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Superfast rotator 2001 OE84 - a large monolithic body

#### Orbit:

Amor,
* q* = 1.201 AU, * a* = 2.279 AU, * i* = 9.4 deg (MPEC 2001-U02)

#### Observations:

This Amor asteroid has been observed by P. Kusnirak and P. Pravec on two nights
of 2001 Oct. 15.1 and 16.0.
The observations revealed a synodic rotation period of (29.190 +/- 0.002) min.
The rotation is so fast that the object cannot be held together by self-gravitation
and it is a body with a non-zero tensile strength (see Pravec *et al.* 2000,
Pravec and Harris 2000).
The composite lightcurve ( Fig. 1) has an amplitude
of 0.60 mag (at phase angle 25 deg) that indicates an elongated figure
with equatorial axis ratio > = 1.4.

Figure 2 shows the observed lightcurve of the second
observational night. A sampling was one point per 140 seconds during most
of the time (130-sec integration time plus a 10-sec readout), therefore
consecutive points fall on fairly different rotation phases.

Figure 3 shows spin rate vs diameter plot for 987
asteroids of all sizes (updated from Pravec *et al.* 2001).
With its estimated size of about 0.9 km (using H=17.7 from
MPEC 2001-U02 and assuming a geometric albedo of 0.18 typical
for near-Earth asteroids),
2001 OE84 is an exceptional
object - all other known superfast monolithic rotators are objects less
than about 0.15 km diameter. All other asteroids greater than ~0.15 km
with known periods are below the critical rotation "barrier" of 12 rev/day
(period > 2 h) that is an evidence that most of them are strengthless
objects, so-called "rubble piles" (e.g., Pravec and Harris 2000).
2001 OE84 is the largest known asteroid with significant tensile
strength, the first known in the km size range. It is an evidence
that coherent bodies exist also among asteroids as large as ~1 km.

#### Fig. 1:

Composite lightcurve of 2001 OE84.
The curve is the best-fit 7th-order Fourier series.
A smoothing of the lightcurve due to the finite integration time (130 sec)
is small; a true amplitude was greater by ~0.05 mag than the observed one.

#### Fig. 2:

Lightcurve of 2001 OE84 taken around October 15.96.
The filled circles are the observed data,
the curve is the 7th-order Fourier series expansion.

#### Fig. 3:

Asteroids' spin rate vs diameter plot. 2001 OE84 is an exceptional
object, a superfast rotating monolithic body in the size range
dominated by strenghtless "rubble piles" that all lie below the
critical spin "barrier" of 12 rev/day (period 2 h) plotted as the
horizontal dashed line.

### References:

Pravec, P., and A. W. Harris 2000.
Fast and slow rotation of asteroids.
* Icarus ***148**, 12-20.

Pravec, P., C. Hergenrother, R. Whiteley, L. Sarounova, P. Kusnirak,
and M. Wolf 2000. Fast rotating asteroids 1999 TY2, 1999 SF10,
and 1998 WB2. * Icarus ***147**, 477-486.

Pravec, P., A. W. Harris, and T. Michalowski 2001.
Asteroid rotations. Chapter submitted to the
*Asteroids III* book.

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