Compact planetary nebulae in the Galactic disk: Analysis of the central stars^⋆,⋆⋆

¹ Institute of Space Sciences (IEEC-CSIC), Meteorites, Minor Bodies and Planetary Science Group, Campus UAB, Carrer de Can Magrans, s/n 08193 Cerdanyola del Vallés, Barcelona, Spain
e-mail: mmoreno@ice.csic.es
² Universidad Autónoma de Madrid, Departamento de Física Teórica, Cantoblanco, 28049 Madrid, Spain
e-mail: eva.villaver@uam.es
³ Finnish Geospatial Research Institute, Department of Geodesy and Geodynamics, Geodeetinrinne 2, 02431 Masala, Finland
⁴ National Optical Astronomy Observatory, 950 N. Cherry Avenue, Tucson, AZ 85719, USA

Received: 25 January 2016
Accepted: 15 June 2016

Abstract

Context. We have obtained multi-wavelength observations of compact Galactic planetary nebulae (PNe) to probe post-asymptotic giant branch (AGB) evolution from the onset of nebular ejection. Here we analyze new observations from HST to derive the masses and evolutionary status of their central stars (CSs).

Aims. Our objective here is to derive the masses of the CSs hosted by compact PNe in order to better understand the relationship between the CS properties and those of the surrounding nebulae. We also compare this sample with others we obtained using the same technique in different metallicity environments: the Large and Small Magellanic Clouds.

Methods. This paper is based on HST/WFC3 images of 51 targets obtained in a snapshot survey (GO–11657). The high spatial resolution of HST allows us to resolve these compact PNe and distinguish the CS emission from that of their surrounding PNe. We derive CS bolometric luminosities and effective temperatures using the Zanstra technique, from a combination of HST photometry and ground-based spectroscopic data. The targets were imaged through the filters F200LP, F350LP, and F814W from which we derive Johnson V and I magnitudes. We infer CS masses by placing the stars on a temperature-luminosity diagram and compare their location with the best available, single star post-AGB evolutionary tracks.

Results. We present new, unique photometric measurements of 50 CSs, and we derive effective temperatures and luminosities for most of them. Central star masses for 23 targets were derived with the evolutionary track technique; the remaining masses were indeterminate most likely because of underestimates of the stellar temperature, or because of substantial errors in the adopted statistical distances to these objects. We expect these problems will be largely overcome when the Gaia distance catalog becomes available. We find that objects with the higher ratios of Zanstra temperatures T(H i)/T( He ii ) tend to have lower-mass progenitors.

Conclusions. The distribution of CS masses in this sample of compact PNe is remarkably different from samples in the LMC and SMC, but with a median mass of 0.59 M_⊙ it is similar to other Galatic samples. Finally, we conclude from the typically advanced evolutionary state of the CSs on the log L, log T_eff plane that the compact nature of many of the PNe is a result of their large distance, rather than their physical dimension.