[FPSPACE] 2 new papers on comets

[LARRY KLAES]

astro-ph/0703220 [abs, ps, pdf, other] :

Title: Two different evolutionary types of comets proved by polarimetric and 
infrared properties of their dust

Authors: L. Kolokolova, H. Kimura, N. Kiselev, V. Rosenbush

Journal-ref: Astronomy and Astrophysics, Volume 463, Issue 3, March I 2007, 
pp.1189-1196, 2007

Comets can be divided into two groups: type I, characterized by high 
gas/dust ratio, low polarization, and a weak or absent 10 micron silicate 
feature, and type II, for which a low gas/dust ratio, high polarization, and 
strong silicate feature are typical. We show that the low polarization is 
the apparent result of depolarization by gas contamination at low dust 
concentration, which, in turn, results from the dust in type I comets being 
concentrated near the nucleus. The simulations of thermal emission show that 
for more porous particles (BCCA), the silicate feature is more pronounced 
than more compact ones (BPCA), for which it even vanishes as the particles 
become larger. We also show that in both types of comets the main 
contribution to light scattering and emission comes from particles larger 
than 10 micron.

Conclusions: The strength of the silicate feature in the cometary infrared 
spectra suggests that the dust in type II comets consists of high-porosity 
aggregates, whereas the dust of type I comets contains low-porosity ones. 
This is consistent with the polarimetric features of these comets, which 
indicate that the dust in type I comets tends to concentrate near the 
nucleus. This may result from the predominance of highly processed particles 
in type I comets, whereas in type II comets we see pristine or 
slightly-processed dust. This conclusion is in accordance with the orbital 
characteristics of the comets. We have found that the strength of the 
silicate feature correlates with the semi-major axis of periodic comets and, 
for short-period comets, with the perihelion distance. Thus, the silicate 
feature weakens due to compaction of aggregate particles if a comet spends 
more time in the vicinity of the Sun, which allows the comet to accumulate a 
mantle on the surface of its nucleus.

http://arxiv.org/abs/astro-ph/0703220


astro-ph/0703221 [abs, pdf] :

Title: Circular polarization in comets: Observations of Comet C/1999 S4 
(LINEAR) and tentative interpretation

Authors: Vera Rosenbush, Ludmilla Kolokolova, Alexander Lazarian, Nikolai 
Shakhovskoyd, Nikolai Kiselev

Journal-ref: Icarus, Volume 186, Issue 2, p. 317-330, 2007

Comet C/1999 S4 (LINEAR) was exceptional in many respects. Its nucleus 
underwent multiple fragmentations culminating in the complete disruption 
around July 20, 2000. We present circular polarization measurements along 
the cuts through the coma and nucleus of the comet during three separate 
observing runs, in June 28 - July 2, July 8 - 9, and July 21 - 22, 2000. The 
circular polarization was detected at a rather high level, up to 0.8%. The 
left-handed as well as right-handed polarization was observed over the coma 
with the left circularly polarized light systematically observed in the 
sunward part of the coma. During our observations the phase angle of the 
comet varied from 61 up to 122 deg., which allowed us to reveal variations 
of circular polarization with the phase angle. Correlation between the 
degree of circular polarization, visual magnitude, water production rate, 
and linear polarization of Comet C/1999 S4 (LINEAR) during its final 
fragmentation in July 2000 was found. The mechanisms that may produce 
circular polarization in comets and specifically in Comet C/1999 S4 (LINEAR) 
are discussed and some tentative interpretation is presented.

http://arxiv.org/abs/astro-ph/0703221