Gaseous Jets in Comet Hale-Bopp (1995 O1)

We report the identification of gas jets in comet Hale-Bopp in OH, NH, CN, C₂ and C₃. This is the first time OH and NH jets without an obvious optical dust jet counterpart have been identified in narrowband comet images. We also confirm the existence of CN jets as reported by Larson et al. (1997) and Mueller et al. (1998). Jet features can be seen in the March and April 1997 datasets, approximately a month before and after perihelion. Our results contribute to the understanding of both the chemical properties of the comet as well as the physical mechanisms necessary to produce these features. This revised version was published online in July 2006 with corrections to the Cover Date.     

Chemical and physical properties of gas jets in comets: II. Modeling OH, CN and C2 jets in Comet C/1995 O1 (Hale-Bopp) one month after perihelion

We present an analysis of OH, CN, and C₂ jets observed in Comet Hale-Bopp during April 22-26, 1997. We conclude that an extended source, which peaks in productivity after a certain amount of time has passed after being released from the nucleus (8.5, 2.5, and 42.6×¹⁰⁴ s, respectively) is responsible for the observed coma jet morphology in all three species. Sub-micron organic grains are the favored explanation for the extended source. Our models indicate that this extended source produces approximately 40% of the OH, 50% of the C₂, and 75% of the CN. The balance for each species is created by a diffuse nuclear gas source. Compared with the nuclear gas source and normalized to the CN abundance, the composition of the extended source is depleted in OH by a factor of ∼6, and depleted in C₂ by a factor of ∼2. The existence of anti-sunward jets do not require production of radicals throughout the cometary night. Instead, our model demonstrates that active areas exposed to near-twilight conditions throughout the comet's rotational period can produce the observed anti-sunward morphology.     

Isophot Observations Of Comet Hale-Bopp

Comet Hale-Bopp has been observed five times with ISOPHOT, the photometer on board the Infrared Space Observatory (ISO), four times before its perihelion passage at heliocentric distances of 4.92, 4.58, 2.93 and 2.81 AU, and at 3.91 AU postperihelion. Each time, multi-filter photometry covering the range between 3.6–175 μm with eight to ten filters was performed to sample the spectral energy distribution of the comet. These measurements were used to determine dust temperatures for the cometary coma. The evolution of the strength of the silicate feature can be followed in the data as well as the flux deficit at longer wavelengths. This revised version was published online in July 2006 with corrections to the Cover Date.     

Physical Characterization of the Binary Edgeworth–Kuiper Belt Object 2001 QT 297

Following our discovery of 2001 QT₂₉₇ as the second knownbinary Edgeworth–Kuiper Belt Object (EKBO) in October of 2001 [IAUC 7733], we havecarried out additional high spatial resolution ground based imaging in October andNovember of 2001 and July, August, and September of 2002. Using the Raymond andBeverly Sackler Magellan Instant Camera (MagIC) on the Baade and Clay 6.5 m telescopesat Las Campanas Observatory in Chile, we have obtained accurate astrometric and photometricmeasurements in the Sloan r’ i’ and g’ filters. Superb seeing conditions andPSF fitting allow an accurate determination of the binary component separation and positionangle over time as well as a detailed study of color and temporal variability of the individualcomponents. Here we present a physical characterization of the individual componentsof 2001 QT₂₉₇ based on these astrometric, color and variability measurements. We findthe primary to exhibit colors about 0.3 magnitudes redder than solar with no evidencefor variability. The secondary component, however, exhibits strong variability(～0.6 magnitudes) with a best fit period of 4.7526 ± 0.0007 h for a single peaklightcurve or 9.505 ± 0.001 h for a dual peaked lightcurve. The colors measured for thesecondary also suggest variability. Based on a preliminary orbit fit for thepair using observations spanning a one year arc, we are able to estimate a system mass of～ 3.2 × 10¹⁸ kg and provide constraints to the surface albedo of 9–14% for assumeddensities between 1 and 2 g/cm³.     

Chemical and physical properties of gas jets in comets: I. Monte Carlo model of an inner cometary coma

We describe a 3-dimensional, time-dependent Monte Carlo model developed to analyze the chemical and physical nature of a cometary gas coma. Our model includes the necessary physics and chemistry to recreate the conditions applicable to Comet Hale-Bopp when the comet was near 1 AU from the Sun. Two base models were designed and are described here. The first is an isotropic model that emits particles (parents of the observed gases) from the entire nucleus; the second is a jet model that ejects parent particles solely from discrete active areas on the surface of the comet nucleus, resulting in coma jets. The two models are combined to produce the final model, which is compared with observations. The physical processes incorporated in both base models include: (1) isotropic ejection of daughter molecules (the observed gases) in the parent's frame of reference, (2) solar radiation pressure, (3) solar insolation effects, (4) collisions of daughter products with other molecules in the coma, and (5) acceleration of the gas in the coma. The observed daughter molecules are produced when a parent decays, which is represented by either an exponential decay distribution (photodissociation of the parent gas) or a triangular distribution (production from a grain extended source). Application of this model to the analysis the OH, C₂ and CN gas jets observed in the coma of Comet Hale-Bopp is the focus of the accompanying paper [Lederer, S.M., Campins, H., Osip, D.J., 2008. Icarus, in press (this issue)].     

Long- and Short-Term Photometric Behavior of Comet Hyakutake (1996 B2)

Narrowband filter photometry observations of Comet Hyakutake (1996 B2) were used to investigate this comet's short-term variability as well as its behavior for the apparition as a whole. Utilizing measurements obtained on a total of 13 nights between February 9, 1996, and April 14, 1996, we find that the heliocentric distance (r_H) dependence of the production rates of OH and NH were much shallower than those for either the carbon-bearing species or the visible dust. Based on the OH measurements, the derived water r_H-dependence was also significantly less steep than expected from a basic water vaporization model and required an effective active surface area of about 29 km² at r_H=1.8 AU, 16 km² at r_H=1 AU, and only 13 km² at r_H=0.6 AU. This decrease in active area may be due to seasonally induced variations of a heterogeneous surface, or due to a decreasing contribution of gas from icy grains in the innermost coma. The relative abundances of the minor gas species place Hyakutake into the “typical” category of comets in the A'Hearn et al. (1995, Icarus118, 223-270) taxonomic classification system. The spectrum is generally redder at shorter wavelengths throughout the apparition; however, the dust color progressively changes from being significantly reddened (37%/1000 Å) at large r_H to near-solar at small r_H. This change of color with distance implies a significant change in grain sizes or a changing proportion between two or more grain populations.A major outburst was initiated near March 19.9, just prior to the comet's close approach to Earth. The characteristic recovery from the outburst differed among the observed species, with OH recovering most rapidly, essentially returning to its baseline values by March 25. The spatial radial fall-off of OH throughout this interval was consistent with the expected nominal spatial distribution, while CN and C₂ displayed fall-offs consistent with a distributed source, and the dust fall-off was significantly less steep than 1/ρ, possibly due to fragmenting grains. Rotational lightcurve amplitudes were largest for the OH, NH, and dust, again consistent with the carbon-bearing species primarily originating from a distributed source. Significant variations were observed in the lightcurve amplitude and phase shifts as functions of aperture size. Finally, a refined value for the rotation period of 0.2614±0.0003 day was determined.