The Peculiar Type II Supernova 1997D: A Case for a Very Low [TSUP]56[/TSUP]N[CLC]i[/CLC] Mass

Abstract

SN 1997D in NGC 1536 is possibly the least luminous and energetic Type II supernova discovered to date. The entire light curve is subluminous, never reaching M V = −14.65. The radioactive tail follows the 56 Co decay slope. In the case of nearly complete trapping of the γ-rays, the 56 Ni mass derived from the tail brightness is extremely small, ∼ 0.002 M ⊙ . At discovery the spectra showed a red continuum and line velocities of the order of 1000 km s −1 . The luminosity and the photospheric expansion velocity suggest that the explosion occurred about 50 days before discovery, and that a plateau probably followed.Model light curves and spectra of the explosion of a 26 M ⊙ star successfully fit the observations. Low mass models are inconsistent with the observations. The radius of the progenitor, constrained by the prediscovery upper limits, islow explosion energy of ∼ 4 × 10 50 ergs is then required in the modeling. The strong Ba II lines in the photospheric spectra are reproduced with a solar abundance and low T eff . A scenario in which the low 56 Ni mass observed in SN 1997D is due to fall-back of material onto the collapsed remnant of the explosion of a 25-40 M ⊙ star appears to be favored over the case of the explosion of an 8-10 M ⊙ star with low 56 Ni production.

Classification of Supernovae

“…High explosion energies are required to explain the high luminosities and kinetic energies of hypernovae (see, e.g., §6) and a large mass of radioactive 56 Ni (0.3 M ⊙ ) has been measured for SN1992am [112]. At the other extreme the faint SN1997D had a considerably smaller explosion energy (∼ few × 10 50 erg) and returned to the ISM a mass of radioactive material as small as M ( 56 Ni) = 0.002 M ⊙ [122]. The discovery of similar objects (e.g., SN1999eu) indicates that faint, under energetic SNII may represent a non-negligible fraction of all core collapse SNe.…”

Optical Light Curves of Supernovae

“…At the Cerro Tololo Inter-American Observatory (CTIO) the Yale-AURA-Lisbon-Ohio (YALO) telescope has regularly provided light curves for nearby supernovae (see, e.g., [54,112]). light curves have been further contributed by the Padova group (see, e.g., [9,97,116,117] and the results of the Harvard-Smithsonian Center for Astrophysics (CfA) team [50,92]). There are additional contributions on SN1993J [8,11,27,64,74,90,120] and more recently SN1998S [32] and SN1999em [45,63].…”

“…From such measurements a rather large range of nickel masses has been derived [99,103,116]. These phases are especially interesting as they may show signatures of significant fallback of the inner explosion material onto the forming compact object, neutron star or black hole, in the explosion [6,10,122].…”

Supernova Types and Rates

“…An extreme case is that of SN 1997D (Fig. 6) which produced only 0.002 M ⊙ of 56 Ni and showed extremely low expansion velocities (Turatto et al 1998). Representative light curves of SN II.…”

“…In most cases, the remnant of the explosion is expected to be a neutron stars, but it has been argued that the unusual properties of SN 1997D may be understood if the collapsed remnant is a black hole (Turatto et al 1998).…”