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IAUC 8555: 2005cs; 161P

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IAUC number

                                                  Circular No. 8555
Central Bureau for Astronomical Telegrams
Mailstop 18, Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
IAUSUBS@CFA.HARVARD.EDU or FAX 617-495-7231 (subscriptions)
URL http://cfa-www.harvard.edu/iau/cbat.html  ISSN 0081-0304
Phone 617-495-7440/7244/7444 (for emergency use only)

     M. Modjaz, R. Kirshner, and P. Challis, Harvard-Smithsonian
Center for Astrophysics, report that a spectrogram (range 340-740
nm) of SN 2005cs (cf. IAUC 8553), obtained by R. Hutchins on June
30.23 UT with the F. L. Whipple Observatory 1.5-m telescope (+
FAST), shows it to be a young type-II supernova.  The spectrum
consists of a blue continuum and P-Cyg profiles of the Balmer and
He lines.  Adopting the NED recession velocity of 600 km/s for the
host galaxy (from Brian et al. 1996, Ap.J. 473, 130), the expansion
velocity derived from the minimum of the H_beta line is about 7500
km/s.  Zero-velocity interstellar Na I D absorption with an
equivalent width of about 0.02 nm is detected in the spectrum of SN
2005cs, indicating gas along the line-of-sight in our galaxy and
thus suggesting some reddening by dust.  Schlegel et al. (1998,
Ap.J. 500, 525) estimate E(B-V) = 0.035 mag of Galactic reddening
along the line-of-sight to M51.  Interstellar Na I D absorption is
detected also at the redshift of the host galaxy, with comparable
equivalent width, suggesting some host-galaxy extinction.
     M. W. Richmond, Rochester Institute of Technology, reports
that deep Hubble Space Telescope images of M51 (Hubble Heritage
Project) taken in January 2005 show a cluster of young stars near
the position of SN 2005cs (cf. IAUC 8553).  The object closest to
the position of the supernova on Itagaki's image is at R.A. =
13h29m52s.803, Decl. = +47o10'36".52 (equinox 2000.0).  This is a
blue star; assuming E(B-V) = 0.10 based on the Na I D absorption
reported by Modjaz et al. (above), the candidate has intrinsic
colors B-V = -0.2 and V-I = -0.4 and, assuming a distance of 8.4
Mpc to M51, an absolute V magnitude of roughly -6.  If this blue
star was indeed the progenitor, one might expect SN 2005cs to
evolve like SN 1987A, another type-II supernova with a blue
     G. M. Hurst, Basingstoke, England, reports that an unfiltered
CCD image of M51 obtained by P. Birtwhistle (Great Shefford,
England, 0.40-m Schmidt-Cassegrain reflector) on June 27.933 UT
shows SN 2005cs at red mag 16.6 and at R.A. = 13h29m52s.81, Decl. =
+47o10'35".3 (equinox 2000.0).

     Total visual magnitude estimates:  June 2.10, 11.8 (J. J.
Gonzalez, Leon, Spain, 0.20-m reflector); 3.07, 11.9 (N. Biver,
Ablis, France, 0.41-m reflector); 6.09, 11.6 (Gonzalez); 16.08,
11.3 (Gonzalez).

                      (C) Copyright 2005 CBAT
2005 July 1                    (8555)            Daniel W. E. Green

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