P/2008 CL94 (Lemmon) and P/2011 S1 (Gibbs): comet-like activity at large heliocentric distances

Based on spectroscopic and photometric observations we analyzed the dust environment of two minor distant objects, P/2008 CL94 (Lemmon) and P/2011 S1 (Gibbs). Both targets demonstrated the comet-like activity beyond the ``zone of water-ice sublimation''. Meanwhile the spectrum of P/2008 CL94 (Lemmon) did not reveal molecular emission features above reflected continuum in a spectral region of 4100-6800 angstrom. Reddening of the continuum is linear along the dispersion with the mean normalized reflectivity gradient equals to 2.0% +/- 0.4%. The normalized reflectivity of P/2011 S1 (Gibbs) derived from the V-R and R-I color indices equals 11% +/- 9% and 26% +/- 6% respectively. Both objects have likely small nuclei (about 2 and 4 km in the radii for P/2008 CL94 and P/2011 S1 respectively), which are consistent with nucleus sizes of `Jupiter-family' comets. The level of physical activity of P/2008 CL94 and S/2011 S1 is characterized by R-Af rho quantity of 106 +/- 3 cm and 76 +/- 8 cm respectively. The Af rho values are resulted in dust production rates of about 1-2 kg/s, assuming the average geometric albedo of grains of 0.1 and the dust outflow velocities between 1 and 10 m/s. (C) 2016 Elsevier Inc. All rights reserved.