185 research outputs found
Optical polarimetry and photometry of comet 17P/Holmes
Comet 17P/Holmes was observed for linear polarisation using the optical
polarimeter mounted on the 1.2m telescope atop Gurushikhar peak near Mt. Abu
during the period November-December 2007. Observations were conducted through
the IHW narrow band (continuum) filters. During the observing run the phase
angle was near at which the comet showed negative polarisation. On
the basis of the observed polarisation data we find comet 17P/Holmes to be a
typical comet with usual dust characteristics. We note that radial rate of
change of brightness in coma in red band is higher than that in blue band; it
has decreased by a factor of 3.6 and 2.5 respectively in red and blue bands
during the November - December run, indicating relative increase in the
abundance of smaller dust particles out ward. Radial brightness variation seen
near the nucleus on November 6 is indicative of the presence of a blob or
shocked region beyond 10" from the nucleus which has gradually smoothened by
December 13. The brightness distribution is found steeper during November 5-7
as compared to on December 13.Comment: 11pages, 7 figures, accepted for publication in MNRA
Evidence of Fragmenting Dust Particles from Near-Simultaneous Optical and Near-IR Photometry and Polarimetry of Comet 73P/Schwassmann-Wachmann 3
We report imaging polarimetry of segments B and C of the Jupiter-family Comet
73P/Schwassmann-Wachmann 3 in the I and H bandpasses at solar phase angles of
approximately 35 and 85deg. The level of polarization was typical for active
comets, but larger than expected for a Jupiter-family comet. The polarimetric
color was slightly red (dP/dL = +1.2 +/- 0.4) at a phase angle of ~ 35deg and
either neutral or slightly blue at a phase angle of ~ 85deg. Observations
during the closest approach from 2006 May 11-13 achieved a resolution of 35 km
at the nucleus. Both segments clearly depart from a 1/rho surface brightness
for the first 50 - 200 km from the nucleus. Simulations of radiation driven
dust dynamics can reproduce some of the observed coma morphology, but only with
a wide distribution of initial dust velocities (at least a factor of 10) for a
given grain radius. Grain aggregate breakup and fragmentation are able to
reproduce the observed profile perpendicular to the Sun-Comet axis, but fit the
observations less well along this axis (into the tail). The required
fragmentation is significant, with a reduction in the mean grain aggregate size
by about a factor of 10. A combination of the two processes could possibly
explain the surface brightness profile of the comet.Comment: 40 pages including 11 figure
Structure, bonding and morphology of hydrothermally synthesised xonotlite
The authors have systematically investigated the role of synthesis conditions upon the structure and morphology of xonotlite. Starting with a mechanochemically prepared, semicrystalline phase with Ca/Si=1, the authors have prepared a series of xonotlite samples hydrothermally, at temperatures between 200 and 250 degrees C. Analysis in each case was by X-ray photoelectron spectroscopy, environmental scanning electron microscopy and X-ray diffraction. The authorsâ use of a much lower water/solid ratio has indirectly confirmed the âthrough solutionâ mechanism of xonotlite formation, where silicate dissolution is a key precursor of xonotlite formation. Concerning the role of temperature, too low a temperature (~200 degrees C) fails to yield xonotlite or leads to increased number of structural defects in the silicate chains of xonotlite and too high a temperature (>250 degrees C) leads to degradation of the xonotlite structure, through leaching of interchain calcium. Synthesis duration meanwhile leads to increased silicate polymerisation due to diminishing of the defects in the silicate chains and more perfect crystal morphologies
ESA F-Class Comet Interceptor: Trajectory design to intercept a yet-to-be-discovered comet
Comet Interceptor (Comet-I) was selected in June 2019 as the first ESA F-Class mission. In 2029+, Comet-I will hitch a ride to a Sun-Earth L2 quasi-halo orbit, as a co-passenger of ESA's M4 ARIEL mission. It will then remain idle at the L2 point until the right departure conditions are met to intercept a yet-to-be-discovered long period comet (or interstellar body). The fact that Comet-I target is thus unidentified becomes a key aspect of the trajectory and mission design. The paper first analyses the long period comet population and concludes that 2 to 3 feasible targets a year should be expected. Yet, Comet-I will only be able to access some of these, depending mostly on the angular distance between the Earth and the closest nodal point to the Earth's orbit radius. A preliminary analysis of the transfer trajectories has been performed to assess the trade-off between the accessible region and the transfer time for a given spacecraft design, including a fully chemical, a fully electric and a hybrid propulsion system. The different Earth escape options also play a paramount role to enhance Comet-I capability to reach possible long period comet targets. Particularly, Earth-leading intercept configurations have the potential to benefit the most from lunar swing-by departures. Finally, a preliminary Monte Carlo analysis shows that Comet-I has a 95â99% likelihood of successfully visit a pristine newly-discovered long period comet in less than 6 years of mission timespan
Dust in Comet C/2007 N3 (Lulin)
We report optical imaging, optical and near-infrared polarimetry, and Spitzer
mid-infrared spectroscopy of comet C/2007 N3 (Lulin). Polarimetric observations
were obtained in R (0.676 micron) at phase angles from 0.44 degrees to 21
degrees with simultaneous observations in H (1.65 micron) at 4.0 degrees,
exploring the negative branch in polarization. Comet C/2007 N3 (Lulin) shows
typical negative polarization in the optical as well as a similar negative
branch near-infrared wavelengths. The 10 micron silicate feature is only weakly
in emission and according to our thermal models, is consistent with emission
from a mixture of silicate and carbon material. We argue that large,
low-porosity (akin to Ballistic Particle Cluster Aggregates) rather absorbing
aggregate dust particles best explain both the polarimetric and the
mid-infrared spectral energy distribution.Comment: 18 pages, 9 figures, 3 table
The First Detections of the Extragalactic Background Light at 3000, 5500, and 8000A (II): Measurement of Foreground Zodiacal Light
We present a measurement of the absolute surface brightness of the zodiacal
light (3900-5100A) toward a fixed extragalactic target at high ecliptic
latitude based on moderate resolution (~1.3A per pixel) spectrophotometry
obtained with the du Pont 2.5m telescope at Las Campanas Observatory in Chile.
This measurement and contemporaneous Hubble Space Telescope data from WFPC2 and
FOS comprise a coordinated program to measure the mean flux of the diffuse
extragalactic background light (EBL). The zodiacal light at optical wavelengths
results from scattering by interplanetary dust, so that the zodiacal light flux
toward any extragalactic target varies seasonally with the position of the
Earth. This measurement of zodiacal light is therefore relevant to the specific
observations (date and target field) under discussion. To obtain this result,
we have developed a technique that uses the strength of the zodiacal Fraunhofer
lines to identify the absolute flux of the zodiacal light in the
multiple-component night sky spectrum. Statistical uncertainties in the result
are 0.6% (1 sigma). However, the dominant source of uncertainty is systematic
errors, which we estimate to be 1.1% (1 sigma). We discuss the contributions
included in this estimate explicitly. The systematic errors in this result
contribute 25% in quadrature to the final error in our coordinated EBL
measurement, which is presented in the first paper of this series.Comment: Accepted for publication in ApJ, 22 pages using emulateapj.sty,
version with higher resolution figures available at
http://www.astro.lsa.umich.edu/~rab/publications.html or at
http://nedwww.ipac.caltech.edu/level5/Sep01/Bernstein2/frames.htm
Polarization Studies of Comet C/2000 WM1 (LINEAR)
Linear polarization observations were carried out on comet C/2000 WM1 with
the 1.2m telescope at Mt. Abu Observatory during November 2001 and March 2002.
The observations in November were at low phase angle (<~22\degr) when the
polarization is negative and where the data for most of the comets are rather
meager. The observations during March were made when the phase angle was
47\degr. Observations were conducted through the IHW narrow band and BVR broad
band filters. Based on these polarization observations we infer that the comet
C/2000 WM1 belongs to high polarization class i.e. the dusty comet family.Comment: 7 pages, 3 figures, accepted for publication in Astron. & Astrop
The Dust Trail of Comet 67P/Churyumov-Gerasimenko
We report the detection of comet 67P/Churyumov-Gerasimenko's dust trail and
nucleus in 24 micron Spitzer Space Telescope images taken February 2004. The
dust trail is not found in optical Palomar images taken June 2003. Both the
optical and infrared images show a distinct neck-line tail structure, offset
from the projected orbit of the comet. We compare our observations to simulated
images using a Monte Carlo approach and a dynamical model for comet dust. We
estimate the trail to be at least one orbit old (6.6 years) and consist of
particles of size >~100 micron. The neck-line is composed of similar sized
particles, particles of size but younger in age. Together, our observations and
simulations suggest grains 100 micron and larger in size dominate the total
mass ejected from the comet. The radiometric effective radius of the nucleus is
1.87 +/- 0.08 km, derived from the Spitzer observation. The Rosetta spacecraft
is expected to arrive at and orbit this comet in 2014. Assuming the trail is
comprised solely of 1 mm radius grains, we compute a low probability (~10^-3)
of a trail grain impacting with Rosetta during approach and orbit insertion.Comment: Accepted for publication in Icaru
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