446 research outputs found
CN Morphology Studies of Comet 103P/Hartley 2
We report on narrowband CN imaging of Comet 103P/Hartley 2 obtained at Lowell
Observatory on 39 nights from 2010 July until 2011 January. We observed two
features, one generally to the north and the other generally to the south. The
CN morphology varied during the apparition: no morphology was seen in July; in
August and September the northern feature dominated and appeared as a mostly
face-on spiral; in October, November, and December the northern and southern
features were roughly equal in brightness and looked like more side-on
corkscrews; in January the southern feature was dominant but the morphology was
indistinct due to very low signal. The morphology changed smoothly during each
night and similar morphology was seen from night to night. However, the
morphology did not exactly repeat each rotation cycle, suggesting that there is
a small non-principal axis rotation. Based on the repetition of the morphology,
we find evidence that the fundamental rotation period was increasing: 16.7 hr
from August 13-17, 17.2 hr from September 10-13, 18.2 hr from October 12-19,
and 18.7 hr from October 31-November 7. We conducted Monte Carlo jet modeling
to constrain the pole orientation and locations of the active regions based on
the observed morphology. Our preliminary, self-consistent pole solution has an
obliquity of 10 deg relative to the comet's orbital plane (i.e., it is centered
near RA = 257 deg and Dec=+67 deg with an uncertainty around this position of
about 15 deg) and has two mid-latitude sources, one in each hemisphere.Comment: Accepted by The Astronomical Journal; 23 pages of text, 2 tables, 8
figure
Spectroscopic Observations of New Oort Cloud Comet 2006 VZ13 and Four Other Comets
Spectral data are presented for comets 2006 VZ13 (LINEAR), 2006 K4 (NEAT),
2006 OF2 (Broughton), 2P/Encke, and 93P/Lovas I, obtained with the Cerro-Tololo
Inter-American Observatory 1.5-m telescope in August 2007. Comet 2006 VZ13 is a
new Oort cloud comet and shows strong lines of CN (3880 angstroms), the Swan
band sequence for C_2 (4740, 5160, and 5630 angstroms), C_3 (4056 angstroms),
and other faint species. Lines are also identified in the spectra of the other
comets. Flux measurements of the CN, C_2 (Delta v = +1,0), and C_3 lines are
recorded for each comet and production rates and ratios are derived. When
considering the comets as a group, there is a correlation of C_2 and C_3
production with CN, but there is no conclusive evidence that the production
rate ratios depend on heliocentric distance. The continuum is also measured,
and the dust production and dust-to-gas ratios are calculated. There is a
general trend, for the group of comets, between the dust-to-gas ratio and
heliocentric distance, but it does not depend on dynamical age or class. Comet
2006 VZ13 is determined to be in the carbon-depleted (or Tempel 1 type) class.Comment: 8 pages, 6 figures, 6 tables; Accepted by MNRA
Thirty Years of Cometary Spectroscopy from McDonald Observatory
We report on the results of a spectroscopic survey of 130 comets that was
conducted at McDonald observatory from 1980 through 2008. Some of the comets
were observed on only one night, while others were observed repeatedly. For 20
of these comets, no molecules were detected. For the remaining 110 comets, some
emission from CN, OH, NH, C, C, CH, and NH molecules were
observed on at least one occasion. We converted the observed molecular column
densities to production rates using a Haser (1957) model. We defined a
restricted data set of comets that had at least 3 nights of observations. The
restricted data set consists of 59 comets. We used ratios of production rates
to study the trends in the data. We find two classes of comets: typical and
carbon-chain depleted comets. Using a very strict definition of depleted
comets, requiring C \underline{and} C to both be depleted, we find
9% of our restricted data set comets to be depleted. Using a more relaxed
definition that requires only C to be below a threshold (similar to other
researchers), we find 25% of the comets are depleted. Two-thirds of the
depleted comets are Jupiter Family comets, while one-third are Long Period
comets. 37% of the Jupiter Family comets are depleted, while 18.5% of the Long
Period comets are depleted. We compare our results with other studies and find
good agreement.Comment: Accepted for Icarus; 15 figures, 9 tables (some multi-page and in
landscape mode
The Highly Unusual Outgassing of Comet 103P/Hartley 2 from Narrowband Photometry and Imaging of the Coma
We report on photometry and imaging of Comet 103P/Hartley 2 obtained at
Lowell Observatory from 1991 through 2011. We acquired photoelectric photometry
on two nights in 1991, four nights in 1997/98, and 13 nights in 2010/11. We
observed a strong secular decrease in water and all other observed species
production in 2010/11 from the 1991 and 1997/98 levels. We see evidence for a
strong asymmetry with respect to perihelion in the production rates of our
usual bandpasses, with peak production occurring ~10 days post-perihelion and
production rates considerably higher post-perihelion. The composition was
"typical", in agreement with the findings of other investigators. We obtained
imaging on 39 nights from 2010 July until 2011 January. We find that, after
accounting for their varying parentage and lifetimes, the C2 and C3 coma
morphology resemble the CN morphology we reported previously. These species
exhibited an hourglass shape in October and November, and the morphology
changed with rotation and evolved over time. The OH and NH coma morphology
showed hints of an hourglass shape near the nucleus, but was also enhanced in
the anti-sunward hemisphere. This tailward brightness enhancement did not vary
significantly with rotation and evolved with the viewing geometry. We conclude
that all five gas species likely originate from the same source regions on the
nucleus, but that OH and NH were derived from small grains of water and ammonia
ice that survived long enough to be affected by radiation pressure and driven
in the anti-sunward direction. We detected the faint, sunward facing dust jet
reported by other authors, and did not detect a corresponding gas feature. This
jet varied little during a night but exhibited some variations from night to
night, suggesting it is located near the total angular momentum vector.Comment: Accepted by Icarus; 20 pages of text (preprint style), 5 tables, 7
figure
Rotation-stimulated structures in the CN and C3 comae of comet 103P/Hartley 2 around the EPOXI encounter
In late 2010 a Jupiter Family comet 103P/Hartley 2 was a subject of an
intensive world-wide investigation. On UT October 20.7 the comet approached the
Earth within only 0.12 AU, and on UT November 4.6 it was visited by NASA's
EPOXI spacecraft. We joined this international effort and organized an
observing campaign. The images of the comet were obtained through narrowband
filters using the 2-m telescope of the Rozhen National Astronomical
Observatory. They were taken during 4 nights around the moment of the EPOXI
encounter. Image processing methods and periodicity analysis techniques were
used to reveal transient coma structures and investigate their repeatability
and kinematics. We observe shells, arc-, jet- and spiral-like patterns, very
similar for the CN and C3 comae. The CN features expanded outwards with the
sky-plane projected velocities between 0.1 to 0.3 km/s. A corkscrew structure,
observed on November 6, evolved with a much higher velocity of 0.66 km/s.
Photometry of the inner coma of CN shows variability with a period of
18.32+/-0.30 h (valid for the middle moment of our run, UT 2010 Nov. 5.0835),
which we attribute to the nucleus rotation. This result is fully consistent
with independent determinations around the same time by other teams. The
pattern of repeatability is, however, not perfect, which is understendable
given the suggested excitation of the rotation state, and the variability
detected in CN correlates well with the cyclic changes in HCN, but only in the
active phases. The revealed coma structures, along with the snapshot of the
nucleus orientation obtained by EPOXI, let us estimate the spin axis
orientation. We obtained RA=122 deg, Dec=+16 deg (epoch J2000.0), neglecting at
this point the rotational excitation.Comment: 9 pages, 10 figures, submitted to Astron. Astrophy
An S2 Fluorescence Model for Interpreting High-Resolution Cometary Spectra. I. Model Description and Initial Results
A new versatile model providing S2 fluorescence spectrum as a function of
time is developed with the aim of interpreting high resolution cometary
spectra. For the S2 molecule, it is important to take into account both
chemical and dynamic processes because S2 has a short lifetime and is confined
in the inner coma where these processes are most important. The combination of
the fluorescence model with a global coma model allows for the comparison with
observations of column densities taken through an aperture and for the analysis
of S2 fluorescence in different parts of the coma. Moreover, the model includes
the rotational structure of the molecule. Such a model is needed for
interpreting recent high spectral resolution observations of cometary S2. A
systematic study of the vibrational-rotational spectrum of S2 is undertaken,
including relevant effects, such as non-equilibrium state superposition and the
number density profile within the coma due to dynamics and chemistry, to
investigate the importance of the above effects on the scale length and
abundance of S2 in comets.Comment: 20 pages, 7 figure
Exploring the volatile composition of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) with ALMA
Comets formed in the outer and cold parts of the disk which eventually
evolved into our Solar System. Assuming that the comets have undergone no major
processing, studying their composition provides insight in the pristine
composition of the Solar Nebula. We derive production rates for a number of
volatile coma species and explore how molecular line ratios can help constrain
the uncertainties of these rates. We analyse observations obtained with the
Atacama Large Millimetre/Submillimetre Array of the volatile composition of the
comae of comets C/2012 F6 (Lemmon) and C/2012 S1 (ISON) at heliocentric
distances of ~1.45 AU and ~0.56 AU, respectively. Assuming a Haser profile with
constant outflow velocity, we model the line intensity of each transition using
a 3D radiative transfer code and derive molecular production rates and parent
scale lengths. We report the first detection of CS in comet ISON obtained with
the ALMA array and derive a parent scale length for CS of ~200 km. Due to the
high spatial resolution of ALMA, resulting in a synthesised beam with a size
slightly smaller than the derived parent scale length, we are able to
tentatively identify CS as a daughter species, i.e., a species produced in the
coma and/or sublimated from icy grains, rather than a parent species. In
addition we report the detection of several CH3OH transitions and confirm the
previously reported detections of HCN, HNC and H2CO as well as dust in the coma
of each comet, and report 3sigma upper limits for HCO+. We derive molecular
production rates relative to water of 0.2% for CS, 0.06-0.1% for HCN,
0.003-0.05% for HNC, 0.1-0.2% for H2CO and 0.5-1.0% for CH3OH, and show that
the modelling uncertainties due to unknown collision rates and kinematic
temperatures are modest and can be mitigated by available observations of
different transitions of HCN.Comment: 10 pages, 4 figures, 2 tables. Accepted for publication in A&
Activity of comet 103P/Hartley 2 at the time of the EPOXI mission fly-by
Comet 103P/Hartley~2 was observed on Nov. 1-6, 2010, coinciding with the
fly-by of the space probe EPOXI. The goal was to connect the large scale
phenomena observed from the ground, with those at small scale observed from the
spacecraft. The comet showed strong activity correlated with the rotation of
its nucleus, also observed by the spacecraft. We report here the
characterization of the solid component produced by this activity, via
observations of the emission in two spectral regions where only grain
scattering of the solar radiation is present. We show that the grains produced
by this activity had a lifetime of the order of 5 hours, compatible with the
spacecraft observations of the large icy chunks. Moreover, the grains produced
by one of the active regions have a very red color. This suggests an organic
component mixed with the ice in the grains.Comment: 11 pages, 7 figures, Icarus in pres
A fully 3-dimensional thermal model of a comet nucleus
A 3-D numerical model of comet nuclei is presented. An implicit numerical
scheme was developed for the thermal evolution of a spherical nucleus composed
of a mixture of ice and dust. The model was tested against analytical
solutions, simplified numerical solutions, and 1-D thermal evolution codes. The
3-D code was applied to comet 67P/Churyumov-Gerasimenko; surface temperature
maps and the internal thermal structure was obtained as function of depth,
longitude and hour angle. The effect of the spin axis tilt on the surface
temperature distribution was studied in detail. It was found that for small
tilt angles, relatively low temperatures may prevail on near-pole areas,
despite lateral heat conduction. A high-resolution run for a comet model of
67P/Churyumov-Gerasimenko with low tilt angle, allowing for crystallization of
amorphous ice, showed that the amorphous/crystalline ice boundary varies
significantly with depth as a function of cometary latitude.Comment: 19 pages, 10 figure
Ensemble Properties of Comets in the Sloan Digital Sky Survey
We present the ensemble properties of 31 comets (27 resolved and 4
unresolved) observed by the Sloan Digital Sky Survey (SDSS). This sample of
comets represents about 1 comet per 10 million SDSS photometric objects.
Five-band (u,g,r,i,z) photometry is used to determine the comets' colors,
sizes, surface brightness profiles, and rates of dust production in terms of
the Af{\rho} formalism. We find that the cumulative luminosity function for the
Jupiter Family Comets in our sample is well fit by a power law of the form N(<
H) \propto 10(0.49\pm0.05)H for H < 18, with evidence of a much shallower fit
N(< H) \propto 10(0.19\pm0.03)H for the faint (14.5 < H < 18) comets. The
resolved comets show an extremely narrow distribution of colors (0.57 \pm 0.05
in g - r for example), which are statistically indistinguishable from that of
the Jupiter Trojans. Further, there is no evidence of correlation between color
and physical, dynamical, or observational parameters for the observed comets.Comment: 19 pages, 8 tables, 11 figures, to appear in Icaru
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