Observations of Comets C/2007 D1 (LINEAR), C/2007 D3 (LINEAR), C/2010 G3 (WISE), C/2010 S1 (LINEAR), and C/2012 K6 (McNaught) at large heliocentric distances

• Broad band CCD photometry of Comets C/2007 D1 (LINEAR), C/2007 D3 (LINEAR), C/2010 G3 (WISE) and C/2010 S1 (LINEAR) with 6 m telescope.
• Broad band CCD photometry of Comet C/2012 K6 (McNaught) with 60-cm Zeiss telescope.
• Spectroscopy of four distant comets: C/2007 D1 (LINEAR), C/2007 D3 (LINEAR), C/2010 G3 (WISE), and C/2010 S1 (LINEAR).
• Determination of their dust production rate and effective radius.
• Determination of potential meteoroid streams crossing the orbits of observed comets.

Photometric and spectroscopic observations of five nearly parabolic comets with eccentricity larger than 0.99 at heliocentric distances greater than 4 AU were performed. No molecular emission was observed for any studied comet and the entire cometary activity in all cases was attributed to dust production. Upper limits of the gas production rates for the main neutral molecules in the cometary comae were calculated. The derived values of dust apparent magnitudes were used to estimate the upper limit of the geometric cross-section of cometary nuclei (upper limits of radii range from 2 km to 28 km). Due to the poor sublimation of water ice at these distances from the Sun, other mechanisms triggering activity in comets are discussed.