tokens: to test this connection more rigorously, but the match is poor, being both too blue and too broad (Fig. 11 ) compared to even the earliest spectra of SN1998bw or SN2002ap (Patat et al. 2001 ; Kinugasa et al. 2002 ), or of the spectrum of SN2008D (Modjaz et al. 2009 ) during its shock-cooling phase. 9 As of yet we have no convincing explanation for the origin of this feature, other than that it implies very fast (nearly relativistic) ejecta. Figure 11. Comparison of an early spectrum of AT 2018cow to the early spectrum of SN 2008D (Ib) and two SNe Ic-BL (top row), and of a later-time spectrum to SN 1998S (IIn) and two tidal disruption events (bottom row). The extremely blue, smooth continuum bears little resemblance to SN Ib/c, even after attempting to subtract the blue continuum. (A power-law plus a constant have been subtracted from AT 2018cow; the reference SN spectra have been subtracted by a constant only. The strength of the features in the SNe after subtraction has been suppressed by a factor of 2.) The later-time spectra are dominated by weak emission features of hydrogen and helium; these features are also present in IIn SNe but are much narrower compared to what is seen in AT 2018cow. These features are seen in known TDEs with similarly broad widths, although typically much greater strengths. The identities of the features seen in later spectra (H and He in broad emission) are secure. In spite of this, these later spectra bear no obvious resemblance to any class of known supernova. The strongest similarities are to Type IIn supernovae (which can also remain hot for several weeks after explosion, and are emission-dominated by definition): in the bottom panel of Fig. 11 , we plot AT 2018cow versus an early spectrum of SN 1998S from Fassia et al. ( 2001 ), which shows a similar blue continuum and most of the same H and He transitions. However, the lines in AT 2018cow are not narrow for most of their evolution ( ∼ 6000 km s −1 , versus a few hundred km s −1 for SN 1998S). Thomson scattering within ionized matter could broaden a line enough to wash out the narrow component, but this would not produce the net redshift in the emission component that we observe. The H and He thus must be in the ejecta itself (and seemingly preferentially in receding ejecta given the net redshift). In fact, the best spectroscopic analogues to AT 2018cow are not supernovae at all. Our spectra bear a striking resemblance to tidal disruption events: the high temperatures, presence of helium and hydrogen features in emission, and moderate velocities all match what is observed for TDEs. The spectral features in AT 2018cow are substantially weaker than in the examples of TDEs that we are aware of (bottom panel of Fig. 11 ; comparison spectra are from Holoien et al. 2014 , 2016 ), but the resemblance to a TDE is much stronger than to any supernova. We summarize the key observational features of AT 2018cow in Table 3 . 5 INTERPRETATION 5.1 Supernova models: A
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