Dynamics and formation of the Oort Cloud, Lille, 28-30 September 2011 The solar twin in Messier 67 A clue to the formation of the Oort Cloud? Andreas Korn Bengt Gustafsson Hans Rickman Uppsala University The Sun as a star Fuhrmann (2008) Fuhrmann The Sun is a very normal, albeit fairly high-mass, thin-disc star. There are many stars like the Sun... A brief history of solar twins • Pioneering work by Hardorp (1978) • A conference on solar analogs (Lowell Obs., 1997) settled for 18 Sco as the best known solar twin • Since then, the HIPPARCOS catalogue has been systematically searched for solar twins, producing dozens of near-twins and HIP 56948 (Meléndez & Ramírez 2007) with a solar-like lithium abundance • Using FLAMES at the VLT, Pasquini et al. (2008) looked for solar-twin candidates in M67: they identified five high-probability solar twins The solar twin in M67 • So far, one star has been followed up with high- resolution (R=47,000) and high S/N ratio (180 pixel-1). • Except for a possibly slightly enhanced metallicity, M67-1194 is a true solar twin (Önehag et al. 2011): • Even its equatorial rotational velocity and its lithium abundance are compatible with the solar values. iron nickel calcium calcium sodium calcium avelength [Å] w iron Comparing the spectra silicon n ormalized flux ormalized Detailed Sun-twin composition Relative to solar twins in the field, the Sun is rich in volatile elements and poor in high- Tcond refractories Meléndez et al. (2009) Detailed Sun-twin composition Relative to solar twins in the field, the Sun is rich in volatile elements and poor in high- Tcond refractories Chemically, M67- 1194 resembles Önehag et al. (2011) the Sun! Age determination for M67 • Most age determinations in the literature scatter around 4 Gyr, both from fitting of the turnoff region and the white dwarf cooling sequence (e.g. Magic et al. 2010; Bellini et al. 2010) • M67-1194 allows us to age-date M67 with a reduced sensitivity to systematic effects: , compatible with the solar age • Using the methods of Baumann et al. (2010), we would derive Given remaining modelling uncertainties, M67 may well be as old as the Sun! M67: orbital parameters U and V velocities fairly compatible M67’s current position with the Sun’s orbit But: W / zmax solar orbit are much larger Caused by interaction with a GMC? To be investigated... Davenport & Sandquist (2010) A working hypothesis The chemical similarity of M67-1194 and the Sun prompts us to explore the possibility that the Sun originated from a stellar cluster with properties much like those of M67. As a starting point for dynamical investigations, we take M67. M67 – the Sun’s birth cluster? Need for a relatively rich birth cluster: – Meteoritic evidence for very short-lived isotopes (26Al, 60Fe etc.) – Photo-evaporation of the solar nebula – Extraction of Sedna? A not too rich birth cluster: – Limit to photo-evaporation efficiency – Preserve planetary orbits Our model for M67 • Current mass ~1400 M, consistent with an initial mass of ~18700 M (Hurley et al. 2005) • Stellar mass distribution from the IMF of Kroupa et al. 1993 <M> = 0.48 M for an even mix of single stars (0.32 M) and binaries (0.64 M) • Half-mass radius ~3.9 pc Plummer model scale radius ~3 pc • Relaxation time ~300 Myr Estimated mass at LHB epoch (~700 Myr after to) ~ 12400 M Constraints on cluster size Adams (2010) ) N P( Save the planetary orbits N According to this crude estimate, M67 would be too rich and massive! Saving the planets • Using a Plummer model for M67 at t=0 and t=700 Myr with the respective masses, we compute for different radial distances: – the local number density of cluster stars – the distribution of relative velocities at close encounters – the resulting encounter flux • Destructive encounters are taken as likely distance those within 250 AU range Residence times ~1 Gyr are OK! Encounter efficiencies Blue: 1000 random stars of the current solar neighborhood Yellow: 10000 random M67 encounters at r = r0 • The impulse imparted to an object at a close encounter goes as M/V – this is very much larger in M67 than in the Galactic disk Saving the Oort Cloud? • Suppose that a primordial Oort Cloud was formed as a by-product of giant planet formation, extending to ~104 AU • During the 700 Myr until the LHB, at r = r0, a closest encounter at ~450 AU is expected • The median value of M/V is ~ 0.4 M/(km/s) • Calculating the solar impulse by hyperbolic deflection, one gets V ~ 2.8 km/s • This is the escape velocity at ~ 225 AU; thus most of the primordial OC escapes the Solar System If the Sun stayed in M67 until the LHB, a primordial Oort Cloud would be destroyed! Creating an Oort Cloud • Typical M67 encounters expected, even in the outer parts, during ~100 Myr at ~1000 AU will strongly perturb scattered disk objects with a ~103-104 AU • Thus, in the Nice Model, the LHB-related formation of a scattered disk in M67 would inevitably lead to the creation of an OC inner core • If the Sun stayed too long in M67, the survival of this OC might be compromised Conclusions One swallow does not make a summer, but... • We are learning more about the Sun and its planetary system (external constraints on the stability of planetary orbits and the Oort Cloud) by studying solar twins • We are learning more about star formation by studying clusters (cf. Ken Croswell (2004): M67 – the Ultimate Survivor) • Sun M67: a reasonable scenario? M67 (HMS Nämdö) Behold M67-1194 .