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Register a new userThe late-time light curve of the type Ia supernova SN 2011fe
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Added by
Georgios Dimitriadis
Publication date
2017
fields
Physics
and research's language is
English
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We present late-time optical $R$-band imaging data from the Palomar Transient Factory (PTF) for the nearby type Ia supernova SN 2011fe. The stacked PTF light curve provides densely sampled coverage down to $Rsimeq22$ mag over 200 to 620 days past explosion. Combining with literature data, we estimate the pseudo-bolometric light curve for this event from 200 to 1600 days after explosion, and constrain the likely near-infrared contribution. This light curve shows a smooth decline consistent with radioactive decay, except over ~450 to ~600 days where the light curve appears to decrease faster than expected based on the radioactive isotopes presumed to be present, before flattening at around 600 days. We model the 200-1600d pseudo-bolometric light curve with the luminosity generated by the radioactive decay chains of $^{56}$Ni, $^{57}$Ni and $^{55}$Co, and find it is not consistent with models that have full positron trapping and no infrared catastrophe (IRC); some additional energy escape other than optical/near-IR photons is required. However, the light curve is consistent with models that allow for positron escape (reaching 75% by day 500) and/or an IRC (with 85% of the flux emerging in non-optical wavelengths by day 600). The presence of the $^{57}$Ni decay chain is robustly detected, but the $^{55}$Co decay chain is not formally required, with an upper mass limit estimated at 0.014 M$_{odot}$. The measurement of the $^{57}$Ni/$^{56}$Ni mass ratio is subject to significant systematic uncertainties, but all of our fits require a high ratio >0.031 (>1.3 in solar abundances).
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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 2011fe is the nearest supernova of Type Ia (SN Ia) discovered in the modern multi-wavelength telescope era, and it also represents the earliest discovery of a SN Ia to date. As a normal SN Ia, SN 2011fe provides an excellent opportunity to decipher long-standing puzzles about the nature of SNe Ia. In this review, we summarize the extensive suite of panchromatic data on SN 2011fe, and gather interpretations of these data to answer four key questions: 1) What explodes in a SN Ia? 2) How does it explode? 3) What is the progenitor of SN 2011fe? and 4) How accurate are SNe Ia as standardizeable candles? Most aspects of SN 2011fe are consistent with the canonical picture of a massive CO white dwarf undergoing a deflagration-to-detonation transition. However, there is minimal evidence for a non-degenerate companion star, so SN 2011fe may have marked the merger of two white dwarfs.
A series of optical and one near-infrared nebular spectra covering the first year of the Type Ia supernova SN 2011fe are presented and modelled. The density profile that proved best for the early optical/ultraviolet spectra, rho-11fe, was extended to lower velocities to include the regions that emit at nebular epochs. Model rho-11fe is intermediate between the fast deflagration model W7 and a low-energy delayed-detonation. Good fits to the nebular spectra are obtained if the innermost ejecta are dominated by neutron-rich, stable Fe-group species, which contribute to cooling but not to heating. The correct thermal balance can thus be reached for the strongest [FeII] and [FeIII] lines to be reproduced with the observed ratio. The 56Ni mass thus obtained is 0.47 +/- 0.05 Mo. The bulk of 56Ni has an outermost velocity of ~8500 km/s. The mass of stable iron is 0.23 +/- 0.03 Mo. Stable Ni has low abundance, ~10^{-2} Mo. This is sufficient to reproduce an observed emission line near 7400 A. A sub-Chandrasekhar explosion model with mass 1.02 Mo and no central stable Fe does not reproduce the observed line ratios. A mock model where neutron-rich Fe-group species are located above 56Ni following recent suggestions is also shown to yield spectra that are less compatible with the observations. The densities and abundances in the inner layers obtained from the nebular analysis, combined with those of the outer layers previously obtained, are used to compute a synthetic bolometric light curve, which compares favourably with the light curve of SN 2011fe.
We report unique EVLA observations of SN 2011fe representing the most sensitive radio study of a Type Ia supernova to date. Our data place direct constraints on the density of the surrounding medium at radii ~10^15-10^16 cm, implying an upper limit on the mass loss rate from the progenitor system of Mdot <~ 6 x 10^-10 Msol/yr (assuming a wind speed of 100 km/s), or expansion into a uniform medium with density n_CSM <~ 6 cm^-3. Drawing from the observed properties of non-conservative mass transfer among accreting white dwarfs, we use these limits on the density of the immediate environs to exclude a phase space of possible progenitors systems for SN 2011fe. We rule out a symbiotic progenitor system and also a system characterized by high accretion rate onto the white dwarf that is expected to give rise to optically-thick accretion winds. Assuming that a small fraction, 1%, of the mass accreted is lost from the progenitor system, we also eliminate much of the potential progenitor parameter space for white dwarfs hosting recurrent novae or undergoing stable nuclear burning. Therefore, we rule out the most popular single degenerate progenitor models for SN 2011fe, leaving a limited phase space inhabited by some double degenerate systems and exotic progenitor scenarios.
We present a new empirical fitting method for the optical light curves of Type Ia supernovae (SNe~Ia). We find that a variant broken-power-law function provides a good fit, with the simple assumption that the optical emission is approximately the blackbody emission of the expanding fireball. This function is mathematically analytic and is derived directly from the photospheric velocity evolution. When deriving the function, we assume that both the blackbody temperature and photospheric velocity are constant, but the final function is able to accommodate these changes during the fitting procedure. Applying it to the case study of SN~2011fe gives a surprisingly good fit that can describe the light curves from the first-light time to a few weeks after peak brightness, as well as over a large range of fluxes ($sim 5$, mag, and even $sim 7$,mag in the $g$ band). Since SNe~Ia share similar light-curve shapes, this fitting method has the potential to fit most other SNe~Ia and characterize their properties in large statistical samples such as those already gathered and in the near future as new facilities become available.
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