Abstract
The discovery of the second interstellar object 2I/Borisov on 2019 August 30 raises the question of whether it was ejected recently from a nearby stellar system. Here we compute the asymptotic incoming trajectory of 2I/Borisov, based on both recent and pre-discovery data extending back to December 2018, using a range of force models that account for cometary outgassing. From Gaia DR2 astrometry and radial velocities, we trace back in time the Galactic orbits of 7.4 million stars to look for close encounters with 2I/Borisov. The closest encounter we find took place 910 kyr ago with the M0V star Ross 573, at a separation of 0.068 pc (90% confidence interval of 0.053–0.091 pc) with a relative velocity of 23 km s−1. This encounter is nine times closer than the closest past encounter identified for the first interstellar object 1I/‘Oumuamua. Ejection of 2I/Borisov via a three-body encounter in a binary or planetary system is possible, although such a large ejection velocity is unlikely to be obtained and Ross 573 shows no signs of binarity. We also identify and discuss some other recent close encounters, recognizing that if 2I/Borisov is more than about 10 Myr old, our search would be unlikely to find its parent system.
Highlights
The discovery of the first interstellar object, 1I/‘Oumuamua, on 2017 October 19 generated an intense period of observation using over 100 hours of medium to large telescope time for the two weeks following its discovery
From Gaia DR2 astrometry and radial velocities, we trace back in time the Galactic orbits of 7.4 million stars to look for close encounters with 2I/Borisov
The closest encounter we find took place 910 kyr ago with the M0V star Ross 573, at a separation of 0.068 pc (90% confidence interval of 0.053–0.091 pc) with a relative velocity of 23 km s−1
Summary
The discovery of the first interstellar object, 1I/‘Oumuamua, on 2017 October 19 generated an intense period of observation using over 100 hours of medium to large telescope time for the two weeks following its discovery. The data obtained on 1I/‘Oumuamua immediately following its discovery focused on the characterization of its physical properties, while the final data, obtained by the Very Large Telescope and Hubble Space Telescope between November 2017 and early January 2018, was an astrometric experiment designed to trace the orbit back to 1I/‘Oumuamua’s parent solar system. From these observations we learned that 1I/‘Oumuamua was small and red, which is typical of solar system comets, but looked asteroidal (without dust or gas, Meech et al 2017; Ye et al 2017; Fitzsimmons et al 2018). The non-Keplerian trajectory was compared with the reconstructed orbits of 7.4 million stars from the Gaia Data Release 2
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