Volume 531, July 2011
|Number of page(s)||5|
|Section||Stellar structure and evolution|
|Published online||06 July 2011|
Discovery of close binary central stars in the planetary nebulae NGC 6326 and NGC 6778⋆
Centre for Astrophysics Research, STRI, University of Hertfordshire, College Lane Campus, Hatfield AL10 9AB, UK
2 South African Astronomical Observatory, PO Box 9, Observatory, 7935, South Africa
3 Southern African Large Telescope Foundation, PO Box 9, Observatory, 7935, South Africa
4 European Southern Observatory, Alonso de Cordova 3107, Casilla 19001, Santiago, Chile
5 Instituto de Astrofísica de Canarias, 38200 La Laguna, Tenrife, Spain
6 Departmento de Astrofísica, Universidad de La Laguna, 38205 La Laguna, Tenerife, Spain
7 Observatorio Astronómico Nacional, Ap 112, 28803 Alcalá de Henares, Spain
8 CAB, INTA-CSIC, Ctra de Torrejón a Ajalvir, km 4, 28850 Torrejón de Ardoz, Madrid, Spain
Received: 14 April 2011
Accepted: 28 May 2011
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-1 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.
Key words: planetary nebulae: individual: PN G338.1−08.3 / planetary nebulae: individual: PN G034.5−06.7 / binaries: general / binaries: eclipsing
© ESO, 2011
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