[BAA Comets] C/2011 L4 (PANSTARRS) update

[Richard Miles]

Just a short note to say that this comet is exhibiting very stable, uniform 
behaviour as it moves away from perihelion. The nucleus is now 10th 
magnitude and fading further. A wide-field image from the night of April 
29/30 shows a fan-shaped tail about 1 degree across as per the following 
image, where cometary dust can be seen reflecting sunlight across the entire 
leftside of the picture.
http://www.britastro.org/~rmiles/Images/C2011L4_20130429_Miles_small.jpg

Looking at the rate this comet is fading, the following plot depicts the 
reduced magnitude in which the range distance from the Earth (delta) is 
adjusted for: this leaves just the effect of the increasing heliocentric 
distance on magnitude:

http://www.britastro.org/~rmiles/Documents/C2011L4_20130314-0429_reduced_mag_Miles.png

The straight-line fit gives a value of 2.5n.log(rh) where n = 4.24, which 
unfortunately means a fairly rapid fade rate. Note the good fit to a linear 
regression obtained using a CCD camera (SXVR-H18) and V filter. It shows 
that the comet nucleus is not undergoing any significant 'mini-outbursts' 
below the level detectable visually.

N.B. A similar photometric result could be obtained using a DSLR camera by 
extracting the 'G' channel, in principle. A key requirement is to avoid 
saturating images of the stars used in the magnitude calibration (or 
saturating the image of the comet nucleus, for that matter).

Richard Miles

P.S. Unfortunately, I cannot attend the Comet Section meeting on May 18 in 
Northampton owing to a long-standing prior engagement. It should be a good 
meeting for sure and I certainly recommend attending if you are interested 
in the subject of observing active small solar system bodies.