Linear and circular polarization of comet C/2009 P1 (Garradd)
© The Society of Geomagnetism and Earth, Planetary and Space Sciences (SGEPSS); The Seismological Society of Japan; The Volcanological Society of Japan; The Geodetic Society of Japan; The Japanese Society for Planetary Sciences; TERRAPUB. 2013
Received: 31 October 2012
Accepted: 24 May 2013
Published: 24 October 2013
We report measurements of linear and circular polarization for the recent comet C/2009 P1 (Garradd) during its approach to the Earth in 2011–2012. Aperture photoelectric observations were carried out on July 29, 2011-April 22, 2012 in the R and WR wide-band filters at the 2.6-m telescope of the Crimean Astrophysical Observatory (Ukraine). Spectropolarimetric measurements of linear polarization in the range 3800–8000 Å and imaging circular polarimetry in the comet continuum filter (λ0 = 6840/90 Å) were also carried out with the SCORPIO-2 focal reducer at the 6-m telescope of the Special Astrophysical Observatory (Russia) on February 2-April 21, 2012. The degree of polarization of light scattered by comet Garradd changes from −2.2 ± 0.2% to 4.9 ± 0.2% at phase angles 13.7° and 35.9°, respectively. These values are in a good agreement with those for dusty comets at the respective phase angles. Left-handed (negative) circular polarization of comet Garradd in the continuum filter was identified. The values of Pc vary noticeably over the coma in the range from −0.03 to −0.08% and from −0.05 to −0.3% on February 14 and April 21, respectively. These results confirm our previous conclusion that the observed circular polarization for comets is predominantly left-handed.
Polarimetry is a powerful tool for probing the physical properties of cometary dust particles; see numerous examples in Mishchenko et al. (2010) and references therein. One of the ways of studying the properties of cometary dust is a comparison of the polarization phase angle dependence for different comets. For this purpose, we created the Database of Comet Polarimetry (Kiselev et al., 2010) which currently contains polarimetric data for 64 comets. In recent years, new data have been collected for a number of comets. Here, we present the results of linear and circular polarimetric observations of the new comet C/2009 P1 (Garradd) (hereafter Garradd) during its approach to the Earth in 2011–2012.
Aperture polarimetric observations of comets in broadband filters are still more frequent because they allow one to observe fainter objects. Such observations using narrowband cometary filters for bright comets are very effective (see, e.g. Kiselev and Velichko, 1997, 1998), although they lose in the spatial resolution as compared with imaging polarimetry. For distant comets, polarimeters with CCD detectors have an advantage only in their penetrating power over photoelectric polarimeters, because observations of fainter comets are carried out with broad-band filters, and the data refer to the average values for the virtual apertures.
In addition to aperture polarimetry of comet Garradd, we present the results of spectropolarimetric observations of the comet with a long slit. Such observations remain scarce (see, e.g. Harrington et al., 2006). The spatial resolution of spectropolarimetric observations of comets can only be high in the region of coma, determined by the size of the slit, but the high spectral resolution reveals the great advantage of such observations, which is especially important taking into account the complexity of cometary spectra.
Unfortunately, to date, observations of comets are usually performed with polarimeters designed to study star-like objects, and there are practically no polarimeters optimally designed for the study of comets. It is obvious that, under such circumstances, combining instruments with different designs can yield improved results. This is exactly what we have done in our work.
The results of aperture observations of linear polarization for comet C/2009 P1 (Garradd).
Date, UT 20011/2012
Pr ± σ P %
θ±σ θ deg
−2.21 ± 0.23
37.6 ± 2.9
−0.27 ± 0.10
110.8 ± 10.7
2.37 ± 0.08
0.7 ± 1.0
2.65 ± 0.25
176.4 ± 2.7 153.4 ± 3.2
2.51 ± 0.28
153.4 ± 3.2
1.67 ± 0.13
58.6 ± 2.3
1.60 ± 0.17
13.0 ± 3.1
The long-slit spectropolarimetric measurements of linear polarization in the range 3800–8000 (the dispersion is 2 Å px−1 in the V band), as well as imaging circular polarime-try in the RC narrow-band cometary continuum filter (λ0 = 6840/90 Å), were carried out with the SCORPIO-2 focal reducer at the 6-m BTA telescope of the Special Astrophysical Observatory on February 2-April 21, 2012. These were the first measurements of cometary polarization ever attempted with this telescope and this instrument, so we describe these measurements in more detail.
The final degree of linear polarization and the position angle of the plane of polarization were calculated from relations similar to Eq. (2). The long-slit observations were carried out with a 3 arcsec × 6 arcmin slit oriented at a position angle of 217.1° and thus perpendicular to the extended Sun-comet radius vector on February 2, 2012.
Observational log of comet C/2009 P1 (Garradd). The 6-m telescope at the SAO.
Date, UT 2012
3. Results and Discussion
3.1 Linear polarization
The scattered light from comets is the superposition of radiation from dust particles and gas molecules. Because of the complexity of cometary spectra, it is difficult to identify the continuum completely free of molecular emissions (Arpigny, 1995), especially for broad-band filters. At phase angles close to 90°, the polarization of light scattered by dust is 25° (Kiselev and Rosenbush, 2004) and is much greater than that for resonance fluorescence of molecules, ~8% (Le Borgne and Crovisier, 1987). Thus, a gaseous contamination causes depolarization of the observed continuum. The contamination is small for dusty comets, but can be significant for gassy comets (i.e., comets with a low dust-to-gas ratio), such as comet Encke (Jockers et al., 2005). Moreover, the dust-to-gas ratio for comets is larger in the near-nucleus region of the coma. All of this leads to significant scatter of polarization data from ~8% to ~20% at large phase angles (α ~ 90°) for comets with low dust-togas ratios (see, e.g., figure 4.2 in Mishchenko et al., 2010). Therefore, there is no good fit for polarization data of the gassy comets, while for the majority of dusty comets the phase dependence of polarization can be represented by the average curve.
Furthermore, some comets exhibit activity in the near-nuclear region of coma in the form of jets, the polarization of which can be higher (Hadamcik et al., 2013). The aperture measurements partly level the difference. In any case, because of differences in activity and dust/gas ratio, or other causes, the linear polarization value of comets may be highly dependent on the aperture.
3.2 Circular polarization
There are three mechanisms potentially responsible for circular cometary polarization: (a) multiple scattering in an asymmetric medium; (b) scattering by non-spherical aligned particles; and (c) scattering by optically-active particles. These mechanisms are discussed in detail by Rosenbush et al. (2007; see also references therein). The latter mechanism is of a special interest. In this case, circular polarization results from intrinsic properties of the dust particles, e.g., chirality of its molecules. However, all the above mechanisms face difficulties when applied to describe the circular polarization for comets. Although cometary circular polarization may shed light on fundamental processes of planetary and life formation, observations of cometary circular polarization are still extremely rare. This factor enhances the value of observations of each new comet, especially those performed with more advanced techniques.
The observations at the 6-m BTA telescope were obtained with the financial support of the Ministry of Education and Science of Russian Federation (state contracts no. 16.552.11.7028, 16.518.11.7073). The authors also express appreciation to the Large Telescope Program Committee of the RAS for the possibility of implementing the program of spectropolari-metric observations at the BTA. N. Kiselev is thankful to SOC and LOC of the conference “Cosmic Dust V“ for financial support. We thank R. Schulz and D. Schleicher for the narrow-band cometary filters.
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