TY - JOUR

T1 - On the aspect ratio of 'Oumuamua

T2 - Regular shape and irregular surface

AU - Vazan, Allona

AU - Sari, Reem

N1 - Publisher Copyright:
© 2020 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society.

PY - 2020/4/1

Y1 - 2020/4/1

N2 - The large brightness variation in the observed light curve of 'Oumuamua is probably related to its shape, i.e.To the ratio between its longest axis and its shortest axis (aspect ratio). Several approaches found the aspect ratio of 'Oumuamua to be unusually elongated. Moreover, the spin axis orientation has to be almost perpendicular to the observer in order to obtain such an extreme light curve, a configuration which is unlikely. However, interstellar 'Oumuamua may have different surface properties than we know in our Solar system. Therefore, in this work we widen the parameter space for surface properties beyond the asteroid-like models and study its effect on 'Oumuamua's light curve. We calculate reflection from a rotating ellipsoidal object for four models: Lambertian reflection, specular reflection, single scattering diffusive, and backscatter. We then calculate the probability to obtain a light-curve ratio larger than the observed, as a function of the object's aspect ratio, assuming an isotopic spin orientation distribution. We find the elongation of 'Oumuamua to be less extreme for the Lambertian and specular reflection models. Consequently, the probability to observe the light-curve ratio of 'Oumuamua given its unknown spin axis orientation is larger for those models. We conclude that different surface reflection properties may suggest alternatives to the extreme shape of 'Oumuamua, relieving the need for complicated formation scenario, extreme albedo variation, or unnatural origin. Although the models suggested here are for ideal ellipsoidal shape and ideal reflection method, the results emphasize the importance of surface properties for the derived aspect ratio.

AB - The large brightness variation in the observed light curve of 'Oumuamua is probably related to its shape, i.e.To the ratio between its longest axis and its shortest axis (aspect ratio). Several approaches found the aspect ratio of 'Oumuamua to be unusually elongated. Moreover, the spin axis orientation has to be almost perpendicular to the observer in order to obtain such an extreme light curve, a configuration which is unlikely. However, interstellar 'Oumuamua may have different surface properties than we know in our Solar system. Therefore, in this work we widen the parameter space for surface properties beyond the asteroid-like models and study its effect on 'Oumuamua's light curve. We calculate reflection from a rotating ellipsoidal object for four models: Lambertian reflection, specular reflection, single scattering diffusive, and backscatter. We then calculate the probability to obtain a light-curve ratio larger than the observed, as a function of the object's aspect ratio, assuming an isotopic spin orientation distribution. We find the elongation of 'Oumuamua to be less extreme for the Lambertian and specular reflection models. Consequently, the probability to observe the light-curve ratio of 'Oumuamua given its unknown spin axis orientation is larger for those models. We conclude that different surface reflection properties may suggest alternatives to the extreme shape of 'Oumuamua, relieving the need for complicated formation scenario, extreme albedo variation, or unnatural origin. Although the models suggested here are for ideal ellipsoidal shape and ideal reflection method, the results emphasize the importance of surface properties for the derived aspect ratio.

KW - asteroids: individual: 'Oumuamua

KW - minor planets

UR - http://www.scopus.com/inward/record.url?scp=85095133216&partnerID=8YFLogxK

U2 - 10.1093/mnras/staa252

DO - 10.1093/mnras/staa252

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AN - SCOPUS:85095133216

SN - 0035-8711

VL - 493

SP - 1546

EP - 1552

JO - Monthly Notices of the Royal Astronomical Society

JF - Monthly Notices of the Royal Astronomical Society

IS - 2

ER -