The Origin of Type Ia Supernovae: A Theoretical Investigation and Observational Evidence

Abstract

The problem of the stellar progenitor of Type Ia supernovae (SNe Ia) has been investigated in order to provide a realistic evolutionary path for producing this kind of phenomenon. The observational evidence suggests that an SN Ia event is produced by the thermonuclear explosion of a carbon-oxygen white dwarf (CO WD) that accretes matter from its companion in a binary system; in this way the accreting structure attains the Chandrasekhar mass limit (MCh) and it ignites C burning at the center in highly degenerate physical conditions, producing an explosive outcome. One first result obtained by performing numerical experiments (S. Cassisi, I. Iben, Jr., & A. Tornambe 1998, ApJ, 496, 376; L. Piersanti, S. Cassisi, I. Iben, Jr., & A. Tornambe 2000, ApJ, 535, 932; L. Piersanti, S. Cassisi, I. Iben, Jr., & A. Tornambe 2000, ApJ, 521, L59) suggests that in the singledegenerate scenario the thermal response of the accreting structure to the mass deposition prevents the CO WD from attaining the Chandrasekhar mass limit. In particular, if H-rich matter is accreted in a recurrent mild H-pulses regime, an He buffer is piled up as a by-product of the H-burning shell. The growth in mass of the He buffer comes to a halt well before the accreting structure attains MCh because of the explosive ignition of He burning in the accreted layers. In this case the final outcome is an event of SN Ia proportion but one that is not able to reproduce the observational evidence (spectra and lightcurve profile). This result is not modified by including the chain in the nuclear network because 14 14 18 N(e , g) C(a, g) O of both the low abundance of N at the bottom of the He shell and the high density ( g cm ) at which e-captures can 6 r ∼ 10 occur (L. Piersanti, S. Cassisi, & A. Tornambe 2001, ApJ, 558, 916). On the other hand, we find that the double-degenerate scenario is a good candidate for SN Ia progenitor system when the lifting effect of rotation is properly accounted for both in modeling the thermal evolution of the accreting CO WD and in the evolution of the accretion disk. In fact, it comes out that in the case of merging of two CO WDs rotation acts as a tuning mechanism of , preventing an off-center C ignition and allowing the acṀ creting structure to attain MCh. In addition, when the CO WD approaches MCh, it becomes a fast rotator ( rad s ) so that q ∼ 1 the structure deforms and the mass distribution becomes asymmetric; as a consequence, the CO WD has to brake for gravitational wave radiation emission (S. L. Shapiro, S. A. Teukolsky, & T. Nakamura 1990, ApJ, 357, L17). The loss of angular momentum and rotational energy induces a strong compression of the structure, which, independently of the braking efficiency, attains the physical conditions for central C burning, thus producing an SN Ia event.