194 research outputs found
Constraints on the Spin-Pole Orientation, Jet Morphology and Rotation of Interstellar Comet 2I/Borisov with Deep HST Imaging
We present high resolution, deep imaging of interstellar comet 2I/Borisov
taken with the Hubble Space Telescope/Wide Field Camera 3 (HST/WFC3) on 2019
December 8 UTC and 2020 January 27 UTC (HST GO 16040, PI Bolin) before and
after its perihelion passage in combination with HST/WFC3 images taken on 2019
October 12 UTC and 2019 November 16 UTC (HST GO/DD 16009, PI Jewitt) before its
outburst and fragmentation of March 2020, thus observing the comet in a
relatively undisrupted state. We locate 1-2\arcsec~long (2,000 - 3,000 km
projected length) jet-like structures near the optocenter of 2I that appear to
change position angles from epoch to epoch. With the assumption that the jet is
located near the rotational pole supported by its stationary appearance on
10-100 h time frames in HST images, we determine that 2I's pole points
near = 32210, = 3710 ( =
341, = 48) and may be in a simple rotation state.
Additionally, we find evidence for possible periodicity in the HST time-series
lightcurve on the time scale of 5.3 h with a small amplitude of
0.05 mag implying a lower limit on its ratio of 1.5 unlike
the large 2 mag lightcurve observed for 1I/`Oumuamua. However, these
small lightcurve variations may not be the result of the rotation of 2I's
nucleus due to its dust-dominated light-scattering cross-section. Therefore,
uniquely constraining the pre-Solar System encounter, pre-outburst rotation
state of 2I may not be possible even with the resolution and sensitivity
provided by HST observations.Comment: 14 pages, 6 figures and 3 tables, accepted for publication in MNRAS
on 23 July 202
Spitzer IRS Spectroscopy of the 10 Myr-old EF Cha Debris Disk: Evidence for Phyllosilicate-Rich Dust in the Terrestrial Zone
We describe Spitzer IRS spectroscopic observations of the 10 Myr-old star, EF
Cha. Compositional modeling of the spectra from 5 {\mu}m to 35 {\mu}m confirms
that it is surrounded by a luminous debris disk with LD/L\star ~ 10-3,
containing dust with temperatures between 225 K and 430 K characteristic of the
terrestrial zone. The EF Cha spectrum shows evidence for many solid-state
features, unlike most cold, low-luminosity debris disks but like some other
10-20 Myr-old luminous, warm debris disks (e.g. HD 113766A). The EF Cha debris
disk is unusually rich in a species or combination of species whose
emissivities resemble that of finely powdered, laboratory-measured
phyllosilicate species (talc, saponite, and smectite), which are likely
produced by aqueous alteration of primordial anhydrous rocky materials. The
dust and, by inference, the parent bodies of the debris also contain abundant
amorphous silicates and metal sulfides, and possibly water ice. The dust's
total olivine to pyroxene ratio of ~ 2 also provides evidence of aqueous
alteration. The large mass volume of grains with sizes comparable to or below
the radiation blow-out limit implies that planetesimals may be colliding at a
rate high enough to yield the emitting dust but not so high as to devolatize
the planetesimals via impact processing. Because phyllosilicates are produced
by the interactions between anhydrous rock and warm, reactive water, EF Cha's
disk is a likely signpost for water delivery to the terrestrial zone of a young
planetary system.Comment: 21 pages, 10 figures, accepted for publication in The Astrophysical
Journa
Measurement of the Cosmic Optical Background using the Long Range Reconnaissance Imager on New Horizons
The cosmic optical background is an important observable that constrains
energy production in stars and more exotic physical processes in the universe,
and provides a crucial cosmological benchmark against which to judge theories
of structure formation. Measurement of the absolute brightness of this
background is complicated by local foregrounds like the Earth's atmosphere and
sunlight reflected from local interplanetary dust, and large discrepancies in
the inferred brightness of the optical background have resulted. Observations
from probes far from the Earth are not affected by these bright foregrounds.
Here we analyze data from the Long Range Reconnaissance Imager (LORRI)
instrument on NASA's New Horizons mission acquired during cruise phase outside
the orbit of Jupiter, and find a statistical upper limit on the optical
background's brightness similar to the integrated light from galaxies. We
conclude that a carefully performed survey with LORRI could yield uncertainties
comparable to those from galaxy counting measurements.Comment: 35 pages, 11 figures, published in Nature Communication
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
The ion-induced charge-exchange X-ray emission of the Jovian Auroras: Magnetospheric or solar wind origin?
A new and more comprehensive model of charge-exchange induced X-ray emission,
due to ions precipitating into the Jovian atmosphere near the poles, has been
used to analyze spectral observations made by the Chandra X-ray Observatory.
The model includes for the first time carbon ions, in addition to the oxygen
and sulfur ions previously considered, in order to account for possible ion
origins from both the solar wind and the Jovian magnetosphere. By comparing the
model spectra with newly reprocessed Chandra observations, we conclude that
carbon ion emission provides a negligible contribution, suggesting that solar
wind ions are not responsible for the observed polar X-rays. In addition,
results of the model fits to observations support the previously estimated
seeding kinetic energies of the precipitating ions (~0.7-2 MeV/u), but infer a
different relative sulfur to oxygen abundance ratio for these Chandra
observations.Comment: 11 pages, 2 figures, 2 tables, submitted to ApJ Lette
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
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