The Excited Spin State of 1I/2017 U1 ‘Oumuamua
Abstract
We show that ‘Oumuamua’s excited spin could be in a highenergy long axis mode (LAM) state, which implies that its shape could be far from the highly elongated shape found in previous studies. CLEAN and ANOVA algorithms are used to analyze ‘Oumuamua’s lightcurve using 818 observations over 29.3 days. Two fundamental periodicities are found at frequencies (2.77 ± 0.11) and (6.42 ± 0.18) cycles/day, corresponding to (8.67 ± 0.34) hr and (3.74 ± 0.11) hr, respectively. The phased data show that the lightcurve does not repeat in a simple manner, but approximately shows a double minimum at 2.77 cycles/day and a single minimum at 6.42 cycles/day. ‘Oumuamua could be spinning in either the LAM or short axis mode (SAM). For both, the long axis precesses around the total angular momentum vector with an average period of (8.67 ± 0.34) hr. For the three LAMs we have found, the possible rotation periods around the long axis are 6.58, 13.15, or 54.48 hr, with 54.48 hr being the most likely. ‘Oumuamua may also be nutating with respective periods of half of these values. We have also found two possible SAM states where ‘Oumuamua oscillates around the long axis with possible periods at 13.15 and 54.48 hr. In this case any nutation occurs with the same periods. Determination of the spin state, the amplitude of the nutation, the direction of the total angular momentum vector (TAMV), and the average total spin period may be possible with a direct model fit to the lightcurve. We find that ‘Oumuamua is “cigarshaped,” if close to its lowest rotational energy, and an extremely oblate spheroid if close to its highest energy state.
 Publication:

The Astrophysical Journal
 Pub Date:
 April 2018
 DOI:
 10.3847/20418213/aab370
 arXiv:
 arXiv:1804.03471
 Bibcode:
 2018ApJ...856L..21B
 Keywords:

 comets: general;
 minor planets;
 asteroids: individual: 1I&2017 U1;
 Astrophysics  Earth and Planetary Astrophysics
 EPrint:
 8 pages, 3 figures