The active nucleus in NGC 6951 has been classified in the literature as a Seyfert 2 (e.g., Boer & Schulz 1993; Ho et al. 1995, 1997b) or as a LINER (Filippenko & Sargent 1985; Muñoz-Tuñón et al. 1989; MM93). Of the different classifications, that of Ho et al. (1997b) is the most accurately performed, because of the data quality and the correction for underlying absorption, that affects somewhat to H1#1, and more to H5#5. The classification is based on the three Veilleux & Osterbrock (1987) diagrams [Oiii]5007/H5#5 vs. [Nii]6583/H1#1, [Oiii]5007/H5#5 vs. [Sii]6716+6731/H1#1, and [Oiii]5007/H5#5 vs. [Oi]6300/H1#1.
We have extracted the central 37#37 arcsec as the most representative spectrum of the nucleus, and we measure [Oiii]5007/H5#5=2.59 (5.89), [Nii]6583/H1#1=7.00 (3.72), and [Sii]6716+6731/H1#1=3.06 (1.63). For the values in parentheses, H5#5 and H1#1 have been corrected for an underlying 1 Å of equivalent width in absorption (as discussed in section 4.1 above); these are the values used in the diagrams. Further, recently we have had access to a lower resolution spectrum, from where we can measure [Oi]6300/H 38#380.35. Using these values in the diagrams, the nucleus of NGC 6951 is located in the very high excitation end of the region of these diagrams filled by the LINER points. This explains the uncertain classification, even when the correction for underlying absorption is taken into account. In fact, because the most critical value is [Oiii]5007/H5#5which is most sensitive to this absorption correction, it makes the precise definite classification somewhat arbitrary. Ho et al. (1997b) classification criteria (cf. their table 5) consider a horizontal boundary between Seyfert 2 and LINERs at [Oiii]5007/H39#393; this puts NGC 6951 in their Seyfert 2 zone. Other authors consider that the boundary between the two classes changes with the value of the abscissa (e.g. Gonçalves et al. 1999), and this would put NGC 6951 in the higher excitation end of the LINER zone. In either case, the value of [Nii]6583/H1#1=3.72 is also very high, that may suggest an overabundance of nitrogen; this will be discussed in a future work on the stellar populations and gas phase abundances.
In summary, the active nucleus in NGC6951 can be considered as a transition object between a very high excitation LINER and a possible nitrogen overabundant Seyfert 2.