446 research outputs found
Dynamics of Large Fragments in the Tail of Active Asteroid P/2010 A2
We examine the motions of large fragments at the head of the dust tail of
active asteroid P/2010 A2. In previous work we showed that these fragments were
ejected from the primary nucleus in early 2009, either following a
hypervelocity impact or by rotationally induced break-up. Here, we follow their
positions through a series of Hubble Space Telescope images taken during the
first half of 2010. The orbital evolution of each fragment allows us to
constrain its velocity relative to the main nucleus after leaving its sphere of
gravitational influence. We find that the fragments constituting a prominent
X-shaped tail feature were emitted in a direction opposite to the motion of the
asteroid and towards the south of its orbital plane. Derived emission
velocities of these primary fragments range between 0.02 and 0.3 m/s,
comparable to the ~0.08 m/s gravitational escape speed from the nucleus. Their
sizes are on the order of decimeters or larger. We obtain the best fits to our
data with ejection velocity vectors lying in a plane that includes the nucleus.
This may suggest that the cause of the disruption of P/2010 A2 is rotational
break-up.Comment: 30 pages, 8 figures, accepted for publication by The Astrophysical
Journa
Velocity-resolved observations of water in Comet Halley
High resolution (lambda/delta lambda approx. = 3 x 10 to the 5th power) near-infrared observations of H2O emission from Comet Halley were acquired at the time of maximum post-perihelion geocentric Doppler shift. The observed widths and absolute positions of the H2O line profiles reveal characteristics of the molecular velocity field in the coma. These results support H2O outflow from a Sun-lit hemisphere or the entire nucleus, but not from a single, narrow jet emanating from the nucleus. The measured pre- and post-perihelion outflow velocities were 0.9 + or - 0.2 and 1.4 + or - 0.2 km/s, respectively. Temporal variations in the kinematic properties of the outflow were inferred from changes in the spectral line shapes. These results are consistent with the release of H2O into the coma from multiple jets
The Extraordinary Multi-Tailed Main-Belt Comet P/2013 P5
Hubble Space Telescope observations of main-belt comet P/2013 P5 reveal an
extraordinary system of six dust tails that distinguish this object from any
other. Observations two weeks apart show dramatic morphological change in the
tails while providing no evidence for secular fading of the object as a whole.
Each tail is associated with a unique ejection date, revealing continued,
episodic mass loss from the 0.24+/-0.04 km radius nucleus over the last five
months. As an inner-belt asteroid and probable Flora family member, the object
is likely to be highly metamorphosed and unlikely to contain ice. The
protracted period of dust release appears inconsistent with an impact origin,
but may be compatible with a body that is losing mass through a rotational
instability. We suggest that P/2013 P5 has been accelerated to breakup speed by
radiation torques.Comment: 13 pages, two figures, three tables, Accepted to ApJ
A binary main belt comet
The asteroids are primitive solar system bodies which evolve both
collisionally and through disruptions due to rapid rotation [1]. These
processes can lead to the formation of binary asteroids [2-4] and to the
release of dust [5], both directly and, in some cases, through uncovering
frozen volatiles. In a sub-set of the asteroids called main-belt comets (MBCs),
the sublimation of excavated volatiles causes transient comet-like activity
[6-8]. Torques exerted by sublimation measurably influence the spin rates of
active comets [9] and might lead to the splitting of bilobate comet nuclei
[10]. The kilometer-sized main-belt asteroid 288P (300163) showed activity for
several months around its perihelion 2011 [11], suspected to be sustained by
the sublimation of water ice [12] and supported by rapid rotation [13], while
at least one component rotates slowly with a period of 16 hours [14]. 288P is
part of a young family of at least 11 asteroids that formed from a ~10km
diameter precursor during a shattering collision 7.5 million years ago [15].
Here we report that 288P is a binary main-belt comet. It is different from the
known asteroid binaries for its combination of wide separation, near-equal
component size, high eccentricity, and comet-like activity. The observations
also provide strong support for sublimation as the driver of activity in 288P
and show that sublimation torques may play a significant role in binary orbit
evolution.Comment: 18 pages, 8 figures, 2 table
Episodic Ejection from Active Asteroid 311P/PANSTARRS
We examine the development of the active asteroid 311P/PANSTARRS (formerly,
2013 P5) in the period from 2013 September to 2014 February using high
resolution images from the Hubble Space Telescope. This multi-tailed object is
characterized by a single, reddish nucleus of absolute magnitude
18.980.10, corresponding to an equal-area sphere of radius 20020
m (for assumed geometric albedo 0.290.09). We set an upper limit to the
radii of possible companion nuclei at 10 m. The nucleus ejected debris in
nine discrete episodes, spread irregularly over a nine month interval, each
time forming a distinct tail. Particles in the tails range from about 10 m
to at least 80 mm in radius, and were ejected at speeds 1 m s. The
ratio of the total ejected dust mass to the nucleus mass is
310, corresponding to a global surface layer 2 mm
thick, or to a deeper layer covering a smaller fraction of the surface. The
observations are incompatible with an origin of the activity by impact or by
the sublimation of entrapped ice. This object appears to be shedding its
regolith by rotational (presumably YORP-driven) instability. Long-term fading
of the photometry (months) is attributed to gradual dissipation of near-nucleus
dust. Photometric variations on short timescales (0.7 hr) are probably
caused by fast rotation of the nucleus. However, because of limited time
coverage and dilution of the nucleus signal by near-nucleus dust, we have not
been able to determine the rotation period.Comment: 37 pages, 12 figure
Hubble and Keck Telescope Observations of Active Asteroid 288P/300163 (2006 VW139)
We present Hubble Space Telescope and Keck 10 meter telescope observations of
active asteroid 288P/300163 (2006 VW139) taken to examine ejected dust. The
nucleus is a C-type object with absolute magnitude = 17.00.1 and
estimated diameter 2.6 km (for assumed visual geometric albedo =
0.04). Variations in the brightness of the nucleus at the 10% to 15% level are
significant in both 2011 December and 2012 October but we possess too few data
to distinguish variations caused by activity from those caused by rotation. The
dust scattering cross-section in 2011 December is 40 km,
corresponding to a dust mass 910 kg (88 m mean particle
radius assumed). The full width at half maximum of the debris sheet varies from
100 km near the nucleus to 1000 km 30arcsec (40,000 km) east of it.
Dust dynamical models indicate ejection speeds between 0.06 and 0.3 m s,
particle sizes between 10 and 300 m and an inverse square-root relation
between particle size and velocity. Overall, the data are most simply explained
by prolonged, low velocity ejection of dust, starting in or before 2011 July
and continuing until at least 2011 October. These properties are consistent
with the sublimation of near-surface ice aided by centrifugal forces. The high
spatial resolution of our HST images (52 km per pixel) reveals details that
remained hidden in previous ground-based observations, such as the
extraordinarily small vertical extent of the dust sheet, ejection speeds well
below the nucleus escape speed, and the possibility of a binary nucleus.Comment: Accepted for publication by A
Disintegrating Asteroid P/2013 R3
Splitting of the nuclei of comets into multiple components has been
frequently observed but, to date, no main-belt asteroid has been observed to
break-up. Using the Hubble Space Telescope, we find that main-belt asteroid
P/2013 R3 consists of 10 or more distinct components, the largest up to 200 m
in radius (assumed geometric albedo of 0.05) each of which produces a coma and
comet-like dust tail. A diffuse debris cloud with total mass roughly 2x10^8 kg
further envelopes the entire system. The velocity dispersion among the
components is about V = 0.2 to 0.5 m/s, is comparable to the gravitational
escape speeds of the largest members, while their extrapolated plane-of-sky
motions suggest break-up between February and September 2013. The broadband
optical colors are those of a C-type asteroid. We find no spectral evidence for
gaseous emission, placing model-dependent upper limits to the water production
rate near 1 kg/s. Breakup may be due to a rotationally induced structural
failure of the precursor body.Comment: 16 pages, 3 figures; accepted by ApJ
Nucleus and Mass Loss from Active Asteroid 313P/Gibbs
We present Hubble Space Telescope observations of active asteroid 313P/Gibbs
(formerly P/2014 S4) taken over the five month interval from 2014 October to
2015 March. This object has been recurrently active near perihelion (at 2.4 AU)
in two different orbits, a property that is naturally explained by the
sublimation of near surface ice but which is difficult to reconcile with other
activity mechanisms. We find that the mass loss peaks near 1 kg s in
October and then declines over the subsequent months by about a factor of five,
at nearly constant heliocentric distance. This decrease is too large to be
caused by the change in heliocentric distance during the period of observation.
However, it is consistent with sublimation from an ice patch shadowed by local
topography, for example in a pit like those observed on the nuclei of
short-period comet 67P/Churyumov-Gerasimenko. While no unique interpretation is
possible, a simple self shadowing model shows that sublimation from a pit with
depth to diameter ratio near 1/2 matches the observed rate of decline of the
activity, while deeper and shallower pits do not. We estimate the nucleus
radius to be 700100 m (geometric albedo 0.05 assumed). Measurements of the
spatial distribution of the dust were obtained from different viewing
geometries. They show that dust was ejected continuously not impulsively, that
the effective particle size is large, 50 , and that the ejection
speed is 2.5 m s. The total dust mass ejected is 10 kg,
corresponding to 10 of the nucleus mass. The observations are
consistent with partially shadowed sublimation from 10 m of ice,
corresponding to 0.2\% of the nucleus surface. For ice to survive in 313P
for billion-year timescales requires that the duty cycle for sublimation be
10.Comment: 34 pages, 11 figures, 4 tables; Astronomical Journal: in pres
The Nucleus of Active Asteroid 311P/(2013 P5) PANSTARRS
The unique inner-belt asteroid 311P/PANSTARRS (formerly P/2013 P5) is notable
for its sporadic, comet-like ejection of dust in nine distinct epochs spread
over 250 days in 2013. This curious behavior has been interpreted as the
product of localized, equator-ward landsliding from the surface of an asteroid
rotating at the brink of instability. We obtained new Hubble Space Telescope
observations to directly measure the nucleus and to search for evidence of its
rapid rotation. However, instead of providing photometric evidence for rapid
nucleus rotation, our data set a lower limit to the lightcurve period,
5.4 hour. The dominant feature of the lightcurve is a V-shaped minimum,
0.3 magnitudes deep, that is suggestive of an eclipsing binary. Under
this interpretation, the time-series data are consistent with a
secondary/primary mass ratio, 1:6, a ratio of separation/primary
radius, 4 and an orbit period 0.8 days. These properties lie
within the range of other asteroid binaries that are thought to be formed by
rotational breakup. While the lightcurve period is long, centripetal dust
ejection is still possible if one or both components rotates rapidly
( 2 hour) and has a small lightcurve variation because of azimuthal
symmetry. Indeed, radar observations of asteroids in critical rotation reveal
"muffin-shaped" morphologies which are closely azimuthally symmetric and which
show minimal lightcurves. Our data are consistent with 311P being a close
binary in which one or both components rotates near the centripetal limit. The
mass loss in 2013 suggests that breakup occurred recently and could even be
on-going. A search for fragments that might have been recently ejected beyond
the Hill sphere reveals none larger than effective radius 10 m.Comment: 37 pages, 9 figures, Astronomical Journal, in pres
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