Discovery of close binary central stars in the planetary nebulae NGC 6326 and NGC 6778

TL;DR

This paper confirms the close binary nature of central stars in two planetary nebulae through photometric and spectroscopic observations, providing evidence of their common-envelope evolution and linking nebular traits to binary history.

Contribution

It presents the first spectroscopic confirmation of NGC 6326's binary central star and highlights the effectiveness of using weak emission lines to identify post-common-envelope nebulae.

Findings

NGC 6326 and NGC 6778 have close binary central stars with periods of 0.372 and 0.1534 days.

Spectroscopic radial velocity shifts in emission lines confirm binary nature.

Traits like low-ionisation filaments and collimated outflows are linked to post-CE evolution.

Abstract

We present observations proving the close binary nature of the central stars belonging to the planetary nebulae (PNe) NGC 6326 and NGC 6778. Photometric monitoring reveals irradiated lightcurves with orbital periods of 0.372 and 0.1534 days, respectively, constituting firm evidence that they passed through a common-envelope (CE) phase. Unlike most surveys for close binary central stars (CSPN) however, the binary nature of NGC 6326 was first revealed spectroscopically and only later did photometry obtain an orbital period. Gemini South observations revealed a large 160 km/s shift between the nebula and emission lines of C III and N III well known to originate from irradiated atmospheres of main-sequence companions. These so-called weak emission lines are fairly common in PNe and measurement of their radial velocity shifts in spectroscopic surveys could facilitate the construction of a statistically significant sample of post-CE nebulae. There is growing evidence that this process can be further accelerated by pre-selecting nebulae with traits of known post-CE nebulae. Both NGC 6326 and NGC 6778 were selected for their rich attribution of low-ionisation filaments and collimated outflows, thereby strengthening the connection between these traits and post-CE CSPN.