A strongly s-process enriched RV Tauri star in the LMC

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1 A strongly s-process enriched RV Tauri star in the LMC Maarten Reyniers, Hans Van Winckel, Leen Decin Instituut voor Sterrenkunde, K.U.Leuven, Belgium Carlos Abia Universidad de Granada, Spain Tom Lloyd Evans St.-Andrews,U.K. Kjell Eriksson Uppsala Astronomical Observatory, Sweden

2 Overview Search for RV Tau stars in the LMC MACHO : basic properties Chemical analysis with UVES spectra spectra atmospheric parameters syntheses and abundance results Intrinsic or extrinsic (post-) carbon star? Tc? Carbon Star Luminosity Function for LMC Comparison CH-stars Comparison C(N)-stars Conclusions A strongly s-process enriched RV Tauri star in the LMC 2

3 Search for RV Tau stars in the LMC RV Tau variability: alternating deep and shallow minima, periods between 30 and 150 days MACHO project: intensive monitoring of LMC with a dedicated 1.27m telescope at Mt.Stromlo during several years Alcock et al. (1998): search in MACHO lightcurves for RV Tau variability ~10 RV Tau stars in the LMC discovered Two bands Macho-Blue (broad V filter) Macho-Red (broad R) A strongly s-process enriched RV Tauri star in the LMC 3

4 Basic properties MACHO Period d (time between two deep minima) ~1.4 mag lightcurve phase diagram Mean absolute visual magn. Mv = -3.9 using distance modulus to LMC of 18.5 Bolometric magn. Mbol ~ -4.2 A strongly s-process enriched RV Tauri star in the LMC 4

5 Basic properties MACHO Other photometric data available, but... taken at different phases very difficult to construct Spectral Energy Distribution DENIS I,J,K SAAO 2MASS J,H,K A strongly s-process enriched RV Tauri star in the LMC 5

6 Basic properties MACHO Spectroscopic data lrs 1 lrs 2 lrs 3 UVES lrs 1 lrs 3 lrs 2 A strongly s-process enriched RV Tauri star in the LMC 6

7 Basic properties MACHO Low-resolution spectra at different phases (Pollard & Evans 2000) Minimum light: strong C2-bands, but weaker CH and CN-bands Sp.T. ~ late G or K0 Maximum light: molecular bands disappear Sp.T. ~ F A strongly s-process enriched RV Tauri star in the LMC 7

8 VLT-UVES spectra Molecules: C2 A strongly s-process enriched RV Tauri star in the LMC 8

9 VLT-UVES spectra Molecules: CN A strongly s-process enriched RV Tauri star in the LMC 9

10 VLT-UVES spectra Balmer Hα A strongly s-process enriched RV Tauri star in the LMC 10

11 VLT-UVES spectra Balmer Hβ A strongly s-process enriched RV Tauri star in the LMC 11

12 VLT-UVES spectra s-process lines A strongly s-process enriched RV Tauri star in the LMC 12

13 VLT-UVES spectra: atmospheric parameters Determination of parameters quite problematic: Strategy: Result: Carbon-rich atmospheric models not publicly available Clean lines are difficult to assess due to strong s-process blending Contribution molecular lines? Calculation of new carbon-rich models by the Uppsala group Most complete atomic line data base VALD and DREAM Molecular line lists from Abia et al. Iterative process of synthesis and adapting parameters yields T eff = 4900K (± 300K) log g = 0.0 (± 0.5) ξ t = 3.5 km s -1 (± 2) [M/H] = -1.4 Confirmed by independent study of iron lines A strongly s-process enriched RV Tauri star in the LMC 13

14 VLT-UVES spectra: C-band syntheses A strongly s-process enriched RV Tauri star in the LMC 14

15 VLT-UVES spectra: C-band syntheses A strongly s-process enriched RV Tauri star in the LMC 15

16 VLT-UVES spectra: C-band syntheses A strongly s-process enriched RV Tauri star in the LMC 16

17 VLT-UVES spectra: atomic lines syntheses A strongly s-process enriched RV Tauri star in the LMC 17

18 VLT-UVES spectra: atomic lines syntheses A strongly s-process enriched RV Tauri star in the LMC 18

19 VLT-UVES spectra: atomic lines syntheses A strongly s-process enriched RV Tauri star in the LMC 19

20 VLT-UVES spectra: atomic lines syntheses A strongly s-process enriched RV Tauri star in the LMC 20

21 VLT-UVES spectra: abundances Summary metals [Fe/H] = -1.4 CNO C/O > 2 12 C/ 13 C = 200±25 log(c/h-o/h)+12 > 8.1 N/H < 2.8 x 10-5 (or < 7.45) α-elements trend not clear LMC evolution? s-process [ls/fe] = +1.2 [hs/fe] = +2.1 [hs/ls] = +0.9 [Pb/hs] 0 A strongly s-process enriched RV Tauri star in the LMC 21

22 Discussion: intrinsic or extrinsic? First thought: look for Tc!... problematic synthesis A strongly s-process enriched RV Tauri star in the LMC 22

23 Discussion: intrinsic or extrinsic? Compare with Carbon Star Luminosity Function of the LMC Curve: recent model (Stancliffe et al 2005) Histogram: observations (Groenewegen) MACHO definitely on the low luminosity side of the (intrinsic) carbon stars! MACHO M bol ~ -4.2 low-luminosity tail has been successfully modelled with binary (extrinsic) C-stars by Izzard et al. (2004) A strongly s-process enriched RV Tauri star in the LMC 23

24 Discussion: intrinsic or extrinsic? Comparison with CH-stars: low-metallicity extrinsically s- process enriched stars with enhanced CH-bands A strongly s-process enriched RV Tauri star in the LMC 24

25 Discussion: intrinsic or extrinsic? [hs/ls] of C(N)-stars compared to the Torino models MACHO Extragalactic C-stars by de Laverny et al. (2006) ST*2 Galactic C-stars by Abia et al. (2002) ST/12 A strongly s-process enriched RV Tauri star in the LMC 25

26 Conclusions MACHO RV Tauri star in LMC discovered by the MACHO experiment Low-res spectra revealed (post-)carbon star classification High-res VLT-spectra analysis T eff 4900K and log(g) 0.0 low metallicity for LMC: [Fe/H] = -1.4 C/O > 2 and 12 C/ 13 C = 200±25 heavily s-process enriched: [ls/fe] = +1.2 and [hs/fe] = +2.1 lead-low star: [Pb/hs] 0.0 Probably intrinsically s-process enriched pulsations RV Tauri like supergiant (low gravity) luminosity quite low, but compatible with low-luminosity tail of the Carbon Star Luminosity Function of the LMC Tc: difficult detection A strongly s-process enriched RV Tauri star in the LMC 26