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TOPIC: Comet 17P/Holmes


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Title: Extinction in the Coma of Comet 17P/Holmes
Authors: Pedro Lacerda, David Jewitt

On 2007 October 29 the outbursting comet 17P/Holmes passed within 0.79 arcsec of a background star. We recorded the event using optical, narrowband photometry and detect a 3% to 4% dip in stellar brightness bracketing the time of closest approach to the comet nucleus. The detected dimming implies an optical depth tau~0.04 at 1.5 arcsec from the nucleus and an optical depth towards the nucleus center tau_n<13.3. At the time of our observations, the coma was optically thick only within rho<~0.01 arcsec from the nucleus. By combining the measured extinction and the scattered light from the coma we estimate a dust red geometric albedo p_d=0.0060.002 at 16 deg phase angle. Our measurements place the most stringent constraints on the extinction optical depth of any cometary coma.

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Title: Near-Nucleus Photometry of Outbursting Comet 17P/Holmes
Authors: Rachel Stevenson, David Jewitt

Comet 17P/Holmes underwent the largest cometary outburst in recorded history on UT 2007 Oct. 23, releasing massive quantities of dust and gas. We used the Canada-France-Hawaii telescope to obtain wide-field images of 17P/Holmes on 15 dates over a period of 3 months following the outburst and employ them here to examine the subsequent activity of the nucleus and the nature of the ejecta closest to the nucleus. Through aperture photometry we observed the inner coma (within 2500 km of the nucleus) to fade from an apparent magnitude of 11.7 mag to 17.6 mag, corresponding to absolute magnitudes of 8.1 mag and 12.4 mag, between UT 2007 Nov. 6 and 2008 Feb. 12. A second much smaller outburst occurred on UT 2007 Nov. 12, three weeks after the original outburst, suggesting that the nucleus remained unstable. The surface brightness profile of the inner coma was consistently shallow relative to the expected steady-state profile, and showed a persistent brightness enhancement within ~ 5000 km of the nucleus. We propose that sublimating ice grains created an ice grain halo around the nucleus, while fragmenting grains were responsible for the shallow surface brightness profile.

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Comet 17P/Holmes
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Title: SuperWASP Observations of the 2007 Outburst of Comet 17P/Holmes
Authors: Henry H. Hsieh, Alan Fitzsimmons, Yogesh Joshi, Damian Christian, Don L. Pollacco

We present wide-field imaging of the 2007 outburst of Comet 17P/Holmes obtained serendipitously by SuperWASP-North on 17 nights over a 42-night period beginning on the night (2007 October 22-23) immediately prior to the outburst. Photometry of 17P's unresolved coma in SuperWASP data taken on the first night of the outburst is consistent with exponential brightening, suggesting that the rapid increase in the scattering cross-section of the coma could be largely due to the progressive fragmentation of ejected material produced on a very short timescale at the time of the initial outburst, with fragmentation timescales decreasing from t(frag)~2x10 s to t(frag)~1x10 s over our observing period. Analysis of the expansion of 17P's coma reveals a velocity gradient suggesting that the outer coma was dominated by material ejected in an instantaneous, explosive manner. We find an expansion velocity at the edge of the dust coma of v(exp) = 0.550.02 km/s and a likely outburst date of t_0=2007 October 23.30.3, consistent with our finding that the comet remained below SuperWASP's detection limit of m(V)~15 mag until at least 2007 October 23.3. Modelling of 17P's gas coma indicates that its outer edge, which was observed to extend past the outer dust coma, is best explained with a single pulse of gas production, consistent with our conclusions concerning the production of the outer dust coma.

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Title: Transient Fragments in Outbursting Comet 17P/Holmes
Authors: Rachel Stevenson, Jan Kleyna, David Jewitt

We present results from a wide-field imaging campaign at the Canada-France-Hawaii Telescope to study the spectacular outburst of comet 17P/Holmes in late 2007. Using image-processing techniques we probe inside the spherical dust coma and find sixteen fragments having both spatial distribution and kinematics consistent with isotropic ejection from the nucleus. Photometry of the fragments is inconsistent with scattering from monolithic, inert bodies. Instead, each detected fragment appears to be an active cometesimal producing its own dust coma. By scaling from the coma of the primary nucleus of 17P/Holmes, assumed to be 1.7 km in radius, we infer that the sixteen fragments have maximum effective radii between ~ 10 m and ~ 100 m on UT 2007 Nov. 6. The fragments subsequently fade at a common rate of ~ 0.2 mag/day, consistent with steady depletion of ices from these bodies in the heat of the Sun. Our characterization of the fragments supports the hypothesis that a large piece of material broke away from the nucleus and crumbled, expelling smaller, icy shards into the larger dust coma around the nucleus.

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Sun's warmth blows comet's icy heart apart

Three years ago, the comet 17P/Holmes exploded with a blast comparable to a small nuclear bomb. Would you believe that an exotic form of ice was responsible?
Comet 17P/Holmes became a million times brighter when it erupted in 2007. A freak collision with an asteroid could have explained that blast, had it been a one-off. But the same comet also exploded in 1892, suggesting something else might be triggering the outbursts. Now William Reach of the California Institute of Technology in Pasadena and his colleagues think the culprit may be an exotic and unstable form of water ice at the comet's heart.

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Holmes.gif
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Title: Explosion of Comet 17P/Holmes as revealed by the Spitzer Space Telescope
Authors: William T. Reach, Jeremie Vaubaillon, Carey M. Lisse, Mikel Holloway, Jeonghee Rho
(Version v2)

An explosion on comet 17P/Holmes occurred on 2007 Oct 23, projecting particulate debris of a wide range of sizes into the interplanetary medium. We observed the comet using the Spitzer spectrograph on 2007 Nov 10 and 2008 Feb 27, and the photometer, on 2008 Mar 13. The fresh ejecta have detailed mineralogical features from small crystalline silicate grains. The 2008 Feb 27 spectra, and the central core of the 2007 Nov 10 spectral map, reveal nearly featureless spectra, due to much larger grains that were ejected from the nucleus more slowly. We break the infrared image into three components (size, speed) that also explain the temporal evolution of the mm-wave flux. Optical images were obtained on multiple dates spanning 2007 Oct 27 to 2008 Mar 10 at the Holloway Comet Observatory and 1.5-m telescope at Palomar Observatory. The orientation of the leading edge of the ejecta shell and the ejecta blob, relative to the nucleus, do not change as the orientation of the Sun changes; instead, the configuration was imprinted by the orientation of the initial explosion. The kinetic energy of the ejecta >1e21 erg is greater than the gravitational binding energy of the nucleus. We model the explosion as being due to crystallisation and release of volatiles from interior amorphous ice within a subsurface cavity; once the pressure in the cavity exceeded the surface strength, the material above the cavity was propelled from the comet. The size of the cavity and the tensile strength of the upper layer of the nucleus are constrained by the observed properties of the ejecta; tensile strengths on >10 m scale must be greater than 10 kPa. The appearance of the 2007 outburst is similar to that witnessed in 1892, but the 1892 explosion was less energetic by a factor of about 20.

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Comet 17P/Holmes
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Like a mushroom shooting out spores, a well-known comet was seen firing multiple "mini comets" that went sailing away at up to 451 kilometres an hour, astronomers have announced.
The fragments were recently revealed in high-resolution images of comet Holmes, a relatively small body discovered in 1892 that mysteriously erupted in 2007.

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Mini-comets with a comet lit up 17P/Holmes during mega-outburst
Astronomers from the University of California, Los Angeles and the University of Hawaii have discovered multiple fragments ejected during the largest cometary outburst ever witnessed. Images and animations showing fragments rapidly flying away from the nucleus of comet 17P/Holmes will be presented by Rachel Stevenson at the European Planetary Science Congress in Potsdam, Germany, on Wednesday September 16.
Stevenson, together with colleagues Jan Kleyna and David Jewitt, began observing comet Holmes in October 2007 soon after it was reported that the small (3.6 km wide) body had brightened by a million times in less than a day. They continued observing for several weeks after the outburst using the Canada- France- Hawaii Telescope in Hawaii and watched as the dust cloud ejected by the comet grew to be larger than the Sun.
The astronomers examined a sequence of images taken over nine nights in November 2007 using a digital filter that enhances sharp discontinuities within images. The filter, called a Laplacian filter, is particularly good at picking out faint small-scale features that would otherwise remain undetected against the bright background of the expanding comet. They found numerous small objects that moved radially away from the nucleus at speeds up to 125 meters per second (280 mph). These objects were too bright to simply be bare rocks, but instead were more like mini-comets creating their own dust clouds as the ice sublimated from their surfaces.
While cometary outbursts are common, their causes are unknown. One possibility is that internal pressure built up as the comet moved closer to the Sun and sub-surface ices evaporated.
The pressure eventually became too great and part of the surface broke away, releasing a huge cloud of dust and gas, as well as larger fragments.
Surprisingly, the solid nucleus of comet Holmes survived the outburst and continued on its orbit, seemingly unperturbed. Holmes takes approximately 6 years to circle the Sun, and travels between the inner edge of the asteroid belt to beyond Jupiter. The comet is now moving away from the Sun but will return to its closest approach to the Sun in 2014, when astronomers will examine it for signs of further outbursts.

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Anatomy of a Busted Comet
Every six years, comet 17P/Holmes speeds away from Jupiter and heads inward toward the sun, travelling the same route typically without incident. However, twice in the last 116 years, in November 1892 and October 2007, comet Holmes mysteriously exploded as it approached the asteroid belt.

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