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

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4. Discussion

We attributed SN 1997D to the SN II-P event characterized by the kinetic energy [FORMULA] erg and ejecta mass [FORMULA]. The ejecta are dominated by H-rich matter and contains 0.02-0.07 [FORMULA] of freshly synthesized oxygen. The estimated 56Ni mass is about 0.002 [FORMULA] in accordance with the value found by Turatto et al. (1998). The pre-SN had a moderate radius of 85 [FORMULA] and possibly a low primordial metallicity, 0.3 cosmic.

At first glance the suggested low metallicity of SN 1997D disagrees with the assumed cosmic abundance of barium which implies a relative overabundance by a factor of three. The latter should not confuse us, however, after SN 1987A in which the relative barium overabundance is about five (Mazzali et al. 1992) for the comparable metallicity of both supernovae. Amazingly, the relatively small pre-SN radius and low primordial metallicity of SN 1997D both are reminiscent of SN 1987A. Possibly it reflects some trend for low metallicity progenitors to have smaller pre-SN radii compared to SNe II-P with cosmic metallicity.

When combined the ejecta mass and the collapsed core mass (presumably 1.4 [FORMULA]), the total pre-SN mass amounts to 7-9 [FORMULA] prior to outburst. The main-sequence progenitor likely was more massive because of a possible wind mass-loss. In the context of general results of stellar evolution theory, the low mass of freshly synthesized oxygen ([FORMULA]) is compelling evidence that the progenitor of SN 1997D was a main-sequence star from the 8-12 [FORMULA] range. These stars are known to end their life with very low amount ([FORMULA]) of synthesized oxygen (Nomoto 1984; Woosley 1986). Ejecta of CCSN produced by such stars must contain very small amount of 56Ni, significantly less than normal CCSN (Woosley 1986), which is also in line with SN 1997D.

In principle, one cannot rule out "nature conspiracy" which might transform the massive star case into the above low-mass case. For this to happen a massive star of [FORMULA] must lose an envelope of [FORMULA] prior to supernova explosion with all the oxygen but a fraction of 1/50 fallen back onto the black hole during the collapse. Note that with the 56Ni mass of [FORMULA] synthesized during the collapse of such a massive star (Woosley & Weaver 1995) the fraction of ejected 56Ni should be roughly the same as for oxygen to accommodate the nebular spectrum. This requires a homogeneous mixing of 56Ni and oxygen before the fall-back of the metal-core material onto the black hole. Such a situation is very unlikely, since 56Ni is synthesized primarily in the silicon mantle at the bottom of the oxygen shell.

The fact that at least some CCSN originating from the 8-12 [FORMULA] mass stars have low kinetic energy ([FORMULA] erg) and eject small amounts of 56Ni ([FORMULA]) modifies a picture of CCSN with "standard" kinetic energy of [FORMULA] erg and 56Ni mass of 0.07-0.1 [FORMULA]. The new situation in the systematics of CCSN is visualized by the [FORMULA] and 56Ni mass-[FORMULA] plots (Fig. 11) which show the position of SN 1997D along with two other well studied CCSN, SN 1987A (Woosley 1988; Shigeyama & Nomoto 1990; Utrobin 1993) and SN 1993J (Bartunov et al. 1994; Shigeyama et al. 1994; Woosley et al. 1994; Utrobin 1996). The primary significance of this plot is that both nearly constant kinetic energy ([FORMULA] erg) and 56Ni mass ([FORMULA]) in the range of progenitor masses between [FORMULA] and [FORMULA] abruptly drop at the low end of massive star range producing CCSN (around 10 [FORMULA]). It would be not unreasonable to consider that SN 1997D is a prototype for a new family of CCSN (below referred to as "weak CCSN") which occupies the same place on the E-[FORMULA] and 56Ni mass-[FORMULA] plots as SN 1997D.

[FIGURE] Fig. 11a and b. Kinetic energy a and 56Ni mass b as a function of progenitor mass for three core-collapse supernovae.

Unfortunately, there are no clear theoretical predictions in regard to weak CCSN. Yet current trends in the core-collapse modelling seem to be generally consistent with Fig. 11. Two explosion mechanisms are related to producing CCSN: prompt (core rebound) and delayed (neutrino-driven mechanism). For 8-10 [FORMULA] progenitors the prompt mechanism attains its highest efficiency (Hillebrandt et al. 1984) with the kinetic energy of ejecta [FORMULA] erg (Baron & Cooperstein 1990), while the delayed mechanism, on the contrary, has the lowest efficiency in this mass range with similar energy [FORMULA] erg (Wilson et al. 1986). Thus both explosion mechanisms remain viable in the context of SN 1997D. However, possibly only the neutrino-driven mechanism is able to account for the kinetic energy increase with the progenitor mass in the range from about 10 [FORMULA] to [FORMULA] (Wilson et al. 1986; Burrows 1998).

How frequent are SN 1997D-like phenomena? The first thought is that they are extremely rare, since among [FORMULA] identified SN II-P events only one such case has been discovered so far. However, with the low absolute luminosity ([FORMULA] mag) and the brief plateau duration (40-50 days) compared to normal SN II-P characteristics ([FORMULA] mag and 80-100 days, respectively) it would not be surprising, if SN 1997D-like events were as frequent as [FORMULA] of normal SN II-P rate. Such a rate might be maintained by progenitors from the mass interval [FORMULA] in the vicinity of main-sequence mass [FORMULA].

Progenitors from the 8-12 [FORMULA] mass range were suggested earlier as counterparts for supernovae with a dense circumstellar wind, low ejecta mass ([FORMULA]), and possibly normal kinetic energy (e.g. SN 1988Z, Chugai & Danziger 1994). The present attribution of SN 1997D to the same mass range introduces some dissonance with the former conjecture. In reality, this controversy is not serious since 8-12 [FORMULA] progenitors are characterized by very complicated evolution of their cores (Nomoto 1984; Woosley 1986) and therefore a different outcome for slightly different initial mass is quite conceivable. Moreover, it may well be that with a normal metallicity presupernova of weak CCSN also vigorously loses mass and explodes in a dense wind thus producing a luminous supernova (possibly SN IIn) due to the ejecta wind interaction.

Another intriguing possibility is that a presupernova of weak CCSN might lose all the hydrogen envelope in a close binary system before the explosion. In this case weak CCSN will be a mini-version of SN Ib with low explosion energy, low amount of 56Ni, and, eventually, low luminosity. Unfortunately it will not be easy to detect such events.

If weak CCSN are as frequent as [FORMULA] of all CCSN, a good fraction of galactic population of SNR may be related to these supernovae. We cannot miss the opportunity to speculate that at least two historical supernovae SN 1054 and SN 1181 might be identified with weak CCSN. Nomoto (1984) already argued that the Crab Nebula was created by CCSN with the progenitor mass around 9 [FORMULA]. The luminosity of SN 1054, which was normal for SN II, could be explained then by the interaction of ejecta with a dense pre-SN wind. This suggestion in fact is a modification of the earlier idea that the initial phase of the light curve of SN 1054 could be powered by the shock wave propagating in the circumstellar ([FORMULA] cm) envelope (Weaver & Woosley 1979). The second possible counterpart of a galactic, weak CCSN is SN 1181. With the absolute luminosity of -13.8 mag at maximum (Green & Gull 1982) and half year period of visibility SN 1181 is the closest analogue of SN 1997D. Low radial velocities ([FORMULA] km s-1) of SN 1181 filaments claimed by Fesen et al. (1988) seem to strengthen this identification. If the association of SN 1181 is correct then we expect to find very low amount of newly synthesized oxygen and iron-peak elements in this supernova remnant.

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Online publication: February 9, 2000