Central Bureau for Astronomical Telegrams
IAUC 5251: 1991T; 1991N
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Circular No. 5251
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
Postal Address: Central Bureau for Astronomical Telegrams
Smithsonian Astrophysical Observatory, Cambridge, MA 02138, U.S.A.
Telephone 617-495-7244/7440/7444 (for emergency use only)
TWX 710-320-6842 ASTROGRAM CAM EASYLINK 62794505
MARSDEN or GREEN@CFA.BITNET MARSDEN or GREEN@CFAPS2.SPAN
SUPERNOVA 1991T IN NGC 4527
M. Hamuy and M. M. Phillips, Cerro Tololo Interamerican
Observatory, report: "A low-dispersion CCD spectrogram (range 310.0-
765.0 nm) of SN 1991T was obtained by Hamuy on Apr. 25.0 UT with the
CTIO 1.5-m telescope. Comparison of this spectrum with a similar
observation made on Apr. 20.0 shows that a broad absorption feature
at an observed wavelength of about 619.0 nm has emerged. The spectrum
has begun to resemble that of a type-Ia event, with the 619.0-
nm absorption presumably due to Si II 635.5-nm. The 424- and 494-nm
absorption features referred to on IAUC 5239 remain strong, but the
wavelengths of the minima have shifted to 428.5 and 498.0 nm in the
Apr. 25.0 spectrum. These features appear to correspond to the Mg
II 448.1-nm and Fe II multiplet-42 absorptions seen in normal type-
Ia events near maximum. Unresolved Na I D absorption (presumably
interstellar in origin) is observed in both spectra at an equivalent
width of about 0.15 nm and a radial velocity of 1975 km/s. Adopting
this value for the supernova rest velocity yields expansion velocities
of 9770 and 15 000 km/s, respectively, for the Si II 635.5-nm
and Mg II 448.1-nm lines. Although we now believe that SN 1990T is
a type-Ia event, we would emphasize that the spectrum remains
peculiar. In particular, the Si II line is still relatively weak in
comparison with the Mg II and Fe II absorption features, and there
is only weak absorption at the expected position of the Ca II H and
K lines, which are normally among the strongest features in the
maximum-light spectra of type-Ia supernovae."
D. Silva, National Optical Astronomy Observatory and Kitt Peak
National Observatory, communicates: "CCD observations of SN 1991T
were obtained on Apr. 22.22 UT under photometric conditions with the
KPNO 0.9-m telescope using Gunn g and r filters. The measured
magnitudes (+/- 0.04) were g = 11.65 and r = 12.08. Note that the
underlying galaxy flux accounts for roughly 15 percent of the local
background and must be taken into account for accurate 'sky'
subtraction."
Further visual magnitude estimates (cf. IAUC 5246): Apr. 22.12
UT, 11.6 (G. Lubcke, Middleton, WI); 22.13, 11.6 (P. Steffey,
Daytona Beach, FL); 23.10, 11.4 (Steffey); 24.15, 11.3 (Lubcke).
SUPERNOVA 1991N IN NGC 3310
Visual magnitude estimates by S. Korth, Dusseldorf, Germany (cf.
IAUC 5234): Apr. 10.85 UT, 14.5; 13.04, 14.6; 19.98, 15.0; 20.92,
15.3.
1991 April 25 (5251) Daniel W. E. Green
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