Optical Spectroscopy and Progenitor Identification of the Low-Luminosity Transient in UGC2773

ATel #2187; E. Berger and R. Foley (Harvard)
on 2 Sep 2009; 23:12 UT
Distributed as an Instant Email Notice Transients
Credential Certification: Edo Berger (eberger@astro.princeton.edu)

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We observed the transient source in UGC 2773 (CBET #1931; hereafter UGC2773 OT2009-1) with the Gemini Multi-Object Spectrograph on the Gemini-North 8-m telescope on 2009 September 2.60 UT. A pair of 1200 sec exposures were obtained with a wavelength coverage of 5000-9000A at a resolution of about 3.5A. The spectrum is dominated by a bright and narrow H-alpha emission line with a width of about 350 km/s, strong CaII IR triplet lines with an apparent P Cygni profile (with the minimum blue-shifted by 365 km/s relative to the systemic velocity of UGC 2773), [CaII] emission lines, and NaI D lines with a P Cygni profile. Additional weaker lines such as OI 7776 are also detected. At the distance of UGC 2773 (m-M = 28.82 mag) the discovery absolute magnitude of UGC2773 OT2009-1 is M ~ -12.7 mag (taking into account a Galactic extinction of A_R = 1.5 mag). Additional extinction within the host galaxy is possible. From the GMOS acquisition image we find that the object is located at the following coordinates (J2000): RA = 03:32:07.22 DEC = +47:47:39.7 We obtained archival Hubble Space Telescope images taken with the Wide Field Planetary Camera 2 instrument in the F606W and F814W filters (600 sec in each filter). Differential astrometry of the GMOS image relative to the WFPC2/F606W image (with an rms of 25 mas in each coordinate) reveals a coincident source in both WFPC2 filters. The HST images at the location of UGC2773 OT2009-1 are available from http://www.cfa.harvard.edu/~eberger/ugc2773_ot2009-1_hst.gif (the circle corresponds to 5-sigma). The spectral properties of UGC2773 OT2009-1 are reminiscent of the recent intermediate-luminosity transient in NGC300 (Berger et al. 2009, ApJ, 699, 1850). However, the progenitor of NGC300 OT2008-1 was completely dust-obscured and visible only in Spitzer images. Furthermore, no P Cygni absorption was seen in any of the lines. Another possibility is that UGC2773 OT2009-1 is a sub-luminous type IIP supernova similar to SN1997D which had a peak M_V>-14.65 mag (Turatto et al. 1998, ApJ, 498, L129). However, even at +1 year, the spectra of SN1997D exhibited H-alpha FWHM of about twice the width we measure for UGC2773 OT2009-1 (Benetti et al. 2001, MNRAS, 322, 361), while early spectra exhibit an H-alpha P Cygni profile, not see here. A final possibility is an LBV outburst, but the strong CaII emission would be unexpected in this case.