Type Ia Supernovae Strongly Interacting with Their Circumstellar Medium
  Owing to their utility for measurements of cosmic acceleration, Type Ia
supernovae (SNe) are perhaps the best-studied class of SNe, yet the progenitor
systems of these explosions largely remain a mystery. A rare subclass of SNe Ia
show evidence of strong interaction with their circumstellar medium (CSM), and
in particular, a hydrogen-rich CSM; we refer to them as SNe Ia-CSM. In the
first systematic search for such systems, we have identified 16 SNe Ia-CSM, and
here we present new spectra of 13 of them. Six SNe Ia-CSM have been
well-studied previously, three were previously known but are analyzed in-depth
for the first time here, and seven are new discoveries from the Palomar
Transient Factory. The spectra of all SNe Ia-CSM are dominated by H{\alpha}
emission (with widths of ~2000 km/s) and exhibit large H{\alpha}/H{\beta}
intensity ratios (perhaps due to collisional excitation of hydrogen via the SN
ejecta overtaking slower-moving CSM shells); moreover, they have an almost
complete lack of He I emission. They also show possible evidence of dust
formation through a decrease in the red wing of H{\alpha} 75-100 d past maximum
brightness, and nearly all SNe Ia-CSM exhibit strong Na I D absorption from the
host galaxy. The absolute magnitudes (uncorrected for host-galaxy extinction)
of SNe Ia-CSM are found to be -21.3 <= M_R <= -19 mag, and they also seem to
show ultraviolet emission at early times and strong infrared emission at late
times (but no detected radio or X-ray emission). Finally, the host galaxies of
SNe Ia-CSM are all late-type spirals similar to the Milky Way, or dwarf
irregulars like the Large Magellanic Cloud, which implies that these objects
come from a relatively young stellar population. This work represents the most
detailed analysis of the SN Ia-CSM class to date.
