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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 • • • • • • • 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 • • • • 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 • • • • • • 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 • • • • • • • 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