Photometric Variations in LMC Planetary Nebulae Dick Shaw

Photometric Variations in LMC Planetary Nebulae Dick Shaw, Armin Rest, Guillermo Damke, R. Chris Smith National Optical Astronomy Observatory Special thanks to collaborators: W. Reid & Q. Parker 21 June 2007 APN 4 Conference Photo Credit, Image of the LMC: S. Points, R. C. Smith, the MCELS Team, and NOAO/AURA/NSF

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 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 m. V ~ 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 21 June 2007 APN 4 Conference

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, Super. MACHO (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. • • • 21 June 2007 The surveys The PN samples A sampler of the variability The remarkable nebula RP 916 Conclusions APN 4 Conference

LMC Time-Domain Surveys Two time-domain surveys of the LMC have enabled this study. They have complementary strengths. Coverage Spatial Super. MACHO 23 deg 2 in bar 40 deg 2 in bar DIQ 0". 8– 2". 0, 0". 27 sampling Astrometry 80 mas RMS accuracy 2". 0 (median), 0". 64 sampling ~1000 mas (? ) Temporal 2 -night period, 3 Nightly: 1992 Jul consecutive dark runs, in 5 through 2000 Jan campaigns: 2001– 2005 Photometric VR (510– 740 nm), plus reference B & I images; m. VR ~ 17. 5 to 23. 5 21 June 2007 B (450– 590 nm), R (590– 780 nm); R ~ 12 to 20 APN 4 Conference Super. MACHO survey footprint

Surveys, Cont. But in some ways, neither survey is ideal for discovering PN variability. • Nebular emission lines in filter bandpasses – Emission lines often >1 mag brighter than CS – Dilutes the signature of stellar variability • Inadequate cadence – Close binaries with 0. 1 d– 7 d periods (Bond 2000) may not be recognized • Inadequate depth – Coverage to m. V~27 is needed for faintest CSs – Sometimes even faint nebulae not detected Filters used for MACHO & Super. MACHO surveys Therefore, variability cannot be ruled out except for the brightest PNe, so the fraction of PNe classified as variable is a lower limit. 21 June 2007 APN 4 Conference

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 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 21 June 2007 APN 4 Conference

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. ) 21 June 2007 APN 4 Conference

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. ) 21 June 2007 APN 4 Conference

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. ) 21 June 2007 APN 4 Conference

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. ) 21 June 2007 APN 4 Conference

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. ) 21 June 2007 APN 4 Conference

Summary of Variability Break-down of variability by sample… HST # Objects Other, Known New RP Total 80 308 448 60 # Variable …and by type Type N Outburst 5 Slow decline 3 Eclipse 3 Likely 12 4 13* 29 Slow variations 3 Possible 14 8* 3* 25 Irregular 39 Totally weird 1 *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! 21 June 2007 APN 4 Conference

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 21 June 2007 APN 4 Conference H (blue) + R (pink); Image courtesy W. Reid

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 21 June 2007 APN 4 Conference H (blue) + R (pink); Image courtesy W. Reid

RP 916: Nebular Variability Difference Images 24 Nov 2001 14 Dec 2002 19 Dec 2003 Template Image 16 Jan 2002 21 June 2007 APN 4 Conference 13 Dec 2004 31 Dec 2005

RP 916: Nebular Variability Difference Images 24 Nov 2001 14 Dec 2002 Possible Model(s): • Binary CS with precessing jet? 19 Dec 2003 13 Dec 2004 31 Dec 2005 Template Image • Analog to He 2 -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 16 Jan 2002 Photometry of east lobe and west lobe. 21 June 2007 APN 4 Conference

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 RP 916 phenomenon? ? 21 June 2007 APN 4 Conference

Acknowledgements Thanks to the APN 4 Organizing Committee! This work was made possible by the Super. Macho 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. 21 June 2007 APN 4 Conference

A Closer Look at SMP 28 LMC-SMP 28 has been declining in flux over the past 15 years: Shaw et al. (2007, in prep. ) 21 June 2007 APN 4 Conference