Henry H. Hsieh

The return of activity in main-belt comet 133P/Elst–Pizarro

Comet 133P/Elst-Pizarro is the first known and currently best-characterized member of the main-belt comets, a recently identified class of objects that exhibit cometary activity but which are dynamically indistinguishable from main-belt asteroids. We report here on the results of a multiyear monitoring campaign from 2003 to 2008, and present observations of the return of activity in 2007. We find a pattern of activity consistent with seasonal activity modulation. Additionally, recomputation of phase function parameters using data in which 133P was inactive yields new IAU parameters of HR = 15.49 ± 0.05 mag and GR = 0.04 ± 0.05, and linear parameters of mR(1, 1, 0) = 15.80 ± 0.05 mag and ? = 0.041 ± 0.005 mag deg-1. The comparison between predicted magnitudes using these new parameters and the comets actual brightnesses during its 2002 and 2007 active periods reveals the presence of unresolved coma during both episodes, of the order of ~0.20 of the nucleus cross-section in 2002 and ~0.25 in 2007. Multifilter observations during 133Ps 2007 active outburst yield mean nucleus colours of B - V = 0.65 ± 0.03, V - R = 0.36 ± 0.01 and R - I = 0.32 ± 0.01, with no indication of significant rotational variation, and similar colours for the trail. Finally, while 133Ps trail appears shorter and weaker in 2007 than in 2002, other measures of activity strength such as dust velocity and coma contamination of nucleus photometry are found to remain approximately constant. We attribute changes in trail strength to the timing of observations and projection effects, thus finding no evidence of any substantial decrease in the activity strength between 2002 and 2007.

ALBEDOS OF MAIN-BELT COMETS 133P/ELST-PIZARRO AND 176P/LINEAR ∗

We present the determination of the geometric R-band albedos of two main-belt comet (MBC) nuclei based on data from the Spitzer Space Telescope and a number of ground-based optical facilities. For 133P/Elst-Pizarro, we find an albedo of pR = 0.05 ± 0.02 and an effective radius of re = 1.9 ± 0.3 km (estimated semiaxes of a ~ 2.3 km and b ~ 1.6 km). For 176P/LINEAR, we find an albedo of pR = 0.06 ± 0.02 and an effective radius of re = 2.0 ± 0.2 km (estimated semiaxes of a ~ 2.6 km and b ~ 1.5 km). In terms of albedo, 133P and 176P are similar to each other and are typical of other Themis family asteroids, C-class asteroids, and other comet nuclei. We find no indication that 133P and 176P are compositionally unique among other dynamically similar (but inactive) members of the Themis family, in agreement with previous assertions that the two objects most likely formed in situ. We also note that low albedo (pR < 0.075) remains a consistent feature of all cometary (i.e., icy) bodies, whether they originate in the inner solar system (the MBCs) or in the outer solar system (all other comets).

SuperWASP Observations of the 2007 Outburst of Comet 17P/Holmes

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 17Ps 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 time-scale at the time of the initial outburst, with fragmentation time-scales decreasing from t frag ∼ 2 × 10 3 to ∼ 1 × 10 3 s over our observing period. Analysis of the expansion of 17Ps 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 υ exp = 0.55 ± 0.02 km s -1 and a likely outburst date of t 0 = 2007 October 23.3 ± 0.3, consistent with our finding that the comet remained below SuperWASPs detection limit of m V ∼ 15 mag until at least 2007 October 23.3. Modelling of 17Ps 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.

Chandra observations of comets 8p/Tuttle and 17p/Holmes during solar minimum

We present results for Chandra X-ray Observatory observations of two comets made during the minimum of solar cycle 24. The two comets, 17P/Holmes (17P) and 8P/Tuttle (8P), were very different in their activity and geometry. 17P was observed, for 30 ks right after its major outburst, on 2007 October 31 (10:07 UT), and comet 8P/Tuttle was observed in 2008 January for 47 ks. 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 was at a relatively high solar latitude (+191) while 8P was observed at a lower solar latitude (34). 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 300-1000 eV flux of 0.5-4.5 × 10–13 erg cm–2 s–1, with over 90% of the emission in the 300-400 eV range. This corresponds to an X-ray luminosity between 0.4 and 3.3 × 1015 erg s–1. However, we cannot distinguish between this significant excess emission and possible instrumental effects, such as incomplete charge transfer across the CCD. 17P is the first comet observed at high latitude during solar minimum. Its lack of X-rays in the 400-1000 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–1 in the 300-1000 eV range. We derive an average X-ray flux in this range of 9.4 × 10–13 erg cm–2 s–1 and an X-ray luminosity for the comet of 1.7 × 1014 erg s–1. The light curve showed a dramatic decrease in flux of over 60% between observations on January 1 and 4. When comparing outer regions of the coma to inner regions, its spectra showed a decrease in ratios of C VI/C V, O VIII/O VII, as predicted by recent solar wind charge exchange (SWCX) 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 SWCX emission in general as a means of remote diagnostics of the interaction of astrophysical plasmas.

U-Band Photometry of Kuiper Belt Objects

We present U-band photometry of Kuiper Belt objects taken to further investigate their color-orbit systematics. As at longer optical and near-infrared wavelengths, the U-band colors of Kuiper Belt objects show a wide range and a unimodal distribution. We find no evidence that color systematics in the Kuiper Belt are strongly wavelength-dependent. This observation is consistent with control of the reflection characteristics by a single (but unidentified) reddening material. No evidence is found for blue/ultraviolet absorption that can arise from charge transfer transitions in hydrated minerals in some primitive (C-type) asteroids. In the classical KBOs alone the U - B and other color indices are most strongly correlated with the Tisserand parameter measured with respect to Neptune.

The Castalia mission to Main Belt Comet 133P/Elst-Pizarro

We describe Castalia, a proposed mission to rendezvous with a Main Belt Comet (MBC), 133P/Elst-Pizarro. MBCs are a recently discovered population of apparently icy bodies within the main asteroid belt between Mars and Jupiter, which may represent the remnants of the population which supplied the early Earth with water. Castalia will perform the first exploration of this population by characterising 133P in detail, solving the puzzle of the MBC’s activity, and making the first in situ measurements of water in the asteroid belt. In many ways a successor to ESA’s highly successful Rosetta mission, Castalia will allow direct comparison between very different classes of comet, including measuring critical isotope ratios, plasma and dust properties. It will also feature the first radar system to visit a minor body, mapping the ice in the interior. Castalia was proposed, in slightly different versions, to the ESA M4 and M5 calls within the Cosmic Vision programme. We describe the science motivation for the mission, the measurements required to achieve the scientific goals, and the proposed instrument payload and spacecraft to achieve these.