Cecilia Kozma

Late Spectral Evolution of SN 1987A. II. Line Emission

Using the temperature and ionization calculated in our previous paper, we model the spectral evolution of SN 1987A. We find that the temperature evolution is directly reflected in the time evolution of the lines. In particular, the IR catastrophe is seen in the metal lines as a transition from thermal to nonthermal excitation, seen most clearly in the [O I] λλ6300, 6364 lines. The good agreement with observations clearly confirms the predicted optical to IR transition. Because the line emissivity is independent of temperature in the nonthermal phase, this phase has a strong potential for estimating the total mass of the most abundant elements. The hydrogen lines arise as a result of recombinations following ionizations in the Balmer continuum during the first ~500 days and later as a result of nonthermal ionizations. The distribution of the different zones, and therefore the gamma-ray deposition, is determined from the line profiles of the most important lines, where possible. We find that hydrogen extends into the core to 700 km s-1. The hydrogen envelope has a density profile close to ρ ∝ V-2 from 2000-5000 km s-1. The total mass of hydrogen-rich gas is ~7.7 M☉, of which ~2.2 M☉ is mixed within 2000 km s-1. The helium mass derived from the line fluxes is sensitive to assumptions about the degree of redistribution in the line. The mass of the helium-dominated zone is consistent with ~1.9 M☉, with a further ~3.9 M☉ of helium residing in the hydrogen component. Most of the oxygen-rich gas is confined to 400-2000 km s-1, with a total mass of ~1.9 M☉. Because of uncertainties in the modeling of the nonthermal excitation of the [O I] lines, the uncertainty in the estimated oxygen mass is considerable. Masses of nitrogen, neon, magnesium, iron, and nickel are also estimated. The dominant contribution to the line luminosity often originates in a different zone from that in which most of the newly synthesized material resides. This applies to, e.g., carbon, calcium, and iron. The [C I] lines, arising mainly in the helium zone, indicate a substantially lower abundance of carbon mixed with helium than given by stellar evolution models, and a more extended zone with CNO-processed gas is also indicated. The [Fe II] lines have in most phases a strong contribution from primordial iron, and at t 600-800 days this component dominates the [Fe II] lines. The wings of the [Fe II] lines may therefore come from primordial iron rather than synthesized iron mixed to high velocity. Lines from ions with low ionization potentials indicate that the UV field below at least 1600 A is severely quenched by dust absorption and resonance scattering.

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

The temperature and ionization of SN 1987A are modeled time-dependently in its nebular phase between 200 and 2000 days. We include all important elements, as well as the primary composition zones in the supernova. The energy input is provided by radioactive decay of 56Co,57Co, and 44Ti. 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 for 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 concerns 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. Freezeout 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 that the most favorable zone for dust condensation is the iron core. The uncertainties introduced by the (in some cases) unknown charge transfer rates are discussed. Especially for ions with low abundances, differences can be substantial.

Hubble Space Telescope and Ground-based Observations of SN 1993J and SN 1998S: CNO Processing in the Progenitors

Ground-based and Hubble Space Telescope observations are presented for SN 1993J and SN 1998S. SN 1998S shows strong, relatively narrow circumstellar emission lines of N III-V and C III-IV, as well as broad lines from the ejecta. Both the broad ultraviolet and optical lines in SN 1998S indicate an expansion velocity of ~7000 km s-1. The broad emission components of Ly? and Mg II are strongly asymmetrical after day 72 past the explosion and differ in shape from H?. Different models based on dust extinction from dust in the ejecta or shock region, in combination with H? from a circumstellar torus, are discussed. It is concluded, however, that the double-peaked line profiles are more likely to arise as a result of optical depth effects in the narrow, cool, dense shell behind the reverse shock than in a torus-like region. The ultraviolet lines of SN 1993J are broad, with a boxlike shape, coming from the ejecta and a cool, dense shell. The shapes of the lines are well fitted by a shell with inner velocity ~7000 km s-1 and outer velocity ~10,000 km s-1. For both SN 1993J and SN 1998S a strong nitrogen enrichment is found, with N/C ? 12.4 in SN 1993J and N/C ? 6.0 in SN 1998S. From a compilation of all supernovae with determined CNO ratios, we discuss the implications of these observations for the structure of the progenitors of Type II supernovae.

The normal Type Ia SN 2003hv out to very late phases

Aims. We study a thermonuclear supernova (SN), emphasizing very late phases. Methods. An extensive dataset for SN 2003hv that covers the flux evolution from maximum light to day +786 is presented. This includes 82 epochs of optical imaging, 24 epochs of near-infrared (NIR) imaging, and 10 epochs of optical spectroscopy. These data are combined with published nebular-phase IR spectra, and the observations are compared to model light curves and synthetic nebular spectra. Results. SN 2003hv is a normal Type Ia supernova (SN Ia) with photometric and spectroscopic properties consistent with its rarely observed B-band decline-rate parameter, Δm15(B) = 1.61 ± 0.02. The blueshift of the most isolated [Fe ii] lines in the nebular-phase optical spectrum appears consistent with those observed in the IR at similar epochs. At late times there is a prevalent color evolution from the optical toward the NIR bands. We present the latest-ever detection of a SN Ia in the NIR in Hubble Space Telescope images. The study of the ultraviolet/optical/infrared (UVOIR) light curve reveals that a substantial fraction of the flux is “missing” at late times. Between 300 and 700 days past maximum brightness, the UVOIR light curve declines linearly following the decay of radioactive 56 Co, assuming full and instantaneous positron trapping. At 700 days we detect a possible slowdown of the decline in optical-bands, mainly in the V-band. Conclusions. The data are incompatible with a dramatic infrared catastrophe (IRC). However, the idea that an IRC occurred in the densest regions before 350 days can explain the missing flux from the UVOIR wavelengths and the flat-topped profiles in the NIR. We argue that such a scenario is possible if the ejecta are clumpy. The observations suggest that positrons are most likely trapped in the ejecta.

Gamma-ray deposition and nonthermal excitation in supernovae

The γ-ray deposition in supernovae is calculated by solving the Spencer-Fano equation. Ionization, excitation, and heating rates are presented for the different chemical composition zones of a core collapse supernova, as well as for a solar composition applicable to for example, active galactic nuclei. The thermalization in pure helium, oxygen, and iron plasmas is also discussed. The latter is of particular interest for Type Ia supernovae. Convenient analytical expressions are given to facilitate the use of these results for the calculation of the physical conditions and emission from supernovae

The late-time light curve of the type Ia supernova 2000cx

We have conducted a systematic and comprehensive monitoring programme of the type Ia supernova 2000cx at late phases using the VLT and HST. The VLT observations cover phases 360 to 480 days past maximum brightness and include photometry in the BVRIJH bands, together with a single epoch in each of U and Ks. While the optical bands decay by about 1.4 mag per 100 days, we find that the near-IR magnitudes stay virtually constant during the observed period. This means that the importance of the near-IR to the bolometric light curve increases with time. The finding is also in agreement with our detailed modeling of a type Ia supernova in the nebular phase. In these models, the increased importance of the near-IR is a temperature effect. We note that this complicates late-time studies where often only the V band is well monitored. In particular, it is not correct to assume that any optical band follows the bolometric light curve at these phases, and any conclusions based on such assumptions, e.g., regarding positron-escape, must be regarded as premature. A very simple model where all positrons are trapped can reasonably well account for the observations. The nickel mass deduced from the positron tail of this light curve is lower than found from the peak brightness, providing an estimate of the fraction of late-time emission that is outside of the observed wavelength range. Our detailed models show the signature of an infrared catastrophe at these epochs, which is not supported by the observations.

SN 1998bw at Late Phases

We present observations of the peculiar supernova SN 1998bw, which was probably associated with GRB 980425. The photometric and spectroscopic evolution is monitored up to 500 days past explosion. We also present modeling based on spherically symmetric, massive progenitor models and very energetic explosions. The models allow line identification and clearly show the importance of mixing. From the late light curves, we estimate that ~0.3-0.9 M☉ of ejected 56Ni is required to power the supernova.

The freeze-out phase of SN 1987A - Implications for the light curve

After ∼800 days, time-dependent effects due to long recombination and cooling times lead to a frozen-in structure of the ejecta of SN 1987A. The result is a higher bolometric luminosity, compared to models where the emitted luminosity is equal to the instantaneous energy input. Good agreement with the observed light curve of SN 1987A is obtained with an initial 57 Ni/ 56 Ni ratio 2 times the solar 57 Fe/ 56 Fe ratio, while steady state models require a factor of 2 more. Consistency with both the 57 Co mass from IR line observations, X-ray, and γ-ray observations, and from models of the nucleosynthesis can thus be obtained

Supernova 1998bw - The final phases ⋆

The probable association with GRB 980425 immediately put SN 1998bw at the forefront of supernova research. Here, we present revised late-time BV RI light curves of the supernova, based on template images taken at the VLT. To follow the supernova to the very last observable phases we have used HST/STIS. Deep images taken in June and November 2000 are compared to images taken in August 2001. The identification of the supernova is firmly established. This allows us to measure the light curve to � 1000 days past explosion. The main features are a rapid decline up to more than 500 days after explosion, with no sign of complete positron trapping from the 56 Co decay. Thereafter, the light curve flattens out significantly. One possible explanation is powering by more long lived radioactive isotopes, if they are abundantly formed in this energetic supernova.

X-ray illumination of the ejecta of supernova 1987A

When a massive star explodes as a supernova, substantial amounts of radioactive elements—primarily 56Ni, 57Ni and 44Ti—are produced. After the initial flash of light from shock heating, the fading light emitted by the supernova is due to the decay of these elements. However, after decades, the energy powering a supernova remnant comes from the shock interaction between the ejecta and the surrounding medium. The transition to this phase has hitherto not been observed: supernovae occur too infrequently in the Milky Way to provide a young example, and extragalactic supernovae are generally too faint and too small. Here we report observations that show this transition in the supernova SN 1987A in the Large Magellanic Cloud. From 1994 to 2001, the ejecta faded owing to radioactive decay of 44Ti as predicted. Then the flux started to increase, more than doubling by the end of 2009. We show that this increase is the result of heat deposited by X-rays produced as the ejecta interacts with the surrounding material. In time, the X-rays will penetrate farther into the ejecta, enabling us to analyse the structure and chemistry of the vanished star.