Progenitor constraints on the Type-Ia supernova SN2011fe from pre-explosion Hubble Space Telescope he II narrow-band observations

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 He II 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 ̃ 1Myr should, in principle, create a He III 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 He III region could appear unresolved, extended, or as a ring, with a range of possible surface brightnesses. We find no trace of He II λ4686Å line emission in the HST data. Using simulations, we set 2σ upper limits on the He II λ4686Å luminosity of L_{He II} < 3.4 × 10³⁴ erg s^-1 for a point source, corresponding to an emission region of radius r < 1.8 pc. The upper limit for an extended source is L_{He II} < 1.7 × 10³⁵ erg s^-1, corresponding to an extended region with r ̃ 11 pc. The largest detectable shell, given an interstellar-medium density of 1 cm^-3, 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 L_bol ≳ 3 × 10³⁷ erg s^-1, or of a super-Eddington accreting WD that produces an outflowing wind capable of producing cavities with radii of 2-6 pc.