A
GLIMPSE
at
CARBON
STARS
Tara Angle
April 18, 2007
Brian Wilhite, University of Chicago
Background
• First recognized by Secchi in 1868
Identified C2 in spectrum
• By 1950’s –
– Molecules CN and CH recognized
– Heavy elements including Tc identified
– Light element Li also abundant
Characteristics
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Typically in the 3000-4000K temperature range
Red in color
Two distinct types – giants and dwarves
Giants are single stars
Dwarves first discovered by Dahn et al (1977)
Binaries
Form by mass transfer with WD companion
But, how do we know
they aren’t M-stars?
M-Star
• Same general temperature
range, but…
• M stars present with metal
oxides such as TiO, VO, etc.
• Carbon stars have C/O ratios
high enough to use all of the
oxygen for CO with plenty of
carbon left over to form carbon
based molecules such as C2,
CN, CH
Carbon Star
Brian Wilhite, University of Chicago
Spectral Class - Classical
• Originally classified by Shane (1928) as R
and N stars
• R0-R3 -> relatively weak C2 and CN bands
• R5-R8 -> strong bands and continuum
down to 3900Å
• N-stars -> also strong bands of C2 and CN
but continuum falls off before 4000Å
(“ultraviolet deficiency”)
Spectral Class - Modern
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Revised by Morgan-Keenan (MK)
C-R
C-N
C-H -> used to be R-peculiar
N4+ C26
Characteristics
T↓
N5 C26
Barnbaum, Stone, & Keenan, 1996
An Odd Couple
• Carbon stars were found to have
– Tc (an unstable species) (Merrill 1952)
And
– Li (McKellar 1940)
HOW?
• Tc has a half-life of 2 X 105 years, so must have
formed in star through neucleosynthesis
• Common Li isotopes do not survive in the stars
which become carbon stars due to proton
capture at high (2 X 106 K) temperatures
**We observe them in the atmospheres due to
dredge-up from deep convective mixing
This also explains the carbon abundance present
13C
Measurements
• Allowed first opportunity to measure carbon
isotopic ratio outside our Solar System
• Terrestrial ratio 12C/13C ~89
• C-N stars –> 30 < 12C/13C < 100 (Lambert et al
1986)
• C-R stars –> 4 < 12C/13C < 9
• C-H stars -> groups which fall into both above
ranges
Magnitudes
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Determined for stars in known distance systems
Globular clusters
Other galaxies (notably the LMC and SMC)
Stars with parallax measures from Hipparcos
<Mv> ≈ 0.76 ± 1.06
Only 3 dC’s measured by parallax, so not
representative of these
Mass
• No known carbon stars in visual binary systems with
measured parallax
• None ever seen to be eclipsed
• Statistical analysis of halo C-H stars yields 0.8 ± 0.1 M☼
(McClure and Woodsworth 1990)
• Not representative of all
• Masses inferred from
• Distribution
• MS turnoff
• Stellar evolution determinations
• Range from 0.8 M☼ to 8 M☼
Temperature
• For C-R and C-H stars, can use
photometry to determine Teff
• R stars ~ 4200-5000K
• Hot C-H stars ~ 4550-5320K
• Cooler C-H stars – large number of bands
and lines in spectra make it difficult to
determine Teff accurately
• N-stars ~ 2200-3300K
Prevalence
• Many giant and supergiant carbon stars
observed in the Magellanic Clouds
• Many dwarf carbon stars (dC) found in the
solar neighborhood (within a few 100
parsecs)
• Seem to be more common than giants in
this region
Spatial Distribution
Barnbaum, Stone, & Keenan, 1996
Variability
• Giant and Supergiant carbon stars can
have a wide range of variability, from Miratypes with periods of hundreds of days to
Cepheid-types with periods of a handful of
days
• Many semi-irregular types also observed
Mdot : Mass Loss Mechanism
• Variable stars are known for mass loss
• Information is mostly empirical for these
types of stars
• Mdot can be as high as 10-5 to 10-6
M☼/year (Paczyński 1970, Schönberner
1983)
Formation Mechanism(s)
• Mentioned that convection brings carbon into the
atmosphere –
• Classical models of giant stars don’t allow for a
convective zone deep enough to dredge-up the
carbon material formed in deeper layers
• BUT – a He shell flash can create a convective
zone, and if hot enough can penetrate the H
shell and bring material to the surface
– “Hot-Bottom convection zone”
References
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Barnbaum, Stone, Keenan, 1996, ApJS,105, 419
Herwig, 2005, ARAA 43, 435
Liebert et al, 2003, AJ 126, 2521
McClure & Woodsworth, 1990, ApJ 352, 709
Schonberner D. ,1983, ApJ 272,708
Wallerstein & Knapp, 1998, ARAA 36, 369
Wilke, Brian , University of Chicago, internet image of spectra