The binary fraction of planetary nebula central stars – I. A high-precision, I-band excess search

Abstract

We still do not know what causes aspherical planetary nebula morphologies. A plausible hypothesis is that they are due to the presence of a close stellar or substellar companion. So far, only ∼40 binary central stars of planetary nebula have been detected, almost all of them with such short periods that their binarity is revealed by photometric variability. Here we have endeavoured to discover binary central stars at any separation, thus determining the unbiased binary fraction of central stars of planetary nebula. This number, when compared to the binary fraction of the presumed parent population can give a first handle on the origin of planetary nebulae. By detecting the central stars in the I band we have searched for cool companions. We have found that 30% of our sample have an I band excess detected between one and a few σ, possibly denoting companions brighter than M3-4V and with separations smaller than ∼1000 AU. By accounting for the undetectable companions, we determine a de-biased binary fraction of 67-78% for all companions at all separations. We compare this number to a main sequence binary fraction of (50±4)% determined for spectral types F6V-G2V, appropriate if the progenitors of today's PN central star population is indeed the F6V-G2V stars. The error on our estimate cannot be constrained tightly, but we determine it to be between 10 and 30%. We conclude that the central star binary fraction may be larger than expected from the putative parent population. However, this result is based on a sample of 27 bona fide central stars and should be considered preliminary. The success of the I band method rests critically on high precision photometry and a reasonably large sample. From a similar analysis, using the more sensitive J band of a subset of 11 central stars, the binary fraction is 54% for companions brighter than ∼M5-6V and with separations smaller than about 900 AU. De-biassing this number in the same way as was done for the I band we obtain a binary fraction of 100-107%. The two numbers should be the same and the discrepancy is likely due to small number statistics. Finally, we note how the previously-derived short period PN binary fraction of 15-20% is far larger than expected based on the main sequence binary fraction and period distribution.As a byproduct of our analysis we present an accurately vetted compilation of observed main sequence star magnitudes, colours and masses, which can serve as a reference for future studies. We also present synthetic colours of hot stars as a function of temperature (20-170kK) and gravity (log g = 6 − 8) for Solar and PG1159 compositions.

Dusty Disks Around Central Stars of Planetary Nebulae

“…As a mimic, this object is similar to EGB 5 and PHL 932 (Geier et al 2011, neither of which is known to have a disk). De Marco et al (2013) detected a 4σ J-band excess for the central star of DeHt 5, consistent with an M5V companion. Bilíková et al (2012) detected point sources in 2MASS, IRAC, and MIPS bands with a clear excess at 8 and 24 μm.…”

“…This is the Ciardullo et al 1999;Liebert et al 2013. d No companion brighter than mid-to-late M (Su et al 2007). e I-and J-band excess binary candidates (De Marco et al 2013). f Close binary (Mendez & Niemela 1981).…”

“…NGC 246, EGB 6, and K 1-22 are wide binaries (where EGB 6 has a known disk around the companion; Liebert et al 2013;Ciardullo et al 1999); NGC 2438 has a detected infrared excess consistent with an M3 V companion (although it could also be due to a warm disk Bilíková et al 2012). DeHt 5, EGB 1, and NGC 6853 have J-band flux excesses consistent with companions with spectral type M5 V (De Marco et al 2013), Sh 2-188 has a low-σ detection of an I-band excess consistent with a companion of spectral type M4 V (De Marco et al 2013). NGC 2346 is a close, single-lined spectroscopic binary with an A5 V companion to a hot star (Mendez & Niemela 1981;Smalley 1997).…”

The H surface brightness-radius relation: a robust statistical distance indicator for planetary nebulae

“…Longmore & Tritton 1980). Absolute magnitudes have been taken from De Marco et al (2013) for main sequence stars, and Schmidt-Kaler (1982) or Jaschek & Gómez (1998) for the evolved stars.…”

“…The binary associations evaluated by CB99 have been reevaluated here using better estimates for the reddening, derived both from unpublished spectroscopic data and from all available CSPN photometry (see F08; De Marco et al 2013). Furthermore, none of CB99's possible or doubtful associations have been considered (cf.…”