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)

True solar analogues in the open cluster M67True solar analogues in the open cluster M67

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 fieldfield: 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!

Introduction to the topics

Introduction to the topics

Why M67?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

Other details: =08:51:18, =+11:48:00; l=215.696, b=+31.896 =1.05±1.96 mas (Hypparcos Catalogue) d≈950 pc E(B–V)=0.041±0.004 (Taylor 2007)

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)D

SS im

age: 6

0’

60

’

≈ 100 targets

Our 100 targets:13m≲V≲15m

0.60≲B–V≲0.75

Yadav et al. (2008)

Membership & Removal of binary stars

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.

59 probable single RV members

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

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).

Effective TemperatureEffective Temperature

Synthetic

spectrum at

5657 KSynthetic solar spectrum (5777

K) Synthetic

spectrum at

5867 K

5477 K

6050 K

GIRAFFE solar

spectrum

Line Depth RatiosLine Depth Ratios

HH wings wings

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, MLT=1.25, Teff=5450–6300 K

We have selected the spectral region in the range between 3 and 5 Å from the H line center as good Teff diagnostics (Cayrel et al. 1985; Fuhrmann et al. 1993; Barklem et al. 2002)

T LDR & T HT LDR & T H

Teff,๏LDR=5792±27 K Teff,๏

H=5717±100 K

Teff,๏Theor=5777 K (Wilson & Hudson

1991)Teff,๏

Phot=5730 K (Alonso et al. 1996)

10 Solar analogues:10 Solar analogues:<Teff

LDR>=–13 K (=60 K)<Teff

H>=–9 K (=58 K)

Lithium AbundanceLithium 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

Solar TwinsSolar Twins

2410 starsYadav’s catalogue

59 starsRVs

750 starsYadav’s PMs

(P>60%, PM<6 mas/yr)

10 solar analoguesT LDR & T H& log(NLi)

(100 K)

5 solar twinsT LDR & T H& log(NLi)

(60 K)90 starsV & B–V

The core is not well reproduced due to: chromosphere NLTE effects

Solar Colour and Cluster Distance

Solar Colour and Cluster Distance

From our 10 solar analogues: B–V=0.692 (=0.020) and <V>=14.583 mag (=0.190)

Correcting for reddening: <B–V>0=0.651

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

B–VB–V Other determinationsOther determinations

0.658 Inverting our fit of all stars using T LDR

0.653 Inverting our fit of all stars using T H

~0.667 Old determinations: e.g., Barry et al. (1978)

0.62–0.64

Recent determinations: e.g., Holmberg et al. (2006)

→ <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

DistancDistance e

modulumoduluss

Other determinationsOther determinations

9.60 Sandquist et al. (2004)

9.61 An et al. (2007)

What we have done ...

What we have done ... What we are doing ...What we are doing ...

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

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