Electronic Telegram No. 2292
Central Bureau for Astronomical Telegrams
INTERNATIONAL ASTRONOMICAL UNION
CBAT Director:  Daniel W. E. Green; Room 209; Dept. of Earth and Planetary
 Sciences; Harvard University; 20 Oxford St.; Cambridge, MA  02138; U.S.A.
e-mail:  cbat@iau.org; cbatiau@eps.harvard.edu
URL http://www.cfa.harvard.edu/iau/cbat.html


2010da IN NGC 300
     Further to CBET 2289, L. A. G. Monard revises his B-band magnitude on
May 24.145 UT to B = 16.4.  He also adds the following magnitudes:  May
25.139, 16.0 (unfiltered CCD); May 25.145, V = 16.3; May 25.146, R = 16.0;
May 25.147, I = 15.8; May 25.148, B = 16.5.
     N. Elias-Rosa, J. C. Mauerhan, and S. D. Van Dyk, Spitzer Science Center,
California Institute of Technology, report that inspection of a spectrogram
(range 400-860 nm) of 2010da, obtained on May 25.4 UT with the SOAR 4.1-m
telescope (+ Goodman spectrograph), shows it to be a young "supernova impostor"
in outburst.  Assuming a recession velocity of 144 km/s (Lauberts and Valentijn
1989, Garching: European Southern Observatory; via NED) for the host galaxy,
the "GELATO" spectrum-comparison code (Harutyunyan et al. 2008, A.Ap. 488,
383; available at https://gelato.tng.iac.es) provides good fits to spectra of
"supernova impostor" 1997bs (Van Dyk et al. 2000, PASP 112, 1532) at several
days after outburst.  The blue continuum is dominated by strong, narrow H-alpha
emission, with a no presence of a P-Cyg profile.  There are also present other
narrow Balmer lines, as well as lines of [N II], Fe II, He I (at about 587.6
and 706.7 nm, with some possible contribution from Na I D at about 589 nm),
possibly a weak [He II] 468.6-nmline, and the [Ca II] near-infrared triplet.
     R. Chornock and E. Berger, Harvard University, report on imaging and
spectroscopy of 2010da, obtained on May 25.4 UT with GMOS on the Gemini-South
telescope.  The spectra (range 350-1000 nm) exhibit strong, narrow emission
lines of the H Balmer and Paschen series.  The FWHM of H-alpha is approximately
660 km/s, although the line profile is not well-fitted by a Gaussian.  The
higher-order Balmer lines exhibit blueshifted P-Cyg absorption components.
Emission lines of He I, Fe II, Ca II H and K, and O I are present, as is He II
468.6-nm.  No strong absorption lines from Na I D are seen, indicative of low
reddening, and consistent with the Swift detections in the ultraviolet
(http://www.astronomerstelegram.org/?read=2633).  In addition, although [Ca II]
729.1-nm falls in a CCD chip gap, [Ca II] 732.4-nm does not appear to be
present in these GMOS spectra.  The Gemini-South acquisition images yield a
magnitude of r = 16.2.  These spectral properties, combined with the low
luminosity relative to supernovae (http://www.astronomerstelegram.org/?read=2632
http://www.astronomerstelegram.org/?read=2633), are indicative of a luminous-
blue-variable-like outburst of a massive star.


NOTE: These 'Central Bureau Electronic Telegrams' are sometimes
      superseded by text appearing later in the printed IAU Circulars.

                         (C) Copyright 2010 CBAT
2010 May 26                      (CBET 2292)              Daniel W. E. Green