143 research outputs found
Hubble Space Telescope Observations of Comet 9P/Tempel 1 during the Deep Impact Encounter
We report on the Hubble Space Telescope program to observe periodic comet
9P/Tempel 1 in conjunction with NASA's Deep Impact mission. Our objectives were
to study the generation and evolution of the coma resulting from the impact and
to obtain wide-band images of the visual outburst generated by the impact. Two
observing campaigns utilizing a total of 17 HST orbits were carried out: the
first occurred on 2005 June 13-14 and fortuitously recorded the appearance of a
new, short-lived fan in the sunward direction on June 14. The principal
campaign began two days before impact and was followed by contiguous orbits
through impact plus several hours and then snapshots one, seven, and twelve
days later. All of the observations were made using the Advanced Camera for
Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a
spatial resolution of 36 km (16 km/pixel) at the comet at the time of impact.
Baseline images of the comet, made prior to impact, photometrically resolved
the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement
with the 6.0 +/- 0.2 km diameter derived from the spacecraft imagers. Following
the impact, the HRC images illustrate the temporal and spatial evolution of the
ejecta cloud and allow for a determination of its expansion velocity
distribution. One day after impact the ejecta cloud had passed out of the
field-of-view of the HRC.Comment: 15 pages, 14 postscript figures. Accepted for publication in Icarus
special issue on Deep Impac
Near-UV OH Prompt Emission in the Innermost Coma of 103P/Hartley 2
The Deep Impact spacecraft fly-by of comet 103P/Hartley 2 occurred on 2010
November 4, one week after perihelion with a closest approach (CA) distance of
about 700 km. We used narrowband images obtained by the Medium Resolution
Imager (MRI) onboard the spacecraft to study the gas and dust in the innermost
coma. We derived an overall dust reddening of 15\%/100 nm between 345 and 749
nm and identified a blue enhancement in the dust coma in the sunward direction
within 5 km from the nucleus, which we interpret as a localized enrichment in
water ice. OH column density maps show an anti-sunward enhancement throughout
the encounter except for the highest resolution images, acquired at CA, where a
radial jet becomes visible in the innermost coma, extending up to 12 km from
the nucleus. The OH distribution in the inner coma is very different from that
expected for a fragment species. Instead, it correlates well with the water
vapor map derived by the HRI-IR instrument onboard Deep Impact
\citep{AHearn2011}. Radial profiles of the OH column density and derived water
production rates show an excess of OH emission during CA that cannot be
explained with pure fluorescence. We attribute this excess to a prompt emission
process where photodissociation of HO directly produces excited
OH*() radicals. Our observations provide the first direct
imaging of Near-UV prompt emission of OH. We therefore suggest the use of a
dedicated filter centered at 318.8 nm to directly trace the water in the coma
of comets.Comment: 21 page
A Distribution of Large Particles in the Coma of Comet 103P/Hartley 2
The coma of comet 103P/Hartley 2 has a significant population of large
particles observed as point sources in images taken by the Deep Impact
spacecraft. We measure their spatial and flux distributions, and attempt to
constrain their composition. The flux distribution of these particles implies a
very steep size distribution with power-law slopes ranging from -6.6 to -4.7.
The radii of the particles extend up to 20 cm, and perhaps up to 2 m, but their
exact sizes depend on their unknown light scattering properties. We consider
two cases: bright icy material, and dark dusty material. The icy case better
describes the particles if water sublimation from the particles causes a
significant rocket force, which we propose as the best method to account for
the observed spatial distribution. Solar radiation is a plausible alternative,
but only if the particles are very low density aggregates. If we treat the
particles as mini-nuclei, we estimate they account for <16-80% of the comet's
total water production rate (within 20.6 km). Dark dusty particles, however,
are not favored based on mass arguments. The water production rate from bright
icy particles is constrained with an upper limit of 0.1 to 0.5% of the total
water production rate of the comet. If indeed icy with a high albedo, these
particles do not appear to account for the comet's large water production rate.
production rate.
Erratum: We have corrected the radii and masses of the large particles of
comet 103P/Hartley 2 and present revised conclusions in the attached erratum.Comment: Original article: 46 pages, 17 figures, 5 tables, published in
Icarus. Erratum: 5 pages, 1 table, accepted for publication in Icaru
GALEX Observations of CS and OH Emission in Comet 9P/Tempel 1 During Deep Impact
GALEX observations of comet 9P/Tempel 1 using the near ultraviolet (NUV)
objective grism were made before, during and after the Deep Impact event that
occurred on 2005 July 4 at 05:52:03 UT when a 370 kg NASA spacecraft was
maneuvered into the path of the comet. The NUV channel provides usable spectral
information in a bandpass covering 2000 - 3400 A with a point source spectral
resolving power of approximately 100. The primary spectral features in this
range include solar continuum scattered from cometary dust and emissions from
OH and CS molecular bands centered near 3085 and 2575 A, respectively. In
particular, we report the only cometary CS emission detected during this event.
The observations allow the evolution of these spectral features to be tracked
over the period of the encounter. In general, the NUV emissions observed from
Tempel 1 are much fainter than those that have been observed by GALEX from
other comets. However, it is possible to derive production rates for the parent
molecules of the species detected by GALEX in Tempel 1 and to determine the
number of these molecules liberated by the impact. The derived quiescent
production rates are Q(H2O) = 6.4e27 molecules/s and Q(CS2) = 6.7e24
molecules/s, while the impact produced an additional 1.6e32 H2O molecules and
1.3e29 CS2 molecules, a similar ratio as in quiescent outgassing.Comment: 15 pages, 4 figures, accepted for publication in the Astrophysical
Journa
A Search for a Sub-Earth Sized Companion to GJ 436 and a Novel Method to Calibrate Warm Spitzer IRAC Observations
We discovered evidence for a possible additional 0.75 R_Earth transiting
planet in the NASA EPOXI observations of the known M dwarf exoplanetary system
GJ 436. Based on an ephemeris determined from the EPOXI data, we predicted a
transit event in an extant Spitzer Space Telescope 8 micron data set of this
star. Our subsequent analysis of those Spitzer data confirmed the signal of the
predicted depth and at the predicted time, but we found that the transit depth
was dependent on the aperture used to perform the photometry. Based on these
suggestive findings, we gathered new Warm Spitzer Observations of GJ 436 at 4.5
microns spanning a time of transit predicted from the EPOXI and Spitzer 8
micron candidate events. The 4.5 micron data permit us to rule out a transit at
high confidence, and we conclude that the earlier candidate transit signals
resulted from correlated noise in the EPOXI and Spitzer 8 micron observations.
In the course of this investigation, we developed a novel method for correcting
the intrapixel sensitivity variations of the 3.6 and 4.5 micron channels of the
Infrared Array Camera (IRAC) instrument. We demonstrate the sensitivity of Warm
Spitzer observations of M dwarfs to confirm sub-Earth sized planets. Our
analysis will inform similar work that will be undertaken to use Warm Spitzer
observations to confirm rocky planets discovered by the Kepler mission.Comment: 22 pages, 8 figures, accepted for publication in PAS
High-Resolution Infrared Spectroscopic Measurements of Comet 2PlEncke: Unusual Organic Composition and Low Rotational Temperatures
We present high-resolution infrared spectroscopic measurements of the ecliptic comet 2P/Encke, observed on 4-6 Nov. 2003 during its close approach to the Earth, using the Near Infrared Echelle Spectrograph on the Keck II telescope. We present flux-calibrated spectra, production rates, and mixing ratios for H2O, CH3OH, HCN, H2CO, C2H2, C2H6, CH4 and CO. Comet 2P/Encke is a dynamical end-member among comets because of its short period of 3.3 years. Relative to "organics-normal" comets, we determined that 2PlEncke is depleted in HCN, H2CO, C2H2, C2H6, CH4 and CO, but it is enriched in CH3OH. We compared mixing ratios of these organic species measured on separate dates, and we see no evidence of macroscopic chemical heterogeneity in the nucleus of 2P/Encke, however, this conclusion is limited by sparse temporal sampling. The depleted abundances of most measured species suggest that 2P/Encke may have formed closer to the young Sun, before its insertion to the Kuiper belt, compared with "organics-normal" comets - as was previously suggested for other depleted comets (e.g. C/1999 S4 (LINEAR)). We measured very low rotational temperatures of 20 - 30 K for H2O, CH3OH and HCN in the near nucleus region of 2P/Encke, which correlate with one of the lowest cometary gas production rates (approx. 2.6 x 10(exp 27) molecules/s) measured thus far in the infrared. This suggests that we are seeing the effects of more efficient radiative cooling, insufficient collisional excitation, and/or inefficient heating by fast H-atoms (and icy grains) in the observed region of the coma. Its extremely short orbital period, very low gas production rate, and classification as an ecliptic comet, make 2PlEncke an important addition to our growing database, and contribute significantly to the establishment of a chemical taxonomy of comets
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