Compact planetary nebulae MaC 2-1 and Sp 4-1: Photoionization models and dust characteristics

We study the characteristics of the planetary nebulae (PNe) MaC 2-1 and Sp 4-1. We use our optical spectra taken with the 2 m Himalayan Chandra Telescope, Spitzer mid-infrared (mid-IR) spectra, HST images, and IR photometric data. These PNe have not previously been individually studied in detail. Both are in the low-to moderate-excitation class. MaC 2-1 shows the presence of silicon carbide (SiC) and magnesium sulphide (MgS) dust. Sp 4-1 hosts polycyclic aromatic hydrocarbon (PAH) molecules. We obtain the plasma properties of the PNe from the optical and mid-IR emission-line fluxes. We compute photoionization models of the PNe for the self-consistent estimation of physical parameters associated with the central star and the nebula, including nebular abundances. From the modelling of the IR data, we obtain the characteristics of dust and molecules formed in the nebulae. From our study, we estimate that the progenitors of MaC 2-1 and Sp 4-1 had masses of 1.2 and 1.55 M·, respectively, and both of them seem to have been born in a metal-poor environment. Both are distant PNe, with estimated distances of 16 and 18 kpc for MaC 2-1 and Sp 4-1, respectively.