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Register a new userCircular polarization in comets: Observations of Comet C/1999 S4 (LINEAR) and tentative interpretation
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Added by
Ludmilla Kolokolova
Publication date
2007
fields
Physics
and research's language is
English
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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.
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We present results from an observing campaign of the molecular content of the coma of comet C/1999 S4 (LINEAR) carried out jointly with the millimeter-arrays of the Berkeley-Illinois-Maryland Association (BIMA) and the Owens Valley Radio Observatory (OVRO). Using the BIMA array in autocorrelation (`single-dish) mode, we detected weak HCN J=1-0 emission from comet C/1999 S4 (LINEAR) at 14 +- 4 mK km/s averaged over the 143 beam. The three days over which emission was detected, 2000 July 21.9-24.2, immediately precede the reported full breakup of the nucleus of this comet. During this same period, we find an upper limit for HCN 1-0 of 144 mJy/beam km/s (203 mK km/s) in the 9x12 synthesized beam of combined observations of BIMA and OVRO in cross-correlation (`imaging) mode. Together with reported values of HCN 1-0 emission in the 28 IRAM 30-meter beam, our data probe the spatial distribution of the HCN emission from radii of 1300 to 19,000 km. Using literature results of HCN excitation in cometary comae, we find that the relative line fluxes in the 12x9, 28 and 143 beams are consistent with expectations for a nuclear source of HCN and expansion of the volatile gases and evaporating icy grains following a Haser model.
On May 23, 2006 we used the ACIS-S instrument on the Chandra X-ray Observatory (CXO) to study the X-ray emission from the B fragment of comet 73P/2006 (Schwassmann-Wachmann 3) (73P/B). We obtained a total of 20 ks of CXO observation time of Fragment B, and also investigated contemporaneous ACE and SOHO solar wind physical data. The CXO data allow us to spatially resolve the detailed structure of the interaction zone between the solar wind and the fragments coma at a resolution of ~ 1,000 km, and to observe the X-ray emission due to multiple comet--like bodies. We detect a change in the spectral signature with the ratio of the CV/OVII line increasing with increasing collisional opacity as predicted by Bodewits e (2007). The line fluxes arise from a combination of solar wind speed, the species that populate the wind and the gas density of the comet. We are able to understand some of the observed X-ray morphology in terms of non-gravitational forces that act upon an actively outgassing comets debris field. We have used the results of the Chandra observations on the highly fragmented 73P/B debris field to re-analyze and interpret the mysterious emission seen from comet C/1999 S4 (LINEAR) on August 1st, 2000, after the comet had completely disrupted. We find the physical situations to be similar in both cases, with extended X-ray emission due to multiple, small outgassing bodies in the field of view. Nevertheless, the two comets interacted with completely different solar winds, resulting in distinctly different spectra.
We measured the degree of linear polarization P of comet C/2018 V1 (Machholz-Fujikawa-Iwamoto) with the broadband Johnson V filter in mid-November of 2018. Within a radius of r{ho}=17,000 km of the inner coma, we detected an extremely low linear polarization at phase angles from 83 to 91.2 degree and constrained the polarization maximum to Pmax = (6.8 +/- 1.8)%. This is the lowest Pmax ever measured in a comet. Using model agglomerated debris particles, we reproduced the polarimetric response of comet C/2018 V1. Four retrieved refractive indices closely match what was experimentally found in Mg-rich silicates with little or no iron content. Moreover, the size distribution of the agglomerated debris particles appears in good quantitative agreement with the in situ findings of comet 1P/Halley. The dust model of polarization of comet C/2018 V1 suggests a strongly negative polarization with amplitude |Pmin| = 5%-7%; whereas, an interpretation based on gaseous emission requires no negative polarization at small phase angles. This dramatic difference could be used to discriminate gaseous-emission and dust explanations in low-Pmax comets in future.
The long-term brightness evolution of the great comet C/1995 O1 (Hale-Bopp) presented a remarkable opportunity to study the behavior of its coma over four years. We used approximately 2200 total visual magnitudes published in the International Comet Quarterly taken from 17 observers during the period of 1995 July - 1999 September to create a secular lightcurve. In order to account for observer differences, we present a novel algorithm to reduce scatter and increase precision in a lightcurve compiled from many sources. It is implemented in a publicly available code, ICQSPLITTER. This code addresses the differences among observers by using a self-consistent statistical approach, leading to a sharper lightcurve, and improving the precision of the measured slopes. To first order, the comets lightcurve approximately follows a r$^{-4}$ response for both pre- and post-perihelion distances. Interestingly, the pre-perihelion data are better fit with a fifth-order polynomial with inflection points at 4.0, 2.6, 2.1 and 1.1 au. We analyze these specific regions and find that they are associated with physical phenomena in the comets evolution. Contrary to other reports, the lightcurve shows no evidence for the comet having been in outburst at discovery. Afrho values derived from the visual lightcurve data are consistent with a r$^{-1.5}$ dependence on heliocentric distance, which is similar in shape to those derived from spectroscopy and narrow-band photometry. We present correlation equations for visual magnitudes and CO and H2O production rates, which are consistent with the pre-perihelion visual magnitudes increasing almost entirely due to CO outgassing until a heliocentric distance of about 2.6 - 3.0 au. We also present two correlation equations that should prove highly useful for observation planning and data analysis, and can be generalized to be applicable to other comets.
We investigated three comets, which are active at large heliocentric distances, using observations obtained at the 6-m BTA telescope (SAO RAS, Russia) in the photometric mode of the focal reducer SCORPIO. The three comets, 29P/Schwassmann-Wachmann 1, C/2003 WT42 (LINEAR), and C/2002 VQ94 (LINEAR), were observed after their perihelion passages at heliocentric distances between 5.5 and 7.08 AU. The dust production rates in terms of Afrho was measured for these comets. Using the retrieved values, an average dust production rate was derived under different model assumptions. A tentative calculation of the total mass loss of the comet nucleus within a certain observation period was executed. We calculated the corresponding thickness of the depleted uppermost layer where high-volatile ices completely sublimated. The results obtained in our study strongly support the idea that the observed activity of Comet SW1 requires a permanent demolition of the upper surface layers.
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