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Type Ibn supernovae (SNe) are a small yet intriguing class of explosionswhose spectra are characterized by low-velocity helium emission lines withlittle to no evidence for hydrogen. The prevailing theory has been that theseare the core-collapse explosions of very massive stars embedded in helium-richcircumstellar material (CSM). We report optical observations of six new SNeIbn: PTF11rfh, PTF12ldy, iPTF14aki, iPTF15ul, SN 2015G, and iPTF15akq. Thisbrings the sample size of such objects in the literature to 22. We also reportnew data, including a near-infrared spectrum, on the Type Ibn SN 2015U. Inorder to characterize the class as a whole, we analyze the photometric andspectroscopic properties of the full Type Ibn sample. We find that, despite theexpectation that CSM interaction would generate a heterogeneous set of lightcurves, as seen in SNe IIn, most Type Ibn light curves are quite similar inshape, declining at rates around 0.1 mag/day during the first month aftermaximum light, with a few significant exceptions. Early spectra of SNe Ibn comein at least two varieties, one that shows narrow P Cygni lines and anotherdominated by broader emission lines, both around maximum light, which may be anindication of differences in the state of the progenitor system at the time ofexplosion. Alternatively, the spectral diversity could arise from viewing-angleeffects or merely from a lack of early spectroscopic coverage. Together, therelative light curve homogeneity and narrow spectral features suggest that theCSM consists of a spatially confined shell of helium surrounded by a less denseextended wind.

Type Ibn Supernovae Show Photometric Homogeneity and Spectral Diversity at Maximum Light