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 1626, or 68 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

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

4
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".82".0, 0".27 sampling 2".0 (median), 0".64 sampling
Astrometry 80 mas RMS accuracy 1000 mas (?)
Temporal 2-night period, 3 consecutive dark runs, in 5 campaigns 20012005 Nightly 1992 Jul through 2000 Jan
Photometric VR (510740 nm), plus reference B I images mVR 17.5 to 23.5 B (450590 nm), R (590780 nm) R 12 to 20
5
Surveys, Cont.
But in some ways, neither survey is ideal for
discovering PN variability.

• Nebular emission lines in filter bandpasses
• Emission lines often gt1mag brighter than CS
• Dilutes the signature of stellar variability
• Inadequate cadence
• Close binaries with 0.1d7d periods (Bond 2000)
  may not be recognized
• Inadequate depth
• Coverage to mV27 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.
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 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

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.)
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.)
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.)
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.)
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.)
12
Summary of Variability
Break-down of variability by sample
and by type
HST Other, Known New RP Total
Objects 60 80 308 448
Variable
Likely 12 4 13 29
Possible 14 8 3 25
Type N
Outburst 5
Slow decline 3
Eclipse 3
Slow variations 3
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!
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
H? (blue) R (pink) Image courtesy W. Reid
VR-band length of arrow is 1 pc _at_ LMC
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 its variable!
30
VR-band length of arrow is 1 pc _at_ LMC
H? (blue) R (pink) Image courtesy W. Reid
15
RP 916 Nebular Variability
Difference Images
16
RP 916 Nebular Variability
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.
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??

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