Figure 1. Rotational light curve of the largest fragment of P/2010 A2.
Time-series g’-band photometry over two nights (upper panel) and phase
based on the best-fit double-peaked period of 11.36 hr (lower panel). A
sine curve with a period of 11.36 hr was plotted in the upper panel
(gray line).
Figure 2. Composite image of asteroid P/2010 A2 constructed from data
from the Gemini Multi-Object Spectrograph on Gemini North. The team used
this data to compare against models of the object’s structure and
dynamics.
A team of Korean astronomers uses imaging from the Gemini Multi-Object
Spectrograph (GMOS) on Gemini North to characterize the rotation of
active asteroid P/2010 A2’s largest fragment. The observations show that
this faint and tiny (about the size of an American football field)
asteroid, which underwent a mass ejection episode, is slowly rotating,
indicative of an impact fragmentation rather a rotational breakup.
In January 2017, the active and fragmented main belt asteroid P/2010 A2
(hereafter A2) made its closest approach to the Earth after its 2010
discovery, when it exhibited a mysterious comet-like dust trail. Prior
to this year’s passage, the fragments had not yet been characterized,
due to the extremely small size (~120 meters in diameter) and faintness
of this object. A Korean team, led by Yoonyoung Kim of Seoul National
University, received time on Gemini North to observe the object’s 2017
close passage when the fragments and associated debris swarm were just
over one astronomical unit away.
According to Kim, a variety of hypotheses have been suggested to explain
the history of this body, including rotational breakup, impact
cratering, or shattering. The team determined a rotation period ~11.36
hours for the largest fragment. If the fragment’s spin period has been
constant after the mass ejection, which Kim says is reasonable to
believe, then it fails to meet the critical spin rate for rotational
breakup. The observations also reveal that the largest fragment has a
highly-elongated shape with about a 2:1 ratio. Looking at the size
distributions of the ejecta and other fragments, the team concludes that
the body likely underwent impact shattering in order to produce the
observed morphology.
The study’s light curve is shown in Figure 1 and presents the largest
fragment’s double-peaked period of 11.36 +/- 0.02 hours. Figure 2
presents a composite from the imaging data revealing the array of
fragments and debris used to determine the mass of the largest fragment
is about 80% of the system’s mass with the other fragments and ejecta
making up the remaining 20%. All figures are from the accepted paper
scheduled for publication in The Astrophysical Journal Letters. A preprint is available here.
Paper Abstract:
We report new observations of the active asteroid P/2010 A2 taken when
it made its closest approach to the Earth (1.06 au in 2017 January)
after its first discovery in 2010. Despite a crucial role of the
rotational period in clarifying its ejection mechanism, the rotational
property of P/2010 A2 has not yet been studied due to the extreme
faintness of this tiny object (∼120 m in diameter). Taking advantage of
the best observing geometry since the discovery, we succeed in obtaining
the rotational light curve of the largest fragment with Gemini/GMOS-N.
We find that (1) the largest fragment has a double-peaked period of
11.36±0.02 hr spinning much slower than its critical spin period; (2)
the largest fragment is a highly elongated object (a/b⩾1.94) with an
effective radius of 61.9+16.8−9.2 m; (3) the size
distribution of the ejecta follows a broken power law (the power indices
of the cumulative size distributions of the dust and fragments are
2.5±0.1 and 5.2±0.1, respectively); (4) the mass ratio of the largest
fragment to the total ejecta is around 0.8; and (5) the dust cloud
morphology is in agreement with the anisotropic ejection model in Kim et
al. These new characteristics of the ejecta obtained in this work are
favorable to the impact shattering hypothesis.
Source: Gemini Observatory
