The Sun Was Not Born in M67

Abstract

Using the most recent proper-motion determination of the old, solar-metallicity, Galactic open cluster M67 in orbital computations in a non-axisymmetric model of the Milky Way, including a bar and three-dimensional spiral arms, we explore the possibility that the Sun once belonged to this cluster. We have performed Monte Carlo numerical simulations to generate the present-day orbital conditions of the Sun and M67, and all the parameters in the Galactic model. We compute 3.5 × 10 5 pairs of orbits Sun-M67 looking for close encounters in the past with a minimum distance approach within the tidal radius of M67. In these encounters we find that the relative velocity between the Sun and M67 is larger than 20 km s −1 . If the Sun had been ejected from M67 with this high velocity by means of a three-body encounter, this interaction would have either destroyed an initial circumstellar disk around the Sun or dispersed its already formed planets. We also find a very low probability, much lower than 10 −7 , that the Sun was ejected from M67 by an encounter of this cluster with a giant molecular cloud. This study also excludes the possibility that the Sun and M67 were born in the same molecular cloud. Our dynamical results convincingly demonstrate that M67 could not have been the birth cluster of our solar system.

Gravitational scattering of stars and clusters and the heating of the Galactic disk

“…(We have also made experiments with modifications of the z-gradient of the potential by ±10% to explore the effects of its uncertainties.) To this we have added two stellar spiral arms, following the recipe of Pichardo et al (2003Pichardo et al ( , 2012, with a pitch angle of 15.5 degrees, a radial scale length exponential mass decrease along the arms of 3.9 kpc, a mass of 4 · 10 9 M and a constant pattern speed of 24 km/s kpc −1 . Each arm is represented numerically by 100 oblate inhomogeneous spheroids with semi-major and minor axes of 1000 pc and 500 pc respectively, and with a mutual distance between the spheroid centres of 500 pc.…”

“…If the early M 67 really would have been the original environment of the Sun, the arguments by Pichardo et al (2012) suggest that the Sun should have been formed in the cluster when it still had an orbit much closer to the Galactic plane and diffused out of the cluster. In some latter scattering process against one or several massive objects the cluster was then accelerated into its present high altitude orbit.…”

“…Its orbit has presently an eccentricity of about 0.13, and its distance from the centre of the Galaxy, now being close to 9 kpc, is estimated to vary between 8 to 10 kpc or possibly 6 to 10 kpc, depending on the mass in spiral arms assumed at the orbit calculations (Pichardo et al (2012)). It has Send offprint requests to: Bengt Gustafsson, e-mail: Bengt.Gustafsson@physics.uu.se a metallicity very close to Solar and an age of about 3.5 -4.8 Gyr (seeÖnehag et al (2011) for references).…”

“…The possibility of a Solar origin in the cluster was, however, refuted by Pichardo et al (2012) who argued that the kicking out of the Sun from the cluster to the rather different Solar orbit would have damaged the outer parts of the Solar system. Pichardo et al (2012) carried out their simulations backwards in time by starting the cluster from the present locus of M 67. Although spiral arms and the Galactic bar were included in the Galactic potential, the more concentrated inhomogeneities in the mass distribution provided by the giant molecular clouds were not represented explicitly.…”

No “Maunder Minimum” Candidates in M67: Mitigating Interstellar Contamination of Chromospheric Emission Lines

“…Its age and composition are nearly indistinguishable from the Sun (Önehag et al 2011), which led some to question whether M67 was the Sun's natal cluster. However, Pichardo et al (2012) concluded that this scenario was unlikely based on a dynamical analysis of the Sun and M67's motion through a realistic Milky Way gravitational potential. Crucially, the membership and multiplicity of M67 have been carefully determined from over 40 years of radial velocity monitoring (Geller et al 2015;Latham 2007).…”