The binary fraction of planetary nebula central stars – II. A larger sample and improved technique for the infrared excess search

MNRAS 448, 3132–3155 (2015) doi:10.1093/mnras/stu2700 The binary fraction of planetary nebula central stars – II. A larger sample and improved technique for the infrared excess search ‹ Dimitri Douchin,1,2,3 Orsola De Marco,1,2 D. J. Frew,1,2 G. H. Jacoby,4 G. Jasniewicz,3 M. Fitzgerald,1,2 Jean-Claude Passy,5 D. Harmer,6 Todd Hillwig7 and Maxwell Moe8 1 Department of Physics and Astronomy, Macquarie University, Sydney, NSW 2109, Australia 2 Astronomy, Astrophysics and Astrophotonics Research Centre, Macquarie University, Sydney, NSW 2109, Australia 3 Laboratoire Univers et Particules, Université Montpellier 2, F-34095 Montpellier Cedex 5, France 5 Department of Physics and Astronomy, University of Victoria, Victoria, BC V8P5C2, Canada 6 Kitt Peak National Observatory, NOAO, PO Box 26732, Tucson, AZ 85719, USA 7 Department of Physics and Astronomy, Valparaiso University, Valparaiso, IN 46383, USA 8 Harvard–Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA Accepted 2014 December 18. Received 2014 December 17; in original form 2014 May 28 ABSTRACT There is no conclusive explanation of why ∼80 per cent of planetary nebulae (PNe) are non-spherical. In the Binary Hypothesis, a binary interaction is a preferred channel to form a non-spherical PN. A fundamental step to corroborate or disprove the Binary Hypothesis is to estimate the binary fraction of central stars of PNe (CSPNe) and compare it with a prediction based on the binary fraction of the progenitor, main-sequence population. In this paper, the second in a series, we search for spatially unresolved I- and J-band flux excess in an extended sample of 34 CSPN by a refined measurement technique with a better quantification of the uncertainties. The detection rate of I- (J-)band flux excess is 32 ± 16 per cent (50 ± 24 per cent). This result is very close to what was obtained in Paper I with a smaller sample. We account conservatively for unobserved cool companions down to brown dwarf luminosities, increasing these fractions to 40 ± 20 per cent (62 ± 30 per cent). This step is very sensitive to the adopted brightness limit of our survey. Accounting for visual companions increases the binary fraction to 46 ± 23 per cent (71 ± 34 per cent). These figures are lower than in Paper I. The error bars are better quantified, but still unacceptably large. Taken at face value, the current CSPN binary fraction is in line with the main-sequence progenitor population binary fraction. However, including white dwarfs companions could increase this fraction by as much as 13 (21) per cent points. Key words: techniques: photometric – surveys – binaries: general – stars: evolution – stars: statistics – planetary nebulae: general. 1 I N T RO D U C T I O N It is not understood yet why a high 80 per cent of planetary nebulae (PNe) are non-spherical (Parker et al. 2006). The Binary Hypothesis – the paradigm in which PNe are preferentially produced by a binary interaction (De Marco 2009) – may enable us to explain such figures. A first important step to test the Binary Hypothesis is to estimate the binary fraction of central stars of PNe (CSPNe)  E-mail: and compare it with the binary fraction of the progenitor population, the main-sequence stars. If the binary fraction of CSPNe were higher than the prediction based on the progenitor population, this would imply that PNe are indeed preferentially formed via a binary channel. The short-period, post-common-envelope binary fraction, 15– 20 per cent, was determined by two-independent photometric closebinary surveys (Bond 2000; Miszalski et al. 2009a,b). This fraction is however limited to very short periods. Estimating the fraction of CSPN that are in binaries with any separation requires an efficient method for detecting binaries, a reasonable sample size and a clear understanding of the intrinsic biases of the method and sample. Our  C 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 4 Giant Magellan Telescope and Carnegie Observatories, Pasadena, CA 91101, USA The binary fraction of planetary nebula central stars – II. 2 O B S E RVAT I O N S A N D DATA R E D U C T I O N Our Johnson–Cousins U, B, V and I images were taken during a seven night observing run at the National Optical Astronomical Observatory (NOAO) 2.1-m telescope at Kitt Peak between 2011 March 11 and 17. Only nights 1, 4 and 6 were partially photometric and the results we present here derive from these photometric data only. During nights 2, 3, 5 and 7 and non-photometric parts of nights 1, 4 and 6 we carried out photometric monitoring of those targets that will be presented in a later paper. We used the optical camera T2KB, with 2048 × 2048 pixels yielding a field of view of 6.5 arcmin × 10 arcmin on the sky after cropping (we used T2KB default sampling of 0.3 arcsec pixel−1 ). The pixel size is 24 µm with a typical readout noise of 4 electrons rms. We used a gain of 1.04 electrons ADU−1 with a pixel saturation of 65 000 ADU. The images have been reduced using the standard ccdproc procedure provided within the software IRAF1 allowing debiasing, overscanning and flat-fielding. A total of 10 bias frames have been taken at the beginning of each night as well as 10 dome-flat images in each filter at the beginning and end of each night. The dark current noise of the T2KB CCD, < 4 electrons h−1 pixel−1 , is negligible in comparison with the other sources of noise expected in our data; therefore, no dark-frame subtraction has been used in the reduction process, although dark images have been acquired for precaution in the morning after each observation. The logs of the photometric observations are provided in Appendix A. 2.1 Target selection As in Paper I, the target list is drawn from the volume-limited sample of Frew (2008) updated by Frew (in preparation). The distances have been determined thanks to an improved Hα surface brightness– radius relation (Frew, Bojičić & Parker 2013), and yields a precision of ∼20 per cent on average. In addition, as in Paper I we have mostly observed CSPN with old, extended (more than 25 arcsec), faint PN around them, while avoiding compact, dense PN for which it is difficult to achieve accurate background subtraction. We have also chosen our targets to have an absolute V magnitude, MV  5, if possible, both to avoid wind-induced variability in intrinsically bright CSPN and to enable the detection of intrinsically faint companions. Objects within ∼10 deg of the Galactic plane have been largely excluded to avoid crowded fields, limiting the possibility of a field star aligning with our targets. When a target was close to the Galactic plane, the field was inspected to insure that the crowding was a minimum. To attempt an unbiased sample, we did not inspect the names of our targets until we had a target list answering to the selection criteria. This means that occasionally a known binary is included in our list. Such inclusi (...truncated)