A revised distance scale of planetary nebulae

Abstract

Context. The advent of large-scale, high-sensitivity surveys of the Galaxy (2MASS, WISE, Spitzer, etc.) has allowed detection of a large number of planetary nebulae (PNe), some of them at large distances. Aims. I propose a new distance scale of planetary nebulae, based on two main quantities: the flux density (or magnitude) at 8 μm and the nebular apparent size. Methods. I used these quantities to calculate the specific intensity of the radiation I8 μm, which decreases as the nebula evolves. The PNe situated in the Large Magellanic Cloud (LMC) with 8 μm photometry obtained by Spitzer and diameter obtained from HST observations were used as calibrators. Results. After calibration, I recalculated distances of PNe in the LMC, and they show a one-sigma spread of 10 kpc (20%). Eventually, I applied the method to a sample of 47 Galactic PNe observed during the Spitzer survey. These results showed agreement within 0.2 dex with the distances determined with another method of calibrating with PNe in the LMC. Conclusions. The main advantage of the present distance scale is that it is nearly “extinction free”, which makes it especially suited to PNe severely affected by interstellar extinction, such as the Galactic disc and bulge. On the other hand, the method requires an unbiased determination of the nebular size, which might be quite uncertain for PNe with extended faint features, such as haloes and ansae.