EPOXI at Comet Hartley 2 A'Hearn, Michael F.; Belton, Michael J. S.; Delamere, W. Alan ...
Science (American Association for the Advancement of Science),
06/2011, Letnik:
332, Številka:
6036
Journal Article
Recenzirano
Understanding how comets work—what drives their activity—is crucial to the use of comets in studying the early solar system. EPOXI (Extrasolar Planet Observation and Deep Impact Extended ...Investigation) flew past comet 103P/Hartley 2, one with an unusually small but very active nucleus, taking both images and spectra. Unlike large, relatively inactive nuclei, this nucleus is outgassing primarily because of CO 2 , which drags chunks of ice out of the nucleus. It also shows substantial differences in the relative abundance of volatiles from various parts of the nucleus.
Eta Carinae, the closest, active, massive binary containing a highly unstable Luminous Blue Variable, exhibits expanding, compressed wind shells, seen in emission, that are spatially and spectrally ...resolved by Hubble Space Telescope/Space Telescope Imaging Spectrograph. Starting in 2009 June, these structures were mapped across its 5.54-yr, highly elliptical, binary orbit to follow temporal changes in the light of Fe iii 4659 Å and Fe ii 4815 Å. The emissions trace portions of fossil wind shells, that were formed by wind–wind interactions across each cycle. Over the high-ionization state, dense arcs, photoionized by far-ultraviolet radiation from the hot secondary, are seen in Fe iii. Other arcs, ionized by mid-ultraviolet radiation from the primary star, are seen in Fe ii. The Fe iii structures tend to be interior to Fe ii structures that trace extensive, less disturbed primary wind. During the brief periastron passage when the secondary plunges deep into the primary's extremely dense wind, on the far side of primary star, high-ionization Fe iii structures fade and reappear in Fe ii. Multiple fossil wind structures were traced across the 5.7-yr monitoring interval. The strong similarity of the expanding Fe ii shells suggests that the wind and photoionization properties of the massive binary have not changed substantially from one orbit to the next over the past several orbital cycles. These observations trace structures that can be used to test 3D hydrodynamical and radiative-transfer models of massive, interacting winds. They also provide a baseline for following future changes in η Car, especially of its winds and photoionization properties.
► Individual large particles are observed at Hartley 2. ► The particles are at least centimeter sized. ► We explore two possible compositions: icy and dusty.
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 20cm, and perhaps up to 2m, 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 0.1–0.5% 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.
► Comet 103P/Hartley 2 has a bi-lobed, nearly axially-symmetric comet nucleus. ► Mass loss rates suggest reduction of the surface by up to several meters per orbit. ► Local processes of deposition ...and erosion compete to form low-slope surfaces.
Data from the Extrasolar Planet Observation and Deep Impact Extended Investigation (EPOXI) mission show Comet 103P/Hartley 2 is a bi-lobed, elongated, nearly axially symmetric comet 2.33km in length. Surface features are primarily small mounds <40m across, irregularly-shaped smooth areas on the two lobes, and a smooth but variegated region forming a “waist” between the two lobes. Assuming parts of the comet body approach the shape of an equipotential surface, the mean density of Hartley 2 is modeled to be 200–400kgm−3. Such a mean density suggests mass loss per orbit of >1%. The shape may be the evolutionary product of insolation, sublimation, and temporary deposition of materials controlled by the object’s complex rotation.
ESA and NASA recently selected two 5 μm cutoff Teledyne H2RG sensor chip assemblies (SCA) for flight on theJames Webb Space Telescope(JWST) Near Infrared Spectrograph (NIRSpec). These HgCdTe SCAs ...incorporate Teledyne’s “improved barrier layer” design that eliminates the degradation that affected earlierJWSTH2RGs. The better indium barrier, together with other design changes that Teledyne phased in from other programs over the years, has improved the performance and reliability ofJWST’s SCAs. In this article, we describe the measured performance characteristics that most directly affect scientific observations including read noise, total noise, dark current, quantum efficiency (QE), and image persistence. As part of measuring QE, we inferred the quantum yield over the full NIRSpec pass band of λ = 0.6–5 μm and found that it exceeds unity for photon energiesE
γ > (2.65 ± .2)E
g
, whereE
g
is the HgCdTe bandgap energy. This corresponds to λ ≲ 2 μm for NIRSpec’s 5 μm cutoff HgCdTe.
The James Webb Space Telescope (JWST) is the successor to the Hubble Space Telescope. JWST will be an infrared-optimized telescope, with an approximately 6.5 m diameter primary mirror, that is ...located at the Sun-Earth L2 Lagrange point. Three of JWST’s four science instruments use Teledyne HgCdTe HAWAII-2RG (H2RG) near infrared detector arrays. During 2010, the JWST Project noticed that a few of its 5 μm cutoff H2RG detectors were degrading during room temperature storage, and NASA chartered a “Detector Degradation Failure Review Board” (DD-FRB) to investigate. The DD-FRB determined that the root cause was a design flaw that allowed indium to interdiffuse with the gold contacts and migrate into the HgCdTe detector layer. Fortunately, Teledyne already had an improved design that eliminated this degradation mechanism. During early 2012, the improved H2RG design was qualified for flight and JWST began making additional H2RGs. In this article, we present the two public DD-FRB “Executive Summaries” that: (1) determined the root cause of the detector degradation and (2) defined tests to determine whether the existing detectors are qualified for flight. We supplement these with a brief introduction to H2RG detector arrays, some recent measurements showing that the performance of the improved design meets JWST requirements, and a discussion of how the JWST Project is using cryogenic storage to retard the degradation rate of the existing flight spare H2RGs.
We present far-UV observations of the Crab Nebula and its pulsar made with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope. Broad, blueshifted absorption arising in the ...nebula is seen in C IV λ1550, reaching a blueward velocity of ~2500 km s-1. This can be interpreted as evidence for a fast outer shell surrounding the Crab Nebula, and we adopt a spherically symmetric model to constrain the properties of such a shell. From the line profile we find that the density appears to decrease outward in the shell. A likely lower limit to the shell mass is ~0.3 Msolar with an accompanying kinetic energy of ~1.5×1049 ergs. A fast massive shell with 1051 ergs cannot be excluded but is less likely if the density profile is much steeper than ρ(R)~R-4 and the maximum velocity is <~6000 km s-1. The observations cover the region 1140-1720 Å, which is further into the ultraviolet than has previously been obtained for the pulsar. With the time-tag mode of the spectrograph we obtain the pulse profile in this spectral regime. The profile is similar to that previously obtained by us in the near-UV, although the primary peak is marginally narrower. Together with the near-UV data, and new optical data from the Nordic Optical Telescope, our spectrum of the Crab pulsar covers the entire region from 1140 to 9250 Å. Dereddening the spectrum with a standard extinction curve we achieve a flat spectrum for the reddening parameters E(B-V)=0.52, R=3.1. This dereddened spectrum of the Crab pulsar can be fitted by a power law with spectral index αν=0.11+/-0.04. The main uncertainty in determining the spectral index is the amount and characteristics of the interstellar reddening, and we have investigated the dependence of αν on E(B-V) and R. In the extended emission covered by our 25''×0.5" slit in the far-UV, we detect C IV λ1550 and He II λ1640 emission lines from the Crab Nebula. Several interstellar absorption lines are detected along the line of sight to the pulsar. The Lyα absorption indicates a column density of (3.0+/-0.5)×1021 cm-2 of neutral hydrogen, which agrees well with our estimate of E(B-V)=0.52 mag. Other lines show no evidence of severe depletion of metals in atomic gas. Based on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS5-26555.
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