Kinematics of the Interstellar Vagabond 1I/‘Oumuamua (A/2017 U1)
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: http://www.minorplanetcenter.net/mpec/K17/K17UI1.html,
‘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
(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;
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
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
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.
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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