Photometric Variations in LMC Planetary Nebulae

1. Photometric Variations inLMC Planetary Nebulae Dick Shaw, Armin Rest, Guillermo Damke, R. Chris Smith National Optical Astronomy Observatory Special thanks to collaborators: W. Reid & Q. Parker Photo Credit, Image of the LMC: S. Points, R.C. Smith, the MCELS Team, and NOAO/AURA/NSF

2. 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 APN4 Conference

3. Roadmap 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. • The surveys • The PN samples • A sampler of the variability • The remarkable nebula RP916 • Conclusions APN4 Conference

4. LMC Time-Domain Surveys SuperMACHO survey footprint Two time-domain surveys of the LMC have enabled this study. They have complementary strengths. APN4 Conference

5. Surveys, Cont. Filters used for MACHO & SuperMACHO surveys But in some ways, neither survey is ideal for discovering PN variability. • 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 Therefore, variability cannot be ruled out except for the brightest PNe, so the fraction of PNe classified as variable is a lower limit. APN4 Conference

6. Samples There are really three PN samples to consider: • 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 withoutHST 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 APN4 Conference

7. Variability: Outbursts Variability in LMC PNe manifests itself in a variety of ways: • Some objects classified by RP (2006) as “true” PNe showed outbursts Shaw et al. (2007, in prep.) APN4 Conference

8. Variability: Slow Decline Variability in LMC PNe manifests itself in a variety of ways: • Some objects classified by RP (2006) as “true” PNe showed outbursts • Some nebulae show a slow decline in flux, over a period of decades Shaw et al. (2007, in prep.) APN4 Conference

9. Variability: Eclipsing Binaries Variability in LMC PNe manifests itself in a variety of ways: • Some objects classified by RP (2006) as “true” PNe showed outbursts • Some nebulae show a slow decline in flux, over a period of decades • Some show signatures of eclipse or occultation Shaw et al. (2007, in prep.) APN4 Conference

10. Variability: Slow Variations Variability in LMC PNe manifests itself in a variety of ways: • Some objects classified by RP (2006) as “true” PNe showed outbursts • Some nebulae show a slow decline in flux, over a period of decades • Some show signatures of stellar eclipse • Some show slow, low-level variations • Obscuration by dust cloud? (a la NGC 2346) Shaw et al. (2007, in prep.) APN4 Conference

11. Variability: Irregular Variability in LMC PNe manifests itself in a variety of ways: • Some objects classified by RP (2006) as “true” PNe showed outbursts • Some nebulae show a slow decline in flux, over a period of decades • 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? Shaw et al. (2007, in prep.) APN4 Conference

12. Summary of Variability Break-down of variability by sample… …and by type *Likely a lower limit The number of PNe in the LMC we find to have likely variability is comparable to the total of such PNe in the Galaxy! APN4 Conference

13. 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” VR-band; length of arrow is 1 pc @ LMC H (blue) + R (pink); Image courtesy W. Reid APN4 Conference

14. 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 And it’s variable! 30” VR-band; length of arrow is 1 pc @ LMC H (blue) + R (pink); Image courtesy W. Reid APN4 Conference

15. RP 916: Nebular Variability 24 Nov 2001 14 Dec 2002 19 Dec 2003 13 Dec 2004 31 Dec 2005 Template Image 16 Jan 2002 Difference Images APN4 Conference

16. RP 916: Nebular Variability 24 Nov 2001 14 Dec 2002 19 Dec 2003 13 Dec 2004 31 Dec 2005 Template Image 16 Jan 2002 Difference Images • 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 Photometry of east lobe and west lobe. APN4 Conference

17. 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?? APN4 Conference

18. 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. APN4 Conference

19. A Closer Look at SMP28 LMC-SMP28 has been declining in flux over the past 15 years: Shaw et al. (2007, in prep.) APN4 Conference