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Astron. Astrophys. 337, 207-215 (1998)

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1. Introduction

Core collapse supernovae (SNe) come in various appearances. During the early phases of the explosions, the emission is dominated by the shock-excited material in the outer layers of the exploding star. Once the ejecta have expanded to become optically thin, we can study the inner layers of the progenitor star and the energy input into the debris. Thus, the classification scheme, which is based on the spectral appearance near maximum light (Harkness & Wheeler 1990, Filippenko 1997), does not necessarily encapsulate the appearance of the supernova at late phases. This has been demonstrated by transition objects like SN 1993J and SN 1987K. These SNe showed hydrogen-dominated spectra, characteristic of Type II supernovae (SNe II), near maximum, but changed to oxygen and calcium dominated spectra in their nebular phase. Their late spectra were thus similar to the spectra of hydrogen-deficient Type Ib and Type Ic supernovae.

The existence of SNe transforming from SNe II to SNe Ib/c clearly indicates a relationship between these subclasses of supernovae. In this scenario, SNe Ib/c are produced by core collapse of massive stars, just as SNe II, except that their progenitors were stripped of their H (SN Ib) and possibly He (SN Ic) envelopes prior to the explosion, either by mass transfer to a companion (Nomoto et al. 1994) or via winds (Woosley et al. 1993). The transition objects are thus believed to have progenitors were most, but not all, hydrogen was removed before the explosion.

Apart from differences in their spectra, supernovae also show variations in their light curves. At late times SN II generally follow the decay rate of 56 Co (Turatto et al. 1990; Patat et al. 1994), whereas some SNe Ib/c decay faster (Wheeler & Harkness 1989; Clocchiatti & Wheeler 1997). This is attributed to differences in optical depth for the [FORMULA]-rays from the 56 Co decay. The faster decline rate of SNe Ib/c is thus due to a lower optical depth, which in turn is related to the smaller ejecta mass of these SNe. So far, however, only few core-collapse supernovae have been observed at late phases ([FORMULA] months after explosion; Patat et al. 1994).

While observations at late phases, when the SN is faint, are clearly more difficult, spectra at late epochs probe deeply into the core of the exploding star, and the expansion leads to an optically thin nebula where the spectral interpretation is easier. For SNe Ib/c the absence of a large hydrogen envelope creates an even cleaner window into the heart of supernova nucleosynthesis.

Supernova 1996N was discovered on March 12.5, 1996, in the large, barred spiral galaxy NGC 1398 (Williams & Martin 1996). The supernova is located [FORMULA] east and [FORMULA] north of the center of the galaxy (Fig. 1). Nothing was visible at this location on February 16. An early spectrum obtained on March 23.4 showed this to be a SN Ib/c about two weeks past maximum (Germany et al. 1996), and Van Dyk et al. (1996) reported a radio detection of the supernova at 3.6 cm on April 2.0 with the VLA. Here we report on optical observations of SN 1996N taken several months after explosion. We have observed the supernova on five occasions with several La Silla telescopes. These observations are part of a long-term program to study core-collapse supernovae at late phases.

[FIGURE] Fig. 1. SN 1996N in NGC 1398. North is up and east to the left. This V-image was taken with the ESO/MPI 2.2m telescope in October 1996, 221 days after discovery. The comparison stars are numbered and the supernova is marked with an arrow.

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© European Southern Observatory (ESO) 1998

Online publication: August 6, 1998