Decay Of 56Co Research Articles

SN 2008S: A COOL SUPER-EDDINGTON WIND IN A SUPERNOVA IMPOSTOR

We present visual-wavelength photometry and spectroscopy of supernova SN2008S. Based on the low peak luminosity for a of M_R = -13.9 mag, photometric and spectral evolution unlike that of low-luminosity SNe, a late-time decline rate slower than 56Co decay, and slow outflow speeds of 600-1000 km/s, we conclude that SN2008S is not a true core-collapse and is probably not an electron-capture SN. Instead, we show that SN2008S more closely resembles a SN impostor event like SN1997bs, analogous to the giant eruptions of LBVs. Its total radiated energy was 1e47.8 ergs, and it may have ejected 0.05-0.2 Msun in the event. We discover an uncanny similarity between the spectrum of 2008S and that of the Galactic hypergiant IRC+10420, which is dominated by narrow H-alpha, [Ca II], and Ca II emission lines formed in an opaque wind. We propose a scenario where the vastly super-Eddington wind of SN2008S partly fails because of reduced opacity due to recombination, as suggested for IRC+10420. The range of initial masses susceptible to eruptive LBV-like mass loss was known to extend down to 20-25 Msun, but estimates for the progenitor of SN2008S (and the similar NGC300 transient) may extend this range to around 15 Msun. As such, SN2008S may have implications for the progenitor of SN1987A.

Late-Time HST Photometry of SN 1994I: Hints of Positron Annihilation Energy Deposition11Based in part on observations with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS 5-26555. This research is primarily associated with proposal GO-5777.

ABSTRACTWe present multicolor Hubble Space Telescope (HST) WFPC2 broadband observations of the Type Ic SN 1994I obtained ∼280 d after maximum light. We measure the brightness of the SN and, relying on the detailed spectroscopic database of SN 1994I, we transform the ground-based photometry obtained at early times to the HST photometric system, deriving light curves for the WFPC2 F439W, F555W, F675W, and F814W passbands that extend from 7 days before to 280 days after maximum. We use the multicolor photometry to build a quasi-bolometric light curve of SN 1994I, and compare it with similarly constructed light curves of other supernovae. In doing so, we propose and test a scaling in energy and time that allows for a more meaningful comparison of the exponential tails of different events. Through comparison with models, we find that the late-time light curve of SN 1994I is consistent with that of spherically symmetric ejecta in homologous expansion, for which the ability to trap the γ-rays produced by the radioactive decay of 56Co diminishes roughly as the inverse of time squared. We also find that by the time of the HST photometry, the light curve was significantly energized by the annihilation of positrons.

Determination of emission probabilities of gamma photons in the decay of 56Co

The emission probabilities of gamma photons in the decay of 56Co were determined at Czech Metrology Institute (CMI) by means of an HPGe detector. This detector was calibrated experimentally and by MCNP-computation in the energy range from 40 to 2754 keV for a point source geometry and source-to-detector distance of 25 cm. Experimental and computed peak and total efficiencies were compared and calibration curves were determined. Full-peak efficiencies were calculated for all 56Co γ-ray energies, and were used to calculate the emission probabilities. A set of point sources was prepared from a 56Co solution. The solution was standardized using the 4π β– γ coincidence method, and an ampoule was sent to international reference system for activity measurement of γ-ray emitting radionuclides (SIR). Each point source was measured with the HPGe detector at a source-to-detector distance of 25 cm. Coincidence emission probabilities of all the gamma photons were calculated and used to determine the summing correction factors.

SN 2006bp: Probing the Shock Breakout of a Type II‐P Supernova

HET optical spectroscopy and unfiltered ROTSE-III photometry spanning the first 11 months since explosion of the Type II-P SN 2006bp are presented. Flux limits from the days before discovery combined with the initial rapid brightening suggest the supernova was first detected just hours after shock breakout. Optical spectra obtained about 2 days after breakout exhibit narrow emission lines corresponding to HeII 4200, HeII 4686, and CIV 5805 in the rest frame, and these features persist in a second observation obtained 5 hours later; however, these emission lines are not detected the following night nor in subsequent observations. We suggest that these lines emanate from material close to the explosion site, possibly in the outer layers of the progenitor that have been ionized by the high energy photons released at shock breakout. A P-Cygni profile is observed around 4450 A in the +2 and +3 day spectra. Previous studies have attributed this feature to high velocity H-beta, but we discuss the possibility that this profile is instead due to HeII 4687. Further HET observations (14 nights in total) covering the spectral evolution across the photometric plateau up to 73 days after breakout and during the nebular phase around day +340 are presented, and expansion velocities are derived for key features. The measured decay slope for the unfiltered light curve is 0.0073 +/- 0.0004 mag/day between days +121 and +335, which is significantly slower than the decay of rate 56Co. We combine our HET measurements with published X-ray, UV, and optical data to obtain a quasi-bolometric light curve through day +60. We see a slow cooling over the first 25 days, but no sign of an early sharp peak; any such feature from the shock breakout must have lasted less than ~1 day.[ABRIDGED]

Monte Carlo study of detector concepts for the MAX Laue lens gamma-ray telescope

MAX is a proposed Laue lens gamma-ray telescope taking advantage of Bragg diffraction in crystals to concentrate incident photons onto a distant detector. The Laue lens and the detector are carried by two separate satellites flying in formation. Significant effort is being devoted to studying different types of crystals that may be suitable for focusing gamma rays in two 100 keV wide energy bands centered on two lines which constitute the prime astrophysical interest of the MAX mission: the 511 keV positron annihilation line, and the broadened 847 keV line from the decay of 56Co copiously produced in Type Ia supernovae. However, to optimize the performance of MAX, it is also necessary to optimize the detector used to collect the source photons concentrated by the lens. We address this need by applying proven Monte Carlo and event reconstruction packages to predict the performance of MAX for three different Ge detector concepts: a standard coaxial detector, a stack of segmented detectors, and a Compton camera consisting of a stack of strip detectors. Each of these exhibits distinct advantages and disadvantages regarding fundamental instrumental characteristics such as detection efficiency or background rejection, which ultimately determine achievable sensitivities. We conclude that the Compton camera is the most promising detector for MAX in particular, and for Laue lens gamma-ray telecopes in general.

Delayed X‐ and Gamma‐Ray Line Emission from Solar Flare Radioactivity

We have studied the radioactive line emission expected from solar active regions after large flares, following the production of long-lived radioisotopes by nuclear interactions of flare-accelerated ions. This delayed X- and gamma-ray line emission can provide unique information on the accelerated particle composition and energy spectrum, as well as on mixing processes in the solar atmosphere. Total cross sections for the formation of the main radioisotopes by proton, 3-He and alpha-particle reactions are evaluated from available data combined with nuclear reaction theory. Thick-target radioisotope yields are provided in tabular form, which can be used to predict fluxes of all of the major delayed lines at any time after a gamma-ray flare. The brightest delayed line for days after the flare is found to be the 511 keV positron-electron annihilation line resulting from the decay of several beta+ radioisotopes. After ~2 days however, the flux of the 511 keV line can become lower than that of the 846.8 keV line from the decay of 56-Co. Our study has revealed other delayed gamma-ray lines that appear to be promising for detection, e.g. at 1434 keV from the radioactivity of both the isomer and the ground state of 52-Mn, 1332 and 1792 keV from 60-Cu, and 931.1 keV from 55-Co. The strongest delayed X-ray line is found to be the Co Kalpha at 6.92 keV, which is produced from both the decay of the isomer of 58-Co by the conversion of a K-shell electron and the decay of 57-Ni by orbital electron capture. Prospects for observation of these lines with RHESSI or future space instruments are discussed.

Growth of Carbon Grains in Supernova Ejecta

We present a chemical reaction network that describes the condensation chemistry of carbon dust grains in an expanding supernova shell. We assume that the region of interest consists solely of gaseous free carbon and oxygen atoms and that the buildup of CO is counteracted by the radioactive decay of 56Co, which breaks up the CO molecule and allows C to condense into solids. Our chemical model takes C to first form linear chains, which, at some critical length, transition into ringed isomers. These isomers are more resistant to oxidation than linear chains. These ringed isomers form the nuclei for the growth of larger carbon solids. The effect of the disruption of CO on grain growth is displayed, leading to a rethinking of previous assumptions on the importance of CO disruption. How the abundance and size distribution of grains are affected by various parameters of the ejecta is also studied, providing insight into the possible sites of grain condensation.

Monte Carlo techniques for time-dependent radiative transfer in 3-D supernovae

Monte Carlo techniques based on indivisible energy packets are described for computing light curves and spectra for 3-D supernovae. The radiative transfer is time-dependent and includes all effects of O(v/c). Monte Carlo quantization is achieved by discretizing the initial distribution of 56Ni into radioactive pellets. Each pellet decays with the emission of a single energy packet comprising gamma-ray photons representing one line from either the 56Ni or the 56Co decay spectrum. Subsequently, these energy packets propagate through the homologously-expanding ejecta with appropriate changes in the nature of their contained energy as they undergo Compton scatterings and pure absorptions. The 3-D code is tested by applying it to a spherically-symmetric SN in which the transfer of optical radiation is treated with a grey absorption coefficient. This 1-D problem is separately solved using Castor's co-moving frame moment equations. Satisfactory agreement is obtained. The Monte Carlo code is a platform onto which more advanced treatments of the interactions of matter and radiation can be added. Some of these have already been developed and tested in previous papers and are summarized here.

Raising the Dead: Clues to Type Ia Supernova Physics from the Remnant 0509−67.5

We present Chandra X-ray observations of the young supernova remnant (SNR) 0509-67.5 in the Large Magellanic Cloud (LMC), believed to be the product of a Type Ia supernova (SN Ia). The remnant is very round in shape, with a distinct clumpy, shell-like structure that extends to an average radius of 148 (3.6 pc) in the X-ray band. Our Chandra data reveal the remnant to be rich in silicon, sulfur, and iron. The yields of our fits to the global spectrum confirm that 0509-67.5 is the remnant of an SN Ia and show a clear preference for delayed-detonation explosion models for SNe Ia. The Chandra spectra extracted from radial rings are in general quite similar; the most significant variation with radius is a drop in the equivalent widths of the strong emission lines right at the edge of the remnant. We study the spectrum of the single brightest isolated knot in the remnant and find that it is enhanced in iron by a factor of roughly 2 relative to the global remnant abundances. This feature, along with similar knots seen in Tycho's SNR, argues for the presence of modest small-scale composition inhomogeneities in SNe Ia. The presence of both Si and Fe, with abundance ratios that vary from knot to knot, indicates that these came from the transition region between the Si- and Fe-rich zones in the exploded star, possibly as a result of energy input to the ejecta at late times because of the radioactive decay of 56Ni and 56Co. Two cases for the continuum emission from the global spectrum were modeled: one in which the continuum is dominated by hydrogen thermal bremsstrahlung radiation, the other in which the continuum arises from nonthermal synchrotron radiation. The former case requires a relatively large value for the ambient density (~1 cm-3). Another estimate of the ambient density comes from using the shell structure of the remnant in the context of dynamical models. This requires a much lower value for the density (<0.05 cm-3) that is more consistent with other evidence known about 0509-67.5. We therefore conclude that the bulk of the continuum emission from 0509-67.5 has a nonthermal origin.

Hypernovae/Gamma-Ray Bursts in the Galactic Center as Possible Sources of Galactic Positrons

The observation of a strong and extended positron-electron line annihilation emission in the central regions of the Galaxy by the SPectrometer on the International Gamma-Ray Astrophysical Laboratory (INTEGRAL/SPI), consistent with the Galactic bulge geometry and without any counterpart in the gamma-ray range, neither at high energy nor in the 1809 keV 26Al decay line, is challenging. Leaving aside the geometrical question, we address the problem of the adequate positron sources, showing the potentiality of a new category of Type Ic supernovae (SNe Ic), exemplified by SN 2003dh, that is associated with a gamma-ray burst (GRB). This kind of supernova/hypernova/GRB event is interpreted as the result of a bipolar Wolf-Rayet explosion, which produces a large amount of 56Ni and ejects it at high velocity along the rotation axis. The bulk of positrons resulting from 56Co decay escapes in the surrounding medium as a result of the rapid thinning of the ejecta in the polar direction. We show that a rate of about 0.02 SN 2003dh-like events per century in the central region of the Galaxy is sufficient to explain the positron flux detected by INTEGRAL/SPI. In order to explain this flux by SN Ia events alone, a rate of 0.5 per century is necessary, much higher than indicated by Galactic evolutionary models applied to the bulge. Further observations of late light curves of SNe Ia and SNe Ic in the bulge of spiral galaxies, together with three-dimensional hydrodynamic calculations of anisotropic ejections of 56Ni in SN Ic/GRB events, will allow us to estimate the separate contributions of SNe Ia and SNe Ic to positron injection.

Modern tools for precise γ-ray spectrometry with Ge detectors

Over the past few years, several data evaluations and method developments have improved the capability to do precise γ-ray spectrometry and some of these are described here. For energy calibration, an evaluation of a large set of consistent γ-ray energies, generally with uncertainties of less than 10ppm, has been published. A program to produce high-quality, well-documented decay data is the Decay Data Evaluation Project (DDEP) which is an informal collaboration of evaluators from six countries with the goal of producing evaluations of the decay data for about 250 nuclides of interest in various applications. As an aid in more precise efficiency calibration, new measurements have improved the relative emission probabilities for the strong γ rays from the decay of 56Co and 66Ga which extend up to 4.8MeV. Also, new studies have demonstrated the use of Monte Carlo calculations along with measured data in the determination of efficiency curves at a precision approaching 0.1% over an extended energy range. A Catalogue of new γ-ray spectra, the associated decay schemes, and the energies and emission probabilities is being made available on the Internet.

A Test for the Nature of the Type Ia Supernova Explosion Mechanism

Currently, popular models for Type Ia supernovae (SNe Ia) fall into two general classes. The —rst comprises explosions of nearly pure carbon/oxygen (C/O) white dwarfs (WDs) at the Chandrasekhar limit which ignite near their centers. The second consists of lower mass C/O cores which are ignited by the detonation of an accreted surface helium layer. Explosions of the latter type produce copious Fe, Co, and Ni Ka emission from 56Ni and 56Co decay in the detonated surface layers, emission which is much weaker from Chandrasekhar-mass models. The presence of this emission provides a simple and unambiguous discriminant between these two models for SNe Ia. Both mechanisms may produce 0.1¨0.6 M _ of 56Ni, making them bright c-ray line emitters. The time to maximum brightness of 56Ni decay lines is distinctly shorter in the class of model (D15 days) than in the model (D30 days), making M M ch M ch c-ray line evolution another direct test of the explosion mechanism. It should just be possible to detect K-shell emission from a explosion from SNe Ia as far away as the Virgo cluster with the XMM sub-M ch observatory. A 1¨ 2m 2 X-ray telescope such as the proposed Constellation-X observatory could observe Ka emission from SNe Ia in the Virgo cluster, providing not just a detection but highM M ch accuracy —ux and kinematic information. S

The Properties of Supernova 1997cy Associated with GRB 970514.

The extraordinary SN 1997cy associated with GRB 970514 has been observed photometrically and spectroscopically for nearly 2 yr. At the time of discovery, SN 1997cy was the brightest supernova (SN) ever observed (MV</=-20.1, vhel=19,140 km s-1, H0=65 km s-1 Mpc-1). Up to the last available observations (600 days after the gamma-ray burst), the total time-integrated flux was equal to or larger than that expected from the complete thermalization of the gamma-rays produced by 2.3 M middle dot in circle of 56Co. However, starting already on day 60 the luminosity decline is slower than the 56Co decay rate, indicating that the SN ejecta was interacting with circumstellar material (CSM). The interaction appeared to weaken around day 550. The spectra of SN 1997cy are dominated at all epochs by Halpha emission, which shows at least three components of different widths, as in SN 1988Z. Several other lines with different widths are also visible, especially at early epochs. The entire light curve of SN 1997cy is reproduced by a model of the interaction of the very energetic (E=3x1052 ergs) ejecta of a massive star (25 M middle dot in circle) with the CSM, with some contribution from radioactive decays. The CSM could have been ejected with a mass-loss rate of M&d2; approximately 4x10-4 M middle dot in circle yr-1 as the progenitor star evolved from a blue to a red supergiant about 104 yr before the explosion. The lack of oxygen and magnesium lines in the spectra at nebular phases poses a problem for models requiring high-mass progenitors. The possibility that most of the core material of the progenitor has fallen onto a massive black hole so that the reverse shock dies at the inner edge of the H/He envelope is discussed.

Hypernovae: SNe 1997ef, 1998bw, and 1997cy

We discuss the properties of the very energetic Type Ic supernovae (SNe Ic) 1998bw and 1997ef, and of Type IIn supernova (SN IIn) 1997cy. SNe Ic 1998bw and 1997ef are characterized by their large luminosity and very broad spectral features. Their observed properties can be explained if they are very energetic SN explosions (EK ≳ 1 × 1052 erg), originating probably from the core collapse of the bare C+O cores of massive stars (~ 30–40M⊙). At late times, both the light curve and the spectra suggest that the explosion may have been asymmetric; this may help us understand the claimed connection with GRBs. Type IIn SN 1997cy is even more luminous than SN 998bw, and the light curve declines more slowly than the 56Co decay. We model such a light curve with circumstellar interaction, which requires the explosion energy of ~ 5 × 1052 erg. Because these kinetic energies of explosion are much larger than in normal core-collapse SNe, we call objects like these SNe “hypernovae”.

Nucleosynthesis and gamma ray-line astronomy

The most energetic part of the electromagnetic spectrum bears the purest clues to the synthesis of atomic nuclei in the universe. The decay of radioactive species, synthesized in stellar environments and ejected into the interstellar medium, gives rise to specific gamma ray lines. The observations gathered up to now show evidence for radioactivities throughout the galactic disk, in young supernova remnants (Cas A, Vela), and in nearby extragalactic supernovae (SN 1987A, SN 1991T and SN1998bu), in the form of specific gamma ray lines resulting, respectively, from the radioactive decay of 26Al, 44Ti and 56Co. The various astrophysical sites of thermal nucleosynthesis of the radioactive nuclei were discussed: AGB and Wolf-Rayet stars, novae, and type Ia and type II supernovae. Nuclear excitations by fast particles also produce gamma ray lines which have been observed in great detail from solar flares, and more hypothetically from active star forming regions where massive supernovae and WR stars abound. This non thermal process and its nucleosynthetic consequences was reviewed. The 511 keV line arising from e+ + e- annihilation also provides important information on explosive nucleosynthesis, as well as on the nature of the interstellar medium where the positrons annihilate. INTEGRAL, the main mission devoted to high resolution nuclear spectroscopy, should lead to important progress in this field.

Bolometric Light Curves of Supernovae and Postexplosion Magnetic Fields

The various effects leading to diversity in the bolometric light curves of supernovae are examined; nucleosynthesis, kinematic differences, ejected mass, degree of mixing, and configuration and intensity of the magnetic field are discussed. In Type Ia supernovae, a departure in the bolometric light curve from the full-trapping decline of 56Co can occur within the 2.5 years after the explosion, depending on the evolutionary path followed by the white dwarf (WD) during the accretion phase. If convection has developed in the WD core during the presupernova evolution, starting several thousand years before the explosion, a tangled magnetic field close to the equipartition value should have grown in the WD. Such an intense magnetic field would confine positrons where they originate from the 56Co decays and preclude a strong departure from the full-trapping decline as the supernova expands. This situation is expected to occur in C + O Chandrasekhar WDs as opposed to edge-lit detonated sub-Chandrasekhar WDs. If the preexplosion magnetic field of the WD is less intense than 105-108 G, a lack of confinement of the positrons emitted in the 56Co decay and a departure from full trapping of their energy would occur. The time at which the departure takes place can provide estimates of the original magnetic field of the WD, its configuration, and also of the mass of the supernova ejecta. In SN 1991BG, the bolometric light curve suggests an absence of a significant tangled magnetic field: its intensity is estimated to be lower than 103 G. Chandrasekhar-mass models do not reproduce the bolometric light curve of this supernova. For SN 1972E, on the contrary, there is evidence for a tangled configuration of the magnetic field and its light curve is well reproduced by a Chandrasekhar WD explosion. A comparison is made for the diagram of absolute magnitude and rate of decline in Type Ia supernovae coming from different explosion mechanisms. The effects of mixing and ejected mass in the bolometric light curve of Type Ib, Ic supernovae are also discussed.

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

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 MV=-14.65. The radioactive tail follows the 56Co decay slope. In the case of a nearly complete trapping of the γ-rays, the 56Ni mass derived from the tail brightness is extremely small, ~0.002 M☉. At discovery, the spectra showed a red continuum and line velocities on 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 fitted the observations. Low-mass models are inconsistent with the observations. The radius of the progenitor, constrained by the prediscovery upper limits, is R0 300 R☉. A low explosion energy of ~4 × 1050 ergs is then required in the modeling. The strong Ba II lines in the photospheric spectra are reproduced with a solar abundance and low Teff. A scenario in which the low 56Ni mass observed in SN 1997D is due to fallback 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 56Ni production.

The 1.05- m feature in the spectrum of the Type Ia supernova 1994D: He in SNe Ia?

A P Cygni profile with absorption at 1.05 μm was observed in three pre-maximum J-band spectra of the Type Ia supernova (SN) 1994D. The feature was not present in two post-maximum spectra. The line was attributed to He 10830 ... or MgII 10926 ..., based on a local thermodynamic equilibrium (LTE) treatment. The detection of He in the ejecta of a SN Ia would be useful for determining the pre-SN evolution and the explosion mechanism of SNe Ia. In this paper, synthetic spectra are presented for both the He and Mg models. The population of the He levels has been computed in non-local thermodynamic equilibrium (NLTE), including non-thermal excitation and ionization effects resulting from the deposition of γ-rays from the decay of 56Ni and 56Co. The J-band feature in the pre-maximum spectra can be reproduced either assuming the presence of a narrow shell, between 10000 and 12500 km s−1, containing about 0.01 M⊙ of He, or increasing the abundance of Mg by about a factor of 5 with respect to the W7 value, implying a Mg mass of about 0.08 M⊙ above 10000 km s−1. Both models are in good agreement with the optical spectrum. In particular, a strong HeI 10830-... line does not imply a strong 5876-... line, because the departure coefficients of the 2p and 2s levels of HeI differ by about an order of magnitude. Unfortunately, neither model is able to reproduce the sudden disappearance of the J-band feature in the post-maximum spectra. Possible explanations are discussed.

Late Spectral Evolution of SN 1987A. I. Temperature and Ionization

The temperature and ionization of SN 1987A is modeled between 200 and 2000 days in its nebular phase, using a time-dependent model. We include all important elements, as well as the primary composition zones in the supernova. The energy input is provided by radioactive decay of Co-56, Co-57, and Ti-44. The thermalization of the resulting gamma-rays and positrons is calculated by solving the Spencer-Fano equation. Both the ionization and the individual level populations are calculated time-dependently. Adiabatic cooling is included in the energy equation. Charge transfer is important for determining the ionization and is included with available and estimated rates. Full, multilevel atoms are used for the observationally important ions. As input models to the calculations we use explosion models for SN 1987A calculated by Woosley et al and Nomoto et al. The most important result in this paper refers to the evolution of the temperature and ionization of the various abundance zones. The metal-rich core undergoes a thermal instability, often referred to as the IR-catastrophe, at 600 - 1000 days. The hydrogen-rich zones evolve adiabatically after 500 - 800 days, while in the helium region both adiabatic cooling and line cooling are of equal importance after ~1000 days. Freeze-out of the recombination is important in the hydrogen and helium zones. Concomitant with the IR-catastrophe, the bulk of the emission shifts from optical and near-IR lines to the mid- and far-IR. After the IR-catastrophe, the cooling is mainly due to far-IR lines and adiabatic expansion. Dust cooling is likely to be important in the zones where dust forms. We find that the dust condensation temperatures occur later than ~500 days in the oxygen-rich zones, and the most favorable zone for dust condensation is the iron core.

A Very Low Mass of56Ni in the Ejecta of SN 1994W

We present spectroscopic and photometric observations of the luminous narrow-line Type IIP (plateau) supernova 1994W. After the plateau phase (t 120 days), the light curve dropped by ~3.5 mag in V in only 12 days. Between 125 and 197 days after explosion, the supernova faded substantially faster than the decay rate of 56Co, and by day 197 it was 3.6 mag less luminous in R than SN 1987A. The low R luminosity could indicate 0.0026+ 0.0017−0.0011 M☉ of 56Ni ejected at the explosion. The emission between 125 and 197 days would in this case be dominated by diffusion of emission from the mantle region, or by an additional power source, presumably circumstellar interaction. Alternatively, the late light curve could have been dominated by 56Co decay. In this case, the mass of the ejected 56Ni would have been 0.015+ 0.012−0.008 M☉, and the rapid fading between 125 and 197 days most likely due to dust formation. Although this value of the mass is higher than in the first case, it is still lower than estimated for any previous Type II supernova. Only progenitors with MZAMS ~ 8-10 M☉ and MZAMS 25 M☉ are expected to eject such low masses of 56Ni. If MZAMS ~ 8-10 M☉, the plateau phase indicates a low explosion energy, while for a progenitor with MZAMS 25 M☉, the energy can be the canonical ~1051 ergs. As SN 1994W was unusually luminous, the low-mass explosion may require an uncomfortably high efficiency in converting explosion energy into radiation. This favors a MZAMS 25 M☉ progenitor. The supernova's narrow (~1000 km s-1) emission lines were excited by the hot supernova spectrum, rather than by a circumstellar shock. The thin shell from which the lines originated was most likely accelerated by radiation from the supernova.