2005 June 29 Planetary Nebulae as Astronomical Tools

Spectroscopic Binaries in Planetary Nebulae

Howard E. Bond

Space Telescope Science Institute

Three Arguments that Many PNe are Ejected from Binary Stars

1. Large majority of PNe have highly non-spherical or bipolar shapes

– Simplest explanation: PN ejection through common-envelope (CE) interactions,

– or at least the ejection process is strongly influenced by companions (tidal spin-up, dynamo generation of magnetic field)

Three Arguments…

2. An observed high incidence of very close binary PNNi

– Photometric monitoring shows that ~10% of PNNi are binaries with periods of a few hours to a few days (Bond & Grauer 1980’s; Bond & Livio 1990; Bond 2000)

– These close systems must have emerged from common-envelope interactions

Common-envelope Interactions• Occur in binaries initially wide enough for

one component to become RG or AGB star before interacting with MS companion

• Companion is engulfed, spirals in, and may eject the envelope, leaving a much closer binary: MS star + hot core.

• Hot core can ionize ejected envelope, producing a PN around the close binary

Common-envelope Interactions• Final orbital period depends on efficiency

with which orbital energy goes into ejecting material from the system…

• …denoted CE

– High CE results in long final period

– Low CE results in short period, or merger

• More discussion will be in following talk (De Marco)

Three Arguments…

3. The 10% of very close pairs may just be the short-period tail of a much larger fraction of binary PNNi

– Population-synthesis studies suggest this is true for a wide range of CE values

– e.g., Yungelson et al. (1993), next slide

Yungelson et al. (1993) Population Synthesis Predicted orbital period distribution of binary PNNi

Yungelson et al. (1993) Population Synthesis Predicted orbital period distribution of binary PNNi

Post-CE systems

Yungelson et al. (1993) Population Synthesis Predicted orbital period distribution of binary PNNi

Post-CE systems

Gap due to binaries moving to shorter P

Yungelson et al. (1993) Population Synthesis Predicted orbital period distribution of binary PNNi

Wide binaries that never interact

Post-CE systems

Gap due to binaries moving to shorter P

Yungelson et al. (1993) Population Synthesis Predicted orbital period distribution of binary PNNi

Wide binaries that never interact

Post-CE systems

A digression on the next slide

Gap due to binaries moving to shorter P

A digression…

• Ciardullo, Bond, et al. (1999 AJ 118, 488) carried out an HST “snapshot” survey for the expected population of visual binaries in PNe

• We found 10 likely and 6 possible resolved binaries, out of 113 examined.

These wide companions are useful for deriving distances to the PNe (MS fitting), but are unlikely to affect PN ejection

HST WFPC2 images

Outcome of CE interaction depends on CE

High CE long periods Low CE short periods & mergers

Yungelson et al. 1993

…but only short-period systems can be found photometrically

Systems that can be found photometrically

The photometric search method

depends on heating effects in

close binaries, so…

Artist: Dana Berry

…the short-period systems

could just be the tip of an iceberg of longer-period

binaries…

…the short-period systems

could just be the tip of an iceberg of longer-period

binaries…

Short periods

LONG PERIODS

…and if so, the total fraction of binary central stars is very high,

and PNe are fundamentally a binary-star phenomenon!

Testing the “iceberg” hypothesis

• Requires radial-velocity (RV) measurements, in order to find wider binaries that lack heating effects

• Results to be reported here—– De Marco, Bond, Harmer, & Fleming: WIYN

3.5m telescope at Kitt Peak– Afsar & Bond: SMARTS 1.5m telescope at

Cerro Tololo

WIYN 3.5-m Program

• Hydra spectrograph, 2002-04.

• Dispersion 0.33 A/pix; resolution ~7500

• RV precision ~3-3.7 km/s

• Scheduling optimized for periods of days to months

• Results of 2002-03 observations were reported by De Marco, Bond, Harmer, & Fleming (ApJ 602,L93,2004)

RV variations are often unequivocal:

RV = 33 km/s in 21 days RV = 27 km/s in 21 days

WIYN result: 10 out of 11 PNNi have variable RVs!

Probability RVis variable

SMARTS 1.5-m Program

• Cassegrain spectrograph, 2003-04.

• Dispersion 0.77 A/pix; resolution ~2000

• RV precision ~10 km/s

• Scheduling (as with WIYN) optimized for periods of days to months

• Part of PhD thesis of Melike Afsar, Ege University (Turkey) & STScI

SMARTS: again many RV variables

P > 0.99

SMARTS: again many RV variablesFound var. in WIYN pgm.

RV survey results

• WIYN (~3.5 km/s): 10 out of 11 PNNi variable

• SMARTS ( ~10 km/s): 7 out of 19

• Sorensen & Pollacco (Asymm PNe III, 2003) ( ~5 km/s?) found 13 out of 33 PNNi have variable RV’s (incl. NGC 6891, also found by WIYN & SMARTS)

Caveats• Measurements are difficult in some PNNi

due to few suitable absorption lines (free of nebular contamination, etc) and low RV amplitudes

• Wind variations could produce line-profile variations that mimic RV variability– But we tried to select against PNNi with strong

UV P Cygni profiles

Searching for periods• Finding orbital periods would strengthen

the case for binarity, but we have not been able to fit a binary period to any of our objects…

…with one possible exception

f(m) = 0.006, implying m2>0.13Msun if m1=0.6Msun

Searching for periods• Finding orbital periods would strengthen

the case for binarity, but we have not been able to fit a binary period to any of our objects…with the possible exception of IC 4593…

• But our observations cover 2-3 years sparsely, which is highly non-optimum for finding periods that now appear to be short (few days)

What next?

• We need an intensive campaign on a few objects with a large telescope, high resolution, and high S/N…

• …in order to distinguish between binary orbital motion and wind-profile variations

• Bond & De Marco had successful 5-night run in May 2005 with Kitt Peak 4m echelle spectrograph.– Concentrated on IC 4593, BD+33 2642, LS IV-12 111,

NGC 6210

• Stay tuned for results!

Other implications of binaries in PNe

• May explain existence of PNe in globular clusters– Post-AGB remnants of low-mass stars evolve

too slowly to produce ionized PNe– But binaries can merge or transfer matter before

the PN stage, producing higher-mass remnants

• PNe at the bright end of the PNLF may be descended from binaries (Ciardullo poster)

The PN in M15 (HST)

Other implications of binaries in PNe, contd.

• Most of the classes of compact binaries are probably descended from binary PNNi via common envelopes:– Pre-cataclysmic red-dwarf/white-dwarf binaries

like V471 Tau– Cataclysmic variables– Low-mass X-ray binaries– SN Ia progenitors

Other implications of binaries in PNe, contd.

• Knowing the overall orbital period distribution of PNNi would help constrain the typical value of CE, which is needed in population-synthesis calculations.

Summary• ~10% of PNNi are very short-period

binaries (hours to a few days) that must have been ejected from CE interactions

• Resolved visual binary PNNi occur about as often as expected

• RV observations are now suggesting that a large population of spectroscopic binaries exists among PNNi, making the total binary fraction very high

Summary contd.• However, additional spectroscopic

observations with large telescopes are needed to verify the suspected high spectroscopic binary fraction

• At present, it appears very plausible that binary-star ejection is a major formation channel for planetary nebulae

Thanks to collaborators!• Orsola De Marco• Di Harmer• Andrew Fleming• Melike Afsar• Robin Ciardullo• Al Grauer• Telescope operators, funding agencies,

SMARTS Consortium, STScI DDRF!

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