Electronic Telegram No. 2300 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 2010dn IN NGC 3184 J. Vinko, University of Szeged; G. H. Marion, Harvard-Smithsonian Center for Astrophysics and University of Texas; J. C. Wheeler, University of Texas; and R. J. Foley and R. P. Kirshner, Harvard-Smithsonian Center for Astrophysics, report that 2010dn (cf. CBET 2299) is probably an outburst of a luminous blue variable (LBV). A spectrum, obtained on June 2.12 UT with the 9.2-m Hobby-Eberly Telescope (+ Marcario Low-Resolution Spectrograph) by S. Rostopchin, shows prominent, narrow Balmer lines in emission, superimposed on a moderately blue continuum corresponding to a blackbody temperature of about 6500 K (assuming no interstellar extinction). No broad spectral features are obvious, but both the Ca II near-infrared triplet and the [Ca II] features at 730 nm are visible. The [Ca II] feature may be associated with circumstellar dust (cf. Foley et al. 2010, http://lanl.arxiv.org/abs/1002.0635) and could explain the non-detection of the progenitor in pre-explosion Hubble Space Telescope images (Berger 2010, http://www.astronomerstelegram.org/?read=2655). The FWHM of H-alpha is 1200 km/s, while the equvalent width of the Na D feature is 0.33 nm, indicative of a relatively large host reddening. According to SNID (Blondin and Tonry 2007, Ap.J. 666, 1024), the spectrum is most similar to several LBV spectra. The faintness of the transient (mag about 17; cf. CBET 2299), corresponding to absolute magnitude -13.4 at the distance of the host (about 12 Mpc), gives further evidence that 2010dn is a giant LBV outburst, and not a supernova. NOTE: These 'Central Bureau Electronic Telegrams' are sometimes superseded by text appearing later in the printed IAU Circulars. (C) Copyright 2010 CBAT 2010 June 2 (CBET 2300) Daniel W. E. Green