Carbon Enhanced Stars in the Sloan Digital Sky Survey ( SDSS ) T. Sivarani, Young Sun Lee, B. Marsteller & T. C. Beers Michigan State University & Joint Institute for Nuclear Astrophysics

Carbon Stars Carbon stars are objects with prominent C 2 molecular bands They are known since 1884 (Duner 1884) Carbon star: Star with C/O > 1

Carbon stars The observed Galactic and extra galactic carbon stars are ==> low mass stars on the AGB phase which has gone through the 3 rd dredge up of He burned products. ==> CH stars, Ba Stars from binary AGB companion ==> Dwarf carbon stars – possibly AGB mass transfer

An increasing interest in this class of objects has arisen from HK survey of metal-deficient stars conducted by Beers, Preston, & Shectman. Hamburg/ESO survey by Christlieb and Co. revealed for the first time an unexpectedly large number of very metal-poor stars with anomalously strong CH and CN bands (~25% in the metallicity range [Fe/H] < -2.5 compared to few percent among stars of higher metal abundances). I Large frequency of carbon enhanced stars at low metallicities

Implications 1. Is the intial mass function of the early galaxy was very different from the present day. Lucatello et al. (2004) 2. Pre Galactic metal enrichment. 3. The evolution at low metallcities are very different. 4. These stars are been accreted from metal poor local satellite galaxies

Carbon Enhanced Metal Poor (CEMP) stars Carbon rich [C/Fe] > 1.0 (Though the classical carbon star at solar metallicity is about 2 times solar). All of them are nitrogen rich (not all the carbon stars are nitrogen rich) Carbon stars in our Galaxy are different from the C-stars in LMC and other local groups. Does metallicity play an important role ?.

Chemical composition of CEMP stars s-process rich r-process rich with no n-capture enrichment. Alpha enhanced mild CEMP stars.

N-capture in CEMP stars Figure from Ryan et al. (2002) S-process rich  AGB mass transfer One with normal Li : LP706-7 CS The only r-process rich carbon star r-only pattern (Sneden et al. 2003) No n-capture elements HE Christlieb et al 2002 CS Aoki et al 2002 HE Cohen et al C/13C ~ 6 – 9 (CNO equilibrium) Also have slightly higher nitrogen abundance compared to s-process rich stars.

n-Capture Processes in CEMP stars [ C/Fe]=0.88, [N/Fe]=1.01, [O/Fe]=0.72 [Mg,Si/Fe] ~ 0.3 [Ba/Fe] = 0.99 [Eu/Fe] = 1.64 [Tb,Dy,Er…./Fe] ~ 1.6 [Ba/Fe] ~ 2.0 [Eu/Fe] ~ 1.5 [Ba/Eu] ~ -0.9 ~ pure r-process value at [Fe/H] ~ -3.0 r-process rich carbon star The only known CS r-only pattern (Sneden et al. 2003) s and r-process 15 objects are known. High [Eu/Ba] compared to pure s-process No n-capture elements HE Christlieb et al 2002 CS Aoki et al 2002 HE Cohen et al C/13C ~ 6 – 9 (CNO equilibrium) s-only stars?

To have a better understanding relative frequency among the various types and origin.  Increasing the no. of similar objects.  High resolution follow up.  radial velocity monitoring.  SDSS DR4 100,000 + SEGUE +SDSS II.....

SDSS DR4 has about 102,714 stars at a resolution R= 1800 from 3800Å to 9000Å Already about 500 Faint high latitute carbon stars have been identified by Morgan et al and Downes et al from SDSS DR1 The propermotions indicate that most of them are near by dwarfs. ==> Young population which had gone through binary mass transfer from an AGB star ==> the AGB star will now be seen as a white dwarf. There have been efforts in search for white dwarfs around these stars. SDSS DR4

Most of the key problems in astrophysics rely on the determination of accurate stellar fundamental parameters 1. Grid of synthetic spectra wavelength range A and 0.01Ang spacing ==> smoothed to SDSS resolution Grid of synthetic colors and lineindices 3. Empirical grid. High resolution spectra with well estimated parameters smoothed to SDSS resolution ==> ELODIE, VLT UVES archive, Cflib, S4N Analysis

Automated methods to estimate stellar parameters Teff, logg, [Fe/H], [C/Fe] … Validation of the estimation with real spectra. Clusters, Standard stars.

Synthetic grid Carbon Enhanced grid. [C/Fe]=0.5,1.0,1.5,2.0,2.5,3.0 At low temperature the stellar atmospheric structure itself changes due to high carbon abundance. ==> Solar scaled models are not appropriate. Teff = 3500K – 10000K, logg = [Fe/H] = – 0.00 Masseron et al. 2005

Synthetic spectra

Results from Artificial neural networks [Fe/H] logg Teff Com_cls True Diff IntErr Com_cls True Diff IntErr Com_cls True Diff IntErr

Now we are ready for the science From the SDSS spectra 1. [C/Fe] versus [Fe/H] 2. [alpha/Fe] versus [Fe/H] 3. s-process and C abundance 4. relative frequency of s-process rich and no n- enrichment How they are distributed in metallicity High resolution follow up C,N and isotopic abundance ==> hydrogen and helium burning s-process abundance (light and heavy) ==> metal poor AGB evolution and nucleosynthesis. r-process abundance ==> SN neucleosynthesis.

S-process among the C-enhanced stars In the figure the straight lines indicate the equivalent width for solar composition of Ba in The metallicity range. The symbols corresponds to Ba equivalent width for the SDSS-DR3 C-enhanced stars

Conclusions ● From SDSS data we can get [C/Fe], [alpha/Fe], [Sr/Fe] and [Ba/Fe] can be obtained. ● Relative frequency of various types of CEMP stars can be found from the Sr and Ba abundances. ● Several hundreds will be suitable for follow up high resolution studies. ● Radial velocity monitoring.