Spectroscopic Distinctions Between the Abundance Patterns of C-Enhanced, Metal-Poor Stars

1. Spectroscopic Distinctions Between the Abundance Patterns of C-Enhanced, Metal-Poor Stars Stelios A. Tsangarides Sean G. Ryan Timothy C. Beers Wako Aoki

2. Contents: • Introduction: abundance trends of C-enhanced, metal-poor stars (CEMPS) until 2002 • Analysis of 6 candidate CEMPS • Possible distinction between s-, hybrid r+s- and r-rich CEMPS • Possible distinction between n-rich and n-poor CEMPS

3. Source: Aoki et al. 2002, ApJ, 567, 1182 Introduction: 5 possible abundance patterns CEMP-s: s-process element enhanced CEMP-r+s: s-process rich; require r-process contribution to Eu, Gd, Dy, Ho and Er? CEMP-r: r-process element rich CEMP-no: n-capture element “normal” CEMP-a: n-capture element “normal”; rich in light a elements

4. Observations • C-program: • 24 candidate CEMPS from the HK survey were selected based on their colour and G-band strength. • WHT/UES spectra were obtained in August 2001 and April 2002 (l/Dl ~ 52,000). • J. E. Norris kindly provided us with spectra of 3 additional HK-survey stars fitting our selection criteria from a run conducted on AAT/UCLES in September 2000 (l/Dl ~ 40,000). • RV-program: • 15 HK-survey stars confirmed to be C-rich or under investigation by other groups were observed with shorter exposures on WHT/UES (April, June and July 2002) and TNG/SARG (May 2002; l/Dl ~ 57,000). • Shorter exposures of C-programme targets were also made during these runs.

5. Data Reduction • IRAF: • Overscan, bias and dark subtraction • Flat-field division • Cosmic-ray removal • Subtraction of sky and extraction of echelle orders • Wavelength calibration using Th-Ar exposures • Continuum normalisation • WHT/UES spectra: • C-program: S/N ≳ 40 per 1.75 km s-1 pixel at 4500 Å • RV-program: 10 ≲ S/N ≲ 20 at 4500 Å • AAT/UCLES spectra: • S/N ≳ 50 per 3.13 km s-1 pixel at 4500 Å • TNG/SARG spectra: • 10 ≲ S/N ≲ 20 per 2.20 km s-1 pixel at 4500 Å

6. Abundance Analysis • 10 targets were analysed (so far …). • Single-line analysis for model atmospheres and abundances of elements which do not exhibit HFS (WIDTH6). • Spectrum synthesis for abundances of elements in bands (C and N) and those which exhibit HFS (ATLAS). • 6 of these stars possess [Fe/H] ≲ -2 and [C/Fe] ≳ +1 and were thereby classified CEMPS:

7. The usual caution … • 1D, LTE Bell et al. model atmospheres were used, and NLTE corrections were not applied to the abundances of Fe. • Ba abundances in CEMPS were obtained from 1 or 2 strong Ba II lines (l4554 Å and l4934 Å), and only 1 weak Ba II line (l4131 Å). • Pb abundances were obtained from a single Pb I (l4058 Å) feature. • S/N in our spectra is at the limit of abundance calculations.

8. CEMPS from the Literature • CEMP-s: • CS 22880-074, CS 29526-110, CS 30301-015, CS 31062-050, HD 196944, LP 625-44, LP 706-7 • Aoki et al. 2001, ApJ, 561, 346; 2002, ApJ, 580, 1149 • CEMP-r+s: • CS 22948-027, CS 29497-030, CS29497-034, HE 2148-1247 • Barbuy et al. 2005, A&A, 429, 1031; Cohen et al. 2003, ApJ, 588, 1082; Ivans et al. 2005, astro-ph/0505002 • CEMP-r: • CS 22892-052, HD 115444 ? • Norris et al. 1997, ApJ, 488, 350; Westin et al. 2000, ApJ, 530, 783. • CEMP-no: • BS 16929-005, CS 22887-001, CS 22957-027, CS 30314-067 • Aoki et al. 2002, ApJ, 567, 1182; Norris et al. 1997, ApJ, 488, 350; 1997 ApJ, 489, L169 • CEMP-a: • CS 22949-037, CS 29498-043 • Aoki et al. 2002, ApJ, 576, L141; Norris et al. 2001, ApJ, 561, 1034 • Non-CEMP but r-process element rich: • BD+17o3248, CS 31082-001 • Cowan et al. 2002, ApJ, 572, 861; Hill et al. 2002, A&A, 387, 560

9. [Eu/Fe] = (0.81 ± 0.36) [Ba/Fe] + (0.76 ± 0.24) Distinction between s-rich, hybrid and r-rich CEMPS. I [Eu/Fe] and [Ba/Fe] follow a strong correlation in all s-rich stars; R2 = 0.873. Thus, Eu and Ba seem to be made by the same process in all s-rich CEMPS, and this seems different to the one producing them in r-rich metal-poor stars. Is Eu produced in the s-process, or are both elements produced by hybrid enrichment mechanisms in s-rich CEMPS? CEMP-s (this work) CEMP-s (literature) CEMP-r+s CEMP-r Non-CEMP but r-rich [Eu/Fe] = (1.34 ± 0.13) [Ba/Fe] + (-1.24 ± 0.26)

10. Some additional caution … • The sample is incomplete with CEMP-s stars  perhaps the correlation can weaken? • Additional r-process elements are observed in r+s stars, which are not discussed here. • No attempt is made to homogenise the data of the different groups. • Observational error could be much higher given S/N and model atmospheres. • [Ba/Fe] for literature stars is based on a single strong line (l4554 Å); La would be a better element to use in these plots (or the mean abundance of Ba, La, Nd and Sm).

11. [Eu/Fe] = (1.17 ± 0.23) [(C+N)/Fe] + (-0.84 ± 0.46) Distinction between s-rich, hybrid and r-rich CEMPS. II [Eu/Fe] correlates wellwith [(C+N)/Fe] in a single trend of s-rich stars; R2 = 0.617. An effect of the s-process? The higher the initial C, the greater the n-to-seed ratio leading to more synthesised Eu, subject to the specific details of s-processing in each AGB progenitor. … but CS 22892-052 and HD 115444 are only 2-3 s away from the trend.

12. Distinction between s-rich, hybrid and r-rich CEMPS. III [Pb/Fe] correlates well with [Eu/Fe] for all s-rich CEMPS (R2 = 0.637), implying that the two elements are made by the same process. Pb is almost completely made in the s-process ... [Pb/Fe] = (0.74 ± 0.14) [Eu/Fe] + (1.66 ± 0.21) If Pb is an s-process element, Eu seems to be synthesised in the s-process in these objects.

13. Distinction between s-rich, hybrid and r-rich CEMPS. IV [Pb/Fe] anti-correlates with [Ba/Eu] in both groups of s-rich CEMPS (R2 = 0.667). An effect of the s-process in metal-poor AGB stars? Since this component synthesises the elements heavier than the second peak, the greater the n-to-seed ratio, the higher the Eu and Pb abundances and the lower the Ba content. [Pb/Fe] = (-1.79 ± 0.32) [Ba/Eu] + (3.70 ± 0.20) The s-process in metal-poor AGB stars seems to favour the synthesis of Pb and Eu (?) over Ba.

14. Distinction between n-rich and n-poor CEMPS If all CEMPS are made by pollution from a metal-poor AGB star (taking place prior to the s-process for n-poor CEMPS), stars with Ba within the entire abundance range should exist. However, we observe a gap in the Ba abundances of n-rich and n-poor CEMPS. CEMPS-no seem to either have enhanced Pb, or not to be polluted by AGB stars.

15. Conclusions and Future Work • Is the hybrid r+s phenomenon real? • The trends of the Eu abundances and Ba/Eu ratios of both groups against their Pb abundances seem to require no contribution from the r-process to be explained. Is Eu synthesised completely in the s-process in CEMPS-r+s? • Future work: Caution requires the use of La instead of Ba, and to consider all the Pb stars with published Eu abundances. • N-rich versus n-poor CEMPS: • N-poor stars exhibit distinct Ba abundances from n-rich CEMPS. • Either they do not share the same nucleosynthetic origin(s) or CEMPS-no stars are enhanced in Pb. • Future: Include all s-rich stars with published Ba.