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

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

Heber, U., 2009, ARA&A, 47, 211

Yi, S.K., 2008, ASPC, 392, 3NGC 2880

K. U. Leuven - Nov. 9., 2012.

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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

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

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

K. U. Leuven - Nov. 9., 2012.

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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?

K. U. Leuven - Nov. 9., 2012.

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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