CCSNe and Type Ia SNe

Radio emission from SNe has been detected from core-collapse SNe (CCSNe) of all types (Type Ib/c, IIp, IIl, IIn), CCSN progenitors are massive (M > 8 Msun) stars, which have ejected copious amounts of mass in the last stages of their lives through intense presupernova winds. The standard interaction model (Chevalier 1982, ApJ 259, L85) provided a theoretical framework that, with modifications, is still valid today in its essence: The material ejected after the SN explosion interacts with the circumstellar medium (CSM), resulting in a shocked shell-like expanding structure. Relativistic electrons, which are continuously reaccelerated in the shock front and are embedded in a significant magnetic field, would emit synchrotron radio emission. 

A systematic radio follow-up of CCSNe has not been possible until recently due to sensitivity limitations, which have prevented the observation of many CCSNe in the local universe. Yet we need a systematic radio monitoring of a large sample of CCSNe (which are expected to emit significantly in the radio) in nearby galaxies to characterize their radio properties, which would be of much use for (i) probing the circumstellar interaction and (ii) typing RSNe in LIRGs and ULIRGs, for which we lack spectroscopic classification. Finally, despite the huge progress in our understanding of how massive stars lose mass, our knowledge of the details on the mass loss history of CCSNe in their immediate stages prior to their catastrophic explosion is scarce.

During the previous project, we were granted time under a ToO programme to observe recently exploded CCSNe with spectroscopic identification (eMERLIN, PI: Pérez-Torres;). Unfortunately, the eMERLIN array is still working below its nominal sensitivities, and not always the observations were of enough quality, and thus were not worth publishing. Still, we published a number of Atels reporting our detections (Radio detection of SN2010P, Herrero- Illana, Romero-Cañizales, Pérez-Torres et al. 2012, Atel #4432; Radio detection of SN2014bc: Argo, Pérez-Torres, Alberdi et al. 2014, Atel #6273).

Thermonuclear runaway supernovae (i.e., Type Ia SNe) are the explosive end-products of white dwarfs. SNe Ia are primary cosmological distance indicators and a major contributor to the chemical evolution of galaxies, yet we do not know what makes a SN Ia. This lack of knowledge makes it difficult to gain a physical understanding of the explosions, so that we 

can model their evolution, and compromises their use as distance indicators. It also means we do not fully understand the timescale over which SNe Ia turn on, adding a large uncertainty to our understanding of the chemical evolution of galaxies.

Unveiling the progenitor scenario for SNe Ia is difficult because white dwarfs (WDs) can, theoretically, reach their Chandrasekhar mass in many ways, and disentangling which is the correct one is challenging from an observational point of view. There are two basic families of models leading to a SN Ia: the single-degenerate model (SD) and the double-degenerate model (DD). In the SD scenario, a WD accretes mass from a hydrogen-rich star companion before reaching a mass close to the Chandrasekhar mass and going off as supernova. In the DD scenario, two WDs merge. In the SD scenario, a prompt radio emission is expected. Initial observations should be performed early after the explosion, since the dense circumstellar medium could be overtaken quickly, making the SN fade away rapidly.

During the period 2012-2014, we had several ToO observing programmes approved in word- leading radio interferometric observatories, aimed at searching for radio emission from any Type Ia SNe within a radius of 20 Mpc (eMERLIN, PI: Pérez-Torres; ATCA, PI: Peter Lundqvist). In 2013, we activated our eMERLIN programme to observe the type Ia SN 2013dy in NGC 7250 (D=13.5 Mpc), which resulted in an upper limit to the putative wind of the progenitor of 2.7e-7 Msol/yr (3 sigma; Pérez-Torres et al. Atel #5619, 2013). 

Yet, the milestone of this subproject was still to come thanks to SN 2014J, which exploded at the end of January 2014 at a mere 3.6 Mpc from us, in M82. We not only activated our eMERLIN programme, but also sent a ToO request to the EVN, which was quickly accepted. SN 2014J was the closest type Ia SN for the last 40 yr. Our observations put the tightest constraints to the radio emission of a type Ia SNe, excepting SN2011fe, which was observed even earlier than SN 2014J, allowing to rule out most of the parameter space occupied by SD scenarios (Pérez-Torres et al. 2014, ApJ, 792, 38; see also the project image, which shows that most parameter space for single-degenerate scenarios is ruled out by out ultra-deep radio observations of SN 2014J).