Herschel limits on far-infrared emission from circumstellar dust around three nearby Type Ia supernovae

We report upper limits on dust emission at far-infrared (IR) wavelengths from three nearby Type Ia supernovae: SNe 2011by, 2011fe and 2012cg. Observations were carried out at 70 and 160 μm with the Photodetector Array Camera and Spectrometer onboard the Herschel Space Observatory. None of the supernovae were detected in the far-IR, allowing us to place upper limits on the amount of pre-existing dust in the circumstellar environment. Due to its proximity, SN 2011fe provides the tightest constraints, M dust 7 × 10−3 M at a 3σ level for dust temperatures T dust ∼ 500 K assuming silicate or graphite dust grains of size a = 0.1 μm. For SNe 2011by and 2012cg the corresponding upper limits are less stringent, with M dust 10−1 M for the same assumptions.