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A GLIMPSE at CARBON STARS

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.

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A GLIMPSE at CARBON STARS

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  1. A GLIMPSE atCARBON STARS Tara Angle April 18, 2007 Brian Wilhite, University of Chicago

  2. 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

  3. 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

  4. 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

  5. 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”)

  6. Spectral Class - Modern • Revised by Morgan-Keenan (MK) • C-R • C-N • C-H -> used to be R-peculiar

  7. Characteristics N4+ C26 T ↓ N5 C26 Barnbaum, Stone, & Keenan, 1996

  8. An Odd Couple • Carbon stars were found to have • Tc (an unstable species) (Merrill 1952) And • Li (McKellar 1940) HOW?

  9. 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

  10. 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

  11. 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

  12. 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☼

  13. 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

  14. 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

  15. Spatial Distribution Barnbaum, Stone, & Keenan, 1996

  16. 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

  17. 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)

  18. 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”

  19. 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

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