Kinematics of the Interstellar Vagabond 1I/\!‘Oumuamua (A/2017 U1)

Kinematics of the Interstellar Vagabond 1I/‘Oumuamua (A/2017 U1)

minor planets, asteroids: individual (1I/‘Oumuamua) – stars: kinematics and dynamics
Corresponding author: Eric

0000-0003-2008-1488]Eric Mamajek \move@AU\move@AF\@affiliationJet Propulsion Laboratory, California Institute of Technology, M/S 321-100, 4800 Oak Grove Drive, Pasadena, CA 91109, USA \move@AU\move@AF\@affiliationDepartment of Physics & Astronomy, University of Rochester, Rochester, NY 14627, USA


The discovery of an asteroid of likely interstellar origin was recently made by the Pan-STARRS survey – A/2017 U1 = 1I/‘Oumuamua333See:, Can ‘Oumuamua’s velocity before it entered the solar system provide any clues to its origin? The best available orbit from the JPL Small-Body Database Browser444 (solution JPL-13 produced by Davide Farnocchia) lists perihelion distance = 0.255287    0.000079 au, eccentricity = 1.19936    0.00021 and semi-major axis = -1.28052    0.00096 au. This value of is consistent with an initial velocity before encountering the solar system of = 26.3209  0.0099 km s, assuming no non-gravitational forces. The ephemeris shows that the object entered the solar system from the direction , = 279.804, +33.997 (0.032, 0.015; 1). This divergent point and value translates to a heliocentric Galactic velocity (Perryman et al., 1998, towards Galactic center) of = -11.457, -22.395, -7.746 km s (0.009, 0.009, 0.011 km s).

Could  ‘Oumuamua  be a member of the Oort Cloud of the Centauri system? Such a scenario might not be unexpected as the tidal radius for the 2.17 triple system (Kervella et al., 2017) is of order 1.7 pc (Mamajek et al., 2013). As the system lies only 1.34 pc away, the solar system may be on the outskirts of Cen’s cometary cloud (see Hills, 1981; Beech, 2011). Kervella et al. (2017) calculated updated heliocentric Galactic velocities for Cen AB of = -29.291, 1.710, 13.589 (0.026, 0.020, 0.013) km s and for Proxima Centauri ( Cen C) of = -29.390, 1.883, 13.777 (0.027, 0.018, 0.009) km s. The velocity difference of 36.80  0.04 km s between ‘Oumuamua  and the Cen system, and the fact they were further apart in the past ( 5 pc 100 kyr ago), argues that it has no relation to Cen. Members of Cen’s cometary cloud would appear to have motions diverging from the vicinity of , = 293, -42 with 32 km s.

The Galactic velocity of  ‘Oumuamua is plotted against those of the nearest stars (parallax 300 mas) in Fig. 1. Besides the velocity of Cen AB and C from Kervella et al. (2017), velocities for the nearest stars are drawn from Anderson & Francis (2012) and Hawley et al. (1997). The velocity for the substellar binary Luhman 16 is calculated using data from Garcia et al. (2017) and Kniazev et al. (2013): = -18.3, -27.5, -6.9 km s. ‘Oumuamua’s velocity is more than 20 km s from any of the stars, and 9 km s off from Luhman 16, so ‘Oumuamua  does not appear to be comoving with any of these nearest systems.

What velocities might be expected of interstellar field objects? We might first suspect that interstellar planetesimals share the velocity distribution of nearby stars. The XHIP catalog (Anderson & Francis, 2012) contains velocities for 1481 stars within 25 pc with distances of 10% accuracy. The XHIP sample has median velocity = -10.5, -18.0, -8.4 km s (33, 24, 17 km s; 1 range), similar to that for volume-limited samples of nearby M dwarfs ( = -9.7, -22.4, -8.9 km s ; 37.9, 26.1, 20.5; 1; Reid et al., 2002). Bland-Hawthorn & Gerhard (2016) provides a recent consensus estimate for the Local Standard of Rest (LSR) of = -10.0, -11.0, -7.0 km s ; 1, 2, 0.5; 1). An object with the median velocity of the local XHIP sample would have speed 22.5 km s coming from , = 273, +33, within only 6 of  ‘Oumuamua’s divergent point. The velocity is very close to the median for the XHIP sample ( 4.5 km s; / = 0.036/3; P = 0.0018), the mean for the local M dwarfs ( 2.1 km s; / = 0.0053/3; P = 0.0001) and the LSR ( 11.5  2.3 km s), compared to the typical 3D velocity of nearby stars. Compared to the LSR, ‘Oumuamua  has negligible radial and vertical Galactic motion555Gaidos et al. (2017) have proposed that the object’s velocity is due to birth in a nearby 40 Myr stellar association., and its sub-Keplerian circular velocity trails by 11 km s.

Robotic reconnaissance of ‘Oumuamua, or future interstellar planetesimals passing through the solar system, might constitute logical precursors to interstellar missions, and provide the opportunity to conduct chemical studies and radiometric dating of extrasolar material which formed around other stars.


figure \hyper@makecurrentfigure

Figure 0. \Hy@raisedlink\hyper@@anchor\@currentHrefGalactic velocities for 1I/‘Oumuamua  (filled triangle), nearby stars (open circles), and LSR (cross).

This research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration. EEM acknowledges support from the NASA NExSS program, and thanks Davide Farnocchia (JPL) for discussions. This work used the JPL Small-Body Database Browser, HORIZONS system, and Vizier.


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