2014MNRAS.442L..28G

We present Hubble Space Telescope (HST) imaging observations of the site of the Type-Ia supernova SN2011fe in the nearby galaxy M101, obtained about 1 yr prior to the event, in a narrow band centred on the HeII λ4686Å emission line. In a `single-degenerate' progenitor scenario, the hard photon flux from an accreting white dwarf (WD), burning hydrogen on its surface over ∼ 1 Myr should, in principle, create a HeIII Strömgren sphere or shell surrounding the WD. Depending on the WD luminosity, the interstellar density, and the velocity of an outflow from the WD, the HeIII region could appear unresolved, extended, or as a ring, with a range of possible surface brightnesses. We find no trace of HeII λ4686Å line emission in the HST data. Using simulations, we set 2σ upper limits on the HeII λ4686Å luminosity of LHeII < 3.4x10³⁴ erg/s for a point source, corresponding to an emission region of radius r < 1.8 pc. The upper limit for an extended source is LHeII < 1.7x10³⁵ erg/s, corresponding to an extended region with r ∼ 11 pc. The largest detectable shell, given an interstellar-medium density of 1/cm3, has a radius of ∼ 6 pc. Our results argue against the presence, within the ∼ 10⁵ yr prior to the explosion, of a supersoft X-ray source of luminosity Lbol ≳ 3x10³⁷ erg/s, or of a super-Eddington accreting WD that produces an outflowing wind capable of producing cavities with radii of 2-6 pc. %K methods: observational