Herschel limits on far-infrared emission from circumstellar dust around
  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 um and 160 um with the Photodetector Array Camera and
Spectrometer (PACS) on board 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_sun at a 3 sigma-level for dust temperatures T_dust ~500 K assuming silicate
or graphite dust grains of size a = 0.1 um. For SNe 2011by and 2012cg the
corresponding upper limits are less stringent, with M_dust < 0.1 M_sun for the
same assumptions.
