Capture of exocomets and the erosion of the Oort cloud due to stellar encounters in the Galaxy

• Published in: Monthly notices of the Royal Astronomical Society Vol. 473; no. 4; pp. 5432 - 5445
• Main Authors: Hanse, J., Jílková, L., Portegies Zwart, S. F., Pelupessy, F. I.
• Format: Journal Article
• Language: English
• Published: London Oxford University Press 01.02.2018
• Subjects: Comets; Erosion; Extrasolar; Galaxies; Oort cloud; Stars; Stars & galaxies; Sun; Tidal effects; comets: general; celestial mechanics; Oort Cloud
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1093/mnras/stx2721

Abstract The Oort cloud (OC) probably formed more than 4 Gyr ago and has been moving with the Sun in the Galaxy since, exposed to external influences, most prominently to the Galactic tide and passing field stars. Theories suggest that other stars might possess exocomets distributed similarly to our OC. We study the erosion of the OC and the possibility for capturing exocomets during the encounters with such field stars. We carry out simulations of flybys, where both stars are surrounded by a cloud of comets. We measure how many exocomets are transferred to the OC, how many OC's comets are lost, and how this depends on the other star's mass, velocity and impact parameter. Exocomets are transferred to the OC only during relatively slow (≲0.5 km s−1) and close (≲105 au) flybys and these are expected to be extremely rare. Assuming that all passing stars are surrounded by a cloud of exocomets, we derive that the fraction of exocomets in the OC has been about 10−5–10−4. Finally, we simulate the OC for the whole lifetime of the Sun, taking into account the encounters and the tidal effects. The OC has lost 25–65 per cent of its mass, mainly due to stellar encounters, and at most 10 per cent (and usually much less) of its mass can be captured. However, exocomets are often lost shortly after the encounter that delivers them, due to the Galactic tide and consecutive encounters.