Gran Telescopio Canarias/OSIRIS Deep Spectroscopy of Galactic Compact Planetary Nebulae: PN G048.5+04.2 and PN G068.7+14.8

Abstract We report deep long-slit spectroscopy of two compact Galactic planetary nebulae (PNe), PN G048.5+04.2 and PN G068.7+14.8, obtained using the 10.4 m Gran Telescopio Canarias (GTC). These spectra cover a broad wavelength range of ∼3630–10370 Å, enabling the detection of numerous emission lines critical for nebular analysis, including [O ii ] λ 3727 and [O iii ] λ 4363 in the blue and [S iii ] λ 6312 and λ 9069 in the red. Also detected in the spectrum of PN G068.7+14.8 are broad C iii and C iv lines probably due to stellar emission, indicating that the central star of this PN is WC type. The GTC optical–near-IR spectra were analyzed in detail in conjunction with archival Spitzer Space Telescope Infrared Spectrograph mid-IR spectra, and the elemental abundances of the two PNe were determined. Photoionization models were established for the two PNe using cloudy , based on the ratios of line fluxes measured from the GTC spectra. Our best photoionization modeling, in combination with state-of-the-art post–asymptotic giant branch (AGB) evolutionary model tracks, shows that both PNe evolved from low-mass progenitors (<2 M ⊙ ) with relatively young ages (<3 Gyr), although their central cores are probably in different evolutionary stages—the central star of PN G068.7+14.8 is still in the process of heating up, while that of PN G048.5+04.2 has already entered the white dwarf cooling track. A comparison with AGB model predictions also constrains the progenitors of both PNe to be of low mass. Moreover, the two PNe are in line with the radial oxygen gradients exhibited by Galactic PN populations, using the most up-to-date distances.