A new faint Type Ia Supernova: SN 1997cn in NGC 5490

Observations of the recent SN 1991bg in the elliptical galaxy NGC 5490 show that this objects closely resembles, both photometrically and spectroscopically, the faint SNIa 1991bg. The two objects have similar light curves, which do not show secondary maxima in the near IR as normal type Ia supernovae. The host galaxy, NGC5490, lies in the Hubble flow. Adopting for SN1997cn a reddening E(B-V)=0, the absolute magnitude is faint: MV = -17.98 using Ho=65 and MV = -17.40 using Ho=85 km/(s Mpc). 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 TiII trough between 4000 and 4500A, the strong CaII IR triplet, the narrow absorption at about 5700A and the slow expansion velocity. In analogy to SN 1991bg the observed spectrum of SN1997cn 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 SNIa. The influence of the distance scale adopted on the input parameters of the best fit model is also discussed. These data demonstrate that peculiar SNIa like SN 1991bg are not once--in--a--lifetime events and that deep SN searches can be contaminated by underluminous SN Ia in a fairly large volume.