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Posts: 131433
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RE: Spitzer Study of Comets
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Title: Water in Comets 71P/Clark and C/2004 B1 (LINEAR) with Spitzer
Authors: Dominique Bockelee-Morvan (1), Charles E. Woodward (2), Michael S. Kelley (3), Diane H. Wooden (4) ((1)LESIA Observatoire de Paris Meudon France, (2) Univ. Minnesota Minneapolis MN, (3) Univ. Central Florida Orlando FL, (4) NASA Ames Moffet Field CA)

We present 5.5 to 7.6 micron spectra of comets 71P/Clark (2006 May 27.56 UT, r_h = 1.57 AU pre-perihelion) and C/2004 B1 (LINEAR) (2005 October 15.22 UT, r_h = 2.21 AU pre-perihelion and 2006 May 16.22 UT, r_h = 2.06 AU post-perihelion) obtained with the Spitzer Space Telescope. The nu_2 vibrational band of water is detected with a signal-to-noise ratio of 11 to 50. Fitting the spectra using a fluorescence model of water emission yields a water rotational temperature of < 18 K for 71P/Clark and approximately less than or equivalent to 14 ±2 K (pre-perihelion) and 23 ±4 K (post-perihelion) for C/2004 B1 (LINEAR). The water ortho-to-para ratio in C/2004 B1 (LINEAR) is measured to be 2.31 ±0.18, which corresponds to a spin temperature of 26^{+3}_{-2} K. Water production rates are derived. The agreement between the water model and the measurements is good, as previously found for Spitzer spectra of C/2003 K4 (LINEAR). The Spitzer spectra of these three comets do not show any evidence for emission from PAHs and carbonate minerals, in contrast to results reported for comets 9P/Tempel~1 and C/1995 O1 (Hale-Bopp).

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Title: Water in Comet 2/2003 K4 (LINEAR) with Spitzer
Authors: Charles E. Woodward, Michael S. Kelley, Dominique Bockelee-Morvan, R.D. Gehrz

We present sensitive 5.5 to 7.6 micron spectra of comet C/2003 K4 (LINEAR) obtained on 16 July 2004 (r_{h} = 1.760 AU, Delta_{Spitzer} = 1.409 AU, phase angle 35.4 degrees) with the Spitzer Space Telescope. The nu_{2} vibrational band of water is detected with a high signal-to-noise ratio (> 50). Model fitting to the best spectrum yields a water ortho-to-para ratio of 2.47 ± 0.27, which corresponds to a spin temperature of 28.5^{+6.5}_{-3.5} K. Spectra acquired at different offset positions show that the rotational temperature decreases with increasing distance from the nucleus, which is consistent with evolution from thermal to fluorescence equilibrium. The inferred water production rate is (2.43 ± 0.25) x 10^{29} molec. s^{-1}. The spectra do not show any evidence for emission from PAHs and carbonate minerals, in contrast to results reported for comets 9P/Tempel 1 and C/1995 O1 (Hale-Bopp). However, residual emission is observed near 7.3 micron the origin of which remains unidentified.

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Posts: 131433
Date:
2P/Encke
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Epoch 2007 Apr. 10.0 TT = JDT 2454200.5
T 2007 Apr. 19.3117 TT                                  MPC
q 0.339269 (2000.0) P Q
n 0.2983680 Peri. 186.5230 -0.9450308 -0.3150640
a 2.218032 Node 334.5714 +0.3084527 -0.7702153
e 0.847040 Incl. 11.7543 +0.1085068 -0.5545296
P 3.30
From 896 observations 1989-2003. Nongravitational parameters A1 = -0.02, A2
= -0.0009.


Date    TT    R. A. (2000) Decl.     Delta      r     Elong.  Phase   m1    m2
2007 03 31    01 55.38   +17 11.1    1.354    0.593    23.6    42.3   8.8  17.7
2007 04 05    02 14.01   +18 04.4    1.236    0.502    23.0    51.1   7.5  17.4
2007 04 10    02 33.15   +18 30.7    1.102    0.420    22.4    65.3   6.1  17.1
2007 04 15    02 49.90   +18 01.1    0.951    0.359    20.9    87.8   4.7  17.3
2007 04 20    02 57.98   +15 53.7    0.796    0.340    17.3   118.7   4.0  18.4
2007 04 25    02 51.63   +11 48.9    0.666    0.372    10.6   150.0   4.2  21.9
2007 04 30    02 32.91   +06 24.2    0.580    0.441     8.3   160.8   5.0  24.8
2007 05 05    02 08.72   +00 45.0    0.534    0.527    17.9   144.0   6.0  21.2
2007 05 10    01 44.37   -04 21.4    0.513    0.618    29.6   126.1   6.9  19.4
2007 05 15    01 22.13   -08 39.6    0.507    0.710    40.9   111.2   7.8  18.5
2007 05 20    01 02.27   -12 15.1    0.508    0.800    51.3    99.0   8.6  18.1
2007 05 25    00 44.17   -15 19.9    0.511    0.887    61.2    88.6   9.3  17.9
2007 05 30    00 26.98   -18 05.2    0.514    0.972    70.6    79.5   9.9  17.8


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Posts: 131433
Date:
Near Earth Asteroid Thermal Model
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Title: Comparing the NEATM with a Rotating, Cratered Thermophysical Asteroid Model
Authors: Edward L. Wright (UCLA)

A cratered asteroid acts somewhat like a retroflector, sending light and infrared radiation back toward the Sun, while thermal inertia in a rotating asteroid causes the infrared radiation to peak over the "afternoon" part. In this paper a rotating, cratered asteroid model is described, and used to generate infrared fluxes which are then interpreted using the Near Earth Asteroid Thermal Model (NEATM). Even though the rotating, cratered model depends on three parameters not available to the NEATM (the dimensionless thermal inertia parameter and pole orientation), the NEATM gives diameter estimates that are accurate to 10 percent RMS for phase angles less than 60 degrees. For larger phase angles, such as back-lit asteroids, the infrared flux depends strongly on these unknown parameter, so the diameter errors are larger; and real world complications such as non-spherical shapes have been ignored.

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Posts: 131433
Date:
Spitzer Study of Comets
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Title: A Spitzer Study of Comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT)
Authors: Michael S. Kelley, Charles E. Woodward, David E. Harker, Diane H. Wooden, Robert D. Gehrz, Humberto Campins, Martha S. Hanner, Susan M. Lederer, David J. Osip, Jana Pittichova, Elisha Polomski

We present infrared images and spectra of comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT) as part of a larger program to observe comets inside of 5 AU from the sun with the Spitzer Space Telescope. The nucleus of comet 2P/Encke was observed at two vastly different phase angles (20 degrees and 63 degrees). Model fits to the spectral energy distributions of the nucleus suggest comet Encke's infrared beaming parameter derived from the near-Earth asteroid thermal model may have a phase angle dependence. The observed emission from comet Encke's dust coma is best-modelled using predominately amorphous carbon grains with a grain size distribution that peaks near 0.4 microns, and the silicate contribution by mass to the sub-micron dust coma is constrained to 31%. Comet 67P/Churyumov-Gerasimenko was observed with distinct coma emission in excess of a model nucleus at a heliocentric distance of 5.0 AU. The coma detection suggests that sublimation processes are still active or grains from recent activity remain near the nucleus.
Comet C/2001 HT50 (LINEAR-NEAT) showed evidence for crystalline silicates in the spectrum obtained at 3.2 AU and we derive a silicate-to-carbon dust ratio of 0.6. The ratio is an order of magnitude lower than that derived for comets 9P/Tempel 1 during the Deep Impact encounter and C/1995 O1 (Hale-Bopp).

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-- Edited by Blobrana at 19:08, 2006-07-27

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