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Observation of the Oort-cloud comet C/2009 P1 (Garradd) at the 6-m SAO RAS telescope

Russian version

    Currently, attempts are underway to establish a taxonomy of comets on the basis of their composition and to link it to the place of their origin. A comparison of physical characteristics of short-period comets with those with long-period and new comets may elucidate which properties of comets are primordial and which are the product of subsequent evolution. Study of polarization properties of different comets may also provide classification of comets and understanding the main processes of dust formation in the protosolar nebula. In addition, polarimetric characteristics of comets may indicate differences in the properties of dust on the surface and inside of the nuclei through observations of jets and different fragments of comets. For these purposes, we conduct systematic polarimetric observations of comets that are available for our instruments. Comet Garradd was unusually dust-rich, and its activity was complex and changed significantly over time. The comet produced lots of dust and gas long before it reached the snow line. This means that its activity was caused by something other than water ice, for example, by sublimation of more volatile components CO or CO2. The water production rate in comet Garradd was highly asymmetric around perihelion. Our post-perihelion observations of the comet Garradd were performed at the 6-m telescope BTA of the Special Astrophysical Observatory (Russia) with the SCORPIO-2 multi-mode focal reducer. Observations of the comet were obtained, when its heliocentric distance increased from 1.65 to 2.23 au. Analysis of photometrical data of the comet allow to isolate two features (dust and gas tails) in cometary coma oriented in the solar and antisolar directions. We estimated the shift of structures with time in cometary coma during our observations we determined the period of rotation of the nucleus as 11.1±0.8 hours. Analysis of spectral observations of the comet allows us to identify the emission bands of neutral molecules such as C2, C3, CN, CH, and NH2 as well as CO+ and H2O+ ions (Fig.2).
    Linear polarization of the comet shows variation, the degree of polarization increases with the increase of wavelength (from 4% for 0.58 μm to 5% at 0.83 μm) and changes with time, during 2 month the polarization decreases by 1%. Our spectropolarimetric observations are essentially unique, because today similar material was obtained only for few comets. The observed character of the polarization variation is likely caused by a change in the comet's activity with time, and change the properties of the dust with the distance from the Sun.
    We found that the degree of polarization which is non-contaminated (due to the continuum) in the C2 emission band is about 3.3% that is slightly higher than the theoretical value. We found the significant left-handed (negative) circular polarization detected at distances up to 3x104 km from the cometary nucleus with values from about -0.06% to -0.5+/-0.02%. There is some systematic increase in the degree of circular polarization to the outer edge of the coma with time.
O.Ivanova, V.Rosenbush, V.Afanasiev, N.Kiselev, Icarus, V.284, 2017, 167-182

Contact - A.Ivanova

Fig.1. Image of the comet C/2009 P1 (Garradd) with isophotes obtained in the -SDSS filter on February 2, 2012 (left-hand panel) and processed with digital filters (right-hand panel) in order to distinguish low-contrast structures in the comet come

Fig.2. Spectrum of the comet obtained on February 2, 2012. Sky background and continuum (the reflected sunlight by cometary dust particles) were subtracted. Separately in the box, the observed (a solid line) and model (a dashed line) spectra of the emission line C2 (Δν = + 1) are shown for comparison