Comet C/2013 US10 (Catalina) Development

Charles Bell, Neil Norman, and Taras Prystavski  (April 30, 2015)

There is a report on comets-ml by Paul Camilleri, Warners Bay Australia, that comet C/2013 US10 (Catalina)  shows no enhancement with a Swan filter. That is what I expect at the comet's current heliocentric distance, 3.142 AU.  C2 generally starts to show emission when a comet reaches 2 AU (or July 30, 2015 for C/2013 US10 (Catalina)). If we could see CN emission at at 3880 Å or 388 nm, we should start to pick it up as the comet passes inward at 3 AU (or May 11, 2015 for C/2013 US10 (Catalina)). The lower end of the visible spectrum is 3900  Å or 390 nm.

Lack of SWAN filter enhancement to me does not mean lack of gas but lack of sufficient gas emission. Gas emission and in particular SWAN band or C2 emission is not yet apparent.

The solar UV flux at the comet's heliocentric distance is not yet sufficient to produce photo dissociation of the parent molecular species of C2 and cause sufficient photon excitation and emission with the common blue green glow we associate with comets.  Theoretically it is the energy of a UV photon that does all these interactions. Flux is different, in that it is the number of photons passing through a unit area with time. As comets approach perihelion, the solar UV flux increases inversely proportional to r squared

We are currently seeing reflected sunlight from dust emitted by this comet. The light reflected may be slightly reddened. The light we see should have nearly the same color indices as sunlight. Photometrically that means the values of B-V, V-R, should be close to standard values of sunlight.  The Johnson V band filter is centered on the C2 emission band at 515 nm wavelength.  Use of such filters have in the past been used to detect gas emission.

Solar color indices:
(B-V) 0.65 
(V-R) 0.35 
(R-I) 0.28
See Section 4.4 in The Sizes, Shapes, Albedos, and Colors of Cometary Nuclei by Philippe L. Lamy, Imre Toth, Yanga R. Fernández, Harold A. Weaver in Comets II

Dust is lifted from the nucleus of the comet by lifting forces as volatile solids sublimate to gaseous phase. There has to be gas flow to cause the drag force to lift the dust.

Comet Spectra by C. Arpigny in Comets: Scientific data and missionsKuiper, G. P. [editor] (Arizona Univ.; Tucson, AZ, United States);Roemer, E. [editor] (Arizona Univ.; Tucson, AZ, United States)Abstract: Proceedings of conference on comets, University of Arizona - Apr. 1970 Publication Date: Jul 01, 1972 NASA-CR-129110; LC-72-619613  Distribution Limits: Unclassified; Publicly available; Unlimited