95 research outputs found
Hubble Space Telescope Observations of 3200 Phaethon At Closest Approach
We present Hubble Space Telescope observations of the active asteroid (and
Geminid stream parent) 3200 Phaethon when at its closest approach to Earth
(separation 0.07 AU) in 2017 December. Images were recorded within
1\degr~of the orbital plane, providing extra sensitivity to low surface
brightness caused by scattering from a large-particle trail. We placed an upper
limit to the apparent surface brightness of such a trail at 27.2 magnitudes
arcsecond, corresponding to an in-plane optical depth . No co-moving sources brighter than absolute magnitude 26.3,
corresponding to circular equivalent radius 12 m (albedo 0.12 assumed),
were detected. Phaethon is too hot for near-surface ice to survive. We briefly
consider the thermodynamic stability of deeply-buried ice, finding that its
survival would require either a very small (regolith-like) thermal diffusivity
( m s), or the unexpectedly recent injection of Phaethon
(timescale 10 yr) into its present orbit, or both.Comment: Improved the discussion of optical depth calculation and corrected an
error in the previous version. 28 pages, 5 figures, Astronomical Journal, in
pres
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
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
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
Anatomy of an Asteroid Break-Up: The Case of P/2013 R3
We present an analysis of new and published data on P/2013 R3, the first
asteroid detected while disintegrating. Thirteen discrete components are
measured in the interval between UT 2013 October 01 and 2014 February 13. We
determine a mean, pair-wise velocity dispersion amongst these components of
m s and find that their separation times are
staggered over an interval of 5 months. Dust enveloping the system has,
in the first observations, a cross-section 30 km but fades
monotonically at a rate consistent with the action of radiation pressure
sweeping. The individual components exhibit comet-like morphologies and also
fade except where secondary fragmentation is accompanied by the release of
additional dust. We find only upper limits to the radii of any embedded solid
nuclei, typically 100 to 200 m (geometric albedo 0.05 assumed). Combined,
the components of P/2013 R3 would form a single spherical body with radius
400 m, which is our best estimate of the size of the precursor
object. The observations are consistent with rotational disruption of a weak
(cohesive strength 50 to 100 N m) parent body, 400 m in
radius. Estimated radiation (YORP) spin-up times of this parent are 1
Myr, shorter than the collisional lifetime. If present, water ice sublimating
at as little as 10 kg s could generate a torque on the parent
body rivaling the YORP torque. Under conservative assumptions about the
frequency of similar disruptions, the inferred asteroid debris production rate
is 10 kg s, which is at least 4% of the rate needed to
maintain the Zodiacal Cloud.Comment: 44 pages, 13 figures, accepted by Astronomical Journa
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