2021MNRAS.503..972P

The analysis of 20 yr of spectrophotometric data of the double-shell planetary nebula (PN) PM 1-188 is presented, aiming to determine the time evolution of the emission lines and the physical conditions of the nebula, as a consequence of the systematic fading of its [WC 10] central star whose brightness has declined by about 10 mag in the past 40 yr. Our main results include that the [O III], [O II], and [N II] line intensities are increasing with time in the inner nebula as a consequence of an increase in electron temperature from 11 000 K in 2005 to more than 14 000 K in 2018, due to shocks. The intensities of the same lines are decreasing in the outer nebula, due to a decrease in temperature, from 13 000 to 7000 K, in the same period. The chemical compositions of the inner and outer shells are derived and they are similar. Both nebulae present subsolar O, S, and Ar abundances, while they are He, N, and Ne rich. For the outer nebula, the values are 12 + log He/H = 11.13 ± 0.05, 12 + log O/H = 8.04 ± 0.04, 12 + log N/H = 7.87 ± 0.06, 12 + log S/H = 7.18 ± 0.10, and 12 + log Ar = 5.33 ± 0.16. The O, S, and Ar abundances are several times lower than the average values found in disc non-Type I PNe, and are reminiscent of some halo PNe. From high-resolution spectra, an outflow in the N-S direction was found in the inner zone. Position-velocity diagrams show that the outflow expands at velocities in the -150 to 100 km s^–1 range, and both shells have expansion velocities of about 40 km s^–1.