C-enhanced stars in the HERES survey: preliminary results

Sara Lucatello-INAF OAPD C-enhanced stars in the HERES survey: preliminary results

Hamburg/ESO R-processe Enhanced Star survey (HERES): Aimed r-process rich metal poor stars: -Cosmo-nucleo-chronology -Allow to study nature of r-process; Very challenging (extra rare objects)!!

HERES observations • Observations of large number of stars needed • Snapshot style approach: • -VLT UVES spectra R≈20,000 and S/N≈30-50; • -Spectral coverage between 3760 and 4980 Å; • -BVRI photometry plus JHK from 2MASS;

HERES: analysis Semi automatic analysis: • Teff from photometry; • Initial guesses for: • [Fe/H] from intermediate resolution spectroscopy; • Log g using Honda et al (2004) relationship; Code optimizes for atmospheric parameters and derive elemental abundances;

HERES: C-rich stars 252 stars successfully analyzed with this technique, provided measurements for the abundance of a wide set of elements; finding 6 r-II and 35 r-I stars; Semi-automatic analysis is not suitable for C-rich stars spectra: blending issues, Teff issues, etc... Manual analysis needed for 95 C-rich HERES stars!

The Approach • EW analysis and spectral synthesis; • Teff from V-K, log g from isochrones, [Fe/H] initial guess as HERES, reddening from Schlegel et al corrected according to Beers et al 2002; • Measured Fe, C, N, Ba, Eu, and (sometimes) Pb abundances for the sample;

The parameter space Teff between ~4200 and 6800K Logg between 0.5 and 4.3

The metallicity distribution 45 objects [Fe/H]≤-2 9 objects [Fe/H]>-1.5 <[Fe/H]>≈-2 dex 21 stars also analyzed with HERES automatic code; “manual” tech-nique yields higher values of [Fe/H]: T scale effect.

C-and N abundance distribution Most stars C-and N rich

C and N abundances HERES C-rich stars are indicated with closed symbols, literature ones with open symbols. Blue dots are CEMP-no. The C and N cor-relation is shared by all CEMP stars

HERES C-stars fraction 45 objects with [Fe/H]≤-2 dex [C/Fe]≥+1.0 216 [Fe/H] ≤-2 dex in HERES C-rich stars fraction in HERES: 18±2 %

C-abundance and evolutionary status Small scatter among evolved stars; [C/H] decreases with decreasing log g (evolutionary effect, mass of the con-vective envelope increases) Should be taken into account when cal-culating C-rich fraction.

N capture elements: Ba and Eu 5 r-rich stars found, (possibly more) including a highly r-enriched; No high [Fe/H] star is r-rich;

Eu abundance for HE1221-1948 T=6600K; [Fe/H]=-2.6 dex [C/Fe]=1.6 dex [Eu/Fe]=2.1 dex

N capture elements: Ba and Eu • Eu abundance correlates with Ba over a large range; • Scatter expected if independent processes • Small r-contribution or related processes?

CEMP subclasses fractions CEMP s-process 0.82 ±0.04 CEMP r-process 0.06 ±0.01 CEMP-no 0.11±0.03 CEMP-no in our sample are slightly more metal poor and less evolved. Most CEMP-s (over 50%) have some degree of Eu overabun- dances with respect to s-process. If we assume that such overabundance is due to the r-process there is a problem from a statistical point of view: should find same fraction of mildly r-enriched stars among C-normal stars, which is not the case, as such value is only about 15-20%

Conclusions • Found 5 new CEMP-r candidates, one very r-enriched; • HERES C-fraction consistent with Rossi et al. work, but needs correction to account for evolutionary effects; • Unexpected correlation between (mainly) s and (mainly) r-process elements. Enrichment mechanisms uncertain.

C-enhanced stars in the HERES survey: preliminary results