Transcript Photometric Variations in LMC Planetary Nebulae Dick Shaw, Armin Rest,

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
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 are advantages to studying PN variability in the LMC:
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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:
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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
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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.
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The surveys
The PN samples
A sampler of the variability
The remarkable nebula RP916
Conclusions
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LMC Time-Domain Surveys
Two time-domain surveys of the LMC have enabled
this study. They have complementary strengths.
Coverage
SuperMACHO
MACHO
23 deg2 in bar
Spatial
40 deg2 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
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
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SuperMACHO survey footprint
Surveys, Cont.
But in some ways, neither survey is ideal for discovering PN variability.
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Nebular emission lines in filter bandpasses
– Emission lines often >1mag brighter than CS
– Dilutes the signature of stellar variability
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Inadequate cadence
– Close binaries with 0.1d–7d periods (Bond
2000) may not be recognized
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Inadequate depth
– Coverage to mV~27 is needed for faintest CSs
– Sometimes even faint nebulae not detected
Filters used for MACHO & SuperMACHO surveys
Therefore, variability cannot be ruled out except for the brightest PNe, so
the fraction of PNe classified as variable is a lower limit.
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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
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Variability: Outbursts
Variability in LMC PNe manifests itself
in a variety of ways:
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Some objects classified by RP (2006)
as “true” PNe showed outbursts
Shaw et al. (2007, in prep.)
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Variability: Slow Decline
Variability in LMC PNe manifests itself
in a variety of ways:
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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.)
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Variability: Eclipsing Binaries
Variability in LMC PNe manifests itself
in a variety of ways:
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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.)
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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
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Obscuration by dust cloud? (a la NGC 2346)
Shaw et al. (2007, in prep.)
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Variability: Irregular
Variability in LMC PNe manifests itself
in a variety of ways:
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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
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•
Obscuration by dust cloud? (a la NGC 2346)
Some have irregular light curves
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Under-sampled temporally?
Shaw et al. (2007, in prep.)
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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!
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The Remarkable PN: RP 916
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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
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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
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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
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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 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
16 Jan 2002
Photometry of east lobe and west lobe.
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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??
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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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A Closer Look at SMP28
LMC-SMP28 has been declining in flux over the past 15 years:
Shaw et al. (2007, in prep.)
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