Abstract
Abstract We investigate the trapping of interstellar objects during the early stages of star and planet formation. Our results show a very wide range of possible values that will be narrowed down as the population of interstellar objects becomes better characterized. When assuming a background number density of 2 · 1015 pc−3 (based on 1I’s detection), a velocity dispersion of 30 km s−1, and an equilibrium size distribution, the number of interstellar objects captured by a molecular cloud and expected to be incorporated to each protoplanetary disk during its formation is O(109) (50 cm–5 m), O(105) (5–50 m), O(102) (50–500 m), O(10−2) (500 m–5 km). After the disk has formed, the number of interstellar objects it can capture from the ISM during its lifetime is 6 · 1011 (50 cm–5 m), 2 · 108 (5–50 m), 6 · 104 (50–500 m), 20 (500 m–5 km); in an open cluster where 1% of stars have undergone planet formation, these values increase by a factor of O(102–103). These trapped interstellar objects might be large enough to rapidly grow into larger planetesimals via the direct accretion of the subcm-sized dust grains in the protoplanetary disk before they drift in due to gas drag, helping overcome the meter-size barrier, acting as “seeds” for planet formation. They should be considered in future star and planet formation models, as well as in the potential spread of biological material across the Galaxy.
Highlights
Stars form from the collapse of dense regions of molecular clouds
After the disk formed, the number of interstellar objects it could capture from the ISM during its lifetime is 6·1011 (50 cm–5 m), 2·108 (5 m–50 m), 6·104 (50 m–500 m), 20 (500 m–5 km); in an open cluster where 1% of stars have undergone planet formation, these values increase by a factor of O(102–103)
If we adopt a cumulative number density of interstellar objects in the interstellar medium of NR ROu = 2·1015 pc−3 and the size distributions given by Equation (30), we find that the number of trapped objects for the different particle sizes are: 2·105–9·1011 (50 cm–5 m), 2·103–9·106 (5 m–50 m), 2·101–9·101 (50 m–500 m), 9·10−4–2·10−1 (500 m–5 km), and 3·10−9–2·10−3 (5 km–50 km)
Summary
Stars form from the collapse of dense regions of molecular clouds. A natural byproduct of this process is the formation of protoplanetary disks, where, according to the leading theory of planet formation, planet formation takes place (Hayashi 1981; Shu et al 1987; Hartmann 2008). As the cm-sized particles grow and become less coupled to the gas, their relative velocities and collisional energies increase, resulting in collisions that, rather than leading to efficient growth, lead to inefficient sticking, bouncing, or fragmentation (Zsom et al 2010); in addition, because the particles are still coupled to the gas, they experience gas drag that results in short inward drift timescales. Weidenschilling (1977) found that a 1 m-sized particles with a density of 3 g·cm−3 located at a radial distance of 5 AU in a solar nebula has a drift lifetime of O(102) years This short drift timescales limits significantly their lifetime in the disk and, their opportunity to grow to sizes unaffected by gas drag (Birnstiel et al 2012). The aeolian erosion that small planetesimals experience in the presence of gas can constitute a significant barrier for the growth of metre-size objects in protoplanetary discs (Rozner et al 2020) and could contribute to a change of the planetesimal mass function and particle concentration at a unique Stokes number (Grishin et al 2020)
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