SN 2013fs and SN 2013fr: exploring the circumstellar-material diversity in Type II supernovae

We present photometry and spectroscopy of SN2013fs and SN2013fr in the first\n100 days post-explosion. Both objects showed transient, relatively narrow\nH$\\alpha$ emission lines characteristic of SNeIIn, but later resembled normal\nSNeII-P or SNeII-L, indicative of fleeting interaction with circumstellar\nmaterial (CSM). SN2013fs was discovered within 8hr of explosion. Its light\ncurve exhibits a plateau, with spectra revealing strong CSM interaction at\nearly times. It is a less luminous version of the transitional SNIIn PTF11iqb,\nfurther demonstrating a continuum of CSM interaction intensity between SNeII-P\nand IIn. It requires dense CSM within 6.5$\\times$10$^{14}$~cm of the\nprogenitor, from a phase of advanced pre-SN mass loss shortly before explosion.\nSpectropolarimetry of SN2013fs shows little continuum polarization, but\nnoticeable line polarization during the plateau phase. SN2013fr morphed from a\nSNIIn at early times to a SNII-L. After the first epoch its narrow lines\nprobably arose from host-galaxy emission, but the bright, narrow H$\\alpha$\nemission at early times may be intrinsic. As for SN2013fs, this would point to\na short-lived phase of strong CSM interaction if proven to be intrinsic,\nsuggesting a continuum between SNeIIn and II-L. It is a low-velocity SNII-L,\nlike SN2009kr but more luminous. SN2013fr also developed an IR excess at later\ntimes, due to warm CSM dust that require a more sustained phase of strong\npre-SN mass loss.\n