II: 180 stars,
steepest-descent with a
constant level structure,
H, He, CNO
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The fitting method
Green: Model, T = 40 000 K, log g = 5.6, log He = -1, log CNO = -2
Red: J2059+4232, T = 20 700 K, log g = 4.5, log He = -0.4
log C = -2.8, log N = -2.9, log O < -2.6
K. U. Leuven - Nov. 9., 2012.
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The fitting method
Green: Model, T = 40 000 K, log g = 5.6, log He = -1, log CNO = -2
Red: J2059+4232, T = 20 700 K, log g = 4.5, log He = -0.4
log C = -2.8, log N = -2.9, log O < -2.6
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Composite spectra
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Temperature – gravity
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Abundances
• Multiple dichotomies
• Can abundance patterns
indicate the evolution or other
properties, like pulsations, of
these stars?
• HST STIS shows high
abundances of iron-peak
elements, but not much Fe.
(O’Toole & Heber, 2006)
• Slow, rapid and hybrid
pulsators are well separated,
but not preictable
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Luminosity distribution
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Spectral evolution?
Canonical
e.g.: Zhang X., Jeffery S. C., 2012, MNRAS, 419, 452
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Hot-flasher
e.g.: Miller Bertolami M. M. et al., 2008, A&A, 491, 253
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Spectral evolution? Complicated.
UV flux induces convection,
turbulence, mixing, wind ...
lots of complications.
(Unglaub, 2008)
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Formation channels
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Canonical
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Common Envelope
Roche Lobe Overflow
WD Mergers
Hot-flasher
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Deep mixing
Shallow mixing
No mixing
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Puzzling questions
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How do subdwarfs form? Which formation
scenarios are viable and what are their
contributions to the observed SD distribution?
What drives the mass-loss on the RGB?
He-sdO  ?  sdB
How clean is the observed population from
ELM WD, post-AGB, CSPN stars?
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The SD1000 Collaboration
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We need spectroscopy for a large sample
Repeat (and later extend) the analysis in a
homogeneous way
Derive homogeneous parameters
Collaborations are important because
subdwarfs link RGs to WDs
GAIA will provide distances and masses
Find binaries
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References
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Østensen, R.H.; Comm. in Asteroseismology, 2008, 159, 75
Heber, U.; ARA&A, 2009, 47, 211
sdB sdO page on Wikipedia
Zhang, X., Jeffery, S. C.; 2012, MNRAS, 419, 452
Miller Bertolami, M. M. et al.; 2008, A&A, 491, 253
Yi, S.K.; 2008, ASPC, 392, 3
O’Toole, S.J; Heber, U.; 2006, A&A, 452, 579
Unglaub, K.; 2008, A&A, 486, 923
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