spectroscopy and the evolution of hot subdwarf stars
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Spectroscopy and the evolution of hot subdwarf stars
Peter NemethAstronomical Institute of the Czech Republic
K. U. Leuven - Nov. 9., 2012.
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Pannon Observatory and Visitor CenterBakonybél
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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
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Globular cluster CMD
Heber, U., 2009, ARA&A, 47, 211
Yi, S.K., 2008, ASPC, 392, 3NGC 2880
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What we know
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 sdO – dominant H and He II absorption lines
sdB – dominant H lines, weak He I absorption lines
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A GALEX sample
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The sample 694 UV-excess objects, NUV-
V < 0.5 7 observing runs, 2007-2011 ~200 targets Low-resolution, optical
spectroscopy Modeling with TLUSTY-
SYNSPEC 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
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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 Hot-flasher
e.g.: Miller Bertolami M. M. et al., 2008, A&A, 491, 253e.g.: Zhang X., Jeffery S. C., 2012, MNRAS, 419, 452
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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
Canonical Common Envelope Roche Lobe Overflow WD Mergers
Hot-flasher Deep mixing Shallow mixing No mixing
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Puzzling questions
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
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
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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