Type Ia Supernovae
We are addressing the issue of the physical origin of the SNIa and their diversity from an observational/theoretical point of view.
In the recent past we have studied the distribution of SNIa absolute magnitudes, ejecta expansion velocities and spectral line ratios in comparison with the expectations from different progenitor scenarios and explosion mechanisms (deflagration, delayed detonation, etc.) Benetti et al 2005 (ApJ 623, 1011) and Mazzali et al 2007 (Science 315, 825). We have found that all SNIa burn roughtly the same amount of mass, which implies that their progenitors had the same mass before explosion. A single explosion scenario, possibly a delayed detonation, may thus explain most SNeIa. The drive for the variations in the explosion observables is likely the strength of the central deflagration and the extent of stellar mass burned before the detonation goes off. The remaining diversity among SNIa could be explained by 3-D effects in the outer envelopes like ejection of blobs of burned material along the line of sight and interaction with the CSM.
Our current goal is thus to increase the sample of nearby SNIa with excellent monitoring from UV to near IR domains, and from photospheric up to nebular phases, to put the results mentioned above on a firm statistical ground. In these effort, we take part in a major international collaborations and we exploit several observing programs running on some of the major ground-based observatories (ESO-VLT, ESO-NTT, TNG) and space missions (Galex, Swift).
Other indications on the nature of the progenitors of SNIa can be derived from the study of CircumStellar Matter. Thanks to high quality and very high resolution multi-epochs spectrograms taken with VLT+UVES we have been able to reveal for the first time signatures of matter unequivocally lost by the progenitor system in the form of NaI shells expanding with velocities of about 50 km/s. These observations favour the Single Degenerate scenario for SNIa explosion ( Patat et al 2007, Science 317, 924) in which the WD companion is a giant star. We are working to enlarge the sample of Type Ia Supernovae observed with this innovative approach.
Recent studies have shown that the extinction curves toward a few, highly-reddened SNIa have values of RV significantly lower than the canonical 3.1 (Elias-Rosa et al. 2006, 2008). These findings rose new questions. Is the peculiar extinction law derived in the direction to these SNIa common to all the highly-reddened objects? Do low-reddening SNIa (used as distance indicators) follow similar extinction laws? What is the effect on the calibration of nearby SNIa? Are the extinction laws the same at low and high redshift? Is this effect "local", i.e. somehow related to the nature of the progenitor system, or due to the overall dust properties of the host galaxy? To answer these questions observational campaigns of highly and moderately reddened SNe of all types over extended wavelength intervals from the UV (SWIFT) to the near IR (TNG, NTT and VLT) are in progress.
A complementary approach to investigate on the progenitors of type Ia SNe consists in studying the rate at which they occur in different contexts. Indeed, different evolutionary paths may lead to the thermonuclear explosion of a CO WD in a close binary system, each corresponding to a different distribution for the delay times (DTD). Evolutionary channels which imply DTDs more skewed at short delay times predict a stronger increase of the SNIa rate per unit mass, when going from old to young stellar populations. Thus, the analysis of the SNIa rate per unit mass (or any other mass tracer) in systems with different age distributions constrains the shape of the DTD, and, in turn, the SNIa progenitor model. Greggio (2005) developped an analytic descriptions of the DTD rooted on stellar evolution, with a built-in parametrization of those astrophysical variables which play an important role in shaping it, e.g. the clock of the explosion and the mass distribution of the binary components in successful systems. These functions are a flexible tool to investigate on the predictions from different progenitor models on various observables. An example for four possible SNIa evolutionary channels is shown in Figure 2, together with the correlation between the specific SNIa rate and the parent galaxy colour which they imply. The constraints on the DTD derived from this kind of correlation appear to depend on the photometric bands used to trace the average age of the stellar populations. We are investigating on a more effective way to derive the star formation history in galaxies,in order to improve on the diagnostic on the DTD from the trend of the SNIa rate with the parent galaxies' properties.
These DTDs are also a handy tool to investigate on the impact of the different SNIa models on the redshift dipendence of the cosmic SNIa rate, on the Fe enrichment timescales in galaxies and in galaxy clusters, on the global Fe budget of galaxy clusters, and of the universe ( Blanc and Greggio, 2008), as well as on the evolution of the cosmic mix of SNIa events if both the DD and SD evolutionary channels are at work ( Greggio, Renzini and Daddi 2008).