A sample of Type Ia-CSM supernovae from the ZTF Bright Transient Survey

Among the supernovae that show strong interaction with the circumstellar medium, there is a rare subclass of Type Ia supernovae — SNe Ia-CSM, that show strong narrow Hydrogen emission lines much like SNe IIn but on top of a diluted over-luminous Type Ia spectrum. In the previous systematic study of this class, 16 objects were identified, 8 historic and 8 from the Palomar Transient Factory (PTF). Now using its successor survey, the Zwicky Transient Facility (ZTF), we have classified 12 more SNe of this type through the systematic Bright Transient Survey (BTS). In this study, we present and analyze the optical and mid-IR light curves, optical spectra and host galaxy properties of this sample. Consistent with the previous studies, we find the objects to have slowly evolving light curves compared to normal SNe Ia, spectra having weak Hβ and He I emission, large Balmer decrements of ~6Å, and strong Ca NIR emission. The hosts tend to be late-type galaxies with recent star-formation. The peak bolometric luminosities of the SNe are a few times 10⁴³ erg s^-1, with peak absolute magnitudes ranging from -17.5 to -21, and the total bolometric energy output is within 10⁵¹ erg. This work nearly doubles the sample of well studied SNe Ia-CSM as a class.