-------------------------------------------------------------------
THE ASTRONOMER Electronic Circular No 2131      2005 Jly 14 18.43UT
Ed:Guy M Hurst, 16,Westminster Close, Kempshott Rise,  Basingstoke,
Hants, RG22 4PP,England.Telephone/FAX(01256)471074Int:+441256471074
INTERNET: GUY@TAHQ.DEMON.CO.UK             GMH at AST.STAR.RL.AC.UK
WORLD WIDE WEB                         http://www.theastronomer.org
-------------------------------------------------------------------
SUPERNOVA 2005cs IN MESSIER 51
Since the identification of the pre-discovery image of this object 
obtained by Peter Birtwhistle, we were asked to submit this to Bruce 
Margon for use in conjunction with Hubble images in the search for the 
supernova progenitor.

Peter has since obtained a further exposure of 2005 July 6.940UT and 
obtain a revised precise position from this unfiltered 30-second 
exposure using Astrometrica and the USNO B1.0 catalogue:

RA 13h 29m 52.770s +/- 0.019s, DEC +47 10' 36.00" +/- 0.16"
(equinox 2000.0) Mag R = +14.25 +/- 0.01

This appears close to the candidate progenitor proposed by W. Li et. al. 
on IAUC 8556. They report that they have isolated a possible progenitor 
for this young type-II supernova in Hubble Space Telescope images of M51 
taken with the Advanced Camera for Surveys (ACS). It is within the 0".15 
uncertainty of the
supernova position; this object was not detected in the F555W images 
(mag > 25.5).  Assuming low reddening to the supernova and a true 
distance modulus to M51 of 29.6 mag the object is consistent with a red 
supergiant with an approximate absolute I magnitude of -5.8 and colours 
V-I > +1.4, implying an initial mass for the progenitor of < about 10 
solar masses. The supernova occurred in a crowded field, and there are 
several other red sources and a bright, blue, possible compact star 
cluster within 0".3 of
the supernova location.
Later Li added that the identification of the progenitor was confirmed 
using CFHT images taken on July 2.28 UT under excellent seeing 
conditions (0".7). Geometrical transformation between the CFHT images 
and the HST/ACS images yields a precision of 0.8
ACS pixel for the supernova location on the ACS images, and their 
proposed progenitor is the only object within the error circle. The 
position of the progenitor is measured to be:
RA 13h 29m 52.760s, DEC +47o 10' 36.11" (2000).

Editor: There is remarkably good agreement between Peter's precise 
position and the that quoted above by Li, which was taken from IAUC 
8556.

Guy M Hurst