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The first high-resolution X-ray spectroscopy of the planetary nebulaBD+30$^\circ$3639 allowed to study X-ray emitting "hot bubbles" (HBs) ofplanetary nebulae in unprecedented detail. We investigate (i) how diagnosticline ratios are affected by the HB thermal structure and chemical profile, (ii)if the HB chemical composition of BD+30$^\circ$3639 is consistent with theH-poor (H for hydrogen) composition of the stellar photosphere, and (iii) ifH-rich nebular matter has been added to this HB by evaporation. We apply ananalytical, 1D model for wind-blown HBs with temperature and density profilesbased on self-similar solutions including thermal conduction. We constructheat-conduction HBs with chemical stratification. The X-ray emission iscomputed using the CHIANTI code. Our HB models are used to re-analyse thehigh-resolution X-ray spectrum of BD+30$^\circ$3639. Our models reproduce theobserved line ratios much better than plasmas with single electrontemperatures. All the temperature- and abundance-sensitive line ratios areconsistent with BD+30$^\circ$3639 X-ray observations for (i) an interveningcolumn density of neutral H, $N_{\rm H} =0.20_{-0.10}^{+0.05}\times\!10^{22}\rm\ cm^{-2}$, (ii) a characteristic HBX-ray temperature of ${T_{\rm X} = 1.8\pm 0.1~ }$MK together with (iii) a veryhigh neon mass fraction of about 0.05, virtually as high as that of oxygen. Forlower values of $N_{\rm H}$, we cannot exclude that the HB of BD+30$^\circ$3639contains a small amount of evaporated (or mixed) H-rich nebular matter. Giventhe possible range of $N_{\rm H}$, the fraction of evaporated H-rich mattercannot exceed 3% of the HB mass. The diffuse X-ray emission fromBD+30$^\circ$3639 can well be explained by models of wind-blown HBs withthermal conduction and a chemical composition equal to that of the H-poor andcarbon-, oxygen-, and neon-rich stellar surface.

Hot bubbles of planetary nebulae with hydrogen-deficient winds - II. Analytical approximations with application to BD+30$^\circ$3639