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High resolution spectra of Comet 8P/Tuttle were obtained in the frequency range 3440.6-3462.6 cm-1 on 3 January 2008 UT using CGS4 with echelle grating on UKIRT. In addition to recording strong solar pumped fluorescent (SPF) lines of H2O, the long integration time (152 miutes on target) enabled eight weaker H2O features to be assigned, most of which had not previously been identified in cometary spectra. These transitions, which are from higher energy upper states, are similar in character to the so-called SH lines recorded in the post Deep Impact spectrum of comet Tempel 1 (Barber et al., 2007). We have identified certain characteristics that these lines have in common, and which in addition to helping to define this new class of cometary line, give some clues to the physical processes involved in their production. Finally, we derive an H2O rotational temperature of 62+/- K and a water production rate of (1.4+/-0.3)E28 molecules/s.
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Comet 8P/Tuttle is a Nearly Isotropic Comet (NIC), whose physical properties are poorly known and could be different from those of Ecliptic Comets (EC) owing to their different origin. Two independent observations have shown that 8P has a bilobate nucleus. Our goal is to determine the physical properties of the nucleus (size, shape, thermal inertia, albedo) and coma (water and dust) of 8P/Tuttle. We observed the inner coma of 8P with the infrared spectrograph (IRS) and the infrared camera (MIPS) of the Spitzer Space Telescope (SST). We obtained one spectrum (5-40 $mu$m) on 2 November 2007 and a set of 19 images at 24 $mu$m on 22-23 June 2008 sampling the nucleus rotational period. The data were interpreted using thermal models for the nucleus and the dust coma, and considering 2 possible shape models of the nucleus derived from respectively Hubble Space Telescope visible and Arecibo radar observations. We favor a nucleus shape model composed of 2 contact spheres with respective radii of 2.7+/-0.1 km and 1.1+/-0.1 km and a pole orientation with RA=285+/-12 deg and DEC=+20+/-5 deg. The nucleus has a thermal inertia in the range 0-100 J/K/m^2/s^0.5 and a R-band geometric albedo of 0.042+/-0.008. The water production rate amounts to 1.1+/-0.2x10^28~molecules/s at 1.6 AU from the Sun pre-perihelion, which corresponds to an active fraction of 9%. At the same distance, the $epsilon f rho$ quantity amounts to 310+/-34 cm at 1.6~AU, and reaches 325+/-36 cm at 2.2~AU post-perihelion. The dust grain temperature is estimated to 258+/-10 K, which is 37 K larger than the thermal equilibrium temperature at 1.6 AU. This indicates that the dust grains contributing to the thermal infrared flux have a typical size of 10 $mu$m. The dust spectrum exhibits broad emissions around 10 $mu$m (1.5-sigma confidence level) and 18 $mu$m (5-sigma confidence level) that we attribute to amorphous pyroxene.
We measured organic volatiles (CH4, CH3OH, C2H6, H2CO), CO, and water in comet 8P/Tuttle, a comet from the Oort cloud reservoir now in a short-period Halley-type orbit. We compare its composition with two other comets in Halley-type orbits, and with comets of the organics-normal and organics-depleted classes. Chemical gradients are expected in the comet-forming region of the proto-planetary disk, and an individual comet should reflect its specific heritage. If Halley-type comets came from the inner Oort cloud as proposed, we see no common characteristics that could distinguish such comets from those that were stored in the outer Oort cloud.
We present results for Chandra observations of comets, 17P/Holmes (17P) and 8P/Tuttle (8P). 17P was observed for 30 ksec right after its major outburst, on 31 Oct 2007 (10:07 UT) and comet 8P/Tuttle was observed in 2008 January for 47 ksec. During the two Chandra observations, 17P was producing at least 100 times more water than 8P but was 2.2 times further away from the Sun. Also, 17P is the first comet observed at high latitude (+19.1 degrees) during solar minimum, while 8P was observed at a lower solar latitude (3.4 degrees). The X-ray spectrum of 17P is unusually soft with little significant emission at energies above 500 eV. Depending on our choice of background, we derive a 300 to 1000 eV flux of 0.5 to 4.5 x 10^-13 ergs/cm2/sec, with over 90% of the emission in the 300 to 400 eV range. This corresponds to an X-ray luminosity between 0.4 to 3.3 x 10^15 ergs/sec. 17Ps lack of X-rays in the 400 to 1000 eV range, in a simple picture, may be attributed to the polar solar wind, which is depleted in highly charged ions. 8P/Tuttle was much brighter, with an average count rate of 0.20 counts/s in the 300 to 1000 eV range. We derive an average X-ray flux in this range of 9.4 x 10^-13 ergs/cm2/sec and an X-ray luminosity for the comet of 1.7 x 10^14 ergs/sec. The light curve showed a dramatic decrease in flux of over 60% between observations on January 1st and 4th. When comparing outer regions of the coma to inner regions, its spectra showed a decrease in ratios of CVI/CV, OVIII/OVII, as predicted by recent solar wind charge exchange emission models. There are remarkable differences between the X-ray emission from these two comets, further demonstrating the qualities of cometary X-ray observations, and solar wind charge exchange emission in more general as a means of remote diagnostics of the interaction of astrophysical plasmas.
Cometary outgassing can produce torques that change the spin state of the nucleus, influencing the evolution and lifetimes of comets (1,2). If these torques spin up the rotation to the point that centripetal forces exceed the material strength of the nucleus, the comet may fragment (3). Torques that slow down the rotation can cause the spin state to become unstable, but if the torques persist, the nucleus may eventually reorient itself and start to spin up again (4). Simulations predict that most comets will go through a short phase of changing spin states, after which changes occur gradually over long times (5). We report on observations of comet 41P/Tuttle-Giacobini-Kresak during its highly favourable close approach to Earth (0.142 au on April 1, 2017) that reveal a dramatic spin-down. Between March and May 2017, the nucleus apparent rotation period increased from 20 hours to over 46 hours, reflecting a rate of change more than an order of magnitude larger than has ever been measured before. This phenomenon must be caused by a fortuitous alignment of the comets gas emission in such a way as to produce an anomalously strong torque that is slowing the nucleus spin rate. The behaviour of 41P suggests that it is in a distinct evolutionary state and that its rotation may be approaching the point of instability.
A search for the near-infrared water-ice absorption band was made in a number of very red OH/IR stars which are known to exhibit the 10um silicate absorption. As a by-product, accurate positions of these highly reddened objects are obtained. We derived a dust mass loss rate for each object by modelling the spectral energy distribution and the gas mass loss rate by solving the equation of motion for the dust drag wind. The derived mass loss rates show a strong correlation with the silicate optical depth as well as that of the water-ice. The stars have a high mass loss rate (> 1.0E-4 Msun/yr) with an average gas-to-dust mass ratio of 110. In objects which show the 3.1um water-ice absorption, the near-IR slope is much steeper than those with no water-ice. Comparison between our calculated mass loss rates and those derived from OH and CO observations indicates that these stars have recently increased their mass loss rates.
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