540 research outputs found
The nucleus of 103P/Hartley 2, target of the EPOXI mission
103P/Hartley 2 was selected as the target comet for the Deep Impact extended
mission, EPOXI, in October 2007. There have been no direct optical observations
of the nucleus of this comet, as it has always been highly active when
previously observed. We aimed to recover the comet near to aphelion, to a)
confirm that it had not broken up and was in the predicted position, b) to
provide astrometry and brightness information for mission planning, and c) to
continue the characterisation of the nucleus. We observed the comet at
heliocentric distances between 5.7 and 5.5 AU, using FORS2 at the VLT, at 4
epochs between May and July 2008. We performed VRI photometry on deep stacked
images to look for activity and measure the absolute magnitude and therefore
estimate the size of the nucleus. We recovered the comet near the expected
position, with a magnitude of m_R = 23.74 \pm 0.06 at the first epoch. The
comet had no visible coma, although comparison of the profile with a stellar
one showed that there was faint activity, or possibly a contribution to the
flux from the dust trail from previous activity. This activity appears to fade
at further epochs, implying that this is a continuation of activity past
aphelion from the previous apparition rather than an early start to activity
before the next perihelion. Our data imply a nucleus radius of \le 1 km for an
assumed 4% albedo; we estimate a ~6% albedo. We measure a colour of (V-R) = 0.
26 \pm 0.09.Comment: 5 pages, 4 figures, accepted for publication in A&
Reduction and analysis of photometric data on Comet Halley
The discovery that periodic variations in the brightness of Comet Halley were characterized by two unrelated frequencies implies that the nucleus is in a complex state of rotation. It either nutates as a result of the random addition of small torque perturbations accumulated over many perihelion passages, or the jet activity torques are so strong that it precesses wildly at each perihelion passage. To diagnose the state of nuclear rotation, researchers began a program to acquire photometric time series of the comet as it recedes from the sun. The intention is to observe the decay of the comet's atmosphere and then, when it is unemcumbered by the light of the coma, follow the light variation of the nucleus itself. The latter will be compared with preperihelion time series and the orientation of the nucleus at the time of Vega and Giotto flybys and an accurate rotational ephemeris constructed. Halley was observed on 38 nights during 1987 and approximately 21 nights in 1988. The comet moved from 5 AU to 8.5 AU during this time. The brightness of the coma was found to rapidly decrease in 1988 as the coma and cometary activity collapses. The magnitude in April 1988 was 19 mag (visual) and it is predicted that the nucleus itself will be the major contributor to the brightness in the 1988 and 1989 season
Thermal evolution and activity of Comet 9P/Tempel 1 and simulation of a deep impact
We use a quasi 3-D thermal evolution model for a spherical comet nucleus,
which takes into account the diurnal and latitudinal variation of the solar
flux, but neglects lateral heat conduction. We model the thermal evolution and
activity of Comet 9P/Tempel 1, in anticipation of the Deep Impact mission
encounter with the comet. We also investigate the possible outcome of a
projectile impact, assuming that all the energy is absorbed as thermal energy.
An interesting result of this investigation, is that the estimated amount of
dust ejected due to the impact is equivalent to 2--2.6 days of activity, during
"quiet" conditions, at perihelion.
We show that production rates of volatiles that are released in the interior
of the nucleus depend strongly on the porous structure, in particular on the
surface to volume ratio of the pores. We develop a more accurate model for
calculating this parameter, based on a distribution of pore sizes, rather than
a single, average pore size.Comment: 25 pages, 8 figures, accepted for publication in PASP (in press). For
fig.xx (composite image, sec.4) and a better resolution of fig.6 see,
http://geophysics.tau.ac.il/personal/gal%5Fsarid
The formation heritage of Jupiter Family Comet 10P/Tempel 2 as revealed by infrared spectroscopy
We present spectral and spatial information for major volatile species in
Comet 10P/Tempel 2, based on high-dispersion infrared spectra acquired on UT
2010 July 26 (heliocentric distance Rh = 1.44 AU) and September 18 (Rh = 1.62
AU), following the comet's perihelion passage on UT 2010 July 04. The total
production rate for water on July 26 was (1.90 +/- 0.12) x 10^28 molecules s-1,
and abundances of six trace gases (relative to water) were: CH3OH (1.58% +/-
0.23), C2H6 (0.39% +/- 0.04), NH3 (0.83% +/- 0.20), and HCN (0.13% +/- 0.02). A
detailed analysis of intensities for water emission lines provided a rotational
temperature of 35 +/- 3 K. The mean OPR is consistent with nuclear spin
populations in statistical equilibrium (OPR = 3.01 +/- 0.18), and the (1-sigma)
lower bound corresponds to a spin temperature > 38 K. Our measurements were
contemporaneous with a jet-like feature observed at optical wavelengths. The
spatial profiles of four primary volatiles display strong enhancements in the
jet direction, which favors release from a localized vent on the nucleus. The
measured IR continuum is much more sharply peaked and is consistent with a
dominant contribution from the nucleus itself. The peak intensities for H2O,
CH3OH, and C2H6 are offset by ~200 km in the jet direction, suggesting the
possible existence of a distributed source, such as the release of icy grains
that subsequently sublimed in the coma. On UT September 18, no obvious emission
lines were present in our spectra, nevertheless we obtained a 3-sigma upper
limit Q(H2O) < 2.86 x 10^27 molecules s-1
Frozen to death? -- Detection of comet Hale-Bopp at 30.7 AU
Comet Hale--Bopp (C/1995 O1) has been the single most significant comet
encountered by modern astronomy, still having displayed significant activity at
25.7 AU solar distance in late 2007. It is a puzzling question when and where
this activity will finally cease. Here we present new observations with the ESO
2.2m telescope at La Silla to check the activity of Hale--Bopp at 30.7 AU solar
distance. On 2010-12-04, 26 CCD images were taken with 180 s exposure times for
photometry and morphology. The comet was detected in R and had a total
brightness of 23.3+-0.2 mag, referring to an absolute brightness of
R(1,1,0)=8.3. The profile of the coma was star-like at a seeing of 1.9",
without any evidence of a coma or tail extending farther than 2.5" (=55,000 km
in projection) and exceeding 26.5 mag/arcs^2 surface brightness. The measured
total brightness corresponds to a relative total reflecting surface, a_RC, of
485 km^2, nine times less than three years before. The calculated a_RC value
would imply a nucleus with 60--65 km radius assuming 4% albedo. This size
estimate is in significant contradiction with the previous results scattering
around 35 km. Therefore we suggest that the comet may still be in a low-level
activity, despite the lack of a prominent coma. Alternatively, if the nucleus
is already dormant, the albedo should be as high as 13%, assuming a radius of
35 km. With this observation, Hale--Bopp has been the most distant comet ever
observed, far beyond the orbit of Neptune.Comment: 4 pages, accepted by A&
Plausible home stars of the interstellar object 'Oumuamua found in Gaia DR2
The first detected interstellar object 'Oumuamua that passed within 0.25au of
the Sun on 2017 September 9 was presumably ejected from a stellar system. We
use its newly determined non-Keplerian trajectory together with the
reconstructed Galactic orbits of 7 million stars from Gaia DR2 to identify past
close encounters. Such an "encounter" could reveal the home system from which
'Oumuamua was ejected. The closest encounter, at 0.60pc (0.53-0.67pc, 90%
confidence interval), was with the M2.5 dwarf HIP 3757 at a relative velocity
of 24.7km/s, 1Myr ago. A more distant encounter (1.6pc) but with a lower
encounter (ejection) velocity of 10.7km/s was with the G5 dwarf HD 292249,
3.8Myr ago. Two more stars have encounter distances and velocities intermediate
to these. The encounter parameters are similar across six different
non-gravitational trajectories for 'Oumuamua. Ejection of 'Oumuamua by
scattering from a giant planet in one of the systems is plausible, but requires
a rather unlikely configuration to achieve the high velocities found. A binary
star system is more likely to produce the observed velocities. None of the four
home candidates have published exoplanets or are known to be binaries. Given
that the 7 million stars in Gaia DR2 with 6D phase space information is just a
small fraction of all stars for which we can eventually reconstruct orbits, it
is a priori unlikely that our current search would find 'Oumuamua's home star
system. As 'Oumuamua is expected to pass within 1pc of about 20 stars and brown
dwarfs every Myr, the plausibility of a home system depends also on an
appropriate (low) encounter velocity.Comment: Accepted to The Astronomical Journa
Photometric observations of distant active comets
We present CCD VR_C observations of 6 distant comets located at heliocentric
distances of 3.4--7.2 AU. Time-series data were obtained on three nights in
July, 2000 covering 16 hours. Each comet was observed after the perihelion,
when a lower activity was expected. Contrary to expectation, we found
well-defined circular comae and extended tails visible out to a few (3-5)
arcminutes. We detected a quasi-cyclic light variation of C/1999 J2, while
C/1999 N4 showed some hints of a more complex variation. C/2000 K1 was constant
to +/-0.04 mag during the observing run. The standard V and R_C data were used
to estimate nuclear diameters, while the colour indices implied a slighly
reddish (V-R=0.68, C/2000 K1), a neutral (V-R=0.47, C/1999 N4) and a slightly
bluish (V-R=0.25, C/1999 J2) coma. Simple fits of the surface brightness
distributions are also presented enabling order of magnitude estimates of
nuclear radii. Beside the time-series observations, further single-shot
observations of three faint comets are briefly described.Comment: 8 pages, 5 figures, accepted for publication in A&
Spectropolarimetry of the Deep Impact target comet 9P/Tempel 1 with HiVIS
High resolution spectropolarimetry of the Deep Impact target, comet 9P/
Tempel 1, was performed during the impact event on July 4th, 2005 with the
HiVIS Spectropolarimeter and the AEOS 3.67m telescope on Haleakala, Maui. We
observed atypical polarization spectra that changed significantly in the few
hours after the impact. The polarization of scattered light as a function of
wavelength is very sensitive to the size and composition (complex refractive
index) of the scattering particles as well as the scattering geometry. As
opposed to most observations of cometary dust, which show an increase in the
linear polarization with the wavelength (at least in the visible domain and for
phase angles greater than about 30%, a red polarization spectrum) observations
of 9P/Tempel 1 at a phase angle of 41 degrees beginning 8 minutes after impact
and centered at 6:30UT showed a polarization of 4% at 650 nm falling to 3% at
950 nm. The next observation, centered an hour later showed a polarization of
7% at 650 nm falling to 2% at 950nm. This corresponds to a spectropolarimetric
gradient, or slope, of -0.9% per 1000 Angstroms 40 minutes after impact,
decreasing to a slope of -2.3% per 1000 Angstroms an hour and a half after
impact. This is an atypical blue polarization slope, which became more blue 1
hour after impact. The polarization values of 4% and 7% at 650nm are typical
for comets at this scattering angle, whereas the low polarization of 2% and 3%
at 950nm is not. We compare observations of comet 9P/Tempel 1 to that of a
typical comet, C/2004 Machholz, at a phase angle of 30 degrees which showed a
typical red slope, rising from 2% at 650nm to 3% at 950nm in two different
observations (+1.0 and +0.9% per 1000 Angstroms).Comment: Icarus Deep Impact special issue, accepted Aug 28 200
WISE/NEOWISE Preliminary Analysis and Highlights of the 67P/Churyumov-Gerasimenko Near Nucleus Environs
On January 18-19 and June 28-29 of 2010, the Wide-field Infrared Survey
Explorer (WISE) spacecraft imaged the Rosetta mission target, comet
67P/Churyumov-Gerasimenko. We present a preliminary analysis of the images,
which provide a characterization of the dust environment at heliocentric
distances similar to those planned for the initial spacecraft encounter, but on
the outbound leg of its orbit rather than the inbound. Broad-band photometry
yields low levels of CO2 production at a comet heliocentric distance of 3.32 AU
and no detectable production at 4.18 AU. We find that at these heliocentric
distances, large dust grains with mean grain diameters on the order of a
millimeter or greater dominate the coma and evolve to populate the tail. This
is further supported by broad-band photometry centered on the nucleus, which
yield an estimated differential dust particle size distribution with a power
law relation that is considerably shallower than average. We set a 3-sigma
upper limit constraint on the albedo of the large-grain dust at <= 0.12. Our
best estimate of the nucleus radius (1.82 +/- 0.20 km) and albedo (0.04 +/-
0.01) are in agreement with measurements previously reported in the literature
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