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Astron. Astrophys. 321, 81-83 (1997)

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3. Discussion

Supernovae typically have peak absolute blue magnitudes of [FORMULA]. The value of [FORMULA] that we measure in the May image is therefore what one might expect if the point source that we see is a SN in IRAS 12112+0305 being observed near its peak (the distance modulus of IRAS 12112+0305 is 37.36 so the absolute magnitude is [FORMULA], correcting for [FORMULA] of Galactic extinction). That it is observed in a galaxy where there is independent evidence (from the high [FORMULA] and the high CO luminosity) for vigorous star formation in progress is further suggestive that we are observing a SN. However, we do not have a spectrum of the candidate SN and so cannot formally rule out this as being some foreground Galactic cataclysmic event. It might even be a distant and compact minor planet or asteroid, as IRAS 12112+0305 lies near the ecliptic. However, given the absolute magnitude of the object were it to lie in IRAS 12112+0305, and the duration of the event, the SN interpretation is overwhelmingly the most likely one.

It is however not possible to identify the type of SN based on this measurement from the May image alone. Type Ia SN have peak magnitudes [FORMULA] ([FORMULA] km s-1 Mpc-1) with scatter of about [FORMULA] (Kirshner 1990). Type II SN typically have peak magnitudes [FORMULA] fainter than this, but the scatter in this peak magnitude is large, and the brighest SN II known have peak magnitudes comparable to those of SN Ia (Kirshner 1990). Therefore this measurement by itself does not distinguish between these two cases. Furthermore, we cannot be certain that we are observing the SN exactly at its peak magnitude in which case we cannot compare our measurement with these numbers. However the value of [FORMULA] is sufficiently bright that we are probably observing the SN very close to the peak in the lightcurve. Type Ib SN, however, generally have peak magnitudes much fainter than what we observe (Uomoto & Kirshner 1986) and so can be ruled out. Finally, our measured magnitude might be affected by internal extinction from dust in the galaxy. Again, however, the observed magnitude of this SN is so close to the maximum observed peak magnitude for both Type I and Type II SN that this extinction is probably very small, even in the B-band. This is perhaps not too surprising as the SN is in an extension of the galaxy well away from its center.

We therefore attempt to determine the type of SN by comparing our May and August data with typical lightcurves. Fig. 2 suggests that we are probably observing a SN II as the fading between our May and August observations is greater than expected for a Type I. For this SN to be a typical Type I we would have to hide a point source [FORMULA] brigher than our estimated limiting magnitude in Sect. 2, which we regard as unlikely. However the scatter around the curves in Fig. 2 is approximately [FORMULA] (Kirshner 1990), so it is still possible that we could be observing a SN Ia, but it would then be a fairly extreme example. Note further that SN II can have colors as red as [FORMULA] at 90 days past maximum (Younger & van den Bergh 1985). If this object is that red, then the limit shown in Fig. 2 is moved down 0.6 mag and the SN II interpretation also becomes dubious.

[FIGURE] Fig. 2. A comparison of our data with SN lightcurves. The lightcurves are those computed by Doggett & Branch (1985) for typical Type I, Type II-P (Plateau), and Type II-L (Linear) SN. Typical scatter of individual SN around these curves is about 1 mag (Kirshner 1990). The point at the origin is our B-band point (May 1995) and the limit at 92 days is from our V-band limit (August 1995). These points were calculated as described in Sect. 2, and plotted assuming that we are observing the SN at peak in May. When comparing the data and the curves, any lateral translation of the points is therefore possible. The limit assumes [FORMULA] at 92 days after peak for both Type I and Type II SN, which we regard as being conservative given the compilation of Younger & van den Bergh (1985). We ignore reddening due to dust within IRAS 12112+0305 because the internal extinction is known to be small from the observed peak magnitude of the SN. We do not plot the March [FORMULA] -band limit, due to the lack of any comparison data from well-studied SN.

In summary, our results suggest that if we have seen a SN, it is probably a SN II, which is what is expected given the environment, in particular the high gas density and huge infrared luminosity of IRAS 12112+0305, which together are suggestive of a high star formation rate. But we do stress that this interpretation is based on a single detection, and so any conclusions drawn from it must be made with caution.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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