Martha S. Hanner

A Spitzer Study of Comets 2P/Encke, 67P/Churyumov-Gerasimenko, and C/2001 HT50 (LINEAR-NEAT)

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° and 63°). Model fits to the spectral energy distributions of the nucleus suggest that comet Enckes infrared beaming parameter derived from the near-Earth asteroid thermal model may have a phase angle dependence. The observed emission from comet Enckes dust coma is best modeled using predominately amorphous carbon grains with a grain size distribution that peaks near 0.4 μm, and the silicate contribution by mass to the submicron 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).

3-14 Micron Spectroscopy of Comets C/2002 O4 (Hönig), C/2002 V1 (NEAT), C/2002 X5 (Kudo-Fujikawa), C/2002 Y1 (Juels-Holvorcem), and 69P/Taylor and the Relationships among Grain Temperature, Silicate Band Strength, and Structure among Comet Families

We report 3-13 � m spectroscopy of four comets observed between 2002 August and 2003 February: C/2002 O4 (Honig) on 2002 August 1, C/2002 V1 (NEAT) on 2003 January 9 and 10, C/2002 X5 (Kudo-Fujikawa) on 2003 January 9 and 10, and C/2002 Y1 (Juels-Holvorcem) on 2003 February 20. In addition, we include data obtained much earlier on 69P/Taylor (1998 February 9) but not previously published. For comets Taylor, Honig, NEAT, and Kudo-Fujikawa, the silicate emission band was detected, being approximately 23%, 12%, 15%, and 10%, respectively, above the continuum. The data for comet Juels-Holvorcem were of insufficient quality to detect the presence of a silicate band of comparable strength to the other three objects, and we place an upper limit of 24% on this feature. The silicate features in both NEAT and Kudo-Fujikawa contained structure indi- cating the presence of crystalline material. The shape of the silicate feature at a projected distance of 1900 km from the nucleus of Kudo-Fujukawa was nearly identical to that centered on the nucleus, indicating that the grain size population had not been measurably modified by the time it had reached that distance. Combining these data with those of other comets, we confirm the correlation between silicate band strength and grain temperature of Gehrz & Ney and Williams and coworkers for dynamically new and long-period comets, but the majority of Jupiter family objects may deviate from this relation. Despite the weakness of the silicate band in Kudo- Fujikawa, its structure resembles the bands seen in dynamically new and long-period objects with substantially stronger features. The limited data available on Jupiter family objects suggest that they may have silicate bands that are slightly different from the former objects. Finally, when compared to the silicate emission bands observed in pre-main-sequence stars, the dynamically new and long-period comets most closely resemble the more evolved stellar systems, while the limited data (in quantity and quality) on Jupiter family objects seem to suggest that these have spectra more like the less evolved stars. Higher quality data on a larger number of Jupiter family objects are needed to confirm (or reject) this trend. Subject heading

Three years of Ulysses dust data: 2005 to 2007

Abstract The Ulysses spacecraft has been orbiting the Sun on a highly inclined ellipse ( i = 79 ∘ , perihelion distance 1.3xa0AU, aphelion distance 5.4xa0AU) since it encountered Jupiter in February 1992. Since then it has made almost three revolutions about the Sun. Here we report on the final three years of data taken by the on-board dust detector. During this time, the dust detector recorded 609 dust impacts of particles with masses 10 − 16 g ⩽ m ⩽ 10 − 7 g , bringing the mission total to 6719 dust data sets. The impact rate varied from a low value of 0.3 per day at high ecliptic latitudes to 1.5 per day in the inner solar system. The impact direction of the majority of impacts between 2005 and 2007 is compatible with particles of interstellar origin; the rest are most likely interplanetary particles. We compare the interstellar dust measurements from 2005/2006 with the data obtained during earlier periods (1993/1994) and (1999/2000) when Ulysses was traversing the same spatial region at southern ecliptic latitudes but the solar cycle was at a different phase. During these three intervals the impact rate of interstellar grains varied by more than a factor of two. Furthermore, in the two earlier periods the grain impact direction was in agreement with the flow direction of the interstellar helium while in 2005/2006 we observed a shift in the approach direction of the grains by approximately 30 ∘ away from the ecliptic plane. The reason for this shift remains unclear but may be connected with the configuration of the interplanetary magnetic field during solar maximum. We also find that the dust measurements are in agreement with the interplanetary flux model of Staubach et al. (1997) which was developed to fit a 5-year span of Ulysses data.

Light of the Night Sky / Lumière Du Ciel Nocturne

The light of the night sky is a difficult to disentangle mixture of tropospherically scattered light, airglow, zodiacal light (including the thermal emission by interplanetary dust particles), unresolved stellar light, diffuse scattering and emission by interstellar dust and gas, and finally an extragalactic component. It has the reputation of being a very traditional field of astronomy, which certainly is true if we look at the long history of the subject. The recent renewed interest in this topic, which continued during this triennium, appears mainly to come from three sources: — first from the impressive results of the IRAS and COBE infrared satellites. They brought to general consciousness the fact that the infrared sky is characterised by strong emission from interplanetary and interstellar dust, and made clear that this emission may interfere with the study of faint interesting sources. — then from the development of sensitive detectors and arrays for essentially all of the wavelength range to be covered in this report, from the Lyman limit to ≈ 300 μm. Now the difficult measurements of the ultraviolet diffuse radiation and of the extragalactic background light in the infrared cosmological windows around 3 μm and 200 μm have become feasible and state of the art projects. — finally, the threat to astronomical observations arising from man-made development and lighting has become important enough to further studies of uncontaminated and contaminated night sky brightnesses. This report will refer mainly to those areas and is meant to highlight noteworthy developments. It was prepared with the help of Drs. Bowyer and Mattila.