photometric variations in lmc planetary nebulae

Photometric Variations inLMC Planetary Nebulae

Dick Shaw, Armin Rest,

Guillermo Damke, R. Chris SmithNational Optical Astronomy Observatory

Photo Credit, Image of the LMC: S. Points, R.C. Smith, the MCELS Team, and NOAO/AURA/NSF

Special thanks to collaborators: W. Reid & Q. Parker

21 June 2007 APN4 Conference

Context

Variability in Planetary Nebulae has been studied for many decades, and a resurgence of interest has taken place over the past several years. Photometric variability yields vital clues to the nature of the source (e.g., Bond 2000), and in particular to the question of binarity and its relevance for the origin, shaping, and evolution of the PN and its central star.

There are advantages to studying PN variability in the LMC:• It is possible to construct an unbiased, volume-complete sample • Distance uncertainties are small (~10%), and extrinsic extinction is low• The population of known PNe is large

This approach is not without its challenges, however: • CSs have mV ~ 16–26, or ~6–8 mag fainter than for a comparable Galactic

sample

• Most PNe are unresolvable from all but the best-equipped telescopes

• Crowding from field stars can be problematic


Roadmap

• The surveys• The PN samples• A sampler of the variability• The remarkable nebula RP916• Conclusions

The biggest constraint in searching for PNe with binary CSs is that it requires an enormous allocation of time and resources to derive a result: most CSs are faint, and only a small fraction have detectible photometric variability.

I will describe some preliminary results from PN photometry of LMC PNe derived from a recent time-domain survey, SuperMACHO (SM), with supporting data from a precursor survey, MACHO. The photometric technique, differential photometry (Alard & Lupton 1998), is exquisitely tuned to generate accurate light curves even for sources in very crowded fields. The results from these surveys may help resolve some long-standing problems in PN research, even as it raises new questions.


LMC Time-Domain Surveys

Two time-domain surveys of the LMC have enabled this study. They have complementary strengths.

Coverage SuperMACHO MACHO

Spatial 23 deg2 in bar 40 deg2 in bar

DIQ 0".8–2".0, 0".27 sampling2".0 (median), 0".64 sampling

Astrometry 80 mas RMS accuracy ~1000 mas (?)

Temporal2-night period, 3 consecutive dark runs, in 5 campaigns: 2001–2005

Nightly: 1992 Jul

through 2000 Jan

Photometric

VR (510–740 nm), plus reference B & I images;

mVR ~ 17.5 to 23.5

B (450–590 nm),

R (590–780 nm);

R ~ 12 to 20

SuperMACHO survey footprint


Surveys, Cont.

• Nebular emission lines in filter bandpasses– Emission lines often >1mag brighter than CS

– Dilutes the signature of stellar variability

• Inadequate cadence– Close binaries with 0.1d–7d periods (Bond

2000) may not be recognized

• Inadequate depth– Coverage to mV~27 is needed for faintest CSs

– Sometimes even faint nebulae not detected

But in some ways, neither survey is ideal for discovering PN variability.

Therefore, variability cannot be ruled out except for the brightest PNe, so the fraction of PNe classified as variable is a lower limit.

Filters used for MACHO & SuperMACHO surveys


Samples

• PNe that have been imaged with HST– Very bright, morphology is known

– CS brightness (or a limit) is known

– Crowded fields are not an issue

• Known PNe without HST images– A little fainter on average, w/very faint central stars

– Nebular morphology, CS brightness are unknown

– Field stars are a worry

• New Reid-Parker (2006) nebulae– Complete PN sample, but…

– Morphology is seldom known, CS brightness is unknown

– Sometimes even the nebulae are too faint to be detected in the SM survey

There are really three PN samples to consider:


Variability: Outbursts

• Some objects classified by RP (2006) as “true” PNe showed outbursts

Variability in LMC PNe manifests itself in a variety of ways:

Shaw et al. (2007, in prep.)


Variability: Slow Decline


• Some nebulae show a slow decline in flux, over a period of decades




Variability: Eclipsing Binaries



• Some show signatures of eclipse or occultation




Variability: Slow Variations



• Some show signatures of stellar eclipse

• Some show slow, low-level variations– Obscuration by dust cloud? (a la NGC 2346)




Variability: Irregular



• Some show signatures of stellar eclipse

• Some show slow, low-level variations– Obscuration by dust cloud? (a la NGC 2346)

• Some have irregular light curves– Under-sampled temporally?




Summary of Variability

Break-down of variability by sample…

HSTOther, Known

New RP

Total

# Objects 60 80 308 448

# Variable

Likely 12 4 13* 29

Possible 14 8* 3* 25

*Likely a lower limit

Type N

Outburst 5

Slow decline 3

Eclipse 3

Slow variations 3

Irregular 39

Totally weird 1

…and by type

The number of PNe in the LMC we find to have likely variability is comparable to the total of such PNe in the Galaxy!


The Remarkable PN: RP 916

• Classified by Reid & Parker (2006) as a “true” PN

• Extreme bipolar morphology, w/central “dust” lane

• Large physical size: ~3.7 1.2 pc; radial velocity of 277 km/s

• Pure nebular emission (i.e., no stellar continuum)

• Modest excitation: no He II, weak [O I] & [O III] 4363, modest N abundance

30”

H (blue) + R (pink); Image courtesy W. Reid

VR-band; length of arrow is 1 pc @ LMC


The Remarkable PN: RP 916

And it’s variable!

• Classified by Reid & Parker (2006) as a “true” PN

• Extreme bipolar morphology, w/central “dust” lane

• Large physical size: ~3.7 1.2 pc; radial velocity of 277 km/s

• Pure nebular emission (i.e., no stellar continuum)

• Modest excitation: no He II, weak [O I] & [O III] 4363, modest N abundance

VR-band; length of arrow is 1 pc @ LMC H (blue) + R (pink); Image courtesy W. Reid

30”


RP 916: Nebular Variability

24 Nov 2001 31 Dec 200514 Dec 2002 13 Dec 200419 Dec 2003

16 Jan 2002

Template Image

Difference Images


RP 916: Nebular Variability

24 Nov 2001 31 Dec 200514 Dec 2002 13 Dec 200419 Dec 2003

16 Jan 2002

Template Image

Difference Images

Photometry of east lobe and west lobe.

Possible Model(s):• Binary CS with precessing jet?

• Analog to He2-104?• Remnant of CE evolution

• during AGB phase of primary• secondary now filling Roche lobe

• Modest N abundance• Lack of N super-enhancement implies HBB did not occur


Conclusions

• Variability studies of PNe in the LMC are not only feasible with current-generation facilities, but are in many ways preferable

– Complete samples can be constructed and studied, unlike in the Galaxy

• Recent time-domain surveys of the LMC are extremely useful– Wide coverage: spatially, temporally, photometrically

– Exquisitely tuned to detecting variability, using difference image photometry

– Provide an excellent basis for follow-up observations

• Variable PNe in the LMC– Are now comparable in number to all known variables in the Galaxy

– The fraction of variable PNe is not less than ~6% (and probably not less than 10%)• in the period-brightness range covered in this survey

• Nebular variability can be a useful indicator of a binary progenitor– Follow-up photometric & spectroscopic surveys would be very valuable

– How common is the RP916 phenomenon??


Acknowledgements

Thanks to the APN4 Organizing Committee!

This work was made possible by the SuperMacho Collaboration:

C. Stubbs (PI), A. Becker, P. Challis, R. Covarrubias, A. Clocchiatti, K. Cook, A. Garg, M. Huber, S. Hawley, S. Keller, A. Miceli, D. Minniti, S. Nikolaev, K. Olsen, J. Prieto, G. Prochtor, A. Rest, B. Schmidt, R. C. Smith, N. Suntzeff, D. Welch

Thanks to NOAO for providing a large time allocation through the NOAO Survey program, and for supporting this research.

Thanks to the MACHO collaboration.

This paper utilizes public domain data originally obtained by the MACHO Project, whose work was performed under the joint auspices of the U.S. Department of Energy, National Nuclear Security Administration by the University of California, Lawrence Livermore National Laboratory under contract No. W-7405-Eng-48, the National Science Foundation through the Center for Particle Astrophysics of the University of California under cooperative agreement AST-8809616, and the Mount Stromlo and Siding Spring Observatory, part of the Australian National University.


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