Interstellar Interloper 1I/2017 U1: Observations from the NOT and WIYN Telescopes

Abstract

We present observations of the interstellar interloper 1I/2017 U1 ('Oumuamua) taken during its 2017 October flyby of Earth. The optical colors B -V=0.70±0.06, V -R=0.45±0.05, overlap those of the D-type Jovian Trojan asteroids and are incompatible with the ultrared objects that are abundant in the Kuiper Belt. With a mean absolute magnitude H V =22.95 and assuming a geometric albedo p V =0.1, we find an average radius of 55 m. No coma is apparent; we deduce a limit to the dust mass production rate of only ∼2×10 −4 kg s, ruling out the existence of exposed ice covering more than a few m 2 of the surface. Volatiles in this body, if they exist, must lie beneath an involatile surface mantle 0.5 m thick, perhaps a product of prolonged cosmic-ray processing in the interstellar medium. The light curve range is unusually large at ∼2.0±0.2 mag. Interpreted as a rotational light curve the body has axis ratio ³ -+ 6.3 1.1 1.3 :1 and semi-axes ∼230 m×35 m. A 6:1 axis ratio is extreme relative to most small solar system asteroids and suggests that albedo variations may additionally contribute to the variability. The light curve is consistent with a two-peaked period ∼8.26 hr, but the period is non-unique as a result of aliasing in the data. Except for its unusually elongated shape, 1I/2017 U1 is a physically unremarkable, sub-kilometer, slightly red, rotating object from another planetary system. The steady-state population of similar, ∼100 m scale interstellar objects inside the orbit of Neptune is ∼10 4 , each with a residence time of ∼10 years.

On the Rotation Period and Shape of the Hyperbolic Asteroid 1I/‘Oumuamua (2017 U1) from Its Lightcurve

“…Measurements of 'Oumuamua's lightcurve allowed us to place lower limits on its rotation period (longer than 3 hr, likely longer than 5 hr), set a lower limit to its elongation of 3:1, and infer an effective diameter of at least 90-180 m. Deep imaging ruled out the presence of any coma to a level below that of the least active comets known. These constraints are consistent with reports made public while this Letter was under review (Bannister et al 2017;Bolin et al 2017;Jewitt et al 2017;Meech 2017): 'Oumuamua is very highly elongated, although there is yet to be consensus on the rotation period. We are optimistic that once the ensemble of data acquired of 'Oumuamua are published, our lightcurve will aide in a conclusive determination of the rotation period.…”

“…Further, while astrometry and snapshot magnitudes in a variety of bandpasses have been regularly reported and we are aware of at least three optical spectra (Fitzsimmons et al 2017;Masiero 2017;Ye et al 2017), no lightcurve measurements, which yield insight into the physical characteristics of the object, had been reported at the time this Letter was submitted. While this Letter was under review, four papers presenting lightcurves were made public (Bannister et al 2017;Bolin et al 2017;Jewitt et al 2017;Meech 2017). However, we have restricted this analysis to our own imaging and lightcurve observations of our solar system's first known visiting minor planet.…”

APO Time-resolved Color Photometry of Highly Elongated Interstellar Object 1I/‘Oumuamua

“…The critical breakup period for a strengthless rotating ellipsoid with an axial ratio is given by (Jewitt et al 2017b;Bannister et al 2017)…”

Investigating the dynamical history of the interstellar object ’Oumuamua

“…Meech et al (2017) estimated its shape to be extremely elongated while Fraser et al (2017) and Drahus et al (2017) determined it to be a tumbling body. An interesting paper 1 https://groups.yahoo.com/neo/groups/mpml/info on the determination of physical parameters for 'Oumuamua has also been presented recently by Jewitt et al (2017).…”