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Register a new userThermonuclear .Ia Supernovae from Helium Shell Detonations: Explosion Models and Observables
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During the early evolution of an AM CVn system, helium is accreted onto the surface of a white dwarf under conditions suitable for unstable thermonuclear ignition. The turbulent motions induced by the convective burning phase in the He envelope become strong enough to influence the propagation of burning fronts and may result in the onset of a detonation. Such an outcome would yield radioactive isotopes and a faint rapidly rising thermonuclear .Ia supernova. In this paper, we present hydrodynamic explosion models and observable outcomes of these He shell detonations for a range of initial core and envelope masses. The peak UVOIR bolometric luminosities range by a factor of 10 (from 5e41 - 5e42 erg/s), and the R-band peak varies from M_R,peak = -15 to -18. The rise times in all bands are very rapid (<10 d), but the decline rate is slower in the red than the blue due to a secondary near-IR brightening. The nucleosynthesis primarily yields heavy alpha-chain elements (40Ca through 56Ni) and unburnt He. Thus, the spectra around peak light lack signs of intermediate mass elements and are dominated by CaII and TiII features, with the caveat that our radiative transfer code does not include the non-thermal effects necessary to produce He features.
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Merging white dwarfs are a possible progenitor of Type Ia supernovae (SNe Ia). While it is not entirely clear if and when an explosion is triggered in such systems, numerical models suggest that a detonation might be initiated before the stars have coalesced to form a single compact object. Here we study such peri-merger detonations by means of numerical simulations, modeling the disruption and nucleosynthesis of the stars until the ejecta reach the coasting phase. Synthetic light curves and spectra are generated for comparison with observations. Three models are considered with primary masses 0.96 Msun, 1.06 Msun, and 1.20 Msun. Of these, the 0.96 Msun dwarf merging with an 0.81 Msun companion, with a Ni56 yield of 0.58 Msun, is the most promising candidate for reproducing common SNe Ia. The more massive mergers produce unusually luminous SNe Ia with peak luminosities approaching those attributed to super-Chandrasekhar mass SNe Ia. While the synthetic light curves and spectra of some of the models resemble observed SNe Ia, the significant asymmetry of the ejecta leads to large orientation effects. The peak bolometric luminosity varies by more than a factor of 2 with the viewing angle, and the velocities of the spectral absorption features are lower when observed from angles where the light curve is brightest. The largest orientation effects are seen in the ultraviolet, where the flux varies by more than an order of magnitude. Despite the large variation with viewing angle, the set of three models roughly obeys a width-luminosity relation, with the brighter light curves declining more slowly in the B-band. Spectral features due to unburned carbon from the secondary star are also seen in some cases.
Merging carbon-oxygen (CO) white dwarfs are a promising progenitor system for Type Ia supernovae (SN Ia), but the underlying physics and timing of the detonation are still debated. If an explosion occurs after the secondary star is fully disrupted, the exploding primary will expand into a dense CO medium that may still have a disk-like structure. This interaction will decelerate and distort the ejecta. Here we carry out multi-dimensional simulations of ``tamped SN Ia models, using both particle and grid-based codes to study the merger and explosion dynamics, and a radiative transfer code to calculate synthetic spectra and light curves. We find that post-merger explosions exhibit an hourglass-shaped asymmetry, leading to strong variations in the light curves with viewing angle. The two most important factors affecting the outcome are the scale-height of the disk, which depends sensitively on the binary mass ratio, and the total ${}^{56}$Ni yield, which is governed by the central density of the remnant core. The synthetic broadband light curves rise and decline very slowly, and the spectra generally look peculiar, with weak features from intermediate mass elements but relatively strong carbon absorption. We also consider the effects of the viscous evolution of the remnant, and show that a longer time delay between merger and explosion probably leads to larger ${}^{56}$Ni yields and more symmetrical remnants. We discuss the relevance of this class of aspherical ``tamped SN Ia for explaining the class of ``super-Chandrasekhar SN Ia.
The double-detonation explosion model has been considered a candidate for explaining astrophysical transients with a wide range of luminosities. In this model, a carbon-oxygen white dwarf star explodes following detonation of a surface layer of helium. One potential signature of this explosion mechanism is the presence of unburned helium in the outer ejecta, left over from the surface helium layer. In this paper we present simple approximations to estimate the optical depths of important He I lines in the ejecta of double-detonation models. We use these approximations to compute synthetic spectra, including the He I lines, for double-detonation models obtained from hydrodynamical explosion simulations. Specifically, we focus on photospheric-phase predictions for the near-infrared 10830 AA~and 2 $mu$m lines of He I. We first consider a double detonation model with a luminosity corresponding roughly to normal SNe Ia. This model has a post-explosion unburned He mass of 0.03 $M_{odot}$ and our calculations suggest that the 2 $mu$m feature is expected to be very weak but that the 10830 AA~feature may have modest opacity in the outer ejecta. Consequently, we suggest that a moderate-to-weak He I 10830 AA~feature may be expected to form in double-detonation explosions at epochs around maximum light. However, the high velocities of unburned helium predicted by the model ($sim 19,000$~km~s$^{-1}$) mean that the He I 10830 AA~feature may be confused or blended with the C I 10690~AA~line forming at lower velocities. We also present calculations for the He I 10830 AA~and 2 $mu$m lines for a lower mass (low luminosity) double detonation model, which has a post-explosion He mass of 0.077 $M_{odot}$. In this case, both the He I features we consider are strong and can provide a clear observational signature of the double-detonation mechanism.
We present predictions for hydrogen and helium emission line luminosities from circumstellar matter around Type Ia supernovae (SNe Ia) using time dependent photoionization modeling. ESO/VLT optical echelle spectra of the SN Ia 2000cx were taken before and up to 70 days after maximum. We detect no hydrogen and helium lines, and place an upper limit on the mass loss rate for the putative wind of less than 1.3EE{-5} solar masses per year, assuming a speed of 10 km/s and solar abundances for the wind. In a helium-enriched case, the best line to constrain the mass loss would be He I 10,830 A. We confirm the details of interstellar Na I and Ca II absorption towards SN 2000cx as discussed by Patat et al., but also find evidence for 6613.56 A Diffuse Interstellar Band (DIB) absorption in the Milky Way. We discuss measurements of the X-ray emission from the interaction between the supernova ejecta and the wind and we re-evaluate observations of SN 1992A obtained 16 days after maximum by Schlegel & Petre. We find an upper limit of 1.3EE{-5} solar masses per year. These results, together with the previous observational work on the normal SNe Ia 1994D and 2001el, disfavour a symbiotic star in the upper mass loss rate regime from being the likely progenitor scenario for these SNe. To constrain hydrogen in late time spectra, we present ESO/VLT and ESO/NTT optical and infrared observations of SNe Ia 1998bu and 2000cx 251-388 days after maximum. We see no hydrogen line emission in SNe 1998bu and 2000cx at these epochs, and we argue from modeling that the mass of such hydrogen-rich gas must be less than 0.03 solar masses for both supernovae. Comparing similar upper limits with recent models of Pan et al., it seems hydrogen-rich donors with a separation of less than 5 times the radius of the donor may be ruled out for the five SNe Ia 1998bu, 2000cx, 2001el, 2005am and 2005cf.
The detonation of a helium shell on top of a carbon-oxygen white dwarf has been argued as a potential explosion mechanism for type Ia supernovae (SNe~Ia). The ash produced during helium shell burning can lead to light curves and spectra that are inconsistent with normal SNe~Ia, but may be viable for some objects showing a light curve bump within the days following explosion. We present a series of radiative transfer models designed to mimic predictions from double detonation explosion models. We consider a range of core and shell masses, and systematically explore multiple post-explosion compositions for the helium shell. We find that a variety of luminosities and timescales for early light curve bumps result from those models with shells containing $^{56}$Ni, $^{52}$Fe, or $^{48}$Cr. Comparing our models to SNe~Ia with light curve bumps, we find that these models can reproduce the shapes of almost all of the bumps observed, but only those objects with red colours around maximum light ($B-V gtrsim 1$) are well matched throughout their evolution. Consistent with previous works, we also show that those models in which the shell does not contain iron-group elements provide good agreement with normal SNe~Ia of different luminosities from shortly after explosion up to maximum light. While our models do not amount to positive evidence in favour of the double detonation scenario, we show that provided the helium shell ash does not contain iron-group elements, it may be viable for a wide range of normal SNe~Ia.
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