SN 2020uem: A Possible Thermonuclear Explosion within A Dense Circumstellar Medium (I) The Nature of Type IIn/Ia-CSM SNe from Photometry and Spectroscopy

Abstract

We have performed intensive follow-up observations of a Type IIn/Ia-CSM SN (SN IIn/Ia-CSM), 2020uem, with photometry, spectroscopy, and polarimetry. In this paper, we report on the results of our observations focusing on optical/near-infrared (NIR) photometry and spectroscopy. The maximum V-band magnitude of SN 2020uem is over $-19.5$ mag. The light curves decline slowly with a rate of $\sim 0.75\mathrm{mag}/100\mathrm{days}$. In the late phase ($\gtrsim 300$ days), the light curves show accelerated decay ($\sim 1.2\mathrm{mag}/100\mathrm{days}$). The optical spectra show prominent hydrogen emission lines and broad features possibly associated with Fe-peak elements. In addition, the $\mathrm{H}\alpha$ profile exhibits a narrow P-Cygni profile with the absorption minimum of $\sim 100\mathrm{km}{\mathrm{s}}^{-1}$. SN 2020uem shows a higher $\mathrm{H}\alpha /H\beta$ ratio ($\sim 7$) than those of SNe IIn, which suggests a denser CSM. The NIR spectrum shows the Paschen and Brackett series with continuum excess in the H and Ks bands. We conclude that the NIR excess emission originates from newly-formed carbon dust. The dust mass ($M_{\mathrm{d}}$) and temperature ($T_{\mathrm{d}}$) are derived to be $(M_{\mathrm{d}},T_{\mathrm{d}})\sim (4-7\times 10^{-5}{\mathrm{M}}_{\odot },1500-1600\mathrm{K})$. We discuss the differences and similarities between the observational properties of SNe IIn/Ia-CSM and those of other SNe Ia and interacting SNe. In particular, spectral features around $\sim 4650$ {\text Å} and $\sim 5900$ {\text Å} of SNe IIn/Ia-CSM are more suppressed than those of SNe Ia; these lines are possibly contributed, at least partly, by \ion{Mg}{1}] and \ion{Na}{1}, and may be suppressed by high ionization behind the reverse shock caused by the massive CSM.

arXiv comment22 pages, 15 figures, 3 tables. Accepted for publication in ApJ

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