224 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
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
The Dust Cloud around the White Dwarf G 29-38. II. Spectrum from 5 to 40 μm and Mid-Infrared Photometric Variability
We model the mineralogy and distribution of dust around the white dwarf G29-39 using the infrared spectrum from 1 to 35 μm. The spectral model for G29-38 dust combines a wide range of materials based on spectral studies of comets and debris disks. In order of their contribution to the mid-infrared emission, the most abundant minerals around G29-38 are amorphous carbon (λ < 8 μm), amorphous and crystalline silicates (5-40 μm), water ice (10-15 and 23-35 μm), and metal sulfides (18-28 μm). The amorphous C can be equivalently replaced by other materials (like metallic Fe) with featureless infrared spectra. The best-fitting crystalline silicate is Fe-rich pyroxene. In order to absorb enough starlight to power the observed emission, the disk must either be much thinner than the stellar radius (so that it can be heated from above and below) or it must have an opening angle wider than 2°. A "moderately optically thick" torus model fits well if the dust extends inward to 50 times the white dwarf radius, all grains hotter than 1100 K are vaporized, the optical depth from the star through the disk is τ║ = 5, and the radial density profile α r ^(–2.7); the total mass of this model disk is 2 × 10^(19) g. A physically thin (less than the white dwarf radius) and optically thick disk can contribute to the near-infrared continuum only; such a disk cannot explain the longer-wavelength continuum or strong emission features. The combination of a physically thin, optically thick inner disk and an outer, physically thick and moderately optically thin cloud or disk produces a reasonably good fit to the spectrum and requires only silicates in the outer cloud. We discuss the mineralogical results in comparison to planetary materials. The silicate composition contains minerals found from cometary spectra and meteorites, but Fe-rich pyroxene is more abundant than enstatite (Mg-rich pyroxene) or forsterite (Mg-rich olivine) in G29-38 dust, in contrast to what is found in most comet or meteorite mineralogies. Enstatite meteorites may be the most similar solar system materials to G29-38 dust. Finally, we suggest the surviving core of a "hot Jupiter" as an alternative (neither cometary nor asteroidal) origin for the debris, though further theoretical work is needed to determine if this hypothesis is viable
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 Measurement of the Cosmic Optical Background and Diffuse Galactic Light Scaling from the R < 50 AU New Horizons-LORRI Data
Direct photometric measurements of the cosmic optical background (COB)
provide an important point of comparison to both other measurement
methodologies and models of cosmic structure formation, and permit a cosmic
consistency test with the potential to reveal additional diffuse sources of
emission. The COB has been challenging to measure from Earth due to the
difficulty of isolating it from the diffuse light scattered from interplanetary
dust in our solar system. We present a measurement of the COB using data taken
by the Long-Range Reconnaissance Imager (LORRI) on NASA's New Horizons mission,
considering all data acquired to 47 AU. We employ a blind methodology where our
analysis choices are developed against a subset of the full data set, which is
then unblinded. Dark current and other instrumental systematics are accounted
for, including a number of sources of scattered light. We fully characterize
and remove structured and diffuse astrophysical foregrounds including bright
stars, the integrated starlight from faint unresolved sources, and diffuse
galactic light. For the full data set, we find the surface brightness of the
COB to be 21.98 1.23 (stat.)
1.36 (cal.) nW m sr. This result supports recent
determinations that find a factor of 2 3 more light than
expected from the integrated light from galaxies and motivate new diffuse
intensity measurements with more capable instruments that can support spectral
measurements over the optical and near-IR.Comment: 36 pages, 22 figures, 8 tables; accepted for publication in Ap
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