Search Results - (Author, Cooperation:M. F. A'Hearn)

2015-07-03

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2015-09-30

Nature Publishing Group (NPG)

0028-0836

1476-4687

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

2015-01-24

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2015-01-24

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2015-01-24

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2011-10-29

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

2011-06-18

American Association for the Advancement of Science (AAAS)

0036-8075

1095-9203

Biology

Chemistry and Pharmacology

Computer Science

Medicine

Natural Sciences in General

Physics

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] At the time of these observations the comet was 0.71 AU from the Sun, 0.61 AU from Earth, and had a total visual magnitude 4. The visual appearance of the comet was diffuse with a weak central condensation and with only a slight trace of an ion tail. The comet could be tracked on its centre of ...

Electronic Resource

1573-0794

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Electronic Resource

1573-0794

Comets ; small ; search program ; planets

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract Arguments are presented for a substantial, unexplored population of comets with radii less than 1 km. Known examples confirm this population and extrapolation of any plausible size-distribution function indicates large numbers. However, their accurate numbers, orbital characteristics, and physical properties are unknown. Thus, even though the small comets may be the most frequent cometary bodies impacting the planets, a quantitative evaluation is not currently possible. We advocate an optimized, dedicated search program to characterize this population.

Electronic Resource

1573-0794

Comet Hale–Bopp ; molecules ; SO and SO2

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract The solution of the statistical equilibrium equations for the SO molecule is used to estimate the expected intensities of microwave lines. The observed fluxes of the microwave lines of SO in Comet Hale–Bopp require approximately 1.8 1028 mol/sec which give fluxes in the A3Π - X3Σ−bands below the observable limit, consistent with the observations.

Electronic Resource

1573-0794

Comets ; infrared ; radio ; rotation ; surfaces ; origin

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract We present infrared imaging and photometry of the bright, giant comet C/1995 O1 (Hale-Bopp). The comet was observed in an extended infrared and optical observing campaign in 1996–1997. The infrared morphology of the comet was observed to change from the 6 to 8 jet “porcupine” structure in 1996 to the “pinwheel” structure seen in 1997; this has implications for the position of the rotational angular momentum vector. Long term light curves taken at 11.3 μm indicate a dust production rate that varies with heliocentric distance as ∶ r−1.4. Short term light curves taken at perihelion indicate a rotational periodicity of 11.3 hours and a projected dust outflow speed of ∶ 0.4 km s−1. The spectral energy distribution of the dust on October 31, 1996 is well modeled by a mixture of 70% silicaceous and 30% carbonaceous non-porous grains, with a small particle dominated size distribution like that seen for comet P/Halley (McDonnell et al., 1991), an overall dust production rate of 2 × 105 kg s−1, a dust-to-gas ratio of ∶5, and an albedo of 39%.

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] Data were obtained during a combined 16 h US1-US2 IUE shift on 18 March 1986 beginning at 11:00 UT. Observations consisted of a large number of relatively short exposures taken at various offset distances from the nucleus with the long-wavelength (LWP) spectrograph to map the spatial distribution ...

Electronic Resource

1476-4687

Nature Archives 1869 - 2009

Biology

Chemistry and Pharmacology

Medicine

Natural Sciences in General

Physics

[Auszug] IUE has been used over the past few years to record the ultraviolet spectrum of nearly 25 comets, characterizing their ultraviolet spectra and performing comparative studies1'3. Since4 1 April 1985, the IUE satellite has monitored the ultraviolet spectrum of comet Halley at regular time intervals. ...

Electronic Resource

1573-0794

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract The impacts of Comet Shoemaker-Levy 9 left spots on Jupiter with diameters on the order of tens of thousands of kilometers, which have the appearance of debris fields strewn upon the Jovian cloud tops. In this note we employ a measurement of the optical depth of the debris at the impact site of fragment G to estimate mass in the debris field and lower limits to the G fragment mass of 4×1012 – 4×1013 g and diameter of 0.1 – 0.3 km. The masses and sizes of the fragments of Comet Shoemaker-Levy 9 are still uncertain, with estimated sizes ranging from 0.1 to 4 km. The size of the cometary body before breakup is believed to have been between 1 and 10 km. (Asphaug & Benz 1994; Solen 1994; Weaver et al. 1994, Scott & Melosh 1993). These estimates were based on pre-impact images of the cometary fragments. A complimentary technique is to use post-impact images of the spots left on Jupiter to infer the sizes and masses of the fragments. Structure in the underlying clouds is clearly visible through spots imaged by the Hubble Space Telescope, implying that the debris fields are relatively thin. Shortly after the G impact, A'Hearn and collaborators (paper in preparation) used the University of Maryland CCD System at the Perth Observatory to image Jupiter in a variety of bandpasses. While a complete reduction is still underway, a preliminary examination of the raw data shows that the spot at the impact site of fragment G, when near the central meridian roughly three hours after impact, had an average optical depth of roughly 0.05 in several bandpasses between 0.62 and 0.73µm. The measured diameter of the spot was approximately D = 15,000 km. In this note we do not present the data for optical depth, but rather we show that measurements of this type can be used to determine the mass of the solid particles in the clouds and thus to set limits on the mass of the impactor. We assume that the spot consisted of a thin layer of dust in the upper atmosphere. Assuming a one-particle layer covering a fraction of 0.05 of the spot area (a valid assumption for an optically thin cloud), the mass of matter in the spot is M = (0.05π/4) ρdD2, where ρ and d are the particle density and diameter. Particle sizes are not directly measured. However, the particle diameters cannot be much less than 1 µm because the CCD observations when compared with HST ultraviolet images show that extinction is not strongly wavelength dependent at optical and near-uv wavelengths. Typical grain sizes in comets and in the zodiacal dust range from 1 to 10 µm. For particle densities of 0.5 g cm−3 and assumed particle diameters in the range 1 – 10 µm, we find masses, M = 4×1012 – 4×1013 g. Assuming an impactor density of 0.5 g cm−3 (Asphaug & Benz 1994), the corresponding fragment diameters are 0.1 – 0.3 km. Larger sizes for the grains would increase the estimated mass. The observed debris may not be actual comet dust. Since temperatures in the fireball are estimated to be several thousand degrees, all the material in the fragment should have been vaporized (Sekanina et al 1995; Takata et al 1994; Zahnle & MacLow 1994). Therefore the debris material could consist of recondensed matter, perhaps organics, from the fireball. An impactor collides with roughly its own mass of atmospheric material before disruption, so the estimates for the impactor mass hold to order of magnitude even if the debris contains matter with contributions from originally atmospheric gases. The estimate of 0.1 – 0.3 km diameter for the G fragment is a lower limit because the object would also contain material, for example ices, that would not appear in the debris field. Furthermore, since the HST images show structure in the spots that is unresolved in the observations used here, the spot may not be optically thin at all points, but only on average, and this leads to our estimate being a lower limit for the mass of particles. As noted above, the particles are unlikely to be much less than 1 µm in size; particles much larger than 10µm would also imply a larger mass of particles. The derived fragment size is comparable to those estimated from pre-impact observations.

Electronic Resource

1573-0794

Comets ; coma ; radio ; sulfur chemistry ; Hale-Bopp

Springer Online Journal Archives 1860-2000

Geosciences

Physics

Abstract The recent availability of bright comets has given us an excellent opportunity to study cometary chemistry. Comet Hale-Bopp (1995 O1)gave us the particularly rare opportunity to study a bright and active comet for almost two years. Our program concentrated on millimeter-wave observations of sulfur-bearing molecules in an effort to understand the total sulfur budget of the comet. Using the National Radio Astronomy Observatory 12-m telescope on Kitt Peak we monitored both the long and short-term variations in H2S, CS, and OCS, as well as observing H2CS and SO. This was the first observation of H2CS in any comet (Figure 1). Additionally, we mapped CS with the BIMA interferometer. Variations in the line profiles and changes in line intensity as large as a factor of two were seen in day to day observations of both H2S and CS. An example for H2S is shown in Figure 2. This is the first time we can attempt to study the entire group of sulfur-bearing molecules. Models of the sulfur coma have thus far largely been based on observations of the daughter products CS and atomic sulfur made over the last 18 years using the International Ultraviolet Explorer (IUE) satellite, coupled with radio observations of CS and H2S in several recent comets. Four new sulfur-bearing species have been observed in comets Hale-Bopp and Hyaku take, three of them parent species. The high resolution maps in CS will also allow spatial information to be included in the sulfur model for the first time. C/Hale-Bopp is the first comet in which so many sulfur species have been observed. Analysis of the abundances of these species in comparison to the total atomic sulfur observed should reveal whether or not we can now account for all of the primary sulfur sources in comets. Perhaps the most interesting question that these observations raised was why C/Hale-Bopp appeared to contain so much more SO and SO2 (as observed by others) than any other comet. This spurred the discovery that the UV fluorescence models of these species were incorrect (S. J. Kim, this issue). Analysis of the data and modeling of the sulfur budget are still underway.

Electronic Resource