Luca Pasquini – European Southern Observatory

1. True solar analogues in the open cluster M67 Luca Pasquini – European Southern Observatory Piercarlo Bonifacio – Trieste Astronomical and Paris Observatories Sofia Randich – Arcetri Astrophysical Observatory Rolly Luigi Bedin – Space Telescope Science Institute Katia BiazzoINAF – Catania Astrophysical Observatory (paper submitted to A&A)

2. Introduction to the topics Specificity of our Sun & Solar System: • How typical is the Sun as age, mass, and chemical composition? • How typical is solar-type stars host planetary systems? • Are the exo-planetary systems similar at all to our Solar System? The quest for solar analogues has been going on for a long time (review: Cayrel de Strobel 1996), and it became even more compelling after the discovery of the first exo-planets (Mayor & Queloz 1995) Recent results in the field: • King et al. (2005): HD 143436 • Meléndez et al. (2006): HD 98618 • Meléndez & Ramirez (2007): HIP 56948 is the best solar twin known to date What are the environments where we can find homogeneous age & chemical composition, common birth & early dynamical environment? Open Clusters!

3. Why M67? • Age (3.5-4.8 Gyr; Yadav et al. 2008) • Chemical composition (in particular, metallicity: [Fe/H]=–0.03±0.03, 0.03±0.01, 0.02±0.03; Tautvaišiene et al. 2000, Randich et al. 2006, Pace et al. 2006) • Lithium depleted G stars (Pasquini et al. 1997) • Rich cluster • Photometry & Astrometry: 2 nights with WFI@MPG/ESO (B,V,I ; Yadav et al. 2008) • Spectroscopy: 3 nights with FLAMES/GIRAFFE@VLT-ESO in MEDUSA mode (R≈17000; Pasquini, Biazzo et al. 2008) Other details: • a=08:51:18, d=+11:48:00; l=215.696, b=+31.896 • p=1.05±1.96 mas(Hypparcos Catalogue)  d≈950 pc • E(B–V)=0.041±0.004 (Taylor 2007) ≈ 100 targets Yadav et al. (2008) DSS image: 60’ 60’ Our 100 targets: 13m≲V≲15m 0.60≲B–V≲0.75

4. Membership & Removal of binary stars Yadav’s Proper Motions (PMs) Our Radial Velocities Retaining the stars showing radial velocity (RV) variations < 1 km/s in three exposures and having mean RV within 2 (≈1.8 km/s) from the median cluster RV, we find 59 probable single RV members. Gaussian fit: <Vrad>=32.9 km/s, =0.73 km/s. Many of the retained stars tend to occupy the fainter side of the MS, where binaries are not expected to be present. On the other hand, our procedure still leaves several stars which are apparently binaries. This is because: • the RV measurements are not of superb quality • the observing time span is of only 18 days 59 probable single RV members

5. Effective Temperature Calibration: • Grid of synthetic spectra computed with SYNTHE • 1D LTE model atmospheres computed with the ATLAS code (Kurucz 1993, 1995) • Opacity Distribution Functions (Castelli & Kurucz 2003) with ξ=1 km/s, [Fe/H]=0, logg=4.4377, aMLT=1.25, Teff=5450–6300 K Line Depth Ratios From the initial sample of ≈100 lines, we have selected 6 line pairs sensitive to temperature and applied a method based on line-depth ratios (LDRs) to derive Teff of the probable members (Gray & Johanson 1991, Catalano et al. 2002, Biazzo et al. 2007). Synthetic spectrum at 5657 K 5477 K Synthetic solar spectrum (5777 K) Ha wings 6050 K Synthetic spectrum at 5867 K We have selected the spectral region in the range between 3 and 5 Å from the Ha line center as good Teff diagnostics (Cayrel et al. 1985; Fuhrmann et al. 1993; Barklem et al. 2002) GIRAFFE solar spectrum

6. DT LDR & DT Ha Teff,๏LDR=5792±27 K Teff,๏Ha=5717±100 K Teff,๏Theor=5777 K (Wilson & Hudson 1991) Teff,๏Phot=5730 K (Alonso et al. 1996) 10 Solar analogues: <DTeffLDR>=–13 K (s=60 K) <DTeffHa>=–9 K (s=58 K)

7. Lithium Abundance • NLi from curve of growth (COG) of Soderblom et al. (1993) • NLTE effects from Carlsson et al. (1994) • Spread present for stars cooler than 6000 K • Stars warmer than 6200 K have a decay (red side of the “Li-gap”?) • Stars with Teff=6000-6200 K don’t show scatter 8 stars stand out of the MS, suggesting a parallel long-period binary sequence not revealed by us

8. Solar Twins 59 stars RVs 2410 stars Yadav’s catalogue 10 solar analogues T LDR & T Ha & log(NLi) (100 K) 750 stars Yadav’s PMs (P>60%, PM<6 mas/yr) 5 solar twins T LDR & T Ha & log(NLi) (60 K) 90 stars V & B–V The core is not well reproduced due to: • chromosphere • NLTE effects

9. Error estimate: • our spread: 0.02/√10=0.006 • [Fe/H] uncertainty: 0.007 • photometry (Yadav et al. 2008): 0.008 • cluster reddening uncertainty (Taylor 2007): 0.004 • binaries! Stellar evolution effects: [Fe/H]=0.01 Solar Colour and Cluster Distance From our 10 solar analogues: B–V=0.692 (s=0.020) and <V>=14.583 mag (s=0.190) Correcting for reddening: <B–V>0=0.651 → <B–V>0=0.649±0.016 Correcting for reddening: <V>0=14.456 mag and <V>0–MV๏(Bessell et al. 1998)=9.65 → <V>0–MV๏=9.63±0.10 Error estimate: • our spread: 0.19/√10=0.060 mag • cluster reddening uncertainty (Taylor 2007): 0.012 mag • [Fe/H] uncertainty: 0.05 mag Stellar evolution effects: [Fe/H]=0.01

10. What we have done ... What we are doing ... • We have observed some solar analogues with SOPHIE@OHP • We have submitted two proposals: HARPS@ESO & SOPHIE@OHP to monitor the “single stars” • We have submitted an UVES@ESO proposal to better define the stellar parameters of our 5 best solar twins • We plan to apply the method to other OCs • Solar twins in M67 (PM, RV, Teff, logNLi) • Solar colour • Cluster distance modulus This work open the possibility to apply our method to other clusters older and younger than M67 for studies of stellar evolution