Near-Infrared Imaging Spectrophotometry Of Comet C/1995 O1 (Hale-Bopp) Near Perihelion     

Harker  David E.  Woodward  Charles E.  McMURTRY  Craig W.  Goetz  J. A.  Pipher  J. L.  Forrest  W. J. 《Earth, Moon, and Planets》1997,78(1-3):259-264

We present 1- to 5-μm broadband and CVF images of comet Hale-Bopp taken 1997 February 10.5 UT, 50 days before perihelion. All the images exhibit a nonspherical coma with a bright “ridge” in the direction of the dust tail approximately 10″ from the coma. Synthetic aperture spectrophotometry implies that the optically important grains are of a radius ≤0.4 μm; smallest radius for any comet seen to date. The variation of the integrated surface brightness with radial distance from the coma (ρ) in all the images closely follows the “steady state” ρ⁻¹ model for comet dust ablation (Gehrz and Ney, 1992). The near-infrared colors taken along the dust tail are not constant implying the dust grain properties vary with coma distance. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Armalcolite stability as a function of pressure and oxygen fugacity     

John J Friel  R.Ian Harker  Gene C Ulmer 《Geochimica et cosmochimica acta》1977,41(3):403-410

The stability of synthetic armalcolite of composition (Fe_0.5Mg_0.5Ti₂O₅ was studied as a function of total pressure up to 15 kbar and 1200°C and also as a function of oxygen fugacity (?O₂) at 1200°C and 1 atm total pressure. The high pressure experiments were carried out in a piston-cylinder apparatus using silver-palladium containers. At 1200°C, armalcolite is stable as a single phase at 10 kbar. With increasing pressure, it breaks down ($\text{dT}\text{dP}\text{= 20°C/}\text{kbar}$), to rutile, a more magnesian armalcolite, and ilmenite solid solution. At 14 kbar, this three-phase assemblage gives way ($\text{dT}\text{dP}\text{= 30°C/}\text{kbar}$) to a two-phase assemblage of rutile plus ilmenite solid solution.A zirconian-armalcolite was synthesized and analyzed; 4 wt % ZrO₂ appears to saturate armalcolite at 1200°C and 1 atm. The breakdown of Zr-armalcolite occurs at pressures of 1–2 kbar less than those required for the breakdown of Zr-free armalcolite. The zirconium partitions approximately equally between rutile and ilmenite phases.The stability of armalcolite as a function of ?O₂ was determined thermogravimetrically at 1200°C and 1 atm by weighing sintered pellets in a controlled atmosphere furnace. Armalcolite, (Fe_0.5Mg_0.5)-Ti₂O₅, is stable over a range ?O₂ from about 10^?9.5to 10^?10.5 atm. Below this range to at least 10^?12.8 atm, ilmenite plus a reduced armalcolite are formed. These products were observed optically and by Mössbauer spectroscopy, and no metallic iron was detected; therefore, some of the titanium must have been reduced to Ti³⁺. This reduction may provide yet another mechanism to explain the common association of ilmenite rims around lunar armalcolites.  相似文献   

Gemini-N mid-IR observations of the dust properties of the ejecta excavated from Comet 9P/Tempel 1 during Deep Impact     

David E. Harker  Charles E. Woodward  R. Scott Fisher 《Icarus》2007,190(2):432-453

We present mid-infrared spectra and images from the Gemini-N (+MICHELLE) observational campaign of Comet 9P/Tempel 1 before, during, and after its encounter with Deep Impact. We use our thermal grain model to probe the 10 μm properties of the dust grains in the coma of the comet. Before impact (3 July 2005 UT), and more than 24 h after impact (5, 16, and 28 July 2005 UT), the comet dust grains were composed mostly of amorphous olivine, and were relatively large (peak of the grain size distribution ). For the night of impact, we extract spectra by centering on the nucleus, and offset 1″ from the nucleus in the direction of the impact ejecta plume. We find small dust grains (∼0.2 μm) of a diverse mineralogy (amorphous olivine, amorphous pyroxene, amorphous carbon, and crystalline olivine) populating the ejecta. The submicron sized dust grains move faster than the other, larger grains (?0.7 μm), with amorphous olivine and amorphous carbon traveling together, and amorphous pyroxene and crystalline olivine dispersing at a similar rate. Deriving a velocity law from a time-of-flight analysis, we find that the material traveled with a velocity law scaled by and with a power of p=0.5. This velocity power-law requires a sustained release of grains for the duration of 45-60 min after impact. Since the mineral species are traveling at different speeds, and there was a sustained release of grains due to a possible “gas-plume,” we conclude that the different minerals did not originate from grain aggregates destroyed by the impact, but instead arise from an inhomogeneous nucleus.  相似文献   

Detection and extraction of signals from the epoch of reionization using higher-order one-point statistics     

Geraint J. A. Harker  Saleem Zaroubi  Rajat M. Thomas  Vibor Jeli&#;  Panagiotis Labropoulos  Garrelt Mellema  Ilian T. Iliev  Gianni Bernardi  Michiel A. Brentjens  A. G. de Bruyn  Benedetta Ciardi  Leon V. E. Koopmans  V. N. Pandey  reas H. Pawlik  Joop Schaye  Sarod Yatawatta 《Monthly notices of the Royal Astronomical Society》2009,393(4):1449-1458

Detecting redshifted 21-cm emission from neutral hydrogen in the early Universe promises to give direct constraints on the epoch of reionization (EoR). It will, though, be very challenging to extract the cosmological signal (CS) from foregrounds and noise which are orders of magnitude larger. Fortunately, the signal has some characteristics which differentiate it from the foregrounds and noise, and we suggest that using the correct statistics may tease out signatures of reionization. We generate mock data cubes simulating the output of the Low Frequency Array (LOFAR) EoR experiment. These cubes combine realistic models for Galactic and extragalactic foregrounds and the noise with three different simulations of the CS. We fit out the foregrounds, which are smooth in the frequency direction, to produce residual images in each frequency band. We denoise these images and study the skewness of the one-point distribution in the images as a function of frequency. We find that, under sufficiently optimistic assumptions, we can recover the main features of the redshift evolution of the skewness in the 21-cm signal. We argue that some of these features – such as a dip at the onset of reionization, followed by a rise towards its later stages – may be generic, and give us a promising route to a statistical detection of reionization.  相似文献   

Fast large-scale reionization simulations     

Rajat M. Thomas  Saleem Zaroubi  Benedetta Ciardi  reas H. Pawlik  Panagiotis Labropoulos  Vibor Jeli&#;  Gianni Bernardi  Michiel A. Brentjens  A. G. de Bruyn  Geraint J. A. Harker  Leon V. E. Koopmans  Garrelt Mellema  V. N. Pandey  Joop Schaye  Sarod Yatawatta 《Monthly notices of the Royal Astronomical Society》2009,393(1):32-48

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Non-parametric foreground subtraction for 21-cm epoch of reionization experiments     

Geraint Harker  Saleem Zaroubi  Gianni Bernardi  Michiel A. Brentjens  A. G. de Bruyn  Benedetta Ciardi  Vibor Jeli&#;  Leon V. E. Koopmans  Panagiotis Labropoulos  Garrelt Mellema  ré Offringa  V. N. Pandey  Joop Schaye  Rajat M. Thomas  Sarod Yatawatta 《Monthly notices of the Royal Astronomical Society》2009,397(2):1138-1152

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Space groups and lattice complexes: by W. Fischer,H. Burzlaff,E. Hellner and J. D. H. Donnay. N.B.S. Monograph 134, U.S. Dept. Commerce, 1973, 184 p. $4.10     

David Harker 《Geochimica et cosmochimica acta》1975,39(2):222

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Radiation from long pulse train electron beams in space plasmas     

K.J. Harker  P.M. Banks 《Planetary and Space Science》1985,33(8):953-963

A previous study of electromagnetic radiation from a finite train of electron pulses is extended to an infinite train of such pulses. The electrons are assumed to follow an idealized helical path through a space plasma in such a manner as to retain their respective position within the beam. This leads to radiation by coherent spontaneous emission. The waves of interest in this region are the whistler slow (compressional) and fast (torsional) Alfvén waves. Although a general theory is developed, analysis is then restricted to two approximations, the short and long electron beam. Formulas for the radiation per unit solid angle from the short beam are presented as a function of both propagation and ray angles, electron beam pulse width and separation and beam current, voltage, and pitch angle. Similar formulas for the total power radiated from the long beam are derived as a function of frequency, propagation angle, and ray angle. Predictions of the power radiated are presented for representative examples as determined by the long beam theory.  相似文献   

Discovery Of Mg-Rich Pyroxenes In Comet C/1995 O1 (Hale-Bopp): Pristine Grains Revealed At Perihelion     

Wooden  Diane H.  Harker  David E.  Woodward  Charles E.  Koike  Chiyoe  Butner  Harold M 《Earth, Moon, and Planets》1997,78(1-3):285-291

The NASA Ames HIFOGS spectrometer observed comet C/1995 O1 (Hale-Bopp) at epochs including 96 Oct 7–14 UT (2.8 AU), 97 Feb 14–15 UT (1.2 AU), 97 Apr 11 UT (0.93 AU), and 97 Jun 22, 25 UT (1.7 AU). The HIFOGS 7.5–13.5 μm spectrophotometry (R = 360 - 180) of the silicate feature at 2.8 AU is identical in shape to the ISO SWS spectra of comet Hale-Bopp (Crovisier et al., 1997); the strong 11.2 μm peak in the structured silicate feature is identified as olivine. Upon close passage to the sun, the HIFOGS spectra at 1.2 AU and 0.93 AU reveals strong peaks at 9.3 μm and 10.0 μm. The post-perihelion 10 μm silicate feature at 1.7 AU is weaker but has nearly the same shape as the pre-perihelion spectra at 1.2 AU, reverting to its pre-perihelion shape: there is no change in the dust chemistry by close passage to the sun. The appearance of the strong peaks at 9.3 μm and 10.0 μm at r_h ≲ 1.7 AU is attributed to the rise in the contribution of pryoxenes (clino-pyroxene and orthopyroxene crystals) to the shape of the feature, and leads to the hypothesis that the pyroxenes are significantly cooler than the olivines. The pyroxenes are radiating on the Wien side of the blackbody at 2.8 AU and transition to the Rayleigh-Jeans tail of the blackbody upon closer approach to the Sun. Composite fits to the observed 10 μm silicate features using IDPs and laboratory minerals shows that a good empirical fit to the spectra is obtained when the pryoxenes are about 150 K cooler than the olivines. The pyroxenes, because they are cooler and contribute signficantly at perihelion, are more abundant than the olivines. The perihelion temperature of the pyroxenes implies that the pyroxenes are more Mg-rich than the other minerals including the olivines, amorphous olivines, and amorphous pyroxenes. The PUMA-1 flyby measurements of comet P/Halley also indicated an overabundance of Mg-rich pryoxenes compared to olivines. Comet Hale-Bopp's pyroxenes are similar to pyroxere IDPs from the ’Spray‘ class, known for their D-richness and their unaltered morphologies: Hale-Bopp's Mg-rich pyroxenes may be pristine relic ISM grains. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献   

Thermal Emission From The Dust Coma Of Comet Hale-Bopp And The Composition Of The Silicate Grains     

Hanner  M. S.  Gehrz  R. D.  Harker  D. E.  Hayward  T. L.  Lynch  D. K.  Mason  C. C.  Russell  R. W.  Williams  D. M.  Wooden  D. H.  Woodward  C. E. 《Earth, Moon, and Planets》1997,79(1-3):247-264

The dust coma of comet Hale-Bopp was observed in the thermal infrared over a wide range in solar heating (R = 4.9–0.9 AU) and over the full wavelength range from 3 μm to 160 μm. Unusual early activity produced an extensive coma containing small warm refractory grains; already at 4.9 AU, the 10 μm silicate emission feature was strong and the color temperature was 30% above the equilibrium blackbody temperature. Near perihelion the high color temperature, strong silicate feature, and high albedo indicated a smaller mean grain size than in other comets. The 8–13 μm spectra revealed a silicate emission feature similar in shape to that seen in P/Halley and several new and long period comets. Detailed spectral structure in the feature was consistent over time and with different instruments; the main peaks occur at 9.3, 10.0 and 11.2 μm. These peaks can be identified with olivine and pyroxene minerals, linking the comet dust to the anhydrous chondritic aggregate interplanetary dust particles. Spectra at 16–40 μm taken with the ISO SWS displayed pronounced emission peaks due to Mg-rich crystalline olivine, consistent with the 11.2 μm peak. This revised version was published online in July 2006 with corrections to the Cover Date.  相似文献