A GLIMPSE

atCARBON STARS

Tara Angle

April 18, 2007Brian 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?• 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

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

Characteristics

Barnbaum, Stone, & Keenan, 1996

N4+ C26

T ↓

N5 C26

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 Mira-types 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