Central Bureau for Astronomical Telegrams
IAUC 5408: 1991bg
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Circular No. 5408
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 1991bg IN NGC 4374
M. Della Valle, J. Danziger, and C. Gouiffes, European Southern
Observatory, communicate: "A CCD spectrogram (range 400-840 nm,
resolution about 2 nm) of SN 1991bg was obtained on Dec. 15.4 UT
with the 3.54-m New Technology Telescope (+ EMMI) at La Silla. The
spectrum is dominated by strong absorptions of Si II (635.5, 597.2,
564.0, 545.4 nm) and Fe II (492.4 nm), indicating a type-Ia supernova.
The expansion velocity deduced from the minima of the Si II
635.5-nm absorption is 10 200 km/s. Preliminary photometric
measurements give V = 14.11 +/- 0.02 and B = 14.87 +/- 0.03. This
implies E(B-V) about 0.7-0.9 and B(0) = 11.9 at maximum light if it is
a normal type-Ia object. This compares with Bmax = 12.2 for SN
1957B in the same galaxy. Alverado and Wenderoth (IIT) report
similar colors obtained from CCD photometry on the 0.90-m Dutch
telescope for Dec. 16.3. If the SN is normal, all these measurements
are consistent (within +/- 0.2 mag), with the age at Dec. 15 being
0-7 days past maximum and with significant reddening."
M. M. Phillips and M. Hamuy, Cerro Tololo Interamerican Observatory,
report: "Low-dispersion CCD spectrophotometry (range 350.0-
730.0 nm) of SN 1991bg was obtained by J. Maza (University of Chile)
on Dec. 15.35 UT with the CTIO 4.0-m telescope. The spectrum
resembles that of a type-Ia event within about 1 week of maximum, but
with clear peculiarities (cf. IAUC 5403, 5405). The following
magnitudes were synthesized from an observation obtained through a 7"
slit: V = 14.03, B-V = +0.86. Direct CCD images were obtained by
N. Caldwell (F. L. Whipple Observatory) on Dec. 17.35 UT with the
CTIO 0.91-m telescope. A preliminary reduction of these data yields
V = 14.05 and B-V = +0.89, suggesting that the supernova is near
maximum. The red color is also unusual for a type-Ia supernova at
maximum. Although NGC 4374 has a small dust lane near its nucleus
(see Gallagher 1986, PASP 98, 81), the supernova is located well
outside this region. Because of this and the lack of evidence for
interstellar Na I D absorption lines in Maza's low-dispersion spectrum,
we consider it unlikely that the supernova magnitudes and colors
have been affected significantly by dust extinction. Hence, if
SN 1991bg has actually reached maximum, it would appear to be as
much as 3 mag fainter in B than the type-I SN 1957B which occurred
in the same galaxy."
Visual magnitude estimates: Dec. 14.167 UT, 13.8 (S. Pesci,
Milan, Italy); 15.52, 13.9 (W. Johnson, Anza, CA).
1991 December 18 (5408) Daniel W. E. Green
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