Quantitative classification of [WC] nuclei of Planetary Nebulae

Abstract

We have carried out spectral classification of 65 [WC] stars, using spectra obtained at ESO in 1994-1995 with a spectral resolution of 1500 and high signal to noise ratio. We quantify the visual classification, by using the line ratios of CIII, CIV, OV, OVI in the 530-580 nm range, correlated with the blue CIII - IV lines. We measured the total strength of the above emission lines and derived the ratios of the emission line strengths. Using the stellar emission line ratios we classified about 32 CSPN as [WC] and about 30 as WELS which show only relatively weak CIV 5806 and CIV 4650 and He II 4686 stellar emission lines. The presence of CIV 5806 and the absence of CIII 5696 the WELS appear to belong to the [WC3] type. However the OV lines are very weak or absent. Some stars are reclassified, and 4 stars are found in the [WC5-7] gap. We classified 34 CSPN as [WC]. The [WC2] CSPN NGC 2867 and PB 6 show narrow and weak He II 5412 absorption feature. Based on the relative strengths of the stellar emission features NGC 2867 [WC 2], PB 6 [WC2] and M1-61 [WC4] appear to be related to WELS (Weak Emission Line Central Stars of Planetary Nebulae). The observed spectral characteristics of the central stars of these PN suggest that they are related to [WC] - PG 1159 type of objects.

Spectroscopic Variability of the Compact Planetary Nebula Hb 12

“…Several objects the change of whose emission-line spectra can be caused by a rise in the temperature of the exciting star, consistent with the idea of rapid evolution in the post-AGB stage, have already been discovered. For example, the compact low-excitation PN Hen 2-260 has shown a change of its spectrum in the last 30 years: in the mid-1980s no nebular emission lines were detected in its spectrum (Acker et al 1991), in 2001 the [O III] λ5007 line flux was 5 % of the Hβ flux (Escudero et al 2004), while in 2012 it was already about 7 % (Hajduk et al 2014). The authors of the latter paper believe that the strengthening of nebular lines is related to the increase in the degree of ionization in the nebula because of a rise in the temperature of the exciting star.…”