a glimpse at carbon stars
DESCRIPTION
A GLIMPSE at CARBON STARS. Tara Angle April 18, 2007. Brian Wilhite, University of Chicago. Background. First recognized by Secchi in 1868 Identified C 2 in spectrum By 1950’s – Molecules CN and CH recognized Heavy elements including Tc identified Light element Li also abundant. - PowerPoint PPT PresentationTRANSCRIPT
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A GLIMPSE
atCARBON STARS
Tara Angle
April 18, 2007Brian Wilhite, University of Chicago
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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
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Characteristics
• 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
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But, how do we know they aren’t M-stars?• 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
M-Star
Carbon Star
Brian Wilhite, University of Chicago
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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”)
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Spectral Class - Modern
• Revised by Morgan-Keenan (MK)
• C-R
• C-N
• C-H -> used to be R-peculiar
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Characteristics
Barnbaum, Stone, & Keenan, 1996
N4+ C26
T ↓
N5 C26
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An Odd Couple
• Carbon stars were found to have– Tc (an unstable species) (Merrill 1952)
And
– Li (McKellar 1940)
HOW?
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• 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
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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
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Magnitudes
• 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
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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☼
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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
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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
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Spatial Distribution
Barnbaum, Stone, & Keenan, 1996
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Variability
• Giant and Supergiant carbon stars can have a wide range of variability, from Mira-types with periods of hundreds of days to Cepheid-types with periods of a handful of days
• Many semi-irregular types also observed
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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)
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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”
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References
• 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