The lost siblings of the Sun

arXiv:0903.0237v1 [astro-ph.GA] 2 Mar 2009 DRAFT VERSION NOVEMBER 1, 2018 Preprint typeset using LATEX style emulateapj v. 04/03/99 THE LOST SIBLINGS OF THE SUN SIMON F. PORTEGIES ZWART Astronomical Institute ‘Anton Pannekoek’, University of Amsterdam, Kruislaan 403, Amsterdam, the Netherlands Institute for Computer Science, University of Amsterdam, Kruislaan 403, Amsterdam, the Netherlands Sterrewacht Leiden University of Leiden, Niels Bohrweg 2, 2333 CA Leiden, the Netherlands Draft version November 1, 2018 ABSTRACT The anomalous chemical abundances and the structure of the Edgewood-Kuiper belt observed in the solar system constrain the initial mass and radius of the star cluster in which the sun was born to M ≃500 to 3000 M⊙ and R ≃1 to 3 pc. When the cluster dissolved the siblings of the sun dispersed through the galaxy, but they remained on a similar orbit around the Galactic center. Today these stars hide among the field stars, but 10 to 60 of them are still present within a distance of ∼100 pc. These siblings of the sun can be identified by accurate measurements of their chemical abundances, positions and their velocities. Finding even a few will strongly constrain the parameters of the parental star cluster and the location in the Galaxy where we were born. 1. INTRODUCTION It is commonly accepted that stars like the sun are born in clusters (Lada et al., 1993). The cluster in which the sun was born is long gone and the sun’s siblings are by now spread over the Galaxy. The structure of the hot Edgeworth-Kuiper belt objects provide evidence of this, as this population can be reproduced by a relatively nearby encounter with another star (Morbidelli & Levison, 2004). Such an encounter is ex- pected to have occurred in the early history of the Solar system (Malhotra, 2008). The existence of a well organized planetary system, however, indicates that the parental cluster cannot have been very dense as otherwise a nearby passing star would also have perturbed the orbits of the planets. Additional evidence for the sun’s dynamic history comes from the discovery of radioactive isotopes and their decay prod- ucts in the proto-solar nebula (Hester et al., 2004), which is ex- plained by a supernova explosion within 1.6 pc of the infant sun (Looney et al., 2006). The combination of arguments enables us to estimate the mass and size of the star cluster in which the sun was born. We follow the orbital evolution of these stars through the Galaxy for the lifetime of the sun and conclude that at least 1% (10 to 60) of the sun’s siblings should still be present within 100 pc, and more than 10% should be within about a kpc along the orbit of the solar system in the Galactic poten- tial. With the Gaia satellite (Perryman et al., 2001) and ground- based searches the radial velocity and distance to the majority of the lost siblings will be determined, and the proper motion will be measured. These constrain the orbit of the proto-solar cluster and enable us to accurately determine the evolution of the Galactic potential and the birth place of the sun. 2. THE PARENTAL STAR CLUSTER The sun and its eight planets were born about 4.57 billion years ago (Bonanno et al., 2002), probably in a star cluster, and it is in due time that our location in the Galaxy became so deso- late. Evidence for this dynamic past comes from meteorite fos- sil records, where the presence of short-lived radioactive iso- topes in primitive meteorites indicate that the 1.8 Myr young sun was polluted by a supernova explosion of a star about 15 to 25 times more massive than the sun within a distance of 0.02 to 1.6 parsec (Looney et al., 2006). Such a massive star lives for 6 to 12 Myr before it sheds the majority of its mass in a super- nova explosion. This massive star must have formed some 4.2 to 10.2 Myr earlier than the sun. Such range in the distribution of stellar ages is also observed in the Orion Nebula where mas- sive stars also tend to be a few to ∼10 Myr younger than the low-mass stars (Palla & Stahler, 1999). The presence of a massive star close to the infant sun puts interesting constraints on its birth environment. Today, such a massive star is not even present within a distance of 100 parsec. By adopting a standard initial mass function (Kroupa & Weidner, 2003) about 1 in 400 stars is sufficiently massive to experience a supernova. Stars of m = 15 to 25 M⊙are less common and would require a star cluster of M >∼500 M⊙(Weidner & Kroupa, 2004). Massive stars in a cluster tend to sink to the center in a frac- tion ∝1/m of the two-body relaxation time scale (trlx). If trlx >∼300 Myr even the most massive stars are unlikely to have reached the cluster center by the time they explode. Massive stars in clusters with smaller trlx tend to populate the central region by the time they explode in supernovae, polluting the nearby young stars and proto planetary disks in the process (Wielen et al., 1996). Low mass stars, like the sun, are not strongly affect

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