Modeling a superluminous supernova
Superluminous supernovae can be up to 100 times brighter than normal supernovae, but there is no consensus on how such bright transients are produced. Jerkstrand et al. identified emission lines of iron in the spectrum of a superluminous supernova that appeared more than a year after the explosion. The authors explored models of several possible mechanisms, finding that only one is consistent with all the observations: a normal Type Ia supernova that interacts with a dense shell of circumstellar material. The shell must have been ejected by the progenitor star less than a century before the explosion, perhaps owing to interaction with a binary partner.
Science, this issue p. 415
Abstract
Superluminous supernovae radiate up to 100 times more energy than normal supernovae. The origin of this energy and the nature of the stellar progenitors of these transients are poorly understood. We identify neutral iron lines in the spectrum of one such supernova, SN 2006gy, and show that they require a large mass of iron (≳0.3 solar masses) expanding at 1500 kilometers per second. By modeling a standard type Ia supernova hitting a shell of circumstellar material, we produce a light curve and late-time iron-dominated spectrum that match the observations of SN 2006gy. In such a scenario, common envelope evolution of a progenitor binary system can synchronize envelope ejection and supernova explosion and may explain these bright transients.
Superluminous supernovae (SNe) are a rare type of astrophysical explosion that emit large amounts of energy, more than can be explained by standard supernova powering mechanisms. One of the first to be observed was SN 2006gy, which showed narrow hydrogen lines (supernova type IIn) indicating interaction with a circumstellar medium (CSM). SN 2006gy radiated about
A spectrum of the supernova at 394 days after explosion (5) revealed a set of emission lines around 8000 Å that could not be identified. Figure 1 shows this spectrum, after removal of light echoes (light from earlier epochs reflected by circumstellar dust) (6). By searching atomic line lists, we determined that these lines all coincide with low-excitation, strong transitions in Fe I (6).
The model also has
These lines are predicted by emission line models for slow-expanding supernova ejecta (7). They arise from the z7D multiplet of Fe I at 2.4 eV above the ground state, which is excited by thermal electron collisions at typical supernova temperatures of a few thousand kelvin. Most supernovae have, however, too little neutral iron and expansion velocities that are too high to exhibit these lines in their spectrum. In addition to these Fe I lines, the spectrum of SN 2006gy shows lines from Ca II and Fe II and is thus dominated by heavy elements, likely produced in explosive oxygen and silicon fusion. The FWHM (full-width-at-half-maximum) of these iron and calcium lines is ~1500 km s
To obtain constraints on the iron producing this emission, we calculated a grid of iron line (Fe I and Fe II) emission models with the spectral synthesis code SUMO (8), varying the iron mass, temperature, ionization, and clumping (degree of compression compared to a uniform distribution) (6). Small masses of iron [
The vertical lines show the minimum Fe masses needed to reproduce the Fe I lines at +394 days for any temperature and ionization. The blue domain outlines where ionization balance holds. The orange and red domains outline where the observed Fe II/Fe I and Fe I cluster emission ratios are reproduced, for unclumped (orange) and clumped (red) ejecta cases. The gray regime outlines where the luminosity is reproduced by the remaining amount of 56Co, if the iron comes from 56Ni/56Co decay. The regime where all constraints are fulfilled at
At +394 days after explosion, SN 2006gy was about 100 times fainter compared with previous observations at +200 days. A fundamental property of a localized CSM is that the shock will traverse the CSM on a time scale of 230 days(R/1016 cm)/(vshock/5000 km s−1), where R is the radius and vshock is the shock speed. Similar drops in brightness have been seen in other luminous type IIn supernovae (9, 10). In its second and third year after explosion, SN 2006gy became dominated by an echo with slower decay than either interaction or radioactive powering (11).
The amount of initial radioactive 56Ni needed to match the estimated luminosity of the supernova at +394 days is
Core-collapse supernovae (arising when the core of a massive star collapses to a neutron star or black hole) produce much less 56Ni, typically
Two model scenarios can explain a 56Ni mass of
We calculated model spectra for the two model scenarios with SUMO and found good agreement for both, as they have similar core structures. Figure 1 shows spectra calculated by using the type Ia explosion model W7 (17, 18), with all velocities in the hydrodynamic model reduced by a factor of 7 to mimic the slowdown due to CSM interaction (leading to higher densities at any given time). We mixed the ejecta with a few solar masses of CSM material; however, the spectrum was not sensitive to this (6). This W7+CSM model reproduces the Fe I lines, the [Ca II] 7291, 7323 Å doublet, and the only ionized iron line seen, [Fe II] 7155 Å. The Ca II triplet at 8500 to 8700 Å is underproduced, possibly because the Ca-rich region is not compact enough; higher density favors a stronger calcium triplet.
The degeneracy between type Ia and PISN models in this late (nebular) phase can be broken by considering the earlier phases of the supernova. We calculated the total amount of light emitted by SN 2006gy using all the spectral and photometric data available in the literature (1, 19, 20). We obtain 9 × 1050 erg, close to that expected in the strong interaction limit of a type Ia supernova where a large fraction of the kinetic energy of (1 to 2) × 1051 erg is converted to radiation (6). Some previous estimates of this number were a factor of 2 to 3 higher, but were based either on single-band data with an assumed bolometric correction (1), or extrapolated blackbodies with high ultraviolet(UV)/blue flux (20). Such UV/blue emission is often blocked by line opacity in supernovae, and the spectra of SN 2006gy show such behavior (6). We used the radiation hydrodynamic code SNEC (21) to calculate light curves arising when a standard Ia SN ejecta (W7), or PISN ejecta, collide with a dense H-rich CSM. The resulting light curves for the Ia case match SN 2006gy if a CSM mass of about
(A) Bolometric luminosity (emission integrated over all frequencies) and (B) velocity of the Ia ejecta (at
Inspection of the Ia-CSM hydrodynamic models shows that the ejecta are decelerated to 1500 km s−1 following interaction with a CSM with properties suitable for reproducing the light curve (Fig. 3). This matches the observed velocities of the Fe I lines at +394 days. The type Ia explosion energy, 1.3 × 1051 erg for the standard scenario (18), is accounted for by about 3 × 1050 erg still in kinetic energy at +394 days (
From the CSM extension and velocity, the CSM material must have been ejected between 10 and 200 years before the supernova explosion. A candidate scenario to explain this is common envelope evolution of a binary progenitor system, in which a white dwarf spirals into a giant or supergiant companion star. This could causally link the processes of envelope ejection and a merger with the core of the other star, producing the explosion. Such synchronization by common envelope evolution has previously been discussed in other contexts (22). The inspiral process has been shown to robustly transfer energy and angular momentum from the orbit to the common envelope, and eject most or all of this, while the orbital separation shrinks by a factor 100 or more (23, 24).
The ejection time scale in SN 2006gy matches the time scales for common envelope ejection obtained in simulations: ~10 years for red giants engulfing WDs (23), and 2 to 200 years for more-massive red supergiants (RSGs) (24). The released orbital energy for a WD of mass
A similar scenario may explain type IIa supernovae, a rare class that have spectra of type Ia at early times but later transition to type IIn (but much less luminous than SN 2006gy). One suggestion put forth is the common envelope ejection in a merger of a WD and an Asymptotic Giant Branch (AGB) star (26). Such a scenario has been criticized on the grounds that the final merger would have to occur by gravitational waves, which would take much longer than decades or centuries (27). However, the last stages of common envelope evolution are not well understood, so that conclusion may be premature.
It is possible that SN 2006gy is an extreme example of the Ia-CSM family, with higher CSM mass located closer to the supernova compared to other cases. This would be more efficient at converting kinetic energy to radiation, over a shorter time scale, leading to the extreme luminosity. It also led to strong ejecta deceleration that trapped gamma rays and produced the distinct narrow Fe lines after a few hundred days. Type IIa supernovae show longer-lasting interactions with a more extended CSM, which would not slow the expanding core sufficiently to produce a distinct signature from the inner ejecta at late times.
Other superluminous type IIn SNe such as SN 2006tf (28), SN 2008fz (29), and SN 2008am (30) share several similarities with SN 2006gy. The total radiated energy in these events is also around 1051 erg, so some may also represent a type Ia SN exploding in a massive common envelope–ejected CSM. These other supernovae were, however, much farther away, with no observable signature similar to the +394-day spectrum of SN 2006gy; attempts at late-time observations yielded either no detections or still-ongoing interaction through broad hydrogen lines (28–30).
SUPPLEMENTARY MATERIALS
science.sciencemag.org/content/367/6476/415/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 to S11
Tables S1 and S2
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