A Spectroscopic Survey of Bright DA(…) White

Dwarfs Alexandros Gianninas, Pierre BergeronUniversité de Montréal

Jean DupuisCanadian Space Agency

Maria Teresa RuizUniversidad de Chile

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Survey

Initial survey aimed at studying ZZ Ceti instability strip (Gianninas et al. 2005, 2006)

Serendipitous discovery of GD 362 (Gianninas et al. 2004)

PhD thesis : expand survey to cover all DA white dwarfs in McCook & Sion catalog with V < 17.5

Motivation : study and understand and all bright (nearby) WD’s (see also posters by Limoges and Giammichele)

Observational sample of 1495 WD’s

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Final tally

Total spectra in final sample = 1334 89% complete, most missing spectra for stars

in the southern hemisphere 706 total spectra obtained specifically for this

project from 2003-2010

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S/N

52% with S/N > 70

78% with S/N > 50

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Analysis

Standard spectroscopic technique using new Balmer-line profiles from Tremblay & Bergeron (2009)

Other types require more in depth analyses: DA+DB

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DA+DB

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Analysis

Standard spectroscopic technique using new Balmer-line profiles from Tremblay & Bergeron (2009)

Other types require more in depth analyses: DA+DB DA+dM (37)

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DA+dM

Combine DA models with spectral templates of Bochanski et al. (2007)

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DA+dM

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Analysis

Standard spectroscopic technique using new Balmer-line profiles from Tremblay & Bergeron (2009)

Other types require more in depth analyses: DA+DB DA+dM (37) DAO (29)

→ Gianninas et al. (2010), ApJ, 720, 581… hot off the press!

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DAO white dwarfs

Distinguished by He II line at 4686 Å

Bergeron et al. (1994) established that stratified atmospheres don’t work, need to use homogeneous models

Most DAO’s in Bergeron et al. (1994) exhibit Balmer-line problem

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Solution to the Balmer-line problem Werner (1996) : add CNO (solar

abundances) to model atmospheres with appropriate Stark broadening, cools upper layers of the atmospheres where core of Balmer lines is formed

TLUSTY : computed new grid of models applying prescribed solution

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DAO white dwarfs

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Balmer-line problem

Bergeron et al. (1994) only detected problem in DAO stars

Only a few mentions of Balmer-line problem in normal DA stars

With our sample: 18 hot DA stars (Teff > 40,000 K) exhibiting the Balmer-line problem

Analagous models without helium

DAO models

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Teff = 65 000 Klog g = 7.5log He/H = -3.0

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Hot DA’s with Balmer-line problem

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Cooling Sequence

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Balmer-line problem : FUSE Must be caused by the presence of metals in

the atmosphere Searched FUSE archive for available FUV

spectra of stars in our sample Selected lines of metallic ions (C, N, O, P, Si,

S, Fe)

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Hot DA white dwarfs

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Hot DA’s with a few metals

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Hot DA’s with Balmer-line problem

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DAO white dwarfs

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DAO phenomenon

Balmer-line problem solved Revised evolutionary scenario One question remains: why do DAO’s have

helium?

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Mass loss

Unglaub & Bues (2000) : mass loss driven by metals can maintain sufficient helium in the atmosphere to explain observed abundances in DAO stars

“Wind limit” at about ~ 80,000 K, but we see DAO’s at cooler temperatures

Helium sinks faster than metals Mass loss does not allow for diffusive equilibrium →

no stratified atmospheres (except PG 1305, ~ 44, 000 K, below wind limit)

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post-AGB evolution

Stellar Wind

Initial abundance of Z

DAO + BP

DA + BP

DA

Teff

Enough Z?

No

Yes

Helium?Yes No