Optical and ultraviolet observations of the Type IIn supernova 1995N at epochs between 321 and 1799 days after the explosion show three distinct velocity components .
The narrow lines come from circumstellar gas and show both low and high ionization .
This component has a low filling factor , and is photoionized by X-rays from the shock .
The intermediate component , which is dominated by newly processed oxygen , originates in a shell with velocity of 2500–5000 ~ { } km~ { } s ^ { -1 } , and most likely comes from the ejecta .
The hydrogen- and helium-dominated gas has a low ionization , a high density , and velocities that extend out to \raise 1.29 pt \hbox { $ > $ } \kern - 7.5 pt { \lower 2.795 pt \hbox { $ \sim$ } } 10 , 000 ~ { } km~% { } s ^ { -1 } .
Strong signatures of Ly \alpha -pumped fluorescence lines of Fe II are seen in the near-infrared and ultraviolet .
The He/H ratio , \sim 0.3 by number , and the nitrogen overabundance provide strong evidence for CNO burning products .
The fluxes of the broad hydrogen and helium lines decrease considerably faster than the oxygen lines .
The H \alpha line profile shows strong evolution , with the red wing decreasing faster than the blue .
Possible scenarios , involving either a clumpy circumstellar medium , or an aspherical distribution of the surrounding gas , are discussed based on the line profiles and physical conditions .
Finally , we propose that Type IIn supernovae have their origin in red supergiants in a superwind phase .