HydrogenDeficient Stars

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Hydrogen-Deficient Stars
Williamina Fleming 1857-1911
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Hydrogen-Deficient Stars

• Simon Jeffery
• Armagh Observatory

Hydrogen-Deficient Stars Discovery Classification
Surveys Distribution Frequency
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? Sgr
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Stars without hydrogen?

• The spectrum of ? Sgr is remarkable since the
  hydrogen lines are very faint and of the same
  intensity as the additional dark lines
• H? completely absent in R CrB
• Hydrogen lines were greatly weakened by partial
  emission in the spectrum of RCrB
• the simultaneous appearance of helium and
  metallic lines might be due to a supernormal
  abundance of helium or to the star being an
  exaggerated form of pseudo-cepheid or giant
• The uniformity of composition of stellar
  atmospheres appears to be an established fact
• R CrB
• ? Sgr
• ? Sgr

• Fleming 1891
• Ludendorff 1906
• Joy Humason 1923
• Plaskett 1927
• Payne 1925
• Berman 1935
• Struve Sherman 1940
• Greenstein 1940

reluctance
irrefutable evidence
...somehow, a very substantial amount of hydrogen
had been lost
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Discovery of helium
Pierre Jules Janssen 1824-1907

• 1868 A bright yellow line at 587.49nm in the
  spectrum of the chromosphere of the Sun
• 1868 A yellow line in the solar spectrum,
  labelled D3,, concluded it was caused by an
  element unknown on earth and labeled it ?????
  (helios).
• 1895 Isolated helium by treating cleveite with
  mineral acids. Actually looking for argon, but
  after removing N and O noticed a bright-yellow
  line that matched the D3 line seen in the Sun.
• Cleveite is an impure variety of uraninite. It
  has the composition UO2 with about 10 of the
  uranium substituted by rare earth elements.
  Helium is created by the alpha radiation of the
  uranium which is trapped (occluded) within the
  mineral
• 1907 Identifies alpha particle with He nucleus

Sir Joseph Norman Lockyer 1836-1920
Sir William Ramsay 1852-1916
cleveite
Lord Rutherford 1871-1937
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Helium Stars?

• Stars with broad emission lines
• HD124448
• FG Sge
• AM CVn
• hydrogen-deficient carbon stars -- a
  portmanteau expression for the lot
• Faint blue stars sdO,sdB
• PG1159, sdOC, sdOD
• He-sdB, He-sdO

• Wolf Rayet1857
• Popper 1940s
• Hofmeister 1940
• Herbig 1968
• Greenstein Matthews
• Bidelman
• Warner 1967
• Greenstein Sargent 1974
• Schmidt, Green Leibert 1986
• EC, HS, SDSS1990 - 2006

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faint blue stars in the Galactic halo
Jesse Greenstein
Aneila Sargent
Greenstein and Sargent 1974, ApJS 28, 157
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The Palomar-Green catalog of uv-excess stellar
objects
Green, Schmidt and Liebert 1986, ApJS 61, 305
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Hydrogen-Deficient Stars in the Galaxy

• recent history
• high mass
• low mass
• degenerates and rejuvenants

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1985 Mysore
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1991
Tutukov 1991, IAU Symp 145, 351
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Population I and massive helium stars

• Helium-rich B stars
• Wolf-Rayet Stars
• SN Ib
• Algols
• Ups Sgr variables

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Normal stellar evolution
He-core burning
Evolution of a 5M star Iben 1967, Ann Rev AA 12,
215
Iben 1967, Ann Rev AA 12, 215
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Helium-rich B orIntermediate He stars

• CP MS B stars
• 24 in catalogue of Drilling Hill 1986
• Helium-variable P1-10d
• ? Ori E
• dipole magnetic field 104 G inclined 90?
• metal-poor magnetic caps
• He-rich patches due to elemental segregation
• corotating clouds?

Bond Levato 1976, PASP 88, 95
Fe-poor
He-rich
clouds
Groote Hunger 1997, AA 331, 250
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Wolf-Rayet stars

• 1867 Charles Wolf and George Rayet at the
  Observatoire de Paris
• Early-type stars with bright broad emission lines
• Disagreement whether they were H-deficient up the
  the early 80s
• Found solely in spiral arms, associations and
  young clusters
• N-rich and C-rich sequences WN and WC
• H detected in about half
• About 230 Wolf-Rayets in the Galaxy
• (227 van der Hucht 2001)
• 159 WRs lt15m
• 100 in the LMC, 12 in the SMC

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Wolf-Rayet abundances

• WC sequence - ionzn of C
• WCE early
• WCL late
• WN sequence - ionzn of N
• WNL late
• WNE-s early strong em
• WNE-w early weak em
• Hydrogen
• detected - solid symbols
• none - open symbols

Hamann 1996, ASPC 96, 127
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WR star evolution

• Massive stars ?gtgt 40 M?
• Convective core plus overshooting
• Radiatively driven mass loss
• Processed material to surface

WNL
WNE
WC
Maeder 1991, IAUS 145, 221
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Type Ib Supernovae

• similar to SN I
• no H lines
• no Si II at maximum
• near star formation sites
• strong He features

Wheeler 1997, Sci.Am.
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SN Ib rates

• Cappellaro et al. (1993) Ia 0.39 /- 0.19
  SnuIb/c 0.27 /- 0.18 Snu II 1.48 /- 0.65
  SNu.

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• SN Ib equivalent to SN II in a massive star with
  helium envelope

SN II
SN Ib
Wheeler 1988, Sci.Am.

• where are progenitors ?

SN II
SN Ib
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?? Sgr

• SpectrumAp
• Campbell 1899, Cannon 1912
• composite variable
• strong helium on metallic spectrum
• H?, H? in emission
• Plaskett 1928, Morgan 1935, Merrill 1939,
  Greenstein 1940 et seq.

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?? Sgr variables

• ? Sgr
• Mp3.00.3M?
• Rp60R?
• Lp105 L?
• nHe/nH104

• ? Sgr
• P138 d
• KS Per
• P360d
• LSS 1922
• P55d
• LSS 4300
• P?
• ? BI Lyn
• P?

• velocities close to circular orbits about
  galactic center
• less than 200 pc from galactic plane
• Pop I helium stars with MgtMChandrasekhar
• SN Ib progenitors ?

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The stellar atmosphere opacity problem

• compare late B stars with ? Sgr
• similar Teff
• similar gravity
• same resolution

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Low-mass helium stars

• R CrB stars
• Extreme helium stars
• He-sdB stars
• He-sdO stars
• H-def PN central stars
• O(He) stars
• PG1159 stars

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1992 St Andrews
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R Coronae Borealis variables

• 35 known in galaxy,17 in the LMC (Claytons
  web page)
• Irregular light fades (5m)
• Low-amplitude pulsations
• Hydrogen-deficient spectrum
• Infrared excess

R CrB
R CrB
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Extreme Helium stars

• Approx. 20 known in galaxy
• Spectrum A- and B-
• Strong HeI
• Narrow lines supergiant
• No Balmer lines
• Strong N and C
• Origin? - clues from
• distribution
• chemical composition
• low-amplitude pulsations

Comparison of spectrum of an extreme helium star
with a helium-rich B star. Jaschek Jaschek,
1987, The classification of stars, Cambridge
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Distribution and kinematics

• concentrated towards gal. center
• do not share galactic rotation
• Galactic bulge
• hence range of Z

Jeffery, Drilling Heber 1987, MNRAS 226, 317
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Helium-rich subdwarfs

• He-sdO/sdB easily confused - need better
  classifications (cf Drilling et al. )
• PG definition (NGP) of sdOD same as for EHe stars
  found by Drilling in survey of OB stars (in
  plane)
• HesdB
• Prototype PG1544488- is a close binary!
• Others JL87, LB1766, - quite heterogeneous

• PG survey
• sdO
• sdOB
• sdOC - He-sdO
• sdOD - He-sdB
• 50 He-rich subdwarfs in 1996 catalogue did not
  discriminate sdB/sdO
• SDSS DR4
• He-sdB 5
• He-sdB 11

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Hdef planetary nebulae central stars

• Spectral-type WC
• H-poor, C very strong
• 50 in 1996 list

NGC6369 - HST/PC
Hamann 1996, ASPC 96, 127
Hamann 1996, ASPC 96, 127
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O(He) stars

• He II absorption
• CIV, NV, OVI emission
• 1996 3
• 1998 4 (32-1)
• GJJC1 He-sdO
• PN / no PN 1
• Same domain as PG1159 stars but considerably
  less metal rich

Rauch et al. 1998, AA
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PG1159 stars

• Spectroscopically unusual in the PG survey
• Very short-period mulit-periodic variables
• Spectra - HeII, highly ionized C, N, in abs and
  emission
• No PN

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Degenerates and Rejuvenants

• H-def white dwarfs
• AM CVn binaries
• Born-again stars

BPM 37093 (actually a DA, but its a neat
picture!)
Image Keck Observatory
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H-deficient white dwarfs

• H-dominated 4367
• DA 4008 H lines, no HeI or metal
• DAx 236 H lines, other weak lines
• DAbin 123 DAms star
• He-dominated 1009
• DB 332 HeI lines, no H or metal
• DBx 65
• DO 32 He II, plus He I or H
• DOx 15
• DQ 91 Carbon lines
• DQx 21
• DZ 61 Metal lines, no H or He I
• DC 358 Continuous spectrum
• DZx 22
• Dx 12 (DD,DF,DG,DH,DK,DX)
• Total 5376
• Similar numbers (?0.3dex) in SDSS DR4 catalogue
  (Eisenstein et al. 2006), but DB gap remains a
  real phenomenon.

• http//www.astronomy.villanova.edu/WDCatalog/index
  .html

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AM CVn stars

• HZ 29 is a peculiar, hydrogen deficient white
  dwarf with broad, apparently double absorption
  lines of He I (Greenstein and Matthews
  1957,1958)
• Interacting binary white dwarfs P17 - 46 min
• Accretion disk seen in high (optically thick) and
  low (thin) states, cf. CVs
• 15 systems known (cf. 6 in 1996!)
• ?(0) ? 1x10-6 ?-1 pc-3 (Roelofs et al. 2007)
• Merger progenitors?
• Probable GWR sources for LISA
• Reviews Warner 1995, Nelemans 2005

Warner Robinson 1972
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Born-again stars

• 3 in 100 years
• Rare?
• 3x107 / Gyr / Galaxy
• Not so rare?
• How does this compare with birth-rate of white
  dwarfs?
• What fraction of p-AGB stars experience a late or
  very late thermal pulse?

• FG Sge
• V605 Aql
• V4334 Sgr

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The story so far photometry

• 1944 Hoffmeister identifies a new variable in
  Sagittae (AN 274, 176)10th magnitude,
  irregular, not reddish
• Star continues to brighten, and to become redder.
• Patrol plates provide history back to 1900
• 1995 van Genderen Gautschy use historical
  brightness and colour gt L,Teff (AA 294, 453)

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FG Sge some puzzles

• Evolution -- Teff

?
high
low
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late thermal pulses

• (Herwig 2001)

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The very late thermal pulse

• Nuclear helium-burning re-ignited at surface of
  e--deg CO core after the H-shell has become
  inactive.
• Energy released forces outer layers to expand in
  1 - 10 years.
• H-rich, He-rich and C-rich material mixed by
  convective overshoot from the helium-shell flash,
  and penetrates through to surface before the star
  cools to develop deep opacity-driven convection
  zone.
• The corollary is that hydrogen is ingested at the
  same time as He, C and s-process elements are
  manifest at the surface.
• (Schönberner 1979, Herwig et al. 1999)

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Hydrogen-Deficient Stars
Williamina Fleming 1857-1911
summary
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Problems to solve

• Astronomy
• statistics
• distribution
• Evolution
• masses
• origin and fate
• links between classes
• Physics
• atmospheres
• pulsations
• mass loss
• convection
• nucleosynthesis