A New Faint Type Ia Supernova: SN 1997cn in NGC 5490^*

Observations of the recent supernova (SN) 1997cn in the elliptical galaxy NGC 5490 show that this object closely resembles, both photometrically and spectroscopically, the faint SN Ia SN 1991bg. The two objects have similar light curves, which do not show secondary maxima in the near-IR as normal type Ia supernova. The host galaxy, NGC 5490, lies in the Hubble flow. Adopting for SN 1997cn a reddening E(B-V) = 0, the absolute magnitude is faint: M_V = -17.98 using H₀ = 65 and M_V = -17.40 using H₀ = 85 km s^-1 Mpc^-1. The latter value is in close agreement with the absolute magnitude of SN 1991bg on the SBF/PNLF/TF distance scale. The photospheric spectra of the two SNe show the same peculiarities, the deep Ti II trough between 4000 and 4500 A, the strong Ca II IR triplet, the narrow absorption at about 5700 A, and the slow expansion velocity. Analogous to SN 1991bg, the observed spectrum of SN 1997cn has been successfully modeled by scaling down the W7 model by a factor of 2, assuming a rise time to B maximum of 18 days, a photospheric velocity, and an effective temperature low compared to normal SNe Ia. The influence of the distance scale adopted on the input parameters of the best-fit model is also discussed.

These data demonstrate that peculiar SNe Ia like SN 1991bg are not once in a lifetime events and that deep SN searches can be contaminated by underluminous SNe Ia in a fairly large volume.