873 research outputs found
The Nucleus of Main-Belt Comet P/2010 R2 (La Sagra)
We present recent observations of main-belt comet P/2010 R2 (La Sagra)
obtained using the Gemini North telescope on five nights in late 2011 and early
2013 during portions of the object's orbit when dust emission was expected to
be minimal or absent. We find that P/La Sagra continues to exhibit a faint dust
trail aligned with its orbit plane as late as 2011 December 31, while no
activity is observed by the time of our next observations on 2013 March 3,
shortly before aphelion. Using only photometry measured when the comet was
observed to be inactive, we find best-fit IAU phase function parameters of
H_R=18.4+/-0.2 mag and G=0.17+/-0.10, corresponding to an effective nucleus
radius of r_N=0.55+/-0.05 km (assuming p_R=0.05). We revisit photometry
obtained when P/La Sagra was observed to be active in 2010 using our revised
determination of the object's nucleus size, finding a peak dust-to-nucleus mass
ratio of M_d/M_N = (5.8+/-1.6)x10^(-4), corresponding to an estimated total
peak dust mass of M_d = (5.3+/-1.5)x10^8 kg. We also compute the inferred peak
total active surface area and active surface fraction for P/La Sagra, finding
A_act ~ 5x10^4 m^2 and f_act ~ 0.01, respectively. Finally, we discuss P/La
Sagra's upcoming perihelion passage, particularly focusing on the available
opportunities to conduct follow-up observations in order to search for
recurrent activity and, if recurrent activity is present, to search for changes
in P/La Sagra's activity strength on successive orbit passages that should
provide insights into the evolution of MBC activity over time.Comment: 20 pages, 8 figures; accepted for publication in Icaru
Search for Dust Emission from (24) Themis Using the Gemini-North Telescope
We report the results of a search for a dust trail aligned with the orbit
plane of the large main-belt asteroid (24) Themis, which has been reported to
have water ice frost on its surface. Observations were obtained with the GMOS
instrument on the Gemini-North Observatory in imaging mode, where we used a
chip gap to block much of the light from the asteroid, allowing us to take long
exposures while avoiding saturation by the object. No dust trail is detected
within 2' of Themis to a 3-sigma limiting surface brightness magnitude of 29.7
mag/arcsec^2, as measured along the expected direction of the dust trail.
Detailed consideration of dust ejection physics indicates that particles large
enough to form a detectable dust trail were unlikely to be ejected as a result
of sublimation from an object as large as Themis. We nonetheless demonstrate
that our observations would have been capable of detecting faint dust emission
as close as 20" from the object, even in a crowded star field. This approach
could be used to conduct future searches for sublimation-generated dust
emission from Themis or other large asteroids closer to perihelion than was
done in this work. It would also be useful for deep imaging of collisionally
generated dust emission from large asteroids at times when the visibility of
dust features are expected to be maximized, such as during orbit plane
crossings, during close approaches to the Earth, or following detected impact
events.Comment: 11 pages, 4 figures, accepted for publication in PAS
Optical and Dynamical Characterization of Comet-Like Main-Belt Asteroid (596) Scheila
We present observations and a dynamical analysis of the comet-like main-belt
object, (596) Scheila. V-band photometry obtained on UT 2010 December 12
indicates that Scheila's dust cloud has a scattering cross-section ~1.4 times
larger than that of the nucleus, corresponding to a dust mass of M_d~3x10^7 kg.
V-R color measurements indicate that both the nucleus and dust are redder than
the Sun, with no significant color differences between the dust cloud's
northern and southern plumes. We also undertake an ultimately unsuccessful
search for CN emission, where we find CN and H2O production rates of Q(CN) <
9x10^23 s^-1 and Q(H2O) < 10^27 s^{-1}. Numerical simulations indicate that
Scheila is dynamically stable for >100 Myr, suggesting that it is likely native
to its current location. We also find that it does not belong to a dynamical
asteroid family of any significance. We consider sublimation-driven scenarios
that could produce the appearance of multiple plumes of dust emission, but
reject them as being physically implausible. Instead, we concur with previous
studies that the unusual morphology of Scheila's dust cloud is most simply
explained by a single oblique impact, meaning this object is likely not a
main-belt comet, but is instead the second disrupted asteroid after P/2010 A2
(LINEAR) to be discovered.Comment: 13 pages, 4 figures, accepted for publication in Ap
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