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Astron. Astrophys. 354, 557-566 (2000)

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1. Introduction

The Type II supernova (SN) 1997D discovered on Jan. 14.15 UT (De Mello & Benetti 1997) is a unique event characterized by extremely low expansion velocity, low luminosity, and very low amount ([FORMULA]) of radioactive 56Ni (Turatto et al. 1998). An analysis of the observational data led Turatto et al. (1998) to conclude that they caught SN 1997D around day 50 after it had exploded as a red supergiant with the mass of 26 [FORMULA] and radius of [FORMULA]. The derived ejecta mass is [FORMULA] and kinetic energy is [FORMULA] erg. They propose a scenario in which the low 56Ni mass in SN 1997D is caused by a fall-back of material onto the collapsed remnant of the explosion of a 25-40 [FORMULA] star. An exciting implication is that SN 1997D might be accompanied by black hole formation (Zampieri et al. 1998).

Here we present arguments for an alternative view on the origin of SN 1997D, which in our opinion was a descendant from the low end of the mass range of core-collapse supernova (CCSN) progenitors. The problem we attempted to solve first was to find the hydrodynamical model, which could reproduce the light curve, the velocity at the photosphere, and the line profiles of major strong lines (Sect. 2). We emphasize the importance of the line profile analysis, since it provides robust information on the velocity at the photosphere. The latter is a crucial parameter for constraining hydrodynamical models. Unexpectedly for us it turned out that Rayleigh scattering in SN 1997D is significant and may be used as a powerful diagnostic tool. The emphatic role of Rayleigh scattering in this case is related to the low energy-to-mass ratio ([FORMULA]) of SN 1997D (Turatto et al. 1998), which results in the higher than normal density at the photospheric epoch for SN II-P. A combination of hydrodynamical modelling and robust analysis of spectra at the photospheric epoch permitted us to impose tight constraints on E, M, and [FORMULA] of SN 1997D.

In addition, we analyzed nebular spectra of SN 1997D using a nebular model (Sect. 3). To make such an analysis as secure as possible we first checked the model taking advantage of the well studied SN 1987A at a similar epoch. We found modelling the nebular spectrum of SN 1997D beneficial in discriminating between low and high-mass options for the ejecta. To our knowledge the present paper is a first attempt of a SN II-P study to simultaneously make use of all data: light curve, photospheric and nebular spectra. Some implications of the low mass and kinetic energy of the SN 1997D ejecta for the systematics of CCSN, explosion mechanism, and galactic population of supernova remnants (SNR) are discussed in the final section. Below we adopt for SN 1997D the dust extinction [FORMULA] mag and the distance 13.43 Mpc following Turatto et al. (1998).

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© European Southern Observatory (ESO) 2000

Online publication: February 9, 2000